半导体工艺-复旦大学-蒋玉龙-Chapter06

Chapter6SiO2,Si/SiO2InterfaceandHigh-k/Low-kdielectricsOutlineI.SiO2---oneofthekeymaterialsforsemiconductortechnologyII.StructureofSiO2filmsIII.GrowthmechanismsofthermaloxideIV.Deal-GrovemodelofoxidationkineticsV.OxidationratedependenceonT,P,OrientationVI.ImpurityeffectsonoxidationrateVII.Si/SiO2interfaceandoxidetrapsVIII.OtherpropertiesofthermalSiO2IX.High-kandlow-kdielectricsI.SiO2−oneofthekeymaterialsforsemiconductortechnology1.Uniquechemicalandphysicalproperties¾Chemicalandthermalstability¾Gooddielectricproperties¾Filmuniformityandmanufacturability¾ExcellentinterfacewithSi2.Varietyofformationmethods¾Thermaloxidation---FoundationforplanarprocessandVLSItechnology¾Anodization(electrochemicaloxidation)¾Plasmaoxidation(RFandmicrowave)¾ChemicalVaporDeposition(CVD)¾ManydifferentCVDtechniques:APCVD;LPCVD;PECVD;HDP-CVD;……SOG(Spin-on-glass)3.Myriadofapplicationsinsemiconductortechnology¾Maskingforimpuritydoping(diffusion&ionimplantation)¾GatedielectricsmaterialforMOSdevices¾Deviceisolation9LOCOSfieldoxide9Refilledtrenches9BuriedinsulatorinSOIwafer¾Electricalisolation9Inter-leveldielectricformultilevelinterconnection9Filmforplanarization9Sidewallspacer¾Surfacepassivation¾Capacitordielectrics¾DevicesfabricationofGaAsandothercompoundsemiconductormaterialsIIStructureofSiO2films1.ThermallygrownSiO2---amorphousfilmconstructedbySi-Obonds¾Nolongrangeorder,buthavingshortrangeorder¾Basicstructuralunittetrahedron2.Quartzcrystal---orderednetworkofpolyhedra(left)Amorphousfilm(silica)---randomnetworkofpolyhedra(right)Six-memberedringstructure*Amorphoussilica-inmetastablestate*Tendencytocrystallineordevitrifiedstablestate(Devitrificationoffurnacetube)---VeryslowprocessfortypicaldeviceoperatingTSiO44-tetrahedra3.Non-bridgingoxygenandimpuritiesinSiO2film¾Lessnon-bridgingoxygen→bettercohesiveness(致密性)¾Substitutionalimpurities:B3+,P5+---Networkformers→Glassystructure(B2O3,P2O5)¾Interstitialimpurities:Na,K,Pb---NetworkmodifiersIIIGrowthmechanismsofthermaloxide1.Chemicalreactionofthermaloxidation¾Dryoxidation:Si(s)+O2(g)→SiO2(s)¾Wetoxidation:Si(s)+2H2O(g)→SiO2(s)+2H2(g)Thesetworeactionsaretypicallycarriedoutat700-1200℃¾OxideQualitydependson:9Substratepurity(wafercleaning)9Gaspurity9Oxidationprocess2.Siconsumption‹tSiNSi=tSiO2NSiO2NSi---SidensityinSi(atoms/cm3)NSiO2---SiO2densityinSiO2(molecules/cm3)‹NSi=5×1022cm-3;N=2.2SiO2×1022cm-3toxSiSiSiO2OriginalsurfaceSioxoxSiNNtt=oxoxsitcmatomscmmoleculestt44.0/105/102.2322322=×ו=3.Diffusionspeciesduringoxidation‹TwopossiblewaysofSiO2growth(1)SidiffusestoSiO2surface(2)OxidantdiffusestoSiO2/Siinterface‹Experiments:OxidationbyisotopeO18→Si-O18bondsInfraredisotopeshiftofSi-OabsorptionpeakMakerexperiment;Radioactivetracer‹Experimentalevidencehasprovedthewayof(2)¾Inwardmigrationofoxidantspecies¾SiO2reactionandgrowthatSiO2/SiinterfaceGasDiffusionSolid-StateDiffusionSiO2FormationSiO2Si-SubstrateGasFlowstagnantLayerOxidantFlow(e.g.O2,orH2O)IVDeal-GrovemodelofOxidationKinetics(developedin1965)1.Kineticfactorsandoxidantfluxequation‹Thekeystepsintheoxidationprocess:(1)DiffusionofOxidantfromgasstreamtoSiO2surface(F1)(2)DiffusionofoxidantthroughSiO2film(F2)(3)ReactionatSiO2/Siinterface(F3)‹F1,F2,F3---fluxes[Numberofatomsormolecules/(cm2·sec)]reactionSilicond0CoCiF1GasF2F3TransportCGOxideCδCsx(1)F1=hG(CG-CS)CG---OxidantconcentrationingasCS---OxidantconcentrationjustoutsideSiO2hG---gas-phasemasstransportcoefficient,relatedtothediffusivity(D)ofoxidantspeciesandthethicknessofthestagnantlayer(δ,滞留层);hG=D/δBytheidealgaslaw:PV=nRTorPV=NkTn---Numberofmoles;N---NumberofatomsormoleculeskTPCcmVNC=→=−)(3;CGkTPG=∴kTPCSs=)(hG1SGPPkTF−=∴d0CoCiF1GasF2F3TransportCGOxideCδCsxReaction‹UseHenry’slaw(relationbetweentheconcentrationofaspeciesinasolidandthepartialpressureofthespeciesinthesurroundinggas)Co=HPs;C*=HPH---Henry’sconstantC*---Equilibriumconcentrationofoxidantintheoxide∴F1=h(C*-Co),h---Newgas-phasemasstransportcoefficient(2)FluxofoxidantacrosstheexistingoxidelayerbyFick’sLawHkThhG=02()ioxDCCCFDXX−∂=−=∂(3)AssumingfirstorderkineticsforreactionattheinterfacesKKs—ReactionrateconstantofthechemicalreactionattheinterfaceInSteadyState:*000()()()ioxisioxDhCCCCXDCCKCX⎧−=−⎪⎪∴⎨⎪−=⎪⎩SolvingtheequationsforCs&Ci*1issoxCCKKXhD=++*0(1)1soxssoxKXCDCKKXhD+=++isCKF=3321FFF==2.Diffusioncontrolledandreactioncontrolledoxidation①DiffusioncontrolledcaseDKsXoxDiffusionfluxgovernedbyDReactionfluxgovernedbyKsCi→0;C0→C*¾Therateofoxidationislimitedbysupplyofoxidizingspecies¾ThiscaseusuallyoccurswhentheoxideisthickXox40-100Åfordry;Xox1000Åforwet¾Parabolictimedependence:GasCxSiO2C*tXox~2②Reactioncontrolledcase¾Morethanenoughoxidantavailableforoxidationreaction¾Oxidegrowthrateiscontrolledbyreactionrate¾AtbeginningofSi-oxidation¾Lineartimedependence:*01isCCCKh=→+⇒;oxxXKDtXOX~3.GrowthrateofoxidationN1—Numberofoxidantmoleculesincorporatedintoaunitvolumeofoxide¾Fordryoxidation:Si+O2→SiO2,N1=2.2×1022/cm3¾Forwetoxidation:Si+2H2O→SiO2+2H2↑N1=4.4×1022/cm3‹Recallthat‹Assumeainitialconditionatt=0,Xox(0)=Xi(Initialoxide)]/[13scmNFdtdXox=*31ssissoxKCFKCK