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The Pennsylvania State University
The Graduate School
College of Engineering
EXPLORING NOVEL GLASS MICROFABRICATION TECHNIQUES
FOR SENSOR APPLICATIONS
A Dissertation in
Electrical Engineering
by
Chenchen Zhang
2017 Chenchen Zhang
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy
December 2017
The dissertaion of Chenchen Zhang was reviewed and approved* by the following:
Srinivas Tadigadapa
Professor of Electrical Engineering and Biomedical Engineering
Dissertation Advisor
Chair of Committee
Zhiwen Liu
Professor of Electrical Engineering
Weihua Guan
Assistant Professor of Electrical Engineering
Saptarshi Das
Assistant Professor of Engineering Science and Mechanics
Kultegin Aydin
Professor of Electrical Engineering
Head of the Department of Electrical Engineering
*Signatures are on file in the Graduate School
iii
ABSTRACT
This work presents the exploration of glass microfabrication techniques for fabricating
novel chip-scale glass based transducers. Inexpensive and readily available, glass materials possess
exceptional properties that include excellent electrical insulation, broad optical transparency, and
biocompatibility. Glass substrates are highly in demand in Microelectromechanical systems
(MEMS) but their use is not as widespread due to the limited availability of microfabrication
processes. The focus of this dissertation is to develop glass microfabrication processes and their
applications for MEMS sensors development.
Plasma etching processes on three compositions of glass substrates are explored using a
modified inductively couple plasma reactive ion etching (ICP-RIE) system for high etch-rate, high
aspect ratio, smooth etching performance, and understanding the fundamental plasma glass etching
mechanism. Using SF6 as the plasma source gas and NF3 and H2O gases introduced downstream
near the surface of the wafer through a diffuser gas inlet, etch rates as high as 1.06 μm/min, 1.04
μm/min, and 0.45 μm/min with surface smoothness of ~2 Å, ~67 Å, ~4 Å are achieved for fused
silica, borosilicate glass, and aluminosilicate glasses respectively after 5 minutes etches. High
aspect ratio etch of 5.2:1, 10:1 and 2:1 are obtained for fused silica, borosilicate glass, and
aluminosilicate glass respectively. Glass etching mechanism is further understood by analyzing the
etch rates and corresponding partial pressure of plasma species detected by in-situ residual gas
analyzer (RGA) with various position of the diffuser gas inlet. Statistical analysis indicates etch
rate is critically influenced by ion flux. Fluorine based radicals and molecular fragments influence
both the etch rate and surface smoothness of fused silica whereas they primarily influence the
surface smoothness for borosilicate glass. The large fraction of impurity atoms of Ca and Al in
aluminosilicate glass form non-volatile fluorides on the etch surface and therefore the etch rate and
surface smoothness of aluminosilicate glass is primarily influenced ion flux and very little by the
iv
fluorine chemistry. We also examine the role of the layout of the metal mask layer on how it
influences the charging of glass substrates during etching and therefore the etch rate.
In the second half of the thesis, chip scale glass blowing technique is explored for novel
sensing and packaging applications. Arrays of on-chip spherical glass shells of hundreds of
micrometers in diameter with ultra-smooth surfaces and sub-micrometer wall thicknesses have
been fabricated and have been shown to sustain optical resonance modes with high Q-factors of
greater than 50 million. The resonators exhibit temperature sensitivity of -1.8 GHz K-1 and can be
configured as ultra-high sensitivity thermal sensors for a broad range of applications. By virtue of
the geometry's strong light-matter interaction, the inner surface provides an excellent on-chip
sensing platform that truly opens up the possibility for reproducible, chip scale, ultra-high
sensitivity microfluidic sensor arrays. As a proof of concept we demonstrate the sensitivity of the
resonance frequency as water is filled inside the microspherical shell and is allowed to evaporate.
By COMSOL modeling, the dependence of this interaction on glass shell thickness is elucidated
and the experimental results of the sensitivity of two different shell thicknesses is explained.
In the last chapter, chip-scale blown, glass microbubbles are explored for encapsulation of
ferrofluid atop a micromachined quartz resonator configured as a magnetometer. The concept of a
ferrofluid based magnetometer has been previously reported where the viscoelastic response of a
thin interfacial ferrofluid layer loaded atop a high frequency shear wave quartz resonator to applied
magnetic field is monitored. The magnetic field can be sensitively quantified by the changes in the
at-resonance admittance characteristics of the resonator. However, under open conditions,
continuous evaporation of the ferrofluid compromises the long term performance of the
magnetometer. In this work, we integrate glass hemispherical microbubbles, used as vessels of
ferrofluid, on the resonator chip to seal and prevent the evaporation of the ferrofluid liquid and
drying out. Using these improvements, a minimum detectable field of 600 nT at 0.5 Hz is achieved.
v
Moreover, comparing with the unsealed ferrofluid device, the lifetime of the glass microbubble
integrated chip packaged device improved significantly from only few hours to over fifty days and
continuing.
vi
TABLE OF CONTENTS
List of Figures .......................................................................................................................... viii
List of Tables ........................................................................................................................... xiv
Acknowledgements .................................................................................................................. xv
Chapter 1 Introduction ............................................................................................................. 1
1.1 Application overview of glass materials .................................................................... 1 1.2 Motivation of exploring glass micro-fabrication techniques ..................................... 9
Chapter 2 Inductive Coupled Plasma – Reactive Ion Etching (ICP-RIE) of Fused Silica
Substrate ........................................................................................................................... 14
2.1 Plasma Etching of Glass Materials: Background ....................................................... 14 2.2 Sample Preparation .................................................................................................... 17 2.3 Conventional ICP-RIE Glass Etching ........................................................................ 18 2.4 Modified ICP-RIE Chamber for Fused Silica Glass Etching ..................................... 22
2.4.1 Chamber Modification 1: Diffuser Ring System ............................................ 23 2.4.2 Chamber Modification 2: Diffuser Tube System ............................................ 34
2.5 High Aspect Ratio Etching of Fused Silica ................................................................ 37 2.6 Skin Layer Formation ................................................................................................ 38 2.7 High Aspect Ratio Etching of Fused Silica ................................................................ 39 2.8 Summary .................................................................................................................... 40
Chapter 3 Inductive Coupled Plasma – Reactive Ion Etching (ICP-RIE) of Borosilicate
and Aluminosilicate Substrates ........................................................................................ 43
3.1 Introduction ................................................................................................................ 43 3.2 Experimental Setup .................................................................................................... 45
3.2.1 Chamber Modification .................................................................................... 45 3.2.2 Sample Preparation ......................................................................................... 46
3.3 Experimental Results & Discussion ........................................................................... 47 3.3.1 Conventional SF6 ICP Etch of Fused Silica and Borosilicate glass ................ 47 3.3.2 Modified ICP-RIE for Various Glass Composition Substrate: Diffuser-
ring Set-up ........................................................................................................ 49 3.3.3 Modified ICP-RIE for Various Glass Composition Substrate: Diffuser-
tube Set-up ........................................................................................................ 53 3.3.4 High Aspect Ratio Glass Etching with Diffuser Gas Inlet .............................. 56 3.3.5 Loading Effect and Charging Effect ............................................................... 63
3.4 Summary .................................................................................................................... 65
vii
Chapter 4 Glass Micro-spherical Shell Based Whispering Gallery Mode (WGM)
Resonator Sensing Platform ............................................................................................. 67
4.1 Introduction of WGM optical resonance and background of WGM based
resonator ................................................................................................................... 67 4.2 Motivation of proposing on-chip glass micro-spherical shell supported WGMs ...... 69 4.3 Fabrication process development roadmap ................................................................ 71 4.4 COMSOL modelling of micro-spherical shell supported WGMs.............................. 77 4.5 Experimental Setup and WGM Resonances .............................................................. 83 4.6 Thermal sensing: experimental results and modelling discussion ............................. 87 4.7 Liquid core sensing: experimental results and modelling discussion ........................ 90 4.8 Additional Preliminary Results .................................................................................. 93
4.8.1 Integrated Microfluidic Devices using Microspherical Shell Optical
Resonators ........................................................................................................ 93 4.8.2 Resonance in serially coupled Optical Resonators .......................................... 94
4.9 Summary .................................................................................................................... 95
Chapter 5 Glass Microbubble Packaged Ferrofluids – Microfabricated Quartz Resonator
Based Magentoviscous Magnetometer ............................................................................ 97
5.1 Introduction and motivation ....................................................................................... 97 5.2 Device Fabrication and Experiment Set-up ............................................................... 101
5.2.1 Quartz Resonator Chip .................................................................................... 101 5.2.2 Glass Microbubble Chip .................................................................................. 102 5.2.3 Ferrofluids Packaging ..................................................................................... 103 5.2.4 Experiment Set-up ........................................................................................... 104
5.3 Results and Discussion ............................................................................................... 105 5.3.1 Characterization of quartz resonator ............................................................... 105 5.3.2 Responds of magnetic field ............................................................................. 107 5.3.3 Lifetime of packaged device ........................................................................... 111
5.4 Summary .................................................................................................................... 112
Chapter 6 Summary and Future Work ..................................................................................... 114
REFERENCE ........................................................................................................................... 118
Appendix NF3 and H2O mass flow controller ................................................................. 128
viii
LIST OF FIGURES
Figure 1-1. Applications of glass in various fields. The center image shows the famous
glass made construction: Louvre Pyramid. The top image shows the application of
glass in solar cell substrate as transparent roof. The left image shows the large scale
floor glass window. The right image shows the glass panel based conceptual
automotive interior. The bottom three images show the applications of glass as
glassware, glass lenses and glass fibers in scientific research. Courtesy: Images are
from google images. ......................................................................................................... 2
Figure 1-2. Glass substrate overall market size in wspy in breakdown per technical
functionalities within 6 years. Courtesy: Image is cited from Yole development
website. ............................................................................................................................. 3
Figure 1-3. 3-D cutaway drawing of a typical CMOS active sensor pixel. Courtesy:
Image is from The Molecule Expressions program website. ........................................... 4
Figure 1-4. Glass wafer with TGVs structures. (b) Glass wafer with micro-holes
structures. (c) Glass wafer with cap structures. (d) Illustration of wafer level
packaging process with glass wafer. Courtesy: images are from Tecnisco, LTD
website. ............................................................................................................................. 5
Figure 1-5. Schematic of the crystal structure of crystalline quartz showing the a- and c-
planes and axes. Taken from http://www.quartzpage.de/gen_struct.html. ...................... 7
Figure 1-6. Sketch of a conventional ICP-RIE etcher showing gas inlet, ICP coils, RF
substrate, magnetic pieces and substrate wafer. ............................................................... 13
Figure 2-1. Schematic process flow for the preparation of glass substrates used in the etch
tests. 15 nm of Cr and 150 nm of Au seed layer were deposited prior to plating 2 - 3
µm of nickel which acts as hard mask in these tests. ....................................................... 18
Figure 2-2. (a) Experimentally obtained etch rate and rms roughness values for various
SF6 flow rates. Since the pumping speed remains constant for these conditions,
increasing flow rate corresponds to increasing pressure in the chamber. Error bars
were obtained by measuring etch rate for several 100 µm features across a single 4”
silica wafer Etch time = 20 min. (b) Residual gas analyzer data for 100 sccm SF6
flow clearly shows the presence of atomic fluorine as well large amounts of SFx
species. Inset shows the sum of the partial pressures of F and SF5+ concentrations for
various SF6 flow rates and the corresponding etch rates of fused silica. ......................... 22
Figure 2-3. Schematic illustration of the modified ICP-RIE systems with ring-diffuser
system connected to NF3 and H2O gas cylinders through the gas panel. ......................... 24
Figure 2-4. (a) Optical photo showing the top view of the machined shower head fitted to
the substrate holder and (b) Optical photo showing an oblique view of the shower
head with zoomed-in image of the showerhead nozzles. ................................................. 25
ix
Figure 2-5. Bar graph showing the composition of the plasma species for the various gas
flow rates. The legend shows the etch rate and pressure in the chamber at which the
etch was performed. For each species the bars correspond to etch rates from fastest
to the slowest from left to right. Etch time in all cases was 15 min. ................................ 27
Figure 2-6. SEM pictures of etched surface by (a) conventional SF6 etch process (left)
and (b) ring diffuser system (right). The etches were performed at PSource = 2000W
and PSubstrate = 400 W and etch time for both cases was 20 minutes. ............................... 29
Figure 2-7. (a) Bar graph showing the chemical composition of the plasma as measured
by the RGA with the etches with smoothest surfaces (red) to roughest (blue) listed
from left to right. Clearly large HF and H2O peaks are seen in the smoothest etches –
whereas the rough surface etches are dominated by large SF5 peaks and do not use
H2O as the process gas. (b) AFM image of the smooth etched surface obtained with
ring active etch corresponding to SF6:NF3:H2O :: 60:100:50. All etches were
performed for 15 minutes. ................................................................................................ 31
Figure 2-8. Bar graph showing the partial pressures of various molecular fragments for
SF6 + H2O plasma. All etches shown here were performed at a constant pressure of
8.5 mTorr. The legend for the graph lists the various gas flow rates, the bias
potential, the obtained etch rates and roughness values, and the percent increase in
the ion flux in comparison to the SF6:NF3:H2O :: 60:0:140 etch. All etches were
performed for 15 minutes. ................................................................................................ 33
Figure 2-9. Aspect ratio dependent etch rates of fused silica features of varying widths.
The data shown here is for 15 minute long etches using SF6:NF3:H2O::60:100:50,
8.5 mTorr pressure, 2000 W source power and 400 W substrate power and etch time
of 15 minutes. ................................................................................................................... 34
Figure 2-10. Sketch of the diffuser tube modified ICP-RIE; the inset picture show the
optical image of diffuser tube. ......................................................................................... 35
Figure 2-11. Obtained etch rates with diffuse tube on fused silica substrates. ........................ 36
Figure 2-12. SEM photograph of a 20 µm wide feature shows a very vertical wall with
sidewall angles of 88.7°. The bottoms of the trenches are flat and do not show any
trenching features indicating the chemical nature of the etch. Although the image
shows a slight bottle like shape, this is likely due to the angled facet of the image
arising during the cleavage of the sample for SEM. Etch time was 150 min. ................. 38
Figure 2-13. (a) Sidewall layer formation can seen in the SEM photograph where ~100
nm thick nickel fluoride/oxide layer is formed on the sidewalls. Inset shows a
broken fragment of the formed sidewall film (b) After stripping the layer in nickel
etchant, a smooth sidewall is obtained. The formation of passivation layer on the
sidewalls is able to provide inhibitor driven anisotropy. ................................................. 39
Figure 3-1. Experimentally obtained etch rate and rms roughness values on fused silica
and borosilicate glass for various SF6 flow rates. Since the pumping speed of the
x
pumping system remains constant for these conditions, increasing flow rate
corresponds to increasing pressure in the chamber. ......................................................... 49
Figure 3-2. 4D plot of the etch rate of (a) fused silica; (b)borosilicate glass; (c)
aluminosilicate glass as a function of the flow rates of SF6 (from source) and NF3
and H2O for diffuser-ring configuration. Color of the circles indicates the etch rate. ..... 51
Figure 3-3. Experimentally measured partial pressure of plasma species in diffuser-ring
modified etch system and conventional ICP etch system respectively by an in-situ
Residual Gas Analyzer (RGA). Identical total pressure was regulated in the RGA
system during mass spectrum acquisition in the two cases. ............................................. 53
Figure 3-4. 4D plot of the etch rate of (a) borosilicate glass; (b) aluminosilicate glass as a
function of the flow rates of SF6 (from source) and NF3 and H2O for 10 cm height
diffuser-tube. Color of the circles indicates the etch rate. ................................................ 54
Figure 3-5. Bar graph of partial pressure percentage of radical species of interest obtained
with 10 cm height diffuser-tube gas inlet and diffuser-ring gas inlet with identical
etch recipe. The etches were processed for 5 minutes. .................................................... 55
Figure 3-6. Bar graph showing the percentages of the various molecular fragments as
measured by the RGA in SF6:NF3::60:20 sccm borosilicate glass etching with
different diffuser tube heights. The magnitude of NFx peak is the sum of magnitudes
of NF, NF2 and NF3 peaks. The magnitude of SFx peak is a sum of magnitudes of
SF, SF2, SF3, SF4 and SF5 peaks. The magnitude of SiFx peak is a sum of magnitudes
of SiF, SiF2 and SiF3 peaks. All etches were performed for 5 minutes. ........................... 56
Figure 3-7. SEM photograph of a borosilicate glass etched using diffuser-ring
modification. The etch was performed under the following conditions: ICP Power =
2000 W, Substrate Power = 400 W, Gas Flow Conditions: SF6:NF3:H2O :: 20:20:25,
Etch Time = 150 mins. The obtained Etch Rate = 0.67 μm/min (50 μm feature) and
the highest aspect ratio is 9.3:1. ....................................................................................... 58
Figure 3-8. SEM Photograph of the cross-sectional profile of borosilicate glass etch using
10 cm height diffuser-tube. The etch was performed under the following conditions:
ICP Power = 2500 W, Substrate Power = 400 W, Gas Flow Conditions: SF6:NF3 ::
60:20, Etch Time = 210 mins. The obtained Etch Rate = 1.06 μm/min (50 μm
feature, after 5 minutes) and the highest aspect ratio is 10.6:1. ....................................... 59
Figure 3-9. SEM photographs of the cross-sectional profiles of the etch features in
aluminosilicate glass using the 10 cm height diffuser-tube modification. The etch
was performed under the following conditions: ICP Power = 2000 W, Substrate
Power = 450 W, Gas Flow Conditions: SF6:NF3 :: 60:20, Etch Time = 210 mins. The
obtained Etch Rate = 0.45 μm/min (50 μm feature, after 5 minutes), aspect ratio: 1.8
: 1. .................................................................................................................................... 63
Figure 3-10. Images of three kinds of patterns used for evaluating the loading and
charging effects. Each pattern consists of different percentages of the overall etched
areas. (a) Etched area ~50%, (b) Etched area ~11%, and (c) Etched area ~10%.
xi
Additionally, pattern shown in (b) consists of nickel pattern (light brow in color) that
is electrically isolated within each patterned squared and does not connect to the
edges of the wafer where the mechanical clamp makes an electrical contact to the
nickel mask layer. ............................................................................................................ 64
Figure 4-1. Chip-scale glass microspherical shells blown on silicon substrate. Inset shows
a near perfect glass microspherical shell with a sphericity of 0.996. ............................... 70
Figure 4-2. (a) Silicon wafer is patterned and plasma etched to a depth of 250 µm to
define circular pits (b) Borosilicate glass wafer is optionally patterned and plasma
etched to define heG µm deep circular features (c) The two wafers are aligned and
anodically bonded. (d) Borosilicate wafer is thinned down to a thickness of t µm in
hydrofluoric acid. (e) Glass microbubble is blown at 775 °C in a vacuum oven
maintained at a pressure of 100 Torr. ............................................................................... 72
Figure 4-3. SEM image of sidewall thickness measurements at the equatorial plane of
glass microspherical (a) #4 and (b) #7. ............................................................................ 76
Figure 4-4. (a) 3D view of simulated WGM resonance modes confined in spherical shell.
(b) The geometry definition of the computational domains. The arc spherical shell
domain is in diameter of 600 µm and thickness of 4 µm, defining with borosilicate
glass properties. The rest domains in the rectangular zone is defined with air
properties. ........................................................................................................................ 79
Figure 4-5. Comsol FEM simulation of whispering gallery resonance modes in
borosilicate micro-spherical shell. The WGM resonance is modeled in a spherical
shell with diameter of 600 µm. The thickness of the glass shell is 4 µm in (a) – (d).
The center wavelength of the couple incident laser is 760 nm. The azimuthal number
is calculated as 3638. The scale bar presents the physical dimension of the cross
section of the spherical shell near equatorial plane. The color bar illustrates the
electric field intensity of the resonate mode. (a) n=1, m=l, p=1 (TE mode), (b) n=1,
m=l, p=1/nr2 (TM mode), (c) n=1, m – l = 1, p=1 (TE mode), (d) n=2, m=l, p=1 (TE
mode). ............................................................................................................................... 82
Figure 4-6. (a) Schematic illustration of the experimental set-up for the measurement for
the WGM resonance in glass bubbles. (b) Optical image showing the light confined
to the equatorial plane of microspherical shell #9 upon evanescent coupling of the
light through the tapered fiber. ......................................................................................... 83
Figure 4-7. Transmission spectrum of the optical resonance in (a) microspherical shell #6
and (b) microspherical shell #5 within 15 GHz frequency span. ..................................... 87
Figure 4-8. (a) Experimentally measured temperature induced resonance frequency shift
of ~107 Q-factor resonance mode in the transmission spectrum of microspherical
shell #7. (b) COMSOL simulation was used to fit the experimentally measured
frequency shift by parametrically tuning the effective value of TCE of the
microspherical shell. Good fit was found for an effective TCE value of 2.19 × 10-6
K-1 for the microspherical shell #7. (c) Measured temperature induced resonance
frequency shift within a finer temperature change for microspherical shell #8 and #9. .. 89
xii
Figure 4-9. Water filled microspherical shell #10. The silicon substrate is wet etched in
TMAH by 250 µm to open the bottom access for filling liquid. The liquid is filled by
immersing the microbubble in the water and pumping the air in a vacuum chamber...... 91
Figure 4-10. (a) Transmission spectrum of resonant modes obtained from microspherical
shell #10 with wall thickness of 4.7 µm. A blue-shift of the resonant modes was
observed as the water-filled microspherical shell core dries out. Inset image shows
0.51 GHz frequency shift observed in a 2.5×106 Q-factor mode. (b) COMSOL
simulated frequency shifts between water-core and air-core microspherical shells
with diameters of 600 µm as a function of the shell thicknesses ranging from 300 nm
to 10 µm. Experimental data for two microspherical shells of thicknesses 4.7 μm and
6.4 μm is also shown. (c)-(d) FEM solved fundamental TE mode showing the spatial
distribution of the electric field intensity in 0.6 µm shell thickness with water and air
core respectively. (e)-(f) Electric field intensity is plotted in logarithmic scale for
water and air cores in 0.6 µm thick shell and clearly exhibits penetration of electric
field into water core in (c). (g)-(h) FEM solved fundamental TE mode in a 8 µm
thick microspherical shell with water and air core respectively. (i)-(j) Electric field
intensity plotted in logarithmic scale for the two cores for the 8 µm thick
microspherical shell. The simulations clearly show that the TE mode electric field
interacts strongly with the fluid in the core of thinner walled microspherical shells
than for thicker shell walls and explains the larger frequency shift obtained for
thinner walled shells. ........................................................................................................ 92
Figure 4-11. Transmission spectrum of resonant modes obtained from the fully silicon
substrate removed glass microbubble in the diameter of 750 µm and initial thickness
about 4 µm. The quality factors of the obtained modes reduce due to the glass
microbubble surface roughening in the KOH releasing process. ..................................... 94
Figure 4-12. Transmission spectrum of single microbubble coupling and double
microbubbles coupling ..................................................................................................... 95
Figure 5-1. (a) Schematically illustration of magnetic particles in ferrofluids. The
diameter of the particle is about 10 nm and the length of the surfactant is about 2
nm. (b) Magnetoviscous effect: the viscosity of the ferrofluids increases as increase
of magnetic field. ............................................................................................................. 98
Figure 5-2. Schematic illustration of the ferrofluid – quartz resonator based
magnetoviscos magnetometer. ......................................................................................... 100
Figure 5-3. (a) – (e) : Schemaic illustration of the design and fabrication of ferrofluid-
μQCR magnetometer. (a) 100 μm thick AT-cut quartz substrate, (b) optimized ICP-
RIE etched 90-95 μm quartz resonating region with deposition and patterning of
15/150 nm thick Cr/Au backside electrode, (c) Deposition and patterning of 15/150
nm thick Cr/Au front side common electrode, (d) Deposition and patterning of 500
nm thick Metglas magnetic flux concentrator. (e) optical image of the fabricated
μQCR. .............................................................................................................................. 102
Figure 5-4. (a) Anodic bonded borosilicate glass wafer and etched silicon wafer. (b)
Glass microbubble formed with thermal annealing process. (c) Schematic illustration
xiii
of the expanded glass microbubble chip. (d) Optical image of microbubble package
chip after drilling holes on the top of microbubbles. (e) Optical image of
microbubble package chip after removing silicon substrate. ........................................... 103
Figure 5-5. (a) Schematic illustration of packaged device. The dimension mismatch of
glass microbubble chip and quartz resonator chip provides the access of wire-
bonding between top-electrode to the ceramic package. (b) Image of glass
microbubble packaged ferrofluid- μQCR device. ............................................................ 104
Figure 5-6. (a) Low noise current source is used to drive Helmholtz coils for modulating
magnetic field in the magnetic shield box. Device Under Test (DUT) is connected
with network analyzer. (b) Image in the shield box: glass microbubble packaged
ferrofluid- μQCR is placed on a stage at the center of Helmholtz coils and connected
with network analyzer through SMA connector. ............................................................. 105
Figure 5-7. Characterization of quartz resonance during the packaging process. .................... 106
Figure 5-8. Obtained high Q-factor resonator after loading ferrofluids in the glass
microbubble. .................................................................................................................... 108
Figure 5-9. (a) Real-time susceptance responds to modulated magnetic field. Modulation
frequency :0.5 Hz; modulation field: 33.6 µT. The scan time is 20 seconds. (b) FFT
is applied to the measured susceptance responds in time spectrum. FFT peak-signal
at the modulation frequency of the magnetic field is tracked to quantify the
susceptance responds. (c) Amptitudes of FFT peak-signals at the modulation
frequency are plotted as a function of intensity of modulation magnetic field. ............... 109
Figure 5-10. Susceptance responds to various modulation frequency as a function
amptitude of magnetic field. ............................................................................................ 110
Figure 5-11. Comparison of susceptance responds of the devices with and without
Metglas® flux concentrator as a function amptitude of magnetic field. .......................... 111
Figure 5-12. Frequency shift of Device 1 and Device 3 under external magnetic field as a
function of time. ............................................................................................................... 112
xiv
LIST OF TABLES
Table 1-1. Bond dissociation energy of oxides in glass materials. .......................................... 10
Table 1-2. Capability, advantages, and disadvantages in various types of etching ................. 10
Table 2-1. Summary of the reported glass etching results from literature. .............................. 15
Table 2-2. Summary of the process parameter space available on the AMS 100 ICP-RIE ..... 19
Table 2-3. Summary of the process parameter space available on the modified ICP-RIE
system being optimized for silica etching. ....................................................................... 25
Table 2-4. Comparison between SF6/NF3/H2O based etching and single SF6 plasma
etching at 400 W substrate power. ................................................................................... 28
Table 2-5. Etch rates and roughness obtained with different lengths tube on fused silica.
Wafers were etched for 5 mins. Etch rates and surface roughness values were
presented as averages of five 100 μm wide feature data which were acquired by
profilometer. ..................................................................................................................... 36
Table 2-6. Mask Dependent Etching Performance 2500W/400W Power ............................... 40
Table 3-1. Comparison between SF6/NF3/H2O based etching and single SF6 plasma
etching at 400 W substrate power. ................................................................................... 51
Table 3-2. Role of relative ion flux and various fluorine radicals and molecules on the
etch rate and surface roughness of the glass substrates etched in this work. The table
lists the value of Pearson correlation coefficients and the P-values for these
parameters based upon 41 independent etches performed in this work. .......................... 61
Table 3-3. Summary of optimized glass etch rates and surface smoothness ........................... 66
Table 4-1. Summary of presented configurations of WGM resonators in literature ................ 68
Table 4-2. Calculated and experimentally measured values of the glass microspherical
shell dimensions for the given glass blowing conditions. For devices where glass
wafer is not etched prior to bonding, r0G and heG are not applicable. ............................... 73
Table 4-3. Appearance order of second order radial TE mode (n=2, m=l, p=1) with
different shell thickness. Diameter of the modeled spherical shell is 600 μm ................. 81
Table 4-4. Optical characteristics of blown microbubbles....................................................... 84
Table 5-1. Resonance characteristics of three fabricated quartz resonators. ........................... 107
Table 5-1. Summary of obtained sensitivity from three devices. ............................................ 111
xv
ACKNOWLEDGEMENTS
This dissertation documents the experience and results of research and development in
Penn State. At the end of the five-year memorable journey, I would like to express my sincere
acknowledge to everyone who is along with me.
Firstly, I would like to express my deepest gratitude to my research adviser, Dr. Srinivas
Tadigadapa, for his inspiration, instruction and support to me in the research projects. Without your
guidance, I could not gain the professional knowledge and achieve those objectives in the research.
But above all, I think I could benefit for my whole life from learning from your personality of the
way of working with students, colleagues and other people. I really enjoyed working with you in
our research group in the last five years.
Secondly, I am indebted to Dr. Zhiwen Liu for your professional and insightful suggestions
to our glass microbubble WGM resonator project. The discussions raised in our group meetings
indeed inspired me and equipped me to tackle the tough questions in the project. Without your help,
I could never easily and quickly access to a new professional field on my own.
Thirdly, I would like to thank Dr. Weihua Guan and Dr. Saptarshi Das for your professional
suggestions and support towards the successful completion of my study in Penn State.
I would like to thank my colleagues in our research group who are always generous to help
me when I have questions in the research.
Thanks to all of my friends in my heart who had had to leave or still being trapped in the
lovely small town.
To my parents and wife.
And the time in State College.
Chapter 1
Introduction
1.1 Application overview of glass materials
As one of the oldest artificial materials, glass is believed to have been accidently produced
by Phoenicia merchants in the region of Syria around 5000 BC. The merchants landed and then
rested cooking pots on blocks of nitrate placed by fire. The intense heat from the fire melted the
nitrate blocks and eventually mixed with the sand of the beach to form opaque beads of glass.
Around 3000 BC, glass was used to graze on pots and vases. The discovery of the new decoration
may have been coincidental with overheating calciferous sand which when combined with sodium
materials in the kiln formed a colored glaze on the ceramics. The new art quickly spread along the
coast of the Mediterranean. Fragments of glass vases have also been independently found in the
region of ancient Mesopotamia, Greece, China, and North Tyrol dating back to 1500 BC. The first
100 years of AD saw rapid developments in glassblowing techniques making it possible to realize
a great diversity of hollow glass structures. By 11th century Venice became the center of glass
industry, where glass craftsmen perfected art of glassblowing techniques to form sheet glass and
glass was used as a commercial industry material on doors and windows. By 18th century, glass
was being extensively used for scientific experiments, in lenses and mirrors in telescopes,
microscopes and other optical devices, and chemistry glassware which began an intense period of
research and study of glass in all its various compositions and morphological .
Glassware is now ubiquitously with food and cooking in the kitchen due to its non-toxic
properties; in scientific research in the laboratory due to its corrosion resistance and low reactivity.
Glass windows are extensively used in the design and construction of high-efficiency and
2
environmentally friendly buildings as well as in the automotive industry. Glass is also an integral
and important element of photovoltaic solar panels. Glass is the material for display and touch
screens in electronic devices such as televisions, computers, cell phones, tablets and so on. With
the requirement of providing better interface functions, the electronic device market is exploring
new generation of display glasses with the capabilities of displaying bright high resolution images
on extremely large displays, while simultaneously being mechanically robust, as well as being
flexible for curved display applications. Figure 1-1 shows some examples of the use of glass in
various fields.
Figure 1-1. Applications of glass in various fields. The center image shows the famous glass made
construction: Louvre Pyramid. The top image shows the application of glass in solar cell substrate
as transparent roof. The left image shows the large scale floor glass window. The right image shows
the glass panel based conceptual automotive interior. The bottom three images show the
applications of glass as glassware, glass lenses and glass fibers in scientific research. Courtesy:
Images are from google images.
In addition to being inexpensive, readily available, high optical transparency, excellent
electrical insulation, high electrical breakdown resistance, good mechanical properties and
3
biocompatibility making it the material of choice in everyday applications, glass in the form of
silicon dioxide has played a pivotal role in evolution of microelectronics industry [1] and is still
continuing to play a critical role in modern microelectronic devices. Until recently, silicon oxide
has been the gate material that controls the charges in the channel of MOSFET transistor which
triggered the era of modern integral circuits. Silicon oxide served as gate material in MOSFET
transistors for about 60 years since MOSFET the initial manufacturing of these devices began in
early 1960’s [2]. Although it is now being gradually replaced by high-k material due to the
exacerbated gate leakage issues in nanoscale transistor, glass material is beginning to play an
increasingly dominant role in the MEMS devices and microscale transducers. Glass substrate
market is projected to grow at a compound annual growth rate of 23% over the next five years.
Related revenues are expected to exceed $594 million by 2022. Figure 1-2 shows overall market
size of glass substrates in wafer shipped per year (wspy) over the next five years and broken
down according to technical functionalities.
Figure 1-2. Glass substrate overall market size in wspy in breakdown per technical functionalities
within 6 years. Courtesy: Image is cited from Yole development website.
4
An important category of the market with a high projected rate of growth, as shown in
Figure 1-2, is the CMOS image sensor (CIS). This includes the realization of reliable glass on-chip
micro-lens, since micro-lenses are indispensable units of every image pixel in CIS. Figure 1-3
shows a cutaway drawing of a typical CMOS active sensor pixel [3]. Another area where glass
plays a pivotal role is in microelectronic chip packaging. Patterned and structured glass realized
through glass etching techniques along with wafer bonding processes are used for realizing devices
with through glass vias (TGV) [4]–[6], and for glass encapsulation [7], [8]. Figure 1-4 (a) – (c)
show patterned and structured glass wafers used for manufacturing glass based devices and
products [9]. Figure 1-4 (d) shows an example of the structured glass products applied in the
electronic device packaging [10]. Due to the matched thermal expansion coefficient of several
formulations of glass with that of silicon, glass has been proposed to be used in the new generation
of fan-out wafer level package (FOWLP) packaging process [11] with an expected market size of
30% of the projected glass substrates market size, shown in Figure 1-2, by 2022.
Figure 1-3. 3-D cutaway drawing of a typical CMOS active sensor pixel. Courtesy: Image is from
The Molecule Expressions program website.
5
Figure 1-4. Glass wafer with TGVs structures. (b) Glass wafer with micro-holes structures. (c)
Glass wafer with cap structures. (d) Illustration of wafer level packaging process with glass wafer.
Courtesy: images are from Tecnisco, LTD website.
From Figure 1-2, it can also be seen that microfluidic applications occupy the largest
portion of the current and projected glass market. Even though there are other polymer based
candidates vying for microfluidic devices market, glass material possesses unique properties such
as high optical transparency over a wide wavelength spectrum and low thermal expansion
characteristics that are highly desirable in opto-fluidic and sensing applications. In the field of
memory circuits, silicon nitride and silicon oxide as layered structure are typically used in 3D flash
cell where silicon oxide serves as a small unit of capacitor to charge and discharge [12]. Thus,
exploration of silicon oxide based microfabrication techniques is critical for future innovations in
the memory circuit market.
The word glass represents amorphous materials, and actually represents various
compositions of materials as long as they consist of silicon oxide or alkane oxide in amorphous
phase. Thus glass substrates can be categorized based on various composition and crystallization
states. In this dissertation, silicon dioxide substrates will be categorized and discussed as following:
Ι. Single crystalline silicon oxide also known as quartz demonstrates excellent piezoelectric
property and is widely used as resonators. ΙΙ. Amorphous silica substrates which include (i) fused
silica, (ii) borosilicate glass and (iii) aluminosilicate glass. Fused silica and borosilicate glass
6
substrates have found extensive use and applications in microelectro-mechanical systems (MEMS),
micro-optical devices, and microfluidic lab-on-a-chip devices, whereas, aluminosilicate glasses are
being increasingly used in display applications.
Fused silica consists of only silicon oxide in amorphous form. The purity of fused silica
provides good transmission characteristics in the ultraviolet is thus used as an excellent material
for making fibers and lenses [13]. The low thermal expansion coefficient also makes fused silica a
useful material for precision mirror substrates [14]. The combination of high transmission
coefficient in ultraviolet and low thermal expansion coefficient also makes fused silica the material
of choice for mask substrates and reticles in photolithography applications where the laser
wavelength is larger than 157 nm [15]. The high melting temperature of fused silica allows for use
as enclosure material in high intensity discharge lamps and as tube inserts in furnaces used in
semiconductor industry [16]. Patterning, metallization, and etching of fused silica is used to
construct high-precision microwave circuits and narrowband filters [17].
Quartz also only consists of silicon oxide but in crystalline in structure as opposed to the
amorphous nature of fused silica. Quartz belongs to the trigonal crystal system. The ideal crystal
shape is a six-sided prism terminating with six-sided pyramids on the top and bottom faces of the
prism as shown in Figure 1-5. Single crystal piezoelectric quartz is an anisotropic material and
depending on the cut angle, quartz crystals show different inherent properties. For certain cuts
such as the Y-cut or the AT-cut quartz that is sandwiched between electrodes, quartz resonates in
thickness shear mode (TSM). In these crystal cuts, application of a sinusoidal electrical field with
the electrodes sets-up the shear wave through the thickness of the quartz, so that the device
exhibits a resonance behavior when the thickness of the quartz slab is half the wavelength of the
shear acoustic wave. As a result, the resonance frequency is inverse proportional to the thickness
of quartz and is extremely sensitive to the loadings on the quartz resonator. Loadings on the
7
resonator surface can be pure elastic loads (solids), pure viscous loads (liquids), viscoelastic loads
(polymer) or a combination of any of these [18].
Figure 1-5. Schematic of the crystal structure of crystalline quartz showing the a- and c- planes and
axes. Taken from http://www.quartzpage.de/gen_struct.html.
Borosilicate glass is another amorphous type of glass which consists of 80% SiO2, 13%
B2O3, 4% Na2O and 3% Al2O3 and Na2O. It is known by the trade names of Borofloat® glass (Schott
Glass Inc.) or Pyrex® glass (Corning Inc.). Compared to the melting temperature of fused silica of
1600 °C, the addition of dopants in the form of boric-oxide and alkaline-oxide lower the melting
point of borosilicate glass to around 850 °C. Therefore, borosilicate is easier melt and reflow for
applications requiring such properties. Borosilicate glass is widely used in laboratory in the virtue
of its chemical inertness and thermal stability, optical clarity, and low cost. Borosilicate is not only
used as laboratory glassware but has also been used in various MEMS application by integration
on silicon substrates through direct silicon-glass anodic bonding process. The resistance of a 500
µm thick borosilicate glass reduces from 2 MΩ to 5 kΩ as glass temperature is increased from 20
°C to 400 °C. With a negative voltage applied on the glass substrate of a glass-silicon stack, the
8
mobile positive alkaline ions in borosilicate move away from the glass-silicon interface creating a
depletion region at the glass-silicon interface with a very large electric field. The negatively charged
volume oxygen ions drift to the bond interface and react with silicon atoms to from SiO2. The
anodic wafer bonding process is commonly used in electronics, microfluidics, and MEMS
packaging applications [19], [20].
Aluminosilicate glass is another group of amorphous glasses that contain less SiO2 and
more alkaline-oxide and aluminum oxide dopants. Generally, alkaline earth boro-aluminosilicate
based glasses are more robust than fused silica and borosilicate glass [21]. The glass is strengthened
by ion exchange during the manufacturing process. Here, the glass is immersed in a molten alkaline
potassium salt bath at a temperature of approximately 400 °C, wherein smaller sodium ions in the
glass are replaced by the larger potassium ions from the salt bath. The larger ions occupy more
volume and thereby create a surface layer of high residual compressive stress at the surface, giving
the glass surface increased strength [21]. Therefore, being more resistive to mechanical tension,
aluminosilicate glass is widely used as display glass in electronic products. As an example of
aluminosilicate glasses, Eagle® glass consists of 55% SiO2, 21% CaO, 10% Al2O3, 7% B2O3, and
1% Na2O. As the first barium-free glass launched by Corning® in 2006 for TFT and LCD market,
Eagle® glass enables panel manufacturers to innovate for thinner and lighter display panels. In
2016, Corning Inc. demonstrated a 0.25 mm-thick, 300 mm×1500 mm size Eagle glass substrate
for curved LCD display. Meanwhile, by adjusting the compositions of oxidants in the glass,
aluminosilicate can be used for dedicated functionalities. For example, Eagle® glass is designed for
TFT and LCD market and is being developed as thin flexible substrate suitable for large area curved
display panels. Lotus™ is another customized glass by Corning® for LCD and OLED display
applications and provides low total pitch variation and high display resolution.
9
1.2 Motivation of exploring glass micro-fabrication techniques
As one of the most promising applications, glass substrate based wafer level packaging for
memory and logic integrated circuits has been proposed for over a decade. Traditionally, silicon
substrate is used for through-substrate vias (TSVs) in 3D IC integration. However, it is challenging
to use silicon through-substrate vias in RF wideband filter applications due to the low dielectric
constant which induces significant energy loss in such devices. In contrast, glass is an excellent
dielectric material and a perfect solution for overcoming this drawback of silicon in high-frequency
RF devices. In addition to the highly desirable dielectric property, glass substrates also exhibit
excellent mechanical properties ideally desired in package applications. A glass substrates with
hermetically sealed TGVs can be fully airtight and provide long-term robust enclosures for MEMS
devices. Fine-pitched TGVs allow reliable conduction of electrical signals and power to and from
the enclosed MEMS device. Fully hermetic 3D wafer level chip size packing can be realized by
placing a glass wafer directly on top of a silicon MEMS wafer. With the feasibility of
manufacturing glass substrates very large areas at low-cost, glass is truly becoming a promising
material in MEMS packaging field.
The potential of applications of glass in so many fields is the primary motivation to develop
robust and reliable microfabrication techniques specifically applicable for glass substrates. As one
of the most important microfabrication processes, the topic of developing effective etching process
suitable for high aspect ratio etching of glass substrates has been investigated for more than three
decades using various approaches. Etching glass is especially challenging because: i) glass is a very
hard and brittle material that can easily chip, crack and break under mechanical forces. So the
traditional machining technique such as computer numerical control (CNC) usually results in very
low process yield and is not feasible for the standard manufacturing with glass. ii) the silicon to
oxide, alumina to oxide, and calcium to oxide bonds found in the various oxides such as SiO2,
10
Al2O3, and CaO comprising glass substrates are extremely strong. Table 1-1 lists the bond
dissociation energy of the oxides [22]. These bonds make the glass a very difficult material to etch,
especially for high speed, high aspect ratio, high selectivity, and deep etches. Table 1-2
demonstrates available glass etching types. Table 1-2 compares the various glass machining
techniques and lists their pros and cons.
Table 1-1. Bond dissociation energy of oxides in glass materials.
Si-Si Si-O Al-O Ca-O
Dissociation
energy (kJ/mol)
310 799.6±13.4 501±10.6 383.3±5.0
Table 1-2. Capability, advantages, and disadvantages in various types of etching
Capability Advantages Disadvantages
San
dbla
stin
g Minimum feature size : 50
µm
Aspect ratio: 2-3:1
Through-hole taper:12-15°
Depth uniformity: < 25µm
Quickly create through
hole
Clean etched initial top
surface
Anisotropic process
Excellent for etch large
areas of materials
Be able to etch different
materials such as
borosilicate, fused silica,
silicon and even sapphire
and silicon carbide
No hearing effects and no
micro-cracking
Low etching selectivity
Low aspect ratio etch
profile
Tapered sidewall
Large feature size
Very rough etch surface
Not directly compatible
with other clean-room
semiconductor process
11
Las
er m
achin
ing Minimum feature size : 50 –
100 µm
Aspect ratio: 10 - 20:1
Through-hole taper: 3-5°
Position tolerance: motion
control equipment
dependent
Easy to create pattern
from a CAD drawing
No mask and tool wear
Programmed automatic
process
Low sidewall taper angle
Can be used for large
piece
Very slow process: only
one hole is processed at
a time
Large features
Creates subsurface
micro-cracks
Heating creation result
in damage at initial top
surface of the etching
feature
Creation of a lip due to
the melting process
makes it impossible to
bond the processed
wafer unless lapped and
repolished.
Uneven etching across
the work-piece
Hard to align to existing
features on the wafer
Large capital
investment
Ult
raso
nic
mac
hin
ing Minimum feature size: 200
µm
Aspect ratio: 25:1
Through-hole taper: 3-5°
Depth uniformity: 50 µm
Can create high aspect
ratio sidewalls
Small taper angle
Slow process
Large features
Large capital
investment
Tool needs redressing
for every 25 -50 pieces
to avoid feature
degradation
Wet
etc
h Minimum feature size: 1 µm
Aspect ratio: 1:1
Fast process
Very small feature
Good at etching thin film
Batch process for
multiple samples
Isotropic process
Undercutting features
Low aspect ratio
Etchant can easily peel
off photoresist mask
12
Co
nv
enti
onal
In
du
ctiv
ely
Co
up
led
Pla
sma
Dee
p R
eact
ive
Ion
Etc
h Minimum feature size: 0.1
µm
Aspect ratio: 10:1
0.5µm/min etch rate
Nanometer level etching
smoothness
Highly anisotropic
process and high glass to
mask selectivity
Good repeatability and
stale process
Compatible with various
glass composition
Compatible with
semiconductor
microfabrication
environment
Unclear plasma glass
etching mechanism
Etch rate is still low for
deep etch like TGVs
Needs to develop
selectively plasma etch
for alkaline-oxide
materials in glass
Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) is a well-established process
technology in the semiconductor industry. It is able to achieve small feature sizes, high aspect ratio,
high selectivity, and high speed for deep silicon etch. In the conventional ICP-RIE system, the
process gases are introduced from the top side of the ICP source. The ICP source is made of one or
several turns of inductor coils and the coils are wound around a dielectric vessel. The substrate with
a capacitor couples with the ICP and forms a complex resonant LC-network. The ICP and substrate
are driven independently by separate radio frequency power sources. The independent RF
frequency can be 380 kHz (low-frequency), 2MHz (mid-frequency), or 13.56 MHz (high-
frequency). Feeding radio frequency power to the coil, a high-frequency oscillating magnetic field
is generated inside the chamber and results in a corresponding oscillating electric field. The electric
field is able to excite the process gases into ions and radicals and drive the ions and radicals to a
high plasma density of ~1018 - 1020 m-3 within the process chamber. The high density plasma is then
driven by the coupled substrate RF power to etch the substrate materials. Figure 1-6 schematically
illustrates a sketch of a conventional ICP-RIE etcher. The high density plasma in the ICP-RIE
system provides two components in terms of etching mechanism. Process gases of large molecules
that are ionized by ICP RF power and driven downwards by the built-in electric field of the
equivalent LC network. Due to the large molecular mass of these ions and molecular fragments and
13
the large built-in electric field, the ions are energized to large momentum, thus physically
bombarding the substrate materials and transferring their physical energy to the substrate atoms.
Simultaneously, the halogen radicals and ions such as F- and Cl- are also able to chemically react
with the thermalized and physically activated surface atoms of the substrate to form volatile
products as chemical etching component.
Figure 1-6. Sketch of a conventional ICP-RIE etcher showing gas inlet, ICP coils, RF substrate,
magnetic pieces and substrate wafer.
However, even though the ICP-RIE is well established for silicon substrate, the mechanism
of ICP-RIE for glass materials is not clear. The conventional ICP-RIE method is hardly able to
deliver the performances for the new requirements of glass etching. In this dissertation, the author
will discuss a modified ICP-RIE glass etching system suitable for various composition glass
substrates.
14
Chapter 2
Inductive Coupled Plasma – Reactive Ion Etching (ICP-RIE) of Fused Silica
Substrate
2.1 Plasma Etching of Glass Materials: Background
Silica substrates in both amorphous glass compositions as well as crystalline forms are
increasingly being used in the construction and chip-scale packaging of various
microelectromechanical systems (MEMS) [19], [23], [24], micro-optical devices [25], and
microfluidic lab-on-a-chip devices [26]–[28]. In addition to being inexpensive and readily
available, silica substrates exhibit several desirable properties including optical transparency,
excellent electrical insulation, high electrical breakdown resistance, good mechanical properties,
and biocompatibility. Glass wafers can be readily bonded to silicon wafers [29], can be blown into
microbubbles [30], [31], and integrated as piezoelectric resonators when used in single crystal
forms [24], [32].
Most of the efforts in silicon dioxide (glass) etching, have been primarily directed towards
realizing features for microelectronics applications such as interconnect vias [33], waveguides [34],
phase shift masks [35], etc. Hence, process optimization has traditionally aimed at increasing the
selectivity of silicon dioxide over silicon substrate [36], reducing gate oxide damage [37],
decreasing sidewall roughness [38], and increasing sidewall angle of the etched features [39]. With
the advent of microelectromechanical systems (MEMS) and microsystems in the last two decades,
the focus has shifted to high aspect ratio etching of silicon dioxide. Many of these applications
require greater than 100 µm of silicon dioxide (glass) etching while maintaining the surface finish,
with rms surface roughness of less than 1 nm [40], [41]. Hence, these applications impose
15
additional new requirements on glass etching processes such as high etch rate, high selectivity to
masking material, high anisotropy, low surface roughness for mirror polish, uniformity of etch
across the wafer and within a pattern [42], etc. Several reports on etching of silica substrates and
borosilicate glass using reactive ion processes have been published using fluorine based
chemistries. In particular, using inductively coupled plasma (ICP) sources, high aspect ratio and
high surface smoothness etching of glass wafers using SF6 and Ar/Xe gases with etch rates
approaching 1 µm/min have been achieved [43]–[45]. In this context the processes developed thus
far rely upon ion bombardment to increase the etch rate while fluorine based gases are used to
provide the reactive component for etching [44]. It has been shown that the use of heavier Xe helps
reduce the re-deposition and more effectively removes any non-volatile residues resulting in
smoother surfaces with an average surface roughness of ~2 nm [43]. Table 1 summarizes the
reported glass etching results in terms of etch rate, surface roughness, mask material, selectivity,
etched depth and aspect ratio since 1999. The reported etch rates on silica and borosilicate
substrates are in the range from 0.5 μm/min to 0.9 μm/min with nanometer level etched surface
roughness and were mostly conducted by using SF6+Ar or C4F8+Ar gas combinations. Without the
introduction of Argon gas, the etch rate and etched surface roughness of glass substrates has been
found to increase with increasing flow rate of SF6 or C4F8 [46]–[50]. Keeping the total gas flow
rate constant and increasing the percentage of Argon gas in the SF6+Ar or C4F8+Ar gas
combination, the etched surface smoothness is found to improve but the etch rate is decreased [46],
[47], [50]. We observed similar trends in etch rate and etched surface roughness using conventional
SF6 + Ar or C4F8 + Ar gas combinations prior to the implementation the modifications to the etch
chamber, to be described next.
Table 2-1. Summary of the reported glass etching results from literature.
16
Reference
Etch rate
(µm/min)
Roughness
(nm)
Mask
Material
Selectivity
Etched
Depth
(µm)
Aspect
ratio
Glass type
Abe et al
[49], 1999
0.5 2 Nickel 30 20 0.05 Quartz
Li et al
[48], 2000
0.6 4 Nickel 20 100 10 Borosilicate
0.5 Nickel Fused silica
Chen et al
[51], 2001
0.55 Nickel Fused silica
Ichiki et al
[44], 2003
0.8
Chromiu
m
23 32 2 Borosilicate
Park et al
[46], 2005
0.75 Nickel 27 2 Borosilicate
Akashi et al
[52], 2006
0.55 Silicon 6.6 430 0.43 Borosilicate
Jung et al
[47], 2006
0.65 Nickel 20 2 Borosilicate
Goyal et al
[45], 2006
0.54 2 Nickel 33 1.65 Borosilicate
Kolari
[53], 2007
0.6 Silicon 3.9 330 1.6 Borosilicate
Queste et
al [20],
2010
0.74 Nickel 27 13 2.9 Quartz
0.9 30 Nickel 18 120 6 Borosilicate
Zhang et al
[54], 2014
1 0.5 Nickel 16 102 5.2 Fused silica
17
Ahamed et
al [55],
2015
0.35 38
Nickel 70
70 7 Fused silica
0.45 62 80 8 Borosilicate
Instead of photoresist, hard masks such as nickel and chromium are commonly used in
glass etchings, especially for deep etches. It results from the fact that glass is typically etched in
low chamber pressure conditions so that the plasma constituents are physically more energetic than
those in the high pressure etching conditions. The energetic plasma easily heats up the glass
substrates which in fluorine environment results in an enhanced attack of the soft mask such as
photoresist and polymers. Secondly, nickel and chromium are selected as hard mask materials for
glass etching out of other candidates like vanadium, tungsten, molybdenum, and magnesium
because of the non-volatile nickel and chromium fluorides they form during fluorine based plasma
etchings. Lastly, nickel can be easily deposited in the laboratory as low-stress thick films (up to 30
µm thick) via electroplating. Chromium is not only fluorine resistant but also excellent adhesive
material to glass substrate so that it is able to be easily patterned with lift-off process with films up
to 1 µm in thickness and critical dimensions of ≤1 µm.
2.2 Sample Preparation
500 μm thick, double polished 4 inch Fused silica (~99.9 % SiO2 and 0.1 % H2O), wafers
were investigated in this work. Wafers were cleaned in Nanostrip® for 30 mins and then deposited
with 15 nm chromium and 150 nm gold seed metal layers using e-beam evaporator. Wafers were
patterned using Shipley™ 1827 photoresist and developed in 25% Microposit™ 351 developer to
form 3.5 μm thick photoresist patterns. 2 - 3 μm nickel was electroplated as hard mask using 1 ms
on and 3 ms off current pulses of 40 mA current to create features of widths ranging from 5 μm to
18
200 μm. SPR 220-7 photoresist was also used to coat 10 μm photoresist for when plating thicker
nickel mask features required for deep etches. Thereafter, the photoresist and the underlying Cr/Au
seed layers were removed from the regions without nickel plating using photoresist striper and
Cr/Au wet etchants respectively in order to obtain clear glass regions to be etched. The process
flow for wafer preparation is schematically shown in Figure 2-1.
Figure 2-1. Schematic process flow for the preparation of glass substrates used in the etch tests. 15 nm of
Cr and 150 nm of Au seed layer were deposited prior to plating 2 - 3 µm of nickel which acts as hard mask
in these tests.
2.3 Conventional ICP-RIE Glass Etching
In our work, an Alcatel AMS 100 ICP-RIE etcher which is schematically illustrated in
Figure1-2 was first examined for glass etch performance using conventional SF6-only plasma
etching methods and thereafter using the modified etch chamber with diffuser ring and diffuser
tube configurations and NF3 based etch chemistry. In order to investigate the effects of diffuser ring
and diffuser tube configurations, it is necessary to benchmark the etch rate and etched surface
roughness by the conventional ICP-RIE approach which feeds SF6 gas through top inlet of the
chamber. The introduced gas is immediately ionized by the 13.56 MHz radio frequency ICP coils
into plasma and the the plasma is driven energetically downwards to the substrate through the use
19
of an RF bias power applied to the substrate. The available etching variables are listed in Table 2-
2.
Table 2-2. Summary of the process parameter space available on the AMS 100 ICP-RIE
Process Parameters Range
ICP Source Gases
SF6 0 – 200 sccm
Ar 0 – 100 sccm
O2 0 – 100 sccm
Physical Parameters
Source Power 0 – 3000 W
Substrate Power 0 – 600 W
Pressure 0 – 75 mTorr
Temperature of
substrate holder
0 – 30 ⁰C
Distance of substrate
holder form ICP
120 – 200 mm
In benchmarking experiments using conventional SF6 only plasma etching for glass, fused
silica and borosilicate wafers were etched for 20 minutes at a source power of 2000 W and substrate
power of 400W. The backside He wafer cooling temperature was set at 20 °C and the wafer position
with respect to the ICP source was set at 120 mm. Etch rate was measured by measuring the depth
of 100 µm feature using a Tencor P-16 profilometer and the surface roughness was measured using
scan size of 500 µm, scan speed of 20 µm/min, sampling rate of 200 Hz, applied force of 2 mg,
vertical scanning range of 64 µm. he etch rate and etched surface roughness of fused silica and
borosilicate glass are plotted as a function of SF6 flow rate and pressure in Figure 2-2. Glass etching
mechanism can be expressed with the equation [56]:
𝑟𝑆𝑖𝑂2= 𝑗+Φ𝑠 +
𝑛𝐹𝑣𝐹̅̅̅̅
4[(𝜖𝐹(𝑆𝑖𝑂2
∗)Θ) + (𝜖𝐹(𝑆𝑖𝑂2)(1 − Θ)] (2.1)
20
where 𝑗+ is the positive ion flux; Φ𝑠 is the average sputtering efficiency; 𝑛𝐹 is the fluorine
concentration; 𝑛𝐹𝑣𝐹̅̅̅̅
4 is the fluorine atom impingement rate; 𝜖𝐹(𝑆𝑖𝑂2
∗) and 𝜖𝐹(𝑆𝑖𝑂2) are the reaction
probabilities and are defined as the number of SiO2 molecules leaving the surface per incident
fluorine atom with surface sensitized and un-sensitized material respectively; and Θ is the fraction
of the surface has been sensitized by ion bombardment which exhibits enhanced reactivity with
fluorine atoms. The expression indicates that the etch rate is contributed by two parts: the left side
of the plus sign in (2.1) describes physical bombarding component to etch glass; the right side of
the plus sign in (2.1) accounts for the chemical component of glass etching due to the flux of
fluorine atoms.
In the conventional etch characteristics of the ICP-RIE chamber using SF6 gas introduced
from the top gas inlet of the ICP source. In these experiments, fused silica wafers were etched for
20 minutes at a source power of 2000 W and substrate power of 400W. The backside He wafer
cooling temperature was set at 20 °C and the wafer position with respect to the ICP source was set
at 120 mm. The etch rate of fused silica is plotted as a function of SF6 flow rate in Figure 2-2(a).
Initially as the flow rate is increased, both the etch rate and the etched surface roughness increase,
reaching a maximum etch rate of ~0.80 µm/min and a corresponding surface roughness of Ra~ 40
nm at a pressure of 4.5 mTorr corresponding to SF6 flow rate of 100 sccm. The error bars were
obtained by measuring the etch rate for several 100 µm features across a single wafer. For higher
than 100 sccm SF6 flow rates, both the etch rate and the surface roughness decrease. Previous work
has shown two possible mechanisms can be proposed for the anisotropic reactive ion etching of
SiO2: (i) damage caused by impinging energetic ions followed by a subsequent reaction of the this
damaged silica layer by fluorine radicals or (ii) sputter desorption (clearance) by impinging ions of
fluorinated silica surface in fluorine rich plasma [56]. While neither mechanism has been clearly
unambiguously proven, based on SF6 etching of silica it is now believed that the positively charged
21
impinging ions essentially orchestrate the breakage of the Si-O bonds thereby freeing oxygen atoms
and the silicon atoms then react with atomic fluorine to form volatile SiF4 and oxyfluoride products
[57], [58]. Thus, SF6 based etch process is dominated by the copious production of atomic fluorine
from electron impact processes as well as by the large SFx+ ion concentrations. The overall etch
rate is considered to be a result of the surface reactions induced in the damaged silica layer by the
physical bombardment by the large SFx ions. In fact this is clearly corroborated with the RGA data
where a roughly linear relationship between the etch rate and the percentage of atomic fluorine and
SF5+ ion concentrations can be found for the various SF6 flow rates as shown in Fig. 2-2(b) Inset.
However, the smoothness of the etch does not directly correlate to the measured SF6+F partial
pressures but instead is found to be best under the conditions of low chamber pressures
corresponding to high ion bombardment energies and also at the higher pressure regions
corresponding to higher flux of impinging etchant species.
22
Figure 2-2. (a) Experimentally obtained etch rate and rms roughness values for various SF6 flow
rates. Since the pumping speed remains constant for these conditions, increasing flow rate
corresponds to increasing pressure in the chamber. Error bars were obtained by measuring etch rate
for several 100 µm features across a single 4” silica wafer Etch time = 20 min. (b) Residual gas
analyzer data for 100 sccm SF6 flow clearly shows the presence of atomic fluorine as well large
amounts of SFx species. Inset shows the sum of the partial pressures of F and SF5+ concentrations
for various SF6 flow rates and the corresponding etch rates of fused silica.
2.4 Modified ICP-RIE Chamber for Fused Silica Glass Etching
In this work, the commercial Alcatel AMS 100 ICP-RIE etch tool was modified. In addition
to conventionally feeding the etch gases through the ICP source, NF3 and H2O vapor etch gases
Atomic Mass (amu)
Inte
nsity x
10
7 (
Arb
Units)
0 20 40 60 80 100 120 140-0.9
0
0.9
1.8
2.7
3.6
4.5
5.4
6.3
SF5
SF SF2
SF4
SF3
OF2O2H2O HFF
F2
a
b
Partial Pressure of F and SF5 (%)
Etc
h R
ate
(
m/m
in)
40 42 44 46 48 50 52 54 56 58 600.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
20 sccm
200 sccm60 sccm
125 sccm
150 sccm
100 sccm
23
were locally introduced in the vicinity of the wafer using two modifications: (i) Diffuser Ring and
(ii) Diffuser Tube which will be described in detail next.
2.4.1 Chamber Modification 1: Diffuser Ring System
Alcatel AMS 100 ICP-RIE etch tool was modified in this work. In addition to
conventionally feeding the etch gases through the ICP source, NF3 and H2O vapor are introduced
in the vicinity of the wafer using a stainless steel gas diffuser ring which is attached to the stainless
steel plate of the mechanical clamping plate of the etcher. Figure 2-3 shows schematically the
modifications made to the etch chamber (indicated within the dashed box) and Fig. 2-4(a) and (b)
show the optical pictures of the gas diffuser showerhead attached to the substrate holder in the
chamber and a zoomed view of the nozzle holes. The diameter of the diffuser ring is 9.6 cm and 1
mm diameter nozzles are placed along the inner side of the diffuser ring with 1 cm spacing between
them. H2O vapor is generated by heating a sealed stainless steel container of water which is 15 cm
high and placed on a hot plate which is maintained at 50 °C. The flow rate of H2O vapor is
controlled using MKS® Type 1150A mass flow controller. NF3 gas is controlled by a separate Unit®
1620 mass flow controller but shares the same inlet tube with H2O vapor into the showerhead
fixture after the mass flow controller. An in-situ residual gas analyzer (RGA) Extorr® XT300
consisting of a quadrupole mass spectrometer complete with a built-in Pirani and ion gauge is
connected to the reactor chamber in order to analyze the plasma and etch reaction species. The
RGA is capable of detecting molecular species of up to 300 amu. Stylus profilometer (Tencor® P-
16) and atomic force microscope (AFM) (PSIA® XE100) was used to characterize the etch
topography and smoothness of the etched features. Auger spectroscopy was used to analyze the
atomic and molecular species on the surface and sidewalls of the etched glass regions. Table 2-3
lists the ranges of power and gas flow rates of individual gases on the modified etch system. The
24
plasma dc self-bias voltage for this system has been measured and reported to be less than ~ 14 V
under typical operating conditions used here [59]. Although the plasma can be obtained up to a
pressure of ~75 mTorr, the glass etch rates, irrespective of gas chemistry used, were found to
monotonically decrease above a pressure of ~12 mTorr reaching an etch rate of 0.05 µm/min for
pressures above 45 mTorr. Therefore, all the optimization results presented here are in the range of
0 – 12 mTorr.
Figure 2-3. Schematic illustration of the modified ICP-RIE systems with ring-diffuser system
connected to NF3 and H2O gas cylinders through the gas panel.
RF Power Supply RF Matching Network
Gas Inlet
ICP Source
Antenna
Magnetic Pieces Circling the Chamber
Diffusion Chamber
Substrate Holder & Mechanical Clamp
NF3 Gas Panel
NF 3
Gas
Cyl
ind
er
Vacuum Sealed Gas Feedthrough
NF3 Gas Diffuser Ring
Wafer
NF3+H2O Gas Panel
Wat
er V
apo
r
RF Substrate Bias
25
Figure 2-4. (a) Optical photo showing the top view of the machined shower head fitted to the
substrate holder and (b) Optical photo showing an oblique view of the shower head with zoomed-
in image of the showerhead nozzles.
Table 2-3. Summary of the process parameter space available on the modified ICP-RIE system
being optimized for silica etching.
Process
Parameters
Range
ICP Source Gases
SF6 0 – 200 sccm
Ar 0 – 100 sccm
O2 0 – 100 sccm
Ring Diffuser Gases
NF3 0 – 250 sccm
H2O 0 – 300 sccm
Physical Parameters
Source Power 0 – 3000 W
Substrate Power 0 – 600 W
Pressure 0 – 75 mTorr
b
a
26
2.4.1.1 Fused Silica Etching Characteristics with Ring Diffuser System
In this section, SF6 gas is used as the ICP source gas and enters the chamber from the top
gas inlet and the plasma is ignited using ICP RF coil, simultaneously NF3 and H2O vapor are
introduced into the chamber through the diffuser ring. Diffuser ring is proposed to locally introduce
NF3 gas and H2O vapor in the vicinity of the wafer. Once the source and substrate RF power are
turned on, the top source power is able to ionize SF6 molecules while, the substrate bias power
drives the positive ions from the plasma towards the NF3 and H2O gas cloud near the glass wafer
and dissociates the highly unstable NF3 gas into NFX radicals. Figure 2-5 shows the bar graph of
the partial pressures of the various gas species of interest in the plasma obtained by in-situ RGA
measurements. The legend in Figure 2-5 provides the corresponding etch rate for various gas flow
rate combinations. Wafers were processed at 2000 W source power and 400 W substrate power.
Each wafer was etched for 15 mins. For each gas species the bars shown correspond from the
highest to the lowest etch rates from left to right. The pressure in the chamber in these etches varied
from 7 mTorr - 12 mTorr. From Figure 2-5 several observations can be made:
(i) In general, faster etch rates (#1-3) are dominated by large partial pressures of fluorine and
SFx radicals/ions – although the relative roles of each is unclear.
(ii) Fast etching can also be achieved in processes which show high concentration of SFx ions
only as can be seen in sample #5 SF6:NF3:H2O :: 200:0:0 etch represented by light olive
green bar in Fig. 5(b). However, as we will show next, etches dominated by large SF5
content result in higher roughness.
HF seems to play a less important role in glass etch rate than atomic fluorine and SFx
ions/radicals as high HF and H2O concentrations correspond to the slowest etch rates obtained
samples #7-10 (blue colored bars). These observations relating to negligible effect of HF on the
etching of glass was also concluded by Yamakawa et al [60]. However, they attribute the high etch
27
rate observed in their work in NF3 + H2O atmospheric pressure microwave plasma to the formation
of ionized HF2− - which is known to vigorously react with SiO2. They propose that the interaction
of ionized HF molecules with the water molecules on hydrated glass surface result in the formation
of ionized HF2− which then dissolves the SiO2. Furthermore, this etch rate is further enhanced by
the low-energy ion bombardment which results in the extraordinarily high etch rates reported in
their work. However, it must be emphasized that in our work, the pressures in the chamber are
about six orders of magnitude smaller than an atmospheric plasma used in their work and in spite
of high water flow rates used we do not expect much hydration of the surface. Thus, the etching
process in an ICP-RIE system is dominated by the positively charged ions and fluorine radicals
available in the chamber. Finally, for all our etches the mass spectrometer showed either no peak
or negligible peak corresponding to HF2 species indicating either their absence or a rapid
decomposition within the etch chamber.
Figure 2-5. Bar graph showing the composition of the plasma species for the various gas flow rates.
The legend shows the etch rate and pressure in the chamber at which the etch was performed. For
each species the bars correspond to etch rates from fastest to the slowest from left to right. Etch
time in all cases was 15 min.
Table. 2-4 shows the etch rate of fused silica glass using the ring diffuser system is higher
by 19% as compared to conventional etching under similar physical operating parameters. The
Pa
rtia
l P
res
su
re (
mT
orr
)
a
-0.3 -0.3
0.3 0.3
0.9 0.9
1.5 1.5
2.1 2.1
2.7 2.7
3.3 3.3
3.9 3.9
4.5 4.5
5.1 5.1
5.7 5.7
F F2 NF2 NF SF5 SF4 SF3 HF H2O
Etch # SF6(sccm)NF3(sccm)
H2O(sccm)
Etch Rate(m/min)
Pressure(mTorr)
1 60 100 50 0.83 8.52 40 100 50 0.80 8.03 100 100 50 0.74 9.04 40 100 100 0.72 9.55 200 0 0 0.70 8.56 20 60 100 0.69 7.07 60 20 100 0.61 8.08 20 60 200 0.53 11.59 60 20 200 0.48 12.0
10 20 20 200 0.36 9.5
28
average etched surface roughness (Ra) was measured using precision scans of the region using the
Tencor® stylus profilometer capable of surface roughness resolution in the Angstrom range.
However, to ensure that the measurements were not artifacts of the measurement system, the most
promising results were verified using atomic force microscope scans. Through these measurements,
with scan rate of 3 Hz and set point of 20 nN at a area of 10 × 10 µm2, it was confirmed that the
achieved surface roughness of the etched areas from the ring diffuser set-up was an order of
magnitude better than conventional etch process.
Table 2-4. Comparison between SF6/NF3/H2O based etching and single SF6 plasma etching at 400
W substrate power.
1Power 2SF6 2NF3 2H2O 3Pressure 4Voltage 5ER 6Roughness*
2000
60 100 50 8.5 78 0.83 2.6
200 0 0 8.5 83 0.70 99.9
2500
60 100 50 8.5 79 0.95 8.3
200 0 0 8.5 83 0.75 1143.7
Units: 1Watt , 2sccm, 3mTorr, 4Volts, 5ER: Etch Rate: µm/min, 6Å. *Roughness is measured using
Tencor® Profilometer.
It is well known that higher source power corresponds to a higher plasma density and
therefore should result in a higher etch rate. To improve the etch rate further the source power was
increased from 2000 W to 2500 W while the substrate power was maintained at 400W. Clearly the
etch rate increased and for some of the larger features we were able to achieve etch rates exceeding
1 µm/min. Table 2-4 shows that the etch rate of fused silica at 2500 W source power using the ring
diffuser is higher by 27% as compared to conventional SF6 based etch and is higher by 14% as
compared to an identical etch performed at 2000 W source power with ring diffuser. The reported
value of 0.95 µm/min in Table II is the average etch rate across several features of different sizes
(2 – 1000 µm width). Furthermore, it can also be observed that increasing source power
dramatically increased the etched surface roughness for both ring diffuser etch as well as for pure
29
SF6 based etch process. Comparisons of the SEM images of etched surface using SF6 based
conventional etch and the ring diffuser process listed in Table 2-4 are shown in Figure 2-6. In both
cases the wafers were etched for 20 mins. Nickel masks were stripped by Transene™ TFB nickel
etchant. The bright edges around the squares arise from the undercutting of Au/Cr seed layers
during the Au/Cr wet etching performed prior to the plasma etch. SEM images reveal that the ring
diffuser etch results in a smooth surface with no surface roughening effects due to micromasking
effects from the nickel hard mask erosion.
Figure 2-6. SEM pictures of etched surface by (a) conventional SF6 etch process (left) and (b) ring diffuser
system (right). The etches were performed at PSource = 2000W and PSubstrate = 400 W and etch time for both
cases was 20 minutes.
Figure 2-7(a) shows the etch roughness data as a function of various gas flow rates and all
etches were 15 minutes in duration. Using the ring diffuser gases it was possible to achieve an
unprecedented roughness of 1.8 Å as measured using the Tencor® surface profilometer. Prior to
this work, the commonly accepted technique for achieving smooth surfaces in glass etching was to
use large ion bombardment as a way to achieve nm level smoothness [48]. The accepted theory
being that bombardment by large ions like Xe result in the effective breakage of Si-O bonds leaving
silicon behind to be attacked by atomic fluorine in the plasma. Micromasking effects arising from
sputter deposition of nickel hard masks in the etched areas are also thought to be the main reason
for the resulting roughness. The use of large ions such as Xe are thought to effectively resputter the
40 m
a
40 m
b
30
inert fluorides due to heavy ion bombardment and thus improve the surface roughness [48]. In
reported literature, etches resulting in smooth surfaces are typically performed at low pressures
where ion energies and ion mean free path are large and therefore correspond to regimes of large
physical etching content and thus lend credibility to the sputtering theory [48]. The etches we report
here have been performed at higher pressures roughly about 2 – 3 times larger than typical ICP
glass etches [45], [48] and no noble gas was used. Only positively charged flux of SFx and NFx ion
fragments acts as the physical sputtering source. Although no obviously clear trends of relationship
between the chemical species present and etch smoothness are evident in Figure 2-7(a), the
following observations can be readily made:
(i) Atomically smooth etches (Sample #1-3; red bars) all have a large content of HF and H2O
in the RGA spectrum. In addition strong SF5, F, and NF2 peaks are observed in these etches.
(ii) Roughest etch surfaces (sample #8-11; blue colored bars) are typically dominated by large
SFx species peak and small HF and H2O peaks.
This suggests that while HF may not directly play a role in etching of fused silica since the
etch rates of these processes are fairly low, however HF in the presence of H2O and SFx ion flux is
able to induce surface nucleation and surface reactions leading to atomically smooth surfaces. An
additional difference is that smooth etches are performed at higher pressures and are likely to be at
lower ion energies and mean free paths lending credibility to a more chemically driven surface etch
than a physically driven process. Finally, we found that the etched surface smoothness is a function
of etch time and increased from Ra 0.3 nm for a 15 minute etch to ~2.8 nm for 150 minute etch
and 100 µm feature, rising monotonically for SF6:NF3:H2O :: 60:100:50, PSource = 2000 W and
PSubstrate = 400W process recipe.
31
Figure 2-7. (a) Bar graph showing the chemical composition of the plasma as measured by the RGA
with the etches with smoothest surfaces (red) to roughest (blue) listed from left to right. Clearly
large HF and H2O peaks are seen in the smoothest etches – whereas the rough surface etches are
dominated by large SF5 peaks and do not use H2O as the process gas. (b) AFM image of the smooth
etched surface obtained with ring active etch corresponding to SF6:NF3:H2O :: 60:100:50. All
etches were performed for 15 minutes.
2.4.1.2 Effect of NF3
In order to further evaluate the effect of NF3 on silica etching, various etches with only
H2O introduced from the diffuser ring were compared with one where both NF3 and H2O are
Pa
rtia
l P
res
su
re (
mT
orr
)
-0.3
0.0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
2.4
2.7
3.0
3.3
3.6
3.9
4.2
4.5
F F2 NF2 NF SF5 SF4 SF3 HF H2O
a
b
Etch #
SF6(sccm)
NF3(sccm)
H2O(sccm)
Roughness(Å)
Pressure(mTorr)
1 20 20 200 1.8 122 60 0 140 2.3 8.53 60 100 50 2.6 8.54 40 20 0 4.3 3.55 20 60 50 16.6 5.56 20 0 25 18.5 2.07 20 100 80 45 8.08 200 0 0 99 8.59 20 100 0 242 7.0
10 100 0 0 389 5.0
11 100 20 0 500 6.5
32
simultaneously introduced through the ring. For all etches SF6 was introduced through the top gas
inlet of the ICP source and the total pressure was maintained at 8.5 mTorr by adjusting the total
amount of gas flow rate to 200 sccm. Wafers were processed at 2000 W source power and 400 W
substrate power for 15 mins. Figure 2-8 summarizes the results and some of the results from Table
2-4 are recalled for easier comparison. For etches #1 – #3 as the SF6 flow rate in relation to H2O
increases, a corresponding increase in etch rate is observed and can be readily explained by the
monotonic increase in the SFx concentrations. Furthermore, as the SF6 flow rate increases, a
decrease in the bias voltage is observed which corresponds to an increase in the positive ion
bombardment of the substrate wafer and explains the increasing etch rate. However, it is important
to note that the level of atomic fluorine is nearly constant for all these etches and does not seem to
affect the etch rate as clearly as SFx concentrations. On the other hand, comparing samples #3 and
#5, shows that for similar fluorine containing gas flow rates and identical H2O vapor flow rates,
NF3 plays a critical role and results in a dramatic increase in the etch rate by 15% and also reduces
the roughness of the etched surface. Since the substrate power in all these etches was held constant
at 400 W, any decrease in the substrate potential reflects an increase in ion current. In order to avoid
over interpretation of the data, we estimated the relative increase in the ion current density
bombarding the substrate with respect to the etch with largest substrate voltage (sample #1). The
legend in Figure 2-8, shows a clear correspondence between the ion flux rate and etch rate – where
for a relative ion flux rate increase from 8.25% to 16.67% the etch rate increases from 0.72 to 0.83
µm/min (Sample #3 and #5). Furthermore, for sample #5 nearly three times larger concentration of
F atoms is also observed as #3 and #4. Additionally, in the case of sample #5, the presence of NFx
molecular fragments could also play a critical role in the overall etching of the silica substrates and
could explain the observed high etch rate.
33
Figure 2-8. Bar graph showing the partial pressures of various molecular fragments for SF6 + H2O
plasma. All etches shown here were performed at a constant pressure of 8.5 mTorr. The legend for
the graph lists the various gas flow rates, the bias potential, the obtained etch rates and roughness
values, and the percent increase in the ion flux in comparison to the SF6:NF3:H2O :: 60:0:140 etch.
All etches were performed for 15 minutes.
2.4.1.3 Aspect Ratio Dependent Etching
Next we examine the etch rates of features of different trench widths performed at 2000 W
and 400 W source and substrate powers respectively. Figure. 2-9 shows the result of aspect ratio
dependent etching performed using SF6:NF3:H2O::60:100:50 etch for 15 minutes. Feature sizes on
the mask ranged from 2 µm to 1000 µm. Results show that the highest etching rate of fused silica
using ring diffuser system was obtained on 100 µm wide features. This can be understood by the
fact that in the narrow features the supply of elemental fluorine is limited by diffusion time constant
(diffusion limited) whereas in wider features the etching is limited by surface reactant depletion
due to rapid consumption of elemental fluorine (loading effect).
Part
ial
Pre
ssu
re (
%)
0
10
20
30
40
50
60
F NF2 NF SF5 SF4 SF3 HF H2O
0 0
3
0 1
37
220 0
20
2
5
25
220 0
46
4
11
7
13
0 0
51
5
13
24
7 6
2
20
25
8
Etch SF6 NF3 H2O Voltage Etch Rate Roughness Relative Ion Flux# (sccm) (V) (m/min) (Å) (%)1 60 0 140 93 0.32 8.0 02 100 0 100 91 0.46 9.5 1.943 150 0 50 85 0.72 10.1 8.254 200 0 0 83 0.70 99.0 10.535 60 100 50 78 0.83 2.6 16.67
34
Figure 2-9. Aspect ratio dependent etch rates of fused silica features of varying widths. The data
shown here is for 15 minute long etches using SF6:NF3:H2O::60:100:50, 8.5 mTorr pressure, 2000
W source power and 400 W substrate power and etch time of 15 minutes.
2.4.2 Chamber Modification 2: Diffuser Tube System
The diffuser ring modified ICP-RIE glass etching system realizes the tuning between
physical bombarding etching component and chemical reacting etching component in the glass
etching mechanism. The achieved fast etch rate and unprecedented smooth etch suggest the
improving chemical etching component in the modified etcher. The RGA data indicates atomic
fluorine, fluorine molecule, and NFx radicals are critical species for fast glass etch. However,
diffuser ring which is adapted on the substrate holder above the wafer may not necessarily be the
optimum position for ionizing richest NFx and Fx radicals. Therefore, the second modification of
the ICP-RIE chamber is raised up as a diffuser tube configuration that a 10 cm stainless steel tube
is adapted on the substrate to introduce NF3 and H2O gases. The modification is sketched in Figure
2-10. The inset image shows the diffuser tube in 10 cm height above the substrate holder.
Feature Size (lateral) (m)
Etc
h R
ate
(
m/m
in)
0
0.2
0.4
0.6
0.8
1
2 20 40 100 250 500 1000
0.720.76 0.79
0.83 0.8 0.79 0.78
35
Figure 2-10. Sketch of the diffuser tube modified ICP-RIE; the inset picture show the optical image
of diffuser tube.
2.4.2.1 Glass Etching Characteristics with Diffuser tube System
In this section, etching characteristics with diffuser tube modification are explored on fused
silica. SF6 gas is still introduced from ICP source and NF3/H2O gas mixture are flow from the
additional inlet through the diffuser tube into the chamber. The substrate stage is 120 mm away
from the ICP source. The NF3/H2O gas outlet point of the diffuser tube is bent to locate in the center
10 cm above the substrate. Figure 2-12 illustrates the obtained etch rates with different
SF6/NF3/H2O combinations on fused silica substrates. Using this modified system, etch rates as
high as 1.06 μm/min, 1.04 μm/min, 0.45 μm/min and 0.45 μm/min with surface smoothness of ~2
Å for fused silica are achieved respectively after 5 minutes etch. The etch rates are within ± 0.01
μm/min variation based on the average of 5 data points.
36
Figure 2-11. Obtained etch rates with diffuse tube on fused silica substrates.
In addition, etch characteristics are also investigated with different tube lengths. Table 2-5
demonstrates the etch rate and roughness of fused silica, borosilicate glass and aluminosilicate glass
in different diffuser tube lengths. In the diffuser tube configuration modification, 1.06 μm/min etch
rates are achieved for etching fused silica, borosilicate glass and aluminosilicate glass respectively.
Etch rates with diffuser tube configuration is 12% higher compared with the optimum etch rates
obtained with diffuser ring configuration and 32.5% higher compared with the conventional SF6
based glass etching.
Table 2-5. Etch rates and roughness obtained with different lengths tube on fused silica. Wafers
were etched for 5 mins. Etch rates and surface roughness values were presented as averages of five
100 μm wide feature data which were acquired by profilometer.
Glass type
Tube
Length (cm)
Recipe:
SF6/NF3/H2O
(sccm)
Etch rate (μm/min) [Roughness (Å)]
Distance between substrate and ICP
source (mm)
105 120
Fused Silica
7
60/100/50
1.05 [3.3] 0.97 [2.1]
10 1.06 [1.7] 1 [1.9]
12 1.04 [2.3] 1 [2.1]
37
Next we examine the etch rates of features of different trench widths performed at 2000 W
and 400 W source and substrate powers respectively. Figure. 2-9 shows the result of aspect ratio
dependent etching performed using SF6:NF3:H2O::60:100:50 etch for 15 minutes. Feature sizes on
the mask ranged from 2 µm to 1000 µm. Results show that the highest etching rate of fused silica
using ring diffuser system was obtained on 100 µm wide features. This can be understood by the
fact that in the narrow features the supply of elemental fluorine is limited by diffusion time constant
(diffusion limited) whereas in wider features the etching is limited by surface reactant depletion
due to rapid consumption of elemental fluorine (loading effect).
2.5 High Aspect Ratio Etching of Fused Silica
Figure. 2-12 shows SEM cross section image of over 100 µm deep etched trench using
process conditions for sample #5 in Figure. 2-9 and a continuous single etch time of 150 minutes.
The effective etch rate dropped from a value of 0.76 µm/min to ~0.68 µm/min for the 20 µm feature.
The nominal decrease in the etch rate can be attributed to the reduced supply of etch species into
the deeper etch feature. Vertical sidewalls with 88.7° can be seen and an aspect ratio of greater than
1:5 was easily obtained. These images are in contrast with highly physically dominated etching
processes where deep trenching features are observed along the edges of the silica walls along with
tapered profiles.
In addition to aspect ratio dependent etching, we studied the loading effect by using two
different masks with open areas of 20% and 50%. For the ring-based etch recipes used here, we
obtained nearly same etch rates for same sized features for the two masks although the uniformity
across the wafer was somewhat changed in the two cases.
38
Figure 2-12. SEM photograph of a 20 µm wide feature shows a very vertical wall with sidewall
angles of 88.7°. The bottoms of the trenches are flat and do not show any trenching features
indicating the chemical nature of the etch. Although the image shows a slight bottle like shape, this
is likely due to the angled facet of the image arising during the cleavage of the sample for SEM.
Etch time was 150 min.
2.6 Skin Layer Formation
Another interesting observation in all etches was the formation of ~100 nm thick layer on
the side walls of the etched trenches. We found that this layer could be readily stripped in
TranseneTM nickel TFB etchant but was completely inert in TranseneTM nickel Type 1 etchant. The
SEM images of the sidewall before and after nickel striping after identical etching using the ring
diffuser etch are shown in Figure. 2-13(a) and (b) respectively. Auger spectroscopic analysis of the
etch sidewalls and bottom surfaces was used to analyze the elemental composition of these regions.
Fluorine, oxygen and nickel peaks were observed in the Auger spectrum of the sidewalls indicating
formation of nickel fluoride or nickel oxide on these surfaces. On the other hand, only silicon and
oxygen peaks were observed at the bottom of etched trenches. This suggests that on the sidewall
where ion bombardment is minimal, a layer of inert nickel fluoride and/or nickel oxide forms due
to the re-deposition of the hard mask material whereas on the bottom of the trenches the large flux
102.3 m
19.8 m
39
of ion bombardment prevents any such inert layer formation. This is in contrast to our
understanding thus far that SF6 based etches as opposed to fluorocarbon gases do not form any
inhibitor layers [61]. After stripping of the sidewall film, extremely smooth surface was observed.
The serrated edge at the top of the sidewall is due to nearly running out mask in this particular
sample.
Figure 2-13. (a) Sidewall layer formation can seen in the SEM photograph where ~100 nm thick
nickel fluoride/oxide layer is formed on the sidewalls. Inset shows a broken fragment of the formed
sidewall film (b) After stripping the layer in nickel etchant, a smooth sidewall is obtained. The
formation of passivation layer on the sidewalls is able to provide inhibitor driven anisotropy.
2.7 High Aspect Ratio Etching of Fused Silica
In order to evaluate the effect of masking material on the etch rate, we prepared a wafer
with a 1 µm thick chromium layer deposited via sputtering as the hard mask layer. Lift-off process
was used to pattern the chromium mask. Sidewall formation was also observed in the case of
chromium mask. Comparison of etching performance between nickel and chromium hard masks is
shown in Table 2-6. In these tests, wafers were etched using SF6:NF3:H2O :: 60:100:50 etch recipe
at 2500 W source power and 400 W substrate power for 15mins.
40
Table 2-6. Mask Dependent Etching Performance 2500W/400W Power
Mask Pressure Voltage Etch Rate Roughness Selectivity
(mTorr) (V) µm/min) (Å)
Nickel 8.5 79 0.95 8.3 15.2
Chromium 8.5 79 0.69 26.4 13.6
2.8 Summary
In conclusion, we were able to successfully demonstrate high aspect ratio fused silica
etching using SF6, NF3, and H2O based etching using a modified ring diffuser modification to the
etch chamber. Using this system we were able to achieve a high etch rates of ~1 µm/min as well as
high aspect ratio etches in fused silica with greater than 1:5 with extremely smooth sidewalls and
flat bottom profiles with Ra ~5 Å.
Due the use of two large fluorine containing gas molecules the plasma gas breakdown
analysis and reaction pathways are fairly complicated. For example, it is well known that SF6 based
ICP plasma processes are dominated by dissociative ionization processes which leads to the
formation of SF5+, SF4
+, SF3+, ions and F through the following reactions [62]
eFFSFSFe
eFSFSFe
eFSFSFe
2
2
2
236
246
56
These reactions are consistent with the dominant peaks obtained from the RGA data in this work.
It is also well known that the relative ionization efficiency of SF3+ is greater than that of SF4
+ and
can explain the consistently lower concentrations of SF4+ found in the RGA data. The addition of
NF3 gas further adds to the elemental fluorine concentration since it is known that NF3 readily
produces ground state fluorine by an electron impact dissociation process such as [63], [64]
41
FNFeNFe
23
which we have shown through RGA measurements is greatly enhanced in all etches containing
NF3. Optical emission data from the plasma chamber was collected using an OceanOptics®
HR4000CG miniature fiber optic spectrometer. Specifically, the amplitude of the peak
corresponding to fluorine excitation at 703.7 nm for varying flow rates of SF6 was measured and
did not show any clear correlation to the corresponding etch rates. This can be explained by the fact
the emission of photon occurs due to the excited fluorine radical relaxing to ground state whereas
the etch rate is dependent upon the concentration of ground state fluorine atoms. Thus, a direct
correspondence between the two need not be necessarily observed.
An additional complication in the analysis of the etch process in using NF3 plasma arises
from the fact that NF3 gas is known to result in a high density of negative ions which have been
shown to react very vigorously in the decomposition of surface chemisorbed species [63]. The
formation of ambipolar plasma and the negative biasing of the substrate with respect to the plasma
by use of the 13.56 MHz substrate source does not allow for the full exploitation of the negative
ion initiated surface reactions. This would be part of the future work.
In summary, the ring diffuser modification to the plasma chamber is able to engineer glass
etches that are dominated by surface chemical reaction processes as opposed to purely physical
driven processes. Although the roles of the individual molecular fragments in the etching were not
uniquely identified, the presence of large elemental fluorine and SF5 peaks have correlated well
with high etch rate processes. Unprecedented smoothness of few Å was obtained using these
processes with corresponding etch rate for fused silica in excess of 1 µm/min. The findings of this
work are in general consistent with the overall silica etching theories with additional chemical
component enhancement achieved through the use of incomplete dissociation of NF3 gas and
creation of NFx and F radicals. The absence of strong trenching effects at the bottom of etched
features, near vertical sidewalls, and highly smooth surfaces indicate a chemically dominated
42
process where micromasking features formed by inert metal fluorides are isotropically undercut
and removed. This is in contrast to ion-bombardment dominated processes where energetic ions
are used to re-sputter the inert metal fluorides formed during the etch and result in an order of
magnitude rougher surface morphology. Our work has also found a strong correlation between
large HF concentration and smoothness although large concentrations of HF had no influence on
the overall etch rate of fused silica at the pressures at which these etches were performed. In
conclusion, the use of NF3 based ring diffuser introduces a new paradigm for etching where reactive
species via partial ionization and dissociation of gas molecules can be realized for engineering
surface chemisorption and subsequent reactions through ion bombardment for rapid and chemically
dominated etching processes.
43
Chapter 3
Inductive Coupled Plasma – Reactive Ion Etching (ICP-RIE) of Borosilicate
and Aluminosilicate Substrates
3.1 Introduction
Being inexpensive and readily available, silica substrates exhibit several desirable
properties including optical transparency, excellent electrical insulation, high dielectric breakdown,
good mechanical properties, and biocompatibility. Silica substrates are available in both crystalline
as well as amorphous forms. Crystalline form of silicon dioxide is known of quartz crystal (~100%
SiO2) and is most commonly used as bulk acoustic wave resonator in frequency reference and
control applications by virtue of its piezoelectric properties [24], [32], [65], [66]. Amorphous form
of silicon dioxide with high chemical composition purity, known as fused silica or glass (99.9%
SiO2, 0.1% H2O) is widely used in optical devices such as micro-optical-electro-mechanical
systems (MOEMS), optical waveguides and fibers, and microlens etc [67]–[69], due to its high
optical transparency and relatively low thermal expansion coefficient. Amorphous borosilicate
glass (80.6% SiO2, 12.6% B2O3, 4.2% Na2O, 2.2% Al2O3, and 0.1% CaO) commonly known by its
trade names Pyrex ® or Borofloat® can be readily bonded to silicon wafers [29] due to its matched
thermal expansion coefficient to silicon. It can be plastically deformed and shaped into 3D
structures such as microspherical shell [30], [31], [70] and can be used in chip-scale packaging of
various microelectromechanical systems (MEMS) [11], [19], [23], [71]–[73]. Another family of
amorphous glass known as aluminosilicates (typically: 55% SiO2, 21.4% CaO, 10.4% Al2O3, 7%
B2O3, 1% Na2O) known by the brand name Eagle ® glass, is a mechanically strengthened glass by
ion exchange process during the manufacture, is extensively used in liquid crystal displays [74] and
44
solar cell panels [75].
As a critical microfabrication process, etching of glass materials have been investigated for
more than thirty years. Initially, most of the efforts in silicon dioxide etching were directed towards
realizing features for micro-optical and microelectronics applications such as waveguides [34],
phase shift masks [35], etc. These glass etching processes therefore primarily focused on
controlling the selectivity between silicon oxide and silicon [36], reducing gate oxide damage [37],
and decreasing sidewall roughness [38]. With the advent of MEMS and microsystems in the last
two decades, requirements of glass etching processes has focused on new etching performances
such as high etch rate, high aspect ratio, low surface roughness, high selectivity to masking
material, and etch uniformity across the wafer. However, the performance metrics of high etch rate
and high aspect ratio micromachining processes for glass wafers have lagged significantly behind
that of silicon. Furthermore, glass etching continues to be of research interest since the plasma
induced physicochemical etching mechanisms during glass etching are still not very well
understood. Glass etching mechanism can be expressed by the equation (2.1) [56]. The expression
indicates that the etch rate is contributed by two parts: the left side of the plus sign in eq. (2.1)
describes physical bombarding component to etch glass; the right side of the plus sign in eq. (2.1)
accounts for the chemical component of glass etching due to the flux of fluorine atoms. In general,
the etch rate of silica glass is dependent upon the number of silicon atoms bonded to four bridging
oxygen atoms. In wet etching of silica it is well known that the replacement of the first oxygen by
a fluorine ion is the slow and rate-determining reaction step whereas the subsequent reaction steps,
to remove the SiF unit from the SiO2 network, are 18-20 times faster [76]
Inductively Coupled Plasma – Reactive Ion Etching (ICP - RIE) system provides excellent
control over plasma density (controlled by ICP power) and energy of etchant ions (controlled by
substrate power) and generates stable plasma under relatively low pressures (0.5 mTorr – 50 mTorr)
which is necessary for removal of etching products. There have been several reports on glass
45
etching using various ICP-RIE processes since the late 1990’s. A summary of the reported glass
etching results in the literature in terms of etch rate, surface roughness, mask materials, silica to
mask selectivity etched depth and aspect ratio is shown in Table 2-1. The table demonstrates the
spurts of research developments in the field leading to reported etch rates of 0.6 – 0.9 µm/min and
surface smoothness in the range of a few nanometers.
High etch rate and high aspect ratio etch process for fused silica substrates is reported in
Chapter 2. The work utilized a diffuser ring gas inlet on the substrate holder in the ICP – RIE etcher
that allowed for supplying NF3 and H2O gases in the vicinity of the glass wafers. In this Chapter
we examine etching of doped glasses such as borosilicate and aluminosilicate glasses in fluorine
plasma using this modification of the etch chamber as well as examine how the position of the
diffuser relative to the plane of the substrate wafer influences the physico-chemical reaction
environment as well as the charge build up on the glass substrates.
3.2 Experimental Setup
3.2.1 Chamber Modification
In this work an Alcatel AMS 100 ICP-RIE etch tool was modified. In addition to
conventionally supplying the etch gases through the ICP source, NF3 and H2O vapor are introduced
in the vicinity of the wafer using a stainless steel diffuser-ring gas inlet which is attached to the
stainless steel plate of the mechanical clamping plate of the etcher. Figure 2-3 schematically
illustrates the modifications made to the etch chamber. Figure 2-4 (a) (b) show the diffuser-ring
showerhead attached to the substrate holder in the chamber and a zoomed view of the nozzle holes.
The diameter of the diffuser-ring is 9.6 cm and 1 mm diameter nozzles are placed along the inner
side of the diffuser-ring with 1 cm spacing between them. Since the diffuser ring is attached to the
46
substrate holder, the distance between them is nominally assumed to be zero cm. In this work, we
also examine the influence of the distance of the diffuser gas inlet with respect to the substrate
surface on the physicochemical etching environment. To achieve varying distance at which etch
gas is introduced with respect to the substrate surface, alternative diffuser-tubes of different heights
were implemented. Figure 1(b) schematically shows the diffuser-tube modified ICP-RIE system,
and the inset picture shows the optical image of the bent diffuser-tube 10 cm above the substrate
holder. H2O vapor is generated by heating a sealed stainless steel container of DI water, 15 cm in
height, and placed on a hot plate which is maintained at 50 °C. The flow rate of H2O vapor was
controlled by MKS® GM50A mass flow controller in the range of 0 – 300 sccm. The flow rate of
NF3 gas was controlled by a separate Unit® 1620 mass flow controller in the range of 0 – 250 sccm,
but shares the same inlet tube with H2O vapor into the diffuser fixture after the mass flow controller
as shown in Figure. 2-3. An in-situ residual gas analyzer (RGA) ExTorr® XT300 consisting of a
quadrupole mass spectrometer connected to the reactor chamber was used to analyze the
downstream plasma and etch reaction species. The RGA is capable of detecting molecular species
of up to 300 amu. Stylus profilometer (Tencor® P-16) was used to characterize the etched depths
and surface smoothness. Auger spectroscopy was used to analyze the atomic and molecular species
on the surface and sidewalls of the etched glass regions.
3.2.2 Sample Preparation
500 μm thick, double side polished 100 mm diameter fused silica, borosilicate glass
(Corning 7740®), and aluminosilicate glass (Eagle XG®) wafers were prepared in this work. Wafers
were cleaned in Nanostrip® for 30 mins and then deposited with 15 nm chromium and 150 nm gold
seed metal layers using e-beam evaporator. Wafers were patterned with feature widths ranging from
5 μm to 200 μm using Shipley™ 1827 photoresist and developed in 25% Microposit™ 351
47
developer to form 3.5 μm thick photoresist patterns. 2-3 μm nickel was electroplated as hard mask
using a pulse generator at 40 mA current and a duty cycle of 25%. For deep etching tests, glass
wafer were coated with SPR 220-7 photoresist to create up to 11 μm thick nickel mask features.
Thereafter, the photoresist and the underlying Cr/Au seed layers were removed using photoresist
striper and Cr/Au wet etchants respectively in order to obtain clean glass regions to be etched. The
process flow for wafer preparation is schematically illustrated in Figure 2-1.
3.3 Experimental Results & Discussion
3.3.1 Conventional SF6 ICP Etch of Fused Silica and Borosilicate glass
In order to investigate the effects of the gas diffuser on the etching of fused silica,
borosilicate glass and aluminosilicate glass, it is necessary to understand the conventional etch
characteristics of the ICP-RIE which introduces fluorine based gas combinations of sulfur
hexafluoride/argon from the top gas inlet of the ICP source. In our experiments, fused silica and
borosilicate glass were etched with SF6 introduced from top gas inlet of the ICP source for 20
minutes at a source power of 2000 W and substrate power of 400 W. Backside He wafer cooling
temperature was set at 20 °C and the wafer position with respect to the ICP source was set to 120
mm. The etch rate and etched surface roughness are plotted as a function of SF6 flow rate in Figure
3-1. The reported etch rate and roughness values are the average of data points measured across
100 µm features and examined at five different locations on the wafer. The five features were
measured with one at center of the wafer and four others in 3 cm apart from the center. Error bar
for the etch rate is within ±0.02 µm/min. In Figure 3-1, as SF6 flow rate is increased, initially the
etch rate and the etched surface roughness increase for both fused silica and borosilicate glass,
reaching their maximum values, and thereafter decrease at the highest flow rates examined. Similar
48
trends in etch rate and roughness on glass substrates have been observed where only a fluorine
based plasma has been employed [45], [46], [48], [49], [52]. In general, low pressures correspond
to large mean free path and therefore higher ion energies and lower flux density whereas higher
pressures lead to smaller mean free path, i.e. lower ion energies, and higher flux densities. The
overall etch rate as described in eq. (2.1) is an interplay between the physical and chemical
contributions. Etches performed at lower pressure are expected to be dominated by higher physical
contribution. However, the overall gains are mitigated by the lower flux density of the bombarding
ions. This can explain the lower etch rates obtained at low pressures. In contrast, at higher pressures
the flux of ions and radicals are increased although the energy of the impinging ions is reduced.
This changes the etching mechanism from a physically dominated process to a chemical process.
However, for the chemical process to be effective, sensitization of the surface atoms via
bombardment by energetic ions is critical. Since at higher pressures, ion energies are reduced, this
results in a less than optimal chemical process thereby reducing the etch rates at the high flow rates.
To summarize, silica etching can be considered as a result of the surface reactions induced in the
damaged silicon oxide layer formed by the physical bombardment by the large SFx ions. From
Figure. 3-1 it can also be seen that the highest etch rate of borosilicate glass occurs at a SF6 flow
rate of 60 sccm corresponding to a chamber pressure of 3 mTorr whereas the maximum etch rate
of fused silica occurs at a SF6 flow rate of 100 sccm corresponding to a chamber pressure of 4
mTorr . This indicates that the presence of impurity atoms such as boron, sodium and aluminum in
borosilicate glass requires higher ion bombardment energies to initiate the sputtering of these
elements from the etched surface since these elements do not form volatile fluorides that can be
pumped away.
49
Figure 3-1. Experimentally obtained etch rate and rms roughness values on fused silica and
borosilicate glass for various SF6 flow rates. Since the pumping speed of the pumping system
remains constant for these conditions, increasing flow rate corresponds to increasing pressure in
the chamber.
3.3.2 Modified ICP-RIE for Various Glass Composition Substrate: Diffuser-ring Set-up
In the modified ICP-RIE glass etching system, SF6 gas is introduced from the top of the
ICP source to create a high density plasma consisting of SFx ions by the ICP RF coils.
Simultaneously NF3 and H2O vapor are introduced into the chamber through the gas diffuser inlet
(diffuser-ring or diffuser-tube). In plasma conditions, NF3 is reported to be highly unstable gas and
possesses smaller dissociation energy than other fluoride gases such as SF6 and CF4 [56], [77] and
the NF3 plasma has been shown to exhibit substantial isotropic etching characteristics with large
undercuts and high etch rates [56]. The ionized SF6 gas by the ICP source, diffuses towards the
substrate where the electric field generated by the substrate RF bias power is able to drive the
positive ions from the plasma towards the NF3 and H2O gas cloud and breaks down the unstable
NF3 gas into NFX radicals. Therefore, by introducing NF3 and H2O gas from the secondary diffuser
gas inlet and adjusting the relative position of the diffuser to the glass substrate, a glass etching
50
system is realized which is capable of tuning between conventional physical bombardment
dominated etching and/or NF3 and H2O gas induced chemical phase reaction dominated etching
mechanism.
Wafers were processed under 2000W of ICP source power and 400W substrate power.
Each wafer was etched for 15mins. Figure 3-2 (a)-(c) are 4D-plots of the etch rate as a function of
the various gas flow rates with the red colored circles indicating the highest etch rates obtained in
fused silica, borosilicate glass and aluminosilicate glass wafers. The highest etch rate of
0.83μm/min was achieved for fused silica glass for SF6:NF3:H2O :: 60:100:50 combination whereas
for borosilicate and aluminosilicate glasses the highest etch rate obtained was 0.68 μm/min and
0.32 μm/min respectively for SF6:NF3:H2O :: 20:20:25 using diffuser ring system. A comparison
of the etches performed on borosilicate glass using the diffuser-ring gas inlet versus conventional
single SF6 etching is presented in Table 3-1. In order to compare the two different etch approaches
we adjusted the flow ranges of the various gases to result in identical chamber pressure. Plasma
source power of 2000 W/2500 W and substrate power 400 W were used in both cases. Identical
pressure and power settings in the two approaches help easier comparison since the plasma density,
the molecular mean free paths and ion flux rates in the two cases should be comparable and any
differences can be discussed in terms of achievable chemistry due to the introduction of NF3 and
H2O gases using the diffuser-ring. Table 3-1 shows the etch rate of borosilicate glass using the
diffuser-ring at 2500 W ICP source power is higher by 15% as compared to conventional etching
under similar physical operating parameters. The average etched surface roughness (Ra) was
measured using precision scans of the region using the Tencor® stylus profilometer capable of
surface roughness resolution in the Angstrom range. Through these measurements, it was
confirmed that the achieved surface roughness of the etched areas from the diffuser-ring set-up was
an order of magnitude better than the conventional etch process. The selectivity obtained using the
diffuser-ring system is almost twice that obtained using the conventional SF6 based etch at 2500 W
51
clearly indicating reduced sputtering of the nickel mask and therefore the improved glass etching
mechanism can be primarily attributed to a dominant chemical reaction component.
Figure 3-2. 4D plot of the etch rate of (a) fused silica; (b)borosilicate glass; (c) aluminosilicate glass
as a function of the flow rates of SF6 (from source) and NF3 and H2O for diffuser-ring configuration.
Color of the circles indicates the etch rate.
Table 3-1. Comparison between SF6/NF3/H2O based etching and single SF6 plasma etching at 400
W substrate power.
52
1Power Diffuser
Ring
2SF6 2NF3 2H2O 3Pressure 4Voltage 5ER 6Roughness* Selectivity
Borosilicate
glass
2000
Yes 20 20 25 3 76 0.68 11 21
No 60 0 0 3 77 0.59 164 NA
2500
Yes 20 20 25 3 74 0.72 3.9 27
No 60 0 0 3 79 0.61 52 14
Units: 1Watt, 2sccm, 3mTorr, 4Volts, 5ER: Etch Rate: μm/min, 6Å. *Roughness is measured using Tencor®
Profilometer. Etching time was 20 minutes.
In the experiments, an in-situ residual gas analyzer (RGA) was used to monitor the plasma
species produced during the etchings. Partial pressure of each observed plasma species was
quantified and examined for understanding the effects of species of interest on etches. Figure 3-3
shows the partial pressure of detected plasma radicals in diffuser-ring set-up and conventional SF6
etch respectively and correspond to the etching conditions and results for 2500 W source power
listed in Table 3-1. Total pressure in the RGA was regulated such that it was the same for the two
cases. Clearly the diffuser-ring based etch has greatly enhanced concentrations of NO, NF, NF2,
and NF2OH radicals. Furthermore, partial pressures of H2O, HF, N2/Si, F2, F, and N2O/SiO are also
dramatically higher in the diffuser-ring etch whereas partial pressure of SFx radicals is suppressed
in the diffuser-ring etch. Since the overall etch rate of the diffuser-ring etch is higher than the
conventional etch, these results suggest the presence of the new radicals and enhanced fluorine
concentrations improve the overall etch rate of glass substrate through improved chemical
reactivity with the silicon oxide to result in fast and Angstrom level smoothness in the etched areas.
53
Figure 3-3. Experimentally measured partial pressure of plasma species in diffuser-ring modified
etch system and conventional ICP etch system respectively by an in-situ Residual Gas Analyzer
(RGA). Identical total pressure was regulated in the RGA system during mass spectrum acquisition
in the two cases.
3.3.3 Modified ICP-RIE for Various Glass Composition Substrate: Diffuser-tube Set-up
The diffuser ring modification as explained in the previous section clearly enhances the
chemical component of the overall etching process and has demonstrated unprecedented etched
surface smoothness as well as enhanced etch rate. However, as designed, the diffuser-ring was
clamped onto the substrate holder just above the wafer with no possibility of adjusting the space
between the place where the cloud of etch gas is released and the substrate surface. It was realized
that dispensing the etch gases right at the surface of the substrate wafer may not necessarily be the
optimum position for maximizing ion bombardment and reactive radical production and interaction
with the substrate for etching. Therefore, the diffuser-tube modification which is schematically
shown in Figure 2-10 was designed where the height of the diffuser gas inlet is raised up in relation
to the substrate surface. Tubes of three different heights of 7 cm, 10 cm, and 12 cm were made and
the etch process for each of these was systematically investigated. In this section, etching
characteristics with diffuser tube modification are explored on fused silica, borosilicate glass, and
aluminosilicate glass.
SF6 gas was introduced as in the earlier experiments from the ICP source and NF3/H2O gas
54
mixture were introduced into the chamber through the diffuser-tube clamped to the substrate holder.
The substrate was set to 120 mm from the ICP source as in the earlier experiments. The NF3 and
H2O gas inlet point of the diffuser-tube was bent so as to locate it above the center of the wafer and
three different tubes with heights of 7 cm, 10 cm, and 12 cm were used in this work. Figure 2-11
and Figure 3-4 (a)-(b) illustrate the obtained etch rates for the various SF6/NF3/H2O combinations
on the three kinds of glass substrates using the 10 cm diffuser-tube. Using this modified system,
etch rates as high as 1.06 μm/min, 1.04 μm/min, and 0.45 μm/min with respective surface
smoothness of ~2 Å, ~67 Å, and ~4 Å for fused silica, borosilicate glass, and aluminosilicate were
achieved after 5 minute etches. The etch rates are within ± 0.01 μm/min variation based on the
average of 5 data points.
Figure 3-4. 4D plot of the etch rate of (a) borosilicate glass; (b) aluminosilicate glass as a function
of the flow rates of SF6 (from source) and NF3 and H2O for 10 cm height diffuser-tube. Color of
the circles indicates the etch rate.
Using the RGA, the plasma species generated during the etches using the 10 cm high
diffuser-tube gas inlet set-up and the diffuser-ring gas inlet set-up are compared for identical etch
recipe using SF6:NF3:H2O :: 20:20:25 sccm flow rates. The measured percentages of the partial
pressure of radicals of interest are shown in Figure 3-5 as a bar graph. The 10 cm high diffuser-
tube modification resulted in 24% enhancement in the obtained etch rate from 0.72 µm/min to 0.89
55
µm/min for borosilicate glass for these etch conditions. It can be seen that higher percentages of
radicals of F, NF, NF2, and NO are observed in the 10 cm height diffuser-tube set-up as are F2 and
NF3. The higher percentage of the volatile SiF3 radical observed in the 10 cm height diffuser-tube
set-up is another evidence of enhanced chemical glass-etching component in the diffuser-tube
modified system.
Figure 3-5. Bar graph of partial pressure percentage of radical species of interest obtained with 10
cm height diffuser-tube gas inlet and diffuser-ring gas inlet with identical etch recipe. The etches
were processed for 5 minutes.
Furthermore, in addition to 10 cm height tube, etches with 7 cm and 12 cm tube heights
were also examined in the experiments. Neither of the two heights achieved higher etch rates than
the fastest etch rate that is reported in Figure 6 for 10 cm high diffuser-tube. The percentages of
partial pressure of plasma species of interest that were obtained for the three diffuser-tube heights
for borosilicate glass etches for SF6:NF3:: 60:20 sccm are shown in Figure 3-6 as a bar graph. that
obtained in the borosilicate etches are plotted as bar graph in Figure 8. Identical recipe was used in
these etches for 5 minutes. Once again it clear that for the 10 cm height tube, the obtained etch rate
was the highest and the RGA data shown the highest percentages of F, NFx, and SiFx radicals and
F2 molecules. Clearly no correlation is observed between the concentration of HF and SFx and the
etch rate. From these experiments it can be concluded that glass etch rate irrespective of the
56
formulation is clearly enhanced by larger concentrations of F and NFx radicals and F2 molecules.
Although, HF is a well-known etchant of glass in aqueous conditions, in plasma conditions no
obvious correlation of etch rate with HF concentration is found. The smoothness of the etched
surfaces indicates the chemical etching is isotropic in nature and therefore laterally undercuts any
micromasked structures that are formed on the surface due to the formation of non-volatile fluorides
of the dopant atoms in the glass. Sensitization of the surface through physical bombardment of
energetic ions is essential for the overall initiation of the etching process.
Figure 3-6. Bar graph showing the percentages of the various molecular fragments as measured by
the RGA in SF6:NF3::60:20 sccm borosilicate glass etching with different diffuser tube heights.
The magnitude of NFx peak is the sum of magnitudes of NF, NF2 and NF3 peaks. The magnitude
of SFx peak is a sum of magnitudes of SF, SF2, SF3, SF4 and SF5 peaks. The magnitude of SiFx peak
is a sum of magnitudes of SiF, SiF2 and SiF3 peaks. All etches were performed for 5 minutes.
3.3.4 High Aspect Ratio Glass Etching with Diffuser Gas Inlet
Next we examine deep etching of borosilicate glass using the diffuser-ring and 10 cm
height diffuser-tube modifications. Figure 3-7 shows the cross-sectional profiles of the etches
obtained using the diffuser-ring using flow rates of SF6:NF3:H2O of 20:20:25 sccm. Figure 3-7
clearly shows that wider features are etched deeper than the narrower features and illustrates the
57
well-known phenomenon of aspect ratio dependent etching. The obtained sidewall angle is between
87° and 88°. It can also be seen that the 8 μm wide feature shows a distinct bottling effect just below
the etch opening. The bottle shaped profile is known as balling effect in high aspect ratio plasma
etching [78]. Since borosilicate glass is a good dielectric material, during etching, the sidewall
surfaces are likely to be negatively charged and the resulting electric field causes the energetic ions
to be deflected towards the sidewalls in the narrow features. The depth at which the bottling effect
occurs depends upon the overall energy of the incident ions and the local electric field from the
charging effect which in turn is a function of the width of the etched feature. As the etch deepens,
the overall energy of the ions is expected to reduce due to collisions while traversing within the
etched feature. The non-volatile oxides of the dopant atoms in borosilicate glass need energetic
ions to be sensitized and subsequent removal through fluorine based chemical reactions. The
lowered energy of the incident ions with depth eventually do not possess sufficient energy to
volatilize the fluorides formed on the sidewalls and therefore result in angled sidewalls. The etched
features show no trenching effect clearly indicating that the overall energy of the ions is likely to
be low in these etches due to the enhanced collisions within the higher local pressure region created
by NF3/H2O gas molecules released from the diffuser-ring. The enhanced chemical nature of the
etch results in extremely smooth etched surfaces as can be seen in the wider features of the etch.
An aspect ratio of >10:1 was obtained indicating that deep high aspect ratio structures with smooth
sidewalls and etch bottoms can be obtained using this process. As a result of enhanced chemical
component of etching in the diffuser-ring modified chamber with NF3, the vertical wall angles of
100 µm deep etches with diffuser-ring set-up on fused silica and borosilicate glass are improved to
88.7⁰ from 87⁰ and 84⁰ respectively in comparison to the work in a recently published article [55].
58
Figure 3-7. SEM photograph of a borosilicate glass etched using diffuser-ring modification. The
etch was performed under the following conditions: ICP Power = 2000 W, Substrate Power = 400
W, Gas Flow Conditions: SF6:NF3:H2O :: 20:20:25, Etch Time = 150 mins. The obtained Etch Rate
= 0.67 μm/min (50 μm feature) and the highest aspect ratio is 9.3:1.
Figure 3-8 shows the SEM cross section of high aspect ratio etches performed using the 10
cm height diffuser-tube modification on borosilicate glass. Once again the bottle effect is seen here
for the 8 and 18 μm features. Since the highest etch rate for borosilicate glass using the diffuser-
tube modifications occurred at SF6:NF3 :: 60:20 sccm, these flow rates were used in this etch. The
lower flow rate of NF3 coupled by the release at a height corresponding to the plasma sheath
boundary likely results in an ion-flux dominated physical etching and a smaller chemical etching
contribution. For the 8 μm feature, the higher energy of the ions results in the maximum width of
the bottling effect to occur at a lower depth from the surface of the glass wafer in the diffuser-tube
(19 μm) as opposed to the diffuser-ring (13 μm). Furthermore, the higher energy ions induce
bottling effect in the wider 18 μm features as well. Overall the etched surface morphology is
rougher and is consistent with etches dominated by energetic ions as seen in SF6 only etches on
both fused silica and borosilicate glass. The maximum etch rate obtained using the diffuser-tube
for borosilicate substrates was 1.06 μm/min and is ~45% higher than that obtained using the
diffuser-ring modification. From these it is clear that borosilicate glass etching is enhanced by
energetic ion bombardment rather than chemical process which is to be expected since only 80%
59
of it consists of SiO2. The dopant atoms in borosilicate glass form non-volatile fluorides with large
bond energies and therefore require physical bombardment by energetic ions for their effective
removal.
Figure 3-8. SEM Photograph of the cross-sectional profile of borosilicate glass etch using 10 cm
height diffuser-tube. The etch was performed under the following conditions: ICP Power = 2500
W, Substrate Power = 400 W, Gas Flow Conditions: SF6:NF3 :: 60:20, Etch Time = 210 mins. The
obtained Etch Rate = 1.06 μm/min (50 μm feature, after 5 minutes) and the highest aspect ratio is
10.6:1.
As compared to the results here, 100% SiO2 fused silica etching reported earlier showed
near 90° sidewalls due to the absence of non-volatile oxides formed from the dopant atoms. There
the lower energy ions are able to sensitize the surface sufficiently such that the chemical reaction
with fluorine species is able to effectively proceed and therefore results in near vertical sidewalls.
It is however interesting to note that the highest overall etch rate obtained for the two substrates is
not significantly different although the gas compositions and conditions of maximum etch rates for
the two substrates are significantly different. In order to quantitatively examine the role of physical
and chemical etching, we examined both the etch rate and the etch smoothness and their dependence
on relative ion flux and the various etch species formed within the chamber such as F, F2, HF, NF,
and NF2. Table 3-2 lists the Pearson Correlation Coefficient and the P-values obtained through
60
multivariate statistical analysis of 41 etches performed over the three types of glass substrates
discussed in this work. The Pearson correlation examination is used to assess the strength and
direction of association between two variables that are linearly related. An absolute value of
Pearson coefficient larger than 0.5 suggests a strong correlation between two variables. Positive
Pearson coefficient indicate a positive relationship while negative values of Pearson coefficient
show an inverse relationship. Since we have listed roughness as the parameter, negative values of
Pearson coefficient are desirable since they imply a smoother etched surface. For each etch, the
relative ion flux density on the substrate was calculated with respect to the etch with lowest ion
flux and based upon the measured substrate bias voltage. The radical/molecular concentrations in
each etch were calculated based upon the corresponding partial pressure of the gas species with
respect to the total pressure during the etch, which were measured using the RGA. From Table 3-
2, the following overall observations can be made. No matter the type of silica substrate ion flux is
a critical factor in their etching. All three substrates have a high Pearson coefficient for relative ion
flux indicating that a high ion flux is desirable and clearly correlated to high etch rates. A smaller
P-value indicates a high probability or confidence of this being true. Overall, the P-values for ion
flux for borosilicate glass and aluminosilicate glass are an order of magnitude smaller than for fuse
silica indicating that ion flux plays the dominant role in the etching of these two doped glasses. For
fused silica it can also be seen that higher values of F and F2 improve the high etch rate while HF
is negatively correlated to increasing the etch rate. This indicates that while ion flux is important in
the etching of fused silica substrates, the presence of fluorine radicals and molecules play a
significant role in its etching. These statistical values are consistent with the experimental
observations of the various etches. High ion flux is found to be directly related to substrate
roughness for fused silica substrates indicating that for smoother etched surfaces it is desirable to
have a lower ion flux. Similarly, high HF concentration is expected to result in smoother etched
surfaces for fused silica however the P-value does not indicate a very high confidence. For
61
borosilicate glass F is the only radical that seems to play a significant role in the chemical etching
whereas aluminosilicate glass seems to be entirely etched through physical sputtering process.
None of the fluorine radicals seem to have any correlations either to the etch rate or to surface
smoothness for this substrate.
Table 3-2. Role of relative ion flux and various fluorine radicals and molecules on the etch rate and
surface roughness of the glass substrates etched in this work. The table lists the value of Pearson
correlation coefficients and the P-values for these parameters based upon 41 independent etches
performed in this work.
Fused Silica Borosilicate Glass Aluminosilicate Glass
Etch Rate Roughness Etch Rate Roughness Etch Rate Roughness
Rel. Ion
Flux
Pearson
Coeff
0.584 0.808 0.752 -0.097 0.797 -0.478
P-value 0.022 0.003 0.003 0.765 0.003 0.415
F
Pearson
Coeff
0.604 -0.376 0.543 0.344 -0.524 -0.329
P-value 0.013 0.255 0.055 0.274 0.098 0.589
F2
Pearson
Coeff
0.663 -0.361 0.114 0.094 -0.734 -0.482
P-value 0.005 0.276 0.711 0.772 0.01 0.411
HF
Pearson
Coeff
-0.546 -0.579 0.043 -0.093 -0.639 -0.648
P-value 0.029 0.062 0.888 0.773 0.034 0.237
NF
Pearson
Coeff
0.481 -0.475 0.213 0.159 -0.71 0.47
P-value 0.059 0.14 0.485 0.622 0.014 0.424
NF2
Pearson
Coeff
0.51 -0.455 0.087 0.057 -0.724 0.365
62
P-value 0.043 0.159 0.776 0.861 0.012 0.545
Total
Fx+NFx
Pearson
Coeff
0.614 -0.406 0.277 0.188 -0.715 -0.413
P-value 0.011 0.215 0.359 0.558 0.013 0.489
For aluminosilicate glass, the 10 cm diffuser-tube set-up demonstrates 41 % higher etch
rate than diffuser-ring set-up for 15 minutes etching. Figure 3-9 shows SEM cross sections of the
high aspect ratio etch profiles obtained for aluminosilicate glass. The etched features shows a
sidewall angle of 73.5° resulting in the distinct V-shape cross-sectional profile with a maximum
aspect ratio of 1:2 as the etch process comes begins to rapidly slow down. The observed etch profile
results from the non-volatile fluoride products that form on the surface during the etch process.
Aluminosilicate glass consists only 55% SiO2, but 21% CaO and 10% Al2O3. The fluorine based
plasma etch products of CaF2 and AlF3 are non-volatile solids that begin to accumulate on the
etched surfaces. Thus, the etch rate of aluminosilicate rapidly deteriorates as the etch goes on.
However, a brief etch in chlorine based plasma like BCl3 on the etched aluminosilicate glass
followed by the fluorine based etch was seen to recover the etch rate since AlCl3 is a volatile product
that can be easily removed during plasma etching. Since AlF3 is a fairly hard ceramic with strong
Al-F bond energies, the use of higher ion energies do not seem to significantly influence the surface
roughness and conversely fluorine based radicals/molecules also seem to have insignificant
influence on the improvement of the overall etch rate of this substrate. Furthermore, the dielectric
constant of aluminosilicate glass (εr = 5.2) is larger than that of fused silica (εr = 3.9) and
borosilicate glass (εr = 4.7), which is expected to allow for larger surface charges to accumulate on
the surface of the glass during etching. The accumulated positive charge on the horizontal etched
surfaces are likely to repel the ions headed downsteam into the etch patterns and therefore also slow
63
down the overall etch rate. Thus, aluminosilicate substrates were seen to etch at the fastest rate of
~0.45 μm/min which about half the value obtained for fused silica and borosilicate glasses.
Figure 3-9. SEM photographs of the cross-sectional profiles of the etch features in aluminosilicate
glass using the 10 cm height diffuser-tube modification. The etch was performed under the
following conditions: ICP Power = 2000 W, Substrate Power = 450 W, Gas Flow Conditions:
SF6:NF3 :: 60:20, Etch Time = 210 mins. The obtained Etch Rate = 0.45 μm/min (50 μm feature,
after 5 minutes), aspect ratio: 1.8 : 1.
3.3.5 Loading Effect and Charging Effect
For a better understanding of the loading, charging, and lagging effect on the overall etch
rate, three different patterns were used on aluminosilicate wafers with the diffuser-ring set-up.
Figure 3-10 shows three patterns used on the wafers. The light brown regions on the wafer are
electroplated regions with nickel which is used as the etching mask while the transparent areas are
etched features. For fractional area of the etched regions for the three patterns are as follows: Figure
3-10(a) corresponds to ~50%, Figure 3-10(b) corresponds to ~11%, and Figure 3-10(c) corresponds
to ~10%. It is important to note that the wafers were mechanically clamped along the edges of the
wafer in the Alcatel etcher used in this work and thus the top nickel mask layer would be electrically
in contact with the aluminum clamping plate. Thus, the etched features are electrical connected to
substrate bias for masks shown in Figure 3-10(a) and Figure 3-10(c), but they are electrical isolated
64
within each square for mask shown in Figure 3-10(b).
Figure 3-10. Images of three kinds of patterns used for evaluating the loading and charging effects.
Each pattern consists of different percentages of the overall etched areas. (a) Etched area ~50%,
(b) Etched area ~11%, and (c) Etched area ~10%. Additionally, pattern shown in (b) consists of
nickel pattern (light brow in color) that is electrically isolated within each patterned squared and
does not connect to the edges of the wafer where the mechanical clamp makes an electrical contact
to the nickel mask layer.
In the experiment, three aluminosilicate glass wafers were processed with identical etch
process consising of gas flow rates of SF6:NF3:H2O :: 20:20:25 sccm using the diffuser-ring system,
Source Power of 2000 W, substrate power of 400 W, and were etch for 20 minutes each. The etch
rates were measured for each of the patterns shown to be Figure 3-10(a): 0.33 μm/min, Figure 3-
10(b): 0.18 μm/min and Figure. 3-10(c): 0.32 μm/min. The electrically isolated features on mask
Figure 3-10(b) were etched nearly at half the etch rate of the other two wafers. It can be speculated
that the electrical isolation of the features, allows for accumulation of positive charge on the surface
of these features due to ion bombardment as the etch continues and thus begins to repel energetic
ions to travel towards the etch surface thus preventing the efficient etching of the substrate. On the
other hand, the availability of electrically connected nickel mask layer to the clamp, allows for
effective discharge of any surface build up of charge on the wafer leaving the incoming flux of
positively charged ions minimally affected for mask patterns of Figure 3-10(a) and Figure 3-10(c).
It must be noted that even though the mask patterns of Figure 3-10(a) and Figure 3-10(c) do not
share similar etching loads, the etch rates are comparable. This suggests that loading effect is less
significant that the charging effect expected in the isolated features.
65
3.4 Summary
In conclusion we were able to successfully demonstrate high aspect ratio glass etching of
fused silica, borosilicate glass and aluminosilicate glass substates using SF6, NF3, and H2O based
chemistries and using a modified ICP-RIE process based upon diffuser gas inlets. Two different
gas diffuser inlets were explore; (i) diffuser-ring modification located on the plane of the substrate
and (ii) diffuser-tube modification located at 7 cm, 10 cm, and 12 cm above the plane of the
substrate. Using the diffuser-ring modified system we were able to achieve a high aspect ratio
etches in fused silica and borosilicate glass > 1:10 with extremely smooth sidewalls and bottom
profiles. Unprecedented angstrom level surface smoothness were observed in the diffuser ring
modified etcher. The proposed modification is able to create etches for which the etch rates and
surface smoothness for fused silica surfaces and surface smoothness only for borosilicate glass that
were largely influenced by the chemical etching processes. Over 1 µm/min etch rate were achieved
for fused silica substrates using diffuser ring modification and for borosilicate glass using the 10
cm height diffuser-tube modified ICP-RIE etcher. The improved etch rates and smoothness of
various glass compositions substrate etching is summarized in Table 3. Although the roles of the
individual molecular fragments in the etching were not uniquely identified, the presence of large
elemental fluorine and fluorine molecule and NFx radicals seem to influence the etch rate of fused
silica. For borosilicate and aluminosilicate glasses the etch was largely influenced by ion
bombardment with borosilicate surface smoothness affected by the fluorine radicals. Etching of
aluminosilicate substrates was dominated by the formation of non-volatile calcium and aluminum
fluoride. The use of the diffuser ring allow for a process flow that is capable of achieve etch rates
of 0.7 µm/min with nm level surface smoothness for borosilicate glass and with aspect ratios of
greater than 10:1. Charging effects on the dielectric substrates seem to have several detrimental
66
effects including a slowdown of the etch rate as well as the well know balling effect resulting in
bottle shaped etch profiles. Table 3-3 summarizes the obtained etch results on glass in this work.
Table 3-3. Summary of optimized glass etch rates and surface smoothness
Conventional etch Diffusing NF3 and H2O etch
Etch rate Smoothness Etch rate Smoothness
Fused silica 0.8 µm/min 10nm 1.06 µm/min 5 Å
Borosilicate glass 0.58 µm/min 16.4nm 1.04 µm/min 11 Å
Aluminosilicate glass 0.17 µm/min 0.45 µm/min
Being inexpensive and readily available, silica substrates exhibit several desirable
properties including optical transparency, excellent
Next we examine deep etching of borosilicate glass using the diffuser-ring and 10 cm
height diffuser-tube modifications.
The diffuser ring modification as explained in the previous section clearly enhances the
chemical component of the overall etching process and has demonstrated unprecedented etched
surface smoothness as well as enhanced etch rate.
In conclusion, we were able to successfully demonstrate high aspect ratio fused silica
etching using SF6, NF3, and H2O based etching using a modified ring diffuser modification to the
etch chamber. Using this system we were able to achieve a high etch rates of ~1 µm/min as well as
high aspect ratio etches in fused silica with greater than 1:5 with extremely smooth sidewalls and
flat bottom profiles with Ra ~5 Å.
67
Chapter 4
Glass Micro-spherical Shell Based Whispering Gallery Mode (WGM)
Resonator Sensing Platform
4.1 Introduction of WGM optical resonance and background of WGM based resonator
Whispering gallery mode (WGM) resonances in optical cavities have been studied for more
than a century ever since the interaction of electromagnetic waves with dielectric spheres was first
observed in late 1900’s [79], [80]. Following the first experimental observation in 1960’s [81], the
WGM optical resonances have been demonstrated to be supported within several structures with
an axis of rotational symmetry such as microdroplets [82], [83], microtubes [84]–[86], microbottles
[87], [88], microspheres [89]–[91], microrings [92], [93], microdiscs [94], [95], microbubbles [96],
[97], and microtoroids [98]. WGM relies upon total internal reflection of light at the external cavity
interface. To induce a resonance mode, an adiabatically tapered fiber is placed in close proximity
to the resonator structure to evanescently couple the light. A large refractive index contrast between
the cavity and the surrounding medium strongly confines the WGMs resulting in resonances with
very high Q-factors of 107 – 109 [99], [100]. Conversely, a low refractive index contrast facilitates
extension of the modal profile beyond the confines of the resonator medium allowing for the optical
radiation to interact with the surrounding medium and thus enabling sensor designs with
exceptionally high sensitivity – albeit at the expense of the Q-factor. In general, changes in either
the cavity geometry or the refractive index contrast between the cavity and surrounding medium
perturb the resonance characteristics of the confined optical modes and can be used for sensing
applications. The extreme level of sensitivity afforded by WGM resonators has elicited intense
research in realizing sensors based on these structures [101]. To date, two kinds of WGM optical
68
resonator configurations have been explored: (i) microsphere, microbottle, and microbubble
structures formed by individually melting or machining and polishing suitable dielectric materials
and allowing the surface tension forces to form highly smooth and axisymmetric structures from
glass fibers and capillaries and other materials; and (ii) on-chip microfabricated microring,
microdisk, and microtoroid structures from suitable dielectric materials. Unlike solid structures
such as spheres and discs, hollow structures such as cylindrical and spherical shells have two
surfaces and offer the advantage of coupling the light through the outer surface whereas the inner
surface can be engineered to induce perturbations for sensing. Microtube, and microbottle based
sensors have been reported in a configuration commonly known as optofluidic ring-resonator
(OFRR) sensors [86], [102], [103] where the analyte fluid interacts with the optical resonance
through the inner surface of the shell. However, until now all OFRR sensors have been fabricated
by glass blowing techniques from individual capillaries where the physical characteristics of these
structures are not easily controlled or reproducible. On the other hand, on-chip microring, microdisc
and microtoroid based sensors are able to leverage the reproducibility afforded by microfabrication
techniques and the economy of wafer scale parallel processing. However, in these resonators it is
much harder to achieve a clean interface with fluidic analyte medium since in most typical
configurations both the resonators and the tapered fiber are exposed to these fluids. Table 4-1
illustrates several configurations of WGM resonators and summarizes the characteristics of each
resonator.
Table 4-1. Summary of presented configurations of WGM resonators in literature
Microsphere
Microtube &
Microbottle
Microring &
Microdisk
Microtoroid
69
Sketch
Fabrica-
tion
method
Individual melting from fiber &
capillary
On-Chip
Quality
Factor
108 105-106 104-105 108
Clean
interface
with fluid
analyte
No Yes No No
4.2 Motivation of proposing on-chip glass micro-spherical shell supported WGMs
Recently, chip-scale glass blowing techniques have been demonstrated to create
hemispherical and toroidal structures from glass and fused silica [31], [104]. These structures
consist of glass microspherical shells with radii ranging from 0.1 mm to > 1 mm and can be used
as WGM resonator structures. What is really significant is that these structures are highly
reproducible and can be integrated with on chip microfluidics to achieve high performance WGM
OFRR structures for sensing applications. Furthermore, the thickness of the spherical bubble
structures on the chip can be precisely tailored to achieve optimal interaction with the fluid within
the spherical shell structure while maintaining very high Q-factor for the optical resonance. Hence,
70
the microspherical shell structures can be utilized for a multitude of optical resonance based sensing
applications including temperature, pressure, (bio)chemicals etc.
In this chapter, the first chip-scale, silica glass microspherical shell, optical resonators with
high-Q factors fabricated by glassblowing techniques are described and the potential of these
structures for on-chip sensing applications is demonstrated. Model of blowing glass microspherical
shells of dimensions from 230 µm to 1.2 mm diameter and shell thicknesses of 300 nm to 10 μm is
presented. Figure 4-1 shows an array of the microfabricated on-chip, near spherical glass
microspherical shells with equatorial planes above the plane of the substrate. On-chip integration
of highly symmetric and smooth surface, closed spherical shell structures, can allow for the
realization of WGM based in-line microfluidic (bio)chemical sensors where the analyte fluid
interacts with the optical resonance through the inner surface of the shell. Here we demonstrate and
model the thermal sensing capability of glass microspherical shell resonators. Furthermore, we
show a proof-of-concept liquid core sensor by sensing the index of refraction change from water
and confirm the phenomenon with a model.
Figure 4-1. Chip-scale glass microspherical shells blown on silicon substrate. Inset shows a near
perfect glass microspherical shell with a sphericity of 0.996.
71
4.3 Fabrication process development roadmap
The glass microbubbles were fabricated on 500 µm thick silicon substrate. First, circular
features were patterned using positive photoresist and the silicon was etched to a depth of heSi =
250 µm using deep silicon etching process to realize cylindrical cavities as schematically shown in
Figure 4-2(a). Second, Corning® 7740 borosilicate glass wafer was optionally patterned with
smaller circles than on silicon using positive photoresist and 4 µm of nickel was electroplated as
an etch mask. After removal of the photoresist in acetone, the borosilicate wafer was etched to a
depth of heG µm using a modified ICP-RIE high-aspect ratio glass etch process [54]. Thereafter, the
nickel, chrome and gold layers were stripped from the borosilicate wafer using wet etchants
resulting in a cross-sectional profile as shown in Fig. 4-2(b). The etched silicon and borosilicate
glass (optionally) wafers were aligned to result in concentric circles and anodically bonded at a
pressure of 1.35 atmosphere (1026 Torr) at 400 °C to form the bonded cavity as shown in Figure
4-2(c). The bonded wafer was diced into chips and the borosilicate layer was thinned down to a
total thickness of t µm from the un-etched side in 49% hydrofluoric acid as shown in Figure 4-2(d).
The bonded chip was thereafter heated on a silicon nitride ceramic heater to a temperature of 775
°C in a vacuum oven maintained at 0.13 atmosphere (100 Torr) for 45 seconds and was rapidly
cooled down to ambient temperature. At this temperature, the borosilicate glass softens and begins
to expand into a spherical shell under the differential pressure between the sealed cavity and the
external pressure created by the high temperature and the external vacuum pressure [31]. The blown
glass microspherical shell is schematically illustrated in Figure 4-2(e). While the dicing step can be
performed after the glass blowing step and the entire process can be done at wafer level, in this
work we fabricated the glass microbubbles at chip scale due to the small sized heater used in this
work.
72
Figure 4-2. (a) Silicon wafer is patterned and plasma etched to a depth of 250 µm to define circular
pits (b) Borosilicate glass wafer is optionally patterned and plasma etched to define heG µm deep
circular features (c) The two wafers are aligned and anodically bonded. (d) Borosilicate wafer is
thinned down to a thickness of t µm in hydrofluoric acid. (e) Glass microbubble is blown at 775 °C
in a vacuum oven maintained at a pressure of 100 Torr.
The final height, hg, that the sphere develops is a function of the heater temperature Tf (in
Kelvin), the pressure in the vacuum oven Pf, the pressure and temperature at which the cavity is
sealed Ps and Ts (in Kelvin) respectively, the etched depth heSi and heG, and the radius r0Si and r0G
of the etched cavity in the silicon and glass wafers respectively and is given by [31]:
31
22260
220
32
22260
93
93
gSig
SigSig
g
VrV
rVrV
h (4-1)
where
2
02
02
0 SieSiGeGSieSisf
fsg rhrhrh
TP
TPV (4-2)
The radius of the glass microsphere rg can now be calculated as
g
Sig
gh
rhr
2
2
0
2
(4-3)
The sphericity of the blown glass microbubbles Ψ is defined as [105]:
73
g
g
A
V 3
2
'3
1
)6( (4-4)
where the Vg' and Ag are the effective volume and surface area respectively of the glass
microbubble region above the top-surface of glass substrate and are expressed in terms of (hg|exp –
t), rg|exp, and t in eq. (5) and eq. (6) as
)(3)(3 expexp
2
exp
' thrthV gggg
(4-5)
)(4)(expexpexp
thrthA gggg (4-6)
Table 4-2. Calculated and experimentally measured values of the glass microspherical shell
dimensions for the given glass blowing conditions. For devices where glass wafer is not etched
prior to bonding, r0G and heG are not applicable.
Bubble
r0Si
(μm)
r0G
(μm)
heG
(µm)
t
(µm)
Tf
(K)
Theory Experiment Wall
Thickness
(µm)
Images of blown
glass
microbubble†
hg* 2rg* hg|exp* 2rg|exp* Ψ
1 250 NA NA 100 1023 1055 1115 1041 1197 0.9640 6.7
2 100 NA NA 100 1048 592 609 651 744 0.9511 8.4
3 75 NA NA 100 1048 491 503 526 614 0.9319 8.6
4 100 NA NA 50 1048 592 609 610 636 0.9875 2.2
74
5 75 NA NA 50 1048 491 503 568 554 0.9960 1.4
6 40 NA NA 50 1048 326 331 361 345 0.9915 1.1
7 150 90 55 85 1048 792 820 729 713 0.9915 0.3
8 75 65 55 85 993 510 520 466 404 NA 1.0
9 40 35 55 85 993 278 281 326 231 0.8286 NA
*hg, hg|exp, rg and rg|exp are given in μm; Pf = 13 kPa, Ts = 673 K, Ps = 135 kPa, heSi = 250 µm; †– red scale bar in the images
represents 250 µm.
Table 4-2 lists the calculated and experimentally measured values of the glass
microspherical shell’s hg and hg|exp and radius rg and rg|exp respectively. A fairly good agreement
between the calculated and experimental values of the height and radii of the blown glass
microspherical shell dimensions is found with a maximum error of < ~20%. The calculated sizes
of the microspherical shell are sensitive function of the temperature and pressure at which these
structures are sealed and blown. The experimental pressure and temperature are measured as global
parameters at the system level of wafer bonder, heater, and vacuum oven pressure. Thus,
uncertainties in the actual temperatures and pressures at the individual microspherical shell level
are considered to be the main reason for the observed discrepancy between the calculated and
observed dimensions of the microbubbles.
75
The position of the equatorial plane of the glass microspherical shell is critical to obtaining
WGM optical resonance. The optical modes are localized on the equatorial plane and are sustained
only when the equatorial plane is above the substrate plane with minimal coupling loss to the
substrate. In our initial experiments, the bonded silicon-glass substrates with sealed cavities were
heated at ambient atmospheric pressure to blow the glass bubbles and resulted in hemispherically
shaped shells. In these devices no optical resonance was obtained due to significant loss into the
substrate. This situation was remedied by changing the glass blowing step to a vacuum ambient
rather than at atmospheric pressure. The vacuum ambient during the glass blowing step raises the
pressure difference relative to the sealed cavity pressure and enhances the expansion of
microspherical shell volume to develop into near spherical structures with the equatorial plane
located above the substrate for all bubble sizes as shown in the last column of Table 4-2. The
sphericity of the blown glass microspherical shells quantifies the relative height of the equatorial
plane with respect to the glass substrate regardless of bubble sizes. Sphericities in the range of
0.985 – 0.996 was measured for the spherical shells 4 – 7 and indicated that near-spherical glass
shells were achieved in this work. Smaller sphericities were observed in glass microspherical shells
1 – 3 blown out of thicker glass substrates. The excess material in these thicker glass substrates was
observed to result in a lateral expansion at the shell-base during the glass reflow process. This
visible lateral expansion at microspherical shell-base could be eliminated by reducing the thickness
of the bonded glass layer which was found to result in near spherical bubbles. Following optical
resonance measurements, microbubbles were cleaved at the equatorial plane and the sidewall
thicknesses were measured using a scanning electron microscope (SEM). For glass microspherical
shells blown from 100 µm thick glass layer, #1 – #3, the thickness of the shell wall thickness ranged
from 6.7 µm – 8.6 µm whereas reducing the thickness of the glass substrate to 50 µm, shells # 4 –
#6, resulted in wall thickness of 1.1 µm – 2.2 µm. Figure 4-3 (a) shows the SEM measurement of
the sidewall thicknesses of microspherical shells #4. Plasma etching of the glass substrate in
76
microspherical shells #7 – #9 followed by the subsequent thinning of the glass substrates to realize
even thinner glass regions of 30 µm resulted in either spherical or vertically elongated shells
depending upon the radius and enclosed cavity volume. Based on the volumetric redistribution of
the glass covering the cavity opening into the spherical shell, the shell wall thickness can be
estimated and agrees well with the measured thicknesses for all microspherical shells. For the
etched glass substrates with a substrate glass thickness of 30 µm, shells with wall thickness as small
as 300 nm were obtained. Figure 4-3 (b) shows the SEM measurement of the sidewall thicknesses
of microspherical shells #7. Furthermore, if a microspherical shell was overblown and was split
open on the top, e.g. the broken shell seen in the background in the image of shell #7, optical
resonance could be sustained, so long as the remaining structure maintained a near spherical profile
around the equatorial plane. Thus, through accurate control of the etched cavity geometries, glass
substrate thickness via micromachining as well as the sealing and blowing conditions wafer level
glass blowing process can be customized to achieve glass microspherical shells of various sizes,
sphericities, and wall thicknesses. The ultra-smooth surfaces obtained through the glass reflowing
process are ideally suited for sustaining ultrahigh-Q optical resonances.
Figure 4-3. SEM image of sidewall thickness measurements at the equatorial plane of glass
microspherical (a) #4 and (b) #7.
77
4.4 COMSOL modelling of micro-spherical shell supported WGMs
Optical resonance modes in WGM resonators occur when the coupled light can
constructively interfere with itself by completing integral number of cycles for each revolution
around the shell’s equatorial circle. Assuming that the mode is tightly confined within the resonator
medium, for a laser wavelength of λ, the condition for WGM resonance in a dielectric annulus of
radius r can be expressed as 2πnrr=mλ, where nr is the mode index; nr = 1.467 for borosilicate glass
is used, and m is the azimuthal mode number and corresponds to integral number of orbital
wavelengths [106].
The optical resonance modes of a dielectric spherical shell can be calculated by solving
Helmholtz equation in spherical coordinates. Helmholtz equation,
(∇2 − 𝑘2𝑛2)𝜓 = 0 (4-7)
where wavenumber 𝑘 =2𝜋
𝜆 and n is the refractive index, in spherical coordinates is given
by:
1
𝑟2
𝜕2
𝜕𝑟2(𝑟𝜓) +
1
𝑟𝑠𝑖𝑛(𝜃)
𝜕
𝜕𝑟(sin(𝜃)
𝜕
𝜕𝜃𝜓) +
1
𝑟2𝑠𝑖𝑛2(𝜃)
𝜕2
𝜕𝜙2 𝜓 − 𝑛2𝑘2𝜓 = 0 (4-8)
where r is radial distance, ϕ is azimuthal angle, and θ is polar angle. Under the assumption
the optical modes can be solved by scalar wave equation approximation, the equation is solved into
either electric in character (TM-case, Eϕ) or magnetic in character (TE-case, Hϕ) alone by the
separation of variables approach as shown in eq. (4-9).
𝐸𝜙 𝑜𝑟 𝐻𝜙 = 𝜓(𝜙, 𝜃, 𝑟) = 𝜓𝜙(𝜙)𝜓𝜃(𝜃)𝜓𝑟(𝑟) (3-9)
The introduced eigenfunctions for the radial, azimuthal and polar fields can be associated
with the radial mode number (n), the azimuthal mode number (m), and the polar mode number (l)
as well as the polarization (p). The azimuthal eigenfunction is given by eq. (4-10):
78
𝜓𝜙 =1
√2𝜋exp (±𝑖𝑚𝜙) (4-10)
By introducing the polar mode number l, the equation for 𝜓𝜃 is given by eq. (4-11):
1
cos(𝜃)
𝑑
𝑑𝜃(cos(𝜃)
𝑑
𝑑𝜃𝜓𝜃) −
𝑚2
cos(𝜃)2 𝜓𝜃 + 𝑙(𝑙 + 1)𝜓𝜃 = 0 (4-11)
And the radial field 𝜓𝑟 is given by eq. (4-12):
𝑑2
𝑑𝑟2 𝜓𝑟 +2
𝑟
𝑑
𝑑𝑟𝜓𝑟 + (𝑘2𝑛(𝑟)2 −
𝑙(𝑙+1)
𝑟2 ) 𝜓𝑟 = 0 (4-12)
The analytical solutions of 𝜓𝜃 and 𝜓𝑟 are generalized Legendre Polynomials 𝑃𝑚𝑙 (𝑐𝑜𝑠𝜃)
which are commonly re-expressed as spherical Harmonics 𝑌𝑚𝑙 (𝜃) and Bessel functions 𝑗𝑙(𝑘𝑟). For
each polar mode number l, the allowed azimuthal mode numbers are in the range of –l < m < l,
resulting in a 2l+1 degeneracy of the azimuthal modes.
The optical resonance of glass spherical shell is modeled with COMSOL software. The
eigenfunctions of the glass spherical shell confined electromagnetic wave are solved by finite
element method (FEM). FEM simulation of 3D structure consumes lots of computational resources
that slows down the modelling process. As a rotationally and axially symmetric geometry, spherical
shell can be modeled with axi-symmetrical FEM so that the simulation of a 3D spherical shell
structure can be simplified to a 2D problem. Meanwhile, being large diameter (small curvature)
sphere it is not necessary to investigate the resonance modes along the whole curvature of the
sphere. The fundamental WGM resonance mode locates at the equator of the sphere and the higher-
order polar modes distribute from the equator to both poles symmetrically. In the preliminary
modelling study, the equatorial section of a 600 µm diameter spherical shell of thickness of 4 µm
is defined within a cylindrical shell with height of 250 µm and width of 50 µm as shown in Figure
4-4(a). In the cross-sectional view the arc of spherical shell is contained within the rectangular
cross-section of the cylinder dividing the entire meshing zone into three sections as shown in Figure
79
4-4(b). The section of the spherical shell domain is defined with glass material properties whereas
the inner and outer two domains are defined with properties of air. Perfect matched layer (PML)
along the boundary of the meshing domain is introduced. The PML is defined as perfect electric
conductor layers and used to simulate radiation tunneling to infinity within a limited domain
calculation space. Triangular shape is used to mesh the spherical shell, inner and outer shell
domains. Maximum element size is set as 0.4 µm. However, for very thin shell less than 1 µm,
domains are suggested to be meshed individually. The shell domain needs to be meshed with
maximum element size of 0.2 µm or even smaller to ensure the shell can be meshed with at least 4
elements in the radial direction.
Figure 4-4. (a) 3D view of simulated WGM resonance modes confined in spherical shell. (b) The
geometry definition of the computational domains. The arc spherical shell domain is in diameter of
600 µm and thickness of 4 µm, defining with borosilicate glass properties. The rest domains in the
rectangular zone is defined with air properties.
In the simulation, the center wavelength of the incident laser is defined as 760 nm and the
azimuthal number was calculated to be 3638 based on the laser center wavelength, diameter of the
modeled sphere, and the refractive index of borosilicate glass listed. With the given material
properties and defined geometry, the electromagnetic wave functions were solved for certain
number of eigenfrequencies in the FEM simulation. Optical resonance modes were obtained at each
solved eigenfrequency and the spatial distribution of the electric field intensity map for each mode
80
was obtained. Within the obtained eigenfrequencies, it is not surprising to find that the
eigenfrequencies with same polarity (TE or TM) are one free spectrum range (𝐹𝑆𝑅 =𝑐
2𝜋𝑛𝑎) away
from each other. c is the speed of light in vacuum; n is the refractive index of borosilicate; and a is
the radius of the sphere. TE and TM modes which possess different polarity quantum number p but
same first three quantum numbers n, m, l are also observed in the simulation results. They exhibit
similar mode patterns confined within the spherical shells but differ in the electric field intensity as
shown in Figure 3-5 (a) and (b). Reference [107] suggests the approximate location of
eigenfrequency in eq. (4-13)
𝜔𝑛𝑚𝑙𝑝 =𝑐
𝑛𝑎𝑅[
𝑙+1
2
𝑛𝑟−
𝑡𝑛0
𝑛𝑟(
𝑙+1
2
2)
1
3
+−𝑝
√𝑛𝑟2−1
+ (𝑙+
1
2
2)
−1
3(𝑡𝑛
0)2
20𝑛𝑟+ 𝑂 (
𝑙+1
2
2)
−2
3
] (4-13)
where 𝑛𝑟 is the relative refractive index 𝑛𝑟 = 𝑛𝑠/𝑛𝑎 (𝑛𝑠 is the refractive index of the
spherical
shell and 𝑛𝑎 is the refractive index of the medium outside the sphere so that 𝑛𝑟 > 1), l is the polar
number, 𝑡𝑛0 is the nth zero of the Airy function Ai(-𝑡𝑛
0) =0 (and corresponds to the nth order radial
mode), p is the polarity number given by eq. (4-14)
𝑝 = {1 𝑇𝐸
1/𝑛𝑟2 𝑇𝑀
} (4-14)
Combination of eq. (3-13) and eq. (3-14) indicates that for the TE and TM modes with
same quantum number n, m, l, the resonance frequency of TE mode is smaller than that of TM
mode. However, comparing TE and TM modes in Figure 4-5 (a) and (b), suggests that the electric
field intensity of TE mode is > 6 times higher than that of TM mode. Figure 4-5 (c) and (d) show
the first radial order second polar order TE mode and second radial order first polar order TE mode
respectively. The modeling of eigen-frequencies, also showed that the thinner the spherical shell,
the later the second radial order mode appears in the series of calculated eigen-freqencies. The
relative appearance of the of the second radial order TE mode (n=2, m=l, p=1) with respect to the
81
first order fundamental radial TE mode (n=1, m=l, p=1), as a function of the spherical shell
thickness is listed in Table 4-3. This implies that the induced confinement in the smaller thickness
shells modulates the appearance order of modes in the series of eigenfrequencies. Shell thickness
is also seen to affect the order of appearance of TE and TM modes. In the thicker shells, the TE
and TM mode appear successively. However, with reducing the shell thickness, the TM modes
appear after several consecutive TE modes.
Table 4-3. Appearance order of second order radial TE mode (n=2, m=l, p=1) with different shell
thickness. Diameter of the modeled spherical shell is 600 μm
Shell
Thickness
4 µm 4.4 µm 4.8 µm 5.5 µm 8 µm 10 µm
Order of
second
radial order
TE mode
54th 48th 46th 45th 44th 44th
82
Figure 4-5. Comsol FEM simulation of whispering gallery resonance modes in borosilicate micro-
spherical shell. The WGM resonance is modeled in a spherical shell with diameter of 600 µm. The
thickness of the glass shell is 4 µm in (a) – (d). The center wavelength of the couple incident laser
is 760 nm. The azimuthal number is calculated as 3638. The scale bar presents the physical
dimension of the cross section of the spherical shell near equatorial plane. The color bar illustrates
the electric field intensity of the resonate mode. (a) n=1, m=l, p=1 (TE mode), (b) n=1, m=l,
p=1/nr2 (TM mode), (c) n=1, m – l = 1, p=1 (TE mode), (d) n=2, m=l, p=1 (TE mode).
As discussed previously, the obtained eigenfrequencies with same polarity (TE or TM) are
one free spectrum range (𝐹𝑆𝑅 =𝑐
2𝜋𝑛𝑎) away from each other. It means each eigenfrequency
(a) (b)
(c) (d)
83
corresponds to a given azimuthal number m but polar number l cannot be distinguished due to the
degeneracy which is discussed in eq. (4-10) – eq. (4-12).
4.5 Experimental Setup and WGM Resonances
The experimental set-up used for characterizing optical resonance in the glass
microspherical shells is shown in Figure 4-6 (a). The excitation source consists of a tunable 760
nm laser (Thorlabs, TLK-L780M). The laser tuning was driven via a triangle wave at 10 Hz and
corresponds to 15 GHz (Δλ = 28.87 pm) shift from the center wavelength of 760 nm. The light was
evanescently coupled to the resonator via a tapered optical fiber. The fiber was fabricated using a
hydrogen torch placed in the middle of the fiber and then being pulled at a constant rate from both
ends. The polarization of the incident laser was adjusted using a fiber polarization controller to
optimize coupling efficiency. After passing by the resonator and the fiber taper, the transmitted
light was monitored using a photodiode (Thorlabs DET36A). Excitation of the resonance modes
sustained in the equatorial plane of the glass microspherical shells manifest as dips in the
transmission spectrum. The full width at half maximum (FWHM) of the transmission dips indicates
the Quality factor of the resonance. Figure 4-6 (b) shows an optical micrograph of microspherical
shell #9 with a mode in which the light is confined to the equatorial plane of the bubble.
Figure 4-6. (a) Schematic illustration of the experimental set-up for the measurement for the WGM
resonance in glass bubbles. (b) Optical image showing the light confined to the equatorial plane of
microspherical shell #9 upon evanescent coupling of the light through the tapered fiber.
84
Table 4-4 lists the physical dimensions of the various microspherical shells studied in this
work, the corresponding experimentally measured highest Q-factor, and the various resonance
parameters calculated through COMSOL® finite element simulation of the optical resonance
characteristics. Azimuthal mode number m is calculated from the equation of 2πnrr=mλ with λ=760
nm. Eigenfrequencies fnml were simulated with azimuthal mode number m, refractive index of bulk
borosilicate glass nr and shell geometry. Effective refractive index neff can be expressed as neff
=mc/(2πr fnml), where fnml is the simulated resonance frequency of fundamental TE mode, and c is
the speed of light in vacuum. The effective refractive index is an indicator of how well the mode is
confined within the glass shell and the thinner the shell thickness, the lower is its value as can be
seen in Table 4-4.
Table 4-4. Optical characteristics of blown microbubbles
Bubble
Experiment COMSOL® Simulation
Diameter
(µm)
Wall
Thickness
(µm)
Highest
Q-factor
Resonance
Frequency
fnml (THz)
Azimuthal
mode
number m
Free Spectral
Range
Finesse
in 103
Effective
Refractive
Index neff
1
1 1197 ± 5 6.7 8.09×106 396.40∓0.02 7259 ± 30 102 pm (54.4 GHz) 1.10
1.46091±
0.00002
2
2 744 ± 5 8.4 4.34×106 397.12∓0.04 4512 ± 30 164 pm (87.5GHz) 0.95
1.45831±
0.00004
3
3 614 ± 5 8.6 4.02×106 397.40∓0.05 3723 ± 30 199 pm (106GHz) 1.07
1.45701±
0.00006
4
4 636 ± 5 2.2 1.15×107 398.11∓0.05 3857 ± 30 192 pm (102GHz) 2.94
1.45465±
0.00004
5
5 554 ± 5 1.4 1.18×107 400.44∓0.05 3359 ± 30 220 pm (117 GHz) 3.45
1.44589±
0.00004
85
6
6 345 ± 5 1.1 5.19×107 403.23∓0.07 2092 ± 30 354 pm (189 GHz) 24.5
1.43589±
0.00005
7
7 713 ± 5 0.3 8.73×106 427.92∓0.03 4324 ± 30 171 pm (91 GHz) 1.99
1.35332±
0.00001
8
8 404 ± 5 1.0 1.46×106 404.29∓0.06 2450 ± 30 302 pm (161 GHz) 0.59
1.43241±
0.00003
9
9 231 ± 5 NA 1.54×106 NA NA 528 pm (282 GHz) 1.09 NA
The small mode volume of microspherical shell #6, diameter of 345 μm and thin sidewall
thickness of 1.1 μm, results in less than 10 observed resonance modes in the transmission spectrum
within the 15 GHz frequency span as shown in Figure 4-7 (a). Asymmetry in the transmission
spectrum was observed upon scanning the laser frequency up and down as shown in Figure 4-7 (a)
and (b) and arises from thermally induced linewidth broadening/compression effect in optical
micro-resonators [108], [109]. The inset image of Figure 4-7 (a) shows that a resonance mode with
a very high Q-factor of 5.19×107 which was deduced by fitting a Lorentzian curve to the
transmission spectrum. For this resonance mode, the calculated finesse was 2.45×104. For the
microspherical shell resonator #5, the resonance spectrum shows equally-spaced resonance
frequencies in the transmission spectrum of as shown in Figure 4-7 (b). Since the free spectral range
for this microbubble was 117 GHz, these peaks with a frequency spacing of 0.76 GHz must arise
due to azimuthal mode splitting. Azimuthal mode splitting typically arises from the removal of
degeneracy of polar quantum number l in the solution of the spherical harmonic mode function
[107] due to eccentricity of the microbubbles. The analytical expression for azimuthal mode
splitting is derived using perturbation method and is given by [110]
86
)1(31
6
2
ll
m
f
f
nml
ecc (3-15)
where ε is the eccentricity of the microspherical shell, fnml is the resonance frequency of the
mode with radial mode number n, azimuthal mode number m, and polar mode number l. For a shell
with polar radius rp and equatorial radius re, eccentricity ɛ is defined as 휀 =𝑟𝑝−𝑟𝑒
𝑟𝑒. Hence, the
azimuthal mode splitting within the free spectral range between successive polar mode numbers
can be approximated as
3
2
11,,
l
mff nml
llmnnmlecc
(3-16)
The resonance frequency of microspherical shell #5 was calculated by COMSOL®
simulation to be fnml = 400.437508 THz under the assumption of an ideal spherical shell of uniform
wall thickness and a radius of 277 μm. Using the equation for resonance condition, for microbubble
#5, and λ = 760 nm, the azimuthal mode number can be calculated to be m = 3359. Under the
assumption, that the observed peaks in Figure 4-7 (b) are due to the splitting of the fundamental TE
azimuthal mode (m ≈ l ≈ 3359), the frequency spacing of 0.76 GHz leads to a corresponding
eccentricity of ɛ ≈ 0.67 %. Dimensional data of microspherical shell #5 from Table 4-2, can be used
to calculate value of eccentricity which gives a value of 2.5%. This is ~4 times larger than the
eccentricity estimated using eq. (4-16). The large uncertainty of ~5 μm in determining the
microspherical shell diameter and height using optical images can easily account for the observed
discrepancy and therefore, the two eccentricities may be considered to be in agreement with the
errors of the measurements.
87
Figure 4-7. Transmission spectrum of the optical resonance in (a) microspherical shell #6 and (b)
microspherical shell #5 within 15 GHz frequency span.
4.6 Thermal sensing: experimental results and modelling discussion
From the WGM resonance condition, it is clear that the resonance frequencies depend on
both the size and refractive index of the resonator. A small change in the size or the refractive index
can cause a significant resonance frequency shift. Since both the refractive index and the size of
the microspherical shells depend upon temperature due to thermo-optic and thermal expansion
effects, a WGM resonator can be configured as a sensitive thermometer. Assuming a linear
dependence of thermal expansion and refractive index for small temperature variations, these can
be expressed as dr/r = αdT and dnr = βdT; where α and β are the temperature coefficient of
expansion (TCE) and thermo-optic coefficient respectively of borosilicate glass. Taking a variation
of the resonance condition, we can now express the fractional change in the wavelength as
rnml
nml
rr
r
nml
nml
ndTf
df
dTnr
dr
n
dn
f
df
(4-17)
88
The frequency shift per unit change in the temperature of the microspherical shell can be
estimated using eq. (4-17) by using borosilicate material properties at λ = 760 nm, i.e., thermo-
optic coefficient β = 3.41×10-6 K-1 [111], [112], temperature coefficient of expansion α = 3.25×10-
6 K-1 [113], and nr = 1.467 [114]. This gives a theoretical frequency shift of 5.574 ppm K-1. The
sensitivity of the microspherical shells to temperature changes was experimentally measured by
placing the device on the hot side of a calibrated Peltier cooler. WGM mode of microspherical shell
#7, with a Q-factor of ~107, was monitored as a function of temperature. As seen in Figure 4-8 (a),
the resonance frequency decreases with increasing temperature and the induced frequency shift as
a function of temperature, Figure. 4-8 (b), shows a linear dependence with an outstanding thermal
sensitivity of -1.81 GHz K-1 (equal to a wavelength shift of -3.48 pm K-1) and corresponds to
temperature sensitivity of 4.58 x 10-6 K-1. Assuming the frequency resolution of measurement
system to be 100 kHz at a Q-factor of 107, the microspherical shell temperature resolution can be
determined to be 55 µK. For microbubble 7, the resonance frequency fnml was first calculated using
COMSOL® modeling at 20 °C. Thereafter, using the temperature coefficient of expansion and the
thermo-optic coefficient, the bubble dimensions and refractive index were changed to the
corresponding values at the increased temperature and the new fnml was modeled. Through this
method, the expected frequency change was modeled through the range of the experimental
temperature values and resulted in a modeled slope of -2.23 GHz K-1. Clearly the ideal model
overestimates the slope in comparison to the obtained experimental slope of -1.81 GHz K-1. It must
be noted that the ultimate change in the microbubble equatorial radius is not only a function of the
TCE of the glass bubble but is also affected by the TCE mismatch between the borosilicate glass
and the bonded silicon substrate at the base. To account for these issues, we parametrized the
effective TCE of glass and modeled the frequency shift to match the experimental data. As shown
in Figure. 4-8 (b), a near ideal fit was obtained by using an effective TCE of borosilicate glass, α|eff
= 2.19 x 10-6 K-1. Figure 4-8 (c) shows the measured thermal sensitivity of microspherical shells 8
89
and 9 performed with a much finer temperature scan. The experimentally obtained linear slopes for
these silica shells of 1.78 GHz K-1 is very similar to that obtained for microspherical shell 7. It must
be noted that both these microspherical shells are located on the same chip and, although of
different dimensions, show similar thermal dependence of resonance frequency shift. This can be
considered as further evidence of the fact that the effective TCE of the microspherical shells
sensitively depends upon the stresses induced in the between the glass and silicon substrates during
bonding as well as the temperature at which the glass shells are blown.
Figure 4-8. (a) Experimentally measured temperature induced resonance frequency shift of ~107
Q-factor resonance mode in the transmission spectrum of microspherical shell #7. (b) COMSOL
simulation was used to fit the experimentally measured frequency shift by parametrically tuning
the effective value of TCE of the microspherical shell. Good fit was found for an effective TCE
value of 2.19 × 10-6 K-1 for the microspherical shell #7. (c) Measured temperature induced
resonance frequency shift within a finer temperature change for microspherical shell #8 and #9.
90
4.7 Liquid core sensing: experimental results and modelling discussion
A major advantage of WGM resonators consisting of hollow shell structures is that fluidic
analyte samples can be introduced and made to interact with the optical resonance mode through
the inner surface of these structures [86]. Through microfabrication processes the thickness and the
diameters of the microspherical shells can be precisely controlled and reproduced. Sensitivity to
the fluid contained in the inner volume of the optofluidic microspherical resonator as a function of
the shell wall thickness was experimentally examined. For these experiments, on-chip glass
microspherical shells were coated and protected with crystal bond epoxy on the outer surface and
the silicon substrate was etched and thinned in potassium hydroxide solution to a thickness of ~250
µm and until a backside access hole to the inner surface of the microspherical shell was obtained.
The crystal bond protective coating was thereafter removed by dissolving it in acetone at 80 °C.
With open access to the shell cavity, the microspherical shell was filled with water (Refractive
index nwater = 1.332986 at 20 °C) in a vacuum chamber. The filled water within the microspherical
shell was held inside the cavity in atmosphere due to surface tension at the small opening. Figure
4-9 shows the water filled microspherical shell #10. The water gradually evaporated and eventually
dried out in the microspherical cavity.
91
Figure 4-9. Water filled microspherical shell #10. The silicon substrate is wet etched in TMAH by
250 µm to open the bottom access for filling liquid. The liquid is filled by immersing the
microbubble in the water and pumping the air in a vacuum chamber.
The water-filled microspherical shells #10 with wall thickness of 4.7 µm and #11 with wall
thickness of 6.4 µm were coupled with fiber taper and the resonance modes were monitored and
tracked as the water dried out in the microspherical shells in real-time. The transmission spectrum
of microspherical shell #10, in Figure 4-10 (a), showed a blue-shift due to the decrease in the
effective refractive index inside the microbubble cavity as a consequence of the water drying out
and being replaced by air. Inset in Figure 4-10 (a) shows a zoomed-in image of a shifted resonance
mode with and without water in the microspherical shell. A frequency shift of 0.51 GHz and an
increase in the Q-factor from 2.51×106 to 2.69×106 was observed as the core changes from water
to air. Resonance frequency shift was barely observed in the transmission spectrum of
microspherical shell #11. The shift in the resonance frequency of the first radial order fundamental
TE mode between water core and air core of a 600 µm diameter microspherical shell resonator was
simulated as a function of the shell wall thicknesses using COMSOL® and is shown in Figure 4-10
(b). Experimentally measured frequency shift of microspherical shell #10 is in good agreement
with the COMSOL® simulations. The very small frequency shift observed for microspherical shell
#11 arises due to the much larger shell wall thickness of 6.4 µm. The electric field confinement in
a 0.6 µm thick shell at the fundamental eigenfrequency is shown in Figure 4-10 (c) for water filled
and in Figure. 4-10 (d) for air filled shell cores. The images in Figure 4-10 (e) and 4-10 (f) plot the
intensity of the electric field on a log scale and clearly show that the electric field clearly penetrates
into the water core in Figure 4-10 (e). On the other hand, Figure 4-10 (g) and 4-10 (h) show that
the electric field is entirely confined to inside the 8 µm thick glass shell with minimal interaction
with the fluid within microspherical shell. Thus, thicker walled shells are expected to show little
sensitivity to any fluidic core changes or interactions.
92
Figure 4-10. (a) Transmission spectrum of resonant modes obtained from microspherical shell #10
with wall thickness of 4.7 µm. A blue-shift of the resonant modes was observed as the water-filled
microspherical shell core dries out. Inset image shows 0.51 GHz frequency shift observed in a
2.5×106 Q-factor mode. (b) COMSOL simulated frequency shifts between water-core and air-core
microspherical shells with diameters of 600 µm as a function of the shell thicknesses ranging from
93
300 nm to 10 µm. Experimental data for two microspherical shells of thicknesses 4.7 μm and 6.4
μm is also shown. (c)-(d) FEM solved fundamental TE mode showing the spatial distribution of
the electric field intensity in 0.6 µm shell thickness with water and air core respectively. (e)-(f)
Electric field intensity is plotted in logarithmic scale for water and air cores in 0.6 µm thick shell
and clearly exhibits penetration of electric field into water core in (c). (g)-(h) FEM solved
fundamental TE mode in a 8 µm thick microspherical shell with water and air core respectively.
(i)-(j) Electric field intensity plotted in logarithmic scale for the two cores for the 8 µm thick
microspherical shell. The simulations clearly show that the TE mode electric field interacts strongly
with the fluid in the core of thinner walled microspherical shells than for thicker shell walls and
explains the larger frequency shift obtained for thinner walled shells.
4.8 Additional Preliminary Results
4.8.1 Integrated Microfluidic Devices using Microspherical Shell Optical Resonators
In order to realize integrated microfluidic devices with microspherical shell optical
resonators with fluidic access to the internal volume, it is necessary to etch the silicon substrate
from the back side of the bubble structure. The silicon substrate of the glass microsphecial shell
was completely removed in 20% KOH at 330 K (57 °C) for 15 hours. The WGM optical resonance
of a fully released glass microbubble, 750 µm diameter and initial shell wall thickness of ~4 µm,
is shown in Figure 4-11. It can be noticed that the Q-factor of the glass microbubble resonator
attenuates after KOH releasing process which etches and roughens the glass microbubble surface.
Resonance Frequency (GHz)
Vo
lta
ge (
V)
-21 -15 -9 -3 3 9 15 215.5
6
6.5
7
7.5
8
8.5
Q=2.1M
94
Figure 4-11. Transmission spectrum of resonant modes obtained from the fully silicon substrate
removed glass microbubble in the diameter of 750 µm and initial thickness about 4 µm. The quality
factors of the obtained modes reduce due to the glass microbubble surface roughening in the KOH
releasing process.
4.8.2 Resonance in serially coupled Optical Resonators
Preliminary experiments were performed to observe the transmission spectrum change,
from a single fiber, upon coupling from one to two microspherical shells located collinearly on a
single chip. In the experiment, two identical microbubbles which were fabricated on one chip were
aligned nearly parallel to the length of the tapered fiber. The two microbubbles were 750 µm in
diameter and 1.3 cm apart from each other. For coupling the two microbubbles, the microbubble
chip was approached towards the taper fiber. The initial parallel alignment ensures that when the
whispering gallery modes of the first microbubble are obtained in the transmission spectrum, the
second microbubble to be coupled onto the taper fiber is very close to the tapered fiber. Upon a
slight further adjustment of the microbubble chip with respect to the fiber, the two bubbles can be
brought to couple to the evanescent field. Figure 4-12 shows the transmission spectrum of coupling
single and dual microbubbles. The result clearly shows that upon coupling to the second
microbubble the transmission signal, photodiode voltage amplitude, is instantly reduced due to the
reduced optical conductance arising from the coupling of the energy to the series arrangement of
the two microbubbles. The density of the observed resonance modes are observed in the dual
microbubble coupling state is clearly increased than the single microbubble coupling – indicating
the convolution of the transmission spectrum of the two bubbles in the output signal.
95
Figure 4-12. Transmission spectrum of single microbubble coupling and double microbubbles
coupling
4.9 Summary
In this chapter, the author demonstrated the first chip-scale, silica glass microspherical
shell, optical resonators with high-Q factors fabricated by chip-scale glassblowing techniques and
demonstrated the potential of these structures for on-chip sensing applications. The author has
demonstrated a glass microfabrication process for realizing microspherical shells with diameters
ranging from 230 µm to 1.2 mm diameters and shell thicknesses ranging from 300 nm to 10 μm.
Arrays of on-chip fabricated, near spherical glass microspherical shells were successfully achieved
with the equatorial planes located above the plane of the substrate. On-chip integration of highly
symmetric and smooth surface, closed spherical shell structures, can allow for the realization of
WGM based in-line microfluidic (bio)chemical sensors where the analyte fluid interacts with the
optical resonance through the inner surface of the shell. The author demonstrated and modeled the
thermal sensing capability of the glass microspherical shell resonator. A proof-of-concept liquid
core sensor was demonstrated by sensing the change in the index of refraction arising from the
evaporation of water from within the microspherical shell and the expected change in frequency
Resonance Frequency (GHz)
Vo
ltag
e (
V)
-21 -18 -15 -12 -9 -6 -3 0 3 6 9 12 15 18 210
2
4
6
8
10
Single microbubble couplingDouble microbubbles coupling
96
was calculated through COMSOL modeling – thus confirming the principle of operation of the
device. Optical resonance of silicon substrate removed glass microspherical shell chip was
characterized to support the future work relating to the direct integration of the microspherical shell
structures into microfluidic structures. The transmission spectrum of two microspherical shells
placed in series and coupled to a single tapered fiber was experimentally measured and showed a
clear convolution of the two spectra. These initial measurements lay the foundations for realizing
integrated microfluidic devices and other sensor concepts using arrays of such coupled resonators.
97
Chapter 5
Glass Microbubble Packaged Ferrofluids – Microfabricated Quartz
Resonator Based Magentoviscous Magnetometer
5.1 Introduction and motivation
Easy to use, low cost, chip-scale, ultra-sensitive magnetometers are attractive for several
applications such as position sensors, orientation sensors, and biomedical imaging and diagnosis
[115]. Many innovative approaches have been proposed and investigated to explore sensitive
magnetometers. Superconducting quantum interference devices (SQUID) is one of the most
sensitive magnetic field sensor which are reported with 4.5fT/Hz resolution [116]. Atomic
magnetometers have also shown comparable sensitivity [117], [118]. However, usually SQUIDs
requires cryogenic cooling systems to operate whereas in atomic magnetometers, the sensor vapor
cell must be heated to high temperatures (>150 °C). Other magnetic field sensors such as fluxgate
sensors [119], giant magnetoresistance (GMR) spin valve [120], magnetoelectric sensors [121],
and magnetoflexoelastic resonator based sensors [66] have been demonstrated with sensitivities
ranging from 10 nT to 100 pT.
Recently, a novel concept for magnetic sensing is demonstrated which is based upon
magnetoviscoelastic effect of ferrofluid [122]. Ferrofluids are emulsions of nanometer sized
ferroparticles suspended in a carrier liquid which are able to organize spontaneously into
columnar structures under the influence of external magnetic fields [123]. The ferrofluid
nanoparticles are illustrated in Figure 5-1 (a) [123]. The aggregation of ferro-particles in response
to external magnetic fields results in large viscosity changes in the ferrofluids and is known as
magnetoviscous effect [124]. The magnetoviscous effect in a ferrofluid (APG513A) is shown in
98
Figure 5-1 (b) [124]. Moreover, not only viscosity changes but also changes in the elastic
properties of the densely agglomerated ferro-particle layer take place in an external magnetic
field.
Figure 5-1. (a) Schematically illustration of magnetic particles in ferrofluids. The diameter of the
particle is about 10 nm and the length of the surfactant is about 2 nm. (b) Magnetoviscous effect:
the viscosity of the ferrofluids increases as increase of magnetic field.
On the other side, micromachined high-resonance frequency quartz resonator demonstrates
high sensitivity to the change of viscoelastic properties of the interfacial layer which is formed at
the surface of quartz resonator in the liquid loading ambient [18]. A thickness shear mode quartz
crystal resonator typically consists of a slab of thin single-crystal, piezoelectric quartz, with very
large lateral dimensions in comparison to its thickness. The slab is sandwiched between two metal
electrodes. When electric field is applied, the quartz generates a shear wave through the thickness
of the quartz. The device exhibits a resonance behavior when the wavelength of shear acoustic
wave is twice the thickness of the quartz slab. The resonance frequency of a quartz resonator is
governed by:
𝑓0 =1
2𝑡𝑞√
𝜇𝑞
𝜌𝑞 (5-1)
Where tq is the thickness of quartz slab, µq and ρq are the shear modulus and density of quartz.
99
Realistically, in addition to the thickness of the quartz slab and the mechanical properties of quartz
materials, the actual resonance frequency of a shear mode quartz crystal resonator also depends on
properties of the ambient loadings of the quartz resonator. The behavior of quartz resonator with
viscous loading was firstly reported by Kanazawa and Gordon in 1985 [125]. Respect to the
resonance frequency of the quartz resonator without loadings, the resonance frequency shift of the
quartz resonator with liquid loading is given by:
Δ𝑓 = −𝑓0
3/2
√𝜋𝜌𝑞𝜇𝑞√𝜌𝐿𝜂𝐿 (5-2)
Where ρL and ηL are density and viscosity of loaded liquid.
Quartz resonator typically samples a layer of thickness equivalent to the decay length which is given
by:
𝛿𝑙𝑖𝑞 = √𝜂𝐿
𝜋𝜌𝐿𝑓0 (5-3)
Therefore, a high resonance frequency resonator is demanded for thin films. For example, a quartz
resonator with 200 MHz resonance frequency can be used for probing a thin layer of 40 nm.
Our previous work proposed the idea of loading ferrofluid atop a high frequency quartz
shear wave resonator and monitor the at-resonance impedance changes of the quartz resonator upon
the application of external magnetic field. The configuration of the proposed device is shown in
Figure 5-2. Ferrofluids is loaded on the quartz and out-of-plane electric field is applied trough the
top and bottom pattered electrodes (yellow layers in the sketch) to drive the quartz slab resonate in
shear mode. A small magnet is used to apply out-of-plane magnetic field at the bottom of the quartz
slab. The out-of-plane bias magnetic field aggregates nano-particles at the surface of the resonator
and forms nano-particles aggregated interfacial layer. In-plane magnetic field is induced by
Helmholtz coils for sensing. The quartz crystal resonance frequency shift resulting from the
deposition of viscoelastic layer in a viscous liquid ambient can be analyzed using a continuum
100
mechanics approach as developed by Kasemo and coworkers [126]. In order to model this situation,
the quartz resonator surface is considered to be in intimate contact with the out-of-plane magnetic
field induced viscoelastic layer with an infinitely thick Newtonian liquid over-layer. Under the
assumption that the thickness of the bulk ferrofluid-liquid layer is much larger than the decay length
of the acoustic wave in the liquid, the frequency changes with respect to only ferro-liquid loading
conditions can then be written as [127]:
𝛥𝑓 = −1
2𝜋𝜌𝑞𝑡𝑞(𝑡𝑣𝑖𝑠𝑐𝜌𝑣𝑖𝑠𝑐𝜔 − 2𝑡𝑣𝑖𝑠𝑐(
𝜂𝑙𝑖𝑞
𝛿𝑙𝑖𝑞)2 𝜂𝑣𝑖𝑠𝑐𝜔2
𝜇𝑣𝑖𝑠𝑐2 +𝜔2𝜂𝑣𝑖𝑠𝑐
2 (5-3)
Where tvisc, ρvisc, µvisc are thickness, viscosity and elastic modulus of the viscoelastic layer. ω = 2πf0.
The proof of concept work demonstrated very promising magnetic sensitivity of 1.5
nT/Hz [122]. However, the lifetime of the device was only a few hours since without any
sealing of the ferrofluids liquid, it continuously evaporates and dried out. In this paper, we will
specifically demonstrate the application of glass microbubble to achieve chip scale device
packaging that allows the ferrofluids to hermetically sealed atop the resonator to achieve a
magnetometer with robust lifetime.
Figure 5-2. Schematic illustration of the ferrofluid – quartz resonator based magnetoviscos
magnetometer.
101
5.2 Device Fabrication and Experiment Set-up
5.2.1 Quartz Resonator Chip
A 100 μm thick, 1’’ diameter polished AT-cut quartz crystal substrate is cleaned in
Nanostrip™ solution for 30 mins as shown in Figure 5-3 (a). 15 nm/150 nm thick Cr/Au seed layers
are deposited on one side of quartz substrate by evaporation and then patterned with SPR-220
photoresist to form 15 μm thick photoresist pattern to define two 1 mm diameter resonator regions.
10 μm nickel is then electroplated as hard mask of etching. After removing photoresist, the
resonator areas then thinned down using deep silicon oxide etching method [45] to a thickness of
8-15 μm. The size of each chip is 9 mm ×9 mm. Following a clean strip of any remaining nickel
mask, Cr (20 nm)/Au (100 nm) layers are evaporated, and lithographically patterned and wet etched
to form bottom electrode as shown in Figure 5-3 (b). Following this, the quartz substrate is flipped
over and is followed by another lithography step to define the top electrode by lift-off of evaporated
films of Cr (20 nm)/Au (100 nm) as shown in Figure 5-3 (c). The patterns of both top and bottom
electrodes are extended to one of the edges of the chip so that they are accessible for wire bonding
after the glass microbubble integrated atop the resonator. Finally, 500 nm thick high relative
permeability Metglas® layer is sputtered on flat (unetched) side of the resonator as shown in Figure
5-3 (d). An optical image of the fabricated micro-quartz-crystal-resonator (μQCR) is shown in
Figure 5-3 (e).
102
Figure 5-3. (a) – (e) : Schemaic illustration of the design and fabrication of ferrofluid-μQCR
magnetometer. (a) 100 μm thick AT-cut quartz substrate, (b) optimized ICP-RIE etched 90-95 μm
quartz resonating region with deposition and patterning of 15/150 nm thick Cr/Au backside
electrode, (c) Deposition and patterning of 15/150 nm thick Cr/Au front side common electrode,
(d) Deposition and patterning of 500 nm thick Metglas magnetic flux concentrator. (e) optical
image of the fabricated μQCR.
5.2.2 Glass Microbubble Chip
A 550 μm thick, 4 inch diameter silicon wafer is patterned and deep reactive ion etched
(DRIE) to form 250 μm deep circular trenches. A 100 μm thick, 4 inch diameter Borofloat® 33 glass
is anodically bonded to the silicon wafer as shown in Figure 5-4 (a). During the bonding, nitrogen
at 1 atm pressure is trapped in the circular trench cavities enclosed by the glass wafer. The bonded
wafer is diced into 8 mm × 9 mm dies and the dies are individually placed in a rapid thermal
annealing (RTA) chamber and heated to 850 °C for 3 mins [31]. Due to the pressure generated from
the volume expansion of trapped nitrogen in the cavity at high temperature, the softened 100 μm
thick borofloat expands into a semi-spherical microbubble as shown in Figure 5-4 (b). The
103
microbubbles are 1.5 mm in diameter. In addition to blowing up the central microbubbles, a
surrounding ring shaped glass is also expanded – creating a moat like structure to trap adhesive
overflow into the central region as shown in Figure 5-4 (c). After thermally shaping the
microbubbles, two holes are drilled on the top of the microbubble by a micro-drill for dropping
ferrofluids liquid later on. Glass microbubbles are shown in Figure 5-4 (d) after micro-drilling. In
the final step, the substrate silicon is dissolved in high selectivity 80 °C 22% (wt.) potassium
hydroxide wet etchant for 30 hours. The glass microbubble chip is shown in Figure 5-4 (e) after
removing silicon substrate.
Figure 5-4. (a) Anodic bonded borosilicate glass wafer and etched silicon wafer. (b) Glass
microbubble formed with thermal annealing process. (c) Schematic illustration of the expanded
glass microbubble chip. (d) Optical image of microbubble package chip after drilling holes on the
top of microbubbles. (e) Optical image of microbubble package chip after removing silicon
substrate.
5.2.3 Ferrofluids Packaging
The quartz resonator chip placed on standard dualin-line ceramic package by using silver
epoxy – which also makes the electrical connection to the bottom electrode. The flat regions of the
glass microbubble chip carefully coated with DevconTM 10 mins epoxy. Two chips are carefully
aligned, placed together and gently pressed to each other. The moat-like structure around the central
104
bubble provides a cavity where the DevconTM epoxy can go when it is pressed, instead of being
squeezed onto the resonating region. About 0.6 µl EMG 911 ferrofluid is loaded through the top
hole of the microbubble by using 25 gauge syringe. Ferrofluids filled microbubble is then sealed
with a piece of dicing tape by using DevconTM epoxy again. The top hole of the microbubble after
being sealed by a small piece of tape and DevconTM epoxy for 24 hours. Figure 5-5 (a) (b) show
the glass microbubble packaged ferrofluid – μQCR device.
Figure 5-5. (a) Schematic illustration of packaged device. The dimension mismatch of glass
microbubble chip and quartz resonator chip provides the access of wire-bonding between top-
electrode to the ceramic package. (b) Image of glass microbubble packaged ferrofluid- μQCR device.
5.2.4 Experiment Set-up
The packaged device is placed in the center of a Helmholtz coil and connected to the
network analyzer using a SMA connector. The entire set-up is placed inside a magnetically shielded
three layer mu-metal box. An out-of-plane directed (perpendicular to the surface of the resonator)
constant bias magnetic field is applied by placing a permanent magnet under ceramic package ~3
mm away from the resonator/ferrofluid interface. The resonance characteristics of the
micromachined quartz resonator were measured using an Agilent E 5061B network analyzer
capable of acquiring 1601 impedance measurements over the set frequency span. Figure 5-6 (a)
shows the experiment setup used for magnetic field modulation and data collection. Figure 5-6 (b)
displays the view of the Helmholtz coil and packaged device in the shield box.
105
Figure 5-6. (a) Low noise current source is used to drive Helmholtz coils for modulating magnetic
field in the magnetic shield box. Device Under Test (DUT) is connected with network analyzer. (b)
Image in the shield box: glass microbubble packaged ferrofluid- μQCR is placed on a stage at the
center of Helmholtz coils and connected with network analyzer through SMA connector.
5.3 Results and Discussion
5.3.1 Characterization of quartz resonator
Quartz resonator was fabricated as the steps described in section 5.2.1. The resonance
performance was carefully examined during the packaging processes in each step. Firstly, a quartz
resonator, which was patterned only with top and bottom electrode without Metglas® flux
concentrator, was wire bonded with a ceramic dual inline package and the package is connected to
network analyzer via SMA connections. The obtained conductance and susceptance are plotted as
a function of frequency in red and blue dot lines in Figure 5-7. The resonance frequency is found
at 146.3209 MHz which indicates the quartz resonator was etched to the thickness of 11.4 µm. The
quality factor of the resonator is calculated as 7726. Next, the glass microbubble was carefully
coated with DevconTM 10 mins epoxy, aligned with the quartz resonator chip, placed and gently
pressed on the resonator chip. The DevconTM epoxy was supposed to cure in 15 minutes. In the
106
experiment, the resonance behavior of the quartz resonator was examined 6 hours later after
attaching glass microbubble on the quartz resonator. The obtained conductance and susceptance
are plotted as orange and light-blue dash-dot lines in Figure 5-7. It shows the resonance peak
slightly shifts to the left direction in the frequency spectrum. It probably results from incomplete
evaporation of epoxy vapor which was trapped in the glass microbubble during the curing process.
But the quality factor of the resonator maintains as the original value after attaching microbubble
chip. At last, ferrofluids was loaded with a small syringe through the drilled top holes and was
sealed with a small piece of dicing tape by DevconTM 10 mins epoxy. The red and blue solid lines
in Figure 5-7 demonstrate the conductance and susceptance shift to the left direction respect to the
original resonance frequency. The new resonance frequency of 146.2337 MHz indicates resonance
frequency shift of 87182.5 Hz due to loading ferrofluid liquid on the resonator. Furthermore, the
quality factor of the resonator damps to 2753 at the new resonance frequency.
Figure 5-7. Characterization of quartz resonance during the packaging process.
107
5.3.2 Responds of magnetic field
Quality factor of micro-fabricated quartz resonator critically determines the magnetic
sensitivity of ferrofluid-µQCR device. Not only the ferrofluid loading, but also inappropriate
magnetic bias, which was supposed to form the interfacial viscoelastic layer, possibly attracts
excessive ferro-particles and deteriorates the quality factor. When the Q-factor deducts under 1000,
the resonance peak turns out too broad to determine the resonance frequency, so the frequency shifts
are difficult to be resolved. Therefore, a high quality factor quartz resonator and optimum magnitude
of bottom magnetic bias are highly desired for sensing low magnetic field with using the ferrofluid-
µQCR device. Table 5-1 summarizes resonance characteristics of three quartz resonators. The
magnetic sensitivity and device lifetime will be discussed later. Figure 5-8 shows an obtained high
Q-factor resonance at 122.6074 MHz after loading ferrofluids. The resonance frequency indicates
the quartz resonator was etched to the thickness of 13.6 µm and the quality factor is calculated as
10779. This device is the one that used to sense magnetic field in the later sections. The resonance
spectrum before loading ferrofluid is missing because the resonance frequency of this resonator was
overestimated initially so the frequency range below 130 MHz was not examined in the frequency
spectrum. However, based on the reduction factors observed in Device 1 and Device 2, the quality
factor of Device 3 is speculated to be around 30000.
Table 5-1. Resonance characteristics of three fabricated quartz resonators.
Device number
Resonance Frequency
Q-factor before ferrofluid packaging
Q-factor after ferrofluid packaging
Resonator thickness
Metglas concentrator
patterns
Bias field
1 131.0 MHz 1500 440 12.7 Yes 5 mT
2 146.3 MHz 7726 2753 11.4 No 10 mT
3 122.6 MHz NA 10779 13.6 Yes 3 mT
108
Figure 5-8. Obtained high Q-factor resonator after loading ferrofluids in the glass microbubble.
As a part of the packaged device, a perpendicular bias field is used to pre-organize the
ferroparticles and introduce spontaneous self-assembly in the ferrofluid/resonator interface layer.
The dense self-assembled ferrofluid interfacial layer can be modeled as a viscoelastic surface load
in modified BVD model [123]. Due to proximity, the applied bias field also magnetizes the Metglas®
layer along the easy axis in the plane of the film causing the interfacial ferroparticles to be very
likely oriented along the in-plane direction of the Metglas layer. Hence, we expect the ferro- particles
ordering on the interfacial layers to be strongly influenced by the in-plane magnetic field of the
Metglas® layer.
Low modulation frequency (0.5 Hz) sense magnetic field is applied by the Helmhloz coils.
The sense field is able to perturb the self-assembled and ordered interfacial viscoelastic ferrofluid
layer. The resulting magnetoviscoelastic changes in this interfacial ferrofluid layer are monitored
by tracking the susceptance at-resonance frequency. Fig. 5-9 (a) shows the real-time response of
the resonator to a sinusiod wave applied magnetic field with peak value of 33.6 µT. The susceptance
responds data in time spectrum is then transferred to frequency spectrum by using fast Fourier
transform (FFT) as shown in Figure 5-9 (b). Figure 5-9 (c) plots the amplitude of FFT peak-signals
at the modulation frequency (0.5 Hz) as a function of applied magnetic field. Noise level is
109
determined by the FFT value at 0.5 Hz frequency without applying magnetic field. It can be seen
that the predicted minimum detectable sensitivity of glass microbubble packaged ferrofluid-µQCR
is 600 nT in Figure 5-9 (c).
Figure 5-9. (a) Real-time susceptance responds to modulated magnetic field. Modulation frequency
:0.5 Hz; modulation field: 33.6 µT. The scan time is 20 seconds. (b) FFT is applied to the measured
susceptance responds in time spectrum. FFT peak-signal at the modulation frequency of the
magnetic field is tracked to quantify the susceptance responds. (c) Amptitudes of FFT peak-signals
at the modulation frequency are plotted as a function of intensity of modulation magnetic field.
The susceptance responds of different modulation frequency of the magnetic field are also
examined in the work. Figure 5-10 plots the susceptance responds as a function of applied magnetic
field in the modulation frequency ranging from 0.2 Hz to 40 Hz. The predicted minimum detectable
magnetic fields in each frequency are listed in the legend of the plot based on the measured noise
level. The result demonstrates the glass microbubble packaged ferrofluid-µQCM is capable of
Time (s)
Su
sc
ep
tan
ce
( S
iem
en
s)
0 2 4 6 8 10 12 14 16 18 200.10288
0.10289
0.1029
0.10291
0.10292
0.10293
0.10294
0.10295
0.10296(a)
Frequency, HertzS
iem
en
s/
Hz
0.05 0.1 0.2 0.5 1 2 3 45 7 10 2030 501E-8
2E-8
5E-8
1E-7
2E-7
5E-7
1E-6
2E-6
5E-6
1E-5
2E-5(b)
Magnetic Field (T)
Sie
me
ns
/H
z
0.5 1 2 3 4 567 10 20 30 50 1005E-7
7E-7
1E-6
2E-6
3E-6
5E-6
7E-6
1E-5
2E-5(c)0.5 HzNoise Floor
110
measuring magnetic modulation frequency ranging from 0.2 Hz to 40Hz. The highest sesitivity of
0.5 µT was obtained with the modulaton frequency of 0.5 Hz. The result is consistent with the fine
scan shown in Figure 5-9 (b). However, a clear trend of modulation frequency dependent minimum
detectable magnetic field is difficult to conclude.
Figure 5-10. Susceptance responds to various modulation frequency as a function amptitude of
magnetic field.
With appling perpendicular bias magnetic field to the ferrofluid, Metglas® thin film is
expected to be magnetized in-plane (along easy axis) and acts as a magnet in close proximity to the
ferrofluid [122]. The patterned bow-tie shape Metglas® film is expected to concentrates the
magnetic flux lines and focuses through the small gap region. The effect of patterned Metglas® flux
concentrator layer is investigated by measuring and calibrating the susceptance responds of Device
2 and Device 3 as a function of magnetic field intensity as shown in Figure 5-11. Comparing to the
minimum detected magnetic field as 2.5 µT in Device 2, the device with Metglas® flux concentrator
demonstrates 4 times higher sensitivity. The modulation frequency is 0.5 Hz in two cases. The
obtained sensitivities from three devices are listed in Table 5-2.
Magnetic Field (T)
Sie
men
s/
Hz
0.5 1 2 3 4 567 10 20 30 50 1002E-73E-7
5E-7
1E-6
2E-63E-6
5E-6
1E-5
2E-53E-5
5E-55E-5
Noise Floor
0.2 Hz1.8 T0.5 Hz0.5 T1 Hz0.8 T2 Hz1.2 T
5Hz2.0 T10 Hz1.9 T20Hz1.5 T40 Hz1.1 T
111
Table 5-2. Summary of obtained sensitivity from three devices.
Device Number Minimum measured magnetic field (µT)
Minimum predicted magnetic field (µT)
Modulation frequency
1 200 15 0.01
2 11.2 2.4 0.5
3 3.4 0.6 0.5
Figure 5-11. Comparison of susceptance responds of the devices with and without Metglas® flux
concentrator as a function amptitude of magnetic field.
5.3.3 Lifetime of packaged device
The idea of loading ferrofluid atop a high frequency quartz shear wave resonator and
monitor the at-resonance impedance changes of the quartz resonator upon the application of
external magnetic field have been realized in [122] and in previous discussion. However, the most
chanllenge issue of this configuration device is that the lifetime of the device was only a few hours
since without any sealing of the ferrofluids liquid, it continuously evaporates and dried out.
Magnetic Field (T)
Sie
me
ns
/H
z
0.5 1 2 3 4 5 7 10 20 30 50 100 2005E-7
7E-7
1E-6
2E-6
3E-6
5E-6
7E-6
1E-5
2E-5with Metglas flux concentratorwithout Metglas flux concentrator
112
Therefore, here we specifically demonstrate the application of glass microbubble to achieve chip
scale device packaging that allows the ferrofluids to hermetically sealed atop the resonator to
achieve a magnetometer with robust lifetime.
With the glass microbubble chip packaging method, to date, Device 2 and Device 3 have
demonstrated reliable magnetic sensitivity for over a week and continuing. Device 1 has
demonstrated reproducible frequency shifts under external magnetic field for more than 50 days.
Figure 5-12 shows repeatable frequency shifts of Device 1 under external magnetic field.
Figure 5-12. Frequency shift of Device 1 and Device 3 under external magnetic field as a function
of time.
5.4 Summary
In this chapter, we presented a new packaging method for improving the lifetime of a
ferrofluid-based magnetoviscous magnetometer. The concept of a ferrofluid based magnetometer
has been previously reported where the viscoelastic response of a thin interfacial ferrofluid layer
loaded atop a high frequency shear wave quartz resonator to applied magnetic field is monitored.
The magnetic field can be sensitively quantified by the changes in the at-resonance admittance
characteristics of the resonator. However, under open conditions, continuous evaporation of the
Day
Fre
qu
en
cy S
hif
t
f (H
z)
0 4 8 12 16 20 24 28 32 36 40 44 480
20
40
60
80
100
120
140
160
180
200
220
674 1348
Day
Sie
men
s/
Hz
1 3 5 7 9 11 13 15 17 19 21 23 251E-6
1.5E-6
2E-6
2.5E-6
3E-6
3.5E-6
4E-63.4 T5.6 T
113
ferrofluid compromises the long term performance of the magnetometer. In this work, we integrated
glass hemispherical microbubbles, used as vessels of ferrofluid, on the resonator chip to seal and
prevent the evaporation of the ferrofluid liquid and drying out. A layer of high relative permeability
thin film Metglas (Fe85B5Si10) is patterned as flux concentrator on the resonator chip to improve the
sensitivity. Using these improvements, a minimum detectable field of 500 nT at 0.5 Hz is achieved.
Moreover, comparing with the unsealed ferrofluid device, the lifetime of the glass microbubble
integrated chip packaged device improved significantly from only few hours to over 50 days.
Furthermore, packaging ferrofluid liquid on miniaturized quartz resonator using glass
microbubble demonstrates an example of realization of chip-scale micro-cavity package for liquid
material. It provides the feasibility of maintaining liquid in micro-liter volume in the potential
applications fields such as microfluids, Bio-MEMS and optical MEMS. Meanwhile, Being
mechanically robust, optical transparent and highly reproducible, on-chip glass microbubble
possesses the capability of wafer-level vacuum packaging for MEMS devices.
114
Chapter 6
Summary and Future Work
This dissertation demonstrated the exploration of glass microfabrication techniques for
fabricating novel chip-scale glass based transducers. Firstly, plasma etching processes on three
compositions of glass substrates were explored using a modified inductively couple reactive ion
etching (ICP-RIE) system for high etch-rate, high aspect ratio, smooth etching performance, and
understanding the fundamental plasma glass etching mechanism. In addition to using SF6 as the
plasma source gas, NF3 and H2O gases were introduced through diffuser-ring gas inlet in the
vicinity of wafer. Unprecedented Angstrom level surface smoothness were observed in the diffuser-
ring modified etcher. 19%, 15% and 88% higher etch rates were achieved in the diffuser-ring
modified etcher than the conventional etching method. Fluorine atom, fluorine molecule and NFx
radicals were speculated as critical radicals for fast etch. Therefore, in order to maximize the
generation of fluorine atom, fluorine molecule and NFx radicals, diffuser-tubes with different
heights were used to introduce NF3 and H2O gases several centimeters above the wafer substrate.
Etch rates as high as 1.06 μm/min, 1.04 μm/min, and 0.45 μm/min with surface smoothness of ~2
Å, ~67 Å, ~4 Å were achieved for fused silica, borosilicate glass, and aluminosilicate glasses
respectively after 5 minutes etches. High aspect ratio etch of 5.2:1, 10:1 and 2:1 are obtained for
fused silica, borosilicate glass, and aluminosilicate glass respectively. Glass etching mechanism
was further understood by statistically analyzing the etch rates and corresponding partial pressure
of plasma species detected by in-situ residual gas analyzer (RGA) with various position of the
diffuser gas inlet. As shown in Table 3.2, statistical analysis confirmed that etch rate of fused silica
is critically influenced by fluorine based radicals and molecular fragments, and the etching
smoothness of fused silica is mostly influenced by HF molecule. Both fluorine atom and ion flux
115
influence on the fast etch of borosilcate. The large fraction of impurity atoms of Ca and Al in
aluminosilicate glass form non-volatile fluorides on the etch surface and therefore the etch rate and
surface smoothness of aluminosilicate glass is primarily influenced ion flux and very little by the
fluorine chemistry. At last, the role of the layout of the metal mask layer on how it influences the
charging of glass substrates was examined during etching and therefore the etch rate.
Plasma glass etching mechanism is further understood in this work. The benchmark of etch
rates and etch roughness with feeding etchant gases from ICP source indicates that the conventional
plasma glass etching results from physical energetic ions bombardment dominated etching
mechanism. The modified ICP-RIE etcher realizes a physical-chemical tunable glass etching
system by introducing NF3 and H2O through diffuser gas inlets. With gas-diffuser modified etching
system, the enhanced chemical etching components such as fluorine atom, fluorine molecule and
NFx radicals lead to faster etch rate and smoother etched surface than the conventional etching
system. The activation of chemical etching follows the step of physical bombardment induced
surface sensitization [56]. However, the enhancement of chemical etching components which
results from enriching fluorine based gas etchants in the etcher conflicts with the conditions for
supporting energetic ion bombardment induced surface sensitization due to the decrease of ion
mean free path. Therefore, an alternate sequence process for plasma glass etching could be
proposed in the future. In the proposal, the step of surface sensitization by energetic ion
bombardment and step of abundant fluorine based chemical etching components can be processed
in alternate pulse sequence. By separately igniting physical energetic ion dominated plasma and
chemical fluorine radicals dominated plasma in two levels of chamber pressure, further increased
glass etch rate might be expected in the new method.
In the second half of the thesis, chip scale glass blowing technique was explored for novel
sensing and packaging applications. Arrays of on-chip spherical glass shells of hundreds of
micrometers in diameter with ultra-smooth surfaces and sub-micrometer wall thicknesses had been
116
fabricated and had been shown to sustain optical resonance modes with high Q-factors of greater
than 50 million. The resonators exhibited temperature sensitivity of -1.8 GHz K-1 and could be
configured as ultra-high sensitivity thermal sensors for a broad range of applications. By virtue of
the geometry's strong light-matter interaction, the inner surface provided an excellent on-chip
sensing platform that truly opened up the possibility for reproducible, chip scale, ultra-high
sensitivity microfluidic sensor arrays. As a proof of concept we demonstrated the sensitivity of the
resonance frequency as water was filled inside the microspherical shell and was allowed to
evaporate. By COMSOL modeling, the dependence of this interaction on glass shell thickness was
elucidated and the experimental results of the sensitivity of two different shell thicknesses was
explained.
As future work, the on-chip glass micro-spherical shell is proposed to be integrated with
microfluid channel substrate for sensing bio-chemical reaction. Secondly, the preliminary
experiments demonstrated the capability of detecting variation in the vapor pressure of water.
Therefore, an experimental setup which ready for making systematic and quantitative
measurements could be proposed in the future.
In the last chapter, chip-scale blown, glass microbubbles were explored for encapsulation
of ferrofluid atop a micromachined quartz resonator configured as a magnetometer. The concept of
a ferrofluid based magnetometer had been previously reported where the viscoelastic response of a
thin interfacial ferrofluid layer loaded atop a high frequency shear wave quartz resonator to applied
magnetic field was monitored. The magnetic field can be sensitively quantified by the changes in
the at-resonance admittance characteristics of the resonator. However, under open conditions,
continuous evaporation of the ferrofluid compromised the long term performance of the
magnetometer. In this work, we integrated glass hemispherical microbubbles, used as vessels of
ferrofluid, on the resonator chip to seal and prevent the evaporation of the ferrofluid liquid and
drying out. Using these improvements, a minimum detectable field of 600 nT at 0.5 Hz was
117
achieved. Moreover, comparing with the unsealed ferrofluid device, the lifetime of the glass
microbubble integrated chip packaged device improved significantly from only few hours to over
fifty days and continuing.
In this work, scalable wafer-level fabricated glass microbubble which was used to package
ferrofluid for magnetometer is demonstrated as an example for potential packaging applications. It
provides feasibilities of on-chip glass microbubble encapsulation for vacuum package, inert gas
package and liquid package etc.
118
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Appendix
NF3 and H2O mass flow controller
NF3 and H2O gas are introduced through diffuser gas inlet in the modified ICP-RIE system.
Flow rate of NF3 is controlled by a Unit 1620 mass flow controller which is capable of introducing
NF3 up to 240 sccm. The calibration of NF3 flow rate is listed as a function of applied voltage in
the table below.
NF3 flow rate 20 sccm 40 sccm 60 sccm 100 sccm 140 sccm 240 sccm
Applied voltage 0.417 V 0.833 V 1.25 V 2.2 V 2.8 V 5 V
H2O vapor is generated by heating up a stainless steel tank of water sitting on a hot plate.
The temperature of the hot plate is preset at 110 ⁰C for maintaining the water temperature of 55 ⁰C.
The flow rate of water vapor is controlled by MKS® Type 1150A mass flow controller in the range
of 0 – 300 sccm. The details of connection pins of the MFC can be found in the manual of MKS®
GM50A online. The mass flow controller is designed and calibrated with downstream pressure of
1-50 mTorr and the upstream pressure of 120 Torr (vapor pressure of water at 55 ⁰C). The
calibration of H2O flow rate is shown as a function of applied voltage in the figure below.
Applied voltage (V)
H2O
ga
s f
low
ra
te (
sc
cm
)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
50
100
150
200
250
300
129
When introducing NF3 and H2O vapor from diffuser gas, a temporization step is required
to preset the flow rate of H2O for stabilization. The temporization step is suggested to be 8 minutes.
When the temporization step starts, the pressure of the chamber is shown as 1ⅹ10-5 mbar at the
processing pressure gauge (high pressure gauge, low vacuum gauge). Then turn up the power
source which controls the flow rate of water vapor to 4.5 V. The reading pressure of the process
gauge is supposed to be 8.8ⅹ10-4 mbar (the actual pressure is one order larger: 8.8ⅹ10-3 mbar). If
the RGA is connected to the chamber, it is supposed to observe an increased partial pressure of N2
in the RGA spectrum. Then wait for 5-6 minute during the temporization step, N2 peak is supposed
to decrease and the partial pressure of H2O is supposed to increase in the RGA spectrum. Then at
6th minute of the temporization step, turn down the power source which controls the flow rate of
water vapor to the actual value (for example 0.95 V for introducing 50 sccm H2O into the chamber).
Both the reading chamber pressure and partial pressure of H2O in the RGA spectrum are supposed
to decrease in the last two minutes of the temporization step. When the temporization step finishes,
turn on all valves for both NF3 and H2O mass flow controllers. The program goes to the next step
and the plasma is ignited with feeding etchant gas from ICP source.
VITA
Chenchen Zhang
Chenchen Zhang was born in Urumqi, Xinjiang, China on February 15th, 1990. He
received his Bachelor in Science degree on Physics from Shanghai Jiao Tong University,
Shanghai, China in 2012. He started his Ph.D study and joined Dr. Srinivas Tadigadapa’s
research group in 2012 Fall in the Electrical Engineering Department in the Pennsylvania State
University, PA, USA. His research interests include MEMS, glass plasma etching and
microelectronic microfabrication process.
