Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-09-16
2001-01-09
Diamond, Alan (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192150, C204S192160, C204S192220, C204S192230, C204S298020, C204S298060, C204S298070, C204S298080, C204S298230, C204S298260, C118S7230AN, C118S7230VE, C118S7230MP, C118S7230ER, C427S569000, C427S575000, C427S576000, C427S577000, C427S578000, C427S580000, C427S585000
Reexamination Certificate
active
06171454
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for coating surfaces using a facility which has at least two electrodes that are spaced apart from one another and arranged inside a process chamber, and which has an inlet for a process gas.
BACKGROUND INFORMATION
German Patent No. 252 205 discloses an atomization device which is suitable for manufacturing thin films by vacuum coating. In particular, it is possible with this device to deposit dielectric films. The atomization device comprises at least two electrodes and magnet systems, the electrodes being electrically connected in such a way that they alternatingly constitute an anode and a cathode. A sine-wave alternating voltage of 50 Hz has proven particularly advantageous. In this device, material ablation occurs from the electrodes in such a way that all the electrodes are ablated uniformly as a result of the alternating voltage.
So-called magnetron sputtering facilities are known to those skilled in the art. They are used, for example, to deposit wear-resistant, metal-containing carbon films. In these, a carbon-containing gas is admitted into a process space or process chamber of the facility, reacted appropriately in the sputtering process on the metal cathode surface being acted upon by a DC voltage, and deposited as a film, together with metal atoms and hydrogen, on the substrates present in the process chamber. The coating plasma which is constituted in the process chamber as a result of the process is located substantially in front of the sputtering cathodes. Because of the large-area cathodes, the coating is highly uniform but also directed. In addition, the coating rate and thus the cost-effectiveness of the method cannot be increased arbitrarily. The coating rate can be increased by adding more of the carbon-containing gas, but this results in the undesirable effect that the surface of the cathodes is increasingly coated in planar fashion with an insulating carbon-containing film. When the surfaces of the cathodes are completely covered, stable DC sputtering operation is no longer possible.
U.S. Pat. No. 3,860,507 discloses a coating method in which two cathodes (targets) located diametrically opposite one another are operated at high frequency (preferably 13.56 MHz), so that a discharge plasma forms between the targets. The targets are connected to two outputs of a secondary coil of an alternating voltage power supply, from which a center tap leads to the wall of the process chamber.
U.S. Pat. No. 5,169,509 furthermore discloses a method in which a reactive coating with electrically insulating films is guaranteed by the fact that two magnetron cathodes are operated with an alternating voltage, while a reactive gas is introduced into the process chamber.
SUMMARY OF THE INVENTION
The method for coating surfaces has, in contrast, the advantages that a high coating rate, stable operation, and a reduction in discharges (microarcs) on the electrode surfaces can be achieved. The method represents a combination of a sputtering method and a plasma-assisted CVD method. This can be achieved by the fact that, for example, two commercially available sputtering cathodes are acted upon by a bipolarly pulsed voltage in such a way that they are alternately operated as cathodes and as anodes, the frequency of this voltage lying in the range from 1 kHz to 1 MHz. The electrodes are arranged, for example, one opposite the other, while the substrates to be coated are located between the electrodes. Also admitted, in addition to the sputtering inert gas, is a reactive process gas which, in known fashion, can partially deposit on the electrode surfaces (electrode poisoning). The use according to the present invention of a bipolarly pulsed voltage guarantees two results:
The first result is that the process can proceed in stable fashion even if the electrodes are partially or completely covered with an electrically insulating film. Consider the case in which one of the electrodes is connected at a given moment to a negative voltage, and is thus being operated as the cathode. In this case, electrode material is being ablated by ion bombardment in accordance with the process, but electrically insulating regions of the surface of the electrode material are also being electrically charged until the ion bombardment drops to zero in those regions due to the charging and the electrostatic repulsion associated therewith. Ablation performance and thus the coating rate are therefore reduced. If the electrically conductive (uncovered) portion of the cathodically connected electrode surface is too small or zero, the plasma will also extinguish and the process will end; stable sputtering operation is no longer possible. According to the present invention, however, the result of reversing the polarity of the applied electrode voltages is that the said electrode acts in a subsequent time period as the anode, and thus an electron current is drawn onto the surface, first electrically discharging the electrically insulating surface regions that have been positively charged by previous ion bombardment, and then negatively charging them. It is possible in this fashion, by appropriately selecting the frequency of the applied bipolar electrode voltage, to achieve stable operation of the electrodes and prevent the plasma from extinguishing.
It is thus possible, as compared with conventional methods, to work with electrodes that are more heavily or indeed completely covered. This means that higher gas flow rates are possible, reaction of the gas on the electrode surfaces can take place to a greater extent, and the coating rate can thus be increased.
Secondly, the use of a pulsed electrode voltage almost completely prevents the formation of so-called microarcs: charging of electrically insulating regions of the cathodically connected electrode surfaces can ultimately cause a voltage discharge to an adjacent conductive region. This discharge on the electrode surfaces, called a “microarc,” is known to interfere with stable operation, and can result in a change in the electrode surface and in the incorporation of foreign particles into the film being deposited onto the substrates. The use of a bipolarly pulsed voltage greatly reduces the probability of microarcing.
As a further and critical advantage, according to the present invention a plasma-assisted CVD film deposition occurs from the plasma moving between the electrodes, in addition to the film deposition by conversion of the reactive gas on the electrode surfaces. This allows a considerable increase in the coating rate, and the creation of new kinds of films.
To achieve this, the parameters of the facility are selected so that the plasmas in front of the respective electrodes are not continually created and extinguished along with the pulsed electrode voltage, but rather that the plasma in front of the electrode that has just been deactivated is pulled to the electrode that is going into operation. A non-extinguishing plasma thus migrates back and forth between the electrodes, which for example are located opposite one another or are in very close proximity, at the defined pulse frequency. The substrates present in this plasma thus also experience, in addition to directed coating by the electrodes, an additional coating from the moving plasma volume, i.e. from the gas phase.
In an advantageous embodiment of the present invention, the duty cycle of the voltage, i.e. the time ratio between the positive and negative voltage values, is set at a value of 1:1. Other duty cycles are also possible.
It has proven advantageous to provide a spacing of 60 cm between two electrodes. Preferably it is also possible to select the spacing between the electrodes at up to 2 m, in particular up to approximately 1 m.
A pressure of 1 to 5×10
−3
mbar is preferably established inside the process chamber, acetylene and argon being used as the process gas, preferably at a ratio of from 1:1 to 10:1.
It is preferably also possible to use other gases instead of acetylene, for example methane for carbon-cont
Lucas Susanne
Voigt Johannes
Weber Thomas
Diamond Alan
Kenyon & Kenyon
Robert & Bosch GmbH
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