Abstract:
An apparatus and method for regulating the plasma characteristics in a plasma processing system that includes a plasma generating assembly that generates a primarily inductively-coupled plasma from a process gas in a processing space to process a substrate. A walled enclosure is provided adjacent the processing space and is configured to generate a capacitively-coupled plasma from process gas therein. The enclosure further includes one or more outlets that permit the capacitively-coupled plasma to enter into the processing space. Charged particles from the capacitively-coupled plasma may be applied to alter the plasma density of the inductively-coupled plasma or to reduce the power required to generate the inductively-coupled plasma.

Description:
FIELD OF THE INVENTION 
     This invention relates to plasma processing of a substrate and, more particularly, to an apparatus and method for improving the plasma density in a plasma processing system. 
     BACKGROUND OF THE INVENTION 
     The fabrication of semiconductors, or integrated circuits, typically consists of multiple distinct processing steps, during which numerous replicas of an integrated circuit are formed on a single substrate or wafer. Generally, the fabrication procedure entails the creation of multiple patterned layers on and into the surface of the semiconductor substrate, ultimately forming the electrically active regions, passive elements and interconnects that comprise the integrated circuit. 
     Modern techniques for fabricating integrated circuits liberally incorporate plasma processes to modify the surface of the semiconductor substrate and to generate the multiple patterned layers. For example, a plasma etching process employs a plasma to selectively remove a layer of extraneous material from the substrate surface or to condition the surface by low-energy ion bombardment. As another example, a physical vapor deposition process may employ a plasma to control the characteristics of the thin films deposited onto the substrate surface from a source of coating material. A high degree of ionization of neutral atoms of coating material has been found to enhance the control over the coating material in transit towards the substrate. 
     Ionized physical vapor deposition (IPVD) is a plasma-enhanced deposition process used to deposit a thin film or coating onto the surface of the semiconductor substrate. A source of a coating material is positioned within a vacuum chamber usually opposite a substrate support holding the substrate. The source generates a flux of atoms or atomic clusters of coating material, such as by the sputtering of a solid target having the desired elemental composition. To deposit as a thin film upon the substrate, the flux of coating material must traverse a high-density plasma confined within a processing space separating the source and the substrate. The high-density plasma may be generated by coupling radio-frequency (RF) energy to a process gas maintained at an operating pressure in the processing space. 
     A significant fraction of the flux of coating material is ionized by collisional interactions with the positive ions of process gas (Penning ionization) and free electrons (impact ionization) constituting the high-density plasma. At high density plasma where the electron density exceeds 10 11  cm −3 , electron impact ionization is the dominant process in IPVD. To preferentially attract positive ions of coating material, the substrate may be negatively biased. The negative bias potential may arise incident to immersion of an ungrounded substrate in the plasma if the substrate support is electrically floating or by directly applying a bias voltage to the substrate support and substrate. The negative bias potential accelerates and steers the trajectories of the positive ions of coating material such that the ions tend to strike the surface of the substrate with a near-normal angle of incidence. As a result, the deposited coating material will more effectively cover the bottoms and sidewalls of submicron features having a high aspect ratio, such as vias, lines, contact holes, and trenches. 
     The RF energy for generating the high-density plasma is supplied by an RF power supply operably connected to an antenna or excitation coil positioned either external or internal to the vacuum chamber. If the excitation coil is externally positioned, a wall of the vacuum chamber may be further provided with a dielectric window which permits RF energy from the coil to ignite and sustain the plasma and isolates the coil from direct contact with the plasma. 
     The dielectric window is typically masked by an electrostatic shield, typically formed of an electrically-conductive material and disposed in the vacuum chamber, which functions as a Faraday shield and as a physical shield. A plurality of openings in the shield permit inductive coupling of RF energy emanating from the excitation coil with the plasma while suppressing the unwanted component of parasitic capacitive coupling. As a physical shield, the shield prevents an unwanted conductive layer of coating material from depositing onto the window by concealing the window from the plasma. If an electrically conductive layer of coating material deposits on the window, RF energy from the excitation coil can no longer couple efficiently with the plasma since inductive RF field is absorbed exponentially with the penetration depth in a conductive material. As a result, the plasma density will be reduced and the deposition process deteriorates or the RF power must increased to compensate for the reduced density. If thickness of the unwanted conductive layer exceeds a frequency-dependent threshold, called the skin depth, then significant RF power loss will occur. 
     A typical coating material source sputters a target, composed of high purity coating material, that is negatively biased with respect to plasma confined close to the target and a chamber anode such as the grounded wall of the vacuum chamber. Usually, the target is operably connected to a direct current power supply that supplies a bias potential for attracting positive ions from the high-density plasma. The source is frequently of a magnetron design which incorporates a magnet structure for creating and confining plasma adjacent the target. 
     Conventional inductively-coupled plasma processing systems have shortcomings and deficiencies that restrict their widespread application for large-area wafer processing. High-density plasmas generated by inductively coupled plasma generating assemblies exhibit significant radial non-uniformities in plasma density. Due to losses near the chamber walls, the plasma in the processing space has a density distribution that is preferentially peaked about the central symmetry axis of the vacuum chamber and depleted of positive ions near the chamber walls. If such a radially non-uniform plasma is used for etching the surface of a substrate, the removed layer will be thinner near the periphery of the substrate due to reduced ion flux that controls etching rate. In an IPVD apparatus, a radially non-uniform distribution in plasma density may affect the properties of the deposited thin film and coverage of features. For example, the thin film thickness may be uniform due to the target geometry, but step coverage may vary across the wafer diameter due to non-uniformities in the ion distribution. Non-uniformities in etching or deposition are most pronounced for substrates having larger diameters. Since the trend in semiconductor fabrication is toward large-area wafers, the presence of non-uniformities in the plasma density will be more significant in future plasma processing systems, such as IPVD systems and plasma etching systems. 
     Plasmas generated by inductively coupled plasma generating assemblies exhibit certain limitations regarding the amount of RF power that must be supplied to initiate an inductively-coupled, high-density plasma. Under certain circumstances, the inductively coupled plasma must be extinguished by reducing the RF power. For example, the RF power must be reduced to load or unload substrates from the vacuum chamber or the RF power supply rendered inoperative if the vacuum chamber is vented to atmosphere pressure. To reinitiate the inductively-coupled component of the plasma, a large amount of RF power must be provided by the RF power supply. For example, the power needed to initiate an inductively-coupled high-density plasma may exceed the power needed to initiate a capacitively-coupled plasma by an order of magnitude, under similar chamber conditions. 
     As a result of the above considerations and problems, there remains a need for an apparatus and method that can supplement the primary, high-density inductively-coupled plasma of an inductively-coupled plasma processing system for increasing plasma uniformity adjacent the substrate and for reducing the RF power required to initiate an inductively-coupled plasma. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides an apparatus and a method for improving the uniformity of the plasma density in an inductively-coupled plasma processing system. The present invention further advantageously provides an apparatus and a method in which a supplemental capacitively-coupled plasma is provided so that a reduced RF power level is required to initiate a high-density, inductively-coupled plasma in a processing system. Moreover, the present invention advantageously provides a more efficient and effective apparatus and method for plasma processing operations that can be incorporated into current plasma processing systems without significantly altering conventional chamber designs. 
     According to the principles of the present invention, one or more hollow anode assemblies are located about the interior of the vacuum chamber of a plasma-processing apparatus that relies upon an inductively-coupled, high-density plasma for processing a substrate. Each hollow anode assembly comprises one or more enclosures or discharge cavities which receive a portion of a process gas residing in the vacuum chamber and which are operable for containing a capacitively-coupled plasma therein. Positive ions of process gas and electrons from the capacitively-coupled plasma exit each cavity through one or more outlets provided therein and enter the vacuum chamber. 
     The vacuum chamber further includes a plasma generating assembly that is configured to deliver energy into the vacuum chamber for generating the inductively-coupled, high-density plasma. An exemplary plasma generating assembly includes an RF power supply operably connected to an excitation coil, which can also provide the energy to initiate and sustain the capacitively-coupled plasma in each cavity. 
     Enclosures may be positioned, for example, adjacent to the plasma generating assembly or adjacent to a substrate support. However, in positions with the vacuum chamber remote from the plasma generating assembly, the coupled energy may be insufficient to generate the capacitively-coupled plasma within each cavity or the excitation coil would require modifications which would add complexity to the processing system. In these cases, the plasma generating assembly may further incorporate an ancillary power supply operable to generate the capacitively-coupled plasma. In one aspect, an electrode may be positioned within the cavity of each enclosure and operably connected to the ancillary power supply. 
     According to the apparatus and method of the present invention, one advantage is that a capacitively coupled plasma can be initiated and sustained in the cavity of each enclosure without the need for an additional plasma generating assembly. Power from the plasma generating assembly, which is principally operable for inductively coupling with process gas in the processing space, can capacitively couple with process gas to generate a plasma in each enclosure. It follows that the enclosure, in certain embodiments, can be a passive element of the plasma processing system without requiring an electrode, an additional power supply, or an electrical feedthrough. 
     Another advantage of the apparatus and method of the present invention is that the density distribution of the inductively coupled plasma in the processing space can be efficaciously modified by emitting ions and electrons from each enclosure and, as a result, the plasma process will achieve results having improved uniformity. 
     Yet another advantage of the apparatus and method of the present invention is that the power required to initiate an inductively coupled plasma in the processing space can be significantly reduced by providing electrons and ions from the capacitively coupled plasma generated within each enclosure prior to ignition of an inductively-coupled plasma in the processing space. 
     The present invention may be incorporated into an existing processing chamber merely by modifying the shield structure to add one or more enclosures. Therefore, the addition of one or more of the enclosures will not significantly alter design of the processing chamber, while optimizing plasma processing and overcoming the difficulties set forth in the background above. 
     These and other advantages of the present invention will be more readily apparent from the following detailed description of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional diagrammatic representation of a plasma-processing apparatus incorporating a hollow anode assembly in accordance with principles of the present invention; 
     FIG. 2A is an enlarged, fragmentary cross-sectional view illustrating an embodiment of the hollow anode assembly of FIG. 1; 
     FIG. 2B is an enlarged, fragmentary cross-sectional view, similar to FIG. 2A, illustrating an alternative embodiment of the hollow anode assembly of FIG. 1; 
     FIG. 3 is a plan view of the section of shield and hollow anode assembly taken along line  3 — 3  of FIG. 2A; 
     FIG. 4 is cross-sectional diagrammatic representation, similar to FIG. 1, of a plasma-processing apparatus incorporating an alternative embodiment of a hollow anode assembly in accordance with principles of the present invention; 
     FIG. 4A is an enlarged view of encircled area “ 4 A” of FIG. 4; 
     FIG. 5 is an fragmentary cross-sectional view illustrating an alternative embodiment of a hollow anode assembly similar to that shown in FIG. 4; 
     FIG. 5A is an enlarged, fragmentary cross-sectional view, similar to FIG. 5, illustrating an alternative embodiment of the hollow anode assembly; and 
     FIG. 5B is an enlarged, fragmentary cross-sectional view, similar to FIG. 5A, illustrating an alternative embodiment of the hollow anode assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A plasma-processing apparatus  10 , according to one embodiment of the present invention, configured for ionized physical vapor deposition (IPVD) is diagrammatically represented in FIG.  1 . The apparatus  10  includes a vacuum chamber  12  having a processing space  13  bounded by a chamber wall  14 . Chamber  12  is provided with a vacuum pumping system  16  for evacuation of the chamber  12  to an ultra-high vacuum (UHV) level of about 10 −8  torr or less. A mass flow control device  18  controls the flow rate of a process gas from a gas supply  19  into chamber  12 . Suitable process gases include inert gases, such as argon, or reactive gases, such as nitrogen or oxygen. A manual or automated vacuum loadlock (not shown) may be provided to transfer substrates into and out of chamber  12  while maintaining a suitable vacuum within the chamber  12  and the loadlock. 
     At one end of the vacuum chamber  12  is situated a substrate support  20  to which a substrate bias power supply  22  is operably connected applying a negative bias on the substrate support  20  and to one or more substrates  24 , such as semiconductor wafers, supported thereon for plasma processing. Bias power supply  22  may comprise a direct current bias supply or an RF bias supply connected through an impedance matching network. 
     At the opposite end of the chamber  12  from the substrate support  20  is situated a coating material source or cathode assembly  26 . The cathode assembly  26  includes a ring-shaped target  28  of the desired coating material and is operably connected to a bias potential furnished by a cathode power supply  30 . The power supply  30  is preferably a DC power supply, but may be an RF supply operably connected to the target  28  through a suitable matching network (not shown). The cathode assembly  26  is electrically isolated from electrically grounded surfaces of the chamber  12  by an insulator ring  34 . The chamber wall  14 , which is usually at ground potential, may serve as an anode. Dark space shield  32  prevents the plasma from eroding or sputtering structure adjacent the target. 
     A non-conductive window  40  is concentrically provided within the central opening of cathode assembly  26  and is configured so as to create a circumferential vacuum-tight seal with an upper edge of the target  28 . The window  40  is constructed of a vacuum-compatible dielectric material, such as alumina or quartz, preferably alumina, and is a generally planar structure having substantially parallel, opposed planar surfaces or faces. 
     An excitation coil, shown schematically as element  42 , is positioned adjacent the outer face of non-conductive window  40 , preferably in close proximity to the window  40 . Coil  42  is operably connected through an impedance matching network  46  to an RF power supply  44 . The RF power supply  44  may be operative in the range of from about 200 kHz to about 80 MHZ and may deliver an RF power between about 100 W and about 10 kW. Since window  40  is electrically non-conductive, RF magnetic energy emanating from coil  42  will not be attenuated during its passage through window  40  into processing space  13 . 
     Suitable excitation coils  42  can be adapted from antennas, for example, as described in U.S. Pat. No. 4,948,458 issued to Ogle, U.S. Pat. No. 5,669,975 issued to Ashtiani, and U.S. patent application Ser. No. 09/277,526 in the name of Josef Brcka entitled Process Apparatus and Method for Improving Plasma Distribution and Performance in an Inductively Coupled Plasma, filed on Mar. 26, 1999. The disclosure of each is hereby expressly incorporated by reference herein in its entirety. 
     Adjacent the inner face of the non-conductive window  40  is a shield  48 , which serves as a combination Faraday shield and physical shield. The shield  48  is mounted a small distance from inner face of the window  40 . As a Faraday shield, the shield  48  promotes inductive coupling of energy from excitation coil  42 , across window  40 , and into the processing space  13  while reducing undesired capacitive coupling between coil  42  and the high-density plasma. As a physical shield, the shield  48  prevents coating material from depositing onto the window  40  by blocking substantially all direct line-of-sight paths between any point on the surface of the target  28  and the window  40 . Shield  48  is preferably composed of a material having a high electrical conductivity, such as copper or aluminum. 
     Shield  48  includes a plurality of slots  49  and may be electrically grounded to, and maintained in good thermal contact for conductive heat flow with, adjacent structures within vacuum chamber  12 , such as chamber wall  14 . The plurality and geometry of slots  49  is configured to avert eddy currents in the shield  48  and allow RF magnetic fields to penetrate shield  48 . 
     In operation, the vacuum chamber  12  is evacuated to a base vacuum level by vacuum pumping system  16 . A flow of a process gas is introduced from gas supply  19  to the chamber  12 . Flow control device  18  meters the gas flow to establish a flow rate of about 5 to about 1000 standard cubic centimeters per minute (sccm) and an operating pressure between about 1 to about 100 mtorr. Power supply  44  is energized to supply RF power through matching network  46  to the excitation coil  42 . A high density primary plasma is initiated and thereafter sustained in the processing space  13  by inductively-coupled RF energy transmitted from coil  42  through non-conductive window  40  and shield  48 . A negative bias potential is supplied by cathode power supply  30  for accelerating positive ions from the processing space toward the target  28  to impact with sufficiently high energy to sputter atoms or atomic clusters of coating material therefrom. Transport of the sputtered coating material is diffusion driven when thermalized and the material flux must pass through the high-density plasma to reach the substrate  24 . A portion of the atoms or atomic clusters become ionized due to collisional interactions with ions and electrons in the high-density plasma. The negative bias applied to substrate  24  by the substrate bias power supply  22  electrostatically attracts and steers positive ions of coating material from the plasma towards the substrate  24  with ion trajectories predominately parallel to the surface normal thereof. 
     In accordance with the principles of the present invention, the periphery of the shield  48  further includes a hollow anode assembly  50 , which is operable to supply positive ions of process gas and electrons from a capacitively-coupled plasma generated therein. Referring to FIGS. 1,  2 A and  3 , the hollow anode assembly  50  comprises an end wall  52  and two opposed, generally parallel side walls  54 ,  56  that collectively define the boundaries of an annular enclosure or discharge cavity  58 . Cavity  58  extends about an outer circumference of, and is substantially concentric with, the shield  48 . Walls  52 ,  54 , and  56  are preferably composed of a material that has a low electrical resistivity, such as copper or aluminum, and may have the same composition as shield  48 . 
     Continuing to refer to FIGS. 1,  2 A and  3 , hollow anode assembly  50  further includes an array of cylindrical outlets or openings  60  that perforate the end wall  52  at substantially equal circumferential spacings. The longitudinal axis of each opening  60  is oriented generally parallel to the longitudinal axis of the chamber  12 . A portion of the process gas in processing space  13  can freely diffuse into discharge cavity  58  through each opening  60 . Hollow anode assembly  50  is not electromagnetically shielded by shield  48  and, therefore, RF energy may be capacitively-coupled from excitation coil  42  across dielectric window  40  so as to generate a capacitively-coupled plasma in the cavity  58 . If the RF power exceeds a predetermined initial threshold, a capacitively-coupled plasma will be initiated in cavity  58  and will be subsequently sustained by capacitive coupling of RF energy. 
     Positive ions and electrons from the capacitively-coupled plasma in discharge cavity  58  spread through each opening  60  into the processing space  13 . Because of a well-known physical phenomenon known in the art of plasma technology, the plasma within the opening  60  has a significantly greater density than the plasma in either cavity  58  or the portion of the processing space  13  adjacent opening  60 . Electrons are accelerated from the capacitively-coupled plasma within cavity  58  toward the portion of the capacitively-coupled plasma within opening  60  and produce enhanced ionization of process gas atoms for enhancing the plasma density within opening  60 . Positive ions of process gas and electrons accelerate out of the opening  60  having initial trajectories generally directed axially toward the base of the vacuum chamber  12 . 
     An advantage of configuring the hollow anode assembly  50  with axially-oriented openings  60  is that the capacitively-coupled plasma produced in cavity  58 , and separated from processing space  13 , can be employed to sustain a low-density plasma in processing space  13  when the RF power from the excitation coil  42  is reduced below a threshold for sustaining an inductively-coupled plasma therein. The RF power to coil  42  must be reduced and the inductively-coupled plasma in processing space  13  can no longer be sustained during, for example, a substrate exchange operation. 
     By way of example, and not by way of limitation, the hollow anode cavity may be sized such that the radial dimension of discharge cavity  58  is about 10 to 30 mm, preferably about 10 mm, and the axial dimension is about 20 mm. The diameter of each opening  60  may be about 1 mm to 10 mm, depending upon the dimensions of discharge cavity  58  and the thickness of end wall  52 . The cross-sectional shape of the hollow anode aperture is not limited to being circular, but may also be rectangular or other geometrical shapes. 
     In accordance with the principles of the present invention and using like reference numbers for features discussed with reference to FIGS. 1 and 2A, FIG. 2B illustrates an alternative embodiment of the hollow anode assembly  70  that is likewise positioned about the circumference of the shield  48 . Hollow anode assembly  70  has an end wall  72  and two side walls  74 ,  76  that define an annular enclosure or a discharge cavity  78 . One or more cylindrical outlets or openings  80  are provided is side wall  76 . The longitudinal axis of each opening  80  is oriented radially inwardly toward the longitudinal axis of vacuum chamber  12  (FIG. 1) and has a generally circular cross-sectional profile. As explained above with regard to hollow anode assembly  50  shown in FIGS. 1,  2 A, and  3 , a capacitively-coupled plasma is generated from process gas received in cavity  78 . Electrons from the capacitively-coupled plasma in cavity  78  are accelerated into each opening  80  for enhancing the density of the portion of the capacitively-coupled plasma therein. Positive ions and electrons expand from the dense plasma in opening  80  into the processing space  13  (FIG.  1 ). Due to the radial orientation of openings  80 , the initial trajectories of the positive ions are generally directed radially inward toward the longitudinal axis of the vacuum chamber  12  and adjacent the inner face of the shield  48 . 
     An advantage of configuring the hollow anode assembly  70  with radially-facing openings  80  is that ions and electrons from the capacitively-coupled plasma can be employed to establish a low-density plasma in processing space  13  before attempting to initiate an inductively-coupled, high-density plasma therein. The presence of the low-density plasma established near the periphery of the shield  48  will reduce the power level required to initiate the inductively-coupled plasma. 
     A plasma presents a non-linear load to the RF power supply  44  (FIG. 1) and a large initial RF power level is required to initiate an inductively-coupled plasma in processing space  13 . A plasma typically cannot be easily initiated by inductive coupling between the excitation coil  42  and the process gas in processing space  13  for the low millitorr operating range of apparatus  10 . The threshold RF power required to initiate an inductively-coupled plasma varies depending upon the vacuum in chamber  12  but has been measured, for example, to range from 250 to 500 watts for a typical plasma-processing system under typical operating conditions. By contrast, the threshold power required to initiate a capacitively-coupled plasma under similar conditions is only on the order of 50 to 100 watts. Therefore, a capacitively-coupled plasma may be initiated in process space  13  at a greatly reduced RF power level. 
     The presence of the shield  48 , however, effectively prevents the capacitively-coupling of energy between the excitation coil  42  and the process gas into processing space  13 . Hollow anode assembly  50  or  70  provides an unshielded discharge cavity  58  or  78 , respectively, in which a capacitively-coupled plasma can be initiated and sustained at a reduced RF power level. Positive ions of process gas and electrons exiting from the capacitively-coupled plasma within opening  60  or  80  in hollow anode assembly  50  or  70 , respectively, will contribute to establishing a low density plasma in processing space  13  and, thereby, reduce the RF power level required to initiate an inductively-coupled plasma therein. 
     FIGS. 4 and 4A diagrammatically represent an alternative embodiment of the plasma processing apparatus  10 , in accordance with the principles of the present invention. Referring to FIGS. 4 and 4A and using like reference numerals for features discussed with reference to other Figures, the plasma processing system  10  further includes a sputter shield  88  circumferentially and coaxially disposed about the interior of the vacuum chamber  12  and a hollow anode assembly  90  attached to chamber wall  14 . Sputter shield  88  is a renewable metallic barrier that prevents ions of the inductively-coupled, high-density plasma in processing space  13  from interacting with the interior surfaces of chamber wall  14 . 
     Hollow anode assembly  90  has an end wall  92 , two side walls  94 ,  96 , and a base  98  that collectively bound an annular enclosure or a discharge cavity  100 . Discharge cavity  100  extends circumferentially about the interior of the vacuum chamber  12 . Relative to the longitudinal axis of chamber  12 , hollow anode assembly  90  is axially positioned adjacent the substrate support  22  and substrate  24 . However, the hollow anode assembly  90  may be located at alternative positions for modifying the plasma density in other volumes of processing space  13 . 
     One or more cylindrical outlets or openings  102  are provided in the end wall  92 . The longitudinal axis of each opening  102  is oriented radially inward toward the longitudinal axis of the chamber  12 . A portion of the process gas introduced into the processing space  13  can freely diffuse through each opening  102  into discharge cavity  100 . 
     An electrode  104  is provided within discharge cavity  100 . Electrode  104  is composed of an electrically-conductive material, such as aluminum or copper, and has an axial dimension that is generally coextensive with the axial dimension of cavity  100 . A non-conductive standoff  116  is interposed between a radially outermost surface of the electrode  104  and an inner surface of the base  98 . Standoff  116  is composed of a suitable dielectric material having a high electrical resistivity, such as alumina, for electrically isolating electrode  104  from the grounded base  98 . 
     Ancillary power supply  110  is operably connected to the electrode  104  via an electrical feedthrough  112 , which is provided in chamber wall  14  and base  98 . The ancillary power supply  110  may constitute a power divider (not shown) that divides the RF power input from the RF power supply  44  to energize the electrode  104  via a transmission line (not shown) and, as discussed above, to energize excitation coil  42  via transmission line  114 . The power supplied to electrode  104  capacitively couples with process gas in the discharge cavity  100  for generating a capacitively-coupled plasma therein. 
     In an alternative embodiment, the ancillary power supply  110  may be completely independent of RF power supply  44  and comprise a separate power supply (not shown) operably connected to electrode  104 . Either a direct current power supply or an independent radio-frequency (RF) power supply are suitable for use as an ancillary power supply  110 . 
     In operation, electrode  104  is energized by ancillary power supply  110  with a power level sufficient to ignite a capacitively-coupled plasma in discharge cavity  100 . Electrons are attracted from the capacitively-coupled plasma in cavity  100  towards each opening  102 , thus enhancing the plasma density within opening  102 . Electrons and positive ions expand from opening  102  into processing space  13 . Initial trajectories of positive ions and electrons exiting each opening  102  are generally directed radially inward toward the longitudinal axis of the vacuum chamber  12  and act to enhance the density of the high-density plasma in processing space  13  at a position adjacent the periphery of the substrate  24 . 
     An advantage of locating hollow anode assembly  70  adjacent the substrate support  22  is that positive ions and electrons from the capacitively-coupled plasma streaming through the opening into the processing space  13  can increase the plasma density near the periphery of the substrate  24 . The uniformity of the plasma density in processing space  13  is affected by the geometry of the vacuum chamber  12  such that the density is peaked about the longitudinal axis of chamber  12  and reduced near the chamber wall  14 . In particular, the proximity of the chamber wall  14  to the substrate support  22  adversely affects the radial uniformity of the plasma density near the substrate  24 . The reduction in plasma density is greatest at the periphery of the substrate  24  which lies closest to the wall. Thus, positive ions and electrons from the hollow anode assembly  70  may advantageously compensate for the non-uniform plasma density, so that a thin film of coating material will be deposited on substrate  24  having a more uniform thickness and a more uniform step coverage radially across the surface thereof. Further, a sputtering or surface conditioning operation will be more radially uniform across the surface of substrate  24  due to the enhanced plasma uniformity. 
     Referring to FIG. 5, in an alternative embodiment of the present invention, hollow anode assembly  120  includes a face plate  124  and a cavity  126  that is integrally provided or embedded within chamber wall  14 . Face plate  124  is received by recessed ledges  128 ,  130  adjacent the radially inward periphery of cavity  126 . Face plate  124  is perforated by an array of gas outlets or cylindrical openings  132 , which are spaced about the circumference and width of the face plate  124  so as to provide a symmetrical injection of positive ions and electrons. Hollow anode assembly  120  is primarily intended for application in a vacuum chamber  12 , as in FIG. 1, that lacks a sputter shield  88  depicted in FIG.  4 . It may be appreciated that the shape of openings  132  is not limited to being circular, but can be rectangles or other shapes, such as slots. 
     An electrode  134  is provided within discharge cavity  126  for receiving excitation power from the ancillary power supply  110  so as to generate a capacitively-coupled plasma from the process gas. Electrode  134  is composed of an electrically-conductive material, such as aluminum or copper, and has an axial dimension that is generally coextensive with the axial dimension of discharge cavity  126 . A non-conductive standoff  136  is interposed between electrode  134  and the chamber wall  14 . The standoff  136  is composed of a suitable dielectric material having a high electrical resistivity, such as alumina, for electrically isolating electrode  134  from the chamber wall  14 . Chamber wall  14  and face plate  124  are electrically grounded so as to form a second electrode. When a capacitively-coupled plasma is initiated in cavity  126  by energizing electrode  134  with power from the ancillary power supply  110 , electrons from the capacitively-coupled plasma are attracted into each opening  132 . As a result, the plasma density is enhanced in openings  132  and positive ions and electrons from that dense plasma spread out into processing space  13 . 
     Referring to FIG.  5 A and using like reference numerals for features discussed with reference to FIG. 5, hollow anode assembly  120  may alternatively include an electrode  140  positioned in discharge cavity  126  and operably connected with ancillary power supply  110 . Electrode  140  has a U-shaped cross-sectional profile and that extends about the circumference of cavity  126 . Electrode  140  includes a base  142  that is oriented generally parallel to the longitudinal axis of the vacuum chamber  12 . Two integral side walls  144 ,  146  extend from opposed ends of the base  142  radially inwardly toward the central longitudinal axis of the vacuum chamber  12 , shown in FIG.  4 . Electrode  140  is composed of an electrically-conductive material, such as aluminum or copper. A non-conductive, U-shaped standoff  146  is interposed between electrode  140  and the chamber wall  14 . The standoff  146  is composed of a suitable dielectric material having a high resistivity, such as alumina, for electrically isolating electrode  140  from the chamber wall  14 . 
     This U-shaped design of electrode  140  better tolerates the deposition of metal ions arriving from the high-density plasma through openings  132 , compared to the electrode  134  depicted in FIG.  5 . The U-shape of electrode  140  also advantageously focuses electrons into the center of cavity  126 , where openings  132  are located, thereby increasing the efficiency of the process gas ionization therein. 
     Referring to FIG.  5 B and using the reference numerals for features discussed with reference to FIG. 5A, another embodiment of hollow anode assembly  120  includes a face plate  148  extending about a plurality of equally-spaced slots  150  that connects discharge cavity  126  for fluid communication with processing space  13  (FIG.  3 ). Slots  150  extend about the circumference of plate  148 , which is received on recessed ledges  128 ,  130 . When a capacitively-coupled plasma is generated in cavity  126  and slots  150 , electrons and positive ions from the plasma within slots  150  are injected into process space  13 . 
     It may be appreciated that electrode  104 ,  134  or  140  may be partitioned into multiple segments that are each provided with excitation power from an ancillary power supply  110 . Each segment may be powered by a separate ancillary power supply or by power divided from a single ancillary power supply. Further, one of ordinary skill in the art of plasma processing would understand that the cavities  100  and  126  may be partitioned into a plurality of individual subcavities, wherein one or more of the subcavities are provided with a distinct electrode, such as electrode  104 ,  134  or  140 , energized by an associated ancillary power supply  110 . 
     If the ancillary power supply  110  is decoupled from RF power supply  44 , hollow anode assembly  90  or  120  is suitable for use in those plasma processing systems where the high-density plasma in processing space  13  is not generated by inductive coupling of RF power from coil  42 . For example, the high-density plasma may be generated in processing space  13  by either a microwave source or an independent RF power source operably connected to a pair of plate electrodes, as in conventional planar electrode or reactive ion etching systems. 
     While the present invention has been illustrated by the description of several embodiment thereof, and while those embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, and representative apparatus and method shown and described. For example, while the explanation of the application of the hollow anode assembly of the present invention is given with respect to an inductively-coupled plasma vapor deposition system, use of the hollow anode assembly is not so limited, and may be used in other materials processing applications as known in the art, such as plasma etching or reactive ion etching. Accordingly, departures may be made from such details without departing from the scope or spirit of applicant&#39;s general inventive concept.