JPH0554809A - Silicon ion source with built-in crucible - Google Patents

Abstract

PURPOSE:To provide a crucible in which no impurities are generated by using a Ta crucible as a crucible for silicon. CONSTITUTION:A Ta crucible filled with silicon is put in a vacuum chamber and the chamber is closed and evacuated. Electricity is transmitted to a cathode filament 4 and an anode filament 5 to heat the filaments. Temperature within the vacuum chamber is raised by heat radiated from the filaments. A space where the filament exist is surrounded by radiation shields 6-8 so that the radiated heat is efficiently confined. As the temperature of each shield rises, the crucible is heated from all directions. The silicon 15 in the crucible 16 is heated by the intense radiated heat and vaporized and arc discharge is generated between the cathode filament and the vacuum chamber. Thermoelectrons from the cathode filament 4 are thereby accelerated and bombarded with silicon vapor and cause ionization, thus generating plasma of the silicon.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon ion beam.
Ion source for creating a membrane. In particular, it relates to an improvement for producing an ion beam of silicon in an ion source having a crucible inside a vacuum chamber.

[0002]

2. Description of the Related Art An ion source is one in which an introduced source gas is excited into plasma by discharge and is extracted as an ion beam by an extraction electrode. There are various uses of ion beams, but one is by applying ions to the target, such as electrons, ions,
This is a case where atoms, X-rays, etc. are emitted and used for measuring physical properties. In this case, the diameter of the ion beam is small and a mass analyzer is attached, so that desired ions can be obtained. The other is when the ion beam is used for modifying the substance.
Beams of large diameter are required to increase throughput. Then, it becomes difficult to provide a mass spectrometer on the outlet side of the ion source. In this case, it is better from the beginning to produce a good quality ion beam containing only desired ions and no impurities.

The object of the present invention is an ion beam of silicon. It is often the case that a small amount of silicon is ion-implanted as a semiconductor dopant, but in many cases, a compound gas such as silane is used as a source gas, so that the inside of the ion source does not need to reach such a high temperature. Therefore, a silicon ion beam can be easily obtained by an ordinary bucket type ion source, Freeman type ion source, Kauffman type ion source, or the like. In this case, since the beam diameter is small and the amount of current is small, mass spectrometry is easy. However, if a silicon beam is used for reforming an iron plate having a large area, an organic sheet, etc., it is no longer appropriate to provide a mass spectrometer at the outlet. Then, a silicon ion beam containing no impurities must be generated from the beginning. Therefore, it is better to be able to generate an ion beam starting from a pure silicon solid rather than a compound from the beginning.

A bucket type ion source is provided with a cathode filament in a vacuum chamber, the filament is heated to emit thermoelectrons, and an arc discharge is generated between the filament as a cathode and the chamber as an anode. The source gas is excited with. The Freeman-type ion source is a linear thick T
A filament is surrounded by a cylindrical anodic chamber to cause a discharge between the filament as a cathode and the anodic chamber, but since a magnetic field is applied parallel to the filament from the outside, electrons emitted from the filament make a spiral motion. Then it collides with the gas and excites it.

Although these were originally those which ionize the raw material gas, a higher purity can be obtained by forming the ion beam using a metal having a high melting point as the raw material. In this case, there are two methods. One is a method of providing an evaporation source that heats a solid to vaporize it separately from the ion source. The other is a method of placing a crucible containing a solid raw material directly in an ion source, heating the crucible to turn it into vapor, and then exciting it into plasma by discharge. The present invention is an improvement relating to the latter type in which the crucible is built in the ion source. This type has not been widely used yet, but it is promising for obtaining a large-diameter ion beam of a high melting point substance.

[0006]

Since a crucible containing silicon solid is placed in an ion source and heated to evaporate, the crucible is required to have a high melting point and physical strength. There are three types of conventional crucibles for silicon. Carbon crucible (C), alumina crucible (Al ₂ O
₃ ) and a boron nitride crucible (BN). However, in the carbon crucible, many compound ions of carbon and silicon such as SiC ⁺ and SiC ₂ ⁺ are generated, and it is difficult to obtain an ion beam of only silicon. S for an alumina crucible
Impurities such as iO ⁺ and SiO ₂ ⁺ ions are generated. In the case of boron nitride, SiN ⁺ ions come out as impurities.

170 because it evaporates silicon
The crucible material is activated at a high temperature of about 0 K and a large amount of impurities are generated. In many cases, the abundance ratio of ions other than silicon reaches 30% or more. Of course, it suffices to place a mass spectrometer on the outlet side of the ion source. However, in the case of a large-diameter ion beam, the mass analyzer becomes too large and heavy, which cannot be realized in terms of cost. If mass spectrometry is not used, a crucible free of impurities is eagerly desired from the beginning. An object of the present invention is to provide a crucible for silicon that does not generate impurities.

[0008]

The present invention uses a Ta (tantalum) crucible as a crucible for silicon. The material of the crucible is Ta. This is the feature of the present invention. In other respects, it is the same as the conventional one. The type of discharge is arbitrary and plasma confinement is also arbitrary. A bucket type ion source, a Freeman type, or a Kauffman type may be used. That is, the silicon ion source with a built-in crucible of the present invention comprises a vacuum chamber which can be evacuated to a vacuum and which provides a space for exciting a raw material gas into plasma by causing direct current discharge, high frequency discharge or microwave discharge. A magnet provided on the outer periphery of the chamber to keep the plasma away from the wall surface of the vacuum chamber, a discharge mechanism for causing discharge in the vacuum chamber, and an ion beam from the plasma provided at the outlet of the vacuum chamber Of the extraction electrode system and a crucible made of Ta which contains silicon as a raw material and is placed inside a vacuum chamber.

[0009]

Since the crucible is made of Ta, it does not form a high vapor pressure molecular compound with silicon and does not generate impurities even when it is strongly heated. Therefore, a highly pure silicon ion beam can be obtained without mass spectrometry. Further, Ta has a large physical strength even at a high temperature and does not soften or melt even at a temperature at which silicon is vaporized.

[0010]

EXAMPLE An ion source according to an example of the present invention will be described with reference to FIG. The cylindrical vacuum chamber 1 is a space in which the inside can be evacuated. This is a combination of a cylindrical side plate 2 and a lid plate 3 that closes one end of the side plate. A cathode filament 4 is provided at a right angle from the cover plate 3 toward the central portion of the vacuum chamber 1. Since this is a cathode potential heated by an external filament power supply, thermoelectrons are emitted from the surface of the filament. Further, an anode filament 5 is provided at a right angle from the cover plate 3 toward the center inside the vacuum chamber 1. This also generates heat from the filament power source and heats the inside of the vacuum chamber. However, because of the anode potential, it is not sputtered by the ions in the plasma. Therefore, it may have a longer life than the cathode filament. Since the inside of the vacuum chamber 1 must be heated to a high temperature, many radiation shields made of metal plates such as Ta and W are provided. There are many cylindrical radiation shields 6 on the inner circumference of the side plate 2 to reflect the heat radiation from the anode filament and the cathode filament. A radiation shield 7 parallel to the cover plate 3 is provided near the base of the filament near the center of the vacuum chamber 1. This prevents radiant heat from escaping in the direction of the lid plate 3. Further, a radiation shield 8 is provided near the outlet of the vacuum chamber 1 in parallel with the lid plate 3.

Further outside the radiation shield 8, there are a positive electrode 9, a negative electrode 10 and a ground electrode 11. These are porous electrode plates. Of course, it may be a single hole. These form the extraction electrode system 12. The positive electrode 9 is also called an acceleration electrode or a plasma electrode. The negative electrode may be a deceleration electrode or an extraction electrode. For example, the positive electrode is 20-30k
V and the negative electrode are about -1 to -5 kV. Several permanent magnets 13 are provided around the side plate 2 of the vacuum chamber 1. The magnets are magnetized in the radial direction and arranged so that the polarities of the adjacent magnets are different. These magnets create a cusp field inside the vacuum chamber. As a result, the plasma does not hit the wall surface of the vacuum chamber and is confined in the central portion.
At the outlet of the vacuum chamber 1, there is a ring-shaped section of an insulator 14, for example, alumina, which is connected to the ion treatment apparatus subsequent to the ion. A crucible 16 made of Ta and containing silicon 15 is placed inside the vacuum chamber 1. This is an important point.

The operation of the above configuration will be described. A Ta crucible filled with silicon is placed in a vacuum chamber and closed. Further, the vacuum chamber is evacuated. The cathode filament 4 and the anode filament 5 are energized to generate heat. The radiant heat raises the temperature inside the vacuum chamber. Since the space in which the filament is present is surrounded by the radiation shields 6, 7, and 8, the radiant heat is efficiently trapped. Eventually, the temperature of the radiation shield rises, too1
It will be 500K-1700K. Then, the radiation from the radiation shield also increases and the crucible is heated from all sides.
The strong radiant heat heats the silicon 15 in the crucible 16 to start evaporation. An arc discharge is generated between the cathode filament and the vacuum chamber. Arc discharge
Thermal electrons are emitted from the defilament 4 and are accelerated and collide with the vapor of silicon, so that silicon is excited and ionized. Since this is repeated, silicon plasma is generated.

The plasma is confined in the center of the vacuum chamber 1 by the action of the permanent magnet 13. Ion beams are extracted from the plasma by the action of the extraction electrode system 12. Ar gas is introduced as an auxiliary when the discharge does not last, but in the case of the present invention, the auxiliary gas is almost unnecessary. This is because the vapor of silicon itself sustains a stable discharge. Although it is an ionic impurity, it does not form a compound with silicon because it is a Ta crucible. In addition, since it is a crucible that does not contain oxygen, it does not create a compound of oxygen and silicon, and eventually does not generate impurity ions other than silicon. In this respect, it is superior to C, Al ₂ O ₃ , BN crucible and the like.

FIG. 2 shows a mass spectrometric spectrum of ions in silicon plasma when a silicon ion beam is generated in the silicon ion source of the present invention. The horizontal axis is the mass of the ion divided by the charge (M / Z). The vertical axis is the intensity distribution. The conditions are that the arc voltage is 50 V and the arc voltage is
The current was 20 A and the temperature of the radiation shield was 1760K. Mostly monovalent Si ⁺ ions, other divalent S
There is i ⁺⁺ . Other than these, there are almost no impurity ions. Since monovalent Si ions have an M / Z of 28 and divalent Si ions are 14, they can be easily separated by mass, but the same Si
Therefore, there is no need for it. Only the energy at the time of acceleration is different, and the action of modifying the physical properties of the target substance is the same. The results obtained here show that the total number of Si ions is 9
It is 9% or more. It turns out that it is a very good crucible. Although the bucket type has been described here, the present invention is also applicable to the Freeman type and the Kauffman type. This is because the main point of the present invention is that the crucible is made of Ta.

[0015]

[Effects of the Invention] S with the crucible inside the vacuum chamber
Since Ta is used as the crucible in the ion source of i, no ionic impurities other than Si are generated, and an extremely high-purity Si ion beam can be obtained. No mass spectrometer required.
Although it is difficult to provide a mass spectrometer when producing a large-area ion beam, the present invention does not require it, which is convenient.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view of a silicon ion source with a built-in crucible according to an embodiment of the present invention.

FIG. 2 is a graph showing the results of mass spectrometric analysis of ion beams generated by an ion source in an example of the present invention. The horizontal axis is M
In / Z, the vertical axis is intensity.

[Explanation of symbols]

 1 Vacuum Chamber 2 Side Plate 3 Lid Plate 4 Cathode Filament 5 Anode Filament 6 Radiation Shield 7 Radiation Shield 8 Radiation Shield 9 Positive Electrode 10 Negative Electrode 11 Grounding Electrode 12 Extraction Electrode System 13 Permanent Magnet 14 Insulator 15 Silicon 16 Crucible

Front page continued (72) Inventor Takatoshi Yamashita, Nisshin Electric Co., Ltd., 47 Umezu Takaunecho, Ukyo-ku, Kyoto (72) Inventor, Shuichi Fujiwara 47 Umezu, Takaune-cho, Ukyo-ku, Kyoto City, Nissin Electric Co., Ltd. (72 ) Inventor Koji Matsunaga Nissin Electric Co., Ltd.

Claims (2)

[Claims] 1. A vacuum chamber which can be evacuated to a vacuum to provide a space for inducing a direct current discharge, a high frequency discharge or a microwave discharge to excite a raw material gas into plasma, and a plasma provided on the outer periphery of the vacuum chamber. Magnet to keep away from the wall surface of the vacuum chamber, a discharge mechanism for causing an electric discharge in the vacuum chamber, an extraction electrode system provided at the outlet of the vacuum chamber for extracting an ion beam from plasma, and a raw material A silicon ion source with a built-in crucible, characterized in that it comprises a crucible made of Ta that contains silicon that is placed inside a vacuum chamber. 2. A vacuum chamber which can be evacuated to a vacuum to provide a space for inducing a direct current discharge to excite a raw material gas into a plasma, and a vacuum chamber provided inside the vacuum chamber, which serves as a cathode for the vacuum chamber and heat A cathode filament that emits electrons and excites the raw material gas, an anode filament that heats the vacuum chamber as a heater at the same potential as the vacuum chamber, and a thermal filament provided along the inner wall of the vacuum chamber to reflect heat radiation. Radiation shield for extraction, an extraction electrode system provided at the outlet of the vacuum chamber for extracting ion beams from the plasma, and a crucible made of Ta which contains silicon as a raw material and is placed inside the vacuum chamber. A silicon ion source with a built-in crucible.