Effect of deposition conditions on internal stresses and microstructure of reactively sputtered tungsten nitride films

A combined investigation of internal stress generation by in situ substrate curvature measurements during the growth of tungsten nitride (WN_x) thin films and of structural properties by ex situ X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electron energy-loss spectroscopy (EELS) is reported. It was found that the properties of the deposited films not only depended on the nitrogen partial pressure in Ar-N₂ gas mixtures but also on the total sputtering-gas pressure. The stress of the films was strongly related to their microstructure, which depended mainly on the incorporation of nitrogen in the films. Annealing of as-deposited films at 600°C or above resulted in crystallization of the amorphous phases, forming either a two-phase structure consisting of W₂N and b.c.c. W or a single-phase structure of W₂N, which was related to the initial nitrogen concentration (C_N) in the films. Cross-sectional TEM studies showed that an average column width for 150-nm-thick films near stoichiometry of W₂N was ~15 nm, whereas the column grains were larger with decreasing C_N. XPS results revealed that W₂N had an ionic bonding character, W^δ+-N^δ-. It was also found that once the W₂N phase was formed, the density, microstructure and bonding feature were similar and insensitive to the total sputtering-gas pressure used in this study.