Nanoimprint Lithography

StephenY.Chou,a)PeterR.Krauss,andPrestonJ.Renstrom

NanoStructureLaboratory,DepartmentofElectricalEngineering,UniversityofMinnesota,Minneapolis,Minnesota55455

͑Received20June1996;accepted17August1996͒

Nanoimprintlithography,ahigh-throughput,low-cost,nonconventionallithographicmethodproposedanddemonstratedrecently,hasbeendevelopedandinvestigatedfurther.Nanoimprintlithographyhasdemonstrated25nmfeaturesize,70nmpitch,verticalandsmoothsidewalls,andnearly90°corners.Furtherexperimentalstudyindicatesthattheultimateresolutionofnanoimprintlithographycouldbesub-10nm,theimprintprocessisrepeatable,andthemoldisdurable.Inaddition,uniformityovera15mmby18mmareawasdemonstratedandtheuniformityareacanbemuchlargerifabetterdesignedpressisused.Nanoimprintlithographyoveranonflatsurfacehasalsobeenachieved.Finally,nanoimprintlithographyhasbeensuccessfullyusedforfabricatingnanoscalephotodetectors,siliconquantum-dot,quantum-wire,andringtransistors.©1996AmericanVacuumSociety.

I.INTRODUCTION

Oneofthemajorroadblocksindevelopingnanostruc-turesisthelackofalow-cost,high-throughputmanufactur-ingtechnology.Thisproblemisparticularlyseriousforstructureswithasizebelow0.1␮m.Numeroustechnologiesareunderdevelopmenttosolvethisproblem.1–6Oneyearago,weproposedanddemonstratedanotherpossiblesolu-tiontonanostructuremanufacturing,namelyanewnoncon-ventionallithographicmethodcallednanoimprintlithography.7Thekeyadvantageofthislithographictech-niqueistheabilitytopatternsub-25nmstructuresoveralargeareawithahigh-throughputandlow-cost.Therefore,nanoimprintlithographyisamanufacturingtechnology.Inthisarticle,wewillpresentrecentprogressindevelopingthislithographictechnique.

II.PRINCIPLEOFIMPRINTLITHOGRAPHY

NanoimprintlithographyhastwobasicstepsasshowninFig.1.Thefirstistheimprintstepinwhichamoldwithnanostructuresonitssurfaceispressedintoathinresistcastonasubstrate,followedbyremovalofthemold.Thisstepduplicatesthenanostructuresonthemoldintheresistfilm.Inotherwords,theimprintstepcreatesathicknesscontrastpatternintheresist.Thesecondstepisthepatterntransferwhereananisotropicetchingprocess,suchasreactiveionetching͑RIE͒,isusedtoremovetheresidualresistinthecompressedarea.Thissteptransfersthethicknesscontrastpatternintotheentireresist.

Duringtheimprintstep,theresistisheatedtoatempera-tureaboveitsglasstransitiontemperature.Atthattempera-ture,theresist,whichisthermoplastic,becomesaviscousliquidandcanflowand,therefore,canbereadilydeformedintotheshapeofthemold.Theresist’sviscositydecreasesasthetemperatureincreases.

Unlikeconventionallithographymethods,imprintlithog-raphyitselfdoesnotuseanyenergeticbeams.Therefore,

a͒

nanoimprintlithography’sresolutionisnotlimitedbytheeffectsofwavediffraction,scatteringandinterferenceinaresist,andbackscatteringfromasubstrate.Furthermore,im-printlithographyisfundamentallydifferentfromstampingusingamonolayerofself-assembledmolecules.8Imprintli-thographyismoreofaphysicalprocessthanachemicalprocess.Itisconceivablethatinthefuture,themoldusedinimprintlithographycanbemadeusingahigh-resolutionbutlow-throughputlithography,andthenimprintlithographycanbeusedforlow-costmassproductionofnanostructures.

III.MOLDS,RESISTS,ANDPROCESSCONDITIONSInourexperiments,silicondioxideandsiliconwereusedasthemoldmaterials.Certainlyothermaterialssuchasmet-alsandceramicscouldalsobeused.Themoldwaspatternedwithdotsandlineswithaminimumlateralfeaturesizeof25nmusingelectronbeamlithographyandRIE.Polymethylmethacrylate͑PMMA͒wasourprimaryresist,althoughwehavehadsuccesswithAZandShipleynovlakresin-basedresistsaswell.ThePMMAshowedexcellentpropertiesforimprintlithography.PMMAhasasmallthermalexpansioncoefficientofϳ5ϫ10Ϫ5per°Candasmallpressureshrink-agecoefficientofϳ3.8ϫ10Ϫ7perpsi.9Moldreleaseagentswereaddedintotheresistsandworkedwelltoreducetheresistadhesiontothemold.Thepressureandtemperaturefortheimprintprocessdependontheresistused.ForPMMA,whichhasaglass-transitiontemperatureofabout105°C,theimprinttemperatureusedinourexperimentsistypicallybe-tween140and180°C,andthepressureisfrom600to1900psi.Forthattemperatureandpressurerange,thePMMAthermalshrinkageislessthan0.8%andthepressureshrink-ageislessthan0.07%͑asmallervolumeatahigherpres-sure͒,therefore,theshapeofthePMMAshouldconformwiththatofthemold.Toreduceairbubbles,theimprintprocessshouldbedoneinavacuum.ThegasusedintheRIEpatterntransfer,whichalsodependsontheresistused,wasoxygenforPMMA.

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4130Chou,Krauss,andRenstrom:NanoimprintlithographyFIG.1.Schematicofnanoimprintlithographyprocess:͑1͒imprintingusingamoldtocreateathicknesscontrastinaresist,͑2͒moldremoval,and͑3͒patterntransferusinganisotropicetchingtoremoveresidueresistinthecompressedareas.

Typically,theintrusionofthemoldisfrom40to200nmandtheaspectratioforthesmallestmoldfeaturesis3:1.Thethicknessoftheresistisfrom50to250nm.Theresistwaskeptthickerthanthemoldintrusiontopreventthemoldfromcontactingthesubstrate.Thisisessentialtoprolongthelife-timeofthemold.

IV.RESULTSANDDISCUSSIONA.Imprint

VariousnanostructureshavebeenimprintedintoPMMAincluding25nmdiamholeswitha120nmperiodand30nmwidetrencheswitha70nmperiod.Figure2showsascan-ningelectronmicrographofimprintedPMMAstripsbefore

FIG.2.SEMmicrographofaperspectiveviewofstripsformedintoaPMMAfilmbyimprint.Thestripsare70nmwideand200nmtall,haveahighaspectratio,asurfaceroughnesslessthan3nm,andnearlyperfect90°corners.

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FIG.3.SEMmicrographofthemoldthatwasusedtoimprintthePMMAstripsshowninFig.2.

RIE.Thestrips,whichare70nmwideand200nmdeep,haveverysmooth͑aroughnesslessthan3nm͒andverticalsidewalls,andnearly90°corners.Thespacingbetweenthestripswasintentionallymadelargetoallowforexaminationofthesidewalls.TheterminalfaceofthePMMAstripsisnotfromcleaving,butdirectlyfromimprinting.Asshownlater,thesmallbendattheendofthePMMAstripsisactuallyduetocurvatureinthemold.B.Comparisonwithmold

Tocomparetheimprintedresistprofileandtheprofileofthemoldfeatures,weexaminedthemoldusingascanningelectronmicroscope͑SEM͒asshowninFig.3.ThePMMAprofileshowninFig.2comesfromtheclosedendofthemoldfingers;therefore,aprecisecomparisonbetweenthemoldshapeandthePMMAprofileisnotfeasible.However,comparisonofthegeneralfeatures,suchasthelinewidth,heights,andslightbendingattheendofeachline,indicatedthatthePMMAprofileconformedtothemold.C.EffectofRIEonlateraldimensionofimprintedPMMApatterns

ToexaminetheeffectsoftheoxygenRIEpatterntransfersteponremovingtheresidueresistinthecompressedareasandonchangingthelateraldimensionofthePMMAfea-tures,thePMMAresiststructurescreatedbyimprintlithog-raphywereusedasthetemplateforaliftoffofmetals.TheRIEprocesswasdonewithapowerof400Wandapressureof90mTorrusingoxygengas.Inthelift-offprocess,5nmTiand15nmAuwerefirstdepositedontotheentiresample,andthenthemetalonthePMMAsurfacewasremovedwhenthePMMAwasdissolvedinacetone.WecomparedtheSEMimageoftheimprintedPMMAtemplatebeforetheoxygen

4131Chou,Krauss,andRenstrom:NanoimprintlithographyFIG.4.SEMmicrographof25nmdiameterand120nmperiodmetaldotsfabricatedbyimprintlithographyandalift-offprocess.

RIEtransfersteptothatofthemetalpatternsaftertheliftoff.Figure4shows25nmdiamdotswitha120nmperiodliftedofffromthePMMAtemplateof25nmdiamholesmadebyimprintlithography.Figure5shows30nmline-widthand70nmpitchmetallinesliftedofffromaPMMAtemplatefabricatedusingimprintlithography.ComparingthesemetalfeatureswiththeimprintedPMMAtemplatesbeforeRIE,therearenonoticeabledifferencesbetweenthe

FIG.5.SEMmicrographof30nmwideand70nmperiodmetallinesfabricatedbyimprintlithographyandalift-offprocess.JVSTB-MicroelectronicsandNanometerStructures

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lift-offmetalstructuresandthePMMApatterns.Thisindi-catesthatduringtheoxygenRIEprocess,thecompressedPMMAareaswerecompletelyremovedwhilethelateralsizeofthePMMAfeaturesexperiencedlittlechange.D.Estimationofultimatelithographyresolution

Theminimumfeaturesizeofimprintlithographyshownintheprevioussectionislimitedbytheminimumfeaturesizeonthemold.Furtherexperimentshaveshownthatafewnanometervariationonthemoldcanbesuccessfullytrans-ferredintothesidewallsofthePMMA,asshowninFig.6͑a͒.Thismeansthatifthepolymerhassufficientmechani-calstrength,imprintlithographyshouldbeabletoproduce10nmfeaturesizeinthepolymer.

E.Processrepeatabilityandmolddurability

Imprintlithographyprocessrepeatabilityandmolddura-bilityaretwokeyissuesinmakingimprintlithographyamanufacturingtechnology.WehaveusedthesamemoldtoimprintPMMAover30timesandexaminedthemoldandthePMMAprofileeverytime.Wedidnotobserveanyno-ticeablechangesineitherthePMMAprofileorthemold.Althoughover30timesimprintingishardlyconsideredarepeatabilityanddurabilitytest,weshouldexpectthepro-cesstohaveagoodrepeatabilityandmolddurability.Thisisbecausemoldreleaseagentsgaveagoodrelease,thePMMAheldaboveglass-transitiontemperaturesisverysoft,andthemoldintrusiondoesnottouchthesubstrate.F.Uniformity

Toexaminetheuniformityofthisprocess,arrayswith30nmwidestripsanda150nmpitchwerefabricatedatthefourcornersandthecenterofamoldthathadasizeof15mmby18mm.AfterimprintlithographyinPMMA,alift-offpro-cessleft30nmwidemetallineswitha150nmpitchonthesubstrate,asshowninFig.7.Figure7clearlyshowsthateventhoughthepressweusedisveryprimitive,imprintlithographycanbeveryuniformoverasignificantlylargearea.Wearequiteconfidentthatwithabetterdesignedpress,gooduniformityoveramuchlargerimprintareacanbeachieved.

G.Imprintlithographyoveranonflatsurface

Thereareatleasttwowaystoapproachtheproblemofimprintlithographyoveranonflatsurface.Thebruteforcemethodistouseathickresist,createalargethicknesscon-trast,andetchthePMMAverydeepintheverticaldirection.AnexampleisgiveninFig.6wherea75nmstepinthesubstratewascoveredwitha300nmresist.Thena200nmthicknesscontractwascreatedinthePMMAandabout150nmofPMMAwasremovedduringthepatterntransfer.Asshown,the75nmstepcanbeseenclearlyafteretching.However,duetoprolongedetching,thelinewidthisreducedfrom60to40nm.

Abetterapproachwouldbetouseathickresisttocreateaflatsurfacefirst.Aftertheimprintstep,amaterialthatis

4132Chou,Krauss,andRenstrom:NanoimprintlithographyFIG.6.ThePMMAlinesimprintedovera75nmstep͑a͒beforeRIEpatterntransferand͑b͒after.Duetothedeepverticaletchrequired,thePMMAlinewidthwasreducedfrom60to40nm.

veryresistanttoRIEiscoatedonlyonthetopsurfaceoftheimprintedpattern.ThenthecoatedmaterialisusedasaRIEmaskintransferringthepatternintotheentireresist.Wearecurrentlydevelopingthistechnology.

H.Fabricationofnanodevicesusingimprintlithography

Inparallelwithdevelopingimprintlithography,wehaveusedimprintlithographytofabricatenanodevices.Oneex-J.Vac.Sci.Technol.B,Vol.14,No.6,Nov/Dec1996

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FIG.7.SEMmicrographsof30nmwidemetalgratingswitha150nmperiodfabricatedusingimprintlithographyandliftoff.Thefivepicturescomefromthefourcornersandthecenterofamoldthathasasizeof15mmby18mm.

ampleismetal–semiconductor–metalphotodetectorsfabri-catedusingimprintlithographyandopticallithography.Inaddition,wefabricatedquantum-wire,quantum-dot,andringtransistorsinsiliconusingimprintlithographyandRIEofsilicon.Quantumeffectsandsingleelectroneffectswereob-servedinthesedevices,whichwillbereportedelsewhere.10V.FUTUREIMPROVEMENTANDCHALLENGESNodoubt,imprintlithographyisstillatitsinfancyandfurtherinvestigationsareneededtomakeitamanufacturingtechnology.Currently,wehavenotfullycharacterizedandfullyunderstoodimprintlithography.Thepressweusedisratherprimitive.Thesurfacestickingproblem,whichhasbeengreatlyreducedinourcurrentwork,stillneedsmoreimprovement.Moldingconditionsarenotoptimizedyet.Theeffectofthermalexpansiononlithographyresolutionhasnotbeenstudied.Moldswithsmallerfeaturesizeareneededtoexploretheultimateresolution.Wealsoneedtoprovethattheareaforasingleimprintcanbemuchlargerthan1sqin.Finally,multilevelalignmentisoneofthebiggestchal-lenges.However,sincethefirstreportonimprintlithographyayearago,manygroupshavestartedlookingintothistech-nology.Weshouldexpectsignificantprogressinthenearfuture.

VI.CONCLUSION

Wehavedemonstratedthatimprintlithographycanachieve25nmfeaturesizeand70nmpitch,verticalandsmoothsidewalls,nearly90°corners,anduniformityoveranareaof15mmby18mminasingleimprint.Ourstudyindicatesthatimprintlithographycanpotentiallyhavea10nmresolutionoveranareamuchgreaterthan1sqin.,andcanhavegoodrepeatabilityanddurability.Therefore,im-printlithographyhashigh-throughputandlow-cost.With

4133Chou,Krauss,andRenstrom:Nanoimprintlithography

furtherdevelopment,imprintlithographycanbecomethetechnologyformanufacturingnanostructures,andcanhaveasignificantimpactinmanyareassuchasintegratedcircuits,biology,andchemistry.Nodoubt,thecurrentstudyofim-printlithographyispreliminary.Yet,thefutureofimprintlithographyseemsverypromising.

ACKNOWLEDGMENT

TheauthorswouldliketothankothermembersoftheNanoStructureLaboratorywhoseeffortshaveprofoundlyaf-fectedthecurrentwork.

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