6.5 GHz BAW Resonators and Filters Fabricated on Single Crystal AlN Template

With the rapid advancement of 5G technology and the extension of WiFi 6E into spectrum beyond 6GHz, it is imperative to develop high-performance bulk-acoustic wave (BAW) resonators and filters with highly crystalline AlN piezoelectric films. In this work, we fabricated single-crystal AlN BAW resonators operating at 6.5 GHz. The full width at half maximum (FWHM) of the (0002) X-ray diffraction (XRD) peak of the AlN films prepared by the two-step method is below 0.5°, significantly outperforming the traditional sputtering technique. The fabricated resonator demonstrates a resonance frequency of 6.5 GHz, achieving 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 811 and an effective electromechanical coupling coefficient $\left({{k_{eff}}^2}\right)$ of 4.9%. Moreover, the fabricated filter centered at 6.5 GHz exhibits a 3 dB bandwidth of 194 MHz, a minimum insertion loss of 3.36 dB and an out-of-band rejection exceeding 22 dB. Due to the implementation of single-crystal AlN piezoelectric film, the devices demonstrate exceptional performance and show great significant potential in ultra-high frequency (UHF) applications.