Structural and mechanical properties of sputtered cubic and hexagonal NbNx thin films

Niobium nitride NbNx films with 0.61⩽x⩽1.06 were prepared by reactive magnetron sputtering of a Nb metal target in an Ar/N2 atmosphere at a total pressure of 0.4 Pa. Different nitrogen partial pressures PN2 and substrate temperatures Ts were used. The films were characterized by X-ray diffraction, electron probe microanalysis, scanning electron microscopy and nano-indentation measurements. The films deposited without substrate heating contain a pure cubic δ-NbN phase for 0.61⩽x<0.98. While, for 0.98⩽x⩽1.06, they include a mixture of hexagonal NbN and cubic δ-NbN phases. The hardness variation correlates with changes in grain size and film morphology. Compact poorly crystallized films exhibit high nanohardness values of approximately 32 GPa. For films deposited at various PN2 with heating the substrate to 400 °C, the crystalline structure was mixed (δ′-NbN+δ-NbN) phases. The concentration of the hexagonal δ′-NbN phase in the film changes with x. The hardness and the Young modulus follow the variation of the concentration of the hexagonal δ′-NbN phase. The films with high concentration of the hexagonal δ′ phase exhibit high hardness and Young modulus. It appears that the hardness of the hexagonal δ′-NbN phase is 40 GPa, whereas the hardness of the cubic δ-NbN phase is 25 GPa.