A 3.98 Ghz Aluminum Nitride Overmoded Bulk Acoustic Wave Resonator for Temperature Sensing Applications

In this work, we present a novel microelectromechanical systems (MEMS) temperature sensor based on an aluminum nitride (AlN) overmoded bulk acoustic wave resonator (OBAR). Utilizing the overmoded operation of a AlN bulk acoustic wave resonator with a thick metal electrode, a large temperature coefficient of frequency (TCF) of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$-105 \text{ppm} /{}^{\circ} \mathrm{C}$</tex> is achieved, while maintaining a high quality factor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(Q)$</tex> of 300 and an electromechanical coupling coefficient <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(k_{t}^{2}\right)$</tex> of 4.34 % at 3.98 GHz. This TCF is approximately 4.5 X higher than that of a regular AlN thin-film bulk acoustic wave resonator (FBAR). Furthermore, based on the dual-oscillator beat frequency sensing topology using OBAR and FBAR resonators characterized in this study, a large <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{TCF}_{\text{beat}}$</tex> of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1,088 \text{ppm} /{}^{\circ} \mathrm{C}$</tex> is obtained through theoretical predictions.