Growth, stress and hardness of reactively sputtered tungsten nitride thin films

Abstract

Tungsten nitride (WNx) thin films were deposited on Si(100) substrates using direct current reactive magnetron sputtering in discharging a mixture of N2 and Ar gas. The effects of nitrogen flow rate (FN2) and substrate bias voltage (Vb) on the composition, phase structure, and mechanical properties for the obtained films were evaluated by means of X-ray photoelectron spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy and nanoindentation. The evolution of phase structure is found closely correlated to N concentration in the films. When Vb=−40V, with increasing FN2, the N/W atomic ratio gradually increases in the film, accompanied by a phase transition from cubic β-W to hexagonal WN through face centered-cubic (fcc)-W2N. At FN2=15sccm, the N/W atomic ratio gradually decreases with increasing the absolute value of Vb, resulting in a transition from fcc-W2N to cubic β-W(N) through a mixture of fcc-W2N+β-W(N). In addition, the increase in implanted nitrogen causes the increase in the compressive stress with increasing FN2. In contrast, although with increasing the absolute value of Vb from 80 to 160V the N/W atomic ratio decreases, the increase of the defects caused by increasing ion bombarding energy, dominates the increase of the compressive stress. Furthermore, the maximum hardness value for the films arrives at 38.9GPa, which is obtained at Vb=−120V when fcc-W2N+β-W(N) mixed structure is formed.