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 (WNx) 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 (CN) 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 CN. XPS results revealed that W2N had an ionic bonding character, Wδ+–Nδ−. 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.