Carbon layers are frequently deposited by plasma-enhanced chemical vapor deposition (PE-CVD). This technology is very promising because of its highly efficient control of film quality, easy integration in current microelectronic technologies, low cost, high efficiency, and reproducibility. Here, we report a study on carbon layer aging under different conditions: photochemical, 400 K in air or N-2 atmosphere, O-2, H2O, and NaCl. We focus specifically on infrared (Fourier transform infrared, or FTIR) and spectroscopic ellipsometry (SE) analyses. Thin films were deposited in a capacitively coupled radiofrequency (RF) plasma in CH4. Six different plasma conditions were used: three different RF plasma powers (20, 80, and 150 W) used on both electrodes [on the polarized one (cathode) and on the floating one (anode)]. Furthermore, films with three different thicknesses (10, 100, and 1000 nm) were deposited. First of all, an interface of about 50 nm on the anode and of about 200 nm on the cathode at 20 W increasing with the RF plasma power appears at the interface between the substrate and the film. Next, the film composition in the ternary diagram Csp(2)-cdp(3)-H shows that thicker films deposited on the anode are rather apparently polymer-like, in contrast with the ones deposited on the cathode, which are rather graphite-like (a-CH double right arrow a-C). Moreover, these films show different aging mechanisms in air. This paper will thus concentrate on the study of the aging mechanisms based on the comparison of films that underwent aging under different conditions. In the case of polymer-like films, aging mechanisms are attributed to oxidation. In contrast, for the films formed at the cathode, these phenomena are attributed to film oxidation and hydrolysis. As aging leads to modifications in electrical and optical properties, a method to avoid these causes is suggested. (c) 2004 Elsevier B.V All rights reserved.