Electric lamp or space discharge component or device manufacturi – Process – With assembly or disassembly
Reexamination Certificate
2001-05-29
2004-03-30
Reichard, Dean A. (Department: 2831)
Electric lamp or space discharge component or device manufacturi
Process
With assembly or disassembly
C445S022000, C445S023000, C445S025000, C445S026000
Reexamination Certificate
active
06712662
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to flat display screens. The present invention more specifically applies to screens provided with an internal space (generally under vacuum) isolated from the outside and defined by the spacing between two plates respectively forming the screen bottom and surface.
2. Discussion of the Related Art
Conventionally, a flat screen of the type to which the present invention relates is formed of two generally rectangular spaced apart external plates, for example made of glass. One plate forms the screen surface while the other one forms the screen bottom generally provided with emission means. The two plates are assembled by means of a peripheral seal. For a field-effect screen (FED), or a screen with microtips, or for a vacuum fluorescent display (VFD), vacuum is created in the space separating the two glass plates. In other cases, this space contains a neutral atmosphere (rare gas).
FIG. 1
schematically shows in a cross-section view, the conventional structure of an example of a flat screen of the type to which the present invention relates.
Such a screen is essentially formed, on a first substrate
1
, for example made of glass, of an electron bombarding cathode and of one or several grids. In
FIG. 1
, the cathode/grid(s) assembly is designated by common reference
2
. This cathode/grid(s) is placed opposite to a cathodoluminescent anode
3
formed on a second substrate
4
, for example made of glass, which is transparent if it forms the screen surface.
An example of a flat screen of the type to which the present invention applies is a microtip screen described, for example, in U.S. Pat. No. 4,940,916 of the Commissariat à l'Energie Atomique.
Cathode/grid(s)
2
and anode
3
are separately formed on the two substrates or plates
1
and
4
, which are then assembled by means of a peripheral seal
5
. An empty space
6
is created between plates
1
and
4
to enable circulation of the electrons emitted by the cathode to the anode. This space is, in what is designated as its thickness, defined by means of spacers
7
of calibrated height.
The spacers of definition of the inter-electrode space may be formed in several ways.
A first known technique consists of using calibrated balls regularly distributed on one of the plates, the diameter of the used balls (for example, of a given value included between 100 micrometers and 2 millimeters) defines the height of the inter-electrode space. An example of a method for positioning such spherical spacers is described in European patent application No. 0,867,912 of the applicant.
Another known technique for the forming of spacers of definition of the inter-electrode space of a flat screen is to use non-spherical spacers having the shape of posts. These may be sections of cylinders or of posts of various cross-sections (square, rectangular, cross-shaped or others). The use of non-spherical elements is often preferred since it enables minimizing the areas forming obstacles against electron travel between the screen cathode and anode.
The present invention more specifically relates to the placing of non-spherical spacers.
An example of a method for assembling plates of a flat display screen using this type of spacers is described in French patent application No. 2,749,105.
Spacers of non-spherical type are generally positioned and maintained, before fastening (gluing or others), on one of the screen plates, in a grid of small thickness (for example, on the order of from 70 to 90 micrometers). Given its small thickness, such a grid is only proper for spacers of relatively small height (in practice, on the order of 200 micrometers), but no longer enables correct pre-positioning before fastening for spacers of greater height (beyond 400 micrometers). Now, the spacer height that defines the thickness of the inter-electrode space conditions the operating voltage of the flat screen. The higher the desired operating voltage, the thicker the inter-electrode space and the higher the spacers must be.
The grids of positioning and temporary hold of the spacers are generally formed by photoetching techniques, either by electroplating of metal, or to etch a full-plate deposited metal layer, or by etching the very grid.
In the case where the spacers to be positioned have a height greater than 400 micrometers, several layers, generally metallic, must conventionally be superposed.
FIG. 2
illustrates, in a simplified cross-section view, what resembles a superposition of positioning grids. The left-hand portion of
FIG. 2
illustrates the superposition of two grids obtained by successive etching of layers
12
deposited full plate, while the right-hand portion of
FIG. 2
illustrates the superposition of two grids formed by successive electroplating of pads
11
. It should be noted that the superposition of the two grids does not correspond to bringing two grids formed separately one onto the other but to successively performing, on a same substrate (not shown), two electroplating or etching steps.
Whatever the used technique, a mask of definition of openings
10
for positioning spacers
7
or defining pads
11
between the holes distributed in the mask, is used. The mask forming generally uses the deposition of a resist layer. This layer is formed over a thickness generally ranging between 70 and 90 microns. This resist is insolated by means of a lithography mask. Then, the resist is developed by a negative or positive etching according to whether the etching of holes
10
(left-hand portion of
FIG. 2
) is desired to be obtained or metal (for example, nickel) is desired to be grown around resist pads at the locations of the future holes
10
(right-hand portion of FIG.
2
).
A first problem which arises has to do with the thickness desired for the grid. Indeed, with such a thickness, it is not possible to obtain an exposure enabling obtaining an isotropic etching of the holes or of the pads in the resist. Accordingly, as illustrated in
FIG. 2
, the etching or electroplating is necessarily performed anisotropically and a minimum diameter of holes
10
corresponding to a diameter greater than the diameter (or than the diameter in which the section is inscribed) of spacers
7
must then be provided. For example, for spacers having a cross-section diameter of approximately 50 microns, a minimum diameter of holes
10
on the order of 60 microns must be provided. As a result, the maximum diameter of holes
10
is much greater.
In the case of an electroplating illustrated by the right-hand portion of
FIG. 2
, the successive layer depositions inevitably come along with an increase of the diameter of holes
10
. In the case illustrated in the left-hand portion of
FIG. 2
, which shows an alternation of steps of full plate deposition of a selectively etchable material
12
and of etching of this material by means of a same exposure mask, the involved thickness inevitably results in anisotropic edges for holes
10
.
A first consequence is that the positioning of spacers
7
in the obtained grid has strong risks of occurring incorrectly.
FIGS. 3A and 3B
illustrate, in simplified cross-section views of a tool for positioning spacers, an example of implementation of a conventional method for positioning and applying spacers on a flat screen plate.
As illustrated in
FIG. 3A
, the obtained pre-positioning grid
15
and
15
′ (
FIG. 2
) is laid on a porous or perforated plate
20
of a vacuum table or the like. Plate
20
is generally formed of a porous support of metal or another adapted material (ceramic, etc.). Space
22
underlying plate
20
is closed by an enclosure
21
partially shown and this space
22
communicates with a pumping opening
23
connected to a vacuum pump (not shown). The suction caused by the pump on plate
20
is transmitted by holes
10
. In a simplified embodiment, a significant volume of spacers
7
is just roughly distributed on the surface of pre-positioning grid
15
or
15
′, after which the vacuum pump is operated so that a spacer
7
is
Duane Morris LLP
Ha Nguyen T.
Pixtech S.A.
Reichard Dean A.
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