High-Overtone Thin Film Ferroelectric AlScN-on-Silicon Composite Resonators

This letter presents the first demonstration of thin-film ferroelectric Aluminum Scandium Nitride (AlScN)-on-silicon composite resonators, targeting high-overtone resonance modes in the sub-6GHz band with a high figure of merit (FoM). The resonators are based on sputtered ferroelectric AlScN films with Sc/(Al+Sc) ratio of 30% and thickness of $\sim 1 {\mu }\text{m}$. Two types of AlScN thickness-extensional (TE) resonators are co-fabricated on the same SOI platform; with and without a $3.55~\mu \text{m}$ -thick Si layer in the resonant stack. We show that although the passive Si device layer underneath the thin piezo-stack results in degradation of the electromechanical coupling coefficient $({k}_{t}^{2})$ , it boosts the quality factor $({Q})$ , provides structural robustness, and improves the overall ${Q} \times {k}_{t}^{2}$ FoM. The resonant frequency spectrum of the high-overtone TE modes of AlScN-on-Si composite resonator is analyzed and the dependency of ${k}_{t}^{2}$ on the Si device layer properties is studied. A high ${k}_{t}^{2}$ value of 11.7% at the 3rd-order TE resonant frequency of 2.4 GHz is reported, yielding a high ${k}_{t}^{2}\times {Q}_{\textit {max}}$ FoM of 84. The reported FoM shows $2\,\, {\times }$ improvement compared to the co-fabricated AlScN-only FBARs.