Determination of gas phase and surface reactions in plasma polymerization

Using macroscopic kinetics, the reactions within the gas phase are governed by the reaction parameter power input per gas flow $W$/$F$, which corresponds to a specific energy, while reactions by energetic particle bombardment at the growing film surface are rather related to power input $W$ alone. Assuming activation reactions, the mass deposition rate per gas flow can be described by an Arrhenius-like approach: \[ \frac{R_m }{F}=G\exp \left( {-\frac{E_a }{W \mathord{\left/ {\vphantom {W F}} \right. \kern-\nulldelimiterspace} F}} \right) \] Mixtures of hydrocarbons (C$_{2}$H$_{4})$ and reactive gases (CO$_{2}$, N$_{2}$+H$_{2})$ were examined within low pressure RF plasmas. Thus, functional a-C:H:O or a-C:H:N plasma coatings result. At increasing energy input it is found that the deposited mass shows a deviation from the above equation, commonly related to energetic particle interactions. However, using the same range of $W$/$F$ with varying power input $W$, it was found that the observed drop in deposition rate scales solely with energy input $W$/$F$ for a-C:H:O, i.e. depending on plasma chemistry. a-C:H:N films, on the other hand, show both chemical and physical influences on the film growth. Hence, gas phase reactions such as a change of film-forming species play a major role in plasma polymerization.