Quadrupolar Illumination for Lithography Scatterometry
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Solution Overview
Problem
Current scatterometry methods using annular illumination for separating zeroth and first diffraction orders are prone to errors due to sensor asymmetries and provide less information, making it challenging to accurately measure substrate characteristics.
Innovation Solution
An inspection apparatus with a radiation projector that uses an illumination profile with four quadrants, where two non-adjacent quadrants are illuminated and the other two are not, allowing for separate detection and reconstruction of zeroth and higher diffracted orders, reducing sensitivity to sensor asymmetries and enhancing measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If annular illumination is used to separate zeroth and first diffraction orders, then diffraction order separation is improved, but measurement precision deteriorates due to sensor asymmetries and loss of information
Solution Approach 1:
The patent applies asymmetry by using a quadrupolar illumination profile where two opposite quadrants are illuminated and the other two are dark, creating an asymmetric illumination pattern that generates characteristic astigmatic diffraction patterns. This asymmetric design allows the system to distinguish between different diffraction orders and orientations while compensating for sensor asymmetries through the specific symmetry of the quadrupolar pattern.
Solution Approach 2:
The patent changes the illumination parameter from conventional annular or full-field illumination to a quadrupolar pattern with specific intensity distribution. By controlling which quadrants are illuminated and adjusting the illumination intensity profile, the system optimizes the separation of diffraction orders while maintaining robustness against sensor asymmetries and maximizing information content in the diffracted light.
2Measurement precision
If annular illumination is used to separate diffraction orders, then diffraction order separation is improved, but information content deteriorates due to lack of near normal incidence beams
Solution Approach 1:
The patent modifies the illumination angular distribution by using a quadrupolar pattern that includes illumination at near normal incidence angles, unlike annular illumination which operates at oblique angles only. This parameter change ensures that valuable information from near normal incidence diffraction is captured while still achieving diffraction order separation through the quadrupolar symmetry pattern.
3Loss of information
If conventional illumination is used, then information content is maximized, but diffraction order separation deteriorates due to overlap
Solution Approach 1:
The patent introduces asymmetric quadrant-based illumination to break the symmetry that causes overlap of diffraction orders in conventional illumination. By illuminating only two opposite quadrants and leaving the other two dark, the system creates a quadrupolar pattern that generates distinct astigmatic signatures for different diffraction orders, enabling their separation while preserving information from multiple illumination angles.
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 robust separation of diffraction orders, improving the accuracy and reproducibility of substrate characteristic measurements by utilizing valuable information from all angles, including those near normal incidence.
Implementation Method 1
a periodic mark on a substrate is simultaneously illuminated at various angles. The light diffracted by this mark is used to measure particular characteristics of that mark
Implementation Method 2
detect the radiation beam reflected from a surface of the substrate and separately detect the zeroth diffracted order and a higher diffracted order
Data Source
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AI summary
For angular resolved spectrometry a radiation beam with an illumination profile having four quadrants is used. The first and third quadrants are illuminated whereas the second and fourth quadrants aren't illuminated. The resulting pupil plane is thus also divided into four quadrants with only the zeroth order diffraction pattern appearing in the first and third quadrants and only the first order diffraction pattern appearing in the second and third quadrants.