Electron optical experiments with a magnetic imaging filter

Abstract

In the CTEM, imaging filter systems consisting of several magnetic prisms permit the removal of inelastically scattered electrons. Such systems transfer the diffraction plane and the intermediate image stigmatically. A prerequisite for a singly achromatic imaging is that the filter produces a radial intermediate image in its plane of symmetry.The symmetry relations of the magnetic fields and the electron paths permit correction of the second-order distortion and of the second-order aperture aberration [1]. The aperture-dependent magnifications, however, remain uncompensated. These aberrations must be tolerated for filter systems which have stigmatic intermediate images [2], If, however, line foci exist between the prisms, hexapoles can be applied for correction (Fig. 1 [3,4]). Such corrections seem imperative in filters which are used for high resolution electron microscopy.The adjustment of a symmetric optical path in our filter system by means of quadrupoles can be controlled by inserting an edge into the symmetry plane and simultaneously observing the image produced by the filter.