A method for fabricating a semiconductor device includes: providing a substrate structure in which a plurality of gate lines are already formed; forming a capping layer over the substrate structure; oxidizing the capping layer; and forming an insulation layer over the oxidized capping layer. The capping layer may include a nitride-based material. The insulation layer may include substantially the same material as the capping layer. The oxidizing of the capping layer may comprise performing a radical oxidation process.