JP3945775B2 - Lead electrode for field emission low-energy electron diffraction and electron diffraction apparatus using the same - Google Patents

Description

  The present invention relates to a field-emission low-energy electron diffraction pulling electrode and an electron diffraction apparatus using the same.

  Conventionally, there is a low-energy electron diffraction method as one of the methods widely used as a surface structural analysis method. Since a low-speed electron beam is used, it is sensitive to the surface structure, and the lattice coordinates in the unit lattice can be determined with high accuracy of about 10 pm. However, the diameter of the electron beam irradiation region is as wide as about 0.1 mm and cannot be applied to the structural analysis of the surface minute region. Thus, field emission low-energy electron diffraction has been devised as a method of irradiating a surface micro-region with a low-energy electron beam. However, in the conventional field emission low-energy electron diffraction, scattered electrons from the surface are extracted and detected in a direction parallel to the sample surface (see, for example, Non-Patent Document 1). As a result, the scattered electric field between the probe and the sample caused many scattered electrons to be returned to the sample again, thereby reducing the detection efficiency.

  Therefore, by attaching a probe shield around the probe, we were able to enhance the ability to pull it out to the vacuum side. A method has been proposed in which scattered electrons are extracted while maintaining symmetry in the direction perpendicular to the sample surface by using a cylindrical probe shield (see, for example, Non-Patent Documents 2 and 3).

  FIG. 2 is a diagram illustrating a configuration example of the field emission electron diffraction device described in Non-Patent Document 3. A field emission electron diffraction apparatus B shown in FIG. 2 includes an ultrahigh vacuum chamber 101, a microchannel plate (MCP) 103 provided in the ultrahigh vacuum chamber 101, a scanner 105 for placing a sample, and electropolishing. The tungsten probe 107 is sharpened by the above, a coaxial cylindrical shield electrode 111 provided outside the tungsten probe 107, and a drawing electrode 115 for drawing scattered electrons. A window 117 is formed on the chamber wall above the MCP 103, and an image appearing on the MCP 103 can be observed and photographed by the video camera 121 provided outside the window 117 through the window 117.

  In the above-described apparatus, a field emission electron beam is irradiated from a sharpened probe to a minute region on the surface of the sample, and scattered and diffracted electrons are drawn upward by the drawing electrode 115 and detected by the video camera 121 through the MCP 103. By doing so, a fine atomic arrangement such as an atomic arrangement of several atomic layers on the sample surface can be examined.

S. Mizuno, Journal of Vacuum Science and Technology B 19 (2001) 1874. S. Mizuno, J. Fukada, H. Tochihara, Surf. Sci. 514 (2002) 291. Mizuno Kiyoyoshi, “Atomic position determination using low-energy electron diffraction for the development of new nanomaterials”, Monthly “Chemical Industry Vol. 53, (2002)”.

  In a field emission low-energy electron diffraction apparatus, it is necessary to extract scattered electrons in a direction perpendicular to the sample surface in order to obtain a diffraction pattern that maintains symmetry of the sample surface. However, since there is a pull-back electric field between the probe and the sample, it is difficult to extract the sample to the vacuum side with sufficient efficiency while maintaining symmetry, and to detect by dispersing according to the scattering angle.

  In addition, even if the probe is used to extract the scattered electrons in the direction perpendicular to the sample, it cannot provide sufficient symmetry and smooth change in the orbit of the scattered electrons, resulting in a clear diffraction pattern. I couldn't get it. As shown in FIG. 2, if a structure in which the lead electrode 115 stands upright in the vertical direction and is separated from the sample 107 (or the probe 111), the lead function is weakened. In addition, if the lead electrode 115 is too close to the sample 107 (or the probe 111), the electric field is suddenly changed, and it becomes difficult to disperse scattered electrons for each scattering angle, or scattered electrons from the sample. Will be blocked. Furthermore, if a hole is made in the lead-in electrode 115 for exchanging the probe and the sample, the electric field is disturbed, and a clear diffraction pattern cannot be obtained.

  An object of the present invention is to provide a technique for giving sufficient symmetry and smooth change to the trajectory of scattered electrons in a field emission low-energy electron diffraction apparatus.

  According to one aspect of the present invention, a probe that irradiates a sample with a low-energy electron beam by field emission, a probe shield electrode that shields the probe, and a method for drawing in scattered electrons emitted from the sample There is provided a low-energy electron diffraction apparatus characterized in that it has a lead-in electrode, and a passage defining the trajectory of the scattered electrons is formed by the probe shield electrode and the lead-in electrode. The defined path is preferably provided as close as possible to the electron trajectory as long as it does not obstruct the scattered electron trajectory. The outer surface of the probe shield electrode preferably has a shape that does not form an electric field as steep as possible, for example, a hemispherical shape. The inner surface of the lead-in electrode preferably has a conical shape, for example.

  According to another aspect of the present invention, a central axis of the probe shield electrode and a central axis of the lead electrode are substantially coaxial, and at least one of the probe shield electrode and the lead electrode is from the sample. There is provided a low-energy electron diffraction apparatus characterized in that the low-energy electron diffraction apparatus is disposed so as to be along the trajectory of emitted scattered electrons.

  Also in this case, it is preferable that at least one of the probe shield electrode and the lead-in electrode is provided at a position as close as possible to the electron trajectory unless the trajectory of the scattered electrons is blocked.

  The lead-in electrode is disposed outside the probe shield electrode, and is a space that accommodates the probe shield so that an inner wall forming the space narrows toward the sample side. It is preferable to have a formed space. This is because the scattered electrons are emitted from the sample at a certain angle and thus tend to spread as they travel.

  Furthermore, it is provided with the collar part which is provided in the said drawing electrode and protrudes toward the said probe shield side. Moreover, it is preferable that the said collar part has a fine adjustment mechanism which can change the relative position with respect to the said drawing electrode or the said probe shield. The track can be adjusted by the collar, and the track can be finely adjusted by the fine adjustment mechanism.

  Furthermore, it is preferable to have a moving mechanism for relatively moving the lead-in electrode and the probe shield or at least one of the sample holders holding the sample. By moving by the moving mechanism, at least one of the probe holder and the sample holder can be exposed, and the sample and the probe can be easily transferred. Further, as compared with a method in which a hole is formed in the lead-in electrode to facilitate delivery of the sample and the probe, the electric field is less disturbed and the scattered electrons can be stably drawn. The lead-in electrode includes, for example, a first electrode part, a second electrode part, and a collar part. Furthermore, at least one of the grid and the MCP may be movable.

  According to the present invention, a sufficient symmetry and a smooth change can be given to the orbit of scattered electrons in a field emission low-energy electron diffraction apparatus. Therefore, the trajectory of scattered electrons that well reflects the diffraction pattern from the sample can be formed in the apparatus.

  Hereinafter, a field emission low-energy electron diffraction apparatus according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a principle diagram of an apparatus using a scanning tunneling microscope probe as a field emission source in the field emission low-energy electron diffraction apparatus according to the first embodiment of the present invention. As shown in FIG. 1, the electrons emitted from the probe 1 into the vacuum 3 are directed to the sample surface while being accelerated by the potential. The electrons scattered back on the sample surface 5 return to the vacuum again. The field emission low energy electron diffractometer detects this electron.

  FIG. 3 corresponds to FIG. 2 and is a cross-sectional view showing a configuration example of almost the entire field emission electron diffraction apparatus according to the present embodiment. FIG. 4 is a diagram illustrating a state in which the configuration illustrated in FIG. 3 is viewed from above. The MCP and video camera shown in FIG. 2 are not shown in FIG.

  As shown in FIGS. 3 and 4, the field emission low-energy electron diffraction apparatus A according to the present embodiment is provided in the vacuum chamber 101 (FIG. 2), and a first electrode portion corresponding to the lead-in electrode shown in FIG. The first electrode portion 15 has an inverted conical inner wall 15 that gradually decreases toward the lower side of the drawing (as it approaches the sample), and is disposed in a space formed inside the inner wall of the first electrode portion 15. The probe shield electrode 17, the arm 21 that supports the probe shield electrode 17, the base 23 that supports the arm 21, and the probe that is held by the probe shield electrode 17 and protrudes downward 25, a sample holder 27 that can place the sample 26 in the vicinity of the probe 25, and a moving mechanism 30 that can move the sample holder 27. Further, a second electrode portion 19 that is insulated by the first electrode portion 15 and the insulating insulator 20 is provided outside the first electrode portion 15.

  Since the first electrode portion 15 has the above shape, the lead-in electrode as the first electrode 15 can be brought close to the probe 17. Furthermore, it has what is called a taper-type inner wall in which the inner wall of the 1st electrode part 15 is located inside the part nearer to the probe 17 in the distance.

  Furthermore, a microscopic area on the sample surface is irradiated with a field emission electron beam from a sharpened probe, and the scattered and diffracted electrons are provided above the vacuum chamber to form a detection unit for detecting scattered electrons. It has a channel plate (hereinafter referred to as “MCP”) and a grid gauge provided on the sample side in the vicinity of the MCP (not shown).

  The MCP is a plate for detecting scattered electrons. For example, a fluorescent screen is attached to the MCP, and a scattering pattern can be observed from outside the vacuum through a window provided in the vacuum chamber. The grid gauge has a function of returning inelastically scattered electrons toward the sample side, and has a role of a kind of filter for allowing only scattered electrons to reach the MCP.

  By detecting the scattered electrons from the sample with the electron diffraction apparatus having the above-described configuration, it is possible to accurately investigate a fine atomic arrangement of the order of several atomic layers on the sample surface.

  FIG. 5 is a view corresponding to FIG. 3, and at least one of the lead electrode (first electrode portion 15, second electrode portion 19, insulating insulator 20) and the probe shield electrode 17 or the sample holder 27 holding the sample 26. It is preferable to have a moving mechanism (not shown) for relatively moving the two. By movement by the moving mechanism, at least one of the probe shield electrode 17 and the sample holder 27 can be exposed, and the sample 26 and the probe 25 can be easily transferred. In addition, according to this method, the electric field is less disturbed than in the conventional method in which a hole is formed in the drawing electrode to facilitate the delivery of the sample and the probe, and scattered electrons can be drawn stably. . The lead-in electrode includes, for example, a first electrode portion 15, a second electrode portion 19, and a collar portion 31 (see FIG. 7 described later). Furthermore, at least one of the grid and the MCP (see FIG. 2) may be movable.

  FIG. 6 is a diagram illustrating an example of a positional relationship among the second electrode unit 19, the probe 25, the probe shield electrode 17, the first electrode 15, the sample 26, and the sample 26 heading toward the MCP side (upward). Based on the potential difference between the probe 25 and the sample 26 (a negative potential is applied to the probe side with respect to the sample), the electrons tunnel through the potential barrier on the probe side as shown in FIG. It jumps out to the vacuum area between the needle and the sample. The field emission electrons travel toward the sample surface while being accelerated by the potential in vacuum. When electrons having kinetic energy are backscattered on the sample surface, they bounce back into the vacuum. At this time, it is possible to obtain information on the structure of the surface microregion by preferably observing a diffraction pattern of only electrons that have undergone only elastic scattering.

  FIG. 7 shows a field emission electron diffraction apparatus according to the second embodiment of the present invention. In addition to the first electrode 15 in the first embodiment, the direction from the inner wall of the first electrode 15 to the probe 25 ( For example, it has a flat plate-shaped flange portion 31 projecting inward. In the field emission electron diffraction apparatus shown in FIG. 7, it is possible to change the length (width) W in the protruding direction of the flange 31 and the position H in the height direction from the upper surface of the sample. Examples of scattered electron trajectories with the extending direction of the probe 25 as the central axis C are schematically shown as B1 to B7. By considering 360 degrees rotation about the C axis, the trajectory of the scattered electrons in the actual apparatus can be estimated.

  As shown in FIG. 7, the orbital positions of scattered electrons are distributed almost uniformly in the plane of the MCP (FIG. 2) as indicated by symbols B1 to B7, and the scattering angle of the electrons from the sample is almost equal. Reflects faithfully. A slight deviation from the scattering angle from the sample can be calibrated in the data processing step after detection. The flange 31 forms part of the second lead-in electrode, and can further bring the electrode closer to the sample side. Furthermore, by adjusting W and H, the potential distribution around the sample can be adjusted, and the trajectory of scattered electrons can be finely adjusted.

  6, the potential of the sample 26 is 50 eV, the probe 25 is 0 eV, the probe shield electrode 17 is 220 eV, the first electrode 15 and the flange 31. Is 180 eV, the second electrode 19 is 20 eV, and the grid gauge potential is 30 eV.

  As shown in FIG. 7, it can be seen that the trajectory of scattered electrons is substantially determined by the shapes of the probe shield electrode 17, the first electrode 15, and the collar portion 31. In other words, the probe shield electrode 17, the first electrode 15, and the flange portion 31 are shaped so as to follow the trajectory of the scattered electrons, thereby exiting from the sample at an angle that directly reflects the structure of the sample. It can be seen that it is preferable that the scattered electrons are not obstructed by the probe shield electrode 17 and the electrodes 15 and 31 and that the electrodes 15 and 31 are as close to the probe 25 as possible. Actually, it can be seen that the above requirements can be satisfied by making the shape of the outer surface of the probe shield electrode 17 and the shape of the inner surface of the electrode 15 as shown in FIG.

  The probe shield electrode has a hemispherical shape or a shape close thereto, and is disposed at a position close to the orbit of scattered electrons, and the shape of the drawing electrode is a hemispherical cavity, a conical cavity, or a shape close to them. By disposing near the orbit of scattered electrons, the scattered electrons can be accurately detected by dispersing the electrons according to the scattering angle while maintaining the symmetry of the scattered electrons.

  As mentioned above, although this invention was demonstrated along embodiment, this invention is not restrict | limited to these. It will be apparent to those skilled in the art that other various modifications, improvements, and combinations can be made.

  Since a fine atomic arrangement such as an atomic arrangement of several atomic layers on the sample surface can be examined, it can be used for analysis of various fine structures in addition to the analysis of nanostructures.

It is a principle figure of the apparatus which used the scanning tunnel microscope probe as a field emission source in the field emission low-energy electron diffraction apparatus by this Embodiment. It is a figure which shows the example of 1 structure of the conventional field emission electron diffraction apparatus. FIG. 3 is a diagram corresponding to FIG. 2, and is a cross-sectional view showing a configuration example of almost the entire field emission electron diffraction device of Example 1 of the present invention. It is a figure which shows a mode that the structure shown in FIG. 3 was seen from the upper surface. FIG. 4 is a view corresponding to FIG. 3, and is a cross-sectional view showing a state in which a lead electrode, a sample, and a probe shield are moved. It is a figure which shows the structural example of the apparatus containing a vacuum chamber, a probe, a probe shield electrode, an electrode, and a sample. FIG. 6 is a diagram showing a field emission electron diffraction apparatus according to a second embodiment of the present invention, and the position of the scattered electrons from the sample toward the MCP side (upward) in the structure in which a collar is loaded on the electrode in addition to FIG. It is a figure which shows the example of a relationship.

Explanation of symbols

DESCRIPTION OF SYMBOLS A ... Field emission low-energy electron diffraction apparatus, 15 ... 1st electrode part, 17 ... Probe shield electrode, 19 ... 2nd electrode part, 25 ... Probe, 26 ... Sample, 27 ... Sample holder, 31 ... Saddle part, 101 ... an ultra-high vacuum chamber.

Claims (6)

A probe that irradiates a sample with a low-energy electron beam by field emission;
A probe shield electrode that shields the probe, and a drawing electrode for drawing scattered electrons emitted from the sample;
The low-energy electron diffraction apparatus , wherein the lead-in electrode is disposed outside the probe shield electrode and has an inner wall that narrows toward the sample side . The low-energy electron diffraction apparatus according to claim 1, wherein a central axis of the probe shield electrode and a central axis of the lead-in electrode are substantially coaxial. The low-energy electron diffraction apparatus according to claim 2, wherein the probe shield electrode has a hemispherical shape on the sample side.   The low-energy electron diffraction apparatus according to any one of claims 1 to 3, wherein the lead-in electrode has a flange protruding toward the probe shield side.   The low-energy electron diffraction apparatus according to claim 4, wherein the flange includes a fine adjustment mechanism that can change a relative position with respect to the lead-in electrode or the probe shield.   6. The apparatus according to claim 3, further comprising a moving mechanism that relatively moves the lead-in electrode and at least one of the probe shield and the sample holder holding the sample. Low-energy electron diffraction device.

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