Role of nitrogen in the formation of hard and elasticCNxthin films by reactive magnetron sputtering

Carbon nitride films, deposited by reactive dc magnetron sputtering in ${\mathrm{A}\mathrm{r}/\mathrm{N}}_2$ discharges, were studied with respect to composition, structure, and mechanical properties. ${\mathrm{CN}}_x$ films, with $0<~x<~0.35,$ were grown onto Si (001) substrates at temperatures between 100 and $550°\mathrm{C}.$ The total pressure was kept constant at 3.0 mTorr with the ${\mathrm{N}}_2$ fraction varied from 0 to 1. As-deposited films were studied by Rutherford-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 $\sim 150°\mathrm{C},$ an amorphous phase forms, the properties of which are essentially unaffected by the nitrogen concentration. For temperatures above $\sim 200°\mathrm{C},$ a transition from a graphitelike phase to a “fullerenelike” phase is observed when the nitrogen concentration increases from ∼5 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 cross-linking between the planes and a distortion of the graphitic structure, and a strong three-dimensional covalently bonded network is formed.