The present disclosure provides a mothed of method of manufacturing a semiconductor device. The method includes steps of forming a dielectric layer on a substrate; etching the dielectric layer to create a plurality of openings in the dielectric layer; applying a sacrificial layer in at least one of the openings to cover at least a portion of the dielectric layer; forming at least one first conductive feature in the openings where the sacrificial layer is disposed and a plurality of bases in the openings where the sacrificial layer is not disposed; removing the sacrificial layer to form at least one air gap in the dielectric layer; and forming a plurality of protrusions on the bases.