Abstract:
An ion beam neutral detector system, an ion implanter system including the detector system and a method of detecting ion beam neutrals that ensures an ion implant is meeting contamination requirements are disclosed. The detector includes an energy contamination monitor positioned with in an ion implanter system. A method of the invention includes implanting the workpiece using an ion beam, and periodically detecting ion beam neutrals in the ion beam such that adjustments to the ion implanter system can be made for optimization.

Description:
CLAIM OF PRIORITY 
   The present invention claims benefit of U.S. Provisional Application Ser. No. 60/544,029 filed on Feb. 12, 2004, entitled, “ION BEAM NEUTRAL DETECTION.” 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The described apparatus and methods relate to detecting energetic neutrals of an ion beam. More particularly, the described apparatus and methods are directed for detecting neutrals of an ion beam in an ion implanter to minimize and prevent energy contamination of a workpiece. 
   2. Related Art 
   Deceleration of an ion beam reasonably close to a workpiece, such as a wafer, is a standard method of improving ion implanter productivity for low energy beams. The main benefit of deceleration is to reduce the distance the beam must travel at low energy where the efficiency of transporting the beam is poor. The closer the deceleration is to the workpiece, the more benefits result as far as increasing the beam current. However, ions that become neutral prior to the deceleration, but within a line of sight of the wafer, will be implanted at their undecelerated energy and are classified as energy contamination.  FIG. 1  illustrates how energy contamination may occur in an ion implanter. As an ion beam  10  propagates through the implanter, a bend magnet  12  may direct beam  10  toward drift space  14 . In drift space  14 , collisions with surfaces and background gas produce energetic neutrals. Through a deceleration stage  16 , ions are decelerated to a final energy in one portion  18  of beam  10 . A small flux of the higher energy neutrals remain as a second portion  20  of beam  10 . The neutrals that pass to a wafer  22  will implant significantly deeper than the ions to cause energy contamination. Only a small amount of energy contaminated ions are allowed to be implanted, typically on the order of 0.2% to 0.5%, before the implantation of the workpiece is adversely effected. 
   Known techniques for limiting energy contamination include an implanter architecture where an electrostatic or magnetic bend is placed between the deceleration stage and the magnet, increased pumping to limit the neutralization of beam ions by residual gas, an aperture and liner design to prevent neutrals formed by collisions with the structures inside the implanter from reaching the workpiece, and limiting the voltage allowed when running deceleration to reduce the implanted depth of the contaminant ions. The implanter may be designed to produce zero energy contamination by extracting the required low energy beams directly from the source, but this inherently runs at much lower beam currents. The other techniques allow for higher currents but do not provide any real time monitoring to ensure that they are effective in preventing energy contamination every time the implanter is run. 
   In view of the foregoing, it is desired to ensure that the implanter is meeting this contamination requirement at all times while the implantation productivity is maximized. 
   SUMMARY OF THE INVENTION 
   The invention includes an ion beam neutral detector system, an ion implanter system including the detector system and a method of detecting ion beam neutrals that ensures an ion implant is meeting contamination requirements. The detector includes an energy contamination monitor positioned within an ion implanter system. A method of the invention includes implanting the workpiece using an ion beam, and periodically detecting ion beam neutrals in the ion beam such that adjustments to the ion implanter system can be made for optimization. 
   A first aspect of the invention is directed to an ion implanter system comprising: a source for generating an ion beam for implanting a workpiece; and an ion beam neutral detector. 
   A second aspect of the invention is directed to a method of implanting a workpiece with an ion implanter system, the method comprising the steps of: implanting the workpiece using an ion beam; and periodically detecting ion beam neutrals in the ion beam. 
   A third aspect of the invention is directed to an ion beam neutral detector system comprising: a detection structure including: a chamber coupled to a bias voltage, a collector plate adjacent the chamber for receiving the ion beam, and a measuring device that measures a current at the collector plate. 
   The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
       FIG. 1  illustrates an ion beam path in a conventional ion implantation system; 
       FIG. 2A  is a schematic diagram of an energy contamination monitor according to an embodiment of the present invention; 
       FIG. 2B  is a cross sectional view of an energy contamination monitor according to an embodiment of the present invention; 
       FIG. 3  is a graph of collector current versus bias voltage of a cylinder for an energy contamination monitor according to an embodiment of the preset invention; and 
       FIG. 4  illustrates an ion implantation system incorporating an energy contamination monitor in an embodiment of the present invention. 
   

   DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention are directed to methods and apparatus for measuring the current of secondary electrons emitted due to the impact of the energetic neutral particles.  FIG. 2A  illustrates a schematic diagram and  FIG. 2B  illustrates a cross sectional view of a detection structure  108  of an energy contamination monitor  110  (also referred to herein as an “ion beam neutral detector system”) according to embodiments of the present invention. Detection structure  108  includes: a chamber  130 , a collector plate  140  and a measuring device  142 . A container  150  may be provided to house and position collector plate  140  adjacent chamber  130  for receiving ion beam  120 , as will be described in more detail below. In addition, as shown in  FIG. 2B , a mesh filter  144  is electrically connected to chamber  130  may be provided in front of collector plate  140 . Mesh filter  144  may include, for example, tungsten or tantalum. 
   As shown in  FIGS. 2A-2B , ion beam  120  of ions and neutrals enters container  150  through an aperture  152  in a shield  160 , which is coupled to container  150 . In one embodiment, detection structure  108  is substantially cylindrical in shape, although this is not necessary. In this case, however, chamber  130  is substantially cylindrical and collector plate  140  is substantially circular. Chamber  130  is connected to a bias voltage supply  132  and collector plate  140  is connected to measuring device  142 , which measures the collector current I col  ( FIG. 3 ) at collector plate  140 . Chamber  130 , collector plate  140  and mesh filter  144  are isolated from container  150  by insulation  154  ( FIG. 2B ). Ion beam  120  includes a flux of ions I ion  at energy E ion  and a flux of neutrals I neutrals . Aperture  152  allows beam  120  to pass toward collector plate  140  and the bias applied to chamber  130  allows portions of the collector current to be distinguished, namely the ion current, I ion  the secondary electrons generated by ions, and the secondary electrons generated by the neutrals, I neutrals . 
   Chamber  130  is designed to a sufficient length and diameter to direct beam  120  to collector plate  140  while subjecting beam  120  to the electric field produced by biasing chamber  130 . Considerations should be made to minimize the completed size of detection structure  108  so that it may fit relatively unobtrusively within a beam-generating device, such as an implanter. For example, chamber  130  may be of a length of about 2″ and a diameter of about 1″ to provide dimensions for meeting these constraints. Also, chamber  130 , collector plate  140 , and shield  160  should be made of materials compatible with the environment. For example, chamber  130  and collector plate  140  may be made of aluminum or like material, and shield  60  may be made of graphite or like material. 
   The bias voltage (V) applied to chamber  130  may be positive, negative or grounded as illustrated in the graph of  FIG. 3 . In region  1 , a negative bias is applied to chamber  130 . In region  1 , the measured collector current (I col ) is due to ions and I l =I ion . At a sufficiently high voltage in region  3 , the measured collector current (I col ) results from neutrals, and I 3 =I neutral ·γ neutral , where γ neutral  is the secondary electron emission coefficient for neutrals. In region  2 , the bias voltage (V) applied to chamber  130  ( FIGS. 2A-2B ) is positive but less than the voltage applied in region  3 , which is required to prevent ions from reaching collector plate  140  ( FIGS. 2A-2B ). The measured collector current (I col ) results from both neutrals and ions such that I 2 =I ion +I ion ·γ ion +I neutral ·γ neutral , where γ ion  is the secondary electron emission coefficient for ions. 
   The currents for each of the operating regions can be manipulated to give: 
   
     
       
         
           
             
               I 
               3 
             
             
               
                 I 
                 2 
               
               - 
               
                 I 
                 1 
               
               - 
               
                 I 
                 3 
               
             
           
           = 
           
             
               
                 γ 
                 neutral 
               
               ⁢ 
               
                 I 
                 neutral 
               
             
             
               
                 γ 
                 ion 
               
               ⁢ 
               
                 I 
                 ion 
               
             
           
         
       
     
   
   By assuming that the secondary electron emission coefficients for neutrals (γ neutral ) and ions (γ ion ) are the same, the relation is derived: 
   
     
       
         
           
             
               I 
               neutral 
             
             
               I 
               ion 
             
           
           = 
           
             
               I 
               3 
             
             
               
                 I 
                 2 
               
               - 
               
                 I 
                 1 
               
               - 
               
                 I 
                 3 
               
             
           
         
       
     
   
   As illustrated in  FIG. 4 , detection structure  108  may be incorporated into an ion implanter system  400  according to an embodiment of the present invention. In this case, ion beam neutral detection system  110  also includes a detector support assembly  460  for moving detection structure  108  between a use position and a non-use position within ion implanter system  400  although configurations may be found that allow the detector to remain in a fixed position. Ion implanter system  400  of the present embodiment includes a source  410  for generating an ion beam. In addition, system  400  may also include a directing magnet  420  of, e.g., 90°, for directing the beam, a deceleration stage  430  for decreasing the energy level of the beam, a corrector magnet  440  for directing the beam, a final stage of deceleration  444  and an end station  450  where the beam is directed to the workpiece. The end station  450  includes detection system  110 , which is connected to support system  460 , which may include, for example, robotics for moving detection system  110  as generally indicated by the double-ended arrow. 
   In operation, detection system  110  is moved toward the use position A for obtaining energy contamination measurements during set up, i.e., prior to implanting, or at predetermined intervals (periods). Detection system  110  is moved away from the workpiece to a non-use position B since energy contamination measurements will not be made when the workpiece is being implanted. Periodic detection or measurements of ion beam neutrals in the ion beam during stoppage of ion implanting can then be input into a computer or processor (not shown) for use in optimizing (i.e., through adjustment) the operation of implanter system  400 , i.e., prior to implanting or during implanting. For example, if the energy contamination measurements are outside of specification values, implanter system  400  can be re-tuned or stopped until the implanter system  400  is brought back to within the specifications. 
   The described invention provides process assurance and maximizes implantation productivity when running in deceleration to low energies. Changing conditions in implanter system  400  ( FIG. 4 ), vacuum pressure and maintenance, for example, may lead to differing levels of contamination when running the same recipe. Present practices limit the output of an implanter system to provide some margin for potential changes in operating conditions. Real time detection of energy contamination minimizes contamination due to poor operating conditions and thereby reduces the margin required for safe operation. 
   Detector system  110  according to the above-described embodiments of the present invention measures the current of secondary electrons emitted due to the impact of the energetic neutral particles. Other neutral detectors could be substituted for the neutral detector based on the same principle or another like calorimetry. 
   Although the methods and systems have been described above relative to specific embodiments, the present invention is not so limited. Obviously, many modifications and variations may become apparent in light of the teachings. Many additional changes in the details, materials, and arrangement of parts, described and illustrated herein, can be made by those skilled in the art.