光刻胶.docx

光刻胶.docx

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光刻胶

HeaderforSPIEuse

157nmImagingUsingThickSingleLayerResists

MichaelK.Crawforda,AndrewE.Feiringa,JeraldFeldmana,RogerH.Frencha,ViacheslavA.Petrova,

FrankL.SchadtIIIb,RobertJ.Smalleya,FredrickC.Zumstega

aDuPontCentralResearchandDevelopmentandbDuPontiTechnologies

Wilmington,DE19880-0356

ABSTRACT

Duringthepastyeartheprobabilitythat157nmlithographywillprecedenextgenerationlithographiessuchasEUVorEPL

hasincreased,partlyduetoencouragingadvancesinthedesignofpolymericmaterials,whichhavesufficienttransparency

at157nmtoserveasplatformsforsinglelayerphotoresists.Wehaveidentifiedseveralfluorinatedresinswhichcanbe

developedinaqueous0.26NTMAH,havereasonableetchresistances（comparabletopoly-parahydroxystyrene）,andcan

beformulatedtoyieldphotoresistswithopticalabsorbanciesat157nmwhicharelowenoughtobeusedatthicknessesof

150-200nm.Wehaveimagedanumberoftheseformulatedresistsat157nmwiththeExitechmicrostepperat

InternationalSematech,andtheresultsforformulatedresistswithopticalabsorptioncoefficients（base10）aslowas2.1per

micronaredescribed.

Keywords:

157nmphotoresists,fluoropolymers,etchresistance,transparency,tetrafluoroethylene

1.INTRODUCTION

Increasingly,157nmlithographyappearstobeaviablestepinthecontinuousevolutionofopticallithography.Originally

plannedforthe100nmnode,itnowappearsthat157nmlithographywillbeintroducedat70nmfeaturesizeandwillbe

useddowntothe50nmnode.Atwavelengthsshorterthan157nmthedifficultyofdevelopingmaterialswithsuitable

opticalpropertiesincreasesrapidlywithdecreasingwavelength,soitmaybethat157nmlithographywillbethefinalstep

intheverysuccessfulhistoryofopticallithography.

Theprimarychallengeinthedesignofchemically-amplifiedresistresinsforuseat157nmisthatoffindingmaterials

whichsimultaneouslypossessthefollowingthreeproperties:

goodopticaltransparency,suitablesolubilityinaqueousbase

afterdeprotection,andgoodetchresistance1.Onesolution,suggestedearlyinthedevelopmentof157nmlithography,was

toturntofluoropolymersastheresistresinsforsinglelayerresists（SLR）,sincesomeexamplesofsuchmaterialswere

foundtobesurprisinglytransparentat157nm2.Ourgoalistodevelopphotoresists,utilizingDuPont’sexpertiseinfluorine

chemistry,whichhaveopticalabsorbancesintherangeof0.5-1.5µm-1,yieldingopticaldensitiesof0.1-0.3foraresist

thicknessof200nm.Herewewilldescribeourprogressdesigningfluoropolymerstoachievethisgoal.

CopolymersbasedonTFEareoneexampleoffluorinatedresinsthat,whensuitablyfunctionalizedandformulated,can

serveas157nmresists3.Thesecopolymersaretypicallyoflowmolecularweight（Mn~2,000–6,000）andapproximately

alternating.Theseandrelatedfluoropolymersarereadilysolubleinorganicsolvents,havehighglasstransition

temperatures,havegoodplasmaetchresistance,andmostimportantlyhavegoodopticaltransparencyat157nm.

Introducingfunctionalitiestoimpartdevelopabilityinaqueousbasegenerallydecreasesetchresistanceandincreasesoptical

absorptionat157nm.Hereweshowthatfluoropolymerscanbesynthesizedandformulatedtobesolubleinaqueousbase

afterdeprotection,whilemaintainingtheopticalabsorptioncoefficientoftheformulatedresistatvaluesaslowas~2.1µm-

1.Furthermore,webelievethatevenloweropticalabsorptionvaluesforformulatedresistsarepossible.Finally,wehave

imagedtheseresistsat157nm,andtheresultsstronglysuggestthat200nmSLRresistsfor157nmlithographyareindeed

achievableusingfluoropolymerresins.

2.1Polymersynthesis

2.EXPERIMENTAL

Caution!

Oneofthemonomersusedtosynthesizethematerialsdescribedbelowistetrafluoroethylene（TFE）,adeflagrating

explosiveandanexperimentalcarcinogen.AllsyntheticworkwithTFEdescribedinthisreportwasconductedwithin

completelybarricadedandventilatedfacilities.

Twobasicpolymerplatformshavebeensynthesizedandstudied.Bothincludethehexafluoroisopropanolfunctionalityto

impartaqueousbasesolubility.ThefirstplatformincludesTFEasacomonomer,comonomerscontainingpolycyclic

entitiestoimproveetchresistanceaswellasothercomonomerstoaffectadhesionandimagingperformance.TheTFE

copolymersaresynthesizedusingstandardfreeradicalpolymerizationmethods.Thesecondplatformiscomposedof

copolymerscontainingprotectedandunprotectednorbornene-fluoroalcohols.Thesepolymersaresynthesizedbymetal-

catalyzedvinyladditionpolymerization.

2.2Resistformulation

Formulationsolventsinclude2-heptanone,PGMEA（propyleneglycolmethyletheracetate）,andcyclohexanone.Standard

oniumphotoacidgenerators（PAGs）wereused.

2.3Opticalproperties

Vacuumultraviolet（VUV）transmissionmeasurementsweremadeusingaMcPhersonspectrometerequippedwitha

deuteriumlamp.Eachresistsamplewasspin-coatedatseveralthicknessesonSisubstratestodeterminetheresistspin-

curve.CaF2substrateswerethenspin-coatedattheappropriatespeedstoachieveresistthicknessesbetween50and200nm.

TheVUVtransmissionspectraofthesesampleswerethenmeasuredandareplottedinthispaperasabsorbance（base10）

normalizedbythefilmthickness.

SpectralellipsometrymeasurementsweremadeatInternationalSematechusingaWoollamVUVVASE.Resistsamples

werespin-coatedonSisubstratesforthesemeasurements.RefractiveindicesandCauchycoefficientswerecalculatedfrom

theellipsometrydataandusedtodetermineresistfilmthicknesses.

Theabsorptioncoefficientsmeasuredbydirecttransmissionandspectralellipsometryweregenerallyingoodagreement（at

leastfortherangesofabsorptioncoefficientsandresistfilmthicknesseswedescribehere）.

2.4Dissolutionandimaging

Dissolution（contrast）curvesweremeasuredusingopenframeexposuresmadewiththeExitechmicrostepperatSematech.

Sampleswerespin-coatedonSiwafersandpost-applybaked（PAB）at120ºCfor120sec.An11x11matrixofexposure

doseswasthenmade,afterwhichthewaferwaspost-exposurebaked（PEB）atvarioustemperatures,followedbypuddle

developmentusingShipleyLDW-26®.Thethicknessofremainingresistwasmeasuredforeachexposuredoseusinga

PrometrixSM300reflectometer.Thesedatawereusedtogeneratecontrastcurvesanddevelopmentratesforthevarious

resistformulationstested.

157nmimagesweremadeusingtheExitech/TropelstepperatInternationalSematech.ThestepperhasaNAof0.6andaσ

of0.7whenusedwiththebinarymask.Imagesusinganalternatingphaseshiftmaskwereobtainedwithaσof0.3.Focus-

exposurematricesweregeneratedforeachresist,andtheresultingimageswereobservedusingaJEOLtiltSEM,andKLA-

Tencortop-downSEMs.Cross-sectionswerealsoobtainedatSematechforselectedwaferregions.Standard0.26N

tetramethylammoniumhydroxidedeveloperwasusedforresistprocessing.

2.5Outgassing

Outgassingmeasurements4forseveralprototypicalfluoropolymerresin-basedresistsweremeasuredatMITLincolnLabs

byirradiatingresistfilmswith157nmexcimerlaserlightandcollectinginacoldtrapanyoutgassedproductsforatimeof

10minutesduringandafterirradiation.Thecollectedgaseswerethenanalyzedbymassspectroscopy.

3.1Opticalproperties

3.1.1VUVabsorptionofresins

3.RESULTSANDDISCUSSION

TheVUVtransmissionofaresinusedinaphotoresistisimportantforseveralreasons.First,theresinisthemajor

componentintheformulatedresistandthereforeprovidesthelargestcontributiontototalopticalabsorption.Second,

opticalabsorptionbytheresindoesnotleadtousefulphotochemistry（excludingthepossibilityofenergytransferfromthe

photoexcitedresintothePAG）andthusservestodecreasetheresistsensitivity.Third,lightabsorptionbytheresinwill

contributetothedegradationofsidewallangles.Finally,157nmphotonsabsorbedbytheresinmaygenerateunwanted

photochemistrysuchascross-linkingreactions,orbondscissionsleadingtoresistoutgassing5.

WehavereportedthatTFEcopolymerscanhaveexcellenttransparencyat157nm3.Ofcourse,suchpolymersmustbe

suitablyfunctionalizedinordertoserveaschemicallyamplifiedphotoresists.For193nmresists,carboxylicacidsand

estershavegenerallybeenusedforthispurpose6.Onewouldexpecttheincorporationofcarboxylicacidgroupstoexerta

negativeimpactupontheopticaltransmissionat157nmsincethesegroupscontaincarbon-oxygendoublebondswhichare

knowntoabsorbatwavelengthsshorterthan200nmduetothepresenceofπ→π*transitions（thelowerenergyoxygen

lonepairtransition,np→π*,typicallyappearsatwavelengthsbetween200and300nm）,σ→π*transitions,andRydberg

transitionsforwhichalonepairoxygenelectronina2porbitalispromotedtohigheroxygenatomicorbitals（suchas3s,3p,

3d,4s,4p,4d,etc.）7,8.Otheracidfunctionalitieswhichhavebeenreportedintheliterature,suchashexafluoroisopropanol

groups9,arefullysaturatedandarethereforeexpectedtoabsorbatshorterwavelengthsthan157nmsincetheydonothave

π→π*transitions.Furthermore,theacidstrengthsofcarboxylicacids（pKa~5）andfluoroalcohols（pKa~9）arealsovery

different.Theseprofounddifferencesinopticalpropertiesandacidityareofobviousimportanceinthedesignof157nm

resins.

InFigure1weshowtheopticalabsorbancespectraoftworesistresins（i.e.resinsthatwhenformulatedwillimageat157

nm）: