Patent Publication Number: US-9839109-B1

Title: Dynamic control band for RF plasma current ratio control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 62/343,069, filed May 30, 2016, which is herein incorporated by reference in its entirety. 
    
    
     FIELD 
     Embodiments of the present disclosure generally relate to plasma processing equipment. 
     BACKGROUND 
     Inductively coupled plasma (ICP) process reactors generally form plasmas by inducing current in a process gas disposed within the process chamber via one or more inductive coils disposed outside of the process chamber. The inductive coils may be disposed externally and separated electrically from the chamber by, for example, a dielectric lid. When radio frequency (RF) current is fed to the inductive coils via an RF feed structure from an RF power supply, an inductively coupled plasma can be formed inside the chamber from an electric field generated by the inductive coils. 
     In some reactor designs, the reactor may be configured to have concentric inner and outer inductive coils. RF power from an RF power source may be split between the two coils via a current divider/variable capacitor, or the like. In some reactors, the power from a RF power source may be coupled through a dynamically tuned matching network (also referred to as a match unit) to an antenna or electrode within the reactor. The RF power is coupled from the antenna or electrode to process gases within the reactor to form a plasma that is used for the etching process. The matching network ensures that the output of the RF source is efficiently coupled to the plasma to maximize the amount of energy coupled to the plasma (e.g., referred to as tuning the RF power delivery). Tuning refers to the process of varying (e.g., tuning) the impedance of the electrical pathway seen by the RF source (i.e., plasma impedance+chamber impedance+matching network impedance) in order to minimize power reflected back to the RF power source from the plasma and maximize efficient coupling of power from the RF power source to the plasma. 
     Existing reactor designs focus on ways to rapidly minimize the amount of reflected power (i.e., controlling the reflected power) to tune the system. By contrast the inventor has recognized that by controlling the current ratio between the two coils, rather than focusing on reflected power, more effective tuning can be achieved in certain situations. However, in existing solutions involving tuning using current ratio control, the limits of the current ratio are fixed for each operating mode (standard/forward current mode and reverse mode). 
     Accordingly, the inventors have devised a plasma process method and apparatus to better control current ratio. 
     SUMMARY 
     Methods and apparatus for plasma processing are provided herein. The method for controlling current ratio in a substrate processing chamber may include (a) providing a first RF signal to a first RF coil and a second RF coil at a first current ratio set point and a first current operating mode, (b) determining a first dynamic control limit for the first current ratio set point based on a value of the first current ratio set point and the first current operating mode, (c) measuring an amount of current supplied to each of the first and second coils, (d) determining the actual current ratio based on the measured amounts of current supplied to each of the first and second coils, (e) determining whether the actual current ratio determined is within the dynamic control limits, and (f) repeating steps (b)-(e) until the actual current ratio determined is within the dynamic control limits. 
     In some embodiments, the method for controlling current ratio in a substrate processing chamber may include (a) providing a first RF signal from an RF source through a first RF coil and a second RF coil at a first current ratio set point and a first divider capacitor position; (b) determining a first dynamic control limit for the first current ratio set point based on a value of the first current ratio set point and the first divider capacitor position, wherein the first dynamic control limit is a derivative of the first current ratio set point versus the first divider capacitor position; (c) measuring an amount of current supplied to each of the first and second coils; (d) determining the actual current ratio based on the measured amounts of current supplied to each of the first and second coils; (e) determining whether the actual current ratio determined is within the dynamic control limits; and (f) repeating steps (b)-(e) until the actual current ratio determined is within the dynamic control limits. 
     In some embodiments, a non-transitory computer readable medium, having instructions stored thereon that, when executed, cause a method for controlling current ratio in a substrate processing chamber is provided. The method include (a) providing a first RF signal to a first RF coil and a second RF coil at a first current ratio set point and a first current operating mode, (b) determining a first dynamic control limit for the first current ratio set point based on a value of the first current ratio set point and the first current operating mode, (c) measuring an amount of current supplied to each of the first and second coils, (d) determining the actual current ratio based on the measured amounts of current supplied to each of the first and second coils, (e) determining whether the actual current ratio determined is within the dynamic control limits, and (f) repeating steps (b)-(e) until the actual current ratio determined is within the dynamic control limits. 
    
    
     
       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. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments. 
         FIG. 1  depicts a process chamber suitable to perform a method for controlling current ratio in accordance with some embodiments of the present disclosure. 
         FIGS. 2A and 2B  depict graphs showing current ratio as a function of capacitor position including dynamic control bands for RF plasma current ratio control in accordance with some embodiments of the present disclosure. 
         FIG. 3  depicts a flow chart of a method for controlling the current ratio in accordance with some 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. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Methods and apparatus for plasma processing are provided herein. The inventive methods and plasma processing apparatus advantageously provides active current ratio control during plasma etch processes. Embodiments of the inventive active current ratio control methods described herein may be used in any RF plasma systems where the current ratio is controlled. For example, the active current ratio control methods may be used in RF plasma systems having dual output sources split by a current divider capacitor. 
     Embodiments of the active current ratio control methods control/optimize the current ratio with respect to a set point. The derivative of current ratio as a function of divider capacitor position, which acts as a power divider, is not a constant across the current ratio range. Thus, to get the best performance in matching the current ratio reading to the current ratio set point, the control limit of a current ratio tuning algorithm needs to be changed in real-time as the current ratio set point is being changed. Embodiments of the inventive active current ratio control method are described below in further detail with respect to  FIGS. 1-3 . 
       FIG. 1  depicts a schematic side view of an exemplary inductively coupled plasma reactor (reactor  100 ) in accordance with some embodiments of the present disclosure. Although embodiments consistent with the present disclosure are described herein with respect to inductively coupled plasma reactor, embodiments consistent with the present disclosure may be used in conjunction with any chamber with a dual output split via variable capacitor (e.g., a divider capacitor). 
     The reactor  100  may be utilized alone or, as a processing module of an integrated semiconductor substrate processing system, or cluster tool, such as a CENTRIS™ SYM3™ integrated semiconductor wafer processing system, available from Applied Materials, Inc. of Santa Clara, Calif. Examples of suitable plasma reactors that may advantageously benefit from modification in accordance with embodiments of the present disclosure include inductively coupled plasma etch reactors such as the MESA™, CENTURA® or DPS® line of semiconductor equipment (such as the DPS®, DPS® II, DPS® AE, DPS® G3 poly etcher, DPS® G5, or the like) also available from Applied Materials, Inc. The above listing of semiconductor equipment is illustrative only, and other etch reactors, and non-etch equipment (such as CVD reactors, or other semiconductor processing equipment) may also be suitably modified in accordance with the present teachings. 
     The reactor  100  includes an inductively coupled plasma apparatus  102  disposed atop a process chamber  104 . The inductively coupled plasma apparatus includes an RF feed structure  106  for coupling an RF power supply  108  to a plurality of RF coils, e.g., a first RF coil  110  and a second RF coil  112 . The plurality of RF coils are coaxially disposed proximate the process chamber  104  (for example, above the process chamber) and are configured to inductively couple RF power into the process chamber  104  to form a plasma from process gases provided within the process chamber  104 . 
     The RF power supply  108  is coupled to the RF feed structure  106  via a match network  114 . A divider capacitor  105  is provided to adjust the RF power respectively delivered to the first and second RF coils  110 ,  112 . The divider capacitor  105  is a variable capacitor that controls the current ratio of the RF current flowing through each of the first and second RF coils  110 ,  112 . The amount of current flowing through each of the first and second RF coils  110 ,  112  may be measured by a current sensor included in one or more of the match network  114 , the divider capacitor  105 , and/or the RF feed structure  106 . The divider capacitor  105  may be coupled between the match network  114  and the RF feed structure  106 . Alternatively, the divider capacitor may be a part of the match network  114 , in which case the match network will have two outputs coupled to the RF feed structure  106 —one corresponding to each RF coil  110 ,  112 . 
     For example, one or more current sensors  107  may be provided as part of the RF feed structure  106 . In some embodiments, a current sensor  107  may be provided for each output that measures the current. The values sensed by the sensors are provided to a controller, such as a controller in the impedance matching network, the controller  140  discussed below, or some other similar controller. The controller  140  calculates the actual current ratio and compares the actual ratio to the desired ratio (for example, a set point from the recipe on the tool). The controller may then adjust the divider capacitor  105  to match the measurement (the actual current ratio) to the set point (the desired current ratio) within the dynamic control limits determined for each set point, as described below in further detail. 
     The RF feed structure  106  couples the RF current from the divider capacitor  105  (or the match network  114  where the divider capacitor is incorporated therein) to the respective RF coils. In some embodiments, the RF feed structure  106  may be configured to provide the RF current to the RF coils in a symmetric manner, such that the RF current is coupled to each coil in a geometrically symmetric configuration with respect to a central axis of the RF coils, such as by a coaxial structure. 
     The reactor  100  generally includes the process chamber  104  having a conductive body (wall)  130  and a dielectric lid  120  (that together define a processing volume), a substrate support pedestal  116  disposed within the processing volume, the inductively coupled plasma apparatus  102 , and a controller  140 . The wall  130  is typically coupled to an electrical ground  134 . In some embodiments, the support pedestal  116  may provide a cathode coupled through a matching network  124  to a biasing power source  122 . The biasing source  122  may illustratively be a source of up to 1000 W at a frequency of approximately 13.56 MHz that is capable of producing either continuous or pulsed power, although other frequencies and powers may be provided as desired for particular applications. In other embodiments, the source  122  may be a DC or pulsed DC source. 
     In some embodiments, a link (not shown) may be provided to couple the RF power supply  108  and the biasing source  122  to facilitate synchronizing the operation of one source to the other. Either RF source may be the lead, or master, RF generator, while the other generator follows, or is the slave. The link may further facilitate operating the RF power supply  108  and the biasing source  122  in perfect synchronization, or in a desired offset, or phase difference. The phase control may be provided by circuitry disposed within either or both of the RF source or within the link between the RF sources. This phase control between the source and bias RF generators (e.g.,  108 ,  122 ) may be provided and controlled independent of the phase control over the RF current flowing in the plurality of RF coils coupled to the RF power supply  108 . 
     In some embodiments, the dielectric lid  120  may be substantially flat. Other modifications of the chamber  104  may have other types of lids such as, for example, a dome-shaped lid or other shapes. The inductively coupled plasma apparatus  102  is typically disposed above the lid  120  and is configured to inductively couple RF power into the process chamber  104 . The inductively coupled plasma apparatus  102  includes the first and second coils  110 ,  112 , disposed above the dielectric lid  120 . The relative position, ratio of diameters of each coil, and/or the number of turns in each coil can each be adjusted as desired to control, for example, the profile or density of the plasma being formed via controlling the inductance on each coil. Each of the first and second coils  110 ,  112  is coupled through the matching network  114  via the RF feed structure  106 , to the RF power supply  108 . The RF power supply  108  may illustratively be capable of producing up to 4000 W at a tunable frequency in a range from 50 kHz to 13.56 MHz, although other frequencies and powers may be provided as desired for particular applications. 
     Returning to  FIG. 1 , optionally, one or more electrodes (not shown) may be electrically coupled to one of the first or second coils  110 ,  112 , such as the first coil  110 . The one or more electrodes may be two electrodes disposed between the first coil  110  and the second coil  112  and proximate the dielectric lid  120 . Each electrode may be electrically coupled to either the first coil  110  or the second coil  112 , and RF power may be provided to the one or more electrodes via the RF power supply  108  via the inductive coil to which they are coupled (e.g., the first coil  110  or the second coil  112 ). 
     In some embodiments, the one or more electrodes may be movably coupled to one of the one or more inductive coils to facilitate the relative positioning of the one or more electrodes with respect to the dielectric lid  120  and/or with respect to each other. For example, one or more positioning mechanisms may be coupled to one or more of the electrodes to control the position thereof. The positioning mechanisms may be any suitable device, manual or automated, that can facilitate the positioning of the one or more electrodes as desired, such as devices including lead screws, linear bearings, stepper motors, wedges, or the like. The electrical connectors coupling the one or more electrodes to a particular inductive coil may be flexible to facilitate such relative movement. For example, in some embodiments, the electrical connector may include one or more flexible mechanisms, such as a braided wire or other conductor. 
     A heater element  121  may be disposed atop the dielectric lid  120  to facilitate heating the interior of the process chamber  104 . The heater element  121  may be disposed between the dielectric lid  120  and the first and second coils  110 ,  112 . In some embodiments, the heater element  121  may include a resistive heating element and may be coupled to a power supply  123 , such as an AC power supply, configured to provide sufficient energy to control the temperature of the heater element  121  to be between about 50 to about 100 degrees Celsius. In some embodiments, the heater element  121  may be an open break heater. In some embodiments, the heater element  121  may comprise a no break heater, such as an annular element, thereby facilitating uniform plasma formation within the process chamber  104 . 
     During operation, a substrate  114  (such as a semiconductor wafer or other substrate suitable for plasma processing) may be placed on the pedestal  116  and process gases may be supplied from a gas panel  138  through entry ports  126  to form a gaseous mixture  150  within the process chamber  104 . The gaseous mixture  150  may be ignited into a plasma  155  in the process chamber  104  by applying power from the RF source  108  to the first and second coils  110 ,  112  and optionally, the one or more electrodes (not shown). In some embodiments, power from the bias source  122  may be also provided to the pedestal  116 . The pressure within the interior of the chamber  104  may be controlled using a throttle valve  127  and a vacuum pump  136 . The temperature of the chamber wall  130  may be controlled using liquid-containing conduits (not shown) that run through the wall  130 . 
     The temperature of the substrate  101  may be controlled by stabilizing a temperature of the support pedestal  116 . In one embodiment, helium gas from a gas source  148  may be provided via a gas conduit  149  to channels defined between the backside of the substrate  101  and grooves (not shown) disposed in the pedestal surface. The helium gas is used to facilitate heat transfer between the pedestal  116  and the substrate  101 . During processing, the pedestal  116  may be heated by a resistive heater (not shown) within the pedestal to a steady state temperature and the helium gas may facilitate uniform heating of the substrate  101 . Using such thermal control, the substrate  101  may illustratively be maintained at a temperature of between 0 and 500 degrees Celsius. 
     The controller  140  comprises a central processing unit (CPU)  144 , a memory  142 , and support circuits  146  for the CPU  144  and facilitates control of the components of the reactor  100  and, as such, of methods of forming a plasma, such as discussed herein. The controller  140  may be one of any form of general-purpose computer processor that can be used in an industrial setting for controlling various chambers and sub-processors. The memory, or computer-readable medium,  142  of the CPU  144  may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or any other form of digital storage, local or remote. The support circuits  446  are coupled to the CPU  144  for supporting the processor in a conventional manner. These circuits include cache, power supplies, clock circuits, input/output circuitry and subsystems, and the like. The inventive method may be stored in the memory  142  as software routine that may be executed or invoked to control the operation of the reactor  100  in the manner described above. The software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU  144 . 
     The controller  140  controls the RF power supply  108 , the matching network  114 , the divider capacitor, and/or elements of the RF feed structure  106  to provide the desired current ratio base on measured current through the first and second RF coils  110 ,  112 . The first and second RF coils  110 ,  112  can be configured such that the phase of the RF current flowing through the first RF coil can be out of phase with respect to the phase of the RF current flowing through the RF second RF coil. As used herein, the term “out of phase” can be understood to mean that the RF current flowing through the first RF coil is flowing in an opposite direction to the RF current flowing through the second RF coil, or that the phase of the RF current flowing through the first RF coil is shifted with respect to the RF current flowing through the second RF coil. 
     As noted above, the divider capacitor  105  is provided between the RF feed structure  106  to control the relative quantity of RF power provided by the RF power supply  108  to the respective first and second coils. For example, as shown in  FIG. 1 , a divider capacitor  105  may be disposed in the line coupling the RF feed structure  106  to the RF power supply  108  for controlling the amount of RF power provided to each coil (thereby facilitating control of plasma characteristics in zones corresponding to the first and second coils). In some embodiments, the divider capacitor  105  may be incorporated into the match network  114 . In some embodiments, after the divider capacitor  105 , RF current flows to flows to the RF feed structure  106  where it is distributed to the first and second RF coils  110 ,  112 . 
     By actively adjusting the current ratio, the inventor has discovered that undesired processing non-uniformities (such as the M-shape etch profile of a substrate surface) may advantageously be controlled. In embodiments consistent with the present disclosure, the active current ratio control methods control/optimize the current ratio with respect to a set point. 
     Specifically,  FIGS. 2A and 2B  depict graphs showing current ratio as a function of capacitor position including dynamic control bands for RF plasma current ratio control in accordance with some embodiments of the present disclosure. More specifically,  FIG. 2A  depicts a plot  202 A of current ratio  200  as a function of capacitor position  201  in standard mode  206 . The divider capacitor will be adjusted to provide a current ratio at, or close to, a first current ratio set point  203   1 . The current ratio set points  203   1 - 203   x  will be used to determine the active dynamic control limits  204   1 - 204   x  that are used to assist in controlling the current ratio in accordance with some embodiments of the present disclosure. Each dynamic control limit  204   1 - 204   x  defines an error range  210  for the control limits which is a derivative of current ratio as a function of divider capacitor position. The higher the derivative of current ratio as a function of divider capacitor position, the larger the control limit error ranges. When the measured current ratio is within the dynamic control limit error range  210 , the controller will stop controlling. As the etch process uses a new current ratio set point (e.g.,  203   2 ), a dynamic control limit  204   2  and associated error range  210   2  will be determined based on the tuning algorithm for that set point and current operating mode (i.e., standard or reverse current operating mode). 
     Similarly,  FIG. 2B  depicts a plot  202 B of current ratio  200  as a function of capacitor position  201  in reverse mode  208 . The divider capacitor will be adjusted to provide a current ratio at, or close to, a first current ratio set point  253   1 . The current ratio set points  253   1 - 253   x  will be used to determine the active dynamic control limits  254   1 - 254   x  that are used to assist in controlling the current ratio in accordance with some embodiments of the present disclosure. Each dynamic control limit  254   1 - 254   x  defines an error range  260  for the control limits which is a derivative of current ratio as a function of divider capacitor position. The higher the derivative of current ratio as a function of divider capacitor position, the larger the control limit error ranges. When the measured current ratio is within the dynamic control limit error range  260 , the controller will stop controlling. As the etch process uses a new current ratio set point (e.g.,  253   2 ), a dynamic control limit  254   2  and associated error range  260   2  will be determined based on the tuning algorithm for that set point and current operating mode (i.e., standard or reverse current operating mode). 
     As shown in  FIGS. 2A and 2B , the derivative of the current ratio as a function of divider capacitor position is not a constant across the current ratio range. Thus, to get the best performance in matching the current ratio reading to the current ratio set point, the control limit of a current ratio tuning algorithm needs to be changed in real-time as the current ratio set point is being changed. The function controlling the dynamic control limits  204   1 - 204   x  and  254   1 - 254   x  depends on the RF source mode (i.e., standard/forward current mode or reverse current mode) and current ratio set point. 
       FIG. 3  depicts a flow chart of a method for controlling the current ratio in accordance with some embodiments of the present disclosure. The method  300  is described below in accordance with embodiments of the disclosure illustrated in  FIGS. 1-3 , however, the method  300  can be applied with any embodiments of the disclosure described herein. The method  300  is performed by the controller  140  which controls the RF power supply  108 , the matching network  114 , the divider capacitor, and/or elements of the RF feed structure  106  to provide the desired current ratio base on measured current through the first and second RF coils  110 ,  112 . 
     The method  300  begins at  302  by providing a first RF signal through the first RF coil  110  and the second RF coil  112  in a first current operating mode (e.g., a standard current operating mode, a reverse current operating mode, etc.). The first RF signal provided will be based on a first current ratio set point that is part of the process recipe. The divider capacitor will be moved to a first divider capacitor position to achieve the desired current ratio set point. The RF signals may be provided at any suitable frequency desired for a particular application. Exemplary frequencies include but are not limited to, a frequency of between about 100 kHz to about 60 MHz. The RF signal may be provided at any suitable power, such as up to about 5000 Watts. 
     At  304 , the derivative of the current ratio versus the divider capacitor position are calculated at the first set point (e.g., at  203   1  in  FIG. 2 ). That is, a first dynamic control limit  204   1  for the first current ratio set point  203   1  are determined/calculated based on the value of the first current ratio set point  203   1 , the first current operating mode, and/or first divider capacitor position. 
     At  306 , the actual currents supplied to each of the first and second coils  110 ,  112  are measured and the actual current ratio is determined. In some embodiments, the currents supplied to each of the first and second coils  110 ,  112  are measured using sensors  107 , and the current ratio may be calculated by controller  140 . 
     At  308 , it is determined whether the actual current ratio determined is within the dynamic control limit  204   1  range. If the actual current ratio determined from the measured currents is within the dynamic control limit  204   1  calculated, the controller  140  stops adjusting the current ratio using the divider capacitor and the process with end at  312 . 
     If the actual current ratio determined from the measured currents is not within the dynamic control limit  204   1  calculated, the controller  140  will adjust the divider capacitor to attempt to achieve the desired first current set point for the process recipe at  310 . In some embodiments, once the divider capacitor position is changed, new dynamic control limits will need to be calculated, so the method returns to  304  and proceeds again until the measured current ratio is within the dynamic control limit  204   1  calculated. 
     In some embodiments, the sensors  107  will continually monitor the currents provided to each coil until the current ratio set point is changed, or until the actual current ratio measured falls outside of the dynamic control limit  204   1  calculated for the desired current ratio set point (e.g., at  203   1 ). The actual current ratio is continually being calculated using updated current values measured by the one or more current sensors. If either of these conditions is sensed/determined, the method begins again at  302  using the second current ratio set point (e.g., at  203   2 ), a second divider capacitor position, and a second current operating mode. 
     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.