Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2002-09-25
2004-01-06
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S306000, C250S307000, C356S128000, C356S904000
Reexamination Certificate
active
06674078
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electron microscope and, more particularly, to a technique that achieves performance comparable to that of a differential interference contrast method in visible light microscopy by forming interfering phases of electrons using a phase plate. This technique permits contrast improvement of electron microscope images and three-dimensional topographical imaging that is a novel method of representation. The invention also relates to a method using this technique.
2. Description of Related Art
Generally, there are four kinds of transmission microscopy that are fundamentally different in imaging method, i.e., a) bright field microscopy, b) dark field microscopy, c) phase contrast microscopy, and d) differential interference contrast microscopy.
Only bright field microscopy, dark field microscopy, and phase contrast microscopy have been realized in transmission electron microscopy for the following reason. In visible light microscopy, complex operations (i.e., splitting into two incident waves and recombination of the waves transmitted through a specimen) have been performed on incident waves in a real space. It is considered that it is technically difficult for the existing electron optical lens system to perform these complex operations. It is known that Schlieren technology is close to differential interference contrast microscopy and consists of inserting a field of view-cutting semicircular aperture into the focal plane behind the objective lens. This technology is also known as single-sideband holography (L. Reimer,
Transmission Electron Microscopy: Physics of Image Formation and Microanalysis
, Ed. 4, Springer, N.Y., 1997). However, this has not been used because half of the spatial-frequency components are discarded and because the resulting image is complex to interpret.
Differential interference contrast microscopy is used in visible light microscopy to topographically image variations in the phases of incident waves caused by a transparent specimen. The difference between the phase contrast microscopy used for a transparent specimen and differential interference contrast microscopy is that images not phases but their derivatives (which ought to be referred to as differences because they are differences in finite displacements) are used. Therefore, two Wollaston prisms are placed on the opposite sides of a specimen. The incident waves are split by the first prism into two beams slightly laterally displaced. After transmission through the specimen, the second prism recombines the two transmitted beams into one on the same optical axis, and the resulting interference is detected. Consequently, the lateral difference of the phase variation (phase difference) due to the specimen is converted into an intensity image. Thus, imaging is achieved.
Where the specimen is thin, the electron beam is little absorbed and almost fully transmitted. That is, it can be considered that what are treated by an electron microscope are transparent specimens. For this reason, the imaging method intrinsic to electron microscopy should be phase contrast microscopy or differential interference contrast microscopy, but neither of them are used today. With respect to the former technology, the principle is understood but the phase plate suffers from the problem of charging effects. On the other hand, where the latter technology is employed, if it is attempted to do work similar to the work done by the aforementioned real-space microscopy technique, it is necessary to combine biprisms and deflectors in a complex manner. Hence, it has been difficult to achieve a simple setup for electron microscopy unlike visible light optics.
SUMMARY OF THE INVENTION
Various operations in a real space can be often replaced by operations in a k-space, i.e., operations on the phases of electron waves at the back focal plane (diffraction plane) of an objective lens, if contrivances are made. One example is installation of a Zernike phase plate at the back focal plane (Japanese Patent Application No. 2000-85493 filed by Nagayama and Danev). Of course, in order that such operations are performed smoothly, the back focal plane is restricted to a point light source illumination system capable of clearly defining a back focal plane. As long as these conditions are met, achievements similar to those achieved by the differential interference contrast microscopy can be accomplished by installing a novel phase plate (i.e., a semicircular phase plate that blocks half of the field of view at the focal plane). This phase plate delays the phases of incident waves by &pgr;. Preferably, the phase plate is made of a thin amorphous film using a light element that shows a small degree of scattering. Examples include a film of amorphous carbon, a film of amorphous aluminum, and a film of amorphous silicon.
This is described in further detail by referring to the accompanying drawings. First, the phase plate is placed at the back focal plane (Pb) of an objective lens placed in a transmission electron microscope as shown in FIG.
11
. Where incident electrons emerging from a point source do not provide strictly parallel illumination immediately ahead of the objective lens, the focus of the incident waves (the circle of least confusion) is shifted above or below the focal plane. In this case, a moving mechanism is necessary to move the phase plate holder into the focal point. The following description is given on the assumption that this moving mechanism is present.
FIG. 1
shows a semicircular phase plate according to the present invention. Normally, the phase plate inserted into the back focal plane (Pb) of the objective lens shown in
FIG. 11
assumes a contour as shown in FIG.
1
. The semicircular phase plate, indicated by numeral
1
, consists of a thin film placed on a circular phase plate support
2
. The semicircular plate
1
and the phase plate support
2
are collectively referred to as the phase plate assembly. It is assumed that the thin film of the phase plate
1
overlies the phase plate support
2
.
FIG. 2
a
shows a plan view and
FIG. 2
b
a side elevation of the phase plate assembly.
FIG. 2
a
of the phase plate assembly is taken from above. The semicircle occupied by the phase plate assembly is referred to as the phase plate semicircle. The remaining semicircle is referred to as the semicircular opening. Transmitted, incident waves are brought to a focus at a point
3
through which the optical axis passes. This point is the zeroth-order diffraction point, and an adjustment is always made to place this point on the side of the semicircular opening. A side elevation of the phase plate assembly is shown in
FIG. 2
b.
The relative arrangement of the two components of the phase plate assembly can be seen from
FIGS. 2
a
and
2
b.
FIG. 3
shows the system of coordinates of the diffraction plane (focal plane) depending on the semicircular phase plate. A system of coordinates as shown in
FIG. 3
is established around the phase plate assembly for correspondence with expansion of the theory. The focal point (zeroth-order diffraction point) of transmitted, incident waves is taken at the origin. Coordinate axes (k
x
, k
y
) corresponding to a Fourier-transformed k-space are placed on the focal plane as shown. The semicircular phase plate covers the half plane on the side k
x
<0 as shown in FIG.
3
. Note that k
x
and k
y
are x- and y-components, respectively, of two-dimensional spatial frequency vector &kgr;. The vector &kgr; is correlated with a real vector r on the focal plane by the following relational equation.
κ
=
r
λ
f
(
1
)
where &lgr; is the wavelength of electron waves and ƒ is the focal distance of the objective lens.
If electron waves pass through a thin film having a uniform thickness and uniform composition, the phases of the waves are shifted according to the following formula (D. Willasch,
Optic
44 (1975) 17):
φ
=
-
π
(
h
/
λ
)
(
V
/
U
0
)
(
1
+
2
Danev Radostin S.
Nagayama Kuniaki
Jeol Ltd.
Lee John R.
Leybourne James J.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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