Abstract:
A coaxial VHF power coupler includes conductive element inside a hollow cylindrical outer conductor of the power coupler and surrounding an axial section of a hollow cylindrical inner conductor of the power coupler. Respective plural motor drives contacting the hollow cylindrical outer conductor are connected to respective locations of the movable conductive element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/370,188, filed Aug. 3, 2010 entitled SYMMETRIC VHF PLASMA POWER COUPLER WITH ACTIVE UNIFORMITY STEERING, by Kenneth S. Collins, et al. 
    
    
     BACKGROUND 
     The disclosure concerns a plasma reactor used in fabrication of microelectronic semiconductor circuits. Such a reactor includes a vacuum chamber for processing a workpiece, such as a semiconductor wafer, an RF power applicator and process gas injection apparatus. 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 as a cylinder so as to optimize uniformity of plasma ion distribution in a radial direction and about an azimuthal direction. As employed herein, the term azimuthal direction refers to a rotational direction around the axis of symmetry of the cylindrical chamber. 
     One problem is that the vacuum chamber itself includes non-symmetrical features that interrupt the cylindrical symmetry of the chamber and therefore create non-uniformities in plasma ion distribution. This may be due, for example, to the effect such asymmetrical features have on the electromagnetic environment of the plasma or on the gas flow distribution within the chamber, or both. Such non-symmetrical features may include a vacuum port in the floor of the chamber and a workpiece (wafer) slit port (“slit valve”) through which the wafer is inserted into and removed from the chamber. These features tend to produce non-uniformities in plasma ion distribution in the azimuthal direction, or azimuthal non-uniformities. 
     That is needed is a way of reducing or eliminating such azimuthal non-uniformities in plasma ion distribution. 
     SUMMARY 
     A plasma reactor includes a vacuum chamber enclosed by a cylindrical side wall, a floor and an overhead ceiling, the overhead ceiling including a ceiling electrode electrically insulated from the side wall. The reactor further includes a coaxial power coupler including: (a) a hollow cylindrical outer conductor having a bottom end coupled to the side wall and a top end, (b) a hollow cylindrical inner conductor coaxial with the outer conductor and having a bottom end coupled to the ceiling electrode and a top end, (c) a conductor electrically contacting the top ends of the inner and outer conductors, and (d) an elongate tap conductor electrically separate from and extending radially through the outer conductor and having a first end connected to the inner conductor and a second end for coupling to an RF power generator. The reactor further includes a movable conductive element inside the hollow cylindrical outer conductor and surrounding an axial section of the hollow cylindrical inner conductor, and respective plural motor drives contacting the hollow cylindrical outer conductor and connected to respective locations of the movable conductive element, the movable conductive element being electrically coupled to the outer conductor through the motor drives. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 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. 
         FIGS. 1 ,  2 A and  2 B depict a first embodiment. 
         FIG. 3  depicts a second embodiment. 
         FIGS. 4 and 5  depict a third embodiment. 
         FIG. 6  depicts a fourth embodiment. 
         FIGS. 7 and 8  depict a fifth embodiment. 
         FIG. 9  depicts a sixth embodiment. 
         FIG. 10  depicts a seventh embodiment. 
     
    
    
     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 
     Referring now to  FIG. 1 , 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 an RF-hot gas distribution plate or showerhead  120  that functions as both a gas distributor and as a ceiling electrode. The showerhead  120  is formed of a conductive material and is supported at its periphery by an insulating annular ring  125  engaged with the side wall  105 . The showerhead  120  receives plasma VHF source power and receives process gases in a manner to be described below. 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 cylindrical side wall  105  has a slit valve opening  107  that extends around only a portion of the circumference of the side wall  105 . The slit valve opening  107  provides ingress and egress to and from the chamber  100  for the workpiece  135 . 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 bottom end of the center conductor  165  may be coupled to respective RF bias power generators  175 ,  180  through an RF impedance match circuit  185 . 
     A transmission line coaxial structure  200 , which may be configured as a shorted coaxial tuning stub, has an RF-hot center conductor  205  and a grounded outer conductor  210 . The center and outer conductors  205 ,  210  may be hollow, although the center conductor  205  may be solid in one embodiment. The bottom end of the center conductor  205  is connected to the showerhead  120 . A shorting device  215  connects the top end of the center conductor  205  to the top end of the outer conductor  210 . While in the illustrated embodiment the shorting device may be a conductor touching the top ends of the center and outer conductors  205 ,  210 , it may be spaced from the top ends by a suitable distance, and may not necessarily be a conductor. In such embodiments, the shorting device  215  may provide capacitive coupling rather than a direct electrical connection. A VHF generator  220  supplying plasma source power is coupled to the center conductor  205  by a radially extending elongate tap conductor  222  at a selected axial location along the length of the center conductor  205 . This axial location may be selected in accordance with published techniques so that an impedance match is obtained at the frequency of the VHF generator  220 . In addition, a separate RF match circuit  225  may be connected between the VHF generator  220  and the tap conductor  222 . 
     The outer conductor  210  has a diameter less than that of the chamber side wall  105 . A conductive expansion section  230 , including a radially extending skirt  235  and an end piece  240 , connects the bottom end of the outer conductor  210  to the grounded chamber side wall  105 . 
     The center conductor  205  may be a hollow cylinder so as to provide a space for utility conduits to the showerhead  120  (e.g., for gas lines, coolant lines and sensors). For example, gas lines may extend through the center conductor  205  from a process gas supply and manifold  247  (which may include flow controllers) to internal gas flow passages (not shown) within the showerhead  120  terminating in gas injection orifices  248  in the bottom surface of the showerhead  120 . In order to accommodate connection of such utility lines in a radial zone of the showerhead  120  greater than the diameter of the center conductor  205 , a conductive expansion section  206  is coupled between the bottom end of the center conductor  205  and the showerhead  120 . 
     In alternative embodiments, process gas is not injected through the showerhead  120  but rather by other means, in which case the showerhead  120  need contain no gas injection orifices, and functions only as a ceiling electrode. 
     As discussed previously herein, non-symmetrical features of the chamber  100 , such as the slit valve opening  107  or the pumping port  117 , may induce azimuthal non-uniformities in plasma ion distribution. Reduction or elimination of such non-uniformities is provided by an active uniformity steering element. 
     In a first embodiment, an active uniformity steering element includes a conductive (or at least semiconductive) ring  250  surrounding a section of the center conductor  205 . Referring to  FIG. 2A , a pair of motor drives  255 ,  260  supported on the outer conductor  210  hold the conductive ring  250  and control its radial location relative to the center conductor  205 . In accordance with one embodiment, the motor drives  255 ,  260  control the non-concentricity of the ring  250  relative to the axis of symmetry of the center conductor  205 .  FIG. 2A  depicts an instance in which the ring  250  is concentric relative to the center conductor  205 , for minimal or no effect on plasma ion distribution. 
     A change in plasma ion distribution is effected by shifting the ring  250  to a non-concentric position relative to the center conductor  205 , as depicted in  FIG. 2B . As indicated in  FIG. 2B , with reference to a stationary X axis, the azimuthal angle A of the direction D of non-concentricity is selected in accordance with the desired change or correction in plasma ion density distribution. This may be selected to compensate for azimuthal (asymmetrical) non-uniformity in plasma ion distribution or etch rate distribution on a workpiece. As discussed above, such asymmetries in plasma ion distribution may be attributable to asymmetrical features of the chamber, such the slit valve opening  107  or the pumping port  117 . As indicated in  FIG. 2A , the motor drive  255  controls the location of the ring  250  in a radial direction along an X-axis, while the motor drive  260  controls the location of the ring  250  in a radial direction along a Y-axis. The ring  250  is electrically connected to the grounded outer conductor through the motor drives  255 ,  260 . Its proximity to the RF-hot center conductor  205  affects capacitive coupling to the center conductor  210 . Varying the location of the ring  250  along the X-axis and Y-axis ( FIGS. 2A and 2B ) varies the azimuthal distribution of capacitive coupling within the transmission line structure  200 , and therefore controls the azimuthal distribution of RF power on the showerhead  120 . By controlling the two motor drives  255 ,  260 , a particular azimuthal asymmetry in RF power distribution on the showerhead  120  may be selected that precisely compensates for azimuthal asymmetry in observed process rate distribution (or plasma ion distribution). Such compensation provides a more uniform distribution of process rate across the workpiece  135 . (Alternatively, the location of the ring  250  may be controlled to achieve a particular desired non-uniformity in process rate distribution that produces a desired effect on the workpiece.) 
     The motor drives  255  and  260  may be identical in structure, each including an electric motor  265  of a conventional type, an axially movable shaft  270  driven by the electric motor  265  having an outer end connected to the ring  250 , and a guide  275  fixed to the outer conductor  210  having an opening through which the axially movable shaft  270  extends. As illustrated in  FIG. 1 , the ring  250  may be configured to have a thin dimension in the radial direction and to have a broad surface in the axial direction of an axial length exceeding the radial dimension (or thickness). However, the cross-section shape of the ring  250  may be other than that depicted in the drawings. The shaft  270  and guide  275  may be conductive to ensure that the ring  250  is electrically grounded. A uniformity distribution controller  280  controls the motor drives  255 ,  260  in accordance with a desired azimuthal distribution of RF power on the showerhead or ceiling electrode  120 . 
       FIG. 3  depicts a modification of the embodiment of  FIG. 2A , in which the ring  250  is divided into 3 or more separate arc sections that are independently controlled by separate drive motors. In the illustrated embodiment, the ring  250  is quartered into four arc sections  250 - 1 ,  250 - 2 ,  250 - 3 ,  250 - 4 , so that each arc section sub-tends an angle of 90 degrees. The four arc sections  250 - 1 ,  250 - 2 ,  250 - 3 , and  250 - 4  are driven by four motor drives  255 ,  256 ,  258 ,  260 , respectively. The ring  250  may be divided into any suitable number of arc sections, and controlled with a corresponding number of motor drives. 
       FIGS. 4 and 5  depict a modification of the embodiment of  FIG. 2A  in which the location of the ring  250  and motor drives  255 ,  260  is shifted downwardly (toward the showerhead), while increasing the diameter of the ring  250  to accommodate the increased diameter of the expansion section  230 .  FIG. 6  depicts a modification of the embodiment of  FIG. 3  in which the plural arc sections  250 - 1 ,  250 - 2 ,  250 - 3 , etc., and the associated motor drives are shifted downwardly (toward the showerhead) similar to the modification depicted in  FIG. 5 . The effective diameter of the plural arc sections may be increased in the embodiment of  FIG. 6  from that of  FIG. 3  to accommodate the increased diameter of the expansion section  230 . 
       FIGS. 7 and 8  depict a modification of the embodiment of  FIG. 4  in which the ring  250  overlies the showerhead  120  and the motor drives  255 ,  260  are oriented to move the ring  250  in the axial (vertical) direction. Each motor drive  255 ,  260  extends through and contacts the expansion section  235  of the outer conductor, thereby coupling the ring  250  to ground. The proximity of different portions of the ring  250  to the showerhead  120  controls azimuthal distribution of RF power in the showerhead  120 . In the embodiment of  FIGS. 7 and 8 , the ring  250  may have a narrow dimension in the axial direction and a greater dimension in the radial direction, so as to present a broad surface to the showerhead  120 . 
       FIG. 9  depicts a modification of the embodiment of  FIG. 6  in which the arc sections  250 - 1  through  250 - 4  overlie the showerhead  120  and the motor drives  255 ,  256 ,  258 ,  260  are oriented to move the arc sections  250 - 1 ,  250 - 2 ,  250 - 3 ,  250 - 4  in the axial (vertical) direction. Each motor drive  255 ,  256 ,  258   260  extends through and contacts the expansion section  235  of the outer conductor  210 , thereby coupling each ring section to ground. The proximity of the different arc sections  250 - 1 ,  250 - 2 ,  250 - 3 , and  250 - 4  to the showerhead  120  controls azimuthal distribution of RF power in the showerhead  120 . In the embodiment of  FIG. 9 , each arc section  250 - 1 ,  250 - 2 ,  250 - 3 ,  250 - 4  may have a narrow dimension in the axial direction and a greater dimension in the radial direction, so as to present a broad surface to the showerhead  120 .  FIG. 10  depicts an embodiment having a movable outer magnet  400  surrounding the expansion section  230  below the outer conductor  210 . The outer magnet  400  is ring-shaped and any number of the motor drives  255 ,  256 ,  258 ,  260  may be employed to govern a tilt angle of the outer magnet  400  about any radial axis of rotation. Each motor drive  255 ,  256 ,  258 ,  260  may be supported on a support ring  410  mounted on the expansion section  230 . 
     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.