Semiconductor device including gate lines with a conductive layer having less nitrogen than an interface layer and method of fabricating the same

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

• forming a device isolation layer defining active regions on a semiconductor substrate;
• forming trenches intersecting the active regions;
• forming a gate insulating pattern on a sidewall and a bottom surface of the trenches;
• forming a conductive layer completely filling each of the trenches on the gate insulating pattern, and the conductive layer is a nitride of a first metal;
• performing an etch back process to recess the conductive layer to fill a lower portion of each of the trenches;
• forming a first interface layer adjacent to an outer surface and a lower surface of the conductive layer, the first interface layer including a nitride of the first metal;
• and
• forming a capping layer filling an upper portion of each of the trenches,
• wherein a nitrogen concentration in the first interface layer is higher than a nitrogen concentration in the conductive layer,
• wherein the forming of the first interface layer includes
• performing a heat treatment process on the conductive layer;
• and
• diffusing nitrogen atoms in the conductive layer toward the outer surface and the lower surface of the conductive layer through the heat treatment process, to form the first interface layer.