Chapter 16 Fluoropolymer Films Deposited by Argon IonBeam Sputtering of Polytetrafluoroethylene 1,4
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Fluorinated Surfaces, Coatings, and Films Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/26/18. For personal use only.
M . A . Golub , B . A . Banks , S. K. Rutledge , and M. C . K i t r a l 1
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N A S A Ames Research Center, Moffett Field, C A 94035-1000 N A S A Lewis Research Center, Cleveland, O H 44135 Cleveland State University, Cleveland, OH 44115 2
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The FT-IR, XPS and U V spectra of fluoropolymer films (SPTFE-I) deposited by argon ion-beam sputtering of polytetrafluoroethylene (PTFE) were obtained and compared with prior corresponding spectra of fluoropolymer films (SPTFE-P) deposited by argon rf plasma sputtering of PTFE. Although the F/C ratios for SPTFE-I and -P (1.63 and 1.51) were similar, the SPTFE-I structure had a much higher concentration of CF groups than the SPTFE-P structure: ca. 61 and 33% of the total carbon contents, respectively. Reflecting that difference, the FT-IR spectra of SPTFE-I showed a distinct doublet at 1210 and 1150 cm whereas SPTFE-P presented a broad, featureless band at ca. 1250 cm . The absorbance of the 1210-cm band in SPTFE-I was proportional to film thickness in the range of 50-400 nm. SPTFE-I was more transparent in the U V than SPTFE-P at comparable thickness. The mechanism for SPTFE-I formation likely involves "chopping off" of oligomeric segments of PTFE as an accompaniment to "plasma" polymerization of TFE monomer generated in situ from PTFE on impact with energetic A r ions. Data are given for SPTFE-I deposits and the associated Ar -bombarded PTFE targets where a fresh target was used for each run or a single target was used for a sequence of runs. 2
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Argon ion-beam sputtering of polytetrafluoroethylene (PTFE) has attracted considerable research interest from the standpoint of the surface modification of the PTFE target (7-5) or the formation of sputter-deposited, fluoropolymer coatings on various substrates (6-70). Regarding surface modification, a strong similarity between the microstructural changes produced in PTFE when subjected to either a low-energy Ai+ ion beam or an Ar plasma had been noted by Wells et al. (3), and later validated (5) by the related work of Tan et al. (2) and Golub et al. (77). Regarding sputter-deposited fluoropolymers, one of us reported recently on the microstructural similarity between plasma-polymerized tetrafluoroethylene (PPTFE) and the fluoropolymer deposits formed from rf plasma sputtering of PTFE (SPTFE-P), first using He or A r as sputtering gas (72), and subsequently using three other inert gases (Ne, Kr and Xe) as sputtering gases (73). Having briefly examined some years ago fluoropolymer deposits formed from Ar ionbeam sputtering of PTFE (SPTFE-I) (7,#), we were prompted to carry out a more complete examination of SPTFE-I using FT-IR, XPS and UV-visible spectroscopy for 3
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Current address: 12,000 4 Street North, #205, St. Petersburg, FL 33716 (retired)
© 2001 American Chemical Society
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214 microstructural characterization as well as for comparison of SPTFE-I and SPTFE-P products. Experimental The ion-beam sputtering technique peformed was described previously (7) and utilized ion-beam sputter cleaning of the target and deposition surfaces prior to sputter deposition. The PTFE targets used in this study were 12.7-cm disks of DuPont Teflon located 21.3 cm downstream of a 2.5-cm beam diameter A r ion source used for ionbeam sputtering. The sputter target was inclined 45° from perpendicular to the ion beam. The deposition surfaces were located 17.1 cm from the sputter target and perpendicular to a line-of-sight between them. Typically, a separate PTFE target was used in each sputtering run, but some runs involved sequential sputtering of a single target. K B r disks (for FT-IR), Si wafers (for XPS), and quartz disks (for U V analyses of SPTFE-I deposits) were used as substrates in the deposition. The Ar ion-beam conditions used to prepare the SPTFE deposits were: Ion energy, 750 eV; ion beam current, 28 mA; chamber pressure, 0.1 mTorr; sputter-deposition times, 2.5-51.0 min. FT-IR absorption spectra of SPTFE were obtained by means of a Mattson Model 6020 spectrometer; UV-visible spectra were obtained with a Cary 3 spectrophotometer, while corresponding XPS spectra were obtained with an SSX-101 spectrometer at Surface Science Laboratories, Mountain View, Calif., using monochromatized ΑΙ Κ α X-rays, the binding energies being referenced to the F i peak at 689.2 eV. s
Resnlts and Discussion Figure 1 compares typical C i XPS spectra of the SPTFE deposits formed by Ar ionbeam sputtering of PTFE (SPTFE-I) in this study and by A r rf plasma sputtering of PTFE (SPTFE-P) as reported previously (12). Both spectra display four characteristic peaks having the following customary assignments (14): 294.0-294.1 (-CF3), 292.0 (-CF -), 289.8-289.9 (>CF-; -CF=) and 287.6-287.7 eV (£CCF-; £CO-), along with a weak, fifth peak at 285.0-285.4 eV that represents hydrocarbon surface contamination (-CH -) and/or crosslinks. The F/C ratios for SPTFE-I and -P, as calculated (12) from the relative intensities of the deconvoluted peaks in the C i spectra, were 1.63 and 1.51, respectively. The small difference between these F/C ratios belies, however, an important structural difference between SPTFE-I and -P, namely, a much higher concentration of C F functionalities in the former material than in the latter (ca. 61 and 33% of the total carbon contents, respectively). Indeed, the C i spectrum of SPTFE-I resembles a composite of the corresponding spectra of PTFE (with a single - C F - peak at 292.0 eV) and of SPTFE-P, or even PPTFE (8) (with their four "finger-like" peaks). In this respect, the C i spectrum of SPTFE-I in Figure 1 (obtained with 750-eV A r ions) resembles closely that of an earlier SPTFE-I (8) (obtained with 1000-eV A r ions) which had an F/C ratio of 1.73 and in which the C F groups represented ca. 70% of the total carbon content. That the prior SPTFE-I (8) had a higher F/C ratio and higher C F content than the present SPTFE-I is in qualitative accord with the results obtained by Quaranta et al. (9) who reported SPTFE-I formed with 700- and 1000-eV A r ions to have F/C ratios of 1.48 and 1 83, and C F contents of 44 and 74%, respectively. Figure 2 compares typical FT-IR spectra of SPTFE-I and -P corresponding to the respective C i spectra presented in Figure 1. In contrast to the broad featureless absorption band centered at ca. 1250 cm" (an overlap of CF, C F and CF3 vibrations) in the FT-IR spectrum of SPTFE-P, there is a distinct doublet centered at the same location in the spectrum of SPTFE-I, the doublet at 1210 and 1150 c m being assignable to the asymmetrical and symmetrical C F stretching modes, respectively, as seen in the FT-IR spectrum of PTFE itself. The same doublet was noted by Quarante et al. (9), while similar doublets were seen at 1197 and 1154 cm* in the FT-IR spectra of s
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218 Ar+ sputter-deposited films from PTFE recently reported by L i et al. (10), and at 1211 and 1152 c m in the FT-IR spectra of pulsed-laser deposited films from PTFE reported by Norton et al. (75). The presence of a distinct doublet in the FT-IR spectrum of SPTFE-I, and its absence in that of SPTFE-P, reinforces the view from XPS that the former deposit is more PTFE-like in structure man the latter. Apart from the intense bands at ca. 1200-1250 cm" , there are weak absorptions at ca. 1720 c m (-CF=CFand >C=0 groups) and at ca. 750 cm" ("amorphous" - C F 2 - structure). The absorbance, A , of the band at 1210 cm" for a series of SPTFE-I deposits was proportional to film thickness, t, in the range of 50-400 nm (Figure 3), and is given by the linear equation A = 4.68 χ l 100 nm. Another experiment involved sputtering four individual ρ χ ρ Ε targets for different periods of time to produce four SPTFE-I deposits of increasing thickness. Here, the F/C ratios for the Ai^-bombarded targets were substantially constant (1.75,1.53,1.83 and 1.64), while the F/C ratios (and thicknesses in nm) for the corresponding deposits were 1.16 (15); 1.27 (30); 1.48 (49) and 1.57 (126). Again, the F/C ratio for SPTFE-I increased with thickness and then levelled off. Our finding that the F/C ratio for the PTFE target remained largely unchanged with time of Ar * bombardment runs counter to the observations in refs. 1 and 3 wherein defluorination of PTFE continued to very low F/C ratios (0.2 and 0.6, respectively), with no indications of any levelling-off after extended Ar * bombardment. As suggested earlier (72), the mechanism for SPTFE-I formation likely results from a relatively large involvement of "chopping o f f of oligomeric segments of PTFE macromolecules as an accompaniment to the "plasma" polymerization of T F E monomer (or other fluorocarbon fragments) generated in situ from PTFE on impact with energetic A r ions. This view is consistent with the appearance of the C i XPS spectra of SPTFE-I which resemble superpositions of the corresponding spectra for PTFE and PPTFE and which exhibit the dominant 292.0-eV peak due to C F groups. A similar mechanism may be considered for SPTFE-P but one where the "chopping o f f of oligomeric segments of PTFE macromolecules is less important than in the case of SPTFE-I. -1
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Figure 4. UV-visible spectra for SPTFE-I (with different thicknesses) and SPTFE-P.
221 Acknowledgment The authors express appreciation to Jeanie Howard, Lockheed Martin Engineering and Sciences, for providing the FT-IR and U V spectra for the various SPTFE samples. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Takahagi, T.; Ishitani, A. Macromolecules 1987, 20, 404. Tan, B. J.; Fessehaie, M.; Suib, S. L. Langmuir 1993, 9, 740. Wells, R. K.; Ryan, M. E.; Badyal, J. P. S. J. Phys. Chem. 1993, 97, 12879. O'Keefe, M. J.; Rigsbee, J. M. J. Appl. Polym. Sci. 1994, 53, 1631. Golub, M. A. Langmuir 1996, 12, 3360. Sovey, J. S. J. Vac. Sci. Technol. 1979, 16, 813. Banks, Β. Α.; Mirtich, M. J.; Rutledge, S. K . ; Swec, D. M. Thin Solid Films. 1985, 127, 107 Wydeven, T.; Golub, Μ. Α.; Lerner, N. R. J. Appl. Polym. Sci. 1989, 37, 3343. Quaranta, F.; Valentini, Α.; Favia, P.; Lamendola, R.; d'Agostino, R. Appl. Phys. 1993, 63, 10. Li, Y . ; Gong, Z.; Long, Z.; Ji, C. Dalian Ligong Daxue Xuebao 1996, 36, 279; Chem. Abst. 1997, 126, 90730. Golub, Μ. Α.; Lopata, E . S.; Finney, L. S. Langmuir 1994, 10, 3629. Golub, Μ. Α.; Wydeven, T.; Johnson, A. L. Langmuir 1998, 14, 2217. Golub, Μ. Α.; Wydeven, T. Polym. Prepr. 1998, 39 (2), 930; also this monograph. Dilks, Α.; Kay, E. Macromolecules 1981, 14, 855. Norton, M. G.; Jiang, W.; Dickinson, J. T.; Hipps, K. W . Appl. Surf. Sci. 1996, 96, 617. Sujimoto, I. Macromolecules 1991, 24, 1480.
