A surface acoustic wave (SAW) filter includes a bottom substrate, a piezoelectric layer disposed above the bottom substrate, the piezoelectric layer having a bottom surface facing the bottom substrate and a top surface opposite to the bottom surface, a lower cavity disposed below the piezoelectric layer, an interdigital transducer (IDT) disposed on the top surface of the piezoelectric layer, and a back electrode disposed on the bottom surface of the piezoelectric layer, and exposed in the lower cavity.