Transversely-excited film bulk acoustic resonator with multiple diaphragm thicknesses and fabrication method

A filter device, comprising:

• a substrate having a surface;
• a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate, portions of the single-crystal piezoelectric plate forming a first diaphragm and a second diaphragm spanning respective cavities in the substrate, wherein
• a first portion of the piezoelectric plate including the first diaphragm has a first thickness, and
• a front surface of a second portion of the piezoelectric plate, which includes the second diaphragm, is recessed relative to the front surface of the first portion of the piezoelectric plate such that the second diaphragm has a second thickness less than the first thickness;
• a first resonator comprising a first interdigital transducer (IDT) with interleaved fingers on a front surface of the first diaphragm;
• and
• a second resonator comprising a second IDT with interleaved fingers on the front surface of the second diaphragm.