Stress induced preferred orientation and phase transition for ternary WC xN y thin films

Abstract

We deposit ternary WC x N y thin films on Si (1 0 0) substrates at 500 °C using direct current (DC) reactive magnetron sputtering in a mixture of CH 4/N 2/Ar discharge, and explore the effects of substrate bias ( V b) on the intrinsic stress, preferred orientation and phase transition for the obtained films by virtue of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and selective area electron diffraction (SAED). We find that with increasing the absolute value of V b up to 200 V the carbon ( x) and nitrogen ( y) atom concentrations of WC x N y films keep almost constant with the values of 0.75 and 0.25, respectively. The XPS and SAED results, combined with the density-functional theory (DFT) calculations on the electronic structure of WC 0.75N 0.25, show our obtained WC x N y films are single-phase of carbonitrides. Furthermore, we find that the compressive stress sharply increases with increasing the absolute value of V b, which leads to a pronounced change in the preferred orientation and phase structure for the film, in which a phase transition from cubic β-WC x N y to hexagonal α-WC x N y occurs as V b is in the range of −40 to −120 V. In order to reveal the relationship between the stress and phase transition as well as preferred orientation, the DFT calculations are used to obtain the elastic constants for β-WC x N y and α-WC x N y . The calculated results show that the preferred orientation is dependent on the competition between strain energy and surface energy, and the phase transition can be attributed to a decrease in the strain energy.