Chapter9

Etching蝕刻EtchingPatternedMaterialSelectivityisImportant!!Un-patternedEtchingDryEtch

（干蝕刻）An-isotropic各向異性dy/dt:dx/dt:6GasPhaseReactionwithvolatileproducts

含揮發性產物的氣相反應FrequentuseofveryreactivespeciesinaPlasma在等離子體中頻繁使用活性物質SiEtchSiO2EtchMetalEtchWetEtch=Dissolution濕蝕刻Isotropicdy/dt:dx/dt:1.2SiEtchStrongHFSiO2EtchStrongNH4OHnotNaOH(Naionisbad)Si3N4EtchPhosphoricAcidMetalEtchAcidSolution(HNO3)PhotoresistSolventH2SO4SolutionxyEtchingWetandDryEtchhaveverydifferentchemicalreactions!WetandDryEtchhavesimilarratedeterminingstepsMassTransferLimitingSurfaceReactionLimitingSimilarmathematicsDissolutionofLayer-WetEtchBL-MassTransferA(l)+bB(s)ABb(l)A=Acidformetal(B)dissolutionredoxreactionBaseforSiO2(B)dissolutionSolventforphotoresist(B)dissolution

EtchReactionsBoundaryLayerMassTransferSurfaceChemicalReactionLikeCatalyticreactionProductdiffusionawayfromsurfaceReactantConcentrationProfileProductConcentrationProfileDissolutionRate/TimeDependsonMassTransferDiffusionCoefficientVelocityalongwafersurfaceSizeofwaferSolubilityDensityoffilmbeingetchedWetEtchReactionWafersinCarriagePlacedinEtchSolutionHowLong??BoundaryLayerMTisRateDeterminingFlowoveraleadingedgeforMTDerivation&MathcadsolutionAlsoa

CfortheConcentrationprofileDryEtchPhysicalEvaporationNottypicallyusedHeatingchipdiffusesdopantsoutofpositionSputteringfromatargetPlasmareactorwithvolatilereactionproductWhyDryEtching?RFPlasmaSputteringforDepositionandforEtchingRF+DCfield

RemovalRateSputteringYield,SS=α(E1/2-Eth1/2)DepositionRate

IoncurrentintoTarget*SputteringYieldFundamentalChargePlasmaFreeElectronsacceleratedbyastrongelectricfieldCollidewithgasmoleculesandejecte-CollisioncreatesmorefreeelectronsFreeelectronscombinewithionstoformfreeradicalsGasIons/FreeRadicalsareveryreactivewithmaterialsatthewafersurfaceIonsnon-selectiveremovalFreeRadicalsPlasmaConditionsReducedPressure~100mtorrFlowofgasesinandoutDCorAC(rf)electricfieldParallelplateelectrodesOthergeometriesPlasmaTemperatureofGasmolecules,TgasPVm/RgTemperatureofElectrons,Te

=e2E2Mg/(6me2

m2kB)AcceleratedbyEfieldbetweencollisionswithgasmolecules

m=momentumcollisionfrequency=Ngvel

m(v)

TeE/Ng

ERgTg/Ptot

>>TgaskBTe>GasIonizationEnergykBTe>MolecularDissociationEnergyPlasmaGasChemistriesReactantGasesPhysicalEtch=SputteringfromchiptargetArChemicalEtchO2CF4HBrCl2CHF3C2F6MixturesCF2deposition(likeateflonpolymerlayer)preventssidewalletchGaseous(Volatile)ProductsSiO(g),SiF4(v),SiCl4(v),SiBr4(v)MFx(v),

MClx(v),MBrx(v),1stIonizationEnergiesO 13.618eVBr 11.814eVCl 12.967eVF 17.422eVH 13.598eVAr 15.759eV PlasmaEtchMechanismPreCleanO2+eO2++2eO2+e2O+eO+eO-O2++e2OO+sO-sO+Si(s)s-SiOSiO-s

SiO(g)Metal(M)EtchCl2+e2Cl+eCl2

Cl2++eCl+sCl-sxCl-M(s)MClx(g)Simultaneouslye+CF4CF3++F+2ee+CF3+CF2+FCF3++CF2(CF2)n+FPolymeronwallofetchNeutralsaremainreactivespecies!!EtchingRecipesICP-RIEwithbetterplasmaorientationICP-RIEetchedSipillarsDeepreactive-ionetching

Deepreactive-ionetching(DRIE)isahighlyanisotropicetchprocessusedtocreatedeeppenetration,steep-sidedholesandtrenchesinwafers/substrates,typicallywithhighaspectratios.CryogenicprocessIncryogenic-DRIE,thewaferischilledto?110°C(163K).Thelowtemperatureslowsdownthechemicalreactionthatproducesisotropicetching.However,ionscontinuetobombardupward-facingsurfacesandetchthemaway.BoschprocessAlternatesrepeatedlybetweentwomodes:1.IsotropicplasmaetchingwithSF6;2.DepositionofachemicallyinertpassivationlayerwithC4F8BoschProcess