Patent Publication Number: US-2023145567-A1

Title: Sensorless rf impedance matching network

Description:
FIELD 
     Embodiments of the present disclosure generally relate to a methods and apparatus for processing a substrate, and for example, to methods and apparatus that use a sensorless radio frequency (RF) impedance matching network. 
     BACKGROUND 
     Impedance matching networks used during plasma substrate (wafer) processing are known. For example, conventional RF impedance matching networks comprise electrical circuitry that is positioned between an RF source and a plasma reactor (chamber) to optimize power efficiency. For example, at a tuned matching point, maximum RF power is delivered (forward power) into the plasma load and near zero power is reflected back (reflected power) to the RF source. Conventional impedance matching networks require one or more sensors for measuring and monitoring impedances. Impedance measurements are, typically, made using a voltage and current probe and/or a magnitude and phase detector at an input and output of the impedance matching networks. In some instances, a sensor may be used at the RF matching network input port. Collected impedance magnitude and phase information are used to control one or more motorized variable capacitors of the impedance matching networks. While conventional impedance matching networks are suitable for delivering maximum power transfer into the plasma, such networks, however, use complex and relatively expensive hardware and require sensor calibration prior to use. 
     SUMMARY 
     Methods and apparatus for plasma processing a substrate are provided herein. In some embodiments, for example, a method for plasma processing a substrate comprises supplying from an RF power source RF power at a first power level suitable for igniting and maintaining a plasma within a processing volume of a plasma processing chamber, measuring at the RF power source a reflected power at the first power level, comparing the measured reflected power to a first threshold, transmitting a result of the comparison to the first threshold to a controller of a matching network, setting at the matching network at least one variable capacitor to a first position based on the comparison of the measured reflected power at the first power level to the first threshold, supplying from the RF power source the RF power at a second power level different from the first power level, the second power level for plasma processing the substrate disposed within the plasma processing chamber, measuring at the RF power source the reflected power at the second power level, comparing the measured reflected power at the second power level to a second threshold different from the first threshold, transmitting a result of the comparison of the second threshold to the controller of the matching network, setting at the matching network the at least one variable capacitor to a second position based on the comparison of the measured reflected power at the second power level to the second threshold, and continuing supplying the RF power at the second power level for plasma processing the substrate. 
     In accordance with at least some embodiments, a non-transitory computer readable storage medium has stored thereon instructions that when executed by a processor perform a method for plasma processing a substrate. The method comprises supplying from an RF power source RF power at a first power level suitable for igniting and maintaining a plasma within a processing volume of a plasma processing chamber, measuring at the RF power source a reflected power at the first power level, comparing the measured reflected power to a first threshold, transmitting a result of the comparison to the first threshold to a controller of a matching network, setting at the matching network at least one variable capacitor to a first position based on the comparison of the measured reflected power at the first power level to the first threshold, supplying from the RF power source the RF power at a second power level different from the first power level, the second power level for plasma processing the substrate disposed within the plasma processing chamber, measuring at the RF power source the reflected power at the second power level, comparing the measured reflected power at the second power level to a second threshold different from the first threshold, transmitting a result of the comparison of the second threshold to the controller of the matching network, setting at the matching network the at least one variable capacitor to a second position based on the comparison of the measured reflected power at the second power level to the second threshold, and continuing supplying the RF power at the second power level for plasma processing the substrate. 
     In accordance with at least some embodiments, a system for processing a substrate comprises an RF power source configured to supply RF power, a gas source configured to supply a process gas into a processing volume of a plasma processing chamber, a matching network configured to set at least one variable capacitor based on a result of a comparison received from the RF power source, and a controller configured to supply from the RF power source RF power at a first power level suitable for igniting and maintaining a plasma within the processing volume of the plasma processing chamber, measure at the RF power source a reflected power at the first power level, compare the measured reflected power to a first threshold, transmit the result of the comparison to the first threshold to a match controller of the matching network, set at the matching network at least one variable capacitor to a first position based on the comparison of the measured reflected power at the first power level to the first threshold, supply from the RF power source the RF power at a second power level different from the first power level, the second power level for plasma processing the substrate disposed within the plasma processing chamber, measure at the RF power source the reflected power at the second power level, compare the measured reflected power at the second power level to a second threshold different from the first threshold, transmit a result of the comparison of the second threshold to the match controller of the matching network, set at the matching network the at least one variable capacitor to a second position based on the comparison of the measured reflected power at the second power level to the second threshold, and continue supplying the RF power at the second power level for plasma processing the substrate. 
     Other and further embodiments of the present disclosure are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure, briefly summarized above and discussed in greater detail below, can be understood by reference to the illustrative embodiments of the disclosure depicted in the appended drawings. However, the appended drawings illustrate only typical embodiments of the disclosure and are therefore not to be considered limiting of scope, for the disclosure may admit to other equally effective embodiments. 
         FIG.  1    is a schematic diagram of an apparatus, in accordance with one or more embodiments of the present disclosure. 
         FIG.  2    is a block diagram of a sensorless impedance matching network configured for use with the apparatus of  FIG.  1   , in accordance with one or more embodiments of the present disclosure. 
         FIG.  3    is a block diagram of a sensorless impedance matching network configured for use with the apparatus of  FIG.  1   , in accordance with one or more embodiments of the present disclosure. 
         FIG.  4    is a flowchart of a method for processing a substrate, in accordance with one or more embodiments of the present disclosure. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. Elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Embodiments of methods and apparatus for processing a substrate are provided herein. For example, in at least some embodiments, apparatus comprises a sensorless RF impedance matching network system for use with plasma process chambers, and methods for matching plasma load impedances of plasma process chambers. The sensorless RF matching networks and automatic matching algorithm described herein use simple, cost-effective hardware that does not require sensor calibration. 
       FIG.  1    is a schematic diagram of an apparatus, in accordance with at least some embodiments of the present disclosure. The apparatus is suitable for etching one or more substrates (wafers) using an electron beam (ebeam). 
     Accordingly, in at least some embodiments, the apparatus is a processing chamber  100  (e.g., a plasma processing chamber, such as an ebeam process chamber) that is configured to perform ebeam induced etching (EBIE). The processing chamber  100  has a chamber body  102  which defines a process volume  101 . In an embodiment, the chamber body  102  has a substantially cylindrical shape and may be fabricated from a material suitable for maintaining a vacuum pressure environment therein, such as metallic materials, for example aluminum or stainless steel. 
     A ceiling  106  is coupled to the chamber body  102  and forms the process volume  101 . The ceiling  106  is formed from an electrically conductive material, such as the materials utilized to fabricate the chamber body  102 . The ceiling  106  is coupled to and supports an electrode  108  (e.g., an upper electrode). In some embodiments, the electrode  108  is coupled to the ceiling  106  such that the electrode  108  is disposed adjacent or within the process volume  101 . The electrode  108  is formed from a process-compatible material having a high secondary electron emission coefficient, e.g., a secondary electron emission coefficient, of about 5 to about 10. Materials having relatively high secondary emission coefficients can include, but are not limited to, silicon, carbon, silicon carbon materials, or silicon-oxide materials. Alternatively, the electrode  108  can be formed from a metal oxide material such as aluminum oxide (Al 2 O 3 ), yttrium oxide (Y 2 O 3 ), or zirconium oxide (ZrO 2 ). A dielectric ring  109 , which is formed from an electrically insulating material, is coupled to the chamber body  102  and surrounds the electrode  108 . As illustrated, the dielectric ring  109  is disposed between the chamber body  102  and the ceiling  106  and supports the electrode  108 . 
     The ceiling  106  can include an insulating layer  150  containing a chucking electrode  152  facing the electrode  108 . In at least some embodiments, a DC voltage power supply  154  can be coupled to the chucking electrode  152  via the feed conductor  155 , for electrostatically clamping the electrode  108  to the ceiling  106 , and to the electrode  108  for applying a DC power (e.g., a voltage potential) thereto. In such embodiments, a DC blocking capacitor  156  can be connected in series with the output of an impedance matching network  124 . A controller  126  functions to control the DC voltage power supply  154 . 
     Mechanical contact between the electrode  108  and the ceiling  106  is sufficient to maintain high thermal conductance between the electrode  108  and the ceiling  106 . Additionally, a force of the mechanical contact can be regulated by the electrostatic clamping force provided by the DC voltage power supply  154 . 
     In one or more embodiments, the ceiling  106  is electrically conductive and in electrical contact with the electrode  108 . Power from the impedance matching network  124  is conducted through the ceiling  106  to the electrode  108 . In one or more embodiments, the chamber body  102  can be maintained at ground potential. 
     In one or more embodiments, grounded internal surfaces (i.e., chamber body  102 ) inside the processing chamber  100  can be coated with a process compatible material such as silicon (Si), carbon (C), silicon carbon (SiC) materials, or silicon-oxide (SiO) materials, aluminum oxide (Al 2 O 3 ), yttrium oxide (Y 2 O 3 ), or zirconium oxide (ZrO 2 ). 
     In some embodiments, internal passages (not shown) for conducting a thermally conductive liquid or media inside the ceiling  106  are connected to a thermal media circulation supply. The thermal media circulation supply acts as a heat sink or a heat source. 
     A pedestal  110  is disposed in the process volume  101 . The pedestal  110  supports a substrate  111  (e.g., semiconductor wafers, such as silicon wafers, or glass panels or other substrates, such as for solar cell, display, or other applications) thereon and has a substrate support surface  110   a  oriented parallel to the electrode  108 . In an embodiment, the pedestal  110  is movable in the axial direction by a lift servo  112 . During operation, an upper electrode, such as the electrode  108 , is maintained at one or more distances (e.g., a process position) from the substrate support surface  110   a . For example, in at least some embodiments, the electrode  108  is maintained from a process position for processing a substrate at a distance from about 1 inch to about 20 inches. For example, in at least some embodiments, the distance can be about 6 inches to about 10 inches. 
     The controller  126  is provided and coupled to various components of the processing chamber  100  to control the operation of the processing chamber  100  for processing a substrate. The controller  126  includes a central processing unit  127 , support circuits  129  and a memory  131 , which can be a non-transitory computer readable storage medium having instructions thereon to perform the methods described herein. The controller  126  is operably coupled to and controls one or more energy sources directly, or via computers (or controllers) associated with the processing chamber  100  and/or support system components. The controller  126  may be any form of general-purpose computer processor that can be used in an industrial setting for controlling various chambers and sub-processors. The memory  131 , or non-transitory computer readable storage medium, of the controller  126  may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, optical storage media (e.g., compact disc or digital video disc), flash drive, or any other form of digital storage, local or remote. The support circuits  129  are coupled to the central processing unit  127  for supporting the central processing unit  127  in a conventional manner. The support circuits  129  include cache, power supplies, clock circuits, input/output circuitry and subsystems, and the like. Inventive methods as described herein, such as the method for processing a substrate (e.g., EBIE of a substrate), may be stored in the memory  131  as software routine  133  that may be executed or invoked to control the operation of the one or more energy sources in the manner described herein. The software routine  133  may also be stored and/or executed by a second central processing unit (not shown) that is remotely located from the hardware being controlled by the central processing unit  127 . 
     In one or more embodiments, the pedestal  110  can include an insulating puck  142  which forms the substrate support surface  110   a , a lower electrode  144  disposed inside the insulating puck  142 , and a chucking voltage supply  148  connected to the electrode  144 . Additionally, in at least some embodiments, a base layer  146  underlying the insulating puck  142  can include one or more internal passages (not shown) for circulating a thermal transfer medium (e.g., a liquid) from a circulation supply. In such embodiments, the circulation supply can function as a heat sink or as a heat source. 
     One or more RF power generators can be coupled to the processing chamber  100 . In at least some embodiments, a high frequency RF power source  120  having a frequency from about 20 MHz to about 200 MHz and a low frequency RF power source  122  having a frequency from about 100 kHz to about 20 MHz are coupled to the electrode  108  through, for example, the impedance matching network  124  via an RF feed conductor  123  (RF transmission line). In at least some embodiments, the one or more RF generators can comprise a single RF source having a frequency from about 100 kHz to about 200 MHz. The RF feed conductor  123  from the impedance matching network  124  can be connected to the electrode support or ceiling  106  rather than being directly connected to the electrode  108 . In such embodiments, RF power from the RF feed conductor  123  can be capacitively coupled from the electrode support to the electrode  108 . The impedance matching network  124  is adapted to provide an impedance match at different frequencies of the high frequency RF power source  120  and the low frequency RF power source  122 , as well as filtering to isolate the high frequency RF power source  120  and the low frequency RF power source  122  from one another. Output power levels of the high frequency RF power source  120  and the low frequency RF power source  122  can be independently controlled by the controller  126 . 
     With the high frequency RF power source  120  and the low frequency RF power source  122 , radial plasma uniformity in the process volume  101  can be controlled by selecting a distance (e.g., from about 6 inches to about 10 inches) between the electrode  108  and pedestal  110 . For example, in some embodiments, a lower VHF frequency produces an edge-high radial distribution of plasma ion density in the process volume  101  and an upper VHF frequency produces a center-high radial distribution of plasma ion density. With such a selection, the power levels of the high frequency RF power source  120  and the low frequency RF power source  122  are capable of generating a plasma with a substantially uniform radial plasma ion density. 
     Upper gas injectors  130  provide process gas into the process volume  101  through a first valve  132 , and lower gas injectors  134  provide process gas into the process volume  101  through a second valve  136 . The upper gas injectors  130  and the lower gas injectors  134  can be disposed in sidewalls of the chamber body  102 . Process gas is supplied from an array of process gas supplies such as gas supplies (e.g., gas source)  138  through an array of valves  140  which are coupled to the first valve  132  and second valve  136 . Process gas species and gas flow rates delivered into the process volume  101  can be independently controllable. For example, gas flow through the upper gas injectors  130  may be different from gas flow through the lower gas injectors  134 . The controller  126  governs the array of valves  140 . 
     In embodiments, one or more inert gases, such as argon (Ar), helium (He) (or other inert gas), and/or one or more reactive gases, such as methane (CH 4 ), acetylene (C 2 H 2 ), hydrogen (H 2 ), hydrogen bromide (HBr), ammonia (NH 3 ), disilane (Si 2 H), nitrogen trifluoride (NF 3 ), tetrafluoromethane (CF 4 ), sulfur hexafluoride (SF 6 ), carbon monoxide (CO), carbonyl sulfide (COS), trifluoromethane (CHF 3 ), hexafluorobutadiene (C 4 F 6 ), chlorine (C 2 ), nitrogen (N 2 ), oxygen (O 2 ), combinations thereof, and the like can be supplied into the process volume  101  through either or both the upper gas injectors  130  and the lower gas injectors  134 . In some embodiments, the process gas delivered to the process volume  101  adjacent the electrode  108  can accelerate secondary electrons toward the substrate  111 , as will be described in greater detail below, and/or buffer the electrode  108  from a reactive plasma formed in the process volume  101 , thus increasing the useful life of the electrode  108 . 
     In accordance with the present disclosure, plasma is generated in the process volume  101  by various bulk and surface processes, for example, by capacitive coupling  170  (e.g., capacitive coupling plasma (CCP)) and/or inductive coupling  172  (e.g., inductive coupling plasma (ICP)). Inductively coupled power or high frequency capacitively coupled power can be used to achieve independent control of plasma density, aside from bias power controlling ion energy. Accordingly, when the processing chamber  100  is configured for use with the capacitive coupling  170  (e.g., configured as a CCP reactor), source power can refer to a higher frequency (compared to bias) power being applied to either a bias electrode (e.g., the electrode  144 ), which supports the substrate  111 , or the upper electrode, e.g., the electrode  108 . Alternatively or additionally, when the processing chamber  100  is configured for use with the inductive coupling  172  (e.g., configured as an ICP reactor), the source power refers to power applied to a coil  173  (shown in phantom in  FIG.  1   ). When the processing chamber  100  is configured as an ICP reactor, a dielectric window  175  (also shown in phantom) is provided on a side of the chamber body  102  of the processing chamber  100 . The dielectric window  175  is configured to provide a vacuum boundary and a window for electromagnetic wave exciting plasma. 
     Ions generated by a CCP or ICP are influenced by an electric field that encourages ion bombardment of the electrode  108  by the ions generated from the plasma, as will be described in greater detail below. Moreover, depending on a mode of operation of the processing chamber  100 , ion bombardment energy of the electrode  108  can be a function of a power supplied to the electrode  108 , e.g., provided by one or more of the DC voltage power supply  154 , the low frequency RF power source  122 , or the high frequency RF power source  120 . For example, in at least some embodiments, ion bombardment energy of the electrode  108  can be provided by application of voltage from one or both the DC voltage power supply  154  and the low frequency RF power source  122 . In at least some embodiments, in addition to using one or both the DC voltage power supply  154  and the low frequency RF power source  122 , the high frequency RF power source  120  can be used to increase plasma density and ebeam flux. 
     When the DC voltage power supply  154  is used to supply power (e.g., bias) to the electrode  108 , the power supplied by the DC voltage power supply  154  can be about 1 W to about 30 kW (e.g., about −1560V to about −1440V). Similarly, when the low frequency RF power source  122  is used to supply power (e.g., bias) to the electrode  108 , the power supplied by the low frequency RF power source  122  can be about 1 W to about 30 KW with a frequency from about 100 kHz and about 20 MHz. Likewise, when the high frequency RF power source  120  is used in conjunction with either or both the DC voltage power supply  154  and the low frequency RF power source  122 , the power supplied by the high frequency RF power source  120  can be about 1 W to about 10 kW with a frequency from about 20 MHz and about 200 MHz. 
     In some embodiments, an RF bias power source  162  can be coupled through an impedance matching network  164  to an electrode  144  of the pedestal  110 . The RF bias power source  162 , if used, is configured to accelerate ions onto the substrate  111 . The RF bias power source  162  can be configured to provide low frequency RF power and/or high frequency RF power. For example, in at least some embodiments, the RF bias power source  162  can be configured to supply 1 W to 30 kW of power to the electrode  144  at one or more frequencies, e.g., of about 100 kHz to about 200 MHz. In some embodiments, for example, the RF bias power source  162  can be configured to supply 1 W to 30 kW of power to the electrode  144  at a frequency of about 100 kHz to about 100 MHz. 
     A waveform tailoring processor  147  may be connected between an output of the impedance matching network  164  and the electrode  144  and/or an output of the impedance matching network  124  and the electrode  108 . The waveform tailoring processor  147  controller can be configured to change a waveform produced by the RF bias power source  162  and/or the high frequency RF power source  120  and the low frequency RF power source  122  to a desired waveform. The ion energy of plasma near the substrate  111  and/or the electrode  108  can be controlled by the waveform tailoring processor  147 . For example, in some embodiments, the waveform tailoring processor  247  produces a waveform in which an amplitude is held during a certain portion of each RF cycle at a level corresponding to a desired ion energy level. The controller  126  controls the waveform tailoring processor  147 . 
     Etching of the substrate  111  can be also influenced by one or more factors. For example, pressure (in addition to ebeam energy, ebeam plasma power, and bias power if used) can influence etching of the substrate  111 . Accordingly, in an embodiment, a pressure maintained in the process volume  101  during EBIE of the substrate  111  can be between about 0.1 mTorr to about 300 mTorr. For example, in at least some embodiments, such as when ebeam neutralization and etch profile control are necessary, a pressure maintained in the process volume  101  during EBIE of the substrate  111  can be between about 0.1 mTorr to about 30 mTorr. Likewise, in at least some embodiments, such as when ebeam neutralization and etch profile control are not necessary and bias power is not needed, a pressure maintained in the process volume  101  during EBIE of the substrate  111  can be between about 0.1 mTorr to about 100 mTorr. The pressure is generated by a vacuum pump  168  which is in fluid communication with the process volume  101 . The pressure is regulated by a gate valve  166  which is disposed between the process volume  101  and the vacuum pump  168 . The controller  126  controls the vacuum pump  168  and/or the gate valve  166 . 
       FIG.  2    is a block diagram of a sensorless impedance matching network  200  (e.g., the impedance matching network  124  or the impedance matching network  164 ) configured for use with the apparatus of  FIG.  1   , in accordance with one or more embodiments of the present disclosure. The sensorless impedance matching network  200  can comprise one or more capacitors and inductors. For example, in at least some embodiments, the sensorless impedance matching network  200  can comprise a shunt variable capacitor  202  and a series variable capacitor  204 . The sensorless impedance matching network  200  can be a component of or connected to an RF power source (e.g., the high frequency RF power source  120  and the low frequency RF power source  122 ) through a 50Ω transmission line  206 . In at least some embodiments, the sensorless impedance matching network  200  can be a component of or connected to the RF bias power source  162 , the V rf  connected to the capacitive coupling  170 , and/or the r connected to inductive coupling  172 , also through a 50Ω transmission line. In at least some embodiments, the sensorless impedance matching network  200  can also comprise an L-type sensorless impedance matching network. For example, in at least some embodiments, one or more fixed inductors can be connected in series with the shunt variable capacitor  202  and the series variable capacitor  204  for tuning range optimization. 
     Unlike conventional matching networks previously described, the sensodess impedance matching network does not use any sensors on the input side or the output and, thus, no sensor calibration data is required to be stored in a memory of a controller. In operation, the sensorless impedance matching network  200  receives forward power signals and reflected power signals directly from the RF generator. For example, the forward power signals and reflected power signals received from the RF source can be transmitted (e.g., via wired or wireless communication) to a match controller  208  of the sensodess impedance matching network  200 . Alternatively or additionally, the match controller  208  can receive the forward power signals and reflected power signals from the controller  126 . The match controller  208  uses the forward power signals and reflected power signals to tune the shunt variable capacitor  202  and the series variable capacitor  204 . For example, in at least some embodiments, the match controller  208  uses a pre-programmed learning-based algorithm—which can be stored in a memory (e.g., the memory  131 )—that comprises one or more parameters, such as, voltage standing wave ratio (VSWR), reflection coefficient, etc., to calculate capacitance values for the shunt variable capacitor  202  and the series variable capacitor  204 . 
       FIG.  3    is a block diagram of a sensorless impedance matching network  300  (e.g., the impedance matching network  124  or the impedance matching network  164 ) configured for use with the apparatus of  FIG.  1   , in accordance with one or more embodiments of the present disclosure. The sensorless impedance matching network  300  is substantially identical to the sensorless impedance matching network  200 . Accordingly, only the features that are unique to the sensorless impedance matching network  300  are described herein. 
     The sensorless impedance matching network  300  comprises an RF circulator  302  comprising one or more terminals. For example, in at least some embodiments, the RF circulator  302  can comprise 3 terminals and is connected between the RF power source and the sensorless impedance matching network  300 , via the 50Ω transmission line  206 . The RF circulator  302  is configured to allow RF power to flow in one direction. For example, the forward power signals and reflected power signals pass through the RF circulator  302  and terminates in a 50Ω dummy load  304 . In at least some embodiments, a power meter  306  can be used to measure the RF reflected power at a desired frequency. The reflected power signal can be transmitted to a tool controller (e.g., the controller  126 ) that is communicatively connected to the match controller  208 . In at least some embodiments, the RF power source and the sensorless impedance matching network  300  can be synchronized and controlled by the tool controller. Alternatively, as noted above, the controller  126  need not be used and the power meter  306  and the dummy load  304  can be connected directly to the match controller  208 . 
     Power meter data, RF power, and capacitor positions of the shunt variable capacitor  202  and the series variable capacitor  204  can be transmitted to the tool controller, and combined with one or more other system processing data, such as temperature, chemistry, and flow rate, thus creating cooperative intelligent real time control during operation. In at least some embodiments, the combined data can be used to teach a learning-based model. 
       FIG.  4    is a flowchart of a method  400  for processing a substrate, in accordance with one or more embodiments of the present disclosure. The method  400  can be performed using, for example, a processing chamber that is configured for performing EBIE of a substrate, e.g., the processing chamber  100 . For illustrative purposes, the processing chamber is assumed configured as a CCP reactor configured for EBIE of a substrate, e.g., the substrate  111 , which can be, for example, a 150 mm, 200 mm, 300 mm, 450 mm substrate, etc. For example, in at least some embodiments, the substrate can be a 300 mm substrate, such as a semiconductor wafer or the like. As can be appreciated, the herein described power/voltages and/or pulsing/duty cycles can be scaled accordingly, e.g., for substrates having diameters greater or less than 300 mm. Initially, one or more of the above described process gases can be introduced into a process volume, e.g., the process volume  101 , of the process chamber. For example, in at least some embodiments, the process gas can be one or more of He, Ar, and the like (or other inert gas), and/or H 2 , HBr, NH 3 , Si 2 H 6 , CH 4 , C 2 H 2 , NF 3 , CF 4 , SF 6 , CO, COS, CHF 3 , C 4 Fe, Cl 2 , N 2 , O 2 , and the like (or other reactive gas). Additionally, the process volume can be maintained at one or more operating pressures from about 0.1 mTorr to about 300 mTorr. 
     In at least some embodiments, the method  400  comprises two main steps, a first pre-learning phase, which uses a relatively low power to ignite and maintain a plasma, and a second tuning phase, which uses a relatively high power to plasma process a substrate. 
     During the first pre-learning phase, at  402 , the method  400  comprises supplying from an RF power source configured to supply RF power at a first power level suitable for igniting and maintaining a plasma within a processing volume of a plasma processing chamber. For example, in at least some embodiments, the RF power source can provide RF power (e.g., 1 W to about 30 kW at a frequency of about 20 kHz to about 20 MHz) to the processing chamber  100  to ignite and maintain a plasma within the processing volume  101  of the processing chamber  100 . For example, in at least some embodiments, the first power level can be about 1 W to about 30 kW and can be supplied at frequency of about 100 kHz to about 20 MHz. 
     Based on a reflected power signal measured from the RF generator, a controller (e.g., the controller  126 ) determines positions of one or more series variable capacitors and one or more shunt variable capacitors. For example, at  404 , the method  400  comprises measuring at the RF power source a reflected power at the first power level. For example, a controller (e.g., the controller  126 ) of the RF power source can measure the reflected power at the high frequency RF power source  120  (and/or the low frequency RF power source  122 ). 
     Next, at  406 , the method  400  comprises comparing the measured reflected power to a first threshold. For example, in at least some embodiments, the first threshold can be about 10% of the reflected power at the first power level. Additionally, comparing the measured reflected power to a first threshold can comprise performing frequency tuning, e.g., at the RF power generator. 
     Next, at  408 , the method  400  comprises transmitting a result of the comparison to the first threshold to a controller of a matching network. For example, the controller of the RF power source transmits the result of the comparison to the first threshold to the match controller  208  of the sensorless impedance matching network  200 . Next, at  410 , the method  400  comprises setting at the matching network at least one variable capacitor to a first position based on the comparison of the measured reflected power at the first power level to the first threshold. For example, the match controller  208  of the sensorless impedance matching network  200  can set one or both of the shunt variable capacitor  202  and the series variable capacitor  204  to one or more positions, which can be stored in a memory of the match controller  208 . In at least some embodiments, the match controller  208  can use one or more optimization algorithms (e.g., gradient based methods, derivative-free methods, and/or model-based methods) to determine the one or more positions that the shunt variable capacitor  202  and the series variable capacitor  204  can be set to. 
     During the second tuning phase (e.g., fine-tuning phase), an algorithm can be implemented using a relatively small threshold value. For example, at  412 , the method  400  comprises supplying from the RF power source the RF power at a second power level, different from the first power level. The second power level used during the second tuning phase is used for plasma processing the substrate disposed within the plasma processing chamber. For example, in at least some embodiments, the RF power source can provide RF power (e.g., 1 W to about 10 kW at a frequency of about 20 MHz to about 200 MHz) to the processing chamber  100  for plasma processing the substrate. In at least some embodiments, the RF power can be supplied in a pulsed mode, including a single level pulse mode or a multilevel pulse mode. Additionally, in at least some embodiments, frequency tuning techniques at the RF plasma source can also be used during the second tuning phase. 
     Next, at  414 , the method  400  comprises measuring at the RF power source the reflected power at the second power level. For example, as noted above, a controller (e.g., the controller  126 ) of the RF power source can measure the reflected power at the high frequency RF power source  120  (and/or the low frequency RF power source  122 ). 
     Next, at  416 , the method  400  comprises comparing the measured reflected power at the second power level to a second threshold different from the first threshold (e.g., the second threshold is less than the first power level). For example, in at least some embodiments, the second threshold can be about 1% to about 9% of the reflected power at the first power level. Additionally, similar to  406  comparing the measured reflected power to a second threshold can comprise performing frequency tuning, e.g., at the RF power generator. 
     Next, at  418 , the method  400  comprises transmitting a result of the comparison of the second threshold to the controller of a matching network. For example, the controller of the RF power source transmits the result of the comparison to the second threshold to the match controller  208  of the sensorless impedance matching network  200 . Next, at  420 , the method  400  comprises setting at the matching network at least one variable capacitor to a second position based on the comparison of the measured reflected power at the second power level to the second threshold. For example, the match controller  208  of the sensorless impedance matching network  200  can set one or both of the shunt variable capacitor  202  and the series variable capacitor  204  to one or more positions, which can be stored in a memory of the match controller  208 . In at least some embodiments, the match controller  208  can use one or more optimization algorithms (e.g., gradient based methods, derivative-free methods, and/or model-based methods) to determine the one or more positions that the shunt variable capacitor  202  and the series variable capacitor  204  can be set to. Next, at  422 , the method  400  comprises continuing supplying the RF power at the second power level for plasma processing the substrate. 
     Operation of the method  400  using the sensorless impedance matching network  300  is substantially identical to operation of the method  400  using the sensorless impedance matching network  200 . For example, instead of using reflected power at the RF power source as described above, the controller  126  can be configured to receive measurements from the power meter  306  that is connected to the RF circulator  302  when performing the method  400 . 
     Additionally, the method  400  can be used in conjunction with an RF bias power source during operation. For example, the method  400  can be used with the RF bias power source  162 . 
     All data, including motor position, forward powers/reflected powers, pressure, chemistry, etc. can be stored in memory (e.g., the memory  131  or a local memory of the sensorless impedance matching network  200  and the sensorless impedance matching network  300  (not shown in  FIGS.  2  and  3   )), and can be accessed by the controller  126  or the match controller  208  in the method  400 . Such data can be used for the learning-based tuning algorithm (which can be based on history results, empirical data, etc.) and tuning trajectory optimization. In some embodiments, the sensorless impedance matching network  200  and the sensorless impedance matching network  300  can tune to new positions based on results from all history runs. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof.