34 MEMS and composites_DSG

null530.352 Materials Selection530.352 Materials SelectionLecture #34 Processing MEMS Monday Dec. 5th, 2004What are MEMS?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 micrometersWhat are MEMS?What are MEMS?MEMS Technologies:MEMS Technologies: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 popular http://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://mems.engr.wisc.edu/what/http://www.mdl.sandia.gov /Micromachine/vision.htmlKlasseen et al., 1995~1 mmSurface micromachining: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 Microsample FabricationMicrosample Fabrication(1) Start with single-side polished bare Si wafer(2) Grow thermal oxide (~3.5 μm) for ~30 hours(3) Pattern with positive photoresist on backside(4) Use KOH anisotropic etchant to remove Si, leaving ~30 – 50 micron layer for frontside patterning(5) Remove oxide on frontside with HF(6) Pattern positive PR on frontside with tensile geometry(7) Sputter Al film using pulsed deposition and liquid nitrogen substrate cooling(8) Liftoff PR and attached Al to define pattern(9) Remove layer of Si using XeF2 dry pulsed etchingMEMS AccelerometerMEMS AccelerometerFrom John Yasaitis, Analog Devices Inc Digital Micromirror Device (DMD)Digital Micromirror Device (DMD)Hornbeck Fig 15 Hornbeck Fig 20 http://www.dlp.com/includes/demo_flash.asp?bhcp=1Up to 2.07 million Al mirrors on one chip!Digital Micromirror Device (DMD)Digital Micromirror Device (DMD)Digital Micromirror DeviceTM (Texas Instruments)What of the fatigue life of the DMD?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, 2004Design perspectivesDesign perspectivesMicroMacropolysiliconsteel“Si is a wonderful structural material”“Si is too brittle and too expensive”Design perspectivesDesign perspectivesFor 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 Why DRIE and LIGA ?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. nullThe NSWC MEMS F/S&A device:The NSWC MEMS F/S&A device:NSWC F/S&A chip and components:NSWC F/S&A chip and components: