Abstract:
Methods and apparatus are provided for plasma doping of a workpiece. The plasma doping apparatus includes a housing defining a plasma doping chamber, a platen for supporting a workpiece in the plasma doping chamber, an anode spaced from the platen in the plasma doping chamber, a process gas source coupled to the plasma doping chamber, a vacuum vessel enclosing the plasma doping chamber and defining an outer chamber, a primary vacuum pump connected to the vacuum vessel, a pulse source for applying pulses to the anode, and a controller. The controller establishes a controlled plasma doping environment in the plasma doping chamber in a first mode, typically a plasma doping mode, and establishes a gas connection between the plasma doping chamber and the outer chamber in a second mode, typically a vacuum pumping and wafer exchange mode.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to plasma doping of semiconductor wafers and, more particularly, to methods and apparatus for plasma doping by anode pulsing, thereby permitting the semiconductor wafer to be grounded.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.  
           [0003]    Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired species, is accelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. Examples of prior art ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 to Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al.; U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.; and U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al.  
           [0004]    A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1,000 Angstroms and may eventually require junction depths on the order of 200 Angstroms or less.  
           [0005]    The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 20 keV to 400 keV, and may not function efficiently at the energies required for a shallow junction implantation. At low implant energies, such as energies of 2 keV and lower, the current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers.  
           [0006]    Plasma doping systems have been studied for forming shallow junctions in semiconductor wafers. In one type of plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode, located in a plasma doping chamber. An ionizable gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode, causing formation of a glow discharge plasma having a plasma sheath in the vicinity of the wafer. The applied voltage pulse causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved. Plasma doping systems are described, for example, in U.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng; U.S. Pat. No. 6,020,592 issued Feb. 1, 2000 to Liebert et al.; and U.S. Pat. No. 6,182,604 issued Feb. 6, 2001 to Goeckner et al.  
           [0007]    In other types of plasma systems, known as plasma immersion systems, a continuous RF voltage is applied between the platen and the anode, thus producing a continuous plasma. At intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.  
           [0008]    Prior art plasma doping systems typically utilize a configuration wherein the anode and the chamber walls are grounded and the cathode is pulsed negative. This configuration has the advantage that only the wafer is implanted with ions from the plasma. A plasma immersion system which utilizes a pulsed anode is disclosed in U.S. Pat. No. 5,911,832 issued Jun. 15, 1999 to Denholm et al. A disadvantage of the disclosed approach is that shielding of the chamber walls is required to limit power consumption and to limit sputtering of the chamber walls and the resulting wafer contamination. It is difficult to effectively shield vacuum pumping ports and vacuum pumping equipment connected to the chamber. Accordingly, there is a need for improved plasma doping systems and methods.  
         SUMMARY OF THE INVENTION  
         [0009]    According to a first aspect of the invention, plasma doping apparatus is provided. The plasma doping apparatus comprises a housing defining a plasma doping chamber, a platen for supporting a workpiece in the plasma doping chamber, the platen being coupled to a reference potential such as ground, an anode spaced from the platen in the plasma doping chamber, a process gas source coupled to the plasma doping chamber, a vacuum vessel enclosing the plasma doping chamber and defining an outer chamber, a primary vacuum pump connected to the vacuum vessel, a pulse source for applying pulses to the anode, and a controller. The controller establishes a controlled plasma doping environment in the plasma doping chamber in a first mode, typically a plasma doping mode, and establishes a gas connection between the plasma doping chamber and the outer chamber in a second mode, typically a vacuum pumping and wafer exchange mode. A plasma containing ions of the process gas is produced in the plasma doping chamber between the anode and the platen. The plasma has a plasma sheath in the vicinity of the workpiece. The pulses applied to the anode accelerate the ions across the plasma sheath toward the platen for implantation into the workpiece.  
           [0010]    In one embodiment, the plasma is produced by the pulses applied to the anode. In some embodiments, the plasma is pulsed. In other embodiments, the plasma is continuous.  
           [0011]    In some embodiments, the plasma doping apparatus further comprises a secondary vacuum pump connected to the plasma doping chamber for pumping the plasma doping chamber in the first mode. The primary vacuum pump may be used for vacuum pumping of the plasma doping chamber in the second mode, and the secondary vacuum pump may be used to maintain a desired pressure of the process gas in the first mode. As a result, the secondary vacuum pump can have a relatively small capacity and can be connected to the plasma doping chamber through a throttle pumping port.  
           [0012]    In other embodiments, the plasma doping apparatus further comprises a controlled conductance aperture between the plasma doping chamber and the outer chamber. The plasma doping chamber is pumped through the controlled conductance aperture in the first mode.  
           [0013]    The platen may be movable between a processing position sealed into the plasma doping chamber and a retracted position removed from the plasma doping chamber. The control device may comprise means for moving the platen between the processing position in the first mode and the retracted position in the second mode.  
           [0014]    The plasma doping apparatus may further comprise a hollow electrode surrounding a space between the platen and the anode. In one embodiment, the hollow electrode is electrically connected to the anode. In another embodiment, the apparatus further comprises a hollow electrode pulse source electrically connected to the hollow electrode. In this embodiment, the plasma is produced by the pulses applied to the anode and the pulses applied to the hollow electrode.  
           [0015]    According to another aspect of the invention, a plasma doping method is provided. The plasma doping method comprises the steps of providing a plasma doping chamber containing a platen coupled to a reference potential and an anode spaced from the platen, supporting the workpiece on the platen, supplying a process gas to the plasma doping chamber, enclosing the plasma doping chamber within a vacuum vessel that defines an outer chamber, vacuum pumping the vacuum vessel with a primary vacuum pump, applying pulses to the anode, and controlling the apparatus to establish a controlled plasma doping environment in the plasma doping chamber in a first mode and to establish a gas connection between the plasma doping chamber and the outer chamber in a second mode. A plasma containing ions of the process gas is produced in the plasma doping chamber between the anode and the platen. The plasma has a plasma sheath in the vicinity of the workpiece. The pulses applied to the anode accelerate the ions across the plasma sheath toward the platen for implantation into the workpiece. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0017]    [0017]FIG. 1A is a top schematic view of a beamline ion implanter suitable for implementation of the present invention;  
         [0018]    [0018]FIG. 1B is a top schematic view of the beamline ion implanter of FIG. 1A, showing beamline components;  
         [0019]    [0019]FIG. 2 is a schematic cross-sectional side view of a process system in accordance with an embodiment of the invention, shown in the beamline ion implant mode;  
         [0020]    [0020]FIG. 3 is a schematic cross-sectional side view of the process system of FIG. 2, shown in the plasma doping mode;  
         [0021]    [0021]FIG. 4 is a schematic block diagram of the process system of FIGS. 2 and 3;  
         [0022]    [0022]FIG. 5 is a schematic block diagram of a first embodiment of the plasma doping module, shown with the platen sealed into the plasma doping chamber;  
         [0023]    [0023]FIG. 6 is a schematic block diagram of the first embodiment of the plasma doping module, shown with the platen removed from the plasma doping chamber; and  
         [0024]    [0024]FIG. 7 is a schematic block diagram of a second embodiment of the plasma doping module, shown with the platen sealed into the plasma doping chamber. 
     
    
     DETAILED DESCRIPTION  
       [0025]    A block diagram of an embodiment of a beamline ion implanter suitable for implementation of the invention is shown in FIGS. 1A and 1B. An ion source  10  generates ions and supplies an ion beam  12 . As known in the art, ion source  10  may include an ion chamber and a gas box containing a gas to be ionized. The gas is supplied to the ion chamber where it is ionized. The ions thus formed are extracted from the ion chamber to form ion beam  12 . Ion beam  12  has an elongated cross-section and is ribbon-shaped, with a long dimension of the beam cross-section preferably having a horizontal orientation. A power supply  14  is connected to an extraction electrode of ion source  10  and provides a voltage that may be adjustable, for example, from about 0.2 to 80 keV. Thus, ions from ion source  10  are accelerated to energies of about 0.2 to 80 keV by the voltage from power supply  14 . The construction and operation of ion sources are well-known to those skilled in the art.  
         [0026]    Ion beam  12  passes through a suppression electrode  20  and a ground electrode  22  to a mass analyzer  30 . The mass analyzer  30  includes a resolving magnet  32  and a masking electrode  34  having a resolving aperture  36 . Analyzing magnet  32  deflects ions in ion beam  12  such that ions of a desired ion species pass through resolving aperture  36 , and undesired ion species do not pass through resolving aperture  36  but are blocked by the masking electrode  34 . In a preferred embodiment, resolving magnet  32  deflects ions of a desired species by 90°.  
         [0027]    Ions of the desired species pass through resolving aperture  36  to a first deceleration stage  50  positioned downstream of mass analyzer  30 . Deceleration stage  50  may include an upstream electrode  52 , a suppression electrode  54  and a downstream electrode  56 . Ions in the ion beam are decelerated by deceleration stage  50  and then pass through an angle corrector magnet  60 . Angle corrector magnet  60  deflects the ions and converts the ion beam from a diverging ion beam to a ribbon beam  62  having substantially parallel ion trajectories. In a preferred embodiment, angle corrector magnet  60  deflects ions of the desired species by 70°. The ion implanter may include a second deceleration stage  80  positioned downstream of angle corrector magnet  60 .  
         [0028]    An end station  70 , or process station, supports one or more semiconductor wafers, such as wafer  72 , in a process chamber  74  such that ions of the desired species are implanted into the semiconductor wafer. Process chamber  74  is enclosed by a vacuum vessel  75 . The end station  70  may include a cooled electrostatic platen  76  and a platen positioner  78  (FIG. 4) for mechanically scanning wafer  72  perpendicular to the long dimension of the ribbon ion beam  62 , so as to distribute the ions over the surface of wafer  72 .  
         [0029]    End station  70 , as shown in FIG. 1A, may include an automated wafer handler  82  for introducing wafers into the ion implanter and for removing wafers after implantation. Wafer handler  82  shown in FIG. 1A includes wafer robots  90  and  92 , a wafer orienter  94  and load locks  100  and  102 . One of the wafer robots removes a wafer from a cassette or other wafer carrier in one of the load locks and transfers the wafer to platen  76 . The wafer may be oriented at wafer orienter  94 . Following processing, the wafer is removed from platen  76  and is returned to its cassette or other wafer carrier by one of the wafer robots.  
         [0030]    End station  70  may also include a dose measuring system, a plasma flood gun or an electron flood gun, and other known components. It will be understood that the entire path traversed by the ion beam is evacuated during ion implantation.  
         [0031]    In accordance with an aspect of the invention, a plasma doping module is combined with a beamline ion implant module to form an integrated processing system. The integrated processing system may be used to process wafers by beamline ion implantation, by plasma doping, or both, depending on the required implant recipe for the wafers. Wafers in a single process chamber may have access to a beamline ion implant module and to a plasma doping module. The integrated processing system may include any beamline ion implant module and any plasma doping module. A variety of different beamline ion implanter architectures are known to those skilled in the art. Various plasma doping architectures are described below.  
         [0032]    The beamline ion implant module may include all or part of a beamline ion implanter. In one embodiment, described below, a plasma doping module is incorporated into the end station of a beamline ion implanter. In another embodiment, the end station of a beamline ion implanter is replaced with a process chamber. The process chamber is connected to a beamline ion implant module and contains or is connected to a plasma doping module.  
         [0033]    In an embodiment shown in FIGS.  2 - 4 , a plasma doping module  110  is incorporated into process chamber  74 , with one or more components of plasma doping module  110  located within vacuum vessel  75  and one or more components of plasma doping module  110  located outside vacuum vessel  75 , as shown in FIG. 4. Plasma doping module  110  may include a plasma doping chamber  120 , a process gas source  124 , a vacuum pump  126 , a chamber positioner  128 , an anode positioner  130  connected to an anode located in plasma processing chamber  120 , and a pulse source  132  connected between platen  76  and the anode in plasma doping chamber  120 . Process gas source  124  and vacuum pump  126  are connected by gas conduits to plasma doping chamber  120 , and chamber positioner  128  is mechanically connected to plasma doping chamber  120 . Additional embodiments of plasma doping module  110  are described below.  
         [0034]    A beamline ion implant module  140  supplies ribbon ion beam  62  to process chamber  74 . Referring to FIGS. 1A and 1B, the components of beamline ion implant module  140  may include ion source  10 , mass analyzer  30 , deceleration stage  50 , angle corrector magnet  60  and second deceleration stage  80 . The beamline ion implant module  140  may employ any beamline ion implanter architecture.  
         [0035]    Additional components of the integrated processing system include vacuum vessel  75 , platen  76 , platen positioner  78  and wafer handler  82 . In a preferred embodiment, platen  76  may be an electrostatic wafer clamp as described for example in U.S. Pat. No. 5,452,177 issued Sep. 19, 1995 to Frutiger. A vacuum pump  142  controls the pressure within process chamber  74 . In the embodiment of FIGS. 2 and 3, vacuum pump  142  comprises a cryogenic pump. Additional vacuum pumps, such as a turbomolecular pump  144 , may be used for increased vacuum pumping capability. A Faraday cup  148  may be positioned in alignment with ribbon ion beam  62  for dose and uniformity measurements. A system controller  150  controls the elements of the integrated processing system. System controller may comprise a programmed general purpose computer, including for example a microprocessor, memory, interfaces to the components of the integrated processing system and peripheral devices, such as a keyboard and a video display terminal.  
         [0036]    Platen  76  holding wafer  72  may be positioned to intercept ribbon ion beam  62  in a beamline implant mode, as shown in FIG. 2, or may be positioned in plasma doping chamber  120  in a plasma doping mode, as shown in FIG. 3. The system thus constitutes an integrated processing system that is capable of beamline ion implantation and plasma doping. The system controller  150  controls the operating mode in response to inputs that define the parameters of each implant.  
         [0037]    Referring to FIGS. 2 and 3, plasma doping chamber  120  defines an enclosed volume  160  in the plasma doping mode. In the plasma doping mode shown in FIG. 3, platen  76  is positioned in an opening  158  in plasma doping chamber  120 , and a platen halo  162  seals platen  76  into plasma doping chamber  120 . Platen  76  thus positions wafer  72  within plasma doping chamber  120 . The platen  76  supports wafer  72  and provides an electrical connection to wafer  72 . An anode  170  is positioned within plasma doping chamber  120  in spaced relation to platen  76 , which functions as a cathode. Anode  170  may be movable by anode positioner  130  (FIG. 4) in a direction perpendicular to the surface of platen  76 . The region between platen  76  and anode  170  may be surrounded by a hollow electrode  172  as described in U.S. Pat. No. 6,182,604 issued Feb. 6, 2001 to Goeckner et al., which is hereby incorporated by reference. A shield ring  174  containing a Faraday beam sensor may surround platen  76  as described in U.S. Pat. No. 6,020,592 issued Feb. 1, 2000 to Liebert et al., which is hereby incorporated by reference. The enclosed volume  160  within plasma doping chamber  120  may be connected by a coaxial gas line  180  to process gas source  124  (FIG. 4). In addition, enclosed volume  160  may be connected through a throttled pumping port  182  to vacuum pump  126  (FIG. 4). Plasma doping chamber  120  is preferably movable by chamber positioner  128  (FIG. 4) between a plasma doping position shown in FIG. 3 and a retracted position shown in FIG. 2. The plasma doping chamber  120  moves upwardly from the plasma doping position to the retracted position.  
         [0038]    The platen positioner  78  (FIG. 4) positions platen  76  in accordance with the operating mode of the processing system. In the beamline implant mode shown in FIG. 2, platen  76  and wafer  72  are oriented vertically in the path of ribbon ion beam  62 , and platen  76  is mechanically scanned upwardly and downwardly by platen positioner  78  to distribute ribbon ion beam  62  over the surface of wafer  72 . Platen positioner  78  may include a tilter  190  for tilting wafer  72  at a desired angle with respect to ribbon ion beam  62 . Preferably, platen  76  is moved below ribbon ion beam  62  during part of the mechanical scan to permit Faraday cup  148  to monitor ion beam current.  
         [0039]    In the plasma doping mode, the platen  76  and wafer  72  may be oriented horizontally. Platen  76  and wafer  72  are moved upwardly into opening  158  in plasma process chamber  120 , and platen halo  162  is sealed to plasma process chamber  120 . Thus, platen  76  and wafer  72  are sealed into plasma doping chamber  120  as shown in FIG. 3. During plasma doping, platen  76  and wafer  72  may remain stationary.  
         [0040]    In a wafer exchange mode, platen  76  and wafer  72  are oriented horizontally and are lowered below the path of ribbon ion beam  62 . Wafer  72  is removed from platen  72  by one of the wafer robots  90 ,  92  (FIG. 1A) and a new wafer is placed on platen  76  for processing. Wafer handling techniques are known to those skilled in the art and are not discussed further.  
         [0041]    In operation, the system controller  150  may receive an implant recipe that specifies the parameters for doping a batch of wafers. The implant recipe may, for example, specify a dopant species, an energy and a dose to be applied to the wafers. The system controller  150  may select an operating mode based on the implant recipe. For example, implant energies greater than 2 keV may utilize the beamline implant mode and energies less than 2 keV may utilize the plasma doping mode.  
         [0042]    When the beamline implant mode is selected by system controller  150 , a wafer of the batch is loaded onto platen  76  by wafer handler  82  and platen  76  is rotated to the vertical position as shown in FIG. 2. The beamline ion implant module  140  is tuned to provide the desired implant parameters and to generate ribbon ion beam  62 . The platen positioner  78  mechanically scans platen  76  and wafer  72  vertically through ribbon ion beam  62 , typically multiple times, until a desired dose and dose uniformity are achieved. Dose and dose uniformity may be monitored by Faraday cup  148 . During the beamline implant mode, the plasma doping chamber  120  remains in the retracted position shown in FIG. 2 in order to provide clearance for mechanical scanning, and the components of plasma doping module  122  are inactivated. Following completion of processing, the wafer  72  may be removed from the process chamber  74  by wafer handler  82 .  
         [0043]    When the plasma doping mode is selected by system controller  150 , the beamline ion implant module  140  is inactivated, and plasma doping chamber  120  is lowered by chamber positioner  128  to the plasma doping position shown in FIG. 3. After a wafer is loaded onto platen  76  by wafer handler  82 , the platen  76  and wafer  72  are raised into the opening  150  in plasma doping chamber  120  and are sealed into plasma doping chamber  120 . The process gas source  124  and the vacuum pump  126  are activated to provide a process gas at the desired pressure within plasma doping chamber  120 . The pulse source  132  is activated, causing formation of a plasma between platen  76  and anode  170  and acceleration of ions toward wafer  72 . For very low energy implants, hollow electrode  172  may be utilized as described below. The applied dose may be monitored by the Faraday beam sensor in shield ring  174 . When the desired dose is achieved, the pulse source  132  and the process gas source  124  are deactivated, and vacuum pump  126  pumps the plasma doping chamber  120  to a desired vacuum level. The platen  76  and wafer  72  are then lowered from plasma doping chamber  120 , and wafer  72  may be removed by wafer handler  82 . The vacuum pump  142  may pump residual gas that escapes into process chamber  74  when platen  76  is lowered from plasma doping chamber  120 . If desired, wafer  72  may be processed by plasma doping and beamline ion implantation without removing wafer  72  from process chamber  74 .  
         [0044]    It will be understood that the plasma doping chamber  120  defines a process environment in enclosed volume  160  that may be very different from the process environment within process chamber  74 . In particular, process chamber  74  is preferably maintained at high vacuum, for example 20 microtorr, during beamline ion implantation. The pressure within plasma doping chamber  120  during operation in the plasma doping mode may be in a range of about 1 millitorr to about 500 millitorr. A process gas, such as BF 3 , N 2 , Ar, PH 3 , AsH 3  or B 2 H 6 , for example, may be used.  
         [0045]    In the embodiment of FIGS. 2 and 3, plasma doping chamber  120  is located within process chamber  74  and is movable between a plasma doping position and a retracted position. In other embodiments, plasma doping chamber  120  may be fixed in position if the platen positioner  78  provides a sufficient range of platen travel to permit mechanical scanning and access to plasma doping chamber  120 . In addition, plasma doping chamber  120  may be located partially or entirely outside process chamber  74 , such that the plasma doping chamber  120  can be accessed from process chamber  74 . For example, plasma processing chamber  120  may be accessed from process chamber  74  through a gate valve.  
         [0046]    Schematic block diagrams of a first embodiment of plasma doping module  110  are shown in FIGS. 5 and 6. A schematic block diagram of a second embodiment of plasma doping module  110  is shown in FIG. 7. Like elements in FIGS.  1 - 7  have the same reference numerals. In FIGS.  5 - 7 , vacuum vessel  75  and vacuum pump  142  are shown because these elements are involved in the operation of the plasma doping module. The other components of the integrated processing system are omitted in FIGS.  5 - 7 .  
         [0047]    In FIG. 5, platen  76  is sealed into plasma doping chamber  120  in the plasma doping mode. In the configuration of FIG. 5, plasma doping chamber  120  is isolated from process chamber  74 , and different environments may be maintained in plasma doping chamber  120  and processing chamber  74 . In FIG. 6, platen  76  is lowered from opening  158  in plasma doping chamber  120 . Thus, plasma doping chamber  120  and process chamber  74  have a common environment. This configuration is applicable to the beamline implant mode and the wafer exchange mode.  
         [0048]    As shown in FIGS. 5 and 6, plasma doping chamber  120  is located within vacuum vessel  75 . Plasma doping chamber  120  is connected to vacuum pump  126 , and vacuum vessel  75  is connected to vacuum pump  142 . Vacuum pump  142  pumps both process chamber  74  and plasma doping chamber  120  when platen  76  is lowered from opening  158  in plasma doping chamber  120 , as shown in FIG. 6. Thus, plasma doping chamber  120  has a relatively low pressure at the time when platen  76  is sealed into plasma doping chamber  120 . After plasma doping chamber  120  is sealed, plasma doping chamber  120  is pumped by vacuum pump  126 . This arrangement permits vacuum pump  126  to have a relatively small pumping capacity, while vacuum pump  142  has a larger pumping capacity sufficient to pump vacuum vessel  75 . Thus, vacuum pump  142  may be considered as a primary vacuum pump and vacuum pump  126  may be considered as a secondary vacuum pump in the embodiment of FIGS. 5 and 6.  
         [0049]    Vacuum pump  142  evacuates plasma doping chamber  120  to a desired pressure level with platen  76  in the lowered position shown in FIG. 6. Platen  76  is then sealed into plasma doping chamber  120  as shown in FIG. 5. Process gas source  124  introduces a process gas to plasma doping chamber  120 , and vacuum pump  126  provides sufficient pumping to maintain a desired pressure of the process gas within plasma doping chamber  120 . Because vacuum pump  126  is not required to pump plasma doping chamber  120  from atmospheric pressure to the process pressure, the port connecting plasma doping chamber  120  to vacuum pump  126  may be throttled, and vacuum pump  126  may have a relatively small capacity. After processing is complete, process gas source  124  is turned off and vacuum pump  126  pumps the remaining process gas from plasma doping chamber  120 . Then, platen  76  is lowered, and vacuum pump  142  provides further vacuum pumping of plasma doping chamber  120 .  
         [0050]    As further shown in FIGS. 5 and 6, platen  76  and the walls of plasma doping chamber  120  may be connected to a reference potential, such as ground, and pulse source  132  may provide a series of pulses to anode  170 . Anode  170  is electrically isolated from plasma doping chamber  120  by an insulator  176  and is electrically isolated from vacuum vessel  75  by an insulator  178 . Hollow electrode  172  is connected by a switch  184  to pulse source  132  or to a hollow electrode pulse source  190 , as described below.  
         [0051]    In the typical case where positive ions are to be implanted into wafer  72 , positive pulses are applied to anode  170 . In the case where the voltage corresponding to the required implant energy is sufficient to initiate a plasma discharge between anode  170  and wafer  72 , pulse source  132  may be used to initiate a plasma discharge and to accelerate ions from the plasma into wafer  72 . The positive pulses accelerate positive ions across the plasma sheath and into wafer  72 . In the case where negative ions are to be implanted into wafer  72 , pulse source  132  applies a negative pulse to anode  170 . Where pulse source  132  is used to initiate a plasma discharge between anode  170  and wafer  72 , hollow electrode  172  is connected to pulse source  132  by placing switch  184  in position  1  shown in FIGS. 5 and 6. In this configuration, the plasma is substantially surrounded, except at wafer  72 , by positively biased anode  170  and hollow electrode  172 , and positively charged ions in the plasma are accelerated to wafer  72 .  
         [0052]    In the case where a very low implant energy is required and the corresponding amplitude of the pulses supplied by pulse source  132  is not sufficient to initiate a plasma discharge between anode  170  and wafer  72 , switch  184  is placed in position  2 , and hollow electrode  172  is connected to hollow electrode pulse source  190 . In the embodiment of FIGS. 5 and 6, a negative pulse is applied to hollow electrode  172  when positive ions are to be implanted into wafer  72 . The negative pulse applied to hollow electrode  172  combined with the positive pulse applied to anode  170  is sufficient to initiate a plasma discharge between anode  170  and wafer  72 , and a relatively small amplitude pulse applied to anode  170  achieves very low implant energy. For example, where singly-charged positive ions having energies of 500 electron volts are to be implanted into wafer  72 , switch  184  is placed in position  2 , pulse source  132  is programmed to generate positive 500 volt pulses, and hollow electrode pulse source  190  is programmed to generate negative 1000 volt pulses. The pulse sources  132  and  190  are synchronized to generate pulses that overlap in time. This results in 1500 volt pulses being applied between anode  170  and hollow electrode  172 , which is sufficient to initiate a plasma discharge. The positive ions in the plasma discharge are accelerated to 500 electron volts by the pulses applied between anode  170  and wafer  72 .  
         [0053]    The plasma doping module shown in FIGS. 5 and 6, wherein wafer  72  and plasma  76  are grounded, has several advantages. Because the wafer is grounded, biasing and dose measurement are simplified. The wafer  76  is substantially surrounded by anode  170  and hollow electrode  172 , and plasma doping chamber  120  is connected through a throttled pumping port to vacuum pump  126 . As a result, contamination of wafer  76  caused by sputtering of chamber walls and vacuum pumping components is limited. In addition, the surface area for collecting ions is limited, thereby reducing the load placed on pulse sources  132  and  190 . To further reduce contamination caused by sputtering, hollow electrode  172  and other exposed elements may be coated with a non-contaminating material, such as silicon in the case of a silicon wafer. The throttled pumping port reduces the tendency for ions in the plasma to enter the pumping port and to be deposited on vacuum pumping components.  
         [0054]    A schematic block diagram of the second embodiment of plasma doping module  110  is shown in FIG. 7. The embodiment of FIG. 7 differs from the embodiment of FIGS. 5 and 6 with respect to grounding and the electrical connections to pulse source  132  and hollow electrode pulse source  190 . In particular, anode  170  is connected to a reference potential, such as ground, and the cathode (platen  76 ) is pulsed negative for implantation of positive ions. Hollow electrode  170  is connected by switch  184  to platen  76  or to hollow electrode pulse source  190 , depending on the required implant energy. In the embodiment of FIG. 7, platen halo  162  is an electrically insulating material to permit electrical isolation between platen  76  and plasma doping chamber  120 .  
         [0055]    The embodiment of FIG. 7 also differs from the embodiment of FIGS. 5 and 6 with respect to the vacuum pumping arrangement. In particular, plasma doping chamber  120  is provided with a controlled conductance aperture  194 , and vacuum pump  126  (FIGS. 5 and 6) is eliminated. The controlled conductance aperture  194  provides a controlled gas flow between the interior volume of plasma doping chamber  120  and process chamber  74 . Thus, plasma doping chamber  120  is vacuum pumped by a controlled gas flow through aperture  194  to vacuum pump  142  when platen  76  is sealed into plasma doping chamber  120 . The controlled conductance aperture  194  may include one or more openings having known gas flow characteristics. In one embodiment, the openings of aperture  194  avoid a direct line of sight between the interior volume of plasma doping chamber  120  and process chamber  74 , to permit gas flow while inhibiting passage of the plasma. For example, aperture  194  may be implemented as a gas conduit having a bend. In the other embodiments, aperture  194  may be fixed, may be opened or closed, or may have an adjustable gas conductance. It will be understood that the vacuum pumping arrangement of FIG. 7 may be used in the embodiment of FIGS. 5 and 6. Further, the vacuum pumping arrangement of FIGS. 5 and 6 may be used in the embodiment of FIG. 7  
         [0056]    The plasma doping systems shown in FIGS.  5 - 7  and described above may be utilized in the integrated processing system shown in FIGS.  2 - 4  and described above. In addition, the embodiments of FIGS.  5 - 7  may be utilized separately or in any processing system having an outer vacuum vessel to provide vacuum pumping of the plasma doping chamber as described above. The outer vacuum vessel may or may not include another processing module.  
         [0057]    Other plasma doping architectures may be utilized within the scope of the invention. For example, the plasma may be pulsed or continuous. The plasma may be generated by a DC voltage, an RF voltage or a microwave voltage, each of which may be pulsed or continuous. Different process gas pressures may be utilized.  
         [0058]    It should be understood that various changes and modifications of the embodiments shown in the drawings described in the specification may be made within the spirit and scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and not in a limiting sense. The invention is limited only as defined in the following claims and the equivalents thereto.