EP0671755B1 - Electron source comprising emissive cathodes with microtips - Google Patents

Description

The present invention relates to an electron source with microtip emissive cathodes ("microtips").

It applies in particular to the manufacture of display devices by cathodoluminescence excited by emission by field effect and, in particular, to the manufacture of flat screens.

It can also be used for the manufacture of electron guns or even vacuum gauges for example.

• (1) FR-A-2593953 correspondant à EP-A-0234989 et à US-A-4857161
• (2) FR-A-2623013 correspondant à EP-A-0316214 et à US-A-4940916
• (3) FR-A-2663462 correspondant à EP-A-0461990 et à US-A-5194780
• (4) FR-A-2687839 correspondant à EP-A-0558393 et à la demande de brevet américain du 26 février 1993, numéro de série 08/022,935 (Leroux et al.).

Sources of electrons with microtip emissive cathodes are already known from the following documents to which reference will be made:

• (1) FR-A-2593953 corresponding to EP-A-0234989 and US-A-4857161
• (2) FR-A-2623013 corresponding to EP-A-0316214 and US-A-4940916
• (3) FR-A-2663462 corresponding to EP-A-0461990 and US-A-5194780
• (4) FR-A-2687839 corresponding to EP-A-0558393 and to the American patent application of February 26, 1993, serial number 08 / 022,935 (Leroux et al.).

Document (1) describes a method for manufacturing a display device by cathodoluminescence excited by field effect emission, the source of microtip electrons being formed on a glass substrate and has a matrix structure.

Documents (2), (3) and (4) describe improvements made to this source described in document (1).

These documents (2) to (4) relate in particular to improving the uniformity of emission by limiting the current in the microtips which emit the most electrons.

This improvement is obtained by introducing an electrical resistance mounted in series with the microtips.

This electrical resistance is formed from a resistive layer which can be continuous or discontinuous.

Figure 1 is a schematic and partial view of a known source of electrons with microtip emissive cathodes, which is described in detail in the document (2) mentioned above.

This known source has a matrix structure and comprises a substrate 2, for example made of glass, on which a thin layer of silica 4 is optionally formed.

This source also includes, on this layer of silica 4, a plurality of electrodes 5 in the form of parallel conductive strips which act as cathode conductors and constitute the columns of the matrix structure.

The cathode conductors are each covered by a resistive layer 7 which can be discontinuous or continuous (except at its ends, to allow the connection of the cathode conductors with polarization means 20).

An electrically insulating layer 8, made of silica, covers the resistive layers 7.

Above the insulating layer 8 are formed a plurality of electrodes 10 also in the form of parallel conductive strips.

These electrodes 10 are generally perpendicular to the electrodes 5 and play the role of grids which constitute the lines of the matrix structure.

A resistive layer can optionally be placed above or below the electrodes 10.

In an improvement on this source known from document (2), at least one of the series of electrodes (cathode conductors or grids) is associated with a resistive layer and each electrode of this series has a lattice structure or mesh structure .

Thus the document (3) recommends using cathode conductors in the form of a lattice so that the microtips are arranged in the openings of the lattices of these cathode conductors.

In this configuration, the breakdown resistance of a microtip no longer depends, in the first order, on the thickness of the resistive layer but on the distance between this microtip and the corresponding cathode conductor.

Another improvement to electron sources with microtip emissive cathodes is provided by document (4).

This further improvement aims to reduce the risk of short circuit between the rows and columns of the source.

To do this, the overlap areas of the two sets of electrodes are reduced as much as possible.

This is schematically and partially illustrated by Figures 2 and 3.

FIG. 2 is a schematic and partial top view of an electron source described in this document (4) and FIG. 3 is an enlarged view in section along the axis III-III of FIG. 2.

This known source with a matrix structure comprises a substrate 1, for example made of glass, and possibly a thin layer 6 of silica on this substrate 1.

On the silica layer 6 is formed a series of parallel electrodes 3, playing the role of cathode conductors, each of these electrodes having a lattice structure.

These are the columns of the matrix structure.

These cathode conductors 3 are covered by a resistive layer 9 made of silicon, itself covered by an electrically insulating layer 11 made of silica.

Above this insulating layer 11 is formed another series of parallel electrodes also having a perforated but different structure, this structure being designed to minimize the areas of overlap with the cathode conductors.

These electrodes formed above the insulating layer 11 are generally perpendicular to the cathode conductors and constitute the grids 13 of the source.

These are the lines of the matrix structure.

Figures 2 and 3 show a detail of one of the grids of this source known from document (4).

This grid, bearing the general reference 13, comprises parallel tracks 14 orthogonally cutting other parallel tracks 15.

At the intersections of tracks 14 and 15, the grid has enlarged zones 17 which here have a square shape.

It can be seen in FIG. 2 that the areas 16 of overlap of a cathode conductor 3 and tracks 14 and 15 of the grid have a very small surface.

The enlarged zones 17 are located in the center of the meshes of the cathode conductor in the form of a lattice.

In the crossing zones of the cathode conductors and the grids, holes or more exactly micro-holes 18 are preferably formed in the thickness of the enlarged zones of the grid and in the thickness of the insulating layer 11.

The microtips 19 of the source are arranged in these holes and rest on the resistive layer 9.

A set consisting of a microtip and a micro-hole forms a micro-emitter of electrons.

The electron microemitters occupy the central regions of the meshes of the mesh of the cathode conductor as well as the enlarged and square zones 17 of the grid.

The mesh of the trellis can have different shapes and different dimensions.

For example, they can be square and have a side of 25 microns.

The number of holes and spikes in each mesh may also vary.

There can for example be 4x4 = 16 points per mesh.

When the source which has been described with reference to FIGS. 2 and 3 is operated, an electrical voltage is applied between the cathode conductor and the grid.

An electric current is thus obtained which passes through the resistive layer, between the cathode conductor and the microtips.

The further the microtips are from the cathode conductor, the longer the distance between them, the higher the electrical resistance (due to the resistive layer) through which these microtips are connected to the cathode conductor and therefore the more current electric powering these microtips is weak.

In FIG. 3, the electrical resistance r1 of the microtips situated at the edge of the group of microtips corresponding to a mesh of the cathode conductor has been represented symbolically, and the electrical resistance r2 of the microtips situated in the center of this group of microtips, r2 being greater than r1 .

It follows from the above that, in the mesh, the microtips located in the center of the group, which are more distant from the cathode conductor than the microtips located at the edge of this group, emit less electrons than the latter.

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

It aims to improve the uniformity of the emission of electrons by the microtips located inside the meshes (or more generally opposite the meshes) of electrodes with lattice structure, in an electron source with emissive cathodes. with microtips.

• une première série d'électrodes parallèles qui sont placées sur un support électriquement isolant, jouent le rôle de conducteurs cathodiques et portent une pluralité de micropointes émettrices d'électrons,
• une deuxième série d'électrodes parallèles, jouant le rôle de grilles, électriquement isolées des conducteurs cathodiques par une couche isolante et faisant un angle avec ceux-ci, ce qui définit des zones de croisement des conducteurs cathodiques et des grilles,

chacune des électrodes d'au moins l'une des séries étant en contact avec une couche résistive et possédant une structure en treillis, comportant des pistes qui se croisent et délimitent des ouvertures appelées mailles, un groupe de micropointes se trouvant en regard de chaque maille,
la source étant caractérisée en ce qu'elle comprend en outre un élément électriquement conducteur en regard de l'intérieur de chaque maille, électriquement isolé des pistes qui se croisent, en regard du groupe de micropointes correspondant à cette maille et en contact avec la couche résistive de façon à uniformiser la résistance d'accès aux micropointes à l'intérieur de chaque maille.Specifically, the subject of the present invention is a source of electrons comprising:

• a first series of parallel electrodes which are placed on an electrically insulating support, act as cathode conductors and carry a plurality of electron-emitting microtips,
• a second series of parallel electrodes, playing the role of grids, electrically isolated from the cathode conductors by an insulating layer and making an angle with them, which defines the crossing zones of the cathode conductors and grids,

each of the electrodes of at least one of the series being in contact with a resistive layer and having a lattice structure, comprising tracks which intersect and delimit openings called meshes, a group of microtips being located opposite each mesh ,
the source being characterized in that it further comprises an electrically conductive element facing the interior of each mesh, electrically isolated from the intersecting tracks, facing the group of microtips corresponding to this mesh and in contact with the layer resistive so as to standardize the access resistance to the microtips inside each mesh.

It is these electrically conductive elements which allow the improvement of the uniformity of emission of electrons in the source.

According to a preferred embodiment of the source object of the invention, each electrically conductive element is located inside the mesh corresponding to this element.

This makes it possible to simplify the manufacture of the source since it is then possible to manufacture the conductive elements during the same step as the lattice structure electrodes with which these elements are associated.

To further simplify this manufacture, it is preferable that the thickness of each electrically conductive element is equal to the thickness of the electrodes having a lattice structure with which this element is associated.

According to a first particular embodiment of the source object of the invention, the electrodes which have the trellis structure and which are associated with the electrically conductive elements are the electrodes of the first series of electrodes.

In this case and when each electrically conductive element is inside the mesh corresponding to this element, preferably, the electrodes having the lattice structure are located under the resistive layer and each electrically conductive element is also under this layer resistive and under the group of microtips corresponding to this element.

According to a second particular embodiment of the source object of the invention, the electrodes which have the lattice structure and which are associated with the electrically conductive elements are the electrodes of the second series of electrodes.

In this case and when each electrically conductive element is inside the mesh corresponding to this element, preferably, the electrodes having the lattice structure are on the resistive layer and each electrically conductive element is also on this layer resistive and above the group of microtips corresponding to this element and includes a hole opposite each microtip of this group.

• la figure 1 est une vue schématique et partielle d'une source connue d'électrons et a déjà été décrite,
• la figure 2 est une vue de dessus schématique et partielle d'une source connue d'électrons à micropointes, dont les conducteurs cathodiques ont une structure en treillis, et a déjà été décrite,
• la figure 3 est une vue en coupe agrandie de la figure 2 selon l'axe III-III et a déjà été décrite,
• la figure 4 est une vue de dessus schématique et partielle d'un mode de réalisation particulier de la source objet de l'invention,
• la figure 5 est une vue en coupe agrandie de la figure 4 selon l'axe V-V,
• la figure 6 est une vue en coupe schématique et partielle d'une source connue d'électrons à micropointes, et
• la figure 7 est une vue en coupe schématique d'un autre mode de réalisation particulier de la source d'électrons à micropointes objet de l'invention.

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 source of electrons and has already been described,
• FIG. 2 is a schematic and partial top view of a known source of microtip electrons, the cathode conductors of which have a lattice structure, and has already been described,
• FIG. 3 is an enlarged sectional view of FIG. 2 along the axis III-III and has already been described,
• FIG. 4 is a schematic and partial top view of a particular embodiment of the source object of the invention,
• FIG. 5 is an enlarged sectional view of FIG. 4 along the axis VV,
• FIG. 6 is a schematic and partial sectional view of a known source of microtip electrons, and
• Figure 7 is a schematic sectional view of another particular embodiment of the microtip electron source object of the invention.

The microtip source according to the invention, which is schematically and partially shown in plan view in FIG. 4 and in enlarged section in FIG. 5 (which is section III-III in FIG. 4) is identical to the source which has been described with reference to Figures 2 and 3 except that it includes in addition to the elements electrically conductive 3a respectively placed inside the meshes of the cathode conductors 3.

These electrically conductive elements 3a aim to improve the uniformity of the emission of electrons by standardizing the access resistance to the microtips inside each mesh.

In the example shown in Figures 4 and 5, each electrically conductive element 3a constitutes an independent plate of electrically conductive material, located in the center of each mesh, under the resistive layer 9, in contact with the silica layer 6 and under the group of microtips 19 corresponding to this mesh.

In addition, this plate 3a preferably occupies a surface slightly larger than that which is covered by this group of microtips as seen in FIGS. 4 and 5.

These plates 3a are advantageously produced during the same photolithography step as that during which the cathode conductors 3 are formed, and from the same photomask and the same metallic layer as those which are used for the manufacture of these cathode conductors (the thickness of the plates 3a thus being equal to the thickness of the cathode conductors).

FIG. 5 shows symbolically the electrical resistances r3 connecting each plate 3a to the tracks of the corresponding lattice as well as the resistors r4 between the microtips and these plates 3a respectively.

The use of plates 3a makes it possible to obtain the same electrical resistance r3 + r4 under each of the microtips (r3 + r4 representing the access resistance to the microtips), hence a better uniformity of electron emission from these microtips.

This electrical resistance of access to the microtips depends, in the first order, on the distance between the conductive plate 3a and the tracks of the corresponding lattice.

For example, for square meshes of 25 µm side and with 4x4 micro-holes of 1.5 µm diameter, spaced from each other by 3 µm, square conductive plates of 15 µm side can be used and 0.4 μm thick (the thickness of the cathode conductors also being 0.40 μm in this example).

In practice, the dimensions of the conductive plates are adjusted as a function of the resistivity and of the thickness of the resistive layer 9 and also as a function of the alignment tolerance between the levels of formation of the cathode conductors and of the micro-holes.

FIGS. 4 and 5 show a grid with an openwork structure, but of course the invention also applies to a source having respectively full grids.

Another example of a microtip electron source is known from document (4) and schematically and partially represented in section in FIG. 6.

In this known source of FIG. 6, it is the grids which have a lattice structure while the cathode conductors form openwork structures with enlarged zones.

More specifically, in the example shown in Figure 6, each cathode conductor 22 is formed on the silica layer 6 and is thus located under the resistive layer 9 and has, in top view, the same forms that the electrode 13 of FIGS. 4 and 5, except that this cathode conductor has no hole at the level of the microtips which are carried by the resistive layer 9.

In the case of FIG. 6, a resistive layer 24 is formed on the insulating layer and provided with holes 26 opposite the microtips, to allow the electrons emitted by them to pass during the excitation of the source.

The grid 28 is formed on this resistive layer 24 and has a lattice structure of which we see, in section, tracks 28a in FIG. 6.

In the case of FIG. 6, instead of using perforated cathode conductors, cathode conductors can be used respectively forming solid strips, parallel to each other.

The present invention also applies to the case of FIG. 6 (with perforated or solid cathode conductors) with a view in particular to standardizing the access resistance to each microtip in each mesh of the grids.

This variant also has the advantage of standardizing the time of application of the grid-cathode conductor voltage around each microtip.

Thus, Figure 7 illustrates schematically and partially, in section, a source according to the invention which is identical to the source described with reference to Figure 6 except that it further comprises an electrically conductive element 30 to l inside each mesh of the grids 28, opposite the group of microtips corresponding to this mesh.

More specifically, in the example shown in FIG. 7, this electrically conductive element forms an independent plate, of square shape, located inside this mesh, on the resistive layer 24, above the microtip group 19.

Each plate 30 comprises holes 32, aligned with the holes 26 and placed respectively opposite the microtips of this group.

Each plate 30 is advantageously produced during the same step as that leading to the formation of the grids, from the same conductive layer, the plates 30 thus having the same thickness as the grids 28.

As in the case of document (3), the cathode conductors with lattice structures of FIG. 5 could be not under the resistive layer 9 but on the latter (all other things being equal).

Likewise, the grids 28 with a lattice structure of FIG. 7 could not be on the resistive layer 24 but under the latter and in contact with the insulating layer 11.

In the latter case, the conductive plates 30 can be either on the resistive layer 24 as seen in FIG. 7, or under this resistive layer 24 and in contact with the insulating layer 11 (these plates 30 then being at the same level as the grids 28, inside the meshes of the latter.

In the context of the invention, it is also possible to use, in the same source, grids and cathode conductors in the form of a lattice associated respectively with conductive elements.

Claims (7)

1. Electron source comprising:

   - a first series of parallel electrodes (3, 22) placed on an electrically insulating support (1), serve as cathode conductors and carry a plurality of electron emitting microtips (19),

   - a second series of parallel electrodes (13, 28) serving as grids, which are electrically insulated from the cathode conductors by an insulating layer (11) and form an angle therewith, which defines intersection areas of the cathode conductors and the grids,

   each of the electrodes of at least one of the series being in contact with a resistive layer (9, 24) and having a lattice structure, incorporating tracks which intersect and define openings called meshes, a group of microtips facing each mesh, the source being characterized in that it also has an electrically conductive element (3a, 30) facing the interior of each mesh, electrically insulated from the intersecting tracks, facing the group of microtips corresponding to said mesh and in contact with the resistive layer (9, 29), so as to render uniform the access resistance to the microtips within each mesh.
2. Source according to claim 1, characterized in that each electrically conductive element (3a, 30) is located within the mesh corresponding to said element.
3. Source according to claim 2, characterized in that the thickness of each electrically conductive element is equal to the thickness of the electrodes having a lattice structure and with which said element is associated.
4. Source according to any one of the claims 1 to 3, characterized in that the electrodes having the lattice structure and associated with the electrically conductive elements are the electrodes (3) of the first series of electrodes.
5. Source according to claims 2 and 4, characterized in that the electrodes having the lattice structure are positioned beneath the resistive layer (9) and in that each electrically conductive element (3a) is also positioned beneath said resistive layer and beneath the group of microtips (19) corresponding to said element.
6. Source according to any one of the claims 1 to 3, characterized in that the electrodes having the lattice structure and which are associated with the electrically conductive elements are the electrodes (28) of the second series of electrodes.
7. Source according to claims 2 and 6, characterized in that each electrically conductive element (30) is located within the mesh corresponding to said element, in that the electrodes having the lattice structure are located on the resistive layer (24) and in that each electrically conductive element (30) is also located on said resistive layer and above the group of microtips (19) corresponding to said element and has a hole (32) facing each microtip of this group.

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