ELSEVIERSurfaceScience301 (1994) 203-213 .......~..:.~::::::::::j::::i::::::j:.: ....::....~:.:.:..:.:.:.:..:.:.:.:.. ~,:,),,, .-..:>:............-.I. . . . . >)>:..>:,):.). . . . . :i :.:.,,.:.:,i_.,.,):,, ,,., ........ Iliil~~,~,~~~:~,~i.::::~:.~:.:.:.:..i.s.i.. . . . . ..,.,,,,,,,,,,, :.:.:._.:.~.:,::;,:_~):~)) {p+ .,..:,:,: ..._i sur f ac esc i enc e ::..::x ..._.,.,., ..~..........................~,....,,,.,.,., ..ii...?....(.::.:.,,.,.,,,,,,,,,,,,, .,.,.>:::::;;;i . .,.....:.:?:::.::::::::::;:::: .:.-~:...-,.:.:............ ,,.,.,.,,i,:,,,,,,.,,, .i...:.:.:.:>:.:.::::.:\::,:.:_~:,.,,_ ..::.:::::i:::::::i:~:::::::::.:::j.,.:.:.~,.,.,:;, .,: (,,,,., Characterizationofthintitaniumoxideadhesionlayersongold: resistivity,morphology,andcomposition K.W.Vogt,P.A.Kohl*,W.B.Carter,R.A.Bell,L.A.Bottomley GeorgiaInstituteofTechnology,Atlanta,GA30332-0100,USA (Received22March1993;acceptedforpublication10August1993) Abstract Group1Bmetalfilms(copper,silverandgold)areattractiveformetallizationsinmultichipmodules(MCM)and integratedcircuitsbecausetheyhavehighelectricalconductivities.Unfortunately,Group1Bmetalsrequire additionalbondinglayersforadhesiontoinsulators(i.e.silicondioxideorpolymers).Inthiswork,thinelectrically insulatingfilmsoftitaniumoxideontitaniumhavebeeninvestigatedasadhesionlayersbetweengoldandawide varietyofinsulators.Theadhesionlayerdoesnotalterthedielectricpropertiesoftheinsulatorsurroundingthe metalbecauseitisthin.Themorphology,composition,andresistivityofthetitaniumoxidefilmswerestudiedwith angleresolvedX-rayphotoelectronspectroscopy(XPS),scanningtunnelingmicroscopy(STM),andelectrical resistancemeasurements.Theresultsshowthatsputter-depfsitedtitaniumfilmsgrowbyanislandgrowth (Volmer-Weber)mechanism.Theislandscoalesceafterlo-20Aoftitaniumdeposition.Followingdeposition,the titaniumfilmswereoxidizedbyexposuretoairatrelativelylowtemperatures(T wereoxidized,alayeredfilmformedwithasub-oxide (TiO)coreandatitaniumdioxidesurfacelayer.Whenthickerfilms(>20A>wereoxidized,alayeredfilmwasalso producedwithatitaniumcoreandtitaniumoxidesurfacelayer. 1.Introduction ThehighconductivityofGroup1Bmetalfilms (copper,goldandsilver)isappealingforusein metallizationsformicroelectronicsinterconnec- tions.Inaddition,theinertnessofgoldmakesit attractiveforhighreliabilityapplications.Transi- tionmetalfilmsarecommonlyusedasadhesion layersformetalswhichdonoteasilyformstable oxides.Theadhesionpropertiesoftransition *Correspondingauthor. metals,particularlyGroupsIVA-VIIIA,have beencorrelatedtotheheatofformationofthe oxide[ll.Group1Bmetals(copper,silver,and gold)donotformoxidesadequateforadhesion. Recently,athinfilmoftitaniumoxideon titanium(hasbeeninvestigatedasan adhesionlayerbetweengoldandsilicondioxide orpolymers[2,3].Thetitaniumoxideisdeposited onthemetallizationtoprovideanadhesivefilm forthesubsequentdepositionofaninsulator.In additiontobeingelectricallyinsulating,titanium oxidefilmsdonotalterthedielectricproperties oftheinsulatoron,orunderthemetalbecause theyarethin.Thetitanium-goldmetallurgyalso 0039-6028/94/$07.0001994ElsevierScienceB.V.Allrightsreserved SSDI0039-6028(93)E0490-L 204K. WVogt r l al . /SwfacrSciencr301 f l YY4) 203- 213 makestitaniumadesirableadhesivematerial. Althoughtitanium-goldinterdiffusionisfast,ti- taniumout-diffusesalonggrainboundariesand quicklyoxidizesatthesurfaceofthegold.The surfaceoxidationcausescontinuedoutdiffusion andachemicalpotentialsink[4].Thus,stable titaniumoxidescanbeformedonthesurfaceof gold.Thisbehaviorisunlikesometransitionmet- alsongoldwhichdiffuseintothegoldandsignifi- cantlyreduceitselectricalconductivity111. Inthis study,thintitaniumadhesionlayerswerecharac- terizedtounderstandthenatureofgrowthand adhesionmechanismofthefilmstogold. Titaniumfilmshavebeendepositedbyseveral techniquesincludingchemicalvapordeposition [5],evaporation[6],andsputtering[7].Inthis study,thepropertiesofthintitaniumfilmsde- positedwithDCmagnetronsputteringhavebeen analyzedbyelectricalresistancemeasurements, XPS,andSTM.Thesetechniqueswillhelpchar- acterizetheresistivity,composition,andmor- phologyofthethintitaniumfilms. 2.Experimental Titaniumandgoldfilmsweredepositedse- quentiallybyDCmagnetronsputtering.The sputteringtargetswere99.99%titaniumand 99.99%gold,andthebasepressurewas2X10ph Torr.Thetemperatureduringthedepositionwas below50Candthed?positionrateforbothgold andtitaniumwas60A/min. Threesetsofsampleswereprepared.Electri- calconductivitymeasurementsweremadeby sputteringtitaniumbetweentwocloselyspaced goldstripsonasilicondioxidesubstrate.The electrodeswere3.5cmlong,3mmthick,andhad 40mmlinesandspaces.Theresistanceofthe titaniumfilmbetweentheelectrodeswasmea- suredasafunctionofpost-processingconditions. Forcompositionandmorphologystudies,3,10, and75Atitaniumfilmswereinvestigated.Identi- caltitaniumfilmswerepreparedonbothevapo- ratedgoldandsputteredgoldosubstrates.The evaporatedgoldfilmwas2500Athickandde- positedonmica,andthesputteredgoldfilmwas 1500Athickanddepositedonsilicon.After oxidationinairatroomtemperaturefortwo weeks,thecompositionofthefilmswasmeasured withsputter-profileXPS,andthemorphology wasevaluatedwithangleresolvedXPSandSTM. ASurfaceScienceLaboratories,Inc.SSX-100 X-rayphotoelectronspectrometerwithanAlKa X-raysource(1486.6eV>wasusedforangle resolvedandsputter-profileXPSstudies.The basepressurewaslowerthan1 xlo-Torr.Ar- gonionsputteringwasusedtoetchawaythe surfaceduringprofiling.TheKratosMini-Beam IIiongunwasoperatedat4keVinanargon pressureof3.7xlo-Torr. STMimageswereobtainedwithaDigitalIn- struments,Inc.NanoscopeIIscanningtunneling microscope.TheSTMtipswerepreparedbyme- chanicallyshearingplatinum/iridiumwire.Im- ageswereobtainedintheconstantcurrentmode atatunnelingcurrentof0.5-1.0nA,andthe tunneljunctionbiaswasbetweenf50andk250 mV.Mostimageswereacquiredatscanningrates between2and3Hz.Theimageswereflattened priortoanalysis. 3.Angle-resolvedXPSmorphologymodel Duringthedepositionoftitanium,filmgrowth canoccurbyseveralmechanisms[8].During Frank-VanderMerwe(layer-by-layer)growth, onemonolayerofcoverageiscompletedbeforea secondbegins.Thesecondpossiblegrowthmech- anismisStranski-Krastanov(layer-plus-island- ing)growth.Afterthesubstratesurfaceiscom- pletelycoveredbythefirstmonolayer,subse- quentdepositionofmaterialformsislands.The thirdmethodisVolmer-Weber(island)growth, whereislandsnucleate,grow,andcoalesceas depositioncontinues. Inthiswork,angleresolvedXPSandSTM havebeenusedtocharacterizeanddistinguish betweenthefilmgrowthmechanisms.Alayer- by-layermodelforangleresolvedXPSwasfirst discussedbyHenke[9]andfurtherdevelopedby Fadley[lo].TheXPSintensityfromasemi-in- finitesubstratecoveredwithanoverlayermate- rialwasmeasuredasafunctionofphotoelectron take-offangle,0.TheintensityoftheXPSsignal K. W.Vogt et al. /SurfaceScience301(1994)203-213205 fromthesubstrate(Isubstrate),wasattenuatedex- ponentiallybytheoverlayer. I substrate= l;bstrateeXP(-t/hsubstratesine>.(1) Theintensityfromasemi-infinite,atomically cleansurfaceofthesubstratematerialwas h:bstrate~thethicknessoftheoverlayerfilmwast, theattenuationlengthforthephotoelectrons fromthesubstratematerialastheytravelthrough theoverlayerwasAsubstrate,andthephotoelectron take-offanglemeasuredfromtheplaneofthe samplewas0. TheXPSintensityoftheoverlayermaterial (loverlayer)wassimultaneouslymeasured.Thepho- toelectronsoriginatingfromtheoverlayerfilm arealsoattenuatedexponentiallywithdepth. I OVerlayer=Z:.erlayer[I-exp(-t/*oerlayersine)]. (2) Theintensityfromasemi-infinite,atomically cleansurfaceoftheoverlayermaterialwas I,,er,ayer,andtheattenuationlengthofthephoto- electronfromtheoverlayerasittravelsthrough theoverlayerfilmwasAoverlayrr. Asingleparameter,R(O),isobtainedforthe layer-by-layermodelafterdividingeq.(2)byeq. (1). I R(O)= oerlayer/Z&_rlayer Z substrdzs%strate 1 -ew( -t/Aoverlayersin0) = exp(-t/hsubstratesine, (3) AmodelfortheVolmer-Weber(island) growthmechanismhasalsobeendeveloped[ll]. Thefilmhasbeenapproximatedasapatched overlayeroftheovercoatmaterial.Inthismodel, theobservedintensityfromthesubstrate (Isubstrate) hascontributionsfromtheovercoated andbareportionsofthesubstrate.TheXPS intensityfromtheovercoatedsubstrate,eq.(4), wasattenuatedbyislandswithuniformheight,h, whereas,thesignalfromthebaresurfacewasnot attenuatedbytheovercoatlayer. I substrate=ZsmubstrateI(1- Y1 +Yexr4-h/bubstratesinQl .(4) Thefractionofthesurfacecoveredbythefilm wasy,andtheuncoveredfractionwas(1 -Y>. Likewise,theXPSintensityfromtheoverlayer (Zoverlayer 1hasanexponentialattenuationwith thicknessbecausethephotoelectronsoriginate fromdifferentdepthsintheoverlayerfilm,as shownbyeq.(5). Z OVerlayer =Y Z:erlayer[ 1 -exp(-VLerlayersine)]. (5) Theanalogousquantitytoeq.(3)(R(B),layer- by-layermodel)fortheislandgrowthmodelis givenbyeq.(6)whichwasobtainedbydividing eq.(4)byeq.(5). Zoverlayer/Zoave*layer R(B)=Z_ substrate/zs:bstrate Y[1-ew(-Woverlayer sine)] [@ -Y)+ Yexp(-h/Asubstratesine>I (6) Thefourquantities,zsubstrate,z;bstrate~ zoverlayer~andGerlayer7 havebeenmeasuredasafunction ofthephotoelectrontake-offangle(substrate= Table1 Measuredresistance(R)andcalculatedresistivitv(4)asafunctionoftitaniumthicknessandamountofoxidation Oxidizedat95C 0min5min10min R(n)p(fi.crn)R(a)p(R.cm)R(0)p(n.crn) lO.&Ti1.70x1014900>9.99x10s>87400>9.99xlox>87400 20.& Ti160000281.30x10829600>9.99x10>175000 30ATi100.002651.60.01441000.0260 206K. W.Vogtetal. /SurfaceScience301(1994)203-213 gold,overlayer=titanium)andfittedtothetwo modelsdiscussedabove.Non-linearregression analysiswasusedtoanalyzethefitofeq.(3)and Ti *Ps/2 A(a) rii(t4) . v Tit?$0) 30seconds 4694th463460457454451 BiidiigEnergykv) rM Ti(+4)Tit+2Ti(O) ++$ cIIIII 4694664634604574544 BiidiigFnergykV) 51. Fig.1.Titanium2pXPSspectrafor(A)the3i\titaniumfilm(B)the75I% film,and(C)the10Afilmasafunctionofargon etching. eq.(6)totheR(0)versus0data.Theoverlayer thickness(t)orislandheight(h)andfractional coverage(y)wereestimatedfromthedeposition processandwereconfirmedfromtheregression analysis.Furtherdiscussionconcerningtheas- sumptionsintheangleresolvedXPSmodelsis includedintheAppendix. 4.Resultsanddiscussion 4.1.Resistance/resistivity measurements Theelectricalresistancebetweentwo3.5cm longelectrodeswith40mmspacingandtitanium overcoatthicknessesof10,20,and30wwere measuredafterheattreatmentinairat95C (4.5%relativehumiditymeasuredat20C)for0, 5,and10min.ThemaximumDCresistanceof themeterwas1GR.Theresistivityofthetita- niumfilmswascalculated,R=pL/A,whereRis themeasuredresistance(a),pistheresistivity K.W. Vogtetal./SurfaceScience301(1994)203-213207 (0.cm),Listhedistancebetweentheanodes (cm),andAis thecross-sectionalarea(thickness timeselectrodelength)ofthefilm(cm2).Asthe filmthicknessincreasedfrom10to30A,the resistivitysignificantlydecreased,asshownin Table1.Second,theresistivityofeachfilmin- creasedwithairexposuretimeat95C.Witha10 minexposuietoairat95Ctheresistanceofthe 10and20Athickfilmswasgreaterthan1GR, themeterslimit.Forthe10Afilm,a5min exposurewassufficienttoproducegreaterthana gigaohmresistance. Theseresultsshowthatoxidationof10 and20 Afilmsfor10minwassufficienttoconvertthe titaniumconductivepathtoa non-conductivepath (i.e.titaniumoxide).Ifthefilmsweregrownwith anislandgrowthmechanism,oxidationofthe outershellofthetitaniumislandswouldhave adequatelyeliminatedanyconductivepaths, whereaswithalayer-by-layergrowthmechanism, thebulkofthefilmwouldhaveoxidized.Thicker films(30A> areeithernotcompletelyoxidized,or thevoidsbetweentheislandsarefilledwithmetal causinganincreaseintheconductivity. 4.2.Sputter-profileX-rayphotoelectron spectroscopy TheTi2pXPSspectrafor3,10,and75A titaniumovercoatsongoldwereobtainedasa functionofargonionsputter@gtime,asshownin Fig.1forthe3and75Atitaniumongold samples.Thepeakpositionshavebeenrefer- encedtotheadventitiousC 1s peakat284.6eV. Inadditiontothecarbonreference,thebinding energyforgoldwasmeasured,anditsposition wasunchanged.Thespectracanbeinterpretedin termsofthreedoubletsassociatedwithTi(zero valencestate),TiO(+2oxidationstate),and TiO,(+4oxidationstate)[12]. Thepeaksineach doubletcorrespondtophotoemissionfromthe titanium2p,,,andtitanium2~i,~energylevels. Thethree2p1,*p eaksarelocatedbetween461 and467eV,andthethree2p3,2peaksarelo- catedbetween453and461eV.Thetitanium 2P 3,2 peaksarecommonlyusedfortheidentifi- cationofthetitaniumoxidationstatesbecauseof theirhighphotoemissionintensity[13]. Thebind- ingenergyofthetitanium2p,,,peakforthe3 A titaniumovercoatsampleoccurredat458eV after0,10,20,and30sofsputteretching,as showninFig.la.Thisbindingenergycorre- spondstoTi4+(i.e.TiO,).Thisindicatesthat withinthesensitivityofXPS,the3Afilmwas TiO,. Fig.lbshowstheXPSspectraforthe75ATi filmat0,200,and1100sofsputteretching.The oxidationstateoftitaniumonthesurfacewas primarilyTi4+,justlikethe3Afilm.After200s ofsputteretching,theTi2p,,,peakshiftedto 454eV,correspondingtoTi.Thepeakat461eV correspondstotheTiO2~,,~signal.After1100s ofsputteretching,onlyasmallquantityoftita- @urnremainedonthegoldsurface.Thus,the75 Afilmwasalayeredstructurewithanoxide surfaceandametalcore.Sinceasub-oxidewas notobservedafterasignificantamountofsputter etching,itwasconcludedthatsputteretchingdid notreducetitaniumoxideoroxidizetitaniumto formasub-oxide(Ti2+l. Fig.lcshowsthattheoxidationstateofthe surfaceofthe10 Atitaniumfilm(priortosputter etching)wasalsoprimarilyTi4+(TiO,),analo- goustothe3Afilm.However,unlikethe3A film,theobservedoxidationstateafter45,90and 135 s ofsputteretchingwasfoundtochangefrom Ti4+toTi2(thebindingenergyoftheTi2p,,, peakshiftedfromto458to456eV).Elemental titanium(bindingenergyat454eV)wasnotob- served.Thus,assumingsputteretchingdoesnot producetitaniumsub-oxide,the10Atitanium filmwasalayeredstructurewithaTiO,surface andasub-oxidecore(TiOorpossiblyTi,O,). Previousstudieshavefoundsimilarresultsfor roomtemperatureFxidation.Evans[14]hassug- gestedthat6-12Aofoxideformontitanium metalwithinthefirstfewsecondsofoxidation, followedbyaslowoxidegrowthwhichceases after70days.Othershaveshownthatoxide growthontitaniumisalogarithmicfunctionof temperaturebelow300C[15,16].Theoxidation quicklybecomesdiffusionlimitedatlowtempera- turesbecauseoftheinwardmovementofoxygen totheoxide-metalinterface[15,17,18].Kofstad [15]hasshownthattheoxidefilmwhichformed ontitaniumhasasub-oxide(TiOandTi,O,) 20XK.KVogtetal./SurfaceScimce30111994)203-213 attheinterfacebetweenthetitaniummetaland thesurfaceTiO,.Similarresultswerefoundby Aouadietal.fortheoxidationoftungstenfilms [191.Inthiscase,WO,wasfoundbetweena tungstenmetalsubstrateandaWO,surfaceox- ide. 4.3.Angleresolued X-rayphotoelectron spectroscopy Inthissection,theangle-resolvedXPSdata will bepresentedandcomparedtothetheoretical predictionsdiscussedearlier.Thetitanium2p 2.5 - I 23 - \ \ \ \ \ L (al -_+_-__- _,___ ------ 102030405060706090 photoeiectron takeoffangle tdeam) \ \ lb) 0*---_-.---t--_-pi_-_-_.__.~+---d 010203040506070Bo90 photoelectron takeoffangle (degrees) 31\ l \ . 2 11 l . *'. .. '. 1+,_. *. I ? 0l-y._____-.._-.-_.__.r ____ 01020304060607060xl photoelectron takeoffangle (degrees) Fig.2.Theintensityratio,R(B),asafunctionofthephotoelectrontake-offangleforthe(a)3iTi/Au/Sifilm,(bt10A Ti/Au/Sifilm,(c)3ATi/Au/micafilm,andCd)10I% Ti/Au/micafilm. K.W.Vogt et al./SurfaceScience301(1994)203-213 0102030405060708090 photoelectron take-off angle (degw1 Fig.2 (continued). 209 andgold4fXPSspectraweremeasuredasa functionofphotoelectrontake-offangle(thean- glebetweentheplaneofthesampleandthe entrancetotheanalyzer).Inthiscase,theover- layerfilmistitanium(oxide)andthesubstrateis gold.Th,emeasurementsweremadefor0,3,10, and75Atitaniumfilmsonevaporatedandsput- teredgoldzubstrates.Thethicksputteredgold film(1500A)wasdepositedonsiliconandthe thickevaporatedgoldfilm(2500A>wasde- positedonmica.Analysisoffilmsonsputtered gold/siliconsubstratesareusefulforstudyof microelectronicsapplications.Evaporatedgoldon micasubstratesformatomicallyflatsurfacesand aresuitedforcharacterizationofadsorbates[201. Nophotoelectronswereobservedfromeitherthe siliconormicabeneaththegold.Theintensities (Zi)ofthetitanium2pandgold4fXPSspectra weredeterminedbynumericallyintegratingthe areaofeachphotoemissionpeakusingalinear backgroundfunction.Theintensityratios,R(8), werecalculatedandplottedversu:thephotoelec- trontake-offanglefor3and10 Atitaniumfilms forbothsputteredgold(sput.Au)substrates(figs. 2aand2b)andforevaporatedgold(evap.Au) substrates(figs.2cand2d).Themeasuredvalue forthicktitanium,ZTi, wasobtainedfromthe75 AtitaniumsampleandthevalueforZiUwas obtainedfromthecleangoldfilms. Theintensityratio(R(0))forthe3ATi/sput. Ausample,Fig.2a,decreasesslightlywithin- creasingtake-offangle.Theintensityratioforthe 10 ATi/sput.Au,Fig.2b,behavessimilarly,but theintensityoftheXPSsignalfromtheovercoat layeris greater.Atsmallerangles,thepatio,Z?(8), againincreases.Similarlyforthe3ATi/evap. AudatainFig.2c,R(B)isnear3forsmall take-offanglesand1fcrlargertake-offangles. Lastly,R(8)forthe10 ATi/evap.Austartsnear 5forsmallanglesanddecreasestoabout2for largeangles. Thelayer-by-layermodel,eq.(3),andtheis- landmodel,eq.(61, wereusedtofitthedataand arealsoincludedinfigs.2a-2d.Theadjustable parametersforthemodelswerelayerthickness 0)forthelayer-by-layermodelandislandheight (h)andfractionalcoverage(y)fortheisland model.Theelectroninelasticmeanfreepaths(A) werecalculatedusingthemethodsofTokutakaet al.[21],SeahandDench[22],andPenn[23]. AveragevatuesofA,,=22A,hTi =21A,and Ario,=27Awereusedinbothmodels.These valuesaresimilartothosepreviouslyreportedfor goldandtitanium[10,12,21,24,251. Thetwomodelsareeasilydistinguishedatlow take-offangles,whentheXPSanalysisdepthis approximatelyequaltothefilmthickness.Atlow angles,theexposedsubstrateintheislandmodel allowsasignificantgoldsignaltobeobserved, whereasthelayer-by-layermodeldoesnot.The calculatedvaluesofR(8)forthetwomodelsare plottedinFig.2.Theislandingmodelveryclosely fitsthedatawhenovercoatthicknessesof10, 42, 12,and20Awereusedforfigs.2a-2d,respec- 210K. W.Vogtetal. /SurfaceScience301( 1994)203- 213 tively.Apossiblesourceofthesmalldeviations fromtheislandingmodelcanbeattributedto shading/roughnesseffectsignoredinthederiva- tionofthemodels(seeAppendix).Atsmalltake- offangles(0SO>, theroughsubstrateaccentedICwerlayer resultinginalargerR(B)thanpredicted,espe- ciallyforthesputteredAu/siliconsubstratewhich hasaroughersurfacethanevaporatedAu/mica. Theislandheight(h)andfractionalcoverage(y) yere28Aand47%forothe3ATi/sput.eu,46 Aand94%for,the10ATi/sput.AU& 18Aand 74%forth:3ATi/evap.Au,and30Aand82% forthe10ATi/evap.Au.Thevaluesofovercoat A thickness,islandheightandfractionalcoverage arehigh,butinsemi-quantitativeagreementwith thequantityoftitaniumdeposited,withinexperi- mentalerror.Theyareviewedassemi-quantita- tivebecausethemodelassumesaspecificshape whichonlyapproximatesthenaturalvariation. Whentitaniumisoxidizedtotitaniumdioxide, thevolumeofthefilmapproximatelydoublesand thedensitydecreases,assumingthattheTiOz formedissimilartobulkanatase.Thedeposition rateoftitaniumwasestimatedtobe1 A/sbased on500-5000sdepositions;however,theactual depositionrateinthefirstsecondsofsputtering couldnotbemeasured.Also,theerrorinthe depositiontimewas+1sduetothesample rotationandshuttermechanisms.However,the Fig.3.STMimagesof(A)cleangold/siliconsubstrate,(B)10Atitaniumongold/silicon,(C)cleangold/micasubstrate,and(D) 10Atitaniumongold/mica. K. W. Vogi etal. /SurfaceScience301 (I 994) 203-213211 factthatalltheapproximationsarehighsuggests thatthiserrorisnotimportant.Theinelastic meanfreepathswerealsoapproximationswhich mayhavebeenoverestimated.Thiswouldresult inelevatedpredictionsforislandheight.Inany case,thelarge;esistivitiesobservedforthick- nessesbelow30Aareduetoincompletetitanium coverage. oxidation)of10 Aoftitanium,50-100Adiame- ter,and20-30Atallislandswereeasilyidenti- fied,asshowninFig.3D.Thevalleysbetween theislandsappeartoreachto(ornearlyto)the goldsubstrate.Theheightofthetitaniumislands correspondwelltothenumericalvaluesusedin theR(B)islandgrowthmodelabove. 4.4.Scanningtunnelingmicroscopy(STM)5.Summary STMimagingoftheTi/Ausampleswasper- formedtoinvestigatethefractionalcoverageand sizeofthetitanium/titaniumoxideislands.Im- agesofthesputteredgoldonsiliconandevapo- ratedgoldonmicasamples(withandwithout titaniumovercoats)wereobtained.Fig.3A,isa 1000 x1000nmSTMimageofclean,sputtered goldonsilicon.Thesurfacewascoveredwith 50-100nmdiameterhillocks.Thethree-dimen- sionaldatawereanalyzedstatisticallyinorderto estimatethesurfaceroughness.Theheightofthe islandswasnormallydistributedwithameanof 3.4nmandavarianceof2.1nm. Theelectricalresistance,XPS,andSTMre- sultsallsupporttheislandgrowthformationof titaniumongold,asillustratedinFig.4.This modelcanbeusedtounderstandhowthethin titaniumfilmsfunctionasadhesionlayerson gold.Thetitaniumnucleatesaadgrowsinis- lands,asevidencedbythe3Atitaniumfilm, whichsparselycoversthesurfacewithTiO,is- lands.Themonolayeroftitaniumdirectlyonthe ASTMimageof10 ATi/sput.Auis shownin Fig.3B.ComparedtoFig.3A,thesurfaceno longerhasuniformmounds;instead,itisrough withislandsofmaterialonthesurface.The heightsoftheislandsarenormallydistributed withameanof4.9nmandavarianceof6.0nm. Thus,thetitaniumformedwithanisland-like morphology.Themeanheightanddistributionof thesurfacetopographysignificantlyincreased overthevaluesforthesubstrate.Ifthetitanium filmhadformedinauniform,conformallayer- by-layerfashion,themeanheightanddistribution wouldnothavechanged. TitaniumDeposition 3A (0.1mg/cm3 114 TitaniumDeposition IOA (0.5mg/cm? 444 Althoughthetitanium/sput.Au/siliconsam- plescomparewithadhesionlayerfilmsinmicro- electronics,atomicresolutionimagesarecompli- catedbytheroughnessofthegoldonsilicon. Imagesofatomicallyflat,evaporatedgoldon micaweretakentomorecloselyexaminethe morphologyofthetitaniumdeposition.Previ- ously,thissubstratehasbeenusedtoobtain atomicscaleimagesofthinadsorbatefilms[22,27]. Fig.3Cshowsatomicallyflatgoldplateauson micawitha3Astep.Afterdeposition(and TitaniumDeposition 20-30A (091.4mg/cm3 411 TitaniumDeposition >30A (>1.4mgkm2) Fig.4.Compositionandmorphologysummaryforthethin titaniumadhesionlayers. nnn 212K. W. Vogt et al. /SurfaceScience301(1994)203-213 goldmaybeintheTi2+oxidationstate.Asthe growthproceeds,asinthecaseofthe10A titaniumfilm,theindividualislandsgrowcloser together.When-oxidized,theislandshaveaTiO core(andgossiblyTilandarecoveredwithTiO,. After20AofTideposition,theislandscoalesce. Thecoreofthetitaniumisnotcompletelyoxi- dized,andthefilmsresistivitydoecreases.After depositionandoxidizationof30Aoftitanium,a layeredfilmformswithatitaniumbaseandoxide surfacelayer.Theoxidegrowthislimited,most likelybythediffusionofoxygenthroughothe oxidelayer.TheXPSdepthprofileofa75ATi filmdemonstratesthatthefilmislayeredwitha titaniumcoreandaoxidesurfacelayer. filmformswheretheislandshaveasuboxidecore andtitaniumdioxidesurfacelayer.Forthicker titaniumfilms(>20A),oxidationproducesa layeredfilmwithatitaniumcorelayerandtita- niumoxidesurfacelayer. Appendix:assumptionsinthelayergrowthmodel Severalassumptionsweremadeinthederiva- tionofthesemodels.Theirvalidityandeffectson resultsarediscussedbelow: Theseresultssuggesthowtheadhesionlayer functions.Themetalcorelayerbondstothegold substrate.Eventhoughonlythinlayersareused, theadhesivelayerinthemonolayerclosesttothe goldisadequateforbonding.Becausethelayers areverythin,theelectricaleffectsofTiO,are negligibleonthepropertiesofthemetalsand insulators.Althoughthinlayersofanadhesive materialareadequateforadhesion,theabilityof titaniumtodiffuseoutofgoldandformapoten- tial-sinkoxideisessential.Incontrasttothis behavior,verythinfilmsofmetalsandmetal- oxideswhichprefertoresidewithinthesubstrate (forexampleatgrainboundaries)wouldnotbe expectedtoservewellaslong-termadhesionlay- ers.Lastly,thetitaniumoxideandtitaniummetal stronglyadheretoeachotherbecausethetita- niumoxideisgrownatlowtemperature[261. Titaniumoxidegrownathightemperaturesis susceptibletocracking,flakingandoxide/metal separation. (1)Thesurfaceofthesubstratewassmooth androughnesseffectswereneglected.Bairdetal. [27]studiedtheeffectsoflargescaleroughness (ontheorderoflo4A>.Ingeneral,theyfound thatroughsubstratesurfacesaccentthesubstrate signalatlowtake-offanglesandaccenttheover- layersignalatlargetake-offangles.However,the effectsofsmallscaleroughness(ontheorderof 50A> werenotobserved,eventhoughsomeef- fectswereexpectedbasedonthetheoretical derivationsofFadley[lo]andWagnerandBrum- mer[28]. EbelandWernisch[29] alsostudiedthe effectofroughnessusinggroovedsurfacesand foundthattheroughnessdepthcanbeatleast tentimesthemeanfreepathoftheincident X-raysbeforeshadingeffectsoccur.Thirdly, Fadley[l11 andKimandHummel1301havestud- iedgoldfilmsandsuggestedthattheycanbe consideredasflatfilms. (2)X-raydiffractionoftheincidentbeamwas neglected.Thisisagoodassumptionforamor- phousandpolycrystallinematerials. (3)TheincidentX-rayfluxwasuniform.Any non-uniformX-rayfluxwasdeterminedexperi- mentallybymeasuringZ~~~_,averandZghstrate.Thus 6.Conclusiontheeffectofanynon-uniformfluxcancels. Ongoldsubstrates(sputteredandevaporated goldfilms),sputter-depositedtitaniumfilmsgrow byanislandgrowth(Volmer-Weber)meochanism. Theislandsgrowtogetherafterlo-20Aoftita- niumdeposition.Afteroxidationofthetitanium filmsinairatlowtemperatures(T, very thintitaniumfilms(3A> oxidizecompletely.For thintitaniumfilms(10A>,alayeredisland-like (4)RefractionandreflectionofX-raysatthe surfacewereneglected.Thisassumptionisvalid forincidentanglesgreaterthanthecriticalangles determinedbyFadley[ll].Typically,thisangleis 2 forgoldsurfaces. (5)Theelectronswereattenuatedexponen- tiallyalongtheirpathlength.Thisisafairas- sumptionintheXPSkineticenergyrange.Re- cently,severalresearchershavefounddeviations K. UC Vogtetal. /SurfaceScience301(1994)203-213213 wheretheattenuationlengthisnotonlyafunc- tionofthematerialbutisalsoafunctionof elasticscattering[31,32,331. (6)BecausetheX-rayattenuationlengths (1000-10000A> weremuchgreaterthanelectron attenuationlengths(10-100A>,X-rayattenua- tionwasneglected. (7)Theelectronpathlengthwasindependent ofthedepthoforiginandangleofescape.Thus, thepathlengthwasonlyafunctionofelectron energyandthematerialinwhichitwastraveling. (8)Electronrefractionandreflectionatthe surfacewerenegligible.Fadley[illhasshown thattheseeffectsoccurmostlyfortake-offangles lessthanlo.Also,theeffectwasminimizedby takingtheratio(loverlayer/l~erlayer)/(Lbstrate/ II substrate . (9)Theeffectsofelasticelectronscatteringon angulardistributionswasneglected.Nefedovet al.[34]investigatedtheeffectsofelasticscatter- ingandonlyfoundsmallchangesinthetake-off angleandmeanfreepath,hi. 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