Mitigation of mask-3D effects by 'vote-taking' exposures

摘要

Extreme ultraviolet lithography (also known as EUV or EUVL) is a lithography technology using a range of extreme ultraviolet (EUV) wavelengths, roughly spanning a 2% FWHM bandwidth of 13.5 nanometres (nm). In EUV lithography, masks comprising multilayer stacks, for example consisting of forty to fifty alternating silicon and molybdenum layers, are required to reflect the EUV light through Bragg diffraction. Accordingly, the multilayer stack is 250 nm to 350 nm thick. On top of the multilayer stack, the mask also includes a capping layer (usually ruthenium-based), followed by a tantalum (Ta) absorber. Typically, the Ta absorber layer is 60 nm thick, thereby creating a 3D-likc feature abutting the mask. In order to prevent an overlap between reflected and incident light, a chief-ray angle of 6 degrees is used. However, this oblique incidence angle leads to shadowing effects in the presence of the 3D-like absorber layer. A consequence of this shadowing effect is that one side of a feature (outside the shadow) would appear brighter than the other side of the feature (within the shadow), resulting in unwanted feature-size dependent focus and pattern placement shifts. These effects are known as mask-3D(M3D) effects.