Development of a 20X Schwarzschild projection optics for principle experiment of EUV at-wavelength interferometry

A 20 × Schwarzschild projection optics (PO) designed for principle experiment of interferometry for extreme ultraviolet lithography (EUVL) is assembled and aligned utilizing a Fizeau-type interferometer. The design goal of the PO is to achieve a wavefront error (WFE) of less than 30mλ rms (λ = 632.8nm ) in an image side field diameter of 0.2mm. This paper presents the detailed design, assembly and alignment procedure of the PO. The PO is designed at 13.4nm wavelength for future EUV at-wavelength experiment and the original design could achieve a resolution of 100nm within a depth of focus of 75nm. Due to fabrication errors, the fabricated PO could only achieve a WFE of 28mλ rms (λ = 632.8nm ) simulated by CODE V in ideal alignment status. Tolerance analysis is performed by CODE V to determine the requirement of assembly mechanism and the tolerance of coarse alignment. In the coarse alignment process, a simple method based on the precise positioning of the curvature center of each mirror in Schwarzschild PO is used. In the fine alignment process, an effective computer aided alignment (CAA) method based on the singular value decomposition (SVD) of sensitivity matrix is used. Finally, alignment experiment is performed and a WFE of 28mλ rms (λ = 632.8nm ) is achieved after fine alignment.