Imaging Optical System with Convex Fourth-Last Mirror
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Solution Overview
Problem
Imaging optical systems for projection exposure installations, particularly in microlithography, face challenges in achieving improved imaging properties such as higher numerical aperture and better correction of imaging errors while simplifying mirror production and reducing the number of optical elements required.
Innovation Solution
The design of an imaging optical system with a convex fourth-last mirror allows for reduced pupil obscuration, enabling a larger central opening and fewer mirrors, which results in improved imaging properties and reduced production complexity, including a numerical aperture of at least 0.4 and a maximum root mean square wavefront error of less than 10 nm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If the intermediate image plane is positioned at the height of the last mirror in known constructions, then the system achieves compact arrangement, but the pupil obscuration increases and light throughput decreases
Solution Approach 1:
The patent positions the intermediate image plane at a distance from the last mirror along the optical axis rather than at the mirror's height, utilizing the longitudinal dimension to resolve the conflict between compact arrangement and reduced pupil obscuration. This spatial reconfiguration allows light to pass through the last mirror with minimal obstruction while maintaining a manageable system layout.
2Manufacturing precision
If more mirrors are used to improve imaging properties and correct errors, then imaging quality improves, but the number of optical elements increases and production complexity increases
Solution Approach 1:
The patent modifies the curvature parameter of the fourth-last mirror by making it convex instead of concave or flat. This parameter change enables the mirror to perform both imaging and aberration correction functions, reducing the need for additional correction mirrors while maintaining high imaging quality.
3Measurement precision
If the numerical aperture is increased to improve resolution, then local resolution improves, but the requirements on mirror production and support structure increase
Solution Approach 1:
The patent extracts the aberration correction function from the support structure and mirror mounting system by incorporating it into the optical design itself through the convex fourth-last mirror. This allows high numerical aperture operation with relaxed mechanical support requirements, as the optical system self-corrects rather than relying on precision mechanical compensation.
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 configuration enhances light throughput, increases the bandwidth of available illumination, and provides high-contrast imaging independently of object structure, facilitating the production of microstructured components with high local resolution across a larger image field.
Implementation Method 1
A convex fourth-last mirror according to claim 2 allows the imaging optical system to be constructed with relatively low pupil obscuration
Implementation Method 2
enabling a larger central opening and thus an intermediate plane separated from the reflection surface of the penultimate mirror, can be tolerated there
Data Source
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AI summary
An imaging optical system (7) comprises a plurality of mirrors (M1 to M8), which image an object field (4) in an object plane (5) into an image field (8) in an image plane (9). At least three of the mirrors (M6, M7, M8) are obscured, and thus have a through-opening (21) for imaging light (15) to pass through. At least one intermediate image plane (23) is present between the object plane (5) and the image plane (9). The intermediate image plane (23) which is closest to the image plane (9) in the light path between the object field (4) and the image field (8), is spatially arranged between the last mirror (M8) in the light path and the image plane (9). These measures lead to an imaging optical system with improved imaging properties and/or reduced production costs.