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Novel plasma polymerization system using a d.c. glow discharge
Thin Solid Films 273 ( 1996) 258-262
Novel plasma polymerization system using a d.c. glow discharge Toshihiro Suwa a,*, Mitsutoshi Jikei a, Masa-aki Kakimoto a, Yoshio Imai a, Akira Tanaka b, Katsumi Yoneda b aDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152 Japan b Nippon Laser Electronics Laboratory, 2318. Hero-cho, Tenpaku-ku, Nagoya 468 Japan
Abstract Plasma polymerization using a direct current (dc.) glow discharge was carried out using several organic compounds. We have developed a plasma-polymerization instrument capable of using not only gaseous monomers but also liquids and solids as the starting materials. The characteristics of films prepared by d.c. glow discharge were examined by Fourier transform infrared spectroscopy and Raman spectroscopies, electron spectroscopy for chemical analysis, and contact angle measurements. The chemical structures of the deposited films were greatly influenced by the applied d.c. voltages during the discharge. Hydrogen-free carbon films were prepared from plasma polymerization of both naphthalene and benzene; the structure of these films, evaluated by Raman spectra, were quite different. Comparison of films deposited in different regions of the plasma, such as in the negative glow-phase region and in the positive column region, were performed. From these experiments similar film compositions were obtained. Keywords:
Plasma processingand deposition; Glow discharge; Organic substances; Optical spectroscopy
1. Introduction Polymeric films prepared by the plasma polymerization of organic monomers are, in many respects, different from those obtained by conventional means. In general, polymerization requires that monomers should have functional bonds such as vinyls, amines, carboxylic acids, etc., and these types of monomers form mostly only linear polymers. On the other hand, plasma polymerization can use monomers having no special functional bonds, i.e. methane, and can make pinholefree films of highly cross-linked polymers. These films, such as diamond films from various hydrocarbons, can also exhibit excellent mechanical properties. Therefore, much attention has been paid to plasma-polymerized films, because of their potential as electronic, optical, and biomedical materials. Plasma is usually induced by low-frequency (50 or 60 Hz), radio frequency (r.f.) ( 13.56 MHz) or microwave (MW) (2.45 GHz) radiation. However, so far, few studies have been made of plasma polymerization in a d.c. glow. One of the authors has developed a plasma-polymerization replica method for use in electron microscopy [ 11. We have developed a plasma-polymerization instrument capable of not only using gaseous monomers but also low boiling point liquids and solids, which can be vaporized under 100 ‘C, as the * Corresponding author. 0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI0040-6090(
95) 07000-
1
starting materials. Different to the typical plasma polymerization conditions, the d.c. plasma process is unique because it uses a constant electric field between anode and cathode. This static field allows the films to grow predominantly on the cathode. In addition, in our apparatus, the applied voltage can be easily controlled between 0 and 3 kV. In this study, we attempted the plasma polymerization of various monomers such as hydrocarbons (naphthalene and benzene) and fluorocarbons (perfluoronaphthalene and perfluorobenzene). The relation between the applied voltage and the resulting chemical structure was studied. The chemical structure was determined by infrared (IR), electron spectroscopy for chemical analysis (ESCA) and contact angle measurements. The difference between hydrogen-free carbon films prepared from naphthalene and from benzene plasma were examined with the aid of Raman spectroscopy.
2. Experimental The substrates where the films were made were usually placed on the cathode. At first, during each deposition, the reaction chamber was pumped down to less than lo-* Pa. Then the reactor was filled with a monomer gas to a pressure of approximately 10 Pa. A d.c. voltage (0.4-2.1 kV) was subsequently applied and the plasma-polymerized film was
T. Suwa et al. /Thin Solid Films 273 (1996) 258-262
formed on the substrate. Immediately after completion of the discharge, the system was evacuated again ( < lo-* Pa) for a few minutes to remove residual monomer, and then vented to atmospheric pressure. The Fourier transform infrared (FTIR) spectra were obtained by a JEOL JIR-MICRO 6000 equipped with a nitrogen-cooled mercury-cadmium-telluride (MCT) detector. In order to evaluate the surface properties of films, the contact angle of water was measured with a contact-angle meter (Kyowa Interface Science Co., Model CA-A) using the sessile drop method. Surface analysis of the chemical structure was performed by ESCA with a ULVAC-PHI-5500MT system. Monochromated Al Ka ( 1486.7 eV) radiation was used at 14 kV and 200 W. The quantification of the elements was accomplished by a system supplied by ULVAC-PHI. Carbonlike films from naphthalene and benzene were compared with help of Raman spectroscopy (JASCO NR-1800). In this analysis, the spectra were obtained using an argon-ion laser (A=5145 nm). A schematic drawing of our deposition apparatus using d.c. glow discharge is depicted in Fig. 1. The stainless steel chamber is 25 cm in height and 21 cm in diameter. The reactor consisted of two parallel disk electrodes (5 cm in diameter), a high-voltage dc. source, a vacuum system, a gas inlet, and a monomer reservoir. The electrodes were arranged so that the column was vertical to the base plate and the vacuum
I
prevention cylinder 4. leakage discharge prevention cylinder 5. eas i&l 6. iigb voltage pcnvcr supply sublimation chamber 8. positive column Faraday dark space
1. sublimation chamber 2. vessel B
6. I. 8. 9. 10.
release handle vessel A heater tbcrmomuplc temperature controller
lb)
Fig. 1. Schematic diagram of the plasma polymerization monomer reservoir (b)
reactor (a) and the
259
system was equipped with a turbo molecular pump to prevent oil contamination. A reservoir for supplying the solid and liquid monomer was also placed in the chamber. As shown in Fig. 1 (b) , the reservoir had two kinds of monomer vessels (vessel A and vessel B) . Vessel A was for low-volatile liquids and solid monomers which were capable of sublimation. Vessel B was used for highly volatile liquids such as benzene. Two tubes (tube X and tube Y) functioned as valves. This reservoir was connected with the reactor via a inlet valve, the introduction tube. When vessel A was used, tube X was raised and then the whole apparatus was pumped down together. Then the tube was closed and the reservoir was tilled with the vaporized gas. Afterward the tube was adjusted so as to keep a constant pressure and the discharge was performed for a certain period of time. On the other hand, when a volatile liquid was used, the liquid was injected into vessel B and tube Y was raised in order to isolate from the reservoir. Next, tube X was also raised and the reactor pressure was reduced in a similar manner. After the pressure was reduced enough, both tubes were lowered and a vaporized monomer filled the reservoir. Subsequent polymerization steps were the same for solid plasma polymerization. In our apparatus gaseous monomers can also be utilized. Two kinds of gasses could be mixed in the chamber with gasinlet flow rates controlled by gas-flow controllers. In addition, since a heater ( < 100 “C) with a thermocouple is built into vessel A, many kinds of monomers which have low vapor pressures in this temperature range were also able to be used. Because of potential chemical corrosion on the inside of the reservoir, it was coated with PTFE and the materials in the reservoir were also made of PTFE except for vessel A (stainless steel) and the thermocouple.
3. Results and discussions A d.c. glow method has the advantage of an easily adjusted applied voltage. It is expected that higher voltages give more energy to the reactive species such as electrons, ions (especially positive ion), and radicals in the glow phase, and that could affect the structure of the resulting deposits. The plasma polymerization, therefore, was carried out using different voltages. Fig. 2(a) and 2(b) show the IR spectra of plasmapolymerized films of naphthalene and benzene, respectively. The films were prepared at the voltages of 0.6 and 2.1 kV. As the spectra indicate, the chemical structure of the films were highly dependent on the voltage used. In each spectrum of films prepared at 0.6 kV, two regions of C-H stretching bands were observed at 3000-3 100 and 2800-3000 cm ’ The former due to the aromatic C-H band, while the latter to the aliphatic C-H band. The existence of aliphatic C-H suggests that the aromatic rings of naphthalene or benzene decomposed to some extent at this voltage. When the applied voltage was raised further, the intensity of all the C-H bands
T. Suwu et ~11./Thin Solid Films 273 (I 996) 258-262
I
I
3000
1,
2500
Wavenumber
L
I
3000
I1
,,
I
,
2000
.,
,.
2500
I
1,,
1000
(cm-‘)
1
,,
1,
2000
Wavenumber Fig. 2. IR spectra of plasma-polymerized
1
1500
1500
.I
their structures are quite different. The naphthalene plasma polymer shows features similar to that of amorphous carbon. The broad absorption of the benzene polymer, on the contrary, is indicative of graphitization. IR analysis, as mentioned above, suggested that the formation of the carbon film is accompanied by decomposition of the hydrocarbon aromaticity and subsequent hydrogen elimination. We cannot currently consistently explain the decomposition of the aromaticity and the growth of the graphitization. At present, the influence on the applied voltage on the Raman spectra of plasma polymerized benzene is being studied in detail. The glow discharge has a negative glow phase, a positive column, and a few other regions. We also compared the structure of products deposited in these different regions. Fig. 4 shows the IR spectra of plasma-polymerized perfluorobenzene prepared in the negative glow phase and in the positive column (the substrate placed in between the elcctrodes). The applied voltage was set at 0.4 kV. The highest peak at 1335 cm _’ in each spectrum is attributed to C-F stretching, C=C stretching is observed around at 15 10 cm I. Although the assignment of all peaks is difficult, no notable differences were found in the spectra. This suggests that the
I
1000
(cm-‘)
naphthalene
(a) and benzene (b)
preparedat different voltages.
2.
.z s -E
decreased gradually and no C-H peak was observed at 2.1 kV. In contrast to the decrease of the C-H band, a new absorption peak appeared in the region of 1600-l 800 cm ~ ’ . This absorption is attributed to C=O stretching. The formation of carbonyl groups means that oxygen was incorporated into the film. ESCA analysis indicated that oxygen existed only in the outermost portion of film. Thus, we can assume that these carbonyl groups resulted from post-reaction of the residual radicals generated during the discharge with atmospheric oxygen or water vapor. If the oxidized surface is ignored, then practically hydrogen-free carbon films were produced when the higher voltages were applied. Raman spectroscopy is a useful technique to evaluate caxbon films, and has been applied frequently for characterization of carbon materials such as diamond-like films [ 2-101. We compared Raman spectra of the carbon films generated by both the naphthalene and benzene plasmas. These spectra are shown in Fig. 3. The spectrum of the naphthalene plasma polymer has a series of bands at around 1300-1400 cm- ‘, while that of benzene has a very broad peak centered at 1550 cm-’ . The former peaks are considered to be associated with a sp’ bonded carbon, and the latter to carbons having sp’ configurations. The two films for Raman spectroscopy analysis were deposited under almost identical conditions, yet
+I 1
1700
1600
1500
1400
1300
1200
Raman Shift ( cm-’ ) Fig. 3. Raman spectra of plasma-polymerized (B) prepared at 2.1 kV.
naphthalene
(A) and benzene
I
I
I. 3000
I
2500
,,
2000
Wavenumber
I
1500
,
I
1000
( cm-’ )
Fig. 4. IR spectra of plasma-polymerized films of perfluorobenzene ited in a negative glow phase and positive column.
depos-
T. Suwu et al. / Thiri Solid Films 273 f 1996) 25X-262
110
110
100
-
90
-
Lob e@
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$ p a z g s
(a)
100
00
.
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so-
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20-
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2.5
3.0
“.a’j’o”’ 0
0.5
1.0
1.5
2.0
2.5
3.0
Applied Voltage (kv)
Fig 5. Relationship of water contact angle and the applied voltage:
(a) plasma-polymerized
process of deposition may not depend on where the substrate was placed in the plasma. ESCA analysis is another useful technique to evaluate the chemical structure of the plasma-polymerized films. Since it is impossible to detect hydrogen by ESCA, we made the films using perfluoronaphthalene. For these measurements, the films were prepared on both the cathode and anode. When the substrates were placed on the cathode, a thin glass plate (micro cover glass, 0.12-0.17 mm in thickness) was put under the substrates for insulation. Thus, the surface of the substrate was in the negative glow phase. The effect of the applied voltage on polymerization was examined as well. The chemical composition calculated from C Is, F Is, and 0 I s core-level spectra is summarized in Table 1. As had been expected from IR analysis, a certain amount of oxygen, which was not an clement contained in the starting monomer, was also detected in the perfluoronaphthalene plasma polymer. Considering similar results seen in our previous work [ I I 1, the oxygen originated from the oxidized surface layer of the films. The table indicates that the chemical compositions depend on neither the voltage nor the electrodes upon which the substrates were set. In the polymerized films the carbon to fluorine atomic ratios are about 3:2, and this roughly approximates that of the monomer (C:F = 5:4). We should note hem that these data are indicative of only the surface region, since the analysis depth of ESCA is only in the order of ten angstroms below the surface. The effect of the applied voltage was seen in contact angle measurements. The contact angle is sensitive to only the
naphthalenc,
(b)
plasma-polyrllcrlled
periluoronaphthalene
outermost region of the film. This measurement, therefore, will give some indication about the surface condition. The relation of water contact angle and the applied voltage for plasma-polymerized naphthalene and perfluoronaphthalene are shown in Fig. 5(a) and 5(b), respectively. The value 01 6” at V=O kV referred to the contact angle of an untreated glass slide. The contact angle of the naphthalene-baseddeposited films shows a slight dependence on the voltage. As the voltage increased, the contact angle decreased slightly. The change in the contact angle is probably related to the degree of film oxidation which could be caused hy residual radicals. The discharge at higher voltage generated more radicals on the film and more oxidation would occur when the system was vented to the atmosphere On the other hand. the contact angles of the perfluoronaphthalene plasma polymer did not depend upon the voltage within experimental error. This does not immediately mean that lilm surface oxidation was successfully prevented. The value of90” was much smaller than that of a typical fluorocarbon polymer like polyperlluoroethylcne ( I IO”). We should consider that in case of the pcrfluoronaphthalene the degree of the oxidation did not depend on the applied voltage.
Acknowledgements
This work has been performed under the support of “Special Coordination Funds for Promoting Science and Technology“ from the Science and Technology Agency of Japan.
I
Atomic concentration of plasma-polymerized
films prepared at drfferent
conditions Electrode
References C:F:O (%) l4kV
I .I kV
2.1 kV
[I ] A. Tanaka M. Yamaguchi. T. lwusaki and K Inyama. (1989)
Cathode Anode
O
-
0 1.5
00
40-
10
1.0
0
0
Applied Voltage (kV)
Table
26 I
S9.0:3 I .4:9.6
51.9:34.6:1.5
52.5:39.08.5
56.7:34.0:9.3
58.4:34.8:6.8
58.3:36.6:5.1
Clrrnr I.c,fr.
1219.
[2] R.O. Dillon, J A. Woollamand
V. Katknnant. Phw Reb.. H2Y (1984)
3482.
[ 31 P. Couderc
and Y. Catherine, Third Solid Films, I4
( 1987) 93.
262
T. Suwa et ul. /Thin Solid Films 273 (I 996) 258-262
[41 T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, Jpn. J. Appl. Phys.. 26 (1987) L1487. [51 M. Ramsteiner
[61M. Yoshikawa, Communications,
and J. Wagner, J. Appl. Phys. Lett., 51 ( 1987) 1355. G. Katagiri, 66 (1988)
H. Ishida and A. Ishitani. Solid State 1177.
[71 P.V. Houg, Diamond RelutedMater.,
I (1991) 33.
[81 D.P. Dowling, M.J. Ahern, T.C. Kelly, B.J. Meenan, N.M.D. Brown, G.M. O’Connor and T.J. Glynn, Surf: Coating Technol.. 53 (1992) 177. [91 V.P. Godbole and J. Narayama, J. Muter. Res., 7 (1992) 2785. [IO] W. Zhu, B.H. Tan and H.S. Tan, Thin Solid Films, 236 ( 1993) 106; [ 1 I I T. Suwa, M. Jikei, M. Kakimoto and Y. Imai, Jp. .I. A&. Phys., 34 ( 1995) in press.
Novel plasma polymerization system using a d.c. glow discharge Toshihiro Suwa a,*, Mitsutoshi Jikei a, Masa-aki Kakimoto a, Yoshio Imai a, Akira Tanaka b, Katsumi Yoneda b aDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, Tokyo 152 Japan b Nippon Laser Electronics Laboratory, 2318. Hero-cho, Tenpaku-ku, Nagoya 468 Japan
Abstract Plasma polymerization using a direct current (dc.) glow discharge was carried out using several organic compounds. We have developed a plasma-polymerization instrument capable of using not only gaseous monomers but also liquids and solids as the starting materials. The characteristics of films prepared by d.c. glow discharge were examined by Fourier transform infrared spectroscopy and Raman spectroscopies, electron spectroscopy for chemical analysis, and contact angle measurements. The chemical structures of the deposited films were greatly influenced by the applied d.c. voltages during the discharge. Hydrogen-free carbon films were prepared from plasma polymerization of both naphthalene and benzene; the structure of these films, evaluated by Raman spectra, were quite different. Comparison of films deposited in different regions of the plasma, such as in the negative glow-phase region and in the positive column region, were performed. From these experiments similar film compositions were obtained. Keywords:
Plasma processingand deposition; Glow discharge; Organic substances; Optical spectroscopy
1. Introduction Polymeric films prepared by the plasma polymerization of organic monomers are, in many respects, different from those obtained by conventional means. In general, polymerization requires that monomers should have functional bonds such as vinyls, amines, carboxylic acids, etc., and these types of monomers form mostly only linear polymers. On the other hand, plasma polymerization can use monomers having no special functional bonds, i.e. methane, and can make pinholefree films of highly cross-linked polymers. These films, such as diamond films from various hydrocarbons, can also exhibit excellent mechanical properties. Therefore, much attention has been paid to plasma-polymerized films, because of their potential as electronic, optical, and biomedical materials. Plasma is usually induced by low-frequency (50 or 60 Hz), radio frequency (r.f.) ( 13.56 MHz) or microwave (MW) (2.45 GHz) radiation. However, so far, few studies have been made of plasma polymerization in a d.c. glow. One of the authors has developed a plasma-polymerization replica method for use in electron microscopy [ 11. We have developed a plasma-polymerization instrument capable of not only using gaseous monomers but also low boiling point liquids and solids, which can be vaporized under 100 ‘C, as the * Corresponding author. 0040-6090/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDI0040-6090(
95) 07000-
1
starting materials. Different to the typical plasma polymerization conditions, the d.c. plasma process is unique because it uses a constant electric field between anode and cathode. This static field allows the films to grow predominantly on the cathode. In addition, in our apparatus, the applied voltage can be easily controlled between 0 and 3 kV. In this study, we attempted the plasma polymerization of various monomers such as hydrocarbons (naphthalene and benzene) and fluorocarbons (perfluoronaphthalene and perfluorobenzene). The relation between the applied voltage and the resulting chemical structure was studied. The chemical structure was determined by infrared (IR), electron spectroscopy for chemical analysis (ESCA) and contact angle measurements. The difference between hydrogen-free carbon films prepared from naphthalene and from benzene plasma were examined with the aid of Raman spectroscopy.
2. Experimental The substrates where the films were made were usually placed on the cathode. At first, during each deposition, the reaction chamber was pumped down to less than lo-* Pa. Then the reactor was filled with a monomer gas to a pressure of approximately 10 Pa. A d.c. voltage (0.4-2.1 kV) was subsequently applied and the plasma-polymerized film was
T. Suwa et al. /Thin Solid Films 273 (1996) 258-262
formed on the substrate. Immediately after completion of the discharge, the system was evacuated again ( < lo-* Pa) for a few minutes to remove residual monomer, and then vented to atmospheric pressure. The Fourier transform infrared (FTIR) spectra were obtained by a JEOL JIR-MICRO 6000 equipped with a nitrogen-cooled mercury-cadmium-telluride (MCT) detector. In order to evaluate the surface properties of films, the contact angle of water was measured with a contact-angle meter (Kyowa Interface Science Co., Model CA-A) using the sessile drop method. Surface analysis of the chemical structure was performed by ESCA with a ULVAC-PHI-5500MT system. Monochromated Al Ka ( 1486.7 eV) radiation was used at 14 kV and 200 W. The quantification of the elements was accomplished by a system supplied by ULVAC-PHI. Carbonlike films from naphthalene and benzene were compared with help of Raman spectroscopy (JASCO NR-1800). In this analysis, the spectra were obtained using an argon-ion laser (A=5145 nm). A schematic drawing of our deposition apparatus using d.c. glow discharge is depicted in Fig. 1. The stainless steel chamber is 25 cm in height and 21 cm in diameter. The reactor consisted of two parallel disk electrodes (5 cm in diameter), a high-voltage dc. source, a vacuum system, a gas inlet, and a monomer reservoir. The electrodes were arranged so that the column was vertical to the base plate and the vacuum
I
prevention cylinder 4. leakage discharge prevention cylinder 5. eas i&l 6. iigb voltage pcnvcr supply sublimation chamber 8. positive column Faraday dark space
1. sublimation chamber 2. vessel B
6. I. 8. 9. 10.
release handle vessel A heater tbcrmomuplc temperature controller
lb)
Fig. 1. Schematic diagram of the plasma polymerization monomer reservoir (b)
reactor (a) and the
259
system was equipped with a turbo molecular pump to prevent oil contamination. A reservoir for supplying the solid and liquid monomer was also placed in the chamber. As shown in Fig. 1 (b) , the reservoir had two kinds of monomer vessels (vessel A and vessel B) . Vessel A was for low-volatile liquids and solid monomers which were capable of sublimation. Vessel B was used for highly volatile liquids such as benzene. Two tubes (tube X and tube Y) functioned as valves. This reservoir was connected with the reactor via a inlet valve, the introduction tube. When vessel A was used, tube X was raised and then the whole apparatus was pumped down together. Then the tube was closed and the reservoir was tilled with the vaporized gas. Afterward the tube was adjusted so as to keep a constant pressure and the discharge was performed for a certain period of time. On the other hand, when a volatile liquid was used, the liquid was injected into vessel B and tube Y was raised in order to isolate from the reservoir. Next, tube X was also raised and the reactor pressure was reduced in a similar manner. After the pressure was reduced enough, both tubes were lowered and a vaporized monomer filled the reservoir. Subsequent polymerization steps were the same for solid plasma polymerization. In our apparatus gaseous monomers can also be utilized. Two kinds of gasses could be mixed in the chamber with gasinlet flow rates controlled by gas-flow controllers. In addition, since a heater ( < 100 “C) with a thermocouple is built into vessel A, many kinds of monomers which have low vapor pressures in this temperature range were also able to be used. Because of potential chemical corrosion on the inside of the reservoir, it was coated with PTFE and the materials in the reservoir were also made of PTFE except for vessel A (stainless steel) and the thermocouple.
3. Results and discussions A d.c. glow method has the advantage of an easily adjusted applied voltage. It is expected that higher voltages give more energy to the reactive species such as electrons, ions (especially positive ion), and radicals in the glow phase, and that could affect the structure of the resulting deposits. The plasma polymerization, therefore, was carried out using different voltages. Fig. 2(a) and 2(b) show the IR spectra of plasmapolymerized films of naphthalene and benzene, respectively. The films were prepared at the voltages of 0.6 and 2.1 kV. As the spectra indicate, the chemical structure of the films were highly dependent on the voltage used. In each spectrum of films prepared at 0.6 kV, two regions of C-H stretching bands were observed at 3000-3 100 and 2800-3000 cm ’ The former due to the aromatic C-H band, while the latter to the aliphatic C-H band. The existence of aliphatic C-H suggests that the aromatic rings of naphthalene or benzene decomposed to some extent at this voltage. When the applied voltage was raised further, the intensity of all the C-H bands
T. Suwu et ~11./Thin Solid Films 273 (I 996) 258-262
I
I
3000
1,
2500
Wavenumber
L
I
3000
I1
,,
I
,
2000
.,
,.
2500
I
1,,
1000
(cm-‘)
1
,,
1,
2000
Wavenumber Fig. 2. IR spectra of plasma-polymerized
1
1500
1500
.I
their structures are quite different. The naphthalene plasma polymer shows features similar to that of amorphous carbon. The broad absorption of the benzene polymer, on the contrary, is indicative of graphitization. IR analysis, as mentioned above, suggested that the formation of the carbon film is accompanied by decomposition of the hydrocarbon aromaticity and subsequent hydrogen elimination. We cannot currently consistently explain the decomposition of the aromaticity and the growth of the graphitization. At present, the influence on the applied voltage on the Raman spectra of plasma polymerized benzene is being studied in detail. The glow discharge has a negative glow phase, a positive column, and a few other regions. We also compared the structure of products deposited in these different regions. Fig. 4 shows the IR spectra of plasma-polymerized perfluorobenzene prepared in the negative glow phase and in the positive column (the substrate placed in between the elcctrodes). The applied voltage was set at 0.4 kV. The highest peak at 1335 cm _’ in each spectrum is attributed to C-F stretching, C=C stretching is observed around at 15 10 cm I. Although the assignment of all peaks is difficult, no notable differences were found in the spectra. This suggests that the
I
1000
(cm-‘)
naphthalene
(a) and benzene (b)
preparedat different voltages.
2.
.z s -E
decreased gradually and no C-H peak was observed at 2.1 kV. In contrast to the decrease of the C-H band, a new absorption peak appeared in the region of 1600-l 800 cm ~ ’ . This absorption is attributed to C=O stretching. The formation of carbonyl groups means that oxygen was incorporated into the film. ESCA analysis indicated that oxygen existed only in the outermost portion of film. Thus, we can assume that these carbonyl groups resulted from post-reaction of the residual radicals generated during the discharge with atmospheric oxygen or water vapor. If the oxidized surface is ignored, then practically hydrogen-free carbon films were produced when the higher voltages were applied. Raman spectroscopy is a useful technique to evaluate caxbon films, and has been applied frequently for characterization of carbon materials such as diamond-like films [ 2-101. We compared Raman spectra of the carbon films generated by both the naphthalene and benzene plasmas. These spectra are shown in Fig. 3. The spectrum of the naphthalene plasma polymer has a series of bands at around 1300-1400 cm- ‘, while that of benzene has a very broad peak centered at 1550 cm-’ . The former peaks are considered to be associated with a sp’ bonded carbon, and the latter to carbons having sp’ configurations. The two films for Raman spectroscopy analysis were deposited under almost identical conditions, yet
+I 1
1700
1600
1500
1400
1300
1200
Raman Shift ( cm-’ ) Fig. 3. Raman spectra of plasma-polymerized (B) prepared at 2.1 kV.
naphthalene
(A) and benzene
I
I
I. 3000
I
2500
,,
2000
Wavenumber
I
1500
,
I
1000
( cm-’ )
Fig. 4. IR spectra of plasma-polymerized films of perfluorobenzene ited in a negative glow phase and positive column.
depos-
T. Suwu et al. / Thiri Solid Films 273 f 1996) 25X-262
110
110
100
-
90
-
Lob e@
70-
$ p a z g s
(a)
100
00
.
0
70-
69.
*
69
50 -
p a 5
so-
30-
2
30-
20-
s
20.
0
0
-
oQ.m’s”.n’B’
10
0
0.5
(b)
-
go80-
3 m g 8 s
40
0
2.0
2.5
3.0
“.a’j’o”’ 0
0.5
1.0
1.5
2.0
2.5
3.0
Applied Voltage (kv)
Fig 5. Relationship of water contact angle and the applied voltage:
(a) plasma-polymerized
process of deposition may not depend on where the substrate was placed in the plasma. ESCA analysis is another useful technique to evaluate the chemical structure of the plasma-polymerized films. Since it is impossible to detect hydrogen by ESCA, we made the films using perfluoronaphthalene. For these measurements, the films were prepared on both the cathode and anode. When the substrates were placed on the cathode, a thin glass plate (micro cover glass, 0.12-0.17 mm in thickness) was put under the substrates for insulation. Thus, the surface of the substrate was in the negative glow phase. The effect of the applied voltage on polymerization was examined as well. The chemical composition calculated from C Is, F Is, and 0 I s core-level spectra is summarized in Table 1. As had been expected from IR analysis, a certain amount of oxygen, which was not an clement contained in the starting monomer, was also detected in the perfluoronaphthalene plasma polymer. Considering similar results seen in our previous work [ I I 1, the oxygen originated from the oxidized surface layer of the films. The table indicates that the chemical compositions depend on neither the voltage nor the electrodes upon which the substrates were set. In the polymerized films the carbon to fluorine atomic ratios are about 3:2, and this roughly approximates that of the monomer (C:F = 5:4). We should note hem that these data are indicative of only the surface region, since the analysis depth of ESCA is only in the order of ten angstroms below the surface. The effect of the applied voltage was seen in contact angle measurements. The contact angle is sensitive to only the
naphthalenc,
(b)
plasma-polyrllcrlled
periluoronaphthalene
outermost region of the film. This measurement, therefore, will give some indication about the surface condition. The relation of water contact angle and the applied voltage for plasma-polymerized naphthalene and perfluoronaphthalene are shown in Fig. 5(a) and 5(b), respectively. The value 01 6” at V=O kV referred to the contact angle of an untreated glass slide. The contact angle of the naphthalene-baseddeposited films shows a slight dependence on the voltage. As the voltage increased, the contact angle decreased slightly. The change in the contact angle is probably related to the degree of film oxidation which could be caused hy residual radicals. The discharge at higher voltage generated more radicals on the film and more oxidation would occur when the system was vented to the atmosphere On the other hand. the contact angles of the perfluoronaphthalene plasma polymer did not depend upon the voltage within experimental error. This does not immediately mean that lilm surface oxidation was successfully prevented. The value of90” was much smaller than that of a typical fluorocarbon polymer like polyperlluoroethylcne ( I IO”). We should consider that in case of the pcrfluoronaphthalene the degree of the oxidation did not depend on the applied voltage.
Acknowledgements
This work has been performed under the support of “Special Coordination Funds for Promoting Science and Technology“ from the Science and Technology Agency of Japan.
I
Atomic concentration of plasma-polymerized
films prepared at drfferent
conditions Electrode
References C:F:O (%) l4kV
I .I kV
2.1 kV
[I ] A. Tanaka M. Yamaguchi. T. lwusaki and K Inyama. (1989)
Cathode Anode
O
-
0 1.5
00
40-
10
1.0
0
0
Applied Voltage (kV)
Table
26 I
S9.0:3 I .4:9.6
51.9:34.6:1.5
52.5:39.08.5
56.7:34.0:9.3
58.4:34.8:6.8
58.3:36.6:5.1
Clrrnr I.c,fr.
1219.
[2] R.O. Dillon, J A. Woollamand
V. Katknnant. Phw Reb.. H2Y (1984)
3482.
[ 31 P. Couderc
and Y. Catherine, Third Solid Films, I4
( 1987) 93.
262
T. Suwa et ul. /Thin Solid Films 273 (I 996) 258-262
[41 T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, Jpn. J. Appl. Phys.. 26 (1987) L1487. [51 M. Ramsteiner
[61M. Yoshikawa, Communications,
and J. Wagner, J. Appl. Phys. Lett., 51 ( 1987) 1355. G. Katagiri, 66 (1988)
H. Ishida and A. Ishitani. Solid State 1177.
[71 P.V. Houg, Diamond RelutedMater.,
I (1991) 33.
[81 D.P. Dowling, M.J. Ahern, T.C. Kelly, B.J. Meenan, N.M.D. Brown, G.M. O’Connor and T.J. Glynn, Surf: Coating Technol.. 53 (1992) 177. [91 V.P. Godbole and J. Narayama, J. Muter. Res., 7 (1992) 2785. [IO] W. Zhu, B.H. Tan and H.S. Tan, Thin Solid Films, 236 ( 1993) 106; [ 1 I I T. Suwa, M. Jikei, M. Kakimoto and Y. Imai, Jp. .I. A&. Phys., 34 ( 1995) in press.