High performance BAW resonators with improved AlN thin films quality based on BaF2 buffer layer

The quality of AlN thin films has an important effect on the performance of bulk acoustic wave (BAW) resonators. In this work, the low lattice mismatch of BaF2 buffer layer with AlN thin films was employed to improve the crystalline quality of AlN thin films. Furthermore, an ethanol assisted epitaxial liftoff (ethanol-ELO) technique based on the BaF2 buffer layer was proposed to lift off AlN thin films from Si substrate, which reduced surface roughness scattering. The ELO technology reduced the damage of AlN thin films and Si wafer during the ELO process due to the selective etching of AlN and BaF2. Utilizing the BaF2 buffer layer, the as-prepared BAW resonators, based on single-crystalline AlN, displayed Q-factor up to 2857, which was 47% higher than that without the BaF2 buffer layer. This study highlights the significant role of the BaF2 buffer layer in enhancing BAW resonators' performance and reducing fabrication costs.