US 20040168771 A1
A method for processing a workpiece is carried out with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation. The apparatus includes an array of electromagnets mounted circumferentially around the plasma chamber. The method comprises generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece, selecting distributions of current signals for the electromagnets, and applying each selected distribution to the electromagnets to impose more than one magnetic field topology on the plasma during the plasma processing operation.
1. A method for processing a workpiece with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation, the apparatus including an array of electromagnets mounted circumferentially around the plasma chamber, the method comprising:
generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece;
selecting distributions of current signals for said electromagnets; and
applying each said selected distribution to said electromagnets to impose more than one magnetic field topology on the plasma during the plasma processing operation.
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15. A method for processing a workpiece with a plasma derived from a process gas in a plasma chamber of a plasma processing apparatus during a plasma processing operation, the apparatus including an array of electromagnets mounted circumferentially around the plasma chamber, the method comprising:
generating a plasma from a process gas within the chamber and causing plasma particles to strike the workpiece; and
supplying a distribution of current signals to said electromagnets so that said electromagnets impose a rotating bucket magnetic field topology on the plasma during the plasma processing operation.
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18. A plasma processing apparatus for processing a workpiece, the plasma processing apparatus comprising:
a plasma chamber including an interior region for supporting a plasma;
a plasma generating source;
a vacuum system in fluidic communication with the interior region of the plasma chamber;
a gas supply system in fluidic communication with the interior region of the plasma chamber;
a plurality of coil magnets mounted circumferentially around the plasma chamber, each coil magnet having an axis extending radially from an axis of the plasma chamber;
a plurality of arbitrary waveform generators, each being electrically communicated to an associated one of the plurality of coil magnets;
a control system electrically coupled to the gas supply system, the vacuum system, the cooling system, and the plurality of arbitrary waveform generators, the control system being configured to operate the arbitrary waveform generators so that the coil magnets impose a magnetic field topology on the plasma during the plasma processing operation.
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 This is a continuation of International Application No. PCT/US02/27978, filed on Sep. 4, 2002, which, in turn, claims the benefit of U.S. Provisional Application No. 60/318,890, filed Sep. 14, 2001, the contents of both of which are incorporated herein in their entirety by reference.
 The present invention relates to plasma processing systems and more particularly to a method and apparatus for using a magnetic field imposed on a plasma to control plasma characteristics to improve plasma processing of a workpiece.
 A plasma is a collection of charged particles that may be used to remove material from or deposit material on a workpiece. Plasma may be used, for example, to etch (i.e., remove) material from or to sputter (i.e., deposit) material on a semiconductor substrate during integrated circuit (IC) fabrication. A plasma may be formed by applying a radio frequency (RF) power signal to a process gas contained in a plasma chamber to ionize the gas particles. The RF source may be coupled to the plasma through a capacitance, through an inductance, or through both a capacitance and an inductance. Magnetic fields may be imposed on the plasma during plasma processing of a workpiece to improve plasma characteristics and thereby increase control over the plasma processing of the workpiece.
 Magnetic fields are sometimes used during the plasma processing of a workpiece to contain the plasma within the chamber or to change plasma properties during plasma processing. Magnetic fields may be used, for example, to contain the plasma within the chamber, thereby reducing plasma loss to the chamber walls, and to increase plasma density. Increasing plasma density increases the number of plasma particles striking the workpiece, which improves the processing of the workpiece by, for example, decreasing the processing time required to etch a workpiece. Containment of the plasma using magnetic fields also prevents plasma particle deposition on surfaces within the chamber such as chamber wall surfaces and electrode surfaces.
 Magnetic fields are also used to increase the uniformity of the distribution of plasma within the chamber. Non-uniform distribution of plasma within a plasma chamber is undesirable because non-uniform distribution may result in non-uniform processing of the workpiece. Non-uniformly distributed plasmas may, in some situations, result in plasma-induced damage to the workpiece being processed in the chamber.
 Arrays of either permanent magnets or electromagnets are sometimes used to impose a magnetic field on the plasma. An array of permanent magnets can be arranged, for example, so that they impose a magnetic field on the plasma within the interior of the chamber, or, alternatively, they can be arranged and moved (by rotation with respect to the chamber, for example) so that they impose a rotating magnetic field on the plasma, which improves plasma uniformity.
 The present invention includes methods and apparatuses for utilizing magnetic fields to control the processing of a workpiece with the plasma.
FIG. 1 is a schematic diagram of an example plasma processing system for illustrating the present invention, the plasma processing system showing a workpiece and plasma within a plasma chamber of a plasma processing apparatus and showing an outer flux conducting structure and an array of electromagnets surrounding the processing chamber;
FIG. 2 is a schematic top plan view of a portion of the apparatus of FIG. 1, FIG. 2 showing the processing chamber, a lower electrode, the outer flux conducting structure and the array of electromagnets surrounding the processing chamber and showing a magnetic cross field topology imposed on the interior of the chamber;
FIG. 3 is identical to FIG. 2 except showing a magnetic bucket field topology imposed on the interior of the chamber;
FIG. 4 is a schematic representation of an example power supply circuit for supplying an array of magnets with electrical power;
FIG. 5 is a schematic representation of a second example power supply circuit for supplying an array of magnets with electrical power;
FIG. 6 is a schematic view similar to FIG. 3 except showing a bucket field topology imposed on the processing chamber by two systems of electromagnets; and
FIG. 7 is a graph showing current flows in four adjacent electromagnets of the apparatus of FIG. 6.
FIG. 1 shows a schematic representation of an example of a plasma processing apparatus (or reactor) 10 of a plasma processing system 12. The plasma processing apparatus 10 includes a plasma chamber 14, which provides an interior region 16 for containing and supporting a plasma. A plurality of electrodes may be mounted within the chamber 14 in plasma generating relation to one another and to a process gas within the chamber 14. The electrodes are energized to generate a plasma from the process gas within the chamber 14. To facilitate the description of the invention, only two electrode assemblies are included in the apparatus 10. Specifically, a first electrode assembly 18 is mounted on a first side of the chamber 14 (in an upper portion of the interior 16 of the chamber 14 in the example apparatus 10). A second electrode assembly in the form of a chuck electrode assembly 20 is mounted on a second side of the chamber 14 opposite the first side of the chamber 14 (in a lower portion of the chamber interior 16 in the example apparatus 10) in a position spaced from the first electrode assembly 18.
 The first electrode assembly 18 may include a plurality of electrode segments, each segment being electrically isolated from the other segments and each segment being independently powered by an associated RF power source and independently supplied with a selected process gas for transmission at a predetermined rate into the interior of the plasma chamber. To facilitate the description of the present invention, however, the first electrode assembly 18 is in the form of a single showerhead-type electrode. The first electrode assembly 18 includes an inner chamber 22 (indicated schematically by a broken line in FIG. 1) that is in pneumatic or fluidic communication with a gas supply system 24 through a gas supply line. A selected gas (or gasses) may be supplied to the electrode assembly 18 to purge the chamber 14, for example, or to serve as a process gas (or source gas) for plasma formation in the chamber interior 16. The process gas is transmitted from the chamber 22 into the interior 16 of the plasma chamber 14 through a plurality of gas ports (not shown). The flow of gas through the ports of the first electrode is indicated by a series of directional arrows G.
 The first and second electrodes 18, 20 are electrically communicated through associated matching networks 30, 32 to respective RF power sources 34, 36 which provide voltage signals VB1, VB2, respectively, to the associated electrodes 18, 20. Matching networks 30, 32 may be inserted between respective RF power sources 34, 36 in order to maximize the power transferred to the plasma by the respective electrode assemblies 18, 20. Alternately, the matching networks 30, 32 may be coupled to control system 60.
 Each electrode assembly 18, 20 may be independently cooled by a fluid that circulates from a cooling system 38 through a fluid chamber 39, 41 (indicated by a broken line) in each electrode assembly 18, 20, respectively, and then back to the cooling system. The plasma processing apparatus 10 further includes a vacuum system 40 in pneumatic or fluidic communication with the plasma chamber 16 through a vacuum line. The plasma processing apparatus 10 optionally includes a voltage probe 44, 46 in the form of a pair of electrodes capacitively coupled to the transmission lines between the associated RF power sources 34, 36, respectively, and the associated electrode assembly 18, 20, respectively. (An example voltage probe is described in detail in commonly assigned pending U.S. application 60/259,862 (filed on Jan. 8, 2001), and it is incorporated in its entirety herein by reference.) The plasma processing apparatus 10 optionally includes an optical probe 48 for determining plasma characteristics and conditions based on spectral and optical properties of the plasma.
 A system or array of electromagnets 51 are mounted circumferentially around the plasma chamber 14. The electromagnets 51 are operable to impose one or more magnetic fields on a plasma during a plasma processing operation on a workpiece. The imposition of a magnetic field improves the condition of the plasma and thereby improves the processing of the workpiece.
FIG. 2 shows an example of an arrangement of the plurality of electromagnets 51 with respect to the plasma chamber 14. The example apparatus 12 includes twelve electromagnets, designated 51A-L. Each electromagnet 51 shown is in the form of a coil magnet that includes a coil of an electrically conductive material. Each coil is in electrical communication with an electrical power source 53 (shown schematically in FIG. 1).
 Each coil magnet 51 of a particular array may be provided by a coil of conductive material wound on an air core (not shown) or, alternatively, may be provided by a coil of conductive material wound around a core 55 (partially visible in FIG. 1) of, for example, a magnetically permeable material. Each core 55 may have a cylindrical cross section (as shown) or, alternatively, may have an arbitrary elongated cross section (with the longer dimension extending in the vertical direction in the example apparatus 10). The axis of each coil magnet 51 is radially aligned with the plasma chamber 14. That is, the axis of each coil magnet 51 extends radially from an imaginary axis that extends (vertically in the example reactor 10) between the electrode assemblies 18, 20 through the center of the plasma chamber 14. An outer flux conducting structure 57 may be mounted in surrounding relation to the array of coil magnets 51 as best seen in FIG. 2. Each coil magnet 51 and each core 55 is in magnetic flux communication with the flux conducting structure 57. An example of the flux conducting structure 57 is an annular wall structure. Both the outer wall structure 57 and the core 55 of each coil magnet 51 may be constructed of a magnetically permeable material such as iron. Each core 55 may be integrally formed on the outer ring structure 57 or may be formed separately from the outer wall structure 57 and then mounted on the outer ring structure 57.
 It can be appreciated from FIG. 2 that each coil magnet 51 and its associated core 55 extends in a radial direction between the outer ring structure 57 and the wall structure 59 of the plasma chamber 14. In the example apparatus 10, the wall structure 59 is cylindrical and comprises the side wall of the processing chamber 14. The wall structure 59 of the plasma chamber 14 may be constructed of either a suitable dielectric material or a suitable metallic material. If the wall structure 59 is constructed of a metallic material, a non-magnetic metallic material is used in the construction so that the wall structure 59 does not interfere with a magnetic field imposed on a plasma within the plasma chamber 14 by the coil magnets 51.
 The array of magnets in the example apparatus 10 is vertically aligned with the plasma in FIG. 1, but this vertical positioning is an example only. The array of magnets could have any vertical position with respect to the processing chamber and the structures (the electrodes, for example) and materials (the workpiece or plasma, for example) contained therein. For example, the apparatus 10 could be constructed and arranged so that the array of magnets are vertically aligned with the top of the workpiece, aligned with the center of the workpiece, slightly above the workpiece, for example, or aligned with the vertical center of the plasma, or slightly above or below the plasma, for example.
 A control system 60 of the plasma processing apparatus 10 is electrically communicated to various components of the apparatus 10 to monitor and/or control the same. The control system 60 is in electrical communication with and may be programmed to control the operation of the gas supply system 24, vacuum system 40, the cooling system 38, the voltage probe 44, 46, the optical probe 48, each RF power source 34, 36 and the power source 53. The control system 60 may send control signals to and receive input signals (feedback signals, for example) from the probes 44, 46, 48 and system components 24, 34, 36, 38, 40, 53. The control system 60 may monitor and control the plasma processing of a workpiece. By controlling the power source 53, the control system 60 is able to control the transfer of electrical power to each coil magnet comprising the array of coil magnets 51, and thereby control the properties of the magnetic field imposed on the plasma.
 The control system 60 may be provided by a computer system that includes a processor, computer memory accessible by the processor (where the memory is suitable for storing instructions and data and may include, for example, primary memory such as random access memory and secondary memory such as a disk drive) and data input and output capability communicated to the processor.
 The methods of the present invention are illustrated with reference to the example plasma processing system 12. The operation of the plasma processing system 12 can be understood with reference to FIG. 1. A workpiece (or substrate) 62 to be processed is placed on a support surface provided by the chuck assembly 20. The control system 60 activates the vacuum system 40 which initially lowers the pressure in the interior 16 of the plasma chamber 14 to a base pressure (typically 10−7 to 10−4 Torr) to assure vacuum integrity and cleanliness for the chamber 14. The control system 60 then raises the chamber pressure to a level suitable for forming a plasma and for processing the workpiece 62 with the plasma (a suitable interior pressure may be, for example, in the range of from about 1 mTorr to about 1000 mTorr). In order to establish a suitable pressure in the chamber interior 16, the control system 60 activates the gas supply system 24 to supply a process gas through the gas inlet line to the chamber interior 16 at a prescribed process flow rate and the vacuum system 40 is throttled, if necessary, using a gate valve (not shown). The process gas may flow through ports in the first electrode assembly as indicated in FIG. 1 by arrows G.
 The particular gas or gases included in the gas supply system 24 depends on the particular plasma processing application. For plasma etching applications, for example, the gas supply system 24 may supply chlorine, hydrogen-bromide, octafluorocyclobutane, or various other gaseous fluorocarbon compounds; for chemical vapor deposition applications, the system 24 may supply silane, ammonia, tungsten-tetrachloride, titanium-tetrachloride, or like gases. A plasma may also be used in chemical vapor deposition (CVD) to form thin films of metals, semiconductors or insulators (that is, conducting, semiconducting or insulating materials) on a semiconductor wafer. Plasma-enhanced CVD uses the plasma to supply the required reaction energy for deposition of the desired materials.
 The control system 60 then activates the RF power sources 34, 36 associated with the first and second electrode assemblies 18, 20. The RF power sources 34, 36 may provide voltages to the associated electrodes 18, 20 at selected frequencies. The control system 60 may, during a plasma processing operation, independently control the RF power sources 34, 36 to adjust, for example, the frequency and/or amplitude of the voltage at which each source 34, 36 drives the associated electrode assembly 18, 20.
 The RF power sources 34, 36 may be operated to convert the low-pressure process gas to a plasma. The power sources 34, 36 may be operated, for example, to cause an alternating electric field to be generated between the first and second electrodes 18, 20 which induces an electron flow between the electrodes 18, 20. Electrons, for example, are accelerated in this electric field and the flow of heated electrons in the field ionizes individual atoms and molecules of the process gas by transferring kinetic energy thereto through multiple collisions between the electrons and the gas atoms and molecules. This process creates a plasma 54 that is confined and supported within the chamber 14.
 Because each RF power source 34, 36 is independently controllable by the control system 60, either power source may be operated to have a relatively low frequency (i.e., a frequency below 550 KHz), an intermediate frequency (i.e., a frequency around 13.56 MHz), or a relatively high frequency, around 60 to 150 MHz. In an example of an etch reactor, the RF power source 34 for the first electrode assembly 18 can be driven at a frequency of 60 MHz and the RF power source 36 for the second electrode assembly 20 can be driven at a frequency of 2 MHz. In order to improve the performance of the aforementioned reactor, or, more generally, a plasma processing device having one or more electrodes that are driven at one or more frequencies, the control system 60 can be programmed and operated to impose one or more magnetic fields on the plasma during processing of the workpiece to control the characteristics (such as, for example, magnetic field topology and orientation, magnetic field strength, magnetic field duration and so on) of the magnetic fields.
 The invention allows a large number of possible magnetic field topologies to be generated using a single array of magnets 51 having no moving parts. FIGS. 2 and 3 show two magnetic field topologies that can be imposed on the plasma 54 (the plasma 54 being shown schematically in FIG. 1 only) using the magnet system. FIG. 2 shows a cross field topology and FIG. 3 shows a magnetic bucket field topology.
 The cross field topology illustrated has nonlinear (i.e., arcuate) magnetic fields lines. The cross field topology may be used to improve the uniformity of the plasma. Increasing the plasma uniformity increases the process uniformity both for a single substrate 62 and also increases process uniformity among a plurality of substrates processed in succession by the apparatus 10. The array of electromagnets 51 may be operated to rotate the cross field topology in a manner described below. A magnetic bucket topology (FIG. 3) may be imposed on the plasma to reduce plasma wall loss and to increase plasma density.
 An example of a circuit 68 for realizing the electrical power source 53 for powering the coil magnets 51A-L to create a desired magnetic field topology is shown schematically in FIG. 4. Specifically, each of a series of arbitrary waveform generators 70A-L may be electrically communicated to a respective coil magnet 51A-L (not shown in FIG. 4) of the system of electromagnets through an associated amplifier 71A-L.
 Each arbitrary waveform generator 70A-L may be electrically communicated to the control system 60 (through electrical connections that are not shown in FIG. 4). The control system 60 can be programmed to control each of the arbitrary waveform generators 70A-L independently of one another to generate from each a current waveform of arbitrary shape, magnitude and phase for transmission to the associated coil magnet 51A-L to polarize the same and to create the magnetic field that is imposed on the plasma. All of the arbitrary waveform generators 70 may be phase locked to a single low power reference signal source 72. Each generator 70 is capable of shifting the phase of its output relative to the reference signal from 72.
 The power source arrangement of FIG. 4 enables the control system 60 (acting through the series of arbitrary waveform generators 70A-L) to supply each coil magnet 51 with a current waveform having a wave shape, amplitude, phase and period that is independent of the current waveforms generated by all other arbitrary waveform generators in the series. Thus, the reference signal from the reference signal source 72 is used to synchronize the current waveforms transmitted from the system of arbitrary waveform generators to the coil magnets 51. The control system 60 can independently program each arbitrary waveform generator 70 to generate a different waveform with the starting phase locked to the reference signal from source 72. This arrangement provides great flexibility in imposing, for example, two or more magnetic field topologies on the plasma. This arrangement allows the control system 60 to impose, for example, two magnetic field topologies in succession with one another during a plasma processing operation on a particular substrate. These two topologies can be the same as one another or can be different from one another. A topology may be stationary or may be rotated.
 For example, this arrangement (that is, using a separate arbitrary waveform generator for each coil magnet) allows an operator to program the control system 60 to impose on the plasma a stationary magnetic field topology (azimuthally, for example) and a rotating magnetic field topology, during a processing operation. Each imposed field topology can be selected to achieve a particular change in the plasma. For example, the rotating cross field topology may be applied to improve plasma uniformity. As another example, this arrangement also allows the waveforms to be generated such that even though the imposed magnetic field is rotating, there is a localized, field (e.g., a low- or a high-field region) imposed at a particular location within the processing chamber. This localized field may be used to correct for azimuthal variations of plasma properties which result from, for example, non-axisymmetric gas injection and the pumping of the plasma, and so on.
 Another circuit 76 that can be used as a power source 53 is shown schematically in FIG. 5. A single arbitrary waveform generator 77 drives a series of amplifiers 71A-L that each supply current to an associated coil magnet (not shown in FIG. 5). A phase delay circuit 78 is coupled between the arbitrary waveform generator and all but one of the amplifiers. Essentially the same signal is sent to each coil magnet 51A-L, the only difference being that the signals may be out of phase with one another because of the presence of the phase delay circuits. Therefore, the circuit 76 can be used in instances in which the current waveforms to be applied to the coil magnets 51 have identical wave shapes and periods but differ from one another in phase. The power supply circuit 76 can provide a rotating magnetic field topology or a magnetic field topology that changes angular orientation. The topology of the field that is produced and rotated by the power source circuit 76 depends on several factors including the shape of the current waveforms transmitted to the coil magnets, the number of and the relative positions of the coil magnets in the coil magnet system, relative field strength of each coil magnet 51 and the phase difference between the current waveform signals. The control system 60 can be programmed to control the arbitrary waveform generator 70 of circuit 76 to produce, for example, a rotating cross field topology having nonlinear (e.g., arcuate) fields lines.
 The control system 60 can produce steady currents in any (or all) coil magnets 51 or can produce a time changing current in any (or all) coil magnets 51A-L (using, for example, the power supply circuitry 68). The distribution of steady and/or time-varying currents passing through the coil magnets 51 determines the topology of the magnetic field imposed on the plasma and determines the change in time of the magnetic field topology. Appropriate current waveforms can be sent to the coil magnets 51 to cause the magnetic field imposed on the plasma to rotate, for example.
 The current waveform applied to each coil magnet 51A-L radially polarizes each coil. During radial polarization, opposite ends of each coil magnet assume respective North and South magnetic polarities. Generally, the magnetic fields lines extend between opposite poles of the coil magnets 51. The direction of current flow in each coil determines the polarity of each coil magnet. The magnitude of the current flowing through the coil magnet determines the strength of the magnetic field produced by each coil magnet, and therefore the strength of the magnetic field imposed on the plasma.
 Other arrangements of the array of magnets are possible. For example, although the axis of each coil magnet 51 extends radially from an imaginary axis that extends between the electrode assemblies 18, 20 in the example reactor 10, other arrangements are possible. For example, each coil magnet 51 could be oriented so that its axis is “tangential” to the reactor 10. Each tangentially oriented coil could be an air coil or could be wound around a core material. When each coil is wound around a core of material, each core could be a separate structure or form part of a continuous structure such as a ring or yoke.
 This tangential arrangement has several disadvantages (relative to the radial alignment in the example reactor 10). For example, when a radially extending array of electromagnets are used to generate magnetic fields, most of the magnetic flux lines enter the chamber. When a tangential arrangement is used, however, most of the magnetic flux field lines tend to flow around the exterior of the plasma chamber 14, particularly when the coils are wound around a yoke surrounding the chamber, and a relatively small amount “leaks” or “fringes” out of the side of each tangentially arranged coil and enters the plasma chamber 14. Thus, the tangential arrangement relies on the fringing fields on one side of each tangential coil to impose a magnetic field on the plasma in the chamber. Because a magnet system that utilizes a tangential arrangement of the coils relies on fringing fields to impose a magnetic field on a plasma, more power is required to create a particular topology having a particular field strength relative to the use of a radial arrangement to create the same field topology having the same strength. A radially arranged magnet system utilizes less current than required by a comparable tangentially arranged magnet system. Because a tangential arrangement relies on field lines emerging from the sides of each coil, each coil emits field lines toward the chamber and field lines out the opposite side, for example, away from the chamber. An outer surrounding structure is also needed to shield the area surrounding the plasma processing apparatus from the magnetic field. When the tangentially oriented electromagnets are wound around a yoke, for example, a second permeable shield is needed if the area surrounding the apparatus is to be shielded from magnetic fields. A second permeable shield or other flux shielding structure is not required in the example arrangement shown in FIGS. 1 and 2, for example, because the structure 57 performs both a flux transmitting function and a shielding function.
FIGS. 2, 3 and 6 illustrate examples of magnetic field topologies that can be imposed on the plasma using the coil magnets 51A-L. The direction of the current flowing through each coil magnet 51A-L is indicated by a directional arrow in FIGS. 2, 3 and 6. The relative magnitude of the current in each coil magnet is roughly indicated by the relative size of the directional arrows in FIGS. 2, 3 and 6. Absence of a directional arrow indicates an instantaneous current of zero magnitude in the associated coil magnet 51. The iron ring structure 57 closes the magnetic field lines for each topology.
 A rotating cross field topology can be imposed on the plasma utilizing, for example, either power supply circuit 68 or 76. For example, a complex current waveform can be fed to each coil magnet 51 that is phase shifted with respect to the previous coil in a rotational direction that is opposite the direction of magnetic field topology rotation. This method allows the cross field topology to be rotated without mechanically moving any of the coil magnets.
FIG. 2 shows a rotating cross field topology at a particular instant in time. At this instant, the coil magnets 51A and 51B have currents that are oppositely directed to one another and are of relatively high magnitude, the coil magnets 51L and 51C have currents that are oppositely directed to one another and are of lesser magnitude than the currents in coil magnets 51A and 51B, and the coil magnet pairs 51K and 51D, 51J and 51E, and 51I and 51F have oppositely directed currents of successively lesser magnitude (as indicated by the relative size of the directional arrows). Nonlinear magnetic fields lines extend generally between the coils of each pair of coil magnets as indicated by the arcuate arrows in the processing chamber 14. The coil magnets 51H and 51G may have instantaneous currents of zero magnitude (depending, for example, on the exact field that one is trying to impose).
 It can also be appreciated from FIG. 2 that magnetic field lines extend generally from coil magnets 51A, 51L, 51K, 51J and 51I on one side of the chamber to respective associated coil magnets 51B, 51C, 51D, 51E and 51F on an opposite side of the chamber. The decreasing magnitude of the currents (on opposite sides of the chamber) creates, in effect, a magnetic field gradient of increasing strength from the approximately eleven o'clock azimuthal position to approximately the five o'clock azimuthal position. This gradient can help compensate for ExB drift. ExB drift can occur if a homogeneous field crosses a plasma chamber 14 parallel to the workpiece while an electric field perpendicular to the workpiece is present in the chamber. The vector product of these electromagnetic fields is parallel to the workpiece and perpendicular to both sets of field lines. This results in having the electrons directed in the direction of the vector product (i.e., the “preferred” direction) which causes the plasma to be denser in one area (or “corner”) of the plasma chamber. This results in a nonuniformity of the processing of the workpiece, which is undesirable. To correct for this ExB drift, the magnetic field topology is rotated. If the magnetic field topology is uniform, however, rotating the field merely causes the “hot spot” (area of relatively high electron density) to rotate around the periphery of the plasma. To correct for this effect, the field lines of the magnetic field topology are curved which causes the electrons to “fan out” sufficiently to reduce the hot spot effect.
FIG. 3 shows a bucket-type field topology (or bucket field topology) which forms a magnetic “bucket” around the walls of the chamber 14. This topology produces arcuate lobes of magnetic field lines that extend toward the center of the chamber. These lobes tend to concentrate the plasma in the center of the chamber. This has a number of benefits including, for example, tending to reduce the number of plasma particles striking the chamber side wall and other surfaces within the chamber 14 and increasing plasma density (by confining it to a smaller volume of space). The greater the plasma density, the faster the rate of etching or deposition, for example. Faster processing of the workpiece increases commercial productivity during, for example, semiconductor fabrication.
 As shown in FIG. 3, the bucket field topology can be achieved by conducting equal currents of opposite polarity (that is, currents of opposite direction) to pairs of adjacent coil magnets 51 of the array. The reactor 12 may also be constructed to provide magnetic field lines having a bucket topology that rotates or oscillates.
 A schematic view of an apparatus 80 for imposing a rotating bucket field topology on the plasma is shown in FIG. 6. The apparatus 80 is identical to apparatus 12 except for the number of coil magnets mounted around the chamber 14 thereof. Identical structures between the two embodiments 12 and 80 are identified with identical reference numbers and are not commented upon further. The number of coil magnets mounted around the chamber 14 determines the resolution of the magnetic field produced by the magnet system. That is, the more coil magnets that are positioned circumferentially around a chamber, the more “finely” the bucket field topology covers the interior of the chamber wall. To better control the “peripheral” magnetic field (that is, the portion of the magnetic field that is adjacent the wall), a relatively large number of relatively smaller coils are mounted around the chamber 14 of apparatus 80. When a fine resolution field is required, the inner ends of adjacent coil magnet 51 cores almost touch one another as shown in FIG. 6. Because the apparatus 80 has twice the number of coil magnets 51 mounted around its chamber compared to apparatus 12, for example, the apparatus 80 can be operated to achieve a bucket field topology that is finer resolution than the bucket field topology achieved using apparatus 12. The number of coils utilized depends on the resolution of the field that is required. Generally, the greater the number of magnets, the finer the field resolution.
 The lobe length can be increased by operating the magnets in pairs, in threes, and so on. That is, when the electromagnets 51 are operated in “pairs” to produce a bucket field topology, at each instant the magnitude and direction of the current in coils 51A and B are identical to one another. Similarly, the magnitude and direction of the current in coils 5 IC and D are identical to one another. Thus, coils 51A and B (and coils 51C and D and so on) function, in effect, as a single coil. The longer the lobes of the bucket field topology extend into the chamber, the more the plasma is “squeezed” into the center of the plasma chamber 14, thereby raising plasma density and reaction rate.
 The apparatus 80 can also be operated (using the circuit 76 of FIG. 5, for example) to produce a “rotating” or oscillating bucket field topology that has the same resolution as the non-rotating bucket field topology illustrated in FIG. 3 but which produces a series of overlapping lobe patterns which tend to more uniformly “squeeze” the plasma (relative to the magnetic field topology of FIG. 3). The bucket field topology produced according to the example method described below is also advantageous because at all times at least some location has a non-zero instantaneous field strength. That is, the imposed field is always non-zero at some location in the processing chamber at each point in time. Oscillating or rotating the bucket field is advantageous because it prevents the magnetic field lines from always striking the wall (or walls) of the processing chamber at the same place (or places). If the bucket magnetic field lines are not rotated and therefore strike a wall, for example, at the same locations, this can cause plasma particles to be directed along the field lines into the wall that these locations which can result in degradation of the wall material at these locations. This local degradation of wall material from a stationary bucket magnetic field can happen, for example, in places between the lobes where the field lines from adjacent lobes enter the chamber wall 14 together. Thus, it can be understood that while the imposed bucket magnetic field can be made to be static, can be made to oscillate or can be made to rotate, it may not be desirable to impose a static bucket (or other type) magnetic field on the plasma for a prolonged period because this may result in localized damage to the walls of the processing chamber. FIG. 6 shows an instantaneous view of the electrical currents and magnetic field in the apparatus 80 when the apparatus 80 is operated to produce a rotating bucket field topology. FIG. 7 shows a graphic representation of the magnitudes of the currents flowing through four coil magnets 51 over time while the example rotating bucket field topology is being produced. The rotating bucket field topology in apparatus 80 has essentially the same field resolution as is imposed utilizing the apparatus 12.
 The coil magnets 51A-X are essentially operated as two separate magnet systems that each provide a bucket field topology independently of the other magnet system. The first magnet system includes 51A, 51C, 51E, 51G, 51I, 51K, 51M, 510, 51Q, 51S, 51U, and 51W, and the second magnet system includes the remaining coil magnets 51. The graph of FIG. 7 shows the currents through coil magnets 51A-D. It can be appreciated that the current waveforms in adjacent coil magnets (51A and 51B, for example) are ninety degrees out of phase with one another. The currents in every other coil magnet (such as coil magnets 51A and C, for example) are one hundred and eighty degrees out of phase with one another.
FIG. 6 shows the magnetic field lines that occur at a time=tx. The time tx is also indicated on the graph of FIG. 7. At time tx one set of a coil magnets (the set that includes 51B and 51D) each have maximum current and the other set of coil magnets (the set that includes 51A and 51B) each have a current of zero magnitude. Adjacent coils in each set (51B and 51D, for example) have oppositely directed currents as indicated by the oppositely directed current directional arrows in FIG. 6 and by the graph of FIG. 7. It can be appreciated from FIG. 7 that each current waveform is sinusoidal. It can also be appreciated from the graph of FIG. 7 that the magnetic field produced by the rotating (or oscillating) bucket field topology does not vanish at any point during the plasma processing operation because the currents are never zero in all coil magnets 51A-X, at any instant.
 The structure and operation of the apparatus 80 is an example only. It is contemplated to construct an apparatus that includes three or more independent magnetic systems to produce, for example, three or more rotating magnetic field topologies.
 One or more magnetic field topologies can be imposed on the plasma during the processing of a particular workpiece (such as a semiconductor, as an example) processing quality and yield. For example, selected magnetic field topologies can be imposed on the plasma during an etching operation (or, alternatively, a deposition operation) in which a pattern is etched on a surface of a wafer of semiconductor material. Because a system of arbitrary waveform generators and coil magnets 51 may be used to create the magnetic fields, and because the arbitrary waveform generators can be controlled by the control system 60, a manufacturer is able to select an appropriate magnetic field topology (or magnetic field topologies) for a particular semiconductor material and a particular semiconductor etching (or deposition) application. The determination of the optimal combination of magnetic field topologies for a particular application may be done experimentally. That is, particular current waveforms can be fed to selected coil magnets of one or more magnet systems during processing of a particular type of wafer and the results examined. The quality of the results of the etching/deposition can be correlated with or examined in light of the magnetic field topologies used in the etching/deposition process. If damage to the workpiece occurs, for example, or if the processing results are not uniform, the distributions of current waveforms fed to the coil magnets 51 can be changed (by reprogramming the control system 60) to, for example, change the topology (or topologies), strength, gradient, period, and so on of the magnetic field topologies imposed on the plasma.
 When a semiconductor is processed in a plasma chamber, the semiconductor is susceptible to damage caused by nonuniform concentrations (either areas of high concentration or low concentration) of electrons in the plasma. Most of the damage that occurs due to nonuniformities in the concentration of the plasma occurs towards the end of a processing operation. Two or more magnetic fields topologies can be used during the processing of a workpiece (such as a semiconductor) to mitigate against the damage that may occur from plasma nonuniformities. During the first portion of a processing operation, when the workpiece is relatively unsusceptible to damage from plasma density nonuniformities, one or more bucket field topologies may be imposed on the plasma to increase plasma density and thereby increase the rate of processing. By increasing plasma density in the early part of a processing operation, therefore, material can be etched away from the workpiece faster, for example, and then, toward the end of the process, when it becomes risky to run at such a high processing rate, another magnetic field or fields can be imposed on the processing chamber to improve plasma uniformity during the final critical stages of the processing operation. As another example, a bucket field topology having relatively large lobes can be imposed on the plasma during the initial stages of processing, then a bucket field topology having intermediate sized lobes can be imposed on the plasma, and then a bucket field topology having relatively small lobes can be imposed on the plasma. By decreasing the size of the lobes (either in steps or continuously over time) of the bucket field topology during a processing operation, the density of the plasma can be gradually reduced as processing occurs. During the final critical stages of plasma processing, a rotating cross field topology having curved field lines can be imposed on the plasma to increase plasma uniformity during the final critical stages of processing.
 Localized nonuniformities can occur in a plasma for a number of known reasons including, for example, because of nonuniform gas injection, nonuniform RF excitation fields being applied to the plasma, nonuniform pumping within the plasma chamber, and so on. Because each coil magnet can be driven by an independent arbitrary waveform generator, the controller can be programmed to control the distribution of currents sent to the array of magnets to compensate for a local nonuniformity in the plasma. Thus, the controller can be programmed to create a rotating field that provides a localized nonuniformity in the imposed magnetic field to compensate for the density nonuniformity occurring in the plasma.
 It will be understood that while the electrodes of a plasma chamber were described as each being driven by an associated voltage source, this does not imply that each electrode has to be driven by the associated voltage source. Thus, for example, it is possible for one or the other of the pair of electrodes 18, 20 of the system 10 to be constantly at ground level or at any other static (i.e., unchanging) voltage level during processing.
 The many features and advantages of the present invention are apparent from the detailed specification and thus, it is intended by the appended claims to cover all such features and advantages of the described method which follow in the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those of ordinary skill in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described. Moreover, the method and apparatus of the present invention, like related apparatus and methods used in the semiconductor arts that are complex in nature, are often best practiced by empirically determining the appropriate values of the operating parameters, or by conducting computer simulations to arrive at best design for a given application. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the invention.