Comparative study of surface acoustic wave based hydrogen sensors with: InOx / SiNx / 36° YX LiTaO3 structure

Layered Surface Acoustic Wave (SAW) based sensors with: InOx/ SiNx / 36 degrees YX LiTaO3 structure were developed for sensing different hydrogen (H-2) concentrations between 0.06% (600ppm) and 1% H-2 in synthetic air. This paper presents a comparative study of the sensors performances in terms of response time, recovery time and response magnitude as a function of operational temperature. The SAW devices consist of metal interdigitated electrodes fabricated on lithium tantalate (LiTaO3) piezoelectric substrate forming the input and output Interdigital Transducers (IDTs). A 1 mu m thick silicon nitride (SiNx) intermediate layer was deposited over these finger pairs, either by Plasma Enhanced Chemical Vapour Deposition (PECVD) or by r.f. magnetron sputtering. A 100 nm thin film of indium oxide (InOx) deposited by r.f magnetron sputtering provides the selectivity towards hydrogen. The highest sensitivity for the sensor with r.f. magnetron sputtered SiNx intermediate layer was recorded at 190 degrees C, when the frequency shift of 361 KHz for 1% H-2 in synthetic air was recorded. However larger responses were obtained for the sensor with the PECVD SiNx intermediate layer at 290 degrees C, when the large frequency shift of 516 KHz was recorded for the same H-2 concentration. Microstructural characterization of the InOx and SiNx films by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) is also presented.