The effect of localized mask density variations on image quality in EUV lithography

Abstract

The impact of Stray Light (also sometimes called 'scattered light' or 'flare') in lithographic exposure tools is one of the key issues in EUV lithography to reduce its level to less than 10%. EUV mask can also be considered as one of the scattering sources because EUV mask used in memory and logic devices has various kinds of patterns with localized density variations, which are determined by patterned multilayer area. The most efficient way to decrease influences of stray light can be a combination of selective biasing and dummy implementation. In this paper, the effect of EUV mask density variations on image quality is investigated in terms of process window, such as LWR, depth of focus, resolution, etc., while mask density surrounding features is varied from dark field to bright field by implementing dummy patterns. As a result, allowable mask background density, which does not affect image quality of surrounding features and decreases effects of stray light, is suggested.