Patent Publication Number: US-11664192-B2

Title: Temperature control for insertable target holder for solid dopant materials

Description:
This application is a continuation of U.S. patent application Ser. No. 16/735,125 filed Jan. 6, 2020, which claims priority of U.S. Provisional Application Ser. No. 62/913,023 filed Oct. 9, 2019, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     Embodiments of the present disclosure relate to an ion source, and more particularly, an ion source with an insertable target holder to hold solid dopant materials, wherein the temperature of the dopant material or the target holder may be measured and optionally controlled. 
     BACKGROUND 
     Various types of ion sources may be used to create the ions that are used in semiconductor processing equipment. For example, an indirectly heated cathode (IHC) ion source operates 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 arc chamber of the ion source. The cathode is disposed at one end of an arc chamber. A repeller may be disposed on the end of the arc chamber opposite the cathode. The cathode and repeller may be biased so as to repel the electrons, directing them back toward the center of the arc chamber. In some embodiments, a magnetic field is used to further confine the electrons within the arc chamber. A plurality of sides is used to connect the two ends of the arc chamber. 
     An extraction aperture is disposed along one of these sides, proximate the center of the arc chamber, through which the ions created in the arc chamber may be extracted. 
     In certain embodiments, it may be desirable to utilize a material that is in solid form as a dopant species. For example, a crucible or target holder may be used to hold the metal such that when the metal liquifies, it remains in the target holder. Use of pure solid metal directly for ion implant increases the beam current available for wafer implant. 
     However, there may be issues associated with the use of a target holder for solid dopant materials. For example, when using metals with low melting and boiling temperatures in a contained target holder, very high temperatures may be problematic. For example, the dopant material may become unstable and be prone to runaway effects that can cause inconsistent beam performance and lead to the undesired accumulation of dopant material in the arc chamber. 
     Therefore, an ion source that may be used with solid dopant materials having low melting temperatures, such as certain metals, and is capable of monitoring and controlling its internal temperature, would be beneficial. 
     SUMMARY 
     An ion source with a target holder for holding a solid dopant material is disclosed. The ion source comprises a thermocouple disposed proximate the target holder to monitor the temperature of the solid dopant material. In certain embodiments, a controller uses this temperature information to vary one or more parameters of the ion source, such as arc voltage, cathode bias voltage, extracted beam current, or the position of the target holder within the arc chamber. Various embodiments showing the connections between the controller and the thermocouple are shown. Further, embodiments showing various placement of the thermocouple on the target holder are also presented. 
     According to one embodiment, an indirectly heated cathode ion source is disclosed. The indirectly heated cathode ion source comprises an arc chamber, comprising a plurality of walls connecting a first end and a second end; an indirectly heated cathode disposed on the first end of the arc chamber; a target holder, having a pocket to hold a dopant material; a thermocouple in contact with the target holder; and a controller in communication with the thermocouple, wherein the controller varies a parameter of the ion source based on a temperature measured by the thermocouple. In certain embodiments, the ion source further comprises an actuator in communication with the target holder to move the target holder between a first position and a second position, and wherein the parameter comprises the position of the target holder. In some embodiments, the parameter is selected from the group consisting of arc voltage, filament current, cathode bias voltage, flow rate of feed gas, and beam extraction current. In certain embodiments, the ion source further comprises a heating element in communication with the target holder, and therein the parameter comprises the current supplied to the heating element. In some embodiments, the ion source further comprises an actuator assembly, the actuator assembly comprising: wires to electrically connect the thermocouple to the controller; a housing, comprising a rear housing, a front housing and an outer housing connecting the rear housing and the front housing; a shaft affixed to the target holder, and having a retaining plate disposed within the housing; a bellows disposed within the housing and affixed to the retaining plate on one end and to the rear housing on an opposite end; and an actuator to linearly translate the shaft. In some embodiments, a connector is mounted in the front housing, and the wires pass from the controller through a space between the outer housing and the bellows, and terminate at the connector. In some further embodiments, a second connector is mated to the connector, and thermocouple wires are disposed between the second connector and the thermocouple. In some embodiments, the thermocouple wires are coiled to allow target holder position adjustment with respect to the arc chamber. In some embodiments, the thermocouple wires are encased in an Inconel braid. In some embodiments, the thermocouple wires are encased in alumina tubes. In certain embodiments, the wires pass through a hollow interior of the shaft. In certain embodiments, the target holder comprises a target base, a crucible plug, a crucible and a porous plug. In some embodiments, the thermocouple is disposed on an outer surface of the crucible. In certain embodiments, a cavity is disposed on an interior surface of the target holder, and the thermocouple is disposed in the cavity on an outer surface of the crucible plug. In some embodiments, potting material is used to hold the thermocouple in place. In some embodiments, a set screw is used to hold the thermocouple in place. In certain embodiments, a spring is disposed in the cavity to hold the thermocouple in place. In some embodiments, the ion source comprises a heating element in communication with the target holder, wherein the heating element comprises resistive wires. In certain embodiments, the resistive wires are in communication with the crucible or the crucible plug. 
     According to another embodiment, an assembly for use with an ion source is disclosed. The assembly comprises a connector; a thermocouple; and wires disposed between the connector and the thermocouple. In certain embodiments, the wires are coiled. In certain embodiments, the wires are individually insulated. In some embodiments, the insulated wires are encased in an Inconel braid. In certain embodiments, the insulated wires are encased in alumina tubes. 
     According to another embodiment, a target holder to hold a dopant material for use in an ion source is disclosed. The target holder comprises a target base; a crucible shaped as a hollow cylinder; a crucible plug to cover one open end of the crucible and disposed proximate the target base; a porous plug to cover an opposite end of the crucible, wherein gaseous dopant material may pass through the porous plug; and a thermocouple in communication with the target holder. In certain embodiments, the thermocouple is disposed on the outer surface of the crucible. In other embodiments, the thermocouple is disposed in a channel in the wall of the crucible. In some embodiments, there is a cavity disposed in the target base proximate the crucible plug and the thermocouple is disposed in the cavity proximate the crucible plug. In certain embodiments, a channel is disposed in the target base, wherein the channel in communication with the cavity to allow wires to be routed to the thermocouple. In some embodiments, potting material is used to hold the thermocouple in place. In certain embodiments, a set screw is used to hold the thermocouple in place. In some embodiments, the target holder comprises a shaft affixed to the target base. In some embodiment, an interior of the shaft is hollow to allow wires to be routed through the hollow interior of the shaft to the thermocouple. 
     According to another embodiment, an indirectly heated cathode ion source is disclosed. The ion source comprises an arc chamber, comprising a plurality of walls connecting a first end and a second end; an indirectly heated cathode disposed on the first end of the arc chamber; a target holder, having a pocket to hold a dopant material, wherein the target holder is movable within the arc chamber; and a controller, wherein the controller varies a position of the target holder within the arc chamber based on a temperature of the dopant material. In some embodiments, the temperature of the dopant material is determined using optical measurements, a pyrometer, color dots, a thermocouple, a wireless thermocouple reader or an RTD (resistance temperature detector). In certain embodiments, the temperature of the dopant material is determined using a thermocouple. In some embodiments, the temperature of the dopant material is estimated based on a temperature of a component within the arc chamber. 
    
    
     
       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 indirectly heated cathode (IHC) ion source with an insertable target holder in accordance with one embodiment; 
         FIG.  2    shows the actuator assembly and target holder according to one embodiment; 
         FIG.  3    shows the actuator assembly and target holder according to a second embodiment; 
         FIG.  4    shows the actuator assembly and target holder according to a third embodiment; 
         FIG.  5    shows the placement of the thermocouple on the target holder according to one embodiment; 
         FIG.  6    shows the placement of the thermocouple on the target holder according to a second embodiment; 
         FIG.  7    shows the placement of the thermocouple on the target holder according to another embodiment; 
         FIGS.  8 A- 8 B  show the placement of the thermocouple on the target holder according to other embodiments; 
         FIGS.  9 A- 9 C  show the placement of the thermocouple on the target holder according to other embodiments; and 
         FIG.  10    shows the actuator assembly and target holder according to one embodiment where resistive wires are used to heat the target holder. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, at very high temperatures, the solid dopant in an ion source may melt too quickly and create unwanted accumulation of dopant in the arc chamber. At low temperatures, the solid dopant may not melt at all. 
       FIG.  1    shows an IHC ion source  10  with a target holder that overcomes these issues. The IHC ion source  10  includes an arc chamber  100 , comprising two opposite ends, and walls  101  connecting to these ends. The walls  101  of the arc chamber  100  may be constructed of an electrically conductive material and may be in electrical communication with one another. In some embodiments, a liner may be disposed proximate one or more of the walls  101 . A cathode  110  is disposed in the arc chamber  100  at a first end  104  of the arc 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  115  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  115  may bias the filament  160  so that it has a voltage that is between, for example, 200V to 1500V more negative than the voltage of the cathode  110 . The voltage difference between the cathode  110  and the filament  160  may be referred to as cathode bias voltage. The cathode  110  then emits thermionic electrons on its front surface into arc chamber  100 . 
     Thus, the filament power supply  165  supplies a current to the filament  160 . The cathode bias power supply  115  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 . In certain embodiments, the cathode  110  may be biased relative to the arc chamber  100 , such as by bias power supply  111 . The voltage difference between the arc chamber  100  and the cathode  110  may be referred to as arc voltage. In other embodiments, the cathode  110  may be electrically connected to the arc chamber  100 , so as to be at the same voltage as the walls  101  of the arc chamber  100 . In these embodiments, bias power supply  111  may not be employed and the cathode  110  may be electrically connected to the walls  101  of the arc chamber  100 . In certain embodiments, the arc chamber  100  is connected to electrical ground. 
     On the second end  105 , which is opposite the first end  104 , a repeller  120  may be disposed. The repeller  120  may be biased relative to the arc chamber  100  by means of a repeller bias power supply  123 . In other embodiments, the repeller  120  may be electrically connected to the arc chamber  100 , so as to be at the same voltage as the walls  101  of the arc chamber  100 . In these embodiments, repeller bias power supply  123  may not be employed and the repeller  120  may be electrically connected to the walls  101  of the arc chamber  100 . In still other embodiments, a repeller  120  is not employed. 
     The cathode  110  and the repeller  120  are each made of an electrically conductive material, such as a metal or graphite. 
     In certain embodiments, a magnetic field is generated in the arc chamber  100 . This magnetic field is intended to confine the electrons along one direction. The magnetic field typically runs parallel to the walls  101  from the first end  104  to the second end  105 . 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). Thus, electrons do not experience any electromagnetic force to move in the y direction. However, movement of the electrons in other directions may experience an electromagnetic force. 
     Disposed on one side of the arc chamber  100 , referred to as the extraction plate  103 , may be an extraction aperture  140 . In  FIG.  1   , the extraction aperture  140  is disposed on a side that is parallel to the Y-Z plane (perpendicular to the page). Further, the IHC ion source  10  also comprises a gas inlet  106  through which the gas to be ionized may be introduced to the arc chamber  100 . 
     In certain embodiments, a first electrode and a second electrode may be disposed on respective opposite walls  101  of the arc chamber  100 , such that the first electrode and the second electrode are within the arc chamber  100  on walls adjacent to the extraction plate  103 . The first electrode and the second electrode may each be biased by a respective power supply. In certain embodiments, the first electrode and the second electrode may be in communication with a common power supply. However, in other embodiments, to allow maximum flexibility and ability to tune the output of the IHC ion source  10 , the first electrode may be in communication with a first electrode power supply and the second electrode may be in communication with a second electrode power supply. 
     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 perform the functions described herein. 
     The IHC ion source  10  also includes a target holder  190 , which can be inserted into and retracted from the arc chamber  100 . In the embodiment of  FIG.  1   , the target holder  190  enters the arc chamber along one of the walls  101  of the arc chamber  100 . In certain embodiments, the target holder  190  may enter the arc chamber  100  at the midplane between the first end  104  and the second end  105 . In another embodiment, the target holder  190  may enter the arc chamber  100  at a location different from the midplane. In the embodiment shown in  FIG.  1   , the target holder  190  enters the arc chamber  100  through the side opposite the extraction aperture  140 . However, in other embodiments, the target holder  190  may enter through the sides that are adjacent to the extraction plate  103 . The target holder  190  may move between a first position and a second position. 
     The target holder  190  has a cavity or pocket  191  into which the dopant material  195  may be disposed. The pocket  191  may have a bottom surface and sidewalls extending upward from the bottom surface. In certain embodiments, the sidewalls may be vertical. In other embodiments, the sidewalls may be slanted outward from the bottom surface. In some embodiments, the sidewalls and the bottom surface meet at a rounded edge. The bottom surface and the sidewalls form a cavity which is closed at the bottom. In other words, much like a traditional cup, the dopant material  195  is inserted or removed via the open top, while the sidewalls and bottom surface form a sealed structure from which the dopant material  195  cannot exit. In another embodiment, the pocket  191  may be enclosed, such that the dopant material  195  is disposed inside the pocket  191 . For example, a hollow cylindrical crucible may be employed to create the pocket  191 . A porous plug  192  may be used to hold the dopant material inside the pocket  191  and to allow vapors to exit the pocket  191 . The porous plug  192  may be graphite foam, for example. The feed rate of the dopant material from the target holder  190  may also be controlled by adding patterned holes of various size to the porous plug  192  or any other wall of the target holder  190 . Any of the walls of the target holder  190  may be a porous material and used for controlled feed of the dopant material into the arc chamber  100 . 
     A dopant material  195 , such as indium, aluminum, antimony or gallium, may be disposed within the pocket  191  of the target holder  190 . The dopant material  195  may be in the form of a solid when placed in the pocket  191 . This may be in the form of a block of material, filings, shavings, balls, or other shapes. In certain embodiments, the dopant material  195  may melt and become a liquid. Therefore, in certain embodiments, the target holder  190  is configured to enter the arc chamber  100  such that the open end is facing upward and the sealed bottom is facing downward so that melted dopant material  195  cannot flow from the target holder  190  into the arc chamber  100 , but rather remains in the target holder  190 . In other words, the IHC ion source  10  and the target holder  190  are oriented such that the dopant material  195  is retained within the pocket  191  by gravity. 
     A thermocouple  198  may be in proximity to the target holder  190  or the dopant material  195 . This thermocouple  198  may be in communication with the controller  180 . The thermocouple  198  may comprise one or more wires  199  that electrically connect the thermocouple  198  to the controller  180 . 
     In certain embodiments, the thermocouple  198  may be fixed to the outside of the target holder  190 . In other embodiments, the thermocouple  198  may include a rigid sheath that may be used to position relative to the target holder. In another embodiment, the thermocouple point of measurement may be directly inside the pocket  191 , holding the dopant material  195 . In these embodiments, the corrosion of thermocouple  198  may be prevented by using a ceramic insulator sheath to the protect the thermocouple wires. 
     During operation, the filament power supply  165  passes a current through the filament  160 , which causes the filament  160  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 arc chamber  100 . These electrons collide with the molecules of gas that are fed into the arc chamber  100  through the gas inlet  106 . A carrier gas, such as argon, or an etching gas, such as fluorine, may be introduced into the arc chamber  100  through a suitably located gas inlet  106 . The combination of electrons from the cathode  110 , the gas and the positive potential creates a plasma. In certain embodiments, the electrons and positive ions may be somewhat confined by a magnetic field. In certain embodiments, the plasma is confined near the center of the arc chamber  100 , proximate the extraction aperture  140 . Chemical etching or sputtering by the plasma transforms the dopant material  195  into the gas phase and causes ionization. The ionized feed material can then be extracted through the extraction aperture  140  and used to prepare an ion beam. 
     Negative ions and neutral atoms that are sputtered or otherwise released from the dopant material  195  are attracted toward the plasma, since the plasma is maintained at a more positive voltage than the target holder  190 . 
     In certain embodiments, the dopant material  195  is heated and vaporized due to the heat created by the plasma. However, in other embodiments, the dopant material  195  may be heated by additional means as well. For example, a heating element  170  may be disposed within or on the target holder  190  to further heat the dopant material  195 . The heating element  170  may be a resistive heating element, or some other type of heater. 
     In certain embodiments, the target holder  190  may be made of a conductive material and may be grounded. In a different embodiment, the target holder  190  may be made of a conductive material and may be electrically floated. In a different embodiment, the target holder  190  may be made of a conductive material and may be maintained at the same voltage as the walls  101 . In other embodiments, the target holder  190  may be made of an insulating material. 
     In yet another embodiment, the target holder  190  may be biased electrically with respect to the arc chamber  100 . For example, the target holder  190  may be made from a conductive material and may be biased by an independent power supply (not shown) so as to be at a different voltage than the walls  101 . This voltage may be more positive or more negative than the voltage applied to the walls  101 . In this way, electrical biasing may be used to sputter the dopant material  195 . 
     The controller  180  may monitor the temperature of the dopant material  195  using the thermocouple  198 . In certain embodiments, the controller  180  may be in communication with a thermocouple  198  and with the heating element  170 . Thus, the controller  180  may control the heating element  170  to maintain the dopant material  195  at a desired or predetermined temperature. In other words, the controller  180  may vary the current through the heating element  170  to maintain a desired temperature, as measured by the thermocouple  198 . This may allow the controller  180  to control the feed rate of dopant material  195  into the arc chamber  100 . In other embodiments, the controller  180  may indirectly measure the temperature of the dopant material  195 , such as by measuring the temperature of the target holder  190  or some other component. 
     The target holder  190  is in communication with one end of shaft  200 . The opposite end of the shaft  200  may be in communication with an actuator assembly  300 . The actuator assembly  300  may be attached directly to one of the walls  101 . In other embodiments, the actuator assembly  300  may be set back from the wall  101  to allow the target holder  190  to be retracted out of the main cylinder of the arc chamber  100 . The actions of the actuator assembly  300  allow the target holder  190  to move linearly within the arc chamber  100 . 
       FIG.  2    shows one embodiment of the actuator assembly  300 . In this embodiment, the actuator assembly  300  includes a rear housing  310  and a front housing  340 . The front housing  340  may be bolted or otherwise connected to one of the walls  101  of the arc chamber  100 . Alternatively, the front housing  340  may be set back from the walls  101 . An outer housing  360  may be used to connect the rear housing  310  and the front housing  340 . 
     Inside the rear housing  310  is an actuator  320 . The actuator  320  may have a drive shaft  325 . In certain embodiments, the actuator  320  is an electric motor, although other types of actuator may be used. In one embodiment, the drive shaft  325  has a threaded distal end  326 . A correspondingly threaded member  330  may be in communication with the threaded distal end  326 . The threaded member  330  may be affixed to the shaft  200 . In this way, when the drive shaft  325  rotates, the threaded member  330  is drawn to the actuator  320  or moved away from the actuator  320 , depending on the direction of rotation. Since the shaft  200  is affixed to the threaded member  330 , the shaft  200  similarly is translated linearly in the X direction by the rotational movement of the drive shaft  325 . This allows the target holder  190  to be disposed in different locations within the arc chamber  100 . 
     In this embodiment, the shaft  200  includes a retaining plate  210 . The retaining plate  210  is disposed inside the actuator assembly  300  behind the front housing  340 . The retaining plate  210  is welded or otherwise connected to bellows  350 . In certain embodiments, the bellows  350  may be metal. The bellows  350  may also be welded or otherwise attached to the rear housing  310 . The bellows  350  and the retaining plate  210  form the barrier between the vacuum conditions in the arc chamber  100  and the atmospheric conditions outside of the arc chamber  100 . Thus, when the drive shaft  325  rotates, the bellows  350  expands and contracts based on the direction of motion of the shaft  200 . 
     Note that the thermocouple  198  is disposed inside the arc chamber  100 , and the wires  199  need to exit the arc chamber  100 , while preserving the integrity of the vacuum conditions. In the embodiment of  FIG.  2   , a first connector  390  is mounted inside the arc chamber  100  on the front housing  340 . Wires  199  extend from outside the actuator assembly  300  to the first connector  390 . In this embodiment, a channel  311  may be created in the rear housing  310  to allow the wires  199  to pass out of the actuator assembly  300 . The wires  199  may then pass in the space between the bellows  350  and the outer housing  360 . This space is part of the vacuum environment, and therefore a vacuum feedthrough  370  is used to maintain the vacuum. A vacuum feedthrough is a member that allows the passage of wires  199 , but maintains the pressure difference between the two sides of the feedthrough. Thus, the wires  199  pass through channel  311  in the rear housing  310 , then pass through the vacuum feedthrough  370 . The wires  199  then pass through the space between the outer housing  360  and the bellows  350  and finally terminate at the first connector  390 . 
     A second connector  391  mates with the first connector  390 . Thermocouple wires  197  extend from the second connector  391  to the thermocouple  198 . The thermocouple  198  may be a K type thermocouple. Further, the thermocouple wires  197  attached to the thermocouple  198  may be insulated. For example, in one embodiment, each of the two thermocouple wires  197  is individually coated with an insulating material. The two thermocouple wires  197  may then be wrapped together in an Inconel braid. In other words, the thermocouple wires  197  are individually coated for electrical insulation and the pair are then wrapped to protect them from the harsh environment in the arc chamber  100 . In another embodiment, the thermocouple wires  197  may be encased in alumina tubes. 
     In certain embodiments, the thermocouple wires  197  are coiled, as shown in  FIG.  2   . In this way, when the target holder  190  is extended and retracted, the thermocouple wires  197  coil and uncoil to compensate for the change in length. 
     In one embodiment, the thermocouple  198 , the thermocouple wires  197  and the second connector  391  may be a replaceable part. Further, as described above, the thermocouple wires  197  in this embodiment may be individually insulated and then wrapped in a braid. Further, the thermocouple wires  197  may be coiled to allow for changes in length without kinking or interference. 
     A heating element  170  may be disposed within or on the target holder  190  to further heat the dopant material  195 . In certain embodiments, wires associated with the heating element  170  are routed with the thermocouple wires  197 . 
     The thermocouple  198  may be attached to the target holder  190  in a plurality of ways, which are described below. 
       FIG.  3    shows a second embodiment of an actuator assembly  300 . Many of the components are the same as shown in  FIG.  2    and have been given identical reference designators. In this embodiment, the shaft  200  may be hollow such that thermocouple wires  197  may be routed through an interior of the shaft  200 . The shaft  200  also has an opening  201  to the hollow interior. The opening  201  may be located on the side of the retaining plate  210  further from the target holder  190 . In this way, the opening  201  is in atmospheric conditions. If the thermocouple  198  is located within the hollow interior of the shaft  200 , a vacuum feedthrough may not be needed. However, if the thermocouple  198  is located on an exterior surface of the target holder  190 , such as shown in  FIG.  4   , a vacuum feedthrough  370  may be used to preserve the vacuum within the arc chamber  100 . The vacuum feedthrough  370  would be disposed at the entrance to the hollow interior of the shaft  200 . 
     The thermocouple wires  197  pass through the opening  201  and may exit through the channel  311  in the rear housing  310 . In certain embodiments, one or more cable mounts  351  may be used to hold the thermocouple wires  197  in place. In certain embodiments, the wires  199  that are in communication with the controller  180  are the same as the thermocouple wires  197  that pass through the hollow interior of the shaft  200 . In other embodiments, a connector may be disposed between the thermocouple  198  and the controller  180  to create two separate wire segments. For example, the portion of the thermocouple wires that are exposed to the plasma may need to be replaced more often. Therefore, this section of the wires may be formed as a replaceable unit by inserting a connector between the thermocouple  198  and the controller  180 . 
     Thus, in this embodiment, the shaft has a hollow interior that is used to route the thermocouple wires  197  from the thermocouple  198  to the interior of the actuator assembly  300 . As stated, a vacuum feedthrough  370  may be employed at the entrance to the interior of the shaft  200  if the thermocouple  198  is disposed in vacuum conditions, as shown in  FIG.  4   . 
     In certain embodiments, the thermocouple wires  197  are individually insulated and then wrapped together in an Inconel braid or alumina tubes. In other embodiments, because the thermocouple wires are protected by the shaft  200 , an Inconel braid is not employed. 
       FIGS.  2 - 4    describe several systems that may be used to route the wires from the controller  180  to the thermocouple  198 .  FIGS.  5 - 9    show various embodiments concerning the placement of the thermocouple  198  on the target holder  190 . 
       FIG.  5    shows an enlarged view of the target holder  190 . In certain embodiments, the target holder  190  comprises a target base  193 , which is affixed or otherwise attached to the shaft  200 . The target holder  190  may also include a crucible  196 . The crucible  196  holds the dopant material  195 . In certain embodiments, the crucible  196  may be made from graphite. In some embodiments, the crucible  196  may be a hollow cylinder having two open ends. A crucible plug  194  may be disposed between the crucible  196  and the target base  193 . The crucible plug  194  is used to plug one of the open ends of the crucible  196 . Clamp  410  may be used to secure the target base  193  to the crucible  196 . As described above, a porous plug  192  may be used to plug the second open end of the crucible  196 . As stated above, this porous plug  192  may be made from graphite foam or another suitable material. 
     In the embodiment of  FIG.  5   , the thermocouple  198  is mounted on an outer surface of the crucible  196 . Potting material  400  may be used to hold the thermocouple  198  in place. The thermocouple wires  197  may be routed along the exterior of the target holder  190 . 
       FIG.  6    shows another embodiment of the target holder  190 . Many of the components are the same as shown in  FIG.  5    and have been given identical reference designators. In this embodiment, a conduit  420  is created in the target base  193  and optionally in the crucible plug  194 . The thermocouple wires  197  pass through the conduit  420  and the thermocouple  198  is mounted on the outer surface of the crucible  196 , as was done in  FIG.  5   . Potting material  400  may be used to hold the thermocouple  198  in place. 
       FIG.  7    shows another embodiment of the target holder  190 . Many of the components are the same as shown in  FIG.  6    and have been given identical reference designators. In this embodiment, rather than using potting material, a set screw  430  is used to hold the thermocouple  198  in place. The set screw  430  may screw into a threaded shallow hole in the crucible  196 . In some embodiments, the threaded shallow hole does not pass through to the interior of the crucible  196 . 
     It is noted that the set screw  430  may be used with the embodiment of  FIG.  5   . In other words, the thermocouple wires  197  may be routed around the exterior of the target holder  190  and be secured to the crucible  196  using set screw  430 . 
     In summary,  FIGS.  5 - 7    show different target holders  190  where the thermocouple  198  is in contact with an outer surface of the crucible  196 . This thermocouple  198  may be affixed to the crucible  196  using a potting material  400  or a set screw  430 . Other fastening techniques may also be employed. 
     Additionally, the thermocouple  198  may be embedded in the wall of the crucible  196 .  FIG.  8 A  shows an embodiment where a channel  440  is created in the wall of the crucible. Many of the components are the same as shown in  FIG.  6    and have been given identical reference designators. The channel  440  is narrower than the width of the wall of the crucible  196 . The thermocouple  198  is inserted into the channel  440 . Potting material (not shown) may be used to hold the thermocouple  198  in place. In this embodiment, the channel  440  may extend through the target base  193  and optionally the crucible plug  194 . In another embodiment, shown in  FIG.  8 B , the channel  440  exits on the outer surface of the crucible  196 . In this embodiment, the channel does not extend through the target base  193  or the crucible plug  194 . 
     In another embodiment, the channel  440  may extend to the pocket  191 , so that the thermocouple  198  is actually in contact with an interior of the pocket  191  and/or the dopant material  195 . In these embodiments, a ceramic insulator sheath may be employed to protect the thermocouple  198  and the thermocouple wires  197 . 
     The thermocouple  198  may also be in contact with the crucible plug  194 , as shown in  FIGS.  9 A- 9 C . Many of the components are the same as shown in  FIG.  6    and have been given identical reference designators. In these embodiments, a cavity  450  is disposed in the target base  193 . The cavity  450  provides a location where the thermocouple  198  can be placed. A channel  460  is created in the target base  193  from an exterior of the target base  193  to the cavity  450 . Thermocouple wires  197  enter the cavity  450  via the channel  460 .  FIG.  9 A  shows the thermocouple  198  held in place through the use of potting material  400 .  FIG.  9 B  shows the thermocouple  198  held in place using a set screw  430 .  FIG.  9 C  shows the thermocouple  198  held in place through the use of a spring  470 . Of course, other force-based means may also be used to hold the thermocouple  198  in place. 
     While the above disclosure describes various apparatus for routing the wires for the thermocouple  198  to the target holder  190 , the same techniques may also be used to route resistive wires to the target holder  190 . These resistive wires may be employed as a heating element  170 . For example, resistive wires may be in contact with all or a portion of the outer surface of the crucible  196 , as shown in  FIGS.  1 - 4   . The resistive wires may be routed using the same means as shown in  FIGS.  2 - 7   . Alternatively, the resistive wires may be embedded in the wall of the crucible, similar to the embodiments shown in  FIGS.  8 A- 8 B . In another embodiment, the resistive wires may be in contact with the crucible plug  194 , such as shown in  FIGS.  9 A- 9 C . When a current is passed through the resistive wires, heat is generated. This may allow the controller  180  another mechanism to control the temperature of the dopant material  195 . 
     In certain embodiments, the resistive wires are bundled with the thermocouple wires  197 . In these embodiments, the resistive wires are routed with the thermocouple wires  197 . 
     In other embodiments, the resistive wires are provided in a separate braid or bundle, and traverse the same path as the thermocouple wires  197 . 
     In yet other embodiments, the resistive wires may be in contact with one part of the target holder  190 , such as the crucible  196 , while the thermocouple  198  is in contact with another part of the target holder  190 , such as the crucible plug  194 .  FIG.  10    shows an embodiment where the resistive wires  500  are separate from the thermocouple wires  197 . This embodiment is similar to  FIG.  2   , but includes a third connector  510  and a fourth connector  520 . The third connector  510  may be mounted to the front housing  340 . Wires  540  from the controller  180  are routed to the third connector  510 . In certain embodiments, the components used for routing the wires  540  are similar to those for routing the wires  199 . For example, a channel  311  and vacuum feedthrough  370  may be employed. The resistive wires  500  may be coiled to allow changes in length and may be connected to an outer surface of the crucible or to the crucible plug. 
     Although  FIG.  10    shows a third and fourth connector, it is understood that the resistive wires  500  may also be routed with the wires  199  and a larger connector may be used. 
     Alternatively, the resistive wires  500  may also be routed through the shaft  200 , similar to the routing of wires  199  shown in  FIGS.  3 - 4   . 
     The above disclosure describes various embodiments that allow the controller  180  to monitor the temperature of the dopant material  195  by measuring the temperature of a component (i.e. the crucible, the crucible plug, etc.) using a thermocouple  198 . The controller  180  may use this information in a variety of ways. 
     It may be advantageous to heat the dopant material  195  to a temperature within a predetermined range. For example, at low temperatures, the dopant material  195  may not melt, and therefore no dopant vapor is released from the target holder  190 . However, at excessively high temperatures, the melt rate of the dopant material may be too great. This may cause accumulation of dopant material in the arc chamber  100 . Additionally, variation in the melt rate may also affect the beam current and other parameters. 
     By monitoring the temperature at or near the target holder  190 , the controller  180  may be able to better regulate the temperature of the dopant material  195 . For example, the controller  180  may monitor the temperature of the dopant material  195 . If the temperature is not within the predetermined range, the controller may change the current through filament power supply  165 , change the arc voltage, change cathode bias voltage, alter the flow rate of gas into the arc chamber  100 , change the position of the target holder  190  in the arc chamber  100 , vary the beam extraction current or perform a combination of these actions. Furthermore, if first and second electrodes are disposed on walls  101 , the voltage applied to these electrodes may also be varied by the controller  180  based on the temperature of the dopant material  195 . Additionally, in embodiments where a heater is employed, such as through the use of resistive wires  500 , the controller  180  may vary the current through the heater to change the temperature of the dopant material  195 . 
     In certain embodiments, the controller  180  may move the position of the target holder  190  within the arc chamber  100  based on the temperature of the dopant material  195 . For example, the target holder  190  may heat to a higher temperature when disposed directly in the cylindrical region defined between the cathode  110  and the repeller  120 . To slow the heating of the dopant material, the target holder  190  may be moved linearly to be outside this cylindrical region. Conversely, to increase the temperature of the dopant material  195 , the target holder  190  may be moved into this cylindrical region. 
     The controller  180  may employ various closed loop algorithms to determine the parameters associated with the IHC ion source  10  based on the temperature obtained by the thermocouple  198 . 
     While the above disclosure describes the use of a thermocouple  198 , other temperature sensors may also be used. For example, optical measurements, a pyrometer, and color dots are all indirect methods of detecting the temperature of the target holder  190 . RTDs (resistance temperature detectors) and wireless thermocouple readers may also be employed. Thus, the above disclosure is not limited to the use of thermocouples. 
     Further, while the above disclosure describes the thermocouple  198  as being in contact with the target holder  190  or the dopant material  195 , other embodiments are also possible. For example, the thermocouple  198  (or other temperature sensor) may measure the temperature of another component within the arc chamber  100  and estimate the temperate of the dopant material based on this measured temperature. This other component may be a wall of the arc chamber  100 , the shaft  200 , the repeller  120 , the front housing  340 , or another component. 
     Further, the control described above may be performed using open loop techniques. For example, empirical data may be collected to determine the temperature of the dopant material as a function of various parameters, such as cathode bias voltage, arc voltage, feed gas flow rate, and position of the target holder. The empirical data may also determine the temperature of the dopant material as a function of time. Using tables or equations, the controller  180  may vary one or more of the parameters to maintain the dopant material  195  within a predetermined range. For example, the controller  180  may move the target holder  190  over time to maintain its temperature within the desired range. 
     While the above disclosure describes the use of a target holder in an indirectly heated cathode ion source, the disclosure is not limited to this embodiment. The target holder, actuator assembly and thermocouple may also be employed in other ion or plasma sources, such as capacitively coupled plasma sources, inductively coupled plasma sources, a Bernas source or another suitable source. 
     The embodiments described above in the present application may have many advantages. Using an insertable target holder allows for pure metal dopants to be used as sputter targets in an environment that exceeds their melting temperature. Traditionally, an oxide/ceramic or other solid compound containing the dopant that has a melting temperature of greater than 1200° C. is used. The use of a dopant-containing compound rather than a pure material severely dilutes the available dopant material. For example, when using Al 2 O 3  as an alternative to pure aluminum, the stoichiometry of the ceramic composition not only introduces impurities into the plasma, possibly introducing undesirable mass coincidences with the dopant of interest, but also leads to lower beam currents than a pure elemental target. Use of Al 2 O 3  may also lead to generation of undesirable byproducts, such as oxides and nitrides, which can be deposited along the beamline and compromise the operation of the ion implanter. For instance, the beam optics may be cleaned chemically to maintain ion beam stability after use of Al 2 O 3 . 
     In one experiment, beam currents of up to 4.7 mA were achieved using a pure Al sputter target, whereas a maximum beam current of less than 2 mA could be achieved using an Al 2 O 3  target. Use of pure metals will also increase the multi charge beam currents by 50%-75% as compared to the beam currents obtained from oxides/ceramics of the same metal species. With the insertable container, access to a large volume of pure metal is available when needed and the solid target can be safely removed from the arc chamber to utilize other species. 
     Further, by monitoring the temperature of the dopant material, the ion source can be controlled to ensure that the melt rate of the dopant material is within a predetermined range. Specifically, without temperature control, it is possible that the dopant feed may become unstable and prone to runaway effect that can cause inconsistent beam performance and lead to undesired accumulation of dopant material in the arc chamber. Thus, temperature control prevents exponential increase in dopant vapor in the arc chamber. It also allows faster tuning of the ion source. 
     Additionally, by monitoring the temperature of the dopant material, it is possible to use this information in the beam tuning process, thus making beam performance more reliable. 
     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.