The surface roughness and scaling behavior of a-C: H films deposited by plasma enhanced chemical vapor deposition from CH4-Ar mixtures were studied using atomic force microscopy. Raman spectroscopy gives some insights about the film microstructure. The film surface roughness is shown to decrease with the increase of deposition negative self-bias, while the presence of Ar ions enhances this effect. An analysis of the film surface and scaling behavior suggests that there is a transition of the mechanism of the film growth from a random deposition with surface diffusion process to a thermal spike based process that occurs upon the increase of the negative self-bias. voltage and the argon bombardment. (c) 2006 American Vacuum Society.