FR2783633A1 - LOW-GAS MICROPOINT CATHODE - Google Patents

Abstract

The invention relates to a microtip cathode (10) for a flat display screen, of the type comprising cathode conductors (3), between a substrate (1) and an insulation layer (4) of a grid (5). , and a layer (20) having a low bonding coefficient, visible at least between lines (13) of the grid.

Description

LOW-GAS MICROPOINT CATHODE

  The present invention relates to a flat display screen.

  microtip reading. It applies more particularly to the production of a constituent electronic emission cathode

  of such a screen, intended to be associated with a cathode anode

luminescent.

  Figures 1 and 2 partially show, respec-

  in cross section and in perspective, the structure of a

  flat screen with microtips of the type to which the invention relates.

  Such a microtip screen is essentially made up of

  killed by a cathode 11 with microtips 10 and a grid 5 provided with holes at the locations of the microtips. Cathode 11 is

  placed opposite a cathodoluminescent anode 12, one of which

  glass trat 2 constitutes the screen surface.

  The operating principle and the detail of the constitution of such a microtip screen are described in the

  American patent N 4940916 from the French Atomic Energy Commission.

  The cathode 11 is organized in columns and is made up of

  killed, on a substrate 1, for example made of glass, of conductors

  cathode 3 organized in mesh from a conductive layer.

  The microtips 10 are produced on a resistive layer (not

  shown) of homogenization of electronic emission, deposition

  placed on (or under) the cathode conductors. The microtips are generally placed inside the meshes defined by the cathode conductors. The cathode 11 is associated with the grid 5 with the interposition of an insulating layer 4 (generally made of silicon oxide SiO2) to isolate the cathode conductors 3 from the grid 5. Holes are respectively made in

  the grid 5 and isolation 4 layers to receive the micro-

  tips 10. The grid 5 is organized in rows or lines 13, the intersection of a row of the grid and a column of the cathode generally defining a pixel. For reasons of clarity, only a few microtips 10 have been shown in FIG. 2, at the intersection of a row 13 and a column 3. In practice, these microtips are several thousand per pixel of screen . Likewise, the mesh of the cathode conductors 3 has not been shown for reasons of clarity. It will simply be noted that, in most cases, the columns 3 and the rows 13 each form, overall, a succession of

  wide sections corresponding to the intersection with, respectively

  vement, a row or a column, and narrower sections of connection. This device uses the electric field created between the cathode 11 and the grid 5 so that electrons are extracted from the microtips 10 towards phosphor elements 8 of the anode

12 by crossing an empty space 6.

  One problem posed by flat microtip screens is that the constituent layers, in particular of the grid cathode

  tend to degas during screen operation.

  Generally, the screen cathode-grid plate is subjected to various debugging and degassing treatments before

  assembly of the screen in order to limit this drawback. It sub-

  however, there are still elements that will degas over time once the screen is closed. This is why, there is provided in any screen an element for trapping impurities (getter) intended for

  absorb the products from this degassing.

  However, even in the presence of a getter, the importance

  degassing harms the lifespan of conventional screens.

  In particular, the layer of silicon oxide insulating the grid of the cathode desorbs pollutants, in particular water, when electrons fall on this layer of SiO2, in

  particular between lines 13 of the grid.

  Less critically, the material (usually niobium) that makes up the grid lines also tends to

  degas when electrons fall on the grid.

  The present invention aims to propose a new

  Low degassing microtip cathode.

  The invention aims more particularly to propose a new cathode minimizing the desorption of pollutants by the

  layer insulating the cathode grid.

  A feature of the present invention is to mask the areas of the cathode that are most prone to

  degas, with a material chosen for its low

  ble degassing when struck by electrons. In particular

  Binding, according to the present invention, the apparent surfaces of silicon oxide (SiO2) of the cathode are covered as much as possible.

  by a material chosen for its low "stickiness coefficient".

  More particularly, the present invention provides a microtip cathode for a flat display screen, of the type comprising cathode conductors, between a substrate and an insulating layer of a grid, comprising a layer with a low coefficient of bonding, apparent to the less between

grid lines.

  According to an embodiment of the present invention,

  the grid and insulation layers are etched to the bottom

  trat according to the pattern for defining the grid lines, the layer with a low bonding coefficient being deposited directly on the substrate, preferably before the cathode conductors are formed. According to an embodiment of the present invention, the layer with a low bonding coefficient covers the entire

insulating layer.

  According to an embodiment of the present invention, the material constituting the layer with a low bonding coefficient is chosen from alumina, zinc oxide, oxide of

chromium and amorphous carbon.

  According to an embodiment of the present invention, the material constituting the layer with a low bonding coefficient and / or its thickness are chosen to preserve

  the isolation between two neighboring grid lines.

  These and other objects, features and advantages of the present invention will be discussed in detail in

  the following description of particular embodiments

  made without limitation in relation to the attached figures, among which: FIGS. 1 and 2 which have been described previously are intended to explain the state of the art and the problem posed; FIG. 3 represents a first embodiment of a cathode-grid with microtips according to the present invention; Figure 4 is a partial sectional view of a variant of the embodiment of Figure 3; FIG. 5 partially shows in section a second embodiment of a microtip grid cathode according to the present invention; FIG. 6 partially shows in section a third embodiment of a microtip grid cathode according to the present invention; and FIG. 7 partially shows in section a fourth embodiment of a microtip grid cathode

according to the present invention.

  The same elements have been designated by the same references

  references to the different figures. For reasons of clarity, the figures are not to scale and only the elements necessary for understanding the invention have been shown in the figures

and will be described later.

  FIG. 3 illustrates a first embodiment of

  the present invention. This figure represents a perspective view

  partially torn pective of a micro-grid cathode

points according to the invention.

  As before, the cathode 11 consists, on a substrate 1, for example made of glass, of mesh conductors 3 associated with a resistive layer (not shown) and defining cathode columns. Microtips 10 are deposited on the resistive layer inside the meshes formed by the

cathode conductors.

  An insulating layer 4, generally made of silicon oxide

  cium, is attached to the cathode conductors and a conductive grid 5 is formed on this insulating layer 4. The grid is, as before, made up of conductive lines 13 perpendicular to the columns 3 of the cathode 11. The intersection of a line 13 of the grid with a column 3 of the cathode generally defines a pixel of the screen. The microtips 10 are deposited in holes 14 formed at least in the lines

13 and in the insulating layer 4.

  According to a preferred and known embodiment of the cathode columns and the grid lines, these do not have a rectilinear line, but alternate wider sections, respectively 3 'and 13', defining the pixels or active areas and narrower connecting sections, respectively 3 "and 13", to electrically connect the pixels in the direction of the column or the line. Such an embodiment minimizes the

  opaque surfaces on the cathode-grid side and thus allows the realization

  tion of a screen whose plate 1 carrying the cathode constitutes the

  screen viewing area.

  According to the present invention, provision is preferably made

  at least between the lines 13 of the grid, an anti-

  degassing 20. This layer 20 is intended to mask the maximum possible surface of the materials which degas strongly in the

  grid cathode under electronic bombardment. These are, in part

  particular, of the silicon oxide constituting the iso- layer

lante 4.

  According to the first embodiment shown in Figure 3, the layer 20 is formed on the grid layer 5 and is etched in a pattern of lines 21 parallel to the lines 13 of the grid 5. The pattern of the lines 21 of the layer 20 is here chosen to completely cover at least the silicon oxide spaces separating two grid lines. The material of the layer 20 is here chosen to maintain the insulation between the lines 13 adjacent to the grid 5, insofar as it is in contact on either side of the line 21 with two lines 13

grid.

  If the anti-degassing material constituting the protective layer 20 does not have sufficient resistivity, for example if the material chosen is chromium oxide (Cr203), it will be preferable to use a variant as illustrated in FIG. 4 which represents a section perpendicular to the direction

overall of lines 13 of the grid.

  According to this variant, a space 22 is provided between the grid lines 13 and the lines 21 of the anti-degassing layer 20 to maintain the electrical discontinuity between two neighboring grid lines due to the weakly conductive nature of

  chromium oxide. The preservation or not of an apparent interstice

  rent in silicon oxide will depend on the thickness with which is deposited the chromium oxide which conditions the resistivity and the

leakage current between two lines.

  Figure 5 is a partial sectional view of a

  second embodiment of the present invention.

  According to this embodiment, the material chosen for layer 20 is an insulating material, for example, alumina (A1203). According to this embodiment, the protective layer 20 is deposited full plate on the insulating layer 4 of silicon oxide, prior to the formation of the grid 5. The layer is then etched at the same time as the grid layer 5 and the insulating layer 4 according to the pattern of the holes 14, before formation

microtips 10.

  An advantage of prior deposition of alumina is that it considerably simplifies the manufacturing process. Indeed, alumina can be deposited by the same technique as oxide of

  silicon, that is to say by chemical vapor deposition.

  In this case, it suffices to change the precursor at the end of the silicon deposition step so as to deposit the alumina on the silicon layer. The alumina layer 20 can thus be deposited with the

  same equipment as the insulating layer 4.

  Figure 6 is a partial sectional view of a three-

  sth embodiment of the invention in which the layer of

  anti-degassing protection 20 is placed full plate on the cover

  che grid in which were previously formed the lines 13. According to this embodiment, the material chosen for the layer 20 is of course insulating. For the implementation of such an embodiment, the constituent layer of the grid is etched according to the pattern of the lines 13. Then, the layer 20 is deposited full plate. The layers 20, 5 and 4 are then etched according to the pattern of the holes 14. Finally, the microtips 10 are

  deposited at the bottom of the wells thus formed.

  Such an embodiment has the advantage of opti-

  use the anti-degassing function. However, this constitutes an improvement which is not essential insofar as the niobium constituting the grid layer degas very clearly

  less than the silicon oxide constituting the insulating layer.

  Figure 7 shows, partially and in section, a fourth embodiment of the present invention. This embodiment is intended for a grid cathode in which the columns 3 of the cathode and the grid lines 13 as well as the insulating layer 4 are etched up to the glass constituting the substrate 1. Such a microtip cathode is described, for example, in European patent application N 0668604 of the applicant. According to this embodiment, the anti-degassing layer is deposited directly on the glass constituting the substrate 1

  before formation of the cathode columns 3.

  According to a variant of this embodiment, the visible sides of the insulating layer 4 may themselves be coated

  as a result of an additional protective layer (not shown

  felt). An advantage of the present invention is that it makes it possible to considerably reduce the degassing during operation of the screen. In fact, the layer of the grid cathode having the most unfavorable bonding coefficient (silicon oxide) is now enclosed and is therefore no longer liable to

  receive electrons falling on the grid.

  Preferably, the material constituting the layer 20 will be chosen, not only for its low bonding coefficient, but also to maintain the insulation between the

neighboring grid lines.

  According to a preferred embodiment of the present invention, the material chosen to constitute the layer 20 will be alumina (A1203) which has several advantages over

chromium oxide (Cr203).

  A first advantage is that alumina constitutes a good dielectric and can therefore be deposited without leaving oxide of

  apparent silicon, which improves the anti-degassing function.

  Another advantage is that alumina is a transparent material which is therefore particularly suitable if the plate 1 carrying the cathodegrid must constitute the surface of the screen. Other materials than those mentioned above may however be chosen to make the layer 20, provided that these

  materials meet the following specifications.

  First, the material should be chosen for its

  suitable bonding coefficient, i.e. for degassing sensi-

  significantly less than silicon oxide and preferably less

  than niobium under electronic impact.

  The material must also be chosen to have the same coefficient of expansion as glass. This prevents layer 20 from being damaged by heat treatments

  subsequent manufacturing of the screen.

  As a particular example, we can choose as

  another material is amorphous carbon or zinc oxide (ZnO).

  Zinc oxide has the same transparency advantage as alumina. It is however a little less resistive. So we have to

  check the line spacing by the thickness of the deposit.

  Of course, the present invention is susceptible of various variants and modifications which will appear to those skilled in the art. In particular, the adaptation of the method for manufacturing a conventional cathode for the implementation of the invention is

  within the reach of those skilled in the art from functional indications

  given above and particular constraints linked to the choice of material. In addition, other materials than those

  indicated for example may be chosen provided that they respect

  have the above functional characteristics.

Claims (5)

  1. Microtip cathode (10) for flat display screen, of the type comprising cathode conductors (3), between a substrate (1) and an insulation layer (4) of a grid (5), characterized in that it comprises a layer (20) with a low bonding coefficient, visible at least between lines (13) of the grid.   2. Cathode according to claim 1, in which the grid (5) and isolation (4) layers are etched to the substrate (1) according to the definition pattern of the grid lines (13), characterized in that the layer (20) with a low bonding coefficient is deposited directly on the substrate,   preferably before formation of the cathode conductors (3).   3. Cathode according to claim 1, characterized in that said layer with a low adhesive coefficient (20) covers the entire insulating layer (4).   4. Cathode according to any one of claims 1   in 3, characterized in that the material constituting the layer with a low bonding coefficient (20) is chosen from alumina,   zinc oxide, chromium oxide and amorphous carbon.   5. Microtip cathode according to any one of   claims 1 to 4, characterized in that the material   the layer with a low bonding coefficient (20) and / or its thickness are chosen to preserve the isolation between two neighboring grid lines (13).