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Overview of MEMS by Dr. Walsh
Citation preview
Kevin M. Walsh, PhD
ECE543
Overview of Microtechnology
and MEMS
1
• BACKGROUND -MEMS definition,
introduction, history, market survey, and
references
• MATERIAL ISSUES
• uFAB and uMACHINING PROCESSES
• APPLICATIONS and EXAMPLES
• CLEANROOM CONSIDERATIONS
• FUTURE DIRECTIONS
Outline
2
Micro-Electro-Mechanical Systems (MEMS) is the
integration of mechanical elements, sensors, actuators, and
electronics on a common substrate through the utilization of
microfabrication technology or “microtechnology”.
So what exactly is MEMS?
3
“Micromachining” formally refers to the bulk
anisotropic etching of crystalline silicon using
traditional batch fabrication techniques.
“Micromachining” informally refers to the complete
combination of processing technologies used to
fabricate miniature MEMS-based devices and systems,
such as LOC systems.
…and how about micromachining?
source: Sandia NL
Source: UofL Source: UofL 4
MEMS and Micro-
machining Examples
pressure sensors
accelerometers
flow sensors
inkjet printers
deformable mirror devices
gas sensors
micromotors
microgears
lab-on-a-chip systems (LOC)
5
MEMS - the next evolutionary step on the microtechnology
ladder.
Microtechnology - refers to the “miniaturization” technology
that was originally developed for the fabrication of electronic
integrated circuits (ICs)
MEMS - resulted when microtechnology was applied to the
production of devices, structures, and systems that were
more than just electronic in functionality (1980)
Source: UW Source: UofL
6
USA - MEMS (micro-electro-mechanical systems)
[name officially adopted in 1989 by a group of 80 researchers at Salt
Lake City for the Micro-Tele-Operated Robotics Workshop]
Europe - MST (micro-systems technology)
Japan - Micromachines
A ROSE is a rose !!
7
Why use micromachining?
• miniaturization
• cost/performance advantages (due to batch fabrication)
• integration with electronics
• faster devices (speed usually scales with size)
• lower power consumption
• improved accuracy, reliability, and reproducibility
• new effects and products due to miniaturization/microfab
8
MEMS/Micromachining Texts
Micromechanics and MEMS: Classic and Seminal Papers to 1990 by W.
Trimmer (editor)
Micromachined Transducers Sourcebook by G. Kovacs
Fundamentals of Microfabrication by Marc J. Madou
Microsensors by Richard S. Muller, Roger T. Howe, Stephen D.
Senturia, R. Smith (editors)
An Introduction to MEMS Engineering by Nadim Maluf
Silicon Micromachining by Elwenspoek and Jansen
MEMS WWW Bookstore: http://mems.isi.edu/bookstore/
Handbook of Microlithography, Micromachining and
Microfabrication (Vol. 2) by P. Rai-Choudhury (editor)
9
MEMS/Micromachining Journals
Sensors and Actuators by Elsevier Science, Netherlands
Journal of MEMS by ASME and IEEE, USA
Sensors Magazine by Helmers Publishing, USA
Journal of Micromechanics and Microengineering by Institute of
Physics, United Kingdom
Micromachine Devices, a companion to R&D Magazine, by Cahners
Business Information, USA
IEEE Sensors Journal by IEEE, USA (new)
10
Dedicated MEMS/Micromachining Issues
Forbes ASAP, April 2, 2001
MRS Bulletin, Vol. 26, No. 4, April 2001
Proceedings of the IEEE, August 1998
Microengineering and MEMS by Dr. Daniel Banks
http://www.dbanks.demon.co.uk/ueng/
UofL MEMS Resources
http://mitghmr.spd.louisville.edu/mems_links.html
MEMS Clearinghouse – http://mems.isi.edu/
MEMS Exchange - http://www.mems-exchange.org/
Various LOC Web Sites
Dedicated MEMS/Micromachining Web Sites
11
Traditional Microelectronic Fabrication Texts
Semiconductor Devices: Physics and Technology by S. M. Sze
Semiconductor Integrated Circuit Processing Technology by W. R.
Runyan and K. E. Bean
Introduction to Microelectronic Fabrication by Richard C. Jaeger
Silicon Processing for the VLSI Era: Volume 1 – Process Technology
by S. Wolf and R. N. Tauber
The Science and Engineering of Microelectronic Fabrication by S.
A. Campbell (includes a chapter on MEMS)
Modern Semiconductor Fabrication Technology by P. Gise and R.
Blanchard
Microchip Fabrication by Peter Zant
12
MEMS…
…evolved from the Microelectronics Revolution
IC Industry Timeline
1999
10 million transistors
1947
single transistor
1958
first IC
History
13
MEMS Timeline 1980
2030
1999
(1.3 million micro-mirrors) TI DMD ?
Bulk micromachined
pressure sensor
14
The Opportunity for MEMS Technology
15
MEMS compared to ICs
Source: Madou
2001 $14B (5% of IC market) $300B
2009 $100B Source: Forbes
- 2000 BMW 740i has over 70 MEMS sensors - 16
MEMS Sectors and Forecast
Source:
Maluf
17
MEMS Technology: Materials Issues
Source: Madou 18
MEMS Technology: Materials Issues
Source: Madou 19
The MicroTechnology/MEMS Tool Set …
cleanroom plus microfab processes
+
So, you’re interested in MEMS; what do you need?
20
Standard IC Processes
Source:
CWRU
Source: Jaeger
1
2 3
21
Standard IC Processes
• Sputtering
• Evaporation
• Thermal Oxidation
• CVD (chemical vapor deposition)
• Spinning
• Epitaxy
(1) Deposit/Grow Thin Films oxidation
spinning sputtering sputtering 22
Standard IC Processes
• Photolithography
• Etching Techniques (wet, dry/RIE)
(2) Pattern Thin Films
RIE system photolithography
patterned wafer
23
• thermal diffusion
• ion implantation
(3) Introduce Dopants - to form electrically-active
regions for resistors, diodes, transistors, etc.
thermal diffusion furnaces solid source doping
Standard IC Processes
24
Example of Deposition
Thermal Oxidation
• dry oxide – slower growth, denser, better quality
• wet oxide – faster growth rate (~10x)
• MEMS applications – barrier masks for etching
and doping, dielectrics for devices
dry
wet
Source:
Jaeger
dry oxidation 25
Oxidation Kinetics
Source: Jaeger
Wet oxidation is faster because water
vapor has a higher solubility in
silicon dioxide than gaseous oxygen
(i.e larger N0 in graph to right and
therefore larger flux, J)
26
Dry Oxidation
Source: Sze
note crystal orientation dependence
(111) Si has highest packing density 27
Wet Oxidation
Source: Sze
28
Techniques for Determining Oxide Thickness
Source: Campbell
• ellipsometry – laser polarization technique (expensive)
• profilometry – mechanical stylus technique (requires a step)
• color chart - inexpensive
29
Sputtering
Thin film deposition technique in which energetic ions, typically Ar+,
bombard a target and displace atoms which are then transported to the
wafer surface, where deposition occurs. Both conductive and insulating
materials can be sputtered.
Source: Jaeger 30
Evaporation
Thin film deposition technique in which material is evaporated
from a solid source in a high vacuum environment using thermal
or e-beam energy.
Source: Sze (Semi Sensors)
31
Comparison of Evaporation and
Sputtering Deposition
Source: Gise
32
Spinning
Thin film deposition technique used for polymers (such as
photoresists), spin-on dopants and SOG (spin-on glass). Low
cost capital equipment $5-10K.
Source: Sze (Semi Sensors)
33
a) oxidation
b) spin resist
c) align/expose
d) develop
e) oxide etch (wet
or dry/RIE)
f) strip resist
g) patterned oxide
“Photolithography”
Source:
Jaeger
Thin Film Patterning
Typical Photolithographic
Steps
34
Photoresist Types
Source:
Zant
• positive – better resolution, less
pinholes, liftoff compatible
• negative – faster exposure, better
adhesion, cheaper
IR mask aligner
35
Other Photolithographic Issues
• photomask polarity – light or dark field
• photoresist polarity – positive or negative
• etch vs liftoff
• aligners - contact vs proximity vs projection (steppers)
• aligners – frontside vs backside (IR)
• non-optical lithography – e-beam, x-ray, soft
IR mask aligner Liftoff 36
Example of Impurity Doping
Thermal Diffusion
• liquid, solid, or gaseous dopant sources
• highest doping concentration at surface
• high temp process – oxide masks
• deep junction depths possible
• MEMS applications – piezoresistive
elements and p+ etch stops
Source:
Jaeger
diffusion furnaces
37
Source:
Jaeger
Thermal Diffusion Process
• MEMS applications – piezoresistive elements and p+ etch stops 38
Example of Impurity Doping
Ion Implantation
Source:
Jaeger
39
Ion Implantation
• precise control of doping profile (through specie, dose, energy)
• low temp process (can use resist as mask)
• buried peak concentrations
• expensive capital equipment
• MEMS applications – piezoresistive elements, p+ etch stops, SIMOX
Source: Jaeger Source: Sze
40
Micromachining Processes
Source: Maluf
41
Micromachining Processes
• subtractive (etching) process
• wet vs dry etching
• isotropic vs anisotropic etching
Bulk Micromachining
42
Typical Micromachining
Etch Profiles
Source: Maluf
“RIE”
RIE - reactive ion etch 43
• Using (100) silicon wafer with SiO2 mask
Bulk Micromachining
44
Typical Bulk Micromachining
Etching Setup
45
Bulk Micromachining
using (100) silicon
Examples
Source: Maluf
Source: Madou
46
Bulk Micromachining Rules for (100) Si – Rule 1 “Features misaligned to the <110> wafer flat will be undercut”
Source: UofC 47
Source: Elwenspoek
Bulk Micromachining Rules for (100) Si – Rule 1b “Any feature will eventually result in the largest inverted pyramidal
rectangular pit that can be circumscribed around that feature”
48
Source: Ristic
Bulk Micromachining Rules for (100) Si – Rule 1b “Any feature will eventually result in the largest inverted pyramidal
rectangular pit that can be circumscribed around that feature”
Source: UofL
49
Bulk Micromachining Rules for (100) Si – Rule 2 “Convex (outside) corners will be undercut due to fast-etching
exposed secondary planes”
Source: Elwenspoek 50
Source: Ristic
Source: Ristic
Source: Ristic Source: Wise
51
Micromachining Processes
Bulk Micromachining – Corner Compensation
Source: Maluf Source: UofL
• Technique for minimizing corner erosion by adding
additional mask features at the corners. Can result in near
perfect mesa structures.
52
• Isotropic
• HNA (hydroflouric, nitric and acetic acid)
• also called “poly-etch”
Most Popular Silicon Bulk
Micromachining Wet Etchants
• Anisotropic
• Potassium Hydroxide (KOH)
• EDP
• Hydrazine
• TMAH
53
Comparison of Silicon Micromachining Etchants
Source: Maluf
54
Etchants for Other MEMS Thin Films
Source: Maluf
55
Micromachining software
ACES – PC based Micromachining Simulation Software
(http://galaxy.ccsm.uiuc.edu/aces/)
56
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Micromachining Processes
• Timed etch stop
• Boron (p+) etch stop
• Electrochemical (pn junction) etch stop
• Buried oxide etch stop (BESOI, SIMOX)
Bulk Micromachining Etch Stop Techniques
57
Micromachining Processes
Boron (p+) Etch Stop Technique
Source: Sze (Semiconductor Sensors)
58
Boron (p+) Etch Stop Examples
Source: Sze (Semi Sensors)
59
Boron (p+) Etch Stop Examples
Source: Kovacs 60
Micromachining Processes
Electrochemical (pn junction) Etch Stop Technique
Source: Kovacs
61
Electrochemical Etch Stop Example
Source: Kovacs
Source: Maluf
62
Micromachining Processes
• Similar to electrochemical etch stop process except the
p+ region is replaced with a buried oxide layer, which
functions as an etch stop to most anisotropic etchants.
• Bonded and Etched-back Silicon On Insulator (BESOI)
• Separation by Ion Implantation of Oxygen (SIMOX)
• Requires no voltage bias.
Buried Oxide Etch Stop Technique
Source: Kovacs
oxide
63
Micromachining Processes
• additive process
• structural & sacrificial layers
Surface Micromachining
Source: Sandia
64
• MUMPS (Cronos)
• SAMPLES (Sandia)
Surface Micromachining Process
Source: Sze (Semi Sensors)
MEMS Foundries
65
Micromachining Processes
• glass-Si anodic bonding
• si-si fusion bonding (SFB)
• eutectic bonding
• low temp glass bonding
• thermal compression
Wafer-Level Bonding
Source: EV
Source: Maluf
Source: UofL
66
• glass-Si field assisted bonding technique
• requirements
• similar TCEs (Corning 7740)
• clean & smooth surfaces
• <1um rms roughness
• slightly conductive glass
• elevated temperature (200-500C)
• high voltage (200-1000V)
Anodic or Electrostatic (ES) Bonding
Source: Sze
Source: UofL
67
Silicon Fusion Bonding (SFB)
Source: Sze
(Semi Sensors)
hydrophilic
surfaces
68
Micromachining Processes
• Single Crystal Reactive
Etching and Metalization
• CMOS compatible
• used by EG&G IC for
accelerometers
• 20-30 um depth limit
SCREAM
Source: Maluf Source: Elwenspoek 69
Micromachining Processes
• high density ICP plasma
• high aspect ratio Si structures
• cost: $500K
• vendors: STS, Alcatel, PlasmaTherm
Deep Reactive Ion Etching (DRIE)
Source: LucasNova
Source: AMMI Source: STS Source: STS 70
DRIE Etching Process – Gas Cycling
Source:
Kovacs
71
Micromachining Processes
• electroless plating and electroplating
techniques for producing thick films
• thick patterned resists used as molds
Plating
Source: Carl Suss
Copper
structures
72
Micromachining Processes
• uses x-ray lithography (PMMA resist), electrodeposition and molding to
produce very high aspect ratio (>100) micro-structures up to 1000 um tall
(1986)
LIGA (lithographie, galvanoformung, abformtechnik)
Source: Madou
Source: Kovacs 73
Micromachining Processes
• uses optical epoxy negative-resist developed by IBM to produce high
aspect ratio micro-structures (1995)
Poor Man’s LIGA
Source: Maluf
UofL Micro-reaction wells: 150 um wide,
120 um tall, 50 um wall thickness
74
Ultra- High-Precision Micromilling
and Microdrilling
Vibration isolated
• 1 ton block of granite
• Hydraulic suspension system
Computer-controlled laser-guided positioning system
• Air bearings
• X-Y stage resolution < 10 nm
• Z-axis resolution >50 nm
• (Linear Encoder)
Spindle speed = 20,000 rpm
G-code programming language
Tools – High Speed Tool Steel or Tungsten Carbide
Source: Dover Instruments 75
Materials Micro-milled/drilled
Plastics:
• PMMA (Top), Lexan,
Teflon, nylon, Epoxy, etc.
Metals:
• Molybdenum (Bottom),
aluminum, copper,
stainless steel, mild steel,
gold, titanium, tantalum,
tungsten, graphite, etc.
Others:
• PZT, silicon, Pyrex.
Source: Dover Instruments 76
• bulk micromachining
• silicon fusion bonding (SFB)
• DRIE
Combining uMachining
Processes
Source: LucasNova
Source: Maluf 77
Micromachining Tricks
Source: Kovacs
78
Other Micromachining Processes
• Chemical Vapor Deposition (CVD, LPCVD, PECVD)
• Epitaxy
• Vapor Phase Etching – xenon diflouride non-plasma
isotropic dry etch
• Laser Machining – laser ablation process for primarily
polymers
• Porous Silicon Formation
• Chemical Mechanical Polishing (CMP)
• Micro-embossing and Micro-stamping
• Inkjet Printing
• Soft Lithography with SAMs
79
MEMS Examples
Conventional Pressure Sensor
Source: Maluf 80
MEMS Examples
Conventional Pressure Sensor
Source: NovaSensor
0
10
20
30
40
50
60
0 20 40 60 80 100 120
Pressure (PSI)
Ou
tpu
t V
olt
ag
e (
mV
)
UofL uFab Course Sensors 81
MEMS Examples
Conventional Pressure Sensor Packaging
Source: Madou
82
MEMS Examples
Pressure Sensor (ultra-miniature)
Source: NovaSensor 83
MEMS Examples
Pressure Sensor (ultra-miniature)
Source: UofL 84
MEMS Examples
• Accelerometers
Source: UofL
85
MEMS Examples
Accelerometers
Sources: Analog Devices, Lucas NovaSensor, and EG&G IC Sensors 86
MEMS Examples
• Flow Sensors
Source: UofL
87
MEMS Examples
• Gas Sensors • MicroMotors
Source: UofL
Source: Berkeley
88
MEMS Examples
Inkjet Technology – “side shooter”
Source: Elwenspoek
(Microsensor)
89
MEMS Examples
Inkjet Technology – “top shooter”
Source: Maluf 90
MEMS Examples
Micromotors
Source: MIT and Berkeley 91
MEMS Examples
Micro-structures using LIGA
Source: UW 92
MEMS Examples
Micro-Grippers
Source: Berkeley 93
MEMS Examples
Micro-Tweezers
Source: MEMS Precision Instruments 94
MEMS Examples Neural Probes
Source: Mich (K. Wise) 95
MEMS Examples
Neural Interface Chip
Source: Stanford 96
MEMS Examples
Lab-on-a-Chip Systems
Source: Caliper
• separation
• dilution
• mixing and dispensing
• analysis
Source: Maluf
97
UofL NSF Lab-on-a-Chip Project
GOAL
Develop true portable microchip analysis systems
with electrochemical detection for broad practical
use
BACKGROUND
Current microanalysis devices with laser induced
fluorescence detection are successful but are neither
fully “micro” nor fully “integrated”
Electrodes can be fabricated directly on a microchip
in almost any size, shape, location, or composition
RESULTS
Developed portable electronics for CE/EC device
Fabricated and tested working CE/EC micro-chips
Modeled microfluidic flow using MEMCAD
TECHNOLOGICAL IMPACT
Produce self-contained lab-on-a-chip instruments
that fully utilize microfabrication technology for
optimum miniaturization, structural sophistication,
and ease of construction
Identify and define optimum chip configuration
Portable Electronics, DataAcquisition and Display Unit
Disposable Micro-CE/ECPlug- In Module
Micromachined GlassWafer
Microfabricated Glass Substrate with Patterned Electrodes
Electrical Connections
Sample Inlet Reservoir
Sample WasteReservoir
Buffer Inlet Reservoir
Buffer Waste Reservoir
PC Interface
“An Integrated Monolithic Capillary Electrophoresis (CE)
System with Electrochemical Detection (ECD)”
98
Operation
Injection mode Separation mode
A
A
A
Injection
Waste Sample
CE
Detection
A
A
Separation
A
Waste
CE
Sample
Detection
99
Results
• Analyte
– Dichlorofluorescein (60 μM, 420.1 g/mole)
• Buffer
– Phosphate (10mM – pH = 6.0)
• Comments
– Injection and separation both at 1 kV (250 V/cm)
– Average plug speed 372 ± 13 um/sec (n=7)
– Higher injection voltages decreased the volume of the plug – did not affect the speed. Spectra Physics 770 Argon-Ion laser (514 nm) at 2.5 W through a 2mm/5mm diverging tip.
injection separation
simulation
100
Separation and EC Detection
• On-chip separation and
electrochemical detection
has been realized
• Photolithographically
patterned Pt electrodes
• Dopamine and Catechol
used as analytes
• Integrated with
miniaturized custom-
made power supply and
detection circuit
101
MEMS Examples
Channels, Nozzles, Flow Structures, and Load Cells
Source: EG&G IC Sensors 102
MEMS Examples
Micromachined Tips for FEDs and AFMs
Source: IBM Source: Micron Technology 103
MEMS Examples
Optical MEMS (MOEMS)
Source: NIST, Simon Fraser, UCLA, and MCNC 104
MEMS Examples Optical MEMS (MOEMS)
Source: IMC (Sweden), Maluf and TI
TI’s DMD
105
• physical sensors: pressure, acceleration, flow
• lab-on-a-chip systems: uTAS using CE/EC
• micro-pumps
• bio-MEMS devices
• RF MEMS for wireless communication
• chemical and CNT sensors
• microhotplates
• MEMS-based microphones
• inkjet microtechnology
• MEOMS: optical MEMS
• smart sensors
• energy harvesting
• nanotechnology
UofL MEMS Group
Research Areas
106
Future Direction
Commercial Successes Emerging Technologies
• pressure sensors
• accelerometers
• gyros
• flow sensors
• radiation sensors
• MEMS microphones
• gas sensors
• ink jet printheads
• DMD micro-mirrors for
projection systems
• LOC
• optical switches
• RF MEMS
• microfluidics
• micro pumps, valves and mixers
• gnat robots
• drug delivery
• tissue engineering
• micro-motors
• NEMS
• new fab technologies
2000 BMW 740i contains
over 70 MEMS sensors !
107
Cleanroom
Standards
Source: Zant
Class 100 – no more than 100
particles of diameter .5 microns or
larger per cubic foot of air space.
Source: Zant
108
Cleanroom Configuration and Activities
Source: Zant
Table from Zant illustrating
increase in particle count due to
various cleanroom activities 109
The End
110