Aberration Correction in CTEM
- 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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By employing an aberration corrector consisting of electrostatic and magnetic quadrupoles and octupoles, an actual reduction of the spatial resolution limit by a factor of approximately three was first achieved by Zach and Haider [1] in an LVSEM (low voltage scanning electron microscope) in 1995. However, it had not been able to use this type of correctors for imaging extended object areas in TEMs due to the large field (off-axial) aberrations of the corrector.

Later on, Haider et al. [2] corrected the aberrations in TEM imaging mode successfully. Figure 4109 shows an example of diffractogram tableaus of the aligned TEM under both uncorrected and corrected conditions. The figures taken under the uncorrected condition demonstrates that the electron beam tilt about the coma-free pivot point introduces primarily a defocus and an axial twofold astigmatism. The strength of these aberrations does not depend on the azimuthal direction of incident beam due to the rotational symmetry of the aligned TEM. After the aberration correction all diffractograms in the tableau exhibit approximately the same appearance revealing the properties of an aplanatic lens. In this case the illumination tilt does neither introduce defocus nor two-fold astigmatism confirming the correction of spherical aberration.

Diffractograms obtained in an aligned microscope without (a) and with (b) correction of the spherical aberration

Figure 4109. Diffractograms obtained in an aligned microscope without (a) and with (b) correction of the spherical aberration. τ stands for tilt angles. [2]

The phase-contrast term -isin2πχ(g) in the contrast transfer function becomes zero if CS=0 and Δf=0, while the amplitude-contrast term cos2πχ(g) is maximum, 1. Therefore, for aberration-corrected microscope, the TEM images present atomic structures by amplitude-contrast rather than by phase-contrast, and thus in HRTEM images the projected atom column are imaged in bright contrast on a dark background.

 

 

 

 

[1] Zach J. and Haider M., Nucl. Instrum. Methods., Phys. Res. A 363, 316 (1995).
[2] Maximilian Haider, Harald Rose, Stephan Uhlemann, Bernd Kabius and Knut Urban, Towards 0.1 nm resolution with the first spherically corrected transmission electron microscope, Journal ofElectron Microscopy 47(5): 395-405 (1998).

 

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