Outline of an imaging magnetic energy filter free of second-order aberrations

Abstract

An imaging magnetic energy filter free of second-order aberrations is proposed for a high-resolution fixed-beam transmission electron microscope. The filter consists of four homogeneous bending magnets symmetrically arranged about the central plane of the filter (see Fig. 1). A distorted stigmatic image of the object plane is located at the midplane (see Fig. 2). Due to the required midplane symmetry of the fields and the Gaussian rays, the dispersion of first degree, the second-order aperture aberration, and the second-order distortion vanish in the image plane. The remaining second-order aberrations in this plane, the inclination of image field, and the field astigmatism consist of three linearly independent components. These aberrations decisively limit the field of view.Energy selection is performed at the diffraction image D2 on the far side of the filter. The field aberrations cause the selected energy to depend on the off-axial position of the object points [1]. To guarantee a large field of view with equally well-resolved image points, all second-order aberrations must be cancelled or sufficiently reduced.