Evidences for liquid encapsulation in PMMA ultra-thin film grown by liquid injection Photo-CVD

To link to this article : DOI:10.1016/j.porgcoat.2013.05.027

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Eprints ID: 14102

To cite this version:

Manole, Claudiu Constantin and Marsan, Olivier and Charvillat, Cedric and

Demetrescu, Ioana and Maury, Francis Evidences for liquid encapsulation in

PMMA ultra-thin film grown by liquid injection Photo-CVD. (2013) Progress in

Organic Coatings, vol. 76 (n° 12). pp. 1846-1850. ISSN 0300-9440

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Evidences

for

liquid

encapsulation

in

PMMA

ultra-thin

film

grown

by

liquid

injection

Photo-CVD

Claudiu

Constantin

Manole

_,

_Olivier

_Marsan

_,

_Cedric

_Charvillat

_,

_Ioana

_Demetrescu

_,

Francis

Maury

a_{CIRIMAT,CNRS/INPT/UPS,4alléeE.Monso,BP44362,31030Toulousecedex4,France} b_{UPB,FacultyofAppliedChemistryandMaterialsScience,1-7Polizu011061,Bucharest,Romania}

Keywords:

Poly(methylmethacrylate) Photo-CVD

DirectLiquidInjection RamanConfocalSpectroscopy AtomicForceSpectroscopy

a

b

s

t

r

a

c

t

Thispaperdealswiththecharacterizationofultra-thinfilmsofPMMAgrownbyanoriginal photo-assistedChemicalVaporDepositionprocessequippedwithapulsedliquidinjectionsystemtodeliver themonomer.Thenanometricthickfilmsshowedagoodabilitytoencapsulatealiquidphaseas micro-dropletsprotectedbyathinpolymerictightmembraneintheformofblisters.Techniquesthatarecapable toanalyzetheseheterogeneousstructuresatmicro-andnanoscopicscalesuchasRamanConfocal Spec-troscopyandAtomicForceMicroscopywereusedtocharacterizethesepolymerfilms.Theliquiddroplets werefoundtobemonomerencapsulatedbyaPMMAfilm.Thespecificpropertiesoftheseultra-thin filmsexhibitself-healingcapabilitiesatmicroscopicscalemakingthemattractiveforfunctionalization ofsurfacesandinterfaces.

1. Introduction

TheChemicalVaporDeposition(CVD)isavaluableand versa-tiletechniquetogrowmicro-andnanostructuredmaterials.Itfinds applicationsinmanynanofabricationareasforinstancetoproduce carbonnanotubes[1],lowdielectrics[2],core–shell nanostruc-tures[3]andnanocompositecoatings[4].TodevelopindustrialCVD processesseveraltechnologiescompetetodeliverthehighflow ratesofreactivegasrequiredforhighgrowthratesandlargescale reactors.Startingfromliquidmolecularcompoundsasprecursors, thisincludesforinstanceAerosolAssistedCVD[5]andpulsedDirect LiquidInjectionCVDcoupledtoaflashvaporizationoftheliquid monomertoformthevaporphase(DLI-CVD)[6].Forthislast tech-niqueawideareaofapplicationsistargetedbyintensiveresearch oninorganicmaterials,eitheroxides[7–9]orcarbides[6,10,11]. OrganicmaterialsarealsodepositedbyCVDprocessesusing vari-ousactivationtechniquesofthereactivegasphasetocompensate theverylowdepositiontemperatureimposedbythesethermally fragilematerials.Thuspolymericthinfilmsweregrownbyinitiated CVD(iCVD),e.g.hot-filamentactivation[12,13],PlasmaEnhanced CVD(PECVD)[14]andtheGrahamprocess[15]usingrespectively chemical,plasmaandthermalactivationmethods.

∗ Correspondingauthorat:FacultyofAppliedChemistryandMaterialsScience, UniversityPolitehnicaBucharest,Str.Polizu1-7,011061,sector6,Bucharest, Roma-nia.Tel.:+40214023930;fax:+40213111796.

E-mailaddress:claudiu.manole@ymail.com(C.C.Manole).

The ultra-thin polymericfilms discussedin this paper were obtainedbyanoriginalCVDprocess.First,theoriginalitycomes fromthephotonactivationofthegasphase(Photo-CVD)without irradiatingthethinlayergrowingonthesubstrate.Uptonowthis techniquewasreportedtodepositinorganicmaterials[16]. Sec-ondly,sincethepolymerizationinvolvesaliquidmonomerasCVD precursoranovelapproachwastouseDLI-CVD.Thisdual inno-vationresultedinthedepositionofultra-thin(nanometric)poly (methylmethacrylate)(PMMA)filmsthat,underparticular condi-tions,encapsulatedaliquidphaseofitsmonomer.PMMAthinfilms werepreviouslydepositedbyiCVDandevidencesforafree-radical mechanismwerefound[13].TheapproachoftheDLIPhoto-CVD processisbasedonthephotonactivationofthegasphase.Ithas theadvantageofdepositingatroomtemperature(orlower), with-outdirectsubstrateirradiationanditallowstoobtainaconformal coverageatlowpressures(oversolidandliquidsurfaces).

Becausethefilmsarenanometricthicktheircharacterization requires specific tools and methodologies. This paper gives an insightintothemeansofcharacterizingsuchpolymericultra-thin films.Theexperimentalresultsdeducedfromthetechniquesused givedirectand indirectevidencesofthemainfeaturesof these PMMAthinfilms.

2. Materialsandmethods

ThecoreofthePhoto-CVDsystemstandsinavertical align-mentperpendiculartothesubstratesurfaceofanInjectionSystem, aVaporizationChamber,anUVactivationchamberofthegasphase

Table1

ExperimentalparametersusedforDLIPhoto-CVDofPMMAfilms.

Parameters Inj-1 Inj-2

Depositiontemperature(◦_{C) 18 18}

Totalpressure(Torr) 9.5 9.5

Injectorconditions

fliq(Hz) 1 1

ton(ms) 1 1

N2carriergasininjector(sccm) N/A 150

N2carriergasinvaporizationchamber(sccm) 108 N/A

Setpoint(%) N/A 10

fgas(Hz) N/A 1

andaSampleReactor.Theinjectorsystemsflashvaporizesthe liq-uidmonomerusedasprecursor.Fortheexperimentsreportedin thispaper,twocommercialinjectionsystemsprovidedby Kem-stream(www.kemstream.com)wereused.

Inthecaseofinjectionsystem1(Inj-1)aliquidinjector commu-nicatesdirectlytoavaporizationchamber.Theamountofliquid injectedis computercontrolledbyanopeningtime (ton)and a

repeatingfrequency(fliq)oftheprocess.Inthevaporization

cham-berthesprayoftheliquidisvaporizedandmixedwithacontinuous streamofN2usedascarriergasmonitoredbyamassflowmeter.

Theinjectorsystem2(Inj-2)isanewgenerationdesignedto improvetheatomizationandthegasflowrateoftheprecursor.It hastwo-stagecomponents.Itcontainsafirstliquidinjectorthat pulsesthemonomertoamixingchamber.Theamountofliquid injectedatthisfirststageiscontrolledbyanopeningtime(ton)and

arepeatingfrequency(fliq)oftheprocess.Thisfirstliquid

injec-torcommunicateswithasecondgasinjectoractingalsoasmixing chamberwiththecarriergas.Thegasinjectormixesthespraywith N2ascarriergasandthenpulsestheliquid/carriergasmixtureinto

theflashvaporizationchamber.Thegasinjectionismonitoredby amassflowmeter.Theamountofgasinjectediscontrolledbythe softwarethatconvertsagivensetpoint(percentofsccm)inopening time.Therecurringpulsedopeningtimeofthegasinjectorisgiven bya gasfrequency(fgas).Bothinjectionsystemsproducesprays

withdifferentcharacteristicsintermsofaveragesizeofdroplets andsizedistribution.

For Inj-1 setup, the monomer waspushed by a pressure of 3.8barofN2towardstheinjectorwhichoperatedatfliq=1Hzand

ton=1ms.IntheInj-2setup,themonomerwaspushedwitha

pres-sureof3.5barandtheN2carriergaspressurewasof2bar,with

Setpoint=10%and fgas=1Hz.Theexperimentalparameterswith

respecttotheinjectingconditionsaresummarizedinTable1. TheUVchamberconsistsoffourHglowpressurelamps emit-tingat254nm(HeraeusmodelNNI60/35XL).Foreachlamp,the radiativefluxat254nminairatroomtemperaturewas20Wand theirradianceatthiswavelengthwas150␮W/cm2_{atadistanceof}

1m.Theyare38cmlongandplacedaroundaquartztubeof5cmin diameterand40cmlong.Thelampsandthetubearesurroundedby ashellandapolishedaluminumreflector.Thetemperatureofthe lampsandthequartztubearemaintainedaround80◦Cbyastream ofcompressedair.Thevaporphaseisphoto-activatedwhenpassing intothisquartztubeandiscarriedtowardsthesamplereactor.For thedeposition,themonomermethylmethacrylate(MMA) (con-taining∼0.004%hydroquinoneasstabilizer) wasmixedwith2% 1-hydroxycyclohexylphenylketoneasphoto-initiator(PI)inorder toaidthepolymerization.TheywerebothpurchasedfromSigma Aldrichandusedasreceived.

The substrates in the growth reactor were placed perpen-dicularly tothemain gasstreamon asample holder equipped witha temperaturecontrolsystem.Theywerekeptatambient temperature(18◦C).Theyweredegreased andcleanedin ultra-sonicbathwithacetoneand thenethanol. During deposition,a substrateareawasmaskedinordertoallowadirectcomparison

between a deposited and an undeposited zone. The masking was performedby placing the polished face of a Si(100) chip (10mm×5mm×0.4mmdirectlyonthepolishedsurfaceofa rect-angular Si(100) wafer (30mm×5mm×0.4mm). The intimate contactbetweenthetwopolishedsurfacespreventsthediffusion ofreactivegasbetweenthemandthereforethecontaminationof themaskedsurfaceofthesubstrateduringtheCVDrun.Inorder tohaveadifferenttypeofsurfaces,a25mm×25mm×1mmglass substratewasalsousedneartheSi(100)substrate.

TheAtomicForceMicroscopy(AFM)datawasobtainedbyan AgilentTechnology5500AFMoperatinginacousticmodewith sil-iconTypeVIIMACLeverfromAgilent.TheAFMmeasurementswere madeataspeedof5nm/satroomtemperatureinambientair.The RamanspectraandinsituOpticalMicrographieswereobtainedat roomtemperaturebyaHoribaLabRAMRamanConfocal Spectrom-eterwitha5mWlaseremittingat532nm.Bothtechniquesoffer directcharacterizationofthesamples.However,withrespectto ultra-thinnanometriclayers,thesecharacterizationscanprovide direct and indirect experimentalresults. TheAFM offers direct resultsovertheultra-thinfilmduetotheatomicforceinteractions thatitmeasures;itisreallyasurfaceanalysistechnique.

TheRamanConfocal Microscopy(RCM)analysisofultra-thin polymericfilmsprovidesbothdirect andindirectresultsinthe sensethatitisnotreallyasurfaceanalysistechniqueanditcan beinvasiveforthematerialanalyzed.Thesamplescanbeanalyzed as-depositedandaftersubsequenttreatmentseitherinsituduring theRamananalysisasaresultoftheinteractionbetweenthelaser beamandthematerial,orvariouspost-treatments.Inthiswork,we focusonpost-thermaltreatmentintheambientairofaMemmert Model550ovenat115◦C.

3. Resultsanddiscussion

ThetwoDLIsystemsallowedencapsulatingdifferentvolumes ofliquidinsideapolymericthinmembraneonthesurfaceofa sub-strate.Thetwoinjectionsystemswithcontinuous(Inj-1)orpulsed (Inj-2)carriergasofferedthepossibilitytoinvestigateultra-thin filmswithdifferentmorphologiesandproperties.

Inafirstseriesofdepositionexperiments,theInj-1 injection systemwasusedtoobtainverythin polymericfilmbytheDLI Photo-CVDprocess.Theformationofananometricthickmembrane isdifficulttoidentifyandcharacterize,inparticulartodetermine itsthicknessandstructuralfeaturesbecauseorganicpolymersare generally nonconductive and thermally fragile materials. Infor-mationwithrespecttothepresenceofanultra-thinfilmcanbe obtaineddirectlybyAFMorindirectlybyextrapolatingRCMdata. TheAFMtopographyofanuncoatedarea(Fig.1a)andacoated area(Fig.1b)overthesamesubstrateisshown inFig.1.Itcan beobservedthatthemaximumheightforacoatedareaincreased byapproximately30nmwithrespecttotheuncoatedone,which revealsthepresenceofanultra-thinfilmcoveringthesurfaceofthe substrate.Bymappingthephaseofthecantileveroscillations,the AFMoffersthepossibilitytoidentifyvariationsinthesample com-position.InFig.1c,thephaseimagingofthecoatedareashowssuch aphasechangeandrevealstwotypesofstructures.First,whatcan beregardedasencapsulatedliquiddroplets(structure#1),looking asblistersca.1␮mindiameterspreadoutoverthesurfaceofthe sampleand,secondly,smallerheterostructureslookingasagood conformalcoverageofsolidnanoparticlesthatcontaminatedthe surfaceoftheSisubstratebyathinfilm.

Afterthedeposition,theblistersobservedonthesurface pre-servetheirshape,sizeanddistributiononthesurfaceevenafter2 monthsinambientairandindark.Theydonotshowanychange duetoeitherchemicalreactivityforinstancewithairorto evap-orationofthevolatileliquid.Indeed,iftheliquidwastheMMA

Fig.1.AFMtopographyforanuncoatedarea(a)andforacoatedarea(b)withtheInj-1system.Thephaseimageofthedepositedareaisalsopresented(c)withpossible liquidencapsulationasblisters(1)andinhomogeneitiesresultingfromconformalcoverageofparticlescontaminatingthesurfaceoftheSi(100)substrate(2).

monomer,itsvaporpressureissufficientlyhigh(29.5Torrat20◦C) tobecompletelyevaporated.Thisindicatestheliquiddropletsare protectedfromtheenvironmentbyaverythinpolymeric mem-brane.Atthisstage,thebestassumptionistoconsiderthatthese blisterslikestructurescontainaliquidthatshouldbethemonomer andthephotoinitiatormoleculesincetheyarethesoleliquid com-poundsinvolvedintheCVDprocess.

Raman analysis of an as-deposited droplet-like structure approximately10␮mwide, revealsthe C C bonds ofboth the monomer(notpresentinthepolymer)andthephotoinitiatorwith bandsat1640cm−1 and1660cm−1,respectively.Thisresultsin abroadcumulativeC Cpeakinthisspectralrange(Fig.2).Even iftherewereonly2%ofPIintheinitialsolutioninjected,itcan bepresent in anamount significantly greater in theseblisters. Indeed,thevolatilityofMMA(29.5Torrat20◦C)comparedtoPI (2.2×10−3Torr)suggeststhatapartofthemonomerislikely evap-oratedduring thedepositionunderreduced pressure (9.5Torr), whichenrichestheproportionofPIintheliquidcondensedonthe substratesurface.

Theissueofseparatingthetwospeciestodeterminethe compo-sitionoftheliquidinthedropletswillberesolvedbyasubsequent post-treatmentofthesample.TheRamantechniquewasreported asa tool for estimating theextentof polymerizationof PMMA after a thermal treatment, highlightingthe 115◦C temperature as optimum temperaturefor monomer polymerization[17].As observedinFig.2,thebandassociatedtothemonomerdecreases graduallywiththeincreaseofthetimeofathermalpost-treatment at115◦Cin air.Thisindicatesthepresenceofliquid MMAinto theobserveddrop-likestructurethatprogressivelypolymerizeby

Fig.2.Ramanspectraofa10␮mencapsulateddropletsobtainedwithInj-1system afteranthermal(115◦_{C)post-treatment.}

thermal activation.Duringand afteralloftheRaman measure-ments,thefilmandthedropletsdepositedusingtheInj-1system werepreservedintact.

ThefurtheruseoftheInj-2injectionsystemwillprovide poly-mericthinfilmswithsimilarfeatures,butwithdifferentproperties. Largerdropletswereincorporatedintoathinfilmgrownusingthe Inj-2injectionsystem.TheAFMforcemicroscopywasperformed ontothreepointsindicatedintheAFMtopographicalimage(Fig.3). TheAtomicForceSpectroscopyinvolvesthreestepsof theAFM cantileverbehavior:(i)anapproachhalfcycle,whenthecantilever tipapproachesthesurface,(ii)acontactwiththesampleand(iii)a retractinghalfcyclewhenthecantileverretracts.Fig.3bshowsthat thebareSisubstrate(thereference)hasashortapproach/retracting cycle(∼0.5s).Duetotheultra-thinpolymericfilmcoveringthe sub-strate,theapproach/retractcyclesarelonger(∼1.5s)thatindicates amoreelasticbehaviorcomparedtothebareSisubstrate.Ifpure liquidwerepresentontothesurfaceasfreedroplets,i.e.without encapsulationinanultra-thinmembraneasblisters,theAFMtip wouldhavepenetratedtheliquideasily,indicatingthusadifferent retractingcyclefortheregion3comparedtoregions1and2.

ItiswellknownthatthefocusedRamanlasergeneratesheat thatcandamagematerialsiftheyabsorbthephotons.Forinstance, thelaser-inducedthermaleffectswerestudiedoncarbon nano-tubesandonmagnetiteoxidation[18,19].Also,theoptimizedlaser energycanbeusedtoachieveweldingofPMMAsheets[20].Here, duringtheRamanMicroscopyanalysesoflayersobtainedby Inj-2system,theultra-thinfilm ontheSisubstrate wasirradiated insitubythelaserbeam.Duringthislaserirradiationfor approx-imately15s,bothadestructiveanda constructiveeffectcanbe extrapolated.

Firstly,theas-depositedblisterapproximately30␮min diam-eter(Fig.4a)onwhichthelaserfocusshowsacatastrophicfailure of the film bythe presence of a crater-like structure (Fig.4b). Thefocal diameterof theRCMlaserwas430nm.Theresulting crater-like geometry after the exposure to laser beam reaches approximately5␮mindiameter.Thedegradationprocesscanbe essentiallydescribedbythefollowingsteps:(i)laserenergy absorp-tionatthethin filminterfacewiththeSisubstrateleadingtoa suddentemperatureincreaseoriginatingessentiallyfrom absorp-tionoftheSi,(ii)initialthermalstrainandpossiblymelting(suchas depolymerizationofPMMA[21])and/orvaporizationoftheliquid species(MMA),and(iii)thermallyinducedpolymerizationfrom encapsulatedmonomerinthelowtemperatureareas(attheedges ofthecrater)[22].InFig.4aandb,thelaserbeamwasfocusedon aheterogeneityembeddedintheblister.Thisheterogeneitylikely originatingfromthesurfacecontaminationoftheSisubstrateis uncoveredafterlaserirradiationatthecenterofthecraterasshown inFig.4b.Forblisterswithoutsuchcontaminationthebottomof thecrateriscleanafterlaserexposureandnoparticleisobserved atthecenter(Fig.4d).

Fig.3.TheAFMtopography(a)witharrowsindicatingtheareaswheretheforcespectroscopydata(b)wereobtainedforthePMMAfilmpolymerizedwiththeInj-2system andforthebareSisubstrate.

Fig.4. InsituopticalmicroscopyimageoffilmsdepositedwiththeInj-2systemacquired(a)(c)beforeand(b)(d)afterirradiationbyfocusinga5mWlaserbeamemitting atawavelengthof532nmoftwoblister-likestructuredepositedover(a)(b)glassand(c)(d)Si(100)substratesduringRamananalysis.

Thepresenceofsuchcrater-likestructuresafterseveralRaman analysessupportsthisdegradationprocess.Ramanlaserprovides a heatgradient acrossthe surfaceof thesample, witha maxi-mumtemperatureatthecenterofthespotandasharpdecrease towardsambienttemperature attheperiphery. Shebanova and LazorobservedthatRamanlaserirradiation(at512nm)induced theoxidationofmagnetite[18].Thisoxidationcouldoccurat tem-peraturescloseto240◦C[18].Theirstudywasorientedtowardsthe determinationofthelocaltemperatureforthelaser-heatedspot.It involvedthevariationofthepowertothesample(7–60mW)over aspotsizeofaround5–10_␮m,forinstance50mW/5_␮m.Forthe opticalobjectiveusedinourexperimentsthecalculatedspotsizeof our5mWlaserisaround0.5␮m.Thereforetheapproximatepower persurfaceareaoftheRCMusedinourexperimentsisofthesame orderofmagnitude(5mW/0.5␮m).At50mWthetemperaturewas correlatedbyShebanovaandLazorataround410◦Cformagnetite

[18].Sisubstratestronglyabsorbsirradiationat532nmanditcan readilytransmititslocalheattothepolymericfilm.Therefore,itcan besuspectedthatattheobservedradiusof2.5␮mfromthecenter ofthecrater(i.e.edgeofthecrater)thetemperaturecouldreachan

optimumvaluenear100◦Cthatleadstoaself-healingmechanism oftheblisterbythermalpolymerization.Thesealingattheedgeof theexposedareacanbeconsideredasaconstructiveeffectoflaser irradiation(Fig.4c).

ThedifferentbehaviorofthesamplespreparedusingtheInj-1 andInj-2 systemsobservedduringRamananalysesmaybedue to a different composition of the liquid (for instancethe ratio MMA/PI)encapsulatedinthepolymerfilm.Thenanometer-thick PMMAfilmsthatencapsulatepolymerizablemonomericspecies showself-healingproperties,withtheabilitytore-sealthearea thatsufferedacatastrophicfailure.Thisisachievedunderparticular conditionsdictatedbytheactivationparametersofthe polymer-izationsuchasthetemperatureinducedbylaserheating.

4. Conclusions

AnoriginalPhoto-CVDprocess wasusedtoobtainultra-thin nanometric PMMA films. After the photo-excitation of thegas phase, the monomer polymerizes over both liquid and solid

surfaces.Twocommercialinjectorsystemsallowedliquid encap-sulationasblisterswithdifferentmorphologiesandproperties.A partoftheliquidmonomerwasencapsulatedasmicrodropletsin theultra-thinpolymericfilm.AFMandRCMtechniqueswereused tocharacterizesuchfilmsandevidencesforliquidencapsulation weregiven.Thefilmpresentsself-healingbehavioratmicrometric scale,apropertythathasthepotentialtobealsoexploitedatlarger scales.

Acknowledgements

The work has been funded by the Sectoral Operational Programme Human Resources Development 2007–2013 of the RomanianMinistryofLabour,FamilyandSocialProtectionthrough theFinancialAgreementPOSDRU/88/1.5/S/61178.

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