JPH0234142B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a transmission electron microscope having an objective lens, an intermediate lens, a projection lens, and a selected area diaphragm, in particular, to obtain a high-quality low-magnification high-field image while suppressing spherical aberration and chromatic aberration. This relates to a transmission electron microscope that has been improved to allow for.

The quality of images obtained in a transmission electron microscope (hereinafter simply referred to as an electron microscope) is strongly influenced by the spherical aberration and chromatic aberration of the imaging lens system. In order to reduce the influence of this spherical aberration and chromatic aberration,
Usually, the objective lens is used with relatively strong excitation. However, on the other hand, the ability to obtain a wide field of view during low-magnification observation and the absence of image blur or distortion at the periphery of the field of view are also important factors that determine the performance of an electron microscope. When the objective lens is used with strong excitation as described above, it is difficult to obtain a high-quality wide-field image at a magnification of several tens to 2,000 times. Therefore, in conventional electron microscopes, the excitation of the objective lens is made extremely weak or zero, and the object plane of an intermediate lens placed behind the objective lens is made to almost match the sample plane to obtain a low-magnification image. It has gained. In this way, when the objective lens is extremely weakly excited to obtain a wide-field image, for example, when a relatively wide area of the sample from 2 mmφ to 0.1 mmφ is irradiated, the electron beam transmitted through the sample is focused on the objective aperture. Therefore, it is not possible to use an objective aperture to increase the contrast of the projected image, and various efforts have been made to solve this problem. Examples of conventional electron microscopes with such a design include those shown in FIGS. 1 and 2, for example.

The electron microscope shown in FIG. 1 includes an objective lens 3 placed behind a condensing lens 1 at the final stage, an intermediate lens 5 placed behind this objective lens 3,
It has an imaging lens 7 composed of a projection lens or an intermediate lens from the second stage onward, and a selected area diaphragm 4 disposed between the objective lens 3 and the intermediate lens 5, and a portion of the objective lens 3. In the microscope structure, which enlarges and projects the sample 2 placed on the projection plane, the objective lens 3 is extremely weakly excited, and the electron beam 8 transmitted through the sample 2 is directed to the selected area aperture 4 position in front of the intermediate lens 5. This is an example of focusing on Since the electron beam flux 8 is focused on the selected area diaphragm 4 in this manner, it becomes possible to operate the selected area diaphragm 4 instead of the objective diaphragm to increase the contrast of the low-magnification wide-field image.

The electron microscope shown in FIG. 2 has the same microscope structure as the electron microscope shown in FIG. Electron beam flux 8 transmitted through sample 2 with appropriate excitation settings
This is an example in which the light is focused at the selected area diaphragm 4 position in front of the intermediate lens 5. In this example, as in the example shown in FIG. 1 above, the contrast of the low magnification wide-field image can be increased by operating the selected area diaphragm 4.

However, all of the imaging methods adopted in the conventional electron microscopes described above make it possible to obtain a wide-field image with relatively little projection image distortion and peripheral blur, but they do not sufficiently suppress spherical aberration and chromatic aberration. The problem was that it could not be made smaller.
That is, as shown in FIG. 1 and FIG. In the field of view method, in which a real image of the stage is formed, relatively greatly enlarged and projected by the imaging lens, the excitation of the objective lens 3 is extremely weak or zero, so the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc are as described above. The aberration coefficients Cs and Cc of the first stage intermediate lens 5 are approximately equal to each other. And these aberration coefficients Cs,
The value of Cc generally increases as the distance between the sample 2 surface and the first stage intermediate lens 5 increases, although it depends on the shape of the lens. For example, in a high-resolution electron microscope with an accelerating voltage of 100 kilovolts, the distance between the sample 2 and the first stage intermediate lens 5 is 100 to
200 mm, Cs is several meters to several tens of meters, and Cc is 200 mm to 500 mm. Deterioration in image quality due to these aberrations is not a big problem when observing images at a magnification of several tens of times, but when observing images at a magnification of about 2,000 times, the image quality is particularly affected by chromatic aberrations. Decrease becomes a problem.

Letting the amount of blur due to chromatic aberration be ζ _c , ζ _c can be roughly expressed as ζ _c =M·Cc·ΔE/Uα (1). Here, M: Magnification Cc: Chromatic aberration coefficient ΔE: Energy loss due to inelastic scattering in the sample U: Accelerating voltage α: Opening angle of the electron beam scattered by the sample.

Therefore, as an example, M=2000, Cc=200mm,
If ΔE=20eV, U=100KV, and α=1×10 ^-3 rad, then ζ _c =80μ ……(2) according to equation (1). The resolution of the exposure film placed on the projection plane is approximately 20 μm, so if an ultra-thinly sliced biological specimen with a thickness of several hundred Å to 1,000 Å is used as sample 2, the image quality of sample 2 will be It is clearly degraded by chromatic aberration.

The amount of blur caused by spherical aberration is smaller than the amount of blur caused by chromatic aberration at magnifications of several tens to thousands of times.
Although this is not normally a problem, if the selected area diaphragm 4 having a large aperture is used, there is a risk that the image quality of the projected image will deteriorate.

In this way, conventional high-resolution electron microscopy
In particular, there was a problem in that high-quality images could not be obtained at magnifications of about 1,000 to 3,000 times.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide an objective lens, a selected area diaphragm placed behind the objective lens, an intermediate lens placed behind the selected area diaphragm, and an intermediate lens placed behind the selected area diaphragm. The excitation of the imaging lens, the setting lens disposed between the objective lens and the selected area diaphragm, and the objective lens are set to zero or extremely weak excitation, while the excitation of the setting lens is set to a value that is transmitted through the sample. By providing an electron microscope in which the electron beam transmitted through the sample is focused on the selected area diaphragm by providing a means for setting the electron beam to a value that causes the electron beam to be focused on the selected area diaphragm, spherical aberration and chromatic aberration can be reduced. The object of the present invention is to solve the above-mentioned conventional problems by making it possible to obtain a high-quality image with a small amount of blur caused by the image forming apparatus.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

FIG. 3 is a diagram showing an embodiment of the present invention. The transmission electron microscope according to this embodiment includes an objective lens 3, a selected area diaphragm 4 disposed behind the objective lens 3, an intermediate lens 5 and an imaging lens 7 disposed behind the selected area diaphragm 4, Setting lens 10 arranged between objective lens 3 and selected area diaphragm 4
Then, the excitation of the objective lens 3 is set to zero or extremely weak excitation, while the excitation of the setting lens 10 is set to a value that focuses the electron beam transmitted through the sample 2 on the selected area diaphragm 4 (not shown). figure)
It is equipped with A focusing lens 1 is arranged in front of the objective lens 3, and the electron beam is focused by the focusing lens 1 and irradiated onto the sample 2. When it is desired to obtain an enlarged image at a relatively high magnification of several tens of times to several thousand times, the excitation of the objective lens 3 is maintained at zero or extremely weak excitation, while the setting lens 10 is also set to a predetermined excitation. The electron beam flux 8 that has passed through the sample 2 is focused onto the selected area aperture 4. This results in
The selected area diaphragm 4 can be operated to limit the opening angle of the scattered electron beam from the sample 2, thereby increasing the contrast of the observed image.

Further, the setting lens 10 and the intermediate lens 5 are set as a pair of lenses at an appropriate interval in order to form a first-stage real image of the sample 2 on the object surface of the imaging lens 7. Therefore, the point on the optical axis Z of sample 2
The electron beam 9 scattered at X _p is first bent by the setting lens 10 (the excitation of the objective lens 3 is zero or extremely weak, so the refraction at the objective lens 3 can be ignored), and the selected field of view is After the aperture angle is limited by the diaphragm 4, the object surface 6 of the imaging lens 7 is refracted by the intermediate lens 5.
A first-stage real image is formed on top.

The spherical aberration coefficient Cs in such a lens system,
and the chromatic aberration coefficient Cc are determined by the following equation.

Cs=e/128MoU ^* ∫ ^ZI _Zp [(3e/MoU ^* Bz ⁴ +8Bz′ ² )・y ⁴
−8Bz ² y ² y′ ² 〕dz……(3) Cc＝−e／8MoU ^* ∫ ^ZI _Zp Bz ² y ² dz……(4) Here, Zo: Sample position (on the optical axis) Z _I : Image plane position (〃) e: Electron charge Mo: Electron mass U ^* : Accelerating voltage (relativistically corrected) Bz: On-axis magnetic flux density y: At Z=Zo, y _p = 0, y _p ′= This is the orbit of an electron beam that satisfies 1 (cited documents include Glaser,
W: Grundlagen der Elektronenoptik).

As already mentioned, since the real image of the first stage of the sample 2 is formed by the setting lens 10 and the intermediate lens 5, the setting lens 10 is positioned between the objective lens 3 and the selected area diaphragm 4. When moved, the excitation of each lens 5, 10 changes, and from equations (3) and (4), Cs,
It becomes clear that Cc changes. Now, the third
In the example shown in the figure, the distance between the sample 2 and the intermediate lens 5 is 100 mm, the distance between the intermediate lens 5 and the image plane of the first real image is 100 mm, and the focal point of the electron beam 8, that is, the selected area aperture 4 position, and the sample 2 Assuming a distance of 80mm,
Cs for the distance Z _L between the sample 2 and the setting lens 10,
I asked for Cc. In this case, setting lens 1
0 means that the electron beam parallel to the optical axis Z is the selected area aperture 4.
The intermediate lens 5 is also excited so that the scattered electron beam 9 is always focused on the image plane of the first real image. Also setting lens 1
The half width d of the magnetic field distribution at 0 is approximately 9 mm, and the intermediate lens 5
The half width of the magnetic field was set to approximately 12 mm.

FIG. 4 shows the values of Cs and Co with respect to the distance Z _L between the sample 2 and the setting lens 10, and the magnification of this lens system, which were determined under these conditions. In this figure, Z _L =0 corresponds to the case where the focusing operation of the electron beam 8 by the setting lens 10 is performed by the objective lens 3, and the configuration is similar to that of the conventional lens system shown in FIG. As is clear from this figure, Z _L
When the setting lens 10 is set at a position of ≒40 mm, that is, the middle position between the sample 2 and the selected area aperture 4, the chromatic aberration coefficient Cc is reduced to 1/4 of Cc when Z _L = 0, that is, about 50 mm. be able to. At this time, the amount of blur due to chromatic aberration is M = 2000, △E = 20eV, as described in relation to equation (1) above.
If U=100KV, α=1×10 ^-3 rad, Cc=50
mm, so from Equation (1), ζ _c = M・Cc・△E/Uα=20μ...(5), and the image quality is significantly improved compared to the result in Equation (2) above. It can be seen that

Furthermore, regarding the spherical aberration coefficient Cg, Z _L ≒ 40
It is clear that Cs at mm can be made 1/10 or less of Cs at Z _L =0, and the magnification increases by about 2.5 times. In addition, in Fig. 4, Z _L =0
_Cs , Cs, and
Although the change in Mag (magnification) is shown, when Z _L > 45 mm, the electron beam flux 8 is focused at the selected area aperture 4 position, and the first one is focused at the set object plane position 6 of the imaging lens 7.
Since it is no longer possible to create a real image of the step, Z _L > 45
Range in mm is not shown.

Also, the changes in Cs, Cc, etc. shown in Figure 4 are the changes when the excitation of the objective lens 3 is set to zero, but the objective lens 3 is made extremely weakly excited, and the objective lens 3 is set with this weakly excited objective lens 3. In addition to converging the electron beam 8 on the selected area diaphragm 4 using the lens 10, it is also possible to image the electron beam 9 scattered from the sample 2 on the object surface 6 of the imaging lens 7 as a first-stage real image. . Under the configuration of such a lens system Z _L = 40mm
By changing the combination of the excitation of the objective lens 3 and the setting lens 10, the magnification of this lens system can be changed from 1 to 2.5 times.

Since the electron beam passing through the sample is focused on the selected area diaphragm, it is possible to increase the contrast of the image by operating the selected area diaphragm. Aberrations, especially chromatic aberrations, can be reduced, and a high-quality, low-magnification image with little blur can be obtained.

[Brief explanation of drawings]

Figure 1 shows the objective lens being extremely weakly excited.
FIG. 2 is a diagram showing a conventional low-magnification imaging method in which an electron beam transmitted through a sample is focused on a selected area diaphragm. In Figure 2, the excitation of the objective lens is set to zero,
FIG. 3 is a diagram illustrating another conventional low magnification imaging method in which a focusing lens focuses an electron beam onto a selected area diaphragm. FIG. 3 is a diagram showing the lens configuration of the electron microscope of the present invention, in which a setting lens is arranged between the objective lens and the selected area diaphragm, and both lenses focus the electron beam onto the selected area diaphragm. It is. FIG. 4 shows that in the electron microscope of the present invention,
FIG. 3 is a graph diagram showing changes in spherical aberration coefficient, chromatic aberration coefficient, and magnification when the placement location of a set lens is changed with respect to a sample. 1... Focusing lens, 2... Sample, 3... Objective lens, 4... Selected area diaphragm, 5... Intermediate lens,
6... First stage real image, 7... Imaging lens, 8...
Electron beam flux transmitted through the sample, 9...Electron beam scattered by the sample, 10... Setting lens.

Claims (1)

[Claims] 1. An objective lens, a selected area diaphragm placed behind the objective lens, an intermediate lens and an imaging lens placed behind the selected area diaphragm, and a setting lens placed between the objective lens and the selected area diaphragm. and a means for setting the excitation of the objective lens to zero or very weak excitation, while setting the excitation of the setting lens to a value that focuses the electron beam transmitted through the sample on the selected area aperture. microscope.