Particle-optical appliance provided with aberration-correcting means
AssigneeFEI Company
InventorAlexander Henstra
Life of this patent
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Abstract
Quadrupole-octupole aberration corrector for application in a TEM, STEM or SEM. A known corrector for correcting third-order and fifth-order aberrations of the objective is embodied with eight quadrupoles and three octupoles. The corrector according to the invention has at least the same aberration-correcting power, but, according to the invention, is embodied with six quadrupoles and three octupoles. By adding octupoles with a relatively weak excitation to a portion of the quadrupoles, correction of the anisotropic coma of the objective lens is also attained. By embodying all quadrupoles, or a portion thereof, to be electromagnetic, chromatic aberrations can also be corrected for.
Claims25
A particle-optical appliance provided with an objective lens and with aberration-correcting means for correcting lens errors of the objective lens, which aberration-correcting means comprise:
- a first group of optical elements, consecutively consisting of a first, a second and a third quadrupole lens and a first octupole;
- a second group of optical elements, consecutively consisting of a second octupole and a fourth, a fifth and a sixth quadrupole lens;
- and
- at least one third octupole, placed outside both groups of optical elements,
- whereby the first and the second octupole and the third and the fourth quadrupole lens are placed between the first and the second quadrupole on the one hand and the fifth and the sixth quadrupole on the other hand,
- in which the quadrupole lenses determine the paths of electrically charged particles in the aberration-correcting means in such a manner as to cause imaging of octupoles upon one another,
- characterized in that
- in a first axial plane, the first and the second octupole are not imaged upon one another, and the second and the third octupole are imaged upon one another;
- in a second axial plane perpendicular to the first axial plane, the first and the second octupole are not imaged upon one another, and the first and the third octupole are imaged upon one another;
- and
- an axial beam in the first axial plane passes through the axial point of the first octupole and in the second axial plane passes through the axial point of the second octupole,
- as a result of which third-order lens errors are corrected, and fifth-order lens errors are, at the least, minimized.
Dependencies are inferred from the claim text.