A first mask layer pattern including a plurality of parallel line portions is formed on an etch target layer on a semiconductor substrate. A sacrificial layer is formed on the first mask layer pattern and portions of the etch target layer between the parallel line portions of the first mask layer pattern. A second mask layer pattern is formed on the sacrificial layer, the second mask layer pattern including respective parallel lines disposed between respective adjacent ones of the parallel line portions of the first mask layer pattern, wherein adjacent line portions of the first mask layer pattern and the second mask layer pattern are separated by the sacrificial layer. A third mask layer pattern is formed including first and second portions covering respective first and second ends of the line portions of the first mask layer pattern and the second mask layer pattern and having an opening at the line portions of the first and second mask layer patterns between the first and second ends. The sacrificial layer and the etch target layer are etched using the third mask layer pattern, the first mask layer pattern and the second mask layer pattern as a mask to thereby form a plurality of parallel trenches in the etch target layer between the line portions of the first and second mask layer patterns. Conductive lines may be formed in the trenches.