Semiconductor DopingWord下载.docx
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Semiconductor DopingWord下载.docx
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AcceptorDopant
GroupIIIA
(P-Type)
Semiconductor
GroupIVA
DonorDopant
GroupVA
(N-Type)
Element
AtomicNumber
Boron(B)
5
Carbon
6
Nitrogen
7
Aluminum
13
Silicon(Si)
14
Phosphorus(P)
15
Gallium
31
Germanium
32
Arsenic(As)
33
Indium
49
Tin
50
Antimony
51
1.3Twotypeofdoping
1.3.1Diffusion
Addingadopingmaterialcanbedoneviadiffusion.Thedopingmaterialfillsemptyspaceswithinthecrystallattice,whileitcomesbetweenthesiliconcompounds.Insomecases,itevenswitchspositionswithsiliconcompounds.Diffusioncanbeachievedviagases(nitrogenandargon)orsolidmaterials,orotherdopingsources.
AsitisshowninFigure1.2(a),amaskinglayer(e.g.PR)isusedtoblockthewafersurfaceexceptwherethedopantsaredesired.Thewaferisplacedinahigh-temperaturefurnace(~1000°
C)wheretheatmospherecontainsthedesiredimpurityingaseousform.Throughtheprocessofdiffusion,impurityatoms,whichareinhighconcentrationintheatmosphere,willdiffuseintothesubstrate,wheretheyhavealowconcentration(initiallyzero).Aftersometime(~0.5–10hours)theimpurityatomsareuniformlydistributedintotheexposedwafersurfaceatashallowdepth(0.5-5mm)ataconcentrationthatcanbereliablycontrolled(~1012-1019cm-3).
1.3.2Ionimplantation
Anotherdopingapproachisionimplantation,whichisveryusefultoalterawaferthathasalreadybeendoped,becauseionimplantationisperformedatambienttemperature.Thispreventsexistingdopantstodiffuse.Itispossibletomaskcertainareasofthewafertotreatonlyveryspecificareas.Thereismuchmoretosayaboutionimplantation,suchasthedepthofpenetration,activationofthedopingadditiveathightemperatures,channeling,diffusingoxidelayers,etc.Thisprocesscanberepeatedmultipletimesduringtheprocess.
Implantationisfunctionallysimilartodiffusion,butheretheatomsare"
shot"
intothewaferathighvelocity(acrossaverystrongelectricfield)andtheyembedthemselvesintothewafersurface(Figure1.2(b)).Ashort(~10min.)annealingstepatelevatedtemperatures(~800°
C)isusedtofitthenewatomsintothesubstratecrystallattice.Implantationismoreuniformacrossthewaferthandiffusionandallowsforveryprecisecontrolofwheretheimpuritieswillbe.Inaddition,itspeakconcentrationcanbebeneaththewafersurface,anditdoesnotrequirealongperiodoftimeathightemperature.However,animplantedjunctionmustremainnearthesurfaceofthewafer(~0.1-2mm)andcannotgoasdeepasadiffusedjunction.Theimpurityconcentrationprofile(concentrationvs.depth)isdifferentfordiffusionandionimplantation,asshowsinFigure1.2.Howeverbotharewellknownandpredictable.
Ionimplantationisveryusefultoalterawaferthathasalreadybeendoped,becauseionimplantationisperformedatambienttemperature.Thispreventsexistingdopantstodiffuse.Itispossibletomaskcertainareasofthewafertotreatonlyveryspecificareas.
Figure1.2Twotypeofdoping:
(a)diffusion;
(b)ionimplanting
1.3DopingprocessofSemiconductorfabricationtechnology
Dopingcanbedoneatthetimethemonocrystalisgrownordonewiththefinalwaferandduringthephotolithographyprocesslateron.Thisallowsforthealterationoftheelectricalpropertiesofspecificareasandspecificlayersinsteadoftheentirecrystalstructure(partialdoping).Mostly,dopingprocessisdoneafterphotolithography.
BasicProcesses:
(1)WaferGrowth:
Creationofsingle-crystalsubstratematerial;
(2)Photolithography:
Definitionofregions(throughamask)onwaferforselectiveprocessing;
(3)Doping:
Additionofdesiredimpuritiesnearthesurfaceofthewafer
(4)Oxidation:
Thermalgrowthofnaturaldielectriclayer(e.g.SiO2)
(5)Deposition:
Additionoflayersofaspecificmaterial(e.g.Si3N4)onthewafersurface
(6)Etching:
Removalofmaterialfromthewafersurface
(7)EpitaxialGrowth:
Reorderingofdepositedmaterialtoformsingle-crystalmaterial
Intheearlytimeofsemiconductordevicemanufacture,diffusionisthemaintoolofdoping.However,asthedecreasingofcharacteristicsizeofsemiconductordevices,ionimplantingisreplacingdiffusinginthemostarea.Figure1.3isthecross-sectiondiagramofaCOMSinverterandthedopingareasareindicated(norpdopingtype,+or–representconcentration).Table1.2showstheparticularprocess(diffusionorionimplantation)ordopantcorrespondstothedopingareainFigure1.3(A~O).AsitisillustratedinTable1.2,ionimplantationmakesupthemajorityofthedopingprocessescomparedtodiffusion.[1]
Figure1.3CMOSStructurewithDopedRegions
Table1.2CommonDopantProcessesinCMOSFabrication
2.Diffusion
2.1CharacteristicofDiffusion
Diffusionisfirstusedtodopesemiconductor,andisgraduallyreplacedbyionimplantationafteritsintroductioninmid-1970s.Nowadays,itisstillwidelyforthedrive-inofdopantandR&
Donultrashallowjunctionformation.Therearesomecharacteristicsofdiffusion:
(1)Isotropicprocess;
(2)Can’tindependentlycontroldopantprofileanddopantconcentration;
(3)Performedinhightemperaturefurnace;
(4)Usingsilicondioxidemask;
(5)Causinglittledamagetothesubstrate.
2.2Diffusionprocess
Mostofthesediffusionprocessesoccurintwosteps:
thepredepositionandthedrive-indiffusion.
2.2.1Predeposition
Inthepredepositionstep,ahighconcentrationofdopantatomsareintroducedatthesiliconsurfacebyavaporthatcontainsthedopantatatemperatureofabout1000°
C(Figure2.1).Atthetemperatureofl000℃,siliconatomsmoveoutoftheirlatticesitescreatingahighdensityofvacanciesandbreakingthebondwiththeneighboringatoms.Theconcentractionofthedopantsnearthesurfaceisaconstant.
Weshouldnotethatpredepositionbydiffusioncanalsobereplacedbyashallowimplantationstep
Figure2.1Predepositionprocessindiffusion
2.2.2Drive-in
Thesecondstepisdriveinprocess,usedtodrivetheimpuritiesdeeperintothesurfacewithoutaddinganymoreimpurities(Figure2.2).Thetotalamountofimpuritiesisaconstant.
Figure2.2thedrive-inprocessindiffusion
Diffusiondepthiscontrolledbythetimeandtemperatureofthedrive-inprocess.Byprecisecontrolofthetimeandtemperature(towithin0.25°
C),accuratejunctiondepthsoffractionofamicroncanbeobtained.
2.2.3Theformationofp-njunction
Thecontrolleddiffusionofdopantsintosilicontoalterthetypeandlevelofconductivityofsemiconductormaterialsisthefoundationofformingap-njunctionandformationofdevicesduringwaferfabrication.
Whendopantofonetype(porn)isdiffusedintoawaferoriginallydopedwithdopantoftheoppositetype,ajunctionwillbeformed,asshowninfigure2.3.Thedistanceofthejunctionfromthesurfaceisknownasthejunctiondepth,xj.Atxjtheconcentrationofthediffuseddopant=theconcentrationofthesubstratedopant(assumedtohaveauniformconcentration),asitisshowninFigure2.3(b).
Itisusedtoformbases,emitters,andresistorsinbipolardevices,aswellasdrainsandsourcesinMOSdevices.Itisalsousedtodopepolysiliconlayers.ItisnotapplicabletoSiCprocessingandotherwidebandgapmaterial.
Figure2.3theformationofp-njunction
2.3Microscopicpointofviewofdopantdiffusion
Thereexistseveraltypesofdiffusionmechanisms.Wetakethediffusionofdopantsinsiliconasanexample.Figure2.4(a)showsaperfectlatticeinsiliconcrystal.Afirstpossiblemechanismconsistsofadopantdirectlydiffusingintoalatticevacancy(Figure2.4(b)).Animpuritycanalsodiffuseintoaninterstitialsiteinthelatticeandcanmovefromtheretoanotherinterstitialsite,asshowninFigure2.4(d).Sometimesasiliconatomcanbeknockedintoaninterstitialsite,leavingavacancyinthelatticewhereadiffusingdopantatomcanfit,whichisnamedasinterstitialdiffusion,asshowninFigure2.4(c).Itisonlyinthecasesthatanimpurityoccupiesavacatedlatticesitethatn-typeorp-typedopingcanoccur.Thepresenceofsuchvacanciesinthelatticecanbeduetodefectsortoheatwhichincreasesatomicvibrationsthusgivingenoughenergytothesiliconatomstomoveoutoftheirequilibriumpositionsintointerstitialsites.Sothediffusionprocessneedshightemperaturesinceweneedtochangetheelectricpropertiesofsemiconductorsthroughdoping.
Figure2.4diffusionmodelsinsilicon
Therearemanydifferenttypesofimpuritiesthatcanbeusedfordiffusion,themost
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