EUV Illumination Optical Unit Hollow Waveguide Homogeneity
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Solution Overview
Problem
Mask inspection systems using EUV illumination light face challenges in achieving high throughput while maintaining illumination homogeneity, leading to variations in illumination dose that are not adequately addressed by existing technologies.
Innovation Solution
An illumination optical unit comprising a hollow waveguide and an imaging mirror optical unit with grazing incidence mirrors, optimized for EUV light, which includes a Wolter telescope configuration with ellipsoid and hyperboloid mirrors, ensuring high reflectivity and compact design to achieve homogeneous illumination with low dose variation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional illumination optical units are used, then the system structure is simple, but the throughput is low and illumination homogeneity cannot be maintained
Solution Approach 1:
The illumination optical unit is divided into two independent functional modules: a hollow waveguide for light transport and mixing, and an imaging mirror optical unit for field illumination. This segmentation allows each module to be optimized independently - the waveguide maximizes light throughput through multiple reflections, while the mirror unit ensures homogeneous illumination distribution, thereby resolving the contradiction between high throughput and structural simplicity.
Solution Approach 2:
The hollow waveguide acts as an intermediary component between the light source and the imaging mirror optical unit. It receives illumination light, performs multiple internal reflections to mix the light uniformly, and transports it to the mirror unit. This intermediary function enables the system to achieve both high throughput (via efficient light transport) and good illumination homogeneity (via mixing), without requiring direct coupling between source and field.
2Productivity
If high throughput is achieved, then more light is transmitted, but illumination homogeneity deteriorates due to dose variation
Solution Approach 1:
By separating the light transport function (waveguide) from the light distribution function (imaging mirror unit), the system can optimize each for its specific purpose. The waveguide maximizes transmission through efficient total internal reflections, while the mirror unit independently controls illumination homogeneity by imaging the exit opening onto the illumination field, achieving dose variation less than 5% perpendicular to the scanning axis.
Solution Approach 2:
The imaging mirror optical unit is designed with specific parameters: at least one mirror with grazing incidence angle greater than 60°, and an image-side numerical aperture less than 0.2. These parameter optimizations ensure that high transmission is maintained while achieving homogeneous illumination distribution across the illumination field, resolving the contradiction between transmission efficiency and illumination uniformity.
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 enables high-efficiency illumination with less than 5% dose variation perpendicular to the scanning axis, achieving a high throughput and maintaining illumination homogeneity across the illumination field, thereby improving the inspection process.
Implementation Method 1
a hollow waveguide for guiding the illumination light, comprising an entry opening for the illumination light and an exit opening for the illumination light
Implementation Method 2
an imaging mirror optical unit, arranged down-stream of the hollow waveguide, for imaging the exit opening into an illumination field
Data Source
AI summary
An illumination optical unit for a mask inspection system is used with EUV illumination light. A hollow waveguide of the illumination optical unit serves for guiding the illumination light. The hollow waveguide has an entry opening for the illumination light and an exit opening for the illumination light. An imaging mirror optical unit, arranged downstream of the hollow waveguide serves to image the exit opening into an illumination field. This results in an illumination optical unit, the throughput of which is optimized for the EUV illumination light.