CMOS Front End Process CMOS前端工艺英文版课件：oxidation 2

3.1

OxideDensity

Ingeneral,highergrowthratelowerdensity.

(Dryoxidationinlowertemperatureproduceshigherdensity.)

Lowdensityofoxide:

lowerrefractiveindex higheretchrateindiluteHFsolution

But,thermaloxidehasamuchhigherdensity,comparedallkindsofCVDoxides.3.OxideProperties3.2OxideStress

Thermaloxideonsiliconisinastateofcompressiononthesurface. (intherangeof~109dyne/cm3)

Viscousflowofthermaloxideoccursataround960C. Abovethistemperature,stressreliefeffectduringoxidationisgreat.

Thisisveryimportantforlocaloxidationofsiliconsinceexceedinglyhighstress levelscanoccurattheoxidationwindowedge. 3.3DopantRedistribution

Asoxidationproceeds,theinterfaceadvancesintothesilicon,anddopantswill redistributeattheinterfaceuntilitschemicalpotentialisthesameoneachsideofthe interface. Thisredistributionmayresultinanabruptchangeinimpurityconcentration acrosstheinterface.

segregationcoefficientmis;

m= segregationintotheoxidewithslowdiffusion(boron)segregationintotheoxidewithfastdiffusion(boroninH2ambient)segregationintothesiliconwithslowdiffusion(P)segregationintothesiliconwithfastdiffusion(Ga) Fortheboron,

generally0.1<m<1for850~1200Chighertemperature:higherm

mofdryoxidation>mofwetoxidationbutmisverysensitivetotheamountof moistureindryox. 20ppmmoistureindryoxidation exhibitsmsimilartothatofwet oxidation. P,As,Sb;mis10orabove.(dopantpile-up)

Ga;dopantisdepletedatinterfaceevenm>1.

becauseofhighdiffusivityofGainoxide.

Boronsegregationisthemostimportantbecauseitreducesfieldthresholdvoltageand evencancausefieldinversioninextremecase. Stackingfaultisa2-Dstructuraldefectinthesiliconlattice.

Insilicon,italwayslieson(111)planes. Atthesurfaceof(100)wafer,SFisseenasaline.

Defectdecorationetchingtechnologycanbeused toobservetheSFonthewafer. (i.e.Secco-etching,Young-etching,…..)3.4Oxidation-inducedStackingFaults(OSF)

BasicmechanismofOSF

ThelatticesiliconatomsneartheSi/SiO2interfaceoxidizetoformSiO2,butleavea substantialnumberofsiliconatomsasinterstitials(SiI)neartheinterface

SiIdiffuseintobulk

coalescenceofexcesssiliconatomsinthesiliconlatticeonnucleationsites growthofSF

IftheSFcapturesimpurities,suchasheavymetals,theSFiselectricallyactive.

thesourceoflargejunctionleakage

PropertiesofOSF

DensityofOSF:byimperfectionsinthestartingmaterial

SizeofOSF:bytheoxidationcondition

ReductionTechniqueofOSF

Tousegoodstartingmaterial reductionofthedensityofOSFToannealinaninertgasambientathightemperature NogenerationofSiI

atinterface.EquilibriumconcentrationofSiIinthebulkishighathightemperature.Then,thedislocationactsasasourceofSiI.Thus,thelengthofOSFshrinks.ToaddChlorineduringoxidation reactionofClwithsiliconinterstitialsattheinterface(formingsiliconchlorides)reducesthesupersaturationofSiIinsiliconlatticeduringoxidationRef.:

A.M.Lin,R.W.Dutton,D.A.Antoniadis,andW.A.Tiller,“Thegrowthofoxidation stackingfaultsandthepointdefectgenerationatSi-SiO2interfaceduringthermal oxidationofsilicon”,JournaloftheElectrochemicalSociety,Vol.128,No.5,pp. 1121-1130,May1981.3.5OxideCharges

Becauseinterface-traplevelsaredistributedacrossthesiliconbandgap,

Dit(interface-trapdensity)isdefinedas (#ofcharges/)

Orderof1010/ .(1)

Interface-trappedchargeQit

originatedfromstructuraldefectrelatedtotheoxidationprocess,metallicimpurities,orbondbreakingprocesses.

locatedattheSi/SiO2interfacewithenergystatesinthesiliconforbiddenbandgapandwhichcanexchangechargeswithsiliconinashorttime.

C-VandG-VaretypicallyusedtodeterminetheQit.

Alowtemperaturehydrogenanneal(post-metallizationanneal)iseffectivein reducingQit. usuallyinforminggas(H2-N2)inthetemperaturerangeof350~500C (2)FixedoxidechargeQf

Originatedfromnonstoichiometricregion(strainedregion)ofSiO2neartheSi/SiO2interface(SiOxwhere1<x<2)Immobileunderanappliedelectricfield.(cannotbechargedordischarged)Forelectricalmeasurement,QfisconsideredasachargesheetattheSi/SiO2interface.(typicallymeasuredbyVFBinC-Vcurve.)Rangedfrom1010/cm2~1012/cm2dependingonoxidationandannealingconditionsaswellassiliconwaferorientation.orientationdependence:Qf(100)<Qf(111),relatingtothenumberofavailablebondsperunitareaofsiliconsurface.thelasthightemperaturethermaltreatmentdeterminesQf

(Usually,highertemperatureproduceslowervalueofQf)fastcoolingfromhightemperatureoxidation rapidcoolingpreventsoxidationatlowtemperaturewhenlowtemperatureoxidationisinevitable(e.g.verythingateoxideformation),in-situsubsequenthightemperatureannealinginaninertgasambientistobeadded.(3)OxidetrappedchargeQot

positively(hole)ornegatively(electron)trappedchargeinthebulkoxideassociatedwithdefectsintheSiO2,andmayresultfromX-rayradiationorhotelectroninjection.canbeannealedout(notperfectly)rangefrom109~1013/cm2

TrappinganddetrappingprocessescausehysteresisinC-Vcurve.MobileionicchargeQM

attributedtoalkaliionssuchassodium,potassium,….andheavymetalsrangefrom1010~1012/cm2

relatedtotheprocessingenvironment,i.e.cleanlinessoftheoxidationprocessincludingsuchcomponentsoftheprocessas:thefurnacetube,processingchemicals,oxidizingambient,gateelectrodematerial,waferhanding,andsoon.AdditionofchlorineduringoxidationneutralizesNaions.Measuredbyatemperature-biasstresstest.Formostrecentdeviceapplication,allsumoftheseoxidechargesshouldbekepttothelow1010cm-2regime.4.OxidationTechnique

Recipechosendependsonthethicknessandoxidepropertiesrequired.

Thickoxide:usuallywetoxidation,

thinoxide:dryoxidationorverylowtemperaturewetoxidation

Dryoxidation Rampingup&downrateshould beslowenoughtoensureno thermalshocktothewaferthat cancausewarpageorslip dislocationonthewafer.

LowflowrateO2duringwafer loadingisonlyforhorizontaltube withoutloadlock.TimeTemp.600~700°CwaferloadingrampingupstabilizationoxidationrampingdownunloadingN2/lowO2N2N2O2/HClorTCA,TCE,DCE

N2N24.1OxidationRecipe

Wetoxidation

Wetoxidationwith in-situannealingTimeTemp.600~700°CwaferloadingrampingupstabilizationrampingdownunloadingN2/lowO2N2N2N2N2dryoxidationdryoxidationWetoxidationO2/HClH2+O2orsteamO2/HClannealingN2rampingupN2TimeTemp.600~700°CwaferloadingrampingupstabilizationrampingdownunloadingN2/lowO2N2N2O2/HClN2N2dryoxidationdryoxidationWetoxidationO2/HClH2+O2orsteam4.2Equipment

HorizontalTubeFurnace

havebeenusedsince1960sforoxidation,

diffusion,

annealing,andvarious

depositionprocesses.

Firstdevelopedfordiffusionprocess,and

tothisdayareoftencalledsimply

diffusionfurnaces.

Theprocessingwafersareplacedona

waferboatandpositionedintheflatzone.

Temperaturedeviationintheflatzone

0.5C

(1)Reactionchamber

Highqualityquartzismostwidelyusedbecauseofitsinherentstabilityathightemperatureanditsbasiccleanliness.

Drawbacks:highlyfragile,sagattemperaturesabove1200C,highdiffusivityofsodiumions

Sophisticatedgateoxidationfurnacesusedoublequartztubesystem,toreducesodiumioncontaminationandtominimizenativeoxidegrowthduringthewaferloadingstage.

Forhightemperatureandlongtimeprocess,SiCtubeisusedbecausesiliconcarbideisstructurallystrongerandhasgoodmechanicalresistancetorepeatedtemperaturecycling.Nosagginginhightemperature.Buthighcostandheavy.

(2)OxidationSource

DryOxidationTokeepthemoisturelevellessthan1ppmindryoxygen,precombustor,coldtrap,andgaspurifiersareusedinthedryoxygenline.WetOxidationInearlydays,bubblerwaswidelyused.Nowadaysthepyrogenict