Vacuum thermally evaporated WO/sub 3/ thin film ozone sensor

The gas sensitivity properties of WO/sub 3/ thin films have been studied for the detection of O/sub 3/ gas (25-200 ppb), and NO/sub 2/ (0.7-5 ppm) in the working temperature range 150-300 /spl deg/C. Thin film sensors have been obtained by evaporating commercial WO/sub 3/ powders on sapphire substrates provided with Pt finger type sputtered electrodes and annealing at 500/spl deg/C for different hours. The film morphology, crystalline phase and chemical composition of the sensors have been characterized through SEM, low angle XRD and XPS. The electrical response has been measured by exposing the annealed films to O/sub 3/, NO/sub 2/ in dry air. The 500/spl deg/C annealed for 24 hours shows the highest sensitivity to O/sub 3/ at 215/spl deg/C (S=30 at 175 ppb O/sub 3/), while the highest NO/sub 2/ gas sensitivity is reached at 250/spl deg/C working temperature (S=45 at 1.7 ppm NO/sub 2/). The annealing time has the effect to enhance the O/sub 3/ and NO/sub 2/ sensitivities.