A method for fabricating a semiconductor device includes forming a plurality of semiconductor structures over a substrate, forming an interlayer dielectric layer over the semiconductor structures, etching the interlayer dielectric layer, and defining open parts between the semiconductor structures to expose a surface of the substrate, forming sacrificial spacers on sidewalls of the open parts, forming conductive layer patterns in the open parts, and causing the conductive layer patterns and the sacrificial spacers to reach each other, and defining air gaps on the sidewalls of the open parts.