Sensors and Actuators B 124 (2007) 24–29 WO 3 sensor response according to operating temperature: Experiment and modeling M. Bendahan ∗ , J. Gu´ erin, R. Boulmani, K. Aguir Laboratoire Mat´ eriaux et Micro´ electronique de Provence, L2MP-CNRS, Universit´ e Paul C´ ezanne, Aix-Marseille III, Facult´ e des Sciences et Techniques de St J´ erˆ ome, France Received 10 July 2006; received in revised form 22 November 2006; accepted 22 November 2006 Available online 20 December 2006 Abstract WO 3 -based sensors are realized in the aim to detect ozone. The thin film of WO 3 is sputtered on a SiO 2 /Si substrate with Pt micro-electrodes. In a previous work, the sensor response dependence on processing parameters has been studied. Now operating temperature of the sensor is investigated and a theoretical model developed by our team confirms experimental measurements. The interaction between the gas and the surface was modeled by Langmuir isotherm and the electrical resistivity was evaluated by solving the transport equations. © 2006 Elsevier B.V. All rights reserved. Keywords: WO 3 ; Gas sensor; Modeling 1. Introduction Electrical properties of semiconductor oxides depend on the composition of the surrounding gas atmosphere. The surface conductivity of the sensor is modified by adsorption of gas species and related space charge effects. In oxidizing atmo- sphere, the oxide surface is covered by negatively charged oxygen adsorbates and the adjacent space charge region is electron-depleted: the oxide layer presents therefore a high resis- tance. Under reducing conditions, the oxygen adsorbates are removed by reaction with the reducing gas species and the elec- trons are re-injected into the space charge layers: as a result, the oxide layer resistance decreases. Recently, gas sensing properties of simple binary metal oxides, such as tin oxide (SnO 2 ) and tungsten trioxide (WO 3 ) [1] have been tested for monitoring pollutant components of atmo- sphere for improving quality of life and enhancing industrial processes [2–4]. Tungsten oxide is an n-type metal oxide semi- conductor with oxygen vacancies, which act as donors. Because the electron density depends on the density of oxygen vacancies, ∗ Corresponding author. Tel.: +33 4 91288973; fax: +33 4 91288970. E-mail address: marc.bendahan@L2MP.fr (M. Bendahan). the vacancies play a significant role in the detection mechanism as in SnO 2 sensors [5]. Many techniques are being used for the fabrication of WO 3 films, including thermal evaporation [6,7], sol–gel [7] and sput- tering [8–10]. Table 1 summarizes the responses (S=R gas /R air ) of various WO 3 -based ozone sensors and fabrication methods. For exam- ple, Qu and Wlodarski [6] studied WO 3 ozone sensors deposited on sapphire substrates by thermal evaporation. The working tem- perature of the sensors was 573 K and the film thickness was about 150 nm. Cantalini et al. [7] reported a study of WO 3 ozone sensors realized on alumina substrates, by sol–gel, sputtering and thermal evaporation techniques. The operating temperature range was 473–673 K. The best ozone sensitivity is obtained with the WO 3 sensors prepared using reactive magnetron sputtering, with an operated temperature of 523 K [8], a film thickness of about 40 nm and a grain size of 40 nm. These results show clearly that the preparation method influences the sensor response. It is now well known that the sensor response depends on physical and chemical properties of sensitive films. In fact, morphology, thickness, chemical composition, and microstructure of WO 3 thin films are very important parameters to obtained stable and sensitive sensors. We have shown that sensor response depends essentially on the grain size and film porosity [9]. These proper- ties can be controlled during film deposition, using rf sputtering 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.11.036