Metrology Focus Height Optimization via Multi-Position Pupil Analysis
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Solution Overview
Problem
In the context of metrology processes, particularly in lithographic apparatuses, determining an optimal focus height is challenging due to the need for precise alignment and focus to accurately measure small metrology targets, especially in areas with limited space, where radiation spots often sample regions outside the target, reducing measurement accuracy.
Innovation Solution
A method is developed to determine the optimal focus height by obtaining measurement data from multiple applications of the metrology process at different nominal focus heights and alignments, analyzing the detected pupil representation to identify a focus height that minimizes sensitivity to alignment and focus errors, thereby improving measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the radiation spot size is reduced to measure small metrology targets in limited space, then measurement capability is improved, but sensitivity to alignment and focus errors increases
Solution Approach 1:
The patent changes the focus position parameter from a single fixed value to multiple discrete focus positions. By measuring at different focus heights and selecting the optimal one, the system reduces sensitivity to alignment and focus errors while maintaining measurement accuracy on small metrology targets.
Solution Approach 2:
The patent performs preliminary measurements at multiple focus positions before selecting the optimal focus height for actual metrology measurements. This preliminary characterization of the radiation spot behavior at different focuses enables the system to compensate for alignment and focus errors in subsequent measurements.
2Measurement precision
If multiple measurements are performed at different focus positions, then optimal focus determination is improved, but measurement time increases
Solution Approach 1:
The patent performs measurements at a limited set of discrete focus positions (e.g., 3-5 positions) rather than continuously scanning through all possible focus values. This partial action approach provides sufficient information to determine optimal focus while minimizing measurement time.
Solution Approach 2:
The multi-position focus characterization is performed as a preliminary step that needs to be done only once or occasionally, rather than before every measurement. This separates the time-consuming focus optimization from routine measurements, reducing overall 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 metrology processes by reducing contamination from surrounding structures and improving the reliability of measurements, even in the presence of alignment and focus errors, leading to more precise determination of parameters like overlay and critical dimension.
Implementation Method 1
detecting radiation redirected by the target
Data Source
AI summary
Methods of determining an optimal focus height are disclosed. In one arrangement, measurement data from a plurality of applications of the metrology process to a target are obtained. Each application of the metrology process includes illuminating the target with a radiation spot and detecting radiation redirected by the target. The applications of the metrology process include applications at different nominal focus heights. The measurement data includes, for each application of the metrology process, at least a component of a detected pupil representation of an optical characteristic of the redirected radiation in a pupil plane. The method includes determining an optimal focus height for the metrology process using the obtained measurement data.