Influence of the annealing and operating temperatures on the gas-sensing properties of rf sputtered WO3 thin-film sensors

The sensing properties of WO3 thin films deposited by rf sputtering onto silicon substrates have been investigated. After deposition, the films underwent an annealing process in dry air that lasted 24 h. Three different annealing temperatures (350, 400 and 450 °C) were used to analyze the influence of this parameter on the sensor response. The morphology and composition of the films were studied by AFM and XPS. The films were found to be essentially inhomogeneous. The oxygen-to-tungsten atomic ratio of the active layers reveals that, irrespective to the annealing temperature used, the WO3 active layers were close to their stoichiometric formulation, although a tendency to loose oxygen atoms at the higher annealing temperatures tested was inferred. The sensitivity of the films to ammonia, nitrogen dioxide, ethanol, benzene, methane and carbon monoxide was studied at three different operating temperatures (300, 350 and 370 °C). By selecting the appropriate annealing and operating temperatures, it was found possible to selectively detect nitrogen dioxide in the presence of ethanol and ammonia, or to detect ammonia in the presence of ethanol.