Influence of the deposition conditions of SnO2 thin films by reactive sputtering on the sensitivity to urban pollutants

Thin films of SnO2 were deposited by r.f. magnetron sputtering from a SnO2 target varying the oxygen concentration from 0 to 20% within the sputtering plasma. The samples were characterized by electrical conductivity measurements for different pollutant gas mixtures. The electrical measurements were performed in a temperature range between 100 and 350 degrees C with NO2 concentrations varying from 50 to 800 ppb and also with interference gases such as CO (10 ppm) and H2O (similar to 1-80%). The aim of the work was to determine the optimum concentration of oxygen introduced in the sputtering plasma to obtain the best response to these gases on using tin oxide as gas sensor. Techniques such as XPS and GAXRD were used to study the influence of the deposition parameters on the stoichiometry and morphology of the SnO2 thin films. It was observed that thin films deposited at room temperature and with the sputtering plasma containing 8% of O-2 showed the best sensitivity values, and therefore, the optimum number of adsorption sites needed for a good gas adsorption. These sensors had a SnO, grain size of similar to 5 nm, all of them were substoichiometric before the thermal treatment and the film growth was columnar. A selective response to NO2 and CO/air mixtures can be achieved by suitable choice of the sensor operating temperature. The results obtained with a relative humidity (similar to 1-80%) on the sensitivity to low NO2 concentrations need further investigation due to the large interference effect of water vapor shown at all operating temperatures. (C) 1997 Elsevier Science S.A. All rights reserved.