Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-12-03
2002-04-02
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S397000
Reexamination Certificate
active
06365896
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 examined by means of the apparatus,
a focusing device for forming a focus of the primary beam in the vicinity of the specimen holder,
scanning means for scanning the specimen by means of the focused beam,
detection means for capturing electrically charged particles originating from the specimen.
An apparatus of this kind is known from U.S. Pat. No. 4,785,182.
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 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 chosen in dependence 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 for 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. Other specimens, however, require a higher acceleration voltage, for example of the order of magnitude of 30 kV.
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 5 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, an SEM is attractively provided with a detector for secondary electrons. These electrons are released at the side of the specimen where the primary beam is incident, after which they travel back, against the direction of incidence of the primary electrons, approximately along the field lines of the focusing lens. Therefore, when a detector (for example, provided with an electrode carrying a positive voltage of 300 V) is arranged in the vicinity of the secondary electrons thus travelling 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.
According to the cited United States patent the specimen to be examined is arranged in an atmosphere of a gas at a pressure of between 0.05 Torr (≈6.5 N/m
2
) and 20 Torr (≈2630 N/m
2
), so a pressure which is many times higher than the pressure at which conventional SEMs operate. The electric field produced by the voltage between the specimen and the electrode of the detector accelerates the secondary electrons emanating from the specimen to such a speed that they are capable of ionizing the atoms of the gas enveloping the specimen. During these ionizations, one or more electrons are released from the gas atoms, which electrons themselves are accelerated and can release further electrons by further ionizations again, etc. The gas surrounding the specimen thus acts as an amplifier for the secondary electron current, so that the current to be detected can in principle be larger than the current caused by the secondary electrons themselves.
Further advantages of a SEM operating with a gas atmosphere (to be referred to hereinafter as an “Environmental SEM” or ESEM) over the conventional SEM consist in that the ESEM enables the formation of electron optical images of humid or non-conductive specimens (for example, biological specimens, synthetic materials, ceramic materials or glass fibers) which are extremely difficult to image in the customary vacuum conditions in the conventional SEM. The ESEM enables the specimen to be maintained in its “natural” condition, without the specimen having to be subjected to the adverse effects of drying, freezing or vacuum coating operations which are normally required for the study by means of electron beams in high vacuum conditions.
Furthermore, because of the comparatively high permissible pressure in the specimen space of the ESEM, the gas ions formed neutralize any electric charging of a non-conductive specimen which could otherwise impede the formation of an image of high resolution. The ESEM also enables direct, real-time observation of phenomena such as transport of liquids, chemical reactions, solution, crystallization and other processes taking place at a comparatively high vapor pressure which is far beyond that permissible in the specimen space of a conventional SEM.
It is to be noted that, generally speaking, ESEMs can operate with an atmosphere in the specimen space whose pressure is also outside the range stated in the cited United States patent. It is notably possible to admit a lower pressure in the specimen space, for example a pressure of 0.01 Torr (≈1.3 N/m
2
).
It is a drawback of the device disclosed in the cited United States patent that a comparatively high voltage is required at the detector electrode so as to obtain a sufficient number of successive ionizations, and that hence the distance between the specimen and the detector electrode cannot become smaller than a comparatively large minimum distance.
It is an object of the invention to provide a scanning particle-optical apparatus in which the number of collisions between the ionizing electrons and the gas atoms becomes substantially higher than in the known particle-optical apparatus while using the same geometry of the specimen and the detector electrode.
To this end, the particle-optical apparatus according to the invention is characterized in that it includes means for producing an additional magnetic field in the space between the detection means and the specimen holder. In the context of the present invention the space between the detection means and the specimen holder is to be understood to mean the space which is traversed by electrically charged particles originating from the specimen (and possibly particles produced by said particles, for example electrons and ions produced by ionizations) before they are captured by a detector electrode.
It is to be noted that in the context of the present invention an additional magnetic field is to be understood to mean a magnetic field which is formed additionally to the magnetic field formed in the focusing device which serves to produce a focus of the primary beam, which field may possibly extend as far as the specimen as in the case of an immersion lens.
As is known, an electron moving in a magnetic field experiences a force which is directed perpendicularly to the direction of movement and also perpendicularly to the magnetic field. In the absence of a magnetic field, a secondary electron travelling from the specimen to the detector electrode will follow a practically straight path to the detector electrode, except for changes of direction due to collisions with gas atoms. In the presence of a magnetic field, therefore, such an electron is deflected away from its direction of movement to the detector electrode and, theoretically speaking, in the case of given field geometries it cannot even reach the detector electrode in the absence of loss of energy. As a result, this electron will travel a substantially longer distance, so that the probability of collisions with the gas atoms is substantially increased. Because of the ionizing collisions with the gas atoms, such an electron loses each time a given amount of energy during its t
Anderson Bruce
Fulbright & Jaworski L.L.P.
Philips Electron Optics B.V.
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