Optical emission spectroscopy of low-discharge-power magnetron sputtering plasmas using pure tungsten target

To study the mechanism of a tungsten oxide (WO₃) thin film using an RF magnetron sputtering method, optical emission spectroscopic (OES) measurements for the RF plasma of a pure W target have been performed. We also examined the crystalline structure and atomic composition rate of the prepared thin film using X-ray photoelectron spectroscopy (XPS). Experimental results indicate that Ar I emission peak intensity slightly increased with increasing Ar gas mixture. On the other hand, W I emission peak intensity rapidly increased with increasing Ar gas mixture. The increase rates of these two emission peak intensities are different, which may be due to the difference in emission mechanism. O I emission peak intensity decreased with increasing Ar gas mixture, indicating that O I emission intensity increased with increasing O₂ gas mixture. Electron density and deposition rate increased with increasing Ar gas mixture, and their dependences on Ar gas mixture were very similar to that of Ar I emission. XPS analyses indicate that the oxidation ratio of the prepared film was slightly decreased with decreasing Ar gas mixture. These results suggest that the W sources of the WO₃ film on the substrate are W atoms and WO_x molecules sputtered from the W target. The plasma phase reaction between W and O atoms and the intermediate-energy O atomic and/or O₂ molecular reaction on the surface of the substrate are considered to be important for WO₃ film production in low-energy magnetron sputtering deposition.