Method of forming a semiconductor device

A method of forming a semiconductor device, the method comprising:

• forming active regions and isolation regions in a semiconductor substrate, the active regions and isolation regions extending in line patterns in a first direction, the active regions and isolation regions being alternately aligned in a second direction across the first direction;
• forming gate electrode grooves in the semiconductor substrate, the gate electrode grooves extending in the second direction and across the active regions and isolation regions;
• forming gate insulating films on inside walls of the gate electrode grooves;
• forming buried gate electrodes on the gate insulating films and in lower portions of the gate electrode grooves;
• forming cap insulating films in upper portions of the gate electrode grooves, the cap insulating films covering the buried gate electrodes;
• forming a first interlayer insulating film on an upper surface of the cap insulating film and on an upper surface of the semiconductor substrate;
• planarizing an upper surface of the first interlayer insulating film;
• and
• forming openings in line shape in the first interlayer insulating film, the openings exposing first active regions and the isolation regions, the first active regions being adjacent to the isolation regions in the second direction, the first active regions being for formation of bit line contacts,
• wherein forming the buried gate electrodes comprises:
• forming a silicon oxide film over the semiconductor substrate having the isolation regions;
• forming a silicon nitride film on the silicon oxide film;
• forming a carbon film on the silicon nitride film;
• patterning a stack of the silicon oxide film, the silicon nitride film, and the carbon film, to form hard masks, the hard masks extending in the second direction, the hard masks extending over the isolation region and the active region;
• selectively etching the isolation region and the active region by using the hard masks to form gate electrode grooves, the gate electrode grooves being in line-shape, the gate electrode grooves extending in the second direction;
• forming a gate insulating film which covers at least inside walls of the gate electrode grooves;
• forming a gate electrode material on the gate insulating film, the gate electrode material filling the gate electrode grooves;
• and
• etching back the gate electrode material to form buried gate electrodes in lower portions of the gate electrode grooves.