The deposition of SiC:H,F films from SiF4-CH4 plasmas is investigated: the emphasis is on the role of H and F atoms in determining the carbon and silicon etching processes. It is found that the H-2 addition to the SiF4-CH4-He mixture affects the relative amounts of the H and F atoms in the plasma phase. The H/F ratio controls the deposition rate, the composition and the structure of silicon carbon alloys.