Stability, sensitivity and selectivity of tungsten trioxide films for gas sensing applications

The stability, sensitivity and selectivity of RF sputtered gold doped tungsten trioxide (WO/sub 3/) films that have been proposed as the sensing element in Taguchi and surface acoustic wave (SAW) gas microsensors have been examined. The WO/sub 3/ film sensing properties such as electrical conductivity and carrier type, concentration and mobility were measured using both two and four point probe Hall effect techniques. It was found that in order for the WO/sub 3/ film to be stable, it was necessary to transform the as-deposited films from an amorphous to a polycrystalline structure through a heat treatment or ´activation´ process. This activation process also dramatically improved the response time and sensitivity of the films to hydrogen sulfide (H/sub 2/S) gas. The authors show that properly prepared and treated gold doped WO/sub 3/ films are extremely stable, sensitive and selective to H/sub 2/S gas in the p.p.b. range.<>