Chemical Analysis of Plasma-polymerized Films

Journal of Electron Spectroscopy and Related Phenomena 121 (2001) 111–129www.elsevier.com/ locate /elspec

Chemical analysis of plasma-polymerized films: The application ofX-ray photoelectron spectroscopy (XPS), X-ray absorption

spectroscopy (NEXAFS) and fourier transform infraredspectroscopy (FTIR)

*I. Retzko, J.F. Friedrich, A. Lippitz, W.E.S. Unger¨ ¨Bundesanstalt f ur Materialforschung - und Prufung (BAM), Rudower Chaussee 5, D-12200 Berlin, Germany

Received 23 April 2001; accepted 30 April 2001

Abstract

Selected FTIR, XPS and NEXAFS spectroscopy results obtained with films deposited with different plasma poly-merization processes and different monomers (styrene, acetylene, ethylene and butadiene) are presented. In detail FTIRspectra, XPS surveys, XPS valence bands and core level signals including shake-up features as well as carbon K-edgeabsorption spectra are qualitatively and, in some cases, semi-quantitatively considered. Information on the film formation,the chemistry of the films and the film stability against air exposure are derived from spectroscopic features. With styrenechemically rather well defined plasma polymer layers can be formed with rather high deposition rates applying a pulseplasma process. Using ethylene, acetylene or butadiene as a monomer in the plasma deposition process the deposition ratesare smaller. Using these monomers plasma polymer films can be obtained with a primary chemical structure which is similarto each other but more or less different from those of the respective conventional polymers. The main difference betweenthese samples is their individual concentration of unsaturated carbon species. Finally, a technologically relevant example, i.e.a plasma deposited polymer barrier layer deposited on the inner wall of a poly(ethylene) vessel is briefly considered. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Plasma polymer; Styrene; Ethylene; Acetylene; Butadiene; XPS; NEXAFS; FTIR; Chemical characterization; Depositionprocess; Stability

1. Introduction plasma polymerization applied to produce suchcoatings. It is of technological interest that, in

Organic thin films with defined chemistry and principle, the deposition of plasma polymer coatingsproperties are required for various applications [1] in is possible on any material of any shape in thefields concerning, e.g., electronics, optics or life desired thickness [2].science. Recently an increasing number of papers The use of plasma polymerization in the ‘continu-have been published which deal with the use of ous wave’ (cw) mode as it was used in former times,

predominately leads to fragmentation of the mono-mers. The resulting polymer films are strongly cross-*Corresponding author. Fax: 149-30-8104-1827.

E-mail address: [email protected] (W.E.S. Unger). linked and chemically rather different from the

0368-2048/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 01 )00330-9

112 I. Retzko et al. / Journal of Electron Spectroscopy and Related Phenomena 121 (2001) 111 –129

respective classic polymer analogues [3]. Since the chains and rings) in polymers can be differentiatedearly 1970s many attempts have been made to from each other. Actually this is a rather complicatedachieve plasma polymers with better defined chemis- task for XPS (cf. Ref. [16]). Molecule specifictry by decreasing the energy load. The Yasuda factor, information can be obtained by NEXAFS spectros-defined by W/FM, where W is the power, F the flow copy because resonant excitations of electrons fromrate and M the molecular weight of the respective core levels into unoccupied molecular levels viamonomer, was introduced in 1978 [4] in order to dipole transitions and their decay are monitored byprovide an empirical measure of the energy load and this method. Recently, the state-of-the-art of NEX-to control it in technological applications. Other AFS spectrometry of polymers was reviewed inattempts to reduce the fragmentation and damage Refs. [17,18]. Basics are discussed in a recom-caused by particle bombardment and UV radiation mended text book [19].[5] are to separate the location of the plasma from In this communication we will concentrate onthat of deposition as for example by remote-, down- selected FTIR, XPS and NEXAFS spectroscopystream- or afterglow techniques [6]. results obtained with films deposited by different

The most suitable method to generate chemically plasma polymerization processes with differentbetter defined plasma polymer films seems to be the monomers (styrene, acetylene, ethylene andasymmetrically pulsed radio-frequency (r.f.) plasma butadiene). Emphasis is given here on thepolymerization, which was established first by asymmetrically pulsed plasma process. InformationYasuda in 1977 [7] and since then developed further on the film formation, the chemistry of the films andalso by other research groups (for examples see Refs. the film stability against air exposure are derived.[8,9]). The rather low power input, that is approxi- Finally a technologically relevant example, i.e., amately 10 times lower than that which is characteris- plasma deposited polymer barrier layer on the innertic of a continuous plasma, combined with a lowered wall of a poly(ethylene) vessel is considered, too.UV damage, which may take place only during thevery short plasma impulses, yields polymer coatingsthat are much more similar to classically produced

2. Experimentalpolymers. It is well known that this holds true whenmonomers with a reactive functionality [10], such as

2.1. Pulse r.f. plasmastyrene, are used.In this work deposited plasma polymer films

A r.f. plasma generator combined with a matchingproduced using different monomers and plasmaunit (CESAR with VM1500, Dressler, Stolberg,modes were characterized by means of XPS (X-rayGermany) were used to establish the plasma in thephotoelectron spectroscopy), NEXAFS (near edgereactor. The generator was used with 13.65 MHz r.f.X-ray absorption fine structure) spectroscopy andfrequency. The pulse frequency can be chosen in aFTIR (Fourier transform infrared spectroscopy) in

4range between 10 and 10 Hz and the duty-cycle oforder to obtain chemical information on the resultingthe pulses is variable between 0.1 and 0.9. Theplasma polymer films.power can be adjusted in a range between 1 and 550Because the probing depth of XPS and NEXAFSW. To reach mild conditions for the polymerization,spectroscopy (electron yield mode) are similar it isthe lowest duty-cycle was used at a power betweenrather attractive to use these spectroscopic techniques30 and 150 W. The duty cycle regime used in thisas complementary tools in the field of surfacestudy is illustrated in Fig. 1. The effective powerchemical analysis of classic polymers as well asW , providing a relevant process parameter, canplasma (modified) polymers. Some of the work effective

be calculated to be in the range of 3–15 W by Eq.which has been done at BAM in this field in the past(1):is published in Refs. [11–15]. The advantages of

XPS are well known. An important advantage oftonNEXAFS spectroscopy is that carbon atoms in ]]] ? W 5 W (1)max effectivet 1 tstandard situations (C–C, unsaturated bonds in on off

I. Retzko et al. / Journal of Electron Spectroscopy and Related Phenomena 121 (2001) 111 –129 113

ethylene (Linde AG, Berlin, Germany). The purity ofall these gases was better than 99.5%. Acetylene isstabilized by acetone. The monomers were dosed at20–21 sccm gas flow into the plasma reactor. Plasmapolymerization was obtained with the help of apulsed r.f. plasma. The duty cycle was 0.1 at a

3frequency of 1*10 Hz. Layers of approximately10–50 nm thickness were deposited at 4 W Weffective

on gold evaporated glass substrates within 20 min.Aged specimens were obtained in the same simpleway described for plasma poly(styrene) in Section2.2.1.

2.3. X-ray photoelectron spectroscopy

Fig. 1. Schematic representation of the duty cycle used for the The main body of the XPS work was done usingpulsed 13.56 MHz r.f. plasma film deposition process applied in

an oil free pumped VG Scientific ESCALAB 200Xthis study.electron spectrometer (VG Scientific, East Grinstead,UK). Spectra were acquired with Al Ka or Mg Ka

2.2. Preparation of plasma polymerized filmsexcitation (15 kV, 20 mA) at a take-off angle of 158

relative to the sample normal. The SCIENTA ESCA-2.2.1. Plasma poly(styrene)

300 software (SCIENTA AB, Uppsala, Sweden) wasLiquid styrene (99% purity specified for synthesis

used for XP spectra analysis, for quantitative XPS¨applications; Merck-Schuchhardt, Munchen, Ger-

the respective VG ECLIPSE data system routine. Formany) was held at 508C in a round bottom flask

an estimation of the relative intensities (atomicwhich was connected to the plasma chamber by a

ratios) and surface concentrations (in %) we usedheated stainless steel gas-pipe. A dosing valve

Scofield’s cross sections [20], inelastic electron meanpermitted to keep the pressure of the styrene vapor in 0.7free path lengths | E and a transmission | Ekin kinthe plasma reactor chamber at 8 Pa. Styrene was

(CRR mode). The uncertainty of the atomic ratios ispolymerized by a pulsed r.f. plasma at a duty cycle

estimated to be better than 610%.3of 0.1 at a frequency of 1*10 Hz. An approximatelyA home made plasma reactor was attached to the

100-nm thick plasma poly(styrene) layer was de-electron spectrometer. Its base pressure was around

posited on a gold evaporated glass substrate at 3 W 2610 Pa. Samples which were plasma coated in thisW within 15 min.effective chamber could be transferred into the UHV analysis

Aged plasma poly(styrene) samples were simplychamber for XPS without exposure to ambient air.

obtained by storing freshly prepared and character-ized films in ambient air for a certain time withoutexposure to light. 2.4. X-ray absorption spectroscopy (NEXAFS)

2.2.2. Plasma poly(acetylene), poly(butadiene) and NEXAFS spectroscopy was carried out on thepoly(ethylene) HE-TGM2 monochromator beam line (33.12) at the

The plasma polymerization of acetylene, butadiene synchrotron radiation source BESSY I (Berlin, Ger-and ethylene was carried out at a pressure of 8 Pa at many). Spectra were acquired at the C K-edge in theconstant monomer flow, which was adjusted by a gas partial electron yield mode (PEY) using a retarding

¨flow controller (MKS, Munchen, Germany). The voltage of 2150 V. The monochromator resolutionmonomers used in this study were acetylene (Mes- at the C 1s edge was better than 0.8 eV. Raw spectraser-Griessheim AG, Berlin, Germany), 1,3-butadiene were divided by the monochromator transmission(Messer-Griessheim AG, Berlin, Germany) and function which was obtained with a freshly sputtered


Au sample. Energy alignment of the energy scale reflectance (GIR) method at an angle of incidence of21was achieved by using the C 1s→p* resonance, 708 and a resolution of 4 cm . Silicon wafers with

measured with a pyrolytic graphite sample (Ad- gold or chromium coatings providing a reflectivevanced Ceramic Corp., Cleveland, USA). It was background were used as a substrate for the plasmafixed to the value of 285.4 eV [21]. Characteristic polymer films. The software OMNIC 5.1 (Nicolet,features in the flux monitor signals were used to Madison, WI, USA) was employed for measurementalign the energy scales of the spectra. Reproducibil- and data reduction as well as for correction of theity of the spectra was carefully checked by multiple background. All spectra are displayed with CO and2

scanning. Spectra are shown with the pre-edge count H O bands subtracted. In order to do this, these2

rate subtracted and after normalization in units of the bands were measured before in the empty infraredabsorption edge jump [19]. NEXAFS spectra were spectrometer.recorded at an angle of 558, measured between thesurface plane of the sample and the direction vector

2.6. Deposition controller (micro balance)of the incident, linearly polarized light beam. Theangle measured between the E vector of the

To estimate the deposition rate a quartz microsynchrotron light and the surface normal of a sample

balance EDWARDS FTM5 (Edwards High Vacuumwas also 558.

International, West Sussex, UK) was placed insidePrincipally, absorption spectra can be measured by

the plasma chamber, next to the sample to be coatedmonitoring the non-radiative decay (electron yield)

by the plasma polymer. The density of the plasmaof core holes, created in the sample by the photo 23polymers was assumed to be 1 g cm .absorption process itself [19]. Photoelectrons as wellas C-KLL series electrons contribute to this electronyield signal measured by a channeltron detector.Surface sensitivity is achieved by using the partial 3. Results and discussionelectron yield mode because low-energy electrons,which may originate from deeper layers, were sup- 3.1. Plasma polymer deposition and polymerizationpressed by the pre-set threshold energy from entering mechanismsthe detector. The information depth, which is definedto be the depth, from which 95% of the detected Deposition rates for the monomers were measured

˚electrons originate [22], is estimated to be ¯30 A with the help of a micro balance. Film thickness vs.for the C K-edge signal [23]. Here it was assumed time of deposition data obtained for different plasmathat the respective Auger electrons dominate the modes are summarized in Fig. 2. Comparison of themeasured partial electron yield signal [19]. pulse plasma mode results measured at equal process

Another home made plasma reactor (base pressure parameters reveals two different groups: there is2510 Pa) was attached to the UHV NEXAFS ap- styrene with a rather high deposition rate and

paratus and a plasma poly(ethylene) sample prepared acetylene, butadiene and ethylene with rates whichby a d.c. plasma process could be transferred into the are considerably lower (of the order of one tenth).analysis chamber without exposure to ambient air. Within the latter group acetylene is characterized byThe analysis chamber comprises an XP analyzer the highest deposition rate.(VG CLAM 2) and a VG double anode X-ray gun. The observation of these two groups is in accord-More details on the layout of the NEXAFS/XPS ance with the results of Yasuda [25] who concludedfacility are given in Ref. [24]. that cyclic, alicyclic and aromatic monomers yield

higher deposition rates than aliphatic monomers2.5. FTIR spectroscopy because they are characterized by a higher poly-

merizability. Considering the low-deposition-rateThe infrared spectra were measured with a group one has to state the higher individual plasma

NICOLET NEXUS infrared spectrometer (Nicolet, polymerizability for acetylene in comparison toMadison WI, USA) using the grazing incidence butadiene and ethylene. Again this observation is in


Fig. 2. Film layer thickness vs. deposition time measured with a quartz micro balance for styrene, acetylene, butadiene and ethylene used asmonomers in a pulse plasma deposition process. r.f. pulse plasma process parameters: monomer flux565 sccm, W 515 W, Yasudaeffective

21 21 21 21factors50.009 J kg (acetylene), 0.008 J kg (ethylene), 0.004 J kg (1,3-butadiene) and 0.002 J kg (styrene). Data taken from a cwr.f. plasma styrene deposition process are given for comparison (plasma process parameters: monomer flux565 sccm, W515 W r.f. power,

21Yasuda factor 0.002 J kg ). Schemes for the suggested polymerization mechanisms are sketched.

good agreement with similar observations in the volved in the formation of these plasma polymers.literature [26], where the following ranking of the Most probably they grow by attachment of acetylenicplasma polymerizabilities of monomers in a correla- fragments via substitution reaction at the triple bond.tion to the degree of saturation of the carbon bond This idea is supported by mass spectrometric analy-involved is derived: sis of the respective plasma phase [30]. Here C ,2

C H and C H species are detected with high2 2 2alkanes , alkenes , alkynes. abundance.The higher rate of deposition of acetylene versus

Styrene preferentially polymerizes via radicals the other two monomers could be a hint for the[27] which are stabilized by the aromatic system. importance of an acetylenic intermediate. In thatThis feature provides the possibility for a radical context the appearance of bands for C≡C-triplegrafting mechanism. Fragmentation and random bonds in the IR spectra of ethylene and acetylenerecombination, known as ‘polyrecombination’ [28] plasma polymers might be interesting (cf. Ref. [31]or ‘atomic polymerization’ [29] of the styrene mono- and Fig. 8 of this study). However, triple bonds maymer during the plasma pulse are less important. also be formed by exposure of the growing plasma

In contrast to that, the three other monomers polymer to vacuum UV radiation.polymerize preferentially (butadiene) or almost ex- Finally, styrene pulse plasma deposition rates areclusively (ethylene and acetylene) via the way of found to be higher than for a cw plasma with thefragmentation /polyrecombination. One important re- same value of W . This should be due to theeffective

action step in this mechanism should be the forma- lower UV radiation load and reduced particle etchtion of acetylenic intermediates. There are some phenomena enabled by the relevant plasma-off timeindications that triple bond species should be in- interval.


Table 1Detailed assignments of relevant features in the atactic reference polystyrene and the plasma polystyrene IR spectra presented in Fig. 4

21Wave number (cm ) Type of vibration Remarks

3027 n C–H aromatic2927 n C–H aliphatic2950 n CH observed only for plasma polymers,s 3

branching results in enhanced end groupconcentrations

2924 n CHas 2

2844 n CHas

2000–1665 ‘five aromatic fingers’1491 n C=C aromatic1446 d CH2

756 v aromatic ring (out-of-plane) mono-substituted702 v aromatic ring (out-of-plane) mono-substituted584 v aromatic ring (out-of-plane) probably multi-substituted ring,

(or d C≡CH) band observed only for plasma polymers

3.2. Chemical characterization of plasma polymer similar IR spectra. The main finger-printing IR bandsfilms by spectroscopy of poly(styrene) are unequivocally found: Both bands

21of the mono-substituted aromatic ring at 702 cm213.2.1. Plasma poly(styrene) and 756 cm , which are assigned to out-of-plane

IR and XP spectra obtained with plasma poly- deformation bending, the ‘five aromatic finger’ bands21(styrene) films prepared by different plasma pro- between 1665 and 2000 cm and the two bands at

21 21cesses and a reference atactic poly(styrene) film 1446 cm and 1491 cm due to dCH and nC=C2

prepared by spin coating are presented in Figs. 4 and aromatic vibrations. Furthermore, the bands related5. Relevant hydrocarbon IR assignments and XP to aromatic and aliphatic C–H stretching occur in the

21 21spectroscopic data are given in Tables 1 and 2, region of 3027 cm and 2927 cm , respectively.respectively. A XPS survey scan of the pulse plasma Exclusively in the spectra of the plasma polymerized

21deposited poly(styrene) film (Fig. 3) shows that it is styrene a band occurs at 584 cm , which can beactually free of oxygen, i.e., the O surface con- interpreted as a ring deformation vibration of acentration is below the detection limit of XPS. Fig. 4 poly-substituted aromatic ring [32]. Another bandverifies that r.f. pulsed plasma as well as cw plasma appears exclusively in the spectra of the plasma

1deposition results in films characterized by very polymers at 2950 cm that is due to asymmetric CH3

Table 2aSpectroscopic features measured with r.f. pulse plasma deposited samples. C 1s analysis was set up following Ref. [16]

Sample FWHM of Shake-up K-edge FWHM of Intensity ofC 1s (eV) intensity p* resonance p* resonance p* resonance

(% of total C 1s energy (eV) (eV) (eV3edgeintensity) jump units)

r.f. pulse plasma 1.37 7.0 (9.5) 285.2 0.74 2.15poly(styrene)r.f. pulse plasma 1.47 – 285.6 1.05 0.93‘poly(ethylene)’r.f. pulse plasma 1.64 – 285.1 1.10 2.45‘poly(acetylene)’r.f. pulse plasma 1.59 2.0 (2.5) 285.2 0.88 2.58‘poly(butadiene)’

a The shake-up quantification numbers given in round brackets are taken from the respective reference polymer sample.


Fig. 3. Waterfall presentation of XPS survey scans of r.f. pulse plasma deposited polymer films (plasma process parameters: pressure58 Pa,21monomer flux521 sccm for acetylene and ethylene, 20 sccm for 1,3-butadiene, W 54 W, Yasuda factors50.007 J kg (for acetyleneeffective

21and ethylene), 0.004 J kg (for 1,3-butadiene)). The films were not exposed to air prior to analysis. The O 1s contribution found with theplasma ‘poly(acetylene)’ is due to a stabilizer (acetone) in the acetylene used in the experiment. The features at high BE are due to C KLLAuger electrons, those at ¯220 eV due to Al Kb X-ray satellite excited C 1s photoelectrons.

vibrations. The most intense band in the region of discussed to be a band mixed from C 2s and 2paliphatic nCH vibrations in the spectrum of the contributions [33]. The other contributions are de-atactic reference polymer sample occurs at 2924 rived from C 2p orbitals. Additionally, an intense C

21cm . This band originating from asymmetric CH 1s shake-up feature (cf. Table 2) was observed with2

vibrations of the backbone is substantially reduced in the plasma film.the spectra of the plasma polymers. The conclusion The IR and XPS results were successfully crossis that the backbone of the plasma polymers should checked by NEXAFS spectroscopy, too. Fig. 6be cross-linked and the number of methyl end-groups presents the respective C K-edge spectra. Bothis increased. characteristic C 1s→p* resonances (Ref. [19], p.

Considering the XPS results, C 1s core level 230) are unequivocally found in the spectrum of the(including its p→p* shake-up feature) and valence plasma polymer. These can be assigned with p*(e )2u

band spectra (Fig. 5), a rather good agreement and p*(b ) according to the respective benzene2g

between the data of the pulse plasma deposited film features. The backbone related feature at 287.5 eV,and the spin coated reference can be stated. The which was discussed to represent a C–H* resonancevalence band features characteristic of poly(styrene) [19] or a 3p Rydberg state [33], and different Cwere unequivocally found (cf. Ref. [33], note that the 1s→s* resonances above 292 eV were observed inauthors incorrectly shifted their spectra by roughly 3 both cases. It cannot be excluded that also someeV). These are indicated in Fig. 5A, B, which are minor carbonyl-related contributions, which occurrelated to C 2s atomic orbitals, and C, which is usually between 288 and 290 eV, are within the


Fig. 4. Waterfall presentation of FTIR absorbance spectra of r.f. pulse and cw plasma deposited poly(styrene) films in comparison to that of21an atactic poly(styrene) reference sample. All absorbance tracks are normalized to the intensity of the band measured at 705 cm . The pulse

21plasma process parameters were monomer flux5100 sccm, W 515 W, Yasuda factor50.001 J kg . cw plasma was used under theeffective21same conditions with a monomer flux5100 sccm, W515 W, Yasuda factor50.001 J kg . The films were exposed to air before analysis.

spectrum. The features in the plasma polymer spec- result is successfully cross-checked by the respectivetrum are broader. This effect is usually observed C 1s shake up intensity monitored by XPS (cf. Tablewith plasma treated or deposited samples. One 2).reason could be here that multiple substitution of the Obviously, the spectroscopic results suggest thataromatic ring, which was concluded from the FTIR the primary chemical structure of the pulse plasmaspectrum of the plasma-deposited samples (cf. Fig. poly(styrene) film is very similar to that of the4), results in a slight variation of the resonance reference poly(styrene) sample. This result is notenergies. unexpected because we were running a rather ‘gen-

Applying the NEXAFS spectroscopy method the tle’ plasma process using a monomer with an easilyretention of aromatic rings within the plasma film polymerizable functionality, which should result in acan be discussed relying on the intensities of the radical grafting mechanism. Comparison of the FTIRrespective p* resonances. The result was that spectra of cw r.f. and pulse r.f. plasma-depositedring1

with the plasma film a p* resonance intensity of samples revealed a reduced number of multiple ringring1

more than 75% of that measured with the atactic substitutions for the pulse plasma polymer samplepoly(styrene) reference sample was reached. This pointing to its higher degree of regularity.


Fig. 5. XPS core level and valence band spectra of a r.f. pulse plasma deposited poly(styrene) film in comparison to those of an atacticpoly(styrene) reference sample. XPS: Al Ka radiation, 300 W, CRR 20 (reference), Mg Ka radiation, 300 W, CAE 10 (plasma polymer). The

21pulse plasma process parameters were pressure58 Pa, monomer flux521 sccm, W 53 W, Yasuda factor50.007 J kg . The plasmaeffective

polymer film was not exposed to air prior to analysis.

3.2.2. Plasma ‘poly(ethylene)’, plasma styrene 4 butadiene . ethylene . acetylene.‘poly(aceytelene)’ and plasma ‘poly(butadiene)’

Acetylene as well as ethylene and butadiene were The influence of the stability or reactivity of thesupposed to react with substantially lower radical respective macroradicals on the radical polymeri-grafting rates as assumed for the case of styrene. sation process has to be considered here, too. In theSeveral factors affect the efficiency of a radical graft case of styrene the resonance stabilization gives apolymerisation process. These are the activation comparably high life time of the respective mac-energy of the polymerizable bond, the reactivity and roradicals thus enabling them to grow continuouslystability of the formed macroradicals, the density of by attachment of monomers from the surroundings.monomer molecules in the surroundings of these On the other hand, radicals derived from acetyleneradicals and certain steric conditions. The poly- and ethylene are highly reactive. Reaction withmerizability via radical grafting of the monomers oxygen, radical–radical recombination or dispropor-used in this study can be ranked using the activation tionations easily quench them. Because the radicalenergies of the respective polymerizable bonds (cf. grafting mechanism is less characteristic of theRef. [34]): polymerization of ethylene, butadiene and acetylene,


Fig. 6. NEXAFS C-K edge spectra of a r.f. pulse plasma deposited poly(styrene) film and an atactic poly(styrene) reference sample. Pulse21plasma process parameters: monomer flux565 sccm, W 57.5 W, Yasuda factor50.001 J kg . The plasma film sample was exposed toeffective

air prior to analysis.


it can be expected to get considerably unsaturated signal as verified by Fig. 3. This is taken as a proofplasma polymer films. To name the resulting samples for a well-controlled deposition process.in the respective figures and tables we used the term The overall result was that using these threepulse plasma ‘poly(monomer name)’. The quotation different monomers plasma polymer films weremarks were used to indicate that the resulting film is obtained characterized by qualitatively very similarnot really this polymer in the literal sense. spectroscopic features. The differences are rather

Because the UHV technology based reactor subtle. Therefore, we discuss these results in onechamber was directly connected to the spectrometer block.XPS analysis could be carried out avoiding any air The respective XPS core level and the valenceexposure of the plasma polymer films prepared from band region spectra are summarized in Fig. 7,ethylene, butadiene and acetylene. This is rather selected spectroscopic data are given in Table 2. Theimportant because these plasma polymers are well C 1s XP spectra suggest that there is actually noknown to be rather unstable in ambient air. The XP oxidation of the samples. However, careful analysisspectra of these samples — except that which was revealed for plasma ‘poly(acetylene)’ a 5% C–Omade from acetylene — did not show any oxygen related contribution on the high energy side of the C

Fig. 7. XPS core level and valence band spectra of r.f. pulse plasma deposited ‘poly(ethylene)’, ‘poly(acetylene)’ and ‘poly(butadiene)’ filmsin comparison to those of commercial poly(propylene) and poly(butadiene) reference samples. XPS on plasma polymer films: 300 Wnon-monochromatized Al Ka radiation, CAE 10 for C 1s and CAE 20 for valence bands; XPS on poly(butadiene): 300 W non-monochromatized Al Ka radiation, CRR 20; XPS on poly(propylene): 300 W non-monochromatized Mg Ka radiation, CRR 40. The pulseplasma process parameters were: pressure58 Pa, monomer flux521 sccm for acetylene and ethylene, 20 sccm for 1,3-butadiene,

21 21W 54 W, Yasuda factors50.007 J kg (acetylene and ethylene), 0.004 J kg (1,3-butadiene). The plasma polymer films were noteffective

exposed to air before analysis.


Fig. 7. (continued)

1s peak. Exclusively in the spectrum of plasma band spectrum of a poly(propylene) reference sam-‘poly(butadiene)’, a p→p* transition related shake- ple, where the side chain methyl C 2s contributionup feature can be found. occurs just at this binding energy [35], one may

Considering the valence band spectra of the conclude, that this could be related to substantialplasma polymers presented in Fig. 7 no significant branching or cross-linking in the plasma polymerdifferences can be found. Comparison to a poly- samples. These phenomena are usually expected for(butadiene) reference sample spectrum reveals a plasma polymers. It should be mentioned that thegood correspondence of the overall shape. However, maximum in the valence band intensity of thewe found a higher density of states around 16 eV in reference poly(butadiene) sample was found at |19the plasma polymer spectra. Relying on the valence eV (cf. Fig. 7). This interpretation might be supported


21by the FTIR spectra of pulse plasma ‘poly(ethylene)’ cm , respectively. Furthermore, other bands charac-21and ‘poly(butadiene)’ in Fig. 8 where at 1264 cm teristic of unsaturated species were found. One of

a band is clearly observed which can be correlated to them is a .C=C=C, stretching vibration in21a skeletal vibration of (CH ) C species. This vi- cumulene units which was observed at 1947 cm3 3

21bration band can be taken to verify branching. The [36,37]. The shoulder around 1600 cm at the21same band but with rather low intensity is also carbonyl band (1720 cm ) can be interpreted as

observed in the FTIR spectrum (cf. Fig. 8) of aged aromatic or cumulated C=C double-bond stretching21pulse plasma ‘poly(acetylene)’. vibrations. Around 2950 cm the symmetric and

The FTIR spectra of the plasma polymer films asymmetric aliphatic CH and CH stretching vi-3 2

made from acetylene (fresh and aged) or ethylene are bration bands are observed, the respective deforma-21 21qualitatively similar. They are presented in Fig. 8. tion vibrations between 1440 cm and 1370 cm .

Each of them is characterized by a band in the region As to be expected for air exposed samples oxygen21of 3300 cm which is assigned to a H–C≡ stretch- functionalities are represented by the OH vibration at

21ing vibration. Further indications for the existence of 3430 cm , the carbonyl band (already mentioned)21C≡C-triple bonds are the R9–C≡C–R and the H– and a band at 1020 cm that could be due to a

21C≡C–R stretching vibrations at 2198 cm and 2102 C–COH stretching vibration.

Fig. 8. FTIR absorbance spectra of r.f. pulse plasma deposited ‘poly(ethylene)’, ‘poly(acetylene)’ and ‘poly(butadiene)’ films. Pulse plasma21 21process parameters: monomer flux5100 sccm, W 57.5 W, Yasuda factors50.004 J kg (acetylene), 0.003 J kg (ethylene) and 0.001effective

21J kg (1,3-butadiene). The nominally fresh ‘poly(acetylene)’ film was exposed for some minutes, the other films for several hours to airbefore analysis.


Many of the IR features discussed above are alsofound in the spectrum of pulse plasma ‘poly-(butadiene)’ in Fig. 8. Additionally, some indicationsfor the existence of aromatic rings in this plasmapolymer film are found: There are two well resolved

21 21bands at 744 cm and 701 cm that can becorrelated to aromatic out-of-plane vibrations. In the‘poly(butadiene)’ spectrum two shoulders are found

21at the C=O vibration. The new one at 1660 cmoccurs in a region where C=C stretching vibrationstypically appear.

The C K-edge NEXAFS spectra clearly confirmthe presence of unsaturated carbon species becauseintense C 1s→p* resonances are measured around285 eV (cf. Fig. 9). This is interesting because nop→p* shake-up features are found in the C 1s XPspectra of plasma ‘poly(ethylene)’ and ‘poly-(acetylene)’. Obviously, the respective intra-bandtransition probabilities seem to be rather low. Itbecomes clear that NEXAFS spectroscopy is thebetter choice when the question of the existence ofunsaturated carbon species has to be answered.Considering the C K-edge spectra of the plasmapolymer films discussed in this section commonfeatures and differences have to be stated. We foundp* resonances in any case, however, with differentintensities, different FWHM and at different reso-nance energies (cf. Table 2). ‘Poly(acetylene)’ and‘poly(butadiene)’ are characterized by intense p*resonances at very similar energies but the FWHM issignificantly higher with the ‘poly(acetylene)’ sam-ple. The p* resonance of ‘poly(ethylene)’ is smalland occurs at ¯0.5 eV higher energy. The conclu-sion is that there must be certain differences in thecharacter, abundance and frequency of unsaturatedcarbon species in the films which contribute to thep* resonance. Principally, these should be theseobserved in the IR spectra, too. Additionally, conju-gation effects may influence the spectroscopic pa-rameters of the p* resonances. Therefore, a simplecorrelation of the concentration of unsaturated car-bon species in the films with the integrated area ofthe p* resonance could be misleading. This problem

Fig. 9. NEXAFS C-K edge spectra of r.f. pulse plasma depositedrequires detailed consideration in the future. How-‘poly(ethylene)’, ‘poly(acetylene)’ and ‘poly(butadiene)’ films.ever, there is no doubt that in the case of ‘poly-Pulse plasma process parameters: monomer flux5100 sccm,

(ethylene)’ the concentration of unsaturated carbon 21W 57.5 W, Yasuda factors50.004 J kg (acetylene), 0.003effective21 21species is significantly lower in comparison to the J kg (ethylene) and 0.001 J kg (1,3-butadiene). The films

other plasma polymer samples. It should be men- were exposed to air prior to analysis.


tioned that in the case of ‘poly(butadiene)’ the carbonyl related contribution is found. The integra-integrated area of the p* resonance reaches that tion of the C 1s→ p* resonance suggestsunsaturated

measured for the poly(butadiene) reference polymer that it contains roughly 50% less non-saturatedsample. carbon species than conventional poly(butadiene)

Other qualitatively common features of the K-edge which is characterized by a saturated to unsaturatedspectra presented in Fig. 9 are the s* resonance carbon atom ratio of 1. The relatively large FWHMregion beginning above 292 eV and the resonances of the C 1s→ p* resonance observed forunsaturated

between 287 and 291 eV which are due to C 1s→C– this plasma polymer sample suggests once again theH* and→p* resonances. The latter occurs be- coexistence of a certain number of different unsatu-C=O

cause the samples were exposed to air prior to rated carbon species with slightly different resonanceanalysis. energies.

In another study a combined NEXAFS/plasmareactor apparatus was used to check out the possi-bility of an estimation of the relative amount of 4. Stability and aging of plasma polymer filmsunsaturated carbon species in a layer deposited fromethylene but applying a low pressure and low power As already mentioned fragmentation is a minord.c. plasma process. Here the samples could be process in the plasma polymerization of styrene, theimmediately transferred without air exposure to the main process is radical grafting. This provides aanalysis position within the apparatus. Again the positive effect on the stability of the resulting plasmaquestion was of interest, whether there are unsatu- polymer, which can be measured as the oxygenrated carbon species or not. The K-edge NEXAFS uptake vs. time of storage at ambient air. Thespectrum gave a clear answer (cf. Fig. 10): Obvious- respective results are presented in Fig. 11a. Actuallyly, this plasma ‘poly(ethylene)’ film contains a the oxygen-free deposited plasma poly(styrene) sam-significant number of non-saturated carbon species, ple is characterized by an oxygen uptake ,5% attoo. The shape of the C K-edge is very similar to that after 1 month storage time. A comparably lowobtained with the r.f. pulse plasma-deposited ‘poly- number of unreacted radicals and reactive species are(ethylene)’ film, but, as to be expected, almost no enclosed in the plasma poly(styrene) film and, there-

fore, its stability is better in comparison to the othermonomers used for plasma polymerization in thisstudy. XPS analysis of the aged plasma poly(styrene)sample reveals only a small number of carbon atomsto be involved into C–O bonds (cf. Fig. 11b andTable 3).

The oxygen uptake of plasma ‘poly(ethylene)’,‘poly(acetylene)’ and ‘poly(butadiene)’, displayed inFig. 11a, reaches as much as 20% oxygen under thesame storage conditions as in the case of plasmapoly(styrene). Note that, as a consequence of the useof acetone stabilized acetylene, the respective plasmapolymer contains already some oxygen, even when itwas measured without contact to ambient air. Theenhanced oxygen uptake is a consequence of the

Fig. 10. C K-edge spectrum taken from a 100-nm cw plasma- other plasma polymerization mechanisms determin-polymerized ‘poly(ethylene)’ film deposited on a Al foil (process ing the character of the layer formation here. Asparameters: 15 Pa ethylene, 14 sccm gas flow and 1.5 W d.c. discussed in Section 3.1, it is more irregular in21power input, Yasuda factor50.004 J kg ) in comparison to those

comparison to classic radical graft polymerization.taken from well defined reference poly(butadiene) and low densityThe polyrecombination of atoms, radicals and frag-poly(ethylene) samples. The plasma polymer sample was not

exposed to air before analysis. ments gives a branched and cross-linked polymer.


Fig. 11. a: Oxygen uptake of r.f. pulse plasma-deposited ‘poly(ethylene)’, ‘poly(acetylene)’, ‘poly(butadiene)’ and poly(styrene) films duringstorage in ambient air as measured by quantitative XPS. b: Analysis of XP C 1s spectra of r.f. pulse plasma deposited ‘poly(ethylene)’,‘poly(acetylene)’, ‘poly(butadiene)’ and poly(styrene) samples which were 1 month aged in ambient air. Fitting these spectra the BE of theC–O species was fixed to 286.5 eV, that of C=O to 287.9 eV and that of COO to 289.3 eV (following Ref. [16]). Relevant fit results are givenin Table 3.


Table 3 regularity of the plasma polymer the lower is theOxidation phenomena of r.f. pulse plasma deposited samples after oxygen uptake.exposure to ambient air for 1 month as derived from XP C 1s peakfitting analysis of the spectra presented in Fig. 11b

Sample Area of C 1s sub peaks as % of total C 1s 5. Analysis of a plasma polymer layer on thespectrum area

inner barrier wall of a PE vessel–C–O– C=O COO Shake-up

r.f. pulse plasma 1 – – 4.1 In another, strictly technologically motivated in-poly(styrene) vestigation the plasma coating process of the innerr.f. pulse plasma 14 3 2 – wall of a poly(ethylene) vessel was investigated by‘poly(ethylene)’

XPS and NEXAFS spectroscopy. Briefly, a plasmar.f. pulse plasma 10 3 2 –polymer was deposited running a multi step r.f.‘poly(acetylene)’

r.f. pulse plasma 13 7 3 – plasma process, comprising hydrogen plasma pre-‘poly(butadiene)’ cleaning and butadiene plasma polymer deposition,

in a pilot process facility. The goal was to produce adense barrier layer with good adhesion to the poly-

Lots of radicals or other vacancies in the resulting (ethylene) substrate and a high barrier effect againstfilm are not saturated or compensated. Air exposure permeation of benzene (cf. Ref. [40]). The idea wasopens the possibility for diffusion of molecular that a three-dimensionally networked polymer layeroxygen into the bulk, which may recombine with provides the required barrier properties. BecauseC-radical sites: networking means here consumption of the double

bonds of the butadiene monomer, methods, whichC ? 1 ? O–O ? → C–O–O ?are able to detect this bond, at least semi-quantita-tively, are required. Moreover, a technologicallyThen peroxy radicals and, subsequently, hydro-important issue was to find out how homogeneousperoxides are formed:the plasma polymer layer is when different locations

C–O–O ? 1 RH → C–O–OH 1 R ? within the vessel are compared. The task was notonly to check the film thickness but also the chemis-

Hydroperoxides are unstable and may decay or try as an important parameter.rearrange giving rise to many kinds of oxygen Fig. 12a presents a typical C K-edge spectrum offunctionalities. Autoxidation takes place. Usually, the plasma–polymer barrier film. It reveals a signifi-peroxy groups bridge polymer chains causing the cant number of non-saturated carbon species and theformation of carboxylic acids, esters, peroxyesters, existence of carbonyl species. Additionally, IR bandsaldehydes, ketones, ethers and alcohols [38]. The characteristic of non-saturated carbon species, i.e.whole ageing process is closely related to the acetylenic as well as aromatic carbon and carbonylphotoxidation of polyethylene or other polyolefins species were found. The latter result was cross[39]. Fig. 11b displays the C 1s XP spectra of the checked by XPS analysis of the C 1s and O 1spulse plasma polymers aged on ambient air. Detailed spectral regions. A semi-quantitative measure of theinformation on the contributions of species character- number of unsaturated carbon species was obtainedized by C–O, C=O and COO bonds are summarized again by relating the area of the p* resonance in thein Table 3. The ageing effect can also be observed C K-edge spectra of the plasma polymer films to thatby FTIR. Fig. 8 presents the example of pulse one measured with a poly(butadiene) reference sam-plasma ‘poly(acetylene)’ analyzed after several min- ple. The respective results are given in Fig. 12butes (called ‘fresh’) and after several hours of air together with XPS surface concentration data for C,exposure. The increase of the relative intensity of the O and N. The O and N signals originate from thecarbonyl band successfully cross checks the XPS plasma processing where air leaks cannot be fullyresult. avoided. The result was that the surface of the barrier

Finally the rule may be derived that the higher the layer seems to be chemically rather homogeneous


Fig. 12. Analytical data obtained with specimens taken from different locations of a poly(ethylene) vessel coated with a thick plasmapolymerized barrier film. The film was deposited using butadiene. Specimens were exposed to air for several hours prior to analysis. (a)Characteristic C K-edge spectrum. (b) Left ordinate: semi-quantitative estimation of the concentration of unsaturated carbon species obtainedby integration of the p* resonance area and its relation to the p* resonance area of the spectrum of a poly(butadiene) reference sampleunsat.

(set to 100%). Right ordinate: XPS C, O and N surface concentrations.

within the coated vessel pointing to a well developed plasma polymer films are derived from the respectivecoating process. However, there is still a consider- p* resonances in the C K-edge spectra. The exist-able number (roughly 60% of that measured for the ence of this kind of species can be cross-checked bypoly(butadiene) reference) of unsaturated carbon IR spectroscopy and XPS where characteristic vi-species in the coating. This suggests that there is a bration bands and shake-up features in the C 1s corepotential for more branching and networking in the level spectra are observed.plasma polymer layer and the deposition process The main result from the point of view of materi-requires further optimization. als science was that with styrene chemically rather

well defined and stable plasma polymer layers can beformed applying a pulse plasma process. Theirspectral features strongly resemble those obtained

6. Conclusion with conventional poly(styrene) suggesting a verysimilar primary chemical structure. Using ethylene,

Applying a multi method approach comprising the acetylene or butadiene as a monomer in a plasmause of XPS, NEXAFS and FTIR spectroscopy a deposition process, however, plasma films can becomprehensive chemical analysis of plasma-poly- obtained with a primary chemical structure, whichmerized films can be obtained. XPS is well suited to seems to be similar to each other. It is, more or less,provide the quantitative elemental analysis of the different from those of the respective conventionalfilms. Bond specific information can be taken from polymers. These films are characterized by relativelyIR spectra, C 1s shake-up features and XPS valence high (‘poly(acetylene)’ and ‘poly(butadiene’) orbands as well as K-edge absorption spectra by lower (‘poly(ethylene)’) concentrations of unsatu-relying on spectral fingerprints obtained with refer- rated carbon species comprising double and tripleence samples. Semi-quantitative information on the carbon bonds, aromatic rings and, most probably,amount of unsaturated carbon species existing in more complex (conjugated) p systems.


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Chemical Analysis of Plasma-polymerized Films