The simulation of optical defect detection on wafers by microscopy requires several approximations. For the simulation with Fourier modal methods, the number of modes is limited by memory and time consumption. Furthermore, the illumination pupil has to be divided into discrete incidence angles for plane waves. We present a way to save the computation of most of these incidence angles. It works only for certain configurations of grating period and illumination wavelength but is an accurate and robust approximation in these cases. We present sample calculations for one- and two-dimensional periodic structures with defects.
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