RADIO FREQUENCY DRIVE GENERATOR FOR PLASMA SOURCES
Technical field[0001] The present invention is related to power drive generators for radio frequency (RF) applications, particularly to drive generators for plasma sources.
Background art[0002] Chemically reactive plasma discharges are widely used to modify the surface properties of materials. In particular, integrated circuit manufacturing processes use plasma processing on a large scale for wafer surface modifications.
[0003] A conventional plasma system 10 as shown diagrammatically in
Fig. 1, comprises a plasma chamber 11, an RF generator 14 and an RF matchbox 13.
Different gases can be supplied to the plasma chamber, depending on the required process and recipe. An antenna 12, which can be a coil or an electrode is coupled to the plasma chamber to excite a gas inside the plasma chamber resulting in the plasma.
Generally, the matchbox 13 is directly or very closely connected to the antenna 12, whereas the RF generator 14 is placed remotely and connected by a transmission line 15, such as a coaxial cable, to the matchbox.
[0004] Conventional RF generators are optimized for operation in a 50Q load. The power is coupled to the matchbox via a 50Q transmission line. The matchbox consists of a network of electronic or mechanically tuned variable capacitors and/or inductors which can be adapted to transform the load impedance to the required 50Q impedance. When the matchbox is “matched”, no power is reflected at the matchbox input and maximum power is delivered from the RF generator to the load.
[0005] The conventional approach has several disadvantages such as high costs of the cable and matchbox, speed limitations, plasma ignition problems, accurate plasma control and limited capability at different pulses. Multiple attempts have been made to remove the matchbox and transmission line.
[0006] US11224116 discloses a matchless plasma source comprising a half-bridge transistor circuit configured to generate an amplified square waveform which is coupled to a reactive circuit having a high-quality factor to negate the reactance of the antenna of a plasma chamber. The switching frequency of the half-bridge transistor circuit is adjusted to control the output power by measuring a phase difference between a complex voltage and a complex current at the output of the transistor circuit to maintain a zero degree phase difference.
[0007] US 2019/0385821 discloses an RF generator including dual drive circuits that operate at different frequencies. At least one of the dual drive circuits is a direct drive circuit that includes a resonant network to cancel out the frequency at which the other drive circuit operates. The direct drive circuit operates at a frequency that is adjusted based on a phase offset between voltage and current at the output node.
[0008] US 2022/0230847 discloses a RF power drive system for an inductively coupled plasma that does not require an impedance matching network and avoids non-essential harmonic attenuation. The RF power amplifier comprises a resonant network tuned to operate over a frequency range. The power amplifier operates at a variable frequency that is selected to ensure zero-voltage switching.
[0009] All the above solutions still require some impedance tuning, such as frequency tuning, to align the phase of the voltage and current delivered to the antenna by using a resonant circuit with a high-quality factor.
Summary[0010] It is an object of the present disclosure to provide a radio frequency drive generator for plasma sources that overcomes the above mentioned disadvantages.
Particularly, it is an object to provide a radio frequency drive generator that allows to operate over a large impedance range without added complexity, particularly without requiring (variable) impedance matching and/or frequency tuning. It is therefore an object to provide a radio frequency drive generator that has reduced complexity while being more agile for operating at higher frequencies.
[0011] According to a first aspect of the present disclosure, there is therefore provided a drive generator for supplying radio frequency energy to a plasma device, as set out in the appended claims. A radio frequency drive generator according to the present disclosure comprises an input section, a switch-mode converter and an output section. The input section is configured to generate a DC supply voltage having at least one and possibly multiple voltage levels and which is fed as an input to the switch-mode converter. The DC supply voltage defines an amplitude of the shaped sinusoidal voltage waveform and can define a power output by the switch-mode converter. The switch-mode converter is configured to generate a shaped sinusoidal voltage waveform at an output. The output section connects the output of the switch- mode converter to an output of the drive generator. In some examples, the output is configured to be connected to an antenna of a plasma device, such as an inductively or capacitively coupled plasma device.
[0012] According to a first sub-aspect, the output section connects the output of the switch-mode converter to the output of the RF drive generator through a reactive network having a fixed reactance.
[0013] According to another sub-aspect, which can be provided in addition, or in the alternative to the previous sub-aspect, the switch-mode converter is a resonant converter, comprising a switching section coupled to a resonator network. The resonator network has advantageously a fixed reactance. As a result, a radio frequency energy path between the switching section and the output of the drive generator has a fixed reactance. Advantageously, the resonator network and the reactive network are tuned to a predetermined impedance operating region, e.g., an impedance range, of the drive generator.
[0014] The switching section is configured to be operated at a constant and possibly fixed switching frequency to generate the shaped sinusoidal voltage waveform.
In some examples, the switching section comprises at least one, possibly multiple modes of operation, each mode of operation configured to generate a different shaped sinusoidal voltage waveform. In each mode of operation the switching section is configured to be operated at a respective fixed or constant switching frequency.
[0015] The provision of a reactive network with fixed reactance and a resonant converter in the RF drive generator of the present disclosure allows to obtain an improved tolerance to impedance variations of the load (i.e., the plasma device). It was surprisingly found that the reactive network and the resonator network of the switch- mode resonant converter can be tuned to the plasma system such that efficient operation of the switch-mode converter can be maintained over a large impedance range. Hence, frequency tuning of the switch-mode converter or variable reactances at the output can be dispensed with, while maintaining a high efficiency of operation. Hence, advantageously, drive generators according to the present disclosure are not configured to (continuously) vary the switching frequency of the switch-mode converter for impedance tuning. This simplifies control and reduces costs considerably and allows to further increase the operating frequency.
[0016] Advantageously, the resonator network and the reactive network together define an impedance operating region of the drive generator. It has been found that the reactance of the resonator network and of the reactive network can be tuned to the impedance trajectory that is effected by the plasma device between an excitation state and an OFF state. As a result, a matchless connection can be made between the drive generator and the antenna for exciting the plasma, which reduces costs and improves static and dynamic plasma stability.
[0017] Advantageously, the switching section of the switch-mode converter comprises a pair of switch devices, particularly transistor devices, arranged in a push- pull configuration with a shared voltage node connected to the source terminals of the pair of switch devices, particularly a shared ground. Advantageously, the resonator network is configured to define a voltage waveform across the switch devices allowing the switching section to operate under zero-voltage switching or zero-current switching.
A push-pull switching pair combined with a properly designed resonator network advantageously results in low voltage and current stress on the switch devices.
[0018] The drive generator according to the present disclosure is advantageously a matchless or direct drive RF drive generator which can directly interface the antenna of the plasma chamber, rendering a (variable) matchbox and possibly a transmission line unnecessary. This reduces costs significantly. Furthermore, the load-tolerant topology of the drive generator according to the present disclosure enables to maintain zero-voltage switching capability over a large RF impedance range and doesn't require frequency control, rendering it significantly more robust. In addition, due to the instant power delivery, there are no speed constraints caused by tuneable matching circuits and plasma ignition issues. The direct and accurate control of the plasma power enable high speed pulsing applications.
[0019] According to a second aspect of the invention, there is provided a plasma system as set out in the appended claims. The plasma system comprises a plasma processing chamber, an antenna (110) and the drive generator according to the first aspect. The drive generator is configured to supply radio frequency energy to the antenna to create a plasma in the plasma processing chamber.
Brief description of the drawings[0020] Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein same reference numerals illustrate same features and wherein:
[0021] Figure 1 represents a diagram of an inductively coupled plasma system;
[0022] Figure 2 represents a diagram of a complex impedance trajectory of an induction coil of an inductively coupled plasma (ICP) chamber;
[0023] Figure 3 represents a schematic diagram of a radio frequency drive generator according to the present disclosure;
[0024] Figure 4 represents a diagram of the input section of the radio frequency drive generator of Fig. 3;
[0025] Figure 5 represents a diagram of a switch-mode converter implemented in a radio frequency drive generator according to the present disclosure;
[0026] Figure 6 represents a circuit diagram of a resonator network of the switch-mode converter; 5 [0027] Figure 7 represents a graph of a rectangular pulse train of a radio frequency plasma excitation signal, illustrating the voltage amplitude envelope and the sinusoidal voltage signal;
[0028] Figure 8 represents a graph of a multi-state pulse train of a radio frequency plasma excitation signal, illustrating the voltage amplitude envelope having alternating between two non-zero voltage levels and the sinusoidal voltage signal;
[0029] Figure 9 represents a diagram of a radio frequency drive generator with multiple switch-mode converter units and a combining network according to an aspect of the present disclosure;
[0030] Figure 10 represents a diagram of a radio frequency drive generator with multiple DC/DC converters and switch-mode converter units and a combining network according to an aspect of the present disclosure;
[0031] Figure 11 represents a diagram of a series combining transformer for use in the drive generators of Figs. 9 and 10;
[0032] Figure 12 represents a diagram of a parallel combining transformer for use in the drive generators of Figs. 9 and 10;
[0033] Figure 13 represents a diagram of a transformer coupled plasma system comprising a pair of coupled coils driven by two respective RF drive generators according to the present disclosure.
Detailed description[0034] Referring to Fig. 1, an exemplary plasma set-up 10 with inductively coupled plasma is shown. The set-up comprises a plasma chamber 11. The plasma is excited using an antenna, particularly a coil 12 or an electrode, which electromagnetically couples to the plasma 16 inside the chamber 11. The coil 12 is inductive and has to be matched to the RF source 14 using a matching box 13. Prior art matching boxes typically comprise a network of variable electrical matching elements. Such a matching box becomes redundant in systems as described in the present disclosure, as will be detailed below.
[0035] The power from the RF source 14 is transferred via a transmission line 15 to the matching box 13. The plasma 16 inside the chamber 11 can be manipulated in view of controlling the ion energy density and ion flux. Typically a DC biasing source 17 is coupled to a bottom electrode 18 which supports the substrate 19 to be processed,
e.g. a silicon wafer, on which various plasma-assisted chemical processes such as low- temperature etching and deposition can be performed.
[0036] When the matching box 13 and the transmission line 15 are removed, the RF source 14 directly sees the impedance of the coil 12 while inducing the plasma 16. The RF source 14 may be optimized for the impedances presented by the coil 12 when the RF source 14 no longer needs to be 50Q matched. The optimization of the RF source 14 for the varying impedance range requires knowledge of the expected impedance trajectory as presented by the coil 12. Such an impedance trajectory can be measured, and one exemplary impedance trajectory 200 is shown in Fig. 2. The impedance trajectory shows the change of impedance across the complex impedance plane over time and excitation power. The impedance trajectory 200 comprises an “OFF- state” value 201. During power increase, the real and complex impedance will vary along a trajectory 202, until an ultimate impedance value 203 is reached at peak power, from which the impedance will revert back to the OFF-state value 201 along a possibly different trajectory 204. The impedance can further be dependent on process conditions (plasma chamber pressure, process gas, gas flow rate, temperature, etc...) and chamber configuration.
[0037] The variability of the plasma impedance requires a constant change of the matching circuit, typically effected in matching box 13. This is typically achieved using a closed-loop system. However, such an adjustable matching circuit is source of inefficiency, added costs and complexity. The design of dedicated matching circuits furthermore poses challenges in transient behaviour (such as in the transient start-up behaviour of a plasma), instability (regressive power transfer) and drift/power tracking.
[0038] Referring to Fig. 3, an RF drive generator 300 according to the present disclosure comprises an input section 310, a switch-mode converter 320 and an output section 330. The input section 310 is configured to receive input power, such as an AC mains supply, at an input 301 and to generate a DC supply voltage Voo which is input to the switch-mode resonant converter 320. Switch-mode converter 320 is advantageously an inverter and is configured to generate a shaped sinusoidal waveform
Ver, advantageously an AC waveform of radio frequency, at an output of the converter.
The output section 330 is configured to adjust an impedance at the output 302 of the RF drive generator 300 and can comprise or consist of a reactive network comprising one or more inductors and/or capacitors. To this end, the output section has a fixed reactance and aids in defining an impedance operating region of the drive generator when directly coupled to an antenna 110, such as a coil or electrode, of a plasma chamber 11. It will be appreciated that the RF drive generator 300 is advantageously configured to be directly coupled to the antenna 110 without requiring a (variable) matchbox or transmission line.
[0039] The RF drive generator 300 further comprises a control unit 340 configured to control the operation of any one or all of the input section 310, switch-mode converter 320 and possibly output section 330, based on a set-point input 303 received e.g. from the plasma process control system through a user interface. A measurement unit 350, such as an IV probe, is configured to measure the complex voltage and current delivered by the switch-mode converter 320 and to feed the measurement to the control unit 340 which can provide feedback control to the input section 310 and/or the switch- mode converter 320.
[0040] Referring to Fig. 4, the input section 310 can comprise an AC/DC converter 311 coupled to a DC/DC converter 312. The AC/DC converter 311 is configured to convert the input signal at input 301, such as a single-phase or a three- phase mains supply to a stable DC voltage Veus on a DC rail 313. The AC/DC converter 311 can be isolated or non-isolated. Multiple topologies as known in the art can be used to achieve the AC/DC conversion. The DC/DC converter 312 is configured to transform the stable DC bus voltage Veus to the supply voltage Voo of the switch-mode converter 320. The DC/DC converter 312 can have an isolated or a non-isolated topology. Suitable examples are a half-bridge (buck) converter with or without interleaving. Advantageously, the DC supply voltage Voo is smaller than the DC voltage Vsus of the DC rail 313 at the input of the DC/DC converter 312. The DC/DC converter 312 therefore is advantageously a step-down (buck) converter.
[0041] The DC supply voltage Vpp defines the output power level of the RF drive generator and therefore the DC/DC converter 312 directly controls the power delivered by the switch-mode converter 320. According to an aspect, the supply voltage
Vbo is adjustable to multiple voltage levels. This can be achieved through a single DC/DC converter having a variable or adjustable Vpp output. Alternatively, multiple DC/DC converter units (e.g., two units) can be provided, connectable to the switch-mode converter, possibly in a mutually exclusive fashion. Each of the multiple DC/DC converter units is connected to Veus and is configured to have a different Voo output. The latter alternative reduces control requirements for the DC/DC converters, yet allows to obtain multi-state pulse trains with very high rise-times at the output of the RF drive generator, at the expense of increased hardware count.
[0042] The DC/DC converter 312 has preferably a high control bandwidth allowing a fast step response, e.g. smaller than or equal to 1 us, which is beneficial for fast pulsing applications. The output capacitance of the DC/DC converter 312 is advantageously sufficient small to allow for fast voltage control of Voo without sacrificing a too high output voltage ripple.
[0043] Alternatively, the input section 310 can be implemented with a direct
AC to DC converter, converting from AC Mains to Voo, hence combining the functions of the AC/DC converter 311 and DC/DC converter 312. One advantage of such an implementation is the reduction of components, with the disadvantage of less flexibility and control.
[0044] Referring to Fig. 5, the switch-mode converter 320 is configured to deliver the required RF power at the plasma excitation frequency, which can be in the range between 2 MHz and 300 MHz, preferably between 2 MHz and 150 MHz, such as 2 MHz, 13.56 MHz, or 27 MHz. The switch-mode converter 320 comprises a switching section 120 coupled to a resonator network 130. The switching section 120 receives the
DC supply voltage Voo, advantageously through a DC biasing network 321 and/or resonator network 130, and is configured to convert the DC supply voltage into an AC output signal Vas which is fed to the resonator network 130.
[0045] The switching section 120 advantageously comprises two switch devices M4, Mz, such as transistor devices, arranged in a push-pull configuration with a shared ground 121. Particularly, when My and M: are transistor devices, these are connected with their source terminal to ground 121 and the drain terminal to the supply voltage Voo. One advantage of the push-pull arrangement compared to half-bridge configurations is the inherent impossibility of shoot-through which is especially challenging for high-frequency converters and when using wide-bandgap transistors.
Furthermore, the connection to the shared ground 121 is preferable over a half-bridge since no galvanic isolation from any heatsink, which is also at ground potential, is needed, resulting in much better cooling possibility and lower electromagnetic emission.
[0046] The controller 340 generates a pair of synchronized drive signals 341, such as a pulse-train of possibly rectangular pulses having the same frequency and possibly same duty-cycle. Alternatively, possibly fixed duty cycle offset between the switch devices Ms and M: can be implemented. The second signal is 180 degrees shifted compared to the first signal, resulting in switching of the switch devices Mi, Mz alternatingly. The drive signals 341 are fed to the gate-driver 322 which buffers and translates the drive signals to the required signal levels of the pair of switch devices Mh,
Ma.
[0047] The switch devices M;, Mz which can be semiconductor switch devices, such as transistors, which can be based on SiC, GaN or silicon semiconductor technology, e.g. Metal Oxide Semiconductor Field Effect Transistor (MOSFET), are configured as a push-pull pair, and are operated in an alternating fashion. Particularly, when switch device M: is turned on, the other switch device M: is turned off and vice versa. Depending on the duty-cycle, and particularly for duty-cycles exceeding 0.5, both switch devices My and Mz can be turned on at the same time for a (small) portion of the switching period. For example, if the duty-cycle is 0.6, both switch devices can be turned on simultaneously for 20% of the switching period. For duty-cycles below 0.5, both switch devices can be turned off simultaneously for a portion of the switching period.
[0048] It will be appreciated that, depending on the drain current that the (transistor) switch devices My, Mz need to switch, use can be made of multiple transistor switches in parallel and/or multiple transistor stages M: / Mz in parallel. Alternatively, or in addition, multiple switching sections 120 can be arranged in parallel and/or in series, with shared or individual DC supply voltage Voo, to provide increased output power and/or voltage levels.
[0049] The DC biasing network 321 is configured to supply the DC supply voltage to the switching section 120 and advantageously behaves as a short at DC. The
DC biasing network 321 can have various suitable topologies for injecting the supply voltage Vpp. One suitable example is through an individual choke inductor for each of the two switch devices My and Mz, or through a shared choke inductor.
[0050] Switched-mode converters can obtain very high efficiency if zero- voltage switching (ZVS) is maintained, i.e., the equivalent capacitance at the drain node of the switching transistor is fully discharged before turning the transistor on. If the transistor is turned on too early, the charge inside the transistor is discharged and energy is lost in every switching cycle. At RF frequencies there are many cycles per second (>MHz) meaning loss of zero-voltage switching causes losses to unbearably increase.
Conventional topologies only maintain zero-voltage switching at small ideal load ranges.
Conversely, in RF drive generators according to the present disclosure, the load impedance range at which zero-voltage switching can be maintained is enlarged by including a resonator network 130. As a result, an improved load tolerance of the switch- mode converter is obtained.
[0051] The resonator network 130 is configured to achieve zero-voltage or zero-current switching conditions for the switching section 120. The resonator network 130 transforms the discrete on/off cycling of the pair of switch devices M:, Mz into a sinusoidal shaped voltage and current, significantly increasing the efficiency and robustness of the converter. The resonator network 130 can comprise or consist of a combination of reactive components such as capacitors and inductors, arranged in one or more resonating circuits, to shape the waveform accordingly. The network can comprise one or more sections, each consisting of a series connection, a parallel connection or a combination of parallel and series connection of reactive components.
The resonator network 130 is advantageously configured to condition the odd and even frequency components of the voltage waveform Vs 1, Vas2 occurring across the switch terminals of the switch devices Mi, M2 respectively (e.g. the drain-source voltage waveform in case of transistor devices), to maintain a zero-voltage switching (or a zero- current switching) of the switch devices Mi, M,. Specifically, once the impedance trajectory 200 of the antenna 110 is known, the resonator network 130 can be designed along with the reactive network 330 to enable an odd mode and an even mode impedance seen by the switch devices My, Mz which enable to maintain zero-voltage switching conditions over the entire impedance trajectory 200 of the antenna 110, thereby maintaining high efficiency of operation at desired power output over the impedance operating region of the load (i.e., the antenna 110).
[0052] Referring to Fig. 6, one example resonator network 130 comprises afirst resonator circuit formed by a T-network 133 of reactances Ls, C: coupled between the input terminals 131 of the resonator network. A third terminal 134 of the T-network is advantageously connected to ground. The input terminals 131 are directly connected to the “drain” voltage terminals of the switch devices M4, Mz of the switching section 120. A pair of shunt capacitors Csnunt can be series connected between the input terminals 131, with a midpoint connected to the third terminal 134 of the T-network 133. Fig. 6 illustrates the T-network 133 formed as a CLC network, but the T-network can alternatively be formed as an LCL network in which the positions of the inductance L: and capacitance
C: are swapped. The T-network is effective in conditioning the even frequency components of the voltage waveform of Vas,1 and Vas2, particularly in shorting the impedance seen by the switch devices M4, Ms at the second harmonic frequency of the switching frequency. It will be appreciated that other reactance networks can be utilized for this purpose instead of the T-network 133, e.g. a pair of series LC circuits connected between a respective one of the first terminal 136 and the second terminal 137 and the midpoint node 138, which can be equipotential with the third terminal 134.
[0053] The resonator network 130 can comprise a second resonator circuit 135 connected parallel to the first resonator circuit, between the input terminals 131 and the output terminals 132. The second resonator circuit 135 can be formed by an LC circuit, which can be a series LC circuit connected between respective input and output terminals of the resonator network 130, or advantageously, as illustrated in Fig. 8, an LC circuit with a capacitor Cs connected parallel to the output terminals 132. The pair of inductors Ls are advantageously connected between the first and second terminals 136,
137 of the T-network (i.e, the input terminals 131) and the output terminals 132. The second resonator circuit 135 is effective in conditioning the odd frequency components of the voltage waveform of Vas: and Vas2, particularly to condition the fundamental switching frequency and the third harmonic of the switching frequency to enable maintaining zero-voltage switching, or alternatively zero-current switching of switch devices My, Mo.
[0054] It will be appreciated that alternative implementations of the resonator network 130 can be utilized for the RF drive generators according to the present disclosure. Suitable example topologies for the switch-mode converter 320 are class-E, Class-F, or Class-EF: push-pull topologies. The transistor push-pull pair (common source) configuration of the switching section 120 combined with a properly designed resonator network 130 advantageously results in low voltage and current stress on the transistors.
[0055] Referring again to Fig. 5, the switch-mode converter 320 can furthermore comprise a transformer 323 coupled to the output of the resonator network 130. The two terminals of the primary coil of transformer 323 are coupled directly or indirectly, via resonating or reactive components, to a respective one of the switch devices My, Mz. Advantageously, the terminals of the primary coil of transformer 323 are connected to the output terminals 132 of the resonator network 130.
[0056] Transformer 323 can be a balun transformer, configured to convert a differential signal to a single-ended signal. In most common plasma applications, the antenna 110 of the plasma chamber accepts a single-ended RF signal at one terminal of the antenna, while the other terminal is connected to ground. One terminal of the secondary coil of transformer 323 can be connected to ground, whereas the other terminal of the secondary coil forms the output 324 of the switch-mode converter 320.
[0057] In some examples, the antenna 110 of the plasma chamber 11 is a differential antenna. A full differential path can thus be utilized from transformer 323 to the antenna 110. One advantage of such implementations is that the effective voltage from antenna terminal to ground can be twice as low compared to a single-ended implementation.
[0058] The transformer 323 can be implemented with coupled resonant coils, or alternatively with coils coupled by ferrite.
[0059] It will be appreciated that the functions of any two or more of: the
DC biasing network 321, resonator network 130 and transformer 323 can be combined.
In some examples, the primary coil of the transformer 323 can be centre-tapped and the supply voltage Voo connected to the centre tap such that the primary winding of the transformer can be utilized for DC biasing. Alternatively, or in addition, the leakage inductance of the transformer 323 can be incorporated as an inductance in the resonator network 130, e.g. as partial or complete replacement of L3 in the network of Fig. 6.
[0060] The output power of the RF drive generator is controlled by adapting the DC supply voltage Vop that is input to the switch-mode converter 320. The switch- mode converter 320, particularly the switch devices My, Mz are configured to operate at a constant, or fixed switching frequency. Minor adaptions can be made to the duty-cycle of the switch devices Ms, My, particularly to optimize the losses within the switch devices over impedance and power range. On/off pulsing of the RF drive generator can be achieved by switching the duty-cycle between nominal value and OFF, or by enabling and disabling the DC/DC converter 312, and hence setting Voo between zero and an operating value.
[0061] The reactive network of the output section 330 comprises or consists of one or a combination of reactive components, such as one or more inductors and/or capacitors, resulting in a reactive network of fixed reactance. The reactive network is advantageously directly connected to the antenna 110 of the plasma chamber 11. The reactive components of the reactive network are selected such that the impedance operating region with and without plasma ignition is within the desired operating range of the switch-mode converter 320. The reactive component(s) should produce a high- quality factor (Q) in combination with the antenna 110 at the plasma excitation frequency.
A high Q makes the ignition of the plasma substantially easier due to the high voltage across, and therewith electromagnetic field of, the antenna 110. In some examples, the plasma system is an Inductive Coupled Plasma (ICP), and the antenna 110 is a large coil with a significant reactance. In such case, the reactive network 330 is advantageously configured as a series capacitor to nullify the majority of the reactive impedance. Alternatively, the reactive network 330 can be configured as a series inductor, particularly when the load is a capacitive coupled plasma. Combinations of a series inductor and parallel capacitor or a parallel inductor and series capacitor as the reactive network are possible. Directly connecting the reactive network 330 to the antenna 110 can greatly reduce the size and costs of the interconnection between the
RF drive generator and the plasma chamber since most reactive current is cancelled locally.
[0062] The measurement unit 350 can be a current and voltage (IV) probe arranged at the output 324 of the switch-mode converter, e.g. between the output 324 (transformer 323) and the reactive network 330. The IV probe is configured to measure the complex voltage and current waveforms and therewith also the true and reactive power and complex load impedance. Alternatively, the IV probe can be arranged between the reactive network 330 and the antenna 110. The latter can be beneficial when the reactive network comprises multiple reactive components, of which some are connected to ground. The measurement unit 350 is connected to the control unit 340 which uses the measured complex parameters (current and voltage) to adapt the operation of the DC/DC converter 312 and/or the switch-mode converter 320.
[0063] The control unit 340 can be configured to translate a set-point of an output parameter, e.g. output power, into control signals for the various sections of the
RF drive generator, such as input section 310 and/or switch-mode converter 320. The set-point can be supplied at an input 303. The control unit further receives as input, the output from the measurement unit 350 (IV probe), particularly the measured complex (output) voltage and current. Based on the desired absorbed power and load impedance, the required level of supply voltage (Von) and duty-cycle of the switch devices M4, Mz is determined and supplied to the DC/DC converter 312 and switch-mode converter 320 respectively.
[0064] The control unit 340 or the measurement unit 350 can comprise high-speed digitizers to sample the complex voltage and currents measured by the IV probe. To perform the calculations at high-speed, a microprocessor, FPGA, DSP or PLD can be used.
[0065] The input section 310 is configured to generate the supply voltage
Vop which has an adjustable voltage level. Vos can either be continuously adjustable, or can be selected from multiple discrete predetermined voltage levels that can be supplied or generated by the DC/DC converter. The supply voltage Von advantageously defines the envelope of the sinusoidal power or voltage signal at the output 302 of the drive generator. This envelope can be shaped as desired by appropriate adjustment of Voo.
Possible envelope shapes are block (rectangular or trapezoidal) pulses, triangular pulses, sine waves, multi-state or multi-level pulses, and combinations thereof. An example of a rectangular pulse train is illustrated in Fig. 7, with Vpp defining the envelope voltage 601 shown in dashed line. The RF output voltage Vr has a frequency equal to the plasma excitation frequency as well as the switching frequency of the switch devices
Mi, M2. The rise-time 602 of each pulse can be specifically controlled, e.g. by adjusting
Voo. An example of a multi-state pulse train having two voltage levels (states) is illustrated in Fig. 8. Arbitrary envelope modulation is possible, essentially limited only by the capability of the DC/DC converter in terms of the speed and accuracy. Through the
DC supply voltage Voo, the DC/DC converter 312 and control unit 340 hence directly control the RF output voltage Vrr generated by the RF drive generator and therewith the power absorbed by the plasma. It will be beneficial when the DC/DC converter is flexible and dynamic to adjust Voo, so as to modulate and control the required output power with a desired speed and accuracy.
[0066] Alternatively, instead of adjusting Vpp to obtain the pulse trains as indicated above, an ON-OFF type pulsing can be performed by controlling the gate signals to the switch devices while Vpp is maintained at a fixed or constant level.
[0067] Referring to Fig. 9, when high power output is required, the switch- mode converter 320 can comprise multiple converter units, such as N converter units 3251 … 3259, N = 2, in parallel. Each converter unit 325, ... 325y can comprise an individual switching section 120 and resonator network 130. The outputs of the resonator networks of the multiple converter units 3251 ... 325n are advantageously connected in parallel through a combining network 326. The combining network can be implemented in several ways, such as a Wilkinson combiner or as a transformer.
[0068] Referring to Fig. 10, the RF drive generator 300 can in addition comprise multiple DC/DC converters 312; ... 312n, with N 2 2 and advantageously equal to the number N converter units 325: n. Each DC/DC converter can be configured to regulate its respective DC supply voltage Voo such that the N converter units 325; output equal power. To this end, it is advantageous to measure the output voltage and/or current of each of the N converter units individually, such as via individual measurement probes 327 provided at respective outputs of the converter units. Based on the measured output power (voltage and/or current) of each converter unit, the respective DC supply voltage Voo can be controlled individually, e.g. via control unit 340, to balance the output power of each converter unit individually. It will be appreciated that the measurement probes 327 can be provided in the RF drive generator of Fig. 9 as well. In the latter case the output power can be adjusted to some extent via individually controlling the duty- cycles of the switch devices of the converter units. It is alternatively possible to provide multiple AC/DC converters instead of the (single) AC/DC converter 311 and the multiple
DC/DC converters 3251. n.
[0069] In some examples, referring to Figs. 11 and 12, the combining network 326 is implemented as a multi-port transformer 70. The primary side of multi- port transformer 70 comprises multiple coils 711 … 71x, each connected to the output of a respective converter unit 3251 … 3255. The secondary side of multi-port transformer 70 can comprise multiple coils 721 … 72n each inductively or magnetically coupled with a respective primary side coil 711 … 71n. To combine the outputs of the converter units 325; … 325 in a single output 324, the secondary side coils 72: … 72x can be series connected as shown in Fig. 10, or parallel connected as shown in Fig. 11. Alternatively,
the secondary side of multi-port transformer 70 can comprise a single coil that is inductively or magnetically coupled with all the primary side coils 714 ... 71n. Yet alternatively, the outputs of the resonator sections 130 of the N converter units can be parallel connected before connecting to a transformer.
[0070] Referring to Fig. 13, another plasma application in which drive generators according to the present disclosure can be advantageously utilized is
Transformer Coupled Plasma (TCP). In a TCP system 500, the antenna 110 comprises two coupled coils 111, 112 to excite the plasma in the plasma chamber 11. Each of the coupled coils 111, 112 is driven by an independent RF drive generator 3001, 3002 respectively. The drive generators 3001, 3002 can have a shared AC/DC converter 311, and hence be coupled to a shared DC rail with bus voltage Vsus, but can alternatively have independent AC/DC converters. The drive generators 3001, 3002 can be controlled by a same control unit 340. Each of the two RF drive generators is configured to independently control the phase and amplitude of the power delivered to the respective coil, e.g. by individual IV probes 350 the measurements of which are individually fed back through control unit 340 to control the respective DC/DC converters 312 and converter units 320. By using two independent drive generators 3001, 3002, the voltage and power for each of the coupled coils 111, 112 can independently be controlled resulting in maximum plasma uniformity control. In addition, since the drive generators operate with constant switching frequency, the control is rendered simpler.