Abstract:
This invention relates to a method of implanting ions in a substrate using an ion beam where instabilities in the ion beam may be present and to an ion implanter for use with such a method. This invention also relates to an ion source for generating an ion beam that can be switched off rapidly. In essence, the invention provides a method of implanting ions comprising switching off the ion beam when an instability has been detected whilst continuing motion of the substrate relative to the ion beam to leave an unimplanted portion of a scan line across the substrate, establishing a stable ion beam once more and finishing the scan line by implanting the unimplanted portion of the path.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a method of implanting ions in a substrate using an ion beam where instabilities in the ion beam may be present and to an ion implanter for use with such a method. This invention also relates to an ion source for generating an ion beam that can be switched off rapidly.  
       BACKGROUND OF THE INVENTION  
       [0002]     Ion implanters are well known and generally conform to a common design as follows. An ion source produces a mixed beam of ions from a pre-cursor gas or the like. Only ions of a particular species are usually required for implantation into a substrate, for example a particular dopant for implantation into a semiconductor wafer. The required ions are selected from the mixed ion beam using a mass-analysing magnet in association with a mass-resolving slit. Hence, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit to be transported to the process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder.  
         [0003]     Frequently, an ion beam used for implantation has a smaller cross-sectional area than the substrate to be implanted. In order to ensure ion implantation across the whole of the substrate, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface. This may be achieved by (a) deflecting the ion beam to scan across the substrate which is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed, or (c) deflecting the ion beam and moving the substrate.  
         [0004]     Substrates are generally implanted serially one after another or as a batch at one time: for serial processing, relative motion between ion beam and substrate is effected such that the ion beam traces a raster pattern on the substrate surface by scanning to and fro across the substrate to form a series of parallel, equally-spaced scan lines; for batch processing, the substrates are held on spokes of a rotating wheel such that the ion beam scans across each substrate in a series of scan lines that form adjacent arcs.  
         [0005]     To achieve uniform implantation, there must be adequate overlap between adjacent scan lines. Put another way, if the spacing between adjacent scan lines (with respect to the ion beam width profile) is too great, “striping” of the substrate will result with periodic bands of increased and decreased doping levels.  
         [0006]     The precautions described above cannot be effected if the ion beam incident on the substrate is not itself uniform over time. Unfortunately, instabilities of the ion beam are inevitable and result from discharges in the ion source area for example. The effect of these instabilities is that there is a “glitch” in the ion beam in that the flux will usually drop significantly within a short period of time. The drop in ion beam flux leads to areas of the semiconductor wafer receiving a lower level of doping that may lead to the production of faulty semiconductor devices. More unusually, a rapid rise is seen in the ion beam flux. Again, this produces incorrect dosing that may lead to faulty devices.  
         [0007]     The above problem is particularly severe for serial processing ion implanters that use mechanically scanned substrate holders, as will now be explained. To create the raster pattern, the substrate holder is moved in a reciprocating fashion and there is a limit to the maximum speed at which this can be done. To date this has been far lower than the scanning speeds that can be achieved with rotating batch substrate holders. Fast scan speeds require the ion beam to make many passes over the substrate to achieve a desired dosing: any instability in the beam during a single pass leads to a small residual dosing error due to dilution by the many subsequent passes. The adverse effects are far more severe in serial processing where the slow scan speeds result in fewer passes to achieve the same dosing.  
         [0008]     The problem of ion beam instabilities has been addressed previously, see The Ion Beam Optics of a Single Wafer High Current Ion Implanter by White et al., Proceedings of the Eleventh International Conference on Ion Implantation Technology, North Holland (1997), pages 396-399. However, this disclosure is made in the context of high-current implantation using a ribbon beam (i.e. a beam with a width wider than the substrate such that scanning in effected in the direction perpendicular to the beam&#39;s width only rather than with two-dimensional, mechanical scanning). Upon detecting a beam instability during a scan, the ion beam is gated off for the rest of the scan. The scan is then repeated in the reverse direction and the ion beam gated off once more upon reaching the position corresponding to where the instability had been detected.  
         [0009]     Hence, there is a demand for methods of addressing the problem of ion beam instability such that a uniform dosing of a substrate can be achieved, particularly for systems using an ion beam of a smaller size than the substrate and also for mechanically-scanned implantation.  
       SUMMARY OF THE INVENTION  
       [0010]     According to a first aspect, the present invention resides in a method of implanting ions in a substrate using an ion beam having cross-sectional dimensions smaller than the substrate comprising the steps of: (a) establishing a stable ion beam with the substrate clear of the ion beam; (b) implanting the substrate by causing relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along at least one path; (c) monitoring the ion beam for instabilities during step (b); (d) upon detecting an ion beam instability, switching off the ion beam as the relative motion continues to leave an unimplanted portion of the path; (e) recording an off position corresponding to the ion beam&#39;s position relative to the substrate when the ion beam is switched off in step (d); (f) establishing a stable ion beam once more; and (g) continuing to implant the substrate by causing relative motion between the ion beam and the substrate along the unimplanted portion of the path.  
         [0011]     Extinguishing the ion beam upon detecting an instability is advantageous as it stops implantation and thus avoids creating an area of non-uniform implantation in the substrate.  
         [0012]     Recording the off position is beneficial as it allows control of further implantation to ensure uniform dosing of the substrate. The off position may be recorded when an action is taken to switch off the ion beam (e.g. interrupting power to an ion source). If this is done, it is clearly advantageous for the ion beam to be switched off rapidly. Where there is a known latency in switching off the ion beam, the off position may be recorded as the position where the action is taken to switch off the ion beam plus the distance corresponding to this latency.  
         [0013]     Alternatively, the ion beam flux may be monitored and the off position may be recorded when the ion beam flux is zero or drops below a threshold. Clearly, the phrase “recording an off position corresponding to the ion beam&#39;s position relative to the substrate when the ion beam is switched off” can be construed to cover these possibilities.  
         [0014]     In addition, a profile of the ion beam may be taken to identify any changes in beam shape of movements in the centre of the beam. Any changes identified may be corrected by tuning the beam or by slightly altering the position of the beam as it follows the path.  
         [0015]     The relative motion may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern.  
         [0016]     The relative motion between ion beam and substrate is preferably controlled to ensure the same dosing as for the previously implanted portion of the path. For example, the same relative speed should be used if the ion beam has the same flux as before it was extinguished. If a difference in ion beam flux is determined, the relative speed may be adjusted to ensure the same dosing (i.e. the relative speed may be measured in response to an increase in ion beam flux).  
         [0017]     According to one embodiment, step (f) comprises establishing a stable ion beam with the substrate clear of the ion beam prior to step (g); step (g) comprises causing relative motion between the ion beam and the substrate such that the ion beam travels along said path in a reverse direction, that is in an opposite direction as for step (b); and switching off the ion beam when the ion beam crosses the off position.  
         [0018]     Restarting the ion beam clear of the substrate avoids non-uniformities in implanting as the ion beam settles to a stable flux. In addition, extinguishing the ion beam can be performed rapidly and so the drop in dosing concentration is abrupt. Moreover, the exact timing of switching the ion beam off as it reaches the off position can be adjusted to optimise overlap of any short tailing-off regions where the ion beam is extinguished. As the ion beam is scanned in the reverse direction, the overlap of the tailing-off regions complement each other to give the desired uniformity.  
         [0019]     According to a second embodiment, step (g) further comprises switching the ion beam on at the off position prior to the ion beam traversing the unimplanted portion of said path in the forward direction, that is the same direction as for step (b). Preferably, step (g) comprises causing relative motion between the ion beam and substrate in the forward direction from a point along said path such that the ion beam is switched on upon crossing the off position. After starting the ion beam, there is a brief period where the ion beam flux increases to its stable value. This behaviour can be determined and the operation of the ion implanter adjusted to ensure the tailing-off region where the ion beam was extinguished complements the ramping-up region where the ion beam is restarted to give uniform dosing. The exact timing of when the relative speeds of ion beam and substrate can be adjusted to provide uniform dosing.  
         [0020]     Where recovery is performed by scanning in the reverse direction, the method may further comprise repeating steps (c), (d) and (e) during step (g) such that, if a second beam instability is detected, a central portion of said path is not implanted; and continuing to implant the substrate once more by causing relative motion between the ion beam and the substrate such that the ion beam travels across the substrate along the central portion of said path. Preferably, the method comprises the steps of commencing the relative motion along said path outside of the central portion, switching the beam on when first crossing an off position and switching the beam off when crossing the other off position. As will be appreciated, this dosing may be performed in either direction.  
         [0021]     From a second aspect, the present invention resides in a method of implanting ions in a substrate held in a substrate holder moveable bidirectionally along a first axis of translation, the method comprising the steps of: (a) establishing a stable ion beam having cross-sectional dimensions smaller than the substrate with the ion beam clear of the substrate in a start position adjacent the substrate along the first axis; (b) implanting the substrate by moving the substrate holder along the first axis such that the ion beam transverses the substrate along a first scan line and continues until clear of the substrate; (c) causing relative motion between the ion beam and the substrate holder along a second axis; (d) repeating steps (b) and (c) to implant a series of scan lines across the substrate; (e) monitoring the ion beam during implantation in step (b) and as repeated according to step (d); (f) upon detecting an ion beam instability, switching off the ion beam as the relative motion continues to leave an unimplanted portion of the scan line; (g) recording an off position corresponding to the position of the substrate holder when the ion beam is switched off in step (f); (h) establishing a stable ion beam once more; (i) completing implantation of the scan line by moving the substrate holder along the first axis so that the ion beam scans over the unimplanted portion of the scan line; and (j) completing implantation of the substrate by repeating steps (b) and (c) to complete the series of scan lines across the substrate.  
         [0022]     Movement along the first axis may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern. The movement may be in one direction along the first axis or may be in both directions along the first axis.  
         [0023]     Preferably step (c) comprises translating the substrate holder along a second axis of translation relative to a fixed ion beam, the first and second axes being perpendicular. Alternatively, the ion beam may be deflected along such a second axis.  
         [0024]     From a third aspect, the present invention resides in an ion implanter controller for an ion implanter operable to generate an ion beam for implanting into a substrate, the controller comprising: ion beam switching means operable to cause the ion beam to switch on and off; scanning means operable to cause relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along at least one path; ion beam monitoring means operable to receive a signal indicative of the ion beam flux and to detect instabilities in the ion beam therefrom during said relative motion; and indexing means operable to determine the position of the ion beam relative to the substrate during said relative motion; wherein the controller is arranged such that: the ion beam switching means is operable to cause the ion beam to switch off during the relative motion when the ion beam monitoring means detects an instability in the ion beam to leave an unimplanted portion of the path; the indexing means records an off position of the ion beam relative to the substrate when the ion beam is switched off; the ion beam switching means is operable to cause the ion beam to switch on once more; and the scanning means is operable to cause relative motion between the ion beam and the substrate such that the ion beam traverses the substrate along the unimplanted part of the path.  
         [0025]     The ion implanter controller may be embodied in hardware or software form, i.e. parts of the controller may be implemented electronically or using software provided on a computer or the like. In fact, a part-hardware and part-software implementation could be followed where some parts are based on electronic components and others are based in software.  
         [0026]     Movement along the first axis may form a series of scan lines that extend in parallel and the scan lines may, optionally, form a raster pattern. The movement may be in one direction along the first axis or may be in both directions along the first axis.  
         [0027]     From a fourth aspect, the present invention resides in an ion implanter for implanting a substrate using an ion beam, including the controller described herein above.  
         [0028]     From a fifth aspect, the present invention resides in an ion source for an ion implanter comprising: a cathode; an anode; biasing means for biasing the anode relative to the cathode; a first switch; and a first electrical path connecting anode to cathode via the biasing means and switch arranged in series; wherein the first switch is operable to make or break the first electrical path. This simple arrangement rapidly isolates the biasing means that otherwise biases the anode relative to the cathode. Hence, an ion beam may be rapidly extinguished when an instability is detected.  
         [0029]     Optionally, the ion source further comprises a second conductor path connecting anode to cathode with at least a portion that extends in parallel across the biasing means, the portion comprising a second switch operable to make or break the second electrical path. Preferably, the first switch is operable in response to a first binary switching signal and the second switch is operable in response to a second binary switching signal that is the complement of the first switching signal. This allows a convenient way of switching the potential of the anode to be biased either relative to the cathode or at the same potential as the cathode. When a potential difference exists, an ion beam is produced: when no potential difference exists, there is no ion beam.  
         [0030]     Preferably, the first switch and/or any second switch is a power semiconductor switch as this allows particularly rapid switching and hence particularly rapid extinction or creation of an ion beam.  
         [0031]     The present invention also extends to an ion implanter including the ion source described herein above and to a method of switching such an ion source comprising the step of operating the first switch to break the first electrical path in response to detection of an instability in the ion beam generated by the ion source.  
         [0032]     This method may be accompanied by the steps of maintaining or increasing the power supplied to the cathode. For example, the ion source may comprise an indirectly heated cathode and three power supplies: a filament supply (for the cathode&#39;s filament), a bias supply (for biasing within the indirectly heated cathode) and an arc supply (for biasing the anode relative to the cathode). Power supplied by the filament supply and the bias supply may be maintained, or may be increased to match the power of the arc supply prior to operating the first switch. This is to minimise any cooling in the ion source, and in the cathode in particular, when arc discharging ceases. Indirectly heated cathodes comprise a filament in front of an end cap. Increasing power supplied by the filament supply generates more electrons to be accelerated into the end cap, whilst increasing the power supplied by the bias supply increases the energy with which the electrons strike the end cap: in either case, the cathode enjoys greater heating from the electrons to compensate for the heating otherwise provided by the arcing.  
         [0033]     Other preferred features of the invention are set forth in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     Examples of the invention will now be described with reference to the accompanying drawings, in which:  
         [0035]      FIG. 1  is a schematic view of an ion implanter having a wafer holder for serial processing of wafers;  
         [0036]      FIG. 2  is a simplified representation of an ion source for use in an ion implanter showing the power supply units used for biasing various parts of the ion source;  
         [0037]      FIG. 3  shows a raster scan of an ion beam across a wafer adopted in serial processing;  
         [0038]      FIGS. 4   a  to  4   d  show an ion beam scanning scheme according to a first embodiment of the present invention for use during ion implantation where a glitch in the ion beam is detected;  
         [0039]      FIGS. 5   a  to  5   d  correspond to  FIGS. 4   a  to  4   d  but for a second embodiment of the present invention;  
         [0040]      FIGS. 6   a  to  6   d  correspond to  FIGS. 4   a  to  4   d  but shows a case where two glitches in the ion beam occur in the same scan line;  
         [0041]      FIG. 7  is a schematic view of an ion implanter including a first embodiment of a return current monitor;  
         [0042]      FIG. 8  is a schematic view of an ion implanter including a second embodiment of a return current monitor; and  
         [0043]      FIG. 9  corresponds to  FIG. 2  but shows a modification of the arc power supply unit arrangement. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0044]      FIG. 1  shows a typical ion implanter  20  comprising an ion beam source  22  such as a Freeman or Bernas ion source that is supplied with a pre-cursor gas for producing an ion beam  23  to be implanted into a wafer. The ions generated in the ion source  22  are extracted by an extraction electrode assembly. The flight tube  24  is electrically isolated from the ion source  22  and a high-tension power supply  26  supplies a potential difference therebetween.  
         [0045]     This potential difference causes positively charged ions to be extracted from the ion source  22  into the flight tube  24 . The flight tube  24  includes a mass-analysis arrangement comprising a mass-analysing magnet  28  and a mass-resolving slit  32 . Upon entering the mass-analysis apparatus within the flight tube  24 , the electrically charged ions are deflected by the magnetic field of the mass-analysis magnet  28 . The radius and curvature of each ion&#39;s flight path is defined, through a constant magnetic field, by the mass/charge ratio of the individual ions.  
         [0046]     The mass-resolving slit  32  ensures that only ions having a chosen mass/charge ratio emerge from the mass analysis arrangement. In fact, the ion source  22  and mass analysing magnet  28  are rotated through 90° when compared to the arrangement of  FIG. 1 , so that the ion beam  23  would initially travel perpendicular to the plane of the paper. The ion beam  23  is then turned by the mass-analysing magnet  28  to travel along the plane of the paper. Ions passing through the mass-resolving slit  32  enter a tube  34  that is electrically connected to and integral with the flight tube  24 . The mass-selected ions exit the tube  34  as an ion beam  23  and strike a semiconductor wafer  36  mounted upon a wafer holder  38 . A beamstop  40  is located behind (i.e. downstream of) the wafer holder  38  to intercept the ion beam  23  when not incident upon the wafer  36  or wafer holder  38 . The wafer holder  38  is a serial processing wafer holder  38  and so only holds a single wafer  36 . The wafer holder  38  is operable to move along X and Y axes, the direction of the ion beam  23  defining the Z axis of a Cartesian coordinate system. As can be seen from  FIG. 1 , the X axis extends parallel to the plane of the paper, whereas the Y axis extends into and out from the plane of the paper.  
         [0047]     To maintain the ion beam current at an acceptable level, an ion extraction energy is set by a regulated high-tension power supply  26 : the flight tube  24  is at a negative potential relative to the ion source  22  by virtue of this power supply  26 . The ions are maintained at this energy throughout the flight tube  24  until they emerge from the tube  34 . It is often desirable for the energy with which the ions impact the wafer  36  to be considerably lower than the extraction energy. In this case, a reverse bias voltage must be applied between the wafer  36  and the flight tube  24 . The wafer holder  38  and beamstop  40  are contained within a process chamber  42  that is mounted relative to the flight tube  24  by insulating standoffs  44 . Both the beamstop  40  and wafer holder  38  are connected to the flight tube  24  via a deceleration power supply  46 . The beamstop  40  and wafer holder  38  are held at a common ground potential so that, to decelerate the positively-charged ions, the deceleration power supply  46  generates a negative potential with respect to the grounded wafer holder  38  and beamstop  40  at the flight tube  24 .  
         [0048]     In some situations, it is desirable to accelerate the ions prior to implantation in the wafer  36 . This is most easily achieved by reversing the polarity of the power supply  46 . In other situations, the ions are left to drift from flight tube  24  to wafer  36 , i.e. without acceleration or deceleration. This can be achieved by providing a switched current path to a short out the power supply  46 .  
         [0049]     Turning now to  FIG.2 , a typical ion source  22  is shown along with its associated power supply units. The ion source  22  comprises an ion source chamber  48  enclosed by chamber walls  50 . The ions are produced in a plasma by emitting electrons from a cathode  52  located within the ion source chamber  48  and by biasing the chamber walls  50  to form an anode. In this ion source  22 , an indirectly heated cathode  52  is used.  
         [0050]     The indirectly heated cathode  52  comprises a filament  54  supplied by a filament power supply unit  56 . The filament supply  56  provides sufficient current to cause thermionic emission of electrons from the filament  54 . The indirectly heated cathode  52  also comprises a tube  58  enclosing the filament  54  that is connected across a bias power supply unit  60  such that the tube  58  is at a positive potential relative to the filament  54 . This ensures that electrons emitted by the filament  54  are attracted and accelerated into the end-cap of the tube  58 . The impacts of the electrons heat the end-cap of the tube  58  such that it emits electrons into the ion source chamber  48 .  
         [0051]     The chamber walls  50  are held at a positive potential relative to the tube  58  by virtue of their connection to an arc power supply unit  62 . Accordingly, electrons emitted by the tube  58  are attracted to the chamber walls  50 . In fact, the motion of the electrons emitted from the cathode  52  is constrained by creating a magnetic field across the ion source  22  using a pair of coils of an associated electromagnet (not shown). The magnetic field created is such that electrons emitted by the cathode  52  follow a spiral path towards the far end of the ion source chamber  48 .  
         [0052]     Located at this far end is a counter-cathode  64  also connected to the bias supply  60  so as to be at the same potential as the tube  58  of the indirectly heated cathode  52 . Accordingly, electrons approaching the counter-cathode  64  are repelled such that they travel back along the spiral path in a reverse direction. This increases the chances of electrons interacting with the pre-cursor gas that fills the ion source chamber  48  thereby creating more ions that may be extracted through an aperture  66  provided in the chamber walls  50  to form the ion beam  23 .  
         [0053]     As described previously, the wafer holder  38  can be moved along the X and Y axes. Movement of the wafer holder  38  is controlled such that the fixed ion beam  23  scans across the wafer  36  according to the raster pattern  68  shown in  FIG. 3 . Although the wafer  36  is scanned relative to a fixed ion beam  23 , the raster pattern  68  of  FIG. 3  is equivalent to the ion beam  23  being scanned over a stationary wafer  36  (and this method is in fact used in some ion implanters). As imagining a scanning ion beam  23  is more intuitive, the following description will follow this convention although in fact the ion beam  23  is stationary and it is the wafer that is scanned.  
         [0054]     The ion beam  23  is scanned over the wafer to form a raster pattern of parallel, spaced scan lines  70 . This is achieved by scanning the ion beam  23  forwards along the X-axis direction to form the first scan line  70  until the ion beam is clear of the wafer  36 , moving the ion beam  23  up along the Y-axis direction as shown at  72 , scanning the ion beam  23  backwards along the X-axis direction until clear of the wafer  36  once more, moving the ion beam  23  up along the Y-axis direction  72 , and so on until the whole wafer  36  has seen the ion beam  23 .  
         [0055]     During scanning of the ion beam  23  across the wafer  36 , the ion beam current is measured such that any glitches in ion beam flux can be detected. A detailed description of how the ion beam current is measured and the conditions that correspond to a glitch follows later. As scanning is performed by moving the wafer holder  38  in a controlled manner, the position of the ion beam  23  relative to the wafer  36  is known at any instant. Hence, the position of the ion beam  23  on the wafer  36  at the instant a glitch is detected or at the instant the ion beam  23  is turned off may be determined.  
         [0056]      FIG. 4   a  shows the initial stages of a raster scan  68  formed during implantation. Seven complete scan lines  70  have been formed on the wafer  36 . However, a glitch in the ion beam  23  is detected during the eighth scan line  74 . The ion implanter  20  responds to detection of the glitch by extinguishing the ion beam  23  as rapidly as possible. Extinguishing the ion beam  23  results in the ion beam  23  switching off at the position shown in  FIG. 4   a  at  76  and this position is duly recorded as an “off” position with reference to the known position of the wafer holder  38 .  
         [0057]     Movement of the wafer holder  38  continues along the scan line when and after the ion beam  23  is extinguished such that the ion beam  23 , were it still switched on, would follow the remainder of the current scan line in a forward direction to end beyond the far side of the wafer  36  at the position  79  (this movement is shown by the dashed line  78  in  FIG. 4   b ). In FIGS.  4  to  6 , a solid line denotes movement of the wafer holder  38  with the ion beam  23  switched on whereas a dashed line denotes movement of the wafer holder  38  with the ion beam  23  switched off.  
         [0058]     In this position  79 , the ion beam  23  is switched on once more and is monitored to detect when stability has been achieved. Upon confirmation of a stable ion beam  23 , the wafer holder  38  is moved once more such that it follows the current scan line, but in the reverse direction as shown by the solid line  80 .  FIG. 4   c  shows lines  78  and  80  offset from each other for the sake of clarity: in fact, the path of the ion beam  23  (whether switched on or off) is usually coincident upon the same scan line  74 . Accordingly, the remainder of the current scan line  74  is implanted. To ensure uniform implantation across the entire scan line  74 , the same rapid extinction of the ion beam  23  is performed at the “off” position  76  where the ion beam  23  was extinguished following detection of the glitch. This is shown in  FIG. 4   c , where upon reaching the “off” position  76 , the wafer holder  38  continues to move along the scan line  70  in a reverse direction, such that, if the ion beam  23  were still switched on, it would scan across the wafer  36  to finish at position  83  adjacent to the edge of the wafer  36  (the movement is shown by the dashed line  82 ).  
         [0059]     The ion beam  23  is restarted once more at  83  and, upon confirmation of a stable ion beam  23 , the remainder of the raster scan  68  is performed as shown in  FIG. 4   d . In this way, uniform implantation across the entire wafer  36  is achieved.  
         [0060]     It is inadvisable to restart the ion beam  23  when it will be incident upon the wafer  36  as this will dose further the wafer  36  at that point. In addition, it is inadvisable to restart the ion beam  23  when it will be incident upon the wafer holder  38  as this may produce contamination. This may be the case as the wafer holder  38  extends adjacent the wafer  36  along the X-axis and so a movement along the X-axis direction alone may not be enough to ensure the ion beam is clear of the wafer holder  58 . Accordingly, after a scan line  70  has been followed with the ion beam  23  switched off following a glitch, the wafer holder  38  is moved in the Y-axis direction prior to restarting the ion beam  23  if it would otherwise strike the wafer holder  38 . Once a stable ion beam  23  is obtained, the wafer holder  38  is moved back along the Y-axis direction and the next movement along a scan line  70  is performed.  
         [0061]     An alternative method for recovering from a glitch in the ion beam  23  is shown in  FIGS. 5   a  to  5   d . The same starting conditions as described for  FIG. 4   a  are assumed and these are reflected in  FIG. 5   a  where the ion beam  23  is extinguished during forward motion along a scan line  74  at the “off” position  76  shown.  
         [0062]     In addition to extinguishing the ion beam  23 , movement of the wafer holder  38  is stopped and then reversed such that, if the ion beam  23  were still switched on, it would follow the current scan line  74  but in the reverse direction to end up clear of the wafer  36  at  79 . This movement is reflected in  FIG. 5   b  by the dashed line  84 .  
         [0063]     Movement of the wafer holder  38  is started once more, with the ion beam  23  still switched off, such that the ion beam  23  would follow the current scan line  74  in the forwards direction as shown by the dashed line  86 . When the “off” position  76  is reached, the ion beam  23  is switched on rapidly while movement of the wafer holder  38  continues to complete the current scan line  70 . This is shown in  FIG. 5   c  by the solid line  88  that ends at  83  and results in a uniform implantation of that scan line  74 . As shown in  FIG. 5   d , scanning can continue to complete the raster scan  68  and therefore achieve uniform implantation of the entire wafer  36 .  
         [0064]     The method of  FIGS. 4   a  to  4   d  is preferred to the method of  FIGS. 5   a  to  5   d . This is because the ion beam  23  can be extinguished faster than it can be turned on, and turning the ion beam  23  on inevitably produces uneven dosing while the ion beam  23  settles.  
         [0065]     Of course, the possibility exists that a further beam instability may occur during a second pass  80 ; 88  along a scan line  74  where a previous glitch is being repaired. Were this to happen in the method described with relation to  FIGS. 5   a  to  5   d , this can easily be overcome by repeating the same method time and time again. Specifically, the wafer holder  38  can be translated  84  back to the start position  79  of the current scan line  70 , the wafer holder  38  moved  84  along the current scan line  70  and the ion beam  23  is switched on rapidly when it reaches the previous “off” position  76 . In this way, the entire scan line  70  is implanted over a number of successive passes in the same direction.  
         [0066]     Clearly the situation is different for the method already described with respect to  FIGS. 4   a  to  4   d . A hybrid method of recovering from two glitches is adopted which will now be described with reference to  FIGS. 6   a  to  6   d .  FIG. 6   a  corresponds to  FIG. 4   b  and so describes the situation where an ion beam  23  glitch has been detected, the ion beam  23  has been switched off at  76  and the wafer holder  38  has been moved such that the ion beam  23 , if it were switched on, travels along line  78  to finish at  79  to the side of the wafer.  
         [0067]      FIG. 6   b  shows the start of the recovery operation where the ion beam  23  is switched on at  79  and, upon confirmation of a stable ion beam  23 , the wafer holder  38  is moved such that implantation occurs along the current scan line  74  in the reverse direction as shown by  80 . However, at the point  90  indicated in  FIG. 6   b  a further glitch is detected and the ion beam  23  is switched off and the second “off” position  90  recorded.  
         [0068]     The ion beam  23  is extinguished while translation of the wafer holder  38  continues such that, if the ion beam  23  were still switched on, it would follow the current scan line  70  along the reverse direction to reach the far side of the wafer  36  at  83  (the movement is shown by the dashed line  92 ). Movement of the wafer holder  38  is then reversed to follow the current scan line  70  in a forwards direction and continues along the entire length of the current scan line  70 . During this movement, initially the ion beam  23  is switched off as shown by  94 , the ion beam  23  is switched on when reaching the first “off” position  76  to form the line  96  and is then switched off upon reaching the second “off” position  90  to continue as dashed line  98 .  
         [0069]     Accordingly, the remaining central portion of the current scan line  70  is implanted thereby forming a complete scan line  70  with uniform implantation. As before, the remainder of the wafer  36  can be implanted using the standard raster pattern  68  as shown in  FIG. 6   d . As recovery from the second ion glitch relies upon the inferior method of restarting the ion beam  23  while scanning across the wafer  36 , it is important to check the stability of the ion beam  23  when first restarting the ion beam  23  at position  79 . Obviously, it is best to avoid the need to recover from two glitches in a single scan line  74 .  
         [0070]     In order to determine when beam glitches occur, the ion beam current is monitored continuously by using a return current monitor. This arrangement will now be described with reference to  FIG. 7 .  
         [0071]     As mentioned previously, in usual operation the deceleration supply  46  generates a negative potential with respect to the grounded wafer holder  38  and beamstop  40  to decelerate positively-charged ions exiting the tube  34 . In order for the deceleration power supply  46  to maintain a regulated voltage between the wafer holder  38 /beamstop  40  and the flight tube  24 , it is important to ensure that a forward current flows through the deceleration power supply  46  to compensate for the positively charged ions flowing between flight tube  24  and the wafer holder  38 /beamstop  40 . This is achieved by connecting a deceleration supply load resistance  122  in parallel with the power supply  46 .  
         [0072]     In order to provide cooling to assemblies in the beam line and ion source areas of the ion implanter  20 , a closed circuit cooling water flow is required from a heat exchanger located at ground potential. The flow and return pipes must cross the post mass acceleration or deceleration voltage gaps. The water is slightly electrically conductive and part of the return current flow from the wafer  36  passes through these pipes. This represents a further effective load resistance in parallel with the deceleration power supply  46 . Although the current through the water used to cool the wafer holder  38  (that is usually deionised) is typically negligible, the current return through the cooling pipes will not necessarily be negligible. For example, when high post-mass acceleration or deceleration voltages are employed, a cooling water current of several mA may arise. To take this into account,  FIG. 7  shows a cooling system resistance  124  placed in parallel with the deceleration supply load resistance  122  and a deceleration power supply  46 .  FIG. 7  also shows a switch  125  that allows the deceleration power supply  46  to be shorted out when operating in ‘drift’ mode (described previously).  
         [0073]     The current flowing through the deceleration supply load resistance  122  will then be the sum of the forward current through deceleration power supply I DECEL  and the net current I BEAM  absorbed by both the wafer  36  and beamstop  40  minus a small cooling system water current.  
         [0074]     The output of the beamstop  40  is monitored by a first current monitor  126  that generates a voltage signal representative of the beamstop current. This voltage signal is connected to one input of a comparator  128 , as will be described below. The ion implanter  20  also contains a second current monitor  130  arranged in the path of the total current (the sum of the beam and deceleration currents) as it returns to the flight tube  24 . The second monitor  130  also generates a voltage signal V TOTAL  that indicates the total returning to the flight tube  24 . In one embodiment, the signal V TOTAL  may be measured directly without comparing it to the beamstop current.  
         [0075]     Alternatively, the signal V TOTAL  is fed to a second input of the comparator  128 . Thus the comparator  128  generates an output V DIFF  representative of the difference of the beamstop current I BEAMSTOP  and the total current I TOTAL  returned to the flight tube  24 .  
         [0076]     This arrangement is described in more detail in our U.S. Pat. No. 6,608,316 that is incorporated herein in its entirety by reference. Briefly, the voltage output of the current monitor  126  is connected to a differential amplifier that fulfils the function of the comparator  128 . The total current from the wafer holder  38  and beamstop  40  passes through the deceleration power supply  46 , deceleration supply load resistance  122  and any cooling systems  124 . The total current I TOTAL  is fed to a second current monitor  130  that operates in a similar manner to the first current monitor  126 .  
         [0077]     The advantages of monitoring the total current returning to the flight tube  24 , instead of or as well as the beamstop  40  is that it is broadly indicative of the ion beam current at the point when it impacts the wafer holder  38 /beamstop  40  assembly. Any arcing, for example, in the ion source  22  will manifest itself as a glitch in the ion beam  23 . This in turn may be monitored by monitoring I TOTAL . At any time during the implantation cycle, a qualitative indication of the ion beam integrity may then be obtained as is required for the method of the present invention. In particular, the voltage signal which is an output of the current monitor  130  allows wide band stability monitoring of the ion beam  23 .  
         [0078]     The arrangement shown in  FIG. 7  is particularly applicable for use with batch processing of wafers  36  because problems in ripples in the current measured by the beamstop  40  are largely avoided. I TOTAL  is slightly distorted due to backstreaming electrons generated when the ion beam  23  is striking the wafers  36 . For positively charged ions, some electrons liberated from the wafers  36  are accelerated away during ion deceleration, thus adding to the current return to the flight tube  24 . The beamstop  40  effectively traps the secondary electrons but, however, there are no backstreaming electrons to augment the current when the wafer holder  38  does not occlude the ion beam  23 . When the ion beam  23  is entirely incident on the beamstop  40 , the beamstop current substantially equals the current beam return to the flight tube  24 , i.e. I BEAMSTOP ≅I TOTAL . Thus, the differential output of the comparator  128  is approximately zero in this instance and so can be used to distinguish the measured beam current as determined by the current beamstop measurement as opposed to the current incident upon the wafers  36 .  
         [0079]     An alternative embodiment of an incident ion beam  23  current measurement arrangement is shown in  FIG. 8 . Many parts correspond to those shown in  FIG. 7  and so are labelled with corresponding reference numerals.  
         [0080]     As shown in  FIG. 8 , rather than employing a deceleration ion power supply  46 , a variable resistance  132  is placed in the current path which returns the ion beam current from the wafer holder  38  and beamstop  40  back to the flight tube  24 . Although the variable resistance  132  may consist of passive devices, it is preferable to use a series of active devices such as field effect transistors (FET&#39;s). The manner of operation of the device of  FIG. 8  is described in more detail in the above mentioned U.S. Pat. No. 6,608,316 and in British Patent Application No. 9523982.8.  
         [0081]     Briefly, the potential difference between the wafer holder  38 /beamstop  40  (normally held at ground potential) and the flight tube  24  is controlled by varying the resistance of a chain of FET&#39;s connected in series between the wafer holder  38 /beamstop  40  (at ground potential) and the flight tube  24 . This is done by measuring the voltage across the FET chain, with a potential divider buffering the voltage and comparing the voltage to a reference voltage (V REF ) using a differential amplifier. The error signal (i.e. the amplified difference between the desired acceleration potential and the active deceleration potential) as measured by the potential divider is used to adjust the effective resistance of the FET chain.  
         [0082]     The potential drop across the FET chain, V TOTAL , is indicative of the total current return to the flight tube  24 . In one embodiment, this is fed through the comparator  128  which may be a differential amplifier. The other input to the comparator  128  is a voltage representative of the beamstop current. This is derived from the beamstop current monitor  126 . The output of the comparator  128  is similar to that already described with reference to  FIG. 7 . With the apparatus shown in  FIG. 7 , the voltage signal V TOTAL  may be measured directly rather than being compared with the beamstop current signal.  
         [0083]     The continuous measurement of the ion beam  23  current is used to determine whether or not a beam glitch has occurred. The continuous beam current is monitored for fast changes to indicate a beam glitch, rather than looking for slow changes. This is because slow changes in the ion beam current frequently occur and may be due to such mechanisms as residual gas neutralisation of the ion beam  23 . A threshold value for the rate of change can be set and this is likely to be dictated by any particular ion implantation recipe.  
         [0084]     Any event which does not meet the slow changing criteria is assumed to indicate instability of the change is above a certain size.  
         [0085]     Quantifying changes in the ion beam current is performed using a comparison to an average ion beam current value. This average is obtained by taking a number of readings of the ion beam current once a stable ion beam  23  has been obtained, e.g. by using a rolling average of the total current obtained by measuring the total current I TOTAL  with a time constant of 50 to 200 ms. Obviously, this method cannot be employed initially and so pre-set average values are used as initial starting conditions. With an average value determined, upper and lower thresholds may be used to test any variation in the ion beam current. The thresholds are measured relative to the average ion beam current and may be offset from that average by differing amounts. The offset may, for example, correspond to a drop of 50%. The thresholds are often specific to a particular implantation recipe. Either every single ion beam current measurement can be compared against the thresholds or a small number of consecutive measurements can themselves be averaged before comparison to the thresholds (e.g. measure I TOTAL  with a short time constant of 1 ms). A further condition may be imposed that consecutive readings (e.g. ten) should exceed the thresholds before the ion beam is switched off.  
         [0086]     As described previously, detection of an ion beam glitch leads to the ion beam  23  being switched off. This may be achieved in any number of ways, although it is clearly advantageous to achieve a rapid extinction of the ion beam  23 . To date, an ion beam  23  has been extinguished by interrupting the power input to the arc power supply unit  62 . However, it is relatively slow, taking in excess of 20 ms. An alternative method of extinguishing the ion beam  23  that is far quicker is now described.  
         [0087]      FIG. 9  shows an ion source  22  akin to that shown in  FIG. 2  and therefore like reference numerals will be used for like parts. In addition, repetitive description will be avoided. Inspection of  FIG. 9  compared to  FIG. 2  shows that the circuit around the arc power supply unit  62  has been modified to include a pair of power semiconductor switches  134   a,b . The power semiconductor switches  134   a,b  allow rapid switching, typically less than 20 ms.  
         [0088]     The power semiconductor switches  134   a,b  are supplied with command signals derived from a common line indicated at  136  in  FIG. 9 . It will be seen that this line  136  bifurcates with one portion  136   a  being supplied to a first switch  134   a  and the other portion  136   b  of the signal being supplied to the second switch  134   b  via a NOT gate  138 . This ensures that the pair of switches  134   a,b  are operated mutually exclusively, i.e. the first switch  134   a  is open when the second switch  134   b  is closed and vice versa. In the configuration shown in  FIG. 9 , the first switch  134   a  is closed and the second switch  134   b  is open such that the ion source  22  is biased by the arc supply  62  to ensure a potential difference between anode  50  and cathode  52 . This ensures ion creation and hence provides an ion beam  23  for implanting the wafer  36 .  
         [0089]     Reversing the signal on line  136  inverts the two switches  134   a,b  so that the first switch  134   a  is open and the second switch  134   b  is closed. This isolates the arc supply  62  to connect directly the chamber walls  50  to the tube  58  of the indirectly heated cathode  52 . The resulting zero potential difference between anode  50  and cathode  52  causes immediate collapse of the plasma and immediate extinction of the ion beam  23 .  
         [0090]     The collapse of the plasma in this way will cause the ion source chamber  38  to cool down. Restarting the ion source  22  from cold will prolong the time for the ion beam  23  to settle to the previous steady flux value. This can be avoided by increasing the power delivered to the filament  54  or across the filament  54  and tube  58  using the bias power supply  60 .  
         [0091]     Reversing the signal on line  136  once more leads to rapid creation of an ion beam  23  because the two switches  134   a,b  are inverted such that the anode  50  is biased relative to the cathode  52  and ions are created by the ion source  22 . This is helped by keeping the chamber  48  hot, as described above.  
         [0092]     As will be appreciated by the skilled person, variations may be made to the embodiments described above without departing from the scope of the amended claims.  
         [0093]     Examples of scanning schemes are presented in FIGS.  4  to  6 , but these are merely examples and the present invention may be employed with other schemes. It will be readily apparent that the present invention may be adapted to any scheme where an ion beam  23  is scanned relative to a substrate along one or more pre-defined paths. The paths may be linear, arcuate or may follow any other shape. For example, a spiral scan may be used where the ion beam follows a spiral path around the wafer. If raster scans are used, the scan lines need not be parallel, for example the ion beam may follow a zig-zag pattern. Where movement along the path may be reciprocated, the method illustrated in  FIGS. 4 and 5  may be used. Where movement may not be reciprocated, the method illustrated in  FIG. 5  may be used.  
         [0094]     The present invention may also be used with different overall scanning schemes. For example, the present invention may be used with an interlaced series of raster scans  68 , i.e. where only certain scan lines  70  are allowed on one pass, other missed scan lines  70  being implanted on the next pass. For example, the first pass may implant the first, fifth, ninth, . . . scan lines  70  of  FIG. 4   a , the second pass may implant the second, sixth, tenth, . . . scan lines  70 , the third pass may implant the third, seventh, eleventh, . . . scan lines  70  and the fourth, eighth, twelfth, . . . scan lines  70 . The wafer  36  may be rotated through 180° between passes. Alternatively a series of raster scans  68  may be performed following the same pattern: the wafer  36  may be rotated (say by 90°, or any other angle) between passes so that each raster pattern  68  is at an angle to the other patterns  68 .  
         [0095]     The above embodiments of the present invention are all used in the context of serial processing of wafers  36  using raster scans  68 . As mentioned previously, scanning may be achieved by (a) translating the wafer  36  relative to a fixed ion beam  23 , (b) deflecting an ion beam  23  across a fixed wafer  36  or (c) using a hybrid method of translating the wafer  36  and deflecting the ion beam  23 . In addition, the present invention may be used with batch processing of wafers  36  where an ion beam  23  scans over each wafer along a plurality of scan lines  70 . For example, the invention may be used with a batch implanter comprising a spoked-wheel wafer holder (i.e. a plurality of wafers are held at the ends of a number of spokes extending from a central hub).  
         [0096]     The method given above for determining the ion beam  23  current is merely one example of doing so. The ion beam  23  current may also be determined by monitoring the beam line power supplies (e.g. the pre-acceleration power supply, the lens voltage power supply, the deceleration power supply), monitoring the current flowing from the chuck to ground or by using a current clamp method. The current clamp method comprises placing a solenoid around a part of the ion beam path  23 . Any change in ion beam current will cause a change in the current flowing through the solenoid. Thus, ion beam glitches can be detected by measuring the current flowing through the solenoid.  
         [0097]     The arrangement shown in  FIG. 9  is particularly well suited to extinguishing and starting the ion beam  23  due to its rapid switching speed. However, it is but one method of turning the ion beam  23  on and off. Other possibilities include changing the pre-acceleration voltage, changing the extraction voltage, changing the magnetic field in the mass-analysing arrangement or closing the mass-resolving slit.  
         [0098]      FIG. 9  shows an ion source  22  having an indirectly heated cathode  52 . The ion source  22  need not use an indirectly heated cathode  52  and could instead be of a single filament  54  design. In this design, a filament  54  is used as the cathode  52  to emit electrons directly into the ion source chamber  48  and is often located directly in front of an electron reflector biased to ensure electrons are accelerated away from filament  54 . In this arrangement, only one power supply unit is needed to supply current to the filament  54 , i.e. the filament supply  56  and bias supply  60  of  FIG. 9  are replaced by a single supply  62  that provides current to the filament  54 . An arc power supply unit is again used to create a potential difference between anode  50  and cathode  52 . Alternatively, a Freeman-type cathode may be used.