Abstract:
The present invention relates to an ion beam extraction assembly for use in an ion beam generation apparatus such as those used, for example, in an ion implanter. An ion beam extraction assembly is provided for mounting within an ion beam generating apparatus comprising an ion source such that the extraction assembly is operable to extract ions from the ion source as an ion beam. The extraction assembly comprises an electrode assembly separate from the ion source, an electrode of the electrode assembly defining at least partly a path through the extraction assembly for passage of an ion beam. At least a part of the electrode assembly adjacent the path is tungsten and at least a part of the electrode assembly that is remote from the path is formed from a less expensive and/or lighter material.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an ion beam extraction assembly for use in an ion beam generation apparatus such as those used, for example, in an ion implanter. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ion implantation techniques, e.g. for modifying the electrical conductivity properties of semiconductor materials, are known in the manufacture of integrated circuit structures in semiconductor wafers. Such ion implanters generally comprise an ion beam generation apparatus having a source of ions of the element to be implanted in the semiconductor wafer, and an extraction assembly for extracting ions from the source and forming a beam of the extracted ions. The ion beam so produced is then passed through a mass analyser and selector for selecting a particular species of ions in the ion beam for onward transmission for implantation in the wafer or target substrate. 
         [0003]    The extraction assembly may be a triode extraction assembly, so called because it involves an arrangement of three electrodes. A triode assembly requires mechanical adjustment of the electrodes to be made in order to optimise or “tune” the ion source for maximum beam current on the wafer. 
         [0004]    In an attempt to simplify this “tuning” operation, it has been proposed to use a tetrode assembly having four electrodes. Such an assembly is disclosed in U.S. Pat. No. 6,559,454. 
         [0005]    The tetrode assembly has four electrodes, each having at least one aperture to allow the passage of the ion beam. The first electrode is a source electrode which generally forms one wall of an arc chamber of the ion source and is at the same potential as the arc chamber. The second electrode immediately adjacent to the first electrode is an extraction electrode which is set at a potential to attract ions out of the ion source. The third electrode is a suppression electrode which operates to prevent electrodes in the ion beam downstream of a fourth, ground electrode from being drawn into the ion source. This ground electrode restricts the penetration of the electric fields between the ground electrode and the ion source into the region downstream of the ground electrode. 
         [0006]    The advantage of a tetrode structure is that the potential between the arc chamber and the extraction electrode can be set independently of the potential between the ion source and the ground electrode. In this way, the energy of the ion beam emerging from the extraction assembly can be determined independently of the potential at which the ions are initially extracted from the arc chamber. This permits the extraction efficiency of the ion source to be optimised and simplifies the “tuning” of the ion source for maximum beam currents. 
         [0007]    In order to provide flexibility for use with a range of ion beam energies, from high-energy beams to low-energy beams, a variable separation between the extraction and suppression electrodes is proposed by U.S. Pat. No. 6,559,454. With this arrangement, the size of the gap between the extraction and suppression electrodes can be decreased for low-energy beams and increased for high-energy beams (to reduce the chances of arcing due to the required higher acceleration voltage difference). 
         [0008]    Further, as changing the gap between the extraction and suppression electrodes alters the focussing effect of the electric field, this arrangement allows better control of the beam shape over a range of beam energies. 
         [0009]    The suppression and ground electrodes may also be moveable relative to the source and extraction electrodes in a lateral direction perpendicular to the beam direction. This provides additional control of the steering of the beam into the subsequent components of the ion implanter. 
         [0010]    The aperture in each electrode is generally an elongate slot. There is a tendency for space-charge expansion to cause the beam to blow up in the direction of elongation of the slot. This causes increased beam strike on the electrodes, and hence a loss of beam current. In order to overcome this problem, at least one of the electrodes may be concave facing away from the ion source in the plane containing the direction of beam travel, and the direction in which the slot is elongate. The concave electrode is often the extraction electrode. This curvature focuses the beam down as it passes through the extraction electrode and into the analyser magnet. The degree of curvature is preferably such that it counteracts the space-charge expansion of the beam in this plane. The source electrode may be concave in addition to the extraction electrode. 
         [0011]    U.S. Pat. No. 6,559,454 suggests that both source and extraction electrodes are curved with a common radius of curvature. U.S. Pat. No. 6,777,882 suggests an improvement may be obtained by having source and extraction electrodes with different radii of curvature and that are arranged concentrically. 
         [0012]    Pentode assemblies are also known. In such assemblies, a further electrode, termed the acceleration electrode is positioned downstream of the extraction electrode to provide an intermediate potential level between the extraction electrode and the ground electrode. This is beneficial in suppressing arc discharge. 
         [0013]    A common feature of the above electrode arrangements is that the end electrode in the downstream position is subject to ion beam erosion and is a significant source of low energy drift particles in the ion beam. For example, the tendency for the ion beam to diverge means that the last electrode sees the highest risk of beam strike. Any beam strike may cause material to be sputtered from the electrode. These contaminants can become entrained within the ion beam. These particles may be implanted in a substrate, either as a result of being directly transported within the ion beam or as a result of one or more cycles of deposition on downstream components followed by subsequent sputtering. Typically, the downstream electrode is a ground electrode, as described above with respect to tetrode assemblies. 
       SUMMARY OF THE INVENTION 
       [0014]    Against this background, the present invention resides in an ion beam extraction assembly for mounting within an ion beam generating apparatus comprising an ion source such that the extraction assembly is operable to extract ions from the ion source as an ion beam. The extraction assembly comprises an electrode assembly separate from the ion source. An electrode of the electrode assembly defines at least partly a path through the extraction assembly for passage of an ion beam. At least a part of the electrode assembly adjacent the path is tungsten and at least a part of the electrode assembly that is remote from the path is formed from a different material. 
         [0015]    One way of overcoming the problem of contamination from beam strike on electrodes within an extraction assembly is to use tungsten. However, such an arrangement would be very heavy, and also very expensive. 
         [0016]    Advantageously, the present invention provides a combination of tungsten and other parts, such as graphite parts. Tungsten is used for parts of the electrode that are prone to beam strike, while graphite or another material is used for parts of the support that are far less prone to beam strike. Thus the benefit of a tungsten electrode is realised, but in an arrangement that may have significant weight and cost savings over an all tungsten arrangement. Thus, the different material should be less expensive than tungsten and/or lighter. 
         [0017]    Preferably, all of an edge of the electrode that defines the ion beam path is formed from tungsten. This is because it is this part of the electrode that is most likely to see beam strike. 
         [0018]    Optionally, the electrode assembly comprises a composite electrode including a first tungsten portion adjacent the ion beam path and a second portion remote from the ion beam path formed of the different material. For example, the electrode assembly may comprise an electrode body formed of the different material that is provided with a tungsten cap that fits over a portion of the electrode body adjacent the ion beam path. 
         [0019]    The electrode assembly may comprise an electrode mounted to a support that, optionally, may be formed of a material other than tungsten. 
         [0020]    Preferably, the extraction assembly comprises a tungsten electrode mounted to a support. The support may be made from a single material, e.g. graphite. Typically, weight savings of more than 50% may be achieved with such an arrangement of a tungsten electrode and graphite support. 
         [0021]    Optionally, the extraction assembly may comprise a plurality of electrode components that together form the electrode, and wherein each electrode component is mounted to the support. The plurality of electrode components may be mounted to a common support. For example, a pair of common supports may be used to mount the plurality of electrode components, with a support disposed at either side of the extraction assembly. The electrode components may span the width of the extraction assembly, such that each electrode component is supported by both supports. Such electrode components may have apertures provided therein to allow passage of the ion beam along its path. Alternatively, pairs of opposed electrode components may be arranged to define the ion beam&#39;s path therebetween. In this arrangement, each electrode component may be supported by only a single support, depending upon which side of the extraction assembly they reside. 
         [0022]    Preferably, the support comprises angled slots arranged to receive the electrode components and to support each of the electrode components at a desired angle. Optionally, the support and each electrode component are provided with complementary features for setting the position of each electrode component. These may comprise lugs that are received within complementary slots. The lugs may be provided on the electrode components or the supports. 
         [0023]    Graphite has been mentioned above as a suitable choice for the different material, for the part of the electrode assembly remote from the ion beam path or for the support. Other suitable choices include stainless steel and Inconel®. 
         [0024]    Optionally, the extraction assembly comprises a series of electrodes defining a path through which an ion beam is intended to pass, and wherein the electrode is disposed at an end of the series. The extraction assembly may comprise a tetrode arrangement, although other arrangements such as triodes and pentodes are contemplated. 
         [0025]    The present invention also extends to an ion beam generating apparatus comprising an ion source and any of the ion beam extraction assemblies described above. The extraction assembly may be mounted within the ion beam generating apparatus such that the extraction assembly is operable to extract ions from the ion source as an ion beam. 
         [0026]    Optionally, the extraction assembly comprises a source electrode and the tungsten electrode, the source electrode being electrically connected so as to operate at the same voltage as the ion source and having an aperture provided therein for allowing passage of the ion beam from the ion source. The next electrode downstream of the source electrode may be an extraction electrode electrically biased to attract ions from the ion source. The next electrode downstream of the extraction electrode may be a suppression electrode electrically biased to suppress electrons from travelling upstream to the ion source. The next electrode downstream of the suppression electrode may be a ground electrode electrically by being biased to suppress electric fields generated by the extraction assembly from extending downstream of the ground electrode. Any or all of the extraction electrode, the suppression electrode or the ground electrode may correspond to the electrode of the electrode assemblies described above. 
         [0027]    As will be appreciated, the electrodes and electrode components described above may be formed as a single piece of tungsten or may comprise any of the composite designs described above, e.g. a graphite electrode body fitted with a tungsten cap. 
         [0028]    The present invention also extends to an ion implanter comprising any of the ion beam generating apparatuses described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    An example of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0030]      FIG. 1  is a schematic view of an ion implanter incorporating the present invention; 
           [0031]      FIG. 2  is a schematic plan view illustrating the arrangement of electrodes of  FIG. 1 ; 
           [0032]      FIG. 3  is a schematic view along line III-III in  FIG. 2 ; 
           [0033]      FIG. 4  is a schematic drawing showing the mounting of the extraction electrode in greater detail than as shown in  FIG. 2 ; 
           [0034]      FIG. 5  corresponds to  FIG. 2 , and shows an electrode arrangement including an electrostatic lens; 
           [0035]      FIG. 6  is a perspective view of one half of a ground electrode assembly according to an embodiment of the present invention; 
           [0036]      FIGS. 7 and 8  are perspective views of the electrodes of  FIG. 6 ; 
           [0037]      FIG. 9  is a perspective view of the end support of  FIG. 6 ; 
           [0038]      FIG. 10  is a detail from  FIG. 9 ; 
           [0039]      FIG. 11  shows the ground electrode assembly of  FIG. 6  mounted within an ion beam generating assembly; 
           [0040]      FIG. 12  is a perspective view of an electrode assembly according to a further embodiment of the present invention; and 
           [0041]      FIG. 13  is an exploded view of the electrode assembly of  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    Referring to  FIG. 1 , a conventional ion implanter is shown schematically at  8 . The ion implanter  8  includes ion beam generation apparatus  9 . The ion beam generation apparatus  9  comprises an ion source  10  with an extraction assembly  11 . The extraction assembly  11  extracts and directs an ion beam  12  through an ion mass selector  13  to impinge on a wafer  14  mounted on a wafer holder  14 A. As is well known to workers in this field, the above elements of the ion implanter  8  are housed in a vacuum housing of which a part  15  only is illustrated in  FIG. 1 . The vacuum housing may be evacuated by a vacuum pump  16 . 
         [0043]    The ion source  10  may comprise any known ion source such as a Freeman source, a Bernas source or an indirectly heated cathode source. The ion source  10  comprises an arc chamber which is fed a supply of a feed gas containing a desired dopant, ions of which are to be implanted in the wafer  14 . The feed gas may be supplied to the arc chamber in gaseous or vapour form, e.g. from a gas bottle  17 . 
         [0044]    The extraction assembly  11  comprises a number of electrodes located immediately adjacent the front of an arc chamber of the ion source  10  so as to extract ions from the arc chamber through an exit aperture in the front face. 
         [0045]    The ion mass selector  13  illustrated in  FIG. 1  comprises a magnetic sector mass analyser  33  operating in conjunction with a mass selecting slit  34 . The magnetic analyser  33  comprises a region of uniform magnetic field in the direction perpendicular to the plane of the paper in  FIG. 1 . In such a magnetic field, all ions of constant energy and having the same mass-to-charge ratio will describe circular paths of uniform radius. The radius of curvature of the path is dependent on the mass-to-charge ratio of the ions, assuming uniform energy. 
         [0046]    As is well known for such magnetic sector analysers, the geometry of such paths tends to bring a cone of ion paths emanating from an origin focus outside the entrance aperture of the analyser  33 , back to a focal point beyond the exit aperture of the analyser  33 . As illustrated in  FIG. 1 , the origin focus or point of origin of the central beam  30  is a point close to, typically just inside, the exit aperture of the arc chamber of the ion source  10 . The beam  30  is brought to a focus in the plane of the mass selection slit  34  beyond the exit aperture of the analyser  33 . 
         [0047]    In  FIG. 1 , the beam  30  is drawn showing only ions of a single mass/charge ratio, so that the beam  30  comes to a single focus at the aperture of the slit  34 , so that the beam of ions of this mass/charge ratio can pass through the slit  34  towards the wafer  14 . In practice, the beam  30  emitted by the ion source  10  will also contain ions of different mass/charge ratio from those desired for implantation in the wafer  14  and these undesired ions will be brought to a focus by the analyser  33  at a point in the plane of the slit  34  either side of the position of the slit  34 , and will therefore be prevented from travelling on towards the wafer  14 . The analyser  33  thus has a dispersion plane in the plane of the drawing. 
         [0048]    Referring to  FIGS. 2 and 3 , the ion beam generation assembly  9  is illustrated schematically. The ion source  10  comprises an arc chamber  10 A mounted to housing  15  by arms  43  as more fully described with reference to  FIG. 2 . A bushing  10 B acts as an insulator to isolate the ion source  10  from the remainder of the housing  15 . Ions formed in the arc chamber  10 A are extracted from the ion source  10  through an exit aperture  21  in a front face  22  of the ion source  10 . The front face  22  of the ion source  10  forms a first apertured source electrode  22  at the potential of the ion source  10  forming part of the extraction assembly  11  ( FIG. 1 ). The rest of the extraction assembly  11  is illustrated in  FIG. 2  by extraction, suppression and ground apertured electrodes  23 ,  24  and  25  respectively. Each of the apertured electrodes  23 ,  24  and  25  comprise a single electrically conductive plate having an aperture through the plate to allow the ion beam emerging from the ion source  10  to pass through. Each aperture has an elongate slot configuration with the direction of elongation being perpendicular to the plane in  FIG. 2  and in the plane of  FIG. 3 . 
         [0049]    For a beam of positive ions, the ion source  10  is maintained by a voltage supply at a positive voltage relative to ground. The ground electrode  25  restricts the penetration of the electric fields between the ground electrode  25  and the ion source  10  into the region to the right (in  FIG. 2 ) of the ground electrode  25 . The energy of the ion beam  30  emerging from the extraction assembly is determined by the voltage supplied to the ion source  10 . A typical value for this voltage is 20 kV, providing an extracted beam energy of 20 keV. However extracted beam energies of 80 keV and higher, or 0.5 keV or lower may also be contemplated. To obtain higher or lower voltages, it is a matter of raising or lowering respectively the source voltage. 
         [0050]    The suppression electrode  24  is biased by a voltage supply to a negative potential relative to ground. The negatively-biased suppression electrode  24 , operates to prevent electrons in the ion beam  30  downstream of the ground electrode  25  (to the right in  FIG. 2 ) from being drawn into the extraction region and into the ion source  10 . As is known to workers in this field, it is important to minimise the loss of electrons from the ion beam  30  in zero electric field regions, so as to maintain ion beam neutralisation. 
         [0051]    For a beam of positive ions, the extraction electrode  23  is maintained by a voltage supply at a potential below the potential of the ion source  10  to extract the ions from the ion source  10 . The potential of the extraction electrode  23  would typically be below the potential of the suppression electrode  24  for a low energy beam and above the potential of the suppression electrode  24  for a high energy beam. In the former case, the ion beam  30  will decelerate between the extraction electrode  23  and the suppression electrode  24 , while in the latter case it will accelerate here. 
         [0052]    The extraction electrode  23 , and the source electrode  22  are curved in the plane of the paper of  FIG. 3  so as to be concave facing away from the ion source  10 . The degree of curvature is sufficient to suppress any divergence of the beam in the direction perpendicular to the plane of the paper on  FIG. 2 . 
         [0053]    An example of how the extraction electrode  23  may be mounted is shown in more detail in  FIG. 4 . The arc chamber  10 A is mounted by a pair of arms  40  to a circular disc  41  having a hole  42  through which the extraction electrode  23  penetrates. The circular disc  41  is itself supported by two arms  43  attached to the housing  15 . The extraction electrode  23  is supported from one of the arms  43  by a pair of insulators  44 . A lead  45  supported through the wall of the housing  15  by an insulator  46  connects the extraction electrode  23  to a voltage supply (not shown). It will be appreciated that the disc  41  provides shielding to prevent contaminants from being deposited on the electrode mounting. The extraction electrode  23  may be mounted so as to allow movement in the beam direction (arrow x) relative to the arc chamber  10 A, for example as described in U.S. Pat. No. 6,777,882, the contents of which is incorporated in its entirety by reference. 
         [0054]    The suppression electrode  24  and ground electrode  25  are mounted as shown in  FIG. 2  so as to be moveable in the beam direction as represented by the arrow x and in a steering direction as represented by arrow y. 
         [0055]    The suppression electrode  24  is mounted so as to be moveable relatively to the extraction electrode  23  in the direction of travel of the ion beam  30  as indicated by the arrow x. The apparatus can be “tuned” such that the gap between the extraction electrode  23  and suppression electrode  24  is larger, the larger the beam energy. The ground electrode  25  may be moveable in the direction  26  together with or independently of the suppression electrode  24 . The electrodes  22 - 25  are further mounted, such that the suppression electrode  24  and ground electrode  25  are moveable relatively laterally in the direction of arrow  27 , namely in the plane of the paper and perpendicular to the ion beam direction  26 , relatively to the extraction electrode  23  and source electrode  22 . 
         [0056]    Further details pertaining to how the suppression electrode  24  and ground electrode  25  may be mounted may be found in U.S. Pat. No. 6,559,454, the contents of which is incorporated in its entirety by reference. 
         [0057]      FIG. 5  generally corresponds to  FIG. 2 , but shows an ion beam extraction assembly  11  where the ground electrode  25  is replaced with an assembly that includes a ground electrode  25  as part of an electrostatic lens  80 . The lens  80  comprises two cylinders of different diameters that are each coaxial about the ion beam axis x. The inner cylinder provides the ground electrode  25 , while the outer cylinder  90  provides a lens electrode. The electrodes  15 ,  90  could be planar rather than cylindrical. The ground electrode  25  is provided with four slots  100 . Each slot  100  is elongate along the ion beam axis x, and the slots  100  are equispaced about the cylinder  25 . The field generated by the lens electrode  90  penetrates through the slots  100  to produce an electrostatic quadrupole. This lens arrangement  80  allows focusing in both z-axis and y-axis directions. Further details of such an arrangement may be found in U.S. Pat. No. 6,777,882. 
         [0058]    An electrode assembly  100  according to an embodiment of the present invention is shown in  FIG. 6 . In this embodiment, the electrode assembly comprises one half of a ground electrode  25 . However, the electrode assembly  100  could function as either the extraction assembly  23  or suppression electrode  24 . The ground electrode  25  comprises two halves, of which the ground electrode assembly  100  shown in  FIG. 6  is one.  FIG. 11  shows the ground electrode assembly  100  mounted on a plate  102  attached to the ion beam generation apparatus  9  relative to the ion source  10 . Although not shown in  FIG. 11 , the ground electrode  25  comprises a second, like ground electrode assembly  100  disposed opposite the assembly  100  shown in  FIG. 11  to form a symmetrical pair. The ion beam  30  passes between the assemblies  100 . For the sake of clarity, the extraction electrode  23  and the suppression electrode  24  have been omitted from  FIG. 11 . 
         [0059]    The ground electrode assembly  100  comprises four electrode plates  104 - 107  mounted to a pair of end supports  108 - 109 . The provision of multiple electrode plates  104 - 107  is for better control of both high and low energy ion beams  30 . Each electrode plate  104 - 107  is generally planar and is made from tungsten. The electrode plates  104 - 107  are received in slots  114 - 117  formed in the end supports  108 - 109  as best seen in  FIG. 9 . The end supports  108 - 109  are made from graphite. The slots  114 - 117  are angled to set the electrode plates  104 - 107  in an orientation to achieve a desired electrostatic field. The outer electrode plates  114  and  117  extend inwardly to converge towards the ion beam  30 . In contrast, the inner electrode plates  115 - 116  diverge as they extend towards the ion beam  30 . This divergence towards the ion beam  30  sees the inner electrode plates  105 - 106  converge within the end supports  108 - 109  such that slots  115 - 116  merge as shown at  118 . 
         [0060]    To fit within the merging slots  115 - 116 , the inner electrode plates  105 - 106  have a tapering form. This can be seen in  FIG. 8  where electrode plate  106  is shown; electrode plate  105  is of an identical design. This tapering form may be contrasted to the constant thickness of electrode plates  104  and  107 .  FIG. 7  shows electrode plate  104 , although electrode plate  107  is of an identical design. 
         [0061]    As can be seen from  FIGS. 7 and 8 , each electrode plate  104 - 107  is provided with a pair of lugs  120  that are sized and shaped to be received within channels  122  provided in end supports  108 - 109 . The combination of the lugs  120  and channels  122  ensure correct positioning of the electrode plates  104 - 107 . In addition, to prevent twisting of the electrode plates  104 - 107 , the channel  122  provided in end support  108  has a chamfered base to receive a lug  120  with a correspondingly-shaped portion. These co-operating shapes ensure that the electrode plates  104 - 107  adopt and maintain a precise position rather than suffering from warping, e.g. due to thermal cycling. To ensure correct assembly of the ground electrode assembly  100 , the two end supports  108 - 109  are indexed by their fixing hole positions. 
         [0062]      FIGS. 12 and 13  show another embodiment of the present invention. An electrode assembly  200  is shown assembled in  FIG. 12  and as an exploded view in  FIG. 13 . The electrode assembly  200  comprises four electrode units  201 - 204 . A first, upstream pair of opposed electrode units  201 - 202  (relative to the ion beam shown at  30 ) together comprise a suppression electrode  24  and a second, downstream pair of opposed electrode units  203 - 204  together comprise a ground electrode  25 . The two pairs of opposed electrode units  201 - 204  form a pair of apertures through which the ion beam  30  passes as it travels through the ion beam extraction assembly  11 . A pair of support members  205 - 206  are provided to hold the electrode units  201 - 204  in place. Suppression electrode unit  201  and ground electrode unit  203  are mounted to support member  205 , and suppression electrode unit  202  and ground electrode unit  204  are mounted to support member  206 . The electrode units  201 - 204  fit within slots  207  provided in the support members  205 - 206  and may be mounted to their respective support members  205 - 206  in any convenient manner, e.g. interference fit, bolt fastenings, screw fastenings, etc. 
         [0063]    As can be seen most clearly from  FIG. 13 , each electrode unit  201 - 204  comprises an electrode body  208  and an electrode cap  209 . Each electrode body  208  is positioned adjacent to one of the support members  205 - 206  such that an edge  210  of the electrode body  208  is received within a slot  207 . Each electrode cap  209  fits over the edge  211  of an electrode body  208  that would otherwise define the aperture through which the ion beam  30  passes. Each electrode cap  209  is hollow with an open face  212  such that the electrode cap  209  may be placed over the edge  211  of the electrode body  208 . The electrode caps  209  may be held in position on the electrode bodies  208  by any convenient means, e.g. a push fit, bolts, screws, etc. The attachment means should not be permanent as the electrode caps  209  will require removal from time to time, either for cleaning or for replacement. 
         [0064]    In accordance with the present invention, the materials are carefully chosen for the components of the electrode assembly  200 . Tungsten is chosen for the electrode caps  209  as these are most likely to see beam strike. The depth of the electrode caps  209  is chosen to ensure that all parts of the electrode units  201 - 204  likely to suffer from beam strike are covered by the tungsten electrode caps  209 . As a result, the electrode bodies  208  do not need to be made from tungsten: instead, less expensive and lighter stainless steel is used, although other materials such as graphite or Inconel® may be used. Also, tungsten need not be used for the support members  205 - 206 . An insulator is used for these support members because of the need to provide electrical insulation between the suppression electrode  24  and the ground electrode  25  (the electrical connections to the suppression electrode  24  and the ground electrode  25  are not shown in  FIGS. 12 and 13  for the sake of clarity). Those skilled in the art will appreciate that other electrical arrangements may be used. For example, the support members  205 - 206  may be made of graphite and insulating sleeves may be used to join the electrode units  201 - 204  to the support members  205 - 206 . 
         [0065]    The electrode assembly  200  of  FIGS. 12 and 13  is advantageous as only the parts exposed to the ion beam  30  are formed from tungsten. These electrode caps  209  are easily fitted and removed such that they may be cleaned or replaced periodically. The other components require servicing less frequently, if at all, and may be left in position when the electrode caps  209  are being refurbished or replaced. 
         [0066]    This design may be simplified by replacing the electrode body  208 /electrode cap  209  combination with a single tungsten electrode piece. For example, caps  209  may be omitted and the bodies  208  may be formed of tungsten. These symmetrical electrode pieces  208  may be reversed in slots  207 , i.e. when one edge gets dirty the electrode piece  208  may be turned around to present a new clean edge to the ion beam  30 . 
         [0067]    Slots  207  may be made deeper so as to allow the position of the electrodes  201 - 204  to be varied, and hence the width of the aperture to be varied. 
         [0068]    As will be appreciated by the person skilled in the art, variations may be made to the above embodiment without departing from the scope of the invention defined by the claims. 
         [0069]    For example, the invention has been described with respect to an embodiment as a ground electrode  25  and an embodiment as a suppression electrode  24 /ground electrode  25  assembly. However, the invention is applicable to any of the electrodes in the extraction assembly  11 . Thus in the context of a tetrode arrangement, any combination of the source electrode  22 , extraction electrode  23 , suppression electrode  24  and ground electrode  25  may be arranged in accordance with the present invention, including any combination of these electrodes  22 - 25 . 
         [0070]    The ground electrode  25  has been described to include four pairs of electrode plates  104 - 107 . However, the ground electrode  25  may comprise a single pair of electrode plates. Moreover, rather than using one or more pairs of opposed electrode plates that form the ion beam path therebetween, one or more single plate electrodes may be provided. Such single plate electrodes usually have a central aperture provided therein defining in part the ion beam&#39;s path. Often, such apertures are elongate. 
         [0071]      FIG. 11  shows electrode plates  104 - 107  mounted to end support  108  that is in turn mounted to plate  102 . However, electrode plates  104 - 107  may be mounted directly to the plate  102 . For example, grooves may be formed in the plate  102  to receive the electrode plates  104 - 107 . The electrode plates  104 - 107  may be held in place by any suitable means. 
         [0072]    The extraction assemblies  11  described above may be adapted for use with any of the well-known ion implanter arrangements, including those described with respect to  FIGS. 1 to 5 . For example, the electrodes  22 - 25  may be fixed in position or may be mounted on mechanisms that allow their relative positions to be moved.