Electron microscope equipped with energy filter

Details Electron microscope equipped with energy filter Electron microscope equipped with energy filter

1999-04-09

2001-11-27

Nguyen, Kiet T. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

Electron microscope type

C250S305000

Reexamination Certificate

active

06323485

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electron microscope equipped with an &OHgr;-filter and, more particularly, to an electron microscope equipped with an energy filter capable of providing a high-energy resolution.
DESCRIPTION OF THE PRIOR ART
In recent years, instruments comprising a transmission electron microscope having an imaging lens system in which an energy filter is mounted have enjoyed wide acceptance. An instrument of this kind is known as an apparatus equipped with an electron spectroscopic imaging (ESI) system. Various types of energy filters are available. An instrument using an &OHgr;-filter or an &agr;-filter is ordinarily known as an in-column energy filter, since the optical axis of the beam incident on the energy filter is in line with the optical axis of the beam going out of the filter.
FIG. 6
schematically shows an in-column ESI system using an &OHgr;-filter. This system comprises an objective lens
1
, an objective aperture
3
, intermediate lenses
5
,
6
,
7
, the &OHgr;-filter
8
, an exit slit
10
, projector lenses
11
,
12
, and an image-recording system
14
. The &OHgr;-filter
8
has an entrance window
9
.
An electron beam emitted from an electron gun (not shown) is converged by a condenser lens (not shown) and focused onto the specimen
4
by the objective lens
1
. A diffraction pattern of the specimen
4
is created at the position of the back focal plane of the objective lens
1
downstream of the specimen
4
(i.e., on the side of the image-recording system
14
). Also, an electron microscope image (hereinafter referred to as EM image) of the specimen is created at a given position. The objective aperture
3
is located at the position of the back focal plane of the objective lens
1
.
The electron beam transmitted through the objective aperture
3
is converged by the action of the intermediate lenses
5
,
6
, and
7
and enters the &OHgr;-filter
8
from its entrance window
9
. Only those electrons of the electron beam which have a certain energy are allowed to go out of the exit slit
10
that is located at the position of the exit window of the &OHgr;-filter
8
.
With the structure shown in
FIG. 6
, an EM image attributed to an electron beam having a certain range of energies passed through the &OHgr;-filter
8
can be created. Furthermore, it is possible to obtain a diffraction pattern owing to the electron beam having the certain range of energies passed through the &OHgr;-filter
8
. In either case, an EM image and a diffraction pattern are required to be formed at given positions in the &OHgr;-filter
8
. In particular, in the former case (to obtain an EM image on the image-recording system
14
), the diffraction pattern needs to be created in the position of the entrance window
9
of the &OHgr;-filter
8
, and the EM image needs to be formed at the position A of the entrance pupil inside the &OHgr;-filter
8
. In the latter case (to obtain a diffraction pattern on the image-recording system
14
), the EM image needs to be created in the position of the entrance window
9
of the &OHgr;-filter
8
, while the diffraction pattern needs to be formed at the position A of the entrance pupil of the &OHgr;-filter
8
.
Namely, the EM image and the diffraction pattern are focused at different positions, depending on whether the EM image or the diffraction pattern is observed. To focus the diffraction pattern or the EM image at the position of the entrance window
9
, three stages of intermediate lenses
5
,
6
, and
7
are mounted. When an EM image should be observed, these lenses
5
-
7
are excited to bring the diffraction pattern formed in the back focal plane of the objective lens
1
into focus at the position of the entrance window
9
and to bring the EM image into focus at the position A of the entrance pupil inside the &OHgr;-filter
8
. When the diffraction pattern is observed, the lenses
5
-
7
are so excited that the diffraction pattern created in the back focal plane of the objective lens
1
is brought into focus at the position A of the entrance pupil inside the &OHgr;-filter
8
and that the EM image is brought into focus at the position of the entrance window
9
.
When an EM image is observed, the diffraction pattern formed at the position of the entrance window
9
is brought into focus at the position of the exit slit
10
by the action of the &OHgr;-filter
8
, the exit slit
10
being positioned immediately behind the exit port of the &OHgr;-filter
8
. The EM image created at the position A of the entrance pupil is brought into focus at the position of the exit pupil B by the action of the &OHgr;-filter
8
. The position of the entrance pupil A and the position of the exit pupil B are symmetrical with respect to the center
0
of the &OHgr;-filter
8
. Similarly, the entrance window
9
and the exit slit
10
are arranged symmetrically with respect to the center O of the &OHgr;-filter
8
.
The EM image or diffraction pattern focused at the position B of the exit pupil in the &OHgr;-filter
8
is magnified and focused onto the image-recording system
14
by the projector lenses
11
and
12
. In this way, the first stage of projector lens
11
takes an image at the position of the exit pupil B of the &OHgr;-filter
8
as an object plane. In this instrument, film, an imaging plate, a TV camera consisting of a CCD camera or the like is used as the image-recording system
14
.
Usually, the &OHgr;-filter
8
is composed of plural magnets. Since the shapes of the individual magnets and their arrangement per se are not essential to the present invention, the individual magnets of &OHgr;-filter
8
are omitted in FIG.
6
. The intermediate lenses
5
,
6
, and
7
will hereinafter be referred to as the intermediate lens system. Also, the projector lenses
11
and
12
will hereinafter be referred to as the projector lens system.
In the electron microscope equipped with the energy filter as shown in
FIG. 6
, all the lenses have variable magnifications. In the past, when a certain magnification is set, the magnification M
PL
of the projector lens system is made fixed, and the combination of exciting currents to the lenses is so set that the necessary magnification range is covered by the total magnification M
i
of the lenses located upstream (i.e., on the side of the electron gun) of the &OHgr;-filter
8
, as described in U.S. Pat. No. 4,812,652. Where the kind, position, or size of the image-recording system
14
is modified, the magnification M
PL
of the projector lens system is varied simply to provide a desired magnification.
Usually, the fixed magnification M
PL
of the projector lens system is set to a range of approximately 20 to 100×. Specifically, where film or an imaging plate is used as the image-recording system
14
, the magnification is set to about 100×. Where a TV camera is employed as the image-recording system
14
, the magnification is set to about 25×. In this way, the magnification of the projector lens system is varied according to the kind of the image-recording system
14
, because the image is recorded in a different area, depending on a different kind of image-recording system.
Let M
OL
be the magnification of the objective lens
1
. Let M
IL
be the magnification of the intermediate lens system. Obviously, the total magnification of the lenses located upstream of the &OHgr;-filter
8
is given by
 M
i
=M
OL
*M
IL
We have discovered, however, that high-energy resolutions cannot be obtained if the magnification M
PL
of the projector lens system is fixed at a value within a range of 20 to 100× as in the prior art technique.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an electron microscope which is equipped with an energy filter and which can provide higher-energy resolution without modifying the energy filter at all by optimally setting the magnification of the projector lens system.
The above-described object is achieved by an electron microscope in accordance with this invention, the

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