Mask defect inspection by detecting polarization variations

State-of-the-art lithography is often severely influenced by defects that are smaller than the resolution limit of the mask inspection system. However, the mask inspection suffers from noises comparable to signal of the small defect, due to illumination nonuniformity, laser speckle, and fluctuation of the sensor signal. In order to overcome these issues, we propose a novel mask defect inspection method that uses detection optics for polarization variation. This inspection method uses the variation of polarization states which are caused by form birefringence in the mask feature. Thus the defect signals in the polarization-variation image can be obtained with sufficient intensity for much smaller defects than the wavelength. However since pattern edges are especially emphasized in the polarization-variation images, the images can not faithfully be acquired the mask pattern. To avoid these problems, we simultaneously use both conventional transmitted inspection images and the polarization variation images. By using numerical simulation, this paper discusses the validity of the mask inspection method that detects the polarization variation. The simulated results show that this new inspection method is quite effective for 20-nm-size defect and smaller ones.