Introduction and Capacitive Sensing Introduction and Capacitive SensingGIEE/NTUTsung Tsung- -Hsien Lin Hsien LinOutlines OutlinesIntroduction to MEMS Introduction to MEMS MEMS fabrication technologies Surface micromachining Bulk micromachining High-aspect ratio micromachining Sensing principle Capacitive p Interface circuitTsung Tsung- -Hsien Lin Hsien Lin 2MEMS/NEMS MEMS/NEMSMEMS MMicro EElectroMMechanical SSystems MEMS MMicro-EElectroMMechanical SSystems NEMS NNano-EElectroMMechanical SSystems Based on the IC manufacturing technology to fabricate planar or 3-D t t structures Sensing and actuating Economic benefit Leverage IC process for mass production Technical benefit Precision, light weigh Integration, small size, Material Mature MEMS products atu e S p oducts Airbag accelerometer Blood pressure monitoring sensors Inkjet printer headTsung Tsung- -Hsien Lin Hsien Lin 3Inkjet printer headDimensions DimensionsTsung Tsung- -Hsien Lin Hsien Lin 4Thoughts . Thoughts . A foot-long stick, cut in half everyday. The last cutting to eternity. Richard Feynman (Dec. 29th, 1959) There is plenty of room at the bottom. (Why cannot we write the entire 24 volumes of the Encyclopedia ( y y pBrittanica on the head of a pin?)Tsung Tsung- -Hsien Lin Hsien Lin 5History HistoryMEMS technology evolves with IC fabrication technologies MEMS technology evolves with IC fabrication technologiesTsung Tsung- -Hsien Lin Hsien Lin 6MEMS Capability MEMS CapabilityWhat can MEMS do? What can MEMS do?Tsung Tsung- -Hsien Lin Hsien Lin 7Market/Application Market/Application Defense Aeronautics Industrial Automobile Medical & Life Science Life Science Telecom ConsumerTsung Tsung- -Hsien Lin Hsien Lin 8Applications (1) Applications (1)Tsung Tsung- -Hsien Lin Hsien Lin 9Applications (2) Applications (2)(and video games)Tsung Tsung- -Hsien Lin Hsien Lin 10Applications (3) Applications (3)Tsung Tsung- -Hsien Lin Hsien Lin 11MEMS on Cars MEMS on CarsExample ExampleTsung Tsung- -Hsien Lin Hsien Lin 12Example: Bio Example: Bio--Medical MedicalSwallow able capsule endoscope system Swallow-able capsule endoscope systemTsung Tsung- -Hsien Lin Hsien Lin 13MEMS Fabrication Methods MEMS Fabrication MethodsBulk micromachining Bulk micromachining Etching of bulk silicon wafers Surface micromachining Deposition of thin layers on silicon surface Selective etching of sacrificial layers High-aspect ratio processes LIGA DRIE (deep reactive ion etching)Tsung Tsung- -Hsien Lin Hsien Lin 14Bulk Micromachining Bulk MicromachiningBulk Micromachining makes micromechanical devices by etching Bulk Micromachining makes micromechanical devices by etching deeply into the silicon wafer.Tsung Tsung- -Hsien Lin Hsien Lin 15 Heavy use of etching (isotropic/anisotropic; wet/dry (RIE))Example: Accelerometer Example: AccelerometerTsung Tsung- -Hsien Lin Hsien Lin 16Surface Micromachining Surface MicromachiningDeveloping structures on surface of a substrate Developing structures on surface of a substrate Etch away the sacrificial layer to release the structurePSG: sacrificial layer Issues:Tsung Tsung- -Hsien Lin Hsien Lin 17Issues: Adhesion of layers; interfacial stress; stictionExamples ExamplesTsung Tsung- -Hsien Lin Hsien Lin 18High High- -Aspect Ratio Process Aspect Ratio ProcessLIGA LIGAX-ray: shorter wavelength, less scattering; higher penetrationTsung Tsung- -Hsien Lin Hsien Lin 19y g , g; g pPMMA: photoresist materialExamples ExamplesTsung Tsung- -Hsien Lin Hsien Lin 20DRIE DRIE RIE: Reactive Ion Etching RIE: Reactive Ion Etching Mechanical (ion) etching in plasma for chemical selectivity Deep RIECreates high aspect ratio patterns narrow and deep Creates high aspect ratio patterns, narrow and deepTsung Tsung- -Hsien Lin Hsien Lin 21MEMS Capacitive Sensing MEMS Capacitive SensingCapacitance is easily available in Si technologies Capacitance is easily available in Si technologies Do not require special materials and process steps Must be dealt with anyway Negligible temperature dependence Easily integrate with electronics Accurate displacement measurement Can be used for sensing and actuating Challenges Small signals parasitics Small signals, parasitics Undesired electrostatic actuationTsung Tsung- -Hsien Lin Hsien Lin 22System Integration System IntegrationIntegration of CMOS electronics with MEMS structures Integration of CMOS electronics with MEMS structuresTsung Tsung- -Hsien Lin Hsien Lin 23CMOS MEMS Integration Platform CMOS MEMS Integration PlatformNational Chip Implementation Center (CIC) Taiwan National Chip Implementation Center (CIC), TaiwanGoldCMOS MEMSCMOS BioMEMS CMOS MEMS CMOS standard process + post-etching CMOS BioMEMS with the addition of Gold layer (electrode/sensor)Tsung Tsung- -Hsien Lin Hsien Lin 24(Source: CIC)Basic Capacitive Sensing/Actuating Basic Capacitive Sensing/ActuatingDetect the physical excitation by sensing the capacitance variation Detect the physical excitation by sensing the capacitance variation Parallel-plate and comb-finger capacitance structures Actuating by means of electrostatic forceTsung Tsung- -Hsien Lin Hsien Lin 25Parallel Parallel--Plate Sensing Plate SensingSingle ended sensing; 2 conductive plates of area A separated by a Single-ended sensing; 2 conductive plates of area A, separated by a distance x0A00xAC c=x0area=AC(x)Ax C = ) ( carea=AxgapC0x x +0) (x0gapx In general:Tsung Tsung- -Hsien Lin Hsien Lin 26Parallel Parallel--Plate Considerations Plate ConsiderationsSensitivity: Sensitivity:( )20) (x xAxx C+ =cc cx xAx C+=0) ( c( )0 x x + Design concerns:Sensitivity depends on x0 Sensitivity depends on x Non-linear function of x Desire some form of compensation or linearization linearizationTsung Tsung- -Hsien Lin Hsien Lin 27Comb Structure Sensing Comb Structure SensingComb finger structure Comb-finger structureLhN C c=L N: # of gaps(h: height of the fingers)LgN C =0g) ( ) ( LhN C c hNx C c c ) (xS iti it (dC/d ) i d d t f) ( ) ( x LghN x C + = cgNx =c ) ( Sensitivity (dC/dx) independent of xTsung Tsung- -Hsien Lin Hsien Lin 28Actuation ActuationActuator: Actuator: A mechanical device for moving or controlling something Common techniques: Thermal force Shape-memory Alloys Piezoelectric effect Electrostatic force Magnetic forceTsung Tsung- -Hsien Lin Hsien Lin 29Electrostatic Actuation Electrostatic ActuationElectrostatic force induced between the two plates: Electrostatic force induced between the two plates:( )2) ( A x C =cc c In a parallel-plate structure( )20 x x x + c( ) 22221 ) (21Vx xAVxx CF+=cc = cx0Varea=Ax Electrostatic force is attractive (pulling plates together) Force depends on x( )02 2 x xx +cTsung Tsung- -Hsien Lin Hsien Lin 30p Force is nonlinear with displacementComb Structure Actuation Comb Structure ActuationIn a comb finger structure In a comb-finger structure) ( ) ( x LhN x C + = c hNx C c=c ) () ( ) ( x LgN x C + =gNx =c Electrostatic force is attractive (pulling fingers together)Tsung Tsung- -Hsien Lin Hsien Lin 31g g ) Force is independent of xActuating Parallel Plates Actuating Parallel PlatesTypically driven by a DC bias (V ) with an AC signal (V) Typically driven by a DC bias (Vb) with an AC signal (Vi) Filters, resonators, etc.Tsung Tsung- -Hsien Lin Hsien Lin 32 Issue: limited range of motion; pull-in limitPull Pull--in inConsider the biased parallel plate capacitor as a spring system; force Consider the biased parallel-plate capacitor as a spring system; force balanced by the electrostatic force and the mechanical structureFxk k F F = = 2VFmechx ( ) x xk k F F mech elec mech elec+= =0kelec/kmechV0 x0FelecC A x C c ) ( c( ) ( ) x x x x x + =+ =c 020) (( )22221 ) (21x xAVVxx CFelec+ =cc = cP ll i h( )02 2 x x x + c( )202) (x xCVxx Fk elecelec+=cc=Pull-in occurs when kelec > kmech x = -x0/3determines maximum V0Tsung Tsung- -Hsien Lin Hsien Lin 33x k F mech mech =Actuating Comb Structure Actuating Comb StructureForce is independent of Force is independent of displacement Wide range of motion Issues: Side-wall stability Pull-in Levitation Ground plane to prevent stick down Asymmetrical field lines result in vertical force vertical force Tsung Tsung- -Hsien Lin Hsien Lin 34Levitation Control Levitation ControlTsung Tsung- -Hsien Lin Hsien Lin 35Capacitive Sensor Interface Capacitive Sensor InterfaceOpen loop topologies: Open-loop topologies: Capacitive divider Transimpedance amplifierCh lifi Charge amplifier Closed-loop topology Force rebalancing methodTsung Tsung- -Hsien Lin Hsien Lin 36Capacitive Divider Topology Capacitive Divider Topology The sensing capacitor is used in an impedance divider network The sensing capacitor is used in an impedance divider network Time-varying reference signal of magnitude Vrefapplied to one node of the sensing capacitance Cs The other node (output) connected to the reference capacitance C The other node (output) connected to the reference capacitance Cref The amplifier stage design should minimize the parasitic resistive and capacitive loadings Output signal is amplified and demodulated with the reference signal VfOutput signal is amplified and demodulated with the reference signal Vref Reference capacitance often formed by a second sensing capacitor for better linearityAv VrefCsDemodCrefVrefTsung Tsung- -Hsien Lin Hsien Lin 37Alternative AlternativeReadout circuit Readout circuit00 0 00; 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