EP0546137B1 - Electrically insulating elements for plasma display panels and a method for producing same - Google Patents

Abstract

Plasma display panel type screens, and in particular electrically insulating elements such as spacers (12, 15) and/or dielectric layers (4,6) made of a polymerizable organic compound, are provided and enable the highest temperature to which the plasma display panels are subjected during manufacture to be lower than the sealing temperature of said panels.

Description

The invention relates to display screens of the plasma panel type, and more particularly to the electrically insulating elements used in these devices.

Plasma panels (or abbreviated as "PAP") are flat display screens that operate on the principle of luminescent discharges in a gas. They include two insulating tiles, joined together so as to define a calibrated space between them. This space is formed in a sealed manner at the periphery of the slabs in order to form a gas space.

The electrical discharges in the gas are obtained using electrodes to which electrical voltages are applied. The electrodes can be distributed on either side of the gas space: in this case most often an array of electrodes is carried by a slab and at least one other array of electrodes is carried by the other slab. The two networks are orthogonal to each other, and an elementary cell or pixel is defined at each intersection of electrodes. However, the electrodes can also be arranged on the same side with respect to the gas space, that is to say be carried by the same slab.

There are different types of plasma panels, in particular panels of the type operating with direct voltage and so-called "alternative" panels. The alternative panels have the advantage of presenting a memory effect which makes it possible to address the useful information only to the pixels whose state one wishes to change (on or off) on the other pixels, the state of the latter is simply maintained by repetition of alternating electrical discharges, called maintenance discharges. This memory effect is obtained by electrically isolating the electrodes from the discharge gas, by covering with a dielectric layer on which accumulate the charged particles generated by the discharge in the gas.

The article in IBM Technical Disclosure Bulletin Volume 24 No 1B of June 1981 gives the structure of plasma panels of the alternative type. The electrodes are covered with a dielectric layer, in particular in alumina or magnesia, which is itself protected against ion bombardment by a second layer, for example in magnesia.

An explanation of how an alternative type panel works can be found in an article by G.W.DICK published in PROCEEDING OF THE SID, volume 27/3 1986, pages 183-187. The structure described in this document relates more particularly to a structure of the coplanar maintenance type. In this type of panel, three electrodes are used to define a pixel: two parallel and coplanar electrodes carry out the maintenance discharges in each pixel; the coplanar electrodes are crossed with so-called addressing electrodes, the function of which is generally only to carry out the addressing in cooperation with one of the coplanar electrodes. It should be noted that the above-mentioned document also mentions the use of discharge barriers, the function of which is to separate the discharges produced in contiguous cells.

Such discharge barriers can also be used in "PAPs" whose cells or pixels are formed at the crossing of only two electrodes, and their presence is practically essential in "PAPs" of the "continuous" type.

Whatever the type of "PAP", the discharge barriers can be constituted by pieces forming shims, called spacers, which define the height of the gas space.

The function of such spacers is illustrated by the figure which shows a plasma panel of the type with two crossed electrodes to define a cell or pixel. The figure is a sectional view parallel to one of these two electrodes.

The panel 1 comprises two tiles 2, 3 each carrying an array of electrodes. The slabs 2, 3 constitute substrates, they commonly have a thickness E1 of the order of 1 to 6 mm.

The first panel 2 carries a first network of electrodes Y1 to Yn parallel. The second panel 3 carries a second array of parallel electrodes represented by an electrode X (shown parallel to the plane of the figure) orthogonal to the electrodes Y1 to Yn.

On the first plate 2, the electrodes Y1 to Yn (seen according to their section) are covered with a dielectric layer 4, the thickness E2 of which is commonly of the order of 20 to 30 micrometers.

The dielectric layer 4 is covered by a protective layer 5 often made of Mg0, the thickness of which is very small, of the order of 0.2 micrometers.

On the second panel 3, the electrodes X of the second network are covered by a second dielectric layer 6 having substantially the same thickness E2 as the first. This second dielectric layer is itself covered with a second protective layer 7 similar to the first 5. On the second slab 3, ends 8 of the electrode X, not covered by the dielectric layer 6, constitute sockets contact.

The two tiles 2, 3 are intended to be assembled so as to provide between them a space 10 which must contain a gas, neon for example, at a pressure of for example 500 mb.

To this end, the panel 1 has sealing joints 11 arranged at the periphery of one of the slabs, the second slab 3 for example. The height H1 of the gas space 10 is defined using spacers 12 called spacers, arranged at the periphery of a slab, of the first slab 2 for example. In the example shown, the spacers 12 are produced on the first dielectric layer 4, and in the bringing together of the two slabs 2, 3, these spacers must come into abutment on the second protective layer 7 these conditions are taken into account to define the height H2 of these spacers 12 in order to give the gas space the desired height H1, height H1 (of the gas space) which is commonly of the order of 100 micrometers.

The sealing joints 11 generally consist of a glass with a low melting point (between 380 ° C. and 450 ° C.). They have a height H3 such that, taking into account the surface on which they are arranged (surface of the second dielectric layer in the example), it is necessary to crush them to bring the spacers 12 into abutment on the second slab 3 , so as to thus seal the gas space 10.

The quality of operation of the "PAP" can be degraded if the height H1 of the gas space shows too great variations. To avoid this defect, it is known to have, between the peripheral spacers or separators 12 and even in central positions, second spacers 15 or central spacers having the same thickness H2 as the first peripheral spacers 12.

It is also possible to use such central spacers 15 to further perform a separation barrier function between the discharges of contiguous pixels.

Each pixel being defined in the area of intersection of electrodes X and Y, it is known to produce such central spacers 15, with a parallelepiped shape for example and to arrange them so as to surround each pixel.

These separators then fulfill both a spacer function and a discharge separation barrier function.

This technique is commonly used, although it has the drawback of requiring a long and delicate implementation. Indeed, the separators or barriers 12, 15 are generally made of mineral glass: walls of mineral glass are formed in several intermediate layers by successive screen printing. These successive serigraphs are followed by a final baking to densify and harden the material. The layers produced by successive screen prints are difficult to superimpose with precision: thus for a layer whose width is for example 50 micrometers, it it is not uncommon for it to overflow 10 micrometers from the previous layer, so that finally these partitions or barriers have variable widths, the dimensions of which are difficult to control. This further results in a degradation of the operation of the plasma panel.

Another drawback of this technique is that during the final firing of the layers forming these spacers or barriers, the temperature can reach, for example, 530 ° C. to 600 ° C. This may result in degradation of the glass which forms the slabs 2, 3 and / or degradation of the conductive deposits which form the electrodes. For example, the glass softens and loses its flatness if it does not rest on a perfectly flat support.

Another method for making spacers (which in this case does not additionally fulfill the discharge barrier function) consists in depositing a dense network of calibrated glass beads, regularly arranged between the electrodes. However, the precision on the diameter of the balls is insufficient to obtain that the greatest number of balls are in contact at the same time with the two slabs or substrates.

For plasma panels of the direct current type, the general structure shown in the figure is the same, the difference being that in this case the dielectric layers 4, 6 and the protective layers 5, 7 do not exist, so that the electrodes X, Y1 to Yn are in contact with the gas contained in the gas space 10.

• une poudre de verre finement broyée est mélangée à un solvant ou à une huile se décomposant à des température supérieures à 400°C ;
• le mélange est ensuite déposé par sérigraphie, ou au trempé ou par "spray" (projection), puis séché sur le substrat ou dalle de verre et les électrodes
• la dalle de verre est ensuite chauffée à des température supérieures à 530°C, et le mélange réagit pour former une couche vitreuse dont l'épaisseur généralement est comprise entre 20 micromètres et 30 micromètres.

In "alternative" type "PAPs", the production of dielectric layers also poses problems. Indeed, to date all the dielectric layers of "PAP" type "alternative" are mineral glass with low melting point (530 ° C to 600 ° C), for example lead oxide glasses. These glass dielectrics can be transparent, white, black or colored and have relative dielectric constants Er compatible with the operation of the alternative panels. (Er typically between 10 and 30). The dielectric layers are made up as follows:

• finely ground glass powder is mixed with a solvent or an oil which decomposes at temperatures above 400 ° C;
• the mixture is then deposited by screen printing, or by dipping or by "spray" (projection), then dried on the substrate or glass slab and the electrodes
• the glass slab is then heated to temperatures above 530 ° C, and the mixture reacts to form a vitreous layer whose thickness is generally between 20 micrometers and 30 micrometers.

During this latter treatment, a drawback lies in the fact that the glass slab must rest on a rectified slab, made of ceramic for example, so as not to be deformed because the glass transition temperature of the glass forming the substrate or slab is around 510 ° C - 520 ° C.

In addition, at these temperatures, the glass begins to react with the conductive or dielectric layers deposited on its surface, and in particular with the materials constituting the electrodes.

On the other hand, this vitreous dielectric offers the advantage of very good mechanical and chemical stability, during the subsequent step of sealing the plasma panel, which step requires temperatures of at least 400 ° C.

Also known from international patent application WO 90/00808 is a flat field emission display panel which comprises between the two panels polyimide spacers.

In order to respond to the various problems mentioned above, posed by the electrically insulating elements such as dielectric layers and spacers and / or discharge barriers for plasma panels, the invention proposes to produce these elements from materials whose implementation work does not require exposing the entire plasma panel to a temperature much higher than that required in the sealing step.

To this end, the invention proposes to produce at least one dielectric layer disposed between the gas space and electrodes in a polymerizable organic compound, and thermostable for temperatures equal to or lower than the sealing temperature of the plasma panel in which it went up.

The resulting advantage is that the highest temperature imposed on the plasma panel is that necessary to effect the sealing.

Spacers and / or discharge barriers can also be produced in a polymerizable organic compound and this compound can be photosensitive, which makes it possible to engrave it in a simple manner by conventional photolithography processes, and to obtain any type of pattern with a excellent resolution and uniform thickness.

The invention therefore relates to a plasma panel as defined in claim 1.

The invention further relates to a method for producing such electrically insulating elements.

The invention will be better understood, and the advantages which it provides will appear more clearly on reading the description which follows, given by way of nonlimiting example with reference to the single appended figure.

The appended figure, already partially described, schematically shows a plasma panel to which the invention can be applied.

In the example shown in the figure, the plasma panel 1 comprises two panels 2, 3 each carrying an array of electrodes X, Y1 to Yn, so that these electrodes are arranged on either side of the gas space 10 formed between the slabs 2, 3. In this case, for an "alternative" panel, at least one dielectric layer 4, 6 is required interposed between each network of electrodes and the gas space 10, ie at least two dielectric layers.

But there are other conventional embodiments (not shown), in which for example all of the electrodes are arranged on the same side of the gas space 10, that is to say carried by the same slab; the latter is in this case generally the so-called "rear slab", that is to say that which is opposite to an observer and which generally comprises the queusot (not shown) which makes it possible to establish in the panel the desired pressure (after the sealing step).

Whatever the embodiment, and the number of dielectric layers such as layers 4, 6, the invention proposes to produce them with a thermostable polymerizable organic compound.

   diamine : NH₂ - AR₂ - NH₂

   où AR₁ et AR₂ sont des chaines aromatiques.Thus, for example, the basic organic compound can be a solution in a suitable solvent (xylene or metacresol for example) of a dianhydride and a diamine (the formulas of which are given below) for obtaining a polyimide:

diamine: NH₂ - AR₂ - NH₂

where AR₁ and AR₂ are aromatic chains.

The organic compound can be deposited by the usual methods of depositing so-called "thick" layers, for example the following methods: spinner, spray (projection), soaking, roller or screen printing; conventionally in itself, the viscosity of the product can be adapted to the method used by varying the fraction of polymer in the solvent.

Then heated gradually to slowly evaporate the solvents and polymerize. The final polymerization temperature should preferably be greater than or equal to the temperature of the panel sealing step. For example, a layer of final thickness of approximately 5 micrometers of polyphenylquinoxaline polymerized at 410 ° C for 10 minutes, will no longer evolve chemically and mechanically during a sealing step at 400 ° C.

It will be recalled that the step of sealing a PAP is the step in which the two slabs 2, 3 are brought together, to obtain the desired height H1 of the gas space 10, and in which deforms the sealing joints 11 to make the seal.

It should be noted that the organic compound can be loaded with mineral and / or metallic compounds, for example in order to modify the dielectric constant and / or to modify the color thereof.

The relative dielectric constant Er of the organic compounds used can be between 2 and 4 for the pure compound (for example a polyimide) and it can be increased to reach values greater than 10.

The thicknesses can vary from less than 1 micrometer to several tens of micrometers, depending on the dielectric capacity desired by the layer.

For example, for the uncharged organic compound (2 <Er <4), correct operation of the "PAP" is obtained for thicknesses E2 of the dielectric layers 4, 6, (after polymerization) of the order of 5 to 6 micrometers.

The possible color of the final deposit can also be adjusted by adding an organic dye or a mineral compound. Black or white deposits can also be obtained in this way.

The thermostable organic compound as defined above, can be polymerized at relatively low temperatures, so as not to cause deformation of the substrate. glass or slab 2, 3, nor degrade the other layers deposited on this substrate. In particular, the organic compound does not react with the electrode material (ITO, metal, etc.).

In addition, the organic compound allows a homogeneous covering of the electrodes and therefore supports high electric fields without showing any phenomenon of electrical breakdown.

Of course, the invention applies as well to the case where the dielectric layers are produced along continuous surfaces as in the case of discontinuous surfaces.

A polymerizable organic compound similar to that indicated above for the dielectric layers, can constitute the basic material for the production of the spacers and barriers 12, 15.

As above, the organic compound can be loaded with mineral and / or metallic compounds, in order to vary the viscosity and / or the color and / or the resistance to crushing after polymerization.

The organic compound can be spread on the substrate or slab 2, 3 by usual methods similar to those mentioned above for the dielectric layers (spin, spray, screen printing, etc.).

Several layers may be necessary to obtain the desired height H2. In this case, a drying operation is inserted between each deposition step.

An important advantage of the use of an organic compound for the production of spices, results from the fact that this organic compound can be (or be made) photosensitive, and then lends itself to being insulated (through a mask) and engraved . Such a material is called "photo-imageable".

Photosensitive organic compounds are commercially available.

If several deposits are necessary to obtain the height H2, it is sufficient to insulate (generally by exposure to ultraviolet radiation) the layer when the last deposit is made, then burn using conventional photoengraving methods.

The exposure and photogravure phase occurs after the last deposit has dried, and before polymerization or following partial polymerization of the organic compound.

The polymerization of the organic compound is obtained by exposing it to a heat treatment and / or by exposure to ultraviolet rays, in a manner in itself conventional.

All of the operations can be repeated to make spacers or multilayer barriers.

The operations described above can be carried out simultaneously for all the spacers which also act as a discharge barrier or not.

However, these operations can also be repeated in particular to obtain a variable geometry and / or mechanical or optical properties in the thickness of the spacer forming a barrier or not. This is indicated in particular when it is desired to produce certain barriers with lower heights, with a view to conditioning the cells (in particular circulation of gas between the cells).

The photo-imageable nature of the organic compound makes it possible to impart, simply and securely, to the spacers and barriers 12, 15, the desired dimensions as well as the desired positions in particular relative to the electrodes X, Y1 to Yn.

This characteristic is particularly advantageous in the case of barriers 15 whose width L, relative to the pitch P of the cells, must remain relatively small, and whose position between the cells is also important.

In addition, spacers or barriers 12, 15 thus produced are thermostable and do not tend to creep: it is therefore possible to obtain ratios of height H2 to width L (H1 / L) greater than 1, for heights H2 greater than 200 micrometers .

The possibility of superimposing intermediate layers to obtain a final layer having the height H2 desired, makes it possible to produce stacks in which at least one intermediate layer, the first produced for example, is colored (with a thin thickness of the order of one to a few micrometers) in order to increase the optical contrast presented by the PAP.

The invention can be applied to the production of any electrically insulating element carried by a PAP slab, whether the latter is of the continuous or alternative type, monochrome or polychrome, whatever the distribution of the electrodes relative to the gas space. , and regardless of the number of electrodes used to define a cell.

Claims (11)

1. Display of the plasma screen-type comprising two plates (2, 3) at least one of which supports electrodes (X, Y1 to Yn), said plates (2, 3) being assembled such that a space (10) is formed between these two plates, said space having the purpose of forming a gaseous space the tightness of which is accomplished by an operation called potting, with at least one dielectric layer (4, 6) being disposed between said gaseous space (10) and said electrodes (X, Y1 to Yn), characterized in that said layer (4, 6) is made of a polymerizable organic compound.
2. Display according to claim 1, characterized in that it comprises at least one spacer (12, 15) defining the height (H1) of said gaseous space (10), also made of a polymerizable organic compound.
3. Display according to claim 1 or claim 2, characterized in that it comprises at least one discharge barrier (15) made of a polymerizable organic compound.
4. Display according to anyone of the preceding claims, characterized in that said polymerizable organic compound is obtained from a mixture of monomers.
5. Display according to anyone of the preceding claims, characterized in that said polymerizable organic compound is of polyimide.
6. Display according to anyone of the preceding claims, characterized in that said organic compound is thermostable up to a temperature at least equal to a temperature produced during said potting operation.
7. Display according to anyone of the preceding claims, characterized in that said organic compound is photosensitive.
8. Display according to anyone of the preceding claims, characterized in that said organic compound is polymerizable at a temperature inferior or substantially equal to a temperature causing a softening of at least one plate (2, 3).
9. Display according to anyone of the preceding claims, characterized in that said organic compound is charged with mineral and/or metallic products or compounds.
10. Process for the manufacture of a display according to anyone of claims 1 to 9, characterized in that it consists in stabilizing said organic compound by insulation against ultra-violet rays.
11. Process for the manufacture of a display according to anyone of claims 1 to 9, characterized in that it consists in stabilizing said organic compound by exposing it to a temperature comprised between the temperature produced during the step of potting and a temperature of softening of at least one of said plates (2, 3).

Images