The low source power is one of the major challenges that hinder the extreme ultraviolet lithography from high volume manufacturing. To alleviate the source development pressure, a high-efficiency illumination system with three mirrors is proposed, based on the authors' knowledge, for the first time. Free-form fly's eye is introduced into the system to get a qualified arc-shaped irradiance distribution on the reticle. A method integrated with a numerical method and optimization to design the free-form surface is given in detail. The transfer efficiency of the system is much higher than that of the four-mirror configuration employed in the EUV exposure platform. Compared with the previous high-efficiency illumination system with two mirrors, this configuration can ensure a good uniformity and will not increase the objective design difficulty or affect the image quality of the objective. Simulation result of the design with three mirrors shows the uniformity on the reticle is about 95.5%, and the energy efficiency is about 25.4%. It indicates that the system is effective in enhancing the efficiency and potential to promote the EUV lithography into high volume manufacturing.

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Abstract. Source-mask optimization (SMO) has emerged as a key technique for 7-nm node and beyond in extreme ultraviolet (EUV) lithography. The pupil required by SMO is usually pixelated, with a free choice of intensity per pixel. However, due to the discrete nature of the EUV illumination system, pupil intensity in current EUV SMO must also be discretized. An illumination system with a freeform fly's eye that is able to generate the pixelated pupil is proposed. Clear apertures of the field facets in the fly's eye are different from each other so that the intensity of each pixel on the pupil can meet the requirements of SMO. Each of the field facets is constructed with a freeform surface to get the required arc-shaped illuminated area on the reticle. A method integrated with a numerical method and an optimization process is used to design the freeform surface of the field facets. The simulation result of the design for a prescribed freeform pixelated pupil shows that the uniformity on the reticle is 96.4%, and the pupil intensity error is approximated to be 0.035. The results indicate that the system is effective in generating the required freeform pixelated pupil and reducing the restrictions imposed on the SMO process in EUV lithography.