Ion source

A cathode holder of a tubular shape is inserted into an opening for a cathode of a plasma generating chamber with a tip of the cathode holder positioned outward from an inner wall surface of the plasma generating chamber. The cathode is held in the cathode holder so that a front surface of the cathode will be positioned outward from the inner wall surface. In the cathode holder is provided a tubular first heat shield surrounding the cathode with a space provided between the first heat shield and the cathode, the tip of the first heat shield positioned outward from the inner wall surface. At a rear side of the cathode is provided a filament. The gap between the cathode holder and the plasma generating chamber is filled with an electrical insulating material.

The present application claims foreign priority based on Japanese Patent Application No. 2005-144376, filed May 17, 2005, the content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates an ion source having a structure where a cathode is heated by a filament to emit thermal electrons for generating plasma into a plasma generating chamber also serving as an anode. Such an ion source is also referred to as an indirectly heated cathode type ion source.

2. Related Art

This type of related art ion source has a structure where a tubular cathode holder is inserted into a plasma generating chamber with a gap between itself and the plasma generating chamber and a cathode is held at the tip of the cathode holder and a filament to heat the cathode is arranged in the cathode holder (for example, refer to JP-2995388, paragraph 0009, FIG. 6; JP-A-10-134718, paragraph 0009, FIG. 7; U.S. Pat. No. 2004/0061668 A1, paragraph 002, FIG. 1).

In the ion source, the tubular cathode holder is inserted into the plasma generating chamber, and an area where the plasma is generated is made smaller by at least the volume of the cathode holder. This lowers the ionization efficiency of a gas for generating plasma in the plasma generating chamber thus degrading the plasma generation efficiency as well as reduces the plasma volume. Therefore, it is difficult to increase the beam current of ion beams to be extracted from the ion source.

The gap between the cathode holder and the plasma generating chamber serves as an escape route of the gas for generating plasma. This lowers the use efficiency of the gas. The gas for generating plasma generally cost high. A reduced use efficiency of the gas leads to a higher operation cost of the ion source. Leakage of gas may contaminate a structure on the periphery of the plasma generating chamber, which shortens the service life of the ion source.

Further, the cathode wears with the operation time of the ion source. Although a larger axial length of the cathode (or depth of the cathode) is advantageous in terms of the service life of the cathode, and thus, the ion source, it is difficult to provide a long cathode in the related art ion source. A longer cathode results in a larger heat loss caused by emission from the side surface of the cathode, which makes it difficult to heat the cathode. Moreover, the cathode holder is heated up to a high temperature and thermal electrons are emitted therefrom. This may cause unwanted electric discharge (arc discharge) between the cathode holder and the plasma generating chamber thus causing a loss as well as contaminating the inside of the plasma generating chamber.

SUMMARY OF THE INVENTION

An object of the invention is to improve the plasma generation efficiency and gas use efficiency as well as ensure a longer service life of an ion source.

However, the present invention need not achieve the above object, and other objects not described herein may also be achieved. Further, the invention may achieve no disclosed objects without affecting the scope of the invention.

A first ion source according to the invention is an ion source having a structure where a cathode is heated by a filament and thermal electrons are emitted from the cathode into a plasma generating chamber also serving as an anode, the ion source comprising: an opening for a cathode provided in the wall surface of the plasma generating chamber; a tubular cathode holder for holding the cathode, the tip of which is inserted into the opening for the cathode from outside the plasma generating chamber so as to leave a gap between the tip and the plasma generating chamber, the tip of the cathode holder positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber; a cathode held in the cathode holder, the front surface of the cathode positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber; a tubular first heat shield arranged to enclose the side surface of the cathode by at least one layer with a gap provided between itself and the side surface of the cathode, the tip of the heat shield positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber; a filament provided in the cathode holder for heating the cathode from its rear surface; and an electrical insulating material provided in the opening for a cathode, the electrical insulating material filling the gap between the cathode holder and the plasma generating chamber.

According to the first ion source, the cathode holder and the cathode are positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber. This enlarges the plasma generation area in the plasma generating chamber thus improving the plasma generation efficiency.

The gap between the cathode holder and the plasma generating chamber is filled with an electrical insulating material. This prevents possible leakage of a gas for generating plasma and improves the gas use efficiency.

Further, the first heat shield suppresses a heat loss caused by emission from the side surface of the cathode. It is thus possible to increase the length of the cathode. This assures a longer life of the cathode, and by extension, the ion source.

The electrical insulating material may be positioned inside the plasma generating chamber and have a labyrinthine structure part having a bent cross section at the part surrounding the tip of the cathode holder.

A second ion source according to the invention is an ion source having a structure where a cathode is heated by a filament and thermal electrons are emitted from the cathode into a plasma generating chamber also serving as an anode, the ion source comprising: an opening for a cathode provided in the wall surface of the plasma generating chamber; a tubular cathode holder for holding the cathode, the tip of which is inserted into the opening for the cathode from outside the plasma generating chamber so as to leave a gap between the tip and the plasma generating chamber, the tip of the cathode holder positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber; a cathode held in the cathode holder, the front surface of the cathode positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the side of the plasma generating chamber; a tubular first heat shield arranged to enclose the side surface of the cathode by at least one layer with a gap provided between itself and the side surface of the cathode, the tip of the heat shield positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber; and a filament provided in the cathode holder for heating the cathode from its rear surface; characterized in that a labyrinthine structure part having a bent cross section is formed in a gap between the cathode holder and the plasma generating chamber.

According to the second ion source, it is possible to improve the plasma generation efficiency and extend the service life of the ion source.

It is possible to reduce the conductance of a gas by way of a labyrinthine structure part formed in a gap between the cathode holder and the plasma generating chamber. This suppresses possible leakage of a gas for generating plasma thereby improving the gas use efficiency.

The member on the cathode holder side forming a labyrinthine structure part between the plasma generating chamber and the cathode holder may be formed of an electrical insulating material.

The cathode holder may include a second tubular heat shield arranged to surround the side surface of the filament by at least one layer with a space provided between the second heat shield and the filament.

The cathode holder may include a third heat shield arranged to cover the rear surface of the filament by at least one layer with a space provided between the third heat shield and the filament.

The cathode may have a male screw part formed at the rear part and is detachably held at a holding part provided in the cathode holder by way of the male screw part and a nut screwed with the male screw part.

The filament may have a heating part in the shape of a flat plate bent along the rear surface of the cathode.

The filament may have a heating part in the shape of a round bar filament material bent along the rear surface of the cathode and the heating part may have a flat surface obtained by machining a round-bar-shaped filament material and the flat surface may be opposed to the rear surface of the cathode.

The ion source may have a heat insulating material covering the part on the outer peripheral surface of the cathode holder, the part positioned outside the plasma generating chamber.

According to a first aspect of the invention, the cathode holder and the cathode are positioned on the same plane with the inner wall surface around the opening for a cathode of the plasma generating chamber or further outward from the plasma generating chamber. This enlarges the plasma generation area in the plasma generating chamber thus improving the plasma generation efficiency, compared with a related art ion source where a tubular cathode holder is inserted into a plasma generating chamber.

As a result, it is made easy to increase the beam current of ion beams to be extracted. It is possible to reduce the amount of the power and gas supplied to generate plasma instead of or while increasing the beam current.

It is possible to prevent the side surface of the cathode holder from being exposed to plasma thus suppressing generation of impurities from the cathode holder caused by ion sputtering in the plasma. This reduces contamination in the plasma generating chamber which ensures a longer service life of an ion source.

The gap between the cathode holder and the plasma generating chamber is filled with an electrical insulating material. This prevents possible leakage of a gas for generating plasma and improves the gas use efficiency. As a result, it is possible to reduce the gas use amount thus reducing the operation cost of an ion source. It is also possible to prevent contamination of a structure on the periphery of the plasma generating chamber caused by a gas leakage, which contributes to a longer life of the ion source.

Further, the first heat shield suppresses a heat loss caused by emission from the side surface of the cathode. It is thus possible to increase the length of the cathode. This assures a longer life of the cathode, and by extension, the ion source.

According to a second aspect of the invention, the electrical insulating material has a labyrinthine structure part. Even when the creepage distance becomes longer and conductive impurities are deposited on the surface of the electrical insulating material to form a conductive film, the film reduces the chance of electrical short between the cathode holder and the plasma generating chamber. As a result, it is possible to assure a longer service life of an ion source.

According to a third aspect of the invention, the advantage due to the configuration except that a labyrinthine structure part is provided instead of an electrical insulating material of the first aspect of the invention is the same as that offered by the first aspect of the invention.

According to the invention, it is possible to lower the conductance of a gas by way of a labyrinthine structure part formed in a gap between the cathode holder and the plasma generating chamber. This suppresses possible leakage of a gas for generating plasma thereby improving the gas use efficiency. It is thus possible to prevent contamination of a structure on the periphery of the plasma generating chamber caused by a gas leakage, which contributes to a longer life of the ion source.

According to a fourth aspect of the invention, the member on the cathode holder side forming a labyrinthine structure part between the plasma generating chamber and the cathode holder is formed of an electrical insulating material. Even in case conductive impurities are deposited on the surface of the gap of the labyrinthine structure part to form a conductive film and the film peels off and thin pieces (flakes) are formed, it is possible to prevent electrical short between the cathode holder and the plasma generating chamber. This contributes to a longer service life of an ion source.

According to a fifth aspect of the invention, it is possible to reduce a heat loss caused by emission from a filament by way of the second heat shield, thus enhancing the heating efficiency of the cathode by the filament.

According to a sixth aspect of the invention, it is possible to reduce a heat loss caused by emission from a filament by way of the third heat shield, thus enhancing the heating efficiency of the cathode by the filament.

According to a seventh aspect of the invention, the cathode is detachably held by its male screw part and a nut. This makes it possible to replace easily a cathode with a new one when it is worn. As a further advantage, the male screw part requires a smaller area of contact with the nut, and by extension, the cathode holder, compared with fit. This reduces a heat loss caused by conduction of heat from the cathode to the cathode holder and enhances the heating efficiency of the cathode.

According to an eighth aspect of the invention, the filament has a heating part of the shape of a flat plate. Thus, the thermal electron emission area from the filament to the cathode is larger than when a round-rod-shaped filament is used. As a result, for example to obtain a thermal electron emission amount equivalent to that of a round-rod-shaped filament, the temperature of the filament may be lowered to extend the service life of the filament. It is also possible to extend the length of between the cathode and the filament, which assures stable operation against thermal expansion of the filament or a member on the periphery of the cathode.

According to a ninth aspect of the invention, the filament has a heating part of a flat surface. Thus, the thermal electron emission area from the filament to the cathode is larger than when a round-rod-shaped filament is used. As a result, for example to obtain a thermal electron emission amount equivalent to that of a round-rod-shaped filament, the temperature of the filament may be lowered to extend the service life of the filament. It is also possible to extend the length of between the cathode and the filament, which assures stable operation against thermal expansion of the filament or a member on the periphery of the cathode.

According to a tenth aspect of the invention, the heat insulating material may reduce emission from the cathode holder thus enhancing the heating efficiency of the cathode. Moreover, it is not necessary to additionally heat the member on the periphery of the cathode holder. This reduces the thermal expansion of the periphery member, maintains the mechanical accuracy between the cathode and the filament, thus stabilizing thermal electron emission from the filament.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is cross-sectional view of an exemplary, non-limiting embodiment of an ion source according to the invention.FIG. 2is an enlarged view of Part C inFIG. 1.

An ion source2has a structure to heat a cathode26by a filament38and emit thermal electrons from the cathode2into a plasma generating chamber also serving as an anode. The ion source2is sometimes called an indirectly heated cathode type ion source.

The plasma generating chamber4is for example of a rectangular parallelepiped. Into the plasma generating chamber4is introduced a desired gas (including in the state of vapor)10for generating plasma6via a gas inlet8. The gas10includes desired elements (for example dopant of B, P, As). To be more specific, the gas may include a material gas such as BF3, PH3, A3H3and B2H6.

In one wall surface of the plasma generating chamber4(on one of the long side walls) is provided an ion extraction port12for extracting ion beams14. The ion extraction port12has the shape of a narrow slit in the longitudinal direction of the wall surface.

In another wall surface of the plasma generating chamber4(on one of the short side walls) is provided an opening20for a cathode for positioning a cathode. The front shape of the opening20for a cathode has the shape of a circle in this example. Inside a wall surface opposed to a wall surface including the opening20for a cathode is held, via an electrical insulating material18, a reflector16for reflecting electrons in the plasma6while opposed to the cathode26.

The reflector16may be put to floating potential without being connected anywhere as in this example. Or, the reflector16may be put to cathode potential while connected to a support body50for a cathode (in other words, a negative electrode end of an arc power supply60).

As shown in this example, a magnetic field80along an axis connecting the cathode26and the reflector16may be applied to the inside of the plasma generating chamber4from a magnet (not shown) for generating/maintaining the plasma6provided outward from the plasma generating chamber4. The orientation of the magnetic field80may be opposite to that shown.

The tip of a cathode holder22in a tubular shape (cylindrical shape in this example) for holding the cathode26is inserted into the opening20for a cathode from outside of the plasma generating chamber4with a gap provided between its tip and the plasma generating chamber4. Note that the gap is filled with an electrical insulating material40. In this example, the tip of the cathode holder22is positioned further outward from the plasma generating chamber4than an inner wall surface5on the periphery of the opening20for a cathode in the plasma generating chamber. Note that the tip of the cathode holder22may be positioned on the same surface as the inner wall surface5. The cathode holder22is composed of molybdenum (Mo) for example. This also holds true for a holding part24, a first heat shield36, a second heat shield44, a third heat shield46, a support body50,52and a filament current conductor54mentioned later.

In the cathode holder22in this example is held the cathode26in the shape of a column (to be more specific, a cylindrical column) with a space provided between its side surface and the cathode holder22. A front surface28of the cathode26is positioned further outward from the plasma generating chamber4than the inner wall surface5on the periphery of the opening20for a cathode in the plasma generating chamber4. Note that the front surface28of the cathode26may be positioned on the same surface as the inner wall surface5. The cathode26is composed of tungsten (W) for example. This also holds true for a nut34and a filament38mentioned later.

The cathode26in this example has a male screw part32formed at the rear part and is detachably held at the holding part24provided in the intermediate part of the cathode holder22by way of the male screw part32and the nut34screwed with the male screw part.

In the cathode holder22is provided the first heat shield36in a tubular shape (cylindrical shape in this example) so as to surround the side surface of the cathode26by at least one layer (two layers in this example) with a space provided between the side surface of the cathode holder26and the first heat shield36. The tip of each first heat shield36is positioned further outward from the plasma generating chamber4than the inner wall surface5on the periphery of the opening20for a cathode in the plasma generating chamber4. Note that the tip of each first heat shield36may be positioned on the same surface as the inner wall surface5. Each first heat shield36is erected integrally to the holding part24of the cathode holder22in this example.

In the vicinity of the rear surface30of the cathode26in the cathode holder22is provided the filament38for heating the cathode26from its rear surface30. A specific example of the filament38will be described later.

In the opening20for a cathode in the plasma generating chamber4is provided an electrical insulating material40filling the gap between the cathode holder22and the plasma generating chamber4. The electrical insulating material40is composed of boron nitride (BN) for example. This also holds true for a heat insulating material48mentioned later.

In this example, the electrical insulating material40has a labyrinthine structure part42having a bent cross section at a part surrounding the tip of the cathode26in a circular fashion while positioned in the plasma generating chamber4. The labyrinthine structure part42has a gap43bent in the shape of a hook on the inner periphery and outer periphery, as shown inFIG. 2.

In this example, a second heat shield44in a tubular shape (cylindrical shape in this example) so as to surround the side surface of the filament38by at least one layer (one layer in this example) with a space provided between the filament38and the second heat shield44. The second heat shield44is erected integrally to the holding part24of the cathode holder22in this example.

In this example, a third heat shield46in a tubular shape (cylindrical shape in this example) so as to cover the rear surface of the filament38by at least one layer (two layers in this example) with a space provided between the filament38and the third heat shield46. The third heat shield46is erected integrally to the tip of the tabular part47.

The cathode holder22is supported in position by the support body50. The filament38is supported in position by two filament current conductors54via its two legs70(or76) (only one of the two conductors and two legs are shown). The third heat shield46is supported in position by one filament current conductor54via the tubular part47and the support body52.

To the ends of the filament38, or to be more specific, to its two legs70(or76) is connected a filament power supply56for heating the filament38. One end of the filament38and the third heat shield46are put at the same potential via the support body52and the tubular part47. The filament power supply56may be a DC poser supply as shown or an AC power supply.

Between the filament38and the cathode26is connected a DC heating power supply58, which accelerates thermal electrons emitted from the filament38to the cathode26and heating the cathode26with the impact of the thermal electrons, via the cathode holder22and with the cathode26serving as a positive pole.

Between the cathode26and the plasma generating chamber4is connected a DC arc power supply60, which accelerates thermal electrons emitted from the cathode26and ionizing the gas10introduced into the plasma generating chamber4as well as causes arc discharge in the plasma generating chamber4to generate plasma6, with the plasma generating chamber4at the positive pole.

According to the ion source2, the filament38is used to heat the cathode26and thermal electrons are emitted from the cathode26into the plasma generating chamber4. The thermal electrons are used to cause arc discharge in the plasma generating chamber4and the gas10introduced into the plasma generating chamber4is ionized to generate the plasma6. From the plasma6, it is possible to extract ion beams14via the ion extraction port12by the action of the electric field. In the vicinity of the exit of the ion extraction port12is generally provided an extraction electrode for extracting the ion beams14.

According to the ion source2, the cathode holder22and the cathode26are positioned on the same plane with the inner wall surface5around the opening20for a cathode of the plasma generating chamber4or further outward from the side of the plasma generating chamber4. It is thus possible to increase the volume of an area where the plasma is generated in the plasma generating chamber4to improve the plasma generation efficiency. In other words, it is possible to prevent the side surface of the cathode holder22from being exposed to the plasma6thus reducing the loss area of the plasma6caused by the contact with the side surface of the cathode holder22, which improves the plasma generation efficiency.

As a result, it is easy to increase the beam current of the ion beams14to be extracted from the ion source2. It is possible to reduce the amount of the power and gas supplied to generate plasma instead of or while increasing the beam current.

It is possible to prevent the side surface of the cathode holder22from being exposed to the plasma6thus suppressing generation of impurities from the cathode holder22caused by ion sputtering in the plasma6. This reduces contamination in the plasma generating chamber4which ensures a longer service life of the ion source2.

The gap between the cathode holder22and the plasma generating chamber4is filled with the electrical insulating material40. This prevents possible leakage of the gas10for generating plasma and improves the use efficiency of the gas10. As a result, it is possible to reduce the use amount of the gas10thus reducing the operation cost of the ion source2. It is also possible to prevent contamination of a structure on the periphery of the plasma generating chamber, for example the support body50and the an insulator or some insulators (not shown) for supporting the filament current conductor54, caused by leakage of the gas10, which contributes to a longer life of the ion source2.

It is possible to suppress a heat loss caused by emission from the side surface of the cathode26by way of the first heat shield36. This ensures a longer service life of the cathode26, and thus, the ion source2. For example, the thickness of a cathode is 5 to 8 mm at most in a related art ion source although the thickness of the cathode26of the ion source2may be as thick as 10 to 15 mm.

According to this embodiment, the electrical insulating material40has the labyrinthine structure part42. Since the creepage distance becomes longer, even when conductive impurities are deposited on the surface of the electrical insulating material40to form a conductive film, it is possible to reduce the chance of electrical short between the cathode holder22and the plasma generating chamber4by the film. As a result, it is possible to assure a longer service life of the ion source2.

It is possible to reduce a heat loss caused by emission from the filament38by way of the second heat shield44. This further enhances the heating efficiency of the cathode26by the filament38.

It is possible to reduce a heat loss caused by emission from the filament38by way of the third heat shield46. This further enhances the heating efficiency of the cathode26by the filament38.

The cathode26is detachably held by its male screw part32and the nut34. This makes it possible to replace the cathode26with a new one when it is worn. As a further advantage, the male screw part32is in the line contact state and requires a smaller area of contact with the nut34, and thus, the cathode holder22(to be more specific, its holding part24), compared with fit. This reduces a heat loss caused by conduction of heat from the cathode26to the cathode holder22and enhances the heating efficiency of the cathode26.

The filament38may have a heating part68in the shape of a flat plate bent along the rear surface30of the cathode26as shown inFIG. 4. Both ends of the heating part68are connected to two legs70.

Use of the filament38expands the area of emission of thermal electrons from the filament38to the cathode26, thus increasing the thermal electron emission amount. As a result, for example, to obtain a thermal electron emission amount equivalent to that of a round-rod-shaped filament, the temperature of the filament38may be lowered to extend the service life of the filament38. It is also possible to increase the length of the distance between the cathode26and the filament38thus stabilizing operation against thermal expansion of a member on the periphery of the filament38and the cathode26.

The filament38has a heating part72in the shape of a round bar filament material bent along the rear surface30of the cathode26as in the example shown inFIGS. 5 and 6. The heating part72has a flat surface74obtained by machining (for example cutting) a round-bar-shaped filament material and the flat surface74may be opposed to the rear surface30of the cathode26. Both ends of the heating part72are connected to two legs76.

When a general round-bar-shaped filament is used, only one end of its circular cross section may be brought into the vicinity of the rear surface of the cathode26and the electric field between the remaining parts and the cathode is weakened with a smaller amount of thermal electrons emitted. Use of the filament38allows its flat surface74to be brought closer to the rear surface30of the cathode26. Compared with the general round-bar-shaped filament, it is possible to increase the area of emission thermal electrons from the filament38to the cathode26, thereby increasing the thermal electron emission amount. As a result, for example, to obtain a thermal electron emission amount equivalent to that of a general round-rod-shaped filament, the temperature of the filament38may be lowered to extend the service life of the filament38. It is also possible to increase the length of the distance between the cathode26and the filament38thus stabilizing operation against thermal expansion of a member on the periphery of the filament38and the cathode26.

Referring toFIG. 1again, as in this embodiment, a heat insulating material48may be provided covering the part on the outer peripheral surface of the cathode holder22, the part positioned outside the plasma generating chamber4. In this example, the entire outer peripheral surface of the cathode holder from the heat insulating material48to the support body50is covered by the heat insulating material48. The heat insulating material48may be also called a heat shielding material or a warm material. This also holds true for the heat insulating material48shown inFIG. 3. The heat insulating material48is composed of boron nitride (BN) for example.

The heat insulating material48reduces an emission heat from the cathode holder22thus enhancing the heating efficiency of the cathode26. Moreover, it is not necessary to additionally heat the member on the periphery of the cathode holder, for example the support body50. This reduces the thermal expansion of the peripheral member, maintains the mechanical accuracy between the cathode26and the filament38, and stabilizes the thermal electron emission from the filament38.

Instead of filling the gap between the cathode holder22and the plasma generating chamber4with the electrical insulating material40, it is possible to form a labyrinthine structure part64having a cross section bent for example in a zigzag shape at the gap62between the cathode holder22and the plasma generating chamber4. While the labyrinthine structure part64is formed by attaching a labyrinth forming member66separate from the cathode holder22on the outer peripheral surface of the tip of the cathode holder22in the example ofFIG. 3, the tip of the cathode holder22may be formed into the same shape as the labyrinth forming member66to form the labyrinthine structure part64.

By forming the labyrinthine structure part64instead of arranging a straight gap between the cathode holder22and the plasma generating chamber4, it is possible to reduce the conductance of a gas at the gap62by forming the labyrinthine structure part64. This suppresses possible leakage of the gas10to improving the use efficiency of the gas10. As a result, it is possible to reduce the use amount of the gas10thus reducing the operation cost of the ion source2. It is also possible to prevent contamination of a structure on the periphery of the plasma generating chamber caused by the leakage of the gas10, which contributes to a longer life of the ion source2.

The labyrinth forming member66on the side of the cathode holder22for forming the labyrinthine structure part64by using an electrical insulating material (such as boron nitride). With this configuration, even in case conductive impurities are deposited on the gap62of the labyrinthine structure part64to form a conductive film and the film peels off and thin pieces (flakes) are formed, it is possible to prevent electrical short between the cathode holder22and the plasma generating chamber4. This contributes to a longer service life of the ion source2.

As in the example shown inFIG. 3, it is possible to form the labyrinth forming member66and the heat insulating material48with a material serving as an electrical insulating material and a heat insulating material, for example an integrated member composed of boron nitride (BN). Or, it is possible to form the flange67and the heat insulating material in the labyrinth forming member66with an integrated member composed of such a material.