Constant-Voltage-Bias-Stress-Testing

IEEETRANSACTIONSONELECTRONDEVICES,VOL.56,NO.7,JULY20091365Constant-Voltage-BiasStressTestingofa-IGZOThin-FilmTransistorsKenHoshino,DavidHong,HaiQ.Chiang,andJohnF.WagerAbstract—Constant-voltage-bias(VDS=VGS=30V)stressmeasurementsareperformedforaperiodof105sonthin-filmtransistors(TFTs)withamorphousindium–gallium–zinc–oxide(IGZO)channellayersfabricatedviaRFsputteringusingapostdepositionannealingtemperatureof200◦C,250◦C,or300◦C.ThermalsilicondioxideisemployedasaTFTbottom-gateinsulator.AllSiO2/IGZOTFTstestedexhibitthefollowing:1)apositiverigidlog(ID)–VGStransfercurveshift;2)acon-tinuousdrain–currentdecreaseovertheentirestressduration;and3)recoveryofthelog(ID)–VGStransfercurvetowardtheprestressedstatewhenthestressedTFTisleftunbiasedinthedarkatroomtemperatureforanextendedperiodoftime.TheSiO2/IGZOTFTssubjectedtoahigherpostdepositionan-nealingtemperaturearemorestable.Asmall(andtypicallynegligible)amountofclockwisehysteresisispresentinthelog(ID)–VGStransfercurvesofIGZOTFTs.TheseinstabilityandhysteresisobservationsareconsistentwithaSiO2/IGZOTFTinstabilitymechanisminvolvingelectrontrappingwithintheIGZOchannellayer.IndexTerms—Amorphousoxidesemiconductor(AOS),galliumcompound,indiumcompound,indium–gallium–zincoxide,stabil-ity,thin-filmtransistor(TFT),zinccompound.I.INTRODUCTIONAMORPHOUSoxidesemiconductors(AOSs)areofcon-siderableinterestfornext-generationthin-filmtransistor(TFT)applications[1]–[3].Inadditiontoitstransparencyinthevisiblerangeoftheelectromagneticspectrum,AOSsexhibitsuperiorelectricalcharacteristicscomparedtohydrogenatedamorphoussilicon[4],whichisthepresent-daydominantTFTtechnologyforlarge-areaapplications[5].OneofthemostpromisingAOScandidatematerialsisindium–gallium–zinc–oxide(IGZO),asproposedbyHosonoetal.oftheTokyoInstituteofTechnologyin2004[6],whichiscurrentlyunderintensedevelopmentworldwide[7]–[12].SeveralreportstodatehavebeendirectedtowardthetopicofIGZOTFTstability[9]–[12].Thepurposeoftheworkpresentedhereinistoreportonaconstant-voltage-biasstressinvestigationofIGZOTFTsfabricatedviaRFsputteringusingthermalsilicondioxideasaTFTbottom-gateinsulator.TheprimaryobjectiveofthisManuscriptreceivedJanuary22,2009;revisedApril3,2009.FirstpublishedMay15,2009;currentversionpublishedJune19,2009.ThisworkwassupportedinpartbyHewlett-PackardCompany,bytheDefenseAdvancedResearchProjectsAgency(DARPA)(MEMS/NEMSScienceandTechnologyFundamentals),bytheArmyResearchLaboratory(W911NF07-7-0083),andbytheFlexTechAlliance.ThereviewofthispaperwasarrangedbyEditorS.Bandyopadhyay.TheauthorswerewiththeSchoolofElectricalEngineeringandComputerScience,OregonStateUniversity,Corvallis,OR97331USA(e-mail:[e-mail];[e-mail]).DigitalObjectIdentifier10.1109/TED.2009.2021339Fig.1.Cross-sectionalviewofthebottom-gateTFTdevicestructure.paperistoattempttoidentifytheSiO2/IGZOTFTinstabilitymechanism.II.SAMPLEPREPARATIONAbottom-gatestaggeredTFTstructureisselectedusingap-typecrystalline-Sisubstratewitha100-nm-thickthermallygrownSiO2layerandatantalum/goldbackcontact(Fig.1).IGZOisdepositedontopofSiO2usingRFmagnetronsputter-ing,followedbyindium–tin–oxideorAlsource/draincontacts(∼200nmthick)depositedviaRFmagnetronsputteringorthermalevaporation,respectively.RFmagnetronsputteringisperformedusinganRFpowerof100Win5mtorrofpureArfor3.33min.Theceramic,whichis3In:Ga:Zn:O=1:1:1:4(Cerac,Inc.),isplaced4fromthesubstratedur-ingdeposition.Filmdefinitionisaccomplishedusingshadowmasks,resultinginTFTwidthandlengthof2000and200μm,respectively.TFTchannellayersarefurnaceannealedinairfor1hat200◦C,250◦C,or300◦C.Thetopsideofthechannellayerisnotpassivatedbutisratherexposedtoair.III.EXPERIMENTALPROCEDUREAnAgilent4155C/4156Csemiconductorparameteranalyzerisusedtomonitorallelectricalcharacteristicsinthedark.Alog(ID)–VGStransfercurveusingVDS=30Vismeasuredbeforethestressing,whichestablishestheprestressedstate.Constant-voltage-biasstresstestingisaccomplishedoveraperiodof105sbyholdingVDS=VGS=30V,duringwhichtimethedraincurrentiscontinuouslymonitored.Anotherlog(ID)–VGStransfercurveusingVDS=30Visextractedim-mediatelyafterthestressingperiodtoestablishthepoststressedstate.Therecoveryprocess,inwhichtheTFTisplacedinthedarkforanextendedperiodoftimewithoutelectricalorthermalstress(no-stressrecovery),occursafterthepoststresstransfercurveextraction.Anotherlog(ID)–VGStransfercurveisobtainedaftertherecoveryprocesstoestablishthepost-recoverystate.0018-9383/$25.00©2009IEEE1366IEEETRANSACTIONSONELECTRONDEVICES,VOL.56,NO.7,JULY2009Fig.2.Log(ID)–VGStransfercurves(VDS=30V)beforeandafterconstant-voltage-biasstresstestingforadurationof105sandafterarecoveryperiodof50days.Asmallamountofclockwisehysteresisispresentforeachcurve.Stressingleadstoapositiverigidshiftinthetransfercurve,whichrecoversbacktotheoriginalprestressstateaftera50-day-longno-stressperiod.Fig.3.Draincurrentasafunctionofstresstimeforaconstant-voltage-biasstresstest,inwhichVDS=VGS=30Viscontinuouslyappliedinthedark.Noticethatthedraincurrentcontinuouslydecreasesforatleast105s.TheSiO2/IGZOTFTusedforthismeasurementwasannealedat300◦C.IV.EXPE