Spherical Aberration Corrector Using Angularly Offset Multipole Pairs
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Solution Overview
Problem
Existing spherical aberration correctors in charged particle beam instruments, such as transmission electron microscopes, cannot fully correct six-fold astigmatism, limiting the improvement of spatial resolution due to their complexity and inability to adjust excitation currents and hexapole element dimensions effectively.
Innovation Solution
A spherical aberration corrector design using two pairs of multipole elements producing three-fold symmetric fields, where the second pair's fields are angularly spaced 30° from the first pair's fields, with a rotation lens or mechanism to align them, effectively canceling out six-fold astigmatism and allowing for improved spatial resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two stages of hexapole elements are used to correct spherical aberration, then negative spherical aberration is produced to suppress positive spherical aberration in the objective lens, but six-fold astigmatism appears as a fifth-order aberration that cannot be corrected
Solution Approach 1:
The correction system is divided into two separate pairs of multipole elements (first pair: 21, 22; second pair: 23, 24) arranged in sequence along the optical axis. Each pair independently produces three-fold symmetric fields that cancel three-fold astigmatism, while the angular offset between pairs addresses six-fold astigmatism. This segmentation allows independent optimization of each pair's parameters.
Solution Approach 2:
The second pair of multipole elements (23, 24) is angularly offset by 30° relative to the first pair (21, 22) about the optical axis. This asymmetric angular arrangement causes the six-fold astigmatism produced by the second pair to be angularly displaced from that of the first pair, enabling cancellation when the fields are superimposed. The asymmetry breaks the symmetry that would otherwise reinforce six-fold astigmatism.
2Measurement precision
If three stages of multipole elements are arranged angularly spaced to suppress six-fold astigmatism, then spatial resolution improves, but the device complexity and adjustment difficulty increase
Solution Approach 1:
The invention extracts and isolates the essential correction function to two pairs of multipole elements rather than using three stages. By removing unnecessary stages and focusing on the minimal configuration needed to correct both three-fold and six-fold astigmatism simultaneously, the design reduces structural complexity while maintaining correction effectiveness.
Solution Approach 2:
The invention changes the operational parameters by allowing independent adjustment of excitation currents for each multipole element pair and enabling angular positioning adjustment (30° offset). This parameter flexibility simplifies the adjustment process compared to fixed three-stage designs, as each pair can be optimized independently for the specific aberration conditions.
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 design simplifies the correction process, reduces development costs, and enhances the spatial resolution of charged particle beam instruments by completely canceling six-fold astigmatism and three-fold astigmatism, addressing the limitations of previous correctors.
Implementation Method 1
a first pair of multipole elements 21, 22 for producing a first pair of three-fold symmetric fields
Implementation Method 2
a rotation lens or mechanism to align them, effectively canceling out six-fold astigmatism
Data Source
AI summary
A spherical aberration corrector and method is offered, which is easy to design and which can correct spherical aberration and even six-fold astigmatism in a charged particle beam instrument. The corrector has a first pair of multipole elements for producing a first pair of three-fold symmetric fields in which three-fold astigmatisms produced mutually are canceled out and a second pair of multipole elements for producing a second pair of three-fold symmetric fields in which three-fold astigmatisms produced mutually are canceled out. The second pair of multipole elements produce six-fold astigmatisms angularly spaced by 30° about an optical axis from six-fold astigmatisms produced by the first pair of multipole elements.