EUV Imaging Optical System Mirror Coating Angle Optimization
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Solution Overview
Problem
Imaging optical systems using mirrors suffer from high reflection losses when dealing with EUV radiation, particularly at wavelengths less than 10 nm, which limits their transmission properties.
Innovation Solution
The design incorporates a multi-layer coating on mirrors with a low acceptance bandwidth for the angle of incidence, achieved by minimizing the ratio of the maximum angle of incidence to the numerical aperture, and using obscured mirrors to facilitate a structural resolution of less than 10 nm, allowing for reflective coatings like B4C/CsI multilayers that enhance reflectivity across varying angles of incidence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a multi-layer coating is applied to mirror surfaces to achieve high reflectivity, then reflection losses are reduced, but the acceptance bandwidth for the angle of incidence becomes very narrow
Solution Approach 1:
The patent minimizes the ratio of the maximum angle of incidence to the numerical aperture (specifically making it less than 33°, and even less than 30.7°) to create favorable conditions for using multi-layer coatings with narrow acceptance bandwidths. This parameter optimization allows the system to achieve high reflectivity while maintaining adequate angular acceptance for EUV radiation below 10 nm wavelength.
2Loss of energy
If the ratio of maximum angle of incidence to numerical aperture is minimized to enable multi-layer coating, then reflectivity increases, but the system complexity increases
Solution Approach 1:
The patent employs an imaging optical system with six mirrors, where at least three mirrors have openings (obscured mirrors) that cause pupil obscuration. This segmentation approach minimizes the maximum angle of incidence for a given image-side numerical aperture, enabling the use of multi-layer coatings while managing system complexity through distributed optical elements.
Solution Approach 2:
The patent applies different reflective coatings to different mirrors in the system. Specifically, B4C/CsI multilayers are used on certain mirrors, with layer thickness increasing parabolically with distance from the optical axis to compensate for varying angles of incidence across the mirror surface, optimizing local reflectivity conditions.
3Manufacturing precision
If obscured mirrors are used to minimize the maximum angle of incidence, then structural resolution improves, but the pupil obscuration reduces light transmission
Solution Approach 1:
The patent accepts pupil obscuration from obscured mirrors as a necessary trade-off to achieve structural resolution of less than 10 nm (and even 6 nm or smaller). The system optimizes the balance between resolution and light transmission by carefully designing the obscuration geometry and minimizing the maximum angle of incidence.
Solution Approach 2:
The patent uses B4C/CsI multilayer coatings with parabolically increasing layer thickness from the optical axis outward. This local variation in coating properties compensates for the reduced light transmission caused by obscured mirrors, maintaining adequate reflectivity across the pupil while achieving high structural resolution.
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
This approach minimizes reflection losses and enables the production of highly integrated microstructured or nanostructured components with resolutions better than 20 nm, suitable for EUV microlithography, by optimizing the mirror coatings and system configuration.
Implementation Method 1
Because of a high reflection loss of the light on mirrors in such imaging optical systems, such systems can have transmission properties which cannot be tolerated when using EUV radiation
Implementation Method 2
B4C/CsI multilayers may be used as reflective coatings. Each multilayer reflective coating may be constructed from 300 B4C/CsI bilayers
Data Source
AI summary
An imaging optical system has a plurality of mirrors. These image an object field in an object plane into an image field in an image plane. In the imaging optical system, the ratio of a maximum angle of incidence of imaging light) on reflection surfaces of the mirrors and an image-side numerical aperture of the imaging optical system is less than 33.8°. This can result in an imaging optical system which offers good conditions for a reflective coating of the mirror, with which a low reflection loss can be achieved for imaging light when passing through the imaging optical system, in particular even at wavelengths in the EUV range of less than 10 nm.