Provided is an apparatus for fabricating a semiconductor device. The apparatus includes a first photomask and a second photomask. The first photomask has a plurality of first features thereon, and the first photomask having a first global pattern density. The second photomask has a plurality of second features thereon, and the second photomask has a second global pattern density. The plurality of first and second features collectively define a layout image of a layer of the semiconductor device. The first and second global pattern densities have a predetermined ratio.