Sandwiched XBAR for third harmonic operation

An acoustic resonator comprising:

• a back surface of a first piezoelectric plate bonded to a top surface of a substrate comprised of a plurality of layers and having at least a first cavity;
• a planarized top surface of the first piezoelectric plate forming a first thickness of the first piezoelectric plate with the back surface of the first piezoelectric plate;
• the first piezoelectric plate spanning the first cavity;
• a first interdigital transducer (IDT) on a front surface of the first piezoelectric plate that is opposite the substrate and having interleaved fingers over the first cavity;
• a first dielectric layer on a top surface of the first piezoelectric plate and covering the first IDT;
• a back surface of a second piezoelectric plate bonded to a top surface of the first dielectric layer that is opposite the first piezoelectric plate;
• and
• a second dielectric layer coupled to a top surface of the second piezoelectric plate that is opposite the back surface without any electrodes therebetween,
• wherein the second piezoelectric plate spans the first cavity.

An acoustic filter device comprising:

• a plurality of acoustic resonators connected in a ladder filter circuit, each acoustic resonator comprising:
• a first single-crystal piezoelectric membrane;
• a first dielectric layer on a surface of the first single-crystal piezoelectric membrane;
• an interdigital transducer (IDT) embedded in the first dielectric layer;
• a second single-crystal piezoelectric membrane bonded to the first dielectric layer;
• and
• a second dielectric layer on a surface of the second single-crystal piezoelectric membrane that is opposite the IDT without any electrodes between the second dielectric layer and the second single-crystal piezoelectric membrane.