Surface relaxation during plasma-enhanced chemical vapor deposition of hydrocarbon films, investigated by in situ ellipsometry

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Abstract
The ion–thin-film interaction during plasma-enhanced chemical vapor deposition of hydrocarbon films (C:H films) from a methane electron cyclotron resonance plasma was investigated by means of in situ ellipsometry. Films were deposited with varying rf bias, resulting in a dc self-bias ranging from floating potential up to 100 V. The ion bombardment during plasma-enhanced chemical vapor deposition of hydrocarbon films leads to a disturbed surface layer, which becomes apparent in the in situ ellipsometric results by optical constants which are higher than those for the bulk material. The optical constants of the modified surface layer increase with ion energy during deposition, but decrease with the addition of hydrogen to the source gas. When using acetylene instead of methane for the deposition we also find a higher value for the optical constants. After switching off the plasma, the increased optical constants of the surface layer relax to its equilibrium state. The sources of this modified film surface and the mechanisms for its relaxation are discussed in this article. The change of the optical constants of the film surface is compared to the dependence of the film properties on the gas composition and the ion bombardment during deposition.