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

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
^nd
and 3
^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
~ 0.3% (using a simple top-electrode-only configuration with a 2-mask process) and a groundbreaking Q ~ 1100 which is the highest ever achieved among piezoelectric resonators above 6 GHz. The f - Q product ~1 × 10
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.