JP2011034888A - Ion source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ion source capable of increasing ion formation amount even though a clean state is maintained and a long service life is made possible. <P>SOLUTION: The ion source includes: a plasma chamber 1; a permanent magnet 3 as a generating means of a magnetic field for cusp magnetic field generation in order to confine plasma in the plasma chamber 1; a permanent magnet 4 as the magnetic field generating means for filter magnetic field generation in order to separate plasma into a high temperature electronic region and a low temperature electronic region; and a plasma electrode 5 which is installed at more downstream side than the permanent magnet 4 and has an open hole part 5d in order that ions pass through the hole part. A diamond thin film 8 is installed on an interior surface of an open hole part electrode 7 mounted on the plasma electrode 5. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an ion source that generates an ion beam using, for example, hydrogen gas as a raw material.

  Conventionally, researches in the fields of nuclear / particle physics, solid state physics, material science, and structural biology have been carried out using beams with large intensity. In recent years, negative ions have attracted attention because they can increase the beam intensity.

  Two methods of generating negative ions are generally known: a surface generation method and a volume generation method. Among these two methods, the volume generation method is in the spotlight because it can generate a large amount of negative ions directly in the collision process in the discharge plasma. For example, when hydrogen gas is used as a raw material, a technique is known that can generate negative hydrogen ions directly in discharge plasma using a dissociative adhesion reaction between excited hydrogen molecules and electrons (for example, Non-Patent Document 1). reference).

According to this non-patent document 1, it is disclosed that a two-stage process exists in the generation of hydrogen negative ions. That is, as shown in FIG. 4, the reaction at each stage is divided by the energy of electrons, and in the ion source, a localized sheet-like transverse magnetic field (filter magnetic field) by the permanent magnet 104 is provided in the plasma chamber 101. The electron energy region is divided into two. Among these regions, in the first stage of the reaction, electrons of high energy (several tens of eV) generated by arc discharge in the region where the filament 102 exists and hydrogen in the high temperature electron region (T = several tens of eV). The molecules (H ₂ ) collide to generate excited hydrogen molecules (H ₂ ^* (ν)). In the second stage of the reaction, low energy (excited hydrogen molecules (H ₂ ^* (ν)) in the low temperature electron region (T ≦ 1 eV) in the space near the opening 105d of the plasma electrode 105 from which ions are extracted (low energy ( It is described that hydrogen negative ions (H ⁻ ) are generated by attaching electrons of 1 eV or less).

  Further, Non-Patent Document 1 describes a method of adding a small amount of cesium to the plasma chamber 101 as a means for increasing the amount of hydrogen negative ions generated in the volume generation method of hydrogen negative ions. For example, the cesium introduction port 120 may be used for the addition of cesium as shown in FIG.

  As described above, cesium is used because cesium has the lowest work function as a simple metal, and for example, when copper is assumed as the material of the plasma electrode 105, the work function of the surface of the plasma electrode 105 is used. Is 1.64 eV in the lowest case (see, for example, Non-Patent Document 2).

Thus, by adding cesium, hydrogen atoms (H) and hydrogen positive ions (H ⁺ ) take in electrons obtained from cesium attached to the inner wall surface of the plasma chamber 101 and the surface of the plasma electrode 105 to generate hydrogen. Negative ions (H ⁻ ) are generated. The effects electrons and hydrogen molecules positive ions obtained from the cesium and (H 3 ^₊₎ is to generate excited hydrogen molecules by reacting ^(H 2 _* (ν)) and a hydrogen atom (H) occurs. This effect is said to increase the amount of hydrogen negative ions (H ⁻ ) produced. The above reactions are collectively shown in the following formulas (1), (2), and (3).
H + e → H ^― ――― Formula (1)
H ⁺ + 2e → H ^― ――― Formula (2)
H ₃ ⁺ + e → H ₂ ^* (ν) + H ――― Equation (3)

Oguri Hidekazu “Study on High Intensity Negative Hydrogen Ion Source”, High Energy Accelerator Research Organization website (http://www.kek.jp/sokendai/intro/ronbun/200309/02 oguri hidetomo / thesis-j. pdf) Junzo Ishikawa, “Ion Source Engineering”, published by Ionics Corporation, pp. 160-163

  However, the cesium vapor causes a problem that the inner wall surface of the plasma chamber 101 and the filament 102 are contaminated. In particular, contamination of the filament 102 is problematic because it leads to a decrease in thermionic emission and a decrease in the lifetime of the filament 102. Therefore, there is also the inconvenience of having to perform regular cleaning. Furthermore, cesium has a high reactivity, there is a problem that it is difficult to handle because it oxidizes in the atmosphere and there is a risk of spontaneous ignition.

  An object of the present invention is to provide an ion source capable of increasing the amount of ions generated while maintaining a clean state and extending the lifetime.

In order to achieve the above object, the invention according to claim 1 of the present invention provides:
A plasma chamber for generating plasma by discharging the introduced gas, and ionizing the plasma by thermoelectrons emitted from a filament heated by passing an electric current;
Magnetic field generating means for generating a cusp magnetic field provided around the plasma chamber in order to confine the plasma in the plasma chamber;
In order to separate the plasma into a high temperature electron region and a low temperature electron region, a magnetic field generation unit for generating a filter magnetic field provided downstream from the magnetic field generation unit for generating the cusp magnetic field,
A plasma electrode provided on the downstream side of the magnetic field generating means for generating the filter magnetic field, and having an aperture for allowing ions to pass therethrough,
An ion source comprising a diamond thin film on at least a part of the surface of the plasma electrode on the aperture side.

  The invention according to claim 2 of the present invention is characterized in that, in the invention according to claim 1, the diamond thin film contains at least one of phosphorus and boron as an impurity.

  The invention according to claim 3 of the present invention is characterized in that, in the invention according to claim 1 or 2, the surface of the diamond thin film is hydrogen-terminated.

  The invention according to claim 4 of the present invention is the invention according to any one of claims 1 to 3, wherein the plasma chamber and the plasma electrode are spaced apart from each other above the opening of the plasma electrode. And a charge electrode having a porosity covered with a diamond thin film.

  The invention described in claim 5 of the present invention is characterized in that, in the invention described in claim 4, the skeleton of the charge electrode is formed of a thin metal wire and has a shape having a number of meshes. To do.

As described above, the present invention
A plasma chamber for generating plasma by discharging the introduced gas, and ionizing the plasma by thermoelectrons emitted from a filament heated by passing an electric current;
Magnetic field generating means for generating a cusp magnetic field provided around the plasma chamber in order to confine the plasma in the plasma chamber;
In order to separate the plasma into a high temperature electron region and a low temperature electron region, a magnetic field generation unit for generating a filter magnetic field provided downstream from the magnetic field generation unit for generating the cusp magnetic field,
A plasma electrode provided on the downstream side of the magnetic field generating means for generating the filter magnetic field, and having an aperture for allowing ions to pass therethrough,
Since the diamond electrode is provided on at least a part of the surface of the plasma electrode on the aperture side, the ions can be maintained in a clean state and can have a long life while increasing the amount of ions generated. A source can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram for demonstrating 1st Embodiment of the ion source which concerns on this invention, Comprising: (a) is a general view, (b) is an expansion perspective view of an aperture part electrode. It is a schematic diagram for demonstrating 2nd Embodiment of the ion source which concerns on this invention, Comprising: (a) is a general view, (b) is the C section enlarged view for demonstrating a charge electrode. It is an expansion perspective view of the charge electrode. It is a schematic diagram for demonstrating the conventional ion source.

  Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

(First embodiment)
FIG. 1 is a schematic diagram for explaining a first embodiment of an ion source according to the present invention, where (a) is an overall view and (b) is an enlarged perspective view of an aperture electrode.

  In FIG. 1, 1 is a plasma chamber made of oxygen-free copper, 2 is a filament, 3 is a permanent magnet as magnetic field generating means for generating a cusp magnetic field provided around the plasma chamber 1, and 4 is provided downstream of the permanent magnet 3. A permanent magnet 5 serving as a magnetic field generating means for generating a filter magnetic field, and a plasma electrode 6 made of oxygen-free copper having a tapered portion whose diameter increases upward from the surface on the side having the opening 5d; 6 Is an insulator made of Teflon (registered trademark) for electrical insulation between the plasma chamber 1 and the plasma electrode 5, and 7 is attached along the inside of the plasma electrode 5 and is removable from the plasma electrode 5 Opening electrode made of oxygen-free copper, 7a is a flange part of the opening part electrode 7, 7b is a mounting hole for attaching the flange part 7a to the plasma electrode 5, 7c is a taper part of the opening part electrode 7, and 7d is open. Hole A hole portion of the electrode 7, 8 is a diamond thin film containing impurities such as phosphorus and boron formed on the surface of the hole electrode 7 by microwave CVD, and the surface is hydrogen-terminated, and 9 is an ion. An extraction electrode for drawing out to the downstream side, 9d is an opening provided in the center of the extraction electrode 9, and 10 is made of Teflon (registered trademark) for electrical insulation between the plasma electrode 5 and the extraction electrode 9. It is an insulator. In this configuration, by attaching the aperture electrode 7 to the plasma electrode 5, the surface on the plasma chamber 1 side is covered with the diamond thin film 8 in the aperture 5 d of the plasma electrode 5.

  Next, an outline of an ion generation principle of the ion source according to the present invention will be described with reference to FIG.

  Plasma is generated by discharging the hydrogen gas introduced into the plasma chamber 1, and the plasma is negatively ionized by thermoelectrons emitted from the filament 2 that is heated by passing an electric current. At this time, the generated plasma is confined in the plasma chamber 1 by the permanent magnet 3.

  Further, the plasma is generated by a permanent sheet 4 transverse magnetic field (filter magnetic field) oriented in a direction perpendicular to the inner wall of the plasma chamber 1 emitted from the permanent magnet 4, and a high temperature electron region (shown in FIG. 1A) and a low temperature. It is separated into an electronic region (shown in FIG. 1B).

In this high temperature electron region, high energy (several tens of eV) electrons generated by arc discharge in the region where the filament 2 exists and hydrogen molecules (H ₂ ) in the high temperature electron region (T = several tens of eV) collide. As a result, excited hydrogen molecules (H ₂ ^* (ν)) are generated. In addition to this reaction, when ions such as H ₂ ⁺ and H ₃ ⁺ are neutralized on the wall surface, some of them may become H ₂ ^* (ν) and H. The above reactions are collectively shown in the following formula (4).
H ₂ ⁺ , H ₃ ⁺ + e → H ₂ ^* (ν), H —— Equation (4)

Then, in the space near the opening 7d of the plasma electrode 5 that draws out negative hydrogen ions in the low temperature electron region, electrons of low energy (1 eV or less) are excited hydrogen molecules (H ₂ ^* ) in the low temperature electron region (T ≦ 1 eV) ^. (Ν)) and hydrogen negative ions (H ⁻ ) are generated.

Furthermore, the electrons emitted from the diamond thin film 8 adhere to the excited hydrogen molecules (H ₂ ^* (ν)) in the low temperature electron region (T ≦ 1 eV), and hydrogen negative ions (H ⁻ ) are additionally generated.

The diamond thin film 8 described above is subjected to collisions of charged particles accelerated by discharge and accelerated negative hydrogen ions (H ⁻ ), but can be used for a long time because of high sputtering resistance and small damage. Further, since there is no contamination with vapor such as cesium, a clean state can be maintained as an ion source including the plasma chamber 1 and the like.

Further, as described above, since an increase in negative hydrogen ions (H ⁻ ) due to electrons supplied from the diamond thin film 8 can be expected, the amount of ions generated can be increased.

  As described above, by adopting the ion source according to the present invention, it is possible to provide an ion source capable of increasing the amount of ions generated while maintaining a clean state and extending the lifetime. .

(Second Embodiment)
2A and 2B are schematic diagrams for explaining a second embodiment of the ion source according to the present invention, in which FIG. 2A is an overall view, and FIG. 2B is an enlarged view of a portion C for explaining a charge electrode. FIG. 3 is an enlarged perspective view of the charge electrode. In the present embodiment, the same components as those shown in FIG. 1 are denoted by the same reference numerals, detailed description thereof is omitted, and only different portions are described in detail.

In FIGS. 2 and 3, 11 is a charge electrode spaced apart above the opening 5d of the plasma electrode 5, 11a is a connection line made of an oxygen-free copper thin wire, 11b, 11c, 11d, and 11e are none. It is a planar part having a large number of meshes formed from oxygen copper thin wires, and the planar parts 11b, 11c, 11d, and 11e composed of four layers are connected to each other by oxygen-free copper thin wires to form a skeleton, It is attached to the connecting wire 11a. Reference numeral 12 denotes a diamond thin film containing impurities such as phosphorus and boron formed on the surface of the charge electrode 11 by microwave CVD or the like and having a hydrogen-terminated surface. The connection lines 11a are connected to the current introduction terminals 13, respectively. Then, a negative voltage of about 1 V or less is applied to the charge electrode 11 from the charge power supply 14 with respect to the aperture electrode 7. Thereby, the electron emission efficiency from the surface of the diamond thin film 12 is increased, and the generated hydrogen negative ions (H ⁻ ) can be more efficiently extracted toward the opening 7d.

Further, the low-energy electrons existing in this region are also accelerated by the electric field between the charge electrode 11 and the aperture electrode 7, but the energy of the electrons is within about 1 eV because the applied voltage is about 1V. Therefore, although the generated hydrogen negative ions (H ⁻ ) have a property of easily disappearing in collision with high energy electrons, they cannot be a factor that hinders the generation of hydrogen negative ions (H ⁻ ) for the above reasons. . When the ion source is operated, the amount of hydrogen negative ions (H ⁻ ) generated may be measured while changing the output voltage of the charge power supply 14 around 1 V and set so that the amount generated is maximized.

Further, the charge electrode 11 has a skeleton having a mesh structure as shown in FIG. 3 so that low energy electrons, excited hydrogen molecules (H ₂ ^* (ν)), and generated hydrogen negative ions (H ⁻ ) It can prevent disappearance. This suppresses the rate at which electrons in the low-temperature electron region, excited hydrogen molecules (H ₂ ^* (ν)), and generated negative hydrogen ions (H ⁻ ) contact the charge electrode 11 and disappear, and the surface of the charge electrode 11 It becomes easy to draw out the hydrogen negative ions (H ⁻ ) generated in step 1 to the opening 9d. For the above reason, it is preferable that the charge electrode 11 uses a thin wire of about 0.1 mm, for example, and the skeleton having the mesh structure has a lattice shape so that spaces are formed at equal intervals.

  As described above, the configuration of the present embodiment (see FIG. 2 and FIG. 3) can reduce the amount of ions generated compared to the configuration described in the first embodiment (see FIG. 1). Further increase is possible.

  In the first and second embodiments, the diameter increases from the surface on the side having the aperture 7d to the upper side so that diamond can be easily formed on the inner surface of the aperture electrode 7. Although the example which has the taper part 7c was demonstrated, it is not necessarily limited to this. For example, a configuration without a tapered portion is also possible. In the first and second embodiments, the example in which the aperture electrode 7 on which the diamond thin film 8 is formed is configured separately so as to be removable from the plasma electrode 5 has been described. However, the present invention is not necessarily limited thereto. It is not a thing. That is, the diamond thin film 8 may be provided directly on the tapered portion whose diameter increases upward from the surface of the plasma electrode 5 where the aperture 5d is present. In the ion source according to the present invention, these technical ideas are generically defined by the phrase “having a diamond thin film on at least a part of the surface of the plasma electrode on the opening portion side”. It is preferable that at least the surface on the plasma chamber 1 side around the aperture 5d is covered with the diamond thin film 8 on the aperture 5d side of the plasma electrode 5.

  In the first and second embodiments, only the diamond that electrons are spontaneously emitted into the vacuum once the electrons are excited in the conduction band has a “conduction band at a position higher than the vacuum level”. In order to make use of the feature of “so-called negative electron affinity”, an example using the diamond thin film 8 containing impurities such as phosphorus and boron and having a hydrogen-terminated surface has been described. However, the present invention is not necessarily limited thereto. It is not a thing. However, n-type diamond doped with phosphorus has a particularly abundant electron in the conduction band, a work function of 1.4 eV, and a negative electron affinity. Therefore, the ion source according to the present invention is more preferable.

  In the first and second embodiments, the example in which hydrogen gas is used as the gas introduced into the plasma chamber 1 has been described. However, the present invention is not necessarily limited thereto.

  In the first and second embodiments, an example in which oxygen-free copper is used for the plasma electrode 5, the aperture electrode 7, and the charge electrode 11 has been described. However, the present invention is not necessarily limited thereto, and the plasma chamber 1 Any material may be used as long as the material can maintain a clean state, can extend the life of the filament 2, and does not hinder the increase in the amount of ions generated.

  In the first and second embodiments, the example of the microwave CVD is described as the film formation of the diamond thin film. However, the present invention is not necessarily limited to this, and various film formation methods can be employed. .

DESCRIPTION OF SYMBOLS 1 Plasma chamber 2 Filament 3, 4 Permanent magnet 5 Plasma electrode 5d Opening part 6, 10 Insulator 7 Opening part electrode 7a Flange part 7b Mounting hole 7c Taper part 7d, 9d Opening part 8, 12 Diamond thin film 9 Lead electrode DESCRIPTION OF SYMBOLS 11 Charge electrode 11a Connection line 11b, 11c, 11d, 11e Planar part which has many meshes 13 Current introduction terminal 14 Charge power supply

Claims (5)

A plasma chamber for generating plasma by discharging the introduced gas, and ionizing the plasma by thermoelectrons emitted from a filament heated by passing an electric current;
Magnetic field generating means for generating a cusp magnetic field provided around the plasma chamber in order to confine the plasma in the plasma chamber;
In order to separate the plasma into a high temperature electron region and a low temperature electron region, a magnetic field generation unit for generating a filter magnetic field provided downstream from the magnetic field generation unit for generating the cusp magnetic field,
A plasma electrode provided on the downstream side of the magnetic field generating means for generating the filter magnetic field, and having an aperture for allowing ions to pass therethrough,
An ion source comprising a diamond thin film on at least a part of the surface of the plasma electrode on the aperture side.   2. The ion source according to claim 1, wherein the diamond thin film contains at least one of phosphorus and boron as an impurity.   3. The ion source according to claim 1, wherein the surface of the diamond thin film is hydrogen-terminated.   A charge electrode is provided that is spaced apart above the opening of the plasma electrode, is electrically insulated from the plasma chamber and the plasma electrode, and has a porosity covered with a diamond thin film. The ion source according to any one of claims 1 to 3.   The ion source according to claim 4, wherein the skeleton of the charge electrode is formed of a thin metal wire and has a large number of meshes.

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