The demand for steadily decreasing dimensions in semiconductor devices is driving the need for increased resolution in optical lithography. The use of phase shift masks (PSMs) is among such resolution enhancement techniques. PSM are well known to show prominent diffraction effects, which cannot be described by the assumption of an infinitely thin mask (Kirchhoff approach) that is used in many commercial photolithography simulators. A correct prediction of sidelobe printability, process window and linearity of an OPC mask requires a rigorous application of diffraction theory. Optical lithography simulation employing a time-domain finite-difference (TDFD) algorithm (TEMPEST) has been used effectively to study the problem of aerial image intensity imbalance through focus with alternating phase shift masks (altPSMs). Using Geometrical Theory of Diffraction (GTD), and the solutions to canonical problems, we obtained a relationship between mask edge and disturbance in an image space. The main objective to develop useful formulations that can be readily applied to diffraction in mask technology. Rigorous analysis of diffraction effect on altPSMs using the GTD, approach is discussed and results show ' that the effect of shifter edge angle is equivalent to that of shifter width. (c) 2005.