Electric heating – Metal heating – By arc
Reexamination Certificate
2001-03-28
2002-09-03
Paschall, Mark (Department: 3742)
Electric heating
Metal heating
By arc
C219S121520, C219S121360, C427S469000, C118S7230ER
Reexamination Certificate
active
06444945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to a plasma source, and in particular to a bipolar plasma source made up of two hollow cathodes to which is applied a bipolar AC signal for driving the hollow cathodes to mutually opposed positive and negative voltages in order to generate a uniform plasma.
The invention also relates to a method of generating plasmas by applying a bipolar AC signal to two hollow cathode structures in order to generate a uniform plasma.
Applications of the bipolar plasma source of the invention include (i) an effusion cell in which heat used to evaporate the evaporant material is directly or indirectly supplied by plasma generated within two opposed hollow cathodes powered by a bipolar power source, the hollow cathodes being situated in a lid of the effusion cell adjacent an exit through which vapor exits the effusion cell, and (ii) a plasma sheet source in which the plasma is generated by two opposed hollow cathode slots powered by a bipolar power source, the plasma being confined by magnetic field lines running parallel to the electric field lines, the plasma sheet source being suitable for use in energizing a gas situated between a vacuum deposition source and a substrate to be coated.
2. Description of Related Art
The present invention seeks to provide a more uniform and stable plasma, in order to enable use of the plasma as a heat or energy source in processes requiring even application of energy or heat over an extended area. Processes to which the plasma source and method of the invention may be applied include coating processes such as vacuum deposition, the plasmas being used as a heat source for an evaporant in an effusion cell and/or to apply energy to a gas situated between the vacuum deposition source and a substrate to be coated.
A. Background Concerning Hollow Cathode Plasma Generation
The present invention achieves the stable and uniform plasma necessary for applications requiring, for example, efficient heating of an evaporant in an effusion cell or uniform energization of a reactant, by using a modification of the conventional plasma generation method known as hollow cathode plasma generation. While uniform plasmas have been generated using closed drift ion sources having a closed path plasma shape, including sputtering magnetrons, such sources are bulky and not readily adaptable for use with conventional vacuum or sputter deposition apparatus. Conventional hollow cathode plasma devices, in contrast, are simple, compact, and efficient, but it has heretofore been impossible to generate a stable and uniform plasma over an extended linear dimension using a hollow cathode plasma generator, and thus use of hollow cathode plasma generators as heat or energy sources for coating processes has been limited.
A hollow cathode is simply a cavity in the form of an opening or slot in a conductive material. When a voltage is applied to the conductive material, the applied voltage will cause electrons in a gas present in the opening or slot to acquire energy from the applied voltage, eventually resulting in formation of a plasma.
Key parameters in the formation of plasmas include the material and pressure of the gas present in the opening or slot that forms the hollow cathode, the shape and dimensions of the opening or slot, and the material of the walls that define the opening or slot. In addition, the internal diameter of a cylindrically shaped opening or the width of the slot required to sustain a discharge depends on the gas pressure, so that the ideal cavity size for the plasma is inversely proportional to the pressure so that the higher the operating pressure the smaller the cavity. Common cavity diameters or slot widths are on the order of 1 mm to 3 mm at gas pressure in the range of 10
−1
to 10
−3
Torr although devices that operate from atmospheric pressure down to the 10
−4
region have been proposed and are intended to be included within the scope of the present invention.
Depending on the above-described parameters, the plasmas formed in the opening or slot may be used in a wide variety of industrial applications, including applications based on the energy transfer involved in plasma formation for such surface treatment applications as cleaning, etching, and activation of compounds present in or on the surface, as well as deposition applications involving transfer of materials, which may include materials from the walls of the cathode, for transport to a surface. The constituents of the plasmas may be transported from the hollow cathode and accelerated through a nozzle by electric fields or other transport phenomena, or the plasmas may be allowed to lose energy shortly after formation through the recombination of electrons with ions.
While the hollow cathode plasma formation technique is well-known and has successfully been used for many years, it is difficult to use the hollow cathode plasma formation technique to form plasmas covering an extended area. In addition, the hollow cathode plasma formation technique has the disadvantage in some applications that the electrodes are subject to contamination by vapors.
The conventional method of generating plasmas covering an extended linear dimensions is to form an array multiple hollow cathodes in a common conductive structure to which cathode the voltage may be applied. Ideally, if the openings or slots are of uniform dimensions, then each cathode should simultaneously generate an identical amount and/or density of plasma, resulting in a relatively uniform formation of plasma over the extended area. However, even under ideal conditions, the hollow cathode array method of generating plasmas suffer from a number of problems:
First, the plasma formed by such an array cannot be completely uniform due to the discrete nature of the cathodes in the array.
Second, a problem arises in that unless the cathodes all light simultaneously, the ignition of some cathodes will cause a voltage drop in the area of other cathodes that may eventually prevent their ignition, making it very difficult in practice to get each hole to ignite uniformly.
Third, in any practical application, the walls of the cathodes will become contaminated, causing variations in the fields generated within the cathodes by applied voltages.
One way to counteract the inherently non-uniform nature of plasmas generated in discrete cathode arrays, and also the effects of differences in the electrical characteristics of the openings, including those resulting from contamination, is to use magnetic fields to cause the plasma to spread out more evenly following formation. The magnetic fields used for this purpose are in addition to any magnetic or electric fields used to manipulate or control the plasmas for a particular application following generation of the plasmas, however, and therefore add to the complexity of the device. In addition, it can be difficult to control over extended areas or beam lengths.
The present application avoids the problems of plasma instability and non-uniformity by utilizing a technique similar to that used to prevent accumulation of materials during sputter deposition, namely the use of two magnetrons to which an AC current is alternately applied, with the positive cycle of the AC current effectively discharging contaminants from the plasma source. In the case of sputter deposition, the technique involves the use of dual magnetrons or alternately energized cathodes, whereas the present invention involves the use of hollow cathodes.
It turns out that during hollow cathode plasma formation, the effect of applying an AC voltage to a pair of cathodes is not only to reduce the effect of contaminant formation in the cathodes (such as unintended arcing), but surprisingly also to increase the uniformity of the plasma over an extended linear area, permitting the formation of plasmas having an indefinite linear dimension. Even though each individual hollow cathode arranged in the manner to be described below may be identical to a conventional hollow cathode, th
Li Jaime
Maschwitz Peter
Bacon & Thomas
CP Films, Inc.
Paschall Mark
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