Abstract:
A system and method for distributing gas to a substrate in a dry etch chamber make use of different flow channels to distribute the gas to different portions of a substrate. A first flow channel can be oriented to distribute gas to an inner portion of the substrate. A second flow channel can be oriented to distribute gas to an outer portion of the substrate. With different flow channels, the system and method enable separate control of gas distribution for different portions of the substrate. In particular, the flow channels allow separate control of gas flow rate, concentration, and flow time for different areas of the substrate. In this manner, gas distribution can be selectively controlled to compensate for different etch rates across the substrate surface. Also, gas distribution can be controlled as a function of etch rate patterns exhibited by different etch gasses used in successive process steps. Thus, etch uniformity can be enhanced, leading to improvement in the quality of the overall fabrication process. In a semiconductor fabrication processes, enhanced etch uniformity can lead to increased device yield.

Description:
[0001]     The present invention relates to dry etch processing, and, more particularly, to techniques for distribution of gas used in a dry etch process.  
         [0002]     Dry etch processing is used to selectively remove patterned material from the surface of a substrate. The dry etch process involves introduction of a gas into an etch chamber. The gas serves as the etch medium. The gas is typically distributed by a set of gas flow conduits coupled to a gas supply. The gas flow conduits are arranged over the substrate and include a pattern of holes that form gas outlets.  
         [0003]     The substrate to be processed is mounted on a cathode. An electric field applied between the cathode and an anode, e.g., the wall of the etch chamber, excites the gas to generate a plasma The plasma breaks down the gas distributed from the gas outlets into chemically reactive species. Also, the plasma ionizes the gas, enhancing the mobility of the gas particles to transport to the substrate under the influence of the electric field.  
         [0004]     The gas ions strike the substrate surface, reacting with the surface material to remove it from the substrate. In some applications, the plasma also creates a sputtering effect. Specifically, the ions strike the substrate surface with sufficient impact energy to cause physical removal of the surface material. In either case, the result is selective removal of the surface material. A molecular pump draws the material out of the chamber.  
         [0005]     Uniformity of the dry etch phase is a critical factor in the quality of the overall fabrication process. In semiconductor device fabrication, for example, significant nonuniformities such as over-etch or under-etch can directly impact device yield. Etch uniformity is particularly troublesome in large area applications, such as flat panel display fabrication. Some large area applications require substrates having surface areas on the order of 600×720 mm. As substrate surface areas increase, the problems of etch uniformity and resulting yield are compounded.  
         [0006]     Etch uniformity across the substrate surface is typically a product of etch rate. For a variety of reasons, an etch gas may etch different portions of the substrate surface at different rates. For example, an etch gas may etch an inner portion of the substrate more quickly than an outer portion, or vice versa. The etch rate may vary, for example, due to cross-substrate differences in mask pattern, gas concentration, gas flow rate, or electric field strength. Moreover, the effect of such variations can be inconsistent for different etch gases or steps. Consequently, in a fabrication process involving multiple dry etch phases with different etch gasses, etch uniformity can differ from layer to layer. The end result can be decreased fabrication quality and yield.  
       SUMMARY  
       [0007]     The present invention is directed to a system and method for distributing gas to a substrate in a dry etch chamber. The gas distribution system and method make use of a first flow channel that is oriented to distribute gas to a first portion of the substrate, and a second flow channel that is oriented to distribute the gas to a second portion of the substrate. The first flow channel, for example, may take the form of an inner flow channel that distributes gas to an inner portion of the substrate. Similarly, the second flow channel may take the form of an outer flow channel that distributes gas to an outer portion of the substrate.  
         [0008]     With first and second flow channels, the system and method enable selective control of gas distribution for different portions of the substrate. In this manner, gas distribution can be controlled to compensate for different etch rates across the substrate, e.g., between the center and edges of the substrate. Thus, etch uniformity can be improved, leading to improvements in the quality of the overall fabrication process. In a semiconductor fabrication process, for example, the enhanced etch uniformity can lead to increased device yield.  
         [0009]     Gas distribution in different portions of the substrate can be separately controlled not only for a single gas, but for different gasses used in multiple etch steps. For example, one gas may exhibit a greater etch rate at an inner portion of the substrate than at an outer portion of the substrate. In contrast, a different gas may act in the opposite manner, exhibiting a greater etch rate at the outer portion for a different etch step or different substrate layer material. Flow through the first and second flow channels can be adjusted for different etch steps to provide targeted compensation for the characteristics of particular gasses or etch steps.  
         [0010]     Differential control of the first and second flow channels can be achieved conveniently using one or more programmable mass flow controllers. Each mass flow controller can be disposed to control flow rate and/or time within one of the flow channels. The mass flow controllers can be programmed with different profiles for each flow channel, as well as for different gasses.  
         [0011]     Also, the mass flow controllers can be programmed to control the composition of the gas distributed to different portions of the substrate via the first and second flow channel. For example, the concentration of the gas distributed via each channel can be controlled in a differential manner as a function of etch rate. As an alternative, flow can be controlled with a single mass flow controller in combination with one or more valves that can be actuated to terminate flow through a selected channel.  
         [0012]     The system and method of the present invention enable convenient compensation of cross-substrate etch rate variation with little added expense, and without impacting process throughput. Consequently, the system and method are capable of achieving enhanced etch uniformity for dry etch processes involving existing and future substrate sizes.  
         [0013]     In addition, the system and method provide flexibility for the varying etch rate characteristics that may arise from different gases, materials, mask patterns, and etch chambers. In particular, the system and method generally eliminate the need to change the setup of the etch chamber or transfer the substrate to a different chamber for a subsequent etch step due to differences in the etch characteristics exhibited by different gasses.  
         [0014]     The present invention provides, in one aspect, a gas distribution system for distributing a gas to a substrate in an etch chamber, the system comprising: a gas supply containing a gas; a first flow channel coupled to the gas supply and oriented to distribute the gas to an inner portion of the substrate; and a second flow channel coupled to the gas supply and oriented to distribute the gas to an outer portion of the substrate.  
         [0015]     In another aspect, the present invention provides a method for distributing a gas to a substrate in an etch chamber, the method comprising: distributing a gas to an inner portion of the substrate via a first flow channel; and distributing the gas to an outer portion of the substrate via a second flow channel.  
         [0016]     Additionally, the present invention provides a gas distribution system for distributing a gas to a substrate in an etch chamber, the system comprising: a gas supply containing a gas; a first flow channel coupled to the gas supply and oriented to distribute the gas to an inner portion of the substrate; a second flow channel coupled to the gas supply and oriented to distribute the gas to an outer portion of the substrate; and a controller that controls the rate of flow of the gas through the first and second flow channels such that the rate of flow through the first flow channel is different than the rate of flow through the second flow channel.  
         [0017]     The present invention further provides a method for distributing gas to a substrate in an etch chamber, the method comprising: distributing a gas to an inner portion of the substrate via a first flow channel; distributing the gas to an outer portion of the substrate via a second flow channel; and controlling the rate of flow of the gas through the first and second flow channels such that the rate of flow through the first flow channel is different than the rate of flow through the second flow channel.  
         [0018]     Also, the present invention provides a method for etching a substrate comprising: distributing gas to an inner portion of the substrate via a first flow channel; distributing the gas to an outer portion of the substrate via a second flow channel; energizing the gas to remove material from the substrate; and controlling the flow of gas through the first and second flow channels to improve etch uniformity between the inner and outer portions of the substrate.  
         [0019]     The present invention also provides a method for etching a substrate comprising: distributing a first gas to an inner portion of a layer of the substrate via a first flow channel; distributing the first gas to an outer portion of the layer of the substrate via a second flow channel; energizing the first gas to remove material from the layer of the substrate; controlling the flow of gas through the first and second flow channels to improve etch uniformity between the innner and outer portions of the layer of the substrate; distributing a second gas to an inner portion of a second layer of the substrate via the first flow channel; distributing the second gas to an outer portion of the second layer of the substrate via the second flow channel; energizing the second gas to remove material from the second layer of the substrate; and controlling the flow of gas through the first and second flow channels to improve etch uniformity between the inner and outer portions of the second layer of the substrate.  
         [0020]     Further, the present invention provides a gas distribution system for distributing a gas to a substrate in an etch chamber, the system comprising: a gas supply containing a gas; a first flow channel coupled to the gas supply and oriented to distribute the gas to a first portion of the substrate; and a second flow channel coupled to the gas supply and oriented to distribute the gas to a second portion of the substrate.  
         [0021]     The present invention, in a further aspect, provides a method for distributing a gas to a substrate in an etch chamber, the method comprising: distributing a gas to a first portion of the substrate via a first flow channel; and distributing the gas to a second portion of the substrate via a second flow channel.  
         [0022]     Other advantages, features, and embodiments of the present invention will become apparent from the following detailed description and claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0023]      FIG. 1  is side view illustrating a dry etch chamber,  
         [0024]      FIG. 2A  is a plan view illustrating a gas distribution manifold;  
         [0025]      FIG. 2B  is a side view of the gas distribution manifold of  FIG. 2B  taken along line  2 B- 2 B;  
         [0026]      FIG. 3  is a functional block diagram of a gas distribution system incorporating a manifold as shown in  FIGS. 2A and 2B ;  
         [0027]      FIG. 4  is a functional block diagram of a second gas distribution system incorporating a manifold as shown in  FIGS. 2A and 2B ; and  
         [0028]      FIG. 5  is a functional block diagram of a third gas distribution system incorporating a manifold as shown in  FIGS. 2A and 2B . 
     
    
       [0029]     Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0030]      FIG. 1  is a side view of a dry etch chamber  10 . As shown in  FIG. 1 , chamber  10  includes: a bottom wall  12 ; opposing side walls  14 ,  16 ; top wall  18 ; and front and back walls (not shown in  FIG. 1 ). Walls  12 ,  14 ,  16 ,  18  enclose a cathode  20 . Cathode  20  is mounted within housing  12  and is configured to receive and support one or more substrates  22 ,  24 . In particular, substrates  22 ,  24  are mounted on opposite sides of cathode  20  with mounting hardware including spacers  26  and clamps  28 . Although substrates  22 ,  24  are shown for purposes of example, etch chamber  10  may be designed for the processing of a single substrate, or two or more substrates.  
         [0031]     Chamber  10  further includes a first gas distribution manifold  30  mounted adjacent side wall  14  and opposite substrate  22  and a second gas distribution manifold  32  mounted adjacent side wall  16  and opposite substrate  24 . Manifolds  30 ,  32  distribute etch gas to substrates  22 ,  24 , respectively. If etch chamber  10  is designed for a single substrate, a single manifold may be used. An external gas supply (not shown in  FIG. 1 ) delivers gas to manifolds  30 ,  32 , as indicated by reference numerals  34 ,  36 , via fittings that extend outside of etch chamber  10 .  
         [0032]     Cathode  20  is coupled to an RF power generator (not shown) via a cable, as indicated in cross-section by reference numeral  38 . Chamber walls  14 ,  16  are coupled to ground and serve as the anode for the etch reaction. During the etch process, the electric potential applied between cathode  20  and chamber side walls  14 ,  16  by the power applied by the RF generator creates an electric field. The electric field excites the gas distributed by manifolds  30 ,  32 , creating a plasma. Cathode  20  may include a conduit, as indicated by reference numeral  40 , for water cooling.  
         [0033]     The plasma breaks down the etch gas into reactive species that remove patterned material from the surfaces of substrates  22 ,  24 . Etch chamber  10  may be configured to provide a sputter etch of substrates  22 ,  24 , if desired. A pump  42  is mounted in an opening in top wall  18 . Pump  42  may be a conventional turbo molecular pump (TMP). Pump  42  draws the removed material out of the interior of chamber  10  and directs it to a recovery reservoir, as indicated by reference numeral  44 . The pumping pressure of pump  42  is set according to the etch rate and resultant rate at which material is removed from substrates  22 ,  24 . In addition, a spectrophotometric etch time sensor (not shown) can be provided to monitor the progression of the etch process by reference to plasma-induced spectral emissions from the etch reactants.  
         [0034]      FIG. 2A  is a schematic diagram illustrating gas manifold  30 .  FIG. 2B  is a side view of gas distribution manifold  30 . Gas manifold  32  may take the same form as manifold  30 , but only one manifold is shown for purposes of illustration.  FIGS. 2A and 2B  illustrate an exemplary structure suitable for practice of a method in accordance with an embodiment the present invention. As shown in  FIG. 2A , manifold  30  includes a first flow channel  46  and a second flow channel  48 . First flow channel  46  is coupled to a gas supply (shown in  FIG. 3 ) via a first flow path  50 , and is oriented to distribute the gas to a fist portion of substrate  22 . Second flow channel  48  is coupled to the gas supply via a second flow path  52  separate from first flow path  50 , and is oriented to distribute the gas to a second portion of substrate  22 .  
         [0035]     Each flow channel  46 ,  48  includes a pattern of holes  53  that serve as gas outlets to distribute gas over substrate  22 . In the embodiment illustrated in  FIGS. 2A and 2B , first flow channel  46  forms an inner flow channel that is oriented to distribute the gas to an inner portion of substrate  22 . Second flow channel  48 , as shown in  FIG. 2A , forms an outer flow channel that is oriented to distribute the gas to an outer portion of substrate  22 . First and second flow channels  46 ,  48  can be used to distribute gas separately to different locations on substrate  22 .  
         [0036]     The gas can be distributed to each portion with different flow rates that are selected to accommodate variations in the etch rates of a given gas across substrate  22 . In this manner, gas can be distributed to an inner portion of substrate  22  at one flow rate, and to an outer or edge portion of substrate  22  at a different flow rate. If desired, manifold  30  could incorporate additional flow channels oriented to distribute gas to particular portions of substrate  22 . For example, each of several flow channels could be oriented to cover particular zones across the surface of substrate  22 . Gas could be distributed to the zones with different flow rates based on the particular etch characteristics within the zones.  
         [0037]     With further reference to  FIG. 2A , inner flow channel  46  may take the form of a plurality of flow conduits  54 ,  56 ,  58 ,  60  arranged in a cross-like pattern. Inner flow channel  46  is susceptible to a variety of arrangements, however, as necessary for effective coverage. Conduits  54 ,  56 ,  58 ,  60  can be coupled together in fluid communication with a central four-way fitting  62 . An additional fitting  64  is provided to receive gas from first flow path  50 . Outer flow channel  48  may take the form of a plurality of flow conduits  66 ,  68 ,  70 ,  72  arranged in a peripheral, rectangular pattern. Conduits  66 ,  68 ,  70 ,  72  can be coupled together in fluid communication with two-way corner fittings  74 ,  76 ,  78 ,  80 . As shown in  FIG. 2B , an additional fitting  82  is provided to receive gas from second flow path  52 .  
         [0038]     As shown in  FIG. 2A , conduits  54 ,  56 ,  58 ,  60  can be coupled at one end to conduits  66 ,  68 ,  70 ,  72  for support via fittings  84 ,  86 ,  88 ,  90 . To provide separate flow channels, conduits  54 ,  56 ,  58 ,  60  preferably are not in fluid communication with conduits  66 ,  68 ,  70 ,  72 . First and second flow channels  46 ,  48  could be coupled, however, via selectively operable valves, as will be described with reference to  FIG. 5 . In this manner, manifold  30  could be selectively configured to provide either a single flow channel that integrates first and second flow channels  46 ,  48 , or separate flow channels. To compensate for etch rate variation between different portions of substrate  22 , separate use of first and second flow channels  46 ,  48  ordinarily will be desirable.  
         [0039]      FIG. 3  is a functional block diagram of a gas distribution system  92  incorporating a manifold  30  as shown in  FIGS. 2A and 2B .  FIG. 3  illustrates exemplary structure suitable for practice of a method in accordance with an embodiment of the present invention. System  92  is described for use with gas manifold  30 , but may be replicated for use with another manifold  32 , if two or more manifolds are incorporated in etch chamber  10 . Alternatively, a single system  92  can be used, provided that parallel flow paths are provided for flow to both manifolds  30 ,  32 . As shown in  FIG. 3 , system  92  includes a controller  94  that controls the rate of flow of the gas through first and second flow channels  46 ,  48 . To accommodate varying etch rates between different portions of substrate  22 , e.g., the inner and edge portions, controller  94  can be configured such that the rate of flow through first flow channel  46  is different than the rate of flow through second flow channel  48 .  
         [0040]     To provide varying flow rates, controller  94  may incorporate first and second programmable mass flow controllers (MFC)  96 ,  98 . First mass flow controller  96  is disposed in first flow path  50  between a gas supply  100  and first flow channel  46 . Second mass flow controller  98  is disposed in second flow path  52  between gas supply  100  and second flow channel  48 . Each mass flow controller  96 ,  98  can be realized by a conventional controller, and appropriately programmed according to the particular characteristics of the selected etch gas and the varying etch rates exhibited by the gas. Also, controllers can be provided to control the composition of the gas distributed by each flow channel  46 ,  48 . In particular, such controllers can control the relative mixture of the gasses, as well as the relative temperatures of the gasses in the flow channels to achieve enhanced etch uniformity.  
         [0041]     If the gas exhibits a faster etch rate in the inner portion of substrate  22  than at the edge, mass flow controller  96 ,  98  can be differentially programmed to provide a higher flow rate via second flow channel  48  or a lower flow rate via first flow channel  46 . Similarly, if the opposite is true, mass flow controllers  96 ,  98  can be programmed to provide a higher flow rate via first flow channel  46  or a lower flow rate via second flow channel  48 . In this manner, mass flow controllers  96 ,  98  can be configured to provide enhanced etch uniformity across the surface of substrate  22 . For more precise uniformity, manifold  30  could include multiple flow channels that cover several zones across the surface of substrate  22 . The rate of gas flow for each zone could be controlled using multiple mass flow controllers that set the respective flow rates based on the particular etch rate profile within the zone.  
         [0042]     Mass flow controllers  96 ,  98  also can be programmed to provide particular flow rate profiles, i.e., variation in flow rate over the etch period. In particular, notwithstanding flow rate, mass flow controllers  96 ,  98  can be programmed to separately control the overall time of flow of gas through the first and second flow channels  46 ,  48 , thereby providing differential gas distribution times at different portions of substrate  22 . Flow time can be controlled in combination with flow rate. The time difference can be selected to produce greater etch uniformity between different portions, e.g., between the inner and edge portions of substrate  22 . Again, substrate  22  can be divided into multiple zones that receive gas from additional flow channels. Multiple mass flow controllers could be configured to provide differential gas flow time across the zones, thereby providing precise areal adjustment of the etch process and enhanced etch uniformity.  
         [0043]     By varying flow rate, mass flow controllers  96 ,  98  also operate to control the concentration of the gas that flows through first and second flow channels  46 ,  48 , given a constant volume and pressure within chamber  10 . For example, the relative flow rates can be adjusted by mass flow controllers  96 ,  98  such that gas can be distributed via second flow channel  48  with a greater concentration than gas distributed via first flow channel  46 , or vice versa. Thus, relative concentrations can be controlled based on differential etch rates. Concentrations can be modulated either alone or in combination with adjustment of relative gas distribution time. Also, concentrations can be controlled not only for inner and outer portions of substrate  22 , but for several individual zones. The end result is greater etch uniformity across the surface of substrate  22 .  
         [0044]     Also, mass flow controllers  96 ,  98  can be programmed separately for each etch gas used in the successive steps of a fabrication process. Specifically, in recognition that different gasses exhibit different etch rate characteristics across the surface of substrate  22 , particularly with different substrate layer materials, mass flow controllers  96 ,  98  can be appropriately programmed to ensure etch uniformity for different etch steps. Consequently, system  92  generally eliminates the need to change the setup of chamber  10  or transfer substrate  22  to a different chamber for a subsequent etch step due to differences in the etch characteristics exhibited by different gasses. Rather, it is only a matter of loading mass flow controllers  94 ,  96  with programs appropriate for the particular etch step and gas to be distributed.  
         [0045]      FIG. 4  is a functional block diagram of a second gas distribution system  102  incorporating a manifold  30  as shown in  FIGS. 2A and 2B . System  102  of  FIG. 4  substantially corresponds to system  92  of  FIG. 3 . Like system  92 , for example, system  102  includes a controller  104  that controls the rate of gas flow through first and second flow channels  46 ,  48  via first and second flow paths  50 ,  52 , respectively. As shown in  FIG. 4 , however, controller  104  includes only a single programmable mass flow controller  106  that controls the rate of flow of gas through both flow paths  50 ,  52 . To compensate for differential etch rates, mass flow controller  106  is programmed to control valves  108 ,  110 .  
         [0046]     Valve  108  is disposed within first flow path  50 , whereas valve  110  is disposed within second flow path  52 . Mass flow controller  106  is programmed to selectively open and close valves  108 ,  110 , as indicated by lines  112 ,  114 , to control the gas flow time within flow paths  50 ,  52  and flow channels  46 ,  48 . Mass flow controller  106  may require modification to control valves  108 ,  110 . In particular, it may be necessary to equip mass flow controller  106  with control outputs for valves  108 ,  110 , along with drive circuitry sufficient to actuate the solenoid, motor, or other mechanism associated with the valves for opening and closing.  
         [0047]     Mass flow controller  106  opens both valves  108 ,  110  to allow flow of gas through flow paths  50 ,  52  at a particular flow rate. Instead of controlling the relative gas flow rates through paths  50 ,  52 , mass flow controller  106  controls gas flow time in a differential mode by selectively closing one of the valves  108 ,  110  during the etch process. If a particular gas exhibits a higher etch rate within the inner portion of substrate  22 , for example, mass flow controller  106  closes valve  108  earlier than valve  110 . Valve  110  remains open for an extended period of time necessary for the etch process at the outer portion of substrate  22  to continue. In other words, the etch process at the outer portion of substrate  22  is allowed to proceed to a point approximately commensurate with the etch process in the inner portion. Thus, the timing at which valves  108 ,  110  are closed relative to one another is determined by reference to the difference in etch rate. Alternatively, valve  108  can be opened later than valve  10 . In this case, valves  108 ,  110  are closed at the same time, but opened at different times to produce a difference in overall gas flow time.  
         [0048]     Like system  92  described with respect to  FIG. 3 , system  102  allows manifold  30  to be used in a differential manner for a variety of etch gasses used in subsequent process steps. In this case, mass flow controller  106  is programmed to provide a particular flow rate, and also to control valves  108 ,  110  to provide differential flow times within flow channels  46 ,  48 , as dictated by the particular gas, substrate layer material, or etch step. System  102  thereby avoids the need for changes to chamber  10  or the transfer of substrate  22  to a different chamber. Instead, the cross-substrate gas flow profile for a particular etch step can be easily changed by loading mass flow controller  106  with a different program.  
         [0049]      FIG. 5  is a functional block diagram of a third gas distribution system  116  incorporating a manifold  30  as shown in  FIGS. 2A and 2B . System  116  substantially corresponds to system  92  of  FIG. 3 . In particular, system  116  includes a flow controller  118  that incorporates a first mass flow controller  120  and a second mass flow controller  122 . Mass flow controller  120  is disposed to control the gas flow rate within first flow path  50  and, consequently, first flow channel  46 . Mass flow controller  122  is disposed to control the gas flow rate within second flow path  52  and second flow channel  48 . In addition to mass flow controllers  120 ,  122 , however, system  116  includes a valve  124  that is, in effect, coupled between first and second flow channels  46 ,  48 .  
         [0050]     Valve  124  is disposed to control flow between mass flow controller  120  and first flow path  50 . Also, valve  124  is disposed to control flow between second flow path  52  and first flow path  50 , as indicated by line  126 . Valve  124  is selectively operable to couple first flow channel  46  and second flow channel  48  in fluid communication with one another. Specifically, mass flow controller  120  is programmed to selectively open and close valve  124 , as indicated by line  128 . As an alternative, mass flow controller  122  could be programmed to open and close valve  124 . In either case, selective actuation of valve  124  allows manifold  30  to be used in either a differential mode or a continuous flow mode.  
         [0051]     In particular, valve  124  can be closed to flow from mass flow controller  120  while receiving flow from mass flow controller  122 . In this mode, mass flow controller  122  distributes gas to both flow paths  50 ,  52  and, consequently, to both flow channels  46 ,  48 . Flow channels  46 ,  48  thereby receive essentially the same gas flow at the same flow rate, and operate in a continuous mode to distribute gas in a generally uniform manner across substrate  22 . It is noted that a substantially continuous mode, i.e., a mode producing a substantially common cross-substrate flow rate, also could be achieved with system  92  of  FIG. 3  by simply programming mass flow controllers  96 ,  98  to deliver gas with the same flow rate to both flow channels  46 ,  48 . However, flow channels  46 ,  48  would remain separated from one another in terms of fluid communication.  
         [0052]     Alternatively, valve  124  can be open to receive flow from mass flow controller  120  but closed to flow from mass flow controller  122 . In this mode, mass flow controller  120  distributes gas to flow path  50  and flow channel  46  at a first flow rate, whereas mass flow controller  122  distributes gas to flow path  52  and flow channel  48  at a second flow rate, which may be different than the first flow rate. Flow channels  46 ,  48  thereby can receive different gas flow at different flow rates, as desired, operating in a differential mode to distribute gas in a nonuniform manner across substrate  22 . In particular, the gas flow rates of mass flow controllers  120 ,  122  are selected to compensate for differential etch rates in different portions of substrate  22 , as described with reference to  FIGS. 3 and 4 .  
         [0053]     As in system  92 , relative gas flow rates can be selected in system  116  for particular gasses, substrate layer materials, and etch steps, to provide enhanced etch uniformity. At the same time, system  116  supports continuous flow regardless of the individual characteristics of a gas or etch step. In summary, system  116  enables gas to be distributed via a single manifold  30  in either a continuous mode in which a common gas flow rate is maintained across substrate  22 , or a differential mode in which different gas flow rates are selected for different portions of substrate  22  based on the cross-substrate etch rate profile for a particular gas, substrate layer materials, or etch step. System  116  thereby offers significant flexibility in the design of the etch process.  
         [0054]     System  116  also supports the control of relative flow times and concentration to achieve enhanced etch uninformity for different portions of substrate  22 . Specifically, as in systems  92 ,  102 , flow rate can be modulated by mass flow controllers  120 ,  122  to achieve varied concentrations between the gas distributed by flow channels  46 ,  48 , respectively. Also, in the differential mode, mass flow controllers  120 ,  122  can be programmed to produce different flow times within flow channels  46 ,  48 .  
         [0055]     The foregoing detailed description has been provided for a better understanding of the invention and is for exemplary purposes only. Modifications may be apparent to those skilled in the art without deviating from the spirit and scope of the appended claims.