In the era of sub-30nm devices, the size of the defects on semiconductor wafer has already exceeded the resolution limit
of optic microscope, but we still can't help using optical inspection tools. Therefore, the contrast enhancement technique
is more useful rather than the resolution itself. The best contrast can be taken by the optimized light conditions such as
wavelength, polarization, incidence angle and so on. However these kinds of parameters are not easily estimated
intuitively because they are strongly dependent on the pattern structures and materials. In this paper, we propose a
simulation methodology to find those optic conditions to detect sub 20nm defect. The simulation is based on FDTD
(Finite Difference Time Domain) calculation and Fourier optics.