Run by run control of chemical-mechanical polishin

1RunbyRunControlofChemical-MechanicalPolishingDuaneBoning,WilliamMoyne,TaberSmithMicrosystemsTechnologyLaboratories,MassachusettsInstituteofTechnology,Cambridge,MA02139JamesMoyne,RolandTelfeyanUniversityofMichigan,CenterforDisplayTechnologyandManufacturing,UniversityofMichigan,AnnArbor,MI48109ArnonHurwitz,ScottShellmanStatisticalMethodsandAdvancedProcessControl,SEMATECH,Austin,TX78741JohnTaylorCompugenesis,Inc.,Austin,TX78739AbstractAprototypehardware/softwaresystemhasbeendevelopedandappliedtothecontrolofsinglewaferchemical-mechanicalpolishing(CMP)processes.Thecontrolmethodologyconsistsofexperimentaldesigntobuildresponsesurfaceandlinearizedcontrolmodelsoftheprocess,andtheuseoffeedbackcontroltochangerecipeparameters(machinesettings)onalotbylotbasis.Acceptableregressionmodelsforasinglewaferpolishingtoolandprocesswereconstructedforaverageremovalrateandnonuniformitywhicharecalculatedbasedonfilmthicknessmeasure-mentatninepointson8”blanketoxidewafers.Forcontrol,anexponentiallyweightedmovingaveragemodeladaptationstrategywasused,coupledtomultivariaterecipegenerationincorporat-inguserweightsontheinputsandoutputs,boundsontheinputranges,anddiscretequantizationinthemachinesettings.Wefoundthatthisstrategysuccessfullycompensatedforsubstantialdriftintheuncontrolledtool’sremovalrate.Itwasalsofoundthattheequipmentmodelgenerateddur-ingtheexperimentaldesignwassurprisinglyrobust;thesamemodelwaseffectiveacrossmorethanoneCMPtool,andoveraseveralmonthperiod.Webelievethatthesamemethodologyisapplicabletopatternedoxidewafers;workisinprogresstodemonstratepatternedwafercontrol,toimprovethecontrol,communication,anddiagnosiscomponentsofthesystem,andtointegratereal-timeinformationintotherunbyruncontroloftheprocess.IEEETrans.CPMT(C),Vol.19,No.4,pp.307-314,Oct.1996.21.0IntroductionThechemical-mechanicalpolishing(CMP)processisofcriticalimportancetocurrentandfuturegenerationinterconnectforintegratedcircuittechnologies.InadditiontoCMPprocessandequipmentdevelopment[1],themodelingofCMPprocessesisanactiveareaofresearch,includ-ingworkonwaferscaledependencies[2],featurescalemodels[3],aswellasbehavioroftheequipmentovermanyruns[4,5].ThechallengesposedbyCMPforbothsensorandcontrolresearcharealsobecomingbetterknown[6,7].Whileagooddealofresearchintorunbyruncon-trolmethodshasbeenreported(see[8]forareview),relativelylittlepracticalexperiencewithCMPcontrolexists[7,9,10].Inthispaper,wepresenttheapplicationofanintegratedhardware/softwarecontrolsystemutilizingrunbyrunmethodstoovercomecommonCMPprocessandequipmentdifficulties.InSection2,webrieflyreviewCMPprocessandequipmentfundamentals,andidentifythedifficultiesaccommodatedthroughrunbyrunprocesscontrol.Section3presentstheoverallcon-trolsystemarchitecture,anddescribesthegradualmoderunbyruncontrolstrategy.Thepolyno-mialresponsesurfaceandlinearcontrolmodelsdevelopedforCMParediscussedinSection4.InSection5,wepresentapairofsimulationandfabricationexperimentsthatdemonstratetheimpor-tanceandeffectivenessofmodeladaptiverunbyruncontrol.Finally,inSection6wedrawcon-clusionsbasedontheseexperiments,andhighlightareaswhereadditionalresearchanddemonstrationareneeded.2.0TheCMPProcessIntheCMPprocess,thewaferisaffixedtoawafercarrier(viaback-pressure),andpressedface-downonarotatingplatenholdingapolishingpad,asillustratedinFig.1.Aslurrywithabra-sivematerial(e.g.silicaparticlesofsizesfrom10nm-200μm)heldinsuspensionisdrippedontotherotatingplatenduringpolish.Thecarrierandplatenrotateatvariablespeeds,typicallyontheorderof30rpm.Toolsdifferinthenumberofwafersthatmaybesimultaneouslypolished;single-wafer,dual-wafer,andothermulti-headedtoolsexist.Theprocessremovesmaterialatthesurfaceofthewaferthroughacombinationofmechanicalandchemicalaction.Atypicalprocessgoalistoachieve“global”planarization(acrosstensof3mm)bypreferentialremovalof“high”materialonthewafer.Theplanarizationofdielectric(sili-condioxide)layersbetweenmultilevelmetallizationstepsisonecommonapplication.Metalpla-narizationisalsooftenperformed.ThecontrolofCMPischronicallypoor,arisingfrompoorunderstandingoftheprocess,deg-radation(wear-out)ofpolishingpads,inconsistencyoftheslurry,variationinpadphysicalprop-eraties,andthelackofin-situsensors.Becausetheprocessincludesmechanicalabrasionofthesurface,thepolishingpadwearsrapidly.Concurrentorsequential“conditioning”isusuallyemployedwherebytheabrasivesurfaceofthepadisrestored(eitherbymechanicaldamagetothesurfaceorremovalofathinsurfacelayer),butthelifetimeofapadremainsontheorderofafewhundredwafers.Inadditiontodifficultiesachievingareliablefilmthickness(becauseofchangingremovalratesovertime),thewithin-waferuniformityofthepolishisdifficulttoachieveandmaintain.Differencesbetweenpolishratesatthecenterandedgeofthewafer(i.e.“bulls-eyepat-terns”)mayariseduetowaferasymmetry(e.g.waferflat),non-constantrelativepadvelocityfromtheedgetothecenter,non-uniformslurryandby-producttransportunderthewafer,waferbowingduetopressure,ormachinedriftintimeofanyoftheseparameters.Asaresultoftheseproblems,itisconventionalpracticetouseanumberofsend-aheadordummywaferstoconditionand/orc