Physical and chemical properties of amorphous hydrogenated carbon films deposited by PECVD in a low self-bias range

Amorphous hydrogenated carbon (a-C:H) films are deposited by plasma enhanced chemical vapor deposition (PECVD) at 13.56 MHz using pure methane gas. Structure, optical properties, stress level, hardness, Young modulus and surface energy are determined using spectroscopic ellipsometry, Rutherford backscattering spectroscopy, elastic recoil detection analysis, infrared experiments, curvature, nanoindentation and contact angle measurements. The present study is focused on the effect of the r.f. substrate bias voltage on the various properties, insisting on the very low bias range. We observe the properties change from polymer-like to graphite-like depending on the energy of the ions impinging on the sample surface. These results are incorporated in the discussion emphasizing on the well-known subimplantation model which describes the promotion of the sp3 CC hybridization in an energy range just above the penetration threshold of the film (100-150 V). At higher self-bias, the properties evolution is explained by the relaxation of the material due to the extra energy dissipated by the impinging ions which encourage the carbon atoms to rearrange themselves in a more stable sp2-bond structure. Despite of such a different structure, the measured surface energies are similar whatever the self-bias. This behavior is in a good agreement with a theory of a-C:H growth recently proposed.