Abstract:
In accordance with one embodiment of the present invention, the ion-beam apparatus takes the form of an end-Hall ion source in which the detachable anode module incorporates the outer pole piece and includes an enclosure around the anode that both minimizes the loss of working gas and confines sputter contamination to the interior of this enclosure. This detachable anode module is substantially smaller than the entire end-Hall ion source, weighs substantially less, and can be duplicated for significantly less cost than the duplication of the entire ion source. In general, the components of the magnetic circuit determine the overall size, weight, and much of the cost of a gridless ion source. The reduced size, weight, and cost of the detachable anode module compared to the entire ion source is due to most of the magnetic circuit being excluded from the detachable module.

Description:
FIELD OF INVENTION 
     This invention relates generally to ion and plasma sources, and more particularly it pertains to gridless or Hall-current ion sources. 
     BACKGROUND ART 
     Industrial ion sources are used for etching, deposition and property modification, as described by Kaufman, et al., in the  Characteristics, Capabilities , and  Applications of Broad - Beam Sources , Commonwealth Scientific Corporation, Alexandria, Va. (1987). 
     Both gridded and gridless ion sources are used in these industrial applications. The ions generated in gridded ion sources are accelerated electrostatically by the electric field between the grids. Only ions are present in the region between the grids and the magnitude of the ion current accelerated is limited by space-charge effects in this region. Gridded ion sources are described in an article by Kaufman, et al., in the  AIAA Journal , Vol. 20 (1982), beginning on page 745. The particular sources described in this article use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge. 
     In gridless ion sources the ions are accelerated by the electric field generated by an electron current interacting with a substantial magnetic field in the discharge region. The overall size and weight of a gridless source is primarily determined by the magnetic circuit to generate this magnetic field. A substantial fraction of the overall cost of a gridless ion source is also associated with the magnetic circuit. In contrast, when a magnetic field is used in a gridded ion source, it is only to contain the 50 eV, or less ionizing electrons. The magnetic circuit in a gridded ion source thus plays a secondary role to the ion optics in determining ion-source size and cost. 
     Because the ion acceleration takes place in a quasineutral plasma, there is no space-charge limitation on the ion current that can be accelerated in a gridless ion source. The lack of a space-charge limitation is most important at low ion energies, where a gridded ion source is severely limited in ion-current capacity. 
     The closed-drift ion source is one type of gridless ion source and is described by Zhurin, et al., in an article in  Plasma Sources Science  &amp;  Technology , Vol. 8, beginning on page R1, while the end-Hall ion source is another type of gridless ion source and is described in U.S. Pat. No. 4,862,032—Kaufman, et al. These publications are incorporated herein by reference. 
     A Hall current of electrons is generated normal to both the applied magnetic field and the electric field generated therein, so that these ion sources have also been called Hall-current sources. Because the neutralized ion beams generated by these ion sources are also quasineutral plasmas, i.e., the electron density is approximately equal to the ion density, they have also been called plasma sources. 
     Gridless ion sources used in industrial applications need routine maintenance. This maintenance can result from the limited lifetimes of certain parts, such as cathodes. The need for maintenance can also result from the contamination of ion-source parts due to sputter deposition within the ion source, or from the contamination with materials present in the particular application in which the ion source is used. The contamination can be in the form of conducting layers on insulators, insulating layers on conducting parts, or deposited films that can peel off to cause electrical shorts or flake off in smaller particles to generate unwanted particulates. 
     Performing the routine maintenance typically involves replacing cathodes and some other parts with limited lifetimes, cleaning the remaining metal parts, and replacing insulators. The ion sources must be substantially disassembled to carry out this maintenance. 
     The expense of performing maintenance on gridless ion sources is not limited to the direct time and materials involved. The downtime for the vacuum chamber and associated hardware often constitutes a major expense. This latter expense can be reduced by purchasing two ion sources, so that maintenance can be performed on one ion source while the other is being used. However, the purchase of an additional ion source is an additional expense that must be balanced against the reduction in downtime expense. 
     SUMMARY OF INVENTION 
     In light of the foregoing, it is a general object of the invention to provide a gridless ion source with a detachable anode module that facilitates rapid and economical maintenance. 
     A specific object of the invention is to provide a gridless ion source with a detachable anode module in which the cost of that module is substantially less than the expense of the entire ion source. 
     Another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the size and weight of that module is substantially less than the size and weight of the entire ion source. 
     A further specific object of the invention is to provide a gridless ion source with a detachable anode module in which the contamination of ion-source parts due to sputter deposition within the ion source, and the associated maintenance, is essentially confined to that module. 
     Yet another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the deposition on ion-source parts due to contamination sources external to the ion source are largely confined to that module. 
     Still another specific object of the invention is to provide a gridless ion source with a detachable anode module in which the loss of working gas is minimized by a gas enclosure surrounding the anode in that module. 
     In accordance with one embodiment of the present invention, the ion-beam apparatus takes the form of an end-Hall ion source in which the detachable anode module incorporates the outer pole piece and includes an enclosure around the anode that both minimizes the loss of working gas and confines sputter contamination to the interior of this enclosure. This detachable anode module is substantially smaller than the entire end-Hall ion source, weighs substantially less, and can be duplicated for significantly less cost than the duplication of the entire ion source. In general, the components of the magnetic circuit determine the overall size, weight, and much of the cost of a gridless ion source. The reduced size, weight, and cost of the detachable anode module compared to the entire ion source is due to most of the magnetic circuit being excluded from the detachable module. 
    
    
     DESCRIPTION OF FIGURES 
     Features of the present invention which are believed to be patentable are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objectives and advantages thereof, may be understood by reference to the following descriptions of specific embodiments thereof taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which: 
     FIG. 1 is a prior-art gridless ion source of the end-Hall type; 
     FIG. 2 shows the prior-art ion source of FIG. 1 with the hot-filament cathode assembly separated from the rest of the ion source; 
     FIG. 3 shows the prior-art ion source of FIGS. 1 and 2, without the hot-filament cathode assembly, but with the ion-source assembly separated from the socket assembly; 
     FIG. 4 shows a cross section of the ion-source assembly of the ion source shown in FIGS. 1,  2 , and  3 ; 
     FIG. 5 is an embodiment of the present invention wherein the gridless ion source is of the end-Hall type; 
     FIG. 6 shows the ion source of FIG. 5 with the hot-filament cathode assembly separated from the rest of the ion source; 
     FIG. 7 shows the ion source of FIGS. 5 and 6, without the hot-filament cathode assembly, but with the detachable anode module separated from the magnetic-circuit module; 
     FIG. 8 a  shows a cross section of the detachable anode module of the ion source of FIGS. 5,  6 , and  7 ; 
     FIG. 8 b  shows a cross section of the magnetic-circuit module of the ion source of FIGS. 5,  6 , and  7 ; 
     FIG. 9 a  shows a partial cross section of the detachable anode module of the ion source of FIGS. 5,  6 , and  7 , showing additional features not shown in FIG. 8 a;    
     FIG. 9 b  shows a partial cross section of the magnetic-circuit module of the ion source of FIGS. 5,  6 , and  7  showing additional features not shown in FIG. 8 b;    
     FIG. 10 a  is a simplified cross section of another embodiment of the present invention wherein the gridless ion source is also of the end-Hall type; 
     FIG. 10 b  is a simplified cross section of the embodiment shown in FIG. 10 a  wherein the anode module is separated from the magnetic-circuit module; 
     FIG. 11 a  is a simplified cross section of yet another embodiment of the present invention wherein the gridless ion source is of the closed-drift type; and 
     FIG. 11 b  is a simplified cross section of the embodiment shown in FIG. 11 a  wherein the anode module is separated from the magnetic-circuit module. 
    
    
     DESCRIPTION OF PRIOR ART 
     Referring to FIG. 1, there is shown a prior-art gridless ion source  10  of the end-Hall type. Ion source  10  is generally of the type described in U.S. Pat. No. 4,862,032—Kaufman, et al. More specifically, it is a Mark II ion source marketed first by Commonwealth Scientific Corporation, Alexandria, Va., and more recently by Veeco Instruments Inc., Plainview, N.Y. Differences of the Mark II ion source from the aforementioned U.S. Pat. No. 4,862,032 include the use of a plug-and-socket design to facilitate removal for maintenance and the use of a permanent magnet in place of the electromagnet to generate the magnetic field. The plug-and-socket concept is generally similar to that shown in the earlier U.S. Pat. No. 4,446,403—Cuomo, et al. 
     Ion source  10  includes ion-source assembly  11 , socket assembly  12 , and cathode assembly  13 . The components of the ion-source assembly shown in FIG. 1 include plug body  14 , outer shell  15 , and outer pole piece  16 , all of which are also parts of the magnetic circuit. Also included in ion-source assembly  11  and shown in FIG. 1 are anode  17 , external anode support  18 , retaining nuts  19  that must be removed to disassemble the ion-source assembly, threaded retainer rods  20  to which nuts  19  attach, and knobs  21  that attach to plug-and-socket retaining rods  22 . When knobs  21  are tightened, ion-source assembly  11  is clamped to socket assembly  12 , establishing both the electrical connections and the gas connection necessary for operation. Cathode assembly  13  includes cathode supports  23 , cathode  24 , and cathode retaining nuts  25 . To separate the cathode assembly from the rest of the ion source, the two cathode supports are grasped with the fingers of two hands and lifted, overcoming the friction with which the cathode supports are attached to the rest of the ion source. 
     Referring to FIG. 2, ion source  10  is shown with cathode assembly  13  separated from the rest of the ion source. At the separated location, the lower ends of cathode supports  23  are exposed to show connectors  26  thereon, with each connector comprised of elastic spring “fingers” to establish an electrical connection with a complementary cylindrical contact. The spring fingers of the connectors also generate the friction that must be overcome in removing the cathode assembly from the ion-source assembly. Ion source assembly  11  can be separated from socket assembly  12  by rotating knobs  21 , thereby removing the threaded ends of plug-and-socket retaining rods  22  from socket assembly  12 . 
     It should be noted that the hot-filament cathode shown in FIGS. 1 and 2, together with its particular installation, is exemplar only. Different mounting arrangements are possible for hot-filament cathodes. Also, end-Hall ion sources have been operated with hot-filament, hollow-cathode, and plasma-bridge types of electron-emitting cathodes. These alternate cathodes are described in “Ion Beam Neutralization,” anon., CSC Technical Note, Commonwealth Scientific Corporation, Alexandria, Va. (1991). This publication is also incorporated herein by reference. 
     Referring to FIG. 3, ion-source assembly  11  is shown separated from socket assembly  12 . The socket assembly is comprised of socket body  30 , openings  31  with socket connectors  32  therein to provide the electrical connections for cathode  24 , opening  33  with socket connector  34  to provide the electrical connection for anode  17 , threaded opening  35  with threaded gas fitting  36  to provide a flow path for the ionizable working gas used in the ion source, and threaded openings  37  for the threaded lower ends of plug-and-socket retaining rods  22  to be threaded into and thereby clamp ion-source assembly  11  to socket assembly  12 . The socket connectors in FIG. 3 are generally similar in function to connectors  26  shown at the lower ends of cathode supports  23 . 
     Referring to FIG. 4, there is shown a cross section of ion-source assembly  11 . Note that the cross section of FIG. 4 is not a particular cross section of the Mark II ion source, but instead is one that has been constructed to include the major design features of that ion source. That is, only one exemplar feature is shown when there are typically a plurality of such features. As an example, only one accommodation is shown for a cathode support, when two cathode supports are normally installed on opposite sides of the ion-source assembly, so that both would normally show in the same cross section through the center line. When the cathode assembly is installed on the ion-source assembly, sockets  26  of cathode assembly  13  (shown in FIG. 2) are electrically connected to cylindrical contacts  40 , which are integral parts of cathode support rods  41 . Cathode support rods  41  are spaced from and located relative to main support plate  42  and plug body  14  by ceramic insulators  43  held in place by nuts  44 . The lower ends of cathode support rods  41  form contacts  45  which, when ion-source assembly  11  is clamped to socket assembly  12 , provide electrical connections with complementary cathode connectors  32  shown in FIG.  3 . It should be noted that to provide an insulative function at high temperature without adverse outgassing, insulators  43  are typically fabricated from a refractory ceramic material such as alumina. 
     Anode  17  is held between external anode support  18  and internal anode support  46 , with the external and internal anode supports in turn held together with screws  47 . The assembly of anode and internal and external anode supports is spaced from and located relative to main support plate  42  by additional ceramic insulators  43  held in place by screws  48 . Reflector  49  is also spaced from and located relative to main support plate  42  by additional ceramic insulators  43  held in place by screws  50  and additional nuts  44 . 
     Still referring to FIG. 4, the anode is connected by conducting wire covered with ceramic insulator beads  52  to anode rod  53  which is spaced from and located relative to plug body  14  by additional ceramic insulators  43  held in place by additional nuts  44 . Contact  54  is an integral part of anode rod  53  and is electrically connected to complementary anode connector  34  in the socket assembly (FIG. 3) when the ion-source assembly is clamped to the socket assembly. Permanent magnet  55  magnetically energizes the magnetically permeable parts of the magnetic circuit, which include plug body  14 , outer shell  15 , and outer pole piece  16 . Parts other than those of the magnetic circuit are constructed of essentially nonmagnetic materials, i.e., parts with a magnetic permeability not significantly different from free space. Main support plate  42  is spaced from and located relative to plug body  14  by threaded retainer rods  20 . 
     The ionizable working gas is introduced through gas fitting  36  which is attached to a gas feed tube (not shown) and installed in threaded opening  35  (see FIG.  3 ). Returning to FIG. 4, when ion-source assembly  11  is clamped to socket assembly  12 , the working gas flows from the socket assembly into volume  57 , through first gas fitting  59 , through tube  58 , through second gas fitting  59 , to circumferential manifold  61 . From this manifold, the working gas flows though apertures  62  in reflector  49  to reach discharge volume  63 , where collisions of energetic electrons emitted from cathode  24  (shown in FIGS. 1 and 2) ionize the working gas. The ions formed by these collisions in volume  63  are accelerated by electric fields in that volume to form an energetic ion beam. A more detailed description of the operation of an end-Hall ion source is included in the aforementioned U.S. Pat. No. 4,862,032—Kaufman, et al., which is included herein by reference. A schematic diagram showing the required power supplies to operate an end-Hall ion source is also included in the aforementioned patent. 
     Those skilled in the art of ion sources will recognize that, similar to other ion sources used in industrial applications, ion source  10  is installed in a vacuum chamber. The vacuum chamber is normally assumed to be ground in the ion-source circuit, and is usually also at earth ground. 
     The magnetic circuit is comprised of those parts that are used to generate a magnetic field between the anode and electron-emitting cathode, i.e., the magnetic field that electrons from the electron-emitting cathode must cross to reach the anode. The magnetic-circuit parts include a magnetic-field energizing means of one or more electromagnets or permanent magnets. It also includes magnetically permeable parts that have a magnetic permeability that is significantly greater than that of free space, preferably greater than one or two orders of magnitude greater than that of free space. The preferred permanent magnet material would be one of the Alnico alloys, which would have a substantial advantage in maximum temperature compared to rare-earth permanent-magnet materials. It should be noted that the magnetic-circuit parts, plug body  14 , outer shell  15 , outer pole piece  16 , and permanent magnet  55 , constitute the largest and heaviest parts of the ion source. The magnetic circuit also accounts for a major fraction of the cost. 
     The need for maintenance can result from the limited lifetime of some parts, usually the cathode and the reflector. Maintenance can also result from insulative coatings on anode  17 . Such coatings can result from the formation of compounds with the working gas (e.g., the formation of oxides or nitrides with oxygen or nitrogen as the working gas). Such coatings can also result from the external sources, such as when an ion source is used in an ion-assist function with the thermal deposition of a dielectric coating. 
     Conductive coatings can be deposited on insulators  43  due to internal sputtering in the ion source from normal operation (from reflector  49  or outer pole piece  16 ). Conductive coatings can also be deposited from occasional arcs that propagate though gap  64  between the anode and main support plate  42  to reach volume  66  external to the anode. As is known to those skilled in the art, the proper use of shadow shielding can reduce the rate at which sputtered coatings are deposited on insulators  43  exposed to volume  66 , but it cannot completely eliminate such coatings. 
     Conductive coatings can also be deposited due to the decomposition of some ionizable working gases, e.g. methane. Such coatings can be found on insulators exposed to the working gas, even if there is no exposure to either the discharge or arcs propagated outside of the discharge region, e.g., volumes  67 . Because the decomposition rate tends to increase with increasing temperature, however, these coatings would be more likely on insulators in physical contact with warmer main support plate  42 , rather than cooler plug body  14 . 
     The deposition of conductive coatings on parts others than the insulators can eventually be a problem because of the possible shorting due to loosened flakes of deposited layers. As described in the Background Art section, the deposited layers can also come off as particulates that adversely affect the thin-film products of the industrial process. 
     Disassembly for maintenance of ion-source assembly  11  starts with the removal of retainer nuts  19  from threaded retainer rods  20 . The anode, together with the external anode support, can be removed for cleaning by removing screws  47 . Removal of screws  48  and  50  then permit removal of internal anode support  46  and reflector  49 . To complete the maintenance, it is often necessary to replace all insulators  43  above main support plate  42 , as well as remove deposited films on all metal parts in the same region. If conducting deposits can come from the working gas, almost all insulators in the entire ion-source assembly may need to be replaced, as well as almost all metal parts cleaned. 
     In addition to the extensive disassembly and maintenance procedures required for the prior-art ion source of FIGS. 1 through 4, there is also the reduced utilization of the working gas that is inherent to the design. The working gas can escape the discharge region through gap  64 . From there the gas can escape through penetrations in external anode support  18  for threaded retainer rods  20  and cathode support rods  41 , as well as through the gap between the external anode support and outer shell  15 . Because of the large diameter of the outer shell compared to the diameter of the other parts in the ion-source assembly, the circumferential leakage area between the external anode support and the outer shell can be substantial. Better containment of the working gas would reduce both the loss of this gas, which results in a greater vacuum pumping requirement, and the deposition of conducting films on insulators when decomposition of the working gas is possible. 
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to FIG. 5, there is shown a gridless ion source  70  of the end-Hall type that is an embodiment of the present invention. Ion source  70  is also generally of the type described in U.S. Pat. No. 4,862,032—Kaufman, et al., although it additionally incorporates a detachable anode module that facilitates rapid and economical maintenance. 
     Ion source  70  includes cathode assembly  13 , detachable anode module  71 , and magnetic-circuit module  72 . Cathode assembly  13  includes cathode supports  23 , cathode  24 , and cathode retaining nuts  25 . The components shown in FIG. 5 for the magnetic-circuit module include outer shell  15  and back plate  73 , both of which are also parts of the magnetic circuit. Also parts of the magnetic-circuit module are threaded retainer rods  74 . 
     Retaining nuts  76  are used to clamp anode module  71  to magnetic-circuit module  72 . Outer pole piece  16  is part of the anode module and also part of the magnetic circuit. Because outer shell  15  remains with the magnetic-circuit module  72 , knobs  77  are attached to outer pole piece  16  to facilitate removal of the anode module from the magnetic-circuit module when the latter is installed in a vacuum chamber. Anode  17 , external anode support  18 , and enclosure retainer screws  78  are also included in the anode module. To separate the cathode assembly from the rest of ion source  70 , the two cathode supports are grasped with the fingers of two hands and lifted, overcoming the friction with which the cathode supports are attached to the rest of the ion source. 
     Referring to FIG. 6, ion source  70  is shown with cathode assembly  13  separated from the rest of the ion source. At the separated location, the lower ends of cathode supports  23  are exposed to show connectors  26  thereon, with each connector again comprised of elastic spring “fingers” to establish an electrical connection with a cylindrical contact. To separate anode module  71  from magnetic-circuit module  72 , retaining nuts  76  are removed and the anode module lifted using knobs  77 . 
     Referring to FIG. 7, there is shown the detachable anode module separated from the magnetic-circuit module. Additional parts shown for anode module  71  are enclosure wall  79  and enclosure internal end  81 . Note that the enclosure is closed on the internal end and open on the external end. Additional parts shown for the magnetic-circuit module are magnet  55 , large support ring  82 , and small support ring  83 . 
     Referring to FIG. 8 a , there is shown a cross section of anode module  71  of ion source  70 . Note that the cross section of FIG. 8 a  is again not a particular cross section of the ion source, but instead is one that has been constructed to include the major design features of that ion source. Parts not shown in FIG. 7, but shown in FIG. 8 include internal anode support  46 , screws  47  for holding the internal and external anode supports together, and reflector  49 . The reflector again has apertures  62  therein. Enclosure internal end  81  has aperture  84  for introducing the ionizable gas into the enclosure formed by enclosure wall  79  and enclosure internal end  81 . The gas flows from aperture  84  to circumferential manifold  86 . The circumferential manifold has cover  87  with apertures  88  therein to circumferentially distribute the gas to apertures  62  in reflector  49 , from which the gas flows to discharge volume  63 . Anode rod  89  electrically connects with anode  17 , while being spaced from and located relative to reflector  49  and enclosure internal end  81  by ceramic insulators  43 . The lower end of anode rod  89  forms anode cylindrical contact  91 . 
     Referring to FIG. 8 b , there is shown a cross section of magnetic-circuit module  72  of ion source  70 . Cathode contacts  40  are integral parts of cathode support rods  92 , which are spaced from and located relative to large support ring  82  by additional insulators  43  held in place by nuts  44 . Electrical connections of the cathode contacts with the cathode power supply (not shown) are provided by conducting wires covered with ceramic insulator beads  93 . The ionizable working gas is provided through tube  94 , which connects to gas fixture  96  with nozzle  97 . Anode connector  98  is connected to the anode supply (not shown) through conducting wire covered with ceramic insulating beads  99 . The anode connector is spaced from and located relative to small support ring  83  by additional insulators  43  held in place with nut  44 . When the anode module is clamped to the magnetic-circuit module, nozzle  97  fits closely into aperture  84 , so that essentially all of the working gas flows into the enclosure formed by enclosure wall  79  and enclosure internal end  81 . In addition, anode contact  91  is inserted into complementary anode connector  98  to electrically connect anode  17  to the anode power supply. 
     Referring to FIG. 9 a , there is shown an additional partial cross section of anode module  71  of ion source  70 . Internal anode support  46  and reflector  49  are shown to be spaced from and located relative to enclosure internal end  81  by screws  101  and additional insulators  43  held in place by additional nuts  44 . There is typically a plurality of screw/insulator/nut assemblies as shown in FIG. 9 a  and only one anode-rod/insulator assembly as shown in FIG. 8 a , so that the clamping function of a nut is not required on the bottom of anode rod  89  in FIG. 8 a . 
     Referring to FIG. 9 b , there is shown an additional partial cross section of magnetic-circuit module  72  of ion source  70 . Threaded retainer rod  74  is screwed into back plate  73 , while locating large support ring  82  relative thereto. Small support ring  83  is located relative to back plate  73  by small ring support  102 . When the anode module is inserted to the magnetic-circuit module, the ends of threaded retainer rods  74  fit though apertures  103  in outer pole piece  16 , so that nuts  76  (shown in FIGS. 5 and 6) on the ends of the threaded retainer rods can clamp the two modules together. 
     It should be apparent to one skilled in the art of ion-source design that there are many arbitrary design features in the embodiment shown in FIGS. 5 through 9 b . Cylindrical contacts and complementary connectors are used to make detachable electrical connections. The locations of these contacts and connectors can generally be exchanged, while still performing as a detachable electrical connection. Or a spring contact and a flat surface may be used instead to make a detachable electrical connection. The locations of a nozzle and an aperture for a detachable gas connection may, in a similar manner, be exchanged, while still performing as such a connection. Alternatively, two flat surfaces with matching apertures may be pressed together to perform as a detachable gas connection. The magnetically energizing means is shown as a permanent magnet, but could have been an electromagnet. The magnetically energizing means could also have been a series of permanent magnets used in place of the outer shell, with the central permanent magnet replaced by a simple magnetically permeable path. 
     To review the maintenance advantages of the apparatus shown in FIGS. 5 through 9 b , the enclosure formed by enclosure wall  79  and enclosure internal end  81  contains both the electrons and ions that constitute the discharge plasma formed during operation. (Additional discussion of the constituents and properties of this discharge plasma can be found in the aforementioned U.S. Pat. No. 4,862,032—Kaufman, et al.) As is known to those skilled in the art of operating gridless ion sources in general and end-Hall ion sources in particular, sputtered particles are generated from parts exposed to the discharge and tend to flow outward in all directions from the sputtered surfaces of these parts. The enclosure contains these sputtered particles, The insulators and other parts that are in region  104 , external to the enclosure but within the magnetic-circuit module when the two modules are clamped together, are thus protected from these sputtered particles. As is also known to those skilled in the plasma-physics art, the containment of the plasma electrons and ions by the enclosure greatly reduces the initiation of discharges and arcs in regions  104 , further reducing the deposits on insulators and other parts in regions  104 . Finally, if conductive deposits can result from the decomposition of the ionizable working gas, the containment of this gas within the enclosure also reduces the deposits in regions  104 . In summary the use of an enclosure surrounding the anode and discharge region limits the required maintenance to essentially the insulators and other parts in the anode module. 
     Compared to carrying out maintenance on the entire ion source, as required in the prior art, the use of modular construction with a removable anode module permits the maintenance to be carried out on the smaller and lighter anode module. In the event that downtime is to be reduced by purchasing a spare unit, only the less expensive anode module need be purchased. The use of modular construction also facilitates maintenance on parts less frequently replaced, e.g., ready access to the magnet in the preferred embodiment compared to essentially complete disassembly to reach the magnet in the prior art. The use of the invention described above thus results in the general advantage of more rapid and economical maintenance. 
     In addition to the maintenance advantages, the modular design of the invention reduces the loss of working gas compared to the prior art. In the prior-art design shown in FIGS. 1 through 4, there is gas leakage between outer shell  15  and external anode support  18 , as well as leakage through the penetrations through the external anode support  18  for the cathode connections, the plug-and-socket retaining rods, and the threaded retainer rods that hold the ion-source assembly together. In the embodiment of this invention shown in FIGS. 5 through 9 b , the smaller mean diameter of the gap between the enclosure wall and the external anode support reduces the circumferential leakage area, and there are no penetrations of the external anode support to add to this leakage. 
     Comparing the invention to the prior art of FIG. 4, openings for the attachment of the cathode assembly in outer pole piece  16  are in the same enclosure formed by the parts of the magnetic circuit and therefore provide additional escape paths for the ionizable working gas. The use of a separate enclosure around the anode (enclosure wall  79  and enclosure internal end  81 ) thus provides improved containment of the working gas. 
     ALTERNATE EMBODIMENTS 
     A simplified cross section of an alternate embodiment of the present invention wherein the gridless ion source is also of the end-Hall type is shown in FIG. 10 a . The simplification is in the omission of the screws, nuts, insulators and other common parts that are required for most ion source hardware, but well understood by those skilled in the design art. For example, there are insulators, screws, and internal and external anode supports used to space the anode from the rest of the anode module, while locating it relative to that module—see FIG. 9 a . As another example, insulators and screws are used to space the reflector from the rest of the anode module, while locating it relative to that module. In a similar manner, the cathode is not shown in FIG. 10 a . Ion source  110  in FIG. 10 a  is again generally of the type described in U.S. Pat. No. 4,862,032—Kaufman, et al. 
     Ion source  110  is comprised of anode module  111  and magnetic-circuit module  112 . The magnetic circuit is made up of permanent magnet  113 , back plate  114 , outer shell  116 , and outer pole piece  117 , all of which are in the magnetic-circuit module. Anode  118 , reflector  119 , and enclosure  121  are all in the anode module. Enclosure  121  is in turn comprised of enclosure wall  121 A and enclosure internal end  121 B. The external end of the enclosure is again open. Other parts of the magnetic-circuit module are nozzle  122  to inject the working gas into enclosure  121  and anode connector  123  to establish the electrical connection to the anode. 
     Referring to FIG. 10 b , there anode module  111  and magnetic-circuit module  112  are shown separated. Aperture  124  into which nozzle  122  fits and anode contact  125  that electrically connects to complementary anode connector  123  are also shown in FIG. 10 b.    
     One difference between the embodiment of FIGS. 5 through 9 b  and that of FIGS. 10 a  and  10   b  is that in the latter the outer pole piece is part of the magnetic-circuit module rather than the anode module. Both embodiments obtain substantial size, weight, and cost benefits from the present invention in that most of the large and heavy magnetic circuit is excluded from the anode module. As shown by the preferred embodiment of FIGS. 5 through 9 b , though, it is not necessary to exclude all of the magnetic-circuit parts from the anode module. 
     A related difference between the embodiment of FIGS. 5 through 9 b  and that of FIGS. 10 a  and  10   b  is that in the latter the entire magnetic circuit is external to enclosure  121 . As shown by the preferred embodiment of FIGS. 5 through 9 b , though, it is not necessary that all the magnetic circuit be external to the enclosure. 
     Referring to FIG. 11 a , there is shown a simplified cross section of an alternate embodiment of the present invention wherein the gridless ion source is of the closed-drift type. Ion source  130  is comprised of anode module  131  and magnetic-circuit module  132 . 
     The magnetic circuit includes inner pole piece  133 , outer pole piece  134 , inner magnetic path  135 , back plate  136 , outer permeable paths  137  (typically four), inner magnetically energizing coil  139 , and outer magnetically energizing coils  141  (also typically four), all of which are parts of the magnetic-circuit module. Although both permanent magnets and electromagnets have been used in closed-drift ion sources, the use of electromagnets is more common. 
     Closed-drift gridless ion source  130  is of the magnetic-layer type, which generally uses an insulating ceramic for discharge-chamber wall  142 —see the aforementioned article by Zhurin, et. al., in  Plasma Sources Science  &amp;  Technology , Vol. 8, beginning on page R1. Anode  143  is of an annular shape with a plurality of apertures  144  for distributing the working gas from internal manifold  145 . Anode  143  connects to gas fitting  146  and electrical connector  147 . Gas fitting  146  and connector  147  are protected from external contamination by shield  148 . A shield enclosing the outside diameter of the magnetic-circuit module would have provided the same protective function, but would also restrict thermal radiation from the outer electromagnets. 
     Referring to FIG. 11 b , there is shown anode module  131  separated from magnetic-circuit module, thereby exposing gas nozzle  149  and electrical contact  150 , with both connected to anode  143 . 
     From the above discussion and FIGS. 11 a  and  11   b , it should be readily apparent that the present invention can utilize a gridless ion source of the closed-drift type. Note that discharge-chamber wall  142  also serves as an enclosure with outer wall  142 A, inner wall  142 B, internal end  142   c , and an open external end. 
     The embodiments shown all implicitly use axially-symmetric configurations or, in the case of the closed-drift ion source with four outer magnetically permeable paths, near-axially-symmetric configurations. However, other shapes for the discharge region such as elongated or “racetrack” shapes. are well known to those skilled in the art of gridless ion sources. See for example the aforementioned U.S. Pat. No. 4,862,032—Kaufman, et al., or the aforementioned article by Zhurin, et. al., in  Plasma Sources Science  &amp;  Technology , Vol. 8, beginning on page R1. The present invention should therefore include embodiments in which the discharge chambers and the ion sources have shapes other than axisymmetric. 
     While particular embodiments of the present invention have been shown and described, and various alternatives have been suggested, it will be obvious to those of ordinary skill in the art that changes and modifications may be made without departing from the invention in its broadest aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of that which is patentable.