Abstract:
The invention is embodied by a plasma reactor for processing a workpiece, including a reactor enclosure defining a processing chamber, a semiconductor window, a base within the chamber for supporting the workpiece during processing thereof, a gas inlet system for admitting a plasma precursor gas into the chamber, and an inductive antenna adjacent a side of the semiconductor window opposite the base for coupling power into the interior of the chamber through the semiconductor window electrode.

Description:
This is a continuation, division, continuation -in-part, of application Ser. No. 08/521,668, filed Aug. 31, 1995, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to a plasma reactor having parallel plates for interposition therebetween of a workpiece to be processed, such as a semiconductor wafer, and an inductive coil antenna coupling RF power through one of the parallel plates into the interior of the reactor. 
     2. Background Art 
     Inductively coupled plasma reactors for processing microelectronic semiconductor wafers, such as the type of reactor disclosed in U.S. Pat. No. 4,948,458 to Ogle, enjoy important advantages over parallel-plate capacitively coupled plasma reactors. For example, inductively coupled plasma reactors achieve higher plasma ion densities (e.g., on the order of 10 11  ions/cm 3 ). Moreover, plasma ion density and plasma ion energy can be independently controlled in an inductively coupled plasma reactor by applying bias power to the workpiece or wafer. In contrast, capacitively coupled reactors typically provide relatively lower plasma ion densities (e.g., on the order of only 10 10  ions/cm 3 ) and generally cannot provide independent control of ion density and ion energy. The superior ion-to-neutral density ratio provided by an inductively coupled plasma etch reactor used to etch silicon dioxide, for example, provides superior performance at small etch geometries (e.g., below 0.5 micron feature size) including better etch anisotropy, etch profile and etch selectivity. In contrast, parallel plate capacitively coupled plasma reactors typically stop etching at feature sizes on the order of about 0.25 microns, or at least exhibit inferior etch selectivity and etch profile due to an inferior ion-to-neutral density ratio. 
     The inductively coupled plasma reactor disclosed in U.S. Pat. No. 4,948,458 referred to above has a planar coil overlying the chamber ceiling and facing the semiconductor wafer being processed, thereby providing an optimally uniform RF induction field over the surface of the wafer. For this purpose, the ceiling, which seals the reactor chamber so that it can be evacuated, must be fairly transmissive to the RF induction field from the coil and is therefore a dielectric, such as quartz. It should be noted here that such a ceiling could be made from dielectric materials other than quartz, such as aluminum oxide. However other materials such as aluminum oxide tend produce greater contamination than quartz due to sputtering. 
     An advantage of capacitively coupled plasma reactors is that the chamber volume can be greatly reduced by reducing the space between the parallel plate electrodes, thereby better confining or concentrating the plasma over the workpiece, while the reactor can be operated at relatively high chamber pressure (e.g., 200 mTorr). In contrast, inductively coupled plasma reactors require a larger volume due to the large skin depth of the RF induction field, and must be operated at a lower chamber pressure (e.g., 10 mTorr) to avoid loss of plasma ions due to recombination and higher pumping speed. In commercial embodiments of the inductively coupled reactor of U.S. Pat. No. 4,948,458 referred to above, the requirement of a large chamber volume is met by a fairly large area side wall. The lack of any other RF ground return (due to the requirement of a dielectric window to admit the RF induction field from the overhead coil) means that the chamber side wall should be conductive and act as the principal ground or RF return plane. However, the side wall is a poor ground plane, as it has many discontinuities, such as a slit valve for wafer ingress and egress, gas distribution ports or apparatus and so forth. Such discontinuities give rise to non-uniform current distribution, which distort plasma ion distribution relative to the wafer surface. The resulting sideways current flow toward the side wall contributes to non-uniform plasma ion distribution relative to the wafer surface. 
     One approach for combining capacitive and inductive coupling is to provide a side coil wound around the side wall of a parallel plate plasma reactor, as disclosed in European Patent Document Publication No. 0 520 519 A1 by Collins et al. For this purpose, the cylindrical chamber side wall must be a nonconductor such as quartz in order to admit the RF induction field of the side coil into the chamber. The main problem with this type of plasma reactor is that it is liable to exhibit processing non-uniformity across the wafer surface. For example, the etch rate is much greater at the wafer periphery and much slower at the wafer center, thereby constricting the process window. In fact, the etch process may actually stop near the wafer center while continuing at the wafer periphery. The disposition of the induction coil antenna along the side wall of the reactor chamber, the relatively short (e.g., 2 cm) skin depth (or depth within which most of the RF power is absorbed) toward the chamber center, and the introduction of the etch precursor gas into the reactor chamber from the side, confine most of the etchant ion and radical production to the vicinity of the chamber side wall or around the wafer periphery. The phrase “etchant ion and radical” as employed in this specification refers to the various chemical species that perform the etch reaction, including fluoro-carbon ions and radicals as well as fluoro-hydrocarbon ions and radicals. The population of free fluorine ions and radicals is preferably minimized by well-known techniques if a selective etch process is desired. Energetic electrons generated by the plasma source power interact with the process precursor gas and thereby produce the required etchant ions and radicals and, furthermore, produce molecular or atomic carbon necessary for polymerization employed in sophisticated etch processes. The etch process near the wafer center is dependent upon such energetic electrons traveling from the vicinity of the chamber side wall and reaching the wafer center before recombining along the way by collisions with neutral species or ions, so that the etch process is not uniform across the wafer surface. These problems are better understood in light of the role polymerization plays in the etch process. 
     Polymerization employing fluoro-carbon (C x F x ) or fluoro-hydrocarbon chemistry is employed in a typical silicon dioxide etch process, for example, to enhance etch anisotropy or profile and etch selectivity, as described in Bariya et al., “A Surface Kinetic Model for Plasma Polymerization with Application to Plasma Etching,”  Journal of the Electrochemical Society , Volume 137, No. 8 (August 1990), pp. 2575-2581 at page 1. An etch precursor gas such as a fluoro-carbon like C 2 F 6  or a fluoro-hydrocarbon introduced into the reactor chamber dissociates by inelastic collisions with energetic electrons in the plasma into etchant ions and radicals as well as carbon. As noted above, such etchant ions and radicals include fluoro-carbon or fluoro-hydrocarbon ions and radicals, for example, and free fluorine ions and radicals. The free fluorine ions and radicals are preferably minimized through scavenging, for example, if the etch process is to be selective with respect to a certain material such as polysilicon. The carbon and at least some of the fluoro-carbon or fluoro-hydrocarbon ions and radicals are polymer-forming. Also present in the plasma are excited neutrals or undissociated species and etch by-products. The polymer-forming radicals and carbon enhance etch profile as follows: By forming only on the side-walls of etch features (formation on the horizontal surfaces being prevented by the energetic downward ion flux from the plasma), polymers can block lateral etching and thereby produce anisotropic (narrow and deep) profiles. The polymer-forming ions and radicals also enhance silicon oxide etch selectivity because polymer generally does not form on the silicon oxide under favorable conditions but does form on silicon or other materials which are not to be etched but which may underlie a silicon oxide layer being etched. Thus, as soon as an overlying silicon oxide layer has completely etched through to expose an underlying polysilicon layer, the polymer-forming ions and radicals in the plasma that contact the exposed polysilicon layer immediately begin to form a polymer layer, inhibiting further etching. 
     Such polymerization during the etch process requires a careful balance of etchant and polymer, the etchant concentration typically being at a depletion level to avoid inhibition of appropriate polymer formation. As a result, a significant proportion of etchant ions and radicals formed near the wafer periphery are consumed before reaching the wafer center, further depleting the etch ion concentration over the wafer center. This leads to a lower etch rate or etch stopping near the wafer center. 
     One reason that there are more ions at the wafer periphery is that the location of the inductive coil at the side wall causes hotter ion-producing electrons to be generated in the vicinity of the side wall, such electrons cooling off and/or being consumed by recombination before reaching the center so that less production of etchant ions and radicals occurs over the wafer center. Moreover, introduction of the etchant precursor gas from the side and coupling of plasma source power from the side produces a non-uniform etchant ion/radical distribution favoring the side. Many of the ions and radicals formed near side (over the wafer periphery) are consumed by etching the quartz side wall and are not available to etch the wafer center, while etchant ion/radical-forming energetic electrons generated near the side are lost to collisions with other species before reaching the wafer center, thus reducing the etchant ion concentration at the wafer center. (It should be noted that the etching of the quartz side wall greatly increases the cost of operating the reactor because it consumes a very expensive item—the quartz side wall, which must be periodically replaced.) The relative lack of etchant ions near the wafer center permits faster formation of polymer at the wafer center, so much so that in some cases the polymer formation overwhelms the etch process and stops it, particularly at feature sizes less than 0.5 microns. Such etch stopping may occur either at larger etch features, at shallower etch depths or at shorter etch times. 
     The converse of the foregoing is that the relative plentitude of etchant ions and radicals near the wafer periphery can, under selected processing conditions, so impede polymerization as to impair etch selectivity, possibly leading to punchthrough of the underlying layer near the wafer periphery, in addition to causing a much higher etch rate at the wafer periphery. A related problem is that the hotter electrons near the chamber side wall/wafer periphery providing more energetic plasma ions in that vicinity, coupled with the oxygen released by the etching of the quartz side wall mentioned above, erodes the edges of the photoresist mask near the wafer periphery. Such erosion leads to faceting, in which the corners defined by the photoresist mask are etched, giving rise to an undesirable tapered etch profile. 
     From the foregoing, it is clear that there is a trade-off between avoiding punchthrough and faceting at the wafer edge and avoiding etch stopping at the wafer center, dictating a very narrow window of processing parameters within which a successful etch process may be realized across the entire wafer surface. To avoid the overetching the wafer periphery, the concentration of etchant ions and radicals in the plasma relative to other particles (e.g., polymer-forming ions or radicals and carbon) may be decreased, which risks etch-stopping at the wafer center. Conversely, to avoid etch-stopping at the wafer center, the concentration of etchant ions in the plasma may be increased, which risks punchthrough or faceting near the wafer periphery. Thus, the process window for successfully etching the entire wafer is very narrow. 
     In the parallel plate plasma reactor, the concentration of free fluorine in the plasma can be controlled by introducing a scavenging article, such as silicon, near or at the top of the reactor chamber. Silicon atoms physically etched (sputtered), chemically etched or reactive ion etched from the scavenging article combine with the fluorine ions and radicals, thereby reducing fluorine ion and radical concentration in the plasma. By controlling the rate at which silicon atoms are physically or chemically etched from the scavenging article, the amount of free fluorine ions and radicals in the plasma may be regulated (e.g., reduced) as desired to meet the narrow processing window mentioned above. The physical or chemical etch rates can be controlled by controlling the temperature of the scavenging article and/or by controlling the rate of ion-bombardment on the scavenging article. The surface of the scavenging article may be activated (to release silicon atoms into the plasma) either by RF power or by heating. By holding the scavenging article&#39;s temperature below the temperature at which polymerization occurs, the polymers accumulate on the scavenging article surface and block any release therefrom of silicon atoms. By raising the scavenging article&#39;s temperature above the condensation temperature, the surface is free from polymers, thus permitting the release of silicon atoms into the plasma. Further increasing the temperature increases the rate at which silicon atoms are released from the scavenging surface into the plasma. As for activating the scavenging article by RF power, the rate of ion bombardment of the scavenging article is affected by the RF potential or bias applied to the top parallel plate electrode adjacent the scavenging article. Reducing the free fluorine concentration in this manner has the effect of not only decreasing etch rate but also enriching the carbon content of the polymer, thus increasing the effect of the polymer on the etch process to guard against punch through at the wafer periphery, but increasing the risk of etch stopping at the wafer center. Conversely, increasing the free fluorine concentration not only increases the etch rate but also depletes the carbon content of the polymer, thus decreasing the effect of polymerization on the etch process, thus decreasing the risk of etch stopping at the wafer center but weakening the protection against punch through at the wafer periphery. 
     The narrow processing window is also met by regulating the polymer-forming ion and radical concentration in the plasma. This is accomplished by regulating the rate at which such polymer-forming radicals and ions are lost from the plasma by polymerization onto the chamber ceiling or sidewalls (or a scavenging article) or the rate at which polymer deposits are sputtered from the ceiling or sidewalls (or scavenging article). The polymerization rate at the ceiling is affected by regulating the ceiling temperature above or below the polymerization temperature. The rate at which such polymer deposits on the ceiling are etched and released into the plasma is affected by the following factors: the RF power applied to the ceiling electrode, temperature, chamber pressure, gas flow rate, inductive source power and other parameters. 
     Thus, in order to meet the narrow processing window, in general the relative concentrations of free fluorine and polymer-forming ions and radicals in the plasma may be controlled by regulating the temperature of the chamber ceiling or side walls or a scavenging article (if any) and/or by regulating the RF power applied to the to overhead/ceiling parallel plate electrode. 
     Thus, it is seen that the parallel-plate plasma reactor with the induction coil wound around its cylindrical side wall has the advantage of providing its ceiling electrode as a uniform ground plane over the entire wafer surface, but confines plasma ion production to the vicinity of the chamber side wall, so that plasma processing is weaker at the wafer center and stronger at the wafer periphery. The overhead planar coil plasma reactor has the advantage of a more uniform RF induction field relative to the wafer surface, so that ion production is not confined to the wafer periphery, but suffers from the lack of any uniform ground plane over the wafer, so that plasma ion current flow to the side walls distorts the plasma. 
     It is an object of the invention to combine the advantages of an inductively coupled plasma reactor having an overhead planar induction coil antenna with the advantages of a parallel plate electrode capacitively coupled plasma reactor in a single reactor without suffering the disadvantages or problems described above. Specifically, it is an object of the invention to provide an inductively coupled parallel plate electrode plasma reactor which exhibits uniform plasma processing across the entire wafer surface, so as to widen the plasma processing window, thus permitting a wider range in processing parameters, such as chamber pressure for example. 
     It is an object of the invention to provide an induction coil antenna whose physical disposition and/or power distribution pattern is relatively uniform with reference to the entire wafer surface so that plasma ion production is not predominantly at the vicinity of the chamber side wall, while at the same time providing a uniform ground plane in close proximity to the entire wafer surface so as to avoid plasma current flow to the chamber side wall. It is a further object of the invention to employ such a ground plane in a manner that effectively confines the plasma closer to the top surface of the wafer so as to minimize interaction with the chamber side wall. 
     It is another object of the invention to eliminate or reduce consumable materials such as quartz or ceramics in the chamber walls, so as to avoid depletion of plasma ions near the chamber walls and consumption of expensive reactor components through etching of such materials. 
     It is a further object of the invention to enhance processing uniformity at the wafer center relative to the wafer periphery in such a reactor by providing a uniform etch and polymer precursor gas distribution. Specifically, it is an object of the invention to introduce such gas from an optimum radial location of the chamber, such as from the chamber center and/or from the chamber periphery, whichever optimizes process uniformity across the wafer surface. For example, where etch rate is low at the wafer center and high at the wafer periphery, the gas is preferably introduced from the center of the ceiling rather than from near the periphery of the ceiling. 
     It is an additional object of the invention to enhance processing uniformity at the wafer center relative to the wafer periphery in such a reactor by enhancing (or reducing, if desired) the RF induction field over the wafer center relative to the RF induction field over the wafer periphery. Specifically, it is an additional object of the invention to provide separate or independent control of the strength of the RF induction field over the wafer center and independent control of the strength of the RF induction field over the wafer periphery, so that the radial distribution of the RF induction field across the wafer surface is adjustable to optimize plasma processing uniformity across the wafer surface. 
     SUMMARY OF THE INVENTION 
     The invention is embodied in a plasma reactor for processing a workpiece, including a reactor enclosure defining a processing chamber, a semiconductor window, a base within the chamber for supporting the workpiece during processing thereof, a gas inlet system for admitting a plasma precursor gas into the chamber, and an inductive antenna adjacent a side of the semiconductor window opposite the base for coupling power into the interior of the chamber through the semiconductor window. The workpiece may be a planar substrate, the semiconductor window (and the inductive antenna) may be either inside the chamber or outside of the chamber. In the latter case the semiconductor window may be a ceiling portion of the reactor enclosure generally parallel to and overlying the planar substrate, and the inductive antenna may overlie the ceiling portion to face the planar substrate through the semiconductor window. Alternatively, the semiconductor window may be a sidewall portion of the reactor enclosure generally perpendicular to and surrounding a periphery of the substrate, the inductive antenna being adjacent the sidewall portion. Preferably, the inductive antenna overlying the ceiling portion includes an arcuately extending elongate conductor disposed generally parallel to the plane of the planar substrate and may be either planar or dome-shaped. The inductive antenna adjacent the sidewall portion may be a conductive coil wound around the sidewall portion. 
     In one embodiment, the semiconductor window, in addition to shielding the inductive antenna, may also be an electrode, in which case an electrical terminal is connected thereto. In this case the semiconductor window is referred to as a semiconductor window electrode. 
     In one embodiment, a bias RF power source is coupled to the substrate, and the electrical terminal of the semiconductor window electrode is connected so as to enable the semiconductor window electrode to be a counter electrode to the bias RF power source coupled to the substrate, by being grounded, for example, thereby providing a uniform ground plane over the workpiece. 
     Another embodiment includes a power splitter having one output coupled to the semiconductor window electrode and another output coupled to the substrate and an input for receiving power from a common source. Yet another embodiment includes a first power source coupled to the semiconductor window electrode and a second power source coupled to the planar substrate. 
     The inductive antenna may include an inner antenna portion overlying a center of the planar substrate and an outer antenna portion overlying a periphery of the planar substrate and electrically separated from the inner antenna portion. This embodiment may include a power splitter having one output coupled to the semiconductor window electrode and another output coupled to the inductive antenna and an input for receiving power from a common source. Alternatively, an RF power splitter may split RF power between the inner and outer inductive antenna portions. 
     In accordance with one feature of the invention, a power splitter has one output coupled to the planar substrate and another output coupled to the inductive antenna and an input for receiving power from a common source. In this case, the electrical terminal of the semiconductor window electrode may be connected to RF ground. Alternatively, an independent RF power generator may be coupled to the electrical terminal of the semiconductor window. 
     The ceiling semiconductor window electrode with the overhead inductive antenna and the sidewall semiconductor window electrode with the side inductive antenna element may be combined in a single reactor. In this case, a power splitter may be employed having one output coupled to the inductive antenna overlying the ceiling portion and another output coupled to the inductive antenna element adjacent the sidewall portion, and an input for receiving power from a common source. 
     The semiconductor window and the workpiece may be separately driven with RF power, while the workpiece may be a counter electrode for the semiconductor window electrode, and the semiconductor window electrode may be a counter electrode for the workpiece. This may be accomplished by employing a first RF power source of a first frequency coupled to the semiconductor window electrode, a second RF power source of a second frequency coupled to the workpiece, a first ground pass filter connected between RF ground and the semiconductor window electrode, the first ground pass filter blocking RF power around the first frequency and passing RF power around the second frequency, and a second ground pass filter coupled between RF ground and the workpiece, the second ground pass filter blocking RF power around the second frequency and passing RF power around the first frequency. Furthermore, a first isolation filter may be connected between the first RF power source and the semiconductor window electrode for blocking RF power near the second frequency, and a second isolation filter may be connected between the second RF power source and the workpiece for block RF power near the first frequency. 
     In accordance with another feature, a conductive backplane lies on an external surface of the semiconductor window electrode, the conductive backplane being connected directly to the electrical terminal, the conductive electrode including plural apertures therein for admitting an induction field of the inductive antenna through the conductive backplane. Preferably, the conductive backplane includes plural conductive radial arms separated by the apertures, and the apertures have a characteristic width on the order of approximately a thickness of the semiconductor window electrode. 
     In accordance with a further feature, a structurally supportive substrate may be bonded to an exterior surface of the semiconductor window electrode. The conductive backplane may be inserted between the window electrode and the supportive substrate if desired, the conductive backplane being connected directly to the electrical terminal. The structurally supportive substrate may be an antenna holder supporting the inductive antenna. The antenna holder may be an insulator or a conductor (insulated from the inductive antenna, however). The conductive antenna holder may serve as a conductive backplane on an exterior surface of the semiconductor window electrode. In one embodiment, the inductive antenna includes an elongate conductor extended in an arcuate path, and the conductive antenna holder includes a elongate groove extending in the arcuate path and holding the elongate conductor, the groove being open at a surface of the conductive antenna holder facing the semiconductor window electrode. In another embodiment, the conductive antenna holder includes an recess in a surface of the conductive antenna holder facing the semiconductor window electrode, and the inductive antenna includes plural elongate conductive turns held within the recess. 
     In accordance with one implementation, the inductive antenna is a non-concentrically arcuately extending elongate conductor. In one case, the non-concentrically arcuately extending elongate conductor includes a center conductive element and plural spirals radiating outwardly from the center conductive element. In another case, the non-concentrically arcuately extending elongate conductor includes a circumferential conductive element and plural spirals radiating inwardly from the circumferential conductive element. 
     In accordance with another implementation, the inductive antenna has a non-planar path such as a three-dimensional helix or dual concentric three-dimensional helical paths or stacked spiral paths. 
     The gas inlet system preferably includes a set of gas inlet ports through the semiconductor window electrode over the planar substrate. These gas inlet ports may be concentrated over the wafer center and/or may be distributed to overlie the wafer periphery. A center gas feed top may be sealed onto an exterior surface of the semiconductor window electrode, forming a gas manifold between the center gas feed top and the semiconductor window electrode, the gas manifold encompassing the gas inlet ports. In one embodiment, a semiconductor baffle extends across the manifold and dividing the manifold into a pair of sub-manifolds, one of the sub-manifolds being adjacent the center gas feed top and the other of the sub-manifolds being adjacent the gas inlet ports, and plural gas feed passages through the semiconductor baffle offset from the gas inlet ports. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a first embodiment of the plasma reactor invention having a planar coil antenna overlying a planar silicon ceiling. 
     FIG. 2 is a graph illustrating the normalized forward voltage transmission coefficient from a transmitting RF coil to a receiving RF coil with a planar silicon window of FIG. 1 in between the transmitting and receiving RF coils. 
     FIG. 3 is a graph illustrating the normalized forward voltage transmission coefficient from a transmitting RF coil to a receiving RF coil with a cylindrical version of the silicon window in between the transmitting and receiving RF coils. 
     FIG. 4 illustrates an embodiment of the invention employing power splitting between the silicon ceiling and the wafer pedestal. 
     FIG. 5 illustrates an embodiment of the invention employing separate RF power sources to drive the wafer pedestal, the overhead induction coil and the silicon ceiling. 
     FIG. 6 illustrates an embodiment of the invention in which the overhead inductive antenna is divided into separately powered concentric inner and outer windings. 
     FIG. 7 illustrates an embodiment employing a side inductive antenna wound around a cylindrical silicon side wall. 
     FIG. 8 illustrates an embodiment corresponding to that of FIG. 7 in which power from a single RF power source is split between the silicon side wall and the wafer pedestal. 
     FIG. 9 illustrates and embodiment employing separate RF generators driving the silicon side wall, the wafer pedestal and the inductive side antenna. 
     FIG. 10 illustrates and embodiment combining the silicon ceiling and overhead inductive antenna with the silicon side wall and the inductive side coil wound thereabout. 
     FIG. 11 illustrates an embodiment corresponding to that of FIG. 10 employing power splitting between the silicon side wall and the wafer pedestal. 
     FIG. 12 illustrates an embodiment in which the silicon ceiling, the silicon side wall, the overhead inductive antenna and the side inductive antenna are separately driven with RF power. 
     FIG. 13A illustrates an embodiment corresponding to that of FIG. 1 employing a dome-shaped silicon ceiling. 
     FIG. 13B illustrates an embodiment corresponding to that of FIG. 4 employing a dome-shaped silicon ceiling. 
     FIG. 13C illustrates an embodiment corresponding to that of FIG. 5 employing a dome-shaped silicon ceiling. 
     FIG. 13D illustrates an embodiment corresponding to that of FIG. 6 employing a dome-shaped silicon ceiling. 
     FIG. 14 illustrates an embodiment having a dome-shaped silicon ceiling and an overlying inductive antenna having a dome-shaped portion overlying the ceiling and continuing into a cylindrical portion wrapped around the cylindrical side wall. 
     FIG. 15 illustrates and embodiment corresponding to that of FIG. 14 but in which the dome-shaped and cylindrical portions of the inductive antenna are isolated from one another and separately driven with RF power. 
     FIG. 16 illustrates an embodiment corresponding to that of FIG. 6 in which power from a common RF generator is split between the inner and outer inductive antennas. 
     FIG. 17A illustrates an embodiment corresponding to that of FIG. 15 in which power from a common RF generator is split between the dome-shaped and cylindrical inductive antenna portions. 
     FIG. 17B illustrates an embodiment having separate inner and outer dome-shaped inductive antennas between which power from a common RF generator is split. 
     FIG. 18 illustrates an embodiment corresponding to that of FIG. 10 in which power from a common RF generator is split between the overhead inductive antenna and the side inductive antenna. 
     FIG. 19 illustrates an embodiment corresponding to that of FIG. 1 in which power from a common RF generator is split between the overhead inductive antenna and the silicon ceiling. 
     FIG. 20 illustrates an embodiment corresponding to that of FIG. 1 in which power from a common RF generator is split between the overhead inductive antenna and the wafer pedestal. 
     FIG. 21 illustrates an embodiment corresponding to that of FIG. 13A having a dome-shaped ceiling in which power from a common RF generator is split between the overhead inductive antenna and the dome-shaped silicon ceiling. 
     FIG. 22 illustrates an embodiment corresponding to that of FIG. 13A having a dome-shaped ceiling in which power from a common RF generator is split between the overhead inductive antenna and the wafer pedestal. 
     FIG. 23 illustrates an embodiment in which the wafer pedestal and the silicon ceiling are separately driven with RF power and each serves as the counter electrode for the other. 
     FIG. 24 illustrates and embodiment corresponding to that of FIG. 23 in which the silicon ceiling is dome-shaped. 
     FIG. 25A is a side view of an embodiment of the silicon ceiling including a conductive back plane. 
     FIG. 25B is a top view of one embodiment of the conductive back plane. 
     FIG. 25C is a top view of another embodiment of the conductive back plane. 
     FIG. 26 is a side view of an embodiment of the silicon ceiling which is bonded to a supportive substrate. 
     FIG. 27 is a side view of an embodiment corresponding to that of FIG. 26 in which the supportive substrate is an insulative holder of the overhead inductive antenna. 
     FIG. 28 is a side view of an embodiment of the invention in which the antenna holder is conductive. 
     FIG. 29 is a side view of an embodiment corresponding to that of FIG. 28 in which the conductive antenna holder has a pair of annular apertures containing the inner and outer overhead inductive antennas. 
     FIG. 30A is a top view of a non-concentric embodiment of the overhead inductive antenna. 
     FIGS. 30B and 30C are side views of alternative implementations of the inductive antenna of FIG. 30A having planar and dome-shaped shapes, respectively. 
     FIG. 31A is a top view of another non-concentric embodiment of the overhead inductive antenna. 
     FIGS. 31B and 31C are side views of alternative implementations of the inductive antenna of FIG. 31A having planar and dome shapes, respectively. 
     FIG. 32 is a cross-sectional side view of a dual cylindrical helix embodiment of the overhead inductive antenna. 
     FIG. 33 is a cross-sectional side view of a pair of concentric cylindrical helix inductive antennas of the type illustrated in FIG. 32 for the inner and outer antennas of FIG.  16 . 
     FIG. 34 is a cross-sectional side view of an embodiment of the overhead inductive antenna consisting a layers of inductive antennas. 
     FIGS. 35A through 35E illustrate one embodiment of a center gas feed silicon ceiling, of which FIG. 35A is a bottom perspective view of the gas feed top, FIG. 35B is a bottom perspective view of an annular seal therefor, FIG. 35C is a cross-sectional view of the seal of FIG. 35B, FIG. 35D is a top perspective view of the silicon ceiling showing the gas feed holes and FIG. 35E is a partial cross-sectional view of the silicon ceiling of FIG.  35 D. 
     FIG. 36A is a cross-sectional view of another embodiment of the center gas feed silicon ceiling having a pair of gas plenums separated by a silicon wafer baffle. 
     FIG. 36B illustrates an alternative embodiment corresponding to FIG.  36 A. 
     FIG. 37A illustrates how a Faraday shield may be included in the embodiment of FIG.  1 . 
     FIG. 37B is a top view of the Faraday shield of FIG.  37 A. 
     FIG. 38A illustrates how a Faraday shield may be included in the embodiment of FIG. 10 having a dome-shaped ceiling. 
     FIG. 38B illustrates how a Faraday shield may be included in the embodiments of FIGS. 7-9 having a cylindrical semiconductor window electrode and inductive antenna. 
     FIG. 39A is a detailed cross-sectional side view of a preferred implementation of the invention. 
     FIG. 39B is an axial cross-sectional view of the conductor employed in the overhead inductive antenna. 
     FIG. 40 illustrates an alternative embodiment in which the semiconductor window and inductive antenna are inside the reactor chamber. 
     FIG. 41A illustrates an embodiment corresponding to FIG. 40 in which the semiconductor window is dome-shaped. 
     FIG. 41B illustrates an embodiments corresponding to FIG. 40 in which the semiconductor window is cylindrical and the inductive coil is cylindrical. 
     FIGS. 42 and 43 illustrate, respectively, side and top views of a segmented version of the semiconductor window electrode. 
     FIG. 44 illustrates an embodiment corresponding to that of FIG. 42 in which the semiconductor window is dome-shaped. 
     FIG. 45 illustrates an embodiment in which RF power is split between the semiconductor window electrode and a sidewall electrode, which itself may be a segmented portion of the semiconductor window electrode. 
     FIGS. 46 and 47 illustrate modifications of the embodiments of FIGS. 42 and 44 in which the outer segment of the semiconductor window electrode is grounded and RF power is split between the center segment of the semiconductor window electrode and the wafer pedestal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Basic Concept of the Invention 
     Referring to FIG. 1, a plasma reactor includes a sealed cylindrical chamber  100  enclosed by a cylindrical side wall  105 , a disk-shaped ceiling  110  and a base  115 . A wafer pedestal  120  supports a semiconductor wafer or workpiece  125  to be processed by the reactor. The wafer pedestal  120  may be an electrostatic chuck assembly, as disclosed in co-pending U.S. patent application Ser. No. 08/507,726 filed Jul. 26, 1995 by Kenneth S. Collins et al. entitled “PLASMA SOURCE WITH AN ELECTRONICALLY VARIABLE DENSITY PROFILE” and assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. A bias RF power generator  130  applies a bias potential to the wafer pedestal  120  through an impedance match circuit  135  of the type well-known in the art. Impedance match circuits are disclosed, for example, in U.S. Pat. No. 5,392,018 to Collins et al. and U.S. Pat. No. 5,187,454 to Collins et al. Gas inlets  137  adjacent the pedestal  120  admit a processing gas such as an etchant precursor gas such as a C x F x  gas. A vacuum pump  140  evacuates the chamber  100  to a desired pressure. An overhead inductive coil antenna  145  held over the ceiling  110  in an insulating antenna holder  147  is connected to a plasma source RF power generator  150  through another conventional impedance match circuit  155  and inductively couples RF power into the chamber through the ceiling  110 . 
     In order to provide a uniform ground return for the bias RF power over the entire surface of the wafer  125  and in order to minimize current flow toward the side wall  105 , the ceiling  110  is grounded. However, this feature requires the ceiling  110  to perform two functions: (a) act as a conductor that can be grounded and (b) act as a non-conductor so that the RF induction field from the overhead coil antenna  145  can pass therethrough. In order to fulfill its dual-function role, the ceiling  110  is a semiconductor such as silicon. The silicon ceiling  110  may be insulated from conductive member of the chamber by insulators  158 . It is felt that the silicon ceiling  110 , as a semiconductor, has sufficient conductive properties to act as an electrode or ground plane. In order to vertically confine the plasma closer to the wafer  125  and bring the uniform overhead ground plane into closer proximity to the wafer  125  (i.e., closer than the side wall  105 ) to minimize sideways plasma current flow to the side wall  105 , the silicon ceiling  110  preferably is placed within a distance of the wafer  125  less than the wafer diameter and as close as only a fraction of the wafer diameter (e.g, within a few centimeters for a 20 cm diameter wafer). This distance can be as great as 20 cm and as little as 1 cm although a preferable range is 2-10 cm. 
     The ceiling  110  is a semiconductor window of a semiconductor material which is preferably silicon. However, other well-known semiconductor materials may be employed, such as silicon carbide, germanium, III-V compound semiconductors such as gallium arsenide or indium phosphide and II-III-V compound semiconductors such as mercury cadmium telluride. The requisite dopant impurity level at room temperature given a desired resistivity value of the silicon window electrode may be obtained from the graph of FIG. 4.14 of Grove,  Physics of Semiconductors , page 113. The temperature of the silicon window  110  must be maintained within a range above which it does not act like a dielectric and below which it does not act like a conductor. The requisite temperature range may be obtained from the graph of FIG. 4.8 of Grove,  Physics of Semiconductors  illustrating the electron concentration in n-type silicon as a function of temperature. This graph shows that below about 100° K., the silicon begins to act like a dielectric while above about 600° K. the silicon begins to act like a conductor. It is therefore preferable to maintain the temperature of the silicon ceiling  110  in a range where the carrier electron concentration is fairly constant with respect to temperature, which is within the range between 100° K. and 600° K. 
     The advantages of the plasma reactor of FIG. 1 include the even distribution across the entire wafer surface of the plasma source power generated by the overhead coil antenna  145  in comparison with the conventional type of reactor having a coil wound around the side wall. Thus, plasma ion generation is not confined to the vicinity of the side wall but is uniformly distributed over the wafer surface. Plasma ion uniformity is enhanced because the grounded ceiling electrode  110  reduces or eliminates plasma current flow to the side wall  105 , in contrast to the sideways current flow in the plasma reactor having an overhead coil antenna over a dielectric ceiling as disclosed in U.S. Pat. No. 4,948,458 referred to above. Also, the close vertical proximity between wafer and ceiling reduces the distance between the wafer and the region in which plasma ions are generated, thereby reducing recombination losses and permitting the chamber pressure to be increased (e.g., to be in the range of 50-200 mTorr, for example). Such an increase in chamber pressure can be desirable for enhancing certain processing performance parameters, such as etch selectivity. The uniformity of plasma ion generation in the embodiment of FIG. 1 (achieved by the overhead coil antenna  145 ) coupled with the reduction in current flow to the chamber side wall (achieved with the grounded silicon ceiling  110  in close proximity to the wafer  125 ) reduces or eliminates differences in processing (e.g., etching) at the wafer center and processing at the wafer periphery, thereby widening the processing window. Specifically, the tendency to overetch or punch through at the wafer periphery is reduced while at the same time the tendency to etch stop at small feature sizes at the wafer center is also reduced. Also, the tendency near the wafer periphery to facet the photoresist mask edges and to generally etch the photoresist giving rise to poor etch profile is reduced or eliminated. Thus, the plasma reactor of FIG. 1 can provide at very small feature sizes aggressive etch performance across the entire wafer surface with superior etch profile and superior etch selectivity with little or no microloading. Moreover, since there is little or no participation of the side wall  105 , consumption of reactor components and the associated operating cost is significantly reduced. Accordingly, the invention provides a revolutionary advance in the art. 
     There are, however, a number of potential problems which, if not resolved, could render the reactor of FIG. 1 impractical. First, there is the problem of whether the silicon ceiling  110  will block the RF induction field of the overhead coil antenna  145  from ever reaching the chamber  100 . For example, the impedance of the silicon ceiling  110  to the RF induction field may be too great to permit transmission therethrough. This may be affected by the dopant impurity concentration in the silicon ceiling  110  or by the temperature of the silicon ceiling  110 . Alternatively, the RF induction field skin depth through the silicon ceiling  110  may not exceed the ceiling thickness, thus preventing transmission therethrough. Also, the RF induction field skin depth through the plasma in the chamber  100  may be greater than the chamber height (i.e., the wafer-top electrode gap), so that the RF induction power may not be efficiently coupled to the plasma. Another problem is that the close proximity of the ceiling  110  to the wafer  125  may be insufficient to separate the plasma sheath near the ceiling  110  from the plasma sheath near the underlying wafer  125 , thereby shorting out the plasma from top to bottom. Also, insertion of the silicon material of the ceiling  110  into the RF return path of the bias RF power generator  130  may occasion significant RF bias power losses. Finally, there may not exist a suitable or practical range of resistivity values for the semiconductor window within which an RF inductive field can be coupled without undue loss or attenuation. These problems are solved or otherwise disposed of in the analyses that follows immediately below. 
     The Induction Field&#39;s Skin Depth Through the Plasma is Sufficiently Short so that Power is Efficiently Absorbed in the Plasma: 
     The skin depth of the RF induction field from the overhead coil antenna  145  is less than the ceiling-to-wafer height (e.g., on the order of the wafer diameter or less), so that the power from the coil antenna  145  is inductively coupled to the plasma efficiently. This may be shown for two cases, namely a collisional skin depth in which the plasma source RF angular frequency is much less than the plasma momentum transfer elastic collision frequency, and a collisionless skin depth in which the plasma source RF angular frequency is much greater than the plasma momentum transfer elastic collision frequency. 
     The collisional skin depth δ c  of the RF induction field through the plasma is computed as follows: 
     
       
         δ c =(2τ m /ω) ½ c[(e 2 n e )/(∈ 0 m e )] −½ ,  (1) 
       
     
     where: 
     τ m =1.4·10 7  sec −1  is the electron-to-neutral momentum transfer collision frequency for an Argon plasma at a temperature of 300° K. and a chamber pressure of 5 mTorr, 
     ω=12.57·10 6  radians/sec is the angular frequency of the induction field RF power source applied to the coil antenna, 
     c=3·10 8  meters/sec is the speed of light, 
     e=1.6022·10 −19  coulomb is the electron charge, 
     n e =5·10 17  meters −3  is the applicable electron density, 
     ∈ 0 =8.85·10 −12  farads/meter is the electrical permittivity of free space, and 
     m e =9.1095·10 −31  kg is the electron mass. Substituting the foregoing values into Equation 1 yields: 
      δ c =1.1 cm  (2) 
     as the collisional skin depth. 
     The collisionless skin depth δ p  of the RF induction field through the plasma is computed as follows: 
     
       
         δ p =c[(e 2 n e )/(∈ 0 m e )] {fraction (−1/2)} .  (3) 
       
     
     Substituting the foregoing values into Equation 3 yields: 
     
       
         δ p =0.8 cm  (4) 
       
     
     as the collisionless skin depth. Thus, in either case the skin depth of the RF induction field is significantly less than the wafer-to-ceiling height, so that the RF source power is efficiently absorbed by the plasma. 
     The Induction Field&#39;s Skin Depth Through the Silicon Ceiling is Greater Than the Ceiling Thickness, So That It Can Extend Through the Ceiling 
     The RF induction field of the coil antenna  145  has a skin depth in silicon that far exceeds the one-inch (2.54 cm) thickness of the silicon ceiling. Therefore, the RF induction field is sufficiently deep to penetrate the silicon ceiling  110 , provided the resistive losses are sufficiently small. This may be closely estimated by computing the skin depth δ of a uniform plane wave incident on an infinite planar silicon slab: 
     
       
         δ=[πfσμ] −½ , 
       
     
     where: 
     f=2 MHz is the frequency of the RF power source  130  connected to the coil antenna  145 , 
     μ=4π·10 −7  Henries/meter is the magnetic permeability of the silicon slab, and 
     σ=3.33Ω −1  meters −1  is the conductance of the 30 Ω-cm resistivity silicon slab. 
     Substituting the foregoing values into the equation for δ yields: 
     
       
         δ=0.195 m, 
       
     
     so that the skin depth is about eight times the thickness of the silicon ceiling  110  and therefore the RF induction field has a penetration depth well into the chamber  10 , provided the silicon ceiling  110  does not pose a high impedance to the RF induction field. 
     The Silicon Ceiling Poses a Low Impedance to the RF Induction Field: 
     Transmission of an RF induction field through an embodiment of the silicon ceiling  110  consisting of a disc-shaped silicon slab of 30 Ω-cm resistivity at room temperature measuring 13.5 inches in diameter and 1 inch in thickness was tested at room temperature by placing induction coils on opposite sides of the slab, connected the coil on one side of the slab to a variable frequency RF source and connecting the coil on the other side to a multichannel analyzer and then sweeping the frequency of the RF source from 1 kHz to 10 MHz. The resulting output of the multichannel analyzer is illustrated in FIG.  2 . For the curve labelled “magnitude”, the vertical axis is the ratio of the magnitudes of the received and transmitted signals and extends from a value of unity at the top of the scale and falls in 0.1 unitless increments, while the horizontal axis is frequency and extends from 1 kHz on the left to 10 MHz on the right. For the curve labelled “phase”, the vertical axis is the difference between the phase angles of the received and transmitted signals and extends from a value of 20° at the top of the scale and falls in 20° increments. The graph of FIG. 2 clearly indicates that there virtually is no loss of RF power through the silicon slab out to 2 MHz and that there is relatively little loss above 2 MHz. 
     While FIG. 2 illustrates the results obtained with a discoid silicon slab, FIG. 3 illustrates results obtained at room temperature with a cylindrical silicon slab having a 12.3 inch outside diameter, a 1 cm wall thickness. A 5-turn transmission coil was wound around the outside of the cylindrical slab for transmitting RF power into the interior volume surrounded by the cylindrical silicon slab. Measurements were taken at the center of the interior volume from an 8-turn receiver coil having a 2.3 inch outside diameter and a 3 inch length. For the curve labelled “magnitude”, the vertical axis is the ratio of the magnitudes of the received and transmitted signals and extends from a value of unity at the top of the scale and falls in 0.1 unitless increments, while the horizontal axis is frequency and extends from 1 kHz on the left to 10 MHz on the right. For the curve labelled “phase”, the vertical axis is the difference between the phase angles of the received and transmitted signals and extends from a value of 0° at the top of the scale and falls in 20° increments. The graph of FIG. 2 clearly indicates that there virtually is no loss of RF power through the silicon slab out to 2 MHz and that there is relatively little loss above 2 MHz. Thus, the silicon ceiling  110  is nearly transparent to the RF induction field. 
     The Silicon Ceiling Poses a Low Impedance to the RF Induction Field Over A Wide Temperature Range 
     The tests described above with reference to FIGS. 2 and 3 were repeated at silicon temperatures of 200° and 250° and similar results were obtained, demonstrating that the temperature of the silicon ceiling  110  may be varied over a wide range to adjust the polymerization and fluorine scavenging processes as desired. However, at temperatures well above 300° C. the silicon ceiling  110  becomes intrinsic and therefore cuts off the RF induction field of the overhead coil antenna  145  from reaching the chamber  100 . It is preferable to employ a high resistivity silicon (e.g., 30 Ω-cm at room temperature) in the ceiling  110 . Otherwise, using, for example, 0.01 Ω-cm resistivity silicon in the ceiling  110  would require reducing the frequency of the RF induction field to the kHz range or below in order to couple through the silicon ceiling. Another option is to reduce the ceiling thickness. 
     The Silicon Ceiling Adds Virtually No Losses in the Return Path of the Bias RF Power Source 
     That the RF power loss occasioned by insertion of the silicon ceiling  110  in the RF return path for the bias power generator  130  is low may be seen by computing the resistance of a silicon slab to a current flux normal to the slab&#39;s flat surface assuming a skin depth much greater than the slab thickness and assuming the presence of a perfect conductor ground plane on the back of the silicon slab. This resistance is shown to be far less than the measured driving point RF impedance at the wafer pedestal  120 , so that the insertion of the silicon ceiling causes only a fractional increase in the overall bias RF power loss. 
     The resistance R slab  of the silicon slab to a current flux normal to the surface is computed as follows: 
     
       
         R slab =Γt(d/2) −2 /π, 
       
     
     where: 
     t=0.0254 m is the slab thickness, 
     d=0.318 m is the slab effective diameter exposed to the current flux, and 
     Γ=0.30 Ω-m is the resistivity of the 30 Ω-cm resistivity silicon at room temperature. 
     Substituting the foregoing values into the equation for R slab  yields for the slab resistance: 
     
       
         R slab =0.096 Ω. 
       
     
     The driving point impedance Z at the wafer pedestal  120  has been measured in typical parallel electrode plasma reactor of the type illustrated in FIG. 1 (with a 2.0 MHz plasma power source RF induction field from a coil antenna and an applied RF bias on the pedestal of 1.8 MHz) as 38.7 Ω at an angle of 50.1°. The real part of this driving point impedance is: 
     
       
         Re(Z)=38.7Ω cos(50.1°(180/π))≈24Ω. 
       
     
     The fractional increase in RF power loss occasioned by the insertion of the silicon ceiling is: 
     
       
         R slab /Re(z)=0.096/24=0.004, 
       
     
     which is a very small fraction. Thus, the insertion of a silicon slab into the RF return path of the RF bias power generator  130  adds only a negligible amount of resistance. 
     The Plasma DC Sheath Thickness is Sufficiently Short to Avoid Shorting Between the Ceiling and the Wafer 
     There is a sheath at the edge of the plasma across which the ion density falls from the plasma ion density value to zero. If the sheath at the wafer surface meets the sheath at the chamber ceiling, there is no plasma in the chamber. The high voltage plasma DC sheath thickness, s, is computed as follows: 
     
       
         s=(2 ½ /3)[(∈ 0 ·T e )/(e·n e )] ½ (2·V 0 /T e ) ¾ , 
       
     
     where: 
     ∈ 0 =8.85·10 −14  farads/cm is the electrical permittivity of free space, 
     T e =5 eV is the mean electron temperature assuming a Maxwellian distribution, 
     e=1.6022·10 −19  coulombs is the electron charge, 
     n e =5·10 11  cm −3  is the average electron density in the bulk plasma adjacent the sheath, and 
     V 0 =300 Volts is the DC voltage across the plasma sheath. 
     Substituting the foregoing values into the equation for s yields: 
     
       
         s=0.04 cm, 
       
     
     so that the plasma sheath thickness is only a small fraction of the wafer-to-ceiling distance and therefore there is no risk of the top and bottom sheaths intersecting. 
     There is a wide range of resistivity values for a semiconductor window electrode through which RF power may be coupled with low attenuation losses: 
     In order to ascertain a range of resistivity values of a semiconductor window through which an inductive RF field is to be coupled or transmitted with low attenuation losses, a thickness of the window must be selected, the frequency of the RF inductive field must be selected and a minimum ratio r of RF skin depth to window thickness must be specified. In a first example, the window thickness is T=0.0254 meter, the RF frequency f is 2·10 6  s −1  and the minimum ratio of RF skin depth to window thickness is r=5. For this first example, the minimum resistivity for the semiconductor window is calculated assuming: 
     μ=4π·10 −7  Henries/meter is the magnetic permeability of the semiconductor window, 
     S=r·T is the skin depth of the RF induction field in the semiconductor window. The minimum resistivity Γ min  is then computed by the following equation: 
     
       
         Γ min =S 2 ·π·f·μ 
       
     
     Substituting the values given above into this equation, the minimum resistivity for this first example is: 
     
       
         Γ min =12.735 Ω-cm. 
       
     
     Next, the maximum resistivity must be computed for this first example. For purposes of the following analysis, it will be assumed that there is a ground plane transmissive to the RF induction field behind the semiconductor window. The real part of the RF bias plasma load impedance must be defined, the acceptable loss L must be defined and the effective plasma interaction diameter d of the semiconductor window must be defined. In this first example, the real part R of the RF bias plasma impedance is: 
     R=−25, 
     the acceptable loss (normalized at 100%=1) is 
     L=0.01, 
     the effective plasma interaction diameter of the semiconductor window is 
     d=0.318, from which the effective plasma interaction area A is computed as A=π(d/2) 2 . 
     The maximum resistivity Γ max  is given by the following equation: 
     
       
         Γ max =L·R·A/T. 
       
     
     Substituting the foregoing values into this equation yields: 
     
       
         Γ max =78.172 Ω-cm. 
       
     
     Therefore, in this first example, the resistivity of the semiconductor may lie anywhere within the range between 12.735 Ω-cm and 78.172 Ω-cm. 
     In a second example, the semiconductor window thickness is reduced by a factor of ten so that T=0.00254 meters. In this case, Γ min  is reduced to 0.127 Ω-cm, while Γ max  increases to 781.719 Ω-cm. 
     In a third example, the parameters of the second example are repeated except that the frequency of the RF induction field is reduced to 100 kHz (f=0.1·10 6 s −1 ). In this case, Γ min  is reduced to 0.006 Ω-cm, while Γ max  is unchanged (from the second example) at 781.719 Ω-cm. 
     In a fourth example, the parameters of the first example are adopted except that the frequency of the RF induction field is increased to 10 MHz (f=10·10 6 s −1 ). In this case, Γ min  is increased to 63.675 Ω-cm, while Γ max  is decreased to 78.172 Ω-cm, thus narrowing the range somewhat relative to the other examples. 
     Thus, the useful range of resistivity values is broad. If the semiconductor window is a 2.54 cm-thick silicon slab and the RF frequency of the induction field is 2 MHz, then the preferred resistivity is 30 Ω-cm. 
     Other Preferred Embodiments of the Invention 
     While the silicon ceiling  110  is grounded in the preferred embodiment of FIG. 1, in the embodiment FIG. 4 power from the RF generator  130  is split by a power splitter  160  between the pedestal  120  and the silicon ceiling  110 . In order to provide a ground return path, the side wall  105  may be a grounded conductor. In FIG. 5, the silicon ceiling  110  is driven independently from the wafer pedestal  120  by a separate RF power generator  165  through a conventional impedance match circuit  170 . While the embodiment of FIG. 1 employs a single coil as the inductor antenna  145 , in the embodiment of FIG. 6 the inductor antenna  145  is comprised of plural (in this case, two) independently driven coils, namely an inner coil  175  overlying the wafer center and an outer coil  180  overlying the wafer periphery. In the implementation of FIG. 6, the inner and outer coils  175 ,  180  are planar concentric coils driven by separate plasma source power generators  185 ,  190 . The advantage is that plasma variations along the radius of the chamber  100  can be compensated by selecting different power levels from the two RF generators  185 ,  190 . In a prototype of the embodiment of FIG. 6, the inner and outer coils consisted of nine turns each. A fixed impedance match was realized using parallel and series capacitors  195 ,  200  connected to the input and return ends, respectively, of the inner coil  175  and parallel and series capacitors  205 ,  210  connected to the input and return ends, respectively, of the outer coil  180 . 
     In a working example, the plasma source RF power generator  155  produced 2600 Watts at 2.0 MHz, the bias RF power generator  130  produced 1600 Watts at 1.8 MHz, C 2 F 6  gas was pumped into the chamber  100  at a rate of 30 sccm with 100 cc of Argon, the chamber pressure was maintained at 6.5 mTorr and the temperature of the silicon ceiling  110  was maintained at 200° C. Under these conditions, the parallel and series capacitors  195 ,  200  of the inner coil  175  had capacitances of 8.19699 nanoFarads and 2.12631 nanoFarads, respectively, while the parallel and series capacitors  205 ,  210  of the outer coil  180  had capacitances of 4.45572 nanoFarads and 1.09085 nanoFarads, respectively. By shorting one of the inner and outer coils  175 ,  180 , the input impedance of the other could be measured. Indeed, with the foregoing capacitances being selected, the complex impedance of the inner coil  175  was measured as 51 Ω at a complex plane angle of −12° and the complex impedance of the outer coil  180  was measured as 51.8 Ω at a complex plane angle of −8.6°. Thus, their impedance nearly matched the standard 50 Ω output of the RF generators  185  and  190 . 
     In the embodiment of FIG. 7, the side wall  105  is replaced by a silicon side wall  215  and inductive coupling is provided by a coil inductor  220  wound around the exterior of the silicon side wall  215  and connected to the plasma source power generator  150 ′ through the impedance match circuit  155 ′. In FIG. 7, the silicon side wall  215  is grounded. As in the embodiment of FIG. 4, FIG. 8 illustrates that the power splitter  160  may split RF power from the bias generator  130  between the pedestal  120  and the silicon side wall  215 . As in the embodiment of FIG. 5, FIG. 9 shows how the silicon side wall  215  may be independently powered by the separated RF generator  165  through the match circuit  170 . FIG. 10 shows how the silicon ceiling  110  and overhead coil antenna  145  of FIG. 1 may be combined with the silicon side wall  215  and side coil antenna  220  of FIG.  7 . FIG. 11 illustrates how the power splitter  160  of FIG. 8 may be employed in the embodiment of FIG. 10 to split power from the bias generator  130  between the side wall  215  and the pedestal  120 . FIG. 12 illustrates the combination of the embodiment of FIG. 9 with the overhead coil antenna  145  and silicon ceiling  110  of FIG.  1 . 
     FIGS. 13A,  13 B,  13 C and  13 D illustrate modifications of the embodiments of FIGS. 1,  4 ,  5  and  6 , respectively, in which the planar silicon ceiling  110  and planar inductive coil antenna  145  have been replaced by a dome-shaped silicon ceiling  230  and a dome-shaped coil antenna  235 , respectively. In one implementation, the dome-shaped coil antenna  235  is wound in a helix. FIG. 14 illustrates an embodiment in which the dome-shaped coil antenna  235  extends from near the top of the dome-shaped ceiling  230  and past the bottom of the dome-shaped ceiling  230  to wind around the side wall  105 . In this case, at least that portion of the side wall  105  surrounded by the bottom portion of the coil antenna  235  would have to be a non-conductor such as quartz or a semiconductor such as silicon. FIG. 15 illustrates an embodiment corresponding to that of FIG. 10 in which the flat silicon ceiling  110  and the flat coil antenna  145  are replaced by the dome-shaped silicon ceiling  230  and the dome-shaped coil antenna  235  of FIG.  13 A. 
     FIG. 16 illustrates how a power splitter  250  can be employed to split power from the plasma source RF power generator  150  between the inner and outer coil antennas  175 ,  180  overlying the silicon ceiling  110  of FIG.  6 . An RF power splitter is disclosed in U.S. Pat. No. 5,349,313 to Collins et al. As disclosed in the co-pending Collins et al. patent application incorporated by reference above, a controller  260  can vary the power ratio between the inner and outer coil antennas  175 ,  180  to compensate for any difference between the plasma ion densities over the wafer center and the plasma ion density over the wafer periphery. FIG. 17A illustrates an embodiment corresponding to that of FIG. 15 in which the power splitter  250  splits RF power from the plasma source power generator  150  between the side coil  220  and the dome-shaped overhead coil  235 . FIG. 17B illustrates an embodiment with the dome-shaped silicon ceiling  230  having an inner (upper) dome-shaped coil  270  and an outer (lower) dome-shaped coil  280 . The power splitter  250  splits RF power from the plasma source power generator  150  between the inner and outer dome-shaped coils  270 ,  280 . The silicon ceiling  230  and the wafer pedestal  120  of FIG. 17B may be connected in accordance with any one of the embodiments of FIGS. 13A,  13 B or  13 C. FIG. 18 shows how the power splitter  250  can split power from the plasma source RF power generator  150  between the overhead coil antenna  145  and the side coil antenna  220  of FIG.  10 . 
     FIG. 19 illustrates how the power splitter  250  can split power from the plasma source RF generator  150  between the overhead coil antenna  145  and the silicon ceiling  110 . In the embodiment of FIG. 19, the wafer pedestal  120  can either be grounded are connected to the bias RF power generator  130  through the impedance match circuit  135 . FIG. 20 illustrates how the power splitter  250  can split power from the RF generator  150  between the overhead coil antenna  145  and the wafer pedestal  120 . In the embodiment of FIG. 20, the silicon ceiling  110  can either be grounded or can be connected to the separate RF power generator  165  through the match circuit  170 . FIG. 21 is an embodiment corresponding to that of FIG. 19 but employing the dome-shaped silicon ceiling  230  and the dome-shaped coil antenna  235  in lieu of the flat silicon ceiling  110  and the planar coil antenna  145  of FIG.  19 . FIG. 22 is an embodiment corresponding to that of FIG. 20 but employing the dome-shaped silicon ceiling  230  and the dome-shaped coil antenna  235  in lieu of the flat silicon ceiling  110  and the planar coil antenna  145  of FIG.  20 . 
     FIG. 23 illustrates an embodiment in which the silicon ceiling  110  and the wafer pedestal  120  are each the RF ground return for the other. The ceiling and pedestal  110 ,  120  are driven through respective impedance match circuits  280 ,  290  by respective independent RF power generators  300 ,  305  of respective RF frequencies f 1  and f 2  through respective RF isolation filters  310 ,  315  and are each connected to ground through respective ground RF filters  320 ,  325 . The isolation filters  310 ,  315  prevent the RF energy from either one of the RF power generators  300 ,  305  from reaching the other. The ground RF filters  320 ,  325  permit each one of the ceiling and pedestal  110 ,  120  to return to ground the RF power radiated across the chamber  100  by the other. At the same time, the ground RF filters  320 ,  325  prevent the RF power applied to either one of the ceiling and pedestal  110 ,  120  from being shorted directly to ground. Specifically, the isolation filter  310  connected between the silicon ceiling  110  and the RF power generator  300  of frequency f 1 , passes RF power at the frequency f 1  and blocks RF power at the frequency f 2 . The isolation filter  315  connected between the wafer pedestal  120  and the RF power generator  305  of frequency f 2  passes RF power at the frequency f 2  and blocks RF power at the frequency f 1 . The ground filter  320  connected between the silicon ceiling  110  and ground passes RF power at the frequency f 2  and blocks RF power at the frequency f 1 . The ground filter  325  connected between the wafer pedestal  120  and ground passes RF power at the frequency f 1  and blocks RF power at the frequency f 2 . 
     The impedance match circuits  280 ,  290  are of the conventional type discussed previously herein and employ voltage and current sensors or impedance match transducers (not shown) in the conventional manner to measure actual input impedance. In order to prevent the RF power from either one of the two RF generators  300 ,  305  from interfering with the operation of the impedance match circuit of the other, respective match isolation filters  330 ,  335  are connected between the impedance match transducers and the transducer inputs to the impedance match circuits  280 ,  290 , respectively. The match isolation filter  330  at the input to the f 1  match circuit  280  passes RF power at the frequency f 1  and blocks RF power at the frequency f 2 . The match isolation filter  335  at the input to the f 2  match circuit  290  passes RF power at the frequency f 2  and blocks RF power at the frequency f 1 . 
     The various RF filters  310 ,  315 ,  320 ,  325 ,  330 ,  335  may be constructed of passive reactive components (capacitors and inductors) using techniques well-known in the art. If the two frequencies f 1 , f 2  are widely separated (e.g., by an octave), then the various RF filters  310 ,  315 ,  320 ,  325 ,  330 ,  335  can be suitable high-pass and low-pass filters with the suitably selected cut-off frequencies. Otherwise, if the two frequencies f 1 , f 2  are not sufficiently separated, then the various RF filters  310 ,  315 ,  320 ,  325 ,  330 ,  335  should be bandpass or band reject filters centered at the appropriate frequencies. 
     FIG. 24 illustrates an embodiment corresponding to that of FIG. 23 but employing the dome-shaped silicon ceiling  230  and dome-shaped coil antenna  235  of FIG. 13A in lieu of the flat silicon ceiling  110  and planar coil antenna  145  of FIG.  23 . 
     FIG. 25A illustrates the use of a conductive backplane  400  on the upper side or back surface of the silicon ceiling  110 . The electrical potential (e.g., ground or, alternatively, the output of an RF generator) to be applied to the silicon ceiling  110  is applied directly to the conductive backplane  400  for uniform distribution across the back surface of the ceiling  110 . The backplane  400  may be of any highly conductive material such as aluminum or copper, for example. Moreover, the backplane  400  must have sufficient openings or apertures to prevent the formation of eddy currents by the inductive RF field of the overhead coil antenna  145  which would block transmission of the RF inductive field through the backplane  400 . For example, the top view of FIG. 25B illustrates that the conductive backplane  400  may be in the shape of a star, with plural conductive arms  405  extending radially from a conductive center  410 . Preferably, the spaces or apertures  415  between the conductive arms  405  are of a characteristic width on the order of the thickness of the silicon ceiling  110 . This feature provides uniform current distribution and vertical current flow through the silicon ceiling  110 . The center connected arms of the embodiment of FIG. 25B may be preferable in the case of silicon laminated on ceramic for ease of fabrication. FIG. 25C illustrates an alternative embodiment of the conductive backplane  400  which consists of an outer conductive annulus  420  with plural conductive arms  425  extending from the conductive annulus  420  radially inwardly toward an imaginary center point  430 . The apertures  435  between the conductive arms  425  are of a characteristic width on the order of the thickness of the silicon ceiling  110 , as shown in FIG.  25 C. Preferably, electrical contact is made along the entire annulus  420 . Preferably, the outer conductive annulus  420  is beyond the effective radial range of the RF inductive field. 
     Preferably, the silicon ceiling is about 1 inch (2.54 cm) thick for a 13 inch (32 cm) diameter wafer to provide structural integrity and an anti-implosion safety factor in excess of 10. However, to significantly reduce the thickness of the silicon ceiling (e.g., to a fraction of an inch),FIG. 26 illustrates how the silicon ceiling  110  and its backplane  400  may be bonded to a strong support substrate  500  such as a ceramic disk. The overhead inductive coil antenna holder  147  is placed over the substrate  500 . In order to control the temperature of the silicon ceiling  110 , a heater layer  510  is placed on top of the antenna holder  147  and a cooling plate  520  is placed on top of the heater layer  510 . FIG. 27 illustrates how the role of the substrate  500  and the role of the antenna holder  147  may be merged into a single member by employing a strong material such as a ceramic in the antenna holder  147  and bonding the silicon ceiling  110  with its conductive backplane  400  directly to the antenna holder  147 . 
     FIG. 28 illustrates an embodiment of the invention employing an alternative antenna holder  147 ′ consisting of a conductor such as aluminum or copper, for example. In this embodiment, an insulating material  146  is provided between the windings of the inductive coil antenna  145  and the conductive antenna holder  147 ′. In order to prevent the conductive antenna holder  147 ′ from blocking the RF inductive field of the coil antenna  145 , each turn of the antenna  145  nests within a groove-shaped aperture  550  in the antenna holder  147 ′, each aperture  550  opening to the bottom surface  147   a  of the conductive antenna holder  147 ′. The conductive antenna holder  147 ′ of FIG. 28 may also serve as the conductive backplane for the silicon ceiling  110  as well as the strong supportive substrate for the ceiling  110 , in which case the silicon ceiling  110  is bonded directly to the conductive antenna holder  147 ′ of FIG.  28 . However, a preferable option is to interpose the star-shaped conductive backplane  400  between the silicon ceiling  110  and the conductive backplane  147 ′ of FIG.  28 . FIG. 29 illustrates another embodiment of the conductive antenna holder  147 ′ having a pair of wide concentric annular apertures  560 ,  565  in which respective ones of the inner and outer coil antennas  175 ,  180  of FIGS. 6 or  16  nest. The annular apertures  560 ,  565  open at the bottom surface  147   a  of the antenna holder so that there is no blockage of the RF induction field. In this embodiment, as FIG. 28, the conductive antenna holder can also serve as the conductive backplane for the silicon antenna (in lieu of the star-shaped conductive backplane  400 ) and as the strong supportive substrate for the silicon ceiling  110  (in lieu of the supportive substrate  500 ) and therefore may bonded directly to the silicon ceiling  110 . Preferably, however, the star-shaped conductive backplane is interposed between the conductive antenna holder  147 ′ and the silicon ceiling  110  and is bonded with the silicon ceiling  110  to the antenna holder  147 ′. 
     The overhead coil antenna  145  has been described above as consisting of a single concentric spiral winding or of inner and outer concentric spiral windings  175 ,  180 . FIGS. 30A,  30 B and  30 C illustrate an alternative design of the overhead coil antenna  145  having non-concentric windings. Specifically, in FIG. 30A there is an outer circular conductor  600  which is of very low inductance and low resistance so as to be at least nearly equipotential along its entire circumference. Plural spiral conductors  610  radiate inwardly in involute paths from the outer conductor  600  to a center point  620  joining all of the spiral conductors  610 . Plasma source RF power (from the RF generator  150 ) is applied between the outer conductor  600  and the center point  620 . As illustrated in FIG. 30B, the involute spiral conductors  610  lie in a plane, while FIG. 30C illustrates an alternative embodiment for use with the dome-shaped ceiling  230  in which the involute spiral conductors  610  form a dome. A dome shape, such as the dome shapes employed in the various embodiments disclosed in this specification may be non-linear or hemispherical or conical or a rotation of some arcuate curve such as a conical section or the combination of two different radii (as disclosed earlier in this specification). 
     FIGS. 31A,  31 B and  31 C illustrate another non-concentric embodiment of the overhead coil antenna. FIG. 31B corresponds to the planar case while FIG. 31C corresponds to the dome-shaped case. An outer circular conductor  700  has plural arcuate conductive arms  710  radiating inwardly therefrom terminating at ends  715 A,  715 B,  715 C. A center point  720  has plural arcuate conductive arms  730  radiating outwardly therefrom and terminating at ends  740 A,  740 B,  740 C. As employed in the embodiment of FIG. 16, RF power from one output of the power splitter  250  of FIG. 16 is applied between the outer circular conductor  700  and the ends  715 , while RF power from the other output of the power splitter  250  is applied between the center point  720  and the ends  740 . 
     The conductor of the inductive antenna may follow any suitable three-dimensional path. For example, FIG. 32 illustrates a non-planar or 3-dimensional embodiment of the coil antenna  145  consisting of dual concentric cylindrical helical windings, namely an outer cylindrical helix  800  and an inner cylindrical helix  810  all formed with the same conductor. FIG. 33 illustrates how a pair of dual concentric cylindrical helical windings of the type illustrated in FIG. 32, namely an inner dual concentric cylindrical helical winding  820  and an outer dual concentric cylindrical helical winding  830  can be employed as the inner and outer windings of the overhead inductive antenna in the embodiment of FIG. 16, in lieu of the inner and outer planar windings  175 ,  180  of FIG.  16 . 
     FIG. 34 illustrates another non-planar embodiment of the inductive antenna  145  of FIG. 1 consisting of stacked layers  840   a ,  840   b ,  840   c  of spiral windings  850 . 
     As mentioned previously herein, one factor that can give rise to plasma etch processing differences between the wafer center and the wafer periphery non-uniform etch precursor gas distribution. Such non-uniformity in gas distribution arises from the introduction of the gas from the side of wafer pedestal through the gas inlets  137 , so that there is relatively more etchant precursor gas near the wafer periphery and relatively less etchant precursor gas near the wafer center. This problem is addressed in the embodiment of the silicon ceiling  110  of FIGS. 35A,  35 B,  35 C,  35 D and  35 E, which includes a center gas feed system built into the silicon ceiling  110  for introducing the etch precursor gas directly over the wafer in a symmetrical manner relative to the wafer center. 
     Referring to FIG. 35D, the 1-inch thick silicon ceiling  110  has an approximately 0.33-inch deep 3.5-inch diameter counterbored opening  900  in its top surface. Preferably, as shown in FIG. 35E, about twenty-two symmetrically placed 0.20-inch diameter holes  910  are drilled down from the top surface of the counterbore opening  900  through about 80% of the thickness of the ceiling  110 . Small 0.030-inch diameter holes  920  concentric with the larger holes  910  are laser-drilled from the bottom surface of the silicon ceiling  110 . As shown in FIGS. 35A and 35B, a disk-shaped gas feed top  930  fits snugly within the counterbore opening  900  in the top surface of the silicon ceiling  110 . The bottom surface of the gas feed top  930  has an approximately 0.01-inch deep 3.3-inch diameter counterbore opening  940  therein which forms a gas distribution manifold. A center gas feed pipe  950  passes through the center of the gas feed top  930  and opens into the counterbore opening  940 . The bottom peripheral corner of the gas feed top has a step  960  cut out therein, the step  960  being 0.143-inch deep and extending 0.075-inch radially inwardly. The step  960  creates a circumferential pocket into which an annular teflon seal  970  having dimensions matching those of the step  960  snugly fits. Preferably, the teflon seal  970  has a U-shaped cross-section, as illustrated in FIG.  35 C. An annular steel wire stiffener  975  within the teflon seal  970  provides stiffness for the seal  970 . 
     The center gas feed silicon ceiling of FIGS. 35A-E can be employed as the silicon ceiling  110  in the embodiment of FIG. 16 in combination with the independently adjustable inner and outer inductive coil antennas  175 ,  180  controlled through the power splitter  250  by the electrical controller  260 . The advantage is that the reactor&#39;s center-to-edge etch uniformity is enhanced by the uniform etchant precursor gas distribution across the wafer surface achieved with the center gas feed silicon ceiling  110  of FIGS. 35A-E, and any residual center-to-edge etch non-uniformity can be precisely compensated by judiciously adjusting, using the controller  260 , the relative proportion of power levels applied to the inner and outer coil antennas  175 ,  180  in the manner disclosed in the co-pending application of Collins et al. referred to above. As a result, the wafer center-to-edge etch uniformity is optimized to a degree not attainable in the prior art without sacrificing etch profile, etch selectivity and etch rate at very small feature sizes. 
     FIG. 36A illustrates another preferred embodiment of the center gas feed silicon ceiling which better protects the gas feed top from the plasma. In this embodiment, a shoulder  980  is provided along the circumferential edge of the counterbore opening  900  in the silicon ceiling  110 . A silicon wafer  985  rests on the shoulder  980  and separates into two separate chambers the counterbore opening  900  in the top of the silicon ceiling  110  and the counterbore opening  940  in the bottom of the gas feed top  930 . The silicon wafer  985  has plural gas feed holes  986  drilled therethrough which are all laterally displaced from the gas feed holes  910  in the silicon ceiling  110 . The interposition of the silicon wafer  985  in this manner eliminates any direct-line path to the gas feed top  930  for plasma ions diffusing upwardly from the chamber through the holes  920 . This feature better protects the top  930  from attack by the plasma. The top  930  is either a material such as a semiconductor or a dielectric which does not appreciably attenuate the RF inductive field, or, if its diameter is less than the diameter of the center null of the inductive antenna, may be a conductor such as stainless steel. 
     In the embodiments of FIGS. 35A-E and  36 A, the gas feed holes  910 ,  920  are grouped about the center of the ceiling  110 . However, in either embodiment the holes  910 ,  920  may be distributed from the center out to the periphery, if desired, or may be grouped about the periphery instead of the center. This is illustrated in FIG. 36B, in which the gas feed holes  910 ,  920  are distributed about the periphery of the ceiling  110 . 
     FIG. 37A illustrates a modification applicable to any of the embodiments employing a flat silicon ceiling  110 , such as, for example, the embodiment of FIG. 1, in which an insulated grounded Faraday shield  990  is interposed between the overhead inductive antenna  145  and the silicon ceiling  110 . The Faraday shield is of the conventional type whose shape is best shown in the top view of FIG.  37 B and is used to reduce capacitive coupling from the inductive antenna  145  to the plasma. FIG. 38A illustrates a modification applicable to any of the embodiments employing a dome-shaped silicon ceiling  230 , such as, for example, the embodiment of FIG. 13A, in which an insulated grounded Faraday shield  990  is interposed between the overhead inductive antenna  145  and the silicon ceiling  230 . FIG. 38B illustrates an embodiment corresponding to the embodiments of FIGS. 7-9 having a cylindrical side semiconductor window electrode  215  and a cylindrical side inductive antenna  220 , with the additional feature of a cylindrical Faraday shield  990 ′ interposed between the inductive antenna  220  and the cylindrical semiconductor window electrode  215 . 
     FIG. 39A is a proportional drawing illustrating a preferred arrangement of the elements described previously herein in a plasma etch reactor. FIG. 39B is a cross-section of a tube-shaped conductor employed in implementing the overhead inductive antenna, the interior volume of the tube being used to pump a coolant such as water. 
     Each of the foregoing embodiments has been described as employing an electrical connection of the semiconductor window to an electrical potential such as an RF power source or ground, thereby employing the semiconductor window as an electrode. However, such an electrical connection and the use of the semiconductor window as an electrode is not necessary. In fact, the semiconductor window may be allowed to float electrically and left unconnected, rather than being employed as an electrode. It would nevertheless provide certain advantages even though not necessarily functioning as an electrode. One advantage is that the semiconductor material (e.g., silicon) of the semiconductor window is less liable to be a source of contamination, in comparison with other materials (e.g., quartz or aluminum) typically employed in or near the ceiling of a typical plasma reactor. Another advantage is that the semiconductor window is a scavenger for fluorine. Thus, the semiconductor window can function simultaneously both as a shield for the inductive antenna and as a scavenger for fluorine. 
     Any of the embodiments described above may be modified by placing both the semiconductor window electrode and its overlying coil inductor inside the chamber. In this modification, the semiconductor window electrode is not part of the chamber enclosure but rather rests under the ceiling of the enclosure. For a planar semiconductor window electrode of the type employed in the embodiments of FIGS. 1,  4 - 12 ,  16 ,  18 - 20 ,  23  and  37 A, FIG. 40 illustrates the planar semiconductor window electrode  110  and the planar inductive antenna  145  inside the chamber  100  under and separate from a chamber ceiling  106 . For a curved or dome-shaped semiconductor window electrode of the type employed in the embodiments of FIGS. 13A-15,  17 A,  17 B,  21 ,  22 ,  24  and  38 , FIG. 41 illustrates the dome-shaped semiconductor window electrode  230  and the conformal inductive antenna  235  inside the chamber  100  under the chamber ceiling  106 . For a cylindrical semiconductor window as employed in the embodiments of FIGS. 7-9, FIG. 41B illustrates a cylindrical semiconductor window  230 ′ and cylindrical inductive antenna  235 ′ inside the chamber  100 . 
     While the semiconductor window of each of the foregoing embodiments has been illustrated as a monolithic structure, in accordance with one modification the semiconductor window may be segmented into plural members. Specifically, in FIGS. 42 and 43 the planar semiconductor window  110 ′ (corresponding to the planar semiconductor window electrode  110  of FIGS. 1 or  40 , for example) is comprised of a center disk  100   a  and a peripheral annulus  110   b  surrounding and concentric with the disk  110   a . Preferably, the disk  110   a  and annulus  110   b  are of the same semiconductor material such as silicon or any one of the other exemplary semiconductor materials suggested above. In one embodiment, the relative center-to-edge etch performance is adjusted by applying different levels of RF power to the annulus  110   a  and disk  110   b  as desired. This is best accomplished by employing a single RF source  150  feeding an RF power splitter  160  having respective RF power outputs applied to the disk  110   a  and annulus  110   b . This requires a third terminal (e.g., a grounded electrode) such as conductive side wall connected to RF ground (not shown in FIG.  42 ). If, for example, the etch rate near the center of the wafer were greater than at that near wafer periphery, the RF power splitter  160  could be adjusted to apply more RF power to the center disk  100   a  and relatively less to the peripheral annulus  110   b  of the semiconductor window. Moreover, for even greater control over the center-to-edge etch performance, the split inner and outer inductive antenna sections of FIGS. 6,  13 D,  16  or  17 B may be combined with the split semiconductor window electrodes  110   a ,  110   b . Specifically, FIG. 42 illustrates the inner and outer inductive antennas  175 ,  180  of FIG. 16 combined with the inner disk  110   a  and outer annulus  110   b  of the semiconductor window electrode. As in FIG. 16, the power splitter  250  has separate RF power outputs connected to respective ones of the inner and outer inductive antennas  175 ,  180 . 
     FIG. 44 illustrates an embodiment corresponding to FIG. 42 in which the split semiconductor window electrode is arcuate or dome-shaped. In FIG. 44, the semiconductor window electrode center disk  110   a  corresponds to the center portion of a dome while the semiconductor window electrode peripheral annulus  110   b  corresponds to the outer portion of a dome and a contiguous cylinder at the circumferential base of the dome. FIG. 45 illustrates a modification of the embodiment of FIG. 44 in which the curvature of the dome nearly disappears so that the center disk  110   a  is virtually planar while the annulus  110   b  is virtually a cylindrical side wall. 
     While the third terminal requisite for the RF power splitting of FIGS. 42-45 has been described as a grounded side wall, in accordance with another modification the third terminal may be the semiconductor window peripheral annulus  110   b  while the RF power is split between the semiconductor window center disk  110   a  and some other entity such as the wafer pedestal  120 . FIGS. 46 and 47 illustrate how this latter modification changes the embodiments of FIGS. 42 and 44, respectively. Such a modification carried out on the embodiment of FIG. 45 results in an embodiment corresponding to that of FIG.  13 B. In the embodiments of FIGS. 46 and 47, the RF power splitter  250  has one of its RF power outputs connected to the semiconductor window center disk  110   a  and its other output connected to the wafer pedestal  120 , while the semiconductor window peripheral annulus  110   b  is grounded. 
     The effective plasma interaction area of the semiconductor window may be changed to change the ratio of the effective plasma interaction areas of the semiconductor window and the wafer/wafer pedestal. There are three regimes for this area ratio: 
     (1) Symmetrical: the effective plasma interaction areas are about the same (the area ratio being about 1), so that both the wafer and the semiconductor window electrode have the same plasma RF current density and the same RF and DC sheath voltage magnitudes; 
     (2) Not fully assymetrical: the area ratio lies in a range from 1 to a factor between 2 and 4, so that the current density and RF and DC sheath voltage magnitudes at the smaller area electrode are greater than at the larger area electrode and change significantly with further increases in the area ratio; 
     (3) Fully assymetrical: the area ratio exceeds a factor between 2 and 4 and the current density and RF and DC sheath voltage magnitudes do not change significantly with further increases in the area ratio, a saturation condition having been reached. 
     In the last case (i.e., case 3), the greater sheath voltage drop and RF current density appears at either the semiconductor window electrode or the wafer/wafer pedestal, whichever one has the smaller effective plasma interaction area. Raising the RF current density and sheath voltage drop in this manner has the same effect as raising the RF bias applied to one element (semiconductor window or wafer/pedestal) whose interaction area was reduced. As for the semiconductor window, such an change affects the rate of sputtering of scavenger material into the plasma and affects the rate of polymerization on the surface of the semiconductor electrode. As for the wafer/pedestal, such a change affects processing parameters normally affected by changes in applied bias RF power such as etch rate, etch profile and etch selectivity, for example. 
     The same principles apply when adjusting the ratio of effective plasma interaction areas in reactors having more than two electrodes. For example, in the embodiment of FIGS. 23 and 24, there are, in effect, four electrodes reduced to two by using electrodes which drive at one frequency and ground at another. Each pair of drive/ground electrodes is analyzed separately, with the change area ratio having the same effects as outlined in the previous paragraph. As another example, in embodiments such as FIGS. 4-12 in which a third electrode such as the cylindrical side wall is connected to a potential such as ground or another RF source, the same analysis applies, except that the apportionment of the ground return current between the grounded electrode (e.g., the cylindrical side wall) and the other terminal connected to an RF power source is determined by dynamic factors such as the phase difference between the two driven terminals as well as the effective plasma interaction area ratio. In order to maintain a constant predetermined phase difference between the two RF-driven electrodes, the invention disclosed in U.S. Pat. No. 5,349,313 by Kenneth S. Collins et al. may be employed. 
     The ability to establish a desired phase relationship between a pair of RF-driven elements (electrodes or inductive antennas) may be employed to apportion RF power thereto. For example, in the embodiments such as FIGS. 16,  17 A,  17 B and  18  employing segmented inductive antenna portions, power apportionment as between the two segmented antenna portions is described above as being accomplished by varying the magnitudes of the RF voltages applied to the respective antenna portions. However, power apportionment may also be changed by changing the phase angle between the RF voltages applied to the different antenna portions. Likewise, in the embodiments such as FIGS. 42-47 employing segmented semiconductor window portions, power apportionment as between the two segmented semiconductor window portions is described above as being accomplished by varying the magnitudes of the RF voltages applied to the respective semiconductor window portions. However, power apportionment may also be changed by changing the phase angle between the RF voltages applied to the different semiconductor window portions. 
     While the invention has been described 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.