A gate structure is formed on a substrate. An insulating interlayer is formed covering the gate structure. The substrate is heat treated while exposing a surface of the insulating interlayer to a hydrogen gas atmosphere. A silicon nitride layer is formed directly on the interlayer insulating layer after the heat treatment and a metal wiring is formed on the insulating interlayer. The metal wiring may include copper. Heat treating the substrate while exposing a surface of the interlayer insulating layer to a hydrogen gas atmosphere may be preceded by forming a plug through the first insulating interlayer that contacts the substrate, and the metal wiring may be electrically connected to the plug. The plug may include tungsten.