Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Frequency of cyclic current or voltage
Reexamination Certificate
2000-06-21
2002-07-16
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Frequency of cyclic current or voltage
C324S071100
Reexamination Certificate
active
06420863
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for monitoring an alternating-voltage discharge on a double electrode, especially between two targets for cathodic sputtering. The method is used to avoid a change-over of the discharge from the state of the anomalous glow discharge to an arc discharge.
2. Discussion of Background Information
In this connection, the double electrode is basically an arrangement for an alternating-voltage discharge between two individual electrodes. The individual electrodes are placed alternatingly on cathodic and anodic potential. The method-related use of the alternating-voltage discharge can serve different purposes. On the one hand, the parameters and the electrode materials can be selected in such a way that essentially the plasma being formed with the discharge is used technically. On the other hand, the parameters and the electrode materials used can be selected in such a way that these materials are always sputtered on the cathodic side by the discharge. The latter method, cathodic sputtering or simply sputtering, is used in this method to deposit the electrode materials as a thin layer on a substrate. In practice, the electrodes are designated in cathodic sputtering as targets, corresponding to the target material.
The frequency, i.e., the phases of switching the electrodes between anodic and cathodic potential, is normally between 10 and 80 kHz in practice.
The progression of the current or voltage curves is symmetrically sinusoidal for the most part, but can also deviate therefrom. The impedance can also be asymmetrical, for example, if different types of target materials are to be deposited in parallel at different speeds (sputtering rates).
The invention can be used for simple cathodic sputtering devices as well as for the more effective magnetic field-supported magnetrons.
The use of double electrodes for cathodic sputtering is preferred in facilities in which very high sputtering rates are required so that, on the other hand, high deposition rates can be achieved when coating selected substrates. For example, these types of devices are operated as double magnetrons in a reactive atmosphere to deposit insulating layers.
Basically, two methods are known from the prior art for generating an alternating-voltage discharge. It is preferred for two direct-current sources with opposed potential to be alternatingly switched on the two electrodes of the double electrode. The method is also designated as Pulse Magnetron Sputter Process (PMS) (Schiller et al. “Pulsed Magnetron Sputter Technology”, Surface Coating Technology, 61, pages 331-337, 1993). On the other hand, the alternating-voltage discharge can be generated with a medium frequency generator, which basically generates sinusoidal current and/or voltage curves.
The invention is used for monitoring the alternating-voltage discharge, especially the monitoring of a discharge in which an alternating-current generator is used as an energy source. However, the method can also be used for monitoring an alternating-voltage discharge with two direct-current sources.
The problem in operating an alternating-voltage discharge on a double electrode with an alternating-current generator is that, in the case of the often very high electrical power used in practice, the discharge must continuously lie in the area of the anomalous glow discharge. A change-over of the discharge to an arc discharge may not take place. The anomalous glow discharge is the prerequisite for the discharge taking place over the entire effective cathode surface. This is the prerequisite for a uniform, error-free, and large-surface coating of the substrate or the substrate arrangement.
While, with an alternating-voltage discharge with two direct-current sources, the cathodic and anodic potentials are made available separately and always start from a zero potential, an alternating-voltage discharge with an alternating-current generator can drift to one electrode. As a result, the glow discharge can assume an asymmetry between the targets in which one target assumes a higher cathodic potential without the character of the sinusoidal current-voltage curve changing considerably. As a result, the danger of forming an arc discharge on the respective target increases.
The causes of a change-over of the discharge from an anomalous glow discharge to an arc discharge are diverse and cannot be completely excluded in practical operation. Thus, the causes of malfunctions could lie, for example, in the formation of flakes of sputtered material that flake off device parts and/or cause local overheating on the electrode surfaces. Malfunctions occur due to the short-term growth of insulating layers on the target. Additional malfunctions can be a function of the device or switching.
The arc discharge, also called vacuum arc discharge, is characterized in that it essentially has a lower impedance than the anomalous glow discharge, i.e., with the same power, the arc drop voltage is considerably lower and the discharge current is correspondingly greater. The arc discharge is connected with a punctiform vaporization of electrode material, which should be avoided in the process technology of cathodic sputtering under discussion. In this connection, the danger of the formation of an arc discharge exists in an increased degree with reactive process management.
Monitoring the power supply for individual magnetrons using measuring techniques is known from the prior art, in which the total current and/or voltage is measured and, when absolute values are exceeded, particularly with respect to the discharge current, the energy supply to the device is briefly interrupted.
Rettich and Wiedemuth (“High power generators for medium frequency sputtering applications”, Internet http://huetinger.com/mf_paper2.doc, /Jun. 11, 1997/) describe the requirements for a power supply for a double magnetron with a medium frequency generator. In this connection, the discharge is monitored by means of measuring the total discharge current and the discharge voltage. A possible change-over of the anomalous glow discharge to an arc discharge is reacted to in two different ways, which are a function of the type of arc. Accordingly, an individual short-term arc (single arc) is tolerable, which, though it causes an increase in current, extinguishes again after a waiting period that can be set in advance. In such a case, the generator operates as an energy source without interruption. If an excessive discharge current is measured via an arc discharge (hard arc), which exceeds the limit for the discharge current and the waiting period, the generator reacts within milliseconds by reducing the output power for a short time. As a result, the arc is extinguished. Afterwards, the normal method conditions are reset. If an arc discharge forms again, the generator is shut down and an error message is issued for the operator.
DE 43 26 100 A1 discloses a method and a device for coating substrates with a device for detecting and suppressing undesired electric arcs. In this connection, recording the measured values in a specific part of the half-wave is emphasized as a special feature that increases the reliability of detecting an arc.
It cannot be determined with the total current measurement of discharge current whether the discharge is occurring symmetrically or drifting asymmetrically to a target without the total current changing considerably. In such a case, the targets are sputtered at different speeds and it can easily come to a change-over of the glow discharge to an arc discharge, for example, due to the overheating of one of the targets.
In general, according to prior art, the discharge voltage, the discharge current or both in combination can be detected and, in the case of an impermissible deviation, the supply of energy to the discharge device can be briefly interrupted.
In this connection, it is problematic that, in the case of high power with frequencies of 10 to 80 kHz, various misinterpretations can occur for the anode/cathode switch-over
Fahland Matthias
Fickert Andreas
Kirchhoff Volker
Milde Falk
Winkler Torsten
Amin Anand B.
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
Greenblum & Bernstein P.L.C.
Sherry Michael J.
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