EFFECTS OF EPITAXIAL LIFT OFF ON THE DC, RF AND TH

PublishedinIEEETransactiononElectronDevicesvol.46,no.7,July1998pp.1407-14131EFFECTSOFEPITAXIALLIFTOFFONTHEDC,RFANDTHERMALPROPERTIESOFMESFETSONVARIOUSHOSTMATERIALSThomasMorf,CharlotteBiber*,WernerBächtold*SwissFederalInstituteofTechnology,(ETH)ElectronicsLaboratory,*LaboratoryforElectromagneticFieldsandMicrowaveElectronics,Gloriastr.35,CH8092Zürich,SwitzerlandTel.++4116323537,FAX++4116321210e-mail:[e-mail]resentanindepthinvestigationontheeffectsofepitaxialliftoff(ELO)onGaAsMESFETs.DCandmicrowavecharacteristicsaswellasthermaleffectsareconsidered.DeviceswerefabricatedonaGaAsfoundryprocessandtransplantedbyELO.ELOisatechnologybywhichepitaxiallygrownlayersareliftedofffromtheirgrowthsubstrateandaresubsequentlyre-attachedtoanewhostsubstrate.HostmaterialsconsideredareInP,quartzandsiliconwithresistivitiesrangingfrom11mΩcmto50Ωcm.PublishedinIEEETransactiononElectronDevicesvol.46,no.7,July1998pp.1407-14132I.INTRODUCTIONELOallowsamonolithicintegrationofpreviouslyincompatiblematerialssuchasGaAsandInPorGaAsandSilicon[1].ThistechnologyopensnewpossibilitiesbyallowingthecombinationofGaAswithnearlyarbitraryothermaterialswithoutconcernforlatticeconstantmismatching.InthispapertheeffectsofELOonMESFETsisdescribedindetail.Deviceperformanceiscomparedbeforeandaftertransplantation.TheeffectsonDC,RFandthermalcharacteristicsarestudied.InadditionRFcharacteristicsasafunctionofsubstrateconductivityarepresented.II.EPITAXIALLIFTOFFGaAsMESFETswerefabricatedusingaslightlymodifiedfoundryprocess.TheprocessoffersdepletionmodeMESFETswithagatelengthof0.7μmaswellasresistors,capacitorsandSchottkydiodes.ThewafermaterialrequiredforELOwasepitaxiallygrown.InordertomakeELOpossible,a50nmthicksacrificialAlAslayerwasgrown800nmbelowthesurfaceofthewafer.OntopofthisAlAslayera600nmthicksemiinsulatingGaAslayeranda200nmthickn-dopedGaAslayerweregrown.ThelatterwillformthechanneloftheMESFETs.TheAlAslayeristheonlynon-standardstep,butisnecessaryforchemicallift-off.ThegrowthoftheAlAslayerissimplesinceitislatticematchedtoGaAs.ThefullyprocessedchipsweretransferredbytheELOprocessontovarioussubstratematerials.AccesstotheburiedAlAslayeriseitherobtainedbycleavingthechipsorbyintroducingdryetchedtrenchesbetweenthechips.SincetheELOlayerthicknessisonly800nm,a300μmthickwaxlayerisdepositedtoprovideprotectionandmechanicalsupportduringtransfer[2,3].ThesacrificialAlAsPublishedinIEEETransactiononElectronDevicesvol.46,no.7,July1998pp.1407-14133layerisundercutbyanextremelyselectiveetchant(10:1dilutedHF).Aslightsurfacetensioninthewaxlayercausesanupwards-curlingofthefilmduringtheetchingoftheAlAs-layer,thusenhancingtheetchingprocess.TheELO-filmisthenreleasedfromitsgrowthsubstrateandcanbesubsequentlyattachedtoanarbitraryhostmaterial,suchasInP,quartzorSi.BondingisassuredbyVanderWaalsforces.Detailsabouttheprocessaredescribedin[2,3,4,5].Toenhanceadhesiontothehostmaterialandplanarizesubstrateirregularitiesa300nmthickpolyimide(PI)layer(DuPont-Pyralin2555)wasdepositedtothehostmaterialbyspincoatingpriorthetransplantation.Oncethefilmisbondedontothehost,thewaxisremovedwithtrichlorethylene.Fromatechnologicalpointofviewthetypeofhostmaterialisuncritical.However,itisadvantageoustousePIasanadhesionlayerforbettermechanicalstrength.An800nmthickfilmofGaAsispartiallytransparentevenforwavelengthsinthevisiblerange(400...800nm).OpticaldevicesonELOfilmscanthereforebeilluminatedfromthebacksideprovidingapplicationsinOEIC.Foran800nmthickfilmredlightistransmittedwithlittleabsorptionwhileontheotherendofthevisualspectrumconsiderableabsorptionisexpected.Figure1showsaphotographofachiptransplantedontoquartz.Thephotographontheleftshowsthechipfromthefrontwhilethephotographontherightshowsthesamechipfromtheback,illuminatedthroughthechip.Onecanclearlyseethemetallizationandetchedstructureonthefrontsideofthechip.ThechipappearsdarkredsincethesewavelengthsareabsorbedlessinGaAs.III.ELOTRANSPLANTEDMESFETsMESFETsweretransplantedontoSIInP,ptypesiliconwithvariousconductivities,andquartz.Fig.2showsthesquarerootofIdss(saturationcurrent)andthetransconductancegmversusVgsforVds=2VPublishedinIEEETransactiononElectronDevicesvol.46,no.7,July1998pp.1407-14134for200μmdevicesbeforeandaftertransplantation.AsmalldecreaseincurrentlevelsaswellasasmallpositiveshiftinVtisobservedforalltransplanteddevices.Inordertoaverageoutprocessvariationsmeasurementshavebeenperformedonalargenumberofdevicesbeforeandaftertransplantation.InFigures3and4themeasuredresultsofIdssandgmaresummarized.DeviceswithvarioussizesbeforeandaftertransplantationontoInP,quartzandSiareshown.AreproducibledropinIdssandgmof14mA/mmand16mA/VmmafterELOtransplantationisvisible.Thesechangesareindependentofdevicesize.TheslightlysmallerdecreaseinIdssaftertransplantationontoInPiscausedbyslightdifferencesinprocessing.Changesinthedevicecharacteristicsarebelievedtobecausedbytwoeffects.First,devicesafterELOoperateathighertemperatureasdescribedlaterinthispaper.Thetemperatureincreasehowever,onlyexplainsabouthalfoftheshiftobserved.Second,anewsurfaceisexposedduringELO.Weatt