Scale Interdigital Transducer-based Micro-Acoustic Resonators into mmWave Applications.

Abstract

Different orders of Lamb wave mode resonators using interdigital transducers (IDTs) and LiNbO₃ thin films are increasingly important due to their large electromechanical coupling (k_t²) and high phase velocities, essential for mmWave miniaturized acoustic filters. In 50 Ω systems, achieving proper impedance matching necessitates large static capacitance. However, this capacitance may interact with self-inductance, leading to multiple electromagnetic (EM) self-resonances in the targeted spectrum, which is one of the major bottlenecks in using acoustic waves for mmWave applications. These resonances will decrease the series quality factor (Q) and alter the capacitive characteristics of the resonator, which significantly degrades the performance in filtering and frequency reference, especially with higher-order Lamb wave modes. Unlike the sub-6 GHz system, a new modeling method is needed to analyze the previously neglected EM-acoustic coupling in the 5G/6G mmWave spectrum. This study proposes new design philosophies for IDTs to reduce self-inductance for mmWave applications, exploring the interactions between acoustic and EM waves within the IDTs and introducing new equivalent circuit models for various scenarios. To verify these methods, devices were fabricated on Y-128° cut LiNbO₃ thin films. Both simulation and experimental results demonstrate the accuracy and efficiency of the proposed approaches. This work enables the effective use of IDT in the mmWave range without sacrificing necessary static capacitance and explains the EM effects based on the proposed multi-physic equivalent circuit models.