Patent Publication Number: US-6982215-B1

Title: N type impurity doping using implantation of P2+ ions or As2+ Ions

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates to the use of an ion implantation beam of P 2 + ions or As 2 + ions to dope N type shallow junction source/drain regions or gate electrodes used in devices with shallow source/drain regions. Use of the P 2 + ions or As 2 + ions uses lower ion density and higher beam energy resulting in improved throughput and ion source life. 
   (2) Description of the Related Art 
   As junctions become very shallow the use of P +  ion beams or As +  ion beams becomes a problem because beam energies must be kept very low while the beam ion densities must be kept very high. These requirements leads to reduced ion source life and reduced wafer throughput. This invention overcomes this problem using P 2 + ion beams or As 2 + ion beams. 
   U.S. Pat. No. 5,155,369 to Current describes a method of using two doses of ions in an ion beam to provide implantation for shallow junction devices. A first dose of ions is implanted to produce a damaged layer through which a second dose of ions is directed. The damaged layer scatters the second dose of ions and channeling is avoided. 
   In their book “Silicon Processing for the VLSI Era, Volume I”, by Wolf and Tauber, Lattice Press, 1990, page 327, Wolf and Tauber discuss ion implantation using doubly charged species. 
   SUMMARY OF THE INVENTION 
   Ion implantation is used frequently in the manufacture of integrated circuits. Ion beams are used to implant impurities into semiconductor wafers to provide doping for source/drain regions, polysilicon electrode patterns, and the like. In ion beam implantation the beam energy and ion density are chosen to provide the desired impurity profile after implantation. One problem encountered as device geometries become smaller and source/drain junction depths become smaller is that the ion beam energy must become lower, in some cases less than 10 KeV. At these low energies it is difficult to obtain sufficient ion beam density resulting in lower throughput rates and increased implant cycle times, which directly impact cost. In addition the ion sources are stressed by these conditions and must be replaced with increased frequency. 
   It is a principle objective of this invention to provide a method of implanting phosphorous in source/drain regions using ion beam implantation with increased beam energy in applications having very shallow junctions. 
   It is another principle objective of this invention to provide a method of implanting arsenic in source/drain regions using ion beam implantation with increased beam energy in applications having very shallow junctions. 
   It is another principle objective of this invention to provide a method of implanting phosphorous in polysilicon electrodes using ion beam implantation with increased beam energy in applications having very shallow junctions. 
   It is another principle objective of this invention to provide a method of implanting arsenic in polysilicon electrodes using ion beam implantation with increased beam energy in applications having very shallow junctions. 
   These objectives are achieved by using ion sources such as solid phosphorous, phosphine gas, solid arsenic, or SDS arsine in the ion beam apparatus. The magnetic analyzer of the ion beam apparatus is then adjusted to select either the P 2 + or the As 2 + isotopes for the ion beam. These ion beams can then be implanted using energies of 20 KeV or greater. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic view of an ion beam apparatus. 
       FIG. 2  shows a cross section view of a part of an integrated circuit wafer showing an ion beam being used to implant impurities into source/drain regions. 
       FIG. 3  shows a cross section view of a part of an integrated circuit wafer showing an ion beam being used to implant impurities into a polysilicon gate electrode. 
       FIG. 4  shows an atomic mass unit spectrum of a solid phosphorous source with a beam energy of 30 KeV showing beam current as a function of atomic mass units. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of this invention will now be described with reference to  FIGS. 1–4 .  FIG. 1  shows a schematic view of an ion beam apparatus. Different ion beam systems may differ from the one shown and described herein but certain key features will be common to all ion beam systems. The ion beam apparatus has an ion source  10  which uses a small accelerating voltage to inject an ion beam  24  into an evacuated enclosure  12 . Ion sources can be solid materials or gasses such as solid phosphorous, phosphine gas, solid arsenic, or SDS arsine. 
   Referring again to  FIG. 1 , the beam then enters a magnetic analyzer  14  which is selected to select particles with a particular mass to charge ratio for the beam  25  exiting the magnetic analyzer. For the example of an ion source  10  using solid phosphorous or phosphine gas the magnetic analyzer  14  is adjusted to select singly charged P 2  ions which have a mass of 62 atomic mass units and a charge equal to the charge of a single electron. These ions will be designated as P 2 + ions. Solid phosphorous or phosphine gas provide an abundance of P 2  isotopes. Solid arsenic or SDS arsine can also be used as the ion source  10 . In this case the magnetic analyzer  14  is adjusted to select singly charged As 2  ions which have a mass of 150 atomic mass units and a charge equal to the charge of a single electron. These ions will be designated as As 2 + ions. Solid arsenic or SDS arsine provide an abundance of As 2  isotopes. 
   As shown in  FIG. 1 , the ion beam  25  exiting the magnetic analyzer  14  is then directed through a voltage accelerator/decelerator  16  where the selected beam energy is imparted to the ion beam  25 . The ion beam  26  exiting the voltage accelerator/decelerator  16  passes through a scanning system  18  which directs the ion beam. The ion beam exiting the scanning system  18  passes through an energy filter  52 , to provide improved energy uniformity, and a plasma flood gun  54 , to neutralize any charge buildup on the wafer during ion implantation. The ion beam  27  exiting the plasma flood gun  54  is directed, by the scanning system  18 , to the proper location on an integrated circuit wafer  30  which is attached to a wafer holder  20 . A coupling mechanism  22  attaches the wafer holder  20  to the evacuated enclosure  12 . In this manner the ion beam  27  exiting the scanning system can be used to implant impurities into source/drain regions or into polysilicon electrodes. 
     FIG. 2  shows a part of the wafer  30  which is held in place by the wafer holder in the evacuated enclosure. The wafer comprises a substrate, in this example a P type silicon substrate, having field oxide isolation regions  34  and a gate oxide layer  36 . A gate electrode  40  is formed on the gate oxide layer  36 . The ion beam  27  is used to implant impurities into the source/drain regions  38 . In this example the ion beam  27  is a P 2 + ion beam having a beam density of between about 4×10 14  and 6×10 14  ions/cm 2  and a beam energy of between about 20 and 48 KeV. The beam density used is one half that required for a beam of P +  ions because two phosphorous atoms are implanted for every P 2 + ion implanted. The beam energy is double that would be required for a beam of P +  ions because each of the P 2 + ions have two phosphorous atoms. After the implantation the wafer is rapidly annealed at an anneal temperature of between about 900° C. and 1100° C. for between about 10 and 20 seconds. This method produces shallow source/drain regions  38  using beam density and energy levels which maintain good throughput and source life. 
     FIG. 3  also shows a part of the wafer  30  which is held in place by the wafer holder in the evacuated enclosure. The wafer comprises a substrate, in this example a P type silicon substrate, having field oxide isolation regions  34  and a gate oxide layer  36 . A polysilicon gate electrode  40  is formed on the gate oxide layer  36 . The ion beam  27  is used to implant impurities into the polysilicon gate electrode  40 . In this example the ion beam  27  is a P 2 + ion beam having a beam density of between about 4×10 14  and 6×10 14  ions/cm 2  and a beam energy of between about 20 and 48 KeV. The beam density used is one half that required for a beam of P +  ions because two phosphorous atoms are implanted for every P 2 + ion implanted. The beam energy is double that would be required for a beam of P +  ions because each of the P 2 + ions have two phosphorous atoms. After the implantation the wafer is annealed at an anneal temperature of between about 900° C. and 1100° C. for between about 10 and 20 seconds. This method produces good conductivity for the polysilicon gate electrode  40  using beam density and energy levels which maintain good throughput and source life. 
   Referring again to  FIG. 2 , an ion beam  27  of As 2 + ions can be used to implant impurities into the source/drain region  38 . The wafer comprises a substrate, in this example a P type silicon substrate, having field oxide isolation regions  34  and a gate oxide layer  36 . In this example the ion beam  27  is an As 2 + ion beam having a beam density of between about 4×10 14  and 6×10 14  ions/cm 2  and a beam energy of between about 20 and 48 KeV. The beam density used is one half that required for a beam of As +  ions because two arsenic atoms are implanted for every As 2 + ion implanted. The beam energy is double that would be required for a beam of As +  ions because each of the As 2 + ions have two arsenic atoms. After the implantation the wafer is annealed at an anneal temperature of between about 900° C. and 1100° C. for between about 10 and 20 seconds. This method produces shallow source/drain regions  38  using beam density and energy levels which maintain good throughput and source life. 
   Referring again to  FIG. 3 , an ion beam  27  of As 2 + ions can be used to implant impurities into the polysilicon gate electrode  40 . The wafer comprises a substrate, in this example a P type silicon substrate, having field oxide isolation regions  34  and a gate oxide layer  36 . In this example the ion beam  27  is an As 2 + ion beam having a beam density of between about 4×10 14  and 6×10 14  ions/cm 2  and a beam energy of between about 20 and 48 KeV. The beam density used is one half that required for a beam of As +  ions because two arsenic atoms are implanted for every As 2 + ion implanted. The beam energy is double that would be required for a beam of As +  ions because each of the As 2 + ions have two arsenic atoms. After the implantation the wafer is annealed at an anneal temperature of between about 900° C. and 1100° C. for between about 10 and 20 seconds. This method produces good conductivity for the polysilicon gate electrode  40  using beam density and energy levels which maintain good throughput and source life. 
     FIG. 4  shows the AMU, atomic mass unit, spectrum for a source of solid phosphorous in an ion beam system using a beam energy of 30 KeV. The spectrum clearly shows a first beam current peak  44  at 31 atomic mass units and a second beam current peak  46  at 62 atomic mass units. This curve clearly shows that when the magnetic analyzer is adjusted to select phosphorous ions having 62 atomic mass units there is ample beam current available. 
   While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.