Magnetic electron lens aberrations due to mechanical defects

Abstract

The electron-optical aberrations due to mechanical defects in magnetic lens pole-pieces are calculated, for several lens geometries and a range of excitations, by a slight modification of the computational procedure of Sturrock. The condition that these aberrations shall not predominate over the combination of spherical aberration and diffraction is taken as a criterion of mechanical tolerance. Tolerances for the alinement of the pole-pieces are shown to be well within the limits of normal workshop practice (for a typical lens: the axes of the two pole-pieces must not be laterally separated by more than 30 μ, nor must the angle between them exceed 0.005 radian). Tolerances for ellipticity of the bore and corrugation of the pole-faces are more stringent (typically: the variation in radius or in distance of the pole-face from the mid-plane must not exceed 0.1 μ). The results are generalized in the form of universal curves, designed to facilitate the assignment of tolerances in the case of lenses of any magnitude or accelerating voltage.