Novel cmWave A1-Mode Filter Design Based on Lithium Niobate On-Chip Capacitors

Abstract

In this work, a novel design scheme of on-chip frequency shifting is proposed and applied to acoustic resonators for the first time. On the same chip with first-order antisymmetric mode (A1-mode) resonators on 128° Y-cut lithium niobate (LiNbO₃ or LN) thin film, LN based capacitors are proposed and realized by rotating the in-plane periodic direction of interdigitated transducer (IDT) electrodes to align the electric field direction at 90° relative to the +X axis. In a ladder-type topology, the series branch achieves an upward shift of the series resonant frequency by connecting an A1-mode resonator in series with an on-chip LN capacitor, while the shunt branch realizes a downward shift of the parallel resonant frequency by connecting an A1-mode resonator in parallel with a capacitor or rotating the IDT electrodes to a specific angle. By aligning these two frequencies through careful design, an acoustic filter is successfully implemented. Compared to traditional A1-mode filters, which rely on differentially localized etching to achieve frequency shifts in the series branch, the proposed novel design scheme reduces at least one photolithography and etching step, thereby greatly lowering process complexity and improving yield. Through this approach, this work demonstrates a high frequency (in the centimeter-wave or cmWave band) filter operating at 14.3 GHz with an insertion loss (IL) of 3.6 dB, a bandwidth of 961 MHz (FBW of 6.7%), and an out-of-band rejection (OoB) of 14 dB. This proves the validity of the pure layout-design based frequency shifting mechanism and enables the Lamb wave resonator technology with better manufacturability for 6G cmWave wireless communications.[2026-0082]