Abstract:
This invention relates to shaped apertures in an ion implanter that may act to clip an ion beam and so adversely affect uniformity of an implant. In particular, the present invention finds application in ion implanters that employ scanning of a substrate to be implanted relative to the ion beam such that the ion beam traces a raster pattern over the substrate. An ion implanter is provided comprising: a substrate scanner arranged to scan a substrate repeatedly through an ion beam in a scanning direction substantially transverse to the ion beam path, thereby forming a series of scan lines across the substrate; and an aperture plate having provided therein an aperture positioned on the ion beam path upstream of the substrate scanner, and wherein the aperture is defined in part by an inwardly-facing projection.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to shaped apertures in an ion implanter that may act to clip an ion beam and so adversely affect uniformity of an implant. In particular, the present invention finds application in ion implanters that employ scanning of a substrate to be implanted relative to the ion beam such that the ion beam traces a raster pattern over the substrate. 
       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]    Ion beams often have approximately circular cross-sectional profiles and are much smaller than the substrate to be implanted. In order to implant the entire surface of the substrate, the ion beam and substrate must be 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. 
         [0004]    Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above. While some steering of the ion beam is possible, the ion implanter is operated such that ion beam follows a fixed path during implantation. Instead, a substrate is held in a substrate holder that is scanned along two orthogonal axes to cause the ion beam to trace over the wafer following a raster pattern like that illustrated in  FIG. 1 . 
         [0005]    The substrate is moved continuously in a single direction (the fast-scan direction) to complete a first scan line. The substrate is then stepped up a short distance orthogonally (in the slow-scan direction), and a second line is then scanned. This combination of reciprocating scan lines and indexed stepwise movement results in scanning of the whole surface of the substrate through the ion beam. The pitch of the scan lines is chosen to be less than the height of the ion beam, such that successive scan lines overlap. The pitch is carefully chosen with reference to the ion beam profile to ensure uniformity of implant: typical profiles see most of the ion beam current at the centre of the ion beam, and the current tails away towards the edges of the ion beam. An ideal profile would be a Gaussian, although such profiles are rarely seen in practice. Overlapping adjacent scan lines may be used to ensure uniform implants due to the smoothly varying profile. 
         [0006]    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, the wafer may be rotated between passes, for example four passes are made with a 90° twist of the substrate between each pass in a quad implant. Our U.S. patent application Ser. No. 11/527,594 provides more details of such scanning techniques. 
       SUMMARY OF THE INVENTION 
       [0007]    It has been realised that the use of overlapping scan lines to ensure uniformity of implant is particularly prone to a problem. Specifically, the success of this technique relies on a smoothly varying profile to the ion beam in the direction transverse to the fast-scan speed across the substrate. Preferably, the variation is a Gaussian variation. However, ion implanters often employ rectangular apertures along the ion beam&#39;s path. It has been appreciated that, should the ion beam clip straight edges of the aperture, it will lose its smooth variation at those clipped edges. In particular, this is severe where the ion beam is clipped by an edge extending in the same direction as the fast scan direction as this leads to a sharp edge on the ion beam that is effectively drawn along the substrate. Thus, a hard edge is formed on the scan lines across the substrate where they overlap, leading to periodic sharp jumps in dose level as you travel across the substrate in the slow scan direction (i.e. as you traverse across the scan lines). This is true irrespective of whether the substrate is scanned or the ion beam is scanned. The effect is described in more detail below with reference to  FIGS. 5 to 7 . 
         [0008]    Against the above background, and from a first aspect, the present invention provides an ion implanter comprising: an ion source arranged to generate an ion beam; ion beam optics arranged to guide the ion beam along an ion beam path; a substrate scanner arranged to scan a substrate relative to the ion beam in a scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate; and an aperture plate having provided therein an aperture defined by internal edges of the aperture plate, the aperture being positioned on the ion beam path upstream of the substrate scanner, and wherein the aperture is defined in part by an edge extending generally in the scanning direction provided with at least one inwardly-facing projection. 
         [0009]    The present application may find application in an ion implanter that uses deflection of the ion beam to effect scanning in the scanning direction. Such scanning is generally performed after the ion beam has cleared the final aperture on the ion beam path, i.e. the ion beam follows a fixed path through the apertures and then is scanned. Nonetheless, if the ion beam is clipped upstream by an aperture, the resulting ion beam profile will have a harder edge where it was clipped that may lead to a loss of uniformity in an implant. Accordingly, it is still useful to use an aperture plate as shaped above. 
         [0010]    However, the present invention is primarily intended for use in ion implanters that use mechanical scanning of the substrate relative to a fixed ion beam. The problem of ion beam clipping is often worse in such implanters because the final apertures tend to be positioned closer to the substrate than for scanned-beam implanters, meaning that any angular variation in the ion beam has less chance to smooth any hard edges imposed by clipping. Hence, preferably the substrate scanner is arranged to scan the substrate repeatedly through the ion beam in the scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate. 
         [0011]    Provision of the inwardly facing projection is advantageous as it addresses the problems of a sharp edge being formed if the ion beam clips that edge. This is because the inwardly facing projection must provide an edge that extends transversely to the scanning direction. Hence, a single edge extending along the scanning direction is avoided. For example, the projection may simply be a tooth that sees a step introduced to the edge that extends in the scanning direction. This alleviates the problem in that there will then be two sharp edges introduced to the ion beam that average as the ion beam is traced across the substrate (i.e. the substrate sees dosing contributions from both edges). As an improvement, the projection may not be a tooth, but may comprise a series of steps. 
         [0012]    Clearly, it is better to present a smoothly varying projection such that contributions to the ion beam&#39;s edge may be made at many positions transverse to the scanning direction. For example, the projection may be arcuate or v-shaped, thereby leading to a better smoothed edge to the ion beam should it clip that edge. Another contemplated shape is for the projection to have sinuous edges. For example, the projection may be generally v-shaped, but have sides that are each s-shaped (i.e. in the shape of an “s” or the mirror image of an “s”). These sides may have the shape of the side of a Gaussian peak, preferably with the peak extending in the fast-scan direction. Put another way, if the projection is provided on a top or bottom edge, the sides may be shaped like the side of a Gaussian peak lying on its side. Both sides may have such a shape such that the projection is symmetrical and adopts the shape of an onion dome, or at least the tapering top half of an onion dome. 
         [0013]    Preferably, the projection is provided centrally on the edge. This is advantageous as the projection is more likely to act on the centreline of the ion beam where the current will be greater. 
         [0014]    Rather than the edge comprising a single projection, it may comprise a plurality of inwardly-facing projections. These projections may all be alike, or they may differ. For example, the edge may comprise a plurality of like onion domes or the edge may comprise a plurality of teeth that extend inwardly to different depths. 
         [0015]    Generally, an aperture will be defined by two edges that extend generally in the scanning direction. If so, both edges are preferably provided with at least one inwardly-facing projection. Optionally, the edges are mirror images. 
         [0016]    From a second aspect, the present invention resides in a method of improving uniformity in an implant made by an ion implanter comprising an ion source arranged to generate an ion beam, ion beam optics arranged to guide the ion beam along an ion beam path, a substrate scanner arranged to scan a substrate relative to the ion beam in a scanning direction substantially transverse to the ion beam path such that the ion beam forms a series of scan lines across the substrate, and an aperture plate having provided therein an aperture defined by internal edges of the aperture plate, the aperture being positioned on the ion beam path upstream of the substrate scanner, the method comprising providing the aperture plate with an edge that partly defines the aperture, and that extends generally in the scanning direction but that is provided with at least a portion that extends in a direction other than the scanning direction. 
         [0017]    From this aspect of the invention, other shapes to the aperture edge are contemplated when trying to address the problem of uniformity in an implant where the ion beam may clip the aperture edge. For example, circular, ovoid, diamond shaped or hexagonal shaped apertures may be used to address uniformity. While not being as effective as an inwardly-facing projection acting on the centreline of an ion beam where the current is highest, these other shapes nonetheless act to smooth the edge if the ion beam is clipped. The method may also comprise providing the edge with the portion that extends over 25%, 50%, 75% or 90% of the length of the edge. Optionally, the portion may be positioned centrally. 
         [0018]    Other preferred features are defined by the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Embodiments of the present invention, along with aspects of the prior art, will now be described with reference to the accompanying drawings, of which: 
           [0020]      FIG. 1  illustrates a raster scan pattern of an ion beam across a wafer; 
           [0021]      FIG. 2  shows a conventional ion implanter; 
           [0022]      FIG. 3   a  shows a conventional aperture plate with an ion beam passing through the aperture without being clipped; 
           [0023]      FIG. 3   b  is an ion beam profile taken along line III-III of  FIG. 3   a , on the downstream side of the aperture plate; 
           [0024]      FIG. 4   a  shows a conventional aperture plate with an ion beam passing through the aperture such that its top and bottom are clipped; 
           [0025]      FIG. 4   b  is an ion beam profile taken along line IV-IV of  FIG. 4   a , on the downstream side of the aperture plate; 
           [0026]      FIG. 5   a  shows schematically an ion beam being scanned across a substrate; 
           [0027]      FIG. 5   b  is an ion beam profile taken along line V-V of  FIG. 5   a  to show a hypothetical top-hat current profile; 
           [0028]      FIG. 6   a  shows schematically an ion beam being scanned across a substrate twice to form two overlapping scan lines; 
           [0029]      FIG. 6   b  shows the dose received by the substrate of  FIG. 6   a  as a function of position across the scan lines; 
           [0030]      FIG. 7   a  shows schematically an ion beam being scanned across a substrate twice to form two separated scan lines; 
           [0031]      FIG. 7   b  shows the dose received by the substrate of  FIG. 7   a  as a function of position across the scan lines; 
           [0032]      FIG. 8  shows an aperture plate according to a first embodiment of the present invention; 
           [0033]      FIG. 9  shows an aperture plate according to a second embodiment of the present invention; 
           [0034]      FIG. 10  shows an aperture plate according to a third embodiment of the present invention; 
           [0035]      FIG. 11  shows an aperture plate according to a fourth embodiment of the present invention; 
           [0036]      FIG. 12  shows an aperture plate according to a fifth embodiment of the present invention; 
           [0037]      FIG. 13  shows an aperture plate according to a sixth embodiment of the present invention; 
           [0038]      FIG. 14  shows an aperture plate according to a seventh embodiment of the present invention; 
           [0039]      FIG. 15  shows an aperture plate according to an eighth embodiment of the present invention; and 
           [0040]      FIG. 16  shows an aperture plate according to a ninth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]      FIG. 2  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 . 
         [0042]    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. 
         [0043]    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. 
         [0044]    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 . 
         [0045]    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 held 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 ). 
         [0046]    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 travelled 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  49  positioned between the mass analysis stage  30  and the substrate position. 
         [0047]    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). 
         [0048]    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 such as raster patterns like that shown in  FIG. 1 . 
         [0049]      FIG. 3   a  shows an aperture plate  52  that may be placed on the ion beam path  34 . For example, the aperture plate  52  may correspond to one of the electrodes in the lens assembly  49  that is used to accelerate or decelerate ions in the ion beam  34  before reaching the substrate  12 . The aperture plate  52  has a conventional rectangular aperture  54  with top and bottom edges  56  and  58  that extend in the fast scan (x axis) direction. 
         [0050]      FIG. 3   a  shows the ion beam  34  passing comfortably through the aperture  54  provided in the aperture plate  52  such that there is clearance between the top edge  56  and the bottom edge  58  of the aperture  54 . 
         [0051]      FIG. 3   a  also indicates axes that are used to define the geometry within the ion implanter  10 . The ion beam  34  is taken to define the z axis, the y axis is defined as the vertical and the x axis is defined as the horizontal. In these embodiments, raster scans are described that see the ion beam  34  trace a series of scan lines horizontally across the substrate  12 , i.e. the x axis defines the fast scan direction and the y axis defines the slow scan direction where the substrate  12  is stepped between successive scan lines. 
         [0052]      FIG. 3   b  shows a profile  60  of the ion beam intensity (i.e. current) taken along a vertical line through the centre of the ion beam  34  at a position immediately downstream of the aperture plate  52 . This line is indicated as III-III in  FIG. 3   a . As the ion beam  34  is not clipped by the aperture plate  52 , a profile obtained immediately upstream of the aperture plate  52  would show good correspondence. As can be seen, the profile  60  approximates a Gaussian and may exhibit some asymmetry. 
         [0053]      FIGS. 4   a  and  4   b  are like  FIGS. 3   a  and  3   b , but instead show an enlarged ion beam  34 ′ that is now taller than the aperture  54 . Accordingly, the top and bottom of the ion beam  34  is clipped by the top edge  56  and bottom edge  58  of the aperture  54 . The corresponding profile  62  taken along line IV-IV shows the effect of the aperture  54  clipping the ion beam  34 . The profile displays sharp edges  64  at its top and bottom. These sharp edges  64  will extend across the profile in the x axis direction for the length of overlap between the ion beam  34  and the aperture  54 . Hence, the sharp edges  64  extend in the same direction as the fast scan direction. Such sharp edges  64  extending along the fast scan direction adversely affects uniformity of the dosing, as will be explained with reference to  FIGS. 5 to 7 . 
         [0054]      FIG. 5   a  shows an ion beam  34  being scanned across a substrate  12  along a first scan line  66 . This is effected by scanning the ion beam  34  across a stationary substrate  12  or by moving the substrate  12  relative to a fixed ion beam  34 .  FIG. 5   b  shows a hypothetical profile for the ion beam  34  taken vertically along line V-V. The profile shown at  68  is top-hat shaped, this shape being chosen as the ultimate demonstration of the effects of sharp edges  64 . 
         [0055]      FIG. 6   a  shows the substrate  12  after the top-hat ion beam  34  has performed two scan lines  66  and  70  of a raster pattern.  FIG. 6   b  shows the dose  72  received by the substrate  12  in the y-axis direction across the two scan lines  66  and  70 . As can be seen, a uniform dose is achieved for the parts of the substrate  12  that see only one pass of the ion beam  34 , but the small slice of the substrate  12  that sees two passes of the ion beam  34  where the scan lines  66  and  70  overlap has a spike  74  equivalent to twice the dose. Hence, completing a full raster scan like that shown in  FIG. 1  in this manner will lead to a substrate  12  with narrow stripes of high dosage, thereby ruining the desired uniformity. 
         [0056]      FIGS. 7   a  and  7   b  correspond to  FIGS. 6   a  and  6   b , but show a situation where a small gap is left between the scan lines  66  and  70 . This produces a dose profile  76  that exhibits a sharp dip  78  for the part of the substrate  12  between the scan lines  66  and  70 , as shown in  FIG. 7   b . So, completing a full raster scan like that shown in  FIG. 1  in this manner will lead to a substrate  12  with narrow stripes, this time of no dosage, again ruining the desired uniformity. 
         [0057]    As will be appreciated, if the two scan lines  66  and  70  can be made to abut perfectly leaving no gap and with no overlap, perfect uniformity may be achieved. However, this is impossible to achieve, meaning that there will always be some overlap or separation leading to striping of the substrate  12 . 
         [0058]    It will also be understood that the problems of sharp edges  64  caused by the ion beam  34  being clipped by the aperture  54  will also lead to a loss of uniformity in implants. This is because, as explained above, the smoothly varying profile of an unclipped ion beam  34  is used to achieve uniform dosing by overlapping adjacent scan lines. Loss of the smoothly varying tails destroys the compensating effect otherwise provided by the overlapping scan lines. 
         [0059]      FIGS. 8 to 16  show nine exemplary designs of aperture plates  52  with apertures  54  shaped to reduce the loss of uniformity in the event that an ion beam clips the top and bottom edges of the aperture  54 . All the apertures  54  are shaped such that the top edge  56  and bottom edge  58  are not linear in the fast-scan direction (along the x axis). Conveniently, this may be achieved by providing one or more inward projections or salients to the top edge  56  and likewise for the bottom edge  56 . 
         [0060]      FIG. 8  shows an aperture plate  52   a  provided with an aperture  54   a  having a top edge  56   a  provided with a broad tooth  57   a  that projects inwardly such that the aperture  54   a  is at its narrowest midway across in the x-axis direction. This narrowing is more pronounced because the bottom edge  58   a  is correspondingly shaped, with a matching tooth  59   a.    
         [0061]    In normal use, the ion beam  34  is intended to pass through the aperture  54   a  without being clipped as shown by the solid hashed cross-section at  34 . However, should the ion beam  34  increase in size, as indicated by the dashed cross-section at  34 ′, the teeth  57   a  and  59   a  provided on the top edge  56   a  and bottom edge  58   a  respectively may clip the ion beam  34 ′. Any single profile taken vertically through the ion beam  34 ′ at any x-axis position will still display sharp edges like those shown at  64  of  FIG. 4   b . However, imagining a succession of slices taken while moving along the x-axis to show successive y-axis profiles demonstrates that the y-axis position of the sharp edges varies between two positions corresponding to the base and end of the teeth  57   a  and  59   a . As the ion beam  34  is scanned in the along each scan line, e.g. those shown at  66  and  70  in  FIGS. 6   a  and  7   a , the substrate  12  sees these two edge positions and effectively averages the two. Hence, the otherwise single sharp edge of the ion beam  34  is to some extent smeared out by the toothed top and bottom edges  56   a  and  58   a  such that a smoother variation is obtained for the top and bottom of the ion beam  34 . 
         [0062]      FIG. 9  shows a similar aperture plate  52   b , this time provided with an aperture  54   b  having a top edge  56   b  provided with two teeth  57   b  and a bottom edge  58   b  similarly provided with two teeth  59   b . Stepped shoulders  61  are also provided in the corners of the aperture  54   b  that extend inwardly as far as the teeth  57  and  59   b . As will be appreciated, this arrangement also provides edges at two positions and so works similarly to the arrangement of  FIG. 8 . An improvement may be made by varying the depth of the teeth  57   b ,  59   b  and the shoulders  61 . In this way, three or four edges may be formed in the ion beam  34 ′ and so will have a greater effect on smearing the otherwise sharp edge of the ion beam  34 ′. 
         [0063]      FIG. 10  shows a further embodiment where the aperture plate  52   c  is provided with a single stepped projection on its top edge  56   c  and a similar stepped projection on its lower edge  58   c . The steps move progressively inwardly to the vertical centreline of the ion beam  34 ′ before moving progressively outwardly. As a result, four edges are introduced into the edge of a clipped ion beam  34 ′. 
         [0064]      FIG. 11  shows an aperture plate  52   d  broadly similar to that of  FIG. 10 , but here the stepped projections to edges  56   d  and  58   d  step inwardly twice before stepping outwardly at the centre and follow the reverse arrangement on the other side of each edge  56   d  and  58   d . Each step in an edge  56   d  or  58   d  is chosen to be at a different height, thereby imparting six edges to the ion beam  34 ′. 
         [0065]    While the above embodiments are effective in addressing problems in uniformity of implant due to clipped ion beams  34 ′, there remains some residual loss of uniformity due to the stepped nature of the projections. Hence, it is preferred to use projections having sides that extend at an angle to the scanning direction so as to provide a continuous range of the depth of the edges  56  and  58  into the aperture  54 .  FIG. 12  shows an embodiment of this concept in an aperture plate  52   e  provided with an aperture  54   e  defined by an arcuate top edge  56   e  that projects inwardly such that the aperture  54   e  is at its narrowest midway across in the x-axis direction. This narrowing is more pronounced because the bottom edge  58   e  is correspondingly shaped, i.e. with an inward arc. 
         [0066]    As before, any single profile taken vertically through the ion beam  34 , at any x-axis position will still display sharp edges like those shown at  64  of  FIG. 4   b . However, imagining a succession of slices taken while moving along the x-axis to show successive y-axis profiles demonstrates this time that the y-axis position of the sharp edges varies continuously, first inwardly as the arcuate projections move inwardly and then outwardly as the arcuate projections move outwardly. As the ion beam  34  is scanned along each scan line, e.g. those shown at  66  and  70  in  FIGS. 6   a  and  7   a , the substrate  12  sees all of this range of varying edge positions. Hence, the otherwise sharp edge of the ion beam  34  is smeared out more successfully by the projecting top and bottom edges  56   a  and  58   a  such that a smooth variation is retained for the top and bottom of the ion beam  34 . 
         [0067]      FIG. 13  shows an alternative arrangement where the aperture plate  52   f  is provided with an aperture  54   f  with a top edge  56   f  and a bottom edge  58   f  shaped to provide inwardly facing v-shaped projections. As will be appreciated, the aperture  54   f  acts in the same way as aperture  54   e , i.e. it smears out smoothly the sharp edges caused by the ion beam  34  being clipped by the top edge  56   f  and the bottom edge  58   f.    
         [0068]    As per the stepped arrangement of  FIG. 9 , multiple projections may be provided on the top edge  56  and bottom edge  58 .  FIG. 14  shows an aperture plate  52   g  having an aperture  54   g  with a top edge  56   g  provided with two symmetrically-disposed v-shaped projections. Likewise, the bottom edge  58   g  is also provided with a pair of inwardly-facing v-shaped projections.  FIG. 15  shows a still further aperture plate  52   h , this time with top and bottom edges  56   h  and  58   h  provided with four v-shaped projections. Although the projections are all shown projecting inwardly to the same depth, this need not be the case. The same use of multiple projections per edge  56 ,  58  may be used with a repeated pattern of the arcuate shape of  FIG. 8 . 
         [0069]    Yet another arrangement is shown in  FIG. 16 . Here, the aperture plate  52   i  has an aperture  54   i  with correspondingly-shaped top and bottom edges  56   i  and  58   i . Each edge  56   i ,  58   i  is shaped to have two projections  80  and smoothly curving shoulders  82 . The projections  80  are defined by pairs of sinuous edges  84  that meet at a rounded tip  86 . The sides  84  curve with the shape of a Gaussian, although rotated through 90° from the vertical such that the projections  80  resemble onion domes (like those seen on churches in Russia). Adjacent projections  80  meet a rounded tip  88 , and the outermost two edges  88  blend smoothly with the shoulders  82 . The shoulders  82  also share the sinuous shape of the projection&#39;s edges  84 , although are larger such that they extend further. 
         [0070]    The skilled person will appreciate that changes may be made to the above-described embodiment without departing from the scope of the present invention defined by the appended claims. 
         [0071]    For example, the number of projections may be varied. The shape of the projections may also be varied, provided the resulting shape still serves to project inwardly. Where multiple projections are used on an edge, these projections need not share a common shape. The depth to which the projections protrude inwardly may be varied according to need. A balance is to be struck between deeper projections having a greater smearing effect and the increased likelihood of deeper projections clipping the ion beam  34 . 
         [0072]    Although described with respect to linear raster scans like that shown in  FIG. 1 , the present invention has application in any scanning that results in a pattern formed of adjacent or overlapping scan lines. For example, in parallel implanters several substrates may be held on spokes of a wheel that is spun while being translated such that a series of arcuate scan lines are formed on each substrate.