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 WNx films prepared by reactive magnetron sputtering of tungsten in an Ar-N-2 gas mixture. XRD theta-2 theta scans combined with plan-view and cross-sectional TEM showed that the as-deposited WNx films were amorphous in structure. Annealing of the as-deposited films at 600 degrees 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 nn 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 degrees C the W2N phase was stable. Nitrogen in the film started to evaporate to vacuum at approximately 820 degrees C and was fully released after 900 degrees C annealing. (C) 2000 Elsevier Science S.A. All rights reserved.