Comparative study of extreme ultraviolet absorber materials using lensless actinic imaging

Abstract. Background: One of the challenges for extreme ultraviolet (EUV) lithography is the mitigation of mask three-dimensional effects arising from the oblique incident angle and the mask topography. As the scanners’ numerical aperture and the pattern aspect ratio increase, these effects become more prominent. A potential solution to reduce them consists in replacing the current TaBN absorber for a higher-k material. Aim: We demonstrate the potential of a mask inspection platform to evaluate the impact of different absorber materials on actinic defect inspection. Approach: We evaluate the performance of a reflective-mode EUV mask scanning microscope (RESCAN), our actinic lensless inspection tool, with three different absorber materials (hydrogen silsesquioxane, TaBN, and Ni). We study the effect of these materials on the image formation and compare the defect maps. Results: The Ni absorber mask exhibits a better contrast compared to the TaBN one, even though the thickness of the layers differs only by 10 nm. Programmed defects are localized and detected with a high signal-to-noise ratio (SNR). Conclusions: The gain in contrast for the Ni absorber being significant, the SNR is higher for a smaller defect in a TaBN absorber photomask. RESCAN allows the evaluation of the performance of absorber materials in defectivity and image formation on small samples.