Optical Pulse Stretcher Using Toroidal Mirrors to Reduce Speckle
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Solution Overview
Problem
Current semiconductor exposure apparatuses face challenges with chromatic aberration due to the large spectral line width of KrF and ArF excimer laser apparatuses, which affects resolution, and existing optical pulse stretchers either require long delay optical paths causing maintenance issues or have limited effectiveness with short paths leading to astigmatism.
Innovation Solution
The use of a first delay optical system with a plurality of concave toroidal mirrors and a beam splitter in an optical pulse stretcher, which configures a first and second optical path to extend pulse width and reduce speckle contrast, while maintaining a compact design and minimizing astigmatism by using concave cylindrical or toroidal mirrors with specific optical path lengths and angles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a long delay optical path is used in the optical pulse stretcher, then the pulse width extension effectiveness is improved, but the device complexity and maintenance difficulty increase
Solution Approach 1:
The patent implements nesting by placing the delay optical path inside the laser resonator cavity. The pulse laser light traverses the delay optical path multiple times through resonant oscillation, effectively achieving a long cumulative delay path while maintaining a compact physical structure. This resolves the contradiction by nesting the functional requirement (long delay path) within the existing resonator structure.
Solution Approach 2:
The delay optical path is configured before the line narrowing module in the optical sequence, allowing the pulse width extension to occur preliminarily before spectral narrowing. This preliminary action enables the system to achieve both long effective delay path and maintained beam quality without the complexity of coordinating multiple modules.
2Device complexity
If a short delay optical path is used in the optical pulse stretcher, then the device complexity is reduced, but the pulse width extension effectiveness deteriorates and astigmatism occurs
Solution Approach 1:
The patent employs curved mirrors (concave mirrors) in the delay optical path instead of flat mirrors. The curved surfaces provide inherent beam focusing and collimating capabilities, which compensate for astigmatism and maintain beam quality. This curvature-based solution allows short physical path length while achieving effective pulse width extension and maintaining beam precision.
Solution Approach 2:
By nesting the delay optical path within the laser resonator, the system achieves multiple passes through a short physical path, effectively multiplying the delay effect without increasing physical complexity or inducing astigmatism.
3Manufacturing precision
If the spectral line width of the laser is narrowed using a line narrowing module, then the chromatic aberration is reduced, but the device complexity increases
Solution Approach 1:
The laser resonator is designed to serve multiple functions simultaneously: it provides optical feedback for laser oscillation, implements pulse width extension through the delay optical path, and performs spectral narrowing through the line narrowing module. By making the resonator multi-functional, the patent avoids adding separate independent modules, thus reducing overall device complexity while achieving spectral narrowing.
4Device complexity
If the pulse laser light is directly output without pulse width extension, then the device complexity is minimized, but the speckle contrast is high
Solution Approach 1:
The patent segments the laser resonator into distinct functional zones: the delay optical path section for pulse width extension and the line narrowing module section for spectral control. This segmentation allows the pulse width extension function to be implemented independently within the resonator, reducing speckle contrast without requiring complex external optical systems.
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 effectively reduces speckle contrast, allows for compact design, and prevents astigmatism, enhancing beam divergence and pointing stability, while enabling efficient pulse width extension without the need for lengthy optical paths.
Implementation Method 1
a beam splitter 175 including a first surface 175a and a second surface 175b, causing a part of pulse laser light B1 incident on the first surface 175a to be transmitted in a first direction and output as a first beam B2 and another part thereof to be reflected in a second direction and enter the first delay optical system 171
Implementation Method 2
a first delay optical system 171 including a plurality of concave toroidal mirrors 171b to 174b
Data Source
AI summary
An optical pulse stretcher includes a first delay optical system including a plurality of concave toroidal mirrors; and a beam splitter including a first surface and a second surface, causing a part of pulse laser light incident on the first surface to be transmitted in a first direction and output as a first beam and another part thereof to be reflected in a second direction and enter the first delay optical system, and causing a part of pulse laser light incident on the second surface from the first delay optical system to be reflected in the first direction and output as a second beam.