Studies have been made of the room temperature deposition of hydrogenated amorphous carbon and hydrogenated amorphous silicon-carbon in a magnetron discharge system at low ion energy (Ei approximately 30 eV), high density (ne approximately 1011 cm-3) and low pressure ( approximately 5 mTorr) with CH4/Ar and CH 4/SiH4/Ar gas mixtures respectively. A gas flow rate modulation technique was introduced to investigate the effect of low energy Ar ion bombardment immediately after the deposition of ultra-thin films. The plasma created a highly reactive environment and the deposition process had high gas conversion efficiency and good uniformity. Both carbon and silicon-carbon films were smooth with good adhesion. Absorption from the C-CH3 mode and some from the C=CH2 mode were found in the IR spectra of the carbon film. The IR spectra of the silicon-carbon film were mainly contributed by the SiHx, Si-CH3 and CHn related modes, and showed higher carbon incorporation than those obtained from the conventional discharge system. The strengths of the SiC and CHn modes and their ratio could be increased by increasing the methane to silane partial pressure ratio or the effective RF power. The index of refraction decreased with the increasing carbon incorporation. The film with alternating deposition of ultra-thin (20-75 AA) a-SixC1-x film and Ar plasma bombardment cycles showed that the post-deposition, low energy ion bombardment does not change the film thickness but is sufficient to crack the hydrocarbon bonds and promote the formation of Si-CHn bonds on the top few tens of monolayers of the ultra-thin film.