FR2647259A1 - ELECTRONIC DISPLAY TUBE WITH BLACK NETWORK BETWEEN LUMINOPHORES - Google Patents

Abstract

The invention concerns display screens and in particular electron beam indexation display tubes. Conventionally, the screen comprises alternate red and blue bands of luminophores separated from one another by black bands (32) providing a clear boundary between the colours. According to the invention, it is proposed to use metal oxides (manganese, chromium, iron) for the black network; these oxides are preferably used as a substrate beneath a chromium reflecting layer (34). The result is both good colour separation and a screen which only slightly reflects the diffuse ambient light. The use of these oxides is, moveover, perfectly compatible with vacuum deposition methods and fine photogravure, making it possible to obtain better resolution than with conventional colour television screens (using shadow masks), which make use of graphite.

Description

ELECTRONIC DISPLAY TUBE
WITH BLACK NETWORK BETWEEN LUMINOPHORES
The invention relates to electronic display tubes, and in particular to so-called electronic beam indexing display tubes.

 An electronic viewing tube includes a viewing screen on which phosphors (in English nphosphors ") are deposited which have the property of emitting light when struck by an electron beam, and all the more so as the intensity of the beam is - important. An electron gun emits an electron beam which scans the screen surface and which is modulated in intensity by a video signal so that each point of the screen is still emitting with a desired intensity.

 The screen is subdivided into image points which are defined by zones of phosphors placed at these points. These areas. image forming phosphors can be distributed in several ways. In beam-indexing tubes, the elementary zones are normally parallel strips of phosphors each extending over the entire height of the screen perpendicular to the scanning lines of the electron beam.

In all cases, the phosphor zones are separated from each other by opaque zones. This is what makes it possible in particular to clearly differentiate colors when luminophores emitting in different colors are
Juxtaposed on the screen.

 FIG. 1 represents the cross section of a screen in the process of being produced, with a glass substrate 10, opaque bands 12 deposited on the front face of this substrate, bands 14 of red (R), green (V) phosphors, blue (B), deposited on the substrate after the formation of the opaque bands and therefore partially covering them. The opaque strips 12 serve in particular to prevent the light emitted by the party. phosphor covering them is not transmitted to the back of the screen. Image resolution would be reduced, in particular for electron beam indexing tubes, whose resolution is essentially based on the sharpness and mutual separation of the color bands. Opaque bands also have the role, in the case of beam indexing tubes, masking the indexing phosphors, so that they do not emit towards the rear face of the screen and therefore towards the observer.

 In the prior art used in color television (consumer televisions), the opaque zones 12 are made of graphite. Graphite has the advantage of being very black, that is to say that it absorbs light well even in a very thin layer. It therefore makes it possible to eliminate the radiation emitted towards the rear relatively well. But graphite is a very polluting product (black powder) that we do not want to introduce in rooms with controlled atmosphere. Furthermore, graphite requires specific deposition methods such as the casting (in English "slurry process") of a graphite suspension and etching by lift off with resins which are not conventional electron microllthography resins and which do not allow a high resolution. The resolution is in particular not comparable to that which can be obtained for the engraving of thin layers deposited by evaporation. Finally, this technique cannot be used at all with flat screens without a peripheral skirt, such as is used for tubes with beam indexing. It is indeed very difficult to properly wet the surface of the screen with a graphite suspension if the screen has no peripheral edge.

 As a reminder, it is recalled that the beam indexing tubes are tubes in which the beam passes during its scanning over indexing bands which are phosphors which do not participate in the formation of the image; these phosphors only emit light (generally in ultraviolet) towards a detector which thus locates the passage of the beam at well determined points so that one can know at any time where the beam is and define according to that the signal video to apply. The beam indexing tubes are used in particular for the reassembly of a color display with a single electron gun.

 For these tubes, a technique that can be used consists in forming the opaque strips by depositing chromium, which has several advantages. First, chromium is highly opaque to light thanks to its strong reflecting power. Then it can be deposited in a thin layer by evaporation under vacuum and engraved with very good resolution (on flat screen). But the image contrast seen from the user side is not optimal.

It is in particular less good than that which one obtains in bulging television screens of the type "shadow mask" using graphite.

 It was found according to the invention that it was possible, without fundamentally modifying the technology used, to considerably improve the contrast of beam indexing tubes, and more generally of tubes whose screen is flat and which can be produced by technologies. of deposit and engravings of thin layers.

 It is proposed according to the invention to re-organize the opaque areas of separation of the phosphors using black materials (strongly absorbing light), which can be deposited by the usual vacuum deposition techniques, these black materials possibly being, for example, black metal oxides, or even oxides charged with metallic particles which make them highly absorbent at visible wavelengths. According to another aspect of the invention, it is proposed to use a combination of a reflective layer (such as chromium) and an absorbent sublayer , this last layer being able to be absorbent either because it is made of a material with strong power of absorption of the light (such as black metallic oxides), or because it is constituted so as to realler an anti effect - reflections seen from the rear side of the screen.

 The black metal oxides which are preferred to use according to the invention are preferably chosen from the following manganese oxide, chromium oxide, iron oxide, these oxides having inter alia the advantage of being inexpensive (they are used usually to resist). It is also advantageous to use silicon oxide charged with metallic particles, in particular chromium (dispersed phase of metallic particles embedded in the oxide). The silicon oxide is transparent but the metallic charge of chromium makes it not only opaque but also absorbent (that is to say non-reflective although the chromium which serves as charge is reflective).

 These materials have the important advantage of being vacuum-depositable and etchable by fine photolithography processes. They are therefore particularly suitable for re-mounting high resolution flat screens, and in particular display tubes with electron beam indexing.

 In addition, these materials were chosen because they remain stable and black at temperatures up to at least 4500C, which is necessary in manufacturing processes where the phosphors are covered with a layer of aluminum and where annealing at 4500C is carried out after deposition of this layer.

 In the case where a metallic layer and an antireflection layer made of an intrinsically poorly absorbent material are used in combination, the thickness of the antireflection layer is a few hundred angstroms, to allow an anti-reflection effect in the visible wavelengths. . In the case where a black absorbent layer is used, the thickness of this layer is of the order of at least several thousand angstroms to allow sufficient absorption of light.

 Finally, we can combine a chromium layer with an anti-reflective sublayer made of a material having a fairly strong intrinsic absorbing power but also having a high refractive index. It is then deposited in a thin layer (of the order of a tenth of the wavelength of light, that is to say 'from 400 to 800 angstroms), and we take advantage of both its absorbing power ( limit since the thickness is very small) and the anti-reflection effect. In this case, we calculate the thickness of the antireflection layer to trap especially in this layer the long lengths of turkey (red color) since the intrinsic absorbing power of the under layer acts rather on the short wavelengths. thus use a layer of manganese oxide or chromium oxide of very low thickness as an under layer below a reflective chromium layer.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the accompanying drawings in which
- Figure 1, already described, shows a section of a display screen during manufacture, according to the prior art;
- Figure 2 shows a display screen according to a first embodiment of the invention;
- Figure 3 shows a display screen according to a second embodiment of the invention.

 The invention will be described more precisely with regard to a beam indexing tube.

 According to a first embodiment (FIG. 1), the screen is produced from a flat glass substrate 20 and a layer of several thousand angstroms of a layer of several thousands of angstroms is deposited on one face of this substrate (front face). simple or compound material having the property of being depositable by conventional thin layer deposition processes under vacuum, in particular by evaporation (as opposed to screen printing processes ("soreen processes" in English). This material is highly absorbent for light and it is desirable that the thickness of the layer is chosen so that the transmission of the visible wavelengths is less than a few percent (transmission from the front face of the screen to the rear face).

The thickness is for example from 0.7 to I micrometer approximately.

 The material chosen according to the invention is a black metal oxide, preferably manganese oxide, or optionally chromium oxide, or iron oxide. It can also be a transparent material such as silicon oxide charged with particles which make it black. This is particularly the case for chromium particles in the dispersed phase in silicon oxide. In the latter case, the absorbent layer is deposited by simultaneous evaporation of chromium and silicon oxide. Manganese oxide is the preferred material.

 The absorbent layer is then etched so as to define narrow strips 22 which will separate the strips of phosphors which will now be deposited. Etching this absorbent layer on a flat screen can make it possible to obtain strips of 20 micrometers wide to more or less 2 micrometers of tolerance over this width. This fineness results in particular from the mode of deposition by evaporation over a very small thickness but sufficient to obtain the desired opacity.

 It should be noted that it is preferable to effect the etching of the opaque layer rather by an llft-off method. In this case, a layer of photosensitive resin is deposited and etched before depositing the absorbent layer; then, the resin is dissolved, thus removing the opaque layer where it rests on the resin and leaving the opaque layer where it rests directly on the screen. The photosensitive resins used are then conventional resins of electron microlithography.

A layer of a phosphor of a first color is then deposited by screen printing. The deposit is made uniformly over the entire surface of the screen. This layer is etched by a photoengraving process. This results in vertical strips 24 of phosphors of this color. The phosphors of the other colors are deposited and etched in the same manner successively. This results in a network of bands 24, 25, 26 of phosphors of alternating colors, each band being separated from the neighboring bands by opaque bands 22;
The rest of the manufacturing process for the beam indexing screen includes the deposition of a planarization substance and then the deposition of a layer of aluminum, the deposition and photogravure of bands of indexing phosphors on the aluminum. , and the annealing of the screen at around 4500C to remove the organic solvents and binders still present in the different layers (especially in the planarization substance).

 Another embodiment is shown in FIG. 3.

In this embodiment, the opaque layer is formed in two parts. First of all, an absorbent sub-layer is deposited which has the same properties and which is deposited in the same way as that which has been described with reference to FIG. 2. It can however be deposited on a less significant thickness since the effect main sought for this layer is the absorption of the ambient light seen from the back of the screen more than the absorption of the light emitted by the phosphors.

The thickness of the absorbent sub-layer remains, however, of the order of a few thousand angstroms.

 Again, the preferred material is manganese oxide.

The other materials mentioned with reference to FIG. 2 can also be used.

 In FIG. 3, the absorbent underlay has the reference 32.

 A cpuche 34 of reflective material is then deposited to suppress the emission towards the user of light coming from the phosphors outside the zones of phosphors delimited by the opaque bands. This material is preferably chromium. The thickness of the deposit can be around 1000 angstroms. Preferably, the deposition is carried out in the same evaporation frame as that which served for the deposition of the absorbent sublayer. In the case where the absorbent sublayer is a silicon oxide loaded with chromium particles, the simultaneous evaporation of silicon oxide and chromium is ended by evaporation of chromium only.

 Then etching successively, or simultaneously if possible, the reflective layer 34 and the absorbent sublayer 32. The etching can also be done by lift off provided that you have deposited - and etched a resin before the deposition of the sublayer absorbent and reflective layer.

 In this embodiment with a reflective material such as chromium, it will be noted that the strips of this material are relatively good conductors of electricity, so that they participate in the elimination of the electric charges generated in the phosphors during scanning. of the screen by the electron beam.

 In yet another embodiment, of general structure similar to that of FIG. 3, the absorbent sublayer 32 is formed so as to derive its power from absorbing light (coming from the back of the screen) not of the intrinsic absorbing power of the material which constitutes it, but of the anti-reflective interference effects which occur in this layer if it is deposited in a thin layer (a few hundred angstroms, that is to say to twist a tenth of the lengths of wave of visible light) and if it has an optical refractive power greater than that of the substrate 20.

 The material of the under-layer 32 can also be one of the oxides mentioned above, in particular manganese oxide or chromium oxide, but deposited on a thickness sufficiently small so that the intrinsic absorption is low before the anti-effect reflection. However, the material of the sub-layer 32 can be another dielectric with a high refractive index, even a transparent one. It can be indium oxide.

 The anti-reflection effect can also be improved by superimposing several anti-reflection layers each having specific absorbed wavelength ranges.

Claims (13)

 1. Electronic display tube comprising a screen carrying on a front face zones (24, 25, 26) of image-forming phosphors separated by zones opaque to the light emitted towards the rear face by the phosphors, characterized in that that these areas comprise an absorbent under layer (32) for the light received by the rear face of the screen, surmounted by an opaque reflecting layer (34).  2. Electronic tube according to claim 1, characterized in that the reflective layer (34) is made of chrome.  3. Electronic tube according to one of claims 1 and 2, characterized in that the absorbent sublayer is a thin dielectric layer made of a transparent or poorly absorbent material but whose thickness is chosen to allow obtaining an anti-reflection effect for visible wavelengths.  4. Electronic tube according to claim 3, characterized in that the screen comprises a glass substrate (20) on which are deposited the opaque zones then the phosphors, characterized in that the sub-layer is a dielectric layer of index optical superior to that of glass, a few hundred angstroms thick  5. Electronic tube according to one of claims 3 and 4, characterized in that the absorbent sublayer is produced by stacking several layers of thickness chosen so as to improve the anti-reflection effect over the entire range of lengths visible wave.  6. Electronic tube according to one of claims 1 and 2, characterized in that the absorbent sub-layer is made of a black material having a high absorption coefficient:  7. Electronic tube according to claim - G, characterized in that the absorbent sublayer has a thickness of a few thousand angstroms.  8. Electronic tube according to one of - claims 6 and 7, characterized in that the absorbent sub-layer is made of a material which can be deposited under vacuum, in particular a metal oxide.  9. Electronic tube according to one of claims 6 to 8, characterized in that the absorbent sublayer is a stable metal oxide and remaining black at a temperature up to about 4500C.  10. Electronic tube according to one of claims 1 to 9, characterized in that the absorbent sublayer is chosen from the following materials  - manganese oxide  - chromium oxide  - iron oxide  - silicon oxide charged with a metal, preferably chromium, forming a dispersed phase of metal in the oxide.  11. Electronic tube according to one of the preceding claims, characterized in that the opaque zones comprise a layer of manganese oxide covered with chromium.  12. Electronic tube according to one of the preceding claims, characterized in that the screen is flat, and in that the tube is an electron beam indexing tube.  13. Electronic display tube comprising a screen bearing on a front face zones of image-forming phosphors separated by zones opaque to the light emitted towards the rear face by the phosphors, characterized in that these zones comprise a layer ( 22) comprising at least one metal oxide strongly absorbing the visible light received both from the rear face and from the front face of the screen.