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 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 layer, the gate dielectric layer, the charge storage layer using the patterned metal gate 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 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.