Patent Publication Number: US-2005116064-A1

Title: Reactors having gas distributors and methods for depositing materials onto micro-device workpieces

Description:
TECHNICAL FIELD  
      The present invention is related to reactors having gas distributors and methods for depositing materials in thin film deposition processes used in the manufacturing of micro-devices.  
     BACKGROUND  
      Thin film deposition techniques are widely used in the manufacturing of micro-devices to form a coating on a workpiece that closely conforms to the surface topography. The size of the individual components in the devices is constantly decreasing, and the number of layers in the devices is increasing. As a result, the density of components and the aspect ratios of depressions (e.g., the ratio of the depth to the size of the opening) are increasing. The size of workpieces is also increasing to provide more real estate for forming more dies (i.e., chips) on a single workpiece. Many fabricators, for example, are transitioning from 200 mm to 300 mm workpieces, and even larger workpieces will likely be used in the future. Thin film deposition techniques accordingly strive to produce highly uniform conformal layers that cover the sidewalls, bottoms, and corners in deep depressions that have very small openings.  
      One widely used thin film deposition technique is Chemical Vapor Deposition (CVD). In a CVD system, one or more precursors that are capable of reacting to form a solid thin film are mixed in a gas or vapor state, and then the precursor mixture is presented to the surface of the workpiece. The surface of the workpiece catalyzes the reaction between the precursors to form a thin solid film at the workpiece surface. The most common way to catalyze the reaction at the surface of the workpiece is to heat the workpiece to a temperature that causes the reaction.  
      Although CVD techniques are useful in many applications, they also have several drawbacks. For example, if the precursors are not highly reactive, then a high workpiece temperature is needed to achieve a reasonable deposition rate. Such high temperatures are not typically desirable because heating the workpiece can be detrimental to the structures and other materials that are already formed on the workpiece. Implanted or doped materials, for example, can migrate in the silicon substrate at higher temperatures. On the other hand, if more reactive precursors are used so that the workpiece temperature can be lower, then reactions may occur prematurely in the gas phase before reaching the substrate. This is not desirable because the film quality and uniformity may suffer, and also because it limits the types of precursors that can be used.  
      One conventional system to prevent premature reactions injects the precursors into the reaction chamber through separate ports. For example, each port of a shower head can be coupled to a dedicated gas line for a single gas. Systems that present the precursors through dedicated ports proximate to the surface of the workpiece, however, may not sufficiently mix the precursors. Accordingly, the precursors may not react properly to form a thin solid film at the workpiece surface. Furthermore, conventional systems also have a jetting effect that produces a higher deposition rate directly below the ports. Thus, conventional CVD systems may not be appropriate for many thin film applications.  
      Atomic Layer Deposition (ALD) is another thin film deposition technique.  FIGS. 1A and 1B  schematically illustrate the basic operation of ALD processes. Referring to  FIG. 1A , a layer of gas molecules A x  coats the surface of a workpiece W. The layer of A x  molecules is formed by exposing the workpiece W to a precursor gas containing A x  molecules, and then purging the chamber with a purge gas to remove excess A x  molecules. This process can form a monolayer of A x  molecules on the surface of the workpiece W because the A x  molecules at the surface are held in place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The layer of A x  molecules is then exposed to another precursor gas containing B y  molecules. The A x  molecules react with the B y  molecules to form an extremely thin layer of solid material on the workpiece W. The chamber is then purged again with a purge gas to remove excess B y  molecules.  
       FIG. 2  illustrates the stages of one cycle for forming a thin solid layer using ALD techniques. A typical cycle includes (a) exposing the workpiece to the first precursor A x , (b) purging excess A x  molecules, (c) exposing the workpiece to the second precursor B y , and then (d) purging excess B y  molecules. In actual processing several cycles are repeated to build a thin film on a workpiece having the desired thickness. For example, each cycle may form a layer having a thickness of approximately 0.5-1.0 Å, and thus it takes approximately 60-120 cycles to form a solid layer having a thickness of approximately 60 Å.  
       FIG. 3  schematically illustrates an ALD reactor  10  having a chamber  20  coupled to a gas supply  30  and a vacuum  40 . The reactor  10  also includes a heater  50  that supports the workpiece W and a gas dispenser  60  in the chamber  20 . The gas dispenser  60  includes a plenum  62  operatively coupled to the gas supply  30  and a distributor plate  70  having a plurality of holes  72 . In operation, the heater  50  heats the workpiece W to a desired temperature, and the gas supply  30  selectively injects the first precursor A x , the purge gas, and the second precursor B y  as shown above in  FIG. 2 . The vacuum  40  maintains a negative pressure in the chamber to draw the gases from the gas dispenser  60  across the workpiece W and then through an outlet of the chamber  20 .  
      One drawback of ALD processing is that it has a relatively low throughput compared to CVD techniques. For example, ALD processing typically takes several seconds to perform each A x -purge-B y -purge cycle. This results in a total process time of several minutes to form a single thin layer of only 60-100 Å. In contrast to ALD processing, CVD techniques require much less time to form similar layers. The low throughput of existing ALD techniques limits the utility of the technology in its current state because ALD may be a bottleneck in the overall manufacturing process. Thus, it would be useful to increase the throughput of ALD techniques so that they can be used in a wider range of applications. Another drawback of ALD processing is that it is difficult to control the uniformity of the deposited films because the holes  72  in the distributor plate  70  also cause a jetting affect that results in a higher deposition rate in-line with the holes  72 . Therefore, a need exists in semiconductor fabrication to increase the deposition uniformity in both CVD and ALD processes.  
     SUMMARY  
      The present invention is directed toward reactors having gas distributors for depositing materials onto micro-device workpieces, systems that include such reactors, and methods for depositing materials onto micro-device workpieces. In one embodiment, a reactor for depositing material onto a micro-device workpiece includes a reaction chamber and a gas distributor in the reaction chamber. The gas distributor includes a first gas conduit having a first injector and a second gas conduit having a second injector. In one aspect of this embodiment, the first injector projects a first gas flow along a first vector and the second injector projects a second gas flow along a second vector that intersects the first vector in a mixing zone. In another aspect of this embodiment, the gas distributor can also include a mixing recess that defines the mixing zone. The mixing recess can have a variety of configurations, such as a conical, cubical, cylindrical, frusto-conical, pyramidical or other configurations. The first injector can project the first gas flow into the mixing recess along the first vector, and the second injector can project the second gas flow into the mixing recess along the second vector. In a further aspect of this embodiment, the first and second injectors are positioned within the mixing recess. The mixing zone can be positioned partially within the mixing recess.  
      In another embodiment, a reactor for depositing material onto a micro-device workpiece includes a reaction chamber, a workpiece support in the reaction chamber, and a gas distributor with a mixing recess in the reaction chamber. The mixing recess is exposed to the workpiece support. The gas distributor includes a first gas conduit having a first injector and a second gas conduit having a second injector. The first injector projects a first gas flow into the mixing recess along a first vector and the second injector projects a second gas flow into the mixing recess along a second vector.  
      These reactors can be used to perform several methods for depositing materials onto micro-device workpieces. In one embodiment, a method includes flowing the first gas through the first injector of the gas distributor along a first vector, and flowing the second gas through the second injector of the gas distributor along a second vector. The second vector intersects the first vector in the mixing zone over the micro-device workpiece. In another embodiment, a method includes flowing the first gas through the first injector of the gas distributor into the mixing recess, and flowing the second gas through the second injector of the gas distributor into the mixing recess over the micro-device workpiece. In a further embodiment, a method includes dispensing a first pulse of the first gas from a first outlet into a recess in the gas distributor, and dispensing a second pulse of the second gas from a second outlet into the recess in the gas distributor after terminating the first pulse of the first gas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  are schematic cross-sectional views of stages in ALD processing in accordance with the prior art.  
       FIG. 2  is a graph illustrating a cycle for forming a layer using ALD in accordance with the prior art.  
       FIG. 3  is a schematic representation of a system including a reactor for depositing a material onto a microelectronic workpiece in accordance with the prior art.  
       FIG. 4  is a schematic representation of a system having a reactor for depositing material onto a micro-device workpiece in accordance with one embodiment of the invention.  
       FIG. 5  is a schematic representation of the gas distributor shown in  FIG. 4  having a plurality of mixing recesses.  
       FIG. 6  is a bottom view of one mixing recess taken substantially along the line A-A of  FIG. 5 .  
       FIGS. 7A-7D  are schematic representations of portions of gas distributors having mixing recesses in accordance with additional embodiments of the invention.  
       FIG. 8  is a schematic representation of a gas distributor in accordance with another embodiment of the invention.  
       FIG. 9  is a schematic representation of a gas distributor in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      The following disclosure describes several embodiments of reactors having gas distributors for depositing material onto micro-device workpieces, systems including such reactors, and methods for depositing materials onto micro-device workpieces. Many specific details of the invention are described below with reference to depositing materials onto micro-device workpieces. The term “micro-device workpiece” is used throughout to include substrates upon which and/or in which microelectronic devices, micromechanical devices, data storage elements, read/write components, and other features are fabricated. For example, micro-device workpieces can be semiconductor wafers, such as silicon or gallium arsenide wafers, glass substrates, insulative substrates, and many other types of materials. The term “gas” is used throughout to include any form of matter that has no fixed shape and will conform in volume to the space available, which specifically includes vapors (i.e., a gas having a temperature less than the critical temperature so that it may be liquefied or solidified by compression at a constant temperature). Several embodiments in accordance with the invention are set forth in  FIGS. 4-9  and the following text to provide a thorough understanding of particular embodiments of the invention. A person skilled in the art will understand, however, that the invention may have additional embodiments, or that the invention may be practiced without several of the details of the embodiments shown in  FIGS. 4-9 .  
      A. Deposition Systems  
       FIG. 4  is a schematic representation of a system  100  for depositing material onto a micro-device workpiece in accordance with one embodiment of the invention. In this embodiment, the system  100  includes a reactor  110  having a reaction chamber  120  coupled to a gas supply  130  and a vacuum  140 . For example, the reaction chamber  120  can have an inlet  122  coupled to the gas supply  130  and an outlet  124  coupled to the vacuum  140 .  
      The gas supply  130  includes a plurality of gas sources  132  (identified individually as  132   a - c ), a valve assembly  133  having a plurality of valves, and a plurality of gas lines  136  and  137 . The gas sources  132  can include a first gas source  132   a  for providing a first precursor A, a second gas source  132   b  for providing a second precursor B, and a third gas source  132   c  for providing a purge gas P. The first and second precursors A and B are the gas or vapor phase constituents that react to form the thin, solid layer on the workpiece W. The purge gas P can be a suitable type of gas that is compatible with the reaction chamber  120  and the workpiece W. The gas supply  130  can include more gas sources  132  for applications that require additional precursors or purge gases in other embodiments. The valve assembly  133  is operated by a controller  142  that generates signals for pulsing the individual gases through the reaction chamber  120 .  
      The reactor  110  in the embodiment illustrated in  FIG. 4  also includes a workpiece support  150  and a gas distributor  160 , such as a shower head, in the reaction chamber  120 . The workpiece support  150  is typically heated to bring the workpiece W to a desired temperature for catalyzing the reaction between the first precursor A and the second precursor B at the surface of the workpiece W. The workpiece support  150  is a plate with a heating element in one embodiment of the reaction chamber  120 . The workpiece support  150 , however, may not be heated in other applications.  
      B. Gas Distributors  
       FIG. 5  is a schematic representation of the gas distributor  160  shown in  FIG. 4  having a plurality of mixing recesses  280 . In this embodiment, the gas distributor  160  has a first surface  262  with mixing recesses  280  that provide zones in which gas flows can mix before flowing to the workpiece W. In CVD applications, the precursors A and B can mix in the recesses  280  before flowing to the workpiece W. In ALD applications, precursor A can mix in the recesses  280  during a pulse and then precursor B can mix in the recesses  280  during a subsequent pulse after alternating purge gas P pulses. The mixing recesses  280  can be spaced uniformly throughout the first surface  262  to provide constant volumes over the entire workpiece W. In this embodiment, the mixing recesses  280  have a generally frusto-conical shape with a first wall  282  defining the side of the conical section and a second wall  284  defining the bottom of the mixing recess  280 . In other embodiments explained below, the mixing recesses  280  can have other shapes, such as those described below with reference to  FIGS. 7A-7D ; in additional embodiments explained below, the gas distributor  160  may not have mixing recesses  280 , such as the embodiment described below with reference to  FIG. 9 .  
      In the embodiment illustrated in  FIG. 5 , the gas distributor  160  includes a plurality of first injectors  270  positioned in the first wall  282 , a plurality of second injectors  272  positioned in the first wall  282  at different locations, and a plurality of third injectors  274  positioned in the second wall  284 . The injectors  270 ,  272 , and  274  are oriented to project gas flows into the mixing recesses  280 . The first injectors  270  are coupled to the first gas source  132   a  by a first gas conduit  232   a.  The first gas conduit  232   a  receives the first precursor A from the gas line  137  at the inlet  122  and distributes the first precursor A throughout the gas distributor  160  to the first injectors  270 . Similarly, the second injectors  272  are coupled to the second gas source  132   b  by a second gas conduit  232   b,  and the third injectors  274  are coupled to the third gas source  132   c  by a third gas conduit  232   c.    
      Each of the first injectors  270  is oriented to project a first gas flow into the mixing recesses  280  along a first vector V 1  at an angle σ with respect to the workpiece W. Each of the second injectors  272  is oriented to project a second gas flow into the mixing recesses  280  along a second vector V 2  at an angle α with respect to the workpiece W. The second vector V 2  forms an angle β with respect to the first vector V 1 . In the illustrated embodiment, the second vector V 2  is transverse (i.e., non-parallel) to the first vector V 1 . In other embodiments, such as the embodiment described below with reference to  FIG. 7A , the second vector V 2  can be generally parallel to the first vector V 1 . The first vector V 1  intersects the second vector V 2  at an intersection point  292  in a mixing zone  290  located proximate to the workpiece W. Each of the third injectors  274  is oriented to project a third gas flow into the mixing recesses  280  along a third vector V 3  at an angle θ with respect to the workpiece W.  
       FIG. 6  is a bottom view of one mixing recess  280  of the gas distributor  160  taken substantially along the line A-A of  FIG. 5 . In the illustrated embodiment, the mixing recess  280  includes a plurality of first injectors  270  (identified individually as  270   a - c ) and a plurality of second injectors  272  (identified individually as  272   a - c ) in the first wall  282  positioned annularly around the third injector  274 . In other embodiments, the first injectors  270 , the second injectors  272 , and/or the third injector  274  can be arranged in different patterns or configurations. For example, the mixing recess  280  can have only one first injector  270 , one second injector  272 , and one third injector  274 , or the mixing recess can have a plurality of third injectors  274  located in the first wall  282  interspersed between the first injectors  270  and the second injectors  272 . In further embodiments, some of the first injectors  270  and/or second injectors  272  can be positioned in the second wall  284 .  
      C. Methods for Depositing Material on Micro-Device Workpieces  
      Referring to  FIG. 5 , in one aspect of the embodiment, the gas distributor  160  can be used in CVD processing. For example, the first injectors  270  can project the first precursor A along the first vector V 1  into the mixing zones  290 , and the second injectors  272  can simultaneously project the second precursor B along the second vector V 2  into the mixing zones  290 . Accordingly, the first and second precursors A and B mix together in the mixing zones  290 . The orientation of the first and second injectors  270  and  272  (and accordingly the first and second vectors V 1  and V 2 ) facilitates the mixing of the first and second precursors A and B by flowing the gases into each other. Consequently, a mixture of the first and second precursors A and B is presented to the workpiece W.  
      In a further aspect of this embodiment, the gas distributor  160  can be used in both continuous flow and pulsed CVD applications. In a pulsed CVD application, a pulse of both the first precursor A and the second precursor B can be dispensed substantially simultaneously. After a pulse of the first and second precursors A and B, the third injector  274  can dispense a pulse of purge gas P along the third vector V 3  into the mixing recesses  280  to purge excess molecules of the first and second precursors A and B. After purging, the process can be repeated with pulses of the first and second precursors A and B. In another pulsed CVD application, the purge gas P flows continuously and pulses of the first and second precursors are injected into the continuous flow of the purge gas. The purge gas P, for example, can flow continuously along the third vector V 3 .  
      In another aspect of this embodiment, the gas distributor  160  can be used in ALD processing. For example, the first injectors  270  can project the first precursor A containing molecules A x  into the mixing recesses  280 . In the illustrative embodiment, the orientation of the first injectors  270  in the mixing recesses  280  causes the first precursor molecules A x  to mix sufficiently to form a uniform layer across the surface of the workpiece W. Next, the third injector  274  can project the purge gas P to purge excess first precursor molecules A x  from the mixing recesses  280 . This process can form a monolayer of A x  molecules on the surface of the workpiece W because the A x  molecules at the surface are held in .place during the purge cycle by physical adsorption forces at moderate temperatures or chemisorption forces at higher temperatures. The second injectors  272  can then project the second precursor B containing B y  molecules into the mixing recesses  280 . The B y  molecules also mix and form a uniform layer across the surface of the workpiece W. The A x  molecules react with the B y  molecules to form an extremely thin solid layer of material on the workpiece W. The mixing recesses  280  are then purged again and the process is repeated.  
      In a further aspect of this embodiment, the first and second injectors  270  and  272  can sequentially project the first and second precursors A and B to induce a vortex within the mixing recesses  280  in order to further increase the mixing. For example, referring to  FIG. 6 , the first injector  270   a  may dispense a first pulse of gas, followed by pulses from the first injector  270   b  and then the first injector  270   c.  In another aspect of this embodiment, the first injector  270   a  and the second injector  272   a  can dispense pulses of gas simultaneously, after which the first and second injectors  270   b  and  272   b  can dispense pulses simultaneously, and then the first and second injectors  270   c  and  272   c  can dispense pulses simultaneously. Accordingly, the first and second injectors  270  and  272  can sequentially project the first and second precursors A and B to increase mixing within the mixing recesses  280 .  
      One advantage of this embodiment with respect to the CVD process is that by using dedicated injectors  270 ,  272  and  274  and gas conduits  232  for each gas, the precursors A and B are kept separate, and accordingly, do not react prematurely. Furthermore, because the precursors A and B do not react prematurely, precursors that are highly reactive can be used, avoiding the need to heat the workpiece W to detrimentally high temperatures. Another advantage of this embodiment with respect to the ALD and CVD processes is that the enhanced mixing of the gases reduces the jetting effect and creates a uniform deposition across the surface of the workpiece W. A further advantage of this embodiment is that the position of the purge gas injectors  274  at the base of the mixing recesses  280  prevents the other gases from being trapped in the mixing recesses  280 . Another advantage of this embodiment is that the flow to each mixing recess can be independently controlled to compensate for nonuniformities on the workpiece W. For example, if the surface at the center of the workpiece W is too thick, the flow of gases from the injectors over the center of the workpiece W can be reduced. Still another advantage is that the chemical composition of the deposited film can be controlled precisely because the mixing at the outlets provides more precise reactions at the workpiece surface.  
      D. Other Gas Distributors  
       FIGS. 7A-7D  are scherriatic representations of portions of gas distributors having mixing recesses and injectors in accordance with additional embodiments of the invention. Each figure illustrates a different mixing recess and a particular arrangement of injectors; however, each arrangement of injectors can be used in conjunction with any of the mixing recesses. For example, the injector arrangements with only first and second injectors, such as those disclosed with reference to  FIGS. 7C and 7D , can be used with any of the mixing recesses.  
       FIG. 7A  illustrates a gas distributor  360  having a mixing recess  380  in accordance with another embodiment of the invention. The mixing recess  380  has a generally cylindrical shape with a first wall  382  defining the side of the cylinder and a second wall  384  defining the bottom of the mixing recess  380 . In another embodiment, the mixing recess  380  could have a different shape, such as a rectangular shape with the first wall  382  being one of the four rectangular sidewalls. In the illustrated embodiment, the gas distributor  360  also includes two first injectors  270  positioned in the first wall  382  at diametrically opposed locations, two second injectors  272  (only one shown) positioned in the first wall  382  offset from the first injector  270  by 90°, and the third injector  274  positioned in the second wall  384 . The first injectors  270  project the first gas flow into the mixing recess  380  along first vectors V 1  generally parallel to the workpiece W (not shown), and the second injectors  272  project the second gas flow into the mixing recess  380  along second vectors V 2  generally parallel to the workpiece W and normal to the first vectors V 1 . The third injector  274  is oriented to project the third gas flow along the third vector V 3  into the mixing recess  380  in a direction generally normal to the workpiece W.  
       FIG. 7B  is a schematic representation of a portion of a gas distributor  460  having a mixing recess  480  in accordance with another embodiment of the invention. The mixing recess  480  has a generally cubical shape with first walls  482   a,    482   b,  and  482   c  defining three sides of the cube and a second wall  484  defining the bottom of the mixing recess  480 . In another embodiment, the mixing recess  480  can have a different shape, such as a pyramidical shape with the first walls  482  being three sidewalls of the pyramid. In the illustrated embodiment, the gas distributor  460  includes first injectors  270  positioned in the first walls  482   a  and  482   c,  second injectors  272  positioned in the first wall  482   b  and a first wall (not shown) opposite the wall  482   b.  The gas distributor  460  also includes a third injector  274  positioned in the second wall  484 . The first injectors  270  project the first gas flow along first vectors V 1  into the mixing recess  480  at the angle σ with respect to the workpiece W (not shown). The second injectors  272  project the second gas flow along second vectors V 2  into the mixing recess  480  at an angle with respect to the workpiece W. The third injector  274  is oriented to project the third gas flow along the third vector V 3  into the mixing recess  480  in a direction generally normal to the workpiece W.  
       FIG. 7C  is a schematic representation of a portion of a gas distributor  560  having a mixing recess  580  in accordance with another embodiment of the invention. The mixing recess  580  has a generally hexagonal shape with first walls  582   a,    582   b,  and  582   c  defining sides of the hexagon and a second wall  584  defining the bottom of the mixing recess  580 . The gas distributor  560  includes the first injector  270  positioned in the second wall  584  and the second injector  272  positioned in the second wall  584 . The first injector is oriented to project the first gas flow along the vector V 1  into the mixing recess  580  at the angle σ with respect to the workpiece W (not shown). The second injector  272  is oriented to project the second gas flow along the second vector V 2  into the mixing recess  580  at the angle α with respect to the workpiece W.  
       FIG. 7D  is a schematic representation of a portion of a gas distributor  660  having a mixing recess  680  in accordance with another embodiment of the invention. The mixing recess  680  has a generally conical shape with a first wall  682  defining the side of the cone. In another embodiment, the mixing recess  680  could have a different shape, such as a pyramidical shape, with the first wall  682  being one of the sidewalls. In the illustrated embodiment, the gas distributor  660  includes the first injector  270  positioned in the first wall  682  and the second injector  272  positioned in the first wall  682  opposite the first injector  270 . The first injector  270  is oriented to project the first gas flow along the first vector V 1  into the mixing recess  680  at the angle σ with respect to the workpiece W (not shown). The second injector  272  is oriented to project the second gas flow along the second vector V 2  into the mixing recess  680  at the angle α with respect to the workpiece W. In other embodiments, the first and second injectors  270  and  272  can be offset individually or in pairs as explained above with reference to  FIG. 7A .  
       FIG. 8  is a schematic representation of a gas distributor  760  in accordance with another embodiment of the invention. The gas distributor  760  has a first wall  764 , a second wall  766 , and a third wall  768  that at least partially define a mixing recess  780 . The mixing recess  780  is positioned over the workpiece W. The gas distributor  760  includes the first injectors  270 , the second injectors  272 , and the third injectors  274 . The first injectors  270  and the second injectors  272  are interspersed along the walls  764 ,  766 , and  768  and are positioned to project gases into the mixing recess  780 . In the illustrated embodiment, many of the injectors  270 ,  272 , and  274  can be oriented at different angles with respect to the workpiece W to facilitate mixing of the gases before deposition onto the workpiece W. In other embodiments, the injectors  270 ,  272 , and  274  can be arranged differently, such as at different angles or positions in the walls  764 ,  766 , and  768 . In other embodiments, the gas distributor  760  can have different shapes or configurations, such as those illustrated in  FIGS. 5-7D .  
       FIG. 9  is a schematic representation of a gas distributor  860  in accordance with another embodiment of the invention. The gas distributor  860  has a first surface  862  from which the first injectors  270  and the second injectors  272  project the individual gas flows. The injectors  270  and  272  can be arranged in pairs (including one first injector  270  and one second injector  272 ) across the first surface  862  of the gas distributor  860 . Each first injector  270  projects the first gas along the first vector V 1  at the angle σ with respect to the workpiece W. Similarly, each second projector  272  projects the second gas along the second vector V 2  at the angle α with respect to the workpiece W. The first and second gases mix in a mixing zone  890  above the workpiece W. In other embodiments, pairs of first injectors  270  can inject a single gas flow along the first and second vectors V 1  and V 2 , and pairs of second injectors  272  can inject another individual gas flow along the first and second vectors V 1  and V 2  in a different mixing zone.  
      From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.