Abstract:
An overhead RF coupling chamber couples RF power to a ceiling electrode of a plasma reactor chamber, the RF coupling chamber having a resonant annular volume defined by coaxial cylindrical conductors, one of which is coupled to an RF power source, the chamber ceiling having an annular gap around the electrode, and the resonant annular volume being aligned with the annular gap so that the resonant annular volume opens into the interior of the main chamber, thereby enhancing the electrical length of the RF coupling chamber.

Description:
[0001]    CROSS-REFERENCE TO RELATED APPLICATIONS 
         [0002]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/638,855, filed Apr. 26, 2012 entitled CAPACITIVELY COUPLED PLASMA SOURCE WITH RF COUPLED GROUNDED ELECTRODE, by Kartik Ramaswamy, et al. 
     
    
     BACKGROUND 
       [0003]    The recent growth in the size semiconductor wafers in integrated circuit fabrication is making it more difficult to obtain the needed degree of uniformity of plasma process rate across the treated wafer surface. The process rate may be an etch rate or a deposition rate, for example. 
         [0004]    Control of plasma ion density distribution within the chamber is essential in order to ensure uniformity of processing, or a uniform distribution of etch (or deposition) rate across the surface of the workpiece. The vacuum chamber is typically configured to have cylindrical symmetry. Plasma may be generated in the chamber by coupling RF source power to a ceiling electrode of the chamber. To optimize uniformity of plasma ion distribution, it is essential to deliver RF power to the ceiling electrode in a uniform symmetrical manner. 
         [0005]    Typically, the ceiling electrode is also a gas distribution plate, requiring the inclusion within the ceiling electrode of various grounded components, such as gas supply conduits, coolant supply conduits and A.C. power lines for internal heaters. Because RF source power typically is applied directly to the ceiling electrode, there is a large voltage difference between the ceiling electrode and the grounded components within it, leading to a significant risk of arcing. 
         [0006]    Another problem is that the presence within the ceiling electrode of grounded components, such as gas supply conduits, coolant supply conduits and A.C. power lines for heaters, may produce non-uniformities in the delivery of RF power to the bottom surface of the ceiling electrode. 
         [0007]    What is need is a way of delivering RF power to the ceiling electrode in a manner that is unaffected by the presence of internal components within the ceiling electrode, and which does not produce a large voltage difference between the ceiling electrode and the grounded components inside the ceiling electrode. 
       SUMMARY 
       [0008]    In accordance with a first embodiment, a plasma reactor includes an RF power source, a vacuum chamber including a ceiling and a cylindrical side wall, a workpiece support pedestal in the chamber, and a ceiling electrode, the ceiling having an annular ceiling gap surrounding the ceiling electrode. The reactor further includes an RF coupling chamber. The RF coupling chamber includes (a) hollow inner, intermediate and outer conductive cylinders coaxial with the ceiling electrode and defining an outer annular volume overlying the annular ceiling gap, and an inner annular volume, the hollow inner conductive cylinder having a bottom end contacting the ceiling electrode; (b) a conductive top disk overlying the inner conductive cylinder and having a top disk peripheral annulus overlying the inner annular volume, the ceiling including an annular ceiling portion extending from about the inner conductive cylinder to the annular ceiling gap and underlying the inner annular volume, and a circular gap between the top disk peripheral annulus and the intermediate conductive cylinder; and (c) a coaxial power distributor coupling the RF power source to the intermediate conductive cylinder. 
         [0009]    In one aspect, the coaxial power distributor includes: an axial center conductor having a top end connected to the RF power source and a bottom end; a conductive member extending radially from the bottom end of the axial center conductor; and plural axial conductive posts extending from the conductive member through the circular gap and to respective locations on the intermediate cylindrical conductor, the plural axial conductive posts being spaced apart. In a related aspect, the intermediate conductive cylinder extends axially from the ceiling toward the top disk peripheral annulus and is terminated at a top edge separated from the top disk peripheral annulus by the circular gap, the axial conductive posts connected to the top edge of the intermediate conductive cylinder. 
         [0010]    In a further related aspect, the conductive member includes a disk-shaped plate. In another embodiment, the conductive member includes plural radial spokes. 
         [0011]    The coupling chamber may further include a radial conduit formed as a shallow cylindrical volume partially enclosing the conductive member. The coaxial power distributor may further include an RF feeder outer conductor surrounding the axial center conductor and coupled to a return potential of the RF power source. The radial conduit may include: a conduit ceiling lying in a radial plane over the conductive member of the coaxial power distributor and having a center opening, the axial center conductor extending through the center opening of the conduit ceiling, the RF feeder outer conductor terminated at the center opening of the conduit ceiling; and a conduit floor including the top disk, the top disk including a top disk hole, the axial center conductor extending through the top disk hole. 
         [0012]    The RF coupling chamber may further include a toroidal shaped ferrite ring coaxial with and between the inner and intermediate hollow cylindrical conductors. 
         [0013]    In accordance with a second embodiment, a plasma reactor includes an RF power source, a vacuum chamber including a ceiling, a workplace support pedestal in the chamber, and a ceiling electrode, the ceiling having an annular ceiling gap surrounding the ceiling electrode. The plasma reactor further includes an RF coupling chamber including: (a) hollow inner and outer conductive cylinders coaxial with the ceiling electrode and defining between the inner and outer conductive cylinders an annular coupling chamber volume overlying the annular ceiling gap, the hollow inner conductive cylinder having a bottom end surrounding the ceiling electrode; (b) a conductive annular cap extending between and electrically contacting respective top edges of the inner and outer conductive cylinders; and (c) a coaxial power distributor connected between the RF power source and the hollow outer conductive cylinder. 
         [0014]    In one aspect, the coaxial power distributor includes an axial center conductor having a top end connected to the RF power source and a bottom end, and plural respective spoke conductors electrically separate from the inner conductive cylinder and extending radially from the bottom end of the axial center conductor through the inner conductive cylinder to respective points on the outer conductive cylinder, the plural respective spoke conductors being spaced apart. 
         [0015]    In a first aspect, there is a slit opening in the inner conductive cylinder, and the plural respective spoke conductors extend through the slit opening. In a second aspect, there are respective holes in the inner conductive cylinder, the plural respective spoke conductors extending through the respective holes. 
         [0016]    The RF coupling chamber may further include a radial conduit formed as a shallow cylindrical volume partially enclosing the plural respective spokes. The coaxial power distributor further includes an RF feeder outer conductor surrounding a portion of the axial center conductor and coupled to a return potential of the RF power source. The radial conduit includes a conduit ceiling lying in a radial plane over the plural respective spokes and having a center opening, the axial center conductor extending through the center opening, the RF feeder outer conductor terminated at the center opening; and a conduit floor lying in a radial plane under the plural respective spokes, the conduit ceiling and conduit, floor terminated at the inner conductive cylinder, the floor including a floor opening, the axial center conductor extending through the floor opening, 
         [0017]    In a further aspect, the RF coupling chamber further includes a toroidal shaped ferrite ring coaxial with and between the inner and outer hollow cylindrical conductors, and located between the annular cap and the coaxial power distributor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0018]    So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarised above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0019]      FIG. 1  is a cut-away elevational view of a plasma reactor in accordance with a first embodiment. 
           [0020]      FIG. 1A  is a cross-sectional plan view taken along lines  1 A- 1 A of  FIG. 1 . 
           [0021]      FIG. 2  is a perspective view corresponding to  FIG. 1 . 
           [0022]      FIG. 3  is a cross-sectional plan view taken along lines  3 - 3  of  FIG. 1 . 
           [0023]      FIG. 4  is an enlarged view corresponding to  FIG. 1 . 
           [0024]      FIG. 5  is a cut-away elevational view of a plasma reactor in accordance with a modification of the embodiment of  FIG. 1 . 
           [0025]      FIG. 6  is a cut-away elevational view of a plasma reactor in accordance with a second embodiment. 
           [0026]      FIG. 7  depicts a modification of the embodiment of  FIG. 6 , in which utility supply lines or conduits enter from a side location. 
           [0027]      FIG. 8  depicts a modification employing plural radial conductive arms instead an RF power distribution plate. 
           [0028]      FIG. 9  is a cut-away elevational view of a plasma reactor in accordance with a further modification of the embodiment of  FIG. 6 . 
       
    
    
       [0029]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION  
       [0030]    Referring to  FIGS. 1 ,  1 A,  2  and  3 , a plasma reactor includes a vacuum chamber  100  enclosed by a cylindrical side wall  105 , a ceiling  110  and a floor  115 . The side wall  105  and floor  115  may be formed of metal and electrically grounded. The floor  115  has an opening or pumping port  117  through which a vacuum pump  119  is coupled, to the interior of the chamber  100 . The ceiling  110  includes a gas distribution plate or showerhead  120  that functions as both a gas distributor and as a ceiling electrode and is referred to herein as the ceiling electrode  120 . The ceiling  110  extends to the side wail  105 , and includes an annular insulating section  110   a  surrounding the ceiling electrode. The ceiling electrode  120  is formed of a conductive material. The ceiling electrode  120  includes an interior gas manifold  121  and an underlying gas distribution layer  122  having an array of gas injection orifices  123 . A workpiece support pedestal  130  is centered, within the chamber  100  to support a workpiece  135 , such as a semiconductor wafer, in facing relationship with the showerhead  120 . The pedestal  130  includes a center post  140  that extends through the floor  115 . An electrically grounded outer layer  145  may enclose the pedestal  130  including the post  140 . An insulated cathode electrode  150  is covered by a top insulating layer  155  and an underlying insulating bed  160 . RF bias power is supplied to the cathode electrode  150  through a center conductor  165 . The center conductor  165  may be separated from the grounded outer layer  145  by a coaxial insulating layer  170 . The center conductor  165  may be coupled to an RF bias power generator  175  through an RF impedance match circuit  185 . 
         [0031]    A coaxial RF feeder  200  has a hollow center conductor  205  and a grounded outer conductor  210 . A utility conduit  206  may extend coaxially through the hollow center conductor  205  while being insulated from the center conductor  205 . As shown in  FIG. 1 , the utility conduit  206  is physically connected to the grounded outer conductor  210  at the top of the grounded outer conductor  210  by a conductive annular cap  210 ′, to provide a field-free region for the utility supply lines entering the conduit  206 . An RF generator  220  supplying plasma source power is coupled to the center conductor  205 . Optionally, the RF generator may be coupled to the center conductor  205  through an RF impedance match circuit  225 . The chassis ground of the RF impedance match circuit  225  (or of the RF generator  220  in absence of the impedance match circuit  225 ) is connected to the outer conductor  210 . RF power from the center conductor  205  is coupled to the ceiling electrode  120  in a manner which will be described below herein. 
         [0032]    The utility conduit  206  within the center conductor  205  may contain one or more utility supply lines. For example, an outlet of a gas supply  247  is connected to gas flow lines inside and extending through the utility conduit  206 . The utility conduit  206  may also contain other utility supply lines, such as electric power conductors to supply AC heaters (not illustrated) inside the ceiling electrode  120 . Optionally, all of these utility supply lines may be fed through the hollow interior of the center conductor  205  without the utility conduit  206 . 
         [0033]      FIG. 4  is an enlarged view of the coaxial RF feeder  200 , depicting in detail the connection of the RF output terminal of the impedance match  225  to the hollow center conductor  205 , the disposition of the utility conduit  206  inside the hollow interior of the center conductor  205 , and the disposition of utility supply lines, including process gas supply lines, inside the hollow utility conduit  206 . In addition,  FIG. 4  depicts an alternative mode, in which the radial spokes  270  extend through individual holes  253  in the inner coaxial wall  252 , while not electrically contacting the inner coaxial wail  252 . 
         [0034]    Referring again to  FIGS. 1 ,  1 A,  2  and  3 , an RF coupling chamber  250  couples RF power from the center conductor  205  to the ceiling electrode  120 . The RF coupling chamber  250  includes inner and outer coaxial wails  252 ,  254  and an annular top  256 , enclosing a coupling chamber annular volume  257 . The RF coupling chamber  250  is sealed at its bottom by the annular insulating section  110   a  of the ceiling  110 . Unless otherwise noted, the elements of the RF coupling chamber  250 , other than the annular insulating section  110   a,  are formed of a metal such as aluminum. The RF coupling chamber  250  is coaxial with the coaxial RF feeder  200 . The coupling chamber annular volume  257  generally is radially outside a circumferential edge  120 - 1  of the ceiling electrode  120 . The coupling chamber annular volume  257  extends above the ceiling  110 . The bottom of the inner coaxial wail  252  surrounds or encloses the ceiling electrode  120 . 
         [0035]    A shallow cylindrical hollow volume  260  (hereinafter referred to as a radial conduit  260 ) is enclosed by a disk-shaped conduit ceiling  262  and a disk-shaped conduit floor  264 . The conduit ceiling  262  has a central opening  262   a  connected to and terminating the grounded outer conductor  210  of the coaxial RF feeder  200 . Generally, the central opening is of the same diameter as the outer conductor  210 . The center conductor  205  of the coaxial RF feeder  200  extends axially to and terminates at a center point  260   a  of the radial conduit  260 . Plural spokes  270  within the interior of the radial conduit  260  lie in the plane of the center point  260   a  and extend radially outwardly from the center conductor  205  to the outer coaxial wail  254  through respective openings  253  in the inner coaxial wall  252 . The plural spokes  270  are angularly spaced at even intervals and electrically contact the outer coaxial wall  254  at uniformly spaced contact points. The assembly including the plural spokes  270  and the center conductor  205  may be referred to as a coaxial power distributor. 
         [0036]    The utility conduit  206  emerges from the bottom end of the hollow center conductor  205  and extends below the radial conduit  260  through a hole  264   a  in the conduit floor  264 , and reaches the gas manifold  121  of the gas distribution plate  120 . Various utility supply lines contained in the utility conduit  206 , such as process gas supply line, coolant supply lines and electrical supply lines, make connection to suitable connection ports on or in the gas distribution plate  120 . The region through which the utility lines extend from the bottom end of the center conductor  205  to the ceiling electrode  110  is enclosed by the inner coaxial wall  252  and is free of electric or RF fields. 
         [0037]    The region of the RF coupling chamber  250  lying above the radial spokes  270  may be referred to as a primary sub-chamber  250 - 1 . The primary sub-chamber  250 - 1  is the volume enclosed by upper portion  252   a  of the inner wall  252 , upper portion  254   a  of the outer wail  254 , the annular top  256  and the radial spokes  270 . 
         [0038]    Coupling of RF power from the center conductor  205  to the ceiling electrode  120  occurs as follows: RF power from the center conductor  205  generates a first RF toroidal current loop  400  flowing on the interior surfaces of the primary sub-chamber  250 - 1 , namely the interior surfaces of the inner wall upper portion  252   a , the outer wall upper portion  254   a,  the annular top  256  and the radial spokes  270 . The first RF toroidal current loop  400  functions as a primary transformer winding. The first RF toroidal current loop induces a second RF toroidal current loop  410  flowing on interior surfaces of the entire length (height) of the RF coupling chamber  250 . The second RF toroidal current loop  410  functions as a secondary transformer winding. The entire RF coupling chamber  250  therefore may be referred to as a secondary chamber containing the secondary winding or second RF current loop  410 . 
         [0039]    The uniformity of azimuthal distribution of the second toroidal RF current loop  410  determines the uniformity of RF power distribution on the ceiling electrode  120 . This uniformity depends upon the uniformity or symmetry of the shape of the RF coupling chamber  250 . The RF coupling chamber is perfectly symmetrical relative to the cylindrical axis of symmetry of the reactor of  FIG. 1 , so that RF power distribution on the ceiling electrode is at least nearly perfectly symmetrical. 
         [0040]    The utility conduit  206  (and the various utility supply lines within the center conductor  205 ) is grounded, and its attachment to the ceiling electrode  120  holds the D.C. potential of the ceiling electrode  120  at ground. However, the second RF current loop  410  produces an RF potential on the ceiling electrode  120  of a high RF voltage, in accordance with the output power level of the RF generator  220 , while allowing the ceiling electrode  120  to remain at D.C. ground. 
         [0041]    The electrical length of the RF coupling chamber  250  (along the cylindrical axis of symmetry) need not necessarily be sufficient to be a resonant length. However, in one implementation, it is resonant or nearly resonant at the frequency of the RF generator  220 . For resonance, the electrical length of the RF coupling chamber  250  may be a selected fraction of the wavelength of the RF voltage supplied by the RF generator  220 , such as a quarter wavelength or a half wavelength, for example. The physical height H 1  of the RF coupling chamber  250  above the ceiling may be less than this length, if desired. 
         [0042]    While the physical length of the RF coupling chamber  250  should be a fraction of the wavelength of the RF generator  220 , such as a quarter wavelength, such a size occupies a significant amount of space, which may be scarce in a crowded production environment.  FIG. 5  depicts a modification of the embodiment of  FIG. 1 , in which the electrical length of the RF coupling chamber  250  is increased without increasing its height HI above the ceiling  110 . As shown in  FIG. 5 , the electrical length is increased by adding a toroidal ferrite  450  (or equivalent magnetically permeable element) in the center of the primary sub-chamber  250 - 1 , and concentric with the cylindrical axis of symmetry of the chamber  100 . Because the addition of the toroidal ferrite  450  provides a longer electrical length of the RF coupling chamber  250  for a given physical length, the physical length (and therefore the height H 1 ) may be decreased to be less than the required electrical length (e.g., a quarter or half wavelength or full wavelength) while the electrical length meets the required fraction of the wavelength. If for example the frequency of the RF generator is about 220 MHz, the wavelength is about 1.25 meters. If it is desired that the length of the RF coupling chamber  250  be a half wavelength (for example), then its physical length (height H 1 ) would have to be one half of 1.25 meters. However, by adding the toroidal ferrite  450  as shown in  FIG. 5 , the physical height HI may be reduced to a significantly shorter length while meeting the requirement of an effective length of half a wavelength. The reduction in length may be in a range of 5%-20%, depending upon the magnetic properties of the toroidal ferrite  450 . 
         [0043]    The ceiling electrode  120  is of the same diameter as the inner coaxial wall  252 . The interior volume enclosed by the inner coaxial wail  252  between the annular cap  256  and the ceiling electrode, as well as the interior of the ceiling electrode  120 , is free of electromagnetic fields. At the same time, the ceiling electrode  120  is at D.C. ground potential. The utility conduit  206  and/or the utility supply lines with the utility conduit are grounded and are electrically connected to the ceiling electrode  120 , holding the ceiling electrode at D.C. ground potential. RF current flow on the ceiling electrode  120  occurs on its exterior surfaces only. The foregoing features prevent undesirable interactions between RF fields and the utility conduit  206  or any other utility supply lines (e.g., creation of non-uniformities in electric field distribution, arcing and the like). 
         [0044]      FIG. 6  depicts a plasma reactor having a folded RF coupling chamber  500 , which is a folded version of the RF coupling chamber  250  of  FIG. 1 . The folded RF coupling chamber  500  can have the same electrical length as the RF coupling chamber  250  of  FIG. 1 , but only about one half the height. Unless otherwise noted, the elements of the folded RF coupling chamber  500  are formed of a suitable metal, such as aluminum. 
         [0045]    In the embodiment of  FIG. 6 , as in the embodiment of  FIG. 1 , the plasma reactor includes a vacuum chamber  100  enclosed by a cylindrical side wall  105 , a ceiling  110  and a floor  115 . The side wall  105  and floor  115  may be formed of metal and electrically grounded. The floor  115  has an opening or pumping port  117  through which a vacuum pump  119  is coupled to the interior of the chamber  100 . The ceiling  110  includes a gas distribution plate or showerhead  120  that functions as both a gas distributor and as a ceiling electrode and may be referred to as the ceiling electrode  120 . The ceiling electrode or showerhead  120  is formed of a conductive material. The ceiling electrode  120  includes an interior gas manifold  121  and an underlying gas distribution layer  122  having an array of gas injection orifices  123 . A workplace support pedestal  130  is centered within the chamber  100  to support a workpiece  135 , such as a semiconductor wafer, in facing relationship with the showerhead  120 . The pedestal  130  includes a center post  140  that extends through the floor  115 . An electrically grounded outer layer  145  may enclose the pedestal  130  including the post  140 . An insulated cathode electrode  150  is covered by a top insulating layer  155  and an underlying insulating bed  160 . RF bias power is supplied to the cathode electrode  150  through a center conductor  165 . The center conductor  165  may be separated from the grounded outer layer  145  by a coaxial insulating layer  170 . The center conductor  165  may be coupled to an RF bias power generator  175  through an RF impedance match circuit  185 . 
         [0046]    In the embodiment of  FIG. 6 , as in the embodiment of  FIG. 1 , a coaxial RF feeder  200  has a hollow center conductor  205  and a grounded outer conductor  210 . A utility conduit  206  extends coaxially through the hollow center conductor  205  while being insulated from the center conductor  205 . An RF generator  220  supplying plasma source power is coupled to the center conductor  205  through an optional RF impedance match circuit  225 . The chassis ground of the RF impedance match circuit  225  (or of the RF generator  220 ) is connected to the outer conductor  210 . RF power from the bottom end of the center conductor  205  is coupled to the ceiling electrode  120  in a manner which will be described below herein. An outlet of a gas supply  247  is connected to gas flow lines inside and extending through the utility conduit  206 . The utility conduit may also contain other utility lines, such as electric power conductors to supply AC heaters (not illustrated) inside the ceiling electrode  120 . 
         [0047]    The folded RF coupling chamber  500  of  FIG. 6  consists of an inner annular chamber  505  and an outer annular chamber  510  with an opening  515  between them. The inner annular chamber  505  is enclosed by inner and intermediate coaxial walls  520 ,  522 , a top disk  524  and an annular portion  110 - 1  of the ceiling  110 . The outer annular chamber  510  is enclosed by the intermediate coaxial wall  522 , an outer coaxial side wall  526  and by the top disk  524 . The outer annular chamber  510  is enclosed at its bottom by an annular insulating section  110 - 2  of the ceiling  110 . The inner coaxial wall  520  surrounds or encloses the ceiling electrode  120 , and therefore the inner annular chamber  505  and the outer annular chamber  510  are radially outside of the ceiling electrode  120 . 
         [0048]    A radial conduit  530  is a shallow cylindrical volume coaxial with the inner and outer chambers  505  and  510 , and is enclosed by a disk-shaped conduit ceiling  532  and by a floor formed by the disk-shaped, top  524 . The conduit ceiling  532  has a central opening  532   a  connected to and terminating the grounded outer conductor  210  of the coaxial RF feeder  200 . The central opening  532   a  and the outer conductor  210  generally are of the same diameter. A disk-shaped RF distribution plate  535  is disposed within the interior of the radial conduit  530  and has a peripheral edge  535   a.  The center conductor  205  of the coaxial RF 1  feeder  200  extends through the central opening  532   a  of the conduit ceiling  532 , and is connected to the center of the RF distribution plate  535 . The center conductor  205  is electrically separated from the conduit ceiling  532 . Plural axial posts  540  extend from the RF distribution plate  535  to a top annular edge  522   a  of the intermediate wail  522 , through respective openings  524 - 2  in the disk-shaped top  524 , each opening  524 - 2  accommodating a respective one of the axial posts  540 . Each opening  524 - 2  is of a sufficient diameter so that the corresponding axial post  540  does not electrically contact the disk-shaped top  524 . The plural posts  540  are angularly spaced at even intervals and electrically contact the intermediate wall  522  at uniformly spaced contact points. 
         [0049]    The assembly including the RF distribution plate  535 , the center conductor  205  and the plural axial posts  540  may be referred to as a coaxial power distributor. 
         [0050]    The utility conduit  206  emerges from the bottom end of the hollow center conductor  205 , extends through a central opening  535 - 1  in the RF distribution plate  535 , and through an opening  524   a.  in the disk-shaped top  524 , and continues toward the gas distribution plate  120 . Various utility supply lines contained in the utility conduit  206 , such as process gas supply line, coolant supply lines and electrical supply lines, make connection to suitable connection ports on or in the gas distribution plate  120 . The region through which the utility lines extend past or below the bottom end of the center conductor  205  is enclosed by the inner coaxial wall  520  and is free of electric or RF fields. 
         [0051]    The ceiling electrode  120  is of the same diameter as the inner coaxial wall  520 . The interior volume enclosed by the inner coaxial wall  520 , as well as the interior of the ceiling electrode  120 , is free of electromagnetic fields. At the same time, the ceiling electrode  120  is at D.C. ground potential. The utility conduit  206  and/or the utility supply lines with the utility conduit are grounded and are electrically connected to the ceiling electrode  120 , holding the ceiling electrode at D.C. ground potential. RF current flow on the ceiling electrode  120  occurs on its exterior surfaces only. The foregoing features prevent undesirable interactions between RF fields and the utility conduit  206  or any other utility supply lines (e.g., creation of non-uniformities in electric field distribution, arcing and the like). 
         [0052]    With the folded RF coupling chamber  500  of  FIG. 6 , coupling of RF power from the center conductor  205  to the ceiling electrode  120  occurs as follows: RF power from the center conductor  205  generates a first RF toroidal current loop  600  flowing on the interior surfaces of the inner annular chamber  505 . The first RF toroidal current loop  600  functions as a primary transformer winding. The first RF toroidal current loop  600  induces a second RF toroidal current loop  610  flowing on interior surfaces of the both the inner and outer annular chambers  505  and  510 . The second RF toroidal current loop  610  functions as a secondary transformer winding. As indicated in  FIG. 6 , the second RF toroidal current loop  610  begins in the inner annular chamber  505  and extends in a spiral path indicated in the drawing through the opening  515  into the outer annular chamber  510 . 
         [0053]    The uniformity of azimuthal distribution of the toroidal RF current loops  600  and  610  determines the uniformity of RF power distribution on the ceiling electrode  120 . This uniformity depends upon the uniformity or symmetry of the shape of the folded RF coupling chamber  500 . The folded RF coupling chamber  500  is perfectly symmetrical relative to the cylindrical axis of symmetry of the reactor of  FIG. 1 , so that RF power distribution on the ceiling electrode  120  is at least nearly perfectly symmetrical. 
         [0054]      FIG. 7  depicts a variation of the embodiment of  FIG. 6 , in which utility supply lines or conduits (gas supply conduits, coolant supply conduits, electrical supply lines for heating, as some examples) enter through the side of the coupling chamber. For this purpose, the disk-shaped top  524  of FIG,  6  is divided into top and bottom planar disks  524   c  and  524   b,  respectively. The top and bottom planar disks are separated by a void  527 . Respective hollow conduits  525  extend between respective holes  524 - 2   a  and  524 - 2   b  formed in the top and bottom planar disks  524   c,    524   b,  respectively. Respective ones of the axial posts  540  extend through respective ones of the hollow conduits  525 . The utility supply conduits or lines access the gas distribution plate  120  through the void  527  along a radial path, as depicted in  FIG. 7 . 
         [0055]      FIG. 8  depicts a modification applicable to either the embodiment of  FIG. 6  or FIG,  7 , in which the RF power distribution plate  535  is replaced by plural radial spokes  536 . All of the spokes  536  are connected to the end of the center conductor  205  and radiate outwardly to the top ends of respective ones of the posts  540 . The spokes  536  are angularly spaced at uniform intervals. 
         [0056]      FIG. 9  depicts an embodiment in which the axial length (height) of the folded RF coupling chamber  500  can be further reduced without reducing its electrical length. For resonances, the electrical length of the folded RF coupling chamber  500  should be a fraction of the wavelength of the RF generator  220 , such as a quarter or half wavelength, or even a full wavelength. However, such a size occupies a significant amount of space, which may be scarce in a crowded production environment. The height of the folded RF coupling chamber  500  may be reduced, without changing its electrical characteristics, by adding a toroidal ferrite  650  (or equivalent magnetic element) in the center of the inner annular chamber  505  concentric with the cylindrical axis of symmetry of the chamber  100 . Because the addition of the toroidal ferrite  650  provides a longer electrical length of the folded RF coupling chamber  500  for a given physical length E 3 , the physical length (height) H 3  may be decreased to be less than the required electrical length while the electrical length meets the required fraction of the wavelength. The reduction in length may be in a range of 5%-20%, depending upon the magnetic properties of the toroidal ferrite  650 . 
         [0057]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.