Abstract
Layered Surface Acoustic Wave (SAW) based sensors with: InO<sub>x</sub> / SiN<sub>x</sub> / 36° YX LiTaO<sub>3</sub> structure were developed for sensing different hydrogen (H<sub>2</sub>) concentrations between 0.06% (600ppm) and 1% H<sub>2</sub> 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 (LiTaO<sub>3</sub>) piezoelectric substrate forming the input and output Interdigital Transducers (IDTs). A 1 μm thick silicon nitride (SiN<sub>x</sub>) 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 (InO<sub>x</sub>) deposited by r.f. magnetron sputtering provides the selectivity towards hydrogen. The highest sensitivity for the sensor with r.f. magnetron sputtered SiN<sub>x</sub> intermediate layer was recorded at 190° C, when the frequency shift of 361 KHz for 1% H<sub>2</sub> in synthetic air was recorded. However larger responses were obtained for the sensor with the PECVD SiN<sub>x</sub> intermediate layer at 290° C, when the large frequency shift of 516 KHz was recorded for the same H<sub>2</sub> concentration. Microstructural characterization of the InO<sub>x</sub> and SiN<sub>x</sub> films by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) is also presented.
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