Impedance-matched High-overtone Bulk Acoustic Resonator

TL;DR

This paper reports the fabrication of a nearly impedance-matched high-quality high-overtone bulk acoustic resonator (HBAR) using epitaxial AlN on SiC, achieving high frequency, high Q-factor, and broadband phonon modes for advanced RF applications.

Contribution

The study introduces a novel impedance-matched HBAR with epitaxial AlN directly grown on SiC, eliminating metal layers and enhancing performance for RF and high-frequency acoustic systems.

Findings

Achieved impedance matching verified by FSR variation.

Reproduced FSR spectra with Mason model.

Realized broadband phonon modes up to 26.5 GHz.

Abstract

A high-overtone bulk acoustic resonator (HBAR), in which a piezoelectric transducer is set on an acoustic cavity, has been attracting attention in both fundamental research and RF applications due to its scalability, high frequency, and high quality factor. The acoustic impedance matching in HBARs is crucial for efficient acoustic power transfer from the piezoelectric transducer to the cavity. However, impedance mismatch remains in most HBARs due to the metal layer insertion between the piezoelectric layer and cavity substrate. In this study, we fabricated a nearly impedance-matched high-quality HBAR using an epitaxial AlN piezoelectric layer directly grown on a conductive SiC cavity substrate with no metal layer insertion. The small impedance mismatch was verified from the variation in the free spectral range (FSR), which is comparable to the best value in previously reported HBARs. The experimentally obtained FSR spectra was greatly reproduced by using the Mason model. Broadband phonon cavity modes up to the K-band (26.5 GHz) were achieved by reducing the thickness of the AlN layer from 800 to 200 nm. The high figure of merit of $f\times\text{Q} \sim 1.3\times 10^{13}\ \textrm{Hz}$ at 10 GHz was also obtained. Our nearly impedance-matched high-quality HBAR will enable the development of RF applications, such as low-phase noise oscillators and acoustic filters, as well as research on high-frequency acoustic systems hybridized with electric, optical, and magnetic systems.