Abstract:
A gas shield assembly for protecting an exposed surface of an injector in a chemical vapor deposition system. The shield assembly includes a back wall which supports perforated sheets to define a plenum. The sheets are held in place by endcaps. A conduit delivers gas to the plenum. The conduit includes a stretch extending along the back wall and has a bend adjacent the ends of the back wall. The conduit includes perforations along the stretch and at the bends to provide substantially uniform flow of gas into the plenum and the ends.

Description:
RELATED APPLICATIONS 
     This application claims priority to Provisional Application Serial No. 60/135,362 filed May 21, 1999, the entire disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to a chemical vapor deposition apparatus and, more particularly, to a protective gas shield apparatus for use during chemical vapor deposition processing. 
     BACKGROUND OF THE INVENTION 
     Chemical vapor deposition (CVD) systems are used to form a thin, uniform layer or film on a substrate such as a semiconductor wafer. During CVD processing, the substrate is exposed to one or more chemical vapors such as silane, phosphane, diborane and the like, and gaseous substances such as oxygen. The gases mix and interact with the other gases and/or the surface of the substrate to produce the desired film. The desired reactions generally occur at elevated temperatures, for example 300-550° C., with the substrate and chamber being heated to the appropriate temperature for a selected process. In many applications including semiconductor processing, film characteristics such as purity and thickness uniformity must meet high quality standards. The substrate is positioned in a clean, isolated reaction chamber to obtain high quality films. CVD systems typically employ injectors which deliver the gaseous substances directly to the surface of the substrate. An exhaust system removes waste products such as unreacted gases and powders formed during the reaction from the reaction chamber. The reaction chamber must be periodically cleaned to remove films which are deposited on the exposed surfaces of the chamber over time, eliminating sources of particulate contamination which may become embedded in the film or degrade the film thickness uniformity. 
     The injection ports are typically positioned less than one inch from the surface of the substrate, for example ⅛ inch to ½ inch. With this limited clearance between the injector and the substrate surface, the surfaces of the injector will become coated with the material produced during the reactions. To reduce the amount of build-up in this area, some CVD systems include protective shields which are positioned in front of the injectors and exhaust port. An inert gas such as nitrogen is delivered through a perforated screen carried by the shield to slow the rate at which materials accumulate on shield. The high temperatures in the CVD reaction chamber create thermal stresses in the perforated screen which may cause the screen to buckle after a period of time. The buckling disrupts the uniform flow of nitrogen through the screen as well as disturbing gas flow within the chamber, leaving portions of the screen unprotected against the accumulation of deposition materials. The ability of the screen to deliver nitrogen to the reaction chamber is further reduced as the screen becomes coated with deposition materials, requiring removal and cleaning or replacement of the shield. 
     A number of protective shield configurations have been disclosed in U.S. Pat. Nos. 5,849,088 and 5,944,900, the entire disclosures of which are hereby incorporated by reference herein. The descriptions provide specific design details that enhance the performance of the shields. In particular, useful life of the shield becomes extended through the use of these inventive ideas. Several of the ideas have been shown to improve the film uniformity. 
     While the protective shields described in these patents provide improved film uniformity, additional improvements are desired. For example, the protective shields as described include inert gas lines that enter the shield plenum near the end of the shield and then run down the length of the shield to the other end. The CVD system configuration typically requires the inert gas line to run down toward the shield, bend and then run along its length, and then bend back upward and exit from the assembly. Typically, the gas line has a series of holes which are usually, but not necessarily, evenly spaced along the length of the shield between the bends of the tube. The holes are not positioned in the bend area. While this design has proven to be superior to earlier designs, the bends in the tube are nonzero in lengths, and the first hole is placed where the tube becomes straight again. This results in low gas flow at the ends of the shield, and increased undesirable buildup of material in this region. 
     As mentioned above, high temperatures in the CVD reaction chamber create thermal stresses in the perforated screen. Over time this causes the screen to buckle and/or break. Prior art designs have addressed this problem. For example, U.S. Pat. No. 5,849,088 describes a protective shield that provides for expansion and contraction of the perforated front wall. While this is a significant advance, the assembly is relatively complex and can be expensive to manufacture. 
     Accordingly, it is desirable to provide an improved protective gas shield apparatus which minimizes build-up of materials at the ends, and which permits expansion and contraction of the perforated front wall in a less costly assembly. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is a object of the present invention to provide a protective gas shield apparatus for use during chemical vapor deposition processing, and in particular to provide a gas shield assembly for protecting the exposed surfaces of a gas injector. 
     It is a further object of the present invention to provide a gas shield assembly which provides substantially uniform inert gas flow along the length of the shield. 
     It is a further object of the present invention to provide a simplified, less costly gas shield assembly which permits expansion and contraction of the perforated front wall. 
     The foregoing and other objects of the invention are achieved by a protective gas shield comprising a shield body having a back wall with flanges extending from the sides and ends of said body with a perforated sheet supported spaced from said back wall by said flanges whereby the back wall and perforated sheet form a plenum. At least one conduit is spaced inwardly from the sides of said shield body extending into said plenum for supplying an inert gas to said plenum whereby the gas flows outwardly through said perforated sheet. A retainer engages each end of said body and the edge of said perforated sheet to hold the perforated sheet on the shield body, while permitting expansion and contraction of the perforated sheet. 
     It is another object of the present invention to provide a shield assembly with removable and replaceable perforated front walls. 
     It is still another object of the present invention to provide a shield assembly which has prolonged life and which can be efficiently and economically manufactured and maintained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when read in conjunction with the drawings of which: 
     FIG. 1 is a schematic view of a processing module or chamber of a CVD system illustrating the gas shield of the present invention. 
     FIG. 2 is a sectional view of the gas shield assembly of the present invention. 
     FIG. 3 is a perspective view of the shield apparatus of the present invention. 
     FIG. 4 is a perspective view of one half of the back wall of the shield apparatus showing the perforated gas delivery tube. 
     FIG. 5 is a perspective view of one perforated sheet section. 
     FIG. 6 shows the perforated sheet retaining cap. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 schematically illustrates a processing module or chamber  11  which is positioned along the path of a CVD processing system such as that described in U.S. Pat. No. 5,849,088 which is incorporated herein by reference in its entirety. The processing module  11  includes an injector  12  such as that shown in Pat. Nos. 5,683,516 or 6,022,414 for injecting chemical reagents or other gaseous substances into a reaction chamber or process area  13  directly below the injector  12 . Wafers  14  to be processed are moved through the reaction chamber by a conveyor belt  17 . The chamber may be heated by a heater  18  placed adjacent the reaction chamber. 
     As the wafer or substrate  14  is moved past the injector, the injected gases react with each other and with the upper surface of the substrate to form a thin uniform layer or film on the surface of the substrate. The waste products including reaction by-products and unreacted gases are exhausted through exhaust vents  21  which are coupled to an exhaust system. 
     As shown in FIG. 1, a protective shield assembly  22  is positioned in front of the exposed surface of the injector  12  and the inlets to the exhaust passage or exhaust vents  21 . The shield separates the injector and exhaust passageways from the reactions occurring within the process chamber  13 , preventing deposition of reaction by-products and other contaminants on the surface of the injector and passageways. The shield assembly also inhibits the accumulation of such by-products and contaminants on the surface of the shield so that most of the by-products are expelled from the process chamber into the exhaust system. 
     As shown particularly in FIGS. 2-5, shield assembly  22  includes a back wall  26  which is positioned adjacent the front face of injector  12  and wraps around the outer edges of the injectors. The back wall includes a slot  28  which provides an inlet port for the flow of reagents into the processing chamber. The size, shape and configuration of the inlet port is dependent upon the design of the injector and its ports. The back wall of the shield is provided with flanges  29  at the slot  28  which define a passage  31 . The side and end edges of the back wall are provided with end and side flanges  32  and  33 . A perforated sheet  34  is positioned on top of the flanges  29 ,  32  and  33 . Preferably, the perforated sheet is in two parts, one part of which is shown in FIG. 5. A cut-out  37  of the two parts cooperates to define a slot for the inlet port  31 . A cut-out  38  accommodates the inert gas inlet conduits to be described. The two parts are placed over the flanges, and are retained by endcaps  41 , as shown in FIG.  6 . The endcaps have the same configuration as the ends of the back wall flanges, and include flanges  42  which engage the back wall and clamp the edges of the perforated sheets to the back wall flanges. The endcaps are retained by screws  43  which mount support  44  to the shield assembly as shown in FIG.  1 . Clamps  46  resiliently clamp the edges of the perforated sheet slot to the inlet port flanges  29 . Differences in thermal expansion of the perforated sheet and the back wall are accommodated by sliding movement of the perforated sheet with respect to the back wall. The space between the back wall and the perforated sheet forms a plenum. Inert gas delivered into the plenum flows through the perforations in the perforated sheets to inhibit deposit of reaction by-products onto the perforated sheet. 
     Referring to FIG. 4, the inert gas is delivered by conduits or pipes  47  located at each edge of the back wall. The pipes have a stretch  48  which extends lengthwise along and secured to the back wall as for example by welding. The stretch  48  is between bends  49  with the ends of the conduits extending upwardly to receive the inert gas from a gas supply. The stretch  48  is perforated  51  to permit the inert gas to flow uniformly along the plenum. In accordance with one feature of the present invention, the bends are also perforated  52  to allow inert gas to flow towards the ends of the plenum to inhibit deposits of by-products near the ends of the shield. 
     Support members  54  are welded on the back wall along the curved portion to prevent the perforated sheet from buckling. The support members extend between the ends of the shield body. A plurality of holes or slots  55  are formed in the support to permit the flow of inert gas along the plenum. It should be understood that the position of the support members, as well as the number of support members, may vary. 
     The shield assembly also includes shield bodies  56  supported outwardly from the upturned edges of the shield assembly  22  by the support  44 . The shield bodies  56  and shield assembly  22  cooperate to form the entrance to the exhaust passage  21 . The shield bodies  56  include a back  61  and a spaced perforated sheet  62  to form plenums therebetween. Inert gas is supplied to the plenums by pipes or conduits  64 . 
     As is apparent from the foregoing, the present invention provides a simple, relatively inexpensive shield. The shield can accommodate changing temperature conditions without severe buckling or breaking of the perforated sheet. The shield assembly can be easily disassembled for cleaning and/or replacement of the perforated sheets. The inert gas flows evenly along the length and ends of the shield assembly. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best use the invention, and various embodiments with various modifications as are suited to the particular use are contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.