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

3.Gateoxidereliability

Dielectricstrengthofanoxide: themaximumelectricfieldstrengththatcanbeappliedbeforeitbreaksdown (inunitsofV/cm).

3.1DielectricStrength

Ramped-voltagetest: avoltagethatincreaseslinearlywithtimeis applieduntiltheoxidebreaksdown. Breakdown definesatthevoltagevalueVBDatwhicha suddenvoltagedropacrosstheoxide(orsudden jumpofcurrent)isobserved.Then,statistical distributionsofdielectricstrengthareplotted.

Ramprateisheldconstant(e.g.,ata valueof0.1or1V/sec).

Thefailureofthegateoxideisclassifiedinto threemodes.

A-modeBreakdown

B-modeBreakdown

C-modeBreakdown

A-modeBreakdown

(Earlybreakdownorinitialfailure,EBD1MV/cm) Cause:physicalshortduetopinholeorparticles,veryhighdensityof permanentelectron/holetraps,severeheavymetalcontamination. Solution:improvementofcleanlinessofenvironment,optimizationofoxidation recipeattheultra-thinoxideregime.

B-modeBreakdown

(Middlefieldbreakdownormediumfailure,1MV/cmEBD8MV/cm) Cause:weekspotofoxide,localthinning,crystaldefectandoxygen precipitationinthesubstratesilicon,electron/holetraps,metalcontamination, siliconsurfaceroughness,electricfieldconcentrationattheLOCOSedge, andsoon. Solution:dependingonthecause.

C-modeBreakdown

(Intrinsicbreakdown,8MV/cmEBD)

Defectfreeoxide Dependingonthephysicalcharacteristicoftheoxide. ToincreaseC-modebreakdownfield,oxidationprocessitselfshouldbe examined.Onepossiblemethodtoenhancetheinherentdielectricstrengthisto useanitridedoxidewhichhaslargerbondstrength.*WeibullPlot Sincethebreakdownphenomenonhasastatistical nature,itmustbeanalyzedwithstatistics. Themostwidelyusedmethodis“Weibullplot”:

Plotofln(-ln(1-F))vs.TBDorQBD

whereF=cumulativefailureprobability. Thebackgroundtheorycanbefoundin“VLSITechnology,byS.M.Sze,McGrawHill,Chapter14”. Thinneroxides =>gentleslope() =>largerTBDdistribution =>needlessdefectstobreakdownRef:T.Nigam,Proc.Int.Rel.Phys.Symp,p.62,1998 3.2Time-DependentDielectricBreakdown(TDDB)

CVST

MonitorthevariationofIGandmeasuretBD(time-to-breakdown) whileaconstantvoltagebeingapplied.

QBDiscalculatedfrom ………………..(3.1) CumulativedistributionofQBDisplottedtoevaluatetheoxidequality.

CCST

MonitorthevariationofVGandmeasuretBD(time-to-breakdown) whileaconstantcurrentbeingapplied.

QBDiscalculatedfrom ………………..(3.2) Thestresscurrentdensityisusually50~100mA/cm2

whenthegateoxidethicknessisintherangeof50~100Å.

Thesloperepresentstrappingrate,andusuallythethinnertheoxideis, thesmalleristhetrappingrate.

Whentheoxideisthickerthan50Å,electron trappingisusuallyseen. Whentheoxidethicknessissmallerthan50 Å,weekholetrappingisseen.

Fortheoxidethinnerthan40Å,neitherhole norelectrontrappingisobserved. Thisnotrappingcharacteristicisattributedto theballistictransportofelectronintheoxide conductionbandafterF-Ntunneling.Time[sec]VG[V]electrontrappingholetrappingnotrappingElectronTrappingwillbedominant.NoTrappingwillbedominant.HoleTrappingwillbedominant.3.3Quasi-breakdown(QB)3.3.1 IntroductionofQuasi-breakdown

FirstreportedandanalyzedinthepaperofIEDM1994(LeeandCho,p.605-608) Itisidentifiedbytwophenomena:

noiseingatesignalduringhighfieldstress

suddenincreaseofgateleakagecurrent Onlyfoundintheoxidethinnerthan50Å. OncetheoxideexperiencesQB,theleakagecurrentlevelistoohightobe acceptedfordeviceapplicationeventhoughtheoxideisnotindestructive breakdownstate. newcriteriafortheoxidereliabilityforthinoxide Itisalsocalled“soft-breakdown(IEEEEDVol.43,pp.1499,1996)”3.3.2NatureofQuasi-breakdown

Whenthestressfieldislow,QBiseasiertobeobserved.SILCshowsexactareadependence–thatmeans,J(A/cm2)isconstantregardlessofgatearea.However,QBcurrentisalmostindependentongatearea–thatmeanscurrentdensityJisnotconstant. ThisimpliesthatQBisalocalizedphenomenon.

Muchhigherleakagecurrentatlowandmiddlefield,butsimilartoF-Ncurrentatveryhighfield. ThisiswhyQBcanbeeasilyoverlookedduringhighfieldstressingtestwhenthebreakdownisdetectedbythesuddenchangeofgatesignal(voltageorcurrent).Whenthegateareaisgettingsmaller,thegatesignalnoise(fluctuation)becomeslarger.Whenthegateareaisverysmall,thenoisesignalshowstwo-orthree-levelfluctuation.WhenQBhappens,suddenincreaseofholetunnelingcurrentfromsubstrateisalwaysobserved. supportsinterfacedamagemodelMOSdeviceisstillfunctioningafterQB,butwithalargegateleakagecurrent.

QBcanberecoveredbytheoppositepolarityelectricalstress.ButtherecoveredoxideisverymuchvulnerabletoQBagainbyadditionalstress.QBoccurswheninterfacetrapdensityreachesacertaincriticalamount. supportsinterfacedamagemodelPrecursorforQBandBDisthesame. supportspercolationmodelTime(sec)(a)(b)(c)I-Vcharacteristicsatvariousstagesofconstantcurrentstressing.AfterQBisattained,thesamepolaritystressingiscontinuedbeforeapplicationofareversebiasstress.GateleakagecurrentafterapplicationofreversebiasshowsreductiontoSILClevel.(Tox=45Å,W/L=10/0.5m,p-MOSFET)3.3.3ModelofQuasi-breakdownInterfacedamagemodel

Whendtisshorterthanelectronmeanfreepath,theelectronstraveltheoxideconductionbandballisticallyandthenreleasemostoftheirenergyattheanodeinterface.

Continuousaccumulationoftheenergyattheanodeinterfacecouldleadtothegenerationofaphysicallydamagedregion.WhenQBhappens,thetotalleakagecurrentisasuperpositionoftheF-NtunnelingthroughtheundamagedregionandthedirecttunnelingthroughPDRregion.ThismodelissupportedbythefactorthatQBoccurswheninterfacetrapdensityreachesacertaincriticalamount(constantDitattheonsetofQB),andcarrierseparationresult. Charge-to-QB(QQB)andinterfacetrapdensities(Nit,QB)at