CMOS Front End Process CMOS前端工藝英文版課件：gate-4

5. Boronpenetrationandnitridedoxiden+-polysiliconhasbeenusedasagateelectrodeforbothn-andp-MOSFETuntilthedesignruleisaround0.3mforthesimplicityofprocess.However,n+-polysilicongateinpMOSrequirescounterdopingofthechannelregion(boronimplantationton-well)toobtainthereasonablethresholdvoltage,whichresultsinburiedchanneldeviceoperationandpoorpunch-throughandsevereshortchanneleffect.

Therefore,indeepsubmicronregime,p+-polysilicongateisrequiredasagateelectrodeforpMOSFET.surfacechannelpMOSFET,symmetricalton-MOSFET.5.1 BoronPenetrationinp+-polyPMOSFETDualpolyapproachcansolvethedeviceperformanceissues,butgenerateseveralproblemsintermsofprocessaswellastheincreasedprocesscomplexity.Highsheetresistanceofp+-poly(~3xhigherthanthatofann+polyofthesamethickness)duetothelowmobilityofholes.PoorVtcontrolduetothepenetrationofboronthroughthethingateoxideBoronpenetrationeffectstartstobeobservedwhenthegateoxideisthinnerthan120?andthethermalbudgetafterboronimplantationishigherthan900Cfor30min.Reducingthethermalbudgetwillhelptheboronpenetrationproblem,butcausesinsufficientactivationandredistributionofboronwithinpolysiliconwhichresultsinseriouspoly-depletionproblem.UseofBF2implantationforshallowp+source/drainformationmakestheboronpenetrationproblemevenworse.Fluorinehasalowsolubilityanddiffusesveryfastinbothsiliconandpolysiliconandsegregatesstronglyintooxide.ThesegregatedFintheoxideintroducesadditionaldefectswhichenhancesborondiffusionintheoxide.Thisisoneofthereasonswhyultra-lowenergyB11implantationispreferredoverBF2ionimplantationforrecentultra-shallowjunctiontechnique.Boronpenetrationbecomes

moreseriouswhenthegateoxidebecomesthinner.5.2 NitrogenIncorporationintoPureOxideIncorporatingnitrogenintopureSiO2filmisoneofthemostimportantandpracticalsolutionsfortheboronpenetrationproblem.Thenitrogenpile-upintheoxidereducesborondiffusivitysignificantly.Otheradvantagesofnitrogenincorporationintosilicondioxide:strongerresistancetointerface-stategeneration(Dit)underhot-carrierstressandimprovedTDDBThenitrogenatomsterminatedanglingSibondsattheSi-SiO2interface,and/ordanglingbondsinthebulkassociatedwithSivacancies.TheSi-NbondsarehardertobreakthanSi-Hbonds.improvedreliabilityabarriertovariouscontaminantsaswellasboronincreaseddielectricconstantforveryhighlevelnitrogenincorporationHowever,thehighlevelincorporationofnitrogenatoxide/substrateinterfaceproducesahighfixedoxidecharge

(Qf)densitywhichresultsinVtshiftandmobilitydegradation.Thereareseveralwaystoincorporatenitrogenintooxide:thermalnitridationofpureoxide,oxynitridegrowth,nitrogendiffusionfrompolysilicon,plasmanitridationofpureoxide,andsoon.NitridationinanNH3ambient

TheeasiestwaytoformnitridedoxideistoannealapuresilicondioxideinaNH3ambientathightemperatures(900-1200C)AreactionbetweenSiO2andNH3occursatthetopsurfaceoftheoxidefilm,butNH3alsorapidlydiffusetotheSi-SiO2interface,whereitreactswiththesilicon.ProblemofNH3nitridation:hydrogen-containingspecies(i.e.,NHx,-H,and-OH)areformedbythedissociationofNH3duringthenitridationstep.WhensuchspeciesareincorporatedintotheSiO2film,theyareknowntocauseelectrontraps.Tosolvethisproblem,are-oxidationstepisusuallyadded.There-oxidationprocessconsistsofexposingthenitridedoxidestoahightemperatureoxidizingambient,whichreducesthehydrogenconcentration,resultinginreductionofdensityofelectrontraps.Aslongasthenitrogen-richlayerattheinterfaceremainsintact,oxygenispreventedfromreachingthesiliconandgrowingmoreSiO2duringthere-oxidizingcycle.nothicknesschangeIfexcessivere-oxidationtakesplace,thedenseinterfacelayermaydissipate,permittingthermaloxidationtoresume.thicknessincreasePureoxideNH3-nitridedoxideN2O-nitridedoxideOxidegrowthinanitrousoxide(N2O)ornitricoxide(NO)ambient

togrowagatedielectricbydirectlyreactingN2OorNOatthesiliconsurfacethefilmisoftencalled“oxynitride”significantlylesshydrogenincorporationthanNH3nitridationbecauseofthehydrogen-freenatureoftheprocessing.ThefilmgrowthinaN2Oambientshowssomewhatself-limitingnature(butnotrealself-limiting)becauseofthebarrierpropertyofincorporatednitrogentodiffusionofoxidantsthusimpedinggrowth.ProcessissuesforN2Ooxynitridation

Limitednitrogenconcentration: Inhightemperature,N2OisdecomposedintoO,NO,andN.SincethenitridationisdonebyNO,notbyN2O,thelevelofnitridationisdependingontheamountofNOproducedbydecompositionofN2O.Inusualprocesscondition,theestimatedNOpercentageat960Cisonly~5%. Asresults,ThepeaknitrogenconcentrationwhichcanbeachievedbyN2Oisonlyaround0.5%whilenitridationusingNH3is2~10%.Hightemperatureprocessing: thedecompositionofN2OrequireshighertemperatureforthesamethicknessofoxidecomparedtodryoxidationSelf-limitingnature: ThefilmgrowthinaN2Oambientshowssomewhatself-limitingnature(butnotrealself-limiting)becauseofthebarrierpropertyofincorporatednitrogentodiffusionofoxidantsthusimpedinggrowthanotherreasonforevenhighertemperatureDifficultprocesscontrol: Theoxidationrateisgreatlydependingonthechamberdesign,pressure,flowrate,etc,becausethedominantfactorwhichdecidesthegrowthrateistheincorporatednitrogenconcentrationattheinterfaceduringtheprocessandthelevelofnitrogenincorporationisalsodependingonthedecompositionrateofN2O.ThisiswhyintheearlystageofN2Ooxynitridationstudy,everyresearchgrouppresenteddifferentgrowthratedata.ToovercometheselimitationsofN2Ooxynitridation,severalalternativesweresuggested.annealingofpre-grownpureoxideinaN2Oambient

cansolvehighthermalbudget,thicknesslimitationstilllownitrogenconcentrationdirectuseofNOgasforoxynitridationofsilicon

lowtemperatureprocessbecauseofnoneedofhightemperaturefordecompositionhighNOconcentrationcanachieveover1%nitrogenincorporationatsiliconinterfacestillthicknessself-limitingproblemSomesafetyissueannealingofpre-grownpureoxide inaNOambient

nothicknessself-limitingproblemhighlevelincorporationofnitrogenHighlevelincorporationofnitrogenattheinterfacewillresultinboronaccumulationintheoxide.degradereliabilityoftheoxideonpMOSFET

Highlevelnitridationoftopsurfaceofoxide:perfectconditionforpreventingboronpenetrationproblemHighlevelnitridationofbottominterfaceofoxide/substratecauses(i)reliabilitydegradationduetoboronaccumulationintheoxide,(ii)mobilitydegradationduetohighfixedoxidecharge,and(iii)higherinterfacetrapdensityButlowlevelnitridation(<1%)canimproveinterfacehardnessagainsthotcarrierinjectionTherefore,thebestconditionishighlevelnitridatio