Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, the method comprising:

• forming a pad oxide film and a pad nitride film over a semiconductor substrate;
• etching the semiconductor substrate with the pad oxide and nitride films as an etching mask to form a trench that defines an active region;
• filling the trench with an insulating film and planarizing the insulating film;
• etching the pad nitride film and the insulating film, wherein a region the pad oxide film that does not include a recess gate region is exposed;
• forming a first hard mask layer over the exposed region of the pad oxide film;
• removing the residual pad nitride film and etching the pad oxide film with the first hard mask layer and the insulating film as an etching mask to expose the semiconductor substrate of the recess gate region;
• etching the exposed semiconductor substrate at a predetermined thickness to form a first recess;
• forming a sidewall insulating film over a sidewall of the pad oxide film, the first hard mask layer, and the first recess;
• isotropic-etching the bottom semiconductor substrate of the first recess with the sidewall insulating film as a mask to form a resulting structure including a second recess having a circular or elliptical shape;
• removing the first hard mask layer and the pad oxide film to form a gate oxide film over the exposed semiconductor substrate;
• forming a gate conductive layer over the resulting structure that fills the second recess;
• forming a gate hard mask layer over the gate conductive layer;
• and
• patterning the gate hard mask layer and the gate conductive layer with a gate etching mask to form a gate.