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Cost-effective gas sensors based on tungsten and tungsten trioxide thin films for H2S detection.
Keywords: tungsten oxide, sensors, hydrogen sulfide
Semiconductor gas sensors are generally cost-effective sensors and dedicated to the detection of hazardous gases such as CO, H2S, NO2 [1]. A semiconductor gas sensor is composed of two distinct elements: the first is the sensing element, generally based on oxide, and the second one is the heating element, based on metallic wires or films which can operate at high temperature. The heater is used to heat the sensing element because the sensing process shows a peak of sensitivity at temperature usually ranging from 200 to 500 °C. The mechanism of gas detection by semiconductor oxides is based on the resistance variation resulting from reactions between the sensing film and the ambient gas molecules [2]. In order to improve the adsorption/desorption process of the gas molecules at the sensing film surface, the operating temperature of the sensor must be optimized. Platinum is widely used for the heating element of industrial semiconductor gas sensors due to its good thermal properties, but its cost is particularly high. In a previous study [3], we have given the evidence that tungsten can be used to realize integrated heaters dedicated to cost- effective semiconductor gas sensors in a limited operating temperature range generally below 500 °C. Transition metal oxides such as SnO2, ZnO, TiO2 and WO3 are commonly used in the fabrication of solid-state devices like gas sensors. Tungsten trioxide is a promising material for gas sensing application. In fact, WO3 thin films have been found to be very sensitive to various gases, such as H2S and NOx [4]. The microstructure, electrical properties and stoichiometry of the WO3 active layer have a strong influence on the sensor characteristics and are influenced by the deposition conditions and the annealing temperature. The resistivity of WO3 is determined by its stoichiometric defects. And therefore, the presence of oxygen vacancies in the material is of great interest for the gas sensing process. Several methods have been used for the elaboration and deposition of tungsten oxide films: thermal evaporation, sol- gel, pulsed electrodeposition, radio- frequency sputtering, among others. RF sputter deposition is a widely used technique for elaborating WO3 thin films. Reactive sputtering from a metallic target gives thin films with a good oxygen stoichiometry. However, oxide films deposited by non-reactive sputtering at low argon pressure from a metal-oxide target are generally oxygen deficient [5]. This phenomenon could be related to the tungsten mass. In fact, tungsten is ten times heavier than oxygen. So, tungsten is therefore able to travel through the background gas more easily during the deposition process. Tungsten trioxide adopts five known distinct crystallographic modifications between 0 K and 1200 K: monoclinic- triclinic-monoclinic-orthorhombic- tetragonal [6]. These crystalline structures are strongly affected by impurities, substrate material and annealing temperature. In most of cases, WO3 thin films deposited by RF sputtering and annealed at temperature ranging from 350 °C to 600 °C present a monoclinic crystalline structure. In the present study, we have investigated the crystallographic structure and the electrical properties of WO3 thin films. These films were deposited by RF non-reactive sputtering on silicon substrates and annealed at two different temperatures, here 400 °C and 500 °C. Tungsten was used as metal for the heating element and the sensor electrodes. All metallic elements were made by using optical lithography and DC sputtering techniques. The WO3 sensing element and the W heater were embedded in commercial cases for the fabrication of complete sensors. And finally, the electrical response and sensitivity of our cost-effective gas sensors were measured under various concentrations of H2S at SIMTRONICS SAS. The short response time of about 20 s, the quite complete recovery after a long exposure to H2S and the very good sensitivity prove that our cost-effective sensors can be applied in industrial conditions. Moreover, the sensors are only composed of low cost materials, tungsten and tungsten trioxide, and have been realized by using an industrial technique, i.e. sputtering. This means that this kind of semiconductor gas sensor can be really cost-effective. Finally, all these characteristics of the WO3 sensors, studied here, show a real possibility to use them in industrial sites such as oil platforms or oil drilling sites for the detection of H2S.
UDSMM, Université Littoral Côte d'Opale

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