Optical: systems and elements – Lens
Reexamination Certificate
2003-07-18
2004-12-28
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
C250S3960ML, C356S123000
Reexamination Certificate
active
06836372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to an electrostatic corrector comprising a rectilinear optical axis for eliminating the chromatic aberration of particle lenses comprising two corrective parts, which are arranged one behind the other along the optical axis and have in each case electrical quadrupole fields and superimposed circular lens fields, the astigmatic intermediate image of one cross-section that is created by an axis point lying in one part of the corrector and the astigmatic intermediate image of the other cross-section, which is perpendicular to said first cross-section, lying in the other part of the corrector.
Particle-optical systems, in particular electron microscopes, are suitable for imaging atomic structures. However, there is a physical limit on the resolution capacity of an optical system, which is determined by the refraction. It has the effect that two separate object points are only reproduced by the optical system as two separate points if their distance apart does is not below the minimum value of d=0.6 &Dgr;&lgr;/&agr;, where &lgr; is the wavelength and &agr; the maximum angle of beam spread of the optical system. To image bodies of extremely small dimensions, this limit d must have the smallest possible values, that is to say the wavelength &lgr; of the imaging beams must be chosen small and the angle of beam spread a must be chosen large. With the changeover from the optical microscope to the electron microscope, a gain in resolution of approximately factor 10
3
is obtained, since the wavelength is small by a factor of about 10
−5
, while the maximum angle of beam spread decreases by about 10
−2
.
However, this improvement in resolution is not sufficient to allow the resolution of atomic dimensions with the electron microscope. For a further increase in resolution capacity, it is therefore necessary to employ systems with a large angle of beam spread &agr;. However, this introduces the problem that image aberrations dependent on the beam spread angle come into play, which in turn reduce the resolution. Considerable efforts have been made in the past to correct these image aberrations, in particular the chromatic aberration and spherical aberration. The best successes to date have been achieved with optical systems in which, instead of rotationally symmetrical fields, multipoles, in particular quadrupoles and octopoles, are used.
2. Description of the Prior Art
In the prior art, a corrector built up from electrical and magnetic multipoles (Nucl. Instr. methods A363 (1995), p 316) is disclosed, in which, in a low-voltage scanning electron microscope, spherical and chromatic aberrations have been completely corrected. The resolution capacity achieved with this arrangement lay at a value of 2 nm, with the applied electron energy being 1 kV. However, the use of magnetic multipoles leads to the disadvantage that, because of the magnetic resonance, the adjustment of the fields can only be reproduced with difficulty.
A corrector for elimination of the chromatic aberration in both cross-sections with purely electrical fields is presented in German Patent DE 199 26 927. The corrector proposed here has two corrective parts, which are arranged one behind the other along the optical axis and have in each case electrical quadrupole fields and superimposed circular lens fields. The principal advantage of this corrector over that mentioned above is that the fields required for correction can be adjusted precisely and, in particular, reproducibly. In practice, however, a disadvantage of this design has emerged that a high resolution is only achieved in regions extremely close to the axis, so that the useful image field size is restricted to only about 20 pixels.
SUMMARY OF THE INVENTION
Against this background, the invention has set itself the object of providing a corrector for eliminating the chromatic aberration in particle lenses, which, firstly, utilises purely electrostatic fields and, secondly, offers an image field that is many times larger.
This object is achieved according to the invention in that
two corrector units are provided
the two corrector units
are designed so as to be of similar technical design
and arranged one behind the other along the optical axis
and rotated through 90 degrees about the optical axis
each corrector unit
has one of the corrective parts formed from at least three quadrupoles
at the input and output side of which two further electrostatic quadrupoles are arranged in each case
and the axial paths are represented telescopically with a 1:1 representation.
The corrector according to the present invention is based on an electrostatic corrector which is known per se and comprises two corrective parts, which are arranged one behind the other along the optical axis. Each of these corrective parts has three electrical quadrupole fields and superimposed circular lens fields, the fields of the first and second corrective part being rotated relative to one another through an angle of 90 degrees about the optical axis. The mode of operation of each corrector is chosen such that the intermediate image of one cross-section generated by a point on the axis comes to lie in one corrective part, and the astigmatic intermediate image of the other cross-section, which is perpendicular to the aforesaid intermediate image, comes to lie in the other corrective part.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the corrective parts are in each case arranged within one corrective unit, at the input and output side of which two further electrostatic quadrupoles are provided in each case The two corrective units are designed so as to be identical to one another and arranged one behind along the optical axis, the second unit being arranged so as to be rotated through 90 degrees with respect to the first. The rays emerging from an object point, after passing through the objective, thus pass through the following fields in succession:
the first corrective unit
at the input side, first the fields of two quadrupoles lying one behind the other
then three successive quadrupoles of the first corrective part with superimposed circular lens field
at the output side also two quadrupole fields again
the second corrective unit
at the input side, the fields of two quadrupoles lying one behind the other
then three successive quadrupoles of the second corrective part with superimposed circular lens field
at the output side also two quadrupole fields again
the fields in the second corrective unit, by virtue of the rotation of this unit, being rotated about the optical axis relative to the fields of the first corrective unit by an angle of 90 degrees.
The ray path of the axial rays in the corrector then appears as follows:
The rays emerging from an object point on the optical axis are thus deflected by the objective toward the image point. With the entry of these rays into the corrector, the two quadrupoles arranged at the input side have the effect that the axial paths extending in the x and y cross-section respectively are deflected in different directions. The particle bundle is thereby focused in a cross-section, for example in the x cross-section, while in the other section, the y section, it is spread out, so that an astigmatic intermediate image is produced. By an appropriate choice of the strength of the electrical potentials at the two quadrupoles, this intermediate image comes to lie in the centre of the corrective part. This also expressly includes those cases in which two astigmatic intermediate images come to lie closely one behind the other in the same cross-section and in the direction of the optical axis. Instead of an astigmatic intermediate image, two closely adjacent intermediate images are thus produced. The fields of the corrective piece thereby act essentially only on the spread-out rays of the y cross-section, their path trajectory being subject to a considerable influence and experiencing a negative contribution to the chromatic aberration. In the x cross-sect
Rose Harald
Uhlemann Stephan
Weissbacker Christoph
CEOS Corrected Electron Optical Systems GmbH
Choi William
Epps Georgia
Schindler Edwin D.
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