Abstract:
A plasma ion implantation system includes a process chamber, a source for generating a plasma in the process chamber, a platen for holding a substrate in the process chamber, an implant pulse source configured to generate implant pulses for accelerating ions from the plasma into the substrate, and an axial electrostatic confinement structure configured to confine electrons in a direction generally orthogonal to a surface of the platen. The confinement structure may include an auxiliary electrode spaced from the platen and a bias source configured to bias the auxiliary electrode at a negative potential relative to the plasma.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to plasma doping systems used for ion implantation of workpieces, such as semiconductor wafers, and, more particularly, to methods and apparatus for plasma ion implantation wherein axial electron confinement is utilized to increase plasma density.  
       BACKGROUND OF THE INVENTION  
       [0002]     Plasma doping systems have been studied for forming shallow junctions in semiconductor wafers and for other applications requiring high current, relatively low energy ions. In a plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and is located in a plasma doping chamber. An ionizable dopant gas is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma containing ions of the dopant gas. The plasma has a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to be accelerated across the plasma sheath and to be implanted into the wafer.  
         [0003]     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.  
         [0004]     In the plasma doping systems described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, a continuous plasma is produced, for example, by inductively-coupled RF power from an antenna located internal or external to the plasma doping chamber. The antenna is connected to an RF power supply. At intervals, voltage pulses are applied between the platen and the anode, causing ions in the plasma to be accelerated toward the wafer.  
         [0005]     In general, plasma doping systems deliver higher current at low energy than beamline ion implantation systems. Nonetheless, increased ion currents are desirable in some applications in order to reduce implant times and to thereby improve throughput. It is known that ion current in a plasma doping system is a function of plasma density. It is also known that plasma density can be increased by increasing the dopant gas pressure in the plasma doping chamber. However, increased gas pressure increases the risk of arcing within the plasma doping chamber.  
         [0006]     U.S. Pat. No. 5,354,381, issued Oct. 11, 1994 to Sheng and U.S. Pat. No. 5,572,038, issued Nov. 5, 1996 to Sheng et al., disclose a plasma immersion ion implantation system which includes an electrode that provides a flow of electrons to the wafer. U.S. Pat. No. 5,911,832, issued Jun. 15, 1999 to Denholm et al., discloses plasma immersion implantation with a pulsed anode. U.S. Pat. No. 6,335,536, issued Jan. 1, 2002 to Goeckner et al., discloses a plasma doping system wherein an ignition voltage pulse is supplied to an ionizable gas and an implantation voltage pulse is applied to the target. The aforementioned U.S. Pat. No. 6,182,604 discloses a plasma doping system which utilizes a hollow cathode. The hollow cathode can be utilized to increase ion current and provides highly satisfactory results. However, some applications require even higher plasma density and/or lower gas pressure than is attainable with the hollow cathode configuration.  
         [0007]     Accordingly, there is a need for improved plasma ion implantation systems and methods.  
       SUMMARY OF THE INVENTION  
       [0008]     According to a first aspect of the invention, a plasma ion implantation system is provided. The plasma ion implantation system comprises a process chamber, a source for generating a plasma in the process chamber, a platen for holding a substrate in the process chamber, an implant pulse source configured to generate implant pulses for accelerating ions from the plasma into the substrate, and an axial electrostatic confinement structure configured to confine electrons in a direction generally orthogonal to a surface of the platen.  
         [0009]     According to a second aspect of the invention, a plasma ion implantation system comprises a process chamber; a source for generating a plasma in the process chamber; a platen for holding a substrate in the process chamber; an implant pulse source configured to generate implant pulses for accelerating ions from the plasma into the substrate; an auxiliary electrode spaced from the platen; and a bias source configured to bias the auxiliary electrode at a potential to confine electrons in a direction generally orthogonal to a surface of the platen.  
         [0010]     According to a third aspect of the invention, a method is provided for plasma ion implantation in a process chamber. The method comprises generating a plasma in the process chamber; holding a substrate in the process chamber; accelerating ions from the plasma into the substrate; and confining electrons in a direction generally orthogonal to a surface of the platen.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0012]      FIG. 1  is a simplified schematic block diagram of a prior art plasma doping system;  
         [0013]      FIG. 2  is a simplified schematic block diagram of a plasma doping system in accordance with a first embodiment of the invention;  
         [0014]      FIG. 3  is a simplified schematic block diagram of a plasma doping system in accordance with a second embodiment of the invention;  
         [0015]      FIG. 4  is a simplified schematic block diagram of a plasma doping system in accordance with a third embodiment of the invention; and  
         [0016]      FIG. 5  is a simplified schematic block diagram of a plasma doping system in accordance with a fourth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]     An example of a prior art plasma ion implantation system is shown schematically in  FIG. 1 . A process chamber  10  defines an enclosed volume  12 . A platen  14  positioned within chamber  10  provides a surface for holding a substrate, such as a semiconductor wafer  20 . The wafer  20  may, for example, be clamped at its periphery to a flat surface of platen  14  or may be electrostatically clamped. In one configuration, the platen has an electrically conductive surface for supporting wafer  20 . In another configuration, the platen includes conductive pins (not shown) for connection to wafer  20 . In addition, platen  14  may be equipped with a heating/cooling system to control wafer/substrate temperature.  
         [0018]     An anode  24  is positioned within chamber  10  in spaced relation to platen  14 . Anode  24  may be movable in a direction, indicated by arrow  26 , perpendicular to platen  14 . The anode is typically connected to electrically conductive walls of chamber  10 , both of which may be connected to ground. In further configurations, both anode  24  and platen  14  may be biased with respect to ground.  
         [0019]     The wafer  20  (via platen  14 ) and the anode  24  are connected to a high voltage pulse source  30 , so that wafer  20  functions as a cathode. The pulse source  30  typically provides pulses in a range of about 20 to 20,000 volts in amplitude, about 1 to 200 microseconds in duration and a pulse repetition rate of about 100 Hz to 20 kHz. It will be understood that these pulse parameter values are given by way of example only and that other values may be utilized.  
         [0020]     The enclosed volume  12  of chamber  10  is coupled through a controllable valve  32  to a vacuum pump  34 . A process gas source  36  is coupled through a mass flow controller  38  to chamber  10 . A pressure sensor  44  located within chamber  10  provides a signal indicative of chamber pressure to a controller  46 . The controller  46  compares the sensed chamber pressure with a desired pressure input and provides a control signal to valve  32  or mass flow controller  38 . The control signal controls valve  32  or mass flow controller  38  so as to minimize the difference between the chamber pressure and the desired pressure. Vacuum pump  34 , valve  32 , mass flow controller  38 , pressure sensor  44  and controller  46  constitute a closed loop pressure control system. The pressure is typically controlled in a range of about 1 millitorr to about 500 millitorr, but is not limited to this range. Gas source  36  supplies an ionizable gas containing a desired dopant for implantation into the workpiece. Examples of ionizable gases include BF 3 , N 2 , Ar, PH 3 , AsH 3 , AsF 5 , PF 3 , Xe and B 2 H 6 . Mass flow controller  38  regulates the rate at which gas is supplied to chamber  10 . The configuration shown in  FIG. 1  provides a continuous flow of process gas at a desired flow rate and constant pressure. The pressure and gas flow rate are preferably regulated to provide repeatable results. In another configuration, the gas flow may be regulated using a valve controlled by controller  46  while valve  32  is kept at a fixed position. Such an arrangement is referred to as upstream pressure control. Other configurations for regulating gas pressure may be utilized.  
         [0021]     The plasma ion implantation system may include a hollow cathode  54  connected to a hollow cathode pulse source  56 . The hollow cathode  54  may comprise a conductive hollow cylinder that surrounds the space between anode  24  and platen  14 . The hollow cathode may be utilized in applications which require very low ion energies. In particular, hollow cathode pulse source  56  provides a pulse voltage that is sufficient to form a plasma within chamber  12 , and pulse source  30  establishes a desired implant voltage. Additional details regarding the use of a hollow cathode are provided in the aforementioned U.S. Pat. No. 6,182,604, which is hereby incorporated by reference.  
         [0022]     One or more Faraday cups may be positioned adjacent to platen  14  for measuring the ion dose implanted into wafer  20 . In the system of  FIG. 1 , Faraday cups  50 ,  52 , etc. are equally spaced around the periphery of wafer  20 . Each Faraday cup comprises a conductive enclosure having an entrance  60  facing plasma  40 . Each Faraday cup is preferably positioned as close as is practical to wafer  20  and intercepts a sample of the positive ions accelerated from plasma  40  toward platen  14 . In another configuration, an annular Faraday cup is positioned around wafer  20  and platen  14 .  
         [0023]     The Faraday cups are electrically connected to a dose processor  70  or other dose monitoring circuit. Positive ions entering each Faraday cup through entrance  60  produce in the electrical circuit connected to the Faraday cup a current that is representative of ion current. The dose processor  70  may process the electrical current to determine ion dose.  
         [0024]     The plasma ion implantation system may include a guard ring  66  that surrounds platen  14 . The guard ring  66  may be biased to improve the uniformity of implanted ion distribution near the edge of wafer  20 . The Faraday cups  50 ,  52  may be positioned within guard ring  66  near the periphery of wafer  20  and platen  14 .  
         [0025]     The plasma ion implantation system may include additional components, depending on the configuration of the system. The system typically includes a process control system (not shown) which controls and monitors the components of the plasma ion implantation system to implement a desired implant process. Systems which utilize continuous or pulsed RF energy include an RF source coupled to an antenna or an induction coil. The system may include magnetic elements which provide magnetic fields that confine electrons and control plasma density and spatial distribution. The use of magnetic elements in plasma ion implantation systems is described, for example, in WO 03/049142, published 12 Jun. 2003, which is hereby incorporated by reference.  
         [0026]     In operation, wafer  20  is positioned on platen  14 . The pressure control system, mass flow controller  38  and gas source  36  produce the desired pressure and gas flow rate within chamber  10 . By way of example, the chamber  10  may operate with BF 3  gas at a pressure of 10 millitorr. The pulse source  30  applies a series of high voltage pulses to wafer  20 , causing formation of plasma  40  in a plasma discharge region  48  between wafer  20  and anode  24 . As known in the art, plasma  40  contains positive ions of the ionizable gas from gas source  36 . Plasma  40  includes a plasma sheath  42  in the vicinity, typically at the surface, of wafer  20 . The electric field that is present between anode  24  and platen  14  during the high voltage pulse accelerates positive ions from plasma  40  across plasma sheath  42  toward platen  14 . The accelerated ions are implanted into wafer  20  to form regions of impurity material. The pulse voltage is selected to implant the positive ions to a desired depth in wafer  20 . The number of pulses and the pulse duration are selected to provide a desired dose of impurity material in wafer  20 . The current per pulse is a function of pulse voltage, pulse width, pulse frequency, gas pressure and species and any variable position of the electrodes. For example, the cathode-to-anode spacing may be adjusted for different voltages.  
         [0027]     In the plasma doping system of  FIG. 1 , anode  24  is connected to ground and platen  14  is pulsed negative to implant ions into wafer  20 . In this configuration, plasma  40  is at ground potential, and electrons in plasma  40  may be incident on anode  24 . Such electrons are collected by anode  24  and do not contribute to further ionization of the dopant gas.  
         [0028]     Simplified schematic block diagrams of plasma ion implantation systems in accordance with embodiments of the invention are shown in  FIGS. 2-5 . The embodiments of  FIGS. 2-5  are described as modifications of the prior art system shown in  FIG. 1  and described above. In  FIGS. 2-5 , like elements have the same reference numerals. System components such as gas source  36 , mass flow controller  38 , valve  32 , vacuum pump  34 , controller  46 , pressure sensor  48 , Faraday cups  50 ,  52  and dose processor  70  have been omitted from  FIGS. 2-5  for simplicity of illustration. The embodiments of  FIGS. 2 and 3  do not include a hollow electrode or a hollow electrode pulse source. Other embodiments described below include a hollow electrode.  
         [0029]     A simplified schematic block diagram of a plasma ion implantation system in accordance with a first embodiment of the invention is shown in  FIG. 2 . As shown in  FIG. 2 , a process chamber  100  defines an enclosed volume  112 . A platen  114  positioned within chamber  100  provides a surface for holding a substrate, such as a semiconductor wafer  120 . Platen  114  is connected to a pulse source  130 , and process chamber  110  is connected to ground. Platen  114  functions as a cathode, and process chamber  110  functions as an anode. Pulse source  130  applies to platen  114  negative implant pulses, as described above in connection with pulse source  30 .  
         [0030]     An auxiliary electrode  122  is positioned within chamber  110  in spaced relation to platen  114 . Auxiliary electrode  122  may be movable in a direction perpendicular to platen  114 . In general, auxiliary electrode  122  may be parallel to and spaced from platen  114  and may have the same physical configuration as anode  24  shown in  FIG. 1  and described above. Auxiliary electrode  122  differs from anode  24  with respect to electrical biasing. As shown in  FIG. 2 , auxiliary electrode  122  is connected to a bias source  128  which applies a negative voltage to auxiliary electrode  122 . The bias voltage may be a DC voltage or a pulsed voltage. In either case, the bias voltage is present on auxiliary electrode  122  during at least a portion of the implant pulse supplied to platen  114  by pulse source  130 .  
         [0031]     In operation, the gas control system, as shown in  FIG. 1  and described above, establishes a desired pressure and gas flow rate within process chamber  110 . Pulse source  130  applies a series of pulses to platen  114 , causing formation of a plasma  140  in a plasma discharge region  148  between wafer  120  and auxiliary electrode  122 . Plasma  140  contains positive ions of the dopant gas and has a plasma sheath  142  in the vicinity, typically at the surface, of wafer  120 . An electric field between plasma  140  and platen  114  produced by the pulses from pulse source  130  accelerates positive ions from plasma  140  across plasma sheath  142  toward platen  114 . The accelerated ions are implanted into wafer  120  to form regions of impurity material. The pulse voltage is selected to implant the positive ions to a desired depth in wafer  120 .  
         [0032]     Plasma  140  also includes electrons which produce ionizing collisions. Each electron may undergo multiple ionizing collisions while it is present in plasma discharge region  148 . Electrons lost from plasma discharge region no longer produce ionizing collisions. Accordingly, it is desirable to confine electrons to the plasma discharge region  148  to thereby increase the number of ionizing collisions and thereby increase the density of plasma  140 . Auxiliary electrode  122  is negatively biased with respect to plasma  140  and causes electrons to be repelled toward plasma  140 , where the electrons undergo additional ionizing collisions. Accordingly, the density of plasma  140  is increased by the presence of auxiliary electrode  122 , which is biased so as to repel electrons toward plasma discharge region  148 . Platen  114  is also negatively biased during ion implantation and thereby repels electrons toward plasma discharge region  148 . The configuration of electron repelling electrodes  122  and  114  thus confines electrons within plasma discharge region  148  and increases the density of plasma  140  in comparison with prior art configurations. The configuration of electrodes  114  and  122  produces electron confinement along an axis  132  generally orthogonal to the wafer support surface of platen  114 , as a result of the electric field normal to these electrodes.  
         [0033]     As described above, the confinement of electrons between electrodes  122  and  114  increases plasma density in plasma discharge region  148 . This result can be utilized to increase the ion current delivered to wafer  120 , to decrease the dopant gas pressure in process chamber  110  while maintaining a desired ion current or a combination of increased ion current and reduced gas pressure. The parameters of a particular implant depend on the bias voltage supplied to auxiliary electrode  122  and the dopant gas pressure in process chamber  110 . By utilizing auxiliary electrode  122  and bias source  128 , the dopant gas pressure in process chamber  112  can be decreased to reduce the risk of arcing, while maintaining a desired ion current.  
         [0034]     A simplified schematic block diagram of a plasma ion implantation system in accordance with a second embodiment of the invention is shown in  FIG. 3 . The embodiment of  FIG. 3  differs from the embodiment of  FIG. 2  in that the pulse source  130  is connected to both platen  114  and auxiliary electrode  122 . Thus, the implant pulses supplied to platen  114  are also applied to auxiliary electrode  122 . Because negative voltages are applied to electrodes  114  and  122 , electrons are axially confined along axis  132  orthogonal to the wafer support surface of platen  114 . The embodiment of  FIG. 3  has the advantage that a single pulse source can be utilized to energize platen  114  and auxiliary electrode  122 . However, this configuration has the disadvantage that independent control of auxiliary electrode  122  is lacking.  
         [0035]     A simplified schematic block diagram of a plasma ion implantation system in accordance with a third embodiment of the invention is shown in  FIG. 4 . In the embodiment of  FIG. 4 , pulse source  130  is connected to platen  114  and an auxiliary pulse source  150  is connected to auxiliary electrode  122 . Pulse source  130  supplies to platen  114  negative implant pulses having a pulse amplitude, a pulse width and a pulse repetition rate selected to perform a desired implantation of dopant ions into wafer  120 . Auxiliary pulse source  150  supplies to auxiliary electrode  122  negative auxiliary pulses having an amplitude selected to provide a desired density of plasma  140 . The pulse width and pulse repetition rate may match the pulse width and pulse repetition rate of the pulses supplied by pulse source  130 . In other embodiments, described below, the pulse widths may be different. Pulse sources  130  and  150  may be controlled by a synchronization device  160  which causes the pulses supplied to platen  114  and auxiliary electrode  122  to be synchronized in time. In other embodiments, pulse source  130  may provide a trigger pulse to pulse source  150 , or vice versa.  
         [0036]     Examples of the implant pulses supplied by pulse source  130  and the auxiliary pulses supplied by pulse source  150  are shown in  FIG. 4A . As shown, the implant pulses have a negative amplitude −V I , which establishes the energy of ions implanted into wafer  120 . The auxiliary pulses have a negative amplitude −V A , which is selected to establish a desired plasma density. In the example of  FIG. 4A , the implant pulses and the auxiliary pulses have the same pulse widths and the same pulse repetition rates. In other embodiments, the implant pulses and the auxiliary pulses may have different pulse widths but should overlap in time at least partially in order to provide the desired increase in plasma density.  
         [0037]     The embodiment of  FIG. 4  includes a hollow electrode  154 . Hollow electrode  154  may be a conductive hollow cylinder that surrounds the space between auxiliary electrode  122  and platen  114 . In the embodiment of  FIG. 4 , hollow electrode  154  is electrically connected to ground and thus serves as the anode for the plasma discharge.  
         [0038]     A simplified schematic block diagram of a plasma ion implantation system in accordance with a fourth embodiment of the invention is shown in  FIG. 5 . The embodiment of  FIG. 5  differs from the embodiment of  FIG. 4  in that hollow electrode  154  is electrically connected to auxiliary pulse source  150 . Thus, auxiliary electrode  122 , platen  114  and hollow electrode  154  all provide confinement of electrons in plasma discharge region  148 . In the embodiment of  FIG. 5 , process chamber  110  serves as the anode for the plasma discharge.  
         [0039]     The features shown in  FIGS. 2-5  and described above can be used in any desired combination. For example, a hollow electrode may be utilized in the embodiments of  FIGS. 2 and 3 . Furthermore, the dual pulse source configuration of  FIGS. 4 and 5  can be utilized without a hollow electrode. When a hollow electrode is used in any of the embodiments, it may be connected to ground or may be connected to auxiliary pulse source  150 . The present invention can be utilized in any configuration of a plasma ion implantation system.  
         [0040]     Having described several embodiments and an example of the invention in detail, various modifications and improvements will readily occur to those skilled in the art. Such modifications and improvements are intended to be within the spirit and the scope of the invention. Furthermore, those skilled in the art would readily appreciate that all parameters listed herein are meant to be exemplary and that actual parameters will depend upon the specific application for which the system of the present invention is used. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined by the following claims and their equivalents.