Influence of temperature stability on the sensing properties of SAW NOx sensor

This paper presents the study on the influence of temperature stability of a SAW sensor on the NOx-sensing properties. A temperature-stable SAW sensor based on a 128° YX LiNbO3 single crystal was developed after integrating with a rf-sputtered amorphous TeO2+y (y = 0–1) thin film that acted as a temperature compensation over-layer. TeO2+y thin films possessing a negative temperature coefficient of delay (TCD) also served as a sensing layer for the detection of NOx gas. It was observed that the temperature-stable TeO2+y (0.045λ)/128° Y–X LiNbO3 SAW-layered structure with a TeO2 over-layer deposited in a sputtering gas mixture (Ar + O2) of 25% O2 content exhibited the maximum sensitivity to a given concentration of NOx. The influence of the sensing layer thickness on the response characteristics was also studied after suppressing the contribution due to the temperature instability of the devices, and a thicker TeO2 film was found to give enhanced sensitivity. The temperature-stable SAW devices are essentially desired to avoid the problem of false alarm, besides the enhancement in sensitivity and reliability.