A method of forming a mask layout includes forming a layout of a first mask including a lower wiring structure pattern and a dummy lower wiring structure pattern. A layout of a second mask overlapping the first mask and including an upper wiring structure pattern and a dummy upper wiring structure pattern is formed. A layout of a third mask including a first via structure pattern and a first dummy via structure pattern is formed. A layout of a fourth mask including a second via structure pattern and a second dummy via structure pattern is formed. The second via structure pattern may commonly overlap the lower wiring structure pattern and the upper wiring structure pattern, and the second dummy via structure pattern may commonly overlap the dummy lower wiring structure pattern and the dummy upper wiring structure pattern. The fourth mask may overlap the third mask.