Acoustic characteristics of the third-order quasi-symmetric Lamb wave mode in an AlN/3C-SiC plate

In this study, the phase velocity and electromechanical coupling coefficient of the third-order quasi-symmetric (QS₃) Lamb wave mode in a composite membrane composed of an AlN film and a cubic silicon carbide (3C-SiC) layer are theoretically and experimentally investigated. According to the theoretical simulation results, the epitaxial 3C-SiC (100) layer enhances the electromechanical coupling coefficient of the QS₃ Lamb wave mode in the composite plate with a proper thickness ratio of AlN film to 3C-SiC layer. In addition, the 3C-SiC layer smoothes the phase velocity dispersion, making the QS₃ mode applicable to the designs for the Lamb wave resonators. The experimental results show that the epitaxial 3C-SiC layer not only boosts the quality factor (Q) but also reduces the temperature coefficient of frequency (TCF) of the QS₃ Lamb wave mode. While the 2.5-μm-thick AlN film and 2.6-μm-thick epitaxial 3C-SiC layer are employed, a Lamb wave resonator utilizing the QS₃ mode exhibits a low motional impedance of 91 ohm and a high Q up to 5510 at a series resonance frequency (f_s) of 2.92 GHz, resulting in a very high f_s·Q product, 1.61×10¹³ Hz.