EUV Collector Mirror with Segmented Facets
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Solution Overview
Problem
Existing collector mirrors in EUV microlithography systems suffer from significant loss of usable rays due to the enlargement of the etendue envelope, caused by surface roughness and variations in imaging scale, leading to heat input from non-EUV spectral range rays, which impair optical properties and exposure quality.
Innovation Solution
A collector mirror design featuring facet surfaces arranged on confocal elliptical shells, reducing the variation in imaging scale and effectively blocking a wide range of remaining rays, including IR and DUV light, by focusing EUV light onto a smaller beam diameter and using a smaller stop aperture, while maintaining a compact form with improved radiation resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If an elliptical mirror surface is used to improve light focusing, then the concentration of EUV light increases, but the variation in imaging scale enlarges the etendue envelope, causing loss of usable rays
Solution Approach 1:
The mirror surface is divided into multiple mirror facets, each with a specific orientation and position on confocal elliptical shells. This segmentation allows different regions of the mirror to be optimized independently, reducing the overall variation in imaging scale while maintaining EUV light concentration.
Solution Approach 2:
Different mirror facets are assigned different local orientations and positions based on their specific location on the confocal elliptical shells. This local optimization ensures that each facet contributes to minimizing etendue envelope variation while maintaining effective EUV light focusing, rather than using a uniform elliptical surface.
2Illumination intensity
If the density of tin plasma increases to improve EUV light generation, then the intensity of EUV light increases, but the plasma frequency increases greatly, causing remaining rays to be reflected and focused onto the second focal point
Solution Approach 1:
The use of multiple mirror facets with different orientations segments the reflection process, allowing selective reflection of EUV light while directing remaining rays at different angles that do not converge at the second focal point, thus reducing harmful reflections.
Solution Approach 2:
The asymmetric arrangement of mirror facets on confocal elliptical shells creates different reflection paths for EUV light versus remaining rays. This asymmetry enables the system to maintain high EUV light intensity while suppressing the reflection and focusing of remaining rays that would otherwise cause harm.
3Reliability
If a binary grating is used to suppress remaining rays, then the spectral purity improves, but the manufacturing precision requirements increase and the etendue envelope variation persists
Solution Approach 1:
Instead of requiring precise control of a continuous binary grating, the mirror surface is segmented into discrete mirror facets with relatively relaxed positional and orientational tolerances. This segmentation simplifies manufacturing while achieving similar spectral purity through the collective effect of multiple facets on confocal elliptical shells.
4Quantity of substance
If the mirror surface area is increased to improve light collection, then the amount of EUV light collected increases, but the variation in imaging scale across the surface enlarges the etendue envelope
Solution Approach 1:
The large mirror surface is divided into multiple smaller mirror facets distributed on confocal elliptical shells. This segmentation allows the system to maintain a large total collecting area while each individual facet contributes to a more uniform imaging scale, thereby reducing the overall etendue envelope variation and minimizing loss of usable rays.
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 design reduces the loss of usable rays, enhances exposure quality, and protects downstream optical units from contamination, allowing for more effective suppression of unwanted spectral ranges and improved EUV light transmission.
Implementation Method 1
the optical grating includes a plurality of mirror facets (14) each having a facet surface (15), wherein the facet surfaces (15) form the mirror surface (17) of the grating
Implementation Method 2
which reflects electromagnetic rays emanating from a first focal point and focuses them onto a second focal point
Implementation Method 3
effectively blocking a wide range of remaining rays, including IR and DUV light, by focusing EUV light onto a smaller beam diameter
Data Source
AI summary
A collector mirror for an EUV microlithography system. The collector mirror includes an optical grating having an optically effective mirror surface, which reflects electromagnetic used rays in an EUV spectral range emanating from a first focal point and focuses them onto a second focal point. The first and second focal points lie on a side of the optical grating facing the mirror surface and define an optical axis. The optical grating is configured, in interaction with a stop arranged at the second focal point, to allow the used rays to pass through the stop and to block electromagnetic remaining rays in a remaining spectral range different than the EUV spectral range. The optical grating includes a blazed grating composed of a plurality of mirror facets, each having a facet surface. The facet surfaces form the mirror surface of the blazed grating.