搜索
_________________________________________________________________2.1Chapter2ElectronBeamLithographyProcesses2.1IntroductionFinelyfocusedelectronbeamsareusedinlithographicprocessestoexposepolymerresistlayers.Verycomplexdevicepatternswithhighresolutionnanometersizecanbecreated.Suchresolutionisingeneralsuperiortopresentlyavailableopticallithographytechniquesbecausetheelectronprobesizemaybemuchsmallerthanthecorrespondingdiffractionlimitedimageinopticallithography.Inanylithographicprocess,theresistimageistheimportantitem.Thereforeitisveryimportanttounderstandhowthatresistimageisproducedandwhatfactorsareinfluencingthequalityoftheimage.InordertoutiliseEBLsuccessfullyinnanometerrange,theinteractionandscatteringofelectronswithintheresistlayerandtheunderlyingsubstratemustbewellunderstood.Forexample,theeffectsofbeamenergy,typeofresist,resistthickness,substratetype,andseveralothervariablesarecriticalinproducinganoptimalpatternintheresist.Theresolutionobtainablewithelectronbeamsisnotlimitedbyprobecharacteristics,butratherispresentlylimitedbyelectronscattering.Itisimportanttounderstandthislimitsimposedbyelectronscatteringaswellasmethodsofminimisingscatteringeffectsandtherebyimprovingresolution._________________________________________________________________2.2InSection2wewillbrieflyoutlinephysicalfoundationsforinteractionsoftheelectronbeamwiththeresist-substratesystem.SomeMonteCarlosimulationsandresultingabsorbedenergydistributionswillbepresented.MoresystematicapproachisaddressedinChapter3ofthisthesis.InSection3wewillintroduceanddetermineparametersoftheresist/substratesystems.Resistrequirementsformodernnanofabricationwillfollow.InSection4theedgecontrastimagedintheresistisanalysed.Themodellingofthedevelopmentprocessispresented.Section5willdefineEBLexposurelatitudeandwillpointoutsomeadditionalfactorsimposinglimitationsontheobtainableresolutioninEBL.2.2Interactionoftheelectronbeamwiththeresist-substratesystem2.2.1IntroductionLetusstartwiththedefinitionofthelithographyitself.Itisdonebyanexampleofthelift-offtechnique,whichisfrequentlyusedinmetrologicalexperiments.Lithographyisagenerictermforaprocessinwhichsomethingisprintedonasurfacethatisreceptiveto“ink”insomeregionsandrepels“ink”inotherregions.Thisisopposedtoaphotographicprocessinwhichtheentiresurfaceissensitivetoacertaintypeofactuator(mostoftenlight)withdifferentdegreesofsensitivity.Thetypeoflithographyusedinthisexperimentislift-offlithography.Thistypeoflithographyisatwostepprocess.First,thematerialonwhichonewantstowrite,the“paper”,mustbepreparedinsuchawaythattheregionsonewanttowriteoncanbemadesensitivetothe“ink”.Onecandothisbyplacingalayerofamaterialuponthe”paper”.Next,oneneedstomakeregionsofthe“paper”sensitivetothe“ink”.Thisisdonebyetchingawaycertainregionsofthematerial_________________________________________________________________2.3whichcovereduptheoriginal“paper”.Third,oneneedstospreadthe“ink”overtheentiresurface,includingboththeregionsof“paper”exposedbytheetchingprocessandthematerialstillcoveringthe“paper”.Finally,onestripsawaytherestofthematerialfromthe“paper”(lift-off).Thisfinalstepleavesthe“ink”onthe“paper”intheregionswheretheetchmarksweremade,anditremovesalloftheexcessmaterial(withtheunwanted“ink”)fromthe“paper”.Oneisoftenconfrontedwiththeanalogyofblockprinting.However,thisistrulynotalift-offlithographicprocess.Thisisbecauseinblockprintingonestripsawaythenegativefromtheblock,“inks”theblock,andprintsthepositiveonto“paper”.Inlithography,oneremovesthepositivefromthe“paper”,“inks”the“paper”,andthenremovesthenegative.Forelectronbeamlithography,EBL,the“paper''isasuitablesubstrate,thematerialcoveringthesubstrateisanelectronsensitiveone,theetchingdeviceistheelectronbeamoftheSEM,the“ink”isamaterialofone'schoice,andthestriperoftheexcess“ink”isachemicalchosentoremovetheelectronsensitivematerial.Figure2.1depictsthestepsoftheprocess.Thematerialsensitivematerialcoveringthesubstrateisreferredtoase-beamresist.Theinteractionofanincidentelectronbeamwiththeresistfilmandthesubstrategivesrisetoanelectronscatteringintheresistwhichcausestheso-calledproximityeffects.Muchefforthasbeendevotedtoclarifythisphenomena.Forpracticalapplicationsinelectronlithography,thespatialdistributionoftheenergydissipationisthemostimportantquantity.Manyexperimental[1-5]andtheoreticalresultshavebeenreportedontheenergydistribution.Amongthetheoreticalresults,somearebasedonanalyticalmodels[6-8]andsomehavebeencarriedoutwithMonteCarlosimulations[2-3,7,9-17].However,noneofthemtakesintoaccounttheeffectofsecondaryelectronproduction.Whenaprimaryelectronenterstheresist,ittransferspartofitsenergytoatomicelectrons,resultinginionisationorexcitation.Anelectron-electroncollisionwithalargeenergytransferwillgenerateasecondaryelectronwithakineticenergyrangingfromnearzerotohalfoftheincidentprimaryelectronenergy.Theslowsecondariesarenotsoimportantbecausetheydonottravelfarfrom_________________________________________________________________2.4theirpositionofgeneration.Oneisinterestedmainlyinthefastsecondaryelectronswithenergyi