Apparatus and method for waveform generation

FIELD OF THE DISCLOSURE [0001] Embodiments of the present disclosure relate generally to systems used in semiconductor device manufacturing. In particular, embodiments of the present disclosure relate to plasma processing systems used to process substrates.

[0002] Reliably producing high aspect ratio features is one of the key technological challenges for next generation semiconductor devices. One method of forming high aspect ratio features uses a plasma assisted etching process to bombard material formed on the surface of a substrate through openings formed in a patterned mask layer formed on the substrate surface.

[0003] As technology nodes progress toward 2 nm, fabrication of smaller features with higher aspect ratios requires atomic precision for plasma processing. In the semiconductor equipment industry, ion energy control has always been difficult in etching processes in which plasma ions play a major role. In a typical plasma-assisted etching process, a substrate is placed on an electrostatic chuck (ESC) located in a processing chamber, a plasma is generated above the substrate, and ions are accelerated from the plasma across a plasma sheath (i.e., an electron-depleted region generated between the plasma and the surface of the substrate) toward the substrate. Conventional RF substrate biasing methods that use sinusoidal RF waveforms to excite the plasma and generate the plasma sheath have not been able to desirably generate these smaller device feature sizes. Recently, it has been discovered that the supply of high-voltage DC pulses to one or more electrodes in the processing chamber can be useful in desirably controlling the plasma sheath generated above the surface of the substrate.

[0004] However, it is difficult to generate high voltage pulses with fast rise and/or fall times. For example, to achieve fast rise and/or fall times (e.g., <2.5 μs) in high voltage pulses (e.g., >5 kV), the slope of the pulse rise and/or fall must be very steep (e.g., 10 V/s). Such steep rise and/or fall times are very difficult, especially in circuits driving small capacitive loads. Such pulses can be particularly difficult to generate in a compact manner using standard electrical components, and/or with pulses having variable pulse widths, voltages, and repetition rates, and/or in applications with capacitive loads, such as the generation of plasma.

[0005] Therefore, there is a need in the art for a pulsed voltage supply and biasing method that enables the completion of a desired plasma-assisted process on a substrate.

[0006] Embodiments provided herein broadly include apparatus, eg, plasma processing systems, and methods for plasma processing a substrate in a processing chamber.

[0007] Some embodiments are directed to a waveform generator. The waveform generator generally includes a first voltage stage having a first voltage source, a first switch, and a second switch, where a first terminal of the first voltage source is coupled to a first terminal of the first switch and a second terminal of the first voltage source is coupled to a first terminal of the second switch. The waveform generator also includes a current stage coupled to a common node between the first switch and the second terminal of the second switch. The current stage includes a current source and a third switch coupled to the current source.

[0008] Some embodiments are directed to a method for waveform generation. The method generally includes incorporating a first voltage source in an output current path of a waveform generator by controlling a plurality of switches during a first mode of operation, and incorporating a current source in the output current path by controlling the plurality of switches during a second mode of operation. The plurality of switches includes a first switch, a second switch, and a third switch coupled in parallel with the current source. A first terminal of the first voltage source is coupled to a first terminal of the first switch. A second terminal of the first voltage source is coupled to a first terminal of the second switch. The third switch is coupled to a common node between the first switch and the second terminal of the second switch.

Some embodiments are directed to an apparatus for waveform generation. The apparatus generally includes a memory and one or more processors coupled to the memory. The memory and the one or more processors are configured to: incorporate a first voltage source in an output current path of the waveform generator by controlling a plurality of switches during a first mode of operation; and incorporate a current source in the output current path by controlling the plurality of switches during a second mode of operation. The plurality of switches includes a first switch, a second switch, and a third switch coupled in parallel with the current source. A first terminal of the first voltage source is coupled to a first terminal of the first switch. A second terminal of the first voltage source is coupled to a first terminal of the second switch. The third switch is coupled to a common node between the first switch and the second terminal of the second switch.

[0010] So that the above-mentioned features of the present disclosure can be understood in detail, a more particular description of the present disclosure briefly summarized above can be made by reference to embodiments, some of which are illustrated in the accompanying drawings. It should be noted, however, that the accompanying drawings depict only exemplary embodiments and therefore should not be considered as limiting the scope of the present disclosure, as other equally effective embodiments may be permitted.

<figref num="1">1 is a schematic cross-sectional view of a processing system according to one or more embodiments configured to perform the methods described herein.</figref><figref num="2">3 shows stray capacitances and substrate support capacitances associated with a processing chamber;</figref><figref num="3">[0013] Figures 3A, 3B, and 3C show example voltage waveforms for plasma processing.</figref><figref num="4">[0014] FIG. 1 illustrates a pulse generator in accordance with certain embodiments of the present disclosure.</figref><figref num="5A">1 illustrates various modes of operation of a pulse generator in accordance with certain embodiments of the present disclosure.</figref><figref num="5B">FIG. 4 is a timing diagram illustrating the states of the switches of a pulse generator during various modes of operation in accordance with certain embodiments of the present disclosure.</figref><figref num="6">1 illustrates a charging circuit used to charge a capacitive element, in accordance with certain aspects of the present disclosure.</figref><figref num="7">[0018] FIG. 1 is a process flow diagram illustrating a method for waveform generation in accordance with certain embodiments of the present disclosure.</figref>

[0019] Certain aspects of the present disclosure are generally directed to techniques for generating a voltage waveform for a plasma processing system. During plasma processing of a substrate, a voltage waveform provided to an electrode disposed in a plasma processing chamber will typically be configured to include a sheath collapse phase and an ion current phase. The sheath collapse phase may be implemented by generating a positive voltage (e.g., 100 volts) that is used to collapse a sheath generated over a surface of a substrate disposed on a substrate support disposed in the processing chamber. During the ion current phase, ions in the processing chamber may begin to flow by generating a negative voltage (e.g., -1600 volts). In some embodiments, the voltage during the ion current phase of the waveform may have a ramp to implement current compensation, as described in more detail herein. The voltage waveform may be generated by selectively incorporating various voltage sources (e.g., capacitive elements) in the output current path of a waveform generator.

<u style="Single">Several embodiments of a plasma processing system</u>1 is a schematic cross-sectional view of a processing system 10 configured to perform one or more of the plasma processing methods described herein. In some embodiments, the processing system 10 is configured for a plasma-assisted etching process, such as a reactive ion etching (RIE) plasma processing. However, it should be noted that the embodiments described herein may also be used with processing systems configured for use in other plasma-assisted processes, such as plasma deposition processes, e.g., plasma enhanced chemical vapor deposition (PECVD) processes, plasma enhanced physical vapor deposition (PEPVD) processes, plasma enhanced atomic layer deposition (PEALD) processes, plasma treatment processes, or plasma-based ion implantation processes (e.g., plasma enhanced doping (PLAD) processes).

[0021] As shown, the processing system 10 is configured to form a capacitively coupled plasma (CCP). In that case, the processing chamber 100 includes an upper electrode (e.g., chamber lid 123) disposed within the processing space 129. The upper electrode faces a lower electrode (e.g., substrate support assembly 136) also disposed within the processing space 129. In a typical capacitively coupled plasma (CCP) processing system, a radio frequency (RF) source (e.g., RF generator 118) is electrically coupled to one of the upper or lower electrodes and provides an RF signal configured to ignite and sustain a plasma (e.g., plasma 101). In this configuration, a plasma is capacitively coupled to each of the upper and lower electrodes and is disposed within the processing region therebetween. Typically, the other of the upper or lower electrode is coupled to ground or a second RF power source. One or more components of the substrate support assembly 136, such as the support base 107, are electrically coupled to a plasma generator assembly 163. The plasma generator assembly 163 includes an RF generator 118, and the chamber lid 123 is electrically coupled to ground. As shown, the processing system 10 includes a processing chamber 100, a support assembly 136, and a system controller 126.

[0022] The processing chamber 100 typically includes a chamber body 113. The chamber body 113 includes a chamber lid 123, one or more sidewalls 122, and a chamber base 124, which collectively define a processing space 129. The one or more sidewalls 122 and the chamber base 124 generally include a material sized and shaped to provide structural support for the elements of the processing chamber 100 and configured to withstand pressure and additional energy applied thereto. Meanwhile, the plasma 101 is generated in a reduced pressure environment that is maintained within the processing space 129 of the processing chamber 100 during processing. In one embodiment, the one or more sidewalls 122 and the chamber base 124 are formed of a metal, such as aluminum, an aluminum alloy, or a stainless steel alloy.

A gas inlet 128 disposed through the chamber lid 123 is used to supply one or more process gases to the process space 129 from a process gas source 119 in fluid communication with the process space 129. The substrate 103 is loaded into and removed from the process space 129 through an opening (not shown) in one of the one or more sidewalls 122 that is sealed by a slit valve (not shown) during plasma processing of the substrate 103.

[0024] The system controller 126, also referred to herein as a process chamber controller, includes a central processing unit (CPU) 133, a memory 134, and support circuits 135. The system controller 126 is used to control the process sequence used to process the substrate 103, including the substrate bias method described herein. The CPU 133 is a general-purpose computer processor configured for use in an industrial setting to control the process chambers and sub-processors associated with the process chambers. The memory 134 described herein is generally non-volatile memory and may include random access memory, read-only memory, floppy or hard disk drives, or other suitable forms of digital storage (local or remote). The support circuits 135 are conventionally coupled to the CPU 133 and include cache, clock circuits, input/output subsystems, power supplies, and the like, and combinations thereof. Software instructions (programs) and data may be coded and stored in the memory 134 to instruct the processor in the CPU 133. The software programs (or computer instructions) readable by the CPU 133 in the system controller 126 specify which operations are executable by the components in the processing system 10.

[0025] Typically, a program is readable by the CPU 133 in the system controller 126 and includes code that, when executed by the processor (CPU 133), performs operations associated with the plasma processing schemes described herein. The program may include instructions that are used to control various hardware and electrical components in the processing system 10 to thereby perform various process operations and various process sequences used to implement the methods described herein. In one embodiment, the program includes instructions that are used to perform one or more of the operations described below in connection with FIG. 7.

[0026] The processing system may include a plasma generator assembly 163, a first PV source assembly 196 for establishing a first pulsed voltage (PV) waveform at the bias electrode 104, and a second PV source assembly 197 for establishing a second PV waveform at the edge control electrode 115. The first PV waveform or the second PV waveform may be generated using a waveform generator, as described in more detail herein with respect to Figures 4, 5A, and 5B. In some embodiments, the plasma generator assembly 163 provides an RF signal to the support base 107 (e.g., an electrode or cathode). The support base 107 may be used to generate (sustain and/or ignite) the plasma 101 in a processing region disposed between the substrate support assembly 136 and the chamber lid 123. In some embodiments, the RF generator 118 is configured to provide an RF signal having a frequency of 1 MHz or more, or about 2 MHz or more, such as about 13.56 MHz or more.

[0027] As described above, in some embodiments, the plasma generator assembly 163 includes the RF generator 118 and the RF generator assembly 160 and is generally configured to supply a desired amount of continuous wave (CW) or pulsed RF power at a desired substantially constant sinusoidal frequency to the support base 107 of the substrate support assembly 136 based on control signals provided from the system controller 126. During processing, the plasma generator assembly 163 is configured to supply RF power (e.g., an RF signal) to the support base 107 disposed proximate to the substrate support 105 and within the substrate support assembly 136. The RF power supplied to the support base 107 is configured to ignite and maintain a processing plasma 101 of a processing gas disposed within the process space 129.

[0028] In some embodiments, support base 107 is an RF electrode that is electrically coupled to RF generator 118 via RF matching circuit 162 and first filter assembly 161, both of which are disposed within RF generator assembly 160. First filter assembly 161 includes one or more electrical elements configured to substantially prevent current generated by the output of PV waveform generator 150 from flowing through RF power supply line 167 and damaging RF generator 118. First filter assembly 161 acts as a high impedance (e.g., high Z) to the PV signal generated from PV pulse generator P1 in PV waveform generator 150, thus blocking the flow of current to RF matching circuit 162 and RF generator 118.

[0029] In some embodiments, the RF generator assembly 160 and the RF generator 118 are used to ignite and sustain the processing plasma 101 using an electric field generated by a process gas disposed in the process space 129 and RF power (RF signal) provided by the RF generator 118 to the support base 107. The process space 129 is fluidly coupled to one or more dedicated vacuum pumps via a vacuum outlet 120. The one or more dedicated vacuum pumps maintain the process space 129 at sub-atmospheric pressure and evacuate the process gas and/or other gases from the process space 129. In some embodiments, a substrate support assembly 136 disposed in the process space 129 is disposed on a support shaft 138 that is grounded and extends through the chamber base 124. However, in some embodiments, the RF generator assembly 160 is configured to provide RF power to a bias electrode 104 disposed in the substrate support 105 relative to the support base 107.

[0030] As briefly mentioned above, the substrate support assembly 136 generally includes a substrate support 105 (e.g., an ESC substrate support) and a support base 107. In some embodiments, the substrate support assembly 136 may further include an insulator plate 111 and a ground plate 112, as further described below. The support base 107 is electrically insulated from the chamber base 124 by the insulator plate 111, and the ground plate 112 is interposed between the insulator plate 111 and the chamber base 124. The substrate support 105 is thermally coupled to and disposed on the support base 107. In some embodiments, the support base 107 is configured to regulate the temperature of the substrate support 105 and the substrate 103 disposed on the substrate support 105 during substrate processing.

[0031] Typically, the substrate support 105 is formed of a dielectric material (e.g., a bulk sintered ceramic material such as a corrosion resistant metal oxide or metal nitride material), for example, aluminum oxide (Al₂O₃), aluminum nitride (AlN), titanium oxide (TiO), titanium nitride (TiN), yttrium oxide (Y₂O₃), a mixture thereof, or a combination thereof. In embodiments herein, the substrate support 105 further comprises a bias electrode 104 embedded within its dielectric material. In some embodiments, one or more characteristics of the RF power used to maintain the plasma 101 in the processing region above the bias electrode 104 are identified and/or monitored by measuring an RF waveform established at the bias electrode 104.

[0032] In one configuration, the bias electrode 104 is a chucking pole that is used to secure (i.e., chuck) the substrate 103 to the substrate support surface 105A of the substrate support 105 and to bias the substrate 103 relative to the processing plasma 101 using one or more of the pulsed voltage biasing schemes described herein. Typically, the bias electrode 104 is formed of one or more conductive components, such as one or more metal meshes, foils, plates, or combinations thereof.

In some embodiments, the bias electrode 104 is electrically coupled to a clamping network 116. The clamping network 116 provides a chucking voltage, such as a static DC voltage between about -5000V and about 5000V, to the bias electrode 104 using an electrical conductor, such as a coaxial power supply line 106 (e.g., a coaxial cable). As described further below, the clamping network 116 includes bias compensation circuitry 116A, a DC power supply 155, and a blocking capacitor C, herein referred to as a blocking capacitor C.₅The bias compensation module also includes a blocking capacitor, C₅is disposed between the output of the pulse voltage (PV) waveform generator 150 and the bias electrode 104.

[0034] The substrate support assembly 136 may further include an edge control electrode 115. The edge control electrode 115 is disposed below the edge ring 114, surrounds the bias electrode 104, and/or is disposed at a distance from the center of the bias electrode 104. In general, in a processing chamber 100 configured to process a circuit board, the edge control electrode 115 is ring-shaped, made of a conductive material, and configured to surround at least a portion of the bias electrode 104. In some embodiments, such as shown in FIG. 1, the edge control electrode 115 is disposed within the region of the substrate support 105. In some embodiments, as shown in FIG. 1, the edge control electrode 115 includes a conductive mesh, foil, and/or plate disposed at a similar distance (i.e., in the Z direction) as the bias electrode 104 from the substrate support surface 105A of the substrate support 105. In some other embodiments, the edge control electrode 115 includes a conductive mesh, foil, and/or plate disposed on or within the region of the quartz tube 110. The quartz tube 110 surrounds at least a portion of the bias electrode 104 and/or the substrate support 105. Alternatively, in some other embodiments (not shown), the edge control electrode 115 is disposed within or coupled to an edge ring 114 that is disposed adjacent to the substrate support 105. In this configuration, the edge ring 114 is formed from a semiconductor or dielectric material (e.g., AlN, etc.).

The edge control electrode 115 may be biased using a different PV waveform generator than the PV waveform generator 150 used to bias the bias electrode 104. In some embodiments, the edge control electrode 115 may be biased using the PV waveform generator 150, which is also used to bias the bias electrode 104 by splitting a portion of the power to the edge control electrode 115. In one configuration, a first PV waveform generator 150 of a first PV source assembly 196 is configured to bias the bias electrode 104, and a second PV waveform generator 150 of a second PV source assembly 197 is configured to bias the edge control electrode 115.

[0036] A power supply line 157 electrically connects the output of the PV waveform generator 150 of the first PV source assembly 196 with the optional filter assembly 151 and the bias electrode 104. The following description primarily discusses the power supply line 157 of the first PV source assembly 196 used to couple the PV waveform generator 150 to the bias electrode 104, but the voltage supply line 158 of the second PV source assembly 197 coupling the PV waveform generator 150 to the edge control electrode 115 will include the same or similar components. The electrical conductor(s) in the various portions of the voltage supply line 157 include one or a combination of the following: (a) a coaxial cable, such as a flexible coaxial cable connected in series with a rigid coaxial cable; (b) insulated high voltage corona resistant circuit wire; (c) bare wire; (d) metal rods; (e) electrical connectors; or (f) any combination of the electrical elements of (a)-(e). Optional filter assembly 151 includes one or more electrical elements configured to substantially prevent current generated by the output of RF generator 118 from flowing through power supply line 157 and damaging PV waveform generator 150. Optional filter assembly 151 acts as a high impedance (e.g., high Z) to the RF signal generated by RF generator 118, thus blocking the flow of current to PV waveform generator 150.

[0037] The second PV source assembly 197 includes a clamping network 116, whereby the bias applied to the edge control electrode 115 may be configured similarly to the bias applied to the bias electrode 104 by the clamping network 116 coupled in the first PV source assembly 196. Applying similarly configured PV waveforms and clamping voltages to the bias electrode 104 and edge control electrode 115 can help improve the uniformity of the plasma across the surface of the substrate being processed, and therefore improve the results of the plasma treatment process.

[0038] In some embodiments, the process chamber 100 further includes a quartz tube 110 or collar that at least partially circumscribes portions of the substrate support assembly 136 to prevent the substrate support 105 and/or support base 107 from contacting corrosive process gases or plasmas, cleaning gases or plasmas, or by-products thereof. Typically, the quartz tube 110, the insulator plate 111, and the ground plate 112 are circumscribed by a liner 108. In some embodiments, a plasma screen 109 is disposed between the cathode liner 108 and the sidewall 122 to prevent plasma generation in a space below the plasma screen 109 between the liner 108 and the one or more sidewalls 122.

<u style="Single">Exemplary representative circuit of a processing chamber</u>[0039] Figure 2 shows the stray capacitance and escape capacitance associated with the process chamber._stray) represents the capacitance between the electrode of the processing chamber and ground, and the substrate support capacitance 202 is also referred to herein as the electrostatic chuck capacitance (C_Esc), which represents the capacitance between the bias electrode 104 and the substrate support surface 105A. As shown, C_Escis the output node (U_out) and a load represented by a resistive element 206. As described in more detail herein,_load) so that the voltage pulse at C_EscThe voltage across and C_strayThe voltage across (for example, U_outThe voltage at the_strayThe current (for example, the compensation current (I_comp)) is C_EscThe load current (I_load) to C_strayand C_EscThe output current (I_out) is I_loadand I_compIt is equal to the sum of Iout and Iload (1+Cstray/Cesc) and can be expressed by the following formula:

<u style="Single">Exemplary Voltage Waveforms for a Processing Chamber</u>[0040] Figure 3A illustrates a voltage waveform that may be established at an electrode disposed in a processing chamber, such as electrode 104 shown in Figure 1. The waveform includes two phases: an ion current phase and a sheath collapse phase. At the start of the ion current phase, a drop in the wafer voltage creates a high voltage sheath above the substrate, accelerating positive ions to the substrate 103. The positive ions place a positive charge on the substrate surface, tending to gradually raise the substrate voltage positive. When a square wave is applied, the ion current toward the substrate creates a positive gradient in the substrate voltage (e.g., U-shaped as shown in Figure 2)._loadAs shown in Figure 3C, the load (e.g., U_loadIn order to square the voltage pulse (at_out3B, a voltage gradient is applied to the electrostatic chuck capacitance element C_EscAt the electrode 104 during the ion current phase and at the electrostatic chuck capacitance C_EscThe ramping of U and U during this phase is generally called current compensation._load, which is used to generate a constant voltage seen at . The voltage difference between the start and end of the ion current phase determines the ion energy distribution function (IEDF) width. The larger the voltage difference, the wider the energy distribution of the ions and therefore the wider the IEDF width. To achieve monoenergetic ions and a narrower IEDF width, a current compensation operation is performed to flatten the substrate voltage waveform during the ion current phase. In some embodiments, the voltage waveform is set at a frequency (1/T_PIn some embodiments, the voltage waveform established at the electrodes may be provided with an ion current period (e.g., the length of an ion current step) and a waveform period T_P(e.g., the length of the sheath collapse phase plus the length of the ion current phase). The on-time is greater than 50%, or greater than 70%, such as 80% or more and 95% or less. In some embodiments, the voltage waveform has a period (T_P) (e.g., about 2.5 μs) and is repeated continuously in waveform bursts having burst durations between about 100 microseconds (μs) and about 10 milliseconds (ms). The PV waveform bursts may have a burst duty cycle that is between about 5% and 100%, e.g., between about 50% and about 95%. The duty cycle is the ratio of the burst period divided by the burst period plus the non-burst periods (i.e., no PV waveform is generated) that separate the burst periods. As shown, the sheath collapse phase has a T that may be about 200 ns._SHThe period may have a duration of .

[0041] 4 illustrates a pulse generator 400 (also referred to herein as a waveform generator) in accordance with certain embodiments of the present disclosure. As illustrated, the pulse generator 400 may include pulse capacitive elements 402, 404, 406, and 408 (labeled C1, C3, C4, and C6), as well as transistors 410, 412, 414, 416, 418, 420, and 422 (labeled transistors Q1, Q2, Q3, Q4, Q6, Q10, and Q12). The transistors 410, 412, 414, 416, 418, 420, and 422 (also referred to herein as switches) may be power transistors (e.g., metal oxide semiconductor field effect transistors (MOSFETs)) with parallel diodes (e.g., body diodes). The transistors 410, 412, 414, 416, 418, 420, 422 may be used to select a current path (also referred to as an output current path) for the pulse generator, as described in more detail herein. The capacitive elements 402, 404, 406, and 408 may function as voltage storage elements that may be charged using a charging circuit such as the circuit shown in Figure 6. The capacitive elements shown in Figures 4, 5A, and 6 effectively act as voltage sources. Although the exemplary pulse generator 400 shows capacitive elements for ease of understanding, any suitable voltage source may be used.

[0042] Resistive element 424 (labeled R1) represents the pulse generator's internal series resistance element coupled to load 426. Load 426 may be plasma generated in a plasma processing chamber and may be represented by capacitive element 428 (labeled C2) and resistive element 430 (labeled R2). As shown, capacitive element 402 and transistors 410, 412 form a first voltage stage 440, and capacitive element 404 and transistors 414, 416 form a second voltage stage 442. Pulse generator 400 also includes a current stage 444 having capacitive element 406, transistor 418, and inductive element 450, and a third voltage stage 446 having capacitive element 408 and transistors 420, 422. Although three voltage stages are implemented in pulse generator 400, aspects of the disclosure may implement one, two, or more than three voltage stages. In some embodiments of the pulse generator 400, one or more of the voltage stages may be duplicated one or more times, such as one configuration including a first voltage stage 440, two or more second voltage stages 442, a current stage 444, and a third voltage stage 446. In that case, the two or more second voltage stages 442 are connected in series between the first voltage stage 440 and the current stage 444.

As shown, each of the capacitive elements 402, 404, 406, and 408 may be charged to a particular voltage depending on the waveform being implemented. For example, the capacitive elements 402, 404, 406 may be charged to 800 volts and the capacitive element 408 may be charged to 100 volts. In some implementations, the capacitive elements 402, 404, 406, 408 may be charged to higher or lower voltages to implement different voltage levels for waveforms appropriate for different implementations. In some embodiments, each of the voltage stages 440, 442, 446, and the current stage 444 may have a modular design to facilitate replacement in the event of failure. The operation of the pulse generator 400 to generate the waveforms shown in FIG. 3A is described in more detail in conjunction with FIG. 5.

[0044] Figure 5A illustrates various modes of operation of pulse generator 400 according to certain embodiments of the present disclosure. The voltage magnitudes associated with the various modes of operation 502, 504, 506 and circuit elements illustrated in Figure 5A are intended to provide examples of voltages that may be established during generation of a pulse waveform and are not intended to limit the scope of the disclosure provided herein. Figure 5B is a timing diagram illustrating the state of each of transistors 410, 412, 414, 416, 418, 420, 422 (e.g., transistors Q1, Q2, Q3, Q4, Q6, Q10, and Q12). During the sheath collapse phase, U_outThe voltage at I may be set to 100 volts as an example. To transition from the sheath collapse phase to the ion current phase, transistors Q1, Q3, Q5, and Q7 may be turned on and transistors Q2, Q4, and Q6 may be turned off to implement a voltage drop from 100 volts to -1600 volts as shown by the mode of operation 502. Turning on transistors Q1, Q3, Q5, and Q7 and turning off transistors Q2, Q4, and Q6 effectively places capacitive elements 402, 404 in the output current path of the pulse generator as shown. In the mode of operation 502, I_outflows from ground through the capacitance elements C2 and C1, the transistor Q1, the capacitance element C3, and the transistors Q3, Q5, and Q7. The capacitance elements C1 and C3 are connected to the U_outThe voltage at U is set to -1600 volts (e.g., -800 volts from capacitive element C1 and -800 volts from capacitive element C3) during the ion current phase. Two voltage stages are used to implement the -1600 volts, with each voltage stage providing -800 volts, although a single voltage stage may also be used. For example, the capacitive elements of a single voltage stage may be set to -1600 volts during the ion current phase._outAs shown, during operation 502 mode, I_outflows across the parallel diode (e.g., body diode) of transistor Q5 and across transistor Q7 back to ground. When transistor Q5 is turned on, a current 560 flows in a loop from the drain to the source of transistor Q5 through capacitive element C4, inductive element L1, and so on.

[0045] U_outWhen the voltage at I reaches 1600 volts, the mode of operation 504 may be implemented. During the mode of operation 504, a current source implemented with capacitive element C4 and inductive element L1 may be incorporated into the output current path of the pulse generator 400. As shown, transistor Q5 may be turned off and I_outwill begin to flow across capacitive element C4 and inductive element L1 (e.g., instead of through the parallel diode of transistor Q5 during mode of operation 502). Capacitive element C4 and inductive element L1 are configured to slope U during the ion current phase for ion current compensation, as described in connection with FIG._outImplement a current source that effectively gradually reduces the voltage at U_outThe voltage at may be reduced to -1600 volts to -2400 volts.

[0046] U_outWhen the voltage at I reaches 2400 volts, a mode of operation 506 may be implemented. During the mode of operation 506, a capacitive element C6 may be incorporated into the output current path of the pulse generator 400. As shown, during the mode of operation 506, transistors Q1, Q3, and Q7 may be turned off and transistors Q2, Q4, Q5, and Q6 may be turned on. Thus, I_outflows through capacitive element C6, transistors Q6, Q5, Q4, Q2, and capacitive element C2. As explained, capacitive element C6 may be charged to 100 volts. Thus, the mode of operation 506, as explained in relation to FIG. 3A, operates in the sheath collapse phase with U_outIn other words, I_outflows in the opposite direction in the mode of operation 506 (e.g., during the sheath collapse phase) compared to the modes of operation 502, 504 (e.g., during the ion current phase), thereby implementing a positive voltage (e.g., 100 volts) during the sheath collapse phase and a negative voltage (e.g., between 1600 volts and 2400 volts) during the ion current phase.

FIG. 6 illustrates a charging circuit 600 used to charge a capacitive element 612 according to certain aspects of the disclosure. The capacitive element 612 may correspond to any one of the capacitive elements 402, 404, 406, and 408. In other words, for each of the capacitive elements 402, 404, 406, and 408, a charging circuit (e.g., similar to the charging circuit 600) may be implemented to charge the capacitive elements to their respective voltages as described herein. The charging circuit 600 may include an inverter 602 for converting a direct current (DC) voltage to an alternating current (AC) voltage. The AC voltage may be provided to a primary winding 606 of a transformer 604. The transformer may generate an AC voltage at a secondary winding 608 having a higher voltage than the AC voltage at the primary winding 606. For example, to charge the capacitive element 402, the AC voltage at the secondary winding 608 may have a peak voltage of 800 volts. The AC voltage on the secondary winding 608 may be provided to a rectifier 610 to produce a DC signal used to charge a capacitive element 612 .

7 is a process flow diagram illustrating a method 700 of waveform generation in accordance with certain embodiments of the present disclosure. The method 700 may be performed by a waveform generation system, including a waveform generator, such as the pulse generator 400, and/or control circuitry, such as the system controller 126.

[0049] At operation 702, the waveform generation system incorporates a first voltage source (e.g., capacitive element 402) in an output current path of a waveform generator (e.g., pulse generator 400) by controlling a plurality of switches during a first mode of operation (e.g., mode of operation 502). At operation 704, the waveform generation system incorporates current sources (e.g., inductive element 450 and capacitive element 406) in the output current path by controlling a plurality of switches during a second mode of operation (e.g., mode of operation 504).

[0050] In some embodiments, the plurality of switches includes a first switch (e.g., transistor 410 or transistor 414) and a second switch (e.g., transistor 412 or transistor 416). A first terminal of a first voltage source (e.g., capacitive element 402 or capacitive element 404) is coupled to a first terminal of the first switch, and a second terminal of the first voltage source is coupled to a first terminal of the second switch. In some embodiments, the plurality of switches also includes a third switch (e.g., transistor 418) coupled in parallel with the current source. The third switch may be coupled to a common node between the second terminals of the first switch and the second switch. In some embodiments, incorporating the first voltage source in the output current path may include closing the first switch, opening the second switch, and closing the third switch. Incorporating the current source in the output current path may include closing the first switch, opening the second switch, and opening the third switch.

[0051] In some embodiments, the waveform generation system incorporates a second voltage source (e.g., capacitive element 404) in the output current path by controlling a plurality of switches during a first mode of operation (e.g., a mode of operation 502). The plurality of switches may further include a fourth switch (e.g., transistor 414) and a fifth switch (e.g., transistor 416). A first terminal of the second voltage source may be coupled to a first terminal of the fourth switch, a second terminal of the second voltage source may be coupled to a first terminal of the fifth switch, and a common node between the second terminals of the fourth switch and the fifth switch may be coupled to the second switch (e.g., transistor 412) or the third switch (e.g., transistor 418). In some embodiments, incorporating the second voltage source in the output current path may include closing the fourth switch and opening the fifth switch.

[0052] In some embodiments, the waveform generation system also incorporates a third voltage source (e.g., capacitive element 408) in the output current path by controlling a plurality of switches during a third mode of operation (e.g., mode of operation 506). The plurality of switches may include a sixth switch (e.g., transistor 420) and a seventh switch (e.g., transistor 422). A first terminal of the third voltage source may be coupled to a first terminal of the sixth switch, a second terminal of the third voltage source may be coupled to a first terminal of the seventh switch, and a common node between the sixth switch and the seventh switch may be coupled to the third switch (e.g., transistor 418). In some embodiments, incorporating the third voltage source in the output current path may include closing the sixth switch and opening the seventh switch. During the first mode of operation and the second mode of operation, the sixth switch may be opened and the seventh switch may be closed. In some embodiments, the voltage associated with the first voltage source or the second voltage source (eg, 600 volts) is greater than the voltage associated with the third voltage source (eg, 100 volts).

[0053] The term "coupled" is used herein to refer to a direct or indirect coupling between two objects. For example, if object A is in physical contact with object B, and object B is in physical contact with object C, then objects A and C may be considered to be coupled to one another even though objects A and C are not in direct physical contact. For example, a first object may be coupled to a second object even though the first object is not in direct physical contact with the second object.

[0054] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, the scope of the disclosure being defined by the following claims.