Design method of a low-angle-of-incidence six-mirror extreme ultraviolet lithography objective system

Abstract

Extreme ultraviolet lithography (EUVL) objective systems must satisfy multiple constraints. These include high numerical aperture, large field of view, low angle of incidence (AOI), and high image quality. Traditional damped least squares methods are highly dependent on the initial structure and have limited efficiency in high-dimensional nonlinear optimization. Additionally, conventional EUVL objective systems with large AOI face several challenges. Deviations from the optimal AOI for multilayer coatings lower reflectivity, while also causing higher-order aberrations. This paper proposes a design method for a low-AOI six-mirror EUVL objective system. A mathematical model is developed based on ray tracing and Seidel aberration theory. By incorporating both aberration and structural constraints, a weighted chief ray AOI constraint is introduced to construct the merit function. Adaptive Sine Cosine-Levy Flight-Quantum Particle Swarm Optimization (ASL-QPSO) is employed to explore the high-dimensional nonlinear solution space. This achieves an initial structure that balances aberrations while satisfying the low AOI requirement. Further optimization is performed using optical design software, yielding a novel EUVL objective system. The final design achieves a numerical aperture of 0.3, an RMS wavefront error of 0.018λ, and a distortion of 0.35 nm. The maximum chief ray AOI is 18.97°. Compared with the previously reported maximum AOI of 24.05°, it is reduced by 21.1%, demonstrating improved compatibility with multilayer coatings and significantly relaxing coating design constraints. Tolerance analysis shows that the RMS wavefront error remains below 0.030λ in 97% of Monte Carlo trials, confirming the engineering feasibility and robustness of the proposed design.