Thickness Dependence of Sensitivity in Thin Film Tin Oxide Gas Sensors Deposited by Vapor Pyrolysis

Transparent SnO2 thin films were deposited on porcelain substrates using a chemical vapor deposition technique based on the hydrolysis of SnCl4 at elevated temperatures. A reduced pressure self-contained evaporation chamber was designed for the process where the pyrolysis of SnCl4 at the presence of water vapor was carried out. Resistive gas sensors were fabricated by providing ohmic contacts on the layers obtained and the installation of a custom made micro-heaterbeneath the substrate. The sensitivity (S = Ra/Rg) of the fabricated sensors to acetone vapor contamination was measured at 270ºC. S increased linearly with contamination level up to 8000PPM, and saturation was observed at higher concentrations. The experimental relationship between S and thickness of the sensitive film was investigated in films obtained by CVD for the first time. It was shown that S was inversely related to the film thickness, and higher S values were recorded for thinner sensors. The upper limit for S was imposed by the conduction instabilities observed in the thinnest films deposited.