Morphological, structural and mechanical properties of NbN thin films deposited by reactive magnetron sputtering

Hexagonal β-Nb2N, cubic δ-NbN and hexagonal δ′-NbN films were deposited by reactive magnetron sputtering from a Nb metal target in Ar + N2 atmosphere at various nitrogen partial pressures. Structural and mechanical properties of these films were investigated by X-ray diffraction, Electron Probe Microanalysis, Transmission Electron Microscopy, Rutherford Backscattering Spectroscopy, residual stress and nanoindentation measurements. TEM studies reveal that the films have a columnar morphology. The hardness of the films depends on the film phase: the hardness reaches 35 GPa and 40 GPa for hexagonal β and δ′ phases, respectively, and 25 GPa for the cubic δ phase. The δ′ phase films show a compressive residual stress of about 3.5 GPa, whereas no residual stress can be measured in δ and β phase films. The relative density (compared with the bulk value) of films depends on the film phase: 0.8 for δ, 0.86 for δ′ and 0.96 for β. It is remarkable that the hardness of the hexagonal δ′ phase films is significantly higher than the hardness of the β phase films, in contrast with generally accepted morphological trends.