Rayleigh Wave Suppression in Al0.6Sc0.4N-on-SiC Resonators

TL;DR

This paper presents a novel hybrid SAW/BAW resonator using Sc-doped AlN on SiC that suppresses Rayleigh modes, with experimental validation and potential for improved acoustic device performance.

Contribution

The study introduces a new AlScN-on-SiC resonator design that effectively suppresses Rayleigh waves and demonstrates tunable mode control through layer thickness and reflectivity modifications.

Findings

Thin piezoelectric layers suppress Rayleigh modes.

Resonator achieves high quality factor with wave confinement.

Reflectivity tuning creates stopbands to prevent unwanted modes.

Abstract

We report on the fabrication of a Hybrid SAW/BAW resonator made of a thin layer of Sc-doped AlN (AlScN) with a Sc concentration of 40 at% on a 4H-SiC substrate. A Sezawa mode, excited by a vertical electric field, exploits the d31 piezoelectric coefficient to propagate a longitudinal acoustic wave in the AlScN. The resonant frequency is determined via the pitch in the interdigitated transducer (IDT) defined by Deep Ultraviolet (DUV) lithography. The resonant mode travels in the piezoelectric layer without leaking in the substrate thanks to the mismatch in acoustic phase velocities between the piezoelectric and substrate materials. We show the impact of the piezoelectric and IDT layers' thickness on the two found modes. Importantly, we show how thin piezoelectric and electrode layers effectively suppress the Rayleigh mode. While some challenges in the deposition of AlScN remain towards a large coupling coefficient k_eff^2, We show how wave confinement in the IDT obtains a good quality factor. We also show how modifying the IDT reflectivity allows us to engineer a stopband to prevent unwanted modes from being excited between resonance and antiresonance frequencies. Finally, we validate the simulation with fabricated and measured devices and present possible improvements to this resonator architecture.