Laser Annealing of Power Devices.docx

Laser Annealing of Power Devices.docx

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LaserAnnealingofPowerDevices

ThediscreteIGBT（InsulatedGateBipolarTransistor）isthemostimportantpowersemiconductordeviceforpowerconversionandcontrolinthemediumpowerrangewithvoltages>400V.NextgenerationIGBTsrequirelowthermalbudgetp-njunctionformationforthebacksidefieldstopandemitterlayer.Therequirementforthelimitedthermalbudgetisduetothefactthatthefrontsidemetallizationdoesnotallowhightemperaturetreatmentfortheactivationofthefieldstoplayer.Therefore,laserannealingexperimentshavebeencarriedoutusingafrequencydoubledYb:

YAGlaseratawavelengthof515nmwithanenergydensityupto4J/cm2.InordertofindoutappropriateprocessconditionsforasinglestepBoronandPhosphorouslaserannealingprocess,parametersforionimplantationaswellasenergydensityandpulsedurationofthelaserhavebeenvaried.ThedopingprofilesofBoronandPhosphorousweremeasuredbySecondaryIonMassSpectroscopy（SIMS）andSpreadingResistanceProbe（SRP）inordertoassessthedopantactivationbehaviour.ThemaininterestwastheactivationofthePhosphorousdopedfieldstoplayerinadepthrangeof1mum.AstrongdependencyofPhosphorousactivationonimplantedBorondosewasobservedwithdopantactivationupto70%.

Thispaperappearsin:

AdvancedThermalProcessingofSemiconductors,2007.RTP2007.15thInternationalConferenceon,IssueDate:

2-5Oct.2007,Writtenby:

Friedrich,Detlef;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 cm −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 cm −2  .Theionimplantationforthefieldstoplayerandemitterwascarriedoutwithamediumcurrentimplanter（VarianE220）.TheemitterwascreatedbyBoronimplantationintherange5×10 14 cm −2  upto1×10 16 cm −2  withanenergyof30keV.ForthefieldstoplayeradualPhosphorousimplantationwasperformedwithadoseof2×10 13 cm −2  atanenergyof200keVand4×10 13 cm −2  with400keV.Thelaserenergydensitywasvariedbetween2.6and4J/cm 2  ,andthepulsedurationofthelaserwasadjustedbetween350and600ns.

ThetestannealswereperformedwithanINNOVAVENTopticalannealingsystemVOLCANOwhichwasoperatedwithalaboratoryASAMAlaseremittingpulsedlaserradiationat515nm.ThiswavelengthhasapenetrationintocrystallineSiofroughly1μm ,andthusallowstobringlaserenergyintothebulkmaterial,incontrasttoUVlaserradiationwhichisabsorbedina<10 nmthinsurfacelayer.ThelaboratoryVOLCANOsystemcreatedalaserlineonthewaferwithalengthof1mmwithatop-hatintensityprofile（within+/−3% variation）andaGaussianprofilewidthof40or80μm FWHM.Figure2showstypicalbeamprofiles.

Fig.2a:

LongaxishomogenizedbeamprofileoftheINNOVAVENTVOLCANOopticalsystem

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Fig.2b:

ShortaxisGaussianbeamprofileoftheINNOVAVENTVOLCANOopticalsystem

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TheASAMAlaserallowsvariationofthepulsedurationbycomputercontrol.Thisuniquefeaturemadeitveryeasytoperformtestannealswithdifferentpulsedurationsbetween250and650ns.Figure3showstwolaserpulseswith300and646ns.Energydensitycouldbevariedbetween<1J/cm 2  upto4J/cm 2  usinganopticalattenuator.

Fig.3:

ContinuouslyvariablepulsedurationoftheASAMAlaser.Twodifferentlaserpulseswithpulseduration300ns（uppertrace）and646ns（lowertrace）areshownasexamples

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MoreinformationabouttheVOLCANOannealingsystemandtheASAMAlasercanbefoundelsewhere[6].

Forthelaserannealingtests,singlewafersweremanuallyalignedandplacedontoamotorizedhighresolutionxystagewhichwasequippedwithavacuumchuck.Thelaserprocessshutterwassynchronizedtothemovementofthestage.Acomputercontrolsystemallowedtoilluminatepartsofthewaferwithdifferentlaserbeamparameters.Thelaserlinewasscannedinthedirectionofitssmalldimensionwithvariablevelocity（variationofpulseoverlap）.Foralltestsreportedherethescanspeedwaschosensothatthelaserpulsesoverlappedby90%withreferencetotheFWHMofthelinewidth.Figure4showsthetypicaloverlapofpulses.Severaloftheselines,each1mmwide,werestitchedsidebysidetoanneallargerareas.Noindicationcouldbefoundfordifferencesoftheannealingresultsinthestitchingareas.

Fig.4:

Pulseoverlapofthelinescanprocess

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Alltestannealswereperformedatroomtemperatureonair.

ForcharacterizationpurposesthedopingprofileshavebeenanalyzedbySecondaryIonMassSpectroscopy（SIMS）andSpreadingResistanceProbe（SRP）.ForSIMSanalysisaCamecaIMS-4fsystemwasusedwithCs +  primaryionsforPhosphorousatanenergyof12,5keV.Instead,forBoronanalysisO 2  +  ionswereusedatthesameenergy.

ThespreadingresistancemeasurementswerecarriedoutbyusingtheSSM150system.

Also,simulationsfortheionimplantationhavebeenperformedasareferencebyusingthecommercialsimulatorAthenafromSilvaco.

∙

oINTRODUCTION

oEXPERIMENTAL

oRESULTS

oTEMPERATURESIMULATIONS

oCONCLUSION

RESULTS

ItwastheaimofthisinvestigationtoverifywhetherasinglesteplaserannealforaBoron/Phosphorousp-njunctiongivessufficientactivateddoseforthen-typefieldstoplayer.ForthispurposetheasimplantedPhosphorousandBoronprofileshavebeenmeasuredbySIMSandcomparedbysimulationsusingtheSilvacoAthenaprocesssimulator.

Boronwasimplantedatahighdoseof1×10 16 cm −2  withanenergyof30keV.ForPhosphorousatwofoldimplantationat400keVwithdoublechargedionsand200keVwasbeingappliedhavingdosevaluesof4×10 13 cm −2  and2×10 13 cm −2  ,respectively.

Asdepictedinfigure5agoodcorrelationbetweentheSIMSandsimulationprofilesisobvious,especiallyfortheBoronprofile.ThePhosphoroustailoftheSIMSmeasureme