Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-12-18
2001-04-17
Arroyo, Teresa M. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S3960ML, C250S397000
Reexamination Certificate
active
06218664
ABSTRACT:
The invention relates to a particle-optical apparatus which includes
a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus,
a specimen holder for a specimen to be irradiated by means of the apparatus,
a focusing device for forming a focus of the primary beam in the vicinity of the specimen holder by means of electrostatic electrodes,
a beam deflection system for deflecting the primary beam,
detection means for detecting electrically charged particles which emanate from the specimen in response to the incidence of the primary beam, which detection means are arranged ahead of the focusing device, viewed in the propagation direction of the electrically charged particles in the primary beam.
A particle-optical apparatus of this kind is known from an article entitled “Design of a high-resolution low-voltage scanning electron microscope” by J. Zach in the magazine “Optik”, Vol. 83, No. 1 (1989), pp. 30-40.
Apparatus of the kind set forth are known as Scanning Electron Microscopes (SEM). In a SEM a region of a specimen to be examined is scanned by means of a focused primary beam of electrically charged particles, usually electrons, which travel along an optical axis of the apparatus. The acceleration voltage for the electron beam in the SEM is preferably chosen to be comparatively high (for example, of the order of magnitude of 30 kV) with a view to ensuring that only slight electron interaction during their travel through the electron-optical column, so that only a slight energy spread occurs in the electron beam due to this effect. Evidently, the choice of the acceleration voltage, however, is also dependent on the nature of the specimen to be examined. This acceleration voltage should have a comparatively low value (of the order of magnitude of 1 kV) so as to minimize charging of the specimen by the primary electron beam. This could take place, for example during the study of electrically insulating layers in integrated electronic circuits or in the case of given biological specimens. Moreover, for some examinations it is desirable that the electrons of the primary beam penetrate the specimen to a small depth only, resulting in a better contrast of the image to be formed. Thus, it is often desirable that the electron beam traverses the electron-optical column with a comparatively high voltage but is subsequently decelerated to a comparatively low voltage just ahead of the specimen.
Irradiation of the specimen to be examined releases electrically charged particles (generally secondary electrons) which have an energy which is substantially lower, for example of the order of magnitude of from 1 to 50 eV. The energy and/or the energy distribution of these secondary electrons offers information as regards the nature and the composition of the specimen. Therefore, a SEM is attractively provided with a detector for secondary electrons. These electrons are released at the side of the specimen at which the primary beam is incident, after which they travel back against the direction of incidence of the primary electrons. Therefore, when a detector (for example, provided with an electrode carrying a positive voltage) is arranged in the path of the secondary electrons thus traveling back, the secondary electrons are captured by this electrode and the detector outputs an electric signal which is proportional to the electric current thus detected. The (secondary electron) image of the specimen is thus formed in known manner. With a view to the quality of the image, notably the speed at which the image is formed and the signal-to-noise ratio, the detected current is preferably as large as possible.
The cited article by Zach discloses (for example, see the
FIGS. 3 and 4
therein) a particle-optical apparatus in the form of a SEM in which the focusing device for forming the focus of the primary beam in the vicinity of the specimen holder is formed by three electrostatic electrodes, the first electrode (viewed in the propagation direction of the electrons in the primary beam) being coincident with a detector.
Nowadays there is a tendency to construct SEMs to be as small as possible. Apart from economical motives (generally speaking, smaller apparatus can be more economically manufactured), such small apparatus offer the advantage that, because of their mobility and small space required, they can be used not only as a laboratory instrument but also as a tool for the formation of small structures, for example as in the production of integrated circuits. In this field a miniaturized SEM can be used for direct production as well as for inspection of products. With a view to direct production, the SEM can be used to write, using electrons, a pattern on the IC to be manufactured. With a view to inspection, the SEM can be used to observe the relevant process during the writing by means of a further particle beam (for example, an ion beam for implantation in the IC to be manufactured), it also being possible to use the SEM for on-line inspection of an IC after execution of a step of the manufacturing process.
For miniaturization of a SEM it is attractive to use an electrostatic objective, because such an objective can be constructed so as to be smaller than-a magnetic lens. This is due to the fact that cooling means (notably cooling ducts for the lens coil) can be dispensed with and that the magnetic (iron) circuit of the lens requires a given minimum volume in order to prevent magnetic saturation. Moreover, because of the temporary requirements as regards high vacuum in the specimen space, electrostatic electrodes (which are constructed as smooth metal surfaces) are more attractive than the surfaces of a magnetic lens which are often provided with coils, wires and/or vacuum rings. Finally, as is generally known in particle optics, an electrical field is a more suitable lens for heavy particles (ions) than a magnetic field.
The arrangement of the detector for the secondary electrons ahead of the focusing device as disclosed in the cited article offers the advantage that when the SEM is used for the observation of ICs, it is also easier to look into pit-shaped irregularities; this is because observation takes place along the same line as that along which the primary beam is incident. Moreover, arranging the detector to the side of the objective and directly above the specimen would have the drawback that the detector would then make it impossible for the distance between the objective and the specimen to be made as small as desirable with a view to the strong reduction of the electron source so as to achieve a size of the scanning electron spot which is sufficiently small with a view to the required resolution. Furthermore, when an electrostatic objective is used in a SEM, it often happens that the electrostatic lens field of the objective extends slightly beyond the physical boundaries of the objective, so possibly as far as the specimen. This would cause secondary electrons emanating from the specimen to be attracted by said field. For example, a detector arranged to the side of the objective would then require a much stronger attractive effect whereby the primary beam would be inadmissibly influenced. This adverse effect is avoided by arranging the detector above the objective.
Even though the cited article by Zach describes a SEM, it does not provide any further details as regards the nature, the appearance and the arrangement of the beam deflection system whereby the scanning motion of the primary beam across the specimen is realized. The cited article does provide some information as regards the detection efficiency for the secondary electrons achieved by the configuration described therein. For the determination of this detection efficiency according to said article (see notably section 3.6 thereof) integration over all energies of the secondary electrons takes place. For a substantially punctiform region of the specimen around the optical axis a value of 61% is found for the detection efficiency thus determined.
It is a
Henstra Alexander
Krans Jan M.
Krijn Marcellinus P. C. M.
Arroyo Teresa M.
Barchall Ann E.
FEI Company
Scheinberg Michael O.
Wells Nikita
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