Background Level in Extreme-Uv Dark-Field Image for Mask Blank Inspection

The background level (BGL) in an extreme-UV (EUV) dark-field image for EUV mask blank inspection was analyzed. The background refers to the light scattered by the multilayer roughness of the mask blank, and the variation in the BGL is attributed to random and fixed factors. We found that the BGL variation due to the fixed factor could be successfully reproduced by simulation using surface information obtained by atomic force microscope measurement with the assumption of a multilayer roughness structure. This means that the fixed factor contributing to the BGL variation was found to be speckle noise due to the multilayer roughness. Moreover, this reproduction procedure was found to be valuable for estimating the multilayer roughness structure up to several layers below the surface.