EP0317392B1 - High luminance colour screen for cathode ray tubes, and process for manufacturing the same - Google Patents

Description

The invention relates to a screen for cathode ray tubes with traces of high luminance and of color adjustable by the acceleration voltage of the beam, it relates particularly to the structure of this screen. It also relates to its manufacturing process.

We know from the prior art luminescent screens of cathode ray tubes, called penetration tubes, the color of the trace of which varies with the acceleration voltage of the electron beam.

These cathodoluminescent screens include two phosphors of different fluorescence, green and red in the following. They are represented schematically in the form of two homogeneous layers, each consisting of one of the phosphors, separated or not by a layer of an inert or non-luminescent material, emitting no light under the effect of a bombardment of electrons.

According to the value of the acceleration voltage V of the beam, that is to say according to the energy of the electrons of the beam, only the first of these layers, which will be assumed to be the red fluorescence layer in what follows, undergoes the excitation of the beam, or, if this tension is sufficient, this layer and all or part of the second, made of a phosphor with green fluorescence undergo the excitation of the beam. Green fluorescence begins to be excited only from a certain value of this voltage, that sufficient for the energy of the electrons to allow them to enter the screen - hence the expression of screens with penetration previous - up to the green phosphor layer, after crossing the red phosphor layer and, possibly, the inert layer; this value of the voltage will be designated in the following by V₀, the green fluorescence predominates and the trace takes on the green color. Between the two values V₀ and V₁ we obtain, depending on the value of V, intermediate colors between green and red according to the proportions in the trace of the two excited fluorescences.

Various penetration screen structures of the prior art are known.

One of them has layers of red and green phosphors separated by a layer of an inert material, all obtained by vacuum evaporation in the form of thin films. (See the patent in the name of Feldman issued in the United States of America under No. 3225238). This kind of screen has the disadvantage of having an extremely low luminance under the bombardment of electrons because of the impossibility for the radiations emitted within these films to emerge from them as a result of the multiple reflections that they suffer there.

Another of these structures comprises a mixture of crystals of the two red and green phosphors, the crystals of the latter having been coated prior to the operation of mixing a film of inert material. Generally, with such mixtures, a luminance substantially greater than that of the preceding structure is obtained.

In a third structure of the prior art (see the patent granted in the United States of America under No. 3714490), the red phosphor is integrated into the structure in the form of small grains surrounding the inert layer covering the grains larger of the green phosphor. This significantly improves the luminance of the red phosphor, but the problem to be solved is actually that of increasing the overall luminance of the screen, that is to say that of all traces and not only that given by the red phosphor. At the same time as we increase the latter, we must increase, in the same proportions, that of the green phosphor, because it is this proportion which fixes the extent of the range of shades obtained between red and green from from the moment when the acceleration voltage of the green fluorescence, that is to say the previous value V₀. We generally want to have this whole range for viewing all of the information. This range is narrowed if the increase in luminance of the red phosphor is not accompanied by that of green phosphor.

• ― un support 1 transparent, en verre, constituant l'écran proprement dit;
• ― une couche 2 d'un luminophore vert constituée de cristaux 20, cette couche étant déposée suivant une technique bien connue de sédimentation;
• ― une couche barrière 3 comprenant un matériau inerte transparent. Cette couche est constituée classiquement soit par du sulfure de zinc (ZnS), soit par de l'oxyde de silicium (SiO₂).
• ― une couche 4 d'un luminophore rouge constituée de cristaux 40, cette couche étant déposée soit par centrifugation, le support 1 tournant autour d'une axe parallèle à son plan, ou en fixant l'écran sur une tournette dont l'axe est confondu avec celui de l'écran. L'épaisseur déposée dépend des tensions de fonctionnement du tube. Les cristaux sont soit du vanadate d'yttrium, soit un oxysulfure d'yttrium ou un oxyde de gadolinium, dopés à l'europium;
• ― et enfin une couche conductrice mince généralement en aluminium. Cette couche est portée à la haute tension du tube et permet d'éliminer les charges électrostatiques. Elle sert également de réflécteur afin d'obtenir un rayonnement unidirectionnel.

In a fourth structure of the prior art as shown in FIG. 1, the screen consists of superimposed layers and, as detailed below, it includes the following elements:

• - A transparent support 1, made of glass, constituting the screen proper;
• - A layer 2 of a green phosphor consisting of crystals 20, this layer being deposited according to a well-known sedimentation technique;
• - A barrier layer 3 comprising a transparent inert material. This layer is conventionally made up either of zinc sulfide (ZnS) or of silicon oxide (SiO₂).
• A layer 4 of a red phosphor made up of crystals 40, this layer being deposited either by centrifugation, the support 1 rotating around an axis parallel to its plane, or by fixing the screen on a spinner whose axis is confused with that of the screen. The thickness deposited depends on the operating voltages of the tube. The crystals are either yttrium vanadate, or a yttrium oxysulfide or a gadolinium oxide, doped with europium;
• - and finally a thin conductive layer generally made of aluminum. This layer is brought to the high voltage of the tube and makes it possible to eliminate the electrostatic charges. It also serves as a reflector in order to obtain unidirectional radiation.

• ― on réalise par des moyens connus un film organique, sur la couche de luminophore vert, ce film servant de support provisoire au matériau inerte constituant la couche barrière 3. On utilise généralement un polymère, le métacrylate de butyle, et sa formation se fait par un dépôt humide sur l'écran d'une solution contenant ce produit. Après évaporation de l'eau, le film est collé sur la couche de luminophore vert. Une autre solution consiste à utiliser un film de nitrocellulose réalisé d'une façon classique suivant l'art connu;
• ― on procède ensuite au dépôt du matériau inerte par évaporation sous vide soit par effet Joule, soit au canon à électrons. La mesure et le réglage de l'épaisseur déposée est faite par des moyens connus afin d'atteindre l'épaisseur voulue qui permet d'obtenir la tension de seuil désirée constituant une barrière pour les électrons qui ont une énergie trop faible.

The barrier layer 3 is deposited after the deposition of the first phosphor 2 in the following manner:

• - An organic film is produced by known means, on the green phosphor layer, this film serving as a temporary support for the inert material constituting the barrier layer 3. A polymer, butyl methacrylate, is generally used, and its formation is carried out by a wet deposit on the screen of a solution containing this product. After the water has evaporated, the film is bonded to the layer of green phosphor. Another solution consists in using a nitrocellulose film produced in a conventional manner according to the known art;
• - The inert material is then deposited by evaporation under vacuum either by the Joule effect or by electron gun. The measurement and adjustment of the deposited thickness is made by known means in order to achieve the desired thickness which makes it possible to obtain the desired threshold voltage constituting a barrier for the electrons which have too low an energy.

The temporary organic film is removed during the subsequent heat treatments applied to the cathode ray tube.

The deposition of this temporary support film is essential so that the inert layer has a flat surface and a thickness as constant as possible throughout its extent in order to obtain a good functioning of the screens.

However, following the manufacture of screens having this latter structure, it has been found that the characteristics of these screens are not as good as expected. In fact, after observing the structure, it has in fact been observed that the barrier layer underwent deterioration during its manufacture, these deteriorations resulting in cracks in the inert material.

Indeed, during evaporation under vacuum, the inert material when it is silicon oxide undergoes stresses in extension which cause cracks and which also cause a tearing of the organic film.

When it comes to zinc sulfide, the stresses undergone by this layer are in compression, and the layer does not present any cracks. This water-sensitive material no longer exhibits good optical transmission after having undergone all of the screen finishing operations.

In addition, the high zinc sulphide index n = 2.3, limits the optical transmission in particular of the red radiation coming from the red luminophore.

The present invention overcomes all these drawbacks. Its subject is a high luminance color screen for cathode ray tubes, in which the barrier layer has all of the following characteristics: good transparency optical, uniform thickness suitable for stopping electrons, good mechanical strength, chemical stability under vacuum, under electronic bombardment and in the presence of various products such as water or organic solvents.

The invention therefore relates to a high luminance color screen for cathode-ray tube comprising two fluorescence phosphors of different colors, the visible trace of which under the effect of electron bombardment has a color adjustable by the acceleration voltage of the beam of said beams. electrons, variable, in operation, between two extreme values V₀ and V₁, in which the two phosphors are arranged on the transparent support of said screen in superimposed layers made of crystal powders of each of said phosphors, separated from each other by a barrier with faces planar, characterized in that the barrier with planar faces comprises a first layer of silicon dioxide, a layer of zinc sulfide and a second layer of silicon dioxide, the layer of zinc sulfide being between the two layers of diozyde silicon.

Another object of the invention consists in producing a barrier in which the layers of silicon dioxide have a thickness substantially equal to 0.1 μm in order to minimize the overall refractive index of the barrier, to increase the optical transmission, to protect the zinc sulfide and to maintain the internal stresses of this barrier in compression.

Another object of the invention consists in producing a screen in which the powder constituting the deepest phosphor layer of the screen, called the first layer, that is to say that in contact with the support, is made phosphor P₁ of the JEDEC specification and has a particle size of 6 micrometers or less, and in that the powder constituting the other phosphor layer, called the second layer, is made of yttrium vanadate doped with europium and has a particle size of 0.6 micrometer, and in that these layers have thicknesses corresponding substantially to respective weights of 1 to 3 mg and 0.15 milligrams per square centimeter.

The invention also relates to a screen in which the minimum operating voltage is from 7 to 10 kV and the maximum voltage from 12 to 17 kV.

The invention also relates to a screen in which the traces have the color green (550 nm) at maximum voltage and the color red (610 nm) at minimum voltage.

• ― dépôt de la première couche de luminophore sur l'écran transparent;
• ― dépôt d'un film organique sur cette première couche de luminophore;
• ― dépôt de la première couche d'oxyde de silicium;
• ― dépôt de la couche de sulfure de zinc sur la couche d'oxyde de silicium;
• ― dépôt de la deuxième couche d'oxyde de silicium sur la couche de sulfure de zinc, ces trois dernières couches formant un matériau inerte constituant une barrière aux électrons qui ont une tension d'accélération inférieure au seuil de tension que constitue cette barrière;
• ― dépôt de la deuxième couche de luminophore sur la deuxième couche de dioxyde de silicium;
• ― dépôt d'une couche conductrice électriquement;
• ― suppression de la couche organique par traitement thermique de la structure.

The invention also consists of a method for producing a high-luminance color screen for cathode-ray tube, characterized in that it consists in successively carrying out the following steps:

• - deposit of the first phosphor layer on the transparent screen;
• - depositing an organic film on this first phosphor layer;
• - deposition of the first layer of silicon oxide;
• - deposition of the zinc sulfide layer on the silicon oxide layer;
• - Deposition of the second layer of silicon oxide on the zinc sulfide layer, these last three layers forming an inert material constituting a barrier to electrons which have an acceleration voltage below the voltage threshold which this barrier constitutes;
• - deposition of the second phosphor layer on the second layer of silicon dioxide;
• - depositing an electrically conductive layer;
• - removal of the organic layer by heat treatment of the structure.

• ― la figure 1 représente le schéma d'une structure d'écran selon l'art antérieur, déjà décrite;
• ― la figure 2 représente le schéma d'une structure d'écran selon l'invention.

Other particularities and advantages will appear better on reading the detailed description which follows and which refers to the appended drawings, given solely by way of nonlimiting example and in which:

• - Figure 1 shows the diagram of a screen structure according to the prior art, already described;
• - Figure 2 shows the diagram of a screen structure according to the invention.

In FIGS. 1 and 2, the same references designate the same elements. These figures show a fragment of a penetrating screen and more particularly the constituent structure of these screens. Found in FIG. 2, a thick glass support 1, a layer made of a green phosphor powder, a barrier 3 made of inert material 3, a layer made of a red phosphor powder 4, and a thin conductive layer 5.

The screen shown in this figure 2 is produced, as will be explained below:
on the thick glass support 1, layer 2 is deposited, of green phosphor powder of type P1 of the JEDEC specification, publication of "Electronic Industry Association", Engineering Department, according to one of the known techniques, sedimentation for example . The powder in question has a particle size of approximately 6 micrometers; the amount deposited is 3 milligrams per square centimeter.

Next, the various layers constituting the barrier 3 are deposited.

First of all, a temporary support consisting of an organic film 35 is deposited. This support 35 makes it possible to obtain a flat surface for the barrier, without this temporary support, there would be during the constitution of the barrier. , infiltrations through the crystals 20 of the green phosphor.

To form the organic film 35, a polymer is used, butyl methacrylate in solution which sticks to the phosphor layer 2 after evaporation of the water.

This temporary organic film 35 is removed during the subsequent heat treatments applied to the cathode ray tube.

There is therefore deposited on this film 35 a first layer 31 made of an inert material, this material being silicon dioxide (SiO₂). The deposition is carried out by a conventional technique of vacuum evaporation of the silicon dioxide using a electrons. This technique also makes it possible to control the uniformity of the deposit and to obtain the desired thickness.

A layer 32 is then deposited consisting of an inert material of a different nature, this material being zinc sulfide. The deposition is also carried out by vacuum evaporation of the zinc sulfide by Joule effect using a molybdenum crucible containing the zinc sulfide. The thickness is also automatically controlled by conventional means. It is this thickness which fixes the barrier threshold for the electrons. By controlling this thickness, the desired threshold V₀ is obtained. The electrons which have an acceleration voltage lower than the voltage V₀ are stopped and the electrons which have a voltage higher than V₀ cross the barrier. A second layer of silicon dioxide 33 is then deposited on this layer of zinc sulfide 32. This deposition is carried out in the same way as the first layer 31, that is to say by vacuum evaporation of the silicon dioxide to using an electron gun.

Above this second layer of silicon dioxide 33, the layer of the second phosphor is deposited. It is a red phosphor powder having a particle size substantially finer than that of layer 2, ie approximately 0.6 μm. This powder consists either of yttrium vanadate, or a yttrium oxysulfide, or a gadolinium oxide, doped with europium.

The deposition takes place by centrifugation, the support 1 rotating around an axis parallel to its plane, or by fixing the support screen on a spindle whose axis coincides with that of the screen. The thickness deposited depends on the operating voltages of the tube. For a voltage V₀ of 10 kV and a barrier of inert material 3 with a thickness of 1.2 μm, the layer of red phosphor 4 is 0.15 mg / cm².

The conductive layer is then deposited forming an aluminum film which is brought into operation at the high voltage of the tube. The screen thus formed operates with a high voltage V1 of 17 kV. These voltages of 10 kV and 17 kV correspond to colors of the red and green traces of 610 nanometers and 550 nanometers respectively.

When the tube is completed, the structure of the penetration screen according to the invention is therefore in the form of superposed polycrystalline layers between which a barrier is placed. This barrier consists of inert material in the form of three layers, a layer of zinc sulphide and two layers of silicon dioxide, the layer of zinc sulphide being taken between the layers of silicon dioxide. The thicknesses of the silicon dioxide layers are practically independent of the operating voltages V₀ and V₁, and are chosen in order to improve the optical transmission of the zinc sulfide layer which has a high refractive index. By choosing a thickness of 100 nm ± 5, the overall index of refraction of the barrier is reduced and the optical transmission is therefore improved.

Claims (5)

1. A color screen with a high luminance for a cathode ray tube, comprising two luminophors having different fluorescence colors, whose trace appearing under the effect of the bombarding electrons has a color which is able to be regulated by the acceleration voltage of the beam of the said electrons with variation, during operation, between two extreme values V₀ and V₁, in which the two luminophors (2 and 4) are arranged on the transparent support (1) of the said screen in superposed layers made of powders of crystals of each of the said luminophors separated from each other by a barrier with plane faces (3), characterized in that the barrier with plane faces (3) comprises a first layer of silicon dioxide (31), a layer of zinc sulfide (32) and a second layer of silicon dioxide (33), the layer of zinc sulfide being comprised between the two layers of silicon dioxide.

2. The color screen with a high luminance as claimed in claim 1, characterized in that the powder constituting the layer of the luminophor (2) which is deepest in the screen, termed the first layer, that is to say in contact with the support (1), is composed of a phosphor P₁ in accordance with the specification J.E.D.E.C. and has a particle size of 6 microns or less, and in that the phosphor constituting the other layer of the luminophor (4), termed the second layer, is composed of yttrium vanadate doped with europium and has a particle size of 0.6 micron and in that the layers have thicknesses corresponding essentially to the respective weights of 1 to 3 mg and 0. 15 mg per cm².

3. The screen as claimed in claim 1 or claim 2, characterized in that each layer of silicon dioxide (31 and 33) of the barrier (3) has a thickness essentially equal to 0.1 micron and tending to improve the refractive index of the barrier (3).

4. The screen as claimed in any one of the preceding claims 1 through 3, characterized in that the minimum voltage of operation is comprised between 7 and 10 kV and the maximum voltage is comprised between 12 and 17 kV.

5. A method for the production of the cathode ray tube screen as claimed in claim 1, characterized in that it comprises the following stages:

― deposition of the first layer of luminophor on the transparent screen,

― deposition of an organic film on this first layer of luminophor,

― deposition of the first layer of silicon oxide,

― deposition of the zinc sulfide layer on the silicon oxide layer,

― deposition of the second silicon oxide layer on the zinc sulfide layer, said three layers constituting an inert material as a barrier,

― deposition of the second layer of luminophor on the second silicon oxide layer,

― deposition of an electrically conductive layer,

― removal of the organic layer by heat treatment of the structure.

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