A method of manufacturing a semiconductor device includes forming a first dielectric layer and a through hole passing through the first dielectric layer over a substrate; forming a plurality of dummy contacts in the through hole; forming a plurality of first dummy wires on the plurality of dummy contacts; filling a second dielectric layer between the plurality of first dummy wires, wherein the second dielectric layer has a first air gap; removing the dummy contacts and the first dummy wires to expose the through hole, thereby forming a first wiring trench over the through hole; and forming a contact and a first wire in the through hole and the first wiring trench.