Influence of negative bias voltage on structural and mechanical properties of nanocrystalline TiNx thin films treated in hot cathode arc discharge plasma system

Ti thin films were grown by DC sputtering on a glass substrate and then nitrided in a hot cathode arc discharge plasma system, which is an effective approach to independently monitoring the plasma and nitriding parameters. The hardness of pristine Ti thin film is found to be ~3.06 GPa, which increases upto ~16.08 GPa with an increase in negative bias voltage to −140 V and then decreases to ~15.05 GPa for higher of −240 V bias voltage. Similar kind of variation has been observed in crystallite size and surface roughness. Crystallite size is found to increase from 11.1 nm (pristine Ti) to 14.8 nm (for −140 V) and then reduces to 11.9 nm for -240 V. Surface roughness increases from 2.78 nm (pristine) to 6.84 nm (for -140 V), which is found to be 4.14 nm for -240 V. Optical and electrical measurements also reveal the strong impact of negative bias voltage on the bandgap and resistivity of the films. Above results are understood on the basis of diffusion of nitrogen ions for lower voltages and saturation of nitrogen ions in the host lattice for high voltages.