Spherical aberration, electromagnetic

As with optical lenses, electromagnetic lenses have aberrations (chromatic aberration, spherical aberration, electron diffraction limit, astigmatism, etc.) each entailing an enlargement of the electron probe expressed by the diameter of a circle of least confusion. Under standard operating conditions, when the astigmatism has been adjusted, only spherical aberration plays a significant role. The expression of the probe diameter becomes ... 

Although there is no inherent phase problem in the HREM technique it is replaced by an "instrumental" phase problem arising from the relatively imperfect nature of electromagnetic lenses. Spherical aberration is the chief limitation, and its effect can be coupled with that of deviation from the Qatissian focus into a phase factor which can be considered to act upon the diffracted amplitudes before re-combination by the lens to produce the initial image amplitude, namely ... 

After decades of work and anticipation, systems that correct for aberrations of the electromagnetic lenses used in electron microscopy have now been developed [44-46]. The resolution of the standard transmission electron microscope (TEM) was described in Section 3.1.4.3, and the spherical aberration coefficient (Cs) appears in... 

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. 

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