Omega energy filter

Details Omega energy filter Omega energy filter

1999-03-09

2001-10-23

Berman, Jack (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

Magnetic lens

C250S307000, C250S308000, C250S310000

Reexamination Certificate

active

06307205

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a three-magnet OMEGA energy filter for continuously deflecting the trajectory of an electron beam from the incident aperture plane to the slit plane in an omega-shaped form.
DESCRIPTION OF THE PRIOR ART
FIG. 10
shows an example of the structure of an electron microscope having electron optics incorporating an OMEGA energy filter.
FIG. 11
illustrates the structure of the A-type OMEGA energy filter.
FIG. 12
illustrates the structure of the B-type OMEGA energy filter. FIGS.
13
(A) and
13
(B) illustrate the fundamental trajectory of the A-type OMEGA energy filter. FIGS.
14
(A) and
14
(B) illustrate the fundamental trajectory of the B-type OMEGA energy filter.
As shown in
FIG. 10
, the electron microscope having electron optics incorporating the OMEGA energy filter has an electron gun
11
emitting an electron beam that is directed to a specimen
14
through condenser lenses
12
and an objective lens
13
. An observable image of the specimen
14
is projected onto a fluorescent screen
20
via an intermediate lens
15
, an entrance aperture
16
, the OMEGA energy filter
17
, a slit
18
, and a projector lens
19
. In this OMEGA-type energy filter, four magnetic field regions 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 omega-shaped trajectory. The electron beam is passed through these magnetic field regions in turn such that the outgoing beam is aligned with the incident beam.
FIGS. 11 and 12
show two examples of the shape of the magnetic polepieces and electron trajectory, including the optical axis.
In this way, an instrument having an OMEGA energy filter inserted in or behind the imaging lens system of a transmission electron microscope for magnifying and projecting an image of the specimen
14
onto the fluorescent screen
20
has received popular acceptance as an apparatus for electron spectroscopic imaging (ESI) in recent years. 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. Therefore, such an OMEGA energy filter is inserted in the center of the imaging lens system and is called an in-column ESI instrument.
A filter in which a single-sector magnet is combined with a multipolar corrector is 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 not inserted in the center of the imaging lens system but behind it, i.e., behind the microscope column. Hence, this filter is called a post-column filter.
An OMEGA energy filter is a typical one of in-column filters. 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 field regions. Then, aberration theory of filters has been investigated in Germany. It has been found that use of four magnetic field regions is more advantageous than use of three magnetic field regions. Subsequent research has been conducted into systems using four magnetic field regions.
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 quadrupole lens action, which focuses the beam in the direction of the magnetic field. The two examples shown in
FIGS. 11 and 12
are designed under different optical conditions. The geometry of
FIG. 11
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. 12
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 different in fundamental optics can be seen from the trajectory diagrams of the types A and B shown in FIGS.
13
(A),
13
(B),
14
(A) and
14
(B), where the optical axis is drawn as a straight line.
In these trajectory diagrams, both trajectories x
&agr;
and y
&bgr;
are trajectories of an image finally focused onto the fluorescent screen
20
. A detector, such as a CCD detector, may be mounted instead of the fluorescent screen
20
. On the other hand, x
&ggr;
and y
&dgr;
are focused onto the entrance window in the filter by the previous stage of lens. After passage through the filter, this dispersion takes place on the trajectory focused onto the slit
18
. This slit
18
serves to cut off beams of other energies except for a part of the dispersed beam. The image on the fluorescent screen
20
or detector is formed by beams having an energy range passed through the slit. If the dispersion is left, a blurring will take place. Therefore, the dispersion must disappear on the pupil plane, which is called the achromatic condition. The OMEGA energy filter has a great feature that the trajectory is made symmetrical with respect to the center plane, canceling out the aperture aberration on the pupil 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 magnetic field region M
2
and the third magnetic field region M
3
is used as a symmetrical plane (center plane). Thus, the beam trajectories before and after the symmetrical plane are rendered symmetrical. In particular, let LL be the distance from the exit pupil plane to the slit plane. The entrance 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 symmetrical plane as shown in FIG.
13
(A). With the type B, relations y
&bgr;
′=0 and y
&dgr;
=0 hold as shown in FIG.
14
(A). 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 symmetrical plane under the same conditions for both beams.
If the initial conditions are selected in this way for the type A, x
&ggr;
trajectory is focused three times and y
&dgr;
trajectory is focused three times, also, as shown in FIG.
13
(B). For type B,x
&ggr;
trajectory is focused three times but Y
&dgr;
trajectory is focused only twice as shown in FIG.
14
(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 OMEGA energy filter, in order to focus the x
&ggr;
trajectory and the y
&dgr;
trajectory onto the slit plane and to focus the x
&agr;
and y
&bgr;
trajectories onto the pupil plane, four adjustment parameters are necessary in total. If all of these adjustments are made with the tilt angle of the end surfaces of the magnetic polepieces, it is convenient to utilize four end surfaces of the two magnets. Although there are four magnets, the symmetry about the center must be maintained. Therefore, once the shapes of two magnets are determined, the conditions for the remaining two magnets are automatically determined. This is the fundamental reason why four magnetic field regions are used.
An ALPHA energy filter is known as an in-column energy filter similar to an OMEGA energy filter. In thi

Affiliated with

Also associated with

Rating

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

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2597953