A microplasma array for the production of low-temperature plasmas at or near atmospheric pressures is described. The walls of holes made in a substrate at regular intervals with respect to one another form hollow electrodes and are coated with metal. The hollow electrodes are supplied individually or as a group from one side of the substrate with an electrical excitation in the GHz-region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States nationalization of International Application No. PCT/EP2006/050157 filed Jan. 11, 2006, which claims priority to German Application Serial No. 10 2005 002 142.5 filed Jan. 12, 2005, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention concerns a microplasma array for producing low-temperature plasmas at or near atmospheric pressures.

BACKGROUND OF THE INVENTION

Plasmas are used in many sedimentation-, etching-, and layer formation processes.

In plasma reactors that produce a low temperature plasma, either a high frequency voltage of 10 to 30 MHz is applied to two parallel plate electrodes, or microwaves in the GHz region are introduced into a vacuum chamber, in which case more than 500 watts are required.

The most recent attempts involve attempts to produce suitable low-temperature plasmas even under non-vacuum conditions. Such reactors operate with corona discharges or glow discharges. An overview of such plasma generators can be found in Laroussi, Nonthermal Decontamination of Biological Media by Atmospheric-Pressure Plasmas: Review, Analysis, and Prospects, IEEE Transactions in Plasma Science, Vol. 30, No. 4, August 2002, pp. 1409-1415, and also in Schuetze et al. The Atmospheric-Pressure Plasma Jet: A Review and Comparison to Other Plasma Sources, loc. cit., Vol. 26, No. 6, December 1998. The plasma reactions described here are to be used, among other applications, in biological and medical purposes. Aside from the costs encountered with plasma reactors that operate under vacuum, the use of low pressures must often be excluded in this region, so that here the use of an atmospheric pressure plasma is required. Likewise, a treatment of vacuum-sensitive materials, such as certain polymers or sensitive foods, is possible with low-temperature plasmas at or near atmospheric pressures.

So-called plasma needles have already been described in which plasmas produced at a high-frequency electrode in a cylinder through which a process gas flows are suited for use in plasma surgery or for plasma dental treatment, among other applications, see Stoffels et al., Plasma needle: a non-destructive atmospheric plasma source for fine surface treatment of (bio)materials, Plasma Sources Sci. Technol. 11 (2002, pp. 383-388.)

Efforts at reducing the dimensions of plasma reactors for biological and medical, as well as for other purposes, has led to so-called microstructured electrode arrays (MSE) that operate at voltages below 400 V in the 10 MHz-region with structured comb-like electrodes, see Baars-Hibbe et al., Micro-structured electrode arrays: Atmospheric pressure plasma process—characterization and new applications, www.icpig.uni-greifswald.de/proceedings/data/Baars-hibbe 1.

Arrays like so-called MHCD Devices (microhollow cathode discharge) are known, in particular, for applications involving light technology (excimer lasers, fluorescent lamps). In this case microholes are formed in a conducting cathode material. The region remaining between the microholes is coated with a dielectric. An anode lies opposite the entire cathode. The microholes are formed as blind holes for specific applications (U.S. Pat. No. 5,686,789) or they penetrate through the cathode and an underlying substrate material (DE 198 21 244). However, the latter do not operate with a sufficient plasma density for coating tasks.

BRIEF SUMMARY

The underlying object of the invention is to produce a microplasma array of the type described in the introduction, which is suitable for activating a gas stream with high efficiency and for generating a higher density plasma that is picked up by the gas stream.

The object is achieved according to the invention by the features of claim1. Advantageous embodiments are described in the subclaims.

The array consists of a substrate in which holes are formed at regular intervals with respect to one another. The walls of the holes made in the substrate forming hollow electrodes are coated with metal. These hollow electrodes are supplied individually or as a group from one side of the substrate with an electrical excitation in the GHz-region.

The use of very high frequencies has the advantage that a high plasma density is achieved and improves the ignition characteristics of the plasma.

The hollow electrodes advantageously have a cylindrical cross section, but can also have other cross sectional shapes. The diameter region of the hollow electrodes should preferentially lie between 1 pm and 1 mm, with the lengths of the hollow electrodes lying between 100 μm and 10 mm.

For special applications, the metallic walls of the hollow electrodes can be provided with an insulating coating, especially when the electrode material might disturb the process chemically. Furthermore, flashover can occur on the electrode. The coating helps to avoid and extinguish this process and thereby reduces electrode wear.

In order to improve the ignition behavior of the plasma array, an additional electrode provided with holes is located on the opposite side of the substrate from the lead for the electrical excitation, these holes having the corresponding location and diameter as the hollow electrodes. The interval between the hollow electrodes and the other electrodes should be as small as possible, and not greater than 500 μm. The interval can, for example, be achieved by an intermediate insulating layer.

A ceramic or a ceramic compound (eg., Duroid, Standard-Microwave-Printed Circuit Board material) can be advantageously be used as the substrate material. For future integration, semiconductor materials are also a possibility. The holes are either formed with a laser, or etched (plasma)chemically by a laser with a photomask. The hollow electrode itself is then galvanically formed or produced by vacuum coating (PVD, CVD).

The oscillators delivering the respective excitation can be integrated with the substrate, and expediently on the side with the leads for the electrical excitation, the hollow electrodes forming a part of the oscillator circuit.

In order that the plasma cells of the microplasma array be controlled individually or as a group, a structured treatment of the surfaces is possible. The plasma can also be moved relative to the surfaces of the object being treated. It is also possible for moving objects to be treated along with the coincident movement of the plasma with the object, so that the plasma acts only in a specific region despite the motion of the object.

A plasma array produced according to the invention can be used, for example, forplasma sources for the in-situ sterilization of (food-) packaging, medical devices, and tissues,

functionalization and coating in the area of biomedical applications and diagnostic systems, eg., in the areas of tissue engineering, medication emitting implantsplasma radiation sources for health-promoting light systems, UV-sources for degermination

mobile applications in the household and leisure areas, eg., for the removal of organic pollutants in household textiles, noise abatement in homes and automobilesfunctional surface finishing of packaging, plastics, glass, and textile tissuesthe manufacture and finishing of nano-powders for diagnostic purposes or for the production of complex materialsthe manufacture of thin layers, eg., roll-up or formed solar cells or thin layer sensors

DETAILED DESCRIPTION

FIG. 1shows an overall view of a plasma array according to the invention with 4×9 individual plasma cells, characterized by hollow electrodes2formed in a substrate1.

FIG. 2shows a cross sectional view of an individual plasma cell. The hollow electrode2is formed by a metallic coating of a microhole made in the substrate1. Formation of the holes and their coating is done with the well-known methods used in semiconductor technology. On the upper side of substrate1the hollow electrode2is connected with a freely-oscillating oscillator3, integrated into substrate1, which is designed, e.g., with a frequency of 5.8 GHz (ISM-frequency). The arrangement of oscillators3directly on the substrate1makes the array into a fully-independent component that only requires a voltage source. The power of an individual plasma cell can be limited to less than 10 watts so that plasmas can be formed with very small energy input, which also enables good scalability, as well as the possibility of group level or even individual control of plasma cells.

The substrate1consists of a 500 μm thick ceramic material, for example, Duroid. The hollow electrodes2have a diameter of 100 μm. They can also, as mentioned above, be passivated by an insulating coating.

In order to assure an immediate plasma ignition in the hollow electrodes2upon excitation, an additional electrode4(anode) is placed on the underside of the substrate1, which has holes at the positions corresponding to the hollow electrodes2, so that a respective spark gap5is formed. The electrode4is connected to ground potential. The connection to ground on the upper side of the substrate occurs advantageously by means of a ground connection7through another hole in substrate1. The ground connection7is manufactured analogously to the manufacture of hollow electrodes2. The ground connection7can be hollow or galvanically filled, or can be filled with solder in a subsequent operational step.

The spark gap5is assured by an additional deposited insulating layer6between the electrode4and the substrate1. the insulating layer6has a thickness of approximately 20 μm, and also consists of a ceramic material.

Upon ignition, free electrons, which are always present in low density, are accelerated so that when they collide with gas molecules they have enough energy to knock additional electrons out of the gas molecules. This gives rise to additional free electrons (avalanche effect) and ions. Under appropriate conditions (type of gas, pressure, field strengths) the ionization increases to an equilibrium value at which the new collision ionization is equal to the electron losses. The electron losses arise by recombination (with light, UV, IR-emissions) and by wall losses (electrons being removed from the plasma region). A high field strength is required for ignition, which can be achieved by a small interval of separation at limited voltage, in this case by the small spark gap5. Following ignition, the plasma forms a conducting medium with an extraordinarily complex behavior. It does not remain localized in the spark gap after ignition, since then the loading of the electrodes would be very large at small volumes and high temperatures, a problem that arises in all “bipolar” arrangements. It should expand into the hollow electrodes2and essentially fill them, thereby forming a coaxial line structure (the electrode is the outer conductor and the plasma is the inner conductor).

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