Sensing characteristics of WOx films deposited by r.f. sputtering for NOx gas sensor

Abstract

WOx thin films for use in detecting NOx gas in automobile exhaust were deposited by r.f. sputtering onto polycrystalline Al2O3 substrates (polished and unpolished). The as-grown films had various thicknesses, deposition temperatures, and annealing temperatures. The relative oxygen to tungsten atomic ratio N-O/N-W of the deposited films was approximately calculated by correcting the oxygen to tungsten XPS peak area intensity ratio I-O/I-W with the observed peak-area ratio of the O-1s to W-4f core-level peaks in the XPS spectrum from a standard WO3 powder of known composition. The as-grown films showed higher voltage drops or sensitivities (R-gas/R-air) at the measuring temperature of 200 degrees C due to dominant desorption of NOx gas at the film surface. Annealing at 600 degrees C for 4 hrs gave the as-grown films their highest sensitivity. The as-grown films anneaed at 600 degrees C on the unpolished Al2O3 showed sensitivities which differed with the deposition temperature. These films showed better sensing properties after annealing at 600 degrees C due to improved crystallinity. As-grown films with different film thicknesses (150, 300, and 450 nm) on the polished Al2O3 showed almost the same sensitivities. Even though the as-grown WOx thin films had high sensitivity, the sensing characteristics could be dramatically disturbed by humidity. These results suggest that WO3 thin films will be very useful for sensing NOx gas if the disturbinge effect of the humidity on the sensing characteristics can be eliminated.