Structure and bonding studies of a-C:H films produced by plasma enhanced CVD using CH4-CO2 mixtures

Hydrogenated carbon films have been deposited on Si substrates in a radio frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) system, using CH4-CO2 mixtures as gas precursor. A multi-technique approach has been used for a thorough characterization of the deposited films. In particular, their chemical composition and their structure were investigated by means of X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR). The film mass density was determined by combining gravimetric and thickness measurements. Besides, positron annihilation spectroscopy (PAS) analysis was performed for the detection of nano-scale open volume defects. The films were also characterized from the mechanical point of view using indentation and profilometric techniques to evaluate their hardness and internal stress. End purpose of this work has been the optimisation of the deposition process for obtaining dense and hard films, by studying the effects of process parameters such as the gas precursor composition, the cathode self-bias (VRF) and the sample negative biasing level. Specifically, the CO2 content in the feed gas was varied from 0 to 50 at.%, VRF from –200V to –500V while the bias voltage applied to the substrate was changed in a variable range which depended on the VRF for insuring film adherence. An increase of the film mass density was observed when rising the CO2 richness of the gas mixture, together with increased hardness and elasticity modulus. The first results indicate an evolution of the film structure from a mainly polymer-like one to that of an amorphous carbon phase. Low VRF values resulted in denser films with a lower amount of open volume defects. Higher film hardness was obtained by negatively biasing the sample