Abstract:
A plasma chamber enclosure structure for use in an RF plasma reactor. The plasma chamber enclosure structure being a single-wall dielectric enclosure structure of an inverted cup-shape configuration and having ceiling with an interior surface of substantially flat conical configuration extending to a centrally located gas inlet. The plasma chamber enclosure structure having a sidewall with a lower cylindrical portion generally transverse to a pedestal when positioned over a reactor base, and a transitional portion between the lower cylindrical portion and the ceiling. The transitional portion extends inwardly from the lower cylindrical portion and includes a radius of curvature. The structure being adapted to cover the base to comprise the RF plasma reactor and to define a plasma-processing volume over the pedestal. The structure being formed of a dielectric material of silicon, silicon carbide, quartz, and/or alumina being capable of transmitting inductive power therethrough from an adjacent antenna.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of Ser. No. 09/970,121 filed Oct. 2, 2001 by Collins et al., herein incorporated by reference, which is a divisional of Ser. No. 09/675,319 filed Sep. 29, 2000 by Kenneth S. Collins et al., herein incorporated by reference, which is a divisional of Ser. No. 08/648,254 filed on May 13, 1996 by Kenneth S. Collins et al., herein incorporated by reference, which is a continuation-in-part of the following co-pending U.S. applications, the disclosures of which are incorporated herein by reference:  
         [0002]    (a) Ser. No. 08/580,026 filed Dec. 20, 1995 by Kenneth S. Collins et al. which is a continuation of Ser. No. 08/041,796 filed Apr. 1, 1993 by Kenneth S. Collins et al. now U.S. Pat. No. 5,556,501 issued Sep. 17, 1996, which is a continuation of Ser. No. 07/772,340 filed Jun. 27, 1991;  
         [0003]    (b) Ser. No. 08/503,467 filed Jul. 18, 1995 by Michael Rice et al. which is a divisional of Ser. No. 08/138,060 filed Oct. 15, 1993, U.S. Pat. No. 5,477,975 issued Dec. 26, 1995; and  
         [0004]    (c) Ser. No. 08/597,577 filed Feb. 2, 1996 by Kenneth Collins, issued as U.S. Pat. No. 6,077,384 on Jun. 20, 2000, which is a continuation-in-part of Ser. No. 08/521,668 filed Aug. 31, 1995 (now abandoned), which is a continuation-in-part of Ser. No. 08/289,336 filed Aug. 11, 1994, which is a continuation of Ser. No. 07/984,045 filed Dec. 1, 1992 (now abandoned). In addition, U.S. application Ser. No. 08/648,256 filed May 13, 1996 by Kenneth S. Collins et al. entitled “Plasma Reactor With Heated Source of a Polymer-Hardening Precursor Material” discloses related subject matter. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0005]    1. Technical Field  
           [0006]    The invention is related to inductively coupled RF plasma reactors of the type having a reactor chamber ceiling overlying a workpiece being processed and an inductive coil antenna adjacent the ceiling.  
           [0007]    2. Background Art  
           [0008]    Inductively coupled RF plasma reactors are employed to perform a variety of processes on workpieces such as semiconductor wafers. Referring to FIG. 1, one type of inductively coupled RF plasma reactor has a reactor chamber  10  including a ceiling  12  and a cylindrical side wall  14 . A pedestal  16  supports the workpiece  18 , such as a semiconductor wafer, so that the workpiece generally lies in a workpiece support plane, and a bias RF power generator is coupled to the pedestal  16 . A generally planar coil antenna  20  overlies the ceiling  12  and is coupled to a plasma source RF power generator  22 . A chief advantage of inductively coupled RF plasma reactors over other types such as capacitively coupled ones, is that a higher ion density can be achieved with the inductively coupled type.  
           [0009]    Adequate etch selectivity is achieved by operating at higher chamber pressure. (The term etch selectivity refers to the ratio of etch rates of two different materials exposed to etching in the reactor.) This is because the polymerization processes typically employed in a high density plasma etch reactor to protect underlying non-oxygen-containing (e.g., silicon, polysilicon or photoresist) layers during etching of an overlying oxygen-containing (e.g., silicon dioxide) layer are more efficient at higher chamber pressures (e.g., above about 20-500 mT) than at lower pressures. Polymer precursor gases (e.g., fluorocarbon or fluorohydrocarbon gases) in the chamber tend to polymerize strongly on non-oxygen-containing surfaces (such as silicon or photoresist), particularly at higher chamber pressures, and only weakly on oxygen-containing surfaces (such as silicon dioxide), so that the non-oxygen-containing surfaces are relatively well-protected from etching while oxygen-containing surfaces (such as silicon dioxide) are relatively unprotected and are etched. Such a polymerization process enhances the oxide-to-silicon etch selectivity better at higher chamber pressures because the polymerization rate is higher at higher pressures such as 100 mT. Therefore, it is desireable to operate at a relatively high chamber pressure when plasma-etching oxygen-containing layers over non-oxygen-containing layers. For example, under certain operating conditions such as a chamber pressure of 5 mT, an oxide-to-photoresist etch selectivity of less than 3:1 was obtained, and raising the pressure to the 50-100 mT range increased the selectivity to over 6:1. The oxide-to-polysilicon etch selectivity exhibited a similar behavior.  
           [0010]    The problem with increasing the chamber pressure (in order to increase etch selectivity) is that plasma ion spatial density distribution across the wafer surface becomes less uniform. There are two reasons this occurs: (1) the electron mean free path in the plasma decreases with pressure; and (2) the inductive field skin depth in the plasma increases with pressure. How these two factors affect plasma ion spatial density distribution will now be explained.  
           [0011]    With regard to item  1  above, the electron-to-neutral species elastic collision mean free path length, which is inversely proportional to chamber pressure, determines the extent to which electrons can avoid recombination with other gas particles and diffuse through the plasma to produce a more uniform electron and ion distribution in the chamber. Typically, electrons are not generated uniformly throughout the chamber (due, for example, to a non-uniform inductive antenna pattern) and electron diffusion through the plasma compensates for this and provides greater electron and plasma ion spatial density distribution uniformity. (Electron spatial density distribution across the wafer surface directly affects plasma ion spatial density distribution because plasma ions are produced by collisions of process gas particles with energetic electrons.) Increasing chamber pressure suppresses electron diffusion in the plasma, thereby reducing (degrading) plasma ion spatial density distribution uniformity.  
           [0012]    This problem may be understood by reference to FIG. 1, in which the inductive antenna  20 , due to its circular symmetry, has an antenna pattern (i.e., a spatial distribution of the magnitude of the induced electric field) with a null or local minimum along the antenna axis of symmetry so that very few if any electrons are produced over the wafer center. At low chamber pressures, electron diffusion into the space (“gap”) between the antenna  20  and the workpiece  18  is sufficient to transport electrons into the region near the wafer center despite the lack of electron production in that region, thereby providing a more uniform plasma distribution at the wafer surface. With increasing pressure, electron diffusion decreases and so plasma ion distribution becomes less uniform.  
           [0013]    A related problem is that the overall plasma density is greater near the ceiling  12  (where the density of hot electrons is greatest) than at the workpiece  18 , and falls off more rapidly away from the ceiling  12  as chamber pressure is increased. For example, the electron mean free path in an argon plasma with a mean electron temperature of 5 eV at a chamber pressure of 1 mT is on the order of 10 cm, at 10 mT it is 1.0 cm and at 100 mT it is 0.1 cm. Thus in a typical application, for a 5 cm ceiling-to-workpiece gap, most of the electrons generated near the ceiling  12  reach the workpiece at a chamber pressure of 1 mT (for a maximum ion density at the workpiece), and a significant number at 10 mT, while at 100 mT few do (for a minimal ion density at the workpiece). Accordingly, it may be said that a high pressure regime is one in which the mean free path length is about 1/10 or more of the ceiling-to-workpiece gap. One way of increasing the overall plasma ion density at the workpiece  18  (in order to increase etch rate and reactor throughput) without decreasing the chamber pressure is to narrow the gap so that the mean free path length becomes a greater fraction of the gap. However, this exacerbates other problems created by increasing chamber pressure, as will be described further below.  
           [0014]    With regard to item (2) above, the inductive field skin depth corresponds to the depth through the plasma—measured downward from the ceiling  12 —within which the inductive field of the antenna  20  is nearly completely absorbed. FIG. 2 illustrates how skin depth in an argon plasma increases with chamber pressure above a threshold pressure of about 0.003 mT (below which the skin depth is virtually constant over pressure). FIG. 2 also illustrates in the dashed-line curve how electron-to-neutral elastic collision mean free path length decreases linearly with increasing pressure. The skin depth function graphed in FIG. 2 assumes a source frequency of 2 MHz and an argon plasma density of 5□10 17  electrons/m 3 . (It should be noted that the corresponding plasma density for an electro-negative gas is less, so that the curve of FIG. 2 would be shifted upward with the introduction of an electro-negative gas.) The graph of FIG. 2 was derived using a collision cross-section for an electron temperature of 5 eV in argon. (It should be noted that with a molecular gas such as C 2 F 6  instead of argon, the collision cross-section is greater so that the skin depth is greater at a given pressure and the entire curve of FIG. 2 is shifted upward.) If the chamber pressure is such that the inductive field is absorbed within a small fraction—e.g., {fraction (1/10)}th—of the ceiling-to-workpiece gap adjacent the ceiling  12  (corresponding to a pressure of 1 mT for a 5 cm gap in the example of FIG. 2), then electron diffusion—throughout the remaining {fraction (9/10)}ths of the gap—produces a more uniform plasma ion distribution at the workpiece surface. However, as pressure increases and skin depth increases—e.g., beyond about {fraction (1/10)}th of the gap, then electron diffusion tends to have less effect. Thus, a measure of a high skin depth regime is that in which the skin depth is at about {fraction (1/10)} or more of the source-to-workpiece gap length. For example, if the pressure is so great that skin depth equals the ceiling-to-workpiece spacing (corresponding to a pressure of about 100 mT for a 5 cm gap in the example of FIG. 2), then any antenna pattern null or local minimum extends to the surface of the workpiece  18 , effectively preventing electron diffusion from compensating for the effects of the antenna pattern null on the processing of the workpiece. Such problems can arise, for example, when the ceiling-to-workpiece spacing is decreased in order to increase overall plasma density at the workpiece surface. A related problem with a small ceiling-to-workpiece spacing and a high chamber pressure is that electrons are lost not only to recombination with particles in the processing gas but are also lost to recombination by collisions with the surface of the ceiling  12  and the workpiece  18 , so that it is even more difficult for electrons generated in other regions to diffuse into the region adjacent the workpiece center.  
           [0015]    In summary, plasma ion density at the wafer can be enhanced by reducing the gap between the axially symmetrical antenna/ceiling  20 ,  12  and the workpiece  18 . But if the gap is reduced so much that the inductive field skin depth becomes a substantial fraction (≧10%) of the gap, then ion density at the workpiece center falls off significantly relative to the edge due to the antenna pattern&#39;s center null. However, for a smaller fraction of skin depth over gap and sufficient electron diffusion (characteristic of a low chamber pressure), electrons produced far from the workpiece center may diffuse into the center region before being lost to gas phase recombination or surface recombination, thereby compensating for the antenna pattern&#39;s center null. But as the gap is reduced (to increase overall plasma density at the workpiece) and chamber pressure is increased (to enhance etch selectivity), then: (1) the induced electric field over the workpiece center approaches a null so that no electrons are produced in that region, and (2) electrons produced in other regions generally cannot diffuse to the workpiece center region due to recombination with gas particles and chamber (e.g., ceiling) surfaces.  
           [0016]    Thus, as the wafer-to-coil distance is decreased by the reactor designer (in order to enhance plasma density near the wafer surface, for example), the plasma ion density decreases at the wafer center and ultimately, at very short wafer-to-antenna distances, becomes a center null giving rise to an unacceptable process non-uniformity. For example, in a plasma etch process carried out in such a reactor, the etch rate at the wafer center may be so much less than elsewhere that it becomes impossible to perform a complete etch across the entire wafer surface without over-etching near the wafer periphery. Conversely, it becomes impossible to avoid over-etching at the wafer periphery without under-etching the wafer center. Thus, the problem is to find a way to decrease the wafer-to-antenna distance without incurring a concomitant penalty in process non-uniformity.  
           [0017]    One approach for solving or at least ameliorating this problem is disclosed in U.S. application Ser. No. 08/507,726 filed Jul. 26, 1995 by Kenneth S. Collins et al. and entitled “Plasma Source with an Electronically Variable Density Profile”, which discloses that an outer generally planar coil antenna  24  coupled to a second independently controlled plasma source RF power generator  26  can be provided over the ceiling  12  concentric with the inner coil antenna  20  of FIG. 1. The efficacy of this solution can be seen from the graphs of FIGS. 3A through 3E. FIG. 3A illustrates the plasma ion density as a function of radius from the center of the workpiece  18  for a workpiece-to-ceiling height of 4 inches (10 cm), the curve labelled A being the ion density produced by the outer coil antenna  24  and the curve labelled B being the ion density produced by the inner coil antenna  20 . The total resulting plasma ion density is the sum of these two curves but is not depicted in the drawing for the sake of simplicity. FIG. 3A shows that at a height of 4 inches (10 cm), the outer coil antenna  24  produces a uniform plasma ion density distribution, the inner coil antenna  20  not being required. FIG. 3B corresponds to FIG. 3A for a reduced workpiece-to-ceiling height of 3 inches (7.5 cm), and shows that a dip in plasma ion density produced by the outer coil antenna  24  is compensated by the center-dominated ion density produced by the inner coil antenna  20 . FIG. 3C corresponds to FIG. 3A for a further reduced workpiece-to-ceiling height of 2.5 inches (6.25 cm), and shows that the compensation by the inner coil  20  for the center dip in the plasma ion density produced by the outer coil  24  remains fairly effective as the workpiece-to-ceiling height is further reduced, although a slight dip in the total resulting plasma ion density near the center would begin to appear below this height. As shown in FIG. 3D, a further reduction in workpiece-to-ceiling height to only 1.25 inches (about 3.2 cm) yields a pronounced dip in the plasma ion densities produced by both the inner and outer coil antennas  20 ,  24 , so that there is very little compensation and the resulting plasma ion density (the sum of the two curves shown) is highly non-uniform. As shown in FIG. 3E, the problem worsens as the height is further reduced to 0.8 inches (2 cm).  
           [0018]    What FIGS.  3 A- 3 E show is that even the use of inner and outer coil antennas to solve the problem of the null in plasma ion density near the workpiece center may lose effectiveness as the workpiece-to-ceiling height is reduced below certain values. Thus, the wafer-to-ceiling height cannot be reduced below a factor of the skin depth without sacrificing process uniformity. On the other hand, unless the wafer-to-ceiling height can be so reduced, plasma density and process performance is limited. Accordingly, there is a need for a way to reduce the workpiece-to-ceiling height without sacrificing process uniformity.  
         SUMMARY  
         [0019]    A plasma chamber enclosure structure for use in an RF plasma reactor which includes a pedestal adapted to support a workpiece to be processed, a reactor base housing the pedestal, and a coil antenna adjacent the reactor and which is adapted to inductively couple RF power into the reactor. The plasma chamber enclosure structure being a single-wall dielectric enclosure structure of an inverted cup-shape configuration. The plasma chamber enclosure structure having a ceiling with an interior surface of substantially flat conical configuration extending to a centrally located gas inlet such that when positioned over the base said interior surface is more distant from the pedestal over a center of the pedestal and closer to the pedestal over a periphery of the pedestal.  
           [0020]    The plasma chamber enclosure having a sidewall with a lower cylindrical portion generally transverse to the pedestal when positioned over the base and a transitional portion between the lower cylindrical portion and the ceiling. The transitional portion extends inwardly from the lower cylindrical portion and includes a radius of curvature.  
           [0021]    The plasma chamber enclosure structure being adapted to cover the reactor base to define a plasma-processing volume over the pedestal and comprise the RF plasma reactor. The plasma chamber enclosure structure being capable of transmitting inductive power therethrough from an adjacent antenna. The plasma chamber enclosure structure being formed of a dielectric material of silicon, silicon carbide, quartz, and/or alumina. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a cut-away side view of an inductively coupled plasma reactor of the type employed in a co-pending U.S. patent application referred to above employing generally planar coil antennas.  
         [0023]    [0023]FIG. 2 is a log-log scale graph of induction field skin depth in a plasma in cm (solid line) and of electron-to-neutral elastic collision mean free path length (dashed line) as functions of pressure in torr (horizontal axis).  
         [0024]    [0024]FIG. 3A is a graph of plasma ion density as a function of radial position relative to the workpiece center in the reactor of FIG. 1 for a workpiece-to-ceiling height of 4 inches, the curves labelled A and B corresponding to plasma ion densities produced by outer and inner coil antennas respectively.  
         [0025]    [0025]FIG. 3B is a graph of plasma ion density as a function of radial position relative to the workpiece center in the reactor of FIG. 1 for a workpiece-to-ceiling height of 3 inches, the curves labelled A and B corresponding to plasma ion densities produced by outer and inner coil antennas respectively.  
         [0026]    [0026]FIG. 3C is a graph of plasma ion density as a function of radial position relative to the workpiece center in the reactor of FIG. 1 for a workpiece-to-ceiling height of 2.5 inches, the curves labelled A and B corresponding to plasma ion densities produced by outer and inner coil antennas respectively.  
         [0027]    [0027]FIG. 3D is a graph of plasma ion density as a function of radial position relative to the workpiece center in the reactor of FIG. 1 for a workpiece-to-ceiling height of 1.25 inches, the curves labelled A and B corresponding to plasma ion densities produced by outer and inner coil antennas respectively.  
         [0028]    [0028]FIG. 3E is a graph of plasma ion density as a function of radial position relative to the workpiece center in the reactor of FIG. 1 for a workpiece-to-ceiling height of 0.8 inches, the curves labelled A and B corresponding to plasma ion densities produced by outer and inner coil antennas respectively.  
         [0029]    [0029]FIG. 4A is a cut-away side view of a plasma reactor in accordance with an alternative embodiment of the invention employing a single three-dimensional center non-planar solenoid winding.  
         [0030]    [0030]FIG. 4B is an enlarged view of a portion of the reactor of FIG. 4A illustrating a preferred way of winding the solenoidal winding.  
         [0031]    [0031]FIG. 4C is a cut-away side view of a plasma reactor corresponding to FIG. 4A but having a dome-shaped ceiling.  
         [0032]    [0032]FIG. 4D is a cut-away side view of a plasma reactor corresponding to FIG. 4A but having a conical ceiling.  
         [0033]    [0033]FIG. 4E is a cut-away side view of a plasma reactor corresponding to FIG. 4D but having a truncated conical ceiling.  
         [0034]    [0034]FIG. 5 is a cut-away side view of a plasma reactor in accordance with the preferred embodiment of the invention employing inner and outer vertical solenoid windings.  
         [0035]    [0035]FIG. 6 is a cut-away side view of a plasma reactor in accordance with a second alternative embodiment of the invention corresponding to FIG. 5 in which the outer winding is flat.  
         [0036]    [0036]FIG. 7A is a cut-away side view of a plasma reactor in accordance with a third alternative embodiment of the invention corresponding to FIG. 4A in which the center solenoid winding consists of plural upright cylindrical windings.  
         [0037]    [0037]FIG. 7B is a detailed view of a first implementation of the embodiment of FIG. 7A.  
         [0038]    [0038]FIG. 7C is a detailed view of a second implementation of the embodiment of FIG. 7A.  
         [0039]    [0039]FIG. 8 is a cut-away side view of a plasma reactor in accordance with a fourth alternative embodiment of the invention corresponding to FIG. 5 in which both the inner and outer windings consist of plural upright cylindrical windings.  
         [0040]    [0040]FIG. 9 is a cut-away side view of a plasma reactor in accordance with a fifth alternative embodiment of the invention corresponding to FIG. 5 in which the inner winding consists of plural upright cylindrical windings and the outer winding consists of a single upright cylindrical winding.  
         [0041]    [0041]FIG. 10 is a cut-away side view of a plasma reactor in accordance with a sixth alternative embodiment of the invention in which a single solenoid winding is placed at an optimum radial position for maximum plasma ion density uniformity.  
         [0042]    [0042]FIG. 11 is a cut-away side view of a plasma reactor in accordance with a seventh alternative embodiment of the invention corresponding to FIG. 4A in which the solenoid winding is an inverted conical shape.  
         [0043]    [0043]FIG. 12 is a cut-away side view of a plasma reactor in accordance with an eighth alternative embodiment of the invention corresponding to FIG. 4A in which the solenoid winding is an upright conical shape.  
         [0044]    [0044]FIG. 13 is a cut-away side view of a plasma reactor in accordance with a ninth alternative embodiment of the invention corresponding to FIG. 4A in which the solenoid winding consists of an inner upright cylindrical portion and an outer flat portion.  
         [0045]    [0045]FIG. 14 is a cut-away side view of a plasma reactor in accordance with a tenth alternative embodiment of the invention corresponding to FIG. 10 in which the solenoid winding includes both an inverted conical portion and a flat portion.  
         [0046]    [0046]FIG. 15 is a cut-away side view of a plasma reactor in accordance with an eleventh alternative embodiment of the invention corresponding to FIG. 12 in which the solenoid winding includes both an upright conical portion and a flat portion.  
         [0047]    [0047]FIG. 16 illustrates another embodiment of the invention employing a combination of planar, conical and dome-shaped ceiling elements.  
         [0048]    [0048]FIG. 17A illustrates an alternative embodiment of the invention employing a separately biased silicon side wall and ceiling and employing electrical heaters.  
         [0049]    [0049]FIG. 17B illustrates an alternative embodiment of the invention employing separately biased inner and outer silicon ceiling portions and employing electrical heaters. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]    In a plasma reactor having a small antenna-to-workpiece gap, in order to minimize the decrease in plasma ion density near the center region of the workpiece corresponding to the inductive antenna pattern center null, it is an object of the invention to increase the magnitude of the induced electric field at the center region. The invention accomplishes this by concentrating the turns of an inductive coil overlying the ceiling near the axis of symmetry of the antenna and maximizing the rate of change (at the RF source frequency) of magnetic flux linkage between the antenna and the plasma in that center region.  
         [0051]    In accordance with the invention, a solenoidal coil around the symmetry axis simultaneously concentrates its inductive coil turns near the axis and maximizes the rate of change of magnetic flux linkage between the antenna and the plasma in the center region adjacent the workpiece. This is because the number of turns is large and the coil radius is small, as required for strong flux linkage and close mutual coupling to the plasma in the center region. (In contrast, a conventional planar coil antenna spreads its inductive field over a wide radial area, pushing the radial power distribution outward toward the periphery.) As understood in this specification, a solenoid-like antenna is one which has plural inductive elements distributed in a non-planar manner relative to a plane of the workpiece or workpiece support surface or overlying chamber ceiling, or spaced at different distances transversely to the workpiece support plane (defined by a workpiece supporting pedestal within the chamber) or spaced at different distances transversely to an overlying chamber ceiling. As understood in this specification, an inductive element is a current-carrying element mutually coupled with the plasma in the chamber and/or with other inductive elements of the antenna.  
         [0052]    A preferred embodiment of the invention includes dual solenoidal coil antennas with one solenoid near the center and another one at an outer peripheral radius. The two solenoids may be driven at different RF frequencies or at the same frequency, in which case they are preferably phase-locked and more preferably phase-locked in such a manner that their fields constructively interact. The greatest practical displacement between the inner and outer solenoid is preferred because it provides the most versatile control of etch rate at the workpiece center relative to etch rate at the workpiece periphery. The skilled worker may readily vary RF power, chamber pressure and electro-negativity of the process gas mixture (by choosing the appropriate ratio of molecular and inert gases) to obtain a wider range or process window in which to optimize (using the present invention) the radial uniformity of the etch rate across the workpiece. Maximum spacing between the separate inner and outer solenoids of the preferred embodiment provides the following advantages:  
         [0053]    (1) maximum uniformity control and adjustment;  
         [0054]    (2) maximum isolation between the inner and outer solenoids, preventing interference of the field from one solenoid with that of the other; and  
         [0055]    (3) maximum space on the ceiling (between the inner and outer solenoids) for temperature control elements to optimize ceiling temperature control.  
         [0056]    [0056]FIG. 4A illustrates a single solenoid embodiment (not the preferred embodiment) of an inductively coupled RF plasma reactor having a short workpiece-to-ceiling gap, meaning that the skin depth of the induction field is on the order of the gap length. As understood in this specification, a skin depth which is on the order of the gap length is that which is within a factor of ten of (i.e., between about one tenth and about ten times) the gap length. FIG. 5 illustrates a dual solenoid embodiment of an inductively coupled RF plasma reactor, and is the preferred embodiment of the invention. Except for the dual solenoid feature, the reactor structure of the embodiments of FIGS. 4A and 5 is nearly the same, and will now be described with reference to FIG. 4A. The reactor includes a cylindrical chamber  40  similar to that of FIG. 1, except that the reactor of FIG. 4A has a non-planar coil antenna  42  whose windings  44  are closely concentrated in non-planar fashion near the antenna symmetry axis  46 . While in the illustrated embodiment the windings  44  are symmetrical and their symmetry axis  46  coincides with the center axis of the chamber, the invention may be carried out differently. For example, the windings may not be symmetrical and/or their axis of symmetry may not coincide. However, in the case of a symmetrical antenna, the antenna has a radiation pattern null near its symmetry axis  46  coinciding with the center of the chamber or the workpiece center. Close concentration of the windings  44  about the center axis  46  compensates for this null and is accomplished by vertically stacking the windings  44  in the manner of a solenoid so that they are each a minimum distance from the chamber center axis  46 . This increases the product of current (I) and coil turns (N) near the chamber center axis  46  where the plasma ion density has been the weakest for short workpiece-to-ceiling heights, as discussed above with reference to FIGS. 3D and 3E. As a result, the RF power applied to the non-planar coil antenna  42  produces greater induction [d/dt][N·I] at the wafer center—at the antenna symmetry axis  46 —(relative to the peripheral regions) and therefore produces greater plasma ion density in that region, so that the resulting plasma ion density is more nearly uniform despite the small workpiece-to-ceiling height. Thus, the invention provides a way for reducing the ceiling height for enhanced plasma process performance without sacrificing process uniformity.  
         [0057]    The drawing of FIG. 4B best shows a preferred implementation of the windings employed in the embodiments of FIGS. 4A and 5. In order that the windings  44  be at least nearly parallel to the plane of the workpiece  56 , they preferably are not wound in the usual manner of a helix but, instead, are preferably wound so that each individual turn is parallel to the (horizontal) plane of the workpiece  56  except at a step or transition  44   a  between turns (from one horizontal plane to the next).  
         [0058]    The cylindrical chamber  40  consists of a cylindrical side wall  50  and a circular ceiling  52  integrally formed with the side wall  50  so that the side wall  50  and ceiling  52  constitute a single piece of material, such as silicon. However, the invention may be carried out with the side wall  50  and ceiling  52  formed as separate pieces, as will be described later in this specification. The circular ceiling  52  may be of any suitable cross-sectional shape such as planar (FIG. 4A), dome (FIG. 4C), conical (FIG. 4D), truncated conical (FIG. 4E), cylindrical or any combination of such shapes or curve of rotation. Such a combination will be discussed later in this specification. Generally, the vertical pitch of the solenoid  42  (i.e., its vertical height divided by its horizontal width) exceeds the vertical pitch of the ceiling  52 , even for ceilings defining 3-dimensional surfaces such as dome, conical, truncated conical and so forth. The purpose for this, at least in the preferred embodiment, is to concentrate the induction of the antenna near the antenna symmetry axis, as discussed previously in this specification. A solenoid having a pitch exceeding that of the ceiling  52  is referred to herein as a non-conformal solenoid, meaning that, in general, its shape does not conform with the shape of the ceiling, and more specifically that its vertical pitch exceeds the vertical pitch of the ceiling. A 2-dimensional or flat ceiling has a vertical pitch of zero, while a 3-dimensional ceiling has a non-zero vertical pitch.  
         [0059]    A pedestal  54  at the bottom of the chamber  40  supports a planar workpiece  56  in a workpiece support plane during processing. The workpiece  56  is typically a semiconductor wafer and the workpiece support plane is generally the plane of the wafer or workpiece  56 . The chamber  40  is evacuated by a pump (not shown in the drawing) through an annular passage  58  to a pumping annulus  60  surrounding the lower portion of the chamber  40 . The interior of the pumping annulus may be lined with a replaceable metal liner  60   a . The annular passage  58  is defined by the bottom edge  50   a  of the cylindrical side wall  50  and a planar ring  62  surrounding the pedestal  54 . Process gas is furnished into the chamber  40  through any one or all of a variety of gas feeds. In order to control process gas flow near the workpiece center, a center gas feed  64   a  can extend downwardly through the center of the ceiling  52  toward the center of the workpiece  56  (or the center of the workpiece support plane). In order to control gas flow near the workpiece periphery (or near the periphery of the workpiece support plane), plural radial gas feeds  64   b , which can be controlled independently of the center gas feed  64   a , extend radially inwardly from the side wall  50  toward the workpiece periphery (or toward the workpiece support plane periphery), or base axial gas feeds  64   c  extend upwardly from near the pedestal  54  toward the workpiece periphery, or ceiling axial gas feeds  64   d  can extend downwardly from the ceiling  52  toward the workpiece periphery. Etch rates at the workpiece center and periphery can be adjusted independently relative to one another to achieve a more radially uniform etch rate distribution across the workpiece by controlling the process gas flow rates toward the workpiece center and periphery through, respectively, the center gas feed  64   a  and any one of the outer gas feeds  64   b - d . This feature of the invention can be carried out with the center gas feed  64   a  and only one of the peripheral gas feeds  64   b - d.    
         [0060]    The solenoidal coil antenna  42  is wound around a housing  66  surrounding the center gas feed  64   a . A plasma source RF power supply  68  is connected across the coil antenna  42  and a bias RF power supply  70  is connected to the pedestal  54 .  
         [0061]    Confinement of the overhead coil antenna  42  to the center region of the ceiling  52  leaves a large portion of the top surface of the ceiling  52  unoccupied and therefore available for direct contact with temperature control apparatus including, for example, plural radiant heaters  72  such as tungsten halogen lamps and a water-cooled cold plate  74  which may be formed of copper or aluminum for example, with coolant passages  74   a  extending therethrough. Preferably the coolant passages  74   a  contain a coolant of a known variety having a high thermal conductivity but a low electrical conductivity, to avoid electrically loading down the antenna or solenoid  42 . The cold plate  74  provides constant cooling of the ceiling  52  while the maximum power of the radiant heaters  72  is selected so as to be able to overwhelm, if necessary, the cooling by the cold plate  74 , facilitating responsive and stable temperature control of the ceiling  52 . The large ceiling area irradiated by the heaters  72  provides greater uniformity and efficiency of temperature control. (It should be noted that radiant heating is not necessarily required in carrying out the invention, and the skilled worker may choose to employ an electric heating element instead, as will be described later in this specification.) If the ceiling  52  is silicon, as disclosed in co-pending U.S. application Ser. No. 08/597,577 filed Feb. 2, 1996 by Kenneth S. Collins et al., then there is a significant advantage to be gained by thus increasing the uniformity and efficiency of the temperature control across the ceiling. Specifically, where a polymer precursor and etchant precursor process gas (e.g., a fluorocarbon gas) is employed and where the etchant (e.g., fluorine) must be scavenged, the rate of polymer deposition across the entire ceiling  52  and/or the rate at which the ceiling  52  furnishes a fluorine etchant scavenger material (silicon) into the plasma is better controlled by increasing the contact area of the ceiling  52  with the temperature control heater  72 . The solenoid antenna  42  increases the available contact area on the ceiling  52  because the solenoid windings  44  are concentrated at the center axis of the ceiling  52 .  
         [0062]    The increase in available area on the ceiling  52  for thermal contact is exploited in a preferred implementation by a highly thermally conductive torus  75  (formed of a ceramic such as aluminum nitride, aluminum oxide or silicon nitride or of a non-ceramic like silicon either lightly doped or undoped) whose bottom surface rests on the ceiling  52  and whose top surface supports the cold plate  74 . One feature of the torus  75  is that it displaces the cold plate  74  well-above the top of the solenoid  42 . This feature substantially mitigates or nearly eliminates the reduction in inductive coupling between the solenoid  42  and the plasma which would otherwise result from a close proximity of the conductive plane of the cold plate  74  to the solenoid  42 . In order to prevent such a reduction in inductive coupling, it is preferable that the distance between the cold plate  74  and the top winding of the solenoid  42  be at least a substantial fraction (e.g., one half) of the total height of the solenoid  42 . Plural axial holes  75   a  extending through the torus  75  are spaced along two concentric circles and hold the plural radiant heaters or lamps  72  and permit them to directly irradiate the ceiling  52 . For greatest lamp efficiency, the hole interior surface may be lined with a reflective (e.g., aluminum) layer. The center gas feed  64   a  of FIG. 4A may be replaced by a radiant heater  72  (as shown in FIG. 5), depending upon the particular reactor design and process conditions. The ceiling temperature is sensed by a sensor such as a thermocouple  76  extending through one of the holes  75   a  not occupied by a lamp heater  72 . For good thermal contact, a highly thermally conductive elastomer  73  such as silicone rubber impregnated with boron nitride is placed between the ceramic torus  75  and the copper cold plate  74  and between the ceramic torus  75  and the silicon ceiling  52 .  
         [0063]    As disclosed in the above-referenced co-pending application, the chamber  40  may be an all-semiconductor chamber, in which case the ceiling  52  and the side wall  50  are both a semiconductor material such as silicon. As described in the above-referenced co-pending application, controlling the temperature of, and RF bias power applied to, either the ceiling  52  or the wall  50  regulates the extent to which it furnishes fluorine scavenger precursor material (silicon) into the plasma or, alternatively, the extent to which it is coated with polymer. The material of the ceiling  52  is not limited to silicon but may be, in the alternative, silicon carbide, silicon dioxide (quartz), silicon nitride or a ceramic.  
         [0064]    As described in the above-referenced co-pending application, the chamber wall or ceiling  50 ,  52  need not be used as the source of a fluorine scavenger material. Instead, a disposable silicon member can be placed inside the chamber  40  and maintained at a sufficiently high temperature to prevent polymer condensation thereon and permit silicon material to be removed therefrom into the plasma as fluorine scavenging material. In this case, the wall  50  and ceiling  52  need not necessarily be silicon, or if they are silicon they may be maintained at a temperature (and/or RF bias) near or below the polymer condensation temperature (and/or a polymer condensation RF bias threshold) so that they are coated with polymer from the plasma so as to be protected from being consumed. While the disposable silicon member may take any appropriate form, in the embodiment of FIG. 4A the disposable silicon member is an annular ring  62  surrounding the pedestal  54 . Preferably, the annular ring  62  is high purity silicon and may be doped to alter its electrical or optical properties. In order to maintain the silicon ring  62  at a sufficient temperature to ensure its favorable participation in the plasma process (e.g., its contribution of silicon material into the plasma for fluorine scavenging), plural radiant (e.g., tungsten halogen lamp) heaters  77  arranged in a circle under the annular ring  62  heat the silicon ring  62  through a quartz window  78 . As described in the above-referenced co-pending application, the heaters  77  are controlled in accordance with the measured temperature of the silicon ring  62  sensed by a temperature sensor  79  which may be a remote sensor such as an optical pyrometer or a fluoro-optical probe. The sensor  79  may extend partially into a very deep hole  62   a  in the ring  62 , the deepness and narrowness of the hole tending at least partially to mask temperature-dependent variations in thermal emissivity of the silicon ring  62 , so that it behaves more like a gray-body radiator for more reliable temperature measurement.  
         [0065]    As described in U.S. application Ser. No. 08/597,577 referred to above, an advantage of an all-semiconductor chamber is that the plasma is free of contact with contaminant producing materials such as metal, for example. For this purpose, plasma confinement magnets  80 ,  82  adjacent the annular opening  58  prevent or reduce plasma flow into the pumping annulus  60 . To the extent any polymer precursor and/or active species succeeds in entering the pumping annulus  60 , any resulting polymer or contaminant deposits on the replaceable interior liner  60   a  may be prevented from re-entering the plasma chamber  40  by maintaining the liner  60   a  at a temperature significantly below the polymer condensation temperature, for example, as disclosed in the referenced co-pending application.  
         [0066]    A wafer slit valve  84  through the exterior wall of the pumping annulus  60  accommodates wafer ingress and egress. The annular opening  58  between the chamber  40  and pumping annulus  60  is larger adjacent the wafer slit valve  84  and smallest on the opposite side by virtue of a slant of the bottom edge  50   a  of the cylindrical side wall  50  so as to make the chamber pressure distribution more symmetrical with a non-symmetrical pump port location.  
         [0067]    Maximum inductance near the chamber center axis  46  is achieved by the vertically stacked solenoidal windings  44 . In the embodiment of FIG. 4A, another winding  45  outside of the vertical stack of windings  44  but in the horizontal plane of the bottom solenoidal winding  44   b  may be added, provided the additional winding  45  is close to the bottom solenoidal winding  44   b.    
         [0068]    Referring specifically now to the preferred dual solenoid embodiment of FIG. 5, a second outer vertical stack or solenoid  90  of windings  92  at an outer location (i.e, against the outer circumferential surface of the thermally conductive torus  75 ) is displaced by a radial distance □R from the inner vertical stack of solenoidal windings  44 . Note that in FIG. 5 confinement of the inner solenoidal antenna  42  to the center and the outer solenoidal antenna  90  to the periphery leaves a large portion of the top surface of the ceiling  52  available for direct contact with the temperature control apparatus  72 ,  74 ,  75 , as in FIG. 4A. An advantage is that the larger surface area contact between the ceiling  52  and the temperature control apparatus provides a more efficient and more uniform temperature control of the ceiling  52 .  
         [0069]    For a reactor in which the side wall  50  and ceiling  52  are formed of a single piece of silicon for example with an inside diameter of 12.6 in (32 cm), the wafer-to-ceiling gap is 3 in (7.5 cm), and the mean diameter of the inner solenoid was 3.75 in (9.3 cm) while the mean diameter of the outer solenoid was 11.75 in (29.3 cm) using {fraction (3/16)} in diameter hollow copper tubing covered with a 0.03 thick teflon insulation layer, each solenoid consisting of four turns and being 1 in (2.54 cm) high. The outer stack or solenoid  90  is energized by a second independently controllable plasma source RF power supply  96 . The purpose is to permit different user-selectable plasma source power levels to be applied at different radial locations relative to the workpiece or wafer  56  to permit compensation for known processing non-uniformities across the wafer surface, a significant advantage. In combination with the independently controllable center gas feed  64   a  and peripheral gas feeds  64   b - d , etch performance at the workpiece center may be adjusted relative to etch performance at the edge by adjusting the RF power applied to the inner solenoid  42  relative to that applied to the outer solenoid  90  and adjusting the gas flow rate through the center gas feed  64   a  relative to the flow rate through the outer gas feeds  64   b - d . While the present invention solves or at least ameliorates the problem of a center null or dip in the inductance field as described above, there may be other plasma processing non-uniformity problems, and these can be compensated in the versatile embodiment of FIG. 5 by adjusting the relative RF power levels applied to the inner and outer antennas  42 ,  90 . For effecting this purpose with greater convenience, the respective RF power supplies  68 ,  96  for the inner and outer solenoids  42 ,  90  may be replaced by a common power supply  97   a  and a power splitter  97   b  which permits the user to change the relative apportionment of power between the inner and outer solenoids  42 ,  90  while preserving a fixed phase relationship between the fields of the inner and outer solenoids  42 ,  90 . This is particularly important where the two solenoids  42 ,  90  receive RF power at the same frequency. Otherwise, if the two independent power supplies  68 ,  96  are employed, then they may be powered at different RF frequencies, in which case it is preferable to install RF filters at the output of each RF power supply  68 ,  96  to avoid off-frequency feedback from coupling between the should be sufficient to time-average out coupling between the two solenoids and, furthermore, should exceed the rejection bandwidth of the RF filters. Another option is to make each frequency independently resonantly matched to the respective solenoid, and each frequency may be varied to follow changes in the plasma impedance (thereby maintaining resonance) in lieu of conventional impedance matching techniques. In other words, the RF frequency applied to the antenna is made to follow the resonant frequency of the antenna as loaded by the impedance of the plasma in the chamber. In such implementations, the frequency ranges of the two solenoids should be mutually exclusive. Preferably, however, the two solenoids are driven at the same RF frequency and in this case it is preferable that the phase relationship between the two be such as to cause constructive interaction or superposition of the fields of the two solenoids. Generally, this requirement will be met by a zero phase angle between the signals applied to the two solenoids if they are both wound in the same sense. Otherwise, if they are oppositely wound, the phase angle is preferably 180°. In any case, coupling between the inner and outer solenoids can be minimized or eliminated by having a relatively large space between the inner and outer solenoids  42 ,  90 , as will be discussed below in this specification.  
         [0070]    The range attainable by such adjustments is increased by increasing the radius of the outer solenoid  90  to increase the spacing between the inner and outer solenoids  42 ,  90 , so that the effects of the two solenoids  42 ,  90  are more confined to the workpiece center and edge, respectively. This permits a greater range of control in respectively. This permits a greater range of control in superimposing the effects of the two solenoids  42 ,  90 . For example, the radius of the inner solenoid  42  should be no greater than about half the workpiece radius and preferably no more than about a third thereof. (The minimum radius of the inner solenoid  42  is affected in part by the diameter of the conductor forming the solenoid  42  and in part by the need to provide a finite non-zero circumference for an arcuate—e.g., circular—current path to produce inductance.) The radius of the outer coil  90  should be at least equal to the workpiece radius and preferably 1.5 or more times the workpiece radius. With such a configuration, the respective center and edge effects of the inner and outer solenoids  42 ,  90  are so pronounced that by increasing power to the inner solenoid the chamber pressure can be raised into the hundreds of mT while providing a uniform plasma, and by increasing power to the outer solenoid  90  the chamber pressure can be reduced to on the order of 0.01 mT while providing a uniform plasma. Another advantage of such a large radius of the outer solenoid  90  is that it minimizes coupling between the inner and outer solenoids  42 ,  90 .  
         [0071]    [0071]FIG. 5 indicates in dashed line that a third solenoid  94  may be added as an option, which is desireable for a very large chamber diameter.  
         [0072]    [0072]FIG. 6 illustrates a variation of the embodiment of FIG. 5 in which the outer solenoid  90  is replaced by a planar winding  100 .  
         [0073]    [0073]FIG. 7A illustrates a variation of the embodiment of FIG. 4A in which the center solenoidal winding includes not only the vertical stack  42  of windings  44  but in addition a second vertical stack  102  of windings  104  closely adjacent to the first stack  42  so that the two stacks constitute a double-wound solenoid  106 . Referring to FIG. 7B, the doubly wound solenoid  106  may consist of two independently wound single solenoids  42 ,  102 , the inner solenoid  42  consisting of the windings  44   a ,  44   b , and so forth and the outer solenoid  102  consisting of the winding  104   a ,  104   b  and so forth. Alternatively, referring to FIG. 7C, the doubly wound solenoid  106  may consist of vertically stacked pairs of at least nearly co-planar windings. In the alternative of FIG. 7C, each pair of nearly co-planar windings (e.g., the pair  44   a ,  104   a  or the pair  44   b ,  104   b ) may be formed by helically winding a single conductor. The term “doubly wound” used herein refers to winding of the type shown in either FIG. 7B or  7 C. In addition, the solenoid winding may not be merely doubly wound but may be triply wound or more and in general it can consists of plural windings at each plane along the axis of symmetry. Such multiple-wound solenoids may be employed in either one or both the inner and outer solenoids  42 ,  90  of the dual-solenoid embodiment of FIG. 5.  
         [0074]    [0074]FIG. 8 illustrates a variation of the embodiment of FIG. 7A in which an outer doubly wound solenoid  110  concentric with the inner doubly wound solenoid  106  is placed at a radial distance δR from the inner solenoid  106 .  
         [0075]    [0075]FIG. 9 illustrates a variation of the embodiment of FIG. 8 in which the outer doubly wound solenoid  110  is replaced by an ordinary outer solenoid  112  corresponding to the outer solenoid employed in the embodiment of FIG. 5.  
         [0076]    [0076]FIG. 10 illustrates another preferred embodiment in which the solenoid  42  of FIG. 5 is placed at a location displaced by a radial distance δR from the center gas feed housing  66 . In the embodiment of FIG. 4A, δR is zero while in the embodiment of FIG. 10 δR is a significant fraction of the radius of the cylindrical side wall  50 . Increasing δR to the extent illustrated in FIG. 10 may be helpful as an alternative to the embodiments of FIGS. 4A, 5,  7 A and  8  for compensating for non-uniformities in addition to the usual center dip in plasma ion density described with reference to FIGS. 3D and 3E. Similarly, the embodiment of FIG. 10 may be helpful where placing the solenoid  42  at the minimum distance from the chamber center axis  46  (as in FIG. 4) would so increase the plasma ion density near the center of the wafer  56  as to over-correct for the usual dip in plasma ion density near the center and create yet another non-uniformity in the plasma process behavior. In such a case, the embodiment of FIG. 10 is preferred where δR is selected to be an optimum value which provides the greatest uniformity in plasma ion density. Ideally in this case, δR is selected to avoid both under-correction and over-correction for the usual center dip in plasma ion density. The determination of the optimum value for δR can be carried out by the skilled worker by trial and error steps of placing the solenoid  42  at different radial locations and employing conventional techniques to determine the radial profile of the plasma ion density at each step.  
         [0077]    [0077]FIG. 11 illustrates an embodiment in which the solenoid  42  has an inverted conical shape while FIG. 12 illustrates an embodiment in which the solenoid  42  has an upright conical shape.  
         [0078]    [0078]FIG. 13 illustrates an embodiment in which the solenoid  42  is combined with a planar helical winding  120 . The planar helical winding has the effect of reducing the severity with which the solenoid winding  42  concentrates the induction field near the center of the workpiece by distributing some of the RF power somewhat away from the center. This feature may be useful in cases where it is necessary to avoid overcorrecting for the usual center null. The extent of such diversion of the induction field away from the center corresponds to the radius of the planar helical winding  120 . FIG. 14 illustrates a variation of the embodiment of FIG. 13 in which the solenoid  42  has an inverted conical shape as in FIG. 11. FIG. 15 illustrates another variation of the embodiment of FIG. 13 in which the solenoid  42  has an upright conical shape as in the embodiment of FIG. 12.  
         [0079]    The RF potential on the ceiling  52  may be increased, for example to prevent polymer deposition thereon, by reducing its effective capacitive electrode area relative to other electrodes of the chamber (e.g., the workpiece and the sidewalls). FIG. 16 illustrates how this can be accomplished by supporting a smaller-area version of the ceiling  52 ′ on an outer annulus  200 , from which the smaller-area ceiling  52 ′ is insulated. The annulus  200  may be formed of the same material (e.g., silicon) as the ceiling  52 ′ and may be of a truncated conical shape (indicated in dashed line) or a truncated dome shape (indicated in solid line). A separate RF power supply  205  may be connected to the annulus  200  to permit more workpiece center versus edge process adjustments.  
         [0080]    [0080]FIG. 17A illustrates a variation of the embodiment of FIG. 5 in which the ceiling  52  and side wall  50  are separate semiconductor (e.g., silicon) pieces insulated from one another having separately controlled RF bias power levels applied to them from respective RF sources  210 ,  212  to enhance control over the center etch rate and selectivity relative to the edge. As set forth in greater detail in above-referenced U.S. application Ser. No. 08/597,577 filed Feb. 2, 1996 by Kenneth S. Collins et al., the ceiling  52  may be a semiconductor (e.g., silicon) material doped so that it will act as an electrode capacitively coupling the RF bias power applied to it into the chamber  40  and simultaneously as a window through which RF power applied to the solenoid  42  may be inductively coupled into the chamber  40 . The advantage of such a window-electrode is that an RF potential may be established directly over the wafer  56  (e.g., for controlling ion energy) while at the same time inductively coupling RF power directly over the wafer  56 . This latter feature, in combination with the separately controlled inner and outer solenoids  42 ,  90  and center and peripheral gas feeds  64   a ,  64   b - d  greatly enhances the ability to adjust various plasma process parameters such as ion density, ion energy, etch rate and etch selectivity at the workpiece center relative to the workpiece edge to achieve an optimum uniformity. In this combination, the respective gas flow rates through individual gas feeds are individually and separately controlled to achieve such optimum uniformity of plasma process parameters.  
         [0081]    [0081]FIG. 17A illustrates how the lamp heaters  72  may be replaced by electric heating elements  72 ′. As in the embodiment of FIG. 4A, the disposable silicon member is an annular ring  62  surrounding the pedestal  54 .  
         [0082]    [0082]FIG. 17B illustrates another variation in which the ceiling  52  itself may be divided into an inner disk  52   a  and an outer annulus  52   b  electrically insulated from one another and separately biased by independent RF power sources  214 ,  216  which may be separate outputs of a single differentially controlled RF power source.  
         [0083]    In accordance with an alternative embodiment, a user-accessible central controller  300  shown in FIGS. 17A and 17B, such as a programmable electronic controller including, for example, a conventional microprocessor and memory, is connected to simultaneously control gas flow rates through the central gas feed  64   a  and the peripheral gas feeds  64   b - d , RF plasma source power levels applied to the inner and outer antennas  42 ,  90  and RF bias power levels applied to the ceiling  52  and side wall  50  respectively (in FIG. 17A) and the RF bias power levels applied to the inner and outer ceiling portions  52   a ,  52   b  (in FIG. 17B), temperature of the ceiling  52  and the temperature of the silicon ring  62 . A ceiling temperature controller  218  governs the power applied by a power source  220  to the heaters  72 ′ by comparing the temperature measured by the ceiling temperature sensor  76  with a desired temperature known to the controller  300 . A ring temperature controller  222  controls the power applied by a heater power source  224  to the heater lamps  77  facing the silicon ring  62  by comparing the ring temperature measured by the ring sensor  79  with a desired ring temperature stored known to the controller  222 . The master controller  300  governs the desired temperatures of the temperature controllers  218  and  222 , the RF power levels of the solenoid power sources  68 ,  96 , the RF power levels of the bias power sources  210 ,  212  (FIG. 17A) or  214 ,  216  (FIG. 17B), the wafer bias level applied by the RF power source  70  and the gas flow rates supplied by the various gas supplies (or separate valves) to the gas inlets  64   a - d . The key to controlling the wafer bias level is the RF potential difference between the wafer pedestal  54  and the ceiling  52 . Thus, either the pedestal RF power source  70  or the ceiling RF power source  210  may be simply a short to RF ground. With such a programmable integrated controller, the user can easily optimize apportionment of RF source power, RF bias power and gas flow rate between the workpiece center and periphery to achieve the greatest center-to-edge process uniformity across the surface of the workpiece (e.g., uniform radial distribution of etch rate and etch selectivity). Also, by adjusting (through the controller  300 ) the RF power applied to the solenoids  42 ,  90  relative to the RF power difference between the pedestal  54  and ceiling  52 , the user can operate the reactor in a predominantly inductively coupled mode or in a predominantly capacitively coupled mode.  
         [0084]    While the various power sources connected in FIG. 17A to the solenoids  42 ,  90 , the ceiling  52 , side wall  50  (or the inner and outer ceiling portions  52   a ,  52   b  as in FIG. 17B) have been described as operating at RF frequencies, the invention is not restricted to any particular range of frequencies, and frequencies other than RF may be selected by the skilled worker in carrying out the invention.  
         [0085]    In a preferred embodiment of the invention, the high thermal conductivity spacer  75 , the ceiling  52  and the side wall  50  are integrally formed together from a single piece of crystalline silicon.  
         [0086]    While the invention has been described as being carried out with a number of separate RF sources, some or all of the RF sources depicted herein may derive their outputs from separate RF generators or from a common RF generator with different outputs at different RF power levels, frequencies and phases synthesized with variable power dividers, frequency multipliers and/or phase delays, as may be appropriate. Moreover, while the invention has been described as being carried out with a number of separate process gas supplies, some or all of the process gas supplies may be derived from a common process gas supply which is divided among the plural separately controlled gas inlets  64 .  
         [0087]    While the invention has been described in detail by specific reference to preferred embodiments, it is understood that variations and modifications thereof may be made without departing from the true spirit and scope of the invention.