Abstract:
The present invention relates to a front plate for an ion source that is suitable for an ion implanter. The front plate according to the invention comprises obverse and reverse sides, an exit aperture for allowing egress of ions from the ion source that extends substantially straight through the front plate between the obverse and reverse sides, and a slot penetrating through the front plate from obverse side to reverse side at a slant for at least part of its depth, the slot extending from a side of the front plate to join the exit aperture. The slot is slanted to occlude line of sight into the ion source when viewed from in front, yet provides an expansion gap.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of co-pending U.S. patent application Ser. No. 11/790,682, filed Apr. 26, 2007, which application claims benefit to United Kingdom Patent Application Serial No. 0608528.6 filed on Apr. 28, 2006, of which both applications are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a front plate for an ion source that is suitable for an ion implanter. 
     BACKGROUND OF THE INVENTION 
     A contemplated application of the present invention is in ion sources in an ion implanter that may be used in the manufacture of semiconductor devices or other materials, although many other applications are possible. In such an application, semiconductor wafers are modified by implanting atoms of-desired dopant species into the body of the wafer to form regions of varying conductivity. Examples of common dopants are boron, phosphorus, arsenic and antimony. These dopants are generated in an ion source. 
     Typically, an ion implanter contains an ion source held under vacuum within a vacuum chamber. The ion source produces ions using a plasma generated within an arc chamber. The plasma in the arc chamber is struck using potential differences and a source of thermal electrons. The thermal electrons may be generated using one of a number of different arrangements such as a Freeman source or a Bernas source (including indirectly heated cathodes). 
     In a typical Bernas source, thermal electrons are emitted from a cathode, accelerated under the influence of an electric field and are constrained by a magnetic field to travel along spiral paths towards a counter-cathode. Interactions with precursor gas molecules within the arc chamber produces the desired plasma. 
     Plasma ions are extracted from the arc chamber via an aperture provided in a front plate. In an “ion shower” mode, the ions travel to implant in a target such as a semiconductor wafer. Alternatively, the extracted ions may be passed through a mass analysis stage such that ions of a desired mass and energy are selected to travel onward to implant in a semiconductor wafer. A more detailed description of an ion implanter can be found in U.S. Pat. No. 4,754,200. 
     The ion source will comprise the arc chamber to contain the plasma. Chamber walls and a front plate like that shown in  FIGS. 1 and 2  enclose the arc chamber. This two-piece construction assembles to form a slot-like aperture to allow ions to be extracted from the arc chamber. Tongue and groove arrangements, shown at A, are provided to facilitate alignment of the two parts of the front plate. An extraction electrode assembly is generally provided in front of the aperture to extract ions from the ion source, and the front plate may form one of the electrodes of that assembly. 
     SUMMARY OF THE INVENTION 
     Against this background, the present invention resides in a front plate for an ion source comprising an exit aperture for allowing egress of ions from the ion source that extends substantially straight through the front plate between the obverse and reverse sides, and a slot penetrating through the front plate from obverse side to reverse side at a slant for at least part of its depth, the slot extending from a side of the front plate to join the exit aperture. 
     The provision of the slanted slot allows expansion of the front plate to be accommodated thereby relieving thermal stress. This is beneficial because the front plate of ion sources may become hot. For example, where the front plate is used with an arc chamber, the heat in the plasma will be transferred to some extent to the front plate and this will expand as a result. As the front plate is typically made from a metal, temperature rises are quick and expansion is pronounced. Graphite is also commonly used for the front plate. 
     The exit aperture allows direct line of sight into the ion source such that ions may be extracted freely from the ion source for subsequent implantation where the present invention is used in an ion implanter. The slanted slot does not present line of sight into the ion source. In addition, the use of a slanted slot increases the path length through the front plate. As a result, the tendency for ions and gas to escape from the ion source through the slot is much reduced. In particular, the provision of a slanted slot effectively prevents the penetration of electric fields into the ion source. These fields may be as a result of an electrode assembly used to extract ions from the ion source. The combination of a straight exit aperture and a slanted slot means that the extraction field penetrates into the ion source through the exit aperture but not through the slot. 
     Optionally, the slot may extend linearly from the side to the exit aperture. The exit aperture may also be linear and may, optionally, be substantially co-linear with the slot. Thus the straight exit aperture and the slanted slot may intersect at a point such that the parts of the front plate to either side of the slot and the exit aperture are not joined, and can move relative to each other as the front plate expands. In a preferred embodiment, the front plate is unitary. For example, the front plate may extend around the end of the exit aperture not joined to the slot so as to form a general C-shape or similar. 
     Fashioning the front plate from a single piece of material is advantageous as alignment of the front plate becomes straightforward compared with multi-piece designs. For instance, alignment of the extraction aperture edges becomes easy to control. Furthermore, the front plate may be precisely shaped and it is far easier to control this shape when machining a single piece of material. The precise shape will be very important where the front plate forms an electrode and so is used to shape carefully an electric field. 
     The slot may be formed at a constant slant through the front plate or it may be formed with a dog-leg as it extends through the front plate. One part of the dog-leg may extend straight through the front plate. 
     All combinations of the above features indicated as optional are also contemplated to form part of the invention. 
     According to further aspects, the present invention resides in an ion source comprising any of the front plate arrangements described above and in an ion implanter comprising any such ion source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a perspective view of an assembled front plate according to the prior art; 
         FIG. 2  is a perspective view of the front plate of  FIG. 1  before assembly; 
         FIG. 3  is a schematic representation of an ion implanter; 
         FIG. 4  is a side view of the ion source of  FIG. 3 ; 
         FIG. 5  is a front view of the front plate of  FIG. 4 ; 
         FIG. 6  is a sectional view along line VI-VI of  FIG. 5 ; 
         FIG. 7  is a side view from line VII-VII of  FIG. 5 ; 
         FIG. 8  is a side view from line VIII-VIII of  FIG. 5 ; 
         FIG. 9  is a perspective view from in front of the front plate of  FIG. 5 ; 
       and 
         FIG. 10  is a perspective view from behind the front plate of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to provide a context for the present invention, an exemplary application is shown in  FIG. 3 , although it will be appreciated that this is merely an example of an application of the present invention and is in no way limiting. 
       FIG. 3  shows a known ion implanter  10  for implanting ions in semiconductor wafers  12  including an ion source  14  and front plate  28  according to the present invention. Ions are generated by the ion source  14  to be extracted and passed, 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 a semiconductor wafer  12 . 
     The ion implanter  10  contains an ion source  14  for generating an ion beam of a desired species that is located within a vacuum chamber  15 . The ion source  14  generally comprises an arc chamber  16  containing a cathode  20  located at one end thereof. The ion source  14  may be operated such that an anode is provided by the walls  18  of the arc chamber  16 . The cathode  20  is heated sufficiently to generate thermal electrons. 
     Thermal electrons emitted by the cathode  20  are attracted to the anode, the adjacent chamber walls  18  in this case. The thermal electrons ionise gas molecules as they traverse the arc chamber  16 , thereby forming a plasma and generating the desired ions. 
     The path followed by the thermal electrons may be controlled to prevent the electrons merely following the shortest path to the chamber walls  18 . A magnet assembly  46  provides a magnetic field extending through the arc chamber  16  such that thermal electrons follow a spiral path along the length of the arc chamber  16  towards a counter-cathode  44  located at the opposite end of the arc chamber  16 . 
     A gas feed  22  fills the arc chamber  16  with the species to be implanted or with a precursor gas species. The arc chamber  16  is 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 created in the plasma will also contain trace amounts of contaminant ions (e.g. generated from the material of the chamber walls). 
     Ions from within the arc chamber  16  are extracted through an exit aperture  28  provided in a front plate  28  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 target, 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 target position. 
     The semiconductor wafer  12  will be mounted on a wafer holder  36 , wafers  12  being successively transferred to and from the wafer holder  36  for serial implantation. Alternatively, parallel processing may be used where many wafers  12  are positioned on a carousel  36  that rotates to present the wafers  12  to the incident ion beam in turn. 
       FIG. 4  shows in greater detail the ion source  14  used in the ion implanter  10  of  FIG. 3 .  FIG. 4  corresponds to an indirectly-heated cathode arrangement, although other arrangements such as a filament or Freeman-type may be used. 
     In  FIG. 4 , a cathode is provided by an end cap  58  of a tube  60  that projects slightly into the arc chamber  16 , the tube  60  containing a heating filament  62 . The heating filament  62  and end cap  58  are kept at different potentials to ensure thermal electrons emitted by the filament  62  are accelerated into the end cap  58 , and a gap is left between the tube  60  and the liner  56  of the arc chamber  16  to maintain electrical isolation. Acceleration of electrons into the end cap  58  transfers energy to the end cap  58  such that it heats up sufficiently to emit thermal electrons into the arc chamber  16 . A counter-cathode  44  is located at the far end of the arc chamber  16 , again with a small separation from the liner  56  to ensure electrical isolation. A magnet assembly  46  (shown only in  FIG. 3 ) is operable to provide a magnetic field that causes electrons emitted from the end cap  58  to follow a spiral path  34  along the length of the arc chamber  16  towards the counter-cathode  44 . The arc chamber  16  is filled with the precursor gas species by a gas feed  22  or by one or more vaporisers  23  that may heat a solid or liquid. 
     The heating filament  62  is held in place by two clamps  48  that are each connected to the body  50  of the ion source  14  using an insulating block  52 . The insulating block  52  is fitted with a shield  54  to prevent any gas molecules escaping from the arc chamber  16  from reaching the insulating block  52 . 
     The arc chamber  16  is formed by walls of which the back, sides, top and bottom are provided with the liner  56 . The front of the arc chamber  16  is formed by the front plate  27  that seals the arc chamber  16  with the exception of the exit aperture  28  through which ions are extracted and a slit  28  to be described. 
       FIGS. 5 to 10  show a front plate  27  according to an embodiment of the present invention. The front plate  27  is machined from a single piece of material to have a front face  70  and a back face  72 . The front plate  27  will be made from a high-melting point material that is electrically conducting. Graphite would be a good choice, as would metals. The front face  70  of the front plate  27  (as viewed when fitted to an ion source) is rectangular with rounded corners  76 . An elongate slot  28  with rounded ends  78  is provided centrally therein to serve as the exit aperture  28 . A narrower slit  80  extends from one end  78   a  of the slot  28  to the adjacent side  82  of the front plate  27 . 
     The back face  72  of the front plate  27  has an upstanding flange  84  that abuts against the sides of the arc chamber  16 . The exit aperture  28  sits within the area enclosed by the flange  84 , whereas the slit  80  extends to meet and then to break through the flange  84 . As can be seen, the exit aperture  28  extends at right angles from the front face  70  of the front plate  27  whereas the slit  80  is angled. Thus, the exit aperture  28  provides direct line of sight into the ion source  14  when viewed from in front whereas the slit  80  does not. As a result of this angle and longer path lengths, ion loss and gas loss from the arc chamber  16  through the slit  80  is minimized. 
     As will be appreciated by the person skilled in the art, variations may be made to the above embodiment without departing from the scope of the invention defined by the claims. 
     For example, the overall shape of the front plate  27  may be varied from the rectangular form shown. In addition, the corners  76  need not be rounded. An elongate exit aperture  28  is not essential and other shapes may be adopted. The exit aperture  28  and slit  80  need not be co-linear. In fact, neither the exit aperture  28  nor the slit  80  need be linear and other shapes may be used. Although a slit  80  is shown that adopts a constant slant, the slant may vary as the slit  80  extends through the front plate  28  and/or the slit  80  may be kinked, to form a dog-leg for example. 
     While an arc chamber  16  is described in a preferred ion source  14 , the present invention also extends to other ion sources  14 . For example, the benefit of the present invention will be enjoyed by any ion source  14  that gets hot as a result of the ionisation process.