Patent Publication Number: US-8122910-B2

Title: Flexible manifold for integrated gas system gas panels

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and claims priority under 35 U.S.C. §120 to U.S. patent application Ser. No. 11/761,326 filed on Jun. 11, 2007 and entitled “Flexible Manifold For Integrated Gas System Gas Panels” which is incorporated by reference herein for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to gas delivery systems. More particularly, the present invention relates to a flexible manifold for gas delivery systems. Even more particularly, the present invention relates to a flexible manifold for integrated gas system (IGS) gas delivery systems. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  illustrates a conventional semiconductor etch processing system  100 . The conventional semiconductor etch processing system  100  includes a gas source  102 , a gas delivery panel  104  connected to the gas source  102  via gas supply lines  106 , and a processing chamber  108  connected to the gas delivery system  104 . The gas delivery panel  104  further includes gas sticks  110  coupled to the gas supply lines  106 . The gas sticks  110  are a series of gas distribution and control components such as a mass flow controller, one or more pressure transducers and/or regulators, a heater, one or more filters or purifiers, and shutoff valves. The components used and their particular arrangement in a gas stick can vary depending upon their design and application, with many component arrangements being known in the art. In a typical semiconductor processing arrangement, over seventeen gases are connected to the chamber via gas supply lines  106 , gas distribution components and substrates, mixing manifold. These are attached to a base plate forming a complete system known as “gas panel” or “gas box”. 
     The conventional semiconductor etch processing system  100  depends on the use of several hazardous and non-hazardous processing gases and carefully measured delivery of over seventeen gases from the gas source  102  to the processing plasma chamber  108  via the gas supply lines  106  in a synchronized mode. Such systems usually require gas delivery panel  104  for coupling high purity gases for semiconductor etch processing systems or other thin film coating processes. 
     In semiconductor manufacturing, processes have become increasingly intolerant of particle contamination as the dimensions of semiconductor devices decrease and there is less room to accommodate more components. One source for particle contamination is the gas stick itself that delivers gases from the source of high purity gases to the semiconductor processing chamber where such particle contaminants commonly get deposited onto the semiconductor devices that are being processed. Another source for particle contamination is the exposure of components in a gas delivery system to room air during maintenance and repair of individual gas delivery components. 
     One approach to eliminating connection parts, such as tubing and couplers, facilitating maintenance of the components of the gas stick, and to reduce contamination is to “down mount” the components on multiple manifold blocks and then onto a base plate. These are also known as IGS or surface mounted gas delivery systems. However, each component of a gas stick typically comprises highly machined parts, making each component relatively expensive to manufacture and replace. When a component fails, the entire component is replaced even though in most instances the failure is mechanical (and in the case of a mass flow sensor, it is the sensor that usually fails). Each component is typically constructed with a mounting block, which in turn is made with multiple machine operations, making the component expensive. Thus, while down mounting the component parts on multiple fixing blocks solves one problem, it still is relatively expensive to replace defective parts. 
     Furthermore, gas panels are typically manufactured with three or more gas sticks since manufacturing less gas sticks is expensive, difficult to mount in the gas cabinet, and uses additional parts that may not be necessary to use. Thus, a user has no option other than having a set number of gas sticks. The minimum number of gas sticks is 6 and the standard number of gas sticks is 9. Gas panels having 12 or 16 sticks are possible. However, if a user has a 9 gas stick gas panel installed and wants to add one or two additional gas sticks, the user would be required to buy a gas panel having a minimum of at least three gas sticks. There is no efficient method of connecting a single gas stick to the existing gas panels without removing the entire gas panel, risking contamination, and/or using additional manual effort and time to remove and reinstall the gas delivery components. 
     Alternatively, should the user have a 9 gas stick gas panel installed and later only needs to use 7 gas sticks, 2 gas sticks would not be used on the gas panel. This would result in excess parts of the gas panel that is not used and removal of the excess gas sticks from the gas panel would not be possible. This situation creates “dead-leg”, a section of conduit or manifold through which gas does not flow. Dead-leg is considered to be a source of contamination. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A flexible gas delivery apparatus having a gas panel with a first extension block having a first section and second section, the first section positioned between a mixing valve and the substrate having an exit port in fluid communication with a pump/purge manifold, and a second extension block having a first section and a second section, the first section positioned between a purge valve and the substrate having a discharge port in fluid communication with a mixing manifold, wherein the second portion of the first and second extension blocks extend outwardly from the gas panel. 
     In another embodiment, the flexible gas delivery apparatus may have a manifold extension block with a plurality of input ports, each plurality of input ports in fluid communication with the substrate, a horizontal common manifold pathway, a plurality of output ports, each plurality of output ports in fluid communication with the common manifold pathway; and a plurality of vertical tube ports configured to receive a tube, each of the plurality of vertical tube ports positioned substantially between each of the plurality of input ports, each of the vertical tube ports in fluid communication with the common manifold pathway. 
     These and other features will be presented in more detail in the following detailed description of the invention and the associated figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments and, together with the detailed description, serve to explain the principles and implementations of the invention. 
       In the drawings: 
         FIG. 1  illustrates a conventional semiconductor etch processing system. 
         FIG. 2  illustrates an exemplary gas stick. 
         FIGS. 3A ,  3 B,  3 C and  3 D illustrate exemplary manifold blocks. 
         FIGS. 4A-4D  illustrate an embodiment of manifold extension block. 
         FIGS. 5A and 5B  illustrates yet another embodiment of an extension block. 
         FIG. 6  illustrates the extension blocks in use with an IGS gas delivery system. 
         FIGS. 7A and 7B  illustrate yet another embodiment of an extension block. 
         FIG. 8  illustrates a side view of one end of an exemplary gas panel. 
         FIG. 9  is a schematic view of an exemplary gas feed device for semiconductor processing. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
     The present invention provides for a flexible gas delivery manifold.  FIG. 2  illustrates an exemplary gas stick. Although illustrated with certain components, the specific components are not intended to be limiting as different components may be used and/or less or more components may be used to form the gas stick. Additionally, although described with a single gas stick, the number of gas sticks is not intended to be limiting. As discussed above, a plurality of gas sticks, form a gas box or panel. In an embodiment, the valve on the components is an integrated surface mount valve. In general, an integrated surface mount component is a gas control component (e.g., valve, filter, etc.) that is connected to other gas control components through channels on a substrate assembly, upon which the gas control components are mounted. This is in contrast to gas control components that are generally attached through bulky conduits with VCR attachments (vacuum coupled ring). 
     The gas stick  200  may have a gas stick input port  202  to input a supply gas. A manual valve  204  may be used for carrying out the supply or isolation of the supply of supply gas. The manual valve  204  may also have a lockout/tagout device  206  above it. Worker safety regulations often mandate that plasma processing manufacturing equipment include activation prevention capability, such as a lockout/tagout mechanism. Generally a lockout is a device that uses positive means such as a lock, either key or combination type, to hold an energy-isolating device in a safe position. A tagout device is generally any prominent warning device, such as a tag and a means of attachment that can be securely fastened to energy-isolating device in accordance with an established procedure. 
     A regulator  208  may be used to regulate the gas pressure or the supply gas and a pressure gauge  210  may be used to monitor the pressure of the supply gas. In one embodiment, the pressure may be preset and not need to be regulated. In another embodiment, a pressure transducer (not illustrated) having a display to display the pressure may be used. The pressure transducer may be positioned next to the regulator  208 . A filter  212  may be used to remove impurities in the supply gas. A primary shut-off valve  214  may be used to prevent any corrosive supply gasses from remaining in the gas stick. The primary shut-off valve  214  may be two-port valve having an automatic pneumatically operated valve assembly that causes the valve to become deactivated (closed), which in turn effectively stops plasma gas flow within the gas stick. Once deactivated, a non-corrosive purge gas, such as nitrogen, may be used to purge the gas stick. The purge valve  216  may have three-ports to provide for the purge process—an entrance port, an exit port and a discharge port. 
     Adjacent the purge valve  216  may be a mass flow controller (“MFC”)  218 . The MFC  218  accurately measures the flow rate of the supply gas. Positioning the purge valve  216  next to the MFC  218  allows a user to purge any corrosive supply gasses in the MFC  218 . A mixing valve  220  next to the MFC  218  may be used to control the amount of supply gas to be mixed with other supply gasses on the gas panel. 
     Each component of the gas stick may be positioned above a manifold block. A plurality of manifold blocks form a substrate  222 , a layer of manifold blocks that creates the flow path of gasses through the gas stick  200 . The gas delivery components may be positioned on the manifold blocks by any known means such as with a pressure fitting sealant (eg., C-seal) and the like. 
       FIGS. 3A ,  3 B,  3 C and  3 D illustrate exemplary manifold blocks. Referring to  FIG. 3A , manifold block  300  may have component ports  302  configured to receive gas stick components. The manifold block  300  may be coupled to a base plate via apertures  304   a ,  304   b ,  304   c ,  304   d . A plurality of manifold blocks may be removably coupled to each other via manifold connector  306  to form a substrate. Thus, although illustrated with one manifold block, the number is not intended to be limiting as any number of manifold blocks may be used as necessary. The manifold block may be formed from a stainless steel material such as  316 L vacuum induction melting or vacuum arc re-melting stainless steel. 
       FIG. 3B  illustrates a cut-away view of  FIG. 3A . Gas passageways may be provided in each of the manifold blocks to fluidly connect each of the gas stick components. The gas passageway may be a horizontal passageway. However, as illustrated below, the passageway may be formed with a “V”, “U” or any other shape. 
       FIG. 3C  illustrates another exemplary manifold block. Manifold assembly  2  may be mounted on a base plate  7  by combining individual manifold blocks to form an operative system. Each of the three manifold blocks  4  may be formed from a stainless steel material such as  316 L vacuum induction melting or vacuum arc re-melting stainless steel. Although illustrated with three manifold blocks, the number is not intended to be limiting as any number of manifold blocks may be used as necessary. Gas passageways may be provided in each of the manifold blocks and as shown in  FIG. 3C , the passageways may have access ports positioned on a common upper surface  38 . For ease of description, the flow path will be considered to extend from left to right although it could just as easily be reversed and, accordingly, the entrance port  6  is shown on the upper surface  38  along with an exit port  8 . The entrance port  6  may be partially on an upper flange member  10  that is cantilevered from a central manifold body portion  12 . A lower flange member  14  may be dimensioned to have a complimentary configuration to match the upper flange member  10  of the immediately adjacent manifold block. Each of the gas delivery components illustrated in  FIG. 2  may be bridged across an entrance port  6  on one manifold block  4  to an exit port  8  on the adjacent manifold block  4 . While not shown in any of the drawings of the present application, the gas stick may be mounted in a sealed housing for safety purposes and to control the purging of any leaking gases from the gas delivery system. 
     Referring to  FIG. 3D , V-shaped gas passages between an entrance port  6  and an exit port  8  may be seen in the phantom lines. Although illustrated as V-shaped, any shape may be used such as U-shaped passages as illustrated below. A pair of appropriate boreholes  16  and  18  on an upper flange member  10  may have a lower beveled surface to enable self-aligning of any bolt fasteners. The respective bores  16  and  18  appropriately align with threaded apertures  20  and  22  on the lower flange member  14  of an adjacent manifold block. 
     A pair of threaded bores  24  and  26  and  28  and  29  may be provided on each perimeter side to enable the fastening of a flange on a gas stick component. Finally, a pair of recesses  30  and  32  on opposite sides may also be provided to accommodate any protrusion of screws for fastening a pressure sealer to the bottom of a gas stick component. The bore opening  16  and  18  may have sufficient depth that when an appropriate fastener is sealed, there may be sufficient vertical room above a fastener to accommodate the protruding head of any fastening screw or bolt associated with the pressure sealer. 
     Again, as seen in  FIGS. 3C and 3D , apertures  34  and  36  may be provided on either side of the lower flange members  14  to thereby accommodate a fastener  31  for attachment to the base plate  7 . The upper flange member  10  may be appropriately tapered or cut to facilitate access, for example, by an Allen wrench, to any such fasteners  31 . 
     Thus, a plurality of manifold blocks  4  may have specific upper flange members  10  and lower flange members  14  that are cantilevered from a central manifold body portion  12  to enable the individual manifold blocks to be interconnected to accommodate a specific fluid distribution system. Each of the manifold blocks  4  may have a fluid passageway with an entrance port  6  and an exit port  8  that access a common upper surface  38 . The dimensions of the upper flange member  10  and the lower flange member  14  may be such that they extend across each other and thereby provide means for removably interlocking a pair of adjacent manifold blocks  4  to operatively permit their respective fluid passageways to be positioned for interconnection. 
     Referring back to  FIG. 2 , as stated above, the lower surface of the substrate  222  may be removably coupled to a base plate  7 . The substrate  222  may have a plurality of fluid passageways  228   a ,  228   b  in fluid communication with a pump/purge manifold  224  and a mixing manifold  226  on top of the base plate  7 . The pump/purge manifold  224  may be used to purge corrosive gasses from the gas stick  200 . The mixing manifold  226  may be in fluid communication with the mixing valve and used to mix the gases in any quantity desired by a user. 
     As stated above, single gas sticks are expensive to manufacture, difficult to mount in the gas cabinet, and uses additional parts that may not be necessary to use which may lead to contamination. However, with an extension block, single gas sticks may be joined to an existing gas panel efficiently and quickly. The use of the extension block makes joining a single gas stick less expensive, easy to mount in the gas cabinet, and does not use excess parts. The extension block may be positioned to operatively permit their respective fluid passageways to be positioned for interconnection with the pump/purge manifold  224  and the mixing manifold  226 . Thus, only the purge and mixing valves need to be removed when joining a single gas stick. 
       FIGS. 4A-4D  illustrate an embodiment of manifold extension block. Referring now to  FIG. 4A and 4B , an embodiment of a manifold extension block having two ports. The extension block  400  may be a single machined block having a first section  402  and a second section  404 . Although illustrated with only two sections, the number of sections is not intended to be limiting as the extension block may manufactured to have a longer length and a plurality of sections to join a plurality of single gas sticks. 
     For exemplary purposes only and not intended to be limiting, the two-port manifold extension block  400  will be discussed in use with the mixing valve. The manifold extension block  400  may have a plurality of apertures  406   a ,  406   b ,  406   c ,  406 n (where n is an integer) to removably couple the mixing valve to the extension block  400  and the extension block  400  to the base plate  7  (See  FIG. 2 ). The extension block  400  may have an entrance port  408  and an exit port  414 . The entrance port  408  may be in fluid communication with one of the gas delivery components, such as the MFC and it respective manifold block, via horizontal passageway  412  to receive the supply gas from the MFC. The exit port  414  may extend vertically through fluid passageway  228   b  in the substrate  222  so that it is in fluid communication with the mixing manifold  226 . The exit port  414  may be in fluid communication with a horizontal passageway  416  which may form a secondary mixing manifold. The exit port  414  may also be in fluid communication with discharge port  418  to remove any unnecessary gasses. It will now be known that any of the horizontal fluid passageways may be removably sealed or plugged when not in use. For example, if discharge port  418  is not in use, the port may be plugged so that there will be less dead leg in the gas stick. In another example, if the user does not want the exit port  414  to extend vertically into the fluid passageway  228   b  in the base plate  7 (See  FIG. 2 ), the user may simply plug the exit port  414  to prevent gasses from entering fluid passageway  228   b.    
       FIGS. 4C and 4D  illustrate another embodiment of an extension block described in use with the purge valve. The extension block  450  may be a single machined block having a first section  403  and a second section  405 . Although illustrated with only two sections, the number of sections is not intended to be limiting as the extension block may manufactured to have a longer length and a plurality of sections to join a plurality of single gas sticks. The extension block  450  may have three ports—an entrance port  408 , exit port  414 , and discharge port  420 . The entrance port  408  may be in fluid communication with one of the gas delivery components, such as the primary shut-off valve and it respective manifold block, via horizontal fluid passageway  412 . Exit port  414  may be in fluid communication with the MFC via horizontal fluid passageway  422 . Discharge port  420  may extend vertically downward to be in fluid communication with fluid passageway  228   a  in the substrate  222 , which is in fluid communication with the pump/purge manifold  224 . Discharge port  420  may also be in fluid communication with a horizontal passageway  424  which forms a secondary pump/purge manifold. 
       FIGS. 5A and 5B  illustrates yet another embodiment of an extension block.  FIG. 5A  illustrates a two port manifold extension block  500  and  FIG. 5B  illustrates a three port manifold extension block  502 . The extension blocks illustrated in  FIG. 5A and 5B  are similar to the extension blocks described in  FIGS. 4A-4D . However, the extension blocks illustrated in  FIGS. 5A and 5B  may not be made of one single machined piece. The extension blocks may be connected via horizontal fluid passageway  504 . Thus, unlike the one-piece extension block illustrated in  FIGS. 4A-4D , these embodiments gives a user flexibility to connect as many extension blocks as needed. The horizontal fluid passageways  504  may be connected through any means known to prevent any gas leakage. 
       FIG. 6  illustrates the extension blocks in use with an IGS gas delivery system. In one embodiment as illustrated, a purge manifold  602  and a mixing manifold  604  may be placed within the base plate  606  parallel to each other. In another embodiment as illustrated in  FIG. 2 , the purge manifold  602  and the mixing manifold  604  may be placed on top of the base plate  7 . In either embodiment, the pump/purge manifold  602  and/or the mixing manifold  604  may be any means to allow for the flow of gasses, such as a tubing and the like. The pump/purge manifold  602  and the mixing manifold  604  may have a plurality of apertures  614  to be in fluid communication with a second plurality of apertures  612  in the substrate  608 . The substrate  608  may be coupled to the base plate by an connecting means such as a screw, and the like. A first section  650  of the purge manifold extension block  660  may be removably coupled on the substrate  608  with a pressure sealer  622 . The discharge port  616  may be in fluid communication with one of the second plurality of apertures  612  in the substrate  608 . This allows the discharge port  616  to be in fluid communication with the pump/purge manifold  602  on the base plate  606 . The purge valve  620  may be placed above the purge manifold extension block  660 . The purge valve  620  may be secured to the purge manifold extension block  660  with a pressure sealer  622 . Any fastening means, such as screws  624 , may be used to secure the purge valve  620  to the base plate  606 . The second section  654  of the purge manifold extension block  660  may extend outwardly from the substrate  608  and configured to receive additional gas components. 
     The first section of a mixing manifold extension block  630  may be similarly coupled to the gas panel. The first section  652  of the mixing manifold extension block  630  may be positioned on the upper surface  610  of the substrate  608  with a pressure sealer  622 . The exit port  632  may be in fluid communication with one of the second plurality of apertures  612  in the substrate  608 . This allows the exit port  632  to be in fluid communication with the mixing manifold  604  on the base plate  606 . The mixing valve  634  may be placed above the mixing manifold extension block  630 . The mixing valve  634  may be secured to the mixing manifold extension block  630  with a pressure sealer  622 . Any fastening means, such as screws  624 , may be used to secure the mixing valve  630  to the base plate  606 . The second section  656  of the mixing manifold extension block  630  may extend outwardly from the substrate  608  and be configured to receive other gas delivery components. 
     Thus, extending the existing purge and mixing manifold is not necessary as the secondary manifold in the extension block may replace the need for a new manifold. This reduces the amount of material and cost to make the single gas stick. Furthermore, the use of the extension blocks allows for the flexibility in adding and/or removing as many single gas sticks as desired. 
     Although illustrated with the use of two extension blocks on one gas stick, it will now be known that other extension blocks may also be positioned on the last gas stick of the gas panel  600 . 
       FIGS. 7A and 7B  illustrate yet another embodiment of an extension block. The extension block  700  may have a first section  702 , a second section  704 , and a third section  706 . Although illustrated with three sections, the number of sections is not intended to be limiting as the extension block may have a plurality of sections a desired. Each section may have an entrance port  708 , an exit port  710 , and a plurality of apertures  712  for removably coupling the extension block to the base plate. The entrance port  708  may be in fluid communication with a gas stick component, such as the MFC, via the manifold block illustrated in  FIGS. 3C and 3D . The exit port  710  may be in fluid communication with a common manifold pathway  714  and the mixing manifold via the base plate (not illustrated). 
     The extension block  700  may also have a tubing port  716  positioned between each of the sections. As illustrated in  FIGS. 7A and 7B , the tube port  716  is positioned between the first section  702  and the second section  704  and between the second section  704  and the third section  706 . The tube port  716  may be in fluid communication with the common manifold pathway  714 . 
     The tube port  716  may be configured to receive a vertical tube or pipe able to be positioned between two of the gas delivery components, such as between two mixing valves. There is currently no method in which to add an additional component between two existing gas delivery components due to the small real estate available on the gas stick. However, the use of a vertical tube is possible and may be used as an additional test port to ensure the gas flow is proper, a sample port to insert sample gasses for testing, an extra purge port, and/or to reassign and/or redirect gas lines to reduce dead leg or redirect the flow of the gasses. 
     In one embodiment, the extension block  700  may be fluidly coupled to the gas panel as discussed above with reference to the exemplary embodiments of extension blocks described above. Thus, sections of the extension block may extend outwardly from the gas panel to allow for the removable connection to additional gas delivery components. 
     In another embodiment, the extension block may be used in place of the mixing manifold  226 .  FIG. 8  illustrates a side view of one end of an exemplary gas panel. The gas panel  800  may have a plurality of gas sticks  802   a ,  802   b ,  802   c ,  802 n, each having a mixing valve  804   a ,  804   b ,  804   c ,  804 n. The mixing valves  804   a ,  804   b ,  804   c ,  804 n, may be in fluid communication and removably coupled to the extension block  806  having a first section  808 , a second section  810 , third section  812 , a fourth section  814 , and an n-section  816 . As illustrated, the n-section  716  extends beyond the gas stick  800  to allow for the removable connection of additional gas delivery components. Vertical tubes  820   a ,  820   b ,  820   c ,  820 n may extend upwardly from the extension block  806  between each mixing valve  804   a ,  804   b ,  804   c ,  804 n. Vertical tubes  820   a ,  820   b ,  820   c ,  820 n may be in fluid communication with a common manifold pathway  822  in the extension block  806 . The common manifold pathway  822  may replace the need for another mixing manifold. This would create a more efficient gas panel as less material would be required, the cost to manufacture is less, and there is less dead leg. 
     It will now be known to those skilled in the art that any number of extension blocks may be used to allow for greater flexibility. For example, a single extension block may be used for gas sticks  802   b ,  802   c  in the center of the gas panel. A second extension block may be positioned on the first gas stick  802   a  and a third extension block may be use positioned on the last gas stick  802 n. Both extension blocks may extend outwardly from the first gas stick  802   a  and the last gas stick  802 n to allow for the addition of additional gas delivery components. This allows for the flexibility and ease of removing or adding multiple single gas sticks to the existing gas panel. 
       FIG. 9  is a schematic view of an exemplary gas feed device for semiconductor processing. A plasma processing chamber  910  is supplied processing gas through gas supply line  914 . The gas supply line  912  may provide process gas to a showerhead or other gas supply arrangement arranged in the upper portion of the chamber. Additionally, gas supply line  914  may supply processing gas to a lower portion of the chamber such as, for example, to a gas distribution ring surrounding the substrate holder or through gas outlets arranged in the substrate support. However, an alternative dual gas feed arrangement can supply gas to the top center and top perimeter of the chamber. Processing gas may be supplied to gas line  914  from gas supplies  916 ,  918 ,  920 ,  930  the process gasses from supplies  916 ,  918 ,  920 ,  930  being supplied to MFC  922 ,  924 ,  926 ,  932  respectively. The MFC  922 ,  924 ,  926 ,  932  supply the process gasses to a mixing manifold  928  after which the mixed gas is directed to gas flow line  914 . 
     In operation, the user may select the fraction of mixed flow to be delivered to the plasma processing chamber. For example, the user might select a flow of 250 sccm Ar/30 sccm C 4 F 8 /15 sccm C 4 F 6 /22 sccm O 2  delivered through line  914 . By comparing the total flow, which in this case could be measured by summing all of the flow readouts of the MFC  922 ,  924 ,  926 ,  932  in the gas box, the controller can adjust the degree of throttling in line  914  to achieve the desired flow distribution. Alternatively, an optional total flow meter could be installed just downstream of the mixing manifold  928  to measure the total flow of mixed gas, rather than determining the total flow by summing the readouts of the MFCs  922 ,  924 ,  926 ,  932  in the gas box. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and various substitute equivalents as fall within the true spirit and scope of the present invention.