第16号2014年
Controlofresidualstressinthinfilmsbysubstratevibration
andtheirmechanicalproperty
AkihitoMatsumuro†,MakotoTakagi† AbstractThepreventionofresidualstressofthinfilmsynthesesisveryimportantinprocessingengineering surfaces.Inthispaper,detailsofresidualstresscontrolbythevibrationofthesubstrateusingPZTatvoltageof O-100Varepresented.ThethinfilmsdepositedarecrystallineTiN,Ti,Cu,AlfilmsandamorphousC,Si films.TheresidualstressesaremeasuredusingStoney'smethod.Theresultsofourexperimentshowthatfor Ti,TiNandCuthinfilms,compressivestressesarechangedtotensilestresses.XRDandAugerspectroscopy resultsverifythatthecompositionandcrystallinestructureofthetreatedanduntreatedcrystallinethinfilmsto bethesame.Butitisshownthatthegrainsizeinthefilmtendtodecreasewithincreasingvibrationamplitude. Fromthechangeofthemicrostructureofthefilm,amodelforthecontrolofresidualstressisproposed.The changeofadhesionstrengthismeasuredbyapin-on-disktribo-testerwithvariationoftheresidualstress.The adhesionstrengthshowsthemaximumvalueatnearzeroresidualstress.Therefore,controlofresidualstressby substratevibrationisshowntobeaneffectivemethodforimprovingthewearlife. 1.Introduction Thedevelopmentofsynthesistechniquesforthinfilm depositionhasgainedconsiderableattentioninrecentyears.In thefieldofsurfacemodificationtechniques,thinfilmsfor mechanicalusessuchasDLC,TiNandCrNfilmshave emergedassuccessfulhardcoatingfilms.However,duetothe residualstress,thefilmsdelaminatefromthesubstrateand remainstobeaproblem.Untilnow,inordertodecreasethe residualstress,thechangeofthedepositionparameters[1]and thesynthesisofbufferlayer[2]havebeencarriedout.Butthese processeshavechangedthecrystallinestructure,composition andtheirmechanicalpropertiesremarkably.Thesemethods havenotonlyworsenedthepropertiesofthefilms,butalso needmuchefforttofindsuitabledepositionparameter. Therefore,thedevelopmentofanewcontrolmethodfor decreasingtheresidualstressisvitalwithsignificantapplication invarioushardcoatingmaterials. †AichiInstituteofTechnology, MechanicalEngineering(ToyotaCity) Departmentof Inthispaper,anexcellentandsimplemethodforcontrolling theresidualstressisproposed.Here,substratevibrationthatis independentofthedepositionconditionisused.Inthisprocess, onlysubstratesarevibratedbyPZTduringthedeposition.The changesofThedevelopmentofsynthesistechniquesforthin filmdepositionhasgainedconsiderableattentioninrecent years.Inthefieldofsurfacemodificationtechniques,thinfilms formechanicalusessuchasDLC,TiNandCrNfilmshave emergedassuccessfulhardcoatingfilms.However,duetothe residualstress,thefilmsdelaminatefromthesubstrateand remainstobeaproblem.Untilnow,inordertodecreasethe residualstress,thechangeofthedepositionparameters[1]and thesynthesisofbufferlayer[2]havebeencarriedout.Butthese processeshavechangedthecrystallinestructure,composition andtheirmechanicalpropertiesremarkably.Thesemethods havenotonlyworsenedthepropertiesofthefilms,butalso needmuchefforttofindsuitabledepositionparameter. Therefore,thedevelopmentofanewcontrolmethodfor decreasingtheresidualstressisvitalwithsignificantapplication invarioushardcoatingmaterials. Inthispaper,anexcellentandsimplemethodforcontrollingtheresidualstressisproposed.Here,substratevibrationthatis Fig.1Schematicofdepositionsystem TablelThethicknessandcrystallineofthefilms Film Thickness (nm) 曾 ・ Deposition rate(nm/min) Crystalline TiN 100,200,400 60 Crystal Ti 300 60 Crystal Cu 1000 30 Crystal Al 1000 100 Crystal c 100 3 Amorphous Si 100 40 Amorphous ( d J ) s s a z S i n p is a o.s 0 一〇.s 1 一 一1.5 102103104105 Frequency(Hz) Fig.2Variationofresidualstress frequencyforTiandTiNfilms. 106 with independentofthedepositionconditionisused.Inthisprocess, onlysubstratesarevibratedbyPZTduringthedeposition.The changesofresidualstressaretestedonTiN,Ti,Cu,Al,Siand Cfilms.Furthermore,therelationshipbetweenresidualstress andthemechanicalpropertiessuchaswearlifehardnessand elasticcoefficientareclearlyshown. 2.Experimental ( ￡ O ) の の ㊤ 魯 の 罵 弓 刷の ㊤ 0.5 0 一〇.5 1 ■ ■ 圏 s Ti ■TiN Aschematicdiagramoftheexperimentalsetupisshownin Fig.1.Thesampleholderhaspiezoelectric(PZT)vibratorthat canvibrateverticallywithamplitudeofO-20nmandfrequency ofO-1MHz.Abiasvoltagevariestheamplitudeattherateof O.2nrn八7.Theheatgeneratedwasnegligiblewithatemperature increaseoflessthan30°C.Theresidualstressthatoccurred duringdepositionisnotrelatedtothethermalstressandis attributedtotheintrinsicstressofthefilms. Electronbeamevaporatorwasusedtodepositvariousfilms ofTiN,Ti,Cu,Al,C,andSi.TiNfilmsweresynthesizedinNz gasflowduringTideposition.Backgroundpresserislessthan 5.OX10.4Pa.ThefilmsweredepositedonSi(100)wafersthat washedbyacetone.TableIshowseachfilm'sthicknessand crystallization.Theeffectofthethicknessandcrystallization wereresearched. XRDandAugerelectronbeamspectroscopyresults 0 20 Fig3Variation voltageforTiandTiNfilms. 406080100 v61tage(V) ofresidualstresswithbias verifythatthecompositionandcrystallinestructure. TheresidualstressofthefilmswasmeasuredusingStoney's method.Thehardnessandtheelasticconstantweremeasured byanano-indentationtechniquewithatrigonaldiamond indenter(Berkovich-typeindenter).Themaximum indentationloadswerechosenintherangesofO.8-3.OmN. Thefrictioncoefficientandfrictionalcharacteristicswere investigatedbyaball-on-disktypetribo-testerwith550mN againstasteelballwithadiameterofsmminanatmospheric pressure.SEMandopticalmicroscopywereusedtoobservethe weartracks.Thesectionalareasofweartracksweremeasured bythestylusprofilesystem. 3.Resultsanddiscussion 3.1Controlofresidualstressbysubstratevibration 3.1.1Relationshipbetweenresidualstressandvibration parameters
( d rJ ) s s a a S i n p is a o.2s 0 一〇.25 一〇.s 0 20 4060 Voltage(V) 80 Fig.4Variationofresidualstresswith biasvoltageforAlandCufilms. 100 3 (∠ 1 0 1 (∠ ■ 一 (e d rJ ) s s a . s je n p l s a g 一3 0 20 4060 Voltage(V) 80 Fig.6Variationofresidualstressandbias voltagein100,200,400nmthicknessofTiN films 100 (￡ O ) の の oお 罵 弓 咽の 。 0 一1 一2 一3 一4 020406080100 Vc〕ltage(V) Fig.SVariationofresidualstresswithbias voltageforCandSifilms. Thevibrationparametersarefrequencyandbiasvoltage. Theseparametersareconsideredtogiveeffectivechangesin theresidualstress.WhenTiandTiNfilmsweredeposited withoutsubstratevibration,thefilmsshowcompressivestress. Onthecontrary,thecompressiveresidualstressdecreasedto zerointernalstressat30Vandreachedtothehighesttensile stressthroughthetransitionrangefromcompressivetotensile stressirrespectiveoffilmmaterialwhenthebiasvoltage changed. Itisclearseenthatthedependenceofthebiasvoltageonthe residualstressarelargerthanoffrequency. TherelationshipbetweenfrequencyandresidualstressforTi andTiNfilmsisshowninFig.2.Here,thebiasvoltageiskept constantat10volts.Variationoffrequencywithresidualstress inbothTiandTiNfilmswereconstant.InTiandTiNfilms therelationshipbetweenbiasvoltageandresidualstresswere showedFig.3.Thefrequencywasconstant(100Hz).When thevoltagewasincreased(10and20V)theresidual compressivestresswasdecrease.Butwhenthevoltagewas (￡ O ) の のo お 罵 弓 刷の o 1 0.5 0 一〇.s 0 20 4060 Voltage( 80 100 Fig.7Variationofresidualstresswithbiasvoltage forSiandSiO2substrate. increased(30V)atransitionregionwasrevealedwherebythe stressbecametensile.Andmoreover,whenthevoltagewas increased(50-100V)theresidualtensilestresswasincreased.In TiNfilmstheresidualstresschangedinasimilarwaytothatof Tifilms.Sothebiasvoltagewasobservedtobetheeffective vibrationparameterforcontrollingtheresidualstress. 3.1.2Effectofkindoffilm TheabovediscussionshowedthattheresidualstressesofTi andTiNfilmswithcrystallinestructurecouldbecontrolledby substratevibration.Inordertoconfirmthevalidityofthesame methodtoAlandCufilmswithcrystallinestructureandCand Siwithamorphousstructure,experimentswerecarriedout.Fig. 4andsshowvariationoftheresidualstresswithbiasvoltage.It isclearlyseenthatthecompressiveresidualstressesofAland Cufilmsweredecreasedwiththebiasvoltage.However,the residualstressesinCandSifilmswithamorphousstrictures werealmostconstantatanyrangeofthebiasvoltage.
( .づ .邸 ) 智 旨 g 三 (i) {￡ O 畜 呂 薯 = 10 9 8 7 6 一3-2-101 R郎idu且IStre呂s(GPa} Fig.10Variationsofhardnesswithresidualstress 500 2 20406080 ∠U更degree, Fig.8XRDpatternsofTifilmsonSi(100)with biasvoltage. (a)(b) _月_A
一
AAハLAA 。 ℃ 首 驚 口 。 嘱も 嘱占 X11 300 200 (i)a
一
100 (iii)a
0 !!ノ !! 佃 TiNlOOnm TiN200nm TiN400nm OTi ,`一 、 、 ◆ 、3 .o. 、 ●● ・。 φ Oo(う 一3-2-101 Residualstress(GPa) Fig.11Resultsofwearlifes 2 3 (1V)十
CompressivestressTensilestress Fig.9Schematicmodelforthefilmgrowthofcrystal. (a)withoutsubstratevibration(b)substratevibration (a) -2.4GPa 0.4GPa(b) Therefore,substratevibrationscouldchangetheresidualstress incrystalfilmsonly. 3.1.3Theeffectofthickness Fig.6showsvariationofresidualstresswithvoltagein differentTiNfilmthickness(100,200and400nm).The residualstressofeachfilmthicknesschangedfromcompressive totensilestressthroughzerostress.Therefore,substrate vibrationcouldchangetheresidualstressofanyfilmthickness. Fig.12SEMimageofweartrack after100cycleinTiN200nm 3.1.4Changeofresidualstresscrystallinestructureofsubstrate materials Fig.7showsvariationofthevoltageandresidualstressofTi filmsonSi(100)(crystalline)andSiO2(amorphous)substrate. TheresidualstressofTifilmsonSiO2substrate,changedfrom compressivetotensilethroughOstresswhenthebiasvoltagewasincreased.Therefore,Thecrystallinestructureofsubstrate materialwasindependentofthechangeoftheresidualstress. 3.1.5Considerationofmechanismforchangeoftheresidual stressbysubstratevibration Variationofresidualstressbysubstratevibrationis independentofthecrystallizationofthesubstrateandfilm thickness.Theresidualstresseswerechangedfrom compressivetotensilestresswhenthebiasvoltageisincreased. Variationofthecompositionandcrystalstructurewithbias voltagewereconstantinallcrystalfilms.Fig.8showsthe resultofXRDforTifilms.Thepeekdoesnotshift,but broadenswhenthebiasvoltageisincreased. Amodelforthecrosssectionofthefilmgrowthisshownin Fig.9.Themodelforthecaseinwhichsubstratevibrationis usedandthatofwithoutvibrationisshowninFig.9(a)and9 (b),respectively.Whenthesubstrateisnotvibratedduringthe filmgrowth,thecrystalsizetendstobefairlylargeleadingto compressivestressonthefilmlayer.Ontheotherhand,when vibrationisusedduringthefilmgrowth,thecrystalsizeis muchsmallerandthusleadstotheoccurrenceoftensilestress onthefilmlayer. 3.2Relationshipbetweenmechanicalpropertiesandresidual stressinTiandTiNfilms. 3.2.1Variationofhardnesswithresidualstress inTiandTiNfilmswithvariousfilmsthickness.InbothTiand TiNfilms,thehardnesswasdecreasedwhentheresidualstress waschangedfromcompressandtensilestress.Thegradients inTiandTiNfilmswereO.6GPa/GPaand1.OGPa/GPa, respectively. magnitudesofresidualstressesinbothcompressiveandtensile stressdecrease,thewearlifeofthefilmsincreasedinbothTi andTiNfilms.Fig.12showsSEMimageoftheweartrackafter 100cycle,TiN200nmfilmswithresidualstressof(a)‐2.4GPa and(b)0.4GPa.Highresidualstressfilms(a)havepeeling areasbutlowresidualstress(b)filmshavenopeelingarea.This clarifiesthatcontrolofresidualofresidual stressbysubstratevibrationimprovethewearlifeofthefilms. 4.Conclusion Inthepresentstudy,thinfilmsofvariousmaterialswere preparedonsubstratethatwasvibratedduringthedeposition process.Theeffectsofsubstratevibrationontheresidual stressesandthemechanicalpropertieswereinvestigated.The followingconclusionsweredrawn: 1 , 2 , M , 4 Theresidualstresseswerechangeduponthesubstrate vibrationthatiscontrolledbythebiasvoltage Changingresidualstressisindependentonfilmthickness andcrystalstrictureofthesubstrate Thehardnessvaluesdecreasedwhentheresidualstress waschangedfromcompressivetotensileforbothTiand TiNfilms. Thewearlifewaslongwhentheresidualstresswas decreasedforbothTiandTiNfilms. 5.References [1]C.A.Carrasco,V.VergaraS,R.BenaventeG.,N.Mingolo, J.C.Rios,MaterialsCharacterization48(2002)81-88 [2]S.Zamir,B.Meyler,E.Zolotoyabko,J.Salzman,Jounalof CrystalGrowth218(2000)181-190 3.2.2Frictionalcoefficient Thefrictionalcoefficientwasdeterminedbyball-on-disk tribo-tester.Theaveragefrictionalcoefficientwascalculated fromteststakenbetween50and100cycles.Thefrictional coefficientsofbothTiandTiNfilmswerenotchangedwiththe residualstresses.Thisdemonstratesthatthefrictional coefficientsarenotaffectedbytheresidualstress. 3.2.3Wearlife Theweartrackswereobserveduponincreasingthe frictionalcoefficienttoO.6,whichcorrespondstofrictional coefficientbetweenSi(100)andsteelball.Figllshows variationofresidualstressandnumberofcycle.Whenthe
