1MEMS Overview, Prof. A. Mason Page 1MEMS OverviewSPEAKER• Andrew Mason, Asst. Professor in Electrical and Computer EngineeringTOPIC• Overview of Micro-Electro-Mechanical Systems (MEMS)OUTLINE• Overview of MEMS & Microsystems Navid Yazdi• Micromachining & MEMS process technology Navid Yazdi• Micro-electro-mechanical devices & microsensors–Inertial sensors Navid Yazdi– Pressure sensors Navid Yazdi–Bio-sensors Andrew Mason–Shock sensors Andrew Mason• Integrated Microsystems Andrew MasonMEMS Overview, Prof. A. Mason Page 2What is MEMS?••MEMS = MicroMEMS = Micro--ElectoElecto--Mechanical SystemsMechanical Systems––creation of 3creation of 3--dimensional structures using integrated circuits dimensional structures using integrated circuits fabrication technologies and special fabrication technologies and special micromachining micromachining processesprocesses––typically done on silicon or glass (SiOtypically done on silicon or glass (SiO22) wafers) wafers••MEMS Devices and StructuresMEMS Devices and Structures––transducerstransducers••microsensors and microsensors and microactuatorsmicroactuators––mechanically functional microstructuresmechanically functional microstructures••microfluidicsmicrofluidics: valves, pumps, flow channels: valves, pumps, flow channels••microenginesmicroengines: gears, turbines, combustion engines: gears, turbines, combustion engines••Integrated MicrosystemsIntegrated Microsystems––integrated circuitry and transducers combined to perform a task integrated circuitry and transducers combined to perform a task autonomously or with the aid of a host computerautonomously or with the aid of a host computer––MEMS components provide interface to nonMEMS components provide interface to non--electrical worldelectrical world• sensors provide inputs from non-electronic events• actuators provide outputs to non-electronic events2MEMS Overview, Prof. A. Mason Page 3Why Use MEMS?••Motivation and BenefitsMotivation and Benefits––Small SizeSmall Size––Light Weight Light Weight ––Enhanced Performance & ReliabilityEnhanced Performance & Reliability••high resolution devices high resolution devices ••array of devicesarray of devices––Low Cost (from batch fabrication)Low Cost (from batch fabrication)••ApplicationsApplications––Automotive SystemAutomotive System––Health Care Health Care ––Automated Manufacturing Automated Manufacturing ––InstrumentationInstrumentation––Environmental Monitoring & ControlEnvironmental Monitoring & Control––Consumer ProductsConsumer Products––AerospaceAerospace••MEMSMEMS--basedbasedMicrosystemsMicrosystems––highly integrated systemshighly integrated systems––sensing sensing ––actuationactuation––computationcomputation––controlcontrol––communicationcommunicationMEMS Overview, Prof. A. Mason Page 4Example MEMS-Based MicrosystemMicrocontrollerRF TransmitterPressureSensorHumiditySensorI/O ConnectorOn/Off SwitchAccelerometerPowerManagementInterfaceChips60mm35mmIntegrated FeaturesIntegrated Features--ControlControl--MicrocontrollerMicrocontroller--Power ManagementPower Management--Communication Communication --RF TransceiverRF Transceiver--SensingSensing--PressurePressure--Humidity Humidity --TemperatureTemperature--Vibration Vibration --PackagingPackaging“Micro Cluster” Environmental Monitoring Microinstrument“Micro Cluster” Environmental Monitoring Microinstrument(developed at U(developed at U--Mich Mich in the 1990s, A. Mason, K. Wise, et. al.)in the 1990s, A. Mason, K. Wise, et. al.)3MEMS Overview, Prof. A. Mason Page 5MEMS Fabrication Technologies• Applying Micromachining to create 3-D structures using 2-D processing• Micromachining Processes– bulk and surface micromachining– isotropic etching–anisotropic etching– dissolved wafer process– deep reactive ion etching– anodic and fusion bonding– micromolding micromachiningmicromachining2-D IC fabrication 3-D structurestechnologyMEMS Overview, Prof. A. Mason Page 6Surface vs. Bulk MicromachiningBulk Micromachining: Backside etchSi SubstrateSurface Micromachined StructureSi SubstrateBulk Micromachining: Front-side Etch pittypically polysilicon4MEMS Overview, Prof. A. Mason Page 7Isotropic Etching of Silicon•Isotropic etchant– etches in all directions– forms rounded pits in surface of wafer• Most common solution–HNA: Mixture of HF, HNO3, Acetic acid (CH3COOH)With agitation: Good reactant mass transportWithout agitationMEMS Overview, Prof. A. Mason Page 8Anisotropic Etching of SiliconAnisotropic wet etching using EDP, KOH: (100) surface- Etch stop on (111) plane(100) Surface orientationSilicon54.74°(111) Surface orientation•Anisotropic etchant– directional-dependant etch; based on crystal planes– forms flat-surface pits in surface of wafer• Common anisotropic etchants–EDP, KOH, TMAH5MEMS Overview, Prof. A. Mason Page 9Anisotropic Etching: Convex vs. Concave CornersTop ViewConvex cornerConcave corner(111)SiliconBuried etch stop layer (SiO2 in SOI wafers)Masking layer Cantilever BeamSide View(100)masking layer not attacked by Si etchantMEMS Overview, Prof. A. Mason Page 10Anisotropic Etching of Silicon: Examplebulk micromachined bulk micromachined silicon proof masssilicon proof mass6MEMS Overview, Prof. A. Mason Page 11Dissolved Wafer ProcessSiliconP++ SiliconDopant selective etch (e.g. EDP)• Structure created by “diffusion masking layer”•heavily p-dope silicon (p++)• Dissolve bulk of silicon to release the p++ structureK.D. Wise, K. Najafi, Univ of MichiganReleased p++ structureMEMS Overview, Prof. A. Mason Page 12Dissolved Wafer Process Example• Shock Switch– weighted cantilever beam with contacts that close by acceleration (shock)• Fabrication Flow– create anchor, weight, support beam, and contact on Si– create cavity and contact on glass– bond wafers and then dissolve the Si waferLPCVD layer depositionfor beamsAnodic bonding glass tosiliconDissolve silicon bulkand release structure7MEMS Overview, Prof. A. Mason Page 13Deep Reactive Ion Etching (DRIE)• Reactive Ion Etching = RIE– mechanical (ion) etching in plasma for chemical selectivity• Deep RIE– creates high aspect ratio patterns, narrow and deep• Trench-Refill process– can fill the etched “trench” with another materialTop viewA-A cross sectionAAmaskSiliconMEMS Overview, Prof. A. Mason Page 14Glass-Si Anodic
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