Transmission electron microscope with a mirror objective free of spherical and axial chromatic aberrations

Abstract

The basic scheme of a transmission electron microscope (TEM) with a mirror objective free of spherical and axial chromatic aberrations, which are the main factors limiting the resolution of electron microscopes, is considered. As an objective, an axisymmetric electrostatic mirror (ASEM), the electrodes of which are coaxial cylinders of equal diameter, separated by gaps of finite width, is considered. Based on previously developed aberration concepts, taking into account relativistic effects, the families of three-electrode ASEM are calculated that satisfy the condition of eliminating spherical and axial chromatic aberrations simultaneously at a fixed focal length and a finite width of the interelectrode gaps. From the whole variety of theoretical data, mirrors have been identified that are most suitable for practical use as an aberration-free TEM mirror objective, the geometric and electrical parameters of which ensure the formation of Gaussian planes of the object and the image outside the mirror field. The dependences of the geometric and electrical parameters of such a mirror objective on the energy of the illuminating beam of relativistic electrons are presented.