JPH05314918A - Microwave antenna for ion source - Google Patents

Abstract

PURPOSE:To simplify structure and effectively form a magnetic field by incorporating a permanent magnet inside, and thereby generating in the vicinity of an antenna a magnetic field for satisfying a microwave resonance condition (ECR condition). CONSTITUTION:A magnet case 12 is positioned underneath an antenna 3, and inside thereof a permanent magnet 11 is built in. An electric discharge chamber 10 is a space to be vacuum-evacuated, and a raw gas is introduced thereinto from a gas inlet. A microwave generated in a magnetron 1 reaches the antenna 3 via a connection conductor 2, and power of the microwave is induced to the inside of the electric discharge chamber 10 by means of the antenna 3 and an exterior conductor 4. Next, a gas inside the chamber 10 is ionized to become plasma, and an electron discharged from the gas because of the microwave is oscillated, and this time a comparatively strong magnetic field is formed in the periphery of the antenna 3 by the magnet 11. Accordingly, high density plasma can be generated and plasma generation efficiency can be increased with simple structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antenna in an ion source for exciting a source gas into plasma by microwaves. The ion source excites the source gas by some means into plasma. Equipped with an extraction electrode system to extract ion beams. The ion beam is used for ion implantation, ion etching, modification of substances with ions, and the like. There are various gas excitation means.

There is one that heats a filament to cause an arc discharge between the filament and the chamber. Some are excited by high frequency glow discharge. Alternatively, some are excited by microwaves. The microwave ion source is, for example, for introducing a microwave of 2.45 GHz into the chamber. There is an advantage that the life is long because a hot cathode for discharging is unnecessary. Microwave ion sources typically consist of magnetrons, waveguides, dielectric windows, chambers, and the like. The microwave generated by the magnetron is guided to the waveguide and directly introduced into the chamber. There is a dielectric window at the entrance to the chamber. This goes through the microwave,
Moreover, this is to maintain the vacuum in the chamber.

[0003]

2. Description of the Related Art An ion source for exciting a source gas by using a microwave is called a microwave ion source. It is usually set so that the lowest order mode can exist as a steady state in the chamber. Microwave resonant absorption is often used to increase the efficiency of plasma generation. It is called an ECR microwave ion source. This places a large coil around the chamber and creates a magnetic field of 875 Gauss inside the chamber, causing cyclotron motion of the electrons. Since this and the microwave of 2.45 GHz resonate, electrons can efficiently absorb energy from the microwave. Since the neutral atom is excited by the movement of electrons, the source gas is excited more efficiently by resonance absorption.
It must provide a large coil around the chamber to create a uniform magnetic field in the axial direction.

When a microwave is guided by using a waveguide, a microwave having a large power can be supplied to the chamber. The cutoff wavelength is determined by the size of the waveguide. Microwaves with a wavelength longer than this cutoff wavelength are not transmitted. Also, the larger the size, the more the microwaves in the mode are transmitted. Therefore, the waveguide becomes considerably large in size. Then, the entire apparatus including the chamber becomes large and bulky. If a coil for absorbing the microwave resonance is provided on the outer circumference of the chamber, the size becomes too large.

There is also a method of guiding microwaves into the chamber without using a waveguide. This uses an antenna. For example, M.Pichot, A.Durandet, J.Pelletier, Y.Arna
l and L. Vallier, "Micowave multipolar Plasmas excit
ed by distributed electron cyclotron resonance: Con
cept and Performance ", Rev. Sci. Instrum.59 (7), p1072
(1988) proposes an ion source in which many antennas are installed along the inner wall of the chamber and twice as many permanent magnets are installed on the outer wall of the chamber. FIG. 4 shows a schematic perspective view of this. Adjacent permanent magnets are arranged so that their polarities are opposite to each other and generate a cusp magnetic field. Many antennas are within the range of this cusp magnetic field and satisfy the microwave resonance condition in a narrow range. That is, it is 875 Gauss in a narrow range near the antenna. This ion source chamber is a multi-cusp type. The power of each antenna can be adjusted independently. Since a large number of antennas are provided inside the chamber, the efficiency of plasma generation is high. However, in the central part of the chamber, microwaves are less absorbed by the plasma.
This is because the cusp magnetic field does not reach the central part of the chamber, so that the resonance condition is not satisfied even if the microwave power density is high.

[0006]

The apparatus of Pichot which uses a large number of antennas has a high degree of freedom of control, the distribution condition of plasma can be freely determined, and the density of plasma can be increased up to the vicinity of the chamber wall. It has advantages. However, this is a large-scale device. This is the same as the one using a waveguide. In the function of carrying microwave power, the antenna does not reach the waveguide. However, using an antenna allows the device to be made smaller.
Even in that case, it is preferable that the number of antennas is small in order to further reduce the size. If only one antenna is used in the device of FIG. 4, there will be one antenna in the center. However, this leaves the antenna away from the cusp field. It is not possible to generate microwave resonance using a cusp magnetic field.

In other words, the cusp magnetic field is often used only for confining the plasma, and it is rare to use it so as to satisfy the ECR condition (microwave resonance absorption condition). To provide the usual ECR conditions, a large coil is placed around the circumference of the chamber to create an area of 875 Gauss in the center of the chamber. This is as described above. It can be said that it is a conventional means in the ECR plasma CVD method, the ECR plasma etching method, etc. to generate the ECR condition inside the chamber by providing the coil on the outer circumference of the chamber.
An external coil is used without exception in a device in which microwaves are introduced into the chamber using a waveguide.

The present invention relates to an ion source of the type in which microwaves are introduced into a chamber by using an antenna, but an advantage of the antenna is that it is suitable for making it smaller than that of a waveguide. If a coil is provided on the outer circumference of the chamber, the advantage of downsizing is lost. We want to generate a magnetic field that satisfies the microwave resonance condition inside the chamber without providing a coil.

The magnetic field generated by the coil requires a large amount of electric power to maintain the magnetic field. From a financial perspective, we want to avoid coils. Microwave resonance is not the magnetic field but the microwave that supplies the energy under the resonance condition. Therefore, the microwave resonance condition (cyclotron resonance condition) should be created by the permanent magnet. Actually, the above-mentioned Pichot also generates a resonance magnetic field of 875 Gauss by the magnet outside the chamber based on such an idea. However, an external magnet cannot create a resonant magnetic field deep inside the chamber. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide a compact antenna using an antenna for an ion source capable of microwave resonance.

[0010]

The ion source antenna of the present invention has a permanent magnet incorporated therein to generate a magnetic field satisfying the microwave resonance condition (ECR condition) in the vicinity of the antenna. While the conventional antenna does not have a permanent magnet, the antenna of the present invention has its own permanent magnet inside. The antenna radiates microwaves to the surroundings and at the same time creates a magnetic field in the surroundings. Since both satisfy the microwave resonance condition, plasma generation is promoted.

[0011]

Since the antenna of the present invention has its own permanent magnet, it can generate a strong magnetic field in the vicinity while introducing microwaves into the chamber. When electrons are emitted from the raw material gas, the electrons make a cyclotron motion by the action of this magnetic field. Since this cycle matches the cycle of microwaves, electrons are accelerated and collide with gas molecules with a stronger impact force. Therefore, the efficiency of plasma generation from neutral atoms is enhanced. The advantage of such a structure is that the region where the microwave intensity is high and the region where the magnetic field intensity is high match. Since both are generated from the antenna, the strength of both increases near the antenna. Therefore, the magnetic field can be effectively used for microwave absorption. That is, the resonance absorption region becomes wider. Furthermore, a large coil is not required on the outer peripheral portion of the chamber. Combined with the fact that the means for introducing microwaves is an antenna, the device can be made compact.

[0012]

1 is a sectional view of a microwave antenna for an ion source according to an embodiment of the present invention. The magnetron 1 is a device that generates microwaves. This is 2.45G, for example
Generates microwave at Hz. The microwave is output to the connection conductor 2 and the outer conductor 4 without passing through the waveguide.
This is the same mode of propagation as a coaxial cable. The connection conductor 2 is electrically connected to the antenna 3 of the present invention. The antenna 3 is an elongated tubular metal member. There is a double-walled cooling water passage inside, which allows cooling water to pass through. Cooling water enters from the cooling water inlet 5 and descends through the central portion to reach the lower end. Here, it is inverted and the outer peripheral region is raised. Then, it is discharged from the cooling water outlet 6 on the upper side. The ceramic spacer 7 supports the antenna 3 below. The outer conductor 4 is attached to the outer wall of the discharge chamber 10 via a flange 8. The antenna 3 is inserted into the discharge chamber through the holes in the flange 8 and the discharge chamber 10. An O-ring 9 is provided between the flange 8 and the discharge chamber 10.

What is important in the present invention is that the permanent magnet 11 is provided inside the antenna. Below the antenna 3, there is a magnet case 12 in which a permanent magnet 11 is built. The discharge chamber 10 is a space that can be evacuated. A raw material gas is introduced here through a gas inlet (not shown). The microwave generated by the magnetron 1 reaches the antenna 3 via the connecting conductor 2. The microwave power is introduced into the discharge chamber 10 by the antenna 3 and the outer conductor 4. The microwave travels through the antenna and enters the interior of the discharge chamber. Then, the gas inside the discharge chamber 10 is ionized into plasma. Electrons emitted from gas vibrate due to microwaves.
This electron collides with a neutral atom and ionizes it. At this time, the permanent magnet 11 forms a relatively strong magnetic field around the antenna 3. A region that satisfies the microwave resonance condition f = qB / 2πm exists near the antenna.
Here, f is the microwave frequency, q is the electronic charge, and m is the electronic mass. If f is 2.45 GHz, the magnetic flux density satisfying this is B = 875 Gauss.

In the region where the microwave resonance condition is satisfied, the movement of electrons becomes strong and the source gas is strongly excited. The efficiency of plasma generation is improved. FIG. 2 is a cross-sectional view of the lower part of the antenna. In this example, the permanent magnets 11 are arranged so that the polarities are reversed between the adjacent magnets. The magnetic force lines are formed in an arc shape from the outer surface of the adjacent magnet to the outer surface. Microwave resonance conditions are met in the appropriate portions of the magnetic field. Even if it deviates from this condition a little, the electron's motion excites the gas due to the magnetic field. Arrangement and number of permanent magnets are arbitrary. FIG. 3 is a cross-sectional view showing an arrangement example of another permanent magnet. Here, the permanent magnet has magnetic poles in the circumferential direction.
The magnetic lines of force exit from one end face and reach the other end face. In addition to these, arrangement of permanent magnets is also conceivable.

In the case of the conventional ECR plasma device in which the coil is provided on the outer circumference of the chamber, the coil current must be considerably increased. Otherwise, the magnetic field in the central part of the chamber cannot be increased to satisfy the microwave resonance condition. This is because the magnetic field created by the coil is inevitable throughout the interior of the chamber and the magnetic field energy must be large. In the present invention, since the permanent magnet is attached to the antenna itself that transmits microwaves, the magnetic field to be formed by the permanent magnet has only to reach a narrow area near the antenna. Therefore, a small permanent magnet can be used.

From the viewpoint of efficiency, it is most desirable that the microwave resonance condition is satisfied in the vicinity of the antenna having the highest microwave density. The present invention can also meet this need. When a longitudinal magnetic field is generated inside the chamber by a coil, a region with the strongest magnetic field occurs in the peripheral part of the chamber instead of the center, but if the microwave resonance condition is satisfied here, the microwave density is low and it is efficient. Not. Instead,
If the microwave resonance condition is satisfied in the central portion of the chamber, the magnetic field in the peripheral portion is larger, so extra power is required.

[0017]

According to the present invention, the microwave is introduced into the chamber by using the antenna having the built-in permanent magnet. A strong local magnetic field can be formed in the central portion of the chamber where the microwave intensity is strong, so that the microwave resonance condition can be satisfied. Electrons are strongly accelerated by the microwave, and the source gas can be actively excited. Therefore, high-density plasma can be generated. A compact and high-performance ion source can be provided. It is easy to incorporate a permanent magnet inside the antenna. The end face magnetic field strength of the permanent magnet does not need to be very high. This is because the magnetic field does not have to extend far.

When a waveguide is used for microwave propagation, a large microwave power can be propagated. A coaxial antenna and a structure using an external conductor can transmit only a maximum power of about 200W. However, the use of an antenna simplifies the structure and reduces the size. Since the antenna is mounted inside the chamber, it can serve as a support for the permanent magnet. This is a great advantage. In the case of using the waveguide, there is nothing that can serve as a support inside the chamber, so that the permanent magnet cannot be easily attached. Of course, the type using the waveguide could also suspend and support the permanent magnet inside the chamber. However, the permanent magnet is heated by plasma and must be cooled. Then a cooling mechanism would have to be added. Since the antenna has a built-in cooling mechanism from the beginning and this can cool the permanent magnet, no new cooling mechanism is required.

Electric power is always required to satisfy the microwave resonance condition with the coil. However, in the present invention, since a permanent magnet is used, electric power is unnecessary. Energy for generating plasma can be saved. The present invention can increase the plasma generation efficiency with a simple structure in which a permanent magnet is incorporated in the antenna. There is no need to install a coil on the outer circumference. It can be applied to any ion source such as a bucket type and a Kauffman type.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of an ion source microwave antenna according to an embodiment of the present invention.

2 is a cross-sectional view of an end portion of the antenna of FIG.

FIG. 3 is a vertical cross-sectional view of an antenna end portion showing another arrangement of permanent magnets.

FIG. 4 is a perspective view of an ion source using multiple antennas by M. Pichot et al.

[Explanation of symbols]

 1 Magnetron 2 Connection Conductor 3 Antenna 4 Outer Conductor 5 Cooling Water Inlet 6 Cooling Water Outlet 7 Ceramics Spacer 8 Flange 9 O-ring 10 Chamber 11 Permanent Magnet 12 Magnet Case

Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location H05H 1/46 9014-2G

Claims (1)

[Claims] 1. A rod-shaped antenna that is connected to a magnetron and is inserted from the outside of the ion source chamber into the inside of the ion source chamber and introduces microwaves into the ion source chamber. The antenna has a cooling water passage and a permanent magnet inside. A microwave antenna for an ion source, wherein the permanent magnet generates a magnetic field satisfying a microwave resonance condition inside the ion source chamber.