Metrology Objective Lens Adjustment for Overlay Coma Reduction
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Solution Overview
Problem
Existing metrology tools face challenges in accurately measuring overlay and other properties due to linear and offset coma aberrations in the objective lens, which can lead to inaccurate measurements and reduced throughput.
Innovation Solution
The method involves rotating the objective lens element to achieve optimal contrast for different portions of the metrology target and using actuators to move optical elements in the z-direction to reduce coma aberrations, specifically addressing offset and linear coma.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If coma aberration correction is performed by rotating the objective lens or moving optical elements, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent implements dynamic adjustment of the objective lens system by enabling rotation of the objective lens around its optical axis and/or axial movement of optical elements. This dynamic capability allows the system to adapt to different coma aberration conditions and achieve optimal measurement precision across varying operational scenarios, directly resolving the contradiction between measurement precision and device complexity.
Solution Approach 2:
The patent changes physical parameters of the objective lens system by rotating it to different angular positions and/or moving optical elements to different axial positions. These parameter changes enable the system to compensate for coma aberrations and achieve best focus at different z-positions for different portions of the measurement target, thereby improving overlay measurement accuracy.
2Measurement precision
If multiple z-positions are used to achieve best contrast for different target portions, then measurement accuracy is improved, but measurement time increases
Solution Approach 1:
The patent performs preliminary action by pre-determining the relationship between objective lens rotation angles and optimal z-positions for different target portions. This allows the system to quickly jump to the correct focus position without iterative searching, significantly reducing measurement time while maintaining high contrast optimization accuracy.
Solution Approach 2:
The patent enables simultaneous dynamic adjustment of both the rotation angle and axial position of the objective lens. This coordinated dynamic adjustment allows the system to rapidly transition between different focus requirements for different target portions, reducing the time penalty associated with achieving best contrast across multiple z-positions.
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 measurement accuracy by minimizing coma-related focus offsets, improving the reliability of overlay and other property measurements, thereby increasing the efficiency and precision of the metrology process.
Implementation Method 1
rotating an objective lens element of the metrology apparatus until a best contrast for the first and second portions is achieved at a single substrate z-position
Implementation Method 2
diffraction based overlay target
Data Source
AI summary
Systems and methods provide the ability to mitigate linear and/or offset coma present in an objective of a metrology tool. A method of reducing an effect of offset coma in a metrology apparatus includes rotating an objective lens element of the metrology apparatus until a best contrast for physically separated first and second portions of a metrology target is determined. A method of reducing an effect of linear coma in a metrology apparatus includes determining an amount of an axially symmetric coma aberration present in a lens system of the metrology device, and moving an optical element of the lens system in an axial z-direction to reduce the determined axially symmetric coma. A lens stop or other lens element may be moved in the z-direction to reduce coma. The two approaches may be combined.