Asymmetric Optical Isolator for EUV Back-Reflection Suppression
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Solution Overview
Problem
Conventional optical isolators used in EUV radiation generating devices are limited in their ability to suppress back-reflected laser radiation, leading to reduced amplification power and increased downtime due to thermal lensing and phase shift limitations, which affects the productivity of EUV lithography systems.
Innovation Solution
An asymmetric optical isolator with a diaphragm design that has a lower plasma ignition threshold for back-reflected laser radiation, utilizing different materials and surface treatments to selectively reduce the plasma ignition threshold for reverse pulsed laser radiation, allowing for effective filtering of back-reflected radiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional optical isolators (Faraday insulators or electro-optical modulators) are used to filter back-reflected laser radiation, then the beam source is protected from damage, but the isolators can only handle limited maximum power and require frequent shutdowns when the power limit is reached
Solution Approach 1:
The invention changes the material parameter of the aperture from conventional materials to a material with lower plasma ignition threshold, enabling the optical isolator to handle higher laser radiation powers without damage. This parameter change allows the system to maintain higher amplification power levels continuously without frequent shutdowns for maintenance or replacement of the isolator components.
2Object-affected harmful factors
If optical filtering elements are used in optical isolators to suppress back-reflected radiation, then the reflected laser radiation is filtered, but thermal lensing occurs in the filtering element that alters beam characteristics
Solution Approach 1:
The invention replaces the conventional optical filtering mechanism (which uses physical filtering elements that cause thermal lensing) with a plasma-based filtering mechanism. The plasma, generated by laser radiation interacting with the aperture material, provides the filtering effect without the thermal lensing problems of solid filtering elements, thereby preserving beam characteristics while still suppressing back-reflected radiation.
3Object-affected harmful factors
If optical isolators based on polarization or phase shift are used, then laser radiation with specific phase jump is suppressed, but the phase jump value is not maintained upon reflection from droplet, preventing complete suppression
Solution Approach 1:
The invention changes the fundamental mechanism of the optical isolator from polarization/phase-shift based (which is sensitive to phase jump variations) to plasma-based filtering. The plasma's optical properties can be dynamically adjusted by controlling the laser radiation parameters, allowing the system to maintain effective suppression even when the phase jump upon reflection varies, thereby improving reliability across different operating conditions.
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
The asymmetric diaphragm design effectively suppresses back-reflected laser radiation by creating a plasma that absorbs or deflects the radiation, reducing the need for frequent system shutdowns and increasing the productivity of EUV lithography systems by maintaining higher amplification power.
Implementation Method 1
an aperture (1) having a diaphragm opening for the passage of laser radiation propagating through the aperture in a first direction, wherein the aperture serves to influence a plasma ignition threshold for the ignition of a plasma in order to suppress the passage of laser radiation propagating through the aperture in a second direction opposite to the first
Implementation Method 2
The asymmetric diaphragm design effectively suppresses back-reflected laser radiation by creating a plasma that absorbs or deflects the radiation
Data Source
Figure 1a~2c
Figure 3a~5b
AI summary
The invention relates to an optical isolator comprising: a stop (1) which has an aperture (3) for passing laser radiation (5), said laser radiation passing through the aperture (3) in a first direction (R1), wherein the stop (1) serves to influence a plasma ignition threshold (IP1, IP2) for igniting a plasma in order to suppress the passage of laser radiation propagating in a second direction, opposite to the first, through the aperture (3). For laser radiation which impinges on the second side (2b) of the stop (1) in a vicinity (6b) of the aperture (3), the stop (1) has a lower plasma ignition threshold (IP2 < IP1) than for laser radiation (5) which impinges on the first side (2a) of the stop (1) in a vicinity (6a) of the aperture (3). The invention also relates to a driver laser arrangement and an EUV radiation production apparatus comprising at least one of such optical isolator.