Phase transformation and morphological evolution of ion-beam sputtered tin oxide films on silicon substrate

Amorphous tin oxide films were deposited on a silicon substrate by ion-beam sputtering (IBS) using a SnO₂ target. Phase transformation and morphological changes of deposited films at different annealing temperatures were studied by X-ray diffraction and scanning electron microscopy. Crystallization of the as-deposited film started at 350°C and SnO and SnO₂ phases formed at 400°C. Disproportionation of SnO into Sn and SnO₂ was observed at 450°C followed by the oxidation of metallic tin at 550°C. Large volume changes accompanying the oxidation of metallic tin at this temperature caused the partial detachment and formation of heavy wrinkles on the film. These results suggest that the oxygen deficiency of tin oxide films should be avoided by optimizing the deposition process, since a drastic morphological change at the phase transformation to SnO₂ during annealing may destroy the integrity of the thin films and degrade the long-term stability of tin oxide films used as gas sensors at high temperatures.