The shrinking of design rule requires the short wavelength light used in the optical inspection system. However, the
existence of the condition that the long wavelength light becomes effective for the defect detection in line/space structure
is known. Calculation results using numerical simulation showed that the probe light can penetrate to the line/space
structure depending on the polarization even though the light has long wavelength. A new model was introduced to make
theoretical explanation of this abnormal behavior of long wavelength light and the mechanism of optical penetration was
clarified. In this model, the averaged extinction coefficient was calculated in consideration of the wavelength and the
period of the line/space structure. Using this model, the transmittance was calculated and compared with simulations.
The fact that the calculation result is agreed with the simulations showed this model's utility. This result shows that the
probe light can reach to the bottom of line/space structure in the inspection system for semiconductor devices even
though the light has long wavelength. It means that the long wavelength light can be used effectively for the defect
detection of the micro periodic structure in the semiconductor inspection system.