Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2002-03-05
2004-09-28
Wells, Nikita (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S305000, C250S311000, C250S397000, C250S398000
Reexamination Certificate
active
06797962
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to an electrostatic corrector for eliminating the chromatic aberration of particle lenses, with a straight optical axis and an electrostatic quadrupole for allocating to an objective lens.
2. Description of the Prior Art
According to the Scherzer theorem (O. Scherzer, Zeitschrift für Physik 101, (1936) 593), it is known that in optical reproduction systems for charged particles, which is understood to mean principally electrons and ions, with the use of static, space-charge-free and rotationally symmetrical fields, the chromatic aberration (colour deviations) and spherical aberration (focussing deviations) do not necessarily disappear. Since these errors limit the capability of the reproductive optical systems and in particular the resolution capability, there has been no lack of attempts to eliminate these image errors. Most success is promised by the departure from rotationally symmetrical lenses, that is to say the use of non-circular lenses in the form of multipoles, in particular quadrupoles, octopoles and the like. By means of a corrector of this kind made up from electrical and magnetic multipoles, the two managing directors of the applicant have succeeded in completely correcting the spherical and chromatic aberration in a low-voltage scanning electron microscope (J. Zach, M. Haider Nucl. Instr. method A363 (1995) 316), wherein it was possible to demonstrate a resolution capability of 2 nm at an electron energy of 1 kV.
The disadvantages of the electromagnetic multipole correctors can be seen in the fact that, because of the remanence, the magnetic fields do not allow fast, precise and reproducible adjustment of the magnetic fields. A demagnetisation also requires removal of the coil cores, which represents considerable effort. Over a relatively long period, a relatively large drift of the magnetic fields occurs. Finally, the strong magnetic fields necessary in ion-optical devices, such as, for example, lithography, because of the large ion masses, can only be implemented with difficulty because of the dependency of focusing on the mass. Correctors for eliminating the chromatic aberration with purely electrical fields in both sections are not known.
On this basis, the object of the invention is to provide a corrector for eliminating the chromatic aberration of particle lenses, which is constructed entirely from electrical fields, that is to say without the use of magnetic fields.
SUMMARY OF THE INVENTION
This object is attained according to the invention in that two corrector pieces are arranged along the optical axis upstream of the quadrupole in the beam direction, each corrector piece has two electrical quadrupole fields with overlaid circular lens field, whose quadrupole fields, however, are rotated relative to one another through an angle of 90° about the optical axis, and the adjustment is carried out such that the astigmatic intermediate image of one section lies in one corrector piece and the astigmatic intermediate image, which is perpendicular thereto, of the other section lies in the other corrector piece, and finally a further electrostatic quadrupole is arranged at the output side.
The term chromatic aberration in the sense of the invention, in exact terminology, means the first order, first degree axial chromatic aberration. Herein, the word “axial” describes the fact that this chromatic aberration is only determined by the fundamental paths emerging from the optical axis in the object point, that is to say the fundamental solutions to the Gaussian optics. The chromatic aberration is thus independent of the extra-axial paths. The order describes the power to which the initial gradient of the fundamental path enters into the chromatic aberration; the first order case there is a linear dependency. The term “first degree” describes the fact that the chromatic aberration has a linear dependency on the relative velocity deviation of the mean velocity of the particles. In the case of monochromatic particles—i.e. particles of equal velocity and therefore also constant wavelength—the relative deviations thus become zero. In this case there is no chromatic aberration. In the language of optics, the chromatic aberration is often also termed “colour deviation”.
The proposed electrostatic corrector, in its preferred construction, comprises four elements arranged one behind the other in the direction of the straight optical axis, namely—in the direction of the optical axis starting from the objective—first of a quadrupole and two corrector pieces arranged one behind the other, and finally—at the output end—of a further quadrupole. The quadrupole fields of the two corrector pieces are rotated with respect to one another about an angle of 90° about the optical axis.
The beam path in the corrector travels as follows; the axial beam path starting from the center of the objects first deflected by the objective lens and, after entry into the corrector, is first deflected by the electrical quadrupole in different manners in the two sections (X and Y section). The particle bundle is thereby focused in one section (e.g., in the X section) and caused to diverge in the other section (Y section), so that an astigmatic intermediate image is produced, which passes through the optical axis and is expediently positioned in the center of the first corrector piece. This corrector piece therefore does not significantly affect the path trajectory in the section in which the intermediate image lies (X section), because the axial path passes close to the optical axis and intersects it, wherein positive chromatic aberrations do occur, but because of the low distance from the axis they are only very small. In the section perpendicular thereto (Y section), on the other hand, the path trajectory, due to the quadrupole fields of the corrector piece, experiences a considerable influence and negative contribution to the chromatic aberration. There is thus an influencing of the chromatic aberration of one section in the first corrector piece and, in an analogous manner, of that of the second section in the second corrector piece. In dependence on the set potentials, an influencing of the chromatic aberration and in the ideal case a compensation of the chromatic aberration of the objective lens follow, so that the entire optical system formed of the objective lens and corrector has reproduction properties free of chromatic aberrations. The last quadrupole serves to re-combine the ray path to rotational symmetry again.
The generation of the astigmatic intermediate image within the corrector piece, i.e. the zero crossing of the corresponding paraxial path can be achieved by appropriate choice of the strength of the electrical quadrupole present at the input of the corrector. Variation of the potential of the electrical quadrupole fields of the corrector piece (circular lens component as well as quadrupole field intensity) with respect to one another, that is to say the opposing field between the quadrupole fields, results in the influencing and setting of the chromatic aberration.
The decisive advantages of the electrostatic corrector consist in a rapid and precise adjustment and setting of the fields, a problem-free handling with reproducible conditions even over a relatively long time period and also in the possible use in ion-optical equipment.
Particularly preferred embodiments are those in which a symmetrical construction and/or symmetrical course within a corrector piece with respect to its center plane and/or a symmetrical construction and/or symmetrical course of the fields of the two corrector pieces, with respect to the center plane between them is provided. Because of the symmetrical/anti-symmetrical course of the paraxial paths within the corrector pieces, numerous error integrals are cancelled out, or are at least made clear in an analytical manner and can be resolved without problem, which substantially contributes to the transparency and understanding of the behaviour of the corrector in different si
Rose Harald
Uhlemann Stephan
Weissbacker Christoph
CEOS Corrected Electron Optical Systems GmbH
El-Shammaa Mary
Schindler Edwin D.
Wells Nikita
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