Electricity: electrical systems and devices – Safety and protection of systems and devices – Impedance insertion
Reexamination Certificate
2001-04-10
2003-09-16
Patel, Rajnikant B. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Impedance insertion
C361S018000, C361S090000
Reexamination Certificate
active
06621674
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electric power supply unit for plasma systems such as plasma-processing or coating devices wherein electric arcs or arcing may occur, particularly between the electrodes, in accordance with the introductory clauses of the independent Patent Claims.
Plasma systems for which the electric power supply units of the claimed general type are intended are suitable for a great number of potential applications and are employed, for instance, for target sputtering. As a rule, they produce an electrical power ranging from roughly a few kW up to more than 100 kW. In typical cases, the operating voltages applied to the electrodes is in the range of 400 VV. Variations in the upward or downward direction—also as a function of the respective application—are, of course, possible.
Specifically in so-called “reactive” sputtering, a technique applied, for instance, for the production of oxide or nitride films, or in the application of reactive gases problems occur as a result of disruptive breakdown and/or the formation of insulating layers on the conducting target, with the consequence that parasitic capacitors may be formed.
PRIOR ART
The most different passive and active circuits have become known which serve to extinguish electric arcs that have been formed as a result of disruptive breakdown. All of these circuits share the common feature that the voltage applied to the electrode for a defined period is disconnected. Reference is made here to the U.S. Pat. Nos. 4,692,230, 5,009,764, 5,015,493 and 5,682,067 just as examples, which are, by the way, also explicitly referred to for an explanation of all particulars and potential applications not described here in more details.
The U.S. Pat. No. 5,682,067 discloses, by the way, that state of prior art that the wording of the introductory clauses of the independent Patent Claims started out from.
The disadvantage of the circuits known from these prior art documents is the fact that after re-activation of the operating voltage the system requires a comparatively long time until it is capable again “to sputter with full power” or to process the target in any other manner.
Analogous findings apply when the system is operated in a so-called asymmetric pulsed operation for preventing insulating layers from forming on the target.
It is moreover known to disconnect the voltage applied to the electrodes—independently of whether disruptive breakdown or arcing occurs—periodically for a short interval. In such an approach the duty cycle must be so selected that during the period in which the voltage is applied disruptive breakdown or an electric arc will not occur. Even though this provision is suitable to prevent the occurrence of disruptive breakdown or electric arcs the periodic disconnection and subsequent re-application of the voltage reduces, however, the efficiency of the system substantially.
BRIEF DESCRIPTION OF THE INVENTION
The present invention is based on the problem of providing an electric supply unit for plasma systems, wherein, after re-application of the operating voltage to the electrodes, the system reaches its full power as quickly as possible, and/or wherein the reduction of efficiency is as low as possible, which is due to the occurrence of electric arcs or disruptive breakdown, e.g. as a result of the “prophylactic” disconnection of the operating voltage.
Inventive solutions to this problem are defined in the Patent claims 1 or 18, respectively. Improvements of the invention are the subject matters of the dependent Claims.
In accordance with the invention the application starts out from an electric power supply unit comprising a source of d.c. voltage or direct current, respectively, whose output terminals are connected via at least one inductive resistor and a power switch to the electrodes of the plasma system, and possibly a circuit for detecting electric arcs or disruptive breakdown, which, upon occurrence of an electric arc or disruptive breakdown, operates the switch in such a way that electric energy is no longer applied to the electrodes.
Starting out from such a known supply unit, the invention consists in the aspects that the inductive resistor(s) is (are) connected to a respective recovery diode and that the switch is a series switch.
Based on this configuration, the inventive supply unit operates as follows:
The switch, that switches the electric power applied to the electrodes (hereinafter also referred to as power switch), enables the current conduction between the d.c. voltage source and the electrodes upon detection of an electric arc. As soon as the power switch is opened the diodes, which are connected in the disabling direction relative to the normal operating voltage, prevent an increase of the voltages at the inductive resistors, producing the simultaneous effect that the current in the inductive resistors will decrease only very slowly. When the power switch is energised again after a short period already—roughly a few milliseconds are typical—the current stored in the inductive resistors is available immediately so that the system reaches its full performance more rapidly than this is the case in prior art. The inventive power supply unit hence presents a power source characteristic when the power switch is switched on again.
Hence the power losses, which occur in the inventive electric supply unit as a result of the disconnection of the operating voltage for extinction of electric arcs, are substantially smaller than those in prior art.
This inventive basic circuit—as defined in claim 1—may be extended in consideration of various aspects which may also be combined with each other:
It is possible in particular to provide an inductive resistor with an associated recovery diode in each line connecting a terminal of the d.c. voltage or direct-current source, respectively, with the respectively associated electrode. As a result, a particularly symmetrical circuit structure is achieved. It is also possible, however, to use only one inductive resistor; in such a case only one recovery diode is required.
The most different power switches can, of course, be used as switches. In view of the high performance, however, the switch is preferably an IGBT element.
Moreover, the most different power supply units may be used as mains supply circuits; equally in view of the high performance it is preferred, however, that the d.c. voltage source is a switching-controller supply unit.
In such a case it is expedient to connect a capacitor between the terminals of the d.c. voltage or direct-current source. This capacitor serves to avoid voltage peaks possibly occurring as a result of the inductive resistors in the feeders in the supply unit. Moreover, this capacitor is charged to the full no-load voltage level in the period in which the power switch is de-energised so that when the power switch is switched through the full voltages are available for exciting the plasma.
The inventive supply unit can be employed for the most different plasma systems such as plasma installations where the first electrode comprises at least one cathode disposed in a plasma chamber while the other electrode is the housing of the plasma chamber. The inventive supply unit presents furthermore the special advantage that it is also suitable for use with plasma systems wherein at least two cathodes are provided separately from each other, whereof each is connected via a series switch to an inductive resistor on the first terminal of the d.c. voltage source, that presents a recovery diode.
In such systems an alternating change-over is possible, for instance, between the cathodes. This provision entails the advantage that electric arcs being “in the incipient phase” will be reliably extinguished in the comparatively long period during which the change-over to the other cathode is performed. It is, of course, also possible, however, to perform sputtering from several cathodes.
In an improvement of the invention a diodes is connected between the electrode terminals or the pairs of electrodes, respectivel
Rettich Thomas
Wiedfmuth Peter
Zahringer Gerhard
Hüttinger Elektronik GmbH & Co. KG
Patel Rajnikant B.
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