Patent Publication Number: US-2023139755-A1

Title: Generator with controllable source impedance

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. § 120 
     The present Application for Patent is a Continuation of patent application Ser. No. 17/093,333, filed Nov. 9, 2020, entitled GENERATOR WITH CONTROLLABLE SOURCE IMPEDANCE which is a continuation of Ser. No. 16/388,574 entitled “SYSTEM AND METHOD FOR CONTROL OF HIGH EFFICIENCY GENERATOR SOURCE IMPEDANCE” filed Apr. 18, 2019, which is related to and claims priority under 35 U.S.C. § 119(e) from U.S. Patent Application No. 62/660,893, filed Apr. 20, 2018, titled “SYSTEM AND METHOD FOR CONTROL OF HIGH EFFICIENCY GENERATOR SOURCE IMPEDANCE,” the entire content of all the above-identified applications are incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     Aspects of the present disclosure relate to controlling power sources and, in particular, to control of interactions between power sources and plasma loads in plasma processing systems. 
     BACKGROUND 
     In the field of semiconductor manufacturing, as well as other fields, a plasma system has various possible uses. For example, plasma-enhanced chemical vapor deposition is a process used to deposit thin films on a substrate using a plasma system. A typical plasma processing system, in high-level terms, involves a processing chamber and a power delivery system that ignites and maintains a plasma inside the chamber. The plasma may be characterized as a load with an impedance that is driven by the power generator. The load impedance of the plasma is typically around 50 Ohms, but it will vary depending on process conditions and other variables. For example, plasma load impedance may vary depending on generator frequency, power, chamber pressure, gas composition, and plasma ignition. These variations in plasma impedance may adversely affect the power delivery from the generator; and may also result in undesired drifts or perturbations in process variables, such as etch or deposition rates, due to changes in the physical properties of the plasma at different power levels. Plasma processing systems equipped with impedance matching mechanisms or circuitry that matches the source impedance of the power delivery system to the plasma load impedance may reduce such adverse effects on the plasma process. 
     It is with these observations in mind, among others, that aspects of the present disclosure were conceived. 
     SUMMARY 
     According to one aspect, a power supply system for providing power to a plasma load includes a first power amplifier including a first amplifier input and a first amplifier output, the first power amplifier having a first controllable output power a first asymmetrical power profile with a first peak power offset in reference to an impedance of a load and a second power amplifier including a second amplifier input and a second amplifier output, the second power amplifier having a second controllable output power a second asymmetrical power profile with a second peak power offset in reference to the impedance of the load. The power supply system may also include a controller in communication with at least one of the first power amplifier and the second power amplifier, the first controllable output power combined with the second controllable output power to define a combined output power, the controller to adjust at least one of the first controllable output power or the second controllable output power to control a source impedance of the combined output power. 
     According to another aspect, method for operating a plasma processing system is provided. The method may include, in a power supply including a first amplifier providing a first power signal with a first power profile and a second amplifier providing a second power signal with a second power profile, and in response to an impedance measurement of a load, controlling at least one of the first power signal and the second power signal to define a combined output power signal based on the impedance measurement of the load. 
     According to yet another aspect, a power supply controller includes a processor and a non-transitory memory comprising instructions that, when executed by the processor, are operable to adjust a source impedance of an output signal of a power supply. The instructions are operable to instruct a first power amplifier to alter, based on a determined load impedance of a load coupled to the power supply, an input power signal from a power generator and provide a first variable output power signal with a first power profile and instruct a second power amplifier to alter, based on the determined load impedance of the load, the input power signal from the power generator and provide a second variable output power signal with a second power profile different than the first power profile. The first variable output power signal and the second variable output power signal are combined to generate a combined output power signal transmitted to the load, the combined output power signal comprising a combined power profile and a source impedance based on the load impedance of the load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the technology of the present disclosure will be apparent from the following description of particular embodiments of those technologies, as illustrated in the accompanying drawings. It should be noted that the drawings are not necessarily to scale; however the emphasis instead is being placed on illustrating the principles of the technological concepts. The drawings depict only typical embodiments of the present disclosure and, therefore, are not to be considered limiting in scope. 
         FIG.  1 A  illustrates a first example plasma processing system. 
         FIG.  1 B  is a three-dimensional illustration of the output power profile with reference to a nominal load impedance of a high efficiency radio frequency (RF) power generator of a plasma processing system. 
         FIG.  2    illustrates a second example plasma processing system with a controllable dual-amplifier high efficiency power generator. 
         FIG.  3    is a three-dimensional illustration of an output power profile generated from the combination of two power amplifiers with equal output power settings. 
         FIG.  4    is a three-dimensional illustration of an output power profile generated from the combination of two power amplifiers with unequal output power settings. 
         FIG.  5    illustrates a third example plasma processing system with a controllable dual-amplifier high efficiency power generator with phase and impedance feedback control. 
         FIG.  6    illustrates a fourth example plasma processing system with a controllable quad-amplifier high efficiency power generator. 
         FIG.  7    is view of a simplified Smith chart of the power profiles of the output power signals from a quad-amplifier high efficiency power generator. 
         FIG.  8    is an example waveform of pulsed power applied to a load of a plasma processing system from a high efficiency radio frequency (RF) power generator. 
         FIG.  9    is a flowchart of a method for controlling a plurality of amplifiers of power generating system to control a source impedance of an output power signal. 
         FIG.  10    illustrates an example computer system according to one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Typical plasma processing systems are driven by a power generator. Controlling the plasma process is provided in real time by variation of two basic parameters of the generator—output power and operational frequency. But for modern plasma processes such two-dimensional power delivery systems cannot provide optimal and stable conditions in the wide range required in many plasma processing conditions. As a result, the need to control an additional fundamental parameter of the power generator in real time—its source impedance—is vital. 
     Embodiments of the present disclosure provide a power supply system that allows control of the source impedance of the generator in real time, thereby extending the range of operational conditions for plasma processes. In one embodiment of the power generator system, two radio frequency (RF) amplifiers may be utilized having asymmetrical power profiles in reference to a nominal load impedance. The second power amplifier generally has a power profile that is diametrically opposite that of the first power profile in reference to the nominal load impedance. Such variations in power profiles can be achieved in various ways. For example, variations in power profiles may be achieved by, among other things, using different topologies for each of the amplifiers or implementing a phase delay network. The output power from the first and second amplifiers may be combined using a combiner circuit or other device and the combined output power is transmitted to a plasma load. In certain implementations, the output power of each amplifier may be independently controlled to alter one or more characteristics of the output power signal provided by the individual amplifiers. By changing the ratio of the output power of the first amplifier to the output power of the second amplified, the source impedance of the generators may be varied in real time and in response to a load impedance so as to match the source impedance of the power signal to the load with the load impedance. 
     Typical high efficiency RF generators for plasma generation applications deliver RF power for a wide range of load impedances. For example,  FIG.  1 A  illustrates a first example plasma processing system  100 , including a power generating system  102  (such as a high efficiency RF power generator) configured to provide power to a plasma chamber  106 . The provided power ignites and sustains a plasma within the plasma chamber  106  for any number and type of plasma processing procedures, such as vapor deposition and etching applications. The power generating system  102  may receive a control signal with a voltage value to control the output power from the power generating system. 
     Such conventional power generators  102  may produce an asymmetrical output power profile with reference to a nominal load  114 .  FIG.  1 B  is a three-dimensional Smith chart (or a reflection coefficient chart) of one example of an asymmetrical output power profile  110  of the RF power generator  102  of the plasma processing system  100 . The horizontal plane of the graph  110  represents the active and inactive load impedance components and the vertical axis represents the output RF power. The nominal reference impedance at 50 Ohms is represented in the graph at line  114 . The nominal load  114  may be the load impedance of the plasma chamber  106 , which is typically around 50 Ohms. However, as explained in more detail below, the load impedance of the plasma chamber  106  may vary depending on process conditions or other variables during the application of the power signal to the load. As can be seen in  FIG.  1 B , the output power profile  110  of the power generating system  102  is asymmetrical in reference to the nominal reference load  114  as the profile  110  includes a peak power  112  towards the edge of the chart without a corresponding peak power on the opposite side of the chart. 
     In many plasma processing systems, small variations in load impedance may cause unacceptable variations in the power delivered by the plasma generator  102  and may result in instability of the plasma process  100 . For example, fluctuations in the load impedance of the system may cause a mismatch of the source impedance to the load impedance, resulting in the power provided to the plasma to rise or fall due to the asymmetric power profile  110  of the provided power. Although the effects of variations in load impedances may be at least partially absorbed by using a specific length delay line between the power generator  102  and the load  106  (illustrated in  FIG.  1 A  as the matching cable  104  through which the power from the power generating system is provided to the plasma chamber), this type of adjustment in response to the variable nature of the load impedance has a narrow dynamic range and generally cannot be controlled in real time in response to the changes in the plasma process. Another method to match the source impedance to the load impedance is to adjust the source power profile to match a peak power delivery (i.e., the peak of the power profile  112 ) with the changing impedance. Therefore, real-time control over the source impedance of the power signal may stabilize the plasma processing system. 
     To provide real-time control over the source impedance of the power signal provided to the load to account for variations in the load impedance of the plasma chamber, alternate high efficiency RF power generating systems are described herein. As mentioned above, matching the output impedance of the power generating system (also referred to as the “source impedance”) to an impedance of the load (also referred to as the “load impedance”) may improve the operation and efficiency of the plasma processing system. For example, large variations in the load impedance may de-stabilize the plasma processing system, resulting in a shutdown of the system in some instances. Thus, systems and methods for rapidly adjusting a source impedance of a power generation system over a wide range of values in response to a load impedance are provided herein. Such systems may utilize a plurality of RF amplifiers or power generators with asymmetrical output power profiles that may be combined to form an overall output power profile that is transmitted to a load. Aspects or characteristics of the power amplifiers and/or the output power signal from each power amplifier may be controlled by a power control system to adjust or generate a target source impedance of the output power signal in response to measured or determined variations in the load impedance of a load, such as a plasma. Controlling the variations of the power profiles of the multiple power amplifiers may be achieved by, among other things, using different topologies for each of the amplifiers or implementing a phase delay network. The output power from the multiple amplifiers may be combined using a combiner component and the output power from the combiner may be delivered to a plasma load. In certain implementations, the output power signal of each amplifier may be independently controlled by a power control system. By changing the ratio of the output power signal of the first amplifier to the output power signal of the second amplifier, the source impedance of the power signal provided to a load may be varied in real time to match variations in load impedance. 
       FIG.  2    shows an example plasma processing system  200  with a controllable, dual-amplifier, high efficiency power generator system  202  for providing a power signal with a target source impedance to a load  206 . Many of the components of the plasma processing system  200  of  FIG.  2    are similar to those described above with reference to  FIG.  1 A . For example, the system  200  includes a plasma load  206  receiving a high efficiency RF power signal from a power generating system  202  via a matching cable  204 . The plasma load  206  and matching cable  204  may operate similarly as described above such that the plasma load may have a load impedance of about 50 Ohms, which may vary due to characteristics or conditions of the system  200 . The power generating system  202  of the plasma processing system  200  may include one or more controllable power amplifiers  210 ,  212  to adjust the output power profile of the power generating system  202  in real time to match the source impedance of the output power signal  216  to the load impedance. 
     The power generating system  202  of the plasma processing system  200  may include, among other components not illustrated to simplify the discussion, an RF power supply  208 , a first amplifier  210 , and a second amplifier  212 . Each amplifier  210 ,  212  may receive an output power signal from the power supply  208  and alter the received power signal. Altering the power signal may include adjustment of any characteristic of the power signal, such as adjusting the frequency, amplitude, or phase of the power signal. The output from each amplifier  210 ,  212  may be provided to a combiner circuit or device  214  configured to combine the two outputs into a single output signal  216 . The combined output signal  216  may be provided to the load  206 , which may be a plasma chamber or plasma load to conduct plasma processing. In some implementations, each amplifier  210 ,  212  of the power generating system  202  may be controlled, either together or independently, by power control system  201 . For example, power control system  201  may transmit one or more control signals  218  to amplifier A  210  and/or amplifier B  212 . The control signals  218  may include one or more instructions to the amplifiers  210 ,  212  to configure the respective amplifiers to alter the power signal from the power supply  208  to generate a controlled output power signal from the amplifiers. For example, the control signals  218  may configure amplifier A  210  to alter the amplitude of the input power signal from the power supply  208  and provide the altered power signal to the combiner  214 . In another example, the control signals  218  may configure or instruct amplifier B  212  to alter the phase of the input power signal from the power supply  208  and provide the altered power signal to combiner  214 . As explained in more detail below, changing the ratio of the output power signal of the first amplifier  210  to the output power signal of the second amplified  212  may alter the source impedance of the combined output power signal  216  of the power generating system  202  as provided to the load  206 . 
     As introduced, high efficient RF power generators typically have asymmetrical power profiles with reference to a nominal or reference load. The output power profile of amplifier A  210  and the output power profile of amplifier B  212  may similarly be asymmetrical with reference to a nominal load. In one implementation of the power generating system  202 , the control system  201  may deliver the output power signals of amplifier A  210  and amplifier B  212  to provide output power signals with equal amplitudes, but with the output power profile of one amplifier, such as amplifier B, being diametrically opposite of the output power profile of the other amplifier, such as amplifier A.  FIG.  3    is a three-dimensional illustration of an output power profile  314  generated from the combination of the output signals of two power amplifiers with equal, but diametrically opposed, output power signals. To generate the output power profile  314  of  FIG.  3   , the power control system  201  may provide one or more instructions to amplifier A  210  and amplifier B  212  to generate the respective output power profiles for the amplifiers which, when combined by combiner  214 , form the combined output power profile  314 . In one implementation, the power profile  304  for amplifier B  212  may be diametrically opposed on the Smith chart in relation to the power profile  302  of amplifier A  210 . For example, power profile  302  illustrated in  FIG.  3    of the amplifier A  210  may be with reference to a nominal load  306 , such as 50 Ohms. Similarly, power profile  304  of the amplifier B  212  may be with reference to a nominal load  310 , which may also be 50 Ohms. To be diametrically opposed, power profile A  302  may include a peak power  308  that is shown on the left of the nominal load reference  306  and power profile B  304  may include a peak power  312  that is on the right of the nominal load reference  310  of the power profile. The peaks  308 ,  312  may be equal in height or magnitude, but diametrically opposed in the Smith chart in reference to the nominal load  306 ,  310 . Generating the equal but opposite power profiles  302 ,  304  may be done by using multiple techniques—such as different topology for the amplifiers  210 ,  212 , phase delay networks, manipulation of the power profile by the control system  201 , etc. 
       FIG.  3    illustrates the combination of the power profiles  302 ,  304  of the power amplifiers  210 ,  212  into power profile  314 . The combined output power  314  may be provided to a plasma load  206  by the combiner circuit  214 . As illustrated, the combined output power profile  314  of amplifier A  302  and amplifier B  304  is symmetrical with a relatively flat peak  316  at the nominal load reference. Control system  201  may control the amplifiers  210 ,  212  to provide the combined peak power  316  at the nominal load impedance (usually 50 Ohms). However, variations of the load impedance may result in less than the peak power level  316  of the RF power being delivered into the load  206  due to impedance mismatch between load and source. 
     As illustrated in the power profile  314 , the further from the nominal load reference  316  the load impedance becomes, the less power that may be provided to the load from the combined outputs  314  of the amplifiers  210 ,  212  (from nominal maximum power delivery, as the load impedance moves away from the peak in any direction, it intersects the power profile downward along the profile). In instances when the load impedance varies far from the nominal load impedance, downward along the profile, the plasma processing system  200  may become unstable as the power provided to the plasma load  206  is reduced, which may result in the system shutting down to prevent damage to the system or in response to the plasma collapsing. To maintain optimal power delivery in the presence of changing load conditions, the control system  201  may adjust one or more of the characteristics of the power profiles of the amplifiers  210 ,  212  to alter the combined profile of the power to the load. In one implementation, the control system  201  may adjust one or more of the characteristics of the power profiles of the amplifiers  210 ,  212  such that the combined profile of the power to the load has a peak power at or near a determined load impedance. For example, the load impedance may vary during operation of the plasma system away from the nominal load impedance  316  of  FIG.  3   . In response, the power profile  302  of amplifier A and/or the power profile  304  of amplifier B may be adjusted by the controller to locate the peak power of the power profile of the combined power signal to be located the determined load impedance. In this manner, the peak power (and source impedance) of the combined output power profile  314  may respond to the load impedance. 
       FIG.  4    is a three-dimensional illustration of an example of a combined output power profile  414  generated from the combination unequal output power profiles  402 ,  404 . In particular, the controller may instruct the first amplifier A to generate power with an output power profile  402 . The profile  402  is an asymmetrical power profile in reference to a nominal load impedance  406 . The controller may instruct the second amplifier B to generate power with an output power profile  404 . The profile  404  is also asymmetrical in reference to a nominal load impedance  410 . Notably, the peak of power profile A  402  of amplifier A occurs at a different load impedance on the Smith chart than the power profile B  404  of amplifier B. Further, the power profiles  402 ,  404  have differing peak power magnitudes. The output power profile  404  of the power profiles of amplifier A and amplifier B includes a diametrically opposite peak  412  from the peak  408  location of the output power profile  402 . The magnitude of the peak  408  is greater than the magnitude of the peak  412 . Thus, unlike the combined profile  314  providing a relatively symmetrical profile about the nominal load impedance  316 , the combined output power profile  414  is asymmetrical in reference to the operational load impedance  416 . As shown, peak power  418  of the combined profile is to the side of the amplifier A with the greater magnitude peak power. Stated differently, the asymmetrical shape of the combined output power profile  414  is achieved by increasing the amplitude or magnitude of the power profile  402  of amplifier A  210  in relation to the amplitude or magnitude of the power profile  404  of amplifier B  212 . This is illustrated in  FIG.  4    by the peak  408  of the power profile  402  of amplifier A  210  being higher or taller than the peak  412  of the power profile  404  of amplifier B  212 . When combined, the output power profile  414  is asymmetrical in reference to the nominal load impedance  416  such that the peak or maximum power  418  is provided by the amplifiers  210 ,  212  at a load impedance away from the nominal or reference load impedance. For example, the peak power  418  of the output power profile  414  of the combined power signals may not occur at the nominal load  416 . In the presence of a changing nominal load impedance, the controller may adjust the position of the peak power of the combined signal by altering the magnitude or other attribute of the profile of one or both of the amplifier in the combined system. So, for example, if the load impedance had moved from  416  to  418 , the controller can adjust the power profiles  402  and  404  to adjust the peak power location  418  to match the change in impedance. 
     In more detail, as a result of the amplitude mismatch between the output signals  402 ,  404  of the amplifiers  210 ,  212 , the resulting output power profile  414  when combined is asymmetrical in reference to the nominal impedance  416  such that the peak power delivery point for the source impedance is moved relative to the previous case (shown in  FIG.  3   ) in which each amplifier has an equal output power magnitude. Accordingly, by changing the ratio of the magnitude of the output powers of the two amplifiers  210 ,  212 , the resulting positon of peak power delivery and effectively the source impedance can be adjusted to accommodate different load impedances, and to match changing load impedances. In other words, the value of the generator output or source impedance (or the magnitude of the power profile slope in the provided graphical representations) is controlled by the difference in values of signals from two amplifiers  210 ,  212 . Such changes or control over the output power signals from the amplifiers may be made in real time to account for dynamic variations in load impedance variation, thereby matching the source impedance, controlling power delivery, and improving the controllability and stability of the plasma process. Control of the output of the amplifiers  210 ,  212  (such as through the power control system  201  providing control signals  218  to the amplifiers to alter the power signal from the power source  208 ) may thus control the characteristics of the combined output signal  216  provided to the load  206 . 
     Although discussed above with relation to adjusting the magnitude of the power profile from the first amplifier  210  and/or the second amplifier  212 , the amplifiers may be controlled by the power control system  201  to alter other characteristics of the power signal from the power source  208  to further generate the combined output power signal  216 . For example, the power control system  201  may transmit control signals to the amplifiers  210 ,  212  to alter the phase of either output to adjust and control the combined output power profile  216 . In relation to the power profile graphs of  FIGS.  3  and  4   , adjusting a phase of the output power profile of amplifier A  210  or amplifier B  212  results in a rotation of the power profile graph about the nominal reference load impedance. This rotation of the power profile provides an additional control over the shape of the combined output power profile  414  to further adjust the combined output power signal  216  provided to the load  206  based on the detected variable load impedance. In general, any aspect of the power profile of output power signal  216  may be controllable by the power control system  201 . For example, a target magnitude and/or slope of the power profile  414  of the combined power signals may be determined and generated via the instructions provided to the amplifiers  210 ,  212  from the power control system  201 . 
     The power generating system  202  may control the combined output power signal  216  based on the dynamic variations in load impedance variation to attempt to match the output source impedance to the changing load impedance. In some instances, feedback information and/or measurements from the output power signal  216  and or the load impedance may be provided as an input to the power control system  201  to control the amplifiers  210 ,  212 . For example,  FIG.  5    is an example plasma processing system  500  with a controllable dual-amplifier high efficiency power generator  502  with a phase and impedance feedback system  516 . Components of the plasma processing system  500  are similar to those described above, with a power generating system  502  providing a power signal to a plasma load  506  via an optional matching cable  504 . The power generating system  502  may include a power source  508  providing an input power signal to a first amplifier  510  and a second power amplifier  512 , both of which are controllable by a power control system  501  through one or more power control signals  518 . In addition to the components described above, the power generating system  502  may also include a feedback system  516 . The feedback system  516  may receive the combined output power signal  520  from the combiner circuit  514  and provide information about the combined output power signal  520  (or the combined output power signal itself) to the power control system  501 . For example, the feedback system  516  may receive the combined output power signal  520  from the combiner  514  and determine a phase of the combined output power signal, which may be provided the phase to the power control system  501  as an input to the system. The power control system  501  may use the input information, such as the phase of the combined output power signal  520 , to control the amplifiers  510 ,  512  to configure the output power signal in response to the feedback information provided by the feedback system. 
     In addition, characteristics of the load  506  may also be obtained by or provided to the feedback system  516 . For example, the load impedance present at the load may be detected and provided to the feedback system  516 , such as with an IV probe. The feedback system  516  may provide the load impedance to the power control system  501 , which may adjust the output of amplifier A  510  and/or amplifier B  512  in response to the received load impedance in an attempt to match the source impedance of the output power signal  520  to the load impedance. In some instances, the load impedance (or other characteristics of the load) may be derived from the output power signal  520  to the load  506 . For example, the load impedance may vary based on the profile of the power signal  520  to the load. The feedback system  516  may then receive the output power signal  520 , analyze the signal to determine load impedance, and provide information of the characteristics of the output power signal to the power control system  501 . The power control system  501  may utilize this information to determine how to adjust the output power signals of amplifier A  510  and amplifier B  512  to match the estimated load impedance. In some instances, the power control system  501  may estimate the load impedance from the combined output power signal  520 . Similarly, determining the phase of the power signal  520  and the effects of applying the power signal to the load  506  may aid the power control system  501  in adjusting the output power signal in response. Any characteristic that may be controlled by the power control system  501  to shape the power profile of the combined output power signal  520  may be received by the feedback system  516  and/or the power control system  501 . 
     In yet another example, more than two amplifiers may be included in the power generating system  202  to provide even more control over the shape of the power profile of the combined output power signal. For example,  FIG.  6    illustrates an example plasma processing system  600  with a controllable quad-amplifier high efficiency power generator  602 . Similar to the above systems, the system  600  of  FIG.  6    may include a power generating system  602  providing an output power signal to a plasma load  606 . The power generating system  602  may include a power source  608  providing an input power signal to any number of amplifiers controlled by a power control system  601  through one or more power control signals  622 . In the example shown, the power generating system  602  includes four amplifiers, amplifiers A-D  610 - 616 . Although four amplifiers  610 - 616  are illustrated in the system  600 , any number of amplifiers may be included in the power generating system  602  to provide additional control over the shape of the power profile signal provided by the power generating system  602 . 
     In some implementations, the four or more amplifiers  610 - 616  of the power generating system  602  may be paired such that control over one amplifier of a pair from the power control system  601  may affect control over the second amplifier of the pair. For example, amplifier A  610  and amplifier B  612  may be controlled by the power control system  601  such that the output signals from the amplifiers are diametrically opposite power profiles, as discussed above with relation to  FIG.  2   . A second pair of amplifiers, amplifier C  614  and amplifier D  616 , may also be controlled such that their output power profiles are diametrically opposed (illustrated as rotated 90 degrees and  270  degrees in  FIG.  6    relative to the output signal of amplifier A). Thus, the power profiles of the second pair of amplifiers  614 ,  616  may be rotated by 90 degrees on the Smith chart impedance plane in reference to the first pair of the power amplifies  610 ,  612 . Output signals from all four amplifies  610 - 616  may be combined with the combiner  618  and provided to a plasma load  606  as discussed above. Such a configuration of combining output power signals of four power amplifiers  610 - 616  may provide four quadrant control of the source impedance of the output power signal or, in other terms, allows independent control of the value and direction of the slope of the output power profile. For example,  FIG.  7    is a simple view of a Smith chart  700  of the power profiles of the output power signals from the four amplifiers  610 - 616  of the circuit of  FIG.  6   . Each output profile  702 - 708  may include a peak output power in a different quadrant of the Smith chart. For example, amplifier A  610  may have an output power profile in which a peak of the power profile occurs within the circle  702  illustrated in the chart  700 . The peak power of the power profile of amplifier A  610  may thus occur within a first quadrant of the Smith chart  700 . Similarly, amplifier B  612  may have an output power profile in which a peak of the power profile occurs within the circle  704  of the chart  700  in a different quadrant of the chart. The output power profiles  702 ,  704  of amplifier A  610  and amplifier B  612  may be diametrically opposed in reference to a nominal impedance  710  in the Smith chart, similar to the combined output power profile discussed above with reference to  FIGS.  3  and  4   . The output power profile  706  of amplifier C  614  may include a peak in yet a third quadrant of the Smith chart  700 , with a diametrically opposed power profile  708  of amplifier D  616 . The output power profiles of the pair of amplifier C  614  and amplifier D  616  may be rotated 90 degrees on the Smith chart in relation to the output power profiles of the pair of amplifier A  610  and amplifier B  612  to provide the power control system  601  four quadrant control in shaping the output power profile of the combined output power signal. Additional amplifiers included in the power generating system  602  may provide even more control over the combined output power signal. For purposes of illustration, the power profiles are each shown as uniform circles in the top view; however, other shapes are possible. Moreover, the profile of each power amplifier is shown as the same shape; however, it is possible for the profiles to define different shapes relatively. 
     Some plasma processing systems apply a pulsed power signal to the plasma chamber to ignite and control the plasma rather than a constant power signal. For example,  FIG.  8    illustrates an example waveform of pulsed power signal  800  applied to a load  206  of a plasma processing system  200  from a high efficiency radio frequency (RF) power generator  202 . The pulsed power signal  800  may include providing a high power signal  802  for a first duration, followed by a low power signal  804  for a second duration. In some instances, the high power signal  802  may include a positive voltage signal and the low power signal  804  may include a negative voltage signal, although any characteristics of the power signals may be used by the system  200  given that the high power signal is greater than the low power signal in the pulsed signal  800 . In addition, the duration of the high power signal  802  and the low power signal  804  may be any length of time based on the condition of the system  200  and the intended effects on the plasma load  206 . In still other instances, additional power levels  806 ,  808  may be provided to the load  206  from the power generating system  202  in the pulsed power signal  800  that may also be active for one or more durations. 
     The power generating system of the circuits described above may control the source impedance of a power signal provided to the load in a plasma processing system in response to the impedance of the load at the various power levels of the pulsed power signal  800 . For example, the power generating system  202  of  FIG.  2    may provide a pulsed power signal  800  similar to that illustrated in  FIG.  8   . As mentioned above, the load impedance may be correlated to a power signal provided by the power generating system  202  such that varying the input power to the load, as illustrated in the waveform  800  of  FIG.  8   , may vary the impedance of the load. As the load impedance varies in relation to the pulsed power waveform, the power generating system  202  may attempt to match a source impedance of the provided power signal to the load impedance. In one implementation, the power control system  201  may include a look-up table, database, or other reference data that provides a target source impedance to match the load impedance at a particular power level of the pulsed power signal. For example, the look-up table of the power control system  201  may include an entry associated with the initial power level  802  of the pulsed power signal. When the power generating system  202  provides the initial power level  802 , the power control system  201  may control the amplifiers  210 ,  212  to provide a source impedance of the combined output power signal based on the information in the look-up table for the initial power level. Similarly, as the power generating system  202  provides the second power level  804  of the pulsed power signal, the power control system  201  may respond and control the amplifiers  210 ,  212  to provide a source impedance of the combined output power signal based on the information in the look-up table for the second power level. The power control system  201  may continue to reference to the look-up table to obtain a target source impedance for power level  806  and power level  808  of the pulsed power signal when those signals are provided to the load  206  from the power generating system  202 . In this manner, the look-up table may provide the target source impedance for the combined power output signal for any power level of the provided power signal. The power control system  201  may then transmit corresponding control signals to the amplifiers  210 ,  212  of the power generating system  202  to generate the combined output power signal with the target source impedance accordingly. 
     In another implementation, the power control system  501  may respond and control the amplifiers  510 ,  512  of the power generating system  502  based on the feedback information received as an input to the system. Thus, the pulsed power signal  800  from the power generating system  502  may be provided to the feedback system  516  and a target source impedance for a current power level of the power signal may be determined and provided to the power control system  501 . The power control system  501  may control the amplifiers  510 ,  512  as described above to generate the combined output power signal with the target source impedance based on the information received from the feedback system  516 . 
       FIG.  9    is a flowchart of a method for controlling a plurality of amplifiers of power generating system to control a source impedance of an output power signal. The operations of the method  900  of  FIG.  9    may be performed by the power generating systems described above. For example, the power control system  201 , amplifier A  210 , amplifier B  212 , and/or combiner  214  may perform one or more of the operations described. The operations may also be performed by other components of the power generating system not discussed. The operations may be performed using software-related programs, hardware configured to perform aspects of the operations, or a combination of both software and hardware components. 
     Beginning in operation  902 , the power generating system  202  may determine a target source impedance of a power signal to provide to a load that matches the load impedance. The target source impedance to match the load impedance may be generated in any manner described herein, including obtaining the target source impedance from a look-up table, receiving feedback information on a power signal provided to the load, receiving signal information from the load system, and the like. Further, the target source impedance may vary during operation of the system  200 , such as when the load impedance varies due to operational conditions or changes occur in the power signal provided to the load. 
     In operation  904 , the power control system  201  may control a first amplifier  210  to generate a first output power signal. The power control system  201  may provide one or more instructions to configure or instruct the first amplifier  210  to alter an input power signal according to the instructions. Similarly, in operation  906 , the power control system  201  may control a second amplifier  212  to generate a second output power signal. The power control system  201  may provide one or more instructions to the second amplifier  212  to configure or instruct the second amplifier to alter an input power signal according to the instructions. The first output power signal and the second output power signal may be generated based on the target source impedance. For example and discussed above, the output power signal of the second amplifier  212  may be generated to be diametrically opposed to the output power signal of the first amplifier  210  such that the combination of the two power output signals may create a symmetrical or asymmetrical output power signal in reference to a nominal load impedance value. In addition, the shape of the power profile of the combined output power signal may be controlled by the magnitude (or other characteristics) of the output power signal of amplifier A  210  and/or amplifier B  212 . The control over the output power signals from amplifier A  210  and/or amplifier B  212  may generate a combined power profile signal with a determined amplitude and slope of a source impedance corresponding to the target source impedance determined above. 
     Thus, in operation  908 , the output power signal from amplifier A  210  may be combined with the output power signal from amplifier B  212 . The combined output power signal may have a source impedance similar to the target source as determined by the power generating system  202 . The source impedance of the combined power signal may match the load impedance of the load system  206  to stabilize the operation of the system  200 . In general, any characteristic of the combined output power signal may be controlled by the power control system  201 , including the magnitude, frequency, and phase of the output signal based on the target source impedance. Further, the output power signals from amplifier A  210  and amplifier B  212  may be combined using a combiner circuit or device. In operation  910 , the combined output power signal with the target source impedance may be provided or transmitted to a load corresponding to the load impedance determined above. The operations of the method  900  of  FIG.  9    may be repeated during operation of the system  200  to adjust the source impedance of the power signal to match or attempt to match the load impedance in real-time, thereby generating a more stable and efficient power signal for operating the system. 
     The description above includes example systems, methods, techniques, instruction sequences, and/or computer program products that embody techniques of the present disclosure. However, it is understood that the described disclosure may be practiced without these specific details. 
     In the present disclosure, the methods disclosed may be implemented as sets of instructions or software readable by a device. Further, it is understood that the specific order or hierarchy of steps in the methods disclosed are instances of example approaches. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the method can be rearranged while remaining within the disclosed subject matter. The accompanying method claims present elements of the various steps in a sample order, and are not necessarily meant to be limited to the specific order or hierarchy presented. 
     The described disclosure may be provided as a computer program product, or software, that may include a machine-readable medium having stored thereon instructions, which may be used to program a computer system (or other electronic devices) to perform a process according to the present disclosure. A machine-readable medium includes any mechanism for storing information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The machine-readable medium may include, but is not limited to, magnetic storage medium (e.g., hard disk drive), optical storage medium (e.g., CD-ROM); magneto-optical storage medium, read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; or other types of medium suitable for storing electronic instructions. 
     For example,  FIG.  10    is a block diagram illustrating an example of a host or computer system  1000  which may be used in implementing the embodiments of the present disclosure, such as the controller  116  as shown in  FIG.  3 B . The computer system (system) includes one or more processors  1002 - 1006 . Processors  1002 - 1006  may include one or more internal levels of cache (not shown) and a bus controller or bus interface unit to direct interaction with the processor bus  1012 . Processor bus  1012 , also known as the host bus or the front side bus, may be used to couple the processors  1002 - 1006  with the system interface  1014 . System interface  1014  may be connected to the processor bus  1012  to interface other components of the system  1000  with the processor bus  1012 . For example, system interface  1014  may include a memory controller  1013  for interfacing a main memory  1016  with the processor bus  1012 . The main memory  616  typically includes one or more memory cards and a control circuit (not shown). System interface  1014  may also include an input/output (I/O) interface  1020  to interface one or more I/O bridges or I/O devices with the processor bus  1012 . One or more I/O controllers and/or I/O devices may be connected with the I/O bus  626 , such as I/O controller  1028  and I/O device  1030 , as illustrated. 
     I/O device  1030  may also include an input device (not shown), such as an alphanumeric input device, including alphanumeric and other keys for communicating information and/or command selections to the processors  1002 - 1006 . Another type of user input device includes cursor control, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to the processors  1002 - 1006  and for controlling cursor movement on the display device. 
     System  1000  may include a dynamic storage device, referred to as main memory  1016 , or a random access memory (RAM) or other computer-readable devices coupled to the processor bus  1012  for storing information and instructions to be executed by the processors  1002 - 1006 . Main memory  1016  also may be used for storing temporary variables or other intermediate information during execution of instructions by the processors  1002 - 1006 . System  1000  may include a read only memory (ROM) and/or other static storage device coupled to the processor bus  1012  for storing static information and instructions for the processors  1002 - 1006 . The system set forth in  FIG.  10    is but one possible example of a computer system that may employ or be configured in accordance with aspects of the present disclosure. 
     According to one embodiment, the above techniques may be performed by computer system  1000  in response to processor  1004  executing one or more sequences of one or more instructions contained in main memory  1016 . These instructions may be read into main memory  1016  from another machine-readable medium, such as a storage device. Execution of the sequences of instructions contained in main memory  1016  may cause processors  1002 - 1006  to perform the process steps described herein. In alternative embodiments, circuitry may be used in place of or in combination with the software instructions. Thus, embodiments of the present disclosure may include both hardware and software components. 
     A computer readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). Such media may take the form of, but is not limited to, non-volatile media and volatile media. Non-volatile media includes optical or magnetic disks. Volatile media includes dynamic memory, such as main memory  1016 . Common forms of machine-readable medium may include, but is not limited to, magnetic storage medium (e.g., hard disk drive); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory; or other types of medium suitable for storing electronic instructions. 
     Embodiments of the present disclosure include various operations or steps, which are described in this specification. The steps may be performed by hardware components or may be embodied in machine-executable instructions, which may be used to cause a general-purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, the steps may be performed by a combination of hardware, software and/or firmware. 
     It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction, and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. 
     While the present disclosure has been described with reference to various embodiments, it will be understood that these embodiments are illustrative and that the scope of the disclosure is not limited to them. Many variations, modifications, additions, and improvements are possible. More generally, embodiments in accordance with the present disclosure have been described in the context of particular implementations. Functionality may be separated or combined in blocks differently in various embodiments of the disclosure or described with different terminology. These and other variations, modifications, additions, and improvements may fall within the scope of the disclosure as defined in the claims that follow.