EP0696045B1 - Cathode of a flat display screen with constant access resistance - Google Patents

Description

The present invention relates to the production of a microtip cathode. It applies more particularly to the realization of a microtip cathode of a flat screen of visualization.

Figure 1 shows the structure of a flat screen microtips of the type to which the invention relates.

Such a microtip screen essentially consists a cathode 1 with microtips 2 and a grid 3 provided of holes 4 corresponding to the locations of the microtips 2. Cathode 1 is placed opposite a cathodoluminescent anode 5 of which a glass substrate 6 constitutes the surface screen.

The operating principle and the detail of the constitution of such a microtip screen are described in the US Patent 4,940,916 to the French Commissariat for Energy Atomic.

Cathode conductors are arranged in columns on a glass substrate 10. The microtips 2 are produced on a resistive layer 11 deposited on the conductors of cathode and are conventionally arranged inside of meshes defined by the cathode conductors. Figure 1 partially representing the interior of a mesh, the conductors cathode do not appear in this figure. The cathode 1 is associated with grid 3 which is organized in lines. The intersection of a line in grid 3 and a column of cathode 1 defines a pixel.

This device uses the electric field created between cathode 1 and grid 3 so that electrons are extracts from microtips 2 to phosphor elements 7 of the anode 5. In the case of a color screen, as shown in Figure 1, the anode 5 is provided with alternating strips of elements phosphors 7, each corresponding to a color (Blue, Red, Green). The bands are separated from each other others by an insulator 8. The phosphor elements 7 are deposited on electrodes 9, made up of corresponding bands a transparent conductive layer such as oxide indium and tin (ITO). The sets of blue, red bands, green are alternately polarized with respect to the cathode 1, so that the electrons extracted from the microtips 2 of a pixel of the cathode / grid are alternately directed towards the phosphor elements 7 opposite each of the colors.

Figures 2A to 2D illustrate an example of a structure of this type, FIGS. 2B and 2D being respectively enlargements of parts of FIGS. 2A and 2C. Many microtips 2, for example sixteen, are arranged in each mesh 12 defined by cathode conductors 13 (figure 2B). The intersection of a line 14 of grid 3 and a column 15 of cathode 1, corresponds here, for example, to sixty-four meshes 12 of a cathode pixel (Figure 2A).

The cathode 1 is generally made up of layers deposited successively on the glass substrate 10. FIGS. 2C and 2D partially represent a sectional view along the line AA 'in FIG. 2B. A conductive layer 13, for example made of niobium, is deposited on the substrate 10. This layer 13 is etched in a pattern of columns 15, each column having meshes 12 surrounded by cathode conductors 13. A resistive layer 11 is then deposited on these cathode conductors 13. The purpose of this resistive layer 11, made for example of amorphous silicon doped with phosphorus, is to protect each microtip 2 against an excess of current at the start of a microtip 2. The affixing of such a resistive layer 11 aims to homogenize the electronic emission of the microtips 2 of a pixel of the cathode 1 and thus increase its lifetime. An insulating layer 16, for example of silicon oxide (SiO ₂ ), is deposited on the resistive layer 11 to isolate the cathode conductors 13 from the grid 3 (FIG. 2D). The grid 3 is formed of a conductive layer, for example of niobium. Holes 4 and wells 17 are respectively made in layers 3 and 16 to receive the microtips 2 which are for example made of molybdenum.

The deposition of microtips 2 in wells 17 is conventionally obtained by spraying molybdenum on a removal layer on the grid 3.

A disadvantage of conventional techniques is that if the resistive layer protects the microtips against excess current, it cannot completely homogenize electronic transmission. In fact, microtips of a mesh are not all equidistant from the conductors cathode, which results in non-uniformity of the emission electronic.

Another drawback is the need to form in each of the cathode columns, meshes of conductors. What imposes the realization of a complex pattern over the entire surface of the cathode.

In addition, the small diameter of the microtips (of 1 to 2 µm) and the need to reproduce them with a high screen pixel density (several thousand per pixel) means that existing processes limit the surface flat screens that can be produced. The disparities that can appear in the regularity of the diameter of the holes and wells for receiving microtips also adversely affect the homogeneity of the electronic emission, causing disparities in the diameter and height of the microtips.

The object of the present invention is to overcome these disadvantages of providing a microtip cathode providing electronic radiation of optimized homogeneity.

To achieve these objects, the present invention provides a microtip cathode for a flat display screen, of the type comprising a substrate, at least one conductor cathode, and microtips arranged on a layer resistive; said cathode conductor being disposed above of the resistive layer, and having circular openings in the center of each of which is arranged a microtip.

According to one embodiment of the invention, the diameter circular openings that the conductor of cathode is greater than the diameter of the base of a microtip.

According to one embodiment of the invention, the cathode is associated with a grid, separated from the conductor cathode by an insulating layer and provided with a hole in the balance of each microtip; the isolation layer and the cathode conductor having a receiving well of a microtip in line with each hole in the grid; and the diameter grid holes being substantially less than diameter of the wells of the insulation and conductor layers cathode.

According to one embodiment of the invention, the cathode has an auxiliary insulating layer between the conductor cathode and insulation layer.

The invention also relates to a method of making of a microtip cathode which consists in producing, on a stack consisting at least of a substrate, a layer resistive, a layer of cathode conductor, a layer of isolation and a layer of grid, an anisotropic etching holes in the grid layer, and a corresponding engraving of larger section wells, in the isolation layers and cathode conductor.

• réalisation de conducteurs de cathode organisés en colonnes sur une couche résistive déposée sur un substrat ;
• préparation de motifs circulaires dans des lignes d'une grille par photolithogravure ;
• réalisation de trous dans les lignes de la grille, et de puits dans les couches d'isolement et de conducteurs de cathode, et dépôt d'une micropointe au centre de chaque puits, sur une couche résistive.

According to one embodiment of the invention, the method consists in carrying out the following phases:

• making cathode conductors organized in columns on a resistive layer deposited on a substrate;
• preparation of circular patterns in lines of a grid by photolithography;
• making holes in the grid lines, and wells in the insulation layers and cathode conductors, and depositing a microtip in the center of each well, on a resistive layer.

• dépôt pleine plaque d'une couche résistive sur le substrat ;
• dépôt pleine plaque d'une fine couche conductrice d'arrêt de gravure ;
• dépôt pleine plaque d'une couche conductrice de conducteurs de cathode ;
• oxydation électrolytique de la couche conductrice de conducteurs de cathode ;
• gravure simultanée, de la couche de conducteurs de cathode et de la couche isolante auxiliaire obtenue par ladite oxydation, selon un motif de colonnes ; et
• élimination de la couche d'arrêt de gravure entre les colonnes définies par les conducteurs de cathode.

According to one embodiment of the invention, the first phase of producing cathode conductors comprises the following steps:

• full plate deposition of a resistive layer on the substrate;
• full plate deposition of a thin conductive etching stop layer;
• full plate deposition of a conductive layer of cathode conductors;
• electrolytic oxidation of the conductive layer of cathode conductors;
• simultaneous etching of the layer of cathode conductors and of the auxiliary insulating layer obtained by said oxidation, in a pattern of columns; and
• elimination of the etching stop layer between the columns defined by the cathode conductors.

According to one embodiment of the invention, the second phase of circular pattern photolithography is made by depositing a layer of resin on the layer of grid, and by insulating this layer of resin, later to a repository of opaque calibrated microbeads for radiation sunstroke.

According to one embodiment of the invention, a pre-insolation step of the resin layer is carried out, prior to the microbead deposition step, by masking grid lines.

• gravure anisotrope et simultanée de trous dans la couche de grille et d'ébauches de puits dans les couches d'isolement et de conducteurs de cathode ;
• élargissement des puits par une gravure isotrope ;
• dépôt de micropointes au centre de chaque puits, sur la fine couche conductrice d'arrêt de gravure ;
• élimination de la couche d'arrêt de gravure dans le fond des puits autour des micropointes.

According to one embodiment of the invention, the third phase of producing a grid and microtips comprises the following steps:

• anisotropic and simultaneous etching of holes in the grid layer and well blanks in the insulation and cathode conductor layers;
• widening of the wells by isotropic etching;
• depositing microtips in the center of each well, on the thin conductive etching stop layer;
• elimination of the etching stop layer in the bottom of the wells around the microtips.

Thus, according to an advantage of the present invention, the access resistance between the cathode and each of the microtips is constant since it corresponds to a resistive region annular of constant dimensions.

les figures 1 et 2 qui ont été décrites précédemment sont destinées à exposer l'état de la technique et le problème posé ;

les figures 3A et 3B représentent partiellement, respectivement en coupe et en vue de dessus, une cathode à micropointes selon l'invention ;

les figures 4A à 4H représentent, schématiquement et en coupe, différentes étapes d'un mode de mise en oeuvre d'une première phase d'un procédé de réalisation d'une cathode selon l'invention ;

les figures 5A à 5C représentent, schématiquement et en coupe, différentes étapes d'un mode de mise en oeuvre d'une deuxième phase d'un procédé de réalisation d'une cathode à micropointes selon l'invention ; et

les figures 6A à 6C représentent, schématiquement et en coupe, différentes étapes d'un mode de mise en oeuvre d'une troisième phase d'un procédé de réalisation d'une cathode à micropointes selon l'invention.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

Figures 1 and 2 which have been described above are intended to show the state of the art and the problem posed;

FIGS. 3A and 3B partially show, respectively in section and in top view, a microtip cathode according to the invention;

FIGS. 4A to 4H represent, diagrammatically and in section, different stages of an embodiment of a first phase of a method for producing a cathode according to the invention;

FIGS. 5A to 5C show, schematically and in section, different stages of an embodiment of a second phase of a method for producing a microtip cathode according to the invention; and

FIGS. 6A to 6C represent, diagrammatically and in section, different stages of an embodiment of a third phase of a method for producing a microtip cathode according to the invention.

For reasons of clarity, the scales have not were respected for the representation of the figures.

Cathode 1, according to the invention, as shown in Figures 3A and 3B, comprises from an insulating substrate 10, a resistive layer 11 supporting microtips 2. Cathode conductors 13 are arranged on the layer resistive 11 with possible interposition of a thin layer conductive 19 of adhesion and etching stop. These drivers cathode 13 are organized in columns each of which has in its width and in its length a large number of microtips, Figure 3A showing only a small portion of a column. In other words, the drivers of cathode 13 are continuous on all columns 15.

Microtips 2 are arranged on the resistive layer 11 at the center of circular openings 17 which each has cathode conductor 13. Each circular opening 17 defines between the microtip 2 which it receives and the conductor cathode 13, an annular resistive region through of layer 11. Thus, all microtips 2 of cathode conductor 13 will be electrically separated from it last, by a resistive region of the same value, provided that the diameter of the circular openings 17 is the same. The diameter of these circular openings 17 is greater than the diameter presented by the bases of the microtips 2.

All microtips 2 are therefore electrically separated from the cathode conductors 13 by a resistor same value. This is an essential characteristic of the present invention which leads to optimize the homogeneity of the cathodic radiation, making the current in the microtips 2.

According to an exemplary embodiment illustrated in the figure 3A, the cathode 1 is associated with a control grid 3. The cathode conductors 13 are then isolated from grid 3 at by means of an insulation layer 16, possibly associated with an auxiliary insulating layer 18. This auxiliary insulating layer 18 is, when provided, arranged between the conductor cathode 13 and the insulating layer 16. It allows to remove the effects of "needle holes" that may present the insulating layer 16 perpendicular to the surface of the cathode conductors 13.

Holes 4 and wells 17 are made in the layers grid 3, insulation 16 and cathode conductors 13 (and if necessary in the auxiliary insulating layer 18) to receive microtips 2. A characteristic of these holes 4 and well 17 is that wells 17 in the layers insulation 16 (and 18) and the cathode conductor 13 have a diameter significantly larger than the holes 4 in the grid layer 3.

2 microtips are deposited on the thin layer conductor 19, if it exists, plumb with holes 4, and this layer 19 is open around each microtip 2, in its free surface. Thus, each microtip 2 is laterally separated from the layer of cathode conductors 13 by a ring of width corresponding approximately to the difference between the diameter of the wells 17 and the holes 4. If the fine conductive layer 19 is not used, the microtips 2 are are found directly on the resistive layer 11, and always annularly separated from the cathode conductors 13.

According to a particular embodiment, the conductors cathode 13 have a width of approximately 300 μm, corresponding to the width of a screen pixel, defined by the intersection a row 14 of grid 3 and a column 15 of cathode 1. The diameter of the holes 4 is 1.3 μm, that of the well 2.6 of 2.6 μm, and the diameter of each microtip 2 is at the base of 1.1 µm.

An example of an implementation mode will be described below. work of a process for producing such a cathode according to the invention.

This process can be implemented in three phases corresponding respectively to the production of conductors cathode 13, patterning at future locations microtips in grid lines 3, and at the realization of grid 3 and microtips 2.

FIGS. 4A to 4H illustrate the implementation of the first phase which corresponds to the realization of the conductors cathode 13.

During a first step (FIG. 4A), we deposit on the substrate 10 a resistive layer 11.

A second step (Figure 4B) is to file a thin conductive layer 19, called etching stop. The role of this layer 19 is double. On the one hand, it constitutes a surface for attaching the next layer (Figure 4C) and microtips. On the other hand, it ensures an engraving stop of the layer of cathode conductors 13. This second role will be better understood later, in relation to the description of Figures 4E, and 6A to 6C.

A third step (Figure 4C) is to file a conductive layer 13. The bonding of this layer 13 is favored by layer 19.

A fourth possible step consists (Figure 4D) in perform an oxidation of the conductive layer 13, to obtain, in the thickness of this layer 13, an insulating layer auxiliary 18. The layer 13 previously deposited is then chosen to have the characteristic of being oxidizable. We will also ensure that the thickness of layer 13, deposited during the third stage, sufficient to allow obtaining an auxiliary insulating layer 18 while retaining sufficient thickness for cathode conductors 13.

The four steps described above are carried out over an entire surface of the substrate 10.

During a fifth step (Figure 4E), we engrave in columns the cathode conductors 13. The layer 19 provides, during this stage, a stop of the engraving which avoids attacking the resistive layer 11. The cathode conductors 13 have, for example, a width of the order of 300 μm.

Then, in a sixth step (Figure 4F), we eliminate layer 19 at the places where layers 13 and 18 were engraved, that is to say between the columns 15 of conductors of cathode 13.

In a seventh step (Figure 4G), we deposit on the structure resulting from the first phase, an insulator 16.

During an eighth step (FIG. 4H), one deposits a conductive grid layer 3. This deposit is for example obtained in the same way as the deposition of the conductor layer cathode 13.

As can be seen, the structure thus obtained according to the invention differs from previous techniques, in particular by the fact that the conductive layer 13 is no longer etched in a pattern of mesh columns, but that the conductors cathode 13 are continuous over an entire column 15.

In addition, the resistive layer 11 is affixed before the conductive layer 13, which allows the formation of a layer auxiliary insulator 18 by oxidation of this conductive layer 13.

FIGS. 5A to 5C illustrate a second phase of the process for producing a microtip cathode according to the invention, corresponding to a phase of delimitation of lines of grid and pattern formation at future locations of microtips in grid lines 3. For reasons of clarity, layers 13, 18, and 19 of the stack from the first phase have been designated, in FIGS. 5A to 5C, by the common reference 15 corresponding to their layout in column.

This second phase uses photolithography circular patterns to define future locations microtips, i.e. holes 4 in lines of grid 3.

In a first step (Figure 5A), a layer of photosensitive resin 20 of negative type is applied to the conductive layer 3.

Any conventional method of photolithography to define the patterns in layer 20 circular lines as well as grid lines 3. The width of the grid lines is, for example, on the order of 300 µm. The diameter of a circular pattern has a given value included, for example between 1 and 2 µm, and the number of patterns is several thousands per screen pixel.

However, we prefer to implement a phase particular photolithography of circular patterns which ensures regular diameter patterns with density regardless of the screen size. This in order to further optimize the homogeneity of electronic radiation.

During a second step (Figure 5B), we pre-insulate the resin layer 20 through a conventional mask 21 for defining lines 14 of grid 3.

Then, in a third step (not shown), microbeads 22 are deposited on the resin layer 20. These microbeads 22 are for example microbeads of glass or plastic. They are opaque to solar radiation for obtain a maximum masking effect on the areas on which they are filed. The distribution of microbeads 22 on the resin layer 20 is random. We have indeed seen that the quality of a screen was linked to the regularity of the density microtips 2 from one screen pixel to another and at regularity of the microtip diameter 2. On the other hand, the difference between two microtips 2 has no influence on the screen quality as long as the density of microtips is high. So the random distribution of patterns in the layer of grid 3 has no consequence on the quality of the screen. It was thus found that a flat screen of good quality with number and diameter of circular patterns in each pixel of the screen which are the same at five for hundred percent, the pixel density of patterns being high to not not harm the screen brightness. A microbead depot 22 calibrated with a given diameter of a value between 1 and 5 µm with a tolerance of 10 percent for the diameter of microbeads 22 achieves this result.

To ensure that the density of the microbeads 22 deposited on layer 20 is sufficient and regular, we can use, according to the invention, several methods of depositing microbeads 22.

A first method is to immerse the stack from the first phase, coated with the resin layer 20, in a bath containing microbeads 22 in solution. The density microbeads 22 in the bath is fixed according to the desired pattern density. The deposit of microbeads 22 is carried out by decantation, the microbeads used being in this case in glass. It is also possible to perform the sunshine step through the bath as soon as the microbeads 22 have settled, which accelerates the execution of the process. Evacuation microbeads 22, after sunshine, takes place here simply by removing the stack and its possible support from the bath.

A second method consists in spraying, on the resin layer 20, a mixture of solvent and microbeads 22 contained in a tank. The solvent is alcohol-based, this which allows its evaporation during spraying. The distribution microbeads 22 on the resin layer 20 present good homogeneity, the density of microbeads 22 being fixed by the duration of the spraying carried out. Here, the microbeads 22 hold on the resin layer 20 by electrostatic effect, resulting from charges acquired during their crossing air between a spray nozzle and the resin layer 20. The evacuation of microbeads 22 after insolation can be made by blowing or any other means. An advantage of this technique is that it is created between microbeads 22, makes their charge a repulsive force which tends to improve the regularity of their distribution.

A third method is to drown microbeads 22 in a viscous material, for example polyvinyl alcohol. The resin layer 20 is covered with a layer of this material for example by scraping or screen printing without pattern. The polyvinyl alcohol is then dried and then manner which will be described below. Subsequently, polyvinyl alcohol is dissolved, for example in water and microbeads 22 are removed at the same time.

Once the microbeads 22 have been deposited on the resin layer 20, this resin layer 20 is exposed by means of a light insulator almost parallel to the course of a fourth step (not shown). The wavelength of radiation of the insolator is chosen according to the resin used and the precision sought, for example in the ultraviolet range. The microbeads 22 are then removed of the resin layer 20 during a fifth step (not shown).

Insolation is only effective in surfaces that were masked during the second stage, pre-sunshine, either inside lines 14 of grid 3 which have been trained. So when developing the resin by means of a conventional process (FIG. 5C), patterns 23 are obtained in the resin layer 20 only in the surface of the lines 14 of grid 3. This makes it possible to position the zones of microtips 2 of cathode 1, limiting the formation of patterns 23 to surfaces that correspond to areas in front receive microtips 2. In FIG. 5C, the plot of the columns 15 of cathode conductors 13, has been shown in mixed lines, and that of the pre-exposed surfaces 14, corresponding in lines 14 of grid 3, has been shown in dotted lines.

In a sixth step (Figure 5C), we develop the resin by implementing a conventional process in conditions compatible with the type of resin used. Of circular patterns 23 are thus formed in the layer of resin 20 at the locations of the microbeads 22. These patterns 23 are then used to engrave holes 4 and blanks corresponding wells 17 in layers 3, 16, 18, and 13, of the stack from the first phase, as we will see subsequently in relation to FIGS. 6A to 6C.

A variant of the insolation step consists of insulating the resin layer 20, still by means of an insulator at quasi-parallel light but by tilting layer 20 by relative to the beam axis, and rotating it around this axis. To do this, the stack from of the first phase, coated with the resin layer 20 on which have been deposited the microbeads 22, on a support rotary inclined at a given angle to the beam axis. Thus, the diameter actually insulated at the base of each microbead 22 is found to be less than the diameter of the microbeads 22. Patterns 23 of diameter are thus obtained smaller than the diameter of the microbeads 22. The ratio between the diameter of the microbeads 22 and the diameter of the patterns 23 obtained depends on the angle of inclination of the support in relation to the axis of the quasi-parallel beam of radiation from the insolator. This variant further improves the resolution obtained by the implementation of the method according to the invention. We can effect use larger microbeads 22 which will present a better uniformity between them. We can by example make patterns 23 with a diameter of 2 µm using microbeads 22 having a diameter of 5 μm.

FIGS. 6A to 6C illustrate an example of setting up work of a third phase of the process according to the invention. This third phase corresponds to the formation of holes 4 in lines 14 of grid 3, and of deposit of microtips 2 in wells 17 directly above these holes 4. For reasons of clarity, the sections of FIGS. 6A to 6C represent a part of a pixel defined by the intersection of a line 14 of the grid 3 and a column 15 of cathode 1.

In a first step (not shown), we engrave in grid layer 3, grid lines 14 as well as holes 4 at future locations of microtips 2, i.e. at the locations of the patterns 23. The engraving of this first step is carried out in such a way that it attacks the material of the grid 3 without attacking the material of the layer insulating 16. In addition, it is preferably a anisotropic etching.

During a second step (FIG. 6A), we carry out reactive ion etching up to the etching stop layer 19. Thus, blanks 17 are etched in the layers insulation 16 (and possibly 18) and conductors cathode 13. This etching is anisotropic so that the blanks wells 17 are aligned with the circular patterns 23. The well blanks 17 have, for example, a diameter 1.3 µm like the holes 4.

In a third step (Figure 6B), we expand the diameter of the wells 17 in the insulation layers 16 (and possibly 18) and of conductors 13. To do this, we performs an isotropic wet etching.

The engravings of the second and third stages are stopped by the etching stop layer 19 so as not to attack the resistive layer 11 on which must be deposited the microtips 2. The etching of the lines 14 of the grid 3 (first step) could also be carried out previously in the second phase. In this case, reactive ion etching of the second step (Figure 6A) can be performed, locations of the patterns 23, simultaneously in the layers 3, 16 (and if applicable 18), and 13. So holes 4 and the well blanks 17 are formed simultaneously. Moreover, the pre-sunstroke step (Figure 5B) of the second phase is no longer necessary since the lines of grid are already formed. We could however use this pre-exposure step to limit the formation of patterns 23 directly above the cathode conductors 13, either inside columns 15.

The microdots 2 are deposited during a fourth step (not shown), conventionally. We uses, for example, an uplift layer (commonly called "lift-off" layer) on which we perform evaporation of a conductive material. This evaporation leads on the one hand to the formation of a residual layer on the uplift elimination layer and on the other hand to the formation of microtips 2 in wells 17. These microtips 2 have, for example, a diameter at the base of 1.1 µm and a height of the order of 1.2 µm. Then we eliminate the residual layer, using the uplift removal layer. We then obtain a structure as shown in Figure 6C.

Finally, in a fifth and final step, we eliminate the etching stop layer 19 surrounding the microtips 2. This elimination leads to forming between each microtip 2 and a cathode conductor 13, via the resistive layer 11, an annular resistance of the same value for all microtips 2.

We then obtain a cathode as shown in Figures 3A and 3B.

We will indicate below a particular example of realization of a microtip cathode by specifying the materials and types of engraving used.

Phase 1:

Step 1: full plate deposition of a resistive layer 11, by sputtering of phosphorus doped amorphous silicon on the glass substrate 10. This resistive layer 11 has, for example, a thickness of 0.3 μm.

Step 2: full plate deposition, by evaporation of chrome, a thin conductive layer 19. The thickness of this layer 19 is for example 0.025 μm.

Step 3: full plate deposition, by evaporation of niobium, a layer of cathode conductors 13. The attachment of this layer 13 is favored by layer 19, niobium difficult to cling to amorphous silicon. The conductive layer 13 has, for example, a thickness of 0.2 to 0.4 µm.

Step 4: Full plate oxidation of the layer 13. This oxidation is for example obtained by subjecting the niobium layer 13 to anodic oxidation in a solution based on ammonium pentaborate and ethylene glycol. To do this, the stack is placed at the anode in an electrolytic bath based on ammonium pentaborate and ethylene glycol. The oxidation thickness depends practically only on the potential at which the electrolysis is carried out. For a potential of 40 V, for example, a thickness of niobium pentoxide (Nb ₂ O ₅ ) of 0.12 μm is obtained, constituting an auxiliary insulating layer 18.

Step 5: plasma etching of sulfur hexafluoride (SF ₆ ) of the insulating 18 and conductive 13 layers, according to a pattern of columns 15. It is preferred to carry out this plasma etching insofar as a chemical (wet) etching of the pentoxide niobium (Nb ₂ O ₅ ) which constitutes layer 18 is difficult to control. On the other hand, this oxide is etched with the same etching plasma as that conventionally used to etch niobium. The plasma employed also etches the amorphous silicon, this is why the layer 19 is said to stop etching and is made of a material chosen to be difficult to attack by the sulfur hexafluoride plasma.

Step 6: elimination of the layer 19, between the columns 15, by masking and chemical etching based on potassium permanganate (KMnO ₄ ) and potassium hydroxide (KOH) which attacks the evaporated chromium without damaging the other surrounding layers.

Step 7: full plate deposition of an insulating layer 16, by chemical vapor deposition (CVD) at ordinary pressure of silicon oxide (SiO ₂ ). The thickness of this insulating layer 16 is for example 1.3 μm.

Step 8: full plate deposition of a conductive layer of grid 3, by evaporation of niobium. The thickness of this grid layer which corresponds to the thickness of the grid 3 is for example from 0.2 to 0.4 μm.

Phase 2:

Step 1: full plate deposition of a layer of photosensitive resin 20.

Step 2: pre-insolently through a shutter mask of lines 14 of grid 3.

Step 3: random deposit of calibrated microbeads 22, on the resin layer 20.

Step 4: exposure of the resin layer 20, coated with microbeads 22.

Step 5: evacuation of the microbeads 22.

Step 6: development of the resin 20, and obtaining of patterns 23 at the future locations of the microtips 2 in rows 14 of grid 3.

Phase 3:

Step 1: plasma etching of sulfur hexafluoride (SF ₆ ) of layer 3, according to the pattern of lines 14, and of holes 4 at the locations of patterns 23. This plasma is chosen to attack the niobium of layer 3 without etching the silicon dioxide (SiO ₂ ) constituting the insulating layer 16.

Step 2: resistive ion etching of blanks well 17 in the insulation layers 16 and 18, and of conductors cathode 13, opposite the holes 4 of the grid 3. This engraving is chosen to be anisotropic.

Step 3: isotropic chemical etching of the wells 17 in the insulation layers 16 and 18, and of conductors cathode 13.

Step 4: depositing an elimination layer by uplift, by electrolytic deposition of nickel on surfaces of the grid layer 3. Realization of microtips 2, by evaporation of molybdenum. Then, elimination by lifting of molybdenum residues.

Step 5: etching of the layer 19 in its free surface, for example by masking and chemical etching based on potassium permanganate (KMnO ₄ ) and potassium hydroxide (KOH).

Of course, the present invention is susceptible various variants and modifications that will appear at one skilled in the art. In particular, each of the constituents described for the layers can be replaced by one or more constituents with the same characteristics and / or fulfilling the same function. In addition, the engraving means described by way of example may be replaced by others means of engraving, dry or wet, allowing to reach the same result.

Similarly, the succession of stages given as example can be modified according to the materials and means of engraving used. For example, the step of obtaining the layer auxiliary insulation 18 (phase 1, step 4) could be postponed after the cathode conductors 13 have been etched, the conductors cathode 13 then also being oxidized on their edges.

The formation of grid lines 14 could be postponed to the end of the process. In this case, the second stage of the second phase, pre-insulating surfaces that correspond to the grid lines. This is to avoid the formation of patterns 23 between lines 14, which would lead to removal of the insulation layer 16 at the places of these motives. The first and second stages of the third phase are in this case simultaneous.

In addition, the dimensional indications given to example title can be changed based on specifications sought for the screen, the materials used, or other. In particular, the diameter of the microbeads 22 used depends on the desired diameter for the grid holes 4 3 and the exposure technique used (vertical or oblique).

Claims (10)

1. A cathode (1) including microtips for flat display screens including a substrate (10), at least one cathode conductor (13), and microtips (2) that are disposed onto a resistive layer (11), characterized in that said cathode conductor (13) is disposed above the resistive layer (11) and has circular apertures (17) of a same diameter in the middle of each of which a microtip (2) is disposed.
2. The microtip cathode of claim 1, characterized in that the diameter of each circular aperture (17) of the cathode conductor (13) is larger than the diameter of the basis of a microtip (2).
3. The microtip cathode of claim 1 or 2, characterized in that it is associated with a gate (3), which is separated from the cathode conductor (13) by an insulating layer (16) and is provided with a hole (4) in front of each microtip (2); the insulating layer (16) and the cathode conductor (13) being provided with a well (17) for accommodating a microtip (2) in front of each hole (4) of gate (3); and the diameter of the holes (4) of gate (3) being substantially smaller than the diameter of the wells (17) of the insulating layer (16) and of the cathode conductive layer (13).
4. The microtip cathode of claim 3, characterized in that it includes an auxiliary insulating layer (18), between the cathode conductor (13) and the insulating layer (16).
5. A method for fabricating a cathode including microtips characterized in that it consists of anisotropically etching, in a pile constituted by at least a substrate (10), a resistive layer (11), a cathode conductive layer (13), an insulating layer (16) and a gate layer (3), holes (4) in the gate layer (3), and etching corresponding larger wells of a same diameter in the insulating layer (16) and the conductive cathode layer (13) under each hole and depositing microtips (2) in the center of each wells (17).
6. The method of claim 5, characterized in that it consists of carrying out the following phases:

   forming cathode conductors (13) arranged in columns (15) onto a resistive layer (11) that is disposed onto a substrate (10);

   photoetching circular patterns (23) in rows (14) of a gate (3);

   etching holes (4) in the rows (14) of gate (3), and corresponding wells (17) in the insulating layer (16) and the cathode conductive layer (13), and depositing a microtip (2) in the middle of each well (17), onto said resistive layer (11).
7. The method of claim 6, characterized in that the first phase of forming cathode conductors (13) comprises the following steps:

   depositing a resistive layer (11) on the substrate (10);

   depositing a thin conductive etch-stop layer (19);

   depositing a conductive layer of cathode conductors (13);

   electrolytically oxidizing the conductive layer of cathode conductors (13);

   simultaneously etching the cathode conductive layer (13) and the auxiliary insulating layer (18) that is obtained by said oxidation, according to a column pattern (15); and

   removing the etch-stop layer (19) between the columns (15) defined by the cathode conductors (13).
8. The method of claim 6 or 7, characterized in that the second phase of photoetching circular patterns (23) is carried out by depositing a resist layer (20) onto the gate layer (3), and by insolating said resist layer (20), after deposition of calibrated microbeads (22) that are opaque to the insolation radiation.
9. The method of claim 8, characterized in that a pre-insolation of the resist layer (20) is carried out, prior to the step of depositing microbeads (22), by masking (21) the gate rows (14).
10. The method of any of claims 6 to 9, characterized in that the third phase for fabricating the gate (3) and the microtips (2) includes the following steps:

    anisotropically and simultaneously etching holes (4) in the gate layer (3) and well preforms (17) in the insulating layers (16, 18) and the cathode conductors (13);

    enlarging holes (17) through isotropic etching;

    depositing microtips (2) in the middle of each well (17), onto the thin conductive etch-stop layer (19);

    removing the etch-stop layer (19) at the bottom of the wells (17) about the microtips (2).

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