Course Development: ME342 MEMS Laboratory Beth Pruitt
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Transcript Course Development: ME342 MEMS Laboratory Beth Pruitt
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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)
AIM Industrial Advisory Committee Meeting 7 April 2004
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
100m displacement range
AIM Industrial Advisory Committee Meeting 7 April 2004
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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)
AIM Industrial Advisory Committee Meeting 7 April 2004
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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-, 10m
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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
AIM Industrial Advisory Committee Meeting 7 April 2004
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