Investigation on Quasi Lamb Wave Modes in AlN-on-Si MEMS Resonators.

Aluminum nitride (AlN)-on-Si MEMS resonators operating in Lamb wave modes have found wide applications for physical sensing and frequency generation. Due to the inherent layered structure, the strain distributions of Lamb wave modes become distorted in certain cases, which could benefit its potential application for surface physical sensing. This paper investigates the strain distributions of fundamental and first-order Lamb wave modes (i.e. S, A, S, A modes) associated with their piezoelectric transductions in a group of AlN-on-Si resonators. The devices were designed with notable change in normalized wavenumber resulting in resonant frequencies ranging from 50 to 500 MHz. It is shown that the strain distributions of four Lamb wave modes vary quite differently as normalized wavenumber changes. In particular, it is found that the strain energy of A-mode resonator tends to concentrate to the top surface of acoustic cavity as the normalized wavenumber increases, while that of S-mode device becomes more confined in the central area. By electrically characterizing the designed devices in four Lamb wave modes, the effects of vibration mode distortion on resonant frequency and piezoelectric transduction were analyzed and compared. It is shown that designing A-mode AlN-on-Si resonator with identical acoustic wavelength and device thickness benefits its surface strain concentration as well as piezoelectric transduction, which are both demanded for surface physical sensing. We herein demonstrate a 500-MHz A-mode AlN-on-Si resonator with decent unloaded quality factor (Q = 1500) and low motional resistance (R = 33 Ω) at atmospheric pressure.