CMOS Front End Process CMOS前端工艺英文版课件：part4 2006 Lithography new

LithographyLithographyprocessreferstotheformationoflatentpatternonapolymerfilm(resist)coatedonwafersurfacewhichisthenpermanentlytransferreddowntotheunderneathsubstratethroughetchingorimplantprocess.Thebasicobjectiveofthelithographystepistotransferthelayoutmaskontothewaferforselectiveprocessing.Thisisachievedbyusingphotoactivematerialsknownasresists.Mostcommonlithographytechniqueisopticallithography.Hencethiswillbediscussedindetails.ElectronBeamLithographyandX-rayLithographywillbetouchedon.PhotolithographyThisstepisrepeatedlyusedforpatterningandiscrucialtoadvancementoftechnology.ProcessingStepsforPatterningWaferCleaningDI(De-ionized)waterusedtoremoveallparticles>0.2umandbacteria.PatterningLayerFormationDoneeitherbyagrowthoradeposition.Thislayeristobeselectivelyremoved.PhotoresistApplicationWaferputonavacuumchuckwithresistdroplets.Waferspunwithspeedsintherangeof1000to5000rpm.Theresistthicknessz=wherepispercentageofsolidinthesolvent,kisaconstantandwisrevolutionspeed.Kgenerallydependsonviscosityoftheresist.Donefor30sto60s.Gives0.5umto2umresistthicknessrange.Resistisverynon-uniformattheedges(~50um). SoftbakeoftheResistPurposeofSoftbake:Toremoveresidualsolventwhichmayadverselyaffecttheexposureofthefilmandthedevelopingprocess.Toannealanystressandremovevoids.goodadhesion.Doneat80cto90cinovenfor1to10minutesinnitrogenorairambient.SoftbaketemperatureandtimecancauseimpactonDOF(depthoffocus)andexposurelatitudeOptimumexposureMaskAlignmentDonewiththealignmentmarks,edgesandequipment.ExposureVioletorUVlight.Doneforbestsensitivity.PostExposureBaking-Hardensphotoresistandpattern.-Improvesadhesion.-StandingWaveEffect:Asaresultofinterferencebetweenreflectedlightfromsubstrateandincidentlightwaves.ReducesstandingwavesinthelatentimagebyPACdiffusion.-Removesremainingsolventsandreducesstressandvoids.-1to10minutes,120cto180cbakingdependingontheresistandisrecommendedbymanufacturers.DevelopmentThereare2methodsofdeveloping:PuddledevelopandSpraydevelop.Bothdevelopingmethodsinvolveintroductionofalkalinedeveloper(TMAH@tetra-methyl-ammonium-hydroxide)toneutralizethecarboxylicpartofexposedPACandDIwatertorinsethepatternedresist.Developingprocessaffects:-FinalCD(criticaldimension)-UniformityofpatternCDHardBakeHardbakeisthefinalbaketoensurehardeningofPRpriortoimplantsteporetchingstep.Theprocessisnormallycarriedoutattemperature110ºC-115ºC.Hardbaketemperaturehasanimpactonfinalpatterndimensionandprofile.EtchingRemovethepatterninglayerbywetorplasmaetchfromexposedregions.ResistRemoval.OpticalResistsForagoodresist,thefollowingpropertiesareneeded:Goodsensitivityi.e.faithfulgenerationofmaskimageonit.Forpositiveresist,itisdefinedtobethedose(energyperunitarea)toproducecompletelysolubleresistwithoutaffectingunexposedparts.GoodresolutionAbilitytoresolveisolatedanddensefeatures.AdhesionEtchresistancewhilesomeothermaterialisselectivelyremoved.Lowdefectdensity,highshelflifeandgoodqualitycontrol.Priorto1985orso,negativeresistwaspopular.Consistsofinertpolyisoprenerubberandphotoactiveagent.Exposedresistformscrosslinkagesbetweenrubbermolecules,makingitlesssoluble.Oxygenistobeavoided.Exposedresistswellsanddistorts.Thusnotgoodforsmallfeatures.PositiveResistConsistsofaresinandaphotoactivecompound(PAC)forUV(i-lineandg-line).Photoactivecompoundisdissolutioninhibitorfordevelopment.Exposuredestroysinhibitor.Moresoluble(exposedresist).Verysensitive(100mJcm-2,300to400nmlight).Noswellingproblemandhencebetterfeatures.DeepUVresistsChemicallyAmplified(CA)Resistsareused.Verysensitive(20mJcm-2,below250nmlight).HerephotonsinteractwithPhotoAcidGenerator(PAG)creatinganacidmolecule.Acidmoleculesarecatalystsforchangingresistpropertiesasrequireddependingonpositiveornegativeresist.Occursduringpostexposurebake(PEB).HencePEBisaverycriticalstephere.Acidmoleculesareregeneratedaftereachchemicalreactionandmayparticipateinfurtherreaction.Hencesensitivityisexcellent.LithographyExposureTechniquesContactPrintingMaskplatepressedagainstanddirectlyincontactwithresist.Athin0.2mmflexiblemaskforbettercontact.Minimumfeaturesizeis wherezistheresistthicknessandisthewavelengthoflight.Hencefor1umthickresistand=400nm, W~0.6umInpractice,upto0.3umfeaturesispossible.Themajorproblemisthatmaskgathersdefects.13defects/cm2after5exposures.37defects/cm2after15exposures.ProximityPrintingMaskseparatedin20to50umrangefromresist.Masklifeincreases.Imageisnotverygood.MinimumfeaturesizeisWheregisspacingbetweenmaskandthewafer(5umto50um).Forg=10um,minimumfeature2um.Notsogood.ProjectionPrintingFancyopticsusedforfocusing.Aberrationsmustbecontrolledinppm.Imageisfocused.ImportantopticalparameterNumericalAperture=NA=abilitytocollectdiffractedrays.Intherangeof0.28to0.42inoldermachines.Now~0.75.MinimumfeatureFor=1,NA=0.28,=0.4um,minimumfeature1.2um.Betterfeaturesarepossible.Stepandrepeatprojection.AdvancesinPhotolithographyAlwaystwocompetingfactors.ResolutionR=k1.Improvefeaturesize.Improvebyreducingk1from0.8to0.6fromgline0.436umtoiline0.365umkrFEx.Laser~0.248umArF(0.193um)F2IncreasingNA0.4to0.5to0.6to0.7.Depthoffocus.Improvesperformancewithnon-planarresist. HencereducesDOF(,NA)ifRimproved.Neededsothatbeamisfocusedinnon-planarwafertoareasonablerange(~1um).TypicaldepthoffocusandprocesslatitudeplotsFirstplotshowsviaCDversesfocus.10%changeacceptable.DOFfordensevias(acontactholebetweentwometallayers)is0.5micronsand0.3micronsforisolatedvias.TechniquestoImproveR&DOFTogetherPhaseShiftMasks(PSM)canmakethemostdramaticimprovementinresolution.e.g.,ifalithographyprocesscanattainaresolutionof0.3musinga248nmKrFlightsourcewithastepperhavinganNA=0.6,thenbyincorporatingphase-shiftingtechniquesonthereticle,theresolutioncanbeimprovedtoasmuchas0.18m.TheconceptofPSMwasfirstintroducedin1982byLevenson,butitstartedtobeusedinmassproductioninthelate90’s.(Evenby1997,onlyabout1%oftheworld’smaskoutcontainedPSMs.)However,beyond0.18mdesignrule,PSMbecamepopularasaresolution-enhancementtechnique.AlternatingPSM(LevensonPSM)MaskMaskAmplitudeatMaskIntensityatWaferAmplitudeatWafer180phaseshiftTheleftsideshowstheelectricfield,,associatedwiththelightjustafteritpassesthroughthemaskandalsoatthewafer,withoutanyphaseshiftinginthemask.Thephotoresistrespondstotheintensityofthelight(field)2andhencetheintensitypatternonthebottomleftisbarelysufficienttoresolvethetwolines.Intherightside,amaterialwhosethicknessandrefractionindexarechosentophaseshiftthelightbyexactly180isaddedtothemask.Sincethelightintensityissquareofthefieldintensity,thequalityoftheimageissignificantlyimproved.Whenaphase-shiftingmaterialisplacedonalternatingclearspacesofamask,thistypeofPSMiscalledanalternatingPSM(ALTPSM)oraLevensonPSM.Introducingaphaseshifteronamaskcanbedonebytwoways:depositionofanappropriatefilm,oretchingofthemaskglassitself.MajordrawbackofALTPSMisthatitisnoteffectiveforsomeothertypesofpatterns.Inthiscase,itisnotpossibletoassignaphasevaluetoeachpatternthatwillmakeit180outofphasewithallitsneighboringfeatures.ALTPSMiswellsuitedformemorycircuits,inwhichtheinterconnectlinesarecloselyandregularlyspaced.rim-shiftPSMAphaseshifterfilmisdepositedontothechromelayer.Afterpatterningthefilm,thechromeisetchedisotropicallysothatitundercutsthephaseshiftingfilm.Inrim-shiftPSM,thephaseshiftingtakesplaceonlyattherimofeachmaskfeature.donotprovideasmuchresolutionimprovementasdotheALTPSMs.However,itdoesnotsufferfromthepatternrestrictions.Soitcanbeusedforarbitrarypatterns,includingisolatedfeatures.

AttenuatedPSM(halftonePSM)Theopaquepartofaconventionalreticleisreplacedwithahalftonefilm,whichispartiallytransmissive,about10%.(e.g.)Fori-line,averythinlayerofchromiumoxynitrideorMo-Si-ON(molybdenumsilicon-oxynitride)hastheproperthicknesstoshiftthephaseofthelightby180.Whilesomelightwillpassthroughthehalftonefilm,itwillbetooweaktoexposethephotoresisttothedegreenecessarytobewashedawayduringdevelopment.However,thenegativeamplitudeofthisweakphase-shiftedlightwilldestructivelyinterferewiththenon-phase-shiftedlight.improvementofresolution.ThehalftonePSMisgenerallybetterthanrim-shiftPSMandmucheasiertofabricate.ThehalftonePSMhasbeenthefirsttypeofPSMtobeadoptedincommercialICfabrication.

Offaxisillustration.Byusingsourceasshownwithanobliqueangleofincidence,0thand1storderdiffractedraysarecollected.ResolutionimprovementswithoutlossinDOFpossible.Showninthefigure.OpticalproximitycorrectionAltermaskshapedependingonsystematicinobservedpatternsorsimulatorsasshown.Possibletogetmuchbetterfeatures.Thelinewidthofisolatedanddenselygroupedlineseventuallyvariesbyasignificantamountfromthatofthepredictedlinewidthwhenthefeaturesizesapproachthewavelengthoftheilluminatinglight.(e.g.assumingtheuseofpositivePR,0.35mwideisolatedlinesare~0.09mnarrowerthan0.35mwidelineswith0.35mspacing)Sincethelinewidthdependsontheproximityofotherfeatures,thiseffectiscalledanopticalproximityeffect.Ifthisdimensionaldifferencecanbepredicted,thewidthofthelinesonthemaskreticlecanbeadjusted.Thisiscalledopticalproximitycorrection(OPC).EventhoughOPChasoriginoncorrectiontheline-sizevariationasafunctionoftheproximityofotherpattern,itisalsoappliedtocorrectcornerroundingandgenerallossofshapefidelityinsmallfeaturescausedbytheinabilityoftheprojectionlenstoresolvedetailssmallerthanthediffractionlimitofthelens.e.g.,asquarecontactholepatternwithasizeclosetotheminimumresolutionlimitwillprintmorenearlylikeacircle.Arectangularshortlinewillbelikeanellipticalshape.Theseeffectsarealsoclassifiedasanotherformofopticalproximityeffect.OPCtechniqueistopre-compensatethereticlepatterntoaccountforexpectedpatterndistortionusingcomputersimulation.e.g.,addingsmallsecondarypatterns,calledserif,tothecornersofasquarecontactholepatternwillproducemoresquare-likepattern.WhileOPCtechniquesprovidesignificantbenefits,theadoptionofOPCincludesthefollowingdrawbacks;Theserifsaddalargenumberofadditionalfeaturestothedatabaseofthereticlepattern,resultinginalargeincreaseinreticlewritingtimeandthuscost.Thesizeoftheserifsaretypicallymuchsmallerthantheminimumfeaturesizeofthereticle,whichcreatesproblemsduringtheinspectionphaseofthereticlemanufacturingprocess.OPCmaynotbefeasibleinallsituations.Dependingonpatternshapeandlayout.(iv) Useofassistfeatures(AF)orScatterBars–inthistechnique,featuresthatcannotbeprintedontheresistbutcanmodifyopticalintensitydistributionaredeliberatelyaddedtoimproveresolution.Foranisolatedfeature,thereareavarietyofschemesasindicatedhereanddependingonavailabilityofspace,canbeoptimizedtogetdesiredresolution.AlternatingPSMnormallygoodforequallinesandspaces.AlternatingPSMwithAssistFeaturesforIsolatedViafeaturecanyieldverygoodresolutionwith248nmwavelengthstepper.Thefollowingschemeofassistfeaturesisstudied(FYPworkbySuryani/ZhaoHui/Mehta/Samudra).DDWLS Carefuloptimizationisneededtoobtainthebestfeature.Clearly,primaryfeatureof180nmX180nmisveryclearlyresolvedwithassistfeaturesandisrobusttochangesinprimaryfeaturesizedownto170nm.Withoutscatterfeatures,theprimaryfeatureisreducedsubstantiallyandresolutionisnotthere.Similarly,evenassistfeatureslikeabovecannothelptoachieve160nmX160nmprimaryfeatureasseenaboveandbelowindicating180nmX180nmislowerboundonprimaryfeaturetogetgoodresolutionifsomeerrortoleranceisneeded.160nmX160nmprimaryfeatureprintsabout110nmX110nmwithassistfeatureswhichisnotgood.Withoutassistfeatures,itdoesnotprintatallasseenabove.AsSeenbelow,160nmX160nmprimaryfeaturewiththeassistfeaturesabovecanatbestprintas110nmX110nmwhichisnotgood.Again,only180nmX180nmprimaryfeatureisresolvedifdistancetoprimaryfeatureDisbelow200nmwithassistfeaturesizeof120nmX120nm. Avarietyofschemestouseassistfeaturesandtheirarrayarepossibleanddependingonscheme,optimizedcontrastandfeaturesize(CD–criticaldimension)canbeobtained. Thetablebelowsummarizesoptimizedresultfromeachmethod.IphereisthepeakintensityandIsisunwantedsidelobeintensity.Featuresdownto145nmpossible.Theactualdimensionsofprimaryfeatureandscatterbarsaredescribedbelow.Design1:singleviawithscatterblockprimaryfeature180/180nmAssistfeatures100/100nmSeparation140nmDesign2:singleviawithscatterblockprimaryfeature170/170nmAssistfeatures120/120nmSeparation150nmDesign3:singleviawithscatterbarPrimaryfeature:180/180nmAssistfeatures:40/110nmSeparationbetweenprimaryfeatureandAF:150nmDesign4:singleviawiththreelayersofscatterboxPrimaryfeature160/160nmAF40nm/respectivelengthSeparationbetweenprimaryfeatureandinnerbar150nmSeparationbetweenfirstandsecondlayerofscatterboxesSeparationbetweensecondandthirdlayerofscatterboxesDesign5:singleviawithcombinationofscatterboxandscatterbarsPrimaryfeature160/160nmInnerlayerofAF40/420nmCenterlayerofAF40/760nmOuterlayerofAF40/1250nmSeparationbetweenprimaryfeatureandinnerlayer150nmSeparationbetweeninnerlayerandcenterlayer140nmSeparationbetweencenterlayerandouterlayer140nmDesignIpIsCD/nmDesign10.50.045165Design20.5340.152160Design30.53290.027160Design40.55470.1488144Design50.54310.1102147(v) DeliberateDefocustoachieveUnusualResolution:(Ph.D.workbyRoy-Mukherjee/Samudra)Althoughthisisnotauniversaltechnique,unusuallybetterresolutioncanbeobtainedasshownbelow.Typically,sidelobeformationisaveryimportantissuethatappearswhenresolutionisstretchedtoitslimit–thisfeaturecouldbemoreprominentthantheprimaryfeaturetobeprintedasseeninthefigurebelow.Toevaluatetheeffectofpitchandfeaturesizeonbestdefocuscondition,densehol