Performance Limiting Factors of 15-GHz Ku-Band FBARs

We study the loss mechanisms limiting the performance of RF filters with epitaxial aluminum nitride (AlN) thin-film bulk acoustic wave resonators (FBARs) operating at the primary mode of 15 GHz. By measuring the dependence of the resonance frequency and quality factor over the temperature range of 79–400 K, we find that electrical loss in the metal electrodes is the primary limiting factor, followed by acoustic loss in the system as well as dielectric loss in the AlN. We model and discuss the measured intrinsic acoustic attenuation mechanisms in the epitaxial AlN layers. From the quantitative analysis of the temperature-dependent small-signal measurements of the filter RF metrics, we extract several temperature-dependent physical quantities, such as the longitudinal stiffness constant
${c}_{{33}}$
of epitaxial AlN, and its longitudinal acoustic phase velocity. A 1% change in the stiffness constant is found to introduce a 100-MHz shift in the resonance frequency. This study identifies roadblocks and provides guidelines for taking the performance of primary mode FBARs to 20 GHz and beyond.