Effect of oxygen on residual stress and structural properties of tungsten nitride films grown by reactive magnetron sputtering

Thin films of W-O-N were produced by reactive d.c. magnetron sputtering of tungsten in an Ar-N₂-O₂ gas mixture. The effects of oxygen incorporation on the residual stress and structural properties of these films as well as the influence of post-deposition annealing have been studied. The films were analyzed in situ by a cantilever beam technique, and ex situ by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electron energy-loss spectroscopy (EELS). It was found that the stoichiometric W₂N films deposited under oxygen-free conditions had a high compressive stress of 1.45 GPa. The compressive stress in W-O-N films decreased significantly with an increase in the oxygen concentration and became slightly tensile for films near 10-15 at.% oxygen. These results can be ascribed to the decrease in the lattice parameter caused by incorporating small oxygen atoms in the lattice sites and the development of an amorphous network in the W-O-N films as the incorporation of oxygen was increased. By high temperature annealing the structural conversion from W₂N to W in oxygen-free films was observed using XRD and the micro structure evolution after conversion was demonstrated using cross-sectional TEM. The effect of oxygen in stabilizing the W₂N structure was also elucidated and explained on the basis of structural and thermodynamic stability.