Overlay Measurement Correction for Grating Imbalance
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Solution Overview
Problem
Lithographic processes face inaccuracies in overlay measurements due to structural asymmetry and stack differences between periodic structures, which are not accounted for in existing methods, leading to errors in diffraction signal analysis.
Innovation Solution
A method is developed to predict the sensitivity of measurement methods to grating imbalance by forming images of target structures using symmetric and asymmetric portions of the diffraction spectrum, allowing for the determination of grating imbalance and its impact on measurement performance, enabling improved overlay measurements through statistical analysis and correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If asymmetry-based measurement methods are used for overlay measurement, then measurement speed and computational simplicity are improved, but measurement accuracy deteriorates due to sensitivity to grating imbalance and structural asymmetry
Solution Approach 1:
The measurement method is segmented into multiple components: a first asymmetry measurement component and a second asymmetry measurement component. By dividing the measurement into separate stages, the system can perform quick asymmetry checks while also capturing additional information about grating imbalance, thus maintaining speed while improving accuracy.
Solution Approach 2:
The patent performs preliminary asymmetry measurements to establish baseline values before conducting full overlay measurements. This preliminary action allows the system to quickly identify potential issues and adjust measurement parameters accordingly, improving both speed and accuracy by avoiding unnecessary full measurements when asymmetry is within acceptable ranges.
2Ease of operation
If simple asymmetry-based overlay measurement is used, then ease of operation and computational simplicity are improved, but measurement reliability deteriorates due to sensitivity to grating imbalance
Solution Approach 1:
The patent introduces an intermediary correction mechanism that accounts for grating imbalance. Instead of directly measuring overlay from asymmetry alone, the system uses the asymmetry measurement as an intermediary step and then applies corrections based on additional measurements, thus maintaining operational simplicity while improving reliability through the intermediary correction process.
Solution Approach 2:
The measurement system incorporates feedback loops where asymmetry measurements are used to adjust and refine subsequent measurements. The system continuously monitors asymmetry values and uses this feedback to correct for grating imbalance effects, thereby maintaining ease of operation while significantly improving measurement reliability through iterative refinement.
3Measurement precision
If additional measurements are performed to account for grating imbalance, then measurement accuracy is improved, but measurement time and processing complexity increase
Solution Approach 1:
The patent merges multiple measurement objectives into a unified measurement approach. By combining asymmetry measurements with grating imbalance characterization in a single integrated process, the system achieves high accuracy without requiring separate measurement steps, thus eliminating the time penalty that would normally accompany additional measurements.
Solution Approach 2:
The system performs partial measurements selectively based on process needs. Rather than always performing full corrected measurements, the system uses asymmetry measurements as a faster partial approach when high accuracy is not critical, and only performs the more time-consuming grating imbalance corrections when needed, thus optimizing the balance between accuracy and measurement time.
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 accuracy of overlay measurements by accounting for grating imbalance, reducing errors caused by structural asymmetry and stack differences, and provides a robust method for high-volume manufacturing by integrating the prediction of measurement performance into existing measurement protocols.
Implementation Method 1
These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation... to obtain a diffraction 'spectrum' from which a property of interest of the target can be determined
Data Source
AI summary
An overlay measurement (OV) is based on asymmetry in a diffraction spectrum of target structures formed by a lithographic process. Stack difference between target structures can be perceived as grating imbalance (GI), and the accuracy of the overlay measurement may be degraded. A method of predicting GI sensitivity is performed using first and second images (45+, 45−) of the target structure using opposite diffraction orders. Regions (ROI) of the same images are used to measure overlay. Multiple local measurements of symmetry (S) and asymmetry (A) of intensity between the opposite diffraction orders are made, each local measurement of symmetry and asymmetry corresponding to a particular location on the target structure. Based on a statistical analysis of the local measurements of symmetry and asymmetry values, a prediction of sensitivity to grating imbalance is obtained. This can be used to select better measurement recipes, and/or to correct errors caused by grating imbalance.