Focus Metrology Target Design with Normalization Sub-Targets
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Solution Overview
Problem
Current focus metrology techniques in EUV lithography are inadequate due to low signal strength and sensitivity to errors, particularly in disentangling focus and dose sensitivity.
Innovation Solution
The method involves using a target design with a combination of focus-sensitive and normalization sub-targets, where the normalization sub-targets have minimal focus sensitivity, allowing for normalized focus measurements and improved signal calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diffraction based focus techniques are used with target structures, then focus measurement capability is provided, but signal strength is low and sensitivity to errors is high
Solution Approach 1:
The target structure is segmented into multiple sub-targets (first sub-target, second sub-target, third sub-target, fourth sub-target) with different focus sensitivities. This segmentation allows the system to separately measure focus-dependent parameters and normalize them against focus-insensitive references, thereby improving measurement precision while reducing error sensitivity through differential measurement.
Solution Approach 2:
Different sub-targets are designed with different local qualities regarding focus sensitivity. The first and second sub-targets have different first focus sensitivities, while the third and fourth sub-targets have different second focus sensitivities. This local differentiation enables selective measurement of focus effects while compensating for systematic errors through normalization.
2Measurement precision
If focus-sensitive target structures are used for metrology, then focus parameter determination is enabled, but disentangling focus and dose sensitivity becomes difficult
Solution Approach 1:
The target is segmented into sub-targets with distinct focus sensitivity characteristics. By measuring multiple sub-targets with different focus sensitivities and performing normalization calculations, the system can mathematically disentangle focus effects from dose effects, enabling accurate focus parameter determination while separating the coupled sensitivities.
Solution Approach 2:
The invention changes the focus sensitivity parameter across different sub-targets. The first sub-target has a different first focus sensitivity than the second sub-target, and similarly for the third and fourth sub-targets. This parameter variation across sub-targets enables the system to isolate and measure focus effects independently from dose effects through comparative analysis.
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 the robustness of focus and dose metrology by reducing sensitivity to errors and improving signal calibration, leading to more accurate focus control in EUV lithography.
Implementation Method 1
A lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate.
Implementation Method 2
Diffraction based focus may use target forming features on the reticle which print targets having a degree of asymmetry which is dependent on the focus and/or dose setting during printing. This degree of asymmetry can then be measured and the focus and/or dose inferred from the asymmetry measurement.
Data Source
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Figure 6(a)~6(c)
AI summary
Disclosed is a method of metrology, comprising obtaining a first measurement parameter value relating to at least a first sub-target; obtaining a third measurement parameter value relating to at least a third sub-target; and determining at least a focus parameter from at least said first measurement parameter value, said determination being normalized using at least said third measurement parameter value. The at least third sub-target comprising a normalization sub-target comprising substantially no or minimal focus sensitivity. Also disclosed is a method for designing a focus target.