Suppression of spurious modes in lateral-excited bulk acoustic wave resonators using piston mode electrodes

Lateral-excited bulk acoustic wave resonators (XBARs) have a large electromechanical coupling coefficient and low mechanical loss. However, XBARs have not yet been commercialized in 5G communications due to spurious modes, high TCF, and low-power handling. This paper presents a lateral-excited bulk acoustic wave resonator with piston mode electrodes named PLBAR. Compared to the conventional interdigital transducer structure, the PLBAR suppresses the transverse waves due to the irregular boundary caused by piston mode electrodes. Higher order modes are also to some extent suppressed by increasing in metallization rate. The fabricated PLBAR achieves a high Keff2 of 26.43% at 5.2 GHz using a 350 nm Z-cut lithium niobate on insulator substrate, effectively suppressing the transversal mode. Additionally, the power durability exceeds +14 dBm due to the increased metallization of the piston mode electrodes. The measured temperature coefficient of PLBAR is −42.55 ppm/°C. The PLBAR addresses some of the limitations of the XBARs and demonstrates significant improvements in performance without requiring additional fabrication steps, making it a promising solution for RF resonators in 5G communication systems.