Reactively sputtered WO_xN_y films

A combined investigation of stress relaxation in WO_xN_y thin films sputter deposited on silicon wafers in an Ar-N₂-O₂ gas mixture by in situ substrate curvature measurements and of structural properties by ex situ x-ray diffraction, x-ray photoelectron spectroscopy, transmission electron microscopy (TEM), electron energy loss spectroscopy, and transmission electron diffraction is reported. It was found that the W₂N films deposited under oxygen-free conditions had a high compressive stress of 1.45 GPa. As the oxygen concentration was increased, the stress became smaller and reached almost zero for films near 10-15 at.% oxygen. These results can be understood in terms of the decrease in the lattice parameter caused by substituting nitrogen atoms with oxygen in the lattice sites and the development of an amorphous network in the WO_xN_y films as the incorporation of oxygen was increased. Plan view and cross-sectional TEM analyses showed that 150-nm-thick oxygen-free crystalline W₂N films had a columnar microstructure with an average column width of 15-20 nm near the film surface, whereas oxygen imbedded in the films provided a finer grain structure. The effect of oxygen in stabilizing the W₂N structure was also elucidated and explained on the basis of structural and thermodynamic stability.