光刻胶的烘烤技术

BakingStepsinPhotoresistsProcessingRevised:2010-01-27Source::[e-mail]sforanunderstandingofthepurposeofthevariousbakingstepsinpho-toresistprocessing,andhowthebakingparameterstemperatureandtimeimpactonthein-dividualprocess.SubstrateHeatingbeforeResistCoatingHeatingthesubstratebeforeresistcoatingcanimprovetheresistadhesionintwoways:From100°Con,H2Opresentonallsurfacesdesorbs,sowerecommendabakingstepof120°Cforfewminutesforthispurpose.Atwo-stepcleaningprocesswithacetone,followedbyisopropylalcohol,hasthesameeffect.From150°Con,alsoOHbondsapparentonanyoxidizedsurfacessuchassilicon,glasses,quartz,orignobelmetals,arethermallycracked.TheseOHbondsotherwiseformahy-drophilicsurfacewithinferiorresistadhesion.ApplyingadhesionpromoterssuchasHMDSorTIPRIMEgivesasimilarresult.Thecoatingshouldbeperformeddirectlyaftercoolingdownofthesubstratesinordertoavoidre-adsorptionofwater.However,thesubstrateshouldagainhaveroomtemperaturebeforeresistcoatingsinceotherwisetheresistfilmthicknesshomogeneitywillsuffer.ThedocumentSubstrateCleaningandAdhesionPromotiongivesmoredetailsonthistopic.SoftbakeAftercoating,theresistfilmcontainsaremainingsolventconcentrationdependingontheresist,thesolvent,theresistfilmthicknessandtheresistcoatingtechnique.Thesoftbakereducestheremainingsolventcontentinorderto:Îavoidmaskcontaminationand/orstickingtothemask,ÎpreventpoppingorfoamingoftheresistbyN2createdduringexposure,Îimproveresistadhesiontothesubstrate,Îminimizedarkerosionduringdevelopment,Îpreventdissolvingoneresistlayerbyafollowingmultiplecoating,andÎpreventbubblingduringsubsequentthermalprocesses(coating,dryetching).Asoftbaketoocoolor/andshortmaycausetheabovementionedproblems.Asoftbaketoohotor/andlongwillthermallydecomposeasignificantfractionofthephotoactivecom-poundinpositiveresists,withalowerdevelopmentrateandhigherdarkerosionasaconse-quence.Negativeresistswillsufferfromthermalcross-linkingduringbaking,whichlowersthedevelopmentrateormakesthrough-developmentimpossible.Generally,werecommendasoftbakeat100°Conahotplatefor1minuteperµmresistfilmthickness.Inanovenisused,itisrecommendedtoaddsomeminutessoftbaketime.Ifsoftbakeisappliedat110°C,oneshouldhalvethesoftbaketime,whileforeach10°Cbelow100°Cthetimeshouldbedoubledinordertosufficientlydecreasetheremainingsolventconcentration.ThedocumentSoftbakeofPhotoresistFilmsgivesmoredetailsonthisbakingstep.Photoresists,developers,remover,adhesionpromoters,etchants,andsolvents...Phone:[e-mail]-2-MicroChemicalsGmbH-BakingStepsinPhotoresistProcessingPostExposureBakeThepostexposurebakePEB(performedafterexposure,butbeforedevelopment)canbeappliedabovethesofteningpointoftheresistwithoutdestroyingthestructurestobedevel-opedduetothestillclosedresistfilm.TherearevariousdifferentpossiblereasonsfortheapplicationofaPEB:ChemicallyAmplifiedResistsInchemicallyamplifiedresists,thePEBcatalyticallyperformsandcompletesthephotoreac-tioninitiatedduringexposure.MostoftheAZ®andTIresistssuppliedbyMicroChemicals®donotbelongtochemicallyamplifiedresists,andthereforedonotrequireaPEBforthispurpose.Typically,110°Cfor2minutesarerecommendedforthisbakingstep.CrosslinkingNegativeResistsInthecaseofmanycrosslinkingresistssuchastheAZ®nLOF2000seriesortheAZ®15nXT,thePEBisessentialforthecrosslinkingmechanisminitiatedduringtheexposure.ThenegativeresistAZ®125nXTdoesnotrequireaPEB,sincethecross-linkingalreadytakesplaceatroomtemperature.Detailsonthesenegativeresistcanbefoundhere.HighlyReflectiveSubstratesThePEBpromotesthethermallyactivateddiffusionofcarboxylicacidformedduringexposurefromthephotoactivecompound.Thisdiffusionstepsmoothensouttheripplingeffectoftheperiodiccarboxylicacidconcentration.Thisripplingeffectisduetostandinglightwavesduringmonochromaticexposure,espe-ciallyincaseofhighlyreflectivesubstrates.Thesepatternsotherwisewouldtransfertotheresistprofilethuse.g.reducingthespatialresolutionoftheresistandthedesiredaspectratio.Alternatively,oradditionally,bottom-layerantireflec-tivecoatingssuchasAZ®Barliimprovetheresolutionandresistprofileonhighlyreflectivesubstratesundermonochromaticexposure.Detailsonanti-reflective-coatingsaresummarizedinthedocumentAnti-Reflec-tiveCoatingsforPhotoresists.MechanicalRelaxationAPEBperformednearthesofteningpointofthepho-toresistreducesmechanicalstressformedduringsoft-bakeandexposureofespeciallythickresistfilmsduetotheexpandingnitrogenandthereforeimprovesre-sistadhesionandreducesunder-etchinginsubse-quentwetchemicaletching.However,acertaindelaybetweenexposureandPEBisrequiredtooutgasN2.Otherwise,duringPEBtheN2intheresistwillexpandandincreasemechanicalstressinthefilm.Thisdelaystronglydependsoftheresistfilmthickness.PEBRequired?Ifnoneofthepreviouslymentionedeffectsarerel-evant,aPEBisgenerallynotnecessaryandcanbeskipped.Thespatialincidentlightintensitydis-tributioninaresistfilmasafu