What is wafer bonding and how does it apply to MEMS?

Ted Chi [TC]: Wafer bonding is literally sticking two different pieces of material together at the wafer level using different techniques. The two wafers might ... WhatiswaferbondingandhowdoesitapplytoMEMS? Learnthefollowinginthispost WhatiswaferbondingandhowdoesitapplytoMEMS? Wheredoeswaferlevelpackaging(WLP)enterthisstory? Whatistheimportanceofbondlinewidth? WhichtypesofbondsareusedmostoftenatAtomica? Whatfactorsaffectthechoiceofbond? WhatdoesAtomicabringtothetableregardingwaferbonding? Waferbondingplaysanintegralpartinmicroelectromechanicalsystems(MEMS).Tohelpunderstandwaferbondinganditsroleintheelectronicsindustry,wesatdownwithseveralmembersofthe Atomicateamtodiscuss:EricSigleristheCEOofAtomica,Dr.BrianMaertzisAtomica’sCTO,Dr.StuartHutchinsonisourDirectorofFieldApplicationsEngineering,andTheodoreChiisourDirectorofGlobalSales.(Note:thisinterviewhasbeencondensedandeditedforclarity.) WhatiswaferbondingandhowdoesitapplytoMEMS? TedChi[TC]:Waferbondingisliterallystickingtwodifferentpiecesofmaterialtogetheratthewaferlevelusingdifferenttechniques.Thetwowafersmightbeseparatedbyaninsulatorlikesilicondioxide,oroneofthewafersmayhaveacavityandtheotherwaferhassomestructurethatyouwanttoprotectunderatmosphereorvacuumbybondingthewaferstogether. Semiconductordevicesaresolid-statedevices.They’reallelectroniccircuits.Therearenomovingparts;Thereisnorequirementforpackagingperse.MEMScanbecomposedofactivemovingdevices.ConsideraMEMSrelay–aonemillimeterbyonemillimeterdevicewithaswitchstructurethatmovesupanddown.Ifitisopentotheenvironment,anykindofparticlecouldgetinandthatwillmakethedeviceuselessaftermaybeafewdays.Whenyouprotectthatstructurehermetically,thenthereliabilityofthedevicewilllast20years,forexample.Inotherapplications,theactualdevicedoesn’tfunctionorperformunlessit’sinanenvironmentlikeavacuum. Wheredoeswaferlevelpackaging(WLP)enterthisstory? BrianMaertz[BM]:Therearetwowaystoachievethathermeticpackagingorvacuumpackaging.Oneistodoitonthedevicelevel–afteryoumakethewafer,youcutupallthepartsandeachdiecanbepackagedseparately.Butthatmeansthateitherapersonorassemblyrobotneedstointeractwithandsealeachindividualdie.Theotherwaytopackageitisonthewaferlevelsoyoucansealtheentirewaferatthesametime.Itcouldbethousandsofdevices,allsealedatthesametime.Yousavetimeandmoney TC:Dielevelpackagingisverysequential,oneatatime.Withwaferlevelpackaging,youcanpackage50,000devicesallatonce.That’sveryimportanttechnologythatwebring. EricSigler[ES]:Historically,thewaypeoplewouldpackagesensorsandothertypesofelectromechanicalcomponentswouldbetoputtheminahermeticcavitylikeagoldTOcanoraceramichermeticpackage.Theabilitytousewaferbondingenablesthismuchsmaller,lowercost,andhigherreliabilitywayofmakingthesedevices.Thesamethingistruewithtimingdevices.Thesearesuchtinydevices,thattheabilitytobondwafersandcreatepackagesatwafer-levelprovidesalotofeconomicandsizeadvantages. BM:Notonlythat,butyouarealsooftenabletotestthedevicesafterthey’vebeenpackagedatthewafer-level.Testingiscommonlyaverylargecontributortothecostofadevice.Ifyou’reabletotestatthewafer-levelandstepacrossandtestmanydevicesveryquickly,that’smuchmorecosteffectivethantestingindividualpackagedparts. ES:That’sagreatpoint.Therewouldbethousandsofdevicesonawafer,butyoucanstillaccessthebondpads.Aprobetipcancomedownrepeatedlyandtouchthesebondpadstoturndevicesonandtestthem.Therearejustmassiveeconomiesofscalebybeingabletotestdeviceswhiletheyarestillintheirwaferforminsteadofsingulatingthemandputtingthemindividuallyonamanualtester. There’sane-bookforthat DiscovermoreaboutWLP,oneofthegreatenablersof3-Dintegration GetyourEbookCopy Whatistheimportanceofbondlinewidth? TC:Thebondlineiswhereyou’recreatingamediumtostickthetwowaferstogether,sortofawindowframeifyouwill.You’realigningthosetwowindowframesagainsteachother. BM:Theimportanceofbondlinewidthisthatit’sunusedspaceforyourdevice.Thelargerthebondlinewidth,thefewerdieyoucangetonawafer.Ifyouhaveaverysmalldevice,thatcanbealargeeffect.Ifyouhaveaverylargedevice,awiderbondlinemightbeOKbecause,relativelyspeaking,itisnottakinguptoomuchspaceonthewafer.Itisalsoacostthinginthedielevelpricingbecausethemoredieyoucangetonawafer,thelessthecostofthedevice. WhichtypesofbondsareusedmostoftenatAtomica? ES:Therearecategoriesatahighlevel.You’vegotatemporarybond,whichissomethingwe’redoingduringthemanufacturingprocessjusttoallowustohandlethewaferorprocessathinwafer,forexample.Andthenyouhavepermanentbondswhicharefunctionalinthefinalproduct. BM:Weusethesesixbondsrelativelyregularly[seethetablebelow].Wehavecurrentprogramsrunningeachofthem.Wedotemporarywaxbondsregularlyaswell.Wehavedoneothertypesofbondsinthepastandwe’veentertaineddoingothersaswell.Butthesesixaretheonesthatweknowthemostaboutandwehavethemostexperiencewith. BondType Hermeticity Vacuum Temp(°C) Difficulty BondLineWidth(μm) BondCost Lowtempmetalalloy Yes High 200 Low 100 $$$ Sifusion Yes High 1050 High - $$ Glassfrit Yes Medium 420-440 Medium 400 $ Anodic Yes High 370 Medium 50+ $$ AuAuthermo-compression Yes High 200-300 High <100 $$$ Polymer No No 200 Medium 200 $$ Lowtempmetalalloy Yes High 200 Low 100 $$$ Alowtempmetalalloybondisaproprietaryonewe’vedevelopedhere.Basically,wehavetwodifferentmetalelementsthatcometogether.Weheatthatduringthebondprocesswhilepressingandtheyformanalloythatismuchstrongerandhasamuchhighermeltingpointthanthebondtemperature.So,webondedaround200C,andthenitcanwithstanduptoabout500Cafterthatwithoutreflowingormoving. Thesiliconfusionbondusuallyinvolvessilicontosiliconormorefrequently,silicontosilicondioxide.Youhavetwosurfacesthatareextremelyflatandextremelylowroughness,youbringthemintocontactwitheachother,andthenyouannealthematahightemp.Theyfuseintoonematerial. Theglassfritbondisapasteofsortsthathasglassembeddedinitaswellassomesolventsandvariousotherchemistries.Youscreen-printthatontothewafer(justlikeyouwouldascreen-printedT-shirt)intothebondlineconfiguration.Youbringthewaferstogetherandheatthem.Thatdrivesoutallthesolventsandturnsitintoamuchmoresolidglassfromthatpastethatyouoriginallystartedwith. Theanodicbondinvolvesasiliconwaferandaspecialkindofglasswafer(borofloat)thathasalotofsaltions.Youbringthewaferstogetherandapplyaveryhighvoltageacrossthosetwowafers.Yougetanionmigrationacrosstheinterfaceandthewafersfusetogether.It’saveryhigh-strengthbondthatcanbedonerelativelyeasily–averygoodbondwhenyoucanallowforoneofthewaferstobeglass. Alotofpeopledogold-gold(Au-Au)thermocompressionbonds.Oneofthechallengesisthatyouhavetoapplyalargeamountofforcetogetthatbondtoworkproperly.Youalsohavetoknowhowtopreparethesurfaceproperly.Wehaveourownproprietarywayofmakingagold-goldthermocompressionbondworkbetterthanjustthekindofstandardapproach.Thebondisgoodbutitdoesaddsomecomplexity,whichiswhythat’soneofthehighercostbonds(alsobecauseit’sgoldwhichisarelativelyexpensivematerial). Withapolymerbond,you’reusingsomekindofpolymer,likeaglue.Youpatternthatpolymerintotheshapeofthebondlinethatyouwanttouse,pressthewaferstogether,anddriveoutthesolventsfromthatgluetosolidifythepolymer. Evenwithinsomeofthesebonds,therearedifferenttypes.Forinstance,therearemanytypesofpolymers.Wehaveexpertiseinchoosingwhichpolymeristhebesttouseandhowyouwouldapplythatpolymertothewafer. Doyouhaveaprojectthatrequiresbonding? SpeaktooneofourtechnicalexpertstoseehowAtomicacanhelp. GetinTouch Whatfactorsaffectthechoiceofbond? TC:Thattableshowssixdifferentbondingtechnologies.Somecanbehermeticbondsorcanholdavacuum.Itshowsbondingtemperature.Theseareveryimportantpiecesofinformationtohavewhenyouhavewaferlevelpackagingapplications.Forexample,microbolometerinfraredcamerasensors–theyhaveacertainmaterialsetthatcan’tbeexposedtotemperaturesabove200°C.Inthatcase,thelowtemperaturemetalalloybondwouldbethecandidatetoexplore.So,thetypeofbonddependsontheendapplicationandmaterialset.Costisaconsiderationaswell. BM:Topographyonthewafersurfaceisalsoafactortoconsider(suchasbondpadscomingoutunderneaththebondline).Somethinglikeasiliconfusionbondisnotgoingtobeabletoovercomeanytopographyonthebondingsurface.Youcanhavezero;itwillnotworkifyouhaveanythingmore.Butapolymerbondoraglassfritbondcanovercomesometopography.Wealsohaveourownproprietarybonds,suchasthelowtemperaturemetalalloybond,whichallowsustoovercomesomeofthattopographywhilestillmaintainingaveryhighvacuum,relativelynarrowbondwidth,andalowtemperature. Wehaveexperiencedevelopingotherbonds.We’veactuallycomeupwithsomebondsthatallowforbothcompliance(withtopography)andrigidity(foraverygood,strongbond)atthesametimeforsomespecificbiologicalapplicationswherethatisnecessary. There’ssomeadditionalcomplexitytosomeofthebondsorconsiderationsthatyouneedtomake.Forexample,forthesiliconfusionbond,youhavetokeepthesurfaceincrediblypristineforthattofunctionproperly.Thatcanpresentanintegrationchallenge,dependingonwhatthedeviceis,tomakesurethatallthestepsbeforethebondleavethesurfaceofthewaferpristine.Therearesomecaseswhereit’srelativelyeasytodobecausemaybeyou’renotdoingtoomuchprocessingtothatsurface.Butincaseswhereyou’remakingacomplicateddeviceanddoingalotofprocessingtothatsurfaceandthenyouhavetoachievethesiliconfusionbond,youjusthavetobecarefulandensurethatyouhaveenoughsurfaceareaforthatbondtoperformwell. TC:Brianmentionedagoodpoint.Itdependsonwhenyouareperformingthebond.Whenthewaferhasgonethroughlithographysteps,metaldeposition,andanyprocesses,thatsurfaceisnotgoingtobepristine.It’sgoingtobehardtokeepitclean. ES:Ithinkit’simportantforpeopletounderstandthateverysituationbringsadifferentsetofconstraints.Sometimesyou’vegotathermalbudgetthatyouhavetostaywithinbecausethere’ssomestructureonthedevicethatcan’tgoaboveacertaintemperature.Sometimesyou’remakingatemporarybondandyouneedtobeabletoremoveiteasilyduringsubsequentprocessing.Sometimesyouaresuperconcernedaboutthediesizeandthereforethecostofthedeviceandsoyoucan’taffordathickbondline.Andsometimesyoumightbeconcernedabouthowfragilethedeviceissoyoucan’tapplyalotofpressure.Youhavetoweighallofthesedifferentconstraintsonthedesignconsiderationstochoosewhat’sthebestbondforaparticularsituation. WhatdoesAtomicabringtothetableregardingwaferbonding? BM:IthinkEric’scommentisagreatsegue–we’veseenallthesedifferentbondsandwe’veseenwhatworksinvarioussituations.Wecanhelpwalkthecustomertonotjustthebestapproachfortheirapplicationbutalsothelowestcostoptionfortheirdevice. ES:Iwouldsaythatmaybehalfofourprograms,maybemore,requiresometypeofbond,eitherafinalbondoratemporarybondduringfabrication.Anytimeyou’remakingsensorsorphotonicsorbiochips,you’regoingtoneedthisexpertisetomakeyourfinalproduct.Whatareotherpeoplethinkingintermsofthepercentofourprograms? StuartHutchinson:Yes,I’dsayatleasthalfifyou’regoingtoincludetemporarybondingaswell.Wouldn’tyousayso,Brian? Atomica’s8inch(200mm)BondingLine BM:Yeah,definitely.Wealsohavesomeproprietaryapproachesthataren’tavailableanywhereelsethatallowforsomeuniquebenefits.Ourproprietarybondsarepatented;theyarenotavailableanywhereelseandprovideauniquesolution. ES:CMOSfoundries,whicharemakingtransistorsinveryhighvolumes,don’tusuallyneedtodobonding;it’sgenerallynotacorecompetencyforthem.Similarly,thesebondsoftenrequirematerialsthatarenotCMOScompatible.Thatlimitsthenumberoffacilitieswhereyouwouldcustomarilymakethesetypesofbonds. Anotherfactor:whatsizewafersareyoudealingwith.It’sonethingtobondwafersatsix-inch–we’vebeendoingthatfor20years–butbeingabletodoitateight-inch.Thisintroducesanotherlevelofcomplexitybecauseyouhavemuchlargersurfaceareasandyou’vegottohavemoreuniformityacrossthewafer.Therearen’tthatmanyplacesintheworldthathavetheexpertise,thebonders,thechemical-mechanicalpolishing(CMP),electroplating,andalltheothertoolsnecessarytocreategoodbondsoneight-inchwafersinaproductionfoundryenvironment. Doyouhaveaprojectthatrequiresbonding? SpeaktooneofourtechnicalexpertstoseehowAtomicacanhelp. GetinTouch Doyouhaveanyprojectswithparticularchallengesthatyouwouldliketomention? ES:Yeah.Brian,youcouldtalkalittlebitaboutProgramXwhichintroducestheneedfortopography,throughsiliconviasthatcanconnecttothedevice,andahighhermeticityrequirement.That’sachallengingsetofconstraintsplusathermalbudget,too. BM:Yeah,alittlebitofeverythinginthatcase.Wehadalotofconstraintsandalsosomecostconcerns.Fittingallthoseconstraintsintoplaceandfiguringouthowtointegrateitintotheirdeviceinawaythat’scost-effective,space-saving,highreliability,andhighyieldingisachallenge.Weworkedthroughthatwiththecustomer,makingsurethatwe’reconsideringalloftheirconstraintsandfindingthebestsolutionforthem. Anyothercommentsinclosing? ES:Yeah.Thesemiconductorindustry,ingeneral,ismovingtoaworldthatwillrequiremoreandmoreofwhatwecall3Dheterogeneousintegration.Moore’slawisslowingdown.We’renowatthepointwherethegateonaleading-edgetransistorsislessthanthreenanometersinsize–that’sonlylike10atomswide!Canyoumakethemmuchsmallerthanthat? Withthislimitationsofcontinuedscalingoftraditionalsemiconductormanufacturing,we’reallstartingtothink…“ifwecan’tgosmallerandsmaller,howdowegovertically”?Howcanwestackdevicessowecancontinuetokeepreducingsizeandtakeadvantageoftheeconomicsandthemanufacturingadvantagesofsemiconductorwaferprocessing?So,we’reseeingmoreandmoredevicesstackedvertically. WedoalotofworkonCMOSwaferswherepost-processthewafertoandeitheraddsomeadditionalfunctionalityorbondsomethingontopofit.Webelievetheindustryisgoingtoneedmoreandmorebondingtocreatethese3Dstructuresthataffordsmallerdevices,morefunctionality,asmallerfootprint,bettereconomics,andallofthethingsthatyouexpectoutoftheelectronicsindustry. 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