JP2002008574A - Energy filter electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an energy filter electron microscope which restrains increase of the length of a lens-barrel by widening magnification range and yet reducing the number of lens steps of an image formation system to a bare minimum. SOLUTION: Magnification is changed, during use of an energy filter 6, by changing the distance LL between an incident window 6a of the energy filter 6 and an incident image surface 6b. But, with the change of this distance LL, focus conditions get out of alignment, ensuing distortion of images. Therefore, a first and a second multipolar sub-lenses are placed in front of the incident window 6a and at the rear of a slit 6d, which make focus conditions deviated to correct distortion of appearing images. With this, a more precise image is to be obtained, even if focal conditions get out of point.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of an energy filter electron microscope (EFTEM) having an in-column type energy filter such as an .OMEGA. Filter in the middle of an imaging system.

[0002]

2. Description of the Related Art In an EFTEM having an in-column type energy filter such as an Ω filter in the middle of an imaging system, a magnification of an intermediate lens system between a sample and a filter and a projection lens system between a filter and a detector are reduced. How to set the magnification is a very important factor in determining the performance of the EFTEM.

[0003] For this reason, conventionally, in an electron microscope mainly for biomedical systems, the entire magnification range is covered by a four-stage system including a three-stage intermediate lens and an objective lens.
An EFTEM using a system in which the projection lens is almost fixed has been proposed. Further, an EFTEM in which the magnification of both the intermediate lens and the projection lens is changed from the viewpoint of the imaging lens system for reducing the non-isochromaticity to realize the optimum incidence condition for the filter has been conventionally proposed.

[0004]

However, in the former case, at least three steps of the intermediate lens and the projection lens 2 are required.
Steps are required, and the latter requires three steps of intermediate lenses and three steps of projection lenses. For this reason,
Further, when the objective lenses are combined, an image forming lens system having a large number of lens stages of 6 to 7 steps is required, so that there is a problem that the length of the lens barrel of the EFTEM becomes long. In addition, these conventional EFs
In any of the TEMs, there is a problem that complicated lens function settings are required for setting the magnification of the lens system, setting the camera length for an electron diffraction image, and the like.

[0005] The present invention has been made in view of such circumstances, and its object is to increase the magnification range,
Moreover, it is an object of the present invention to provide an energy filter electron microscope capable of minimizing the number of lens stages of the image forming system and suppressing an increase in the length of a lens barrel, and further simplifying setting of a lens function.

[0006]

According to a first aspect of the present invention, the energy filter is disposed downstream of an objective lens of an imaging system, and an entrance window surface and an entrance pupil surface are provided. changing the distance (L _L) between, further characterized in that on at least one multipole lens between the upstream and downstream of the energy filter is provided. According to a second aspect of the present invention, there is provided an energy filter electron microscope having an imaging system including an energy filter, wherein the energy filter is disposed downstream of an objective lens of the imaging system, and a lens immediately upstream of the energy filter. Is characterized in that the distance between the main surface and the incident surface is set to 25 mm or less. Further, the invention of claim 3 is characterized in that a two-stage intermediate lens is provided.

[0007]

In the energy filter electron microscope of the present invention having such a configuration, the magnification of the electron microscope changes by changing the distance between the entrance window surface of the energy filter and the incident image plane (pupil plane). Become like As described above, since the magnification of the electron microscope can be changed simply by changing the above-described distance, the number of lens stages of the imaging system of the electron microscope can be minimized. Therefore, even if the magnification of the electron microscope can be changed over a wide range, the number of lens stages is suppressed, so that the length of the lens barrel is shortened, the magnification of the lens system is set, and the camera length for electron diffraction images, etc. The lens function for the setting can be easily set. In particular, distortion caused by a change in the distance between the entrance window surface of the energy filter and the incident image plane is corrected by the multipole lens.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an EFTEM according to the present invention.
It is a figure which shows typically the imaging system containing the energy filter in an example of 1st Embodiment. As shown in FIG. 1, an imaging optical system 1 including an EFTEM energy filter of this example has objectives arranged sequentially in a downstream direction as viewed from the flow of an electron beam emitted from an electron gun (not shown). A lens (hereinafter, also referred to as OL) 2 and first and second intermediate lenses [hereinafter, IL (1) and IL (1), respectively]
(Also referred to as (2))], 3, 4 and the first-multipole lens 5
An energy filter 6, a second multipole lens 7,
First projection lens [hereinafter also referred to as PL (1)] 8
And a second projection lens (hereinafter also referred to as PL (2))
9 and a screen (corresponding to the image detection and recording means of the present invention) 1
0.

The first and second multipole lenses 5 and 7 have the same configuration. As shown in FIG. 2, four arc-shaped electromagnets are arranged in an Ω shape, and an electron beam is emitted. The trajectory is arranged before and after the energy filter 6 whose trajectory has an Ω shape. These first and second multipole lenses 5, 7 are both for correcting image distortion. The first and second multipole lenses 5, 7 are symmetrically installed, and their operating conditions are set to be the same. Therefore, each control system of the first and second multipole lenses 5, 7 can be constituted by a common control system, but may be constituted by independent control systems to correct various asymmetric components. it can.

The in-column type energy filter 6
Is constituted by an in-column type energy filter such as a conventional well-known Ω filter or α filter shown in FIG. 2. For example, when observing an enlarged image of a sample, an electron diffraction pattern is formed on the surface of the entrance window 6a. Is focused, the image of the sample is focused on the incident image plane (pupil plane) 6b. After the electron beam passes through the energy filter 6, this image is focused again on the exit image plane (pupil plane) 6c. Further, after only the electron beam of a predetermined energy is selected by the slit 6d, the exit image plane (pupil plane) 6 is selected.
The image of only a predetermined energy of the image on c is downstream PL (1)
8 and PL (2) are enlarged and projected on the screen 10. Since the electron beam is actually deflected in the energy filter 6, an image is formed at the bent position. Other OL2, IL (1) 3, I
L (2) 4, PL (1) 8, and PL (2) 9 are all conventionally known.

The incident window 6 of the energy filter 6
Assuming that the distance between a and the incident image plane (entrance pupil plane) 6b is L _L , in a conventional EFTEM, this distance L is usually
_L is assumed to be fixed, and the energy filter 6 and IL (1) 4, I
Lens systems L (2) 5, PL (1) 8 and PL (2) 9 have been designed.

[0012] In the first embodiment of the EFTEM of the present invention, unlike the conventional EFTEM, so that changing the magnification in use of the filter by changing the distance L _L. Now, while using this filter, the distance L _L
Consider whether it is possible to change First, considering the surface of the entrance window 6a, the slit 6d is provided at a position symmetrical with respect to the surface of the entrance window 6a and the center plane as in the conventional case, so that the position of the entrance window 6a is mechanically fixed. ing. In addition, with respect to the incident image plane 6b, an incident aperture that defines the size of the field of view is usually placed near the incident image plane 6b, and the position of the incident aperture does not always exactly coincide with the incident image plane 6b. Therefore, it is placed on the actual trajectory near the incident image plane 6b, and nothing mechanically prevents the distance _LL from changing while the energy filter is in use. Therefore, by changing the position of the incident image plane 6b with respect to the incident window 6a, it is possible to change the distance L _L.

However, a change in the distance L _L shifts the focus condition. Therefore, the first and second multipole lenses 5 and 7 are respectively installed in front of the entrance window 6a and behind the slit 6d, and the focus condition is set by the first and second multipole lenses 5 and 7. The image distortion caused by the displacement is corrected. As a result, an accurate image can be obtained even if the focus condition is shifted.

OL2 and two IL (1) 3 and IL
(2) In a system including (4), assuming that the distance _LL is fixed now, the focus condition of IL (2) is fixed.
Because the focus position created by IL (2) is the surface of the entrance window 6a of the energy filter 6, the image plane is changed to the incident image plane 6a.
This is because b must be matched. Therefore, only IL (1) 3 is responsible for the magnification change, and OL2 is used for focusing.

An actual test was performed to show that changing the distance L _L changes the magnification. FIG. 3 shows the results of this test, in which the focal length f _{0 of} OL2 is set to a distance f ₀ = 2.3 mm, and two ILs (between OL 2 and energy filter 6) as shown in FIG. 1) 3 and I
Using L (2) 4, the distance L between OL2 and IL (1) 3
(0) = 216.4 mm, distance L (1) = 110 mm between IL (1) 3 and IL (2) 4, IL (2) and energy filter 6
FIG. 10 is a diagram showing a relationship between the distance _LL and the exciting ampere turn (NI) of the IL (1) 3 when the distance L (2) = 21 mm from the surface of the entrance window 6a of FIG. In this case, the total magnification made by the OL 2 and each of the intermediate lenses 3 and 4 is changed as a parameter.

As can be seen from FIG. 3, when the magnification M is small, the real image mode (curve in which the NI value decreases with an increase in _{LL in} the figure) and the virtual image mode (L in the figure) for each magnification. Curve with NI value increasing with increasing _L ) 2
One of the cases there is less further change in IL (1) 3 excitation ampere-turns (NI) when changing the distance L _L in each mode. However, the change in ampere-turns NI relative to the distance L _L as the magnification M is increased becomes larger, the virtual image region in M = 800 times exists only above L _L = 130 mm. At M = 1500, there is only a real image area, and at M = 6000, the real image area is also L _L > 150 mm
It appears only above. Therefore, from this graph, L _L = 1
It is understood that M = 6000 can be realized by setting the distance to 50 mm or more. When the magnification M shown here is multiplied by the magnification of each of the projection lenses 8 and 9 at the subsequent stage of the energy filter 6, the total magnification is obtained. For example, if the magnification of each of the projection lenses 8, 9 is 200, the maximum magnification is 6000.
× 200 = 1.2 million times.

However, the problem here is that
Distance When designing L _L energy filter 6 as a large value, since there is the size of the energy filter 6 becomes large, better to set the distance L _L small value is advantageous as a design of the energy filter 6 . Also,
If the energy filter 6 is designed to be small while keeping the distance L _L at a large value, the aberration will increase. Therefore, for example, if L _L = 90 mm is set, M
= 2000 times.

Therefore, a possible practical use is L _L
= 90 mm (or L _L = 60 mm), and the energy filter 6 is used as it is when the magnification is low, and M = 6
This is a method of setting L _L = 150 mm only at a high magnification such as 000 times. In the case of high magnification, even if the aberration coefficient shows a large value, only the electron beam near the central axis of the energy filter 6 is actually used, so that the aberration does not enter the observed image. The above is the first embodiment of the present invention.

In the second embodiment of the present invention, if the distance L (2) is shortened without changing the distance L _L , the magnification between M and 6000 times can be changed. This is realized by a two-stage lens system.

FIG. 4 shows that L (1) = 110 mm and L (2) =
It is a graph showing the relationship between the distance L _L and IL (1) 3 of ampere-turns in the case of 13 mm. Note that the parameter magnification M is the same as in FIG. Since the value of the ampere-turn is a function of the lens cap length and the hole diameter, it is relative. However, in the case of setting such that 5300AT is the maximum value, in the case of this figure, it is impossible to realize M = 6000 times if L _L = 90 mm or less. However, as compared with the case of FIG.
It can be seen that simply by shortening (2), the value of L _L capable of realizing M = 6000 times decreases.

FIG. 5 shows that L (1) = 137 mm and M = 600.
It is a figure which shows only the case of 0 times changing L (2). FIG.
As shown in the figure, when the distance L (2) is 35 mm, the distance L (2) does not appear in the range shown in the figure, and M = 6000 is not realized. Conventionally, the pole piece of the IL (2) 4 was located near the middle of the yoke of the lens, a deflection system or a stigmator was inserted between the IL (2) 4 and the energy filter 6, and ), The advantage of shortening the length is unknown.
= 35 mm, it is common to use a relatively large value such as M = 60 in the conventional EFTEM.
00 times could not be realized.

Further, from FIG. 5, it can be seen that in the practical range of L _L ≦ 100 mm, M = 6000 times can be realized by L (2) <20 mm
It can be seen that this is the case. Of course, if the distance L (1) is made very long, it is possible to realize M = 6000 times outside the range of L (2) <20 mm. However, this is one of the effects of reducing the number of lens systems. Although the number of power supplies and the number of mechanical parts can be reduced, the length of the lens barrel cannot be reduced. Therefore, it is desirable to set L (2) <25 mm without substantially changing the distance L (1) in order to realize M = 6000 times.
More preferably, it is better to set L (2) <20 mm. Of course, in the case of the second embodiment,
The first multipole lens 5 and the second multipole lens 7 of FIG. 1 become unnecessary.

[0023]

As is apparent from the above description, according to the energy filter electron microscope of the present invention, by changing the distance between the entrance window surface of the energy filter and the incident image plane (pupil plane), Since the magnification is changed, the number of lens stages for the magnification of the imaging system of the electron microscope can be minimized. Therefore, the magnification of the electron microscope can be varied over a wide range, and the number of lens stages can be reduced, so that the length of the lens barrel can be shortened, and the magnification of the lens system and the camera length for electron diffraction images can be set. Lens function can be easily set.

In particular, distortion caused by a change in the distance between the entrance window surface of the energy filter and the incident image plane can be corrected by the multipole lens. Therefore, an accurate sample image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating an imaging system including an energy filter according to an example of an EFTEM according to the present invention.

FIG. 2 is a diagram schematically illustrating an example of a positional relationship between an energy filter used in the imaging system illustrated in FIG. 1 and a multipole to be added;

FIG. 3 shows the distance (L _L ) between the entrance window of the energy filter and the incident image plane, and the excitation ampere turn (N
It is a figure which shows an example of the relationship with I).

FIG. 4 shows the distance (L _L ) between the entrance window of the energy filter and the entrance image plane, and the excitation ampere turn (N
It is a figure showing other examples of the relation with I).

FIG. 5 shows the distance (L _L ) between the entrance window of the energy filter and the entrance image plane and the excitation ampere turn (N
It is a figure which shows another example of the relationship with I).

[Explanation of symbols]

1... An imaging system including an EFTEM energy filter, 2.
... objective lens, 3 ... first intermediate lens, 4 ... second intermediate lens, 5 ... first-multipole lens, 6 ... energy filter,
6a: entrance window, 6b: entrance image plane (pupil plane), 6c: exit image plane, 6d: slit, 7: second-multipole lens, 8: first projection lens, 9: second projection lens, 10 ... screen

Claims (3)

[Claims] 1. An energy filter electron microscope having an imaging system including an energy filter, wherein the energy filter is arranged downstream of an objective lens of the imaging system, and has a distance (L) between an entrance window surface and an entrance pupil surface. _L ) is varied, and further, a multipole lens is provided on at least one of the upstream and downstream of the energy filter. 2. An energy filter electron microscope having an imaging system including an energy filter, wherein the energy filter is disposed downstream of an objective lens of the imaging system, and is connected to a main surface of a lens immediately upstream of the energy filter. An energy filter electron microscope, wherein a distance from the incident surface is set to 25 mm or less. 3. The energy filter electron microscope according to claim 1, wherein a two-stage intermediate lens is provided between the objective lens and the energy filter.