EUV Imaging Optics with Penultimate Mirror for Compact High-NA Design
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Solution Overview
Problem
Conventional imaging optics face challenges in achieving a high numerical aperture and compact design while maintaining low aberrations and high light throughput, particularly for EUV wavelengths, due to limitations in mirror size and optical refractive power distribution.
Innovation Solution
The design incorporates a penultimate mirror and a last mirror with a through-opening, where the penultimate mirror images an intermediate image onto a further intermediate image, allowing the last mirror to have increased optical refractive power and a smaller diameter, and utilizing free form surfaces to enhance design flexibility and reduce aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the last mirror is designed with high optical refractive power to image the intermediate image onto the image field, then the numerical aperture is improved, but the mirror diameter decreases leading to increased sagitta and manufacturing difficulty
Solution Approach 1:
A further intermediate image is introduced between the penultimate mirror and the last mirror. This intermediary imaging step allows the last mirror to operate with higher refractive power and smaller diameter while the penultimate mirror handles the intermediate imaging, distributing the optical burden and enabling better manufacturing precision for the last mirror
Solution Approach 2:
The second partial objective is divided into two separate imaging stages: the penultimate mirror images the intermediate image onto a further intermediate image, and the last mirror images the further intermediate image onto the image field. This segmentation allows each mirror to be optimized independently, with the last mirror having smaller diameter and higher curvature
2Volume of moving object
If the last mirror diameter is reduced to make the optics more compact, then the volume is improved, but the optical refractive power must be increased leading to larger sagitta
Solution Approach 1:
The further intermediate image acts as an intermediary that enables the last mirror to be positioned closer to the image plane with smaller diameter, achieving compact optics volume while the penultimate mirror compensates by providing the intermediate imaging function
Solution Approach 2:
The optical path is reconfigured by introducing an additional imaging stage in the longitudinal dimension, allowing the last mirror to have reduced radial size (smaller diameter) while maintaining imaging capability through the intermediate image formation
3Manufacturing precision
If free form surfaces are used on the mirrors to increase design flexibility, then the manufacturing precision is improved, but the device complexity increases
Solution Approach 1:
The mirror surfaces are defined by free form equations with multiple parameters (c, k, Cj coefficients) that can be optimized to correct aberrations. This parametric approach allows precise control of surface shape to achieve high manufacturing precision while maintaining a systematic design methodology
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 increases the numerical aperture, reduces mirror size, and enhances light throughput, enabling high-resolution imaging with reduced aberrations and a more compact optics system, particularly suitable for EUV microlithography.
Implementation Method 1
The penultimate mirror and the last mirror are designed so that the penultimate mirror images the intermediate image onto a further intermediate image
Implementation Method 2
the last mirror images the further intermediate image onto the image field
Data Source
AI summary
An imaging optics has a plurality of mirrors to image an object field in an object plane into an image field in an image plane. The imaging optics includes a first partial objective to image the object field onto an intermediate image, and the imaging optics includes a second partial objective to image the intermediate image onto the image field. The second partial objective includes a penultimate mirror in the beam path of imaging light between the object field and the image field, and the second partial objective includes a last mirror in the beam path. The penultimate mirror images the intermediate image onto a further intermediate image, and the last mirror images the further intermediate image onto the image field.