Electrostatic correction of the chromatic and of the spherical aberration of charged-particle lenses (Part II)

A doubly symmetric electrostatic corrector which compensates for the axial chromatic and the axial third-order aberration of charged-particle lenses is outlined. Due to the double symmetry the corrector does not introduce linear off-axis aberrations and yields in combination with a round objective lens an electron-optical aplanat. The principle of the electrostatic correction of the axial chromatic aberration is explained in mathematical terms. The geometry of the electrodes of a suitable corrector is optimized with respect to the chromatic correction, the maximum strength of the electric field, and the residual higher-order aberrations which limit the resolution. The resulting aplanat achieves a resolution limit of about 2 nm for an image field with a diameter of 1500 nm. The required stabilities of the electric power supplies are discussed in detail.