Reflective Diffraction Grating Stray Light Reduction
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Solution Overview
Problem
Conventional mirrors with grating patterns for soft X-rays (SXR) and extreme ultraviolet (EUV) radiation struggle to minimize stray light outside well-defined diffraction orders, particularly around the zeroth order, which affects the accuracy of metrology measurements.
Innovation Solution
A method for manufacturing reflective diffraction gratings with specific substructures and sidewall configurations to optimize specular reflection and diffraction, using a combination of ridge and trench structures with controlled aspect ratios and pitches to minimize stray light, ensuring that rays reflecting into the zeroth order are obscured by the sidewall, thereby enhancing measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional mirror surfaces are used for SXR and EUV radiation, then the mirror can reflect radiation, but stray light is generated outside well-defined diffraction orders which reduces measurement accuracy
Solution Approach 1:
The mirror surface is segmented into periodic grating structures with distinct substructures (ridges and trenches). This segmentation creates well-defined diffraction orders while confining stray light to specific angular regions, thereby improving measurement accuracy by separating useful diffracted radiation from harmful stray light.
Solution Approach 2:
Different regions of the grating structure are given different local qualities: ridges are optimized for specular reflection while trenches are designed to obscure stray light paths. The sidewalls of trenches are specifically configured with particular aspect ratios to block stray light while maintaining diffraction efficiency, applying local quality optimization to resolve the contradiction.
2Measurement precision
If grating structures are added to mirrors to reduce stray light, then measurement accuracy improves, but the manufacturing complexity increases
Solution Approach 1:
The stray light blocking function is extracted as a separate geometric constraint (sidewall configuration of trenches) rather than being integrated into the basic grating pattern. This allows the grating structure to be designed primarily for diffraction efficiency, while the sidewall geometry is independently optimized to obscure stray light paths, simplifying the overall design process.
Solution Approach 2:
Specific parameter ranges are established for trench aspect ratios and sidewall angles that simultaneously achieve stray light obscuration and manufacturability. By defining optimal parameter ranges rather than requiring exact geometric configurations, the manufacturing complexity is reduced while maintaining the stray light reduction functionality.
3Object-generated harmful factors
If trench depth is increased to obscure stray light paths, then stray light reduction improves, but manufacturing difficulty increases
Solution Approach 1:
The grating structure employs asymmetric trench designs where the sidewall angles and depths are specifically optimized to provide stray light obscuration at minimal depths. The asymmetric configuration allows the trench to block stray light paths effectively while maintaining a shallower profile that is easier to manufacture compared to symmetric deep trench designs.
Solution Approach 2:
Instead of requiring complete obscuration of all potential stray light paths through excessive trench depth, the design uses partial obscuration with optimally positioned sidewalls. This partial action approach achieves sufficient stray light reduction while keeping trench depths within manufacturable limits, avoiding the need for excessively deep trenches.
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 reduces stray light, improving the accuracy of diffraction spectrum measurements and achieving efficient specular reflection, which is crucial for metrology applications in the SXR and EUV domains.
Implementation Method 1
determining a configuration of the first substructure to specularly reflect the beam, incident at a grazing angle of incidence
Implementation Method 2
determining a fixed or varying pitch configuration of grating periods including the second substructure to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction
Data Source
AI summary
A grating is provided on a mirror for specularly reflecting and diffracting a grazing-incidence beam of radiation and has a periodic structure with a grating period comprising first (ridge) and second (trench) substructures either side of a sidewall 806 facing the incident beam 800. The ridge is configured to specularly reflect the beam from the flat top 808 of the ridge into a specularly reflected beam 810 in a zeroth-order direction β′=β. The grating is configured with fixed or varying pitch to diffract the beam from the grating periods in one or more non-zero-diffraction-order direction β′≠β. The shape of the trench may be is described by structural parameters top width and depth that define the aspect ratio of the trench. The shape is determined such that any rays (and optionally diffraction) of the beam that reflect once from the trench floor in the zeroth-order direction are obscured by the sidewall.