Thin-film gas sensors operating in a perpendicular current mode

Thin-film gas sensors tend to deteriorate with an increase in micro-cracks. These micro-cracks are usually generated perpendicular to the surface of the sensing film by the shrinkage associated with the sintering of the film during the high-temperature annealing process. Some cracks are also created by the contraction and expansion caused by the difference of the thermal expansion coefficients between the sensing film and the substrate during periodic heat cycle. As one of the schemes to avoid the adverse effects of cracks, thin-film gas sensors operating in a perpendicular current mode are presented in this paper. To achieve this mode, a porous top electrode is deposited on a sensing film instead of an electrode that is normally deposited on a substrate. The sensor current flows perpendicularly to the surface of the thin film and thus is not blocked by the micro-cracks. The resistance change in time of the sensor is stable compared to a traditional sensor operating in a parallel current mode. The experimental results show that the sensor is sensitive and selective to hydrogen with relatively fast response and recovery times when a WO3-based thin-film and a porous Pt film are used for the sensing film and the top electrode, respectively. In particular, the porous Pt top electrode is effective in enhancing the response and selectivity to hydrogen by acting as a catalyst.