Electric heating – Metal heating – By arc
Patent
1997-08-11
1999-12-21
Paschall, Mark H.
Electric heating
Metal heating
By arc
21912157, 21912152, B23K 1000
Patent
active
06005218&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to a process for the bipolar pulse-shaped feeding of energy into an electric low-pressure discharge as well as to the pertaining circuit for implementing the process. It is preferably used in systems and apparatuses of plasma and surface treatment technology which operate with pole changing frequencies of up to 200 kHz. A typical application is the process of reactive bipolar pulse sputtering for depositing electrically insulating thin layers on workpieces; for example, for the purpose of providing a mechanical protection, the reduction of wear or the improvement of sliding characteristics.
It is known to operate low-pressure discharges with a high frequency of 1 MHz to 100 MHz (German Patent document DE 39 42 560 A1). A disadvantage of using such high frequencies is that they present great technical difficulties in coupling the high frequency onto large surfaces (electrodes), and the coating rate is lower by a factor of from 10 to 100 than using frequencies in the kHz range.
It is known to operate electric low-pressure discharges in a bipolar manner by means of a pole change frequency of up to 100 kHz. When such discharges, for example, by means of two magnetron sources, are used for reactive sputtering, the required stability of the discharge can also be reached in this manner when electrically highly insulating layers are deposited. By cyclically changing the polarity, the discharge of surfaces is caused. The surfaces are completely or partially covered with insulating layers and are connected with the plasma when the pole changing frequency is sufficiently high (German Patent Documents DD 252 205 A1; DE 40 42 287; DE 41 06 770 A1).
From the above references, it is also known to feed energy into such discharges by means of alternating-current generators, particularly sine wave generators. This form of energy feeding has the following significant disadvantages. (1) It is very difficult or even impossible to feed the same power at both polarities because the impedance of the electric low-pressure discharge is unequal in the two pole directions. This is particularly true when, for example, the electrodes are formed by sputtering cathodes made of a different material or are covered with oxides in a different manner. As a result, the stable operation of the discharge at an operating point required for implementing the plasma process is impossible. (2) Furthermore, by means of sine wave generators, it is not possible to feed a predetermined ratio of the power for both pole directions, as required, for example, for magnetron sources with several galvanically separated electrodes.
It was suggested to control the feeding time of the energy as the operating time for the glow process for the generator during the positive and/or negative half-wave such that no energy is fed at least for the period of time of the half-wave (German Patent document P 43 24 683.4). This process for adapting the sine wave generator was tested only in the lab operation and has not yet been used. One disadvantage when feeding the discharge by means of a sine wave generator is the occurrence of arcing. It is not possible to lower the residual energy of the pertaining transformer or oscillating circuit, which is unintentionally converted in such an arcing, below a specific value.
Furthermore, a circuit is known which improves the switch-off action (German Patent document DE 41 27 505). However, its expenditures are too high in the required frequency range. The main disadvantage during the feeding of a low-pressure gas discharge by means of a sine wave generator is the considerable increase of the impedance of the gas discharge during each pole change which, on the whole, results in a faulty adaptation of the generator. In addition to a deterioration of the efficiency, this leads to high discharge voltages with the known danger of spark-overs.
It is also known to feed low-pressure gas discharges by means of bipolar rectangular pulse generators (European Patent document EP 0 534 0
REFERENCES:
patent: 3917992 (1995-11-01), Vladimirovich et al.
patent: 5082546 (1992-01-01), Szczyrbowski et al.
patent: 5393954 (1995-02-01), Pasquini et al.
patent: 5513087 (1996-04-01), Kay
Dunne Schichten 1992, entitled Mit bipolarem Pulsplasma in eine neue Plasera by Gunter Mark, Thomas Linz, pp. 13-15.
Frach Peter
Goedicke Klaus
Kirchhoff Volker
Reschke Jonathan
Walde Hendrik
Fraunhofer-Gesellschaft zup Foerderung der angewandten Forschung
Paschall Mark H.
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