beth pruitt assistant professor dept. of mechanical engineering stanford university

AIM Industrial Advisory Committee Meeting 7 April 2004

Beth PruittAssistant Professor

Dept. of Mechanical Engineering

Stanford University

http://me342.stanford.edu

Course Development:ME342 MEMS Laboratory


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


Course Structure: project based course

• Two quarter sequence–Spring

Predesigned masks, device and processLab teams assigned for diversity of majors and backgroundsQualify on equipment in Stanford Nanofab

–SummerDefined 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 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


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


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


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


ME342A MEMS LaboratoryQ1 Project: Fabrication and Testing of Piezoresistive Cantilevers for nN-mN Force Measurement

Beth PruittDept. of Mechanical Engineering

Stanford University


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


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


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


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)

• 4-wire contact resistance

measurement

• Measure force and contact

resistance simultaneously

Gold Pad

Piezoresistor

measurement leads


Complete Experimental Setup:Force-Displacement Contact Measurements

Piezoactuator and controller

GPIB cardLaptop with Labview

DAQ card

Voltage Measurements(7 Channels)


Design

• Cantilever Beam– Equivalent spring constant, K (N/m)

• Goal: maximize range and sensitivity

• Constraints100 micron travel in 5nm steps (actuator selection)

w

t

L

P

001.06

2≤=

wEtLPε

1.06

3

2

≤=EwtPLθ

3

3

4L

wEtK =

Piezoresistor linearity with strain (Matsuda & Kanda)

Linear elastic beam equations (Young)

z

x

P=Kz


1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

1E-04 1E-03width(m)

Design Space

A = require L > w

B= piezo ε limited

C= linear elastic θ limited

D = cantilever design 1

800µm x 3mm x 40µm

K= 85 N/m

1E+01

1E+00

1E-01

1E-02

1E-03

1E-04

Kmin (N/m)

L max(m)

B

C

D

40µm thick cantileverPmax @ 100 µm =10mN

A

L max(m) Kmin (N/m)


1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E-04 1E-03width(m)

Design Space

A = require L > w

B= piezo ε limited

C= linear elastic θ limited

E = cantilever design 2

400µm x 6mm x 25µm

K=1.3 N/m

Kmin (N/m)

L max(m)

B

25µm thick cantileverK ~ 1.3 N/mPmax = 0.6mN

A

L max(m) Kmin (N/m)

E


Comparison to AFM cantilever

Park Scientific dlevers ™K from 1.3 to 16 N/m

Small displacement range

3.6mm

1.6mm

L = 180 mW = 35 mt = 2 m

K = 1.3 N/m

L= 6 mmW= 400 mt = 25 m

K = 1.3 N/m

L

W

Custom CantileversK from 1.3 to 85 N/m

100m displacement range


Cantilever Fabrication (omit gold pads!)

doped piezoresistor, B+

doped conductor, B++ aluminum

siliconSiO2

silicon

aluminum

piezoresistor

conductor

7 mask process: 25 micron SOI, 300micron handle


Processing: alignment

Pattern resist and light Si etch (3000 angstroms) to define alignment patterns

KEY:SiliconOxideResistPiezoresistor dopingConductor dopingInterconnect Metallization (Al)


Processing: protective oxide

Strip resist

Grow protective screeening oxide ~250 angstroms



Processing: piezoresistors

Pattern resist

50 keV boron implant for piezoresistors, e.g. dose = 1e15 ions/cm2



Processing: conductors

Pattern resist

50 keV boron implant for piezoresistors, dose = 1e16 ions/cm2



Processing: oxide/anneal

Strip damaged oxide

Wet Oxidation 900C, ~2500A, 2 m depth, piezo ~ 130 / , conductors ~ 45 /



Processing: contacts

Open oxide

Strip Resist

Sputter 0.5 m Aluminum

Pattern and etch Al



Processing: DRIE

Frontside Etch- 1.6 m resist, open oxide, etch Si to buried oxide, 1.6 m resist frontside protect


Backside Etch-, 10m resist, open oxide, etch Si to buried oxide, wet etch box


Cantilever Fabrication (shown w/ gold)

doped piezoresistor, B+

doped conductor, B++ aluminumgold

siliconSiO2

aluminum

conductor piezo

gold


Cantilever SEM


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!


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


Interconnect Levels: wire bonding to dip package

0th level interconnect1st level interconnect

2nd level interconnect

Printed circuit board

Silicon die

Package


Cantilever Calibration

• 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

15V

Laser vibrometer

Signal analyzer

Vdisplacement

Vstrain


Cantilever Calibration: time & frequency

effn m

K=ω

dceff mmm 24.0+=

ωn = 1st resonance

K = spring constant mc= concentrated mass

md= distributed mass

ω0

ω1

ω2

ω3


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


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


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)


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


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

beth pruitt assistant professor dept. of mechanical engineering stanford university

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