A semiconductor device and related method of manufacture are disclosed. The device comprises; a trench having a corner portion formed in the semiconductor substrate, a first oxide film formed on an inner wall of the trench and having an upper end portion exposing the corner portion of the semiconductor substrate, a nitride liner formed on the first oxide film, a second oxide film formed in contact with the upper end of the first oxide film and on the exposed corner portion and an upper surface of the semiconductor substrate, a field insulating film formed on the nitride liner to substantially fill the trench, and a field protecting film formed in contact with the second oxide film and filling a trench edge recess formed between the field insulating film and the second oxide film.