sequentially stacking a first electrode, a first piezoelectric layer, a second electrode, an insulating layer, a third electrode, a second piezoelectric layer, and a fourth electrode on a surface of a substrate;

sequentially patterning the fourth electrode, the second piezoelectric layer, the third electrode, the insulating layer, the second electrode, the first piezoelectric layer, and the first electrode to expose first predetermined areas of the first, second, and third electrodes;

forming a plurality of connection electrodes respectively connected to the first predetermined areas of the first, second, and third electrodes and a second predetermined area of the fourth electrode; and

etching a third predetermined area of the substrate underneath the first electrode to form an air gap.