Metrology Target Layout for Robust Overlay Measurement
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Solution Overview
Problem
Existing metrology targets in lithographic processes face challenges in minimizing overlay error and ensuring robustness against process variations, particularly due to process-induced asymmetries and variations in film thickness, etch biases, and geometry asymmetries.
Innovation Solution
A method for designing metrology targets involves determining the sensitivity of target parameters to process perturbations and evaluating their robustness by calculating a performance indicator based on the product of sensitivities and perturbations, allowing for efficient simulation and selection of targets resistant to process variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional metrology targets are used in lithographic processes, then overlay measurements can be performed, but measurement precision deteriorates due to process-induced asymmetries and variations in film thickness, etch biases, and geometry asymmetries
Solution Approach 1:
The patent employs asymmetric metrology target designs where the target structure intentionally incorporates asymmetric features that are sensitive to overlay errors. The target includes asymmetric grating patterns or geometric figures that produce measurable signal variations when overlay deviations occur, enabling precise detection of alignment errors between layers while being resistant to symmetric process variations
Solution Approach 2:
The patent utilizes parameter changes by varying the geometric parameters of the metrology target (such as grating line widths, spacing, orientation, and pattern density) to optimize sensitivity to overlay errors. By carefully selecting and adjusting these parameters, the target becomes highly responsive to overlay deviations while minimizing sensitivity to common process variations like film thickness variations and etch biases
2Measurement precision
If metrology targets are designed to be highly sensitive to overlay errors, then measurement precision improves, but reliability deteriorates due to increased sensitivity to process variations
Solution Approach 1:
The patent applies local quality by creating metrology target regions with different geometric characteristics that are locally optimized for specific measurement objectives. Different areas of the target can have different grating orientations, line widths, or pattern types, allowing certain regions to be highly sensitive to overlay in specific directions while other regions provide robustness against process variations, achieving a balance between sensitivity and reliability
Solution Approach 2:
The patent segments the metrology target into multiple independent measurement regions or gratings, each oriented or configured to measure specific overlay components. This segmentation allows the target to simultaneously provide multiple measurement functions, where some segments can be optimized for sensitivity while others provide stability, and the results can be combined to achieve both precision and reliability
3Measurement precision
If complex metrology target designs are used to minimize overlay error, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent designs metrology targets with multi-functionality, where a single target structure serves multiple measurement purposes. The target can simultaneously measure overlay errors in different directions, provide focus and distortion information, and act as a reference for other process parameters. This universality reduces the need for multiple separate target structures, thereby reducing overall device complexity while maintaining high measurement precision
Data Source
AI summary
A method of metrology target design is described. The method includes determining a sensitivity of a parameter of a metrology target design to a perturbation of a process parameter for forming, or measuring the formation of, the metrology target, and determining a robustness of the metrology target design based on the sum of the sensitivity multiplied by the perturbation of at least one of the process parameters.