Role of Nitrogen in the Formation of Hard and Elastic CNx Thin Films by Reactive Magnetron Sputtering

Carbon nitride films, deposited by reactive dc magnetron sputtering in Ar/N2 discharges, were studied with respect to composition, structure, and mechanical properties. CNx films, with 0\u3c~x\u3c~0.35, were grown onto Si (001) substrates at temperatures between 100 and 550°C. The total pressure was kept constant at 3.0 mTorr with the N2 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 ∼150°C, an amorphous phase forms, the properties of which are essentially unaffected by the nitrogen concentration. For temperatures above ∼200°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