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Ar/HMDSO/O2 Fed Atmospheric Pressure DBDs: Thin Film Deposition and GC-MS Investigation of By-Products

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Ar/HMDSO/O2 Fed Atmospheric Pressure DBDs: Thin Film Deposition and GC-MS Investigation of By-Products
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  Ar/HMDSO/O 2  Fed Atmospheric Pressure DBDs:Thin Film Deposition and GC-MS Investigationof By-Products Fiorenza Fanelli,* Sara Lovascio, Riccardo d’Agostino,Farzaneh Arefi-Khonsari, Francesco Fracassi Introduction In the last decades the interest for organosilicon and silica-likethinfilmshascontinuouslyincreasedfortheirpotentialutilization in many technological fields [1] such as micro-electronics, [2–5] packaging, [6,7] scratch-resistant materi-als, [8] corrosion protection, [9–12] and biomaterials. [13] Plasma-enhanced chemical vapor deposition (PECVD) hasturned out to be a very attractive preparation method forthese films since it is compatible with most materials, alsosensitivetotemperatureincrease(e.g.,plastics,naturalandsynthetic fabrics, etc.), it allows to control film thickness,conformity, chemical composition and properties, etc.Low pressure PECVD from organosilicon precursors is awell-established technology since many papers andpatents have been published so far, [1–10,13–26] unfortu-natelythehighcostofvacuumequipmentsandthedifficultintegration in continuous production lines do not allow awideutilization ofthisapproach inlargeareamanufactur-ing.Inordertoovercomethesedifficulties,manyacademicandindustrialresearchgroupsarestudyingthePECVDfromorganosilicon and other precursors in non-equilibriumplasma at atmospheric pressure. [27] Full Paper F. Fanelli, S. Lovascio, R. d’Agostino, F. FracassiDipartimento di Chimica, Universita` degli Studi di Bari AldoMoro  IMIP CNR, via Orabona 4, 70126 Bari, ItalyFax: ( þ 39) 0805443405; E-mail: fiorenzafanelli@chimica.uniba.itF. Arefi-KhonsariLaboratoire de Ge´nie des Proce´de´s Plasmas et Traitements deSurfaces, EA3492, Universite´ Pierre et Marie Curie ENSCP, 11 ruePierre et Marie Curie, Paris 75005, France The thin film deposition in DBDs fed with Ar/HMDSO/O 2  mixtures was studied by comparingthe FT-IR spectra of the deposits with the GC-MS analyses of the exhaust gas. Under theexperimental conditions investigated, oxygen addition does not enhance the activation of themonomer while it highly influences the chemical composition and structure of the depositedcoating as well as the quali-quantitative distribution of by-products in the exhaust. Withoutoxygen addition a coating with high monomer struc-ture retention is obtained and the exhaust containsseveralby-productssuchassilanes,silanols,andlinearand cyclic siloxanes. The dimethylsiloxane unit seemsto be the most important building block of oligomers.Oxygen addition to the feed is responsible for anintense reduction of the organic character of the coat-ing as well as for a steep decrease, below the quanti-fication limit, of the concentration of all by-productsexcept silanols. Some evidences induce to claim thatthesilanolgroupscontainedinthedepositsareformedthrough heterogeneous (plasma-surface) reactions. Plasma Process. Polym.  2010 ,  7   ,  535–543   2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  DOI: 10.1002/ppap.200900159  535  Among the various experimental ways to generate non-equilibrium plasmas at atmospheric pressure, dielectricbarrier discharge (DBD) technology is one of the mostpopularapproachforthinfilmdepositionandinparticularfor the production of SiO x  coatings (see, for instance,refs. [11,12,28–43] ). The critical point of the DBD technology isthatuniform discharges (glow or Townsend regime [44] ) aredifficult to obtain since in most cases inhomogeneousfilamentary discharges are generated. [45] Homogeneousdischarges, similar to those obtained at low pressure, existonly in a narrow range of working parameters, that is, gasmixture composition, precursor concentration, frequency,appliedvoltage,etc.GenerallyDBDstendtobefilamentaryand hence intrinsically inhomogeneous, they can conse-quently produce non-uniform and damaged coat-ings. [29,30,36,37] For instance, it has been reported thatwith N 2 /HMDSO/N 2 O mixtures the Townsend regime canbe successfully obtained at hexamethyldisiloxane(HMDSO) concentrations lower than few tens of ppm(e.g., 20ppm [33] ) and that a stable homogeneous dischargecanbegeneratedatamaximumconcentrationofoxygeninnitrogen of 400ppm. [46] In order to obtain uniform andpinhole-free coatings in DBDs fed with organosiliconprecursors the following approaches are reported: i) thedeposition in a homogeneous regime [28,32–34,38,39] andii) the optimization of filamentary discharges. [29,35,41,43] Due to its non-toxic character, chemical inertness, andrelatively high vapor pressure even at room temperature,HMDSOisoneofthemostwidelyused‘‘monomers’’forthedepositionoforganosiliconandsilica-likethinfilmsbothinlow and atmospheric pressure plasmas. HMDSO reactivityinlowpressureRFplasmahasbeeninvestigatedwithmanydiagnostic techniques such as Fourier transform infraredabsorption spectroscopy (FT-IRAS), optical emission spec-troscopy (OES), and mass spectrometry (MS). [7,15,16,18,20,22–26] It was reported that the main electron impact dissociationpath of HMDSO consists in a methyl loss and Si  O bondbreaking. [15,16,18] The oxygen addition to the gas mixturepromotes homogeneous and heterogeneous oxidationproducingpartiallyoxidizedfragmentsthatcancontributetothefilmgrowth. [7,15,16,18] Thediagnosticstudiesallowedto outline an overall deposition mechanism and tosuccessfully correlate the plasma chemistry of HMDSO/O 2 -containing low pressure plasmas with the chemicalstructure and final properties of the deposited coatings.Organic silicone-like coatings and inorganic SiO 2 -like thinfilms can be in fact deposited by simply changing theoxygen content in the feed. [7,25] Anotherdiagnostictool,usefultoincreasetheknowledgeon the deposition mechanism, is the analysis of exhaustgases by means of gas chromatography with massspectrometry detection (GC-MS). Although an indirectanalytical technique, not compatible with on-line andcontinuous sampling, GC-MS is a powerful tool whichallows the evaluation of the precursor reactivity, theidentificationandeventuallythequantificationofthemostabundant stable by-products generated by plasma activa-tion. Besides the pioneering work of Wro´bel and co-workers, [1,17] who widely used this technique for studyingthinfilmdepositioninlowpressureremoteplasmafedwithsiloxanes and silanes, few authors have reported on thistechnique. Among them Sarmadi et al. [14] and Fracassiet al. [21] investigated the exhaust gases of low pressure RFplasma fed with HMDSO/O 2 . Light hydrocarbons [14] weredetected along with different organosilicon compounds,most of them contained one or more dimethylsiloxane(  Me 2 SiO  ) groups, [21] confirming the importance of thisunit as building block in film growth.Interesting results have also been published for atmo-sphericpressurecoldplasmascontainingHMDSOwiththeaimofcorrelatingtheplasmachemistrywiththechemicalcomposition and structure of the coating. Vinogradovetal. [40,41] performedtheFT-IRASandOESanalysisofDBDsfed with Ar/HMDSO/O 2  and He/HMDSO/O 2 . In particularthe investigation of the plasma phase with FT-IRASsuggested that monomer fragmentation mainly resultsin the production of four radicals: (CH 3 ) 3 SiO, Si(CH 3 ) 3 ,(CH 3 ) 3 SiOSi(CH 3 ) 2 ,andCH 3 ;thesereactivefragmentscanberesponsible for the formation of pentamethyldisiloxane,trimethylsilane, and methane. The concentration of thesespecies decreases with oxygen addition, with the produc-tion of CO, CO 2 , H 2 CO, O 3 , and HCOOH.Also GC-MS was utilized to investigate organosilicon-containing atmospheric pressure plasmas. Sonnenfeldet al. [31] studied HMDSO- and TEOS-fed filamentarydischarges sustained in Ar, N 2 , and He. It was reportedthat, in HMDSO-plasma, methyl loss with formation of pentamethyldisiloxane is the main reaction path inmonomer activation along with Si  O bond breaking andformation of (CH 3 ) 3 SiO and (CH 3 ) 3 Si units. Since only smallamounts of unidentified oligomers were detected, theauthorsassumedthatthepolymerizationprocessesmainlytake place at the surface of the growing polymer.ThepresentworkreportsadetailedGC-MSinvestigationof the exhaust of DBDs fed with Ar/HMDSO/O 2  gasmixtures.Inparticulartheevolutionofthemostimportantspecies detected in the exhaust gas as a function of theoxygen-to-monomer feed ratio is compared with the FT-IRfeatures of the deposits. The results allow to draw someimportant conclusions on the monomer activation, on theeffect of the oxygen content in the feed, and on the silanolgroups formation in the deposit. Experimental Part Plasma processes were carried out in the home-made DBD reactorschematicallyshowninFigure1.Thedischargecell,enclosedinan F. Fanelli, S. Lovascio, R. d’Agostino, F. Arefi-Khonsari, F. Fracassi 536 Plasma Process. Polym.  2010  ,  7  , 535–543   2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  DOI: 10.1002/ppap.200900159  airtight Plexiglas chamber (volume of about 14L), consists of two50  50mm 2 parallel plate electrodes both covered by a 2.54mmthick 70  70mm 2 Al 2 O 3  plate (CoorsTek, 96% purity). Theinterelectrode distance, that can be regulated with spacers, isfixed at 2mm. The electrodes are connected to an AC HV (highvoltage) power supply (SG2 STT Calvatron), composed of a coronagenerator and an HV transformer, working in the 15–50kHzfrequency range. Discharges were driven at fixed excitationfrequency and voltage of 30kHz and 2.5kV rms , respectively. Theapplied voltage ( V  ) was measured by an HV probe (TektronixP6015A); the current (  I  ) and the charge ( Q ) were evaluated bymeasuringwitha TektronixP2200probethevoltagedropacrossa50 V  resistor and a 4.7nF capacitor connected in series with thegroundedelectrode,respectively.Thedatawererecordedbymeansof a digital oscilloscope (Tektronix TDS2014B). The powerdissipated in the discharge was evaluated employing the Manleymethod and in particular the voltage–charge ( V  – Q ) Lissajousfigure. [45] Thedissipatedpowerwasexpressedasspecificpowerperunit of electrode surface.Ar/HMDSO/O 2  mixtures were longitudinally injected in thedischargegapthroughagasinletslitandpumpedout,bymeansof amembranepump,throughaslitplacedattheoppositesideofthegap. Gas flow canalization along the electrode length was assuredbytwoglassspacerswhichlaterallyconfinetheinterelectrodegap.The pressure in the chamber, measured by an MKS baratron, waskept constant at 760Torr by pumping speed regulation with aneedle valve.ArandO 2 (AirLiquideArgonCandOxygenC)gasflowrateswerecontrolledbyMKSelectronicmassflowcontrollers;HMDSO(Fluka,98.5% purity) vapors were introduced by an Ar stream bubblingthrough a liquid HMDSO reservoir kept at 30 8 C. The effectiveamount of precursor admitted into the reactor was evaluated byreservoirweightvariationperunittimeand,assuminganidealgasbehavior, it was converted to flow rate expressed in sccm.Experiments were performed by keeping constant the Arand HMDSO flow rates at 4000sccm and 1sccm, respectively,and changing the O 2  flow rate in order to vary the O 2 -to-HMDSOfeed ratio in the range 0–40. Under these conditions the gasresidencetimeintheinterelectrodezonewasequaltoabout80ms.Beforeeachexperiment,thePlexiglaschamberwaspurgedwith4000sccm of Ar for 20min to remove air contaminations. Thedeposition processes were carried out for 5min.The deposition rate was evaluated by measuring the filmsthicknessbyanAlpha-Step500KLATencorSurfaceProfilometeronpartially masked thin glass slides placed on the bottom groundelectrode. Measurements were performed at different positionsalong the gas flow, i.e., at different gas residence times; for eachexperimental condition the mean value in the region between 20and 30mm from the gas entrance inside the discharge area wasconsidered. [47] The bulk chemical characterization of the coatings wasperformed by Fourier transform infrared spectroscopy (FT-IR).Filmsdepositedonto1cm 2 ,0.7mmthickc-Si(100)substrateswereanalyzed with a commercial Brucker Equinox 554 FTIR Inter-ferometerin400–4000cm  1 range,witharesolutionof4cm  1 .Tominimize water vapor and carbon dioxide interferences thespectrometer optical path was purged with a continuous N 2  flowfor 10min between each measurement. The analyses wereperformedonsamplespositionedonthealuminaplatethatcoversthe ground electrode both inside the discharge zone (20–30mmfromthe gasentrance)anddownstreamof the electrodearea (50–60mm from the gas entrance) (Figure 1).In order to collect stable by-products formed by plasmaactivation, the exhaust gas was sampled for 30min with astainlesssteelliquidnitrogentraplocatedbetweenthereactorandthepump(Figure1).Aftersampling,thetrapwasisolatedfromthesystem, the condensate was dissolved in acetone (Sigma–Aldrich,99.8%purity),andthesolutionwasfilteredandanalyzedbymeansof a GC 8000 Top gas chromatographer (Thermoquest Corporation)coupledwithadifferentialpumpedquadrupolemassspectrometer(Voyager, Thermoquest Corporation). A Grace AT-1MS fused silicacapillary column (polydimethylsiloxane 0.25 m m thick stationaryphase, length of 30m, internal diameter of 0.25mm) was utilizedwith He as carrier gas (2sccm) under the following conditions:injector temperature of 200 8 C, column temperature programmedfrom30to200 8 C(1minat30 8 C,linearheatingrateof10 8 C  min  1 ,1min at 200 8 C). Separated products were analyzed at the GC-MSinterface and mass spectrometer source temperature of 250 and200 8 C,respectively.Mass spectrawere recordedin full-scanmodein the  m / z  range 15–500amu at the standard ionizing electronenergy of 70eV. Stable by-products were identified by means of available libraries, [48] some species were tentatively identifiedthrough the interpretation of their mass spectra according to thetypical fragmentations pattern of organosilicon compounds. Theidentification of some products was confirmed by the comparisonof retention time and mass spectrum with standard compounds.Nonane(Aldrich,99%purity)wasusedasinternalstandard(IS)forquantitativeanalysisofidentifiedspecies;calibrationcurveswerecalculatedinthelinearrangeutilizingtheareaofthecorrespondingpeaks in the chromatogram acquired in total ion current. Themeasured amounts have then been converted in flow rate. Theextent of reacted HMDSO, namely the HMDSO depletion percen-tage (HMDSO depletion ), was evaluated according to Equation (1):HMDSO depletion ð % Þ¼  HMDSO off  ð sccm Þ  HMDSO on ð sccm Þ HMDSO off  ð sccm Þ   100 (1)where HMDSO off   and HMDSO on  are the precursor flow ratesdetected in the exhaust in plasma off and plasma on conditions,respectively.Consideringtheoverallprocedureutilized(sampling, Ar/HMDSO/O 2  Fed Atmospheric Pressure . . . Figure 1.  Schematic of the experimental apparatus. Plasma Process. Polym.  2010 ,  7   ,  535–543   2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plasma-polymers.org  537  GC-MS analyses conditions, etc.) the limit of quantification (LOQ)of by-products in the exhaust was 0.0001sccm. Results and Discussion Underthe experimentalconditions explored in thiswork afilamentary DBD was obtained. In fact, as appears inFigure 2, the current signals at various O 2 /HMDSO feedratios show several peaks characteristic of filamentarydischarges. [45] The filamentary character seemsto increasewiththeoxygencontentinthefeedgassincethenumberof currentpeaksincreaseswithineachhalf-cycle.Inparticularat an O 2 -to-HMDSO feed ratio of 0 the discharge current isformed by a quasi-periodical multipeak signal and thefilamentary discharge is characterized by a quasi-homo-geneous appearance ascribed to stochastically distributedmicrodischarges; under this condition only few filaments(definedinref. [49] asafamilyofstreamerswhichrepeatedlygenerateinthesamespot)wereobservedinthegasgap.Atan O 2 -to-HMDSO ratio of 25 the typical current signal of afilamentary DBD characterized by intense and well-distinguished filaments is observed.WithincreasingtheO 2 -to-HMDSOfeedratiofrom0to40theaveragespecificdischargepowerincreasedfrom0.20to0.33W  cm  2 .Transparent and compact coatings without appreciablepowder formation were deposited with Ar/HMDSO/O 2 feeds, while without oxygen an oily film was obtained.Powder deposition occurred downstream of the electroderegion especially at high O 2 /HMDSO ratios. Since powderformationin thedischargezonehasbeenreportedforDBDfed with O 2  and HMDSO, [30,38] it is reasonable to assumethat under our experimental conditions, due to the highflow rate (i.e., low residence time) the gas phase reactionsresponsible of powder formation occur outside thedischarge zone and/or that the processes responsible of powder formation and deposition are scarcely efficient inthe plasma zone. The latter possibility is supported by theworkofBorra [50] whereitisreportedthatthedepositionof charged nanoparticles is prevented in parallel plate DBDsdrivenatafrequencyhigherthan10kHz(ourDBDisdrivenat 30kHz), due to poor collection efficiency.The deposition rate varies in the 120–150nm  min  1 range, and it is not significantly affected by O 2  content. Onthe contrary the films chemistry is markedly affected byoxygenaddition.InFigure3thenormalizedFT-IRspectraof coatings deposited at O 2 -to-HMDSO ratios 0 and 25 areshown (Figure 3a and c). For both conditions also the FT-IRspectra of the deposit collected on a silicon substratepositioneddownstreamoftheelectroderegionarereportedfor comparison (Figure 3b and d).The film deposited inside the discharge region withoutoxygen (Figure 3a) shows the typical features of silicone-like films: the intense Si  O  Si asymmetric stretchingband at 1042cm  1 , the Si  (CH 3 ) x  symmetric bendingat 1258cm  1 , and the CH x  absorptions in the 2850–3000cm  1 region (i.e., intense CH 3  asymmetric stretchingat 2959cm  1 , weak CH 3  symmetric stretching at2874cm  1 , and CH 2  asymmetric stretching at2900cm  1 ). [1,2,4,5,7,19,24,25] The absorptions in the 750–900cm  1 region suggest the presence of di- and tri-substituted Si  (CH 3 ) x  moieties. [1,2,4,5,7,19,24,25] The intensepeak at 841cm  1 can be assigned to the Si  C rocking inSi  (CH 3 ) 3 ; the strong absorption at 796cm  1 (which alsocontainsacontributionduetoSi  O  Sibendingreportedinliterature at 800cm  1 ) is due to Si  C rocking in Si  (CH 3 ) 2 .The significant presence of Si  (CH 3 ) 2 , i.e., chain-propa-gatingunits,andSi  (CH 3 ) 3 ,i.e.,chain-terminatingunits,isfurtherconfirmedbythepositionofSi  (CH 3 ) x absorptionat1258cm  1 .Ithasbeenreported,infact,thatthepositionof Si  (CH 3 ) x  signal shifts at lower wavenumbers as thenumber of methyl groups bonded to silicon increases. [2,4,5] The absorptions due to mono-substituted Si  CH 3 , di-substituted Si  (CH 3 ) 2 , and tri-substituted Si  (CH 3 ) 3  are F. Fanelli, S. Lovascio, R. d’Agostino, F. Arefi-Khonsari, F. Fracassi Figure 2.  Current and voltage waveforms of the DBD fed with Ar/HMDSO/O 2  gas mixtures, at different O 2 /HMDSO feed ratios:a) 0, b) 1, and c) 25. 538 Plasma Process. Polym.  2010  ,  7  , 535–543   2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  DOI: 10.1002/ppap.200900159  in fact generally observed at about 1275, 1260, and1255cm  1 , [2,4,5] respectively.ThefactthatinthisworktheSi  (CH 3 ) x  band was found at 1258cm  1 suggests thedeposition of a poorly crosslinked coating with highmonomer structure retention, in fact oily films areobtained.The film also contains some Si  H units asconfirmed by the presence of the Si  H stretching at2124cm  1[1,2,4,5,7,19,24] and, since the typical OHabsorption in the 3200–3600cm  1 region is notevident, the small shoulder at 907cm  1 can be attributedto H  Si  O hybrid vibrations [2] and not to Si  OHbending. [1,7,19,25] As expected, in the FT-IR spectra of coatings depositedinside the discharge zone at O 2 -to-HMDSO ratio of 25, amarked reduction ofabsorptions dueto carbon-containinggroups (i.e., CH x  and Si(CH 3 ) x ) is observed (Figure 3c). TheCH 3  asymmetric stretching at 2970cm  1 shifts to higherwavenumbersforthemoreoxidizedchemicalenvironmentand the Si  (CH 3 ) x  absorption at 1274cm  1 suggests theprevalenceofmono-substitutedSi  CH 3 unitsandthereforea higher crosslinking of the deposited coating. This is alsoconfirmed by the reduced absorptions of Si  C rocking inSi  (CH 3 ) 3 at841cm  1 andSi  (CH 3 ) 2 at800cm  1 .ThebroadOHabsorptionappearsinthe3200–3600cm  1 regionand,since any Si  H can be detected, the intense signal at905cm  1 can be ascribed to silanol (Si  OH) groups. Theintense Si  O  Si asymmetric stretching slightly moves tohigher wavenumbers, 1050cm  1 , with a shoulder around1123cm  1 likely due to short Si  O  Si chains. [2,4,5] ThepositionofSi  O  Siasymmetricstretchingisinagreementwith the presence of carbon-containing groups since inSiO 2 -like coating this absorption falls at 1070cm  1 andshift at lower wavenumbers as the carbon contentincreases. [4,7,19] Thusitcanbeconcludedthat,underourexperimentalconditions,also at high O 2 -to-HMDSO feed ratio anappreciableamountofresidualcarbonisstill present in the deposit even thoughthe reduced IR absorption of carbon-containing groups and the predomi-nanceofmono-substitutedSi  CH 3 unitssuggeststheformationofmoreoxidizedand crosslinked coatings.Figure 3b and d shows the FT-IRspectra of the downstream deposits.Without oxygen addition, no significantdifferences with respect to the filmdeposited inside the discharge zone canbedetected,whileatO 2 -to-HMDSOratioof 25 (Figure 3d), the deposit consists of powders and higher absorptions of CH x and Si  (CH 3 ) x  groups are evident ascompared to the film deposited insidethe discharge zone. Also a different shape of Si  O  Siasymmetric stretching can be appreciated due to themarkedincreaseoftheshoulderat1123cm  1 thatcouldberelatedtoalessdense,lessorderednetworkofthecollectedpowders with respect to the coating deposited in thedischarge zone. [4,5] FT-IR spectra allow making some considerations on theHMDSOdepositionmechanisminDBDs.Inagreementwithpublished data, [7,11,14,19,24,25,30,40,41] without oxygen addi-tion the deposit is mainly polydimethylsiloxane-like withHMDSO structure retention; thus, as also confirmed by FT-IRspectra,(CH 3 ) 3 Si  O  Si(CH 3 ) 2 ,Si  (CH 3 ) 3 ,Si(CH 3 ) x O( x ¼ 2,3,  . . . ) units could be considered representative of themain film precursors chemical structure. At high oxygencontent in the feed a partial oxidation of these reactivefragments occurs. The lower organic character of the filmdeposited inside the discharge region with respect to thepowders collected downstream of the electrode regionsuggests that part of the oxidation reactions in thedischarge zone occurs on the surface of the growing film.Aquitesimilarcarboncontentshouldbeexpectedbothforthe film deposited in the discharge zone and for thedownstream powder without heterogeneous oxidation inthe discharge zone.The GC-MS investigation of by-products showed thatunderalltheexperimentalconditionsexploredtheamountofreactedHMDSO(HMDSO depletion )wasalwayshigherthan50%. As reported in Figure 4, oxygen addition to the gasfeed does not improve monomer activation/utilizationsince HMDSO depletion is always lower than withoutoxygen. Moreover, the increase of the specific powerobserved as a function of the oxygen content in thefeed does not result in an increase of the monomerutilization. Ar/HMDSO/O 2  Fed Atmospheric Pressure . . . Figure 3. FT-IRspectraofdepositsobtainedinsidethedischargezoneanddownstreamof the electrode region at O 2 /HMDSO ratios 0 and 25 (a) discharge zone at O 2 /HMDSO ¼ 0,b)downstreamatO 2 /HMDSOfeedratio ¼ 0,c)dischargezoneatO 2 /HMDSO ¼ 25,andd)downstream at O 2 /HMDSO ¼ 25. Plasma Process. Polym.  2010 ,  7   ,  535–543   2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plasma-polymers.org  539
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