Development of SAW gas sensor for monitoring SOX gas

We have developed a new type SO₂ gas sensor applying a particular inorganic thin film on SAW devices. The sensor consists of twin SAW oscillators of the center frequency of 54 MHz fabricated on LiTaO₃ piezoelectric single crystals. One delay line of the sensor is coated with a CdS thin film that selectively adsorbs and desorbs SO₂, while the other is uncoated for use as a stable reference. Deposition of CdS thin films has been carried out by the spray pyrolysis method using an ultrasonic nozzle. Mass loading induced by the SO₂ gas adsorbed onto the CdS film result in corresponding frequency shifts directively proportional to the gas concentration. The relative change in the frequency of the two oscillators is monitored with a digital signal processing circuit to measure the concentration of the gas. SO₂ sensor properties investigated include selectivity, sensitivity, response time, and repeatability. The sensor shows promising performance as a micro sensing tool and is capable of measuring concentrations in air less than 100 parts per billion of SO₂. The optimum substrate temperature is found to be 65°C for proper operation of the sensor