A method for forming a semiconductor device includes forming a metal gate stack having a gate dielectric layer and a gate electrode disposed over the gate dielectric layer. The gate electrode includes a first metal layer and a second metal layer. The method further includes performing a plasma treatment to a top surface of the metal gate stack and forming a conductive layer over the treated top surface of the metal gate stack. A top portion of the conductive layer is formed above a top surface of the gate dielectric layer, and a bottom portion of the conductive layer penetrates into the first and the second metal layers of the gate electrode at different distances.