Abstract:
A plasma processing system for etching a semiconductor wafer comprises: 1) a plasma chamber in which the semiconductor wafer may be mounted; 2) an upper ring capable of being mounted on an upper opening of the plasma chamber, wherein a central portion of the upper ring forms a hole; and 3) an electrode plate having a plurality of vias therethrough. The electrode plate is disposed in the hole in the upper ring, wherein the central portion of the upper ring further forms a shelf for supporting the electrode plate in the hole.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to plasma processing systems and, more specifically, to a plasma reaction chamber with a captive silicon electrode for dry plasma etching of semiconductor wafers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Plasma processing techniques, such as dry plasma etching, reactive ion etching, and ion milling techniques, provide numerous advantages over traditional chemical etching of semiconductor wafers. For example, plasma etching has a vertical etch rate that is much greater than the horizontal etch rate. This provides good control over the resulting aspect ratio (i.e., the height to width ratio of the resulting notch) of the etched features. Thus, plasma etching forms very fine features with high aspect ratios in very thin films.  
         [0003]     During the plasma etching process, large amounts of energy are added to a gas at relatively low pressure, thereby ionizing the gas. This forms plasma above the masked surface of the substrate (i.e., the semiconductor wafer). An electrical field is established between an electrode at the top of the etching chamber and the semiconductor wafer at the bottom of the etching chamber. The electrical potential of the substrate is adjusted so that charged particles in the plasma are propelled towards the substrate and collide substantially perpendicularly upon the wafer surface. The energy of the impact removes materials in the unmasked regions of the wafer surface. Reactive ion etching improves this process by using gases that are chemically reactive with the material being etched. Reactive ion etching combines the kinetic etching effects of the plasma particles with the chemical etching effect of the gas.  
         [0004]     The effectiveness of the etching process is greatly affected by the components of the etching chamber. Uniform etching rates may be achieved across the surface of the wafer by evenly distributing the plasma over the wafer surface. U.S. Pat. Nos. 4,595,484, 4,792,378, 4,820,371, and 4,960,488 disclose showerhead electrodes for distributing gas through holes in the electrodes. These patents generally describe gas dispersion disks having an arrangement of apertures tailored to provide a uniform flow of gas vapor to a semiconductor wafer.  
         [0005]     However, the showerhead electrodes that are commonly used in the industry are prohibitively expensive. For example, LAM Research Corporation provides a one-piece showerhead assembly comprising an electrode and a retaining ring. The showerhead assembly is inserted into the top of the etching chamber and costs about $4500. This is a consumable item that greatly increases the cost of using a dry plasma etching system.  
         [0006]     Therefore, there is a need in the art for an improved dry plasma etching system that costs less to operate than conventional dry plasma etching systems. In particular, there is a need in the art for an improved dry plasma etching system that uses a less expensive showerhead electrode.  
       SUMMARY OF THE INVENTION  
       [0007]     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a plasma processing system for etching a semiconductor wafer. According to an advantageous embodiment of the present invention, the plasma processing system comprises: 1) a plasma chamber in which the semiconductor wafer may be mounted; 2) an upper ring capable of being mounted on an upper opening of the plasma chamber, wherein a central portion of the upper ring forms a hole; and 3) an electrode plate having a plurality of vias therethrough. The electrode plate is disposed in the hole in the upper ring, wherein the central portion of the upper ring further forms a shelf for supporting the electrode plate in the hole.  
         [0008]     According to one embodiment of the present invention, the shelf is formed below the hole on the side of the upper ring towards the interior of the plasma chamber.  
         [0009]     According to another embodiment of the present invention, the shelf encircles the hole and projects inward towards a center of the hole.  
         [0010]     According to still another embodiment of the present invention, the shelf has an upper surface capable of supporting a perimeter region of a lower surface of the electrode plate when the electrode plate is inserted into the hole.  
         [0011]     According to yet another embodiment of the present invention, the plasma processing system further comprises a retaining ring disposed on an upper surface of the upper ring, wherein the retaining ring encircles and overlaps the hole and holds the electrode plate in place in the hole.  
         [0012]     According to a further embodiment of the present invention, the retaining ring is made of aluminum.  
         [0013]     According to a still further embodiment of the present invention, the electrode plate is made of a semiconductor material.  
         [0014]     According to a yet further embodiment of the present invention, the plasma processing system further comprises an O-ring capable of forming a gas-tight seal between the retaining ring and the electrode plate.  
         [0015]     In one embodiment of the present invention, the O-ring is disposed in a groove formed in the retaining ring.  
         [0016]     In another embodiment of the present invention, the O-ring is disposed in a groove formed in the electrode plate.  
         [0017]     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:  
         [0019]      FIG. 1  illustrates a cross-sectional view of selected portions of a conventional dry plasma etching system according to an exemplary embodiment of the prior art;  
         [0020]      FIG. 2  illustrates a cross-sectional view of selected portions of an improved dry plasma etching system and an improved showerhead electrode assembly according to an exemplary embodiment of the present invention;  
         [0021]      FIG. 3  illustrates in greater detail the cross-sectional view of selected portions of the dry plasma etching system and the improved showerhead electrode assembly in  FIG. 2  according to a first exemplary embodiment of the present invention; and  
         [0022]      FIG. 4  illustrates in greater detail the cross-sectional view of selected portions of the dry plasma etching system and the improved showerhead electrode assembly according to a second exemplary embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged plasma processing system.  
         [0024]      FIG. 1  illustrates a cross-sectional view of selected portions of conventional plasma etching system  100  according to an exemplary embodiment of the prior art. Plasma etching system  100  comprises plasma chamber  105 , which encloses empty space  106 . Semiconductor wafer  115  is mounted on support  120  on the bottom wall (floor) of plasma chamber  105 . Plasma etching system  100  further comprises showerhead electrode assembly  130  and housing  110 . Showerhead electrode assembly  130  comprises electrode plate  131  (diagonal line shading) and graphite ring  132  (lattice shading), which are bonded together to form showerhead electrode assembly  130  as a single unit. Showerhead electrode assemblies similar to showerhead electrode assembly  130  are well-known to those skilled in the art and may include, for example, the showerhead electrode assemblies provided by LAM Research Corporation as part of the 2300 Exelan® dielectric etch system.  
         [0025]     Upper ring  107  of plasma chamber  105  comprises inner (or lower) surface  108  and outer (or upper) surface  109 . Upper ring  107  may comprise, for example a ring assembly of quartz and/or silicon carbide and/or ceramic. Upper ring  107  may be removably mounted on the top of plasma chamber  105  to thereby close the chamber. A portion of the center of upper ring  107  forms a circular opening into which showerhead electrode assembly  130  is inserted. The circular opening in upper ring  107  is encircled by retaining ring  135  (dotted shading). When showerhead electrode assembly  130  is inserted into the circular opening, graphite ring  132  is pressed between retaining ring  135  and housing  110 . Graphite ring  132  and retaining ring  135  are mounted on, and held in place by, a plurality of threaded bolts  136 , including exemplary bolts  136   a  and  136   b , mounted up to housing  110 . Bolts  136  hold showerhead electrode assembly  130  in place in the circular opening in upper ring  107  and provide a gas-tight seal between housing  110  and graphite ring  132 . Screws  142 , including exemplary screws  142   a  and  142   b  hold upper ring  107  to housing  110 .  
         [0026]     Multiple baffle plates (dotted shading) are disposed on top of electrode plate  131 , including exemplary baffle plate  140 . Holes or vias  141  are formed in baffle plate  140 . Similarly, holes or vias  133  are formed in electrode plate  131 . Vias  133  and  141  enable ionizing gas, enclosed in space  111  above baffle plate  140  by housing  110 , to flow through the baffle plates and electrode plate  131  into space  106  enclosed by plasma chamber  105 . The movement of the ionizing gas from space  111  to space  106  is indicated by the dotted arrows pointing downward in  FIG. 1 .  
         [0027]     Vias  141  in baffle plate  140  are not aligned with the vias in the baffle plate below baffle plate  140 . Similarly, the vias in the lower baffle plate are not aligned with vias  133  in electrode plate  131 . This misalignment forces the ionizing gas to disperse evenly as the gas flows through the two baffle plates and electrode plate  131 . The ionized gas forms plasma in the region above semiconductor wafer  115  in space  106 . An electrical potential is introduced between electrode plate  131  and semiconductor wafer  115 . This electrical potential is adjusted so that charged particles in the plasma are propelled towards the upper surface of semiconductor wafer  115  and collide substantially perpendicularly upon the upper surface of semiconductor wafer  115 . The energy of the impact removes materials in the unmasked regions of the upper surface of semiconductor wafer  115 .  
         [0028]     For convenience, the upper components of plasma etching system  100  may be assembled in an upside down position prior to being placed on top of plasma chamber  105 . Initially, housing  110  may be placed upside down and the baffle plates are put in place. Next, showerhead electrode assembly  130  is placed on top of the baffle plates. Retaining ring  135  is then placed on top of showerhead electrode assembly  130  and is bolted into housing  110  by bolts  136 . Finally, upper ring  107  is placed on top of retaining ring  135  and is bolted to housing  110  by bolts  142 . The assembled upper components are then flipped over and mounted on top of plasma chamber  105 . In that position, upper ring  107  and retaining ring  135  prevent showerhead assembly  130  from dropping into plasma chamber  105 .  
         [0029]     Electrode plate  131  is worn away throughout the process, thereby requiring replacement. Showerhead electrode assembly  130  is therefore a consumable that increases the cost of operating plasma etching system  100 . The present invention improves upon plasma etching system  100  by reducing the cost of showerhead electrode assembly  130 .  
         [0030]      FIG. 2  illustrates a cross-sectional view of selected portions of improved dry plasma etching system  200  and an improved showerhead electrode assembly according to an exemplary embodiment of the present invention. Plasma etching system  200  comprises plasma chamber  205 , which encloses empty space  206 . Semiconductor wafer  115  is mounted on support  120  on the bottom wall (floor) of plasma chamber  205 . Plasma etching system  200  further comprises a two-piece showerhead electrode assembly and housing  210 . The two-piece showerhead electrode assembly comprises electrode plate  231  (diagonal line shading) and retaining ring  235  (dotted shading) Retaining ring  235  is preferably made from aluminum.  
         [0031]     Upper ring  207  of plasma chamber  205  comprises inner (or lower) surface  208  and outer (or upper) surface  209 . A portion of the center of upper ring  207  forms a circular opening into which electrode plate  231  is lowered. However, upper ring  207  also forms circular shelf  250 , which encircles, and projects into, the circular opening in upper ring  207 . The outer perimeter region of the bottom surface of electrode plate  231  rests upon, and is supported by, the upper surface of shelf  250 .  
         [0032]     Retaining ring  235  (dotted shading) is mounted on housing  210  and is held in place by a plurality of threaded bolts  236 , including exemplary bolts  236   a  and  236   b . Electrode plate  231  is then placed in upper ring  207  and upper ring  207  is bolted to housing  210  by a plurality of threaded bolts  242 , including exemplary bolts  242   a  and  242   b . Bolts  242  tighten electrode plate  231  between retaining ring  235  and upper ring  207 , providing a gas-tight seal between retaining ring  235  and electrode plate  231 .  
         [0033]     As in  FIG. 1 , baffle plates are disposed on top of electrode plate  231 , including exemplary baffle plate  140 . Holes or vias  141  are formed in baffle plate  140 . Similarly, holes or vias  233  are formed in electrode plate  231 . Vias  233  and  141  enable ionizing gas, enclosed in space  211  above baffle plate  140  by housing  210 , to flow through the two baffle plates and electrode plate  231  into space  206  enclosed by plasma chamber  205 . The movement of the ionizing gas from space  211  to space  206  is indicated by the dotted arrows pointing downward in  FIG. 2 .  
         [0034]     As before, vias  141  in baffle plate  140  are not aligned with the vias in the baffle plate below baffle plate  140 . Similarly, the vias in the lower baffle plate are not aligned with vias  233  in electrode plate  231 . This misalignment forces the ionizing gas to disperse evenly as the gas flows through the baffle plates and electrode plate  231 . The ionized gas forms plasma in the region above semiconductor wafer  115  in space  206 . As in  FIG. 1 , an electrical potential is introduced between electrode plate  231  and semiconductor wafer  115 . This electrical potential is adjusted so that charged particles in the plasma are propelled towards the upper surface of semiconductor wafer  115  and collide substantially perpendicularly upon the upper surface of semiconductor wafer  115 . The energy of the impact removes materials in the unmasked regions of the upper surface of semiconductor wafer  115 .  
         [0035]      FIG. 3  illustrates in greater detail the cross-sectional view of selected portions of dry plasma etching system  200  and the improved showerhead electrode assembly according to a first exemplary embodiment of the present invention. O-ring  305  is placed in a groove formed in retaining ring  235 . When retaining ring  235  is pressed down on silicon electrode plate  231 , O-ring  305  forms a seal that captures any process gas and channels the captured gas through vias  233  in silicon electrode plate  231 . The groove in which O-ring  305  is disposed is recessed so that, when compressed, silicon electrode plate  231  contacts retaining ring  235 , which is grounded. Retaining ring  235  is anodized in such a way that silicon electrode plate  231  conducts to a ground plane.  
         [0036]      FIG. 4  illustrates in greater detail the cross-sectional view of selected portions of dry plasma etching system  200  and the improved showerhead electrode assembly according to a second exemplary embodiment of the present invention.  FIG. 4  is identical in most respect to  FIG. 3 , except that O-ring  405  is placed in a groove formed in silicon electrode plate  231 . When retaining ring  235  is pressed down on silicon electrode plate  231 , O-ring  405  forms a seal that captures any process gas and channels the captured gas through vias  233  in silicon electrode plate  231 . As in  FIG. 3 , the groove in which O-ring  405  is disposed is recessed so that, when compressed, silicon electrode plate  231  contacts retaining ring  235 , which is grounded.  
         [0037]     Although the present invention has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.