Abstract:
This invention relates to a method of scanning a substrate through an ion beam in an ion implanter to provide uniform dosing of the substrate. The method comprises causing relative motion between the substrate and the ion beam such that the ion beam passes over all of the substrate and rotating the substrate substantially about its centre while causing the relative motion. Rotating the substrate while causing the relative motion between the substrate and the ion beam has several advantages including avoiding problematic angular effects, increasing uniformity, increasing throughput and allowing a greater range of ion beam profiles to be tolerated.

Description:
FIELD OF THE INVENTION 
   This invention relates to a method of scanning a substrate through an ion beam in an ion implanter to provide uniform dosing of the substrate. The invention also relates to an ion implanter arranged to perform this method of scanning a substrate. 
   BACKGROUND OF THE INVENTION 
   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 precursor gas or the like. Only ions of a particular species are usually required for implantation in a substrate, for example a particular dopant for implantation in 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 a process chamber where the ion beam is incident on a substrate held in place in the ion beam path by a substrate holder. 
   Ion beams of different shapes have been used in the past. Ribbon beams are well known and generally have a major axis that is greater in dimension than the substrate to be implanted and a minor axis much smaller than the substrate. Another common type of ion beam is the spot ion beam where the cross-sectional profile of the ion beam is much smaller in all directions than the substrate to be implanted. With either type of ion beam, the ion beam and substrate are moved relative to one another such that the ion beam scans the entire substrate surface with the aim of achieving a uniform ion implant across the whole of the substrate. For a ribbon beam, only one scan across the substrate is required, whereas multiple scans are required for a spot beam. Scanning may be achieved by (a) deflecting an ion beam to scan across a substrate that is held in a fixed position, (b) mechanically moving a substrate whilst keeping an ion beam path fixed or (c) a combination of deflecting an ion beam and moving a substrate. 
   Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above that uses a spot beam. While some steering of the ion beam is possible, the implanter is operated such that ion beam follows a fixed path during implantation. Instead, a wafer is held in a substrate holder that is moved along two orthogonal directions to cause the ion beam to trace over the wafer following a raster pattern like that illustrated in  FIG. 1 . First, the wafer is moved continuously in a single direction (the fast-scan direction) to complete a first scan line. The substrate is then stepped down a short distance orthogonally (in the slow-scan direction), and then moved back along the fast-scan direction to form a second scan line across the wafer to overlap with the first scan line. This process is then repeated such that the combination of tracing scan lines punctuated by the stepwise movement results in the whole surface of the wafer seeing the ion beam. The series of scan lines that leads to a complete dosing of the wafer is referred to herein as a “pass”. An implant may comprise multiple passes over the wafer. 
   Further improvements may be made to improve the uniformity of implants made using such raster scans. For example, multiple passes over the substrate may be made and interlacing may be effected (e.g. make a first pass implanting the first, fifth, ninth, etc. scan lines, then make a second pass implanting the second, sixth, tenth, etc. scan lines, then make a third pass, etc.). Also a problem of angular effects (i.e. off-normal incidence of the ion beam or asymmetries in the ion beam) may be addressed by making multiple passes with rotation of the wafer between passes. For example, in a quad implant four (or a multiple of four) passes are made with a 90° twist of the substrate between each pass. Changing the orientation of the wafer clearly helps alleviate such angular effects. Our U.S. patent application Ser. No. 11/527,594 (U.S. Patent Application Publication No. 2007/0105355) provides more details of such scanning techniques. While such techniques offer excellent uniformity in dosing, the need to perform multiple passes has an associated time overhead that reduces the throughput of the ion implanted. 
   U.S. Patent Application Publication No. 2001/0032937 describes a very different method of scanning a substrate that does not rely solely on linear movement of the substrate relative to the ion beam. Instead, as illustrated in  FIG. 2 , a substrate is spun about its central axis with a constant angular velocity, while also being translated across a fixed position, elongate spot ion beam. Movement of the substrate effectively sees the ion beam travel through the centre of the substrate. The ion beam is unusual in that it is nether a conventional spot beam, nor is it a ribbon beam. Rather, it is elongated such that it has a longer major axis that is smaller than the width of the substrate. 
   As the ion beam first clips the substrate, the rotation of the substrate sees the ion beam implant the periphery of the substrate: as the substrate is translated across the ion beam, the implanted region grows in width and spirals into the centre of the substrate before spiraling out and moving off the periphery of the substrate. However, the linear speed of the edge of the spinning substrate is much faster than the linear speed of the centre of the substrate. To compensate for this effect, the substrate is translated at a variable velocity through the ion beam such that its speed is greatest at the centre of the substrate. 
   In practice, such a technique is difficult to implement. The control law for the translational velocity is complex and achieving accurate control of this varying velocity is problematic. Worse still, to achieve high uniformity of implant across the substrate requires exceptional uniformity in the ion beam. 
   SUMMARY OF THE INVENTION 
   Against this background, and from a first aspect, the present invention resides in a method of scanning a substrate through an ion beam in an ion implanter, comprising: causing relative motion between the substrate and the ion beam such that the ion beam passes over all of the substrate; and rotating the substrate substantially about its centre while causing the relative motion. The relative motion is caused such that the ion beam would pass over all of the substrate even if the substrate were not rotating. 
   Rotating the substrate while causing the relative motion between the substrate and the ion beam has several advantages. Briefly, problems of angular effects are overcome, uniformity is improved, throughput may be increased, and a greater range of ion beam profiles may be tolerated. More specifically, the velocity control law problem inherent in the arrangement of US2001/0032937 may be avoided. 
   Where the ion beam is a ribbon beam, the method may comprise causing a relative motion between the substrate and the ribbon beam such that all of the substrate passes through the ribbon beam. This relative motion may be performed in one pass. Hence, a single scan line is formed. This method has some similarities with US2001/0032937. Instead of a tall spot beam, a ribbon beam is used. However this simple change has a huge benefit, namely that the relative motion between the substrate and the ion beam may be effected with a constant speed while still achieving a uniform implant. Hence, the complex velocity control law of US2001/0032937 is avoided. In addition to maintaining a constant speed of the relative motion, a constant rotational speed is also preferably employed. 
   Where the ion beam is a spot ion beam, the method may comprise causing the relative motion between the substrate and the ion beam such that the ion beam passes over all of the substrate by causing a series of translations of the substrate relative to the ion beam such that the ion beam traces a series of scan lines over the substrate. 
   Compared to US2001/0032937, forming multiple scan lines to cover the substrate as if it were not rotating sees a uniform implant when the relative motion is effected as a constant speed. Hence, the complex velocity control laws are avoided. 
   These scan lines may be formed so as to be parallel or substantially parallel. The scan lines may all be formed in a common direction or may be formed by a reciprocal motion such that the scan lines extend back and forth. The scan lines may be arcuate. Alternatively, the scan lines may be linear such that a raster scan or saw-tooth scan is formed. The raster scan may be formed by performing a reciprocal motion in a fast-scan direction while performing an intermittent step-wise motion in a slow-scan direction. The saw-tooth scan may be formed by performing a reciprocal motion in a fast-scan direction while performing a continuous motion in a slow-scan direction. 
   Preferably, scan lines are arranged so as to overlap. By this, it is meant that if the substrate were not to be spun, a series of overlapping regions of the substrate that are implanted during each movement along a scan line would result. Advantageously, the scan lines may be arranged so as to have minimal overlap. For example, if the scan lines have a pitch P and the ion beam a. dimension D in the pitch direction, the pitch may be just less than that dimension. As an example, the pitch P may be 5% or less than that dimension D. It may be greater than 0.9 times the dimension D. Put another way, the pitch may be approximately equal to the ion beam&#39;s dimension in the pitch direction, say 50 mm each. These values work well with disk-like substrates of 300 mm diameter, as are typical in the semiconductor wafer industry. 
   With either type of ion beam, a motion is imparted between substrate and ion beam that sees a translational motion and a rotational motion. The translational motion may be achieved by translating the substrate or by scanning the ion beam (e.g. by electrostatic deflection), or by a combination of the two. 
   The rotation preferably occurs for each scan line. The rotational motion has several advantages. First, the rotation helps overcome the problematic angular effects described above. Whereas quad implants alleviate these problems by using orientations of the substrate at four angles, the present invention provides a continuous range of angles in a single pass. Hence the need for time-consuming multiple passes is avoided. Preferably, the substrate is rotated such that it performs at least a complete revolution as the ion beam scans across a or each scan line. More preferably, the substrate is rotated so as to complete fifteen to twenty revolutions as the ion beam scans across a or each scan line. This means that the ion beam traces a spiral across the substrate that has a reasonable number of revolutions. 
   Where the ion beam is a spot beam, the method may comprise rotating the substrate and/or causing the relative motion between the substrate and the ion beam such that the resulting spirals traced by the ion beam over the substrate overlap on adjacent revolutions. 
   Other advantages are obtained by rotating the substrate while causing the relative motion between the substrate and ion beam. Uniformity of implant benefits greatly from this method, namely the uniformity of the dose received by different parts of the substrate. In addition, excellent uniformity may be achieved even where a large pitch is used between scan lines. To illustrate this, when performing a traditional raster scan where no rotation of the substrate is performed, pitches are chosen so as to provide a large overlap between adjacent scan lines in order to ensure good uniformity. This results in a large number of scan lines that lengthens the implant process. In contrast, only minimal overlap is required for the present invention. As a result, the pitch can be increased such that fewer scan lines are needed. This of course means that substrates may be implanted more quickly, thereby increasing the throughput of the ion implanter. 
   Another advantage that follows from the general improvements in uniformity is that a larger range of beam profiles and imperfections may be tolerated. 
   Optionally, the method may comprise causing relative motion between the substrate and the ion beam to form a scan line such that a point in the ion beam, having an average value of the total ion beam current along a section taken through the ion beam orthogonal to the direction of relative motion, passes over the centre of the substrate. Where multiple scan lines are formed, the positions of the other scan lines may be determined by the position of the scan line that passes through the centre of the substrate. For example, fixing the position of the central scan line and ensuring a desired pitch will dictate the position of the other scan lines. 
   From further aspects, the present invention resides in a controller in an ion implanter arranged to implement the above methods and an ion implanter comprising such a controller. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings, of which: 
       FIG. 1  illustrates a raster scan pattern of an ion beam across a wafer; 
       FIG. 2  illustrates a method of scanning a substrate through an ion beam according to the prior art; 
       FIG. 3  shows a conventional ion implanter; 
       FIG. 4  is a schematic showing a substrate being scanned through an ion beam in accordance with an embodiment of the present invention; and 
       FIG. 5  shows how multiple scan lines add to form a virtual ribbon beam; 
       FIG. 6  is a graphical representation of a wafer being spun and moved towards a virtual ribbon beam; 
       FIG. 7  is a graphical representation of the wafer of  FIG. 6  being moved into the virtual ribbon beam to show a position where the virtual ribbon beam first makes contact with the wafer, and also illustrates the part of the virtual ribbon beam seen by a point on the edge of the wafer as it is spun through the virtual ribbon beam; 
       FIG. 8  continues the motion indicated in  FIG. 6  and shows the virtual ribbon beam part way across the wafer; 
       FIG. 9  continues the motion indicated in  FIG. 6  and shows the virtual ribbon beam half way across the wafer; 
       FIG. 10  is a graphical representation of the wafer and virtual ribbon beam of  FIG. 6 , and shows the part of the virtual ribbon beam seen by a point on the wafer displaced inwardly from the edge as the virtual ribbon beam scans over the wafer; 
       FIG. 11  is a graphical representation of the wafer and virtual ribbon beam of  FIG. 6 , and shows the part of the virtual ribbon beam seen by a point close to the centre of the wafer as the virtual ribbon beam scans over the wafer; 
       FIGS. 12 and 13  shows schematically how scan lines can be positioned relative to the centre of the wafer such that any point on the wafer will effectively see an average current as it scans the resulting ripple in the virtual ribbon beam, and hence illustrates how to ensure uniform dosing at the centre of the wafer; 
       FIG. 14  illustrates how a saw-tooth raster scan can be thought of as two scans through a virtual ribbon beam and hence how uniformity is preserved; and 
       FIGS. 15   a  and  15   b  illustrate how spin speed and translation speed may be determined with respect to the width of the in beam. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a known ion implanter  10  for implanting ions in substrates  12 , and that may be used to implement the present invention. Ions are generated by the ion source  14  to be extracted and follow an ion path  34  that passes, in this embodiment, through a mass analysis stage  30 . Ions of a desired mass are selected to pass through a mass-resolving slit  32  and then to strike the semiconductor substrate  12 . 
   The ion implanter  10  contains an ion source  14  for generating an ion beam of a desired species that is located within a vacuum chamber  15  evacuated by pump  24 . The ion source  14  generally comprises an arc chamber  16  containing a cathode  20  located at one end thereof. The ion source  14  may be operated such that an anode is provided by the walls  18  of the arc chamber  16 . The cathode  20  is heated sufficiently to generate thermal electrons. 
   Thermal electrons emitted by the cathode  20  are attracted to the anode, the adjacent chamber walls  18  in this case. The thermal electrons ionise gas molecules as they traverse the arc chamber  16 , thereby forming a plasma and generating the desired ions. 
   The path followed by the thermal electrons may be controlled to prevent the electrons merely following the shortest path to the chamber walls  18 . A magnet assembly  46  provides a magnetic field extending through the arc chamber  16  such that thermal electrons follow a spiral path along the length of the arc chamber  16  towards a counter-cathode  44  located at the opposite end of the arc chamber  16 . 
   A gas feed  22  fills the arc chamber  16  with the species to be implanted or with a precursor gas species. The arc chamber  16  is maintained at a reduced pressure within the vacuum chamber  15 . The thermal electrons travelling through the arc chamber  16  ionise the gas molecules present in the arc chamber  16  and may also crack molecules. The ions (that may comprise a mixture of ions) created in the plasma will also contain trace amounts of contaminant ions (e.g. generated from the material of the chamber walls  18 ). 
   Ions from within the arc chamber  16  are extracted through an exit aperture  28  provided in a front plate of the arc chamber  16  using a negatively-biased (relative to ground) extraction electrode  26 . A potential difference is applied between the ion source  14  and the following mass analysis stage  30  by a power supply  21  to accelerate extracted ions, the ion source  14  and mass analysis stage  30  being electrically isolated from each other by an insulator (not shown). The mixture of extracted ions are then passed through the mass analysis stage  30  so that they pass around a curved path under the influence of a magnetic field. The radius of curvature traveled by any ion is determined by its mass, charge state and energy, and the magnetic field is controlled so that, for a set beam energy, only those ions with a desired mass to charge ratio and energy exit along a path coincident with the mass-resolving slit  32 . The emergent ion beam is then transported to the process chamber  40  where the target is located, i.e. the substrate  12  to be implanted or a beam stop  38  when there is no substrate  12  in the target position. In other modes, the beam may also be accelerated or decelerated using a lens assembly positioned between the mass analysis stage  30  and the substrate position. 
   The substrate  12  is mounted on a substrate holder  36 , substrates  12  being successively transferred to and from the substrate holder  36 , for example through a load lock (not shown). The substrate holder  36  may be of any conventional design that provides linear translation of the substrate  12  in both x- and y-axis directions (the ion beam path  34  defining the z axis, and the x axis is taken to be horizontal and the y axis vertical), while also providing rotation of the substrate  12  about its centre. For example the possibilities include: a cantilevered scanning arm that effects linear movements like that described in U.S. Pat. No. 6,956,223 which is incorporated herein in its entirety; a scanning arm provided with rotary joints that are moved to effect scanning like those described in our co-pending U.S. patent application Ser. No. 11/588,432 which is incorporated herein in its entirety; or a reactive mass scanning arrangements like that described in our co-pending U.S. patent application Ser. No. 11/589,312 which is incorporated herein in its entirety. 
   The ion implanter  10  operates under the management of a controller, such as a suitably programmed computer  50 . The controller  50  controls scanning of the wafer  12  through the ion beam  34  to effect desired scanning patterns. 
     FIG. 4  shows the motion described by the substrate  12  as it is scanned through the ion beam  34  by the controller  50 . The substrate  12 , in this embodiment, is a standard 300 mm silicon wafer commonly used in the semiconductor industry. Of course, other sizes and types of wafers may be used. The ion beam  34  is a typical spot beam with a reasonably uniform diameter of 50 mm. The controller  50  may manage operation of the ion implanter  10  to control, to a certain extent, the size and shape of the ion beam  34 . For example, the controller  50  may vary operational properties of the ion source  14  or of ion optics that guide the ion beam  34  through the ion implanter  10 . Some steering of the ion beam  34  is possible, although generally the ion beam  34  will be fixed as the wafer  12  is mechanically scanned therethrough. 
   The controller  50  directs the substrate holder  36  to scan the wafer  12  through the ion beam  34  to follow the raster scan indicated at  52 . The raster scan  52  comprises a series of scan lines  54  formed in alternate directions by reciprocal motion of the wafer  12  in the fast scan direction (left and right along the x-axis direction), separated by steps  56  formed by periodic stepwise motion in the slow scan direction (downwardly in the y-axis direction). Hence, the motion of the wafer  12  in the fast scan direction must be reversed between successive scan lines  54 , while its motion in the slow scan direction is in the same direction. Simultaneously, the controller  50  directs the substrate holder  36  to spin the wafer  12  about its centre. The direction of spin is indicated in  FIG. 4  and is kept the same for all scan lines  54  (and this can be either clockwise or anti-clockwise, as is desired). The wafer  12  is translated at 360 mm/sec and is rotated at 1200 rpm, leading to approximately twenty revolutions per scan line  54 . Other values may be chosen. For example, the spin speed may be varied although a speed sufficient to allow 15 to 20 revolutions of the wafer  12  along each scan line  54  is preferred. 
   An advantage of rotating the wafer  12  while moving the wafer  12  through the ion beam  34  along each scan line  54  is that a larger pitch between adjacent scan lines  54  may be realised without compromising uniformity of implant: in this embodiment a pitch of 50 mm was used. In a conventional raster scan, where no rotation of the wafer  12  is performed, such a pitch would lead to there being only minimal overlap between the stripes of wafer  12  dosed as adjacent scan lines  54  are formed. 
   A further advantage of rotating the wafer  12  while moving the wafer  12  along each scan line  54  is that the problematic angular effects described above are avoided. This is because rotating the wafer  12  ensures that the wafer  12  sees the ion beam  34  over the full range of 360°. 
   It is not so straightforward to envisage how the combined translation and rotation of the wafer  12  provides another advantage, namely uniformity of implant of the wafer  12 . However, an understanding becomes readily apparent when the implant process is thought of in the following way. 
   Rather than considering the scan lines  54  of the raster pattern  52  being formed successively, they may be thought of as being formed concurrently, i.e. all scan lines  54  are formed in one pass of a plurality of spot ion beams  34  over the wafer  12 . The size of each ion beam  34  and the pitch used means that the plurality of spot ion beams  34  overlap and so may be regarded as a virtual ribbon beam  60 , as shown in  FIG. 6 .  FIG. 5  shows how the intensity of this virtual ribbon beam  60  in the slow scan direction is derived from the individual spot beams  34 . The ion beam current profiles  62  of the spot ion beams  34  at each scan line  54  is shown, and adding these individual contributions provides the current profile  64  of the virtual ribbon beam  60 . The resulting profile  64  of the virtual ribbon beam  60  has a broadly flat top that extends over the height of the wafer  12 . However, in practice the individual profiles  62  do not add to form a perfectly flat top, but instead the top exhibits a periodic ripple  66 . 
   To demonstrate that spinning the wafer  12  does not have a detrimental effect on dosing uniformity, first consider a hypothetical perfect ribbon beam, i.e. a ribbon beam exhibiting no ripple  66  but instead having a perfect flat top in the region that passes over the wafer  12 . Clearly, passing a wafer  12  through this perfect ribbon beam without spinning the wafer  12  will lead to a perfect uniform implant. It is easy to see that spinning the wafer  12  while it passes through the perfect ribbon beam will have no detrimental effect as all points on the wafer  12  will still see exactly the same total amount of ion beam current. 
   To appreciate that spinning the wafer  12  in fact has a beneficial effect on the uniformity of implant, we should return to the rippled virtual ion beam  60  that is equivalent to our multiple passes of a spot ion beam  34  over the wafer  12 . Passing the wafer  12  through this virtual ion beam  60  without spinning the wafer  12  results in any particular point on the wafer  12  seeing only one particular part of the ripple  66  on the virtual ion beam  60 . For example, a first point may pass through a peak in the ripple  66  and a second point may only pass through a trough in the ripple  66 . Hence, the first point will receive a greater dose than the second point. Considering the wafer  12  as a whole, the dose it receives will exhibit stripes extending in the fast-scan direction. Put another way, the wafer  12  will have stripes due to lines of high dose corresponding to peaks in the ripple  66  and lines of low dose corresponding to troughs in the ripple  66 : the high doses correspond to points on the wafer  12  that see the centre of the spot ion beam  34  pass overhead and the low doses correspond to points on the wafer  12  that see the outer edges of the spot ion beam pass overhead. 
   Turning now to a combination of translating and spinning the wafer  12 , it can be appreciated that any point on the wafer  12  (the centre point aside) will see different parts of the virtual ion beam  60  as it spins with the wafer  12 .  FIG. 6  shows schematically the equivalent movement of the wafer  12  through the virtual ribbon beam  60 , with arrow  68  indicating the translation of wafer  12  and arrow  70  indicating the rotation of wafer  12 . 
     FIG. 7  shows the wafer  12  further translated in the fast-scan direction towards virtual ribbon beam  60 , at a point where it just contacts the beam  60 . Taking a point P 1  on the periphery of the wafer  12 , it will rotate with the wafer  12  in the direction  72 . In  FIG. 7 , point P 1  just clips the central part of virtual ribbon beam  60 . The ripple  66  in the virtual ribbon beam  60  is shown to the right in  FIG. 7 . As the point P 1  rotates to clip the centre of the virtual ribbon beam  60 , it effectively scans down a central segment  74  of the ripple  66 , as indicated in  FIG. 7 . As will be understood, segment  74  is initially very small as the wafer  12  first clips the virtual ribbon beam  60 , and segment  74  then expands as the wafer  12  is driven further into the virtual ribbon beam  60 . 
     FIG. 8  shows the wafer  12  further translated in direction  68  such that the leading edge of the wafer  12  is now a little way clear of the far side of virtual ribbon beam  60 . Point P 1  spinning on the edge of the wafer  12  now passes through the virtual ribbon beam  60  before emerging on the far side of the beam  60 , before passing back through the beam  60  once more. As shown to the right in  FIG. 8 , this results in point P 1  scanning down two separate segments  76  and  78  of the ripple  66 . 
   Considering the gradual movement of the wafer  12  into the virtual ribbon beam  60  that will occure between the stages shown in  FIGS. 7 and 8 , the single segment  74  in the ripple  66  of  FIG. 7  expands until the leading edge of the wafer  12  breaks clear of the far side of the virtual ribbon beam  60  at which point the single segment  74  divides into the two segments  76  and  78 . As the wafer  12  continues its motion, segments  76  and  78  move outwardly along the ripple  66  as the point P 1  intercepts the beam  60  further and further towards the beam&#39;s edges. 
     FIG. 9  shows the wafer  12  driven into the virtual ribbon beam  60  such that the beam  60  extends across the centre of the wafer  12 . In this position, the segments  76  and  78  have moved outwardly along the ripple  66  to be at their extremes. As motion of the wafer  12  through the virtual ribbon beam  60  continues, the segments  76  and  78  move back inwardly across the ripple  66  to join once more as a central segment  74 . 
   Thus, the overall movement of the wafer  12  through the virtual ribbon beam  60  is such that point P 1  sees virtually all the ripple  66 . The effect of point P 1  scanning over all the peaks and troughs in the ripple  66  is that point P 1  sees what will be close to the average ion beam current rather than just seeing a single value as was the case described above where the wafer  12  is not spun. Clearly, the more peaks and troughs that are sampled, the closer the dose seen by point P 1  will be to the average. 
     FIG. 10  shows the wafer  12  and virtual ribbon beam  60  once more, but this time considers a second point P 2  that resides inward of the edge of the wafer  12 , in this case about a quarter of the way in a long a radius. As wafer  12  spins, point P 2  follows the path indicated by the circle  80 . As the wafer  12  passes through the virtual ribbon beam  60 , point P 2  first scans the central segment  74  of the ripple  66  and then the pair of segments  76  and  78 . After the pass of the wafer  12  through the virtual ribbon beam  60 , point P 2  has seen the segment  82  of the ripple  66  indicated in  FIG. 10 . As point P 2  is inset from the edge of the wafer  12 , segment  82  extends across only a fraction of ripple  66  with a width corresponding to the diameter of circle  80 . As a result, point P 2  samples a smaller part of the ripple  66  than point P 1  that was on the edge of the wafer  12 . Nonetheless, point P 2  still sees multiple peaks and troughs in the ripple  66  and so sees an averaged amount of ion beam current. 
   As we take other points that reside closer and closer towards the centre of the wafer  12 , the segment  82  of the ripple  66  scanned by the point gets ever and ever smaller such that the number of peaks and troughs seen by the point decreases. As a result, the averaging effect works less and less well. Eventually, points are reached that do not see a full cycle of the ripple  66 .  FIG. 11  shows such a point P 3  that resides just a small distance from the centre of the wafer  12 . 
   Point P 3  follows circle  80  as the wafer  12  spins, and mapping the diameter of circle  80  to the ripple  66  shown to the right in  FIG. 11  shows that only a small segment  82  of the ripple  66  is seen by point P 3 . The detail from the ripple  66  shown in the far right of  FIG. 11  demonstrates that, in this embodiment, the segment  82  seen by point P 3  sits in a trough of the ripple  66 . As a result, point P 3  always receives ion beam current that sits below the average ion beam current value shown at  84  in the detail. Hence, point P 3  receives a below average dose during the implant. 
   This potential problem may be overcome by careful selection of which part of the ion beam  34  traces across the centre of the wafer  12 .  FIG. 12  corresponds to the detail of  FIG. 11 , but shows that ripple  66  shifted such that the centre of the wafer  12  (indicated by line  86 ) passes through the ripple  66  where its value is equal to the average current value  84 . Hence, the centre point of the wafer  12  will see the average current value (as it effectively traces a line through the virtual ribbon beam  60 ). Moreover, points further and further away radially from the centre of the wafer  12  will see the segments  82  of the ripple  66  that they sample expand across the ripple  66  symmetrically, as indicated by arrow  88 , such that they are symmetric about the average ion beam current. Hence, each point sees an average amount of ion beam current irrespective of its position on the wafer  12 . 
     FIG. 13  shows how this notional aligning of the average current  84  of the ripple  66  with the centre  86  of the wafer  12  relates to the actual situation of scanning the wafer  12  relative to the spot beam  34  along the series of scan lines  54 . The left hand side of  FIG. 13  shows a series of scan lines  54  extending across the wafer  12 , each scan line  54  corresponding to the centre of the spot ion beam  34  and hence the peak in its current profile  62 . The centre of the wafer  12  is also shown by line  86 . The right hand side of  FIG. 13  corresponds to  FIG. 5  and shows the virtual ribbon beam  60  created by the spot ion beam  34  as it travels along the scan lines  54 . The ripple  66  on the virtual ribbon beam  60  is correctly aligned such that it is at its average value  84  at the centre line  86 . As can be seen, the peaks in the ripple  66  correspond to the peaks in the individual current profiles  62 , and hence the position of scan lines  54  shown across the wafer  12 . The troughs in the ripple  66  correspond to the midpoints between scan lines  54 . Typically, the average current value  84  occurs half-way between a peak and a trough and so corresponds to a quarter of the way from one scan line  54  to the next  54 . Hence, if the scan lines  54  are spaced with a pitch T, the scan line  54  closest to the centre of the wafer  12  should be formed by scanning the wafer  12  such that the centre of the spot ion beam  34  passes along a line offset by T/4 from the centreline  86  of the wafer  12 . The remaining scan lines  54  may be arranged according to the pitch spacing T. 
   The present invention may be used with scan patterns other than the raster pattern illustrated in  FIG. 4 . For example, the controller  50  may direct the substrate holder  36  to effect a constant motion in the slow-scan direction rather than the stepped motion described above. Implementing the same reciprocal motion in the fast scan direction causes the ion beam  34  to trace a saw-tooth scan pattern  90  like that shown in  FIG. 14 . As before, spinning the substrate  12  while performing this saw-tooth scan also results in uniform dosing of the wafer  12 . This can be appreciated by equating the single saw-tooth scan pattern  90  to a pair of scan patterns  90   a  and  90   b  that are rotated relative to one another as shown in  FIG. 14 . Each scan pattern  90   a  and  90   b  comprises a series of parallel scan lines akin to the raster pattern  52  already described. Each scan pattern  90   a  and  90   b  will see uniform dosing of the wafer  12  in the same way as for the raster patterns  52 . Of course, performing these two uniform scan patterns  90   a  and  90   b  will result in an overall uniform scan pattern  90 . 
   The skilled person will appreciate that changes may be made to the above-described embodiment without departing from the scope of the present invention. 
   For example, the dimensions of the substrate  12  and the ion beam  34  may be changed, as may the pitch spacing, the angular velocity of the spinning substrate  12  and the scanning speed of the substrate  12 . 
   For example, the invention may be used with a variety of substrates  12 . For example, substrate material and substrate shape and dimensions may be varied without departing from the scope of the present invention. 
   The dimensions of the ion beam  34  may also be varied. Varying the dimensions of the ion beam  34  may require a consideration of at least some of the other parameters affecting the scanning. For example, changing the height of the ion beam  34  (i.e. the dimension in the slow-scan direction) may necessitate a change in the pitch spacing. Remembering how the virtual ribbon beam  60  is formed by the overlapping ion beams  34  corresponding to the ion beam&#39;s position at adjacent scan lines  52  shown in  FIG. 5 , decreasing the height of the ion beam  34  may necessitate a reduction in pitch to ensure overlap between adjacent scan lines  52 . Conversely, increasing the ion beam height may allow the pitch spacing to be increased without adversely affecting dose uniformity. 
   Varying the width of the ion beam  34  (i.e. the dimension in the fast-scan direction) may necessitate a change in the spin speed and/or translation speed of the wafer  12 . This is illustrated in  FIGS. 15   a  and  15   b .  FIG. 15   a  shows a wafer  12  being translated along line  68  through a wide ion beam  34  while the wafer  12  is spun in direction  72 . Wide ion beam  34  traces a wide spiral  92  over the wafer  12  during motion (only part of the spiral  92  traced during a single revolution is shown in  FIG. 15   a ). The spin speed of the wafer  12  is selected such that adjacent parts of the spiral  92  overlap, as will be clear from  FIG. 15   a .  FIG. 15   b  corresponds to  FIG. 15   a , but shows a narrower ion beam  34 . Translating the wafer  12  at the same speed and spinning the wafer  12  at the same speed results in adjacent parts of the spiral  92  no longer overlapping. To ensure that overlap is acquired, either the wafer  12  should be spun more quickly or the wafer  12  should be translated more slowly. 
   Obviously, the reverse of the above situations is true, namely that a change in pitch, translation speed or spin speed may require adjustment of the ion beam dimensions in the fast and slow scan directions. 
   A further parameter that may be varied is the offset of the centre of the ion beam  34  from the centre of the wafer  12 , and the consequent arrangement of adjacent scan lines  52 . The above arrangement demonstrates how uniform dosing may be achieved by ensuring a point in the ion beam  34  having average current passes through the centre line  86  of the wafer  12 . Where this average current resides in the ion beam  34  is of course dependent upon the ion beam profile itself. Determining ion beam profiles is well known in the art, see for example our co-pending U.S. patent application Ser. Nos. 11/589,156 and 11/029,004. Determining the average position from such a profile is straightforward. 
   Variations in how the relative movement between ion beam  34  and wafer  12  is effected are possible. For instance, the ion beam  34  may be made to move relative to the substrate  12  rather than the arrangement described above. Realistically, the substrate  12  will be rotated rather than trying to spin the ion beam  34  around the substrate  12 , but the ion beam  34  may be scanned in a raster pattern  52  across a spinning substrate  12 . 
   The scan pattern achieved using relative motion between the wafer  12  and ion beam  34  may also be varied. A traditional square raster pattern  52  is shown in  FIG. 4 , and a saw-tooth pattern  90  is shown in  FIG. 14 , but others are possible. For example, a series of arcuate scan lines is possible, whether those arcs correspond to a series of concentric, variable radius arcs (like those that may be created using the scanning arm of our co-pending U.S. application Ser. No. 11/588,432) or to a series of non-concentric arcs of fixed radius. The latter pattern results in a series of arcs that are not parallel, but are considered as being substantially parallel. 
   The above embodiments describe using a spot ion beam  34  to form a series of scan lines  52  by translating the substrate  12  while also rotating the substrate  12 . This creates a virtual ribbon beam  60 . However, the present invention may be implemented with an actual ribbon beam. For example, a substrate  12  may be translated through a ribbon beam in the usual way but, at the same time, the substrate  12  may be rotated. Thus, any non-uniformities in the ribbon beam will be averaged out over the spinning substrate  12  such that improved uniformity is achieved.