the device isolation insulating film is planarized until the semiconductor substrate 310 is exposed to form a device isolation region 330 fig4 is a cross sectional diagram illustrating a semiconductor device according to an embodiment of the present invention fig4 i is a cross sectional diagram taken along i i u2032 of fig1 and fig4 ii is a cross sectional diagram taken along ii ii u2032 of fig1 in comparison with fig2 g a liner nitride film 420 is not formed on trench sidewalls 416 i e sidewalls located at both sides of the gate region 104 in a second direction intersected by the first direction that is in one embodiment of the present invention a liner nitride film is formed only on the sidewalls 416 in the first direction the semiconductor device of fig4 may be fabricated by the method of fig2 a to 2 h except that a mask pattern is formed to expose the liner nitride film 420 not in the first direction but in the second direction fig5 is a cross sectional diagram illustrating a semiconductor device according to an embodiment of the present invention fig5 i is a cross sectional diagram taken along i i u2032 of fig1 and fig5 ii is a cross sectional diagram taken along ii ii u2032 of fig1 in comparison with fig3 c a liner nitride film 420 is not formed on one of trench sidewalls 516 i e sidewalls located at both sides of the gate region 104 in a second direction the liner nitride film is not formed on one of the trench sidewalls 516 in the first direction or the second direction for example a liner nitride film may.