Defocus/Defocus Aberrationin Electron Microscopes
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The third- and fifth-order spherical aberration coefficients and defocus are present in a perfect, round electromagnetic lens, while all other higher-order coherent aberration coefficients in Table 3740 are caused by lens imperfections. Note that adjusting defocus and defocus instability, in CTEMs with LaB6 or W electron guns and FE-EMs, affects the intensity of diffractogram and real images.

Note that the intensities of the electron diffraction (ED) pattern from a crystalline specimen are independent of defocus and the spherical aberration of the electron-optical system.

Figure 3737a shows the misalignment of an objective lens, where the off-axis electron beam enters the objective lens. For a specific lens setting, the object ABC forms an image A'B'C'. The image will be changed to A"B"C" (smaller than A'B'C') with a center (B' or B”) formed by the unchanged beam (BB' or BB”) if the objective lens setting is changed. In this case, the back focal plane of the objective lens is also changed. Furthermore, as shown in Figure 3737a (b) this change induces a rotation of the image because of the helical path of the electrons through the lens. This can happen, for instance, when the objective lens current is changed.

Alignment of Objective Lens Axis

Figure 3737a. Misalignment of the objective lens: (a) Sweep of the image, and (b) Combination of sweep and rotation of the image on the viewing screen in the EM.

The beam misalignment can be corrected either by tilting the incident illumination as shown in Figure 3737b (a), or by adjusting the physical position of the objective lens pole pieces as shown in Figure 3737b (b). In Figure 3737b (b) the lower pole piece in TEM is moved so that both upper and lower pole pieces are aligned to the optical axis.

correction of non-axial objective lens illumination

Figure 3737b. The correction of non-axial objective lens illumination by: (a) Beam tilt, and (b) Movement of the lower objective lens pole piece.

Finally, the beam alignment can be conformed by viewing the rotation center of the image on the viewing screen when the defocus of the objective lens is adjusted. If the rotation center of the image matches the center of the viewing screen, the beam alignment around the objective lens is correct.

Because the intensity of Ronchigram, formed at the Fraunhofer diffraction plane, varies significantly with angle, and this variation is a very sensitive function of lens aberrations and defocus [1], Ronchigram is a very useful way to characterize and optimize the electron probe in STEM mode.

If the spherical aberration (Cs) is zero in a Cs-corrected microscope, positive phase contrast of a weak-phase object can still be obtained by the defocus,

         Defocus/Defocus Aberrationin Electron Microscopes ----------------------------------- [3737]

This condition presents a pass band up to the information limit (gmax) at the contrast delocalization of R0 = 1/gmax.






[1] J.M. Cowley, Electron diffraction phenomena observed with a high resolution STEM instrument, J. Electron. Microsc. Tech. 3 (1986) 25-44.




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