EUV Droplet Detection via Forward Scattered Light
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Solution Overview
Problem
Current droplet detection systems in EUV light sources face limitations due to the layout of the vessel, which reduces the numerical aperture of collection optics and requires high-power illumination, making it difficult to detect smaller droplets and limiting the focus of the illumination module, especially for droplets approaching the wavelength of the illumination.
Innovation Solution
A metrology system that utilizes forward scattered light to detect the position, size, and trajectory of droplets, incorporating an occlusion to block direct illumination and employing a homodyne method with stray light to refine measurements, allowing for the detection of smaller droplets such as satellites and subcoalesced droplets and facilitating in-line tuning of the droplet generator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If darkfield illumination with backscatter collection is used, then droplet detection is achieved, but the numerical aperture of collection optics is reduced and high-power illumination is required
Solution Approach 1:
The patent inverts the traditional darkfield illumination approach by collecting forward scattered light instead of backscattered light. The detection optics are positioned to receive light scattered in the forward direction (0-90 degrees relative to illumination), rather than collecting light scattered backward. This inversion allows use of the much stronger forward scattered signal, enabling detection with lower illumination power while maintaining or improving detection precision.
Solution Approach 2:
The patent introduces an occlusion element positioned in the optical path to block direct illumination from reaching the detector. This intermediary component allows the detection system to selectively receive only scattered light (forwards scattered by the droplet) while rejecting the intense direct beam, thereby enabling sensitive detection without requiring excessive illumination power.
2Measurement precision
If darkfield illumination with backscatter collection is used, then droplet detection is achieved, but the focus of the illumination module is limited
Solution Approach 1:
By inverting the collection geometry to capture forward scattered light, the patent relaxes the focusing requirements of the illumination module. Forward scattered light naturally diverges in a cone, making it easier to collect over a larger area without requiring tight focusing, thereby simplifying the illumination module design while maintaining detection precision.
3Measurement precision
If forward scattered light is detected, then smaller droplets such as satellites and subcoalesced droplets can be detected, but direct illumination must be blocked
Solution Approach 1:
The occlusion element serves as an intermediary that selectively blocks direct illumination while permitting forward scattered light to reach the detector. This simple geometric blocking mechanism enables detection of small droplets that would otherwise be obscured by the intense direct beam, without requiring complex filtering or modulation systems.
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 approach increases the signal availability, enabling the detection of smaller droplets and improving the numerical aperture of the collection optics, allowing for lower power illumination and more precise droplet positioning, thus enhancing the efficiency and accuracy of the droplet detection process.
Implementation Method 1
detect radiation forward scattered by the droplet
Data Source
AI summary
Disclosed is an apparatus for and method of detecting a droplet of target material in a system for generating EUV radiation in which an illumination system is used to illuminate the droplet of a target material and a detector is arranged to detect radiation from the illumination system that has been forward or side scattered by the droplet of target material.