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Budoküman@superonline.comveyildirimalper@yahoo.comadreslerindenulaşabilirsiniz.Websitesi:(chips)whicharethenassembledintopackageswhichcanbehandledinthefinalapplications.Fullfunctionalelectricaltestisperformedatbothwaferandpackageleveltoensureoutgoingquality.Thewafergrowingprocessissimilartocrystalgrowingexperimentsthatmostofusdidatschool.Aseedcrystalofsiliconisimmersedinabathofmoltensilicon.Itisslowlypulledout,andbecausecrystalgrowthoccursuniformlyinalldirections,thecrosssectioniscircularand,asitispulled,itformsacylindricalingotofpuresilicon.Theingotpullingprocesslastsforalmost24hoursandproducesacylinderofdiameterlargerthanisdesired.Theingotisgrounddowntotherequireddiameter,andthenisslicedintoindividualwafers.Atthisstage,thewafershavearoughtextureandneedtobefinelypolishedtomeetthesurfaceflatnessandthicknessspecifications.Togiveyouanideaofhowcriticaltheflatnessspecificationis,considerthefollowinganalogy.IfanairplaneweretoflyacrossthePacificOceanandholditsaltitudetowithin30feetoftheoceansurface,thenthiswouldbeequivalenttotheflatnessspecificationofawafer.Thewholesemiconductormanufacturingprocesshingesontheuseofaphotographicprocesstocreatethefinefeaturedpatternsoftheintegratedcircuit.Eachlayerofthechipisdefinedbyaspecificmask,andthereare16to24masklayersineachIC.Themaskissomewhatlikeaphotographicnegative,whichismadebypatterningafilmofchromiumonapurequartzglassplate.Thesefinishedplatesarecalledreticles.Reticlesaremanufacturedbyverysophisticatedandexpensivepatterngenerationequipment,whichisdrivenfromthechipdesigndatabase.Thepatternsareformedonthechromiumplatedquartzplatedbyremovingthechromiumwitheitherlaserorelectron-beamdriventools.Astransistorfeaturesarereduced,morecomponentsareplacedoneachchip,requiringlarger,morecomplexpatternstobedrawn.Thisalltakeslongertowritethemasks.Eachnewdesignordieshrinkrequiresnewmasktooling,sothissegmentisdrivenbynewproductacceleration.CrystallineorcontaminatedefectswillkilltheoperationofanIC,soitisimperativethatthesiliconisultra-pure.Inordertocreatethebestpossiblequalityofsilicon,apurelayerofsiliconisgrownontherawwaferviaanepitaxialgrowthprocess.Thisisknownastheepi-layer.Thislayerisverythin-approximately3percentorlessofthewaferthickness.Asdevicecomplexitygrows,theneedforepi-wafersincreases.Photographyisthebestanalogytodescribethephotolithographyprocess.Thestepperislikeaphotographicenlargerwherealightsourceprojectsanimagethroughalenssystemontophotographicpaper.Inthesemiconductorcase,thelightbulbusedtobeamercuryarclamp,butforDUVlithographyhasbeenreplacedbyexcimerlaserlightsources.Theimagecomesfromourreticle,andthisisthenprojectedthroughaverycomplexquartzglasslenssystemontothewaferwhichhasbeencoated(spun-on)withanultra-thinlayerofphotoresistmaterial.Thereticleimageiseitherprinted1:1insizeorreducedby4:1or10:1,dependingontheparticularstepper.Clearly,fora4:1reduction,thefeaturesonthereticleneedonlybeone-fourthofthoserequiredonthewafer.Theexposuretimeisdependantonmanyvariableincludingthesensitivityoftheresist,lensaperture,etc.Themachineusedtodoallofthisiscalledastepperbecauseitliterallydoesonedieorafewdieatatime,thenstepstothenextdieorsetofdieuntilithasexposedtheentirewafer.Nowwewillstarttoconstructthetransistorsonthewafer.Thefirststepistoheattheepi-wafertogrowanoxidelayer-silicondioxide.Atthispoint,wegotothelithographyprocess,spinonphotoresist,bakeittomakeitharder,exposethereticlestep-bystepoverthewafer,thendeveloptheresisttocreatethepatternonthewafer.Thedeposition,diffusionorimplantprocessesthatfollowwouldactuallydestroythephotoresist,sothenextstepistotransferthepatternfromthephotoresisttothetougheroxidelayer.....Thewaferwithpatternedphotoresististhenputintoanoxideetchprocesstoremovetheoxidewherethereisnopattern.Thishastheeffectoftransferringthepatterntotheoxide,creatingbarriersofoxidewherewedonotwantsubsequentprocessestoimpactthesiliconbelow.Theetchmaybeeitheraclassicwetchemistryoradryetchwhichusesgasexcitedbyaradiofrequencygeneratortoandexcitedplasmas