EP0657914A1 - Anode consisting of conducting stripes which are addressed individually - Google Patents

Abstract

The subject matter of the invention is an electron collector including an anode consisting of a substrate (21, 40, 41) on which conductive tracks (23, 43) are deposited. A layer of dielectric material is deposited on at least one of the edges of each conductive track. <IMAGE>

Description

Technical area

The subject of the present invention is an electron collector comprising anodes which can be controlled independently of one another.

State of the art

An electron collector is encountered, for example, in fluorescent microtip screens, such as those described in patents FR-A-2,633,763, FR-A-2,633,765 and US-A-4,763,187, as well that in the VFD type structures described in the article by K. MORIMOTO et al. ("Proceedings of Japan Display 86", pages 516 to 519), entitled "320x200 - Pixel Color Graphic FLV FD".

The structure and operating principle of a collector according to the prior art are shown in Figures 1a, 1b and 2, the example taken being that of a fluorescent microtip screen.

Figure 1a recalls, in perspective, the constituent elements of an example of such a screen, essentially an anode and a cathode arranged opposite. The cathode comprises a first substrate 10, made of glass for example, on which are arranged conductive columns 12 (cathode conductors of tin and indium oxide for example) supporting metallic microtips 14, in molybdenum for example. The columns 12 cross perforated conductive lines 16 (grids), made of niobium for example.

All the microtips 14 positioned at the intersection of a line 16 and a conductive column 12 have their apex substantially opposite a perforation of the line 16. The cathode conductors 12 and the grids 16 are separated by an insulating layer 18 in silica for example, provided with openings allowing the passage of microtips 14.

The anode comprises a second insulating substrate 20, made of glass for example, which supports conductive strips 22, covered with luminescent materials.

At least one of the two substrates (10 or 20) must be transparent.

A first series of these bands 22 is covered by a luminescent material 24 in red, Y₂O₂S doped Eu for example, a second series of these bands 22 is covered by a luminescent material 26 in green, ZnS doped CuAl for example, the last series of bands 22 is covered by a material 28 luminescent in blue, ZnS doped Ag for example.

The bands 22 of the different series are alternated and distant from each other, for example equidistant.

Each triplet formed by an anode in each series is approximately opposite a grid 16 (line). Each crossing of a grid 16 and a cathode conductor 12 forms a three-color pixel.

Advantageously, a resistive layer (not shown) can be interposed between the microtips and the cathode conductors.

Figure 1b is a sectional representation along aa '(Figure 1a).

Figure 2 specifies the operation of such an assembly. The three colors are addressed successively, for example according to a sequential mode. To address the green color, the corresponding bands 22 are polarized, for example at 200 volts, while the bands covered with luminescent materials emitting in the colors red 24 and blue 28 are kept at a potential below their threshold of excitation. We then do the same for red then for blue. The voltages indicated in Figure 2 for column 12 (0 V) and line 16 (+70 V), are obviously by way of example. The addressing corresponds to the focusing of the electrons emitted by the cathode on the selected anode strip 22, which causes the fluorescence of the corresponding material 26. This type of operation is called "switched anode operation".

• un nombre réduit de circuits d'adressage de conducteurs cathodiques (1 conducteur cathodique pour trois bandes rouge-vert-bleu d'anodes au lieu de 1 conducteur cathodique par bande d'anode),
• une moindre précision nécessaire pour l'alignement entre anode et cathode,
• une plus grande résolution.

Compared to the classic structure in which the three colors are addressed simultaneously, the structure described above has three essential advantages:

• a reduced number of cathode conductor addressing circuits (1 cathode conductor for three red-green-blue anode bands instead of 1 cathode conductor per anode band),
• less precision required for alignment between anode and cathode,
• higher resolution.

The major drawback of this structure is that the excitation voltage of the anode (or anode voltage) that it is possible to apply is limited due to the risks of breakdown existing between the polarized color (that polarized at 200 V in the example in FIG. 2) and the other colors maintained at a potential close to that of the tips (those polarized at 0 V in the example in FIG. 2).

This limitation is troublesome because, on the other hand, it is well known to those skilled in the art that the light efficiency of the conductive strips 22 of the anode is higher the higher their excitation voltage.

Another drawback, illustrated in FIG. 2, where the references 1 to 8 designate electron trajectories, is that part of the electrons does not go on the luminescent material 26, but directly on the edges of the conductive tracks 22 (see Figure 2, trajectories 1, 7 and 8). The light output of the screen is thus weakened.

These problems are not specific to fluorescent microtip screens. Reference will be made, by way of illustration, to FIG. 3, more generally, in which an electron collector according to the prior art is shown in section. It comprises a substrate 20, which extends in the direction x'x, but also perpendicular to the plane of the figure.

• 1. Risque de claquage entre deux pistes conductrices voisines, lorsque l'une est dans un état collecteur d'électrons et l'autre est dans un état non collecteur, donc lorsque la différence de potentiel entre les deux est élevée.
  Sur la figure 3, on a représenté quelques lignes de champ 30 entre ces deux bandes. Les claquages sont plus spécialement amorcés du fait d'un resserrement des lignes de champ sur le bord A des pistes conductrices 22. Le vide entre les bandes conductrices 22 est particulièrement propice à la propagation de tels claquages, puisque la constante diélectrique du vide est égale à 1.
• 2. Etalement des trajectoires électroniques sur toute la largeur de la piste collectrice, et même jusque sur les bords de la piste, là où celle-ci s'incline vers le substrat (également dans la zone A).

Two conductive strips 22-1 and 22-2 are shown there (they also extend perpendicular to FIG. 3), which are each brought to a variable potential making it possible to define, for each strip, a collector or non-collector state d 'electrons. The latter are emitted by a cathode situated opposite the strips 22, but not shown in FIG. 3. For this type of collector, we encounter the same two problems as in the case of the fluorescent screen:

• 1. Risk of breakdown between two neighboring conductive tracks, when one is in an electron collector state and the other is in a non-collector state, therefore when the potential difference between the two is high.
  In FIG. 3, a few field lines 30 are shown between these two strips. The breakdowns are more particularly initiated due to a tightening of the field lines on the edge A of the conductive tracks 22. The vacuum between the conductive strips 22 is particularly conducive to the propagation of such breakdowns, since the dielectric constant of the vacuum is equal at 1.
• 2. Spread of the electronic trajectories over the entire width of the collector track, and even up to the edges of the track, where it inclines towards the substrate (also in zone A).

This leads to poorer "focusing" of the electron beam on the selected track and, when the track is covered with a target material, to a weakened electronic efficiency.

Statement of the invention

The object of the present invention is precisely to remedy these drawbacks by improving the structure of such an electron collector. The practical result, in the particular case of fluorescent screens, is an improvement in the brightness of the screen. It is also possible to obtain an improvement in contrast when the screen is subjected to external illumination.

Specifically, the present invention relates to an electron collector comprising an anode consisting of a substrate on which are deposited conductive strips, each of these strips comprising a central part delimited by edges, characterized in that a layer of dielectric material is deposited on at least one edge of each conductive strip.

Advantageously, such a collector is characterized in that a layer of dielectric material is deposited on each edge of each conductive strip.

Where the dielectric material (also called a shield) is deposited in contact with a conductive strip, two essential technical effects are obtained.

, où ε représente la constante diélectrique du bouclier et ε_o la constante diélectrique du vide. Ceci réduit d'autant les risques de claquage. Par conséquent, une tension d'anode plus élevée peut être appliquée.First, the electric field on the edge of the conductive strip in contact with the dielectric material is reduced relative to the electric field in the same place, in a device according to the prior art (i.e. without dielectric material ). The reduction is in a ratio close to $$\frac{\text{ε}}{{\text{ε}}_{\text{o}}}$$

, where ε represents the dielectric constant of the shield and ε _o the dielectric constant of vacuum. This further reduces the risk of breakdown. Therefore, a higher anode voltage can be applied.

Second, where the dielectric material is in contact with the edge of a conductive strip, the electrons emitted by the cathode go directly to the part of the track not covered by the shield (for example zone B in FIG. 3) or, in the case of a fluorescent screen, on the luminescent material which covers this track, since the edges of the conductive strip are protected by the dielectric. The efficiency of electron collection is thus improved and a better "focusing" can be obtained on the selected band.

This effect is further enhanced if the layer of dielectric material extends beyond the edge of the conductive strip on which it is deposited, and encroaches on the central part of this strip.

Preferably, a collector as described above is characterized in that the layer of dielectric material also extends from the edge of the strip with which it is in contact, to the edge of the neighboring strip.

According to a preferred embodiment, the dielectric shield is made of a material having a high dielectric constant.

In particular, the dielectric shield is made of silica.

• les figures 1a et 1b, déjà décrites, montrent la structure d'un collecteur d'électrons selon l'art antérieur, dans le cas d'un écran fluorescent à micropointes ;
• la figure 2, déjà décrite, montre les trajectoires des électrons émis par la cathode, dans le cas d'un collecteur d'électrons selon l'art antérieur ;
• la figure 3 montre la répartition des lignes de champ entre deux anodes voisines dans le cas d'un collecteur d'électrons selon l'art antérieur ;
• les figures 4a à 4e montrent plusieurs structures possibles d'un collecteur d'électrons selon la présente invention ;
• la figure 5 montre la structure d'un collecteur d'électrons selon la présente invention, incorporé dans un écran fluorescent à micropointes ;
• la figure 6 montre un mode de réalisation d'une couche diélectrique entre deux bandes conductrices adjacentes de l'anode d'un collecteur d'électrons selon l'invention ;
• la figure 7 montre un exemple de réalisation d'une couche diélectrique entre deux bandes conductrices adjacentes de l'anode d'un collecteur d'électrons selon l'invention ;
• la figure 8 montre un autre exemple de réalisation d'une couche diélectrique entre deux bandes conductrices adjacentes de l'anode d'un collecteur d'électrons selon l'invention.

Other characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to exemplary embodiments, given by way of explanation and without limitation. It also refers to attached drawings, in which:

• FIGS. 1a and 1b, already described, show the structure of an electron collector according to the prior art, in the case of a fluorescent microtip screen;
• FIG. 2, already described, shows the trajectories of the electrons emitted by the cathode, in the case of an electron collector according to the prior art;
• FIG. 3 shows the distribution of the field lines between two neighboring anodes in the case of an electron collector according to the prior art;
• FIGS. 4a to 4e show several possible structures of an electron collector according to the present invention;
• FIG. 5 shows the structure of an electron collector according to the present invention, incorporated in a fluorescent microtip screen;
• Figure 6 shows an embodiment of a dielectric layer between two strips adjacent conductors of the anode of an electron collector according to the invention;
• FIG. 7 shows an exemplary embodiment of a dielectric layer between two adjacent conductive strips of the anode of an electron collector according to the invention;
• FIG. 8 shows another embodiment of a dielectric layer between two adjacent conductive strips of the anode of an electron collector according to the invention.

Detailed description of the invention

A preferred embodiment of an electron collector according to the invention is shown in section in Figure 4a and in perspective in Figure 4b. Obviously, only part of the collector has been shown, the pattern in FIGS. 4a and 4b being able to be repeated identical to itself in the direction x'x. The collector, or anode, comprises an insulating substrate 21 (made of glass, for example) which extends in the direction x'x and perpendicular to the plane of the figure. The substrate supports conductive strips 23-1 and 23-2 also called conductive tracks, which extend perpendicular to the plane of Figure 4a. Each strip has a central part (zone B in FIG. 4a) delimited by two edges (zone A in FIG. 4a). Those skilled in the art will be able to choose, depending on the application envisaged, the dimensions of the substrate and of the tracks. Typically, for an application to fluorescent screens, the substrate has a thickness of approximately 1 mm and the tracks have a thickness of the order of magnitude of a micrometer, or one tenth of a micrometer. Tracks having a width of approximately 100 µm for a distance between two tracks of around 50 µm has already been achieved.

According to the invention, a layer 25 of a dielectric material is deposited on at least one of the edges of each track 23-1 and 23-2 and advantageously on each of the edges. In the case of application to fluorescent screens, the luminescent materials are then deposited on the conductive tracks by any known method. The layer 25 extends in a direction perpendicular to the plane of Figure 4a, along the track on the edge of which it is deposited (see perspective representation in Figure 4b). It can also extend along x'x, in the inter-track space and thus cover part of the space between two neighboring tracks, as illustrated in FIG. 4a. A variant is illustrated in FIG. 4c which must also (as well as FIGS. 4d and 4e) be understood as a sectional view of the electron collector, the substrate-tracks-dielectric layer assembly extending, here again, perpendicularly in the plane of the figure. In this FIG. 4c, where the references 21, 23-1, 23-2 have the same meaning as in FIG. 4a, the layer of dielectric material 27 covers the edges of the tracks 23-1 and 23-2 as well as all of the space between these tracks.

Another preferred embodiment of the invention is shown in FIG. 4d, where the layer 29 of dielectric material extends beyond the edge of the track (23-1, 23-2) on which it is deposited, and encroaches on the central part thereof. This encroachment must nevertheless be limited so as not to reduce too much the surface covered by luminescent materials (typically, it will be between one micron and a few microns).

A variant is shown in FIG. 4e, where the layer 31 of dielectric material extends over the entire space between the two tracks 23-1 and 23-2 and beyond the edge of each track, in the direction of its part. central.

In all the cases described above, if one chooses for dielectric material 25, 27, 29, 31 a material having a high dielectric constant, one obtains a reduction of the electric field prevailing at the edge of any conductive track 23-1, 23- 2 in contact with the dielectric material (zone A in FIGS. 4a, 4c to 4e) in a ratio equal to ε / ε _o , where ε is the dielectric constant of the dielectric layer and ε _o the dielectric constant of the vacuum. This further reduces the risk of breakdowns, and a higher voltage difference can therefore be applied between tracks 23-1 and 23-2.

If we choose for example silica for dielectric material, we will have ε / ε _o ≈4.

It will be even more advantageous to work preferably with ε / ε _o ≧ 10.

On the other hand, due to the contact of the layer of dielectric material with at least one edge of the corresponding track 23 and advantageously with the two edges, the electrons emitted by the cathode go directly to the part of the conductive track parallel to the substrate. (zone B in FIG. 4a or 4c) and the "edge" effect, that is to say the collection of electrons by the part of the conductive track which inclines towards the substrate, is considerably reduced. We have, in a way, an improvement in the focusing of the electrons on the conductive track.

These two effects (reduction of the risk of breakdown and better focusing) are reinforced when the dielectric layer projects slightly over the conductive tracks (Figures 4e, 4d).

In the particular non-limiting case where the electron collector according to the invention is applied to a fluorescent microtip screen, the structure illustrated in FIG. 5 is obtained, which represents a sectional view of the screen, the latter s 'extending in the direction x'x and perpendicular to the plane of the figure. The screen consists of an anode and a cathode, the latter having the same structure as in the prior art described above: the cathode comprises a first substrate 11, conductive columns 13 supporting metal microtips 15 and crossing perforated conductive lines 17 (grids). The columns and the grids are separated by an insulating layer 19 (silica, for example), provided with openings allowing the passage of the microtips 15. Advantageously, a resistive layer can be interposed between the microtips and the cathode conductors.

The anode comprises a second insulating substrate 40 (glass, for example), which supports conductive strips 43.

At least one of the two substrates (11) and (40) must be transparent.

A first series of these bands 43 is covered by a luminescent material 45 in red, Y₂O₂S doped Eu for example, a second series of these bands 43 is covered by a luminescent material 47 in green, ZnS doped CuAl for example, the last series of bands 43 is covered by a material 49 luminescent in blue, ZnS doped Ag for example. In addition, a layer of dielectric material 33 is disposed between the conductive tracks 43 of the anode.

Preferably, this dielectric layer 33 extends slightly beyond the edge of each conductive track 43, in the direction of the central part of the latter (as illustrated in FIG. 4d).

FIG. 5 also represents the focusing effect in the case of the fluorescent microtip screen: all the electrons emitted by the cathode go to the luminescent material 47 of the selected track 43 and the luminous efficiency of the screen is found improved.

Still in the case of the fluorescent screen, and in order to improve the contrast under external lighting, it is possible to seek to reduce the reflectivity of the anode. This is obtained by choosing for the dielectric layer 33, preferably, a material absorbing visible light. This can be useful, in particular for the application of the invention to display screens. If the layer 33 is transparent, it will advantageously be covered, as illustrated in FIG. 6, with an absorbent material 51. It will be noted that this material 51 is not necessarily an insulating material. FIG. 6 also shows two conductive tracks 43 and, on each, a layer 47, 49 of luminescent material.

Another embodiment of the dielectric layer is described in FIG. 7. On the glass substrate 41, conductive tracks 43 made of indium oxide doped with tin (ITO) are deposited, a layer 47, 49 of material luminescent being deposited on each of these tracks. The thickness of the tracks 43 is approximately 0.2 μm.

To form these tracks, we proceed as follows. A layer of indium oxide doped with tin (ITO) is deposited, for example by spraying cathodic. Parallel strips are then produced in this layer of ITO using techniques known elsewhere, such as for example photolithography through a mask and chemical etching. In the latter case, a mixture of FeCl₃-HCl can be used at 50 ° C, the etching time being, under these conditions, of the order of 2 minutes. The width of the strips and the distance between two strips depends on the desired application. For example, in the case of fluorescent screens, these parameters depend on the resolution that one wants to achieve. by way of indication, strips having a width of 100 μm have been produced with an interband distance of the order of 50 μm.

The dielectric layer 53 is made of silica and has a thickness of approximately 1 μm. The silica covers the edges of the ITO over a width of 10 µm plus or minus 5 µm (Figure 7). Silica is for example deposited by chemical vapor deposition (CVD) on the entire surface. Its geometry is then defined using well known techniques of photolithography and chemical etching from an HF-NH₄F mixture.

Luminescent materials can then be deposited on the conductive tracks by any known method.

According to another embodiment of the shield (same structure of the layers as in FIG. 7), the substrate 41 is made of glass, the conductive tracks 43 are made of ITO, or of aluminum (Al). The method of forming the strips 43 is the same as that which has just been described above. The dielectric layer 53 is made of absorbent black glass with a low melting point. It is deposited by screen printing on the entire surface and its thickness is approximately 5 µm. Its geometry is also defined there by photolithography and chemical etching techniques.

According to a third exemplary embodiment (FIG. 8), the substrate 41 is made of glass, the conductive tracks 43 made of ITO (see above for the formation process, and the dielectric layer 55 made of silica, with a thickness of approximately This layer 55 is covered with a layer 57 of black chromium oxide approximately 1 μm thick. The silica is first deposited by CVD (chemical vapor deposition) over the entire Then a layer 57 of black chromium oxide is deposited on the silica, for example by a sputtering technique. This oxide layer has a thickness of approximately 1 μm. Then by lithography and chemical etching, a pattern such as that shown in Figure 8.

Claims (10)

Electron collector comprising an anode consisting of a substrate (21, 40, 41) on which are deposited conductive strips (23, 43) each brought to a variable potential which makes it possible to put them in a collector or non-collector state, each of these strips comprising a central part delimited by edges, characterized in that a layer (25, 27, 29, 31, 33, 53, 55) of dielectric material is deposited on at least one edge of each conductive strip. Electron collector according to claim 1, characterized in that a layer of dielectric material is deposited on each edge of each conductive strip. Electron collector according to one of Claims 1 or 2, characterized in that the layer of dielectric material (25, 27, 29, 31, 33, 53, 55) extends beyond the edge of the conductive strip on which it is deposited, and encroaches on the central part of this strip. Electron collector according to one of claims 1 to 3, characterized in that the layer of dielectric material also extends from the edge of the strip with which it is in contact, to the edge of the neighboring strip. Electron collector according to one of the preceding claims, characterized in that the layer of dielectric material (25, 27, 29, 31, 33, 53, 55) consists of a material having a dielectric constant allowing a reduction in the electric field at the edge of the conductive strips on which it is deposited. Electron collector according to claim 5, characterized in that the layer of dielectric material (25, 27, 29, 31, 33, 53, 55) is silica. Electron collector according to one of Claims 1 to 6, characterized in that the layer of dielectric material (25, 27, 29, 31, 33, 53, 55) also has properties which absorb visible light. . Electron collector according to one of Claims 1 to 6, characterized in that a material absorbing visible light (51, 57) is deposited on the layer of dielectric material (25, 27, 29, 31, 33, 53, 55). Electron collector according to claim 8, characterized in that the absorbent material (51, 57) is black chromium oxide. Matrix display microtip fluorescent screen comprising an electron emitting cathode and an electron collector according to one of the preceding claims.

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