Method of forming flat wiring layer

A method of fabricating a semiconductor device, comprising the steps of:

• a) providing a substrate having an insulating layer thereon;
• b) forming a connection hole including a first sub-hole and a second sub-hole mutually aligned and connected in the insulating layer, wherein the first sub-hole has a first diameter and the second sub-hole has a second diameter larger than the first diameter;
• c) forming a first conductive layer over the substrate;
• d) removing the first conductive layer to a thickness so as to form a side wall spacer film on a side wall of the second sub-hole and a plug film in the first sub-hole from the first conductive layer, respectively, wherein the plug film has a seam therein;
• e) forming a barrier metal layer over the substrate to cover the side wall spacer and the plug film;
• f) forming a second conductive layer over the substrate to fill space in the connection hole;
• and
• g) planarizing the second conductive layer to provide a damascene structure in the connection hole.