dc cathodic polymerization of trimethylsilane (TMS). in a closed reactor system was investigated. The composition of reactive species in the gas phase was monitored during the deposition process by a residual gas analyzer. It was found that, in such a closed plasma system, the deposition of TMS plasma polymers could be visualized as three consecutive, time-delayed, consecutive three fundamental processes. In the early stage of plasma deposition (< 60 s after the initiation of plasma), the deposition of plasma polymers was dominated by the polymerization of silicon-based species because the silicon-based species polymerized much quicker than carbon-based species. In the second stage of plasma deposition (between 60 and 120 s), the deposition was then dominated by carbon-based species due to the consumption of silicon in the early stage. In the final stage (more than 120 s), because of the total consumption of all the polymerizable species in the system, the deposition stopped and the deposited plasma polymer surface was continuously treated by nonpolymer forming gas plasma. As a result, the TMS plasma coatings obtained under such operations have a unique chemical structure that gradually changes from a lower carbon (C/Si ratio of similar to1.7 at the film/substrate interface) to carbon rich (C/Si ratio of similar to4.7 on the surface), as identified by x-ray photoelectron spectroscopy analysis.