Laterally Excited Bulk Acoustic Resonators With Grooves Between Interdigital Electrodes

Laterally excited bulk acoustic resonators (XBARs) with adjustable piezoelectric coupling coefficient (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{2}$ </tex-math></inline-formula>) are a hot spot in radio frequency (RF) filters for the flexible adjustment of bandwidth. In this letter, the XBARs with grooves (G-XBARs) achieve adjustable <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{2}$ </tex-math></inline-formula> by changing the groove size. The equivalent series capacitor (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C_{\text {r}}$ </tex-math></inline-formula>) is introduced due to the etched grooves, and consequently, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{2}$ </tex-math></inline-formula> is adjusted. The designed G-XBAR is fabricated using a 300-nm Z-cut LiNbO3 on an insulator (LNOI) wafer, where the grooves in LiNbO3 thin films are formed by ion beam etching (IBE) at an etch rate of 15.7 nm/min. The measured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$K^{2}$ </tex-math></inline-formula> of G-XBAR is adjusted from 7.54% to 33.69%, with a wide range of 77.6% over 6 GHz. Furthermore, the power-handling capability of G-XBAR is measured over +19 dBm. The G-XBAR demonstrated in this work exhibits a wide adjusting range, showing great potential for high-performance RF filters with tunable bandwidth in multiband applications.