Radiant energy – Inspection of solids or liquids by charged particles – Electron microscope type
Reexamination Certificate
2001-02-20
2003-06-03
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron microscope type
C250S306000, C250S307000, C250S308000, C250S310000, C250S398000, C250S3960ML, C250S3960ML
Reexamination Certificate
active
06573501
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a holography electron microscope comprising a transmission electron microscope (TEM) equipped with an electron biprism.
2. Description of the Related Art
A holography electron microscope comprises a TEM having electron optics in which an electron biprism is inserted. This instrument is constructed as shown in FIG.
7
. Shown in this figure are an electron gun
1
, an illumination lens system
2
, a specimen
3
, a specimen holder
4
, an objective lens
5
, an auxiliary objective lens
6
, an electron biprism
7
, intermediate lenses
8
, a projector lens
9
, and an observation/recording device
10
. This observation/recording device
10
is made of a fluorescent screen, a photography device, or a TV camera.
When the electron biprism
7
is not operated, the instrument functions as a normal TEM. That is, the electron beam produced by the electron gun
1
is directed onto the specimen
3
via the illumination lens system
2
. The electron beam transmitted through the specimen
3
is magnified by a magnification-and-projection system comprising the objective lens
5
, the auxiliary objective lens
6
, the intermediate lenses
8
, and the projector lens
9
. Finally, the electron beam is projected as a TEM image onto the observation/recording device
10
.
The structure and operation of the electron biprism
7
are next described. As shown in
FIG. 8
, the electron biprism
7
comprises a filament
11
and two electrodes
12
and
13
located on the opposite sides of the filament
11
. In
FIG. 8
, the crossover position that is the position of the back focal plane of the objective lens
5
is indicated by C. The image plane of the objective lens
5
is indicated by IP.
The filament
11
is normally made of thin wire of platinum having a diameter of about 1 &mgr;m. During operation, a positive voltage of hundreds of volts is applied to the filament
11
. The electrodes
12
and
13
are grounded. Sometimes, none of the electrodes
12
and
13
may be mounted. In this example, it is assumed that the instrument is fitted with these electrodes
12
and
13
.
When a given voltage is applied to the filament
11
to operate the electron biprism
7
, the electron beam transmitted through the specimen
3
is divided into two by the filament
11
. The resulting two portions of the electron beam overlap each other as shown, producing interference. Interference fringes are created in the region in which interference occurs. In
FIG. 8
, the width of the overlap (hereinafter referred to as the interference region) is indicated by D. The auxiliary objective lens
6
is omitted in FIG.
8
.
In this case, the specimen
3
is examined as follows. One of the two portions obtained by dividing the electron beam by the filament
11
passes through a space in which the specimen does not exist (i.e., a vacuum). The other portion is transmitted through the specimen. In this way, the interference fringes created in the interference region by the two portions of the electron beam contain information about the phase shift that electrons transmitted through the specimen undergo during passage through the specimen.
Since the electron beam transmitted through the specimen contains information about an object of interest, the beam is known as an object wave. The electron beam transmitted through a vacuum is not affected at all and, therefore, can provide a reference. Hence, this beam is termed a reference wave. The interference fringes created in the interference region are known as an electron hologram. Procedures for analyzing the phase of the object wave based on the electron hologram and analyzing information about the specimen are referred to as electron holography.
The electron hologram created by the electron biprism
7
is magnified by the rear intermediate lenses
8
and projector lens
9
and focused onto the observation/recording device
10
. The image plane of the objective lens
5
, i.e., the object plane of the intermediate lenses
8
, is made noncoincident with the position of the electron biprism
7
. In particular, the image plane of the objective lens
5
is normally placed in position below the electron biprism
7
.
The spacing I between the interference fringes of the electron hologram and the width D of the interference region are next described. The spacing between the interference fringes depends on the voltage applied to the filament
11
of the electron biprism
7
and on the position of the image plane IP of the objective lens
5
. The spacing I between the interference fringes at the image plane IP of the objective lens
5
is given by:
I
=
(
a
+
b
)
λ
2
a
ϕ
V
f
(
1
)
where a is the distance from the crossover position C of
FIG. 8
to the filament
11
, b is the distance from the filament
11
to the image plane IP, &lgr; is the wavelength of electrons, V
ƒ
is the voltage applied to the filament
11
, and &PHgr; is the deflection sensitivity of the electron biprism
7
. The width D of the interference region from which the thickness of the filament
11
itself of the electron biprism
7
is subtracted is given by:
D
=
2
b
ϕ
V
f
-
2
(
a
+
b
)
r
a
(
2
)
where r is the radius of the filament
11
of the electron biprism
7
.
In electron holography, the spacing I between the interference fringes of the electron hologram and the value of the width D of the interference region converted onto the specimen surface are important. The spacing I
s
between the interference fringes converted onto the specimen surface is given by:
I
s
=
(
a
+
b
)
λ
2
aM
ϕ
V
f
(
3
)
The width of the interference region D
s
converted onto the specimen plane is given by:
Ds
=
2
b
ϕ
V
f
M
-
2
(
a
+
b
)
r
Ma
(
4
)
The magnification M of the objective lens
5
at the image plane IP is used in Eqs. (3) and (4). This magnification M is now discussed. The objective lens
5
is normally excited strongly. Its focal distance ƒ can be made sufficiently smaller than the sum of the distances (a+b) and so the magnification can be given by:
M
=
a
+
b
f
(
5
)
Accordingly, where the objective lens
5
is excited strongly and its focal distance ƒ is made sufficiently smaller than the sum of the distances (a+b), the interference fringe spacing I
s
and the interference region width D
s
converted onto the specimen surface can be given by Eqs. (6) and (7), respectively, by substituting Eq. (5) into Eqs. (3) and (4).
I
s
=
fλ
2
a
ϕ
V
f
(
6
)
Ds
=
2
fb
ϕ
V
f
a
+
b
-
2
fr
a
(
7
)
An example of the calculation is now given. It is assumed that the accelerating voltage for the electron beam is 200 kV and that a=150 mm. Normally, the distance b is about 10 mm. These are practical values. In this case, the deflection sensitivity &PHgr; of the electron biprism
7
is roughly 1×10
−6
rad/V. If we assume that V
ƒ
=200 V,ƒ=2 mm, and r=0.3 &mgr;m, then I
s
=0.084 nm and D
s
=42 nm.
It is assumed that the resolution of a recorder set in the observation/recording device 10 is 20 &mgr;m. To record an electron hologram, the final image magnification (total magnification) needs to be in excess of 250,000 times. The width of the interference region of the electron hologram recorded when the final image magnification (total magnification) is 250,000 times is as narrow as 10 mm. To secure a sufficient field of view by increasing the width of the interference region, the voltage V
ƒ
applied to the filament
11
may be increased. However, as can be easily seen from Eq. (1), if the voltage V
ƒ
applied to the filament
11
is increased, the spacing I between the interference fringes at the image plane IP of the objective lens
5
narrows.
Kaneyama Toshikatsu
Nunome Hiromi
Takeguchi Masaki
JEOL Ltd.
Lee John R.
Vanore David A.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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