JP4845022B2 - Ion beam source device - Google Patents

Description

  The present invention relates to all ion beam or neutral particle beam generators, particularly ion doping, material surface modification, material synthesis, etc. using a beam generator with a low energy of about 1000 eV or less and a high current density. The present invention relates to an ion beam source apparatus used in the field.

  An ion beam source device that draws only ions from an electric field using a mesh structure or an electrode with one or many holes from a plasma source in which ions and electrons coexist to maintain almost electrical neutrality In FIG. 2, space charges are formed by the extracted ions, and when the extraction current density increases, a large electric field proportional to the generated electric field is generated, and as a result, the ion beam self-diverges. When the beam energy is large and the speed in the beam extraction direction is large, the effect of divergence is small, but when extracting a low-energy beam, the effect of divergence is large, and the current density is only a little away from the electrode. It becomes impossible to obtain a highly focused beam.

  To suppress this divergence, (1) a method in which a filament using a tungsten wire or the like is provided near the electrode, the filament is energized and heated, and the charge of ions is neutralized by the resulting thermoelectrons ( Patent Documents 1 and 2), (2) A method in which an electron gun is provided at a distance from the electrode, and an electron beam emitted from the electron gun is injected into an ion beam to neutralize charges (see Patent Document 3). Has been devised.

  However, in the above method (1), in order to avoid excessive heat input to the electrode, a certain distance is required between the electrode and the filament for generating thermoelectrons, and when the energy of the ion beam is reduced, The divergence of the ion beam in the meantime becomes a non-negligible magnitude, and since it is unavoidable that a part of the ion beam drawn from the electrode collides with the filament, the ion beam current that can be finally extracted is reduced, There are problems such as damage to the filament due to the ion beam, shortening the life of the filament, and contamination of the ion beam with the filament material. In addition, the temperature of the electrode, the vessel wall, and the like due to heat radiation to the periphery of the filament is increased, and heating of the filament requires a large amount of power, which may increase the operating cost.

  In the method (2), the speed of the electron beam is generally about two orders of magnitude higher than the speed of the ion beam, the incident direction of the electron beam cannot be matched with the extraction direction of the ion beam, Since it is difficult to match the density distribution of the ion beam with the density distribution of the ion beam, it is difficult to completely neutralize the charge during the entire process from the extraction of the ion beam to the irradiation of the target. There is a problem that beam divergence cannot be suppressed.

In addition, the electron beam used to neutralize the ion charge is scattered on the ions and electrode surface to form thermionic electrons, but the energy of the thermal electrons is close to the energy of the incident electron beam (usually around 100 eV). Therefore, there is a problem that it is a great obstacle to the realization of ion beam irradiation under a low electron temperature (about 10 eV or less) condition required by a plasma process apparatus.
Japanese Patent No. 3076843 JP-A-9-115850 Japanese Patent Laid-Open No. 7-273072

  The present invention is an ion that draws only ions from a plasma source in which ions and electrons coexist and is almost electrically neutral using a mesh structure or an electrode having one or many holes. In the beam source apparatus, space charges are formed by the extracted ions, and when the extraction current density increases, a large electric field proportional to the generated electric field is generated, and as a result, the problem of self-divergence of the ion beam is eliminated. And

The above problem is solved by the following means.
(1) In an ion beam source apparatus for extracting ions from a plasma source using an electrode having a mesh structure or one or many holes, light or X-rays are emitted from a beam extraction direction opposite to the plasma source. By irradiating the back surface of the electrode, photoelectrons are emitted from the back surface of the irradiated electrode, neutralizing the space charge of ions extracted from the electrode hole, and suppressing ion beam divergence. Source equipment.
(2) By adjusting the amount of photoelectrons generated by changing the intensity of light or X-rays and controlling the degree of charge neutralization, the magnitude of the electric field due to space charge is changed, and the ion extraction energy, The ion beam source device according to (1), wherein the magnitude of the extraction current is variable.
(3) Efficient photoelectron emission can be achieved by using a material having a high quantum efficiency and a large photoelectron emission coefficient on the back surface of the electrode or coating a material having a high quantum efficiency and a large photoelectron emission coefficient on the surface of the electrode back surface. The ion beam source device according to (1) or (2), wherein
(4) For a surface other than the back surface of the electrode that does not need to emit photoelectrons, use a material having a low quantum efficiency and a low photoemission coefficient, or a surface having a low quantum efficiency and a low photoemission coefficient. The ion beam source device according to (1) or (2), wherein the unnecessary photoelectron emission is minimized by coating on the ion beam source.
(5) Utilizing the fact that the wavelength dependence of the emission coefficient of photoelectrons differs depending on the material, and by selecting light of a specific wavelength, the emission coefficient is high on the surface where photoelectron emission is required for that light Use a material with a value that is as close to zero as possible for the surface that does not require photoemission, or coat the surface of the surface that does not require photoemission with a material that has an emission coefficient as close to zero as possible. Thus, the ion beam source device according to (1) or (2), which simultaneously realizes efficient photoelectron emission and suppression of unnecessary photoelectron emission.

The effects of the present invention are as follows.
(1) According to the present invention, neutralization of the charge of an ion beam can be realized by a simple method of light irradiation.
(2) Neutralization of the space charge of the ion beam can be performed immediately after the generation and extraction of the ion beam, and during the entire process of the beam. It becomes possible to suppress.
(3) Since a device that generates filaments and other impurities is not required, it is possible to generate a high-purity beam.
(4) Since no filament is used, there is no need to consider the temperature rise of peripheral devices due to heating of the filament, and a stable operation can be performed.
(5) Since the ionic charge can be neutralized without using consumable parts such as a filament and an electron gun, maintenance and management of the apparatus are facilitated.
(6) As a result, it is possible to generate a high purity ion beam having a high current density in a low energy region of 100 eV or less, which has been difficult until now.
(7) By using a high-purity, high-current-density, low-energy ion beam, it becomes possible to synthesize and modify various materials that could not be performed because of high beam energy.
(8) Since no electron beam is used, the electron temperature can be lowered. This enables application to an ion beam irradiation apparatus (for example, a plasma process apparatus) that needs to keep the electron temperature low.

The principle of the present invention is as follows.
In the present invention, the surface of the electrode for extracting the ion beam of the ion beam source device that faces the side where the extracted beam is transmitted and irradiated onto the target (hereinafter, this surface is referred to as “electrode back surface”). Further, the principle is to emit photoelectrons from the back surface of the electrode by irradiating light or X-rays, and to neutralize the space charge of the ion beam extracted by the photoelectrons.
The electrode itself is made of a material that emits a large amount of photoelectrons, or if for some reason it is not possible to use a material that emits a large amount of photoelectrons, the back surface of the electrode is coated with a material that emits a large amount of photoelectrons. Thus, it is possible to neutralize the space charge with the intensity of light using a commercially available light source.

  For example, the quantum efficiency of the material used for the light-receiving surface of the photomultiplier tube is usually about 20 to 30% in the range from visible light to ultraviolet light, but the use state of the present invention is ideal like a photomultiplier tube. In view of deviating from the use environment, it is considered possible to coat and use a material having a quantum efficiency of about one-tenth, approximately 2%.

Using an example calculation, an ion beam current density of 1 mA / sq cm corresponds to an ion flux of 6.24 x 10 ¹⁵ ions / sq cm / s, while a 1 W / sq cm argon laser (wavelength 514.5 nm → 2.41 eV / photon) corresponds to a photon flux of 2.59 x 10 ¹⁸ photons / square cm / s. Assuming that the quantum efficiency is 2%, the above-mentioned one-fifth electron flux is emitted from the back of the electrode by irradiation with an argon laser of 1 W / square cm. Therefore, an ion dose of 120 mW / square cm is possible to neutralize an ion beam of 1 mA / square cm. Considering the fact that normal incidence is difficult and other effects such as light attenuation, an irradiation dose of about 250 mW / square cm is sufficient to neutralize an ion beam of 1 mA / square cm. it can. This value can be easily realized using a commercially available laser transmitter.

  Furthermore, for surfaces that do not require photoelectron generation, the generation of unnecessary photoelectrons may cause discharge breakdown between the electrodes, which may interfere with the normal operation of the device. The coating of a material with low quantum efficiency and a small photoelectron emission coefficient minimizes unnecessary photoelectron emission.

  On the other hand, when photoelectron emission is required on a surface other than the back surface of the electrode, a necessary amount of photoelectrons can be generated by using or coating a material having an appropriate light emission coefficient.

The ion beam source apparatus using the present invention based on the above principle can solve the problems of the conventional methods described above.
First, in the present invention, the release of photoelectrons that neutralize the ion charge is performed on the back surface of the electrode from which the ion beam is extracted, so that the charge of the ion is neutralized immediately after the ion is extracted from the electrode, thereby generating the space charge. It is possible to suppress immediately. Further, since it is not necessary to insert a filament into the ion beam, beam attenuation, filament damage, and ion beam contamination due to collision of ions with the filament are essentially impossible. Further, there is no problem of heat dissipation due to the filament. Further, the initial velocity of the emitted photoelectrons is small, and after the emission, the ions are attracted by the charge of the ions and move together with the ions, so that the ion charges can be neutralized in the entire process of the ion beam. For these reasons, it is possible to maintain high beam focusing by a simple method of irradiating the back surface of the electrode with light or X-rays.

Embodiments of the present invention will be described below in detail with reference to the drawings.
An embodiment of the present invention is shown in FIG. The inside of the plasma source container of the ion beam source device of FIG. 1 is evacuated by an exhaust device (not shown). A cusp magnetic field may be applied around the container by a permanent magnet (not shown). In a normal ion beam source device, a plasma composed of ions and electrons is supplied into the plasma source container by an ion supply device (not shown) and a gas is supplied from a gas supply device (not shown). There are both. These plasmas or gases are ionized and heated by high-frequency power fed into a container from a high-frequency supply device (not shown), arc discharge between a filament electrode (not shown) and the plasma source container wall, or energy supply by other methods. Then, it is confined in the plasma source vessel wall as a high-density plasma.

  In the case of the ion beam source apparatus shown here, an ion beam is extracted from this plasma by the three beam extraction electrodes (acceleration electrode, deceleration electrode, ground electrode) shown in FIG. The ion beam is self-diverging due to the effect of the self-electric field due to the charge of the ions, and a beam with good focusability cannot be obtained.

  In the invention of the present application, as shown in FIG. 1, the back surface of the outermost ground electrode is irradiated with light such as an argon laser or a mercury lamp, or X-rays to generate photoelectrons. The positive charge is neutralized immediately after the beam is extracted to suppress the generation of the electric field, thereby eliminating the beam divergence and realizing a highly focused beam. Since the initial velocity of the emitted photoelectrons is not large, it is pulled by the electrostatic force due to the positive charge of the ion beam and moves following the beam ions, so that it is uniform throughout the ion beam until it is irradiated to the target. Charge neutralization can be performed.

When the ion beam current density is high and generation of a large amount of photoelectrons is required, a material having a high quantum efficiency and a large photoemission coefficient is used for the back surface of the electrode, or the quantum efficiency is high for the surface of the back surface of the electrode. By coating a material having a large photoelectron emission coefficient, efficient photoelectron emission can be performed. In particular, since the thickness of the coating affects the photoelectron emission coefficient, an optimum thickness is selected.
Also, for surfaces that do not require the generation of photoelectrons, the generation of unnecessary photoelectrons may cause discharge breakdown between the electrodes, which may hinder the normal operation of the device. Thus, use of a material having a low quantum efficiency and a small photoelectron emission coefficient, or coating a surface with a material having a low quantum efficiency and a low photoelectron emission coefficient, minimizes unnecessary photoelectron emission.

  Furthermore, since the emission coefficient of photoelectrons has a wavelength dependency that varies depending on the material, by selecting light of a specific wavelength, a high emission coefficient value can be obtained for the surface that requires photoelectron emission. By using a material that has an emission coefficient as close to zero as possible for the surface that does not require photoemission, or by coating the surface of the surface that does not require photoemission with a material that has an emission coefficient as close to zero as possible. It is possible to simultaneously achieve efficient photoelectron emission and unnecessary emission suppression.

It is a schematic diagram which shows the principle of neutralization of the ionic charge by the photoelectron emission from an electrode back surface.

Claims (5)

With a mesh structure or an electrode having a large number of holes, in the ion beam source device extracting ions from the plasma source, the plasma source is irradiated from the opposite beam extraction direction, light or X-rays to the electrode backside by, by emitting photoelectrons from irradiated the electrode back, to neutralize the ion space charge immediately after being drawn out from the electrode hole of the electrode, ions, characterized in that to suppress the divergence of the ion beam Beam source device.   By adjusting the amount of photoelectrons generated by changing the intensity of light or X-rays and controlling the degree of charge neutralization, the magnitude of the electric field due to space charge is changed, the ion extraction energy, and the extraction current The ion beam source apparatus according to claim 1, wherein the size of the ion beam source is variable.   Efficient photoemission is enabled by using a material with high quantum efficiency and a large photoemission coefficient on the back surface of the electrode or coating a material with high quantum efficiency and a large photoemission coefficient on the surface of the electrode backside. The ion beam source device according to claim 1, wherein   For surfaces other than the back surface of the electrode that do not need to emit photoelectrons, use a material with low quantum efficiency and a low photoelectron emission coefficient, or coat the surface with a material with low quantum efficiency and a low photoelectron emission coefficient. The ion beam source apparatus according to claim 1, wherein unnecessary photoelectron emission is minimized.   Utilizing the fact that the wavelength dependence of the emission coefficient of photoelectrons differs depending on the material, by selecting light of a specific wavelength, the surface where photoelectron emission is required has a high emission coefficient for that light. Efficiency is improved by using a material with an emission coefficient as close to zero as possible for surfaces that do not require photoemission, or by coating a surface with an emission coefficient as close to zero as possible to the surface that does not require photoemission. The ion beam source device according to claim 1 or 2, wherein the emission of a typical photoelectron and the suppression of unnecessary photoelectron emission are simultaneously realized.