Abstract:
In accordance with the present invention, an ion implanter including a rotatable support disposed in an implantation chamber of an ion beam implanter for supporting a plurality of wafer workpieces. The rotatable support includes a hub adapted to be rotated about an axis of rotation substantially parallel to a direction of an ion beam beam line entering the implantation chamber. The rotatable support further includes a plurality of wafer support members adapted to be attached to the hub, each wafer support member adapted to support at least one of the wafer workpieces. Each wafer support member includes an attachment structure for affixing the support to the rotating member and a wafer support pad extending from the attachment structure and passing through the beam line as the hub rotates. The wafer support pad includes a wafer support surface facing the beam line that includes a concave portion. Preferably, the concave portion of the wafer support surface is cylindrically shaped and a central axis of an imaginary cylinder partially formed by the concave portion intersects an axis of rotation of the hub. A radius of curvature of the concave portion is large, for a 300 mm. disk shaped wafer, the radius of curvature is 7 meters. Each wafer support member further includes a clamp for releasably securing a wafer workpiece to the wafer support pad. Upon rotation of the hub at a predetermined angular velocity, the workpiece conforms to a shape of the concave portion due to a component of centrifugal force normal to a surface of the wafer support surface.

Description:
SUMMARY OF THE INVENTION 
     The present invention relates generally to an ion beam implanter and, more particularly, to an ion beam implanter including a rotatable workpiece support having cylindrically shaped workpiece support surfaces to minimize the variation in the angle of incidence of the ion beam across the semiconductor wafer workpieces 
     BACKGROUND OF THE INVENTION 
     Ion implantation has become the technology preferred by industry to dope semiconductor wafers with impurities in the large scale manufacture of integrated circuits. Ion dose and ion energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable of up to about 1 mA beam current) are used for lower dose applications. 
     Ion energy is used to control junction depth in semiconductor devices. The energy levels of the ions comprising the ion beam determine the depth of implantation of the ions into the wafer workpieces. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below I thousand electron volts (1 KeV). 
     The continuing trend to smaller and smaller semiconductor devices requires a ion beam beam line construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application. 
     In high current and high energy implanters, semiconductor wafer workpieces are mounted near the periphery of a rotatable workpiece support. As the support rotates, the workpieces pass through the ion beam and are implanted with ions. When implanting wafers, if the angle of incidence of the ion beam (implantation angle) is perpendicular or normal to the surface of the workpiece, an effect called “channeling” has been found to occur. When channeling occurs, the ions of the ion beam pass into the crystal lattice structure of the semiconductor wafers and achieve greater penetration depth than is normally the case. The effective tilt angle (ETA) is defined as the angle between the ion beam and a ray extending perpendicularly from the surface of the wafer workpieces. An ETA=0 degrees defines a channeling implantation. 
     If channeling not desired, the effective tilt angle ETA is increased slightly, usually in the range of 1-10 degrees so that the ion beam beam line is not exactly perpendicular to the workpiece surface. This is accomplished by tilting the workpiece support with respect to the ion beam beam line direction. 
     In some implantation applications, channeling is useful. However, in such channeling applications, that is, ETA=0 degrees, implantation depth is very sensitive to implantation angle variation across the workpiece. As the implantation angle varies across the workpiece, the depth of ion penetration into the semiconductor wafer workpieces changes markedly. 
     If implantation depth is to be accurately controlled, the implantation angle must not change significantly over the surface of the wafer. In some applications, for example, in channeling implants the maximum allowable variation in the implantation angle is 0.2 degrees. 
     However, current art implanters wherein the workpiece support rotates and the workpieces lie flat on a flat workpiece support pad, a variation in the implantation angle of over 1 degree with a 300 millimeter (mm.) (30 cm.) diameter wafer workpiece at an ETA=0 degrees (channeling implant) is usual. 
     What is needed is a wafer support apparatus that minimizes the variation of implantation angle over a range of effective tilt angles ETA from 0 degrees (channeling implantation) and greater (non channeling implantation). 
     SUMMARY OF THE INVENTION 
     The present invention concerns an ion beam implanter for treating a plurality of semiconductor wafer workpieces is disclosed. The ion implanter includes an implantation station defining an implantation chamber and further includes an ion source for generating the ion beam and beam forming and directing apparatus defining an interior region through which the ion beam passes from the ion source to the implantation station. A pump system is provided for pressurizing and depressurizing the interior region. 
     The implanter includes a novel rotatable support disposed in the implantation chamber for supporting a plurality of wafer workpieces. The rotatable support includes a hub adapted to be rotated about an axis of rotation substantially parallel to a direction of an ion beam beam line entering the implantation chamber. The rotatable support further includes a plurality of wafer support members adapted to be attached to the hub, each wafer support member adapted to support at least one of the wafer workpieces. Each wafer support member includes an attachment structure for affixing the support member to the hub and a wafer support pad extending from the attachment structure and passing through the beam line as the hub rotates. 
     The wafer support pad has a wafer support surface that includes a concave portion being concave in shape. In one preferred embodiment, the concave portion is cylindrical and a central axis of an imaginary cylinder corresponding to the cylindrically shaped concave portion passes substantially through an axis of rotation of the hub. Each wafer support member further includes a clamp for releasably securing a wafer workpiece to the wafer support pad. Upon rotation of the hub at a predetermined angular velocity, the workpiece conforms to a shape of the concave portion due to a component of centrifugal force normal to a surface of the wafer support surface. 
    
    
     These and other objects, features and advantages of the invention will become better understood from the detailed description of the preferred embodiments of the invention which are described in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of an ion beam implanter including a rotatable wafer workpiece support of the present invention; 
     FIG. 2 is a front elevation view of the rotatable wafer workpiece support of the present invention; 
     FIG. 3 is a sectional view of the rotatable support of FIG. 2 as seen from a plane indicated by the line  3 — 3  in FIG. 2; 
     FIG. 4 is a schematic depiction of a portion of a cooling structure for the rotatable support of FIG. 2; 
     FIG. 5 is a front elevation view of a wafer support member of the rotatable support of FIG. 2; 
     FIG. 6 is a sectional view of the wafer support member of FIG. 5 as seen from a plane indicated by the line  6 — 6  in FIG. 5; 
     FIG. 6A is a section view of the wafer support member of FIG. 5 as seen from a plane indicated by the line  6 A— 6 A in FIG. 6; 
     FIG. 7 is another sectional view of the wafer support member of FIG. 5 as seen from a plane indicated by the line  7 — 7  in FIG. 5; 
     FIG. 8 is a side elevation view of the wafer support member of FIG. 5 as seen from a plane indicated by the line  8 — 8  in FIG. 5; 
     FIG. 9 is a back elevation view of the wafer support member of FIG. 5; 
     FIG. 10 is a schematic side elevation view of the wafer support member of FIG. 5; 
     FIG. 11A is a schematic depiction of an ion beam beam line impinging upon an upper portion of a wafer workpiece supported on a cylindrically curved support; 
     FIG. 11B is a two dimension schematic representation of the depiction of FIG. 11B; 
     FIG. 11C is a two dimensional schematic representation of an ion beam beam line impinging upon a central portion of the cylindrically curved workpiece of FIG. 11A; 
     FIG. 11D is a two dimensional schematic representation of an ion beam beam line impinging upon a lower portion of the cylindrically curved workpiece of FIG. 11A; 
     FIG. 12 is a graph showing ion beam implant angle variation as a function of a distance from the wafer center for five different radii of curvature; 
     FIG. 13 is a graph showing maximum ion beam implant angle variation as a function of tilt angle for a flat wafer support pad surface and a cylindrically curved wafer support pad surface having a radius of curvature of 7 meters; 
     FIG. 14A is a schematic representation of the support and a semiconductor workpiece mounted on the support to illustrate the support tilt angle and the effective tilt angle for a non-channeling implantation; and 
     FIG. 14B is a schematic representation of the support and a semiconductor workpiece mounted on the support to illustrate the support tilt angle and the effective tilt angle for a channeling implantation. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, an ion implanter is shown generally at  10  in FIG.  1 . The specific implanter shown is a low energy ion implanter, but it should be understood that the present invention is useful and applicable to both low and high energy ion implanters, that is, ion implanters having energies anywhere in the range of 0.2 kiloelectron volts to several million volts. The ion implanter  10  includes an ion source  12 , a mass analyzing magnet  24  and an implantation or end station  16 . The implantation station  16  defines an implantation chamber  17 . The ion source  12  generates an ion beam  18  which impacts semiconductor wafer workpieces  100  disposed on a rotating and translating disk-shaped workpiece support  110  in the implantation chamber  17 . 
     In a low energy ion implanter, to minimize the tendency of a low energy ion beam  18  to diffuse (i.e., “blow-up”) as it traverses the distance between the ion source  12  and the implantation station support  110 , the distance from the ion source  12  is kept to a minimum (approximately 3 meters). 
     The ion source  12  is mounted to an L-shaped frame  19  and includes a housing  21  defining an interior region. The housing  21  supports a plasma arc chamber  20 . The plasma arc chamber  20  defines an interior region into which source materials are injected. The source materials may include an ionizable gas or vaporized source material. Source material in solid form is deposited into a vaporizer which is then injected into the plasma chamber  28 . If an n type extrinsic wafer material is desired, boron (B), gallium (Ga) or indium (In) will be used. Gallium and indium are solid source materials, while boron is injected into the plasma chamber as a gas, typically boron trifluoride (BF 3 ) or diborane (B 2 H 6 ), because boron&#39;s vapor pressure is too low to result in a usable pressure by simply heating solid boron. 
     If a p type extrinsic material is to be produced suitable source materials include source gases arsine (AsH 3 )and phosphine (H 3 P) and vaporized solid antimony (Sb). Energy is applied to the source materials to generate positively charged ions in the plasma chamber  20 . Generally, positive ions are generated, although the present invention is applicable to systems wherein negative ions are generated by the source. The positively charged ions exit the plasma chamber  20  through an elliptical arc slit in a cover plate overlying an open side of the plasma chamber  20 . 
     The ion beam beam line  18  is formed and directed by beam forming and directing structure  22 . The beam forming and directing structure  22  includes a mass analyzing or resolving magnet  24  and a set of extraction electrodes  26 . The positive ions are extracted through a slit in the plasma chamber  20  by the set of extraction electrodes  26 . The electrodes  26  are charged with negative potential voltages, increasing in magnitude as the distance from the plasma chamber slit increases. The plasma chamber ions are accelerated by the set of electrodes  26  adjacent the plasma chamber  20  into a region bounded by the mass analyzing magnet  24 . An ion beam path through the magnet region is bounded by an aluminum beam guide  28 . During production runs, an interior region  30  defined by the beam forming and directing structure  22  is evacuated by a pressure regulation system including a pump  31 . 
     Ions that make up the ion beam  18  move from the ion source  12  into a magnetic field generated by the mass analyzing magnet  24 . The strength and orientation of the magnetic field produced by the analyzing magnet  24  is controlled by the control electronics  32  coupled to a magnet connector  33  for adjusting a current through the magnet&#39;s field windings. 
     The mass analyzing magnet  24  causes only those ions having an appropriate mass to charge ratio to reach the ion implantation station  16 . The ionization of source materials in the plasma chamber  20  generates a species of positively charged ions having a desired atomic mass. However, in addition to the desired species of ions, the ionization process will also generate a proportion of ions having other than the proper atomic mass. Ions having an atomic mass above or below the proper atomic mass are not suitable for implantation. The mass analysis magnet  24  functions to pass only ions of an appropriate charge-to-mass ratio to a resolving housing  34 . The magnetic field generated by the mass analyzing magnet  24  causes the ions in the ion beam  18  to move in a curved trajectory. The magnetic field that is established by the control electronics  32  is such that only ions having an atomic mass equal to the atomic mass of the desired ion species traverse the curved beam path to the implantation chamber  17 . Ions that do not have the proper charge-to-mass ratios are deflected away from the beam path  18  and collide with walls of the beam guide  28  as shown by the beam paths labeled  18 ′ and  18 ″. The mass analysis magnet  24  includes a curved beam path which is defined by the aluminum beam guide  28 . The ion beam  18  which propagates along this path is affected by the magnetic field generated by the mass analysis magnet  24 . Partial focusing of the ion beam  18  by the magnet  24  is achieved in the “dispersive” plane (the plane of the curved portion of the beam path  18 ) only by establishing a gradient in the magnetic field (i.e., “indexing”), or by rotating the entrance or exit poles of the magnet  24 . The magnetic field causes the ion beam  18  to move along the curved beam path, from a first or entrance trajectory near the ion source  12  to a second or exit trajectory near the resolving housing  34 . 
     The entrance and exit trajectories of the ion beam  18 , being in the plane of the curved beam path  29 , lie in the dispersive plane. The “non-dispersive” plane is defined herein as the plane which resides perpendicular to both the dispersive plane and the target plane. Accordingly, the magnet  24  performs mass analysis in the dispersive plane by removing from the beam ions of inappropriate charge-to-mass ratio, and redirecting the beam from the entrance trajectory to the exit trajectory and toward the target plane in which the workpiece wafers  100  lie. 
     The beam forming and directing structure  22  includes the resolving housing  34  and a beam neutralizer or electron shower  45 . The resolving housing  34  supports an electrostatic lens  36 , which mass resolves and focuses the ion beam  18  output by the magnet  24 . The resolving housing  34  defines a chamber  41  in which resides the electrostatic lens  36 , as well as a dosimetry indicator such as a Faraday flag  42 . The chamber  41  is evacuated by a vacuum pump  43  which is part of the pressure regulation system. The adjacent beam neutralizer  45  is supported within a chamber  44  defined by an ion neutralizer housing  49 . The electron shower  45  neutralizes the positive charge which would otherwise accumulate on the target workpieces  100  as a result of being implanted by the positively charged ion beam  18 . Such a net positive charge on a wafer workpiece has undesirable effects. The implantation station  16  is pivotable with respect to the neutralizer housing  49  and is attached to the housing  49  by a flexible bellows  50 . 
     During a production run, that is, when semiconductor wafer workpieces  100  are being impinged upon by the ion beam  18  and thereby being implanted with ions, the ion beam  18  travels through an evacuated path from the ion source  12  to the implantation chamber  17 , which is also evacuated. Evacuation of the beam path is provided by the pressure regulation system including the vacuum pumps  31 ,  43 . 
     Workpiece Support  110   
     Supported within the implantation chamber  17  is the disk-shaped semiconductor wafer workpiece support  110 . During ion beam implantation operations, the support  110  is rotated at a constant angular velocity by a motor  111  about an axis of rotation labeled CL in FIGS. 1,  14 A and  14 B. An output shaft of the motor  111  is coupled to a drive shaft of the support  110  via a belt. A stepper motor also drives a lead screw to translate the support  110  vertically. Ion dosage received by the workpieces  100  is determined by a combination of the velocity of the rotation and translation of the support  110  which is under the control of the control electronics  32 . 
     The implantation station  16  is pivotable with respect to the remainder of the implanter  10 . Particularly, the implantation station  16  is pivotable with respect to the ion beam beam line  18  to change a tilt angle of the support  110  and, thereby, change an angle of implantation at which the ion beam  18  strikes the workpieces  100  as the workpieces move though the beam line  18 . The term STA will be used hereafter to refer to the support tilt angle, that is, the angle between the beam line  18  and the center line CL of the support  110 . The portion of the beam line  18  that traverses the distance between the ion neutralizer housing  49  and the support  110  is labeled BL. As can be seen in the exaggerated schematic views of the support  110  in FIGS. 14A and 14B, the semiconductor wafer workpieces  100  are supported on wafer workpiece support surfaces  132 . These wafer support surfaces  132  are disposed at a 5 degree offset angle (labeled OA FIG. 7) with respect to the generally planer surface of the support  110  (the 5 degree angle of the wafer support surfaces  132  has been exaggerated greatly in FIGS. 14A and 14B for illustrative purposes). The term ETA will be used hereafter to refer to the effective tilt angle, that is, the angle between the beam line portion BL and an imaginary line extending normal to an implantation surface of the wafer workpiece  100  being implanted. Since the wafer support surfaces  132  are at a 5 degree angle with respect to the front planar surface of the support  110 , the relationship between the station tilt angle STA and the effective tilt angle ETA is given by the equation: 
     
       
         ETA=STA−5 degrees 
       
     
     FIG. 14A illustrates an non-channeling implantation wherein: 
     
       
         Support tilt angle=STA=0 degrees 
       
     
     
       
         Effective tilt angle=ETA=5 degrees 
       
     
     FIG. 14B illustrates a channeling implantation wherein: 
     
       
         Support tilt angle=STA=5 degrees 
       
     
     
       
         Effective tilt angle=ETA=0 degrees 
       
     
     FIG. 1 illustrates a support tilt angle STA=0 degrees and an effective tilt angle ETA=5 degrees in a less exaggerated manner than FIG.  14 A. For illustration purposes, a line labeled CL′ which is parallel to the support axis of rotation CL is depicted in FIG.  1 . 
     As can best be seen in FIGS. 2 and 3, the support  110  includes a center disk or hub  112 . Affixed to the hub  112  and extending radially outwardly from the hub are thirteen adjacently spaced wafer support members  120 . Both the hub  112  and the wafer support members  120  are comprised of aluminum. Other portions of the support  110  are not shown for clarity. Each wafer support member  120  includes a wafer support pad  130  which extends from an attachment portion  150  for affixing the wafer support member to the hub  112 . In the instant embodiment, each of the wafer support members  120  includes a wafer support pad  130  having a wafer support surface  132  sized to receive a 300 millimeter (mm.) (30 cm.) diameter disk-shaped semiconductor wafer workpiece  100  (workpiece  100  is shown in phantom in FIG.  5 ). The distance from the center axis CL of the hub  112  to the center of the 30 cm. diameter workpiece is approximately 61 centimeters (cm.). 
     The wafer support surface  132  is machined to have a concave surface and, specifically a cylindrically-shaped concave surface (best seen in FIG.  10 ). The surface  132  faces in the direction of the ion beam  18  as it enters the implantation chamber and is concave with respect to the direction of the ion beam  18 . The concave wafer support surface  132  forms a portion of an outer wall of an imaginary cylinder IC (partially shown in dashed line in FIGS. 11B,  11 C, and  11 D) having a radius of curvature R (FIGS. 10,  11 B,  11 C, and  11 D) of approximately 7 meters. 
     A central axis of the imaginary cylinder IC passes through the axis of rotation CL of the hub  112 . For implanting disk shaped wafer workpieces having a 20 cm. diameter, the wafer support members would be modified to have a 20 cm. diameter wafer support surface. With a 45 cm. radius to the center of the workpiece  100  and a 5 degree support surface angle, the support surface would be machined to have a radius of curvature of 5.12 meters. 
     Each wafer support member  120  is affixed to the central portion  114  of the hub  112  by four bolts  118 . The bolts  118  fit into counterbored openings  152  (FIG. 8) in a flat peripheral portion  154  (FIG. 5) of the attachment portion  150  and extend through aligned threaded openings in the hub  112 . As can be seen in FIG. 3, the flat peripheral portion  154  of the attachment portion  150  seats against a recessed outer portion  119  of the hub  112 . 
     An upper surface  122  of the wafer support member  120  is best seen in FIG.  5 . Extending between the wafer support surface  132  and the flat peripheral portion  154  are a pair of thinner recessed portions  156  flanking a thicker raised center portion  158 . The raised center portion  158  terminates at an inner periphery  136  (FIG. 5) of the wafer support surface  132 . A lower surface  124  of the wafer support member is best seen in FIG.  9 . Extending downwardly from the flat peripheral portion  152  of the attachment portion  152  and a flat bottom surface  134  of the support pad  130  is a central portion  135 . As can best be seen in FIGS. 5,  6  and  9 , the central portion  135  provides a path for a pair of cooling channels  180  extending through an interior region of the wafer support member  120 . The channels  180  are angled and have a common vertex. As can be seen in FIG. 9, the downwardly extending central portion  135  terminates in a V-shaped portion  136  under the support surface  132 . The channels  180  are part of a cooling structure  170  of the wafer support member  120 . 
     As can best be seen in FIG. 6A a divider  181  is disposed in each channel  180  terminating short of a distal end of each channel (see FIG. 6) such that the channel provides a cooling liquid inflow path  182  and an outflow path  184  for routing coolant in proximity to a workpiece support surface  132  of the pad  130 . The preferred cooling fluid is water or, preferably, deionized water. 
     The channels  180  terminate at a vertex in a fluid coupling  175  (only shown in FIG. 9) seated against an inclined portion  155  transitioning between flat peripheral portion  152  and the downwardly extending central portion  135 . The coupling  175  mates with an “o” ring on the hub  112  to seal against cooling fluid leakage. Shown schematically in FIG. 4 is a representation of the “daisy chain” flow of coolant through three of the support members  120  and the hub  112 . 
     The workpiece  100  is held in place on the wafer support surface  132  by a workpiece clamping assembly  200  including three spring loaded clamps: a peripheral clamp  210  disposed on a distal peripheral portion  138  of the support pad  130  and two interior clamps  220 ,  230  disposed on an inner periphery  136  of the support pad  130  spaced outwardly and equidistantly from the intersection of the support pad and the raised portion  158  (FIG.  5 ). The peripheral clamp  210  includes a base  211  (FIG. 9) affixed to the bottom support pad surface  134  and a spring loaded clamping member  212  extending above the support surface  132  at the distal peripheral portion  138  to push radially inwardly on the circular shaped workpiece  100 . The interior clamps  220 ,  230  include respective bases  222 ,  232  affixed to the downwardly extending portion  135 . Portions of the clamps  220 ,  230  extend upward through small holes in the thin portions  156  and are disposed adjacent slight indentations in the support surface periphery (best seen in FIG.  5 ). Spring loaded clamping members  224 ,  234  of the clamps  220 ,  230  extend above the support surface  132  to push radially inwardly on the workpiece  100 . It should be noted that when the support  110  is rotating during a production run at approximately 1200 revolutions per minute, the interior clamps  220 ,  230  are no longer needed to hold the workpiece in place as centrifugal force pushes the workpiece outwardly against the peripheral clamp  210 . However, in the present embodiment the clamps are left in place, clamping the workpiece  100 . 
     As can be seen in FIG. 7, there is an offset angle, OA, of 5 degrees between the flat attachment portion  150  attached to the hub central portion  114  and the workpiece support surface  132 . 
     The cylindrical shape of the workpiece support surface  132  is shown in FIGS. 10,  11 A,  11 B,  11 C and  11 D. As the support  110  rotates, the centrifugal force acting on the workpiece  100  bends the workpiece to make it conform to the cylindrical shape of the support surface  132 . Because the support surface  132  is at an angle with respect to the axis of rotation CL of the support  110 , a component of the outward centrifugal force acts on the workpiece  100  pushing the workpiece flush against the support surface  132 . In FIGS. 11A,  11 B,  11 C and  11 D, the radius of curvature of the support surface  132  is greatly exaggerated to illustrate the principle that the curvature of the support surface  132  causes the rectangular shaped ion beam BL exiting the ion neutralizer housing  49  to have an angle of implantation A (that is, the angle of incidence of the ion beam  18  on the workpiece) that is substantially perpendicular to the workpiece implantation surface. As can be seen in FIGS. 11B,  11 C and  11 D, as the support  110  rotates and the workpiece  100  passes through the beam line BL, the implantation angle A remains at 90 degrees, whereas, if the workpiece was flat, the angle of implantation would change markedly as the workpiece passes through the beam line BL. The variation of implantation angle if the workpiece  100  were supported on a flat surface is shown by the angle labeled B in FIGS. 11B,  11 C and  11 D. As will be explained below, there is some variation of the implantation angle for the cylindrically shaped wafer support surface  132  of the present invention, but, under a range of tilt angles, the implantation angle variation is less than 0.2 degrees which is a marked improvement over the implantation angle variation of a flat wafer support surface for a range of tilt angles. 
     For channeling implantation, ETA=0 degrees, a radius of curvature of R=7 meters provides for less that 0.2 degrees of implant angle variation across a 30 cm. (300 mm.) wafer workpiece  100  wherein the center of the workpiece  100  is 61 cm. from the rotational center line CL of the rotatable support  120 . FIG. 12 shows the implant angle variation across a 30 cm. diameter (radius of 15 cm.) wafer workpiece for cylindrical radii of curvature varying from 5 meters to 100 meters. Note that the limit of the horizontal axis is 15 cm. which corresponds to the outer edge of the 30 cm. diameter circular workpiece  100 . The 100 meter radius of curvature is so large that it can be viewed as an approximation of a flat wafer workpiece, that is, a workpiece on a flat wafer support surface. As can be seen, the minimum variation occurs for a radius of curvature of 7 meters (275.6″). For an effective tilt angle ETA=0 degrees, the maximum implant angle variation for the R=7 meter radius cylindrical support surface  132  is only 0.15 degrees. For the 100 meter radius of curvature, which approximates prior art flat workpiece support surfaces, the implant angle variation is 1.2 degrees for a 30 cm. diameter workpiece. 
     Since the pressure to deflect the wafers into the pad is much less than the available centrifugal pressure, performance of the cooling structure  170  is substantially unaffected, that is, sufficient coolant flows through the channels  180  to provide cooling at acceptable coolant pressure levels. Stress on the wafer workpiece  100  is well within acceptable limits, being only about 4% of the minimum breaking stress. The imaginary central axis of a cylinder that includes the cylindrical support surface  132  and passes through the rotatable support axis of rotation CL. 
     FIG. 13 shows the maximum implant angle variation as a function of the effective tilt angle, ETA, for a flat wafer support surface and for the wafer support surface  132  of the present invention having a radius of curvature, R=7 meters. As can be seen in FIG. 13, in a channeling implantation, ETA=0 degrees, for a flat wafer support surface, the maximum variation of implantation angle has been determined to be 1.2 degrees across the 30 cm. wafer workpiece  100 . This implantation angle variation is unacceptably large for channeling implantations, where minimizing implantation angle variation is especially critical. For the cylindrical wafer support surface  132  having a radius of curvature R=7 meters, the maximum variation of implantation angle is a nearly constant value just less than 0.2 degrees across the 30 cm. wafer workpiece for a range of effective tilt angles ETA from 0 degrees to +10 degrees. 
     A flat workpiece support surface provides a constant implant angle when the beam is parallel to the rotation axis, thus, the maximum variation in implantation angle is essentially zero at an ETA=5 degrees. At all other implant angles the implantation variation is non-zero. The maximum implant angle variation occurs at ETA=0 degrees. In contrast, the cylindrical surface  132  of the present invention provides a maximum implant variation that is more or less constant over the range of interest. 
     It should be recognized that, while the cylindrically shaped concave workpiece support surface  132  provides an acceptable maximum implantation angle variation over a range of utilized tilt angles, TA, there exist other non-cylindrically shaped concave support surfaces that will provide even lower maximum implantation angle variation over certain ranges of tilt angles. It is the intent of the present invention to cover all such concave workpiece support surfaces, in addition, to the cylindrically shaped support surface  132  specifically disclosed. The advantage of the cylindrically shaped surface is that such a cylindrical surface is easier to machine than more complex concave surfaces while still providing acceptable maximum implantation angle variation over a robust range of tilt angles. Additionally, the force required to deform the wafer workpieces  100  to such non-cylindrically shaped concave support surfaces will be greater, thereby decreasing contact pressure between the workpiece and the workpiece support. With most commonly employed workpiece cooling schemes currently in use, this will also decrease the workpiece cooling. 
     While the invention has been described herein in its currently preferred embodiment or embodiments, those skilled in the art will recognize that other modifications may be made without departing from the invention and it is intended to claim all modifications and variations as fall within the scope of the invention.