Arc-suppressing pulse plasma system and method for suppressing a developing arc

Abstract

An arc-suppressing pulse plasma system comprising a cathode (2) and an anode (3) in a vacuum chamber, wherein the cathode and the anode are connected to a generator (4) for generating a plasma in the vacuum chamber, said generator supplying a pulsed direct voltage, a current measuring device (7) for determining the electric current flowing through the cathode (2) and the anode (3), a generator switch (46) for switching the generator (4) on and off, and an evaluation circuit (40) to which the current measuring device (7) is connected in order to receive its measurement signals, and the generator switch (46) is connected in order to output a switching signal thereto, characterized in that the evaluation circuit (40) has a memory (42) for storing a gradient threshold value, a module for determining the temporal change in the current intensity, and an evaluation unit (41).which compares the temporal change in the current intensity with the gradient threshold value, wherein the evaluation circuit (40) is arranged such that a switch-off signal for the generator switch (46) is generated when the gradient threshold value is exceeded.

Description

[0001] The invention relates to an arc-suppressing pulse plasma system with a cathode and an anode in a vacuum chamber, wherein the cathode and the anode are connected to a generator for generating a plasma in the vacuum chamber, which generator supplies a pulsed direct voltage, with an ammeter for determining the electrical current flowing through the cathode and the anode, and with a generator switch for switching the generator on and off, as well as with an evaluation circuit to which the ammeter is connected in order to receive its measuring signals, and to which the generator switch is connected in order to output a switching signal.

[0002] One possible application of a pulsed plasma system with a pulsed direct current is described in DE 198 15 019 B4. The system is used to produce aluminum nitride coatings on a component. The plasma is generated by a pulsed direct current between 420 and 1,000 V in the frequency range between 5 and 50 kHz. This method is also known as plasma nitriding.

[0003] US 5,608,297 A discloses a plasma switch with a residual current interrupter. US 4,514,661 A discloses an arc suppressor for an electron gun. Finally, US 2007 / 0000772 A1 discloses a method for operating a pulsed arc source.

[0004] The general problem with plasma systems is that the cathode heats up considerably due to the plasma, causing electrons to escape from it through thermionic emission, which leads to the formation of an arc.

[0005] However, the formation of an arc is not desirable, as it can damage the system or a substrate that is to be treated in the plasma.

[0006] DE 41 27 504 A1 therefore proposes continuously measuring the voltage at the electrodes (cathode, anode) in a plasma system operated with a continuous direct current, and briefly shutting down the generator when a predetermined lower limit for the voltage drop gradient and a predetermined minimum duration of the voltage drop are reached. The system has a choke with high inductance, so that when a sharp current change occurs, it builds up a countervoltage that quickly extinguishes any arc that may occur. To provide the necessary inductance, the choke is a relatively large and expensive component.

[0007] According to DE 100 34 895 A1, in a plasma system whose electrodes are applied with an alternating voltage by means of a resonant circuit inverter, the time and value of the ignition voltage for each half-cycle are determined by differentiating the voltage over time. The ignition voltage is the value at which an arc is ignited. The current value of the ignition voltage is compared with the value of the ignition voltage in the previous half-cycle or with an average value of the ignition voltage over a certain number of cycles. By repeatedly determining the ignition voltage, flashover can be detected very quickly, since the ignition voltage increases in this case.

[0008] DE 10 2006 043 900 A1 addresses the question of when re-ignition should occur after the generator has been shut down. For this purpose, the decaying current through the electrodes is observed after the generator has been shut down. As soon as this current falls below a specified limit, this is considered a sign that the arc has been extinguished. After the arc has been extinguished, the interruption of the power supply to the generator can be resumed after a certain waiting period.

[0009] DE 10 2006 043 898 A1 also deals with the question of when the generator can be switched on again after an arc has occurred.

[0010] DE 43 26 100 A1 also refers to a plasma system operated with alternating current. In each half-cycle, a check is made to determine whether a condition exists in which the voltage drops to a small value, namely the arc voltage, and the current remains at a certain level or even increases. If several such signals are measured, the generator is shut down.

[0011] The invention is therefore based on the task of creating a pulse plasma system in which the formation of arcs is detected at an early stage and prevented by briefly switching off the generator.

[0012] To solve the problem, the invention provides that the evaluation circuit has a memory for storing a gradient threshold value, a module for determining the temporal change in the current intensity and an evaluation unit which compares the temporal change in the current intensity with the gradient threshold value, wherein the evaluation circuit is set up in such a way that a switch-off signal for the generator switch is generated when the gradient threshold value is exceeded.

[0013] In contrast to previous proposals, the current flowing through the electrodes is not considered, but rather the voltage at the electrodes. Since this allows the formation of an arc to be detected very early and its complete formation prevented, a choke with high inductance is not required.

[0014] As already mentioned, in an arc the electrodes originate from the cathode, whereby the number of electrons on the way to the anode increases greatly due to impact ionization.

[0015] This creates a strong current in a short time, with both the current intensity and the current gradient being characteristic for the formation of an arc.

[0016] If the current is measured with sufficiently short cycle times and the current gradient - the speed of the current increase - is determined from this, it can be determined by comparing it with corresponding threshold values ​​whether an arc is forming or is threatening to form.

[0017] By interrupting the power supply to the electrodes, the complete formation of an arc can be prevented at an early stage.

[0018] The cycle times used to measure the current also depend on the size of the vacuum chamber, which determines the length of a fully developed arc. Since the arc propagates at approximately 10 cm / µs, a fully developed arc is present after just a few µs in conventional vacuum chamber sizes. The cycle times for measuring the current must therefore be significantly lower. Typically, they are no more than 8 ns, which corresponds to a frequency of 125 MHz. This short cycle time allows for a sufficiently accurate determination of the current gradient.

[0019] Even better detection is achieved if not only the current gradient but also a maximum current value is observed. If this value exceeds the value of the undisturbed plasma current, a shutdown should occur.

[0020] For this purpose, the evaluation circuit has a memory for storing a maximum current threshold value and a further evaluation unit which compares the current intensity with the maximum current threshold value, whereby the evaluation circuit is set up in such a way that a switch-off signal for the generator switch is generated when the gradient threshold value and the maximum current threshold value are exceeded.

[0021] The above-mentioned thresholds must, of course, be individually adapted to a specific system. For this purpose, the evaluation circuit includes an input module for manually changing the stored values ​​for the gradient threshold and the maximum current threshold.

[0022] To avoid false positives, the stored values ​​for the gradient threshold and the maximum current threshold are dynamically adaptable to the pulse profile. Pulse profile refers to the time course within a pulse. In particular, the threshold at the beginning of the pulse can be lowered, since even during normal operation, the current increases relatively quickly to the current present in the formed plasma. Taking the current increase at the beginning of a pulse into account would result in the false detection of the formation of an arc.

[0023] The measuring devices and the evaluation circuit must be designed to allow measurements to be taken in short time windows that are significantly shorter than the pulse duration. To this end, the invention provides for the evaluation circuit to have a response time of 200-600 ns.

[0024] The invention further relates to a method for suppressing a developing arc in a pulse plasma system, wherein the method provides that the generator of the pulse plasma system is switched off when an arc forms. In particular, the invention provides that the current flowing through the electrodes of the pulse plasma system is measured, a temporal change in a time window is determined, and the change is compared with a gradient threshold value. Exceeding the gradient threshold value is evaluated as an indication of the formation of an arc, and the generator of the pulse plasma system is therefore briefly switched off.

[0025] Furthermore, the current current is also measured in a time window and compared with a maximum current threshold value. If the gradient threshold value and the maximum current threshold value are exceeded, this is interpreted as an indication of the formation of an arc and the generator of the pulse plasma system is therefore switched off.

[0026] The threshold values ​​mentioned can be dynamically adjusted to the pulse curve.

[0027] The size of the time window is between 200-600 ns.

[0028] The invention will be explained in more detail below using an exemplary embodiment. Fig. 1 a circuit diagram of a pulse plasma system operated with direct current, Fig. 2 a diagram illustrating the current intensity curve in the formation of a plasma during a pulse, Fig. 3 the current intensity curve of an arc that forms at the beginning of a pulse, Fig. 4 a diagram showing the current intensity of an arc that is generated during a pulse, and Fig. 5 a circuit diagram for detecting arcs.

[0029] First, the focus will be on Fig. 1: In a plasma chamber 1 there are a cathode 2 and an anode 3, between which a plasma forms when a sufficiently high voltage is applied to the electrodes 2, 3 in a vacuum in the plasma chamber 1. For this purpose, a pulse generator 4 is provided, the poles of which are connected via a line 5, 6 to the cathode 2 and the anode 3 in the plasma chamber 1, respectively.

[0030] The two lines 5 and 6 form inherent, unavoidable inductances, which are represented by the circuit symbol for inductance in the lines. This is not a separate high-inductance choke as described above in the prior art.

[0031] In one line 5, here from the positive pole of the generator 4 to the cathode 2, an ammeter 7 is arranged, which measures the current in the line 5.

[0032] Current meter 7 is a device that responds quickly to changes. This means that currents are measured at cycle times of at least 6 ns.

[0033] A feedback loop 8 is intended to indicate that the measurement of the current measuring device 7 influences the switching state of the generator 4, i.e., that the generator 4 may be temporarily switched off to prevent the formation of an arc. The feedback loop has a response time of 200-600 ns.

[0034] To illustrate the operation of the invention, the Fig. Figure 2 shows the current curve during a voltage pulse 11, which is not disturbed by an arc, using line 10. The current intensity is shown as negative. However, since the absolute values ​​ultimately count, falling edges of line 10 of the current intensity are treated as current increases, and rising edges as current decreases.

[0035] When a voltage is applied, free electrodes in the plasma chamber are accelerated, releasing additional electrons through impact ionization. This leads to a rapid current increase, which is, however, limited by recombination in the plasma. At the end of the pulse, the current drops back to a minimum value. This process repeats in time with the pulse generator's pulses.

[0036] The Fig. Figure 3 shows the current waveform 20 during the formation of an arc. This can occur at the beginning 22 of a pulse. The current increases rapidly, but does not - as in the Fig. 2 - into a "saturation", but runs almost straight, limited only by the cable inductances, towards a maximum value, at the same time the voltage at the electrodes 2, 3 drops suddenly.

[0037] The rapid increase in current is a clear indication of the formation of an arc. If such an increase is observed, the pulse generator is immediately shut down and the pulse is interrupted.

[0038] There - like the Fig. 2 shows that even when a plasma is formed, there is initially a strong current increase, the application of the threshold value for assessing whether there is an arc-induced rapid current increase is limited to a period of time which is offset by an offset time from the beginning 22 of a pulse.

[0039] The Fig. 4 shows the typical current curve 30 when the arc forms during a pulse.

[0040] The current curve 30 initially follows the course as shown in the Fig. 2 can be seen. With the formation of an arc at time 32, the current intensity increases sharply.

[0041] To detect such a surge in a timely manner, the current measuring device must have a small measurement time window and take measurements at a rapid rate. An evaluation circuit, which will be described below, must also be able to follow this rate.

[0042] In Fig. Finally, in section 5, a circuit will be explained which is used to detect an arc.

[0043] The evaluation circuit 40 has an evaluation unit 41, a memory 42 for a gradient threshold value, and a memory 43 for a maximum current threshold value. The measured values ​​of the current measuring device 7 are made available to the evaluation unit 41 via a signal line.

[0044] In a differentiation circuit 44, the measured current values ​​are differentiated over time to obtain a current gradient value. This value is compared with the gradient threshold of the memory 42. If the actual current gradient value is greater than the threshold, a switching signal is output at output 45, which commands the generator switch 46 to briefly interrupt the generator 4.

[0045] The threshold values ​​in the memories 42, 43 can be manually changed by means of an input module 47 and adapted to the respective system.

[0046] Dynamic adjustment of the threshold values ​​to the pulse profile is also conceivable. Pulse profile refers to the temporal progression within a pulse. In particular, the threshold values ​​are raised at the beginning of a pulse so that no shutdown signal is generated even with sharp current increases. As the pulse progresses, namely when a constant plasma current has developed in an arc-free plasma, the threshold values ​​are lowered. List of reference symbols 1 plasma chamber 2 Cathode 3 Anode 4 Pulse generator 5 Line 6 Line 7 ammeter 8 Feedback 10 Line 11 Voltage pulse 20 Current flow 22 Pulse start 30 Current curve 32 Time 40 Evaluation circuit 41 Evaluation unit 42 memory (gradient) 43 Memory (Absolute) 44 Differentiation circuit 45 Exit 46 generator switches 47 Input module

Claims

[1] Arc-suppressing pulse plasma system with a cathode (2) and an anode (3) in a vacuum chamber, wherein the cathode and the anode are connected to a generator (4) for generating a plasma in the vacuum chamber, which generator supplies a pulsed direct voltage, with an ammeter (7) for determining the electric current flowing through the cathode (2) and the anode (3), and with a generator switch (46) for switching the generator (4) on and off, as well as with an evaluation circuit (40) to which the ammeter (7) is connected in order to receive its measuring signals, and the generator switch (46) is connected in order to output a switching signal thereto, characterized bythat the evaluation circuit (40) has a memory (42) for storing a gradient threshold value, a module for determining the temporal change in the current intensity and an evaluation unit (41) which compares the temporal change in the current intensity with the gradient threshold value, wherein the evaluation circuit (40) is set up such that a switch-off signal for the generator switch (46) is generated when the gradient threshold value is exceeded. [2] Pulse plasma system according to claim 1, characterized by that the evaluation circuit (40) has a memory (43) for storing a maximum current threshold value and a further evaluation unit (41) which compares the current current intensity with the maximum current value, wherein the evaluation circuit (40) is set up such that when the gradient threshold value and the maximum current threshold value are exceeded, a switch-off signal for the generator switch (46) is generated. [3] Pulse plasma system according to claim 1, characterized by that the evaluation circuit (40) has an input module in order to be able to manually change the stored values ​​for the gradient threshold value and the maximum current threshold value. [4] Pulse plasma system according to claim 1, characterized by that the stored values ​​for the gradient threshold and the maximum current threshold can be dynamically adapted to the pulse curve. [5] Pulse plasma system according to claim 1, characterized by that the evaluation circuit (40) has a response time of 200 to 600 ns. [6] Method for suppressing a developing arc in a pulse plasma system, the method providing that when an arc is formed, the generator (4) of the pulse plasma system is switched off, characterized bythat the current flowing through the electrodes of the pulse plasma system is measured, its temporal change is determined in a time window and compared with a gradient threshold value, whereby exceeding the gradient threshold value is evaluated as an indication of the formation of an arc and therefore the generator (4) of the pulse plasma system is switched off. [7] Method according to claim 6, characterized by that the current current intensity is measured in a time window and compared with a maximum current threshold value, whereby exceeding the gradient threshold value and the maximum current threshold value is evaluated as an indication of the formation of an arc and therefore the generator (4) of the pulse plasma system is switched off. [8] Method according to claim 6 or 7, characterized by that the gradient threshold and the maximum current threshold are dynamically adapted to the pulse curve. [9] Method according to claim 6, 7 or 8, characterized by that the size of the time window is between 200 and 600 ns.

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