StephenY.Chou,PeterR.Krauss,andPrestonJ.Renstrom
NanoStructureLaboratory,DepartmentofElectricalEngineering,UniversityofMinnesota,Minneapolis,Minnesota55455
͑Received17July1995;acceptedforpublication14September1995͒
Ananoimprintprocessthatpressesamoldintoathinthermoplasticpolymerfilmonasubstratetocreateviasandtrencheswithaminimumsizeof25nmandadepthof100nminthepolymerhasbeendemonstrated.Furthermore,theimprintprocesshasbeenusedasalithographyprocesstofabricatesub-25nmdiametermetaldotarraysofa100nmperiodinalift-offprocess.Itwasfoundthatthenanostructuresimprintedinthepolymersconformcompletelywiththegeometryofthemold.Atpresent,theimprintedsizeislimitedbythesizeofthemoldbeingused;withasuitablemold,theimprintprocessshouldmoldsub-10nmstructureswithahighaspectratioinpolymers.Thenanoimprintprocessoffersalowcostmethodformassproducingsub-25nmstructuresandhasthepotentialtobecomeakeynanolithographymethodforfuturemanufacturingofintegratedcircuitsandintegratedoptics.©1995AmericanInstituteofPhysics.
Thereisagreatneedtodeveloplow-costtechnologiesformassproducingsub-50nmstructuressincesuchtechnol-ogycanbringenormousimpacttomanyareasofengineeringandscience.Notonlywillthefutureofsemiconductorinte-gratedcircuitsbeaffected,butalsothecommercializationofmanyinnovativedeviceswhicharefarsuperiortocurrentdevicesishingedonthepossibilityofsuchtechnology.Scan-ningelectronbeamlithographyhasdemonstrated10nmli-thographyresolution.1,2But,usingitformassproductionofsub-50nmstructuresseemseconomicallyimpracticalduetoinherentlowthroughputinaserialprocessingtool.X-raylithography,whichcanhaveahighthroughput,hasdemon-strated50nmlithographyresolution,3butthex-raylithogra-phytoolsareratherexpensiveanditsabilityformasspro-ducingsub-25nmstructuresisyettobeseen.Furthermore,lithographiesbasedonscanningprobeshaveproduced10nmstructuresinaverythinlayerofmaterials.However,thepracticalityofsuchlithographiesasamanufacturingtoolishardtojudgeatthispoint.
Imprinttechnologyusingcompressionmoldingofther-moplasticpolymersisalowcostmassmanufacturingtech-nologyandhasbeenaroundforseveraldecades.Featureswithsizesgreaterthan1mhavebeenroutinelyimprintedinplastics.Compactdiskswhicharebasedonimprintingofpolycarbonateareoneexample.Otherexamplesareim-printedpolymethylmethacrylate͑PMMA͒structureswithafeaturesizeontheorderof10m͑Ref.4͒andmoldedpolyesterpatternswithfeaturedimensionsofseveraltensofmicrons.5However,ithasneverbeentestedwhethersub-25nmstructureswithhighaspectratioscanbeimprintedintopolymers.Furthermore,thepossibilityofreplacingthecon-ventionallithographiesusedinsemiconductorintegratedcir-cuitmanufacturingwithimprinttechnologywasneverraised.
Inthispaper,wewilldemonstratethatimprinttechnol-ogycanproduceviasandtrencheswith25nmminimumfeaturesizeand100nmdepthinthinpolymers,hasthepotentialtoproduce10nmstructures,andcanbeusedasthenanolithographyprocessinintegratedcircuitfabrication.
AsshowninFig.1,inthenanoimprintprocess,amoldis
3114
Appl.Phys.Lett.67(21),20November1995
pressedintoathinthermoplasticpolymerfilmonasubstratethatisheatedaboveitsglasstransitiontemperature.Abovethattemperaturethepolymerbehavesasaviscousliquidandcanflowunderapressure,therebyconformingtothemold.Themoldcanbemadeofmetals,dielectrics,orsemiconduc-tors.Inourexperiments,silicondioxidemoldswereusedexclusively.Themoldconsistsofathicklayerofsilicondioxideonasiliconsubstrate.Themoldispatternedwithdotsandlineswithaminimumlateralfeaturesizeof25nmusingelectronbeamlithographyandetched250nmintotheSiO2layerusingreactiveionetching͑RIE͒.
Thepolymerusedforthenanoimprintexperimentisa55nmthickPMMAfilmspunonasiliconwafer.PMMAwaschosenforseveralreasons.First,PMMAdoesnotadherewelltotheSiO2moldduetoitshydrophilicsurface.Goodmoldreleasepropertiesareessentialforfabricatingnano-scalefeatures.Second,theshrinkageofPMMAislessthan0.5%forlargechangesoftemperatureandpressure.6
Duringtheimprinting,boththemoldandPMMAwerefirstheatedtoatemperatureof200°CwhichishigherthantheglasstransitiontemperatureofPMMA,105°C.4ThenthemoldwascompressedagainstthesampleandheldthereuntilthetemperaturedroppedbelowthePMMA’sglasstransitiontemperature.Variouspressureshavebeentested.Itwasfoundthattheoptimumpressureisabout1900psi.Atthatpressure,thepatternonthemoldcanbefullytransferredintothePMMA.Atmuchlowerpressures,thepatterncannotbetransferredintothePMMAcompletely.Usinghigherpres-surecausedboththesubstrateandthemoldtobowinward,resultingincontactbetweenthePMMAandconcaveregionsoftheSiO2mold.Also,atsignificantlyhigherpressuresthenanoscaleSiO2featuresonthemoldwerefoundtobreakoff.
Figure2showsascanningelectronmicroscope͑SEM͒imageof25nmdiameterdotswitha120nmperiodim-printedintoaPMMAfilm.Wehavenotbeenabletodetectanyvariationbetweenmoldfeaturesizeandimprintedfea-turesizeduetoshrinkageofPMMA.Inadditiontothenanometerfeatures,themoldalsohasfeaturesaslargeastensofmicronswhichhavebeenimprintednicelyintothePMMAaswell.
©1995AmericanInstituteofPhysics
0003-6951/95/67(21)/3114/3/$6.00
Downloaded¬08¬Mar¬2009¬to¬210.13.109.85.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/apl/copyright.jsp
FIG.3.SEMmicrographofa60nmwidetrenchimprintedintoPMMA.ThePMMAlinesare100nmtall.
FIG.1.Schematicofnanoimprintlithographyprocess.
Figure3showsthecrosssectionofa60nmwidetrench100nmdeepimprintedintoaPMMAfilm.TheinitialPMMAfilmwas55nmthick,butmoldingdisplacedPMMAresultinginathickerPMMAfilmoutsideoftheviasandtrenches.Moreover,PMMAlineswith50nmwidth,175nmspacingand150nmheighthavebeenimprinted,butfelloverduetopooradhesiontothesiliconsubstrate.Nonetheless,SEMexaminationshowsthatthesidewallsofimprintedPMMAlinesconformwiththemoldandarethereforeverystraight.Fromourobservations,itisclearthatthefeaturesizeimprintedislimitedbyourmoldsize.FromthetextureoftheimprintedPMMA,itappearsthat10nmfeaturescanbeimprinted.
ThePMMAfilmpatternedwithnanoimprinttechnologywasusedtoreplacethenanolithographyinthefabricationofnanoscalemetalfeaturesthroughalift-offtechnique.AfternanoimprintofPMMA,O2RIEwasusedtoremove10nmofthePMMAtomakesurethebottomsoftheimprintedviaandtrenchregionswerefreeofanyPMMAresidue.Next,5nmoftitaniumand15nmofgoldwereevaporatedontothewafers.ThewaferswerethensoakedinacetonetoliftoffmetalswhichwereontheremainingPMMA.Figure4showsanSEMmicrographof25nmdiameterTi/Audotswitha120nmperiodwhichwerefabricatedusingnanoimprintli-thography.Dotarrayswith100nmperiodhavealsobeenfabricated,buthavea5nmvariationindotsizeduetovaria-tionsofthemold.
Theuseofnanoimprintasalithographyprocessisverysignificant.First,asdiscussedpreviously,thereisnootherlow-costandhighthroughputlithographytechnologywiththeresolutionofthenanoimprintlithography.Second,nanoimprintlithographywillnothavethebackscatteringand
FIG.2.SEMmicrographofdotpatternimprintedintoPMMA.Thedotshavea25nmdiameterand120nmperiod.
Appl.Phys.Lett.,Vol.67,No.21,20November1995
FIG.4.SEMmicrographofTi/Audotpatternonasiliconsubstratefabri-catedusingnanoimprintlithographyandalift-offprocess.Thedotshavea25nmdiameterand120nmperiod.
Chou,Krauss,andRenstrom
3115
Downloaded¬08¬Mar¬2009¬to¬210.13.109.85.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/apl/copyright.jsp
interferenceproblemsencounteredinotherlithographieswhichdegradetheresolution.Therefore,nanoimprintlithog-raphyissuitablefornanolithographyonhighatomicnumbermaterialsubstrates.Webelievethatwithaproperselectionofthepolymerandmoldmaterialsandanoptimizationofthepressingconditions,thestickinganddefectproblemsassoci-atedwiththetraditionalcontactprintingcanbeavoided,makingthenanoimprintlithographyaviablemanufacturingtechnology.
Insummary,wehavedemonstratedananoimprintingtechnologythatcanimprintsub-25nmviasandtrenchesinaPMMAfilmusingnanoimprintlithographywithaSiO2mold.Nanoimprinthasreplacedlithographytoliftoffarraysof25nmdiameterTi/Audots.Thenanoimprintprocessshouldbeabletoproduce10nmfeatureswithhighaspect
ratiosandprovideauniquelowcosttechnologyinmassproducingsub-25nmstructures.Finally,nanoimprintlithog-raphyhasthepotentialtobeusedasakeynanolithographyprocessinfutureintegratedcircuitsandintegratedoptics.
1
A.N.Broers,J.M.Harper,andW.W.Molzen,Appl.Phys.Lett.33,392͑1978͒.2
P.B.FischerandS.Y.Chou,Appl.Phys.Lett.62,2989͑1993͒.3
K.Early,M.L.Schattenburg,andH.I.Smith,MicroelectronicEng.11,317͑1990͒.4
M.Harmening,W.Bacher,P.Bley,A.El-Kholi,H.Kalb,B.Kowanz,W.Menz,A.Michel,andJ.Mohr,inProceedings,IEEEMicroElectroMe-chanicalSystems͑1992͒,p.202.5
H.LiandS.D.Senturia,inProceedingsof199213thIEEE/CHMTIn-ternationalElectronicManufacturingTechnologySymposium͑1992͒,p.145.6
I.Rubin,InjectionMolding͑Wiley,NewYork,1972͒.
3116Appl.Phys.Lett.,Vol.67,No.21,20November1995Chou,Krauss,andRenstrom
Downloaded¬08¬Mar¬2009¬to¬210.13.109.85.¬Redistribution¬subject¬to¬AIP¬license¬or¬copyright;¬see¬http://apl.aip.org/apl/copyright.jsp
因篇幅问题不能全部显示，请点此查看更多更全内容