A Ku-Band 2^nd Order Mode FBAR Based on Bilayer Polarity-Inverted Single Crystal AlN Films

The pursuit of enhancing the performance of Aluminum Nitride (AlN) Film Bulk Acoustic Resonators (FBARs) poses significant challenges in meeting the application demands of 6G and satellite communications, particularly at frequencies beyond 10 GHz. In this work, we present a two-step growth method for the fabrication of single-crystal, polarity-inverted AlN piezoelectric film stacks, which are subsequently utilized to fabricate Bulk Acoustic Wave (BAW) resonators operating in the Ku-band. The fabricated resonator exhibits a single TE2 mode resonant response at 14.5 GHz, with an effective electromechanical coupling coefficient $\left( {{k_{eff}}^2} \right)$ of 5.13% and a maximum quality factor (Q<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf>) of 551, demonstrating superior performance in the Ku-band. Our results highlight the great potential of the polarity-inverted single-crystal AlN piezoelectric films for the development of high-frequency acoustic filters.