Laser Annealing of Power DevicesWord文档下载推荐.docx
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Bernt,Helmut;
Hanssen,Henning;
Oesterlin,Peter;
Schmidt,Henning
INTRODUCTION
IGBTsareusedformostpowerswitchingapplicationsinthefieldsofe.g.automotive,industryandconsumerelectronics.ThemainadvantageofIGBTsistheexcellentconductionbehaviourcombinedwithhighrobustness,soIGBTsarewidelyusedforallkindsofmotorcontrolpurposes.DiscreteIGBTsareverticaltransistors,meansthecurrentflowisdirectedfromthefrontsideofthedevicetothebacksideIGBTshavebeencontinuouslyimprovedovertheyearswithspecialfocusonreductionofswitchingandconductionlosses.Therefore,oneofthemainactivitiesisthereductionofthesubstratethicknessdowntoablockingvoltagedependentminimalvaluewhichensurestheOff-statewithoutvoltagebreakdown.Fora600VIGBTthedevicephysicallimitforthesubstratethicknesswillbereachedwiththenextgenerationsoftrenchfieldstopIGBTshavingasubstratethicknessbelow50μm
.TherealizationofultrathinIGBTswithlowconductionlossesispossibleonlybyimplementingafieldstoplayerfromthebacksideofthewafer[2][3]asshowninfigure1.
Fig.1:
Schematiccrosssectionofa600VtrenchfieldstopIGBThavingaPhosphorousfieldstopandBoronemitterimplantedfromthewaferbackside
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SincehomogeneouslydopedFZsubstratesarepreferredtobeusedinsteadofmoreexpensiveepitaxialwafersthefieldstopandemitterlayershavetobecreatedbyionimplantationusingPhosphorousandBoron,respectivelywithlowthermalbudgetactivation.ThechallengeinfabricationofultrathinfieldstopIGBTsisthehandlingofsubstratesinthethicknessrangeof50μm
afterbacksidegrindingandthelimitedthermalbudgetforbacksideprocessinginthecasethefrontsidehasbeenfinalizedbefore.Here,therequirementforthelimitedthermalbudgetisduetothefactthatthefrontsidemetallizationoftheIGBTdoesnotallowhightemperaturetreatmentfortheactivationoftheimplantedspecies.
Therefore,Yb:
YAGlaserannealingatawavelengthof515nmwithmaximumenergydensityupto4J/cm
2
andpulsedurationsintherangeof350–600nsechasbeeninvestigated.SufficientdopantactivationoftheimplantedfieldstoplayerdowntoaSi-depthof1μm
isrequiredwhichisinaccordancewiththepenetrationdepthsofalaserfor515nmwavelength.Duetothisexcellentmatchingbetweenthepenetrationdepthsandthedepthsofthefieldstoplayeralaserwavelengthof515nmispreferentialcomparedtoshorterwavelengths.Sincetheminimumactivateddoserequiredforafieldstoplayerisabout3×
10
12
cm
−2
thedepthsofthefieldstoplayercanbeinthe1μm
rangeorevenbelowaslongthedopantconcentrationisintherangeof10
17
−3
.Theimplantationofboth,fieldstoplayerandemitterwithsubsequentlaserannealingispreferredsinceinthiscasehomogeneouslydopedfloatzonesubstratescanbeusedinsteadofsubstrateswithepitaxialdriftzonelayers.
Activationofdopantswithfrequencydoubledpulsedsolidstatelasershasbeenproposedandpursuedsinceseveralyears[4][5].EspeciallyKudoandWakabayashi[5]investigateddopantactivationasafunctionoflaserpulseparametersindetail.Theywereabletoshowgoodactivationrate.However,theirdatawereforsingleimplantsonly,eitherBoronimplantsfortheemitterorPhosphorousimplantsforthefieldstoplayer.Inthisworkwedemonstratethatbothimplantscanbeactivatedsimultaneouslyinasingleprocessstepwithsufficientactivationrate.
∙
oINTRODUCTION
oEXPERIMENTAL
oRESULTS
oTEMPERATURESIMULATIONS
oCONCLUSION
EXPERIMENTAL
Toexplorethepotentialofpulsedlaserannealingat515nm,testannealsofplainwaferswithBoronandPhosphorousdopantswereperformed.Forthesamplepreparation6inchFloatZone(FZ)wafershavebeenusedwithan-typePhosphorousconcentrationof10
14
.Theionimplantationforthefieldstoplayerandemitterwascarriedoutwithamediumcurrentimplanter(VarianE220).TheemitterwascreatedbyBoronimplantationintherange5×
upto1×
16
withanenergyof30keV.ForthefieldstoplayeradualPhosphorousimplantationwasperformedwithadoseof2×
13
atanenergyof200keVand4×
with400keV.Thelaserenergydensitywasvariedbetween2.6and4J/cm
andthepulsedurationofthelaserwasadjustedbetween350and600ns.
ThetestannealswereperformedwithanINNOVAVENTopticalannealingsystemVOLCANOwhichwasoperatedwithalaboratoryASAMAlaseremittingpulsedlaserradiationat515nm.ThiswavelengthhasapenetrationintocrystallineSiofroughly1μm
andthusallowstobringlaserenergyintothebulkmaterial,incontrasttoUVlaserradiationwhichisabsorbedina<
nmthinsurfacelayer.ThelaboratoryVOLCANOsystemcreatedalaserlineonthewaferwithalengthof1mmwithatop-hatintensityprofile(within+/−3%
variation)andaGaussianprofilewidthof40or80μm
FWHM.Figure2showstypicalbeamprofiles.
Fig.2a:
LongaxishomogenizedbeamprofileoftheINNOVAVENTVOLCANOopticalsystem
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Fig.2b:
ShortaxisGaussianbeamprofileoftheINNOVAVENTVOLCANOopticalsystem
TheASAMAlaserallowsvariationofthepulsedurationbycomputercontrol.Thisuniquefeaturemadeitveryeasytoperformtestannealswithdifferentpulsedurationsbetween250and650ns.Figure3showstwolaserpulseswith300and646ns.Energydensitycouldbevariedbetween<
1J/cm
upto4J/cm
usinganopticalattenuator.
Fig.3:
ContinuouslyvariablepulsedurationoftheASAMAlaser.Twodifferentlaserpulseswithpulseduration300ns(uppertrace)and646ns(lowertrace)areshownasexamples
MoreinformationabouttheVOLCANOannealingsystemandtheASAMAlasercanbefoundelsewhere[6].
Forthelaserannealingtests,singlewafersweremanuallyalignedandplacedontoamotorizedhighresolutionxystagewhichwasequippedwithavacuumchuck.Thelaserprocessshutterwassynchronizedtothemovementofthestage.Acomputercontrolsystemallowedtoilluminatepartsofthewaferwithdifferentlaserbeamparameters.Thelaserlinewasscannedinthedirectionofitssmalldimensionwithvariablevelocity(variationofpulseoverlap).Foralltestsreportedherethescanspeedwaschosensothatthelaserpulsesoverlappedby90%withreferencetotheFWHMofthelinewidth.Figure4showsthetypicaloverlapofpulses.Severaloftheselines,each1mmwide,werestitchedsidebysidetoanneallargerareas.Noindicationcouldbefoundfordifferencesoftheannealingresultsinthestitchingareas.
Fig.4:
Pulseoverlapofthelinescanprocess
Alltestannealswereperformedatroomtemperatureonair.
ForcharacterizationpurposesthedopingprofileshavebeenanalyzedbySecondaryIonMassSpectroscopy(SIMS)andSpreadingResistanceProbe(SRP).ForSIMSanalysisaCamecaIMS-4fsystemwasusedwithCs
+
primaryionsforPhosphorousatanenergyof12,5keV.Instead,forBoronanalysisO
ionswereusedatthesameenergy.
ThespreadingresistancemeasurementswerecarriedoutbyusingtheSSM150system.
Also,simulationsfortheionimplantationhavebeenperformedasareferencebyusingthecommercialsimulatorAthenafromSilvaco.
oEXPERIMENTAL
oRESULTS
RESULTS
ItwastheaimofthisinvestigationtoverifywhetherasinglesteplaserannealforaBoron/Phosphorousp-njunctiongivessufficientactivateddoseforthen-typefieldstoplayer.ForthispurposetheasimplantedPhosphorousandBoronprofileshavebeenmeasuredbySIMSandcomparedbysimulationsusingtheSilvacoAthenaprocesssimulator.
Boronwasimplantedatahighdoseof1×
withanenergyof30keV.ForPhosphorousatwofoldimplantationat400keVwithdoublechargedionsand200keVwasbeingappliedhavingdosevaluesof4×
and2×
respectively.
Asdepictedinfigure5agoodcorrelationbetweentheSIMSandsimulationprofilesisobvious,especiallyfortheBoronprofile.ThePhosphoroustailoftheSIMSmeasureme
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