Design method for an off-axis reflective anamorphic optical system with aberration balance and constraint control

Area of Science:

• Optical Engineering
• Lithography Technology
• System Design

Background:

• Off-axis reflective anamorphic optical systems (ORAOS) are crucial for advanced lithography.
• Balancing aberrations and constraints in ORAOS design is complex.
• Existing methods may lack efficiency or adaptability.

Purpose of the Study:

• To propose and validate a new design methodology for ORAOS.
• To enable the development of high-performance anamorphic optical systems.
• To address the challenges in aberration balancing and structural optimization.

Main Methods:

• Vector aberration theory for mathematical modeling.
• Spatial ray tracing and grouping design for error function construction.
• Automatic weight factor adjustment for dynamic aberration balancing.
• Particle swarm simulated annealing for iterative structure calculation.

Main Results:

• A design method for ORAOS is successfully established.
• An extreme ultraviolet (EUV) lithographic projection objective was designed as a verification case.
• The designed objective features off-axis six-reflective anamorphic mirrors with magnifications ${\beta _x} = 1/4$ and ${\beta _y} = 1/8$.

Conclusions:

• The proposed design method is effective for ORAOS.
• The achieved EUV lithographic anamorphic objective has a numerical aperture of 0.55.
• The system demonstrates a root mean square wavefront error better than $1/30\lambda$ (13.5 nm).