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MEMS:Sensors & ActuatorsPart I: Introduction to MEMSJune 2, 2003What is MEMS?• In US: Microdynamics, Micromachining• In Europe: Micro Systems• In Japan: Micromachine, MicroRobotsA Micro-Electro-Mechanical System (MEMS) is aminiature system that containscomponents with characteristic sizes ranging from nanometers to millimeters.Otherwise known as:Machine Sizenmµmmm mkmConventionalMachinesMolecularMachinesQuantityMachine Sizenmµmmm mkmConventionalMachinesMEMSMolecularMachinesQuantityThere’s Plenty of Room At the Bottom(Miniaturization)Richard P. Feynman, APS Meeting, Caltech, 1959… I’d like to describe a field, in which little has been done, but in which enormous amount can be done.… it might tell us … strange phenomena that occur in complex situations.… it would have an enormous number of technical applications.… equipment won’t simply be scaled down; it has to be redesigned.Micromachining?Micromachining(Microfabrication)LithographyOxidationEvaporationDepositionPlasma etchingChemical etchingBulk µ-machiningSurface µ-machiningLIGA...MicrostructuresPlatesHolesBeamsDiaphragmsTipsPostsChannelsNozzlesCavities...MEMSMicroelectronicsMicrosensorsMicroactuators(M3systems)MEMS Technologies• Bulk Micromachining• Surface Micromachining• LIGA/SLIGA and LIGA-Like• Wafer Bonding•Others–Micro EDM– 3-D Stero Lithography– Laser MicromachiningHistory of Micro-Electronics(1940’s) (1970’s) (1990’s)Transistor (10 cm) IC (0.1mm)CPU (0.18 µm)A Brief MEMS History1950 1960 1970 1980 1990 2,000HNAAnodic BondingEDPPressure Sensor (Honeywell)KOHSi Pressure Sensor(Motorola)Si as a mechanical material (Petersen)Thermo-pneumatic valve (Redwood)SFBSFB Pressure Sensor (NovaSensor)TMAHDRIE !!XeF2/BrF3Metal sacrificial process (US Patent)RGT (Nathanson et al)Metal Light Valve (RCA)PolySi beams (Howe, Muller)PolySi Micromotor(Tai, Muller)IR imager (Honeywell)PolySi Comb Drive (Tang, Howe)ADXL AccelerometerDMD (TI)Si Gyro (Draper)LIGABJT TransistorICRF MEMSBio MEMSBulk MicromachiningDefinitionBulk micromachining employs orientation-dependent chemical etches such as EDP, KOH, and hydrazine in conjunction with several etch-stop techniques to sculpt microstructures from the silicon substrate ------ NSF report, 1988.In bulk micromachining, mechanical structures are created withinthe confines of a silicon wafer by selectively removing wafer material ------ J. Bryzek et al, 1994.Bulk micromachining uses chemical or plasma selective etching of silicon wafers, with the help of masking films, to form micromechanical structures ------ Y.C. Tai, 1995Isotropic EtchingSiMasking materialCross SectionGas Chromatograph on a wafer• Capillary Column200 µm × 40 µm × 1.5 m• Gas Control Valve5 - 15 µm Nickel Membrane• Thermal Detector(Terry, Jerman, and Angell, Stanford, 1979)Isotropic Etching Example<111>54.74º<100> Surface Orientation<111><110> Surface OrientationAnisotropic EtchingAnisotropic EtchingFabrication Process(Petersen et. al., NovaSensor, 1988)Silicon Fusion Bonded Pressure Sensor(100) ExampleHoneywell Pressure Sensor1stsilicon micromachined sensor (1970)(Kendall et. al, 1985) (Tuckerman et. al, Standford, 1981)Area = 1 cm2• Ww= Wc= 57 mm, Z = 365 mm• Water Flow: 10 cm3/s• Power Dissipation Capability: 600 W/cm2• Thermal Resistance: 0.1 °C/W(110) ExampleToPumpBrF3XeN2XeF2VaporReservoirReactionChamberGas-Phase Si Etching• Spontaneous etching• Room temperature• No surface tension Force• No charging effects• Isotropic• Highly Selective• Simple SystemXeF2 (Solid)1962 Synthesized1979 Etching Si (IBM)1984 Etching Si Model (Bell Lab)1995 MEMS (UCLA)2XeF2 + Si → 2Xe + SiF4(Cost $150 to etch 1 gram Si)XeF2 & BrF3BrF3 (Liquid)1984 Etching Si (Bell Lab)1996 (Br2+F2) Etching Si(U.Kohler,et.al,)1997 MEMS (Caltech)4BrF3 + 3Si → 2Br2+ 3SiF4(Cost $16 to etch 1 gram Si)Similar Etching Rate 1~10µm/min in Pulse ModeDeep RIE• RIE - Dry Plasma-Based Etch• Deep RIE (STS, Plasma-Therm, Alcatel)– Excellent Selectivity to Mask Material (>30:1)– High Etch Rate (1-10 microns/min)– High Aspect Ratio (>20:1)20 µmTrenches - Surface Technology Systems Spring - Klaassen, et al 1995DefinitionSurface micromachining is the carving of layers put down sequentiallyon the substrate by using selective etching of sacrificial thin films toform free-standing or even completely released thin-film microstructures------ NSF report, 1988.Surface MicromachiningSurface MicromachiningOxide Poly-Si CantileverBaseSiSiSiSacrificial Layers1. PSG -- phosphosilicate glass2. Polysilicon3. Polymers -- photoresist and polyimide4. Metals, etc.Polysilicon MechanismsPin jointSpringed pin-jointJoint LeversCombined mechanisms(Fan, Tai and Muller, 1988)Cross-sectionPolysilicon wire embeddedin 1.2 µm nitride diaphragmPolysilicon wire(3 x 150 x 0.5 µm3)Vacuum cavity(200 x 200 x 2 µm3)Polysilicon wireNitride diaphragmVacuum cavitySi substrateMetal lead Metal lead(Jiang et. Al., Caltech, MEMS’97)Vacuum-Insulated Shear Stress Sensor“Method of preparing electrostatic shutter mosaics,” US Patent 2,749,598(Filed Feb. 1, 1952, issued June 12, 1956)SubstrateSacrificial mesaMetal flapshuttersThe First Surface MicromachiningResonant Gate Transistor (RGT)(Nathanson and Wickstrom, 1965)The First Polysilicon Beams(Howe & Muller, 1982)(Howe & Muller, 1984)230 nm thick polysilicon Resonant-bridge vapor sensorPolysilicon Micromotors(Tai and Muller, 1989)Micromotor: 60-µm-Diameter(Tai & Muller, 1988)Polysilicon Micro Bridge6 µm(Tai and Muller, 1985)Spiral Springs(Fan, Tai & Muller, 1987)Micro Ratchet(Tai & Muller, 1987)Polysilicon Comb DrivesLinear ActuatorTorsional Actuator(Tang, Nguyen and Howe, 1989)MEMS Lock For Safing & Arming(Sandia Lab, 1998)LIGALIthographieGalvanoformungAdformungProcess(a) X-ray exposure(b) Develop(c) Electroplating(d) Mold formation(e) Injection molding(f) Molding separation(g) Electroplating(h) Part separationSynchrotron radiationMask membraneAbsorber patternThick photoresistConductive base3D ProcessSLIGA AccelerometerC. Burbaum, J. Mohr, P. Bley, and W. Ehrfeld, Sensors and Actuators. A–25 (1991) pp 559—563.SuspensionProof massElectrodesAnchorWafer Bonding• Permanently bond two or more wafers together• To form patterned “3-D” structures• To form hermetically sealed chamber• Three most common approaches: – Si-Si fusion bonding– Si-pyrex anodic bonding– Eutectic bondingFusion Bonding—SequenceC. Harendt, W. Appel, H.-G. Graf, B.


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