Transversely-excited film bulk acoustic resonator with etch-stop layer

A filter device, comprising:

• a substrate having a surface;
• a single-crystal piezoelectric plate having front and back surfaces;
• an etch-stop layer sandwiched between the surface of the substrate and the back surface of the single-crystal piezoelectric plate, portions of the single-crystal piezoelectric plate and the etch-stop layer forming two or more diaphragms spanning respective cavities in the substrate;
• a conductor pattern formed on the front surface, the conductor pattern including a plurality of interdigital transducers (IDTs) of a respective plurality of acoustic resonators including a shunt resonator and a series resonator, interleaved fingers of each of the plurality of IDTs disposed on a respective diaphragm of the two or more diaphragms;
• and
• a frequency setting dielectric layer disposed on the front surface between the fingers of the IDT of the shunt resonator, wherein
• the etch-stop layer is impervious to an etch process used to form the cavities.

A method of fabricating an acoustic resonator device comprising:

• forming an aluminum oxide etch-stop layer sandwiched between a surface of a device substrate and a first surface of a single-crystal piezoelectric plate having a second surface attached to a sacrificial substrate;
• removing the sacrificial substrate to expose the second surface of the single-crystal piezoelectric plate;
• using an etch process to form a cavity in the device substrate, a portion of the single-crystal piezoelectric plate and the aluminum oxide etch-stop layer forming a diaphragm spanning the cavity;
• and
• forming an interdigital transducer (IDT) on the second surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm, wherein
• the aluminum oxide etch-stop layer is impervious to the etch process used to form the cavity.