EUV Projection Objective with Freeform Mirrors
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Solution Overview
Problem
Conventional projection objectives for microlithography face challenges in total transmission, unwanted apodization, and space requirements, particularly when used with EUV illumination light, and require improvements in compactness and imaging performance.
Innovation Solution
The use of a projection objective with at least six mirrors, including one with a freeform surface, and an intermediate image plane to minimize mirror dimensions and maintain a large working distance, allowing for reduced apodization effects and improved imaging performance, while also optimizing the reflective coating for high peak reflection across the mirror surface.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If conventional projection objectives are used with EUV illumination light, then imaging function is provided, but total transmission is insufficient and unwanted apodization occurs
Solution Approach 1:
The projection objective is divided into multiple mirror segments (at least six mirrors) arranged in a specific configuration. Each mirror is optimized for specific portions of the beam path, allowing the system to achieve high total transmission while maintaining imaging quality through cumulative reflection effects.
Solution Approach 2:
At least one mirror is equipped with a freeform surface having locally optimized properties. The reflecting surface is designed with specific curvature and orientation variations to control beam paths locally, reducing apodization effects and improving overall transmission efficiency in critical regions.
2Volume of moving object
If conventional projection objectives are used, then imaging function is provided, but space requirements are excessive
Solution Approach 1:
The projection objective utilizes oblique beam paths and folded optical configurations where mirrors are arranged to redirect light through three-dimensional space. This dimensional rearrangement allows the system to achieve compact footprint while maintaining the necessary optical path length and imaging precision.
Solution Approach 2:
Multiple mirror elements are nested or closely integrated in the optical path, with later mirrors positioned to receive beams from earlier mirrors in a compact arrangement. This nested configuration reduces the overall volume required while preserving the sequential optical functions needed for high-precision imaging.
3Manufacturing precision
If mirrors with large radii of curvature are used, then imaging errors are reduced, but the overall length of the projection objective increases
Solution Approach 1:
The optical design optimizes the curvature radius parameter of individual mirrors to achieve the desired balance between imaging precision and compact length. By carefully selecting and varying the curvature parameters across different mirrors, the system achieves sufficient error correction without excessive overall length.
Solution Approach 2:
The projection objective employs asymmetric mirror configurations where mirrors have different curvature radii and orientations optimized for their specific positions in the beam path. This asymmetric design allows the system to achieve adequate imaging precision while minimizing the overall length through non-uniform optical element dimensions.
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 total transmission, reduces apodization, and allows for compact designs with minimal imaging errors, ensuring effective EUV light guidance and improved spatial resolution in microlithography.
Implementation Method 1
a projection objective for imaging an object field into an image field, wherein the projection objective comprises at least six mirrors
Data Source
AI summary
A projection objective for microlithography is used for imaging an object field in an object plane into an image field in an image plane. The projection objective comprises at least six mirrors of which at least one mirror has a freeform reflecting surface. The ratio between an overall length (T) of the projection objective and an object image shift (dOIS) can be smaller than 12. The image plane is the first field plane of the projection objective downstream of the object plane. The projection objective can have a plurality of mirrors, wherein the ratio between an overall length (T) and an object image shift (dOIS) is smaller than 2.