Design and optimization of magnetic lenses and deflection systems for electron beams

Abstract

Methods are described for computing the optical properties of any combination of magnetic lenses and deflection yokes, including the most general case in which the lens and deflector fields may be physically superimposed. These techniques can handle either toroidal or saddle deflection yokes, wound on either nonmangetic of ferromagnetic formers, and can handle cases where the magnetic materials of the lenses directly influence the deflection fields. The basic program for calculating the properties of any given lens and deflection system has been combined with an optimization program, which systematically searches (subject to given physical constraints) for the arrangement which minimizes the deflection aberrations for any specified field size and aperture angle. Illustrative computed results are presented. It appears that conventional postlens single-deflection systems can have better properties than conventional prelens double-deflection systems. However, the performance of double-deflection systems can be improved dramatically by placing the second yoke inside the lens and rotating it relative to the first yoke. An arrangement has been found, which, at the corners of a 5×5-mm deflection field with 0.005-rad aperture and 1 in 104 beam voltage ripple, produces a total aberration disk of 0.45 μm before dynamic corrections, or 0.15 μm after dynamic corrections. The properties of in-lens single-deflection systems have also been investigated. Such systems offer the possibility, for the same operating conditions as quoted above, of producing a total aberration disk of less than 0.2 μm after dynamic corrections. By introducing a ’’predeflection coil’’ before the main deflection coil, this value can be reduced to less than 0.1 μm.