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Fundamental Principles of Optical Lithography The Science of Microfabrication.pdf

Fundamental Principles of Optic…

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简介:本文档为《Fundamental Principles of Optical Lithography The Science of Microfabricationpdf》,可适用于高等教育领域,主题内容包含IntroductiontoSemiconductorLithographyThefabricationofanintegratedcircuit(符等。

IntroductiontoSemiconductorLithographyThefabricationofanintegratedcircuit(IC)involvesagreatvarietyofphysicalandchemicalprocessesperformedonasemiconductor(egsilicon)substrateIngeneral,thevariousprocessesusedtomakeanICfallintothreecategories:filmdeposition,patterningandsemiconductordopingFilmsofbothconductors(suchaspolysilicon,aluminum,tungstenandcopper)andinsulators(variousformsofsilicondioxide,siliconnitrideandothers)areusedtoconnectandisolatetransistorsandtheircomponentsSelectivedopingofvariousregionsofsiliconallowstheconductivityofthesilicontobechangedwiththeapplicationofvoltageBycreatingstructuresofthesevariouscomponents,millions(orevenbillions)oftransistorscanbebuiltandwiredtogethertoformthecomplexcircuitryofamodernmicroelectronicdeviceFundamentaltoalloftheseprocessesislithography,ietheformationofthreedimensional(D)reliefimagesonthesubstrateforsubsequenttransferofthepatternintothesubstrateThewordlithographycomesfromtheGreeklithos,meaningstones,andgraphia,meaningtowriteItmeansquiteliterallywritingonstonesInthecaseofsemiconductorlithography,ourstonesaresiliconwafersandourpatternsarewrittenwithalightsensitivepolymercalledaphotoresistTobuildthecomplexstructuresthatmakeupatransistorandthemanywiresthatconnectthemillionsoftransistorsofacircuit,lithographyandetchpatterntransferstepsarerepeatedatleasttimes,butmoretypicallytotimestomakeonecircuitEachpatternbeingprintedonthewaferisalignedtothepreviouslyformedpatternsasslowlytheconductors,insulatorsandselectivelydopedregionsarebuiltuptoformthefinaldeviceTheimportanceoflithographycanbeappreciatedintwowaysFirst,duetothelargenumberoflithographystepsneededinICmanufacturing,lithographytypicallyaccountsMaskWaferMaskWaferFundamentalPrinciplesofOpticalLithography:TheScienceofMicrofabricationChrisMackJohnWileySons,LtdISBN:FundamentalPrinciplesofOpticalLithographyforaboutofthecostofmanufacturingachipAsaresult,ICfabricationfactories(‘fabs’)aredesignedtokeeplithographyasthethroughputbottleneckAnydropinoutputofthelithographyprocessisadropinoutputfortheentirefactorySecond,lithographytendstobethetechnicallimiterforfurtheradvancesintransistorsizereductionandthuschipperformanceandareaObviously,onemustcarefullyunderstandthetradeoffsbetweencostandcapabilitywhendevelopingalithographyprocessformanufacturingAlthoughlithographyiscertainlynottheonlytechnicallyimportantandchallengingprocessintheICmanufacturingflow,historically,advancesinlithographyhavegatedadvancesinICcostandperformanceBasicsofICFabricationAsemiconductorisnot,asitsnamemightimply,amaterialwithpropertiesbetweenanelectricalconductorandaninsulatorInstead,itisamaterialwhoseconductivitycanbereadilychangedbyseveralordersofmagnitudeHeat,light,impuritydopingandtheapplicationofanelectricfieldcanallcausefairlydramaticchangesintheelectricalconductivityofasemiconductorThelasttwocanbeappliedlocallyandformthebasisofatransistor:byapplyinganelectricfieldtoadopedregionofasemiconductormaterial,thatregioncanbechangedfromagoodtoapoorconductorofelectricity,orviceversaIneffect,thetransistorworksasanelectricallycontrolledswitch,andtheseswitchescanbeconnectedtogethertoformdigitallogiccircuitsInaddition,semiconductorscanbemadetoamplifyanelectricalsignal,thusformingthebasisofanalogsolidstatecircuitsByfarthemostcommonsemiconductorinuseissilicon,duetoanumberoffactorssuchascost,formationofastablenativeoxideandvastexperience(thefirstsiliconICwasbuiltinabout)Awaferofsinglecrystalsiliconanywherefrom–mmindiameterandabout–mmthickservesasthesubstrateforthefabricationandinterconnectionofplanartransistorsintoanICThemostadvancedcircuitsarebuiltonandmmdiameterwafersThewafersarefarlargerthantheICsbeingmadesothateachwaferholdsafewhundred(anduptoafewthousand)ICdevicesWafersareprocessedinlotsofaboutwafersatatime,andlargefabscanhavethroughputsofgreaterthanwafersperweekThecycletimeformakingachip,fromstartingbaresiliconwaferstoafinishedwaferreadyfordicingandpackaging,istypically–daysSemiconductorprocessing(orICfabrication)involvestwomajortasks:•Creatingsmall,interconnectedDstructuresofinsulatorsandconductorsinordertomanipulatelocalelectricfieldsandcurrents•Selectivelydopingregionsofthesemiconductor(tocreatep–njunctionsandotherelectricalcomponents)inordertomanipulatethelocalconcentrationofchargecarriersPatterningTheDmicrostructuresarecreatedwithaprocesscalledpatterningThecommonsubtractivepatterningprocess(Figure)involvesthreesteps:()depositionofauniformfilmofmaterialonthewafer()lithographytocreateapositiveimageofthepatternIntroductiontoSemiconductorLithographythatisdesiredinthefilmand()etchtotransferthatpatternintothewaferAnadditiveprocess(suchaselectroplating)changestheorderofthesesteps:()lithographytocreatenegativeimageofthepatternthatisdesiredand()selectivedepositionofmaterialintotheareasnotprotectedbythelithographicallyproducedpatternCopperisoftenpatternedadditivelyusingthedamasceneprocess(namedforauniquedecorativemetalfillprocessappliedtoswordsanddevelopedinDamascusaboutyearsago)Depositioncanusemanydifferenttechnologiesdependentonthematerialandthedesiredpropertiesofthefilm:oxidegrowth(directoxidationofthesilicon),chemicalvapordeposition(CVD),physicalvapordeposition(PVD),evaporationandsputteringCommonfilmsincludeinsulators(silicondioxide,siliconnitride,phosphorousdopedglass,etc)andconductors(aluminum,copper,tungsten,titanium,polycrystallinesilicon,etc)Lithography,ofcourse,isthesubjectofthisbookandwillbediscussedatgreatlengthinthepagesthatfollowPhotoresistsareclassedaspositive,whereexposuretolightcausestheresisttoberemoved,andnegative,whereexposedpatternsremainafterdevelopmentThegoalofthephotoresististoresistetchingafterithasbeenpatternedsothatthepatterncanbetransferredintothefilmEtchingEtchinvolvesbothchemicalandmechanicalmechanismsforremovalofthematerialnotprotectedbythephotoresistWetetch,perhapsthesimplestformofetch,usesanetchantsolutionsuchasanacidthatchemicallyattackstheunderlyingfilmwhileleavingthephotoresistintactThisformofetchingisisotropicandthuscanleadtoundercuttingasthefilmisetchedfromunderneaththephotoresistIfanisotropicetchingisdesired(andmostalwaysitis),directionalitymustbeinducedintotheetchprocessPlasmaetchingreplacestheliquidetchantwithaplasma–anionizedgasApplyinganelectricfieldcausestheionstobeaccelerateddownwardtowardthewaferTheresultingetchisamixofchemicaletchingduetoreactionofthefilmwiththeplasmaandphysicalsputteringduetothedirectionalbombardmentoftheionshittingthewaferThechemicalnatureoftheetchcanleadtogoodetchselectivityofthefilmwithrespecttotheresist(selectivitybeingdefinedastheratioofthefilmetchratetotheresistetchrate)andwithrespecttothesubstratebelowthefilm,butisessentiallyisotropicPhysicalsputteringisveryWaferFilmResistDepositionLithographyEtchResistStripFigureAsimplesubtractivepatterningprocessFundamentalPrinciplesofOpticalLithographydirectional(etchingisessentiallyverticalonly),butnotveryselective(theresistetchesataboutthesamerateasthefilmtobeetched)Reactiveionetchingcombinesbotheffectstogivegoodenoughselectivityanddirectionality–theacceleratedionsprovideenergytodriveachemicaletchingreactionThephotoresistpropertyofgreatestinterestforetchingistheetchselectivity,whichisdependentbothonthephotoresistmaterialpropertiesandthenatureoftheetchprocessforthespecificfilmGoodetchprocessesoftenhaveetchselectivitiesinexcessof(forexample,polysiliconwithanovolacresist),whereaspoorselectivitiescanbeaslowas(forexample,whenetchinganorganicbottomantireflectioncoating)EtchselectivityandthethicknessofthefilmtobeetcheddeterminetheminimumrequiredresistthicknessMechanicalpropertiesoftheresist,suchasadhesiontothesubstrateandresistancetomechanicaldeformationsuchasbendingofthepattern,alsoplayaroleduringetchingForanetchprocesswithoutperfectselectivity,theshapeandsizeofthefinaletchedpatternwilldependonnotonlythesizeoftheresistpatternbutitsshapeaswellConsideraresistfeaturewhosestraightsidewallsmakeanangleqwithrespecttothesubstrate(Figure)GivenverticalandhorizontaletchratesoftheresistRVandRH,respectively,therateatwhichthecriticaldimension(CD)shrinkswillbeddHVCDtRR=(cot)θ()Thus,therateatwhichtheresistCDchangesduringtheetchisafunctionoftheresistsidewallangleAstheangleapproaches,theverticalcomponentceasestocontributeandtherateofCDchangeisatitsminimumInfact,Equation()showsthreewaystominimizethechangeinresistCDduringtheetch:improvedetchselectivity(makingbothRHandRVsmaller),improvedanisotropy(makingRHRVsmaller)andasidewallclosetovertical(makingcotqsmaller)Theexampleabove,whilesimple,showsclearlyhowresistprofileshapecanaffectpatterntransferIngeneral,theidealphotoresistshapehasperfectlyverticalsidewallsOthernonidealprofileshapes,suchasroundingofthetopoftheresistandresistfooting,willalsoaffectpatterntransferResistRHRVqFigureErosionofaphotoresistlineduringetching,showingtheverticalandhorizontaletchratecomponentsIntroductiontoSemiconductorLithographyIonImplantationSelectivedopingofcertainregionsofthesemiconductorbeginswithapatterningstep(Figure)Regionsofthesemiconductorthatarenotcoveredbyphotoresistareexposedtoadopantimpurityptypedopantslikeboronhavethreeoutershellelectronsandwheninsertedintothecrystallatticeinplaceofsilicon(whichhasfourouterelectrons)createmobileholes(emptyspotsinthelatticewhereanelectroncouldgo)ntypedopantssuchasphosphorous,arsenicandantimonyhavefiveouterelectrons,whichcreateexcessmobileelectronswhenusedtodopesiliconTheinterfacebetweenptyperegionsandntyperegionsofsiliconiscalledap–njunctionandisoneofthefoundationalstructuresinthebuildingofsemiconductordevicesThemostcommonwayofdopingsiliconiswithionimplantationThedopantisionizedinahighvacuumenvironmentandacceleratedintothewaferbyanelectricfield(voltagesofhundredsofkilovoltsarecommon)Thedepthofpenetrationoftheionsintothewaferisafunctionoftheionenergy,whichiscontrolledbytheelectricfieldTheforceoftheimpactoftheseionswilldestroythecrystalstructureofthesilicon,whichthenmustberestoredbyahightemperatureannealingstep,whichallowsthecrystaltoreform(butalsocausesdiffusionofthedopant)Sincetheresistmustblocktheionsintheregionswheredopantsarenotdesired(thatis,intheregionscoveredbytheresist),theresistthicknessmustexceedthepenetrationdepthoftheionsIonimplantationpenetrationdepthisoftenmodeledasaGaussiandistributionofdepths,ietheresultingconcentrationprofileofimplanteddopantsfollowsaGaussianshapeThemeanofthedistribution(thepeakoftheconcentrationprofile)occursatadepthcalledtheprojectedrange,RpThestandarddeviationofthedepthprofileiscalledthestraggle,RpForphotoresists,theprojectedrangevariesapproximatelylinearlywithimplantenergy,andinverselywiththeatomicnumberofthedopant(Figurea)Amoreaccuratepowerlawmodel(asshowninFigurea)isdescribedinTableThestragglevariesapproximatelyasthesquarerootofimplantenergy,andisaboutindependentofthedopant(Figureb)Forhigherenergies(MeVandabove),higheratomicnumberdopantsproducemorestraggleWhenmoredetailedpredictionsareneeded,MonteCarloimplantationsimulatorsarefrequentlyusedResistLithographyIonImplantationResistStripWaferDopantIonsFigurePatterningasameansofselectivedopingusingionimplantationFundamentalPrinciplesofOpticalLithographyInordertomasktheunderlyinglayersfromimplant,theresistthicknessmustbesettoatleastresistthicknessRmRpp()wheremissettoachieveacertainlevelofdopantpenetrationthroughtheresistForexample,ifthedopantconcentrationatthebottomoftheresistcannotbemorethantimesthepeakconcentration(atypicalrequirement),thenmshouldbesettoWhiletheresolutionrequirementsfortheimplantlayerstendnottobechallenging,oftenthethicknessrequiredforadequatestoppingpowerdoesposerealchallengestothelithographerCarbonizationoftheresistduringhighenergyandhighdoseimplantation(aswellasduringplasmaetching)canalsoresultinafilmthatisverydifficulttostripawayattheendProcessIntegrationThecombinationofpatterningandselectivedopingallowsthebuildupofthestructuresrequiredtomaketransistorsFigureshowsadiagrammaticalexampleofapairofCMOS(complementarymetaloxidesemiconductor)transistorsSubsequentmetallayers(uptometallevelsarenotuncommon)canconnectthemanytransistorsintoafullIonEnergy(keV)ProjectedRange(nm)BPAsIonEnergy(keV)Straggle(nm)BPAs(b)(a)FigureMeasuredandfittedionimplantationpenetrationdepthsforboron,phosphorousandarsenicinAZresist:(a)projectedrangeand(b)straggleSymbolsaredataandcurvesarepowerlawfitstothedataasdescribedinTableForthestraggledata,theempiricalmodelfitisRp=EwhereEistheionenergyinkeVandRpisthestraggleinnmTableEmpiricalmodelofionimplantedprojectedrange(Rp,innm)intophotoresistversusionenergy(E,inkeV)asRp=aEbDopantCoefficientaPowerbBoronPhosphorousArsenicIntroductiontoSemiconductorLithographycircuitandthefinalmetallayerwillprovideconnectionstotheexternalpinsofthedevicepackageManylithographiclevelsarerequiredtofabricateanIC,butaboutoftheselevelsareconsidered‘critical’,meaningthatthoselevelshavechallenginglithographicrequirementsWhichlevelsarecriticaldependsontheprocesstechnology(CMOSlogic,DRAM,BiCMOS,etc)ThemostcommoncriticallevelsofaCMOSprocessareactivearea,shallowtrenchisolation(STI),polysilicongate,contact(betweenmetalandpoly)andvia(betweenmetallayers),andmetal(thefirstorbottommostmetallayer)Foralargelogicchipwithlayersofmetal,thefirstthreewillbe‘’,meaningthedimensionsareatornearlyattheminimummetaldimensionsThenextthreemetallayerswillbe‘’,withdimensionsabouttwiceasbigasthemetallevelsThenextfewmetallevelswillbe,withthelastfewlevelsaslargeasForaDRAMdevice,someofthecriticallevelsareknownasstorage,isolation,wordlineandbitlinecontact(seeFigureforexampledesignpatternsforthesefourlevels)Moore’sLawandtheSemiconductorIndustryTheimpactofsemiconductorICsonmodernlifeishardtooverstateFromcomputerstocommunication,entertainmenttoeducation,thegrowthofelectronicstechnology,fueledSiliconWaferpwellnwellppOxidePolysiliconMetalnnFigureCrosssectionofapairofCMOStransistorsshowingmostofthelayersthroughmetal(a)Storage(b)Isolation(c)Wordline(d)BitlineContactFigureCriticalmasklevelpatternsforaGbDRAMchipEachpatternrepeatsinbothxandymanytimestocreatetheDRAMarrayFundamentalPrinciplesofOpticalLithographybyadvancesinsemiconductorchips,hasbeenphenomenalTheimpacthasbeensoprofoundthatitisnowoftentakenforgranted:consumershavecometoexpectincreasinglysophisticatedelectronicsproductsateverlowerprices,andsemiconductorcompaniesexpectgrowthandprofitstoimprovecontinuallyTheroleofopticallithographyinthesetrendshasbeen,andwillcontinuetobe,vitalTheremarkableevolutionofsemiconductortechnologyfromcrudesingletransistorstobilliontransistormicroprocessorsandmemorychipsisafascinatingstoryOneofthefirst‘reviews’ofprogressinthesemiconductorindustrywaswrittenbyGordonMoore,afounderofFairchildSemiconductorandlaterIntel,forthethanniversaryissueofElectronicsmagazineinAfteronlyyearssincetheintroductionofthefirstcommercialplanartransistorin,Mooreobservedanastoundingtrend–thenumberofelectricalcomponentsperICchipwasdoublingeveryyear,reachingabouttransistorsinExtrapolatingthistrendforadecade,Moorepredictedthatchipswithcomponentswouldbeavailableby!Althoughextrapolatinganytrendbythreeordersofmagnitudecanbequiterisky,whatisnowknownasMoore’sLawprovedamazinglyaccurateSomeimportantdetailsofMoore’sremarkablepaperhavebecomelostintheloreofMoore’sLawFirst,MooredescribedthenumberofcomponentsperIC,whichincludedresistorsandcapacitors,notjusttransistorsLater,asthedigitalagereducedthepredominanceofanalogcircuitry,transistorcountbecameamoreusefulmeasureofICcomplexityFurther,Mooreclearlydefinedthemeaningofthe‘numberofcomponentsperchip’asthenumberwhichminimizedthecostpercomponentForanygivenlevelofmanufacturingtechnology,onecanalwaysaddmorecomponents–theproblembeingareductioninyieldandthusanincreaseinthecostpercomponentAsanymodernICmanufacturerknows,crammingmorecomponentsontoICsonlymakessenseiftheresultingmanufacturingyieldallowscoststhatresultinmorecommerciallydesirablechipsThis‘minimumcostpercomponent’conceptisinfacttheultimatedrivingforcebehindtheeconomicsofMoore’sLawConsideraverysimplecostmodelforchipmanufacturingasafunctionoflithographicfeaturesizeForagivenprocess,thecostofmakingachipisproportionaltotheareaofsiliconconsumeddividedbythefinalyieldofthechipsWillshrinkingthefeaturesizeson

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