Abstract:
This invention relates to a substrate holding apparatus for use in ion implanters. In particular, the present invention relates to a substrate holding system comprising two or more substrate holders that can adopt interchangeable positions, thereby allowing one substrate holder to scan a substrate through an ion beam while substrates can be swapped on the other substrate holder. The substrate holder assembly comprises a base rotatable about a first axis and at least two support arms extending from the base to ends provided with substrate holders. Rotating the base allows the substrate holders to move between designated positions. One designated position may correspond to a position for implanting a substrate and another designated position may correspond to a loading/unloading station.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a substrate holding apparatus for use in ion implanters. In particular, the present invention relates to a substrate holding system comprising two or more substrate holders that can adopt interchangeable positions, thereby allowing one substrate holder to scan a substrate through an ion beam while substrates can be swapped on the other substrate holder. 
       BACKGROUND OF THE INVENTION 
       [0002]    Although the present invention is not limited to the field of ion implanters, this field corresponds to a contemplated application and provides a useful context for understanding the invention. Hence there follows a description of ion implanters. 
         [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 ion beam is incident on a substrate held in place in the ion beam path by a substrate holder. 
         [0004]    Often, the cross-sectional profile of the ion beam is smaller than the substrate to be implanted. For example, the ion beam may be a ribbon beam smaller than the substrate in one axial direction or a spot beam smaller than the substrate in both axial directions. 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. For a spot beam, relative motion is generally effected such that the ion beam traces a raster pattern on the substrate. 
         [0005]    Our U.S. Pat. No. 6,956,223 describes an ion implanter of the general design described above. A single wafer 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 wafer holder is moved along two orthogonal axes to cause the ion beam to scan over the wafer following a raster pattern. 
         [0006]    The above design is particularly suitable for serial processing of wafers where a robot must unload a processed wafer before loading a new wafer to be implanted. Loading and unloading wafers between each implant causes an undesirable delay. 
         [0007]    Our U.S. Pat. No. 6,555,825 describes an ion implanter having a twin scanning arm arrangement shown in  FIG. 1 . Each scanning arm has a motorised hub unit A that can rotate about an axis X 1  between a scanning position shown on the right and a loading position shown on the left. A hollow arm B is rotatably mounted to the hub unit at one of its ends so as to be able to turn about axis X 2  to effect scanning of a wafer through an ion beam. The other end of the arm is provided with a wafer holder C. Wafer holder C can rotate about axis X 3  to allow the orientation of the wafer to be varied. 
         [0008]    The construction and arrangement of each scanning arm is such that when the arm is in the loading position, it is above the axis X 1  and therefore above the path of the ion beam D. This has particular significance because the wafer holder can be loaded and unloaded without any undesired effects due to the presence of the ion beam. At the same time, the hub unit can be rotated through 90° to convey the wafer holder to the scanning position where a wafer may be scanned through the path of the ion beam. As shown in dotted outline, the wafer may be moved on the arm from position P 1  down through lower positions to position P 2 . In this ion implanter, a ribbon ion beam is used such that this movement sees the entire wafer implanted. 
         [0009]    Provision of two such scanning arms allows one scanning arm to be used to scan a wafer while the other scanning arm may be positioned for concurrent loading and unloading of wafers. Hence, as soon as an implant is complete for one wafer, another wafer is ready for implant on the other scanning arm. Such a scanning arm arrangement is not capable of producing linear raster scans. 
         [0010]    Another disadvantage of such radial scanning arrangements is the resultant non-uniform dose characteristics arising from differences in scan speed across the width of the wafer. This is because the closest edge of the wafer to the pivot scans more slowly than the outer edge, causing a higher dose on that side of the wafer. 
       SUMMARY OF THE INVENTION 
       [0011]    Against this background, the present invention resides in a substrate holder assembly for holding substrates to be exposed to an ion beam during implantation in an ion implanter, the substrate holder assembly comprising a base rotatable about a first axis and at least two support arms extending from the base to ends provided with substrate holders. Rotation of the base allows the substrate holders to adopt designated positions, with the at least two support arms extending from points displaced from the axis of rotation by substantially equal distances and separated by a substantially equal separation angle such that rotation of the base through the separation angle causes the support arms to move between designated positions. 
         [0012]    Thus, two or more sample holders may be provided that may be rotated to swap positions. For example, one position may correspond to a substrate scanning position and another position may correspond to a substrate loading/unloading position. Then, the substrate holder assembly may comprise a pair of support arms disposed on opposite sides of the axis of rotation of the base. When rotating the base between designated positions, one support arm moves from the scanning position to the loading position and the other support arm moves from the loading position to the scanning position. 
         [0013]    More than two support arms may be used. For example, three support arms may be used, arranged with a separation of 120°, or four support arms may be used arranged 90° apart. Clearly, the provision of three support arms leads to three designated positions and, likewise, four support arms gives four designated positions. Various processes may be performed at each designated position, e.g. unloading a substrate, loading a substrate, both unloading and loading a substrate, implanting, cleaning, etching, annealing, deposition, etc. One or more designated positions may not have associated processes, i.e. the substrate may merely be parked at this position while other substrates are processed at one or more other designated positions. 
         [0014]    Preferably, the substrate holders are each provided with a support surface for supporting the substrate aligned substantially normal to the support arm&#39;s longitudinal axis, and the at least two support arms are rotatable about their longitudinal axes. Optionally, the support surface is rotatable about its centre axis. This allows the orientation of a substrate held by the substrate holder to be varied. 
         [0015]    Preferably, the at least two support arms are moveable along their longitudinal axes such that the distance of each substrate holder from the base may be varied. This allows a substrate to be moved into and out of a beam, and to be scanned through an ion beam in this direction. 
         [0016]    The present invention also resides in an ion implanter comprising an ion source, optics operable to guide ions produced by the ion source along an ion beam path to a process chamber for implantation in a substrate, a substrate transfer mechanism, and a substrate holder assembly as described above to hold substrates in the process chamber. The at least two support arms extend substantially normal to the ion beam path. A first of the designated positions corresponds to the substrate holder being disposed to face into the ion beam. A second of the designated positions corresponds to the substrate holder being disposed to be clear of the ion beam path and to co-operate with the substrate transfer mechanism thereby allowing substrates to be placed on and removed from the substrate holder. 
         [0017]    Preferably, the at least two support arms are moveable along their longitudinal axes such that the distance of each substrate holder from the base may be varied. Optionally, the base may have an associated scanning unit that is operable to scan the support arm back and forth along its longitudinal axis. Also, the scanning unit may be operable to scan the support arm in a direction substantially normal to both the longitudinal axis of the support arm and the ion beam path. This allows a substrate to be scanned relative to a fixed ion beam, for example according to a raster pattern. Alternatively, the substrate may be held in position while an ion beam is scanned across the substrate or a hybrid system may be used where both substrate and ion beam are moved. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    In order that the present invention may be better understood, a preferred embodiment will now be described with reference to the accompanying drawings, in which: 
           [0019]      FIG. 1  is a perspective view of a twin scanning arm apparatus according to the prior art; 
           [0020]      FIG. 2  is a schematic representation of a conventional ion implanter; 
           [0021]      FIG. 3  is a perspective view of a substrate holder assembly according to an embodiment of the present invention; and 
           [0022]      FIG. 4  is a section taken through the process chamber containing the substrate holder assembly of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In order to provide a context for the present invention, an exemplary application is shown in  FIG. 2 , although it will be appreciated that this is merely an example of an application of the present invention and is in no way limiting. 
         [0024]      FIG. 2  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 then to strike the semiconductor wafer  12 . 
         [0025]    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. 
         [0026]    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. 
         [0027]    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 . 
         [0028]    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 ). 
         [0029]    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 (riot 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 wafer  12  to be implanted or a beam stop  38  when there is no wafer  12  in the target position. In other modes, the beam may also be accelerated or decelerated using a lens assembly positioned between the mass analysis stage  30  and the wafer position. 
         [0030]    The semiconductor 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). 
         [0031]    The ion implanter  10  operates under the management of a controller, such as a suitably programmed computer  50 . The computer  50  controls scanning of the wafer  12  through the ion beam  34  to effect desired scanning patterns. These scanning patterns may comprise raster scans, including interlaced patterns, as is well known in the art. 
         [0032]      FIG. 3  shows apparatus  100  for holding two wafers  102   a,b  in an ion implanter  10  like the one shown in  FIG. 2 .  FIG. 4  shows the apparatus  100  in position in a process chamber  40 , its sectional view corresponding to a section taken a little above the plane of the ion beam  34 . As will be appreciated, the apparatus  100  is mounted to a wall  130  of the process chamber  40  and is used either to hold a wafer  102   a,b  in position while the ion beam  34  is scanned or to scan a wafer  102   a,b  through an ion beam  34 . The apparatus  100  also cooperates with a robot  132  during loading and unloading of wafers  102   a,b.    
         [0033]    The apparatus  100  comprises a turntable  106  that mates with a rotary actuator  134  that provides full range of rotation of the turntable  106  in the direction indicated by arrow  110 . The turntable  106  is supported by a crossed-roller bearing and sealing is provided by floating air bearing labyrinth seal units with differential pumping. The rotary actuator  134  has an associated flange for mounting the apparatus to the process chamber  40 . 
         [0034]    Two scanning arms  114   a,b  are provided, each with a wafer holder  116   a,b  at its end. Each scanning arm  114   a,b  extends from its wafer holder  116   a,b  back towards the supporting turntable  106 , and in fact extends through the turntable  106  to be received by a linear actuator  118   a,b  that is mounted to the rear of the turntable  106 . The scanning arms  114   a,b  are supported at the turntable  106  by sealed bushings  120   a,b  that allow the scanning arms  114   a,b  to be driven through the turntable  106  by the linear actuator  118   a,b.  In this way, the distance of the wafer holder  116   a,b  from the turntable  106  can be varied, i.e. the wafer  102   a,b  can be driven into and out of the process chamber in the direction indicated by arrow  122 . 
         [0035]    In addition to driving the scanning arm  114   a,b  into and out of the process chamber  40 , the linear actuator  118   a,b  also drives rotary motion of the scanning arm  114   a,b  such that it rotates about its longitudinal axis as indicated by the arrow  124 . 
         [0036]    Each scanning arm  114   a,b  is hollow so as to provide routing for services to the wafer holder  116   a,b.  This includes a drive mechanism that allows a chuck  126   a,b  that supports the wafer  102   a,b  to be rotated about its centre, as indicated by arrow  128 . The chuck  126   a,b  holds the wafer  102   a,b  firmly in place electrostatically, such that rotation of the chuck  126   a,b  causes corresponding rotation of the wafer  102 a,b. Other methods of mounting the wafer  102   a,b  to the chuck  126   a,b  are equally possible. 
         [0037]      FIGS. 3 and 4  show the apparatus  100  with turntable  106  set so that wafer holder  116   a  occupies the loading position while wafer holder  116   b  occupies the scanning position. Rotating the turntable  106  through 180° swaps the positions occupied by the wafer holders  116   a,b.    
         [0038]    In the loading position, the scanning arm  114   a  is rotated about direction  124  to ensure that the wafer  102   a  faces upwards. The electrostatic force holding wafer  102   a  to the chuck  126   a  can be interrupted and the wafer  102   a  removed from the chuck  126   a  by the robot  132 . A simple robot  132  is shown in  FIG. 4  that comprises a pair of hinged jaws  136  that can open and close by rotation of cam  138 . Jaws  136  and cam  138  are cantilevered on an arm  140  that is mounted to shaft  142 . Shaft  142  allows vertical movement (i.e. in and out of the plane of the paper) and rotation as indicated by arrow  144 . Thus, the robot  132  may be rotated such that jaws  136  move over wafer  102   a,  the jaws  136  may be lowered on shaft  142  and closed to grip the wafer  102   a.  The robot  132  may then be raised on shaft  142  and subsequently rotated in direction  144  such that the wafer  102 a in jaws  136  swings out of the process chamber  40  through gate valve  146 . Due to vacuum considerations, a load lock is preferable to the single gate valve  146  shown in  FIG. 4 . The reverse procedure may then be used to load a new wafer  102   a  onto wafer holder  126   a.  As will be apparent to those skilled in the art, many other alternative robot arrangements may be used to load and unload wafers. 
         [0039]    Once a new wafer  102   a  is loaded onto the chuck  126   a  and held in place by electrostatic force, it is ready for the scanning arm  114   a  to be rotated. As will be appreciated from  FIG. 3 , the loading position sees the wafer  102   a  held away from the ion beam path  34 . The scanning arm  114   a  holds the wafer  102   a  above the level of the ion beam path  34  and also the scanning arm  114   a  is retracted towards the turntable  106  along direction  122  thereby moving the wafer  102   a  back from the ion beam path  34 . 
         [0040]    In the scanning position, the scanning arm  114   b  is extended away from the turntable  106  in direction  122  and is rotated in direction  124  such that the wafer  102   b  is held upright to face the incoming ion beam  34 . In some applications, it will be preferable for the ion beam  34  to strike the wafer  102   b  at an angle rather than perpendicularly, for example when implanting trench walls or when using grazing angles to achieve shallower implants. This is easily accomplished by rotating the scanning arm  114   b  in the direction  124  such that the wafer  102   b  adopts the desired angle to the ion beam  34 . In addition, the chuck  126   b  can be rotated about direction  128  to achieve a desired orientation of the wafer  102   b.  In this way, features on the wafer  102   b  such as trench walls can be correctly aligned with the ion beam  34 . 
         [0041]    In this embodiment, the wafer  102   b  is held in the scanning position while a ribbon beam  34  is scanned across the static wafer  102   b.  However, other arrangements are possible. For example, a ribbon beam need not be used, and a spot beam may be scanned across the wafer  102   b  using a raster pattern for example. Also, the wafer  102   b  may be scanned in addition to or as an alternative to scanning the ion beam  34 . Scanning in the x direction (taken to be the horizontal here) is readily achieved using the linear actuator  118   a  to drive the scanning arm  114   b  back and forth in the x direction. 
         [0042]    To move wafer holders  116   a,b  between the loading and scanning positions, three movements are required: (1) the turntable must be rotated through 180° about direction  110 , either clockwise or anticlockwise as desired; (2) scanning arms  114   a,b  must be rotated through 90° about direction  124 , scanning arm  114   a  in an anticlockwise direction and scanning arm  114   b  in a clockwise direction; and (3) scanning arms  114   a,b  must be moved along direction  122 , scanning arm  114   a  into the process chamber  40  away from the turntable  106  and scanning arm  114   b  towards the turntable  106 . Generally, these movements will be effected by the controller  50 , but how it coordinates these three different movements is a matter of preference. Clearly, it is advantageous to effect the movements concurrently to avoid delay and maximise throughput of wafers  102   a,b  through the ion implanter  10 . The scanning arms  114   a,b  are separated sufficiently to ensure that the wafers  102   a,b  cannot collide as they are rotated during movement between loading and scanning positions, even when larger 300 mm wafers are being processed. 
         [0043]    The skilled person will appreciate that changes may be made to the above-described embodiment without departing from the scope of the present invention. 
         [0044]    For example, the above embodiment exemplifies the present invention for use in the field of implanting semiconductor wafers. However, the present invention enjoys far wider applicability. For example, the present invention may be used in any type of ion implanter, whether that be for implanting semiconductor wafers or any other type of substrate. Also, the present invention may be used in any other type of apparatus that requires workpiece manipulation, where workpieces are rotated between a number of designated positions. 
         [0045]    The above embodiment describes a twin scanning arm apparatus  100 . However, three or more scanning arms  114   a,b  may be used. Advantageously, the scanning arms  114   a,b  are equally spaced from the centre of rotation of the turntable such that the wafer holders  116   a,b  adopt the same position as they are moved between positions. Also, it is advantageous for the scanning arms  114   a,b  to be separated by substantially equal angles such that the scanning arms  114   a,b  merely rotate between successive positions. Each position may correspond to a station where a processing step is performed, e.g. implanting, loading/unloading, etching, annealing, deposition, cleaning, etc. Loading and unloading may be split into two separate actions performed at separate stations, i.e. a wafer  102   a,b  or other substrate is unloaded at a station before a new wafer  102   a,b  or other substrate is loaded at the next station. One or more positions may be idle positions where the wafer  102   a,b  or other substrate dwells before moving on to the next position. An idle station may be useful where limited space precludes the installation of processing apparatus adjacent that position. 
         [0046]    The embodiment of the present invention described above enjoys many degrees of movement to provide a flexible system. However, the apparatus  100  need not be provided with such capability. For example, an apparatus  100  enjoying only rotation about direction  110  to allow the scanning arms  114   a,b  to swap positions is possible. The loading robot would need to be adapted to load and unload wafers  102   a,b  from a vertical orientation, but such adaptation is straightforward. Scanning arms  114   a,b  need not be mobile in direction  122 , i.e. towards and away from the turntable  106 , because the wafer  102   a,b  is rotated clear of the ion beam  34 . 
         [0047]    While  FIG. 3  shows the scanning arms  114   a,b  to extend through the turntable  106 , telescopic arms may be provided that do not need to penetrate the turntable  106 .