Systems and methods for controlling plasma sheath characteristics - Patents.com

Abstract

Systems and methods for controlling a plasma sheath characteristic are described. One of the methods includes determining a first value of a plasma sheath characteristic of a plasma sheath formed in a plasma chamber. The method further includes determining whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic. The method also includes modifying a variable of a radio frequency (RF) generator connected to the plasma chamber via an impedance matching circuit upon determining that the first value is not within the predetermined range from the set value. The act of modifying the variable of the RF generator is performed until it is determined that the first value of the plasma sheath characteristic is within the predetermined range from the set value.

Description

[Technical field]

[0001] The present embodiments relate to systems and methods for controlling plasma sheath properties. [Background technology]

[0002] The plasma tool includes a radio frequency (RF) generator, an impedance matching network, and a plasma chamber, the RF generator connected to the impedance matching network, which is further connected to the plasma chamber.

[0003] A semiconductor wafer is placed in a plasma chamber. An RF signal is supplied from an RF generator through an impedance matching network to the plasma chamber to process the semiconductor wafer. The substrate must be precisely processed to achieve the desired processing results.

[0004] It is in this context that the embodiments described in this disclosure arose.

[0005] The Background Art provided herein is intended to present the contents of the present disclosure generally, and the inventions of the presently named inventors are not admitted expressly or impliedly as prior art to the present disclosure to the extent that they are described in this Background Art section and in a manner that is not prior art at the time of filing. Summary of the Invention

[0006] The embodiments of the present disclosure provide a system, an apparatus, a method, and a computer program for controlling plasma sheath properties. It should be appreciated that the embodiments can be implemented in several ways, such as a process, an apparatus, a system, an apparatus, or a method by a computer readable medium. Several embodiments are described below.

[0007] One method involves controlling radio frequency (RF) energy coupled into a dual frequency capacitively coupled plasma (CCP). This method offers advantages including high RF power efficiency, low RF generator stress, reduced RF matching network component requirements, and improved process uniformity. A CCP plasma operating at multiple frequencies uses low frequency (LF) to control ion energy and high frequency (HF) to control plasma density. As LF high voltage (HV) RF is applied to the electrodes, a sheath of increasing and decreasing width is formed as the LF RF voltage magnitude and polarity change during each cycle of the LF RF voltage. Each cycle of the LF RF voltage is referred to herein as a LF RF cycle.

[0008] This change in sheath width creates a change in the capacitance from the electrode to the plasma, resulting in a dynamic change in the plasma impedance of the HF RF system. This dynamically changing plasma impedance results in insufficient HF RF coupling to the plasma due to the fixed tuning positions established by the matching network. The fixed tuning positions are selected to provide a large amount of RF coupling and a small amount of reflected power as the plasma impedance dynamically changes with the sheath width.

[0009] The matching network is adjusted to the impedance average value as the plasma impedance changes dynamically during each LF RF cycle. The fixed adjustment position is typically established using an auto-tuner that sets the optimal adjustment point that results in a small amount of reflected power. The fixed adjustment position is optimized for the impedance average value, but since there is no ability to adjust to the dynamically changing impedance during each LF RF cycle, there can be a large amount of reflected power. This can result in low RF power efficiency and high voltages and currents on the RF components, increasing component costs and space requirements.

[0010] The method described herein facilitates coupling of HF RF power to the plasma as the plasma impedance changes dynamically in synchronization with the LF RF cycle. The method includes dynamically adjusting an HF RF generator in response to changes in sheath width during the LF RF cycle. The dynamic adjustment to changes in sheath width is accomplished by synchronizing an adjustment position of the HF RF generator to the LF RF voltage. The method increases HF RF coupling to the plasma. A further benefit of improved process uniformity can be obtained using the method by controlling the amount of HF RF power coupled to the plasma as a function of the sheath width.

[0011] Tuning of the HF RF generator during the LF RF cycle is accomplished by changing the frequency of the HF RF generator. The change in HF RF frequency to improve tuning can be synchronized to the LF RF cycle. The change in HF RF frequency can occur in continuous or non-continuous steps. The frequency of the HF RF generator is used to optimize the HF RF frequency to control the sheath width continuously or multiple times during each LF RF cycle.

[0012] In one embodiment, a method for controlling a plasma sheath characteristic is described. The method includes determining a first value of a plasma sheath characteristic of a plasma sheath formed in a plasma chamber. The method further includes determining whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic. The method also includes modifying a variable of an RF generator coupled to the plasma chamber via an impedance matching circuit upon determining that the first value is not within the predetermined range from the set value. The act of modifying the variable of the RF generator is performed until it is determined that the first value of the plasma sheath characteristic is within the predetermined range from the set value.

[0013] In an embodiment, a computer system for controlling a plasma sheath characteristic is described. The computer system includes a processor and a storage device. The processor determines a first value of a plasma sheath characteristic of a plasma sheath formed in the plasma chamber. The processor further determines whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic. When the processor determines that the first value is not within the predetermined range from the set value, the processor modifies a variable of an RF generator connected to the plasma chamber via an impedance matching circuit. The variable of the RF generator is modified until it is determined that the first value of the plasma sheath characteristic is within the predetermined range from the set value. The storage device stores the set value.

[0014] In one embodiment, a plasma system for controlling a plasma sheath characteristic is described. The plasma system includes an RF generator that generates an RF signal. The RF generator has an output. The plasma system further includes an impedance matching circuit having an input and an output. The input of the impedance matching circuit is connected to the output of the RF generator. The plasma system includes a plasma chamber having an input. The input of the plasma chamber is connected to the output of the impedance matching circuit. The plasma system includes a controller connected to the RF generator. The controller determines a first value of a plasma sheath characteristic of a plasma sheath formed in the plasma chamber. The controller also determines whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic. If the controller determines that the first value is not within the predetermined range from the set value, the controller modifies a variable of the RF generator. The variable of the RF generator is modified until it is determined that the first value of the plasma sheath characteristic is within the predetermined range from the set value.

[0015] Some advantages of the systems and methods for controlling plasma sheath properties described herein include achieving similarity in the processing of multiple substrates: by controlling the plasma sheath properties to be within a range of predefined plasma sheath properties, the substrates can be processed in the same way.

[0016] Additional advantages of the systems and methods described herein include controlling the processing of a substrate based on plasma sheath characteristics. The processing of a substrate can be controlled more precisely by measuring the plasma sheath characteristics than when factors other than the plasma sheath characteristics are measured to control the processing of the substrate. For example, the bottom plasma sheath is closer to the substrate than the plasma. Thus, controlling the bottom plasma sheath provides more precise control during substrate processing than controlling the impedance of the plasma to process the substrate.

[0017] Further advantages of the systems and methods described herein include controlling ion flux and ion energy by controlling plasma sheath characteristics. Ion energy and ion flux are used to achieve an etch rate. The etch rate is controlled by controlling plasma sheath characteristics. For example, the power, or frequency, or both, of one or more RF signals are controlled to achieve a predetermined plasma sheath characteristic. By controlling the power, or frequency, or both, the ion energy and ion flux are controlled to further control the etch rate.

[0018] Other aspects will become apparent from the following detailed description taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0019] The embodiments will be better understood by reference to the following description taken in conjunction with the accompanying drawings.

[0020] [Figure 1A] FIG. 2 is a diagram of an embodiment of a system for measuring the sheath width of the bottom plasma sheath.

[0021] [Figure 1B] FIG. 1 illustrates an embodiment of a system for measuring the voltage of the bottom plasma sheath.

[0022] [Diagram 2]FIG. 13 is an embodiment of a table illustrating control of a variable of a low frequency (LF) radio frequency (RF) signal based on the width of the bottom plasma sheath.

[0023] [Figure 3A] FIG. 13 is an embodiment of a table illustrating control of a high frequency (HF) RF signal variable based on the width of the bottom plasma sheath.

[0024] [Figure 3B] 11 illustrates an embodiment of a graph showing bins during each cycle of a parameter of a LF RF signal.

[0025] [Figure 4] FIG. 2 is a diagram of an embodiment of an RF transmission line.

[0026] [Diagram 5] FIG. 2 is a diagram of an embodiment of a system showing details of an LF RF generator and an HF RF generator. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The following embodiments describe systems and methods for controlling plasma sheath properties. It will be apparent that the embodiments may be practiced without some or all of these specific details. In other instances, well-known operations have not been described in detail so as not to unnecessarily obscure the embodiments.

[0028] 1A is a diagram of an embodiment of a system 100 illustrating a method for measuring a sheath width. The system 100 includes a low frequency (LF) radio frequency (RF) generator 102, a high frequency (HF) RF generator 104, a matching box 106, a plasma chamber 110, and a host computer 111.

[0029] An example of the LF RF generator 102 is a generator having an operating frequency of 400 kilohertz (kHz) or 2 megahertz (MHz). For example, a 400 kHz RF generator has a frequency tuning range of 330 kHz to 440 kHz. As another example, a 2 MHz RF generator has a frequency tuning range of 1 MHz to 3 MHz. An example of the HF RF generator 104 is a generator having an operating frequency of 13.56 MHz, 27 MHz, or 60 MHz. For example, a 13.56 MHz HF RF generator has a frequency tuning range of 12 MHz to 14 MHz. As another example, a 60 MHz HF RF generator has a frequency tuning range of 54 MHz to 63 MHz. The operating frequency of the HF RF generator 104 is greater than the operating frequency of the LF RF generator 102. In some embodiments, the operating frequency may also be referred to herein as the frequency of operation.

[0030] The matching circuit 106 includes a network of circuit components such as capacitors, inductors, and resistors. For example, the matching circuit 106 includes a plurality of series components and a plurality of shunt components. Each series component includes a capacitor, an inductor, a resistor, or a combination thereof. Similarly, each shunt component includes a capacitor, an inductor, a resistor, or a combination thereof. Each shunt component is connected to a ground potential.

[0031] The plasma chamber 110 includes an electrostatic chuck (ESC) 112 and an upper electrode 114. As an example, the ESC 112 includes a base plate and a dielectric layer located on top of the base plate. In this example, the base plate functions as a lower electrode. As another example, the ESC 112 includes a lower electrode embedded in its dielectric layer (such as a ceramic layer). As an example, each of the upper electrode 114 and the lower electrode is made of a metal such as aluminum or an aluminum alloy. The upper electrode 114 faces the ESC 112 and is located above the ESC 112 to form a gap 116 between the upper electrode 114 and the ESC 112. The upper electrode 114 is connected to a ground potential and is an example of a counter electrode. The lower electrode may be referred to herein as a powered electrode.

[0032] The plasma chamber 110 has a side wall 122, a top wall 124, and a bottom wall 126. The side wall 122 is connected to the top wall 124 and the bottom wall 126 and is located between the walls 124 and 126. Inside the side wall 122, there is a window 128 made of transparent glass. As an example, the window 128 is embedded in the lower half of the side wall 122. For example, when the bottom plasma sheath 130 is formed in the gap 116, the window 128 faces the bottom plasma sheath 130. As a further example, the window 128 is fixed to the side wall 122 so as to face the gap between the plasma 150 and the top surface 155 of the substrate SU.

[0033] The optical sensor 136 is positioned adjacent to the window 128 such that it can see through the bottom plasma sheath 130. For example, the optical sensor 136 is attached to the window 128. As another example, the optical sensor 136 is attached to the side wall 122 and is mounted on a shelf below the window 128. When the optical sensor 126 is mounted on the shelf, the optical sensor 136 sees through the window 128 to the bottom plasma sheath 130. For example, the optical sensor 136 sees through the bottom plasma sheath 130 and the bottom portion 150A of the plasma 150. As another example, the optical sensor 136 sees through the bottom plasma sheath 130 without seeing through the bottom portion 150A of the plasma 150. The plasma 150 is shown as being evenly divided between a bottom portion 150A and a top portion 150B. The top portion 150B is located above the bottom portion 150A. Examples of light sensor 126 include a laser sensor, an optical emission spectrometer (OES), a charge-coupled device (CCD) camera, a CCD spectrometer, and a low-light CCD (ICCD) camera.

[0034] The host computer 111 includes a processor 118 and a storage device 120. Examples of processors used herein include a central processing unit, an application specific integrated circuit (ASIC), a programmable logic device (PLD), and a microprocessor. Examples of storage devices used herein include a random access memory (RAM) and a read only memory (ROM). For example, the storage device is a flash memory. The processor 118 is connected to the storage device 120. Examples of the host computer 111 include a desktop computer, a laptop computer, a tablet, a smartphone, and a controller. For example, the controller is an integrated controller integrated into a system on chip (SoC).

[0035] The LF RF generator 102 has an output O1 connected to an input I1 of the matcher 106 via an RF cable RFC1. And the HF RF generator 104 has an output O2 connected to an input I2 of the matcher 106 via an RF cable RFC2. The output O3 of the matcher 106 is connected to an input I3 of the plasma chamber 110 via an RF transmission line RFT. An example of the input I3 is a point on the RF rod of the RF transmission line RFT. The input I3 is connected to the bottom electrode of the ESC 112 within the plasma chamber 110 via a portion of the RF transmission line RFT.

[0036] The processor 118 is connected to the LF RF generator 102 via a transmission cable 132 and to the HF RF generator 104 via a transmission cable 134. The optical sensor 136 is also connected to the processor 118 via a transmission cable 156. Examples of transmission cables are cables that transmit data serially, or cables that transmit data in parallel, or cables that transmit data using the Universal Serial Bus (USB) protocol.

[0037] The processor 118 generates and transmits a recipe signal 138 to the LF RF generator 102 via the transmission cable 132, and generates and transmits a recipe signal 140 to the HF RF generator 104 via the transmission cable 134. The recipe signal 138 includes recipe information (e.g., power and frequency) for the LF RF signal 144 generated by the LF RF generator 102. The recipe signal 140 also includes recipe information (e.g., power and frequency) for the HF RF signal 146 generated by the HF RF generator 104. After transmitting the recipe signals 138 and 140, the processor 118 generates and transmits a trigger signal 142 to the LF RF generator 102 via the transmission cable 132, and transmits the trigger signal 142 to the HF RF generator 104 via the transmission cable 134.

[0038] In response to receiving the trigger signal 142, the LF RF generator 102 executes the recipe information received in the recipe signal 138 to generate an LF RF signal 144 having the power and frequency of the recipe information. Similarly, in response to receiving the trigger signal 142, the HF RF generator 104 executes the recipe information received in the recipe signal 140 to generate an HF RF signal 146 having the power and frequency of the recipe information. The LF RF generator 102 provides the LF RF signal 144 via output O1 and RF cable RFC1 to an input I1 of the matcher 106. The HF RF generator 104 also provides the HF RF signal 146 via output O2 and RF cable RFC2 to an input I2 of the matcher 106.

[0039] The matching circuit 106 receives RF signals 144 and 146. When the LF RF signal 144 is transmitted through a first branch of the matching circuit 106, the impedance of the LF RF signal 144 is modified by the first branch to output a first modified RF signal. When the HF RF signal 146 is transmitted through a second branch of the matching circuit 106, the impedance of the HF RF signal 146 is modified by the second branch to output a second modified RF signal. The first branch includes a number of circuit components, and the second branch includes a number of circuit components. An input I1 is connected to the first branch, and an input I2 is connected to the second branch. The first and second branches are connected to each other at an output O3.

[0040] The matcher 106 combines the first and second modified RF signals at an output O3 to output a modified RF signal 148 at O3, and transmits the modified RF signal 148 through the output O3, the RF bars of the RF transmission line RFT, and the input I3 to the lower electrode of the ESC 112. In addition to providing the modified RF signal 148 to the lower electrode, when one or more process gases (such as a fluorine-containing gas, or an oxygen-containing gas, or a combination thereof) are provided to the gap 116, the one or more process gases are ignited to generate a plasma 150 or remain ignited to maintain the plasma 150 in the gap 116.

[0041] The plasma 150 in the gap 116 is used to process a substrate SU disposed on the ESC 112 in the plasma chamber 110. Examples of substrate SU processing include depositing material on the substrate SU, etching the substrate SU, cleaning the substrate SU, and sputtering the substrate SU. The substrate SU is disposed on a top surface 154 of the ESC 112. Examples of substrates used herein include wafers, semiconductor substrates, and the like. For example, integrated circuits are fabricated on a silicon layer of the substrate.

[0042] The plasma 150 is bounded by the bottom plasma sheath 130 at or adjacent to its bottom side BS and by the top plasma sheath 152 at or adjacent to its top side TS. For example, the top plasma sheath 152 is located above the plasma 150, and the bottom plasma sheath 130 is located directly below the plasma 150. As another example, each plasma sheath 130 and 152 is darker in color than the plasma 150. For example, each plasma sheath 130 and 152 includes particles such as positive ions and neutrons, and the plasma 150 includes positive ions, neutrons, and electrons. For example, the bottom plasma sheath 130 extends from the top surface 155 of the substrate SU to the plasma 150, and the sheath width of the bottom plasma sheath 130 is the distance between the top surface 155 of the substrate SU and the plasma 150. A top surface 155 of the substrate SU is not adjacent to and does not contact the top surface 154 of the ESC 112. Rather, a bottom surface 157 of the substrate SU is adjacent to and contacts the top surface 154 of the ESC 154. The bottom surface 157 of the substrate SU is located opposite the top surface 155 of the substrate SU. By way of example, when there is no substrate in the plasma chamber 110, the bottom plasma sheath 130 extends from the top surface 154 of the ESC 112 to the plasma 150, and the sheath width of the bottom plasma sheath 130 is the distance between the top surface 154 of the ESC 112 and the plasma 150.

[0043] As an example, each plasma sheath 130 and 152 contains fewer electrons than the number of electrons in the plasma 150. As another example, each plasma sheath 130 and 152 excludes electrons. In this example, the electrons in the plasma 150 are lighter than the positive ions, so they escape from the boundary (e.g., gap) between the ESC 112 and the plasma 150 to form the bottom plasma sheath 130. In this example, as soon as the electrons leave the boundary, the bottom plasma sheath 130 with positive ions and neutrals is formed.

[0044] After the plasma 150 is formed in the gap 116, the optical sensor 136 measures the light emitted by the bottom plasma sheath 130. For example, the optical sensor 136 detects the light emitted by the bottom plasma sheath 130 and the bottom portion 150A of the plasma 150 to generate an electrical signal having an optical emission profile of the bottom plasma sheath 130. The optical emission profile includes the intensity of the light emitted by the bottom plasma sheath 130 and the bottom portion 150A of the plasma 150 toward the optical sensor 136. The optical sensor 136 provides the electrical signal to the processor 118 through the transmission cable 156. Upon receiving the electrical signal, the processor 118 creates a plot of the light intensity versus the distance of the intensity from the top surface 155 of the substrate SU to create an intensity profile. The processor 118 creates a first tangent to a portion of the plot based on the intensity of the light emitted by the bottom portion 150A and a second tangent to a portion of the plot based on the intensity of the light emitted by the bottom plasma sheath 130. The processor 118 further determines an intersection point between the first tangent and the second tangent. The processor 118 determines that the width of the bottom plasma sheath 130 is the distance between the top surface 155 of the substrate SU and the intersection point. The width of the bottom plasma sheath 130 is an example of a plasma sheath property. By way of example, the width (e.g., thickness) of the bottom plasma sheath 130 is measured in millimeters (mm). For example, the thickness of the bottom plasma sheath 130 extends from the top surface 155 of the substrate SU to the bottom 150A of the plasma 150.

[0045] In embodiments, the terms matcher, matching housing, impedance matching circuit, and impedance matching network are used interchangeably herein.

[0046] In one embodiment, the top electrode 114 is sometimes referred to herein as the upper electrode.

[0047] In an embodiment, the RF transmission line RFT is connected to the top electrode 114 instead of being connected to the ESC 112, and the bottom electrode of the ESC 112 is connected to ground potential. In this embodiment, a modified RF signal 148 is provided to the top electrode 114. In this embodiment, the top electrode 114 is an example of a powered electrode, and the base plate of the ESC 112 is an example of a counter electrode.

[0048] 1B is a diagram of an embodiment of a system 160 illustrating a method for measuring RF voltage at and RF current through the plasma sheath 130. System 160 is the same as system 100 (FIG. 1A), except that system 160 uses a probe 170 instead of optical sensor 136. An example of probe 170 includes a combined voltage and current (V&I) sensor. Additionally, the system uses a Langmuir probe 125. Langmuir probe 125 has a pin that extends into plasma 150. Langmuir probe 125 is connected to processor 118 via transmission cable 163.

[0049] The probe 170 is connected to an RF rod of the RF transmission line RFT. The probe 170 is connected closer to the input I3 of the plasma chamber 110 than the output O3 of the matching device 106. As an example, the probe 170 is installed in the housing of the plasma chamber 110 and connected to a point on the RF rod of the RF transmission line. The housing of the plasma chamber 110 is formed by the walls 122, 124, and 126. As another example, the probe 170 is connected to a bottom surface 164 of the base plate of the ESC 112. The bottom surface 164 faces vertically downward, and the top surface 154 of the ESC 112 faces vertically upward. As yet another example, the probe 170 is connected to a point 166 on the RF rod of the RF transmission line RFT. The point 166 is closer to the input I3 than the output O3 of the matching device 106. For example, the distance between the probe 170 and the input I3 is shorter than the distance between the probe 170 and the output O3. The probe 170 is connected to the processor 118 via the transmission cable 162 .

[0050] When plasma 150 is generated in gap 116 in plasma chamber 110, probe 170 measures the voltage (e.g., a voltage signal) at point 164 of the RF rod and provides the voltage to processor 118 through transmission cable 162. The voltage is an example of a plasma sheath characteristic.

[0051] The Langmuir probe 125 is placed within the plasma 150 in close proximity to the bottom plasma sheath 130 to measure the ion saturation current (e.g., positive ion saturation current) and electron temperature of the bottom plasma sheath 130. For example, the Langmuir probe 125 is placed closer to the bottom plasma sheath 130 than the top plasma sheath 152. The Langmuir probe 125 provides the ion saturation current and electron temperature to the processor 118 through a transmission cable 163.

[0052] In one embodiment, the probe 170 is connected to the bottom surface 164 of the base plate of the ESC 112 instead of being connected to the RF rod of the RF transmission line RFT.

[0053] In an embodiment, a current-voltage probe (e.g., a Langmuir probe) is placed in the region of the bottom plasma sheath 130 to measure the flux rate of the bottom plasma sheath 130. The current-voltage probe is connected to an ammeter and a voltmeter. The ammeter and the voltmeter are connected to the processor 118. The current-voltage probe measures the flux rate of positive ions and neutrons in the bottom plasma sheath 130 to generate a current. The current is measured by the ammeter, and the voltage generated by the current is measured by the voltmeter. The voltage and current measurements of the bottom plasma sheath 130 are provided to the processor 118. The processor 118 calculates the capacitance of the bottom plasma sheath 130. The capacitance is calculated based on the voltage measurement of the bottom plasma sheath 130 and the total charge of the particle mass. As an example, the processor 118 calculates the capacitance to be the ratio of the total charge to the voltage. The total charge of the particle mass of the bottom plasma sheath 130 is calculated by the processor 118. For example, the total charge is calculated to be equal to half the product of the amount of charge carried by each positive ion and the volume of the bottom plasma sheath 130. The volume of the bottom plasma sheath 130 is calculated by the processor 118 from the width and radius of the bottom plasma sheath 130. For example, the volume of the bottom plasma sheath 130 is calculated to be πr 2 h, where r is the radius of the bottom plasma sheath 130 and h is its width. The radius of the bottom plasma sheath 130 is equal to the radius of the bottom wall 126. In this example, the plasma chamber 110 is cylindrical. In this example, it is assumed that half of the bottom plasma sheath 130 contains positive ions and the other half contains neutrons. Capacitance is another example of a plasma sheath property.

[0054] In one embodiment, the probe 170 measures the RF current, RF voltage, and phase between the RF current and the RF voltage in the ESC 112. The Langmuir probe 125 also measures the electron temperature and ion saturation current of the bottom plasma sheath 130. The RF current, RF voltage, and phase are provided from the probe 170 to the processor 118 through the transmission cable 162. The density of the bottom plasma sheath 130 is determined by the processor 118 as a function of the charge of one positive ion in the bottom plasma sheath 130. The density is further determined based on the effective area (e.g., pin area) of the Langmuir probe 125 in the bottom plasma sheath 130, the ion saturation current measured by the Langmuir probe 125, the effective ion mass of one positive ion, and the electron temperature. The processor 118 determines that the ion saturation current is the amount of current received from the Langmuir probe 125 during the period when the voltage sensed by the probe 170 is negative. The density is another example of a plasma sheath characteristic.

[0055] In an embodiment, the processor 118 determines that the thickness of the bottom plasma sheath 130 is a function of the RF voltage, RF current, and phase received from the probe 170, as well as the ion saturation current and electron temperature received from the Langmuir probe 125.

[0056] 2 is an illustration of an embodiment of a table 200 illustrating control of a variable of the LF RF signal 144 (FIG. 1A) based on the width of the bottom plasma sheath 130 (FIG. 1A). Example parameters include voltage and power. Example variables of the LF RF signal 144 include a parameter of the LF RF signal 144, or frequency, or a combination thereof. Table 200 is generated by processor 118 and stored in storage device 120. Table 200 is an example of a database.

[0057] Table 200 includes a listing of widths of bottom plasma sheath 130 calculated by processor 118 based on signals received from the sensors. For example, table 200 includes a number of values ​​SWA, SWB, and SWC, a default sheath width PSW, a number of parameter values ​​LPRA, LPRB, and LPRC of LF RF signal 144, and a number of frequency values ​​LFA, LFB, and LFC of LF RF signal 144. The default sheath width, sometimes referred to herein as a set sheath width, is a value in millimeters.

[0058] The processor 118 determines the width of the bottom plasma sheath 130 (FIG. 1A). For example, during each cycle of the parameters of the LF RF signal 144 (FIG. 1A) provided by the LF RF generator 102 (FIG. 1A), the processor 118 calculates the width of the bottom plasma sheath 130. For example, during a first cycle (e.g., cycle 1) of the parameters of the LF RF signal 144, the processor 118 receives a first electrical signal from the optical sensor 136 (FIG. 1A) having a first optical emission profile of the bottom plasma sheath 130. The processor 118 determines the width SWA of the bottom plasma sheath 130 from the first electrical signal. Similarly, during a second cycle (e.g., cycle 2) of the parameters of the LF RF signal 144, the processor 118 receives a second electrical signal from the optical sensor 136 having a second optical emission profile of the bottom plasma sheath 130. The processor 118 determines the width SWB of the bottom plasma sheath 130 from the second electrical signal. Also, during a third cycle (e.g., cycle 3) of the parameters of the LF RF signal 144, the processor 118 receives a third electrical signal from the optical sensor 136 having a third emission profile of the bottom plasma sheath 130. The third cycle follows the second cycle, which follows the first cycle. For example, there are no other cycles between the first and second cycles or between the second and third cycles. The processor 118 determines the width SWC of the bottom plasma sheath 130 from the third electrical signal. In this example, the probe 170 is connected to a point 166 (FIG. 1B) on the RF rod of the RF transmission line RFT of the system 100 (FIG. 1A). The probe 170 is also connected to the processor 118 of the system 100 via a transmission cable 162 (FIG. 1B). The probe 170 provides a voltage signal to the processor 118 through the transmission cable 162. The processor 118 utilizes a Fourier transform to convert the voltage signal from the time domain to the frequency domain. In the frequency domain, the processor 118 identifies a portion of the voltage signal having a frequency that is within the operating frequency range of the LF RF generator 102. The processor 118 identifies first through third cycles of a parameter of the LF RF signal 144 from the portion of the voltage signal having a frequency that is within the operating frequency range of the LF RF generator 102.An example of a portion of a voltage signal is represented as voltage signal 352 (FIG. 3B).

[0059] As another example, the processor 118 determines the sheath width SWA as a statistical value (e.g., average or median) calculated from a first set of sheath widths during a first cycle of the parameters of the LF RF signal 144. The first set of sheath widths is determined by the processor 118 from a plurality of electrical signals having a first set of optical emission profile of the bottom plasma sheath 130 received from the optical sensor 136 during a first cycle of the parameters of the LF RF signal 144. Also in this example, the processor 118 determines the sheath width SWB as a statistical value calculated from a second set of sheath widths. The second set of sheath widths is determined by the processor 118 from a plurality of electrical signals having a second set of optical emission profile of the bottom plasma sheath 130 received from the optical sensor 136 during a second cycle of the parameters of the LF RF signal 144.

[0060] If during the first cycle of the parameters of the LF RF signal 144, it is determined that the sheath width SWA is not within a predetermined range from the default sheath width PSW, then the processor 118 modifies the variables (e.g., parameters and / or frequency of the LF RF signal 144) during the first cycle to achieve the default sheath width PSW. For example, in response to determining that the sheath width SWA is not within a predetermined range from the default sheath width PSW, the processor 118 modifies the parameter setpoints (e.g., parameter values) of the recipe signal 138 (FIG. 1A) to the parameter value LPRA to achieve the default sheath width PSW. For example, when the parameters of the LF RF signal 144 are changed from the parameter setpoints to the parameter value LPRA, the sheath width SWA is modified during the first cycle of the parameters of the LF RF signal 144 to be within a predetermined range from the default sheath width PSW. The default range is stored in the storage device 120. It is noted that the parameters of the LF RF signal 144 are modified to change the shape of the bottom plasma sheath 130. For example, the shape of the bottom plasma sheath 130 resembles the shape of the peak-to-peak value of the parameter of the LF RF signal 144. For example, when the shape of the peak-to-peak value of the parameter of the LF RF signal 144 is pulsed, the shape of the bottom plasma sheath 130 is also pulsed. As another example, when the shape of the peak-to-peak value of the parameter of the LF RF signal 144 is defined by a continuous wave, the shape of the bottom plasma sheath 130 is also defined by a continuous wave. As another example, when the shape of the peak-to-peak value of the parameter of the LF RF signal 144 is defined by an arbitrary waveform, the shape of the bottom plasma sheath 130 is also defined by an arbitrary waveform.

[0061] Meanwhile, processor 118, in response to determining that sheath width SWA is within a predetermined range from the default sheath width PSW during the first cycle of the parameters of LF RF signal 144, does not modify the parameter setpoint of recipe signal 138 to parameter value LPRA during the first cycle to achieve the default sheath width PSW. For example, when the parameter of LF RF signal 144 is not changed from the parameter setpoint to parameter value LPRA, sheath width SWA is not modified during the first cycle of the parameters of LF RF signal 144 and is within a predetermined range from the default sheath width PSW.

[0062] Similarly, as another example, processor 118 changes the frequency setpoint (e.g., frequency value) of recipe signal 138 to frequency value LFA to achieve the default sheath width PSW during the first cycle of parameters of LF RF signal 144 in response to determining that sheath width SWA is not within a default range from default sheath width PSW. For example, when the frequency of LF RF signal 144 is changed from the frequency setpoint to frequency value LFA, sheath width SWA is modified to be within a default range from default sheath width PSW during the first cycle of parameters of LF RF signal 144.

[0063] Meanwhile, processor 118 does not modify the frequency setpoint of recipe signal 138 to frequency value LFA to achieve the default sheath width PSW in response to determining that sheath width SWA is within the default range from default sheath width PSW during the first cycle of parameters of LF RF signal 144. For example, when the frequency of LF RF signal 144 is not changed from the frequency setpoint to frequency value LFA, sheath width SWA is not modified during the first cycle of parameters of LF RF signal 144 and sheath width SWA is within the default range from default sheath width PSW.

[0064] The processor 118 controls the LF RF generator 102 to generate an adjusted LF RF signal having a parameter value LPRA, or a frequency value LFA, or a combination thereof, during a first cycle of the parameter of the LF RF signal 144 to change the sheath width SWA to the default sheath width PSW. For example, the processor 118 changes a parameter set point of the parameter of the RF signal 144 to a value LPRA to generate an adjusted parameter recipe signal and transmits the adjusted parameter recipe signal to the LF RF generator 102. In response to receiving the adjusted parameter recipe signal having the parameter value LPRA, the LF RF generator 102 generates an adjusted parameter RF signal based on the parameter value LPRA of the adjusted parameter recipe signal and provides the adjusted parameter RF signal having the parameter value LPRA to the matcher 106 (FIG. 1A) through the RF cable RFC1. Upon receiving the adjusted parameter RF signal having the parameter value LPRA and the RF signal 146, the matcher 106 adjusts the impedance of the adjusted parameter RF signal to output a first adjusted parameter modified RF signal. The matcher 106 combines the first adjustment parameter modified RF signal and the second modified RF signal generated based on the RF signal 146 (FIG. 1A) to output an adjustment parameter combined RF signal, and transmits the adjustment parameter combined RF signal to the ESC 112 through the RF transmission line RFT to achieve the predetermined sheath width PSW. In this example, during cycle 1 of the parameters of the LF RF signal 144, after the adjustment parameter combined RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicating a modified optical emission profile of the bottom plasma sheath 130. In this example, during cycle 1 of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile, and further determines that the modified sheath width is within a predetermined range from the predetermined sheath width PSW.

[0065] As another example, the processor 118 changes the frequency set point of the frequency of the RF signal 144 to a value LFA to generate an adjusted frequency recipe signal, and transmits the adjusted frequency recipe signal to the LF RF generator 102. In response to receiving the adjusted frequency recipe signal having the frequency value LFA, the LF RF generator 102 generates an adjusted frequency RF signal based on the frequency value LFA of the adjusted frequency recipe signal, and provides the adjusted frequency RF signal having the frequency value LFA to the matcher 106 (FIG. 1A) through the RF cable RFC1. Upon receiving the adjusted frequency RF signal having the frequency value LFA and the RF signal 146, the matcher 106 adjusts the impedance of the adjusted frequency RF signal to output a first adjusted frequency modified RF signal. The matcher 106 combines the first adjusted frequency modified RF signal with a second modified RF signal generated based on the RF signal 146 (FIG. 1A) to output an adjusted frequency synthesized RF signal, and transmits the adjusted frequency synthesized RF signal to the ESC 112 through the RF transmission line RFT to achieve the predetermined sheath width PSW. In this example, during cycle 1 of the parameters of the LF RF signal 144, after the adjusted frequency synthesized RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicative of a modified optical emission profile of the bottom plasma sheath 130. In this example, during cycle 1 of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile and further determines that the modified sheath width is within a predetermined range from the default sheath width PSW.

[0066] As yet another example, processor 118 changes a parameter set point of a parameter of RF signal 144 to value LPRA and a frequency set point of a frequency of RF signal 144 to value LFA to generate an adjusted recipe signal and transmits the adjusted recipe signal to LF RF generator 102. In response to receiving the adjusted recipe signal having values ​​LPRA and LFA, LF RF generator 102 generates an adjusted LF RF signal based on the values ​​LPRA and LFA of the adjusted recipe signal and provides the adjusted LF RF signal having values ​​LPRA and LFA to matcher 106 (FIG. 1A) over RF cable RFC1. Upon receiving the adjusted LF RF signal having values ​​LPRA and LFA as well as RF signal 146, matcher 106 adjusts the impedance of the adjusted LF RF signal to output a first adjusted modified RF signal. The matcher 106 outputs an adjusted combined RF signal by combining the first adjusted modified RF signal and the second modified RF signal generated based on the RF signal 146 (FIG. 1A) to change the sheath width SWA to be within a predetermined range from the default sheath width PSW, and transmits the adjusted combined RF signal to the ESC 112 through the RF transmission line RFT. In this example, during cycle 1 of the parameters of the LF RF signal 144, after the adjusted combined RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicating a modified optical emission profile of the bottom plasma sheath 130. In this example, during cycle 1 of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile, and further determines that the modified sheath width is within a predetermined range from the default sheath width PSW. An example of an adjusted LF RF signal is the LF RF signal 144 with modifications to the frequency, or parameters, or both, of the LF RF signal 144.

[0067] Similarly, upon determining during the second cycle of the parameters of the LF RF signal 144 that the sheath width SWB is not within the predetermined range from the predetermined sheath width PSW, the processor 118 changes the parameter value LPRA to the parameter value LPRB, or changes the frequency value LFA to the frequency value LFB, or changes the parameter value LPRA to the parameter value LPRB and the frequency value LFA to the frequency value LFB to achieve the predetermined sheath width PSW. During the second cycle of the parameters of the LF RF signal 144, the processor 118 controls the LF RF generator 102 to generate an adjusted LF RF signal having a parameter value LPRB, or a frequency value LFB, or both, to change the sheath width SWB to be within the predetermined range from the predetermined sheath width PSW. In this example, after the adjusted LF RF signal is generated by the LF RF generator 102 during cycle 2 of the parameters of the LF RF signal 144, the processor 118 receives a modified electrical signal indicative of a modified emission profile of the bottom plasma sheath 130. In this example, during cycle 2 of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified emission profile and further determines that the modified sheath width is within a predetermined range from the default sheath width PSW.

[0068] Meanwhile, in response to determining during the second cycle of the parameters of the LF RF signal 144 that the sheath width SWA is within the predetermined range from the default sheath width PSW, the processor 118 does not modify the parameter value LPRA to the parameter value LPRB and does not modify the frequency value LFA to the frequency value LFB. For example, when the frequency value LFA of the LF RF signal 144 is not changed to the frequency value LFB and the parameter value LPRA of the LF RF signal 144 is not changed to the parameter value LPRB, the sheath width SWB is not modified during the second cycle of the parameters of the LF RF signal 144 and is within the predetermined range from the default sheath width PSW.

[0069] Similarly, in response to determining during the third cycle of the parameters of the LF RF signal 144 that the sheath width SWC is not within the predetermined range from the default sheath width PSW, the processor 118 changes the parameter value LPRB to the parameter value LPRC, or changes the frequency value LFB to the frequency value LFC, or changes the parameter value LPRB to the parameter value LPRC and the frequency value LFB to the frequency value LFC to achieve the default sheath width PSW. During the third cycle of the parameters of the LF RF signal 144, the processor 118 controls the LF RF generator 102 to generate an adjusted LF RF signal having the parameter value LPRC, the frequency value LFC, or both, to change the sheath width SWC to be within the predetermined range from the default sheath width PSW.

[0070] Meanwhile, in response to determining during the third cycle of the parameters of the LF RF signal 144 that the sheath width SWC is within the predetermined range from the default sheath width PSW, the processor 118 does not modify the parameter value LPRB to the parameter value LPRC and does not modify the frequency value LFB to the frequency value LFC. For example, when the frequency value LFB of the LF RF signal 144 is not changed to the frequency value LFBC and the parameter value LPRB of the LF RF signal 144 is not changed to the parameter value LPRC, the sheath width SWC is not modified. The sheath width SWC is not modified during the third cycle of the parameters of the LF RF signal 144 and is within the predetermined range from the default sheath width PSW.

[0071] It should be noted that the processor 118 determines the period of each cycle of the parameter of the LF RF signal 144. For example, the processor 118 determines the period of each cycle of the parameter of the RF signal 144 based on the voltage signal received from the probe 170. For example, the processor 118 receives the voltage signal measured by the probe 170 and converts the voltage signal from the time domain to the frequency domain using a Fourier transform on the voltage signal. The processor 118 identifies a low frequency range having a low frequency of the LF RF generator 102 and a high frequency range having a high frequency of the HF RF generator 104 from the voltage signal in the frequency domain by distinguishing between low and high frequencies of the voltage signal. To distinguish between low and high frequencies, the processor 118 determines that the frequency tuning ranges of a 100 kHz RF generator, a 400 kHz RF generator, and a 2 MHz RF generator are examples of the low frequency range, and the frequency ranges of a 13.56 MHz RF generator, a 27 MHz RF generator, and a 60 MHz RF generator are examples of the high frequency range. The low frequency range identified from the voltage signal measured by the probe 170 may be referred to herein as the LF voltage signal (e.g., LF voltage signal 352 (FIG. 3B)), and the high frequency range identified from the voltage signal measured by the probe 170 may be referred to herein as the HF voltage signal. To identify cycle 1 of the LF voltage signal, the processor 118 determines from the LF voltage signal that the LF voltage signal achieves a negative voltage value at time t0 and then achieves the same negative voltage value at time t12. To identify cycle 2 of the LF voltage signal, the processor 118 also determines from the LF voltage signal that the LF voltage signal achieves a negative voltage value at time t12 and then achieves the same negative voltage value at time t24. The processor 118 determines the period between time t0 and time t12 as the cycle 1 period of the parameter of the LF RF signal 144. The processor 118 further determines the period between time t12 and time t24 as the cycle 2 period of the parameter of the LF RF signal 144.

[0072] As another example of determining the duration of each cycle of a parameter of LF RF signal 144, a voltage probe can be connected to output O1 of LF RF generator 102 to measure the LF voltage signal at output O1. The voltage probe is connected to processor 118. Upon receiving the LF voltage signal from output O1, processor 118 determines cycle 1 and cycle 2 of the LF voltage signal in the same manner as shown in the previous example, but without the need to use a Fourier transform.

[0073] 3A is an illustration of an embodiment of a table 300 illustrating control of variables of the HF RF signal 146 (FIG. 1A). The variables are controlled based on the width of the bottom plasma sheath 130 (FIG. 1A). The table 300 is generated by the processor 118 and stored in the storage device 120. The table 300 is an example of a database.

[0074] The table 300 includes a listing of widths of the bottom plasma sheath 130 calculated by the processor 118 based on signals received from the sensors. For example, the table 300 includes a plurality of values ​​SW1, SW2, etc., up to a value SW(n-1), where n is a positive integer. The table 300 further includes values ​​SWn, SW(n+1), etc., up to a value SW(p-1) and SWp, where p is a positive integer greater than n. The table 300 also includes a default sheath width PSW. The table 300 includes a plurality of parameter values ​​HPR1, HPR2, etc., up to a parameter value HPR(n-1). The table 300 also includes parameter values ​​HPRn, HPR(n+1), etc., up to a parameter value HPR(p-1) and HPRp of the HF RF signal 146. The table 300 includes a plurality of frequency values ​​HF1, HF2, etc., up to a frequency value HF(n-1). The table 300 also includes frequency values ​​HFn, HF(n+1), etc., up to frequency values ​​HF(p-1) and HFp of the HF RF signal 146.

[0075] The processor 118 determines a width of the bottom plasma sheath 130 (FIG. 1A) for each bin of the parameter of the LF RF signal 144. For example, during each bin of the parameter of the LF RF signal 144 provided by the LF RF generator 102 (FIG. 1A), the processor 118 calculates a width of the bottom plasma sheath 130. For example, during bin 1 of the first cycle of the parameter of the LF RF signal 144, the processor 118 receives a first electrical signal from the optical sensor 136 (FIG. 1A) having a first optical emission profile of the bottom plasma sheath 130. The processor 118 determines a width SW1 of the bottom plasma sheath 130 from the first electrical signal. Similarly, during bin 2 of the first cycle of the parameter of the LF RF signal 144, the processor 118 receives a second electrical signal from the optical sensor 136 having a second optical emission profile of the bottom plasma sheath 130. The processor 118 determines a width SW2 of the bottom plasma sheath 130 from the second electrical signal. Also, during bin n of the first cycle of the parameter of the LF RF signal 144, the processor 118 receives an nth electrical signal from the optical sensor 136 having an nth optical emission profile of the bottom plasma sheath 130. The processor 118 determines a width SWn of the bottom plasma sheath 130 from the nth electrical signal. Also, during bin p of the first cycle of the parameter of the LF RF signal 144, the processor 118 receives a pth electrical signal from the optical sensor 136 having a pth optical emission profile of the bottom plasma sheath 130. The processor 118 determines a width SWp of the bottom plasma sheath 130 from the nth electrical signal. In this example, the probe 170 is connected to a point 166 (FIG. 1B) on the RF rod of the RF transmission line RFT of the system 100 (FIG. 1A), and the voltage signal from the probe 170 is used by the processor 118 to identify the first cycle of the parameter of the LF RF signal 144.

[0076] As another example, the processor 118 determines the sheath width SW1 as a statistical value (e.g., average or median) calculated from a first set of sheath widths during bin 1 of a first cycle of a parameter of the LF RF signal 144. The first set of sheath widths is determined by the processor 118 from a plurality of electrical signals having a first set of optical emission profiles. The first set of optical emission profiles is of the bottom plasma sheath 130 and is determined based on electrical signals received from the optical sensor 136. The electrical signals are received during bin 1 of a first cycle of a parameter of the LF RF signal 144. Also in this example, the processor 118 determines the sheath width SW2 as a statistical value calculated from a second set of sheath widths during bin 2 of a first cycle of a parameter of the LF RF signal 144. The second set of sheath widths is determined by the processor 118 from a plurality of electrical signals having a second set of optical emission profiles of the bottom plasma sheath 130 received from the optical sensor 136. A second set of emission profiles is received during bin 2 of the first cycle of the LF RF signal 144 parameters.

[0077] Upon determining that the sheath width SW1 is not within a predetermined range from the default sheath width PSW during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 alters the variables of the HF RF signal 146 to achieve the default sheath width PSW. For example, in response to determining that the sheath width SW1 is not within a predetermined range from the default sheath width PSW, the processor 118 alters a parameter setpoint (e.g., parameter value) of the recipe signal 140 (FIG. 1A) to a parameter value HPR1 to achieve the default sheath width PSW. For example, when the parameters of the HF RF signal 146 are altered from the parameter setpoint to the parameter value HPR1, the sheath width SW1 is modified during bin 1 of the first cycle of the parameters of the LF RF signal 144 to be within a predetermined range from the default sheath width PSW.

[0078] Meanwhile, in response to determining that the sheath width SW1 is within the predetermined range from the default sheath width PSW during bin 1 of the first cycle of the parameter of the LF RF signal 144, the processor 118 does not modify the parameter set point of the recipe signal 140 to the parameter value HPR1. For example, when the parameter set point of the HF RF signal 146 is not changed to the parameter value HPR1, the sheath width SW1 is not modified during bin 1 of the parameter of the LF RF signal 146, and the sheath width SW1 is within the predetermined range from the default sheath width PSW.

[0079] Similarly, as another example, in response to determining that sheath width SW1 is not within a predetermined range from the default sheath width PSW, processor 118 changes the frequency setpoint (e.g., frequency value) of recipe signal 140 to frequency value HF1 to achieve the default sheath width PSW. For example, when the frequency of HF RF signal 146 is changed from the frequency setpoint to frequency value HF1, sheath width SW1 is modified during bin 1 of the first cycle of parameters of LF RF signal 144 to be within a predetermined range from the default sheath width PSW.

[0080] Meanwhile, in response to determining that the sheath width SW1 is within the predetermined range from the default sheath width PSW during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 does not modify the frequency set point of the recipe signal 140 to the frequency value HF1. ​​For example, when the frequency set point of the HF RF signal 146 is not changed to the frequency value HF1, the sheath width SW1 is not modified during bin 1 of the parameters of the LF RF signal 146, and the sheath width SW1 is within the predetermined range from the default sheath width PSW.

[0081] The processor 118 controls the HF RF generator 104 to generate an adjusted HF RF signal having a parameter value HPR1, or a frequency value HF1, or a combination thereof, during bin 1 of the first cycle of the parameter of the LF RF signal 144 to change the sheath width SW1 to the default sheath width PSW. For example, the processor 118 changes a parameter set point of the parameter of the HF RF signal 146 to a value HPR1 to generate an adjusted parameter recipe signal and transmits the adjusted parameter recipe signal to the HF RF generator 104. In response to receiving the adjusted parameter recipe signal having the parameter value HPR1, the HF RF generator 104 generates an adjusted parameter RF signal based on the parameter value HPR1 of the adjusted parameter recipe signal and provides the adjusted parameter RF signal having the parameter value HPR1 to the matcher 106 (FIG. 1A) through the RF cable RFC2. Upon receiving the adjusted parameter RF signal having the parameter value HPR1 and the RF signal 144, the matcher 106 adjusts the impedance of the adjusted parameter RF signal to output a first adjusted parameter modified RF signal. The matcher 106 combines the first adjustment parameter modified RF signal with the first modified RF signal generated based on the RF signal 144 (FIG. 1A) to achieve the preset sheath width PSW, outputs the adjusted parameter combined RF signal, and transmits the adjusted parameter combined RF signal to the ESC 112 through the RF transmission line RFT. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, after the adjusted parameter combined RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicative of a modified emission profile of the bottom plasma sheath 130. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath from the modified emission profile, and further determines that the modified sheath width is within a preset range from the preset sheath width PSW.

[0082] As another example, the processor 118 changes the frequency set point of the frequency of the HF RF signal 146 to a value HF1 to generate an adjusted frequency recipe signal, and transmits the adjusted frequency recipe signal to the HF RF generator 104. In response to receiving the adjusted frequency recipe signal having a frequency value HF1, the HF RF generator 104 generates an adjusted frequency RF signal based on the frequency value HF1 of the adjusted frequency recipe signal, and provides the adjusted frequency RF signal having the frequency value HF1 to the matcher 106 (FIG. 1A) through the RF cable RFC1. In response to receiving the adjusted frequency RF signal having the frequency value HF1 and the RF signal 144, the matcher 106 adjusts the impedance of the adjusted frequency RF signal to output a first adjusted frequency modified RF signal. The matcher 106 combines the first adjusted frequency modified RF signal with the first modified RF signal generated based on the RF signal 144 (FIG. 1A) to achieve the predetermined sheath width PSW, and transmits the adjusted frequency modified RF signal to the ESC 112 through the RF transmission line RFT. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, after the adjusted frequency synthesized RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicative of a modified optical emission profile of the bottom plasma sheath 130. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile and further determines that the modified sheath width is within a predetermined range from the predetermined sheath width PSW.

[0083] As yet another example, processor 118 changes a parameter set point of HF RF signal 146 to parameter value HPR1 and changes a frequency set point of HF RF signal 146 to value HF1 to generate an adjusted recipe signal and transmits the adjusted recipe signal to HF RF generator 104. In response to receiving the adjusted recipe signal having values ​​HPR1 and HF1, HF RF generator 104 generates an adjusted HF RF signal based on the values ​​HPR1 and HF1 of the adjusted recipe signal and provides the adjusted HF RF signal having values ​​HPR1 and HF1 to matcher 106 (FIG. 1A) through RF cable RFC2. Upon receiving the HF RF signal having values ​​HPR1 and HF1 as well as RF signal 144, matcher 106 adjusts the impedance of the adjusted HF RF signal to output a first adjusted modified RF signal. The matcher 106 combines the first adjusted modified RF signal with the first modified RF signal generated based on the RF signal 144 (FIG. 1A) to output an adjusted combined RF signal to change the sheath width SWA to be within a predetermined range from the default sheath width PSW, and transmits the adjusted combined RF signal to the ESC 112 through the RF transmission line RFT. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, after the adjusted combined RF signal is provided to the ESC 112, the processor 118 receives a modified electrical signal indicative of a modified optical emission profile of the bottom plasma sheath 130. In this example, during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile, and further determines that the modified sheath width is within a predetermined range from the default sheath width PSW.

[0084] Similarly, in response to determining that the sheath width SWn is not within a predetermined range from the default sheath width PSW during bin n of the first cycle of the parameter of the LF RF signal 144, the processor 118 changes the parameter value HPR(n-1) to the parameter value HPRn, or changes from the frequency value HF(n-1) to the frequency value HFn, or changes the parameter value HPR(n-1) to the parameter value HPRn and from the frequency value HF(n-1) to the frequency value HFn to achieve the default sheath width PSW. During bin n of the first cycle of the parameter of the LF RF signal 144, the processor 118 controls the HF RF generator 104 to generate an adjusted HF RF signal having the parameter value HPRn or the frequency value HFn, or both the values ​​HPRn and HFn, to change the sheath width SWn to be within a predetermined range from the default sheath width PSW. For example, after the adjusted HF RF signal is generated by the HF RF generator 104 during bin n of the first cycle of the parameters of the LF RF signal 144, the processor 118 receives a modified electrical signal indicative of a modified optical emission profile of the bottom plasma sheath 130. In this example, during bin n of the first cycle of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile and further determines that the modified sheath width is within a predetermined range from the predetermined sheath width PSW.

[0085] Meanwhile, in response to determining that the sheath width SWn is within the predetermined range from the default sheath width PSW during bin n of the first cycle of the parameter of the LF RF signal 144, the processor 118 does not modify the frequency value HF(n-1) of the HF RF signal 146 to the frequency value HFn. Also, the processor 118 does not modify the parameter value HPR(n-1) of the HF RF signal 146 to the parameter value HPRn. For example, when the frequency value HF(n-1) of the HF RF signal 146 is not changed to the frequency value HFn and the parameter value HPR(n-1) of the HF RF signal 146 is not changed to the parameter value HPRn, the sheath width SWn is not modified during bin n of the parameter of the LF RF signal 146 and is within the predetermined range from the default sheath width PSW.

[0086] Also, in response to determining that the sheath width SWp is not within a predetermined range from the default sheath width PSW during bin p of the first cycle of the parameter of the LF RF signal 144, the processor 118 changes the parameter value HPR(p-1) to the parameter value HPRp, or changes the frequency value HF(p-1) to the frequency value HFp, or changes the parameter value HPR(p-1) to the parameter value HPRp and changes the frequency value HF(p-1) to the frequency value HFp to achieve the default sheath width PSW. During bin p of the first cycle of the parameter of the LF RF signal 144, the processor 118 controls the HF RF generator 104 to generate an adjusted HF RF signal having the parameter value HPRp or the frequency value HFp, or having both the value HPRp and the value HFp, to change the sheath width SWp to be within a predetermined range from the default sheath width PSW. For example, during bin p of the first cycle of the parameters of the LF RF signal 144, after the adjusted HF RF signal is generated by the HF RF generator 104, the processor 118 receives a modified electrical signal indicative of a modified optical emission profile of the bottom plasma sheath 130. In this example, during bin p of the first cycle of the parameters of the LF RF signal 144, the processor 118 determines a modified sheath width from the modified optical emission profile and further determines that the modified sheath width is within a predetermined range from the predetermined sheath width PSW.

[0087] On the other hand, in response to determining that the sheath width SWp is within the predetermined range from the default sheath width PSW during bin p of the first cycle of the parameter of the LF RF signal 144, the processor 118 does not modify the frequency value HF(p-1) of the HF RF signal 146 to the frequency value HFp and does not modify the parameter value HPR(p-1) of the HF RF signal 146 to the parameter value HPRp. For example, when the frequency value HF(p-1) of the HF RF signal 146 is not changed to the frequency value HFp and the parameter value HPR(p-1) of the HF RF signal 146 is not changed to the parameter value HPRp, the sheath width SW(p-1) is not modified during bin p of the parameter of the LF RF signal 144 and the sheath width Swp is within the predetermined range from the default sheath width PSW.

[0088] 3B is an embodiment of a graph 350 illustrating bins 1 through p during a first cycle of a parameter of LF RF signal 144 (FIG. 1A). Graph 350 plots voltage (e.g., LF voltage signal 352) versus time t. LF voltage signal 352 is representative of the voltage of LF RF signal 144 generated by LF RF generator 102 (FIG. 1A). For example, processor 118 (FIG. 1A) determines LF voltage signal 352 from a voltage measured by probe 170.

[0089] The voltage of the LF voltage signal 352 is plotted on the y-axis and time t is plotted on the x-axis. Time t includes times t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, and t10. Time t includes times t11, t12, t13, t14, t15, etc., up to times t23 and t24. The duration between any two consecutive times on the x-axis is equal. For example, the duration between times t0 and t1 is equal to the duration between times t1 and t2. The voltage signal 352 is shown using a dashed line in the graph 350. The graph 350 further includes the voltage 354 of the bottom plasma sheath 130 (FIG. 1A). The voltage 354 is shown using a solid line in the graph 350. Note that the voltage signal 352 and the voltage 354 are offset from zero voltage by a negative voltage value indicated as -Vdc.

[0090] Processor 118 divides each cycle of LF voltage signal 352 into a predetermined number of bins 1 through p. For example, after identifying each cycle of LF voltage signal 352, processor 118 divides cycle 1 of LF voltage signal 352 into bins 1 through p. Processor 118 also divides cycle 2 of LF voltage signal 352 into bins 1 through p. For example, the period between time t0 of cycle 1 of LF voltage signal 352 and time t12 of cycle 1 of LF voltage signal 352 is divided by processor 118 into bins 1 through p, and the period between time t12 of cycle 2 and time 24 of cycle 2 is divided by processor 118 into bins 1 through p.

[0091] Bin 1 extends from time t0 to time t1, bin 2 extends from time t1 to time t2, bin(n-1) extends from time t4 to time t5, bin n extends from time t5 to time t6, bin(n+1) extends from time t6 to time t7, bin(p-1) extends from time t10 to time t11, and bin p extends from time t11 to time t12.

[0092] In one embodiment, each bin has an equal duration. For example, all bins of the parameter of the LF RF signal 144 extend over an equal duration. For example, bin 1 of cycle 1 of the parameter of the LF RF signal extends over a first duration from time t0 to time t1. And, bin 2 of cycle 1 of the parameter of the LF RF signal extends over a second duration from time t1 to time t2. The first duration is equal to the second duration. As another example, bin (n-1) of cycle 1 of the parameter of the LF RF signal extends over a first duration from time t4 to time t5. And, bin n of cycle 1 of the parameter of the LF RF signal extends over a second duration from time t5 to time t6. The first duration is equal to the second duration. As yet another example, bin (p-1) of cycle 1 of the parameter of the LF RF signal extends over a first duration from time t10 to time t11. Also, bin p of cycle 1 of the parameter of the LF RF signal extends over a second period of time from time t11 to time t12, the first period of time being equal to the second period of time.

[0093] In an embodiment, no negative offset -Vdc is applied to the voltage signal 352. In this embodiment, the voltage signal 352 has zero voltage at times t0, t12, and t24.

[0094] In an embodiment, the processor 118 controls the HF RF generator 104 during one or more of the bins 1-n until the corresponding one or more sheath widths SW1-SWn are within a predetermined range from the default sheath width PSW. For example, the processor 118 controls the frequency value, or the power value, or a combination thereof, of the HF RF signal 146 until the corresponding one or more sheath widths SW1-SWn are within a predetermined range from the default sheath width PSW. For example, the processor 118 increases the power value of the HF RF signal 146 during the bins 1-n of the voltage signal 352 to achieve the default sheath width PSW. As another example, the processor 118 decreases the power value of the HF RF signal 146 during the bins 1-n of the voltage signal 352 to achieve the default sheath width PSW. As yet another example, the processor 118 maintains the power value of the HF RF signal 146 constant during the bins 1-n of the voltage signal 352 to maintain the default sheath width PSW. Note that during the bins 1-n, the voltage signal 352 has a value greater than the negative potential -Vdc.

[0095] In an embodiment, the processor 118 controls the HF RF generator 104 during one or more of the bins (n+1) through p until a corresponding one or more of the sheath widths SW(n+1) through SWp are within a predetermined range from the default sheath width PSW. For example, the processor 118 controls the frequency value, or the power value, or a combination thereof, of the HF RF signal 146 until the sheath widths SW(n+1) through SWp are within a predetermined range from the default sheath width PSW. For example, the processor 118 increases the power value of the HF RF signal 146 during the bins (n+1) through p of the voltage signal 352 to achieve the default sheath width PSW. As another example, the processor 118 decreases the power value of the HF RF signal 146 during the bins (n+1) through p of the voltage signal 352 to achieve the default sheath width PSW. As yet another example, the processor 118 maintains the power value of the HF RF signal 146 constant during the bins (n+1) through p of the voltage signal 352 to maintain the default sheath width PSW. Note that between bins (n+1) through p, the voltage signal 352 has a value less than the negative potential −Vdc.

[0096] In one embodiment, the processor 118 controls the HF RF generator 104 during one or more bins occurring during the negative transition of the voltage signal 352. For example, the processor 118 controls the HF RF generator 104 during one or more of cycle 1 bin (n-2), cycle 1 bin (n-1), cycle 1 bin n, cycle 1 bin (n+1), cycle 1 bin (n+2), and cycle 1 bin (n+3) until the corresponding one or more of sheath widths SW(n-2), sheath width (n-1), sheath width n, sheath width SW(n+1), sheath width SW(n+2), and sheath width SW(n+3) are within a predetermined range from the predetermined sheath width PSW. For example, the processor 118 controls the frequency value, or power value, or a combination thereof, of the HR RF signal 146 until the sheath widths SWn and SW(n+1) are within a predetermined range from the predetermined sheath width SPW. In a further example, the processor 118 increases the power value of the HF RF signal 146 between bins (n+1) and n of the voltage signal 352 to achieve the predetermined sheath width PSW. As yet another example, the processor 118 decreases the power value of the HF RF signal 146 between bins (n+1) and n of the voltage signal 352 to achieve the predetermined sheath width PSW. As yet another example, the processor 118 maintains the power value of the HF RF signal 146 constant between bins (n+1) and n of the voltage signal 352 to maintain the predetermined sheath width PSW. Note that bin (n-2) extends over a period starting at time t3 and ending at time t4. Bin (n+2) extends over a period starting at time t7 and ending at time t8. Bin (n+3) extends over a period starting at time t8 and ending at time t9.

[0097] Also note that the voltage signal 352 periodically transitions from a value V1 to a value −V1. The value V1 is greater than the zero voltage of the voltage signal 352. The transition from the value V1 to the negative value −V1 is an example of a negative transition. As an example, the negative transition from the value V1 to the value −V1 includes a period that includes bin (n-2), bin (n-1), bin n, bin (n+1), bin (n+2), and bin (n+3).

[0098] In one embodiment, the processor 118 controls the HF RF generator 104 during one or more bins occurring during a positive transition of the voltage signal 352. For example, the processor 118 controls the HF RF generator 104 during one or more of cycle 1 bin (p-2), cycle 1 bin (p-1), cycle 1 bin p, cycle 2 bin 1, cycle 2 bin 2, and cycle 2 bin 3 until a corresponding one or more of cycle 1 sheath width SW(p-2), cycle 1 sheath width SW(p-1), cycle 1 sheath width p, cycle 2 sheath width SW11, cycle 2 sheath width SW12, and cycle 2 sheath width SW13 are within a predetermined range from the preset sheath width PSW. For example, the processor 118 controls a frequency value, or a power value, or a combination thereof, of the HF RF signal 146 until cycle 1 sheath width SWp and cycle 2 sheath width SW11 are within a predetermined range from the preset sheath width PSW. In a further example, the processor 118 increases the power value of the HF RF signal 146 during bin p of cycle 1 and bin 1 of cycle 2 of the voltage signal 352 to achieve the predetermined sheath width PSW. As yet another example, the processor 118 decreases the power value of the HF RF signal 146 during bin p of cycle 1 and bin 1 of cycle 2 of the voltage signal 352 to achieve the predetermined sheath width PSW. As yet another example, the processor 118 maintains the power value of the HF RF signal 146 constant during p of cycle 1 and bin 1 of cycle 2 of the voltage signal 352 to maintain the predetermined sheath width PSW. Note that bin (p-2) of cycle 1 of the voltage signal 352 extends for a period starting at time t9 and ending at time t10. Bin 1 of cycle 2 extends for a period starting at time t12 and ending at time t13. Bin 2 of cycle 2 extends for a period starting at time t13 and ending at time t14. Bin 3 of cycle 2 extends over a period beginning at time t14 and ending at time t15.

[0099] It is also noted that voltage signal 352 periodically transitions from a value −V1 to a value V1. A transition from a negative value −V1 to a positive value V1 is an example of a positive transition. By way of example, a positive transition from a value −V1 to a value V1 includes a period that includes bin (p-2) of cycle 1, bin (p-1) of cycle 1, bin p of cycle 1, bin 1 of cycle 2, bin 2 of cycle 2, and bin 3 of cycle 2.

[0100] In one embodiment, instead of modifying the variables of the HF RF signal 146 during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 modifies the variables of the HF RF signal 146 during bin 1 of the second cycle of the parameters of the LF RF signal 144. The variables are modified to achieve a predetermined sheath width PSW. The variables are modified after determining that the sheath width SW1 is not within a predetermined range from the predetermined sheath width PSW during bin 1 of the first cycle of the parameters of the LF RF signal 144. For example, in response to determining that the sheath width SW1 is not within a predetermined range from the predetermined sheath width PSW, the processor 118 changes a parameter set point of the recipe signal 140 (FIG. 1A) to a parameter value HPR1 during bin 1 of the second cycle of the parameters of the LF RF signal 144 to achieve the predetermined sheath width PSW. For example, when the parameter of HF RF signal 146 is changed from the parameter setpoint to parameter value HPR1 during bin 1 of the second cycle of the parameter of LF RF signal 144, the sheath width SW1 is modified to be within a predetermined range from the default sheath width PSW during bin 1 of the second cycle of the parameter of LF RF signal 144.

[0101] Meanwhile, in response to determining that the sheath width SW1 is within the predetermined range from the default sheath width PSW during bin 1 of the first cycle of the parameter of the LF RF signal 144, the processor 118 does not modify the parameter set point of the recipe signal 140 to the parameter value HPR1 during bin 1 of the second cycle of the parameter of the LF RF signal 144. For example, when the parameter set point of the HF RF signal 146 is not changed to the parameter value HPR1, the sheath width SW1 is not modified during bin 1 of the second cycle of the parameter of the LF RF signal 146 and the sheath width SW1 is within the predetermined range from the default sheath width PSW.

[0102] Similarly, as another example, in response to determining that the sheath width SW1 is not within a predetermined range from the default sheath width PSW, the processor 118 changes the frequency setpoint (e.g., frequency value) of the recipe signal 140 during bin 1 of the second cycle of the parameters of the LF RF signal 144 to the frequency value HF1 to achieve the default sheath width PSW during bin 1 of the second cycle of the parameters of the LF RF signal 144. For example, when the frequency of the HF RF signal 146 is changed from the frequency setpoint to the frequency value HF1 during bin 1 of the second cycle of the parameters of the LF RF signal 144, the sheath width SW1 is modified to be within a predetermined range from the default sheath width PSW during bin 1 of the second cycle of the parameters of the LF RF signal 144.

[0103] Meanwhile, in response to determining that the sheath width SW1 is within the predetermined range from the default sheath width PSW during bin 1 of the first cycle of the parameters of the LF RF signal 144, the processor 118 does not modify the frequency set point of the recipe signal 140 to the frequency value HF1 during bin 1 of the second cycle of the parameters of the LF RF signal 144. For example, when the frequency set point of the HF RF signal 146 is not changed to the frequency value HF1 during bin 1 of the second cycle of the parameters of the LF RF signal 144, the sheath width SW1 is not modified during bin 1 of the second cycle of the parameters of the LF RF signal 144, and the sheath width SW1 is within the predetermined range from the default sheath width PSW. The processor 118 controls the HF RF generator 104 to generate an adjusted HF RF signal having a parameter value HPR1, or a frequency value HF1, or a combination thereof, during bin 1 of the second cycle of the parameter of the LF RF signal 144, and changes the sheath width SW1 to the default sheath width PSW in the same manner as described above for controlling the HF RF generator 104 during bin 1 of the first cycle of the parameter of the LF RF signal 144.

[0104] Similarly, instead of modifying the variables of the HF RF signal 146 during bin p of the first cycle of the parameters of the LF RF signal 144, the processor 118 modifies the variables of the HF RF signal 146 during bin p of the second cycle of the parameters of the LF RF signal 144. The variables are modified to achieve a predetermined sheath width PSW. The variables are modified after determining that the sheath width SWp is not within a predetermined range from the predetermined sheath width PSW during bin p of the first cycle of the parameters of the LF RF signal 144. For example, in response to determining that the sheath width SWp is not within a predetermined range from the predetermined sheath width PSW, the processor 118 changes the parameter value HPR(p-1) of the recipe signal 140 (FIG. 1A) to the parameter value HPRp during bin p of the second cycle of the parameters of the LF RF signal 144 to achieve the predetermined sheath width PSW. For example, when the parameter of HF RF signal 146 is changed from parameter value HPR(p-1) to parameter value HPRp during bin p of the second cycle of the parameter of LF RF signal 144, the sheath width SWp is modified to be within a predetermined range from the default sheath width PSW during bin p of the second cycle of the parameter of LF RF signal 144.

[0105] Meanwhile, in response to determining that the sheath width SWp is within the predetermined range from the default sheath width PSW during bin p of the first cycle of the parameter of the LF RF signal 144, the processor 118 does not modify the parameter value HPR(p-1) of the recipe signal 140 to the parameter value HPRp during bin p of the second cycle of the parameter of the LF RF signal 144. For example, when the parameter setpoint of the HF RF signal 146 is not changed to the parameter value HPRp, the sheath width SWp is not modified during bin p of the second cycle of the parameter of the LF RF signal 144 and the sheath width SWp is within the predetermined range from the default sheath width PSW.

[0106] Similarly, as another example, in response to determining that the sheath width SWp is not within a predetermined range from the default sheath width PSW, the processor 118 changes the frequency value HF(p-1) of the recipe signal 140 to a frequency value HFp during bin p of the second cycle of the parameters of the LF RF signal 144 to achieve the default sheath width PSW during bin p of the second cycle of the parameters of the LF RF signal 144. For example, when the frequency of the HF RF signal 144 is changed from the frequency value HF(p-1) to the frequency value HFp during bin p of the second cycle of the parameters of the LF RF signal 144, the sheath width SWp is modified to be within a predetermined range from the default sheath width PSW during bin p of the first cycle of the parameters of the LF RF signal 144.

[0107] Meanwhile, in response to determining that the sheath width SWp is within the predetermined range from the default sheath width PSW during bin p of the first cycle of the parameters of the LF RF signal 144, the processor 118 does not modify the frequency value HF(p-1) of the recipe signal 140 to the frequency value HFp during bin p of the second cycle of the parameters of the LF RF signal 144. For example, when the frequency value HF(p-1) of the HF RF signal 146 is not changed to the frequency value HF1 during bin p of the second cycle of the parameters of the LF RF signal 144, the sheath width SWp is not modified during bin p of the second cycle of the parameters of the LF RF signal 144, and the sheath width SWp is within the predetermined range from the default sheath width PSW. The processor 118 controls the HF RF generator 104 to generate an adjusted HF RF signal having a parameter value HPRp, or a frequency value HFp, or a combination thereof, during bin p of the second cycle of the parameter of the LF RF signal 144, to change the sheath width SWp to the default sheath width PSW in the same manner as described above for controlling the HF RF generator 104 during bin p of the first cycle of the parameter of the LF RF signal 144.

[0108] FIG. 4 is a diagram of an embodiment of an RF transmission line RFT. The RF transmission line RFT comprises an RF rod 402 and an RF sheath 404. An insulating material is placed between the RF rod 402 and the RF sheath 404. The insulating material surrounds the RF rod 402. The RF rod 402 is made of a metal such as copper or aluminum. The probe 170 is connected to the RF rod 402 at a point 166. The RF rod 402 provides a forward path for providing the modified RF signal 148. The RF sheath 404 provides a return path for RF power from the plasma chamber 110 (FIG. 1A) to the RF generators 102 and 104 (FIG. 1A) through the housing of the impedance matching circuit 106 (FIG. 1A) and the RF cables RFC1 and RFC2 (FIG. 1A).

[0109] 5 is a diagram of an embodiment of a system 500 showing details of the LF RF generator 102 and the HF RF generator 104. The system 500 includes a host computer 111, the LF RF generator 102, and the HF RF generator 104.

[0110] The LF RF generator 102 includes a digital signal processor (DSP) DSPL, an automatic frequency tuner AFTL, a parameter controller PRTL, a driver DRVRL, and an RF power supply 502. The HF RF generator 104 includes a digital signal processor DSPH. The HF RF generator 104 further includes an automatic frequency tuner (AFT) for each bin and a parameter controller for each bin. For example, the HF RF generator 104 includes a plurality of automatic frequency tuners AFTbin1, AFTbin2, etc. (collectively referred to as automatic frequency tuners AFTH) up to an automatic frequency tuner AFTbinp. The HF RF generator 104 also includes a plurality of parameter controllers PRbin1, PRbin2, etc. (collectively referred to as parameter controllers PRH) up to a parameter controller PRbinp. The HF RF generator 104 includes a driver DRVRH and an RF power supply 504.

[0111] An example of a controller as used herein includes a processor and a memory device. The controller's processor is connected to the controller's memory device. For example, the controller is a microcontroller or an integrated controller integrated with an ASIC or PLD. An example of an automatic frequency tuner as used herein includes a processor and a memory device. The automatic frequency tuner's processor is connected to the automatic frequency tuner's memory device. An example of a driver as used herein includes one or more transistors. The one or more transistors are connected to each other. An example of an RF power source includes an electronic oscillator, which is an electric circuit that generates a periodic frequency electric signal, such as a sine wave.

[0112] The digital signal processor DSPL is connected to the processor 118 via a transmission cable 132. The digital signal processor DSPL is connected to an automatic frequency tuner AFTL and a parameter controller PRL. The tuner AFTL and the controller PRL are connected to a driver DRVRL, which is connected to an RF power supply 502. The RF power supply 502 is connected to an RF cable RFC1 at an output O1.

[0113] Similarly, the digital signal processor DSPH is connected to the processor 118 via the transmission cable 134. The digital signal processor DSPH is connected to the automatic frequency tuners AFTbin1-AFTbinp and to the parameter controllers PRbin1-PRbinp. The controllers PRbin1-PRbinp are connected to the driver DRVRH, which is connected to the RF power supply 504. The RF power supply 504 is connected to the RF cable RFC2 at the output O2. The automatic frequency tuners AFTbin1-AFTbinp are connected to the driver DRVRH.

[0114] Processor 118 transmits recipe signal 138 to digital signal processor DSPL over transmission cable 132 and recipe signal 140 to digital signal processor DSPH over transmission cable 134. Recipe signal 138 includes frequency and power setpoints for RF signal 144, and recipe signal 140 includes frequency and power setpoints for RF signal 146.

[0115] Upon receiving the recipe signal 138, the digital signal processor DSPL sends the parameter set points of the RF signal 144 to the parameter controller PRL and sends the frequency set point of the RF signal 144 to the automatic frequency tuner AFTL. The processor of the parameter controller PRL stores the parameter set points of the RF signal 144 in a memory device of the parameter controller PRL, and the processor of the automatic frequency tuner AFTL stores the frequency set point of the RF signal 144 in a memory device of the automatic frequency tuner AFTL.

[0116] The processor 118 generates a trigger signal 142 and transmits it to the digital signal processor DSPL via the transmission cable 132. The processor 118 also transmits the trigger signal 142 to the digital signal processor DSPH via the transmission cable 134.

[0117] Upon receiving the trigger signal 142, the digital signal processor DSPL sends an instruction to the parameter controller PRL and the automatic frequency tuner AFTL. Upon receiving an instruction from the digital signal processor DSPL, the parameter controller PRL generates a parameter command signal based on the parameter set point of the RF signal 144 and sends the parameter command signal to the driver DRVRL. Similarly, upon receiving an instruction from the digital signal processor DSPL, the automatic frequency tuner AFTL generates a frequency command signal based on the frequency set point of the RF signal 144 and sends the frequency command signal to the driver DRVRL. Upon receiving the parameter command signal and the frequency command signal, the driver DRVRL generates a current signal and sends the current signal to the RF power supply 502. Upon receiving the current signal, the RF power supply 502 generates an LF RF signal 144 having a frequency set point and a parameter set point, and outputs the LF RF signal 144 to the RF cable RFC1 at the output O1.

[0118] Similarly, upon receiving recipe signal 140, digital signal processor DSPH transmits parameter set points for RF signal 146 to power controller PRH and transmits a frequency set point for RF signal 146 to automatic frequency tuner AFTH. The processor of power controller PRH stores the parameter set points for HF RF signal 146 in a memory device of power controller PRH, and the processor of automatic frequency tuner AFTH stores the frequency set point for HF RF signal 1446 in a memory device of automatic frequency tuner AFTH.

[0119] Further, upon receiving the trigger signal 142, the digital signal processor DSPH sends instructions to the parameter controller PRH and the automatic frequency tuner AFTH. Upon receiving an instruction from the digital signal processor DSPH, the parameter controller PRH generates a parameter command signal based on the parameter set point of the HF RF signal 146 and sends the parameter command signal to the driver DRVRH. Similarly, upon receiving an instruction from the digital signal processor DSPH, the automatic frequency tuner AFTH generates a frequency command signal based on the frequency set point of the HF RF signal 146 and sends the frequency command signal to the driver DRVRH. Upon receiving the parameter command signal and the frequency command signal, the driver DRVRH generates a current signal and sends the current signal to the RF power supply 504. Upon receiving the current signal, the RF power supply 504 generates the HF RF signal 146 having the frequency set point and the parameter set point, and outputs the HF RF signal 146 to the RF cable RFC2 at the output O2.

[0120] Upon generating the LF RF signal 144, the processor 118 determines to change variable values ​​(e.g., change parameter values, or change frequency values, or a combination thereof) during cycles M of the LF RF signal 144 to achieve the predetermined sheath width PSW, where M is a positive integer. For example, after generating the LF RF signal 144, the processor 118 determines to change a parameter set point, or a frequency set point, or a combination thereof, of the LF RF signal 144 provided in the recipe signal 138. As another example, after generating the LF RF signal 144, the processor 118 determines to change a parameter, or a frequency, or a combination thereof, of the LF RF signal 144 during a cycle (e.g., cycle 1, cycle 2, or cycle 3) of the LF RF signal 144.

[0121] The processor 118 transmits the modified variable value of the LF RF signal 144 to the digital signal processor DSPL through the transmission cable 132 during cycle M of the LF RF signal 144. Upon receiving the modified variable value of the LF RF signal 144 during cycle M of the LF RF signal 144, the digital signal processor DSPL determines whether the modified variable value includes a modification of the frequency of the LF RF signal 144, or a modification of a parameter of the LF RF signal 144, or both. Upon determining that the modified variable value of the LF RF signal 144 includes a modification of a parameter of the LF RF signal 144 during cycle M of the LF RF signal 144, the digital signal processor DSPL transmits the modified parameter value of the LF RF signal 144 to the parameter controller PRL. Also, in response to determining that the modified variable value of the LF RF signal 144 includes a modification of the frequency of the LF RF signal 144 during cycle M of the LF RF signal 144, the digital signal processor DSPL transmits the modified frequency value of the LF RF signal 144 to the automatic frequency tuner AFTL.

[0122] Upon receiving a modified parameter value of the LF RF signal 144 during cycle M of the LF RF signal 144, a processor of the parameter controller PRL generates a modified parameter command signal during cycle M based on the modified parameter value and sends the modified parameter command signal to the driver DRVRL. Also, upon receiving a modified frequency value of the LF RF signal 144 during cycle M of the LF RF signal 144, a processor of the automatic frequency tuner AFTL generates a modified frequency command signal during cycle M based on the modified frequency value and sends the modified frequency command signal to the driver DRVRL.

[0123] In response to receiving the modified parameter command signal, or the modified frequency command signal, or both, during cycle M of the LF RF signal 144, the driver DRVRL generates a modified current signal during cycle M based on the modified parameter command signal, or the modified frequency command signal, or a combination thereof, and transmits the modified current signal to the RF power supply 502. When the modified current signal is received during cycle M, the RF power supply 502 outputs an adjusted LF RF signal by modifying a parameter, or a frequency, or a combination thereof, of the LF RF signal 144 to achieve the predetermined sheath width PSW.

[0124] Also, upon generating HF RF signal 146, processor 118 determines a modified variable value (e.g., modified parameter value or modified frequency value) during bin N of cycle M of parameters of LF RF signal 144 to achieve the predetermined sheath width PSW (bin N being one of bins 1-p). For example, after generating HF RF signal 146, processor 118 determines a modified parameter set point, or a modified frequency set point, or a combination thereof, of HF RF signal 146 provided in recipe signal 140 during bin 1 of cycle 1 of parameters of LF RF signal 144. As another example, after generating HF RF signal 146, processor 118 determines a modified parameter, or a modified frequency, or a combination thereof, of HF RF signal 146 during bin N of cycle M of parameters of LF RF signal 144.

[0125] The processor 118 transmits the modified variable value of the HF RF signal 146 to the digital signal processor DSPH through the transmission cable 134 during bin N of the cycle M of the parameter of the LF RF signal 144. Upon receiving the modified variable value of the HF RF signal 146 during bin N of the LF RF signal 144, the digital signal processor DSPH determines whether the modified variable value includes a modification of the frequency of the HF RF signal 146, or a modification of the parameter of the HF RF signal 146, or both. Upon determining that the modified variable value of the HF RF signal 146 during bin N of the LF RF signal 144 includes a modification of the parameter of the HF RF signal 146, the digital signal processor DSPH transmits the modified parameter value of the HF RF signal 146 to the corresponding parameter controller PRbinN. The parameter controller PRbinN is one of the parameter controllers PRbin1 to PRbinp. For example, upon determining that the modified variable value of the HF RF signal 146 during bin (p-1) of the LF RF signal 144 includes a modified parameter of the HF RF signal 146, the digital signal processor DSPH transmits to the parameter controller PRbin(p-1) the modified parameter value of the HF RF signal 146. As another example, upon determining that the modified variable value of the HF RF signal 146 during bin p of the LF RF signal 144 includes a modified parameter of the HF RF signal 146, the digital signal processor DSPH transmits to the parameter controller PRbinp the modified parameter value of the HF RF signal 146.

[0126] Also, in response to determining that the modification variable value of the HF RF signal 146 during bin N of the LF RF signal 144 includes a modification of the frequency of the HF RF signal 146, the digital signal processor DSPL transmits the modified frequency value of the HF RF signal 146 to the corresponding automatic frequency tuner AFTbinN. The automatic frequency tuner AFTbinN is one of the automatic frequency tuners AFTbin1 to AFTbinp. For example, upon determining that the modification variable value of the HF RF signal 146 during bin (p-1) of the LF RF signal 144 includes a modification of the frequency of the HF RF signal 146, the digital signal processor DSPH transmits the modified frequency value of the HF RF signal 146 to the automatic frequency tuner AFTbin(p-1). As another example, upon determining that the modification variable value of the HF RF signal 146 during bin p of the LF RF signal 144 includes a modification of a parameter of the HF RF signal 146, the digital signal processor DSPH transmits the modified frequency value of the HF RF signal 146 to the automatic frequency tuner AFTbinp.

[0127] Upon receiving a modified parameter value for the HF RF signal 146 during bin N of the LF RF signal 144, a processor in the automatic frequency tuner AFTbinN generates a modified parameter command signal during bin N based on the modified parameter value and transmits the modified parameter command signal to the driver DRVRH. Also, upon receiving a modified frequency value for the HF RF signal 146 during bin N of the LF RF signal 144, a processor in the automatic frequency tuner AFTbinN generates a modified frequency command signal during bin N based on the modified frequency value and transmits the modified frequency command signal to the driver DRVRH.

[0128] In response to receiving the modified parameter command signal, or the modified frequency command signal, or both, during bin N of the LF RF signal 144, the driver DRVRH generates a modified current signal during bin N based on the modified parameter command signal, or the modified frequency command signal, or a combination thereof, and transmits the modified current signal to the RF power supply 504. When the modified current signal is received during bin N, the RF power supply 504 outputs an adjusted HF RF signal by modifying a parameter, or a frequency, or a combination thereof, of the HF RF signal 146 to achieve the predetermined sheath width PSW.

[0129] In one embodiment, instead of a driver, a driver amplifier system is used in the RF generator. The driver amplifier system includes a driver and an amplifier. The driver is connected to the amplifier. The modified current signal is sent from the driver to the amplifier. The amplifier amplifies (e.g., increases the magnitude of) the modified current signal to output an amplified current signal. The amplifier sends the amplified current signal to an RF power supply connected to the amplifier. The RF power supply generates an RF signal based on the amplified current signal.

[0130] It should be noted that the above embodiments are described with respect to the plasma sheath width of the bottom plasma sheath 130. In some embodiments, the above embodiments apply to other plasma sheath characteristics (e.g., capacitance and voltage) as well. For example, instead of the predetermined sheath width PSW, a predetermined capacitance of the bottom plasma sheath 130, or a predetermined voltage of the bottom plasma sheath 130, or a predetermined density of the bottom plasma sheath 130 is used.

[0131] The embodiments described herein may be practiced with a variety of computer system configurations, including handheld hardware units, microprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, etc. The embodiments may also be practiced in distributed computing environments where tasks are performed by remote processing hardware that is linked through a network.

[0132] In some embodiments, the controller is part of a system that may be part of the examples described above. Such systems include semiconductor processing equipment with processing tools, chambers, processing platforms, and / or specific processing components (wafer pedestals, gas flow systems, etc.). These systems are integrated with electronics to control pre-, during, and post-processing operations of semiconductor wafers or substrates. By electronics is meant a "controller" that can control various components or sub-components of the system. The controller is programmed to control the processes disclosed herein, including supply of process gases, temperature settings (e.g., heating and / or cooling), pressure settings, vacuum settings, power settings, RF generator settings, RF matching circuit settings, frequency settings, flow settings, fluid supply settings, position operation settings, wafer loading and unloading from tools and other transport tools and / or load locks connected or coupled to the specific system, depending on the processing requirements and / or type of system.

[0133] In general, in various embodiments, a controller is defined as an electronic device having various integrated circuits, logic, memory, and / or software that receive instructions, issue instructions, control operations, enable cleaning operations, enable endpoint measurements, etc. Integrated circuits include chips in firmware format that store program instructions, chips defined as digital signal processors (DSPs), ASICs, PLDs, and / or one or more microprocessors or microcontrollers that execute program instructions (e.g., software). Program instructions are instructions communicated to the controller in the form of various individual settings (or program files) that define parameters, elements, variables, etc. for performing a particular process on or for a semiconductor wafer or for a system. In some embodiments, the program instructions are part of a recipe defined by a process engineer to accomplish one or more processing steps in the manufacture of one or more layers, materials, metals, oxides, silicon, silicon dioxide, surfaces, circuits, and / or wafer dies.

[0134] In some embodiments, the controller is part of or connected to a computer that is integrated with or connected to the system, or otherwise networked or combined with the system. For example, the controller is in the "cloud" that enables remote access of wafer processing, or is all or part of a fab host computer system. The computer enables remote access to the system to monitor the progress of manufacturing operations, review the history of past manufacturing operations, review trends or performance metrics from multiple manufacturing operations, and modify parameters of a current process or set up processing steps following a current process, or initiate a new process.

[0135] In some examples, a remote computer (e.g., a server) provides a process recipe to the system over a network, including a local network or the Internet. The remote computer includes a user interface that allows entry or programming of parameters and / or settings that are then communicated to the system from the remote computer. In some examples, the controller receives instructions in the form of data that specify parameters, factors, and / or variables of each processing step performed during one or more operations. It should be understood that the parameters, factors, and / or variables are specific to the type of process being performed and the type of tool the controller is configured to connect to or control. Thus, as described above, a controller may be distributed, for example, by including one or more separate controllers networked together and working together toward a common purpose, such as the process and control described herein. An example of a controller distributed for such purposes includes one or more integrated circuits of a chamber that are located remotely (e.g., at the platform level or as part of a remote computer) and communicate with one or more integrated circuits that work together to control the process in the chamber.

[0136] Without being limiting, in various embodiments, exemplary systems to which the methods may be applied include plasma etch chambers or modules, deposition chambers or modules, spin rinse chambers or modules, metal plating chambers or modules, cleaning chambers or modules, bevel edge etch chambers or modules, physical vapor deposition (PVD) chambers or modules, chemical vapor deposition (CVD) chambers or modules, atomic layer deposition (ALD) chambers or modules, atomic layer etch (ALE) chambers or modules, ion implantation chambers or modules, track chambers or modules, and any other semiconductor processing systems related to or used in the fabrication and / or manufacturing of semiconductor wafers.

[0137] It is further noted that in some embodiments, the above operations apply to several types of plasma chambers, such as, for example, inductively coupled plasma (ICP) reactors, transformer coupled plasma chambers, plasma chambers including conductor tools, dielectric tools, plasma chambers including electron cyclotron resonance (ECR) reactors, etc. For example, one or more RF generators are connected to an inductor in an ICP reactor. Examples of inductor shapes include a solenoid, a dome coil, a flat coil, etc.

[0138] As described above, depending on the processing steps being performed by the tool, the host computer communicates with one or more of other tool circuits or modules, other tool components, cluster tools, other tool interfaces, adjacent tools, nearby tools, tools installed throughout the factory, a main computer, another controller, or tools used to transport materials to and from tool locations and / or load ports in a semiconductor manufacturing factory.

[0139] With the above embodiments in mind, it should be understood that some embodiments employ various computer-implemented operations on data stored in computer systems. These operations are physical manipulations of physical quantities. The operations described herein that form part of the embodiments are usefully machine operations.

[0140] Some embodiments also relate to a hardware unit or apparatus for performing these operations, which is specially made for a special purpose computer. When a computer is defined as a special purpose computer, it operates for a special purpose, yet performs other processes, program execution, or routines that are not part of the special purpose.

[0141] In some embodiments, the operations may be processed by a computer selectively activated or configured by one or more computer programs stored in a computer memory, cache, or obtained over a computer network. When data is obtained over a computer network, the data may be processed by other computers on the computer network (e.g., cloud computing resources).

[0142] One or more embodiments may also be created as computer readable code on a non-transitory computer readable medium. The non-transitory computer readable medium is any data storage hardware unit (e.g., storage device, etc.) that stores data that is subsequently read into a computer system. Examples of non-transitory computer readable media include hard drives, network attached storage devices (NAS), ROM, RAM, compact disk ROM (CD-ROM), recordable CD (CD-R), rewritable CD (CD-RW), magnetic tape, and other optical and non-optical data storage hardware units. In some embodiments, the non-transitory computer readable medium includes computer readable tangible media distributed in a networked computer system such that the computer readable code is stored and executed in a distributed manner.

[0143] Although the method operations above have been described in a particular order, it should be understood that in various embodiments, other preparatory operations are performed between the operations, or the method operations may be adjusted to occur at slightly different times, or distributed in a system that allows them to occur at various intervals, or performed in an order different than that described above.

[0144] It is further noted that in embodiments, one or more features of the above-described embodiments may be combined with one or more features of other embodiments also described above without departing from the scope of the various embodiments described in this disclosure.

[0145] It should also be noted that in embodiments, one or more features of the above examples may be combined with one or more features of other examples similarly described above without departing from the scope of the various embodiments described in this disclosure.

[0146] It should also be noted that in an embodiment, one or more features of the above examples may be combined with one or more features of the embodiments described above without departing from the scope of the various embodiments described in this disclosure.

[0147] Although the above embodiments have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications can be practiced within the scope of the appended claims. The present embodiments are therefore to be regarded as illustrative and not restrictive, and are not to be limited to the details set forth herein.

Claims

1. 1. A method for controlling plasma sheath characteristics, comprising: determining a first value of the plasma sheath property of a plasma sheath formed in a plasma chamber; determining whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic; modifying a parameter of a radio frequency (RF) generator connected to the plasma chamber through an impedance matching circuit upon determining that the first value is not within the predetermined range from the set value, the modifying the parameter of the RF generator being performed until the first value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; A method comprising:

2. 2. The method of claim 1 , the RF generator is a high frequency RF generator, the impedance matching circuit is connected to the high frequency RF generator and a low frequency RF generator, the low frequency RF generator having an operating frequency lower than an operating frequency of the high frequency RF generator, and the method further comprises: determining a second value of the plasma sheath property associated with the plasma sheath; determining whether the second value of the plasma sheath characteristic is within the predetermined range from a set value of the plasma sheath characteristic; and upon determining that the second value is not within the predetermined range from the set value, altering the variable of the RF generator, the altering being performed until the second value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; The method of claim 1, wherein the first value is determined based on a first measurement value measured during a first bin of an operating cycle of the low frequency RF generator and the second value is determined based on a second measurement value measured during a second bin of the operating cycle of the low frequency RF generator.

3. 3. The method of claim 2, A method in which the first value and the second value are determined based on an optical signal detected by an optical sensor, the plasma chamber having a window, and the optical signal is detected by the optical sensor through the window.

4. 3. The method of claim 2, The method of claim 1, wherein the first and second values ​​are determined based on a voltage signal measured by a sensor connected to an RF transmission line connecting an output of the impedance matching circuit to an input of the plasma chamber, the RF transmission line connecting the output of the impedance matching circuit to an input of the plasma chamber, and the sensor connected closer to the input of the plasma chamber than the output of the impedance matching circuit.

5. 2. The method of claim 1 , The method, wherein the plasma sheath characteristic is related to a width of the plasma sheath.

6. 6. The method of claim 5, The method of claim 1, wherein the first value of the plasma sheath property is determined based on measurements taken during bins of an operating cycle of the low frequency RF generator.

7. 6. The method of claim 5, The method, wherein the first value of the plasma sheath characteristic is determined based on measurements taken during a bin of an operating cycle of the low frequency RF generator, and the modifying step is performed during the bin of the operating cycle of the low frequency RF generator.

8. 6. The method of claim 5, The method, wherein the first value of the plasma sheath characteristic is determined based on measurements taken during a bin of a first operating cycle of the low frequency RF generator, and the modifying step is performed during a corresponding bin of a second operating cycle of the low frequency RF generator.

9. 9. The method of claim 8, The method, wherein the second cycle follows the first cycle.

10. 2. The method of claim 1 , The method, wherein the plasma sheath characteristic is the width of the plasma sheath or the capacitance of the plasma sheath.

11. 2. The method of claim 1 , The method, wherein the variable is power, or voltage, or frequency.

12. 1. A computer system for controlling plasma sheath characteristics, comprising:

1. A processor comprising: determining a first value of the plasma sheath property of a plasma sheath formed in a plasma chamber; determining whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic; a processor configured to, upon determining that the first value is not within the predetermined range from the set value, modify a variable of a radio frequency (RF) generator coupled to the plasma chamber through an impedance matching circuit, the variable of the RF generator being modified until the first value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; a storage device coupled to the processor and configured to store the setting value; A computer system comprising:

13. 13. The computer system of claim 12, the RF generator is a high frequency RF generator, the impedance matching circuit is connected to the high frequency RF generator and a low frequency RF generator, the low frequency RF generator having an operating frequency lower than an operating frequency of the high frequency RF generator, and the processor: determining a second value of the plasma sheath property; determining whether the second value of the plasma sheath characteristic is within the predetermined range from the set value of the plasma sheath characteristic; configured to vary the parameter of the RF generator upon determining that the second value is not within the predetermined range from the set value, and the parameter of the RF generator is varied until the second value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; The first value is determined based on a first measurement value measured during a first bin of a first operating cycle of the low frequency RF generator, and the second value is determined based on a second measurement value measured during a second bin of the first operating cycle of the low frequency RF generator.

14. 13. The computer system of claim 12, The plasma sheath characteristic is related to a width of the plasma sheath.

15. 15. The computer system of claim 14, The first value is determined based on measurements taken during a bin of an operating cycle of the low frequency RF generator, and the variable is modified during the bin of the operating cycle of the low frequency RF generator.

16. 15. The computer system of claim 14, The first value is determined based on measurements taken during a bin of a first operating cycle of the low frequency RF generator, and the variable is modified during a corresponding bin of a second operating cycle of the low frequency RF generator.

17. 1. A plasma system for controlling plasma sheath characteristics, comprising: a radio frequency (RF) generator configured to generate an RF signal, the RF generator having an output; an impedance matching circuit having an input and an output, the input of the impedance matching circuit being connected to the output of the RF generator; a plasma chamber having an input, the input of the plasma chamber being connected to the output of the impedance matching circuit; a controller coupled to the RF generator, determining a first value of the plasma sheath property of a plasma sheath formed in the plasma chamber; determining whether the first value of the plasma sheath characteristic is within a predetermined range from a set value of the plasma sheath characteristic; a controller configured to modify a variable of the RF generator upon determining that the first value is not within the predetermined range from the set value, the variable of the RF generator being modified until the first value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; A plasma system comprising:

18. 20. The plasma system of claim 17, the RF generator is a high frequency RF generator, the plasma system further comprises a low frequency RF generator, the impedance matching circuit is connected to the low frequency RF generator, the low frequency RF generator has an operating frequency lower than an operating frequency of the high frequency RF generator, and the controller determining a second value of the plasma sheath property; determining whether the second value of the plasma sheath characteristic is within the predetermined range from the set value of the plasma sheath characteristic; configured to vary the variable of the RF generator upon determining that the second value is not within the predetermined range from the set value, the variable being varied until the second value of the plasma sheath characteristic is determined to be within the predetermined range from the set value; The first value is determined based on a first measurement value measured during a first bin of a first operating cycle of the low frequency RF generator, and the second value is determined based on a second measurement value measured during a second bin of the first operating cycle of the low frequency RF generator.

19. 20. The plasma system of claim 17, A plasma system, wherein the plasma sheath characteristic is related to a width of the plasma sheath, the first value being determined based on measurements taken during a bin of an operating cycle of the low frequency RF generator, and the variable being modified during the bin of the operating cycle of the low frequency RF generator.

20. 20. The plasma system of claim 17, The first value is determined based on measurements taken during a bin of an operating cycle of the low frequency RF generator, and the variable is modified during a corresponding bin of a second operating cycle of the low frequency RF generator, the second operating cycle of the low frequency RF generator being consecutive to the first operating cycle.

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