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 straidistributions of Lamb wave modes become distorted in certain cases, which could benefit its potential application for surface physical sensing. This article investigates the strain distributions of fundamental and 1st-order Lambwave modes (i.e.,S-0,A(0),S-1, andA(1)modes) associated with their piezoelectric transductions in a group of AlN-on-Si resonators. The devices were designed with notablechanges 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 theA(1)-mode resonator tends to concentrate on the top surface of the acoustic cavity as the normalized wavenumberincreases, while that of theS(0)-mode device becomesmore confined in the central area. By electrically charac-terizing the designed devices in four Lamb wave modes,the effects of vibration mode distortion on resonant fre-quency and piezoelectric transduction were analyzed andcompared. It is shown that designingA1-mode AlN-on-Siresonator with identical acoustic wavelength and devicethickness benefits its surface strain concentration as wellas piezoelectric transduction, which are both demanded for surface physical sensing. We herein demonstrate a500-MHzA1-mode AlN-on-Si resonator with a decent unloaded quality factor (Q(u)=1500) and low motional resistance(R-m=33 Omega)at atmospheric pressure