776 Journal of The Electrochemical Society, 147 (2) 776-779 (2000) S0013-4651(99)07-105-0 CCC: $7.00 © The Electrochemical Society, Inc. Most of the chemical sensors described in the literature are based on inorganic semiconducting oxides. In some cases, the main oxide is modified by doping with small amounts of additives such as other oxides and/or metals. The sensing principle is based on the change in the conductance undergone by the oxide film when gases are adsorbed and react on its surface. A survey of typical sensor materi- als for detecting different gases in characteristic temperature ranges can be found elsewhere. 1,2 In recent years, some gas-sensitivity stud- ies with tungsten trioxide (WO 3 ) based semiconductors have been reported. Pure or doped tungsten oxide is a promising material for the detection of nitrogen oxides (NO and NO 2 ) 3-6 and sulfur diox- ide, 7 two substances which are considered to be responsible for ambient degradation together with carbon oxides and hydrocarbons. The gas-sensing properties of WO 3 are highly dependent on the deposition method. Tungsten oxide is generally deposited by reac- tive rf sputtering. 8-10 However, other techniques such as thermal evaporation, 11,12 sol-gel methods, 13 screen printing, 14 or photo- chemical production 6 have been reported to grow thin or thick active films of WO 3 . Recent studies have shown that simple and mixed tungsten oxides are very sensitive to ammonia vapors. 15,16 Ammonia emissions such as those produced by agricultural activities (e.g., livestock buildings) are a major environmental problem, particularly in the neighborhood of urban settlements. To measure the ammonia emissions and the quality of indoor air from individual livestock buildings, cheap and reliable ammonia sensors are required. This paper examines the gas sensing properties of resistive tung- sten oxide. The films were grown by depositing an inorganic precur- sor onto a microelectronic substrate by drop coating. 17 Two series of tungsten oxide devices annealed at different temperatures were made. The sensors showed very promising results which suggested that sensitivity, selectivity, and good stability could all be good. Experimental The sensor substrates were fabricated on silicon wafers by using silicon technology. In the first stage, a layer of SiO 2 was grown by thermal oxidation. Then a heating resistor and six contact pads were patterned using thermally evaporated NiCr and a photolithographic process with wet etching. Finally, Au was thermally evaporated and a pair of interdigitated gold electrodes and a temperature sensing gold meander were patterned by using photolithography and wet etching. The whole process required the use of two masks. Figure 1 shows a picture of a sensor substrate. The heating resistor, the tem- perature sensor, and the pair of interdigitated electrodes are coplanar. The typical value of the temperature measuring resistor is 500  100% with a temperature coefficient of 780 ppm/°C. The value of the heating resistor is 45  4% with a low temperature coefficient. The WO 3 was deposited by drop coating with a micropipette. A saturated aqueous solution of recrystallized ammonium wolframate (99.999% pure) was used as a precursor. Once the WO 3 was deposit- ed, the sensor was dried in air at 60°C. Finally, the sensor was annealed in air at 300 or 400°C for 3 h. The composition of the WO 3 films was investigated by X-ray photoelectron spectroscopy (XPS) and the morphology of the surface was studied by scanning electron microscopy (SEM). To investigate the gas-sensing properties, the devices were introduced into a thermally controlled test chamber (1°C). All contaminants and water (to obtain the desired moisture level) were injected into the test chamber by using high precision chromatographic syringes. Dry air was used both as a reference gas and as a diluting gas to obtain the desired concentrations of the test gases. The gases tested were 0-1000 ppm ammonia, 0-1000 ppm toluene, 0-1000 ppm benzene, 0-10000 ppm methane, 0-1000 ppm ethanol, and water vapor (20-80% relative humidity). The procedure for creating the desired concentration in the test chamber is described elsewhere. 18,19 The electrical resistance of the sensors was Fabrication of Highly Selective Tungsten Oxide Ammonia Sensors E. Llobet, a,z G. Molas, a P. Molinàs, b J. Calderer, b X. Vilanova, a J. Brezmes, a J. E. Sueiras, c and X. Correig a a Departament of Electronic Engineering, Universitat Rovira i Virgili, 43006 Tarragona, Spain b Departament of Electronic Engineering, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain c Departament of Chemical Engineering, Universitat Rovira i Virgili, 43006 Tarragona, Spain Tungsten oxide is shown to be a very promising material for the fabrication of highly selective ammonia sensors. Films of WO 3 were deposited onto a silicon substrate by means of the drop-coating method. Then, the films were annealed in dry air at two dif- ferent temperatures (300 and 400°C). X-ray photoelectron spectroscopy was used to investigate the composition of the films. Tung- sten appeared both in WO 2 and WO 3 oxidation states, but the second state was clearly dominant. Scanning electron microscopy results showed that the oxide was amorphous or nanocrystalline. The WO 3 -based devices were sensitive to ammonia vapors when operated between 250 and 350°C. The optimal operating temperature for the highest sensitivity to ammonia was 300°C. Further- more, when the devices were operated at 300°C, their sensitivity to other reducing species such as ethanol, methane, toluene, and water vapor was significantly lower, and this resulted in a high selectivity to ammonia. A model for the sensing mechanisms of the fabricated sensors is proposed. © 2000 The Electrochemical Society. S0013-4651(99)07-105-0. All rights reserved. Manuscript received July 26, 1999. z E-mail: ellobet@etse.urv.es Figure 1. Picture of a sensor substrate.