Abstract:
There are provided a method and apparatus for forming by chemical vapor deposition on large diameter (e.g., 300 mm) semiconductive wafers thin insulating layers of silicon oxide (SiO 2 ) having high uniformity from rim to rim across any diameter through the centers of the wafers. Such high degree of uniformity of the layers is obtained by directing separately a first reactive gas stream and a second reactive gas stream into close proximity to an exposed surface of a wafer to a be coated by the gasses with an insulating layer, the gas streams when mixed together reacting with each other to deposit an insulating layer on a wafer; forming a whirlpool-like swirling mixture of the first and second gas streams to thoroughly mix together the gasses thereof; forming a highly uniform mixture of the reactive gasses; and promptly flowing the mixture of reactive gasses over and upon the surface of the wafer. The apparatus also provides dual wafer processing chamber cavities.

Description:
FIELD OF THE INVENTION 
     This invention relates to the processing of large diameter semiconductive wafers into integrated circuits (and similar devices) wherein the wafers are put through a series of processing steps, one or more of which steps involve depositing on an exposed surface of each wafer a thin, uniform insulating layer or layers of silicon oxide (SiO 2 ) by means of chemical reactions of mixed gasses (well known in the art) to which the wafers are exposed. A commonly used process is known as sub-atmospheric chemical vapor deposition (SACVD)™. 
     BACKGROUND OF THE INVENTION 
     The processing of semiconductive wafers (e.g., thin discs of single-crystal silicon) into various integrated circuits (and similar devices) is well known in the art. To this end manufacturers offer throughout the world various makes and designs of equipment for this purpose. Because of the precision in construction and of operation required of such equipment, and the uniformity in processing necessary to obtain a high yield within specifications of devices being produced, the equipment is expensive to build and to operate. It is highly desirable therefore that the capital and operating costs of such equipment, for a given production throughput, be reduced as much as possible. 
     Recently semiconductive wafers with a diameter of 300 mm (0.3 meter) have become available to the manufacturers of integrated circuits. Compared to previously available 200 mm wafers (or even smaller ones), a 300 mm wafer offers a potential gain in productivity of more than two to one. Use of 300 mm wafers is thus highly attractive from a cost standpoint. 
     In a SACVD™ process step where silicon oxide is being deposited as insulation on a wafer, reactive gasses (well known in the art such as an organic vapor in helium or nitrogen, and ozone) are separately mixed together very close to where they will be used, then immediately introduced into a hermetically sealed chamber. The mixed gasses flow into a chamber at desired pressure and flow rate and are continuously exhausted from the chamber by a pump. A wafer within the chamber is held at a desired temperature (e.g., in the range of 200 to 800° C.) while the reactive gasses flow over an exposed surface of the wafer and in so doing deposit thereon a thin layer of silicon oxide insulation. Since the layer of silicon oxide being deposited onto the wafer should be as uniform as possible over the entire wafer surface from center to rim, the reactive gas stream should have its component gasses thoroughly mixed together before impinging on the wafer, and the mixed gasses should flow with perfect, or near perfect, uniformity over the entire area of the exposed surface of the wafer. 
     Non-uniformity in mixing and/or flow of the reactive gasses results in an insulating layer (SiO 2 ) being deposited unevenly onto the wafer. The resulting layer is thus thicker, or thinner, in some places than in others. When even small peaks and/or valleys begin to show up in an insulating layer the integrated circuits (or similar devices) which are being produced on the wafer can be rendered defective and thus become scrap. It becomes however, more and more difficult to achieve absolute uniformity in the mixing and flowing of larger volumes of the reactive gasses as the area of a wafer is made larger and larger (e.g., from a diameter of 200 mm to a diameter of 300 mm or greater). Thus, in practical effect, processing apparatus intended for 200 mm wafers cannot merely be scaled up in size so that it is big enough to handle 300 mm wafers and still produce integrated circuits having zero, or nearly zero defects. Substantial modifications in the apparatus are required. The present invention in one of its aspects provides an effective and economical solution to this problem of achieving uniform processing in chambers for large diameter wafer (e.g., 300 mm). 
     Previously, where wafer diameters were much smaller, there have been attempts to combine two wafer-processing chamber cavites into a single piece of equipment. Thus common usage could be made of certain elements of equipment such as housing, platform, gas supplies, control circuits, etc. The provision of dual-cavity chamber equipment would therefore offer increased production throughput along with substantial savings in capital cost. But problems of uniform processing, as discussed above have, among other reasons, precluded dual-cavity chamber equipment suitable for 300 mm wafers. The present invention in another of its aspects makes possible dual-cavity chamber apparatus capable of processing two such semiconductor wafers simultaneously. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention there is provided a method of mixing together two separate streams of gasses which react together, and then promptly flowing them into a wafer-processing chamber in such a way that the reacting gasses will result in the deposition of a highly uniform layer of silicon oxide insulation onto a large diameter semiconductive wafer within the chamber. The wafer lies upon a heating element in the chamber and is maintained at a suitable elevated temperature while the walls of the chamber are kept at a much lower temperature by coolant fluid pumped around and within the walls thereof. 
     To achieve immediate and intimate mixing of the gasses in the separate streams, one stream is injected tangentially into a mixing block having a small vertical cavity, and the other stream is injected tangentially into the cavity in the opposite direction. This results in a vigorous stirring and mixing of the two gas streams as they enter the cavity. The now-mixed reactive gasses continually flow out of the mixing cavity down through a plurality of perforated gas dispersion plates, which evenly spread the gasses into a highly uniform mixture flowing over an area slightly larger than the area of the wafer. The dispersion plates are specially configured and mounted with respect to each other, the mixing cavity, and a wafer in order to obtain gas flow over the wafer with the necessary high degree of uniformity. The reactive gasses flow down upon and over an exposed upper surface of the wafer and are exhausted from the bottom of the chamber by an evacuation pump. After an insulating layer of desired thickness (and virtually perfect uniformity) has been deposited on the wafer, the wafer is removed from the chamber by an automatic mechanism (well known in the art) and cleaning gas is pumped into the chamber. The cleaning gas passes through the mixing cavity, the gas dispersion plates, and down through and out of the chamber. Chemical residues left over from a previous processing step or steps of forming an insulating layer are thus removed from the passages and walls of the chamber, and the equipment is thus readied for another wafer-processing step. 
     Viewed from one process aspect, the present invention is directed to a method of forming a layer of uniform thickness on a surface of a semiconductive wafer by chemical vapor deposition from a mixture of reactive gasses. The method comprises the steps of: forming from separate streams of first and second gases a whirlpool-like swirling mixture of the gases in close proximity to the semiconductive wafer on which a layer of uniform thickness is to be deposited; and forming from the mixture of gases a uniform mixture of the reactive gasses; and flowing the mixed reactive gasses over and upon the surface of the wafer so as to form a layer of uniform thickness on the surface of the semiconductive wafer. 
     Viewed from one apparatus aspect, the present invention is directed to apparatus for forming a layer of uniform thickness on a surface of a semiconductive wafer from reactive gasses. The apparatus comprises a housing defining a chamber therein configured to contain a semiconductive wafer during processing and a mixing block defining a gas mixing cavity. A first entrance of the mixing cavity receives a first reactive gas in one direction tangentially into the mixing cavity. A second entrance of the mixing cavity receives a second reactive gas into the mixing chamber in the opposite direction tangentially into the mixing cavity such that gasses flowing through the first and second entrances swirl around and mix together in the cavity. An exit of the mixing chamber is in close proximity to the semiconductor wafer. In a preferred embodiment the apparatus further comprises a blocker plate and shower head comprising first and second dispersion plates with each dispersion plate defining a plurality of holes therethrough, the first dispersion plate not having a perforation located at the center thereof. The first dispersion plate has perforations thereof located in close proximity to the exit of the mixing cavity and is located in close proximity to the second dispersion plate. The second dispersion plate has outlets in close proximity to the surface of the semiconductive wafer such that there is formed a layer of uniform thickness on the surface of the semiconductive wafer. 
     The invention will be better understood from the following more detailed description taken with the accompanying drawings and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view, partially broken away, of a dual-cavity chamber, large diameter wafer-processing apparatus in accordance with the present invention; 
     FIG. 2 is an exploded perspective view of a gas mixing block in accordance with the present invention; 
     FIG. 3 is a schematic cross-sectional view of a center part of the mixing block showing a gas mixing cavity and separate reactive gas feed lines connected thereto; 
     FIG. 4 is a schematic cross-section of a portion of the apparatus of FIG. 1 showing a mixing block and a respective set of gas diffusion plates attached to the underside of the lid, and also showing a heater assembly with a wafer thereon for processing within a chamber of the apparatus; 
     FIG. 5 is a schematic plan view of the upper one of the gas diffusion plates of FIG. 4; 
     FIG. 6 is a schematic plane view of the lower one of the gas diffusion plates of FIG. 4; and 
     FIG. 7 is an enlarged vertical cross-section view of a hole through the center of the plate of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, there is shown apparatus  10  in accordance with the present invention. Apparatus  10  is useful for processing semiconductor wafers and comprises a chamber housing  12  (partially broken away) having a two cavity chamber (otherwise not shown), a lid assembly  14 , and a platform  16  (partially broken away) containing an automatic mechanism (not shown). This mechanism inserts wafers into the respective chamber through horizontal slots (not shown) in the rear of the housing  12  and removes the wafers after a processing step. Such mechanism is well known and is not further described herein. 
     The lid assembly  14  comprises a frame plate  18 , gas mixing blocks  20  and  22 , a cleaning gas supply line  24  and a similar cleaning gas supply line  26 , a reactive gas supply conduit  30  and a similar reactive gas supply conduit  32 . The lid assembly  14 , which is shown in closed position by solid lines and in open position by dashed lines, is hinged at  34  along its rear to the housing  12 , and when closed (down position) provides a hermetic seal for the two separate wafer-processing cavities of the chamber (not shown) within the housing  12 . 
     Each of the gas mixing blocks  20  and  22  is centered vertically over a respective chamber cavity within the housing  12 . The block  20  is connected at its top to the gas line  24  which supplies (when required) a flow of cleaning gas (e.g., ionized NF 3 ) to the mixing block  20  and thence to a respective cavity of the chamber (not shown) beneath the lid assembly  14 . A similar cleaning gas line  26  is connected to the top of the block  22 . The other ends of the gas lines  24  and  26  are connected together to a common supply line  36  which is connected to a source (not shown) of ionized gas. Gas conduits  30  and  32  are connected to mixing blocks  20  and  22 , respectively, and supply each with two separate streams of reactive gasses. Within each conduit  30  and  32  are a pair of separate gas lines (not shown) which apply the gasses unmixed to each block  20  and  22 , as will be explained shortly. The other ends of the conduits  30  and  32  (and their respective internal gas lines) are connected to gas sources (not shown). 
     Referring now to FIG. 2, there is shown an exploded view of one of the gas mixing blocks (i.e., the block  20 ). It is to be understood that the other block (i.e., the block  22 ) is substantially identical, though a mirror image. The block  20  comprises a top portion  37 , a lower portion  38  which has a lower, hollow stub  40 , an “O ” ring  42 , and a cylindrical member  44 . The latter will nest within the lower block portion  38  when the portions  37  and  38  are put together along a vertical axis  46 , as will be further explained shortly. 
     As seen in FIG. 2, the upper portion  37  of the mixing block  20  has a flat surface  50  which mates with an end of the cleaning gas line  24  (not shown here but shown in FIG.  1 ). An orifice  52  in the face  50  provides entrance to an internal passage (not shown here) in the upper portion  37  so that cleaning gas can enter into the mixing block, as indicated by an arrow  54 , and flow down along the axis  46  into and through a respective wafer-processing chamber of the apparatus  10 . The lower portion  38  of the mixing block  20  has an opening  55  which mates with an end of the gas conduit  30  (not shown here but shown in FIG.  1 ). A first orifice  56  and a second orifice  58  in the opening  55  of lower portion  38  provide entrances to separate internal passages (not shown here) in the lower portion  38 . The reactive gasses supplied by the separate gas lines in the gas conduit  30  flow into these orifices  56  and  58 , as indicated by the respective arrows  60  and  62 . 
     Still referring to FIG. 2, the member  44  has a vertical cylindrical wall  64 , and a central, gas mixing cavity  65  centered along the axis  46 . Near the lower end of the member  44 , on opposite sides thereof, are a first wall cutout  66  and a second wall cutout  68 . Each of the cutouts  66  and  68  provides a tangential opening through the wall  64  into the gas mixing cavity  65 . The respective streams of reactive gasses (indicated by the arrows  60  and  62  ) flow tangentially through these cutouts  66  and  68  and into the cavity  65  where the gasses are vigorously mixed together. The mixed gasses then immediately flow down through the hollow stub  40 , as indicated by an arrow  69 . 
     Referring now to FIG. 3, there is shown a schematic cross-section of the nested cylindrical member  44  of the now assembled mixing block  20 . The flow of one reactive gas stream, indicated by the arrow  60 , is along an internal passage  70 , which reverses the flow of this gas stream just before it tangentially flows through the cutout  66  (see also FIG. 2) in the member  44  into the cavity  65 . The flow of the other reactive gas stream indicated by the arrow  62  is along a short internal passage  72  and flows tangentially through the cutout  68  into the cavity  65  in a direction opposite to that of the first stream. This gives rise to a vigorous whirlpool-like mixing and stirring of the two gas streams, which upon mixing immediately flow down through the hollow stub  40  as indicated by the arrow  69  in FIG.  2 . 
     Referring now to FIG. 4, there is shown a schematic cross-section of a portion of the apparatus  10 . There is shown a portion of the lid frame  18 , the mixing block  20  and components thereof (see also FIG.  2 ), the gas line  24 , and the gas conduit  30  which have previously been described. Also shown in FIG. 4 are a first gas diffusion (blocker) plate  80  perforated with holes  81 , a second gas diffusion (face) plate (shower head),  82 , perforated with holes  83  and a center hole  85 , a heater assembly  84 , and a large diameter semiconductive wafer W positioned on a top face  86  of the heater assembly  84 . The blocker plate  80  and the face plate (shower head)  82  together serve as a means for flowing a highly uniform mixture of reactive gasses down onto a wafer W, as will be further explained hereinafter. It is to be noted that two face plates  82  are seen in dashed outline (lid open) in FIG. 1, there being a respective face plate  82  (and shower head) for each of the two chamber cavities (not shown) within the apparatus  10 . 
     As seen in FIG. 4 the blocker plate  80  and the face plate  82  are attached by suitable means not otherwise described to an underside of the lid frame  18 , and are centered on the vertical axis  46 . The wafer W is automatically centered on this axis  46  by a tappered shoulder  88  around the rim of the heater face  86  which abuts the rim of the wafer W. The heater assembly  84  is in an “up” position so that the wafer W is properly held closely beneath the face plate  82  during processing. After a given processing step, the heater assembly is moved to a “down” position, as indicated by an arrow  89 , (by a mechanism not shown) so that the wafer W may be removed from the chamber cavity and another wafer inserted. The heater assembly  84  has three or more lift fingers  90  (only two actually shown) underneath the wafer W which are raised up from the “down” position shown (by a mechanism not shown) to lift the wafer W above the heater face  86  and permit it to be easily removed from the chamber cavity in a way previously mentioned. 
     The reactive gasses, after being mixed together in the cavity  65 , flow downward as indicated by the arrow  69  and are initially spread by the blocker plate  80  and its holes  81  over an area defined by the wafer W. The face plate  82 , which has a much larger number of holes  83  through it than does the blocker plate  80 , then further spreads the reactive gasses into a uniform mixture flowing down upon an upper, exposed face of the wafer W. The flowing reactive gasses are exhausted from a lower part of a respective chamber by a pump (not shown). The walls of each chamber, and the lid assembly  14  and lid frame  18 , are maintained at a much lower temperature than that of the heater assembly  84  and the wafer W by the flow of coolant liquid through pipes and passages not shown. 
     Referring now to FIG. 5, there is shown a schematic plan view of the blocker plate  80  and the holes  81  through it (see also FIG.  4 ). The holes  81  are schematically illustrated here but in fact they are arranged on concentric circles in a specific pattern, provided in accordance with an aspect of the invention. The blocker plate  80  has a zero (“0”) index indicated at  92  to which the holes  81  are referenced. The following Chart 1 gives the radial and angular positions, and the numbers of holes  81  in each circle in the blocker plate  80  in a specific embodiment of apparatus of the invention designed for processing 300 mm wafers W. The holes  81  are approximately 28 mils (thousandths of an inch) in diameter extending through the plate  80 , which is about 0.3 inch thick. There are approximately 1310 of the holes  81  in total, but there is no hole in the center of the blocker plate  80 . In an illustrative embodiment the holes are arranged evenly spaced along 28 concentric circles, the diameters of which are given in inches. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Hole Pattern Chart 1 
               
             
          
           
               
                   
                   
                   
                   
                 Angle Offset Counter 
               
               
                   
                   
                 Diameter 
                   
                 Clockwise in degrees 
               
               
                   
                 Circle No. 
                 (inches) 
                 # of Holes 
                 from 0 Index 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 0.750 
                 8 
                 22.50 
               
               
                   
                 2 
                 1.250 
                 8 
                 0 
               
               
                   
                 3 
                 1.500 
                 7 
                 6.00 
               
               
                   
                 4 
                 1.750 
                 8 
                 0 
               
               
                   
                 5 
                 2.000 
                 9 
                 10.00 
               
               
                   
                 6 
                 2.250 
                 11 
                 0 
               
               
                   
                 7 
                 2.500 
                 11 
                 16.50 
               
               
                   
                 8 
                 2.750 
                 14 
                 6.00 
               
               
                   
                 9 
                 3.050 
                 17 
                 10.50 
               
               
                   
                 10 
                 3.350 
                 19 
                 0 
               
               
                   
                 11 
                 3.650 
                 20 
                 9.00 
               
               
                   
                 12 
                 3.950 
                 26 
                 0 
               
               
                   
                 13 
                 4.350 
                 33 
                 5.50 
               
               
                   
                 14 
                 4.750 
                 36 
                 4.00 
               
               
                   
                 15 
                 5.150 
                 38 
                 5.00 
               
               
                   
                 16 
                 5.550 
                 44 
                 0 
               
               
                   
                 17 
                 6.000 
                 51 
                 2.00 
               
               
                   
                 18 
                 6.450 
                 54 
                 0 
               
               
                   
                 19 
                 6.900 
                 58 
                 3.00 
               
               
                   
                 20 
                 7.350 
                 62 
                 0 
               
               
                   
                 21 
                 7.850 
                 73 
                 5.00 
               
               
                   
                 22 
                 8.350 
                 78 
                 0 
               
               
                   
                 23 
                 8.850 
                 82 
                 2.00 
               
               
                   
                 24 
                 9.350 
                 87 
                 0 
               
               
                   
                 25 
                 9.900 
                 101 
                 3.00 
               
               
                   
                 26 
                 10.450 
                 107 
                 0 
               
               
                   
                 27 
                 11.000 
                 124 
                 5.00 
               
               
                   
                 28 
                 11.550 
                 130 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     Referring now to FIG. 6, there is shown a schematic plan view of the face plate  82  and the holes  83 , and the center hole  85  there through (see also FIG.  4 ). The holes  83  (and hole  85  ) are schematically illustrated here but in fact they are arranged on concentric circles in a specific pattern, also provided in accordance with an aspect of the invention. The face plate  82  has a zero (“0”) index indicated at  93  to which the holes  83  are referenced. This index  93  and the index  92  of the blocker plate  80  are aligned with each other when the face plate  82  and the blocker plate  80  are assembled together underneath the lid frame  18  (see FIG.  4 ). The following Chart 2 gives the radial and angular positions, and the numbers of holes  83  in each circle in the face plate  82  in the specific embodiment of apparatus  10  designed for processing 300 mm wafers W. In an illustrative embodiment the holes  83  are approximately 28 mils (thousandths of an inch) in diameter through the plate  82 , which is about 0.6 inch thick. There are approximately 7350 of the holes  83  in total. The center hole  85  (of smaller diameter) passes through the center of the face plate  82  and is aligned with the vertical axis  46  (see FIG.  4 ). The holes  83  and the center hole  85  are arranged evenly spaced along 50 concentric circles (including the center), the diameters of which are given in inches. The holes  83  and the center hole  85  will be described in greater detail hereinafter. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Hole Pattern Chart 2 
               
             
          
           
               
                   
                   
                   
                   
                 Angle Offset Counter 
               
               
                   
                   
                 Diameter 
                   
                 Clockwise in degrees 
               
               
                   
                 Circle No. 
                 (inches) 
                 # of Holes 
                 from 0 Index 
               
               
                   
                   
               
             
          
           
               
                   
                 1 
                 center 
                 1 
                 0 
               
               
                   
                 2 
                 0.250 
                 6 
                 0 
               
               
                   
                 3 
                 0.500 
                 12 
                 5 
               
               
                   
                 4 
                 0.750 
                 18 
                 0 
               
               
                   
                 5 
                 1.000 
                 24 
                 10 
               
               
                   
                 6 
                 1.250 
                 30 
                 0 
               
               
                   
                 7 
                 1.500 
                 36 
                 3 
               
               
                   
                 8 
                 1.750 
                 42 
                 0 
               
               
                   
                 9 
                 2.000 
                 48 
                 1 
               
               
                   
                 10 
                 2.250 
                 54 
                 0 
               
               
                   
                 11 
                 2.500 
                 60 
                 1 
               
               
                   
                 12 
                 2.750 
                 66 
                 0 
               
               
                   
                 11 
                 2.500 
                 60 
                 1 
               
               
                   
                 13 
                 3.000 
                 72 
                 0 
               
               
                   
                 14 
                 3.250 
                 78 
                 1 
               
               
                   
                 15 
                 3.500 
                 84 
                 1 
               
               
                   
                 16 
                 3.750 
                 90 
                 0 
               
               
                   
                 17 
                 4.000 
                 95 
                 1 
               
               
                   
                 18 
                 4.250 
                 102 
                 0 
               
               
                   
                 19 
                 4.500 
                 108 
                 2 
               
               
                   
                 20 
                 4.750 
                 114 
                 0 
               
               
                   
                 21 
                 5.000 
                 120 
                 1 
               
               
                   
                 22 
                 5.250 
                 126 
                 0 
               
               
                   
                 23 
                 5.500 
                 132 
                 1 
               
               
                   
                 24 
                 5.750 
                 138 
                 0 
               
               
                   
                 25 
                 6.000 
                 144 
                 2 
               
               
                   
                 26 
                 6.250 
                 150 
                 0 
               
               
                   
                 27 
                 6.500 
                 156 
                 1 
               
               
                   
                 28 
                 6.750 
                 162 
                 0 
               
               
                   
                 29 
                 7.000 
                 168 
                 2 
               
               
                   
                 30 
                 7.250 
                 174 
                 0 
               
               
                   
                 31 
                 7.500 
                 180 
                 1 
               
               
                   
                 32 
                 7.750 
                 186 
                 0 
               
               
                   
                 33 
                 8.000 
                 192 
                 1 
               
               
                   
                 34 
                 8.250 
                 198 
                 0 
               
               
                   
                 35 
                 8.500 
                 204 
                 2 
               
               
                   
                 36 
                 8.750 
                 210 
                 0 
               
               
                   
                 37 
                 9.000 
                 216 
                 0 
               
               
                   
                 38 
                 9.250 
                 222 
                 0 
               
               
                   
                 39 
                 9.500 
                 228 
                 1 
               
               
                   
                 40 
                 9.750 
                 234 
                 0 
               
               
                   
                 41 
                 10.000 
                 240 
                 2 
               
               
                   
                 42 
                 10.250 
                 246 
                 0 
               
               
                   
                 43 
                 10.500 
                 252 
                 1 
               
               
                   
                 44 
                 10.750 
                 255 
                 0 
               
               
                   
                 45 
                 11.000 
                 264 
                 2 
               
               
                   
                 46 
                 11.250 
                 270 
                 0 
               
               
                   
                 47 
                 11.500 
                 276 
                 1 
               
               
                   
                 48 
                 11.750 
                 282 
                 0 
               
               
                   
                 49 
                 12.000 
                 288 
                 1 
               
               
                   
                 50 
                 12.250 
                 294 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     It should be noted that the last two hole circles, numbers 49 and 50, in the Chart 2 extend somewhat beyond the rim of a 300 mm diameter wafer W. This insures uniform flow of reactive gasses even beyond the rim of such wafers and is important in the depositing of highly uniform insulating layers by the reactive gasses. By way of example, a wafer W is held during processing about 50 mils below the bottom of the face plate  82 . 
     Referring now to FIG. 7, there are shown in an enlarged vertical cross-section through a portion of the center of the face plate  82  details of the center hole  85 . An upper part of the hole  85  has a first diameter bore at  95  extending through most but not all of the plate  82 . The lower part of the hole  85  has a second diameter bore  96  through a remaining thickness of the plate  82 . The first bore diameter is larger than the second bore diameter. The axis of the hole  85  coincides with the vertical axis  46 . The smaller bore  96  of the hole  85  lies in the lower part of the face plate  82  just above a wafer W (see FIG.  4 ). By way of example, the smaller bore  96  of the center hole  85  has a diameter of about 23 mils, and the larger bore  95  a diameter of about twice this. The smaller bore  96  extends through about 0.1 inch thickness of the plate  82 , the total thickness of which is about 0.6 inch. The holes  83  in the face plate  82  are closely similar in shape to that of the center hole  85 , but the diameter of the smaller bore of the holes  83  is slightly larger (e.g., about 28 mils) than that of the bore  96  of the center hole  85 . This dual-diameter somewhat funnel shape of the holes  83  and the center hole  85  insures high precision in the shape and exact locations of the holes, and this precision in turn contributes to obtaining virtually perfect uniformity in the insulating layers deposited on wafers W. Making the diameter of the smaller bore  96  of the center hole  85  slightly smaller than the corresponding diameter of the smaller bores of the holes  83  (e.g., 23 mils versus 28 mils) further contributes to obtaining uniformity of the insulating layers across the diameters of the wafers W. 
     The above description is intended in illustration and not in limitation of the invention. Various minor changes in the method and apparatus described may occur to those skilled in the art, and can be made without departing from the spirit or scope of the invention as set forth in the accompanying claims. For example, the invention is not limited to the processing of two wafers W at the same time or solely to the processing of wafers of 300 mm diameter. Also the exact numbers, sizes and shapes of the holes through the gas dispersion plates can be modified slightly to accommodate minor variations in wafer processing conditions. Furthermore, the exact sizes and shapes of the gas mixing blocks can be changed somewhat to accommodate such variations in processing conditions.