Internal Split Faraday Shield for an Inductively Coupled Plasma Source

Abstract

An inductively coupled plasma source for a focused charged particle beam system includes a conductive shield within the plasma chamber in order to reduce capacitative coupling to the plasma. The internal conductive shield is maintained at substantially the same potential as the plasma source by a biasing electrode or by the plasma. The internal shield allows for a wider variety of cooling methods on the exterior of the plasma chamber.

We claim as follows: 
     
         1 . A charged particle beam system, comprising:
 a plasma source including:
 a plasma chamber for containing a plasma; 
 a conductor for providing radio frequency energy into the plasma chamber; and 
 a conductive shield for reducing capacitative coupling between the conductor providing radio frequency energy and the plasma, wherein the conductive shield is positioned within the plasma chamber; and 
   one or more focusing lenses for focusing charged particles from the plasma source onto a sample.   
     
     
         2 . The charged particle beam system of  claim 1  further comprising a biasing electrode for electrically biasing the conductive shield to a desired voltage; and 
     
     
         3 . The charged particle beam system of  claim 1  in which the conductive shield comprises a layer coated onto the interior wall of the plasma chamber. 
     
     
         4 . The charged particle beam system of  claim 1  in which the conductive shield comprises a thin conductive foil inserted within the plasma chamber. 
     
     
         5 . The charged particle beam system of  claim 1  further comprising a cooling fluid surrounding and in thermal contact with at least a portion of the plasma chamber. 
     
     
         6 . The charged particle beam system of  claim 5  in which the cooling fluid comprises air or a liquid. 
     
     
         7 . The charged particle beam system of  claim 1  in which the conductive shield is maintained at substantially the same voltage as the exterior boundary of the plasma sheath. 
     
     
         8 . The charged particle beam system of  claim 1  in which the conductive shield is electrically isolated and during operation is at the same electrical potential as the external boundary of the plasma sheath. 
     
     
         9 . The charged particle beam system of  claim 1  in which the conductive shield is maintained at a voltage having a magnitude of between 500V and 100 kV. 
     
     
         10 . The charged particle beam system of  claim 1  in which the conductive shield is maintained at a voltage having a magnitude of between 5000V and 50000V. 
     
     
         11 . The charged particle beam system of  claim 1  in which the plasma is biased to a potential to produce a landing energy of the charged particles of between 500 eV and 100 keV. 
     
     
         12 . The charged particle beam system of  claim 1  in which in operation the plasma in the plasma chamber and the conductive shield are maintained at an electrical potential different from ground potential. 
     
     
         13 . The charged particle beam system of  claim 1  in which the conductive shield is a split Faraday shield. 
     
     
         14 . The charged particle beam system of  claim 13  in which the split Faraday shield is wrapped around to form a cylindrical shape within the plasma chamber. 
     
     
         15 . The charged particle beam system of  claim 1  in which the plasma temperature is kept low enough to avoid sputtering of the conductive shield within the plasma chamber. 
     
     
         16 . The charged particle beam system of  claim 1  in which the conductive shield distributes heat from the plasma to the walls of the plasma chamber. 
     
     
         17 . The charged particle beam system of  claim 1  in which the conductor for providing radio frequency energy into the plasma chamber includes internal passages for passage of a cooling fluid. 
     
     
         18 . A method of operating an inductively coupled plasma source including a plasma chamber comprising:
 providing radio frequency energy into the plasma chamber from at least one conductive coil to maintain a plasma in the plasma chamber;   providing a conductive shield to reduce capacitative coupling between the source of the radio frequency and the plasma;   maintaining the plasma and the conductive shield at electrical potentials different from ground potential;   extracting charged particles from the plasma chamber; and   focusing the charged particles into a beam and directing the beam onto or near a workpiece.   
     
     
         19 . The method of  claim 18  further comprising providing a cooling fluid to cool the plasma chamber. 
     
     
         20 . The method of  claim 18  in which providing a conductive shield includes providing a conductive shield coated onto the interior wall of the plasma chamber. 
     
     
         21 . The method of  claim 18  in which providing a conductive shield includes providing a conductive material inserted into the interior of the plasma chamber.