Abstract:
Methods and apparatus for plasma implantation of a workpiece, such as a semiconductor wafer, are provided. A method includes introducing into a plasma doping chamber a dopant gas selected from the group consisting of PF 3 , AsF 3 , AsF 5  and mixtures thereof, forming in the plasma doping chamber a plasma containing ions of the dopant gas, the plasma having a plasma sheath at or near a surface of the workpiece, and accelerating the dopant gas ions across the plasma sheath toward the workpiece, wherein the dopant gas ions are implanted into the workpiece. The selected dopant gas limits deposition of neutral particles on the workpiece.

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 limiting deposition of surface layers on semiconductor wafers during plasma implantation.  
       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 doping 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. 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.  
         [0003]     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.  
         [0004]     Dopant gas species used for plasma implantation may decompose or dissociate during the implant process into atomic or molecular fragments which may be deposited on the surface of the wafer. Atomic or molecular fragments that result from dissociation of dopant gas molecules are referred to herein as “neutral particles”. Examples of dopant gas species which dissociate during the implant process include AsH 3 , PH 3  and B 2 H 6 . For example, arsine gas (AsH 3 ) may dissociate into As, AsH and AsH 2 , which may be deposited on the surface of the wafer being implanted. These deposited surface layers can cause a number of problems, including dose nonrepeatability, poor dose uniformity and dose measurement problems. In particular, the neutral particles that form the deposited surface layers are not measured by the dose measurement system. Further, the depth profile of the dopant is altered by the deposited surface layer itself and by its effect on implanted ions. In addition, the deposited surface layers can cause contamination of other equipment, such as annealers, when the wafers are subsequently processed in such equipment.  
         [0005]     A method for limiting the formation of a deposited surface layer on a workpiece during plasma implantation is disclosed in International Publication No. WO 2004/013371 A2, published Feb. 12, 2004. A dopant gas and a dilution gas are introduced into a plasma doping chamber for ionization. The dopant gas ions are implanted into the workpiece, and the dilution gas ions remove a deposited surface layer from the workpiece. In another approach, the substrate is heated to promote evaporation of the deposited material. These approaches, while generally satisfactory, increase the cost and complexity of the plasma implantation process.  
         [0006]     Accordingly, there is a need for improved methods and apparatus for limiting formation of deposited surface layers during plasma implantation.  
       SUMMARY OF THE INVENTION  
       [0007]     According to a first aspect of the invention, a method is provided for plasma implantation of a workpiece. The method comprises introducing into a plasma doping chamber a dopant gas selected from the group consisting of PF 3 , AsF 3 , AsF 5  and mixtures thereof, forming in the plasma doping chamber a plasma containing ions of the dopant gas, the plasma having a plasma sheath at or near a surface of the workpiece, and accelerating the dopant gas ions across the plasma sheath toward the workpiece, wherein the dopant gas ions are implanted into the workpiece.  
         [0008]     According to a second aspect of the invention, a method is provided for plasma implantation of a workpiece. The method comprises introducing into a plasma doping chamber a dopant gas selected to limit deposition of neutral particles on the workpiece, forming in the plasma doping chamber a plasma containing ions of the dopant gas, the plasma having a plasma sheath at or near a surface of the workpiece, and accelerating the dopant gas ions across the plasma sheath toward the workpiece, wherein the dopant gas ions are implanted into the workpiece.  
         [0009]     According to a third aspect of the invention, a method is provided for plasma implantation of a substrate. The method comprises selecting a dopant gas to limit deposition of neutral particles of the dopant gas on the substrate, introducing the selected dopant gas into a plasma doping chamber, forming in the plasma doping chamber a plasma containing ions of the dopant gas, the plasma having a plasma sheath at or near a surface of the workpiece, and accelerating the dopant gas ions across the plasma sheath toward the substrate, wherein the dopant gas ions are implanted into the substrate.  
         [0010]     According to a fourth aspect of the invention, plasma doping apparatus comprises a plasma doping chamber, a platen located in the plasma doping chamber for supporting a workpiece, a process gas source coupled to the plasma doping chamber for introducing into the plasma doping chamber a dopant gas selected from the group consisting of PF 3 , AsF 3 , AsF 5  and mixtures thereof, a plasma source coupled to the plasma doping chamber for producing a plasma containing ions of the dopant gas, and a pulse source for accelerating the dopant gas ions from the plasma toward the workpiece, wherein the dopant gas ions are implanted into the workpiece.  
         [0011]     According to a fifth aspect of the invention, plasma doping apparatus comprises a plasma doping chamber, a platen located in said plasma doping chamber for supporting a workpiece, an anode spaced from said platen in said plasma doping chamber, a process gas source coupled to said plasma doping chamber for introducing into said plasma doping chamber a dopant gas selected from the group consisting of PF 3 , AsF 3 , AsF 5  and mixtures thereof, wherein a plasma containing ions of the dopant gas is produced in a plasma discharge region between said anode and said platen, and a pulse source for applying pulses between said platen and said anode for accelerating the dopant gas ions from the plasma toward the workpiece, wherein the dopant gas ions are implanted into the workpiece.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:  
         [0013]      FIG. 1  is a simplified schematic block diagram of a plasma doping system; and  
         [0014]      FIG. 2  is a graph of SIMS dose v. nominal dose for PLAD implants with different species. 
     
    
     DETAILED DESCRIPTION  
       [0015]     An example of a plasma ion implantation system suitable for implementation of the present invention 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 embodiment, the platen has an electrically conductive surface for supporting wafer  20 . In another embodiment, 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.  
         [0016]     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 another embodiment, platen  14  is connected to ground, and anode  24  is pulsed to a negative voltage. In further embodiments, both anode  24  and platen  14  may be biased with respect to ground.  
         [0017]     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 within the scope of the invention.  
         [0018]     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. 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 embodiment, 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.  
         [0019]     The plasma ion implantation system may include a hollow cathode  54  connected to a hollow cathode pulse source  56 . In one embodiment, the hollow cathode  54  comprises 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.  
         [0020]     One or more Faraday cups may be positioned adjacent to platen  14  for measuring the ion dose implanted into wafer  20 . In the embodiment 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 embodiment, an annular Faraday cup is positioned around wafer  20  and platen  14 .  
         [0021]     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.  
         [0022]     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 .  
         [0023]     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.  
         [0024]     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.  
         [0025]     As noted above, dopant gas species typically used for plasma implantation may dissociate into neutral particles during the implant process and form deposited surface layers on wafer  20 . Examples of dopant gas species which form deposited surface layers include AsH 3  (arsine), PH 3  (phosphine) and B 2 H 6 . For example, arsine gas may dissociate into As, AsH and AsH 2 , which may be deposited on the surface of wafer  20 . These deposited surface layers cause dose non-repeatability, poor dose uniformity and metrology problems.  
         [0026]     In accordance with an aspect of the invention, dopant species are selected which exhibit improved ionization efficiency and reduced dissociation to form neutral particles in comparison with conventional dopant gas species, such as the hydrides of dopant materials including PH 3  and AsH 3 . As a result of more efficient ionization, a larger percentage of the dopant gas is ionized and implanted into the wafer and a lower percentage of the dopant gas is deposited on the wafer surface in the form of neutral particles. Examples of suitable dopant gas species include halogen-containing dopant gas species. The halogen-containing dopant gas promotes chemical etching of the implanted surface, which removes dopant material deposited on the surface of the wafer. For example, fluorides and chlorides of the dopant materials may be utilized. Specific examples include PF 3 , AsF 3 , and AsF 5.    
         [0027]     Species such as PF 3 , AsF 3 , and AsF 5  may be used for plasma implantation in a manner similar to prior art dopant species such as AsH 3  and PH 3 . In order to obtain an equivalent implantation depth, the implant energy is adjusted to compensate for the mass of the dopant gas species. As known in the art, a dopant species with a higher mass requires higher energy in order to obtain the same implant depth in the semiconductor wafer. The pressure levels utilized for plasma implantation with dopant gas species according to embodiments of the invention are substantially similar to those used in prior art plasma implantation systems. As noted above, the pressure is typically controlled in a range of about 1 millitorr to about 500 millitorr.  
         [0028]     A comparison of results obtained using embodiments of the invention and results obtained using prior art dopant gas species is shown in  FIG. 2 . In  FIG. 2 , SIMS (Secondary Ion Mass Spectrometry) integrated dose is plotted as a function of nominal PLAD (Plasma Doping) implant dose. The nominal implant dose is the implant dose measured by the Faraday system of the plasma implantation system, such as Faraday cups  50  and  52  shown in  FIG. 1  and described above. The SIMS measurement is a well-known technique for determining dose implanted into the wafer by analysis of the wafer. The nominal implant dose represents a measurement of charged particles, whereas the SIMS measurement represents a measurement of charged particles and neutral particles, including deposited surface layers. In a system with no deposition of neutral particles, the two measurements should yield equal values.  
         [0029]     In  FIG. 2 , curve  100  represents AsH 3  integrated dose, curve  102  represents a AsF 5  integrated dose, and curve  104  represents BF 3  integrated dose. Curve  104  is shown for reference. The units on each axis of  FIG. 2  are atoms per cubic centimeter (cm 3 ). Thus for example, the notation “1E +14” refers to a dose of 1×10 14  atoms per cm 3 . A logarithmic scale is used on both axes of  FIG. 2 .  
         [0030]     Referring to curve  104 , which represents BF 3 , at a nominal implant dose of 1E+14, the SIMS integrated dose is slightly less than 1E+14. The close agreement between the nominal implant dose and the SIMS integrated dose indicates little or no deposition on the surface of the wafer. By contrast, for AsH 3  (curve  100 ) at a nominal implant dose of 1E+14, the SIMS integrated dose exceeds 1E+15, as indicated at point  110  on curve  100 . This difference indicates that the neutral particle deposition on the surface of the wafer exceeds the implanted ion dose by a factor of about 10. In the case of AsF 5 , according to an embodiment of the invention, for a nominal implant dose of 1E+14, the SIMS integrated dose is on the order of 1.3 E+14, as indicated by point  112  on curve  102 . Thus, curve  102  is indicative of relatively low deposition of neutral particles on the wafer surface when the dopant gas species is AsF 5.    
         [0031]     As noted above, formation of a deposited surface layer on a workpiece may be limited by utilizing a dopant gas and a dilution gas. Dopant gas ions are implanted into the wafer and dilution gas ions remove a deposited surface layer from the wafer. The dopant gas species described above may be utilized with a dilution gas to limit formation of deposited surface layers during plasma ion implantation.  
         [0032]     The atomic masses of the dopant gas and the dilution gas may be similar to achieve efficient removal of the deposited surface layers. The ratio of the dilution gas to the dopant gas is selected to remove the deposited surface layers as they are formed. For example, an inert dilution gas, such as krypton or xenon, may be utilized. In other examples, dilution gases such as argon or neon may be utilized. In further examples, the dilution gas may include a chemically active component, such as a halogen and more particularly, may include fluorine or chlorine. Additional details regarding the use of a dilution gas with a dopant gas are disclosed in International Publication No. WO/2004/013371 A2, which is hereby incorporated by reference.  
         [0033]     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.