Resonators with different membrane thicknesses on the same die

A method of forming a filter device comprising:

• bonding a first piezoelectric plate to a substrate over a location in the substrate for a first cavity and a location in the substrate for a second cavity;
• forming a bonding layer on the first plate over the location for the first cavity and the location for the second cavity;
• bonding a second piezoelectric plate to the bonding layer over the location for the first cavity and the location for the second cavity;
• etching away a portion of the second piezoelectric plate spanning the location for the second cavity;
• forming a first interdigital transducer (IDT) on a front surface of the first piezoelectric plate over the location for the first cavity;
• and
• forming a second IDT on a front surface of the bonding layer over the location for the second cavity.

A method of forming a filter system comprising:

• bonding a first piezoelectric plate to a substrate;
• the first piezoelectric plate spanning a location in the substrate for a first cavity and a location in the substrate for a second cavity;
• forming a bonding layer on the first piezoelectric plate;
• the bonding layer spanning the location for the first cavity and the location for the second cavity;
• bonding a second piezoelectric plate to the bonding layer;
• the second piezoelectric plate spanning the location for the first cavity and the location for the second cavity;
• and
• etching away a portion of the second piezoelectric plate spanning the location for the second cavity.