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Studies of the earliest stages of plasma-enhanced chemical vapor deposition of SiO2 on polymeric substrates
Thin Solid Films 382 Ž2001. 1᎐3
Letter
Studies of the earliest stages of plasma-enhanced chemical vapor deposition of SiO 2 on polymeric substrates G. Dennler a , A. Houdayer b , M. Latreche ` c , Y. Segui ´ d , M.R. Wertheimer a,U a
Groupe des Couches Minces (GCM) and Department of Engineering Physics and Materials Engineering, Ecole Polytechnique, C.P. 6079, Station Centre-Ville, Montreal, QC H3C 3A7 Canada b Groupe des Couches Minces (GCM) and Department of Physics, Uni¨ ersite´ de Montreal, ´ Montreal, QC H3C 3J7 Canada c Polyplasma Inc., 3740 Jean Brillant, Montreal, QC H3T 1P1 Canada d Laboratoire de Genie ´ Electrique de Toulouse (LGET), Uni¨ ersite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France Received 17 July 2000; received in revised form 7 September 2000; accepted 27 September 2000
Abstract We have studied the structure of hyper-thin Žthickness, dF 10 nm. SiO 2 coatings deposited by plasma-enhanced chemical vapor deposition ŽPE-CVD. on various polymers Žpolypropylene, polyimide, polyethyleneterephthalate .. Rutherford backscattering spectroscopy ŽRBS. has shown that the concentration of silicon atoms per unit area is a linear function of the deposition time, t, for t G 0.5 s. Using reactive ion etching ŽRIE. in O 2 plasma, we observe that the coatings are continuous, not island-like, even for df 2 nm; this is confirmed by X-ray photoelectron spectroscopy ŽXPS., at high values of the take-off angle. In conclusion, PE-CVD film growth on polymers occurs in a layer-by-layer ŽFrank-van der Merwe., not in a Volmer᎐Weber Žisland coalescence . mode. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma deposition; Polymers; Silicon oxide; Growth mechanism
Transparent barriers such as SiO 2 against oxygen andror water vapor permeation through polymers are the object of increasing interest in the food and pharmaceutical packaging industries, and more recently in encapsulation of organic-based displays. Plasma-enhanced chemical vapor deposition ŽPE-CVD. is one of the preferred routes to deposit thin SiO 2 coatings on plastic webs with high throughput w1x. Their usual thickness, d, which provides an adequate barrier is approximately 20 nm on polyester ŽPET., e.g. close to the so-called ‘critical thickness’, d c , approximately 15 nm in this case w2x. For dF d c , the oxygen transmission U
Corresponding author. Tel.: q1-514-340-4749; fax: q1-514-3403218. E-mail address: [email protected] Ž M.R. Wertheimer..
rate ŽOTR, in standard cm3rm2 per day per bar. is roughly that of the bare polymer, while for dG d c OTR is reduced by up to three orders of magnitude, attaining an asymptotic limiting value w2x. In earlier work in this laboratory, A.S. da Silva Sobrinho et al. w3x correlated OTR and the number density, n, of defects per unit surface area of the coating, the latter being due mostly to dust particles and anti-block agents on the polymer surface. It was also shown that n and OTR have exactly the same qualitative variation with d, even for d close to, but below d c w3x. This, along with numerical agreement of calculated OTR values, confirmed that permeation is basically controlled by the number of defects in the coating. The present study is the first to investigate the region dF d c systematically; other authors w4x interpreted the
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 7 8 1 - 8
2
G. Dennler et al. r Thin Solid Films 382 (2001) 1᎐3
absence of barrier here by invoking a nucleation phase. According to this interpretation, Volmer-Weber growth takes place w5x, resulting in island-like, discontinuous SiO 2 coatings. In order to test this interpretation, we have deposited hyper-thin SiO 2 coatings in a radiofrequency ŽRF. PE-CVD reactor described in detail elsewhere w2x, using hexamethyldisiloxane ŽHMDSO., oxygen and argon feed-gas mixture in the proportions 1:6:3, respectively. Various polymers were used as substrates, namely polyimide ŽPI, 50 m, Dupont KaptonH 䊛 ., polyethyleneterephthalate ŽPET, 50 m, Dupont Mylar 䊛 ., and polypropylene ŽPP, 38 m, Hercules.. Since the same d c values Žf 15 nm. were found for SiO 2 deposits on PI and PET, all film thicknesses discussed hereafter are in the range 1 F d- 15 nm. Knowing the pseudo-Brewster angle for the particular polymers, we are able to measure d by variable angle spectroscopic ellipsometry ŽVASE, J.A. Woollam Co.. w6x directly on the polymer, but only for dG 10 nm. For dF d c , VASE measurements were performed on silicon witness platelets placed on the polymers during deposition. We also used X-ray fluorescence and Rutherford backscattering spectroscopy ŽRBS. to measure d, the latter to evaluate the elemental silicon concentration per unit area vs. deposition time: 1 MeV ␣ particles ŽTandetron, University of Montreal. were made to impinge at normal incidence on the substrate surface, and backscattered particles were collected at an angle of 30⬚ to the normal. We determined the ratio A Si rAC , where A Si is the area under the silicon RBS peak and A C is that of a window chosen at small energy values beneath the carbon RBS signal. A Si rAC was found to be independent of counting time, indicating that the thin SiO 2 layer on the polymer is not appreciably modified by the ␣ particle beam. Fig. 1, a plot of A Si rA C vs. deposition time for various film samples of PI, shows the two quantities to be proportional, with a correlation coefficient R s 0.999. This implies that the sticking coefficient of Si precursors is independent of the substrate chemistry, since it is seen to be constant during the entire growth process. In previous articles w2,7x, we have described a new method to detect and characterize submicrometer breaches in SiO 2 on polymers, a technique based on reactive ion etching ŽRIE. with an oxygen plasma, under relatively mild ion bombardment conditions. Since the SiO 2 coating is inert towards atomic oxygen ŽAO., while polymers are rapidly etched w8x, the latter occurs on portions of the substrate which are not coated. The surface topography is then greatly enhanced, which can readily be observed by scanning electron microscopy ŽSEM.. Fig. 2 is a SEM image of a 2-nm-thin SiO 2 coating on PET, after the sample had been exposed to oxygen plasma for 10 min. Clearly, no evidence for island-like surface coverage is observable,
Fig. 1. A Si rAC Žsee text. vs. deposition time, t, evaluated by RBS.
the bright round spots being free-standing SiO 2 film over cavities etched in the polymer. The tiny black points in the center of each bright zone are the primary ‘pinhole’ defects in the coating, through which the AO penetrated to the polymer substrate w2,7x. Evidently, this is the exact opposite of an island structure, resulting from Volmer᎐Weber growth, where the resulting nuclei Žfor 2 nm nominal film thickness . would not be detectable at this relatively low level of magnification. However, the fact that we can observe the free-standing film over the cavities allows us to definitively state that this 2 nm SiO 2 coating is continuous, albeit with many small holes. Manifestly, ion bombardment during RIE was insufficiently intense to cause sputter-removal of the SiO 2 , but it cannot be excluded that it contributed to the formation of the holes in this exceedingly thin SiO 2 film. The same experiments have also been conducted with PI and PP substrates, and precisely the same behavior was observed. To further confirm these conclusions, we also obtained X-ray photoelectron spectroscopy data ŽXPS,
Fig. 2. Scanning electron micrograph of a 2-nm-thin SiO 2 coating on PET, following 10 min of exposure to oxygen plasma.
G. Dennler et al. r Thin Solid Films 382 (2001) 1᎐3
Fig. 3. Silicon, carbon and nitrogen high resolution XPS spectra of a 2-nm-thin SiO 2 coating on Kapton 䊛 polyimide; the full and dotted spectra correspond to take-off angles ŽTOA. of 0 and 80⬚, respectively.
VG ESCALAB 3 MKII, non-monochromated MgK ␣ radiation. from hyper-thin coatings on PI. This polymer, being composed of the elements carbon, hydrogen and oxygen, but also nitrogen, one can distinguish the substrate from the SiO 2 film, where the latter may contain a few percent of carbon, by varying the take-off angle ŽTOA. between 0 and 80⬚. Fig. 3 shows high resolution spectra of a 2-nm-thin SiO 2 film on PI, the three data sets being Si 2s, C 1s and N 1s spectra. For a normal Ž0⬚. TOA, the three elements are clearly detected and their concentrations are found to be 44.0% C, 14.2% Si, 38.8% O and 3.0% N, while those of the uncoated substrate are 79.5% C, 14.3% O and 6.2% N, regardless of the TOA. However, at TOAs 80⬚, the N signal is seen to disappear completely, as does the 288 eV C 1s subpeak, the latter being assigned to the carboxyl linkages of the PMDA group in Kapton 䊛 PI w9x. This combined XPS information from Fig. 3, of course, indicates that the topmost layer is structurally completely different from the PI substrate, even though it is rich in C. This result harmonizes with the ones described above, namely that for dF d c , no evidence for nucleation and Volmer᎐Weber-type growth can be observed: hyper-thin SiO 2 Žf 2 nm. coatings deposited by PE-CVD on polymers do not manifest island-like structure; instead, they completely cover the substrate, which allows us to confirm a layer-by-layer ŽFrank-van der Merwe. growth mechanism w5x. The fact that this surprising deduction can be made for various structurally very different polymers, all of which initially possess low surface energies, can be rationalized as follows. In plasma pre-treatment of polymers for purposes of raising the surface energy, and thereby the adhesive bond strength of a subsequent overlayer, it is known that a residence time in the active plasma zone
3
as short as tens of milliseconds is sufficient w10x. In the presently-used ŽPE-CVD. plasma, which comprises abundant AO, we may therefore surmise that the main effect of the plasma on the polymer surface immediately following initial ignition of the plasma, is to raise the surface energy, i.e. to render this surface ‘wettable’. Moreover, as already pointed out above, RBS measurements have shown the sticking coefficient of SiO 2 precursor species to be independent of the chemical structure of the surface, during all phases of growth Ž0 F d F 100 nm.. Finally, we now have convincing evidence that the absence of barrier properties for dF d c is largely due to the presence of vast numbers of tiny defects, the surface density of which can be evaluated at n f 10 9 cmy2 for the three kinds of polymers investigated so far. As we have shown elsewhere w3,11x such large n values result in total absence of barrier properties, even if the remaining ) 99% of the polymer surface is covered with a well-adhering, impermeable coating. Acknowledgements The authors wish to thank Dr G. Czeremuszkin and Professors L. Martinu and P. Desjardins for useful discussions and comments, and Remi Poirier for his valuable assistance on the RBS measurements. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada ŽNSERC.. G.D. gratefully acknowledges a postgraduate scholarship from CNRS ŽFrance.. References w1x H. Chatham, Surf. Coat. Technol. 78 Ž1996. 1. w2x A.S. da Silva Sobrinho, M. Latreche, G. Gzeremuszkin, J.E. ` Klemberg-Sapieha, M.R. Wertheimer, J. Vac. Sci. Technol. A 16 Ž1998. 3190. w3x A.S da Silva Sobrinho, G. Gzeremuszkin, M. Latreche, M.R. ` Wertheimer, J. Vac. Sci. Technol. A 18 Ž2000. 149. w4x J.T. Felts, Society of Vacuum Coaters, Proceedings of the 34th Annual Technical Conference, Philadelphia, USA, March 17᎐22, Ž1991. 184. w5x M. Ohring, The Materials Science of Thin Films, Academic Press, Boston, MA, 1992. w6x A. Bergeron, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol. A 16 Ž1998. 3227. w7x A.S. da Silva Sobrinho, M. Latreche, G. Gzeremuszkin, G. ` Dennler, M.R. Wertheimer, Surf. Coat. Technol. 116᎐119 Ž1999. 1204. w8x A.M. Wrobel, B. Lamontagne, M.R. Wertheimer, Plasma Chem. Plasma Process. 8 Ž1988. 315. w9x G. Beamson, D. Briggs, High Resolution XPS of Organic Polymer, Wiley, New York, 1992. w10x C. Bichler, M. Bischoff, H.C. Langowsky, U. Moosheimer, Society of Vacuum Coaters, Proceedings of the 39th Annual Technical Conference, Philadelphia, USA, May 5᎐10, Ž1996. 378. w11x G. Gzeremuszkin, M. Latreche, M.R. Wertheimer, Society of ` Vacuum Coaters, Proceedings of the 43rd Annual Technical Conference, Denver, USA, May 17᎐21, 2000, 8.
Letter
Studies of the earliest stages of plasma-enhanced chemical vapor deposition of SiO 2 on polymeric substrates G. Dennler a , A. Houdayer b , M. Latreche ` c , Y. Segui ´ d , M.R. Wertheimer a,U a
Groupe des Couches Minces (GCM) and Department of Engineering Physics and Materials Engineering, Ecole Polytechnique, C.P. 6079, Station Centre-Ville, Montreal, QC H3C 3A7 Canada b Groupe des Couches Minces (GCM) and Department of Physics, Uni¨ ersite´ de Montreal, ´ Montreal, QC H3C 3J7 Canada c Polyplasma Inc., 3740 Jean Brillant, Montreal, QC H3T 1P1 Canada d Laboratoire de Genie ´ Electrique de Toulouse (LGET), Uni¨ ersite´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France Received 17 July 2000; received in revised form 7 September 2000; accepted 27 September 2000
Abstract We have studied the structure of hyper-thin Žthickness, dF 10 nm. SiO 2 coatings deposited by plasma-enhanced chemical vapor deposition ŽPE-CVD. on various polymers Žpolypropylene, polyimide, polyethyleneterephthalate .. Rutherford backscattering spectroscopy ŽRBS. has shown that the concentration of silicon atoms per unit area is a linear function of the deposition time, t, for t G 0.5 s. Using reactive ion etching ŽRIE. in O 2 plasma, we observe that the coatings are continuous, not island-like, even for df 2 nm; this is confirmed by X-ray photoelectron spectroscopy ŽXPS., at high values of the take-off angle. In conclusion, PE-CVD film growth on polymers occurs in a layer-by-layer ŽFrank-van der Merwe., not in a Volmer᎐Weber Žisland coalescence . mode. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma deposition; Polymers; Silicon oxide; Growth mechanism
Transparent barriers such as SiO 2 against oxygen andror water vapor permeation through polymers are the object of increasing interest in the food and pharmaceutical packaging industries, and more recently in encapsulation of organic-based displays. Plasma-enhanced chemical vapor deposition ŽPE-CVD. is one of the preferred routes to deposit thin SiO 2 coatings on plastic webs with high throughput w1x. Their usual thickness, d, which provides an adequate barrier is approximately 20 nm on polyester ŽPET., e.g. close to the so-called ‘critical thickness’, d c , approximately 15 nm in this case w2x. For dF d c , the oxygen transmission U
Corresponding author. Tel.: q1-514-340-4749; fax: q1-514-3403218. E-mail address: [email protected] Ž M.R. Wertheimer..
rate ŽOTR, in standard cm3rm2 per day per bar. is roughly that of the bare polymer, while for dG d c OTR is reduced by up to three orders of magnitude, attaining an asymptotic limiting value w2x. In earlier work in this laboratory, A.S. da Silva Sobrinho et al. w3x correlated OTR and the number density, n, of defects per unit surface area of the coating, the latter being due mostly to dust particles and anti-block agents on the polymer surface. It was also shown that n and OTR have exactly the same qualitative variation with d, even for d close to, but below d c w3x. This, along with numerical agreement of calculated OTR values, confirmed that permeation is basically controlled by the number of defects in the coating. The present study is the first to investigate the region dF d c systematically; other authors w4x interpreted the
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 7 8 1 - 8
2
G. Dennler et al. r Thin Solid Films 382 (2001) 1᎐3
absence of barrier here by invoking a nucleation phase. According to this interpretation, Volmer-Weber growth takes place w5x, resulting in island-like, discontinuous SiO 2 coatings. In order to test this interpretation, we have deposited hyper-thin SiO 2 coatings in a radiofrequency ŽRF. PE-CVD reactor described in detail elsewhere w2x, using hexamethyldisiloxane ŽHMDSO., oxygen and argon feed-gas mixture in the proportions 1:6:3, respectively. Various polymers were used as substrates, namely polyimide ŽPI, 50 m, Dupont KaptonH 䊛 ., polyethyleneterephthalate ŽPET, 50 m, Dupont Mylar 䊛 ., and polypropylene ŽPP, 38 m, Hercules.. Since the same d c values Žf 15 nm. were found for SiO 2 deposits on PI and PET, all film thicknesses discussed hereafter are in the range 1 F d- 15 nm. Knowing the pseudo-Brewster angle for the particular polymers, we are able to measure d by variable angle spectroscopic ellipsometry ŽVASE, J.A. Woollam Co.. w6x directly on the polymer, but only for dG 10 nm. For dF d c , VASE measurements were performed on silicon witness platelets placed on the polymers during deposition. We also used X-ray fluorescence and Rutherford backscattering spectroscopy ŽRBS. to measure d, the latter to evaluate the elemental silicon concentration per unit area vs. deposition time: 1 MeV ␣ particles ŽTandetron, University of Montreal. were made to impinge at normal incidence on the substrate surface, and backscattered particles were collected at an angle of 30⬚ to the normal. We determined the ratio A Si rAC , where A Si is the area under the silicon RBS peak and A C is that of a window chosen at small energy values beneath the carbon RBS signal. A Si rAC was found to be independent of counting time, indicating that the thin SiO 2 layer on the polymer is not appreciably modified by the ␣ particle beam. Fig. 1, a plot of A Si rA C vs. deposition time for various film samples of PI, shows the two quantities to be proportional, with a correlation coefficient R s 0.999. This implies that the sticking coefficient of Si precursors is independent of the substrate chemistry, since it is seen to be constant during the entire growth process. In previous articles w2,7x, we have described a new method to detect and characterize submicrometer breaches in SiO 2 on polymers, a technique based on reactive ion etching ŽRIE. with an oxygen plasma, under relatively mild ion bombardment conditions. Since the SiO 2 coating is inert towards atomic oxygen ŽAO., while polymers are rapidly etched w8x, the latter occurs on portions of the substrate which are not coated. The surface topography is then greatly enhanced, which can readily be observed by scanning electron microscopy ŽSEM.. Fig. 2 is a SEM image of a 2-nm-thin SiO 2 coating on PET, after the sample had been exposed to oxygen plasma for 10 min. Clearly, no evidence for island-like surface coverage is observable,
Fig. 1. A Si rAC Žsee text. vs. deposition time, t, evaluated by RBS.
the bright round spots being free-standing SiO 2 film over cavities etched in the polymer. The tiny black points in the center of each bright zone are the primary ‘pinhole’ defects in the coating, through which the AO penetrated to the polymer substrate w2,7x. Evidently, this is the exact opposite of an island structure, resulting from Volmer᎐Weber growth, where the resulting nuclei Žfor 2 nm nominal film thickness . would not be detectable at this relatively low level of magnification. However, the fact that we can observe the free-standing film over the cavities allows us to definitively state that this 2 nm SiO 2 coating is continuous, albeit with many small holes. Manifestly, ion bombardment during RIE was insufficiently intense to cause sputter-removal of the SiO 2 , but it cannot be excluded that it contributed to the formation of the holes in this exceedingly thin SiO 2 film. The same experiments have also been conducted with PI and PP substrates, and precisely the same behavior was observed. To further confirm these conclusions, we also obtained X-ray photoelectron spectroscopy data ŽXPS,
Fig. 2. Scanning electron micrograph of a 2-nm-thin SiO 2 coating on PET, following 10 min of exposure to oxygen plasma.
G. Dennler et al. r Thin Solid Films 382 (2001) 1᎐3
Fig. 3. Silicon, carbon and nitrogen high resolution XPS spectra of a 2-nm-thin SiO 2 coating on Kapton 䊛 polyimide; the full and dotted spectra correspond to take-off angles ŽTOA. of 0 and 80⬚, respectively.
VG ESCALAB 3 MKII, non-monochromated MgK ␣ radiation. from hyper-thin coatings on PI. This polymer, being composed of the elements carbon, hydrogen and oxygen, but also nitrogen, one can distinguish the substrate from the SiO 2 film, where the latter may contain a few percent of carbon, by varying the take-off angle ŽTOA. between 0 and 80⬚. Fig. 3 shows high resolution spectra of a 2-nm-thin SiO 2 film on PI, the three data sets being Si 2s, C 1s and N 1s spectra. For a normal Ž0⬚. TOA, the three elements are clearly detected and their concentrations are found to be 44.0% C, 14.2% Si, 38.8% O and 3.0% N, while those of the uncoated substrate are 79.5% C, 14.3% O and 6.2% N, regardless of the TOA. However, at TOAs 80⬚, the N signal is seen to disappear completely, as does the 288 eV C 1s subpeak, the latter being assigned to the carboxyl linkages of the PMDA group in Kapton 䊛 PI w9x. This combined XPS information from Fig. 3, of course, indicates that the topmost layer is structurally completely different from the PI substrate, even though it is rich in C. This result harmonizes with the ones described above, namely that for dF d c , no evidence for nucleation and Volmer᎐Weber-type growth can be observed: hyper-thin SiO 2 Žf 2 nm. coatings deposited by PE-CVD on polymers do not manifest island-like structure; instead, they completely cover the substrate, which allows us to confirm a layer-by-layer ŽFrank-van der Merwe. growth mechanism w5x. The fact that this surprising deduction can be made for various structurally very different polymers, all of which initially possess low surface energies, can be rationalized as follows. In plasma pre-treatment of polymers for purposes of raising the surface energy, and thereby the adhesive bond strength of a subsequent overlayer, it is known that a residence time in the active plasma zone
3
as short as tens of milliseconds is sufficient w10x. In the presently-used ŽPE-CVD. plasma, which comprises abundant AO, we may therefore surmise that the main effect of the plasma on the polymer surface immediately following initial ignition of the plasma, is to raise the surface energy, i.e. to render this surface ‘wettable’. Moreover, as already pointed out above, RBS measurements have shown the sticking coefficient of SiO 2 precursor species to be independent of the chemical structure of the surface, during all phases of growth Ž0 F d F 100 nm.. Finally, we now have convincing evidence that the absence of barrier properties for dF d c is largely due to the presence of vast numbers of tiny defects, the surface density of which can be evaluated at n f 10 9 cmy2 for the three kinds of polymers investigated so far. As we have shown elsewhere w3,11x such large n values result in total absence of barrier properties, even if the remaining ) 99% of the polymer surface is covered with a well-adhering, impermeable coating. Acknowledgements The authors wish to thank Dr G. Czeremuszkin and Professors L. Martinu and P. Desjardins for useful discussions and comments, and Remi Poirier for his valuable assistance on the RBS measurements. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada ŽNSERC.. G.D. gratefully acknowledges a postgraduate scholarship from CNRS ŽFrance.. References w1x H. Chatham, Surf. Coat. Technol. 78 Ž1996. 1. w2x A.S. da Silva Sobrinho, M. Latreche, G. Gzeremuszkin, J.E. ` Klemberg-Sapieha, M.R. Wertheimer, J. Vac. Sci. Technol. A 16 Ž1998. 3190. w3x A.S da Silva Sobrinho, G. Gzeremuszkin, M. Latreche, M.R. ` Wertheimer, J. Vac. Sci. Technol. A 18 Ž2000. 149. w4x J.T. Felts, Society of Vacuum Coaters, Proceedings of the 34th Annual Technical Conference, Philadelphia, USA, March 17᎐22, Ž1991. 184. w5x M. Ohring, The Materials Science of Thin Films, Academic Press, Boston, MA, 1992. w6x A. Bergeron, J.E. Klemberg-Sapieha, L. Martinu, J. Vac. Sci. Technol. A 16 Ž1998. 3227. w7x A.S. da Silva Sobrinho, M. Latreche, G. Gzeremuszkin, G. ` Dennler, M.R. Wertheimer, Surf. Coat. Technol. 116᎐119 Ž1999. 1204. w8x A.M. Wrobel, B. Lamontagne, M.R. Wertheimer, Plasma Chem. Plasma Process. 8 Ž1988. 315. w9x G. Beamson, D. Briggs, High Resolution XPS of Organic Polymer, Wiley, New York, 1992. w10x C. Bichler, M. Bischoff, H.C. Langowsky, U. Moosheimer, Society of Vacuum Coaters, Proceedings of the 39th Annual Technical Conference, Philadelphia, USA, May 5᎐10, Ž1996. 378. w11x G. Gzeremuszkin, M. Latreche, M.R. Wertheimer, Society of ` Vacuum Coaters, Proceedings of the 43rd Annual Technical Conference, Denver, USA, May 17᎐21, 2000, 8.