Carbon coatings with high concentrations of silicon deposited by RF PECVD method at relatively high self-bias

This paper presents properties of a-C:H:Si coatings as a function of incorporated silicon atoms. The chemical composition of the examined coatings was controlled by tetramethylsilane (Si(CH3)4) to methane (CH4) flow ratio during RF PECVD (Radio Frequency Plasma Enhanced Chemical Vapour Deposition) process conducted at two values of negative substrate bias. The results have shown two stages in behavior of hardness, refractive index and density of deposited coatings, measured using the nanoindentation, ellipsometry and X-ray reflectivity techniques, respectively. The critical concentration of incorporated silicon, dividing the two stages, was established to about 15 at.%. Below this concentration the analyzed parameters decrease with the increasing amount of dopant, whereas above this value, just after reaching the minimum, further increase in silicon concentration results in increase of hardness, refractive index and density. We also registered a high consistency of results from both ellipsometry and X-ray reflectivity methods.