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530.352 Materials Selection

530.352 Materials Selection. Lecture #29 Processing MEMS Tuesday Nov. 21 th , 2006. What are MEMS?. MICROELECTROMECHANICAL SYSTEMS (MEMS) Small structures, sensors, actuators, machines, and robots Maximum dimensions on the order of millimeters

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530.352 Materials Selection

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  1. 530.352 Materials Selection Lecture #29 Processing MEMS Tuesday Nov. 21th, 2006

  2. What are MEMS? MICROELECTROMECHANICAL SYSTEMS (MEMS) Small structures, sensors, actuators, machines, and robots Maximum dimensions on the order of millimeters Minimum features on the order of micrometers

  3. What are MEMS?

  4. What are MEMS? Sandia National Laboratory http://www.sandia.gov/mstc/technologies/micromachines/movies/index.html

  5. What are MEMS?

  6. J OHNS HOPKINS E N G I N E E R I N G MEMS Technologies: ~1 mm • Surface micromachining • LPCVD polysilicon used for vast majority of MEMS applications. • Thickness limited to 3-5 microns.http://www.memsrus.com/documents/PolyMUMPs.flow.show.pps • Bulk micromachining • Allows for “thicker” structures • DRIE of single crystalline Si wafers is most popularhttp://www.memsrus.com/figs/bulk.micro.dcr • Electrodeposited (LIGA) • Allows for “thicker” metallic structures • Ni most popular http://www.memsrus.com/figs/liga.dcr http://www.mdl.sandia.gov /Micromachine/vision.html Klasseen et al., 1995 http://mems.engr.wisc.edu/what/

  7. Surface micromachining: • Like IC processing. • Most MEMS devices are made from LPCVD polysilicon (low pressure chemical vapor deposition) • Must selectively etch and build up layers. • See for example: • http://www.mems-exchange.org/ • http://www.memsrus.com/CIMSmain2ie.html http://www.memsrus.com/figs/mumps.pps http://www.memsrus.com/svcsrules.html • http://mems.sandia.gov/scripts/SAMPLE_summit.asp • http://bsac.eecs.berkeley.edu/ • http://mems.cwru.edu/SiC/Pages/surface.html • http://www.csa.com/hottopics/mems/oview.html

  8. {111} planes 1 cm 600 μm 4 mm 1 cm Microsample Fabrication (1) Start with single-side polished bare Si wafer (6) Pattern positive PR on frontside with tensile geometry (2) Grow thermal oxide (~3.5 μm) for ~30 hours (7) Sputter Al film using pulsed deposition and liquid nitrogen substrate cooling (3) Pattern with positive photoresist on backside (8) Liftoff PR and attached Al to define pattern (4) Use KOH anisotropic etchant to remove Si, leaving ~30 – 50 micron layer for frontside patterning (9) Remove layer of Si using XeF2 dry pulsed etching (5) Remove oxide on frontside with HF

  9. MEMS Accelerometer From John Yasaitis, Analog Devices Inc

  10. Digital Micromirror Device (DMD) Up to 2.07 million Al mirrors on one chip! Hornbeck Fig 15 Hornbeck Fig 20 http://www.dlp.com/includes/demo_flash.asp?bhcp=1

  11. Digital Micromirror Device (DMD) Digital Micromirror DeviceTM (Texas Instruments)

  12. What of the fatigue life of the DMD? • Many thin film materials being utilized in component: • TiN/Si • SiO2/Si • Al alloy/Si • TiN/Al alloy/Si • SiO2/TiN/Al alloy/Si • Possible failure modes • Fatigue of hinges • Fatigue of films from contact • Mirrors breaking from handling • Lifetime from high operating temperatures? • Lifetime from intense light exposure • Mirrors switch every 200 μs • Need 5 years of life • 1000 operating hours / year • Requires 9 x 1010 cycles! Wei, Bhushan, and Jacobs, J. Vac. Sci. Technol. A, 2004

  13. Design perspectives Macro Micro polysilicon steel “Si is a wonderful structuralmaterial” “Si is too brittle and too expensive”

  14. Design perspectives For polysilicon: • Macro world defects ~ 0.1 mm • sf = KIc/(pc)1/2 = 0.9 MPa m1/2 / (p 10-4 m)1/2 • =50 MPa • MEMS world defects ~0.1 mm sf = KIc/(pc)1/2 = 0.9 MPa m1/2 / (p 10-7 m)1/2 = 1,600 MPa

  15. Why DRIE and LIGA ? • Need for thicker structures. • LIGA • DRIE single crystal silicon • LIGA offers wide range of materials with full-balance of mechanical properties. • Metals tougher than Si below ~600°C • Plastic MEMS available through molding and embossing.

  16. MEMS Chip J OHNS HOPKINS E N G I N E E R I N G The NSWC MEMS F/S&A device:

  17. Locks, actuators G-sensor Slider NSWC F/S&A chip and components:

  18. Courses at JHU Intro to MEMS Prof. Sharpe Microfabrication Lab Prof. Wang, Prof. Andreou

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