Catoptric Objective with Refractive Sealing for Broadband Imaging
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Solution Overview
Problem
Current imaging optics for sample inspection face challenges in achieving high resolution and manufacturability for wavelengths below 250 nm due to increased dispersion in optical materials and limited Anti-Reflective (AR) coating availability, while also dealing with contamination risks and limited simultaneous imaging capabilities across multiple wavelength bands and modes.
Innovation Solution
The development of an optical system with a 4 or 6 mirror objective configuration, incorporating refractive elements for improved manufacturability and photo-contamination control, and utilizing refractive pupil relay optics to enhance flexibility and aberration compensation, allowing for broadband imaging with multiple wavelength bands and simultaneous inspection modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an all-reflective optical system is used for wavelengths below 250 nm, then manufacturability and design are substantially improved, but contamination risk increases due to the opening in the mirror adjacent to the wafer
Solution Approach 1:
A refractive element is introduced as an intermediary component in the optical path, positioned to seal the opening in the outermost mirror. This refractive element serves as a mediator that blocks contaminants from reaching the wafer and objective while still allowing optical radiation to pass through, thus resolving the contamination risk without compromising the all-reflective system's manufacturability benefits
2Object-affected harmful factors
If a large flow through the opening is provided to reduce contamination risk, then contamination protection is improved, but mechanical instabilities occur at or near the opening
Solution Approach 1:
The refractive element acts as a physical barrier that seals the opening, eliminating the need for large gas flows to protect against contamination. This sealing approach provides contamination protection while avoiding the mechanical instabilities that would result from high-velocity gas flows through the opening
Solution Approach 2:
The refractive element creates a sealed environment that can be maintained with controlled atmospheric conditions, providing contamination protection through environmental control rather than through high-velocity gas flows that would cause mechanical instability
3Object-affected harmful factors
If a thin window is used for gas purge system, then contamination protection is improved, but color correction and AR performance issues occur and mechanical instability increases
Solution Approach 1:
Instead of using a thin window that would be mechanically unstable and optically problematic, the invention changes the parameter of the sealing element to be a thicker, more robust refractive element. This parameter change provides adequate contamination protection while maintaining mechanical stability and allowing for proper optical coatings and correction
4Adaptability or versatility
If multiple imaging paths are implemented, then simultaneous inspection across multiple wavelength bands is improved, but device complexity increases
Solution Approach 1:
The all-reflective objective design provides a universal platform that can support multiple imaging paths for different wavelength bands. The reflective optics inherently handle broadband radiation well, allowing a single objective to serve multiple functions across UV, DUV, and VUV ranges, thereby enabling simultaneous inspection capabilities without proportionally increasing complexity
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 solution enables high numerical aperture and large field of view imaging with reduced central obscuration, improved manufacturability, and simultaneous defect inspection across multiple wavelength bands and modes, while minimizing contamination risks and optimizing AR performance.
Implementation Method 1
an objective having at least four mirrors including a concave outermost mirror located closest to the inspected sample among the at least four mirrors
Implementation Method 2
The pupil relay optics include one or more refractive optical elements and is centered on the optical axis
Data Source
Figure 1a~1b
Figure 1c
Figure 1d
AI summary
An optical system may include an objective having at least four mirrors including an outermost mirror with aspect ratio < 20:1 and focusing optics including a refractive optical element. The objective provides imaging at numerical aperture > 0.7, central obscuration < 35% in pupil. An objective may have two or more mirrors, one with a refractive module that seals off an outermost mirror's central opening. A broad band imaging system may include one objective and two or more imaging paths that provide imaging at numerical aperture > 0.7 and field of view > 0.8 mm. An optical imaging system may comprise an objective and two or more imaging paths. The imaging paths may provide two or more simultaneous broadband images of a sample in two or more modes. The modes may have different illumination and/or collection pupil apertures or different pixel sizes at the sample.