Aluminum Nitride Combined Overtone Resonators for the 5G High Frequency Bands

Abstract

This work presents a new acoustic MEMS resonator technology, dubbed Aluminum Nitride (AlN) Combined Overtone Resonator (COR), capable of addressing the filter requirements for the 5G high frequency bands in the 6-40GHz range. The COR exploits the multimodal excitation of two higher-order Lamb waves ( $2^{\mathbf {nd}}$ and $3^{\mathbf {rd}}$ order Asymmetrical Lamb Waves) in a suspended thin-film AlN plate to transduce a 2-dimensional vibration mode with high electromechanical coupling coefficient $k_{t}^{2}$ (up to 1.9%) and quality factor $Q>1100 $ at twice the frequency of a fundamental thickness-extensional mode in the same structure. Analytical and finite-element method (FEM) models are developed to describe the working principle of the COR technology and predict the achievable $k_{t}^{2}$ , Q and lithographic frequency tunability. An 8.8 GHz COR prototype was fabricated showing a high $k_{t}^{2}~\sim ~0.3$ % (using a simple top-electrode-only configuration with a 2-mask process) and a groundbreaking $Q\sim {1100}$ which is the highest ever achieved among piezoelectric resonators above 6 GHz. The $f - Q$ product $\sim 1\times 10 ^{\mathbf {13}}$ is the highest among all demonstrated piezoelectric resonators with metallic coverage >50%. Additionally, the capability of the COR technology to deliver contiguous filters with bandwidths between 355 and 592 MHz (aggregated BW >2GHz) in the mmWave spectrum, with relaxed lithographic requirements, is demonstrated by FEM. [2019-0229]