Measurement Region Optimization for Small Metrology Targets
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Solution Overview
Problem
As metrology targets decrease in size, edge effects from light leakage distort intensity readings, affecting the accuracy and throughput of parameter inference in lithographic processes, particularly in determining measurement regions for small targets.
Innovation Solution
A method is developed to determine a measurement region by obtaining variation maps from metrology targets, defining a cost function that optimizes the measurement region based on signal variations and repeatability, using a computer program to refine the selection process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If metrology targets are reduced in size to fit among product features, then target positioning flexibility is improved, but edge effects from light leakage increase causing measurement distortion
Solution Approach 1:
The patent divides the target into multiple gratings (e.g., four gratings arranged in a square pattern) and the measurement spot is divided into multiple measurement regions, each associated with a specific grating. This segmentation allows the system to measure each grating separately and combine results, reducing the impact of edge effects on any single measurement while maintaining small overall target size.
Solution Approach 2:
The patent assigns different measurement regions to different gratings within the target, where each measurement region is optimized for its corresponding grating. This local optimization ensures that each grating is measured in a region where edge effects are minimized, thereby improving overall measurement accuracy while maintaining small target dimensions.
2Reliability
If measurement region is enlarged to improve signal strength, then measurement repeatability is improved, but edge effects from light leakage increase distorting intensity readings
Solution Approach 1:
Instead of using a single large measurement region that would include edge effects, the patent segments the measurement into multiple smaller regions, each dedicated to a specific grating. The sum of these regions provides sufficient signal strength while avoiding the distortion caused by including edge regions in a single large measurement area.
Solution Approach 2:
The patent introduces an optimization process that acts as an intermediary between measurement region size and edge effect mitigation. This optimization determines the appropriate measurement region size by evaluating multiple factors including signal strength, edge effects, and measurement repeatability, thereby finding the optimal balance point.
3Measurement precision
If optimization process is added to determine measurement region, then measurement accuracy is improved, but processing complexity increases
Solution Approach 1:
The patent performs preliminary optimization of measurement regions during the target design and fabrication stage, storing the optimized measurement region parameters in a database. This preliminary action eliminates the need for complex real-time optimization during actual measurements, reducing processing complexity while maintaining measurement accuracy.
Solution Approach 2:
The patent uses pre-determined measurement region parameters that are copied from optimization results obtained during target design. These copied parameters are then applied consistently across multiple measurements, avoiding the need to perform complex optimization calculations for each individual measurement while maintaining high accuracy.
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
Improves the accuracy and efficiency of parameter inference by minimizing edge effects and optimizing the measurement region, enhancing the precision of lithographic process control.
Implementation Method 1
Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets
Implementation Method 2
These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation - e.g., intensity at a single angle of reflection as a function of wavelength
Data Source
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Figure 5(a)~5(d)
AI summary
Disclosed is a method of determining a measurement region comprising: obtaining at least one variation map derived from signals of a plurality of metrology targets, the at least one variation map comprising a plurality of data points, indicating a variation of a component of the signals collected from a corresponding location of the plurality of metrology targets, the component of the signals not being indicative of a parameter of interest; defining a cost function for optimizing a measurement region in images of the plurality of metrology targets, comprising a first term relating to a variation of the data points whose corresponding locations of the metrology targets falling within a default measurement region of the at least one variation map, a second term relating to an impact of a size of the default measurement region on measurement repeatability; and using the cost function to determine the measurement region of the images.