Liquid Lithium Jet EUV Source for Mask Inspection
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Solution Overview
Problem
Current EUV light sources for lithographic mask inspection face challenges such as high cost, complexity, and limited brightness, with existing technologies requiring frequent replacement of components and suffering from contamination and erosion issues, which hinder efficient zero defect control in nanolithography.
Innovation Solution
A high-brightness EUV light source is developed using a continuous liquid lithium jet as the laser target, circulated in a closed loop system, with a collector mirror placed outside the vacuum chamber in an inert gas environment, and an EUV spectral purity filter to prevent contamination and extend the lifetime of optical components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a Z-pinch discharge is used as EUV light source, then the source is simple and compact, but the brightness is insufficient (B13,5 ~10 W/mm²sr) and the ceramic bush erodes requiring frequent replacement
Solution Approach 1:
The patent replaces the mechanical Z-pinch discharge system with a laser-produced plasma (LPP) system. The laser beam creates plasma in a gas jet, eliminating the need for complex electrical discharge mechanisms and ceramic bushes. This substitution achieves higher brightness (B13,5 ≥30 W/mm²sr) while maintaining structural simplicity, as the laser system requires no moving parts or erodible components in the interaction zone.
2Illumination intensity
If xenon is used as target material in LPP, then high brightness is achieved, but the system becomes complicated with frequent mirror protection needs and high operating costs
Solution Approach 1:
The patent changes the target material parameter from xenon to a lighter gas such as helium, neon, or argon. This parameter change fundamentally alters the plasma characteristics, reducing the formation of heavy ion contaminants that require complex protection and recycling systems. The lighter gas targets achieve sufficient brightness while dramatically simplifying the overall system architecture and reducing operational complexity.
3Power
If tin droplets are used as laser target, then powerful EUV light is generated, but the system requires complex pre-pulse laser synchronization and frequent target replacement
Solution Approach 1:
The patent replaces the solid tin droplet target system with a gas jet system. The gas flows through a nozzle to create a controlled jet that interacts with the laser beam. This pneumatic approach eliminates the need for complex droplet generation, acceleration, and synchronization mechanisms. The gas jet provides a continuous, stable target that simplifies the laser system requirements while maintaining high EUV power output.
4Length of stationary object
If the collector mirror is placed inside the vacuum chamber, then the light path is short, but the mirror suffers from contamination and erosion reducing its lifetime
Solution Approach 1:
The patent introduces a differential pumping system with an intermediate pressure region between the vacuum chamber and the collector mirror environment. This intermediary pressure zone acts as a barrier, allowing the mirror to be protected from direct exposure to the harsh vacuum plasma environment while maintaining the necessary light path. The differential pumping creates a pressure gradient that prevents contaminant migration to the mirror surface, extending its operational lifetime.
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 solution provides a reliable, cost-effective, and stable EUV light source with high spatial and energy stability, enabling efficient actinic inspection of lithographic masks and extending the operational life of optical components.
Implementation Method 1
irradiating at high repetition rate the continuous liquid lithium jet by a laser beam introduced into the interaction zone through an input window of the vacuum chamber
Implementation Method 2
producing a laser target as a continuous liquid lithium jet... irradiating... by a laser beam... led out EUV light from the laser-produced plasma
Data Source
AI summary
The invention provides a method and apparatus for a commercially viable EUV light source for EUV metrology and actinic inspection of EUV lithography masks. The invention is carried out using a laser target in the form of a continuous jet of liquid Lithium, circulated in a closed loop system by means of a high temperature pump. The collector mirror is placed outside the vacuum chamber in an environment filled with an inert gas and EUV output to a collector mirror is provided through the spectral purity filter, configured as an EUV exit window for the vacuum chamber. In the vacuum chamber, the input window for the laser beam is coated with a screening optical element. Evaporative cleaning of the EUV spectral purity filter and the screening optical element is provided. The protective shield with a temperature higher than 180° C. may be adjusted around the target jet.