Patent ID: 7015481

Claim:
A charged-particle optical system for focusing a beam of charged particles onto a specimen, said optical system comprising: (1) four stages of multipole elements arranged along an optical axis of the beam of charged particles; (2) independently variable power supplies capable of supplying six or more independent octopole electric or magnetic potentials, said independently variable power supplies applying electric or magnetic potentials for generating normal octopole fields to at least three stages of multipole elements of said four stages of multipole elements independently and applying electric or magnetic potentials for generating skew octopole fields to at least two stages of the multipole elements independently; (3) fixed or non-independently variable power supplies capable of supplying one or more octopole electric or magnetic potentials, said non-independently variable power supplies applying electric or magnetic potentials for generating normal or skew octopole fields having values that are constant or dependent on any value of said independently variable power supplies to at least one stage of multipole elements of said four stages of multipole elements, said skew fields having functions equivalent to those of a structure obtained by rotating the normal field by one half the pitch between multipole elements; and (4) a control means for correcting third-order aperture aberrations by independently adjusting the octopole electric or magnetic potentials of said five or more independently variable power supplies.