专英2.docx
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专英2.docx
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专英2
Microelectromechanicalsystems(MEMS)isatechnologydevelopedfromthe
integratedcircuit(IC)industrytocreateminiaturesensorsandactuators.Originally,thesesemiconductor
processesandmaterialswereusedto
buildelectricalandmechanicalsystems
buthavenowexpandedtoincludebiological,optical,fluidic,magnetic,and
othersystemsaswell.MEMS,theterm
originatedintheUnitedStates,typically
containmovingordeformableobjects.
Engineersinotherpartsoftheworldmay
refertothesedevicesasmicrosystems
technologyormicrostructurestechnology
(MST).MEMSmergethefunctionsof
sensingandactuatingwithcomputation
andcommunicationtolocallycontrol
physicalparametersatthemicroscale
level.Figure1showsthemaincomponentsofMEMS[1].Inadditiontoreducingsizeandmass,otheradvantagesof
MEMS-basedsystemsincludelowcost
throughmassproduction.
Inordertounderstandthedesignof
MEMSdevices,areviewoffabricationtechnologies,materials,and
powersuppliesisprovided.
FabricationofMEMScomprises
stepssimilartothoseusedbysemiconductormanufacturingprocesses
andfallsintothreecategories:
➤surfacemicromachining
➤bulkmicromachining
➤moldingprocess.
Surfacemicromachininginvolves
thebuildupofmicromechanical
structuresonthesurfaceofasubstratebydeposition,patterning,and
etchingprocesses.Oneoftheprocessingstepsinvolvestheselective
removalofanunderlyingfilm
referredtoasasacrificiallayer,
withoutattackingtheoverlyingfilm,
referredtoasthestructurallayer.
Figure2illustratesthetypicalsurfacemicromachiningprocess[1].
Asanexample,surfacemicromachiningcanbeusedtomanufacture
inertialsensors—primarilydevices
formeasuringlinearandrotational
acceleration[2].Figure3illustrates
thestructureofaMEMSrotational
accelerometerdevicethatcanbeused
inharddiskdrivestocompensatefor
vibrationsorinvehiclestabilitycontrolsystemstodetectunwantedyaw
axismovements.
Themicromachinedstructureconsistsofa“rotor”freetomoveasmall
fractionofadegreewithinastator
structure.Whenthedeviceisrotated,
therotortendstoremainwhereitwas
soitmovesrelativetothestator,
changingthecapacitanceofthe
rotor/statorcombination.Asensitive
capacitancemeasuringcircuittranslatedthecapacitanceintoadigitaloutput
signal.InertialMEMSdevices,such
astheonepresentedinFigure3,are
typicallymanufacturedbythesame
basicprocesshighlightedinFigure2.
Generallytheprocessbeginswiththe
growthofalayerofsilicondioxideon
thesiliconwafer.Thislayeriscalled
sacrificialbecauselateritwillbe
mostlyremovedtofreethemoving
parts.Intothesacrificiallayerholes
areetchedatpointscorrespondingto
thesupportsforfixedelementsand
anchorsformobileelements.A
thickerepitaxialpolysiliconlayer
isgrownontopofthisandinto
thislayertheformofthefixed
andmovingelementsisetched.
Finallythesacrificialoxidelayer
beneaththestructuresisremoved
byanisotropicetchingoperation
tofreethemovingparts.Theopen
spacearoundtheMEMSstructuresisfilledwithgas,usuallydry
nitrogen,toavoidthestiction
effectsduetohumidityorvariationsingasdensitythatwould
affecttheresonancefrequencies.
Thestepsaboveareshownin
moredetailinFigure4[2].
Comparedtorivaltechnologies
likepiezoelectricmaterials,siliconMEMSsensorsachievemuch
greatersensitivityanduniformity
ofperformancewhileatthesametimetheycanbemanufacturedatvery
competitivecost,inlargequantities.
Asaconsequenceitislikelythat
MEMSaccelerometerswillbeadoptedmuchmorewidelyinmedical
devicesthatdetectthepatients’level
ofactivitytoeffectivelycontroltheir
heartrate,suchascardiacpacemakers
andimplantablecardioverterdefibrillators(ICDs).Bulkmicromachining
useswetoradryplasmaprocessesto
etchintothesubstratetoproduce
MEMSstructures.Theetchingcanbe
isotropicoranisotropic.Byexploiting
thepredictableanisotropicetching
characteristicsofsingle-crystalsilicon,manyhigh-precisioncomplex
three-dimensionalshapes,suchasVgrooves,channels,andnozzles,canbe
formed.Figure5isanexampleof
bulkmicromachiningalongcrystallographicplanes.Deepreactiveion
etchingisaplasmaprocessthatis
usedincreasinglytomakeMEMS,
withstructuresthatareovertentimes
asdeepastheyarewide.Thisisan
importantconsiderationinMEMS,
wherehighermechanicalpowerof
forcelevelsisdesiredorinapplicationsinvolvingfluidssuchasnozzles.
Thethirdfabricationprocessusedin
thecreationofthemechanicalelements
ofthedeviceisthedepositionofmaterialintomicrofabricatedmolds.The
mostwidespreaduseofthisprocessis
theLIGA(Germanacronym—lithography,galvanoforming,molding).It
involvesX-raylithographyformask
exposure;galvanoformingtoform
metallicparts;andmoldingtoproduce
micropartswithplastic,metal,ceramics,andtheircombination.Laserinducedetchinganddepositionof
materialsaswellasultrasonicandelectrondischargemillingareotheralternativetechnologiesthatcanbeusedas
partofthisprocess[1].
Theprincipalmaterialsusedin
MEMSmanufacturingincludedoped
singlecrystalsiliconwafersasthe
semiconductorsubstrateanddeposited
layersofpolycrystallinesiliconfor
resistiveelements;aluminum(orcopperorgold)astheprincipalconductor;
andsiliconoxide,siliconnitride,and
titaniumnitrideforelectricalisolation
andprotection.Recently,newmaterialshavebeendeveloped:
theshape
memoryalloysthatareusedforactuatorsareoneexample.Piezolectric
materialshavebecomeveryusefulin
MEMSdevicesbecauseoftheirelectrical-mechanicalreciprocity.Piezoelectricmaterialsarecapableofvery
highenergyandpowerdensitiesat
microscales.Thehighfrequencyof
operationinherentinMEMSdevices
matcheswellwiththerelativelyhighfrequencycapabilityofpiezoelectric
materials.Themostcommonlyusedpiezo-materialsinMEMSdevicesare
leadzirconatetitanate(PZT),zinc
oxide(ZnO),andaluminumnitride
(AlN).Recentadvancementsinenvironmentalmonitoring,especiallyin
theareaofchemicalandbiological
sensors,havegivenrisetonewmaterialsapplicationsinMEMSdesign.
Biocompatibilitytestingisattheforefrontofevaluatingnewmaterialsfor
MEMSapplicationsinmedicine.
Voskericianetal.studiedbiocompatibilityofmaterialsusedforMEMS
drugdeliverydevices[3].Theyfound
thatgold,siliconnitride,silicondioxide,SU-8photoresist,andsiliconwere
biocompatible.Gold,siliconnitride,
silicondioxide,andSU-8photoresist
alsoshowedreducedbiofouling.
Theneedtoprovideenergytoeffect
sensingandactuationcallsfortheintegratedpowersupplyintotheMEMS
device.Theapplicationofembedded
microsensorsentailsburyingthemin
thestructureswithnophysicalconnectiontotheoutsideworld.Insuchcases,
electricpowercanbeobtainedfromthe
environmentbyextractingenergyfrommechanicalmotionandvibrationby
usingpiezoelectricmaterials;air/liquid
flowbyusingaminiatureairturbine
generator;temperaturegradientsbyusingthermopiles;pHgradientby
usingchemicalelectrodes;andparticle
radiationbyusingp-njunctionorother
converters[4].EfficientMEMSpowersuppliesshouldhavealowrecurring
cost,ausableservicelifethatcommensurateswiththeinstrumentedstructure,
andtheabilitytoaccommodateavaryingnumberofdifferenttypesofsensorsincloseproximity.Effortshave
beendedicatedtodevelopwireless
technologiesthatcancommunicate
withandalsoprovidepowertoMEMSbiodevices[5],[6].Figure6illustrates
thisconcept[6].
Thefrequencyrangeforthedevice
presentedinthisexampleisfrom
200–700MHz,withinadistanceofup
to10cm[6].
Nanogeneratorsrepresentadifferent
andnewconceptofprovidingpowerto
batterylessMEMSdevice[7].Thenanogeneratorsdevelopedatthe
GeorgiaInstituteofTechnology
producecurrentbybendingandthen
releasingzincoxidenanowires—which
arebothpiezoelectricandsemiconducting.Theconceptbehindnanogenerators
isillustratedinFigure7.Arraysofzinc
oxidenanowiresaregrown.Atomicforcemicroscope(AFM)tipsdeflect
individualwires.Asawireiscontacted
anddeflectedbythetip,stretchingon
onesideofthestructureandcompressionontheothersidecreatesacharge
separation—positiveonthestretched
sideandnegativeonthecompressed
side—duetothepiezoelectriceffect.
Thechargesarepreservedinthe
nanowirebecauseaSchottkybarrieris
formedbetweentheAFMtipandthe
nanowire.Thecouplingbetweensemiconductingandpiezoelectricproperties
resultsinthecharginganddischarging
processwhenthetipscansacrossthe
nanowire.Byconvertingmechanical
energyfrombodymovement,muscle
stretching,orwaterflowintoelectricity,thesenanogeneratorscouldmakepossibleanewclassofself-powered
implantablemedicaldevices,sensors,
andportableelectronics.
MEMSApplications
inMedicineandBiology
WhileacomprehensivelistofMEMS
usesinmedicineisbeyondthescopeof
thisarticle,severalmorerecentapplicationsarediscussedbelow.
PressureSensors
MEMStechnologyhasbeenutilizedto
realizeawidevarietyofdifferential,
gauge,andabsolutepressuremicrosensorsbasedondifferenttransduction
principles.Typically,thesensingelementconsistsofaflexiblediaphragm
thatdeformsduetoapressuredifferentialacrossit.Theextentofthe
diaphragmdeformationisconvertedto
arepresentativeelectricalsignal,which
appearsatthesensoroutput
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