Plasma processes are used, e.g., in plasma processing and coating devices. Arcing frequently occurs in DC sputtering systems. During arcing, the current seeks an electrically conducting channel in the plasma. Such arcing occurs quite often, in particular, during the so-called reactive coating of a substrate. The reason for the frequent arcing is that not only the substrate but also parts of the sputter system, such as, e.g., the inner wall of the process chamber or parts of apertures, are coated with electrically non-conducting or poorly conducting materials, which are charged up to a breakdown voltage. Upon the occurrence of an arc, and to prevent serious damage to the sputter system, the current supply can be disconnected, the current can be temporarily interrupted, or the voltage at the plasma chamber can be short-circuited or commutated. In order to introduce these measures, however, the arc must first be detected. Therefore, arrangements for arc discharge detection (arc detection) and for deleting arc discharges (arcs) are often part of the plasma current supply.
Reliable detection of arcs is thereby important. A voltage drop or a current increase at the output of the plasma current supply are indications of an arc. Conventionally, a threshold value is manually preset for the voltage measured at the output of the plasma current supply. An arc is detected when the voltage falls below the threshold value. In another conventional method, a threshold value is manually preset for the current measured at the output of the plasma current supply. An arc is detected when this threshold value is exceeded.
DE 41 27 504 A1 describes a circuit arrangement for suppressing arcs, wherein the instantaneous value of the voltage of the plasma path is compared with a voltage value that corresponds to an average plasma voltage detected over a predetermined time period. If the difference between the instantaneous value and the detected average value exceeds a predetermined value, an arc is detected and the current supply to the plasma chamber is interrupted.
The predetermined value at which an arc is detected should be sufficiently large to prevent the residual ripple of the output voltage from being erroneously interpreted as an arc. Plasma processes often cover a wide output voltage range. The residual ripple may depend on the output voltage. The predetermined value at which an arc is detected should be sufficiently large to ensure that this value will not be reached, even with maximum residual ripple, and that no erroneous arc message is generated.
When a current supply is switched on, the plasma is initially not ignited. At a voltage depending on process parameters, such as pressure and geometry, the plasma is ignited and the load resistance changes from very large to low values. This results in a voltage drop that could be erroneously interpreted as arc. Also, the value that is associated with the occurrence of an arc should be sufficiently large to prevent erroneous detection of an arc. The predetermined time period during which the average value is formed may alternatively be varied to prevent erroneous detection of an arc.
For this reason, values that should actually be adjusted to ensure reliable detection of an arc, depend on other parameters and can no longer be optimally selected.
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