Gas sensors based on improoved SAW device

A surface acoustic wave (SAW) hydrogen sensor with improved performance is described. It is based on the velocity changes of a surface acoustic wave propagating on a delay path coated with a palladium film. Adsorption and desorption of hydrogen in the palladium film produce a change both in density and elastic properties causing surface acoustic wave velocity variations which are detected as frequency shifts at the output of a SAW oscillator. An improvement in the sensitivity, together with a higher stability of the device was obtained by using a STx-quartz plate as piezoelectric substrate and a double delay line differential configuration was used in order to reduce common mode error sources such as temperature and pressure.Sensitivity, selectivity and time response of the device are given and discussed. The use of a monolitic operational amplifier in the resonant loop of the oscillator has been experimented aiming at an integration of the sensor in a silicon structure.