Voltage Centering Alignment in TEMs
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Coma-free alignment is almost influenced only by the objective lens, while the voltage-center is influenced by the whole lens system in the microscope. In HRTEM measurements, the voltage-centering alignment is applied to reduce the image intensity attenuation due to chromatic aberration of the projector lens (PL) or of the intermediate lens (IL). If coma-free alignment in TEM operations is needed, it is done before the alignment of the voltage centering or current centering.

In many commercial TEMs, both coma-free and voltage-centering alignments use the same beam deflectors above the objective lens for adjusting the direction of incident beam, the coma-free alignment is achievable only at the expense of the voltage-center alignment, and vice versa. However, it was also proposed [1] that the coma-free alignment can be obtained by using beam defectors above the objective lens, while the voltage-centering alignment is adjusted by beam deflectors below the objective lens. The object point corresponding to the viewing-screen center (image plane) is not generally a point along the axis of the objective lens-field due to the leakage-field as shown in Figure 1922. In this design, the deflector is introduced into the location of the image plane of the objective lens.

Voltage Centering in TEMs

Figure 1922. Schematic illustration of voltage-centering alignment using deflectors.




[1] T. Yanaka. K. Shirota, A. Yonezawa and Y. Arai, in: Proc. 8th Int. Cong. on Electron Microscopy. Canberra. 1974. Vol. I. p. 128.



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