“Actinic-only” Defects in Extreme Ultraviolet Lithography Mask Blanks —Native Defects at the Detection Limit of Visible-Light Inspection Tools

We present recent experimental results from an actinic defect inspection system for extreme ultraviolet (EUV) lithography mask blanks. The current actinic inspection system has demonstrated the ability to detect 50 nm defect in cross correlation experiments with visible-light inspection tools. We found that native defects as small as 60 nm with only 3 nm height were detectable by the actinic tool. These defects are just below the detection limit of current commercial visible-light inspection tools. A new class of defect was discovered, which is quite large, in the several micrometer range, and shows suppressed non-specular EUV scattering intensity as compared to the intrinsic background scatter from the multilayer blanks. Despite their large physical dimensions, these defects are also near the detection limit of current visible-light inspection tools.