1. A method for designing an alternating phase shifting mask (altPSM) for projecting an image on an image plane, the method comprising: providing a circuit layout; identifying a critical element of said circuit layout, said critical element having a layout dimension (LW), said layout dimension corresponding to a target image dimension in the image plane; providing a relationship between phase shape width and said target image dimension; selecting an optimal phase shape width so that said relationship has an optimal value; and generating a phase shape disposed adjacent to said layout dimension wherein said phase shape has said optimal phase shape width.