Magnetic energy filter

Details Magnetic energy filter Magnetic energy filter

2000-06-01

2002-08-27

Lee, John R. (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

Magnetic lens

C250S3960ML, C250S305000

Reexamination Certificate

active

06441378

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic energy filter having plural magnetic fields designed to deflect the trajectory of the electron beam from the entrance window to an exit slit.
2. Description of the Related Art
FIG. 4
shows an example of the structure of an electron microscope having electron optics incorporating an OMEGA energy filter.
FIG. 5
illustrates the structure of the A-type OMEGA energy filter.
FIG. 6
illustrates the structure of the B-type OMEGA energy filter. FIGS.
7
(
a
) and
7
(
b
) illustrate the fundamental trajectory in the A-type OMEGA energy filter. FIGS.
8
(
a
) and
8
(
b
) illustrate the fundamental trajectory in the B-type OMEGA energy filter.
In-column energy filters, such as OMEGA energy filters and Castain-Henry filters, are often used as energy filters connected with electron microscopes in use because the microscope column can be incorporated in the microscope while maintaining the column straight. In an electron microscope having electron optics incorporating the OMEGA energy filter, an electron gun
11
emits an electron beam that is directed to a specimen
14
through condenser lenses
12
, as shown in FIG.
4
. An observable image of the specimen is projected onto a fluorescent screen
20
via an objective lens
13
, an intermediate lens
15
, an entrance window
16
, an OMEGA energy filter
17
, an exit slit
18
, and a projector lens
19
. In this OMEGA-type energy filter, four magnetic fields M
1
, M
2
, M
3
, and M
4
, where the beam has radii of curvature R
1
, R
2
, R
3
, and R
4
, respectively, are arranged to form an &OHgr;-shaped trajectory. The electron beam is passed through these magnets in turn such that the outgoing beam is aligned with the incident beam.
FIGS. 5 and 6
show two examples of geometry of the magnetic polepieces and electron trajectory. A straight line on which the incident beam and the outgoing beam are aligned with each other or a straight line passing through both entrance window and exit slit is referred to as the “straight axis” herein. The center trajectory of the beam deflected by the magnetic fields of the filter, as shown in
FIGS. 5 and 6
, is referred to as the “optical axis indicating the center trajectory of the filter” herein.
In this way, an instrument having an OMEGA energy filter inserted in or behind the imaging lens system of a transmission electron microscope is used as an apparatus for electron spectroscopic imaging (ESI). In OMEGA energy filters, ALPHA energy filters, and so on, the optical axis of the incident beam is in line with the optical axis of the outgoing beam. Plural magnetic fields are developed. Therefore, such an OMEGA energy filter is inserted in the imaging lens system to deflect the trajectory of the electron beam from the entrance window to the exit slit, and this is called an in-column ESI instrument. On the other hand, a filter in which a single-sector magnet is combined with a multipolar corrector is also available. In this filter, the optical axis of the outgoing beam makes an angle of about 90° to the incident beam. Therefore, this filter is mounted behind the microscope column and known as a post-column filter.
An OMEGA energy filter is a typical in-column filter. The prototype of this filter was manufactured by combining a magnetic field prism, an electrostatic mirror, and another magnetic field prism to form an in-column filter (originally known as the Castain-Henry filter) and replacing the electrostatic mirror by a magnetic field prism so that all the deflecting elements were made of magnetic fields. This filter was developed in the 1970s in France and consists of three magnetic fields. Since then, aberration theories of filters have been investigated in Germany. It has been found that use of four magnets is more advantageous than use of three magnets. Subsequent researches have been conducted into systems using four magnets.
In a sector-shaped magnet having a uniform magnetic field, the beam is focused in a direction x parallel to the plane of the magnetic polepieces in which energy dispersion takes place. However, no focusing action occurs in the direction of the magnetic field y. Accordingly, in the case of an OMEGA energy filter, the end surfaces of the magnetic polepieces are tilted to produce a quadruple lens action which focuses the beam in the direction of the magnetic field. The two examples shown in
FIGS. 5 and 6
are designed under different optical conditions. The geometry of
FIG. 5
is called type A in which three focusing actions take place in the direction x parallel to the plane of the magnetic polepieces and also in the direction of magnetic field y. The geometry of
FIG. 6
is called type B in which three focusing actions take place in the direction x parallel to the plane of the magnetic polepieces, and two focusing actions occur in the direction of the magnetic field y. Their differences in fundamental optics can be seen from the trajectory diagrams of the types A and B shown in
FIGS. 7 and 8
, respectively, where the optical axis indicating the center trajectory of the filter is drawn as a straight line.
In these trajectory diagrams, both trajectories x
&agr;
and y
&bgr;
are trajectories of an electron beam that will finally form a focused electron microscope image on the fluorescent screen. On the other hand, trajectories x
&ggr;
and y
&dgr;
are trajectories of an electron beam focused onto the entrance window plane of the filter by the previous stage of lens. After passage through the filter, these trajectories x
&ggr;
and y
&dgr;
are focused onto the exit slit plane. On reaching the exit slit plane, the electron beam is sufficiently dispersed according to its energy. The exit slit selects only a desired energy range of the beam. The image on the fluorescent screen is formed by an energy range of the beam passed through the exit slit. If the dispersion is left behind, a blurring will take place. Therefore, the dispersion must disappear on the image plane, which is called the achromatic condition. The OMEGA energy filter has a great feature in that the trajectory is made symmetrical with respect to the center plane, canceling out the aperture aberration on the image plane and distortions.
In the OMEGA energy filter, in order to make some second-order aberrations zero and to reduce the remaining aberrations, the plane between the second magnet M
2
and the third magnet M
3
is used as a plane of symmetry (center plane). Thus, the beam trajectories before and after the plane of symmetry are rendered symmetrical. In particular, let LL be the distance from the image plane (pupil plane) to the exit slit plane. The image plane (pupil plane) of the incident beam is adjusted to be at a distance of LL from the entrance window plane. Under these conditions, types A and B differ on the trajectory in the y-direction (in the direction of the magnetic field) as follows. With the type A, relations y
&bgr;
=0 and y
&dgr;
′=0 hold on the plane of symmetry as shown in FIGS.
7
(
a
) and
7
(
b
). With the type B, relations y
&bgr;
′=0 and y
&dgr;
=0 hold on the plane of symmetry as shown in FIGS.
8
(
a
) and
8
(
b
). Note that ′″″ indicates differentiation with respect to z, i.e., the gradient of the trajectory. In either type, the x trajectory gives x
&agr;
=0 and x
&ggr;
′=0 on the plane of symmetry under the same conditions for both beams.
If the initial conditions are selected in this way for the type A, then trajectory x
&ggr;
is focused three times and trajectory y
&dgr;
is also focused three times, as shown in FIGS.
7
(
a
) and
7
(
b
). The focused points are indicated by the arrows, respectively, in the figure. For type B, trajectory x
&ggr;
is focused three times but trajectory y
&dgr;
is focused only twice, as shown in FIGS.
8
(
a
) and
8
(
b
). That is, the image is turned over. The presence of these two types of OMEGA energy filters has been known for many years.
In the case of an electro

Affiliated with

Also associated with

Rating

Magnetic energy filter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Magnetic energy filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic energy filter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2952027