Use of periodic trenches in SMR-type XBAR for suppression of transverse mode resonances and lateral leakage

Abstract

This paper discusses the use of periodic trenches to the top SiO2 layer of the Bragg reflector in solidly-mounted-resonator type laterally-excited-bulk-wave-resonators for the slowness manipulation to suppress both transverse mode resonances and lateral leakage. First, it is shown that the SAW slowness surface can be flattened by the application of periodic trenches, and this is owing to the coupling of the propagating SAW mode with resonances of periodic trench walls. It is also shown that flattening the SAW slowness surface enables us to suppress transverse mode resonances well. However, its impact on lateral leakage is not obvious. Then, the broadband piston mode design is applied, and its effectiveness is demonstrated for the suppression of both transverse mode resonances and lateral leakage. Deposition of a dielectric layer to the gap regions is also proposed as an alternative for leakage suppression, and it is shown that Si3N4 is appropriate as a dielectric material.