Record-High ${K_{T}}^{2}$ Exceeding 7.4% Through Low-Impedance Lithographically Defined Resonant Rods in Aluminum Nitride Thin Plates

We demonstrate the first prototype of a novel class of Aluminum Nitride resonators, labeled Two-Dimensional-Resonant-Rods (2DRRs), for operation in the radiofrequency (RF) range. 2DRRs rely, for the first time, on the excitation of an array of locally resonant rods, micro-fabricated in thin-film AlN plates. In this work, we built a low-impedance ((ωC0)-1 ≈ 208Ω) 2DRR, operating at 2.35 GHz and showing an unprecedented electromechanical coupling coefficient ( k
_t
²
), for lithographically defined AlN resonators, exceeding 7.4%. Such a high k
_t
²
-value, that matches closely the predicted value through Finite Element Methods (FEM), can be attained while relying on an unpatterned bottom metal plate. This unique feature permits to simultaneously attain large capacitance values and an optimal AlN crystalline orientation. Moreover, as verified through FEM, the resonance frequency of 2DRRs can be lithographically tuned without a substantial degradation of k
_t
²
.