Cryogenic Loss Limits in Microwave Epitaxial AlN Acoustic Resonators

TL;DR

This study investigates cryogenic loss limits in epitaxial AlN FBARs, developing a physics-based model to predict quality factors at low temperatures, relevant for quantum and communication technologies.

Contribution

The paper introduces a comprehensive, physics-based model that estimates cryogenic quality factor limits in AlN FBARs, validated against experimental data and benchmarked with other resonators.

Findings

Quality factor decreases from ~1589 at 6.5 K to 363 at 294 K.

The model accurately predicts temperature-dependent Q limits.

Validation against a 23 GHz HBAR confirms model transferability.

Abstract

Aluminum nitride (AlN)-based thin-film bulk acoustic wave resonators (FBARs) are promising compact platforms for 6G communications and quantum memory hardware, enabled by their integrable acoustic modes with high quality factors. However, temperature-dependent acoustic dissipation ultimately limits device performance. In this work, we fabricated a 16 GHz epitaxial AlN FBAR as a test platform, performed small-signal RF measurements from 6.5 K to 300 K, and developed a physics-based model to estimate the fundamental quality-factor limits of FBARs to cryogenic temperatures. The proposed model incorporates both intrinsic and extrinsic loss mechanisms, including an analytical anchor-radiation loss model for bulk acoustic wave resonators, rather than relying solely on finite-element simulations. Measured loaded quality factor (Q) decreases monotonically with temperature, from Qmax of approximately 1589 (Qf=24.79 THz) at 6.5 K to 363 at 294K (Qf=5.66 THz). This trend is consistent with the theoretical limit based on the resonator geometry and the chosen Metal-Insulator-Metal (MIM) stack. To demonstrate the generality of the physics-based framework, we further validate it by benchmarking against a 23 GHz high-overtone bulk acoustic resonator (HBAR) using previously reported data. The validated model provides a practical, transferable framework to interpret Q(T) limits in low-loss resonators by quantifying the temperature-dependent mechanisms that constrain Q, enabling the design of cryogenic microwave filter elements for superconducting quantum hardware.