JP2020085873A - Photoemission electron microscope - Google Patents

Abstract

To provide a photoemission electron microscope that has high spatial resolving power capable of observing a bulk sample.SOLUTION: A photoemission electron microscope is composed of: an objective lens 6 that is an electric field magnetic field overlapping type characterized in which a tip 13 of a yoke cone part 15 facing a sample 9 is shaped into a ring (an external diameter r1 and inner diameter r2), the external diameter r1 is more than three times a distance g between the tip 13 and sample 9, an angle (cone angle) 14 formed with the yoke cone part 15 with respect to a surface 13 (in this fig., a horizontally arranged sample stand 30) vertical to an optical axis 12 is equal to or more than 20°, and in a yoke outer peripheral part 18, a boring part 17 is provided symmetrically relative to an axis at even portions; and a two-stage mirror aberration corrector in which an aberration corrector 32 has two pairs of mirror type aberration correctors coaxially faced and arranged.SELECTED DRAWING: Figure 3

Description

The present invention relates to a photoelectron microscope capable of observing emitted photoelectrons by illuminating the surface of a bulk material with an objective lens and an aberration correcting means at atomic resolution.

There are various types of electron microscopes such as PEEM (photoelectron microscope), LEEM (low energy electron microscope), TEM (transmission electron microscope), and SEM (scanning electron microscope). From the viewpoint of electron optics, PEEM is common with LEEM. , TEM/SEM is recognized as being significantly different.

In PEEM/LEEM, the large aberration of the lens used is not so much of a problem, rather, the inside of the device is kept in an ultra-high vacuum state, and the surface aspect analysis is the main component rather than the image observation. The resolution was considerably lower than that of TEM/SEM.

On the other hand, in the TEM, a high resolution type in which the accelerating voltage between the electron gun 1 and the image observation unit (fluorescent plate) 2 is increased to 200 kV or more to shorten the wavelength of the electron beam and the atomic image can be observed is on the market (Fig. 2 (See (a)). Further, by correcting the aberration of the lens, atomic resolution is realized even at a low acceleration voltage such as 40 kV. However, in the case of TEM, there is a restriction that the sample 3 is made thin, and it has been desired that atoms are observed in the bulk form which is the original form of the sample.

Although SEM is used for observing the bulk sample, as shown in FIG. 1B, incident electrons diffuse inside the sample, and secondary electrons 26 are generated from the periphery of the incident position, which causes blurring of the image. However, the resolution was lower than that of TEM by one digit or more, and it was difficult to observe even an atomic image.
In addition, the atomic force microscope displays the voltage for keeping the atomic force constant by scanning the probe so that the generated atomic force is kept constant by bringing the probe close to the sample. Although it can observe the arrangement of atoms, it cannot observe the atomic images of all samples, and it is difficult to observe at low and medium magnifications, and composition analysis is not possible. Its use is limited because it is scarce.

On the other hand, unlike other electron microscopes, PEEM is an electron microscope that observes a bulk sample using light as an incident radiation source, and since the incident beam is light, the only diffusion of electrons in the sample is Since the thickness remains the same as the TEM sample thickness, as shown in FIG. 1A, the sample is only illuminated and does not diffuse into the sample.

As a PEEM for observing the surface of a bulk material, a general-purpose PEEM using an electrostatic lens as an objective lens is commercially available and disclosed. However, this PEEM is characterized by observing the crystal structure of the sample, the guaranteed resolution remains at 300 nm, and it was not intended for high resolution. (Non-Patent Document 1, Patent Document 1)
Also, SPECS employs an aberration correction method that uses a mirror, but since it is a continuous electron beam, it is necessary to distinguish between going and returning by a beam selector, which causes a new aberration and hinders the correction function. It was (Non-patent document 2)
On the other hand, Koike et al. proposed a new aberration correction method using a two-step mirror (Non-Patent Document 3), but it was intended for TEM and was not directly applied to PEEM.

Japanese Patent No. 5690610 (paragraph number 0008, FIG. 6)

Catalog “Light Emission Electron Microscope MyPEEM”, Suga Seisakusho Co., Ltd., p. Two Catalog "FE-Low Energy Electron Microscope/Photoelectron Microscope", Tokyo Instruments, Inc., p. 1 Hiromi Koike, Proposal of Cc/Cs Simultaneous Correction Method by 2-Stage Mirror, The 67th Academic Meeting of the Microscopy Society of Japan, 16Aam_I1-5

The present invention has been made in view of such a point, and an object thereof is an invention technology for improving the resolution of PEEM, which has been delayed to 300 nm in the past, to the same level as that of TEM. The surface can be observed with atomic resolution.

A PEEM according to the present invention is a PEEM that obtains a magnified image of a photoelectron emitted from a sample by irradiating the sample with light from a light source through an objective lens and an aberration corrector.
(1) In the objective lens 6, as shown in FIG. 3, the tip 13 of the yoke conical portion 15 facing the sample 9 has a ring shape (outer diameter r1, inner diameter r2), and the outer shape r1 has the tip 13 and the sample 9. And the angle g (cone angle) 14 with respect to the surface 13 perpendicular to the optical axis 12 (the sample table 30 arranged horizontally in this figure) that is 3 times the distance g or more and 20° or more. age,
A perforated portion 17 is provided on the outer peripheral portion 18 of the yoke. However, in order to prevent the occurrence of astigmatism of the image due to the shape imbalance, the perforated portion 17 is provided in an axially symmetric even-numbered place. Objective lens of
(2) The aberration corrector 32, as shown in FIG. 5, corrects an electrostatic plate (mirror portion) 20 that reflects an incident electron beam, an electrostatic plate (concave mirror forming portion) 21 that forms a concave mirror, and spherical aberration. The electrostatic plate (for chromatic aberration correction) 23 faces two sets of mirror aberration correctors, each of which includes one electrostatic plate (for spherical aberration correction) 22 and an electrostatic plate (for chromatic aberration correction) 23 for correcting chromatic aberration. And a two-stage mirror aberration corrector which is arranged coaxially as described above.

The aberration simulation result of the objective lens of the present invention is shown in FIG. 3B, and the aberration simulation result of the conventional objective lens (Patent Document 1, FIG. 4) is shown in FIG. 4B for comparison. It has been found that the objective lens of the present invention has a resolution improved by two digits (62/4 times) as compared with the conventional objective lens. However,
The simulation is conducted by the following process.
(1) The equipotential lines generated in the space around the lens by the voltage applied to each part of the lens are shown in the figure by the finite element method.
(2) Similarly, the lines of isomagnetic force generated in the periphery by the magnetism applied to the lens are shown in the figure.
(3) The figure shows how the photoelectron beam generated by the material flies in the electric field and magnetic field space by changing the position and angle.
(4) Based on the result, the performance of the lens action is calculated by the degree of aberration. In the case of FIG. 3B, the value was 4 μm.

In conventional PEEM (Patent Document 1), an electrostatic lens was often adopted as an objective lens, but this was changed to a lens-shaped magnetic lens capable of exerting a magnetic field on a sample as shown in FIG. By adopting the method of increasing the yield of photoelectrons by the superposition action with the electric field for extraction, the resolution is improved three times.

Moreover, the simultaneous correction method of spherical aberration and chromatic aberration using a two-stage mirror proposed by Koike et al. is adopted, and as shown in FIG. Aberration correction is completed without maintaining the linear optical path. This leads to unnecessary generation of the new aberration shown in Non-Patent Document 2. This improves resolution by 2-3 times

By making the above improvements, it is possible to realize sub-nm or less from the current PEEM resolution of 300 nm, which makes it possible to observe atoms having a sub-nm size. Further, if the objective lens is provided with a discharge countermeasure and the accelerating voltage between the sample 9 and the image observation unit (detector) 10 in FIG. 2B is increased from the normal 10 kV to 100 kV or more, the wavelength of the photoelectron beam is shortened. Therefore, the calculation improves the resolution by about 4 times, and makes it easier to observe atoms.
The structure of the entire photoelectron microscope is shown in FIG.

FIG. 3 is a diffusion diagram in the sample of (a) light and (b) electron beam, which is the scientific basis of the present invention. It is an electron beam (ray) figure of a (a) transmission electron microscope and a (b) photoelectron microscope for comparison. It is (a) sectional drawing of the objective lens of this invention, and (b) aberration simulation result. It is (a) sectional drawing and the (b) aberration simulation result of the objective lens for conventional PEEMs. It is (a) block diagram of a mirror type|mold aberration correction mechanism, and (b) electron beam diagram. It is a figure of the photoelectron convergence of the electric field magnetic field superposition type objective lens. It is a device block diagram of this invention.

1. Electron beam source 2. Image observation part (fluorescent plate)
3. Sample (thin film)
4. First focusing lens 5. Second focusing lens 6. Objective lens 7. Projection lens 8. Light source 9. Sample (bulk material)
10. Image observation unit (detector)
11. York 12. Optical axis 13. Yoke tip 14. Cone angle 15. Yoke conical section 16. Simulation diagram (bottom plate shape)
17. Perforated portion 18. Outer portion of yoke 19. Simulation diagram (conical)
20. Electrostatic plate (mirror part)
21. Electrostatic plate (concave mirror forming part)
22. Electrostatic plate (for spherical aberration correction)
23. Electrostatic plate (for chromatic aberration correction)
24. Electron beam 25. Voltage variable unit 26. Secondary electron 27. Photoelectron 28. Magnetic field 29. Electric field 30. Sample table 31. Slit 32. Aberration corrector 33. Imaging system g Sample-Bottom of objective lens Distance (gap)
r1 Objective ring bottom ring outer diameter r2 Objective lens bottom ring inner diameter

Claims (3)

A photoelectron microscope that obtains a magnified image by irradiating a sample with light from a light source to emit photoelectrons emitted from the sample through an objective lens and an aberration corrector.
In the objective lens, the tip of the yoke conical portion facing the sample has a ring shape (outer diameter r1, inner diameter r2), and the outer diameter r1 is 3 times or more the distance g between the tip and the sample table, An objective lens having an angle (cone angle) with the yoke cone of the plane perpendicular to the optical axis (the sample stage) of 20° or more;
The aberration corrector includes an electrostatic plate (mirror unit) that reflects an incident electron beam, an electrostatic plate (concave mirror forming unit) that forms a concave mirror, an electrostatic plate (for spherical aberration correction) that corrects spherical aberration, and chromatic aberration. Two stages, wherein two sets of mirror-type aberration correctors, one set of which is an electrostatic plate (for chromatic aberration correction) to be corrected, are coaxially arranged so that the electrostatic plates (for chromatic aberration correction) face each other. A mirror type aberration corrector,
A photoelectron microscope comprising: 2. The PEEM according to claim 1, wherein the objective lens is provided with perforated portions on an outer peripheral portion of the yoke in axially symmetrical even-numbered locations. 3. The photoelectron microscope according to claim 1, wherein the objective lens is an electric field/magnetic field type superimposing type objective lens.