EP0616356A1 - Micropoint display device and method of fabrication - Google Patents

Abstract

According to the invention, a cathodoluminescent anode (4, 6) is formed on an insulating substrate (2), and cathodic conductors (10), an insulating layer (12), a gate layer (14) intended for forming gates, holes (17) in the insulating layer and the gate layer, and micropoints (16) in these holes are formed on another insulating substrate (8). In addition, an insulating thin film (20) is formed on the gate layer, in order to limit the current which can flow between the anode and the gates. This thin film is associated with means (21) capable of preventing this electrically insulating thin film from disturbing the electric field created between the micropoints and the gates. Application to flat screens. <IMAGE>

Description

The present invention relates to a microtip display device and a method of manufacturing this device.

It applies in particular to the field of visualization and, more particularly, to flat screens.

• (1) Demande de brevet français n° 8601024 du 24 Janvier 1986, correspondant à EP-A-0234989 et à US-A-4,857,161
• (2) Demande de brevet français n° 8715432 du 6 novembre 1987, correspondant à US-A-4,940,916
• (3) Demande de brevet français n° 9007347 du 13 juin 1990, correspondant à EP-A-0461990.

Microtip display devices are already known from the following documents, to which reference will be made:

• (1) French patent application No. 8601024 of January 24, 1986, corresponding to EP-A-0234989 and US-A-4,857,161
• (2) French patent application n ° 8715432 of November 6, 1987, corresponding to US-A-4,940,916
• (3) French patent application no. 9007347 of June 13, 1990, corresponding to EP-A-0461990.

A microtip display device comprises a source of electrons with microtip emissive cathodes and a cathodoluminescent anode comprising a layer of cathodoluminescent material and placed opposite the source of electrons with emissive microtip cathodes which is more simply called "cathode".

• 1) La distance entre cette anode et la cathode est faible (quelques dizaines à quelques centaines de µm, typiquement 200 µm).
• 2) Le matériau cathodoluminescent de l'anode est généralement sous forme de poudre dont l'adhérence est incertaine.
• 3) Des espaceurs sont disposés entre l'anode et la cathode pour maintenir la rigidité du dispositif d'affichage au moment où le vide est fait dans celui-ci ; ces espaceurs sont préférentiellement sous forme de billes électriquement isolantes ; ces espaceurs sont susceptibles de constituer des points faibles vis-à-vis de l'isolation électrique.

Electrical insulation faults are likely to appear between this cathode and the cathodoluminescent anode for the following reasons:

• 1) The distance between this anode and the cathode is small (a few tens to a few hundred microns, typically 200 microns).
• 2) The cathodoluminescent material of the anode is generally in the form of powder whose adhesion is uncertain.
• 3) Spacers are arranged between the anode and the cathode to maintain the rigidity of the display device when the vacuum is created therein; these spacers are preferably in the form of electrically insulating balls; these spacers are likely to constitute weak points vis-à-vis the electrical insulation.

In particular, detachment of powder, local degassing, an electrically charged spacer can trigger a regime of electric arc between the anode and the cathode, which results in the destruction of the display device over a more or less large area.

This arc regime phenomenon is all the more likely to occur when the anode voltage which is applied is high and the distance between the anode and the cathode is small.

However, to improve the performance of the display device, it is precisely desirable to increase this anode voltage (to increase the brightness of the screen of the device) and to decrease the space between the anode and the cathode ( to be able to use smaller spacers and therefore less visible and / or to improve the resolution).

In known microtip display devices, microtip sources include parallel cathode conductors and grids which are parallel and form an angle with the cathode conductors.

These grids are generally metallic and, in the event of a short circuit or an arc regime between the cathodoluminescent anode and the microtip source of a device, nothing limits the electric current between the anode and the grids and the device. then risks being destroyed.

The object of the present invention is to remedy this drawback.

• une première série d'électrodes parallèles jouant le rôle de conducteurs cathodiques et portant des micropointes ("microtips") en matériau émetteur d'électrons,
• une couche électriquement isolante sur ces conducteurs cathodiques,
• une deuxième série d'électrodes parallèles jouant le rôle de grilles, placées sur cette couche isolante et faisant un angle avec les conducteurs cathodiques, des trous étant formés dans la couche isolante et les grilles pour le passage des micropointes,

ce dispositif étant caractérisé en ce qu'il comprend en outre, sur les grilles, une couche mince électriquement isolante, pour limiter le courant électrique susceptible de circuler entre l'anode cathodoluminescente et les grilles et empêcher la survenance d'un arc électrique entre cette anode et ces grilles, cette couche mince comportant aussi des trous en regard des micropointes, et en ce que ladite couche mince électriquement isolante est associée à des moyens aptes à éviter la perturbation, par cette couche mince électriquement isolante, du champ électrique créé entre les micropointes et les grilles.It firstly relates to a microtip display device, this device comprising a first electrically insulating substrate carrying a cathodoluminescent anode and a second electrically insulating substrate carrying, opposite the first substrate:

• a first series of parallel electrodes acting as cathode conductors and carrying microtips ("microtips") made of electron emitting material,
• an electrically insulating layer on these cathode conductors,
• a second series of parallel electrodes acting as grids, placed on this insulating layer and at an angle with the cathode conductors, holes being formed in the insulating layer and the grids for the passage of the microtips,

this device being characterized in that it further comprises, on the grids, an electrically insulating thin layer, in order to limit the electric current capable of flowing between the cathodoluminescent anode and the grids and prevent the occurrence of an electric arc between this anode and these grids, this thin layer also comprising holes opposite the microtips, and in that said electrically insulating thin layer is associated with means capable of avoiding disturbance, by this electrically insulating thin layer, of the electric field created between the microtips and the grids.

The use of such a thin layer on the grids of the device makes it possible to greatly reduce the risks of the latter malfunctioning even in the event of an electrical fault.

Preferably, the means capable of avoiding the disturbance of the electric field comprise another thin layer which covers said thin insulating layer and which is sufficiently conductive to allow the flow of parasitic electrical charges which may be created during the operation of the device and which also has holes opposite the microtips.

This other thin layer which has an electrical conductivity sufficient to allow the flow of charges can be conductive but, preferably, it is resistive to allow only this flow.

The total thickness of the thin layer or layers formed on the grids can be for example between a few tens of nanometers and a few hundred nanometers.

Preferably also, the diameter of the holes formed in said thin insulating layer is greater than the diameter of the holes formed in the grids to avoid disturbance of the electric field created between the microtips and the grids, this thin insulating layer thus being over-etched.

Thus, said thin insulating layer can be over-etched and / or covered with the layer sufficiently conductive to allow the flow of parasitic electrical charges.

This avoids the accumulation of parasitic charges in the vicinity of the microtips during the operation of the device.

The device which is the subject of the invention may comprise, on the grids, a thin insulating layer, for example made of silica or silicon nitride, and a resistive layer, for example made of resistive silicon or of SnO₂.

According to a preferred embodiment of the device which is the subject of the invention, this device also comprises a resistive layer which is interposed between each cathode conductor and the corresponding microtips, the latter thus resting on this resistive layer.

Such a resistive layer is of the kind described in documents (2) and (3) mentioned above.

The present invention also relates to a method of manufacturing the microtip display device which is also the subject of the invention, method according to which said cathodoluminescent anode is formed on the first substrate, and the cathode conductors, said electrically insulating layer, a grid layer intended for forming the grids, the holes and then the microtips, this process being characterized in that said thin electrically insulating layer is further formed on the grid layer and in that said electrically insulating thin layer is associated with means capable of avoiding the disturbance, by this electrically insulating thin layer, of the electric field created between the microtips and the grids.

In the process which is the subject of the invention, the grid layer is etched to form the grids, advantageously before the holes and the thin layer are formed.

According to a particular embodiment of the process which is the subject of the invention, said thin electrically insulating layer is formed before the holes.

It is also possible to form, on said thin electrically insulating layer, another thin layer which is sufficiently conductive to allow the flow of parasitic electrical charges likely to be created during the operation of the device.

This other thin layer, which is sufficiently conductive to allow the flow of parasitic electrical charges, can be formed before or advantageously after the step of forming the holes.

A protective layer can be formed on said thin electrically insulating layer either directly or over said other thin layer sufficiently conductive to allow the flow of charges when it exists.

This protective layer can be deposited before or advantageously after the holes have been formed.

This protective layer can be removed by etching after the step of forming the microtips.

As a variant, this protective layer is not removed or is only partially removed after the step of forming the microtips.

The layer or layers formed over the grids, which are resistive or conductive, can be deposited after the holes have been made.

• la figure 1 est une vue schématique et partielle d'un dispositif d'affichage à micropointes connu,
• la figure 2 est une vue schématique et partielle d'un dispositif d'affichage à micropointes conforme à la présente invention,
• la figure 3 est une vue schématique et partielle d'un autre dispositif conforme à la présente invention, dans lequel une couche résistive est formée sur les conducteurs cathodiques,
• la figure 4 est une vue schématique et partielle d'un autre dispositif conforme à l'invention dans lequel la couche isolante qui est formée sur les grilles est surgravée, et
• la figure 5 est une vue schématique et partielle d'un autre dispositif conforme à l'invention dans lequel une fine couche électriquement conductrice est déposée après gravure des trous du dispositif.

The present invention will be better understood on reading the description of exemplary embodiments given below by way of purely indicative and in no way limiting, with reference to the appended drawings in which:

• FIG. 1 is a schematic and partial view of a known microtip display device,
• FIG. 2 is a schematic and partial view of a microtip display device according to the present invention,
• FIG. 3 is a schematic and partial view of another device according to the present invention, in which a resistive layer is formed on the cathode conductors,
• FIG. 4 is a schematic and partial view of another device according to the invention in which the insulating layer which is formed on the grids is over-etched, and
• Figure 5 is a schematic and partial view of another device according to the invention in which a thin electrically conductive layer is deposited after etching the holes in the device.

In Figure 1, there is shown schematically and partially a known microtip display device.

This known device comprises a cathodoluminescent anode formed on a glass substrate 2 and comprising a conductive and transparent layer 4, for example made of ITO and, on this layer 4, a layer 6 of luminescent powder.

The device of FIG. 1 also comprises a source of microtip electrons formed on another insulating substrate 8 and comprising cathode conductors such as the conductor 10, a layer insulating 12 formed on these cathode conductors and grids such as the grid 14, formed on the insulating layer 12 and perpendicular to the cathode conductors 10.

Microtips such as microtip 16 are formed on the latter, in holes 17 made in the grids and the insulating layer 12.

In addition, spacers such as the spacer 18 are disposed between the cathodoluminescent anode and the grids to maintain the rigidity of the device when a vacuum is created between the cathodoluminescent anode and the source of microtip electrons.

In this device, nothing is provided, in the event of an electrical insulation fault, to limit the electric current I between the anode and the grids, between the anode and the microtips, and between the microtips and the grids.

Such a device is extremely sensitive to short circuits, very unstable and difficult to control.

The device according to the invention, which is schematically and partially shown in FIG. 2, differs from the device in FIG. 1 in that it further comprises an electrically insulating layer 20, formed on the grids and pierced opposite the microtips. , this layer 20 being provided to limit the current between the anode and the grids.

The device of FIG. 2 also comprises a layer 21 which covers the layer 20 and which is sufficiently conductive to allow the flow of parasitic electrical charges likely to be created during the operation of the device and which also has holes facing the microtips. .

This layer 21 avoids the disturbance, by layer 20, of the electric field created between the microtips and the grids when the device is operating.

In a variant not shown, the layer 21 does not exist and, to avoid disturbance of the electric field, the diameter of the holes formed in the layer 20 is greater than that of the holes formed in the grids.

It is also possible to produce a device according to the invention in which both the layer 21 exists and this condition on the diameters is fulfilled.

The limited current i between the anode and the grids has been symbolized by a dotted arrow in FIG. 2.

This limitation already constitutes a very significant improvement.

However, nothing is yet planned to limit the current between the anode and the microtips and between the grids and the microtips.

This is why the invention is preferably applied to microtip display devices whose electron source comprises a resistive layer between the cathode conductors and the microtips which rest on this resistive layer.

Such an electron source is described in documents (2) and (3) mentioned above.

Figure 3 is a schematic and partial view of a device according to the invention which comprises such a resistive layer between the cathode conductors and the microtips.

This device of Figure 3 differs from the device of Figure 2 in that it comprises in addition a resistive layer 22 between the insulating layer 12 and the cathode conductors 24 which are here meshed as in document (3).

Thanks to the insulating layer 20 formed on the grids and to the resistive layer formed on the cathode conductors, all the currents i (between the anode and the grids, between the anode and the microtips and between these microtips and the grids) are controlled and limited.

The device is thus protected against any risk of short circuit.

The most important advantage of the invention is that it makes it possible to increase the anode voltage and, possibly, to decrease the space between the anode and the source of microtip electrons without risk of electrical accident liable to destroy the device.

A few examples of a method for manufacturing microtip display devices according to the invention are given below, purely by way of non-limiting indication, with reference to FIGS. 4 and 5.

• les conducteurs cathodiques, tels que le conducteur cathodique 24 sont en niobium, ont une épaisseur de 0,2 µm et une structure en treillis avec par exemple des mailles carrées dont le pas vaut 25 µm et ces conducteurs cathodiques sont gravés pour former les colonnes du dispositif,
• la couche résistive 22 est en silicium amorphe dopé au phosphore, elle est déposée sur les conducteurs cathodiques et l'épaisseur de cette couche résistive est de l'ordre de 1 µm,
• la couche isolante 12 est en silice, elle est déposée sur la couche résistive 22 en silicium et l'épaisseur de cette couche isolante 12 est également de l'ordre de 1 µm, et
• une couche métallique 14 en niobium formant la couche de grille est déposée sur cette couche isolante 12 en silice et l'épaisseur de cette couche métallique est de l'ordre de 0,4 µm, cette couche métallique en niobium étant gravée pour former les grilles suivant les lignes du dispositif.

In these examples:

• the cathode conductors, such as the cathode conductor 24 are made of niobium, have a thickness of 0.2 μm and a lattice structure with for example square meshes whose pitch is equal to 25 μm and these cathode conductors are etched to form the columns of the device,
• the resistive layer 22 is made of amorphous silicon doped with phosphorus, it is deposited on the cathode conductors and the thickness of this resistive layer is of the order of 1 μm,
• the insulating layer 12 is made of silica, it is deposited on the resistive layer 22 of silicon and the thickness of this insulating layer 12 is also of the order of 1 μm, and
• a metallic layer 14 made of niobium forming the grid layer is deposited on this insulating layer 12 made of silica and the thickness of this metallic layer is of the order of 0.4 μm, this metallic layer of niobium being etched to form the grids along the lines of the device.

In a first example of the method, an insulating layer 26 of silica (FIG. 4) is deposited on the grids.

The thickness of this layer 26 is for example equal to 0.2 μm.

This layer 26 can be produced by chemical vapor deposition, by sputtering or by any other method of depositing thin layers.

A thin sufficiently conductive layer 28, for example made of niobium, molybdenum or SnO₂, is deposited on this silica layer 26 to allow the creation of microtips and possibly the flow of parasitic charges during operation of the device when this layer is kept.

The thickness of this layer 28 is for example equal to 50 nm.

This layer 28 is preferably formed by evaporation by means of an electron gun or by spraying.

In the example considered, where cathode conductors forming meshes are used, holes, the diameter of which is of the order of 1.4 μm, are etched in the thin conductive layer 28, the insulating layer 12, the grid layer 14 and insulating layer 26, inside the mesh of the conductors cathodic, or more exactly plumb with the domains delimited by these meshes.

A wet etching process or a dry etching process can be used.

Preferably, a reactive ion etching process is used to etch the metal layers and the insulating layers.

Preferably also, a chemical over-etching of the silica of the layer 12 is carried out, this over-etching being for example a few hundred nanometers (length e in FIG. 4), which makes it possible to enlarge the holes at the level of this layer 12.

Such an over-etching process is known and makes it possible to avoid metallization of the edges of the holes in the silica during the production of the microtips.

Advantageously, an over-etching of the layer 26 is carried out in order to allow the grids to be released around the holes 30 and therefore to avoid a disturbance of the electric field (during operation of the device) between the microtips 16 and the grids, this disturbance being caused by a charging phenomenon of the insulating layer 26.

It is possible to produce the over-etchings of layers 12 and 26 at the same time when they are made of the same material, which is the case in the example described.

The microtips 16 are then produced according to the method described in the document (1) mentioned above.

• une meilleure adhérence d'une couche de nickel (non représentée) qui est utilisée lors de l'élaboration des micropointes (voir le document (1))
• l'assurance de la continuité électrique pendant la phase de dissolution électrochimique du nickel.

The thin conductive protective layer 28 allows

• better adhesion of a nickel layer (not shown) which is used during the development of the microtips (see document (1))
• ensuring electrical continuity during the electrochemical dissolution phase of nickel.

Once the microtips have been made, the contact points for the row conductors and the column conductors are released if necessary.

The thin conductive layer 28 can optionally be removed by appropriate etching.

Another example of the process will now be described with reference to FIG. 5.

In this example, the thin conductive layer (sufficiently conductive to allow the flow of charges) can be deposited after etching the holes, in which case the bottom of the holes is covered by this layer.

FIG. 5 schematically and partially illustrates this case where the thin conductive layer 28 is deposited after etching the holes and it can be seen that the bottom of the holes is covered by this fine conductive layer 28 (which covers the insulating layer 26 of silica already mentioned in the description of Figure 4).

It can also be seen in FIG. 5 that the microtips 16 are thus above the thin conductive layer.

The deposition of the thin conductive layer, after etching the holes, makes it possible to avoid etching of this layer.

Claims (9)

Microtip display device, this device comprising a first electrically insulating substrate (2) carrying a cathodoluminescent anode (4, 6) and a second electrically insulating substrate (8) carrying, opposite the first substrate (2): - a first series of parallel electrodes acting as cathode conductors (10, 24) and carrying microtips (16) made of electron emitting material, - an electrically insulating layer (12) on these cathode conductors, - A second series of parallel electrodes acting as grids (14), placed on this insulating layer (12) and at an angle with the cathode conductors, holes (17, 30) being formed in the insulating layer and the grids for the passage of microtips, this device being characterized in that it further comprises, on the grids, a thin electrically insulating layer (20, 26), in order to limit the electric current liable to circulate between the cathodoluminescent anode and the grids and prevent the occurrence of an electric arc between this anode and these grids, this thin layer also comprising holes opposite the microtips and in that said electrically insulating thin layer is associated with means capable of avoiding the disturbance, by this electrically insulating thin layer, of the field created between the microtips and the grids. Device according to claim 1, characterized in that the means capable of avoiding the disturbance of the electric field comprise another thin layer (28) which covers said thin layer insulating and which is sufficiently conductive to allow the flow of parasitic electrical charges likely to be created during the operation of the device and which also has holes opposite the microtips. Device according to any one of claims 1 and 2, characterized in that the diameter of the holes formed in said thin insulating layer (26) is greater than the diameter of the holes formed in the grids, to avoid disturbance of the electric field created between the microtips and grids, this thin insulating layer thus being over-etched. Device according to any one of Claims 1 to 3, characterized in that it further comprises a resistive layer (22) which is interposed between each cathode conductor and the corresponding microtips (16), the latter thus resting on this resistive layer . Method of manufacturing the microtip display device according to claim 1, method according to which said cathodoluminescent anode (4, 6) is formed on the first substrate (2), and the cathode conductors (8) are formed on the second substrate (8) 10, 24), said electrically insulating layer (12), a grid layer (14) intended for the formation of grids, the holes (17, 30) then the microtips (16), this process being characterized in that further forms said electrically insulating thin layer (20, 26) on the grid layer and in that said electrically insulating thin layer is associated with means capable of avoiding the disturbance, by this electrically insulating thin layer, of the electric field created between microtips and grids. Method according to claim 5, characterized in that said thin electrically insulating layer (26) is formed before the holes. A method according to claim 6, characterized in that a further thin layer (28) is formed on said thin electrically insulating layer (26) which is sufficiently conductive to allow the flow of parasitic electrical charges liable to be created during the operation of the device. Process according to either of Claims 6 and 7, characterized in that a protective layer is formed on the outermost thin layer produced on the grids. Method according to either of Claims 7 and 8, characterized in that the layer or layers formed over the grids, which are resistive or conductive, are deposited after the holes have been made.

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