Transversely-excited film bulk acoustic resonator with low thermal impedance

A method of forming a filter device with low thermal impedance, the method comprising:

• forming a bonding oxide (BOX) layer on a surface of a substrate;
• bonding a piezoelectric layer to the BOX layer;
• removing the piezoelectric layer and the BOX layer from at least a portion of the surface of the substrate;
• forming a conductor pattern at the piezoelectric layer and that contacts the portion of the surface of the substrate where the piezoelectric layer and BOX layer were removed;
• and
• forming a cavity, such that a portion of the piezoelectric layer forms a diaphragm over the cavity and the conductor pattern includes an interdigital transducer (IDT) with interleaved fingers disposed on the diaphragm over the cavity.