Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
1998-12-04
2001-02-06
Anderson, Bruce C. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S398000, C250S397000, C250S3960ML, C250S492300
Reexamination Certificate
active
06184525
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 (
4
) of the apparatus,
a specimen holder for a specimen to be irradiated by means of the apparatus,
an immersion lens 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 detecting signals originating from the specimen in response to the incidence of the primary beam, which detection means include an electrostatic detection electrode for generating an electric field in the space between the detection electrode and the specimen holder.
An apparatus of this kind is known from the abstract No. 5-174768 (A) of Japanese patent application No. 3-53811, published on 13.7.93.
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 the 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 provides 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 where the primary beam is incident, after which they travel back against the direction of incidence of the primary electrons. 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.
In a so-called Environmental SEM (ESEM) the specimen is arranged in an atmosphere of a gas at a pressure of between 0.01 Torr (≈1.3 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. An electric field, produced by the voltage between the specimen and an is electrostatic detection electrode associated with the detection means for detecting signals originating from the specimen in response to the incidence of the primary beam, 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 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 nonconductive 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 of a conventional SEM.
According to the cited abstract No. 5-174768(A) the primary beam is focused on the specimen by an immersion lens. As is known, an immersion lens is a magnetic lens which produces a magnetic field in the space between the poleshoes of the lens and the specimen. The electrons released from the specimen by the primary beam then travel back from the specimen to the electrostatic detection electrode of the detector while following approximately the field lines of the immersion lens. The electrostatic detection electrode disclosed in the cited abstract is an annular electrode which generates an electric field between the specimen (carrying a voltage which is lower than that present at this electrode) and this electrode. In order to achieve a sufficiently high current amplification effect by the gas atmosphere in the ESEM, however, a comparatively high voltage is required for the detector electrode and, because of the risk of electric breakdowns, the distance between the specimen and the detector electrode may not be smaller than a comparatively large minimum distance. Consequently, the number of successive ionizations is limited, and hence also the current amplification.
It is an object of the invention to provide an ESEM having a current amplification which is higher than that of the known ESEM. To this end, the particle-optical apparatus according to the invention is characterized in that the detection means are arranged to produce an electric multipole field around the optical axis which extends transversely of the optical axis in the same space as the magnetic field of the immersion lens.
The invention utilizes the property of electric multipoles that at a given field strength at the optical axis of the multipole the electric field strength outside the optical axis may be substantially higher. Thus, while exerting a slight effect only on the primary electron beam, a strong detector field can nevertheless be provided so that the secondary electrons to be accelerated receive adequate energy so as to realize numerous multipole ionizations, and hence a high current amplification in the gas atmosphere around the specimen. Moreover, the space angle at which the specimen is “perceived” by the electrode assembly producing the electric multipole field is very large.
The invention is based on the recognition of the fact that, 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
Anderson Bruce C.
Fulbright & Jaworski LLP
Philips Electron Optics B.V.
Wells Nikita
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