Effect of Post-Deposition Annealing on the Optical and Mechanical Properties of Amorphous Tungsten Oxynitride Thin Films

The effect of post-deposition thermal treatment on the optical and mechanical properties of WON thin films is reported. WON thin films were deposited onto silicon (100) and quartz substrates utilizing direct current (DC) sputtering under the variable flow of reactive gas mixture. Post-deposition annealing was performed at a temperature (Ta) of 400°C under inert (argon) and oxygen (air) atmosphere. For annealing in an inert atmosphere, spectroscopic ellipsometry (SE) revealed changes to the dispersion profiles and magnitudes of the index of refraction (n) and extinction coefficient (k) as a result of thermal treatment. X-ray photoelectron spectroscopy (XPS) measurements revealed the loss of both nitrogen and oxygen during the annealing process in an inert atmosphere while the samples remain amorphous as evidenced in X-ray diffraction measurements. The chemical changes correlate with the changes seen in the optical constants for annealed WON films. Furthermore, the physical changes during post thermal treatment probed by calculating the film-density using the Lorentz-Lorenz relation indicate a decrease in density that supports the changes in chemical composition and optical properties. Density values for annealed WON films vary in the range ~9–12g/cm3, the low-to-high end of density values are noted for low and high values of nitrogen flow rate, respectively, employed during deposition. Mechanical properties of WON films were demonstrated a drastic improvement as a result of post thermal treatment. The hardness values generally were improved but the WON films annealed in air showed the significant improvement, up to ~10GPa. XRD measurements indicated the amorphous-to-crystalline transition with m-WO3 phase as dominant component for samples annealed in an air due to the abundance of oxygen absorbed during annealing.