Negative ion source and negative ion generation method

Abstract

To provide a negative ion source and a negative ion generation method that can achieve high negative ion generation efficiency.SOLUTION: A housing 108 includes: an introduction port 106 that introduces a sample; a plasma generation region 113 that communicates with the introduction port 106 and generates plasma by electric discharge; a negative ion generation region 114 in which particles dissociated or excited by the reaction between the generated plasma and the sample are negatively ionized; and a draw-out port 107 that communicates with the negative ion generation region 114 and draws out the generated negative ions to the outside. The negative ion generation region 114 is filled with a thermoelectron emitting substance 116 that generates thermoelectron by high-frequency heating.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a negative ion source and a negative ion generation method, and more specifically, to a negative ion source and a negative ion generation method using a hot electron emitting material that generates hot electrons by heating.

Background Art

[0002] Conventionally, negative ion sources for generating negative ion beams have been used in medical radiation fields such as ion beam analysis, accelerator mass spectrometry, positron emission tomography (PET), and industrial fields such as the manufacturing process of semiconductor integrated circuits. In particular, negative ions can maintain a high charge conversion efficiency from negative charge to neutral or positive charge at high energies compared to positive ions, and thus research and development have been increasingly promoted in recent years.

[0003] Examples of conventional technologies related to such negative ion sources include the following. In Non-Patent Document 1, a filament-driven negative ion source is described in which a current is passed through a sintered lanthanum hexaboride (LaB 6 ) in a filament shape for heating, and plasma is generated by arc discharge using this as a cathode to create negative ions.

[0004] In Patent Document 1, as a negative ion source assumed to be used in a neutral beam injection device, which is one of the heating devices for fusion plasma, it is described that a ceramic with a low work function such as LaB 6 is used as a charge converter for negative ion generation.

[0005] Patent Document 2 describes an ion source in which microwaves and gas are supplied into a plasma chamber in a magnetic field, the gas is ionized in the plasma chamber, and the ions are led out to the outside. The ion source includes a gas introduction pipe provided in communication with the plasma chamber for introducing gas, and a hot electron generating material provided at the tip of the gas introduction pipe.

[0006] In addition, Non-Patent Document 2 describes a negative ion generation mechanism in which a plasma generation region and a negative ion generation region are spatially separated. In this generation mechanism, first, positive ions are generated by a plasma with a relatively high density of several eV by electron cyclotron resonance, and these positive ions are accelerated and guided into a gas cell in a beam line, and negative ions are generated by charge conversion due to collisions with gas atoms in the gas cell.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0008]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the negative ion generation methods described in Patent Documents 1 and 2 and Non-Patent Document 1, negative ions are generated and extracted in a plasma with an electron temperature of several eV. However, considering that the electron affinity of the substances contained in the ionized gas is around 1 eV, electron detachment occurs due to collisions between the generated negative ions and plasma electrons, and the loss of negative ions cannot be ignored.

[0010] In order to suppress this, as in Non-Patent Document 2, it is conceivable to generate positive ions in the plasma region and then guide the positive ions into a gas cell different from this plasma region to perform negative ionization.

[0011] However, in Non-Patent Document 2, in addition to the low negative ion generation efficiency of about less than 10% when using a gas cell, another problem occurs in that the angular dispersion of negative ions increases due to the collision between negative ions and gas atoms in the gas cell, and the extraction efficiency of the generated negative ions also decreases.

[0012] Furthermore, in the prior art, for example, as in Non-Patent Document 1, a thermionic emission material of LaB 6 was used in the form of a filament. This is because the thermionic emission material is heated by passing an electric current through the filament-shaped thermionic emission material. Therefore, the shape of the thermionic emission material is limited to a shape in which a conductive state is ensured like an electric wire that forms part of an electric circuit. In order to extract a large amount of thermions to increase the negative ion generation efficiency, it is preferable to increase the surface area of the thermionic emission material. However, in addition to the difficulty of increasing the surface area by making the thermionic emission material thin and long or thin and long while maintaining the filament shape, there is also a risk that the filament may break due to constantly flowing an electric current.

[0013] As described above, in the conventional negative ion source, there has been a problem that sufficient negative ion generation efficiency cannot be obtained due to various factors.

[0014] The present invention has been made in view of the above problems, and an object thereof is to provide a negative ion source and a negative ion generation method capable of suppressing the loss of generated negative ions, ensuring a sufficient negative ion generation area, and as a result, achieving a high negative ion generation efficiency.

Means for Solving the Problems

[0015] To solve the above problems, a negative ion source according to an embodiment of the present invention includes an inlet through which a sample is introduced, a plasma generation region that communicates with the inlet and in which plasma is generated by discharge, a negative ion generation region in which particles dissociated or excited by the reaction between the generated plasma and the sample are negatively ionized, and an outlet that communicates with the negative ion generation region and through which the generated negative ions are drawn out to the outside, and has a housing. The negative ion generation region is filled with a hot electron emitting substance capable of generating hot electrons by heating.

[0016] Further, a negative ion generation method according to an embodiment of the present invention uses the above negative ion source, and includes a step of irradiating electromagnetic waves on the plasma generation region and the negative ion generation region, introducing a sample from the inlet into the plasma generation region, and passing the generated dissociated particles and excited particles through the negative ion generation region.

Advantages of the Invention

[0017] According to the present invention, the generation of hot electrons in the hot electron emitting substance is significantly promoted compared to the prior art, and the loss of the generated negative ions is also suppressed. Therefore, it is possible to greatly improve the generation efficiency of negative ions.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0019] Hereinafter, embodiments will be described with reference to the drawings.

[0020] FIG. 1 is a schematic diagram showing the overall configuration of a negative ion beam generation apparatus 1 to which a negative ion source according to an embodiment of the present invention is applied. In this embodiment, a gas sample of carbon dioxide used in the field of mass spectrometry is used as a sample, and a mass spectrum of ions containing carbon ( 12 C, 13 C and 12 CH, etc.) will be described as an example. Note that, as the sample, in addition to the gas sample of carbon dioxide, a gas sample of hydrogen useful in the fields of ion beam analysis and medical radiation, as well as various other gas samples, can also be used. Further, the present invention can also be applied to liquid samples and solid samples by making them into fine particles using, for example, spraying, sputtering, laser ablation, etc.

[0021] The negative ion beam generation apparatus 1 shown in FIG. 1 includes a negative ion source 100 that generates negative ions, an extraction unit 200 that extracts the negative ions as a negative ion beam, and a measurement unit 300 that measures the mass spectrum of ions containing carbon in the negative ion beam. Further, a carbon dioxide gas cylinder 401 for supplying carbon dioxide gas to the negative ion source 100 is connected to a carbon dioxide gas supply source 403 via a carbon dioxide gas introduction pipe 402, and carbon dioxide gas is supplied from the carbon dioxide gas supply source 403 into the negative ion source 100. The negative ion beam generation apparatus 1 is placed in a cavity 404. The horizontal dimension L1 of the cavity 404 is, for example, 850 mm.

[0022] The negative ion source 100 includes a negative ion source main part 101 where negative ions are generated, and an electromagnetic wave irradiation part 102 that irradiates the negative ion source main part 101 with electromagnetic waves. In the negative ion source main part 101, plasma is generated by the electromagnetic waves irradiated from the electromagnetic wave irradiation part 102, and negative ions are generated by thermoelectrons attaching to the particles generated by the reaction between this plasma and carbon dioxide gas. The details thereof will be described later.

[0023] The electromagnetic wave irradiation unit 102 includes an electromagnetic wave oscillator 103 that generates electromagnetic waves, an electromagnetic wave waveguide unit 104 that propagates the generated electromagnetic waves to the main part of the negative ion source, and an electromagnetic wave cavity 105 that serves to confine the electromagnetic waves irradiated on the housing.

[0024] As the electromagnetic wave oscillator 103, for example, an oscillator having a magnetron can be adopted. In this embodiment, in order to efficiently heat the thermionic emission material, electromagnetic waves having a frequency in the microwave frequency band are used as the electromagnetic waves. The frequency of the microwave is, for example, 2.45 GHz. However, the frequency is not limited to this, and any frequency band that can efficiently heat the thermionic emission material may be used. This frequency band is closely related to the particle diameter of the thermionic emission material described later, and based on this, it is generally 10 MHz to 10 GHz. This frequency band corresponds to the short wave band, the ultra short wave band, and the microwave band.

[0025] The electromagnetic wave waveguide unit 104 is usually composed of a power monitor, a tuner, and a circulator. The material of the electromagnetic wave waveguide unit 104 is not limited in terms of material and shape as long as it can propagate the electromagnetic waves without attenuation. For example, it can be composed of a cylindrical member made of an aluminum alloy.

[0026] The electromagnetic wave cavity 105 is not limited in terms of material and shape as long as it can confine the microwaves. However, in order to hold the electromagnetic waves in the cavity and suppress the attenuation of the electromagnetic waves, it is preferably made of a material and shape that can efficiently reflect the electromagnetic waves. For example, using a rectangular parallelepiped housing with a high conductivity such as aluminum is a typical method. Also, in order to efficiently confine the electromagnetic waves in the electromagnetic wave cavity 105, it is preferably shaped so that the electromagnetic waves are likely to resonate. For example, by providing a movable plunger 1051, a part of the wall surface constituting the electromagnetic wave cavity 105 can be made movable, and it is preferable to make the volume and shape in the electromagnetic wave cavity 105 adjustable.

[0027] The extraction section 200 accelerates the negative ions generated in the negative ion source 100 by an electric field, extracts them as a negative ion beam, and emits them to the measurement section 300. The configuration of the extraction section 200 will also be described later.

[0028] The measurement section 300 includes a magnetic field deflector 301, a chamber 303, a Faraday cup 302 disposed in the chamber, and a vacuum evacuation system 304 for evacuating the inside of the chamber.

[0029] The processing performed by the measurement section 300 will be described. When the negative ion beam extracted by the extraction section 200 passes through the magnetic field deflector 301 with a magnetic field applied, it is bent in different directions according to the radius of curvature corresponding to the mass (m) - to - charge (q) ratio (m / q) of the ions. Then, in the chamber 303 evacuated by the vacuum evacuation system 304, their currents are measured by the Faraday cup 302 installed based on the assumed trajectory of the desired ions. By measuring the current of the Faraday cup 302 while changing the magnetic field strength, the mass spectrum is measured.

[0030] Next, the negative ion source main part 101 and the extraction section 200 according to the embodiment of the present invention will be described in detail with reference to FIGS. 2 and 3. In the following description, it is assumed that a rectangular waveguide type is adopted as the electromagnetic wave cavity 105. The electromagnetic wave propagation mode in the electromagnetic wave cavity 105 is the Transverse Electric (TE) mode. The propagation direction is the z - direction, the long - side direction of the rectangular cross - section of the waveguide is the x - direction, and the short - side direction is the y - direction. Further, when the number of electric field antinodes is 1 in the x - direction, 0 in the y - direction, and 2 in the z - direction, the resonance mode is TE 102 is defined as such. Also, the frequency of the electromagnetic wave is 2.45 GHz. The sample gas is a carbon dioxide gas sample. The thermionic emission material is lanthanum hexaboride (LaB 6 )

[0031] [Configuration of the negative ion source main part 101] As shown in Fig. 2, the main part 101 of the negative ion source is fixed in the chamber 303 by a fixing member 120, and includes a housing 108 having an inlet 106 through which carbon dioxide is introduced and an outlet 107 through which negative ions are drawn out to the outside, and a generation electrode 109 disposed at the outlet 107.

[0032] Inside the housing 108, further, by a partition wall 112 and a separation mesh 110, it is divided into a plasma generation region 113 that communicates with the inlet 106 through which the sample gas is introduced and where plasma is generated, and a negative ion generation region 114 that communicates with the outlet 107 through which the generated negative ions are drawn out and where negative ions are generated.

[0033] The housing 108 having the plasma generation region 113 and the negative ion generation region 114 inside is preferably made of a dielectric with weak electromagnetic wave absorption and a high melting point so that an excitation electromagnetic field can be generated inside by electromagnetic wave irradiation. For example, a quartz tube can be adopted. In this embodiment, the length L2 inside the electromagnetic wave cavity 105 of the housing 108 is 54.6 mm.

[0034] Fig. 3 is a schematic diagram showing the positional relationship between the housing 108 and the electromagnetic wave cavity 105, and the electromagnetic field generated inside the electromagnetic wave cavity 105 at a certain point in time. The axial direction of the housing 108 is the y direction (parallel to the short side of the rectangular cross-section of the waveguide).

[0035] Also, in this embodiment, the position of the housing 108 in the x-z plane of the electromagnetic wave cavity 105 is, in the x direction, at half (a / 2) of the length of the long side as a, and in the z direction, at half (λ / 2) of the length L3 in the z direction of the electromagnetic wave cavity 105 defined by the electromagnetic wave resonance mode TE 102 (one wavelength λ of the propagation wave). When the microwave frequency is 2.45 GHz as in this embodiment, the propagation wavelength λ is, for example, about 15 cm. This position is where the intensity of the alternating magnetic field of the microwave is maximum. Therefore, in this embodiment, as will be described later, the thermionic emission material is mainly heated by the microwave magnetic field. Lanthanum hexaboride (LaB 6) is a conductive material (with a conductivity similar to that of stainless steel). Therefore, it is preferable to use a microwave magnetic field for efficient heating.

[0036] Cavity resonance mode TE 102 For the electromagnetic field, when viewed from the center plane (x = a / 2) of the waveguide, an electric field EF is formed in the ±y directions, and a magnetic field MF is formed in the ±x directions. Note that the illustrated magnetic field MF shows the state where it reaches its maximum 1 / 4 cycle after the electric field EF reaches its maximum.

[0037] Also, the shape and dimensions of the housing 108 are not particularly limited. However, when a quartz tube is adopted as the housing 108, if the volume of the quartz tube is increased, the energy of the electromagnetic wave will be absorbed by the quartz tube accordingly, and the plasma generation efficiency will deteriorate. Therefore, it is preferable to design considering various factors such as the energy of the electromagnetic wave used, the type and amount of the hot electron emitting material, etc.

[0038] Referring to FIG. 2 again. The partition wall 112 that divides the plasma generation region 113 and the negative ion generation region 114 has, near its center, an opening 115 through which particles dissociated and excited by the reaction of carbon dioxide introduced into the plasma generation region with the plasma, such as carbon atoms and oxygen atoms, can pass.

[0039] The opening 115 has an isolation mesh 110 with voids smaller than the average particle diameter of the hot electron emitting material described later in order to prevent the hot electron emitting material 116 filled in the negative ion generation region of the housing 108 from moving from the negative ion generation region to the plasma generation region through the opening 115.

[0040] The size of the opening 115 can be appropriately designed as long as the particles dissociated or excited by the plasma can pass through as described above. However, when the size of the opening 115 increases and the edge of the opening 115 approaches the tube wall, the number of particles of the sample gas passing through the vicinity of the edge of the opening 115 and entering the negative ion generation region increases. Then, these particles are less likely to pass through the relatively high-temperature and high-density plasma in the center of the tube and are less likely to be dissociated or excited. Since the particles that are not dissociated or excited do not contribute to negative ion generation, it is necessary to suppress their number. From this perspective, it is preferable that the size of the opening 115 is at most about half of the inner diameter of the tube.

[0041] Also, as the material of the isolation mesh 110 provided in contact with the opening 115, for example, high-melting-point ceramics are possible. However, as will be described later, in this embodiment, since the particle diameter of the hot electron emitting material is about 20 μm to 30 μm, it is preferable to use tungsten, a high-melting-point metal, as the isolation mesh 110 that can generate a large number of voids smaller than this particle diameter.

[0042] In this embodiment, since the quartz tube is inserted in the y direction, the surface of the isolation mesh 110 is perpendicular to the y direction (that is, parallel to the x-z plane). The position of the housing 108 in the x-z plane is approximately at the center (a / 2, λ / 2) on the x-z plane of the electromagnetic wave cavity 105 as described above. The direction of the magnetic field at this position is parallel to the x-z plane, and most of it is the x component. Therefore, the magnitude of the magnetic field passing through the isolation mesh 110 (that is, the product of the cross-sectional area of the isolation mesh 110 and the magnetic field) can be minimized, and the absorption of microwaves by the isolation mesh 110 can be most suppressed. This arrangement is preferable for suppressing the heating of the isolation mesh 110 and enhancing the controllability of plasma generation and the controllability of heating of the hot electron emitting material. In this embodiment, the isolation mesh 110 is provided on the surface of the partition wall 112 on the negative ion generation region 114 side, but it may also be provided on the surface on the plasma generation region 113 side.

[0043] The partition wall 112 is made of a material that does not cause physical / chemical interference with the plasma, such as quartz of the same material as the housing 108, but is preferably made of the same material as the thermoelectron-emitting substance from the viewpoint of securing a larger negative ion generation area.

[0044] The partition wall 112 is also fixed by a tubular member 117 inserted into the plasma generation region 113. The tubular member 117 has a diameter smaller than the inner diameter of the housing 108, and the end on the partition wall 112 side communicates with at least the opening 115 of the partition wall 112. Note that the tubular member 117 is preferably formed of the same material as the housing 108. In this case, the housing 108 has a double structure in the plasma generation region 113 by the tubular member 117.

[0045] The plasma generation region 113 is a space where plasma is generated by heating the gas in the region with electromagnetic waves. In this embodiment, the housing 108 is sealed against the outside air, and therefore the generated plasma is plasma in a vacuum.

[0046] The negative ion generation region 114 is a region where particles dissociated and excited in the plasma generation region 113 are given electrons to generate negative ions. The mechanism of generating negative ions will be described later.

[0047] And in this embodiment, the negative ion generation region 114 is filled with an aggregate of small pieces of lanthanum hexaboride (LaB 6 ). As the thermoelectron-emitting substance 116, it is required that the work function indicating the ease of electron emission is low, it is chemically stable, it has a high melting point, it has a high conductivity, and so on. The chemical stability and melting point of LaB 6 are about the same as those of tungsten, which has been conventionally used frequently. However, compared with the work function of tungsten of 4.5 eV, the work function of LaB 6 is far superior at 2.5 eV and is suitable as a thermoelectron-emitting substance.

[0048] In addition, cesium was often used as an electron source in the past. However, cesium is highly reactive and has problems with chemical stability, such as being prone to spontaneous ignition. In addition to lanthanum hexaboride (LaB 6 ), crystals of 12CaO·7Al 2 O 3 and C12A7 electride, which is an isotypic compound having the same crystal structure as this, can be adopted. Also, if there is a material suitable as a thermionic emission substance such as having a low work function, it can be used as a material to replace LaB 6 or C12A7 electride.

[0049] In this embodiment, the flaky substance filled in the negative ion generation region 114 is granular. Regarding the average particle diameter of the flaky substance, it is preferably about 0.01 mm to 10.0 mm, for example, but it can be designed in consideration of various factors such as the size of the housing 108 and the reactivity with dissociation / excitation particles. Note that the flakes are preferably granular, but the shape is not limited to granular, and any shape that increases the contact area with the dissociated / excited particles may be used. In addition to the granular shape, shapes such as powder, plate, and cylinder can be adopted. Based on the fact that the low work function substance containing lanthanum hexaboride (LaB 6 ) in this embodiment is a conductor, based on the findings described in Non-Patent Document 3, a guideline for the particle diameter can be obtained as follows. In the case of a conductor, the particle diameter at which the heating efficiency is highest is about twice the penetration length of electromagnetic waves (also called the skin depth) into the conductor in the microwave frequency region. The penetration length of electromagnetic waves (denoted as δ) is expressed as δ = [2 / (σωμ)] 0.5 . Here, σ is the conductivity, μ is the magnetic permeability of vacuum (4π×10 -7 H / m), ω is the angular frequency of the electromagnetic wave, and using the frequency f, ω = 2πf. In this embodiment, when the conductivity σ of LaB 6 is 8.3×10 5 (Ωm) -1 and the microwave frequency f = 2.45 GHz are used, δ = 11 μm. Therefore, 22 μm (0.022 mm), which is twice that, is obtained as a guideline for the particle diameter. Based on this result, in this embodiment, the particle diameter of LaB 6 is such that for LaB6 Taking into account the ease of acquisition, it is set to 20 μm to 30 μm (0.02 mm to 0.03 mm), which can be said to be the most preferable average particle diameter. The measurement of the average particle diameter shall be based on observation with a scanning electron microscope.

[0050] Also, a converging member 118 is disposed on the outlet side of the negative ion generation region 114. The converging member 118 is for accurately extracting the negative ions generated in the negative ion generation region 114 to the outside. The end on the outlet 107 side is at least in communication with the outlet 107, and the other end has a diameter substantially the same as the diameter of the negative ion generation region 114. The converging member 118 guides the negative ions in the negative ion generation region 114 to move toward the outlet 107. Note that, the smaller the diameter of the outlet 107, the more accurately the extracted negative ion beam can be extracted. However, the narrower it is, the lower the efficiency of extracting the negative ion beam. Therefore, it is preferably set to about 1 to 2 mm.

[0051] Also, in order to suppress the thermionic emission material 116 filled in the negative ion generation region of the housing 108 from passing through the outlet 107 and moving from the negative ion generation region to the extraction part 200, an electrode mesh 111 having voids smaller than the aforementioned average particle diameter of the thermionic emission material is provided at the outlet 107.

[0052] The electrode mesh 111 is preferably composed of a conductor in order to serve as an extraction electrode for negative ions. The potential of the electrode mesh 111 is wired so as to be adjustable from the atmosphere side outside the chamber 303 (not shown). In this embodiment, the electrode mesh 111 is in contact with the outlet 107 structurally and is electrically connected to the electromagnetic wave cavity 105. Therefore, the potential of the electrode mesh 111 is the same as the potential of the electromagnetic wave cavity 105.

[0053] Since the electrode mesh 111 is in contact with the thermionic emission material heated to a high temperature, a high melting point metal such as tungsten is suitable.

[0054] [Configuration of the extraction part 200] The extraction section 200 includes an extraction electrode 201 facing the generation electrode 109, a permanent magnet 202 disposed so as to surround the extraction electrode 201, and an emission port 203 from which the negative ion beam 406 is emitted. The extraction electrode 201 is electrically insulated from the chamber 303, and a positive voltage is applied to the extraction electrode 201 with respect to the generation electrode 109. The permanent magnet 202 is used to remove electrons contained in the negative ion beam 406.

[0055] [Principle of Negative Ion Beam Generation] The principle of generating a negative ion beam in this embodiment will be described. First, plasma 119 is generated in the plasma generation region 113 as follows. The electromagnetic wave irradiated from the electromagnetic wave irradiation unit 102 and propagated in the electromagnetic wave waveguide unit 104 is confined in the electromagnetic wave cavity 105 to form a standing wave. By arranging the housing 108 inside this as described above, an excited electromagnetic field is generated inside the housing 108. Due to this excited electromagnetic field, a discharge occurs and ionization occurs in the gas inside the housing 108. In this embodiment, since the housing 108 is arranged at the position where the magnetic field is maximum in the electromagnetic wave cavity, plasma 119 at several tens of thousands of degrees Celsius is generated mainly by the induced electromotive force due to the fluctuating magnetic field.

[0056] When carbon dioxide gas 405 is introduced into the plasma generation region 113 in the state where the plasma 119 is generated, the carbon dioxide molecules dissociate into carbon atoms and oxygen atoms, or into carbon monoxide molecules and oxygen atoms. Or an excited state of the carbon dioxide molecules and carbon monoxide molecules is generated. These dissociated or excited particles are then introduced into the negative ion generation region 114 through the opening 115 of the partition wall 112.

[0057] Also in the negative ion generation region 114, similar to the plasma generation region 113, an excitation electromagnetic field is formed by the electromagnetic wave propagated from the electromagnetic wave irradiation unit 102. By this excitation electromagnetic field, the hot electron emitting material 116 is heated to about 1200 °C, and the binding of the electrons of the hot electron emitting material is loosened, or the energy of the electrons is released beyond the vacuum level. That is, the hot electron emitting material generates and emits hot electrons by heating. Incidentally, electrons in these states are sometimes called thermal electrons. The generated and emitted hot electrons are rich in reactivity and become electrons that attach to dissociated / excited particles in the negative ion generation region 114. In this embodiment, the heating of the hot electron emitting material 116 is based on eddy current caused by a fluctuating magnetic field. Joule heating is generated by this eddy current. According to Non-Patent Document 3, the electromagnetic wave penetration length is several μm to several tens of μm as described above for individual conductive particles, but it is known that it reaches several cm on the aggregate of the particles, that is, on the macroscale. Therefore, in the case of this embodiment, the macroscale size of the hot electron emitting material 116 is limited to about several cm at most.

[0058] Then, when the dissociated / excited atoms introduced from the plasma generation region 113 as described above pass through the negative ion generation region 114, electrons that are free on or near the surface of the hot electron emitting material 116 attach to the dissociated / excited atoms and are negatively ionized. Further, with respect to the excited molecules introduced from the plasma generation region 113, dissociation occurs and negative ions are generated when hot electrons on or near the surface of the hot electron emitting material 116 attach to the excited molecules.

[0059] The generated negative ions become a negative ion beam 406 to which velocity and energy are imparted by a draw-out electrode 201 to which a positive voltage is applied with respect to a generation electrode 109 disposed at the draw-out port 107. Then, after electrons that interfere with the measurement are removed by the permanent magnet 202, they are emitted to the measurement unit 300 through the discharge port 203.

[0060] In addition, carbon-14 in mass spectrometry ( 14C) From the perspective of measurement, the plasma generated within the plasma generation region 113 14 also contributes to dissociating and removing molecules of approximately the same mass that interfere with the measurement of C ( 12 CH 2 and 13 CH). In addition, it also helps in creating negative ions through the dissociation process of excited electron-attached molecules. The reason for measuring negative ions in this example is 14 to separate C from its isotope nitrogen-14 ( 14 N) ( 14 Since N does not ionize negatively, 14 it can be separated from C).

[0061] The embodiments of the present invention have been described above. As in this embodiment, by filling the negative ion generation region 114 with an aggregate of small pieces of the hot electron emitting material 116, each of the small pieces of the hot electron emitting material 116 is heated by the excitation electromagnetic field, creating or emitting hot electrons that are weakly bound to the hot electron emitting material. Therefore, compared to the case of using a filamentous hot electron emitting material as in the prior art, the area of the hot electron emitting material that is exposed increases, and accordingly, the number of hot electrons that can attach to particles significantly increases. With this increase in hot electrons, the negative ion ionization efficiency, in which the dissociated and excited particles generated in the plasma generation region 113 attach to the hot electrons and are ionized negatively within the negative ion generation region 114, is significantly improved.

[0062] Furthermore, in this embodiment, the plasma generation region 113 for dissociating and exciting carbon dioxide by high-temperature plasma and the negative ion generation region 114 for negatively ionizing the dissociated and excited particles are separated. As a result, it is possible to suppress the generated negative ions from being ionized by the high-temperature plasma and the loss of negative ions. Note that, in the negative ion generation region 114 as well, a relatively low-temperature (about 1000°C to 2000°C) plasma that is assumed from the thermally equilibrium state that can be established between the hot electron emitting material 116 and the plasma may be generated. However, since the electron temperature of this low-temperature plasma is low, it is considered that the generated negative ions are hardly ionized. In this embodiment, high-frequency discharge using an electromagnetic wave cavity is used as the plasma generation method. However, various other methods such as DC discharge and laser ablation are also applicable.

[0063] Also, according to the configuration of the present invention, there is no need to use cesium as an electron source used in conventional negative ion sources due to the hot electron emitting material. Since cesium is highly reactive and spontaneously ignites in air, the present invention is also useful from the viewpoint of safety.

[0064] In the above-described embodiment, an example using carbon dioxide as a sample has been described. However, in addition to carbon dioxide, boron, phosphorus, arsenic, etc. can also be adopted as samples. These are used in ion implantation in the semiconductor manufacturing process, and it can be expected to suppress damage to the semiconductor surface compared to the positive ions usually used.

[0065] FIG. 4 shows a negative ion beam generation apparatus 1 described with reference to FIG. 1, in which the electromagnetic wave irradiation unit 102 has a different configuration. The electromagnetic wave irradiation unit 102' in FIG. 4 includes a first electromagnetic wave oscillator 103a that generates a first electromagnetic wave, a first waveguide unit 104a that propagates the first electromagnetic wave to the plasma generation region 113, and a first electromagnetic wave cavity 105a that confines the first electromagnetic wave to the plasma generation region 113, and a second electromagnetic wave oscillator 103b that generates a second electromagnetic wave, a second waveguide unit 104b that propagates the second electromagnetic wave to the negative ion generation region 114, and a second electromagnetic wave cavity 105b that confines the second electromagnetic wave to the negative ion generation region. This is different from the electromagnetic wave irradiation unit 102 described in FIG. 1. Note that the other configurations are the same as those in FIG. 1, and the description thereof is omitted. In addition, plungers 1051a and 1051b for adjusting the volume inside the cavities are provided in the first and second electromagnetic wave cavities 105a and 105b, respectively.

[0066] The above configuration means that the electromagnetic waves irradiated to the plasma generation region 113 and the electromagnetic waves irradiated to the negative ion generation region 114 can be controlled separately. For example, by setting the frequencies of the electromagnetic waves to be the same (e.g., 2.45 GHz) and varying the outputs (e.g., the maximum output of the first electromagnetic wave oscillator is 1 kW and the maximum output of the second electromagnetic wave oscillator is 1.5 kW), it becomes easy to adjust the input power to be optimal for dissociation / excitation in the plasma generation region 113 and for the generation of hot electrons and electron attachment in the negative ion generation region 114.

[0067] The embodiments of the present invention have been described above with reference to the drawings. The technical scope of the present invention is not limited to the scope described in the above embodiments, and various modifications are included without departing from the main features of the present invention. Therefore, the above-described embodiments are merely illustrative and should not be construed in a limiting sense. In addition, it is possible to add, delete, or replace a part of the configuration of each embodiment with other configurations, and all of them are within the scope of the present invention.

Description of Reference Numerals

[0068] 100 Negative ion source, 101 Main part of negative ion source, 102 Electromagnetic wave irradiation part, 103, 103a, 103b Electromagnetic wave oscillator, 104 Electromagnetic wave waveguide part, 104a, 104b Electromagnetic wave waveguide part, 105, 105a, 105b Electromagnetic wave cavity, 106 Inlet, 107 Outlet, 108 Quartz tube (housing), 112 Partition wall, 113 Plasma generation region, 114 Negative ion generation region, 115 Opening, 116 Thermoelectron emission material, 117 Tubular member, 118 Converging member, 119 Plasma, 200 Extraction part, 201 Extraction electrode, 202 Permanent magnet, 203 Discharge port, 300 Measurement part, 301 Magnetic field deflector, 302 Faraday cup, 303 Chamber, 304 Vacuum exhaust system, 401 Carbon dioxide gas cylinder, 402 Carbon dioxide gas introduction pipe, 403 Carbon dioxide gas supply source, 404 Cavity, 405 Carbon dioxide gas (sample gas), 406 Negative ion beam

Claims

1. A housing having an inlet through which a sample is introduced, a plasma generation region communicating with the inlet where plasma is generated by discharge, a negative ion generation region where particles dissociated or excited by the reaction of the generated plasma and the sample are negatively ionized, and an outlet communicating with the negative ion generation region through which the generated negative ions are drawn out to the outside, wherein the negative ion generation region is filled with a hot electron emitting material that generates hot electrons by heating. A negative ion source characterized by the above.

2. The negative ion source according to claim 1, further comprising an electromagnetic wave irradiation unit that generates electromagnetic waves and irradiates the housing with the electromagnetic waves. A negative ion source characterized by the above.

3. The negative ion source according to claim 2, wherein the electromagnetic wave irradiation unit includes a first electromagnetic wave oscillator that generates a first electromagnetic wave and a first waveguide that propagates the first electromagnetic wave to the plasma generation region, and a second electromagnetic wave oscillator that generates a second electromagnetic wave and a second waveguide that propagates the second electromagnetic wave to the negative ion generation region. A negative ion source characterized by the above.

4. The negative ion source according to any one of claims 1 to 3, further comprising a partition provided at the boundary between the plasma generation region and the negative ion generation region, wherein the partition has an opening through which the particles can pass. A negative ion source characterized by the above.

5. The negative ion source according to claim 4, wherein the partition is made of the same material as the hot electron emitting material. A negative ion source characterized by the above.

6. The negative ion source according to claim 4 or 5, wherein a mesh member covering at least the opening is disposed on the surface of the partition. A negative ion source characterized by the above.

7. The negative ion source according to any one of claims 1 to 6, wherein on the outlet side of the negative ion generation region, a converging member is provided having an outlet side opening at one end communicating with the outlet and having a diameter smaller than the inner diameter of the housing, and a negative ion generation region side opening having the same inner diameter as the housing at the other end. A negative ion source characterized by the above.

8. The negative ion source according to any one of claims 1 to 7, wherein the hot electron emitting material is lanthanum hexaboride. A negative ion source characterized by the above.

9. The negative ion source according to claim 2 or 3, wherein the frequency of the electromagnetic wave is in the frequency band of short wave, ultra short wave, or microwave. ​ ​ ​ ​ ​ ​ ​ ​ A negative ion source characterized by the following.

10. The negative ion source according to any one of Claims 1 to 9, wherein the sample is carbon dioxide or hydrogen, characterized by the following.

11. The negative ion source according to any one of Claims 1 to 10, wherein the hot electron emitting material is an aggregate of small pieces, and the small pieces are granular, characterized by the following.

12. The negative ion source according to Claim 11, wherein the average particle diameter of the small pieces is 0.01 mm or more and 10.0 mm or less, characterized by the following.

13. A method for generating negative ions using the negative ion source according to any one of Claims 1 to 12, comprising irradiating the plasma generation region and the negative ion generation region with electromagnetic waves, introducing the sample from the inlet into the plasma generation region, and passing the generated particles through the negative ion generation region, characterized by the following.

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