Optimized Acoustic Mirror Stack for Layered LLSAW Devices Using Thin LiNbO₃ Plates

This paper discusses the design of acoustic Bragg mirrors (ABMs) for mitigating out-of-band spurious modes in layered LLSAW devices employing X-cut lithium niobate as the piezoelectric layer. First, a matrix-form simulation tool was developed to predict the performance of ABMs in layered SAW devices. Acoustic reflection analysis reveals that the transmission curve of ABMs for SAW devices exhibits a more complex form compared to that of BAW-SMR devices, due to the presence of cut-off frequencies and mode conversion. Additionally, we found that the out-of-band "mirror mode," which is primarily trapped in the high acoustic impedance (HAI) layer within the ABM, is strongly correlated with the longitudinal cut-off frequency of the HAI layer. Based on these analyses, several ABM stack design approaches are discussed to mitigate these out-of-band responses. The effectiveness of these approaches was validated by experimental results using improved ABM design with a Pt-SiO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> stack, demonstrating mitigation and wider separation of these spurious modes while maintaining k<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and Bode Q values comparable to our previous reports.