HOLLOW CATHODE ION SOURCES

Abstract: A hollow cathode ion source comprising a cylindrical cathode (1) through one end of which at least a discharge maintaining gas or said discharge maintaining gas and a metal vapor is or are introduced, and an anode (2) provided on the other end of said cylindrical cathode (1) and having an ion extraction opening (2a), said gas or said gas and metal vapor being ionized by a discharge between said cylindrical cathode (1) and said anode (2) to produce ions which are extracted through said ion extraction opening (2a) in the axial direction of said cylindrical cathodes (1). The cylindrical cathode in the ion source has a large diameter and may be directly cooled.

Description: This invention relates to a hollow cathode ion source which may be used in a plasma processing, an ion implantation or an analysis and as described in the first part of claim 1. Such an ion source is known from DE-A-3.038.575. Generally, it is known that a hollow cathode ion source has such features, that its electrode structure is simple, a plasma of relatively high density is produced, and the operation is stable for a long time, thereby obtaining stable ion beams. Various types of hollow cathode ion sources have been proposed. For example, DE-A-3 038 575 discloses an ion source comprising a hollow cathode, which has one end closed and is cooled. DD-A-217 082 discloses an ion source comprising a hollow cathode having one end closed, an extraction electrode disposed adjacent the closed one end of the hollow cathode and another electrode disposed on the opposite side of the cathode. Japanese Patent Kokai No. JP-A-62-73542 which was published after the first priority date discloses a hollow cathode ion source of a cold cathode type, which comprises a cylindrical discharge chamber having a major portion formed as a cathode, anodes attached to the cathode electrode via an electrically insulating member, an inlet for a discharge maintaining gas provided on the cylindrical discharge chamber, an ion extraction opening provided on the cathode portion of the cylindrical discharge chamber, and means for cooling the cathode portion of the cylindrical discharge chamber. A sample gas (or metal vapor) is introduced through the gas inlet into the cylindrical discharge chamber and is ionized by a discharge between the anodes and the cathode to produce desired ions. The produced ions are extracted through the ion extraction opening in a direction perpendicular to the axial line of the cathode. By feeding a coolant such as pure water or the like to the cooling means, the sputtering of the cathode material may be accelerated. In the hollow cathode ion source of this type, however, since the ion extraction opening is provided on the cathode portion of the cylindrical discharge chamber, there can not be obtained a ion beam having a circular cross section of a considerably large diameter. Further, the ions are accelerated near the cathode to cause the ion beam having irregular energy to be naturally flown out through the extraction opening on the cathode and therefore some drawbacks may be involved when the ion beam is to be used for an analysis. In Japanese Patent Application No. JP-A-278767/86 we, the inventors, have proposed a hollow cathode ion source in which an ion extraction slit and an opening for introducing a carrier gas are respectively provided on the front and back surfaces of a cylindrical hollow cathode, an extraction electrode is disposed in front of the ion extraction slit, a floating electrode is interposed between the ion extraction slit and the floating electrode and is provided with a slit aligned with the ion extraction slit, and an anode is interposed between the floating electrode and the extraction electrode and is provided with a slit communicating with the slit of the floating electrode. Argon gas or other carrier gas is introduced through the opening into the cylindrical hollow cathode. The introduced carrier gas is passed through the ion extraction slit and the slits of the floating electrode and the anode, and a discharge is generated in those slits by applying a suitable discharging voltage thereto, thereby forming a plasma of relatively high density in the slits. The previously proposed hollow cathode ion source has drawbacks that the discharge may be readily occurred between the extraction electrode and the anode when the degree of vacuum in the cylindrical hollow cathode is insufficient, and thus it is impossible to apply a higher voltage to the extraction portion. In order to improve the degree of vacuum in the cylindrical hollow cathode to be able to apply the higher voltage to the extraction portion, it is necessary to reduce the quantity of the gas to be introduced into the cylindrical hollow cathode. However, if the quantity of the gas to be introduced is reduced, the gas pressure in the cylindrical hollow cathode decreases and then the mean free pass of electrons is lengthened. Therefore, the probability that the electrons arrive at the surface of the cylindrical hollow cathode is higher than the probability that the electrons collide with the gas and metal atoms. As a result, the reduction of the gas quantity makes it difficult to maintain the discharge in the cylindrical hollow cathode. Thus, the previously proposed hollow cathode ion source has not provided sufficient gas efficiency. It is, therefore, an object of this invention to provide a hollow cathode ion source in which the drawbacks of the above-mentioned conventional ion source can be overcome and an ion beam having a circular cross section and a uniform energy can be obtained with better efficiency. Another object of this invention is to provide a hollow cathode ion source which can maintain a higher discharging voltage and provide a high ionization rate. Still another object of this invention is to provide a hollow cathode ion source which is provided with multistage floating electrode for enhancing a plasma density. A further object of this invention is to provide a hollow cathode ion source which is provided with means for increasing a sputtering rate and converging the plasma in a discharging pass. According to this invention, there is provided a hollow cathode ion source having a discharge chamber into which a discharge maintaining gas introduced through one end thereof so as to produce ions which are extracted through the other end in the axial direction of the chamber, and which is characterized in that it comprises a cylindrical cathode for defining said discharge chamber into which a gaseous medium including said discharge maintaining gas is introduced, a first anode disposed on one end of said cylindrical cathode and provided with an ion extraction opening for extracting the ions produced in said discharge chamber in the axial direction of said cylindrical cathode, a second anode disposed on the other end of said cylindrical cathode and having at least one opening for introducing said gaseous medium into said discharge chamber, electrical insulator means each being disposed between said cylindrical cathode and said first anode and between said cylindrical cathode and said second anode and means cooling said cylindrical cathode. The size of the ion extraction opening on the first anode may be steplessly selected up to the size of the inner diameter of the cylindrical cathode, and the ion extraction opening may be formed not only as a single opening, but also as a multi-aperture opening. The surface area and hence the diameter of the hollow cathode may be selected to be able to maintain the discharging even if the gas quantity to be introduced therein is reduced. By increasing the surface area or diameter of the hollow cathode, electron emission amount from the surface of the hollow cathode may be increased to readily maintain the discharge. The quantity of metal atoms may also be increased by the increase in the surface area or diameter of the hollow cathode, thereby resulting in an increase in the metal ions. The temperature raising of the hollow cathode may be suppressed by directly cooling it, and a high discharging voltage necessary for cathode sputtering can be maintained. In a preferred embodiment, the electrical insulator means disposed between the cylindrical cathode and the first anode may comprise multi-stage insulator means, and multi-stage floating electrode means may be provided each floating electrode being arranged between arranged between the adjacent insulators and provided with an ion passage for guiding the ions from said cathode to the ion extraction opening of the first anode along the axial direction of the cathode and floating between the one end of the cathode and the first anode. The multi-stage floating electrode may be operated to limit the flow of the discharge maintaining gas in the extraction portion and to converge the plasma at the portrion of the anode and the multi-stage floating electrode, thereby enhancing the plasma ionization to increase the ionization rate. In another embodiment, therte may be provided means for applying a magnetic field to an assembly of the cathode, the first anode and the multi-stage floating electrode in the axial direction of the cathode. By applying the external magnetic field, the plasma in the discharge path may be more strongly converged, thereby resulting in an increase of the ionization rate. In order that this invention may be more clearly understood, it will now be disclosed in greater detail with reference to the accompanying drawings, wherein: Fig. 1 is a longitudinal sectional view showing a principle of a hollow cathode ion source according to this invention; Fig. 2 is a longitudinal sectional view schematically showing an embodiment of this invention; Fig. 3 is a longitudinal sectional view schematically showing another embodiment of this invention; Fig. 4 is a longitudinal sectional view schematically showing still another embodiment of this invention; Fig. 5 is a longitudinal sectional view schematically showing a further embodiment of this invention. Reference is now made to Fig. 1 of the drawings wherein the principle of a hollow cathode ion source according to this invention is shown. Reference numeral 1 designates a cylindrical cathode having an upper and lower ends on which an upper and lower circular anodes 2 and 3 are provided via annular electrical insulators 4 and 5, respectively. The upper circular anodes 2 is provided with an ion extraction opening 2a at substantially the center thereof, and the lower circular anode 3 is provided with a metal vapor inlet 3a and a sample gas inlet 3b. To the outer periphery of the cylindrical hollow cathode 1 is attached a cylindrical heat shield 6 for supporting the cylindrical hollow cathode 1. In a cold cathode type, the cylindrical heat shield 6 and the cylindrical hollow cathode 1 may be cooled by providing a cooling pipe 7 for circulating coolant such as pure water or the like around the heat shield 9 as designated by broken lines in Fig. 1. Gas introduced through the gas inlet 3b or this gas and metal vapor introduced through the metal vapor inlet 3a is or are ionized by a discharge which occurs between the hollow cathode 1 and each of the upper and lower circular anodes 2 and 3. The ions thus produced are extracted through the ion extraction opening 2a of the upper circular anode 2. In this ion source, the ion extraction opening 2a is provided on the upper circular anode 2 to extract the ions in the axial direction of the cylindrical hollow cathode 1. Thus, the diameter of the ion extraction opening 2a can be steplessly selected up to the size of the inner diameter of the cylindrical hollow cathode 1 at the maximum, and the extracted ion beam has a uniform energy. In Fig. 2 there is illustrated a hollow cathode ion source according to an embodiment of this invention. the illustrated ion source comprises a cylindrical hollow cathode 10 which is supported by a cylindrical cathode shield 11 attached to the outer periphery thereof. The cathode shield 11 has a edge portion 11a engaged with the upper end of the hollow cathode 10 and a flange 11b positioned substantially in the same level as the lower end of the hollow cathode 10. The hollow cathode 10 and the cathode shield 11 are supported on a supporting member 12. An upper anode 13 having a diameter larger than that of the cathode shield 11 is of a cylindrical cap-like, and is provided with an ion extraction opening 13a at the center of the upper end wall and a flange 13b at the lower end. The upper anode 13 is supported onto the flange 11b of the cathode shield 11 via an annular insulator 14 A disk-like lower anode 15 has an annular projection 15a on the upper surface, a metal vapor inlet 15b at the center and a sample gas inlet 15c formed at the position displaced slightly from the center. The lower anode 15 supports the supporting member 12, the cathode 10, the cathode shield 11 and the upper anode 13 via an annular insulator 16. In case the ion source is to be used as a cold cathode type, it comprises, as shown by broken lines in Fig. 2, a coolant circulating pipe 17 for circulating coolant such as pure water or the like around the upper anode 13 to cool it, and the cathode 10 is cooled by means of heat radiation. In the illustrated hollow cathode ion source, the gas and optionally metal vapor are introduced through the gas inlet 15c and the metal vapor inlet 15b on the lower anode 15 into the hollow cathode 10, and is ionized by a discharge which occurs between a cathode assembly of the cathode 10, cathode shield 11 and the supporting member 12, and each of the upper and lower anodes 13 and 15 to produce the ions which are extracted the ion extraction opening 13a on the upper anode 13 in the axial direction of the hollow cathode 10. When used as a cold cathode type, coolant such as pure water or the like is fed into the pipe 17 wound around the upper anode 13 to cool it. In this case, when the cathode 10 and the cathode shield 11 are made of metal containing a required ion seed such as Mo, W, Ni and the like, the atoms of the metal are fed into the the ion source by sputtering, and are ionized by the discharge between the cathode assembly of the cathode 10, cathode shield 11 and the supporting member 12, and each of the upper and lower anodes 13 and 15. Fig. 3 schematically shows a hollow cathode ion source according to another embodiment of this invention. Reference numeral 20 designates a hollow cathode body which is made of nickel, molybdenum, tungsten and the like. Upper and lower flanges 21 and 22 made of the same material as the hollow cathode body 20 are integrally provided at the upper and lower ends of the hollow cathode body 20 to form a part of the cathode. On the upper flange 21 is provided a circular upper anode 23 via an annular insulator 24. This upper anode 23 is provided with an ion extraction opening 23a at the position passing substantially the center thereof, i.e., the axis of the hollow cathode body 20. On the lower flange 22 is provided a circular lower anode 25 via an annular insulator 26. The lower anode 25 is provided with a discharge maintaining gas inlet 25a and a metal vapor inlet 25b as shown in Fig. 3. A cylindrical shield member 27 is mounted on the outer periphery of the hollow cathode body 20, and is surrounded by a cooling pipe 28 for circulating coolant such as pure water or the like. In the operation of the illustrated ion source as described above, discharge maintaining gas such as argon gas or the like and metal vapor ( for example Na ) to be ionized are introduced into the hollow cathode body 20 through the gas inlet 25a and the metal vapor inlet 25b, respectively, and a suitable discharge voltage is applied between the hollow cathode body 20 and each of the upper and lower anodes 23 and 25 so as to start the discharge in the hollow cathode body 20. Thus, the introduced metal vapor and gas are ionized by the discharge between the hollow cathode body 20 and each of the upper and lower anodes 23 and 25. In this case, in order to maintain the high discharge voltage by improving the degree of vacuum in the hollow cathode body 20, the quantity of the metal vapor to be introduced through the metal vapor inlet 25b is relatively reduced, and therefore the mean free pass of electrons is lengthened to increase the probability that the electrons do not collide with the gas and the metal element but arrive at the surface of the cathode body 20, but due to the large diameter of the hollow cathode body 20, the electrons have a tendency to collide with the gas and the metal element before arriving at the surface of the cathode body 20. thereby resulting in maintaining the discharge. The metal ions thus produced are extracted through the ion extraction opening 23a on the upper anode 23. In the embodiment of Fig. 3, it should be understood that since the metal by which the hollow cathode body 20 is formed is sputtered and ionized, it should be constructed by the same metal as the metal vapor to be introduced through the metal vapor inlet 25b. Fig. 4 shows a hollow cathode ion source according to still another embodiment of this invention. This hollow cathode ion source comprises a hollow cathode body 30 which is made of nickel, molybdenum, tungsten and the like. The hollow cathode body 30 has a flange 31 made of the same material as the hollow cathode body 30 at the upper end thereof, and is closed at the lower end by a terminal plate 32 which is also made of the same material as the hollow cathode body 30. The terminal plate 32 is provided with a discharge maintaining gas inlet 32a and a metal vapor inlet 32b. On the flange 31 of the cathode body 30 are provided three upper floating electrodes 33, 34 and 35 with annular insulators 36, 37 and 38. The floating electrode 33 disposed directly above the flange 31 has a tapered or convergent opening 33a extending along the inner inclined edge of the flange 31 as shown in Fig. 4, and the other two floating electrodes 34 and 35 have openings 34a and 35a coaxial with the convergent opening 33a of the floating electrode 33. An anode 39 is provided on the uppermost floating electrode 35 via an annular insulator 40, and has an ion extraction opening 39a substantially at the center. the ion extraction opening 39a communicates with the interior of the hollow cathode body 30 through the respective openings 33a, 34a and 35a of the floating electrodes 33, 34 and 35. A lower floating electrode 41 is mounted on the cathode terminal plate 32 via an annular insulator 42, and is provided with openings 41a and 41b which communicate with the gas inlet 32a and the metal vapor inlet 32b, respectively. To the outer periphery of the hollow cathode body 30 is attached a cylindrical shield member 43 similarly to the case of the embodiment in Fig. 3, and a cooling pipe 44 for circulating coolant such as pure water or the like is spirally wound on the shield member 43. The operation of the ion source illustrated in Fig. 4 will now be described. Carrier gas such as argon gas or the like and metal vapor to be ionized are introduced into the hollow cathode body 30 through the openings 41a and 41b of the lower anode 41, and the gas inlet 32a and the metal vapor inlet 32b, respectively, and the discharge is commenced by initially setting the upper floating electrodes 33, 34 and 35 and the upper anode 39 to the same potential and then applying a voltage between the hollow cathode body 30 and the upper anode 39. Then, the connections of each of the upper floating electrodes 33, 34 and 35 and the upper anode 39 are disconnected sequentially from the side of the hollow cathode body 30, and when all the upper floating electrodes 33, 34 and 35 are disconnected from the upper anode 39, the discharge is established between the hollow cathode body 30 and the inner surface of the ion extraction opening 39a in the upper anode 39. The produced plasma is naturally flowed out through the openings of the floating electrodes 33, 34 and 35 due to the difference between the external pressure and the inner pressure of the hollow cathode body 30. In this connection, since the openings of the floating electrodes are fine or narrow, the gas scarcely flows through those openings so that a high plasma density can be obtained. Thus, the ionization rate may be improved to obtain a dense ion beam. Fig. 5 shows further embodiment of this invention in which the ionization rate can be further improved by applying a magnetic field to the ion source of Fig. 4, and the same components as those in the ion source of Fig. 4 are designated by the same reference numerals as those in Fig. 4. In the embodiment in Fig. 5, on the outside of an assembly of the hollow cathode body 30, the upper floating electrodes 33, 34 and 35 and the upper anode 39 is provided means 45 for applying the magnetic field thereto in the direction of an ion extraction. The magnetic field applying means 45 may be formed of a suitable electromagnet assembly or a permanent magnet assembly. The magnetic flux density of the applied magnetic field is enhanced along the openings of the upper floating electrodes 33, 34 and 35 as shown in graphs at the upper and left sides in Fig. 5. Thus, the discharge voltage may be raised so that the sputtering rate is increased and the plasma in the discharge path is converged, thereby further increasing the ionization rate. In the embodiments shown in Figs. 4 and 5, three floating electrodes have been used as the multi-stage floating electrode. It is, however, appreciated that the number of the floating electrodes can be arbitrarily set as required. Further, the openings of the floating electrodes and the ion extraction opening are circular in the sectional shape. However, they may be formed in any other sectional shape in accordance with the object for use. In the embodiment shown in Fig. 3, the hollow cathode body is provided with the anodes at the both sides thereof. However, it should be understood that as in the embodiments in Figs. 4 and 5 the anode may be provided only at the upper side of the hollow cathode body. According to this invention as described above, since the ion extraction opening is provided on the upper anode, the diameter of the ion beam to be extracted can be continuously selected up to the size of the inner diameter of the hollow cathode, and a single ion extraction opening or a multi-aperture opening can be provided. Since the extraction of the produced ions is performed through the anode, a beam having uniform energy can be obtained to be used not only for an analysis but also for any other application. With the use of the hollow cathode having a large diameter and the direct cooling of the hollow cathode, even if the quantity of the gas to be introduced is reduced, the discharge can be maintained, the raising of the temperature of the hollow cathode can be effectively suppressed, and therefore the degree of vacuum in the ion source can be improved so that the high discharging voltage required for cathode sputtering can be easily maintained and a high extraction voltage can be applied to the ion extraction portion. By the provision of the multi-stage floating electrode interposed between the hollow cathode and the anode, the flow of the gas in the extraction portion can be limited, and the plasma ionization can be enhanced by the discharge at the anode and the multi-stage floating electrode, thereby increasing the ionization rate and obtaining a dense ion beam. Further, with the provision of the magnetic field applying means, the discharge voltage can be raised so that the sputtering rate can be more increased and the plasma in the discharge path can be effectively converged. The embodiments of this invention described herein are for purposes of illustration and the scope of this invention is intended to be limited only by the scope of the following claims.

Claims: A hollow cathode ion source having a discharge chamber in which ions are produced by introducing at least a discharge maintaining gas into the discharge chamber through one end thereof and are extracted through the other end in the axial direction of the chamber, characterized therein that it comprises a cylindrical cathode (1; 10; 20; 30) for defining said discharge chamber into which a gaseous medium including said discharge maintaining gas is introduced, a first anode (2; 13; 23; 39) disposed on one end of said cylindrical cathode (1; 10; 20; 30) and provided with an ion extraction opening (2a; 13a; 23a; 39a) for extracting the ions produced in said discharge chamber in the axial direction of said cylindrical cathode (1; 10; 20; 30), a second anode (3; 15; 25; 41) disposed on the other end of said cylindrical cathode (1; 10; 20; 30) and having at least one opening (3a, 3b; 15b, 15c; 25a, 25b; 41a, 41b) for introducing said gaseous medium into said discharge chamber, electrical insulator means (4, 5; 14, 16; 24, 26; 40, 42) each being disposed between said cylindrical cathode (1; 10; 20; 30) and said first anode (2; 13; 23; 39) and between said cylindrical cathode (1; 10; 20; 30) and said second anode (3; 15; 25; 41), and means (7, 17, 28, 44) for cooling said cylindrical cathode (1; 10; 20; 30). A hollow cathode ion source as claimed in claim 1, wherein said ion extracting opening is of multi-aperture type. A hollow cathode ion source as claimed in claim 1, wherein said cooling means (7, 17; 28; 44) is intended to cool said first anode (2; 13; 23; 39) and said cylindrical cathode (1; 10; 20; 30) by circulating a coolant around them. A hollow cathode ion source as claimed in claim 1, wherein said gaseous medium includes a metal vapor to be ionized. A hollow cathode ion source as claimed in claim 1, wherein said gaseous medium introducing opening (3a, 3b; 15b, 15c; 25a, 25b; 41a, 41b) comprises a discharge maintaining gas inlet (3b; 15c; 25a; 41a) and a metal vapor inlet (3a; 15b; 25b; 41b). A hollow cathode ion source as claimed in claim 1, wherein said electrical insulator means disposed between said cylindrical cathode (30) and said first anode (39) comprises multi-stage insulator means (36, 37, 38, 40), and multi-stage floating electrode means (33, 34, 35), each floating electrode being arranged between the adjacent insulators (36, 37, 38, 40) and provided with an ion passage (33a, 34a, 35a) for guiding the ions from said cathode (30) to the ion extraction opening (39a) of said first anode (39) along the axial direction of said cathode (30) and floating between said one end of the cathode (30) and said first anode (39). A hollow cathode ion source as claimed in claim 1, wherein it further comprises means (46) for applying a magnetic field to an assembly of said cathode (30), said first anode (39) and said multi-stage floating electrode means (33, 34, 35) in the axial direction of said cathode (30).

Inventors: FUKUI, RYOTA, KIKUCHI, RIICHI, TAKAGI, KENICHI, TAKAYAMA, KAZUO, C.I.HEIGHTS D-501, TONEGAWA, AKIRA DAIICHI COOP IZUMI 205
Applicants: NIHON SHINKU GIJUTSU KABUSHIKI KAISHA
Application year: 1988
Main classification: H01J27
IPC codes: H01J27