1U. Srinivasan ©EE C245Dr. Thara SrinivasanLecture 26Picture credit: Alien TechnologyAssembly Techniques for MEMS2U. Srinivasan ©EE C245Lecture Outline• Reading from reader• Cohn, M. B., Howe, R. T. et al., “Microassembly Technologies for MEMS,” pp. 2-16.• Srinivasan, U. et al., “MEMS: Some Self-Assembly Required,” pp. 20-24, 56.• Sign up for poster session ( Dec.11 or 12) today or e-mail me• Today’s Lecture• Assembly of Hinged MEMS Structures• Assembly of MEMS With Other Microdevices• Serial Assembly (pick & place)• Parallel Assembly Processes– Guided Transfer– Self-Assembly3U. Srinivasan ©EE C245Assembly and MEMS• “This new technology, originating in microelectronics, offers the possibility of fabricating fully assembled, low-cost mechanical devices and systems …” (emphasis added)• Small Machines, Large Opportunities. NSF Workshop on MEMS Research, 1987-1988 (K. Gabriel, J. Jarvis, and W. Trimmer, eds.)M. S. Rodgers and J. Sniegowski,Sandia National LabsTransducers ‘994U. Srinivasan ©EE C245Assembling Hinged Structures• Enable assembly of 3-D structures from surface micromachined parts• Manual assembly, fluidic agitation• On-chip MEMS actuators• Parallel external methods First hinge : K. Pister, et al. 1992Corner Cube Reflector : V. Hsu, 199925U. Srinivasan ©EE C245On-chip Actuation Methods• Use actuators (i.e., comb drives, vibromotors, scratch drives) to push hinges into assembled position• Fix hinges using ohmic heating, friction• Vibromotor results• Step size 0.27 ± 0.16 µm; near resonance, step size can change with small frequency variations• Closed loop control necessaryDaneman et al., BSAC6U. Srinivasan ©EE C245Using Beam Buckling• Use in-plane forces to buckle clamped-clamped support beam out of plane• Can generate few µN, displace few tens of µm• Challenges• Real estate requirements• Need closed-loop controlGarcia et al., Sandia Labs7U. Srinivasan ©EE C245Parallel External Methods• Fluidic agitation• Ultrasonic forces• Magnetic deflection• Polymer shrinkage• Surface tension of dropletsYi and Liu, U of Illinois8U. Srinivasan ©EE C245Syms, Imperial CollegeParallel External Methods• Polymer shrinkage• Surface tension of dropletsE. Smela group Univ. of MD39U. Srinivasan ©EE C245Pop-Up MEMS• Complex structures; one simple assembly step• Sandia SUMMiT 4-level process used to fabricate pop-up MEMSE. Hui, Howe group10U. Srinivasan ©EE C245Closed Box AssemblyE. Hui, Howe group11U. Srinivasan ©EE C245Lecture Outline• Assembly of Hinged MEMS Structures• Assembly of MEMS With Other Microdevices• Serial Assembly (pick & place)• Parallel Assembly Processes• Guided Transfer• Self-Assembly12U. Srinivasan ©EE C245Integration with Circuitry• MEMS/CMOS co-fabrication• MEMS/CMOS mixed • “Boutique processes” (Analog Devices)• MEMS first, CMOS last • Need to own or control CMOS fab (Sandia)• CMOS first, MEMS last • Can use CMOS foundry; thermal budget for MEMS is a challenge• Challenges• CMOS and MEMS wafer sizes• Material and process incompatibilities• Yield losses from high mask counts• MEMS-specific dicing and packaging• Microassemble finished components instead?413U. Srinivasan ©EE C245K. Pister group, BSAC, now on leave at Dust, Inc., BerkeleySmart Dust Mote14U. Srinivasan ©EE C245Motivation for IntegrationBrittle star on SEM of brittle star “eyes”Joanna Aizenberg et al. Nature 2002• Advantages• increased functionality• higher performance• lower cost• Next generation of microsystems• MEMS, microelectronic, microoptical, microfluidic components on a single substrate15U. Srinivasan ©EE C245Why Microassembly?• Assembly vs. co-fabrication• Avoid materials and process incompatibilities and optimize elements separately• Reduce materials cost• Reduce yield losses• Technique requirements• Massively parallel (low cost, high speed)• Microscale positioning precision• Mechanical bonding and electrical interconnection availableAdapted from K. Böhringer, et al, May 199816U. Srinivasan ©EE C245Assembly Classification• Lecture Outline• Assembly of Hinged MEMS Structures• Assembly of MEMS With Other Microdevices• Serial Assembly (pick & place)• Parallel Assembly Processes• Guided Transfer• Self-Assembly• Serial microassembly ~ “pick and place” on the microscale• Parallel microassembly ~ multiple parts assembled simultaneously• Guided: Pre-determined destination for parts ensured by guiding• Self-Assembling: Parts initially in random positions; energy minimization determines part destinations without active intervention517U. Srinivasan ©EE C245Robotic MicroassemblyC. Keller and R.T.Howe, MEMS ‘97• Serial robotic “pick & place” assembly• Hierarchy of adhesive forces must be establishedMEMS PI18U. Srinivasan ©EE C245Microassembly System• Commercial tooling advancing for high-density consumer electronics (e.g., cell phones …5 x 106 chip capacitors in a coffee mug (Murata, 2002)• Suss MicroTEC bonder can achieve submicron accuracy in x, y, z• Imaging ~ optical mixing of two images• Robotic bonding arm ~ parts as small as 250×300 µm2, range of bonding forces available (10 g to 200 kg), 6 DOF motion, temp. up to 450°C,.SUSS FC 250 Production Device Bonder19U. Srinivasan ©EE C245SUSS Microassembly• Challenges• High cost, low speed• Surface effects do lead to unwanted adhesion• Compromised electrical performance20U. Srinivasan ©EE C245MEMS Actuator Arrays• Use MEMS cooperatively• Regular grid of motion pixels move parts in parallel until they reach potential energy minima• Microcilia arrays built on top of CMOS, Bohringer et al., 2000• Each actuator curls into/out of substrate plane due to CTE differences of 2 polyimide layers • Linear translation, diagonal motion, and vector field operations like centering and squeeze field manipulations possible621U. Srinivasan ©EE C245Wafer-to-Wafer Transfer• Parallel assembly• Wafer-to-wafer transfer• Self-assembly• Wafer-to-wafer transfer• Microstructures held on donor wafer by break-away tethers• Transferred occurs between aligned donor and target wafers• Pros and Cons + Parallel assembly+ Smaller area electrical connections– Inefficient use of materials– Yield lossesCohn PhD 1998, Heck PhD 2001Howe group, BSACHexsil CapTarget Wafer (Sandia IMEMS)Donor WaferHexsil Cap22U. Srinivasan ©EE
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