Plasma organo-silicones

Primary interest was in the barrier properties obtained from plasma organo-silicones and from inorganic "SIN" coatings. Spectral grade HMDSO was used in the former case, while mixtures of SiH and NH were used to produce the SIN structures. The substrate in much or the work was DuPont Kapton type H polylmide film, 51 pm thick. Substrate temperatures extended to 450 C, as described earlier (6). The thickness of plasma-polymer deposits was about 0.5 pm. Moisture permeation was evaluated by the routine of ASTME-96-53 T (water vapor transmission of materials in sheet form). Additional, more precise data, were obtained with both a Dohrmann Envirotech Polymer Permeation Analyser, modified as previously described (6), and a Mocon "Permatran W" moisture permeation apparatus. 

As stated, the capability of plasma deposits to reduce the access of water to corrosion-sensitive surfaces may be an important motivation for their application in corrosion protection. In order to study this property, Kapton polyimide film was selected as the substrate because of its high inherent permeability to water and its ability to resist elevated temperatures. The response of Kapton film overcoated by PPHMDSO to the permeation of water vapor is shown in Fig. 1. Clearly, the presence of the organo-silicone plasma film greatly reduces water permeation. The magnitude of the effect is much enhanced when plasma polymers are produced at high T and p. 

The implied capability of these plasma deposits to inhibit corrosion at metal surfaces may be of practical as well as of basic importance. An important consideration in this respect is the rapid rate of deposition for such protective coatings attainable at micro-wave frequencies. Since plasma technology is still in a process of evolution, optimum deposition kinetics cannot yet be stated however, the marked effect of excitation frequency on the deposition of organo-silicones can be documented (10), as in Fig. 3. Here, using terminology and comparative data due to Yasuda et al. (2). it is shown that deposition rates in microwave plasmas exceed those at lower (e.g. radio) frequencies by about an order of magnitude. 

Figure 3. Deposition rates (D) for organo-silicones, normalized to flow rate (F) and monomer molecular weight (M), as function of plasma power (P). Shaded develope 13.56 MH plasmas data points are for 2.45 (M plasmas at monomer pressure 0.1 Torr (A) 0.2...

In view of the evident structural changes produced in organo-silicon plasma polymer films as a result of thermal post-treat-... 

Plasma-based CVD may also be used to deposit polymer films (plasma polymerization). In this case the precursor vapor is a monomer that becomes crosslinked in the plasma and on the surface to form an organic or inorganic polymer film. These films have very low porosity and excellent surface coverage. When plasma depositing films from organo-silane precursors, oxygen can be added to the plasma to oxidize some of the silicon in the film. 

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