A method for forming a semiconductor device includes receiving a substrate having a first opening and a second opening formed thereon, wherein the first opening has a first width, and the second opening has a second width less than the first width; forming a protecting layer to cover the first opening and expose the second opening; performing a wet etching to widen the second opening with an etchant, wherein the second opening has a third width after the performing of the wet etching, and the third width of the second opening is substantially equal to the first width of the first opening; and performing a photolithography to transfer the first opening and the second opening to a target layer.