Process for the manufacture of a plasma panel

Details Process for the manufacture of a plasma panel Process for the manufacture of a plasma panel

2000-04-13

2003-03-04

Patel, Ashok (Department: 2879)

Electric lamp or space discharge component or device manufacturi

Process

With assembly or disassembly

C313S493000

Reexamination Certificate

active

06527606

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to plasma panels (PP), that is to say flat display screens in which the displayed image consists of a number of light-discharge points. The light discharges are produced in a gas contained between two insulating tiles, each point corresponding to an intersection in electrode arrays borne by at least one of the tiles.
The present invention relates more particularly to a process for the manufacture of barriers on at least one of the tiles of the panel, these barriers themselves being structural elements well known in the PP field.
DESCRIPTION OF THE PRIOR ART
It is known that a PP comprises a two-dimensional matrix of cells organized in rows and columns, which is traced to the geometry of the electrode arrays. In this case, the barriers are relief elements intended to separate the rows or the columns of cells. In some panels, the barriers may also separate both the columns and the rows of cells, therefore forming a chequerboard pattern of the latter. The role of the barriers is multipurpose. Thus, by partitioning the space of each cell at least in the direction of the rows or of the columns, the barriers prevent a discharge in one cell from inducing undesirable discharges in neighbouring cells by the ionization effect. They thus prevent cross-torque phenomena.
Moreover, the barriers constitute optical screens between the neighbouring cells, allowing good confinement of the radiation emitted by each cell. This role is particularly important in colour PPs in which the neighbouring cells constitute dots of different colours, in order to form triads for example. In this case, the barriers ensure good saturation of the colours.
Furthermore, the barriers often act as spacers between the tiles of the panel. Thus, the fact that the barriers may have a height corresponding to the required separation between the two tiles may be exploited. In this case, the tile not provided with barriers rests on the tops of the barriers present on the other tile.
The barriers may have various structures. However, if they are intended to be supporting, they are conventionally made of a dense and hardened material. These supporting barriers must be able to withstand the considerable pressure exerted by one tile on the other. This is because, during the operation of vacuum-pumping the space between the two facing tiles, prior to introduction of the low-pressure discharge gas, the force exerted per unit area of barrier may be as much as 10
6
pascals (10 kg/cm
2
), depending on the ratio of the area of the barriers to the total area of the panel. In the current state of the art, the barriers are composed of a dense material, generally a glassy phase, which is sufficiently crush-resistant to maintain a constant space between the two tiles. These barriers are produced, for example, by screen-printing (in 10 to 20 successive layers) a paste containing a glass frit or by blasting a layer containing a glass frit. After producing the geometry of the barriers, these layers are fired at temperatures of between 450° C. and 600° C. (typically 550° C.) so as to densify the material and make it mechanically strong. However, the densified material always exhibits porosity throughout it and this porosity cannot be easily pumped during the operation of vacuum-pumping the panel, which lasts only a few hours (generally 4 to 15 hours at 150° C. to 350° C.). Even if this porosity is low, and even if the surface of the barriers is perfectly vitrified, outgassing may occur over the few tens of thousands of hours that constitute the lifetime of a plasma panel. Any contamination of the gas phase in a PP causes operational variations which may be manifested either in terms of the operating voltages or on the luminous efficiency or on their lifetime. To remedy this drawback, it has been proposed in French Patent Application No. 98/16093 in the name of Thomson Plasma to produce the barriers from a material giving them substantially open porosity, the porosity being advantageously also relatively high. For this purpose, the Applicant has discovered that if barriers with a high porosity are produced, it is possible to remove from them, during the vacuum pumping, practically all the molecules capable of outgassing, so that the risk of the panels subsequently outgassing hardly exists any more. This technical effect is all the more remarkable in that the duration of the vacuum-pumping step can be reduced from several hours to less than one hour, or even only thirty minutes, without the performance characteristics of the PP being affected thereby.
In patent application Ser. No. 98/16093, the barriers are produced by using conventional manufacturing processes, such as screen printing, blasting and photolithography. Thus, as illustrated in
FIGS. 1
a
to
1
c
, the barriers are produced on a tile
1
having address electrodes X
1
, X
2
. . . X
5
. . . . For example, these barriers have, at the end of the manufacturing process, a 400 &mgr;m pitch, a 100 &mgr;m width and a 180 &mgr;m height, for a plasma panel having a working area corresponding to a 106 cm diagonal with TV resolution (560 rows, 700 columns). In a known manner, a thick layer of dielectric
2
and a thin layer of magnesium oxide or MgO have been deposited using conventional techniques on the tile
1
covered with the address electrodes.
The barriers are produced by photolithography of a pasty layer
10
′ deposited by screen printing on the thin MgO layer
3
. The composition of the paste forming the layer is as follows:
a mineral filler in the form of alumina particles having a mean particle diameter of 5 microns with a narrow particle size distribution;
a glassy phase, which may be lead borosilicate or bismuth borosilicate at a level of 10% of the mass of the alumina and a photosensitive resin of the negative type, constituting 50% of the volume of the paste.
Using a doctor blade
20
, the paste
10
′ is spread uniformly over the MgO layer
3
through a screen-printing mask
21
having an aperture corresponding to the aspect ratio of the working area of the tile, as illustrated in
FIG. 1
a
. The layer of paste
10
′ is dried at 80° C. After this operation, it has a thickness of about 20 &mgr;m.
Next, a photolithography mask
22
is laid over the layer of paste
10
′. The mask has an elongate-aperture pattern corresponding to the pattern of barriers to be printed on the MgO layer
3
. Those parts of the layer which are revealed by the mask are exposed to ultraviolet radiation so as to make them resistant to the development, as illustrated in
FIG. 1
b.
Next, the layer
10
′ thus exposed is deposited in water or in water to which sodium carbonate has been added, depending on the type of resin used, and then the surface is dried using an air knife.
A first layer of barrier material
10
′ with an elementary height of approximately 20 &mgr;m is then obtained, as illustrated in
FIG. 1
c.
The above steps are repeated in succession until the total required height for the barriers is obtained. Each new deposition of paste
10
′, by screen printing, completely covers the working area of the tile, including the tops of the barriers being formed. Depending on the number of iterations of these steps, the vertical position of the screen-printing mask
21
or the depth of the latter is modified, depending on the variation in the deposited layers existing on the tile.
The process described in patent application Ser. No. 98/16093 requires several passes in order to be able to produce barriers having the required height. Typically, the process requires from 3 to 5 deposition operations since the individual thicknesses are small, of about 15 to 30 &mgr;m. In order to deposit barriers with a height of 150 &mgr;m, at least 5 layers are therefore required, with intermediate drying steps and a final firing at 400° C. to 520° C. for 20 to 60 minutes in order to stabilize the deposited structure and to burn off the organic compounds.
SUMMARY OF THE INVENTION
The object of the present invention i

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