High-power generator and method for supplying high-power pulses
The high-power generator system addresses voltage handling limitations in plasma processing by employing a network of low-power generators and switching units to deliver high-power pulses with fast transitions and reduced losses.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- TRUMPF HUETTINGER SP ZOO
- Filing Date
- 2023-02-28
- Publication Date
- 2026-06-04
Smart Images
Figure 0007869982000001 
Figure 0007869982000002 
Figure 0007869982000003
Abstract
Description
Technical Field
[0001] The present invention relates to a high-power (HP) generator configured to deliver pulsed high-power at high voltage values and / or high current to capacitive loads, particularly plasma processes. The present invention also relates to a method of supplying high-power pulses to a plasma process.
Background Art
[0002] A capacitive load means a load having a capacitive part, which means that the voltage rise at this load suggests high-current characteristics. The capacitive part can be, in such cases, at least 100 pF, preferably 200 pF or more, for example about 500 pF. This can be used as a load in, for example, a plasma process as a plasma processing application.
[0003] In some plasma processing applications such as etching and film formation, the supply of a high-voltage (HV), high-frequency (HF) rectangular asymmetric pulse voltage is required. The voltage value often exceeds the voltage handling capacity of individual semiconductor switches, especially when high-frequency operation is necessary.
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Patent Document 2
Patent Document 3
Patent Document 4
[0005] Summary of the Invention The objective of the present invention is to provide a high-power generator and a method for supplying high-power pulses to a plasma, which enable the supply of rapidly changing power (pulses) to a plasma process.
[0006] According to a first aspect, the present invention relates to a high-power (HP) generator configured to deliver high power with high voltage and / or high current values to a load, particularly a plasma process, which, • Some low-power (LP) generators, Each low-power (LP) generator includes an energy storage component, and during use, the energy storage component is charged to a predetermined value related to the energy storage component. • While each low-power (LP) generator is in use, it supplies an LP generator value corresponding to the value of the energy storage component built into each LP generator at its output. Several low-power generators, A coupling in which the LP generator is electrically connected such that the LP generator corresponds to the output value of the HP generator and, during use, a coupling value can be obtained at the output of the coupling that is higher than the LP generator value at one of the LP generator outputs in at least several states of the HP generator. A control unit configured to select the contribution of the LP generator to the HP generator's output value during power delivery of the HP generator in order to cause the rise and / or decay of pulses at the coupling output, Includes.
[0007] In one embodiment, the control unit may include a switching unit having a current-increasing capability of at least 10 A / μs. Such a high current-increasing capability makes it possible to charge a capacitive load sufficiently quickly.
[0008] In one embodiment, the control unit may include a switching unit capable of withstanding voltages of approximately 0.5 kV or more with a voltage change rate of 15 kV / μs or more. This may enable the realization of extreme pulses with extremely clear voltage transitions. This is a highly desirable feature for many applications, such as plasma processes, particularly semiconductor plasma processes.
[0009] According to one embodiment, the output of the coupling and / or the output of the HP generator (10) is a stepped waveform, in particular a true stepped waveform without a continuous slope, during use.
[0010] In one embodiment, the LP generators are activated sequentially within a single pulse. This means that not all LP generators are activated at once at the start of the pulse. For example, if the pulse length is 0.5 μs to 2 μs, the first LP generator or a stack of several LP generators is activated at the start of the pulse. After several tens or hundreds of nanometers, one or more additional LP generators are activated, thereby causing the pulse at the HP generator output to rise. Then, after another several tens or hundreds of nanometers, the next LP generator or stack of LP generators is additionally activated, causing the pulse to rise further. For pulse decay, one or more of the LP generators are similarly deactivated in stages. "Activating one or more LP generators" means that the control unit selects these LP generators to contribute to the HP generator output value during power delivery of the HP generator. In other words, the control unit may be configured to select the contribution of each LP generator to the HP generator output value multiple times within a single pulse at the HP generator output.
[0011] According to one embodiment, the coupling includes five or more electrically connected LP generators, particularly six or more LP generators, preferably ten or more, and most preferably fifteen or more LP generators. Such a large number of LP generators makes it possible to provide a true stepped waveform without a continuous gradient.
[0012] In one embodiment, the control unit is configured to select the contribution of the LP generator such that at least one amplitude step is less than 500V and / or a generator with a continuous gradient output is not required.
[0013] This solution eliminates the need for a generator that produces continuous gradients, making the HP generator more efficient.
[0014] In one embodiment, the coupling and control unit are configured to connect the LP generator by switching only.
[0015] In one embodiment, the control unit may be configured to select the contribution of the LP generator to the output value of the HP generator to form a step-line pulse waveform at the rising and / or falling edges of the pulse.
[0016] In one embodiment, the number of LP generators is large enough to form a step line pulse waveform at the output of the coupling, having a voltage rise and / or fall that is equal to or greater than the sum of the values of the multiple LP generators, and a step that corresponds to a value equal to or greater than the value of one or more of the LP generators.
[0017] According to one aspect, the charging energy of the LP generator is supplied through a transformer having a primary winding and a secondary winding for each LP generator, the secondary winding is connected to a rectifier, and each rectifier is connected to an energy storage component of the corresponding LP generator. The rectifier may include at least one semiconductor element such as a diode. The rectifier may include four diodes connected in the manner of a bridge rectifier. The rectifier and / or the transformer may be part of the generator.
[0018] In some cases, particularly each LP generator may have a corresponding transformer.
[0019] In some cases, particularly each transformer corresponding to one LP generator may have its own magnetic core.
[0020] In some cases, particularly each transformer corresponding to one LP generator may include a balance winding, preferably two balance windings.
[0021] This allows the AC supply current to flow freely along the transformer, supplying charge to the stage with load, i.e., the LP generator connected to the load, and preventing overcharging of the LP generators without load, i.e., the LP generators not connected to the load and thus not contributing to the output of the HP generator. The same concept can be used to supply power to the driver circuit.
[0022] One balance winding of one transformer may be connected to the balance windings of different transformers.
[0023] Thus, in some cases, particularly each transformer corresponding to one LP generator may be connected in an open-chain configuration.
[0024] Some of the primary windings, particularly each primary winding of the transformer corresponding to one LP generator, may be connected in series. This may mean that one wire is routed through all the transformers in series configured to receive the AC supply current.
[0025] High voltages and currents can be efficiently and effectively separated if each transformer has its own magnetic core and the only shared element is the primary winding, which can be fabricated from a single common wire.
[0026] Each transformer, also called a "stage" and corresponding to several, particularly one LP generator, can be implemented as a ring core transformer.
[0027] In one embodiment, a balancing circuit is connected between two LP generators, which includes a component that allows current to flow in only one direction, in particular a diode or a component that behaves like a diode. For example, several energy storage components, particularly the negative outputs of capacitors, of each LP generator are connected to the same potential as the preceding LP generator having such a balancing circuit. This may allow a switch in parallel with the load to be operated so that when the value of the energy storage component of the lower LP generator exceeds that of the higher LP generator, the charge from the energy storage component of the lower LP generator is transferred to the energy storage component of the higher LP generator. "Operated" means "switched on" or "switched to low impedance at its output." "Switch in parallel with the load" means a switching element that, when operated, disconnects the corresponding LP generator from nearby connected LP generators. In Figures 2a, 2b, 4, and 5, these are switching element S1. In Figure 3, these are switching element S2. A connecting switch means a switching element that, when operated, connects the corresponding LP generators. In Figures 2a, 2b, 4, and 5, these are switching elements S2. In Figure 3, these are switching elements S1. The energy storage component of the tallest LP generators accumulates excess charge from the entire stack. Therefore, these tallest LP generators can be controlled in such a way that they provide more power to the load than other LP generators in order to remove the excess charge, which is advantageous.
[0028] In one embodiment, damping circuits may be positioned between some, in particular all, of the LP generators in an open-chain configuration. The open-chain configuration means that such damping circuits are positioned between each LP generator and its adjacent, higher LP generator, rather than between the highest and lowest. The damping circuits may include resistors and / or inductors if the energy storage component is a capacitor. The damping circuits between LP generators may preferably be positioned in series with a balance circuit. The damping circuits may be used to eliminate vibrations on parasitic components of the charge handling path.
[0029] The entire HP generator can be directly liquid-cooled. This can be done using an immersion dielectric coolant, and the LP generator in particular is immersed in the dielectric coolant. All components of the HP generator, including the transformer, can be immersed in a dielectric coolant that provides high cooling capacity while reducing clearance and creepage requirements. This allows for better utilization of the safe operating area of all components, resulting in further miniaturization and reduced parasitic inductance and capacitance. This leads to a smaller overall HP generator and improved efficiency.
[0030] Some, and especially all, LP generators and control units may be housed in a single housing, at least partially, and at least together with the switching unit. Couplings may be housed in the same housing. Transformers and rectifiers may be housed in that housing. The housing may be waterproof. Dielectric coolant may flow through the housing, thereby improving cooling and isolation. Distances can be reduced, allowing for the use of shorter wire lengths, which reduces intrinsic inductance and thus makes the switching function faster. Temperature differences between different switches and other components and / or couplings of the LP generator can be reduced by this type of series fluid arrangement, thus allowing for improved balance.
[0031] In one embodiment, the control unit may be configured to sequentially select the contribution of an LP generator from among multiple LP generators. In particular, since each pulse sequence may begin with a different LP generator, all n LP generators will receive an equal load after n pulses. This leads to better load balancing between LP generator stacks and, therefore, a better balance of power losses between switching elements.
[0032] In one embodiment, the control unit may be configured to select the contribution of the LP generators (14, 16, 18) in such a manner that it reduces voltage overshoot at specific locations of the HP generator output and / or load, particularly in the substrate of the plasma process. This can be done, for example, by having the first step in the HP generator output be only a portion, particularly half, of the value of the second step, which may be the entire amount.
[0033] In one embodiment, at least one, preferably several, most preferably all, LP generators include an LP generator value limiting circuit, such as a voltage limiting circuit or a current limiting circuit. This may be an additional switching element with an additional resistor or an electrically controllable variable impedance with a power consumption unit. By using such a circuit, the HP generator becomes much more reliable.
[0034] In one embodiment, the control unit is configured to select the contribution of LP generators via galvanically isolated connections, particularly optical fiber connections or magnetic coupling. Each of several, preferably, LP generators may be selected by the control unit through the control of a switching unit. The connection between the data processor output and the switching unit may be achieved by optical fiber or magnetic coupling. This helps to switch all LP generators effectively, quickly, and without delay, for example, to form pulses with sharp voltage edges. Several switching elements of a single switching unit can be connected by a single galvanically isolated connection. Several LP generators with corresponding switching units can be grouped together, configured such that this group is always operated or deactivated only together. Then, for such a group, it is possible to use only one galvanically isolated connection. Reducing the number of galvanically isolated connections reduces costs and makes the LP generators more reliable due to fewer components at risk of failure. An advantage is that each switch has its own galvanically isolated connection. However, several transmitters, if grouped together, can be controlled by a single control signal. This reduces the number of I / O ports required.
[0035] In one embodiment, the voltages on all LP generators, also called "stages," are equal even if their loads are unbalanced. Furthermore, primary-secondary winding isolation (ground-HV) is easily achieved by positioning the primary path near the center of the ring core transformer and keeping the secondary winding near the core, away from the primary winding. A balancing winding can be placed near the secondary winding, as this only needs to provide stage-to-stage voltage isolation.
[0036] The phrase "The control unit is configured to select the contribution of the LP generator to the output value of the HP generator" should be understood as the capability and configuration of the HP generator circuit, in combination with the control unit's program, to allow the HP generator to select a choice of LP generators having their respective LP generator values to supply its contribution to the output value of the HP generator.
[0037] The control unit may include, for example, a data processor implemented as an embedded microphone processor having a program memory implemented as, for example, a non-volatile data memory and a data memory implemented as, for example, a volatile data memory. Such a control unit typically includes a data interface for incoming data, such as measurement data, control input data, and others, as well as outgoing data, such as control signals, information signals, warning signals, and others. Such a control unit typically includes a data calculation unit, a data comparison function, and similar circuits that enable calculating new data from input data, finding decisions regarding the input and / or calculated data, and outputting relevant data signals for controlling other electrical circuits of the HP generator, etc.
[0038] The output of the coupling can be the output of the HP generator. However, it is also possible to connect some small additional voltage or current waveform shaping components between the output of the coupling and the output of the HP generator. These small additional voltage or current waveform shaping components can be filters or small voltage, current, or power sources of much smaller amounts than the output value of the coupling.
[0039] "HP generator state" refers to the different states that the HP generator may be in during use, depending on the operation of different LP generators and / or switching units and / or switching elements within the HP generator.
[0040] High power means 10kW or more during pulse operation. High voltage means 4kV or more. High current means 10A or more. The HP generator may be configured to influence the voltage and / or charge carrier movement between the plasma potential and the substrate in and / or during the plasma process. Energy storage components may be capacitors or inductors and / or other energy storage components. This should be understood as an energy storage component configured to store a certain amount of energy, which may be a reasonable portion of the output energy of one pulse of the HP generator, i.e., at least 1 / 200, preferably at least 1 / 100, and more preferably at least 1 / 50 of the energy of one pulse of the HP generator. Continuously charging the energy storage component includes charging the capacitor to a predetermined voltage value or the inductor to a predetermined current value. The electrical connection of the LP generators in the coupling may be in series, parallel, or a combination of both. The coupling value may be voltage, current, or power. The control unit may determine which of the LP generators contributes to the output power of the HP generator. The control unit may be configured to control the LP generators such that at least two LP generators contribute to the output of the HP generator, that is, so that the output values of at least two LP generators are added together to form an HP output value.
[0041] The primary advantage is that the energy storage component charges independently of the control unit's state. This means there are no limitations on the duty cycle of the generated output value, particularly the output pulses, their width, and consequently, the average output power. The second advantage is that each LP generator has its own energy storage component, thereby protecting the switches in the control unit from overvoltage or overcurrent. The third advantage is that all components are relatively inexpensive because they are low-power components.
[0042] The control unit may include a switching unit, one of which is associated with each LP generator. Each switching unit may include one or two transistors, in particular MOSFETs. MOSFETs provide high switching capability, can be used with currents of 50A or more, and enable switching to voltages of 500V or more. When MOSFETs are used, antiparallel diodes may be provided for each MOSFET, in particular in parallel with each MOSFET. The MOSFETs may be silicon carbide (SIC) or gallium nitride (GaN) type MOSFETs suitable for high-speed switching voltages of 500V or more with fast voltage rise and fall times (15kV / μs or more), as well as high currents (50A or more) with fast current rise and fall times (10A / μs or more).
[0043] Each switching unit may include a half-bridge circuit comprising two switching elements connected in series. One transistor may be configured to connect two of the LP generators when closed such that the values of both LP generators affect the value at the coupling output. In particular, the output values of both LP generators may be added together. The other transistor may be configured to short-circuit the load when activated, i.e., when the transistor is closed, i.e., in a conductive state.
[0044] A half-bridge circuit can be clamped by its storage component, such as a capacitor. This means the storage component is directly connected in parallel to the half-bridge. This configuration protects both switches from overvoltage up to the capacitor's voltage. The storage component may also be an inductor. This configuration then protects both switches from overcurrent up to the current in the inductor. To prevent failures related to overvoltage or overcurrent, separate safety voltages / currents should be maintained on each LP generator. Thus, each LP generator maintains its operating level within its own level. This creates a self-holding safety state.
[0045] The control unit may include a driver. The driver may be configured to drive a switching unit, in particular a transistor, in particular a MOSFET. The control unit, in particular a switching unit, may be part of the respective LP generator.
[0046] At least six, preferably ten or more, and preferably fifteen or more LP generators can be coupled to a single coupling. At least two LP generators may provide equal LP generator values. Preferably, most, and even all, LP generators provide equal values. It is also possible that at least two LP generators provide different LP generator values, and in particular, all LP generators may provide different values. One LP generator may provide an LP generator value that is half the LP generator value of another LP generator.
[0047] A capacitor may be provided as an energy storage component in parallel with the switching unit, particularly the half-bridge. This allows for stabilization of the coupling's output value. Each LP generator may have its own capacitor charged to an appropriate voltage level. Activating one switch on the associated switching unit connects the load, such as a plasma process, to the capacitor. Activating the other switch on the switching unit shortens the load. The output of the entire HP generator can become a true dynamic voltage source during each pulse, enabling very large output currents and the necessary fast voltage transitions.
[0048] An inverter, including a full-bridge circuit, and preferably a buck converter, may be connected to the primary winding. This enables efficient power delivery to the LP generator.
[0049] In another embodiment, the present invention relates to a method for supplying high-power pulses of different amplitudes to a plasma process, provided by a high-power (HP) generator, which is: a. A step of continuously charging the energy storage components of multiple low-power (LP) generators to a predetermined value, b. A step of selectively combining the output values of at least several LP generators by controlling the contribution of the LP generator to the output of the HP generator in order to obtain a desired output value of the HP generator corresponding to a pulse having a desired amplitude, Includes.
[0050] By using multiple low-power generators, each containing an energy storage component, in a coupling configuration, it is possible to select how many low-power generators contribute to generating the output pulse. This solution is relatively safe, universal, and inexpensive. It is safe because each low-power generator can be configured to protect its own associated switch from power higher than the power value of its energy storage component. This solution is universal because it is possible to rapidly change the amplitude of the output pulse with a resolution equal to the number of low-power generators. It is inexpensive because many low-power generators and low-voltage switches can be used, which is usually cheaper than one high-power generator and a high-power pulse unit.
[0051] In one embodiment, LP generators, also called "stages," can be controlled individually or in groups. Control signals can be supplied to the switching unit's driver, preferably by optical fiber, but other means may also be used. Not only can all switches in a switching unit be controlled independently of a single main control board, but a single signal can control an entire stage (for example, control signals for the switching unit's connection switches may be supplied from the main control board, signals for the switching unit's load parallel switches may be generated on the connection switch driver board), a single signal can control multiple switches / stages in a collective control, and so on. The total output value is the sum of the output values of only the activated / selected LP generators. By activating or selecting a different number of LP generators for each pulse, the total output value can be varied pulse by pulse.
[0052] In one embodiment, the output value of an LP generator, i.e., the LP generator value, can be dynamically stabilized. Each LP generator may have its own capacitor, charged to an appropriate voltage level. Activating one switch connects the load to the capacitor. Activating another switch shortens the load. The output of the entire LP generator becomes a true dynamic generator during each pulse, allowing for very large output currents and the necessary fast voltage transitions.
[0053] A two-step transition can be achieved instead of a one-step output value transition. Current and voltage oscillations are inevitable when a low-impedance voltage source is connected to a capacitive load via any connector impedance. The load peak voltage can reach twice the applied voltage level. Resistive damping circuits consume a large amount of power. By applying a two-step transition, power losses can be significantly reduced. This is possible with the presented topology. A two-step transition results in a nearly rectangular voltage waveform, lower overshoot, and significantly reduced power losses.
[0054] Several stages, also called "LP generators," can be activated sequentially during a continuous output pulse. Activating an LP generator means activating the corresponding connected switching element. To compensate for the attenuating wafer voltage, the output voltage can be increased in several steps. At each step, its voltage is added to the total output voltage value. If the voltages of all stages are equal, the resolution of each step is 1 / n times the voltage of the entire LP generator stack. Another stack can be built to handle the compensation function. This can be controlled separately, resulting in a much finer step resolution, thereby providing a more stable wafer voltage and a better ion energy distribution.
[0055] Other features and advantages of the present invention are derived from the following detailed description of embodiments of the invention based on the drawings illustrating essential details of the invention, and from the claims. The features shown herein should not necessarily be understood to be at exact scale, but are shown in a manner that clearly shows the special features of the present invention. Various features can be implemented individually or in any combination in the various embodiments of the present invention.
[0056] Examples of various stages of use of the present invention are shown in the schematic diagrams and will be explained in more detail in the following description. [Brief explanation of the drawing]
[0057] [Figure 1] This shows pulse sequences with different amplitudes. [Figure 2a] A first embodiment of the HP generator is shown. [Figure 2b] A second embodiment of the HP generator is shown. [Figure 3] A first embodiment of the portion of the HP generator for generating positive voltage pulses is shown. [Figure 4] A second embodiment of the portion of the HP generator for generating positive voltage pulses is shown. [Figure 5] An embodiment of an HP generator having a power supply for supplying power to an LP generator is shown. [Figure 6] An embodiment of an HP generator having a balancing and damping circuit is shown. [Figure 7] This shows waveform shaping and power loss during one-step and two-step transitions. [Figure 8] The waveforms obtained from the sequential operation of the LP generator are shown. [Figure 9] The results of a periodic control signal sequence via an LP generator are shown. [Figure 10] This shows a plasma processing system having a plasma chamber. [Figure 11] An alternative embodiment of the HP generator is shown. [Figure 12] An alternative embodiment of the HP generator is shown. [Figure 13] An alternative embodiment of the HP generator is shown. [Figure 14] An alternative embodiment of the HP generator is shown. [Figure 15] An example of an LP generator value limiting circuit is shown. [Figure 16] An embodiment of an HP generator having a balance circuit is shown. [Figure 17] An embodiment of an HP generator with a liquid cooling function is shown. [Figure 18a] An embodiment of an HP generator having a galvanically isolated driver is shown. [Figure 18b] An embodiment of an HP generator having a galvanically isolated driver is shown. [Modes for carrying out the invention]
[0058] Figure 1 shows the sequence of pulses 1 required for plasma applications. While the amplitude of pulse 1 must be constant for some pulses, it can be seen that there may be amplitude changes from pulse 1 to pulse 1a.
[0059] Figure 2a shows a first embodiment of a high-power (HP) generator 10 suitable for generating high-power pulses. The HP generator has a positive output 32 and a negative output 12. The HP generator 10 includes several low-power (LP) generators 14, 16, and 18, each containing energy storage components C1, C2, and Cn, which are implemented in this case as capacitors. During use, the energy storage component C1 is continuously charged to a predetermined value related to the energy storage component C1, thereby maintaining a constant amount of energy stored within it. During use, each LP generator 14, 16, and 18 supplies an LP generator value at its output corresponding to the value of the energy storage component C1 incorporated within each LP generator 14, 16, and 18. The LP generators 14, 16, and 18 are electrically connected in a coupling 20, at the output of the coupling 20, a coupling value corresponding to the output value of the HP generator 10 can be obtained, which is higher than the LP generator value at the output of one of the LP generators 14, 16, and 18. The control unit 22 is configured to select the contributions of LP generators 14, 16, and 18 to the output value of HP generator 10 during power delivery of HP generator 10. For clarity, only three LP generators are shown. In practice, many more LP generators, typically six or more, even ten or more, and especially fifteen or more, are used to produce a step-like transition of output that can be smooth to the extent required for the load.
[0060] The control unit 22 includes switching units 24, 26, and 28 associated with each of the LP generators 14, 16, and 18. Each of the switching units 24, 26, and 28 includes switching elements S1 and S2. In the embodiment shown in the figure, the switching elements S1 and S2 are connected in series, thus forming a half-bridge. The switching units 24, 26, and 28 and therefore the switching elements S1 and S2 are driven by the driver 30 of the control unit 22.
[0061] The positive output 32 of the HP generator is connected to earth, also known as ground potential PE. All LP generators are connected to a power supply 34, which includes a transformer, a rectifier, and an inverter, which is connected to the power grid and itself includes a full-bridge and buck converter, as will be explained in more detail later.
[0062] Figure 2b is almost identical to Figure 2a. Therefore, similar elements are given similar numbers. The difference between Figure 2a and Figure 2b is that in Figure 2b, the energy storage components are implemented as inductances L1, L2, and Ln. Therefore, the electrical connections / wiring within the coupling 20 are slightly different. In general, the energy stored in an inductor becomes a current source. Therefore, the energy storage component is connected in parallel with other current sources, leading to current amplification.
[0063] In both Figure 2a and Figure 2b, the ground potential, also called the earth potential PE, can be connected to one of the outputs of the LP generators 14, 16, and 18. This makes it possible to form voltage signals with positive and negative voltages relative to earth, for example, as shown in Figure 10 of (Patent Document 1) or Figure 21 of (Patent Document 2). The ground potential can also be connected to the positive output 32 or the negative output 12. In Figures 2a and 2b, this is connected to the positive output 32 as an example.
[0064] Figure 3 shows a simplified embodiment of a portion of the HP generator 10 having multiple LP generators 14, 16, and 18. It is advantageous to construct a push-pull switch to generate high-voltage pulses, meaning that the switching unit should have at least two switching elements S1, S2, i.e., a "push" (switching element S1) used to charge the capacitance of the plasma reactor and a "pull" (S2) used to discharge the capacitance of the plasma reactor. To generate pulses with different amplitudes while allowing rapid amplitude changes between pulses (as shown in Figures 1, 7, and 8), the outputs of two or more LP generators 14, 16, and 18 must be combined. The LP generator values of LP generators 14, 16, and 18, in this case the voltage amplitudes, may be the same or different. In Figure 3, the ground potential, also called the earth potential PE, is connected to the negative output of LP generator 18.
[0065] In certain situations, if neither switching element S1 nor S2 is switched on (during the transition state between the two operating states), and the LP generators 14, 16, and 18 are connected in series, and also in series with the load (e.g., a plasma reactor), then switching elements S1 and S2 may be destroyed by overvoltage. For this reason, it is advantageous to protect all switching elements S1 and S2 from voltages higher than the voltage stored in their respective energy storage components by adding diodes D1 and D2 in parallel. This is especially true when switching elements S1 and S2 are implemented as MOSFETs. When switching elements are implemented as unipolar transistors, the diodes are already mounted as part of the transistor or are an integral part of the transistor die, as is typically the case with MOSFETs. A die, in the context of an integrated circuit, is a small block of semiconductor material on which a functional circuit is built. In this case, these diodes can be used, and there is no need to add them separately.
[0066] A ground potential can be generated at the output of the HP generator (the load is short-circuited to the ground potential PE). This state is achieved by the switch-on switching element S2 when the ground potential is connected to the negative output 12 or positive output 32 of the HP generator. In this state, the switching element S1 is opened (switched off). To generate (voltage) pulses to the load, it is necessary to switch on an appropriate number of switching elements S1. LP generators 14, 16, and 18 can provide the same or different LP generator values, in this case, voltage amplitudes. By switching on several switching elements S1, the associated LP generators, e.g., LP generators 14 and 16, are connected in series, and the amplitude of the voltage pulse to the load becomes equal to the LP generator values, e.g., the sum of their output voltages.
[0067] The complementary switching elements S2 associated with the LP generators 14 and 16 should be switched off to avoid short circuits within the LP generators 14 and 16.
[0068] The switching element S1 associated with the LP generator 18 (or another LP generator) should be switched off, while the switching element S2 can be switched on or off (in either case, current flows through the diode D2).
[0069] The configuration of the switching elements, which can be switched on and off, can be changed quickly, which leads to a rapid change in the amplitude of the pulses generated at the output of the HP generator 10. The number of LP generators 14, 16, and 18 supplying power to the load can be quickly changed to any feasible number (0 to n).
[0070] The only difference between Figure 4 and Figure 3 is the wiring, namely the interconnections of the LP generators 14, 16, and 18, and the connection points between the load and PE. However, the same results as in Figure 3 can be obtained; that is, by selecting the LP generators 14, 16, and 18 that contribute to the output voltage pulse by appropriately driving the switching elements S1 and S2, voltage pulses of different amplitudes can be realized. In Figure 4, the ground potential, also called the earth potential PE, is connected to the negative output of the HP generator 10.
[0071] Figure 5 shows the power supply for LP generators 14, 16, and 18 in more detail. The energy storage components C1, C2, and Cn of LP generators 14, 16, and 18 are continuously charged by transformers T1, T2, and Tn, respectively, and their associated rectifiers X1, X2, and Xn, respectively. It is much simpler and cheaper if all transformers T1, T2, and Tn have their own magnetic cores, and the only common element is the primary windings P1 and P2, which can be made from a single common wire. A balance winding BW is added to have the same voltage regardless of the load. The balance winding BW equalizes the magnetic flux in each of the transformers T1, T2, and Tn. If the number of wires in the secondary windings SW1, SW2, and SWn is the same, the voltage on the energy storage component C1 is the same. By changing the number of turns in the secondary windings SW1, SW2, and SWn, the charging voltage of the energy storage component C1 can be changed. In this way, it is possible to ensure that each energy storage component C1 is charged to a different voltage.
[0072] A balanced winding BW in transformers T1, T2, and Tn operates in this manner, and if the magnetic flux in the adjacent transformers T1, T2, and Tn is the same, both windings BW have the same voltage, and therefore no current flows between the windings BW. If for some reason the magnetic flux in one core is different from the next core (for example, higher), the induced voltages in the different transformers T1, T2, and Tn will be different, and therefore some current will begin to flow between these windings BW. This current will cause the transformer T1, T2, and Tn with the higher magnetic flux to be subjected to a greater load, and the magnetic flux of the next transformer will also increase due to this current. By making the magnetic flux in the magnetic cores equal, the induced voltages become equal. In a balanced winding BW, the number of turns in both connected transformers should be the same using a magnetic flux equalization method.
[0073] The primary windings P1, P2, and Pn are connected to the inverter 40, which is connected to the power grid 42. The inverter 40 is part of the power supply 34 and includes a full bridge 44 and a buck converter 46 upstream of the full bridge 44, which is connected to a rectifier 48.
[0074] Figure 6 shows, for example, an HP generator 10 similar to that in Figure 2a, where a balancing circuit is connected between the two LP generators 14, 16, and 18, with components that allow current flow in only one direction, in particular diode D or components that act like diodes. Switching units 24, 26, and 28 include half-bridge switching elements. These switching elements are shown as transistors, more precisely as bipolar transistors. For faster switching times, a more preferred type of transistor would be a MOSFET, particularly a SiC or GaN-based MOSFET. In this embodiment, the negative terminal of each LP generator 14, 16, and 18, also called a stage, is connected to the same potential as the preceding LP generator 14, 16, and 18, which has a diode D or a circuit including a diode. This allows energy stored in the lower stage energy storage component to be transferred to the higher stage energy storage component when the voltage of the lower stage energy storage component exceeds that of the higher stage when the switching element is activated. The charge on the highest stage 14 accumulates excess charge from the entire stack. The highest stage 14 is therefore controlled to provide most of the power to the load in order to eliminate excess charge. The voltage distribution along the LP generator stack is improved by using diode D or a circuit including a diode. A damping circuit 50 may be used, in particular, in series with a balancing circuit, here diode D, to eliminate oscillations on parasitic components of the charge handling path. The damping circuit may include an inductor L and / or resistor R connected in parallel, in particular as shown in Figure 6.
[0075] By connecting a low-impedance voltage source, such as a capacitive voltage source with a capacitive load, through any connector inductance, current and voltage oscillations inevitably occur. This can cause the load peak voltage to reach twice the applied voltage level. This is highly undesirable as it can damage the load. Resistive snubber circuits are often used to attenuate the peak voltage. These loss circuits consume a large amount of power and therefore generate a large amount of heat, which is also highly undesirable. By applying a two-step transition, this overshoot can be controlled, reduced, and even avoided. Energy consumption in the loss circuit can be dramatically reduced, or even avoided. This allows for a much smaller or completely avoidable energy-consuming component. Power loss can be significantly reduced. This is possible in the presented topology, shown in Figure 7. Waveform 70 shows a one-step transition from 0kV to 10kV at the output of coupling 20, by activating all 15 stages instantaneously, for example. This results in a voltage overshoot, as seen in waveform 74. As shown by waveform 71, a stepwise transition is obtained with, for example, a first step from 0V to 5.5kV by activating only 8 of the 15 stages, and a second step from 5.5kV to 10kV by activating the remaining 15 stages after several tens or hundreds of nms, for example. The resulting waveform 73 has a much lower overshoot peak voltage, and the voltage rise is slower, but still fast enough for the process. As can be seen from waveform 73, the two-step transition results in a nearly rectangular output waveform and lower overshoot, and therefore little power dissipation, whereas the one-step transition leads to waveform 74 showing voltage overshoot and, consequently, large power loss 75. The same overshoot peak and its decline can be seen, for example, in the downward slope after 1 μs and 3.5 μs.
[0076] Figure 8 shows the stepwise increase in the amplitude of pulse 80 by sequentially activating LP generators 14, 16, and 18 to contribute to the HP output signal. Waveform 81 shows the voltage at the output of the HP generator, and waveform 82 shows the voltage on the wafer in the plasma chamber.
[0077] Figure 9 shows the result of circulating a control signal sequence through an LP generator stack containing 15 LP generators #01 to #015. This figure shows a timeline t on the horizontal axis, as in Figure 8. The vertical axis shows the operation of 15 switching elements corresponding to the 15 LP generators #01...#15. The shaded area 92 is the portion where the connected switching elements are activated. The white area 91 is the portion where the switching elements in parallel with the load are activated. Therefore, in time frames without shaded areas, there are no pulses in the output. In the first column, LP generators #01 to #06 are switched along with their corresponding connected switching elements. In the second column, LP generators #01 to #10 are switched along with their corresponding connected switching elements. In the third column, LP generators #01 to #11 are switched along with their corresponding connected switching elements. In the fourth column, LP generators #01 to #12 are switched along with their corresponding connected switching elements, and so on. In this way, the pulse has the shape of a stepped waveform. In the ninth column, the pulse ends and all connected switching elements are deactivated. Then, in the tenth column, the next pulse begins, this time using LP generators #02 to #07 instead of LP generators #01 to #06. In this example, all LP generators have the same output value, but the output value at the output of coupling 20 is the same as it was for the first pulse. The advantage is that each pulse sequence starts with a different LP generator stack, so that all 15 LP generators and their corresponding switching elements receive an equal load after 15 pulses. This results in better load balancing between the LP generator stacks and, therefore, an improved balance of power loss between the switching elements.
[0078] Figure 10 shows a plasma processing system having a plasma chamber 100 in which plasma 101 is generated within a plasma space. Such or similar systems are shown and described, for example, in (Patent Document 3), (Patent Document 1), or (Patent Document 2). An upper electrode 103 may be positioned within the plasma chamber. Gas inlets and outlets, in particular gas supply pipes 104, may be installed from the outside to the inside of the plasma chamber 100 and connected in particular to the electrode 103. A substrate 102, in particular a semiconductor wafer, may be placed inside the plasma chamber 100 on a support 105 including a substrate holder. During use, the substrate 102 can be processed by the plasma 101 in processes such as etching, ashing, or in particular deposition by atomic layer deposition. The etching process can be very difficult, for example, when the ratio of the diameter to the length of the etching hole is extremely low, for example, <1 / 100, as is often required in deep etching. A conductive electrode 106 may be placed inside the plasma chamber 100, particularly near the substrate 102, for example, around the substrate 102. This conductive electrode 106 may be an edge ring, also known as a focus ring. This conductive electrode 106 may be connected to a first power supply 114 via a first connection line 115. The first power supply 114 may be a DC pulse power supply, in which case the pulses may have different lengths, different amplitudes, and waveforms, for example, as shown in Figures 11, 14 of (Patent Document 4) or Figure 2 of (Patent Document 1). By controlling the first power supply 114, the conductive electrode 106 may be additionally or alternatively used as an ion energy and / or ion acceleration direction control means, also described in (Patent Document 4) or (Patent Document 1). A first high-frequency (RF) power supply 118 may be electrically connected to the support 105 via a first power supply rod 119 and a first matching unit 116 and a first connection unit 117. A second high-frequency (RF) power supply 108 may be electrically connected to the upper electrode 103 via a second power supply rod 109 and a second matching unit 110 and a second connection unit 111. Electrode 107 may be positioned in or near the support 105 and electrically connected to a second power supply 112 via a second connection line 113.The second power supply 112 may be a DC pulse power supply, in which case the pulses may have different lengths, different amplitudes, and waveforms, for example, as shown in Figures 11, 14 of (Patent Document 4) or Figure 2 of (Patent Document 1). The substrate 102 may be fixed to the support 105 via electrodes 107 which can function as electrostatic chucks. By controlling the second power supply 112, the electrodes 107 may be additionally or alternatively used as ion energy and / or ion acceleration direction control means, also described in (Patent Document 4) or (Patent Document 1).
[0079] Some plasma processing applications, such as etching and thin-film deposition, require the supply of high-voltage (HV) and high-frequency (HF) rectangular asymmetric pulse voltages. In many cases, the voltage values significantly exceed the voltage handling capabilities of individual semiconductor switches, especially when high-frequency operation is required.
[0080] Some plasma applications require not only pulsed operation but also inter-pulse amplitude variation. In some plasma applications, the power supply needs to deliver high peak current to obtain short voltage transition times. In most plasma applications, a load is presented, which includes capacitive components. Significant power loss is related to the pulse-by-pulse charging and discharging of the capacitance of this load. Some plasma applications require pulse shaping, such as shown in Figures 11 and 14 of (Patent Document 4) or Figure 2 of (Patent Document 1).
[0081] Therefore, connecting such switches in series is one possible solution. A series connection requires a voltage balancing mechanism. These voltage balancing mechanisms are not easily implemented in HF operation.
[0082] Figure 11 shows an alternative embodiment of the HP generator stack shown in Figure 5. The difference between the HP generators shown in Figure 5 and Figure 11 is that in Figure 11, the primary windings P1, P2, ... Pn are connected in parallel to the transformers T1, T2, ... Tn, and a balance winding BW is not provided.
[0083] Figure 12 shows another alternative embodiment of the HP generator shown in Figure 5. The difference between the HP generators shown in Figure 5 and Figure 12 is that in Figure 12, all the transformers T1, T2, and Tn in Figure 5 are integrated as a single main transformer T1 with only one core. Only one primary winding P is required, and a balance winding Bw is not provided.
[0084] Figure 13 shows another alternative embodiment of the HP generator shown in Figure 5. The difference between the HP generators shown in Figure 5 and Figure 13 is that in Figure 13, the primary windings P1, P2, ... Pn are connected in parallel to the transformers T1, T2, ... Tn. A balance winding is shown, but it is not absolutely necessary in this configuration.
[0085] Figure 14 shows an alternative embodiment of the HP generator shown in Figure 5. The difference between the HP generators shown in Figure 5 and Figure 14 is that in Figure 14, the primary winding P1 is connected only to transformer T1. The balance winding BW is used to transport energy from transformer T1 to T2, and from transformer T2 to subsequent transformers.
[0086] Figure 15 shows an example of an LP generator value limiting circuit 151. This LP generator value limiting circuit 151 is a voltage limiting circuit. It can be connected in parallel at its terminals 152a, 152b to one, some, or all of the LP generators 14, 16, and 18. It can also be connected in parallel at its terminals 152a, 152b to one, some, or all of the switching units 24, 26, and 28. This LP generator value limiting circuit 151 includes a series circuit of diode D15 and capacitor C15, which clamps the overvoltage present at its terminals 152, 153 to the voltage to which capacitor C15 is charged. The voltage across capacitor C15 is controlled by a discharge circuit 153. The discharge circuit 153 includes a transistor T15, in this case a MOSFET, and a resistor R15, in this case a power resistor, configured to be cooled very effectively. Transistor T15 is controlled to be switchable on or off, and therefore in pulse mode. Therefore, transistor T15 does not dissipate much energy into heat and is kept very cold. When transistor T15 is switched on, the capacitor discharges, and the discharge current flows through resistor R15, dissipating energy into heat. Most of the heat is generated within resistor R15, which is simpler and cheaper to use than transistor T15 for keeping it sufficiently cold. Transistor T15 is controlled by a voltage control circuit 154 with respect to its control input G15. The voltage control circuit 154 includes a series connection of a first high-voltage Zener diode 155, a current-limiting resistor 156, and a second low-voltage Zener diode 158. The connection point between the current-limiting resistor 156 and the second low-voltage Zener diode 158 is connected to the control input G15 of transistor T15. The voltage control circuit 154 may further include a driver 157. The driver 157 may be connected at its input to the connection point between the current-limiting resistor 156 and the second low-voltage Zener diode 158. The driver 157 may be connected at its output to the control input G15 of transistor T15. A voltage divider 159 may be connected between the output of driver 157 and the control input G15 of transistor T15.
[0087] The driver has hysteresis at its input and can therefore be implemented as a Schmitt trigger. This generates a pulsed signal to drive the discharge transistor T15 at a frequency of approximately 2–20 kHz. In this configuration, more than 90% of the power is consumed within resistor R15. This keeps the transistor cool enough to operate reliably over long periods.
[0088] Figure 16 shows an embodiment of an HP generator 10 in which a balancing circuit, including a component that allows current to flow in only one direction, in particular a diode or a component that acts like a diode, is connected between two LP generators 14, 16, and 18. The balancing circuit, in particular diode D, is connected between the two negative outputs of LP generators 14, 16, and 18. This allows a switching element S1 to be activated so that when the voltage of the lower stage energy storage component exceeds that of the higher stage, energy from the lower stage energy storage component is transported to the higher stage energy storage component. The charge on the highest stage 14 accumulates excess charge from the entire stack. Therefore, the highest stage 14 should be controlled to provide most of the power to the load in order to eliminate the excess charge. By using diode D or a circuit including a diode, the voltage distribution along the LP generator stack is improved.
[0089] Figure 17 shows an embodiment of an HP generator 10 with liquid cooling. All the electronic components of the HP generator 10, such as LP generators 14, 16, 18, switching units 24, 26, 28, transformers T1, T2, Tn, and inverter 40, are housed in a waterproof housing 171. A liquid reservoir 174 is located near the housing. Between the housing and the reservoir 174 is a liquid pump 173 for circulating the liquid 171 between the housing and the reservoir 174 as a closed liquid loop. The reservoir is in thermal contact with a heat exchanger 175 to transfer heat from the cooling liquid closed loop to the environment.
[0090] Figures 18a and 18b show an embodiment of the HP generator 10 having a galvanically isolated connection 181 between the driver 30 and the switching elements S1 and S2.
[0091] Therefore, the control unit 22 is configured to select the contributions of the LP generators 14, 16, and 18, particularly by optical fiber connection or magnetic coupling, so as to be galvanically isolated. The connection between the output of the data processor and the switching units 24, 26, and 28 can be achieved by optical fiber or by magnetic coupling. This helps to switch all the LP generators 14, 16, and 18 effectively, quickly, and without delay to form pulses with sharp edges, for example, in voltage. Several switching elements S1, S2 of one switching unit 24, 26, and 28 can be connected by one galvanically isolated connection. Several LP generators 14, 16, and 18 can be grouped together with their corresponding switching units 24, 26, and 28, and this can be configured such that this group is always operated or deactivated together. Then, only one galvanically isolated connection 181 can be used for such a group. Galvanic isolation (transmitter and receiver) can be used for each switch. The group of transmitters can be controlled simultaneously, reducing the number of control signals required. Reducing the number of galvanically isolated connections 181 lowers costs and makes the HP generator 10 more reliable by reducing the number of components at risk of failure. [Explanation of Symbols]
[0092] 1 pulse 10 High-Power (HP) Generators Negative output of a 12 HP generator 14, 16, 18 Low-Power (LP) Generators 20 Couplings 22 Control Unit 24, 26, 28 Switching Units 30 drivers Positive output of a 32 HP generator 34 Power supply 40 Inverters 42 Power grid 44 Full Bridge 46 Buck converter 48 Rectifier 50 Damping Circuit 70 waveforms 71 Waveform 73 Waveform 74 Waveform 75 Power loss 80 pulses 81 Waveform 82 Waveform 91 areas 92 areas 100 Plasma Chambers 101 Plasma 102 circuit boards 103 Electrode 104 Gas supply pipe 105 Support 106 Conductive electrode 107 Electrode 108 Power supply 109 Second power supply rod 110 Second Matching Unit 111 Second connection unit 112 Second power supply 113 Second connecting line 114 First power supply 115 First connecting line 116 First Matching Unit 117 First connection unit 118 Power supply 119 First power supply rod 151 LP Generator Value Limiting Circuit 152 edge 152a end 152b end 153 Discharge circuit 154 Voltage Control Circuit 155 First high-voltage Zener diode 156 Current-limiting resistor 157 Drivers 158 Second Low Voltage Zener Diode 159 Voltage divider 171 Waterproof Housing 173 Liquid pump 174 Reservoir 175 Heat exchanger 181 connections BW Balanced Winding C1, C2, Cn capacitors D1, D2 diodes L1, L2, Ln inductors P1, P2, Pn Primary winding PE Earth potential R resistor S1, S2 switching elements SW1, SW2, SWn secondary winding T1, T2, Tn transformers X1, X2, Xn rectifier
Claims
1. In a high-power (HP) generator (10) configured to deliver pulsed high power of high voltage and / or high current values to a capacitive load, - Several low-power (LP) generators (14, 16, 18), Each low-power (LP) generator (14, 16, 18) includes an energy storage component (C1, L1), and during use, the energy storage components (C1, L1) are charged to a predetermined value related to the energy storage component. - While each low-power (LP) generator (14, 16, 18) is in use, its output supplies an LP generator value corresponding to the values of the energy storage components (C1, C2, Cn, L1, L2, Ln) incorporated into each of the LP generators (14, 16, 18). Several LP generators (14, 16, 18) and - A coupling (20) to which the LP generators (14, 16, 18) are electrically connected, such that a coupling value at the output of the coupling (20) can be obtained that corresponds to the output value of the HP generator (10) and is higher than the LP generator value at the output of one of the LP generators (14, 16, 18) during use in at least several states of the HP generator (10), - A control unit (22) configured to select the contribution of the LP generators (14, 16, 18) to the output value of the HP generator (10) during power delivery of the HP generator (10) in order to cause the rise and / or decay of the pulse at the output of the coupling (20), Includes, - The aforementioned LP generators (14, 16, 18) operate sequentially within one pulse. - The control unit (22) further includes switching units (24, 26, 28), and the switching units (24, 26, 28) are - Having a current increase capability of at least 10 A / μs, and / or - It has the ability to withstand voltages of 0.5kV or higher with a voltage change rate of 15kV / μs or higher. - At least one LP generator (14, 16, 18) includes an LP generator value limiting circuit (151), High-power (HP) generator (10).
2. The control unit (22) further utilizes the contribution of the LP generators (14, 16, 18) with the following characteristics: - The output of the coupling (20) and / or the output value of the HP generator (10) are stepped waveforms. • At least one amplitude step is less than 1 kV. The LP generators (14, 16, 18) are connected by switching only. Configured to select in such a way that one or a combination of the following is achieved, The HP generator according to claim 1.
3. The coupling (20) includes five or more electrically connected LP generators (14, 16, 18) and / or - The number of LP generators (14, 16, 18) is such that, at the output of the coupling (20), - A sufficiently large number of values to form pulses having voltage rises and / or falls equal to or greater than the sum of several values of the LP generators (14, 16, 18), The HP generator according to claim 1 or 2.
4. The charging energy for the LP generators (14, 16, 18) is supplied on transformers (T1, T2, ...Tn) having primary windings (P1, P2, ...Pn) and secondary windings (SW1, SW2, ...SWn) for each LP generator (14, 16, 18). a. The secondary windings (SW1, SW2, .... SWn) are connected to rectifiers (X1, X2, .... Xn), and each rectifier (X1, X2, .... Xn) is connected to the corresponding energy storage components (C1, C2, Cn, L1, L2, Ln) of the LP generators (14, 16, 18), and / or b. Some of the primary windings (P1, P2, .....Pn) are connected in series. The HP generator according to claim 1 or 2.
5. The HP generator according to claim 1 or 2, wherein the charging energy of the LP generators (14, 16, 18) is supplied on transformers (T1, T2, ..., Tn), and some of the transformers include balance windings (BW).
6. The HP generator according to claim 1 or 2, wherein a balance circuit including a component that causes current to flow in only one direction is connected between two LP generators.
7. The HP generator according to claim 1 or 2, wherein the damping circuit (50) is positioned in an open chain configuration between some of the LP generators (14, 16, 18).
8. The HP generator according to claim 1 or 2, which is at least partially directly liquid-cooled.
9. The HP generator according to claim 1 or 2, wherein the control unit (22) is configured to sequentially select the contributions of the LP generators (14, 16, 18) through the LP generators (14, 16, 18).
10. The HP generator according to claim 1 or 2, wherein the control unit (22) is configured to select the contribution of the LP generators (14, 16, 18) in such a manner that it reduces voltage overshoot in the output value of the HP generator (10) and / or at a specific location on the load.
11. The HP generator according to claim 1 or 2, wherein the control unit (22) is configured to select the contribution of the LP generators (14, 16, 18) by optical fiber or magnetic coupling so as to be galvanically isolated.
12. The HP generator according to claim 1 or 2, wherein the energy storage components (C1, L1) include capacitances, and the capacitances are provided in parallel with the switching units (24, 26, 28).
13. The HP generator according to claim 4, wherein an inverter (40) including a full-bridge circuit (44) and a buck converter (46) is connected to the primary windings (P1, P2, .....Pn).
14. A method for supplying high-power pulses of different amplitudes to a plasma process, provided by a high-power (HP) generator (10), a. A step of continuously charging the energy storage components (C1, L1) of multiple low-power (LP) generators (14, 16, 18) to predetermined values, b. The steps of controlling the LP generators (14, 16, 18) to selectively couple the output values of at least some of the LP generators (14, 16, 18) in order to obtain a desired output value of the HP generator (10) corresponding to a pulse having a desired amplitude, c. The step of increasing the current by at least 10 A / μs in order to supply the high-power pulse, d. A step of increasing the voltage to 15 kV / μs or more in order to supply the high-power pulse, Includes, The aforementioned multiple LP generators (14, 16, 18) operate sequentially within one pulse. At least one LP generator (14, 16, 18) includes an LP generator value limiting circuit (151), method.