Phase-shifting masks (PSM) achieve improved wafer printability with higher resolution and increased DOF (Depth of Focus), by controlling the phase shift and the transmission rate of the light transmission through the photomask. This is a standard technology for lithography in which line width being printed is smaller than the exposure wave length. Most well known PSM photomasks are called Halftone masks (Attenuated PSM) and Levenson masks (Alternating PSM).
