HEMT Millimeter-wave Monolithic IC Technology for

153FUJITSUSci.Tech.J.,34,2,pp.153-161(December1998)UDC621.385:621.396.967HEMTMillimeter-waveMonolithicICTechnologyfor76-GHzAutomotiveRadarVYuuWatanabeVNaofumiOkubo(ManuscriptreceivedJune8,1998)ThispaperdescribesmonolithicICtechnologiesfordevelopingpracticalautomotiveradarsystems,coveringtheHEMTdevicestructure,ICfabricationprocess,flip-chipassembling,andcircuitdesign.InGaP/InGaAsHEMTswitha0.15-μmgatewereusedinmillimeter-wavemonolithicICsfortheW-band,providingamaximumstablegainof9dBat76GHz.Height-controlledflip-chipbondingwithpillarinterconnectionwasdemonstratedasalow-costassemblymethod.Ade-embeddingtechniquetomodelaface-downco-planarwaveguidewasproposed.Achipsetwasdesignedwiththistech-nique,consistingofa76-GHzamplifier,76-GHzmixer,76-GHzSPDTswitches,38/76-GHzdoubler,38-GHzvoltage-controlledoscillator,and38-GHzbufferamplifier.Thefabricatedchipsetshowedhighperformanceforautomotiveradarsystems.1.IntroductionInlinewithrecentadvancesinmillimeter-wavesystemsbasedonsemiconductortechnolo-gydevelopment,theapplicationofthesesystemsisbeingextendedtocommercialfields.1),2)Oneofthemostpromisingareasisautomotiveradartoprovideactivesafetyfordriving.Researchonau-tomotiveradarstartedintheearly1970sintheUnitedStates.In1981,Fujitsudevelopeda50-GHzradarsensor.Theseefforts,however,didnotmeetthedemandsofthemarketduetocostandsize.Inthelate1980s,severalnationalprogramspromotedresearchonmillimeter-wavesystems,whileduringthesameperiodGaAssemiconduc-tormanufacturinggrewstronglyduetotherapidspreadofwirelesscommunications.Thesecircum-stancesstimulatedcommercialinterestinmilli-meter-wavesystems,includingautomotiveradar.Frequencyallocationisakeytothemarketofanysystemutilizingelectromagneticwaves.TheJapaneseMinistryofPostsandTelecommu-nications(MPT)assignedthe60-GHzbandforlow-powermillimeterradarsystemsin1995.Fol-lowingtheallocationofthe76-GHzbandforra-darintheU.S.andEurope,theMPTdecidedin1997toassignthesamebandtoautomotivera-darsystemsinadditiontothe60-GHzband.Al-thoughthisactionpromotedaglobalstandardforcommercialuse,itgavearisetochallengingtech-nicalissues.Torealizepractical,reasonablypricedradarsystemsforpassengercars,thefrequencyperfor-manceofthedevicesmustbeimprovedandcostsreduced.Monolithicintegrationistheusualap-proachtocostreductionandisstillimportantfortheW-band.3)Muchefforthasrecentlybeende-votedtothedevelopmentofmonolithicintegrat-edcircuits,andithasbeenshownthatperipheralelementsplayamoreimportantroleinthisfre-quencyregion.Diepositioningandwiringinapackageseriouslyaffectthecircuitperformanceandyield,resultinginhighercosts.Flip-chipscanbeusedtoavoidthesedifficulties,aswellasdigi-talcircuits.4)Usingflip-chipbonding,chipposi-tioningcanbeautomatedwithgreateraccuracy.Thewaferprocessforflip-chipswillbesimpler154FUJITSUSci.Tech.J.,34,2,(December1998)Y.Watanabeetal.:HEMTMillimeter-waveMonolithicICTechnologyfor76-GHzAutomotiveRadarthanthatforconventionalmicrowavedevices.Tomakeuseoftheseadvantagesinmillimeter-waveapplications,problemsassociatedwiththeshortwavelengthmustbeovercome.5),6)Thispaperdescribesamillimeter-wavemonolithicIC(MMIC)technologyforthedevel-opmentof76-GHzautomotiveradar,includinga0.15-μmHEMTICprocess,flip-chipwithpillarinterconnection,andW-bandface-downcircuitde-sign.UsinganInGaP/InGaAs/GaAsheterostruc-turewithelectronbeamlithography,theHEMTexhibitedgoodimprovementcomparedtothecon-ventionalstructure.Thepillarinterconnectiontechniquewasemployedinthemillimeter-wavemonolithicICforflip-chips.Wehavedemonstrat-edthegoodelectricalpropertiesoftheflip-chipandshowedthedifficultyofdesignduetotheface-downstructure.SeveralMMICsweresuccessful-lydesignedandfabricatedforthe76-GHzauto-motiveradarsensor.Theseface-downcircuitsexhibitedgoodperformanceintheW-band.2.DevicetechnologyTheoperationfrequencyof76GHzinMMICsischallengingfortransistortechnology.Thetran-sistorsinthecircuitmustprovidesufficientgainat76GHz,whichconventionalsemiconductorcir-cuitshavenotreached.HeterojunctiondevicessuchasHEMTshavefundamentaladvantagesoverothertypesoftransistorsintermsofhigh-frequencyperformance,becauseofthebandgapdifferenceintheirstructure.Fujitsuhasalreadydevelopeda0.25-μmgateHEMTtechnologyus-ingphotolithographyandsidewalltechniques,whichiscommerciallyavailableforseveralmilli-meter-wavesystems.However,evenwithquar-ter-micronHEMTs,thefrequencyperformanceisnotsufficientfor76-GHzautomotiveradarsys-tems;high-performancetransistorswithanfmaxexceeding150GHzarerequired.Thereductionofgatecapacitanceandin-creaseintransconductanceareimportanttoim-provetransistorcharacteristics.Scalingisthetra-ditionalapproachforFETtechnologyandiseffectiveforimprovingperformanceeveninthesub-micronregion,butparasiticfactorscannotbeneglectedforhigh-performancedevices.Suchphe-nomenaastheshortchanneleffect,increaseingateresistanceandfringingcapacitancedegradethetransistorperformance.Figure1showsthenewstructureemployedforthe0.15-μmHEMT.TheT-shapedgatewaspatternedbyelectronbeamlithography.Thismakesthegateelectrodefreefromthesidewall,thusreducingthefringingcapacitance.Thei-GaAs/i-InGaAs/n-