Tilted Aperture Stop for EUV Lithography NA Homogenization
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Solution Overview
Problem
In EUV projection lithography, existing imaging optical units face challenges in maintaining a uniform numerical aperture across the image plane, leading to variations in structure resolution depending on the field location and orientation of the image-side plane of incidence.
Innovation Solution
The implementation of a tilted circular aperture stop, which is oriented relative to the normal plane by at least 1°, allows for the homogenization of numerical apertures (NAx and NAy) across the image field by adjusting the maximum illumination angle independently of the incidence direction and position, thereby ensuring consistent structure resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a conventional aperture stop perpendicular to the optical axis is used, then the structure is simple and easy to manufacture, but the numerical aperture varies across the image plane leading to non-uniform structure resolution
Solution Approach 1:
The aperture stop is tilted by at least 1° relative to the normal plane perpendicular to the optical axis, breaking the conventional symmetric configuration. This asymmetric tilt enables independent control of maximum illumination angle in different directions, achieving homogenized numerical aperture (deviation < 0.003) across the image plane while maintaining a simple circular stop contour that is easy to manufacture
2Manufacturing precision
If the aperture stop is tilted relative to the normal plane, then the numerical aperture is homogenized across the field, but the device complexity increases
Solution Approach 1:
The tilt angle of the aperture stop is optimized to at least 1° relative to the normal plane, transforming the illumination geometry to achieve uniform numerical aperture across the image field. This parameter change enables the maximum illumination angle to be set independently of incidence direction and image point position, achieving NA homogeneity (deviation < 0.003) without requiring complex adaptive mechanisms
3Ease of manufacture
If a circular aperture stop is used instead of an elliptical stop, then the manufacturing cost is reduced, but the ability to correct imaging parameters such as ellipticity may be limited
Solution Approach 1:
Instead of using an elliptical stop shape to correct imaging parameters, the invention tilts a simple circular aperture stop by at least 1° relative to the normal plane. This dimensional change (from in-plane ellipticity to out-of-plane tilt) achieves the same correction effect for parameters like ellipticity and trefoil while maintaining the manufacturing simplicity and cost-effectiveness of a circular stop
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 solution achieves a deviation of less than 0.003 in the numerical aperture averaged over the image field points, improving imaging parameters such as telecentricity, ellipticity, and trefoil, and enabling consistent high-resolution imaging across the field.
Implementation Method 1
a deviation of a numerical aperture NAx measured in one of these planes from a numerical aperture NAy measured in the other one of these two planes is less than 0.003, averaged over the field points of the image field
Implementation Method 2
a plurality of mirrors for guiding imaging light from the object field to the image field
Data Source
AI summary
An imaging optical unit for EUV projection lithography serves to image an object field into an image field. Mirrors guide imaging light from the object field to the image field. An aperture stop is tilted by at least 1° in relation to a normal plane which is perpendicular to an optical axis. The aperture stop has a circular stop contour. In mutually perpendicular planes, a deviation of a numerical aperture NAx measured in one plane from a numerical aperture NAy measured in the other plane is less than 0.003, averaged over the field points of the image field. What emerges is an imaging optical unit, in which homogenization of an image-side numerical aperture is ensured so that an unchanging high structure resolution in the image plane is made possible, independently of an orientation of a plane of incidence of the imaging light in the image field.