Microfabrication by electrochemical metal removalWord文件下载.docx

Microfabrication by electrochemical metal removalWord文件下载.docx

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Microfabricationbyelectrochemicalmetalremoval@#@Microfabricationbyelectrochemicalmetalremoval@#@Datta,M@#@Recentadvancesinthedevelopmentofelectrochemicalmetal-removalprocessesformicrofabricationarereviewedinthispaper.Afterabriefdescriptionoftheprocess,severalimportantparametersareidentifiedthatdeterminethematerial-removalrate,shapecontrol,surfacefinishing,anduniformity.Theinfluenceofsurfacefilmproperties,masstransport,andcurrentdistributiononmicrofabricationperformancearediscussed.Severalexamplesofmicroelectroniccomponentfabricationarepresented.Theseexamplesdemonstratethechallengesandopportunitiesofferedbyelectrochemicalmetalremovalinmicrofabrication.Introduction@#@Material-removaltechniquesareamongthekeyprocessingtechnologiesthatareusedinthefabricationofmicroelectroniccomponents[1].Thesemethodsarepopularlyknownasetchingtechniques.Dryvacuumprocessesforthin-filmetchingarebasedonplasmaassistedprocessesandincludeionetching,plasmaetching,andreactiveionetching[1].Ionetchingisaphysicalprocess,whereasplasmaetchinginvolvesachemicalreaction.Reactiveionetchingisacombinationofbothphysicalandchemicaleffectscontributingtomaterialremoval.Inionetching,ionsareextractedfromagaseousplasmaandacceleratedtothesubstrate,wherethesurfaceiserodedbymomentumtransfer.Plasmaetchingemploysaglowdischargetogenerateactivespeciessuchasatomsorfreeradicals.Theactivespeciesdiffusetothesubstrate,wheretheyreactwiththesurfacetoproducevolatileproducts.Additionofreactivegasestotheionsource（reactiveionetching）enhancesthephysicaletchrateandintroduceschemicaletchingaswell.Theseprocessesareparticularlyemployedinthesemiconductorindustryforultralarge-scaleintegration（ULSI）becauseoftheirabilitytoremovematerialwithprecision.However,someofthedisadvantagesthatareinherentindry-etchingtechniquesincludehighequipmentcost,lackofselectivity,andproblemsarisingfromredepositiononthesampleanddepositiononthevacuumchamber.Dry-etchingtechniquesareusedforprecisionetchingofthinfilmsinvolvingverysmallamountsofmaterialremoval.Lately,concernhasintensifiedaboutthesafety,environmentalimpact,anddisposalofthetoxicgasesusedinplasmaassisteddryetching.@#@Wetchemicaletchinginvolvesremovalofunwantedmaterialbytheexposureoftheworkpiecetoanetchant.Theexposedmaterialisoxidizedbythereactivityoftheetchanttoyieldreactionproductsthataretransportedfromthesurfacebythemedium.Chemicaletchingconvertsasolidinsolublematerialtoasolubleformbydissolvingtheextendedlatticeofmetalatomssothattheseatomscanenterthesolutionassolublecompounds.Thisisaccompaniedbyremovalofelectronsfromthemetal（oxidation）.Theseelectronsareacceptedbytheetchant,whichactsasanoxidizingagentinchemicaletching.Themetal-removalreactiontypicallyinvolvesseveralsequentialsteps,thedissolutionkineticsbeingcontrolledeitherbythechemicalreactivityofthespeciesinvolved（activation-controlled）orbythespeedatwhichthereactionproductisremovedfromthesurfaceandthefreshreactantissuppliedtothesurface（diffusion-ormass-transport-controlled）.Temperaturevariationsalsoprofoundlyinfluencethekineticsofmetal-removalreaction.@#@Wetchemicaletchingbathscontainchemicalsthataregenerallyaggressiveandtoxic[2],thusposingsafetyanddisposalproblems.Inmanywet-etchingmanufacturingprocesses,wastetreatmentanddisposalcostsoftensurpassactualetchingprocessingcosts.Theeverincreasingcostofincinerationandtheimpositionoflandfillingrestrictionsarethemainreasonsbehindtheneedfordevelopingalternativeprocesses.@#@Electrochemicalmetalremovalisanalternativewetetchingprocesswheretheworkpieceismadeananodeinanelectrolyticcellinwhichasaltsolutionisusedasanelectrolyteandcontrolledmetalremovaltakesplacebyapplicationofanexternalcurrent.Severalnonconventionalmachiningprocessessuchaselectrochemicalmachining（ECM）andelectropolishingarebasedontheprincipleofelectrochemicalmetalremoval[3,4].TheECMprocesshasbeenusedpredominantlyintheproductionofturbineenginepartsandforotheraerospaceapplications,butapplicationsofECMalsoexistintheproductionofautomotivecomponents,medicalimplants,applianceparts,artilleryprojectiles,gun-barrelrifling,etc.becauseofitsabilitytomachinecomplexfeaturesandcomplicatedcontourswithoutmachiningmarks,burrs,orsurfacestresses.ECMisusedtoperformmachiningoperationsanalogoustobroaching,turning,anddiesinking,whilestaticmachiningwithastationarytoolisusedtodeburr,polish,and/orradiuscomponents.Therapidmetalremovalratealongwiththeadvantageofnonconsumedtoolingmakesitanattractiveprocessformetalshapingandfinishing.However,electrochemicalmetalremovalhasreceivedlittleattentionsofarinthemicroelectronicsindustry.Recentinvestigationsofthedevelopmentofadvancedelectrochemicalmetal-removalprocessesdemonstratethattheECMconceptscanbeeffectivelyusedintheremovalandpatterningofconductingfilmsthatareofinterestintheelectronicsindustry[5,6].@#@Electrochemicalmicromachining（EMM）@#@ApplicationofECMintheprocessingofthinfilmsandinthefabricationofmicrostructuresisreferredtoaselectrochemicalmicromachining（EMM）.Comparedtopredominantlyusedchemicaletching,theEMMprocessoffersbettercontrolandflexibility,requiresverylittlemonitoringandcontrol,andhasminimumsafetyandenvironmentalconcerns[5,6].AvarietyofmetalsandalloyscanbemachinedbyEMM.EMMisnowreceivingconsiderableattentionintheelectronicsandotherhightechnologyindustries,particularlyasanalternate,"@#@greener"@#@methodofprocessingmetallicparts[5,6].@#@Mostofthethinfilmsofmetalsandalloys,includingconductingceramicsandhighlycorrosion-resistantalloys,thatareofinterestinthemicroelectronicsindustrycanbeanodicallydissolvedinneutralsaltelectrolytessuchassodiumnitrate,sulfate,orchloride.Intheseelectrolytes,thedissolvedmetalionsformhydroxideprecipitateswhichremaininsuspendedforminsolutionandcanbeeasilyfiltered,thussignificantlyminimizingproblemsofsafetyandwastedisposal.Hydrogenevolutionisgenerallythemaincathodicreaction.Thecathode,therefore,remainsunaltered.AccumulationofreactionproductsinsolutionanddepletionofbathcomponentsareoflittleconcerninEMM,thusmakingitasimplerandmoreenvironmentallyfriendlymanufacturingprocess.@#@MicrofabricationbyEMMmayinvolvemasklessorthrough-maskmaterialremoval.Thin-filmpatterningbymasklessEMMrequireshighlylocalizedmaterialremovalinducedbytheimpingementofafineelectrolyticjet[5,7].Investigationofjetandlaser-jetEMMdemonstratedthatneutralsaltsolutionscanbeeffectivelyusedforhighspeedmicromachiningofmanymetalsandalloys.AnexampleofsuchaninvestigationisshowninFigure1,whichdemonstratesthefeasibilityofemployinganelectrolyticjetforgeneratingcomplicatedpatternsinmetallicfoilsandsubstrates.OtherexamplesofmasklessEMMincludemicrofinishingofcomponentsandremovalofunwantedlayersofthinfilmsbyelectromilling[8].@#@EMMinconjunctionwithaphotoresistmaskisofconsiderableinterestinmicroelectronicfabrication.Through-maskEMMinvolvesselectivemetaldissolutionfromunprotectedareasofaone-ortwo-sidedphotoresistpatternedworkpiece.Through-maskmetalremovalbywetetchingisaccompaniedbyundercuttingofthephotoresistandisgenerallyisotropicinnature.Inisotropicetching,thematerialisremovedbothverticallyandlaterallyatthesamerate.Thisisparticularlythecaseinchemicaletching,wheretheetchboundaryusuallyrecedesata45deganglerelativetothesurface[2].InEMM,however,themetal-removalrateinthelateraldirectionmaybesignificantlyreducedthroughproperconsiderationofmasstransportandcurrentdistribution[5].Etchfactorisdefinedastheratiooftheamountofstraight-throughetchtotheamountofundercut[6].Forapplicationsrequiringhighaspectratio,minimizedundercuttingofthephotoresistandahighvalueofetchfactoraredesirable.@#@EMMperformancecriteria@#@Themetal-removalrate,microfeatureprofile,surfacefinish,anduniformityofmetalremovalaresomeoftheperformancecriteriathatdeterminethetechnicalfeasibilityofametal-removalprocess.InEMM,thesecriteriaaredependentontheabilityofthesystemtoprovidedesiredmass-transportrates,currentdistribution,andsurfacefilmpropertiesattheactivesurface（Figure2）.Anunderstandingofthemetal-electrolyteinteractionunderhigh-rateanodicdissolutionconditionsisaprerequisiteforoptimizingprocessparameterssuchaselectrolytecompositionandvoltage/current.Thedevelopmentofprecisiontoolsrequiresanunderstandingoftheinfluenceofhydrodynamics,currentdistribution,andprocessparametersontheEMMperformance.Aprecisiontoolshouldprovideconditionsofdesiredcurrentdistributionandahighrateofuniformmasstransportatthedissolvingsurface[5].@#@Inthrough-maskprocesses,additionalissuesrelatedtolithographyprocessingarecriticaltoachievingdesiredperformance.Productionofthemasterartwork,surfacepreparation,choiceofproperphotoresist,andimagingareextremelyimportantinthesuccessfulimplementationofanetchingprocess.Sincepartsproducedbythisprocessareadirectreflectionofthemasterartwork,itisessentialthatallaspectsofpreparingth