Abstract:
An apparatus and method for the detection of and recovery from a plurality of electrical transients in a plasma generator system is disclosed. The plasma generator system comprises a plasma chamber and a power amplifier is provided, wherein the apparatus comprises a sensor, the sensor detects an electrical transient in the electrical transients and outputs a first signal when the electrical transient is detected. The system also includes a first circuitry for receiving the first signal, the circuitry controlling said power output from said power amplifier in response to the first signal and outputting a second signal communicating the controlled power output, and a second circuitry for receiving the second signal, the second circuitry outputting a reset signal to the first circuitry, wherein the first circuitry receives the reset signal and resets the power output in response to the reset signal.

Description:
FIELD OF THE INVENTION 
     This invention relates to plasma generation equipment and techniques, and it is particularly directed to a method and apparatus for the quick protection and recovery from load transients, and protecting circuitry in plasma generation equipment from transient current and voltage peaks originating in the plasma chamber. 
     BACKGROUND OF THE INVENTION 
     It has long been understood in the field of plasma generation that, extremely short and high power load transients may occur. The design of plasma generators, therefore, must afford the generator circuitry protection from such destructive transients. Moreover, such a system must be capable of reacting extremely quickly in recognizing and recovering from such transients, as those transients may be extremely brief. It is similarly advantageous for such a system to guarantee that the control system associated with the plasma generator is informed each and every time a transient occurs. This would allow the control system of the plasma generator to restore output power to its pre-transient set-point as rapidly as possible. 
     In a typical RF plasma generator arrangement, a generator supplies forward power to the plasma chamber. A portion of the forward power is reflected back towards the generator, as reverse, or reflected power. Conditions may occur, however, that can cause transient peaks in the reflected power. These transient peaks can cause excessive current or voltage peaks in the generator. These peaks are unpredictable, and are of such amplitude that they pose a threat to the integrity of circuits within the plasma generation equipment. Detection, effective countermeasures, and quick recovery from such transient peaks, therefore, is an important aspect of the design of plasma generation equipment. 
     Typical plasma generator protection schemes, however, merely focus on the cut-off of the input power (and therefore the output) to the plasma generator in order to protect the generator circuitry from damage. As will be discussed below, this protection scheme suffers from several disadvantages. 
     FIG. 1 depicts a block diagram of a typical transient protection scheme. This protection circuitry works on the aforementioned principle of simply shutting down the drive to the power amplifier  150  upon detection of a transient. When the sensor  110  detects a transient, it will turn off the drive to the power amplifier  150  via the switch  160 . 
     Although this design may operate to protect the power amplifier circuitry from damage due to transients in the plasma chamber, it suffers from several shortcomings. Most important of these shortcomings is that, due to the rapid rise and short duration of some transients, there is a substantial probability that the controller  120  will not be able to react quickly enough to acknowledge that a transient has occurred. Accordingly, the response time of the protection system, from the time the transient occurs to the time the protection system restores power output to its previous levels, may be unacceptably long. 
     Additionally, the inability of present systems to acknowledge all transients may have damaging effects on the plasma generator&#39;s circuitry. For example, if the controller  120  does not react quickly enough to realize that a transient has occurred, the software will also be unaware that a transient has occurred. Thus, the control software  130  will only realize that the power output from the power amplifier  150  has either diminished, or has shut off completely, since the input power to the power amplifier  150  has been cut by sensor  110 . Accordingly, and in reaction to the above situation, a cascade effect will occur that may damage the circuitry of the system. That is, the software  130 , unaware that input power was cut-off due to the transient, will increase its own internal parameters in an attempt to produce more power out of the generator in order to meet its present power set-point. Thus, when the transient actually does subside and the sensor  110  restores input power to the power amplifier, the software&#39;s  130  internal control parameters will have been driven to a very high level; much higher than before the transient occurred. This may result in a second transient, which may destroy the circuit elements of the plasma generator. 
     Furthermore, since there is no guarantee that the software will be capable of reacting to each and every transient, accurate reporting of these transients is impossible. Thus, although it would be desirable to report on the frequency of transients within the system, such reporting is not possible using a protection scheme that cannot guarantee recognition of virtually each and every transient. 
     Accordingly, although present cut-off protection schemes offer at least some level of protection to power amplifier circuitry from the occurrence of transients, they are lacking in several respects. Most importantly, they are incapable of reacting quickly enough to realize that a transient has even occurred, and will have an unacceptable delay time from the point the transient is detected, to the point where output power from the power amplifier is turned off. Additionally, since present detection systems are incapable of reliably detecting transients, they can react incorrectly to the protective cut-off and produce a second equally destructive transient. Also, they cannot provide accurate reports of the occurrence of the transients, which would greatly aid in the design and improvement of plasma generator and plasma chamber design. 
     It is thus an object of the present application to solve these and many other problems. This object, and many other features and advantages of this invention will become apparent from the ensuing description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a transient protection scheme; 
     FIG. 2 is a general block diagram of the present invention. 
     FIG. 3 is a block diagram of a first embodiment of the present invention; 
     FIG. 4 is a block diagram of a second embodiment of the present invention; 
     FIG. 5 is a block diagram of a third embodiment of the present invention, and, 
    
    
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, an apparatus is disclosed for the detection of and recovery from a plurality of electrical transients in a plasma generator system, said plasma generator system comprising a plasma chamber and a power amplifier is provided, wherein said apparatus comprises a sensor, said sensor detecting an electrical transient in said plurality of electrical transients and outputting a first signal when said electrical transient is detected, a first circuitry for receiving said first signal, said first circuitry controlling said power output from said power amplifier in response to the first signal and outputting a second signal communicating the controlled power output, and a second circuitry for receiving said second signal, said second circuitry outputting a reset signal to said first circuitry, wherein said first circuitry receives said reset signal and resets said power output in response to said reset signal. 
     A method is also disclosed for the detection of and recovery from a plurality of electrical transients in a plasma generator system is provided, wherein said plasma generator system comprises a plasma chamber and a power amplifier, said method comprising the steps of detecting an electrical transient in said plurality of electrical transients in said plasma generator system, controlling the output power so as to protect the system, signaling a processor that said transient has occurred, sending a signal from said processor to a control software program, said signal from said processor to said control software program being sent each time a transient is detected in said plurality of transients; said control software program controlling the output of said plasma generator system until said output is stabilized, said control software program outputting a reset signal to said processor, said reset signal indicating that said output of said plasma generator is stabilized. 
     DETAILED DESCRIPTION 
     Referring now to FIG. 2, a general block diagram of the present invention is shown. Generally, the Fast Transient Recovery system (FTR) of the present invention comprises three main systems; a sensor circuit  200 , which may comprise one or more sensors monitoring for harmful transient conditions, a fast response circuit  210  and a control system  240 . The sensor circuit  200  detects a transient condition within the system based on set-point parameters for the general operation of the plasma generator system. The sensor circuit communicates this condition to the fast response circuit  210  to override  220  the control output  250  and modify the control output signal. The fast response circuit  210  then signals main control system  240  as an inform output  280  that its has acted to modify the output, this inform signal  280  is processed by a reactive portion  270  of the main control system  240 , which may be a “smart” system, i.e. digital microprocessor based application running pursuant to a software algorithm or an analog circuit. Based on this signal from the fast response system  210 , main control system  240  then operates to recover from the transient event and to re-initiate the original signal, which may be in turn modified depending whether a “soft” or “hard” start-up is desired. In so doing, main control system  240  sends a recover signal  290  to fast response system  210  to clear the override condition and reassert control over the control output signal. 
     As is more particularly depicted in FIG. 3, a block diagram of a first embodiment of the present invention, the sensor circuit  200  generally comprises a power sensor  301 , and a fast comparator  302 . The control override portion  220  generally comprises a latch, a Processor (CPU)  315 , a signal generator  316 , and a switch  317 . 
     The operation of the embodiment of the present invention depicted in FIG. 3 is as follows. The power sensor  301  in the sensor circuitry  200  is capable of detecting a fast reverse peak condition in the reverse power. The power sensor  301  may be any sensor device capable of detecting a fast reverse peak condition; for example, the power sensor  301  may be any suitable type of commercially available directional coupler. The sensor circuitry also comprises a fast comparator  302 . In a preferred embodiment, the fast comparator  302  may be a National Semiconductor LT 1116, available from Linear Technologies. The comparator may be pre-set for any threshold value, depending on the type of generator to be protected. Thus, the FTR of the present invention may be tailored to meet the protection needs of different types of generators by being adjusted relative to the load conditions and power level of the plasma generator equipment. Upon detection of a transient signal that exceeds the threshold value of the comparator, the comparator  302  will output a signal to the latch  313 . This indicates that a transient is occurring. The latch can be any suitable type of commercially available latch. Subsequently, once a transient of sufficient amplitude has been detected, the latch will output a signal to the switch, which will in turn cut-off the drive signal to the power amplifier  350 , thereby protecting the circuits of the plasma generator. 
     Meanwhile, once the latch has been set by the occurrence of a transient, it stays in its ‘set’ state until it is reset. Only the CPU  315  can reset the latch. The CPU may be any suitable, commercially available CPU. By giving control of the reset of the latch  313  to the CPU  315 , the CPU  315  is guaranteed to be notified of all transients, regardless of their brevity. This in turn ensures that the software  340  is also aware that a transient has occurred, each and every time one occurs. This allows the software  340  to modify its own internal parameters accordingly throughout the duration of the transient. The CPU  15  will retain control over the latch  313  until it receives notification from the software  340  that the output power of the power amplifier has been stabilized. By doing so, this allows the software  340  enough control to most efficiently adjust its internal control parameters. Once the CPU  315  has sent a reset signal to the latch  313 , the latch  313  outputs a signal to the switch  317  which restarts the input to the power amplifier  350 . 
     For example, upon receiving a signal from the sensor  401  of sufficient amplitude to meet its pre-set thresholds, the comparator  302  will output a signal to the latch  313 . The latch  313  is set once it receives the signal from the fast comparator  311 , which indicates that a transient is occurring. The latch will not be reset until it receives a reset signal from the CPU  315 . The comparator  302  also outputs a signal to the signal generator  316 , which causes the switch  317  to cut input power to the power amplifier  350 . The latch  313  outputs a signal to the CPU  315  which indicates that the transient has occurred. Upon receiving the signal from the latch  313 , the CPU  315  outputs a signal to the software  340 . The software is now aware that a transient is occurring, and can adjust its control parameters accordingly. 
     Once the transient has subsided, the FTR system of the present invention can quickly stabilize the power output to its pre-transient set-point, and output a signal to the switch  317  to reestablish input power to the power amplifier  350 . As discussed above, without the FTR system of the present invention, the transient may have occurred far too quickly for the CPU to react. Thus, the software would not have been informed that a cut-off of input power had occurred, which could have caused the software to ramp up its control parameters in response to the cut off situation. This, in turn, would have caused a secondary spike once the transient had subsided, and recovery was attempted. In the FTR system of the present application, the CPU has sole control over the resetting of the latch  313 . Accordingly, the software  340  is guaranteed to be informed of each and every transient as it occurs, and will level its control parameters through the transient. Thus, since the control parameters of the software  340  will not be ‘ramped up’ (i.e., the control parameters will not be unintelligently reacting to a cut-off situation, thereby requesting substantially more power than is actually required), quick recovery to normal power levels once the transient has subsided is possible. 
     Furthermore, and more importantly, the time in which the FTR system of the present invention can restore output to its pre-transient set-point level after the transient has occurred is extremely fast. This is due to the fact that the design of the present invention allows CPU and software to intelligently manage their internal parameters during the transient such that they can recover quickly after the transient. 
     The speed of recovery of the present invention has been illustrated by experimentation. For the purposes of one experiment power amplifier  350  is run at full power, or 5 kilowatts, into a 50Ω load. The output is connected through a length of coaxial cable to a large relay in series with the 50Ω load. In order to simulate the harshest possible transient, at 2 milliseconds into the experiment, a load transient is simulated by opening the relay. 
     Shortly after 2 milliseconds the power clamps close to 0 kilowatts, as a result of the initial transient. However, in less than 2 milliseconds, the FTR system has enabled the software control algorithms to stabilize the power output of the generator, even though its output is terminated in an open circuit. At this point the relay is closed, and the output is switched back to a 50Ω load. The FTR system, thus, allows the software to stabilize the output of the plasma generator within roughly 5 milliseconds. 
     The FTR system of the present invention also includes several other distinct advantages. Due to the speed in which the FTR system can react to transients, individual circuit elements are protected from excessive voltages and currents, thus increasing their longevity and reliability. Moreover, the elements of the present system are frequency independent, and adaptable to any power level facilitating manufacture and universal acceptance. Additionally, since the software is guaranteed to receive notification each and every time a transient occurs, the software can accurately report on the number of times the FTR is used to recover from a transient. This is advantageous, since it may be desirable to compare the number of times the FTR system is used to chamber arcing, for example, in order to optimize operation of the plasma chamber. An accurate correlation could only be made using a system that is guaranteed to sense and record every transient. 
     FIG. 4 depicts a first alternative embodiment of the present invention. In this embodiment the power sensor  301  of FIG. 3 is replaced with current sensor  401 . Although the operation of the FTR System remains essentially the same, replacing the power sensor  301  with the current sensor  401  allows the added versatility of sensing for an over current condition in the power supply, providing protection against an over current condition, and providing quick recovery after the condition has subsided. 
     FIG. 5 depicts yet another alternative embodiment of the present invention. In this embodiment of the present invention, protection against both over current and reverse peak transients is provided, through the addition of logical OR gate  501 . A current or reverse power transient of sufficient magnitude will cause an output signal from their respective comparators. This signal will in turn be output to the logical OR gate  501 . The logical OR gate will then output the signal to the latch  313 , indicating that a transient in the reverse power or in the supplied current, or both, has occurred. Thus, the FTR system will be able to react to and recover from either an over current or reverse power transient. 
     While the invention has been described in respect to the above embodiments of the invention, it should be understood that the invention is not limited to these precise embodiments. Rather, many modifications and variations will present themselves to persons skilled in the art without departure from the scope and spirit of the inventions, which is defined in the appended claims.