Abstract:
An apparatus and method of improving impedance matching between a RF signal and a multi-segmented electrode in a plasma reactor powered by the RF signal. The apparatus and method phase shifts the RF signal driving one or more electrode segment of the multi-segmented electrode, amplifies the RF signal, and matches an impedance of the RF signal with an impedance of the electrode segment, where the RF signal is modulated prior to matching of the impedance of the RF signal. The apparatus and method directionally couples an output of the matching of the impedance of the RF signal and the electrode segment, and adjusts the output of the matching of the impedance of the RF signal such that a directionally coupled output signal and a reference signal representing the RF signal at the output of the master RF oscillator produces a demodulated signal of minimal amplitude.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to an apparatus and method of improving impedance matching between an RF signal and a multi-segmented electrode in a plasma reactor powered by the RF signal.  
           [0003]    2. Discussion of the Background  
           [0004]    Manufacturers of semiconductor integrated circuits (IC) are faced with severe competitive pressure to improve their products. This pressure in turn is driving the manufacturers of the equipment used by IC manufacturers to improve the performance of their equipment. One particular type of tool that is widely used, and that is therefore particularly susceptible to these competitive pressures, is the plasma reactor. These reactors can be used to remove material, or, with modifications, the reactors can be used to deposit material. The mechanisms for either deposition or removal are complex, but in either case, it is essential to control the physical processes at the surface of the wafer. Control of these processes is the focus of significant technological development.  
           [0005]    One of the key factors that determine the yield and overall quality of an IC is the uniformity of processes, such as etching, at the surface of the wafer. In plasma reactors, uniformity is governed by the design of the overall system, and in particular by the design of the RF feed electronics and the associated control circuitry.  
         SUMMARY OF THE INVENTION  
         [0006]    The inventors have recognized that one important method of providing a uniform process involves measuring inter-electrode coupling in plasma reactors with multi-segmented electrodes, and adjusting the process accordingly. The inventors have identified various difficulties in measuring the inter-electrode coupling in plasma reactors, as will be described in detail below, and the inventors have provided an apparatus. Thus, a method is intended to address these difficulties and improve impedance matching between an RF signal and a multi-segmented electrode in a plasma reactor powered by the RF signal.  
           [0007]    An embodiment of the present invention advantageously provides an apparatus that includes an electrode segment adapted to be connected to the RF signal by a circuit. The circuit includes a phase shifter adapted to be connected to the RF signal, an amplifier connected to an output of the phase shifter, and a matching network connected to an output of the amplifier, where the matching network is configured to match an impedance of the RF signal with an impedance of the electrode segment. The circuit further includes a modulation source configured to modulate the RF signal prior to receipt by the matching network, a directional coupler connected to an output of the matching network and having an output connected to the electrode segment, and a demodulator configured to receive an output signal from the directional coupler and a reference signal representing the RF signal at the output of the master RF oscillator. The circuit also includes a control system connected to an output of the demodulator, where the control system is configured to control the output of the matching network such that the output signal of the demodulator is minimized.  
           [0008]    The apparatus preferably provides that the modulation source is connected to an input terminal of the phase shifter, or, alternatively, the modulation source is connected to an input terminal of the amplifier. The apparatus preferably provides that the modulation source is adjustable. In alternative embodiments, the reference signal is received by the demodulator from the output of the master RF oscillator. In alternative embodiments, the demodulator is synchronous, or the demodulator is asynchronous (non-synchronous). The apparatus preferably provides that the control system is configured to control the phase shifter and the amplifier. The apparatus preferably includes a filter connected to an output of the directional coupler, where the filter has an output connected to the electrode segment.  
           [0009]    The apparatus preferably further includes an additional electrode segment adapted to be connected to the RF signal by the circuit. The circuit further includes an additional phase shifter adapted to be connected to the RF signal, an additional amplifier connected to an output of the additional phase shifter, an additional matching network connected to an output of the additional amplifier. The additional matching network is configured to match an impedance of the RF signal with an impedance of the additional electrode segment, and the modulation source is selectively configured to modulate the RF signal prior to receipt by the additional matching network. The circuit further includes an additional directional coupler connected to an output of the additional matching network, where the additional directional coupler has an output connected to the additional electrode segment. The demodulator is configured to selectively receive (1) an output signal from the additional directional coupler and (2) an additional reference signal representing the RF signal at the output of the master RF oscillator. The control system is configured to control the output of the additional matching network such that the output signal of the additional demodulator is minimized.  
           [0010]    An alternative embodiment of the apparatus preferably includes an additional electrode segment adapted to be connected to the RF signal by the circuit. The circuit includes an additional phase shifter adapted to be connected to the RF signal, an additional amplifier connected to an output of the additional phase shifter, an additional matching network connected to an output of the additional amplifier, where the additional matching network is configured to match an impedance of the RF signal with an impedance of the additional electrode segment, an additional modulation source configured to modulate the RF signal prior to receipt by the additional matching network, and an additional directional coupler connected to an output of the additional matching network, where the additional directional coupler has an output connected to the additional electrode segment. In this embodiment the demodulator is configured to receive an output signal from the additional directional coupler and an additional reference signal representing the RF signal at the output of the master RF oscillator, and the control system is connected to the output of the demodulator, where the control system is configured to control the output of the additional matching network such that the output signal of the additional demodulator is minimized.  
           [0011]    The apparatus of the present invention is preferably incorporated into a plasma reactor including a process chamber; a multi-segmented electrode within the process chamber, and an RF power supply system configured to generate an RF signal to drive an electrode segment of the multi-segmented electrode, where the circuit connects the electrode segment to the RF signal.  
           [0012]    The present invention further advantageously provides a method including the steps of phase shifting the RF signal driving an electrode segment of the multi-segmented electrode, amplifying the RF signal, and matching an impedance of the RF signal with an impedance of the electrode segment, where the RF signal is modulated prior to matching of the impedance of the RF signal. The method further includes the steps of directionally coupling an output of the matching of the impedance of the RF signal and the electrode segment, and adjusting the output of the matching of the impedance of the RF signal such that a directionally coupled output signal and a reference signal representing the RF signal at the output of the master RF oscillator produces a minimized output signal from the demodulator.  
           [0013]    The method preferably provides that the step of adjusting the output of the matching of the impedance of the RF signal includes the steps of demodulating the directionally coupled output signal and the reference signal, and controlling the output of the matching of the impedance of the RF signal such that the demodulated directionally coupled output signal is minimized. In alternative embodiments, the RF signal is phase modulated prior to matching of the impedance of the RF signal, or the RF signal is amplitude modulated prior to matching of the impedance of the RF signal. Further, in alternative embodiments, the reference signal represents a RF signal output from the master RF oscillator. The method preferably includes the step of filtering the RF signal after the RF signal is directionally coupled and before the RF signal is received by the electrode segment.  
           [0014]    The method preferably includes the steps of phase shifting an additional RF signal driving an additional electrode segment of the multi-segmented electrode, amplifying the additional RF signal, matching an impedance of the additional RF signal with an impedance of the additional electrode segment, where the additional RF signal is modulated prior to matching of the impedance of the additional RF signal, directionally coupling an output of the matching of the impedance of the additional RF signal and the additional electrode segment, and adjusting the output of the matching of the impedance of the additional RF signal such that an additional directionally coupled output signal and an additional reference signal representing an additional RF signal at the output of the master RF oscillator prior to modulation produce a minimized signal at the output of an additional demodulator. In alternative embodiments, the RF signal and the additional RF signal are selectively modulated using a single modulation source, or the RF signal and the additional RF signal are modulated using independent modulation sources. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which:  
         [0016]    [0016]FIG. 1 is an electrical equivalent circuit representing electrical connections to electrode segments and inter-electrode coupling impedances for a five-electrode reactor;  
         [0017]    [0017]FIG. 2 is a schematic representation of a plasma reactor system according to the present invention;  
         [0018]    [0018]FIG. 3 is a diagram of an embodiment of the present invention depicting a phase modulation circuit with a single modulation source for the electrode segments;  
         [0019]    [0019]FIG. 4 is a diagram of an alternative embodiment of the present invention depicting an amplitude modulation circuit with a single modulation source for the electrode segments;  
         [0020]    [0020]FIG. 5A is a diagram of an embodiment of the present invention depicting a phase modulation circuit with independent modulation sources for each electrode segment;  
         [0021]    [0021]FIG. 5B is a diagram describing an array of filters to be used with the embodiments presented in FIGS. 5A and 6; and  
         [0022]    [0022]FIG. 6 is a diagram of an alternative embodiment of the present invention depicting an amplitude modulation circuit with independent modulation sources for each electrode segment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The inventors have identified problems with conventional processing reactors and methods of using those reactors that are solved by the present invention. Therefore, the present invention provides an apparatus and a method for improved impedance matching between an RF signal and a multi-segmented electrode in a plasma reactor powered by the RF signal which overcomes the shortcomings identified with regard to the conventional processing reactors.  
         [0024]    The inventors recognized the difficulty in measuring the inter-electrode coupling in plasma reactors with multi-segmented electrodes. There is considerable coupling between the electrodes of plasma reactors with multi-segmented electrodes. The coupling is due to mutual capacitance between the electrodes, mutual lead inductances, imperfect isolation between the RF generators that drive each electrode, and coupling between the matching networks. However, when the reactor is energized and plasma is present, the coupling between the electrodes increases dramatically due to the presence of the plasma. Thus, measurements of the coupling between electrodes that are made when the reactor is not operating are not valid when the reactor is being used and there is plasma present. What is needed is a way to measure the coupling between electrodes in plasma reactors with multi-segmented electrodes in order to facilitate the achievement of improved impedance matching for each electrode.  
         [0025]    The difficulty is measuring the inter-electrode coupling in plasma reactors with multi-segmented electrodes while the process evolves. As the process conditions in the reactor change during use, the coupling between the electrodes also changes. What is needed is a way to measure, as a process proceeds, the changes in the coupling between the electrodes that routinely occur during normal reactor use as a consequence of electrode erosion, process changes, etc. Similarly, for the application of RF power to an electrode with a consistent impedance match, the ability to differentiate the contributions to a measured RF signal at the electrode from inter-electrode coupling and plasma reflections of the signal applied to the electrode provides a considerable advantage for repeatable process conditions.  
         [0026]    Additionally, it is important to measure the inter-electrode coupling in plasma reactors with multi-segmented electrodes without interfering with the process. Plasma based process dynamics are sensitive to parasitic or stray capacitances and inductances whose impedances depend on process parameters. Thus, a change in the plasma due to a measurement affects the process, which can adversely affect the performance of the reactor. What is needed is a way to measure the inter-electrode coupling in plasma reactors with multi-segrnented electrodes without interfering with the process. The method is based on modulating, in a way that is benign to the process, the RF signal being used to power the electrodes, and afterwards demodulating the ensuing signal.  
         [0027]    [0027]FIG. 1 depicts an electrical equivalent circuit that represents the electrical or electrode connections  10  to the electrode segments, S 1 , S 2 , . . . S 5  for a five-electrode system, and the twenty (although only ten are shown) interelectrode coupling impedances Z 12 , Z 13  Z 14 , Z 15 , Z 23 , Z 24 , Z 25 , Z 34 , Z 35 , and Z 45  (Z 21 , Z 31 , Z 41 , Z 51 , Z 32 , Z 42 , Z 52 , Z 43 , Z 53 , and Z 54  are not depicted). In general there may be fewer than or greater than five electrodes. For an N electrode system, the impedances Z ij , where i,j=1, 2 . . . N, subject to the condition that i is not equal to j, represent the inter-electrode coupling impedances. For the case considered above, N=5. For a linear circuit Z ij =Z ji , and, in general, the total number of inter-electrode coupling impedances is N(N−1)/2. Thus, for an electrode with more than 3 segments, the number of unknown coupling parameters is larger than the number of accessible terminals, and the segments, in general, have to be excited in N(N−1)/2 different ways to determine all of the interactions, unless the symmetry of the segmented electrode would allow one to infer that certain inter-electrode impedances are equal. For a non-linear circuit, the total number of inter-electrode coupling impedances is N(N−1). What is needed is a way to measure the inter-electrode coupling in plasma reactors with multi-segmented electrodes or a plurality of electrodes.  
         [0028]    It is even more difficult to measure the inter-electrode coupling in plasma reactors with multi-segmented electrodes when there are non-linear elements present in the network. The presence of the non-linear plasma introduces significant added complexity to the problem of determining the inter-electrode coupling in plasma reactors with multi-segmented electrodes. This complexity is significantly less than it once was because of tremendous advances that have been made in understanding the dynamics of non-linear systems. However, even now only simplified cases usually lend themselves to analysis. Such cases often lack the generality of plasma systems as a whole, and actual commercially available plasma reactors in particular. What is needed is a way to measure the inter-electrode coupling in actual plasma reactors with multi-segmented electrodes when non-linear elements are present in the RF network.  
         [0029]    The preferred embodiment of a plasma reactor system for processing a substrate or wafer according to the present invention is depicted in FIG. 2. The system generally includes a plasma chamber  20  that contains two or more capacitively-coupled, electrode segments  22  and a wafer chuck  14  for supporting a wafer  16 , and a wafer handling and robotics system  24  that moves wafers into and out of the plasma chamber  20 . The system further includes a multiple segment inject electrode RF power supply system  26  that includes separate RF generation systems to drive each segment  22  of the segmented upper electrode and an inter-electrode coupling impedance measuring capability, and a cooling system  28  that circulates a coolant through coolant lines  30  and  32  to and from an enclosed cooling chamber  34  that is part of the plasma chamber  20 . The system also includes a vacuum pump system  36 , a gas supply system  38  that delivers gas to the plasma chamber  20 , and a control system  40  that oversees the operation of the tool and the interactions of the various parts. The RF power supply system  26  includes a separate RF generation system to drive each segment  22  of the upper electrode.  
         [0030]    [0030]FIG. 3 is a diagram of an embodiment of the present invention depicting a phase modulation circuit with a single modulation source for the electrode segments. The embodiment of FIG. 3 is based on phase modulation with a drive electrode having three segments. In general, the electrode may include any number of segments, N. In the discussion of FIG. 3 that follows, the uppermost electrode segment is referred to as electrode segment A. The circuit associated with electrode segment A, which is denoted by  160 A in FIG. 3 (and denoted generally by  22  in FIG. 2), will be described. The circuits associated with the remaining N−1 electrode segments (two additional electrode segments are depicted in FIG. 3) are identical, and corresponding circuit elements are denoted by the same number with an appropriate letter appended after the number (for example,  160 A,  160 B, and  160 C). All of the electrode segments are immersed in plasma  270 .  
         [0031]    A master RF oscillator  105  from within the RF power supply system  26  (see FIG. 2) provides a common RF signal as the input for the drive circuit for each electrode segment. The power output and frequency of RF oscillator  105  is to be compatible with phase shifters  110 , amplifiers  130  and de-modulator  140 . The RF oscillator preferably has a frequency of 60 MHz, but other frequencies can alternatively be used. The power output of the amplifiers  130  is preferably on the order of hundreds of watts.  
         [0032]    With regard to the drive circuit for electrode segment  160 A, the output of RF oscillator  105  is electrically coupled to the input terminal of voltage-controlled phase shifter  110 A. The phase difference between the RF voltage at the output terminal of voltage-controlled phase shifter  110 A and the RF voltage at the input terminal of voltage-controlled phase shifter  110 A is controlled by the sum of the output voltages of phase controller  112  and modulation source  120 , both of which are electrically connected to the phase control terminal of phase shifter  110 A by means of a summer or adder (not depicted). The phase difference is variable over a range of 2π radians or, equivalently, 360°.  
         [0033]    The frequency and amplitude of modulation voltage source  120  are adjustable, however a frequency in the range from about 1 kHz to 10 kHz is preferred. The amplitude of the output voltage of modulation source  120  determines the amplitude of the AC phase variation produced by phase shifter  110  to which it is electrically connected. The modulation source  120  can be electrically connected to any of the phase shifters  110  by using a selector switch  175  (e.g., a ganged selector switch. FIG. 3 depicts modulation source  120  connected by the selector switch  175  to phase shifter  110 A. The selector switch  175  alternatively can be an electronically controlled device, rather than a mechanical device as depicted in FIG. 3. The output terminal of phase shifter  110 A is electrically coupled to the input terminal of voltage-controlled RF amplifier  130 A. The control terminal of voltage-controlled RF amplifier  130 A is electrically coupled to the output terminal of gain controller  125 . The output voltage of gain controller  125  controls the gain of the voltage-controlled RF amplifier  130 A.  
         [0034]    The output of voltage-controlled RF amplifier  130 A is electrically coupled to match network  135 A, which automatically matches the input impedance presented by the electrode segment  160 A at the frequency determined by RF oscillator  105  by a match controller  170 , and the output of matching network  135 A is electrically connected to the power input port  146 A of directional coupler  145 A. The power output port  147 A of directional coupler  145 A is electrically connected to the input terminal of low-pass (or band-pass) filter  150 A, which is intended to prevent power generated in the plasma at any harmonics of the RF frequency produced by RF oscillator  105  from reaching match network  135 A. Finally, the output terminal of low-pass (or band-pass) filter  150 A is electrically connected to electrode segment  160 A.  
         [0035]    The amount of RF power emerging from electrode segment  160 A is directed via terminal  148 A of directional coupler  145 A to one terminal of a switch  185 . In fact, the RF signal emerging from terminal  148 A of directional coupler  145 A is an attenuated RF voltage comprising a first component associated with the reflection of the RF signal incident on the power coupling structure (i.e. electrode  160 A) as well as additional RF components associated with the coupling of RF power between electrodes within the multi-electrode system. The switch  185  can alternatively be an electronically controlled device, rather than a mechanical device as depicted in FIG. 3. Corresponding terminals  148  of directional couplers  145  are electrically connected, respectively, to corresponding terminals of switch  185 . By actuating the switch  185 , the RF power emerging from any electrode  160  may be connected to the input terminal  195  of synchronous demodulator  140 . FIG. 3 depicts electrode segment  160 A connected by the switch  185  to the input terminal  195  of synchronous demodulator  140 .  
         [0036]    The reference terminal  190  of synchronous demodulator  140  is also depicted in FIG. 3 as electrically connected to the output of the master RF oscillator  105 . Although the demodulator of this embodiment is a synchronous demodulator  140 , a simpler nonsynchronous demodulator may be used instead, albeit with somewhat reduced immunity to noise. In addition, the amount of RF power incident upon electrode segment  160 A from match network  135 A may be measured at terminal  180 A of directional coupler  145 A (by a detection system not depicted).  
         [0037]    One such demodulator  140  could be an I-Q demodulator, wherein provided a RF input  195  and a local (reference) oscillator input  190 , it outputs two signals at the modulation frequency, namely, I(t)=Icos(ω mt ) and Q(t)=Qsin(ω mt ), where ω m  is the modulation frequency. The amplitudes I and Q may be used to compute (I 2 +Q 2 ) 1/2  which is proportional to the desired power to be measured. When, for instance, selector switch  175  is set such that the RF signal incident on electrode  160 A is phase-modulated and selector switch  185  is set such that it receives a RF signal from output  148 A of directional coupler  145 A, then the output (described above) from demodulator  140  is proportional to the RF power associated with the reflections of the corresponding RF power incident at electrode  160 A through match network  135 A (and measured at node  180 A). Furthermore, while keeping the selector switch  175  unchanged, selector switch  185  may be changed to receive the RF signal from output  148 B of directional coupler  145 B and then the output from demodulator  140  is proportional to the power coupled from electrode  160 Aa to electrode  160 B. Similarly, this may be carried out through the Nth electrode. Therefore, by carrying out this operation for each electrode, one may obtain N 2  outputs which, in addition to monitoring the reflected incident power at each electrode (N signals), one also provides sufficient information (the remaining N(N−1) signals) to understand the coupling between electrodes of a N-electrode system as described in FIG. 1.  
         [0038]    Moreover, the output from the demodulator  140  (as described above) can be calibrated (if necessary) by performing the N 2  measurements described above while only delivering RF power to a single electrode. For example, phase-modulated RF power is delivered to a first electrode (in the multi-electrode system) while sequentially recording the output from demodulator  140  for each selector switch  185  setting (i.e. each electrode) and, using a second directional coupler and detector (not shown) in line with the first directional coupler  145 , sequentially recording the power emerging from the respective electrode  160  in a direction from the electrode  160  to the match network  135 . Simply by increasing the power to the given electrode, one may increase the reflected power and power coupled to other electrodes, wherein a calibration curve for the output of demodulator  140  and power can be assembled. This sequence of measurements is then performed for each of the remaining electrodes. Moreover, the calibration may be repeated while delivering power to multiple electrodes to check the significance of nonlinear effects.  
         [0039]    Finally, a processor or computer  200  controls, by using appropriate software, a phase controller  112 , a gain controller  125 , a match controller  170 , and, preferably, switches  175  and  185 . Input to the computer  200  by an operator is affected by an input device  250 , which is, for example, a keyboard or a touch-sensitive screen. The computer  200  includes a computer monitor  260 , upon which data obtained by the measurement system is displayed. In one embodiment, the computer  200  may serve to provide the match network controller with the above-mentioned information, particularly the measurement of the reflected incident power at each electrode. In turn, the match network controller may be programmed to minimize the reflected incident power at each electrode and serve the purpose of providing a repeatable plasma process using the multi-electrode system. Similar to controllers in the prior art, this system provides information on the forward and reflected incident power at each electrode in a multi-electrode configuration to the control algorithm of the match network controller.  
         [0040]    [0040]FIG. 4 is a diagram of an alternative embodiment of the present invention depicting an amplitude modulation circuit with a single modulation source for the electrode segments. The embodiment depicted in FIG. 4 is based on amplitude modulation with a drive electrode having three segments. In general, the electrode may include N segments. In the discussion of FIG. 4 that follows, the uppermost electrode segment is referred to as electrode segment Aa. The circuit associated with electrode segment A, which is denoted by  160 A in FIG. 4 (and denoted generally by  22  in FIG. 2), will be described. The circuits associated with the remaining N−1 electrode segments (two additional electrode segments are depicted in FIG. 4) are identical, and corresponding circuit elements are denoted by the same number with an appropriate letter appended after the number.  
         [0041]    A master RF oscillator  105  from within the RF power supply system  26  (see FIG. 2) provides a common RF signal as the input for the drive circuit for each electrode segment. The power output and frequency of RF oscillator  105  is compatible with phase shifters  110 , amplifiers  130  and demodulator  140 . The RF oscillator  105  has a frequency that is preferably 60 MHz, but other frequencies are possible. The power output of the amplifiers  130  is preferably on the order of hundreds of watts.  
         [0042]    With regard to the drive circuit for electrode segment  160 A, the output of RF oscillator  105  is electrically coupled to the input terminal of voltage-controlled phase shifter  110 A. The phase difference between the RF voltage at the output terminal of voltage-controlled phase shifter  110 A and the RF voltage at the input terminal of voltage-controlled phase shifter  110 A is controlled by the output voltage of phase controller  112  which is electrically connected to the phase control terminal of phase shifter  110 A. The phase difference is variable over a range of 2π radians or, equivalently, 360°. The output terminal of phase shifter  10 A is electrically connected to the input terminal of voltage-controlled RF amplifier  130 A.  
         [0043]    The modulation source  120  may be electrically connected to any of the voltage-controlled RF amplifiers  130  by a selector switch  175 . The frequency and amplitude of modulation voltage source  120  are adjustable, however a frequency in the range from about 1 kHz to 10 kHz is preferred. The amplitude of the output voltage of modulation source  120  determines the amplitude of the AC variation (ripple) of the output voltage produced by amplifier  130 A to which it is electrically connected. The instantaneous voltage gain of amplifier  130 A is controlled by the sum of the DC output voltage of gain controller  125  and the AC output voltage of modulation source  120 , both of which are electrically connected to the gain control terminal of voltage-controlled RF amplifier  130 A by a summer or adder (not depicted). FIG. 4 depicts modulation source  120  connected by the selector switch  175  to the control terminal of voltage-controlled RF amplifier  130 A. The selector switch  175  can be an electronically controlled device, rather than a mechanical device as depicted in FIG. 4.  
         [0044]    The output of voltage-controlled RF amplifier  130 A is electrically coupled to a match network  135 A, which automatically matches the input impedance at the electrode segment  160 A at the frequency determined by RF oscillator  105  using a match controller  170 , and the output of matching network  135 A is electrically connected to the power input port  146 A of directional coupler  145 A. The power output port  147 A of directional coupler  145 A is electrically connected to the input terminal of low-pass (or band-pass) filter  150 A, which is intended to prevent power generated in the plasma at any harmonics of the RF frequency produced by RF oscillator  105  from reaching match network  135 A. Finally, the output terminal of low-pass (or bandpass) filter  150 A is electrically connected to electrode segment  160 A.  
         [0045]    The amount of RF power emerging from electrode segment  160 A is directed via terminal  148 A of directional coupler  145 A to one terminal of a switch  185 . In fact, the RF signal emerging from terminal  148 A of directional coupler  145 A is an attenuated RF voltage comprising a first component associated with the reflection of the RF signal incident on the power coupling structure (i.e. electrode  160 A) as well as additional RF components associated with the coupling of RF power between electrodes within the multi-electrode system. The switch  185  can be an electronically controlled device, rather than a mechanical device as depicted in FIG. 4. Corresponding terminals  148  of directional couplers  145  are electrically connected, respectively, to corresponding terminals of the switch  185 . The RF power emerging from any electrode  160  can be connected to the input terminal  195  of synchronous demodulator  140  by using the switch  185 . FIG. 4 depicts electrode segment  160 A connected by the switch  185  to the input terminal  195  of synchronous demodulator  140 .  
         [0046]    The reference terminal  190  of synchronous demodulator  140  is also depicted in FIG. 4 as electrically connected to the output of the master RF oscillator  105 . Although the demodulator of this embodiment is a synchronous demodulator  140 , a simpler nonsynchronous demodulator may be used instead, albeit with somewhat reduced immunity to noise.  
         [0047]    In addition, the amount of RF power incident upon electrode segment  160 A from match network  135 A may be measured at terminal  180 A of directional coupler  145 A (by using a detection system that is not depicted).  
         [0048]    One such demodulator  140  could be an I-Q demodulator, wherein provided a RF input  195  and a local (reference) oscillator input  190 , it outputs two signals at the modulation frequency, namely, I(t)=Icos(ω m t) and Q(t)=Qsin(ω m t), where ω m  is the modulation frequency. The amplitudes I and Q may be used to compute (I 2 +Q 2 ) 1/2  which is proportional to the desired power to be measured. When, for instance, selector switch  175  is set such that the RF signal incident on electrode  160 A is phase-modulated and selector switch  185  is set such that it receives a RF signal from output  148 A of directional coupler  145 A, then the output (described above) from demodulator  140  is proportional to the RF power associated with the reflections of the corresponding RF power incident at electrode  160 A through match network  135 A (and measured at node  180 A). Furthermore, while keeping the selector switch  175  unchanged, selector switch  185  may be changed to receive the RF signal from output  148 B of directional coupler  145 B and then the output from demodulator  140  is proportional to the power coupled from electrode  160 A to electrode  160 B. Similarly, this may be carried out through the Nth electrode. Therefore, by carrying out this operation for each electrode, one may obtain N 2  outputs which, in addition to monitoring the reflected incident power at each electrode (N signals), one also provides sufficient information (the remaining N(N−1) signals) to understand the coupling between electrodes of a N-electrode system as described in FIG. 1.  
         [0049]    Moreover, the output from the demodulator  140  (as described above) can be calibrated (if necessary) by performing the N 2  measurements described above while only delivering RF power to a single electrode. For example, amplitude-modulated RF power is delivered to a first electrode (in the multi-electrode system) while sequentially recording the output from demodulator  140  for each selector switch  185  setting (i.e. each electrode) and, using a second directional coupler and detector (not shown) in line with the first directional coupler  145 , sequentially recording the power emerging from the respective electrode  160  in a direction from the electrode  160  to the match network  135 . Simply by increasing the power to the given electrode, one may increase the reflected power and power coupled to other electrodes, wherein a calibration curve for the output of demodulator  140  and power can be assembled. This sequence of measurements is then performed for each of the remaining electrodes. Moreover, the calibration may be repeated while delivering power to multiple electrodes to check the significance of nonlinear effects. Finally, a computer or processor  200  controls, using appropriate software, a phase controller  112 , a gain controller  125 , a match controller  170 , and, preferably, switches  175  and  185 . Input to the computer  200  by an operator is affected by using an input device  250 , which is, for example, a keyboard or a touch-sensitive screen. The computer  200  preferably includes a computer monitor  260 , by which data obtained by the measurement system can be displayed. In one embodiment, the computer  200  may serve to provide the match network controller with the above-mentioned information, particularly the measurement of the reflected incident power at each electrode. In turn, the match network controller may be programmed to minimize the reflected incident power at each electrode and serve the purpose of providing a repeatable plasma process using the multi-electrode system. Similar to controllers in the prior art, this system provides information on the forward and reflected incident power at each electrode in a multi-electrode configuration to the control algorithm of the match network controller.  
         [0050]    [0050]FIG. 5A is a diagram of an embodiment of the present invention depicting a phase modulation circuit with independent modulation sources for each electrode segment. The embodiment depicted in FIG. 5A is based on phase modulation with a drive electrode having 3 segments. In general, the electrode may include N segments. In the discussion of FIG. 5A that follows, the uppermost electrode segment is referred to as electrode segment A. The circuit associated with electrode segment A, which is denoted by  160 A in FIG. 5A (and denoted generally by  22  in FIG. 2), will be described. The circuits associated with the remaining N−1 electrode segments (two additional electrode segments are shown in FIG. 5A) are identical, and corresponding circuit elements are denoted by the same number with an appropriate letter appended after the number. All of the electrode segments are immersed in plasma  270 .  
         [0051]    A master RF oscillator  105  from within the RF power supply system  26  (see FIG. 2) provides a common RF signal as the input for the drive circuit for each electrode segment. The power output and frequency of RF oscillator  105  is to be compatible with phase shifters  110 , amplifiers  130 , and demodulator  140 . The oscillator has a frequency that is preferably 60 MHz, but other frequencies are possible. The power output of the amplifiers  130  is typically of the order of hundreds of watts.  
         [0052]    With regard to the drive circuit for electrode segment  160 A, the output of RF oscillator  105  is electrically coupled to the input terminal of voltage-controlled phase shifter  110 A. The phase difference between the RF voltage at the output terminal of voltage-controlled phase shifter  110 A and the RF voltage at the input terminal of voltage-controlled phase shifter  110 A is controlled by the sum of the output voltages of phase controller  112  and modulation source  120 A, both of which are electrically connected to the phase control terminal of phase shifter  110 A by a summer or adder (not depicted). The phase difference is variable over a range of 2π radians or, equivalently, 360°.  
         [0053]    The frequency and amplitude of modulation voltage source  120 A are adjustable, and a frequency in the range from about 1 kHz to 10 kHz is preferred. In this embodiment, the frequencies of modulation sources  120 A,  120 B, and  120 C must be unique and the frequency ratios should preferably not be rational numbers. The amplitude of the output voltage of modulation source  120 A determines the amplitude of the AC phase variation produced by phase shifter  110 A to which it is electrically connected. The output terminal of phase shifter  110 A is electrically coupled to the input terminal of voltage-controlled RF amplifier  130 A. The control terminal of voltage-controlled RF amplifier  130 A is electrically coupled to the output terminal of gain controller  125 . The output voltage of gain controller  125  controls the gain of the voltage-controlled RF amplifier  130 A.  
         [0054]    The output of voltage-controlled RF amplifier  130 A is electrically coupled to match network  135 A, which automatically matches the input impedance presented by the electrode segment  160 A at the frequency determined by RF oscillator  105  by using a match controller  170 , and the output of matching network  135 A is electrically connected to the power input port  146 A of directional coupler  145 A. The power output port  147 A of directional coupler  145 A is electrically connected to the input terminal of low-pass (or band-pass) filter  150 A, which is intended to prevent power generated in the plasma at any harmonics of the RF frequency produced by RF oscillator  105  from reaching match network  135 A. Finally, the output terminal of low-pass (or band-pass) filter  150 A is electrically connected to electrode segment  160 A.  
         [0055]    The amount of RF power emerging from electrode segment  160 A is directed via terminal  148 A of directional coupler  145 A to one terminal of a switch  185 . In fact, the RF signal emerging from terminal  148 A of directional coupler  145 A is an attenuated RF voltage comprising a first component associated with the reflection of the RF signal incident on the power coupling structure (i.e. electrode  160 A) as well as additional RF components associated with the coupling of RF power between electrodes within the multi-electrode system. The switch  185  can be an electronically controlled device, rather than a mechanical device as depicted in FIG. 5A. The respective terminals  148  of directional couplers  145  are electrically connected to respective terminals of the switch  185 . The RF power emerging from any electrode  160  can be connected to the input terminal  195  of synchronous demodulator  140  by using the switch  185 . FIG. 5A depicts electrode segment  160 A connected by the switch  185  to the input terminal  195  of synchronous demodulator  140 .  
         [0056]    The reference terminal  190  of synchronous demodulator  140  is also depicted in FIG. 5A as electrically connected to the output of the master RF oscillator  105 . The preferred demodulator for this embodiment is a synchronous demodulator  140 , but a simpler non-synchronous demodulator can be used instead, albeit with somewhat reduced sensitivity and immunity to noise.  
         [0057]    In addition, the amount of RF power incident upon electrode segment  160 A from match network  135 A can be measured at terminal  180 A of directional coupler  145 A (by a detection system not depicted).  
         [0058]    One such demodulator  140  could be an I-Q demodulator, wherein provided a RF input  195  and a local (reference) oscillator input  190 , it outputs two signals at the modulation frequencies, namely, I(t)=I a cos(ω ma t)+I b cos(ω mb t)+I c cos(ω mc t) and Q(t)=Q a sin(ω ma t)+Q b sin(ω mb t)+Q c sin(ω mc t), where ω ma , ω mb  and ω mc  are the modulation frequencies for each electrode  160 A,  160 B and  160 C from modulation sources  120 A,  120 B and  120 C, and I a , I b , I c , Q a , Q b  and Q c  are the corresponding amplitudes of the respective modulation frequencies in the I and Q signals. The signals output from demodulator  140 , I(t) and Q(t), may then be filtered digitally within the computer processor  200  via Fourier analysis to extract a signal at each modulation frequency, or the I(t) and Q(t) signals may be filtered externally using an array of band-pass filters  280  prior to A/D conversion at the computer processor  200  as shown in FIG. 5B. The amplitudes I j  and Q j  for the jth modulation frequency ω mj  may be used to compute (I j   2 +Q j   2 ) 1/2  which is proportional to the desired power to be measured. When, for instance, selector switch  185  is set such that it receives a RF signal from output  148 A of directional coupler  145 A, then the amplitude (I a   2 +Q a   2 ) 1/2  of filtered output at modulation frequency ω ma  (described above) from demodulator  140  is proportional to the RF power associated with the reflections of the corresponding RF power incident at electrode  160 A through match network  135 A (and measured at node  180 A), the amplitude (I b   2 +Q b   2 ) 1/2  of filtered output at modulation frequency ω mb  from demodulator  140  is proportional to power from electrode  160 B coupled to electrode  160 A, and the amplitude (I c   2 +Q c   2 ) 1/2  of filtered output at modulation frequency ω mc  from demodulator  140  is proportional to power from electrode  160 C coupled to electrode  160 A. Similarly, this may be carried out through the Nth electrode by changing the setting for selector switch  185 . Therefore, by carrying out this operation for each electrode, one may obtain N 2  outputs which, in addition to monitoring the reflected incident power at each electrode (N signals), one also provides sufficient information (the remaining N(N−1) signals) to understand the coupling between electrodes of a N-electrode system as described in FIG. 1.  
         [0059]    Moreover, the output from the demodulator  140  (as described above) can be calibrated (if necessary) by performing the N 2  measurements described above while only delivering RF power to a single electrode. For example, phase-modulated RF power is delivered to a first electrode (in the multi-electrode system) while sequentially recording the output from demodulator  140  for each selector switch  185  setting (i.e. each electrode) and, using a second directional coupler and detector (not shown) in line with the first directional coupler  145 , sequentially recording the power emerging from the respective electrode  160  in a direction from the electrode  160  to the match network  135 . Simply by increasing the power to the given electrode, one may increase the reflected power and power coupled to other electrodes, wherein a calibration curve for the output of demodulator  140  and power can be assembled. This sequence of measurements is then performed for each of the remaining electrodes. Moreover, the calibration may be repeated while delivering power to multiple electrodes to check the significance of nonlinear effects.  
         [0060]    Finally, a computer or processor  200  controls, by using appropriate software, a phase controller  112 , a gain controller  125 , a match controller  170 , and, perhaps, switches  175  and  185 . Input to the computer  200  by an operator is affected by using an input device  250 , which is, for example, a keyboard or a touch-sensitive screen. The computer  200  includes a computer monitor  260 , by which data obtained by the measurement system is displayed. In one embodiment, the computer  200  may serve to provide the match network controller with the above-mentioned information, particularly the measurement of the reflected incident power at each electrode. In turn, the match network controller may be programmed to minimize the reflected incident power at each electrode and serve the purpose of providing a repeatable plasma process using the multi-electrode system. Similar to controllers in the prior art, this system provides information on the forward and reflected incident power at each electrode in a multi-electrode configuration to the control algorithm of the match network controller.  
         [0061]    [0061]FIG. 6 is a diagram of an alternative embodiment of the present invention depicting an amplitude modulation circuit with independent modulation sources for each electrode segment. The embodiment of the present invention depicted in FIG. 6 is based on amplitude modulation with a drive electrode having three segments. In general, the electrode includes N segments. In the discussion of FIG. 6 that follows, the uppermost electrode segment is referred to as electrode segment A. The circuit associated with electrode segment A, which is denoted by  160 A in FIG. 6 (and denoted generally by  22  in FIG. 2), will be described. The circuits associated with the remaining N−1 electrode segments (two additional electrode segments are depicted in FIG. 6) are identical, and corresponding circuit elements are denoted by the same number with an appropriate letter appended after the number. All of the electrode segments are immersed in plasma  270 .  
         [0062]    A master RF oscillator  105  from within the RF power supply system  26  (see FIG. 2) provides a common RF signal as the input for the drive circuit for each electrode segment. The power output and frequency of RF oscillator  105  is compatible with phase shifters  110 , amplifiers  130 , and demodulator  140 . The oscillator  105  has a frequency that is preferably 60 MHz, but other frequencies are possible. The power output of the amplifiers  130  is preferably on the order of hundreds of watts.  
         [0063]    With regard to the drive circuit for electrode segment  160 A, the output of RF oscillator  105  is electrically coupled to the input terminal of voltage-controlled phase shifter  110 A. The phase difference between the RF voltage at the output terminal of voltage-controlled phase shifter  110 A and the RF voltage at the input terminal of voltage-controlled phase shifter  110 A is controlled by the output voltage of phase controller  112  which is electrically connected to the phase control terminal of phase shifter  110 A. The phase difference is variable over a range of 2π radians or, equivalently, 360°. The output terminal of phase shifter  10 A is electrically connected to the input terminal of voltage-controlled RF amplifier  130 A.  
         [0064]    In this embodiment, the frequencies of modulation sources  120 A,  120 B, and  120 C must be unique and the frequency ratios should preferably not be rational numbers. The amplitude of the output voltage of modulation source  120  determines the amplitude of the AC variation (ripple) of the output voltage produced by amplifier  130 A to which it is electrically connected. The instantaneous voltage gain of amplifier  130 A is controlled by the sum of the DC output voltage of gain controller  125  and the AC output voltage of modulation source  120 A, both of which can be electrically connected to the gain control terminal of voltage-controlled RF amplifier  130 A by a summer or adder (not depicted).  
         [0065]    The output of voltage-controlled RF amplifier  130 A is electrically coupled to a match network  135 A, which automatically matches the input impedance at the electrode segment  160 A at the frequency determined by RF oscillator  105  by using a match controller  170 , and the output of matching network  135 A is electrically connected to the power input port  146 A of directional coupler  145 A. The power output port  147 A of directional coupler  145 A is electrically connected to the input terminal of low-pass (or band-pass) filter  150 A, which is intended to prevent power generated in the plasma at any harmonics of the RF frequency produced by RF oscillator  105  from reaching match network  135 A. Finally, the output terminal of low-pass (or bandpass) filter  150 A is electrically connected to electrode segment  160 A.  
         [0066]    The amount of RF power emerging from electrode segment  160 A is directed via a terminal  148 A of directional coupler  145 A to one terminal of a switch  185 . In fact, the RF signal emerging from terminal  148 A of directional coupler  145 A is an attenuated RF voltage comprising a first component associated with the reflection of the RF signal incident on the power coupling structure (i.e. electrode  160 A) as well as additional RF components associated with the coupling of RF power between electrodes within the multi-electrode system. The switch  185  can be an electronically controlled device, rather than a mechanical device as depicted in FIG. 6. Corresponding terminals  148  of directional couplers  145  are electrically connected, respectively, to corresponding terminals of the switch  185 . The RF power emerging from any electrode  160  can be connected to the input terminal  195  of synchronous demodulator  140  by using the switch  185 . FIG. 6 depicts electrode segment  160 A connected by the switch  185  to the input terminal  195  of synchronous demodulator  140 .  
         [0067]    The reference terminal  190  of synchronous demodulator  140  is also depicted in FIG. 6 as electrically connected to the output of the master RF oscillator  105 . Although the demodulator of this embodiment is a synchronous demodulator  140 , a simpler non-synchronous demodulator can be used instead, albeit with somewhat reduced immunity to noise.  
         [0068]    In addition, the amount of RF power incident upon electrode segment  160 A from the match network  135 A may be measured at terminal  180 A of directional coupler  145 A (by a detection system not depicted).  
         [0069]    One such demodulator  140  could be an I-Q demodulator, wherein provided a RF input  195  and a local (reference) oscillator input  190 , it outputs two signals at the modulation frequencies, namely, I(t)=I a cos(ω ma t)+I b cos(ω mb t)+I c cos(ω mc t) and Q(t)=Q a sin(ω ma t)+Q b sin(ω mb t)+Q c sin(ω mc t), where ω ma , ω mb  and ω mc  are the modulation frequencies for each electrode  160 A,  160 B and  160 C from modulation sources  120 A,  120 B and  120 C, and I a , I b , I c , Q a , Q b  and Q c  are the corresponding amplitudes of the respective modulation frequencies in the I and Q signals. The signals output from demodulator  140 , I(t) and Q(t), may then be filtered digitally within the computer processor  200  via Fourier analysis to extract a signal at each modulation frequency, or the I(t) and Q(t) signals may be filtered externally using an array of band-pass filters  280  prior to A/D conversion at the computer processor  200  as shown in FIG. 5B. The amplitudes I j  and Q j  for the jth modulation frequency ω mj  may be used to compute (I j   2 +Q j   2 ) 1/2  which is proportional to the desired power to be measured. When, for instance, selector switch  185  is set such that it receives a RF signal from output  148 A of directional coupler  145 A, then the amplitude (I a   2 +Q a   2 ) 1/2  of filtered output at modulation frequency coma (described above) from demodulator  140  is proportional to the RF power associated with the reflections of the corresponding RF power incident at electrode  160 A through match network  135 A (and measured at node  180 A), the amplitude (I b   2 +Q b   2 ) 1/2  of filtered output at modulation frequency ω mb  from demodulator  140  is proportional to power from electrode  160 B coupled to electrode  160 A, and the amplitude (I c   2 +Q c   2 ) 1/2  of filtered output at modulation frequency ω mc  from demodulator  140  is proportional to power from electrode  160 C coupled to electrode  160 A. Similarly, this may be carried out through the Nth electrode by changing the setting for selector switch  185 . Therefore, by carrying out this operation for each electrode, one may obtain N 2  outputs which, in addition to monitoring the reflected incident power at each electrode (N signals), one also provides sufficient information (the remaining N(N−1) signals) to understand the coupling between electrodes of a N-electrode system as described in FIG. 1.  
         [0070]    Moreover, the output from the demodulator  140  (as described above) can be calibrated (if necessary) by performing the N 2  measurements described above while only delivering RF power to a single electrode. For example, amplitude-modulated RF power is delivered to a first electrode (in the multi-electrode system) while sequentially recording the output from demodulator  140  for each selector switch  185  setting (i.e. each electrode) and, using a second directional coupler and detector (not shown) in line with the first directional coupler  145 , sequentially recording the power emerging from the respective electrode  160  in a direction from the electrode  160  to the match network  135 . Simply by increasing the power to the given electrode, one may increase the reflected power and power coupled to other electrodes, wherein a calibration curve for the output of demodulator  140  and power can be assembled. This sequence of measurements is then performed for each of the remaining electrodes. Moreover, the calibration may be repeated while delivering power to multiple electrodes to check the significance of nonlinear effects.  
         [0071]    Finally, a computer or processor  200  controls, by using appropriate software, a phase controller  112 , a gain controller  125 , a match controller  170 , and, perhaps, switches  175  and  185 . Input to the computer  200  by an operator is affected by an input device  250 , which is, for example, a keyboard or a touch-sensitive screen. The computer  200  includes a computer monitor  260 , by which data obtained by the measurement system is displayed. In one embodiment, the computer  200  may serve to provide the match network controller with the above-mentioned information, particularly the measurement of the reflected incident power at each electrode. In turn, the match network controller may be programmed to minimize the reflected incident power at each electrode and serve the purpose of providing a repeatable plasma process using the multi-electrode system. Similar to controllers in the prior art, this system provides information on the forward and reflected incident power at each electrode in a multi-electrode configuration to the control algorithm of the match network controller.  
         [0072]    All of the embodiments described in FIGS. 3 through 6 may further include an isolator (or circulator with a load) connected to the output of amplifier  130  and input of match network  135 . The isolator can protect the amplifier  130  from reflected RF power and RF power coupled from other electrodes.  
         [0073]    With regard to the embodiments depicted in FIGS.  3 - 6 , the match network components of a given circuit arm (e.g.,  135 A) can be adjusted via input to the match network controller  170  from the computer  200  (and from the demodulator  140 ) to reduce the reflected power at electrode  160 A associated with a reflection of the power incident on the electrode  160 A from a signal generated by oscillator  105  and amplified by amplifier  130 A. In other words, the apparatus and methods described in the above disclosed embodiments are capable of dissecting the RF power measured at coupler  180  and dissociating the reflected “incident” signal from those signals associated with coupling from the other electrodes. In this manner, a repeatable impedance match can be obtained for each electrode in a multi-electrode system, which can, in turn, contribute to repeatable process results.  
         [0074]    It should be noted that the exemplary embodiments depicted and described herein set forth the preferred embodiments of the present invention, and are not meant to limit the scope of the claims hereto in any way.  
         [0075]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.