A method for manufacturing a semiconductor device includes providing a substrate structure having an active region, a gate insulating layer, a charge storage layer, a gate dielectric layer, and a gate electrode layer sequentially formed on the active region. The method also includes forming a patterned metal layer on the substrate structure, removing a respective portion of the gate electrode layer, the gate dielectric layer, the charge storage layer using the patterned metal gate electrode layer as a mask to form multiple gate structures separated from each other by a space. The gate structures each include a stack containing a second portion of the charge storage layer, the gate dielectric layer, the gate electrode layer, and one of the gate lines. The method further includes forming an interlayer dielectric layer on a surface of the gate structures stretching over the space while forming an air gap in the space.