Optimization of LN/SiO₂/LN Structures for Resonators with Improved Thermal Stability Utilizing High-Order Plate Modes

In response to the congestion of the Wi-Fi spectrum, multilayered stacks composed of LiNbO₃ (LN) and SiO₂ thin layers have gained attention as materials for acoustic resonators with high-quality factors and improved thermal stability required for radio-frequency (RF) filters with enhanced selectivity. Electromechanical coupling exceeding 20% and a temperature coefficient of frequency (TCF) of -28 ppm/°C have been recently reported for mode A3 laterally excited in an experimental tri-layered LN/SiO₂/LN structure with equal layer thicknesses. In this study, higher-order modes propagating in LN/SiO₂/LN stacks with variable thickness ratios were investigated to clarify the maximum achievable coupling and identify the optimal structures offering high coupling and low TCF. The analytical model recently proposed for the analysis of high-order modes in aperiodic structures with polarity-inverted layers (APPF) was extended to resonators incorporating dissimilar middle layers. It was revealed that the frequency-versus-thickness-ratio dependence oscillates about a flat line with a period dependent on the mode order. This dependence was confirmed through numerical simulations. Owing to the frequency oscillations, the coupling achievable for the A5, A7, and A9 modes in the LN/SiO₂/LN structures is comparable to or exceeds the maximum coupling in the APPF structures, and a near-zero TCF can be achieved simultaneously at resonance and TCF_R=TCF_A=-3 ppm/C for the A5 mode. The optimal structures found for A5 and A7 modes are most promising for RF filters with good thermal stability operating at cm-Wave frequencies.