AIM Industrial Advisory Committee Meeting 7 April 2004 Beth Pruitt Assistant Professor Dept. of Mechanical Engineering Stanford University

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1 AIM Industrial Advisory Committee Meeting 7 April 2004 Beth Pruitt Assistant Professor Dept. of Mechanical Engineering Stanford University http://me342.stanford.edu Course Development: ME342 MEMS Laboratory

2 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

3 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)

4 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

5 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

6 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

7 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

8 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

9 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

10 AIM Industrial Advisory Committee Meeting 7 April 2004 MicroContact example under study: Formfactor MicroSpring TM Interconnects 1 st and 2 nd level interconnect – pressure connection from the die to the printed circuit board, e.g. 2-sided memory module with permission

11 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

12 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) 4-wire contact resistance measurement Measure force and contact resistance simultaneously Gold Pad Piezoresistor measurement leads

13 AIM Industrial Advisory Committee Meeting 7 April 2004 Complete Experimental Setup: Force-Displacement Contact Measurements Piezoactuator and controller GPIB card Laptop with Labview DAQ card Voltage Measurements (7 Channels)

14 AIM Industrial Advisory Committee Meeting 7 April 2004 Design Cantilever Beam – Equivalent spring constant, K (N/m) Goal: maximize range and sensitivity Constraints 100 micron travel in 5nm steps (actuator selection) w t L P Piezoresistor linearity with strain (Matsuda & Kanda) Linear elastic beam equations (Young) z x P=Kz

15 AIM Industrial Advisory Committee Meeting 7 April 2004 Design Space A = require L > w B= piezo  limited C= linear elastic  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 K min (N/m) L max (m) B C D 40µm thick cantilever Pmax @ 100 µm =10mN A L max (m) K min (N/m)

16 AIM Industrial Advisory Committee Meeting 7 April 2004 Design Space A = require L > w B= piezo  limited C= linear elastic  limited E = cantilever design 2 400µm x 6mm x 25µm K=1.3 N/m K min (N/m) L max (m) B 25µm thick cantilever K ~ 1.3 N/m P max = 0.6mN A L max (m) K min (N/m) E

17 AIM Industrial Advisory Committee Meeting 7 April 2004 Comparison to AFM cantilever Park Scientific dlevers ™ K from 1.3 to 16 N/m Small displacement range 3.6mm 1.6mm L = 180  m W = 35  m t = 2  m K = 1.3 N/m L= 6 mm W= 400  m t = 25  m K = 1.3 N/m L W Custom Cantilevers K from 1.3 to 85 N/m 100  m displacement range

18 AIM Industrial Advisory Committee Meeting 7 April 2004 Cantilever Fabrication (omit gold pads!) doped piezoresistor, B + doped conductor, B ++ aluminum silicon SiO 2 silicon aluminum piezoresistor conductor 7 mask process: 25 micron SOI, 300micron handle

19 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: alignment Pattern resist and light Si etch (3000 angstroms) to define alignment patterns

20 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: protective oxide Strip resist Grow protective screeening oxide ~250 angstroms

21 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: piezoresistors Pattern resist 50 keV boron implant for piezoresistors, e.g. dose = 1e15 ions/cm 2

22 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: conductors Pattern resist 50 keV boron implant for piezoresistors, dose = 1e16 ions/cm 2

23 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: oxide/anneal Strip damaged oxide Wet Oxidation 900C, ~2500A, 2  m depth,  piezo ~ 130  / ,  conductors ~ 45  / 

24 AIM Industrial Advisory Committee Meeting 7 April 2004 Processing: contacts Open oxide Strip Resist Sputter 0.5  m Aluminum Pattern and etch Al

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

26 AIM Industrial Advisory Committee Meeting 7 April 2004 Cantilever Fabrication (shown w/ gold) doped piezoresistor, B + doped conductor, B ++ aluminum gold silicon SiO 2 aluminum conductor piezo gold

27 AIM Industrial Advisory Committee Meeting 7 April 2004 Cantilever SEM

28 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!

29 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

30 AIM Industrial Advisory Committee Meeting 7 April 2004 Interconnect Levels: wire bonding to dip package 0 th level interconnect 1 st level interconnect 2 nd level interconnect Printed circuit board Silicon die Package

31 AIM Industrial Advisory Committee Meeting 7 April 2004 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 V displacemen t V strain

32 AIM Industrial Advisory Committee Meeting 7 April 2004 Cantilever Calibration: time & frequency  n = 1 st resonance K = spring constant m c = concentrated mass m d = distributed mass 00 11 22 33

33 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

34 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

35 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)

36 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

37 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