Abstract:
This invention relates to an ion beam monitoring arrangement for use in an ion implanter where it is desirable to monitor the floating potential across an ion beam used for implantation. The invention provides a ion beam monitoring arrangement comprising a device configured to measure the floating potential of an ion beam when incident thereon, wherein the device is coupled to a substrate support so as to face outwardly in a position so as not to be obscured by a substrate of the contemplated size when held by the substrate holder. Thus, measurements of the floating potential may be taken with a substrate held in place. The ion beam monitoring arrangement may be used to move the device into the ion beam in much the same way as it used to scan a substrate through the ion beam.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an ion beam monitoring arrangement for use in an ion implanter where it is desirable to monitor the floating potential across an ion beam used for implantation. This invention also relates to an ion implanter including such a ion beam monitoring arrangement, and to methods of measuring the floating potential of an ion beam and of implanting a substrate. 
         [0002]    In particular, the present invention relates to measuring the floating potential in and around an ion beam at or close to the plane of a substrate being implanted so as to be able to control the ion beam to minimise charge accumulation caused by the ion beam. This may be performed through ion beam tuning and/or a plasma flood system. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 a substrate is held in place by a substrate holder. Often the ion beam size 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. 10/119,290 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]    Ion beams often have a residual positive charge that is detrimental for at least two reasons. First, it causes space-charge blow-up of the ion beam, particularly at the substrate where the ion beam tends to be at its least energetic. Second, the net positive charge in and around the ion beam seen at the plane of the substrate is transferred to the substrate during implantation, and this accumulation of charge can damage devices being fabricated on the substrate. This is a particular problem in mechanically-scanned serial implanters where scan speeds are slow. In order to address the problem of charge accumulation, the ion implanter may be used to tune the ion beam and/or an electron source may be used to introduce neutralising electrons to the ion beam. The electron source may comprise a plasma flood system placed immediately upstream of the substrate. 
         [0006]    It is often desirable to measure the floating potential in and around an ion beam in order to improve control of the implantation process. For example, the floating potential of the ion beam is a measure of the net positive charge carried by the ion beam. As such, a measurement of the floating potential can be used to control beam tuning and operation of the plasma flood system (or other electron source) to improve ion beam neutrality. For example, where the measurement indicates that the net positive charge of the ion beam is unacceptably high, the plasma flood system may be adjusted to produce more electrons. 
         [0007]    WO02/23583 describes a system for taking measurements of the floating potential of an ion beam at the wafer plane. A test wafer is provided that may be loaded onto a wafer holder and held in the ion beam. The test wafer comprises an array of sensors formed on its surface. The sensors essentially comprise conductive tracks extending from the front face of the test wafer, through the test wafer, to a ring of contacts provided around the periphery of the back face of the test wafer. An interface device connects with the contacts to take the electrical signals to external circuitry used to measure the floating potential of the ion beam. Essentially, the circuitry comprises a high input impedance voltmeter. By measuring the floating potential at each sensor, a map of the floating potentials across the ion beam can be derived. 
         [0008]    By the very nature of the test wafer, measurements cannot be made during an implant. In particular, for a measurement to be taken, an implant must stop, a wafer must be unloaded from the wafer holder, the test wafer loaded, the measurements taken, the test wafer unloaded, a new wafer loaded and only then can implanting resume. Clearly, this is a time-consuming process that reduces the throughput of processed wafers through the ion implanter. 
       SUMMARY OF THE INVENTION 
       [0009]    Against this background, and from a first aspect, the present invention resides in a ion beam monitoring arrangement for an ion implanter, the ion beam monitoring arrangement comprising: a substrate support comprising a support arm and, mounted thereto, a substrate holder for holding a substrate of contemplated size; an actuator arranged to cause relative motion between the substrate holder and the ion beam; and a device configured to measure the floating potential of an ion beam when incident thereon; wherein the device is coupled to the substrate support so as to face outwardly in a position so as not to be obscured by a substrate of the contemplated size when held by the substrate holder. 
         [0010]    Of course, the relative motion may correspond to moving the substrate support relative to a fixed ion beam, scanning the ion beam relative to a fixed substrate support, or a mixture of the two. Advantageously, the device is located on the substrate support such that it is not obscured when a substrate is in position on the substrate holder. For example, when used in an ion implanter, the device will still enjoy line of sight with the ion beam, even when a substrate such as a semiconductor wafer is in position on the substrate holder. 
         [0011]    As measurements of the floating potential of the ion beam may be taken with a substrate in situ, the present invention removes the need to unload and load the substrate before and after measurements are taken, thus increasing the throughput of the associated machine, be it an ion implanter or any other type of equipment. A further advantage is that measurements may be taken during processing of the substrate. For example, in an ion implanter, measurements may be taken between implanting along scan lines. 
         [0012]    The device may be located in a variety of positions on the substrate support, and may be positioned to face a variety of directions. For example, the device may be positioned so as to face in the same direction as a substrate when held by the substrate holder, or in the opposite direction or at 90° to that direction. Possible arrangements include coupling the device to the support arm or to the substrate holder. 
         [0013]    Optionally, the device is mounted within the support arm so as to face outwardly through an aperture provided in the support arm. Alternatively, the device may be mounted to the substrate holder, for example on the back face to face away from the substrate or on an edge to face at 90° to the substrate. Where it is desired for the device to face in the same direction as the substrate, the device may be mounted to a part of the substrate holder that extends beyond the extent of a substrate of the contemplated size when held by the substrate holder. This part may be either integral with the substrate holder, or may be attached to the substrate holder. Optionally, the device is mounted within the substrate holder so as to face outwardly through an aperture provided in the substrate holder. 
         [0014]    Preferably, the actuator is arranged to drive the support arm thereby causing the substrate holder to move through the ion beam. Associating the device with the a substrate support that effects mechanical scanning of the substrate through the ion beam is convenient as the substrate support may be used to drive the device into a desired position for one or measurements of the floating potential to be taken. 
         [0015]    Optionally, the support arm is rotatable about its longitudinal axis. This provides a convenient way of rotating the support arm where the device does not face in the same direction as a substrate held in position on the substrate holder. This may be used, for example, to make the device face an ion beam in an ion implanter. 
         [0016]    Preferably, the actuator has a part operable to drive the support arm along its length. Additionally, or alternatively, the actuator may have a part operable to drive the support arm perpendicularly to its length. This provides a convenient mechanism for allowing the device to be moved to different positions in and around the ion beam where measurements of the floating potential in and around the ion beam may be taken. 
         [0017]    The device may be integral with the substrate support. Optionally, the device comprises a front, insulating plate with an aperture provided therein. The device may be attached to the substrate support using the insulating plate. The device may comprise a sensor positioned behind the aperture of the insulating plate. In a contemplated embodiment, the sensor is connected electrically to a resistor and a current meter. Alternatively, a non-contacting voltmeter may be used as the device. 
         [0018]    Optionally, the device is one of an array of like devices. Using an array of devices means that a profile may be obtained of the floating potential of an ion beam quickly, in that the actuator need only be used to cause relative motion such that the support arm is in a single position relative to the ion beam where the array of sensors cover the ion beam. The ion beam may then be sampled, with each device providing a measurement of the floating potential of the ion beam at its position. Of course, the trade-off against this increased speed is the expense of providing the additional devices, plus the corresponding means for processing the many signals either in parallel or sequentially. 
         [0019]    From a second aspect, the present invention resides in an ion implanter comprising one of the ion beam monitoring arrangements described above. 
         [0020]    From a third aspect, the present invention resides in a method of measuring the floating potential of an ion beam using an ion implanter including any of the ion beam monitoring arrangements described above, the method comprising: using the actuator to cause relative motion between the support arm and the ion beam such that the device adopts a first measuring position in the ion beam; and using the device to measure the floating potential at the first measuring position. A display may be provided of the floating potential, either graphically or as text. 
         [0021]    The actuator may be used such that the device adopts an array of measuring positions in the ion beam, and using the device to measure the floating potential at each measuring position. The floating potentials so measured may be displayed to provide a profile of the floating potential of the ion beam. This is best done graphically, for example using colour coding to represent the magnitude of values. Further computation may be performed to provide continuous data rather than the array of values measured. For example, interpolation may be used to derive a contour plot of floating potentials in the ion beam. 
         [0022]    The actuator may be used to cause relative motion such that the ion beam traces any number of paths over the substrate, such as a raster pattern. An infinite variety of the number of data points and their arrangement is possible. It will be obvious that some are more useful than others. For example, regularly spaced arrays of points are useful, as our some non-uniform arrays such as those where a greater density of points exists at and around the centre of the ion beam. 
         [0023]    Preferably, the actuator is used to cause the substrate support to move relative to a fixed ion beam. 
         [0024]    From a fourth aspect, the present invention resides in a method of implanting a substrate using an ion implanter including any of the ion beam monitoring arrangements described above, the method comprising: generating an ion beam using an ion source of the ion implanter; guiding the ion beam through the ion implanter towards the substrate holder using ion optics of the ion implanter; using the actuator to cause relative motion between a substrate held by the substrate holder and the ion beam to form a first scan line as the ion beam passes over the substrate; repeating the above step of using the actuator so as to cause the ion beam to scan over the substrate repeatedly to form a series of scan lines; and, between two successive steps of using the actuator to form scan lines across the substrate, using the actuator to cause relative motion such that the device adopts a first measuring position in the ion beam and using the device to measure the floating potential at the first measuring position. 
         [0025]    Such a method is advantageous as it allows measurements to be made of the floating potential part way through an implant. This allows corrections to be made, e.g. the ion beam may be adjusted or the operation of a plasma flood system may be adjusted to improve ion beam neutrality as measured by the device. Of course, such adjustments may still be made whenever measurements of the floating potential are made, and not just when they are made between scan lines. For example, measurements of the floating potential may be made between multiple passes over the substrate during an implant. 
         [0026]    Moreover, there is no need to remove the substrate during measurement of the floating potential; this would be particularly undesirable and extremely difficult to implement where the measurement is taken part way through an implant such that only some of the necessary scan lines had been traced. 
         [0027]    Of course, an array of measurements may be taken between scan lines by using the actuator such that the device adopts an array of measuring positions in the ion beam, and using the device to measure the floating potential at each measuring position. 
         [0028]    Preferably, the actuator is used to cause the substrate support to move relative to a fixed ion beam. 
         [0029]    The present invention also extends to a controller arranged to implement any of the above methods, to a computer programmed to implement any of the above methods, to a computer program that when loaded and executed on a computer, causes that computer to implement any of the above methods, and to a computer readable medium carrying such a computer program. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Examples of the invention will now be described with reference to the accompanying drawings, in which: 
           [0031]      FIG. 1  is a schematic representation of a conventional ion implanter; 
           [0032]      FIG. 2   a  shows a schematic side view of an ion implanter in which a substrate is mounted on a substrate support; 
           [0033]      FIG. 2   b  shows a part section along line AA of  FIG. 1   a;    
           [0034]      FIG. 3  is a schematic representation of a device for measuring the floating potential of an ion beam according to an embodiment of the present invention; 
           [0035]      FIG. 4  is a perspective view of a substrate support comprising the device of  FIG. 3  located-on the arm; 
           [0036]      FIG. 5  is a perspective view of a substrate support comprising the device of  FIG. 3  located on a tab mounted to the edge of the wafer holder; 
           [0037]      FIG. 6  is a perspective view of a substrate support comprising the device of  FIG. 3  located on the reverse of the wafer holder; and 
           [0038]      FIG. 7  is a perspective view of a substrate support comprising the device of  FIG. 3  located on the edge of the wafer holder. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    In order to provide a context for the present invention, an exemplary application is shown in  FIG. 1 , although it will be appreciated that this is merely an example of an application of the present invention and is in no way limiting. 
         [0040]      FIG. 1  shows a known ion implanter  10  for implanting ions in semiconductor wafers  12 . 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 carry on eventually to strike the semiconductor wafer  12 . 
         [0041]    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. 
         [0042]    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. 
         [0043]    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 . 
         [0044]    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 ). 
         [0045]    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). This high energy beam of ions is less susceptible to space-charge blow-up. 
         [0046]    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  34  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 wafer  12  to be implanted or a beam stop  38  when there is no wafer  12  or wafer holder in the target position. Prior to arriving at the wafer  12 , the beam is decelerated using a deceleration lens assembly  35  positioned between the mass analysis stage  30  and the target position. 
         [0047]    The ion beam  34  then passes through a plasma flood system  37  located immediately in front of the wafer  12 . The plasma flood system  37  operates to introduce electrons into the ion beam  34  therein to neutralise any net positive charge in the ion beam  34 , and to ensure the wafer  12  does not charge due to the incident ion beam  34 . 
         [0048]    Firstly, the ion beam  34  strikes the wafer  12  that is held by a scanning arm assembly  51  (as shown in  FIG. 1 ). The wafer  12  is mounted on a wafer holder  36 , wafers  12  being successively transferred to and from the wafer holder  36 , for example through a load lock (not shown). 
         [0049]    The ion implanter  10  operates under the management of a controller, such as a suitably programmed computer  50 , that also receives diagnostic measurements from the ion implanter  10  during its operation. The controller  50  optimises performance of the ion implanter  10  and implements selected implant recipes. 
         [0050]    A simplified schematic side view of the ion implanter  10  of  FIG. 1  is shown in  FIG. 2   a  and a corresponding part sectional view along the line AA of  FIG. 2   a  is shown in  FIG. 2   b . The ion implanter  10  includes ion source  14  arranged to generate ion beam  34  that passes through the mass analyser  30 . The ions  34  exiting the mass analyser  30  are decelerated (the deceleration lens assembly  35  and the plasma flood system  37  are omitted from  FIG. 2   a  or  2   b ). The process chamber  40  contains a wafer  12  to be implanted, as may be seen in  FIG. 2   b , and beam stop  38 . 
         [0051]    A scanning arm assembly  51  is shown in  FIGS. 2   a  and  2   b  (explained in more detail below) that is of the type that mechanically scans the wafer  12 , although it is to be remembered that the present invention may be used in an ion implanter  10  where the ion beam  34  is scanned relative to a fixed wafer holder  36 . The scanning arm assembly  51  of  FIGS. 2   a  and  2   b  permits movement of the wafer  12  in multiple directions, while the ion beam  34  is maintained along a fixed path relative to the process chamber  40  during implant. 
         [0052]    The wafer  12  is mounted electrostatically upon the wafer holder or chuck  36  of the scanning arm assembly  51  that also comprises an elongate support arm  52  to which the chuck  36  is connected. The chuck  36  and support arm  52  together comprise a substrate support. The support arm  52  extends out through the wall of the process chamber  40  in a direction generally perpendicular with the direction of the ion beam  34 . The support arm  52  passes through a slot  54  (see  FIG. 1   b ) in a rotor plate  56  which is mounted adjacent to a side wall of the process chamber  40 . The end of the support arm  52  is mounted through a sledge  58 . The support arm  52  is substantially fixed relative to the sledge  58  in the Y-direction as shown in  FIGS. 2   a  and  2   b . The sledge  58  is movable in a reciprocating manner relative to the rotor plate  56  in the direction Y shown in  FIGS. 2   a  and  2   b . This permits movement, also in a reciprocating manner, of the wafer  12  in the process chamber  40 . 
         [0053]    To effect mechanical scanning in the orthogonal, X-direction (that is, into and out of the plane of the paper in  FIG. 2   a  and left to right in  FIG. 2   b ), the support arm  52  is mounted within a support structure. The support structure comprises a pair of linear motors  60  that are spaced from the longitudinal axis of the support arm  52  above and below it as viewed in  FIG. 2   a . Preferably, the motors  60  are mounted around the longitudinal axis so as to cause the force to coincide with the centre of mass of the support structure. However, this is not essential and it will of course be understood that a single motor may instead be employed to reduce weight and/or cost. 
         [0054]    The support structure also includes a slide  62  which is mounted in fixed relation to the sledge  58 . Movement of the linear motors  60  along tracks (not shown in  FIG. 2   a  or  2   b ) disposed from left to right in  FIG. 2   b  causes the support arm  52  likewise to reciprocate from left to right as viewed in  FIG. 2   b . The support arm  52  reciprocates relative to the slide  62  upon a series of bearings. 
         [0055]    With this arrangement, the wafer  12  is movable in two orthogonal directions (X and Y) relative to the axis of the ion beam (Z) such that the whole wafer  12  can be passed across the fixed direction ion beam  34 . 
         [0056]      FIG. 2   a  shows the sledge  58  in a vertical position such that the surface of the wafer  12  is perpendicular to the axis of the incident ion beam  34 . However, it may be desirable to implant ions into the wafer  12  at an angle to the ion beam  34 . For this reason, the rotor plate  56  is rotatable about an axis defined through its centre, relative to the fixed wall of the process chamber  40 . In other words, the rotor plate  56  is able to rotate in the direction of the arrows R shown in  FIG. 2   a  thereby causing the wafer  12  to rotate in the same sense. 
         [0057]    Further details of the above arrangement can be found in our U.S. Pat. No. 6,956,223, the contents of which are incorporated herein in their entirety. 
         [0058]    In a preferred arrangement, the chuck  36  is controlled to move according to a sequence of linear movements across the ion beam  34  in the X-coordinate direction, with each linear movement separated by a stepwise movement in the Y-coordinate direction. The resulting scan corresponds to a raster pattern. The reciprocating scanning action of the wafer  12  ensures that all parts of the wafer  12  are exposed to the ion beam  34 . 
         [0059]      FIG. 3  provides a schematic representation of a device  64  for measuring the floating potential in and around an ion beam  34 . The device  100  may be provided on the scanning arm assembly  51  at various locations, as will be described in further detail with reference to  FIGS. 4 to 7 . 
         [0060]    The device  64  is mounted to a surface  66  of the scanning arm assembly  51  via an insulating plate  68 . The surface  66  is at ground potential. Concentric apertures  70  and  72  are provided in the surface  66  and insulating plate  68  respectively. The insulating plate  68  is circular in cross-section and has a diameter of 30 mm. The diameter of the aperture  72  provided in the insulating plate  68  is 8 mm. The diameter of the aperture  70  provided on the surface  66  of the scanning arm assembly  51  is larger. A sensor  74  with a narrow tip  76  is housed within the scanning arm assembly  51  such that the sensor tip  76  terminates at the front face of the insulator plate  68  and to leave a void around the sensor  74 . The sensor tip  76  has a diameter of 5 mm. The sensor  74 , including tip  76 , is preferably made from graphite or silicon. 
         [0061]    The sensor  74  is electrically connected in series to a resistor  78 , a current meter  80 , and then to ground  82 . The insulator plate  68  allows the space surrounding the sensor  74  to float to the potential of the ion beam  34 . The sensor  74  receives charge from the ion beam  34 , either net positive or negative, until the current saturates. The current flow may be measured by the current meter  80 . When a measurement is taken, the scanning arm assembly  51  is moved such that device  74  occupies the desired position. Once the current has saturated, the value provided by the current meter  80  is used by the controller  50  to calculate the floating potential at that point. This potential is merely the product of the measured current and the resistance of the resistor  78  (this value is stored in an associated memory of the controller  50 ). In this embodiment, the resistance is chosen to be 10 MΩ as this is believed to be an optimum considering the net charge flow and the potential range of about 10 V. 
         [0062]    As mentioned above, the diameter of the sensor tip  76  is 5 mm. This compares to a typical ion beam diameter of 30-100 mm that exerts a floating potential over a region in the order of 120-150 mm across. Hence, the sensor  74  effectively samples a point from within the ion beam  34 . A profile of the floating potential over a cross-section through the ion beam  34  can be provided by moving the sensor  74  through the ion beam  34 . As the sensor  74  is provided on the scanning arm assembly  51 , the assembly  51  can be used to move the sensor  74 . As explained above, the scanning arm assembly  51  is usually moved along X and Y axes to implement a raster scan. This same procedure may be used to obtain measurements from a two-dimensional array of positions within and adjacent to the ion beam  34 . As it is the controller  50  that effects movement of the scanning arm assembly  51  via encoders, the controller  50  can determine both the sensor position and the measured floating potential at that position, and so provide a display of the potential profile of the ion beam  34 . 
         [0063]    The display could take any one of a number of forms and may show the raw data, filtered data or further data derived by interpolation for example (e.g. so as to show a contour plot). As will be appreciated, measurements may be taken at different positions within the ion beam  34  to produce any desired array of points, and need not be acquired following the raster scan suggested above. The number of points used may be determined as a balance between the resolution of the resulting profile and the time required to collect the measurements. For example, regularly-spaced arrays of 32×32 or 64×64 data points may be used. 
         [0064]    The controller may also use the measurements of the floating potential of the ion beam to implement control changes. For example, if the measurements indicate a net positive charge, the controller  50  may increase the electron current from the plasma flood system  37 . Conversely, an indication of a net negative charge may result in the controller  50  decreasing the electron current from the plasma flood system  37 . Beam shaping may also be performed in response to the measured potentials, for example to increase uniformity across the ion beam  34  as well as reducing the absolute values of the potentials seen. 
         [0065]      FIG. 4  shows the device  64  for measuring the floating potential of the ion beam  34  provided in the support arm  52  so as to face outwardly in the same direction as the wafer  12  when mounted on chuck  36 . Thus, the insulator plate  68  is attached to the support arm  52 , with the sensor  74  extending into the support arm  52 . A profile of the ion beam  34  may be obtained by driving the scanning arm assembly  51  such that the sensor  74  provided on the support arm  52  travels through the ion beam  34 . For this embodiment, the sensor tip  78  is brought to the plane of the wafer  12  by the thickness of the insulator plate  68 . 
         [0066]    An alternative arrangement is shown in  FIG. 5  where the device  64  is provided in a tab  84  extending from the edge of the chuck  36 . The insulator plate  68  is provided on the front face of the tab  84  such that the sensor  74  faces forwards in the same direction as the wafer  12 . The tab  84  is positioned such that the sensor tip  78  is at the plane of the wafer  12 . Measurements are taken by driving the sensor  74  in the tab  84  through the ion beam  34  using the scanning arm assembly  51 . 
         [0067]    Yet another embodiment of the present invention is shown in  FIG. 6  where the device  64  is provided in the back of the chuck  36  such that the sensor  74  faces in the opposite direction to wafer  12 . The insulating plate  68  is mounted to the back of the chuck  36  at any suitable location. Measurements are taken by first rotating the support arm  52  so that the wafer  12  is turned away from the ion beam  34  and so that the back of the chuck  36  faces the ion beam  34 . The scanning arm assembly  51  is then used to drive the sensor  74  through the ion beam  34 . The device  64  may be positioned such that, when rotated to face the ion beam  34 , the sensor tip  78  resides at the plane of the wafer  12  during implants. 
         [0068]    A still further embodiment is shown in  FIG. 7  where the device  64  is provided in the edge of the chuck  36  such that when the wafer  12  is held in the implant position, the sensor  74  faces downwards. Thus, the support arm  53  is rotated through 90° to make the sensor  74  face the ion beam  34  before measures are taken. The scanning arm assembly  51  is used, as before, to drive the sensor  74  through the ion beam  34 . 
         [0069]    As will be evident to the skilled person, changes may be made to the above embodiments without departing from the scope of the present invention as defined by the appended claims. 
         [0070]      FIGS. 4 to 7  show the device  64  for measuring the floating potential of the ion beam  34  in a variety of positions on the scanning arm assembly  51 . It will be clear that other positions may be used with success. As illustrated in  FIGS. 4 to 7 , the sensor  74  can be arranged to face in a variety of directions relative to the wafer  12 , for example in the same direction, in the opposite direction or at 90° to the wafer  12 . This may be achieved using different parts of the scanning arm assembly  51 . Clearly, any angle may be adopted by placing the insulator plate  68  accordingly on the cylindrical support arm  52 . Different directions may be achieved using the back and edge of the chuck  36 . Similarly, the front, back and lower edge of the tab  84  of  FIG. 5  may be used to align the sensor  74  as required. 
         [0071]    While acquiring measurements, the controller  50  may be used to drive the scanning arm assembly  51  to whatever positions are desired and following whatever path is desired. Regularly spaced arrays of points is an obvious choice. The arrays may be rectangular or may be arranged to join concentric circles, bearing in mind the usual rotational symmetry of the ion beam  34 . The data points need not be regularly spaced, for example the data points may be arranged more densely at the centre of the ion beam  34  where the ion beam flux is greatest. 
         [0072]    As mentioned above, an array of devices  64  may be used rather than just a single device  64 . In this case, the readings necessary to derive an array of floating potential measurements may be obtained in one sampling period, rather than having to move repeatedly the device  64  from one measuring position to the next. It will be straightforward for one skilled in the art to determine how such an array may be mounted to the scanning arm assembly  51 , and how to manage the signals arriving from each device  64 . 
         [0073]    All embodiments described above relate to scanning arm assemblies  51  that effect mechanical scanning of the wafer  12 . This is a preferred arrangement in that such scanning arm assemblies  51  provide greater flexibility in positioning the device  64 . For example, the available rotation of the support arm  52  allows the device  64  to face in a different direction to that of the wafer  12 . Nonetheless, it will be evident how the above embodiments are readily adaptable to an ion implanter  10  that scans an ion beam  34  relative to a wafer  12  that is held in a fixed position or to a hybrid ion implanter  10  that uses a mixture of ion beam movement and wafer movement. Also, it will be appreciated that the present invention may be used with batch processing implanters, for example those that use a spoked wheel, each spoke comprising a support arm and a wafer holder attached to the remote end of the support arm. 
         [0074]    The actual form of the device  64  for measuring the floating potential of the ion beam  34  may be varied from that shown in  FIG. 3 . Several well-known arrangements are available, for example a non-contacting voltmeter that has an appropriately fast response. While more expensive, such a voltmeter should provide better accuracy than the arrangement of  FIG. 3 . WO02/23583 describes an alternative device 64.