EP0668604B1 - Method of manufacturing a cathode of a microtip fluorescent display and its product - Google Patents

Description

The subject of the present invention is a method for manufacturing a cathode fluorescent screen with microtips, as well as a fluorescent screen with microtips.

It relates to the industrial sector of flat display screens with row-column matrix addressing, and more particularly of display screens using microtip technology, that is to say made up of a vacuum tube formed by two plates of thin glass, the back plate, or cathode plate, comprising a matrix array of field effect emitters deposited by thin film techniques, and the front plate, or anode plate, covered on its internal face with a transparent conductive layer carrying phosphors.

In this type of screens, each light point (pixel) of the anode is associated with an emissive surface located opposite and made up of a large number of microtips. This emissive surface is defined by the intersection of a line (grid) and a column (cathode conductor) of the matrix (eg EP-A-0 234 989).

The cathode of a conventional microtip screen consists essentially of four layers deposited successively on a glass or silicon substrate, then etched, namely: a conductive layer playing the role of cathode "column conductors", a resistive layer generally made of silicon intended to limit the value of the emission current, an insulating layer and finally a second conductive layer constituting the "line conductors" of the grid. After depositing these layers, holes are formed in the grid and the insulating layer in which the microtips are then deposited.

In the methods known to date, the production of the constituent layers of the cathode requires at least four, and preferably five, photolithographic masking and etching operations, namely an etching of the cathode columns, an etching of the holes, an etching grid lines, etching of cathode contacts, and preferably a partial etching of the resistive layer between the cathode columns to avoid leaks and couplings between columns.

Furthermore, the images formed by a microtip screen are observed through the anode plate and it is the side of the phosphors opposite to that which receives the electrons which is seen, that is to say the least bright.

An object of the present invention is to simplify the manufacture of the cathode of microtip fluorescent screens by reducing the number of masking levels to three instead of five.

Another object of the present invention is to make said cathode transparent to obtain an improvement in light efficiency by allowing the observation of the luminescent material from the side where the electrons strike it, through the cathode.

These objects are achieved by etching together the three upper layers of the cathode (the grid, the grid insulator and the resistive layer), according to a single openwork pattern which defines both the grid lines and the access resistances to the microtips by the resistive layer. The column conductors can consist of fine metallic meshes.

The method of manufacturing a microtip fluorescent screen cathode according to the invention is defined in claim 1. The fluorescent microtip screen according to the invention is defined in claim 7.

• la figure 1 représente, vu de dessus et de façon schématique, un point image (pixel) défini comme le croisement d'une ligne et d'une colonne du réseau matriciel,
• la figure 2 est une vue en perspective cavalière illustrant schématiquement une structure de cathode selon la présente invention, et
• les figures 3 à 8 illustrent des étapes successives de fabrication de cathode selon l'invention.

In the appended drawings, given by way of nonlimiting example of one of the embodiments of the object of the present invention:

• FIG. 1 represents, seen from above and schematically, an image point (pixel) defined as the crossing of a line and a column of the matrix network,
• FIG. 2 is a perspective view showing schematically a cathode structure according to the present invention, and
• Figures 3 to 8 illustrate successive stages of cathode manufacturing according to the invention.

• des "conducteurs colonnes" 2 constitués de bandes ajourées (ou maillées) d'une couche de niobium, d'aluminium ou autre conducteur,
• une couche résistive 3 sur laquelle seront formées les micropointes 4,
• une couche isolante 5 (SiO₂ par exemple) qui constitue l'isolant de la grille,
• une couche conductrice, en niobium ou autre, qui constitue la grille 6 formant les conducteurs lignes.

Figures 1 and 2 show a microtip screen cathode structure according to the present invention. This structure comprises successively on a support plate 1 consisting of a glass plate:

• "column conductors" 2 made up of perforated (or mesh) strips of a layer of niobium, aluminum or other conductor,
• a resistive layer 3 on which the microtips 4 will be formed,
• an insulating layer 5 (SiO ₂ for example) which constitutes the grid insulator,
• a conductive layer, made of niobium or the like, which constitutes the grid 6 forming the line conductors.

The intersection of a row and a column defines an image point 7 or pixel (Figure 1).

A column conductor 2 consists of an openwork or mesh strip. Each grid line is made of meshes consisting of square conductive elements 6 linked together by fine conductive bridges 8 (for the sake of simplification in FIG. 2, only the longitudinal bridges have been represented; it is clear that there are also two transverse bridges for each square as shown in Figure 8). The microtips 4 are located in the grid squares and not in the conductive bridges. Each image point 7 is made up of several squares (four in FIG. 1 but much more in practice). Each square carries several microtips (four in the figure but often 16 in a real device).

The respective dimensions of the meshes of the column conductors 2 and of the squares constituting the grid 6 are determined so as to provide empty areas 9 between said squares and each column conductor. We can thus observe the anode phosphors through the cathode plate.

The electrical connection between the base of each microtip and the four sides of a cathode mesh is ensured by the passage of current through the thickness of each of the four resistive bars 10 and through the thickness of the resistive square 3.

The access resistance to the microtips is therefore essentially controlled by the geometry of the bars as well as by the resistivity of the resistive layer.

This access resistance must be high enough to standardize and limit the emission current of the tips while introducing only a few volts of voltage drop.

A screen was produced according to the invention. As an indication, the resistive layer was made of amorphous silicon offering a resistance of 100 megohms per square, four bars allowed access to each square mesh of 25 micrometers side; the bars had a length to width ratio of 2. Under 80 volts of grid / cathode polarization, the emission measured was 500 mA per dm ² . Results of the same order of magnitude can be obtained with neighboring values.

Figure 2 also shows that the electrical continuity along a grid line, from mesh to mesh is ensured by four conductive bridges 8 covering four insulating bars and four resistive bars 10. Since the mask which was used to engrave them is unique , the conductive bars ensuring the continuity of the grid lines and the resistive bars ensuring the access of the cathode current to the microtips have the same width and length.

The same shape of the bars must allow the passage of a large current in the grid lines and only allow the passage of a negligible leakage current from one column to another. This leakage current is inversely proportional to the resistance of the resistive layer 3 while the current of polarization of the grids is inversely proportional to the resistance of the upper conductive layer. However, as already mentioned, a resistance per square of 100 megohms is enough to guarantee the emission rate required for a screen. Furthermore, the resistance of the gate metal, in comparison is very low: 1 ohm per square in the case of the device produced, thanks to a niobium grid 6 400 nm thick. The resistance ratio in the device was therefore 10 ⁸ . It was then experimentally demonstrated that, for a multiplexing rate of several hundred lines, an image refresh at 60 Hz, a number of gray levels greater than 256 per color, the image obtained is free from visual defects of the column-to-column or row-to-row coupling type. It is also possible to replace niobium with a more conductive metal (aluminum is 10 times more conductive).

• dépôt d'une couche métallique 11 de cathode (figure 3),
• gravure des colonnes maillées de conducteurs de cathode 2 au moyen d'un premier masque (figure 4),
• dépôt des couches résistive 3, isolante 5, et de grille 6 (figure 5),
• gravure des trous 12 des micropointes 4 grâce à un deuxième masque (figure 6),
• dépôt des micropointes (figure 7),
• gravure simultanée des grilles 6 et des couches isolante 5 et résistive 3 en lignes ajourées (figure 8) au moyen d'un troisième et dernier masque qui sert également à définir les zones de contact de lignes et de colonnes.

Figures 3 to 8 illustrate successive stages of a manufacturing process according to the invention:

• depositing a metal layer 11 of cathode (FIG. 3),
• etching of the mesh columns of cathode conductors 2 by means of a first mask (FIG. 4),
• deposition of the resistive 3, insulating 5, and grid 6 layers (FIG. 5),
• etching of the holes 12 of the microtips 4 using a second mask (FIG. 6),
• microtip deposition (Figure 7),
• simultaneous etching of the grids 6 and of the insulating 5 and resistive 3 layers in openwork lines (FIG. 8) by means of a third and last mask which also serves to define the contact zones of rows and columns.

Thus, with only three etching steps, one obtains a result as good as with five etching steps in the prior art. In addition, an observable screen is obtained from the cathode.

Claims (9)

1. A method for manufacturing a fluorescent display screen cathode including microtips, including a supporting plate (1) bearing cathode conductors in meshed columns (2), a resistive layer (3) bearing microtips (4), an insulating layer (5) and a grid conductive layer (6) in rows, characterized in that the three upper grid layer (6), insulating layer (5) and resistive layer (3) are simultaneously etched according to a single perforated pattern defining both the grid rows and the access resistance to the microtips (4) through said resistive layer.
2. The method of claim 1, characterized in that it includes only three etching steps.
3. The method of claim 2, charaterized in that it includes the following steps:

   - depositing a cathode metal layer (11),

   - etching meshed columns of conductors (2) in said metal layer through a first mask,

   - depositing a resistive layer (3), an insulating layer (5), and a grid conductive layer (6),

   - etching holes (12) for forming microtips (4) with a second mask,

   - depositing microtips (4),

   - simultaneously etching the grid layer (6), the insulating layer (5) and the resistive layer (3) in perforated rows with a third and last mask which also serves to define the contact areas of the rows and columns.
4. The method of any of the former claims, charaterized in that the pattern of the grid conductive layer (6), insulating layer (5) and resistive layer (3) is formed by squares mutually interconnected by thin bridges thus forming meshes, each pixel (7) being constituted by a plurality of meshes.
5. The method of claim 4, characterized in that the path of the current flowing through the resistive layer (3), from the thin meshing formint a column conductor (2) to the base of a microtip (4), is defined by four resistive bars (10) surrounding each mesh and formed simultaneously as the conductive bridges (8) of the grid (6).
6. The method of claim 4, characterized in that the respective sizes of the meshes of the column conductors (2) and of the grid squares (6) are selected so that, after etching empty areas (9) remain between said squares and said meshes allowing the observation, through the cathode structure, of the phosphor elements formed on an anode plate.
7. A microtip fluorescent display screen having a cathode plate including a transparent supporting plate (1) bearing cathode conductors in meshed columns (2), a resistive layer (3) bearing microtips (4), an insulating layer (5) and a grid conductive layer (6) disposed in rows, charaterized in that the grid layer (6), insulating layer (5) and resistive layer (3) are etched according to a single pattern forming predetermined elements so that empty areas (9) remain between said elements and the column meshes, thus allowing the observation of the phosphor elements of an anode plate through the cathode plate.
8. The microtip fluorescent display screen of claim 7, charaterized in that the elements constituting the pattern of the grid (6), the insulating layer (5) and the resistive layer (3) are squares into which the microtips (4) are positioned, the squares being interconnected through thin conductive bridges (8) for the grid (6) and through resistive or insulating bars (10) to form meshes, each pixel (7) being formed by a plurality of meshes, and each meshes bearing several microtips (4).
9. The microtip fluorescent display screen of claim 8, charaterized in that the grid (6) is made of niobium or aluminum having a thickness of approximately 400 nm, and wherein the resistive layer (3) is made of amorphous silicon having a resistance of approximately 100 megohms per square, four respective bars (10) with a length/width ratio of 2 providing on access to each square mesh, the sides of the squares being approximately 25 micrometers.

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