Patent Publication Number: US-2004045506-A1

Title: Inductive plasma processor method

Description:
TECHNICAL FIELD  
       [0001] The present invention relates generally to inductive plasma|workpiece processors including an RF plasma excitation coil and, more particularly, to such processors wherein the coil includes plural windings (1)having different amounts of RF power supplied to them, and (2)arranged to supply RF magnetic fields having different flux magnitudes to plasma in the chamber to control plasma flux distribution incident on a processed workpiece, and to a method of operating same. The invention also relates to an inductive plasma workpiece processor including a coil having plural parallel electrically short windings connected to impedance arrangements for controlling the amplitude of current flowing in the windings. The invention also relates to a method of making an inductive plasma processor wherein one winding of a coil is positioned relative to another winding of the coil for optimum workpiece processing.  
       BACKGROUND ART  
       [0002] An inductive plasma processor treats workpieces with an RF plasma in a vacuum chamber and includes a coil responsive to an RF source. The coil, which can be planar or spherical or dome shaped, is driven by the RF source to generate electromagnetic fields that excite ionizable gas in the chamber to produce a plasma. Usually the coil is on or adjacent to a dielectric window that extends in a direction generally parallel to a planar horizontally extending surface of the processed workpiece. The excited plasma interacts with the workpiece in the chamber to etch the workpiece or to deposit material on it. The workpiece is typically a semiconductor wafer having a planar circular surface or a solid dielectric plate, e.g., a rectangular glass substrate used in flat panel displays, or a metal plate.  
       [0003] Ogle, U.S. Pat. No. 4,948,458 discloses a multi-turn spiral planar coil for achieving the above results. The spiral, which is generally of the Archimedes type, extends radially and circumferentially between its interior and exterior terminals connected to the RF source via an impedance matching network. Coils produce oscillating RF fields having magnetic and electric field components that penetrate through the dielectric window to excite electrons and ions in a portion of the plasma chamber close to the window. The spatial distribution of the magnetic field in the plasma portion close to the window is a function of the sum of individual magnetic field components produced by the current at each point of the coils. The inductive component of the electric field is produced by the time varying magnetic field, while the capacitive component of the electric field is produced by the RF voltage in the coils. The inductive electric field is azimuthal while the capacitive electric field is vertical to the workpiece. The current and voltage differ at different points because of transmission line effects of the coil at the frequency of the RF source.  
       [0004] For spiral designs as disclosed by and based on the Ogle &#39;458 patent, the RF currents in the spiral coil are distributed to produce a toroidal shaped electric field resulting in a toroidal plasma close to the window, which is where power is absorbed by the gas to excite the gas to a plasma. The toroidal shaped magnetic field is accompanied by a ring shaped electric field which generates a toroidal shaped plasma distribution. At low pressures, in the 1.0 to 10 mTorr range, diffusion of the plasma from the toroidal shaped region where plasma density is peaked tends to smear out plasma non-uniformity and increases plasma density in the chamber center just above the center of the workpiece. However, the diffusion alone generally can not sufficiently compensate plasma losses to the chamber walls and plasma density around the workpiece periphery can not be changed independently. At intermediate pressure ranges, in the 10 to 100 mTorr range, gas phase collisions of electrons, ions, and neutrals in the plasma further prevent substantial diffusion of the plasma charged particles from the toroidal region. As a result, there is a relatively high plasma density in a ring like region of the workpiece but low plasma densities in the center and peripheral workpiece portions.  
       [0005] These different operating conditions result in substantially large plasma flux (i.e., plasma density) variations between inside the toroid and outside the toroid, as well as at different azimuthal angles with respect to a center line of the chamber that is at right angles to the plane of the workpiece holder (i.e., chamber axis). These plasma flux variations result in a substantial standard deviation, i.e., in excess of six percent, of the plasma flux incident on the workpiece. The substantial standard deviation of the plasma flux incident on the workpiece has a tendency to cause non-uniform workpiece processing, i.e, different portions of the workpiece are etched to different extents and/or have different amounts of materials deposited on them.  
       [0006] Many coils have been designed to improve the uniformity of the plasma. The commonly assigned U.S. Pat. No. 5,759,280, Holland et al., issued Jun. 2, 1998, discloses a coil which, in the commercial embodiment, has a diameter of 12 inches and is operated in conjunction with a vacuum chamber having a 14.0 inch inner wall circular diameter. The coil applies magnetic and electric fields to the chamber interior via a quartz window having a 14.7 inch diameter and 0.8 inch uniform thickness. Circular semiconductor wafer workpieces are positioned on a workpiece holder about 4.7 inches below a bottom face of the window so the center of each workpiece is coincident with a center line of the coil and the chamber center line.  
       [0007] The coil of the &#39;280 patent produces considerably smaller plasma flux variations across the workpiece than the coil of the &#39;458 patent. The standard deviation of the plasma flux produced by the coil of the &#39;280 patent on a 200 mm wafer in such a chamber operating at 5 milliTorr is a considerable improvement over the standard deviation for a coil of the &#39;458 patent operating under the same conditions. The coil of the &#39;280 patent causes the magnetic field to be such that the plasma density in the center of the workpiece is greater than in an intermediate part of the workpiece, which in turn exceeds the plasma density in the periphery of the workpiece. The plasma density variations in the different portions of the chamber for the coil of the &#39;280 patent are much smaller than those of the coil of the &#39;458 patent for the same operating conditions as produce the lower standard deviation.  
       [0008] Other arrangements directed to improving the uniformity of the plasma density incident on a workpiece have also concentrated on geometric principles, usually concerning coil geometry. See, e.g., U.S. Pat. Nos. 5,304,279; 5,277,751; 5,226,967; 5,368,710; 5,800,619; 5,401,350; 5,558,722, 5,795,429, 5,847,074 and 6,028,395. However, these coils have generally been designed to provide improved radial plasma flux uniformity and to a large extent have ignored azimuthal plasma flux uniformity. In addition, the fixed geometry of these coils does not permit the plasma flux distribution to be changed for different processing recipes. While we are aware that the commonly assigned co-pending U.S. application of John Holland for “Plasma Processor with Coil Responsive to Variable Amplitude RF Envelope,” Ser. No. 09/343,246, filed Jun. 30, 1999, and Gates U.S. Pat. No. 5,731,565 disclose electronic arrangements for at will controlling plasma flux uniformity for different recipes, the Holland and Gates inventions are concerned primarily with radial, rather than azimuthal, plasma flux uniformity. In the Holland invention, control of the plasma flux uniformity is achieved by controlling a variable amplitude envelope the RF excitation source applies to the coil. In the Gates invention, a switch or a capacitor shunts an interior portion of a spiral-like RF plasma excitation coil.  
       [0009] The frequency, i.e., reciprocal of wavelength, (typically 13.56 MHz) of the RF power source driving the coil and the lengths of the coil are such that there are significant standing wave current and voltage variations along the length of a particular winding. Voltage magnitude can change from about 1,000 volts (rms) to nearly zero volts, while the standing wave current can change nearly 50%. Hence, there are peak voltage and current somewhere along the length of each winding. However, we are aware of prior art including an RF source that drives an electrically short plasma excitation coil.  
       [0010] Our U.S. Pat. No. 6,164,241, entitled “Multiple Coil Antenna for Inductively-Coupled Plasma Generation Systems,” discloses another coil including two concentric electrically parallel windings each having first and second terminals, which can be considered input and output terminals of each winding. Each first terminal is connected via a first series capacitor to an output terminal of a matching network driven by an RF power source. Each second terminal is connected via a second series capacitor to a common ground terminal of the matching network and RF source. Each winding can include a single turn or multiple turns that extend circumferentially and radially in a spiral-like manner relative to a common axis of the two windings. Each winding is planar or three-dimensional (i.e., spherical or dome-shaped) or separate turns of a single winding can be stacked relative to each other to augment the amount of magnetic flux coupled by a particular winding to the plasma.  
       [0011] The value of the second capacitor connected between the second terminal of each winding and ground sets the locations of the voltage and current extrema (i.e., maximum and minimum) in each winding, as disclosed in Holland et al., U.S. Pat. No. 5,759,280, commonly assigned with the present invention. Controlling the value of the second capacitor of each winding controls the distribution of magnetic flux produced by the coil to the plasma and the plasma flux incident on the workpiece because the value of the capacitor determines the location of the maximum values of the RF standing wave current and voltage in each respective winding. The value of the first capacitor determines the maximum magnitude of the current and voltage standing waves in each winding. The values of the first capacitors are also adjusted to help maintain a tuned condition between the RF source and the load it drives, which is primarily the coil and the plasma load coupled to the coil. Adjusting the maximum magnitude and location of the standing wave current in each winding controls the plasma density in different radial and azimuthal regions of the chamber.  
       [0012] It is desirable, in certain instances, to maintain the current in one of the windings relatively constant while changing the current in the remainder of the coil. The RF current generates the magnetic field, and the time varying magnetic field in free space produces the inductive electric field, which in turn generates the plasma and induces a plasma “image” current which is the mirror image of the driving RF current. By maintaining the current in one of the windings relatively constant, the electric field produced by that winding and supplied to the plasma in the chamber remains relatively constant, despite variations in the electric field produced by the remainder of the coil and supplied to the plasma. Maintaining the electric field produced by one of the windings relatively constant while varying the electric field produced by the remainder of the coil and supplied to the plasma provides substantial control for the plasma density incident on the workpiece. Such control is particularly advantageous in connection with processing chambers operating with different recipes, which are performed without opening the vacuum chamber. Such chambers operate at different times under differing conditions. Examples of the different conditions are different processing gases, different pressures and different workpieces.  
       [0013] Consider a coil having first and second parallel, concentric windings respectively close to (1) the chamber periphery and (2) the chamber axis. The first and second windings respectively couple ring shaped electric fields to the peripheral portions of the chamber (close to the chamber wall) and to the chamber center. It is desirable, in certain instances, to maintain the current flowing in the outer winding substantially constant at times, while differing currents flow in the inner windings. This causes the outer winding to produce a substantially constant electric field in the chamber peripheral portions while the inner winding generates different electric fields in the chamber central region. Such a result is attained by simultaneously adjusting the overall impedance in each winding and the total power since these windings are closely coupled to both windings. Since these windings are closely coupled, the change of the overall impedance in each winding causes change in current splitting as well as power splitting between these windings. The current in each winding is changed as the impedance in any winding changes. Therefore, the current in one winding can be compensated by changing the total power in order to maintain constant current in that winding. The ability to maintain a constant electric field in the chamber peripheral portion results in an extra process control knob to maintain constant power deposition in that region and further maintain constant processing results (e.g. etch rate or deposition rate) on the peripheral portion of a workpiece. This process control is particularly useful to compensate changes due to process conditions. In other situations, particularly for other pressures, as discussed supra, it is desirable to maintain the electric field in the chamber center substantially constant at times while the amplitude of the electric field in a peripheral portion of the chamber is changed. This process control capability is particularly useful to compensate the plasma loss to chamber walls and electric coupling to the grounded portion of chamber walls.  
       [0014] It is accordingly an object of the present invention to provide a new and improved vacuum plasma processor and method of operating same wherein the plasma density incident on the workpiece can be controlled at will.  
       [0015] An additional object of the present invention is to provide a new and improved vacuum plasma processor and method of operating same wherein the plasma density incident on a workpiece has relatively high uniformity.  
       [0016] An additional object of the present invention to provide a new and improved vacuum plasma processor and method of operating same wherein plasma density incident on a workpiece of the processor has relatively high azimuthal uniformity.  
       [0017] A further object of the invention is to provide a new and improved vacuum plasma processor including a coil with plural parallel windings driven by a single RF source via a single matching network and having improved control for the electric and magnetic fields that are produced by the coil and coupled to plasma in the chamber.  
       SUMMARY OF THE INVENTION  
       [0018] In accordance with one aspect of the invention, there is an improved control method and apparatus for the distribution of electromagnetic fields coupled by a plasma excitation coil to a plasma in a vacuum plasma processor for processing a workpiece, wherein the coil includes plural parallel windings for coupling inductive electric fields to plasma in the chamber. In the method and apparatus, (a) the total amount of RF power applied to the plural windings is controlled so that for different distributions of electromagnetic fields, different amounts of total RF power are applied to the plural windings and (b) the amount of RF current applied to individual plural windings is controlled so that for different distributions of the electromagnetic fields, different amounts of RF current are applied to the individual windings.  
       [0019] In a preferred embodiment, the windings are arranged so (a) one of the windings is an exterior winding located so the electromagnetic fields generated by it are in proximity to a peripheral wall of the chamber, and (b) electromagnetic fields generated by the remainder of the coil are remote from the chamber peripheral wall. The RF current applied to the exterior winding is controlled so the electromagnetic fields generated by the exterior winding exceeds the electromagnetic fields generated by the remainder of the coil in one arrangement. In a second arrangement, the electromagnetic fields generated by the exterior winding is less than electromagnetic fields generated by the remainder of the coil. In another arrangement, which results in a very nearly uniform plasma density on the workpiece, the RF currents applied to the exterior winding and the remainder of the coil are somewhat equal.  
       [0020] By controlling the currents, the RF power coupled to one of the windings is maintained substantially constant for different electromagnetic fields distributions and the RF power coupled to another of the windings is changed for the different distributions. The power maintaining and changing operations are preferably performed by controlling the values of impedances associated with the individual windings and the total power applied to the coil. In one embodiment, each of the plural windings includes first and second terminals such that the first terminal is connected via a first series capacitor to an output terminal of a matching network driven by a source of the RF power and the second terminal is connected via a second series capacitor to the common ground terminal of the matching network. The values of the impedances are controlled by controlling the capacitance of at least one capacitor associated with each individual winding.  
       [0021] In one embodiment, the RF power has a frequency and the windings have lengths such that there are substantial standing wave current variations along the lengths of the individual windings. In such a configuration, the value of at least one capacitor associated with each winding is adjusted so that adjacent windings have standing wave RF current maxima that are radially opposite to each other.  
       [0022] In another aspect of the invention, the RF power has a frequency and the windings have lengths such that there are no substantial standing wave current variations along the lengths of the individual windings. In such a configuration, only one capacitor need be associated with each winding and its capacitance is adjusted to control the current amplitude flowing in the winding.  
       [0023] A further aspect of the invention involves positioning an exterior winding of the coil relative to the remainder of the coil to achieve substantially uniform plasma density on the workpiece. In particular, the exterior winding is turned about an axis of the coil, relative to the remainder of the coil which is substantially concentric with and inside the exterior winding.  
       [0024] The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed descriptions of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0025]FIG. 1 is a schematic diagram of a vacuum plasma processor of the type employed in connection with the present invention;  
     [0026]FIG. 2 is an electrical schematic diagram of a coil included in the processor of FIG. 1 in combination with an RF source, a matching network, and electronic control circuitry for driving the coil and for controlling (1) the capacitances of variable capacitors connected to the coil and (2) the total power supplied to the coil;  
     [0027]FIG. 3 includes calculated amplitudes of standing wave currents in the windings of the coil illustrated in FIG. 2 for excitations at 13.56 MHz and 4.0 MHz;  
     [0028]FIG. 4 is a circuit diagram of the matching network with current sensors for driving the coil of FIG. 2; and  
     [0029]FIG. 5 is a sputter rate contour plot resulting from use of the apparatus of FIGS. 1, 2 and  4 . 
    
    
     DETAILED DESCRIPTION OF THE DRAWING  
     [0030] The vacuum plasma workpiece processor of FIG. 1 of the drawing includes vacuum chamber  10 , shaped as a cylinder having grounded metal wall  12 , metal bottom end plate  14 , and circular top plate structure  18 , consisting of a dielectric window structure  19 , having the same thickness from its center to its periphery. Sealing of vacuum chamber  10  is provided by conventional gaskets (not shown). The processor of FIG. 1 can be used for etching a semiconductor, dielectric or metal substrate or for depositing materials on such substrates.  
     [0031] A suitable gas that can be excited to a plasma state is supplied to the interior of chamber  10  from a gas source (not shown) via port  20  in side wall  12  and further distributed uniformly through a gas distribution mechanism (not shown). The interior of the chamber is maintained in a vacuum condition, at a pressure that can vary in the range of 1-1000 milliTorr, by a vacuum pump (not shown) connected to port  22  in end plate  14 .  
     [0032] The gas in chamber  10  is excited by a suitable electric source to a plasma having a controlled spatial density. The electric source includes a planar or spherical or dome like coil  24 , mounted immediately above window  19  and excited by variable power RF generator  26 , typically having a fixed frequency of 13.56 MHz.  
     [0033] Impedance matching network  28 , connected between output terminals of RF generator  26  and excitation terminals of coil  24 , couples RF power from the generator to the coil. Impedance matching network  28  includes variable reactances which controller  29  varies in a known manner in response to indications of the amplitude and phase angle of the voltage reflected to the input terminals of the matching network, as sensed by detector  43 . Controller  29  varies the values of the reactances in network  28  to achieve impedance matching between source  26  and a load including coil  24  and the plasma load the coil drives.  
     [0034] Controller  29  also responds to input device  41  to control variable reactances coupled to coil  24 . Input device  41  can be a manual device, such as a potentiometer or keys of a key pad, or a microprocessor responsive to signals stored in a computer memory for different processing recipes of workpiece  32 . Variables of the recipes include (1) species of gases flowing through port  20  into chamber  10 , (2) pressure in chamber  10  controlled by the vacuum pump connected to port  22 , (3) the total output power of RF source  26 , which is substantially equal to the power, supplied to coil  24 , and (4) the values of capacitors connected to coil  24 .  
     [0035] Workpiece  32  is fixedly mounted in chamber  10  to a surface of workpiece holder (i.e., platen or chuck)  30 ; the surface of holder  30  carrying workpiece  32  is parallel to the surface of window  19 . Workpiece  32  is usually electrostatically clamped to the surface of holder  30  by a DC potential that a DC power supply (not shown) applies to a chuck electrode (not shown) of holder  30 . RF source  45  supplies the radio frequency electromagnetic wave to impedance matching network  47 , that includes variable reactances (not shown). Matching network  47  couples the output of source  45  to holder  30 . Controller  29  responds to signals that amplitude and phase detector  49  derives to control the variable reactances of matching network  47  to match the impedance of source  45  to the impedance of an electrode (not shown) of holder  30 . The load coupled to the electrode in holder  30  is primarily the plasma in chamber  10 . As is well known the RF voltage source  45  applies to the electrode of holder  30  interacts with charge particles in the plasma to produce a DC bias on workpiece  32 .  
     [0036] Surrounding coil  24  and extending above top end plate  18  is a metal tube or can-like shield  34  having an inner diameter somewhat greater than the inner diameter of wall  12 . Shield  34  decouples electromagnetic fields originating in coil  24  from the surrounding environment. The diameter of cylindrically shaped chamber  10  defines the boundary for the electromagnetic fields generated by coil  24 . The diameter of dielectric window structure  19  is greater than the diameter of chamber  10  to such an extent that the entire upper surface of chamber  10  is comprised of dielectric window structure  19 .  
     [0037] The distance between the treated surface of workpiece  32  and the bottom surface of dielectric window structure  19  is chosen to provide the most uniform plasma flux on the exposed, processed surface of the workpiece. For a preferred embodiment of the invention, the distance between the workpiece processed surface and the bottom of the dielectric window is approximately 0.2 to 0.4 times the diameter of chamber  10 .  
     [0038] Coil  24  includes plural parallel windings each of which is electrically long enough at the 13.56 MHz frequency of source  26  to function as a transmission line having a total electric length of about 30 to 45° to produce standing wave patterns along the length of the winding. The standing wave patterns result in variations in the magnitude of standing wave RF voltages and currents along the lengths of the windings. The dependence of the magnetic fluxes generated by the windings on the magnitude of these RF currents results in different plasma density being produced in different portions of chamber  10  beneath different windings of coil  24 .  
     [0039] The variations in the RF current magnitude flowing in different windings of the coil are spatially averaged to assist in controlling plasma density spatial distribution. Spatially averaging these different current values in the different windings of the coil can substantially prevent azimuthal asymmetries in the plasma density, particularly at regions of low RF current in the windings. Alternatively, the frequency of generator  26  is 4.0 MHz, in which case the windings of coil  24  are electrically short, about 10° to 15°, causing the standing wave currents and voltages in the windings to be substantially constant.  
     [0040] Controller  29  includes microprocessor  33  (FIG. 2) which responds to (1) input control  41 , (2) voltage amplitude and phase angle signals that detector  31  derives, and (3) memory system  35  that stores programs for controlling microprocessor  33  as well as signals controlling the values of variable capacitors connected to coil  24  and the output power of RF generator  26 . Among the programs memory system  35  stores are control programs for the values of the variable reactances of matching networks  28  and  47 . The output power of source  26  and the values of capacitors connected to coil  24  can also be pre-set at the time the processor is made or installed, particularly if the processor is dedicated to a single recipe.  
     [0041] As illustrated in FIG. 2, coil  24  includes two parallel windings  40  and  42 , both of which are generally concentric with center coil axis  44  and include multiple spiral-like turns that extend radially and circumferentially with respect to axis  44 . Interior winding  40  is wholly within exterior winding  42 , such that winding  42  completely surrounds winding  40 . Winding  40  includes interior terminal  46  and exterior terminal  48 , while winding  42  includes exterior terminal  50  and interior terminal  52 .  
     [0042] Interior winding  40  includes three concentric turns  54 ,  56  and  58  having different radii, as well as two straight segments  60  and  62 . Each of turns  54 ,  56  and  58  is a segment of a circle centered on axis  44  and having an angular extent of about 340°. Adjacent ends of turns  54  and  56  are connected to each other by straight segment  60 , while straight segment  62  interconnects adjacent ends of turns  56  and  58  to each other. Straight segments  60  and  62  extend radially and circumferentially along substantially parallel paths.  
     [0043] Exterior winding  42  includes two concentric turns  64  and  66  having different radii, as well as straight segment  68 . Each of turns  64  and  66  is a segment of a circle centered on axis  44  and having an angular extent of about 340°. Straight segment  68  extends radially and circumferentially to connect adjacent ends of turns  64  and  66  to each other.  
     [0044] The sum of the lengths of turns  54 ,  56  and  58  and sectors  60  and  62  of winding  40  is about equal to the sum of the lengths of turns  64  and  66 , as well as sector  68  of winding  42 . Because windings  40  and  42  have substantially equal lengths, they have standing wave voltage and current variations along their length which are substantially the same, regardless of the frequency that generator  26  supplies to them.  
     [0045] Windings  40  and  42  of coil  24  are driven in parallel by RF current derived by a single fixed frequency RF generator  26 , having a controlled variable output power. As described infra, at either the low (e.g. 4.0 MHz) or high (e.g. 13.56 MHz) frequency of generator  26 , there is a single current maximum in each of windings  40  and  42 . At the high frequency, the current maxima are at locations that are about half-way between the terminals of each winding. The current maxima occur at radially opposite points of the windings  40  and  42  relative to axis  44  to provide approximate azimuthal symmetry to the ring shaped electric field resulting from RF excitation of windings  40  and  42  by generator  26 .  
     [0046] Windings  40  and  42  are respectively in separate parallel circuit branches  81  and  83 . Branch  81  includes series connected winding  40  and variable capacitors  80  and  84 , while branch  83  includes series connected winding  42  and variable capacitors  82  and  86 . The turns of windings  40  and  42  of coil  24  are arranged so that input terminals  46  and  50 , which are driven in the parallel by power from the output terminal of matching network  28 , are on opposite sides of coil axis  44  so current flows in the same direction from terminals  46  and  50  into the remainder of windings  40  and  42 . Terminal  46  is on the smallest radius turn  54  of coil  24  and terminal  58  is on the largest radius turn  66 . Terminals  46  and  50  are respectively connected by series variable capacitors  80  and  82  to the output terminal of matching network  28 .  
     [0047] Output terminals  48  and  52  of coil  24 , which are diametrically opposite to each other relative to axis  44 , are connected to the ground common terminals via series variable capacitors  84  and  86 .  
     [0048] For the high frequency output of source  26 , the values of capacitors  84  and  86  are set such that the standing wave currents in windings  40  and  42  have minimum amplitudes at the input and output terminals  46  and  48  of winding  40  and at terminals  50  and  52  of winding  42 , where the standing wave voltages are at maxima. The standing wave currents in windings  40  and  42  have maximum values at radially opposite points of windings  40  and  42 , were the standing wave voltages are at minima, a result achieved by adjusting the values of capacitors  84  and  86 . The standing wave current maximum can be located by monitoring the standing wave voltages. The current maximum occurs at a place where the voltage is a minimum (close to zero volt). Locating the standing wave current maxima in windings  40  and  42  to be radially opposite to each other assists in providing azimuthally symmetric plasma density.  
     [0049] The values of capacitors  80  and  82  help keep the impedance of each of windings  40  and  42  tuned to matching network  28 . The maximum amplitudes of the standing wave currents in windings  40  and  42  are respectively controlled by the values of capacitors  80  and  82 . The physical configuration of windings  40  and  42  and the location of terminals  46 ,  48 ,  50  and  52  affect the positions of the maximum standing wave currents in windings  40  and  42 .  
     [0050] Proper control of the values of capacitors  80 ,  82 ,  84  and  86 , as well as the total output power of generator  26 , i.e., the power that generator  26  applies in parallel to windings  40  and  42 , enables the current in one of windings  40  or  42  to remain substantially constant, while providing changes of the current in the other winding. The ability to vary the total power while maintaining the current in one of windings  40  or  42  substantially constant provides substantial control over the electromagnetic field distribution resulting from energization of the windings. By maintaining the current in one of windings  40  or  42  substantially constant, the electromagnetic field produced by that winding, and supplied to the plasma in chamber  10  remains relatively constant. Changing the current in the other winding  40  or  42  causes the electromagnetic field that winding supplies to the plasma in chamber  10  to vary. As described previously, different workpiece processing recipes require the electromagnetic power deposited by winding  40  to remain substantially constant and the power that winding  42  couples to the plasma to be varied. For other recipes, it is desirable for the power distribution that winding  42  supplies to the plasma in chamber  10  to remain constant and the power that branch  40  supplies to the plasma in chamber  10  to be varied.  
     [0051] The values of capacitors  80 ,  82 ,  84  and  86 , as well as the output power of generator  26 , are controlled for different recipes by manual adjustment of these parts or by automatic adjustment thereof in response to signals stored in memory system  35  being read out by microprocessor  33  in response to recipe signals from input controller  41 . Alternatively, if a particular coil always operates in connection with a processor that always operates with the same recipe, the values of capacitors  80 ,  82 ,  84  and  86 , as well as the output power of generator  26 , can be set at the factory, at the time the processor is manufactured, or during installation of the processor.  
     [0052] Assume each of windings  40  and  42  typically has a resistance of 6 ohms, which enables the RMS (root mean squared) current in winding  42  to be maintained substantially constant and the RMS current in winding  40  to be varied by adjusting the output power of generator  26  and the total reactances (X 1 ) and (X 2 ) of branches  81  and  83  to be in accordance with Table I:  
               TABLE 1                          (R 1  = R 2  = 6Ω)                                     ↓ Cases   P tot (W)   X 1 (Ω)   X 2 (Ω)   I 1 (A)   I 2 (A)                                                 a)   Equal currents   1000   40   40   9.13   9.13           in 40 and 42       b)   Larger current   1570   20   30   13.36   9.12           in 40 than 42       c)   Lower current   850   60   50   7.63   9.14           in 40 than 42                  
 
     [0053] Similarly, if it is desired to maintain a substantially constant current in interior winding  40  and a variable current in outer winding  42 , the reactances of branch  81  (X 1 ) and branch  82  (X 2 ) and the output power of generator  26  are adjusted in accordance with Table II.  
               TABLE II                          (R 1  = R 2  = 6Ω)                                     ↓ Cases   P tot (W)   X 1 (Ω)   X 2 (Ω)   I 1 (A)   I 2 (A)                                                 a)   Equal currents   1000   40   40   9.13   9.13           in 40 and 42       b)   Larger current   1570   30   20   9.12   13.36           in 40 than 42       c)   Lower current   850   50   60   9.14   7.63           in 40 than 42                  
 
     [0054] By varying the values of capacitors  80 ,  82 ,  84  and  86 , as well as the power of source  26 , control of the plasma density incident on workpiece in both the azimuthal and radial coordinate directions is achieved.  
     [0055] The following analysis of branches  81  and  82  provides a quantitative insight into deriving appropriate values for impedances associated with the branches.  
     [0056] Assume the currents and the impedances are I 1 , and z 1 , respectively for branch  81 , and are I 2  and z 2 , respectively for branch  83 . Since each branch consists of the series combination of an input capacitor, a winding and an output capacitor, the impedance z 1  or z 2  is the lump sum of all the impedances from the input (C 1  or C 2 ) and the output (C 3  or C 4 ) capacitors, and the winding (L 1 or L 2 ) for branch  81  or branch  83 . Thus z 1 =R 1 +j[ωL 1 −1/(ωC 1 )=1/(ωC 3 ]= 1 +jX 1 , where R 1  and X 1 =ωL 1 −1/(ωC 1 )−1/(ωC 3 ) respectively represent the real (resistive) and imaginary (reactive) parts of impedance z 1 . Similarly, z 2 =R 2 +j X 2 , where R 2  and X 2 =ωL 2 −1(ωC 2 )−1/(ωC 4 ) respectively represent the resistive and reactive parts of impedance z 2 . Let V be the RF voltage across either branch; I be the total current supplied to branches  81  and  83 ; P tot  be the total power dissipated in the two branches, i.e., the output power of source  26 ; and z be the overall impedance of the two branches. Because branches  81  and  83  are in parallel  
             z   =           z   1          z   2           z   1     +     z   2         =         (       R   1     +     j                   X   1         )          (       R   2     +     j                   X   2         )           (       R   1     +     R   2       )     +     j        (       X   1     +     X   2       )                     (   1   )                       
 
     [0057] The impedance given by Equation (1) can be rewritten as z=|z|e jφ =R+jX, where R is the overall real component of coil  24 , i.e., of windings  40  and  42  in parallel with each other and is obtained from Equation (1) as:  
             R   =           (       R   1     +     R   2       )          (         R   1          R   2       -       X   1          X   2         )       +       (       X   1     +     X   2       )          (         R   1          X   2       +       R   2          X   1         )               (       R   1     +     R   2       )     2     +       (       X   1     +     X   2       )     2                 (   2   )                            =           R   1          (       R   2   2     +     X   2   2       )       +       R   2          (       R   1   2     +     X   1   2       )               (       R   1     +     R   2       )     2     +       (       X   1     +     X   2       )     2                   (     2      a     )                       
 
     [0058] From Equation 2(a), R is more sensitive to changes of R 1  and R 2  than to changes of X 1  and X 2 .  
     [0059] Then P tot  is given by  
               P   tot     =         1   2          V   o          I   o        cos                 ϕ     =         1   2          I   o   2             z           cos                 ϕ     =         1   2          I   o   2        R     =       I     r                 m                 s     2        R                   (   3   )                       
 
     [0060] where V o  and I o  are respectively peak amplitudes of the voltage and current matching network  28  applies to coil  24 , I rms  is the rms current I matching network  28  applies to coil  24 , and φ is the phase difference between the voltage and current matching network  28  applies to coil  24  since V/I=z=|z|e jφ . Moreover V o −I o |z| and I rms =I o /{square root}{square root over (2)}.  
     [0061] The current I 1  in branch  81  can be calculated from Equations (1) and (3), as  
               I   1     =       V     z   1       =         I   z       z   1       =         z   2         z   1     +     z   2            I                 (   4   )                       
 
     [0062] The rms value of I 1  is obtained by substituting Equations (2) and (3) into (4)  
                   I   1          (     r                 m                 s     )       =                z   2         z   1     +     z   2                   I     r                 m                 s         =             R   2   2     +     X   2   2             (       R   1     +     R   2       )     2     +       (       X   1     +     X   2       )     2         ·       P   tot     R                  
          Similarly   ,             (   5   )                   I   2          (     r                 m                 s     )       =                z   1         z   1     +     z   2                   I     r                 m                 s         =             R   1   2     +     X   1   2             (       R   1     +     R   2       )     2     +       (       X   1     +     X   2       )     2         ·       P   tot     R                   (   6   )                       
 
     [0063] Equations (5) and (6) clearly show that the currents in branches  81  and  83 , and windings  40  and  42  are coupled. For a given constant total power P of source  26 , as X 1  increases (by decreasing the value of the input capacitor  80  in branch  81 ), I 1  decreases while I 2  increases. Since R (from Equation 2(a)) changes very little as X 1  or X 2  changes, R can be approximately treated as a constant in the discussion here.  
     [0064] For simplicity, assume the frequency of RF source  26  is 13.56 MHz and the electrical length of each of branches  40  and  42  is 77° at 13.56 MHz and the values of capacitors  80 ,  82 ,  84  and  86  are properly adjusted so there are equal currents in windings  40  and  42 . For such a situation, the standing wave current amplitudes along the lengths of each of windings  40  and  42  are as depicted by curve  90 , FIG. 3. Curve  90  has a sinusoidal-like variation between the input and output terminals of each of windings  40  and  42 . Curve  90  has a peak value of approximately 14.5 amperes RMS at the midpoint of the curve, i.e., at 38°, and minimum equal values of about 10.7 amperes RMS at the input and output terminals of windings  40  and  42 . Thus, the maximum standing wave current in each of windings  40  and  42  exceeds the minimum standing wave current by approximately 3.8 amperes RMS, i.e., by about 21%.  
     [0065] A possible problem with operating the arrangement of FIG. 2 at a frequency of 13.56 MHz is that capacitors  80 ,  82 ,  84  and  86  might have to be adjusted simultaneously or in an iterative manner to maintain the desired relationship for the electromagnetic field distributions derived from windings  40  and  42 . For example, to maintain an azimuthally symmetric density on the workpiece, requires the maximum currents in each coil to be located radially opposite to each other relative to axis  44 . This is achieved by adjusting the values of capacitors  84  and  86  connected between the output terminals of windings  40  and  42  and ground. Adjusting the values of capacitors  84  and  86  may require adjustment of capacitors  80  and  82  to provide the desired values of standing wave current to achieve the values indicated in Tables I and II. However, adjusting the values of capacitors  80  and  82  can cause a further shift in the current standing wave patterns in windings  40  and  42 , whereby the maxima of the current standing wave patterns are no longer diametrically opposed relative to coil axis  44 . If the current standing wave maxima are shifted in this manner, further adjustment of the values of capacitors  84  and  86  may be necessary.  
     [0066] To overcome this problem, we have realized that if the current variations along windings  40  and  42  can be substantially reduced, such that the location of the standing wave current maxima in windings  40  and  42  is not critical, i.e., the maxima do not have to be on diametrically opposite sides of coil axis  44 , that only a single variable capacitor need be connected to each of windings  40  and  42 . In other words, the necessity to simultaneously or iteratively adjust all four capacitors  80 ,  82 ,  84  and  86  would be obviated.  
     [0067] To these ends, one embodiment of the invention involves reducing the frequency of RF source  26  so that the transmission line effects of windings  40  and  42  are substantially reduced. If the electrical length of each of windings  40  and  42  is substantially less than about 45°, the percent change between the maximum and minimum values of the standing wave current is reduced sufficiently to enable only a single variable capacitor  84  and  86  to be connected in series with windings  40  and  42 , respectively, and the need for any capacitor to be connected between each winding input terminal and the power output terminal of matching network  28  is obviated.  
     [0068] As mentioned previously, in one preferred embodiment, the frequency of RF source  26  is reduced to 4.0 MHz from 13.56 MHz, resulting in a decrease in the electrical length of windings  40  and  42  by a factor of 3.4. Curve  92 , FIG. 3, represents the situation of capacitors  84  and  86  being adjusted so equal standing wave currents are in windings  40  and  42 . The same physical windings that are analyzed at 13.56 MHz (shown in curve  90  ) are re-analyzed at 4.0 MHz (shown in curve  92  ). The electrical length of each of branches  40  and  42  becomes 22.6° (i.e., 77°/3.4). Curve  92  has standing wave current minima of approximately 25.7 amperes RMS at the input and output terminals of windings  40  and  42  and a maximum standing wave current of approximately 26 amperes RMS at the centers of the windings. Despite the fact that substantially larger current flows in windings  40  and  42  for the short transmission line situation of curve  92  than for the long transmission line situation of curve  90 , the output power of source  24  was the same, 2400 watts, for both situations. For the exemplary equal current curves  90  and  92  of FIG. 3, the capacitances of capacitors  84  and  86  are equal to each other and have a value of 137 picofarads (pF) for the 13.56 MHz frequency of source  26 , while the values of capacitors  84  and  86  are 1808 pF for the 4.0 MHz excitation of source  26 .  
     [0069] The percentage change between the maxima and maximum standing wave currents of curve  92  is about 2%, in contrast with the 21% change of curve  90 . Because a relatively low frequency of excitation source  26  results in a relatively small change between the minima and maximum standing wave currents of windings  40  and  42 , there is a relatively uniform azimuthal electric field produced by each of windings  40  and  42  to the plasma in chamber  10 . Consequently, the need to include capacitors  80  and  82 , to adjust the position of the maximum standing wave currents in windings  40  and  42  does not exist. Tables I and II provide the information necessary for the low frequency excitation to adjust the capacitances of capacitors  84  and  86  and the output power of RF source  26  to achieve constant currents in coils  40  and  42 , respectively.  
     [0070] The ratio (I 1 /I 2 )of the maximum standing wave currents in windings  40  and  42  can be varied continuously from 20:1 to 1:1, then from 1:1 to 1:10, for the 4.0 MHz excitation power of source  26 , by adjusting the value of capacitor  84 , while maintaining the value of capacitor  86  constant, and then by adjusting the value of capacitor  86 , while maintaining the value of capacitor  84  constant, where I 1  is the maximum standing wave current in winding  40  and I 2  is the maximum standing wave current in winding  42 . As the values of capacitors  84  and  86  are varied, the output power of source  26  is varied to provide the same effects as indicated by Tables I and II.  
     [0071] To control the values of capacitors  80 ,  82 ,  84  and  86 , in response to output signals of microprocessor  33 , each of the capacitors is driven by a different one of DC motors  87 . Each of motors  87  responds to a different output signal of microprocessor  33 . The signals microprocessor  33  supplies to motors  87  have values commensurate with the amount that the output shafts of the motors are to be turned to achieve the desired capacitance values of capacitors  80 ,  82 ,  84  and  86 . Matching network  28  includes variable reactances (preferably capacitors, FIG. 4) which are driven by motors  88 . Motors  88  respond to different signals microprocessor  33  derives in response to signals derived by a program stored by memory system  35  and detector  43 . Detector  43  derives signals representing (1) the voltage amplitude reflected by matching network  28  toward generator  26  and (2) the difference in phase between the reflected voltage and current. Microprocessor  33  supplies a suitable DC signal to generator  26  to control the generator output power. Microprocessor  33  responds to signals indicative of the voltage applied in parallel to branches  81  and  83  and by RF source  26  and matching network  28 , as well as signals indicative of the standing wave currents at the output terminals  48  and  52  of branches  81  and  83 , as derived by circuitry described in connection with FIG. 4.  
     [0072] Reference is now made to FIG. 4 of the drawing, a circuit diagram of a preferred embodiment of electronic circuitry associated with 4.0 MHz drive of coil  24 . RF source  26  drives matching circuit  28  via phase and magnitude detectors  43  and fixed series capacitor  100 , preferably having a capacitance of 2000 pF. Matching network  28  includes variable shunt capacitor  102  and variable series capacitor  104  having capacitance values which are varied by motors  88 .  
     [0073] The output power of matching circuit  28  is coupled in parallel to branches  81  and  83  via series inductor  106 , RF voltage detector  108  and phase detector  109 . RF voltage detector  108  derives a DC voltage indicative of the peak amplitude of the RF voltage at the joint input terminals of branches  81  and  83 , while phase detector  109  derives a DC voltage indicative of the difference in phase between the RF voltage and current at the joint input terminals of branches  81  and  83 . The output voltages of detectors  108  and  109  are fed back to microprocessor  33  which in turn controls motors  87  and the output power of generator  26  to achieve the previously discussed results. The currents flowing through branches  81  and  83  are respectively coupled to ground via variable capacitors  84  and  86 .  
     [0074] The magnitudes of standing wave currents at output terminals of branches  81  and  83  are respectively detected by current amplitude sensors  110  and  112 , respectively inductively coupled to wire leads  111  and  113  that are connected between the low voltage electrodes of capacitors  84  and  86  and ground. Each of current sensors  110  and  112  includes a current transformer including toroidal coil  109  having a central opening through which the wire leads  111  and  113  extend to provide the inductive coupling. Each of current sensors  110  and  112  also includes a rectifier and low-pass filter for supplying to microprocessor  33  a DC current indicative of the currents respectively flowing through terminals  48  and  52 .  
     [0075] Grounded electromagnetic shields  114  and  116  are respectively interposed between current sensors  110  and  112  and capacitors  84  and  86  to minimize electromagnetic interference from RF fields of the remaining apparatus, particularly from windings  40  and  42 . Shield  114  or  116  consists of a ring-shaped metal plate  119  and shield  121 . Shield  119  has an opening for lead  111  or  113  to run through. Shield  121  is a metal cylinder which horizontally encloses sensor  110  or  112  and lead  111  or  113 . Together with shield  119  and plate  115 , which vertically sandwich the sensor, sensor  110  and  112  and lead  111  or  113  are completely shielded from ambient RF fields, thereby greatly improving the accuracy of the current sensor. Shields  119  and  121  are preferably made of silver-plated copper. Shield  121  is mechanically and electrically connected only to plate  115 . All the voltages at the output terminals of windings  40  and  42  are across capacitors  84  and  86  so end plates  142  of the capacitors connected to leads  111  and  113  are virtually at ground. Shields  114  and  116  and current detectors  110  and  112  are arranged together with detector  43 , capacitors  100 ,  102  and  104 , coil  106  and detectors  108  and  109  in metal housing  117 . The details of the current sensors  110  and  112  and shields  114  and  116  are described by our co-pending application entitled “INDUCTIVE PLASMA PROCESSOR INCLUDING CURRENT SENSOR FOR PLASMA EXCITATION COIL” (Lowe Hauptman Gilman and Berner Docket No. 2328-051).  
     [0076] Each of capacitors  84 ,  86 ,  100 ,  102  and  104  is a vacuum capacitor capable of handling relatively large currents which flow from RF source  26  to windings  40  and  42 . Because of the relatively short electrical length of each of windings  40  and  42  at 4 MHz, relatively large capacitance values are required for capacitors  84  and  86 , with typical maximum values of the capacitors being 2500 pF. Shunt load capacitor  102  has a relatively large maximum value of 1400 pF to match the low impedance of parallel branches  81  and  83 . Series capacitor  104  is a relatively large capacitor, having a maximum value of 1500 pF to tune the low inductive reactances of parallel branches  81  and  83 .  
     [0077] Fixed input series connected capacitor  100 , preferably having a value of 200 pF, provides part of the impedance transformation between source  26  and the parallel windings  40  and  42  of coil  24 . Capacitor  100  is included to enable shunt, load capacitor  102  to have a more reasonable value; otherwise, capacitor  102  would have a considerably higher capacitance value than the values associated with a capacitor having a maximum value of 1400 pF. Fixed capacitor  100  also provides better tuning resolution, to attain better resonant tuning of matching circuit  28  with parallel windings  40  and  42  of coil  24 .  
     [0078] Fixed inductor  106 , preferably having a relatively large value of 3.5 microhenries, extends the tuning range of matching network  28 . Inductor  110 , which is outside housing  117  and is optionally connected to interior winding  40 , can be employed to provide substantially equal impedances for the parallel branches  81  and  83  associated with windings  40  and  42 . Inductor  110  is used if winding  42  has an inductance substantially greater than the inductance of winding  40 .  
     [0079] Voltage detector  108  and current sensors  110  and  112  supply signals to microprocessor  33 . Microprocessor  33  responds to the signals from voltage detector  108 , current sensors  110  and  112  and the phase indication detector  109  and derives the total output power RF source  26 . The indication of total power is used to control the output power of RF generator  26  to enable the powers indicated by Tables I and II to be achieved. The signals that current sensors  110  and  112  derive are used by microprocessor  33  to control the motors which vary the capacitances of capacitors  84  and  86  to assure that the correct currents are flowing in windings  40  and  42  to achieve the currents specified in Tables I and II.  
     [0080]FIG. 5 is a contour plot of sputter rate on an 8-inch semiconductor wafer uniformly covered by an oxide layer. The plot resulted from an inductive discharge using windings  40  and  42  shown in FIG. 2. Windings  40  and  42  were driven by a 1500 watt output of generator  26 , at a frequency of 4 MHz. The electrode in chuck  30  was driven by a 1400 watt output of source  45  at a frequency of 4 MHz, creating a DC bias of −375V in the chemistries of 85 sccm (cm 3 /minute) of Ar and 100 sccm of O 2  with a total pressure of 5 mTorr. Capacitors  80  and  82  were omitted and capacitors  84  and  86  were adjusted so the ratio of standing wave maximum currents in inner winding  40  (I 1 ) to that in outer winding  42  (I 2 ) is fixed at I 1 /(I 2 )=1.4:1.9. The powers dissipated in the inner winding and in the outer winding are roughly balanced. The sputter rate contour plot indicates a uniform plasma density. The spatially averaged sputter rate indicated by line  170  is 1211 angstroms/minute and standard deviation is 3.2%. The “+” sign in FIG. 5 indicates a sputter rate higher than the average while the “−” sign denotes a sputter rate lower than the average. The equipment that produced the contour plot of FIG. 5 produces contours for each 50 angstroms of etched material. Since there is only one contour line in FIG. 5, etching was within±50 angstroms of average contour line  170 . By changing the ratio (I 1 I/ 2 ) of current in the inner and the outer windings  40  and  42  so it is higher than unity, about unity and less than unity, the plasma density incident on the workpiece is varied radially from dense in the center, to uniform, and to dense in the outer edge.  
     [0081] When the processor is being made, interior winding  40  is turned relative to exterior winding  42  to assist in controlling the azimuthal electric field distribution and the azimuthal plasma density distribution. Winding  40  is turned about axis  44  so terminals  46  and  48  are at different positions relative to terminals  50  and  52 . In other words, terminals  46  and  48  can be at locations different from those illustrated in FIG. 2. Winding  40  can be turned to a predetermined position if the processors of the same type have consistent azimuthal electric field and plasma density distributions from processor to processor. If, however, different processors of the same type have differing azimuthal electric field and plasma density distributions from processor to processor, winding  40  is turned relative to winding  42  until tests indicate optimum uniform plasma distribution is achieved in each particular processor.  
     [0082] While there have been described and illustrated specific embodiments of the invention, it will be clear that variations in the details of the embodiments specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, many of the principles of the invention are not limited to coils having two concentric windings but are applicable to coils having three or more windings.