High-resolution x-ray reflectivity (XRR) and heavy-ion elastic recoil detection were employed to study the role of hydrogen on the softening behavior observed in Si-incorporated hydrogenated amorphous carbon (Si-a-C:H) films synthesized by plasma-enhanced chemical-vapor deposition using tetramethylsilane (TMS) precursor in C2H2/Ar plasma. An enhancement of the optical band gap and a massive reduction in the density of the films prepared at high TMS flow rate were revealed, respectively, by spectroscopic ellipsometry and XRR analysis with the development of a double critical angle. A hydrogenation process was responsible for a rise in the density of voids and an associated reduction in the connectivity of the carbon network and the release of its residual stress. (c) 2005 American Institute of Physics.