Semiconductor device having gate trenches and manufacturing method thereof

A method of manufacturing a semiconductor device comprising:

• forming STI trenches in a semiconductor substrate to define an active region extending in a first direction in the semiconductor substrate;
• burying the STI trenches with an insulating film;
• forming a first gate trench, a second gate trench, and a dummy gate trench in the active region;
• forming a first gate electrode, a second gate electrode, and a dummy gate electrode extending in a second direction crossing the first direction so that at least a part of the first gate electrode, the second gate electrode, and the dummy gate electrode are buried in the first gate trench, the second gate trench, and the dummy gate trench, respectively;
• and
• forming first and second diffusion layers in the active region on both sides of the first gate electrode, respectively so as to form a first transistor constituted by the first gate electrode and the first and second diffusion layers, and forming third and fourth diffusion layers in the active region on both sides of the second gate electrode, respectively so as to form a second transistor constituted by the second gate electrode and the third and fourth diffusion layers, wherein
• the dummy gate electrode is arranged between the second diffusion layer and the third diffusion layer, and electrically isolates the first transistor from the second transistor.