Layered Surface Acoustic Wave (SAW) based sensors with: InOx / SiNx / 36° YX LiTaO3 structure were developed for sensing different hydrogen (H2) concentrations between 0.06% (600ppm) and 1% H2 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 μm 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° C, when the frequency shift of 361 KHz for 1% H2 in synthetic air was recorded. However larger responses were obtained for the sensor with the PECVD SiNx intermediate layer at 290° C, when the large frequency shift of 516 KHz was recorded for the same H2 concentration. Microstructural characterization of the InOx and SiNx films by Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) is also presented.
A layered Surface Acoustic Wave (SAW) device based on an InOx/Si3N4/36° YX LiTaO3 structure is investigated for sensing ozone in air at different operating temperatures and concentrations. These concentrations are between 25 ppb and 150 ppb. Layered SAW devices are of a great interest as they show a remarkable performance for liquid and gas sensing applications. This structure is a single delay line SAW device with 64 input and output finger pairs, having periodicity of 24 μm. They were fabricated on a 36° Y-cut X-propagating lithium tantalate (LiTaO3) piezoelectric substrate. A 1 μm thick silicon nitride (Si3N4) layer was deposited over the finger pairs and a 100 nm indium oxide (InOx) sensing layer was deposited over the Si3N4 layer. Both layers were deposited by RF magnetron sputtering. InOx was chosen as it has a remarkable sensitivity towards ozone. Si3N4 was chosen as it is inert and has stable characteristics at high temperature. The sensor performance is analysed in terms of response time, recovery time and response magnitude as a function of operational temperature. The operational temperature ranges between 185°C and 205°C. The sensor shows repeatability, reversibility, fast response and recovery time. At approximately 190°C the highest sensitivity was observed. A frequency shift of 5.0 kHz at 25 ppb, 6.5 kHz at 50 ppb ozone was recorded. The presented results show this structure is promising for gas sensing applications.