Metal cavity for transversely-excited film bulk acoustic resonator (XBAR)

An acoustic resonator comprising:

• a plurality of series resonators comprising:
• a first piezoelectric layer that includes a portion that is over a cavity in a metal layer over a substrate;
• a first interdigital transducer on a surface of the first piezoelectric layer, the first interdigital transducer including interleaved fingers on the portion of the first piezoelectric layer that is over the cavity in the metal layer;
• and
• a metal-to-metal bond configured to bond the first piezoelectric layer to the metal layer such that the interleaved fingers of the first IDT are disposed over the cavity;
• and
• a plurality of shunt resonators comprising:
• a second piezoelectric layer that includes a portion that is over a cavity in a metal layer over a substrate, the second piezoelectric layer being different than the first piezoelectric layer;
• a second interdigital transducer on a surface of the second piezoelectric layer, the second interdigital transducer including interleaved fingers on the portion of the second piezoelectric layer that is over the cavity in the metal layer;
• and
• a metal-to-metal bond configured to bond the second piezoelectric layer to the metal layer such that the interleaved fingers of the second IDT are disposed over the cavity,
• wherein the plurality of series resonators and the plurality of shunt resonators configure a band-pass filter.

An acoustic resonator device comprising:

• a piezoelectric layer having front and back surfaces, the back surface attached to a first metal layer having a first cavity except for a portion of the piezoelectric layer that forms a diaphragm that is over the first cavity;
• a substrate having front and back surfaces, the front surface of the substrate attached to a second metal layer having a second cavity except for a portion of the substrate that spans the second cavity;
• and
• an interdigital transducer (IDT) on the front surface of the piezoelectric layer such that interleaved fingers of the IDT are disposed on the diaphragm.