Beth Pruitt Assistant Professor Dept. of Mechanical Engineering Stanford University

1. Beth Pruitt Assistant Professor Dept. of Mechanical Engineering Stanford University http://me342.stanford.edu Course Development:ME342 MEMS Laboratory

2. 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. 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 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. 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. 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. 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. ME342A MEMS LaboratoryQ1 Project: Fabrication and Testing of Piezoresistive Cantilevers for nN-mN Force Measurement • Beth Pruitt Dept. of Mechanical Engineering Stanford University

9. 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. 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

11. 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. 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 measurement leads Piezoresistor

13. Complete Experimental Setup:Force-Displacement Contact Measurements Piezoactuator and controller Laptop with Labview GPIB card Voltage Measurements (7 Channels) DAQ card

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

15. Design Space 40µm thick cantilever Pmax @ 100 µm =10mN L max(m) Kmin (N/m) 1E+01 1E+00 1E-01 1E-02 1E-03 1E-04 A = require L > w B= piezo  limited C= linear elastic  limited D = cantilever design 1 800µm x 3mm x 40µm K= 85 N/m B Kmin (N/m) C D L max(m) A

16. Design Space 25µm thick cantilever K ~ 1.3 N/m Pmax = 0.6mN L max(m) Kmin (N/m) 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 B Kmin (N/m) E L max(m) A

17. Comparison to AFM cantilever W L 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 3.6mm 1.6mm Custom Cantilevers K from 1.3 to 85 N/m 100m displacement range Park Scientific dlevers ™ K from 1.3 to 16 N/m Small displacement range

18. Cantilever Fabrication (omit gold pads!) aluminum doped conductor, B++ silicon SiO2 doped piezoresistor, B+ aluminum piezoresistor silicon conductor 7 mask process: 25 micron SOI, 300micron handle

19. Processing: alignment Pattern resist and light Si etch (3000 angstroms) to define alignment patterns

20. Processing: protective oxide Strip resist Grow protective screeening oxide ~250 angstroms

21. Processing: piezoresistors Pattern resist 50 keV boron implant for piezoresistors, e.g. dose = 1e15 ions/cm2

22. Processing: conductors Pattern resist 50 keV boron implant for piezoresistors, dose = 1e16 ions/cm2

23. Processing: oxide/anneal Strip damaged oxide Wet Oxidation 900C, ~2500A, 2 m depth, piezo ~ 130 / , conductors ~ 45 / 

24. Processing: contacts Open oxide Strip Resist Sputter 0.5 m Aluminum Pattern and etch Al

25. 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. Cantilever Fabrication (shown w/ gold) aluminum doped conductor, B++ gold silicon SiO2 doped piezoresistor, B+ aluminum conductor piezo gold

27. Cantilever SEM

28. 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. 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. Interconnect Levels: wire bonding to dip package 0th level interconnect 1st level interconnect 2nd level interconnect Silicon die Package Printed circuit board

31. 15V Cantilever Calibration Signal analyzer Laser vibrometer • 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 Vdisplacement Vstrain

32. Cantilever Calibration: time & frequency 0 1 3 2 n = 1st resonance K = spring constant mc= concentrated mass md= distributed mass

33. 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. 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. 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. 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. 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