Course Development:
ME342 MEMS Laboratory
Beth Pruitt
Assistant Professor
Dept. of Mechanical Engineering
Stanford University
http://me342.stanford.edu
AIM Industrial Advisory Committee Meeting 7 April 2004
Course Goal: Multidisciplinary learning
and entrepreneurship
• Micro/nanotechnology
–Scaling laws
–Transduction mechanisms
• Design/manufacturing
–Processes and tolerances
–Material selection and limitations
–Innovation
• Biomedical device engineering
–Biocompatibility
–Safety/Ethics
• Multidisciplinary language
AIM Industrial Advisory Committee Meeting 7 April 2004
Course Structure: project based course
• Two quarter sequence
–Spring
 Predesigned
masks, device and process
 Lab teams assigned for diversity of majors and backgrounds
 Qualify on equipment in Stanford Nanofab
–Summer
 Defined
projects with partners (design starts early May)
 Complete design, fabricate, and test cycle
• Partners
–Internal research collaboration needs (e.g.
Cardiology, Material Science, Cell Physiology)
–Industry defined challenges (e.g. Intel, Honeywell)
AIM Industrial Advisory Committee Meeting 7 April 2004
AIM Course Development Funding
• $10,000 grant to help start this course
–Winter quarter TA support to debug the process and
prepare course materials
–Prototyping supplies (wafers, masks, etc.)
–Thank you!
• I gratefully acknowledged assistance this
quarter that also came from:
–Nu Ions: donation of ion implant service for course
–Center for Integrated Systems: new user grants to
fund team clean room charges
• Goal is self-sustaining course model
AIM Industrial Advisory Committee Meeting 7 April 2004
Day 1
• About 70 students attended the first class
• 20 students were admitted based on questionnaires
of background and interests
• 4 teams of 5 (max. capacity this year) formed with at
least 1 EE, 1 Med/Phys/Chem/MSE, and 2-3 ME
students (will cross-list in EE, not advertised this time)
• 1 team of 5 “overqualified” applicants accepted to
audit A and participate fully in B
• Very tough to turn students away, an exciting amount
of interest in microfabricated solutions for new areas
of research exists at Stanford
AIM Industrial Advisory Committee Meeting 7 April 2004
Week 1
• Safety training sessions for all new students to
obtain clean room access
• Safety tours of SNF (Stanford NanoFab Facility)
• Written safety test
• Cleanliness training
• Instill sense of MEMS/clean room community
AIM Industrial Advisory Committee Meeting 7 April 2004
Week 2-6: Processing
• Fabrication in earnest under wing of senior MEMS
research students for 4 weeks
• Incredible SNF staff support to ensure thorough
qualification of students as users
• 2 weeks and 2 masks as independent users (with
support net of teaching team)
• Analysis/simulation in parallel with fabrication
Week 7-9: Measurements
• Package, test, signal condition and calibrate
• Compare theory and experiment
AIM Industrial Advisory Committee Meeting 7 April 2004
ME342A MEMS Laboratory
Q1 Project: Fabrication and Testing of Piezoresistive
Cantilevers for nN-mN Force Measurement
Beth Pruitt
Dept. of Mechanical Engineering
Stanford University
AIM Industrial Advisory Committee Meeting 7 April 2004
Background for Project
• Sensors designed as part of a MEMS based
system for force-displacement measurements
of electrical microcontacts
• Sensors originally incorporated gold contact
pad at tip to study thin gold films as
MEMS/micro-electrical contacts
AIM Industrial Advisory Committee Meeting 7 April 2004
MicroContact example under study:
Formfactor MicroSpringTM Interconnects
• 1st and 2nd level interconnect
–pressure connection from the die to the printed
circuit board, e.g. 2-sided memory module
with permission
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Trends and opportunities:
Separable Contacts for Packaging, Testing, Switching
• Shrinking interconnect pitch and size
– Smaller probes for test
– Smaller off-chip interconnects
• Thinner wafers and organic dielectrics
– Low force probing
– Thinner metal stackups
• To support continued miniaturization need low force,
small size, and low contact resistance
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Design of Contact Characterization Sensors
• Measurement over 6! orders
of magnitude (2 designs)
• Fabrication of thin film metals
in-situ with standard
processing (evaporated,
sputtered, plated)
Gold Pad
measurement
leads
• 4-wire contact resistance
measurement
• Measure force and contact
resistance simultaneously
Piezoresistor
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Complete Experimental Setup:
Force-Displacement Contact Measurements
Piezoactuator and controller
GPIB card
Voltage
Measurements
(7 Channels)
DAQ card
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Laptop
with
Labview
Design
• Cantilever Beam
– Equivalent spring constant, K (N/m)
Et 3 w
K 
4L3
P
z
P=Kz
t
x
w
• Goal: maximize range and sensitivity
L
• Constraints
100 micron travel in 5nm steps (actuator selection)

6 LP
 0.001
2
Et w
6 PL2

 0.1
3
Ewt
Piezoresistor linearity with strain (Matsuda & Kanda)
Linear elastic beam equations (Young)
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Design Space
40µm thick cantilever
Pmax @ 100 µm =10mN
Kmin (N/m)
L max(m)
1E+01
1E+04
A = require L > w
1E+03
B= piezo  limited
1E+02
C= linear elastic  limited
D = cantilever design 1
K= 85 N/m
Kmin (N/m)
C
1E+01
1E+00
800µm x 3mm x 40µm
1E+00
B
D
L max(m)
1E-01
1E-04
width(m)
AIM Industrial Advisory Committee Meeting 7 April 2004
1E-01
1E-02
1E-03
A
1E-04
1E-03
Design Space
25µm thick cantilever
K ~ 1.3 N/m
L max(m)
Pmax = 0.6mN
K (N/m)
min
1E+01
A = require L > w
1E+00
B= piezo  limited
B
Kmin (N/m)
1E-01
C= linear elastic  limited
E
1E-02
E = cantilever design 2
1E-03
L max(m)
A
400µm x 6mm x 25µm
K=1.3 N/m
1E-04
1E-04
width(m)
AIM Industrial Advisory Committee Meeting 7 April 2004
1E-03
Comparison to AFM cantilever
W
L
3.6mm
L = 180 m
W = 35 m
t = 2 m
L= 6 mm
W= 400 m
t = 25 m
K = 1.3 N/m
K = 1.3 N/m
1.6mm
Park Scientific dlevers ™
K from 1.3 to 16 N/m
Small displacement range
Custom Cantilevers
K from 1.3 to 85 N/m
100m displacement range
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Cantilever Fabrication (omit gold pads!)
aluminum
doped conductor, B++
silicon
SiO2
doped piezoresistor, B+
aluminum
silicon
piezoresistor
conductor
7 mask process: 25 micron SOI, 300micron handle
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Processing: alignment
Pattern resist and
light Si etch (3000
angstroms) to
define alignment
patterns
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
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Processing: protective oxide
Strip resist
Grow protective
screeening oxide
~250 angstroms
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
AIM Industrial Advisory Committee Meeting 7 April 2004
Processing: piezoresistors
Pattern resist
50 keV boron implant
for piezoresistors, e.g.
dose = 1e15 ions/cm2
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
AIM Industrial Advisory Committee Meeting 7 April 2004
Processing: conductors
Pattern resist
50 keV boron implant
for piezoresistors,
dose = 1e16 ions/cm2
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
AIM Industrial Advisory Committee Meeting 7 April 2004
Processing: oxide/anneal
Strip damaged oxide
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
Wet Oxidation 900C,
~2500A, 2 m depth,
piezo ~ 130 / ,
conductors ~ 45 / 
AIM Industrial Advisory Committee Meeting 7 April 2004
Processing: contacts
Open oxide
Strip Resist
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
Sputter 0.5 m
Aluminum
Pattern and etch Al
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Processing: DRIE
Frontside Etch- 1.6 m
resist, open oxide, etch
Si to buried oxide,
1.6 m resist frontside
protect
KEY:
Silicon
Oxide
Resist
Piezo resist or doping
Conductor doping
Interconnec t Metalli zation (Al)
Backside Etch-, 10m
resist, open oxide,
etch Si to buried
oxide, wet etch box
AIM Industrial Advisory Committee Meeting 7 April 2004
Cantilever Fabrication (shown w/ gold)
doped conductor, B++
SiO2
aluminum
gold
silicon
doped piezoresistor, B+
aluminum
conductor
piezo
gold
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Cantilever SEM
AIM Industrial Advisory Committee Meeting 7 April 2004
ME342 Cantilevers-7 Masks, no Gold
• Mask Levels 1-3 completed by TA’s
–Alignment Marks/Cantilever outline
–Conductive Interconnect Implants
–Piezoresistive Region Implants
• Team Processing Mask Levels 4-7
–Complete in Labs 2-6 plus some time outside of lab
for levels 6 and 7
–Qualify individually on wetbenches, litho, DRIE
during labs of ME342
–Note: team stuck at mask 5 until all team members
qualify on required equipment!
AIM Industrial Advisory Committee Meeting 7 April 2004
ME342 Processing
• Each team completes processing with same
mask set
• Each team has 5-6 wafers to process
–2 SOI wafers fully released by DRIE (300µm)
–3 test wafers partially processed (Noise only)
• Sensor measurements, 2 die per person
–Packaging and Signal Conditioning
–Testing and Measurements (Sensitivity & Noise)
• Analysis
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Interconnect Levels:
wire bonding to dip package
0th level interconnect
1st level interconnect
2nd level
interconnect
Silicon die
Package
Printed circuit board
AIM Industrial Advisory Committee Meeting 7 April 2004
Cantilever Calibration
Signal analyzer
Laser vibrometer
Vdisplacemen
t
15V
Vstrain
• Piezoresistor Bridge Voltage vs. Displacement
– Measure at resonant frequency of cantilever
– Typical sensitivity ~ 1mV/µm
• Noise spectrum of piezoresistor
– < 0.1µV/Hz or ~80pN/  Hz at 1Hz
AIM Industrial Advisory Committee Meeting 7 April 2004
Cantilever Calibration: time & frequency
0
1
2
n 
K
meff
meff  mc  0.24md
3
n = 1st resonance
K = spring constant
mc= concentrated mass
md= distributed mass
AIM Industrial Advisory Committee Meeting 7 April 2004
ME342A Analysis
• Simulate piezoresistor values (TSUPREM4)
–Each wafer receives different dose/anneal set, each
student assigned a particular wafer to analyze
• Predict spring constant and gage factor
• Determine sensitivity and noise of cantilevers
–compare analysis by beam equations and noise
characteristics to measurements
• Comparisons and Conclusions
–15 min. talk 6/3, short report of results
AIM Industrial Advisory Committee Meeting 7 April 2004
ME342B Design Projects
• Project and team assignments early May
• Initial designs due end of May
• Mask designs must be submitted before start
of summer quarter!
• Processing and testing completed in ME342B
• Seminars, team meetings and lots of lab time
in summer quarter
• Project results = Conference papers???
–e.g. MEMS’05, ASME’05, send 1 author per paper
AIM Industrial Advisory Committee Meeting 7 April 2004
Potential Projects for ME342B 2004
• Radial 100% strain gage for measuring deformation in animal model
blood vessels, e.g. rat aorta (Taylor, ME/cardiology)
• Integrated touch sensitivity system for neurological examination
(Goodman, molecular & cell physiology)
• Out-of-plane actuated stage (Intel mirror steering)
• Active thermal isolation package (Honeywell chip scale atomic clock)
• Implanted piezoresistor design rule formulation (Pruitt)
• Optimization of miniature blood pressure sensor sensitivity by
process and geometry (Feinstein, pediatric cardiology)
• Coupled beam microresonators for molecular assay (Melosh, MSE)
AIM Industrial Advisory Committee Meeting 7 April 2004
9 weeks to go and the whole Summer!
• A class full of enthusiasm
• The best teaching assistants anyone one
could ask for
• A supportive clean room environment and
technical staff
• A rich tradition of innovation in manufacturing
and design
• Cool projects inspired by local industry and
my Bio-X collaborators
AIM Industrial Advisory Committee Meeting 7 April 2004
Thank you AIM for your help and support!
• 2004-2005 MEMS projects wanted!
• Team of 3-4 multidisciplinary students May
plus summer
• Innovative ideas, unique facilities, excellent
coaching from faculty and industry
• Projects on the margin, something a company
would like to try or know if it works but doesn’t
have manpower, expertise, or resources for it
AIM Industrial Advisory Committee Meeting 7 April 2004