Metrology Targets with Reflection-Symmetric Asymmetric Structures
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Solution Overview
Problem
Current Advanced Imaging Metrology (AIM) targets suffer from significant suppression of high harmonics, making it difficult to distinguish displacement and asymmetry, leading to inaccurate overlay measurements due to their symmetric features and optical system limitations.
Innovation Solution
The introduction of reflection-symmetric pairs of reflection-asymmetric structures in metrology targets, which allow for higher information content and accuracy in measurements by breaking symmetry and preventing harmonic suppression, enabling unique determination of the center of symmetry and reducing sensitivity to random perturbations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If symmetric features are used in AIM targets, then the target structure is simple and easy to manufacture, but high harmonics are suppressed by the optical system making displacement and asymmetry indistinguishable
Solution Approach 1:
The patent applies asymmetry by introducing reflection-asymmetric structures within reflection-symmetric pairs. Each asymmetric structure has a specific asymmetry parameter that breaks the symmetry in a controlled manner, allowing the generation of high harmonics that were previously suppressed. This enables the optical system to distinguish displacement from asymmetry, thereby improving overlay measurement accuracy while maintaining a relatively simple target structure.
2Ease of operation
If periodic structures with single frequency are used, then sub-pixel interpolation is convenient and noise reduction is achieved, but high harmonics are suppressed making accurate overlay measurement unverifiable
Solution Approach 1:
The patent applies local quality by making each individual structure within the periodic array asymmetric, while maintaining the overall periodic arrangement. The asymmetry parameter varies locally across different structures in a controlled manner, allowing high harmonics to be generated from each local element. This preserves the periodicity benefits for sub-pixel interpolation and noise reduction while enabling verification of accurate overlay measurement through the presence of high harmonics.
3Device complexity
If symmetric target elements are used, then the target design is simple, but image analysis algorithms are vulnerable to errors due to exclusion of particular diffraction orders
Solution Approach 1:
The patent introduces controlled asymmetry in target elements by defining asymmetric shapes with specific asymmetry parameters. This breaks the symmetry that causes exclusion rules for certain diffraction orders, allowing more complete information to be captured in the image. The asymmetric structures generate additional harmonic content that makes image analysis algorithms more robust and less vulnerable to errors, while the overall target design remains relatively simple through the use of regular periodic arrangements.
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 enhances measurement accuracy by increasing the information content of the target images, allowing for precise determination of the center of symmetry and reducing errors from random and process variations, thereby improving the reliability of overlay measurements.
Implementation Method 1
symmetric features, based on their extent and particular details of the light scattering, often exclude generation of particular diffraction orders
Data Source
AI summary
Metrology targets, design files, and design and production methods thereof are provided. Metrology targets comprising at least one reflection-symmetric pair of reflection-asymmetric structures are disclosed. The structures may or may not be periodic, may comprise a plurality of unevenly-spaced target elements, which may or may not be segmented. The asymmetry may be with respect to target element segmentation or structural dimensions. Also, target design files and metrology measurements of the various metrology targets are disclosed.