Off-axis EUV Projection Mirrors Aberration Control
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Solution Overview
Problem
Conventional on-axis projection optical systems for EUV lithography require a shielding member to block stray light, which causes diffraction, scattering, and flare, and cannot efficiently reduce third-order aberrations.
Innovation Solution
An off-axis projection optical system is designed with first and second mirrors arranged off-axially, where the tangential and sagittal radii of curvature and incident angles satisfy specific equations, allowing for the elimination of a shielding member and reduction of third-order aberration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If an on-axis projection optical system is used, then the optical path is simple and easy to align, but stray light cannot be effectively blocked and third-order aberrations cannot be reduced
Solution Approach 1:
The patent applies asymmetry by transitioning from a conventional on-axis symmetric optical system to an off-axis asymmetric system. The first and second mirrors are positioned at different locations relative to the optical axis, with the first mirror having its apex at a first position and the second mirror at a second position, creating an asymmetric light path that naturally blocks stray light while maintaining optical performance.
2Object-affected harmful factors
If a shielding member is added to block stray light, then stray light is reduced, but diffraction, scattering, and flare are caused
Solution Approach 1:
The patent converts the potential harm of the off-axis configuration into benefit by designing specific radius of curvature relationships. The radii of curvature of the first and second mirrors are carefully controlled to satisfy specific equations, transforming what could be aberration-inducing asymmetry into a solution that reduces third-order aberrations while blocking stray light without requiring additional shielding members.
3Manufacturing precision
If the radii of curvature of mirrors are increased to reduce aberrations, then third-order aberration is reduced, but the field size is limited
Solution Approach 1:
The patent employs parameter changes by establishing specific mathematical relationships between the radii of curvature and incident angles of the mirrors. The radius of curvature of the first mirror in the tangential direction and the radius of curvature of the second mirror in the tangential direction are related through the incident angles, allowing optimization of both aberration control and field size through parameter coordination.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The off-axis system achieves a large field size with reduced aberrations and eliminates the need for a shielding member, resulting in improved image quality and reduced diffraction limits, with spot sizes smaller than 20 nm and RMS wavefront error values less than 0.1λ.
Implementation Method 1
a first mirror for reflecting a EUV beam and a second mirror for reflecting the EUV beam
Data Source
AI summary
Example embodiments are directed to an off-axis projection optical system including first and second mirrors that are off-axially arranged. The tangential and sagittal radii of curvature of the first mirror may be R1t and R1s, respectively. The tangential and sagittal radii of curvature of the second mirror may be R2t and R2s, respectively. The incident angle of the beam from an object point to the first mirror 10 may be i1, and an incident angle of the beam reflected from the first mirror 10 to the second mirror 30 is i2. The values of R1t, R1s, R2t, R2s, i1 and i2 may satisfy the following EquationR1t cos i1=R2t cos i2R1s=R1t cos2i1R2s=R2t cos2i2.