SputteringTutorial:ThinFilmMetallizationonAlNandAl2O3

CMCLaboratories,Inc.

ReviewoftheSputteringProcess

KeyCharacteristicsofSputtering

• Atom-by-atomdepositiontechnique• Atomsgenerallyhavelittleenergywhendeposited- easyto

producenon-equilibriumphases(atomstrappedattheirdepositionsite)

• Almostanysolidmaterialcanbesputtered- veryversatile• Filmmorphology(grainsizeandshape,porosity,surface

roughness)isverydependentondepositionconditions

BasicSputterDepositionSteps

• AhighvacuumisgeneratedinthesputtertooltoeliminateresidualN2 andO2.Goal10-7 Torrrange.

• ThechamberisbackfilledwithArgasinthe10-3torrrange• AnArplasmaisignited(Ar+ +e-)byapplyinganelectricfield• Atargetcomposedoftheintendedmetalfordepositionisbiased

negatively• Ar+ ionsattractedtothetargetcollideanddislodgeatomsofthetarget

material• Someoftheseatomscollidewiththesubstratematerialandbond

formingadepositedfilm• ThefilmisalsobombardedwithAratoms

TargetSputteringSchematic

SputterToolProcessSchematic

SputterToolProcessSchematic- Details

ThinFilmDepositionProcessSchematic

EK =½MAV2

E=KT

T =TSubstrate +TA

• Depositingatomshavekineticenergyfromthesputterprocess• Theycollidewiththesubstrate• Somelocalizedheatiscreated• Atomsdiffuseonthesurfacetolowenergysites• Diffusionisdrivenbythesubstratetemperatureandlocalizedheatfromcollisions• Smallnucleiformthatgrowandcoalesceintoafilm• Aratomsalsobombardthesurface

ArAr

SurfaceDiffusionProcess

1

2

3

EnergyofD

eposite

dAtom

1 2 3

LocationonSubstrate

IfTsubstrate istoolow,surfacediffusiondoesnotoccur,anddepositedatomtrappedathigherenergysites1or2.Thisresultsindefectformation(suchaspores)orintrinsicfilmstress.DiffusionscalesasTsubstrate/TM.

FilmStresswithSubstrateTemperature

• IntrinsicfilmstressdecreaseswithincreasingTsubstrate• RelevantparametertomeasurediffusionisT/TM•WithhigherTsubstrate,TCEstressbetweenthefilmandsubstrateincreases• Trade-offbetweenthese2factorsmustbedeterminedexperimentally

FromThornton,J.A.andHoffman,D.W.ThinFilmSolids,171:5-31(1989)

FilmGrowthMechanismsvs.ArgonPressure

Arbombardmentduringfilmgrowth• ForlowArpressure,timebetweenAr/Arcollisionsislong• Armean-free-pathislong• Arenergybombardingthefilmsurfaceishigh• “Atomicpeeningeffect”increasesfilmdensity• Producesfilmswithcompressivestressandsmoothsurfaces• For highArpressure,Armean-free-pathisshort• Arenergyislow• Noatomicpeening• Filmsarelowdensity,tensilestress,roughsurfaces

FilmMicrostructureandStressRelationship

CompressiveStress

TensileStress

FilmStressvs.ArgonPressure

StressTransitionPressureforDifferentMetals

KeyParametersintheSputterProcess- Tsubstrate

Tsubstrate/TMLowextrinsicCTEfilmstressduetosmallΔTfromTsubstrate to25C

Stronglayeradhesion.HighTsubstrate resultsinsurfacediffusionofdepositingatomstohighestbondstrengthsites.

BalancePoint

CriticalFilmProperties

KeyParametersintheSputterProcess- ArgonPressure

PargonLowPargon:Compressivefilmstress.Filmcanbuckle.

HighPargon:Tensilefilmstress.Filmcanpeel.Porousmicrostructure

BalancePoint

CriticalFilmProperties

Lowfilmstress.Densedeposit.

SputteredFilmMicrostructureMapping

SurfacePreparationPriortoSputterDeposition

• Cleanoffallcontamination– Organics– Polishingorlappingresidue

• Etchorannealawayanydamagedlayers– Micro-cracks– Createdduringlapping– Createdduringlaserprocessing

SurfacePreparationPriortoSputterDeposition

PreparationStage ProcessingApproach Example

CleanOrganics Plasma Clean O2 plasmaSiwafers

Cleanlappingsolutionsorhandlingcontamination

Alkaline soak Anodexcleaner

EliminateMicro-cracks Etchawaythedamagedlayer

Buffered oxideetchforSi

Eliminate Micro-cracks Heat treatmentnearsinteringtemperaturetoannealawaycracks

Al2O3 T>1600C,forAlNT>1750C

ThinFilmDepositiononAlNandAl2O3

GoalsforThinFilmDepositiononAlNandAl2O3

• Thinfilmadhesiontooxideornitrideceramicsurface• Strongadhesionbetweenmultiplethinfilmlayers• Acceptablefilmstress(minimizeimpactonadhesion)• Preferredfailuremode:ceramicfracture• DiffusionbarriertopreventTiorWonsoldersurface• Oxidefreesolderlayer• Compatibilitywithmultiplesolders:PbSn,SAC,AuSn

AlNandAl2O3 CeramicSurfaceRequirements

Al2O3SurfaceFinish

As-Fired(Ticoatingonpartofsample)

Lapped

Al2O3 SurfacePreparation

• As-firedorPolished– Alkalineclean– Aciddip– ThoroughDIH2Orinse

• Lapped– Adjustlappingtominimizedamagelayerextent– Annealiffeasible– Alkalineclean,aciddip,DIH2Orinse

AlNSurfaceCharacterization

• As-firedsurfaces– Nomicro-crackspresent– Surfaceiscontouredbutsmooth– Needtocleanoffanycontaminants– Expectgoodthinfilmadhesion

• Lappedsurfaces– Micro-cracksandinter-granularfracture– Contaminationfromlappingmedia

AlNSurfaceFinish- AsFired

AlNSurfaceFinish- AsFired

AlNSurfaceFinish- AsFired

AlNSurfaceFinish- lapped

AlNSurfaceFinish- lapped

AdhesionLayer:Al2O3 andALNThinFilmMetallization

WhatisRequiredforTwoMaterialstoStickTogether?

• Thetwomaterialsmustformabond.Thismeansthetotalenergyofthebondedmaterialislowerthanthesumoftheenergiesoftheindividualnon-bondedmaterials

EAB <EA +EB• Forchemicalbonds,thisenergydifferenceisthechangeinGibbs

FreeEnergy(ΔG).NegativeΔGmeansbondingisfavorable.• TherearetwocomponentsinΔG,EnthalpyΔHwhichischemical

energy,andEntropyΔS.• IfΔGisnegative,thenextrequirementisthatthematerialsare

broughtintointimatecontact witheachothersothatabondcanform.Thisinvolvesreactingandmixingonanatomiclevel.Italsorequiresdiffusionwhichisenhancedbytemperature

• EvenfornegativeΔG,materialsrequireanactivationenergyforabondtobegintoform.Thisenergyusuallycomesfromappliedheat.

AdhesiontoAl2O3 - ΔG

Metal Oxide ΔG(Kcal/mole)200C

Ti TiO2 -210

Ti Ti2O3 -225

Cr Cr2O3 -160

V V2O3 -180

Ta Ta2O5 -175

Nb NbO -175

Zr ZrO2 -240

Hf HfO2 -240

W WO3 -115

AdhesiontoAlN- ΔG

Metal Nitride ΔG(Kcal/mole)200C

Ti TiN -140

Zr ZrN -140

Nb Nb2N -90

Cr Cr2N,CrN -35

V VN -65

Ta TaN -100

OO

Ti

O

OAlO

AdhesionLayerSurfaceReaction- TiReactionwithOxygenandAluminum

Ti

OAl Al

AlAl Al Al Al

O

OO O

Al Al

O O

O O

Ti Ti Ti

O OTi=O=Al

Ti Ti TiTi

TiTi Ti

OOO

OAlO

PoorBaseVacuum- TiIsOxidized- NoAdhesionReaction

OAl Al

AlAl Al Al Al

O

OO O

Al Al

O O

O OO O

TiO O

Ti

Ti Ti

O

O

O

O

O

O

Ti/WandTiAdhesionLayerComparison

• TiandTI/Warethemostcommonlyusedbondinglayermaterials• Tihasstrongerchemicalbonding(larger-ΔG)withbothoxide(Al2O3)and

nitride(AlN)surfaces• WhashigheratomicmassthanTi(183gr/molvs.47gr/mol)andthus

higherkineticenergyuponimpactwiththesubstrate.Thisprovidesadditionalactivationenergyforbonding.

• TifilmsaremuchmoresensitivetobasevacuumbecauseofTihighaffinityforoxygen.Adhesionisdegradedforvacuums>10-6 torr.

• Ti/WfilmshavehigherresidualstressthanTiandfilmpropertiesaremuchmoredependentonArgonpressurethanTifilms

• Ti/WfilmscanbeetchwithH2O2 andthusdonotrequireHF.

Ti/WandTiAdhesionLayerComparison

• BothTi/WandTicanbeusedeffectively• ForTi,basevacuum<1x 10-6 torrtogetgoodadhesionto

Al2O3 andtosubsequentmetallayers• ForTi/W,Argonpressuremustbe7x 10-3 torrforlowfilm

stress• BecauseTi/Whasbothstrongchemicalbondingandhigher

depositionenergy,theprocesswindowislarger

ImpactofSubstrateTemperatureonTiAdhesiontoAl2O3

EnergyofD

eposite

dAtom

Ti AlAl

Al

O

Ti AlAl

O

O

Ti AlAl

Al

O

ModerateAdhesion

ModerateAdhesion

StrongAdhesion

Ti EB

KTsubstrate >EB forTitodiffusetostrongadhesionsite

ImpactofSubstrateTemperatureonTiAdhesiontoAl2O3

• IfTsubstrate istoolow,fractionofTiatpoorbondingsites(fewsurroundingOorNatoms)

• AdhesionofTilayeriscompromised• CMCexperience,Tsubstrate >150C(CMCused200C)• ThisisthesameforotherbondinglayerssuchasNitoTi,or

CutoTi.

ThinFilmBarrierLayersonAl2O2 andAlN

BarrierLayers

• Role(s)ofthebarrierlayer:– Preventdiffusionoftheadhesionlayeratoms(Ti,W,Cr)tothe

solderinterface– Actasasolderinterface– Actasseedmetalforsubsequentplatedlayers

• CommonBarrierlayermaterials:NiV,Pd,Pt• CommonBarrierSeedlayermaterials:NiCu,Cu• Keyrequirementsforsputteredbarrierlayer

– Bondwelltoadhesionlayer– Formalowstressdeposit– Formadensedeposit(nopores)– Wetsoldermaterialswell– Notformextensiveintermetalliccompoundswhenreacting

withsolder– Forseedmetalapplications:providelowelectricalresistance

BondingoftheBarrierLayertotheAdhesionLayer

Ti/Ni,Ti/Cu,Ti/Pd,Ti/Ptallhave(–ΔG)

Firstrequirementforbonding,negativeGibbsFreeEnergy(–ΔG)

O OOOO

O

BondingoftheBarrierLayertotheAdhesionLayer

Secondrequirementforbonding:bondingmaterialsinintimatecontact(nocontamination).Forsputteredlayers,thisrequiresalowbasevacuum(10-7 rangeforTi).Caseofapoorbasevacuumisillustratedbelow.

Ti Ti Ti

TiTi Ti

Ti Ti Ti

TiTi Ti

O O

OO O O O O

CuCu

Cu

Cu

Cu Tiispassivatedandnon-reactive.

BondingoftheBarrierLayertotheAdhesionLayer

Thirdrequirementforbondingmaterialscaninter-diffusetofindoptimumbondingsites

EnergyofD

eposite

dAtom

CuCu

Ti

Cu

ModerateAdhesion

ModerateAdhesion

StrongAdhesion

EBCu

CuCu

Ti

Ti

CuCu

Ti

Ti

KTsubstrate >EBSubstrateTemperaturemustbe>150Cduringdepositionforstrongestbondstoform

TypicalSputteringParametersforAl2O3 andAlN

TypicalConditionsforSputteringonAl2O3 andAlN

• Basevacuum– Inthe10-7 torrrangeforTiadhesionlayer– Below2x 10-6 torrforTi/W

• Argonpressure3-7x 10-3 torr– LesscriticalforTilayers– ForTi/W,5-7x 10-3 toreducestress

• Substratetemperature150-200C– Criticalparameter– Needstobeoptimizedforspecificmaterialsandsputtersystem– CMCused200CforTi/WonAlNwithexcellentresults– Needstobeconstantduringrun

• Power2KW

ThinFilmAdhesionTesting

• Therearenouniversalacceptedtestsformeasuringthinfilmadhesion

• MostusedtestisPeelTesting(ASTMD3167)• Mostadhesiontestsinfertheadhesivestrengthbysubjectingthe

filmtoanexternalloadandmeasuringthecriticalvaluewherefailureisobserved

• Usefulforroutinequalitycontrol• Mosttestmethodsmeasureadhesionbydelaminatingafilmfrom

thesubstrate– Energytobreakthethinfilmandsubstratebonds– Energytoplasticallydeformthethinfilmlayerandsubstratelayerasthe

filmispulledaway– Frictionofthethinfilmsurfaceandsubstratesurface

SchematicPeelTest

EnergytodeformthinfilmlayersM1 isincludedintheenergyrequiredtopeeloffthethinfilm.Ifthethinfilmisrelativelythick,ormadeofastiffmaterial,theplasticdeformationenergycanbecomparabletotheadhesionterm.Whatisactuallymeasuredintheinterfacialfracturetoughness.CouldbeasignificantfactorforDPCadhesionbecauseofthick,ductileCulayer.

Regionsdeformingplasticallyduringthepeeltestareshaded.

FundamentalTestsforMeasuringAdhesion

MeasurethestrainenergyreleaserateU(J/m2)orworkrequiredtopropagateacrackalongtheinterface

Indentationinitiatesaninterfacialcrack.Widthofresultingdelaminationblisterismeasured.MechanicalpropertiesoffilmmaterialsareusedtoderiveU.

FundamentalTestsforMeasuringAdhesion

Thesubstratelayeriscutandabendtestisusedtodriveacrackalongtheinterface.

FundamentalTests

• DevelopedatIntel,StanfordUandMotorolaforevaluatingsemiconductormetallizationschemes

• Requiresmathematicalmodelstointerpretdata• Value:

– Allowsoptimizationofadhesionseparatedfromothereffectssuchasfilmdeformation

– Canprovidefundamentalinformationthatcanbeusedtopredictreliability

– Allowsmeaningfulcomparisonofdifferentmaterials

PracticalAdhesionTesting

• Measureadhesionstrength+energytodeformlayerduringtesting

• Goodrelativemeasureofadhesioniffilmcharacteristicsareconstant

• Forexample,usefulforDPCifCuthicknessisuniform.

PeelTestApparatus

•Mechanicaltester(Instron)• Loadcell• Peelfixturedesignedforconstant90° peelangle• Peelforcevs Displacement

PeelTesting- TypicalMetaltoMetalPeel

FailureatCu/CrInterface

ExamplefromCMC:CeramicFailure

• CuNi/TiWonAlN• BrazedLeadPeel• Ceramicfailure• Ceramicfailureisdictatedbyflawpopulation

PracticalAdhesionTests- UsefulforWhat?

• ProcessOptimization• Specificproductspecificationsuchasaleadpullvalue

• PackageQualification– CharacterizeQualificationsamplesforeachcustomer– Adhesionvaluescanbetiedtocustomerestablishedreliability

data– Canevaluateadhesionatanytimerelativetoqualifiedsamples– Canuseadhesiondatafromqualificationtoestablishprocess

changesandalsotointroduceequipmentchanges• PeriodicQualityControl

TroubleshootingGuideforSputteringonAl2O3 orAlN

Issue LocationofIssue PotentialFactorstoAddress

Pooradhesion Ceramictometal Surface contaminationonceramic

Micro-crackdamagelayerpresentonceramic(intra ceramicandceramictometalfailure)

Thinfilmlayer withhighstress

Thinfilmdepositionsubstratetemperature toolow

Oxygencontaminationofadhesionlayerduringdepositionbecausebasevacuumistoohigh

Chemicalattack frompostthinfilmprocessing.Poresinthinfilm.

HighTemperature inducedCTEstress.Thinfilmdepositiontemperaturetoohigh.

TroubleshootingGuideforSputteringonAl2O3 orAlN

Issue LocationofIssue PotentialFactorstoAddress

Pooradhesion Adhesionlayertobarrierlayer

Oxygencontaminationofadhesionlayerduringdepositionbecausebasevacuumistoohigh

Barrierlayerwithhighstress.

Thinfilmdepositionsubstratetemperaturetoolow

Chemicalattack frompostthinfilmprocessing.Poresinbarrierthinfilmlayer.

HighTemperature inducedCTEstress.Thinfilmdepositiontemperaturetoohigh.

TroubleshootingGuideforSputteringonAl2O3 orAlN

Issue LocationofIssue PotentialFactorstoAddress

Blistersformafterexposuretoheat>300C

Ceramic andadhesionlayerdelamination

Pores intheceramicsurfaceresultinaporeinthethinfilmlayer.Afterexposuretopost-thinfilmchemicals,theseporestrapliquidsorgas.Uponheatexpose,theliquidsorgasexpandformingablister.

All ofthefactorsinslide61thatleadtopooroveralladhesionmayalsoleadtomorelocalizedblistering.

AdhesionLayerandBarrierLayerDelamination

All ofthefactorsinslide62thatleadtopooroveralladhesionmayalsoleadtomorelocalizedblistering