Influence of bias voltage on the structure and deposition mechanism of diamond-like carbon films produced by RF (13.56 MHz) CH4 plasma

Plasma enhanced chemical vapour deposition technique (PECVD) was used to grow diamond-like carbon films using pure methane gas plasma. Structural, optical and mechanical properties of the obtained a-C:H films were investigated as a function of bias voltage in the range 120-270 V, using different techniques. Elastic recoil detection analysis (ERDA) was employed to determine the hydrogen content and Fourier transform infrared spectroscopy (FTIR) was used to analyse the absorption of optically active hydrogen in the deposited films. The relative concentrations of sp ² and sp ³ groups were determined from fitting of both X-ray photoelectron spectroscopy (XPS) and FTIR spectra. Mechanical hardness and optical transmission were determined using nanoindentation and spectrophotometry, respectively. The results showed that the structure and properties of the films formed strongly depended on the applied bias voltage. In the range of energy considered the growth of the films was governed by a competition between both chemical and physical processes, with a dominance of physical process (subplantation) above 240 V, the energy at which more than 90% sp ³ hybridization was obtained. Nanoindentation tests revealed hardness and Young's modulus of the films ranging from 12-15 and 116-155 GP _a, respectively. The optical gap values deduced from the optical transmission spectra varied between 1.13 and 1.60 _eV.