Patent Publication Number: US-2011056746-A1

Title: Electric field modification about a conductive structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of prior application Ser. No. 11/527,842, filed Sep. 27, 2006, the contents of which are incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to conductive structures and, more particularly, to electric field modification about a conductive structure. 
     BACKGROUND 
     Ion implantation is a standard technique for introducing impurities into semiconductor wafers. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material. The ion beam may be distributed over the wafer area by beam movement, by wafer movement, or by a combination of beam and wafer movement. 
     An ion implanter may have a terminal structure. The terminal structure may sometimes be referred to in the art as a “terminal” or “high voltage terminal” and is fabricated of conductive material such as metal. The terminal structure may have varying geometries that define a cavity and the ion source is at least partially disposed within the cavity. The terminal structure may be energized to a terminal voltage to assist with acceleration of the ions from the ion source. The terminal structure, as well as other components and sub-systems of the ion implanter, are typically surrounded by a grounded enclosure. The grounded enclosure thus protects personnel from high voltage dangers when the ion implanter is running and protects the components and sub-systems of the ion implanter. 
     Air has conventionally been used to insulate the terminal structure from the grounded enclosure. However, there is a constraint on the distance of the air gap between the terminal structure and the grounded enclosure since the size of the grounded enclosure is limited in the volume manufacturing of semiconductors. Accordingly, most conventional ion implanters have limited the voltage of the terminal structure to about 200 kV. 
     Accordingly, there is a need in the art for a terminal structure of an ion implanter capable of modifying an electric field about the terminal structure in order to energize the terminal structure to high voltages within a reasonably sized enclosure footprint. 
     SUMMARY 
     According to an aspect of the disclosures, an apparatus is provided. The apparatus includes a conductive structure, and an insulated conductor disposed proximate an exterior portion of the conductive structure to modify an electric field about the conductive structure. The insulated conductor has an insulator with a dielectric strength greater than 75 kilovolts (kV)/inch disposed about a conductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which: 
         FIG. 1  is a top view of a block diagram of an ion implanter; 
         FIG. 2  is a perspective view of the terminal structure of the ion implanter of  FIG. 1 ; 
         FIG. 3  is a cross sectional view of one embodiment of the insulated conductor taken along the line A-A of  FIG. 2 ; 
         FIG. 4  is a plot of equipotential lines for an embodiment consistent with  FIG. 3 ; 
         FIGS. 5A and 5B  are cross sectional views of additional embodiments of the insulated conductor taken along the line A-A of  FIG. 2  having a plurality of grading conductors; 
         FIG. 6  is a plot of equipotential lines for an embodiment consistent with  FIG. 5A ; 
         FIG. 7  is a cross sectional view of another embodiment of a terminal structure illustrating positioning of insulated conductors relative to exterior portions of the terminal structure; 
         FIGS. 8 and 9  are cross sectional views of additional embodiments having a plurality of insulated conductors; 
         FIG. 10  is a plot of equipotential lines for an embodiment consistent with  FIG. 9 ; 
         FIGS. 11 and 12  are cross sectional views of further embodiments having a plurality of insulated conductors; 
         FIG. 13  is a cross sectional view of another embodiment of an insulated conductor having one tubular member; 
         FIG. 14  is a cross sectional view of another embodiment of an insulated conductor having two tubular members; 
         FIG. 15  is a cross sectional view of another embodiment of an insulated conductor having three tubular members; 
         FIG. 16  is a cross sectional view of another embodiment of an insulated conductor having two tubular members and a composite filler; and 
         FIG. 17  is a cross sectional view of another embodiment of an insulated conductor. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is described herein in connection with a terminal structure of an ion implanter. However, the invention can be used with other apparatus for modifying the electric field about a conductive structure. Thus, the invention is not limited to the specific embodiments described below. 
       FIG. 1  illustrates a block diagram of an ion implanter  100  including a terminal structure  104  which may sometimes be referred to in the art as a “terminal” or a “high voltage terminal.” The terminal structure  104  may be fabricated of a conductive material such as metal. The ion implanter  100  may also include an insulated conductor  103  disposed proximate an exterior portion of the terminal structure  104  to modify an electric field about the terminal structure. The terminal structure  104  and the associated insulated conductor  103  may be utilized in many different ion implanters known to those skilled in the art. Thus, the ion implanter  100  of  FIG. 1  is but one embodiment of an ion implanter. 
     The ion implanter  100  may further include an ion source  102 , a gas box  106 , a mass analyzer  120 , a resolving aperture  122 , a scanner  124 , an angle corrector magnet  126 , an end station  128 , and a controller  118 . The ion source  102  is configured to provide an ion beam  152 . The ion source  102  may generate ions and may include an ion chamber that accepts gas from the gas box  106 . The gas box  106  may provide a source of gas to be ionized to the ion chamber. In addition, the gas box  106  may also contain other components known in the art such as power supplies. The power supplies may include arc, filament, and bias power supplies for running the ion source  102 . The construction and operation of ion sources and the gas box are well known to those skilled in the art. 
     The mass analyzer  120  may include a resolving magnet that deflects ions so that ions of a desired species pass through the resolving aperture  122  and undesired species do not pass through the resolving aperture  122 . Although showing about a 45 degree deflection for clarity of illustration, the mass analyzer  120  may deflect ions of the desired species by 90 degrees and deflect ions of undesired species by differing amounts due to their different masses. A scanner  124  positioned downstream from the resolving aperture  122  may include scanning electrodes for scanning the ion beam. The angle corrector magnet  126  deflects ions of the desired ion species to convert diverging ion beam paths to nearly collimated ion beam paths having substantial parallel ion trajectories. In one embodiment, the angle corrector magnet  126  may deflect ions of the desired ion species by 45 degrees. 
     The end station  128  may support one or more wafers in the path of the ion beam  152  such that ions of the desired species are implanted into the wafer  140 . The wafer  140  may be supported by a platen  142 . The end station  128  may include other components and sub-systems known in the art such as a wafer handling system  150  to physically move the wafer  140  to and from the platen  142  from various holding areas. When the wafer handling system  150  moves the wafer  140  to the platen  142  from a holding area, the wafer  140  may be clamped to the platen  142  using known techniques, e.g., electrostatic wafer clamping where the wafer is clamped to the platen with electrostatic forces. The end station  128  may also include a platen drive system  152  as is known in the art to move the platen  142  in a desired way. The platen drive system  152  may be referred to in the art as a mechanical scan system. 
     The controller  118  may receive input data from components of the ion implanter  100  and control the same. For clarity of illustration, input/output paths from the controller  118  to components of the ion implanter  100  are not illustrated in  FIG. 1 . The controller  118  can be or include a general-purpose computer or network of general-purpose computers that may be programmed to perform desired input/output functions. The controller  118  can also include other electronic circuitry or components, such as application specific integrated circuits, other hardwired or programmable electronic devices, discrete element circuits, etc. The controller  118  may also include user interface devices such as touch screens, user pointing devices, displays, printers, etc. to allow a user to input commands and/or data and/or to monitor the ion implantation system  100 . The controller  118  may also include communication devices and data storage devices. 
     The ion beam  152  provided to a surface of the wafer  140  may be a scanned ion beam. Other ion implantation systems may provide a spot beam or a ribbon beam. The spot beam in one instance may have an approximately circular cross-section of a particular diameter depending on the characteristics of the spot beam. The ribbon beam may have a large width/height aspect ratio and may be at least as wide as the wafer  140 . The scanner  124  would not be required for systems using a ribbon beam or a stationary spot beam. The ion beam  152  can be any type of charged particle beam, such as an energetic ion beam used to implant the wafer  140 . The wafer  140  can take various physical shapes such as a common disk shape. The wafer  140  can be a semiconductor wafer fabricated from any type of semiconductor material such as silicon or any other material that is to be implanted using the ion beam  152 . 
     The ion source  102 , the gas box  106 , and the terminal electronics  105  may be positioned within the cavity  110  defined by the terminal structure  104 . The terminal electronics  105  may control operation of the components within the terminal structure  104  and may also be capable of communicating with the controller  118 . An extraction power supply  107  may be coupled to the ion source  102 . The extraction power supply  107  may provide a voltage level (Vx) to accelerate and extract ions from the ion source  102 . In one embodiment, the extraction power supply may provide a voltage (Vx) in the range of 20 kV to 120 kV. 
     An additional acceleration power supply  109  may be coupled between the terminal structure  104  and the grounded enclosure  112  so as to bias the terminal structure  104  at a positive voltage (Va) with respect to ground. In one embodiment, the acceleration power supply  109  may provide an additional voltage level (Va) that may have a maximum voltage in the range of 200 kV to 1,000 kV, and may be at least 400 kV in one embodiment. Accordingly, the terminal structure  104  may be energized, in some instances, to a high voltage between 200 kV and 1,000 kV. In other instances, the terminal structure  104  may not be energized at all or energized to nominal values only depending on the desired energy of the ion beam  152 . Although only one acceleration power supply  109  is illustrated for clarity of illustration, two or more power supplies may be utilized to provide the desired maximum high voltage level (Va). 
     During operation of the ion implanter  100 , the terminal structure  104  may be energized, in some instances, to at least 400 kV, e.g., 670 kV in one embodiment. The insulated conductor  103  is disposed proximate an exterior portion of the terminal structure  104  to modify an electric field about the terminal structure  104 . The insulated conductor  103  includes an insulator with a dielectric strength greater than 75 kilovolts (kV)/inch disposed about a conductor. The insulated conductor  103  may drop a high proportion of the terminal voltage within the insulated conductor  103 . Hence, the insulated conductor  103  reduces the electric stress in the air gap  111  and helps to promote a more uniform electric field within the air gap  111  compared to terminal structures with no such insulated conductors. In other words, the insulated conductor  103  may function as an electrical stress shield. Therefore, the terminal structure  104  may be energized to higher voltage levels, e.g., at least 600 kV as opposed to 200 kV, within the same reasonably sized grounded enclosure  112 . Alternatively, for operation at the same lower terminal voltage of about 200 kV and less, the insulated conductor  103  can enable the air gap  111  to be reduced compared to air only insulation schemes. 
     Turning to  FIG. 2 , a perspective view of the terminal structure  104  of  FIG. 1  is illustrated. The terminal structure  104  may include a base, one or more upstanding sidewalls coupled to the base, and a top  202  coupled to the one or more upstanding sidewalls. One upstanding sidewall  204  may have a door  240  with a handle  242  to provide personnel access to the internal cavity of the terminal structure  104 . The terminal structure  104  may have one upstanding sidewall manufactured of one solid material piece or any plurality of separate pieces. Although illustrated as a solid piece, the top  202  of the terminal structure may also be fabricated of a plurality of spaced conductors forming a type of conductor mesh to allow air to flow through the openings of the mesh. 
     In general, one or more insulated conductors may be disposed about portions of the exterior surface of the terminal structure  104  that have excess electric stress. In the embodiment of  FIG. 2 , a top insulated conductor  103  is disposed proximate the entire periphery of a top edge  270  of the terminal structure  104 , and a bottom insulated conductor  203  is disposed proximate the entire periphery of a bottom edge  272 . Although the top and bottom insulated conductors  103  and  203  are positioned about an entirety of the periphery of the respective edges  270 ,  272 , alternative embodiments may have additional or alternative exterior portions where insulated conductors may be positioned. These portions may include, but not be limited to, horizontal edges, vertical edges, corners, and openings or interfaces where the terminal structure  104  interfaces with external parts. Some external parts may include a motor, a generator, or a utility interface. In one example, a sphere shaped insulated conductor may be positioned about a corner of the terminal structure. 
     A plurality of brackets may be coupled to the terminal structure  104  and the associated insulated conductors  103  and  203  to support the insulated conductors  103  and  203  proximate an exterior portion of the terminal structure. The number and position of the brackets depends on the characteristics of the insulated conductors  103  and  203 , the geometry of the terminal structure  104 , and the type of bracket. The brackets may have a length to enable the insulated conductors  103  and  203  to be positioned a desired distance from an exterior portion of the terminal structure  104 . The desired distance may range from almost zero (nearly touching) to a maximum distance permitted by the surrounding air gap. In one embodiment, the desired distance is at least 1.5 inches. The brackets, e.g., bracket  208 , may be fabricated of either conductive of nonconductive material. 
     Turning to  FIG. 3 , a cross sectional view of one embodiment of the insulated conductor  103  taken along the line A-A of  FIG. 2  is illustrated. The insulated conductor  103  includes an insulator  205  with a dielectric strength greater than 75 kV/inch disposed about a conductor  302 . In one embodiment, the insulator  205  may be a solid insulator. The solid insulator may include, but not be limited to, syntactic foam, polytetrafluoroethylene (PTFE), chlorinated polyvinyl chloride (CPVC), polyvinylidene difluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), polytetrafluoroethylene (PTFE), or a polyimide (e.g., kapton). The syntactic foam may include hollow glass spheres and/or polymer pellets dispersed about a filling compound such as epoxy, silicone, or resin. In one embodiment, a syntactic foam insulator had an average dielectric strength of about 300 kV/inch on test samples about 0.25 inches thick. Some other solid insulators had a dielectric strength greater than 150 kV/inch. Alternatively, the insulator  205  may have a chamber wall that defines an internal cavity and the internal cavity may be filled with a liquid insulator or a gas insulator. The liquid insulator may include, but not be limited to, oil. The gas insulator may include, but not be limited to, carbon dioxide (CO 2 ), sulphur hexafluoride (SF 6 ), or pressurized air. Some gases may not need to be pressurized depending on their non pressurized dielectric strength. Vacuum insulation and/or any combination to form a composite insulation may also be utilized. The conductor  302  may be a high voltage conductor having a solid cross section. The insulated conductor  103  can be fabricated as a single entity or composed and joined from segments of insulated conductors. 
     A power supply  310  may energize the terminal structure  104  and the conductor  302 . In one embodiment, the power supply  310  is configured to energize the conductor  302  to a first voltage and the terminal structure  104  to the same first voltage, e.g., a terminal voltage (Vt). This terminal voltage may be at least 400 kV in one embodiment. This terminal voltage may also be a DC voltage in one instance. If the bracket  208  is fabricated of a conductive material, the power supply  310  may provide the voltage to the terminal structure  104  which may also be provided to the conductor  302  via the conductive bracket. If the bracket  208  is fabricated of a nonconductive material, a separate conductor  308  may electrically couple the power supply  310  to the conductor  302 . The conductor  308  may be fed through an opening in the bracket  208 . Having a separate conductor  308  also enables the conductor  302  to be energized to a different voltage level than the terminal voltage (Vt). 
     Non-pressurized air may be present within the grounded enclosure  112  about the terminal structure  104  and insulated conductor  103 . Non-pressurized air may have a dielectric strength of less than or equal to about 75 kV/inch under assumed conditions. This dielectric strength may change with relative humidity, altitude above sea level of the particular location of the ion implanter (i.e., air pressure), separation distance, and electrode surface finish. Temperature also impacts the breakdown strength of air. Essentially the temperature and pressure (PV=nRT) changes show that what is actually changing is the air density. Air density impacts breakdown strength through pressure and temperature. 
     As a safety measure to account for such variations, a dielectric strength of less than or equal to about 45 kV/inch for air may be utilized as a design rule in one instance. In any event, it would be desirable to have the electric field stress at the exterior of the insulated conductor  103  reduced to a value consistent with the selected design rule for air, even if the terminal structure is energized to 600 to 1,000 kV. In this way, the remaining air gap between the insulated conductor  103  and the grounded enclosure  112  (e.g., distance D 1 ) would be adequate to insulate the terminal structure  104  without electrical breakdown, e.g., arcing. 
     The geometry of the insulated conductor  103  may therefore be selected so the electric field stress at the exterior surface of the insulated conductor is less than the selected design rule for air. In some embodiments, the outside diameter (OD 1 ) of the insulator  205  may range between 8 inches and 16 inches depending on the dielectric strength of the particular insulator selected. The conductor  302  may have a center that is offset from the center  319  of the insulator  205  by an offset distance (OS 1 ) ranging from 0 to about 3.0 inches depending on the available space within the insulator  205  and the diameter of the conductor  302 . In one particular embodiment, the insulator  205  is syntactic foam having an outside diameter (OD 1 ) of 11 inches, the conductor  302  has a diameter of 4 inches spaced an offset distance (OS 1 ) of 3.0 inches from the center of the insulator  205 , and the bracket has a length to enable the insulated conductor  103  to be positioned a distance (D 2 ) of 1.5 inches from the terminal structure  104 . 
       FIG. 4  illustrates plots of equipotential lines for an embodiment consistent with  FIG. 3  where the conductor  302  and the terminal structure  104  are both energized to 600 kV. The insulated conductor  103  modifies the electric field about the terminal structure  104  so that areas of greater electric stress are positioned within the insulator  205  designed to accommodate this electric stress. The insulated conductor  103  also helps to reduce electric stress within the air gap  406  between the terminal structure  104  and the grounded enclosure  112  and to promote a more uniform electric field within the air gap  406  compared to terminal structures with no such insulated conductors. 
     The insulated conductor  103  thus operates as a type of electrical stress shield. Therefore, the terminal structure  104  may be energized to higher voltage levels, e.g., at least 600 kV as opposed to 200 kV ion one instance, within the same reasonably sized grounded enclosure  112 . Alternatively, for operation at the same lower terminal voltage of about 200 kV and less as in conventional ion implanters, the insulated conductor  103  can enable the air gap  406  to be reduced compared to air only insulation schemes. 
     Triple point stress regions  402 ,  404  form where three different mediums contact each other (e.g., air, insulator  205 , and a conductor where the bracket  208  is fabricated of conductive material). When the conductor  302  and the terminal structure  104  are energized to the same terminal voltage, an isopotential is created between the terminal structure and the conductor  302  and hence the electric stress at triple point stress regions  402 ,  404  is relatively low. This enables the mounting bracket  208  to be affixed to its location which would otherwise be a difficult connection to make in an area of higher electric stress absent the insulated conductor  103 . 
       FIG. 5A  illustrates a cross sectional view another embodiment of an insulated conductor  500  having a plurality of grading conductors  502 ,  504 ,  506 . Other components of  FIG. 5A  similar to earlier Figures are labeled similarly and hence any repetitive description is omitted herein for clarity. Although three grading conductors  502 ,  504 ,  506  are illustrated, the embodiment is not limited to only three grading conductors as one or more grading conductors may be utilized. In general, the grading conductors  502 ,  504 ,  506  are a means to drop more voltage inside the insulated conductor  500  than without the grading conductors. The grading conductors  502 ,  504 ,  506  thus increase flexibility to increase the operating voltage of the terminal structure  104  or reduce the footprint of the grounded enclosure  112 . 
     The grading conductors  502 ,  504 ,  506  may be radially disposed varying radial distances r 1 , r 2 , and r 3  respectively from the conductor  302 , wherein r 1 &lt;r 2 &lt;r 3 . Each of the grading conductors  502 ,  504 ,  506  may have an arcuate shape where the curved bow of the shape is generally consistent with a segment of an outside edge of the terminal structure  104  proximate the insulated conductor  500 . The arcuate shape has a length that can be similar or different for each grading conductor. In the embodiment of  FIG. 5A , the first and second grading conductors  502  and  504  have a similar arcuate length while the third grading conductor  506  is comparatively shorter. 
     The grading conductors  502 ,  504 ,  506  may be either passive or active. For passive grading, the grading conductors  502 ,  504 ,  506  are not connected to the external power supply  310 , and the grading conductors are electrically floating. For active grading, the grading conductors  502 ,  504 ,  506  are connected to the external power supply  310 . For clarity of illustration, the connections from the power supply  310  are illustrated as phantom lines and could be derived in any number of ways, e.g., a high voltage power supply could be divided by a resistor chain to ground with differing voltages being tapped off different positions in the resistor chain. The conductor  302 , as well as the terminal structure  104 , may receive a terminal voltage (Vt) from the power supply  310 . The first grading conductor  502  may receive a first grading voltage (V 1 ), the second grading conductor  504  may receive a second grading voltage (V 2 ), and the third grading conductor  506  may receive a third grading voltage (V 3 ) from the power supply  310 , where Vt&gt;V 1 &gt;V 2 &gt;V 3 . In one embodiment, Vt=600 kV, V 1 =500 kV, V 2 =400 kV, and V 3 =300 kV. 
       FIG. 6  illustrates plots of equipotential lines for an embodiment consistent with  FIG. 5A  having active grading to illustrate how the insulated conductor  500  modifies an electric field about the terminal structure  104 . The equipotential lines are concentrated in the insulator  505  which is designed to handle the electric stress. The grading conductors  502 ,  504 ,  506  enable more voltage to be dropped within the insulator  505  than without the grading conductors. In addition, the electric stress at the surface of the insulator  505  is reduced and a more uniform electrical field within the air gap between the insulated conductor  500  and the grounded enclosure  112  is promoted. 
     Turning to  FIG. 5B , another embodiment of an insulated conductor  550  with three grading conductors  522 ,  524 ,  526  is illustrated. In general, the size and angular spacing between the grading conductors may vary depending on the particular application. In the embodiment of  FIG. 5B , the three grading conductors  522 ,  524 ,  526  are radially disposed a respective radial distance r 1 , r 2 , and r 3  from the conductor  302 . The first grading conductor  522  has a first arcuate length (L 1 ), the second grading conductor  534  has a second arcuate length (L 2 ), and the third grading conductor has a third arcuate length (L 3 ), where L 1 &gt;L 2 &gt;L 3 . In addition, radial lines from the conductor  302  to a center of each grading conductor  522 ,  524 ,  526  are at different angles from each other. 
     Turning to  FIG. 7 , a cross sectional view of another embodiment of a terminal structure  104  having differing insulated conductors positioned relative to differing exterior portions of the terminal structure  104  is illustrated. Again in general, one or more insulated conductors may be disposed about portions of the exterior surface of the terminal structure  104  that have excess electric stress. In the embodiment of  FIG. 7 , a top insulated conductor  703  is disposed proximate a periphery of a top edge  733  of the terminal structure  104 . A bottom insulated conductor  705  is disposed proximate a periphery of a bottom edge  735 . A first  702  and second  704  intermediate insulated conductor may be further disposed about a periphery of the terminal structure  104  between the top edge  733  and bottom edge  735 . Although each of the insulated conductors  703 ,  702 ,  704 ,  705  is illustrated as having three grading conductors, other embodiments may not utilize any grading conductors. 
       FIGS. 8-12  illustrate embodiments having a plurality of insulated conductors arranged in different configurations about the terminal structure  104 . The quantity of insulated conductors, the size of the insulated conductors, and the clearance between each insulated conductor depend on the maximum operating voltage of the terminal structure  104 , the dielectric strength of each insulator, and the mechanical support structure. The different configurations of  FIGS. 8-12  are flexible to upgrade the system to accommodate differing terminal voltage levels. For instance, a higher operating voltage of the terminal structure may be accommodated by adding additional insulated conductors as necessary, and a lower terminal voltage may be accommodated by removing one or more of the insulated conductors. Each of the insulated conductors of  FIGS. 8-12  may be consistent with any of the embodiments of insulated conductors detailed herein or may be commercially available or custom made high voltage cables. 
       FIG. 8  illustrates a plurality of insulated conductors  802 ,  804 ,  806 , and  808  disposed in a linear array  800  extending outwardly from an exterior portion of the terminal structure  104 . The exterior portion of the terminal structure may be a top edge of the terminal structure where a sidewall and a top of the terminal structure meet. This edge may have an arc shape and the linear array  800  of insulated conductors may extend radially outward from the arc. Although  FIG. 8  illustrates four insulated conductors  802 ,  804 ,  806 , and  808 , any plurality of insulated conductors may be used and insulated conductors may be added or removed to accommodate differing maximum operating voltages of the terminal structure. 
     The linear array  800  may be arranged for passive grading or active grading. For passive grading, conductor  803  and the terminal structure  104  may both receive a terminal voltage (Vt), e.g., 600 kV in one embodiment. The other conductors  805 ,  807 , and  809  may be left electrically floating. For active grading, conductor  803  and the terminal structure  104  may again receive the terminal voltage (Vt), while conductors  805 ,  807 , and  809  may receive respective voltage levels V 1 , V 2 , and V 3 , where Vt&gt;V 1 &gt;V 2 &gt;V 3 . 
       FIG. 9  illustrates another embodiment having a plurality of insulated conductors  902 ,  904 ,  802 ,  804 , and  806 . Insulated conductors  802 ,  804 , and  806  are arranged in a linear array similar to the embodiment of  FIG. 8 . Insulated conductors  902 ,  802 , and  904  are arranged in arc similar to the arcuate shape of the edge of the exterior portion of the terminal structure  104  proximate the three insulated conductors  902 ,  802 , and  904 . For active grading, conductors  903 ,  803 , and  905  may receive the terminal voltage (Vt), while conductors  805  and  807  may receive respective voltage levels V 1 , V 2 , where Vt &gt;V 1 &gt;V 2 . In yet another embodiment, the insulated conductors  804  and  806  may not be present leaving only insulated conductors  902 ,  802 , and  904  arranged in the illustrated arc about an exterior portion of the terminal structure  104 . 
       FIG. 10  illustrates plots of equipotential lines of one simulated active grading embodiment consistent with  FIG. 9  to illustrate how the embodiment of  FIG. 9  modifies an electric field about the terminal structure  104 . In this simulated embodiment, each insulated conductor  902 ,  802 ,  904 ,  804 ,  806  is a high voltage cable with a 0.25 inch diameter conductor disposed within a 4.0 inch diameter solid insulator. The terminal structure  104  and the conductors  903 ,  803 ,  905  closest to the terminal structure were each energized to 600 kV. Conductor  805  was energized to 80% of the terminal voltage or 480 kV, while conductor  807  was energized to 60% of the terminal voltage or 360 kV. The simulated results revealed a maximum electrical stress in the insulators closest to the terminal structure of 148 kV/inch, and the maximum electrical stress in air was 45 kV/inch. A good quality plastic insulator can withstand the 148 kV/inch electrical stress, which would otherwise be far too high for air to withstand. 
       FIG. 11  illustrates yet another embodiment where the insulated conductors  1102 ,  804 , and  1104  are arranged in an arc similar to the arc of the insulated conductors  902 ,  802 ,  904  of  FIG. 9 , yet displaced a further distance from the terminal structure than the embodiment of  FIG. 9 . 
       FIG. 12  illustrates yet another embodiment where at least one insulated conductor is supported by the grounded enclosure  112 . For example, in the embodiment of  FIG. 12 , six insulated conductors  1230 ,  1232 ,  1234 ,  1236 ,  1238 , and  1240  may be supported by the grounded enclosure  112 . For active grading, the conductor of the insulted conductor  802  may receive the terminal voltage (Vt), the conductors of the insulated conductors  1102 ,  804 , and  1104  may receive voltage V 1 , the conductor of the insulated conductor  806  may receive voltage V 2 , and the conductors of the insulated conductors  1230 ,  1232 ,  1234 ,  1236 ,  1238 , and  1240  receive voltage Vn, where Vt&gt;V 1 &gt;V 2 &gt;Vn. 
     Turning to  FIG. 13 , a cross sectional view of an embodiment of an insulated conductor  1300  having a tubular member  1305  is illustrated. The tubular member  1305  defines an interior portion having a conductor  1302  disposed therein. The tubular member  1305  may be fabricated of chlorinated polyvinyl chloride (CPVC), polyvinylidene difluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), polytetrafluoroethylene (PTFE), or a polyimide (e.g., kapton). The CPVC, PVDF, and ECTFE plastic materials meet flammability specifications of different organizations such as Factory Mutual Specification 4910 (FM4910), and Underwriters Laboratory Specification 94-VO (UL94-V0), which both specify standards to reduce the risk of fire and smoke. The conductor  1302  may also be a tubular member and may be fabricated of any variety of conductive materials such as metal. 
     A filler insulator  1312  may be disposed within the interior portion defined by the tubular member  1305 . The filler insulator  312  may completely fill or partially fill the interior portion and combinations of differing filler insulators may be utilized. If the filler  1312  is completely sealed within a FM4910 conforming tubular member  1305 , the filler  1312  does not need to be in conformance with the FM4910 specification. 
     One filler insulator  1312  may be a solid material such as syntactic foam. The syntactic foam can include hollow glass spheres and/or polymer pellets dispersed about a filling compound such as epoxy, silicone, or resin. Another filler insulator  1312  may include a gas. In one embodiment, the gas may be pressurized air. In some instances, a gas such as carbon dioxide (CO 2 ) or sulphur hexafluoride (SF 6 ) may be utilized and such gases may not need to be pressurized depending on their non pressurized dielectric strength. One combination of filler insulators  1312  may include a solid such as syntactic foam contacting the conductor  1302 , with a gas such as pressurized air or SF 6  filling the remainder of the interior volume defined by the tubular member  1305 . 
     The geometry of the tubular member  1305  may be selected so the electric field stress at the exterior surface of the tubular member  1305  is less than or equal to a selected design limit for air to safely accommodate, e.g., 45 kV/inch in one instance. The outside diameter (OD 1 ) of the tubular member  1305  may range between 8 inches and 16 inches in some embodiments. The outside diameter (OD 1 ) may also be selected to have a common size such as 12.7 inches with a wall thickness (T 1 ) of 0.687 inches to be compliant with a size Schedule 80 pipe. The outside diameter of the conductor  1302  may range in size from 0.25 inches to 6.0 inches in some embodiments. The conductor  1302  may have a center that is offset from the center  1319  of the tubular member  1305  by an offset distance (OS 1 ) ranging from 0 to about 3.0 inches depending on the available space within the tubular member  1305  and the diameter of the conductor  1302 . 
     In one particular embodiment, the outside diameter (OD 1 ) of a CPVC tubular member  1305  is 11 inches with a wall thickness (T 1 ) of 0.5 inches. The outside diameter of the conductor  1302  in this particular embodiment is  4  inches and the offset distance (OS 1 ) is 2.5 inches. Therefore, the distance (D 3 ) between the conductor  1302  and the interior surface of the tubular member  1305  is only 0.5 inches and the distance (D 4 ) between the conductor  1302  and an opposing interior surface of the tubular member is 5.5 inches in this particular embodiment. The bracket  208  may have a length to enable the insulated conductor  1300  to be positioned a distance (D 2 ) of 1.5 inches from an exterior portion of the terminal structure  104 . 
       FIG. 14  is a cross sectional view of another embodiment of an insulated conductor  1400 . The insulated conductor  1400  has a first tubular member  1305  and a second tubular member  1404  disposed within the interior portion defined by the first tubular member  1305 . Components of  FIG. 14  similar to  FIG. 13  and other embodiments are labeled similarly and hence any repetitive description is omitted herein for clarity. 
     The second tubular member  1404  may be a solid insulator fabricated of CPVC, PVDF, ECTFE, PTFE, or a polyimide. The conductor  1302  may have an outside diameter slightly less than the inside diameter of the second tubular member  1404  so the conductor  1302  touches the interior surface of the second tubular member  1404 . It is desirable to have no air gaps between the outside of the conductor  1302  and the inside of the second tubular member  1404 . In practice, a tubular member  1404  may be selected and the inside of the tubular member  1404  may be coated with a conductive paint. This may sometimes be referred to as “metalizing” the interior surface of the tubular member  1404 . The second tubular member  1404  may reduce the electric stress at the exterior surface of the conductor  1302  to be much less than 80 kV/inch. 
       FIG. 15  is a cross sectional view of another embodiment of an insulated conductor  1500 . The insulated conductor  1500  has a first tubular member  1305 , a second tubular member  1404  and a third tubular member  1502 . Other components of  FIG. 15  similar to  FIGS. 13 and 14  and other embodiments are labeled similarly and hence any repetitive description is omitted herein for clarity. 
     The third tubular member  1502  may be disposed within the interior portion defined by the first tubular member  1305 , and the second tubular member  1404  may be further disposed within the third tubular member  1502 . A first filler insulator  1504  may be disposed within the interior portion defined by the third tubular member  1502 , and a second filler insulator  1512  may be disposed within the interior portion defined by the first tubular member  1305 . In one embodiment, a pressurized gas, e.g., air, nitrogen, SF 6 , etc., may be utilized as the filler for the first  1504  and second  1512  filler insulator. The outside diameter of the third tubular member  1502  may then be selected so that the volume of gas needed for the first filler insulator  1504  is less than the volume of gas needed for the second filler insulator  1512 , and enables a higher gas pressure to be utilized for the first filler  1504  without invoking stringent pressure vessel code requirements. 
       FIG. 16  illustrates yet another embodiment of an insulated conductor  1600 . The insulated conductor  1600  may include a composite insulator including solid fillers  1602  dispersed in a pressurized gas. The solid fillers  1602  may include a plurality of solid modules, e.g., halar granules and/or glass spheres, which serves as an insulator and also reduce the remaining volume  1604  to be filled with a pressurized gas. The pressurized gas can include pressurized air or SF 6  above 1.5 atm absolute without invoking stringent pressure vessel code requirements given the reduced volume of gas needed in the volume  1604 . 
       FIG. 17  illustrates yet another embodiment of an insulator conductor  1700 . The insulated conductor  1700  is similar to the insulated conductor of  FIG. 14  except that it has a plurality of high voltage cables  1702 ,  1704 ,  1706  disposed within an interior volume defined by the second tubular member  1404  as opposed to the individual conductor  1302  of  FIG. 14 . Other components of  FIG. 17  similar to  FIG. 14  and other embodiments are labeled similarly and hence any repetitive description is omitted herein for clarity. Each of the high voltage cables  1702 ,  1704 ,  1706  includes a conductor enclosed in an associated insulator. The high voltage cables may be commercially available or custom made high voltage cables. Since the high voltage cables  1702 ,  1704 ,  1706  are individually insulated, this provides another layer of insulation protection against other elements having compromised dielectric strength. For example, the high voltage cables  1702 ,  1704 ,  1706  may be utilized with welded tubular members, e.g., members  1404  and  1305 , having compromised dielectric strength at the welding junctions. 
     A first filler insulator  1722  may be disposed within the interior portion defined by the second tubular member  1404  and a second filler insulator  1712  may be disposed within the interior portion defined by the first tubular member  1305 . The first filler insulator  1722  may include non pressurized air, SF 6 , microballoons, granules, etc. and the second filler insulator  1712  may include the same. 
     Having thus described at least one illustrative embodiment, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within and scope of the disclosure. Accordingly, the foregoing description is by way of example only and is not intended as limiting.