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

Abstract

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–N 2 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 W 2N and b.c.c. W or a single-phase structure of W 2N, 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 W 2N was ∼15 nm, whereas the column grains were larger with decreasing C N. XPS results revealed that W 2N had an ionic bonding character, W δ+–N δ−. It was also found that once the W 2N phase was formed, the density, microstructure and bonding feature were similar and insensitive to the total sputtering-gas pressure used in this study.