Multi-Wavelength Overlay Metrology for Layered Grating Targets
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Solution Overview
Problem
Existing overlay metrology systems face challenges in accurately measuring multiple layers of a sample due to material properties and sample morphology, as a single wavelength often fails to penetrate through all layers, limiting the generation of high signal-to-noise signals and impacting the accuracy of overlay measurements.
Innovation Solution
An overlay metrology system using two or more illumination beams with different wavelengths to illuminate a grating-over-grating structure, capturing time-varying interference signals from photodetectors in the pupil plane to determine overlay measurements, allowing for equal signal-to-noise ratios and improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single wavelength is used to measure multiple layers, then the device complexity is reduced, but the measurement precision deteriorates because the wavelength may not penetrate through all layers
Solution Approach 1:
The illumination system is segmented into multiple wavelength channels, each optimized for measuring specific layers. The first wavelength targets the current layer (photo resist) while the second wavelength penetrates to reach the previous layer, allowing independent optimization for each measurement task.
Solution Approach 2:
The illumination wavelength parameter is changed to match the optical properties of different layers. By selecting specific wavelengths that correspond to the absorption and transmission characteristics of each layer material, the system achieves high signal-to-noise ratios for overlay measurements across multiple layers.
2Ease of operation
If a single wavelength is used, then the system is simpler to operate, but the signal-to-noise ratio deteriorates for certain layers
Solution Approach 1:
The wavelength parameter is optimized for each layer being measured. The first wavelength is selected to match the current layer properties while the second wavelength is selected to penetrate through to the previous layer, ensuring high signal-to-noise ratios for both measurements simultaneously.
3Measurement precision
If multiple wavelengths are used to measure all layers, then the measurement precision improves, but the device complexity increases
Solution Approach 1:
The multi-wavelength illumination system is designed to measure multiple layers simultaneously using a unified measurement process. The grating-over-grating structure with different pitches allows both wavelengths to contribute to overlay measurement in a single operation, achieving multi-functionality 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
The system enables high-throughput, sensitive overlay metrology by generating high signal-to-noise signals from both top and bottom gratings, overcoming the limitations of single-wavelength systems and providing accurate overlay measurements.
Implementation Method 1
two or more photodetectors located in a pupil plane to capture at least one diffraction order of a first illumination beam from the first-layer grating feature
Implementation Method 2
capture at least one diffraction order of a second illumination beam from the second-layer grating feature
Implementation Method 3
receive time-varying interference signals from the two or more photodetectors associated with the first-layer grating feature and the second-layer grating feature
Data Source
AI summary
A method may include illuminating an overlay target on a sample as the sample is translated along a stage-scan direction with two or more illumination beams. The two or more illumination beams include at least a first illumination beam having a first wavelength and a second illumination beam having a second wavelength, where the first wavelength is different than the second wavelength. The method may further include receiving time-varying interference signals from two or more photodetectors associated with a first-layer grating feature and a second-layer grating feature of a grating-over-grating structure. The first-layer grating feature may have a first pitch and the second-layer grating feature may have a second pitch different than the first pitch. The method may further include determining an overlay measurement between one of the first-layer grating feature and the second-layer grating feature of the sample based on the time-varying interference signals.