Abstract:
An apparatus that forms a source bushing, while comprehending the possible formation of electrically conductive films thereon, is disclosed. Such an apparatus may advantageously be used to isolate an ion source from other components within the ion implanter, as these components may be at different electrical potentials. In one embodiment, the source bushing is constructed from a material having a lower electrical resistance than is currently used. By constructing the bushing in this manner, the effects of the applied lower resistance films is reduced, as the change in effective resistance is reduced. In other embodiments, the source bushing is purposely lined with an electrically semiconducting material, so that the effects of the later applied lining are minimized. In either case, the electrical potential between the two devices that are being isolated by the bushing is more evenly applied across the bushing.

Description:
[0001]    This application claims priority of U.S. Provisional Patent Application No. 61/111,949, filed Nov. 6, 2008, the disclosure of which is herein incorporated by reference in its entirety. 
     
    
     FIELD  
       [0002]    This disclosure relates to insulators, and more particularly to a conductive contamination resistant insulator. 
       BACKGROUND 
       [0003]    Insulators are used in many different applications. Typically, they are used to separate regions or components having different electrical potentials. In one application, an insulator (which may be referred to herein as a “source bushing”) may be used to electrically insulate an ion source from other components of an ion implanter. An ion source is a critical component of an ion implanter. The ion source generates an ion beam, which passes through the beamline of the ion implanter and is delivered to a target workpiece such as a semiconductor wafer. In order to generate this ion beam, the ion source is typically biased at a voltage significantly higher than the surrounding components, such as 80 kV. The ion source is required to generate a stable, well-defined ion beam for a variety of different ion species and extraction voltages. It is desirable to operate the ion source in a production facility for extended periods of time without the need for maintenance or repair. 
         [0004]    To electrically insulate the ion source from the remaining components, an insulator, such as a source bushing is used to separate the components. Typically, a source bushing made using materials having an extremely higher resistivity, such as epoxy resin with ceramic filler. Such a material may have a resistance of greater than 50 GΩ (Giga Ohms) between the ion source and the grounded attached components. 
         [0005]    An ion source typically generates an ion beam by ionizing within an arc chamber a dopant gas containing a desired dopant element. The ion beam is extracted through an extraction aperture of the ion source by an extraction electrode located proximate the extraction aperture. Different dopant gases are selected in response to the desired dopant element. Some dopant gases are fluorine containing gases such as boron trifluoride (BF 3 ), germanium tetrafluoride (GeF 4 ), and silicon tetrafluoride (SiF 4 ). Because of the temperatures, voltages and ions to which it is exposed, tungsten is typically used to construct the ion chamber. 
         [0006]    While tungsten is an excellent material to use for the ion chamber, it does tend to react with fluorine, especially after prolonged exposure, according to the equation: 
         [0000]      W+3F 2 →WF 6    
         [0007]    Tungsten hexafluoride is a gaseous compound that can permeate the region in communication with the ion source. Consequently, a drawback with a conventional ion implanter utilizing fluorine containing dopant gases such as BF 3 , GeF 4 , and SiF 4  is that conductive tungsten films may form on the source bushing. For example, a conductive tungsten film forms on the source bushing when the dopant gas is BF 3 , since fluorine creates WF 6  gas in the ion source, which selectively chemically attacks the source bushing material. The tungsten film grows from one end of the source bushing and after a short time (which could be on the order of about 40 to 100 hours) could lead to electrical breakdown across the uncoated end of the source bushing. 
         [0008]      FIG. 7  shows a simplified cross sectional view of an ion source  700 , a source bushing  701  and a component  702 . The component  702  may be a mounting flange or vacuum chamber, and may be made of an electrical conductive material, such as a metal or metal alloy. The component  702  is at a different electrical potential than the ion source  700 , and in some embodiments, is grounded. In some embodiments, a metal shield  703  is attached to the component  702 , so as to minimize electrical fields created by the ion source  700 . The source bushing  701  may have one or more convolutions formed therein. These convolutions increase the surface creep path and break down voltage along the bushing ( 701 ) between the ion source ( 700 ) and component ( 702 ), on both the atmosphere and vacuum sides. In some embodiments, gases from the ion source  700 , such as WF 6 , emerge from the ion source  700  and pass through the gap  705  between the ion source  700  and the shield  703 . Tungsten hexafluoride is relatively non-reactive with metal, and therefore migrates toward the source bushing  701 , which may be made using epoxy. Due to higher chemical reactivity of the epoxy, the gas chemically interacts with the bushing  701  and begins to form a film  710 , starting near the ion source  700  and extending toward the component  702  (i.e. moving from left to right in  FIG. 7 ). This film  710  has a lower resistance than the source bushing  701 , and therefore affects the distribution of the voltage potential across the bushing  701 . 
         [0009]      FIG. 1  is an equivalent circuit diagram to illustrate drawbacks of a conventional source bushing having a partial coating of a tungsten film. Although this disclosure describes the film as being of tungsten, other materials can also form a conductive layer on the insulator. These materials include other metals, water or moisture. The electrical model example of  FIG. 1  is based on actual measurements from a failed source bushing, which had an applied film as shown in  FIG. 7 . The source bushing has four convolutions, such as is shown in  FIG. 7 . In this embodiment and that of  FIG. 5 , the term “convolution” is used to describe the protrusions on the vacuum side of the source bushing. Thus,  FIGS. 5 and 7  show a bushing having four convolutions. In contrast, the bushing of  FIG. 3  has three convolutions, in the conventional sense. As seen in  FIG. 7 , the film grows so as to cover three of the four convolutions. Typically, once the film has reached point  711 , arcing will begin to occur. The electrical resistance of this tungsten film is about 1.5 GΩ (Giga Ohms), which is relatively low compared to the surface resistance of the remaining electrical gap in the creepage path along the bushing surface (&gt;20 GΩ). This creepage path is defined by the bushing surface between point of arcing  711  and the component  702 , and is the portion of the bushing that remain uncoated when the arcing occurs. Therefore, the tungsten film and the remaining uncoated source bushing form a voltage divider, which applies nearly all of the extraction voltage across the uncoated part of the source bushing. For example, when the extraction voltage is 80 kilovolts (kV), nearly all (or &gt;74 kV) of the 80 kV extraction voltage is applied across the uncoated part of the bushing. It can be seen, using these ratios, that more than 91% (or about 20 GΩ/21.5 GΩ) of the voltage is applied across the uncoated portion of the source bushing. This implies that point  711  is at 74 kV, where nearby components, such as shield  703  and component  702  are at ground. This large potential over such a small region can lead to arcing and electrical breakdown of the source bushing. As the film  710  approaches the component  702 , there are multiple arc discharge paths that develop. Typically, as the film grows, it reaches a convolution point  711 . Due to the shape of the bushing  701 , arcing is more likely to originate at one of the points on the bushing. In one embodiment, the voltage arcs from point  711  to the tip of shield  703 , as shown by dotted line  714 . In another embodiment, the voltage arcs from point  711  to the extending point on the grounded component  702 , such as a screw or fastener, as shown by dotted line  715 . Once arcing begins, the ion implanter must be taken offline, so that the source bushing  702  can be replaced. 
         [0010]    Accordingly, there is a need for an improved source bushing configuration which overcomes the above-described inadequacies and shortcomings. A source bushing, designed with the knowledge that a conductive film may develop on it, would be beneficial in these applications so as to prevent arcing and electrical discharge. Furthermore, it would improve the operational time (i.e. uptime) of the ion implanter, especially when fluorine-based gases are being used. 
       SUMMARY  
       [0011]    An apparatus that forms a source bushing, while comprehending the possible formation of electrically conductive films thereon, is disclosed. Such an apparatus may advantageously be used to isolate an ion source from other components within the ion implanter, as these components may be at different electrical potentials. By preventing unwanted electrical discharge, the ion implanter may perform longer without need for preventative maintenance or repair. 
         [0012]    In one embodiment, the source bushing is constructed from a material having a lower electrical resistance than is currently used. By constructing the bushing in this manner, the effects of the applied lower resistance films is reduced, as the change in effective resistance is reduced. In other embodiments, the source bushing is purposely lined with an electrically semiconducting material, so that the effects of the later applied lining are minimized. In either case, the electrical potential between the two devices that are being isolated by the bushing is more evenly applied across the bushing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which: 
           [0014]      FIG. 1  is a circuit diagram to illustrate a drawback of a conventional source bushing; 
           [0015]      FIG. 2  is a simplified block diagram of an ion implanter having an ion source and a source bushing consistent with an embodiment of the present disclosure; 
           [0016]      FIG. 3  is a cross sectional view of a source bushing consistent with an embodiment of the present disclosure; 
           [0017]      FIG. 4  is a circuit diagram to illustrate characteristics of a source bushing consistent with the present disclosure; 
           [0018]      FIG. 5  is a cross sectional view of another embodiment of a source bushing consistent with the present disclosure; 
           [0019]      FIG. 6  is a general circuit diagram to illustrate characteristics of a source bushing consistent with the present disclosure; and 
           [0020]      FIG. 7  is a simplified cross sectional view of a conventional source bushing and ion source. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In general, the present disclosure is directed at a source bushing and compensating for the growth of film, such as a tungsten film, on the source bushing to minimize arcing and/or electrical breakdown of the source bushing. 
         [0022]    Turning to  FIG. 2 , a simplified block diagram of an ion implanter  100  is illustrated. The ion implanter  100  includes a high voltage supply  101 , source flange  102 , ion source bushing  103 , beamline  104 , end station  105  and ion source  106 . The walls of items  102  to  105  form a vacuum tight vessel that encloses ion source  106  and one or more work pieces  107  in end station  105 . Ion source  106  generates ion beam  108  which is directed via beamline  104  to workpiece  107 . Ion beam  108  is accelerated by the electrostatic high voltage potential that power supply  101  creates between ion source  106  and earth ground at beamline  104 , end station  105  and work piece  107 . Source bushing  103  provides electrical isolation between flange  102  and beamline  104  which prevents shorting of power supply  101 . 
         [0023]    The beam line components  104  may include components known to those skilled in art to control and direct the ion beam  108  towards the workpiece  107 . Some examples of such beam line components  104  include, but are not limited to, a mass analyzing magnet, a resolving aperture, ion beam acceleration and/or deceleration columns, a collimator magnet, lenses, and electrodes. Those skilled in the art will recognize alternative and/or additional beam line components  104  that may be utilized in the ion implanter  100 . 
         [0024]    The end station  105  supports one or more workpieces, such as workpiece  107 , in the path of ion beam  108  such that ions of the desired species are implanted into workpiece  107 . The workpiece  107  may be, for example, a semiconductor wafer or another object requiring ion implantation. The end station  105  may include a platen (not illustrated) to support the workpiece  107 . The platen may secure the workpiece  107  using electrostatic forces. The end station  105  may also include a scanner (not illustrated) for moving the workpiece  110  in a desired direction. 
         [0025]    The end station  105  may also include additional components known to those skilled in the art. For example, the end station  105  typically includes automated workpiece handling equipment for introducing workpieces into the ion implanter  100  and for removing workpieces after ion implantation. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The ion implanter  100  may also have a controller (not illustrated) to control a variety of subsystems and components of the ion implanter  100 . 
         [0026]    The source bushing  103  may be coupled between the ion source  102  and other components of the ion implanter such as a vacuum vessel. The two termini of the bushing are maintained at different electrical potentials. For example, the ion source may be energized to a high extraction potential of about 80 kV in some instances and even higher in other instances by an extraction power supply (not illustrated). The other terminus of the source bushing  103  may be coupled to a component at ground potential. Advantageously, the source bushing  103  is configured to compensate for the growth of an electrically conductive film, such as a tungsten film, on the source bushing  103  to minimize electrical breakdown of the source bushing. 
         [0027]    Throughout this disclosure, the term “tungsten film” is used to describe a film that adheres to the source bushing during the operation of the ion implanter  100 . This tungsten film has a much lower electrical resistance than the conventional bulk material used to construct the source bushing. “Much lower” is defined as more than ten times less resistance. 
         [0028]    In a first embodiment, the bulk material of the source bushing  103  may be modified by additives to give the bulk material a limited conductivity. For example, it is customary to use alumina (Al 2 O 3 ) at high concentrations to form an insulator. Pure alumina is a very good insulator, but dopants can give it sufficient conductivity to control the resistance of the bushing by introducing a current path through the grains of alumina. In other embodiments, a more conductive additive, such as a semiconducting compound, or silver or graphite powder, may be added in low concentrations to modify the resistance of the bushing by forming a current path that bypasses the alumina and epoxy. Semiconductors may be defined as that class of materials having a bulk resistance below 1E3 ohm-cm. Any semiconductor may be used as the conductive additive described above. In addition, there are other semiconducting materials with high resistivity that could also be used, such as Tungsten-Carbide or Gallium-Arsenide. This class of materials may be referred to as semi-insulators, indicating a bulk resistance from 1E3 to 1E10 ohm-cm. Other materials in this class can also be used as the additive. In yet other embodiments, an organo-metallic material may be added. A suitable organo-metallic substance must be tailored to the specific epoxy. The organo-metallic substances impart conductivity to the epoxy itself with conduction at the molecular level. This is different from the methods described above, which create current paths that bypass the epoxy through connected grains. In practice, the source bushing is made using a mold. Material, such as epoxy, is mixed with an appropriate additive (to achieve the desired resistance), so as to create a nearly homogeneous mixture. This mixture is then poured into the mold where it is hardened, such as by heating, into a source bushing. Other methods of creating an epoxy-based insulator are also known and are within the scope of this disclosure. 
         [0029]    Since the insulator connects components or devices at different potentials, a lower resistance value will increase the flow of current between the component of higher potential and that of lower potential. For example, as stated earlier, the ion source may be at 80 kV, while the other component, such as a vacuum element, may be at ground potential. If the insulator has an overall resistance of 1 MΩ, the power supply generating the 80 kV potential for the ion source will have to provide an additional 80 mA to account for the flow of current through the insulator. Therefore, the resistance of the bulk material may preferably be high enough so that the additional load on the extraction power supply is insignificant, but small enough compared to the resistance of the developing film, so that the strength of electric fields across the surface of the bushing and surrounding space stays within safe limits at all times. Commonly accepted safe values for these limits for a source bushing operating in an ion implanter, are 10 kV/inch across the surface and 85 kV/inch across the interface between bushing and vacuum. These values include safety margins and an actual design may use higher or lower limits depending on required reliability. In order to minimize the load on the power supply, a specific bushing design should make the bulk resistance of the bushing as high as possible while staying below these safe limits for the expected film growth under worst case conditions.  FIG. 6  gives the equation that relates bushing resistance (X), film resistance (Y), resistance of bushing section covered by film (nX), high voltage potential across the bushing (Vext), and high voltage potential (Vp) at the edge of the growing film. With knowledge of the equation shown in  FIG. 6  and the desired geometry of the ion source assembly those skilled in the art can derive the maximum value for the bushing resistance X at which the bushing is still compliant with the limits of 10 kV/inch across the surface and 85 kV/inch across the interface between bushing and vacuum. This maximum value for X is the ideal resistance for the specific design 
         [0030]    In some embodiments, the deposited film has a resistance of 1 GΩ or greater, and the bulk material of the source bushing has a resistance of between 1 and 100 MΩ. In other embodiments, the source bushing may have a resistance as high as 4 GΩ. Advantageously, a growing tungsten film does not significantly affect the potential distribution across the bushing and therefore limits and/or prevents arcing that may otherwise occur over the last, uncoated portion of the source bushing. 
         [0031]      FIG. 3  is a more detailed cross sectional view of one source bushing  301  in more detail. The source bushing  301  may have an annular shape and be coupled between an ion source at an extraction potential and a vacuum vessel  310  at ground potential. The source bushing  301  has a vacuum side  306  and at atmosphere side  304 . A tungsten film (not illustrated in  FIG. 3 ) may form on the vacuum side  306  of the source bushing  301 , as explained in reference to  FIG. 7 . In addition, there may be an electrical shield  307 , attached to the vacuum vessel  310 . The source bushing  301  may have a bulk material modified by additives to give the bulk material a limited conductivity consistent with the first embodiment or have a conductive liner on the vacuum side  306  of the source bushing consistent with the second embodiment of  FIG. 5 . 
         [0032]    Turning to  FIG. 4 , a circuit diagram illustrates an electric model of a source bushing, e.g., source bushings  112  or  301 , having a bulk material with a limited conductivity consistent with an embodiment of the disclosure. Like  FIG. 1 , this circuit diagram assumes an extraction voltage of 80 kV. Furthermore, it assumes that the tungsten film extends across  3  convolutions. Finally, it assumes that the bulk resistance exists across the last convolution. However, in contrast to the circuit diagram of  FIG. 1 , less than 21 kV (as opposed to &gt;74 kV) is applied across the uncoated portion of the source bushing. In other words, point  711  would be at a potential of 21 kV, rather than the 74 kV as occurs with conventional bushings. Hence arcing across the source bushing may be limited and/or prevented. 
         [0033]    Turning to  FIG. 5 , a cross sectional view of a source bushing  502  consistent with a second embodiment is illustrated. The source bushing  502  may have a conductive liner  504  affixed to the vacuum side of the source bushing  502  where the film forms. In contrast to the first embodiment of a bulk conductive bushing, the bulk material of the bushing of the second embodiment remains non-conductive. The conductive liner  504  effectively controls the electrical potential distribution so that the model of the potential distribution is similar to that of  FIG. 4 . 
         [0034]    In one embodiment, the conductive liner  504  may be cast from a material with a much lower bulk resistance than that used for casting a fully conductive bushing, due to the difference in thickness. The thickness of the cast liner can be varied to more finely adjust electrical properties of the bushing and shape the electrical field. Materials and additives as described above can be used to create this liner. An advantage of the liner may be that it is feasible to work with materials of lower bulk resistance or more expensive materials. This increases the number of material choices. Compared to a thin surface film (e.g. CVD deposits), a liner is thicker (e.g. ¼″), so it can be cleaned easily by abrasive methods. 
         [0035]    An alternative method to create the conductive liner  504  is to expose the bushing to WF 6  gas, a plasma process, or a chemical vapor deposition (CVD) process that forms a tungsten film from end to end with controlled thickness. The Tungsten film functions as a thin-film resistor that stabilizes the potential across the bushing. In some embodiments, the resistance of the resulting tungsten film may be too low. In these scenarios, plasma or CVD can create higher resistance by combining materials, such as tungsten silicide. For example, mixing WF 6  with silane or dichlorosilane will create a tungsten silicide film. 
         [0036]    Having reduced the overall bulk resistance of the source bushing allows film formation to occur without impacting the performance of the machine. As described in conjunction with  FIG. 7 , the film starts forming on the source bushing near the high voltage end and migrates toward the grounded component. This behavior will still occur with the new bushing. However, when the film reaches point  711 , the potential is much lower, and therefore will not arc to the shield  703  or grounded component  702 . Since arcing does not occur, the ion implanter remains functional. As the implanter continues to be used, the film continues growing, until it covers the entire inner surface of the bushing. 
         [0037]    Once the film, such as tungsten film, has grown across the entire source bushing, it may add to stability of the electrical potential, even if it has a gradient. The tungsten film thickness growth may be self-limiting since metallic tungsten does not react with WF 6  near room temperature. Thus, once the bushing has been covered with tungsten, it resists further interaction with the WF 6  gas. Under favorable conditions (extraction potential low compared to bushing design potential), the tungsten film can grow to a homogeneous thickness while the bushing stabilizes the potential during growth. This may then give the source bushing an indefinite life. 
         [0038]      FIG. 6  shows a generalized circuit diagram in accordance with the disclosure. In this circuit, the source bushing is used to couple two components, where one of the components is operating at a voltage of V ext  while the second is operating at ground potential. Other voltages may also possible. For example, the second component may be at a voltage other than ground. This circuit assumes that the voltage difference between the two components is V ext . In addition, the source bushing has an overall resistance of X, of which nX (where n is less than 1) is in covered with the tungsten film. Furthermore, the tungsten film has a resistance of Y. 
         [0039]    Assuming no other elements, the voltage at the uncoated source bushing (V p ) can be given by: 
         [0000]    
       
         
           
             
               V 
               p 
             
             = 
             
               
                 V 
                 ext 
               
               · 
               
                 
                   
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         [0040]    In looking at this equation, it is clear that if Y is small relative to X, the expression tends toward: 
         [0000]    
       
         
           
             
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         [0041]    In other words, the entire potential difference appears at the uncoated bushing, since the film effectively negates the inherent resistance of the bushing. This is the situation that exists in conventional bushings. In contrast, if X is small relative to Y, the expression tends toward: 
         [0000]    
       
         
           
             
               V 
               p 
             
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         [0042]    In this case, the effect of the film is small, such that its presence does not change the fundamental voltage divider. Thus, by varying the relationship between X and Y, it is possible to regulate the maximum voltage that is applied to the uncoated bushing. Since the resistance of the tungsten film cannot be easily affected, the resistance changes must occur to the source bushing. Thus, decreasing the resistance of the bushing, by either adding additives to increase its conductivity or by applying a conductive coating to it, relative to the resistance of the film, it is possible to negate the effect of the conductive film that is deposited on the source bushing. The maximum allowable resistance of the source bushing can be determined based on the maximum electrical potential that the uncoated bushing can withstand without breaking down. The minimum allowable resistance of the source bushing is based on the maximum amount of excess current that can be supplied by the extraction voltage supply, as lower resistance values imply higher currents. 
         [0043]    As an example, assume that point  711  may be at a voltage of 40 kV without any arcing occurring. Also assume that point  711  is located in a position such that 75% of the bushing gets coated in order to reach point  711 . Furthermore, assume that the tungsten has a resistance of 1.5 GΩ and the voltage difference between the ion implanter and the second component is 80 kV. Substituting these values into the equation above, it can be determined that a bulk resistance of 4 GΩ would allow point  711  to be at 40 kV. Therefore, the bulk resistance of the source bushing can be no greater than 4 GΩ. Its lower limit is set by the current capability of the power supply. In another example, if the maximum voltage without arcing were determined to be 30 kV, the bulk resistance must be no greater than 1.6 GΩ. 
         [0044]    Thus, to eliminate arcing in this situation, a number of steps can be performed. First, the position from which the arc emanates must be determined, as well as the opposite point to which the voltage arcs. There may be a plurality of possible arc paths that may be considered. Based on the length of these paths, and the medium through which the arc travels (air, vacuum, etc), a determination of the maximum voltage difference between the points on the source bushing and the component  702  can be made. Additionally, the resistance of the film from the ion source  700  to the position from which the arc emanates can be estimated, calculated or measured empirically. Similarly, the percentage of the source bushing that is coated by the film can also be estimated, calculated or measured empirically. Using these values, it is possible to calculate the maximum bulk resistance of the source bushing to eliminate arcing. Once the maximum bulk resistance is determined, the proper additive and its concentration can be calculated, thereby allowing the creation of a source bushing that eliminates arcing. Alternatively, this maximum resistance value can be used to determine the material to be used to construct the conductive liner as described in  FIG. 5 . 
         [0045]    Accordingly, there has been provided a source bushing to compensate for the growth of a tungsten film thereon to minimize arcing and electrical breakdown of the source bushing. Furthermore, there has been provided a method for determining the bulk resistance of a source bushing such that it can operate within the desired environment. 
         [0046]    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. Further, 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.