Patent Publication Number: US-9887060-B2

Title: Ceramic ion source chamber

Description:
This application is a continuation of U.S. patent application Ser. No. 15/009,904 filed Jan. 29, 2016, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Embodiments of the present disclosure relate to an indirectly heated cathode (IHC) ion source, and more particularly, an IHC ion source chamber made from a ceramic material. 
     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. 
     The ion source chamber is typically made of an electrically conductive material, which has good electrical conductivity and a high melting point. The ion source chamber may be maintained at a certain electrical potential. Additionally, the cathode and the repeller are disposed within the ion source chamber, and are typically maintained at electrical potentials that are different from the ion source chamber. Further, apertures are created in the walls of the ion source chamber to allow electrical connections to the cathode and the repeller. These apertures are sized such that arcing does not occur between the wall of the ion source chamber and the electrical connections to the cathode and repeller. These apertures, however, also allow feed gas, which is introduced into the ion source chamber, to escape. 
     Additionally, the materials used to make the ion source chamber may also have good thermal conductivity as one function of the ion source chamber may be to remove heat from within the chamber via conduction to a cooler surface. 
     Thus, the materials used for the ion source chamber typically have high melting points, good electrical conductivity and good thermal conductivity. In some embodiments, materials such as tungsten and molybdenum are used to construct the ion source chamber. 
     One issue associated with IHC ion sources is that the material used to construct the ion source chamber may be expensive and difficult to machine. Additionally, the ions generated within the ion source chamber may cause particles of the ion source chamber to be removed and introduced into the extracted ion beam. Thus, the material used to create the ion source chamber may introduce contamination into the extracted ion beam. Further, feed gas is lost through the apertures that are created to allow electrical connections to the cathode and repeller. 
     Therefore, an IHC ion source in which the material used to construct the ion source chamber did not contaminate the ion beam would be advantageous. Further, it would be beneficial if the openings used to provide electrical connection to the cathode and repeller could be reduced in size or eliminated, so as to reduce the flow of feed gas escaping from the ion source chamber. 
     SUMMARY 
     The IHC ion source comprises an ion source chamber having a cathode and a repeller on opposite ends. The ion source chamber is constructed of a ceramic material having very low electrical conductivity. An electrically conductive liner may be inserted into the ion source chamber and may cover at least three sides of the ion source chamber. The liner may be electrically connected to the faceplate, which contains the extraction aperture. The electrical connections for the cathode and repeller pass through apertures in the ceramic material. In this way, the apertures may be made smaller than otherwise possible as there is no risk of shorting or arcing. In certain embodiments, electrically conductive pieces are molded into the ion source chamber or are press fit in the apertures. Further, the ceramic material used for the ion source chamber is more durable and introduces less contaminants to the extracted ion beam. 
     According to one embodiment, an indirectly heated cathode ion source is disclosed. The indirectly heated cathode comprises an ion source chamber into which a gas is introduced, the ion source chamber constructed of an electrically insulating material and having a bottom, two opposite ends, and two sides; a cathode disposed on one of the two opposite ends of the ion source chamber; a repeller disposed at a second of the two opposite ends of the ion source chamber; an electrically conductive liner covering at least one of the bottom and the two sides of the ion source chamber; and a faceplate having an extraction aperture disposed opposite the bottom of the ion source chamber. In certain embodiments, the faceplate is electrically conductive, and the electrically conductive liner is in electrical contact with the faceplate. In certain embodiments, the electrically conductive liner is in electrical contact with the cathode. In certain embodiments, the electrically conductive liner is in electrical contact with the repeller. In certain embodiments, the indirectly heated cathode ion source comprises a liner power supply, wherein the electrically conductive liner is in electrical contact with the liner power supply. In certain embodiment, the electrically insulating material comprises a ceramic material. In certain embodiments, the ceramic material comprises aluminum nitride. In other embodiments, the ceramic material is selected from the group consisting of silicon carbide, zirconium, yttrified-zirconium carbide, and zirconium oxide. Further, in certain embodiments, the electrically conductive liner comprises three planar segments. In certain embodiments, the electrically conductive liner has a “U” shape. 
     According to another 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, the ion source chamber constructed of a ceramic material and having a bottom, two opposite ends, and two sides; a cathode disposed on one of the two opposite ends of the ion source chamber; a repeller disposed at a second of the two opposite ends of the ion source chamber; an electrically conductive liner covering the bottom and two sides of the ion source chamber; and an electrically conductive faceplate having an extraction aperture disposed opposite the bottom of the ion source chamber and in electrical communication with the electrically conductive liner. 
     In another embodiment, an apparatus for use with an indirectly heated cathode ion source is disclosed. The apparatus comprises an ion source chamber constructed of an electrically insulating material and having a bottom, two opposite ends, and two sides; an electrically conductive liner covering at least one of the bottom and the two sides of the ion source chamber; and a faceplate having an extraction aperture disposed opposite the bottom of the ion source chamber. In certain embodiments, the electrically conductive liner covers the bottom and the two sides of the ion source 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 ion source in accordance with one embodiment; 
         FIG. 2A  is an end view of the ion source of  FIG. 1  having a liner according to a first embodiment; 
         FIG. 2B  is an end view of the ion source of  FIG. 1  having a liner according to a second embodiment; 
         FIG. 3  is an ion source in accordance with another embodiment; 
         FIG. 4  is an ion source in accordance with a third embodiment; 
         FIG. 5  is an ion source in accordance with a fourth embodiment; 
         FIG. 6A  shows a cross-section view of the repeller and its electrical connection according to one embodiment; and 
         FIG. 6B  shows a cross-section view of the repeller and its electrical connection according to a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, indirectly heated cathode ion sources may be susceptible to contamination due to the material used to construct the ion source chamber. Further, apertures in the ion source chamber, which are used to supply electrical connections to the cathode and repeller, allow feed gas to escape. 
       FIG. 1  shows a first embodiment of an IHC ion source  10  that overcomes these issues. The IHC ion source  10  includes an ion source chamber  100 , having two opposite ends, and sides  102 ,  103  connecting to these ends. The ion source chamber  100  may be constructed of an electrically insulating material, such as a ceramic material. An electrically conductive liner  130  is disposed within the ion source chamber  100  may cover at least two surfaces of the ion source chamber  100 . For example, the electrically conductive liner  130  may cover the sides  102 ,  103  that connect the opposite ends of the ion source chamber  100 . The electrically conductive liner  130  may also cover the bottom  101  of the ion source chamber  100 . A cathode  110  is disposed inside the ion source chamber  100  at one of the two opposite 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 electrically conductive liner  130 . In certain embodiments, the cathode power supply  115  may negatively bias the cathode  110  relative to the electrically conductive liner  130 . 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 electrically conductive liner  130  of 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 electrically conductive liner  130  disposed within 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 electrically conductive liner  130  disposed within 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 electrically conductive liner  130  in 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 electrically conductive liner  130  disposed within 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 the top of the ion source chamber  100  may be a faceplate  140  including an extraction aperture  145 . In  FIG. 1 , the extraction aperture  145  is disposed on the faceplate  140  that is parallel to the X-Y plane (parallel to the page). The faceplate  140  may be an electrically conductive material, such as tungsten. 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  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 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  145 . The ions are then extracted through the extraction aperture as an ion beam. 
       FIG. 2A  shows an end view showing a first embodiment of an electrically conductive liner  130 . In this embodiment, the electrically conductive liner  130  covers two sides  102 ,  103  of the ion source chamber  100 , and also covers the bottom  101 . The bottom  101  is the surface opposite the faceplate  140 . In this embodiment, the electrically conductive liner  130  is formed using three planar segments  131 ,  132 ,  133 . These segments may form a unitary piece or may be separate pieces. Planar segments  131 ,  132 , which cover the two sides  102 ,  103 , are in contact with the faceplate  140  and are also in contact with planar segment  133 , which covers the bottom  101 . Thus, all segments are at the same electrical potential as the faceplate  140 . In the embodiment where the segments are individual pieces, electrical connection between the planar segments may be insured through the use of interference fits, springs, or other mechanisms. The connection between the faceplate  140  and the planar segments  131 ,  132  may be achieved in the same manner. The faceplate  140  may be an electrically conductive material, such as tungsten. Thus, by electrically biasing the faceplate  140 , the electrically conductive liner  130  may also be biased at the same electrical potential. 
     Thus, while  FIG. 1  shows the cathode power supply  115  and the repeller power supply  125  in contact with the electrically conductive liner  130 , in some embodiments, these power supplies are actually in electrical contact with the faceplate  140 . 
       FIG. 2B  shows a second embodiment of an electrically conductive liner  135 . In this embodiment, the electrically conductive liner  135  may be “U” shaped, such that the liner covers the sides  102 ,  103  and the bottom  101  of the ion source chamber  100 . As seen in the figure, the rounded portion of the electrically conductive liner  135  is proximate the bottom  101  of the ion source chamber  100 . As described above, the electrically conductive liner  135  may be in electrical contact with the faceplate  140 , and thus is maintained at the same electrical potential as the faceplate  140 . 
     The electrically conductive liners illustrated in  FIGS. 2A-2B  may cover two sides  102 ,  103  and the bottom  101 , but not the two ends of the ion source chamber  100 . Since the cathode  110  is disposed on one end and the repeller  120  is disposed on the other end of the ion source chamber  100 , the small area of exposed ceramic material will not have a deleterious effect on the plasma  150 . Further, in certain embodiments, the electrically conductive liner may cover less than these three surfaces. For example, the electrically conductive liner may cover at least one of the bottom  101  and the two sides  102 ,  103 . 
     While the above disclosure describes a configuration where the electrically conductive liner  130  is in electrical communication with the faceplate  140 , other embodiments are also possible. 
     For example, in one embodiment, one or more segments of the electrically conductive liner  130  are electrically connected to the cathode  110 . In other words, rather than connecting the electrically conductive liner  130  to the faceplate  140 , the electrically conductive liner  130  is connected to the cathode  110 . The connection between the electrically conductive liner  130  and the cathode  110  may be made in a number of ways, including interference fits, springs, or other mechanisms. In certain embodiments, an insulating material may be disposed along the top of the ion source chamber  100  to insure that the electrically conductive liner  130  does not contact the faceplate  140 . In another embodiment, the electrically conductive liner  135  having a “U” shape is used and electrically connected to the cathode  110 .  FIG. 3  shows an embodiment where the cathode power supply  115  is referenced to ground, and is used to provide an electrical potential to the cathode  110  and the electrically conductive liner  130 . The repeller power supply  125  may still be referenced to the electrically conductive liner  130 , or may be referenced to another voltage. 
     In another embodiment, one or more segments of the electrically conductive liner  130  are electrically connected to the repeller  120 . Again, in certain embodiments, an insulating material may be disposed along the top of the ion source chamber  100  to insure that the electrically conductive liner  130  does not contact the faceplate  140 . In another embodiment, the electrically conductive liner  135  having a “U” shape is used and electrically connected to the repeller  120 .  FIG. 4  shows an embodiment where the repeller power supply  125  is referenced to ground, and is used to provide an electrical potential to the repeller  120  and the electrically conductive liner  130 . The cathode power supply  115  may still be referenced to the electrically conductive liner  130 , or may be referenced to another voltage. 
     In yet other embodiments, the planar segments of the electrically conductive liner  130  may be connected to different voltages. For example, one or more segments may be connected to the faceplate  140 , the cathode  110  or the repeller  120 . Another of the segments may be connected to another of the faceplate  140 , the cathode  110  or the repeller  120 . 
     Additionally, in certain embodiments, the electrically conductive liner  130  may be connector to a voltage that is different than the faceplate  140 , the cathode  110  or the repeller  120 . For example, there may be a liner power supply  137 , which is in communication with the electrically conductive liner  130 , such as through an aperture  136  in the ion source chamber  100 , as shown in  FIG. 5 . 
     As described above, the ion source chamber  100  may be constructed from an electrically insulating material, such as a ceramic material. In some embodiments, the ceramic material may be selected such that it has a melting point of at least 2000° C. to withstand the extreme temperatures experienced within the ion source chamber  100 . 
     Additionally, ceramic materials typically have high hardness values, such as 7 or more on the Mhos scale. This hardness allows the ceramic material to withstand repeated aggressive cleanings. Further, this may reduce the amount of contaminants introduced by the ion source chamber  100 . 
     Further, in certain embodiments, the ceramic material is selected such that it has a thermal conductivity similar to that of traditional materials used to construct the ion source chamber  100 , such as tungsten or molybdenum. These metals have a thermal conductivity of between 135 and 175 W/mK. This may allow the ion source chamber to quickly remove heat via convection to a cooled surface. 
     In one embodiment, the ceramic material may be aluminum nitride (AlN), which has a thermal conductivity of 140-180 W/mK. Of course, other ceramic materials, such as alumina (Al 2 O 3 ), silicon carbide, zirconium, yttrified-zirconium carbide, and zirconium oxide may also be used. 
     The ceramic material used for the ion source chamber  100  has much higher electrical resistivity than the metals that are traditionally used, such as 1e14 Ω-cm or more. Thus, the apertures in the ion source chamber  100  used to accommodate the electrical connections for the cathode  110  and the repeller  120  may be made much smaller than would be otherwise possible. This is because there is no risk of arcing or shorting between the ion source chamber  100  and the electrical connection. 
     In one embodiment, the aperture in the ion source chamber  100  is dimensioned such that its diameter is substantially equal to the diameter of the electrical connection or electrically conductive material passing through the aperture. For example, as shown in  FIG. 6A , the repeller  120  may have a stem  122  that passes through an aperture  105  in the ion source chamber  100 . The stem  122  may have a first diameter, while the aperture  105  may have a second diameter which is substantially equal to the first diameter. For example, in some embodiments, the interface between the stem  122  and the aperture  105  may be a press fit or an interference fit. 
       FIG. 6B  shows another embodiment. In this embodiment, the stem  122  is molded or otherwise formed as part of the ion source chamber  100 , such that there is no aperture at all. In this embodiment, feed gas cannot escape from the ion source chamber  100 , as there is no opening in the ion source chamber  100 . 
     While  FIGS. 6A-6B  show the repeller  120 , the electrical connections for the cathode  110  and the filament  160  may be accommodated in the same manner. Thus, by using an electrically insulating material to construct the ion source chamber  100 , the apertures used for electrical connections may be reduced in size or removed, reducing or possibly eliminating the flow of feed gas escaping from the ion source chamber  100 . For example, an electrically conductive material may be molded into the ion source chamber  100 . Connections may be made to the electrically conductive material on both sides of the ion source chamber  100  to complete the electrical circuit. 
     Thus, in certain embodiments, the IHC ion source  10  includes an ion source chamber  100 , constructed from an electrically insulating material. The ion source chamber  100  has a bottom  101 , two sides  102 ,  103 , and opposite ends. The cathode  110  and the repeller  120  are disposed on respective ends of the ion source chamber  100 . An electrically conductive liner is used to cover at least one of the sides  102 ,  103  and the bottom  101  of the ion source chamber. The liner may optionally also cover at least a portion of the ends of the ion source chamber  100 . In certain embodiments, a faceplate  140 , which is electrically conductive, is disposed on the top of the ion source chamber  100 , and is in electrical contact with the electrically conductive liner. Thus, in this way, electrical potentials may be established along the sides and bottom of the ion source chamber  100 , even though the ion source chamber  100  itself is not conductive. Further, apertures in the ion source chamber  100 , which allow the passage of electrical connections or electrically conductive materials to the cathode  110  and the repeller  120 , may be made smaller or eliminated since there is no risk of shorting or arcing. 
     In other embodiments, the electrically conductive liner may be electrically connected to a different voltage. For example, there may be a separate liner power supply that provides the electrical potential to the electrically conductive liner. In other embodiments, one or more portions of the electrically conductive liner may be electrically connected to the repeller  120  or the cathode  110 . 
     Thus, the IHC ion source includes an ion source chamber made of an electrically insulating material having a bottom, two sides and two opposite ends. An electrically conductive liner is disposed so as to cover at least one of the bottom and the two sides. A faceplate having an extraction aperture is disposed opposite the bottom of the ion source chamber. The electrically conductive liner is connected to a power supply. 
     The embodiments described above in the present application may have many advantages. First, the use of ceramic materials for the ion source chamber may reduce the introduction of contaminants into the extracted ion beam, as compared to metal ion source chambers. Further, these ceramic materials may be less expensive than the metal currently used for the ion source chamber. Additionally, these ceramic materials may be able to withstand more aggressive cleanings than traditional materials. Lastly, the use of an electrically insulating ion source chamber allows an elimination or a reduction in the size of the apertures through which the electrical connections to the cathode and repeller pass. This may reduce the amount of feed gas that escapes through these apertures. 
     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.