Abstract:
The present invention relates to components in an ion implanter that may see incidence of the ion beam, such as a beam dump or a beam stop. Such components will be prone to the ions sputtering material from their surfaces, and sputtered material may become entrained in the ion beam. This entrained material is a source of contamination. The present invention provides an ion implanter comprising power supply apparatus and an ion-receiving component. The component has an opening that receives ions from an ion beam such that ions strike an internal surface. The power supply apparatus is arranged to provide an electrical bias to the internal surface to decelerate the ions prior to their striking the surface, thereby mitigating the problem of material being sputtered from the surface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to components in an ion implanter that may see incidence of the ion beam, such as a beam dump or a beam stop. 
     BACKGROUND OF THE INVENTION 
     Ion implanters are used in the manufacture of semiconductor devices and other materials. In such ion implanters, semiconductor wafers or other substrates are modified by implanting atoms of a desired species into the body of the wafer, for example to form regions of varying conductivity. 
     Ion implanters are well known and generally conform to a common design as follows. An ion source generally comprises an arc chamber in which a hot plasma is generated. The plasma will contain ions of a desired species to be implanted. 
     An extraction lens assembly produces an electric field that extracts ions from the ion source and forms a mixed beam of ions. Only ions of a particular species are usually required for implantation in a wafer or other substrate, for example a particular dopant for implantation in a semiconductor wafer. The required ions are selected from the mixed ion beam that emerges from the ion source by using a mass analysing magnet in association with a mass-resolving slit. By setting appropriate operational parameters on the mass-analysing magnet and the ion optics associated therewith, an ion beam containing almost exclusively the required ion species emerges from the mass-resolving slit. The ions travel along a flight tube as they pass through the mass-analysing magnet. 
     The ion beam is transported along a beam line 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. The substrate may be a semiconductor wafer. 
     The various parts of the ion implanter are operated under the management of a controller, typically a suitably trained person, a programmed computer, or the like. A more detailed description of an ion implanter of this general type can be found in U.S. Pat. No. 4,754,200. 
     Ions may strike some components within the ion implanter relatively frequently (other than the substrate to be implanted). For example, ions with a large mass-to-charge ratio will not be deflected sufficiently by the mass-resolving magnet to pass through the mass-resolving slit. As a result, a beam dump may be provided to adsorb such ions. These ions striking the beam dump may cause sputtering of material. Care must be taken though, as material sputtered from the beam dump may become entrained within the ion beam and so contaminate the substrate. 
     In addition, there are times when the ion beam may be dumped into the beam dump on purpose. For example, instability in the ion beam may require that implantation of a wafer be stopped as quickly as possible. One way of achieving this is to switch off the mass-analysing magnet. With the magnet switched off, the ions merely follow a straight path rather than the usual curved path through the flight tube. The beam dump is positioned to absorb the ion beam when it is dumped in this way. Such a beam strike of the whole beam is likely to sputter more material. Although the material can no longer become entrained within the ion beam, there remains a problem in that the beam dump often has line of sight to the substrate. Consequently, material sputtered from the beam dump may still contaminate the substrate. 
     A further example of a component that frequently sees beam strike is the beam stop that resides downstream of the substrate. The ion beam may strike the beam stop when the substrate is moved away from the ion beam path, e.g. during mechanical scanning of the wafer during implants with a spot beam. 
     Unwanted material that has been sputtered from components such as a beam dump may travel to the substrate and subsequently the material may strike the substrate causing contamination or even damage to the devices being formed on the substrate. Moreover, sputtered material may adhere to another surface within the ion implanter. Surfaces adjacent to the ion beam are the most prone to receiving such deposits. As the amount of material deposited accumulates, the chances of the deposits delaminating to form flakes or particles increases. These flakes or particles frequently detach from their host surface and may become entrained in the ion beam. As a result, the flakes or particles contain sputtered material that still ultimately reaches the substrate. 
     SUMMARY OF THE INVENTION 
     Against this background, and from a first aspect, the present invention resides in a method of operating an ion implanter comprising: producing an ion beam; receiving ions from the ion beam in a component having an entrance opening and an internal surface for absorbing ions that have passed through the entrance opening; providing an electrical bias on the internal surface so as to decelerate the ions prior to them striking the internal surface. 
     From a second aspect the present invention resides in an ion implanter comprising power supply apparatus and an ion-receiving component with an entrance opening providing line of sight to an internal surface. The component is arranged to receive ions from an ion beam through the entrance opening such that ions strike the internal surface. The power supply apparatus is arranged to provide an electrical bias to the surface to decelerate the ions prior to their striking the surface. 
     Biasing the surface in this way is advantageous in that it reduces the energy of the ions before they strike the internal surface. Thus, with their energy reduced, the ions will pose less of a problem in sputtering material from the surface. Preferably, the power supply apparatus is arranged to bias the internal surface to be at substantially the same potential as the ion beam. 
     The component may further comprise an array of electrodes disposed between the surface and entrance opening. This allows further electrical control. For example, the array of electrodes may comprise one or more upstream electrodes disposed adjacent the opening. The one or more upstream electrodes may be electrically biased by the power supply to be at substantially the same potential as the ion beam. This is beneficial in that it stops ions within the ion beam travelling past the beam dump, but not travelling into the beam dump, from seeing the potential of the surface. Thus, such ions are not disturbed in their flight by the repulsive electrical field exerted by the surface. 
     In addition, or as an alternative, the array of electrodes may further comprise one or more downstream electrodes positioned adjacent the surface. The one or more downstream electrodes may be electrically biased to repel electrons liberated from the surface. This suppresses these electrons that may otherwise neutralise ions in the beam. 
     The ion receiving component may be a beamstop or it may be part of a flight tube of a mass resolving analyser. The ion receiving component may be used elsewhere in an ion implanter, preferably in positions where it may receive ions, either the ion beam itself or ions that are lost from the ion beam. 
     Other preferred, but optional features, are to be found in the appended claims. 
    
    
     
       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 schematic representation of an ion implanter; 
         FIG. 2  is a cross-section through a first embodiment of a beam dump of the ion implanter of  FIG. 1 ; 
         FIG. 3  is a schematic representation of a beam dump within a flight tube of the ion implanter of  FIG. 1 ; 
         FIG. 4  is a perspective view of a beam dump according to a second embodiment of the present invention; 
         FIG. 5  is a section taken through line IV-IV of  FIG. 4 ; and 
         FIG. 6  is an exploded perspective view of the beam dump of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to provide a context for the present invention, an exemplary application is shown in  FIG. 1 , although it will be appreciated this is merely an example of an application of the present invention and is in no way limiting. 
       FIG. 1  shows an ion implanter  10  for implanting ions in semiconductor wafers  12  that may be used in accordance with the present invention. The ion implanter  10  comprises a vacuum chamber  15  pumped through valve  24 . Ions are generated by ion source  14  and are extracted by an extraction lens assembly  26  to form an ion beam  34 . In this embodiment this ion beam  34  is steered and shaped through the ion implanter  10  such that the ion beam  34  passes through a mass analysis stage  30 . Ions of a desired mass are selected to pass through a mass resolving slit  32  and then conveyed onward along an ion beam path  34  towards the semiconductor wafer  12 . In this embodiment, the ions are decelerated before reaching the semiconductor wafer  12  by deceleration lens assembly  48  and pass through a plasma flood system  49  that acts to neutralise the ion beam  34 . 
     Ions formed within the ion source  14  are extracted through an exit aperture  28  using a negatively-biased (relative to ground) extraction electrode  26 . A potential difference is created between the ion source  14  and the following mass analysis stage  30  by a power supply  21  such that the extracted ions are accelerated. The ion source  14  and mass analysis stage  30  are 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 the mixture passes around a curved path through a flight tube  46  under the influence of a magnetic field. A beam dump  100  resides within the flight tube  46 . The radius of curvature traveled by any ion is determined by its mass, charge state and energy. The magnetic field is controlled so that, for a set beam energy, only those ions with a desired mass-to-charge ratio energy exit along a path coincident with the mass-resolving slit  32 . 
     The ion beam  34  is then transported to the wafer  12  to be implanted (or other substrate) or to a beam stop  38  when there is no wafer  12  in the target position. Before arriving at the wafer  12  or beamstop  38 , the ions may be decelerated using a deceleration lens assembly like that shown at  48  positioned between the mass analysis stage  30  and upstream of the wafer  12 . The deceleration lens assembly  48  is followed by a plasma flood system  49  that operates to produce a flood of electrons that are available to the semiconductor wafer  12  to neutralise the effect of the incident positive ions. 
     The semiconductor wafer  12  is 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  34  in turn. 
     A controller is shown at  50  that comprises a suitably programmed computer. The controller  50  is provided with software for managing operation of the ion implanter  10 . 
     A first embodiment of a ion implanter component according to the present invention is shown in  FIG. 2 . The component shown is a beam dump  60  that may be placed at various location within an ion implanter, such as the one shown in  FIG. 1 , to receive the ion beam. For example, the beam dump  60  may be used as a beamstop  38  positioned downstream of the wafer  12 , so as to receive the ion beam  34  when the wafer  12  is not in the implant position. As another example, the beam dump  60  may be used in a flight tube  46  so as to receive the ion beam  34  when the magnet of the mass analyser  30  is switched off. Also, such a beam dump  60  may be used to receive ions that do not follow the ion beam path  34  through the flight tube  46 , i.e. to receive ions not having the desired mass to charge ratio. 
     The beam dump has a generally box-like shape defined by a top  61 , a bottom  62 , a back wall  63 , a front wall  64  and a pair of end walls  65  (only one of which is visible in  FIG. 2 ). The front wall  64  is provided with a central aperture  66  that penetrates through the front wall  64 . 
     The beam dump  60  is positioned such that the aperture  66  faces the ion beam  34 , so as to receive the ion beam  34  as shown in  FIG. 2 . The ion beam  34  passes through the aperture  66  and passes a pair of opposed suppression electrodes  67  positioned just beyond the aperture  66 . The purpose of the suppression electrodes will be described below. Once past the suppression electrodes  67 , the ion beam  34  enters and strikes a cup  68  comprising a base  69  and a cylindrical wall  70 . The cup  38  need not be cylindrical, but could be other shapes. The cup  38  is electrically biased, as will now be described. 
     The ion beam  34  has a beam energy equal to the potential set on the ion source  14 , e.g. if the ion source  14  is set at +10 kV, ions within the ion beam will typically have an energy of 10 keV. Such high-energy beams  34  are commonly used within ion implanters  10  to reduce the problems of space charge blow-up. Ions striking the cup  68  of the beam dump  60  causes sputtering of material and the problem of material being sputtered from the beam dump  100  worsens the greater the energy of the incident ions. This problem is mitigated by using a power supply unit  71  to place a potential on the cup  68  that decelerates ions in the ion beam  34  before they strike the cup  68 . 
     The potential set on the cup  68  is matched to the beam energy and so chosen to be at or preferably just below the potential of the ion source  14 . For example, the cup  68  may be biased to be +9.9 kV. In this way, the incoming ions are decelerated to near-zero energy prior to striking the cup  68 . Thus, the problem of material being sputtered from the cup  68  is lessened. An alternative to using a power supply unit  71  to provide the decelerating potential is to connect electrically the cup  68  to the ion source  14 , such that both are at the same potential. Setting the cup  68  to be at the same potential as the ion source  14  may cause some ions to be reflected by the cup  68 , hence a slightly lower potential is preferred. 
     As shown in  FIG. 2 , the decelerating ion beam  34  has an ever-increasing tendency to blow-up due to space charge effects. 
     A power supply unit  72  is used to set a potential on the suppression electrodes  67 . Power supply units  71  and  72  may be combined if desired. The suppression electrodes  114  are set at a high negative potential, for example −5 kV. This is to suppress electron travel in either direction. In particular, the suppression electrodes  114  suppress any electrons liberated from the cup  68  from travelling back out of the beam dump  60 . Such an electron beam may otherwise cause damage within the ion implanter  10 . For example, the electron beam may cause heating of any part it impacts and this can be extreme enough to cause melting. Obviously, the potential for any electron beam striking the wafer  12  to cause serious damage is considerable. Electron impact may also cause x-ray emission. 
       FIG. 3  shows a representation of the mass analyser  30  of  FIG. 1 , along with the path  34  of ions through a flight tube  46  defined by the mass analyser  30 . The solid line  34  shows the path of ions having the desired mass-to-charge ratio and describes a smooth quarter-turn through the mass analyser  30 . While the beam dump  60  of  FIG. 2  may be used in this flight tube  46 ,  FIG. 3  shows an alternative embodiment of a beam dump  100 . The beam dump  100  is provided for ions having a greater mass-to-charge ratio than desired, and for instances when the ion beam  34  is dumped. Ions having a greater mass-to-charge ratio than desired may strike the beam dump  100  as shown at  101 - 104 . The path that the ion beam follows when the magnet of the mass analyser  30  is switched off is shown at  105 . Ions having a lesser mass-to-charge ratio than desired will turn inwardly from the ion beam path  34 . Although not shown, a further beam dump  60 ,  100  may be provided on the inner radius of the path  34  to receive such lighter ions. 
     Ions that strike the beam dump  100  may sputter material. Typically, beam dump  100  will be made from graphite and so there is a danger that graphite will become entrained in the ion beam  34  as it passes through the mass analyser  30 . This entrained material may be deposited on nearby parts, causing deposited coatings that can then flake off, generating particulates. These particulates can then be transported to the wafer  12 , causing contamination. 
       FIGS. 4 to 6  show beam dump  100  in greater detail. The beam bump  100  is broadly box like and has a dog-legged shape  106 . The beam dump  100  comprises side walls  107  and  108 , a base  109  and two back walls  110  and  111 . Thus, the beam dump  100  has an open front face entrance opening)  112  to allow entry of ions from the ion beam  34 . 
     A graphite dump plate  113  is attached to the back walls  110  and  111  by any convenient means, e.g. screws, bolts, etc. The dump plate  113  has the same dog-leg shape to conform to the shape of the back walls  110 - 111 . Sitting in front of the dump plate  113  within the beam dump  100  are two sets of electrodes  114  and  115 . The electrodes may be made from tungsten, or other materials such a graphite, stainless steel, etc. Each set of electrodes  114 - 115  comprises four identical generally planar electrodes  114   a - d  and  115   a - d  that are arranged one above another. Each electrode  114   a - d  and  115   a - d  extends from one side wall  107  to the other side wall  108 , and has the common dog-leg shape. The electrodes may be fixed in place in any convenient manner. Electrodes from each set are paired with one another, such that electrode  114   a  resides adjacent to electrode  115   a , and so on. As the electrodes  114 - 115  extend from near top to near bottom of the beam dump  100 , they effectively present a grill to ions entering the beam dump  100 , i.e., their front edges face the entrance opening  112 . How the electrodes  114 - 115  and the dump plate  113  are advantageously biased will now be described. For the sake of clarity, the power supplies for and the electrical connections to the electrodes  114 - 115  and dump plate  113  are not shown in  FIGS. 4 to 6 . Nonetheless, the person skilled in the art will readily identify many different ways of arranging the electrical connections and supplies. 
     As described above, the ion beam  34  has a beam energy equal to the potential set on the ion source  14 , e.g. 10 keV The potential set on the dump plate  113  is matched to the beam energy and so chosen to be at or preferably just below the potential of the ion source  14 , e.g. +9.9 kV. Hence, the incoming ions are decelerated to near-zero energy prior to striking the dump plate  113  and so the problem of material being sputtered from the dump plate  113  is lessened. 
     To ensure that the potential of the dump plate  113  is not seen by ions before they enter the beam dump  100 , the potential set on the front set of electrodes  115  is the same as the surrounding beamline. This may be achieved most easily by linking the potential of the front set of electrodes  115  to that of the flight tube  46  or the surrounding parts. Ensuring that the potential of the dump plate  113  is not seen by ions before they enter the beam dump  100  is important from the point of view of ions within the ion beam  34  that have the desired mass-to-charge ratio, as they should pass through the mass analyser  30  undisturbed by stray electric fields. 
     The back set of electrodes  114  are used to suppress electron travel and so is set at a high negative potential (relative to the front set of electrodes  115 ), for example −2 kV. In particular, the back set of electrodes  114  suppresses any electrons liberated from the dump plate  113  from travelling back out of the beam dump  100 . As described above, such an electron beam may otherwise cause damage within the ion implanter  10  or to the wafer  12 . 
     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, it will be realised that the terms front, back, sides and base used above are merely relative and that the beam dump  60 ,  100  may be used in any orientation. As a result, the terms may need to be changed according to the particular orientation of the beam dump  60 ,  100  chosen. 
     Various features of the beam dumps  60 ,  100  may be interchanged between the two designs. For example, one or more screening electrodes may be used in the beam dump  60  of  FIG. 2 : an array of electrodes akin to the front set of electrodes  115  of  FIGS. 4 to 6  may be used, or either a single such electrode or pair of electrodes may be used. The screening electrodes should have a negative potential to suppress electron travel. 
     As another example, the cup  68  of the beam dump  60  of  FIG. 2  may be used in place of the dump plate  113  of  FIGS. 4 to 6 . Such a cup  68  may be advantageous as it reduces the risk of the ion beam  34  missing the dump plate  113  (remembering that the ion beam  34  is prone to blow-up as it approaches the dump plate  113 , in the manner shown in  FIG. 2 ). 
       FIGS. 1 and 3  show an ion implanter  10  with a single beam dump  100  provided in the flight tube  46 . However, two or more beam dumps  100  may be provided. For example, a series of beam dumps  60 ,  100  may be provided around the outer radius of the ion beam path  34  through the flight tube  46 . The beam dumps  60 ,  100  may be progressively angled to follow approximately the ion beam path  34  through the flight tube  46 . 
     Although, beam dumps  60 ,  100  have been described in use as a beamstop  38  and within a flight tube  46 , the beam dumps  60 ,  100  may be used in any position within an ion implanter  10  that may receive ions from the ion beam  34 . 
     The dog-leg design of the beam dump  100  of  FIGS. 4 to 6  need not be used: as well as the linear beam dump  60 , other shapes may be used.