Electron optical analysis of a retarding Wien filter for electron spectroscopic imaging

Abstract

A new Wien filter has been designed for electron spectroscopic imaging. The shape of the electrodes and the magnetic pole pieces of the filter has fourfold symmetry and therefore the Wien condition giving a straight optical axis is satisfied even in the fringing field region. The second-order aberration coefficients of the filter are formulated in a simple form. The filter is applied to a spectroscopic low-energy reflection electron microscope in a retarding condition. The optics of the image-forming lens system is analyzed using the transfer matrix method. The achromatic focus is achieved when the distances of the objective plane and the imaginary image plane to the filter are equal. A condition for obtaining the image free from second-order aberrations is found. The total magnification of the instrument is 3000–14000 times; energy resolution of 2 eV is attained for an initial beam divergence angle of 10 mrad.