This paper describes a new approach to making the active elements of gas sensors using a photochemical deposition method. The method involves making amorphous films of photochemically active precursors. These precursors are then exposed to light and, in air, convert to metal oxides. Amorphous films constructed of W(CO)4(Et,-en) are deposited on interdigitated microelectrodes. Photolysis of these films, in air at room temperature, results in the deposition of amorphous films of tungsten oxide. This forms the sensing element of an NO,, sensor. Films thus prepared were also annealed to yield sensors whose active elements were crystalline tungsten oxide. An investigation of the conduction mechanism in the sensor materials was performed. The conduction in the polycrystalline materials is controlled by grain boundaries through thermionic emission. Absorption of NO2 leads to a modification of the grain boundary resulting in the change in current. In contrast, the amorphous materials conduct via variable range hopping. In this case, absorption of NO2 leads to a reduction in the number of carriers and a change in conductivity of the material. The response of the amorphous materials has been fit to a model based on a Langmuir isotherm. InfroductionIn this paper we present a room temperature fabrication method to construct an NO2 sensor based on amorphous tungsten oxide. We have been developing a methodology to deposit amorphous metal oxides at low temperature using a photochemical technique.1-" It became apparent during these studies that the physical properties of the materials deposited may be altered by the method of deposition. In view of this, we have initiated a study of some of the electronic and physical properties of these materials. One of the important properties we are interested in is the use of these materials as chemical sensors.Many chemical sensors are based on metal oxides'2'13 such as TiO3, ' SnO3, 15 and WO3. 15-19 In many cases the main component is modified by doping with other metals.25-24 The methods of construction of the active elements vary from pressed powders25-2' to films of various thicknesses.25-3' Some recently developed sensors are fabricated by depositing thin films on more complex devices.'6 Microchemical sensors have been made by combining process steps from the microelectronics industry, such as photolithography with low-pressure chemical vapor deposition (LPCVD), to construct a monolithic device.32 The fabrication of thin-film metal oxide sensor arrays offers a solution to the problem of obtaining specific responses of metal oxide sensors to different gases.33 Gas-sensitive field-effect devices may lead to small reproducible sensors.34'35The basic methodology involves the deposition of a thin film of an inorganic or organometaliic precursor onto a substrate by spin coating from a solvent.'-" The resultant film is photosensitive and upon irradiation in the presence of air converts to the target metal oxide. In many cases these films are amorphous oxides with no detectable contamination...

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