Patent ID: 8314402
Filing Date: 2012-11-20
Classification: H01J

Abstract:
1. A corrector for chromatic and aperture aberration correction in an electron microscope with multipoles which are disposed in an optical path along an optical axis, one after the other and symmetrically with respect to a symmetry plane, the corrector comprising: a first multipole disposed upstream of the symmetry plane, said first multipole generating a first quadrupole field and a first octupole field; a second multipole disposed downstream of said first multipole and upstream of the symmetry plane, said second multipole generating a second quadrupole field and a second octupole field; a third multipole disposed downstream of said second multipole and upstream of the symmetry plane, said third multipole generating a third magnetic and electric quadrupole field, a third octupole field and a third additional octupole field; a fourth multipole disposed downstream of said first multipole and downstream of the symmetry plane, said fourth multipole generating a fourth magnetic and electric quadrupole field, a fourth octupole field and a fourth additional octupole field; a fifth multipole disposed downstream of said fourth multipole and downstream of the symmetry plane, said fifth multipole generating a fifth quadrupole field and a fifth octupole field; a sixth multipole disposed downstream of said fifth multipole and downstream of the symmetry plane, said sixth multipole generating a sixth quadrupole field and a sixth octupole field, wherein said first, second, third, fourth, fifth and sixth quadrupole fields are consecutively rotated through 90° with respect to one another and are point-symmetrical with respect to a point of intersection between the optical axis and the symmetry plane, an exchange symmetry of axial fundamental rays being generated, with the symmetry plane being a mirror plane for the exchange symmetry, both axial and off-axial fundamental rays being reunited at an end of the corrector, with chromatic aberration thereby being corrected through cooperation of said third and said fourth magnetic and electric quadrupole fields, wherein aperture aberrations and off-axial aberrations produced by the corrector itself are corrected by adjustment of said third and said fourth additional octupole fields, said third and said fourth additional octupole fields thereby acting on electrons with a same direction of force as said first, second, third, fourth, fifth and sixth quadrupole fields in principal sections of the corrector; and a double multipole disposed between said third and said fourth multipoles, said double multipole having a first multipole element disposed upstream of the symmetry plane and an adjoining second multipole element disposed downstream of the symmetry plane, said first multipole element generating a first double multipole octupole field and said second multipole element generating a second double multipole octupole field, said first and second double multipole octupole fields exerting a force on electrons in said principal sections in a same direction as said first, second, third, fourth, fifth and sixth quadrupole fields, wherein said first multipole element additionally generates a third double multipole octupole field and said second multipole element additionally generates a fourth double multipole octupole field, said third and said fourth double multipole octupole fields having different polarities and exerting a force on electrons in a direction which is rotated through 90° with respect to the optical axis in said principal sections and relative to said first and second double multipole octupole fields, wherein said third and said fourth double multipole octupole fields are mirror-symmetrical with respect to the point of intersection between the optical axis and the symmetry plane and act on electrons in said principal sections with a force having a direction that corresponds to that of said first and second double multipole octupole fields, however rotated through 45° relative thereto, said first, second, third, fourth, fifth and sixth octupole fields thereby generating a force on electrons in said principal sections in a direction rotated through 90° with respect to the optical axis, in correspondence with said third and fourth double multipole octupole fields and which are point-symmetrical with respect to the point of intersection between the optical axis and the symmetry plane with regard to field strength and sign, wherein an adjustment of poles and field strengths of said first, second, third, fourth, fifth and sixth octupole fields is used for correcting an azimuthal coma while substantially preventing generation of other aberrations, the corrector thereby being arranged in the optical path in such a fashion that an electron beam is convergently irradiated with respect to the optical axis, with said axial fundamental rays only having maxima in regions of said third and said fourth multipoles.