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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