A method of forming a target pattern includes forming a plurality of lines over a substrate with a first mask and forming a first spacer layer over the substrate, over the plurality of lines, and onto sidewalls of the plurality of lines. The plurality of lines is removed, thereby providing a patterned first spacer layer over the substrate. The method further includes forming a second spacer layer over the substrate, over the patterned first spacer layer, and onto sidewalls of the patterned first spacer layer, and forming a patterned material layer over the second spacer layer with a second mask. Whereby, the patterned material layer and the second spacer layer collectively define a plurality of trenches.