Electric lamp and discharge devices – With gas or vapor – Electrode composition
Reexamination Certificate
2000-09-22
2004-08-31
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With gas or vapor
Electrode composition
C313S631000, C313S582000
Reexamination Certificate
active
06784618
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a plate comprising a glass substrate on which at least one electrode made of a conducting material is produced. It relates more particularly to the material for producing the electrodes, especially when the plate is used in the manufacture of display panels, such as plasma display panels.
To simplify the description and to make the problem posed more easily understood, the present invention will be described with reference to the manufacture of plasma display panels. However, it is obvious to a person skilled in the art that the present invention is not limited to the process for manufacturing plasma display panels, but can be used in all types of processes requiring materials of the same type under similar conditions.
DESCRIPTION OF PRIOR ART
As is known from the prior art, plasma display panels (PDPs) are display screens of the flat screen type. There are several types of PDP, all operating on the same principle of an electrical discharge in a gas accompanied by the emission of light. In general, PDPs consist of two insulating plates made of glass, conventionally glass of the soda-lime type, each supporting at least one array of conducting electrodes and defining between them a gas space. The plates are joined together so that the arrays of electrodes are orthogonal, each electrode intersection defining an elementary light cell to which a gas space corresponds.
The electrodes of a plasma display panel must exhibit a certain number of characteristics. Thus, they must have a low electrical resistivity. This is because, since the electrodes supply thousands of cells, a high current flows in the electrode, possibly going up to an instantaneous 500 mA to 1 A. Furthermore, since plasma display panels have a large size, possibly with a diagonal of up to 60 inches, the length of the electrodes is great. Under these conditions, too high a resistance may result in a significant loss of luminous efficiency due to the voltage drop associated with the flow of current through the electrodes.
Usually in plasma display panels the array of electrodes is covered with a thick layer of a dielectric, in general borosilicate glass. The electrodes must therefore have a high corrosion resistance, particularly during baking of the dielectric layer; this is because, during this phase of the process, the reactions between the dielectric layer and the electrode, or even between the glass of the plate and the electrode, cause an increase in the electrical resistance of the electrode and the products of these reactions result in a reduction in the optical transmission, in the dielectric constant and in the breakdown voltage of the dielectric layer.
At the present time, there are two techniques used for producing the electrodes of a plasma display panel. The first technique consists in depositing a paste or ink based on silver, gold or a similar material. This conductive paste is deposited, generally with a thickness greater than or equal to 5 &mgr;m, by various screen printing, vapour deposition and coating processes. In this case, the electrodes are obtained directly during deposition or by a photogravure process. This thick-film technology makes it possible to obtain low electrode resistances that are unaffected by the annealing of the dielectric layer, namely R=4 to 6 m&OHgr; in the case of electrodes made of silver paste from 4 to 6 &mgr;m in thickness, deposited by screen printing. However, this technique requires a specific anneal at a temperature above 500° C. in order to obtain conduction and requires the use of several specific dielectric layers in order to minimize the diffusion of the electrode materials into the dielectric, such diffusion being likely to degrade the electrical and optical characteristics of the panel.
The second technique consists of thin-film deposition of metal. In this case, the thickness of the layers is from a few hundred ångströms to a few microns. The electrodes are generally obtained by photolithography or “lift-off” of a thin layer of copper or aluminium deposited by vacuum evaporation or by sputtering. This thin-film technology does not require annealing to obtain electrode conduction. It makes it possible to obtain an electrode resistance R=5 to 12 m&OHgr; depending on the materials used for electrodes having a thickness of 2 to 5 &mgr;m. However, the materials used in this case, although having a high conductivity, react with the glass substrate and the dielectric layer during its baking, thereby resulting in an increase in the resistance of the electrodes and in the performance of the dielectric layer being impaired owing to the diffusion into the dielectric of the products arising from the reaction between the electrode material and the dielectric layer. The formation of strings of bubbles that reduce the transparency of the dielectric layer, its dielectric constant and its breakdown voltage is observed. To remedy this drawback, it has been proposed to deposit multilayers consisting, for example, of Al—Cr, Cr—Al—Cr or Cr—Cu—Cr multilayer stacks. These multilayers make it possible to limit the degradation of the dielectric layer and the increase in the electrode resistance during baking of the said dielectric layer. However, this technique has a number of drawbacks. It requires the implementation of a more complex chemical etching process, with the use of at least two different etching solutions. After the chemical etching, the width of each of the layers of the stack may then be different, giving very irregular electrode sidewalls, which encourages the bubbles to become trapped during baking of the dielectric layer.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to remedy the abovementioned drawbacks of the thin-film deposition technique by providing a novel material for producing an array of electrodes on a glass substrate.
Thus, the subject of the present invention is a plate comprising a glass substrate on which at least one electrode of a conducting material is produced, characterized in that, at least at the interface between the said electrodes and the glass and/or at least at the interface between the said electrodes and the dielectric layer, the conducting material of the electrodes consists of an aluminium-based and/or zinc-based metal alloy having a melting point above 700° C.
Moreover, the aluminium-based and/or zinc-based metal alloy includes at least 0.01% by weight of at least one dopant whose nature and proportions in the alloy are tailored so that the said alloy has a melting point above 700° C.; preferably, the nature of the dopant is tailored so that the corresponding alloy does not have an eutectic; preferably, this dopant is chosen from the group comprising titanium, zirconium, vanadium, chromium, molybdenum, tungsten, manganese, iron (zinc-based alloy) and antimony. By using such an alloy to produce the electrodes it is possible to increase the temperature difference between the melting point of the material producing the array of electrodes and the temperature at which the dielectric layer deposited on the electrodes is based, this temperature generally being between 500° C. and 600° C.; consequently, especially during the step of baking the dielectric layer, the deleterious effects resulting from the reactions of the electrode material with the materials of the dielectric layer, or even with the glass of the substrate, are considerably reduced.
The dopant is preferably chosen so as to obtain an alloy having an electrical resistivity as close as possible to that of the pure conducting material.
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Berthier Luc
Creusot Jean-Pierre
Moi Agide
Patel Vip
Syngenta Biotechnology Inc.
Syngenta Participations (AG)
Zimmerman Glenn
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