Abstract:
Methods and apparatus are provided for generating an inductively coupled plasma using induced eddy currents. An inductively coupled plasma source of the invention generally comprises a body constructed substantially of a conductive material interrupted by at least one dielectric gap. Radio frequency power is coupled from a current carrier into the conductive body. The one or more dielectric interruptions in the conductive body are disposed so as to cause eddy currents to circulate about portions of the body and thereby couple RF power into a plasma in proximity to the conductive body. By utilizing induced eddy currents to couple power into a plasma, the invention allows for substantial bodies of conductive materials, such as structural metals, to be interposed between the induction coils that receive power from a power generator and the plasma.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to plasma processing sources, and more particularly to apparatus and methods for inductively coupled plasma processing.  
         [0003]     2. Brief Description of the Prior Art  
         [0004]     Inductively coupled plasma sources in a variety of configurations are employed in a broad range of industrial applications. Inductively coupled plasma processing chambers are used abundantly for modifying the surface properties of materials, as for example in the manufacture of modern integrated circuits. Inductively coupled plasma sources may also operate as remote sources of activated gas species for downstream processing operations, or as abatement devices for treatment of toxic or environmentally harmful materials.  
         [0005]     In one form of well known inductively coupled plasma source, radio frequency (RF) power is coupled from inductive coils into a plasma contained within a dielectric enclosure. For example, the source may comprise a cylindrical dielectric discharge tube wrapped by an inductive source coil. When energized by an RF power generator, the source operates like an air core transformer with the inductive coil as the primary circuit and the plasma within the tube as the secondary circuit. Alternatively, induction coils may be disposed in a planar or conformal helix configuration adjacent to a dielectric discharge chamber for coupling of RF power into a plasma contained within the chamber.  
         [0006]     The use of dielectric chamber materials to separate induction coils from the plasma discharge body can significantly limit the scale and operational range of an inductively coupled plasma source. Structural dielectric materials, such as quartz or sapphire, typically suffer from mechanical and thermal constraints when used in high power density and chemically reactive applications. The need to extract and dissipate thermal energy transferred from the plasma to the chamber walls is also more challenging when the chamber is constructed of dielectric materials. Cooling mechanisms such as forced air or circulating fluids are not only complicated and expensive to implement, but also typically result in reduced coupling efficiency of power to the plasma. Moreover, electrostatic coupling between the induction coils and the plasma can result in localized ion bombardment of the chamber walls, which not only exacerbates the problem of chamber heat extraction but may over time impair the structural integrity of the chamber itself. Faraday shielding can be employed to decrease the capacitive coupling between the source coils and the plasma, thereby reducing ion sputtering of the chamber walls. A Faraday shield or cage employed for this purpose is typically designed so as to suppress or minimize eddy currents within the shield.  
         [0007]     In another form of inductively coupled plasma source, RF power is coupled from inductive coils through a high permeability core material to a ring discharge plasma. In this configuration, the source operates as a magnetic core transformer with the ring plasma acting as a single-turn secondary circuit. The ring plasma discharge may be confined within a chamber of closed-path topology, such as a torus, as described for example in U.S. Pat. Nos. 3,500,118 and 4,431,898. The discharge chamber may be comprised of a dielectric material to ensure that currents are coupled into the plasma rather than within the body of the chamber itself. The chamber may, however, be comprised substantially of a conductive material provided that at least one insulating gap or break is provided along the major circumference of the torus to prevent the chamber itself from acting as a short-circuited turn, as described for example in U.S. Pat. No. 3,109,801. By permitting use of a nearly all-metal chamber, which may be fluid cooled, issues of thermal management are simplified. As a result, magnetic core inductively coupled plasma sources are useful for generating charged particles and chemically active species at relatively high densities and power levels. A topologically toroidal plasma source is a complex apparatus, however, that does not lend itself to simple design and manufacturing for commercial applications. Moreover, the performance a toroidal source is limited by the quality, expense, and ability to cool the high permeability ferrite materials that must typically be employed for operation with RF power sources in medium to high frequency ranges.  
         [0008]     It would be desirable to construct an inductively coupled plasma source having a relatively simple configuration, such as a discharge tube, but without the attendant disadvantages of a plasma tube or chamber constructed substantially of dielectric materials. It would be further desirable if the plasma source were not dependent for its operation upon expensive ferrite transformer materials.  
       SUMMARY OF THE INVENTION  
       [0009]     This invention provides methods and apparatus for creating an inductively coupled plasma using induced eddy currents. The invention generally comprises a body constructed substantially of a conductive material interrupted by at least one dielectric break. Alternating current power is inductively coupled from a current carrier, such as an induction coil, into the conductive body. The dielectric gap or gaps in the conductive body are disposed so as to cause eddy currents to circulate about portions of the conductive body and thereby couple RF power into an adjacent plasma.  
         [0010]     In one embodiment of the invention, a plasma chamber comprises conductive segments aligned longitudinally to form a hollow tube, and separated by dielectric breaks or gaps. An induction coil is disposed coaxially about the outer perimeter of the chamber formed of the conductive segments. A power supply provides alternating current to the induction coil, which creates alternating magnetic fields in the space occupied by the chamber. Because of the dielectric separation between the conductive chamber segments, the alternating magnetic fields induce eddy currents that circulate radially along the surfaces of the individual segments, which are thick relative to the surface current skin depth. Net alternating currents are thereby induced along the interior conductive surfaces of the discharge tube. These net currents in turn couple power into a plasma contained within the hollow interior portion of the chamber.  
         [0011]     By utilizing induced eddy currents to couple power into a plasma, the invention allows for substantial bodies of conductive materials to be interposed between the induction coils that receive power from a power generator and the plasma. Thus, in one embodiment of the invention, an inductively coupled plasma source may be constructed in the form of a simple linear or solenoidal discharge tube, but wherein the tube is composed almost entirely of a conductive material such as a metal. The use of a nearly all-metal plasma chamber can have many advantages, including simplified manufacturability and thermal management. A plasma chamber that is substantially conductive also largely avoids the problem of ion bombardment of the chamber walls by reducing or eliminating capacitive coupling between the induction coils and the plasma. As a result, an inductively coupled plasma source of the invention has enhanced performance and durability compared to sources that rely substantially upon structural dielectric materials for confinement of the plasma.  
         [0012]     In one embodiment of the invention, conductive chamber segments are separated by air gaps. Depending upon the application, a dielectric window material may also be provided between chamber segments in order to maintain vacuum integrity or to confine the plasma. In another embodiment, the conductive chamber segments are constructed so that adjoining surfaces of the segments mate flush with each other. An insulating coating or treatment, such as an anodization layer, is applied to the adjoining surfaces. Alternatively, a dielectric adhesive or filler is disposed between the adjoining surfaces of the conductive segments. When the conductive segments are assembled, the resulting discharge chamber is a nearly seamless and unitary metallic article that has embedded within it the dielectric breaks needed for formation of induced eddy currents within the chamber body.  
         [0013]     In forming a plasma chamber of conductive segments, the dielectric breaks between segments may extend along the entire length of the chamber. The chamber may also be formed by joining the segments at their longitudinal ends using caps or rings of dielectric material. Alternatively, the conductive segments may be joined at their longitudinal ends with a conductive material. Although this provides a leakage current path that reduces the power coupled from the induction coils into the plasma, power loss may be minimized by making the path of the leakage current substantially longer than that of the eddy currents.  
         [0014]     Conductive chamber segments may be configured to form a plasma chamber having any cross-sectional shape, including circular or rectangular. Conductive segments may also be disposed in other configurations in accordance with the present invention so as to couple RF energy into a plasma by means of eddy currents induced within the segments. In one embodiment, a planar fixture comprised of radially disposed conductive segments separated by dielectric gaps is provided between a helical induction coil and a plasma. In another embodiment, radially disposed conductive segments form a conformal dome between an induction coil and a plasma. In appropriate configurations, plasma chambers of the invention are suitable for use in numerous plasma processing applications including inline abatement, dissociation, or processing of working gases; remote production of activated gases for downstream processing; plasma modification of surface properties of a workpiece; glass cleaning, etching, or coating; physical or chemical vapor deposition of materials upon a process substrate; etching, coating, stripping or ashing of a substrate surface, as in production of integrated circuit wafers or memory disks; and the like.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  illustrates an inductively coupled plasma source in accordance with one embodiment of the invention.  
         [0016]      FIG. 2  is an orthographic view of the plasma discharge tube of the embodiment depicted in  FIG. 1 .  
         [0017]      FIG. 3  is a cross-sectional view of the plasma discharge tube of the embodiment depicted in  FIG. 1 .  
         [0018]      FIGS. 4   a ,  4   b , and  4   c  illustrate the plasma discharge tube of a further embodiment of the invention.  
         [0019]      FIG. 5  illustrates an inductively coupled plasma source adapted for use in a chemical vapor deposition (CVD) application in accordance with a further embodiment of the invention.  
         [0020]      FIGS. 6   a  and  6   b  illustrate inductively coupled plasma chambers in accordance with further embodiments of the invention.  
         [0021]      FIG. 7  illustrates an inductively coupled plasma source having an external plasma discharge in accordance with another embodiment of the invention.  
         [0022]      FIG. 8  illustrates an alternative embodiment of the invention having an external plasma discharge.  
     
    
     DETAILED DESCRIPTION  
       [0023]      FIG. 1  illustrates an inductively coupled plasma source  10  in accordance with one embodiment of the invention. An RF power source  12  furnishes alternating current to induction coils  14  disposed coaxially about a substantially metallic plasma discharge tube  16  containing a plasma within. As illustrated in the embodiment of  FIG. 1 , plasma discharge tube  16  is configured as a hollow cylinder open at both ends  18  to allow for gas inlet and exhaust, as for example in an inline gas processing application. Alternatively, the plasma tube may be configured as a sealed vacuum chamber having metered inlet and exhaust ports for feed and processing gases. Although not shown, the apparatus may also comprise impedance matching elements or circuitry disposed between RF power source  12  and induction coils  14 , as well as measurement and feedback circuitry to regulate operation of the device. Also not shown are other features that may typically be included in a plasma processing system such as vacuum pumping manifolds, gas delivery connections or manifolds, fluid cooling apparatus, plasma ignition electrodes or other devices, and mechanisms for workpiece mounting, transfer, or electrical biasing.  
         [0024]      FIGS. 2 and 3  represent orthographic and cross-sectional views, respectively, of the plasma discharge tube  16  of  FIG. 1 . In this embodiment, plasma discharge tube  16  is formed of a metal cylinder having longitudinal grooves  22  through the body of the cylinder. A gastight dielectric seal comprising gas seal  24  and dielectric cover  26  is disposed across each groove  22  in order to preserve the gas confinement integrity of the discharge tube  16 . The longitudinal grooves  22  thus divide the walls of plasma discharge tube  16  into longitudinally aligned conductive segments  28  interrupted by dielectric breaks.  
         [0025]     Alternating current  32  applied to induction coils  14  causes time-varying magnetic fields to develop in the space occupied by the chamber  16 . Conductive chamber segments  28  are of a thickness that is greater than the skin depth as determined by the material properties of the segments  28  and the operating frequency of the RF power source  12 . Eddy currents  34  thus develop that circulate radially along the surfaces of each conductive chamber segment  28 . As a result, a virtual current loop  36  is established along the interior conductive surfaces of the chamber  16 . The virtual current loop  36  further creates time-varying magnetic fields in the interior plasma containment portion of chamber  16 , inducing currents within and thereby coupling power into the plasma  50 .  
         [0026]     Only one dielectric gap  22  need be provided in order to create the eddy currents within the conductive chamber body needed to couple power into the plasma within. In principle, the chamber may be comprised of any number of conductive segments  28  separated by dielectric gaps, provided that the resulting segments are of sufficiently substantial dimension to carry the required eddy currents and create the virtual current loop  36 . The conductive segments  28  may be comprised of a common structural metal such as aluminum or stainless steel, or any other conductive material suitable to the thermal and chemical environments of a particular plasma processing application. Preferably, each conductive segment  28  is also sufficiently substantial to have embedded within it one or more cooling channels  40  through which cooling fluids may circulate, while retaining such structural properties as may be required of the segment. Fittings  42  may be provided for connection of the cooling channels  40  to a source of chilled water or other cooling fluid (not shown) for thermal management of the plasma source apparatus.  
         [0027]     Dielectric gaps  22  need only be of sufficient width and dielectric strength to resist the peak-to-peak breakdown voltages that develop across conductive segments  28  upon application of RF power to the induction coils  14 . In the embodiment of  FIG. 2 , the dielectric gaps  22  do not extend the entire length of the discharge tube  16 . As a result, a leakage current path exists that reduces the power coupled from the induction coils into the plasma. This power loss may be minimized to an acceptable level by making the discharge tube  16  substantially greater in overall length than the region occupied by induction coils  14 , thus making the path of the leakage current substantially longer than that of the eddy currents that couple power into the plasma. Alternatively, the leakage current may be reduced or eliminated by forming one or more of the dielectric gaps of a structural insulating material that extends the length of the chamber, or by joining conductive segments at their longitudinal ends using caps or rings of a structural dielectric material.  
         [0028]     By transferring the RF power furnished to induction coils  14  into a virtual current loop within the plasma discharge tube, the electromagnetic fields applied to the plasma are concentrated and coupling of power to the plasma is improved. Due also to the enhanced durability and thermal properties of a nearly all-metal plasma chamber, significantly greater power densities can be realized with a plasma source of the invention as compared with a conventional discharge tube apparatus of similar scale.  
         [0029]      FIGS. 4   a ,  4   b , and  4   c  illustrate a plasma discharge tube in accordance with another embodiment of the invention. Conductive discharge tube segments  128  comprise mating surfaces  122  treated with an electrically insulating layer  124 . The insulating layers  124  may be provided by anodization or similar treatment of the conductive surface, or by application of a dielectric coating material such as an epoxy adhesive. As shown in  FIGS. 4   b  and  4   c , conductive segments  128  assemble to form a hollow cylindrical discharge tube  120  having embedded longitudinal dielectric interruptions  126  and cooling channels  140 . Mating surfaces  122  may be made optically flat so that additional gas sealing between segments  128  is not required. Alternatively, gas sealing may be accomplished through use of a dielectric filler or adhesive between segments, such a high temperature epoxy resin or refractory ceramic paste.  
         [0030]     When alternating current  132  is applied to induction coils  114 , induced eddy currents  134  develop within conductive chamber segments  128  and create virtual current loop  136 . The virtual current loop  136  induces currents within a plasma  150  contained within the hollow portion of discharge chamber  120 .  
         [0031]      FIG. 5  illustrates an embodiment of the invention adapted for use in a chemical vapor deposition (CVD) application. Plasma chamber  516  is a conductive hollow body having one or more feed gas inlets  530  at one end  518  of the body and a substantially open discharge region at opposing end  520 . Also provided near the discharge end of plasma chamber are ports  532  for one or more precursor gases  534  to be injected into the process zone. The cross-sectional aspect ratio of plasma chamber  516  is optimized for dispersal of CVD reaction precursors in the vicinity of a translating workpiece  536 .  
         [0032]     A plurality of longitudinal grooves  522  is provided through the conductive body of plasma chamber  516 , creating a series of longitudinally aligned conductive segments  528  separated by dielectric breaks. If needed, dielectric covers and gas seals may be provided across the grooves  522 . Disposed about the chamber body are induction coils  514  oriented transversely to the conductive segments  528 . When energized by RF current, the induction coils induce eddy currents in the conductive segments, which in turn couple RF power into a plasma  550  contained within the hollow plasma chamber  516 . As an example, the plasma source of this embodiment may be used to generate a plasma from an oxygen feed gas injected at first gas inlets  530 . A silane or other silicon-bearing precursor is injected into the plasma  550  at second inlets  532  where it dissociates and reacts to form a Si X O Y  compound, such as SiO 2 , which is deposited as a solid film upon the translating substrate  536 .  
         [0033]     In accordance with alternative embodiments of the invention, an inductively coupled plasma is generated by inducing eddy currents in conductive bodies that form only a portion of a plasma confinement chamber, or that are ancillary to the chamber. In  FIG. 6   a , plasma processing chamber  602  is an enclosed cylinder containing a workpiece (not shown). Disposed atop processing chamber  602  is a conductive disk  604  having a plurality of radial grooves  606 , creating an array of radially disposed conductive segments  608 . Adjacent to conductive disk  604  are helical induction coils  610 . When energized by RF current, the induction coils  610  induce eddy currents in the conductive segments  608 , which in turn couple RF power into a plasma contained within processing chamber  602  and that acts upon the workpiece. The same principle is illustrated in the embodiment of  FIG. 6   b , wherein radially disposed conductive segments form a conformal dome between a helical induction coil and a plasma.  
         [0034]      FIG. 7  illustrates an embodiment of the invention that generates an external inductively coupled plasma. A substantially conductive body is a hollow cylindrical tube that comprises longitudinally aligned conductive segments  728  interrupted by dielectric breaks  722 . Disposed within the conductive body are induction coils  714  wound transversely to the conductive segments  728 . A flux concentrating magnetic material (not shown) such as a ferrite core may be disposed within induction coils  714  to enhance magnetic fields generated by the coils. When energized by RF current, induction coils  714  induce eddy currents  734  in the conductive segments and create virtual current loop  736  external to the cylindrical tube. The virtual current loop  736  induces currents within a coaxial plasma  750  external to the cylindrical tube. Plasma  750  may be provided as an exposed external discharge, or alternatively may be confined within an outer coaxial enclosure (not shown). If a confined plasma is to be subatmospheric, gastight dielectric windows  724  may also be added to seal dielectric breaks  722 .  
         [0035]     An alternative embodiment of the invention that generates an external inductively coupled plasma is illustrated in  FIG. 8 . Conductive body  820  is disposed adjacent to a current carrier  814 . In cross section, conductive body  820  is formed so as to have a conductive portion  828  that surrounds a hollow cavity with a wall that is interrupted by a dielectric air gap  822 . When current carrier  814  is energized by RF current, eddy currents  834  are induced in conductive portion  828  and create virtual current loop  836 . The virtual current loop  836  induces currents within a plasma  850  in the hollow interior cavity of conductive body  820 . Due to the position of air gap  822 , however, the plasma  850  is not confined within conductive body  820  but may appear as an external discharge. In the embodiment of  FIG. 8 , conductive body  820  is disposed as a body of revolution about current carrier  814 , resulting in coaxial ring plasma discharge  850 .  
         [0036]     Although there is illustrated and described herein specific structure and details of operation, it is to be understood that these descriptions are exemplary and that alternative embodiments and equivalents may be readily made by those skilled in the art without departing from the spirit and the scope of this invention. Accordingly, the invention is intended to embrace all such alternatives and equivalents that fall within the spirit and scope of the appended claims.