In a method, a dummy gate layer structure and a mask layer are formed on a substrate. The mask layer is patterned to form masks. Spacers are formed on sidewalls of the mask. A dummy gate mask is formed between the spacers. The dummy gate layer structure is patterned using the dummy gate mask to form dummy gate structures. The dummy gate structure is replaced with a gate structure. When the mask is formed, an initial layout of masks extending in a first direction is designed. An offset bias in a second direction is provided for a specific region of the initial layout to design a final layout having a width in the second direction varying along the first direction. The mask layer is patterned according to the final layout to form the masks having a width varying along the first direction.