A method for fabricating a semiconductor device is described. A gate dielectric layer is formed on a substrate. A plurality of gate structures are formed on the gate dielectric layer. Each of the gate structures is composed of a stacked structure and a spacer. Each stacked structure includes a gate conductive layer and a cap layer. The spacer includes a first dielectric layer and a second dielectric layer. A barrier layer is formed over the substrate covering conformally the gate structures and the gate dielectric layer. A dielectric layer is formed on the barrier layer. A self-aligned contact window etching process is conducted to form a contact window opening. A SEG process is conducted to grow an epitaxial silicon layer to form a contact window and an air gap in the opening.