Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Plasma generating
Reexamination Certificate
2000-09-12
2002-08-27
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Plasma generating
C118S050100, C118S7230ER, C118S7230AN, C219S121360
Reexamination Certificate
active
06441553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to methods and apparatus for producing plasma; in particular, the invention relates to an electrode for establishing a steady-state glow-discharge plasma at atmospheric pressure and low temperatures.
2. Description of the Related Art
Plasma is an ionized form of gas that can be obtained by ionizing a gas or liquid medium using an AC or DC power source. A plasma, commonly referred to as the fourth state of matter, is an ensemble of randomly moving charged particles with sufficient density to remain, on average, electrically neutral. Plasmas are used in very diverse processing applications, ranging from the manufacture of integrated circuits for the microelectronics industry, to the treatment of fabric and the destruction of toxic wastes.
In particular, plasmas are widely used for the treatment of organic and inorganic surfaces to promote adhesion between various materials. For example, polymers that have chemically inert surfaces with low surface energies do not allow good bonding with coatings and adhesives. Thus, these surfaces need to be treated in some way, such as by chemical treatment, corona treatment, flame treatment, and vacuum plasma treatment, to make them receptive to bonding with other substrates, coatings, adhesives and printing inks. Corona discharge, physical sputtering, plasma etching, reactive ion etching, sputter deposition, plasma-enhanced chemical vapor deposition, ashing, ion plating, reactive sputter deposition, and a range of ion beam-based techniques, all rely on the formation and properties of plasmas.
Corona discharges are widely used in particular for treating plastic films, foils, papers, etc. to promote adhesion with other materials by increasing the surface energy of the film. A corona discharge is established between two electrodes by applying a high voltage to one of the electrodes while the other is connected to ground at typical frequencies in the order of 10-50 kHz. These conditions produce locally concentrated discharges known in the art as streamers, which lead to some non-uniformity in the treatment of film surfaces and can also damage the film by producing low molecular weight species that adversely affect adhesion to the surface. Furthermore, the streamers of corona treatment can produce backside effects on the film being treated, which is undesirable in many applications. Nevertheless, corona treatment is extensively used in the industry for improving the surface energy of materials.
Glow-discharge plasma treatment is also an effective method of treating surfaces to increase their wettability and adhesion to various materials. Glow discharge provides a more uniform and homogenous plasma that produces a more consistent surface treatment than corona treatment, thereby avoiding unintentional back treatment of the film. Glow-discharge plasma is characterized by high-energy electrons that collide with, dissociate and ionize low-temperature neutrals, creating highly reactive free radicals and ions. These reactive species enable many chemical processes to occur with otherwise unreactive low-temperature feed stock and substrates. Based on these properties, low-density glow-discharge plasmas are usually utilized for low material-throughput processes involving surface modification. These plasmas are typically formed by partially ionizing a gas at a pressure well below atmosphere. For the most part, these plasmas are weakly ionized, with an ionization fraction of 10
−5
to 10
−1
, established with AC or DC power in systems with varied geometries. These systems always require vacuum chambers and pumps to maintain a low pressure, which increases operating costs and maintenance.
There has been an extensive effort to develop plasma systems capable of operating at atmospheric pressure for surface treatment of polymer films, foils, and paper, in order to avoid capital and maintenance expenditures for vacuum chambers and pumps. It is known that atmospheric plasma can be generated at relatively low temperatures with a proper power source, the insertion of a dielectric layer between the electrodes, and the use of an appropriate gas mixture as plasma medium. For surface treatment of polymer films, fabrics, paper, etc., atmospheric plasma can be established between two electrodes using an inert gas such as helium under particular operating conditions. Usually one electrode is attached to a high voltage power supply, and a rotating drum is grounded and acts as the other electrode. One electrode is coated with a ceramic layer and the plasma gas is injected between electrodes.
Examples of glow-discharge plasma systems operating at atmospheric pressure are described in U.S. Pat. Nos. 5,387,842, 5,403,453, 5,414,324, 5,456,972, 5,558,843, 5,669,583, 5,714,308, 5,767,469, and 5,789,145. Co-owned U.S. Pat. No. 6,118,218, incorporated herein by reference, disclosed a plasma treatment system capable of producing a steady glow discharge at atmospheric pressure with different gas mixtures operating at frequencies as low as 60 Hz. The invention consists of incorporating a porous metallic layer in one of the electrodes of a plasma treatment system. A plasma gas is injected into the electrode at substantially atmospheric pressure and allowed to diffuse through the porous layer, thereby forming a uniform glow-discharge plasma. As in prior-art devices, the film material to be treated is exposed to the plasma created between this electrode and a second electrode covered by a dielectric layer. Because of the micron size of the pores of the porous metal, each pore also produces a hollow cathode effect that facilitates the ionization of the plasma gas. As a result, a steady-state glow-discharge plasma is produced at atmospheric pressure and at power frequencies as low as 60 Hz. The inventors discovered that, in order for the electrode holes to operate effectively for producing an optimal glow discharge, their size must approach the mean free path of the plasma gas at the system's operating pressure.
Thus, the ability to produce a reliable and uniform glow-discharge plasma at atmospheric pressure has greatly improved the flexibility of operation of plasma treatment processes, but a serious constraint remains as a result of the required geometry of the apparatus. As illustrated in
FIG. 1
, conventional plasma treatment systems consists substantially of a plasma treater
10
mounted on a roller
12
. A film
14
of material to be treated is passed through the system between the plasma treater and the roller. The roller
12
is grounded and coated with a dielectric material
16
, such as polyethylene teraphthalate (PET). The plasma treater
10
contains at least one electrode which is connected, through a cable
18
, to an AC power supply
20
. The treater is held in place by a holding bracket
22
designed to maintain a distance of 1-2 mm between the roller
12
and the treater
10
. This distance, which varies with operating conditions, plasma medium composition, and electrode configuration, is important in establishing a steady plasma flow; therefore, it is very desirable to maintain a gap determined to be optimal. Plasma gas, such as helium, argon, and mixtures of an inert gas with nitrogen, oxygen, air, carbon dioxide, methane, acetylene, propane, ammonia, or mixtures thereof, are used to sustain a uniform and steady plasma. The gas is supplied to the treater
10
through a manifold
24
that feeds the electrode of the apparatus.
As a result of this conventional configuration of plasma treaters, the film substrate 14 being treated is necessarily always bound by the two electrodes of the system (that is, the plasma electrode and the grounded roller). Since plasma formation is achieved only within a limited range of spacing between the two electrodes, the substrate to be treated is obviously also limited in thickness and shape. Therefore, it would be very desirable to have plasma treatment apparatus capable of treating the surface of substrates regardless of their thickness
Decker Wolfgang
Mikhael Michael G.
Pirzada Shahid A.
Yializis Angelo
Durando Antonio R.
Durando Birdwell & Janke PLC
Sigma Technologies International, Inc.
Tran Thuy Vinh
Wong Don
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