Abstract:
An ion implanter incorporates an r.f. accelerator assembly to provide ions for implant at high energies. The accelerator assembly includes electrodes mounted in the vacuum chamber so as to be movable between an operational position for generating and accelerating electric field and a non operational position within the vacuum chamber displaced clear of the beam path. An Actuator moves the electrode between the operational and non operation positions. For energy implanting, the electrodes are in the operational position and for low energy implants the actuator moves the electrodes to the non operational position clear of the beam path.

Description:
FIELD OF THE INVENTION 
   The invention is concerned with ion implanters and with a method of ion implantation. 
   BACKGROUND OF THE INVENTION 
   Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting, in the wafer, ions of a required dopant. Typical ionic species used for this purpose are boron, phosphorus, arsenic and antimony. However, other ionic species are also used for other purposes, including oxygen for example. 
   The depth to which implanted ions penetrate the surface of the wafer is largely dependent on the energy of the ions in the ion beam. The semiconductor industry requires both very shallow implants, for example for very fine structures having a small feature size, and relatively deep implants, for example for buried layers etc. It is also a general requirement of the semiconductor processing industry that process times should be as short as possible which implies that the quantity of ions being implanted per unit area and per unit time into a semiconductor wafer should be as high as possible. This implies that ion implantation is conducted with a high beam current, being a measure of the number of required ions in the beam reaching the wafer surface per unit time. There is also the requirement that implantation should be cost effective. 
   Beam energies up to about 200 keV (for singly charged ions) can quite readily be obtained using electrostatic acceleration systems, in which the source of ions is held at a fixed voltage relative to the wafer to be implanted, the fixed voltage defining the energy of the ions in the beam on implantation. 
   In most ion beam type ion implanters, a mass selection stage is required to select from the beam from the ion source those ionic species required for implantation. Typically mass selection is performed using a mass analysing sector magnet combined with a mass resolving slit downstream of the magnet. It is common practice in implanters using electrostatic acceleration systems for the full beam energy to be delivered to the ions of the beam prior to entering the mass analyser. However, post mass analysis electrostatic acceleration and deceleration are known, using additional electrostatically biased electrodes between the mass resolving slit and the substrate. Examples include U.S. Pat. No. 5,389,793 and U.S. Pat. No. 5,969,366. 
   For higher implant energies radio frequency acceleration systems have been employed, usually post mass analysis. Examples include U.S. Pat. No. 6,423,976 and U.S. Pat. No. 4,667,111 describing the use of r.f. linear accelerators, and U.S. Pat. No. 5,301,488 describing the use of r.f. quadrupole accelerator. 
   It is a known practice to operate ion implanters having post mass analysis accelerators (or decelerators), without energising the accelerators (or decelerators), in so-called drift mode. This practice allows the implanter to operate at lower energies (or higher for post decelerators), using the beam energy directly from the mass analyser. U.S. Pat. No. 6,423,976 describes drift mode operation of a r.f. linear type accelerator. However, the beam current available for implanting when operating in drift mode can be disappointing. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an improved ion implanter which can be used for producing high energy ion beams as well as permitting efficient transport of significantly lower energy beams therethrough, so that the implanter in which an accelerator assembly is installed can be operated efficiently across a wide spectrum of ion implantation energies. 
   The present invention provides an ion implanter comprising an ion beam generator for generating a beam of ions to be implanted in which said ions are at a first energy, and an accelerator assembly having a vacuum chamber and operative when energised to accelerate ions of said beam to a second energy along a beam path through the vacuum chamber of the assembly, the assembly comprising at least one electrode mounted in the vacuum chamber to be movable between a respective operational position for generating an accelerating electric field to accelerate said ions along said beam path, and a respective non-operational position within the vacuum chamber displaced clear of said beam path, and an actuator to move said electrode between said operational and non-operational positions. 
   The accelerator assembly may be a radio frequency (r.f.) accelerator, for example a linear accelerator. 
   The accelerator assembly itself may comprise at least one r.f. booster stage comprising entrance and exit electrodes and at least one intermediate r.f. electrode. Preferably said electrodes of said booster stage are mounted to be movable together transversely of said beam path between respective said operational and non-operational positions. 
   Because the actuator can move the electrode or electrodes of the accelerator assembly out of the beam path through the vacuum chamber, drift mode operation (with no voltages applied to the accelerator) permits significantly increased beam current to be delivered to the substrate. 
   A typical accelerator assembly comprises at least first and second said r.f. booster stages in tandem along said beam path, said first booster stage being upstream of said second booster stage relative to said beam direction, and said electrodes of second booster stage being movable between respective said operational and non-operational positions independently of said electrodes of said first booster stage. 
   In this way, the implanter may be operated with only the first booster stage energised to accelerate beam ions, and with the second stage de-energised with its electrodes clear of the beam path. The resulting beam current can then be higher. 
   In a preferred ion implanter according to the present invention, said at least one intermediate r.f. electrode of the accelerator assembly is movable between said operational and non-operational positions, and the accelerator assembly includes at least one inductive coil electrically connected to said at least one intermediate r.f. electrode, and an electrically conductive enclosure around said coil; said coil, said at least one electrode and said conductive enclosure forming together a r.f. tank circuit having a predetermined resonant frequency when the at least one r.f. electrode is in said operational position; said coil being mounted to move with the at least one r.f. electrode. This conductive enclosure can be mounted to be movable with said coil and the at least one r.f. electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There now follows by way of example a detailed description of an ion implanter embodying the present invention. 
     In the accompanying drawings: 
       FIG. 1  is a general schematic plan view of an ion implanter which embodies the present invention; 
       FIGS. 2A and 2B  are side sectional views of an r.f. accelerator assembly embodying the present invention showing first and second stages thereof with electrodes thereof raised and lowered respectively; 
       FIGS. 3A and 3B  are sectional views showing greater detail of the encircled portions D and E of  FIGS. 2A and 2B  respectively, of the electrodes of the second stage of the accelerator assembly and of parts of the actuator of the accelerator assembly for moving the electrodes thereof; 
       FIGS. 4A and 4B  are enlarged cross-sectional axially oriented views corresponding to  FIGS. 3A and 3B  respectively, showing the disposition of the first r.f. electrode of the second stage of the assembly when raised and lowered respectively; 
       FIG. 5  is an enlarged sectional view, corresponding to  FIG. 3A , of part of the actuator of the illustrated accelerator assembly according to the invention for raising and lowering electrodes of a stage of the assembly into and from the beam path through the assembly, but showing the electrodes in their raised positions; 
       FIG. 6  is a still closer cross-sectional view, similar to  FIG. 4A , but showing more detail, of the electrodes, and part of the actuator for raising and lowering them, in raised position; 
       FIGS. 7A and 7B  are end views of a mechanism of the actuator of an accelerator assembly according to the present invention for raising and lowering the electrodes into and from the beam path of the accelerator assembly, with  FIG. 7  showing the electrodes in raised position and  FIG. 8  showing them in lowered position; 
       FIGS. 8A and 8B  are side sectional views of parts of the mechanism for raising and lowering part of the actuator therefor shown in  FIGS. 5  to  7 ; 
       FIG. 9  is a plan view of a frame member of the actuator for raising and lowering the electrodes; 
       FIG. 10  is a schematic circuit diagram of an r.f. accelerator assembly embodying and illustrating various features of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Aspects of the invention may be employed in many different kinds of ion implanters, including both implanters designed for simultaneously processing a batch of wafers, and single wafer implanters designed for processing single wafers one after the other. 
     FIG. 1  illustrates schematically a single wafer implanter incorporating a radio frequency (r.f.) linear accelerator assembly shown generally and schematically at  10 . In the arrangement shown in the simplified diagram of  FIG. 1 , the general construction of an implanter is shown also to comprise an ion source  11  directing a beam of ions at a predetermined energy E into an analyser magnet  12 . Only ions of the required velocity times mass/charge (m/e) ratio pass through a mass selection slit  13  at the exit of the analyser magnet  12 , and enter as a beam  14 , still at energy E, into the radio frequency accelerator assembly  10 . 
   The beam exiting the r.f. accelerator assembly  10  then enters a beam scanning device  15  which is arranged to scan the ion beam to and fro in a direction  16  transverse to the beam direction. The scanning device  15  may be either electrostatic or electromagnetic. Electromagnetic scanning systems are preferred in applications especially for high current beams. A suitable electromagnetic scanning system is disclosed in U.S. Pat. No. 5,393,984. 
   The scanned beam then enters a process chamber  17  in which a semiconductor substrate  18  is held on a holder  19 . The holder  19  is mounted on a mechanical scanning mechanism shown generally at  20  which can be actuated to reciprocate the wafer in a direction normal to the plane of the paper in FIG.  1  and across the plane of the scanned beam. The combination of scanning of the beam and mechanical scanning of the wafer holder  19  allows the beam to scan over all parts of the wafer during an implant process. Processed wafers are removed from the holder  19  and passed out of the process chamber  17 , and fresh wafers for processing are brought into the chamber  17  and mounted on the holder  19  one at a time, via a load lock  21 , and using robot handling mechanisms which are not shown in this drawing for simplicity. 
   Further details of single wafer implanters can be determined from U.S. Pat. Nos. 5,003,183 and 5,229,615, and of a preferred form of process chamber from U.S. Pat. No. 5,898,179. The specific details of the ion source, the mass selection magnet and the scanning and processing mechanisms of the implanter are not crucial to this embodiment of the present invention. 
   It should be understood that the invention is equally applicable to batch implanters, which typically rely solely on mechanical scanning to process a batch of semiconductor wafers simultaneously. The wafers are usually mounted around the periphery of a rotating wheel, which rotates to bring the wafers one by one across the line of the ion beam. Meanwhile, the axis of rotation of the wheel is reciprocated to and fro to complete the scanning in the orthogonal direction. 
   The earlier referenced U.S. Pat. No. 4,667,111 describes such a batch type implanter. Reference may also be made to U.S. Pat. No. 5,389,793 for further details of a typical batch type implanter. 
   Referring again to  FIG. 1 , the r.f. accelerator assembly  10  is schematically illustrated in the form of a two-stage accelerator assembly in which each stage  10   a ,  10   b  is a three-gap accelerator stage wherein an r.f. voltage of opposite polarity is applied from a respective source  22   a ,  22   b  to respective ones of the two centre electrodes of each stage. The two sources  22   a ,  22   b  are controlled from a control unit  1  so that the two sources are synchronised to accelerate ions through the assembly. 
   In the example illustrated, a buncher  23  is incorporated in front of the accelerator assembly  10  to form and deliver bunches of ions at the injection energy to the accelerator to increase the proportion of ions from the unbunched beam which may be accelerated by the accelerator assembly. Such bunchers are known, and generally produce a controlled energy spread in beam ions so that the ions become physically bunched on entry into the accelerator assembly. Known bunchers are designed to capture for bunching a maximum proportion of unbunched beam ions, without providing any overall increase in average energy to the bunched ions. In  FIG. 1 , the buncher  23  is illustrated as a two gap device having a central electrode energised from an r.f. supply  24 . The purpose and operation of bunchers is described in “Theory of Linear Accelerators”, by A. D. Vlasov, Chapter 2.5, published in English translation in 1968. 
   The r.f. accelerator assembly  10  is followed, along the beam direction, by an energy filter, illustrated generally in  FIG. 1  at  25 . The use of such an energy filter following an r.f. accelerator assembly in ion implanters is well known, see for example “Production of High Energy Ion Implanters Using Radio Frequency Acceleration” by Glavish et al, Nuclear Instruments and Methods in Physics Research, B21(1987) 264-269. The energy filter is used to limit the range of energies of ions from the accelerator which proceed to be implanted in the semiconductor substrate. 
   The energy filter may take any known form such as an electrostatic inflector or an analyser magnet. 
   Referring now to  FIGS. 2A and 2B , many components of the accelerator assembly illustrated are the same as described in U.S. Pat. No. 6,423,976. The ion beam from the analyser magnet enters the accelerator assembly from the left in the direction of arrow  30  and passes through the accelerator assembly generally along the line of an axis  31 . 
   The accelerator assembly is, as previously mentioned, formed by two, i.e. first and second, accelerator stages  10   a  and  10   b , also known as booster stages, each in the form of two three gap r.f. booster cavities in tandem and illustrated generally at  32  and  33 . It will be clearly understood by those conversant with the art, that an accelerator may be constructed with only a single accelerator stage or more than two, depending upon requirements. 
     FIGS. 2A ,  2 B are generally side sectional views of the accelerator assembly, parts of the outer walls of the vacuum chamber of that assembly having been broken away showing the location of the electrodes of the two acceleration stages represented by the cavities  32  and  33 . In  FIGS. 2A ,  2 B, inspection hatches  68 ,  69  which are provided for gaining access to electrodes within the assembly have also been shown with cover plates removed to show the positions of the electrodes. These electrodes and the general construction of the two acceleration stages are shown in section in  FIGS. 2A and 2B  initially and in greater detail in the ensuing Figures. 
   The booster cavity  32  has an entrance electrode  35  and an exit electrode  36  and the cavity  33  has entrance electrode  40  and exit electrode  41 . 
   These entrance and exit electrodes  35 , 36  are held at the same constant potential, usually ground potential. Between the electrodes  35  and  36  are the first and second r.f. electrodes  37  and  38  of the first stage of the accelerator assembly, and, between entrance and exit electrodes  40 ,  41 , the first and second r.f. electrodes  42 ,  43  of the second stage. 
   The r.f. electrodes  37  and  38  of the first stage  10   a  are mounted to be electrically insulated from the walls of the vacuum chamber, and it can be seen that the four electrodes  35  to  38  between them define three successive gaps along the beam direction  30 . As will become apparent, each of the electrodes  35  to  38  defines an aperture on the axis  31  through which the beam can pass. Generally speaking, the axis  31  can also be considered as the centre line of the ion beam as it passes through the accelerator assembly. As will also be explained later herein, as the beam travels across the gaps between the electrodes when the accelerator stage is operating and these electrodes are energised, ions in the beam are accelerated by an r.f. field in the gaps produced by r.f. voltages applied to the first and second electrodes  37  and  38 . 
   In the embodiment of the present invention as illustrated in  FIGS. 2A ,  2 B, the electrodes  35 ,  37 ,  38 ,  36  of the first stage of the illustrated assembly are mounted so as to be movable in order to move them from alignment with the beam path generally along the axis  31 , as shown in  FIGS. 2A ,  3 A,  4 A, to a position in which they are clear of the beam path, as shown in  FIGS. 2B ,  3 B,  4 B. Though hereinafter described in detail, it will be clearly visible from comparison of  FIG. 2A  with  FIG. 2B  for example that, in  FIG. 2A , the apertures in the electrodes  35 ,  37 ,  38  and  36  are all in line with the axis  31  whereas, from  FIG. 2B , it can be seen that all the electrodes have been lowered, so that all of the electrodes are clear of the axis  31  and of the beam path, together with other elements of the assembly, as described below. 
   The second accelerator stage  10   b  of the accelerator assembly shown in the Figures has a similar construction to the first stage  10   a , with the entrance and exit electrodes  40  (shown specifically in  FIGS. 2A ,  2 B) and  41  and intermediate r.f. electrodes  42  and  43 , defining between them three accelerating gaps along the beam direction  30 . The accelerator stages  10   a  and  10   b  lie in juxtaposed tandem relationship and the electrodes of both stages and their associated supporting structure, described below, are aligned. 
   The structure associated with each of the electrodes  37  and  38  of the accelerator stage  10   a  of the assembly shown in the Figures, for mounting the r.f. electrodes, comprises a respective conductor  45 ,  46  which leads out of the chamber enclosing the ion beam and into a resonant tank chamber  47 . Inside the tank chamber  47 , the conductors  45  and  46  are formed as coils  45   a ,  46   a  and are connected to ground. The combination of the electrodes  37  and  38 , the coils  45   a ,  46   a  in the tank chamber  47 , the grounded metal components of the vacuum chamber surrounding the electrodes  37 ,  38  and the tank chamber  47  itself, which is also connected to ground, forms a resonant tank circuit which is designed to be resonant at a desired operating frequency of the accelerator, typically in the range 10 to 50 MHZ; preferably the operating frequency is about 20 MHZ. 
   The interior of the resonant tank chamber  47  is open to the interior of the vacuum chamber containing the electrodes  37  and  38 , so that the interior of the tank chamber  47  is also at a vacuum. 
   The electrodes  42  and  43  of the second accelerator stage  33  of the accelerator assembly are also shown in  FIGS. 2A ,  2 B and are similarly connected by conductors  44 ,  49  to coils  44   a ,  49   a  within a similar resonant tank chamber  48  to chamber  47 . The tank circuit formed by the chamber  48 , the electrodes  42  and  43 , conductors  44 ,  49  and coils  44   a ,  49   a , is similarly arranged to have the same resonant frequency as the resonant cavity  32  of the first stage. 
   In operation of the assembly, r.f. power is supplied to the resonant circuits formed by the booster cavities of the two stages  32  and  33  with associated tank chambers  47  and  48 , so that the r.f. electrodes  37 ,  38  and  42 ,  43  are energised with opposite polarity at the resonant frequency. Bunches of ions from the ion beam along the axis  31  are then accelerated as they traverse the gaps between the electrodes in the two resonant cavities so as to emerge from the accelerator assembly with increased energy. 
     FIGS. 2A ,  2 B also illustrate the location of magnetic quadrupoles along the beam axis  31  at  50 ,  51 ,  52  and  53  in each of the two stages. Magnetic quadrupoles are used to control expansion of an ion beam and bring the beam back to a required focus or waist as it traverses the accelerator assembly. The magnetic quadrupoles  50  to  53  are used to control the expansion of the beam as it passes through the r.f. accelerator assembly. 
   The r.f. accelerator assembly may be constructed using a unitary block of metal as illustrated generally at  60  in  FIGS. 2A ,  2 B. The block  60  provides a housing (not shown) of the r.f. accelerator assembly whose interior is maintained under vacuum. 
     FIGS. 3A ,  3 B,  4 A,  4 B and  5  illustrate the construction of, and the mounting supports for the electrodes  40 ,  41 ,  42  and  43  of the second accelerator cavity  33  of the assembly of  FIGS. 2A ,  2 B. The arrangement of the electrodes  35 ,  36 ,  37  and  38  of the first cavity  32  is similar, except that the electrodes of the second cavity are longer in the direction of the axis  31  to allow for increase in the velocity of the ions in the beam. Each of the r.f electrodes  42  and  43  of the cavity  33  is mounted on a respective metal shaft  100 , typically of copper. The shafts  100  are themselves mounted within a rigid throat structure  403  which is rigidly connected to and movable with the main body  130  of the tank chamber. Thus, each of mounting shafts  100  is securely held in an insulator  70  which is itself rigidly connected across the opening defined by the throat structure  403 , as best seen in  FIGS. 3A and 3B . The insulator  70  holds the electrodes  42  and  43  rigidly aligned with the entrance and exit electrodes  40  and  41 , within the throat structure  403 . 
   The conductors  44  and  49  leading to the coils  44 A and  49 A within the tank chamber are connected to ends of the shafts  100  below the insulating member  70  by means of sliding fit connections  71  and  72  as illustrated. Each sliding fit connection incorporates a respective compressable annular interconnecting piece to ensure ohmic connection between the conductors  44 , 49  and the respective shafts  100  at the applicable r.f. frequencies. As can be seen in  FIGS. 4A and 4B , the insulating member  70  comprises a bar, e.g. of appropriate ceramic material, extending across the aperture of the throat structure  403 , generally in a direction parallel to the axis  31  of the accelerator assembly. The bar  70  provides openings on either side as illustrated in  FIGS. 4A and 4B , so that the interior of the tank chamber  130  is in free communication with the interior of the accelerator assembly. 
   The form and structure of the electrodes is (except as discussed above), generally the same as disclosed in U.S. Pat. No. 6,423,976. 
   The electrodes of the assembly disclosed in U.S. Pat. No. 6,423,976 are fixed in position, so that the apertures therethrough are permanently aligned with the beam path  31 . While this construction is completely satisfactory for producing an ion beam comprising high energy ions accelerated through the accelerator assembly, it is less suitable for lower energy ion beams which have to drift through the assembly when the electrodes are not energised. In this drift mode it is difficult to obtain the higher beam currents for implantation which are desirable. 
   In consequence, it has been appreciated by the inventors that it is necessary to address this issue if, indeed, an ion implanter comprising an accelerator assembly of the type with which this invention is concerned, i.e. a linear r.f. accelerator assembly, is to be truly multi-functional and be useful across a wide range of energies of ion implantation. The alternative to provision of a truly multi-functional instrument is to provide separate implanters, one for high energy ions and another for lower energy ions. Provision of separate implanters, however, is exceedingly costly. 
   In the above description, reference has only been made to mounting of the r.f. electrodes  37 ,  38 ,  42 ,  43 . However, the entrance and exit electrodes  35 ,  36 ,  40  and  41  are also mounted for movement with the r.f. electrodes, as will be apparent in the following description of the actuator for moving the electrodes. The arrangement for permitting movement of the electrodes, and indeed the entire tank circuit of each stage of the accelerator assembly, is shown in  FIG. 3A  onwards. 
   Turning then to  FIGS. 3  to  9 , the arrangement for permitting movement of the electrodes  42 ,  43  is required to permit maintenance of the rigidity of structure of the resonant tank and the coils within it and of the relationship to the electrodes. Thus, the means or arrangement permitting that movement must permit movement of the resonant tank together with the coils and the electrodes as a single rigid unit. In the ensuing description, it is to be understood that the construction and arrangement for the electrodes  37 ,  38  of the first accelerator assembly  32  is substantially the same as that described for the electrodes of the second assembly  33 . 
   The tank chamber  130  has an opening  130   a  in its uppermost section at which the tank chamber is secured to a platform  401  to form a vacuum seal therewith. The perimeter of the opening  130   a  is of L-shaped section to provide an internal shoulder  402  ( FIG. 3B ) to make a vacuum tight seal with the platform  401 . The platform  401  carries the rigid throat structure  403  which may be of circular cross section when viewed in plan. The structure  403  provides an annular sleeve portion  404  (FIG.  3 B), and is suspended beneath the accelerator block  60  in the manner and for the purpose hereinafter described. 
   The sleeve portion  404  has a uniform internal cross-section but its external surface is stepped at  405  (shown in  FIG. 6 ) and provides a shallow channel  406  which is intended to receive a ribbed edge  407  of a differentially pumped sliding seal in the form of an annular skirt  408  which extends entirely around the sleeve  404 . The ribbed edge  407  is located in the channel  406  and clamped in position by an exterior sheath  409  which forms a tight sliding fit on the exterior of the sleeve  404  and is, at its lower edge, provided with an exterior rebate  410 . The thickness of the sheath  409  and the dimensions of the rebate  410  are such that the ribbed edge  407  can be trapped in the rebate with the material of the skirt  408  tightly wedged and so trapped between the sheath  409  and the interior surface of the channel  406  to provide a seal. The material of the skirt is impermeable to the passage of gas therethrough. 
   The opposite end of the skirt  408  is similarly formed to provide an annular ribbed edge  411  trapped in an annular collar  412  which is arranged to envelop the sleeve  404  and the skirt  408  and permit movement thereof. 
   A rectangular frame member  413  is secured to and mounted beneath the block  60 . As can be seen from the plan view of the frame member  413  shown in  FIG. 9 , the frame member is rectangular in shape having a cutout  413   a  formed therein at one end. (The platform  401  is of substantially similar dimensions to the frame member when viewed in plan.) 
   A fixed collet portion  414  ( FIG. 6 ) of the frame member  413  defines an aperture in the frame member  413  and extends through the wall of the block  60 . The block  60  is in sealed vacuum tight engagement with the frame member  413 . 
   The collet portion  414  has a downwardly extending flange portion  415  which, when each tank chamber is raised so that the electrode apertures are aligned with the beam path, has a lower annular face  416  that abuts against the upper surface of the platform  404  to thereby define the upper limit of movement of the tank chamber and the electrodes. 
   The collet portion  414  is formed with a first annular body portion  417  which at its lower end provides the flange portion  415 , the flange portion  415  extending around an annular recess  418  formed in the lower end face of the body portion  417  and whose purpose is described below. 
   Around a waist portion of the first annular body portion  417  is formed an annular recess  420 , whose function will also be described below. 
   Internally of the body portion  417  is the annular collar  412  which has a sealing sleeve  419  thereon which forms a vacuum tight sliding fit within the body portion  417 . The collar  412  has an annular shoulder  422  formed internally at its lower end portion and, at its opposite, upper, end portion, which is of reduced internal diameter relative to the main part of the body portion  417 , provides an upper end face  424 . 
   The upper end face  424  abuts against an annular surface  423  provided by a first internal annular overhang  426  formed at the top end of the body portion  417 , and with a second internal annular overhang  427  defines an undercut annular channel  428 , whose purpose will be described shortly. 
   The collar  412  is held in position, when the arrangement is assembled, by an annular clamping ring  429  which seats in the annular recess  418  and is fastened to the body portion  417  by bolts  430 . The clamping ring  429  has an annular neck portion  432  which, with the shoulder  422  of the collar  412 , defines an undercut channel  433  within which the second, ribbed, edge  411  of the differential pressure seal can be secured. 
   As is visible in each of  FIGS. 2A ,  3 A,  4 A and  5  but which can be best seen from  FIG. 6 , an annular gap exists between the collar  412  and the external surface of the annular sleeve portion  404  to permit movement of the skirt  408  when the sleeve portion  404  moves within the liner  421 . The seal provided by the skirt  408  is entrained by its upper and lower annular ribbed edge portions  407  and  411  with one edge portion  407  sealingly secured in and to the slidable sleeve  404  and the other ribbed edge portion  411  sealingly secured to the fixed part of the arrangement and thus to the fixed structure of the block  60  of the linear accelerator assembly. As the tank chamber structure, comprising the tank chamber  48 , electrodes  44 ,  49  and coils  44   a ,  49   a , is raised or lowered, as hereinafter described, the ribbed edge portion  407  of the seal is also raised and lowered relative to the opposite ribbed edge portion  411 , thereby causing the seal to fold and unfold between the position shown in FIG.  4 A and the position shown in  FIG. 4B , so that effectively, the skirt inverts and turns itself inside out. 
   Above the first seal provided by the skirt  408  is a second similar annular seal generally indicated at  438  comprising a second skirt  441  having ribbed edge portions  442 ,  444 . One edge portion  442  is engaged in the undercut annular channel  428 , defined between the annular surface  423  and an annular portion  443  of the columnar structure  403 , during assembly, and the other edge portion is engaged between an upper flange portion  446  of the sheath  409  and a shouldered annular end portion  448  of a second sheath  450  which is fitted onto the exterior of the sleeve  408  so as to define a channel  452  between the flange portion  446  and the sheath  409 . The annular portion  443  provides a guide for the skirt  441 , between which and the sheath  409  exists an annular gap similar to that within the collar  412 . 
   The second seal  438  is identical to the first seal  436  and is constrained to be flexed and to move in the same manner and with the same degree of motion as the first seal. 
   The integral columnar structure  403  of the platform and the sheaths  409  and  450  can slide freely within the confines of the annular body portion  417  of the frame member  413  and are entrained to do so by the mechanism which controls movement of the platform  401  up to and away from the frame member  413  to raise and lower the electrodes rigidly mounted from the floor of the tank chamber. 
   From the arrangement just described, it can be seen that the two seals ensure that there is completely sealed engagement between the tank chamber and the platform  401  on which it is mounted, between that platform  401  and the frame member  413  mounted under the block  60  and between that frame member  413  and the block  60 , thus enabling the reduced pressure, or vacuum, maintained within the accelerator to also be maintained within the tank chamber while permitting movement of the tank chamber, to thereby move the electrodes into and out of the path of an ion beam passing through the accelerator. 
   Reference was made above to the entrance and exit electrodes  35 ,  36 ,  40 ,  41  of the two stages  10   a  and  10   b  and to the fact that these electrodes are moved with the r.f. electrodes  37 ,  38 ,  42 ,  43 . To this end, as can be seen from  FIG. 5  in particular, the second sheath  450  is of a height (or length measured perpendicular to the axis  31 ) such that, when it is mounted on the sleeve portion  404  in abutment with the sheath  409 , its upper end face is coplanar horizontally with the upper end face of the annular sleeve  404 . These coplanar end faces provide a seat for supporting the entrance and exit electrodes  40 ,  41  (and similarly electrodes  35 ,  36 ). Electrodes  40 ,  41  are shown most clearly in FIG.  5 . Each electrode has a seating surface  453  and a leg portion  454  whereby the electrode can be seated on and braced against an inner surface of the annular sleeve  404 . The two electrodes  40 ,  41  are, when the assembly is being assembled, aligned with the r.f. electrodes  42 ,  43  and then fixed in position by screw fastenings or the like (not shown). As an alternative to screw fastening, the annular sleeve  404  may be formed with rebated slots, each to accommodate a correspondingly shaped portion of the leg portion  454  and thereby retain the respective electrode in situ. Whatever the manner of mounting these entrance and exit electrodes, it is important that they should be as readily demountable as the r.f. electrodes  42 ,  43  when it becomes necessary to replace them. 
   From the above description, it can be seen that the platform can be moved vertically as shown in FIG.  6  and that this movement, up or down as the case may be, causes the two skirts  408  and  441  to ‘peel’ and ‘unpeel’ as the platform  401  is moved relative to the block  60  and the frame member  413  mounted therebeneath. 
   The uppermost and lowermost positions of the platform and the associated tank circuits are shown most clearly in  FIGS. 2A ,  3 A,  4 A and  2 B,  3 B,  4 B respectively, as viewed in side sectional elevation. 
   The manner of mounting the tank chamber  47  to permit movement thereof, while maintaining the sealed relationship between the tank circuits and the interior of the assembly has been explained. However, in practice, and as shown in  FIGS. 2A ,  2 B, the tank chambers  47 ,  48  are mounted in tandem. For each of the two stages, the manner of mounting each tank chamber is the same. 
   In an alternative embodiment of the present invention, one or both of the skirts  408 ,  441  may be replaced by a bellows arrangement where upper and lower edge portions of such a bellows are entwined between the frame member  413  and/or the block  60  on the one hand and the movable platform  404  on the other, in sealing engagement therewith to maintain a vacuum within the respective tank chamber  48 ,  49 . 
   As a further alternative, a seal can be maintained between the block  60 /frame member  413  and the platform  404  and respective tank chamber by a telescopic concentric sleeve arrangement in which one sleeve, or a collar, is mounted in sealed engagement on the frame member  413 /block  60 , and a further sleeve mounted in sealed engagement on the platform  404  can slide in sealed relationship to that mounted on the frame member or block in telescopic fashion, with, if necessary, one or more intermediate concentric telescopic sleeves therebetween, also in sealed engagement with the inner and outer sleeves. 
   To provide an accelerator having as great a flexibility of use as possible, it is also useful to be able to move one set of electrodes independently of the other. 
   In  FIGS. 7A ,  7 B,  8 A,  8 B and  9 , there is shown the mechanism for moving one or both tank circuits of the illustrated accelerator, between a first position in which the electrode apertures of one or both stages of the assembly are aligned with the beam path, and a second position in which all of the electrodes are themselves entirely clear of the beam path. In the disclosed embodiment, each set of electrodes is movable independently of the other, though, as will be explained later, with this same arrangement, it is also possible to move both sets of electrodes together. 
   Four vertically downwardly extending shafts  460  and two spindles  460   a  are mounted so as to depend from the frame member  413 , the shafts  460  being mounted for rotation in bearings (not shown) in the rectangular frame member  413 , with one shaft at or adjacent each corner of the frame. 
   Each shaft  460  and each spindle  460   a  has mounted thereon a toothed pulley wheel or sprocket wheel  461 , and all of the wheels are mounted so that they lie in a common horizontal plane parallel to the axis  31 . Hereinafter, these will be referred to for the sake of clarity as wheels though it is to be clearly understood that any suitable form of rotatable element capable of co-operating with an endless drive belt (or chain or the like) is meant by the term wheel, including gear wheels and friction rollers for example. An endless drive belt  462  extends under tension around the six wheels  461  as shown in dotted line in  FIG. 9 ; though not shown, between adjacent wheels, one or more spring-biased idler wheels can be provided to guide the belt and maintain it under correct tension. 
   One of the four ‘corner’ shafts,  460   b , is coupled by gearing  463  to a reversible drive motor  464  for driving the belt in one direction or the other. Each of the four shafts  460 , including the shaft  460   b , is externally-threaded along its upper length and extends, in threaded engagement, through a respective internally-threaded insert  465  secured to the underside of the frame member  413 . Each shaft passes through a cup  466  which is secured to the underside of the platform  401  from which the tank chamber  47  also depends, and each shaft has a central part  467  of enlarged diameter which is seated within the cup and holds the respective shaft in situ relative to the platform. The four shafts  460 , including shaft  460   b , are, of course, similarly threaded. By rotating the shaft  460   b  and thus the shafts  460  via the drive belt transmission  462 , the platform  401  can be raised or lowered as required relative to the frame member  413 , thereby to move the electrodes, associated coils and tank chamber. 
   The accelerator assembly illustrated in the Figures comprises two sets of electrodes and thus associated coils and tank chambers. It is quite feasible that both units (i.e. electrodes, coils, tank chambers) can be driven together from a single drive motor with a single transmission drive belt extending around and in engagement with all of the wheels  461 . However, to provide greater flexibility of operation, it may be required to raise/lower only one set of electrodes at a time and, for this purpose, separate drives are provides for the two units. 
   It is then very simple to combine and co-ordinate the operation of the drives simply by controlling the power supply to the two motors. 
   As described herein, each set of electrodes is moved into alignment with the beam path or clear of the beam path by rotation of the threaded shafts  460  and consequent vertical movement of the platform up or down those shafts according to the direction of rotation thereof. 
   As an alternative to such an arrangement, a further embodiment of the invention employs fixed externally-threaded shafts and internally-threaded sprockets or toothed pulley wheels which can be driven from a drive sprocket or pulley wheel which is coupled to the output shaft of a drive motor so that rotation thereof causes rotation of the threaded sprockets or pulley wheels to move them up or down the fixed threaded shafts as required. As with the above illustrated embodiment, the sprockets/pulley wheels and drive sprocket would all be mounted on a moveable frame, platform or housing which supported the tank circuit and thus the electrodes. With such an arrangement, it would be possible to mount the threaded shafts directly on the accelerator block instead of in a frame member below the block. 
   As further alternatives, movement of the tank circuit and thus of the electrodes may also be effected by hydraulic or pneumatic arrangements, whereby a plurality of pistons or cylinders are mounted to raise and lower the tank circuit relative to the beam path with the pistons or the cylinders coupled to the tank circuit, and the co-operating member fixed relative to the beam path. 
   As a still further alternative manner of effecting movement of the electrodes into alignment with and clear of the beam path, and in contrast to the above-described solutions, the electrodes may be pivotally moved out of alignment with the beam path, although such pivotal movement would normally only be considered where the structural relationship and disposition of the electrodes and remaining elements of the tank circuit or its equivalent could be sustained. 
   In  FIG. 10 , there is shown a schematic circuit diagram of an r.f. accelerator assembly according to the invention. this circuit, except for the following description, is fully described in U.S. Pat. No. 6,423,976 which is incorporated herein by reference and will not therefore be further described herein. 
   The implant process as a whole is controlled by a micro processor based implant controller  290 . The implant controller may control a number of operating parameters of the implanter but for the purposes of illustrating the present invention, the controller  290  is shown as connected to control circuits  470  which control the operation of the motors  464  to raise and lower the platforms  401 . As can be readily appreciated from the foregoing description, the control circuits can be operated from the implant controller  290  independently of each other or simultaneously so that one or other or both of the platforms can be moved. 
   In the preferred embodiment, the r.f. accelerator assembly as illustrated has dimensions which are similar to those of the assembly disclosed in the aforementioned U.S. Pat. No. 6,423,976.