Patent Publication Number: US-2015075717-A1

Title: Inductively coupled spatially discrete multi-loop rf-driven plasma source

Description:
BACKGROUND 
     1. Technical Field 
     The disclosure relates to RF-driven plasma sources for reactors employed in plasma processing of workpieces such as semiconductor wafers. 
     2. Background Discussion 
     In plasma processing of workpiece, such as a semiconductor wafer, there is a need for a plasma source capable of providing a high plasma ion density and, simultaneously, a low plasma sheath ion energy to an extent that is currently unavailable. A high plasma ion density is needed for improved processing rate and productivity. A reduced plasma ion energy is needed for reduced plasma ion energy in order to prevent contamination from ion bombardment of metal surfaces near the plasma sheath. Reduced ion energy may also reduce ion bombardment damage to semiconductor device features. Such features are becoming extremely small and more susceptible to such damage, thus requiring reduction in plasma electron energy. 
     A basic problem is that plasma sources capable of providing high density plasma also produce relatively high energy plasma ions. The reason is that such sources couple relatively high electric fields to the plasma, raising the plasma sheath voltage. High plasma sheath voltages impart high energy to plasma ions in the plasma sheath. This produces ion bombardment of metal surfaces adjacent the plasma sheath, which produces metal contamination. An inductively coupled plasma source employs an RF-driven coil antenna, which has a capacitance that couples a high voltage to the plasma, contributing to the high plasma sheath voltage. A capacitively coupled plasma source employs an RF-driven electrode which has an even greater tendency to couple high voltage to the plasma. Toroidal plasma sources produce plasma densities somewhat less than inductively coupled plasma sources. 
     What is needed is a plasma source capable of producing a plasma having an ion density as great as or exceeding that of a conventional inductively coupled plasma source, and with a minimum plasma ion energy less than (or not exceeding) that of conventional plasma sources. 
     SUMMARY 
     A plasma reactor comprises a processing chamber and a resonator having an axis of symmetry transverse to the ceiling and comprising a hollow driven cylinder and a hollow return cylinder enclosing the hollow driven cylinder, the hollow driven cylinder and the hollow return cylinder comprising respect bottom edges contacting the ceiling. An RF power generator comprises an output power terminal coupled to the hollow driven cylinder and a return terminal coupled to the hollow return cylinder. The reactor further comprises plural reentrant conduits on a side of the ceiling external of the processing chamber, each of the plural reentrant conduits communicating with the processing chamber. 
     In an embodiment, each of the plural reentrant conduits encloses a path extending in a radial direction. In one embodiment, the plural reentrant conduits are arranged in a circle. 
     In a related embodiment, the ceiling comprises, for each one of the plural reentrant conduits, a pair of ports extending through the ceiling and coupled to the ends of respective ones of the plural reentrant conduits. 
     In one embodiment, the ceiling comprises an internal gas manifold and gas injection orifices coupled to the gas manifold, while the plasma reactor further comprises a process gas supply, and a gas supply conduit coupled to the internal manifold and extending axially from the internal manifold and through an interior volume of the hollow driven cylinder to the process gas supply. The gas injection orifices may comprise openings facing an interior of the processing chamber. 
     In a further embodiment, the plasma reactor further comprises a coolant supply, internal recirculation passages in the ceiling, and a coolant supply conduit coupled to the internal recirculation passages and extending axially from the ceiling and through an interior volume of the hollow driven cylinder to the coolant supply. 
     In accordance with an embodiment, each of the plural reentrant conduits comprises a conductive main portion and an insulating ring-shaped break. 
     In one embodiment, each of the ports has a width along a direction transverse to the path that exceeds a diameter of the respective one of the plural reentrant conduits. In a further embodiment, each of the reentrant ports has a cross-sectional shape that is one of: circular, oval, rectangular, kidney-shaped. 
     In an embodiment, the resonator has an axial length corresponding to one wavelength of RF current or RF voltage produced by the RF power generator. 
     In one embodiment, a conductive disk-shaped cap covers or contacts a top edge of the hollow return cylinder opposite the ceiling. The hollow driven cylinder may be terminated at a height below the conductive disk-shaped cap so as form a gap between the top edge of the hollow driven cylinder and the conductive disk-shaped cap. 
     In one embodiment, an RF bias power generator having an output terminal coupled to the workpiece support and a return terminal coupled to the ceiling. In a related embodiment, a first radial conductor is connected between the output power terminal of the RF power generator and a tap point at an axial location on the driven hollow cylinder, the axial location corresponding to an impedance match between the resonator and the RF power generator. In a further related embodiment, the first radial conductor extends through the hollow return cylinder without electrically contacting the hollow return cylinder. 
     In an embodiment in which the resonator is folded, the resonator comprises an inner hollow return cylinder surrounded by the hollow return cylinder. In a related embodiment, the inner hollow return cylinder comprises a top edge contacting the conductive disk-shaped cap and a bottom edge separated from the ceiling by a second gap. In a related embodiment, the conductive disk-shaped cap is at a height above the ceiling corresponding to a half wavelength of RF current or RF voltage of the RF power generator. 
     In one embodiment, the ceiling comprises a center disk-shaped portion and an outer cylindrical-shaped portion contacting the hollow return cylinder, the plural reentrant conduits located on the cylindrical-shaped portion of the ceiling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the exemplary embodiments of the present invention are attained can be understood in detail, a more particular description of the invention, briefly summarized 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. 
         FIG. 1A  is an elevational cut-away view of a first embodiment. 
         FIG. 1B  is an enlarged view of an upper portion of the embodiment of  FIG. 1A . 
         FIG. 1C  is a cross-sectional view along lines  1 C- 1 C of  FIG. 1A . 
         FIG. 1D  is an orthographic projection corresponding to  FIG. 1C . 
         FIGS. 1E-1H  are views of toroidal channels of different cross-sectional shapes. 
         FIG. 2  is an elevational cut-away view of a second embodiment. 
         FIG. 3  is an elevational cut-away view of a third embodiment. 
         FIG. 4A  is an elevational cut-away view of a fourth embodiment. 
         FIG. 4B  is an orthographic projection corresponding to  FIG. 4A . 
         FIG. 4C  is an enlarged view of an upper portion of the embodiment of  FIG. 4A . 
         FIG. 4D  is a cross-sectional view along lines  4 D- 4 D of  FIG. 4A . 
         FIG. 5A  is an elevational cut-away view of a fifth embodiment. 
         FIG. 5B  is a cross-sectional view along lines  5 B- 5 B of  FIG. 5A . 
     
    
    
     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 
     Embodiments of the invention meet the need for an extremely high ion density plasma source with an extremely low plasma ion energy, by employing an RF-driven coaxial resonator whose shorted end is adjacent a plasma chamber wall (e.g., the ceiling) and whose open end is away from the plasma. A circular array of reentrant toroidal conduits is provided on the ceiling. Process gas in the chamber fills each of the toroidal conduits, and is ionized by RF power to produce a plasma as follows: Each toroidal conduit has a radial path direction, each of which is orthogonal to an azimuthal (circumferential) RF magnetic field produced at the shorted end of the coaxial resonator. This azimuthal RF magnetic field produces a radial RF electric field that is parallel to the path of each toroidal conduit. This azimuthal RF magnetic field is maximum at the coaxial resonator shorted end where the toroidal conduits are located, for maximum coupling to RF currents in the array of toroidal conduits, producing maximum plasma ion density. Furthermore, the electric field is minimum at the coaxial resonator shorted end, for coupling of minimum voltage to the bulk plasma, to minimize electric field in the bulk plasma. Minimizing the electric field in the bulk plasma reduces plasma electron temperature. Minimizing electric field in the bulk plasma also increases plasma ion density. Minimizing (or reducing) the plasma electron temperature and increasing the plasma ion density minimizes (reduces) the plasma sheath voltage. This reduces the energy of ions bombarding metal surfaces near the plasma sheath, which reduces metal contamination caused by sputtering of the metal surfaces. The reactor is characterized by very low metal contamination. 
     Each of the reentrant toroidal conduits is “external” in that each one is on the side of the chamber wall or ceiling that is opposite (outside of) the plasma chamber. The reentrant toroidal conduits are arranged in a circle, each conduit lying along a radial direction with respect to the coaxial resonator. The current path direction in each toroidal conduit is orthogonal to the azimuthal RF magnetic field at the coaxial resonator shorted end. The azimuthal RF magnetic field produces a radial RF electric field. The reentrant toroidal conduits, lying in the radial direction, are parallel with the radial RF electric field, which maximizes coupling to RF currents in the toroidal conduits. 
     The coaxial resonator has an inner RF-driven conductive cylinder and an outer hollow return cylinder surrounding the inner RF-driven conductive cylinder and coupled to an RF return potential (e.g., ground). The inner and outer conductive cylinders are electrically shorted together by the chamber ceiling, so that the shorted end of the coaxial resonator is adjacent the plasma. An RF generator is coupled to the inner RF driven element. The electrical length of the coaxial resonator is related to the wavelength of the RF generator, and is typically one wavelength, a half wavelength or a quarter wavelength. In one embodiment, the space above the chamber ceiling occupied by the coaxial resonator may be reduced by folding the resonator while preserving its electrical length. 
     Referring to  FIGS. 1A ,  1 B,  1 C and  1 D, a processing chamber  100  is enclosed by a cylindrical side wall  102 , a floor  104  and a cover plate  106  that serves as a ceiling of the processing chamber  100 . A vacuum pump  108  evacuates the chamber  100 . A workpiece support pedestal  110  within the processing chamber  100  includes a workpiece support surface  112  for holding a workpiece  114  in facing relationship with the cover plate  106 . A gas injection plate  116  on the bottom surface of the cover plate  106  includes an internal gas manifold  118  having an array of gas injection orifices  120  facing the workpiece support surface  112 . A gas supply conduit  122  coupled to the internal gas manifold  118  extends upwardly from the gas injection plate  116 . A pair of coolant supply conduits  124  extend to internal coolant circulation passages  126  within the cover plate  106 . The cover plate  106  and the gas injection plate  116  may be formed separately or as a single piece, and either one or both may serve as the ceiling of the processing chamber  100 . Optionally, an RF bias power generator  127  is coupled through an impedance match  128  to an electrode  129  underlying the workpiece support surface  112 . The cover plate  106  may be connected to the return potential of the RF power generator  127 . 
     A coaxial resonator  130  formed of conductive structural elements overlies the processing chamber  100 . The cover plate  106  serves as a base of the coaxial resonator  130 . The coaxial resonator  130  has a hollow driven cylinder  132  surrounded by a hollow return cylinder  134 . The hollow driven cylinder  132  and the hollow return cylinder  134  are the coaxial inner and outer conductors, respectively, of the coaxial resonator  130 . A disk-shaped inner cap  133  covers the top of the hollow driven cylinder  132 . A disk-shaped outer cap  135  covers the top of the hollow return cylinder  134 . An RF generator  136  is coupled by a center conductor  138  to the hollow driven cylinder at a tap point  140  on the hollow driven cylinder  132 . The RF generator  136  has a frequency and wavelength corresponding to a resonance wavelength of the coaxial resonator  130 . The axial location of the tap point  140  may be chosen to match the load impedance to the impedance of the RF generator  136 . In this way, the coaxial resonator  130  acts as an impedance match for the RF generator  136 , performing an impedance match function. This impedance match function can permit the wavelength of the RF generator  136  to differ from the resonance wavelength while maintaining resonance. 
     A hollow cylindrical shield  142  surrounds the center conductor  138  and is grounded or connected to the return potential of the RF generator  136 . The bottom edges of the hollow driven cylinder  132  and the hollow return cylinder  134  are shorted together by the cover plate  106 , forming the shorted end of the coaxial resonator  130  adjacent the processing chamber  100 . At the opposite or top end of the coaxial resonator  130 , the hollow driven cylinder  132  and the hollow return cylinder  134  are not connected together, and the top end of the coaxial resonator  130  is referred to as the open end. RF current is maximum and RF voltage is minimum at the shorted end, while RF current is minimum and RF voltage is maximum at the open end. 
     The gas supply conduit  122  extends through the interior of the hollow driven cylinder  132 , through the inner cap  133  and through the outer cap  135  to an external gas supply  123 . The interior of the hollow driven cylinder  132  may be a field-free region. The pair of coolant supply conduits  124  extend through the interior of the hollow driven cylinder  132 , through the inner cap  133  and through the outer cap  135  to an external coolant supply  125 . 
     An array of toroidal channels  150  is provided on the top side of the cover plate  106 , e.g., the side of the cover plate  106  external of processing chamber  100 . Each of the toroidal channels  150  forms a reentrant path. Each one of the toroidal channels  150  includes a reentrant conduit  152  that is external of the processing chamber  100 . Each reentrant conduit  152  has a pair of ends  152 - 1 ,  152 - 2  coupled to the interior of the processing chamber  100  through a pair of respective ports  154 - 1 ,  154 - 2  through the cover plate  106 . The cross-sectional shape of each of the pair of ports  154 - 1 ,  154 - 2  and the reentrant conduit  152  may be circular ( FIG. 1E ), rectangular ( FIG. 1F ), kidney-shaped ( FIG. 1G ) or elliptical ( FIG. 1H ). In the embodiment of  FIG. 1C , there are four uniformly spaced toroidal channels  150  each defining a reentrant path lying along a predominantly radial direction. However, any suitable number of toroidal channels  150  may be employed. In the embodiment of  FIG. 1A , the array of toroidal channels  150  is confined on the cover plate  106  within an annular zone between the driven hollow cylinder  132  and the hollow return cylinder  134 . 
     Each reentrant conduit  152  may include a D.C. break  153 , to prevent generation of currents that could otherwise interfere with inductive coupling. The D.C. break  153  may be an annular gap filled with dielectric material. 
     Each reentrant conduit  152  has a radial path direction D, which is orthogonal to an azimuthal (circular) RF magnetic field M present at the shorted end of the coaxial resonator  130 . The azimuthal RF magnetic field M produces a radial RF electric field E that is parallel to the path direction D of each reentrant conduit  152 . This azimuthal RF magnetic field is maximum at the coaxial resonator shorted end, e.g., at the cover plate  106 , where the reentrant conduits  152  are located, for maximum coupling to RF currents in the array of reentrant conduits  152 , producing maximum plasma ion density. Furthermore, the electric field is minimum at the coaxial resonator shorted end, for coupling of minimum voltage to the bulk plasma, to minimize electron temperature. As explained above, minimizing electron temperature and maximizing plasma ion density reduces metal contamination. 
       FIG. 2  depicts an embodiment in which the toroidal channels  150  extend beyond the diameter of the hollow return cylinder  134 . In the embodiment of  FIG. 2 , the toroidal channels  150  extend through the hollow return cylinder  134 . In one implementation of the embodiment of  FIG. 2 , the toroidal channels  150  may be insulated from the hollow return cylinder  134 . 
       FIG. 3  depicts an embodiment in which the circular processing chamber  100  includes an annular upper chamber  100 - 1  surrounding a lower portion  134 - 1  of the hollow return cylinder  134 , and a cylindrical lower main chamber  100 - 2 . The array of toroidal channels  150  is replaced by an array of radially-facing toroidal channels  150 ′ formed on the lower portion  134 - 1  of the hollow return cylinder  134 . The lower portion  134 - 1  may be considered as an orthogonal extension or portion of the cover plate  106  or ceiling. The radially-facing toroidal channels  150 ′ are open to the annular upper chamber  100 - 1 . 
       FIGS. 4A ,  4 B,  4 C and  4 D depict an embodiment in which a folded coaxial resonator  230  overlies the processing chamber  100 . The folded coaxial resonator  230  includes a hollow inner ground cylinder  231 , a hollow outer ground cylinder  234  surrounding the hollow inner ground cylinder  231 , and a hollow RF drive cylinder  232  between the inner and outer ground cylinders  231  and  234 . The bottom edges of the hollow outer ground cylinder  234  and the hollow RF drive cylinder  232  contact the cover plate  106 , forming the shorted end of the folded coaxial resonator  230 . The bottom edge  231   a  of the hollow inner ground cylinder  231  is separated from the cover plate  106  by a gap G1. The top edges of the inner and outer ground cylinders  231 ,  234  are capped by an annular cover  235  that encloses the annular volume between the inner and outer grounded cylinders  231 ,  234 . The top edge  232   a  of the hollow RF drive cylinder  232  is below the annular cover  235  and separated therefrom by a gap G2. The sizes of the gaps G1 and G2 may be selected to avoid arcing. 
     An RF generator  236  is connected to the hollow RF drive cylinder  232  via discrete RF feed conductors  238  connected to two (or more) uniformly spaced points on the top edge  232   a  of the hollow RF drive cylinder  232 . The RF feed conductors  238  pass through openings in the annular cover  235 , and are surrounded by tubular shields  239  contacting the annular cover  235 . 
     In the embodiment of  FIGS. 4A through 4D , the array of toroidal channels  150  on the cover plate  106  are confined within an annular zone between the hollow outer grounded cylinder  234  and the hollow RF drive cylinder  232 . Each of the toroidal channels  150  in the embodiment of  FIGS. 4A through 4D  may be of the structure described above with reference to  FIGS. 1A-1D , and may have any of the cross-sectional shapes of  FIGS. 1E-1H . 
     As depicted in  FIG. 4C , the gas supply conduit  122  and the coolant supply conduits  124  extend upwardly from the cover plate  106  through the interior of the hollow inner grounded cylinder  231  to the gas supply  123  and the coolant supply  125 , respectively. The interior of the hollow inner grounded cylinder  231  may be a field-free region.  FIG. 4D  depicts an embodiment employing four toroidal channels (solid line), which optionally may be supplemented by four additional toroidal channels (dashed line). Any suitable number of toroidal channels may be employed. 
       FIGS. 5A and 5B  depict an embodiment with a multiple zone folded coaxial resonator  300  overlying the cover plate  106 . The multiple zone coaxial resonator  300  includes an inner return cylinder  310 , an intermediate return cylinder  315 , an outer return cylinder  320  and a disk-shaped cap  322 . The inner return cylinder  310  and outer return cylinder  320  extend from the disk-shaped cap  322  to the cover plate  106 . The intermediate return cylinder  315  extends downwardly from the cap  322  and has a bottom edge  315   a  separated from the cover plate  106  by a gap G1. An inner zone driven cylinder  330  is surrounded by the intermediate return cylinder  315 . The inner zone driven cylinder  330  has a top edge  330   a  separated from the disk-shaped cap  322  by a gap G2. An outer zone driven cylinder  335  surrounds the intermediate return cylinder  315 . The outer zone driven cylinder  335  has a top edge  335   a  separated from the disk-shaped cap  322  by a gap G3. In the illustrated embodiment, the gaps G2 and G3 are of different sizes for ease of illustration, although in general they may be of the same size. 
     The inner zone driven cylinder  330  is coupled at its top edge  330   a  to an inner zone RF generator  350  through RF feed conductors  360  surrounded by shielding  365  contacting the disk-shaped cap  322 . The outer zone driven cylinder  335  is coupled at its top edge to an outer zone RF generator  355  through RF feed conductors  370  surrounded by shielding  375  contacting the disk-shaped cap  322 . A controller  337  governs the ratio between the RF output power levels of the inner zone RF generator  350  and the outer zone RF generator  355 . The controller  337  controls the radial distribution of plasma ion density among the inner and outer zones of the chamber  100  coinciding with the inner zone driven cylinder  330  and the outer zone driven cylinder  335 . 
     As depicted in  FIGS. 5A and 5B , the RF feed conductor  360  contacts the top edge  330   a  at plural uniformly spaced points  331 , while the RF feed conductor  370  contacts the top edge  335   a  at plural uniformly spaced points  336 . 
     As shown in  FIG. 5B , an inner annular zone  380  of the cover plate  106  supports toroidal channels  150 - 1  through  150 - 4 , while an outer annular zone  385  of the cover plate  106  supports toroidal channels  150 - 5  through  150 - 8 . In the illustrated embodiment there are four uniformly spaced toroidal channels in each zone  380 ,  385 . Any other suitable number of toroidal channels may be provided in each zone. For example,  FIG. 5B  depicts in dashed line the optional inclusion of four additional toroidal channels in the outer zone  385 . Each of the toroidal channels  150 - 1  through  150 - 8  may be of the structure described above with reference to  FIGS. 1A-1D . In the illustrated embodiment, the inner zone  380  lies between the inner return cylinder  310  and the intermediate return cylinder  315 , while the outer zone  385  lies between the outer return cylinder  320  and the intermediate return cylinder  315 . 
     As in the embodiment of  FIGS. 1A-1D , in  FIG. 5A  a gas injection plate  116  on the bottom surface of the cover plate  106  includes an internal gas manifold  118  having an array of gas injection orifices  120  facing the workpiece support surface  112 . A gas supply conduit  122  coupled to the internal gas manifold  118  extends upwardly from the gas injection plate  116 . A pair of coolant circulation conduits  124  extend to internal coolant circulation passages  126  within the cover plate  106 . The gas supply conduit  122  extends through the interior of the inner return cylinder  310  to an external gas supply. The interior of the inner return cylinder  310  may be a field-free region. A pair of coolant circulation conduits  124  extend through the interior of the inner return cylinder  310  from an external coolant supply, to coolant passages  126  within the cover plate  106 . 
     Embodiments may be employed for sequential processing, in which the gas distribution plate  118  of  FIG. 1  is divided into four separate sections (e.g., quadrants) corresponding to the four toroidal channels  150  of  FIG. 1 . Each quadrant of the gas distribution plate is supplied with a different process gas, so that each toroidal channel  150  provides a plasma of different species. The workpiece support surface  112  may be rotatable, so that different sections (e.g., quadrants) of the workpiece are exposed to the different plasmas at different times. While such sequential processing is described here with reference to an equal number of toroidal channels and sections of the gas distribution plate  118  in which the number is four, any other suitable number of toroidal channels and gas distribution plate sections may be employed. 
     While the foregoing embodiments have been described with reference to a coaxial resonator ( 130 ,  230  or  300 ) having an effective length corresponding to a wavelength at the RF power generator frequency, it is not required that the generator wavelength exactly match the coaxial resonator length. If the RF power generator wavelength differs from the coaxial resonator length, then an impedance matching function performed by the coaxial resonator  130 ,  230  or  300  compensates for the difference. 
     Each of the embodiments described can provide one or more of the following characteristics: ability to generate a high density plasma with minimum capacitive effects, which minimizes plasma ion energy at metal surfaces adjacent the plasma sheath; a grounded conductive chamber ceiling, to which process gases and coolant flow may be provided through a field-free region, and which provides a uniform RF ground reference for an optional RF bias power generator; and, immunity from influence by chamber grounds, because the plasma current closes a current loop on its own. 
     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.