wafer to wafer bonding

TInvitedPaperWafer-to-waferbondingpsesarecatego-rizedintodirectbonds,anodicbonds,sesanKeywords—bonding,Anodicsiliconbonding,fusionbonding,MEMS,wafermicromachining,UCTIONMicromachiningencompassesabroadrangeoftechnolo-gifractionofthemicromachiningtechnologiesarespecifictothesiliconmaterialsystemprincipallyduetotheoriginsofthefield,namely,thesiliconintegratedcircuit(IC)iliconmicro-machiningfield,therehavebeentwodominantfabricationmethods,broadlyclassifiedasbulkmicromachining(etch-ingdeepfeaturesintoawafer)andsurfacemicromachining(depositing,patterning,andselectiveetchingoffilmsonawafer).Fundamentally,emen-tary,additive,micromachiningtechnologyisthePROCEEDINGSwafer-to-waferbondingofpatternedsubstrates,ically,someoftheearliafer-to-waferbondswereperformedatlowtemperatures(lessthan450OFTHEIEEE,VOL.86

olapressuresensorusingglassfritwaferbondingforpackaging[9].lowerwaferisetchedtoformaninletport,andthustheroleofthelowerwaferbondrevertsfromthatofavacuumsealtoafiercase,isesanimportantpoint,namely,adilyacknowledged,partic-ularlyinthesensorsindustry,thatpackagingcanbeadominantcomponentinthefievingpackagefunctionatthewaferlevel,itispossibletorealizetremendousoverallsavingsincostsincethisenablesthepackagingofamultitudeofsensorsoractuatorssimultaneously,rexampleoftheuseofwaferbondinginthemanufactureofpressuresensorsisshowninFig.2[10].Here,thepressuresensorisformedbyahigh-temperaturesilicondirectbond,arisonwiththeconventionalbulkmicromachinedpressuresensorofFig.1,thispressuresenerbondingproducesareferencecavityfortheabsolutepressuresensorasinthepreviouscase;however,thecavitydepthcanbemadeconsiderablysmaller(1–10500200

(a)(b)onwafer-bondedpressuresensor.(a)Diephoto.(CourtesyofLucasNovaSensor[10].)(b)RIESOFWAFER-TO-WAFERBONDINGThetypesofwaferbondingthataremostcommonlyemployedinmicrostructurefabricationcanbeplacedinthreecategories:BondsWafersaredirectlycontactedwithouttheassistanceofsignificantpressureoranyintermediatelayersorfiondingschemesrelyonthetendencyforsmoothsurfacestoadhere,andalwaysutilizesomeT:WAFER-TO-WAFERBONDINGThemaximumtemperatureoftheannealdefinesthebond-ingasbeinglowtemperature(distyediate-LayerBondsThiscategoryincludesallbondinuldincludeeutecticbonds,polymers,solders,thesewafer-levelbondsareascale-upofbondsthathavebeenroutinelyperformedatthedielevelinthepackagingofIC’oundThedirect-bondmethodreliesonforcesthatnaturallyattractsurfacestogetherwhentheyareverysmoothandflofmechallknownthatsmoothmetalsurfaces,ifatomicallyclean,ocessisoftenreferredtoas“coldwelding”andistypicallyachievedbyclendusuallyreaseofmostdirectbondsthathavebeenperformedformicrostructures,,hydration,oxygenplasmaexposure)isconductedprisometimesassistedbyamodedisusuallyfollowedbyathermalcycle,ollowingsections,wewillbeginwithadescriptionofthesilicondirectbondingprocess,discusscharacterizationmethodsforthesiliconbondthatgeneralizetoallbondingmethods,nBondingProcessExtensivereviewarticleshavebeenwrittenonthesiliconwafer-bondingprocess,particularlyasitpertainstoelec-tronicdevicefabrication[2],[4],[20]–[22].c-tionwillsimplysummarizethemajorpiconwafer-bondingprocessconsistsofthreebasicsteps:surfacepreparation,contacting,rtingwafersmustbesmoothandflavebeenstudies1577

accelerometerusingwaferbonding[12].(CourtesyofLucasNovaSensor.)annelsformedbysilicon–ecessarysurfacequalityforwaferbonding[23],[24],andingeneral,ithasbeenexperimentallyobservedthatthewafersshouldhavearoughnessofnogreaterthan˚andabowoflessthan5wafer).about10AAlso,protrusionsfromthesurface(resultingfromprevious˚canproduceproblemsinprocessing)heprocessstepsareconductedinacleanroomenvironment,althoughG¨oselehasproposedapowerful“microcleanroom”conceptthatdoesnotrequireacleanroom[25].Thesurface-preparationstepinvolvescleaningthemirror-smooth,flavebeenstudiesofthedifferencesbetweenhydrophobicandhydrophilicsurfacesonthefinalbondinterface[22].Followingthispreparation,thewafersarecontactedinacleanenvfacescomeintocontactattctwaveisinitiatedatthispointandsweepsacrossthewafersurfaces,tactingprocessctoriginoftheattractiveforcethatpromotesthecontactwaveisnotuniversallyagreedupon[22]anddependstoacertainextentonwhetherthebondisSi–SiorSi–finalstepinthebondingprocessisanelevatedtem-peratureannealofthecontactedpairattemperaturesany-wherefromroomtemperatureto1200C).Measurementsofthebondstrefirstregion,forannealtemperatureslessthan300C,esumedthatanSi–O–Sibrieraturesgreaterthan800Corgreater,thcally,thebondstrengthathightemperatures(

erallylumpedintotwocategories:rinsicvoidsarethosecreatedbyparticles,protru-sionsonthewafersurface,oidsareusuallyobservedoncontactanddonotchangesignifi.6showsinfrared(IR)transmissionimagesofwaferswithvariousformsofextrinsicvoids(theimagingmethodsaredescribedinthenextsection).Intrinsicvoidsarevoidsthataregeneratedduringtheannealcycle[4].Aftercontact,r,astheannealtemperatureisincreased,dsusuallyareonlyseenwhenbond-ingsilicontosiliconwithoutanintermediateoxide,andtheyareoftenattributedtohydrocarboncontamination,althoughthereisnotaconsensusontheirorigin.G¨oselehasprovidedsomedetaileddiscussionofthisissue[67].Ithasbeenobservedthatcavitiesinthewaferscanserveto“getter”thesemicrovoids,thusminimizingtheproblem[27],[28].aracterizationSeveralnondestructiveanddestructivesection,wewilldiscussthemethodsasappliedtosilicondirectbonding,btcommontechniquesarebondimaging,cross-sectionalanalysis,gingmethodsarenondestructiveandcanbeusedasin-processmonitors,whilethecross-sectionalanalysisandbond-strengthmeasueedominantmethodsforimagingabondedpairofsiliconwafersareinfraredtransmission,ultrasonic,esoftheimagesobtaiifiistsofanIRsource(typicallyanincandescentlightbulb)oncharge-coupleddevicecamerahassufficientsensitivityinthenear-IRrangethatitcanbeusedwhenoutfittedwithafinbondedregions(“voids”)appearwithacharacteristic“Newton’sRings”agingmethodgenerallycannotimagevoidswithasepanatypicalparticlevoid,thistranslatestoaspatialresolutionofseveralmillimeters[24].Fig.8,whichcomparesthethreeimagingmethodsusingthesamewafer,clearlyillustratesvoidsnotpresentintheIRimage,,thistechnidopedlayers,IRabsorbingfilms,orroughsurfaces(backsideofwafer)eofthisresolutionlimit,theIRmethodhastheadvantageSCHMIDT:rontherighgsimple,fast,ertwoimagingmethodsofferhigherresolutionattheexpenseofspeed,cost,-sectionngelectronmicro-scope(SEM)andtransmissionelectronmicroscopetech-ntudieshavehelpedtounderstandthecompositionofthebondedinterface[22].Additionally,itispossibletogainagreatdealofinforma-tionaboutlgroupshavedemonstrated1579

(a)(b)(c)isonofthreemethodsforvisualizationofthesamebondedwaferpair.(a)X-raytopograph.(b)Ultrasonic.(c)IRtransmission.(,SEH-Japan.)thebenefitofthisapproach,particularlyforvisualizationofvoidsontheorderoftensofmicrometers(“microvoids”)[29].rebursttests[Fig.9(a)]canoftenyieldanumberthathasengineeringsignificanceinthedesignofsensorsbutyieldslittleinformationaboutthele/sheartestsample[Fig.9(b)]givesbetterinformationonthebondedinterfacebutisoftenlimitedbydifficulties1580(a)(b)(c)ingandsamplehandling[30].Theknife-edge,ordoublecantilever,technique[Fig.9(c)]hastheadvantageofcreatingaverywell-defiofdefinedthicknessisiRimagingmethods,thelengthofthecrackismeasured,fromwhichthesurfaceenergycanbeinferredthroughaknowledgeofthesampleandbladethicknessesandtheelasticpropertiesofthewafer[31].eennoted,however,thatthecracklengthistime(andhumidity)dependent,andthuscautionmustbeexercisedinconductingthismeasurement[67].Unfortunately,thesurfaceenergyisfourthpowerdependentonthecracklength,andthusuncertaintiesintheethodsbasedonpatternedsampleshavebeenproposedtoeliminatethisproblemforsilicondirectbonding[32]andsilicon–glassanodicbonding[33].CavityEffectsatHighTemperatureNearlyallmicromechanicalapplicationsofsiliconwaferbondingrequirebondingofwaferswithcavitiesetchedinoneortheotherwafer,ureofgasesthatexistinthecavitiescanbeveryimportant,eenshownthatwhenwafersarecontactedinair,andsubsequentlyannealedathightemperature,theoxygeninthecavitycanreactwiththesiliconsurfaceandcreateapartialvacuum[34].PROCEEDINGSOFTHEIEEE,VOL.86,NO.8,AUGUST1998

(a)(b)Fig.10.(a)Amethodforintegrationofelectronicsandmechanicaldevicesusingwaferbonding.(b)SEMmicrographofatestchipcontainingaccelerometers,pressuresensors,andcomplementarymetal–oxide–semiconductorintegratedcircuitsbythisprocess.(swaran.)Whentheoxygeniscompletelyconsumed(forshallowcavities),theresultantpressureinsidethecavityis0.8atm,consistentwiththeconsumptionofthe20%esultsindicatethatthebightemperatures,theresidualgasestrappedinthecavitiescaninduceplasticdeformationinthinsiliconmembranesasthegasesexpand[34].ThisproblemcanbereducedoreliminatedbycontrollingtheambientunderSCHMIDT:demonstratedthatthepressureinsidethecavitycanbereducedbybondingthewafersinanoxygen-richambient[35].Alternatively,rollingthebondingambient,awiderangeofmi-crostructurescampleisthefor-neralprocesshasbeensuc-1581

cessfullydemonstratedbyanumberofgroups[36]–[38].Recently,themergerofdeepreactiveionetchingandsealedcavitywaferbondinghasopenedanexcitingareaofmicroelectromechanicalsystemsdevices[39].gofNonsiliconMaterialsWhiletheprevioussectionshavediscussedbondingofsiliconwafers,thesamebasicprocessstepscanbeappliedtobondingavarietyofmaterials[67].Quartzwaferscanbebondedbythismethod[14].ExamplesofbondingofdissimilarmaterialsincludethebondingofGaAstoSi[40]andSitoglass[41].Providedthesurfacesaremirrorsmoothandcanbehydrated,thebondingproceedsinafashionidenticaltoSi–gofIII–Vcompoundshasbeenachievedattemperaturesof500Cwhileavoltageofapproximately200–uchahighfieldandelevatedtemperature,themobileNaionsintheglassmigrateawayfromthebondedinterface,leavingbehindfixedchargeintheglassthatcreatesahighelectricfietemperaturesandelectricfields,thesameconditionsforcontactingfromsilicondirectbondingapplytothisprocess,althoughtheprocessismoretolerantofsurfaceroughnessduetothehighforcesofattractioncreatedbytheelectricfimaryvariablesthatcontrolthisprocessaretemperature,time,ermalcoefficientofexpansionfilglassfabricatorsnowmarketglassesthatarespecifihodtominimizethethermalstresseffectsistoutilizeasputteredglasslayeronasiliconwafer,suchthatthesubstratesbeingbondedarebothsilicon,andthusthethermalmismatchwouldonlybecausedbythethinglass[1],[59].Thesiliconwaferwithsputteredglassisthuscontactedtoabaresiliconwafer,itionalbenefitofthesputteredglasslayerapproachisthattheappliedvoltagemaybereduced,whichissometimescrittiontotheexamplescitedintheearliersectionforapplicationsofthisbondingapproach,theanodicbondhasbeencriticaltothedissolvedwaferprocesspioneeredattheUniversityofMichigan[60].Thedissolvedwaferprocesshasbeenusedsuccessfullyforawiderangeofdevices,suchaspressuresensors,flowsensors,EDIATE-LAYERBONDINGAwiderangeofintermediatelayerspproachesinclude:•eutectic(usingAuthinfilms)[1],[61];•solder(usingthin-filmdepositedsoldersorpreforms)[1],[62];•polymer[63];•lowmeltingtemperatureglasses(includingglassfrits)[1],[4];•thermocompression(usingsoftmetalthinfilms)[35],[64],[65].Theeutecticbondwasexpliftmechanismswereidentifiedinpressuresensorsthatwereattributedtothisbond[1].Solderbondinghasbeenemployedsuccessfullyforseveraldifferentapplications,includingrecentlyanovelschemeformicroshellencap-sulation[62].Polymerbondshavebeenusedextensivelyastemporarybondingagents,asin[63].Glasseswithalowmeltingtemperatureareademonstratedproduction-worthybondforpressuresensorsinwhichtheglassfritistypicallyscreenprinted[1].PROCEEDINGSOFTHEIEEE,VOL.86,NO.8,AUGUST1998

Thethermocompressionbondisarelativelylittle-usedwafer-levelbondingmechanismthatisintriguingforlow-temperaturevacuumbondingapplicationssinceitinprin-ciplewouldhaveverylittleoutgassing,ascomparedtoglasses,solders,cessisanalogoustothecoldweldingprocess,buttemperature,pressure,andlowhardingprocessstartswiththedepositionofapproximately1-C[35],[64].Thebondhasbeely,ametalstructuretransferprocesshasbeendemonstratedthatutilizesasimilarprocess[65].NGESANDFUTUREDIRECTIONSAgreatdealofprogresshasbeenmadeinrecenttimestoadvancewafer-to-waferbondescribedinthebeginningofthispaper,theprimarydriverforconsiderationofwafer-to-wentialbenefitofthisismostsignifirbenefitofwafer-to-waferbondingisintherealizationofamechanicalstructurethatisotherwiseunattainable,suchassomeofthehighlizethepotentialofwafer-to-waferbondedstructuresfully,,ahealthyinfrastructureofequipmentvendorsthatcansupplywaftly,therearetwocompaniesthatofferhigh-qualitywafer-bondingprocesstools(KarlSussandElectronicVisions).Typically,thesetoolspermitalignmentofwaferstowithinafewmi-crometersandcontactingandannealingunderahostofconditions:pressure,temperature,andcontrolledambient(Fig.12).Standardizationofprocessesforthesetools,andcharacterizationofinteractionsbetweentheprocessandthetools,areneededtodevelopconfiry-standardbedareaoffocusisinthedevelopmentofrobustcriteriafor“bondability.”Thereexistsacertainamountofconflictinginformationinthisconfusionresultsfromdifferentmeasurementstandardsforbonding(anddifferentrequirements).Astandardizedsetofmetrologymethodswouldgoalongwaytowardpermittingrationalcomparisonofprocessesforwafer-to-SCHMIDT:rcialwafer-bondingtool,whichincludesachamberforcontrolledambientbondingwithtemperatureandpressure.(CourtesyofElectronicVisionsCo.)ample,-tunately,manyofthesemethodsarestronglyinfluencedbyhandlingandmountingprocedures,thuspreventingstatisticallysignifiareaofworkisthecharacterizationofstressesinbondedstructures,andthelong-ffectsareofprimaryimportanceinhigh-performanceapplica-tions,andlimitedworkhasbeendonetohaocementofthermalstressesandoptimiz,thereisagreatopportunityforwafer-to-waferbondingthepotentialmarketforinertialdevices,arobustmeanstopackagetheseinvacuumwhenneederkhasbeendoneinthisareaalready,butchallengesexisttodrivethevacuumlevelsto1mTorrandbelowforlonglifetimes(tenyears).Esashiandcoworkershaverecentlyshownsomeinterestingresultsbyutilizingnonevaporablegetters[66].Thisapproachopensnewchallengesinbondingprocesses,getterfabricationandmaterials,SIONThewafer-to-waferbondingprocessesthathavebeendevelopedformicrostructurefabricationhaveaverysig-nififer-to-waferbondingprocessesarealreadywell-establishedproductionprocesses,andothers1583

tsignifingesremaintoconvertsomeofthenew,LEDGMENTManyoftheinsightsintowaferbondingexpressedinthispaperhavebeentheresultofsignificantresearchinteractionswiththestudentsandstaffatMITthathavebeeninvolvedinthewafer-bondingresearchprogramsthere,aswellastechnicalinteractionswiththestaff’onally,theauthorwishestothankDr.U.G¨MITforherassistNCES[1],o,,“Bondingtechniquesformicrosensors,”inMicromachiningandMicropackagingofTransducers,,,,g,dam:TheNetherlands:Elsevier,1985,[2]a,“Silicon-on-insulatorbywaferbonding:A[3]review,”org,ochem.“SiliconwaferSoc.,g138,techniquesno.1,p.341,omechanicalelements,”rkshop,Nara,[4]Japan,,1991,Ed.,,MA:[5]ArtechProc.1stHouse,.1994,nductor5/onding:Science,[6]Technology,ations,SemiconductorPhoenix,WaferAZ,Bonding:e,[7]Technology,ations,SemiconductorHawaii,g:Science,[8]Technology,ations,SemiconductorReno,NV,g:Science,[9]Technology,,ations,ey,Paris,,,gton,“Compensationandcalibrationofamonolithicfourterminalsiliconpressuretransducer,”lid-StateSensorWorkshop,HiltonHead,SC,June1984,pp.[10]21–en,,k,,,“Siliconfusionbondingforpressuresensors,”lid-StateSensorandActuatorWorkshop,HiltonHead,[11]SC,,1988,p.“achinedpressuretransducers,”Sen-[12],Actuators,madi,vol.28,p.R.133,Mayer,k,en,“Amonolithicsiliconaccelerometerwithintegralairdampingandover-rangeprotection,”lid-State[13],andActuator“AchronologyWorkshop,ofHiltonthermalHead,inkjetSC,structures,”1988,-StateSensorandActuatorWorkshop,Hilton[14]Head,,SC,1996,ia,,“Microfabricatedfusedsilicaflowchambersforflowcytometry,”-StateSensorActuatorWorkshop,HiltonHead,SC,[15]on,,,,,“Mi-cromachinedchemicalandbiochemicalanalysisandreactionssystemsonglasssubstrates,”SensorsActuators,vol.B33,pp.[16]105–109,,nstein,jerg,“MicrochipCoulterparticlecounter,”-StateSensorsandActuators,Chicago,IL,1997,p.1319.1584[17]t,“Fabrication,packaging,andtestingofawafer-bondedmicrovalve,”lid-StateSensorandActuatorWorkshop,HiltonHead,SC,[18]1992,,,,d,T.C.O’Brien,n,to,ell,,t,“Microsystemtechnologyforchemicalandbiologicalmicroreactors,”DECHEMAMonographs,vol.132,no.51,[19]n,ia,suresh,,,K.-,,,i,nry,on,,C.-,,a,,,,t,y,ng,,Y.-,,“PowerMEMSandmicroengines,”.[20],erings,Actuators,U.K.1997,nn,,“Silicon-on-insulatorwaferbonding-waferthinningtechnologicalevaluations,”.,vol.28,no.8,[21]p.[22],1426,son,Ed.,1989.“.,1995,g:49,nos.A1/ofinterfacialpropertiesandapplications,”als,[23],21,,8,p.841,,1992.“SiliconwaferbondingprocesstechnologyforSOIstructures,”n-[24]a,TechnologyB.-,s,Yamada,1990,yi,rt,,“Roleofsurfacemorphologyinwaferbonding,”.,vol.69,no.1,p.257,Jan.[25],K.-,andU.G¨osele,“Bubble-freesiliconwaferbondinginanoncleanroomenvironment,”.,[26]vol.Q.-Y.27,Tong,no.12,,L2364,fiteanu,andU.G¨osele,“Lowtemperaturewaferdirectbonding,”.,[27]ger3,no.1,29,ger,1994.“Microstrucuturesforperfectwaferbondingindifferenttemperatureranges,”nductorWaferBonding:Science,Technology,[28]andApplications,,“V