Analysis of piezoelectric boundary acoustic wave in Cu electrode/Y-cut X-propagating LiNbO3 substrate structure partially covered with SiO2

This paper describes the existence of piezoelectric boundary acoustic wave (PBAW) propagating in a Cu electrode/Y-cut X-propagating (YX) LiNbO₃ substrate structure partially covered with a SiO₂ layer. In the analysis, two types of structures are taken into consideration: one is the so-called slotted structure with SiO₂ pillars placed in the grating slots; the other is the so-called topped structure with SiO₂ pillars placed on the top of grating electrodes. The top surface could be fully covered with an additional layer (like epoxy) to bridge the grating slots for encapsulation. Results show that SH-type PBAW begins to propagate in the slotted structure when the SiO₂ thickness exceeds 0.3 wavelength. Strong electromechanical coupling factor K² of 21%, and temperature coefficient of velocity (TCV) of -33 ppm/°C are obtained. In the topped structure, on the other hand, the boundary acoustic wave mode is not supported. Instead, the thickness resonance modes in the SiO₂ pillar do exist. Comparison of the obtained results with those in the structure fully covered with the SiO₂ layer indicates that, as for the PBAW mode, the slotted structure offers improved K² but with worse TCV compared with the fully covered SiO₂ structure.