Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2000-06-19
2004-02-17
Lee, John R. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML, C250S491100, C250S492100, C250S492200, C250S493100, C250S42300F, C250S398000, C250S310000, C250S311000
Reexamination Certificate
active
06693282
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a particle-optical apparatus which includes a source assembly for producing a beam of accelerated, electrically charged particles, which source assembly includes successively a particle emitter, a particle lens and an anode and also includes a beam-limiting diaphragm.
DESCRIPTION OF THE PRIOR ART
A source assembly of the kind set forth is known from European patent No. 0 502 401, notably from the
FIGS. 2 and 3
a
and the associated description. Particle sources of this kind are customarily used in electron microscopes or apparatus for the manufacture of integrated circuits by means of electron beams or ion beams, i.e. so-called lithography apparatus. The source assembly disclosed in the cited European patent is arranged to produce an electron beam which emanates from the particle emitter and is accelerated by the anode voltage. The particle emitter in the known source assembly is formed by the combination of an electron-emissive surface and a directly subsequent extractor electrode. The extractor electrode may be provided with a beam-limiting diaphragm for defining the angle of aperture of the electron beam produced. The beam-limiting diaphragm transmits only a minority of the electrons emitted by the particle emitter.
Downstream from the particle emitter there is arranged an electrode for focusing the electron beam. Said beam-limiting diaphragm may alternatively be provided in said focusing electrode. The electrostatic field generated by said focusing electrode in cooperation with the extractor electrode thus constitutes a particle lens forming part of the source assembly. Finally, the anode accelerates electron beam produced by the particle to the acceleration voltage required for the relevant application of the particle optical apparatus.
Because of the fixed arrangement of the particle lens relative to the other elements of the source assembly and because of the location and the dimensions of the beam-limiting diaphragm, the beam current and brightness produced by the source assembly are either fixed (for a given electron current produced by the particle emitter) or can hardly be varied for all practical purposes. It is to be noted that in the context of the present invention the term “brightness” is to be understood to mean the beam current which is emitted per unit of solid angular measure and per unit of emissive surface area. Generally speaking, it is not possible to realize a large beam current and a high brightness of the electron beam at the same time. This is due to the so-called Coulomb interaction (repulsion) between the electrons of the beam which increases the apparent dimensions of the emissive surface in the case of a large current, and hence reduces the brightness. However, it is sometimes desirable to have either a large beam current or a high brightness of the particle beam, depending on the relevant application of the particle-optical apparatus. It could be attempted to realize such a selection possibility by installing a beam-limiting diaphragm of different dimensions or by arranging this diaphragm in a different location in the source assembly. However, this approach has considerable drawbacks as will be described hereinafter.
Arranging the beam-limiting diaphragm in front of the anode, i.e. in front of the accelerating high-voltage field, has a number of drawbacks. A first drawback is due to the fact that the anode in a particle-optical apparatus, such as an electron microscope, carries ground potential so that the electron source carries the negative acceleration high voltage which may be of the order of magnitude of −300 kV. Consequently, the diaphragm (since it is situated in or in front of the accelerating field) also carries approximately this high voltage, so that the manipulability of this diaphragm is significantly impeded and practically non-existent in practice.
A second drawback is due to the fact that many electron microscopes are provided with a high-voltage insulating envelope which surrounds the space in which the electron source is situated and contains an insulating gas such as sulphur fluoride (SF
6
). The accessibility of the diaphragm is thus strongly reduced. The formation of passages, either mechanical or electrical, through this gas-filled space gives rise to problems as regards gastightness and also as regards electrical insulation of the passages. (The existing electrical connections to the source assembly are fed in via a high-voltage cable of standard design which has been taken into account for the design of the microscope. However, this standard cable is not suitable to conduct electrical signals other than those for which it has been designed).
A third drawback is due to the fact that it would be necessary to make holes for passages in the microscope column of an electron microscope which may already have been installed at a customer. This would necessitate complete disassembly, involving contamination of the vacuum space of the microscope, and also transport of heavy precision tools.
In the described circumstances notably the alignment of the various components of the source assembly relative to one another and relative to the remainder of the electron microscope will be seriously hampered by the above problems.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a particle source of the kind set forth in which switching over between a large beam current and a high brightness of the particle beam can take place in operational conditions, without mechanical modifications of the source assembly being required.
To this end, the particle-optical apparatus according to the invention is characterized in that
the source assembly is provided with at least one further particle lens which is arranged between the first-mentioned particle lens and the anode,
the beam-limiting diaphragm is arranged at the area of or downstream from the further particle lens, and
the apparatus is provided with energizing means for the mutually independent energizing of the first-mentioned lens and the further particle lens.
It is now possible to energize the first particle lens (i.e. the lens situated nearest to the particle emitter) and not the further particle lens. The particle beam emanating from the emissive surface is then already parallelized more or less immediately upon leaving said surface, so that this parallel beam has a comparatively small cross-section so that a comparatively large number of particles will traverse the beam-limiting diaphragm which is arranged at that area or downstream from the further particle lens (i.e. directly in front of, in or behind said further lens). Thus, a comparatively large beam current is obtained in this situation. However, it may also be elected to energize the further particle lens (situated comparatively far from the particle emitter) and to abstain from energizing the first particle lens. The particle beam emanating from the emissive surface is then more or less parallelized comparatively far from said surface, so that this parallel beam has a comparatively large cross-section and hence comparatively few particles will traverse the beam-limiting diaphragm. In this situation, therefore, a comparatively high brightness is obtained in conjunction with a smaller beam current, because only little current is left behind the diaphragm, so that no or hardly any Coulomb repulsion occurs and hence the virtual emissive surface area will not be enlarged.
The further particle lens in an embodiment of the invention is constructed as an electrostatic lens. The compact construction of such lenses makes this embodiment particularly suitable for building into the source assembly according to the invention.
The particle emitter in another embodiment of the invention is constructed as a field emission particle emitter, an extraction electrode being arranged between the particle emitter and the first-mentioned particle lens which consists of a combination of the extraction electrode and a second electrode. The idea of the invention becomes particularl
FEI Company
Lee John R.
Scheinberg Michael O.
Vanore David A.
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