A semiconductor device includes a first insulating interlayer on a substrate, metal lines in the first insulating interlayer, a first air gap between the metal lines in a first region of the substrate and a second air gap between the first insulating interlayer and at least one of the metal lines in a second region of the substrate, a liner layer covering top surfaces and side walls of the metal lines and a top surface and a side wall of the first insulating interlayer, adjacent to the first and second air gaps, and a second insulating interlayer on the liner layer and contacting the liner layer.