JP6150705B2 - Microwave ion source - Google Patents

Description

  The present invention relates to a microwave ion source.

  Ion sources that use microwaves for plasma generation are known. Microwave is introduced into the vacuum plasma chamber. The source gas supplied to the plasma chamber is excited by microwaves to generate plasma. Ions are extracted from the plasma. The ions thus extracted from the ion source are used, for example, for an ion implantation process. Also known is a method for efficiently generating plasma by applying a magnetic field for causing electron cyclotron resonance to the plasma chamber.

Japanese Patent Laid-Open No. 7-14535 Japanese Unexamined Patent Publication No. 4-351538

  When charged particles reach the inner wall of the plasma chamber, they can be lost from the plasma. In order to draw more ions out and use them, it is desirable to suppress such plasma loss.

  One exemplary objective of certain aspects of the present invention is to generate a high density plasma in a microwave ion source.

  According to an aspect of the present invention, a microwave ion source includes a microwave introduction unit, an ion extraction unit, and a sidewall unit that connects the microwave introduction unit and the ion extraction unit so as to surround a plasma generation space; A plasma chamber is provided. At least one of the side wall portion and the ion extraction portion includes a secondary electron emission material layer having a surface exposed to the plasma generation space, and unevenness is formed on at least a part of the exposed surface.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to the present invention, high-density plasma can be generated in a microwave ion source.

1 is a cross-sectional view schematically illustrating a microwave ion source according to an embodiment of the present invention. The cross section of the plasma chamber in the dashed-dotted line A shown in FIG. It is a figure which illustrates the magnetic field which arises in a plasma chamber by the magnetic field generator in the microwave ion source which concerns on one embodiment of this invention. It is sectional drawing which shows schematically a part of side wall part of the plasma chamber which concerns on other embodiment of this invention.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description, the same elements are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. Moreover, the structure described below is an illustration and does not limit the scope of the present invention at all.

  FIG. 1 is a cross-sectional view schematically illustrating a microwave ion source 10 according to an embodiment of the present invention. A microwave ion source 10 generates high-density plasma by inputting microwave power in the direction of magnetic field into a plasma chamber 12 to which a magnetic field satisfying electron cyclotron resonance (ECR) or a magnetic field higher than the magnetic field is applied to generate ions with high density. Ion source to be extracted. The microwave ion source 10 is configured to generate a plasma of a source gas by the interaction between a magnetic field and a microwave, and to extract ions from the plasma to the outside of the plasma chamber 12.

  As is well known, the strength of the magnetic field that satisfies the ECR condition is uniquely determined with respect to the microwave frequency used, and a magnetic field of 87.5 mT (875 gauss) is required when the microwave frequency is 2.45 GHz. It is. Hereinafter, for convenience of explanation, a magnetic field that satisfies the ECR condition may be referred to as a resonance magnetic field.

  The microwave ion source 10 is used, for example, as an ion source for an ion implantation apparatus or a particle beam therapy apparatus. The microwave ion source 10 is used as a monovalent ion source, for example. The microwave ion source 10 can also be used as an ion source for a proton accelerator or an X-ray source.

  The plasma chamber 12 is accommodated in a vacuum chamber (not shown). Therefore, a vacuum environment for generating and maintaining plasma is provided in the internal space of the plasma chamber 12. Hereinafter, the internal space of the plasma chamber 12 may be referred to as a plasma generation space 14.

  The plasma chamber 12 has a cylindrical shape having both ends. Hereinafter, the direction from one end to the other end of the plasma chamber 12 may be referred to as an axial direction for convenience. In addition, a direction orthogonal to the axial direction may be referred to as a radial direction, and a direction surrounding the axial direction may be referred to as a circumferential direction. However, these do not necessarily mean that the plasma chamber 12 has a rotationally symmetric shape. The axial length of the plasma chamber 12 may be longer or shorter than the radial length of the end portion of the plasma chamber 12.

  The plasma chamber 12 includes a microwave introduction unit 16 and an ion extraction unit 18. The microwave introduction part 16 and the ion extraction part 18 are opposed to each other with the plasma generation space 14 in between.

  The plasma chamber 12 includes a side wall 20 that connects the microwave introduction unit 16 and the ion extraction unit 18 and surrounds the plasma generation space 14. Therefore, the inner surface of the side wall portion 20 is exposed to the plasma generation space 14. Side wall portions 20 are fixed to the outer peripheral portions of the microwave introduction portion 16 and the ion extraction portion 18. At least a main body portion of the side wall portion 20 is formed of a nonmagnetic metal material such as stainless steel or aluminum.

  A plasma generation space 14 is defined in the plasma chamber 12 by the microwave introduction part 16, the ion extraction part 18, and the side wall part 20. Note that the plasma chamber 12 is configured such that the microwave introduction part 16, the ion extraction part 18, and the side wall part 20 integrally surround the plasma generation space 14, and thus the side wall part 20 connected to the ion extraction part 18. The end can also be regarded as part of the ion extraction portion 18. Similarly, the end of the side wall portion 20 connected to the microwave introduction portion 16 can be regarded as a part of the microwave introduction portion 16.

  The plasma chamber 12 has, for example, a cylindrical shape. In this case, the microwave introduction part 16 and the ion extraction part 18 are substantially disk-shaped, and the side wall part 20 is substantially cylindrical. The plasma chamber 12 may have any shape as long as plasma can be appropriately accommodated.

  The side wall portion 20 includes a secondary electron emission material layer 50 on the inner surface layer portion thereof. Therefore, the secondary electron emission material layer 50 has a surface exposed to the plasma generation space 14. The secondary electron emission material layer 50 is provided on the side of the ion extraction portion 18 in the side wall portion 20. The exposed surface of the secondary electron emission material layer 50 is flat with the exposed surface of the side wall portion 20 to the plasma generation space 14. In FIG. 1 and FIG. 3, the secondary electron emission material layer 50 is indicated by a bold line for convenience of understanding.

  In the present embodiment, the secondary electron emission material layer 50 is formed of a material having a high secondary electron emission coefficient, such as alumina, magnesium oxide, or boron nitride. Here, a material having a high secondary electron emission coefficient means at least one electron when ions are incident on the material at an energy level (for example, several eV to several tens eV, or about 10 eV) required for ions in this application. A material that releases Thus, the secondary electron emission material layer 50 is higher than the portion of the inner surface of the plasma chamber 12 that is not covered with the layer (for example, the region where the main body portion of the side wall portion 20 is exposed to the plasma generation space 14). Secondary electron emission capability.

  The secondary electron emission material may be an alkaline earth metal oxide containing at least two alkaline earth metal elements. Such alkaline earth metal oxides can have a high secondary electron emission capability. Therefore, it is desirable that the alkaline earth metal element is an element having a large number of electrons. Therefore, the first element of at least two alkaline earth metal elements may be strontium or barium. In addition, the second element of the at least two alkaline earth metal elements may be selected from the group consisting of magnesium, calcium, strontium, and barium, and may be an element different from the first element. Alternatively, the at least two alkaline earth metal elements may include strontium and an element selected from the group consisting of magnesium, calcium, and barium. The alkaline earth metal oxide may be strontium calcium oxide or strontium magnesium oxide.

  FIG. 2 shows a cross section of the plasma chamber 12 taken along one-dot chain line A shown in FIG. Irregularities are formed on at least part of the secondary electron emission material layer 50. As shown in FIG. 2, the unevenness includes a plurality of grooves 52 that are continuous with the outer periphery of the plasma generation space 14. The plurality of groove portions 52 are provided at equal intervals in the circumferential direction. Each of the plurality of grooves 52 is a recess extending along the microwave traveling direction P (see FIG. 1) (that is, in the axial direction of the plasma chamber 12). In the illustrated example, the width of each groove 52 is equal. Moreover, the depth of each groove part 52 is equal.

  Although details will be described later with reference to FIG. 3, the magnetic field 54 of the magnetic field generator 40 intersects at least a part of the exposed surface of the secondary electron emission material layer 50. The uneven shape of the secondary electron emission material layer 50 is formed in the region of the exposed surface that intersects the magnetic field 54.

  Such irregularities may be formed by processing the secondary electron emission material layer 50 when the secondary electron emission material layer 50 is relatively thick. When the secondary electron emission material layer 50 is relatively thin, it is formed by processing the main body portion of the side wall portion 20, and this main body portion is covered with the secondary electron emission material layer 50. Irregularities may be formed on the exposed surface of the material layer 50.

  In addition, in order to protect the side wall part 20 of the plasma chamber 12 from a plasma, the liner (for example, boron nitride) which coat | covers the inner surface of the side wall part 20 may be provided. When the liner is formed of a material having a high secondary electron emission coefficient, the liner corresponds to the secondary electron emission material layer 50. Further, the side wall part 20 may include a cooling part for cooling the side wall part 20. This cooling part may be built in the side wall part 20 or may be attached outside the side wall part 20.

  As shown in FIG. 1, the microwave introduction unit 16 includes a vacuum window 22 and a vacuum window support 24 that supports the vacuum window 22. The vacuum window 22 seals the inside of the plasma chamber 12 to a vacuum. The vacuum window 22 is sometimes called a microwave introduction window. One side of the vacuum window 22 faces the plasma generation space 14, and the other side of the vacuum window 22 is directed to the microwave supply system or the waveguide 26. The traveling direction P of the microwave is perpendicular to the vacuum window 22. The vacuum window 22 receives the microwave from the waveguide 26 into the plasma generation space 14. In the present embodiment, the vacuum window 22 occupies the entire microwave introducing portion 16, but the vacuum window 22 may be formed in a part (for example, the central portion) of the microwave introducing portion 16.

  The microwave incident power to the plasma chamber 12 is greater than about 100 W, for example. Alternatively, the microwave incident power to the plasma chamber 12 may be about 500 W or more, or about 1 kW or more. The waveguide 26 and the vacuum window 22 are suitable for supplying such a high-power microwave compared to other supply means such as a coaxial line.

  The vacuum window 22 is made of a dielectric such as alumina or boron nitride. In this embodiment, the vacuum window 22 has a two-layer structure, for example, a boron nitride window inner layer 28 faces the plasma generation space 14, and an alumina window outer layer 30 is adjacent to the window inner layer 28. The surface of the window inner layer 28 facing the plasma generation space 14 is flat. The window inner layer 28 covers the window outer layer 30 in order to protect the window outer layer 30 from electrons flowing back to the plasma chamber 12 from the outside of the plasma chamber 12 through the ion extraction opening 32. The vacuum window support 24 is made of a nonmagnetic metal material such as stainless steel or aluminum. The vacuum window support 24 may be a flange formed at one end of the side wall 20.

  On the other hand, at least one ion extraction opening 32 is formed in the ion extraction portion 18. The ion extraction opening 32 is, for example, a slit elongated in a direction perpendicular to the paper surface. The ion extraction opening 32 is formed in the central portion of the ion extraction portion 18. The ion extraction opening 32 is formed at a position facing the vacuum window 22 across the plasma generation space 14. The vacuum window 22, the plasma generation space 14, and the ion extraction opening 32 are arranged along the central axis of the plasma chamber 12. Ions are extracted from the plasma generation space 14 through the ion extraction opening 32. Of the inner wall surface of the ion extraction portion 18, the portion facing the microwave introduction portion 16 may be referred to as an ion beam extraction surface 34. The ion beam extraction surface 34 is exposed to the plasma generation space 14.

  The ion extraction portion 18 is made of a nonmagnetic metal material such as stainless steel or aluminum. The ion extraction part 18 may be a high melting point metal such as tungsten or a high melting point metalloid conductor such as graphite. Therefore, the non-magnetic, conductive, and high melting point material is exposed on the ion beam extraction surface 34. Such a material is generally inferior in secondary electron emission capability compared to the secondary electron emission material of the side wall portion 20.

  A power supply 48 is provided to apply a positive high voltage to the plasma chamber 12. When a high voltage is applied to the plasma chamber 12, the same high voltage is also applied to the ion extraction unit 18, which is a part of the plasma chamber 12, so that the ion extraction unit 18 may be called a plasma electrode.

  An extraction electrode system including at least one extraction electrode 36 for extracting ions to the outside of the plasma chamber 12 is provided outside the ion extraction opening 32 in the axial direction. The extraction electrode 36 is opposed to the ion extraction portion 18 with an extraction gap 38 in the axial direction. The extraction electrodes 36 are each formed, for example, in an annular shape, and have an opening at the center for allowing ions extracted from the plasma chamber 12 to pass through. Further, the extraction electrode system includes an extraction power source (not shown) for applying a potential to the extraction electrode 36.

  As shown in FIG. 1, the microwave ion source 10 includes a magnetic field generator 40 for generating a magnetic field in the plasma chamber 12. The magnetic field generator 40 is disposed so as to surround the side wall portion 20 of the plasma chamber 12 in order to generate a magnetic field directed in the axial direction on the central axis of the plasma chamber 12. The magnetic force line direction M is indicated by an arrow in FIG. The magnetic force line direction M by the magnetic field generator 40 is the same direction as the microwave traveling direction P. Further, this magnetic field has a resonance magnetic field or higher intensity at least at a part on the central axis of the plasma chamber 12. However, the magnetic field generator 40 can generate a magnetic field lower than the resonance magnetic field in at least a part on the axis of the plasma chamber 12.

  The magnetic field generator 40 includes a donut-shaped coil 42 surrounding the plasma chamber 12 and a yoke 44 attached to the coil 42. The coil 42 is formed in an annular shape, and a conducting wire is wound in the circumferential direction of the plasma chamber 12. The yoke 44 is provided adjacent to the outer periphery of the coil 42 and both ends in the axial direction. Further, the magnetic field generator 40 includes a coil power supply (not shown) for causing a current to flow through the coil 42.

  The magnetic field generator 40 may include a plurality of coils arranged along the axial direction of the plasma chamber 12 instead of including one coil 42 as illustrated. Further, the magnetic field generator 40 may include a permanent magnet together with the one or more coils or instead of the one or more coils.

  The magnetic field generator 40 may include a magnet support (not shown) for supporting the magnetic field generator 40 with respect to the plasma chamber 12. In this case, the magnetic field generator 40 is disposed so as to surround the plasma chamber 12 by the magnet support portion. In the present embodiment, the magnetic field generator 40 is supported so that a radial gap 46 is formed between the plasma chamber 12 and the magnetic field generator 40. When the magnetic field generator 40 and the plasma chamber 12 are thus separated, different potentials may be applied to the magnetic field generator 40 and the plasma chamber 12. For this purpose, the microwave ion source 10 may include a high voltage power source (not shown) for applying a potential to each of the plasma chamber 12 and the magnetic field generator 40. The magnetic field generator 40 may be directly attached to the outer surface of the plasma chamber 12.

  The operation of the microwave ion source 10 according to this embodiment will be described. Microwaves are introduced into the plasma chamber 12 through the vacuum window 22. The source gas is excited by the microwave, and plasma is generated in the plasma chamber 12. The generated plasma is confined by the magnetic field 54 generated in the plasma chamber 12 by the magnetic field generator 40. Ions are extracted out of the plasma chamber 12 through the ion extraction opening 32.

  A magnetic field 54 generated in the plasma chamber 12 by the magnetic field generator 40 is illustrated in FIG. The magnetic field 54 is directed in the microwave traveling direction P (see FIG. 1). The magnetic field 54 crosses the ion beam extraction surface 34 on and near the central axis of the plasma chamber 12. The secondary electron emission material layer 50 is crossed on the outer peripheral side of the plasma chamber 12 away from the central axis. As described above, the secondary electron emission material layer 50 is provided on the side of the ion extraction portion 18 in the side wall portion 20.

  The charged particles in the plasma move along the magnetic field 54 (for example, so as to wrap around the magnetic field 54). A part of the charged particles hits the ion beam extraction surface 34 and heats the ion beam extraction surface 34. Such charged particles can be trapped on the ion beam extraction surface 34 and lost from the plasma. Further, when electrons in the plasma are trapped by the wall surface, the plasma potential rises, and ions can be attracted to the wall and lost from the plasma.

  On the other hand, some charged particles (for example, ions) strike the secondary electron emission material layer 50. When charged particles collide with the secondary electron emission material layer 50, secondary electrons are emitted to the plasma generation space 14, and electrons are replenished to the plasma. Since the secondary electron emission material layer 50 is adjacent to the ion extraction portion 18, the electron density in the region of the plasma generation space 14 adjacent to the ion extraction portion 18 is increased, so that the region approaches the electrical equilibrium state. Ions flow in. Thus, the plasma density of the ion extraction unit 18 can be improved, and the amount of ions extracted from the microwave ion source 10 (that is, the amount of ion beam current that can be extracted) can be increased.

  In the present embodiment, irregularities are formed on the exposed surface of the secondary electron emission material layer 50. By forming the irregularities, the surface area of the secondary electron emission material layer 50 is geometrically increased. By increasing the area of the region where secondary electrons can be emitted in this manner, the amount of secondary electrons supplied can be increased. In particular, irregularities are formed in the wall portion intersecting with the magnetic field 54, and thus the surface area of a region where more charged particles are incident is increased. Therefore, the supply amount of secondary electrons can be further increased.

  In the present embodiment, the secondary electron emission material layer 50 is provided on the sidewall 20. In most cases, the inner surface area of the side wall portion 20 is larger than the area of the end surface of the plasma chamber 12 (for example, the ion beam extraction surface 34). Therefore, if the irregularities are formed with the same density, the amount of increase in the surface area becomes larger when the irregularities are formed on the side wall portion 20. In general, since the side wall portion 20 and the end portion of the plasma chamber 12 (for example, the ion extraction portion 18) are separate members, it is easier to manufacture by processing only one of the members than by processing irregularities on both members. is there.

  Furthermore, in this embodiment, the magnetic field 54 is directed in the microwave traveling direction P, and the groove 52 is formed along that direction. By aligning the directions of the magnetic field 54 and the groove 52 in this way, the surface of the groove 52 can be effectively used as a whole. In a certain other embodiment, as shown in FIG. 4, the groove part 56 extended in the circumferential direction in the side wall part 20 may be formed. In this case, the first side surface 58 of the groove 56 faces the magnetic field 54, but the other second side surface 60 is in the opposite direction, so that charged particles are less likely to enter. The second side surface 60 is a shade. Therefore, the contribution of the second side surface 60 as a source of secondary electrons is relatively small. Such shade does not occur when the directions of the magnetic field 54 and the groove 52 match as shown in FIG.

  Moreover, since the side wall part 20 is a cylinder, the groove part 52 of an axial direction also has the advantage that a process is easy compared with the groove part of the circumferential direction or another direction.

  In the above, this invention was demonstrated based on the Example. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiment, and various design changes are possible, various modifications are possible, and such modifications are within the scope of the present invention. By the way.

  For example, the secondary electron emission material layer 50 may be provided on at least one of the side wall part 20 and the ion extraction part 18. The secondary electron emission material layer 50 may cover part or all of the side wall part 20 or may cover part or all of the ion extraction part 18. In an embodiment, a secondary electron emission material layer may be provided on at least a part of the ion beam extraction surface 34. In this case, secondary electrons can be emitted by charged particles incident on the ion beam extraction surface 34 along the magnetic field 54.

  In the above-described embodiment, irregularities are formed on a part of the exposed surface of the side wall portion 20. In an embodiment, irregularities may be formed over the entire surface exposed to the plasma generation space 14. For example, the groove part 52 of the side wall part 20 may extend from the ion extraction part 18 to the microwave introduction part 16.

  The unevenness is not limited to the groove 52. The shape of the unevenness is arbitrary, and may be, for example, a large number of protrusions distributed two-dimensionally on the surface. In the embodiment described above, the irregularities have dimensions that are visible, but in certain embodiments the irregularities may be a porous surface or a rough surface, in which case the irregularities may not be visible to the human eye. .

  10 microwave ion source, 12 plasma chamber, 14 plasma generation space, 16 microwave introduction part, 18 ion extraction part, 20 side wall part, 26 waveguide, 40 magnetic field generator, 50 secondary electron emission material layer, 52 groove part 54 Magnetic field.

Claims (5)

A plasma chamber comprising: a microwave introduction part; an ion extraction part; and a side wall part connecting the microwave introduction part and the ion extraction part so as to surround the plasma generation space ;
A magnetic field generator for generating a magnetic field directed in the traveling direction of microwaves in the plasma chamber ,
At least one of the side wall portion and the ion extraction portion includes a secondary electron emission material layer having a surface exposed to the plasma generation space, and unevenness is formed on at least a part of the exposed surface ,
The at least part of the exposed surface of the secondary electron emission material layer includes a region where the magnetic field intersects the exposed surface;
The microwave ion source , wherein the unevenness includes a groove portion extending along a traveling direction of the microwave. The secondary electron emission material layer is provided on the ion extraction part side in the side wall part,
2. The microwave ion source according to claim 1, wherein the side wall portion has a portion that is not covered with the secondary electron emission material layer on the microwave introduction portion side . The side wall portion has a main body portion that has irregularities and is covered with the secondary electron emission material layer, whereby irregularities are formed on at least a part of the exposed surface of the secondary electron emission material layer. The microwave ion source according to claim 1 or 2.   The microwave ion source according to any one of claims 1 to 3, wherein the side wall portion includes the secondary electron emission material layer.   5. The microwave ion source according to claim 1, wherein the microwave introduction unit includes a microwave introduction window for receiving a microwave from a waveguide in the plasma generation space. 6.

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