A method for manufacturing a semiconductor memory device comprises providing a substrate, etching a portion of the substrate that forms a trench therein, forming an element isolation film that fills the trench and defines an active area, herein the element isolation film includes a first liner that covers an inner sidewall and a bottom surface of the trench, wherein the first liner is recessed and exposes a corner portion of the substrate, doping nitrogen into the substrate, and forming a pre-gate insulating film that extends along and on the exposed corner portion of the substrate and an upper surface of the substrate. The pre-gate insulating film includes a first portion on the upper surface of the substrate, and a second portion on the corner portion of the substrate. A thickness of the first portion is less than a thickness of the second portion.