Abstract:
A PECVD reactor for processing a single wafer. The reactor has a susceptor for holding a wafer horizontally, an apparatus for lifting the wafer from the susceptor for loading and unloading. The horizontally positioned thermal plate is positioned above the susceptor for uniform transfer of radiant heat energy from heat lamps to the wafer. The thermal plate also serves as an RF plate, being constructed of an electrically conductive material and connected to an RF transmission line and connector for receiving RF energy from an RF generator for the purpose of providing an RF field for plasma enhancement. The thermal plate is configured thinner near its edges, so as to space the plate further from the susceptor and thicker near the center, placing it closer to the susceptor.

Description:
This application claims the benefit of U.S. Provisional Application No. 06/071,572 filing date Jan. 15, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to methods and apparatus for plasma enhanced chemical vapor deposition (PECVD), and more particularly to a method and apparatus for PECVD in a single wafer reactor that provides control of reactant gas over the surface of a wafer for more uniform processing, and provides for introduction of added dopant gases, and for wafer susceptor biasing to control the rate of ion incident on a wafer 
     2. Brief Description of the Prior Art 
     There are a large number of plasma enhanced chemical vapor deposition processes that are performed inside of enclosed chambers wherein the pressure, temperature, composition gases and application of radio frequency (RF) power are controlled to produce the desired thin film deposition of various materials onto substrates such as semiconductor wafers, flat panel displays and others. For convenience, the term “wafer” will be used in the following disclosure with the understanding that it also applies to the manufacture of flat panel and other types of substrates or devices wherein PECVD processes are employed. For example, silicon nitride is typically deposited via plasma enhanced chemical vapor deposition (PECVD) on top of metal layers on a semiconductor wafer. 
     A main feature of the PECVD process is that it can be carried out at low substrate temperatures as described by S. Wolf and R. N. Tauber, “Silicon Processing for the VLSI Era”, Volume 1—Process Technology, Lattice Press, 1986, pp. 171-174. FIG. 1 shows a prior art chamber  10  having a rotating susceptor  12  capable of holding a plurality of substrates  14 . RF energy is applied to an upper electrode  16  to create a plasma (glow discharge), creating free electrons within the plasma region. The electrons gain sufficient energy from the electric field so that when they collide with gas molecules, gas-phase dissociation and ionization of the reactant gases (e.g. silane and nitrogen) occurs. The energetic species are then absorbed on the film surface of the wafers. 
     There are a variety of single wafer PECVD chamber designs available in the marketplace. There are also a variety of commercially available multiple wafer chambers, similar to the one shown in FIG. 1, wherein the wafers are all supported by a susceptor on a single horizontal plane. 
     The single wafer and horizontal multiple wafer PECVD chamber designs of the prior art, as described above, are problematic for numerous reasons. Single wafer designs suffer from the need to operate at relatively high pressures (&gt;0.8 Torr) in order to achieve acceptable deposition rates and uniform film thickness across a wafer. Multiple wafer horizontal designs pose extreme difficulties in connection with the incorporation of automatic robotic wafer loading and unloading. Also, horizontal multiple wafer designs can process only a limited number of wafers before the chamber becomes so large in area that it becomes very difficult to maintain the necessary plasma uniformity and gas flow control. 
     It is apparent from the above description of the prior art that there is a need for a PECVD chamber that provides an improved material deposition rate and uniform thickness across a wafer, as well as facility for automatic robotic wafer loading and unloading. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a PECVD reactor yielding improved uniformity of material deposition across a wafer. 
     It is a further object of the present invention to provide a PECVD reactor having provision for application of RF energy for plasma enhancement, and provision for retarding incident ions on a wafer. 
     It is a still further objective of the present invention to provide a PECVD reactor having provision for introducing a controlled concentration of dopant across a wafer. 
     Briefly, a preferred embodiment of the present invention includes a PECVD reactor providing enhanced uniformity of deposition on a single wafer. The reactor has a susceptor for holding a wafer horizontally, and an apparatus for lifting the wafer from the susceptor for loading and unloading. A horizontally positioned thermal plate is positioned above the susceptor for uniform transfer of radiant heat energy from heat lamps to the wafer. The thermal plate also serves as an RF plate, being constructed of an electrically conductive material and connected to an RF transmission line and connector for receiving RF energy from an RF generator, for the purpose of providing an RF field for plasma enhancement. The thermal plate is configured thinner near its edges, so as to space the plate further from the susceptor and thicker near the center, placing it closer to the susceptor. The result of this variation is an increase in gas velocity as the gas moves toward the center of the susceptor and therefore wafer, decreasing the boundary layer between the reactant gas and the wafer surface wherein reactant products accumulate, overcoming gas depletion effects at the wafer center, and achieving a more uniform deposition or a wafer surface. The thermal plate also has small diameter holes drilled through to allow injection of dopant gases toward a wafer. Further control of deposition is achieved by applying an electrical bias on the susceptor for controlling the rate of incidence of ions on a wafer. 
    
    
     IN THE DRAWING 
     FIG. 1 shows a prior art chamber having a rotating susceptor; 
     FIG. 2 illustrates a preferred embodiment of the present invention; 
     FIG. 3 shows details of construction of an RF feedthrough; 
     FIG. 4 shows further detail of the RF transmission line of FIG. 3; 
     FIG. 5 is an enlargened view showing support structure for the thermal plate; 
     FIG. 6 shows construction of a shield forming a dark space around the RF transmission line; 
     FIG. 7 shows an alternate thermal plate for controlling reactant gas velocity; 
     FIG. 8 is a cross sectional view showing holes through a thermal plate for injecting additional gases over the susceptor; 
     FIG. 9 illustrates application of a bias to the susceptor; and 
     FIG. 10 is a view of a rotating electrical connection. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 2 of the drawing, a preferred embodiment of the reactor of the present invention is shown. The reactor  22  is similar in design and construction to the reaction chamber described in U.S. Pat. No. 5,551,985 entitled “Method and Apparatus for Cold Wall Chemical Vapor Deposition”, and is incorporated herein by reference. In chamber  22 , the wafer susceptor  24 , and thereby also a wafer  26 , is rotated while being heated from above by heaters  28 , and below by radiant heaters  30 . A reactant gas supply is connected to input  32 , and directed to open space  34  between the thermal plate  36  and wafer  26  by injectors  38 . Other gases, including inert gases, can be injected through inlet  44  to open space  40  between quartz window  42  and plate  36 . The reactant gases are exhausted out the other side  46  through an outlet not shown. Reactant gas injection and exhaust apparatus details are explained in detail in U.S. Pat. No. 5,551,985. 
     An RF connector  48  and transmission line  50  are provided to apply RF energy to the thermal plate  36 , constructed of electrically conductive material. 
     In operation, the reactant gas flow from the injector  38  is generally parallel to the wafer  26  surface, and is at a relatively high velocity due to the short distance between wafer  26  and the thermal plate  36 . The exact distance between wafer  26  and thermal plate  36  for a particular process can be varied through use of the vertical motion mechanism  52 . A high velocity reactant gas flow is beneficial in that wafer reactant gas by-products are quickly exhausted, and fresh reactant gas supplied, resulting in an increased deposition rate. The application of RF energy to plate  36  causes an RF field in the reactant gas open space  34 , resulting in the creation of a plasma. 
     The details of construction of the RF feedthrough  48  are more clearly illustrated in FIG. 3, an enlargened view of section A of FIG.  2 . The feedthrough  48  has a center conductor  54  connected to a conductive clamping strap  56 . In addition to conducting RF energy to the plate  36 , the strap  56  also serves to help hold the thermal plate  36  in place against an isolating quartz ring  58 . The strap  56  is bolted to the grounded top plate  60  via bolt  62 . 
     The construction of the RF feedthrough and the strap apparatus is more clearly illustrated in FIG. 4, an enlarged view of section B of FIG.  3 . The connecting bolt  62  is shown to be electrically insulated from strap  56  by an insulating washer  64  and insulating shoulder washer  66 . Additional clips made from the same material as strap  56  and having similar shape are bolted with insulated bolts around the periphery of the thermal plate  36  for solid support. An example of this is shown in FIG. 5, which is an enlargened view of section C of FIG.  3 . Unlike the strap  56 , these clips  68  extend only slightly beyond the bolts  62  in the direction away from the thermal plate  36 . Also, instead of insulating the bolt from the clip  68 , the clip is spaced from the plate  36  with an insulated spacer  70 . 
     An alternate embodiment of the construction of the RF feedthrough and transmission line  50  is illustrated in FIG. 6 which is an enlargened view of an area similar to section B of FIG.  3 . To prevent the conductive clamping strap  56  of FIG. 4 from having a plasma created along the surfaces of the strap itself, which could cause the sputtering off of the strap&#39;s material onto the wafer, a strap  72  is provided as shown in FIG.  6 . The strap  72  is insulated on its upper side by an insulator  74 , made from suitable material such as ceramic or quartz. Surrounding the bottom and side edges of strap  72 , and the bottom nut  76  of RF feedthrough  48  and the bolt  62  is an electrically conductive shield  78  that is directly bolted to the grounded top plate  60  by bolt(s)  80 . The shape of the shield  78  is such that there is a small gap  81 , typically 0.04 to 0.06 inches, between the shield  78  and the RF powered strap  72  and bottom nut of RF feedthrough  48 . The small gap creates a “dark space” wherein a plasma cannot occur at typical operating pressures. The shield  78  is shown to extend slightly inward toward the center of thermal plate  36  to aid in confining the plasma to the area away from the sharp corner of the outer periphery of thermal plate  36 . Referring again to FIG. 5, surrounding the remaining peripheral area of thermal plate  36  is a shaped conductive ring  82 . In this case, the cross-sectional view of FIG. 5 is used to illustrate a ring  82 , that is continuous around the thermal plate  36 , except for a gap (not shown) to accommodate the shield  78 . The ring  82  is directly bolted to the ground to the grounded top plate  60  via a plurality of bolts  62 . The illustration of FIG. 5 is a valid view for either a clip  68  or a ring  82  since both have a similar cross section. Between the ring  82  and the thermal plate  36  is a ring of insulating material  84  made from a suitable material such as quartz. Again, the cross section of insulator  70  as shown in FIG. 5 is the same as ring  84  in FIG.  5 . This ring of insulating material  84  also has a gap (not shown) to accommodate the shield  78 . The combination of the grounded ring  82  and the insulating ring  84  prevents a plasma from occurring around the rest of the periphery of thermal plate  36 . 
     FIG. 7, an enlargened view of the area of section D of FIG. 3, shows an alternate thermal plate  86 , a version of thermal plate  36 , shaped in such a way that the distance “d” between the thermal plate  36  at its center  88  is shorter than the distance away from the center at  90 . This has the effect of increasing the gas velocity as the gas moves toward the center  88 , the increased gas flow decreasing a layer of reactant products that accumulate over the wafer surface, the layer known as a “boundary layer”, and thereby overcoming gas depletion effects, a result that is important under certain process conditions for achieving a uniform thickness of deposited material across the wafer  26  surface. 
     FIG. 8, including an area approximately equivalent to section D of FIG. 3, shows yet another thermal plate  92  version of thermal plate  36  wherein the thermal plate  92  has a number of small diameter holes  94  drilled through the plate  92  to allow the injection of gases from the open space  96  between the quartz window and plate  92 , into the open space  34  above the susceptor and wafer. For example, dopant gases such a phosphine can be injected through the holes  94  in the direction toward the wafer  26 , while a reactant gas such as silane (for the formation of amorphous or polysilicon) is injected by injector  38  toward the rotating wafer  26 , forming the bulk of the deposited film. The dopant gas is injected into the space  96  between the thermal plate  92  and the quartz window  98  via the passageway  128  of FIG. 5 of Torrex U.S. Pat. No. 5,551,985 and is more fully described therein. A dopant gas such as phosphine will not cause a deposit on the heated quartz window  98 , and will serve in the same way as an inert gas to prevent reactive gases such as silane from getting into space  96  from space  34  and reacting with the heated window to cause a deposit thereon. The spacing and size of the holes  94  in thermal plate  92  can be varied radially from the center to achieve a uniform concentration of the dopant in the bulk of the deposited film across the entire surface of the wafer  26 . 
     Referring now to FIG. 9, (an enlargened view of an area approximately that of section C of FIG.  2 ), for some processes it is desirable to bias the susceptor  100  (wafer holder) and/or the wafer  26 . For example, in the PECVD of expitaxial silicon, a positive DC bias can be applied to the susceptor or wafer to retard incident ions. In other cases, the susceptor  100  and/or the wafer  26  can be biased by putting a variable LC circuit  102  (illustrated schematically) between the susceptor  100  and a ground point  104 . In addition, still other processes can benefit by applying to the susceptor  100  RF energy of a different frequency than that of the RF energy applied to the main electrode (thermal plate  36 ). To permit such biasing and/or the application of RF energy to the susceptor, the susceptor  100  can be made of conductive material such as graphite. The susceptor  100  rests upon the rotating susceptor holder  106 , which is also electrically conductive. The DC bias or connections of resonant circuit  102 , and/or connections of RF energy can be coupled to the susceptor holder  106  via a commercial rotating electrical connector such as supplied by Mercotac, Inc., Carlsbad, Calif. or via the type of rotating RF feedthrough of FIG. 10, fully described in U.S. Pat. Ser. No. 60/071,571 entitled “Vertical Plasma Enhanced Process Apparatus and Method”, and incorporated herein by reference. 
     Referring now to FIG. 10, a rotating electrical connection apparatus  108 , as described in U.S. Ser. No. 60/071,571, includes an electrical connector  110 . 
     If the bias circuit  102  is used, one side  112  is connected to a center conductor  114  of the connector  110 , and the return side  116  is connected to a ground point  118 , connected to the connector outer conductor  120 . Alternatively, a bias supply  122  is connected as shown instead of the circuit  102 . The rotating connection  108  connects center conductor  114  to the electrically conductive shaft  123 , which is connected mechanically and electrically to the shaft  124  of FIG. 9 connected to the susceptor  100 . The details of a vertical rotating mechanical and electrical connection are given in U.S. Ser. No. 60/071,571. 
     Although a preferred embodiment of the present invention has been described above, it will be appreciated that certain alterations and modifications thereof will be apparent to those skilled in the art. It is therefore intended that the appended claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.