Abstract:
This invention relates to a method of implanting a substrate comprising scanning an ion beam relative to a substrate along a series of scan lines extending in a first direction, causing relative rotation between the substrate and the ion beam, scanning the ion beam along a second series of scan lines in a different direction. The implant recipe is changed during scanning in each direction such that different regions are produced during each scanning step. The regions so formed during the two scanning steps overlap such that different parts of the substrate receive different doses according to different recipes during the implantation process. The different recipes may result in different dopant concentrations, doping depths or even different dopant species.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a method of implanting a substrate using an ion beam where there is relative motion between the substrate and the ion beam.  
       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 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.  
         [0003]     Often, the cross-sectional profile of the ion beam is smaller 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 that is held in a fixed position, (b) mechanically moving the substrate whilst keeping the ion beam path fixed or (c) a combination of deflecting the ion beam and moving the substrate. Generally, relative motion is effected such that the ion beam traces a raster pattern on the substrate.  
         [0004]     Our co-pending U.S. patent application Ser. No. 10/119290 describes an ion implanter of the general design described above. A single substrate is held in a moveable substrate holder. While some steering of the ion beam is possible, the implanter is operated such that ion beam follows a fixed path during implantation. Instead, the substrate holder is moved along two orthogonal axes to cause the ion beam to scan over the substrate following a raster pattern.  
         [0005]     The above design is suitable for serial processing of wafers. Alternatively, wafers may be batch processed and this is most frequently done by placing the wafers on the arms of a spoked wheel that is then rotated. The rotating wafers pass through the ion beam which is scanned in a radial direction to ensure the entire wafer is implanted.  
         [0006]     The substrate may be a semiconductor wafer upon which thousands of semiconductor devices are fabricated. Generally, each wafer will be implanted to dope the wafer according to a particular recipe. This ensures that each of the devices on the wafer receives a uniform dosing, such that each device on the wafer will be identical to the others. Experiments may be performed to deduce the optimal parameters that define a particular recipe to achieve the desired doping of devices (or even indirectly, i.e. to determine a particular recipe to achieve desired operational characteristics of a device).  
         [0007]     Implanting each wafer to produce identical devices has the disadvantage that many more devices may be produced than may be required, and each semiconductor device may be worth hundreds of dollars As each device is so expensive, such waste is highly undesirable.  
         [0008]     To address this problem, it is known to implant different areas of a substrate separately, i.e. to trace a raster pattern over a fraction of the substrate. However, this requires the ion beam to be stopped and turned around on the substrate. This slowed motion results in that part of the substrate receiving an increased dose, contrary to requirements.  
         [0009]     An alternative approach is provided by EP-A-1,306,879 that describes a hybrid ion implanter. The ion beam is scanned across the substrate to form a raster pattern where the ion beam is scanned electromagnetically in one direction and the substrate is scanned mechanically in the other direction. The speed of the scan is changed in one of two ways. In a first mode, the first half of the substrate is scanned at a first speed, and the speed of the scan changed during the middle scan line such that the second half of the substrate is scanned at a second speed. In a second mode the speed of the scan is changed across the midpoint of each scan line.  
         [0010]     With either mode, a substrate results that has different doping levels in two halves according to the different scan speeds, but is otherwise identical. The wafer may then be rotated and the process repeated. For example, the substrate may be rotated through 90° thereby arriving at a substrate with four quadrants having different doping levels. EP-A-1,306,879 still suffers a disadvantage in that the ion beam is on the substrate as the scan speed is changed. Accordingly, a small portion of the substrate remains that has been exposed to a varying doping level that will not meet that specified for either of the types of device being produced.  
       SUMMARY OF THE INVENTION  
       [0011]     Against this background, and from a first aspect, the present invention resides in a method of implanting a substrate using an ion beam, comprising: causing relative movement between the substrate and the ion beam such that the ion beam scans across the substrate along a series of scan lines that extend in a first direction relative to the substrate, thereby to implant the substrate in a first pass; causing relative rotation between the substrate and the ion beam; and repeating the step of causing relative movement between the substrate and the ion beam such that the ion beam now follows a series of scan lines that extend in a second, different direction relative to the substrate, thereby to implant the substrate in a second pass.  
         [0012]     The method further comprises: performing a first part of the first pass according to a first implant recipe, changing a first property of the ion beam or substrate, and performing a second part of the first pass according to a second implant recipe thereby forming first and second regions of the substrate; and performing a first part of the second pass according to a third implant recipe, changing a second, different property of the ion beam or substrate, and performing a second part of the second pass according to a fourth implant recipe thereby forming third and fourth regions of the substrate. Both the third and fourth regions overlap the first and second regions.  
         [0013]     Thus, the above method allows different parts of the substrate to receive different doses according to different recipes during the implantation process. The different recipes may result in different dopant concentrations, doping depths or even different dopant species. Accordingly, if the substrate comprises a plurality of individual devices, not all devices will have the same properties. This allows processing of a single wafer to produce different devices.  
         [0014]     Furthermore, the method may be used in experiments to determine the effect on devices of changes in the recipe that follow from the changes in the two or more properties of the ion beam. Such a method is particularly useful where two interdependent properties are changed. Where the rotation is through 90°, the substrate is effectively divided by Cartesian co-ordinates, with variation of each property corresponding to each axis. Thus the substrate effectively becomes a map relating the variation of the two properties.  
         [0015]     The properties to be changed may be selected from a variety of experimental parameters. The requirement for the property is that by altering it, the resultant dosing of the substrate is changed. Examples of the property and how it may be changed are as follows. The position of the substrate may be changed, e.g. the substrate may be tilted relative to the ion beam such that the angle of incidence varies between the first and second relative motions. This affects implant depth and also how different features are implanted (i.e. shadowing behind edges on the wafer). How the regions are arranged across the substrate to be implanted may be changed. Overlapping regions ensure different areas of the substrate receive different doses because of the different directions of the scan lines during the first and second relative motions. The ion beam current may be varied to adjust the dosing level, although this may also be effected by changing the scanning speed of the ion beam/substrate holder or the overlap between adjacent scan lines where the ion beam is scanned relative to the substrate in a raster pattern. The ion beam energy may be altered to adjust the depth of implant. Other examples include changing the ion beam species, the ion beam profile or the ion beam divergence. In addition, where a plasma flood system is used in front of the substrate, the operational settings of the plasma flood system may be varied.  
         [0016]     There may be some commonality between the first, second, third and fourth recipes. For example, in a contemplated embodiment, the first and third implant recipes are the same. Of course, a pass may comprise forming more than two regions, each region being implanted according to a different recipe or some regions being implanted according to a shared recipe. The regions may be of equal size or may be differently sized. More than two passes may be used to implant the substrate.  
         [0017]     According to a second aspect, the present invention resides in a method of implanting a substrate using an ion beam, comprising: causing relative movement between the substrate and the ion beam such that the ion beam scans across the substrate along a series of scan lines that extend in a first direction relative to the substrate, thereby to implant the substrate in a first pass; causing relative rotation between the substrate and the ion beam; and repeating the step of causing relative movement between the substrate and the ion beam such that the ion beam now follows a series of scan lines that extend in a second, different direction relative to the substrate, thereby to implant the substrate in a second pass.  
         [0018]     The method further comprises: performing a first part of the first pass according to a first implant depth, changing a property of the ion beam or substrate, and performing a second part of the first pass according to a second implant depth thereby forming first and second regions of the substrate; and performing a first part of the second pass according to a third implant depth, changing a property of the ion beam or substrate, and performing a second part of the second pass according to a fourth implant depth thereby forming third and fourth regions of the substrate. Both the third and fourth regions overlap the first and second regions.  
         [0019]     In this way, different regions are formed on the substrate with different doping depth profiles. For example, some regions may have doping only at a narrow range of depths, whereas other regions may have doping over a far greater range of depths. Moreover, when combined with changing other properties of the ion beam and/or substrate, other variations with depth may be achieved. As an example, some depths may be more heavily doped than other depths, or the dopant may be varied between different doping depths.  
         [0020]     There may be some commonality between the implant depths, say the third implant depth is the same as the first implant depth.  
         [0021]     Optionally, the method according to either aspect may comprise causing a relative rotation of 90°, for example by rotating the substrate through 90°. However, other angles may be chosen. The ion beam may be scanned in the same direction relative to the substrate, but the substrate may be rotated between the first and second relative movements to ensure that they are not parallel. Optionally, causing relative movement comprises mechanically scanning the substrate relative to a substantially fixed ion beam. Preferably, causing relative movement makes the ion beam scan across the substrate according to a raster. Adjacent scan lines may be traced in the same direction or reverse directions.  
         [0022]     The present invention also extends to an ion implanter arranged to operate in accordance with any of the methods described above. Operation of the ion implanter may conveniently be controlled by a controller that is arranged to implement any of the methods described above. The controller may take a hardware or software form, e.g. the controller may be provided by a suitably-programmed computer. Hence, the present invention also extends to a computer program comprising program instructions that, when loaded into the controller, cause the controller to control the ion implanter to operate in accordance with any of the methods described above. The present invention also extends to a computer-readable medium having such a computer program recorded thereon. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     In order that the invention may be more readily understood, exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, in which:  
         [0024]      FIG. 1  is a schematic view of an ion implanter having a wafer holder for serial processing of wafers;  
         [0025]      FIG. 2  is a schematic representation showing an ion beam scanning across a wafer;  
         [0026]      FIG. 3  shows a wafer divided into three dosing stripes;  
         [0027]      FIG. 4  shows a wafer subjected to two passes of three dosing stripes, wherein the wafer has been rotated through 90° between passes thereby providing nine implant areas on the wafer;  
         [0028]      FIG. 5  corresponds to  FIG. 4 , but shows a wafer subjected to different dosing recipes; and  
         [0029]      FIG. 6  shows a wafer divided into three dosing stripes, wherein the dosing is gradually varied during each dosing stripe.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]      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  36 . 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.  
         [0031]     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.  
         [0032]     The mass-resolving slit  32  ensures that only ions having a chosen mass/charge ratio emerge from the mass analysis arrangement. 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.  
         [0033]     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 .  
         [0034]     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 short out the power supply  46 .  
         [0035]     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  50  shown in  FIG. 2 . Although the wafer  36  is scanned relative to a fixed ion beam  23 , the raster pattern  50  of  FIG. 2  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  36  that is scanned.  
         [0036]     The ion beam  23  is scanned over the wafer  36  to form a raster pattern  50  of parallel, spaced scan lines  52   1  to  52   n , where n is the number of scan lines. Each movement along a scan line  52   n  will be referred to herein as a ‘scan’, whilst each complete raster scan  50  will be referred to herein as a ‘pass’. Each wafer implant process is likely to comprise many individual ‘passes’.  
         [0037]     The ion beam  23  has a typical diameter of 50 mm, whereas the wafer  36  has a diameter of 300 mm (200 mm also being common for semiconductor wafers). In this example, a pitch of 2 mm in the Y-axis direction is chosen, leading to a total of 175 scan lines (i.e. n=175) to ensure the full extent of the ion beam  23  is scanned over the full extent of the wafer  36 . Only 21 scan lines are shown in  FIG. 2  for the sake of clarity.  
         [0038]     The raster pattern  50  for each pass is formed by scanning the ion beam  23  forwards along the X-axis direction to form the first scan line  52   1  until the ion beam is completely clear of the wafer  36 , by moving the ion beam  23  up along the Y-axis direction as shown at  72 , by scanning the ion beam  23  backwards along the X-axis direction until completely clear of the wafer  36  once more to form scan line  52   2 , by moving the ion beam  23  up along the Y-axis direction, and so on until the whole wafer  36  has seen the ion beam  23  thus completing the pass. As can be seen, each scan line  52   n  is of a common length, the length being sufficient such that the ion beam  23  is completely clear of the wafer  36  at the start and end of the middle scan line  52   n/2  that corresponds to the fullest width of the wafer  36 . Using scan lines  52   n  of a common length is not essential; shorter scan lines  52   n  may be used at the top and bottom of the wafer  36  that still ensure the ion beam  23  is completely clear of the wafer  36  at the start and end of each scan line  52   n .  
         [0039]     Obviously, other scan patterns are possible. For example, scan lines may be formed in only one direction, i.e. always left to right or always right to left. Also, a single pass may only implant some of the scan lines  52   n , i.e. a first pass may implant the odd scan lines  52   1, 3, 5, . . .  and a second pass may implant the even scan lines  52   2, 4, 6, . . .  . Thus, a complete implant may comprises a series of interlaced passes. Although straight scan lines are preferred, curved paths may be used. This in fact is the case for batch processing where multiple wafers are rotated on a spoked wheel while the ion beam in scanned radially: a raster pattern of a series of arcuate scan lines extending in the same direction (i.e. right to left or left to right depending upon the rotation of the spoked wheel) is formed across each wafer.  
         [0040]      FIG. 3  shows a wafer  36  that has been divided into three dosing stripes  100   a ,  100   b  and  100   c  that correspond to the top, middle and bottom thirds of the wafer  36  respectively. Each dosing stripe  100  corresponds to an area of the wafer  36  to be implanted according to a unique recipe. Put another way, the dosing recipe is changed between dosing stripes  100  such that a property of the ion beam  23  or wafer  36  is changed to affect the dosing that the next dosing stripe  100  receives.  
         [0041]     Each dosing stripe  100  occupies substantially one-third of the wafer  36 .  FIG. 3  shows the dosing stripes  100  to be separated vertically. This separation is exaggerated for the sake of clarity. In fact, only a small gap is left between each dosing stripe  100  to allow for inaccuracies in the implant process. Alternatively, each dosing stripe  100  may abut its neighbour(s) such that there is no gaps between the dosing stripes  100 .  
         [0042]     As will be evident from the foregoing description, each of the three dosing stripes  100  that occupies substantially one third of the wafer  36  comprises a plurality of scan lines  52   n . Put another way, to complete dosing of each scanning stripe  100 , the ion beam  23  is scanned across the wafer  36  along many scan lines  52   n  to dose the entire dosing stripe  100 .  
         [0043]     Once the bottom dosing stripe  100   c  is implanted, a property of the implant is adjusted with the ion beam  23  clear of the wafer  36 . If this is a property of the ion beam  23 , the ion beam  23  is allowed to settle before implanting the next dosing stripe  100   b  is started. Once dosing the middle dosing stripe  100   b  has been completed, the process is repeated such that a property of the implant is changed before the top dosing stripe  100   a  is implanted.  
         [0044]     Once this pass that sees the entire wafer  36  implanted is complete, the wafer is rotated and the process may be repeated such that dosing stripes are implanted once more. However, the scan lines  52   n  comprising each dosing stripe in the second pass will be in a different, non-parallel direction.  
         [0045]      FIG. 4  provides a specific example of such an implant. Similarly to  FIG. 3 ,  FIG. 4  uses three dosing stripes  100   a ,  100   b ,  100   c  that divide the wafer  36  into thirds. The first, bottom dosing stripe  100   c  is implanted using a first ion beam current to provide a relative dose of 55%. Once the ion beam  23  is clear of the wafer  36 , the ion beam current is decreased to a second value to provide a relative dose of 50%. The ion beam  23  is allowed to settle and checked using a Faraday detector or the like that may be provided by the beamstop  40 . Once the ion beam  23  has settled, the middle dosing stripe  100   b  is implanted. Once the ion beam  23  is clear of the wafer  36 , the ion beam current is again decreased to provide a relative dose of 45% and the ion beam  23  allowed to settle once more. Then, the top dosing stripe  100   a  is implanted.  
         [0046]     The wafer  36  is then rotated through 90° clockwise, as indicated at  102 , to allow further dosing to be performed by scanning the ion beam  23  in the X direction. The rotation of the wafer  36  means that the dosing stripes  100 ′ of the second pass, and hence the scan lines  52   n , extend orthogonally to the dosing stripes  100  of the first pass.  
         [0047]     The implantation process is repeated for the second pass, i.e. using three dosing stripes  100 ′ that divide the wafer  35  into thirds. This time the ion beam currents are kept the same, but the scanning speeds are changed between each dosing stripe  100 ′ to provide different relative doses. The bottom dosing stripe  100   c ′ is exposed to a relative dose of 60%, the middle dosing stripe  100   b ′ is exposed to a relative dose of 50% and the top dosing stripe  100   c ′ is exposed to a relative dose of 40%.  
         [0048]     The pattern formed by the two orthogonal passes, each pass comprising three dosing stripes, divides the wafer  36  into nine implant areas  104   1-9 . Each implant area corresponds to the overlap between two dosing stripes  100  and  100 ′. The varying relative doses across the dosing stripes  100  and  100 ′ produces implant areas that have a different total relative doses as follows: 
        implant area  104   1  receives 85%;     implant area  104   4  receives 90%;     implant areas  104   2  and  104   7  receive 95%;     implant area  104   5  receives 100%;     implant areas  104   3  and  104   8  receive 105%;     implant area  104   6  receives 110%; and     implant area  104   9  receives 115%.        
 
         [0056]     Thus, the wafer  36  provides a map of differently dosed devices, with decreasing doses along each axis. One axis will correspond to decreasing ion beam current and the other axis will correspond to increasing scan speeds.  
         [0057]     As mentioned above, the gap between each dosing stripe  100  and  100 ′ has been exaggerated in  FIGS. 3 and 4 . Hence, the gaps between the implant areas  104  are also exaggerated.  
         [0058]      FIG. 5  corresponds to  FIG. 4  in that it shows a wafer  36  dosed in two passes, each pass comprising three horizontal dosing stripes  100 ,  100 ′, and with a 90° rotation of the wafer  36  between passes. Again this creates a wafer  36  having nine different implant areas  104 .  
         [0059]     The first pass is performed using halo implants for each dosing stripe  100  (indicated by “halo” in  FIG. 5 ). The ion beam energy is varied between each dosing stripe  100 , thereby altering the depth of implant. The three energies are indicated by E 1 , E 2  and E 3  for dosing stripes  100   a ,  100   b  and  100   c  respectively. After rotation of the wafer  36 , the second pass is performed using source drain implants for the three dosing stripes  100 ′ (indicated by “SD”). Again, the three different energies are used for each dosing stripe (E 1 , E 2  and E 3  for  100   a ′,  100   b ′ and  100   c ′ respectively). Accordingly, a wafer  36  is obtained that has nine different implant areas. The nine different areas  104  can be identified as: 
         104   1  is Halo_E 1 /SD_E 1 ;      104   2  is Halo_E 1 /SD_E 2 ;      104   3  is Halo_E 1 /SD_E 3 ;      104   4  is Halo_E 2 /SD_E 1 ;      104   5  is Halo_E 2 /SD_E 2 ;      104   5  is Halo_E 2 /SD_E 3 ;      104   7  is Halo_E 3 /SD_E 1 ;      104   8  is Halo_E 3 /SD_E 2 ; and      104   9  is Halo_E 3 /SD_E 3 .        
 
         [0069]     A further variation on the above-described embodiments is that, in addition to changing a property of the implant between dosing stripes  100  and  100 ′, a property may be changed during all or some of the dosing stripes  100  and  100 ′. Such an embodiment is shown in  FIG. 6  where the wafer is once more divided into three dosing stripes  100 . For each dosing stripe  100 , the ion beam current is steadily increased from left to right. The top dosing stripe  100   a  is exposed to a relative ion beam current that varies from 40% to 70%; the middle dosing stripe  100   b  is exposed to a relative ion beam current that varies from 50% to 80%; and the bottom dosing stripe  100   c  is exposed to a relative ion beam current that varies from 30% to 60%. In this way a map may be formed on a wafer  36  that shows gradual variation of recipe properties across the wafer  36  rather than the incremental variation previously described.  
         [0070]     This implant may be achieved using the raster scan  50  of alternating scan line directions shown in  FIG. 2 . The ion beam current is gradually increased and next gradually decreased for successive scan lines  52   n . Alternatively, all scan lines  52   n  may be performed in the same direction in which case the ion beam current is always increased or only decreased across each scan line  52   n .  
         [0071]     This implant arrangement results in the right-hand side of the wafer  36  receiving a higher dose than the left. Other properties may be changed such that, for example, the implant depth varies across each dosing stripe  100 . This method may be repeated for multiple passes (with a rotation of the wafer  36  between passes) or some passes may be performed using uniform dosing across each dosing stripe  100 ,  100 ′, etc.  
         [0072]     It will be evident to the person skilled in the art that variations may be made to the above embodiments without departing from the scope of the invention defined by the accompanying claims.  
         [0073]     For example, only three different properties have been described in the above embodiments, namely ion beam current, ion beam energy and implant type (halo and source/drain). Other properties of the implant may also be varied between dosing stripes  100  and  100 ′. Examples include the position of the substrate (e.g. the substrate may be tilted relative to the ion beam such that the angle of incidence varies), the scanning speed of the ion beam/substrate holder, the overlap between adjacent scan lines, the ion beam species, the ion beam profile, the ion beam divergence or the operational settings of the plasma flood system. Only one property may be varied between dosing stripes  100  and  100 ′ or between passes, or more than one property may be varied.  
         [0074]     All of the above embodiments have been described using an example of three dosing stripes  100  and  100 ′. However, any number of dosing stripes  100  and  100 ′ may be chosen. In addition to having multiple dosing stripes  100  and  100 ′, only a single dosing stripe  100  and  100 ′ that does not cover the entire wafer  36  may be used. For example, the entire wafer  36  may be subjected to a uniform implant (say the whole wafer  36  is implanted using two dosing stripes  100  with relative ion beam currents of 25% and 50%): the wafer  36  is then rotated through 90° before being implanted with a single dosing stripe  100 ′ with a relative beam current of 50%. This will create four implant regions  104  with total relative ion beam currents of 25%, 50%, 75% and 100%.  
         [0075]     The term “stripe” is appropriate for the embodiments described above where the wafer  36  is notionally divided into horizontal bands. However, the wafer  36  may be divided into any arbitrary shapes and these shapes need not correspond to stripes.  
         [0076]     As will be understood, the present invention requires dosing to be performed in two non-parallel directions using different dosing recipes. The above embodiments use 90° rotations between implants, but this need not be the case. Any angle may be chosen, other than 180°, 360°, etc. that would result in a parallel direction. The rotation is relative between the wafer  36  and the ion beam  23 . While the above embodiments describe a rotation of the wafer  36  while keeping the ion beam scanning directions fixed, the reverse arrangement could be used. Specifically, the wafer  36  may be kept fixed, but the ion beam  23  may be controlled to scan across the wafer  36  in the Y direction before shifting in the X direction to scan once more in the Y direction. This would effectively see the raster pattern  50  shown in  FIG. 2  rotate through 90°.  
         [0077]     Moreover, the relative motion between ion beam  23  and wafer  36  may be varied between (i) keeping the wafer  36  fixed and scanning the ion beam  23 , (ii) keeping the ion beam  23  fixed and moving the wafer  36 , and (iii) a hybrid of scanning both wafer  36  and ion beam  23 .  
         [0078]     The number of passes may also be varied from the two described above. Any number of passes may be chosen to suit needs. In addition, the raster pattern  50  may be varied as has already been described.  
         [0079]     Whilst the embodiments have been described in the context of serial processing of a single wafer  36 , the present invention may also be applied to batch processing of wafers  36 . When applied to the spoked wheel arrangement described previously, a series of arcuate scan lines  52   n  results for each pass. After each pass, the wafers  36  may be rotated around their own axis at the end of each spoke and the spoked wheel then rotated once more to form another pass of arcuate scan lines  52   n  that cross those of the first pass at the angle through which the wafer  36  was rotated. Moreover, each pass may be divided into dosing stripes  100  and  100 ′ by stopping rotation of the wheel part-way through a pass, changing a property of the implant, and then starting rotation of the wheel and continuing the pass.  
         [0080]     The present invention has application generally across the many different fields of implant in addition to the semiconductor wafer processing described above.