Electron Beam Aberration Correction Using Wien Filters
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Solution Overview
Problem
Conventional electron beam apparatuses face challenges in achieving high throughput and minimizing aberrations, particularly due to the space charge effect and difficulties in converging beams to high current densities, which result in blurred focus and increased off-axis aberrations when the filter length is not equal to the object plane-image plane distance.
Innovation Solution
The use of a cathode with a concave electron emission surface and a drawing electrode with a convex surface, along with multi-polar Wien filters arranged to ensure double symmetry and bidirectional focus, helps to compensate for aberrations and improve beam convergence, allowing for higher current densities and reduced aberrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional electron gun with flat or convex cathode is used, then the structure is simple and easy to manufacture, but the beam convergence is poor and current density cannot reach 500 A/cm² or higher
Solution Approach 1:
The patent employs a concave cathode surface with a specific radius of curvature (Rc) and a convex drawing electrode surface with radius of curvature (Ra), where the relationship (Rc-Ra)×0.5 < L satisfies a specific inequality. This curved geometry configuration enables the electron beam to converge to a focal point with high current density (≥500 A/cm²), resolving the contradiction between manufacturing simplicity and beam convergence precision.
2Device complexity
If the filter length is not equal to the object plane-image plane distance, then the device complexity is reduced, but off-axis aberrations increase and focus becomes blurred
Solution Approach 1:
The patent changes the key parameter L (distance between cathode and drawing electrode) to satisfy the inequality (Rc-Ra)×0.5 < L, where Rc and Ra are the radii of curvature of the cathode and drawing electrode respectively. This parameter optimization enables the filter length to differ from the object plane-image plane distance while maintaining sharp focus and minimizing off-axis aberrations, thus reducing device complexity without sacrificing focus precision.
3Manufacturing precision
If multi-polar Wien filters are used for aberration correction, then aberration compensation is improved, but the device complexity and alignment requirements increase
Solution Approach 1:
The patent positions the multi-polar Wien filters asymmetrically at the 1⁄4 and 3⁄4 plane locations along the optical path between the object plane and image plane. This asymmetric positioning, combined with the curved electrode surfaces, achieves effective aberration correction while simplifying the overall device configuration and reducing alignment complexity compared to symmetric arrangements.
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 configuration enhances the resolution and throughput of the electron beam system by ensuring double symmetry and reducing aberrations, enabling higher current densities without increasing illumination current, thus improving the overall performance of the electron beam apparatus.
Implementation Method 1
an electron gun having a cathode and a drawing electrode
Implementation Method 2
an aberration correction optical apparatus comprising a plurality of multi-polar Wien filters
Data Source
AI summary
An electron beam apparatus for providing an evaluation of a sample, such as a semiconductor wafer, that includes a micro-pattern with a minimum line width not greater than 0.1 μm with high throughput. A primary electron beam generated by an electron gun is irradiated onto a sample and secondary electrons emanating from the sample are formed into an image on a detector by an image projection optical system. An electron gun 61 has a cathode 1 and a drawing electrode 3, and an electron emission surface 1a of the cathode defines a concave surface. The drawing electrode 3 has a convex surface 3a composed of a partial outer surface of a second sphere facing the electron emission surface 1a of the cathode and an aperture 73 formed through the convex surface for passage of the electrons. An aberration correction optical apparatus comprises two identically sized multi-polar Wien filters arranged such that their centers are in alignment with a ¼ plane position and a ¾ plane position, respectively, along an object plane-image plane segment in the aberration correction optical apparatus, and optical elements having bidirectional focus disposed in an object plane position, an intermediate image-formation plane position and an image plane position, respectively, in the aberration correction optical apparatus.