JPH09115455A - Microwave ion source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a microwave window structure not affected by plasma impendance to stably derive ion beam therefrom by forming the center and the periphery of the microwave guide window of specific material and specifying the thickness of the microwave guide window. SOLUTION: A microwave guide window 3 to a plasma chamber 2 is in a two-layered structure and a silica disc 3A is functioned as a vacuum sealing window to propagate microwaves in the air into a vacuum plasma chamber. The guide window on the plasma contacting side is divided in the radial direction, with the center 3B formed of boron nitride and the periphery 3C formed as the silica disc. A drawing electrode is a porous drawing electrode with ten pores open. Oxygen gas and 500W-1000W microwaves are guided thereto to obtain large current ion beam of oxygen ion drawing current 200mA or more from an ion source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a microwave ion source used as an ion source of a high current ion implanter,
In particular, the present invention relates to a microwave ion source suitable for stably extracting a large current oxygen ion beam of 100 to 200 mA class.

[0002]

2. Description of the Related Art In recent years, the demand for manufacturing semiconductor devices on a silicon substrate having an oxide film embedded therein has been increasing. As a method for forming a buried oxide film, a method of implanting 100 mA-class high-current oxygen ions into a silicon substrate is used. In order to increase the implantation processing capability, it is required to realize ion implantation in which the beam current exceeds 200 mA. As an ion source for such a high-current oxygen ion implanter, a magnetic field is applied to a plasma chamber having a cylindrical structure, low-pressure oxygen gas is caused to flow, and a microwave is introduced into this to create high-density oxygen plasma, A microwave ion source that extracts an ion beam from this plasma using an extraction electrode is used. An example of a conventional microwave ion source for producing high-current oxygen ions (Japanese Patent Publication No. 7-46588)
2) is shown in FIG. A 2.45 GHz microwave generated by a microwave oscillator (not shown) is introduced into the plasma chamber 2 via the waveguide 1. The microwave introduction window 3 has two or three layers (3 in the figure).
layer). An axial magnetic field is applied to the plasma chamber 2 by the coil 4. A large current oxygen ion beam 6 is extracted from the plasma chamber using the ion extraction electrode system 5. The conventional example of FIG. 2 is an example of a microwave ion source used for extracting an oxygen beam of 100 mA level.

In FIG. 2, the microwave introduction window 3 is plural in the axial direction for the following reason. Conventional tens of mA
In the microwave ion source for the purpose of extracting ions of the class, the microwave introduction window was a single window made of quartz or alumina. However, in the case of the 100 mA class large current oxygen ion beam shown in FIG. 2, there were problems caused by the large current and problems peculiar to oxygen.

Generally, when an ion beam is extracted from plasma, electrons or secondary electrons generated by collision of ions and gas in the space of the extraction electrode system with some of the beams collide with the extraction electrode, and the like, are equal to the extraction voltage. It becomes high-energy electrons and flows back to the plasma chamber side. The high-energy electrons collide with the microwave introduction window, and there is a problem that the microwave introduction window is damaged by thermal shock. For this reason, in the example of FIG. 2 for drawing out a 100 mA class oxygen beam, the window is multi-layered and the introduction window facing the plasma chamber is made of a dielectric material that is resistant to thermal shock. Furthermore, in order to efficiently absorb microwaves into the plasma through the multi-layered dielectric, the thickness of each dielectric should be increased,
From the viewpoint of the microwave circuit, the thickness is set so that impedance matching with plasma can be achieved. As a problem inherent to the extraction of oxygen ions, irradiation of backflow electrons accelerates the chemical reaction between plasma oxygen and the insulating material, causing rapid etching of the dielectric without cracking due to thermal shock, resulting in the formation of through holes in the dielectric. When I opened it, the introduction window of the base was finally damaged.

In the case of the conventional example shown in FIG. 2, the impact of backflow electrons is concentrated on the central portion of the microwave introduction window. Since the microwave window is fixed by the peripheral flange and there is no part that escapes the thermal shock in the radial direction, raising the extraction current from the conventional 100 mA class to a higher value of 200 mA causes a problem that even a dielectric material resistant to thermal shock will be damaged. there were. The opening of the through-hole also occurs at a double speed when the drawing exceeds 200 mA. When a crack occurs or a through hole is opened, it is necessary to immediately replace the dielectric material in contact with the plasma in order to avoid the subsequent damage of the underlying introduction window. On the other hand, the dielectric is generally expensive, and the introduction window structure that can be exchanged at a low frequency and can be exchanged at a low cost was necessary for increasing the oxygen drawing current to the level of 200 mA.

[0006] As other problems, there are the following problems.

In the conventional example shown in FIG. 2 for the purpose of drawing 100 mA class, the impedance of the introduction window must be designed in consideration of its own dielectric constant and plasma dielectric constant. However, since the impedance of plasma changes depending on the absorbed microwave power, plasma conditions (plasma density, electron temperature, etc.) that can achieve impedance matching are limited. Furthermore, the dimensions of the plasma chamber (diameter 90
It was necessary to change the material and structure of the introduction window in order to achieve impedance matching with high-density plasma capable of drawing out 200 mA or more (about mm).

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a structure for facilitating thermal shock to escape in the radial direction when the oxygen extraction beam current is increased so as to prevent damage to the microwave introduction window in contact with the plasma. It is in. Another object of the present invention is to provide a microwave window structure that is not affected by plasma impedance in order to stably extract an ion beam in a wide current range.

[0009]

The part that is most strongly subjected to thermal shock is the central part, which is also coincident with the part where the through hole opens. Therefore, if the central portion is removed and replaced so that it can be loosely attached, thermal damage is reduced, the frequency of replacement is reduced, and the material can be made inexpensive. That is, in order to solve the above problems, the dielectric window in contact with the plasma may be divided in the radial direction and made of dielectrics having different permittivities.

In order to construct a microwave introduction window that is not affected by plasma impedance, the thickness of the introduction window is set to a thin thickness of 1/4 or less of the microwave wavelength, and the existence of the introduction window ensures efficient microwave propagation. The effect on the impedance matching condition of 1 may be made relatively small as compared with the conventional example, and can be ignored.

The dielectric window in contact with the plasma is divided in the radial direction, and only the central portion where the backflow electrons collide is made a dielectric material resistant to thermal shock. If this is installed mechanically loosely, the The shock can escape and thermal shock damage can be avoided even when a large current of 200 mA or more is drawn.

The microwave is introduced into the plasma chamber by using a waveguide. When the thickness of the introduction window is ¼ or more of the guide wavelength, the microwave electric field strength is from zero to the maximum in the thickness direction of the dielectric. Since the part up to is inside the dielectric body, it is necessary to design the device with due consideration of its influence on the absorption and reflection of microwaves. Specifically, it is necessary to select the thickness in consideration of impedance matching with plasma. The influence of such a window material can be reduced by reducing the thickness. Practically, it is effective to set the wavelength to 1/4 or less of the guide wavelength. A structure that does not frequently cause damage and replacement of the window even if the window thickness is reduced may be used.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The contents of the present invention will be described below with reference to embodiments. FIG. 1 shows an embodiment according to the present invention. In the figure, two microwave introduction windows are stacked, and 3A is the heat of 5 to 1
The 0 mm quartz disk functions as a vacuum sealing window for transmitting microwaves in the atmosphere to the plasma chamber in vacuum. The introduction window on the side in contact with plasma is divided into two in the radial direction, the central portion 3B is made of boron nitride, and the peripheral portion 3C is made.
Is a quartz disk. The thickness of boron nitride in the central portion 3B is 5
It is in the range of 10 mm. The diameter was about 30 mm. On the other hand, although the outer diameter of the quartz disk in the peripheral portion changes depending on the diameter of the plasma chamber, it is set to about 80 mm in this embodiment.

The distance between the microwave introduction window of the plasma chamber and the extraction electrode, that is, the length of the plasma chamber is 120 mm and the inner diameter is in the range of 80 to 120 mm. In the extraction electrode system composed of three extraction electrodes, a voltage of 40 to 50 kV was applied to the acceleration electrode in contact with plasma, and a voltage of -1.5 to -3.0 kV was applied to the intermediate electrode.
The third electrode is at ground potential. Diameter of extraction electrode is 5mm
It is a porous extraction electrode having 10 holes. Oxygen gas was introduced, and a microwave of 500 W to 1000 W was introduced, and the oxygen ion extraction current value from the ion source was 210 m.
The value of A was stably obtained. In the case of the present embodiment of FIG. 1, the microwave introduction windows 3B and 3C in contact with the plasma could be stably used without being damaged for several tens to several hundred hours. On the other hand, as a conventional example,
When a single alumina disk or boron nitride disk without dividing the microwave introduction window in contact with the plasma shown in FIG. 1 is used, the beam can be stably generated for only a few tens of hours at the time of drawing a large current exceeding 200 mA. It could not be pulled out and it was damaged due to thermal shock.

In FIG. 1, a boron nitride disk is used for the insulator 3B in the center part and a quartz plate is used for the peripheral part 3C. When a quartz plate is used in the center part, high-speed etching of the quartz plate by backflow electrons is performed. Occurred and it was difficult to operate for a long time. In addition to boron nitride, alumina and alumina nitride were effective as the insulator at the center. Table 1 summarizes the quality of the combination of the materials of the insulator 3B (disk) in the central portion and the insulator 3C (annular) in the peripheral portion at the time of withdrawing 200 mA or more. The quality was judged based on whether stable withdrawal for several tens of hours or more was possible.

[0016]

[Table 1]

In the embodiment shown in FIG. 1, the total thickness of the microwave introduction window is in the range of 10 to 15 mm. 2.45 GH
The guide wavelength for z depends on the structure of the waveguide, but is about 14 to 16 cm in a normal waveguide. Therefore, the total thickness of the introduction window is sufficiently smaller than 1/4 of the guide wavelength. In the experiment in which the thickness is about 30 mm, which is close to the value in the conventional example of FIG.
It took 10 hours or more for adjustment until the drawing of 00 mA. This is because it took time to generate plasma having a plasma parameter suitable for impedance matching. In fact, when microwaves are introduced, an impurity gas or the like is generated for a long time due to the heating of the introduction window, so that oxygen plasma having an impedance determined by a predetermined plasma parameter is not generated until the influence of these impurity plasmas disappears. is there. Further, the introduction window is also heated by the plasma irradiation, but since the dielectric constant changes due to the heating, impedance mismatch occurs depending on the degree of heating, and the plasma state changes again. Therefore, in the case of a thick microwave introduction window,
It is considered that it took a long time to adjust the stable withdrawal to 200 mA.

On the other hand, in the embodiment of FIG. 1, since the introduction window is thin and the influence of the dielectric substance on the impedance matching of microwaves is small, it is possible to stably withdraw 200 mA within a few hours.

[0019]

According to the present invention, when a 200 mA-class high-current oxygen ion beam is extracted from a cylindrical microwave plasma chamber, it is possible to easily and stably extract ions without damaging the microwave introduction window. When applied as an ion source for an implanter, simple and stable ion implantation becomes possible, and 100 to 2 at 40 to 50 keV or more.
The effect is remarkably large in that the ion implantation of 00 mA is practical.

In the embodiment, the example of the division in the radial direction is shown, but it is clear from the essence of the invention that the effect is obtained even if it is divided into three or more. Further, although the oxygen ion beam is taken as an example in the embodiment, it is clear that it is also effective for extraction of other ion species exceeding 200 mA.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional microwave ion source.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Microwave waveguide, 2 ... Plasma chamber, 3 ... Microwave introduction window, 3A ... Insulator for vacuum sealing, 3B ... Insulation in central part of introduction window, 3C ... Insulator in peripheral part of introduction window, 4 ... coil, 5 ... extraction electrode system, 6 ... ion beam, 7 ... water cooling pipe.

Claims (3)

[Claims] 1. An insulator having a plurality of microwave introduction windows for generating plasma by microwave discharge in a magnetic field, extracting an ion beam using an ion extraction electrode, and introducing microwaves from a microwave oscillator into a plasma generation chamber. A microwave ion source having a microwave introduction window formed by stacking the above with each other, wherein the introduction window in contact with the plasma is composed of a plurality of types of insulators having different permittivities in the radial direction. 2. The microwave ion source according to claim 1, wherein the total thickness of the plurality of introduction windows including the introduction windows divided in the radial direction is set to ¼ or less of the guide wavelength of the microwave. 3. The structure of the introduction window in contact with plasma according to claim 1, wherein the central portion is made of boron nitride and the other portions are made of quartz, and a microwave introduction window having a dielectric constant varied in the radial direction is provided. Microwave ion source.