A semiconductor device manufacturing method of the this invention having the step of forming an interlayer insulating film on a semiconductor substrate, the step of making interconnection groove in the interlayer insulating film, the step of filling the inside of the interconnection groove with a conductive film which is made of a first substance and is thicker than the depth of the interconnection groove, the step of thermally stabilizing the size of crystal grains in an Al film either at the same time or after the Al film has been formed, the step of forming a Cu film on the Al film, the step of selectively forming null phase layers in a crystal grain boundary of the Al film by causing Cu to selectively diffuse into the crystal grain boundary of Al film and of allowing the null phase layers to divide the Al film in the interconnection groove into fine Al interconnections shorter than the Blech critical length, and the step of removing the Al film and Cu film outside the interconnection groove.