Aberration Corrector and Method of Aberration Correction
First Claim
1. An aberration corrector for correcting aberration produced in a lens disposed in the direction of motion of a spreading electron beam which is cylindrically symmetrical, said aberration corrector comprising:
- a first quadrupole field-producing multipole element on which the electron beam not yet corrected for the aberration impinges, the first quadrupole field-producing multipole element producing a first quadrupole field; and
a second quadrupole field-producing multipole element from which the electron beam corrected for the aberration exits, the second quadrupole field-producing multipole element producing a second quadrupole field,wherein each of said first and second quadrupole fields produces an optical system having a two-fold astigmatic effect of a primary term of the quadrupole field and a focusing effect in the direction of divergence of a cylindrically symmetrical type, said focusing effect being produced by higher-order terms of the quadrupole field which have magnitudes increasing at higher rates than the primary term when the length in the direction of motion of the electron beam is increased, andwherein said first and second quadrupole fields are set to mutually opposite polarities, whereby the two-fold astigmatic effects of the first and second quadrupole fields on the cylindrically symmetrical electron beam having the spread are canceled out and only the focusing effect in the direction of divergence of the cylindrically symmetrical type is used for correction of the aberration in the lens.
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Abstract
A chromatic aberration corrector has a relatively simple correction system capable of correcting the electron optical system of a transmission electron microscope for chromatic aberration. Furthermore, high-resolution imaging is enabled. The correction system uses a first multipole element for producing a quadrupole field to produce a concave lens effect. However, surplus two-fold astigmatism is left behind. This two-fold astigmatism is removed by inserting a second multipole element for producing a second quadrupole field. The first and second multipole elements are set to mutually opposite polarities. That is, the two-fold astigmatism is canceled out by combining the first and second multipole elements. A cylindrically symmetrical electron beam having spread can be corrected for chromatic aberration by the concave lens effect.
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Citations
13 Claims
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1. An aberration corrector for correcting aberration produced in a lens disposed in the direction of motion of a spreading electron beam which is cylindrically symmetrical, said aberration corrector comprising:
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a first quadrupole field-producing multipole element on which the electron beam not yet corrected for the aberration impinges, the first quadrupole field-producing multipole element producing a first quadrupole field; and a second quadrupole field-producing multipole element from which the electron beam corrected for the aberration exits, the second quadrupole field-producing multipole element producing a second quadrupole field, wherein each of said first and second quadrupole fields produces an optical system having a two-fold astigmatic effect of a primary term of the quadrupole field and a focusing effect in the direction of divergence of a cylindrically symmetrical type, said focusing effect being produced by higher-order terms of the quadrupole field which have magnitudes increasing at higher rates than the primary term when the length in the direction of motion of the electron beam is increased, and wherein said first and second quadrupole fields are set to mutually opposite polarities, whereby the two-fold astigmatic effects of the first and second quadrupole fields on the cylindrically symmetrical electron beam having the spread are canceled out and only the focusing effect in the direction of divergence of the cylindrically symmetrical type is used for correction of the aberration in the lens. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. A method of correcting aberration produced in a lens disposed in the direction of motion of an electron beam, said method comprising the steps of:
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preparing two multipole elements each producing a quadrupole field having a length in the direction of motion of the electron beam; causing each of the multipole elements to produce a two-fold astigmatic effect and a focusing effect in the direction of divergence of a cylindrically symmetrical type, said two-fold astigmatic effect increasing in magnitude as the length of the quadrupole field in the direction of motion of the beam is increased, said focusing effect being produced by higher-order terms of the quadrupole field; and combining the two quadrupole fields produced by the multipole elements such that the two-fold astigmatic effects produced by the two quadrupole fields are canceled out, whereby only the focusing effects in the direction of divergence of the cylindrically symmetrical type are used to correct the lens for the aberration for the cylindrically symmetrical electron beam having spread.
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Specification