Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1998-12-10
2001-02-20
Arroyo, Teresa M. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06191423
ABSTRACT:
The invention relates to a particle-optical apparatus which includes a particle source for producing a beam of electrically charged particles which travel along an optical axis of the apparatus in order to irradiate an object to be irradiated in the apparatus by means of the particle beam, a focusing lens for focusing the beam of electrically charged particles, and a correction device for correcting the spherical aberration of the focusing lens, which correction device includes a correction unit which is provided with at least two hexapoles wherebetween a first imaging transmission lens system is arranged in order to image one hexapole onto the other hexapole, which correction device also includes a second transmission lens system for imaging a coma-free plane of the focusing lens onto the entrance of the correction unit.
The invention also relates to a correction device for use in such an apparatus.
A correction device of this kind for use in such an apparatus is known from U.S. Pat. No. 5,084,622.
Generally speaking, particle-optical apparatus, such as electron microscopes or electron lithography apparatus, are arranged to irradiate an object to be studied or treated by means of a beam of electrically charged particles (usually an electron beam) which is produced by means of a particle source such as a thermal electron source or an electron source of the field emission type. The aim of the irradiation of the object may be to image these objects to be studied in such apparatus (specimens in electron microscope) or to form very small structures on the object, for example for microelectronics (electron lithography apparatus). In both cases focusing lenses are required to focus the electron beam.
The electron beam can in principle be focused in two ways. According to the first method, a specimen to be examined is more or less uniformly irradiated by the electron beam and an enlarged image of the specimen is formed by means of the focusing lens. The focusing lens is in that case the objective lens of an imaging lens system; the resolution of the objective lens then decides the resolution of the apparatus. Apparatus of this kind are known as Transmission Electron Microscopes (TEM). According to a second focusing method, the emissive surface of the electron source, or a part thereof, is imaged, usually at a strongly reduced scale, on the specimen to be examined (in the Scanning Electron Microscope or SEM or in the Scanning Transmission Electron Microscope or STEM) or on an object on which the relevant microstructure is to be provided (in the lithography apparatus). The image of the electron source (the “spot” which is displaced across the object by means of, for example deflection coils) is again formed by means of an imaging lens system. In the latter case the focusing lens is formed by the objective lens of the spot forming lens system; the resolution of this objective lens decides the spot size of the beam and hence the resolution of the apparatus.
The lenses used in all apparatus of this kind are usually magnetic lenses, but may also be electrostatic lenses. Both types of lens are practically always rotationally symmetrical. Such lenses inevitably have a non-ideal behavior, i.e. they have lens defects, among which the so-called spherical aberration and the chromatic aberration are usually decisive in respect of the resolution of the lens; these lens defects thus determine the limit of the resolution of the known electron optical apparatus. According to a theorem of particle-optics, such lens defects cannot be eliminated by compensation by means of rotationally symmetrical electrical or magnetic fields.
In order to enhance the resolution of the particle-optical apparatus nevertheless, it is known from the cited U.S. Pat. No. 5,084,622 to reduce said lens defects by means of a correction device having a structure which is not rotationally symmetrical. In this structure a coma-free plane of the focusing lens to be corrected is imaged on the input of the correction device by means of a transmission lens system. This correction unit is formed by two hexapoles wherebetween there is arranged an imaging transmission lens system for imaging one hexapole onto the other. The entrance of the correction unit is then formed by the center of the first hexapole, viewed in the direction of the incident electrons.
A configuration of this kind must satisfy very severe requirements as regards manufacturing tolerances, mechanical stability (inter alia with a view to thermal drift) and alignment of the various elements relative to one another. Therefore, the aim is to minimize the number of separate structural components so that the requirements as regards manufacturing tolerances, mechanical stability and alignment can be satisfied as readily as possible.
It is an object of the invention to provide a correction device for correcting spherical aberration whose construction is simpler than that of the known correction device. To this end, the particle-optical apparatus according to the invention is characterized in that the second transmission lens system (i.e. the transmission lens system which images the coma-free plane of the focusing lens to be corrected onto the entrance of the correction unit) consists of one lens. The invention is based on the recognition of the fact that the severe requirements in respect of resolution of such a correction device can be satisfied by means of a transmission lens system constructed as a single lens instead of a transmission lens system constructed as a doublet. The number of components to be aligned is thus reduced by one.
It is to be noted that the single lens which replaces the second transmission lens system in conformity with the invention may also be constructed as an assembly of quadrupoles. It is known per se that an assembly of quadrupoles has the same effect as a rotationally symmetrical lens. Furthermore, not only the effect of the single lens can be realized by means of a system of quadrupoles, but also the effect of the first transmission lens system. Both possibilities are known per se, for example from the book “Electron Optics” by P. Grivet, Pergamon Press, 1965, section 10.4.2. It is also possible to integrate two quadrupoles of said quadrupole systems, situated between the hexapoles, with the units generating the hexapoles. This is a technique which is known per se, the hexapole field then being generated by means of a configuration of a number of physical poles which is larger than the required number of six, for example eight or twelve. The desired hexapole field is then obtained by specific excitation of the physical poles, the desired quadrupole field being obtained by likewise specific excitation of the physical poles which is added to the hexapole excitation.
In a preferred embodiment of the invention, the imaging transmission lens system arranged between the hexapoles of the correction unit consists of one lens. The number of components to be aligned is thus further reduced. In a system of this kind electrons traveling through the first hexapole at a given distance from the optical axis will also travel through the second hexapole at the same distance from the optical axis, but not at the same angle relative to the axis. The latter phenomenon causes a (usually small) second-order image defect. These second-order image defects are sometimes negligibly small, depending on the requirements imposed on the correction device.
If these second-order image defects are not negligibly small, correction can be made by exciting one hexapole slightly different with respect to the other hexapole. In an embodiment of the invention, the difference between the excitations of the hexapoles does not exceed 10%. Experiments have shown that the undesirable second-order aberration can be adequately corrected by means of this method of excitation.
In a further embodiment of the invention, the two hexapoles of the correction unit are identical. This results in a high degree of symmetry of the correction unit so that the correction unit is particularly suitable
Krijn Marcellinus P. C. M.
VAN DER Mast Karel Diederick
Arroyo Teresa M.
Fulbright & Jaworski LLP
Philips Electron Optics B.V.
Wells Nikita
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