The efficiency of a methane-based plasma ash process (CH4/Ar chemistry) has been studied. Using Fourier transform infrared, X-ray photoelectron spectroscopy, and Rutherford backscattering techniques, we have shown that the formation of a carbon-rich SiOC surface layer during plasma exposure has a protective effect against moisture uptake and metal barrier diffusion. The methane-based chemistry induces a slight increase of the dielectric constant (10%) despite loss of the methyl group during the process. This chemistry proposes an interesting trade-off between the modification of the film required to prevent the metal diffusion and the modification of the dielectric constant. (C) 2005 The Electrochemical Society.