EUV Diffraction Grating Aperture Ratio for Aberration Measurement
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Solution Overview
Problem
Current aberration measurement systems for EUV lithographic systems face inaccuracies due to unwanted interference beams generated by pinhole arrays, leading to errors in wavefront reconstruction and system performance.
Innovation Solution
A two-dimensional diffraction grating with a specific geometry, featuring a ratio of circular aperture radius to center-to-center distance between 0.34 and 0.38, is used to minimize gain and cross-talk errors in aberration map reconstruction, reducing the number of interference beams contributing to the phase stepping signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a pinhole array is used in the diffraction grating, then the device can be manufactured with standard techniques, but unwanted interference beams are generated that reduce measurement precision
Solution Approach 1:
The patent applies parameter changes by optimizing the ratio of aperture diameter to pitch (distance between adjacent apertures) to fall within the specific range of 0.34 to 0.38. This parameter optimization minimizes the generation of unwanted interference beams while maintaining manufacturability through standard pinhole array fabrication techniques, thereby resolving the contradiction between ease of manufacture and measurement precision
2Quantity of substance
If the aperture pitch is reduced to increase aperture density, then more diffraction beams are captured, but gain errors and cross-talk errors increase in the aberration reconstruction
Solution Approach 1:
The patent resolves this contradiction by optimizing the aperture pitch and aperture diameter parameters simultaneously. The specific ratio range of 0.34 to 0.38 ensures that apertures are densely packed to capture sufficient diffraction beams while maintaining proper spacing to minimize gain errors and cross-talk errors in the wavefront reconstruction algorithm
3Device complexity
If a traditional diffraction grating geometry is used, then the device complexity is low, but the number of interference beams contributing to the phase stepping signal is high, reducing measurement accuracy
Solution Approach 1:
The patent maintains the simple two-dimensional array geometry of traditional diffraction gratings (avoiding complex structures) while optimizing the critical parameters of aperture diameter and pitch ratio to 0.34-0.38. This parameter optimization reduces the number of unwanted interference beams in the phase stepping signal without increasing device complexity, as the fundamental grating structure remains unchanged
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 enhances the accuracy of aberration measurement by reducing gain and cross-talk errors, improving imaging, overlay, and focus performance in EUV lithographic systems.
Implementation Method 1
The reticle level patterning device is illuminated with radiation to form a plurality of first diffraction beams, the first diffraction beams being separated in a shearing direction
Implementation Method 2
The projection system at least partially captures the plurality of first diffraction beams and images these onto the patterning device of the wafer level sensor
Implementation Method 3
A measurement system for determining an aberration map for a projection system may be used to measure aberrations caused by the projection system. Such a measurement system may be used within a lithographic apparatus and may comprise a shearing interferometer
Data Source
AI summary
Diffraction gratings for a phase-stepping measurement system for determining an aberration map for a projection system are disclosed. The gratings are two-dimensional diffraction gratings for use as wafer level gratings in an EUV lithographic apparatus. In particular, the diffraction gratings include a substrate provided with a two-dimensional array of circular through-apertures and are self-supporting. In some embodiments, a ratio of the radius of the circular apertures to the distance between the centers of adjacent apertures may be selected to minimize the gain and cross-talk errors of a wavefront reconstruction algorithm. For example, the ratio may be between 0.34 and 0.38. In some embodiments, the circular apertures are distributed such that a distance between the centers of adjacent apertures is non-uniform and varies across the diffraction grating. For example, a local pitch of the grating may vary randomly across the diffraction grating.