Electric lamp and discharge devices – With positive or negative ion acceleration
Reexamination Certificate
1998-11-24
2001-05-29
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
C313S360100, C315S505000, C315S506000
Reexamination Certificate
active
06239541
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an RFQ accelerator and an ion implanter provided with thereof for use in ion irradiation and ion implantation, and particularly relates to an improvement in electrode configuration of such an RFQ accelerator.
BACKGROUND OF THE INVENTION
An RFQ accelerator is capable of accelerating an ion beam with a focusing force and thus capable of accelerating an ion beam of large current without divergence, and therefore is used as a high energy ion acceleration tube of a high energy ion implanter. The RFQ accelerator also has its application as an ion accelerator of experimental, analytical, and medical use.
As representative accelerators of charged particles, circular accelerator and linear accelerator are available. The circular accelerator, such as cyclotron, accelerates a beam in a circular motion and the linear accelerator accelerates a beam in a linear motion. The RFQ accelerator is an example of the latter. The linear accelerator works under the principle that ions are accelerated by application of a DC (Direct Current) high voltage between hollow electrodes. In this case, when the acceleration energy is qV, a DC power source capable of generating a high voltage V is required. Thus, in order to realize acceleration of several MeV, which is required in high energy ion implanters, a high voltage power source in the order of MV is required, and the power source section alone takes up a large space. Also, in such an accelerator for carrying out high energy acceleration, a vacuum chamber for passing a beam is required to have a large volume. As a result, the high energy accelerator such as above is inevitably large and expensive.
Meanwhile, in recent years, a demand for ion implantation with a high energy of several MeV has been increasing in semiconductor industry. However, to realize production in an industrial setting, a large device, which takes no account of costs, fails to meet such a demand, and there is a need for new and smaller accelerator capable of high energy acceleration.
As an accelerator suitable for such purpose, an RFQ accelerator has been getting an attention. The RFQ accelerator is one of relatively newer linear accelerators, and has a schematic arrangement wherein four electrodes are placed on a position corresponding to vertices of a square, and the electrodes on a diagonal line are connected to each other, and a radio-frequency voltage is induced between adjacent electrodes.
Namely, the four electrodes constitute a quadrupole, and a radio-frequency is applied between adjacent electrodes. Instead of applying a DC high voltage between electrodes which are separated from one another in a beam propagation direction, radio-frequency is induced between four electrodes parallel to the beam propagation direction. The radio-frequency is applied to quadrupole electrodes in this manner, thus the name RFQ, which stands for Radio-Frequency Quadrupole.
The RFQ accelerator was first proposed by Kapchinskii and Teplyakov (I. Kapchinskii and V. Teplyakov Prib. Tekh, Eksp.2 (1970) p.19). Then, it was first confirmed in 1981 that the RFQ accelerator is actually capable of carrying out acceleration in the Los Alamos National Laboratory of the United States (J. E. Stovall, K. R. Crandall and R. W. Hamm, IEEE Trans. Nucl. Sci, NS-28 (1981) P.1508).
Such an RFQ accelerator has a schematic structure wherein four electrodes (for example, A, B, C, and D in counterclockwise direction) are placed on a position corresponding to vertices of a square on a plane perpendicular to a beam propagation direction (z direction). On each of the four electrode rods are formed crest and trough portions in the lengthwise direction, and the electrodes are oriented such that the crest portions of a pair of electrodes, for example, electrodes A and C, correspond to the trough portions of the adjacent other pair of electrodes B and D, and that the trough portions of the pair of electrodes A and C correspond to the crest portions of the other pair of electrodes B and D. By inducing a radio-frequency voltage between each pair of electrodes A and C and the electrodes B and D, an accelerating electric field is generated in the beam propagation direction and a converging electric field is generated in a direction perpendicular to the beam propagation direction. A period between the crest and the trough of the electrode is called a cell.
Then, the time w/v in which ions travel over a distance w of a cell is set to be equal to a half-period T/2 of the radio-frequency. Namely, when the wavelength of the radio-frequency is &lgr;, the distance w=vT/2=(v/c) (cT/2)=&bgr;&lgr;/2. When the distance between adjacent crests is determined in this manner, ions pass through a cell per alternation of the accelerating electric field in the z direction. Thus, ions are accelerated by being subjected to electric field per cell. The RFQ accelerator functions as a linear accelerator because the propagation of ions and the alternation of the radio-frequency are synchronized in this manner. As ions are accelerated, v increases, and accordingly &bgr;=v/c is also increased. Thus, the electrodes are designed such that the cell length increases progressively by small increments along the lengthwise direction of the electrodes.
As described above, the RFQ accelerator accelerates ions under the principle that is completely different from that of the conventional linear accelerator in which ions are accelerated linearly by application of a DC high voltage between electrodes which are separated from one another in the beam propagation direction. Thus, even through the RFQ accelerator is categorized as a linear accelerator by the fact that ions are accelerated in a straight line trajectory, the RFQ accelerator is largely different from the conventional linear accelerator in the arrangement of the electrodes and in the acceleration voltage, for which radio-frequency is used instead of direct current.
The RFQ accelerator has various advantages. First, it is not required to provide a large power source of a DC high voltage, instead a small radio-frequency power source is provided, thus reducing the volume of the power source section.
Secondly, the dimensions of the acceleration tube can be made compact. The cell length of the four electrodes is very small, and beam bore radius R
1
is 4 mm. Thus, because the gap between the electrodes is narrow and the dimension in a direction perpendicular to the beam propagation direction is small, the cylindrical vacuum chamber surrounding the electrodes can be made sufficiently compact with a diameter of, for example, 600 mm. Further, the length in the direction of beam axis can be made short. For example, the length of the chamber is from 1 m to 3 m.
Thus, the RFQ accelerator is highly appealing in view of the power source requirement and the size of a vacuum chamber, and unlike the conventional linear accelerator of DC type, has a potential of realizing a practical accelerator in an industrial setting, such as manufacturing of semiconductors.
In the RFQ accelerator having the described arrangement, the present invention concerns the configuration of the four RFQ electrodes of A, B, C, and D, and their proportional relations to one another. The electrodes A, B, C, and D are provided extending in the beam propagation direction, and are rods each having crest and trough portions which are 180° off-phase between adjacent electrodes (A and B, B and C, C and D, and D and A). Several points on the electrodes A, B, C, and D are supported by components called posts.
Posts provide a mechanical support of the electrodes A, B, C, and D to the inner wall of a tank (vacuum chamber), and form a resonance circuit in the tank. The electrodes A, B, C, and D and the posts generate a large amount of heat as a result of a large amount of radio-frequency current flowing on them. Thus, the electrodes A, B, C, and D are made of material having high electric and thermal conductivity, and a coolant is flown therein. In order to allow sufficient flow o
Nissin Electric Co. Ltd.
Patel Nimeshkumar D.
Staas & Halsey , LLP
Williams Joseph
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