Abstract:
A fluid bearing vacuum seal assembly comprises an annular stator with first and second opposed surfaces, at least part of the first surface defining a first bearing surface. The stator also defines an aperture having a wall extending between the first and second surfaces. The assembly also comprises a rotor with first and second opposed surfaces, the second surface defining in part a second bearing surface which is supported relative to the first bearing surface in use so that the rotor is rotatable relative to the stator. A cylindrical wall projects axially from the second surface of the rotor through the aperture in the stator. An annular flange projects radially outwardly from the cylindrical wall adjacent to the second surface of the stator. At least one annular differential pumping channel is defined in each of the first and second surfaces of the stator and the wall which connects the first and second surfaces. This configuration allows the differential pumping channels to be spaced apart to a greater extent, improving the performance of the vacuum seal and allowing a better vacuum to be achieved.

Description:
FIELD OF THE INVENTION 
     This invention relates to a fluid bearing vacuum seal assembly. The invention relates in particular to an ion implanter having such an assembly. 
     BACKGROUND OF THE INVENTION 
     As will be familiar to those skilled in the art, in a typical ion implanter a relatively small cross-section beam of dopant ions is scanned relate to a silicon wafer. Traditionally, a batch of wafers was mechanically scanned in two directions relative to a fixed direction ion beam. 
     With the advent of larger wafers, up to 300 mm in diameter, processing of a single wafer at a time has become advantageous in terms of cost, reduced wastage etc. Accordingly, it is now desirable to scan an ion beam relative to a silicon wafer by mechanically scanning the wafer in a first direction and electrostatically or electromagnetically scanning or fanning the ion beam in a second direction. 
     There are a number of different configurations of single wafer processing machines. One example is described in WO99/13488 and other configurations are described in U.S. Pat. Nos. 5,003,183 and 5,229,615. In WO99/13488, the wafer is mounted upon a substrate holder in a process chamber of an implantation device. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm and hence the substrate holder relative to the process chamber. 
     To facilitate movement of the moving parts of the scanning mechanism, one or more gas bearings are provided. For example, the end of the arm distal from the substrate support may be attached to a first bearing member which moves reciprocally relative to a second bearing member. This allows the wafer to be mechanically scanned in a plane orthogonal to the ion beam of the ion implanter. Movement of the first bearing member relative to the second bearing member is facilitated via a first gas bearing. 
     Likewise, the second bearing member may itself be rotatable relative to the process chamber to allow tilting of the substrate support relative to the direction of the ion beam. The second bearing member rotates against a stator mounted upon a flange adjacent the aperture in the wall of the process chamber; a second gas bearing is employed between the stator and the surface of the second bearing member to facilitate this rotation. 
     Since the process chamber is evacuated to a high vacuum and the exterior of the chamber is subject to atmospheric pressure, a large pressure differential exists across the second gas bearing. As is known in the art, in order to permit a vacuum to be maintained adjacent a gas bearing a series of differentially pumped channels are provided in one bearing surface. The vacuum which can be achieved in the process chamber depends on the gas flow leakage between adjacent channels. Thus, a greater vacuum can be achieved by increasing the distance between the channels. However, this also leads to an increase in the outside diameter of the bearing and greater vacuum forces on the movable part of the bearing. 
     It is an object of the present invention to address this problem. More generally, it is an object of the invention to provide an improved differentially pumped gas bearing vacuum seal assembly. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved by the provision of an apparatus comprising: an annular stator having first and second opposed surfaces, at least part of the first surface defining a first bearing surface, the stator also defining an aperture having a wall extending between the first and second opposed surfaces; a rotor having first and second opposed surfaces, the second surface defining in part a second bearing surface supported relative to the first bearing surface so that the rotor is rotatable relative to the stator; a cylindrical wall projecting axially from the second surface of the rotor through the aperture of the stator; an annular flange projecting radially outwardly from the cylindrical wall adjacent the second surface of the stator; and at least one annular channel defined in each of the first and second surfaces of the stator and the wall of the stator. 
     Preferably the stator is spaced from the rotor, in use, by a fluid bearing layer. 
     Typically the spacing between the first and second bearing surfaces, in use, may be in the range of about 10 μm to 15 μm. 
     The spacing between the second surface of the stator and the annular flange, in use, may be in the range of about 10 μm to about 30 μm. 
     The spacing between the cylindrical wall and the wall of the stator is substantially greater than the spacing between the first and second bearing surfaces and between the second surface of the stator and the flange. 
     Preferably, the channel defined in the wall of the stator has a substantially greater cross section than that of the channels defined in the first and second surfaces of the stator. 
     Conveniently, the fluid of said fluid bearing layer is compressed air. 
     In another aspect of the invention there is provided an ion implanter comprising: an ion beam generator to generate a beam of ions to be implanted; a process chamber into which the ion beam is directed; an annular stator mounted upon the process chamber, the stator having first and second opposed surfaces, at least part of the first surface defining a first bearing surface and the stator also defining an aperture having a wall extending between the first and second surfaces; a rotor having first and second opposed surfaces, the second surface defining in part a second bearing surface supported relative to the first bearing surface so that the rotor is rotatable relative to the stator; a cylindrical wall projecting axially from the second surface of the rotor through the aperture defined by the stator; an annular flange projecting radially outwardly from the cylindrical wall adjacent the second surface of the stator; and at least one annular channel defined in each of the first and second surfaces of the stator and the wall of the stator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be put into practice in a number of ways, one embodiment of which will now be described by way of example only and with reference to the accompanying figures in which: 
     FIG. 1 shows a schematic side view of an ion implanter including a process chamber; 
     FIG. 2 shows a sectional view along the line A—A in the process chamber of FIG. 1, the process chamber including prior art apparatus; and 
     FIG. 3 shows a sectional view along the line A—A of the process chamber of FIG. 1, the process chamber including apparatus according to a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a schematic side view of an ion implanter is shown. The ion implanter includes an ion source  10  which is arranged to generate a (typically collimated) ion beam  15 . The ion beam  15  is directed into a mass analyser  20  where ions of a desired mass/charge ratio are selected electromagnetically. Such techniques are well-known to those skilled in the art and will not be detailed further. 
     The ion beam  15  exits the mass analyser  20  in a generally collimated stream. The ion beam exiting the mass analyser may be subject to electrostatic acceleration or deceleration of the ions, depending upon the type of ions to be implanted and the desired implantation depth. 
     Downstream of the mass analyser is a process chamber  40  containing a wafer to be implanted. In the present embodiment, the wafer is typically a large single wafer, approximately 300 mm in diameter. 
     The ion beam which exits the mass analyser  20  generally has a beam width and height which is substantially smaller than the diameter of the wafer to be implanted. It is for this reason that the beam needs to be scanned relative to the wafer. In the preferred embodiment, the ion beam is scanned electrostatically or electromagnetically in the first plane via an electrostatic/electromagnetic scanner  30 . In the present example, the ion beam is scanned in a single plane which extends into and out of the page when viewing FIG.  1 . The wafer itself is scanned mechanically in a second direction orthogonal to the direction of scanning of the ion beam. To scan the wafer mechanically, the wafer is mounted upon a substrate support. This consists of a plate onto which the wafer is mounted within the process chamber  40 , and an elongate arm connected to the plate. 
     The elongate arm extends out through the wall of the process chamber in a direction generally parallel with the scanning plane of the ion beam. The arm passes through a slot (not shown) in a rotor plate  50  which is mounted adjacent to a side wall of the process chamber  40 . The end  60  of the scanning arm is mounted within a scanning member  70 . To effect mechanical scanning of the scanning arm (and hence the wafer mounted upon the plate) relative to the electrostatically/electromagnetically scanned ion beam, the scanning member  70  is movable in a reciprocating manner in the direction X shown in FIG.  1 . To facilitate this scanning, the undersurface of the scanning member  70  is spaced from the upper surface of the rotor plate  50  by a cushion of compressed air which acts as an air bearing. 
     The scanning member  70  in FIG. 1 is shown in a vertical position such that the surface of the wafer is perpendicular to the plane of the scanned or fanned instant ion beam. However, it may desirable to implant ions from the ion beam into the wafer at an angle. For this reason, the rotor plate  50  is rotatable about an axis defined through its centre, relative to the fixed wall of the process chamber  40 . In other words, the rotor plate  50  is able to rotate in the direction R shown in FIG.  1 . 
     As with the scanning member  70 , movement of the rotor plate  50  relative to the wall of the process chamber is facilitated with an air bearing which lies between the lower surface of the rotor plate  50  and the upper surface of a stator (not shown in FIG. 1) mounted upon a flange extending from a wall of the process chamber  40 . Radial movement of the rotor plate is constrained by a series of guide wheels  80  arranged around the circumference of the rotor plate  50 . Unwanted axial movement of the rotor plate is prevented in use by the pressure differential between the two faces of the rotor plate. In particular, the inside of the process chamber is evacuated to prevent contamination of the wafer and ion beam and a large force due to atmospheric pressure accordingly acts to hold the rotor plate against the stator. 
     The mechanical scanning arrangement described above is that described in the aforementioned WO99/13488, assigned to a common assignee, the contents of which are hereby incorporated in their entirety by reference. 
     Referring now to FIG. 2, a sectional view along the line A—A of the process chamber  40  of FIG. 1 is shown, in the case where the rotor plate and stator are mounted upon a flange extending from the wall of the process chamber  40  using a prior art arrangement. 
     The wall of the process chamber  40  has a generally circular aperture (indicated by reference numeral  85  in FIG. 2) in it. An annular flange  45  extends around the edge of the circular aperture  85  in the wall of the process chamber  40 . An annular stator  90 , whose purpose will be described below, having an aperture  95  defined by a wall  93 , is affixed to the flange  45 , the stator  90  and aperture  95  being substantially coaxial with the axis of the circular aperture  85 . Fixing of the stator  90  to the flange  45  is achieved by a mounting fastener  100 , such as a bolt. This passes through an opening in the flange  45  and into a corresponding threaded opening in a lower surface of the stator  90 . It will be understood that, in order to effect clamping of the stator  90  to the flange  45 , a plurality of mounting fasteners  100  are employed about the circumference of the flange. 
     The rotor plate  50  lies above an upper surface (as viewed in FIG.  2 )of the stator  90 . The rotor plate  50  acts as a closure for the process chamber  40 . The lower surface  110  (as viewed in FIG. 2) of the rotor plate  50  acts as a first bearing surface, and the upper surface  92  of the stator  90  acts as a second bearing surface. A supply of compressed air (not shown in FIG. 2) is connected to a series of compressed air channels in the stator  90  which are indicated schematically by arrows  130  in FIG.  2 . Application of compressed air to the compressed air channels  130  creates a compressed air bearing  120  between the bearing surfaces  110 , 92  of the rotor plate  50  and the stator  90 . 
     The process chamber  40  is evacuated. The upper surface  112  of the rotor plate  50  is, however, at atmospheric pressure. To allow rotational movement of the rotor plate  50  relative to the stator  90  on the compressed air bearing  120 , whilst maintaining a vacuum within the process chamber  40 , a vent V to atmosphere and a series of differential pumping channels P 1 ,P 2 ,P 3  are provided. The channels allow a graded pressure differential to be obtained between a first differential pumping channel P 1 , which is at relatively close to atmospheric pressure, and a last differential pumping channel P 3  which is at high vacuum. 
     The ultimate vacuum which can be achieved within the process chamber  40  is determined by the gas flow between each of the differential pumping channels P 1 ,P 2  and P 3 . The gas flow is a function of the gap G between the rotor plate  50  and the stator  90  and the distances L 1  and L 2  between the channels P 1 ,P 2 ,P 3  and the distance L 3  between the channel P 3  and the aperture  95 . For a given gap G, the vacuum performance is improved if the distances L 1 -L 3  can be increased. However, as these distances are increased, the outside diameter of the stator  90  and rotor plate  50  and the vacuum forces on the rotor plate  40  also increase. When this happens, the rotor plate  50  may be subject to distortion due to the pressure differential across it arising from the vacuum within the chamber  40  and this may bring the bearing surface  110  of the rotor plate  50  parallel out of alignment with the bearing surface  92  of the stator  90 . This in turn requires the gap G to be increased to ensure the bearing surfaces  92 , 110  do not contact each other which, as mentioned above, increases the gas flow between the pumping channels P 1 ,P 2 ,P 3  and requires a greater pressure of compressed air to be supplied to create the gas bearing  120 . 
     Alternatively, if the outside diameter of the stator  90  and rotor plate  50  is fixed then increasing the distances L 1 -L 3  reduces the radius R of the aperture  85 . 
     Turning now to FIG. 3, a section along the line A—A of the process chamber of FIG. 1 is shown, with an apparatus embodying the present invention employed. Features common to FIGS. 2 and 3 are labelled with like reference numerals. 
     In accordance with the present invention, a tube  140  is secured to the lower surface  110  of the rotor plate  50  and extends through the aperture  95  defined by the stator  90 . A ring  150  is secured to the lower end of the tube  140 , to create a flange which projects radially outwardly beneath the lower surface  94  of the stator  90 . The length of the tube  140  is very precisely dimensioned so as to provide a given gap G 2  between the lower surface  94  of the stator  90  and the upper surface of the ring  150  when the gas bearing  120  is in operation to provide a given gap G 1  between the bearing surface  92  of the stator  90  and the bearing surface  110  of the rotor plate  50 . Typically, G 1  is in the order of 10-15 μm while G 2  is in the order of 10-30 μm. 
     Furthermore, in the arrangement of the present invention the first differential pumping channel P 1  is provided in the upper surface  92  of the stator  90 . The second differential pumping channel P 2  is provided in the wall  93  joining the upper and lower surfaces  92 , 94  of the stator  90 , facing radially inwardly. The third differential pumping channel P 3  is provided in the lower surface  94  of the stator  90 . As shown, the second differential pumping channel P 2  has a considerably larger cross section than the first and third differential pumping channels P 1  and P 3 . The gap G 3  between the wall  93  of the stator  90  and the radially outer surface  142  of the tube  140  is also relatively large compared with the gaps G 1  and G 2  so as to take account of differential expansion problems of the various components. 
     It will be appreciated if the arrangement of the present invention allows the spacing between the differential pumping channels to be increased to considerably larger values than in the prior art arrangement of FIG. 2, without the need to increase the outside diameter of the stator  90  and rotor plate  50 . Thus for a given outside diameter the radius R of the aperture  95  can be maximised. 
     Additionally, the tube  140  and ring  150  serve to educe the ingress of contaminants such as particulate matter into the gap between the bearing surfaces  92 , 110 . 
     An embodiment of the invention has been described in connection with the rotor plate arranged to rotate upon an air bearing above a stator which is in turn mounted upon a flange in an ion implanter. However, it will be appreciated that the invention may be employed in other situations in which a vacuum seal is required adjacent a gas bearing between two relatively rotatable parts. Indeed, the invention is not restricted to ion implanters and may be useful in other situations involving a vacuum chamber and a gas bearing between adjacent components. 
     Similarly, although the embodiment described uses compressed air in the gas bearing, any suitable fluid may be used. 
     Although the present invention has been described with reference to a preferred embodiment, those skilled in the art will recognise that changes may be made in form and detail without departing from the spirit and scope of the invention, which is to be determined in accordance with the appended claims.