A semiconductor device and a method of forming the same, the semiconductor device including an insulating structure having an opening; a conductive pattern disposed in the opening; a barrier structure covering a bottom surface of the conductive pattern, the barrier structure extending between the conductive pattern and side walls of the opening; and a nucleation structure disposed between the conductive pattern and the barrier structure. The nucleation structure includes a first nucleation layer that contacts the barrier structure, and a second nucleation layer that contacts the conductive pattern, and a top end portion of the second nucleation layer is higher than a top end portion of the first nucleation layer.