CMOS Front End Process CMOS前端工藝英文版課件：gate-5

6.High-KgatedielectricsItisknownthataminimumof7?ofSiO2isrequiredtoobtainbulkproperties(i.e.bandgapofSiO2).ThisisanabsolutephysicalthicknesslimitofSiO2.willfacethelimitationofscalingat2007forhighperformancedevice.However,practicallimitationcomesearlier.IthasbeendemonstratedthatCMOSFETswith13–15?operatessatisfactorily.Althoughhighleakagecurrentdensityof1–10A/cm2atVDDaremeasured,transistorsintendedforhighperformancemicroprocessorapplicationscansustainthesecurrent.But,itisreportedthatscalingofgateoxidethinnerthan10–12?resultsinnofurthergainsindrivecurrent,whichsetsapracticallimitationforreducingtheSiO2thickness.6.1WhyHighKDielectrics?L=140nmL=70nm(IEDM1997,G.Timp)L=140nmL=70nmIncontrasttothehighperformancedevice,low-powerapplicationdevicesrequiremuchlowergateleakagecurrent(~3ordersofmagnitudelower).Deviceparametersforthe90nmCMOSTechnology Sincethehugegateleakagecurrentisduetodirecttunneling,theonlywaytoreduce thegateleakagecurrentistousephysicallythickerdielectrics. Theuseofhighkgatedielectricsallowstousephysicallythickerfilmtoreducedirect tunnelingcurrentwhilemaintainsthesameEOT.

………………..(6.1)

e.g.,kvalueofoxideis~3.9,HfO2is~25.Then,theoreticallyabout6times thickerfilmcanbeusedforthesameEOT.ProjectionfortheyearoftheintroductionofhighkforlowpowerapplicationThedesirablepropertiesforhighkdielectricsare:higherkvaluelargebandgapandlargeconductionbandoffsetthermallystableduringthethermalbudgetfortherestoffabricationprocessgoodinterfacequality(lowQf,lowDit)processcompatibilityLowdiffusivityforoxygen,boron,andothercontaminatorsGoodreliability6.2 ProcessIssuesofHighKDielectricMaterial6.2.1 kvalueandbandgap

Higherkvalueisalwaysdesirable,butthereisatrade-offbetweenkvalueandbandgap.Conductionbandoffset(EC)iscriticalforthetunnelingleakagecurrent.Bandoffsetforseveralhighkdielectricmaterials6.2.2 Thermalstability

Mostofhighkdielectricisamorphouswhenitisfirstdeposited.Butduringthesubsequentthermalprocess,thefilmbecomespoly-crystallized.Thisresultsinincreaseofthicknessnon-uniformity,increaseofleakagecurrent(leakagepaththroughthegrainboundary),anddecreaseofkvalue.Thecriticaltemperatureforstartingofcrystallizationtemperaturevariesaccordingtothefilmcomposition,filmthickness,interfacestatus.Al2O3

hasshownthebestthermalstabilityamongallhighkdielectricsreporteduptodate6.2.3 InterfaciallayerandinterfacechargesHighkdielectricshavemuchpoorerinterfacequalitycomparedtoSiO2/Siinterface:alargeamountofQfandDit.Thisresultsinseriousdegradationofcarriermobility.nMOSFET.

Normalizedtransconductanceiswithin67%oftheSiO2controlpMOSFET.

Normalizedtransconductanceiswithin51%oftheSiO2controlAl2O3filmwithoutinterfacialoxidelayershowsa

largeamountofnegativeQf.

oneofthemostseriousdrawbackofAl2O3film.Therefore,theproperthicknessofinterfacialoxidelayeriscriticallyimportantinhighkdielectricprocess.Withinterfacialoxidelayer,thetotalEOTis

………………..(6.2)Asimplifiedbanddiagramwillbe:Thehighertinterface,thebetterelectricalperformanceduetobetterinterfacequalityandlargebandgapattheinterface.However,theexistenceofinterfaciallayerlimitstheEOTscaling

acarefulprocessoptimizationisrequired.IncaseofHfO2,inter-diffusionofHfandSitakesplaceattheinterfaceduringsubsequentthermalprocessing,whichresultsintheformationofHf-silicate.TheformationofHf-silicatereducesthetotalkvalue.SinceoxygendiffusivityinHfO2isverylarge,interfacialoxidelayerkeepsgrowingduringeachthermalsteps.DuetotheoxygendiffusionthroughHfO2andinter-diffusionofHfandSi,theinterfaciallayerthicknesschangesaftereverythermalsteps.Tominimizethisproblem,surfacenitridationinanNH3ambientpriortoHfO2depositioniswidelyused.ButthisprocessagainintroduceshighQfduetothelargeamountofnitrogenattheinterface.6.2.4 MeasurementofEOTandInterfacialLayerthickness

ThemeasurementofEOTforhighkdielectricisusuallydonebyusingcurvefittingbetweensimulatedidealC-VcurveandmeasuredC-Vcurve.ThesimulationmustincludeQMeffectcorrection,otherwisethethicknessobtainedwillbeCET.Eq.(6.2)canbeusedtoevaluateinterfaciallayerthicknessaswellaskvaluebyplottingtEOTvs.thigh-kforseveraldifferenthighkdielectricthickness(physical).They-axisintersectistheinterfaciallayerthickness.Fromtheslopeofthegraph,wecancalculatethekvalueofthedielectric.IftCET,accvs.thigh-kisplotted,theintersectwithy-axiswillbeinterfaciallayerthickness+thicknessincrementduetoquantummechanicaleffectunderaccumulation.…………..(6.3)6.2.5 Comparisonsofpossiblecandidates(1) Al2O3Advantages:largebandgap(~8.7eV)comparabletoSiO2(~8.9eV)excellentthermalstability:nocrystallizationafter1100CspikingannealexcellentdiffusionblockingforoxygenDisadvantages:Relativelylowdielectricconstant(~9):forthesameEOT,theleakagecurrentislargercomparedotherdielectricswithhigherkvalue.NegativeQf:makesVtcontroldifficult(2) HfO2Advantages:higherkvalue(~25)abilitytolowerleakagecurrentbyatleast103times.Acceptableconductionbandoffset(~1.5eV)Disadvantages:Relativelylowcrystallizationtemperature(500~600C)Highoxygendiffusivity

difficulttocontrolinterfaciallayerthicknessHighpositiveQfvalue(butbettersituationthannegativeQfvalueofAl2O3)Inter-diffusionwithsiliconincreaseofEOTandreductionofkvalueZrO2SimilarpropertiestoHfO2,butthecrystallizationtemperatureisevenlower(~300C).becomeslessinterested.6.2.6 RecentProgressinHighKDielectricStudy(1) HfO2-Al2O3mixtureMixtureofAlintoHfO2

improvesthermalstability

leadstoreductionofleakagecurrent afterhightemperatureprocessing.Energybandgapisalsoincreased.Iftheratioiswellcontrolled,thenatureofnegativeQf

ofAluminumoxidecanbeusedtocancelout thepositiveQfofHfO2.(2) HfO2instrainedsilicon

Mobilitydegradationisoneofthemajorconcernsfortheintroductionofhighkdielectric.BiaxialtensilestraininSisplitstheconductionbanddegeneracy,reducingtheintervalleyphononscatteringMobilitydegradationinHfO2isbytheCoulombscatteringduetoQf.ButstrainedsiliconenhancesthemobilityintheintermediatetohighEeffwherethemobilityislimitedbyphononandsurfacescattering.Ref:VLSITech.symp.2002,K.Rim“MobilityenhancementinstrainedSinMOSFETswithHfO2gatedielectrics”,IBM.

7.MetalGateBandbendingandchargedistributioninthepolysiliconlayerresultinafinite,bias-dependentvalueofCp,causingpolysilicondepletion.ToomuchBdopingtoreduceCpcausesBpenetrationandVtinstability.Therefore,metalgateprovidesaclearadvantageinreductionofCET.Thelowresistivityisadditionaladvantage.Metalgatehaslesscompatibilityproblemwithhighkmetaloxide.7.1 MotivationsforMetalGatesasGateElectrodesPoly-Si

GateSiSDGate

OxideCPCOXCS0102030Cox(fF/μm2)Cgate(fF/μm2)1020300IdealMetalGatePolyGate(2×1020cm-3)PolyGate(5×1019cm-3)GoodthermalstabilityInterfacecontrolbetweenmetalgateandhigh-Kgatedielectrics: lessinter-diffusionorreactionduringsubsequentthermalprocessesProcesscompatibility:metaldeposition,metaletchingProperworkfunctionForSingleGate,itneedstobenearmidgapofSi

ForDualGates,bothneedstobenearbandedgesofSi(within0.2eV)7.2 ConsiderationsofChoiceofMaterial7.3 WorkFunctionofMetalGateVacuumHfZrTiTaWMoCuRuCoNiIrPtWorkfunction(eV)

MidgapMetal

DualMetals~5.25eV(p+Poly)nMOS

Ti,Ta,Zr,Hf,TaNpMOSMo,Ni,Pt,RuO2TiN,W,WN,TaNx,Mo,Ru~4.15eV(n+Poly)4.1eV5.3eV4.7eVsinglemetalgatewithmidgapworkfunctionmetalSymmetricalthresholdvoltage(VT)SimpleprocessHigherVTduetohighworkfunction.Atleast0.5Visnecessary,but0.5Vistoolargeforsub100nmdesignruletechnology.Needslightdopingorcounterdopingtoreducethresholdvoltage,butmakeshortchanneleffectworse.DualmetalgateEasyandseparatecontrolofthresholdvoltage(VT)forbothnandpMOSFETs

Theworkfunctionvaluesarerequiredtobewithin0.2eVofECandEVforbulkCMOS.Processintegrationiss