JAE-HYUNGCHO,A.D.ROLLETT,J.-S.CHO,Y.-J.PARK,J.-T.MOON,andK.H.OHCopperbondingwireswerecharacterizedusingelectronbackscatterdiffraction(EBSD).Duringdrawing,shearcomponentsaremainlylocatedunderthesurfaceand,111.and,100.fibertexturecomponentsdevelopwithsimilarvolumefractions.Grainaveragemisorientation(GAM)andscalarorientationspread(SOS)ofthe,100.componentarelowerthanthoseofthe,111.orotherorientations.Also,,100.componentsgrowintoothertextureorientationsduringrecrys-tallization.Copperwiresexperiencethreestagesofmicrostructurechangeduringannealing.Thefirststageissubgraingrowthtokeepelongatedgrainshapesoverallandtobevariedinaspectratio.Thegrainsizesofthe,111.and,100.componentsincrease.Thevolumefractionofthe,100.componentincreases,whereasthatofthe,111.decreases.Thesecondstageisrecrystallization,duringwhichequiaxedgrainsappearandcoexistwithelongatedones.Thethirdstageisgraingrowth,whicheliminatestheelongatedgrains.The,111.and,100.grainscompetewitheachother,andthe,111.grainsgrowfasterthanthe,100.grainsduringthethirdstage.Comparisonofrecrys-tallizationandgraingrowthprocessesincopperandgoldwiresrevealsmanycommonmicrostruc-turalfeatures.
I.INTRODUCTION
copperbondingwirehasattractedatten-tionasanalternativetogoldbondingwire.Copperhasbetterelectricalconductivitythangold,anditischeapertoproduce.However,fabricationofcopperwirestillneedsmoreresearchondrawingandannealingprocessesinordertooptimizethequalityofthebondingwire.
Axisymmetricallydrawnproductsofface-centered-cubic(fcc)metalswithmediumandhighstackingfaultenergieshavetypicaltexturesconsistingofdoublefiberswithamajority,111.componentandaminority,100..[1–8]Itisknownthatthe,100.fiberbecomesdominantafterrecrystallizationbutchangesbacktothe,111.compo-nentuponfurtherannealingathighertemperatures.Thistexturetransitionhasbeenreportedincopperwire.[8]Intheearlystageoftheannealingprocess,the,100.com-ponentisprevalentduetorecrystallization.Inthelaterstage,the,111.or,112.componentbecomesdomi-nantbecauseofabnormalgraingrowth.Theseinvestiga-tionsarebasedonbothX-raydiffractionandelectronbackscatterdiffraction(EBSD)measurement.
RecrystallizationandgraingrowthofgoldbondingwireswerepreviouslyinvestigatedusingEBSD.[9]The,100.componentgrowsintothe,111.fiberduringrecrystalli-zation.Inaddition,the,100.and,111.fibersconsumeothertexturecomponents.Ascoarseningtakesplace,theaveragegrainsizeofboth,100.and,111.orientedgrainsincreases.Grainboundarieswithhighmisorientationanglesbetweenthe,100.and,111.orientationstendtomigrateintothe,111.,andtheoveralltextureshowsahighervolumefractionofthe,100.componentafter
JAE-HYUNGCHOandK.H.OH,MSEs,arewithSeoulNationalUni-versity,Seoul,SouthKorea151-742.Contacte-mail:kyuhwan@snu.ac.krA.D.ROLLETT,MSE,iswithCarnegieMellonUniversity,Pittsburgh,PA15213.J.-S.CHO,Y.-J.PARK,andJ.-T.MOON,MKEResearchLab,arewithMKElectron,Pogok-Myeon,Yongin-Si,Kyunggi-Do,SouthKorea151-742.
ManuscriptsubmittedFebruary12,2006.
METALLURGICALANDMATERIALSTRANSACTIONSA
RECENTLY,
recrystallization.Thisisexplainedbyanenergyadvantageforthe,100.fiber.
Duringplasticdeformation,gradientsintheplasticstraingiverisetogeometricallynecessarydislocations(GNDs)inordertomaintainlatticecontinuity.[10,11,12]TheenergystoredthroughcoldworkmustincludeafractionstoredasGNDs.SmalllatticerotationsduetoGNDscanbemeas-uredandcharacterizedusinglocalorientationmeasure-mentswithEBSD.[13]Thus,theinternallystoredenergyinamaterialcanbeatleastpartiallymeasuredviaintra-grainmisorientationangles,forexample,withgrainaver-agemisorientation(GAM)orscalarorientationspread(SOS).TheSOSorGAMofthe,100.componentwasmeasuredtobelowerthanthatofthe,111.ingoldwires.[9]Thisdifferenceinstoredenergiesprovidesdrivingforceforthe,100.componenttogrowintothe,111.orotherorientationcomponents.
Inthebeginningofannealing,subgraingrowth(orthemigrationoflow-anglegrainboundaries(LAGBs))occurredwithineachfibertexturecomponent.[9]Low-angletwistboundariesbetweengrainsinthesamefiberthatshareacommoncrystallographicaxisconsistofdislocationnet-works.[9]Recoveryresultedintherapideliminationofsuchboundariesandaconsequentincreaseinaspectratio.Fur-therannealingresultedindecreaseinaspectratioasregulargraingrowthtookplaceandthegrainsbecamemorecom-pactinshape.[9]The,100.componentindrawnfccmetals(Al,Cu,Au,Ag,etc.)growsattheexpenseofthe,111.fiberduringrecrystallization.Avarietyofmodelshavebeenusedtoexplainthispatternoftexturedevelopment.[14,15]The,100.componentreducesafterrecrystallizationandfur-therannealinggivesrisetothegrowthtexture,whichisdominatedbythe,111.fiber.Inaluminum,theaveragemobilityof,111.tiltboundariesishigherthanthatof,100.tiltboundariesat400°C.[16,17]The,111.texturemightbeattributedtothe,111.tiltboundarieshavingahigheraveragemobilitythanthe,100.tiltboundariesinfccmetals.Shinetal.[15]discussedthedeformationand
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annealingtextureofsilverwire.Silverisatypicalmaterialinwhichthemajortexturecomponentis,111.atlowreductionsinarea(RAs)(lessthanabout90pctRA),asobservedforotherfccmetals,butchangesto,100.fiberathigherreductionsinarea(99pctRA).
ACu-7.3pctAlalloy,whichhasaverylowstackingfaultenergy,wasdrawntoa70pctRA.[18]Theorientationdensityratioof,100./,111.duringdrawingdecreaseswithincreasingRAupto50pctandthenincreaseswithfurtherRA.Ingoldandcopper,asRAincreasesduringdrawing,andthe,111.componentincreasesattheexpenseofthe,100.fiberandothercomponents.
ThemicrostructuralevolutionofcopperbondingwireshasbeencharacterizedusingEBSDinasimilarwaytopreviousworkongoldbondingwires.[9]Theeffectsofdrawingdeformationandintermediateannealingprocesseswereinvestigatedonthevolumefractionsandmicrostruc-tureof,111.and,100.fibercomponents.Theroleofthreemicrostructuralstagesduringisothermalannealingforbothgoldandcopperbondingwiresisdiscussed,suchassubgraingrowthwithineachcomponent,continuous/dis-continuousrecrystallizationbetween,111.and,100.grains,andgeneralgraingrowth.Itseemsthatcontinuousrecrystallizationbetween,111.and,100.grainsisapartofextendedsubgraingrowth.[19]II.
EXPERIMENTAL
incompletepolefigures,(111),(200),and(220),weremeasuredonacrosssectionofthecastingbarusingtheback-reflectionX-raydiffractionmethodwithCuKaradiationonaSeifert3000PTSdiffractometer(RICH,SEIFERT&Co.,Germany).Textureandmicrostructureofwiressmallerthan1000-mmdiameterweremeasuredandcharacterizedusingEBSD.SpecimensofcopperwiresforEBSDmeasurementswerepreparedinthesamewayaspreviouslydescribedforgoldbondingwires.[9]Thecopperwiresweremountedinepoxyandthensectionedandpol-ished.Thepolishedspecimenswerecleanedwithionmill-ing.High-resolutionEBSD(usingaJEOL*6500Fscanning
*JEOLisatrademarkofJapanElectronOpticsLtd.,Tokyo.
CoppercastingbarswithsomeintentionaldopantofAg,Mg,Ca,andZrwerecolddrawnthroughseveraldiesetsandweremadeintofinebondingwireswith25-mmdiam-eter.Thecopperwas99.99pctpure.Intermediateannealsbetweendrawingstepswereaddedtofacilitatethewiredrawingprocess.Theseintermediateandfinalannealingstepsgivemoreflexibilityforcontrollingthemicrostructureandtextureofthecopperbondingwireswithoptimalprop-erties.Experimentaldetailsondrawingprocesses(RA)andannealingconditions(timeandtemperature)aresummar-izedinTableI.Accordingtothedrawingandannealingsteps,textureandmicrostructureevolutionwereinvesti-gated.Inaddition,isothermalannealingforthecopperwireswith25-mmdiameterwasalsocarriedoutfor1min,10min,60min,and1day.Topreventcopperwiresfromoxidationduringisothermalannealing,afurnacewithprotectivenitrogenatmospherewasused.Atleasttwoorthreedistin-guishablewireswereselectedandusedforEBSDworksforeachtemperatureandtimecondition.Theseisothermalannealingexperimentsfocusedonrecrystallizationandgraingrowthinthebondingwire.
Theinitialtextureofthecoppercastingbarwith7-mmdiameterwasmeasuredwithX-raydiffraction.Three
TableI.
electronmicroscopewithanINCA/OXFORDEBSDsys-tem)wasusedformeasurement,andthedataanalysiswasperformedusingtheREDSsystem.[20]Thefinite-elementscanningelectronmicroscopy–EBSDhasaspatialresolu-tionof1.5nmandangularresolutionof0.5deg.Theoper-atingvoltagefortheEBSDmeasurementswas20kVandtheprobecurrentwas4nA.Arectangulargridwasusedandthesizeofthepixelwasvariedwithwirediameters.The25-mmwirewasmeasuredwitha0.239-mmstepsize.TheEBSDmapsweremeasuredalongthelongitudinalsec-tionsofwires.Inversepolefigure(IPF)mapsfromEBSDwereusedfortextureandmicrostructurecharacterization.Misorientationanglesbetweenadjacentpixelswereusedforgrainidentification(ID).AnytwoadjacentpixelswithagrainIDanglesmallerthanthecut-offvalueareconsideredasapartofthesamegrain.Mostdeformedorrecrystallizedgrainshavesubgrainmicrostructures,andtheiroverallstructurescanbedescribedbymisorientationmeasurescalculatedoverasetofpixelscontainedwithinthegrain.Therearethreetypesofmisorientationmeasurescom-monlyusedinagrain:grainorientationspread(GOS),SOS,andgrainaveragemisorientation(GAM).[21–24]Thefirst,GOS,characterizesthemagnitudeofmisorientationbetweenallpairsofpixelsinagrain.Thesecond,SOS,iscalculatedbetweeneachpixelandtheaverageorientation.Thethird,GAM,iscomputedforadjacentpixelsonly,andgivesinformationaboutthenearestneighborcorrelations.TheGAMvalueisgenerallysmallerthaneithertheGOSorSOS.ConsideringPiasanorientationatapoint(xi)andPjasanotherorientationatadjacentpoint(xj)inagrain,theGAMcanbecalculatedwithamisorientationangle,whichisgivenfortwoadjacentorientationsatPiandPj,
+umisi
GAM¼
in
n
[1]
FabricationofCopperBondingWiresthroughIntermediateAnnealingandDrawingProcesses;CastBarwith7-mm
DiameterwasDrawndownto25mm
Start
1
1000mm
RA97.9pctnoannealingRA97.9pct600°C1h
2105mmRA99.98pctRA98.9pct
3105mm700°C,0.5s650°C,0.5s
448mmRA79.1pctRA79.1pct
525mmRA94.3pctRA94.3pct
625mm555°C,0.5s555°C,0.5s
DiameterCase1Case27000mmcastbarcastbar
Note:Reductioninarea(RA)isoverallreductionafterannealing.
3086—VOLUME37A,OCTOBER2006METALLURGICALANDMATERIALSTRANSACTIONSA
\"
whereumis¼minacos
traceððPiÁ
PÀj1Þ
ÁSÞÀ1
!#
2
Here,Sisthesymmetryoperatorsbelongingtotheappro-priatecrystalclassconcernedandsubscriptsinrotationP
referonlytoposition.[25,26]Tocalculategrainsize,thenum-berofdatapointsorpixelsinagrainarecalculated,and,usingtheknownpixelstepsize,thegrainareaisdeter-mined.Themostconvenientmeasureofgrainsizefromgrainareaistheequivalentcirclediameter.[27]III.
RESULTS
Figure1showsa(111)X-raypolefiguremeasuredincrosssectionwiththeaxialdirection(AD),paralleltothecastingaxis.ThetermsRDandTDaretheradialdirectionand,is100theclose.transversetotexturedirection,respectively.Awell-developedtherotatedisevidentcube,and{100}the,main011.texture.Thiscomponent(111)polefigureissimilartothatofgoldcastbarinReference28,whichwasalsoprovidedbyMKELaboratoryforresearchanddevelopment.Wehavepointedoutthateitherequiaxedor,ture,110columnarand.//RDgrainsthisandalso,ofseems100the.goldcastbarwithalignmentofto//ADapplycouldtotheresultcopperinsuchcastabar.tex-A.IntermediateAnnealingduringDrawing
Cold-drawncopperwireswerecharacterizedwithEBSDaftereachdrawingstep.Intermediateannealing(IA)andfinalannealing(FA)stepswerecarriedoutintheannealingfurnaceforabout0.5seconds.Themicrostructureandtex-tureofthedrawnwiresduringdrawingandannealingpro-cessesareshowninFigure2.TherearetwodifferentcasesinFigure2.Caseoneistheprocesswithoutanintermediateannealingstepforthewirewithadiameterof1000mm,whereascase2doesincludeanintermediateannealingforthewirewithadiameterof1000mm.ThevalueofRAdependsonbothannealinganddrawingprocesses.InFigure2(a),theEBSDmapforthe1000-mm-diameterwire(Figure2(a)-1)showsthat,100.grainsarelocatedunderthesurfaceand,111.grainsoccurmainlyinthecenter.Intermediateannealingat700°Cfordrawn105-mmwire(Figure2(a)-2)increasesthe,100.component,andsuchgrainsarelocatedinthecenterandunderthesurface,asfoundinFigure2(a)-3.Furtherdrawing(Figures2(a)-4and5)resultedintheelongationofgrainsandanintermixingof,the,100.,,111.,anddrawing,100.fiberwhiledecreasesthe,111or.remainsotherfibertheincreasessamecomponents.duringsteadilyfurtherThewithincreasingRA.
Figure2(b)showsanothercaseinwhichintermediateannealingwascarriedoutatawirediameterof1000mm;itsmicrostructurehasequiaxed,111.and,100.grainsandmanyannealingtwins(Figure2(b)-1).Afterdrawingdowntoadiameterof105mm(Figure2(b)-2),thegrainswereelongatedintolongbands.OrientationdistributionsintheEBSDmapdisplaylongerandthickergrainbandsinFigure2(b)-2thaninFigure2(a)-2.Thegrainsizeofthe105mmwireafterintermediateannealingat650°Cwas
METALLURGICALANDMATERIALSTRANSACTIONSA
Fig.1—(a)Aschematicdiagramforwiresectionsmeasured.RDdenotestheradialdirection,TDthetransversedirection,andthecastingaxisisparalleltotheAD,axialdirection.(b)111polefigureofcoppercastingbarof7-mmdiametermeasuredfromcrosssection.
smaller(Figure2(b)-3)thanthatat700°C(Figure2(a)-3).Themicrostructureafterintermediateannealing(Figure2(a)-3or2(b)-3)isdifferentfromthatafterfinalannealing(Figures2(a)-6or2(b)-6).Figure2(a)-3showsamixedmicrostructurewithevidenceofsubgraingrowthanddis-continuousrecrystallization,whileFigure2(a)-6displaysamicrostructureresultingfromgraingrowthwithequiaxedgrains.Wewilldiscussmicrostructureevolutionfurtherbasedonisothermalannealingexperimentsatlowertemper-aturesof300°Cand400°C.Duringisothermalannealing,
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Fig.2—Inversepolefiguremaps(grainIDangle,15deg)ofcopperwiresduringdrawingandannealing.Eachmapisnotifiedbythereductionareaanditsprocessstepnumber.Stepsfrom2to6showthefullsectionsofwiresbut1000mmshowshalfsections.IA:intermediateannealing,andFA:finalannealing.(a)7f/1000mm(1)/105mm(2)/IA(3)/48mm(4)/25mm(5)/FA(6).(b)7f/1000mm/IA(1)/105mm(2)/IA(3)/48mm(4)/25mm(5)/FA(6).
Fig.5—Inversepolefiguremaps(grainIDangle,15deg)ofcopperwireduringisothermalannealingat(a)300°Cand(b)400°C.ExamplesofHAGBsforcontinuousrecrystallizationarespecifiedwithwhiterectanglesin(a),10minand60min.Wiresare25-mmdiameter.
3088—VOLUME37A,OCTOBER2006
METALLURGICALANDMATERIALSTRANSACTIONSA
itbecameclearthatsomegrainsinthe,100.and,111.fibersgrowatthebeginningofrecrystallization.Inparticular,the,100.componentgrowsfasterthanthe,111..Duringgraingrowthathighertemperatures,however,the,111.grainsgrowfasterthanthe,100..Thevolumefractionsof,111.,,100.,andothercomponentswerecalculatedfromEBSDdatainFigure3.Overtheentirecrosssection,similarvolumefractionsofthe,111.and,100.areobservedwithincreasingRA;othercomponentsdecrease.‘‘Othercomponents’’includeallgrainsexceptthosewithin15degofhaving,111.or,100.fibersparalleltothewireaxis.Theseorientationsareusuallyconcentratedinthenear-surfaceregionratherthaninthecenterregion.Thisisaconsequenceoftheinhomogeneousdeformationthatarisesfromthefrictionbetweenthedieandcopperwire.Whenwedefinetherel-ativeradialposition,orratioofs/so,wheresisthedistancefromthewirecenterandsoisthewireradius,thecenterisequivalenttos50andthesurfaceiss51,respectively.Overall,thesurfaceregioninthestudycorrespondstotheratiobetween0.7#s#1.0,andthecenterregiontheratiobetween0#s#0.3.Inthecenterregion,thedeformationis
Fig.3—Variationsinvolumefractionofcopperwireasafunctionofprocessstep.IA:intermediateannealing,andFA:finalannealing.ThenumbersinthefiguresarethelinktotheprocessesofwirefabricationshowninFig.2:(a)wholesection,(b)surfaceregion,(c)centerregionforFig.2(a),(d)wholesection,(e)surfaceregion,and(f)centerregionforFig.2(b).
Fig.4—Variationsingrainsizeandaspectratioofcopperwiresasafunctionofprocessstep.TheprocessstepnumberisthelinktotheprocessesinFig.2.(a)GrainsizeandaspectratioforFig.2(a).(b)GrainsizeandaspectratioforFig.2(b).
METALLURGICALANDMATERIALSTRANSACTIONSA
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Fig.6—Variationsingrainpropertiesduringannealingat300°C.Thetransitionsfromthestageofsubgraingrowthviarecrystallizationtograingrowthareshownbyverticaldashedlines.GAMandSOSaregivenindegrees:(a)grainsize,(b)aspectratio,(c)volumefraction,(d)GAM,and(e)SOS.
morehomogeneousthanunderthesurfaceand,therefore,the,111.componentisstronger.AsRAincreases,the,111.componentstrengthens,whereasthe,100.com-ponentdecreasesorremainsthesame.Intermediateanneal-ingchangestheratioofthe,100.and,111.components.Usually,the,100.componentincreasesandthe,111.decreases.Afterfinalannealingofthe25-mmwires,theratioofvolumefractionsofthe,100.and,111.goestounity.VariationsingrainsizeandaspectratiowithprocessstepsareplottedinFigure4.Increasingdrawingdeformationresultsinincreasingaspectratioanddecreasinggrainsize,whereasannealingdecreasestheaspectratioandincreasesthegrainsizeforbothcases.B.IsothermalAnnealingafterDrawing
CopperbondingwiresfabricatedfromtheprocessofFigure2(a),orcase1withoutfinalannealingforthe1-mm-3090—VOLUME37A,OCTOBER2006
diameterwire,wereusedtocarryouttheisothermalannealingexperimentfortheinvestigationonrecrystalliza-tionandgraingrowth.AfterthedrawingprocessesinFigure2,thetotalRAofcopperwirefromacastbartoa25-mmwireis99.999pct.ThenetRAis94.3pctifoneaccountsfortheeffectofintermediateannealing.
Figure5showsEBSDmapsofcopperwiresduringisothermalannealingat300°Cand400°Cfor1minute,10minutes,60minutes,and1day.TheEBSDmapafterannealingat300°Cfor10minutesshowsthatmostgrainsstillhaveelongatedshapes.Afterannealingat300°Cfor60minutes,someelongatedgrainshavebothwidenedandlengthenedandsomeequiaxedgrainsaremovingintotheelongatedgrains.Afterannealingat300°Cfor1day,allelongatedgrainshavebeeneliminated.Amicrostructurewithbothelongatedandequiaxedgrains(asisfoundafterannealingat300°Cfor60minutes)isalsofoundafterannealingat400°Cfor1minute.Thisisasexpected
METALLURGICALANDMATERIALSTRANSACTIONSA
Fig.7—Variationsingrainpropertiesduringannealingat400°C.Thetransitionsfromthestageofsubgraingrowthwithrecrystallizationtograingrowthareshownbyverticaldashedlines.GAMandSOSaregivenindegrees:(a)grainsize,(b)aspectratio,(c)volumefraction,(d)GAM,and(e)SOS.
becausehighertemperatureacceleratestheannealingproc-ess.TheEBSDmapsinFigure5implythattherearethreestagesofmicrostructuralevolutioninthecopperbondingwiresduringannealing.Inthefirststage,apparentforannealingat300°C,subgraingrowthandcontinuousrecrystallizationaredominantfor10minutesandthegrainshaperemainselongated.Inthesecondstage,equiaxedgrainsfromdiscontinuousrecrystallizationgrowintotheelongateddeformedstructure(afterannealingfor60minutes).TheEBSDmapfor1dayat300°Cshowsonlyequiaxedgrainsandgraingrowthoccursthereafterasthethirdstage.ThiswillbediscussedfurtherinSectionIV.Itisalsonotablethatthesecondstageofdiscontinuousrecrys-tallizationclearlynucleatesinthesubsurfaceregionandspreadstotherestofthecrosssection,asobservedbyWaryobaetal.[29]Theas-deformedcopperwireof25-mmdiameterhasboth,111.and,100.fibercomponents.Duringanneal-METALLURGICALANDMATERIALSTRANSACTIONSA
ing,textureandmicrostructurechange.Figures6and7showvariationsingrainsize,aspectratio,GAM,SOS,andvolumefractionsoffibercomponentsduringannealingat300°Cand400°C,respectively.Basedonthemicro-structuralevolutiondisplayedinFigure5,threestagesofsubgraingrowth,recrystallization,andgraingrowtharespecifiedinFigures6and7.Grainsizeincreaseswithannealingtimeand,100.grainsareusuallylargerthantheothersinFigures6(a)and7(a).Theonlyexceptionisforthelongestannealing(1day)at400°C,Figure7(a),whichisaconsequenceofextendedgraingrowth.Itisclearthat,100.grainsgrowfasterthanothersduringrecrys-tallizationatboth300°Cand400°C.
Aspectratiosduringannealingat300°C(Figure6(b))increaseslightlyafter1minuteandthendecrease.Thisbehaviorwasalsoobservedingoldbondingwiresandisrelatedtosubgraingrowthalongthedrawingaxis.[9]Grainsalongthedrawingaxishavepredominantlypuretwistgrain
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Fig.8—Misorientationdistributionsofcopperwires:(a)300°Cand(b)400°C.
boundaries.Thegrainswithtwistboundariesshareacom-monaxis,i.e.,,111.or,100..Thissuggeststhatthetwistboundarieshavehighermobilitythantheboundariesbetweentheelongatedgrains,atleastduringthesubgraincoarseningstage.Thepeakinaspectratioat400°Cwasprobablymissedbecauseoffasterrecrystallizationthanat300°C(Figure7(b)).
Variationsinthevolumefractionat300°Cshowthatthe,111.fiberdecreaseswhereasthe,100.increasesupto60minutes(Figure6(c)).Howeverthe,111.componentincreasesagainafter60minutesattheexpenseofthe,100.component.Typically,,100.isknownastherecrystallizationtexturecomponent,anditgrowsduringrecrystallizationinordertolowerthestoredenergyofdeformation.[14]Itisnecessarytodistinguishcontinuousrecrystallizationfromdiscontinuousrecrystallization.The,100.componentincreasesmainlyduringcontinuousrecrystallization.Duringdiscontinuousrecrystallization,however,theelongated,100.grainsdecreaseinfre-quency.The,111.componentisfavoredbygraingrowth(athighertemperaturesthanthoserequiredforrecrystalli-zation).Duringgraingrowth,thegrainboundarymobilityisthemajorfactorthatdeterminesthetextureandmicro-structureevolution.Similarly,duringannealingat400°Cfor1minute(Figure7(c)),themicrostructureisalreadyinthegraingrowthstageso,111.increaseswhile,100.decreases.Misorientationdistributionsshowthat,100.orientedgrainshavealowervalueofGAMorSOSthan,111.grainsinthecold-drawndata.ThesetrendsseemtocontinueinFigures6(d)and(e)for300°CandFigures7(d)and(e)for400°Cduringrecrystallization.
Figure8showsthemisorientationangledistributionsforvariouscases,basedona5-degcut-offangle.Theas-drawnwirehasahighfrequencyofbothlowermisor-ientationanglesthan15degandhighermisorientationanglesthan50deg.Duringannealingat300°Cand400°C,thefrequencyofLAGBsdecreasesandapeakappearsaround60deg,whichismainlyfromS3boundaries.Annealingat300°CresultsinastrongerS3peakthanthatat400°C.
3092—VOLUME37A,OCTOBER2006
IV.DISCUSSION
AsimilarstudyofrecrystallizationandgraingrowthingoldbondingwireswasreportedinReference9.Bothgoldandcopperarefccmetalswithlow-mediumstackingfaultenergyandthematerialsinboththeformerandthecurrentstudieshavepartspermillion(ppm)leveldopants.There-fore,itisreasonabletoexpectsimilarbehaviorinthetwodifferentmaterials.Comparisonofisothermalannealingexperimentsforgoldandcopperbondingwireswith25-mmdiameterrevealssimilarmicrostructuralevolution,asanticipated.
Thevolumefractionratioof,100.to,111.fiberforcold-drawncopperwiresis0.34:0.32(Figures6and7)and0.1:0.8ingoldwiresinReference9.Thecopperwirehasmore,100.componentthanthegoldwireduringdraw-ing.The,100.fractionincreasesinthebeginningoftheannealingprocessinbothcopperandgold.Coalescencealongthewireaxisbetween,100.orientedgrainsoccursbytheeliminationoftwistboundaries.Thesameprocessoccursinthe,111.component.Grainboundarymigra-tiontakesplacebetween,111.and,100.orientedgrains,whichisbiasedtowardmovementinto(i.e.,con-sumptionof)the,111.component.Duringthefirststageofannealing,thetwistgrainboundariesmovefasterthanothers,whichresultsinamaximumoftheaspectratioofgrains(Figures6(b)and7(b)).Theseprocessesarefoundinthebeginningofannealing,whichisasubgraincoarseningphase.
TypicalmicrostructureandtextureofthecopperandgoldwiresareshowninFigure9.Theinitiallyelongatedgrainsinacopperwirearereplacedbyamixtureofelon-gatedandequiaxedgrainsafterannealingat400°Cfor1minute(Figure9(b)).Theelongatedgrainsarewiderthanthoseoftheas-drawnwires.Equiaxed,111.grainsgrowintotheelongated,100.grains.Aspointedoutprevi-ously,themixtureofelongatedandnewlygrowngrainsshowsthatbothsubgraingrowth(orcontinuousrecrystal-lization)anddiscontinuousrecrystallizationhavetakenplace.Unlikecopperwires,Figure9(e),however,shows
METALLURGICALANDMATERIALSTRANSACTIONSA
Fig.9—Comparisonoftypicalmicrostructureandtextureof25-mm-diametercopperandgoldwires.CSLsareshownin(c)and(f)withyellowlinesforS3,greenforS5,andblueforS7.(a)EBSDmapforasdrawn,(b)and(c)EBSDmapandCSLforannealedcopperwire,(d)EBSDmapforasdrawn,and(e)and(f)EBSDmapandCSLforannealedgoldwireinFig.8ofRef.9.
Fig.13—SchematicdiagramsforHAGBsandmicrostructureduringstage2.TheIPFmapisthemicrostructureimagefromannealingat400°Cfor1min.(a)DiagramofHAGBsfordiscontinuousrecrystallization.(b)IPFmapfordiscontinuousrecrystallization.
thatcoalescencecontinuestooccuralongthewireaxisinboththe,111.and,100.componentsingold.There-fore,most,111.and,100.grainsretainelongatedshapes.Theslowerkineticsobservedinthegoldsuggeststhattheoverallmobilityofgrainboundariesislowerin
METALLURGICALANDMATERIALSTRANSACTIONSA
goldthanincopperatthesametemperature.Thismaybeaconsequenceofdifferentimpurity(dopant)levelsandtypesinthetwomaterials.
Mostboundariesareeither,111.or,100.tiltboundariesaftercoalescencehaseliminatedmosttwist
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Fig.10—CSLdistributionsasafunctionofannealingtimeandtemperaturealongtheverticalandhorizontaldirectionsinthebondingwireaxis.(a)As-drawn,(b)1min,(c)60min,and(d)1dayat300°Cforcopperwire;and(e)as-drawn,(f)1min,(g)60min,and(h)1dayat300°Cforgoldwire.
boundaries.S3,7,13b,21a,31atypesandnear-coincidentsitelatticeboundaries(CSLs)with,111.misorientationaxesarefoundfrequentlyinthe,111.fiberregions.ThemostfrequentCSLsinthe,100.regionsareS5,13a,17a,25a,and29awith,100.axes,asexpected.Bounda-riesbetweengrainswiththesamefibershowsmallermisor-ientationanglesonaveragethanboundariesbetweengrains
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withdifferentfibers,againasexpected.Almostalloftheboundariesbetweenthe,111.and,100.componentshavemisorientationanglesabove40degsothoseCSLtypesaremainlyS3,9,11,17b,25b,31b,and33c.Figure10showsthedistributionsofCSLboundariesforbothcop-perandgold.Separateplotsofboundarieswithnormalsparallel(horizontal)toandperpendicular(vertical)tothe
METALLURGICALANDMATERIALSTRANSACTIONSA
Fig.11—Variationsinaspectratiowithannealingtimeandtemperature:(a)goldwireand(b)copperwire.
Fig.12—Schematicdiagramsforpatternsofgrainboundarymigrationduringstage1.LAGB:low-anglegrainboundary,andHABG:high-anglegrainboundary.(a)Subgraingrowthbetweenthesamefibersalongthewireaxis,(b)subgraingrowthbetween,100.fibersalongthetransversedirection,(c)subgraingrowthbetween,111.fibersalongthetransversedirection,and(d)continuousrecrystallizationwithgrowthofa,100.grainintoaneighboring,111.grain.
METALLURGICALANDMATERIALSTRANSACTIONSA
wireaxisareshown.Thecopperwiresexhibitalmostneg-ligiblefrequenciesofCSLboundariesotherthanS3,whereasthegoldwiresshowstrongpeaksforboundarieswithS,111.misorientationaxes,i.e.,S3,S7,S13b,andures21a.10(a)Thesethroughpeaksdecrease(d)).Thisastrendannealingismoretimeobviouspassesin(Fig-thegoldwire(Figures10(e)through(h)).
Thechangeinaspectratioisobviouslyrelatedtograinshapechangeduringsubgraingrowth,recrystallization,andgraingrowth.Bothcopperandgoldshowaslightincreaseinaspectratioatshorttimesat300°C(Figure11).Althoughtheaspectratiodependstosomeextentonthechoiceofthecut-offangleusedforgrainidentification,thetrendofincreasefollowedbydecreaseduringannealingat300°Cisconsistentlypresent.Asthecutoffangleusedforgrainidentificationdecreasesfrom5to1deg,theaspectratioalsodecreases.AspointedoutinReference9,LAGBsarelocatedalongthelongitudinaldirection,soemployingasmallergraincut-offangleresultsinasmalleraspectratio.Mostofthelow-angleboundariesunder5degintheas-drawnwiresconsistofpuretwistboundaries.Theseareasourceofsubgraingrowthfromdislocationtanglesatthebeginningofrecoveryduringannealing.Thepresenceofdislocationtangleswasalsoconfirmedbytransmissionelectronmicroscopyinthegoldwires.[9]Theincreaseoftheaspectratioisaconsequenceofsubgraingrowthorcoalescencealongthelongitudinaldirection.
Figures6and7showthevariationsingrainsize,aspectratio,andthevolumefractionswithannealingtimeinthecopperwire,whichrevealresultsofinterest.Both,111.and,100.grainsizesofcopperin300°Cand400°Cincreaseduringannealing(recrystallizationandgraingrowth,processes).Thevolumesame111.,trend,however,wasfoundmovesininthethefractionof,100.andgoldoppositewire.directions.[9]At300The°(Figure111.6)andand,the100volume.grainsfractionbothofgrowthe,up111to.decreases24hoursC,monotonically.After24hours,the,111.fractionincreasesagain.Thevolumefractionofthe,100.com-ponentshowsexactlytheoppositebehavior.At400°C,thegrainsizeof,111.and,100.grainsincreasesduringannealing(Figure7).Bycontrasttothelowertemperature,however,thevolumefractionofthe,111.increasesandthatofthe,100.decreasesafter1minuteofannealing.Thishappensbecauserecrystallizationiscompleteaftera
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shorttimeandthe,111.fractionincreasesduringsub-sequentgraingrowth.
Figure12showsdiagramsforcharacteristicpatternsofgrainboundarymigrationbetweenthevariouscombina-tionsof,111.and,100.orientedgrains.Atshortannealingtimes,subgraingrowthoccursthateliminatesLAGBsbetweengrainsbelongingtothesamefiber.Con-tinuousrecrystallizationinvolvingmigrationofhigh-anglegrainboundaries(HAGBs)betweendifferentfibersalsooccurs.Thisisdefinedasstage1duringannealing.Topo-logically,mostgrainsstillhaveelongatedgrainshapes.Subgraingrowthoccursbetweenthesamegrainsinthesamefiberalongboththelongitudinalandtransversedirec-tions.MigrationofLAGBscausesthegrainsizetoincreaseduringannealingbutdoesnotchangethevolumefractionofeitherthe,111.orthe,100.component.TheincreaseinaspectratioduringannealingisaconsequenceofLAGBmigrationalongthelongitudinaldirection.MigrationofHAGBsismainlyresponsibleforchangesinthevolumefractionof,111.and,100.grains.Continuousrecrys-tallizationshowninFigure12(d)contributestoincreasesinthe,100.fiberanddecreasesinthe,111..Duringcontinuousrecrystallization,theoverallelongatedgrainshapesareremained.ExamplesofHAGBsforcontinuousrecrystallizationarespecifiedwithwhiterectanglesinFigure5(a),10and60min.
AnothertypeofHAGBsismeasuredafter60minutesat300°Cor1minuteat400°Cincopperwires.Newlygrownandequiaxed,111.grainscomeintocontactwithelon-gated,100.grains.Figure13showstheschematicdia-gramforthistypeofHAGBs.ThemotionoftheseboundariesresultsindiscontinuousrecrystallizationandthefractionofHAGBdecreasesexceptforS3.Thisisdefinedasstage2duringannealing,duringwhichmostoftheelongatedgrainsdisappearandequiaxedgrainsgrow(Figures5(a)from60minutesto1day;andFigures5(b)from1minuteto10minutes).Afterstage2,normalgraingrowthofequiaxedgrainsoccurs,whichisdefinedasstage3.Thevolumefractionof,100.increasesduringstages1and2,whereas,111.increasesduringstage3.Infact,itisdifficulttoseparatethesestepsclearlyandthemicro-structuralevolutionincopperandgoldwiresshowsalltheprocessesofsubgraingrowth/continuousrecrystalliza-tion,discontinuousrecrystallization,andgraingrowthatvariouspointsduringannealing.
V.CONCLUSIONS
MicrostructureandtextureincopperbondingwireswerecharacterizedbyEBSDandcomparedwithpreviousworkingoldbondingwiresduringthedrawingandannealingprocesses.
1.Duringdrawingofcopperandgold,theshearcompo-nents,gold111wires,.arecomponentlocatedmainlywhereasdevelopsinthethe,111morenear-surfaceregion.The.componentthanthe,has100a.sim-inilarvolumefractionto,100.incopperwires.
2.Isothermal,annealingat300°Cof100annealing.grainsforgrowbothfastergoldthanand,and111400copper..at°theCshowsthatThebeginning,111.
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componentgrowsfasterthan,100.inthefinalgraingrowthstage,however.
3.Theaspectratioofgrainshapeat300°Cincreasesslightlyatshortannealingtimesforbothgoldandcopper.Thisistheresultofsubgraingrowthalongthelongitudinaldirection.
4.Orientation,spread,measuredbyGAMandother100.componentscomponentinistheloweras-drawnthanstate.intheThis,SOS,111provides.inandtheadrivingforce(basedondifferencesinstoredenergy)forthe,100.componenttogrowintotheothertexturecomponentsinbothgoldandcopperduringrecrystalli-zation.
5.Duringannealing,threestagescanbedefinedforbothcopperandgoldwires.Stage1isdefinedbythepersis-tenceofelongatedgrainshapesandincreasinggrainsizein,both,111.and,100.is100defined.,increasesbytheappearancewhereasof,.111Theamixture.volumedecreases.fractionofnewlyStageofrecrys-2tallizedgrainsandelongatedones.Stage3isgraingrowth.Onlyequiaxedgrainsarefound.The,111.componentgrowsattheexpenseofthe,100.atthisstage.
ACKNOWLEDGMENTS
ThisresearchissupportedbytheBK21projectoftheMinistryofEducationandHumanResourcesDevelopmentinSouthKoreaandMKEElectron.PartialsupportoftheMesoscaleInterfaceMappingProjectatCarnegieMellonUniversityunderNSFGrantNo.DMR-0520425isacknowledged.
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