A new correction lens

A new electrostatic lens capable of correcting third-order aperture aberration is proposed. The new lens is called a self-aligned quadrupole correction lens and consists of an electrostatic quadrupole and an aperture electrode. An octopole field component is automatically created within the quadrupole field by excitation of the aperture electrode. The potential distributions of this lens are calculated by successive over-relaxation techniques involving the numerical solution of Laplace's equation in three dimensions. Correction properties for all the aperture aberration coefficients are obtained by using the potential distributions. The aperture aberrations have been accurately measured by means of the shadow-image technique, and compared with those calculated using the above potential distributions. It was confirmed experimentally that the aberration in the convergent plane can be cancelled completely by the new correction method.