Reduction Mechanism for Spherical and Chromatic Aberration Coefficients of Magnetic Lens and Retarding Electric Fields

Aberrations generated by the magnetic lens, the aperture lens and the retarding field lens are calculated separately using the electron beam trajectories. Most of the chromatic aberrations for the magnetic lens and retarding electric field system are generated by the magnetic lens. The chromatic aberration coefficient for the system is reduced by the factor (landing beam energy/initial beam energy), because the beam is not yet retarded in the magnetic lens. The spherical aberration coefficient is reduced because the beam semiangle is increased by the retarding field lens. The aperture lens generates negative-sign aberration and reduced the total aberration for the system; however, its absolute value is the smallest among those generated by the other two lenses.