Method of forming a semiconductor memory device

A method of forming a semiconductor memory device includes forming an etch target layer on a substrate, forming a sacrificial layer having preliminary openings on the etch target layer, forming assistance spacers in the preliminary openings, respectively, removing the sacrificial layer, such that the assistance spacers remain on the etch target layer, forming first mask spacers covering inner sidewalls of the assistance spacers, respectively, the first mask spacers respectively defining first openings, forming a second mask spacer covering outer sidewalls of the assistance spacers, the second mask spacer defining second openings between the first openings, the first and second openings being adjacent to each other along a first direction, and etching the etch target layer exposed by the first openings and the second openings to form holes in the etch target layer.