Sensing characteristics of dc reactive sputtered WO 3 thin films as an NO x gas sensor

Abstract

In order to apply WO 3 thin films to the NO x gas sensor, WO 3 thin films (3000 Å) were fabricated by using dc reactive sputtering method on alumina substrate and assembled as a unit of an NO x gas sensor by adopting a patterned heater on the back side of substrate. The deposition temperatures of WO 3 thin film were changed from 200°C to 500°C, and then post-annealed for the crystallization for 4 h at 600°C. There were no WO 3 phases at the substrate temperature of 200°C, but the crystalline phases of WO 3 thin film were appeared with the increase of substrate temperature from 200°C to 500°C. The post-annealing of as-deposited WO 3 thin films at 600°C resulted in the enhancements of crystallinity, but it was observed that the quality of the final phases severely depends on the initial formation of phase during deposition. From the SEM images, crack free morphologies were found, which was different from the room temperature growth films. The sensitivity ( R gas/ R air) of as-deposited thin films was ranged from 4 to 10 for the 5 ppm NO test gas at the measuring temperature of 200°C. However, after post-annealing process at the temperature of 600°C, the sensitivities were increased around the values of 70–180 at the same test condition. These results show the WO 3 thin films need to be processed at least at the temperature of 600°C for the well-improved sensitivity against NO x gas. It was also observed that the recovery rate of a sensing signal after measuring sensitivity was faster in the in-situ sputtered films than in the evaporated films or room temperature sputtered films.