Chapter 15 Fluoropolymer Films Deposited by rf Plasma Sputtering of Polytetrafluoroethylene Using Inert Gases 1,3
M . A . Golub
Downloaded by STONY BROOK UNIV SUNY on March 30, 2018 | https://pubs.acs.org Publication Date: February 15, 2001 | doi: 10.1021/bk-2001-0787.ch015
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and T. Wydeven
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N A S A Ames Research Center and Lockheed Martin Engineering and Sciences, Moffett Field, CA 94035-1000
The FT-IR, U V and XPS spectra of fluoropolymer films (SPTFE) deposited by rf plasma sputtering of polytetrafluoroethylene (PTFE), using Ne, Kr and Xe as sputtering gases, were obtained and compared with prior spectra for SPTFE formed using He and Ar. The F/C ratios for SPTFE films (1.44-1.55), obtained at an rf power of 10 W, were essentially the same for all five rare gases, with no trend of decreasing fluorine content in the SPTFE product with increasing atomic weight of the sputtering gas. Increasing rf power from 10 to 50 W resulted in successively lower F/C ratios for SPTFE (e.g., from 1.55 to 1.21 in the case of Xe plasma-sputtered PTFE), accompanied by sputtering of the Pyrex glass reactor and deposition of elements of the glass occurring at 40 W and above. In order to achieve a "Teflon-like" SPTFE structure (i.e., products with as high an F/C ratio as possible) in a given plasma reactor, an optimum rf power must be found, which in the present case was approximately 10 W.
The FT-IR, XPS and U V spectra of plasma-polymerized tetrafluoroethylene (PPTFE) and of the fluoropolymer deposits (SPTFE) formed from rf plasma sputtering of polytetrafluoroethylene (PTFE) using He or A r as sputtering gas were reported previously (i). In apparently the first study to involve preparation of PPTFE and SPTFE deposits in the same reactor and under comparable low-power (10 W) plasma conditions, the general similarity of PPTFE and SPTFE microstructures—noted in prior literature (2-5) on the basis of their IR and/or XPS spectra—has been reconfirmed, with some differences observed. Inasmuch as tetrafluoroethylene (TFE) monomer is the main product formed in the He plasma-induced decomposition of PTFE (6), it is not surprising that in-situ generation of TFE in rf plasma sputtering of PTFE yielded polymeric deposits that were similar to those obtained by plasma polymerization of TFE. From Cis XPS spectra, the average values and standard deviations for the fluorineto-carbon (F/C) ratios for a series of SPTFE deposits formed using either He or Ar (SPTFE-H or -A) as the sputtering gas were 1.56 ± 0.03 and 1.49 ± 0.04, respectively (i), in contrast to the theoretical value of 2.0 for pristine PTFE. These F/C ratios are within the range of literature values (1.3-1.6) (5,7-10) for SPTFE, and 3
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Current address: 12,000 4 Street North, #205, St. Petersburg, FL 33716 (retired)
© 2001 American Chemical Society
Castner and Grainger; Fluorinated Surfaces, Coatings, and Films ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
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204 also comparable to F/C ratios (1.2-1.6) for PPTFE (i). However, the F/C ratios for SPTFE-H and -A are distinctly higher than the values reported by Hishmeh et al. (11) for SPTFE generated in an A r plasma (0.89), and by Ryan et al. (12) for SPTFE deposits formed in He, Ne and A r glow discharges (1.02,0.82 and 0.78, respectively). The latter workers attributed the progressive drop in F/C ratio in moving from He to Ne to A r to momentum transfer phenomena related to the different atomic masses of the impinging rare gases. Since the difference between F/C ratios for Ryan et al.'s SPTFE deposits obtained by He and A r sputtering (0.24) was much greater than the corresponding difference for our SPTFE-H and -A deposits (0.07), we were prompted to extend our spectroscopic study of SPTFE (1) to include three other rare gases (Ne, K r and Xe) as sputtering agent, to test the momentum transfer concept. Moreover, since Ryan et al. carried out their rf plasma sputtering at a power of 50 W, while ours was done at 10 W, the new work with Ne, Kr and Xe included some runs at 20 to 50 W in addition to the desired, comparative runs at 10 W. In this way, we aimed to examine also the effect of rf power on the F/C ratios for SPTFE and, at the same time, try to account for the striking differences noted between the respective FT-IR and C i XPS spectra, as well as F/C ratios, for SPTFE products in our prior work (1) and in that of Ryan et al. (12). s
Experimental The apparatus employed for rf plasma sputtering of PTFE film (0.076 mm thick; Chemfab), using high-purity Ne, K r or Xe as the sputtering agent, and the analytical equipment were the same as those used previously ( i ) . A fresh, cleaned PTFE target sheet (25 cm χ 10 cm) was used in each sputtering run and deployed as described earlier (1). K B r disks, small pieces of Si wafers and quartz windows served as substrates for FT-IR, XPS and U V spectral analyses of SPTFE deposits. XPS spectra were obtained with an SSX-100 spectrometer at Surface Science Laboratories, Mountain View, C A . While most of the sputtering runs were carried out with an rf power of 10 Watts, several runs with Ne, K r and Xe were also conducted at 20,30, 40 and 50 Watts. The initial pressure (25 mTorr) and flow rate (0.5 c m STP/min) of the rare gas were the same in all runs, with sputter deposition times of up to 2 h. 3
Results and Discussion Rf plasma sputtering at 10 Watts. Figure 1 compares typical Q XPS spectra of SPTFE formed using Ne (SPTFE-N), K r (SPTFE-K) or Xe (SPTFE-X) as the sputtering gas. These spectra are not only virtually identical in shape or appearance, they are also essentially indistinguishable from the C i XPS spectra of SPTFE-H and -A reported previously (Fig. 2 in réf. 1). In common with the latter spectra, those of SPTRE-N, - K and - X exhibited four prominent "finger-like" peaks having the following, customary assignments (1): 294.0-294.1 ( - C F ) , 292.0 (~CF ~)* 290 (>GF~; -CF=), and 287.8 eV ($CCF
