INFLUENCE OF THE DEPOSITION CONDITIONS OF SNO2 THIN-FILMS BY REACTIVESPUTTERING 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 conduc tivity measurements for different pollutant gas mixtures. The electric al measurements were performed in a temperature range between 100 and 350 degrees C with NO2 concentrations varying from 50 to 800 ppb and a lso 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 o f oxygen introduced in the sputtering plasma to obtain the best respon se 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 para meters on the stoichiometry and morphology of the SnO2 thin films. It was observed that thin films deposited at room temperature and with th e sputtering plasma containing 8% of O-2 showed the best sensitivity v alues, and therefore, the optimum number of adsorption sites needed fo r a good gas adsorption. These sensors had a SnO, grain size of simila r to 5 nm, all of them were substoichiometric before the thermal treat ment and the film growth was columnar. A selective response to NO2 and CO/air mixtures can be achieved by suitable choice of the sensor oper ating temperature. The results obtained with a relative humidity (simi lar to 1-80%) on the sensitivity to low NO2 concentrations need furthe r investigation due to the large interference effect of water vapor sh own at all operating temperatures. (C) 1997 Elsevier Science S.A. All rights reserved.