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In this paper, a method for solving the initial structure of an off-axismulti-mirror system applied to extreme ultraviolet (EUV) lithography using aparaxial ray-tracing algorithm based on Seidel aberration theory is proposed.By tracing the characteristic rays in the reflection system, the height andparaxial angle on each surface can be obtained, then through the relationshipbetween the Seidel aberration coefficient and these parameters, the initialstructure with good aberration performance can be solved. We can obtaindifferent initial structures by adding different initial condition constraints.In this paper, we have solved two different initial structures by assigningdifferent optical powers as different initial structures, and on this basis, wehave optimized two off-axis six-mirror systems with numerical aperture (NA) of0.25. Their wavefront aberration RMS value is about 0.04 wavelength, and theabsolute distortion is less than 1.2nm, with good imaging quality. We believethat this method can greatly improve the design efficiency and optimizationeffect of complex multi-mirror systems.

The method of solving initial structure by Seidel aberration theory for extreme ultraviolet lithography objective