High-Q Wireless SAW Sensors Based on AlN/Sapphire Bilayer Structure, Operating at 2.45 GHz Range for High-Temperature Applications

This letter deals with the potential of the Al/AlN/Sapphire surface acoustic wave (SAW) structure as a wireless sensor operating in the 2.45 GHz ISM band for high-temperature applications up to 500 degrees C. A first design was used to make SAW resonators by e-beam lithography and characterize them between room temperature and 500 degrees C in a wired configuration. Frequency variation with temperature showed good stability, repeatability, linearity, and sensitivity with a measured temperature coefficient (TCF) of frequency of -47 ppm/degrees C. The figure of merit value, defined by the product of the electromechanical coupling coefficient and the quality factor, varies between 0.8 and 2 throughout the whole temperature range, proving the potential of the structure for wireless interrogation at high temperatures. Consequently, the SAW resonator was successfully wirelessly interrogated up to 500 degrees C at a distance of 1 m. Moreover, wireless interrogation of the sensor was possible up to 3 m at room temperature, with additional path losses of 19 dB, which could prove troublesome for operating temperatures above 400 degrees C. An optimization of the SAW resonator design was then subsequently carried out by simulation means in order to improve the figure of merit and allow wireless interrogation at higher temperatures and greater distances.