Resonators with different membrane thicknesses on the same die

A filter system comprising:

• a first bulk acoustic resonator on a substrate comprising:
• a composite piezoelectric wafer attached to the substrate either directly or via one or more intermediate layers, the composite piezoelectric wafer comprising a first piezoelectric layer and a second piezoelectric layer;
• a first interdigital transducer (IDT) on a surface of the composite piezoelectric wafer;
• and
• a second bulk acoustic resonator on the substrate comprising:
• a second IDT over the second piezoelectric layer but not the first piezoelectric layer.

A filter device comprising:

• a substrate;
• a first piezoelectric layer disposed above the substrate and over at least a first cavity and a second cavity;
• a bonding layer on the first piezoelectric layer and opposite the substrate, the bonding layer over the first cavity and the second cavity;
• and
• a second piezoelectric layer on the bonding layer opposite the first piezoelectric layer, the second piezoelectric layer over the first cavity but not the second cavity;
• a first interdigital transducer (IDT) over the second piezoelectric layer and over the first cavity;
• and
• a second IDT over the bonding layer and the first piezoelectric layer and over the second cavity.