A new type of birefringence monitor based on phase shift masks is proposed and initially characterized for optical lithography using simulation of the electromagnetic artifacts and of the images that result. Birefringence is important in steppers at 193 nm due to its inherent existence in crystal structures. The technique developed in this article employs a set of crossed polarizers with one located in a pinhole within an opaque layer in the back side of the photomask and the other just above the image plane. The small amount of rotation due to birefringence in the projection optics will create a fairly small transmission in the presence of birefringence. The detection of the small amount of birefringence is greatly enhanced by providing a circularly polarized reference wave from a subresolution chromeless grating with which the birefringent component interferometrically interacts with. Simulation studies show a theoretical sensitivity of this technique of 1.3% of the clear field intensity per nanometer of birefringence, where the signal is dependent on fast axis orientation. Implementation of other variations of birefringence monitors is discussed using as a key enabler a chromeless grating of subresolution pitch in place of a rotating wave plate. (c) 2006 American Vacuum Society.