Metrology Target Design for Overlay Measurement Accuracy
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Solution Overview
Problem
In lithographic processes, diffraction-based overlay metrology faces challenges with stack sensitivity being proportional to the wavelength of light used, leading to decreased periodicity and difficulties in selecting and adjusting light sources, which affects the accuracy and robustness of overlay measurements.
Innovation Solution
A method for designing a metrology target that optimizes parameters such as pitch, CD, angle, and duty cycle of gratings, and adjusts illumination parameters like wavelength and polarization to enhance measurement accuracy and robustness, using a simulation package to maximize overlay sensitivity and achieve optimal metrology measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If diffraction-based overlay metrology uses longer wavelength light, then measurement range increases, but stack sensitivity decreases leading to reduced measurement accuracy
Solution Approach 1:
The patent changes the geometric parameters of the metrology target (grating pitch, line width, grating angle, duty cycle) to optimize the diffraction pattern and overlay sensitivity for specific wavelength ranges. By adjusting these parameters, the system maintains high measurement accuracy across different wavelength conditions, effectively decoupling the wavelength choice from sensitivity degradation.
Solution Approach 2:
The patent implements dynamic adjustment of illumination parameters (wavelength, polarization state, incident angle) and target parameters to adapt to different measurement conditions. This allows the system to maintain optimal stack sensitivity regardless of the wavelength used, transforming a static sensitivity problem into a dynamically adjustable measurement system.
2Measurement precision
If metrology target parameters are optimized for specific illumination conditions, then measurement accuracy improves, but adaptability to varying process conditions decreases
Solution Approach 1:
The patent designs metrology targets with multi-functional characteristics that enable accurate measurements under diverse illumination and process conditions. By incorporating multiple grating structures with different orientations and periods, the target can be measured accurately regardless of wavelength variations, polarization changes, or process-induced dimensional variations, achieving both precision and robustness simultaneously.
Solution Approach 2:
The patent pre-compensates for process variations by designing targets with built-in tolerance to dimensional changes, focus variations, and illumination parameter drifts. The target geometry is specifically engineered so that small process variations do not significantly impact the diffraction signal, effectively cushioning against measurement errors before they occur.
3Reliability
If grating pitch and duty cycle are increased to improve signal strength, then measurement robustness improves, but manufacturing precision requirements increase
Solution Approach 1:
The patent employs different grating structures in different regions of the metrology target, with each region optimized for specific measurement functions. Some gratings have larger pitches for robustness, while others have smaller pitches for manufacturing feasibility. This local optimization allows the overall target to achieve high measurement robustness without requiring all features to meet stringent manufacturing tolerances.
Solution Approach 2:
The patent uses composite grating structures combining multiple materials or layer configurations to achieve desired optical properties while relaxing manufacturing constraints. By using material composition and layer structure to enhance signal strength, the system can maintain measurement robustness without requiring extremely precise dimensional control of individual grating features.
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 enables accurate and robust measurement of lithographic characteristics by optimizing metrology target design and illumination parameters, improving the reliability of overlay measurements across varying process conditions.
Implementation Method 1
In a diffraction based overlay measurement, the overlay is extracted from a difference in the light intensity for the first positive and negative first diffracted order.
Data Source
AI summary
A method and apparatus are described for providing an accurate and robust measurement of a lithographic characteristic or metrology parameter. The method includes providing a range or a plurality of values for each of a plurality of metrology parameters of a metrology target, providing a constraint for each of the plurality of metrology parameters, and calculating, by a processor to optimize/modify these parameters within the range of the plurality of values, resulting in a plurality of metrology target designs having metrology parameters meeting the constraints.