Correction of 3^{rd} Order Star Aberration (S_{3})  Practical Electron Microscopy and Database   An Online Book  

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Figure 3338a shows the simulated intensity distribution patterns of 200 keVelectron probes at 100 nm of B_{2} (secondorder axial coma), 100 nm of A_{2} (threefold axial astigmatism), 1 µm of S_{3} (axial star aberration of the 3rd order), and 2 µm of A_{3} (fourfold axial astigmatism). Figure 3338a. Simulated intensity distribution patterns of 200 keVelectron probes: (a) B_{2} = 100 nm, (b) A_{2} = 100 nm, (c) S_{3} = 1 µm, and (d) A_{3} = 2 µm. In this simulation, the defocus C_{1} was set to 3 nm, the imaginary parts of the aberrations to zero, and the illumination semiangle to 30 mrad. [1] For STEM mode, Figure 3338b shows the change of Ronchigram taken at overfocus of 50 nm during correction of 3^{rd} order star aberration (S_{3}). The characteristic feature of the aberration marked by white broken lines disappeared after the aberration correction. Figure 3338b. Change of Ronchigram taken at overfocus of 50 nm during correction of 3^{rd} order star aberration (S_{3}). Adapted from [2] For TEM mode, Figure 3338c shows the schematic comparison of Zemlin (diffractogram)tableau characteristics for the axial aberrations up to third orders. Firstorder aberration (e.g. defocus and twofold astigmatism, A_{1}) shows the elliptical distortion even without electron beam tilting because the impact of firstorder aberrations does not depend on the tilt angle. For the aberrations of higher orders (n≥2), such as secondorder axial coma B_{2}, threefold astigmatism A_{2}, thirdorder spherical aberration C_{3} (>0), thirdorder star aberration S_{3} and fourfold astigmatism A_{3}, there is no elliptical distortion observable for the untilted case. In these cases, only at illumination tilts the characteristic distortion due to the aberrations becomes discernible. Note that the higherorder aberrations have equal symmetries to the ones in Figure 3338c as discussed in page3740. Figure 3338c. Schematic representation of Zemlin (diffractogram)tableau characteristics for the axial aberrations up to third order.
[1] Simulation of Rolf Erni.


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