Spurious Mode Free 3.5GHz AlN Plate Mode Resonator with High FoM

This work reports a spurious mode free AlN plate mode resonator with high figure-of-merit (FoM) operating at 3.5GHz. The reported resonator, which comprises of two sets of $0.27\ \mu \text{m}$ -thick top molybdenum (Mo) interdigitated electrodes (IDE) fingers attached to a layer of $1\ \mu \text{m}$ -thick AlN, has an IDE pitch $(p)$ of 1m and a finger width $(w)$ of $0.5\ \mu \text{m}$ defined by optical photolithography. Besides reducing the fabrication process complexity, having only IDE on the top surface of AlN in the fabrication process also increases the acoustic phase velocity $(v_{p})$ as a result of the exclusion of bottom Mo IDE which has a lower acoustic phase velocity than AlN, thus relaxing the lithographic and etching requirement of IDE pitch. Measurement results show that the resonant frequency $(f)$ is around 3.5GHz, with a spurious mode free frequency response over a 1GHz frequency range from $3GHz$ to $4\text{GHz}$ . The effective electromechanical coupling coefficient $(k_{eff^{2}})$ is 3.15% and the Q factor $(Q)$ is 1000, yielding an $f\cdot Q$ product of $3.5\times 10^{12}$ and $k_{eff^{2}}\cdot Q$ of 31.5, respectively. Together with the series resistance of $3.11\Omega$ and the parallel resistance of $874\Omega$ , the proposed plate mode resonator is a promising candidate for mobile Band 42 applications.