Transversely-excited film bulk acoustic resonators with improved edge effects

A filter device comprising:

• a piezoelectric layer having front and back surfaces;
• a conductor pattern on the front surface, the conductor pattern comprising a plurality of interdigital transducers (IDTs) of a respective plurality of resonators, each of the plurality of IDTs comprising interleaved fingers extending from opposing busbars, an overlapping distance of the interleaved fingers of each of the plurality of IDTs defining an aperture of a respective resonator of the plurality of resonators,
• wherein at least one resonator of the plurality of resonators comprises a modified acoustic velocity region proximate an edge of the aperture, and
• wherein the at least one resonator comprises a dielectric layer on the piezoelectric layer and in the modified acoustic velocity region, the dielectric layer being thinner in the modified acoustic velocity region than in a central portion of the aperture of the at least one resonator,
• wherein the at least one resonator further comprises a slow region adjacent the modified acoustic velocity region and proximate the edge of the aperture, and a strip of dielectric material on the piezoelectric layer over a portion of the interleaved fingers in the slow region,
• wherein the IDT of the at least one resonator further comprises at least one dummy finger in the modified acoustic velocity region between at least one interleaved finger of the interleaved fingers and one busbar of the opposing busbars, wherein the at least one dummy finger extends from the one busbar of the opposing busbars, and wherein a length of the at least one dummy finger corresponds to a distance between the at least one dummy finger and the at least one interleaved finger,
• wherein, for the at least one resonator, the interleaved fingers are disposed at a first portion of the piezoelectric layer and each of the opposing busbars are disposed at a second portion of the piezoelectric layer, and
• wherein, for the at least one resonator, a device thickness of the modified acoustic velocity region is thinner than device thicknesses of each of the first portion and the second portion.

A method of fabricating an acoustic resonator device comprising:

• forming an interdigital transducer (IDT) on a front side of a piezoelectric layer, the IDT comprising interleaved fingers extending from opposing busbars, an overlapping distance of the interleaved fingers defining an aperture of an acoustic resonator device;
• forming a modified acoustic velocity region proximate an edge of the aperture;
• forming a dielectric layer on the piezoelectric layer and in the modified acoustic velocity region;
• and
• forming a slow region adjacent the modified acoustic velocity region and proximate the edge of the aperture, and a strip of dielectric material on the piezoelectric layer over a portion of the interleaved fingers in the slow region,
• wherein the dielectric layer is thinner in the modified acoustic velocity region than in a central portion of the aperture of the acoustic resonator device,
• wherein the IDT is formed to further comprise at least one dummy finger in the modified acoustic velocity region between at least one interleaved finger of the interleaved fingers and one busbar of the opposing busbars, wherein the at least one dummy finger extends from the one busbar of the opposing busbars, and wherein a length of the at least one dummy finger corresponds to a distance between the at least one dummy finger and the at least one interleaved finger,
• wherein the interleaved fingers are disposed at a first portion of the piezoelectric layer and each of the opposing busbars are disposed at a second portion of the piezoelectric layer, and
• wherein a device thickness of the modified acoustic velocity region is thinner than device thicknesses of each of the first portion and the second portion.