Corrector for correcting first-order chromatic aberrations of the first degree

摘要

The invention is directed to a corrector for correcting energy-dependent first-order aberrations of the first degree as well as third-order spherical aberrations of electron-optical lens systems. The corrector includes at least one quadropole septuplet (S 1 ) having seven quadrupoles (Q 1 to Q 7 ). The quadrupoles are mounted symmetrically to a center plane (ZS) so as to permit excitation along a linear axis. The corrector furthermore includes at least five octopoles (O 1 to O 7 ) which can be excited within the quadrupole septuplet. In an advantageous embodiment, two quadrupole septuplets are mounted in series one behind the other. The quadrupole fields of the two quadrupole septuplets are excited antisymmetrically to a center plane lying between the two quadrupole septuplets. With such a system, all geometric third-order aberrations and additional energy-dependent first-order aberrations of the third degree and geometric fifth-order aberrations of a lens system can be corrected in addition to the axial and off-axial first-order chromatic aberrations of the first degree.