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 .