EP0234989A1 - Method of manufacturing an imaging device using field emission cathodoluminescence - Google Patents

Abstract

Method for manufacturing a cathodoluminescence display device, excited by field emission.

This process consists in forming parallel cathodes (8) on a glass substrate (6), depositing a silica layer (12) on the cathodes, then a conductive layer, making a matrix of holes (16) in the conductive layer and the silica layer, deposit a fourth layer (23) not covering the holes on the conductive layer pierced, then deposit a layer of electron-emitting material on the entire structure, remove the fourth layer in order to expose microemitters (18), form in the conductive layer of the grids (10) crossing the cathodes and have above the grids an anode (20) covered with a cathodoluminescent layer (22).

Description

The present invention relates to a method of manufacturing a display device by cathodoluminescence excited by field emission or light emission. It applies in particular to the production of simple matrix displays, allowing the visualization of fixed images, and to the production of complex multiplexed screens, allowing the visualization of animated images, of the television image type.

A display device by cathodoluminescence excited by field emission was described in patent application No. 84 11986 of July 27, 1984, filed in the name of the applicant. FIG. 1 shows an exploded perspective view of the display device described in this document.

This display device comprises a display cell 2, sealed and evacuated, comprising two glass walls 4 and 6, located opposite one another. The lower wall 6 of the cell 2 is equipped with a first series of conductive strips 8, mutually parallel, playing the role of cathodes and a second series of conductive strips 10, parallel to each other, playing the role of grids. The conductive strips 10 are oriented perpendicular to the conductive strips 8 and isolated from the conductive strips 8 by an insulating and continuous layer 12, in particular made of silica.

The extreme regions 9 of the cathodes 8, not covered with insulation and not intercepting the grids 10, allow electrical contact to be made on the cathodes.

The conductive strips 8 and 10 respectively represent columns and rows. Each crossing of a row and a column corresponds to an elementary display point 14.

The conductive strips or grids 10 and the insulating layer 12 are pierced with a large number of holes 16 in which are housed microemitters or electron microchannels. Each elementary display point 14 corresponds to a multitude of micro-transmitters.

These microemitters, as shown in FIG. 2, each consist of a metal cone 18 emitting electrons when a suitable electric field is applied to them. These metal cones 18 rest by their base directly on the cathodes 8 and the top of these cones is substantially at the level of the conductive strips 10. The base diameter of the cones and their height are for example of the order of 1 μm.

The upper wall 4 of the cell 2, as shown in FIG. 1, is provided with a continuous conductive layer 20 acting as an anode. This anode 20 is covered with a layer 22 made of a material emitting light when it is subjected to an electronic bombardment coming from the microemitters 18.

The emission of electrons by a microemitter 18 can be achieved by simultaneously polarizing the cathode 8 and the grids 10 located opposite, as well as the anode 20. The anode 20 can in particular be brought to ground, the grids 10 are , either brought to the potential of the anode, or negatively polarized with respect to the latter using a voltage source 24. The cathodes 8 are negatively polarized with respect to the grid using a source of tension 26. The cathodes 8 and the grids 10 can be polarized sequentially in order to make appear a point by point image on the display cell 2. The image is observed on the side of the upper wall 4 of the cell.

The number of microemitters 18 per display point 14, that is to say by crossing a cathode and a grid, is generally high, which makes it possible to have a more uniform emission characteristic of one display point to another (average effect); this gives a certain redundancy of the microemitters making it possible to tolerate a certain proportion of microemitters not functioning.

In practice, the number of microemitters is between 10⁴ and 10⁵ transmitters per mm². Consequently, traditional manufacturing, requiring precise positioning of the microemitters facing the cathodes and grids, would be complex and would increase the cost of the display device.

The object of the present invention is precisely a relatively simple and inexpensive method for manufacturing a display device operating by cathodoluminescence excited by field effect as described above.

More specifically, the subject of the invention is a method for manufacturing a cathodoluminescence display device, characterized in that it comprises the following successive steps:
- deposit of a first conductive layer on an insulating substrate,
- etching of the first layer to form first parallel conductive strips playing the role of cathodes,
- deposit of a second insulating layer on the structure obtained,
- deposit of a third conductive layer on the second layer,
- Hole openings opening into the third and second layers, these holes being distributed over the entire surface of the third and second layers.
- deposit on the third etched layer of a fourth layer not covering the holes,
- deposit on the entire structure obtained with a fifth layer of an electron emitting material,
elimination of the fourth layer resulting in the elimination of the electron-emitting material surmounting said fourth layer and the retention of said emitter material in the holes,
- etching of the third and second layers to expose at least one of the ends of the first conductive strips,
- etching of the third layer to form second parallel conductive strips acting as grids, the first and second strips being crossed, and
- Production of an anode and a cathodoluminescent material opposite the second conductive strips.

By “holes distributed over the entire surface”, it is necessary to understand holes made opposite the cathodes as well as opposite the intercathode spaces.

This method has the advantage of simple implementation. In particular, it allows the production of electron microemitters in the holes formed in the second and third layers, distributed over the entire display device, without requiring precise positioning with respect to the cathodes and grids. Only microemitters located at an intersection of a cathode and a grid are effectively active.

To improve the grip of the conductors cathode on the insulating substrate, there is advantageously interposed between the substrate and the first conductive layer, in which the cathodes are made, an insulating intermediate layer.

In order to minimize the resistance to access to the microemitters, the first conductive layer must be made of a material that conducts electricity well. Furthermore, this first conductive layer must have good compatibility with the second insulating layer and in particular good adhesion and must be inert with respect to the etching method of this second insulating layer. Advantageously, the first conductive layer is made of a material chosen from indium oxide, tin oxide and aluminum.

Indium oxide and tin oxide are preferably used for producing screens of small dimensions and of low complexity such as screens used for viewing still images. On the other hand, aluminum is preferably used when producing complex multiplexed screens of large dimensions, used in particular for viewing animated images of the television image type.

In order to minimize the capacitances between the cathodes and the grids, and therefore to minimize the response time of the microemitters, the second insulating layer must have as low a dielectric constant as possible. To this end, this second insulating layer is preferably made of silicon oxide (SiO₂) or silica.

This silicon oxide layer can be deposited by the chemical vapor deposition (CVD) technique, by sputtering or by vacuum evaporation. However, the chemical phase deposition technique is preferably used. steam, a technique allowing an oxide layer of uniform quality and constant thickness to be obtained.

The opening of the holes in the insulating layer, in particular of silicon oxide, can be carried out by dry or wet etching techniques well known to those skilled in the art.

The third conductive layer in which the grids are formed must be made of a material having good adhesion to the second insulating layer, for example made of silicon oxide, as well as good chemical resistance to the various products used to make the microemitters. To this end, the third conductive layer is preferably made of a metal chosen from niobium, tantalum and aluminum.

In order to reproducibly obtain holes in this third conductive layer, of a size close to one micron, the formation of these holes is advantageously carried out by an anisotropic dry etching technique.

In order to ensure a good definition of the microemitters, the fourth layer playing the role of mask for the deposition of the fifth layer is made of metal and in particular nickel. The deposition of this fourth layer of nickel is advantageously carried out by evaporation under vacuum at a grazing incidence so as not to cover the holes made in the second and third layers. Furthermore, the elimination of this metallic layer is advantageously carried out by electrochemical dissolution.

The choice of the material of the fifth layer is essentially dictated by these properties with respect to the emission by field effect or cold emission as well as its chemical resistance to the techniques of deposition and elimination of the fourth layer used for the production of microemitters. In particular, the electron-emitting material can be hafnium, niobium, molybdenum, zirconium, lanthanum hexaboride (LaB₆), titanium carbide, tantalum carbide, hafnium carbide, carbide of zirconium, etc. We choose for example molybdenum.

Other characteristics and advantages of the invention will emerge more clearly from the description which follows, given by way of illustration and without limitation.

• - la figure 1, déjà décrite, représente schématiquement, en perspective et vue éclatée, un dispositif de visualisation par cathodoluminescence,
• - la figure 2 déjà décrite, représente une partie agrandie de la figure 1, montrant un micro­émetteur,
• - les figures 3 à 12 illustrent les différentes étapes du procédé selon l'invention, les figures 3 à 6 et 10 à 12 sont des vues générales et les figures 7 à 9 des vues agrandies montrant un microémetteur.

The description refers to the appended figures in which:

• FIG. 1, already described, schematically represents, in perspective and exploded view, a display device by cathodoluminescence,
• FIG. 2, already described, represents an enlarged part of FIG. 1, showing a microemitter,
• - Figures 3 to 12 illustrate the different steps of the method according to the invention, Figures 3 to 6 and 10 to 12 are general views and Figures 7 to 9 enlarged views showing a micro-transmitter.

With reference to FIG. 3, the cleaning of the lower substrate 6 is first of all carried out in order to obtain good flatness and a good surface condition to allow optimized production of the microemitters. The substrate 6 can be a glass or ceramic plate. On the substrate 6, an oxide layer is then deposited by sputtering silicon (SiO₂) 7 of about 100 nm. The insulating layer 7 is then covered with a conductive layer 8a of indium oxide in which the cathodes 8 will be produced. This layer of indium oxide has a thickness of 160 nm and can be deposited by sputtering.

Then formed by conventional photolithography processes (deposition, irradiation, development) a positive resin mask 11 representing the image of the cathodes to be produced. Through this mask 11, the layer of indium oxide 8a is etched to form, as shown in FIG. 4, cathodes 8 0.7 mm wide at a pitch P of 1 mm. The etching of the layer 8a is carried out by chemical attack with orthophosphoric acid brought to 110 ° C. The etching of the indium oxide layer 8a is carried out over the entire thickness of the layer. The resin mask is then removed by chemical dissolution.

On the structure obtained, that is to say on the cathodes 8 and the exposed regions of the insulating layer 7, the silicon oxide layer 12 is then deposited, as shown in FIG. 5, by the technique chemical vapor deposition from silane, phosphine and oxygen gases. This oxide layer 12 has a thickness of 1 μm. The oxide layer 12 is then completely covered with a conductive layer 10a in which the grids will be produced subsequently. This layer 10a is deposited by vacuum evaporation. It has a thickness of 0.4 µm and is made of niobium.

Then formed on the conductive layer 10a a resin mask 13 by conventional photolithography methods (resin deposition, irradiation, development). This resin mask 13 represents the image positive holes to be made in the grid layer 10a and the insulating layer 12.

According to the invention, no precise positioning of these holes is necessary given their high number. A resin mask 13 is therefore produced, comprising openings 15 distributed over the entire surface of the mask, and in particular in regions 17 situated outside the zones 14 reserved for display (elementary display points defined at the intersection). cathodes and grids). This facilitates the production of the photomask 19 used for the exposure 21 of the resin 13 as well as its positioning above the structure.

Next, through the resin mask 13 in FIG. 6, the holes 16 are made in the layer of grid material 10a and the insulating layer 12. These holes 16 pass right through the layers 10a and 12. The etchings of layers 10a and 12 are carried out successively. The etching of the layer 10a is carried out by a reactive ion etching (GIR) process using a sulfur hexafluoride plasma (SF₆). The holes 16 made in the conductive layer 10a have a diameter equal to 1.3 μm to ± 0.1 μm. The holes in the silica layer 12 are produced, for example by chemical attack by immersing the structure in an attack solution of hydrofluoric acid and ammonium fluoride. Then, the resin mask 13 is chemically removed. The profile of the holes 16 thus produced is illustrated in FIG. 7.

We will now describe the process for manufacturing a microemitter. On the layer 10a, pierced with the holes 16, a nickel layer 23 is first deposited by evaporation under vacuum at a grazing incidence relative to the surface of the structure; the angle α formed between the axis of evaporation and the surface of layer 10a is close to 15 °. The nickel layer 23 has a thickness of 150 nm. This deposition technique makes it possible not to plug the holes 16.

Next, as shown in FIG. 8, a layer of molybdenum 18a is deposited on the entire structure. This layer 18a has a thickness of 1.8 μm. It is deposited under normal incidence relative to the surface of the structure; this deposition technique makes it possible to obtain cones 18 of molybdenum housed in the holes 16 having a height of 1.2 to 1.5 μm. Selective dissolution of the nickel layer 23 is then carried out by an electrochemical process so as to release, as shown in FIG. 9, the perforated niobium layer 10a and to reveal the microtips 18 emitting electrons.

Then, as shown in FIG. 10, an etching of the layer 10a and an etching of the insulating layer 12 is carried out in order to free the ends 9 of the cathodes 8 so as to subsequently allow electrical contact to be made on these cathodes. This etching is carried out through a resin mask (not shown), obtained according to conventional photolithography methods, the resin forming the mask must have a sufficiently high viscosity in order to cover all the holes 16 formed in the niobium layer 10a and the silicon oxide layer 12. The etching of the niobium layer 10a is carried out as previously by a reactive ion etching process and the etching of the silica layer 12 by chemical attack.

A resin mask 25 is then produced on the structure obtained representing the image of the grids 10 to be produced in the niobium layer 10a. This resin mask is produced according to the processes photolithography classics. Then carried out, through the mask 25, a dry etching of the reactive ionic type with SF₆ so as to release the conductive strips 10 perpendicular to the conductive strips 8. The resin mask 25 is then removed by chemical attack. The structure obtained after elimination of the mask 25 is that shown in FIG. 11.

On the other hand, on a glass substrate 4, as illustrated in FIG. 12, a conductive layer 20 made of indium oxide (In₂O₃) or tin oxide (SnO₂) is deposited by sputtering corresponding to the anode of the display cell 2. This layer 20 has a thickness of the order of 100 nm. The anode 20 is then covered with a cathodoluminescent layer 22 by sputtering. This layer 22 is made of zinc oxide and has a thickness of 1 μm.

The substrate 4 covered with the anode 20 and the cathodoluminescent material 22 is then presented above the grids 10. A space of 30 to 50 μm is maintained between the cathodoluminescent material 22 and the grids 10 by means of glass spacers 27 randomly distributed. The periphery of the anode 20 is hermetically welded to the lower part of the cell, by means of a fusible glass 29. The assembly obtained is then placed under vacuum.

The description given above has of course been given for information only, any modification, without departing from the scope of the invention, which may be envisaged. In particular, the thickness and the nature of the layers can be modified. In addition, some engravings and deposition techniques can be changed.

The various stages of the process of the invention have the advantage of being simple to implement and are well mastered by those skilled in the art, which allows good reproducibility and homogeneity in obtaining display devices. Furthermore, the fact of producing the transmitters on the whole of the cell without precise positioning with respect to the cathodes and the grids, makes the manufacture of the display device particularly easy.

Claims (11)

1. Method for manufacturing a cathodoluminescence display device, characterized in that it comprises the following successive steps:
- depositing a first conductive layer (8a) on an insulating substrate (6),
- etching the first layer (8a) to form first parallel conductive strips (8) playing the role of cathodes,
- deposition of a second insulating layer (12) on the structure obtained,
- deposit of a third conductive layer (10a) on the second layer (12),
- hole openings (16) opening into the third (10a) and second (12) layers, these holes (16) being distributed over the entire surface of the third and second layers,
- deposition on the third etched layer (10a) of a fourth layer (23) not covering the holes,
- deposition on the entire structure obtained of a fifth layer (18a) of an electron-emitting material,
- elimination of the fourth layer (23) resulting in the elimination of the electron-emitting material surmounting said fourth layer and the retention of said emitting material in the holes,
- etching of the third (10a) and second layers (12) to expose at least one of the ends (9) of the first conductive strips (8),
- etching of the third layer (10a) to form second parallel conductive strips playing the role of grids (10), the first and second strips being crossed, and
- Production of an anode (20) and of a cathodoluminescent material (22) opposite the second conductive strips (10). 2. Manufacturing method according to claim 1, characterized in that one inserts between the substrate (6) and the first layer (18a) an insulating intermediate layer (7). 3. The manufacturing method according to claim 1 or 2, characterized in that the first layer (18a) is made of a material chosen from indium oxide, tin oxide and aluminum. 4. Manufacturing process according to any one of claims 1 to 3, characterized in that the second layer (12) is silicon oxide (SiO₂). 5. Manufacturing method according to any one of claims 1 to 4, characterized in that the second layer (12) is deposited by the chemical vapor deposition technique. 6. Manufacturing method according to any one of claims 1 to 5, characterized in that the third layer (10a) is made of a metal chosen from niobium, tantalum and aluminum. 7. The manufacturing method according to any one of claims 1 to 6, characterized in that the holes (16) are formed in the third (10a) layer by an anisotropic dry etching technique. 8. Manufacturing process according to any one of claims 1 to 7, characterized in that the fourth layer (23) is made of nickel and in that the elimination of this fourth layer (23) is carried out by electrochemical dissolution. 9. Manufacturing method according to any one of claims 1 to 8, characterized in that the fourth layer (23) is deposited by evaporation under vacuum at a grazing incidence (α) relative to the surface of the structure. 10. The manufacturing method according to any one of claims 1 to 9, characterized in that the fifth layer (18a) is obtained by evaporation of molybdenum under vacuum. 11. The manufacturing method according to any one of claims 1 to 10, characterized in that the anode (20) is formed of a continuous conductive layer, covered with a continuous layer of cathodoluminescent material (22), the anode (20) being deposited on a transparent insulating support (4).

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