Radiant energy – Electron energy analysis
Patent
1997-06-02
1998-11-17
Berman, Jack I.
Radiant energy
Electron energy analysis
H01J 3705, H01J 3704
Patent
active
058380043
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a particle-optical apparatus, comprising a particle source for producing a primary beam of electrically charged particles, which particle source comprises high-voltage means for establishing within the particle source an accelerating high-voltage field which is to be traversed by the particles, and a monochromator filter assembly which is situated substantially completely ahead of the high-voltage field within the source so as to select from the primary beam a sub-beam with an energy dispersion which is less than that of the primary beam.
An apparatus of this kind is known from U.S. Pat. No. 5,300,775.
In a particle-optical apparatus, for example an electron microscope, a low energy dispersion in the particle beam (electron beam) is generally desirable. This is because the energy dispersion in the electron beam, in conjunction with the chromatic aberration of the imaging lens or lenses, degrades the resolution of the image of the electron microscope. A low energy dispersion is desired notably in electron microscopes in which electron spectroscopy is carried out, i.e. electron microscopes in which the energy loss of the electron beam in the sample to be studied is determined in dependence on the location on the sample. Conclusions regarding the composition of the sample can be drawn on the basis thereof. The contrast in the image of the sample can also be enhanced by selecting exclusively electrons with a given energy loss to participate in imaging. For these situations it is necessary to have an irradiating electron beam with a low energy dispersion available. This can be achieved by arranging an energy-dispersive element, also referred to as a filter assembly, in the irradiating beam. This enables energy selection by deflecting the electrons, i.e. by spatially separating the electrons, in dependence on their energy and, if necessary, by selecting the electrons of the desired energy.
The cited United States Patent document discloses an electron microscope which comprises an energy-dispersive unit (a filter assembly) for energy selection in an electron beam. As is customary in such apparatus, the electron microscope comprises an electron gun provided with an electron source for producing a primary electron beam, i.e. an electron beam which has not yet been subjected to energy selection. Moreover, the gun can be introduced into an accelerating potential (i.e. an accelerating high-voltage field) in order to accelerate the electrons in the beam. The filter assembly is arranged in the gun in the vicinity of the electron source, so that the electrons enter the filter assembly with a comparatively low energy (for example, of the order of magnitude of 3 kV). This has the advantage that the filter may have a comparatively compact construction because the dimension of a filter for an electron beam is determined, generally speaking by the energy of the beam to be filtered. The compact construction of such a filter enables it to be built into an electron microscope of an existing design, without extensive modification of the apparatus being required.
The described arrangement of the filter assembly ahead of the high-voltage field, however, also has a number of drawbacks. A first drawback is due to the fact that the accelerating electrode (the anode) in the electron gun of an electron microscope carries ground potential, so that the electron source carries the negative accelerating high voltage of the order of magnitude of -300 kV. Consequently, the filter assembly also carries approximately this high voltage (since it is arranged ahead of the accelerating field), so that manipulation of the filter component is substantially obstructed and almost impossible in practice.
A second drawback is due to the fact that for high-voltage insulation many electron microscopes comprise an envelope around the gun space containing an insulating gas, such as sulphur fluoride (SF.sub.6). The accessibility of the filter assembly is thus strongly reduced. The formation of passages, either mechanical or electrical
REFERENCES:
patent: 3308293 (1967-03-01), Mathams
patent: 3979590 (1976-09-01), Andersen
patent: 5386115 (1995-01-01), Freidhoff et al.
patent: 5448063 (1995-09-01), DeJong et al.
Chmelik J.
Kruit Pieter
Tiemeijer Peter Christiaan
Berman Jack I.
Scheinberg Michael O.
U.S. Philips Corporation
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