Professor N Cheung, U.C. BerkeleyLecture 25EE143 F20101Capacitive sensors• Typically used to measure displacement• C ~= e0A/dSeparation (d)Area (A)C(x)=C(x(P))Example: Pressure TransducerProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F20102CO Microsensor Process FlowSource: http://www.itri.org.tw/mems/Professor N Cheung, U.C. BerkeleyLecture 25EE143 F2010MEMS Mechanical SwitchDrainSource Gate(s)• Contact Areas: 0.4x0.4 um2to 8x8 um2• Devices from 50 um to 250 um longDrainProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F2010Mechanical Switch Process FlowIsolationElec0 LayerSi SubstrateDimple holeMain Sacrificial (LTO)Pattern Elec0Pre-alignmentIsolation Growth6000A Low Temp Oxide1000A Stoichiometric Silicon NitridePoly 0 DepositionPoly 0 FormationRIE to isolation w/ overetchMain Sacrificial Deposition5500A Low Temp OxideDimple FormationDRIE to Isolation or timed DRIEProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F2010Fine refill sacrificial (HTO)Anchor holesElec1 LayerSacrificial ReleaseFine Sacrificial Deposition650A High Temp OxideAnchor FormationDRIE to isolation layerPoly 1 Deposition5500A @ 615C n-dopedPoly 1 FormationRIE etch to Main SacSacrificial ReleaseHF:HCl 20’ then critical pt. dryProcess FinishedMechanical Switch Process FlowProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F20106 (gold) = 14 x 10-6/oC (Si) = 2.6 x 10-6/oCExample of Thermal Bimorph ActuatorProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201070 volts- open15 volts-closedProcess Flow of Micro-tweezers by selective CVD TungstenProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F20108Process Flow of MEMS Rotating MechanismsMicro-turbineEngineIn-Plane MovementProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F20109Example : Lateral ResonatorProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201010Using electrostatic actuator, convenient voltage is limited to ~ 200VPaschen Curve for gas breakdownProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201011Electrostatic Gated Pneumatic ValveProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201012Process Flow of Electrostatic Gated Pneumatic ValveProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201013SCREAM ProcessAlelectrodesQuestion:Why 2ndSi etch ?Professor N Cheung, U.C. BerkeleyLecture 25EE143 F201014Monolithic MEMS/IC Integration Approaches• Interleaved process– Processing steps for MEMS and IC are interleaved in the process flow.• Modular process– Allows for separate development & optimization of MEMS and electronics components– Use of IC and/or MEMS foundries a possibilityProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201015Challenges of Modular processes• MEMS-first Approach– Topography is an issue.– Degradation of MEMS devices during high-temperature IC process steps.– Electronics must be protected during HF release-etch.• IC-first Approach– MEMS must be low temperature process– Electronics must be protected during HF release-etch.Professor N Cheung, U.C. BerkeleyLecture 25EE143 F201016Interleaved Process Example(Analog Devices Inc.)• Metallization process steps performed last• Performance of MEMS and electronics compromisedProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201017MEMS-first Process Example(Sandia National Lab)• MEMS fabricated in 12mm-deep trench – Filled with SiO2and planarized using CMP• Modular process, but IC foundries waryProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201018MEMS-Last Process Example(UC-Berkeley)• Non-standard refractory W metallization – melting point > 3000oC!• IC foundries not interestedProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201019New MEMS-Last Technology(A. Franke, UC-Berkeley)• Standard CMOS process (Al-based metallization)• Poly-SiGe as structural material– processing temperatures < 500C possible• Amorphous-Si protects CMOS during HF release-etchProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F2010MEMS-Last Sensor 20The cavities are formed after formation of the lower electrode, when a thick SiO2layer and the upper electrode are deposited. In the upper electrode, multiple holes are etched, a plasma or wet etchant for SiO2is applied through the holes to attack the thick SiO2layer, the SiO2under the holes is removed and small cavities of SiO2are formed and connected gradually. Finally the connected cavities produce a large cavity under the upper electrode. The final cavity size depends on the hole size and etching conditions, including time, gas flow rate and gas components. Source: Hitachi, 2009Professor N Cheung, U.C. BerkeleyLecture 25EE143 F201021Professor N Cheung, U.C. BerkeleyLecture 25EE143 F201022DNA analysis in MicrochannelsEntropic trap – Han & Craighead (Cornell, MIT)75nmStretching DNA and sequencing– Cao et al. (Princeton)30µm30µmProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201023Optical observation of single molecules in their natural state – Cornell Univ.• Light-impeding Al holes• Zero-modewaveguidesProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201024Source, Madou , Lab Chip, 3, 26-28N (2003)Responsive Drug Delivery SysttemProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F201025Heterogeneous Integration of MicrosystemsProfessor Nathan Cheung, EECS-III-VLaser ArrayCPUMicro mirrors1mmMicro pumpMicro-fluidic channelsLaserEmitterArraysLED displayMOS IC-Spectrometer III-VLaser ArrayCPUOptical Modulator1mm1mmMicro pump200 mm200 mmMicro-fluidic channelsLaserEmitterArraysLED displayMOS ICEncapsulatedPower source5mmGreen LEDBlue LED5mm5mm5mmGreen LEDBlue LEDBioMEMS Emitters /Filter/DetectorsEncapsulated battery with switch/LEDSi microfluidic channels200 mmInGaN LEDs on Si 200 mm200 mm200 mmInGaN LEDs on Si Si-Ge Low PowerCMOSGeOIFor reference onlyProfessor N Cheung, U.C. BerkeleyLecture 25EE143 F2010Summary of MEMS Processing Module26•Stress, Strain, Young’s Modulus, Poisson Ratio, Yield Stress•Thin-film stress (intrinsic, thermal expansion, external)•Substrate warpage calculations – Stoney’s Equation•Stiction problems and solutions•Bonding and Molding (qualitative)•Principle of MEMS sensing and actuation (qualitative) •Given a MEMS structure, design the process flow•MEMS-CMOS Integration Sequence - Interleaved, MEMS-first,
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