1.1 Semiconductor Fabrication Processes - iue.tuwien.ac.at

1.1 Semiconductor Fabrication Processes · 1.1.1 Lithography · 1.1.2 Etching · 1.1.3 Deposition · 1.1.4 Chemical Mechanical Planarization · 1.1.5 Oxidation · 1.1.6 Ion ... Next:1.2IsolationTechniques Up:1.Introduction Previous:1.Introduction Subsections 1.1.1Lithography 1.1.2Etching 1.1.3Deposition 1.1.4ChemicalMechanicalPlanarization 1.1.5Oxidation 1.1.6IonImplantation 1.1.7Diffusion 1.1SemiconductorFabricationProcesses Startingwithanuniformlydopedsiliconwafer,thefabricationofintegratedcircuits(IC's)needshundredsofsequentialprocesssteps.Themostimportantprocessstepsusedinthesemiconductorfabricationare [1]: 1.1.1Lithography Lithographyisusedtotransferapatternfromaphotomasktothesurfaceofthewafer.ForexamplethegateareaofaMOStransistorisdefinedbyaspecificpattern.Thepatterninformationisrecordedonalayerofphotoresistwhichisappliedonthetopofthewafer.Thephotoresistchangesitsphysicalpropertieswhenexposedtolight(oftenultraviolet)oranothersourceofillumination(e.g.X-ray).Thephotoresistiseitherdevelopedby(wetordry)etchingorbyconversiontovolatilecompoundsthroughtheexposureitself.Thepatterndefinedbythemaskiseitherremovedorremainedafterdevelopment,dependingifthetypeofresistispositiveornegative.Forexamplethedevelopedphotoresistcanactasanetchingmaskfortheunderlyinglayers. 1.1.2Etching Etchingisusedtoremovematerialselectivelyinordertocreatepatterns.Thepatternisdefinedbytheetchingmask,becausethepartsofthematerial,whichshouldremain,areprotectedbythemask.Theunmaskedmaterialcanberemovedeitherbywet(chemical)ordry(physical)etching.Wetetchingisstronglyisotropicwhichlimitsitsapplicationandtheetchingtimecanbecontrolleddifficultly.Becauseoftheso-calledunder-etcheffect,wetetchingisnotsuitedtotransferpatternswithsub-micronfeaturesize.However,wetetchinghasahighselectivity(theetchratestronglydependsonthematerial)anditdoesnotdamagethematerial.Ontheothersidedryetchingishighlyanisotropicbutlessselective.Butitismorecapablefortransferingsmallstructures. 1.1.3Deposition AmultitudeoflayersofdifferentmaterialshavetobedepositedduringtheICfabricationprocess.Thetwomostimportantdepositionmethodsarethephysicalvapordeposition(PVD)andthechemicalvapordeposition(CVD).DuringPVDacceleratedgasionssputterparticlesfromasputtertargetinalowpressureplasmachamber.TheprincipleofCVDisachemicalreactionofagasmixtureonthesubstratesurfaceathightemperatures.TheneedofhightemperaturesisthemostrestrictingfactorforapplyingCVD.Thisproblemcanbeavoidedwithplasmaenhancedchemicalvapordeposition(PECVD),wherethechemicalreactionisenhancedwithradiofrequenciesinsteadofhightemperatures.Animportantaspectforthistechniqueistheuniformityofthedepositedmaterial,especiallythelayerthickness.CVDhasabetteruniformitythanPVD. 1.1.4ChemicalMechanicalPlanarization Processeslikeetching,deposition,oroxidation,whichmodifythetopographyofthewafersurfaceleadtoanon-planarsurface.Chemicalmechanicalplanarization(CMP)isusedtoplanethewafersurfacewiththehelpofachemicalslurry.First,aplanarsurfaceisnecessaryforlithographyduetoacorrectpatterntransfer.Furthermore,CMPenablesindirectpattering,becausethematerialremovalalwaysstartsonthehighestareasofthewafersurface.Thismeansthatatdefinedlowerlyingregionslikeatrenchthematerialcanbeleft.Togetherwiththedepositionofnon-planarlayers,CMPisaneffectivemethodtobuildupICstructures. 1.1.5Oxidation Oxidationisaprocesswhichconvertssilicononthewaferintosilicondioxide.Thechemicalreactionofsiliconandoxygenalreadystartsatroomtemperaturebutstopsafteraverythinnativeoxidefilm.Foraneffectiveoxidationratethewafermustbesettledtoafurnacewithoxygenorwatervaporatelevatedtemperatures.Silicondioxidelayersareusedashigh-qualityinsulatorsormasksforionimplantation.Theabilityofsilicontoformhighqualitysilicondioxideisanimportantreason,whysiliconisstillthedominatingmaterialinICfabrication. 1.1.6IonImplantation Ionimplantationisthedominanttechniquetointroducedopantimpuritiesintocrystallinesilicon.Thisisperformedwithanelectricfieldwhichacceleratestheionizedatomsormoleculessothattheseparticlespenetrateintothetargetmaterialuntiltheycometorestbecauseofinteractionswiththesiliconatoms.Ionimplantationisabletocontrolexactlythedistributionanddoseofthedopantsinsilicon,becausethepenetrationdepthdependsonthekineticenergyoftheionswhichisproportionaltotheelectricfield.Thedopantdosecanbecontrolledbyvaryingtheionsource.Unfortunately,afterionimplantationthecrystalstructureisdamagedwhichimpliesworseelectricalproperties.Anotherproblemisthattheimplanteddopantsareelectricallyinactive,becausetheyaresituatedoninterstitialsites.Thereforeafterionimplantationathermalprocessstepisnecessarywhichrepairsthecrystaldamageandactivatesthedopants. 1.1.7Diffusion Diffusionisthemovementofimpurityatomsinasemiconductormaterialathightemperatures.Thedrivingforceofdiffusionistheconcentrationgradient.Thereisawiderangeofdiffusivitiesforthevariousdopantspecies,whichdependonhoweasytherespectivedopantimpuritycanmovethroughthematerial.Diffusionisappliedtoannealthecrystaldefectsafterionimplantationortointroducedopantatomsintosiliconfromachemicalvaporsource.Inthelastcasethediffusiontimeandtemperaturedeterminethedepthofdopantpenetration.Diffusionisusedtoformthesource,drain,andchannelregionsinaMOStransistor.Butdiffusioncanalsobeanunwantedparasiticeffect,becauseittakesplaceduringallhightemperatureprocesssteps. Next:1.2IsolationTechniques Up:1.Introduction Previous:1.Introduction Ch.Hollauer:ModelingofThermalOxidationandStressEffects