Lateral chromatic aberration

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. 

Assuming that the sample has the same refraction index as suprasil (about 1.455), the objective has a magnification of 100 in lateral and a 8000 in axial direction. The numerical aperture is NA = 0.722, but a central part with an aperture of NA = 0.329 is lost due to the Cassegrain mirror design. Its theoretical resolution is 0.52 pm. Small spherical and chromatic aberrations reduce the resolution to 1.0 pm. 

In the mid-IR, these objectives deliver diffraction-limited performance over an area extending 100-200 pm from the optic axis. Having no refractive components, they are also free from chromatic aberrations. However, the design causes the central portion of the objective s aperture to be obscured, losing up to 25% of the aperture area. As we will show later, this obscuration leads to significant diffraction effects when compared to a conventional microscope objective. 

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