An alternating phase-shifting mask (Alt-PSM) comprising a 0° phase portion having a first width and a 180° phase portion having a second width greater than the first width. Example differences between the width of the 180° phase portion and the 0° phase portion may be 10 nm, 15 nm, or 20 nm. An Alt-PSM having phase portions of different widths can have an aerial image intensity transmission graph that is symmetric, for example, at 0.2-0.3 intensity.