Characteristics of the Foil Lens for the Correction of the Spherical Aberration of the Strongly Excited Magnetic Lens

The foil lens, in which the voltage is applied between an aperture diaphragm and a conducting foil, acts as a concave lens and produces the negative spherical aberration. This negative spherical aberration can be utilized for the correction of the positive spherical aberration of a magnetic lens. The foil lens has been applied to the prefield lens formed by the magnetic field before the center of the strongly excited objective of a standard electron microscope. The spherical aberration of the lens system composed of the foil lens and the prefield lens has been studied experimentally, and it has been found that the correction is possible, at the voltage of 260 volts applied in the foil lens, up to 1.0 x 10 ' rad in terms of the angle of the beam focusing at the center of the objective. It has been shown as well that the experi-mental result agrees satisfactorily with numerical evaluations calculated from the third and the fifth order spherical aberrations of the lens system