An etching target layer is formed on a substrate. An upper mask layer is formed on the etching target layer. A plurality of preliminary mask patterns is formed on the upper mask layer. The plurality of preliminary mask patterns is arranged at a first pitch. Two neighboring preliminary mask patterns of the plurality of preliminary mask patterns define a preliminary opening. An ion beam etching process is performed on the upper mask layer using the plurality of preliminary mask patterns as an etch mask to form a first preliminary-interim-mask pattern and a pair of second preliminary-interim-mask patterns. The first preliminary-interim-mask pattern is formed between one of the pair of second preliminary-interim-mask patterns and the other of the pair of second preliminary-interim-mask patterns.