Abstract:
Components in an ion implanter that may see incidence of the ion beam include a chamber having an elongate slot opening defined by edges so that a central portion of the ion beam enters the component through the opening with the edges clipping at least a peripheral portion of the ion beam. The arrangement mitigates the problem of sputtered material escaping back out from the component and becoming entrained in the ion beam.

Description:
FIELD OF THE INVENTION 
     The present invention relates to components in ion implanters having surfaces, such as graphite surfaces, adjacent to the path of the ion beam through the ion implanter. Such surfaces will be prone to sputtering, and sputtered material may become entrained in the ion beam. The present invention primarily addresses this problem of sputtering and entrainment of material. 
     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 resulting 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. When a substrate is held clear of the ion beam path, the ion beam strikes a beamstop. 
     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. 
     During normal operation of an ion implanter, unwanted material may become entrained in the ion beam. This material may strike the substrate causing contamination or even damage to the devices being formed on the substrate. A major source of contaminants is material from ion implanter components that surround the ion beam path. If the ion beam strikes such components, material may be sputtered from that surface. As the surfaces surrounding the ion beam path are typically made from graphite, graphite is a major component in the material entrained in the ion beam. 
     Entrained material may be conveyed directly to the substrate to be implanted, or it may adhere to another surface in the ion implanter. Surfaces adjacent to the ion beam are 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. Consequently, the flakes or particles contain sputtered material that still ultimately reaches the substrate. 
     Our co-pending U.S. patent application Ser. No. 11/651,107 and US Patent Application Publication No. 2007/0102652 address the problem of material depositing on downstream surfaces to form large flakes. The present invention is concerned with the initial sputtering of material and how to reduce the amount of material entrained in the ion beam. In particular, the present invention is concerned with sputtering of material from the flight tube. 
     A further problem is the liberation of material from the beamstop when it is struck by the ion beam. The close proximity of the substrate to the beamstop exacerbates the problem of contamination from the beamstop. 
     The present invention also finds application in so-called beam parks or flag Faradays, devices located part way along a beam line in an ion implanter to act as a beam dump. 
     SUMMARY OF THE INVENTION 
     Against this background, and from a first aspect, the present invention resides in an ion implanter comprising an ion source, a substrate holder arranged to hold a substrate in an implant position, an ion beam path extending from the ion source to the implant position, and a component positioned adjacent (laterally of) the ion beam path. The component comprises a chamber with an elongate slot opening defined by edges provided in a forward surface that faces the ion beam path such that the opening is positioned so as to receive ions from the ion beam. The component is arranged such that, in use, a central portion of the ion beam enters the component through the opening with the edges clipping at least a peripheral portion of the ion beam. 
     When the ion implanter is in use, the central part of the ion beam that contains the highest beam current may pass though the opening into the chamber. When these ions strike a surface within the chamber, the material that is sputtered from that surface is generally retained within the chamber and so does not become entrained in the ion beam. On the other hand, the relatively low-current peripheral part of the ion beam strikes the edges of the opening. Thus, the edges may define a narrow opening that opens into the chamber, thereby promoting retention of sputtered material. There is a balance to be struck: the further the edges penetrate inwardly, the better they are at retaining sputtered material, but the greater the portion of the ion beam they clip (and that could back-sputter material into the ion beam). 
     The edges may be defined by one or more walls having a forward face and a rearward face, the rearward face advantageously acting to absorb sputter material. The one or more walls may be set back from the front of the component. 
     Preferably, the component comprises a rear surface having line of sight to the ion beam path through the opening, and wherein the rear surface is oriented such that ions strike the surface substantially normally. Such an orientation minimises sputtering of material. Optionally, the distance from the opening to the rear surface is at least three times greater than the width of the opening, although depths of at least five times, at least ten times and at least twenty times the width are also contemplated. 
     Preferably, the slot is elongate and may have a large aspect ratio. For example, the slot may be at least three times, at least five times, at least ten times or at least twenty times as long as it is wide. 
     Ensuring that material does not escape back through the opening towards the ion beam may be assisted by shaping the back surface to include an angled face that extends generally along the length of the opening in the front face. This angled face will tend to see material sputtered at an angle away from the opening. The back surface may be provided with a pair of angled faces that meet at a ridge, thereby a substantially v-shaped projection into the chamber. This v-shaped projection may comprise the rear surface in its entirety, or may be formed on the rear surface, for example as a rib that extends from the rear surface. 
     The chamber may comprise further surfaces that join the front surface to the back surface, e.g. top and bottom surfaces. Any walls defining the opening may extend from these top and bottom surfaces. These surfaces may be provided with at least one ridge that extends generally in the direction of the length of the opening. Such a ridge may project into the component thereby forming a surface that assists in trapping sputtered material within the component. Optionally, these surfaces are provided with a series of ridges that extend generally in the direction of the length of the opening. 
     Many components in ion implanters are formed of graphite. The component may comprise a surface coating having a greater resistance to sputtering than graphite. Preferably, the coating is tungsten, tungsten carbide, tantalum carbide, titanium carbide or silicon carbide. 
     The components described above may comprise a further modification of one or more of its surfaces. One or more of the surfaces, including all of the faces, may have been roughened. Thus, an arrangement is achieved that resists deposition of material. This provides a two-pronged attack: the initial deposition of material into large flakes is resisted, and entrainment of any sputtered material into the ion beam is also resisted. 
     The faces may be roughened so as to provide a pattern of surface features, such as a regular pattern of surface features. Optionally, the faces may have been roughened to provide surface features defined at least in part by sharp changes in orientation of adjacent parts of the face, for example to provide surface features defined at least in part by adjacent faces that meet at a sharp edge. 
     The surface may have been roughened to provide a series of grooves. The grooves may have a depth in a range of 0.1 mm to 10 mm, 0.25 mm to 7.5 mm, or 0.5 mm to 5 mm. The faces may have been roughened to provide a series of side-by-side grooves, for example to have a regular spacing in a range of: 0.1 mm to 10 mm, 0.25 mm to 7.5 mm, or 0.5 mm to 5 mm. Consequently, the grooves on each individual face are on a smaller scale than the scale of the faces themselves. The grooves may have any of the following cross-sectional profiles: v-shape, u-shaped, saw tooth or box-like trenches. 
     Preferably, the faces have been roughened to provide at least two intersecting series of grooves. The first series and second series may be disposed orthogonally. The first and second series may comprise v-shaped grooves that intersect to form an array of tetrahedra. 
     Optionally, the component is a beamstop. For example, the component may be positioned downstream of the implant position so as to receive the ion beam when the substrate and substrate holder are clear of the ion beam as it passes through the implant position. 
     Alternatively, the component may be positioned within a flight tube of a mass analyser. In this way, the component may be used to receive ions with mass-to-charge ratios outside those chosen for mass selection. The component may also be positioned in a “straight through” position so as to receive the ion beam if the magnet of the mass analyser is switched off. In this way, the component may act as a beam dump. Such a flight tube may be provided with multiple such components. 
     A further alternative is to provide the component in association with a scanner magnet or a collimator magnet so as to receive stray ions and also to receive the ion beam when in extreme positions, i.e. when the beam would otherwise clip an aperture. 
     In any application, the provision of a chamber behind the forward face means that material sputtered from the ions striking a rear face of the chamber is retained within the chamber. Thus, the material does not become entrained within the ion beam and cannot contaminate the substrate. 
    
    
     
       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 schematic representation of a flight tube of the ion implanter of  FIG. 1 ; 
         FIG. 3  is a simplified schematic of a component according to the present invention that may be used in the flight tube of  FIG. 2 ; 
         FIG. 4  is a perspective view of a component according to a first embodiment of the present invention that may be used in the flight tube of  FIG. 2 ; 
         FIG. 5  is a side sectional view of the component of  FIG. 4 ; 
         FIG. 6  is a side sectional view of a component like that of  FIG. 4 , but modified in accordance with a second embodiment of the present invention; 
         FIG. 7  is a side sectional view of a component like that of  FIGS. 4 and 6 , but modified in accordance with a third embodiment of the present invention; 
         FIG. 8  is a perspective view of the component of  FIG. 7 ; 
         FIG. 9  is a schematic representation of a flight tube of the ion implanter of  FIG. 1  including multiple components like that shown in  FIGS. 7 and 8 ; 
         FIG. 10  is a side sectional view of a beamstop in accordance with an embodiment of the present invention; and 
         FIG. 11  is an end sectional view of the beamstop of  FIG. 10 . 
     
    
    
     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 the 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  (or other substrates) 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 the 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 . The ions then 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. The radius of curvature travelled 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 target, i.e. the substrate wafer  12  to be implanted or a beamstop  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. As an alternative, 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. 
     At times when the wafer(s)  12  is clear of the ion beam path, the ion beam  34  strikes a beamstop  38  where it is adsorbed. This beam strike may liberate material and electrons from the beamstop  38 . 
     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 . 
       FIG. 2  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 . Ions having a greater mass-to-charge ratio may strike an outer part  102  of the mass analyser  30 , as shown at  104 . Conversely, ions having a lesser mass-to-charge ratio will turn inwardly and may strike an inner part  106  of the mass analyser  30 , as shown at  108 . 
     Ions that strike the mass analyser  30  in these ways may sputter material from those parts  102 ,  106 . Typically, these parts  102 ,  106  will be made from graphite and so there is a danger that graphite will become entrained in the ion beam 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. 
       FIG. 3  shows a simplified representation of a component  110  of the flight tube  46 . The component  110  is used in the flight tube in portions that may receive some or all of the ion beam  34 . For example, one or more of the components  110  may be used to form the outer part  102  or inner part  106  shown in  FIG. 2 . 
     The component  110  comprises a chamber  114  formed behind a front face  116  of the outer part  102 . The front face comprises walls  118  and  119  separated by a central slot  120 . The chamber  114  comprises a pair of rear surfaces  122  and  124  set at an angle to one another. Although omitted for the sake of clarity, a top surface and a bottom surface extend between the rear surfaces  122 - 124  to meet the walls  118 - 119  and so enclose chamber  114 . 
     Ion beam  34  passes through the flight tube  46  as indicated schematically in  FIG. 2 . The slot  120  extends generally in the direction of the ion beam path  34 , although the component  110  is angled to present the rear surface  122  to be normal to the ion beam  34 . Slot  120  is much wider than the ion beam  34  is tall. Thus, slot  120  has a large aspect ratio, for example as low as 3:1 or as high as 20:1. 
     Ion beam  34  passes through the flight tube  46  as indicated schematically in  FIG. 3 . The slot  120  extends generally in the direction of the ion beam path  34 , although the component  110  is angled to present the rear surface  122  to be normal to the ion beam  34 . Slot  120  is much wider than the ion beam  34  is tall. Thus, slot  120  has a large aspect ratio, for example as low as 3:1 or as high as 20:1. 
     Accordingly, any heavier ions like those shown at  104  in  FIG. 2  will pass through the slot  120  and will then strike one of the rear surfaces  122 - 124 . Material sputtered from the rear surfaces  122 - 124  as a result of such beam strike is prevented from becoming entrained in the ion beam  34  by the walls  118  and  119 . Instead, sputtered material will collect on the surfaces that surround the chamber  114 . 
     Walls  118  and  119  have a depth such that ions formerly from the central part of the ion beam  34  pass through the slot  120 , whereas ions formerly from the top and bottom parts of the ion beam  34  strike the walls  118  and  119 . Relatively few ions are expected to strike the walls  118  and  119  due to the low current of the edges of the ion beam. However, the deepened walls  118  and  119  will offer improved retention of material sputtered from the rear surfaces  122 - 124 . 
     The energy released as ions strike the flight tube  46  causes the flight tube  46  to warm. As sputtering increases with temperature, cooling may be provided to the flight tube  46  and to the rear surfaces  122 - 124  in particular. Any suitable form of cooling may be used, such as water cooling through, for example, channels formed in the walls behind the rear surfaces  122 - 124 . 
       FIG. 4  shows a perspective view of a component  110 . In common with the component  110  of  FIG. 3 , this component  110  comprises back surfaces  122 - 124 , walls  118 - 119  defining slot  120  therebetween, top surface  126 , and bottom surface  128  to define chamber  114 . In this embodiment, the walls  118 - 119  are angled to correspond to the back surfaces  122 - 124 . In addition, the walls  118 - 119  are re-entrant to point into chamber  114  and have curved front faces  130 - 132  to define a taper to sharp ridges  134 - 136  that define the slot  120 . 
     Top and bottom surfaces  126 - 128  are each provided with a series of three ridges  138 - 140  that extend along the length of the chamber  114 . The ridges  138 - 140  present an upright face that faces towards the rear surfaces  122 - 124  and an angle face that faces generally towards the adjacent wall  118 - 119 . 
     The rear surfaces  122 - 124  are provided with a central rib  142  that extends along the length of the chamber  114 . The top of the rib  142  comprises a sharp ridge defined by two sides that meet at an acute angle, e.g. 40°. The rib  142  also comprises a shallower base defined by sides set at an oblique angle to one another, e.g. 70°. 
       FIG. 5  shows how the component  110  is configured to trap material sputtered as a result of beam strike. A central portion of the ion beam  34  is seen to enter the chamber  114  through slot  120  and strike the rib  142 . The angled faces of rib  142  end material sputtered into the chamber  114 , as shown at  144 . This material is trapped by the walls  118 - 119  or by the ridges  138 - 140  on the top and bottom surfaces  126 - 128 . 
     The rib  142  may be omitted to arrive at the component  110  shown in  FIG. 6 . Although slightly inferior to the component  110  of  FIG. 5 , it will still perform satisfactorily. 
     Another simplified arrangement is shown in  FIGS. 7 and 8 . Here, component  110  is generally box-like with only a single rear surface  122 . The component  110  is provided with a rib  142  that fixes to rear surface  122  to occupy all of the rear surface  122  between top and bottom surfaces  126 - 128 . Walls  118 - 119  are provided to project from the top and bottom surfaces  126 - 128 , but are set slightly back from the front edge of these surfaces  126 - 128 . In addition, the walls  118 - 119  have a tapering form such that their inner surfaces  134 - 136  diverge to provide an aperture  120  that widens as it passes through the walls  118 - 119 . 
       FIG. 9  shows how a series of outer parts  110  may be positioned within a flight tube  46  so as to receive ions from the ion beam  34 . Three components  110  are placed to receive ions with a relatively large mass-to-charge ratio, as indicated at  104 . One of the components  110 ′ may act as a beam dump to receive the ion beam  34  when the magnet is switched off such that the ions follow the straight path shown at  34 ′. Two further components  110 ″ are positioned to receive ions with relatively low mass-to-charge ratios as indicated at  108 . As will be apparent from  FIG. 9 , each of the components  110  is angled to ensure the rear surface  122  faces the incoming ions (or at least does so for ions striking the centre of the component  110 , with minimal angles away from the normal for ions striking away from the centre). 
       FIG. 9  shows the present invention as it may be applied to the inner part  106  of the flight tube  46 . It will be understood that any of the features described in the embodiments above and below may be used in conjunction with the inner part  106  of the flight tube  46 . 
     The above components  110  tackle the problem of sputtered material becoming entrained in the ion beam  34  by trapping sputtered material. Another approach to reduce the problem of contamination is to reduce the amount of sputtered material that is generated in the first place. To this end, components  110  may be formed of graphite and treated so as to provide a coating of another material. Ideal candidates are tungsten, tungsten carbide, tantalum carbide, titanium carbide and silicon carbide. Providing such components  110  with a thin coating of one such material has been found to reduce particle and flake formation and has also been seen to extend the life of the graphite part. The tungsten, tungsten carbide, tantalum carbide, titanium carbide or silicon carbide coating can be applied to the graphite part using chemical vapour deposition, sputtering or plasma spraying. Other methods of coating will be apparent to the person skilled in the art. 
     The above embodiments, are described in the context of a flight tube  46  in a mass analyser  30 . However, the idea of a chamber  114  adjacent the ion beam  34  to trap particles may be employed on other components within the ion implanter  10 . 
     For example, the present invention may be applied to the beamstop  38 , and such an embodiment is shown in  FIGS. 10 and 11 . The beamstop  38  comprises a cylindrical side wall  200 , a circular end wall  202  and an open front face  204  that together define a bore  205 . A beam-collecting surface  206  is provided on the end wall  202 . Set halfway along the bore  205  is a pair of walls  208  and  209  that define a narrow slot  210  therebetween. A chamber  207  is defined behind the walls  208 - 209  and by the end wall  202  and side wall  200 . A magnet  214  provides a magnetic field that suppresses the emission of electrons from the beamstop  38 . An electric field may be used for this purpose instead. 
     Walls  208  and  209  extend from opposite sides of the side wall  200  to terminate at a pair of parallel straight edges  211  and  212 . The narrow slot  210  that they define is ideally suited for use with a ribbon beam  34  that extends in the same direction as slot  210 . As such, the slot  210  should have a high aspect ratio. The open face  204  is large enough to allow all the ribbon beam  34  to enter into the bore  205  without any striking the end of the side wall  200 . The slot  210  is considerably smaller such that it allows most but not all the ribbon beam  34  to pass through. For example, the slot may typically allow 75% of the ribbon beam  34  to pass. The remaining 25% will clip the walls  208 - 209 . Any electrons ejected as a result cannot travel back out from the beamstop  38  by virtue of the suppression magnetic field described above. 
     The bulk of the ribbon beam  34  passes through to the chamber  207  where it strikes the beam-collecting surface  206 . Although the beam-collecting surface  206  is designed to reduce contamination, the beam  34  striking the beam-collecting surface  206  may sputter material from that surface  206  along as liberating electrons. The walls  208 - 209  act as baffles to stop the sputtered material escaping from the beamstop  38  and possibly contaminating the adjacent wafer  12 . The beam current may be measured by monitoring the charge accumulating on the rear surface  202  and walls  208 - 209  with any convenient means. 
     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. 
     The flight tubes  46  of  FIGS. 3 and 4  are shown to have rear surfaces  122 - 124  that meet at an angle and, in the case of  FIG. 4 , walls  118 - 119  that also meet at an angle. However, this need not be the case. For example, a single linear rear surface  122 - 124  may be used or a curved rear surface  122 - 125  may be used. The same treatment may be applied to the walls  118 - 119 . 
     The present invention may be used with magnets other than that present in the flight tube  46 . For example, a component  110  like those shown above may be used in conjunction with an aperture associated with a scanner magnet or collimator magnet. Thus any beam strike on the edges of the aperture may be mitigated by providing components  110  to define the aperture. 
     Where the present invention is applied to downstream components, there is a greater likelihood of deposition of sputtered material ablated from components upstream in the ion implanter  10 . As mentioned previously, our co-pending patent application U.S. Ser. No. 11/651,107 describes a way of mitigating against flake delamination of such deposited material (and the entire contents of U.S. Ser. No. 11/651,107 are incorporated herein by reference). In this application, surfaces are patterned with grooves to prevent flake formation. This technique may be used in conjunction with the present invention in that surfaces may be patterned in any of the ways described therein, rather than just being the plain surfaces shown in the figures. For example, the front faces  130 - 132  of the walls  118 - 119  may be provided with a series of tetrahedra formed by patterning the faces  130 - 132  with two series of grooves that intersect at right angles. The tetrahedra assist in preventing delamination of larger flakes of material.