A method for manufacturing a semiconductor device includes providing a substrate structure including a substrate, a high-k dielectric layer on the substrate, a capping layer on the high-k dielectric layer, forming a first N-type work function metal layer on the capping layer, forming a second N-type work function metal layer on the first N-type work function metal layer, and forming a metal electrode layer on the second N-type work function metal layer. The second N-type work function metal layer has a Ti/Al atomic ratio greater than the Ti/Al atomic ratio of the first N-type work function metal layer. The second work function metal layer having a higher Ti/Al atomic ratio will not absorb appreciable oxygen from the atmosphere, so that oxygen will not be available to the first work function metal layer, thereby reducing the oxidation level of the first work function metal layer.