Multi-Beam Astigmatism Correction Using Aperture-Scanned Beam Images
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Solution Overview
Problem
Existing multi charged particle beam writing apparatuses face difficulties in accurately adjusting focusing and astigmatism, especially when blur or large astigmatism is present, making analysis and correction challenging.
Innovation Solution
A multi charged particle beam writing apparatus that includes an objective lens for focus adjustment, an astigmatism correction element, a stage with an inspection aperture, a current detector, and a control unit that calculates and sets parameters for the astigmatism correction element based on feature amounts derived from scanning the beam image in different directions, allowing for precise astigmatism correction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If multiple beams are used for writing, then productivity is greatly increased, but adjustment of optical system becomes more complex and difficult
Solution Approach 1:
The patent segments the optical system adjustment into two independent parts: focus adjustment and astigmatism adjustment. Focus adjustment is performed by scanning a line-shaped reflection mark and detecting reflected electrons while changing focus position. Astigmatism adjustment is performed by scanning an inspection aperture with multiple beams to form a beam image and controlling excitation current for astigmatism correction coil. This segmentation makes the complex adjustment process manageable and systematic.
Solution Approach 2:
The patent employs feedback mechanisms for both focus and astigmatism adjustment. For focus adjustment, reflected electrons are detected and used to determine optimal focus position. For astigmatism adjustment, the beam image is formed and its feature amounts are analyzed to control the excitation current of the astigmatism correction coil. This feedback ensures precise adjustment despite the complexity of multiple beams.
2Measurement precision
If ellipse fitting is used for astigmatism correction, then correction accuracy is improved, but fitting may fail when blur is large or astigmatism amount is large
Solution Approach 1:
The patent changes the measurement parameter from geometric ellipse fitting to intensity-based feature amount analysis. Instead of fitting an ellipse to the beam contour, the patent scans an inspection aperture with multiple beams to form a beam image, then calculates feature amounts from the intensity distribution. This parameter change allows reliable measurement even when blur is large or astigmatism amount is large, as it does not depend on the beam contour being clearly elliptical.
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
Enables high-accuracy astigmatism correction and focusing, improving the writing accuracy of patterns on substrates by determining the optimal astigmatism correction coil value and lens settings, even in conditions with significant distortions.
Implementation Method 1
an objective lens 16 which adjusts focus positions of the plurality of beams
Implementation Method 2
an astigmatism correction element 18 which corrects astigmatism of the plurality of beams
Implementation Method 3
a current detector 50 which detects beam currents of the individual plurality of beams after passing through the inspection aperture 40
Data Source
AI summary
In one embodiment, a multi charged particle beam writing apparatus includes an objective lens adjusting focus positions of multiple beams, an astigmatism correction element correcting astigmatism of the multiple beams, an inspection aperture allowing one of the multiple beams to pass therethrough, a deflector deflecting the multiple beams and causing the multiple beams to scan over the inspection aperture, a current detector detecting beam currents of the individual multiple beams after passing through the inspection aperture, a beam image formation unit forming a beam image based on the detected beam currents, a feature amount calculation unit generating a first waveform and a second waveform by adding brightnesses of the beam image in a first direction and in a second direction, and calculating a first and a second feature amounts from the first and the second waveforms, and a parameter calculation unit calculating an exciting parameter that is to be set for the astigmatism correction element based on the first feature amount and the second feature amount.