Reactive direct current magnetron sputtering of tungsten oxide: A correlation between film properties and deposition pressure

Gasochromic WO x-films with a Pt catalyst cap layer can serve as optically switchable coatings for a variety of applications. In this study the influence of the total pressure during deposition on film growth rate, properties and switching kinetics has been analyzed by X-ray reflectometry and optical spectroscopy. The dependency of density and deposition rate upon total pressure shows the same non-monotone behavior. While the decrease of the rate at higher pressures can be simulated via the Keller-Simmons model, the deviation observed at low pressures is indicative for backsputtering. A similar behavior is observed for the density, which is further evidence for backsputtering attributed to oxygen ion bombardment. To verify that not only surface atoms but also subsurface atoms are sputtered back, TRIM (Transport of Ions in Matter) calculations for high energetic oxygen ion bombardment on a WO3-film have been performed which confirm the experimental results. Finally switching kinetics have been shown to increase with decreasing film density. This is important since films with lower density can be prepared at low deposition pressure demonstrating a possibility to deposit WO x-films at better controllable deposition pressures