Tribological and optical properties of hydrogen-free amorphous carbon films with varying sp³/sp² composition

Unhydrogenated amorphous carbon films were deposited at different temperatures on a single silicon wafer by a process known as Filtered Cathodic Vacuum Arc. A transition from diamond-like properties to graphitic properties was observed at the substrate temperature of 200 °C. This change of properties upon transition includes a change in compressive stress, frictional coefficient and density. The variation of results is consistent with the subplantation model, which emphasizes the role of carbon atoms trapped in the subsurface layers in the evolution of a dense sp³-rich phase at room temperature. At higher temperature (>200 °C) diffusion of carbon atoms to the surface of the evolving film releases internal stress and leads to the formation of a graphitic sp² phase. Based on x-ray reflectivity measurements, it was found that a thin interlayer formed between the tetrahedral amorphous carbon film and the silicon substrate contributes greatly to the good fitting of the spectra. The thickness of this interlayer is independent of the temperature. The optical properties were found to undergo a more gradual transition with the deposition temperature.