Abstract:
A plasma discharge device is provided having features for enhanced thermal management and protection of dielectric materials in the device. The invention generally comprises a plasma confinement chamber constructed at least in part of dielectric materials, with a cooling instrument disposed in contact with the outer dielectric surfaces of the chamber for substantially uniform heat extraction. The cooling instrument may be embedded within an encapsulating material that enhances the uniformity of heat extraction from a dielectric plasma chamber. By improving the uniformity of heat extraction from the dielectric chamber of a plasma discharge device, the invention permits reliable operation of a plasma discharge device at significantly improved power levels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to plasma discharge devices, and more particularly to thermal management and protection of dielectric materials in a plasma discharge device.  
         [0003]     2. Brief Description of the Prior Art  
         [0004]     A persistent challenge in the engineering of plasma discharge devices is control and removal of heat generated by the plasma. The ability of materials exposed to a plasma to withstand the thermal environment of the discharge often significantly restricts the performance, range, reliability, or other operating characteristics of a plasma device. Problems of thermal management are especially difficult in devices having dielectric materials in proximity to the plasma, particularly for structural purposes, owing to the poor thermal conductivity of most dielectrics. While certain dielectric materials such as ceramics may be tolerant of elevated temperatures in bulk, hot spots resulting from non-uniform cooling can lead to high internal stresses in dielectric components due to differential thermal expansions. It is not uncommon for these temperature-induced stresses to result in cracks in dielectric materials, leading in turn to premature failure of the plasma device.  
         [0005]     A dielectric plasma containment vessel may be cooled using a conformal jacket or sheath that permits a cooling fluid to flow over the surface of the dielectric plasma vessel, as described for example in U.S. Pat. No. 5,200,595. For some applications, however, this approach presents an unacceptable risk that cooling fluid could enter the plasma chamber in the event of a break or crack and catastrophically contaminate processes occurring in or downstream of the chamber. A conformal cooling jacket may also impede the ability to provide inductive coupling of electromagnetic energy into the plasma unless the cooling jacket assembly itself and the cooling fluid therein are themselves dielectric.  
         [0006]     One approach to cooling the dielectric chamber of an inductively coupled plasma device provides an arrangement of metal cooling tubes in proximity to the outer surface of the chamber, as for example a helical cooling coil disposed coaxially about a cylindrical chamber. Metal cooling tubes are commonly available and provide an efficient heat extraction path where placed in contact with the chamber body. Even where a plasma chamber has a relatively simple geometry, however, such as a cylinder, providing a metal cooling coil that is exactly conformal to the surface of the chamber is a manufacturing challenge. For example, a cooling coil that is prefabricated to have an inner major diameter larger that the outer diameter of the plasma chamber will be easy to assemble, but the gaps that necessarily exist between the coil and chamber wall will impair the uniformity and resistance of heat transfer from the chamber body to the cooling medium. On the other hand, a prefabricated coil having near-zero tolerance to the outer chamber wall will be difficult to mate to the chamber, and forced assembly of the article can still result in bunching or gapping of the coil along the length of the chamber as well as damage to the chamber wall. If the coil is not prefabricated but instead wound in place around the plasma tube, imperfect contact invariably results due to eccentricity of the winding and relaxation of the coil. Even small gaps resulting from these manufacturing imperfections lead to uneven cooling of the dielectric chamber, and consequently to hot spots in the chamber walls that limit the performance and reliability of the device. This process can also crack or damage the chamber during the manufacturing process.  
         [0007]     In U.S. Pat. No. 6,156,667, a method of removing heat from a dielectric plasma chamber is described using a heat moderating material between the chamber and a cooling instrument. In this approach, the heat moderating material moderates the heat transfer between the dielectric and the cooling instrument to provide a temperature gradient through the dielectric material that minimizes failures such as breakage induced by thermal stress. The heat moderating material may also function as a heat spreader that maintains the surface of the dielectric at a cooler and more uniform temperature. A means is required of interposing the heat moderating material between the chamber and cooling instrument at a desired thickness during assembly of the article.  
         [0008]     Others have addressed issues of temperature management in dielectric plasma chambers through the use of cooled or uncooled dielectric shields, or thin-walled metal cooling structures, located inside the dielectric confinement chamber. It would be desirable to improve the efficiency and uniformity of heat extraction from dielectric components of a plasma discharge device, and thereby to improve performance and reliability of the device, while retaining the benefits of a simple, sturdy, and cost-effective design.  
       SUMMARY OF THE INVENTION  
       [0009]     This invention provides a plasma discharge device having features for enhanced thermal management and protection of dielectric materials in the device. The invention generally comprises a plasma confinement chamber constructed at least in part of dielectric materials. A cooling instrument is disposed in contact with the outer dielectric surfaces of the chamber for substantially uniform heat extraction. Substantially direct and uniform contact between the dielectric surfaces and the cooling instrument is made possible by creating a temporary physical gap between the chamber and cooling instrument through mechanical, hydraulic, thermal or other means, then collapsing the gap so as to couple firmly heat extraction surfaces of the cooling instrument to outer surfaces of the chamber.  
         [0010]     In one embodiment of the invention, a plasma source apparatus comprises a cylindrical plasma discharge tube that confines a plasma within. A helical coil constructed of square metal tubing is disposed coaxially about the outer surface of the dielectric discharge tube. The inward facing flat surfaces of the helical coil are in substantially direct and uniform contact with the outer surface of the dielectric discharge tube. A cooling fluid is flowed through the helical coil to extract heat transferred from the discharge tube to the metal coil. The turns of the cooling coil are spaced apart and electrically connected to an RF power source, thus allowing the cooling coil to function also as an inductive winding that couples RF power into the plasma within the discharge tube.  
         [0011]     To assemble the cooling coil to the dielectric discharge tube, a temporary physical gap is created between the coil and the discharge tube. In one embodiment, the helical cooling coil is fabricated to have an inner major diameter that is slightly smaller than the outer diameter of the dielectric discharge tube. The cooling coil is placed into a fixture that allows it to be expanded by mechanical force until its inner major diameter is slightly larger than the outer diameter of the dielectric discharge tube. While the coil is expanded, the discharge tube is inserted into the space within the coil. The coil is then relaxed, causing the inward facing flat surfaces of the coil to come firmly into compressive contact with the outer surface of the discharge tube. In other embodiments of the invention, a helical cooling coil is twisted onto the body of a cylindrical dielectric plasma tube by applying torque in the direction of the turns of the helix. When the full length of the coil has been twisted onto the plasma tube the torque is released, causing the coil to fit firmly to the plasma tube surface. Alternatively, a temporary physical gap is created between the coil and the discharge tube by pressurizing the cooling coil, or by differentially heating the coil and/or cooling the dielectric tube.  
         [0012]     In another aspect of the invention, the cooling instrument of a plasma discharge device is embedded within an encapsulation material that enhances the uniformity of heat extraction from a dielectric plasma chamber. The encapsulation material preferably has a low viscosity so as to displace residual air pockets between the dielectric chamber and the cooling instrument, along with a thermal conductivity that facilitates heat extraction from the chamber.  
         [0013]     By improving the uniformity of heat extraction from the dielectric chamber of a plasma discharge device, the invention reduces hot spots within the chamber wall during operation that would limit the performance and reliability of the device. As a result, the features of the invention permit safe and reliable operation of a plasma discharge device at significantly improved power levels.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  illustrates a plasma source device in accordance with one embodiment of the invention.  
         [0015]      FIG. 2  illustrates assembly of a cooling coil to the discharge tube of a plasma source device in accordance with one embodiment of the invention.  
         [0016]      FIG. 3  illustrates use of an encapsulation material in accordance with another aspect of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 1  illustrates a plasma source device in accordance with one embodiment of the invention. Plasma source  100  comprises cylindrical discharge tube  102  containing a plasma within. Discharge tube  102  is constructed substantially of a dielectric material such as quartz, alumina, aluminum nitride, or other structural dielectric suitable to the chemistry of the discharge environment within the tube. Discharge tube  102  is open at both ends  104  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. Not shown are other features that may typically be included in a plasma processing device such as vacuum pumping manifolds, gas delivery connections or manifolds, plasma ignition electrodes or other devices, and mechanisms for workpiece mounting, transfer, or electrical biasing.  
         [0018]     Disposed coaxially about discharge tube  102  is helical metal cooling coil  110  constructed of square copper tubing. In the embodiment illustrated in  FIG. 1 , coil  110  functions both as a cooling instrument as well as an inductive winding that couples RF power into the plasma within discharge tube  102 . An RF power generator (not shown) provides alternating current power to coil  110  through electrical taps  112  affixed to coil  110 . When energized by an RF power source, plasma source  100  operates like an air core transformer with the coil  110  as the primary circuit and the plasma within discharge tube  102  as the secondary circuit. Insulating gaps  114  are maintained between the windings of coil  110 , and taps  112  are located so as to provide a turns ratio desired on the primary circuit of the transformer-coupled source. Coil  110  is also provided with fittings  116  for connection of the coil to a source of coolant fluid (not shown).  
         [0019]      FIG. 2  illustrates assembly of coil  110  to discharge tube  102 . Coil  110  is constructed having an inner major diameter  118  in its relaxed state that is smaller than the outer diameter  106  of cylindrical discharge tube  102 . Coil  110  is placed into coil expansion fixture  140 , where fitting ends  120  of coil engage and are temporarily fastened into mating grooves in upper and lower plates  142  and  144  of fixture  140 . Torque is applied using fixture handles  146 , radially expanding the coil until inner major diameter  118  of the coil is slightly larger than the outer diameter  106  of the discharge tube. While the coil is expanded, discharge tube  102  is inserted into expanded space  122  within the coil. Torque on handles  146  is released and the coil radially contracts, causing the inward facing flat surfaces of coil  110  to come firmly into compressive contact with the outer surface of discharge tube  102 . In its assembled state, the coil thus exerts a residual compressive force upon the outer surface of discharge tube  102 . In a preferred embodiment, the inner major diameter of coil  118  in its relaxed state is approximately 0.5-1.0% smaller than the outer diameter of dielectric discharge tube  102 , which is held to a tolerance of ±0.001 inch. These dimensions are found to provide substantially direct contact between the dielectric discharge tube and cooling coil without undue compressive stresses on the tube.  
         [0020]     Flat faces  124  of coil square tubing provide ample contact area between each turn of the coil winding and discharge tube  102 . Alternatively, the coil is constructed of tubing having any cross sectional shape that provides substantially direct and uniform contact between the coil and discharge tube, while permitting coolant flow within the coil. Heat from discharge tube  102  is conducted through the contacting portions of cooling coil  110  and into a fluid coolant that flows therein. Preferably, coil  110  is fabricated of copper or other metal, but may be constructed of any resilient material that is both thermally and electrically conductive, and that is not vulnerable to cracking or fatigue due to thermal stresses.  
         [0021]      FIG. 3  illustrates the use of an encapsulation material in accordance with another aspect of the invention. In one embodiment, a cylindrical shell  150  is disposed coaxially about the discharge tube  102  with conformal cooling coils  110  of plasma source  100 . Preferably, shell  150  is composed of polycarbonate or other polymeric material that is both flexible and visually transparent for ease of manufacture. Flanges  152  at each end of tube  102  seal the coaxial space between the shell  150  and tube  102 . Through a window or other opening in the shell or flanges, an encapsulation material  154  is introduced into the coaxial space, embedding cooling coils  110 . In its liquid phase, encapsulation material  154  has sufficiently low viscosity to displace virtually all air voids within the coaxial space, including between the turns of cooling coils  110  and any residual gaps that may exist between coils  110  and dielectric tube  102 . Preferably, vacuum potting techniques are used to aid in removal of air pockets. Encapsulation material  154  is then cured into a rigid or solid state, at which time shell  150  may be left in place or removed.  
         [0022]     In order to achieve the objectives of the invention, encapsulation material  154  has a unique combination of properties. The material must be dielectric to maintain electrical separation between windings of coil  110 , should bond well to the dielectric surface of discharge tube  102 , and should be flexible and exhibit minimal shrinkage in its cured state. Preferably, the material has high thermal conductivity to aid in thermal transfer from the discharge tube to the coils. Most importantly, the material must have sufficiently low viscosity in its liquid (pre-cured) state so as to displace trapped air in any small gaps between the cooling coil and discharge tube that would impede thermal transfer in these critical spaces. In a preferred embodiment of the invention, a two-part heat cured silicone adhesive is used as an encapsulation material.  
         [0023]     In an alternative embodiment of the invention, a helical cooling coil is twisted onto the body of a cylindrical dielectric plasma tube by applying torque in the direction of the turns of the helix. When the full length of the coil has been twisted onto the plasma tube the torque is released, causing the coil to fit firmly to the plasma tube surface. Alternatively, a temporary assembly gap is created between a cooling coil and discharge tube by hydraulic or thermal means. In one embodiment, a fluid is injected into the coil under elevated hydrostatic pressure, which expands the coil radially. In another embodiment, the cooling tube is heated, causing it to expand, or the discharge tube is chilled, causing it to contract. In either case, following release of the mechanical, hydraulic or thermal assembly force(s), the parts return to their relaxed state and the coil mates firmly to the outer surface of discharge tube.  
         [0024]     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.