Catoptric Objective with Refractive Seal for DUV Imaging
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Solution Overview
Problem
Current imaging optics for defect inspection in the DUV range face challenges with dispersion characteristics of optical materials and limited availability of anti-reflective coatings, leading to difficulties in designing and manufacturing systems that support broadband illumination and detection below 250 nm, while also risking contamination and mechanical instability.
Innovation Solution
The development of an optical system with a 4 or 6 mirror objective configuration that includes refractive elements to improve manufacturability, reduce contamination risks, and provide simultaneous imaging across multiple wavelength bands and modes, utilizing refractive pupil relay optics to enhance flexibility and efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an all-reflective optical system is used for broadband illumination below 250 nm, then the dispersion characteristics and AR coating limitations are overcome, but the risk of contamination deposition and diffusion increases due to the opening in the mirror adjacent to the wafer
Solution Approach 1:
A refractive element is introduced as an intermediary component within the opening of the outermost mirror. This refractive element serves as a mediator that allows light to pass through while blocking contaminants from reaching the wafer and objective, thus resolving the contradiction between maintaining an all-reflective broadband optical path and preventing contamination
Solution Approach 2:
The optical system transitions from a purely reflective design to a composite catadioptric system that combines reflective mirrors with a refractive element. This composite approach allows the system to maintain broadband illumination capabilities while the refractive element provides contamination protection, resolving the contradiction between manufacturability and contamination risk
2Object-affected harmful factors
If the opening in the mirror is reduced to minimize contamination risk, then contamination deposition is reduced, but mechanical instabilities occur at or near the opening
Solution Approach 1:
The refractive element acts as a structural intermediary that fills and supports the opening area, allowing the opening to be minimized for contamination control while the refractive element provides mechanical support to prevent instabilities
Solution Approach 2:
The refractive element is placed locally within the opening area, providing localized contamination protection and mechanical support exactly where needed, without affecting the overall reflective optical path or requiring changes to the entire mirror structure
3Object-affected harmful factors
If a gas purge system is implemented to block contaminants, then contamination protection is improved, but color correction and AR performance issues arise, and mechanical instability occurs if the window is made too thin
Solution Approach 1:
The gas purge system is extracted and replaced by a solid refractive element that provides contamination protection through its physical presence rather than gas flow, eliminating the associated optical interference and mechanical complexity
Solution Approach 2:
The refractive element serves as a simple, robust contamination barrier that does not require complex gas delivery infrastructure, maintaining systems and reducing operational complexity compared to a gas purge system
4Measurement precision
If an all-refractive or catadioptric optical system is designed to support broadband illumination below 250 nm, then the detection capability is improved, but the dispersion characteristics of optical materials and limited AR coating availability make design and manufacture extremely difficult
Solution Approach 1:
Refractive optical elements are extracted from the broadband illumination path below 250 nm, eliminating the need for specialized materials and AR coatings in that wavelength range, thus simplifying design and manufacture while maintaining detection capability through the reflective path
Solution Approach 2:
The optical system is segmented into different functional zones: the all-reflective path handles broadband illumination below 250 nm, while a localized refractive element handles contamination protection, allowing each segment to be optimized independently for its specific function
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 achieves high numerical aperture and field of view with low central obscuration, improved manufacturability, and effective photo-contamination control, enabling efficient broadband imaging and simultaneous defect inspection across multiple wavelength bands and modes.
Implementation Method 1
The refractive optical element is disposed in the aperture in such a way that the aperture is sealed against transmission of contaminants
Implementation Method 2
an outermost mirror of an objective for an imaging system such as a wafer inspection system may have improved manufacturability by decreasing its aspect ratio at a sample side to between about 10:1 and about 20:1
Data Source
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.