Plasma Polymerization in Direct Current Glow - ACS Publications

Chapter 31

Plasma Polymerization in Direct Current Glow: Characterization of Plasma-Polymerized Films of Benzene and Fluorinated Derivatives 1

Downloaded by IMPERIAL COLLEGE LONDON on May 1, 2018 | https://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0614.ch031

Toshihiro Suwa, Mitsutoshi Jikei, Masa-aki Kakimoto , and Yoshio Imai Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Meguro-ku, Tokyo 152, Japan The plasma polymerization by direct current (D.C.) glow discharge method was performed with benzene, fluorobenzene, 1,2,3trifluorobenezene, and perfluorobenezene. The plasma polymers were characterized by FT-IR and Raman spectroscopies, ESCA, ellipsometry, and contact angle measurements. The applied voltage was significant factor for the chemical structure of the products. The formation of CF2 or CF3 owing to molecular rearrangements was hardly observed by this method. Plasma polymerization is expected to provide unique films since the reaction mechanism is quite different from conventional polymerization. Because no special functions in the starting materials are required, even methane undergoes polymerization under certain conditions and forms highly cross-linked, pinhole-free films. The plasma reaction has been extensively applied for the modification of surface properties. The plasma polymerized films prepared using fluorine-contained monomer usually lowers the surface energy of the materials (1). These monomers were also utilizedfrequentlyfor evaluating the mechanism of plasma polymerization by the use of electron spectroscopy for chemical analysis (ESCA). A strong carbonfluorine chemical bonding causes considerable chemical shifts in the Cis core-level spectra. The degree of this effect depends upon the number offluorineatoms which bonded to carbon, and hence it would be expected to facilitate quantification of various carbon components such as C-H, CF, CF2, and CF3. The plasma polymerization of a series offluorinatedcompounds in inductively coupled RF plasma have been widely investigated by D. T. Clark et al. (2,3). They reported that the polymerization of highly fluorinated compounds generally proceeded accompanying eliminations, fragmentations, and rearrangements of initial monomers. Thus, perfluorobenzene plasma, for example, generated CF2, CF3, etc. components, which did not exist in the starting monomers. Plasma for the glow discharge is usually generated by low frequency (50 or 60 Hz), radiofrequency(R.F.; 13.56 Hz), or microwave (M.W.; 2.45 GHz). We have recently shown the plasma polymerization of naphthalene and perfluoronaphthalene by a direct current (D.C.) glow discharge (Suwa, T. Jpn. J. Apply. Phys. in press.). The films prepared by this method, as indicated there, had 1

Corresponding author 0097-6156/95/0614-0471$12.00/0 © 1995 American Chemical Society

Reichmanis et al.; Microelectronics Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

MICROELECTRONICS TECHNOLOGY


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31. SUWA ET AL.

Plasma Polymerization in Direct Current Glow 473

such properties as high film growth and low rearrangement, compared with conventional plasma polymerization. The D.C. glow polymerization have developed coupled with the application as replica films in election microscopy field (4,5). However, only few attempts have so far been made to explore the chemical properties of the film prepared by this method. In our previous work on naphthalene derivatives, some unique properties, which were unusual in case of normal plasma reaction, were observed. In this paper, therefore, we confirm that the peculiar results of previous work came from either the characteristics of D.C. glow discharge or the nature of selected monomers. For the purpose of comparison and facility in Elemental Analysis for Chemical Analysis (ESCA), we are concerned with benzene and its fluorinated derivatives, such asfiuorobenzene,1,2,3-trifluorobenezene, and perfluorobenezene. Experimental Materials. Fluorinated monomers for this study were purchased from Tokyo Kasei Co. and used without further purification. Since commercial benzene contained thiophen, it was purified by distillation after the treatment with concentrated H2SO4. According to the measurement, glass slides, silicon wafers, and silver-coated glass slides were used appropriately as the substrates. Plasma Deposition. A schematic diagram of thereactionchamber for D.C. glow discharge was shown in Figure 1. The bell-jar-typereactorconsisted of two parallel disk electrodes (5 cm in diameter) which were placed vertically to base plate, a high voltage D.C. source, monomer reservoir, a pressure gauge, and a vacuum system. The substrates were placed on the cathode, which was the lower side electrode of the two. For each run, the experimental procedure employed was as follows. First the reaction chamber was pumped down to less than K H Pa. Then thereactorwas filled with the monomer gas to a pressure of approximately 10 Pa by the vaporization of liquid monomer. A D.C. voltage (0.4-1.5 kV) was subsequently applied for 5-120 seconds and a plasma polymerized film was formed on the substrate, a surface of which was located in a negative glow phase. Finally the system was evacuated again (< 10~ Pa) for 1 minute and ventedtoatmospheric pressure. 2

Analysis. Hie Fourier transform infrared (FT-IR) spectra were measured by the reflection absorption mode (RAS) with a JEOL JIR-MICRO 6000 equipped with a nitrogen-cooled mercury-cadmium-telluride (MCI) detector. For these measurements, the films were deposited onto 100-nm-thick Ag coated glass slides. Thickness of deposited films were determined by an ellipsometer (Nippon Infrared Instrumental Co., EL-101). The films were prepared on silicon wafer. Electron spectroscopy for chemical analysis (ESCA) was carried out with a ULVAC-PHI-5500MT system. The spectra were acquired using monochromated ΑΙ Κα (1486.7 eV) radiation at 14 kV and 200 W. The component analysis of the elements was accomplished by data system supplied by ULVAC-PHI, assuming Gaussian peaks with 10% Lorentzian tails. All spectra were cariblated by adjusting the decomposed Cis curve of C-H and/or C-C at energy of 284.6 eV. Hie sputtering offilmsurface was done with Ar ions at 2.5 kV and 25 mA. In order to study physical properties of the surface, the contact angles of water and formamide were measured with a contact angle meter (Kyowa Interface Science Co., Model CA-A). The measurements were performed by the sessile drop method. Theresultingvalues were the means of at least six measurements withrespectiveerrors of ±1*. A carbon-like film prepared from benzene plasma was preliminarily analyzed by using Raman Spectroscopy. Hie spectrum was recorded on a JASCO NR-1800 employing an argon ion laser (λ=514.5 nm). +



474 Results and Discussion

IR Spectra. A D.C. glow method has the advantage of setting the different applied voltages easily. It is considered that higher voltage gives more energy to reactive species in the glow phase such as electrons, ions, andradicalsand affects the structure of resulting deposits. Hie plasma polymerization, therefore, was performed by using various applied voltages. Figure 2a-d show the IR spectra of plasma polymerized films of benzene (PPB), perfluorobenzene (PPPB), fluorobenzene (PPFB), and trifluorobenzene (PPTB), respectively. On the whole, spectra varied with changes in the applied voltage during discharge. In case of benzene plasma (Figure 2a), two regions of C-H stretching bands were observed around 3000 cm- . A small peak at 3000-3100 c m is due to the aromatic C-H band, and the latter peak to the aliphatic one. Needlesstosay, a starting material-benzene has only aromatic C-H bonding. So this result suggests that the aromatic ring of benzene was easily destroyed to a considerable extent even at relatively low voltages. A series of spectra changed gradually with arisein the applied voltage. On the other hand, new absorption bands appeared in the region of 1600-1800 and 1000-1500 cm . The former region is probably attributed to C=0 stretching. The other one, which was composed of a sequence of absorption, became broader with the increase of the applied voltage. Although definite assignment of them was difficult to makefromthese spectra, it was presumed that C-H bending vibration was involved around 1400 cm . The formation of carbonyi group means insertion of oxygen in the chemical structure. This phenomenon was also observed in our previous work about naphthalene and we revealed the oxygen existed only in the outermost portion of the films. That was considered to result from the post-reaction of residualradicalsgenerated by a plasma with atmospheric oxygen or water vapor. Strange pointtonote is that O-H stretching, which appeared at 3300 an for naphthalene plasma polymer, was not able to observed. It might comefromthe difference of plasma polymerization between these two kinds of aromatic hydrocarbon. This would be supported by Raman spectrum later. The IR spectra of PPPF were depicted in Figure 2b. As the applied voltage rose, the C-F peak at 1338 GOT decreased gradually, while new peak centered on 1220 cm* began to appear in the wide region. Considering the decrease of CF=CF at 1546 cm- , this spectral variation presumably owed to change of aromatic C-F group into aliphatic one. The C-F groups in various environments made their spectra very vague. Another broad peak in the region of 1600-1800 cm- , was assigned to C=0 stretching. The carbonyi carbon in higher wavenumber region (around 1800 cm- ) would bond to fluorine, whereas fluorine was consideredtobe eliminatedfromcarbon atoms at higher voltages. Figure 2c and 2d arc IR spectra of PPFB and PPTB, respectively. These spectra have similar tendency with benzene and perfluorobenzene plasma polymer. Both CF=CF stretching of fluorobenzene (1489, 1608 cm ) and of trifluorobenzene (1513, 1623 cm ) plasma polymer became small as the applied voltage rose. In addition, a small C-H stretching were observed at lower voltage of each spectra. Hie existence of carbonyi group was also common phenomenon through four kinds of plasma polymer.


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ESCA Spectra. ESCA analysis was performed in order to evaluate the composition of the films. Our interest was particularly the comparison of film surface and die bulk region. First of all, therefore, a depth profile analysis was carried out for all monomers. Figure 3 shows a depth profiles of PPPF films (approximately 24 nm) prepared at 1.5 kV. The calculated sputtering rate was roughly 0.1 nm/s. For the ESCA analysis, silicon wafer was used as substrate. The profile was composed of


31. SUWA ET AL.


carbon,fluorine,oxygen, and silicon. The profile exhibits that oxygen exists at the surface of as-prepared film and is scarcely found within the film. We can recognized that the film is composed of only carbon andfluorinein the bulk, and that oxygen, the existence of which was suggested by IR spectra as well, remained in the neighborhood of the surface. The profiles also indicates that atomic ratio of F/C is much higher at the surface, compared wit the bulk region. A highfluorineconcentration in the outermost region is presumably a result of post-reaction. To make this point clear, high resolution measurements were done before and after the argon ion sputtering. The Cis core-level spectrum measured before Ar sputtering is revealed in Figure 4a. There observed a variety of components in the spectrum. They are composed of C-C., £-CF, CF, £F-CF, CF2 (CFO), CF3 and a π-π* shake-up. Since the CFO component, which was detected by IR analysis, was predicted to appear at similar region with CF2, further separation of these components was not attempted. Argon ion sputtering changed the spectrum remarkably and the peaks in higher binding energy region became small. The quantitative details are summarized in Table I. Similarly to the previous study about plasma polymerized perfluoronaphthalene in D.C. glow, the contribution of CF2 and CF3 components to the spectra was very small even in the unsputtered sample. This means the rearrangement of bondings, which has been regarded as usual for the plasma polymerization of fluorocarbon by R.F. or microwave etc. (2,3,6), hardly take place in D.C. glow method.


+

Table I. The Quantitative Details in C i Spectrum of PPPB Measured before Ar+ Sputtering Cis Components Binding Energy (eV) Relative Concentration(fo) Q-C 33. 9 284. 6 £-CF 286. 8 36. 9 £F 287. 6 7. 7 CF-CF 289. ,3 18. 7 291. 8 1. 7 £F2(CFO) 293. 1 0. 7 CF3 294. 7 0. 4 π-π* s

We also carried out high resolution measurement with the sample prepared at lower voltage (0.4 kV). No particular difference was observed between the two (spectra not shown). We may therefore, reasonably say, that the diminution of the CF2 and CF3 peaks in this spectrum is characteristics of the D.C. glow polymerization of fluorocarbons. Let us now return to an interpretation of high concentration of fluorine at the surface region. The critical difference in spectra measured before and after the sputtering is the disappearance of most of CF and CJF-CF components. These components reflect nature of starting monomer, QFe. The chemical structure of the surface is probably close to that of monomer. It is thought that the outermost region of the film was mainly formed by a post-reaction of the residual radicals with monomer gas that remained in the chamber. Interestingly the depth profile displayed that the proportion of fluorine slightly increased again at thefilm/substrateinterface. This could be caused by the difference in plasma condition between initial discharge and the subsequent steady state. Around the interface, trace amount of oxygen was detected as well. Since this peak increased with the appearance of silicon peak, it seems reasonable to suppose that oxygen camefromsilicon oxide, which had been formed on the surface of aie substrate. ESCA analysis of fluorobenzene and trifluorobenzene plasma polymer was employed in a similar manner as above. The behavior of their depth profiles were, as



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31. SUWA ET AL.



expected, not so different from that of perfluorobenzene except for the atomic concentration of fluorine. These values varied in proportion to the number of fluorine atoms which monomers contain. Figure 5 and 6 show the C i s core-level spectra of the PPFB and PPTB, respectively. Hiey were measured before and after argon bombardment. Compared with the spectra of PPPB, the intensity in higher binding energy region was small even before the bombardment. As can be seen from each spectra, they further decreased in size after the treatment. These results also support that the formation of CF2 and CF3 components hardly occur by D.C. glow. Atomic concentration offluorinein thefilmsis presented in Table II.

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Table II. Concentration of Fluorine in the Plasma Polymerized Films Measured before and after Sputtering Concentration of Fluorine (%) Plasma Polymer ° Before After 5 2 PPFB PPTB 22 14 PPPB 60 20-30 A l l films were prepared at 1.1 kV.

Finally PPB was examined. We could not find the difference in the spectra measured before and after the sputtering (Figure 7). Since IR spectra revealed that elimination of the hydrogenfromthe products proceeded to a considerable extent at higher voltage, It is considered that the film is almost compose of carbon. Surface Properties of the Plasma Polymerized Films. Contact angle measurement is convenient technique to evaluate the surface properties offilms.We measured contact angles of water (θψ) and formamide (%) on each films which were prepared at 1.1 kV. From the obtained contact angles the surface free energy of each plasma polymerized film was estimated according to Kaelble method (7,8). All the results were presented in Table III. Table III. Surface Properties of the Plasma Polymerized Films Prepared at 1.1 kV Total Surface Energy Plasma Yf θψ Ys F Polymer (degree)(degree)(mJIm ) (mJIm ) (mJIm ) PPB 67.0 42.0 33.7 10.9 44.6 PPFB 71.5 47.3 32.8 8.8 41.6 PPTB 78.7 54.0 32.9 2.4 35.4 PPPB 95.7 72.9 26.5 1.5 28.0

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As shown in the table, both contact angles, as expected, increase with increasing the number offluorineatoms which monomer possess. The surfaces of the films reflect the structure of starting monomers, that is, the amount of surface fluorination are proportion to the F/Cratioof monomer and surface energy is fairly connected with die degree of fluorination. Surface energy are composed of a polar (Ys ) and a dispersive component (Vs^. They are presented separately in the table. Because of the presence of polar groups on the film surface, γ is expected to depend on how many polar groups have been incorporated. The results suggest more polar p

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Binding Energy (eV) Figure 5. Hie Cis core-level spectra of PPFB film measured before (solid line) and after (dotted line) Ar+ sputtering.

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31. SUWA ET AL.



groups, like C=0, were formed on the benzene polymer than on the fluorinated polymers. This is probably related to the difference of bonding strength between C-H and C-F. The C-H bondings were broken more easily and subsequently formed more polar groups. Contact angle measurements were also employed as a function of applied voltage. However, no particular tendency was observed from them (data not shown). Film Growth. The deposition rate was very sensitive to the discharge factors such as an applied voltage and a gas pressure. Hie data we present here were prepared at two kinds of general conditions. As we had noted the effect of the applied voltage, 0.4 kV and 1.5 kV were selected at first. Then target pressures were adjusted at 27 Pa (0.2 Ton) and 4.0 Pa (0.03 Torr), respectively, in order to maintain a stable discharge. To put it more concretely, when the applied voltage was set at 0.4 kV the discharge did not occur at 4.0 Pa. On the other hand, in case of 1.5 kV, the discharge was too vigorous at 27 Pa. Thus these two conditions have different pressure and cannot be compared directly. Figure 8a shows film growth against dischargetimeat relatively low voltage (0.4 kV). As the diagram indicates, all monomers polymerized approximately at the samerates(0.1-0.3 nm/s). It is interesting to note that these extrapolated lines deviate farfromthe origin. It admits of two interpretations. One explanation for it is that film growth proceeded morerapidlyduring initial state of discharge. At this period not so muchfragmentationof monomer would take place as subsequent stable discharge period and polymerizationreactionwould proceed easily. But very soon thereactionwould go into the equilibrium state between polymerization and ablation, and it would make therateof film growth slow down. The other reason is the large amount of absorption of monomer gases on the substrates before the discharge. It is thought that these absorbed molecules were fixed at the substrates by plasma-induced polymerization. ESCA depth profile analysis, as we have seen, indicates that the fluorine concentration aroundfilm/substrateinterface was different from that of bulk region. By ESCA data we cannot conclude, however, which reason is responsible for the results. When applied voltage was set at 1.5 kV, specific film growth (2.4 nm/s) was observed in case of perfluorobenzene (Figure 8b). The other three monomers were polymerized at the almost samerates(0.2-0.7 nm/s). These values are comparable to that of common plasma polymerizations induced by radiofrequencyand microwave. In addition, the extrapolated lines of these data deviatefromthe origin as well as the that of polymerization at low voltage. It suggests that the large deviation from the origin is probably not due to absorption of monomer molecules on the substrates but due to therapidpolymerization containing littlefragmentationand ablation, because the pressure dependence of the absorption was not observed. Although film growth of PPPB was reproducible, we can not so far explain that clearly. Therefractiveindex of films, which was also obtainedfromellipsometric measurements, may give a clue to explain the specificity of perfluorobenezene plasma polymer. Therefractiveindexes of the plasma polymerized films from benzene,fluorobenzene,and trifluorobenezene were approximately 2.0. On the other hand, the index decreased to 1.6 for PPPB. We can, therefore, imagine as follows. Hie C-F bonding strength is stronger than CH and large amount offluorineremainedin the film for PPPB. This would prevent the cross-linking of carbon atoms and result in die formation ofrelativelylow density film. It would be closely linked to therapidfilmgrowth. Raman Spectrum. Judgingfromthe results which we have hitherto mentioned, the plasma polymerized films prepared at higher voltage, except for the oxidized surface, were composed of only carbon. Raman spectroscopy has beenfrequentlyused for the characterization of carbon films. The spectrum of PPB prepared at 2.1 kV is shown in Figure 9. We can observe a very broad peak centered at 1550 cm . The peak around 1



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31.

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Raman Shift ( cm" ) Figure 9. Raman spectrum of PPB prepared at 2.1 kV. this regain is common for /-carbon films. Although this peak can be associated with a sp bonded carbon in an amorphous material (9-16), an unambiguous assignment has not been determined. In contrast to the plasma polymerized naphthalene which had peaks at around 1300-1400 cnr , it is interesting that PPB has a peak in other region. Details on these difference of carbonfilmswill be presented in future publications. 2

1

Conclusion D.C. glow discharge was used to prepare plasma polymerized films. The IR spectra showed that the chemical structure of die products were affected by the applied voltage during discharge and that plasma polymerization of benzene at higher voltage produced hydrogen-free carbon film. ESCA analysis revealed that molecular rearrangements forming CF2 or CF3fluorinatedmonomers, which has been considered as familiar reaction in case of conventional plasma polymerization, did not occur by this method. The surfaces of films were morefluorinatedthan the bulk regions and the surface energy depended on starting monomers. Acknowledgments This work has been performed under the support of "Special Coordination Funds for Promoting Science and Technology" from die Science and Technology Agency of Japan. Literature Cited 1. 2. 3. 4. 5. 6. 7.

Inagaki, N.; Nakanishi, T.; Katsuura, K. Polym. Bull., 1983, 9, 502. Clark, D. T.; Shuttleworth, D. J. Polym. Sci., Polym. Chem., 1979, 17, 1317. Clark, D. T.; Shuttleworth, D. J. Polym. Sci., Polym. Chem., 1980, 18, 27. Tanaka, Α.; Sekiguchi, Y.; Kuroda, S. J. Electron Microscopy, 1978, 27, 378. Tanaka, Α.; Yamaguchi,M.;Iwasaki, T.; Iriyama, K. Chem. Lett., 1989, 1219. Munro, H. S.; Till, C. J. Polym. Sci., Polym. Chem., 1984, 22, 3933. Kaelble, D. H. Proc. 23rd Int. Cong. of Pure and Appl. Chem.8; Butterworths: London, 1971; pp.265-302.


484 8. 9. 10. 11. 12. 13.


14. 15.

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Kaelble, D. H.; Dynes, P. J.; H. Cirlin, E.J. Adhession, 1974, 6, 23. Dillon, R. O.; Woollam, J. Α.; Katkanant, V. Phys. Rev., 1984, B29, 3482. Couderc P.; Catherine, Y. Thin Solid Films, 1987, 146, 93. Sato, T.; Furuno, S.; Iguchi, S.; Hanabusa, M. Jpn. J. Appl. Phys., 1987, 26, L1487. Ramsteiner, M.; Wagner, J. J. Appl. Phys. Lett., 1987, 51, 1355. Yoshikawa, M.; Katagiri, G.; Ishida, H.; Ishitani, A. Solid State Communications, 1988, 66, 1177. Houg, P. V. Diamond Related Mater., 1991, 1, 33. Dowling, D. P.; Ahem, M. J.; Kelly, T. C.; Meenan, B. J.; Brown, N. M. D.; O'Connor, G. M.; Glynn, T. J. Surface and Coating Technology, 1992, 53, 177. Godbole, V. P.; Narayama, J. J. Mater. Res., 1992,

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Plasma Polymerization in Direct Current Glow - ACS Publications

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