A semiconductor device includes a first bit line and a second bit line disposed over a semiconductor substrate, and a dielectric structure disposed over a sidewall of the first bit line. The first bit line is between the second bit line and the dielectric structure, and the first bit line is separated from the second bit line by an air gap. A method for preparing a semiconductor device includes forming a first dielectric structure and a second dielectric structure over a semiconductor substrate, and forming a conductive material over the first and the second dielectric structures. The conductive material extends into a first opening between the first and the second dielectric structures. The method also includes partially removing the conductive material to form a first bit line and a second bit line in the first opening.