a substrate including a first silicon layer, a first oxide layer and a second silicon layer sequentially stacked from bottom to top, wherein a back chamber is formed in the substrate, the back chamber sequentially penetrates through the first silicon layer and the first oxide layer, and the second silicon layer is exposed by the back chamber;

a second oxide layer formed on the substrate;

a piezoelectric unit formed on the second oxide layer and including a first electrode layer, a piezoelectric layer and a second electrode layer sequentially stacked from bottom to top;

a slit formed in the middle of the second electrode layer and penetrating through the second electrode layer, the piezoelectric layer, the first electrode layer, the second oxide layer and the second silicon layer;

an opening formed at an edge of the second electrode layer and penetrating through the second electrode layer and the piezoelectric layer, wherein the first electrode layer is exposed by the opening;

a metal pad stacked on the first electrode layer at the opening; and

an additional film layer including a first part, a second part and a third part, wherein the first part is formed in the slit, a side wall of the first part is attached to an inner wall surface of the slit, a bottom wall of the first part covers a bottom opening of the slit, and the side wall and the bottom wall of the first part surround to form a groove having a top opening, the second part is formed on the second electrode layer, and the third part is formed in the opening and wraps the metal pad, a through slot is formed penetrating through the third part and corresponding to the metal pad, and the metal pad is exposed by the through slot.