Patent Publication Number: US-2004050325-A1

Title: Apparatus and method for delivering process gas to a substrate processing system

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates generally to the field of semiconductor processing and more specifically to a method and apparatus for delivering process gas to a substrate processing system.  
       BACKGROUND OF THE INVENTION  
       [0002] Semiconductor devices such as microprocessors and memories are fabricated by various processes, such as depositing a film on a substrate or etching portions of an existing film on a substrate. Of principal concern in many semiconductor manufacturing processes is the difficulty of maintaining process uniformity. For example, a layer deposited on a substrate may exhibit thickness variations across the substrate as well as composition variations within the deposited layer itself. However, as integrated circuit feature sizes become smaller, it is increasingly important to minimize these variations in order to achieve a deposited layer which exhibits very high thickness and composition uniformities.  
       [0003] Many semiconductor fabrication processes are activated thermally and/or via mass transport. As a result, maintaining optimal process uniformity typically requires adjustments to substrate temperature uniformity and/or gas flow distribution across the surface of the substrate. Prior art semiconductor processing equipment has utilized multi-zone heat sources to adjust the temperature distribution across a substrate in order to compensate for non-uniform mass transport effects. Additionally, prior art semiconductor processing equipment has featured means for distributing process gases according to a desired flow pattern in order to minimize mass transport effects across the surface of a substrate.  
       [0004] Chemical vapor deposition (CVD) processes are commonly used in semiconductor manufacturing to deposit a layer of material onto the surface of a substrate. In an epitaxial silicon-germanium (SiGe) deposition process, doped or undoped silicon-germanium layers are typically deposited onto a substrate using a low-pressure CVD process. In this process, a reactant gas mixture including a source of silicon and germanium is heated and passed over a substrate to deposit a silicon-germanium film on the substrate surface. The silicon source may be monosilane, disilane, dichlorosilane, trichlorosilane, or tetrachlorosilane; the germanium source may be germane. The reactant gas mixture may also include a dopant gas, such as phosphine, arsine or diborane. Other silicon sources, germane sources, and dopants may also be used. In some instances, a non-reactant carrier gas, such as hydrogen, is also injected into the processing chamber, together with either or both of the reactant or dopant gases.  
       [0005] Typically, the temperature dependence of the germanium (Ge) incorporation is reversed as compared to the temperature dependence of the silicon-germanium deposition rate. As a result, simultaneous tuning of the deposited silicon-germanium film thickness and germanium concentration uniformities may be problematic.  
       [0006] In a doped or undoped polysilicon deposition process, the crystallographic nature of the deposited silicon is a function of the deposition temperature. At low reaction temperatures, the deposited silicon is predominantly amorphous. However, when higher deposition temperatures are employed, a mixture of amorphous silicon and polysilicon, or polysilicon alone, is deposited. Additionally, in a doped polysilicon deposition process, the temperature dependence of dopant incorporation into the film is reversed as compared to the temperature dependence of the polysilicon deposition rate. As a result, adjusting the temperature distribution across a substrate to optimize the thickness uniformity of a doped polysilicon layer may result in non-uniform dopant incorporation within the polysilicon layer. In other CVD processes, adjusting the temperature distribution across a substrate may result in detrimental changes to electrical and/or physical properties of a deposited film.  
       [0007] U.S. Pat. No. 5,916,369 to Anderson et al. discloses a method and apparatus for controlling the flow rate and composition of a mixture comprising a silicon source gas and a dopant gas across a substrate surface. Referencing FIG. 2, a gas mixture containing a silicon source and a hydrogen carrier gas is injected into chamber  218  from gas sources  202  and  204 . Mass flow controllers  203  and  205  independently control the flow rate of the silicon source and the hydrogen carrier gas to chamber  218 . The gas mixture flows through two metering valves  211  and  212  which operate as variable restrictors to apportion the flow of silicon bearing gas between different gas inlet ports of chamber  218 . A dopant gas is fed from gas source  214 , through mass flow controllers  216  and  220 , and into the silicon source and hydrogen carrier gas mixture downstream of metering valves  211  and  212 . Mass flow controllers  216  and  220  may be used to independently control the dopant gas concentration flowing into different gas inlet ports of chamber  218 .  
       [0008] In Anderson et al., the dopant gas is mixed with the silicon source gas after the silicon source gas passes through metering valves  211  and  212 . Metering valves  211  and  212  may be adjusted to alter the apportionment of silicon bearing gas to the gas inlet ports of chamber  218 . If such an adjustment occurs, mass flow controllers  216  and  220  may require substantial readjustment and tuning, resulting in excessive system downtime. Additionally, a mass flow controller must be provided to control the flow of each dopant gas at each gas inlet port. In FIG. 2, a single dopant gas is fed into two inlet ports, thereby requiring two mass flow controllers. However, in the case of two dopant gases provided to three gas inlet ports, six mass flow controllers are required, resulting in excessive complexity and high cost of ownership.  
       [0009] Accordingly, a need has arisen for a system of supplying process gases to a semiconductor processing system which overcomes these problems. Such a gas delivery system may be useful in several different fabrication processes such as chemical vapor deposition, physical vapor deposition, etching, thermal annealing, thermal oxidation, and other such processes as are commonly used in the manufacture of integrated circuit devices.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings.  
     [0011]FIG. 1 is a schematic diagram illustrating one embodiment of an apparatus for delivering process fluids to a substrate processing system.  
     [0012]FIG. 2 is a schematic diagram illustrating one embodiment of an apparatus for delivering process fluids to a substrate processing system.  
     [0013]FIG. 3 is a schematic diagram illustrating one embodiment of a substrate processing system.  
     [0014]FIG. 4 is a schematic diagram illustrating one embodiment of a substrate processing chamber.  
     [0015]FIG. 5 is a schematic diagram illustrating one embodiment of a gas interface adapted to provide gas flow into a process chamber.  
     [0016]FIG. 6 is a schematic diagram illustrating one embodiment of a gas interface adapted to provide gas flow into a process chamber.  
     [0017]FIG. 7 is a schematic diagram illustrating one embodiment of a gas interface adapted to provide gas flow into a process chamber.  
     [0018]FIG. 8 is a schematic diagram illustrating one embodiment of a substrate processing chamber.  
     [0019]FIG. 9 is a schematic diagram illustrating one embodiment of a showerhead adapted to provide gas flow into a process chamber.  
     [0020]FIG. 10 is a schematic diagram illustrating one embodiment of an apparatus for delivering process fluids to a substrate processing system.  
     [0021]FIG. 11 is a schematic diagram illustrating one embodiment of an apparatus for delivering process fluids to a substrate processing system.  
     [0022]FIG. 12A is a graph illustrating thickness uniformity across deposited SiGe layers.  
     [0023]FIG. 12B is a graph illustrating Ge concentration across deposited SiGe layers.  
    
    
     SUMMARY OF THE INVENTION  
     [0024] A method and apparatus for delivering process fluids to a substrate processing system is described herein. In one embodiment, the fluid delivery system may include a first conduit for coupling a first fluid to the substrate processing system with a first flow controller for controlling the flow of the first fluid through the first conduit; a second conduit for coupling a second fluid to the substrate processing system with a second flow controller for controlling the flow of the second fluid through the second conduit; and a third conduit for coupling the second fluid to the substrate processing system with a third flow controller for controlling the flow of the second fluid through the third conduit. The fluid delivery system may be used to deliver processing fluids to a substrate processing system during semiconductor fabrication.  
     DETAILED DESCRIPTION OF THE INVENTION  
     [0025] The present invention describes a method and apparatus for delivering process fluids to a substrate processing system. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. One skilled in the art will appreciate that these specific details are not necessary in order to practice the present invention. In other instances, well known equipment features and processes have not been set forth in detail in order to not unnecessarily obscure the present invention.  
     [0026] A processing system having a gas delivery system is described herein. The processing system may include a number of chambers for performing various processes involved in semiconductor fabrication. The processing system may include a process chamber for depositing layers of material onto a surface of a substrate held within the process chamber. The layers may be deposited, for example, by a process such as chemical vapor deposition. During a chemical vapor deposition process, a process gas may be directed into an interior portion of a process chamber and over a surface of a substrate while the temperature of the substrate is maintained at a particular level, such that a layer is formed on the substrate as the process gas passes over the substrate.  
     [0027] A gas delivery system may be used to control the composition and distribution of gases within a process chamber during substrate processing. For example, the gas delivery system may be used to control the concentration and flow rate of one or more process gases flowing over the surface of a substrate during a chemical vapor deposition process, thereby minimizing thickness and composition variations within a deposited layer.  
     [0028] The gas delivery system may direct gases into two or more gas channels contained within an inlet manifold coupled to a process chamber. The gas channels may subsequently direct the gases into an interior portion of the process chamber and across a surface of a substrate. Flow controllers and isolation valves may be used to control the composition and distribution of gases within the gas channels and across the surface of the substrate.  
     [0029] The gas delivery system may provide a gas mixture comprising gases from two or more gas sources to a plurality of gas channels. The composition and flow rate of the gas mixture may be controlled using flow controllers coupled to each gas source. Each flow controller coupled to each gas source may be operated independently of the flow controllers coupled to other gas sources.  
     [0030] The gas delivery system may include a bypass for selectively directing gas from a particular gas source into a gas channel independently of the gas mixture entering that channel. The gas flow rate through the bypass may be controlled using a flow controller coupled to the bypass. As a result, the total flow of gas from a particular gas source may be controlled by two flow controllers: one flow controller may control the gas flow entering the gas mixture and another flow controller may control the gas flow passing through a bypass. Each of the two flow controllers may operate independently of the other flow controller.  
     [0031] The bypass may be coupled to two or more gas channels. The bypass may include an isolation valve for each gas channel coupled to the bypass, and each isolation valve may be used to control the flow of gas from the bypass into a gas channel. Each isolation valve may operate independently of the other isolation valves. Consequently, the bypass may be used to selectively control the flow of a gas into a particular gas channel independently of the flow of gas into other gas channels coupled to the bypass.  
     [0032] The gas delivery system may be structured such that the flow controllers and isolation valves described above are computer controlled flow controllers and isolation valves. A system controller may execute a process recipe which contains settings for controlling the gas delivery system. The system controller may automatically control the computer controlled flow controller and isolation valve setpoints based upon settings contained within the process recipe. Consequently, the gas delivery system may be used to automatically alter the composition and flow rate of gases passing through the gas channels and across different portions of a substrate during processing. The composition and flow rate of gases passing through the gas channels may be altered between steps in a single process recipe or between different process recipes.  
     [0033] The gas delivery system may be used to enhance the control of two or more process gas flows within a process chamber. Alternatively, the gas delivery system may be used to enhance the control of one or more process gas flows and one or more inert gas flows within a process chamber. The gas delivery system may be structured such that the composition and flow rate of gases passing through a particular gas channel may be varied independently of the composition and flow rate of gases passing through other gas channels. Additionally, the gas delivery system may be structured such that the composition and flow rate of gases passing through each gas channel may be varied independently of the composition and flow rate of gases passing through all other gas channels. Consequently, the present invention may provide significant benefits to a wide variety of processes commonly used in the manufacture of electronic devices. For example, in one embodiment the gas distribution system may be integrated with a chemical vapor deposition (CVD) processing system to control the concentration and flow rate of process gases over the surface of a substrate, thereby minimizing mass transport effects during processing and enhancing thickness and/or composition uniformity of a deposited layer. In alternative embodiments, the gas distribution system may be integrated with other types of processes, such as physical vapor deposition (PVD), etch, thermal anneal, thermal oxidation, and others to improve various process parameters.  
     [0034] Processing System  
     [0035]FIG. 3 is a schematic diagram illustrating one embodiment of a substrate processing system  300  having a gas distribution system which is described herein. Processing system  300  may be a cluster processing tool, such as a Centura or Endura processing system manufactured by Applied Materials of Santa Clara, Calif. Processing system  300  may include one or more load-lock chambers  304 , one or more process chambers  306 ,  308 , and  310 , and a cooldown chamber  314  attached to a central transfer chamber  302 . Processing system  300  may further include a system controller  325  for controlling various operations of processing system  300 , power supplies  350  for supplying various forms of energy to processing system  300 , and pumps  375  for evacuating various vacuum chambers contained within processing system  300 .  
     [0036] System controller  325  may control the operation of processing system  300 , including the operation of load-lock chambers  304 ; process chambers  306 ,  308 , and  310 ; metrology chamber  312 ; cooldown chamber  314 ; central transfer chamber  302 ; power supplies  350 ; and pumps  375 . System controller  325  may also control the operation of computer controlled mass flow controllers and isolation valves structured to the computer controlled gas delivery system.  
     [0037] System controller  325  may include a single board computer (SBC) comprising a processor and memory. The SBC processor may include a central processing unit (CPU) such as a Pentium microprocessor manufactured by Intel Corporation of Santa Clara, Calif. In some embodiments, the SBC processor may include an application specific integrated circuit (ASIC) to operate one or more specific components of processing system  300 . For example, the SBC processor may include an ASIC to operate computer-controlled mass flow controllers. The SBC memory may include various volatile and non-volatile memory devices, such as RAM or EPROMs.  
     [0038] System controller  325  may also include one or more memory storage devices, such as a hard disk drive, a floppy disk drive, or a CD-ROM drive. System controller  325  may further include one or more input/output (I/O) devices, such as a CRT monitor and keyboard; analog input/output boards; digital input/output boards; interface boards; and stepper motor controller boards. The SBC processor, SBC memory, memory storage devices, and input/output devices may communicate via a communications bus.  
     [0039] System controller  325  may control all of the activities of the processing system  300  according to an instruction set defined by system control software. The system control software may be stored in a computer-readable medium and executed by system controller  325 . Preferably, system control software is stored on a hard disk drive, but system control software may also be stored on a floppy disk, RAM, a CD-ROM or other types of memory storage devices. The system control software may be written in any conventional programming language, including but not limited to 68000 assembly language, C, C++, Pascal, or Fortran. In a preferred embodiment, the system control software comprises Legacy software developed by Applied Materials of Santa Clara, Calif.  
     [0040] The system control software typically contains instructions for managing all operational aspects of processing system  300 . For example, the system control software may include a chamber manager subroutine for controlling the various chamber components necessary to carry out a particular process on a substrate, such as process gas control valves, susceptor positioning actuators, and power supplies. In operation, the chamber manager subroutine may monitor the various chamber components, determine which components need to be operated based on the process parameters for the process set to be executed, and direct the control of those components responsive to the monitoring and determining steps. The system control software may manage other operational aspects of processing system  300 , such as the movement of wafer transfer mechanisms and the opening and closing of vacuum pump valves.  
     [0041] Instructions for directing a chamber to perform a specific process on a substrate may be contained within a process recipe which is stored in memory and executed by the SBC processor. A process recipe may comprise one or more sequential process steps. Each process step may contain a set of variables that dictate various process parameters for that recipe step, such as step duration, gas flow, chamber pressure, substrate temperature, power supply output, and susceptor position. Process parameters may be changed between process steps to vary the processing environment within a process chamber. To execute a process recipe, the process recipe is read into SBC memory and executed by the SBC processor to perform the tasks identified within the process recipe steps.  
     [0042] Instructions for directing processing system  300  to perform a series of processes on a substrate are contained within a process sequence. Like the system control software and process recipe, a process sequence may be stored in a computer-readable medium such as a memory. A process sequence may direct processing system  300  to perform a series of processes on a substrate in several different chambers within processing system  300 . For example, a process sequence may direct process system  300  to transfer a wafer from load-lock chamber  304  to process chamber  306 . The sequence may then direct process chamber  306  to perform a first process on the wafer as governed by a first process recipe. The sequence may then direct process system  300  to transfer the wafer from process chamber  306  to process chamber  308  in order to perform a second process on the wafer as governed by a second process recipe. The process sequence may then direct processing system  300  to transfer the wafer to cooldown chamber  314  to be processed according to a cooldown recipe. Finally, the process sequence may direct processing system  300  to return the wafer to load-lock chamber  304 .  
     [0043] A process sequence may be assigned to each substrate in a lot of substrates prior to processing. Each substrate within a lot of substrates may be assigned the same process sequence, in which case each substrate is processed identically within processing system  300 . Alternatively, substrates within a lot of substrates may be assigned different process sequences, in which case substrates within the lot of substrates are processed differently according to their assigned process sequence.  
     [0044] Prior to performing a process sequence, a lot of wafers is placed within load-lock chamber  304 . The atmosphere within load-lock chamber  304  is subsequently evacuated, thereby removing a majority of atmospheric gases from the interior of load-lock chamber  304 . Upon initiating a process sequence, a wafer transfer robot located within transfer chamber  302  sequentially transfers wafers to a series of chambers as defined in the process sequence. For example, the transfer robot may transfer a wafer to an orienter chamber; one or more process chambers  306 ,  308 , and  310 ; a cooldown chamber  314 ; and then back to load-lock chamber  304 . Process chambers  306 ,  308 , and  310  may perform various processes on the wafer as required, such as deposition, etching, or annealing. Cooldown chamber  314  may be used to cool each wafer before returning the wafer to load-lock chamber  304 . After the lot of wafers has been processed, load-lock chamber  304  may be vented to atmospheric pressure, opened, and the wafers may be removed for subsequent processing in other wafer processing systems.  
     [0045] Process Chamber  
     [0046] Referencing FIG. 3, process chambers  306 ,  308 , and  310  may include a process chamber used to deposit layers over a substrate. The layers may be deposited by numerous processes such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or other such processes as are commonly used in the fabrication of electronic devices. The gas distribution system of the present invention may be incorporated into a variety of substrate processing systems in order to enhance the control of two or more process gas flows within a process chamber. For example, the gas distribution system may be integrated with a chemical vapor deposition (CVD) processing system to control the flow of process gases over the surface of a substrate, thereby enhancing thickness and/or composition uniformity of a deposited layer. Alternatively, the gas distribution system may be used to enhance the control of one or more process gases flows and one or more inert gas flows within a process chamber. In the present invention, a process gas is defined as a gas or gas mixture which acts to deposit, remove, or treat a film on a substrate placed in a processing chamber. An inert gas is defined as a gas which is substantially non-reactive with chamber features and substrates placed in a deposition chamber at particular process temperatures.  
     [0047] For illustrative purposes, the gas distribution system of the present invention will be described herein in reference to a CVD processing system. However, the gas distribution system may also be integrated with a physical vapor deposition (PVD) processing system, an etch processing system, or any of a variety of other substrate processing systems as are commonly used in the manufacture of electronic devices. In a typical CVD process, a process gas is passed through a process chamber and over a substrate. The substrate is maintained at a particular temperature such that a layer is formed on the substrate as the process gas passes over the substrate.  
     [0048] CVD Process Chamber with Side Gas Injection  
     [0049]FIG. 4 is a schematic diagram illustrating one embodiment of a CVD process chamber  400 . Process chamber  400  may be substantially similar to process chambers  306 ,  308 , and/or  310  described above in reference to FIG. 3. Process chamber  400  may include an upper dome  402 , a lower dome  404 , and a sidewall  406  positioned between upper dome  402  and lower dome  404 . Cooling fluid may be circulated through sidewall  406  to cool o-rings which seal upper dome  402  and lower dome  404  to sidewall  406 . An upper liner  408  and a lower liner  410  may be mounted against an inside surface of sidewall  406 . Upper dome  402  and lower dome  404  may be formed from a transparent material to allow heating light to pass through into process chamber  400 . An upper clamping ring  412  may extend around the periphery of an outer surface of upper dome  402 . A lower clamping ring  414  may extend around the periphery of an outer surface of lower dome  404 . Upper clamping ring  412  and lower clamping ring  414  may be secured together so as to clamp upper dome  402  and lower dome  404  to sidewall  406 .  
     [0050] A susceptor  416  may be located within process chamber  400 . Susceptor  416  may be adapted to removeably support a wafer in an approximately horizontal position. Susceptor  416  may extend transversely across process chamber  400  to divide process chamber  400  into an upper portion  418  above susceptor  416 , and a lower portion  420  below susceptor  416 . Susceptor  416  may be mounted on a shaft  422  that extends vertically downward from the center of the bottom surface of susceptor  416 . Shaft  422  may be connected to a motor that rotates shaft  422  and thereby rotates susceptor  416  and a wafer supported by susceptor  416 . An annular preheat ring  424  may be connected at its outer periphery to the inner periphery of lower liner  410  and may extend around susceptor  416 . Annular preheat ring  424  may be in the same plane as susceptor  416 , with the inner periphery of annular preheat ring  424  separated by a gap from the outer periphery of susceptor  416 .  
     [0051] In one embodiment, a plurality of lamps  426  may be mounted around process chamber  400 . Reflectors  428  may be located around lamps  426  to prevent energy radiated by lamps  426  from radiating away from process chamber  400 . Reflectors  428  may also be formed to reflect radiant energy towards upper dome  402  and lower dome  404 . Lamps  426  may radiate energy through the upper dome  402  and lower dome  404  to heat susceptor  416  and annular preheat ring  424 . Upper dome  402  and lower dome  404  may be made of a transparent material, such as quartz, so that energy radiated by lamps  426  may pass through upper dome  402  and lower dome  404 . In other embodiments, heating devices other than lamps, such as resistance heaters or RF inductive heaters, may be used to heat susceptor  416  and annular preheat ring  424 .  
     [0052] Susceptor  416  and annular preheat ring  424  may be formed from a material that is opaque to radiation emitted by lamps  426 , such as silicon carbide coated graphite. Thus, susceptor  416  and annular preheat ring  424  may be more readily heated by energy radiated from lamps  426 . A lower infrared temperature sensor  430 , such as a pyrometer, may be mounted below lower dome  404 , and may face the bottom surface of susceptor  416  through lower dome  404 . Lower infrared temperature sensor  430  may be used to monitor the temperature of susceptor  416  by receiving infrared radiation emitted from susceptor  416  when susceptor  416  is heated. An upper infrared temperature sensor  432  may be mounted above upper dome  402  facing the top surface of susceptor  416  through upper dome  402 . Upper infrared temperature sensor  432  may be used to monitor the temperature of a wafer supported by susceptor  416 .  
     [0053] Process chamber  400  may be a “cold wall” reactor wherein sidewall  406 , upper liner  408 , and lower liner  410  are at a substantially lower temperature than preheat ring  424  and susceptor  416  during processing. For example, in a process to deposit an epitaxial silicon film on a wafer, susceptor  416  and a wafer supported by susceptor  416  may be heated to a temperature of between 400-1200° C. The sidewall and liners may be maintained at a lower temperature of approximately 200-600° C. by cooling fluid circulated through sidewall  406 .  
     [0054] Process chamber  400  may include a gas interface  434  positioned in a side of process chamber  400 . Gas interface  434  may be adapted to transmit gases from one or more gas sources  436  into process chamber  400 . Gas sources  436  may include process gases and inert gases. Gas interface  434  may include a connector cap  440 , a baffle  442 , and an insert plate  444  positioned within sidewall  406 . Upper and lower fluid conduits  441  and  466  may be formed in connector cap  440  and insert plate  444 . Process chamber  400  may further include a passage  456  formed between upper liner  408  and lower liner  410 . Passage  456  may be fluidly connected to upper portion  418  of process chamber  400 . Process gas from gas sources  436  may pass through connector cap  440 , baffle  442 , insert plate  444 , and passage  456  into upper portion  418  of process chamber  400 .  
     [0055] As shown in FIG. 4, gas sources  436  may be connected to gas interface  434  by gas supply conduit  427 . However, typically, each gas source has an independent gas supply conduit from the gas source to a gas distribution panel located on or adjacent to processing system  300 . Additional gas supply conduits may be structured to connect gas interface  434  to the gas distribution panel. Consequently, gases from gas sources  436  may be directed to a gas distribution panel which subsequently directs the gases to gas interface  434 .  
     [0056] During operation, one or more gases are supplied to gas interface  434  by means of inlet ports  450 . Gases from inlet ports  450  flow through connector cap  440  and bank against the upstream surface of baffle  442 . The gases are directed through holes formed in baffle  442  into upper and lower conduits  441  and  466  formed in insert plate  444 . Inlet ports  450 , connector cap  440 , baffle  442 , and upper and lower conduits  441  and  466  may form independent flow pathways for each gas entering process chamber  400 . As a result, each gas flowing into each inlet port and through connector cap  440 , baffle  442 , and insert plate  444  along upper and lower conduits  441  and  466  may be kept separate from other gases entering process chamber  400 . From upper conduits  441 , gases may flow across preheat ring  424 , susceptor  416  and a wafer supported by susceptor  416  in the direction indicated by arrows  486 . The gas flow profile from upper conduits  441 , across preheat ring  424  and a wafer may be predominantly laminar.  
     [0057] In one embodiment, process gases from lower conduits  466  and upper conduits  441  may both be directed into upper portion  418  of process chamber  400 . In an alternative embodiment, an inert gas may be directed through lower conduits  466  into lower portion  420  of process chamber  400 . For example, an inert purge gas such as hydrogen or nitrogen may be directed into lower portion  420  of process chamber  400  in order to prevent deposition on the back side of susceptor  416 . An inert purge gas may be fed into lower portion  420  at a rate which develops a positive pressure within lower portion  420  with respect to the process gas pressure in upper portion  418 , thereby preventing process gas from entering lower portion  420 .  
     [0058] Gases entering process chamber  400  from upper and lower conduits  441  and  466  may be evacuated from process chamber  400  through outlet  468 . Outlet  468  may be positioned in the side of process chamber  400  opposite gas interface  434 . Outlet  468  may include an exhaust passage  478  which extends from the upper chamber portion  418  to the outside diameter of sidewall  406 . Exhaust passage  478  may be coupled to outlet connector  490  on the exterior of sidewall  406 . Outlet connector  490  may be coupled to a vacuum source, such as a pump, by means of an exhaust foreline. The vacuum source may be used to create low or reduced pressure in chamber  400  during processing. Thus, process gas fed into process chamber  400  may be evacuated through exhaust passage  478  and outlet connector  490  into an exhaust foreline.  
     [0059]FIG. 5 illustrates one embodiment of gas interface  434  adapted to provide two gas flow channels into upper portion  418  of process chamber  400 . In this embodiment, gas interface  434  may include a first inlet port  505  and a second inlet port  510  connected to a first channel  507  and a second channel  512 , respectively. During substrate processing, a first gas flow entering first inlet port  505  may flow through first channel  507  and across a first portion of a substrate positioned on susceptor  416 . Similarly, a second gas flow entering second inlet port  510  may flow through second channel  512  and across a second portion of the substrate.  
     [0060] In one embodiment, the composition of the gas mixture entering first channel  507  may be controlled independently of the composition of the gas mixture entering second channel  512 . Consequently, the composition of gas mixtures flowing across first and second portions of a substrate positioned on susceptor  416  may be varied to more accurately control the uniformity of a layer deposited on the substrate. For example, the gas flow passing through first channel  507  may contain a higher concentration of a gas than the gas flow passing through second channel  512  in order to increase the thickness uniformity of a particular deposited layer. Alternatively, the gas flow passing through first channel  507  may contain a lower concentration of a gas than the gas flow passing through second channel  512 .  
     [0061]FIG. 6 illustrates another embodiment of gas interface  434  adapted to provide three gas flow channels into upper portion  418  of process chamber  400 . In this embodiment, gas interface  434  may include a central inlet port  605 , a first outside inlet port  610 , and a second outside inlet port  615  connect to a central channel  607 , a first outside channel  612 , and a second outside channel  617 , respectively. During substrate processing, a first gas flow entering central inlet port  605  may flow through central channel  607  and across a central portion of a substrate positioned on susceptor  416 . A second gas flow entering first outside inlet port  610  may flow through first outside channel  612  and across a first outside portion of the substrate. And a third gas flow entering second outside inlet port  615  may flow through second outside channel  617  and across a second outside portion of the substrate.  
     [0062] In one embodiment, the composition of the gas mixture entering central channel  607  may be controlled independently from the composition of the gas mixture entering first outside channel  612  and second outside channel  617 . Consequently, the composition of the gas mixture flowing across the central portion of a substrate positioned on susceptor  416  may be varied with respect to the composition of the gas mixtures flowing across the first and second outside portions of the substrate to more accurately control the uniformity of a layer deposited on the substrate. For example, the gas flow passing through central channel  607  may contain a higher concentration of a gas than the gas flow passing through first outside channel  612  and second outside channel  617  in order to increase the thickness uniformity of a particular deposited layer. Alternatively, the gas flow passing through central channel  607  may contain a lower concentration of a gas than the gas flow passing through first outside channel  612  and second outside channel  617 .  
     [0063]FIG. 7 illustrates yet another embodiment of gas interface  434  adapted to provide five gas flow channels into upper portion  418  of process chamber  400 . In this embodiment, gas interface  434  may include a central inlet port  705 , a first middle inlet port  710 , a second middle inlet port  715 , a first outside inlet port  720 , and a second outside inlet port  725  connected to a central channel  707 , a first middle channel  712 , a second middle channel  717 , a first outside channel  722 , and a second outside channel  727 , respectively. During substrate processing, a first gas flow entering central inlet port  705  may flow through central channel  707  and across a central portion of a substrate positioned on susceptor  416 . A second gas flow entering first middle inlet port  710  may flow through first middle channel  712  and across a first middle portion of the substrate. A third gas flow entering second middle inlet port  715  may flow through second middle channel  717  and across a second middle portion of the substrate. A fourth gas flow entering first outside inlet port  720  may flow through first outside channel  722  and across a first outside portion of the substrate. A fifth gas flow entering second outside inlet port  725  may flow through second outside channel  727  and across a second outside portion of the substrate.  
     [0064] In one embodiment, the composition of the gas mixture entering central channel  707  may be controlled independently of the composition of the gas mixtures entering first middle channel  712 , second middle channel  717 , first outside channel  722 , and second outside channel  727 . Similarly, the composition of the gas mixtures entering first middle channel  712  and second middle channel  717  may be controlled independently of the composition of the gas mixtures entering central channel  707 , first outside channel  722 , and second outside channel  727 . Additionally, the composition of the gas mixtures entering first outside channel  722 , and second outside channel  727  may be controlled independently of the composition of the gas mixtures entering central channel  707 , first middle channel  712 , and second middle channel  717 .  
     [0065] Consequently, the composition of a gas mixture flowing across the central portion of a substrate positioned on susceptor  416  may be varied with respect to the composition of the gas mixtures flowing across the first middle, second middle, first outside, and second outside portions of the substrate; the composition of the gas mixtures flowing across the first middle and second middle portions of the substrate may be varied with respect to the composition of the gas mixtures flowing across the central, first outside, and second outside portions of the substrate; and the composition of the gas mixtures flowing across the first outside and second outside portions of the substrate may be varied with respect to the composition of the gas mixtures flowing across the central, first middle, and second middle portions of the substrate to more accurately control the uniformity of a layer deposited on the substrate.  
     [0066] For example, the gas flow passing through central channel  707  may contain a higher concentration of a gas than a gas flow passing through first middle channel  712 , second middle channel  717 , first outside channel  722 , and second outside channel  727  in order to increase the thickness uniformity of a particular deposited layer. Alternatively, the gas flow passing through central channel  707  may contain a lower concentration of a gas than a gas flow passing through first middle channel  712 , second middle channel  717 , first outside channel  722 , and second outside channel  727 . The gas flow passing through first middle channel  712  and second middle channel  717  may similarly contain a higher or lower concentration of a gas than the gas flows passing through central channel  707  and/or first outside channel  722  and second outside channel  727 . And the gas flow passing through first outside channel  722  and second outside channel  727  may similarly contain a higher or lower concentration of a gas than the gas flows passing through central channel  707  and/or first middle channel  712  and second middle channel  717 .  
     [0067] The embodiments illustrated in FIGS. 5, 6, and  7  should not be interpreted as limiting as one of ordinary skill in the art will recognize that gas interface  434  may be structured to provide any number of gas flow channels into upper portion  420  of process chamber  400 . Additionally, the described gas flows and comparative gas concentrations are merely exemplary and other gas flows and concentrations may be directed to different gas flow channels as required for particular processes.  
     [0068] CVD Process Chamber with Showerhead Gas Injection  
     [0069]FIG. 8 illustrates process chamber  800 , an alternative embodiment of a CVD process chamber. Process chamber  800  may be substantially similar to process chambers  306 ,  308 , and/or  310  described above in reference to FIG. 3. Process chamber  800  may include showerhead  815 , lower chamber wall  810 , and a sidewall  825  between showerhead  815  and lower chamber wall  810 . Cooling fluid may be circulated through sidewall  825  to cool o-rings which seal showerhead  815  and lower chamber wall  810  to sidewall  825 . An upper liner  830  and a lower liner  835  may be mounted against an inside surface of sidewall  825 . An upper clamping ring  840  may extend around the periphery of an outer surface of showerhead  815 . A lower clamping ring  845  may extend around the periphery of an outer surface of lower chamber wall  820 . Upper clamping ring  840  and lower clamping ring  845  may be secured together so as to clamp showerhead  815  and lower chamber wall  810  to sidewall  825 .  
     [0070] A susceptor  822  may be located within process chamber  800 . Susceptor  822  may be adapted to removeably support wafer  820  in an approximately horizontal position. Susceptor  822  may extend transversely across process chamber  800  to divide process chamber  800  into an upper portion  818  above susceptor  822 , and a lower portion  828  below susceptor  822 . Susceptor  822  may be mounted on a shaft  824  that extends vertically downward from the center of the bottom surface of susceptor  822 . An annular preheat ring  824  may be connected at its outer periphery to the inner periphery of lower liner  835  and may extend around susceptor  822 . Annular preheat ring  824  may be in the same plane as susceptor  822 , with the inner periphery of annular preheat ring  824  separated by a gap from the outer periphery of susceptor  822 . In one embodiment, susceptor  822  and annular preheat ring  824  may be heated by means of a resistance heater contained within susceptor  822 . In other embodiments, RF inductive heaters, lamps, or other such heating devices may be used to heat susceptor  822  and annular preheat ring  824 . The temperature of susceptor  822  may be monitored by means of a thermocouple embedded within susceptor  822 .  
     [0071] One or more process gases may be injected into upper portion  818  of process chamber  800  through a plurality of orifices  850  extending through a lower surface  855  of showerhead  815 . Orifices  850  may be arranged in a plurality of regions or zones on lower surface  855  of showerhead  815 . As shown in FIG. 9, orifices  850  may be arranged in a center region  905 , a middle region  910 , and an outer region  915 . Middle region  910  may be arranged in an annular configuration encircling center region  905  and outer region  915  may be arranged in an annular configuration encircling middle region  910  and extending adjacent to an outer periphery  920  of showerhead  815 .  
     [0072] Showerhead  815  may further include center passageway  907 , middle passageway  912  and outer passageway  917 . Orifices contained within center region  905  of showerhead  815  may connect with center passageway  907 . Similarly, orifices contained within middle region  910  may connect with middle passageway  912 . In like fashion, orifices contained within outer region  915  may connect with outer passageway  917 .  
     [0073] Process chamber  800  may further include a gas interface  875  positioned in a top portion of process chamber  800  and connected to showerhead  815 . Gas interface  875  may be adapted to direct gas from one or more gas sources through showerhead  815  and into upper portion  818  of process chamber  800 . Referencing FIG. 9, gas interface  875  may include center conduit  925 , middle conduit  930 , and outer conduit  935 . Center passageway  907  may be connected to center conduit  925 ; middle passageway  912  may be connected to middle conduit  930 ; and outer passageway  917  may be connected to outer conduit  935 . Center conduit  925  may be arranged coaxially along a portion of middle conduit  930  and outer conduit  935 . Similarly, middle conduit  930  may be arranged coaxially along a portion of outer conduit  935 .  
     [0074] Gas interface  875  may further include center inlet port  940 , middle inlet port  945 , and outer inlet port  950 . Center inlet port  940 , middle inlet port  945 , and outer inlet port  950  may be structured and arranged to provide process gas from one or more gas sources to gas interface  875 . Center inlet port may be connected to center conduit  925 ; middle inlet port  945  may be connected to middle conduit  930 ; and outer inlet port  950  may be connected to outer conduit  935 . Center inlet port  940 , middle inlet port  945 , and outer inlet port  950  may be connected to one or more gas supply lines, which are in turn connected to gas sources, such as gas cylinders.  
     [0075] As in the previous embodiment, process chamber  800  may be a “cold wall” reactor wherein sidewall  825 , upper liner  830 , and lower liner  835  are at a substantially lower temperature than preheat ring  824  and susceptor  822  during processing. Additionally, one or more channels  990  having an inlet  992  and an outlet  994  may be formed in showerhead  815 . A fluid may be directed into inlet  992 , through channels  990 , and out of outlet  994  to heat or cool showerhead  815  during operation of process chamber  800 .  
     [0076] In operation, one or more gases may be supplied to gas interface  875  through center inlet port  940 , middle inlet port  945 , and outer inlet port  950 . Gas from center inlet port  940  may flow through center conduit  925 , center passageway  907 , and orifices in center region  905  into upper portion  818  of process chamber  800 . Gas from middle inlet port  945  may flow through middle conduit  930 , middle passageway  912 , and orifices in middle region  910  into upper portion  818  of process chamber  800 . Gas from outer inlet port  950  may flow through outer conduit  935 , outer passageway  917 , and orifices in outer region  915  into upper portion  818  of process chamber  800 . Inlet ports  940 ,  945 , and  950 ; conduits  925 ,  930 , and  935 ; and passageways  907 ,  912 , and  917  may form independent flow pathways for each gas entering process chamber  800 . As a result, each gas flowing into each inlet port and through each conduit and passageway may be kept separate until the gases enter upper portion  818  of process chamber  800 .  
     [0077] Gases entering process chamber  800  from showerhead  815  may be evacuated from process chamber  800  through outlet  816 . Outlet  816  may be formed in lower chamber wall  810  of process chamber  800 . Outlet  816  may include an exhaust passage  804  which extends from lower chamber portion  828  to the lower surface of lower chamber wall  810 . Exhaust passage  804  may be coupled to outlet connector  806  on the exterior of lower chamber wall  810 . Outlet connector  806  may be coupled to a vacuum source, such as a pump, by means of an exhaust foreline. The vacuum source may be used to create low or reduced pressure in chamber  800  during processing. Thus, process gas fed into process chamber  800  may be evacuated through exhaust passage  804  and outlet connector  806  into an exhaust foreline.  
     [0078] Gas entering center inlet port  940  may initially contact a central portion of a substrate positioned on susceptor  416 ; gas entering middle inlet port  945  may initially contact a middle annular portion of the substrate; and gas entering outer inlet port  950  may initially contact an outer annular portion of the substrate. After entering upper portion  818  of process chamber  800 , process gases may flow radially across wafer  820 , susceptor  822 , and preheat ring  824 .  
     [0079] In one embodiment, the composition of the gas mixtures entering center inlet port  940  and outer inlet port  945  may be controlled independently from the composition of the gas mixtures entering middle inlet port  945 . Consequently, the composition of the gas mixtures flowing across the central and outer annular portions of a substrate positioned on susceptor  822  may be varied with respect to the composition of the gas mixtures flowing across the middle annular portion of the substrate to more accurately control the uniformity of a layer deposited on the substrate. For example, the gas flows passing through center inlet port  940  and outer inlet port  945  may contain a higher concentration of a gas than the gas flow passing through middle inlet port  945  in order to increase the thickness uniformity of a particular deposited layer.  
     [0080]FIG. 8 should not be interpreted as limiting as one of ordinary skill in the art will recognize that gas interface  875  may be structured to provide any number of gas flow channels into upper portion  818  of process chamber  800 . Additionally, the described gas flows and gas concentrations are merely exemplary and other gas flows and concentrations may be directed to different gas flow channels as required for particular processes.  
     [0081] Gas Delivery System  
     [0082] As previously discussed, a process chamber may include a gas interface adapted to provide multiple gas flow channels to an interior portion of a process chamber. For example, FIG. 5 illustrates one embodiment of gas interface  434  adapted to provide two gas flow channels, FIG. 6 illustrates another embodiment of gas interface  434  adapted to provide three gas flow channels, and FIG. 7 illustrates yet another embodiment of gas interface  434  adapted to provide five gas flow channels. Similarly, FIG. 9 illustrates an embodiment of a gas interface  875  adapted to provide three gas flow regions within process chamber  800 .  
     [0083] In each of these examples, a gas delivery system may be arranged to direct one or more gases into each gas flow channel. The gas delivery system may provide a mixture of gases from two or more gas sources to the channels. The composition and flow rate of the mixture of gases may be controlled using flow controllers coupled to each gas source. Each flow controller coupled to each gas source may be operated independently of the flow controllers coupled to other gas sources.  
     [0084] The gas delivery system may include a bypass for selectively directing gas from a particular gas source into a gas channel independently of the gas mixture entering that channel. The gas flow rate through the bypass may be controlled using a flow controller coupled to the bypass. The bypass may be coupled to two or more gas channels, and the bypass may include an isolation valve for each gas channel coupled to the bypass. Each isolation valve may be operated independently from other bypass isolation valves. As a result, gas from the bypass may be selectively directed into each gas channel. Hence, the bypass may be used to selectively control the flow of a gas into a particular gas channel independently of the flow of gas into other gas channels coupled to the bypass.  
     [0085] In one embodiment, the gas delivery system may allow the composition and flow rate of gases passing through a particular gas flow channel to be varied independently of the composition and flow rate of gases passing through other gas flow channels. In another embodiment, the gas delivery system may allow the composition and flow rate of gases passing through each gas flow channel to be varied independently of the composition and flow rate of gases passing through all other gas flow channels. In some embodiments, the flow controllers and isolation valves described above may be computer controlled flow controllers and computer controlled isolation valves.  
     [0086] In the following descriptions, the term “manifold” is generally used to describe a plurality of conduits arranged to combine two or more fluid flow inlets into a single fluid flow outlet, or a plurality of conduits arranged to divide a single fluid flow inlet into two or more fluid flow outlets. Fluid flow conduits used to construct a manifold may be formed from a variety of materials as are commonly employed in semiconductor manufacturing systems, such as stainless steel gaslines.  
     [0087] Gas Delivery System I  
     [0088]FIG. 10 shows a schematic diagram illustrating one embodiment of a gas delivery system  1000  for controlling the flow of gas to gas interface  1005 . Gas interface  1005  may be adapted to flow gas to a variety of process chambers. For example, gas interface  1005  may be substantially similar to gas interface  434  illustrated in FIG. 5, which is structured to provide two gas flow channels into upper portion  418  of process chamber  400 . Consequently, during substrate processing, a first gas flow entering a first inlet port  1006  may be directed to flow across a first portion of a substrate contained within a process chamber and a second gas flow entering a second inlet port  1007  may be directed to flow across a second portion of the substrate.  
     [0089] Gas delivery system  1000  may include a first gas source  1010 , a second gas source  1015 , a first manifold  1030 , a second manifold  1050 , a third manifold  1070 , and gas interface  1005 . First manifold  1030  may include a first inlet  1032 , a second inlet  1034 , and a first outlet  1036 . Second manifold  1050  may include a third inlet  1052 , a second outlet  1054 , and a third outlet  1056 . Third manifold  1070  may include a fourth inlet  1072 , a fourth outlet  1074 , and a fifth outlet  1076 . Gas interface  1005  may include first inlet port  1006  and second inlet port  1007 .  
     [0090] First inlet  1032  and second inlet  1034  of first manifold  1030  may be coupled to first gas source  1010  and second gas source  1015 , respectively. First outlet  1036  of first manifold  1030  may be coupled to third inlet  1052  of second manifold  1050 . Second outlet  1054  and third outlet  1056  of second manifold  1050  may be coupled to first inlet port  1006  and second inlet port  1007  of gas interface  1005 , respectively. Fourth inlet  1072  of third manifold  1070  may be coupled to second inlet  1034  of first manifold  1030 . Fourth outlet  1074  and fifth outlet  1076  of third manifold  1070  may be coupled to second outlet  1054  and third outlet  1056  of second manifold  1050 , respectively.  
     [0091] Flow controllers may be structured to gas delivery system  1000  to manipulate the flow of gas through gas delivery system  1000 . A first flow controller  1012  may be positioned inline with first inlet  1032  to control the flow rate of gas from first gas source  1010  through first manifold  1030 . A second flow controller  1017  may be positioned inline with second inlet  1034  and downstream of fourth inlet  1072  to control the flow rate of gas from second gas source  1015  through first manifold  1030 . A third flow controller  1019  may be positioned inline with fourth inlet  1072  to control the flow rate of gas from second gas source  1015  through third manifold  1070 .  
     [0092] As described above, first flow controller  1012  and second flow controller  1017  may be adapted to control the flow rate of gases passing through first manifold  1030  and third flow controller  1019  may be adapted to control the flow rate of gases passing through third manifold  1070 . In one embodiment, first flow controller  1012 , second flow controller  1017  and third flow controller  1019  each may comprise a valve containing a variable orifice which is manually adjusted to control the flow rate of gas passing through the valve body. For example, flow controllers  1012 ,  1017 , and  1019  each may comprise a needle valve which is adjusted to permit or restrict gas flow by the movement of a pointed plug or needle in an orifice or tapered orifice in the valve body. A wide variety of needle valves are commonly available to accommodate various fluid properties and fluid flow rates.  
     [0093] In another embodiment, first flow controller  1012 , second flow controller  1017  and third flow controller  1019  each may comprise an automatic flow controller which provides closed loop flow control of gases passing through the automatic flow controller. For example, flow controllers  1012 ,  1017 , and  1019  may each comprise a computer controlled mass flow controller (MFC). An MFC typically comprises an electronic control board, a thermal sensor, and a control valve. During operation, system controller  325  may direct an input signal representing an MFC setpoint to the electronic control board. The input signal received from system controller  325  causes the electronic control board to open the control valve, thereby allowing gas flow through the MFC. A portion of the gas flow through the MFC is directed across the thermal sensor, which generates an output signal proportional to the flow rate of the gas flowing through the MFC. The electronic control board monitors the thermal sensor output signal, compares it to the MFC setpoint, and adjusts the control valve to a setting that provides equalization between the setpoint and the thermal sensor output. Thus, an MFC provides a regulated and highly repeatable flow of gas by means of a closed loop mass flow control system. A wide variety of mass flow controllers are commonly available through manufacturers such as MKS, Horiba, and others to accommodate various fluid properties and fluid flow rates.  
     [0094] In yet another embodiment, first flow controller  1012 , second flow controller  1017  and third flow controller  1019  may comprise a combination of manually adjusted flow control valves and automatic flow controllers. For example, first flow controller  1012  and second flow controller  1017  may be structured as mass flow controllers and third flow controller  1019  may be structured as a needle valve. Alternatively, first flow controller  1012  and third flow controller  1019  may be structured as mass flow controllers and second flow controller  1017  may be structured as a needle valve.  
     [0095] Gas delivery system  1000  may further include one or more isolation valves for controlling the flow of gas through portions of gas delivery system  1000 . The term “isolation valve” in the following descriptions is generally used to describe a valve which may be configured to either an ON or an OFF condition. An isolation valve configured to an ON position allows for the passage of gas through the valve. Conversely, an isolation valve configured to an OFF position prevents the passage of gas through the valve. An isolation valve may be a computer controlled isolation valve. A computer controlled isolation valve is typically configured to an ON or OFF condition by means of a pneumatic or electrical input signal received from a computer, such as system controller  325 . An isolation valve may be either normally closed or normally open. A normally closed isolation valve is configured to an OFF condition in the absence of an input signal. A normally open isolation valve is configured to an ON condition in the absence of an input signal.  
     [0096] Isolation valves  1040 ,  1042 , and  1044  may be arranged inline with first inlet  1032 , second inlet  1034 , and fourth inlet  1072  immediately upstream and immediately downstream of flow controllers  1012 ,  1017 , and  1019 , respectively. Accordingly, isolation valves  1040 ,  1042 , and  1044  may be configured to control the flow of gas from first gas source  1010  and second gas source  1015  to downstream portions of gas delivery system  1000 . More specifically, isolation valves  1040 ,  1042 , and  1044  may each be selectively configured to an ON condition to allow for the passage of gas or to an OFF condition to prevent the passage of gas to downstream portions of gas delivery system  1000 . Additionally, isolation valves  1046  and  1048  may be arranged inline with fourth outlet  1074  and fifth outlet  1076  of third manifold  1070 , respectively. Isolation valves  1046  and  1048  may be selectively configured to control the flow of gas from second gas source  1015  through third manifold  1070  to second outlet  1054  and third outlet  1056  of second manifold  1050 , respectively.  
     [0097] During substrate processing, isolation valves  1040  and  1042  may each be configured to an ON condition, thereby allowing gas to flow from first gas source  1010  and second gas source  1015  through first flow controller  1012  and second flow controller  1017 , respectively. First flow controller  1012  may be configured to a first flow setpoint and second flow controller  1017  may be configured to a second flow setpoint, thereby controlling the flow rate and composition of gases passing through first manifold  1030  and into second manifold  1050 . Gas from first gas source  1010  and second gas source  1015  may be mixed together within first manifold  1030  and subsequently directed to third inlet  1036  of second manifold  1050 . The gas mixture comprising gas from first gas source  1010  and second gas source  1015  may then be directed into second outlet  1054  and third outlet  1056  of second manifold  1050 .  
     [0098] Isolation valves  1044  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1015  through third flow controller  1019 . Third flow controller  1019  may be configured to a third flow setpoint, thereby controlling the flow rate of gas from second gas source  1015  passing through third manifold  1070 . Isolation valve  1046  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1015  through fourth gas outlet  1074  into second gas outlet  1054  of second manifold  1050 . Similarly, isolation valve  1048  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1015  through fifth gas outlet  1076  into third gas outlet  1056  of second manifold  1050 . Gas flows directed into second gas outlet  1054  and third gas outlet  1056  from first manifold  1030  and third manifold  1070  may be subsequently directed into first inlet port  1006  and second inlet port  1007  of gas interface  1005 .  
     [0099] Isolation valves  1046 , and  1048  may be independently configurable such that one valve may be configured to an ON condition while another valve is configured to an OFF condition, or both valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of gas from second gas source  1015  through third manifold  1070  may be directed to either second outlet  1054  or third outlet  1056 , or to both second and third outlets simultaneously. As a result, third flow controller  1019  may be used to alter the concentration of gas from second gas source  1015  passing through second outlet  1054  or third outlet  1056 .  
     [0100] Gas delivery system  1000  allows the composition and flow rate of gases passing through second outlet  1054  to be varied independently of the composition and flow rate of gases passing through third outlet  1056 . Conversely, the composition and flow rate of gases passing through third outlet  1056  may be varied independently of the composition and flow rate of gases passing through second outlet  1054 . As a result, the composition and flow rate of the gas mixture passing through second outlet  1054  and/or third outlet  1056  may be “tuned” by altering the flow setpoint of third flow controller  1019 . Consequently, gas delivery system  1000  may be used to control process gas flows across two different portions of a substrate in a process chamber, thereby providing a means for minimizing mass transport effects across the surface of a substrate during processing.  
     [0101] In one embodiment gas delivery system  1000  may be integrated with a CVD processing system to control the composition and flow rate of a mixture of monosilane (SiH 4 ) and phosphine (PH 3 ) across two different portions of a silicon substrate. For example, first gas source  1010  may comprise monosilane and second gas source  1015  may comprise phosphine. During substrate processing, isolation valves  1040  and  1042  may each be configured to an ON condition, thereby allowing monosilane to flow from first gas source  1010  and phosphine to flow from second gas source  1015  through first flow controller  1012  and second flow controller  1017 , respectively. First flow controller  1012  may be configured to a first flow setpoint and second flow controller  1017  may be configured to a second flow setpoint, thereby controlling the flow rate and concentration of monosilane and phosphine passing through first manifold  1030  and into second manifold  1050 . Monosilane from first gas source  1010  and phosphine from second gas source  1015  may be mixed together within first manifold  1030  and subsequently directed to third inlet  1036  of second manifold  1050 . The monosilane and phosphine gas mixture may then be directed into second outlet  1054  and third outlet  1056  of second manifold  1050 .  
     [0102] In this embodiment, isolation valves  1044  may be configured to an ON condition, thereby allowing phosphine to flow from second gas source  1015  through third flow controller  1019 . Third flow controller  1019  may be configured to a third flow setpoint, thereby controlling the flow rate of phosphine from second gas source  1015  passing through third manifold  1070 . Isolation valve  1046  may be configured to an ON condition, thereby allowing phosphine to flow from second gas source  1015  through fourth gas outlet  1074  into second gas outlet  1054  of second manifold  1050 . Similarly, isolation valve  1048  may be configured to an ON condition, thereby allowing phosphine to flow from second gas source  1015  through fifth gas outlet  1076  into third gas outlet  1056  of second manifold  1050 . Monosilane and phosphine directed into second gas outlet  1054  and third gas outlet  1056  from first manifold  1030  and third manifold  1070  may be subsequently directed into first inlet port  1006  and second inlet port  1007  of gas interface  1005  and across the surface of a substrate.  
     [0103] Isolation valves  1046 , and  1048  may be independently configurable such that one valve may be configured to an ON condition while another valve is configured to an OFF condition, or both valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of phosphine from second gas source  1015  through third manifold  1070  may be directed to either second outlet  1054  or third outlet  1056 , or to both second and third outlets simultaneously. As a result, third flow controller  1019  may be used to alter the concentration of phosphine passing through second outlet  1054  or third outlet  1056 .  
     [0104] In alternative embodiments, first gas source  1010  may comprise an alternative source of silicon, such as dichlorosilane (SiH 2 Cl 2 ) or trichlorosilane (HSiCl 3 ) and second gas source  1015  may comprise germane. In other embodiments, gas delivery system  1000  may be integrated with a CVD processing system to control the composition and flow rate of a gas mixture comprising a silicon source and an inert gas across two different portions of a substrate. For example, first gas source  1010  may comprise monosilane, dichlorosilane, or trichlorosilane and second gas source  1015  may comprise hydrogen.  
     [0105] In the above description, gas delivery system  1000  is structured to a gas interface  1005  comprising two inlet ports  1006  and  1007 . However, it is to be noted that gas delivery system  1000  may be adapted to flow one or more gases to a variety of gas interfaces corresponding to various process chamber configurations.  
     [0106] For example, in one embodiment gas delivery system  1000  may be adapted to a gas interface such as gas interface  434  in FIG. 6 by dividing second gas outlet  1054  into two conduits coupled to first outside inlet port  610  and second outside inlet port  615 , and coupling third gas outlet  1056  to central inlet port  605 . Alternatively third gas outlet  1056  may be divided into two conduits which are coupled to first outside inlet port  610  and second outside inlet port  615  and second gas outlet  1054  may be coupled to central inlet port  605 . In either configuration, third flow controller  1019  may be used to alter the concentration of gas from second gas source  1015  passing through second gas outlet  1054  and third gas outlet  1056 , thereby increasing or decreasing the concentration of gas from second gas source  1015  in the gas flows passing across a central portion and first and second outside portions of a substrate.  
     [0107] In another embodiment, gas delivery system  1000  may be adapted to a gas interface such as gas interface  434  in FIG. 7 by dividing second gas outlet  1054  into three conduits which are coupled to first outside inlet port  720 , second outside inlet port  725 , and central inlet port  705 ; and dividing third gas outlet  1056  into two conduits which are coupled to first middle inlet port  710  and second middle inlet port  715 . Alternatively, third gas outlet  1056  may be divided into three conduits which are coupled to first outside inlet port  720 , second outside inlet port  725 , and central inlet port  705 ; and second gas outlet  1054  may be divided into two conduits which are coupled to first middle inlet port  710  and second middle inlet port  715 . In either configuration, third flow controller  1019  may be used to alter the concentration of gas from second gas source  1015  passing through second gas outlet  1054  and third gas outlet  1056 , thereby increasing or decreasing the concentration of gas from second gas source  1015  in the gas flows passing across central, first outside, second outside, first middle, and second middle portions of a substrate.  
     [0108] In yet another embodiment, gas delivery system  1000  may be adapted to a gas interface such as gas interface  875  in FIG. 8 by dividing second gas outlet  1054  into two conduits coupled to center inlet port  940  and outer inlet port  950 , and coupling third gas outlet  1056  to middle inlet port  945 . Alternatively third gas outlet  1056  may be divided into two conduits which are coupled to center inlet port  940  and outer inlet port  950 , and second gas outlet  1054  may be coupled to middle inlet port  945 . In either configuration, third flow controller  1019  may be used to alter the concentration of gas from second gas source  1015  passing through second gas outlet  1054  and third gas outlet  1056 , thereby increasing or decreasing the amount of gas passing across a central portion and middle and outer annular portions of a substrate.  
     [0109] Gas delivery system  1000  may also include a variety of inline filters, purifiers, pressure transducers, and other such devices as are commonly used on substrate processing systems. These types of components have been omitted for illustrative purposes so as to not obscure the description of the present invention.  
     [0110] Gas Delivery System II  
     [0111]FIG. 11 shows a schematic diagram illustrating a second embodiment of a gas delivery system  1100  for controlling the flow of gas to gas interface  1105 . Gas interface  1105  may be adapted to flow gas to a variety of process chambers. For example, gas interface  1105  may be substantially similar to gas interface  434  illustrated in FIG. 5, which is structured to provide two gas flow channels into upper portion  418  of process chamber  400 . Consequently, during substrate processing, a first gas flow entering a first inlet port  1106  may be directed to flow across a first portion of a substrate contained within a process chamber, and a second gas flow entering a second inlet port  1107  may be directed to flow across a second portion of the substrate.  
     [0112] Gas delivery system  1100  may include a first gas source  1110 , a second gas source  1115 , a third gas source  1120 , a first manifold  1130 , a second manifold  1150 , a third manifold  1170 , and a fourth manifold  1180 . First manifold  1130  may include a first inlet  1132 , a second inlet  1134 , a third inlet  1135 , and a first outlet  1136 . Second manifold  1150  may include a fourth inlet  1152 , a second outlet  1154 , and a third outlet  1156 . Third manifold  1170  may include a fifth inlet  1172 , a fourth outlet  1174 , and a fifth outlet  1176 . Fourth manifold  1180  may include a sixth inlet  1182 , a sixth outlet  1184 , and a seventh outlet  1186 . Gas interface  1105  may include first inlet port  1106  and second inlet port  1107 .  
     [0113] First inlet  1132 , second inlet  1134 , and third inlet  1135  of first manifold  1130  may be coupled to first gas source  1110 , second gas source  1115 , and third gas source  1120 , respectively. First outlet  1136  of first manifold  1130  may be coupled to fourth inlet  1152  of second manifold  1150 . Second outlet  1154  and third outlet  1156  of second manifold  1150  may be coupled to first inlet port  1106  and second inlet port  1107  of gas interface  1105 , respectively. Fifth inlet  1172  of third manifold  1170  may be coupled to second inlet  1134  of first manifold  1130 . Fourth outlet  1174  and fifth outlet  1176  of third manifold  1170  may be coupled to second outlet  1154  and third outlet  1156  of second manifold  1150 , respectively. Sixth inlet  1182  of fourth manifold  1180  may be coupled to third inlet  1135  of first manifold  1130 . Sixth outlet  1184  and seventh outlet  1186  of fourth manifold  1180  may be coupled to second outlet  1154  and third outlet  1156  of second manifold  1150 , respectively.  
     [0114]FIG. 11 shows sixth outlet  1184  of fourth manifold  1180  as being coupled to second outlet  1154  of second manifold  1150  downstream of the point at which fourth outlet  1174  of third manifold  1170  is coupled to second outlet  1154  of second manifold  1150 . Similarly, FIG. 11 shows seventh outlet  1186  of fourth manifold  1180  as being coupled to second outlet  1156  of second manifold  1150  downstream of the point at which fifth outlet  1176  of third manifold  1170  is coupled to third outlet  1156  of second manifold  1150 . In alternative embodiments, sixth outlet  1184  of fourth manifold  1180  may be coupled to second outlet  1154  of second manifold  1150  upstream of the point at which fourth outlet  1174  of third manifold  1170  is coupled to second outlet  1154  of second manifold  1150 . Similarly, seventh outlet  1186  of fourth manifold  1180  may be coupled to second outlet  1154  of second manifold  1150  upstream of the point at which fifth outlet  1176  of third manifold  1170  is coupled to third outlet  1156  of second manifold  1150 .  
     [0115] Flow controllers may be structured to gas delivery system  1100  to manipulate the flow of gas through gas delivery system  1100 . A first flow controller  1112  may be positioned inline with first inlet  1132  to control the flow rate of gas from first gas source  1140  through first manifold  1130 . A second flow controller  1115  may be positioned inline with second inlet  1134  and downstream of fifth inlet  1172  to control the flow rate of gas from second gas source  1115  through first manifold  1130 . A third flow controller may be positioned inline with third inlet  1135  to control the flow rate of gas from third gas source  1120  through first manifold  1130 . A fourth flow controller  1119  may be positioned inline with fifth inlet  1172  to control the flow rate of gas from second gas source  1115  through third manifold  1170 . A fifth flow controller  1124  may be positioned inline with sixth inlet  1182  to control the flow rate of gas from third gas source  1120  through fourth manifold  1180 .  
     [0116] In one embodiment, first flow controller  1112 , second flow controller  1117 , third flow controller  1122 , fourth flow controller  1119 , and fifth flow controller  1124  each may comprise a valve containing a variable orifice which is manually adjusted to control the flow rate of gas passing through the valve body. For example, flow controllers  1112 ,  1117 ,  1122 ,  1119 , and  1124  each may comprise a needle valve which is adjusted to permit or restrict gas flow. In another embodiment, flow controllers  1112 ,  1117 ,  1122 ,  1119 , and  1124  each may comprise an automatic flow controller, such as a computer controlled mass flow controller, which provides closed loop flow control. In yet another embodiment, flow controllers  1112 ,  1117 ,  1122 ,  1119 , and  1124  may comprise a combination of manually adjusted flow control valves and automatic flow controllers. For example, first flow controller  1112 , second flow controller  1117 , and third flow controller  1122  may be structured as mass flow controllers; and fourth flow controller  1119  and fifth flow controller  1124  may be structured as needle valves. Alternatively, first flow controller  1112 , fourth flow controller  1119 , and fifth flow controller  1124  may be structured as mass flow controllers; and second flow controller  1117  and third flow controller  1122  may be structured as a needle valves.  
     [0117] Gas delivery system  1100  may further include one or more isolation valves for controlling the flow of gas through portions of gas delivery system  1100 . Isolation valves  1140 ,  1142 , and  1162  may be arranged inline with first inlet  1132 , second inlet  1134 , and third inlet  1135  of first manifold  1130  immediately upstream and immediately downstream of flow controllers  1112 ,  1117 , and  1122 , respectively. Additionally, isolation valves  1144  and  1164  may be arranged inline with fourth inlet  1172  of third manifold  1170  and fifth inlet  1182  of fourth manifold  1180  immediately upstream and immediately downstream of flow controllers  1119 , and  1124 , respectively. Accordingly, isolation valves  1140 ,  1142 ,  1144 ,  1162 , and  1164  may be configured to control the flow of gas from first gas source  1110 , second gas source  1115 , and third gas source  1120  to downstream portions of gas delivery system  1100 . More specifically, isolation valves  1140 ,  1142 ,  1144 ,  1162 , and  1164  may each be selectively configured to an ON condition to allow for the passage of gas or to an OFF condition to prevent the passage of gas to downstream portions of gas delivery system  1100 .  
     [0118] Isolation valves  1146  and  1148  may be arranged inline with fourth outlet  1174  and fifth outlet  1176  of third manifold  1170 , respectively. Isolation valves  1146  and  1148  may be selectively configured to control the flow of gas from second gas source  1115  through third manifold  1170  to second outlet  1154  and third outlet  1156  of second manifold  1150 . Isolation valves  1166  and  1168  may be arranged inline with sixth outlet  1184  and seventh outlet  1186  of fourth manifold  1180 , respectively. Isolation valves  1166  and  1168  may be selectively configured to control the flow of gas from third gas source  1120  through fourth manifold  1180  to second outlet  1154  and third outlet  1156  of second manifold  1150 .  
     [0119] During substrate processing, isolation valves  1140 ,  1142 , and  1162  may each be configured to an ON condition, thereby allowing gas to flow from first gas source  1110 , second gas source  1115 , and third gas source  1120  through first flow controller  1112 , second flow controller  1117 , and third flow controller  1122 , respectively. First flow controller  1112  may be configured to a first flow setpoint, second flow controller  1117  may be configured to a second flow setpoint, and third flow controller  1122  may be configured to a third flow setpoint, thereby controlling the flow rate and composition of gases passing through first manifold  1130  and into second manifold  1150 . Gases from first gas source  1110 , second gas source  1115 , and third gas source  1120  may mix together within first manifold  1130  and subsequently enter fourth inlet  1152  of second manifold  1150 . The gas mixture comprising gas from first gas source  1110 , second gas source  1115 , and third gas source  1120  may then flow into second outlet  1154  and third outlet  1156  of second manifold  1150 .  
     [0120] Isolation valves  1144  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1115  through fourth flow controller  1119 . Fourth flow controller  1119  may be configured to a fourth flow setpoint, thereby controlling the flow rate of gas from second gas source  1115  passing through third manifold  1170 . Isolation valve  1146  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1115  through fourth gas outlet  1174  into second gas outlet  1154  of second manifold  1150 . Similarly, isolation valve  1148  may be configured to an ON condition, thereby allowing gas to flow from second gas source  1115  through fifth gas outlet  1176  into third gas outlet  1156  of second manifold  1150 . Isolation valves  1146  and  1148  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of gas from second gas source  1115  through third manifold  1170  may be directed to either second outlet  1154  or third outlet  1156 , or to both second and third outlets simultaneously.  
     [0121] As above, isolation valves  1166  and  1168  may be configured to an ON condition, thereby allowing gas to flow from third gas source  1120  through fifth flow controller  1124 . Fifth flow controller  1124  may be configured to a fifth flow setpoint, thereby controlling the flow rate of gas from third gas source  1120  passing through fourth manifold  1180 . Isolation valve  1166  may be configured to an ON condition, thereby allowing gas flow from third gas source  1120  through sixth gas outlet  1184  into second gas outlet  1154  of second manifold  1150 . Similarly, isolation valve  1168  may be configured to an ON condition, thereby allowing gas flow from third gas source  1120  through seventh gas outlet  1186  into third gas outlet  1156  of second manifold  1150 . Isolation valves  1166  and  1168  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of gas from third gas source  1120  through fourth manifold  1180  may be directed to either second outlet  1154  or third outlet  1156 , or to both second and third outlets simultaneously. Gas flows directed into second gas outlet  1154  and third gas outlet  1156  from first manifold  1130 , third manifold  1170 , and fourth manifold  1180  are subsequently directed into first inlet port  1106  and second inlet port  1107  of gas interface  1105 .  
     [0122] The flow of gas from second gas source  1115  through third manifold  1170  and/or the flow of gas from third gas source  1120  through fourth manifold  1180  may be directed to either second outlet  1154  or third outlet  1156 , or to both second and third outlets simultaneously. Hence, the composition and flow rate of gases passing through second outlet  1154  may be varied independently of the composition and flow rate of gases passing through third outlet  1156 , and the composition and flow rate of gases passing through third outlet  1156  may be varied independently of the composition and flow rate of gases passing through second outlet  1154 . As a result, the composition and flow rate of the gas mixture passing through second outlet  1154  and/or third outlet  1156  may be “tuned” by altering the flow setpoint of fourth flow controller  1119  and fifth flow controller  1124 . Consequently, gas delivery system  1100  may be used to control process gas flows across two different portions of a substrate in a process chamber, thereby providing a means for minimizing mass transport effects across the surface of a substrate during processing.  
     [0123] In one embodiment, gas delivery system  1100  may be integrated with a CVD processing system to control the composition and flow rate of a mixture of monosilane (SiH 4 ), germane (GeH 4 ), and diborane (B 2 H 6 ) across two different portions of a silicon wafer. For example, first gas source  1110  may comprise monosilane, second gas source  1115  may comprise germane, and third gas source  1120  may comprise diborane. These gases may also be diluted by an inert carrier gas, such as hydrogen (H 2 ). During substrate processing, isolation valves  1140 ,  1142 , and  1162  may each be configured to an ON condition, thereby allowing monosilane to flow from first gas source  1110 , germane to flow from second gas source  1115 , and diborane to flow from third gas source  1120  through first flow controller  1112 , second flow controller  1117 , and third flow controller  1122 , respectively. First flow controller  1112  may be configured to a first flow setpoint, second flow controller  1117  may be configured to a second flow setpoint, and third flow controller  1122  may be configured to a third flow setpoint, thereby controlling the flow rate and composition of monosilane, germane, and diborane passing through first manifold  1130  and into second manifold  1150 . Monosilane from first gas source  1110 , germane from second gas source  1115 , and diborane from third gas source  1120  may mix together within first manifold  1130  and subsequently enter fourth inlet  1152  of second manifold  1150 . The gas mixture comprising monosilane, germane, and diborane may then flow into second outlet  1154  and third outlet  1156  of second manifold  1150 .  
     [0124] Isolation valves  1144  may be configured to an ON condition, thereby allowing germane to flow from second gas source  1115  through fourth flow controller  1119 . Fourth flow controller  1119  may be configured to a fourth flow setpoint, thereby controlling the flow rate of germane from second gas source  1115  passing through third manifold  1170 . Isolation valve  1146  may be configured to an ON condition, thereby allowing germane to flow from second gas source  1115  through fourth gas outlet  1174  into second gas outlet  1154  of second manifold  1150 . Similarly, isolation valve  1148  may be configured to an ON condition, thereby allowing germane to flow from second gas source  1115  through fifth gas outlet  1176  into third gas outlet  1156  of second manifold  1150 . Isolation valves  1146  and  1148  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of germane from second gas source  1115  through third manifold  1170  may be directed to either second outlet  1154  or third outlet  1156 , or to both second and third outlets simultaneously. As a result, fourth flow controller  1119  may be used to alter the concentration of germane passing through second outlet  1154  and/or third outlet  1156 .  
     [0125] Similarly, isolation valves  1164  may be configured to an ON condition, thereby allowing diborane to flow from third gas source  1120  through fifth flow controller  1124 . Fifth flow controller  1124  may be configured to a fifth flow setpoint, thereby controlling the flow rate of diborane from third gas source  1120  passing through fourth manifold  1180 . Isolation valve  1166  may be configured to an ON condition, thereby allowing diborane to flow from third gas source  1120  through sixth gas outlet  1184  into second gas outlet  1154  of second manifold  1150 . Similarly, isolation valve  1168  may be configured to an ON condition, thereby allowing diborane to flow from third gas source  1120  through seventh gas outlet  1186  into third gas outlet  1156  of second manifold  1150 . Isolation valves  1166  and  1168  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of diborane from third gas source  1120  through fourth manifold  1180  may be directed to either second outlet  1154  or third outlet  1156 , or to both second and third outlets simultaneously. As a result, fifth flow controller  1124  may be used to alter the concentration of diborane passing through second outlet  1154  and/or third outlet  1156 .  
     [0126] The flow of monosilane, germane, and diborane directed into second gas outlet  1154  and third gas outlet  1156  from first manifold  1130 , third manifold  1170 , and fourth manifold  1180  may be subsequently directed into first inlet port  1106  and second inlet port  1107  of gas interface  1105  and across the surface of a substrate. In alternative embodiments, first gas source  1110  may comprise an alternative source of silicon, such as dichlorosilane (SiH 2 Cl 2 ) or trichlorosilane (HSiCl 3 ), second gas source  1115  may comprise germane, and third gas source  1120  may comprise diborane.  
     [0127] In the above description, gas delivery system  1100  is structured to a gas interface  1105  comprising two inlet ports  1106  and  1107 . However, it is to be noted that gas delivery system  1100  may be adapted to flow one or more gases to a variety of gas interfaces corresponding to various process chamber configurations.  
     [0128] For example, in one embodiment gas delivery system  1100  may be adapted to a gas interface such as gas interface  434  in FIG. 6 by dividing second gas outlet  1154  into two conduits coupled to first outside inlet port  610  and second outside inlet port  615 , and coupling third gas outlet  1156  to central inlet port  605 . Alternatively third gas outlet  1156  may be divided into two conduits which are coupled to first outside inlet port  610  and second outside inlet port  615 , and second gas outlet  1154  may be coupled to central inlet port  605 . In either configuration, fourth flow controller  1119  may be used to alter the concentration of gas from second gas source  1115  passing through second gas outlet  1154  and third gas outlet  1156 , thereby increasing or decreasing the concentration of gas from second gas source  1115  in the gas flows passing across a central portion and first and second outside portions of a substrate. Similarly, fifth flow controller  1124  may be used to alter the concentration of gas from third gas source  1120  passing through second gas outlet  1154  and third gas outlet  1156 , thereby increasing or decreasing the concentration of gas from third gas source  1120  in the gas flows passing across a central portion and first and second outside portions of a substrate.  
     [0129] In another embodiment, gas delivery system  1100  may be adapted to a gas interface such as gas interface  434  in FIG. 7 by dividing second gas outlet  1154  into three conduits which are coupled to first outside inlet port  720 , second outside inlet port  725 , and central inlet port  705 ; and dividing third gas outlet  1156  into two conduits which are coupled to first middle inlet port  710  and second middle inlet port  715 . Alternatively, third gas outlet  1156  may be divided into three conduits which are coupled to first outside inlet port  720 , second outside inlet port  725 , and central inlet port  705 ; and second gas outlet  1154  may be divided into two conduits which are coupled to first middle inlet port  710  and second middle inlet port  715 . In either configuration, fourth flow controller  1119  may be used to alter the concentration of gas from second gas source  1115  passing through second gas outlet  1154  and third gas outlet  1156 , thereby increasing or decreasing the concentration of gas from second gas source  1115  in the gas flows passing across central, first outside, second outside, first middle, and second middle portions of a substrate. Similarly, fifth flow controller  1124  may be used to alter the concentration of gas from third gas source  1120  passing through second gas outlet  1154  and third gas outlet  1156 , thereby increasing or decreasing the concentration of gas from second gas source  1115  in the gas flows passing across central, first outside, second outside, first middle, and second middle portions of a substrate.  
     [0130] In yet another embodiment, gas delivery system  1100  may be adapted to a gas interface such as gas interface  875  in FIG. 8 by dividing second gas outlet  1154  into two conduits coupled to center inlet port  940  and outer inlet port  950 , and coupling third gas outlet  1156  to middle inlet port  945 . Alternatively third gas outlet  1156  may be divided into two conduits which are coupled to center inlet port  940  and outer inlet port  950 , and second gas outlet  1154  may be coupled to middle inlet port  945 . In either configuration, fourth flow controller  1119  may be used to alter the concentration of gas from second gas source  1115  passing through second gas outlet  1054  and third gas outlet  1056 , thereby increasing or decreasing the concentration of gas from gas source  1115  passing across a central portion and middle and outer annular portions of a substrate. Similarly, fifth flow controller  1124  may be used to alter the concentration of gas from third gas source  1120  passing through second gas outlet  1154  and third gas outlet  1156 , thereby increasing or decreasing the concentration of gas from third gas source  1120  passing across a central portion and middle and outer annular portions of a substrate.  
     [0131] Gas delivery system  1100  may also include a variety of inline filters, purifiers, pressure transducers, and other such devices as are commonly used on substrate processing systems. These types of components have been omitted for illustrative purposes so as to not obscure the description of the present invention.  
     [0132] Gas Delivery System III  
     [0133]FIG. 1 shows a schematic diagram illustrating a preferred embodiment of a gas delivery system  100  for controlling the flow of gas to gas interface  105 . Gas interface  105  may be adapted to flow gas to a variety of process chambers. For example, gas interface  105  may be substantially similar to gas interface  434  illustrated in FIG. 6, which is structured to provide three gas flow channels into upper portion  418  of process chamber  400 . Consequently, during substrate processing, a first gas flow entering a first inlet port  106  may be directed to flow across a first outside portion of a substrate contained within a process chamber, a second gas flow entering a second inlet port  107  may be directed to flow across a second outside portion of the substrate, and a third gas flow entering a third inlet port  108  may be directed to flow across a central portion of the substrate.  
     [0134] Gas delivery system  100  may include a first gas source  110 , a second gas source  120 , a third gas source  130 , a fourth gas source  140 , a fifth gas source  150 , a first manifold  160 , a second manifold  170 , a third manifold  175 , a fourth manifold  185 , a fifth manifold  190 , a sixth manifold  125 , and a seventh manifold  195 . First manifold  160  may include a first inlet  161 , a second inlet  163 , a third inlet  165 , a fourth inlet  167 , and a first outlet  169 . Second manifold  170  may include a fifth inlet  171 , a second outlet  172 , and a third outlet  173 . Third manifold  175  may include a sixth inlet  176 , a fourth outlet  180 , and a fifth outlet  181 . Fourth manifold  185  may include a seventh inlet  184 , a sixth outlet  186 , and a seventh outlet  187 . Fifth manifold  190  may include an eighth inlet  191 , a ninth inlet  192 , and an eighth outlet  193 . Sixth manifold  125  may include a tenth inlet  126 , a ninth outlet  127 , and a tenth outlet  128 . Seventh manifold  195  may include an eleventh inlet  196 , an eleventh outlet  197 , and a twelfth outlet  198 . Gas interface  105  may include first inlet port  106 , second inlet port  108 , and third inlet port  107 .  
     [0135] First inlet  161 , second inlet  163 , third inlet  165 , and fourth inlet  167  of first manifold  160  may be coupled to first gas source  110 , second gas source  120 , third gas source  130 , and ninth outlet  127  of sixth manifold  125 , respectively. First outlet  169  of first manifold  160  may be coupled to fifth inlet  171  of second manifold  170 . Second outlet  172  of second manifold  170  may be coupled to sixth inlet  176  of third manifold  175 ; third outlet  173  of second manifold  170  may be coupled to third inlet port  108 . Fourth outlet  180  and fifth outlet  181  of third manifold  175  may be coupled to first inlet port  106  and second inlet port  107 , respectively.  
     [0136] Seventh inlet  184  of fourth manifold  185  may be coupled to third inlet  165  of first manifold  160 . Sixth outlet  186  of fourth manifold  185  may be coupled to second outlet  172  of second manifold  170 . Similarly, seventh outlet  187  of fourth manifold  185  may be coupled to third outlet  173  of second manifold  170 . Eighth inlet  191  and ninth inlet  192  of fifth manifold  190  may be coupled to fourth gas source  140  and fifth gas source  150 , respectively. Eighth outlet  193  of fifth manifold  190  may be coupled to tenth inlet  126  of sixth manifold  125 . Ninth outlet  127  of sixth manifold  125  may be coupled to fourth inlet  167  of first manifold  160 . Tenth outlet  128  of sixth manifold  125  may be coupled to eleventh inlet  196  of seventh manifold  195 . Eleventh outlet  197  of seventh manifold  195  may be coupled to second outlet  172  of second manifold  170 . Twelfth outlet  198  of seventh manifold  195  may be coupled to third outlet  173  of second manifold  170 .  
     [0137] Gas delivery system  100  may further include flow controllers to manipulate the flow of gas through gas delivery system  100 . A first flow controller  112  may be positioned inline with first inlet  161  to control the flow rate of gas from first gas source  110  through first manifold  160 . A second flow controller  122  may be positioned inline with second inlet  163  to control the flow rate of gas from second gas source  120  through first manifold  160 . A third flow controller  132  may be positioned inline with third inlet  165  to control the flow rate of gas from third gas source  130  through first manifold  160 ; a fourth flow controller  134  may be positioned inline with seventh inlet  184  to control the flow rate of gas from third gas source  130  through fourth manifold  185 . A fifth flow controller  142  may be positioned inline with fourth inlet  167  to control the flow rate of gas from fourth gas source  140  and/or fifth gas source  150  through first manifold  160 . A sixth flow controller  152  may be positioned inline with eleventh inlet  196  to control the flow rate of gas from fourth gas source  140  and/or fifth gas source  150  through seventh manifold  195 . Flow controllers  112 ,  122 ,  132 ,  134 ,  142 , and  152  are preferably computer controlled mass flow controllers, such as Series 8100 and Series 1660 mass flow controllers manufactured by the UNIT Corporation.  
     [0138] Gas delivery system  100  may further include a plurality of isolation valves for controlling the flow of gas through portions of gas delivery system  100 . Isolation valves  113 ,  123 ,  133 , and  143  may be arranged inline with first inlet  161 , second inlet  163 , third inlet  165 , and fourth inlet  167  of first manifold  160  immediately upstream and immediately downstream of flow controllers  112 ,  122 ,  132 , and  142 , respectively. Isolation valves  135  may be may be arranged inline with seventh inlet  184  of fourth manifold  185  immediately upstream and immediately downstream of flow controller  134 ; isolation valves  137  and  139  may be arranged inline with sixth outlet  186  and seventh outlet  187  of fourth manifold  185 , respectively. Isolation valve  137  may be configured to control the flow of gas from third gas source  130  through sixth outlet  186  of fourth manifold  185  to second outlet  172  of second manifold  170 . Similarly, isolation valve  139  may be configured to control the flow of gas from third gas source  130  through seventh outlet  187  of fourth manifold  185  to third outlet  173  of second manifold  170 .  
     [0139] Isolation valves  145  and  155  may be arranged inline with eighth inlet  191  and ninth inlet  192  of fifth manifold  190 . Isolation valves  153  may be arranged inline with eleventh inlet  196  immediately upstream and immediately downstream of flow controller  152 ; isolation valves  157  and  159  may be arranged inline with eleventh outlet  197  and twelfth outlet  198  of seventh manifold  195 , respectively. Isolation valve  157  may be configured to control the flow of gas from fourth gas source  140  and/or fifth gas source  150  through seventh manifold  195  to sixth inlet  176  of third manifold  175 . Similarly, isolation valve  159  may be configured to control the flow of gas from fourth gas source  140  and/or fifth gas source  150  through seventh manifold  195  to third outlet  173  of second manifold  170 .  
     [0140] Isolation valves  113 ,  123 ,  133 ,  135 ,  143 ,  137 ,  139 ,  145 ,  155 ,  153 ,  157 , and  159  are preferably Veriflo Series 944, 945, and 955 pneumatic diaphragm valves manufactured by the Parker Hannifin Corporation. Additionally, isolation valves  113 ,  123 ,  133 ,  135 ,  143 ,  137 ,  139 ,  145 ,  155 ,  153 ,  157 , and  159  are preferably computer controlled isolation valves controlled, for example, by system controller  325 .  
     [0141] During substrate processing, isolation valves  113 ,  123 ,  133  may each be configured to an ON condition, thereby allowing gas to flow from first gas source  110 , second gas source  120 , and third gas source  130  through first flow controller  112 , second flow controller  122 , and third flow controller  132 , respectively. Additionally, isolation valves  143 ,  145  and/or  155  may be configured to an ON condition, thereby allowing gas to flow from fourth gas source  140  and/or fifth gas source  150  through fifth flow controller  142 . First flow controller  112  may be configured to a first flow setpoint, second flow controller  122  may be configured to a second flow setpoint, third flow controller  132  may be configured to a third flow setpoint, and fifth flow controller  142  may be configured to a fifth flow setpoint, thereby controlling the flow rate and composition of gases passing through first manifold  160  and into second manifold  170 . Gases from first gas source  110 , second gas source  120 , third gas source  130 , fourth gas source  140 , and/or fifth gas source  150  may mix together within first manifold  160  and subsequently enter fifth inlet  171  of second manifold  170 . The gas mixture comprising gas from first gas source  110 , second gas source  120 , third gas source  130 , fourth gas source  140 , and/or fifth gas source  150  may then flow into second outlet  172  and third outlet  173  of second manifold  170 . From second outlet  172  of second manifold  170 , the gas mixture may flow into sixth inlet  176  of third manifold  175 . From sixth inlet  176 , the gas mixture may flow through fourth outlet  180  and fifth outlet  181  of third manifold  175  into first inlet port  106  and second inlet port  107 , respectively.  
     [0142] Isolation valves  135  may be configured to an ON condition, thereby allowing gas to flow from third gas source  130  through fourth flow controller  134 . Fourth flow controller  134  may be configured to a fourth flow setpoint, thereby controlling the flow rate of gas from third gas source  130  passing through fourth manifold  185 . Isolation valve  137  may be configured to an ON condition, thereby allowing gas to flow from third gas source  130  through sixth outlet  186 . Isolation valve  137  may be configured to an ON condition, thereby allowing gas to flow from third gas source  130  through sixth outlet  186  of fourth manifold  185  into second outlet  172  of second manifold  170 . Similarly, isolation valve  139  may be configured to an ON condition, thereby allowing gas to flow from third gas source  130  through seventh gas outlet  187  into third outlet  173  of second manifold  170 . Isolation valves  137  and  139  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of gas from third gas source  130  through fourth manifold  185  may be directed separately to second outlet  172  or to third outlet  173 . Alternatively the flow of gas from third gas source  130  through fourth manifold  185  may be directed to second outlet  172  and to third outlet  173  simultaneously.  
     [0143] Isolation valves  153 ,  145  and/or  155  may be configured to an ON condition, thereby allowing gas to flow from fourth gas source  140  and/or fifth gas source  150  through sixth flow controller  152 . Sixth flow controller  152  may be configured to a sixth flow setpoint, thereby controlling the flow rate and composition of gases passing through seventh manifold  195 . Isolation valve  157  may be configured to an ON condition, thereby allowing gas to flow from fourth gas source  140  and/or fifth gas source  150  through eleventh outlet  197  of seventh manifold  195  into sixth inlet  176  of third manifold  175 . Similarly, isolation valve  159  may be configured to an ON condition, thereby allowing gas to flow from fourth gas source  140  and/or fifth gas source  150  through twelfth outlet  198  of seventh manifold  195  into third outlet  173  of second manifold  170 . Isolation valves  157  and  159  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow of gas from fourth gas source  140  and/or fifth gas source  150  through seventh manifold  195  may be directed separately to sixth inlet  176  or to third outlet  173 . Alternatively the flow of gas from third gas source  130  through fourth manifold  185  may be directed to sixth inlet  176  and to third outlet  173  simultaneously.  
     [0144] Gas delivery system  100  may further include a first metering valve  178  and a second metering valve  179  positioned inline with second outlet  172  and third outlet  173  of second manifold  170 . Metering valves  178  and  179  may be used to proportion the flow of gases passing through second manifold  170  between second outlet  172  and third outlet  173 . For example, first metering valve  178  may be adjusted to have a greater flow restriction than second metering valve  179  such that a greater proportion of gases from fifth inlet  171  will be diverted into third outlet  173  than second outlet  172 . Alternatively, second metering valve  179  may be adjusted to have a greater flow restriction than first metering valve  178  such that a greater proportion of gases from fifth inlet  171  will be diverted into second outlet  172  than third outlet  173 . In a preferred embodiment, metering valves  178  and  179  are computer controlled flowPoint valves manufactured by Applied Precision of Issaquah, Wash., such as flowpoint valve part number 53-710150-000. Metering valves  178  and  179  may be controlled, for example, by an input signal generated by system controller  325 .  
     [0145] As discussed above, the flow of gas from third gas source  130  through fourth manifold  185  may be directed to fourth and fifth outlets  180  and  181  or to third outlet  173 . Hence, the composition and flow rate of the gas mixture passing through fourth and fifth outlets  180  and  181  or third outlet  173  may be altered by varying the flow setpoint of fourth flow controller  134 . Similarly, the flow of gas from fourth gas source  140  and/or fifth gas source  150  may be directed to fourth and fifth outlets  180  and  181  or third outlet  173 . Hence, the composition and flow rate of the gas mixture passing through fourth and fifth outlets  180  and  181  or third outlet  173  may also be altered by varying the flow setpoint of sixth flow controller  152 . As shown in FIG. 1, fourth outlet  180 , fifth outlet  181 , and third outlet  173  may be connected to first inlet port  106 , second inlet port  107 , and third inlet port  108 , respectively. As a result, the composition and flow rate of the gas mixture passing through first inlet port  106 , second inlet port  107 , and third inlet port  108  may be “tuned” by altering the flow setpoint of fourth flow controller  134  and sixth flow controller  152 . Consequently, gas delivery system  100  may be used to control process gas flows across three different portions of a substrate in a process chamber.  
     [0146] In one embodiment, gas delivery system  100  may be integrated with a CVD processing system to control the composition and flow rate of a mixture of hydrogen (H 2 ), dichlorosilane (SiH 2 Cl 2 ), and a 10% mixture of germane (GeH 4 ) in hydrogen across three different portions of a silicon wafer in order to deposit a layer of epitaxial SiGe onto the surface of a substrate. For example, first gas source  110  may comprise hydrogen, second gas source  120  may comprise dichlorosilane, and third gas source  130  may comprise a 10% mixture of germane in hydrogen. During substrate processing, isolation valves  113 ,  123 ,  133  may each be configured to an ON condition, thereby allowing hydrogen to flow from first gas source  110 , dichlorosilane to flow from second gas source  120 , and a mixture of germane and hydrogen to flow from third gas source  130  through first flow controller  112 , second flow controller  122 , and third flow controller  132 , respectively. Additionally, isolation valves  143 ,  145  and/or  155  may be configured to an ON condition, thereby allowing gas to flow from fourth gas source  140  and/or fifth gas source  150  through fifth flow controller  142 . First flow controller  112  may be configured to a first flow setpoint, second flow controller  122  may be configured to a second flow setpoint, third flow controller  132  may be configured to a third flow setpoint, and fifth flow controller  142  may be configured to a fifth flow setpoint, thereby controlling the flow rate and composition of gases passing through first manifold  160  and into second manifold  170 . Hydrogen from first gas source  110 , dichlorosilane from second gas source  120 , germane and hydrogen from third gas source  130 , and gases from fourth gas source  140  and/or fifth gas source  150  may mix together within first manifold  160  and subsequently enter fifth inlet  171 . The gas mixture may then flow into second outlet  172  and third outlet  173 . From second outlet  172 , the gas mixture may flow through sixth inlet  176 , fourth outlet  180 , and fifth outlet  181  into first inlet port  106  and second inlet port  107 .  
     [0147] In this embodiment, isolation valves  135  may be configured to an ON condition, thereby allowing germane and hydrogen from third gas source  130  to flow through fourth flow controller  134 . Fourth flow controller  134  may be configured to a fourth flow setpoint, thereby controlling the flow rate of germane and hydrogen from third gas source  130  passing through fourth manifold  185 . Isolation valve  137  may be configured to an ON condition, thereby allowing germane and hydrogen to pass through sixth outlet  186 . Isolation valve  137  may be configured to an ON condition, thereby allowing germane and hydrogen to pass through sixth outlet  186  into second outlet  172 . Similarly, isolation valve  139  may be configured to an ON condition, thereby allowing germane and hydrogen to pass through seventh gas outlet  187  into third outlet  173 . Isolation valves  137  and  139  may be independently configurable such that one valve may be configured to an ON condition while the other valve is configured to an OFF condition, or both isolation valves may be configured to an ON or OFF condition simultaneously. Consequently, the flow germane and hydrogen through fourth manifold  185  may be directed separately to second outlet  172  or to third outlet  173 . Alternatively the flow of germane and hydrogen through fourth manifold  185  may be directed to second outlet  172  and to third outlet  173  simultaneously.  
     [0148] In this embodiment, first metering valve  178  and second metering valve  179  may be used to proportion the flow of gases passing through second manifold  170  between second outlet  172  and third outlet  173 . For example, first metering valve  178  may be adjusted to have a greater flow restriction than second metering valve  179  such that a greater proportion of gases from fifth inlet  171  will be diverted into third outlet  173  than second outlet  172 . Alternatively, second metering valve  179  may be adjusted to have a greater flow restriction than first metering valve  178  such that a greater proportion of gases from fifth inlet  171  will be diverted into second outlet  172  than third outlet  173 .  
     [0149] As discussed above, the flow of germane and hydrogen from third gas source  130  through fourth manifold  185  may be directed to fourth and fifth outlets  180  and  181  or to third outlet  173 . Hence, the concentration of germane and hydrogen passing through fourth and fifth outlets  180  and  181  or third outlet  173  may be altered by varying the flow setpoint of fourth flow controller  134 . As shown in FIG. 1, fourth outlet  180 , fifth outlet  181 , and third outlet  173  may be connected to first inlet port  106 , second inlet port  107 , and third inlet port  108 , respectively. As a result, the concentration of germane and hydrogen passing through first inlet port  106 , second inlet port  107 , and third inlet port  108  may be “tuned” by altering the flow setpoint of fourth flow controller  134 . Consequently, gas delivery system  100  may be used to control process gas flows across three different portions of a substrate in a process chamber.  
     [0150] In the above description, gas delivery system  100  is structured to a gas interface  105  comprising three inlet ports  106 ,  107 , and  108 . However, it is to be noted that gas delivery system  1100  may be adapted to flow one or more gases to a variety of gas interfaces corresponding to various process chamber configurations.  
     [0151] For example, in one embodiment gas delivery system  100  may be adapted to a gas interface such as gas interface  434  in FIG. 7 by coupling third outlet  173  to central inlet port  705 ; dividing fourth outlet  180  into two conduits which are coupled to first outside inlet port  720  and second outside inlet port  725 ; and dividing fifth outlet  181  into two conduits which are coupled to first middle inlet port  710  and second middle inlet port  715 . In this embodiment, fourth flow controller  134  may be used to alter the concentration of gas from third gas source  130  passing through third outlet  173 , thereby increasing or decreasing the concentration of gas from third gas source  130  in the gas flow passing across a central portion of a substrate. Similarly, fourth flow controller  134  may also be used to alter the concentration of gas from third gas source  130  passing through fourth outlet  180  and fifth outlet  181 , thereby increasing or decreasing the concentration of gas from third gas source  130  in the gas flows passing across first outside, second outside, first middle, and second middle portions of a substrate.  
     [0152] In yet another embodiment, gas delivery system  100  may be adapted to a gas interface such as gas interface  875  in FIG. 8 by coupling third outlet  173  to middle inlet port  945 , coupling fourth outlet  180  to center inlet port  940 , and coupling fifth outlet  181  to outer inlet port  950 . In this embodiment, fourth flow controller  134  may be used to alter the concentration of gas from third gas source  130  passing through third outlet  173 , thereby increasing or decreasing the concentration of gas from third gas source  130  in the gas flow passing across a middle annular portion of a substrate. Similarly, fourth flow controller  134  may also be used to alter the concentration of gas from third gas source  130  passing through fourth outlet  180  and fifth outlet  181 , thereby increasing or decreasing the concentration of gas from third gas source  130  in the gas flow passing across a central portion and outer annular portions of a substrate.  
     [0153] Gas delivery system  100  may also include a variety of inline filters, purifiers, pressure transducers, and other such devices as are commonly used on substrate processing systems. These types of components have been omitted for illustrative purposes so as to not obscure the description of the present invention.  
     [0154] Experimental Data  
     [0155] In the embodiment described above, hydrogen (H 2 ), dichlorosilane (SiH 2 Cl 2 ), and a 10% mixture of germane (GeH 4 ) in hydrogen (H 2 ) may be pre-mixed and distributed among inner and outer injection zones of a deposition chamber in order to deposit a layer of epitaxial SiGe onto the surface of a substrate. Referencing FIG. 1, first gas source  110  may contain hydrogen, second gas source  120  may contain dichlorosilane, and third gas source  130  may contain a 10% mixture of germane in hydrogen. First inlet port  106  and second inlet port  107  may direct process gases into a process chamber and across an outer periphery of a substrate, and third inlet port  108  may direct process gasses into a process chamber and across a central portion of a substrate. First metering valve  178  and second metering valve  179  may be adjusted to a fully open setpoint, and isolation valves  113 ,  123 ,  133 , and  135  may be configured to an ON position, thereby allowing hydrogen, dichlorosilane, and the 10% mixture of germane in hydrogen to flow from gas sources  110 ,  120 , and  130 , respectively. First flow controller  112  may be adjusted to flow 30 slm of hydrogen, second flow controller  122  may be adjusted to flow 0.2 slm of dichlorosilane, and third flow controller  132  may be adjusted to flow 0.03 slm of the 10% mixture of germane in hydrogen. In this particular embodiment, fourth gas source  140  and fifth gas source  150  may not be utilized, and isolation valves  145 ,  155 ,  143 ,  153 ,  157 , and  159  may be configured to an OFF condition.  
     [0156]FIG. 12A shows examples of deposited SiGe film thickness uniformity across Test 1 and Test 2 substrates, each substrate comprising a 200 mm diameter silicon wafer. FIG. 12B shows examples of Ge concentration within the deposited SiGe film across the same substrates.  
     [0157] For the Test 1 substrate, isolation valve  137  and isolation valve  139  were each configured to an OFF condition during substrate processing. As shown in FIGS. 12A and 12B, both SiGe thickness and Ge concentration are lower at the edges of the Test 1 substrate than in the center. The SiGe thickness uniformity and Ge concentration uniformity for 3 mm edge exclusion (1-sigma deviation) for the Test 1 substrate are approximately 2.4% and 2.6%, respectively.  
     [0158] As previously discussed, the thickness and concentration uniformity of a deposited SiGe film across the surface of a substrate may each be altered by varying the temperature of different portions of the substrate. However, this method cannot be used to improve thickness and concentration uniformities simultaneously. Increasing the temperature across an outer periphery of a substrate will increase the edge thickness of a deposited SiGe layer relative to the thickness at the center due to increased SiGe growth rate at higher temperatures. However, the Ge concentration at the outer periphery of the substrate will decrease relative to the Ge concentration at the center because Ge incorporation within a deposited film decreases as temperature increases, assuming all other process conditions are fixed.  
     [0159] For the Test 2 substrate, isolation valve  137  was configured to an ON condition, isolation valve  139  was configured to an OFF condition, and the flow of the 10% mixture of germane in hydrogen through third flow controller  132  was 0.03 slm. As demonstrated by the Test 2 substrate data in FIGS. 12A and 12B, this method allows both the SiGe thickness and Ge concentration uniformities to be improved simultaneously such that the SiGe thickness uniformity and Ge concentration uniformity for 3 mm edge exclusion (1-sigma deviation) for the Test 1 substrate are approximately 1.1% and 0.9%, respectively. The Ge concentration at the outer periphery of the substrate is increased relative to the center of the substrate because the concentration of Ge directed to first inlet port  106  and second inlet port  107  was increased. The thickness uniformity is similarly improved because increasing the Ge concentration increases the SiGe growth rate.  
     [0160] In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. However, it should be evident to one skilled in the art that various modifications and changes may be made without departing from the broader spirit and scope of the invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.