MICROSTRUCTURAL EFFECTS IN WO3 GAS-SENSING FILMS

The microstructure of 50 nm thick WO3 films used in surface acoustic w ave (SAW) gas sensors was characterized by atomic force microscopy (AF M), X-ray diffraction (XRD) and conductivity measurements. The films w ere deposited by r.f magnetron sputtering onto piezoelectric yz-cut Li NbO3 substrates at 200 degrees C and then subjected to annealing treat ments in air. As-deposited films are amorphous, have an r.m.s. roughne ss of 0.6 nm and are easily eroded with the AFM tip. Annealing above 3 15 degrees C causes the films to crystallize into a distorted rhenium oxide structure. This crystallization is accompanied by a decrease in film conductivity and a surface roughening due to polycrystalline grai n growth. The crystalline films are more sensitive for H2S gas detecti on and have faster response characteristics than the amorphous films.