Layered InOx/Si3N4/36YX LiTaO3 surface acoustic wave based hydrogen sensor

Layered Surface Acoustic Wave (SAW) devices are of enormous interest for liquid and gas sensing applications. In this paper, a multilayer structure consisting of an InOx / Si3N4 / 36° YX LiTaO3 is investigated for sensing hydrogen concentrations as low as 600 ppm in air at different operating temperatures. In each of the input and output ports 64 metal finger pairs were fabricated with a periodicity of 24 ìm. A 1 ìm silicon nitride intermediate layer (Si3N4) and a 100 nm thick indium oxide (InOx) sensitive layer were deposited over these finger pairs by RF magnetron sputtering. The sensor performance was analyzed in terms of response time, recovery time and response magnitude as a function of operational temperature at different hydrogen concentrations. The operating temperature range of the sensor was between 145 °C and 310 °C. However highest sensitivity was observed at around 190 °C. Large frequency shifts of 361 kHz for 1% H2 and 290 kHz at 0.25% H2 in synthetic air were recorded.