Glass manufacturing – Processes – Devitrifying glass or vitrifying crystalline glass
Reexamination Certificate
1999-12-07
2002-11-12
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Devitrifying glass or vitrifying crystalline glass
C065S033600, C065S044000, C065S060200, C065S060800
Reexamination Certificate
active
06477863
ABSTRACT:
The present invention relates to a process for the fabrication, on a plasma display panel sheet, of a dielectric layer with raised designs. The application of the invention is particularly advantageous when used with plasma panels of the alternative type.
Plasma panels (abbreviated to “PP” in the rest of this description) are image display screens of the “flat screen” type that operate using the principle of a discharge in gases.
PPs generally include two insulating sheets, each one carrying one or more networks of electrodes, the space between the sheets being gas-filled. The sheets are assembled together such that their networks of electrodes are mutually orthogonal. Each intersection of electrodes defines a cell corresponding to a gas space in which an electric discharge is produced when the cell is activated.
FIG. 1
represents by way of example, in a partial and simplified manner, a classic structure of a color alternative PP. Various types of alternative PPs are found, among which we can mention for example: those of the type using only two crossed electrodes to define and control a cell, as described notably in a French patent no. 2 417 848, and those of the so-called “coplanar structure” type whose structure and operation are described for example in the European patent EP-A- 0.135.382. Alternative PPs have a common characteristic, in that they have an internal memory effect in operation, owing to the fact that their electrodes are separated from the gas and the discharge by a layer of dielectric material.
In the example of
FIG. 1
, the PP is of the type having two crossed electrodes defining a cell. It is composed of two substrates or sheets
2
,
3
, of which one is a front sheet
2
, i.e. the sheet that is on the same side as an observer (not shown); this sheet carries a first network of electrodes called “line electrodes”, of which only 3 electrodes Y
1
, Y
2
, Y
3
are shown. The line electrodes Y
1
to Y
3
are covered with a layer
5
of a dielectric material. The second sheet
3
forms the rear sheet. It is on the opposite side from the observer and carries a second network of electrodes called “column electrodes”, of which only 5 electrodes X
1
to X
5
are shown. The two sheets
2
,
3
, are of a same material, generally glass; they are destined to be assembled together such that the networks of line and column electrodes are mutually orthogonal.
On the rear sheet
3
, the column electrodes X
1
to X
5
are positioned at intervals of P, whose value (for example from 100 &mgr;m to 500 &mgr;m) depends on the definition of the image. They are they also covered with a layer
6
of dielectric material, whose thickness e
1
is generally about 20 &mgr;m to 30 &mgr;m. In the example shown, the dielectric layer
6
is itself covered with layers of luminiferous materials forming bands
7
,
8
,
9
that correspond for example to the colors green, red and blue respectively. The rear sheet
3
also includes a network of barriers
11
, parallel to the column electrodes X
1
to X
5
and to the luminiferous bands
7
to
9
. These barriers
11
are placed between adjacent luminiferous bands so as to separate them.
The PP is formed by the assembly of the front and rear sheets
2
,
3
, this operation forming a matrix of cells C
1
to Cn. The cells are defined at each intersection between a line electrode Y
1
to Y
3
and a column electrode X
1
to X
5
, and each cell has a discharge zone whose section corresponds substantially to so-called “useful” areas formed by the surfaces facing the two crossed electrodes. The cells C
1
to Cn are illustrated in the figure by cavities Ep
1
to Epn made in the luminiferous bands
7
to
9
. In the example shown, intersections made by the first line electrode Y
1
with the column electrodes X
1
to X
5
define a line of cells C
1
to C
5
, illustrated respectively by cavities Ep
1
to Ep
5
. For each cell, the discharge in the gas causes electric charges to cumulate on the dielectrics
5
,
6
facing the line and column electrodes, in other words at the positions of the cavities Ep
1
to Epn.
The dielectric layers
5
,
6
therefore have a particularly important function. They are generally made by stoving a glass frit: the stoving increases the density of the material until a glass is formed. Unfortunately, this method frequently leaves defects in the glass, such as bubbles and depressions (resulting in insufficient thickness), or even holes. The ability of the dielectric to hold electrical tension is weakened at these defects. In the case for example of the dielectric layer
6
, the dielectric must have breakdown voltages of a few hundred volts.
Another difficulty in the fabrication of PPs is the making of the barriers
11
. These barriers commonly act as spacers: they determine the separation distance between the front sheet
2
and the rear sheet
3
. This spacing distance is then defined by the height H
1
of the barriers
11
, commonly from 50 &mgr;m to 150 &mgr;m depending on the applications. This requires a high level of precision in the height H
1
in order to assure optimal discharge characteristics, and very small dispersion in the value of the heights H
1
of the different barriers. The barriers must also have a suitable geometry in order to enhance the luminous efficiency of the structure. We note that the barriers
11
may also have another function known as “confinement”, assuring that the cells are “isolated” from each other.
These various criteria are difficult to respect using classical methods of fabrication.
FIG. 2
represents a barrier made in the classic manner by superposed layers: a sheet
20
carries electrodes
21
which are themselves covered with a dielectric layer
22
; a barrier
23
is formed on the layer
22
by means of a number N of successive serigraphy operations, each one producing a layer Cs
1
, . . . , CsN; the number N may be for example between 10 and 20, depending on the height H
1
required.
One disadvantage of this method is the large number of serigraphy operations necessary to obtain the height H
1
. Another disadvantage is the irregular profile of the sides of the barrier
23
which is due to the impossibility of perfectly superimposing the successive layers Cs
1
to CsN. Finally, another disadvantage is that it is difficult to achieve the required precision in the height of the barrier, since the layers Cs
1
to CsN are not of uniform thickness.
Another classic method of fabrication of the barriers makes use of blasting operations (not shown). This consists in protecting with a mask the zones that are to form the barriers, then blasting the surface to erode the material in the unprotected zones. One of the problems of this method is that the geometry of the barriers is limited, notably the sides are necessarily entirely vertical, which does not favor high luminous efficiency. Another drawback is the risk of damage to the underlying dielectric layer during the blasting operation, which imposes many particularly onerous precautions. Finally, a serious disadvantage of this method is that it makes use of large quantities of grit contaminated by heavy metals contained in the layers subject to blasting, and which must therefore be reprocessed.
The present invention proposes a process that enables a dielectric layer with raised designs, such as for example the network of barriers described previously, to be made on a PP sheet, in a simple manner and avoiding the defects and disadvantages mentioned previously.
The invention is therefore a process for fabrication of a dielectric layer having raised designs on a plasma display panel sheet, consisting in depositing on the sheet a layer containing a glass frit, then vitrifying this layer which becomes a “vitreous layer”, characterized in that a mold carrying raised designs is then brought to bear on the vitreous layer, this mold and the sheet bearing the vitreous layer being heated until a creep effect occurs in the vitreous layer enabling it to adopt the shape of the mold.
The term “raised design” with respect to the surface of the dielectric layer
Irlbeck Dennis H.
Thomson Multimedia
Tripoli Joseph S.
Vincent Sean
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