At the highest level, the model consists of a scalar propagator that handles the difraction from the mask to the pupil, and a vector propagator that handles the propagation of light in the vicinity of the edges of the ∼ 2 micron thick sidewalls of the mask. The vector model results in two orthogonally polarized felds that are combined with the scalar feld as described below. Once the light is propagated to the pupil, a simple Fast Fourier Transform algorithm is used to propagate the light to the
image plane.