A method for fabricating a semiconductor device includes the steps of forming an opening defined by an inner wall surface in an insulation film, forming a Cu—Mn alloy layer in the opening, depositing a Cu layer on the Cu—Mn alloy layer and filling the opening with the Cu layer, and forming a barrier layer as a result of reaction between Mn atoms in the Cu—Mn alloy layer and the insulation film, wherein the step of forming the barrier layer is conducted by exposing the Cu layer to an ambient that forms a gaseous reaction product when reacted with Mn.