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

Abstract Tungsten nitride (WN x ) thin films were deposited on Si(100) substrates using direct current reactive magnetron sputtering in discharging a mixture of N 2 and Ar gas. The effects of nitrogen flow rate (F N2 ) and substrate bias voltage (V b ) 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 V b  = −40 V, with increasing F N2 , 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)-W 2 N. At F N2  = 15 sccm, the N/W atomic ratio gradually decreases with increasing the absolute value of V b , resulting in a transition from fcc-W 2 N to cubic β-W(N) through a mixture of fcc-W 2 N + β-W(N). In addition, the increase in implanted nitrogen causes the increase in the compressive stress with increasing F N2 . In contrast, although with increasing the absolute value of V b from 80 to 160 V 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 38.9 GPa, which is obtained at V b  = −120 V when fcc-W 2 N + β-W(N) mixed structure is formed.