Radiant energy – Electron energy analysis
Reexamination Certificate
2000-03-17
2002-11-19
Lee, John R. (Department: 2881)
Radiant energy
Electron energy analysis
C250S3960ML
Reexamination Certificate
active
06483110
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an energy filter consisting of at least one electron-deflecting magnet assembly to pass incident electrons which have a certain energy.
2. Description of the Related Art
Electron microscopes having electron optics incorporating an energy filter have been developed. Such a conventional electron microscope is shown in
FIG. 6
, in which the microscope is indicated by numeral
1
. This microscope has an electron gun
2
emitting a beam of electrons e. The beam is directed to a specimen
5
via a condenser lens system
3
. The beam transmitted through the specimen
5
is projected onto a fluorescent screen
11
via an objective lens
4
, an intermediate lens
6
, an entrance aperture
7
, a spectrometer
8
, a slit
9
, and a projector lens
10
. Thus, a transmission image of the specimen is observed. The entrance aperture
7
, the spectrometer
8
, and the slit
9
constitute an energy filter
12
, known as an &OHgr;-filter.
The spectrometer
8
incorporated in the energy filter
12
of the electron microscope
1
is equipped with at least one electron-deflecting magnet assembly. One example of the electron-deflecting magnet assembly is shown in FIGS.
7
(
a
) and
7
(
b
), where the magnet assembly, indicated by
13
, comprises a pair of opposed magnetic polepiece bases
100
,
101
. Coil grooves
22
and
24
are formed adjacent to each other in one surface of the magnetic polepiece base
100
. Thus, those portions which are surrounded by the coil grooves
22
and
24
form magnetic polepieces
14
and
16
, respectively. Coils
18
and
20
are received in the coil grooves
22
and
24
, respectively. Similarly, the other magnetic polepiece base
101
is provided with coil grooves
23
and
25
formed adjacent to each other. Thus, those portions which are surrounded by the coil grooves
22
and
24
form polepieces
15
and
17
, respectively. Coils
19
and
21
are received in the coil grooves
23
and
25
, respectively. The polepiece bases
100
and
101
are so positioned that the formed polepieces
14
and
16
are located opposite to the polepieces
15
and
17
, respectively. Those portions of the polepieces
14
-
17
that are surrounded by the coil grooves
22
-
25
are recessed as viewed from the other portions. Gaps
26
and
27
are formed between them and in communication with each other via a passage
28
. These gaps
26
,
27
, and passage
28
together form an electron passage
29
.
Electrical current is supplied from a current source (not shown) to the coils
18
-
21
to produce magnetic fields in the gaps
26
and
27
between the polepieces
14
and
15
and between the polepieces
16
and
17
, respectively. Shunts (not shown) are mounted at the entrance and exit surfaces of the gaps
26
and
27
to prevent ooze or spreading of the magnetic fields. Using these shunts, the distributions of the magnetic fields developed in the gaps
26
and
27
between the polepieces are tightly controlled. Electrons are caused to pass through these magnetic fields. This gives good electron optical characteristics to the electron-deflecting magnet assembly
13
acting to deflect electrons.
Electrons react with molecules within air and are lost rapidly. It is necessary to evacuate the coil grooves
22
-
25
and the electron passage
29
within the electron-deflection magnet assembly
13
to create a low-pressure condition. In the past, therefore, the electron-deflecting magnet assembly
13
itself has been accommodated within a vacuum chamber. With this method for evacuating the electron-deflecting magnet assembly
13
, however, it is very difficult to pump down the inside of the magnet assembly
13
because the components of the magnet assembly
13
, such as the coils
18
-
21
, have large surface areas. Where there is a large amount of residual gas, the electron microscope
1
fitted with the energy filter
12
suffers from various problems, such as instability of the accelerating voltage and specimen contamination due to electron irradiation.
In an attempt to solve these problems, the following two methods have been adopted. A first method consists of placing a tube
30
along an electron passage
29
as shown in FIG.
8
and evacuating only the inside of the tube
30
. With this first method, it can be expected that the aforementioned problems will be solved at the highest efficiency, since the volume evacuated is smallest.
A second method consists of covering the coils
18
-
21
with vacuum-resistant packs
31
-
34
, respectively, as shown in
FIG. 9
, to suppress degassing from the coils
18
-
21
. With this second method, intrusion of gas into the electron passage
29
is suppressed, the gas escaping from the coils
18
-
21
. Therefore, the aforementioned problems can be effectively solved.
With the first method, it is necessary to accurately shape the tube
30
. Since the tube
30
is very complex in shape, it is very difficult to shape the tube
30
accurately. Furthermore, it is necessary to clean the inside of the tube
30
. However, it is not easy to finish the interior of the tube
30
with a high degree of cleanliness.
To put the tube
30
in the electron passage
29
, the gaps
26
and
27
between the polepieces
14
and
15
and between the polepieces
16
and
17
are inevitably set large. If these gaps are made large, a larger power supply is necessary to produce a given magnitude of magnetic field. In addition, the aberrations of the deflecting magnetic field increase. Accordingly, limitations are imposed on increase of the gaps
26
and
27
.
In the second method described above, the coils
18
-
21
are separately covered with the vacuum-resistant packs
31
-
34
, respectively. Therefore, the coil grooves
22
-
25
in the coils
18
-
21
must have large space. This increases the size and complexity of the electron-deflecting magnet assembly
13
. Additionally, the gap between each shunt and the corresponding polepieces, such as
14
-
17
, is increased to secure spaces to accommodate the coils
18
-
21
.
SUMMARY OF THE INVENTION
In view of the foregoing, the present invention has been made.
It is an object of the present invention to provide an energy filter that can be designed compactly without increasing the gaps between polepieces or spaces to accommodate coils and has an electron passage capable of being evacuated more reliably.
An energy filter built in accordance with a first embodiment of the present invention solves the foregoing problems and comprises at least one magnet assembly mounted in a vacuum created within an electron microscope, the magnet assembly being designed to pass only incident electrons which have a certain energy. The magnet assembly comprises a pair of polepiece bases located opposite to each other, polepieces and coil grooves formed in respective surfaces of the polepiece bases, coils inserted in the coil grooves, respectively, a pair of spacers interposed between the polepiece bases, and a yoke fixedly mounted to side surfaces of the polepiece bases. The coil grooves are located opposite to each other. The spacers are provided with sealing grooves to accommodate hermetic seals, respectively, for hermetically sealing the coils received in the coil grooves in the opposite polepiece bases, respectively. At least one electron passage gap is between the spacers to form an electron passage. Seal members are inserted in the sealing grooves, respectively, to permit the coils to be located outside the vacuum described above.
An energy filter in accordance with a second embodiment of the present invention is based on the energy filter in accordance with the first embodiment and further characterized in that the polepieces have bulges swelling outward from the coil grooves, respectively, to form shunts for preventing ooze of magnetic fields.
An energy filter in accordance with a third embodiment of the present invention is based on the energy filter in accordance with the first or second embodiment and further characterized in that at least one magn
JEOL Ltd.
Lee John R.
Vanore David A.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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