ISSN 1063-7850, Technical Physics Letters, 2006, Vol. 32, No. 8, pp. 725–728. © Pleiades Publishing, Inc., 2006. Original Russian Text © V.V. Simakov, O.V. Yakusheva, A.S. Voroshilov, A.I. Grebennikov, V.V. Kisin, 2006, published in Pis’ma v Zhurnal Tekhnicheskoœ Fiziki, 2006, Vol. 32, No. 16, pp. 75–83. 725 The effect of adsorbed gases on the resistance of thin films of metal oxides is widely used in gas sen- sors [1, 2] and the systems intended to identify odors and gas mixture compositions [3, 4]. The response sig- nal usually represents the ratio of a change in the film sensor conductivity upon sampling to the value of con- ductivity in pure air [5]. However, this ratio depends both on the amount of each impurity in air and on the type of this impurity, the two contributions being diffi- cult to separate. This Letter presents the results of investigations of the kinetics of relaxation of the conductivity of a thin film of tin dioxide (SnO 2 ) in response to a stepwise change in the gas phase composition. Based on these data, a new parameter is introduced, which is indepen- dent of the impurity concentration in the gas phase. Experimental. The experiments with stepwise gas sampling were performed using thin SnO 2 films on polycrystalline sapphire (Polycor) substrates. The films were deposited by RF magnetron sputtering of a sto- ichiometric SnO 2 target in an Ar–O 2 atmosphere [6]. The sensor structures were provided with electric con- tacts, representing 700-μm-long platinum (Pt) stripes spaced by 50 μm. In addition, thin-film Pt heaters and Pt thermoresistors were formed on the rear and front sides of the substrate, respectively. The oxide film thickness was ~1 μm, as measured by an SE-400 Model 15/42 ellipsometer (Sentech Instruments GmbH, Germany). The heaters and ther- moresistors were calibrated with the aid of an IR cam- era of the TH 3100MR type (NEC Instruments Ltd., Japan) and allowed the film temperature to be main- tained at 300°C over the entire area with a lateral inho- mogeneity below 2%. The conductivity was measured using an electronic- nose system of the KAMINA type (Forschungszentrum Karlsruhe GmbH, Germany). Controlled atmospheres were prepared from certified gas mixtures (Messer Griesheim GmbH, Germany) in a special gas inlet sys- tem, which was capable of controlling the content of each component to within 0.1 ppm. The gas phase humidity was monitored by a Testo Model 915 hygrom- eter (Testo AG, Germany) and controlled at a 50% level in all experiments. The kinetics of conductivity variation in gas-sensi- tive SnO 2 based thin-film structures was studied using one of the sample gas impurities representing carbon monoxide (CO), ammonia (NH 3 ), and propane (C 3 H 8 ). Gas mixtures with a preset impurity concentration C were prepared by mixing synthetic air (19.5% O 2 in N 2 ) with standard gas mixtures containing rated amounts of CO, NH 3 , or C 3 H 8 . The measurements were performed at a gas flow rate of 500 cm 3 /min in the working cham- ber with a volume of 4 cm 3 . Accordingly, the time of gas phase renewal in the chamber did not exceed 1 s. The impurity content in the gas mixture admitted to the working chamber was sequentially increased. The exposure time for each sample mixture was 1 h, after which the gas inlet system and the working chamber were purged for 1 h with a flow of synthetic air at a rate of 500 cm 3 /min. Results and discussion. The conductivity of gas- sensitive SnO 2 based thin-film structures exhibited long-term increase in all gas mixtures. The time of Variation of the Conductivity of a Thin Film of Tin Dioxide in Response to Stepwise Gas Sampling V. V. Simakov, O. V. Yakusheva, A. S. Voroshilov, A. I. Grebennikov, and V. V. Kisin* Saratov State Technical University, Saratov, Russia “Sintez” Company, Saratov, Russia * e-mail: kisin@sstu.ru Received March 2, 2006 Abstract—Variation of the conductivity of a thin film of tin dioxide (SnO 2 ) in response to a stepwise change in the content of various impurities present in the gas phase has been studied in a broad range of concentrations. The experimental data are interpreted within the framework of the Roginskii–Elovich model of adsorption kinetics. A gas-pressure-independent parameter characterizing the sensor conductivity variation is established. Using the phenomenon under consideration, it is possible to identify adsorbed gas species. PACS numbers: 68.60.–p DOI: 10.1134/S1063785006080256