Boosting Qs of AlN Resonators by Redefining Acoustic Boundaries

This paper reports on a quality factor ( Q)-boosting technique for aluminum nitride (AlN) Lamb wave resonators by lithographically defining their acoustic boundaries. The technique has been demonstrated to enhance resonator Q from 980 to 3000 without compromising the electromechanical coupling (kt
²
=1.75%), consequently resulting in a high figure of merit of 52.25 (FoM=kt
²
·Q). Our approach derives from our quantitative analysis of the reflection coefficient at the acoustic boundaries of a conventional Lamb wave AlN resonator, which suggests inadequate energy confinement from a reflection coefficient of only 0.47. A two-prong approach, which involves first defining the release regions lithographically and then adding a Pt layer regionally over the anchors, is adopted to create a more significant impedance mismatch at the acoustic boundaries and reduce energy leakage to the surrounding.