Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2002-02-08
2004-06-01
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S310000, C359S237000, C359S279000
Reexamination Certificate
active
06744048
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lens system used for a phase plate that is used in a transmission electron microscope (TEM) and also to a TEM.
2. Description of Related Art
Where a specimen is observed with a conventional TEM, two major causes of the contrast in the resulting image are scattering contrast and phase contrast. These two kinds of contrast are normally discussed regarding elastically scattered electrons. In the following description, “scattering” means “elastic scattering” unless otherwise specifically stated.
Various portions of a specimen scatter an electron beam to different extents. A scattering contrast technique is a method of causing these different intensities (quantitative variations) of scattering to be reflected in the contrast displayed in an image. Those specimen portions which are composed of a heavy element (having a large atomic number) scatter electrons more strongly. Those specimen portions which are made of a light element (having a smaller atomic number) scatter electrons more weakly. An objective aperture placed around an optical axis between the specimen and the image plane acts to pass only electrons that are weakly scattered. As a result, variations in amount of scattered electrons blocked by the objective aperture result in variations in brightness on the image plane. As the element constituting the specimen becomes heavier, the scattering contrast increases. Also, as the specimen becomes thicker in the forward direction of travel of electrons, the scattering contrast increases. However, as the thickness of the specimen is increased, electrons are inelastically scattered more strongly. In consequence, chromatic aberration blurs the image.
Where an observation is made with a TEM, if the specimen is thick in the penetration direction of electrons, the electrons are inelastically scattered, thus blurring the image. To reduce inelastic scattering of electrons, the specimen is made sufficiently thin. Therefore, the specimen causes weak elastic scattering. This makes it difficult to obtain scattering contrast that is enough for imaging. This tendency is especially conspicuous in sections of biological specimens. Accordingly, in the case of a section of a biological specimen, a certain portion of the specimen is chemically bonded to heavy metals (known as staining). This increases the scattering contrast in this portion. In this way, an image owing to scattering contrast is obtained. In the case of sections of biological specimens, scattering contrast is intrinsically hard to obtain. A specimen that should be observed intact in itself is stained (i.e., artifacts are intentionally introduced) to make an observation by making use of scattering contrast.
On the other hand, a phase contrast technique is a method of causing phase variations of electron waves to be reflected in the contrast in an image, the electron waves undergoing such variations after passing through a specimen. Electrons transmitted through the specimen without being scattered (i.e., without being affected by the specimen at all) are herein referred to as transmitted electrons. When transmitted electrons and scattered electrons interfere with each other at the image plane, a phase difference is created between them. Thus, contrast is produced. In the case of phase contrast, if the specimen is made thinner within a practically processable range, the specimen structure is more easily reflected in the image, unlike the case of scattering contrast. Accordingly, if phase contrast can be utilized for observation of sections of biological specimens, it is expected that specimen images will be easily obtained without manual operations, such as staining. In normal TEM, however, phase contrast is not easily produced under low to medium magnification observations, for the following reasons.
It is known that the Fourier component I(k) of phase contrast in a sufficiently thin specimen is given by
I
(
k
)=&sgr;
V
(
k
)·
B
(
k
) (1)
where
B
(
k
)=sin(&khgr;(
k
))·
E
(
k
) (2)
where &sgr; is a constant used to convert the electrostatic potential of the specimen into phase shift and is determined by the energy of electrons, V(k) is the Fourier transform of the electrostatic potential distribution in the specimen, B(k) is a function indicating the manner in which information about the specimen is transferred, &khgr;(k) is a wavefront aberration function, E(k) is attenuation of contrast due to partial coherence and chromatic aberration, and k is a spatial frequency.
Eq. (1) indicates that the structure V(k) of the specimen is reflected in the image via the function B(k). If the function B(k) is kept at unity over the whole spatial frequency range, the specimen structure will be precisely reflected in the image. Actual function B(k) attenuates while oscillating between positive and negative domains as shown in
FIG. 1
, where the spatial frequency k is plotted on the horizontal axis. The center of the vertical axis is indicated by zero. Numerical values attached to the scale on the horizontal axis indicate periods of a real space, in Angstrom unit, corresponding to the spatial frequency k. The function B(k) attenuates toward zero in a region of an inverse space where the spatial frequency k is large (corresponding to a small structure in a real space) because of attenuation E(k). This corresponds to the fact that there is an information limit in electron microscopy and a structure smaller than a certain limit cannot be viewed. During attenuation of B(k) toward zero, the function B(k) oscillates across zero. The mode of this oscillation is determined by the accelerating voltage of the TEM, the spherical aberration coefficient, and the amount of defocus. Among them, what the TEM operator can set at will is only the amount of defocus. At spatial frequencies close to the spatial frequency where the B(k) is zero, the corresponding specimen structure is partially lost from the image. To prevent the information from being lost, some contrivance is necessary. For example, the amount of defocus is appropriately varied so that the function B(k) does not assume a value of zero in the vicinity of the spatial frequency corresponding to the desired structure.
In normal TEM, phase contrast is prevalent only under high-magnification observation of crystal lattice images. On the other hand, at observation magnifications suitable for sections of biological specimens, phase contrast is not prevalent because phase contrast does not participate in imaging and, therefore, one has to depend on scattering contrast to get the images. More specifically, as can be seen from
FIG. 1
, B(k) is a sinusoidal function of &khgr;(k), and &khgr;(k)=0 where k=0. Therefore, in the vicinity of k=0, B(k) remains close to zero. The spatial frequencies k close to zero are components bearing large structures of a specimen, and are in a region where a mild structure of the image is reflected. That is, information about low frequencies is lost, because B(k) is a sinusoidal function of &khgr;(k). Therefore, an image created by phase contrast can reflect only microscopic specimen structures of the order of nanometers or less. Consequently, these structures cannot be viewed unless a high magnification is accomplished. Conversely, relatively large structures to be observed at low to medium magnifications cannot be imaged via phase contrast. Because of these circumstances, where sections of biological specimens are observed at low to medium magnifications, the TEM operator normally has to rely on scattering contrast.
Accordingly, we have already proposed an improved electron microscope and filed for a patent (Japanese Patent Publication No. 2001-273866) on this microscope for alleviating the drawback with phase contrast (i.e., large structures are not imaged under low to medium magnification observations), and for eliminating the drawback under high magnification observations (i.e., information is lost from the frequency regions withi
Danev Radostin S.
Hosokawa Fumio
Nagayama Kuniaki
Fernandez Kalimah
Jeol Ltd.
Lee John R.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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