The stability of tin and indium oxide thin film gas sensors

Tin and indium oxide reactively sputtered thin films, surface doped with palladium and platinum, are evaluated as sensors for carbon monoxide, methane, propane, n-butane and ethanol. The sensors are subjected to continuous heating to 450 degrees C - cooling to 150 degrees C cycles, in order to determine their degradation with continuous use. A simple nonparametric technique is used in order to determine the stability, interdependence and selectivity of the sensors. Useful results are obtained concerning the stability of sensors with different sensing layers and different catalysts deposited onto their surfaces. Sensors with different thicknesses of the same additive are found to be strongly correlated (i.e., they exhibit similar responses to the same test gases at the same temperatures; this is often referred to as collinearity in the sense that the response vector of one sensor can be derived from the response vector of another sensor by simply multiplying with a constant), while those with different additives are found to be uncorrelated. Finally, platinum-doped sensors were found to be selective, i.e., to exhibit different responses in the presence of different gases, while undoped or palladium-doped ones were found to be nonselective. Such results derived from simple techniques can be used to evaluate sensors incorporated in a multisensor pattern recognition array.