An integrated circuit device includes: a substrate including active regions; a device isolation film defining the active regions; a word line arranged over the active regions and the device isolation film and extending in a first horizontal direction; and a gate dielectric film arranged between the substrate and the word line and between the device isolation film and the word line, in which, in a second horizontal direction orthogonal to the first horizontal direction, a width of a second portion of the word line over the device isolation film is greater than a width of a first portion of the word line over the active regions. To manufacture the integrated circuit device, an impurity region is formed in the substrate and the device isolation film by implanting dopant ions into the substrate and the device isolation film, and a thickness of a portion of the impurity region is reduced.