Abstract:
A plasma reactor employs an e-beam source to generate plasma, and the e-beam source has a configurable magnetic shield.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/549,362, filed Oct. 20, 2011 entitled ELECTRON BEAM PLASMA SOURCE WITH PROFILED MAGNET SHIELD FOR UNIFORM PLASMA GENERATION, by Kallol Bera, et al. 
     
    
     BACKGROUND 
       [0002]    A plasma reactor for processing a workplace can employ an electron beam as a plasma source. Such a plasma reactor can exhibit non-uniform distribution of processing results (e.g., distribution of etch rate across the surface of a workplace) due to non-uniform density distribution of the electron beam. Such non-uniformities can be distributed in a direction transverse to the beam propagation direction. 
       SUMMARY 
       [0003]    A plasma reactor for processing a workpiece includes a workpiece processing chamber having a processing chamber comprising a chamber ceiling and a chamber side wall and an electron beam opening in the chamber side wall, a workplace support pedestal in the processing chamber having a workpiece support surface facing the chamber ceiling and defining a workpiece processing region between the workpiece support surface and the chamber ceiling, the electron beam opening facing the workpiece processing region. The reactor further includes an electron beam source chamber comprising a source enclosure, the source enclosure defining an electron beam propagation path along a longitudinal direction extending into the workpiece processing region. An electromagnet surrounds the source chamber, the source enclosure and the electron beam opening extending along a transverse direction that is non-parallel to the longitudinal direction. A magnetic shield is disposed between the scarce chamber and the electromagnet, the shield having an edge defining a shield length, the edge having a profile corresponding to a distribution of the shield length along the transverse direction. The distribution of the shield length corresponds to a measured distribution in electron beam density along the transverse direction, that is corrected by the profiling of the shield length. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0005]      FIGS. 1A ,  1 B and  1 C depict a plasma reactor in accordance with one embodiment in which a magnetic shield is profiled along a transverse direction. 
           [0006]      FIG. 1D  depicts a plasma reactor in accordance with a second embodiment employing a different shield profile. 
           [0007]      FIGS. 2A and 2B  are orthographic projections of profiled shields of the embodiments of  FIGS. 1C and 1D  respectively. 
           [0008]      FIGS. 3A and 3B  depict modifications of the embodiments of  FIGS. 2A and 2B  respectively, employing stepped profiling. 
           [0009]      FIG. 4  depicts an embodiment that may be configured according to the profiles of  FIGS. 3A and 3B . 
           [0010]      FIGS. 5 and 6  depict different embodiments for overlapping adjacent shield slats. 
           [0011]      FIGS. 7 and 8  are orthographic projections of the shield of  FIG. 4  configured into profiles corresponding to  FIGS. 3A and 3B  respectively, 
       
    
    
       [0012]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION 
       [0013]    Referring to  FIGS. 1A ,  1 B and  1 C, a plasma reactor has an electron beam plasma source. The reactor includes a process chamber  100  enclosed by a cylindrical side wall  102 , a floor  104  and a ceiling  106 . A workplace support pedestal  108  supports a workpiece  110 , such as a semiconductor wafer, the pedestal  108  being movable in the axial (e.g., vertical) direction. A gas distribution plate  112  is integrated with or mounted on the ceiling  106 , and receives process gas from a process gas supply  114 . A vacuum pump  116  evacuates the chamber through the floor  104 . A process region  118  is defined between the workpiece  110  and the gas distribution plate  112 . Within the process region  118 , the process gas is ionized to produce a plasma for processing of the workpiece  110 . 
         [0014]    The plasma is generated in process region  118  by an electron beam from an electron beam source  120 . The electron beam source  120  includes a plasma generation chamber  122  outside of the process chamber  100  and having a conductive enclosure  124 . The conductive enclosure  124  may be rectangular and include side walls  124   a  and  124   b,  a ceiling  124   c,  a floor  124   d  and a back wall  124   e.  The conductive enclosure  124  has a gas inlet or neck  125 . An electron beam source gas supply  127  is coupled to the gas inlet  125 . The conductive enclosure  124  has an opening  124 - 1  facing the process region  118  through an opening  102   a  in the sidewall  102  of the process chamber  100 . 
         [0015]    The electron beam source  120  includes an extraction grid  126  between the opening  124 - 1  and the plasma generation chamber  122 , and an acceleration grid  128  between the extraction grid  126  and the process region  118 , best seen in the enlarged view of  FIG. 1B . The extraction grid  126  and the acceleration grid  128  may be formed as separate conductive meshes, for example. The extraction grid  126  and the acceleration grid  128  are mounted with insulators  130 ,  132 , respectively, so as to be electrically insulated from one another and from the conductive enclosure  124 . However, the acceleration grid  128  is in electrical contact with the side wall  102  of the chamber  100 . The openings  124 - 1  and  102   a  and the extraction and acceleration grids  126 ,  128  can be mutually congruent, and define a thin wide beam flow path for an electron beam into the processing region  118 . The width of the beam flow path is about the diameter of the workplace  110  (e.g., 100-500 mm) while the height of the beam flow path is less than about two inches. 
         [0016]    The electron beam source  120  further includes a pair of electromagnets  134 - 1  and  134 - 2  adjacent opposite sides of the chamber  100 , the electromagnet  134 - 1  surrounding the electron beam source  120 . The two electromagnets  134 - 1  and  134 - 2  may be symmetrical along the direction of beam propagation, and produce a magnetic field parallel to the direction of the electron beam along an electron beam path. The electromagnets  134 - 1  and  134 - 2  may be rectangular in shape, and may be oriented parallel with the rectangular conductive enclosure  124 . The electron beam flows across the processing region  118  over the workpiece  110 , and is absorbed on the opposite side of the processing region  118  by a beam dump  136 . The beam dump  136  is a conductive body having a shape adapted, to capture the wide thin electron beam. 
         [0017]    A plasma D.C. discharge voltage supply  140  is coupled to the conductive cathode enclosure  124 . One terminal of an electron beam acceleration voltage supply  142  is connected to the extraction grid  126  and referenced to the ground potential of the sidewall  102  of the process chamber  100 . A coil current supply  146  is coupled to the electromagnets  134 - 1  and  134 - 2 . Plasma is generated within the chamber  122  of the electron beam source  120  by a D.C. gas discharge produced by power from the voltage supply  140 , to produce a plasma throughout the chamber  122 . This D.C. gas discharge is the plasma source of the electron beam source  120 . Electrons are extracted from the plasma in the chamber  122  through the extraction grid  126  and accelerated through the acceleration grid  128  due to a voltage difference between the acceleration grid and the extraction grid to produce an electron beam that flows into the processing chamber  100 . 
         [0018]    Outer shields  400  surround the electromagnets  134 - 1  and  134 - 2  and may be formed of a magnetically permeable material. Each outer shield  400  may be rectangular and aligned with the electromagnet  134 - 1 . 
         [0019]    The distribution of electron density along the width of the beam (along the X-axis or direction transverse to beam travel) affects the uniformity of plasma density distribution in the processing region  118 . The electron beam may have a non-uniform distribution. Such non-uniformity may be caused by electron drift due to the interaction of the bias electric field with the magnetic field, divergence of electron beam due to electron-electron interactions and/or electron collision with neutral gas in the process chamber. Such non-uniformity may also be caused by fringing of an electric field at the edge of the electron beam. The distribution of electron density along the width of the beam (along the X-axis or direction transverse to beam travel) is liable to exhibit non-uniformities due to the foregoing causes. 
         [0020]    An inner shield  500  is placed between the conductive enclosure  124  and the electromagnet  134 - 1 . The inner shield  500  includes a top shield  510  overlying the ceiling  124   c  of the conductive housing  124 , and a bottom shield  520  underlying the floor  124   d  of the conductive enclosure  124 . Optionally, as shown in  FIG. 1B , the top and bottom shields  510 ,  520  may be connected together by a back portion  522 , although this feature may not be required. A leading edge  510   a  of the top shield  510  defines a shielded region of the plasma generation chamber  122  having a length L ( FIG. 1C ) extending from a plane of the back wall  124   e  to the leading edge  510   a.  The length L may be referred to as shield length, because it defines the extent of the shielded portion of the plasma generation chamber  122 . The plasma generation chamber  122  has a chamber length extending from the back wall  124   e  to the extraction grid  126 . To the extent that the shield length L is less than the chamber length, a portion of the plasma generation chamber  122  is unshielded by the top shield  510  while the remainder is shielded. In  FIG. 1C  the leading edge  510   a  is profiled in that it is curved, although in other embodiments it may be profiled by being stepped, as will be described below. As a result, the shield length L ( FIG. 10 ), which lies along the beam propagation direction (the Y axis or axial direction) is profiled, along the transverse direction (X-axis). Similarly, the bottom shield  520  may be profiled along the transverse direction. In the embodiment of  FIG. 1C , the profile of the top shield  510  is convex, that is, it is longest near its center and shorter at each side edge.  FIG. 2A  is an orthographic projection of the convex shield  500  of  FIG. 1C , in an embodiment in which the top and bottom shields  510  and  520  are profiled in a similar manner. In the embodiment of  FIG. 1D , the profile of the top shield is concave. Specifically, it has the shortest shield length L near its center and the longest shield length at each side edge.  FIG. 2B  is an orthographic projection of the concave shield  500  of  FIG. 1D . The bottom shield  520  may be profiled in the same manner as the top shield  510 , as depicted in  FIGS. 2A and 2B , or it may have a different profile or it may have a straight leading edge so as to have no profile. Alternatively, the top shield  510  may have a straight leading edge while the bottom shield  520  may have a profiled leading edge. 
         [0021]    Profiling of the magnetic shields  510 ,  520  affects the distribution of magnetic field, flux density along the transverse direction. The magnetic field of the electromagnets  134 - 1  and  134 - 2  confines the electron beam, enhancing its plasma electron density. The shields  510  and  520  attenuate the magnetic field, thereby attenuating plasma electron density in the electron beam. Profiling of either or both shields  510  and  520  attenuates the beam density in those regions that are shielded, while leaving the electron beam density in the unshielded regions unattenuated and therefore greater). This creates a corresponding profile of electron density distribution along the transverse direction. For example, a longer shield, length L at a certain point along the transverse direction reduces plasma electron density at that point relative to other locations where the shield length L is shorter. In the embodiment of  FIG. 1C , the ratio between the shield, length L and the chamber length at the center of the X-axis is nearly 100% (for maximum electron beam density attenuation), while this ratio is approximately 70% near each of side walls  124   a  and  124   b  (for least electron beam density attenuation). Thus, the convex shape of the top shield  510  tends to render plasma electron distribution along the transverse direction center low and edge high, and is therefore suitable when the uncorrected distribution is center high. In the embodiment of  FIG. 1D , the concave shape of the top shield  510  tends to render plasma electron distribution along the transverse direction center high and edge low, and is therefore suitable when the uncorrected distribution is center low. In some cases, the desired effect may be achieved when the profile is fiat, neither convex nor concave. 
         [0022]      FIGS. 3A and 3B  depict embodiments in which the profiling of  FIGS. 1C and 2D , respectively, is implemented in a stepped manner. 
         [0023]      FIG. 4  depicts an embodiment that may be transformed between different stepped configurations, including the stepped configurations of  FIGS. 3A and 3B . In  FIG. 4 , the top shield  510  is divided into elongated slats  515  which may be moved back and forth relative to one another along the direction of beam propagation. Although not shown in  FIG. 4 , the bottom shield  520  is similarly divided into elongated slats movable relative to one another along the direction of beam propagation. An actuator array  600  is linked by individually actuated arms  610  to the individual slats  515 . A controller  620  governing the actuator array  600  enables a user to configure the slats  515  in any stepped profile, including the convex profile of  FIG. 3A  and the concave profile of  FIG. 3B , for example. The magnetic shield configuration as defined by the slat positions can be changed independently with time from one process step to another process step or within one process step as necessary using the actuator array. 
         [0024]      FIG. 5  depicts an embodiment in which the individual slats  515  overlap one another.  FIG. 6  depicts and embodiment in which the individual slats  515  are interlaced.  FIG. 7  is an orthographic projection of the shield  500  of  FIG. 4  configured in a convex profile.  FIG. 8  is an orthographic projection of the shield  500  configured in a concave profile. 
         [0025]    While the main plasma source in the electron beam source  120  is a D.C. gas discharge produced by the voltage supply  140 , any other suitable plasma source may be employed instead as the main plasma source. For example, the main plasma source of the electron beam source  120  may be a toroidal RF plasma source, a capacitively coupled RF plasma source, or an inductively coupled RF plasma source. 
         [0026]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may foe devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.