108

CHAPTER 4

FABRICATION AND TESTING

4.1 INTRODUCTION

Based on the design guidelines arrived at in the second and third

chapters the TOF has been realized in SOI wafers using DRIE. The SEM of

the TOF is shown in the Figure 4.1.

Figure 4.1 SEM showing the tilted top view (left) and top view of the

full device (right)

This chapter deals with the microfabrication of the TOF in SOI

wafer and its electromechanical and optical characterizations. Several fixed

PC filters with fixed period, but varying defect widths have been fabricated in

<110> oriented silicon wafers using relatively simpler wet anisotropic

109

etching. These filters are fabricated along with fiber grooves for the easy

alignment of lensed optical fibers for light coupling. The SEM of the top view

of the fixed PC filter fabricated by wet anisotropic etching of <110> oriented

silicon is shown in the Figure 4.2. These fixed filters are used for the

experimental validation of the theoretical predictions of optical

characteristics, minimum insertion loss that can be achieved and also the

thermally induced spectral shift in the transfer function. Therefore this chapter

begins with the fabrication and optical characterization of these fixed filters.

(a) (b)

Figure4.2 (a) Virtual prototype of the fixed PC filter and (b) SEM (top

view) of the fabricated fixed filter

Various fabrication issues related to the high aspect ratio dry and

wet etchings are discussed in this chapter. A novel method for the transfer of

lithographically generated patterns with better CD uniformity and sidewall

verticality on to the etch mask is also described in this chapter.

4.2 FABRICATION OF FIXED PC FILTERS

Fixed PC filters are fabricated in (110) planar silicon wafer by

anisotropic wet etching. The PC layers are defined by etching vertical slits in

to the silicon wafer to form alternating Si/air layers. The silicon wafer

specifications are given in the Table 4.1.

110

Table 4.1 Wafer specifications

Type Orientation Doping concentration Size Thickness Quality

p-type <110> 1013

cm-3

3” 300µm DSP

DSP – Double side polished

In addition to this grating structure, V-grooves are also fabricated to

facilitate on chip fiber alignment for wave propagation normal to the device.

The wet anisotropic etching is carried out using 44% wt. aqueous solution of

KOH. A thermally grown SiO2 layer is used as hard mask during KOH

etching. Several etching trials have been carried out with different

concentrations and temperatures to optimize this recipe, which gives maximum

etch rate with better anisotropy and etch selectivity between silicon and SiO2 etch

mask. The fabrication is a single layer process by which both the PC layers and the

fiber grooves are fabricated simultaneously and is detailed in Figure. 4.3.

Figure 4.3 Single-layer process flow optimized for the fabrication of

PBG structures with fiber grooves.

111

In this process direct laser lithography is used for the pattern

generation and transfer on to the photoresist (AZ5214E). Then this pattern is

transferred on to the SiO2 etch mask by wet etching using buffered

hydrofluoric acid (BHF). Table 4.2 gives the different etch parameters

optimized for the fabrication.

4.2.1 Fabrication issues

Several issues have to be addressed for the optimization of the etch

parameters like etch time, etch mask thickness and strip and gap widths in the

design file before going for the final fabrication. Some of the important fabrication

issues related to wet etching of silicon and SiO2 are described below.

Table 4.2 Different etch parameters optimized using dummy samples

Material Etchant Etch rate (µm/min.) Etch selectivity

(110) 1.6Si

(111)KOH

0.01696.3

SiO2 BHF 0.3 123

4.2.1.1 Wet etching of SiO2 in BHF

The SiO2 etching in BHF is isotropic i.e. the vertical and lateral

etch rate are same and hence it produce large undercut of the same dimension

as that of the etch depth as shown in the Figure 4.4

Figure4.4 Lateral etching of SiO2 in BHF produce large undercut

undesirable for MEMS devices

112

This broadening of the opening due to the lateral etching of SiO2

etch mask should be taken in to account while making the design file i.e. the

strip width in the design file should be adjusted to compensate for this

widening. We used fiber collimators of spot size 25µm to reduce the angular

spectrum entering in to the device. This necessitates PC layers with high

aspect ratio. Besides this fiber grooves must be deep enough to collimate the

light on to the device. The typical fiber diameter is 125µm and taking account

of the spot size of 25µm, the fiber grooves must have 80µm depth. Since this

is a single layer process and taking account of the SiO2 etch rate of 13nm/min.

in the 44% wt. aqueous solution of KOH (increasing temperature leads to

increased oxide etch rate); the oxide thickness should be more than 650nm to

etch 80µm depth in to the silicon substrate. The final dimensions arrived at for

the fabrication is listed in the Table.4.3.

4.2.1.2 Anisotropic etching of different crystal planes in silicon

Wet etching of single crystal silicon using KOH is anisotropic i.e.

different crystal planes etch at different etch rates as discussed in the previous

chapter. This property of (110) silicon is exploited in this work for the

fabrication of PC structures with smooth vertical sidewalls. KOH etching of

(110) silicon exposes three pairs of (111) planes out of which two pairs form

the vertical sidewalls of a parallelogram and the remaining one pair is

projected out 35.260 from the wafer flat as shown in the Figure 4.5.

Figure 4.5 Wet etching of (110) silicon in KOH exposes 3 pairs of (111)

planes

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The two slanted (111) planes grows as etch depth increases and it

stops etching further once the two planes make contact each other. Therefore

the etch depth is limited by these slanted planes and this depth depends on the

length of the opening in the etch mask through the equation (3.18). Therefore

PC layers must be long enough to give sufficient depth after etching, to

accommodate the large spot size of the incoming light. The final dimensions

of the device set in the design file are listed in the Table 4.4.

4.2.1.3 Mask alignment

In a (110) wafer (110) plane will etch faster than the (111) plane

leaving the former exposed as vertical sidewalls (anisotropy of more than

300). Therefore, in order to etch features with smooth vertical sidewalls, they

must be aligned parallel to the (111) plane during pattern transfer. Optical

flatness and sidewall verticality are the crucial factors which determine the

optical performance of the PC based optical channel drop filter. Therefore the

length of the PC layers must be aligned exactly parallel to the (111) plane

which forms the sidewall during lithography as shown in the Figure 4.6. The fiber

grooves are running perpendicular to the PC layers so that they are not aligned

with the (111) plane and hence is etched with irregular and rough sidewalls.

(a) (b)

Figure 4.6 (a) Parallelogram shaped samples with (111) sidewalls can

be cleaved in (110) wafer and (b) features aligned with the

(111) plane during pattern transfer

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The etch parameters are set based on the various optical

requirements such as high aspect ratio PC layers needed for accommodating

light beams of 25µm spot size and deep trenches for mounting optical fibers

of diameter 125µm. The design specifications and the required etch

parameters to achieve these specifications are listed in the Table 4.3.

Table 4.3 Device specifications and etch parameters

Device specification Etch parameters

Etch depth (µm)

PC layers Fiber groovesEtching time (s) SiO2 hard mask thickness (µm)

>30 80 50 1

The SiO2 etch mask has a thickness of 1µm and in BHF etching (which opens

the etch window for silicon bulk etching using KOH) the opening will

broaden by 1µm from both sides expanding the window size by 2µm. The

measured etch rate of silicon (111) plane and SiO2 is 16nm/min. and

13nm/min respectively in KOH solution. Therefore during the bulk etching,

the silicon layer (which forms the PC) will further thin down. This finally

requires a thicker width to be set in the mask for regions (gap widths in table

4.4) corresponding to silicon layers. Taking all these into account, the PC

layer dimensions are set to 10µm period with 50% filling fraction which still

gives PBG in the C and L bands. Therefore mask design needs to be

optimized as it has to (i) adjust the length of PC layers to ensure clear vertical

etch depth of more than 30 m (ii) compensate for the undercutting of SiO2

layer during isotropic BHF etching (iii) compensate for the lateral etching of

SiO2 in 44% wt. aqueous solution of KOH at 850C and (iii) compensate for

the lateral etching of Si PC layers in KOH. The mask parameters set for the

fabrication and is listed in the Table 4.4.

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Table 4.4 Design parameters

Strip width in the design file 3µm

Gap width in the design file 7µm

Length of the PC layers 200µm

Width of the fiber grooves 100µm

Length of the fiber grooves 5mm

The strips and gaps in the design file form the air and silicon layers

for the PC structure respectively. Strip width cannot be reduced further,

because the minimum resolution of the lithography equipment used for this

work is 3µm and hence the final silicon layer thickness will be even lesser

than the expected value of 5µm. The SEM of the fabricated fixed PC filters is

shown in the Figure 4.7.

Figure 4.7 SEM of the fabricated PC filters with fiber grooves

The cross-section image (left) exposes the slanted (111) plane. The depth of

the PC layers and fiber grooves obtained after the fabrication is listed in the

116

Table 4.5. The final PC layer and defect layer thicknesses obtained after all

processes are listed in the Table 4.6.

Table 4.5 Etch depth after fabrication

Etch depth (µm)

PC layers 57.7

Fiber grooves 80

4.3 OPTICAL TESTING OF THE FIXED PC FILTERS

Several fixed filters with varying central defect widths have been

fabricated and the optical results of the best four devices are discussed in this

section. Apart from the irregularities during lithography, the lateral etching of

both the SiO2 and Si layers in the KOH solution is dependent on the gap on

either side of each layer and is hence different for the Si layers defining the

defect region when compared to that for the central Si layer in each PC. This

difference, at the end of 50minutes, will increase with larger defect

dimensions and this was not taken care of in the mask design. Therefore, all

effects put together, the silicon is thinned down non-uniformly leaving the

system highly asymmetric. As discussed in the previous section, the system

can then be treated as asymmetric FP cavity in which the cavity is formed by

two asymmetric or geometrically chirped 1d PC structures say PC1 to the left

of the cavity and PC2 to the right of the FP cavity as shown in the Figure 4.8.

In some of the samples the thickness is found varying along the length of the

silicon layer.

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Figure4.8 SEM of fabricated fixed filter shows the PC layer thickness

variation across the PC layers and along the length of the Si

layers

This is attributed to the slight misalignment between the critical

features (PC layers) and the silicon (111) plane during lithography. The

thickness of the PC layers and the percentage variation of lattice constant

from 10µm and filling fraction between the two constituting PC halves of the

four tested filters are listed in the Table 4.6

Table 4.6 Structural details of the samples

Sample PCSi

m)

Air

m)

Si

m)

Air

m)

Si

m)

% Change in

period of bilayer

from 10 m

% Change in

filling

fraction

Defect

( m)

PC1 3.92 5.32 3.84 6.31 3.51 2-8 3F1

PC2 3.65 6.77 3.34 6.43 3.57 3-5 17.3

PC1 3.93 6.66 3.48 6.35 3.66 2-6 2F2

PC2 3.72 6.57 3.52 6.46 3.21 1-3 37.6

PC1 3.34 5.99 4.29 6.12 3.97 4-6 5F3

PC2 3.23 6.35 3.57 6.56 3.25 1-4 28

PC1 3.02 6.49 4.07 6.63 3.25 4-7 10F4

PC2 2.93 6.17 3.38 5.98 2.65 6-9 412.34

118

H and L in the Figure 4.8 represent the higher (silicon) and lower

(air) refractive index materials respectively.

The optical testing is carried out using a tunable laser of

wavelength range 1506nm-1616nm (Thorlabs INTUN ™TL 1550-B SM)

and an optical spectrum analyzer (OSA) of resolution 0.02nm

(YOKOGAWA AQ6370). The schematic of the optical characterization set

up is shown in the Figure 4.9.

Figure 4.9 Schematic of the optical characterization set up

The SEM images of the four tested filters are depicted in the Figure 4.10.

Figure 4.10 SEM images of the PC filters F1, F2, F3 and F4

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Both anti reflection (AR) coated micro GRIN lensed and

hemispherical ball lensed fiber collimators with spot size of 25µm are used

for the light coupling in to and out of the devices. The AR coated micro GRIN

lensed fiber collimators provide low insertion loss compared to the uncoated

hemispherical fiber collimators. The microscopic images of the samples with

both AR coated micro GRIN lensed and hemispherical ball lensed fiber

collimators taken during the process of insertion and alignment of fiber

collimators close to PC layers are shown in the Figure 4.11.

(a) (b)

Figure4.11 Microscopic images (100X) of the samples with (a) GRIN

and (b) hemispherical ball lensed fiber collimators

The experimentally measured and theoretically estimated

normalized transmissions of the four PC filters are shown in the Figures

4.12(a) and (b) respectively.

120

(a) (b)

Figure4.12 The normalized transfer functions of the PC filters (a)

experimental and (b) theoretical

A comparative plot between the experimental and theoretical

transfer functions of the filters are shown in the Figure 4.13.

Figure4.13 Comparative plot between the theoretical (line only) and

measured (line with symbols) transfer function of the filters

The experimentally measured and theoretically estimated

specifications of the four filters are listed in the Table 4.7.

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Table 4.7 Optical properties of the fixed filters

Peak of the dropped wavelength

channels (nm)FWHM (nm)Name

FP

cavity

width

m)Experimental Theoretical

Experimental Theoretical

F1 7.3 10.8 4.78 1540 1540

F2 7.6 8.2 5.58 1603 1603

F3 8 6 7.11 1520 1520

F4 12.34 8 4.11 1592 1592

The experimentally measured and theoretically calculated dropped

wavelength peaks are same but the experimentally measured pass band width

at FWHM of each filter differs from that estimated theoretically. This is

attributed to the thickness variation along the length of the silicon layers in

the PC structures which is not incorporated in to the mathematical model of

the filter. This variation also accounts for the weaker transmission peaks in

the experimental transfer curve. This is illustrated in the Figure 4.14, by

comparing the experimental transmission of the filters F3 and F4 with the

theoretical transmission after incorporating the thickness variation.

(a) (b)

Figure 4.14 Normalized transmittance of the samples simulated using

the smaller thickness values shows the emergence of

transmission bands in the (a) 1580-1600nm region for

sample F3 and (b) 1530-1560nm region for sample F4

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Thickness variation of each layer along its length is evident from

the SEM pictures of Figure 4.10 and this variation is also not the same for

symmetrically placed layers on either side of the defect. The optical beam

whose spot size covers most of this filter structure will hence see different

layer thicknesses at different points along the length and therefore the weak

transmission peaks are indicative of smaller fraction of the light encountering

a different thickness giving rise to a different passband. The emergence of

such undesired transmission peaks due to thickness variation has been

confirmed by doing the theoretical simulations with slightly varying

thicknesses. The thickness values used in theoretical simulation for the curves

depicted in Figure 4.14 is an average value of thickness of each layer at

different points along the length of the layer obtained from the SEM analysis

of the samples. Therefore this mathematical model based on the asymmetric

FP cavity approach can effectively predict the resonant mode and the free

spectral range (FSR) of the FP cavity formed by two PC structures.

The experimental and theoretical studies illustrate two different

properties of the PC arising from the asymmetric geometrical variations. First

one is the large frequency range with minimum transmission except for the

defect modes which appear inside this stop band and the second one is the

intensity variation of the defect modes at different frequencies inside the stop

band. Both these aspects have strong impact on the performance if the

structure is configured as an optical component such as a channel drop filter.

For the sake of abstraction we explain these properties using the samples F3

and F4.

4.3.1 Large frequency range with minimum transmission

The transmittance of PC1 (line with squares), PC2 (line with

circles) and the combined system (PC1 + PC2 + defect, line with star) for the

samples F3 and F4 are shown in Figures 4.15(a) and (b) respectively.

123

(a) (b)

Figure4.15 Spectral response of the two halves PC1 (line with squares)

and PC2 (line with circles) and the total response (line with

solid star) of the combined system for (a) F3 and (b) F4

Since the two halves of the combined system have different lattice

properties (refer to the Table 4.6), they posses distinct spectral characteristics

and in both cases the pass band of PC1 lies inside the stop band of PC2 and

vice versa resulting in a large spectral range with minimum transmission,

except for the defect modes which appear inside this stop band.

Table 4.8 gives the stop and pass bands of PC1, PC2 and the

combined system of PC1, PC2 and the central defect layer for both samples.

In both cases, the transmission peaks of PC1 lie in the stop band of PC2 and

vice versa, effectively lowering the transmission coefficient at the band edges

of the combined system which in turn provides large frequency range of 90

nm with low transmission. In the combined system, only the defect mode has

maximum transmittance approaching 0.9 for those near the band edges (line

with solid star in both Figures 4.15(a) and (b)) and all band edge

transmissions are suppressed. Therefore large FSR can be realized by using

geometrically chirped PCs. Realization of the same FSR using ideal PC’s

would require lower thickness of Si layers i.e. lower filling factors and will

necessitate the use of advanced lithography techniques during the fabrication.

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Table 4.8 Stop band, band edges and defect modes of the structures

Sample Main Stop bands Band edges Defect mode

PC11514nm-1534nm,

1540nm-1600nm1,2,5 (Fig 4.15(a))

PC21506nm-1560nm

1570nm-1590nm3,4 (Fig 4.15(a))

F3

PC1+PC21506nm-1535nm

1540nm-1595nm

1535nm-1540nm

1595nm-1605nm

(Weak transmission <0.4)

1520nm

PC11510nm-1585nm

1588nm-1616nm1,4 (Fig 4.15(b))

PC21520nm-1547nm

1552nm-1602nm

2,3(Fig 4.15(b))

1602nm-1616nm(weak transmission<0.4)F4

PC1+PC2 1517nm-1616 nm1510nm-1517nm

(weak transmission<0.2)1592nm

4.3.2 Defect mode intensity variation

In an ideal PC, with central defect layer and geometrically

symmetric structures on either side of the defect layer, the normalized

transmission intensity of the defect mode is unity and is independent of the

defect mode frequency. In the case of samples F3 and F4, which are

geometrically asymmetric with respect to the central defect layer, the intensity

of the defect mode varies with defect mode frequency (refer to Figures 4.16(a)

and (b)). The graphs shown in Figures 4.16(a) and (b) depicting the

theoretical transfer functions of the structures similar to F3 and F4 except for

a change in the width of the central defect layer, confirm that the observed

pass band at 1520 nm for F3 and 1592 nm for F4 are indeed defect modes of

the structures. It is found that the peak of the defect mode shifts towards the

higher wavelength (red shift) as the defect size increases which is as expected

in any Fabry–Perot resonator.

125

(a) (b)

Figure4.16 Frequency shift of the defect modes and the transmission

intensity variation inside the stop band of the samples (a) F3

and (b) F4 when the width of the defect layer varies.

It can be seen that the defect mode intensity varies across the band

gap and drops to minimum at the center of the band edges and it appears as a

notch in the transmission spectra (marked by circles in the two Figures 4.16

(a) and (b)).

Figure 4.17 The normalized transmission intensity of defect mode is

maximum at frequencies (marked by black squares) for

which the two PC halves have equal transmittance

(sample F3)

126

Figure 4.18 The normalized transmission intensity of defect mode is

maximum at frequencies (marked by black squares) for

which the two PC halves have equal transmittance

(sample F4)

In the Figures 4.17 and 4.18, shaded region represents the map of

the normalized defect mode transmissions of the sample F3 and F4

respectively for various defect layer widths. The curves, black solid line and

black dotted line, represent the normalized transmittance of the two halves

PC1 and PC2 of each sample respectively. It is clear that the normalized

transmission intensity of defect modes is maximum at frequencies (marked by

black squares) for which the two PC halves of each sample F3 and F4 have

equal transmittance and minimum at frequencies for which the two PC halves

have large variation in the transmittance and it appears as notch (marked by

black circles in Figures. 4.16–18) in the defect mode transmission spectrum.

Therefore this study reveals that the geometrical asymmetry in the

structure with respect to the central defect layer leads to (i) wider stop band

ranges with suppressed band edges but also (ii) causes large variation in the

transmission intensities of the defect modes. Therefore tight control of

symmetry has to be ensured in the fabrication process of such PC structures

127

for application as low loss channel drop filters in a WDM network. The

intensity variation across the stop band can be predicted by a knowledge of

the transfer profiles of the individual PC’s. Even if a small asymmetry can be

induced in one half of the structure, then the same filter can be reconfigured

to function as a channel drop filter or as a drop and continue filter suitable for

broadcast modes.

4.3.3 Insertion loss

The insertion loss of the device is the sum of loss due to the

geometrical chirping described in the previous section, angular spread of the

light entering in to the device, scattering and Fresnel reflection losses at the

fiber interfaces and coupling loss due to the fiber misalignment. The insertion

loss of all the four filters have been measured as a function of axial distance

and the filter F4 shows lesser insertion loss (drop wavelength at 1592nm) and

is described here. The input fiber is fixed at 150 m (working distance of the

fiber collimator) away from the PC layers and the position of the output fiber

is varied by an increment of 30 m using a nano positioner. Then the insertion

loss was measured in log scale by taking the difference between the input and

output power. The fiber – to – fiber insertion loss of the filter includes the

insertion loss of the collimating fibers also. The insertion loss of the filter as a

function of axial distance is shown in the Figure 4.19 and it can be seen that

the filter has minimum insertion loss of 2.2dB for an axial separation of 60-

120µm and the loss increases as the axial separation increases. This is because

of the weak coupling in to the output fiber due to the divergence of the light

beam.

128

Figure 4.19 Insertion loss of the filter F4 as a function of axial distance

of the output fiber from the PC layers

4.3.4 Spectral shift due to temperature variation

The spectral shift of the pass band of one of the filters has been

measured experimentally by changing the ambient temperature around the

device. This is illustrated in the Figure 4.20. The temperature is varied from

250C to 106

0C.

Figure 4.20 Spectral shift of the passband due to temperature variation

129

The set up used for this experiment is shown in the Figure 4.21.

The pass band shift is measured to be 0.04nm/K. This is attributed to the

combined thermo-optic effect and thermal expansion of silicon layers

described in the second chapter.

Figure 4.21 Test set up used for the thermal experiment, inset is the

microscopic image of the device under test (DUT)

The experimentally observed device specifications are listed in the

Table. 4.9.

Table 4.9 Experimental results

Insertion loss 2.2dB

Stopband width >90nm

Passband width <10nm

Thermal drift 0.04nm/K

130

4.4 FABRICATION OF THE TOF

The 1D Si/air based TOF is realized by micromachining the device

layer of an SOI wafer and the detailed process flow is described in the Figure

4.22. The SOI wafer specification is given in the Table 4.10.

Table 4.10 SOI wafer specifications

Name Type Orientation Resistivity ( cm) Size Thickness (µm) Quality

Device layer n-type 100 3-10 4” 30 Polished

Box layer ---------- ------- 1018 4” 3 ---------

Handle wafer n-type 100 3-10 4” 350 Polished

The micro-fabrication comprised of the following steps

Pattern generation and transfer on to a photosensitive material

(lithography)

Pattern transfer on to an etch mask (RIE)

Device layer etching (DRIE)

Dicing

Wet releasing (CPD)

Defining the ohmic contact using shadow mask technique

131

Figure 4.22 Fabrication process flow which uses e-beam lithography for

pattern generation and DRIE for device layer etching

4.4.1 Fabrication issues

To achieve the anticipated device performance as mentioned in the

chapters 2 and 3, the TOF has to be fabricated with the following design

specifications:

Silicon layer thickness : 1.4µm

Air layer thickness : 2µm

Initial cavity width : 4µm

CD non-uniformity : <40nm

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Roughness (scalloping) : <20nm

Sidewall angle : <0.10

Actuation method : Gap tuning by electrostatic

comb drives

In order to achieve these device specifications, the fabrication is

carried out using e-beam lithography for pattern generation, RIE for pattern

transfer on to the etch hard mask and DRIE for device layer etching. Several

trials have been carried out to optimize the DRIE parameters to meet the

fabrication requirements. The general issues related to DRIE have already

been discussed in the previous chapter. As far as the device is concerned, the

size of the etch window varies between 1µm and 4µm across the sample and

it introduces ARDE. Besides we used small samples of size (2cm×2cm) fixed

on a 4” support wafer using cool grease for DRIE. This leads to the oxide

hard mask erosion due to overheating of the samples during DRIE. By taking

all these issues in to account the following DRIE recipe is optimized for the

fabrication (Table 4.11). We used STS advanced silicon etcher for this

purpose.

Table 4.11 DRIE recipe

SF6 200Gas flow rate (sccm)

C4F8 80

Etching time (second) 1

Passivation time (second) 1

The etch parameters obtained by etching a dummy sample using

this recipe is listed in the Table 4.12.

133

Table 4.12 DRIE parameters

Name Etch rate (µm/minute)

4µm window size 2Silicon

1µm window size 1.6

SiO2 0.037

Due to ARDE, 4µm opening etches faster than 1µm opening. Since

the 1µm opening defines the air layer in the PC structures, it doesn’t require

etching down to the box layer but the 4µm window is designed for releasing

the structure therefore it requires etching down to the box layer. The oxide

etch rate is 37nm/minute due to the overheating of samples during DRIE and

this fixes the hard mask thickness required for 30µm device layer etching.

The Table 4.13 gives the optimized etch parameters.

Table 4.13 Optimized etch parameters

Total process time 16 minutes

SiO2 hard mask thickness 900nm

The other issues are the cleaning and roughness treatment of the

samples after DRIE and finally the stiction during wet releasing.

4.4.1.1 RIE patterning

The positive photoresist PMMA950K is used for the e-beam

lithography. The thickness of the resist layer is 100nm. The etch window is

opened on the SiO2 hard mask using RIE and the 100nm thick PMMA cannot

be an etch mask during this process. Therefore a thin layer of sputtered

Chromium has been used as an etch mask during this process. The etch

window can be opened on Cr layer using Cr etchant. This leads to broadening

of the etch window due to the isotropic etching of Cr in chrome etchant. The

134

RIE recipe for SiO2 etching is given in the Table 4.14, which is optimized as

per the design requirements using dummy samples. An ANNELVA made

DEM451 RIE system has been used for this purpose.

Table 4.14 RIE recipe

CF4 50Gas flow rate (sccm)

O2 5

RF power (W) 150

Champer pressure (mTorr) 40

The etch parameters obtained by etching a dummy sample using

this recipe is shown in the Table 4.15.

Table 4.15 RIE parameters

SiO2 etch rate (nm/minute) 45

Cr etch rate (nm/minute) 2.8

Therefore the thickness of the Cr layer and time required to etch

900nm of SiO2 is fixed based on these RIE parameters and are listed in the

Table 4.16.

Table 4.16 Optimized etch parameters

Total process time (minutes) 20

Thickness of the Cr layer (nm) 100

The wet etching of Cr layer using chrome etchant leads to the

broadening of the etch window from both sides by the equal amount of the

thickness of the Cr layer. The window further broadens during the RIE of the

SiO2 hard mask. Therefore in the design file the strip width is set to 800nm,

135

which corresponds to the air layer in the PC mirror and the gap width is set to

2.6µm which corresponds to the silicon layer in the PC mirror. This reverse

engineering is expected to yield the desired silicon and air layer thicknesses.

The other advantage of using thin layer of Cr as etch mask in RIE for pattern

transfer is the improved sidewall verticality and CD uniformity across the wafer.

The shape of the etched sidewall depends on the profile shape of

the etch window also. A slanted etch window (as in the case of thick

photoresist) converge the plasma at the center of the etch window making the

ion flux density non-uniform across the opening, while in the case of vertical

etch window the ion flux density is uniform across the etch window and the

etching will be more directional than the former case and hence the etched

sidewall also will be vertical as shown in the Figure 4.23.

Figure 4.23 The influence of the shape of the etch window on the flow

dynamics of the plasma. (a) a qualitative plot showing the

concentration of flux density at the center of the opening, (b)

schematic representation of the flux density across an

opening and (c) crossectional view shows the trend (dotted

line) of the etching process

136

The Figures 4.24 (a) and (b) compare the samples with 1µm thick

SiO2 layer etched using photoresist and Cr as etch mask in RIE respectively.

(a) (b)

Figure 4.24 Crossectional view of the samples etched using (a)

photoresist and (b) Chromium as etch mask in RIE

From the figures it can be seen that the sample etched using thin

layer of Cr etch mask has vertical sidewall compared to that etched using

photoresist etch mask. This is attributed to the profile shape of the etch

window as described in the Figure 4.23.

4.4.1.2 Cleaning and roughness treatment after DRIE

In DRIE a passivation layer is applied on the etched surfaces to

protect it from being etched in the next cycle of isotropic etching. In our

process C4F8 is used for this purpose, it leaves a polymer coating on the

sidewall after the DRIE process. This can be removed by a short oxygen

plasma descom at high platen power using a conventional RIE system.

Another issue of the DRIE etched surface is the roughness due to the inherent

sidewall scalloping arising from the cyclic nature of BOSCH process. This

can be reduced by decreasing the etching and passivation time during the

etching process without the need for added steps. To further smoothen the

surface, post DRIE treatments are required at the cost of added steps and

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complexity. Usually oxidation and subsequent removal of the oxide is

suggested for the roughness treatment of the DRIE etched samples.

In this work, the same Annelva made DEM451 system was used for

the oxygen plasma descom at 350W for one hour. The roughness is removed

by growing a 100nm thick oxide layer using dry oxidation and subsequently

removing it by wet etching using BHF solution. This is done along with the

releasing process.

4.4.1.3 Stiction during wet releasing

In micromachining using wet processes, closely spaced delicate

membranes or released structures will tend to deflect and stick together due to

the surface tension induced by the trapped liquid droplets attached to the

surfaces during the final rinsing and drying operations.

Figure 4.25 SEM shows some of the critical parts of the released

structure (clockwise from top, folded spring connected to

the anchors and the comb drive, interdigitated comb fingers

and PC layers with comb actuators)

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This phenomenon is known as stiction. This is prominent in MEMS

devices having released structures with large area. This can be minimized by

using dry releasing or critical point drying. In our fabrication process the wet

releasing of the movable PC mirrors is performed using CPD with liquid CO2

at 310C and 1100psi. The SEM of the released TOF is shown in the Figure

4.25and the SEM of the smooth vertical PC layers is shown in the Figure 4.26.

Figure4.26 SEM of the PC layers (tilted top view) shows smooth

sidewalls after dry oxidation followed by BHF dip

The critical device dimensions finally achieved in the fabricated

device is reported in Table 4.17.

Table 4.17 Structural dimensions of the PC layers and FP cavity

Design ( m) Fabricated device ( m)Name

Si Air Cavity Si Air Si Air Si Cavity

PC1 1.4 2 1.65 1.65 1.69 1.87 1.78

PC2 1.4 24

1.74 1.87 1.69 1.65 1.693.9

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In our device the electrostatic comb drive actuators have a total of

50 comb fingers (25 in each pair) and the initial overlap go between them is

m with an inter finger gap x of 1 m in the overlapping region. The

structural dimension of the actuator part after fabrication is given in the Table

4.18.

Table 4.18 Structural dimensions of the actuator parts

Name PC layers Comb fingers Support springs

Length ( m) 60 23 250

Breadth ( m) 9 1.8 1.8

Thickness ( m) 30 30 30

Gap ( m) 3.9 1 4

The final step of the fabrication is the dicing of the sample,

patterning of metal pads for ohmic contact and simple packaging. The metal

pads of 500µm diameter are defined by shadow mask and RF sputtering. A

Ti/Au layer of thickness 100nm/300nm is sputtered using an Annelva made

RF sputtering system.

Figure 4.27 Photograph of the MEMS chip containing eight TOF’s

Finally the chip of dimension 16mm×8mm containing 7 devices separated

from each other is mounted on a 16 head electronic package and wire bonding

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between electronic heads and metal pads are done for electric contact. The

photograph of the MEMS chip is shown in the Figure 4.27.

4.5 ELECTROMECHANICAL TESTING OF THE TOF

Since this is a capacitive system, the displacement of the PC

mirrors has been extracted from the CV analysis, which was carried out by

sweeping the voltage from 1V to 4V with an increment of 1V and the graph

shown in the Figure 4.28(a) depicts the capacitance of the system at different

voltages. Hence the amount by which the FP cavity width shrunk was

calculated using the equation C = o rA/d. This gives the total displacement

caused by the movement of the two PC mirrors. Here A = 370 m× 30 m is the

effective area of interaction between the two moving plates and d = 3.9 m is

the initial cavity width. The graph shown in the Figure 4.28(b) depicts the

experimentally measured displacement (black line with squares) and that

estimated using the analytical expression for the total actuator force

mentioned in the previous chapter (red line with circles).

(a) (b)

Figure 4.28 (a) Capacitance of the system at different voltages

(CV characteristics) and (b) experimentally measured

(black line with squares) and theoretically estimated (red

line with circles) displacement versus voltage curve

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The measured and theoretically estimated displacement and cavity

widths are listed in the Table 4.19. The lateral stiffness (k) of the folded

support springs was extracted from the finite element model of the structure

simulated using commercial MEMS CAD tool and it is found to be 0.42N/m.

The geometry of the fabricated structure obtained from SEM analysis (refer to

Tables 4.17 and 4.18) was used for this modeling.

Table 4.19 Results of mechanical analysis

Displacement (nm) Cavity width ( m)Voltage (V) C (fF)

Expt. Theoretical Expt. Theoretical

1 25.66 35.99 31.96 3.864 3.868

2 26.42 146.41 127.88 3.754 3.772

3 27.57 303.04 287.96 3.597 3.61

4 29.38 525.33 509.61 3.375 3.39

The microscopic images of the device taken during actuation are

shown in the Figures 4.29, 4.30 and 4.31.

(a) (b)

Figure 4.29 The PC based FP cavity of the TOF (a) with load and (b)

without load

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(a) (b)

Figure 4.30 Microscopic images of the comb drives (a) with and (b)

without load

(a) (b)

Figure 4.31 Microscopic images of the support beams during actuation

(a) support beams attached to the anchors and (b) support

beams attached to the comb drives

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4.6 OPTICAL TESTING OF THE TOF

The schematic and the photograph of the optical test set up is

shown in the Figure 4.32(a) and (b) respectively.

(a) (b)

Figure 4.32 The optical test set (a) schematic showing tunable laser

source and OSA detector and (b) photograph showing

lensed fiber collimators for light coupling

A tunable laser (INTUN TL1550B-SM, THORLABS) is used as

the optical source and an optical spectrum analyzer (AQ6370, YOKOGAWA)

is used as detector. Two micro ball lensed fibers (SMF28E, LASEOPTICS)

are used for the light coupling to and from the device. An external electrical

power supply is used to electrostatically tune the device. There are seven

devices in the chip and unfortunately only two of them are useful for optical

testing since the electronic heads block the insertion of fibers for the rest of

the devices and best one of the results is presented here.

The device was tested for a wavelength range of 110nm starting

from 1506nm to 1616nm at different bias voltages with a wavelength step of

1nm. The voltage was swept from 0V to 3.4V with an increment of 0.1V. Five

wavelength channels were continuously tuned for a voltage range of 3 – 3.4V

with an increment of 100mV and are shown in the Figure 4.33. It was

observed that, the wavelength shifts from higher value to lower value (blue

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shift), due to the decrease in the resonant cavity width as the two PC mirrors

move towards each other under actuation.

(a) (b)

Figure 4.33 (a) Experimental measurement shows five wavelength

channels dropped for voltage increments in steps of 0.1V in

the 3-3.4 V range and (b) Comparison between the

simulated (line alone) and experimentally measured (line

with symbols) transfer functions at various voltages

The numerical model of the filter is constructed using the

asymmetric FP cavity approach and the information given in the Tables 4.17

and 4.18 and using the analytic expressions arrived at for the total actuator

force described in the third chapter in the general framework of TMM. Figure

4.33(b) compares the experimentally measured transfer function (line with

symbols) to that estimated theoretically (line alone) at various applied

voltages. In this figure the theoretical curve incorporates the coupling loss

which is extracted from the experimental transfer function. Since the DRIE

process yields angled surfaces, the silicon layers will not be parallel to each

other inside the PC structures. Therefore an etch angle of 0.060 was

introduced in to the theoretical simulation to match with the experimental

peaks. The other discrepancies between theory and experiment are mainly due

to the fabrication process induced structural asymmetries like variation of

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thickness along the length of the same Si layer, which has not been

incorporated in to the theory. It can also be seen from the figures 4.33(a) and

(b) that the intensity of the dropped channels varies across the band gap. This

is attributed to the geometrical chirping of the PC layers and has already been

discussed in the case of fixed PC filters.

A comparison between the experimental and theoretical

transmittance is given in the Table 4.20. The pass band width for each

dropped channel is less than 8nm.

Table 4.20 Results of optical analysis

Peak wavelength (nm) Pass band width (nm)Voltage (V)

Experiment Simulation Experiment Simulation

3 1609 1609 7.64 8.93

3.1 1600 1600 6.38 4.74

3.2 1591 1591 4.47 3.57

3.3 1582 1581 5.76 3.1

3.4 1574 1572 4.83 2.57

In our design the light is first coupled to the input waveguide of

length 4mm using lensed fiber and then coupled to the device. From the

device the light is coupled to the output waveguide of same length and then to

the lensed fiber. This introduces large coupling loss at the waveguide edges.

This along with the scattering and Fresnel reflection losses in the waveguide

contributed to the high loss (>20dB) reported in the Figure 4.33(b). Therefore

the filter will have to be necessarily fabricated using a two step process with

in-plane fiber grooves to facilitate free space coupling of light in to and out of

the device using AR coated micro GRIN lensed fibers.

It can be seen from the Table 4.16 that the two PC halves on either

side of the resonant cavity are asymmetric, i.e. PC1 has an average filling

fraction of 0.483 and that of PC2 is 0.501, i.e. a filling fraction change of

1.8% between the two PC mirrors which is lesser than that obtained using

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anisotropic wet etching. This geometrical asymmetry within the PC structures

(and hence with respect to the defect layer) will induce coupling variation

between the eigen modes of the defect layer (or the resonant modes of the FP

cavity) and those at the band edges of the constituent PC’s. As observed

theoretically and also for the fixed filter , the asymmetry in the structure with

respect to the defect layer, leads to wider stop band ranges with suppressed

band edges. It is also found that the defect mode intensity varies across the

band gap and will have maximum values at frequencies for which the two

constituting PC halves on either side of the defect layer have equal

transmittance. The defect mode intensity drops to the minimum value at the

center of the suppressed band edges where the difference in transmittance of

the two PC halves is maximum, and this appears as a notch in the defect mode

transmission spectrum. This is detailed in the Figure 4.34.

Figure4.34 Simulation results show that the defect modes have

maximum transmission if the difference in transmittance of

PC1 and PC2 is minimum and vice versa (green shaded

region is the map of the defect modes, red line is the

transmittance of PC1 and blue line is that of PC2, magenta

line represents the difference in transmittance of PC1 and

PC2 and black lines are defect modes at various voltages)

147

To conclude, when compared with the similar designs reported

earlier by Lipson et al (2007), Saadany et al (2006) and Pruessner et al (2008),

we have designed and fabricated PC filters with small filling factor of 3.4µm

which ensures large PBG and hence large tuning range. We have

demonstrated more than 70nm tuning range in the C and L band with a high

wavelength tuning sensitivity of 9nm/100mV. The pass band width of our

filter is greater than that reported in the above mentioned references due to the

geometrical chirping of the PC layers. The important device parameters

achieved with this TOF is listed in the Table 4.21.

Table 4.21 Device specifications

Tuning range 70nm (1550nm-1620nm)

Tuning efficiency 0.09nm/mV

Actuation voltage <4V

Reconfiguration time (theoretical prediction) <100µs

Passband width <8nm

Insertion loss >20dB

The Figure 4.35 depicts the simulated transfer function of the TOF

for the entire CWDM grid using the geometry extracted from the SEM

analysis (Table 4.17 and 4.18) for an applied voltage range of 3V – 3.4V with

an increment of 0.1V. The shaded region represents the experimentally tested

region of the spectrum.

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Figure 4.35 Simulated results shows the transmission of the filter for the

complete CWDM range

The simulation using the SEM geometry of the TOF shows the

tunable pass bands in the voltage range of 0-3V (Figure 4.36(a)) and 3.5V-7V

(Figure 4.36(b)).

(a) (b)

Figure 4.36 Simulation results of the TOF for (a) 0V-3V and (b) 3.5V-7V

voltage ranges with 0.1 voltage increment

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4.7 CONCLUSION

The fabrication and characterization of fixed and tunable PC optical

channel drop filters have been discussed in this chapter. Wet anisotropic etching

of (110) silicon was used for the fabrication of fixed filters with fiber grooves. It

guarantees smooth vertical sidewalls crucial for the low loss performance of the

filter. It can be achieved by manipulating a few etch parameters such as etching

temperature and etchant concentration. While in the case of anisotropic DRIE

using BOSCH process used for the fabrication of TOF, requires the manipulation

of a lot of process parameters to achieve the design specifications and even post

DRIE treatments are required to achieve optical flatness. The minimum insertion

loss achieved with the wet anisotropic etching is 2.2dB. The insertion loss of

more than 20dB reported for the TOF is mainly due to the signal loss in the

waveguide due to Fresnel reflection and scattering due to structural

imperfections. The TOF has wide tuning range of 70nm in the L bands of the

CWDM grid. The tuning sensitivity is 0.09nm/mV which brings down the

requirement of actuation voltage. The pass band width is 8nm, which will be

required in the CWDM networks operated by uncooled semiconductor lasers.

Therefore the wide pass band width can accommodate the peak wavelength shift

of the laser due to temperature variation and ageing and also the thermally

induced spectral shift of the filter itself. The design modification required for the

improvement of the optical as well as electromechanical performance will be

discussed in the next chapter.