Correlation between Optical Properties and Hardness of Diamond-Like Carbon Films

For this study, four kinds of DLC films were prepared using the ionized evaporation method. The substrate voltage was controlled, from 1 to 3 kV. Three kinds of films were prepared with benzene, and one with cyclohexane as the source gas. The DLC structure could be controlled using these parameters. This is related to hardness and optical properties. Various types of DLC films, which were used in round-robin testing for standardization were also analyzed in this study. These DLC films were deposited on Si substrates using the ionized evaporation method, the sputtering method, the arc method, and the plasma CVD method. The hardness of DLC films was determined using the nano-indentation technique. The refractive index and the extinction coefficient of DLC films were evaluated using a spectroscopic ellipsometer. The refractive index of tetrahedral amorphous carbon (ta-C) was found to be relatively high, from 2.50 to 2.74, whereas the extinction coefficient was low, from 0.04 to 0.50. For hydrogenated tetrahedral amorphous carbon (ta-C:H), the refractive index was lower than ta-C. Hydrogenated amorphous carbon (a-C:H) showed its own unique extinction coefficient and refractive index range, as did amorphous carbon (a-C). The refractive index was found to be proportional to the hardness of DLC films. The refractive index and the extinction coefficient were different depending on the kind of DLC, which were in turn related to the deposition method and DLC structure. Using optical properties, such as the refractive index and the extinction coefficient, we can distinguish the type of DLC film.