Laser Sustained Plasma VUV Illumination with Purged Gas Path
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Solution Overview
Problem
Current laser-sustained plasma light sources face limitations due to the absorption of short-wavelength light by fused silica bulbs, leading to rapid degradation and overheating, and pulsed plasma systems encounter difficulties with registration, alignment, and data combination, particularly in the 190-260 nm wavelength range.
Innovation Solution
A system and method utilizing a laser sustained plasma (LSP) illumination sub-system that generates broadband radiation by focusing pumping illumination into a gas volume, emitting wavelengths suitable for VUV inspection, with a purged chamber to minimize signal degradation and separate plasma pumping illumination from the short-wavelength output, enabling efficient imaging of samples with improved optical transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fused silica bulbs are used in laser-sustained plasma sources, then the system can contain the plasma and transmit light, but the bulbs rapidly degrade and overheat due to absorption of short-wavelength light below 185-190 nm
Solution Approach 1:
The patent removes the fused silica bulb from the optical path entirely. Instead of containing plasma in a bulb, the system uses an open gas cell where plasma is generated and maintained by laser pumping. This extraction eliminates the bulb's absorption problem at short wavelengths while still allowing plasma containment through magnetic fields and precise laser focusing.
Solution Approach 2:
The patent introduces a nitrogen-purged environment to replace air in the optical path. Nitrogen is transparent to VUV radiation and prevents absorption and scattering that would occur with atmospheric gases. This creates an inert optical environment that allows efficient transmission of short-wavelength light without degradation.
2Illumination intensity
If pulsed plasma sources are used for VUV inspection, then short-wavelength radiation can be generated, but difficulties arise with registration, alignment, and data combination due to timing synchronization requirements
Solution Approach 1:
The patent transitions from pulsed plasma operation to continuous wave (CW) laser pumping that sustains plasma continuously. This eliminates the need for precise timing synchronization between pulses, detectors, and stage motion. The continuous plasma provides steady-state VUV radiation that simplifies data acquisition and processing while maintaining high illumination intensity.
Solution Approach 2:
The patent uses periodic modulation of the CW laser pump power to control plasma emission intensity on demand. This allows the system to operate in a controlled manner similar to pulsed systems but with the simplicity of CW operation, enabling flexible illumination control without the complexity of pulse synchronization.
3Power
If laser-sustained plasma sources operate in the 190-260 nm wavelength range, then powerful broadband radiation can be generated, but the fused silica bulb absorbs this light causing rapid degradation
Solution Approach 1:
The patent removes the fused silica bulb that causes absorption in the 190-260 nm range. By using an open gas cell configuration with magnetic confinement, the system allows broadband plasma radiation to escape directly into the nitrogen-purged optical path without encountering absorbing bulb material, preserving optical transmission capability.
Solution Approach 2:
The patent changes the optical path parameters by replacing air with nitrogen gas and removing the bulb window. These parameter changes eliminate absorption coefficients at 190-260 nm, allowing powerful broadband radiation to transmit efficiently through the optical system without degradation of transmission capability.
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 solution allows for effective imaging of samples with short-wavelength radiation, such as VUV light, by generating and separating the plasma-generated broadband radiation from the pumping illumination, thereby overcoming the limitations of existing systems and enhancing the durability and performance of optical inspection systems.
Implementation Method 1
Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting light. This effect is typically referred to as 'pumping' the plasma.
Implementation Method 2
the plasma emits broadband radiation including one or more second selected wavelengths
Implementation Method 3
a collector configured to focus the pumping illumination from the pumping source into the volume of gas contained within the gas containment element in order to generate a plasma within the volume of gas
Implementation Method 4
a purged chamber containing a selected purge gas and configured to purge at least a portion of the illumination pathway and the collection pathway
Data Source
AI summary
The inspection of a sample with VUV light from a laser sustained plasma includes generating pumping illumination including a first selected wavelength, or range of wavelength, containing a volume of gas suitable for plasma generation, generating broadband radiation including a second selected wavelength, or range of wavelengths, by forming a plasma within the volume of gas by focusing the pumping illumination into the volume of gas, illuminating a surface of a sample with the broadband radiation emitted from the plasma via an illumination pathway, collecting illumination from a surface of the sample, focusing the collected illumination onto a detector via a collection pathway to form an image of at least a portion of the surface of the sample and purging the illumination pathway and/or the collection pathway with a selected purge gas.