A mask is disclosed which includes a plurality of first phase shift regions disposed on a first side of the mask, and a plurality of second phase shift regions disposed on a second side of the mask. The first phase shift regions and second phase shift regions may be alternating phase shift regions in which phase shift of the first phase shift regions is out of phase, for instance by 180 degrees, from phase shift of the second phase shift regions. A method for forming the mask, and a semiconductor device fabrication method using the mask is also disclosed.