Aberration Determinations by Image
Shift due to Tilting Illumination
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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The aberrations of a TEM system can be evaluated by tilting the illumination. The TEM image will shift with tilting the illumination, therefore, the new position of the image after shifting depends on all the aberrations and thus the aberrations can be measured from the distance of image shift compared with an axial image. In other words, the new images will appear to have different aberrations compared with the axial images when the illumination is tilted. Simply speaking, these new aberrations will depend on the original, axial, aberrations and the tilt. Note that we do not need any amorphous materials in the TEM sample to measure the image shift, therefore, different from the aberration measurement method based on diffractogram, this method can be applied to various materials. However, the accuracy of this technique can be influenced by the TEM specimen drift because it is hard to separate the image shift from specimen drift. [1]

Under axial illumination conditions it is not possible to measure all aberration coefficients. In this case, only the defocus and the twofold astigmatism can be determined by the defocus-based techniques. The other aberration coefficients, e.g. spherical aberration, need to be provided by the microscope manufacturer or to be determined by different methods. However, those aberration coefficients, such as the axial coma and the threefold astigmatism, cannot be ignored at high spatial resolutions approaching 0.1 nm. Therefore, the measurements of the other aberration coefficients should be done by other methods such as tilting the illumination. Note that we need to realize that tilting illumination also changes the defocus, astigmatism, and the original aberrations.

 

[1] A. Steinecker and W. Mader, “Measurement of lens aberrations by means of image displacements in beam tilt series,” Ultramicroscopy, vol. 81, pp. 149–161, 2000.

 

 

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