Abstract:
The IHC ion source comprises an ion source chamber having a cathode and a repeller on opposite ends. The repeller is made of two discrete parts, each comprising a different material. The repeller includes a repeller head, which may be a disc shaped component, and a stem to support the head. The repeller head is made from a conductive material having a higher thermal conductivity than the stem. In this way, the temperature of the repeller head is maintained at a higher temperature than would otherwise be possible. The higher temperature limits the build-up of material on the repeller head, which improves the performance of the IHC ion source. In certain embodiments, the repeller head and the stem are connected using a press fit. Differences in the coefficient of thermal expansion of the repeller head and the stem may cause the press fit to become tighter at higher temperatures.

Description:
FIELD 
     Embodiments of the present disclosure relate to an indirectly heated cathode (IHC) ion source, and more particularly, an IHC ion source having a repeller made of two different materials. 
     BACKGROUND 
     Indirectly heated cathode (IHC) ion sources operate by supplying a current to a filament disposed behind a cathode. The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the ion source chamber. The cathode is disposed at one end of the ion source chamber. A repeller is typically disposed on the end of the ion source chamber opposite the cathode. The repeller may be biased so as to repel the electrons, directing them back toward the center of the ion source chamber. In some embodiments, a magnetic field is used to further confine the electrons within the ion source chamber. The electrons cause a plasma to be created. Ions are then extracted from the ion source chamber through an extraction aperture. 
     One issue associated with IHC ion sources is that the cathode and repeller may have a limited lifetime. The cathode is subjected to bombardment from electrons on its back surface, and by positively charged ions on its front surface. This bombardment results in sputtering, which causes erosion of the cathode. 
     Further, in some embodiments, tungsten or carbon like material may grow on the surface of the repeller. These deposits may reduce the efficiency of the ion source, or may lead to issues with the plasma, such as, for example, non-uniformity of extracted ribbon ion beams. Further, these deposits may also introduce contaminants into the extracted ion beam and reduce the life of the ion source. 
     Therefore, an IHC ion source in which material did not build up on the repeller may be beneficial. This IHC ion source may have improved life, performance and beam uniformity. 
     SUMMARY 
     The IHC ion source comprises an ion source chamber having a cathode and a repeller on opposite ends. The repeller is made of two discrete parts, each comprising a different material. The repeller includes a repeller head, which may be a disc shaped component, and a stem to support the head. The repeller head is made from a conductive material having a higher thermal conductivity than the stem. In this way, the temperature of the repeller head is maintained at a higher temperature than would otherwise be possible. The higher temperature limits the build-up of material on the repeller head, which improves the performance of the IHC ion source. In certain embodiments, the repeller head and the stem are connected using a press fit or an interference fit. Differences in the coefficient of thermal expansion of the repeller head and the stem may cause the press fit to become tighter at higher temperatures. 
     According to one embodiment, an indirectly heated cathode ion source is disclosed. The indirectly heated cathode ion source comprises an ion source chamber into which a gas is introduced; a cathode disposed on one end of the ion source chamber; and a repeller disposed at an opposite end of the ion source chamber, the repeller comprising a repeller head disposed within the ion source chamber and a stem that supports the repeller head and exits the ion source chamber through an opening; wherein the repeller head is made of a first material and the stem is made from a second material, different than the first material. In certain embodiments, the first material has a first thermal conductivity and the second material has a second thermal conductivity and the first thermal conductivity is greater than the second thermal conductivity. In some embodiments, the second thermal conductivity is less than half of the first thermal conductivity. In some embodiments, the second thermal conductivity is less than a third of the first thermal conductivity. In certain embodiments, the repeller head and the stem are connected using a press fit. In some embodiments, the repeller head comprises a cavity disposed on a back surface, and wherein the stem is inserted into the cavity. In other embodiments, the repeller head comprises a post disposed on a back surface, and a cavity is disposed at an end of the stem, and the post is inserted into the cavity. 
     According to a second embodiment, a repeller for use within an ion source chamber is disclosed. The repeller comprises a repeller head disposed within the ion source chamber; and a stem that supports the repeller head and exits the ion source chamber through an opening; wherein the repeller head is made of a first material and the stem is made from a second material, different than the first material, wherein the first material has a higher thermal conductivity than the second material. In some embodiments, the repeller head comprises tungsten. In certain embodiments, the stem is in electrical communication with a repeller power supply to supply a voltage to the repeller head. 
     According to a third embodiment, a repeller for use within an ion source chamber is disclosed. The repeller comprises a disc-shaped repeller head disposed within the ion source chamber and biased at a voltage; and a stem attached to a back surface of the disc-shaped repeller head and exiting the ion source chamber through an opening; wherein the disc-shaped repeller head and the stem are both electrically conductive and made from materials having a melting point greater than 1000° C., and wherein a thermal conductivity of the disc-shaped repeller head is at least twice as great as a thermal conductivity of the stem. In certain embodiments, the stem is made from a material selected from the group consisting of tantalum, titanium, rhenium, hafnium, stainless steel, KOVAR® and INVAR®. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
         FIG. 1  is an ion source in accordance with one embodiment; 
         FIGS. 2A-2D  show views of the connection between the repeller head and the stem according to various embodiments; 
         FIG. 3  shows a view of the connection between the repeller head and the stem according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, indirectly heated cathode ion sources may be susceptible to performance issues due to material build-up on the surface of the repeller. As the material grows on the surface of the repeller, the uniformity of the extracted ribbon ion beam may be degraded. 
       FIG. 1  shows an IHC ion source  10  that overcomes this issue. The IHC ion source  10  includes an ion source chamber  100 , having two opposite ends, and sides connecting to these ends. The ion source chamber  100  may be constructed of an electrically conductive material. A cathode  110  is disposed inside the ion source chamber  100  at one of the ends of the ion source chamber  100 . This cathode  110  is in communication with a cathode power supply  115 , which serves to bias the cathode  110  with respect to the ion source chamber  100 . In certain embodiments, the cathode power supply  115  may negatively bias the cathode  110  relative to the ion source chamber  100 . For example, the cathode power supply  115  may have an output in the range of 0 to −150V, although other voltages may be used. In certain embodiments, the cathode  110  is biased at between 0 and −40V relative to the ion source chamber  100 . A filament  160  is disposed behind the cathode  110 . The filament  160  is in communication with a filament power supply  165 . The filament power supply  165  is configured to pass a current through the filament  160 , such that the filament  160  emits thermionic electrons. Cathode bias power supply  116  biases filament  160  negatively relative to the cathode  110 , so these thermionic electrons are accelerated from the filament  160  toward the cathode  110  and heat the cathode  110  when they strike the back surface of cathode  110 . The cathode bias power supply  116  may bias the filament  160  so that it has a voltage that is between, for example, 300V to 600V more negative than the voltage of the cathode  110 . The cathode  110  then emits thermionic electrons on its front surface into ion source chamber  100 . 
     Thus, the filament power supply  165  supplies a current to the filament  160 . The cathode bias power supply  116  biases the filament  160  so that it is more negative than the cathode  110 , so that electrons are attracted toward the cathode  110  from the filament  160 . Finally, the cathode power supply  115  biases the cathode  110  more negatively than the ion source chamber  100 . 
     A repeller  120  is disposed inside the ion source chamber  100  on the end of the ion source chamber  100  opposite the cathode  110 . The repeller  120  may be in communication with repeller power supply  125 . As the name suggests, the repeller  120  serves to repel the electrons emitted from the cathode  110  back toward the center of the ion source chamber  100 . For example, the repeller  120  may be biased at a negative voltage relative to the walls of the ion source chamber  100  to repel the electrons. Like the cathode power supply  115 , the repeller power supply  125  may negatively bias the repeller  120  relative to the walls of the ion source chamber  100 . For example, the repeller power supply  125  may have an output in the range of 0 to −150V, although other voltages may be used. In certain embodiments, the repeller  120  is biased at between 0 and −40V relative to the walls of the ion source chamber  100 . 
     In certain embodiments, the cathode  110  and the repeller  120  may be connected to a common power supply. Thus, in this embodiment, the cathode power supply  115  and repeller power supply  125  are the same power supply. 
     Although not shown, in certain embodiments, a magnetic field is generated in the ion source chamber  100 . This magnetic field is intended to confine the electrons along one direction. For example, electrons may be confined in a column that is parallel to the direction from the cathode  110  to the repeller  120  (i.e. the y direction). 
     Disposed on another side of the ion source chamber  100  may be a faceplate including an extraction aperture  140 . In  FIG. 1 , the extraction aperture  140  is disposed on a side that is parallel to the X-Y plane (parallel to the page). Further, while not shown, the IHC ion source  10  also comprises a gas inlet through which the gas to be ionized is introduced into the ion source chamber  100 . 
     A controller  180  may be in communication with one or more of the power supplies such that the voltage or current supplied by these power supplies may be modified. The controller  180  may include a processing unit, such as a microcontroller, a personal computer, a special purpose controller, or another suitable processing unit. The controller  180  may also include a non-transitory storage element, such as a semiconductor memory, a magnetic memory, or another suitable memory. This non-transitory storage element may contain instructions and other data that allows the controller  180  to maintain appropriate voltages for the filament  160 , the cathode  110  and the repeller  120 . 
     During operation, the filament power supply  165  passes a current through the filament  160 , which causes the filament to emit thermionic electrons. These electrons strike the back surface of the cathode  110 , which may be more positive than the filament  160 , causing the cathode  110  to heat, which in turn causes the cathode  110  to emit electrons into the ion source chamber  100 . These electrons collide with the molecules of gas that are fed into the ion source chamber  100  through the gas inlet. These collisions create ions, which form a plasma  150 . The plasma  150  may be confined and manipulated by the electrical fields created by the cathode  110 , and the repeller  120 . In certain embodiments, the plasma  150  is confined near the center of the ion source chamber  100 , proximate the extraction aperture  140 . The ions are then extracted through the extraction aperture as an ion beam. 
     The repeller  120  is made up of a repeller head  121  and a stem  122 . The repeller head  121  may be a disc-shaped structure which is disposed within the ion source chamber  100 . The stem  122  is attached to the repeller head  121  and exits through an opening in the ion source chamber  100  to allow connection of the repeller  120  to the repeller power supply  125 . In certain embodiments, the stem  122  may be held in place by a clamp (not shown) on the exterior of the ion source chamber  100 , which may be constructed from molybdenum or a molybdenum alloy, such as, for example, TZM, which comprises titanium, zirconium, carbon with the balance being molybdenum. The stem  122  has a much smaller cross-sectional area than the repeller head  121 . The repeller head  121  is intended to provide a charged surface to repel electrons. In contrast, the stem  122  is intended to provide mechanical support and electrical conductivity between the repeller head  121  and the exterior of the ion source chamber  100 . Thus, to minimize the size of the opening in the ion source chamber  100 , the cross-sectional area of the stem  122  may be minimized. 
     The repeller head  121  may be made of a first electrically conductive material, having a first thermal conductivity. The stem  122  may be made of a second electrically conductive material, different from the first electrically conductive material, and having a second thermal conductivity less than the first thermal conductivity. 
     In some embodiments, the second thermal conductivity is less than half of the first thermal conductivity. In certain embodiments, the second thermal conductivity is less than a third of the first thermal conductivity. 
     In operation, the repeller head  121  is heated by the energy introduced into the ion source chamber  100 . For example, the plasma  150  may have an elevated temperature. Further, the repeller head  121  may be struck by energetic ions or electrons disposed inside the ion source chamber  100 . Radiation of the plasma  150  and the other components in the ion source chamber  100  will also transfer heat to the repeller  120 . These various phenomena serve to heat the repeller head  121 . Some of this heat is removed by thermal conduction through the stem  122  to the components external to the ion source chamber  100 . By using a second material having a lower thermal conductivity than the repeller head  121 , the amount of heat that is removed from the repeller head  121  may be reduced. 
     For example, traditionally, the repeller head  121  and the stem  122  are both constructed from tungsten. During operation, the repeller head may maintain a first temperature of about 600° C. during normal operation, and a second temperature of about 800° C. during high power operation. By replacing the tungsten stem, which has a thermal conductivity of around 150 W m −1  K −1 , with a stem made of tantalum, for example, which has a thermal conductivity of around 50 W m −1  K −1 , the temperature of the repeller head  121  increases to 720° C. during normal operation and 1100° C. during high power operation. Thus, a material having a thermal conductivity that is about a third that of tungsten causes a significant increase in the temperature of the repeller head  121 . 
     Increased temperature of the repeller head  121  may reduce the rate and amount of material that build up on the surface of the repeller head  121 . For example, it has been observed that less material builds up on the cathode  110 , which is known to be at a higher temperature than the repeller  120 . 
     The repeller head  121  and the stem  122  may be joined using a press fit. For example, one of the repeller head  121  and the stem  122  may include a cavity, while the other comprises a post that may be inserted into the cavity.  FIG. 2A  shows a first embodiment where a hole  126  is drilled through the repeller head  121 . The stem  122  is pressed into the hole  126 . 
       FIG. 2B  shows a second embodiment illustrating the connection between the repeller head  121  and the stem  122 . In this embodiment, a recessed cavity  123  is created within the back surface of the repeller head  121 , such that the recessed cavity  123  does not extend to the front surface of the repeller head  121 . In this disclosure, the front surface of the repeller head is that surface that faces toward the center of the ion source chamber  100 . The back surface of the repeller head  121  is that surface that faces toward an end of the ion source chamber  100 . The stem  122  is then inserted into the recessed cavity  123 . 
       FIG. 2C  shows a third embodiment illustrating the connection between the repeller head  121  and the stem  122 . In this embodiment, a cavity  124  is created on the back surface of the repeller head  121  by extending the material such that it forms a raised annular ring  131 . The stem  122  then is pressed into the cavity  124 . 
     In another embodiment, the embodiments of  FIGS. 2B and 2C  may be combined such that there is a raised annular ring  131  and a recessed cavity  123 . This embodiment is illustrated in  FIG. 2D . 
     In each of these embodiments, it may be desirable that the coefficient of thermal expansion of the stem  122  is greater than that of the repeller head  121 . In this way, as the repeller  120  heats, the stem  122  expands more than the cavity, which tightens the fit. 
     Further, in certain embodiments, the repeller head  121  may be made of tungsten. Thus, for the embodiments of the  FIGS. 2A-2D , the stem  122  may have a lower thermal conductivity than tungsten and a higher coefficient of thermal expansion than tungsten. Table 1 illustrates some materials that have these properties. Additionally, each of these materials is electrically conductive. The first row of Table 1 shows the characteristics of tungsten for comparison purposes. It is noted that this table is not intended to be exhaustive; rather it simply illustrates several possible materials that may be used for the stem  122  in these embodiments where the repeller head  121  is made of tungsten. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Coefficient of 
                   
               
               
                   
                 Thermal 
                 Thermal 
               
               
                   
                 Conductivity 
                 Expansion 
                 Melting 
               
               
                 Material 
                 (W/mK) 
                 (ppm/K) 
                 Point (° C.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Tungsten 
                 174 
                 4.5 
                 3422 
               
               
                 Tantalum 
                 57 
                 6.3 
                 3017 
               
               
                 Titanium 
                 22 
                 8.6 
                 1668 
               
               
                 Rhenium 
                 48 
                 6.2 
                 3192 
               
               
                 Hafnium 
                 23 
                 5.9 
                 2233 
               
               
                 300 Series SST 
                 16.4 
                 17-18 
                 1400 
               
               
                 KOVAR ® 
                 17 
                 5.3 
                 1449 
               
               
                   
               
             
          
         
       
     
     Of course, this table is only illustrative, as the repeller head  121  may be constructed of a different material, such as molybdenum, tantalum, rhenium or another metal. Regardless of the material used for the repeller head  121 , the material for the stem  122  is selected so as to have a lower thermal conductivity than the repeller head  121 . 
     In certain embodiments, there may be a minimum acceptable melting temperature for the first material and the second material to allow proper operation within the IHC ion source  10 . In some embodiments, this minimum melting temperature may be 1000° C. In other embodiments, this minimum melting temperature may be 1400° C. Each of the materials listed in Table 1 satisfy this limitation. 
     Other connections between the repeller head  121  and the stem  122  are also possible. For example,  FIG. 3  shows an embodiment where the repeller head  121  has a post  127  extending from its back surface. The stem  122  has an annular ring  128  extending from its distal end, creating a cavity  129  at the end of the stem  122 . In this embodiment, the post  127  from the repeller head  121  extends into the cavity  129  created by the annular ring  128  on the end of the stem  122 . 
     In this embodiment, it may be beneficial for the repeller head  121  to have a greater coefficient of thermal expansion than the stem  122 , such that the post  127  expands more than the cavity  129 . Table 2 shows a possible material that may be used for the embodiment shown in  FIG. 3  when the repeller head  121  is made of tungsten. It is noted that this table is not intended to be exhaustive, rather it simply illustrates one possible material that may be used for the stem  122  in this embodiment. As described above, this material is also electrically conductive. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Coefficient of 
                   
               
               
                   
                   
                 Thermal 
                 Thermal 
               
               
                   
                   
                 Conductivity 
                 Expansion 
                 Melting 
               
               
                   
                 Material 
                 (W/mK) 
                 (ppm/K) 
                 Point (° C.) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Tungsten 
                 174 
                 4.5 
                 3422 
               
               
                   
                 INVAR ® 
                 10 
                 0.6 
                 1427 
               
               
                   
                   
               
             
          
         
       
     
     As described above, in certain embodiments, there may be a minimum acceptable melting temperature for the second material to allow proper operation within the IHC ion source  10 . In some embodiments, this minimum melting temperature may be 1000° C. In other embodiments, this minimum melting temperature may be 1400° C. The material listed in Table 2 satisfies this limitation. 
     While the previous description discloses a press fit between the post and the cavity, other configurations are also possible. For example, in certain embodiments, the post may be cooled while the cavity is heated during the insertion process, such that an interference fit is created when the post and cavity reach a common temperature. In other embodiments, only the post is cooled prior to insertion. In yet other embodiments, only the cavity is heated prior to insertion. In each of these embodiments, the temperatures of the post and cavity are manipulated to allow the post to fit within the cavity during insertion. After thermal equilibrium is reached, an interference fit is created. Thus, an interference fit is a special type of press fit. 
     In yet other embodiments, the repeller head  121  and the stem  122  may be welded, soldered or otherwise joined together. 
     The embodiments described above in the present application may have many advantages. As described above, IHC ion sources are susceptible to short life and performance degradation due to the material build-up on the repeller. By reducing the thermal conductivity of the stem  122 , the repeller head  121  retains more of the heat imparted to it by the plasma and energetic electrons and ions. This serves to raise the temperature of the repeller head  121 , which reduces the build-up of material on its front surface. In certain embodiments, the temperature of the repeller head  121  may increase 150-250° C. through the use of a stem  122  that is made of a second material, having a thermal conductivity that is one third that of tungsten. 
     The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.