Abstract:
A chamber housing ( 2 ) enclosing a plasma region ( 20 ) in a large area plasma source used for performing plasma assisted processes in large area substrates, the chamber housing ( 2 ) being composed of: a housing member ( 2 ) constituting a substantially vertically extending wall ( 4 ) surrounding a space ( 6 ) corresponding to the plasma region ( 10 ), the housing member ( 2 ) having a plurality of openings ( 32 ) and electrically conducive elements forming an electrostatic shield around the space; a plurality of dielectric members ( 36 ) each having a peripheral edge and each disposed to close a respective opening ( 23 ); and sealing members ( 40, 40′, 42, 42′ ) forming a hermetic seal between said housing member and said peripheral edge of each of said dielectric members ( 36 ).

Description:
This application is the National Phase of International Application PCT/US99/27928 filed Dec. 10, 1999 which designated the U.S. and that International Application was published under PCT Article 21(2) in English. This application also claims priority from U.S. provisional application No. 60/114,454 filed on Dec. 30, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to plasma sources for use in the performance of plasma-assisted processes, including deposition and etching processes performed on substrates in processing chambers. The invention particularly relates to plasma sources which allow processing of large area substrates. 
     There is a demand for plasma sources that will enable processes of the above-mentioned type to be performed on large size wafers and even more so for flat panel display processing. There are indications in the industry that efforts will be made to manufacture flat panel displays measuring 1 meter on a side and plasma-assisted processing of such substrates will require higher plasma ion density levels than are produced in existing systems. Plasma-assisted processing of such large area substrates requires both high plasma density and high pumping speed to achieve high processing rates. 
     In plasma sources of the type described above, the plasma deposition or etching rate will depend on the ion flux, or ion density, as long as the process gas throughput, or pumping speed, satisfies the processing chamber requirements. Therefore, the achievement of satisfactory processing rates for large area substrates requires both the gas throughput and the ion flux be sufficiently high. 
     In addition, a plasma source having the requisite large dimensions must withstand a considerable force from atmospheric pressure and must be capable of providing an optimum geometry for creation of an electric field that will provide a uniform plasma inside the processing chamber of the source. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a large area plasma source which has the above-mentioned capabilities. 
     Another object of the invention is to provide a large area plasma source housing capable of supporting atmospheric pressure forces while providing a requisite electrostatic shield for the plasma confined within the housing and permitting transmission of RF electromagnetic field energy to the plasma. 
     The invention achieves these and other objects by providing a plasma source housing having side walls made of: metal electrostatic shield members that provide support against atmospheric pressure; or a ridged dielectric wall that is capable of supporting atmospheric pressure and is combined with electrically conductive elements that provide the electrostatic shield function; or a combination of the two. These walls can be shaped according to any vertical geometry including, but not limited to, straight, tapering in or out, curved in or out, etc. Therefore, a plasma source housing can be constructed to have virtually any dimensions and shape needed, while allowing RF energy to be supplied to the plasma through the housing wall. In addition, this housing will readily accommodate a system for cooling the processing chamber walls. 
     A further object of the invention is to achieve a high degree of plasma uniformity within the processing chamber. Because the RF electric field which creates and maintains the plasma originates in the region which surrounds the processing chamber, plasma uniformity is attained by diffusion, with gas species, or processing gas, flow and plasma gradient combining to provide process uniformity. Therefore, at any pressure and RF power level, plasma uniformity is a function of the aspect ratio of the processing chamber, i.e., the ratio of the square root of the cross sectional area to the height of the processing chamber. The cross-sectional area is the area of a horizontal plane at a location where the chamber has an average cross-sectional area. 
     It presently appears that by applying the principles to be disclosed herein together with standard testing procedures, a high degree of plasma uniformity can be achieved. 
     The above and other objects are achieved, according to the present invention, by a chamber housing enclosing a plasma region in a large area plasma source used for performing plasma assisted processes on large area substrates, the chamber housing comprising: 
     a housing member constituting a substantially vertically extending wall surrounding a space corresponding to the plasma region, the housing member having a plurality of openings, and electrically conductive elements forming an electrostatic shield around the space; 
     a plurality of dielectric members each having a peripheral edge and each disposed to close a respective opening; and 
     sealing means forming a hermetic seal between the housing member and the peripheral edge of each of the dielectric members. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a perspective view of a first embodiment of a large area plasma source according the present invention. 
     FIG. 2 is a view similar to that of FIG. 1 showing a second embodiment of the plasma source according to the invention. 
     FIG. 3 is a perspective, detailed view of a portion of one component of the embodiment shown in FIG. 1, with several elements shown in exploded form. 
     FIG. 4 is a cross-sectional, elevational view taken along line  4 — 4  of FIG.  1 . 
     FIG. 5 is a view similar to that of FIG. 3, showing a second embodiment of the component of the source shown in FIG.  1 . 
     FIG. 6A is a view similar to that of FIG. 3, showing a third embodiment of the component of the source shown in FIG.  1 . 
     FIG. 6B is a view similar to that of FIG. 6A showing a modified form of the embodiment of FIG.  6 A. 
     FIG. 6C is a cross-sectional detail view of a portion of the structure shown in FIG.  6 B. 
     FIG. 7 is a cross-sectional plan view of an embodiment of the source illustrated in FIG. 1, looking upwardly from within the chamber housing. 
     FIGS. 8A and 8B are, respectively, an elevational, cross-sectional detailed view and a bottom plan detailed view of components of the embodiments shown in FIG.  1 . 
     FIG. 9 is an elevational, pictorial view illustrating the principle of operation of a component illustrated in FIG.  7 . 
     FIGS. 10A and 10B are velocity distribution diagrams illustrating different modes of operation of the components shown in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a first embodiment of a processing apparatus according to the invention. This apparatus is composed essentially of a chamber housing  2  and an enclosure  4  which surrounds housing  2 . Housing  2  and enclosure  4  cooperate to delimit an annular space  6  which, in this embodiment, has a rectangular cross-section in a horizontal plane. This is an appropriate shape for processing substrates constituting flat panel displays. However, other cross sections can be provided and, for some applications, e.g., semiconductor wafer processing, a circular cross section will not only correspond to the substrate shape but also provides better structural integrity than a rectangular shape. 
     Space  6  is filled with a liquid coolant and contains an RF coil  8  that is supplied with an RF current which generates an electric field in the region enclosed by housing  2  in order to ignite and sustain a plasma in processing region  10  enclosed by housing  2  and by upper and lower walls of enclosure  4 . Upper wall  12  of enclosure  4  carries coolant supply and return lines  16 , vacuum pump assemblies  18  for pumping gas molecules and ions out of processing region  10  and maintaining a desired vacuum pressure therein, and passages (not shown) for couplings used to introduce fresh processing gas into region  10 . 
     Wall  12  additionally carries a fast match assembly, or match network,  20 , which is a component that is known per se and that is typically made up of an L-network of two variable capacitors and an inductor wherein the variable capacitors are mechanically adjusted by an automatic control network. The purpose of network  20  is to equilibrate the source impedance of the RF generator with the load impedance as seen by the generator looking into the match network and plasma source. Typically, the source impedance of the RF generator is 50Ω and, hence, the variable match network components are varied such that the output impedance of the match network is the complex conjugate of the input impedance to the plasma source. During matched conditions, the forward power at the match network juncture is maximized and the reflected power is minimized. Match network designs, although different in speed, robustness and controllability, are all based upon the same fundamental principles and are often found described in the prior art. 
     As will be described in greater detail below, chamber housing  2  provides the vertical bounding walls for region  10  and is constructed to withstand the forces acting on walls  12  and  14  due to the difference between the atmospheric pressure acting on the outer surfaces of the relatively large area walls  12  and  14  of enclosure  4  and the vacuum pressure established in region  10  and acting on the inner surfaces of those walls. Chamber housing  2  is further constructed to provide an electrostatic shield for region  10  and allow transmission of RF energy from coil  8  to region  10 . The vertical wall of enclosure  4  may, but need not, be constructed to assist in withstanding the above-mentioned forces imposed on walls  12  and  14  by the pressure differentials between opposed surfaces of each wall. 
     Each vacuum pump  18  is part of a vacuum pump assembly that includes a respective gate valve or throttle,  22  mounted on wall  12  with the aid of a coupling flange  24 . When gate valve  22  is left wide open, maximum pumping speed can be achieved. However, partial closure can permit a spatial variation of the pumping speed by means of varied flow restriction through the distributed pumping orifice. Gate valve  24  can be of a known design. 
     Flange  24  is a cylindrical part which provides a flow path between a respective through bore  28  in wall  12  and the inlet end of a respective pump  18 . Bores  28  are pumping ports which will each communicate with the inlet of a respective vacuum pump  18 . By suitable positioning, and selection of the number, of bores  28 , along with suitable selection and control of the operation of pumps  18 , the exhaust gas flow can be tuned for uniform gas exit from the region  10 . Selection and control of pumps and the arrangement of bores  28  can be effected on the basis of principles and practices already well known in the art. 
     Attainment of the desired process uniformity also requires appropriate control of gas injection. This aspect of the invention will be described, infra. 
     In addition, the bottom of the source will be provided with a suitable substrate support and means for applying a bias voltage, for example, an RF bias to the support. Here again, such a substrate support can be constructed and installed in the source in accordance with principles and practices already well known in the art. 
     FIG. 2 is a view similar to that of FIG. 1 showing a second embodiment of a source according to the invention which differs from the source in FIG. 1 essentially with respect to geometric configuration. Whereas chamber  10  of the embodiment shown in FIG. 1 has the form of a parallellapiped, chamber  10 ′ of FIG. 2 has the form of a pyramidal frustum. Thus, the side walls of housing  2 ′ are inclined with respect to a vertical axis of the source as are the side walls of enclosure  4 ′. 
     FIG. 3 is a perspective, detail view showing a portion of housing  2 , which is made of a conductive material, such as aluminum, and is given a wall thickness sufficient to provide the requisite compressive and tensile strengths. When the interior of enclosure  4  is under a vacuum, integrated pressure forces directed toward the interior thereof will generate a bending moment within the material, hence creating separate regions of enclosure  4  that are under tension or compression. 
     Being made of a conductive material, housing  2  constitutes an electrostatic shield. Housing  2  is provided with a series of vertically elongated recesses  32  that are spaced at uniform intervals about the housing periphery. At the center of each recess  32 , there is a narrow, elongated slot  34  which extends through the remaining thickness of housing  2  to communicate with region  10 . Each recess  32  is provided with an insert  36  made of a dielectric material, such as alumina, with a projecting portion that extends into slot  34  of its respective recess  32 . Each insert  36  is provided with four elastomeric vacuum seals  40  and  40 ′ at the inwardly facing surface of insert  36  and  42  and  42 ′ at the outside of insert  36 . Each insert  36  is covered by a respective frame  46  which holds its associated insert  36  and seals  40 ,  42  in place in recess  32 , with the aid of a plurality of screws (not shown) that extend through screw holes  48 . Between the two seals  40  and  40 ′ there is provided at least one passage to atmosphere, shown at  52  in FIG.  4 . This passage is fabricated to extend to a location outside of enclosure  4  for access when the system is totally assembled. Passage  52  allows for leak checking both seals  40  and  40 ′. Thus, both sealing with respect to the coolant fluid  54  between chamber  2  and wall  4  and sealing with respect to the vacuum in region  10  can be checked with one port. 
     RF energy can flow from coil  8  into region  10  through inserts  36  and slots  34 . 
     FIG. 5 shows another form of construction of the embodiment of FIG.  1 . In the form of construction shown in FIG. 5, housing  2  is provided with vertically elongated slots  60  that extend through the entire wall thickness of housing  2 . In the illustrated embodiment, each slot  60  has an outwardly diverging, or flaring, portion adjacent the exterior surface of housing  2 , and a portion  64  having a surface that is perpendicular to the vertical walls of housing  2 . A dielectric window  66  is installed in portion  64  of each slot  60  and is secured to portion  64  by a metal band  68  which is brazed to both portion  64  and the peripheral edge of dielectric window  66 . Band  68  serves to compensate for thermal expansion differences between dielectric window  66  and housing  2 . FIG. 5 shows a broken-away portion of the outer vertical surface of housing  2 . The reason for this portion is to better illustrate the cross-sectional form of the regions between adjacent slots  60 . 
     Band  68  may be made of Kovar™, which is a trade-name for a metal alloy containing 54% iron, 29% nickel and 17% cobalt. The coefficient of thermal expansion for Kovar is between that of the metal housing and dielectric window. The use of such a material is common to the industry. 
     In the embodiment of FIG. 5, each slot  60  can be given a larger area than each slot  34  of the embodiment shown in FIGS. 3 and 4, so that the FIG. 5 embodiment is capable of providing a larger total effective dielectric window area for passage of RF energy into region  10 . In addition, the structure, or structural area, necessary to fasten the dielectric window to the housing walls of the plasma source is minimized in this embodiment. 
     FIG. 6A illustrates a further embodiment of a chamber housing  72  according to the invention. Housing  72  has the same general form as housing  2  of FIG. 1, but is provided on each side with a large area opening  74  surrounded by a recessed portion  76  that frames opening  74 . Each opening  74  is completely covered by a rigid dielectric panel  78  made, for example, of alumina. Each panel  78  is a one-piece dielectric body composed of a flat base portion  80  and a plurality of vertically extending ribs  82  that project at right angles from base portion  80 . Panel  78  is dimensioned to extend entirely across opening  74  and recessed portion  76 . 
     Housing  72  is further provided with a plurality of elongated load supporting members  86  made of electrically conducting material, such as aluminum. Each member  86  has a T-shaped cross section, is seated between two adjacent ribs  82  and is securely connected at its upper and lower ends to top and bottom edges of housing  72 . Members  86  function as the conductive members of the electrostatic shield and it is important these members have good RF electrical contact to housing  72  both at the top and bottom. Satisfactory contact, and a sound mechanical connection, can be provided by using machine screws (not shown) to secure the upper and lower ends of each member  86  to the top and bottom edges of housing  72 . Because members  86  are made of metal and are therefore relatively inelastic, a layer of an elastic material is preferably disposed between each member  86  and its associated part of base portion  80 . One such member  88  is shown in broken lines in FIG.  6 A. 
     As an alternative to the embodiment illustrated in FIG. 6A, the electrostatic shielding may be provided in the form of a metal coating on the external surface of each panel  78 , as shown in FIG.  6 B. This coating will be in the form of individual strips each located between two adjacent ribs  82 . In this embodiment, ribs  82  are shortened so that their ends are spaced from the upper and lower edges of each panel  78 . Upper and lower window braces  89  (only upper brace  89  is shown in FIG. 6B) extend along the upper and lower edges of each panel  78 , are provided with notches that receive the ends of ribs  82  and with tabs which interlock with the ribs  82 . These tabs are in direct contact with the applied metal coating. Window braces  89  secure the entire perimeter of each dielectric panel  78  to the associated opening in housing  72 . Braces  89  are bolted to housing  72  as shown in FIG.  6 B. 
     Each brace  89  has certain built in features that are shown in FIG.  6 C. In particular, FIG. 6C shows O-ring seals  40  and  40 ″ and leak testing port  52 ′. In both embodiments presented in FIGS. 6A and 6B, each dielectric panel  78  must be capable of withstanding the bending moments imposed due to inwardly directed pressure forces present when a chamber vacuum exists. This the primary purpose of ribs  82 . 
     In the case of the embodiment shown in FIG.  6 A and the alternative thereto described above, each dielectric panel  78  is hermetically sealed to its associated recessed portion  76  by at least two O-rings held in grooves  41 . Preferably, sealing is achieved by the provision of dual elastomeric seals, like rings  40  and  40 ′, separated by a space which is coupled by a series of passages, like passage  52 , internal to the housing to the outside to allow technicians to sense liquid leaks or vacuum leaks. 
     FIG. 7 is a cross-sectional plan view showing the wall of chamber  2  and the bottom surface of wall  12  when one looks upwardly from within chamber region  10 . Wall  12  carries an array of processing gas introduction tubes  90  having vertically extending inlet portions  92  that extend through, and are supported by, dielectric inserts  96  that are secured in openings  98  in wall  12 . Inserts  96  and other dielectric parts of this assembly may be made of, for example, PTFE. Each tube  90  has a horizontally extending outlet portion  94  that extends between its associated inlet portions  92 . Outlet portion  94  of each tube  90  is provided with a row of outlet holes, or injection nozzles,  94 ′ (FIGS. 8A and 8B) extending along the length thereof. 
     Outlet portions  94  of tubes  90  can be placed at a height above the substrate to be treated that allows the optimum gas species to arrive at the substrate. As the distance between outlet portions  94  and the substrate is increased, the spacing between tubes  90  can be increased while the density of ionized gas reaching the substrate remains approximately uniform. Of course, an increase in the spacing between tubes  90  results in a reduction in the number of tubes. For certain processes, however, it may be desirable to bring the outlet portions of tubes  90  closer to the substrate. This may be done, for example, if it is desired to reduce the time between gas ionization and contact of the resulting ions with the substrate. 
     Each gas injection tube input portion is connected to a flow regulating valve or an individual mass flow controller to control the injection of gas from an inlet manifold (not shown). By controlling the flow of gas into each end of each tube  90 , a variety of gas injection profiles are made possible, as will be described below. 
     At the center of wall  12  there is provided a viewport  99  in the form of a funnel-shaped passage. This funnel-shaped passage is angled such that it provides a field of view encompassing the entire substrate being processed. Viewport  99  may be used simply for visual inspection of the chamber and its process, or it may accommodate a diagnostic system requiring optical access to the inner chamber. 
     The exterior surfaces of gas introduction tubes  90  will become coated with residue of the processing gasses over the course of time. According to a further feature of the invention, these coatings can be removed from tubes  90  by applying RF bias to tubes  90  during cleaning of the interior of housing  2 . 
     Such cleaning is conventionally performed periodically in present day etch or deposition chambers by a separate cleaning process wherein the chamber is cleaned with a substrate installed in region  10 . Region  10  is filled with a gas which, when ionized in a plasma, is capable of removing residue coating from surfaces within housing  2  and a plasma generating RF field is created in region  10 . In wafer processing, this cleaning process is often conducted at very much higher pressure than the normal process pressure to improve the chemical process rate by increasing the number of atoms, or ions, in the plasma. Applying RF bias to parts of housing  2  can also increase the residue removal rate. 
     It is also known to install a metal electrode outside the housing and behind the dielectric wall of the housing and to apply a voltage to the electrode in order to provide bias to the wall and increase the cleaning rate. An arrangement of this type is disclosed, for example, in pending provisional application No. 60/065,794, by Wayne Johnson, entitled ALL RF BIASABLE AND/OR SURFACE TEMPERATURE CONTROLLED ESRF. 
     Application of RF bias to tubes  90  can increase the energy of ion bombardment within tubes  90  and thus increase the rate and effectiveness with which residues are etched away from their interior walls. Ion bombardment can be thought of as increasing the surface temperature of surfaces being bombarded and hence can increase the chemical reaction rates. 
     Preferably, injection tubes  90  are made of anodized aluminum or are composed of a metal tube component sheathed in dielectric tubing made of quartz or alumina. 
     To allow for application of a cleaning RF bias voltage to tubes  90 , it is necessary to isolate tubes  90  electrically from the walls of enclosure  4 . Such isolation is needed so that plasma is not created inside the tubes that deliver gas. This bias will not be applied during normal operation, but only during periodic cleaning cycles. The RF bias to the gas injection tubes is applied periodically to clean the exterior surface (or process side). During etch process, contaminants may build up on the tube surface. To minimize long-term particulate contamination, the exterior surface of the injection tubes (in addition to all surfaces within the chamber) must be cleaned during a cleaning cycle. The RF bias will generate a DC self-bias (and resultant average voltage difference across the sheath) which in-turn affects the average ion energy delivered to the injection tube surface. 
     If the tubes are made of conductive material, then a capacitor is needed to allow them to charge by self-bias. 
     In order to prevent the generation of a plasma within the interior volume of the injection tubes during RF bias application, it is possible to use strictures inside the tube that minimize breakdown by use of dielectric surface area. One example is a bundle of capillary tubes of dielectric material (quartz) in the gas flow path as has been used in the semiconductor industry to deliver process gas to an upper electrode-inject plate. 
     FIGS. 8A and 8B show details of an arrangement for supplying RF voltage to an injection tube  90 . FIG. 8A is a cross-sectional view and FIG. 8B is a bottom plan view of the entrance region of one injection tube  90  via which process gas is supplied and RF bias is applied. This assembly resides within upper wall  12 . Process gas is fed from a standard gas line and fitted to the gas injection system using a standard fitting  102 , as shown. Dielectric inserts  96  isolate fitting  102  from a conductive base ring  104  which surrounds inlet portion  92 . A RF voltage is applied to ring  104  through a standard connection flange made at the output of the respective match network, an RF connection interface  108  and a RF inner conductor  110  which forms a unit with base ring  104 . The RF feed input to the gas injection RF bias assembly is a standard feed consisting of inner conductor  110 , an outer conductor  114  and a dielectric  116  sandwiched between the conductors. Inner conductor  110  attaches to base ring  104  that is in immediate contact with gas injection tube  90 . Outer conductor  114  is integral with a support plate  120 . Dielectric inserts  96  and  116  isolate outer conductor  114  and its support plate  120  from inner conductor  110  and ring  104 . 
     According to the invention, the distribution of processing gas within region  10 , using the injection tube arrangement shown in FIG. 7, can be controlled by suitable selection of one or more of gas inlet pressure, gas flow rate and the total area of the injection nozzles ( 94 ′) in each tube  90 . The relationships involved in effecting this control will be described with reference to FIGS. 7,  9 ,  10 A and  10 B. 
     As shown in FIG. 7, processing gas enters region  10  via the array of injection tubes  90  whose outlet portions  94  lie in a common horizontal plane located at a selected vertical distance below enclosure wall  12 , which wall constitutes a pump manifold plate. However, it is not necessary that all injection tubes  90  lie in a common horizontal plane. In fact, it may be beneficial to vary their vertical spacing relative to the wafer plane. 
     Injection tubes  90  are equally spaced (with spacing d) in one horizontal direction. As illustrated in FIG. 9, outlet portion  94  of each tube  90  has a length 2 L in a horizontal direction across region  10 . As noted earlier herein, the number of tubes  90 , and their spacing, d, can be varied to achieve a selected processing result. Equally, length 2 L is selected on the basis of the desired processing result. Each injection tube  90  has a cross-sectional area A 1  along its entire length, i.e., along its inlet and outlet portions, and outlet portion  94  of each tube  90  is provided with N injection nozzles ( 94 ′ in FIGS. 8A and 8B) each having a cross-sectional area A 2 . Thus, the injection nozzles in one tube  90  have a total outlet area A 2T =NA 2 . The injection nozzles are uniformly spaced apart with a spacing Δl between the center lines of adjacent injection nozzles. Gas is fed to both ends of a tube  90  with an inlet pressure P t  and a volume flow rate Q at each end, i.e., at each inlet portion. This is also elaborated in the discussion of FIG.  9 . The cross-sectional area of each tube  90  need not be constant nor do the injection nozzles need to be equally spaced. Clustering of the injection nozzles may be beneficial for additional gas injection control. 
     The injection system is designed to introduce the processing gas to a large volume chamber region  10  in which a vacuum pressure P c  is maintained. The gas is introduced at subsonic speeds, i.e., M=v/a&lt;0.3-0.5 where M is the Mach number, v is the gas velocity at each exit orifice and a is the local speed of sound. 
     According to the invention, the distribution, or gradient, of gas exit velocities across the length of output portion  94  of a tube  90  can be controlled by proper selection of the flow rate Q and the inlet total pressure P t  at both ends of that tube  90 . The particular gradient established will influence the uniformity of the plasma-assisted treatment across the substrate surface. 
     When the flow rate Q into an injection tube  90  is high, an exit velocity distribution can be obtained such that the velocity is greatest at the mid-point of the length of the outlet portion of the tube and decreases progressively toward the ends thereof. Alternatively, when the flow rate Q is low, the velocity distribution is such that the greatest velocities are achieved at the ends of tube outlet portion  94 . Hence, it is possible to control the velocity distribution along the length of the outlet portion of a tube  90 , or the span-wise velocity distribution, by adjusting the inlet volume flow rate Q. 
     For the sake of clarity, the terms “high flow rate” and “low flow rate” will be defined. A “high flow rate” is one in which the gas momentum is large relative to the difference in pressure between the gas in the injection tube and the chamber pressure. Similarly, a “low flow rate” is one in which the relative gas momentum is small. In the case of a high flow rate, the lateral pressure gradient is unable to sufficiently bend the “high” momentum fluid and accelerate it through the adjacent injection nozzle; therefore, the predominant mechanism is deceleration of the gas to a stagnation pressure at the midpoint of the length of the outlet portion  94  of the tube  90  where momentum is cancelled and a sufficient pressure difference can be achieved. The stagnation flow at the outlet portion midpoint is simply a consequence of introducing process gas at both ends of the injection tube. In the case of a low flow rate, the gas momentum is such that gas will tend to exit via openings at the ends of the outlet portion of the injection tube under the effect of the differential between the inlet pressure and the pressure in chamber region  10 ; as gas exits through successive injection nozzles  94 ′, the pressure within outlet portion  94  of a tube  90  decreases. 
     Taking a more rigorous approach, the above explanation can be substantiated in a clearer manner. Consider the transverse equation of momentum for the coordinate system indicated in FIG.  9 . Assuming the flow to be steady and two-dimensional and neglecting the viscous terms, the transverse equation of momentum becomes:                    -   1     ρ                       ∂   P       ∂   r         =         v   z                       ∂     v   r         ∂   z         +       ∂     v   r           v   r          ∂   r                   (   1   )                                
     The radial pressure gradient is balanced by two terms; the first of which represents the transfer of stream-wise momentum (in the direction z) into radial momentum (in the direction r), and the second of which represents the radial acceleration of the radial flow. The injection tube design depends on an independent set of parameters including ρ 0 , Q, P t , P c , A 1 , A 2t , Δl and L; refer to FIG.  9 . Note that this parameter list excludes the number N of injection nozzles  94 ′ and their respective cross-sectional areas A 2  since N=2 L/Δl and A 2 =A 2T /N). Neglecting compressibility effects (a good assumption for M&lt;0.3), the radial equation of momentum is non-dimensionalized using the following relationships                z   *     =     z     Δ                 l               (2a)                 r   *     =         A     2      T          r       2        A   1        Δ                 l               (2b)                 v   z   *     =         A   1          v   z       Q             (2c)                 v   r   *     =         A     2      T            v   r         2      Q               (2d)                 P   *     =     P     Δ                 P               (2e)                 ρ   *     =     ρ     ρ   0               (2f)                                
     where ΔP=P t −P c ; ρ represents local density, and ρ 0  represents density at stagnation conditions. The radial length and velocity scales are obtained by virtue of continuity. Hence, the non dimensional radial equation of momentum is obtained,                  ∂     P   *           ρ   *          ∂     r   *           =       B   *          (         v   z   *                       ∂     v   r   *         ∂     z   *           +       v   r   *                       ∂     v   r   *         ∂     r   *             )               (   3   )                                
     which identifies the non-dimensional parameter                B   *     =       4                   ρ   0          Q   2           A     2      T     2        Δ                 P               (   4   )                                
     When B*&gt;&gt;1, this corresponds to high flow rates at which the pressure gradient is inadequate to substantially turn the stream-wise momentum and, hence, larger velocities exit at the midpoint, or center, of the outlet portion of the injection tube. Conversely, B*&lt;&lt;1 corresponds to low flow rates at which the opposite is true. The resulting velocity distributions are depicted in FIGS. 10A and 10B. 
     Thus, if B*=1, the gas exit velocity will be constant along the length of outlet portion  94  of tube  90 . For many processes, this will be the preferred exit velocity distribution. However, there may be situations in which it is preferable that B*≠1. For example, the RF field generated in region  10  may vary in intensity in radial directions perpendicular to the vertical center axis of region  10 . In such a case, a gas flow rate variation having a form shown in one of FIGS. 10A and 10B may be used to compensate for the RF field variation in order to produce a processing result which is uniform across the surface of the substrate. 
     Close inspection of the definition of B* provides insight into the design of injection tubes  90 . For example, the condition B*&gt;&gt;1 can be achieved by performing any one of the following actions while holding all other parameters constant: increase Q (increase the gas momentum ρ 0 V); decrease ΔP (reduce the turning force); and decrease A 2T  (provide greater flow resistance). 
     Typically, a fixed relation will exist between a given value for P c , inlet pressure and flow rate. However, it might be possible to independently control the inlet total pressure and the mass flow rate. This would require adjusting the total pressure losses in the system using throttle valves. For example, the throttle valve upstream of the turbo-molecular pump can adjust the chamber pressure and pressure regulators upstream of the injection tubes can regulate the total pressure. 
     Considering the list of independent dimensional parameters listed previously, it is sufficient to define a parameter for uniformity u=P(z=0)−P(z=L) such that the non-dimensional uniformity u*=u/ΔP takes the form                u   *     =       u   *          (       B   *     ,       Δ                 l     L     ,       Δ                 P       P   c       ,       A   2       A   1       ,       A   1       L   2         )               (   5   )                                
     We consider the asymptotic limit where the four latter parameters go to zero, i.e., the number of injection nozzles is large (Δl/L→0), the pressure difference is small relative to the absolute value (ΔP/P c →0), each injection nozzle area A 2  is small relative to the injection tube cross-sectional area (A 2 /A 1 →0), and the injection tube is long relative to its diameter (A 1 /L 2 →0). Nominal conditions for B*˜1 are: Δl=1.0 cm, L=50 cm, N=100, A 1 =1.77 cm 2 , A 2 =0.0079 cm 2 , P c =500 mTorr, P t =600 mTorr and Q=160 sccm (or Q tot =320 sccm). 
     According to various alternatives made possible by the invention, it may be desirable to allow the gas flow to choke at the gas injection nozzles  94 ′. When the pressure ratio across an injection nozzles  94 ′ (i.e., the ratio of the total pressure inside the injection tube to the ambient chamber pressure beyond the exit of the injection nozzle) is sufficiently large, the injection nozzle reaches a “choked” condition wherein the volume flow rate is invariant with either further reduction of the back pressure (or chamber pressure) or increase of the inlet total pressure. In fact, the mass flow can only be increased further by increasing the total inlet pressure (hence affecting the gas density). Since the volume flow rate at an injection nozzle exit is invariant and the injection nozzle exit area is constant, this implies that the exit velocity is constant. However, one may redistribute the injection nozzles  94 ′ in injection tube  90  in order to affect the mass flow distribution entering the chamber. Hence, the mass flow distribution may be designed to behave in either manner as described in FIGS. 10A and 10B. For example, if injection nozzles  94 ′ are clustered towards the ends of injection tubes  90 , then a mass flow distribution similar to FIG. 10B can be obtained. Conversely, if injection nozzles  94 ′ are clustered towards the center of the injection tubes, then a mass flow distribution similar to FIG. 10A can be obtained. Additionally, far from the injection plane (approximately 10 to 20 injection nozzle diameters) the velocity distribution will behave similar to these distribution forms. 
     There are several advantages to the use of the gas injection tubes, namely: RF bias of the gas injection tubes allows periodic cleaning of the exterior surface; variable placement of adjacent tubes in separate vertical and/or horizontal planes, allowing each injection tube to be located at a different vertical height above the substrate for improved process control; selectable injection nozzle distribution for modification of inlet mass flow distribution; supersonic or subsonic injection capability; and adjustment of gas momentum in the injection tubes for mass flow redistribution subsonic injection. 
     As mentioned earlier herein with reference to FIGS. 1 and 2, a plasma source according to the invention includes a plurality of pumps  18  that continuously withdraw gas from region  10  during a processing operation. Each pump  18  acts primarily on an associated section of region  10 . Processing at high rates requires high gas throughput to get the desired number of surface chemical reactions to occur. Processing needs high gas throughput and high plasma density. The large number of pumps employed in the embodiments disclosed herein provide the high pumping capacity needed to achieve high processing rates for large area substrates. Each pump is fitted with a throttle control valve, as shown at  22  in FIG. 1, that can regulate the pumping speed of that pump. By individually controlling the pumping speed of each pump  18 , a wide variety of pumping speed profiles are possible. Each pump  18  may be constituted by any type of pump currently employed in plasma processing apparatus. Solely by way of non-limiting example, each pump  18  may be a turbomolecular pump, or turbo-pump, with or without a backing pump. 
     As indicated in FIGS. 1 and 2, apparatus according to the invention may be equipped with sixteen (4×4) pumps  18 . Each pump may be a 1000 liter/sec turbo-pump positioned atop wall  12 . Processing gas is directed towards the substrate from injection nozzles  94 ′ in injection tubes  90  by the momentum created by the velocity at which the processing gas exits from the injection nozzles. After interacting with the substrate, unused process gas and volatile reaction products are removed via pumps  18 . In order to minimize the interaction of reaction products with incoming process gas, an outward pressure gradient is established to bias the reaction products to flow to the outer walls and then upward to the pumps. This is accomplished by reducing the pumping rate of the pumps  18  that are proximate to the center of plate  12 , for example by slightly closing the valves of the those pumps to decrease their pump conductance. In this manner lower chamber pressures can be achieved towards the walls of the chamber housing  2 ,  2 ′. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     A plasma source according to the invention may include a conventional chuck for holding the substrate, or wafer, to be processed. The chuck would be typical of most conventional plasma sources. In addition to holding the substrate, it should be capable of applying a RF bias to and heating the substrate. Therefore, for large area processing, this chuck could consist of multiple segments. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.