A method for manufacturing a semiconductor device includes the following steps. At first, two gate stack layers are formed on a semiconductor substrate, wherein each of the gate stack layers includes a top surface and two side surfaces. A conductive material layer is deposited to conformally cover the top surface and the two side surfaces of each of the gate stack layers. Then, a cap layer is deposited to conformally cover the conductive material layer. Finally, the cap layer and the conductive material layer above the top surface of each of the gate stack layers are removed to leave the cap layer adjacent to the two side surfaces of each of the gate stack layers and covering a portion of the conductive material layer.