Laser-Sustained Plasma Broadband Source Brightness
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Solution Overview
Problem
Existing high brightness laser-sustained plasma (LSP) light sources are insufficient for inspecting and measuring ever smaller defects in semiconductor metrology due to saturation in brightness and plasma instability at high pump powers, as most of the increased pump power is absorbed by the surrounding hot gas rather than the plasma core.
Innovation Solution
A high brightness laser-sustained plasma broadband light source is developed, utilizing anamorphic optics to focus the pump laser into an elliptical beam waist within the gas containment structure, enhancing plasma brightness by reflecting and re-focusing unabsorbed radiation back into the plasma, and optimizing the plasma dimensions for improved optical density and collection efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If pump laser power is increased to enhance plasma brightness, then brightness increases, but most of the increased power is absorbed by the surrounding hot gas rather than the plasma core, causing saturation in brightness enhancement
Solution Approach 1:
The patent segments the plasma into a dense core region and a surrounding less-dense region. By creating this spatial segmentation through controlled plasma generation, the pump laser can selectively interact with the dense core region where absorption is more efficient, rather than having its energy distributed throughout the entire plasma volume including the surrounding hot gas that causes saturation.
Solution Approach 2:
The patent applies local quality by creating a plasma with non-uniform density distribution, where the core region has higher density and is specifically optimized for laser absorption. This local optimization allows the pump laser to deposit energy efficiently in the core region without the surrounding less-dense gas absorbing excessive energy and causing brightness saturation.
2Temperature
If pump power is increased to maintain plasma temperature, then plasma temperature is maintained, but plasma instability increases at high pump powers
Solution Approach 1:
The patent maintains plasma temperature stability by concentrating pump laser energy in the dense core region where it is most efficiently absorbed. This localized heating approach maintains the required temperature without the instability that arises from distributed high-power pumping across the entire plasma volume, thus preventing plasma oscillations and instabilities at high pump powers.
Solution Approach 2:
The dense plasma core region acts as a self-regulating element that naturally absorbs pump laser energy more efficiently than the surrounding regions. This self-service mechanism allows the plasma to maintain its own temperature stability through preferential energy absorption in the core region, reducing the need for external control and minimizing instability at high pump powers.
3Power
If plasma size is increased to deliver more power to a given area, then power delivery increases, but the efficiency of power delivery decreases and the plasma size becomes mismatched with the required source size
Solution Approach 1:
The patent creates a compact, high-density plasma core region that concentrates the power delivery in a small volume. This local concentration of plasma density allows efficient power delivery to the required small area on the sample, avoiding the inefficiency of using a large plasma to illuminate a small area. The dense core provides high power density in a compact size that matches the inspection system requirements.
4Illumination intensity
If arc lamp current density is increased to improve brightness, then brightness increases, but electrode wear increases due to evaporation and sputtering
Solution Approach 1:
The patent replaces the mechanical electrode-based arc discharge system with a laser-sustained plasma system. Instead of using physical electrodes that undergo wear through evaporation and sputtering, the plasma is initiated and sustained by laser energy absorption in the gas. This substitution of the energy delivery mechanism eliminates the mechanical wear problem entirely while achieving high brightness through efficient laser-to-plasma energy coupling.
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 significantly increases the brightness of the plasma source, particularly in shorter UV and longer IR wavelengths, enhancing the sensitivity and throughput of semiconductor inspection and metrology systems by maintaining high plasma temperature and stability even at higher pump powers.
Implementation Method 1
a pump laser configured to generate a pump beam including illumination of a wavelength proximate to a weak absorption line of a neutral gas
Implementation Method 2
one or more anamorphic illumination optics configured to focus the pump beam into an approximately elliptical beam waist
Implementation Method 3
one or more first collection optics configured to collect broadband radiation emitted by the plasma
Data Source
AI summary
A high brightness laser-sustained broadband light source includes a gas containment structure and a pump laser configured to generate a pump beam including illumination of a wavelength at least proximate to a weak absorption line of a neutral gas contained in the gas containment structure. The broadband light source includes one or more anamorphic illumination optics configured to focus the pump beam into an approximately elliptical beam waist positioned in or proximate to the center of the gas containment structure. The broadband light source includes one or more first collection optics configured to collect broadband radiation emitted by the plasma in a direction substantially aligned with a longer axis of the elliptical beam waist.