Semiconductor device with recessed gate and method for fabricating the same

A method for fabricating a semiconductor device, comprising:

• providing a substrate comprising a first peripheral region and a second peripheral region;
• forming a bottom hard mask layer on the substrate;
• forming a mandrel layer on the bottom hard mask layer and above the first peripheral region;
• conformally forming a layer of spacer material on the bottom hard mask layer and covering the mandrel layer;
• performing a spacer etching process to turn the layer of spacer material into a plurality of sacrificial spacers on sides of the mandrel layer;
• forming an under layer on the bottom hard mask layer and covering the mandrel layer and the plurality of sacrificial spacers;
• recessing the under layer to expose the mandrel layer and the plurality of sacrificial spacers;
• selectively removing the plurality of sacrificial spacers to form a plurality of openings exposing the bottom hard mask layer;
• deepening the plurality of openings to expose the first peripheral region of the substrate;
• forming a plurality of gate recesses in the first peripheral region;
• and
• forming a plurality of recessed gates on the plurality of gate recesses, forming a plurality of sidewall spacers covering sidewalls of each of the recessed gates, and forming an air gap between pairs of the sidewall spacers.