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Preparationandpropertiesofnano-silicamodifiednegativeacrylate
photoresist
Chih-KangLee,Trong-MingDon,Wei-ChiLai,Chin-ChungChen,Dar-JongLin,
Liao-PingCheng⁎
DepartmentofChemicalandMaterialsEngineering,TamkangUniversity,Taipei,25137,TaiwanReceived30May2007;receivedinrevisedform25February2008;accepted7April2008
Availableonline12April2008
Abstract
Aseriesofsilicamodifiedphotoresistshadbeensuccessfullydevelopedthroughincorporationofaparticularnanoparticlessuspension.Freeradicalpolymerizationwasemployedtosynthesizethebinder,anacrylatecopolymerresinofbenzylmethacrylate,methacrylicacidand2-hydroxyethylmethacrylate,ofthephotoresist.Theacidvalue,viscosity,molecularweightandthermalpropertiesoftheformedbindersweremeasured.Then,surface-modifiedsilicaparticlespreparedbythesol–gelmethodwereintroducedtothephotoresist.Becausethemodifiedsilicaparticlescontainedconsiderableamountofreactivedoublebonds(CfC)ontheirsurfaces,theywouldreactwiththepolyfunctionalmonomersinthephotoresisttoformanorganic–inorganicnanohybrid.Fouriertransforminfraredspectrometerwasusedtoanalyzetheevolutionofchemicalbondsatvariousstagesofthepreparationprocess.Thermalanalysesincludingthermalgravimetricanalyzer,differentialscanningcalorimeterandthermo-mechanicalanalyzerwereusedtoevaluatethelevelofenhancementonthermalanddimensionalstabilitiesofthephotoresistduetosilicaincorporation.
©2008ElsevierB.V.Allrightsreserved.
Keywords:Photoresist;Binder;Sol–gelmethod;Organic–inorganicnanohybrid
1.Introduction
Photoresistsarewidelyusedforthemanufactureofmicroelectronics,silkscreenprintings,printedcircuitboards,opticaldisks,colorfilterresistsandsoon.Therecipeofacommonnegative-typephotoresistconsistsofabinder,photo-sensitivepolyfunctionalmonomer,photoinitiator,solventsandpigments[1–5].Foraphotoresistappliedincolorfilter,thebinderplaysavitalrole,asitdeterminestheadhesionstrength,hardness,heatresistance,anddevelopabilityofthephotoresist[6].Inthisresearch,aseriesofbinderswassynthesizedfromfreeradicalcopolymerizationofthreekindsofacrylatemonomersbenzylmethacrylate(BZMA),methacrylicacid(MAA),and2-hydroxyethylmethacrylate(2-HEMA).ThesyntheticrouteofthesebindersisoutlinedinScheme1.Thethermalbehaviors,molecularweights,andacidvaluesofthepreparedbinderswere
⁎Correspondingauthor.Tel.:+886226215656x2614or2725;fax:+886226209887.
E-mailaddress:lpcheng@mail.tku.edu.tw(L.-P.Cheng).0040-6090/$-seefrontmatter©2008ElsevierB.V.Allrightsreserved.doi:10.1016/j.tsf.2008.04.051
measuredandcomparedsoastooptimizethethermalmechanicalandchemicalstabilitypropertiesofthephotoresist.Organic–inorganichybridmaterialshavebeenextensivelyinvestigatedinrecentyears[7–13].Organicpolymers,ascharacterizedbygoodflexibility,ductilityandprocessability,havelongbeenappliedinvariousindustries.Incontrast,inorganicmaterialspossessproperties,suchashighrigidity,mechanicalstrength,andthermalstabilitythatarenotachievablebypolymers.Combinationoftheadvantagesofthesetwoclassesofmaterialstoyieldacompositewithsuperiorpropertiesthatmeettheindustrialstandardsisnowadayshighlydemanded.Topreparesuchorganic–inorganichybridmaterials,amodifiedsol–gelprocessisgenerallyused.Sol–gelprocesswasoriginallydevelopedforthelow-temperaturesynthesisofglassorceramicmaterials,whichinvolvesconsecutivehydro-lysisandcondensationofanalkoxysilanetoformsiliconeoxideparticlesthatsuspendinanaqueous/alcoholicsolution;e.g.,silicacanbeformedfromhydrolysisandcondensationoftetraethoxysilane(TEOS).Therelatedtheoriesarebuiltuponcolloidalscience,andthebasicprinciplescanbefoundin
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Scheme1.Preparationofpoly(BZMA–MAA–2-HEMA)byfreeradicalpolymerization.
previousliterature[14,15].Furthermore,thecompositematerialisopticallytransparent,andisthereforesuitedtooptoelectronicapplications[16–18].
Inthisresearch,anano-silicamodifiedphotoresistwasdevelopedbyintroductionthroughcovalentbondingofsurfaced-modifiedsilicaparticlesintothepolymermatrixofthephotoresist.Themodifiedsilicaparticlescontainedconsiderableamountofreactivedoublebonds(CfC)ontheirsurfaces,whichwouldreactwiththepolyfunctionalmonomersinthephotoresisttoformanorganic–inorganichybridduringtheUV-curingstepforphotoresistprocessing.Therouteforthesynthesisofsurface-modifiedsilicaparticlesisoutlinedinScheme2.Tsushimaetal.[19]demonstratedthatthephotoresistconsistingofpolysilaneandsilicagelexhibitedhigherheatresistancethanthosewithoutusingsilica.Siliantypecouplingagentsusedforimprovingtheadhesionandhardnessonglasssubstratewerereportedintheliterature[4].Inthecaseof
conventionalacrylicphotoresistappliedincolorfilter,therequiredresolutionandheatstabilitywere10∼20μmand220∼300°C,aswasreportedbySabnisetal.[6].Theorganic–inorganichybridphotoresistdevelopedinthepresentworkdemonstratedimprovedmechanicalstrength,thermalanddimensionalstabilitycomparedwithconventionalorganic-basedphotoresists,whileholdingtheresolutionb10μm.Theresolution,heatresistance,hardness,andadhesionmeettheindustrialstandardsofphotoresistsappliedincolorfilters.2.Experimentaldetails2.1.Materials
Materialsforthepreparationofpoly(acrylate)/SiO2nano-particlesphotoresistincludesBZMA,MAA,2-HEMA,2,2′-azobisisobutyronitrile(AIBN),1-dodecanethiol
Scheme2.Preparationofsilicasol(a)andMSiO2nanoparticlessuspension(b)byhydrolysisandcondensationfromTEOSandMSMA.
C.-K.Leeetal./ThinSolidFilms516(2008)8399–84078401
(Thiol),tetraethoxylsilane(TEOS),3-(trimethoxysilyl)propylmethacrylate(MSMA),2-propanol(IPA),ethyl3-ethoxypro-pionate(EEP),cyclohexanone(ANONE),propyleneglycolmethyletheracetate(PMA),anddipentaerythritolhexaacrylate(DPHA).AllofthemarereagentgradeandpurchasedfromAldrichChemicalCo.,USA.Photoinitiators,α-aminoketone(Irgacure369)andisopropylthioxanthone(ITX),weresuppliedbyCiba-GeigyLtd.Allmaterialswereusedasreceived.2.2.Preparationofbinders
ThesolutionsofbinderwerepreparedbyfreeradicalcopolymerizationofthreekindsofacrylatemonomersBZMA,MAA,and/or2-HEMAincyclohexanone(cf.,Scheme1;solidcontent=40wt.%).Appropriateamountsofthemonomersweredissolvedinthesolvent;thecompositionsareshowninTable1.Thesolutionwaspreheatedat100°Candstirredundernitrogenatmosphere.Theinitiator(AIBN,1wt.%ofmonomers)andchain-transferagent(Thiol,1wt.%ofmonomers)werealsodissolvedinthesamesolventbeforedroppingintothemonomersolution.Thefreeradicalpolymerizationwascarriedoutfor6h,andthentheproductwascooledtoroomtemperature.Theobtainedacrylateresin,i.e.,poly(BZMA–MAA–2-HEMA)dissolvedincyclohexanone,wasusedasthebinderofthephotoresist.Topreparesamplesforthermalpropertymeasure-mentsandchemicalstructureidentifications,thebindersolutionwasdilutedwithtetrahydrofuran(THF)andthenpouredintohexanetoinducepolymerprecipitation.Theprecipitationprocedurewasrepeatedtwicetoensurefreeofresidualmonomers.Finally,thepolymericwhite-powdersweresepa-ratedbyfiltrationandweredriedinvacuoat60°C.ThecompositionsforvariousbindersarelistedinTable1.2.3.PreparationofMSiO2nanoparticlessuspensionandphotoresists
ThesilicasolwaspreparedbyhydrolysisandcondensationofTEOSinIPAatroomtemperature.ThemolarratioofIPA/TEOSwas1.1,andhydrochloricacidaqueoussolutionatpH1.3wasusedasacatalyst.Thereactantswerestirredfor3h,andthenthecouplingagent,MSMA,withmolarratioofTEOS/MSMA=4.5,wasdroppedintothesilicasol.Afterreactionforanother3h,surface-modifiedsilica(calledMSiO2)solwasobtained.Toremovetheresidualbyproducts(e.g.,ethanol,methanol,IPA,waterandHCl),thepreparedMSiO2solwas
Table1
CompositionsandpropertiesofthesynthesizedbindersBindercode
Composition(mol%)BZMA
B80M20H0B72M18H10B64M16H20B56M14H30B63M27H10B54M36H10
amixedwithEEP,usedasadispersantforMSiO2nanoparticles.Aftervacuumdistilledat30°Cfor1h,atransparentMSiO2suspensionwasobtained(cf.,Scheme2).
Subsequently,anegativephotoresistwaspreparedbyaddingappropriateamountsofMSiO2nanoparticlessuspensiontotheconventionalpolymeric-photoresist.Theorganic–inorganichybridphotoresistwascomposedofbinder(8wt.%),polyfunctionalmonomerDPHA(8wt.%),photoinitiatorandphotosensitizer(0.6wt.%,Irgacure369/ITX=5),andvariousratiosofMSiO2suspension(5–20wt.%oftheorganicmoietyofphotoresist)mixedinatri-solvent.Thefinalcompositionofthetri-solventwasadjustedtoANONE/EEP/PMA=0.87/1/1inmolarratio.Theviscosityofphotoresistcouldbeadjustedbytheamountoftri-solventtogiveasatisfactorycoatability.2.4.Characterization
Severalmethodswereadoptedtocharacterizethebindersandthephotoresists:
1.Gelpermeationchromatography(GPC,Waters1515Iso-craticHPLCPump)wasperformedwithTHFasthemobilephaseat40°Cwithapolystyrenegelcolumn(AMGPCGel).Molecularweightsofthebindersarereportedbasedonpolystyrenestandards.
2.InfraredabsorptionspectraofthebinderandMSiO2solsweretakenusingaFouriertransforminfraredspectro-photometer(Nicoletspectrometer550,USA).PowdersofbinderweregroundwithKBrtoformadisc(1:50)forFTIRscanning.BothsilicaandMSiO2sampleswerepreparedbydroppingappropriateamountsofthesolsontoaKBrdisc,andthenthesolventwasevaporatedat80°Cfor10min.Forallscans,thespectrawerecollectedoverthewavenumberrangeof400–4000cm−1witharesolutionof4cm−1.
3.Acidvaluesofthepreparedbindersweredeterminedbytitrationusinga0.1mol/dm3potassiumhydroxide(KOH)solution.ThedefinitionofacidvalueistheamountofKOHinmilligramsneededtoreachtheequivalentpointfor1gofthepreparedbinder(mg/g).
4.Thermalgravimetricanalyzer(TGA),Hi-ResTGA2950fromTAinstrumentsLtd.,USA,wasusedtomeasurethethermaldecompositiontemperature(Td)ofthebinderandthecuredfilmsofthephotoresists.Samples(8–12mg)wereheatedfromroomtemperatureto600°Cwithaheatingrateof10°/minundernitrogenflow.
MAA201816142736
2-HEMA01020301010
Averagemolecularweight(Mw)2.30×1042.19×1042.16×1042.05×1042.11×1041.99×104PDIThermalpropertiesTg(°C)
Td(°C)331.6328.8314.4307.9329.7329.8
Acidvaluea76.270.263.358.1108.1152.3
8072645663541.191.231.201.181.161.1687.687.186.586.299.9120.4
MilligramsofKOH/gofbinderpolymer.
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5.Differentialscanningcalorimeter(DSC)wasemployedtomeasuretheglasstransitiontemperature(Tg)ofthebinderandthefilmsofphotoresist.DSC,model2010,TAInstrumentLtd.,USA,wasfirstcalibratedwithindiumstandardbeforerunningthetests.Anappropriateamountofadriedsamplewassealedinanaluminumpanandplacedintheheatingchambertogetherwithanemptyreferencepan.Thetemperaturewasraisedfrom50°Cto170°Cataconstantrateof20°C/minundernitrogenflow.Tgofthesamplewasdeterminedfromthethermogramofthesecondheatingcycle.6.TheparticlesizeofMSiO2wasobtainedfromthedynamiclightscatteringmeasurementusingaZetasizer(Malvern,DTS1060)at25°C.Theinstrumentwasequippedwithamonochromaticcoherentheliumneonlaser(633nm)asthelightsource.A4mlMSiO2solsolutionwasinjectedintothequartzcuvetteandthescatteredlightwasrecordedatanangleof90°.
7.Thethermaldimensionalstabilityofthepreparedfilmswasstudiedwithathermo-mechanicalanalyzer,TMA,modelTMAQ400fromTAinstrumentsLtd.,USA.Thecoeffi-cientsofthermalexpansion(CTE),α1andα2,wereobtainedbymeasuringthelineardimensionalvariations(ΔL)atdifferenttemperatures.TheTgofasamplewasidentifiedastheinterceptofthetwotangentlineswhereachangeinslopeoccurred,i.e.,aboveandbelowTg.Samples(4×8mm)wereheatedfrom30°Cto180°Cwithaheatingrateof10°C/mininanitrogenatmosphere.
8.Tapetest,alsocalledpeeltest,wascarriedouttoevaluatetheadhesionofthefilmscoatedonaglasssubstrate.Thedegreeofadhesionbetweenthefilmandtheglasssubstrateincreasesasthepercentageoftheresidualfilmontheglassafterthetapetest[20].Thehardnessoftheformedfilmswasexaminedbytheindustrialpencilhardnesstestwithpencilsofdifferenthardness[21].
9.Resolutionofthenegative-typephotoresistwasdeterminedbyexaminingthedevelopedcircuitdiagramundermicro-scope.Theexposure-developmentprocedurewasasfollows:a)Thenegative-typephotoresistwasuniformlyspreadonaglasssubstrateusingaspin-coator(1500rpm),andthenprebakedat80°Cfor5min.
b)FollowingUVirradiationunderaphotomaskat250mJ/cm2(UVsource:broadband,235–400nm,localmaximaat257,313,and365nm,GroupUpInd.Co.,Japan),itwasthendevelopedwithadeveloperliquid,IPA/H2O/Na2CO3=3/2/0.02.
c)Beforeusingopticalmicroscopetoexaminetheresolution,thephotoresistwaspostbakedat200°Cfor1htoobtainacuredcoating(about1–2μminthickness).3.Resultsanddiscussion3.1.Characterizationofbinders
3.1.1.Molecularweightmeasurements
Inthisstudy,AIBNwasusedasinitiatorandThiolwasusedaschain-transferagentforbindersynthesis.Theycontributednotonlytocontrolthemolecularweightofthesynthesized
copolymerbutalsotogiveabinderwithrelativelynarrowpolydispersity[22].Ifthemolecularweightofthepreparedbinderwastoosmall,thedevelopabilityofthephotoresistwouldbeexcessivelyrapid.Thismadeitdifficulttocontrolpatternshapeduringthedevelopmentprocess.Evenifpatternscouldbeformed,problemssuchasareductionoffinalcoatingthicknesscouldoccur.Onthecontrary,asthemolecularweightofthebinderwastoolarge,theviscosityofthephotoresistsolutionwastoohighandthiscouldaffectthecoatability.Furthermore,thedevelopabilitymightalsobedeterioratedmakingitdifficulttoformsharppatterns[1].Theweightaveragemolecularweight(Mw)andpolydispersityindex(PDI)valuesofthepreparedbinderswasmeasuredusingGPC,aslistedinTable1.Mwofallpreparedbinderswasfoundtoberangingfrom2.0×104to2.3×104g/mole,andPDIfrom1.16to1.23.ItindicatesthatbothAIBNandthiolcontentsrestrictedto1wt.%ofmonomerswerereasonableinthepresentsynthesisprocess.
3.1.2.Thermalproperties
3.1.2.1.Glasstransitiontemperature.Glasstransitiontem-peratures(Tg)ofpurepoly(benzylmethacrylate),poly(2-hydroxyethylmethacrylate)andpoly(methacrylicacid)are54,55and166°C,respectively,asobtainedfromtheliterature[23,24].AttemptshavebeenmadetomeasureTgofthepreparedbindersbymeansofDSC.ThethermogramsofthepreparedbindersareshowninFig.1.ItappearsthatTgofthebindersfromB80M20H0toB56M14H30(BZMA/MAA=4,molarratio)wereverycloseandrangedfrom86to88°C,asindicatedinTable1.Thevalueswereverycloseevenwhenthe2-HEMAcontentwas30mol%.However,TgoftheB80M20H0stillslightlyhigherthanthatofB56M14H30,ca.1.4°C,duetothehighercontentofMAA.Ontheotherhand,as2-HEMAcontentwaslimitedto10mol%,e.g.,B72M18H10,B63M27H10andB54M36H10,TgincreasedobviouslywithincreasingMAAcontent,inaccordancewiththeresultsfromliterature[25].3.1.2.2.Thermaldegradation.TGAwasutilizedtomeasurethethermaldecompositionbehaviorsofvariousbinders.Fig.2
Fig.1.DSCthermogramsofthepreparedbinders.
C.-K.Leeetal./ThinSolidFilms516(2008)8399–84078403
Fig.2.TGAthermogramsofthepreparedbinderpolymers.(a)2-HEMAcontentis10mol%;(b)molarratioofBZMA/MAAis4.
showsTGAthermogramsofthepreparedbinders,inwhichthermaldecompositionofthepreparedbindersgenerallyfollowsathree-stagepattern.Forallbinders,thefirst,secondandthethirdstagedecompositionsoccurredatabout180–270°C,270–370°Cand370–470°C,respectively.TheweightlossinthefirststagecouldbeattributedtothereactionsofMAAwithadjacentMAAor2-HEMAmonomerunit,forwhichwaterwaslostfromtheformationofanhydrideorestergroup,aspointedoutbyMansuretal.[23,25].Becausetherangesofthermaldecom-positionsofpoly(2-HEMA)andpoly(BZMA)were290–400°Cand270–370°C,respectively,thesecondstagedecompositionwasattributedtoboth2-HEMAandBZMAsegments.There-fore,theweightlossofthebindercontaining72%BZMA,B72M18H10,inFig.2(a)wasmostseriousoverthetemperaturerangeof270–370°C.Forthethirdstagedecomposition,theweightlosscouldlargelybeattributedtotheMAAunits(thethermaldecompositionrangeofpoly(MAA)is370–470°C)inthepreparedbinders.ItwasfoundthatthethirdstagedegradationtemperatureincreasedastheMAAcontentincreased.Thisisprobablyduetotheincreasingcontentofanhydridegroupsformedinthefirststageofdegradation.Fig.2(a)demonstratesthatthebinderB54M36H10,MAAcon-tent=36%,hasthemostweightlossofthefirststageandthehighestdegradationtemperatureofthethirdstage.Fig.2(b)
showsthattheextentofthefirststagedecompositionincreasedasthetotalamountofboth2-HEMAandMAAcontentincreased(dehydrationreaction);furthermore,thedecomposi-tionextentofthebinder“B56M14H30”wasthehighestoverthetemperaturerangeof290–400°Cduetoitshigh2-HEMAcontent.Becausethermaldecompositionbehaviorsofthepreparedbinderswerecomplicateandinvolvedthreestages,thetemperaturecorrespondingto10wt.%losswasidentifiedasthethermaldegradationtemperature,Td,foreasycomparison.TheresultsarelistedinTable3.
3.1.3.Chemicalstructurecharacterization
Fig.3(a)illustratestheFTIRspectrumofthepreparedbinder,B54M36H10.Theabsorptionpeaksat3500and3261cm−1wereduetothe–OHstretchingbandofhydroxylgroupandcarboxylgroup,respectively.ThefC\\Hstretchingbandofthearomaticring(BZMAunit)wasobservedfrom2950to3065cm−1,andtheCfOstretchingbandofthecarbonylgroupat1729cm−1(MAAunit)and1708cm−1(2-HEMAunit)couldalsobeobserved.Forthe2-HEMAunit,thestretchingbandoftheestergroup(C\\O\\C)wasobservedat1146cm−1.Furthermore,twoC\\Hvibrationbandsofmonosubstitutedbenzene(BZMAunit)wereobservedat701and748cm−1.Thespectrumsuggeststhatthethreekindsofacrylatemonomerswerelinkedtogethertoformabinderinthefreeradicalpolymerization.Fig.3(b)showsFTIRspectrumofthebinder,B54M36H10,thatwasbakedat220°Cfor1h.TheasymmetricandsymmetricstretchingvibrationsofthetwoCfOgroupsofaformedanhydridewereobservedat1805cm−1and1762cm−1,respectively;furthermore,theacidanhydridealsohasastrongbandat1019cm−1duetotheC\\O\\Cstretchingvibration.Thisresultindicatesthattheanhydridewasproducedafterthebinderwasbakedat220°Cfor1h.Thatis,asthebindercontainsmoreMAAor2-HEMA,theweightlossinthefirst-stagedegradationofTGAexperimentwouldbemorepronounced,inaccordancewiththeTGAresultshowninFig.2(a).
3.1.4.Acidvaluemeasurement
Thecarboxylgroupsofthebinderprovideacidvalueinanegative-typephotoresist.Iftheacidvalueofthebinderistoosmall,thesolubilityoftheprebakedphotoresistinanalkaline
Fig.3.FTIRspectraofthebinder,B54M36H10.(a)before;(b)afterbakedat220°Cfor1h.
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developerliquidwillbetoolow,whichcausesanexcessamountofdevelopmentresidue,i.e.,poordevelopability.Also,pooradhesiontothesubstratemayoccurwhentheacidvalueistoolow.Ontheotherhand,whentheacidvalueistoolarge,thefilmmaydissolvetoomuchinthedeveloperliquid[26].AcidvaluesofthepreparedbindersareshowninTable1.Asexpected,theacidvalueincreasesastheMAAcontentincreases.Thatis,thesolubilityofabinderinanalkalinedeveloperliquidisdirectlyrelatedtotheMAAcontentofthebinder.Basedonthethermalpropertiesandtheacidvalues,thebinders,B63M27H10andB54M36H10,werechosentopreparethenano-silicamodifiedphotoresistsinthesubsequentexperimentalprocess,andtheformedphotoresistsweretermedPR63andPR54series,respectively,inthediscussionthatfollows.
3.2.Modificationofsilicananoparticles
3.2.1.Characterization
Evolutionofchemicalbondsduringthesol–gelprocessshowninScheme2wasanalyzedusingFTIR.InFig.4,thespectraofMSiO2andsilicasolsobtainedbyasol–gelprocessaredemonstrated.Forthesilicasol,cf.Fig.4(a),theadsorptionbandsat1073and797cm−1representtheasymmetricandsymmetricstretchingvibrationsoftheSi\\O\\Sibond,respectively.The454cm−1bandisthebendingvibrationofthisbond.Thebandsfor\\Si\\O\\C\\andSi\\OHstretchingvibrationsarelocatedat1163cm−1and3326cm−1,respec-tively.Theadsorptionbandat945cm−1isassignedtotheSi\\OHbond[27].FromexaminationofthespectrumofMSiO2solinFig.4(b),condensationreactionbetweenMSMAandsilicatoproduceMSiO2nanoparticlescanbeconfirmed.ThefirstfeaturecomesfromtheSi\\OHabsorptionbandofMSiO2solat945cm−1.ItsintensityislowerthansilicasolinFig.4(a)duetothedecreaseofSi\\OHgroupscontentonthesurfaceofsilicaparticles.Thesecondfeatureistheadsorptionbandsof−1CfCandCfOappearingat1630cm−1and1700cm,respectively.ThisindicatesthatafterhydrolysisandcondensationofMSMAwiththesilicasol,themodifiednanoparticlescontainconsiderableamountofreactivedoublebonds(CfC)ontheirsurfaces,whichcanreactwiththepolyfunctionalmonomer,DPHA,inthephotoresisttoformanorganic–inorganicnanohybrid.
Fig.4.FTIRspectraof(a)silicasol(b)MSiO2sol.
Fig.5.ParticlesizeanalysisoftheMSiO2solstoredat4°Cfor(a)1dayand(b)1–30days.
3.2.2.Particlesize
ThemodifiedMSiO2particlesmaygrowandaggregatetobecomelargerparticlesduringthecourseofstorage.ParticlesizeofthepreparedMSiO2solmeasuredatdifferentstoragetime(1–30days,storedat4°C)isshowninFig.5.ItcanbeseenthatthemeanparticlesizeofMSiO2solwasabout5–6nmwithin24hinstorage,cf.Fig.5(a).Asthestoragetimeincreased,themeanparticlesizeofMSiO2increasedaswell;itsfinalparticlesizewasabout12nmafter30days.Fig.5(b)revealsthatthesizeoftheMSiO2nanoparticlescanbemaintainedsmallerthan10nmifthestoragetimeislessthanca.24days.3.3.Characterizationofnano-silicamodifiedphotoresistPhotoresistgenerallyconsistsofbinder,polyfunctionalmonomer,photoinitiator,andphotosensitizer.Inthisresearchthebinders,B63M27H10andB54M36H10,wereselectedtopreparethenano-silicamodifiedphotoresists.TheformedphotoresistswithdifferentcompositionsofMSiO2nanoparticlesweretermedtheseries“PR63-x”and“PR54-x”,andtheircompositionsarelistedinTable2.Afterthephotoresistwasappliedontheglasssubstrate,acuredfilmwasobtainedfollowingtheindustrialprocesswhichincludedprebaking,UVexposureandpostbaking.Thenano-silicamodifiedphotoresistisexpected
C.-K.Leeetal./ThinSolidFilms516(2008)8399–8407
8405
Table2
ThermalpropertiesofvariousphotoresistsSampleMSiO2contentCTE
TdTg(°C)name
(wt.%)
(μm/m°C)(°C)
α1α2TMADSCPR63
0221.6462.3387.8101.799.1PR63-MSi55194.7371.6388.2103.6100.3PR63-MSi1010173.5368.1395.7104.7103.6PR63-MSi1515164.3291.4396.5108.6104.9PR63-MSi2020160.9260.1398.8110.3109.3PR54
0203.9614.5384.9113.6113.8PR54-MSi55197.3561.9389.1114.8116.7PR54-MSi1010177.6492.4399.4119.1121.7PR54-MSi1515163.9325.8402.4121.6122.9PR54-MSi20
20
147.1
275.3
406.1124.7
123.6
toexhibitimprovedhardness,adhesiveness,andthermalendurancewithrespecttoconventionalpolymeric-photoresists.3.3.1.Thermalproperties
3.3.1.1.Glasstransitiontemperature.Theglasstransitiontemperatureofthecuredfilmsofnano-silicamodifiedphotoresistwasdeterminedbyDSC,andtheobtainedthermogramsoftheseriesPR63-xandPR54-xareshowninFig.6.ForthecuredphotoresistsPR63andPR54,freeofMSiO2content,Tgareidentifiedtobe99.1and113.8°C,
Fig.6.DSCthermogramsofthephotoresists.(a)PR63-x;(b)PR54-x.respectively.NotethatTgofthebinderB54M36H10ishigherthanthatofB63M27H10,whichimpliesthatTgofthephotoresistdependsonTgofthebinder.Ontheotherhand,Tgofthenano-silicamodifiedphotoresistsincreasedprogres-sivelywithincreasingMSiO2content,andwhentheMSiO2contentreached20wt.%,Tgoftheformedfilmcanincreaseby10°Cforbothseriesofphotoresists,asshowninTable2.Thus,itcanbeconcludedthatTgofthephotoresistscouldbeeffectivelyincreasedeitherbyusingbinderwithahighTgorMSiO2nanoparticlesinthephotoresist.
3.3.1.2.Thermaldegradation.Thermaldecompositionbeha-viorsofthecuredphotoresistswereinvestigatedusingTGA.Thetemperaturecorrespondingto10wt.%losswasdefinedastheinitialthermaldegradationtemperature.Fig.7showsthethermogramsofthePR63-xandPR54-xseriesofnano-silicamodifiedphotoresists.Itcanbeseenthatallofthesamplesremainrelativelystableagainstthermaldecompositionevenwhenthetemperaturewasraiseduptoabout250°C;theweightlosswassmallerthan2%.Furthermore,TdincreasedmildlywithincreasingMSiO2contentforbothseriesofsamples,assummarizedinTable2.Intheextremecase,PR54-MSi20,itsinitialthermaldegradationtemperaturewasca.21°higherthanthatofPR54(cf.Fig.7(b)).Inaddition,thecharyieldalsoincreasedwithincreasinginorganicnano-silicacontent,asexpected[27].
Fig.7.TGAthermogramsofthephotoresists.(a)PR63-x;(b)PR54-x.
8406C.-K.Leeetal./ThinSolidFilms516(2008)8399–8407
3.3.1.3.Thermalexpansioncoefficient.Thelinearthermalexpansioncoefficients(α)andTgofvariousphotoresistsweremeasuredusingTMAandtheresultsareshowninFig.8andTable2.Asanexample,Fig.8showsthethermogramsofthecuredphotoresists,PR54andPR54-MSi20,intermsoflineardimensionalchangeversustemperature.ForsamplesfreeofMSiO2,Fig.8(a)indicatesthatitsthermalexpansioncoefficientsbefore(α1)andafter(α2)glasstransitionwere204and615μm/m°C,respectively.Tgofthissample,asdeterminedbytheintersectionofthetwostraightlinesaboveandbelowthetransition,was114°C,avalueveryclosetotheresultmeasuredusingDSC.Thethermalexpansioncoeffi-cientsofpolymersarenormallyveryhigh,andthislimitstheirindustrialapplicationsascoatingmaterials.Thisshortcomingcanbeovercomebyincorporationofinorganicnano-silicaintothepolymerhost.FromTable2,itcanbeseenthatα1andα2oftheformedphotoresistfilmsdecreasedwithincreasingnano-silicacontent.Specifically,α1andα2ofthenano-silicamodifiedphotoresist“PR54-MSi20”werereducedto147and275μm/m°C,respectively(cf.Fig.8(b)).Inotherwords,thethermaldimensionalstabilityofthesamplehasbeenenhancedduetotherigidityofferedbyMSiO2.
Fig.8.TMAthermogramsofthephotoresists.(a)PR54;(b)PR54-MSi20.
Table3
Hardness,adhesion,andresolutionofvariousphotoresistsSampleMSiO2contentPencilAdhesionResolutionname(wt.%)hardness(%)(μm)PR63
02H80b10PR63-MSi553H100b10PR63-MSi10103H100b10PR63-MSi15154H100b10PR63-MSi20205H100b10PR54
02H90b10PR54-MSi553H100b10PR54-MSi10103H100b10PR54-MSi15154H100b10PR54-MSi20
20
5H
100
b10
3.3.2.Adhesion,hardness,andresolutiontests
Thehardnessofthephotoresistfilmcoatedonglasssubstratewasexaminedusingtheindustrialpenciltest,whereastheadhesionstrengthbetweenphotoresistandglasswasdeterminedbythetapetest[20].Foralltestedsamples,thethicknessofthedriedcoatinglayerwasabout1–2μmandthetestedresultsaresummarizedinTable3.ThehardnessofbothPR63andPR54was2H,andthevalueincreasedwithincreasingMSiO2content.Averyhighhardnessof5Hcouldbeattainedwhenthephotoresistcontained20%MSiO2.Astotheadhesionstrength,allphotoresistsshowedperfectadhesiononthetapetest,exceptsamplesPR63andPR54,whichwerefreeofMSiO2.Itfollows
Fig.9.OpticalmicroscopicimageofthepatternsformedusingphotoresistPR54.(a)circuitdiagram;(b)resolutiontest.
C.-K.Leeetal./ThinSolidFilms516(2008)8399–84078407
Fig.10.OpticalmicroscopicimageofthepatternsformedusingphotoresistPR54-MSi20.(a)mosaicdiagram;(b)resolutiontest.
thattheadhesionstrengthwasalsoimprovedbytheadditionofMSiO2nanoparticlesinthephotoresists.
Finally,itisimportanttoknowhowhightheresolutionthenano-silicamodifiedphotoresistcanachieveafterastandardexposure-developmentprocess,incasethatindustrialapplica-tionistargeted.Figs.9and10showthemicroscopicimagesofthedevelopedphotoresists,PR54andPR54-MSi20,respectively.ItcanbeseenthattheresolutionofPR54isbetterthan10μm.TheresolutionofPR54-MSi20alsoreachesto10μm(cf.Fig.10(b)).Ingeneral,incorporationofnano-silicaintothephotoresistwouldreducethedevelopabilityofthephotoresist.However,thisproblemcouldbesolvedbyadjustmentofthecompositionofthedeveloperliquid.Inthepresentresearch,wefoundthataddingasuitableamountofIPA(IPA/H2O/Na2CO3=3:2:0.02,w/w/w)givesasatisfactoryresultwithadevelopmenttimeof60s.Theresolutionofallotherphotoresistsiswithin10μm,cf.Table3.Inotherwords,thenano-silicamodifiedphotoresistshavearesolutionmeet-ingtheindustrialrequirement.4.Conclusions
Anano-silicafilledphotoresistsweresynthesized.Theirchemical,mechanicalandthermalpropertieshavebeenstudied.Severalconclusionscanbedrawnfromtheexperi-
mentalobservations.ThemodifiedMSiO2nanoparticles,particlesizesmallerthan10nm,containaconsiderableamountofreactivedoublebonds(CfC)ontheirsurfaces.Theywouldnotonlyreactwiththepolyfunctionalmonomersinthephotoresistbutalsoincreasethecompatibilitybetweenorganicandinorganiccomponentsinthephotoresist.IncorporationofMSiO2nanoparticlesenhancedthethermalstabilityofthepreparedphotoresist:TdandTgwerefoundtoincrease,whereasα1andα2decreasewithincreasingMSiO2content.Nano-silicamodifiedphotoresistcoatedonglasssubstratedemonstratesahighhardness(5H)andastrongadhesion(100%)witharesolutionreaching10μm.Forthedevelopmentprocess,theoptimalweightratioofIPN/H2O/Na2CO3indeveloperliquidisfoundtobe3/2/0.02,withwhichthenano-silicamodifiedphotoresistcanbeeasilydevelopedwithin60s.Acknowledgements
TheauthorswishtoexpresstheirthankstoProf.Jui-TangChenofMing-HsinUniversityofScienceandTechnologyforprovidingphotomaskforresolutionstudies.References
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