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.
(10)Theeffectofdiffractionoftheemitted
electronswasneglected.Theapproximationis
validforamorphousandpolycrystallinesamples.
(11)Theattenuationeffectofadventitious
carbonwasneglected.Bairdetal.[27] foundthat
thisassumptionwasavalidfirstapproximation
whentheratioofthesubstrateandoverlayer
intensitieswasused.YabeandYamashina[35]
experimentallydeterminedtheattenuationeffect
ofsurfacecarboncontaminationontheXPSsig-
nalandfoundthatitwaslessthan2.5%. References
[llJ.M.Poate,K.N.TuandJ.W.Mayer,ThinFilms-
InterdiffusionandReactions(Wiley,NewYork,1978).
[2]T.A.Cloud,M.R.Houston,P.A.KohlandS.A.Bidstrup,
EvaluationoftheProcessingandPerformanceofNoble
MetalMCMs,InternationalConferenceonMultichip
ModulesProceedings,ISHM(1993)p.451.
[3]T.A.Cloud,M.R.Houston,P.A.KohlandS.A.Bidstrup,
IEEECHMT,submitted.
[4]J.M.Poate,P.A.Turner,W.J.DeBonteandJ.Yahalom,
J.Appl.Phys.46(1975)427. 151 161 [71 IS1 191 [lOI 1111 1121 [131
[141 [151 [I61 [171 1181 [191 [201 1211 1221 [231 [241 (251 [261
[271 I281 [291 1301 [311 [321 [331 [341 [351
N.RauschandE.P.Burte,Microelectron.Eng.19(19921 725.
C.P.LoftonandW.E.Swartz,ThinSolidFilms52(1978) 271.
J.A.Thorton,J.Vat.Sci.Technol.11(1974)666.
L.C.FeldmanandJ.W.Mayer,FundamentalsofSurface
andThinFilmAnalysis(North-Holland,NewYork.1986) p.136.
B.L.Henke,Phys.Rev.A6(1972)94.
C.S.Fadley,J.ElectronSpectrosc.Relat.Phenom.5 (1974)125.
C.S.Fadley,Prog.Surf.Sci.16(1984)275.
J.F.Moulder,W.F.Stickle,P.E.SobolandK.D.Bomben,
HandbookofPhotoelectronSpectroscopy(Perkin-Elmer,
EdenPrairie,MN,1992). M.J.Vasile,A.B.EmersonandF.A.Baiocchi,J.Vat.
Sci.Technol.A8(1990)99. U.R.Evans,TheCorrosionandOxidationofMetals:
ScientificPrinciplesandPracticalApplications(Arnold, London,1960).
P.KofstadandK.Hauffe,Werkst.Korros.7(1956)642.
T.Smith,Surf.Sci.38(1973)292.
M.J.MindelandS.R.Pollack,ActaMetall.17(1969) 1441.
W.D.Sylwestowicz,J.Electrochem.Sot.122(1975)1504.
M.S.Aouadi,R.R.Parsons,P.C.WangandK.A.R.
Mitchell,J.Vat.Sci.Technol.A10(1992)273.
J.Inukai,W.Mizutani,K.Saito,H.ShimizuandY.
Iwasawa,Jpn.J.Appl.Phys.30(1991)3476.
H.Tokutaka,K.NishimoriandH.Hayashi,Surf.Sci.149 (1985)349.
M.P.SeahandW.A.Dench,Surf.InterfaceAnal.1 (1979)2.
D.R.Penn,J.ElectronSpectrosc.Relat.Phenom.9 (1976)29.
N.Wagnerand0.Brummer,Exp.Tech.Phys.29(1981) 571.
J.SzajmanandR.C.G.Leckey,J.ElectronSpectrosc.
Relat.Phenom.23(1981)83.
U.R.Evans,AnIntroductiontoMetallicCorrosion,3rd
ed.(Arnold,London,1982). R.Baird,C.S.Fadley,S.KawamotoandM.Mehta,
Chem.Phys.Lett.34(1975)49.
N.Wagnerand0.Brummer,Exp.Tech.Phys.29(1981) 571.
M.F.EbelandJ.Wernisch,Surf.InterfaceAnal.3(1981) 193.
J.KimandR.Hummel,Phys.StatusSolidi(al122(1990) 255.
W.S.M.Werner,Surf.InterfaceAnal.18(1992)217.
A.JablonskiandS.Tougaard,J.Vat.Sci.Technol.A8 (1990)106.
W.S.M.Werner,W.H.GriesandH.Stori,J.Vat.Sci. Technol.A9(1991)21.
V.Nefedov,N.Sergushin,I.BandandM.Trzhaskowska-
ya,J.ElectronSpectrosc.Relat.Phenom.2(1973)383.
K.YabeandT.Yamashina,Appl.Surf.Sci.8(1981)387.
