On the residual six-fold astigmatism in DCOR/ASCOR
P Hartel1, V Gerheim1, M Linck1
1Corrected Electron Optical Systems GmbH, Englerstr. 28, Heidelberg 69126, Germany.
Ultramicroscopy
|August 23, 2019
Summary
Advanced hexapole correctors significantly reduce astigmatism in scanning transmission electron microscopes (STEM). This allows for larger aperture angles and improved STEM resolution across a wide range of electron energies.
Area of Science:
- Electron Microscopy
- Optics and Instrumentation
Background:
- Hexapole Cs-correctors have advanced scanning transmission electron microscopy (STEM).
- A key challenge has been the reduction of six-fold astigmatism (A5).
Purpose of the Study:
- Derive simple expressions for optimal hexapole strength to minimize A5.
- Determine the residual A5 size based on hexapole and lens properties.
Main Methods:
- Analysis of hexapole correctors (DCOR/ASCOR) and transfer lens doublets.
- Derivation of theoretical expressions for hexapole strength and residual astigmatism.
Main Results:
- Optimized hexapole strength scales inversely with electron wavelength.
- Residual A5 is independent of electron energy but dependent on spherical aberration (Cs).
- A5 remains low (0.03–0.4 mm) across typical Cs values and acceleration voltages.
Conclusions:
- Advanced hexapole designs enable large semi-aperture angles (>40 mrad) without compromising STEM resolution.
- The findings provide a theoretical basis for optimizing STEM aberration correction.
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