Unlocking the Top-Space Multi-Functionality of Thin-Film Acoustic Wave Devices

Abstract

Thin-film acoustic wave devices have substantially improved performance through bottom substrate-stack engineering, which leaves the vulnerability of interdigital transducers (IDTs) a critical challenge, especially under high-power excitation and top-space perturbations. By introducing cladding multilayers above the IDTs on a piezoelectric thin film, layered acoustic wave (LAW) devices with a symmetric acoustic-reflection structure provide strong vertical energy confinement and environmental isolation for the target modes. This architecture unlocks the top space above the IDTs for flexible functionalization. As a representative demonstration, this work presents a polymer-functionalized LAW prototype, showing that mature packaging pipelines can be simplified when inherently suppressing high-frequency, higher-order bulk wave spurious modes. With dispersion analysis and simulation, the low-acoustic-velocity and high-damping polymer is verified to be an effective medium for guiding and absorbing leaked bulk waves. The fabricated resonators provide dual functionality: spurious suppression and enhancement of the quality factor ( ${Q}\text {)}$ without degrading electromechanical coupling of the targeted mode. This work highlights the strong potential of versatile top-space stacking on LAW devices beyond conventional thin-film surface acoustic wave platforms.