Lithography Projection Objective Mirror Defect Correction
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Solution Overview
Problem
Catadioptric projection objectives in lithography face challenges due to narrow tolerance requirements for mirror surfaces, leading to wavefront errors and imaging defects, especially with the increased optical effect of mirror deformations compared to lenses, making direct correction of mirror surfaces complex and often impractical.
Innovation Solution
The method involves determining the ratio of principal ray height to marginal ray height at the defective mirror surface and selecting an optically operative lens surface for correction, which is approximately conjugate to the mirror surface, allowing for positional adjustments or aspherization to compensate for image defects without directly correcting the mirror.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If mirrors are used in catadioptric projection objectives to reduce lens count and achieve dispersion-free power, then the optical design becomes more economical and compact, but the tolerance requirements for mirror surface accuracy become extremely narrow because mirror deformations have more than twice the optical effect compared to lens surface deformations
Solution Approach 1:
The patent introduces a corrective optical element (such as a lens or optical component) as an intermediary between the defective mirror and the image plane. This corrective element compensates for the wavefront errors introduced by the mirror surface deformations, effectively mediating the optical path to achieve the desired imaging quality without requiring the mirror itself to be perfectly accurate.
Solution Approach 2:
The patent employs adjustable parameters of the corrective optical element (such as position, orientation, or shape) to dynamically compensate for mirror surface deformations. By changing these parameters, the system can adapt to different deformation patterns and maintain image quality despite variations in mirror manufacturing precision.
2Ease of manufacture
If mirrors with different coefficients of thermal expansion are used for coating and substrate, then the mirror can be manufactured with available materials and processes, but layer stresses cause shape changes upon light irradiation leading to wavefront errors
Solution Approach 1:
The patent explicitly addresses thermal expansion effects by designing the mirror system to compensate for shape changes caused by differential thermal expansion between the coating and substrate. The corrective optical element is positioned and configured to counteract the thermal deformation-induced wavefront errors, maintaining imaging stability despite the inherent thermal expansion mismatch in the mirror structure.
3Adaptability or versatility
If folding mirrors are used for beam guidance in catadioptric projection objectives, then the optical path can be compactly arranged according to design requirements, but the mirrors require precise repositioning after removal and installation which is difficult to ensure
Solution Approach 1:
The patent creates an optical copy or equivalent function through the corrective optical element that compensates for the position variations of the folding mirrors. Instead of requiring the folding mirrors to be precisely repositioned, the corrective element replicates the necessary optical correction, making the system tolerant to positioning variations while maintaining the flexible optical path configuration.
4Manufacturing precision
If direct correction of mirror surfaces is attempted through polishing or ion beam etching, then the mirror surface accuracy can be improved, but the process becomes extremely complex and requires multiple removals and reinstallations with high adjustment requirements
Solution Approach 1:
Instead of directly correcting the mirror surface through complex polishing or etching processes, the patent introduces a separate corrective optical element as an intermediary. This element can be adjusted or manufactured independently to compensate for mirror defects, avoiding the need for repeated mirror removals and complex surface modification processes.
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 approach effectively compensates for image defects caused by mirror deformations by selecting a lens surface that mimics the mirror's optical effect, enabling precise correction of wavefront errors and improving imaging quality without the need for direct mirror adjustment, thus enhancing the reliability of semiconductor production.
Implementation Method 1
an optical arrangement composed of a plurality of lenses and at least one mirror
Implementation Method 2
an optical arrangement composed of a plurality of lenses and at least one mirror
Implementation Method 3
selecting an optically operative lens surface for correction, which is approximately conjugate to the mirror surface
Data Source
AI summary
A method for correcting at least one image defect of a projection objective of a lithography projection exposure machine, the projection objective comprising an optical arrangement composed of a plurality of lenses and at least one mirror, the at least one mirror having an optically operative surface that can be defective and is thus responsible for the at least one image defect, comprises the steps of: at least approximately determining a ratio VM of principal ray height hMH to marginal ray height hMR at the optically operative surface of the at least one mirror, at least approximately determining at least one optically operative lens surface among the lens surfaces of the lenses, at which the magnitude of a ratio VL of principal ray height hLH to marginal ray height hLR comes at least closest to the ratio VM, and selecting the at least one determined lens surface for the correction of the image defect.