Condenser astigmatism

Once the primary electron beam is created, it must be demagnified with condenser lenses and then focused onto the sample with objective lenses. These electron lenses are electromagnetic in nature and use electric and magnetic fields to steer the electrons. Such lenses are subject to severe spherical and chromatic aberrations. Therefore, a point primary beam source is blurred into a primary beam disk to an extent dependent on the energy and energy spread of the primary electrons. In addition, these lenses are also subject to astigmatism. AH three of these effects ultimately limit the primary beam spot size and hence, the lateral resolution achievable with sem. 

Since the color center laser is operated at cryogenic temperatures, the crystal must be enclosed in a vacuum chamber to provide thermal insulation and to prevent condensation on the crystal surfaces. The focusing optics. Mi and M2 of Fig. 12, are usually located inside the vacuum chamber to avoid the astigmatism effects of windows. These mirrors must be carefully prealigned prior to operation. The crystal is usually mounted on a translatable cold finger so that the position of the beamwaist on the crystal can be adjusted. In this way, any local defects or scratches in the crystal can be avoided. 

Diffraction astigmatism It is also called intermediate lens astigmatism. Use the largest condenser aperture and spread the beam with brightness fully clockwise. In the diffraction mode, a caustic image can be obtained. Adjust the intermediate lens stigmator to obtain a caustic triangle (similar to the one as shown in Fig. 5.2). Finally adjust the diffraction focus to obtain a very small and distinctly visible spot. 

The Ronchigram can be also used to adjust the condenser astigmatism. When there is a condenser stigmatism, one can see some parallel diffusion lines in the middle as shown in Eig. 5.12. Changing cross-over of the beam above or below the... 

Fig. 5.12 Ronchigrams taken under conditions (a) under-focus, (b) over-focus when the condenser astigmatism is not corrected...

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