Method and apparatus for preventing instabilities in radio-frequency plasma processing

A method and apparatus for controlling a power supply to prevent instabilities due to dynamic loads in RF plasma processing systems, operating at frequencies of from 1 MHz and up, uses a feedforward type of control loop to tightly regulate the power supplied to the dynamic electrical load, such as loads caused by variable and inconsistent plasma impedance. A feedback control loop can also be used in combination with the feedforward loop, but at a slower rate, to help regulate the amount of power provided to the load.

FIELD OF THE INVENTION

The invention relates generally to control systems and methods, and more particularly to a method and system for controlling power supplied to a plasma load.

BACKGROUND OF THE INVENTION

Radio frequency or microwave (hereinafter “RF”) plasma generation equipment is widely used in semiconductor and industrial plasma processing. Plasma processing supports a wide variety of applications, including etching of materials from substrates, deposition of materials onto substrates, cleaning of substrate surfaces, and modification of substrate surfaces. The frequency and power levels employed vary widely, from about 10 kHz to 10 GHz, and from a few Watts to as much as 100 kW or greater. For semiconductor processing applications, the range of frequencies and powers presently used in plasma processing equipment is somewhat narrower, ranging from about 10 KHz to 2.45 GHz and 10 W to 30 kW, respectively.

Plasma processing equipment typically requires a precision RF signal generator, a matching network, cabling, and metrology equipment. In addition, precision instrumentation is usually required to control the actual power reaching the plasma. The impedance of loads associated with a plasma can vary considerably in response to variations in the gas recipe, various gas supply parameters, plasma density, delivered RF power, pressure and other variables. The dynamic electrical load presented by the varying impedance of a plasma load, and these other variables, can create significant stability control problems for plasma generation equipment.

In today's plasma processing equipment, such as semiconductor tools, it is often observed that the process becomes unstable, exhibiting oscillations in plasma density and/or loss of plasma altogether. Thus, a need exists to prevent these instabilities by changing the way RF power supply is regulated.

A known plasma generation system including a typical plasma processing tool and an RF power supply, familiar to those skilled in the art, is depicted inFIG. 1. An RF power supply10delivers RF power through a filter12and to the process vacuum chamber15via cables13and a load matching circuit14. RF power, both forward and reflected, is measured at output power measurement point18, near where the power enters the process vacuum chamber15. Feedback from the output power measurement18is sent back to the power supply10, to complete the control loop.

Power regulation control circuitry is generally designed to operate as fast as possible, to maximize control stability. A typical power supply control loop can operate as fast as a few hundreds of microseconds, and is usually designed and optimized in view of a fixed load, or perhaps with a range of possible fixed loads in mind. The speed of the control loop, e.g., in a plasma generation system, is limited by filter and matching network delays, power measurement time and the internal reaction speed of the power supply. As described above, the plasma introduces additional delays in the control loop that may ultimately result in unstable system behavior. These delays are not easily predictable, are process dependent, and are inconsistent.

However, many power regulation control instabilities can be attributed to having the plasma as a part of the power regulation control loop, since plasma impedance is not constant. More specifically, plasma impedance is a function of the amount of power delivered, the plasma gas pressure, and the chemical composition of the plasma gas mixture. Moreover, the reaction chemistry and gas pressure variables are dependent upon plasma density and temperature, and also have inertia (latent time dependencies) of their own. These inertia times can be in the range of from microseconds to milliseconds, and are determined by various processes, such as diffusion rates, pumping speed, ionization and chemical reaction rates, etc.

Another complicating factor is introduced by the gas supply system20, which oftentimes has a control loop of its own. This gas supply control loop22commonly controls the gas supply to the chamber15based on a chamber pressure measurement23. This separate control loop22can extend the specific and system inertia (delay) times of the power control system to hundreds and thousand of milliseconds. Thus, any power supply that uses only a feedback control loop25with a response time interval that is faster or comparable to the system or plasma inertia, i.e., the response time of the electrical load (such as the plasma), is potentially prone to instability.

FIG. 2illustrates the power section of a known power supply technology that can be used with a system such as the plasma processing system illustrated inFIG. 1. A power supply30(e.g., a DC switching power supply) is designed to deliver a constant power output value to a load40at the output of the power supply. DC power32, e.g., from a rectified bus or a DC power source, feeds switching transistors and reactive elements such as inductors and capacitors, as is known to those of skill in the art. Power supply30generally delivers constant current or constant voltage on a time scale (i.e., at a time interval) faster than the response time of its control loop33. The control circuit35adjusts a control signal to the power supply30based on information received from the feedback control loop33. Of course, power supply30can also deliver constant current or power when the control loop is frozen (open). For reference, such power supplies are referred to as “constant current” or “constant voltage” types, below. The control circuit35uses feedback33from the plasma power measurement circuit18to modify the output voltage or current of the power supply30, to maintain the desired power output. In some situations this can result in stable control of the load40(e.g., a plasma load).

In practice, one example of known technology is a “constant current” DC power supply which is used to power an arc-type plasma load. Since plasma resistance drops at higher temperatures (often referred to as “negative resistance”), it is known that supplying a constant current to such a system maintains a stable plasma. In such systems, the power supply functions somewhat as a virtual ballast resistor.

More commonly, many DC switching power supplies use a pulse width modulation (“PWM”) control method. These behave as voltage sources and thus are unable to effectively sustain a negative-resistance plasma without a feedback control loop. Moreover, when plasma systems are powered with an RF power supply control problems are exacerbated, since the phase of the reflected power is shifted by cables (e.g.,13) and filters (e.g.,12) to such an extent that it is difficult or impossible to ensure either positive or negative apparent plasma resistance, as viewed from the power supply. Such problems are especially pronounced for systems operating at high frequencies, e.g., in the RF region (i.e., in the range of 1 MHz and up).

SUMMARY OF THE INVENTION

To overcome these problems, what is needed is more than a power supply that merely provides constant voltage or constant current, but instead is capable of delivering essentially constant power to an open loop system, whereby stable operation of the plasma can be maintained regardless of reflection phase and impedance of the plasma.

More specifically, what is needed is a method and apparatus that can be used to precisely control the amount of power provided to a dynamic load, such as a plasma load, in spite of varying load impedance and resistance caused by the plasma and/or other system components.

The present invention overcomes these problems using a power supply that comprises a feedforward loop, and optionally a slow feedback loop that provides a constant power supply to an electrical load, such as a plasma in a plasma generator.

One aspect of the invention features an RF power supply for providing power to a dynamic load, such as a plasma, comprising a power circuit for converting an input power to output RF power. The power circuit has low stored energy, generally on the order of a few microseconds, such that the amount of stored energy in the circuit does not interfere with the control stability. The control circuit provides a control signal to the power circuit to regulate an input power consumption by, e.g., a DC section of a power converter that supplies an RF generator. It also includes a first circuit that provides a measurement of an input power, which is used to adjust the control signal to the power circuit at a first rate. The input power to the RF power supply is thus maintained at a constant, or a substantially constant amount.

Embodiments include a second circuit for measuring an output power at an output of the RF power supply, which is used to compensate the power control signal at a second rate. The first rate is greater than the second rate. The first rate can correspond to a frequency of a switching power supply that feeds DC current to the RF power supply. Furthermore, a weighting factor can be used to cause the first rate to be greater than the second rate. The output of the RF power supply can be used to energize a plasma load.

Embodiments can include at least one of a resonant inverter, a buck-type switching regulator, a boost regulator, a DC switching power supply, and a gate drive circuit of an inverter or a switching power supply. The input power can be measured on a frequency corresponding to each cycle of the switching power supply. The output power signal can be measured at a different rate, e.g., not for each cycle of the switching power supply. In some embodiments the control signal is updated each cycle of the switching power supply, while in others it is not.

The input power measurement can be determined by measuring a total charge that flows through an input terminal of the switching power supply during each cycle. The first rate can adjust the power control signal once per cycle, and the second rate is slower than the first rate and should also be slower than a response time of the electrical load. The second rate can also be slower than a response time of a gas supply system that supplies the electrical load.

Another aspect of the invention features an RF power supply for providing power to a variable-impedance load that includes a control circuit providing a control signal to the RF power supply to regulate a power output level from the RF power supply and a first circuit that provides a measurement of an input power to the RF power supply. The first circuit adjusts the control signal to the RF power supply at a first rate to maintain substantially constant input power. The RF power supply can include a second circuit for measuring an output power at an output of the RF power supply, which is used to compensate the control signal to the RF power supply at a second rate. The first rate should be greater than the second rate. In some embodiments, the input power is constant.

A weighting factor can be used to cause the first rate to be greater than the second rate, and an output of the RF power supply can be used to energize a plasma load. A matching circuit can be used, located between the RF generator and the load (e.g., the plasma load). The power supply can include a resonant inverter, a buck-type switching regulator, a boost regulator, a DC switching power supply, and/or a gate drive circuit of an inverter or switching power supply. The input power can be measured for each cycle of the power supply, and in some embodiments the output power is not measured for each cycle of the power supply. The control signal can be updated for each cycle of the power supply, but in some embodiments it is not.

The RF power supply can deliver power to a plasma load. Further, the control signal can control a pulse width modulation of the power supply, and the amount of pulse width modulation can be determined on a cycle by cycle basis. In some embodiments the first rate adjusts the control signal once per cycle, and the second rate is slower than the first rate and should also be slower than a response time of the electrical load. The second rate can also be slower than a response time of a gas supply system that supplies the electrical load.

Another aspect of the invention features an RF plasma generator comprising a power source and an AC switching power supply. The switching power supply includes a control circuit that provides a control signal to the power supply to regulate a power output level from the power supply and a first circuit that provides a measurement of an input power to the power supply, which is used to adjust the control signal to the power supply at a first rate, thereby maintaining substantially constant input power. It also includes a a second circuit for measuring an output power at an output of the power supply, which is used to compensate the control signal to the power supply at a second rate, such that the first rate is greater than the second rate. An output of the switching power supply supplies an RF generation section. The first rate can correspond to a switching frequency of the switching power supply.

A weighting factor can be used to cause the first rate to be greater than the second rate, and an output of the RF generation section can energize a plasma load. A matching circuit can be used, located between the RF generation section and the load. The RF generator can include a resonant inverter, a buck-type switching regulator, a boost regulator, a DC switching power supply, and/or a gate drive circuit of an inverter or switching power supply. The input power can be measured for each cycle of the power supply, and in some embodiments the output power is not measured for each cycle of the power supply. The control signal can be updated for each cycle of the switching power supply, but in some embodiments it is not.

The input power of the RF generator can be determined by measuring a total charge that flows through an input terminal of the switching power supply during each cycle, and the RF generator can include a plasma generation system. The control signal can a pulse width modulation of the switching power supply and the amount of pulse width modulation can be determined on a cycle by cycle basis. In some embodiments the first rate adjusts the control signal once per cycle, and the second rate is slower than the first rate and should also be slower than a response time of the electrical load. The second rate can also be slower than a response time of a gas supply system that supplies the electrical load.

Yet another aspect of the invention features a method of regulating power to a variable-impedance load, comprising the steps of providing a control circuit that provides a control signal to a switching power supply, to regulate a power output level from an RF generator. The control signal to the switching power supply is adjusted at a first rate based on a first power measured at an input side of the switching power supply. It is also compensated at a second rate, which is based on a second power that is measured at an output side of the RF generator. The first rate is greater (faster) than the second rate. The first rate can correspond to a frequency of the switching power supply.

A weighting factor can be used to cause the first rate to be greater than the second rate, and an output of the RF generator can energize a plasma load. A matching circuit can be used, located between the RF generator and the load. The power supply of the method can include a resonant inverter, a buck-type switching regulator, a boost regulator, a DC switching power supply, and/or a gate drive. The input power can be measured for each cycle of the switching power supply, and in some embodiments the output power is not measured for each cycle of the power supply. The first power can be determined by measuring a total charge that flows through an input terminal of the switching power supply during each cycle. Further, the control signal can be updated for each cycle of the switching power supply, but in some embodiments it is not.

The method can include having an electrical load that comprises a plasma generation system, and the control signal can control a pulse width modulation of the power supply. The amount of pulse width modulation can be determined on a cycle by cycle basis. In some embodiments the first rate adjusts the control signal once per cycle, and the second rate is slower than the first rate and should also be slower than a response time of the electrical load. The second rate can also be slower than a response time of a gas supply system that supplies the electrical load.

DETAILED DESCRIPTION

FIG. 3illustrates an embodiment of the invention that can be used to overcome the problems described above. Providing a constant amount of power to an electrical load, such as a plasma, can result in stable operation of the system. Unfortunately, the feedback-only systems described above do not respond quickly enough to the dynamic operating conditions of plasma generation systems. Even if the response time of feedback loop33is faster than the impedance perturbations and other ongoing variations of a dynamic plasma load, such systems do not compensate for varying input signals.

The power supply30ofFIG. 3overcomes these problems by controlling input power instead of output power, taking advantage of high efficiency and low stored energy achievable with presently available technologies of switching power supplies. Using this technology, the invention supplies constant power regardless of changes in plasma loading transients, on a time scale basis that is slower than stored energy divided by suppplied power, which can be of the order of several microseconds. To accomplish this the power supply30ofFIG. 3uses a fast input measurement52on the DC power30feeding the power supply30. This input power measurement52feeds a fast feedforward control loop53that provides information to the control circuit35. The control circuit can be an actual circuit (e.g., on a circuit board), an analog controller, any of various digital control systems known to those in the art, or combinations of these. Using this technology, the invention controls the interaction between the power supply and the plasma by tightly controlling the amount of power delivered to the power supply30. This results in tight control of the power sent to the load because the dynamic plasma parameters are not as sensitive to current or voltage waveforms provided by the power supply, as they are to the amount of power delivered, as averaged over very short time intervals (e.g., a few microseconds).

Accordingly, the invention includes power supply topologies that can deliver almost constant power into a plasma load on very fast time scales, such as time intervals on the order of a few microseconds. In some embodiments, these time intervals can be sufficiently long in duration to approach the dynamic response times of plasma or gas supply responses. However, in all embodiments the power supply output is maintained at a steady amount, in a time scale that is set by the ratio of stored energy to power, regardless of changes in plasma impedance or other load dynamics.

Feedback control33alone is not capable of responding fast enough to load and power input changes to accomplish the precise and consistent power output of the invention. Generally, to maintain stability, the feedback control response rate must either be much faster or much slower than all possible plasma responses and gas supply responses. To achieve the precise power output control of the invention, not only must stabilization of the system be provided on a fast time scale, but the control signal38of the control circuit35can be allowed to only weakly (i.e., slowly) depend upon the output power measurement. Thus, the invention uses only a slow response feedback control loop when compensating the control signal38to the power supply. An additional benefit of such a power supply is the easy and stable ignition and build-up of the plasma.

To achieve the results of the invention, a DC power source32is connected to the power section of a power supply30. Preferably, the system is designed to dissipate as little power as possible, and thus incorporates mostly reactive or switching devices. It is desirable to minimize stored energy in the system, thus reducing or virtually eliminating any lag time or time constant of the control system. This design principle is preferred for both DC switching power supplies and lower frequency RF generators incorporating the principles of the invention, which can operate in a switching mode (e.g., Class D or Class F). At higher frequencies, where RF transistors operate in an almost linear regime, this design feature is less important. Nonetheless, it is always preferred to minimize energy losses in the power section of the power supply30.

In the embodiment shown inFIG. 3, the input power to the power module is measured via input measurement52. The input power measurement52can be located in front of the power supply, but is should be located downstream of any input filter capacitor (not shown). Alternatively, the input power measurement52can be located at some point inside the power supply30, as long as a measurement of input current, voltage, and/or power is provided, as is known to those of skill in the art. The input power measuring means52sends this information to the control circuit35, which then adjusts power control signal38to maintain the input power measurement32at a substantially constant value. Since the efficiency of the power module is designed to be high, this method of regulation results in nearly constant power output to the load40.

The power supply30can include small amounts of stored energy in output filters and reactive components of the power section, which cannot be changed instantaneously. As discussed above, the design of the system minimizes the amount of energy stored in these components, and thus the amount of energy present in these components is sufficient to power the system for the equivalent of not more than a few microseconds. Having such low stored energy is beneficial to reducing a lag time, or time constant, of the system and is beneficial in maintaining stable plasma control. Since the stored energy amount is small compared with the amount of energy stored in the plasma, it does not have a significant adverse impact on the control of the system and does not cause the plasma to be lost, to become unstable, or to be changed significantly. The RF power thus tracks the input power with very little lag time. As the amount of stored power increases the likelihood of plasma stability control problems also increases.

Embodiments also include a feedback control loop33. In order to maintain precise power output control to the load40, the feedback control loop33should be tuned such that it has only a slow effect on the control equations of the control circuit35. The slow response of the feedback control loop33ensures that load impedance changes do not destabilize the control loop.

FIG. 4illustrates a partial view of a power supply representing one embodiment of the invention. A buck-type switching regulator72can be used to supply an RF generator75. The input power to the RF generator is measured using an input measurement52, an integrator73, and a comparator74which provide an error signal to the control circuit35. The top switch76is an actively driven transistor. The bottom switch77can be passive, e.g., a diode, or it can also be a transistor. If the operating frequency of the buck converter72is fixed, then the power input is proportional to the amount of charge that passes through the top switch76each cycle. Accordingly, a current transducer (e.g.,52) and an integrator73can be used to determine the amount of work (energy) provided each cycle. The end of the switching cycle can then be triggered when the amount of work reaches a desired value. During fixed frequency operation, this is indicative of the amount of work provided per unit time, i.e., the amount of power being provided. This measurement can then be conveniently used as a rapid input power measurement52to the power supply, upon which the feedforward control loop53can be based.

Further, when switching frequency of the DC section is not constant, as is the case in resonant inverter topology, the desired value of integrated charge per period can be determined as a function of both the input voltage and switching frequency in such a way that it can be used to maintain the input power that is entering the power supply30via its input terminals at a constant value. Such feed-forward regulation can be performed on a pulse-by-pulse basis, and lags the regulator by only one period (i.e., one cycle of the power supply). The time interval of such a measurement can thus be as short as, e.g., one or two microseconds. One way to compensate for frequency change is to send per-cycle integrated charge information to a comparator, using a ramping reference rather than a constant. The trip point of the comparator is reached when the integrated charge is proportional to the time that has elapsed since the start of the cycle. This technique can be used to maintain constant power on a cycle-by-cycle basis.

Based on the above description, alternative embodiments will become apparent to those of ordinary skill in the art. For example, embodiments include boost regulators, for which constant input power requires that the inductor current be maintained constant. Further, other PWM switching topologies can be used, as well as frequency-controlled topologies such as resonant inverters. These can include an isolation transformer controlled in similar fashion, maintaining constant input power on a pulse-by-pulse basis.

FIG. 5illustrates a detailed embodiment of the invention, which includes additional detail and optional components. The DC power source32provides power to a DC power section30, such as a DC switching power supply. The input current measurement52is illustrated as being located on the input side of the power supply30, but it can also be located on the output side, e.g., between the power section30and the converter91. As described with respect toFIG. 4, the input measurement signal from52can be routed through an integrator73and a set point comparator74.

Embodiments include different comparator74functionalities. For example, the input power from input measurement signal52can be compared with a power setpoint to determine a calculated feedforward error, and this error can be used by the control circuit35. This error is used to adjust the control signal38to the power supply. In other embodiments, the input signal52can be compared, e.g., with an algorithm or a predetermined table of values whereby the output result from the comparator is determined by a predetermined functionality, in accordance with control principles and technologies known to the skilled artisan. This input power regulation can operate very quickly, including as frequently as once per each cycle of the power supply.

The control circuit35can also receive a feedback power signal from the output power measurement18. This feedback signal can be compared with the power output setpoint to determine a calculated feedback error, and this feedback error and the calculated feedforward error can be summed by the control circuit35to determine the control signal38. The power output setpoint for the feedback error calculation can be the same as the power outpoint setpoint for the feedforward error calculation, or it can be different. In either case, weighting of the feedforward and feedback signal can then be performed using various techniques.

Unlike many conventional control systems, the feedforward control loop53should be weighted to have more effect on the output signal (e.g., the control signal38) than the feedback control loop (e.g., feedback control loop33). This result can be achieved by measurement sampling and then using the results of the sampled feedforward signal more frequently (at a faster rate), e.g., each cycle of the power supply, and using the results of the feedback loop at a second, slower rate such as every cycle, or 10th, or 100thcycle. Using this technique the control signal thus reflects the contribution of the feedforward loop more frequently than the feedback loop, and thus the feedforward measurement is prevalent in the control result38.

Another technique for favoring the feedforward measurement52over the feedback measurement18is to sample each loop at the same frequency, but to incorporate a weighting multiplier that increases the feedforward signal or error result, and/or decreases the feedback signal or error result. The error result from the feedback loop could thus always be made to have less effect than the error from the feedforward loop, when determining the control signal38, even if both loops were sampled at the same frequency. In this manner the weighting factor can cause the rate of the adjusted feedward loop to be greater than the rate of the compensating feedback control loop, even though the feedforward measurement signal52and the feedback power output signal are sampled at the same frequency. In some embodiments, the feedback control loop18can be weighted to such an extent that it has zero effect on the control signal38. Other techniques to accomplish the different types of weighting results described above will also become apparent to the skilled artisan, such as averaging, using timed mean values, various sampling techniques, statistical techniques, and others. The result of these methods is that the control circuit35outputs a control signal38to the power supply based mostly on the input signal, but also on the feedback power signal18.

The response of the power supply30to the control signal38is faster than either the feedforward or feedback control loops. The control signal38can regulate the power supply30using pulse width modulation technologies (for a constant frequency power supply), using frequency modulation, combinations thereof, or using other known DC switching power supply control technologies.

The DC power section30generally feeds a converter section91, such as a buck converter or a resonant converter in combination with a rectifier. Of course, other converter topologies can also be used. Embodiments of the invention include feeding a plasma load from the buck converter without an intermediate RF power generator (described below). Referring again toFIG. 5, output of the converter optionally passes through an output filter92, and then can be feed to an RF generator75. The output of the RF generator75can pass through the output power measurement device18and then to an electrical load40, such as a plasma in a plasma chamber. Optionally, a matching network96can be placed between the RF generator75and the load40.

Although the input measurement52is illustrated as being on the supply side of the DC power section30, it can also be placed in other locations. For example, this feedforward measurement signal can be taken between the power supply30and the converter91, or even further downline, between the converter91and the output filter92(if an output filter is present).

Similarly, the feedback power measurement18can also be located in other positions. For example, the best power measurement reading is obtained when the feedback power measurement device18is located directly upstream of the load, i.e., between the matching network (if present) and the load. Due to cost, however, it is often located at the output of the RF generator, i.e., between the RF generator75and the matching network96(if present). However, embodiments of the invention include having the feedback signal measurement functionally positioned between the output filter92(if present) and the RF generator75, or even between the converter section91and the output filter92. Of course, different combinations of these various input and output signal measurement locations can be used.

Suitable power output measurement techniques can include voltage and/or current measurements, as discussed above with respect to the input measurement52. However, the output power measurement can also use known techniques and technologies to measure forward and reflected power to accomplish the objectives of the invention.

The techniques of the invention have broader applicability than the embodiment depicted inFIG. 5. AlthoughFIG. 5depicts the feedforward measurement on the input side of the DC switching power supply30and the power output measurement18at the output of the RF generator, the invention can be used in other ways. For example, an embodiment includes a feedforward measurement52at the inlet of a DC switching power supply30and an output power measurement18at the outlet of the power supply30, e.g., after a converter91. In some applications, the output of the converter can then be routed directly to a load. In other embodiments, the supply to an RF generator does not have to be provided by a DC switching power supply. In such embodiments the input measurement52can be taken at the input to the RF generator and the power output measurement18can be positioned at the outlet of the RF generator75. The output of the RF generator can then be fed to a load, e.g., a plasma chamber, either with or without a matching network96.

Although such embodiments are within the scope of the invention, as noted above, the invention is particularly useful in solving plasma stability problems for systems using high frequency power supplies, and in particular for RF power supplies (i.e., 1 MHz to 10 GHz).