Yan Xu, Li Yanqiu, Liu Lihui, Liu Ke
Appl Opt. 2022 Jan 20;61(3):806-811. doi: 10.1364/AO.446729.
High-numerical-aperture (NA) anamorphic extreme ultraviolet lithography (EUVL) is the next-generation technology to achieve a 3 nm node and below. Current EUVL anamorphic projection objectives in publications are designed with small off-axis distance of the field of view (FOV). Further, the ray path of the illumination system will be obscured by the last mirror of the objective. In fact, as the NA increases, the diameter of the last mirror of the objective becomes larger. Thus, the off-axis distance of the FOV should be larger to ensure that the ray path of the illumination system is not obscured by the objective. It is well known that the design of an anamorphic EUVL objective with large off-axis distance is very challenging. In this paper, a high-NA anamorphic EUVL objective with large off-axis distance is designed. The value of the large off-axis distance that satisfies the unobscured constraint is first analyzed. Then an 8× demagnification isomorphic objective with large off-axis distance is designed by a grouping design process. Finally, the XY polynomial surface is adopted to achieve a 4× demagnification perpendicular to the scanning direction. This method is implemented to design a NA 0.55 anamorphic EUVL objective with large off-axis distance. The performance of the designed objective demonstrates that this design method is very effective and will provide a good structure of the objective for the illumination system design.
