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 ~1μm. Two types of AlScN thickness-extensional (TE) resonators are co-fabricated on the same SOI platform; with and without a 3.55μm-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 (kt2), it boosts the quality factor (Q), provides structural robustness, and improves the overall Q×kt2 FoM. The resonant frequency spectrum of the high-overtone TE modes of AlScN-on-Si composite resonator is analyzed and the dependency of kt2 on the Si device layer properties is studied. A high kt2 value of 11.7% at the 3rd-order TE resonant frequency of 2.4 GHz is reported, yielding a high kt2×Qmax FoM of 84. The reported FoM shows 2× improvement compared to the co-fabricated AlScN-only FBARs.