JPH0662449U - Negative ion beam generator - Google Patents

Abstract

(57) [Abstract] [Purpose] Negative ion beam generator for generating negative ions by attaching cesium vapor to the target electrode 4 and converting the cesium vapor into plasma to form positive ion beams. The electrons are accelerated together with the negative ions and hit the ground electrode 5 to generate secondary electrons 14, which causes a breakdown between the electrostatic lens 6 and the secondary electrons 14. Ground electrode 5
There is provided a negative ion beam generator that does not cause a breakdown between the electrostatic lens 6 and the electrostatic lens 6. [Structure] Permanent magnets 1 having different polarities on the outer circumference of the ground electrode 5
1 are arranged side by side to generate a cusp magnetic field 12 inside the ground electrode 5. Due to the action of the cusp magnetic field 12, the secondary electrons 14 cannot go out and disappear.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 This invention relates to a negative ion beam generator. In particular, the present invention relates to a negative ion beam generator that prevents breakdown between the ground electrode and the electrostatic lens and enables stable beam transport.

The negative ion beam generator uses a target as a target for negative ions and applies a negative high pressure to the target to cause cesium vapor to adhere to lower the work function of the target surface. -The target material is sputtered by hitting the target with a beam to hit the surface, and electrons are transferred from the target whose work function is lowered by electron affinity to the sputtered particles to form negative ions, which are accelerated by the electric field to generate negative ion beams. -It is designed to be pulled out as a message.

[0003]

[Prior art]

 A negative ion beam generator according to a conventional example will be described with reference to FIG. The members shown in FIG. 3 are in a vacuum, but the vacuum container is not shown. The plasma generation chamber 1, the plasma electrode 2, the extraction electrode 3, the target electrode 4, the ground electrode 5, and the electrostatic lens 6 are arranged in a straight line. These electrodes have openings 22, 23, 24, 25 along the central axis.

As shown in the voltage distribution below, the plasma generation chamber 1 and the plasma electrode 2 are 0 V or slightly positive voltage or negative voltage. The ground electrode 5 is, of course, 0V. The intermediate extraction electrode 3 and the target electrode 4 have negative high voltages -Ve and -Vt. First, a positive ion beam is created and hits the target with this to generate a negative ion beam to be taken out to the outside, so that a high voltage is negative in the middle.

The operation will be described. Although not shown, a cesium evaporation source is connected to the plasma generation chamber 1. This produces cesium vapor and enters the plasma generation chamber 1. Here, a gas state is excited by an arc discharge or a microwave discharge to generate plasma. Plasma is a collection of positive ions and electrons. Electrodes and magnets for generating and maintaining plasma are provided in the plasma generation chamber 1, but they are not shown here for simplicity. Not all cesium becomes ions, but some become ions.

Most other cesium is neutral vapor. Neutral cesium leaks to the outside from the opening 22 of the plasma generation chamber 1 due to the difference in vapor pressure, slowly advances along the axis, and reaches the target electrode 4. Here, the vapor comes into contact with the surface of the target electrode 4, so that it is cooled and returns to a liquid or solid state.

A thin cesium film is formed on the surface of the target electrode 4. This lowers the work function of the target. In other words, the Fermi level on the target surface rises.

By the way, a part of cesium becomes positive ions in the plasma generation chamber 1. When positive ions leave the plasma electrode 2 of the plasma generation chamber 1, they are accelerated by the strong electric field existing between the plasma electrode 2 and the extraction electrode 3. This is the positive ion beam 7 shown by the solid line. The positive ion beam 7 passes through the opening 23 of the extraction electrode 3 and collides with the target electrode 4 with a strong impulse.

The lattice structure is partially destroyed by mechanical energy, and the constituent materials are sputtered from the target. At this time, it is neutral, but the sputtered particles attract electrons from the target having a lowered work function to become negative ions due to the dominant electron affinity. When the negative ions pass through the opening 24 of the target electrode 4, they are accelerated by the strong electric field between the target electrode 4 and the ground electrode 5. As a result, the negative ion beam 9 is formed.

The negative ion beam further passes through the opening 25 of the ground electrode 5, is narrowed down by the electrostatic lens 6, and is extracted to the outside. The electrostatic lens 6 is an Einzel lens or the like. Three cylindrical electrodes are combined in the longitudinal direction, and positive or negative ions can be converged by the action of the electric field generated between the electrodes. Most of the negative ion beams are squeezed out here, and then go out and used for some purpose.

[0011]

[Problems to be solved by the device]

 However, an electric field exists between the target electrode 4 and the ground electrode 5 in the beam flow direction. This accelerates the negative ion beam in the flow direction, but at the same time, accelerates the electron, which is a negative charged particle, and carries it to the ground electrode 5 beyond. As described above, the target electrode 4 has a reduced work function due to the adhesion of cesium, and many secondary electrons are generated when positive ions collide. The secondary electrons have little energy, but some of them may pass through the opening 24. Then, a strong electric field is felt and it is accelerated toward the ground electrode.

The acceleration energy when passing through the ground electrode 5 is the same as that of negative ions. Many negative ions and electrons are narrowed down by the electrostatic lens 6 and go out. Although they are charged particles at high speed, they are not neutral but have negative charges, and they tend to spread in a direction perpendicular to the axial direction that exerts a Coulomb repulsive force on each other. This appears remarkably in electrons with small mass. Therefore, some electrons and negative ions are deflected to the outside and collide with the inner wall of the ground electrode 5 and the inner wall of the electrostatic lens 6. Since particles having considerable energy collide with each other, secondary electrons are ejected from the inner wall of the ground electrode 5 and the electrostatic lens 6. Since the energy is large, one electron knocks out many secondary electrons.

Since the secondary electrons initially have an energy of several eV, they are localized in the vicinity of the ground electrode 5 and the electrostatic lens 6. Since a high voltage exists between these parts, the secondary electrons are accelerated by the electric field and travel to the opposing electrodes and collide with them. A high-density current flows due to the flow of such secondary electrons. This is discharge. The electrostatic lens 6 and the ground electrode 5 are short-circuited by the discharge. This is called a breakdown. An appropriate voltage will not be applied to the electrostatic lens 6. As a result, the convergence of the path of the negative ion beam deteriorates and the beam is disturbed. Therefore, efficient ion beam transport cannot be achieved.

In this case, the power supply automatically lowers the voltage to 0 V, interrupts the discharge, and raises the voltage again. Alternatively, the power may be turned off manually. It would be good if the recovery from the occurrence of a breakdown could be done instantaneously, but that is not the case. Although it depends on the power supply performance, it takes time to recover from the breakdown.

The ion source must be capable of generating an ion beam stably for many hours without interruption. Don't let the ion beam stop appearing on the way. That is, it is necessary to prevent breakdown between the electrodes.

An object of the present invention is to provide a negative ion beam generator which prevents breakdown due to secondary electrons generated by negative ions and electrons emitted from the target electrode 4. .

[0017]

[Means for Solving the Problems]

 In the negative ion beam generator of the present invention, a plurality of permanent magnets are arranged around the ground electrode so that the poles are different from each other to generate a cusp magnetic field inside the ground electrode, and secondary electrons generated by the ground electrode are generated. Prevent you from getting out of the magnetic field. Since secondary electrons do not flow toward the electrostatic lens, no breakdown occurs between the electrostatic lens and the ground electrode.

[0018]

[Action]

 Since a plurality of permanent magnets are arranged equidistantly around the ground electrode and the poles are opposite to each other, a cusp magnetic field is generated inside the ground electrode 5. The cusp magnetic field has magnetic field lines that connect the end faces of adjacent permanent magnets with a smooth curve, and is localized so that the magnetic field does not exist near the center of the cusp magnetic field (center of the ground electrode). It is a magnetic field. The end face of the permanent magnet becomes the input / output point of the magnetic force line, and the magnetic force line is sharpened here, so it is called the cusp magnetic field. Since the poles of adjacent magnets are placed so as to be reversed, the magnetic fields cancel each other out and do not reach far.

The cusp magnetic field is often used for plasma confinement. It is often practiced to provide a number of permanent magnets along the outer wall of the ion source chamber to confine the plasma inside the chamber. The plasma is confined in the center of the chamber where there is no magnetic field. If the ions move toward the magnetic field, they will experience Faraday forces due to the magnetic field lines. As a result, even if the magnetic field lines are rotated half a turn, they are returned to the original central space. It will not hit the wall. If the cusp field is not present, the ions hit the wall, but then lose energy and charge and are no longer ions. A cusp magnetic field is used in the ion source chamber to prevent this and maintain a high-density plasma.

The use of the cusp magnetic field in the present invention is to limit the movement of charged particles, which is the opposite of the conventional method. In the past, charged particles outside the cusp magnetic field were prevented from entering the inside of the magnetic field, thereby confining the charged particles in the part without the magnetic field.

However, in the present invention, this is not the case, and secondary electrons of low energy, which are formed on the inner wall of the ground electrode 5 and are in the vicinity thereof, are kept out of the magnetic field. This is the opposite. High-speed electrons and negative ions coming from the target electrode 4 collide with the ground electrode 5 to generate a large number of secondary electrons, which are in the cusp magnetic field from the beginning. The secondary electrons cannot get out of the cusp magnetic field because they are trapped in the magnetic field lines of the strong cusp magnetic field. The magnetic field lines are wound around the magnetic field lines and move in a spiral with a small Larmor radius. However, since the magnetic field lines cross the wall of the ground electrode 5, the secondary electrons eventually hit the wall. When hit, it is absorbed by the wall and the secondary electrons disappear.

That is, the cusp magnetic field functions so as not to let secondary electrons go out. Since it does not go out to the outside, the secondary electrons do not induce a breakdown with the electrostatic lens 6. Stable ion beam generation can be continued. It can be said that the action of the cusp magnetic field is an extremely interesting use unlike the conventional one.

It may be feared that a cusp magnetic field is formed inside the ground electrode 5, which affects the beam path. However, the cusp magnetic field produces a strong magnetic field only in a very small area in the peripheral area, but the magnetic field in the central area is small. There is little effect on the beam flying axially in the center. Especially, there is almost no influence on an ion beam having a large mass.

[0024]

【Example】

 A negative ion beam generator according to an embodiment of the present invention will be described with reference to FIG. The plasma generation chamber 1, the plasma electrode 2, the extraction electrode 3, the target electrode 4, the ground electrode 5, and the electrostatic lens 6 are aligned on a straight line. The electrodes have openings 22, 23, 24 and 25 respectively in the center so that ion beams can pass therethrough.

An evaporation source of cesium is connected to the plasma generation chamber 1, and cesium vapor is introduced into the plasma generation chamber 1. Here, the cesium vapor is excited by arc discharge and microwave discharge, and part of it becomes plasma.

Neutral cesium also exists in the plasma generation chamber 1, and this leaks from the opening 22 due to the difference in vapor pressure and adheres to the inward conical surface of the target electrode 4. Due to the adhesion of cesium, the work function of the target material decreases and the surface Fermi level rises.

Although plasma exists in the plasma generation chamber 1, positive ions of cesium leave the opening 22 of the plasma electrode 2 and are accelerated by the action of the electric field existing between the positive electrode 7 and the extraction electrode 3. Becomes When a high-speed cesium beam collides with the target electrode 4, it sputters the target material. Neutral target material is knocked out. Since the distance between the sputtered particles and the target atoms is narrow and the work function of the target is lowered, electrons move from the target to the sputtered particles due to the tunnel effect. Sputtered particles become negative ions.

In the vicinity of the target electrode 4, there is an electric field from the ground electrode 5 toward the extraction electrode 3, which carries negative ions to the ground electrode 5. After passing through the opening 24 of the target electrode 4, negative ions are further accelerated by the strong electric field between the target electrode 4 and the ground electrode 5 and exit from the opening 25 of the ground electrode 5. This is converged by the electrostatic lens 6 and flies to a target object (not shown).

The problem is that the secondary ions 14 generated by the negative ion beam and the electron beam 8 colliding with the ground electrode 5 induce the breakdown. In the present invention, since the permanent magnets 11 are arranged on the outer periphery of the ground electrode 5 so that the poles are opposite to each other, the cusp magnetic field 12 is generated inside the ground electrode 5. This is as shown in FIG.

The cusp magnetic field 12 captures the secondary electrons 14 and causes them to be lost to the cusp 15. The secondary electrons 14 have low energy and spiral around the lines of magnetic force with a small Larmor radius. However, since the lines of magnetic force are always distributed so as to cut the inner wall of the ground electrode 5, the secondary electrons do not move after the spiral motion. Eventually, it collides with the inner wall of the ground electrode 5 and is absorbed by this.

Since the secondary electrons are absorbed by the original ground electrode 5 and disappear, the secondary electrons do not fly to the electrostatic lens 6 and cause discharge between the two electrodes. That is, no breakdown occurs between the electrostatic lens 6 and the ground electrode 5. This is due to the confinement effect of secondary electrons in the cusp magnetic field.

The cusp magnetic field is usually used for confining plasma. The plasma is localized in the region of no magnetic field to prevent the plasma from colliding with the wall surface and disappearing. However, in the present invention, the cusp magnetic field is used in a completely different manner. In this method, the secondary electrons generated inside the magnetic field are confined inside the magnetic field and collide with the wall surface so that they disappear.

[0033]

[Effect of device]

 In the present invention, a large number of permanent magnets are provided on the outer circumference of the ground electrode to form a cusp magnetic field inside the ground electrode. Even if high-speed electrons or negative ions collide with the ground electrode and generate secondary electrons, the secondary electrons are caught in the magnetic field and cannot spread, and eventually disappear. Therefore, no breakdown occurs between the ground electrode and the electrostatic lens. It is possible to perform stable ion beam transportation for a long time. It is possible to provide a highly reliable negative ion beam generator capable of stably irradiating an object with a negative ion beam for a long time.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a negative ion beam generator according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA of FIG.

FIG. 3 is a schematic sectional view of a negative ion beam generator according to a conventional example.

[Explanation of symbols]

 1 Plasma Generation Chamber 2 Plasma Electrode 3 Extraction Electrode 4 Target Electrode 5 Grounding Electrode 6 Electrostatic Lens 7 Positive Ion Beam 8 Electron Beam 9 Negative Ion Beam 11 Permanent Magnet 12 Cusp Magnetic Field 14 Secondary Electron 15 Cusp

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Creator Kobe Hitoshi, Nisshin Electric Co., Ltd. 47 Umezu Takaunecho, Ukyo-ku, Kyoto City

Claims (1)

[Scope of utility model registration request] 1. A plasma electrode having a plasma generation chamber 1 which is in vacuum and introduces cesium vapor into a plasma by discharging a part thereof, and an opening 22 provided at the exit of the plasma generation chamber 1 for passing positive ions. 2, an extraction electrode 3 provided in a vacuum in front of the plasma electrode 2 for extracting positive ions and having an opening 23 biased to a negative high voltage, and an opening having at least a front surface of a substance desired to be negative ions and passing negative ions. Extraction electrode 3 having 24 and being biased to a negative voltage
A target electrode 4 provided in a vacuum in front of the
A ground electrode 5 provided in front of the get electrode 4 and having an opening 25 for passing a negative ion beam to bias negative ions to accelerate negative ions, and an electrostatic provided in front of the ground electrode 5 to converge the negative ion beam. It is composed of a lens 6 and a plurality of permanent magnets 11 provided around the ground electrode 5 so as to have different polarities. Inside the ground electrode 5, a cusp magnetic field 12 is formed.
And a negative ion beam generator.