Effect of the thickness of reactively sputtered WO3 submicron thin films used for NO2 detection

Tungsten trioxide (WO3) thin films were prepared by magnetron sputtering onto silicon micro-machined substrates. The dependence of the thin film thickness (varied from 37.5 to 400 nm) on NO2 gas sensing properties was investigated. The structure, grain size and chemical composition of the deposited WO3 films have been studied by X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS), respectively. Our results show that the films are formed by grains crystallized in the WO3 monoclinic phase with preferential orientations along the c axis. The grain size increased with the film thickness. The gas sensing characteristics of the sensors such as sensitivity (normalized response), response time, recovery time and aging were investigated for NO2 detection and related to the film thickness. For example, a maximum response of 200 was obtained for 37.5 nm thick films operated at 350 °C.