Nitrogen-Doped DLC Films: Correlation between Optical and Mechanical Properties

Optical and mechanical properties of nitrogen-containing diamond-like carbon (NC-DLC) films deposited by rf plasma decomposition of a CH4:h2:n2 gas mixture have been investigated. Nitrogen was incorporated into DLC films both during the film growth and after the deposition of the film by implantation of nitrogen ions. It could be shown that both optical and mechanical properties of the films strongly depend on their nitrogen content. Films deposited in low n2 concentration gas mixtures have a high refractive index (up to 2.4), a hardness of up to 10 GPa and a low optical gap (less than 2.0 eV). An increase in the nitrogen content of the gas mixture results in a lowering of the hardness and refractive index values because the incorporated nitrogen causes more polymerlike films. It could also be shown that intrinsic stresses are substantially reduced by nitrogen incorporation in the films. A significant increase in microhardness and Young&s;s modulus values (up to 3 times) of above mentioned films was observed after nitrogen ion implantation. The mechanisms of the effects mentioned are also discussed.