Beam Tilt in Aberration Corrected TEM
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In an aberration-corrected TEM, a tilt of the incident beam does neither introduce a strong defocus nor astigmatism, which is different from the case in an uncorreaed instrument.

Note that in Cs-corrected EMs, due to the absence of spherical aberration it is not possible anymore to correct the residual axial coma by tilting the illumination beam. In this case an appropriate coma compensator is needed to eliminate the coma.

Later on, Haider et al. [1] corrected the aberrations in TEM imaging mode successfully. Figure 2767 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 2767. Diffractograms obtained in an aligned microscope without (a) and with (b) correction of the spherical aberration. τ stands for tilt angles. [1]

 


[1] 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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