Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1999-08-09
2001-12-11
Anderson, Bruce (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML, C250S398000, C250S311000
Reexamination Certificate
active
06329659
ABSTRACT:
FIELD OF THE INVENTION
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 lens defects of the focusing lens, which correction device includes a correction unit which is provided with at least two hexapoles wherebetween a first imaging transmission system is arranged in order to image one hexapole onto the other hexapole, and which correction device also includes a second transmission 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.
DESCRIPTION OF PRIOR ART
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 irradiation of the object may be aimed at imaging the objects to be studied in such apparatus (specimens in electron microscopes) or at forming very small structures on the object, for example for micro-electronics (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 Transition 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 such a structure a coma-free plane of the focusing lens to be corrected is imaged onto the entrance of the correction unit by means of a transmission lens system consisting of rotationally symmetrical lenses. This correction unit is formed by two hexapoles wherebetween there is arranged an imaging transmission lens system which consists of rotationally symmetrical lenses and serves to image 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 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 easily as possible.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a correction device for correcting lens defects 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 first transmission system consists of at least four quadrupoles having substantially equal quadrupole strengths, the quadrupole effects of neighboring quadrupoles each time being directed perpendicularly to one another.
The invention is based on the recognition of the fact that the severe requirements imposed in respect of the resolution of such a correction device can be satisfied by means of a transmission system which is constructed in the form of four quadrupoles instead of a transmission lens system constructed as a doublet of round lenses. Using such a configuration, the spherical aberration of the objective can be corrected to a high degree. Taking these steps offers a number of advantages. First of all, as opposed to rotationally symmetrical lenses, the images formed by an imaging quadrupole system are rotation-free relative to the imaged object. This facilitates the adjustment and alignment of the entire optical system of the particle-optical apparatus. Secondly, quadrupoles are so-called strong lenses, which means that for a given focal distance a much smaller energizing current is required in comparison with corresponding rotationally symmetrical (round) lenses. This offers the advantage that the cooling of such quadrupoles is much easier than that of round lenses, and that thermal drift of a particle-optical apparatus which is not in a state of thermal equilibrium is strongly reduced.
In a preferred embodiment of the invention, the second transmission system consists of at least four quadrupoles having substantially equal quadrupole strength, the quadrupole effects of neighboring quadrupoles each time being directed perpendicularly to one another. The quadrupoles in the second transmission system then yield the same advantages as the quadrupoles in the correction unit.
In a further embodiment of the invention, the two hexapoles of the correction unit are substantially identical. This results in a high degree of symmetry of the correction unit, so that the correction unit is particularly suitable for a SEM (in which the electrons travel through the correction unit to the lens to be corrected) as well as for a TEM (in which the electrons travel through the lens to be corrected to the correction unit).
In a further embodiment of the invention, each of the two hexapoles is constructed as a hexapole doublet. An axial shift of the centers of the hexapoles can thus be realized without physical displacement of these elements; this makes it easier to satisfy the requirements imposed as regards mechanical precision and alignment.
In a further embodiment of the invention, the central quadrupoles of the first transmission system are constructed as a multipole unit having at least eight physical poles, at least four of which can be adjusted to an electric potential in a mutually independent manner. When the electric voltages on the pol
Krijn Marcellinus PC. M.
Mentink Sjoerd A. M.
Anderson Bruce
Philips Electron Optics B.V.
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