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, wherein the slit is communicated with the back chamber;

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 and a second part, wherein the first part is lay on the second electrode layer and covers the slit, and the second part is lay on the metal pad, a through slot is formed penetrating through the second part and corresponding to the metal pad, and the metal pad is exposed by the through slot.