A method for forming a semiconductor device includes receiving a first bonded to a second substrate by a dielectric layer, wherein a conductive layer is disposed in the dielectric layer and a cavity is formed between the first substrate, the second substrate and the dielectric layer; forming a via opening in the second substrate to expose the conductive layer and a vent hole in the substrate to couple to the cavity; forming a first buffer layer covering sidewalls of the via opening and a second buffer layer covering sidewalls of the vent hole; and forming a connecting structure in the via opening and a sealing structure to seal the vent hole.