Structural properties and nitrogen-loss characteristics in sputtered tungsten nitride films

A combination of X-ray photoelectron spectroscopy (XPS), parallel electron energy-loss spectroscopy (PEELS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and transmission electron diffraction (TED) were used to investigate structural properties and nitrogen-loss characteristics of thin WN_x films prepared by reactive magnetron sputtering of tungsten in an Ar-N₂ gas mixture. XRD θ-2θ scans combined with plan-view and cross-sectional TEM showed that the as-deposited WN_x films were amorphous in structure. Annealing of the 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 in the films. The 150-nm thick crystalline films near a stoichiometry of W₂N had a columnar microstructure with an average column width of 15-20 nm near the film surface, whereas the column grains were larger for substoichiometric films. Thermal stability and nitrogen-loss characteristics of nitride films were also studied by in situ annealing in the TEM and PEELS system. The results indicate that between 600 and 800 °C the W₂N phase was stable. Nitrogen in the film started to evaporate to vacuum at approximately 820 °C and was fully released after 900 °C annealing.