Patent Publication Number: US-2022230847-A1

Title: Power delivery to a plasma via inductive coupling

Description:
CLAIM OF PRIORITY UNDER 35 U.S.C. § 119 
     The present Application for Patent claims priority to Provisional Application No. 63,139,488 entitled “Power Delivery to a Plasma via Inductive Coupling” filed Jan. 20, 2021, and is assigned to the assignee hereof and hereby expressly incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to plasma processing. In particular, but not by way of limitation, the present invention relates to systems, methods and apparatuses for applying and distributing power to inductively coupled plasmas. 
     BACKGROUND OF THE INVENTION 
     Inductively coupled plasma (ICP) processing systems are utilized to create high density plasmas at relatively low gas pressures and perform a variety of processes including etching, chemical vapor deposition, and gas abatement. In many implementations, inductive coil antennas are wound around a reactive chamber and actively driven by radio frequency (RF) power to prompt ignition of (and to maintain) a high density plasma in the chamber. In some implementations, multiple RF-powered inductively coupled coils may be used in the same reactive chamber to optimize ignition, increase plasma stability, adjust the etch or deposition profile, and extend the range of the process window. 
     There are many challenges and issues in typical ICP power systems such as cost, complexity, and precise control of RF power or current in one or more ICP coils to maintain a repeatable etch or deposition profile to achieve a desired uniformity. 
     Some existing power solutions utilize fixed frequency, 50 ohm, RF power sources that are each coupled to an ICP coil through separate RF match circuits. Challenges with these solutions include high costs; slow impedance matching tuning time (e.g., hundreds of milliseconds to several seconds); and non-repeatable power delivery run-to-run because power regulation at a 50 ohm point neglects variation of losses through an impedance matching network; thus, there is a variation of power delivered into the ICP coils to maintain a repeatable etch or deposition profile. 
     Other solutions use variable frequency, 50 ohm, RF power sources that are each coupled to an ICP coil through separate RF match circuits. Challenges with these solutions include high costs; variable frequency tuning time (e.g., hundreds of microseconds to tens of milliseconds); and non-repeatable power delivery run-to-run because power regulation at the 50 ohm point neglects variation of losses through an impedance matching network, thus, there is a variation in the power delivered into the ICP coils to maintain a repeatable etch or deposition profile. 
     In yet other solutions, a single fixed-frequency, 50 ohm, power source is coupled to multiple ICP coils through an RF match circuit whose output is reactively split to control a desired current through each of the coils, or a desired current ratio between each of the coils. In an alternate implementation, the ICP coils may be terminated by a tunable reactive circuit, allowing the magnetic coupling between multiple coils and their reactive terminations to control the magnitude of current flowing through each ICP coil, or a desired current ratio between each of the coils. But the challenges with these solutions include slow tuning time (e.g., hundreds of milliseconds to several seconds); limited range of current control between ICP coils; and maintaining good repeatability through a reactive current-splitting circuit to maintain a repeatable etch or deposition profile. 
     Variable frequency, direct-drive of one or more ICP coils has also been employed, but variable frequency tuning time may be slow for some process recipes (e.g., hundreds of microseconds to several milliseconds) and there are challenges with the accuracy and run-to-run repeatability of non-50 ohm power regulation at the input of the ICP coils. 
     SUMMARY 
     Aspects of the present disclosure may be characterized as an inductively coupled plasma (ICP) RF power delivery system that comprises two or more DC voltage sources, two or more corresponding RF resonant power amplifiers, and a controller. Each of the two or more RF resonant power amplifiers is tuned to operate over a different range of frequencies that are coupled into a common plasma, and each of the RF resonant power amplifiers comprises a switch network comprising at least one switch and a resonant tank circuit comprising a reactive network comprising at least one capacitive component and an ICP coil connected to the reactive network. A controller is configured to control the power into each ICP coil by both, varying the corresponding DC voltage source and simultaneously varying operating frequency into each ICP coil by allowing a resonant voltage waveform across a corresponding open switch network to rise and then fall to substantially zero volts before closing the corresponding switch network for a remainder of an RF cycle. 
     Another aspect of the present disclosure may be characterized as an inductively coupled plasma (ICP) RF power delivery system for controlling power into two or more ICP coils coupled into a common plasma. The ICP RF power delivery system comprises a DC voltage source connected to an RF resonant power amplifier that comprises a first ICP coil, and the RF resonant power amplifier is tuned to operate over a range of frequencies. The RF resonant power amplifier comprises a switch network and a resonant tank circuit tuned to a frequency range. The resonant tank circuit includes a reactive network comprising at least one capacitive component and the first ICP coil. At least one passive ICP coil is arranged and configured to be coupled to the first ICP coil, and each passive ICP coil is terminated by an independently adjustable impedance. A controller is configured to control power applied to the first ICP coil by varying the DC voltage source and simultaneously varying an operating frequency into the first ICP coil by allowing a resonant voltage waveform across an open state of the switch network to rise and then fall to substantially zero volts before closing the switch network for a remainder of an RF cycle. Current into each of the passive ICP coils is controlled by adjusting a termination impedance for each of the corresponding passive coils. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an example of an ICP power delivery system; 
         FIGS. 2A, 2B, and 2C  are graphs depicting a resonant voltage waveform relative to switching time; 
         FIG. 2D  is a graph depicting an example of a cycle of a voltage waveform applied to a source load that is effectuated by a resonant voltage waveform; 
         FIG. 3  is a schematic representation of another example of an ICP power delivery system; 
         FIG. 4  is a schematic representation of yet another example of an ICP power delivery system; 
         FIG. 5  is a schematic representation of yet another example of an ICP power delivery system; 
         FIG. 6  is a block diagram depicting an example of one approach to controlling power applied to directly driven ICP coils; 
         FIG. 7  is a block diagram depicting another example of an approach to controlling power applied to directly driven ICP coils; 
         FIG. 8  is a block diagram depicting an example of an approach to controlling power applied to a directly driven ICP coil and a passive ICP coil; 
         FIG. 9  depicts variations of the types of variable impedance that may be utilized as the terminating impedance of  FIGS. 4, 5, and 8 ; 
         FIG. 10  is a schematic diagram depicting an example of a switch network that may be used to realize the switch networks disclosed herein; and 
         FIGS. 11A, 11B, and 11C  are schematic diagrams depicting simplified examples of a buck converter, a boost converter, and a buck-boost converter, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, an aspect of many embodiments disclosed herein is a direct current (DC) controlled power amplifier (PA) that comprises a resonant tank that includes at least one ICP coil. Another aspect of many embodiments is that one or more PAs do not require impedance matching, signal filtering, and in the simplest case, do not require output RF metrology/sensors. For example, if the RF PA losses can be controlled (as discussed further herein), and therefore sufficiently approximated, power delivery into each ICP coil can be also be approximated by simply taking the product of the DC voltage of the DC supply and its output current. 
     To further simplify, some implementations (discussed further herein) utilize a lower cost/complexity, remote fixed-DC power supply, that would require only the RF PAs and a simplified DC/DC power converter, such as a DC buck circuit, to be mounted on the plasma chamber. Precise regulation of power or current into each ICP coil, or its approximation by taking the product of the DC voltage and current from the DC/DC power converter, may be used to enable control of field/deposition/etch uniformity. 
     These embodiments and methods, when combined with passive terminations (discussed further herein) to undriven ICP coils that are inductively coupled to a primary driven coil, may also further simplify the solutions. Regulation of any differences of current between the multiple ICP coils may be effectuated by accurate measurement, and control of current through each coil may be made by changing the terminating impedance to each undriven coil to control uniformity. 
     An example of an ICP RF power delivery system  100  is depicted in  FIG. 1 , which utilizes n RF resonant power amplifiers  102  in connection with n ICP coils  104  where each of the ICP coils  104  is a portion of both, a corresponding RF resonant power amplifier  102  and an ICP source  101 , and n is greater than or equal to 2. The RF resonant power amplifier  102  is a tuned RF power amplifier, realized in combination with the corresponding ICP coil  104  as its load, and it can be optimized to operate similar to a high-efficiency class E amplifier. As shown, each RF resonant power amplifier  102  comprises a switch network  106  and resonant tank  108 . The switch network  106  may be realized by a single switch (as shown in  FIG. 1 ) or multiple switches that function as a single switch (as shown in  FIG. 10 ). 
     As shown, the collection of ICP coils  104  is inductively coupled via mutual coupling  107  (either passively between the coils themselves and/or through the plasma body), and the collection of ICP coils  104  drives a singular plasma body  109 . Within the plasma body  109  control of power to each ICP coil provides a separate control over field uniformity, deposition uniformity, and/or etch uniformity. 
     Although not shown in the drawing figures, those of ordinary skill in the art will readily appreciate that the plasma body  109  may be contained in a plasma chamber, and the ICP coils  104  may be coupled to the chamber (e.g., to ignite and sustain the plasma body  109 ) in a variety of different configurations known to those of ordinary skill in the art. As shown, the resonant tank  108  includes a reactive network  110  that comprises at least one capacitive component such as one of, or both of, a series capacitance  112  and a parallel capacitance  114 . The switching device technology of the switch or switches in the switch network  106  is not-essential (e.g., BJT, FET, silicon, Si—C, and GaN technology may be used), so long as the switch network  106  can be switched substantially fully on and off and the output capacitance characteristics of the switch network  106 , combined with any required external parallel capacitance  114  in the reactive network  110 , can create a resonant voltage waveform that will rise and then fall to substantially zero volts while the switch network  106  is off, and then (when the resonant voltage waveform is approximately zero volts) the switch network  106  is turned on. The series capacitance  112  can be physically a part of the RF resonant power amplifier  102 , part of an assembly (including the corresponding RF resonant power amplifier  102  and corresponding ICP coil  104 ), or divided between. 
     In the implementation depicted in  FIG. 1 , DC voltage sources  116  are implemented as variable voltage sources, but in other implementations discussed further herein, fixed output voltage DC power supplies in connection with DC/DC conversion (e.g., buck convertors) may be utilized. The simplified RF system depicted in  FIG. 1  (and the other systems described herein) may vary its switching time until, as illustrated in  FIGS. 2A, 2B , and  2 C, the resonant voltage waveform, V d , (shown at node  118  in  FIG. 1  as Vd A  and Vd B ) substantially equals zero, at which point, the switch network  106  is turned on (closed). The time that the switch network  106  is off, t off , nominally equals one half-cycle of the instantaneous operating frequency of the RF resonant power amplifier  102 , hence the frequency tuning time also equals nominally one-half cycle of the same operating frequency (e.g., a few tens of nanoseconds for typical ICP applications). This improvement in tuning speed may be appreciated by contrasting the tens of RF cycles (of embodiments disclosed herein) to several thousands of RF cycles of existing RF power architectures driving multiple ICP coils; thus, the tuning speed of the embodiments disclosed herein are many orders of magnitude faster than existing solutions. 
     As shown in  FIGS. 2A-2C , the switch network  106  is nominally closed at a time, t n . The time, t n , is the time during which the switch network  106  would nominally be in an off state to effectuate an RF frequency at a “center” frequency of a frequency range for the RF resonant amplifier  102 . As discussed above, and further herein, the time the switch network is off, t off , is adjusted to maintain zero-voltage switching. As a consequence, t off , may be shorter than t n  in some instances (as shown in  FIG. 2B ), and t off , may be longer than t n  in other instances (as shown in  FIG. 2C ) to adjust the frequency (and hence, power) into the ICP coil  104 . But regardless of the RF frequency that is effectuated at the ICP coil  104 , in many modes of operation the switch network  106  is closed when the resonant voltage waveform, V d , across the switch network  106  is substantially zero. But in some instances, aspects of the source load prevents the resonant voltage waveform, V d , from reaching zero volts (at least within a viable amount of time), so in these instances, the switch network  106  is sub-optimally closed if a pre-determined maximum time for t off  is reached. For example, the predetermined maximum time may be established based upon calculations and/or an empirical assessment of the performance of the RF resonant power amplifier  102 . This may be done to protect the RF resonant power amplifier  102  and/or to ensure the frequency range of one power amplifier does not overlap with a frequency range of another RF resonant power amplifier  102 . 
       FIG. 2A  also depicts current, I S , through the switch network  106  during an RF cycle of the RF voltage waveform (realized by the combination of waveforms through the resonant tank  108 ) that is applied across the source load. The source load is comprised of the combination of each ICP coil  104 , their mutual coupling between adjacent ICP coils  104  and into the plasma body  109 , and the plasma load itself, which is depicted in  FIG. 1  and  FIGS. 3-8  in a simplified model as a resistor in series with each ICP coil  104 . Although it is understood that each ICP coil  104  has its own internal resistance, both the desired and predominant power from the RF power delivery system is principally coupled into the bulk plasma or plasma body  109 . 
     As shown, the resonant voltage waveform, V d , across the switch network  106  rises (when the switch network  106  is in an open state) and then falls to substantially zero volts before the switch network  106  is closed (clamping the resonant voltage waveform, V d  to zero volts) for a remainder of the RF cycle. In effect, the RF cycle of RF voltage waveform (applied to the source load) is forced to be roughly double the time needed to ring up the resonant voltage waveform, V d  and return it again to zero (during t off ). As shown, the current, I S , through the switch network  106  rises and falls during a second half of the RF cycle. It should be noted that the time, t off , that the switch network  106  is off (open) may be approximately the same as the time, t on , the switch network  106  is on (closed). 
     As shown in  FIG. 2D , which depicts an example of an RF voltage waveform applied to the source load, each cycle of the RF voltage waveform is substantially equal to the time, t on , that the switch network  106  is closed plus the time, t off , the switch network  106  is open. Because t on  is roughly equal to t off ,  FIG. 2D  further emphasizes that controlling when the switch network  106  is turned on controls the length of t off , and hence, the length of the RF cycle of the RF voltage waveform. 
     In operation, each of the RF resonant power amplifiers  102  is tuned to operate over a different (e.g., non-overlapping) range of frequencies than the other RF resonant power amplifiers  102  connected to a common plasma body  109 . In many implementations, the range of frequencies that each RF resonant power amplifier  102  operates across is no more than a ratio of 1.8:1 so that the highest frequency of a frequency range is no more than 1.8 times the lowest frequency of the range. As shown in  FIG. 1 , a first RF resonant power amplifier  102  may be tuned to operate at a center frequency, fa, and a second RF resonant power amplifier  102  may be tuned to operate at a center frequency, fb. Although referred to as “center” frequencies, fa and fb are not intended to be limited to a midpoint of a frequency range. 
     In some implementations, fa may be, for example and without limitation, about 13.56 MHz and fb may be about 27.12 MHz. In these implementations, the first RF resonant power amplifier  102  may operate over a range of 10.1 MHz to 18.2 MHz and the second RF resonant power amplifier  102  may operate over a range of 20.2 MHz to 36.3 MHz so that each frequency range is not overlapping. These frequencies and frequency ranges are only examples, and other narrower frequency ranges and other “center” frequencies may be utilized. For example, the first RF resonant power amplifier  102  may operate over a range of 11.56 MHz to 15.56 MHz and the second RF resonant power amplifier  102  may operate over a range of 25.12 MHz to 29.12 MHz, and a third RF resonant power amplifier  102  (not shown) may operate over a range of 1.8 MHz to 2.2 MHz. 
     To substantially eliminate switching losses, the switch network  106  is switched on (closed) at substantially zero volts. With no to low switching losses in the primary switch network  106 , the predominant power loss mechanism in the RF resonant power amplifier  102  is only the internal resistance of the one or more switching devices (in the switch network  106 ) when the switch network  106  is turned on. The internal resistance of the switching device(s) is determined by the properties of the device channel of each switching device, and this loss mechanism is substantially independent across the range of operating frequencies. As a consequence, in the simplest implementation, power regulation of each of the RF resonant power amplifiers  102  into a corresponding ICP coil  104  may be based on the output power of the corresponding DC voltage source  116 , which is the product of the DC voltage of the DC voltage source  116  and its output current. This product is substantially proportional to the RF power driving the ICP coil  104  corresponding to the DC voltage source  116 . Alternatively, as discussed further herein, the system can be regulated from the output RF current driving the ICP coil  104  or the RF power measured at node  120 , although this would require an additional RF sensor placed at the output of the simplified RF system, including its resulting costs/complexities. 
     Referring to  FIG. 3 , shown is another example of an ICP RF power delivery system  300  that utilizes a fixed DC voltage source  316  to apply power to n DC/DC converters  320  (where n is greater than or equal to 2), and each of the n DC/DC converters  320  is coupled to a corresponding one of n RF resonant power amplifiers  102 , and in turn, each of the n RF resonant power amplifiers  102  is coupled to one of n ICP coils  104 . Each of the DC/DC converters  320  may be separately controlled to regulate the DC voltage in each branch of the system (where each branch comprises a DC/DC converter  320 , RF resonant power amplifier  102 , and an ICP coil  104 ). In the system depicted in  FIG. 3 , each DC/DC converter  320  operates as a DC voltage source with inputs driven by the fixed voltage source  316 . 
     Beneficially, the fixed DC voltage source  316  may be remotely located from the plasma chamber and simplified RF resonant power amplifiers  102  and their corresponding DC/DC converters  320  may be implemented in close connection with the plasma chamber. The reduced complexity of on-tool RF systems allows multiple like-solutions to be packaged together to drive an integrated, multi-coil ICP source. With sufficient separation between source frequencies from each of the separate RF resonant power amplifiers  102 , controllable power and current may be used to influence field/deposition/etch uniformity. 
     Turning to  FIG. 4 , shown is yet another ICP RF power delivery system  400 , which comprises both (a driven) ICP coil  104  a passive ICP coil  404 . As shown, the passive ICP  404  coil may be terminated by an adjustable impedance  406  (shown as a variable capacitor in  FIGS. 4, 5, and 8 ) to enable control over the current through the passive ICP coil  404 , which is inductively coupled from the primary RF powered ICP coil  104  through the plasma. Although a single passive ICP coil  404  is depicted in  FIG. 4 , there may be multiple undriven ICP coils  404 . As described further herein, regulation of any differences of current between the multiple passive ICP coils  404  may be effectuated by accurate measurement and control of current through each passive ICP coil  404 . For example, the adjustable terminating impedance  406  may be adjusted to control field/deposition/etch uniformity (or any desired nonuniformity). 
       FIG. 5  depicts another ICP RF power delivery system  500  that comprises both a driven ICP coil  104  and a passive ICP coil  404 . As shown, the system depicted in  FIG. 5  operates in much the same way as the system in  FIG. 4  except that the system in  FIG. 5  comprises a DC/DC converter  320  (with inputs driven by the fixed voltage source  316 ) that functions as a DC voltage source. The fixed DC voltage source  316  may be remotely located from the RF resonant power amplifier  102  or integrated with the RF resonant power amplifier  102 . 
     Implementations that utilize a fixed DC source (such as those depicted in  FIGS. 3 and 5 ) may implement the fixed DC source remotely from the plasma chamber; thus, reducing complexity for tool-mounted solutions by allowing localized DC voltage regulation (e.g., buck conversion) from each separate RF resonant power amplifier  102 . 
     Beneficially, the several variations of power delivery systems disclosed herein enable a significant reduction in cost and complexity of on-tool RF systems. It is unnecessary to match the impedance of the RF resonant power amplifiers  102  to 50 ohms and non-essential harmonic attenuation may be avoided when powering ICP loads. Neither a match network nor attenuation is essential/necessary in many of the implementations disclosed herein. In addition, the depicted RF resonant power amplifiers  102  may be simplified to exclude an on-board resonant tank circuit, as this function is realized by affirmatively utilizing the ICP coil  104  as part of the resonant tank  108 . 
     Frequency operation is set by utilizing zero-voltage switching, thereby eliminating the requirement for a frequency tuning algorithm. As such, the frequency “tune time” in this implementation may be considered to nominally equal half the primary RF operating frequency cycle (e.g., half a cycle of f a  or f b ). And with zero-voltage switching, the primary loss mechanism between the PA switching device and the reactive network is the resistance of the switching device, and these losses are largely independent of the operating frequency within the frequency range. This enables a simplification of control over power delivery into the ICP loads due to its proportionality to the product of DC voltage and current sourced to the PA. 
     Referring to  FIG. 6 , for example, shown is an implementation in which power applied to an ICP coil  104  is regulated based upon a voltage, V DCS , at the output of the DC voltage source  616  and current, iDCS, at the output of the DC voltage source  616 . More specifically, to regulate power applied to the ICP coil  104 , a controller  630  may simultaneously vary a voltage setpoint, V set , of the DC voltage source  616  (to vary V DCS  and control power that is applied to the ICP coil) and a frequency setpoint, f set , of the RF resonant power amplifier  102  (to vary an operating frequency into the ICP coil to maintain zero-voltage switching) based upon a product of a sensed current  632  and a sensed voltage  635  at the output of the DC voltage source  616 . Those of ordinary skill in the art, in view of this disclosure, will recognize that adjusting the DC voltage of the DC voltage source  616  may be used to adjust power applied to the ICP coil  104  and that as the frequency is adjusted (e.g., by adjusting t off ) to maintain zero voltage switching, the power applied by the RF resonant amplifier  102  may change, which prompts a readjustment the DC voltage of the DC voltage source  616  to compensate. It should be recognized that only a single branch (comprising a single RF resonant power amplifier  102  and a single ICP coil  104 ) is depicted for purposes of simplicity and clarity, but in actual implementation, two or more branches are controlled in the same way (by one or more controllers  630 ) so that each branch may be tuned to operate at a different power level and over a different range of frequencies than the other branches that are coupled to the common plasma. 
     As discussed above, by virtue of the switching device losses (of the switch network  106 ) being the predominant less mechanism between the switching device and the reactive network, the actual power applied to the ICP coil  104  is substantially proportional to the DC power applied at an output  636  of the DC voltage source  616 . But there are still losses in each branch that render the actual power applied to the ICP coil  104  different than the power applied at the output  636  of the DC voltage source. As a consequence, optional calibration data  634  may be used by the controller to scale the measured power at the output  636  so that the scaled version of the measured power is substantially the same as the actual power applied to the ICP coil  104 . Those of ordinary skill in the art will appreciate that the optional calibration data  634  may be obtained by using precision metrology instruments to measure the actual power applied to the ICP coil  104  to obtain one or more adjustment factors or an equation that can be used to adjust the power measured at the output  636  so that it is substantially the same as the power actually applied to the ICP coil  104 . 
     Alternatively, regulating on output current, as opposed to power measured directly at the output, also reduces complexity and enables improved repeatability. Referring to  FIG. 7 , for example, shown is a control configuration that regulates current applied to the ICP coil  104  based on a sensed current  732  (indicative of output current, i out ). As shown, a sensed voltage  734  (indicative of the RF voltage waveform, V O , applied to the ICP coil  104 ) may also be optionally measured and used (in conjunction with the output current i out ) to regulate power that is applied to the ICP coil  104 . 
     As discussed above with reference to  FIGS. 4 and 5 , driving a multi-element ICP source with even a single RF resonant power amplifier  102  can be complemented with passive adjustment of a termination impedance connected to undriven ICP coils  404 . This allows the same frequency to couple controllable currents into the adjacent coils through the common plasma to influence field/deposition/etch uniformity. Referring to  FIG. 8 , shown is a block diagram depicting aspects of control in a system that includes both a first (direct driven) ICP coil  104  and a passive ICP coil  404 . As shown, the controller  630  may receive a sensed-current signal  832  via a current sensor to sense current, i out , that flows through the direct driven ICP coil  104  and receive a sensed-current signal  833  via a current sensor that is positioned to sense current, I O2 , that flows through the adjustable impedance  406  and the passive ICP coil  404 . In response, the controller  630  may control the current,  102 , into the passive ICP coil  404  by adjusting the termination impedance. In addition, similar to other embodiments discussed herein, the controller  630  is configured to control power applied to the direct driven ICP coil  104  by varying the DC voltage source  616  and simultaneously varying an operating frequency into the first ICP coil  104  by allowing a resonant voltage waveform across an open state of the switch network  106  to rise and then fall to substantially zero volts before closing the switch network to complete an RF cycle. 
     The controller  630  may be implemented directly in hardware, in one or more software modules executed by a processor, or in a combination of the two. A software module may reside as non-transitory processor executable instructions in non-volatile memory, which is coupled to the processor so the processor can read information from, and write information to, the non-volatile memory. The steps of the methods described above may be implemented as processor-executable instructions stored in the non-volatile memory, and the optional calibration data  634  may be stored in non-volatile memory (which may be distributed among several memory components). The processor may be a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) and/or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. 
       FIG. 9  depicts variations of the types of adjustable impedance  406  that may be utilized as the terminating impedance (for the passive ICP coils) in the embodiments depicted in  FIGS. 4, 5, and 8 . As shown, the adjustable impedance  406  may be realized by a variable capacitor, a variable inductor, or a variable reactance. 
       FIG. 10  depicts a switch network  1006  (that is one example of the switch network  106 ), which comprises a plurality of series-arranged switches that are operated to function as a single switch. More specifically, all of the series-arranged switches in the switch network  1006  are simultaneously opened when the switched network is off or open, and all of the series-arranged switches in the switch network  1006  are simultaneously closed when the switched network is on or closed. Those of ordinary skill in the art will readily appreciate, in view of this disclosure, that other types of switch networks may be implemented. For example, two parallel switches may replace any of the single switch devices depicted herein (e.g., to allow for higher current flow with lower-current-rated switch devices) and may open and close simultaneously to operate as a single switch. 
       FIGS. 11A, 11B, and 11C  each depict an example of the DC/DC converter  320 . More specifically,  FIG. 11A  depicts an example of a simplified buck converter;  FIG. 11B  depicts an example of a simplified boost converter; and  FIG. 11C  depicts an example of a simplified buck-boost converter. Those of ordinary skill in the art will readily appreciate, in view of this disclosure, that other topologies may be utilized for the types of DC/DC converters depicted in  FIGS. 11A-11C . 
     Disclosed herein are a variety of different embodiments and implementation designs. It should be recognized that the constituent components of the RF resonant power amplifiers  102  including the ICP coils  104 ,  404  may be integrated into a unitary system or may be distributed among separable components. When distributed, for example, the RF resonant power amplifiers  102  and the voltage sources  116 ,  316  may be separable components (and sold and distributed separately from) the ICP coils  104  and plasma chamber. It is further contemplated that the RF resonant power amplifiers  102  including the ICP coils  104 ,  404  may be physically integrated with a plasma chamber and sold a distributed as an integrated ICP source. 
     As used herein, the recitation of “at least one of A, B and C” or “at least one of A, B or C” is intended to mean “either A, B, C or any combination of A, B and C.” The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.