CMOS Front End Process CMOS前端工藝英文版課件：part3 2006 isolatin

IsolationIsolationisveryimportantwhenseveraldevicesareintegratedonasinglesubstrateorwafer.Itappearswhereverthereisnosourceordrainimplant,mostofitbeingtheinterconnectregion.Allisolationmethodsusesiliconnitrideasamaskforpreventingoxidationwherevernitridecoverssiliconsurface.Nitridebeingmuchdense,doesnotallowoxygenorsteamtodiffusethroughtoreachsilicon,preventingoxidationfromoccurring. LocalOxidationofSilicon(LOCOS)Processingsteps:OnSi,20-30nmoxideisgrown.100nmnitrideisdepositedontopofthisandpatternedtoopenisolationareas.NitrideNitrideOxideOxidePPSisubstrateNitrideNitrideOxideOxidePSiPSiUsuallyafieldimplantisdoneheretoraisefieldthresholdifneeded.Fieldimplantcouldalsobedoneaftergrowingtheoxidetoovercomeeffectsduetosegregation.Resistisnowstripped.Awetoxidationtogrow500to1000nmisolationoxide.Finally,maskingnitridelayerisetchedwithphosphoricacid.Arealisticsimulatedlocosprofileisasshown.Fieldoxidationbeginsfrom3.9mpoint.Nitridethicknessis70nmhere.Fieldimplantofboronhas50keVenergyanddoseof5x1012cm-2.Bird’sbeakstructureisclearlyseen.ThisphenomenonoccursbecausesteamdiffusessidewaysandthenverticallythroughpadoxidetoSiandoxidizesit.Thenitridebendsupwardduetothisgrowth.Thisallowsevenfurthersteamdiffusion.Theencroachmentinthedeviceregioninthisexampleisabout0.8m.Themostcontrolledencroachmentis0.4mwiththickernitride.Moresevereconsequencesinthewidthdimension.Theoxidelayerbelownitrideisforreleaseofstress.NitrideexertsastrongtensilestressonSiifdirectlydepositedandwillproducedefectsoredgedislocationsSi.Oxidecushionsthisstress.HencethickeroxideisbetterforcushionbutthenthickeroxideallowsmoresteamtoreachSiincreasingbird’sbeakencroachment.Thickernitridegiveslessbird’sbeakencroachment,butmaygeneratedislocations.Anoptimumcombinationistobedesignedforaprocess.Borondepletesnearoxideinterfaceduetosegregationandmaylowerfieldthreshold.Also,boronencroachesintosourcedrainregionsonthewidthedgesnarrowingtheeffectivechannel.Thebiggestproblemisthebird’sbeakencroachment.Asimplebutundesirablemethodtogetreadofthebeakisbyetchingawaytheencroachedoxidethickness.Otherpracticalusefulwaytoreducebird’sbeakto0.2mlevelisbyusingpolybufferedlocos.PolyBufferedLOCOSInthismethod,a50-100nmpolysiliconbufferlayerisdepositedontopofthepadoxide.Thislayerformsanothercushionagainststress.Thereforepadoxidethicknesscanbereducedto10nmandnitridethicknessincreasedto400nmstillmaintainingdefectfreeSi.RealisticpolybufferedLOCOSsimulationresultisasshown.Herepadoxideis20nm,polyis40nm,2nmoxideontopofpolyandnitrideis200nmthick.Thefieldedgeisat1m.Thedecreasedstepinoxideisclear.Bird’sbeakencroachmentisabout0.3m.Howeverunevensurfacetopographyandbird’sbeakproblemsremainedwhichmademultilevelmetallizationandcompactisolationstructureaproblem.Hencetechnologynodesat0.13mhaveallmovedtoshallowtrenchisolation(STI)Darkregion–nitride,polyjustbelownitrideandabovethinoxide.ShallowTrenchIsolationBecamepopularduetoplanarityandcompactness.Processsequencestartslikelocoswiththinnerpadoxidein10-20nmrangeandthinnernitridein50-100nmrangeasnothickthermaloxideisgrownhere.Afteropeningisolationareas,Sisubstrateisetchedtodepthintherange0.3to0.8mwithangledwalls.Angledwallsneededsothateffectsassociatedwithsharpcornerscanbeavoided.Athin10-30nmoxideisnowgrownathightemperaturelike11000CtogivegoodSiinterface,cornerroundingatthebasecornerwithvisco-elasticflowofoxide.Thenathickoxideisdeposited.ThisisthenfollowedbyoxideetchbackandChemicalMechanicalPolishing(CMP)toplanarizethestructure.Theremaybeslight