In a method of manufacturing a metal wiring structure, a first metal wiring and a first barrier layer are formed on a substrate, and the first barrier layer is nitridated. An insulating interlayer is formed on the substrate so as to extend over the first metal wiring and the first barrier layer. Part of the insulating interlayer is removed to form a hole exposing at least part of the first metal wiring and part of the first barrier layer. A nitridation plasma treatment is performed on the exposed portion of the first barrier layer. A second barrier layer is formed along the bottom and sides of the hole. A plug is formed on the second barrier layer to fill the hole.