Role of nitrogen in the formation of hard and elastic CNx thin films by reactive magnetron sputtering

Carbon nitride films, deposited by reactive de magnetron sputtering in Ar/N-2 discharges, were studied with respect to composition, structure, and mechanical properties. CN, films, with 0 less than or equal to x less than or equal to 0.35, were grown onto Si (001) substrates at temperatures between 100 and 550 degrees C. The total pressure was kept constant at 3.0 mTorr with the N-2 fraction varied from 0 to 1. As-deposited films were studied by Rurherford-backscattering spectroscopy, x-ray photoelectron spectroscopy, electron-energy loss spectroscopy, Raman and Fourier transform infrared spectroscopy, and nanoindentation. Three characteristic film structures could be identified: For temperatures below similar to 150 degrees C, an amorphous phase forms, the properties of which are essentially unaffected by the nitrogen concentration. For temperatures above similar to 200 degrees C, a transition from a graphite-like phase to a "fullerenelike" phase is observed when the nitrogen concentration increases from similar to 5 to similar to 15 at. %. This fullerenelike phase exhibits high hardness values and extreme elasticity, as measured by nanoindentation. A "defected-graphite" model, where nitrogen atoms goes into substitutional graphite sites, is suggested for explaining this structural transformation. When a sufficient number of nitrogen atoms is incorporated, formation of pentagons is promoted, leading to curving of the basal planes. This facilitates crosslinking between the planes and a distortion of the graphitic structure, and a strong three-dimensional covalently bonded network is formed. [S0163-1829(99)03808-4].