Loss mechanisms in cryogenic microwave epitaxial AlN resonators

Epitaxial aluminum nitride (AlN) thin-film bulk acoustic resonators (FBARs) enable low-loss filtering for future 6G systems. They also provide a compact approach for qubit sensing at cryogenic temperatures. However, these devices are rarely characterized systematically from room temperature to cryogenic temperatures, and the mechanisms that limit their cryogenic performance remain unclear. In this work, we study a 15.6 GHz epitaxial AlN FBAR from room temperature to cryogenic temperatures to identify losses from the AlN film and those introduced by the electrodes, anchors, and other device layers. Small-signal RF measurements from 294 K down to 6.5 K show an increase in the unloaded Qₘₐₓ from 245 to 1729. A temperature-dependent model that includes phonon–phonon scattering, thermoelastic damping, dielectric loss, electrical loss, and anchor loss helps explain the measured Q(T) trend and identifies a transition from the Landau–Rumer to the Akhiezer regime near 270 K. The model indicates that acoustic energy leakage through the anchors and electrical loss set upper bounds on Q. Energy transfer from the main resonant mode to spurious acoustic modes can also lower the measured Q. Improving anchor isolation, suppressing spurious modes, and reducing electrode loss, including through superconducting electrodes, increase the cryogenic Q. These results guide the design of higher-Q acoustic resonators for high-frequency 6G filters and cryogenic quantum-microwave circuits.