A suppression method of second order mode spurious responses for boundary acoustic wave filters

In this paper, we describe a suppression method of spurious responses for boundary acoustic wave filters, which employ a shear-horizontal type boundary acoustic wave in SiN/SiO₂/electrodes/LiNbO₃ structure. In this structure, the second order mode of boundary acoustic wave is excited strongly and causes a spurious response which deteriorates out-of-band attenuation. We studied the excitation mechanism of the second order mode and found that the intensity of the second order mode depends highly on the relation between the acoustic velocity of the second order mode and the fast shear bulk wave velocity of LiNbO₃. Moreover, we found that the change of the propagation direction from the X-axis on rotated V-cut LiNbO₃ makes the fast shear bulk wave velocity lower. As a combination of these results, a new suppression method of the spurious responses was proposed. Using this method, a boundary acoustic wave filter without spurious responses was successfully realized.