Movable Mirror Retroreflector for EUV Aberration Correction
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Solution Overview
Problem
In microlithographic projection exposure apparatuses, especially those operating in the EUV range, the correction of optical aberrations during the exposure process is challenging due to the sensitivity of image position movements relative to mirror movements, leading to undesired image shifts on the wafer plane, which cannot be easily rectified by adapting the wafer position.
Innovation Solution
An optical system with a first and a second reflective surface in the optical beam path, where the relative position of these surfaces is maintained stable during travel movements, either directly succeeding each other or with only reflective optical elements in between, allowing for aberration correction without causing image movement on the wafer plane, utilizing control loops for precise positioning and sensor feedback.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a mirror within the projection lens is actively manipulated for aberration correction, then the aberration correction capability is improved, but the image position stability deteriorates due to simultaneous image movement in the wafer plane
Solution Approach 1:
A retroreflector is introduced as an intermediary component between the movable mirror and the optical beam path. The retroreflector receives the reflected beam and redirects it back through the same optical path, ensuring that any lateral displacement of the mirror does not translate to image position changes on the wafer plane. This mediator decouples the aberration correction function from the image position stability requirement.
Solution Approach 2:
The optical beam is configured to pass through the movable mirror assembly multiple times in a rapid sequence - traveling to the mirror, reflecting off it, passing through the retroreflector, and returning through the same path. This rapid round-trip traversal allows the system to compensate for aberrations while minimizing the time during which image position instability could occur, effectively skipping through the problematic interaction zone.
2Manufacturing precision
If the first reflective surface is moved for aberration correction, then the aberration correction is improved, but the relative position stability between the first and second reflective surfaces deteriorates
Solution Approach 1:
The first and second reflective surfaces are merged into a single rigid mirror assembly or mechanically coupled structure. This merging ensures that when the first reflective surface moves for aberration correction, the second reflective surface moves identically, maintaining their relative position stability. The combined structure allows aberration correction while preserving the fixed geometric relationship between the two surfaces.
3Stability of the object's composition
If wafer position adaptation is used to correct image shifts, then the image position stability is improved, but the productivity deteriorates due to exposure process interruptions
Solution Approach 1:
The retroreflector and movable mirror assembly are pre-configured to provide automatic real-time compensation for image position shifts during the exposure process. This preliminary setup eliminates the need for interrupting the exposure to manually adapt wafer position, as the system proactively maintains image stability throughout continuous operation.
Solution Approach 2:
The aberration correction mechanism operates continuously throughout the exposure process without interruption. The movable mirror and retroreflector work in real-time to maintain image position stability, allowing the exposure process to proceed without pauses for wafer repositioning, thus maintaining continuous productive action.
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 enables effective correction of aberrations during the microlithographic exposure process without inducing image shifts on the wafer plane, reducing the complexity of control engineering and sensor technology requirements, and maintaining image stability across mirror movements.
Implementation Method 1
a first reflective surface, which is arranged in the optical beam path of the optical system and is movable for the correction of an aberration that occurs during the operation of the optical system; and at least one second reflective surface, arranged in the optical beam path of the optical system
Data Source
AI summary
A microlithographic projection exposure apparatus optical 22 system includes a first reflective surface and at least one second reflective surface, each in the optical beam path. The first reflective surface is movable for the correction of an aberration that occurs during the operation of the optical system. The optical system is configured in so that, during the travel movement of the first reflective surface, the relative position of the first reflective surface and of the second reflective surface is maintainable in a stable manner. Either the first reflective surface and the second reflective surface directly succeed one another in the optical beam path, or there are only reflective optical elements between the first reflective surface and the second reflective surface.