Laser-Driven Plasma Photon Source Brightness
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Solution Overview
Problem
Current photon sources, such as xenon arc-discharge lamps, face challenges in achieving high brightness for metrology applications in lithographic processes, particularly in delivering increased spectral bandwidth with lower transmittance while maintaining or reducing measurement time, due to the small optical etendue at the target side of the metrology apparatus.
Innovation Solution
A plasma-based photon source apparatus is developed, comprising a container for a gaseous atmosphere, a driving system to generate and focus radiation into an elongate plasma zone, and a collecting optical system to align with the plasma's longitudinal direction, enhancing brightness by capturing photons emitted along the plasma's length rather than its transverse direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a conventional xenon arc-discharge lamp is used as the light source, then the apparatus can provide broadband radiation covering near infrared, visible and ultraviolet bands, but the brightness is insufficient to meet future requirements for increased spectral bandwidth and lower transmittance while maintaining measurement time
Solution Approach 1:
The invention changes the fundamental parameters of the light source by transitioning from a conventional xenon arc-discharge lamp to a laser-driven plasma source. This involves changing the excitation mechanism (from electrical discharge to laser-induced plasma), the plasma composition (using gas jets with controlled composition), and the operational parameters (laser wavelength, pulse duration, power) to achieve both high brightness and broad spectral coverage simultaneously
Solution Approach 2:
The invention exploits the temporal dimension by using pulsed laser excitation to generate plasma that emits radiation in short, intense bursts. This time-domain approach allows the system to deliver extremely high peak brightness while maintaining broad spectral bandwidth, effectively adding a temporal dimension to the light source characteristics that resolves the contradiction between brightness and spectral capability
2Illumination intensity
If the total source power is increased to achieve higher brightness, then more energy is available, but the optical etendue remains very small due to the small spot size, preventing real increase in usable brightness
Solution Approach 1:
The invention applies local quality by creating a plasma source with highly non-uniform spatial distribution of radiation emission. The plasma is confined to a small region (maintaining small etendue) but emits radiation with extremely high intensity locally. The gas jet geometry and laser focusing create localized zones of enhanced emission that deliver high usable brightness to the small spot size required by the metrology apparatus
Solution Approach 2:
The pulsed laser operation ensures continuous delivery of useful radiation energy to the target. By operating in a pulse regime with appropriate repetition rates, the system maintains continuous illumination of the measurement spot while allowing the plasma to form and decay between pulses. This ensures that every unit of energy put into the system contributes to useful brightness at the target, maximizing energy delivery efficiency
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 significantly increases the brightness of the photon source, as demonstrated by experimental data showing a factor of 5 or more improvement in visible and ultraviolet spectra, and up to a factor of 15 with optimized plasma dimensions, effectively addressing the limitations of existing sources.
Implementation Method 1
a driving system for generating driving radiation and forming the driving radiation into at least one beam focused on a plasma forming zone within the container thereby to generate a plasma in the plasma forming zone
Implementation Method 2
a collecting optical system for collecting photons emitted by the plasma and forming the collected photons into at least one beam of output radiation
Data Source
Figure 1~2
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Figure 5~6
AI summary
A laser driven light source comprises laser (52) and focusing optics (54). These produce a beam of radiation focused on a plasma forming zone within a container (40) containing a gas (e.g., Xe). Collection optics (44) collects photons emitted by a plasma (42) maintained by the laser radiation to form a beam of output radiation (46). The plasma has an elongate form (L > d) and the collecting optics is configured to collect photons emerging in the longitudinal direction from the plasma. The brightness of the plasma is increased compared with sources which collect radiation emerging transversely from the plasma. A metrology apparatus using the light source can achieve greater accuracy and/or throughput as a result of the increased brightness. Back reflectors may be provided. Microwave radiation may be used instead of laser radiation to form the plasma.