Optical Return Beam Filtering for EUV Ghost Reflection Suppression
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Solution Overview
Problem
Existing optical systems in EUV lithography face challenges in accurately measuring primary return beams due to contamination by ghost reflections, which complicate the assessment of EUV generation effectiveness and require complex adjustments.
Innovation Solution
An optical system and method that utilizes an OFF-droplet and ON-droplet condition to measure and calibrate return beam measurements, distinguishing and suppressing ghost reflections by curvature analysis and polarization, enabling accurate primary return beam detection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If transmissive elements are used in the optical system, then light transmission efficiency is improved, but ghost reflections are generated that contaminate the return beam measurements
Solution Approach 1:
The patent extracts and isolates the harmful ghost reflections from the useful return beam by using a spatial filter. The spatial filter is positioned to selectively block the ghost reflection path while allowing the return beam to pass through, thereby removing the contamination without affecting the main measurement function.
Solution Approach 2:
The patent introduces a spatial filter as an intermediary element between the transmissive elements and the detector. This intermediary component mediates the interaction by selectively blocking ghost reflections while permitting the return beam to reach the detector, thus resolving the contradiction between maintaining light transmission and eliminating harmful reflections.
2Device complexity
If ghost reflections are not filtered out, then the optical system remains simple, but measurement accuracy of the primary return beam deteriorates
Solution Approach 1:
A spatial filter is introduced as an intermediary component to block ghost reflections while allowing the return beam to pass. This simple additive element provides the necessary measurement accuracy without requiring complex system redesign.
Solution Approach 2:
The spatial filter applies local quality control by selectively blocking only the specific spatial region where ghost reflections occur, while leaving the rest of the optical path unchanged. This localized intervention maintains overall system simplicity while improving measurement precision.
3Device complexity
If the return beam detector measures both primary return beam and ghost reflections, then the detection system remains simple, but the reliability of EUV generation assessment deteriorates
Solution Approach 1:
The spatial filter extracts and removes the harmful ghost reflection component from the detected signal, leaving only the primary return beam information. This extraction process ensures that the detector measures only the relevant signal for EUV generation assessment, thereby improving reliability without significantly increasing system complexity.
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
Accurately separates and filters out ghost reflections, providing reliable and precise measurements of primary return beams, enhancing EUV beam assessment and system control.
Implementation Method 1
The forward propagating beam is partially reflected on one or more of the transmissive elements and/or the droplet to form a return beam
Implementation Method 2
distinguishing and suppressing ghost reflections by curvature analysis
Implementation Method 3
distinguishing and suppressing ghost reflections by curvature analysis and polarization
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
The present invention provides an optical system comprising a plurality of transmissive and reflective elements arranged to transmit light of a unique wavelength from a source to an intended destination. The forward propagating beam follows a path through the transmissive elements and towards a droplet. The forward propagating beam is partially reflected on one or more of the transmissive elements and/or the droplet to form a return beam travelling along a path in a generally opposing direction to the forward propagating beam. The optical system further comprises is configured to estimate an amount of stray ghosting light in the return beam.