Optics: measuring and testing – Focal position of light source
Reexamination Certificate
1998-11-19
2001-02-06
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Focal position of light source
C250S3960ML
Reexamination Certificate
active
06184975
ABSTRACT:
I FIELD OF THE INVENTION
The invention relates to a particle-optical apparatus which includes:
a particle source for emitting a beam of electrically charged particles which travel along an optical axis of the apparatus in order to expose 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 chromatic aberration of the focusing lens,
which correction device includes correction elements, each of which is provided with electrodes for producing electric quadrupole fields, said electrodes being arranged in successive layers along the optical axis, the quadrupole fields produced by the electrodes in the layers being rotated through an angle of substantially 90° about the optical axis relative to the quadrupole field produced by the electrodes in an adjacent layer.
The invention also relates to a correction device for use in such an apparatus, and to a method of operating a correction device for correcting the chromatic aberration of a focusing lens of a particle-optical apparatus.
A correction system for correcting lens defects in particle-optical apparatus is described in an article entitled “A Possible Chromatic Correction System for Electronic Lenses” by G. D. Archard, Proc. Phys. Soc. London, 1955, pp. 817-829, notably in the sections 6 and 7 and FIG. 2
b.
II GENERAL STATE OF THE ART
Particle-optical apparatus, such as electron microscopes or electron lithography apparatus, generally 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 purpose of the irradiation of the object may be to image the object to be studied (specimens in electron microscopes) or to produce very small structures on the object, for example for micro-electronics (apparatus for electron lithography). In both cases focusing lenses are required for focusing the electron beam.
The focusing of the electron beam can in principle be performed in two ways. According to a first method, the zone to be imaged of a specimen to be examined is more or less uniformly exposed by means of 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 formed by 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 in significantly reduced form, on the specimen to be examined (in the Scanning Electron Microscope or SEM) or on an object on which the relevant microstructure is to be provided (in lithography apparatus). The image of the electron source (the “spot” which is moved across the object by means of, for example deflection coils) is again formed by means of an imaging lens system. The focusing lens is then the objective lens of the spot-forming lens system; the resolution of this objective lens i then decides the spot size of the beam and hence the resolution of the apparatus.
The lenses used in all such apparatus are usually magnetic lenses, but may also be electrostatic lenses. The lenses of both types are practically always rotationally symmetrical. Such lenses inevitably have a non-ideal behavior, i.e. they inherently have lens defects which limit the resolution of the lens; its so-called spherical aberration and chromatic aberration are usually decisive for 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, said lens defects cannot be eliminated by compensation by means of rotationally symmetrical electric or magnetic fields.
In contemporary electron optical apparatus, notably in scanning particle-optical apparatus having a spot-forming objective lens (the so-called scanning electron microscope (SEM)) there is a tendency to select a value for the acceleration voltage of the electron beam which is lower than was customary thus far, i.e. of the order of magnitude of from 0.5 kV to 5 kV instead of the previously customary voltage of the order of magnitude of 30 kV or more. This is because the charging of non-conductive specimens (for example, photoresist material for the manufacture of electronic integrated circuits) is substantially reduced at such comparatively low acceleration voltages and because a substantial improvement of the so-called topographic contrast is also achieved at these low voltages. For such low acceleration voltages, the chromatic aberration is the predominant lens defect, so the factor deciding the resolution of the particle-optical apparatus. (This can be readily understood considering the fact that the chromatic aberration is proportional to &Dgr;&PHgr;/&PHgr;, where &Dgr;&PHgr; the non-variable energy spread in the electron beam and &PHgr; is the nominal acceleration voltage; this factor, therefore, increases as &PHgr; is reduced.)
In order to enhance the resolution of the particle-optical apparatus nevertheless, it has already been proposed to reduce said lens defects by means of a correction device (also referred to as a corrector) having a structure which is not rotationally symmetrical.
III BACKGROUND OF THE STATE OF THE ART
The approach that could be followed so as to attempt making the chromatic aberration of an imaging system equal to zero by means of optical elements which are not rotationally symmetrical has already been described in an article entitled “Sphärische und chromatische Korrektur von Elektronen-Linsen”, Optik 2, pp. 114-132, by O. Scherzer, notably pp. 114-119.
The non-rotationally symmetrical optical elements described in the cited article (notably in the sections 1a, 1b and 1c) consist of cylinder lenses and quadrupole fields and monopole fields which act as correction members. The cylinder lenses form an astigmatic path for the electron beams and the correction members, consisting of a combination of quadrupole fields and monopole fields, are arranged in said astigmatic path. The chromatic aberration of the electron path in this known structure is corrected in a first plane which contains the optical axis (the so-called x-z plane), and subsequently the same is done in a second plane which extends perpendicularly thereto (the y-z plane). At the area where the electron path in the x-z plane is subject to chromatic correction, the distance between the electron beam and the optical axis in the y-z plane equals zero for electrons of nominal energy, and vice versa. Because an energy which deviates from the nominal energy occurs in the electron beam, the electron rays having this deviating energy follow a path other than the rays of nominal energy. The electron rays having this deviating energy thus traverse the correction member along a path which deviates from the nominal path; consequently, at said areas with a distance zero from the axis, the distance from the axis is not equal to zero for these rays. However, in the cited article it is assumed that for the rays of deviating energy this distance from the optical axis is so small that the deflecting effect thereof is negligibly small. (In this respect see notably section 1c of the cited article.)
III-1 The Problem Stemming from the State of the Art
Generally speaking, the configuration disclosed in the article by Scherzer cannot be simply used in a particle-optical apparatus. The configuration according to Scherzer constitutes an imaging system, whereas a particle optical apparatus such as a SEM needs a correction system which corrects only the chromatic aberration and has no or hardly any effect on the strength of the imaging lens (the objective). It would be feasible to replace the system known from the ci
Henstra Alexander
Krijn Marcellinus P. C. M.
Font Frank G.
Fulbright & Jaworski LLP
Nguyen Tu T.
Philips Electron Optics B.V.
LandOfFree
Electrostatic device for correcting chromatic aberration in... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electrostatic device for correcting chromatic aberration in..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electrostatic device for correcting chromatic aberration in... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2609726