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Chapter2,Microfabrication(ElectronicsManufacturing)1LearningObjectivesBeabletodescribethebasicprocessesofmicrofabricationBeabletoexplaintheprinciplesofphotolithography.Beabletodescribethebasicmechanismsoftheadditiveprocesses,includingrelativecomparisonsamongthem.PhysicalVaporDeposition(evaporation,sputtering)ChemicalVaporDepositionBeabletodescribethebasicmechanismsofthesubtractiveprocesses,includingrelativecomparisonsamongthem.WetEtching(isotropic,anisotropic)DryEtching(physical,chemical,physical-chemical)Beabletodescribetheprocessofbondingandpackaging2WhatisLIGA?LIGAisafabricationprocessforhighaspectratiomicrostructuresconsistingofthreemajorprocesssteps.X-raylithography(LI=lithographie)togenerateprimarymicrostructures.(DXRL=deepX-raylithography,UDXRL=ultra-deepX-raylithography)Electroplating/Electrodeposition(G=Galvanik)toproducemicrostructuresinmetal.Molding(A=Abformung)tobatchproducesecondarymicrostructuresinpolymers,metals,ceramics…3WhyIsLIGAInteresting?VeryhighaspectratiostructurescanbeachievedHeight(typical)20-500μm,thiscannotbeachievedbystateofartsiliconsurfacemicromachiningHavemorematerialselectivityinfinalproductsCouldbemetal,polymer,andevenceramics.VerticalandbettersurfaceroughnessinsidewallVerticalslope<1μm/mm,Surfaceroughness0.03-0.05μm(max.peaktovalley)ItcanbeappliedinbothMEMSandtraditionalprecisionmanufacturing.5LIGAProcess—X-raylithography6LIGAProcess—Electroplating7LIGAMaterialsPolymersUsedinX-raylithographyperiod.ThematerialsmustbeabletoallowphotochemicalreactionunderexposureunderX-rayThickPMMAisapopularchoiceMetalsUsedinelectrodepositionphasetoformamouldNickelisapopularchoice.PolymeragainUsedinmoldingperiodThefinalLIGAproductisusuallymadeofpolymers9LIGAApplicationshighaspectratiomicrostructures

Microturbinerotor,KfK10LIGAApplications

SpinneretCapillary,IMM11LIGA-LikeProcessOverview13DRIESystemPlasmalabSystem100ModularICP-RIEEtchingSystemOxfordInstruments14OverviewofDRIEReactiveionetching(RIE)isadryetchingmethodwhichcombinesplasmaetchingandionbeametchingprinciples.ChoiceformostadvancedproductlineIt’snotwellsuitablefordeepetching(>10μm)Inductively

coupledplasma(ICP)reactorshavebeenintroducedforsiliconRIEprocessleadingtothedeepreactiveionetching(DRIE)technique.HigherplasmadensityHigheretchingrate,eitherforanisotropicandisotropicetchingHigheraspectratio(AR)Reductionofparasiticeffects15DRIEApplicationsThesiliconstructureswerepartofaninnovativeescapemechanismoftheUlysseNardin"Freak"watch.(CSEMSA,IMT)Siliconparts:LighterLowerfrictioncoefficientInsensitivetomagneticfields17SiliconReviewInaperfectcrystal,eachofsilicon’sfourouterelectronsformcovalentbonds,resultinginpoorelectronmobility(i.e.insulating)Dopingsiliconwithimpuritiesalterselectronmobility(i.e.semiconducting)Extraelectron(“N-type”,withphosphorous,forexample)Missingelectron(“P-type”,withboron,forexample)18SiliconCircuitsSiliconmakesthetransistorsTransistorscanbemadetoverylargescaleintegration(VLSI)andultralargescaleintegration(ULSI)APentium4processcontains42milliontransistors19MicrofabricationSiliconcrystalstructureisregular,well-understood,andtoalargeextentcontrollable.Itisallaboutcontrol:thesizeofatransistoris1m,thedopingmustthereforelessthanhaveofthatHowtocontrol?21MicrofabricationTechniques22ProcessofMicrofabricationSinglecrystalgrowingWaferslicingFilmdepositionOxidationDiffusionIonimplantationEtchingLithographyMetallizationBondingPackagingTesting23CrystalGrowing25WaferSlicingThisstepincludesSlicetheingotintoslicesusingadiamondsawPolishthesurface,andSort26FilmDepositsThisstepisusedtoaddaspeciallayeronthesurfaceofthesiliconformaskingManytypesoffilmsareusedforinsulating/conducting,includingpolysilicon,siliconnitride,silicondioxide,tungsten,andtitanium.Filmsmaybedepositedusingvariousmethod,includingEvaporationSputteringChemicalVaporDeposition(CVD)27PhotolithographyPhotolithographyisaprocessbywhichanimageisopticallytransferredfromonesurfacetoanother,mostcommonlybytheprojectionoflightthroughamaskontoaphotosensitivematerial.Photoresistisamaterialthatchangesmolecularstructurewhenexposedtoradiation(e.g.ultravioletlight).Ittypicallyconsistsofapolymerresin,aradiationsensitizer,andacarriersolvent.29PhotolithographyAddingaphotoresistlayeronthewaferAphotomaskistypicallymanifestedasaglassplatewithathinmetallayer,thatisselectivelypatternedtodefineopaqueandtransparentregions.30PhotolithographyApositivephotoresistisweakenedbyradiationexposure,sotheremainingpatternafterbeingsubjecttoadevelopersolutionlooksjustliketheopaqueregionsofthemaskAnegativephotoresistisstrengthenedbyradiationexposure,sotheremainingpatternafterbeingsubjecttoadevelopersolutionappearsastheinverseoftheopaqueregionsofthemask.31Etching–MaskforaMaskThemaskmustwithstandthechemicalenvironment.Atypicalmask/substratecombinationisoxideonsilicon.Resilientmasksaretypicallygrownordepositedinwholefilms,andmustthereforebepatternedthroughaphotosensitivemaskReusablemaskPhotoresistcoatingFunctionalmask32DryEtchingMechanismsPhysicalRemovalbasedonimpact&momentumtransferPoormaterialselectivityGooddirectionalcontrolHighexcitationenergyLowerpressure,<100mTorrChemicalHighestremovalrateGoodmaterialselectivityGenerallyisotropicHigherpressure,>100mTorrPhysical/ChemicalGooddirectionalcontrolIntermediatepressure,~100mTorr33IsotropicWetEtchingEtchoccursinallcrystallographicdirectionsatthesamerate.Mostcommonformulationismixtureofhydrofluoric,nitricandaceticacids(“HNA”:HF+HNO3+CH3COOH).Etchratemaybeveryfast,manymicronsperminute.Masksareundercut.Highaspectratiodifficultbecauseofdiffusionlimits.Stirringenhancesisotropy.Isotropicwetetchingisapplicabletomanymaterialsbesidessilicon34AnisotropicWetEtchingEtchoccursatdifferentratesdependingonexposedcrystalUsuallyinalkalinesolutions(KOH,TMAH).Heatingtypicallyrequiredforratecontrol(e.g.>80oC).Etchratetypically~1μm/min,limitedbyreactionsratherthandiffusion.Maintainsmaskboundarieswithoutundercut.Anglesdeterminedbycrystalstructure(e.g.54.7o).Possibletogetperfectorthogonalshapesoutlinesusing1-0-0wafe