Langmuir 1998, 14, 2217-2220
2217
Notes Similarity of Plasma-Polymerized Tetrafluoroethylene and Fluoropolymer Films Deposited by rf Sputtering of Poly(tetrafluoroethylene) Morton A. Golub,*,† Theodore Wydeven,‡ and Allen L. Johnson§ NASA and Lockheed Martin Engineering and Sciences, Ames Research Center, Moffett Field, California 94035-1000, and Surface Science Laboratories, 625-B Clyde Avenue, Mountain View, California 94043 Received October 9, 1997. In Final Form: January 29, 1998
Introduction Considerable literature exists on the infrared (IR) or X-ray photoelectron spectroscopic (XPS) characterization of the products obtained from plasma polymerization of tetrafluoroethylene (TFE),1-12 and a somewhat smaller body of literature exists on the IR or XPS characterization of the fluoropolymer deposits formed in the rf plasma sputtering of poly(tetrafluoroethylene) (PTFE).9,10,13-21 Several references have noted that the structures of the fluoropolymer deposits arising from rf-sputtered PTFE (SPTFE) were similar to those of plasma-polymerized TFE (PPTFE).8-10,16,20 In none of these earlier studies were SPTFE and PPTFE prepared in the same plasma reactor. Tibbitt et al.,9 who apparently first reported the remarkable similarity of the IR spectra of PPTFE and SPTFE, †
NASA, Ames Research Center. Lockheed Martin Engineering and Sciences, Ames Research Center. § Surface Science Laboratories. ‡
(1) Giegengack, H.; Hinze, D. Phys. Status Solidi 1971, A8, 513. (2) Rice, D. W.; O’Kane, D. F. J. Electrochem. Soc. 1976, 123, 1308. (3) Hetzler, U.; Kay, E. J. Appl. Phys. 1978, 49, 5617. (4) Hozumi, K.; Kitamura, K.; Kitade, T. Bull. Chem. Soc. Jpn. 1981, 54, 1392. (5) Buzzard, P. D.; Soong, D. S.; Bell, A. T. J. Appl. Polym. Sci. 1982, 27, 3965. (6) Golub, M. A.; Wydeven, T.; Cormia, R. D. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 2683. (7) Golub, M. A.; Wydeven, T.; Finney, L. S. Plasmas Polym. 1996, 1, 173. (8) Dilks, A.; Kay, E. Macromolecules 1981, 14, 855. (9) Tibbitt, J. M.; Shen, M.; Bell, A. T. Thin Solid Films 1975, 29, L43. (10) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985; pp 184-185. (11) Wydeven, T.; Golub, M. A.; Lerner, N. R. J. Appl. Polym. Sci. 1989, 37, 3343. (12) Kaplan, S.; Dilks, A. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1984, 38, 105. (13) Pratt, I. H.; Lausman, T. C. Thin Solid Films 1972, 10, 151. (14) Sugimoto, I.; Miyake, S. J. Appl. Phys. 1988, 64, 2700. (15) Sugimoto, I.; Miyake, S. J. Appl. Phys. 1991, 70, 2618. (16) Yamada, Y.; Kurobe, T. Jpn. J. Appl. Phys. 1993, 32, 5090. (17) Horie, M. J. Vac. Sci. Technol. A 1995, 13, 2490. (18) Pireaux, J. J.; Delrue, J. P.; Hecq, A.; Duchot, J. P. In Physicochemical Aspects of Polymer Surfaces; Mittal, K. L., Ed.; Plenum Press: New York, 1983; pp 53-81. (19) Ryan, M. E.; Fonseca, J. L. C.; Tasker, S.; Badyal, J. P. S. J. Phys. Chem. 1995, 99, 7060. (20) Hishmeh, G. A.; Barr, T. L.; Sklyarov, A.; Hardcastle, S. J. Vac. Sci. Technol. 1996, A14, 1330. (21) Mare´chal, N.; Pauleau, Y. J. Vac. Sci. Technol. 1992, A10, 477.
suggested that the mechanism of rf sputtering of PTFE resembled that of plasma polymerization of TFE. Since the TFE monomer (CF2dCF2) is the main product in the helium plasma-induced decomposition of PTFE,22 it is not surprising that in-situ generation of TFE during rf plasma sputtering of PTFE would yield fluoropolymer deposits that were similar to those resulting from the plasma polymerization of TFE. Holland et al.,23 on the other hand, claimed that Tibbitt et al.’s contention that PPTFE and SPTFE had similar structures by virtue of their similar IR spectra was an oversimplification; that claim, however, may be dismissed since it was based on a comparison of UV-visible absorption spectra of SPTFE and plasma-polymerized carbon tetrafluoride (CF4) and notsas it should have beenson the spectra of SPTFE and PPTFE. In a different kind of sputtering, which was brought about by pulsed laser ablation of bulk PTFE targets in the presence of argon background gas,24 the deposited films approached the structure of PTFE itself, as evidenced by XPS, with a fluorine-to-carbon (F/C) ratio as high as 1.9 versus 2.0 for pristine PTFE. Still another kind of sputtering of PTFE, which was induced by an argon ion beam, yielded an SPTFE structure which was considered to be a 4-5:1 blend of PTFE- and PPTFE-type structures,11 based on its C1s XPS spectrum and an F/C ratio of 1.73. Consistent with the view that PPTFE and SPTFE have similar structures is the fact that most of the XPS-derived F/C ratios published for SPTFE (1.3-1.6)16-18,21,25 match the ratios for PPTFE (1.2-1.6).6-8,10,11 Two recent papers, however, have presented distinctly lower F/C values for SPTFE: Ryan et al.19 reported F/C ratios for SPTFE produced with various inert gases to decrease on moving from He to Ne to Ar (1.02, 0.82, and 0.78, respectively)swhich they ascribed to momentum transfer phenomena related to the different atomic masses of the impinging gasesswhile Hishmeh et al.20 obtained an F/C ratio of 0.89 for SPTFE generated in an Ar plasma. Reflecting these lower F/C ratios, the C1s XPS spectra for SPTFE obtained with an Ar glow discharge shown in the latter two papers,19,20 although similar to each other, differed considerably in shape from corresponding spectra in refs 14 and 16-18 by each having as its major peak the one associated with carbons not directly attached to fluorine. This contrasted with prior C1s XPS spectra for SPTFE14,16-18 which presented four distinct peaks (due to CFx; x ) 0-3), the most prominent peak being that for x ) 2. On the other hand, the FT-IR spectrum for SPTFE obtained by Hishmeh et al. resembled the corresponding IR spectra shown in refs 9, 10, 13-15, and 21, while the FT-IR spectra for SPTFE deposits obtained with He, Ne, or Ar obtained by Ryan et al. differed from prior spectra (22) Mathias, E.; Miller, G. H. J. Phys. Chem. 1967, 71, 2671. (23) Holland, L.; Biederman, H.; Ojha, S. M. Thin Solid Films 1976, 35, L19. (24) Shah, S. I.; Blanchet, G. B. Mater. Res. Soc. Symp. Proc. 1993, 285, 599. (25) The C1s XPS spectrum for (conventional) Ar-sputtered PTFE obtained by Sugimoto and Miyake14 is similar to that of SPTFE-A shown in Figure 2; since those workers did not report an F/C ratio for their sputtered material, we estimated the F/C ratio to be ca. 1.45 based on the deconvoluted C1s XPS spectrum.
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2218 Langmuir, Vol. 14, No. 8, 1998
in displaying intense, extraneous absorption bands in the 1400-1800 cm-1 region in addition to the characteristic C-F absorption band centered at ca. 1250 cm-1. Although the variances between F/C values previously reported for SPTFE16-18,21,25 and those of Ryan et al.19 and of Hishmeh et al.20 are likely attributable to the particular rf powers used in their glow discharges (50 and 500 W, respectively) and to the design and operating parameters of their reactors, it is noteworthy that the consistent, prior XPS and IR literature for SPTFE involved rf powers that ranged from 50 to 250 W. Given the indications8-10,16,20 noted above of IR and XPS similarities between PPTFE and SPTFE formed in an rf glow discharge, it appeared desirable to reexamine this question of their similarity by preparing PPTFE and SPTFE in the same apparatus and under comparable low-power discharge conditions and by characterizing the resulting deposits. A low rf power was optimal for the plasma reactor in which we had previously prepared numerous PPTFE samples.6,7,11 Having previously examined Ar ion-sputtered PTFE,11 we were prompted to extend that work to Ar rf plasma-sputtered PTFE. This paper presents new FT-IR, C1s XPS, and UVvisible spectra for PPTFE and SPTFE deposits, the latter formed in an rf glow discharge using Ar or He as the sputtering gas. Experimental Section Plasma polymerization of TFE monomer inhibited with limonene-d (Scott Specialty Gases) was performed in the apparatus described previously.6 The conditions used to prepare PPTFE deposits were as follows: rf (13.56 MHz) power, 10 W; initial pressure, 100 mTorr; TFE flow rate, 3.0 cm3(STP)/min; and deposition times of up to 3 h. The same apparatus was employed for rf plasma sputtering of PTFE film (0.076 mm thick; Chemfab), using high-purity Ar or He as the sputtering agent. The sputtering target was a sheet (25 cm × 10 cm) cut from that PTFE film (cleaned sequentially with dichloromethane, acetone, and distilled water for 15 min in a sonicator and dried in air), then rolled into a cylindrical shape (7.6 cm in diameter), and used to line the inner wall of the tubular reactor beneath the external electrodes. A fresh PTFE sheet was used in each sputtering run. KBr disks (for FT-IR), small pieces of Si wafers (1 cm square, for XPS), and quartz windows (for UV-visible spectra) were supported by a Pyrex glass plate situated along the axis of the PTFE “cylinder” midway between the leading and trailing edges of the electrodes during the sputtering runs. The conditions used to prepare SPTFE deposits were as follows: rf power, 10 W; initial pressure, 25 mTorr; Ar or He flow rate, 0.5 cm3(STP)/min; and sputter-deposition times of up to 2 h. FT-IR spectra of PPTFE or SPTFE (both formed “in the glow” of the plasma reactor) were obtained with a Mattson Model 6020 spectrometer; XPS spectra were obtained with an SSX-100 spectrometer, using monochromatized Al KR X-rays, the binding energies being referenced to the F1s peak at 689.2 eV; and UVvisible spectra were obtained with a Cary 3 spectrometer.
Results and Discussion Figure 1 compares typical FT-IR spectra obtained for SPTFE formed with Ar or He (SPTFE-A or SPTFE-H) as sputtering gas and for PPTFE. Not only are these spectra very similar, they are at once virtually the same as the IR spectra of SPTFE and PPTFE presented by Tibbitt et al.9 and quite comparable to the IR spectra of SPTFE13-15,21 and PPTFE1,3-7 reported by other workers employing different reactors and varied plasma parameters. Figure 1 exhibits for either SPTFE or PPTFE a characteristic strong, broad absorption band centered at ca. 1250 cm-1 (an overlap of CF, CF2, and CF3 vibrations), plus weak absorptions at ca. 750 cm-1 (“amorphous” -CF2- structure)3 and at ca. 1720 cm-1 (-CFdCF- and >CdO
Notes
Figure 1. Typical FT-IR spectra of (A) SPTFE-A, (B) SPTFEH, and (C) PPTFE deposits.
groups).6,11 Figure 1 lacks the strong, broad extraneous absorptions in the 1400-1800 cm-1 range seen in the FTIR spectra of Ryan et al.’s SPTFE (Figure 3 in ref 19)sabsorptions that have not appeared in any prior IR spectra of SPTFE or PPTFE. Figure 2 compares the C1s XPS spectra of SPTFE-A, SPTFE-H, and PPTFE corresponding to the respective FT-IR spectra in Figure 1. Although the FT-IR spectra of these products are nearly indistinguishable, their C1s spectra, while similar in displaying four prominent peaks and a very weak, fifth peak, exhibit a definite difference between PPTFE and SPTFE-A or SPTFE-H and only a slight difference between the latter two products. These differences, which are illustrated by the quantitative assessments of the various functionalities given in Table 1, may be summarized as a relatively larger -CF2content (C1s peak at 292.0 eV) in the SPTFE structures than in PPTFE. The three other prominent C1s peaks have the following customary assignments:6,8 294.0-294.1 eV (-CF3), 289.6-289.9 (>CF-; -CF)), and 287.5-287.7 (mCCFCF-
mCCFx
-CH2-
total C1s
F1s
O1s
N1s
F/C ratiosb
SPTFE-A
8.0 (20.4 8.3 (21.5 9.4 (22.1
12.9 32.9 12.8 33.4 10.5 24.9
9.5 24.2 9.0 23.4 10.1 23.9
7.9 20.1 7.6 19.7 11.7 27.7
0.9 2.4)c 0.8 2.0)c 0.6 1.4)c
39.1
58.9
0.9
1.1
1.51 (1.51)
38.5
59.9
0.8
0.9
1.56 (1.55)
42.3
56.2
1.0
0.5
1.33 (1.40)
SPTFE-H PPTFE
a Only the major functionalities contributing to the five C peaks are noted in the table; other functionalities are indicated in the text. 1s F/C ratios derived from F1s/C1s intensity ratios from XPS survey scans; values in parentheses are corresponding F/C ratios obtained by deconvolution of C1s spectra, using the relation8 F/C ) (3CF3 + 2CF2 + CF)/(total C1s). c Values in parentheses are percentages of C1s peaks used to determine F/C ratios by deconvolutionb of C1s spectra.
b
Figure 3. Typical UV-visible spectra of SPTFE-H (‚‚‚), SPTFE-A (- -), and PPTFE (s) deposits (all ca. 429 nm thick), and of the uncoated (blank) quartz window (o o o).
Figure 2. Typical C1s XPS spectra of (A) SPTFE-A, (B) SPTFEH, and (C) PPTFE deposits. The binding energies are given in electronvolt (eV) units.
within the range of prior F/C ratios for SPTFE (1.31.6),16-18,21,25 but they are distinctly higher than the F/C ratios of 0.78 and 1.02 observed by Ryan et al.19 for Arand He-sputtered PTFE, respectively, or the F/C ratio of 0.89 obtained by Hishmeh et al.20 for Ar-sputtered PTFE. In work to be reported elsewhere,26 we have obtained F/C ratios for SPTFE films formed using Ne, Kr, and Xe as sputtering gases (also at a power of 10 W) that were similar to those for SPTFE-A and -H, namely, 1.48 ( 0.07, 1.44 ( 0.05, and 1.55 ( 0.01, respectively. In an effort to account for the significantly lower F/C values for SPTFE obtained by both Ryan et al. and Hishmeh et al.,20 and aware that increasing rf power for the sputtering process in a particular reactor could lead to enhanced defluorination of PTFE, we performed limited (26) Golub, M. A.; Wydeven, T. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), submitted for publication.
experiments at 20-50 W and found the F/C ratio for SPTFE-H to indeed decrease as follows: 1.37 (20 W); 1.23 (30 W); 1.24 (40 W); 1.08 (50 W), the 50 W deposit being contaminated by substantial cosputtering of the Pyrex glass reactor! The C1s spectrum for SPTFE-H at 50 W (similar to the corresponding spectrum for SPTFE-X formed using Xe as sputtering gas26), while beginning to approach the shape of that of Ryan et al., presents the same four prominent peaks seen in Figure 2 but with different relative intensities. At the same time, the corresponding FT-IR spectrum presents C-F absorptions in the 1100-1300 cm-1 range with none of the extraneous absorptions in the 1400-1800 cm-1 range seen in the work of Ryan et al. At any rate, we infer that the FT-IR and XPS results of Ryan et al. and Hishmeh et al., while at variance with the bulk of the relevant literature, may be due to their use of higher-than-optimum powers or other plasma parameters for their particular reactors. The point to emphasize here is that in order to achieve SPTFE structures with high F/C ratios, an optimum rf power is needed for a given plasma reactor, and in our case that power was 10 W. Figure 3 compares typical UV spectra of SPTFE-A, SPTFE-H, and PPTFE deposits having the same thickness of ca. 430 nm. Both SPTFE films are more transparent than the PPTFE film, with SPTFE-H being only slightly more transparent than SPTFE-A. The transmittances for SPTFE-H and SPTFE-A at 350 nm, are, respectively, ca. 12 and 9% greater than that for PPTFE; this situation is the opposite of that seen in ref 23, where the transmit-
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tance of Ar-sputtered PTFE at 350 nm is ca. 28% smaller than that for plasma-polymerized CF4 (PPTFM). As noted above, PPTFE should have been used instead of PPTFM in ref 23. The observed order of decreasing transmittance for SPTFE-H, SPTFE-A, and PPTFE parallels the order of decreasing F/C ratios (1.56, 1.49, and 1.33, respectively). Since the smooth, band-free nature of the UV-visible absorption curves in Figure 3 is indicative of conjugated double bonds,27 that order of decreasing F/C ratios may reflect increasing conjugated unsaturation associated with increasing defluorination. The rf plasma sputtering of PTFE is considered to involve, in part, scission of -(CF2)n- chains to yield oligomeric segments, -(CF2)m- (m
