Method of forming metal wiring

The invention relates to a method of forming a metal layer (for instance, a buried copper (Cu) layer). In a method of manufacturing a semiconductor device, a semiconductor substrate including an insulating layer is provided. A groove is formed on the insulating layer. An additive-containing barrier layer is formed on the insulating layer. A metal seed layer and a metal layer are formed on the barrier layer. Then, the metal layer is subjected to a first heat treatment at a first temperature that is capable of promoting grain growth of the metal seed layer and the metal layer. The barrier layer, the metal seed layer and the metal layer are partially removed so that a conductive layer including the metal seed layer and the metal layer is formed in the groove. Finally, the conductive layer is subjected to a second heat treatment at a second temperature that is higher than the first temperature and allows an additive element in the barrier layer to diffuse into the metal layer.