JP3014210B2 - Direct mapping reflection electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct mapping type reflection electron microscope for directly mapping backscattered electrons generated when a bulk sample is irradiated with an electron beam.

[0002]

2. Description of the Related Art In recent years, reflected electrons generated when a bulk sample is irradiated with an electron beam are directly mapped, and at the same time, the energy of the reflected electrons is selected as necessary. Direct mapping reflection electron microscope (LEEM: Low Energy Electro
n Reflection Microscope) has been proposed. FIG. 7 shows an example of the configuration.

In FIG. 7, a primary electron beam from an electron gun 1 is accelerated to about 10 kV, converged by an irradiation lens system 2, further deflected by a magnetic field of a bending magnet 3, passes through electrodes 5 and 6, 4 perpendicularly. Here, a predetermined voltage is applied to the sample 4 and the electrodes 5 and 6, respectively, whereby an emission lens 11 is formed.
The primary electron beam is decelerated by 1 and enters the sample 4 at an acceleration voltage of about 100V. The irradiation lens system 2 can include a deflection coil for scanning a desired region of the sample 4 with a primary electron beam. Reflected electrons are generated from the sample 4 by the incidence of the primary electron beam, and the reflected electrons are accelerated by the emission lens 11 and finally accelerated to the same accelerating voltage as the primary electron beam. The reflected electrons accelerated by the emission lens 11 are deflected by the bending magnet 3 and separated from the optical path of the primary electron beam. The reflected electrons deflected in a direction different from the primary electron beam by the bending magnet 3 are separated in energy by an energy analyzer 8, only reflected electrons having a predetermined energy are selected, and a predetermined electron is selected by a lens 9. The image is enlarged to the magnification and formed on the screen 10 to obtain a monochromatic image. LEEM as above
By obtaining a monochromatic image using and analyzing it,
Various analyzes such as the shape of the surface of the bulk sample, surface potential (work function), crystal structure, electronic state, and elemental analysis can be performed.

[0004]

However, the above configuration has a problem that the image of the reflected electrons is distorted because the optical axis of the reflected electrons is curved rather than straight.
Further, in the backscattered electron imaging system of the LEEM, an optical axis-symmetric optical characteristic is required to obtain an image of the sample. In FIG. 7, the optical path of the primary electron beam and the optical path of the reflected electrons are separated. A bending magnet 3 is usually a sector magnet, which is an optical system that is asymmetric with respect to the optical axis, and prevents electrons passing in a direction parallel to the paper surface and a direction perpendicular to the paper surface. All the characteristics are greatly different. Therefore, regarding the backscattered electron image, the focal length must be adjusted in the direction parallel to the paper surface and in the direction perpendicular to the paper surface, the magnification must be equal, and the aberration must be reduced. It was very difficult to determine the shape of the magnet.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a direct-mapping reflection electron microscope capable of obtaining an image with very little distortion by making the imaging system substantially optically symmetric. It is intended to provide.

[0006]

Means for Solving the Problems In LEEM, in order to make the imaging system of reflected electrons substantially optical axis symmetric, the optical axis of the reflected electrons may be made linear, thereby forming an image with little distortion. What you can get is easy to understand. For this purpose, the direct-mapping reflection electron microscope of the present invention comprises an electron beam source, an irradiation lens system for irradiating a primary electron beam from the electron beam source toward a sample, and an electron beam source. A first Wien filter for deflecting a primary electron beam from a beam source and vertically incident on the sample, and for straightening electrons generated from the sample; and a first Wien filter disposed between the first Wien filter and the sample; An electron optical member that forms an emission lens together with the sample, and a second Wien filter that selects only electrons having a predetermined energy from the electrons that have passed through the first Wien filter.

[0007]

The primary electron beam from the electron beam source is converged by the irradiation lens system, deflected by the first Wien filter, and vertically incident on the sample. When energy analysis is performed, electrons generated from the sample go straight through the first Wien filter and enter the second Wien filter.
In the second Wien filter, only electrons having a predetermined energy are selected from electrons having various energies. The electrons selected by the second Wien filter go straight through the second Wien filter.

[0008]

Embodiments will be described below with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an embodiment of a direct-mapping reflection electron microscope according to the present invention, in which 13 is a Wien filter, 14 is an imaging lens system, 15 is a screen, 16 is a Wien filter, 17 Denotes an imaging lens system, and 18 denotes a screen. In FIG. 1, the same parts as those in the conventional configuration of FIG. 7 are denoted by the same reference numerals.

First, the overall configuration of the direct mapping reflection electron microscope according to the present invention will be described. In FIG. 1, a Wien filter 13 separates the optical path of the primary electron beam from the optical path of the reflected electrons from the sample 4.
An X-type energy filter, whose electric field vector E and magnetic field vector B are set such that the primary electron beam is perpendicularly incident on the sample 4 and that the reflected electrons satisfy the Wien condition. You. It has been confirmed that such a condition can be actually achieved by making the Wien filter 13 weakly exciting and increasing the length L of the filter. Here, the filter length L
Is the charge amount and mass of the charged particles e and m, and the electric field strength and magnetic field strength of the Wien filter 13 are E and B, respectively.
Is a value given by the following equation (1). L = πEm / eB ² (1) As described above, the primary electron beam is largely deflected by the Wien filter 13, is vertically incident on the sample 4, is generated from the sample 4, and is emitted from the emission lens 1.
The reflected electrons accelerated by 1 travel straight through the Wien filter 13 without receiving any deflecting action. In FIG. 1, what is indicated by θ is the angle between the optical axis of the primary electron beam emitted from the electron gun 1 and the optical axis of the reflected electrons, and this angle θ
Is preferably about 30 ° to 45 °.

The reflected electrons generated from the sample 4, accelerated by the emission lens 11, and proceed straight through the Wien filter 13 pass through the imaging lens system 14. The imaging lens system 14 is
When an image of the backscattered electrons is projected onto the screen 15, some of them are excited so as to function as a projection lens.
Since the image is enlarged to a predetermined magnification by 4 and projected on the screen 15, the operator can observe the reflected electron image formed on the screen 15 from the direction of the arrow 19. However, when the screen 15 is deviated from the optical axis and the reflected electrons are guided to the Wien filter 16 for energy analysis, the imaging lens system 14 is not excited or excited to function as a converging lens. You.

When energy analysis is performed, reflected electrons are incident on the Wien filter 16 from the imaging lens system 14. The Wien filter 16 is arranged for performing energy analysis, and a known configuration such as a retarding Wien filter can be used. The Wien filter 16 has sufficient intensity so that an image or diffraction image formed immediately before the Wien filter 16 is formed immediately after the Wien filter 16, that is, the Wien filter 16 has sufficient intensity to function as one lens. Used in. Therefore, the filter length L of the Wien filter 16 is shortened.

As a result, only the reflected electrons having a predetermined energy are selected from among the reflected electrons having various energies. The selected reflected electrons travel straight through the Wien filter 16, and the image thereof is formed by the imaging lens system 17. The image is enlarged to a predetermined size and formed on the screen 18. Thereby, a monochromatic image can be observed.

Next, the magnetic pole shape of the Wien filter 13 will be described. Now, the Wien filter 13 can have the configuration shown in FIGS. 2 and 3, for example. FIG. 2 is a cross-sectional view in a plane orthogonal to the optical axis, and FIG. 3 is a cross-sectional view along AA in FIG. In the figure, 20 is a magnetic pole, 21 is a coil,
Reference numeral 22 denotes an electrode, and 23 denotes a mirror for restricting the magnetic field distribution of the fringe field. 2 and 3, a uniform magnetic field is formed by the magnetic poles 20 and 21 forming a pair, and a uniform electric field is formed by the electrodes 24 and 25 forming a pair. Naturally, the electric and magnetic fields satisfy the conditions described above.

However, in this configuration, an aberration does not occur for an electron beam traveling straight to the Wien filter 13 like a reflected electron, but for an electron beam incident obliquely like a primary electron beam. Has a problem that first-order astigmatism occurs. That is, as described above, the Wien filter 13 satisfies the Wien condition with respect to the reflected electrons, and since it is used in a weak excitation, it is considered that almost no aberration occurs in the reflected electron beam. On the other hand, for the primary electron beam, the Wien filter 13 is a mere bending magnet and an electric field deflector superimposed, and it is inevitable that aberration will occur. Although it is true that the primary electron beam merely irradiates the observation area of the sample, there is a view that the aberration of the primary electron beam is not a major problem. In the case of LEEM, the aberration of the primary electron beam determines the quality of the finally obtained reflected electron image, and therefore cannot easily be ignored. I can understand.

According to the first-order optical theory relating to the uniform magnetic field sector, the uniform magnetic field sector does not have a convergence effect in the direction of the magnetic pole, but the edge of the magnetic field sector, that is, the plane on which the beam enters and the beam exits. It is known that the magnetic field sector can be made to converge in the direction of the magnetic pole by giving an appropriate angle to the surface. That is, assuming that A is an object plane and B is an image plane in FIG. 4, if tan φ ₁ = ₂ tan ε ′ (1) tan φ ₂ = ₂ tan ε ″ (2) is satisfied, the generation of first-order astigmatism is prevented. It should be noted that in FIG. 4, 30 indicates a magnetic field sector, ρ ₀ indicates the radius of gyration of the trajectory of the beam,
O indicates the center of rotation of the beam. Also, in FIG. 4, it is clear that φ ₀ = φ ₁ + φ ₂ + ε′−ε ″ (3) holds.

Now, the magnetic sector 30 having the configuration shown in FIG.
It is apparent that ε ″ = 0 must be satisfied from the function required for the Wien filter 13 when the ″ is applied to the Wien filter 13 because the reflected electrons from the sample enter the magnetic field sector 30 with B as the object plane. However, at that time, it is necessary to perpendicularly enter the magnetic field sector 30. Therefore, from equation (2), φ ₂ = 0, and
If the deflection angle φ ₀ is given, ε ′ can be obtained.
Actually, the calculation is performed by setting the distance l ₁ from the object plane A to the incident side edge of the magnetic field sector 30 and the distance l ₂ from the exit side edge of the magnetic field sector 30 to the image plane B to desired values. When,
phi ₀ is when the 45 ° epsilon 'is 26.6 °, when phi ₀ is 36 ° epsilon' is 20 °, φ ₀ is when the 30 ° epsilon 'was 16.1 °.

As a result of the simulation,
In the configuration shown in FIGS. 2 and 3, the primary electron beam is distorted into an elliptical shape due to astigmatism as shown in FIG. 5A at the position of the object surface B, that is, at a position at or near the sample surface. When the edge of the magnetic field sector on the side where the beam is incident is angled using the obtained value of ε ′,
The shape of the primary electron beam at the position of the object plane B was substantially circular as shown in FIG. 5B, and it was confirmed that astigmatism could be corrected.

FIG. 6 shows an example of the Wien filter 13 configured based on the above results. In FIG. 6, coils and electrodes wound around the magnetic poles are omitted. The paired magnetic poles 31 and 32 form a uniform magnetic field, but the edge on the side where the primary electron beam of the magnetic pole 31 is incident has a predetermined angle.
Is cut out by ε 'to form a tapered shape. Here, it is needless to say that ε 'is the angle obtained as described above. Accordingly, the mirror 33 on the side where the primary electron beam is incident is also inclined by the same angle. This is because the gap between the mirror 33 and the edge of the magnetic pole 31 needs to be constant. The portion of the magnetic pole 31 to be tapered may be only the portion where the primary electron beam is incident. This is because if the portion of the magnetic pole 31 where the reflected electrons are emitted is also tapered, the reflected electrons do not go straight.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications are possible. For example, in the above embodiment, the Einzel lens type is used as the emission lens, but a magnetic field superposition type may be used. As the screen 18, a photographic film, a TV camera, or the like can be used instead of the fluorescent screen.

Further, in the above embodiment, the case where the reflected electrons generated from the sample are subjected to energy analysis has been described.
The present invention can be applied to an electron microscope that handles secondary electrons, photoelectrons, and thermoelectrons. Further, in the above embodiment, only the primary astigmatism of the primary electron beam is removed. However, when the so-called hexapole component is included in the distortion of the primary electron beam, the primary electron beam of the Wien filter 13 is removed. May be formed in a circular arc shape or a quadratic curved surface.

[0021]

As is apparent from the above description, according to the present invention, a Wien filter is used to separate the optical path of a primary electron beam from the optical path of electrons generated from a sample, and the energy analyzer is used as an energy analyzer. Since the Wien filter is used, the optical axis of electrons generated from the sample, such as reflected electrons, can be made linear, and the imaging system can be made substantially optical axis symmetric, so that axis alignment and focusing can be performed. In addition to facilitating various adjustments such as distance and magnification, the aberration of the imaging system can be extremely reduced, so that an image with very little distortion can be obtained. In addition, since no astigmatism occurs in the primary electron beam, a high-quality image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention.

FIG. 2 is a diagram showing a structural example of a Wien filter that can be used as a Wien filter 13;

FIG. 3 is a sectional view taken along line AA of FIG. 2;

FIG. 4 is a diagram for explaining a method of correcting primary astigmatism of a primary electron beam.

FIG. 5 is a diagram for explaining a simulation result of astigmatism correction.

FIG. 6 is a diagram showing a configuration example of a Wien filter 13;

FIG. 7 is a diagram illustrating a configuration example of a conventional LEEM.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Irradiation lens system, 4 ... Sample, 5 and 6 ... Electrode, 7 ... Power supply, 11 ... Emission lens, 13 ... Wien filter, 14 ... Imaging lens system, 15 ... Screen, 16 ... Wien filter , 17 ... imaging lens system, 1
8 ... Screen.

──────────────────────────────────────────────────続 き Continuation of the front page (56) Reference Jikken Sho 49-41798 (JP, Y1) Patent 2810797 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 37/00 -49/48

Claims (4)

(57) [Claims] 1. An electron beam source, an irradiation lens system for irradiating a primary electron beam from the electron beam source toward a sample, and an electron beam source disposed in front of the sample, wherein the primary electron beam from the electron beam source is emitted from the electron beam source. A first Wien filter for deflecting the light to be perpendicularly incident on the sample and for straightly moving electrons generated from the sample, and an electron disposed between the first Wien filter and the sample to form an emission lens together with the sample. A direct-mapping reflection electron microscope, comprising: an optical member; and a second Wien filter that selects only electrons having a predetermined energy from electrons that have passed through the first Wien filter. 2. The direct-mapping reflection electron microscope according to claim 1, wherein the first Wien filter is used in a weak excitation. 3. The first Wien filter according to claim 1, wherein the end surface of the magnetic pole surface of the first Wien filter on the primary electron beam incident side is tapered at a predetermined angle so that aberration of the primary electron beam is reduced. Item 3. A direct mapping reflection electron microscope according to item 1 or 2. 4. A mirror for restricting the magnetic field distribution of the fringe field is arranged on the primary electron beam incident side of the first Wien filter, and an end face of the mirror on the primary electron beam incident side is provided. 4. The direct-mapping reflection electron microscope according to claim 3, wherein the angle is parallel to the tapered surface.