EP0143714B1 - Luminescent screen and process for manfacturing the same - Google Patents

Description

The present invention relates to luminescent screens. It also relates to a method of manufacturing these luminescent screens.

The screens in question in the present invention comprise, in particular, several layers of luminescent material, in the form of grains, which are deposited on a transparent support; generally, it is a glass substrate, with parallel faces.

The luminescent material may be cathodoluminescent, that is to say that it becomes luminescent when subjected to the bombardment of an electron beam. Such cathodoluminescent screens are used for example in cathode ray tubes, radiological image intensifiers. The luminescent material can also be, for example, electroluminescent, that is to say that it becomes luminescent under the action of an electric field.

In the following description, the problems to be solved and the solutions provided by the invention will be described in the case of cathodoluminescent screens used in radiological or IIR image intensifiers, but it is understood that the invention applies to all types of screens mentioned above.

In FIG. 1, an IIR has been represented schematically. This tube has a primary screen which converts the X photons it receives into light photons, then into photoelectrons. Electronic optics, which are not shown, ensure the focusing of the electronic trajectories and the energy gain of the electrons. Finally, a cathodoluminescent secondary screen ensures the conversion of electrons into visible photons. It is this secondary screen that will be discussed later.

In FIGS. 2a and b, an embodiment of the secondary screen in FIG. 1 has been seen in cross section.

On the glass substrate 1, there is a deposit 2 of several layers of crystals of a cathodoluminophore body, the first layer of which is in direct contact with the substrate. The last layer of crystals is covered with a metallic film 3, made of aluminum for example. This film is used to reflect the light created in the screen towards the observer and to apply an acceleration voltage to the incident electrons. Figure 2b shows, enlarged, the screen area surrounded by a circle in Figure 2a.

The cathodoluminophore body used can be, for example zinc sulfide doped with silver. The diameter of the grains can vary for example between 1 and 3 μm, depending on the desired resolution.

The thickness of the glass substrate 1 is for example, from 1 to 3 mm approximately, while the thickness of luminescent material is approximately 10 μm.

It is known that the screens just mentioned present a phenomenon called halos. When the screen is excited at a point, we observe around this point, rendered luminous, luminous rings or halos, centered on this point, which are equidistant at a distance close to twice the thickness of the substrate and whose intensity decreases when you move away from the central point of light.

This phenomenon is illustrated by Figures 3a and b.

In Figure 3a, we see the screen seen in profile and receiving an electronic impact directed along the axis XX '.

In Figure 3b, we see the central light point due to this impact and three of the halos created.

The explanation of this halo phenomenon will be recalled with reference to Figures 4 and 5.

In FIG. 4, there is shown a sectional view of the luminescent screen, the thickness of the metallic film 3 and of the layers of luminescent material 2 has been greatly increased in FIG. 4 relative to the thickness of the substrate 1.

Any light ray which is generated in a grain A which is not in contact with the substrate, passes through the substrate 1 as if it were a blade with parallel faces and gives a ray A _i output. It is the same for the light rays generated in grains which are in contact with the substrate but which emerge from the grains in another place than the point of contact of the grain with the substrate. This is the case for radius B _o from grain B.

Let us now consider the case of the light rays emitted by the grain B in contact with the substrate and which in addition penetrate into the substrate by the point of contact of the grain and the substrate.

Everything happens for these rays as if they were created by a light source in intimate optical contact with the substrate. When the angle of incidence 6 of these rays on the internal exit face of the substrate is less than the angle 0 ₀ such that sin 0 ₀ = 1 / n, with n the optical index of the substrate, these rays pass through the substrate towards the observer. This is the case of rays B ₁ and B ₂ in Figure 4. On the other hand when the angle of incidence is greater than or equal to 6 ₀ , there is a phenomenon of total reflection and rays such as B ₃ on Figure 4 are returned to the internal input face of the substrate. These rays are returned laterally to a grain C in contact with the substrate and distant from the grain B by the distance: D = 2e. 0 ₀ = 2nd, because with a glass substrate, n = 1, ₅ and 6 ₀ = 42 °. By diffraction or scattering, the rays coming to touch the grain C are re-emitted, some, such as C ₁ , towards the observer and others such as C ₂ , by total reflection, are returned to another grain D distant from '' a distance equal to about 2e of the grain C.

This phenomenon continues gradually until exhaustion of the light intensity and in all directions around the point B. It thus appears around a light spot centered in B, a multitude of rings, separated by a distance D and decreasing luminous intensity I, I ₁ , I ₂ , I ₃ . The other points such as C or D are the seat of halo phenomena, minus brighter than the halos centered at point B.

In FIG. 5, the substrate is represented seen in section as well as the path of the light rays, and in particular of those which undergo a total reflection. FIG. 5 also shows the variations in intensity 1 observed and corresponding to the central spot and to the different halos.

We will expose in the following description, and with reference to Figures 6 to 10, various known techniques which seek to eliminate this phenomenon of halos. This phenomenon is very annoying because it gives rise to information which parasites useful information. In addition, it produces a decrease in the contrast of the screen.

The problem that arises is that the known techniques are not satisfactory. In particular, these techniques improve the contrast but cause the light output to drop. Some of these techniques produce a decrease in resolution.

The present invention makes it possible to solve this problem and makes it possible, as will be explained in detail below, to obtain an optimized contrast screen without the gain dropping too much and without the resolution being reduced.

The present invention, as characterized in claim 1, relates to a luminescent screen comprising in particular several layers of luminescent material in the form of grains, deposited on a transparent support, characterized by the presence of blocks, arranged between the grains of the first layer of material and the support, these blocks having a cross section at most equal to the cross section of the grains and having an optical transparency of less than 1.

• a) on dépose sur le support transparent une couche mince ayant la transparence optique recherchée ;
• b) on dépose sur cette couche une première couche de grains en matériau luminescent ;
• c) on réalise une attaque sélective par plasma de la couche mince en utilisant les grains de la première couche comme masque de façon à obtenir les pavés :
• d) on dépose les autres couches de grains luminescents, et on termine l'écran de la façon habituelle.

The present invention, as characterized in claim 8, also relates to a method for manufacturing a luminescent screen, comprises the following steps:

• a) a thin layer having the desired optical transparency is deposited on the transparent support;
• b) a first layer of grains of luminescent material is deposited on this layer;
• c) a selective attack by plasma of the thin layer is carried out using the grains of the first layer as a mask so as to obtain the blocks:
• d) the other layers of luminescent grains are deposited, and the screen is terminated in the usual way.

• la figure 1, le schéma d'un intensificateur d'image radiologique ;
• les figures 2a et b. des vues en coupe d'un écran luminescent ;
• les figures 3a et b et 4 et 5, des schémas illustrant le phénomène de halos observé dans les écrans luminescents ;
• les figures 6 à 10, des schémas illustrant les techniques connues utilisées contre le phénomène de halos ;
• la figure 11, une vue en coupe d'un mode de réalisation de l'écran selon l'invention ;
• les figures 12a à d, des schémas illustrant les différentes étapes d'un procédé de fabrication d'un mode de réalisation d'un écran selon l'invention ;
• les figures 13a. b, c, des schémas montrant le pavé associé à chaque grain de la première couche.

Other objects, characteristics and results of the invention will emerge from the following description, given by way of nonlimiting example and illustrated by the appended figures which represent:

• Figure 1, the diagram of a radiological image intensifier;
• Figures 2a and b. sectional views of a luminescent screen;
• FIGS. 3a and b and 4 and 5, diagrams illustrating the phenomenon of halos observed in luminescent screens;
• Figures 6 to 10, diagrams illustrating the known techniques used against the phenomenon of halos;
• Figure 11, a sectional view of an embodiment of the screen according to the invention;
• FIGS. 12a to d, diagrams illustrating the different stages of a method of manufacturing an embodiment of a screen according to the invention;
• Figures 13a. b, c, diagrams showing the paving stone associated with each grain of the first layer.

In the different figures, the same references designate the same elements, but, for reasons of clarity, the dimensions and proportions of various elements are not observed.

We will briefly recall with reference to Figures 6 to 10 the solutions used in the prior art against the phenomenon of halos.

A first solution consists in using a mass-tinted glass substrate whose optical transparency T _l is less than 1.

In Figure 6, there is shown in section such a screen. The intensity rays AT ₁ and BT ₁ do not participate in the formation of halos as is the case for the intensity ray CT, ³ which undergoes total reflection.

The gain G (or light output) of this screen is expressed as a function of the gain G ₀ of a screen comprising a transparent glass substrate by the relation G, = G _o - T ₁ . Remember that the gain is the ratio between the light power emitted by the screen and the electric power it receives.

The contrast Ci of this screen is expressed as a function of the contrast C _o of a screen with a transparent glass substrate by the relation: C, = C _o - (1 / T ₁ ² ). It is recalled that the contrast is the ratio of the luminances of an excited screen area and an unexcited screen area.

This solution therefore increases the contrast but in return reduces the gain. A compromise must be established in the choice of transparency T, so that the minimum gain acceptable to users is respected.

A second solution consists in rejecting the halos outside the useful area of the screen by increasing the thickness e of the screen. If we call φ the diameter of the useful area delimited by a cover 4 in FIG. 7, it is clear that for all the halos to be located outside this area it suffices that the following relation is verified: 2e »φ .

We are limited in increasing the thickness e of the substrate by practical reasons. Too large an increase in this thickness would modify the optical path available at the output of the IIR towards the use of the image.

The two other solutions which will now be described include the use of an intermediate layer 5 between the substrate 1 and the first layer of luminescent material.

FIG. 8 illustrates the solution where this intermediate layer is metallic, of transparency T ₂ . The gain G ₂ and the contrast C _{2 are} expressed by the same type of relationship as when a tinted glass substrate is used:

There are therefore the same drawbacks of increasing the contrast and reducing the gain as in the case of a tinted glass substrate.

An additional drawback of the metallic intermediate layer is that for example in the case of the intensity ray A in FIG. 8, there is transmission to the observer of a ray of intensity AT ₂ and reflection on the metallic layer d '' a ray of intensity A · (1 - T ₂ ) which is finally transmitted to the observer but contributes to the decrease in the resolution of the screen, because it increases the diameter of the central spot corresponding to the impact of the electron beam.

The other known solution uses an intermediate layer 5 which has been described in European patent application No. 0 018666. This intermediate layer can be made up of alternating layers of silicon oxide and titanium oxide. FIG. 9 shows the reflection coefficient R of this layer as a function of the angle of incidence 6. When the angle of incidence is less than the total reflection angle 8 ₀ , the reflection coefficient is substantially zero. This reflection coefficient becomes substantially equal to 1 for an angle of incidence greater than 6 ₀ .

Consequently, this intermediate layer has a practically total transparency T (with T = 1 - R) for rays such as radius A in FIG. 10, the angle of incidence of which is less than θ ₀ . On the other hand, this layer prevents the exit to the observer of the rays which contribute to the halos. In FIG. 10, it can be seen that the ray B, whose angle of incidence is equal to 6 ₀ , propagates laterally in the substrate without exiting towards the observer. This ray B undergoes successive total reflections on the two faces of the substrate.

This intermediate layer has the disadvantage of causing a drop in resolution by the same phenomenon as that explained for the metallic layer. In addition, it is difficult and expensive to carry out.

In Figure 11, there is shown in section an embodiment of a screen according to the invention. Between the grains of the first luminescent material layer and the transparent support 1, there are blocks 6, having a section at most equal to the section of the grains and having an optical transparency T _{3 of} less than 1.

We see in Figure 11 that the light rays generated in a grain which is not in contact with the substrate emerge towards the observer without being attenuated, this is the case of ray A.

The light rays generated in the grains of the first layer but which emerge from these grains in a place other than the point of contact of the grain with the substrate may have to pass through a block 6 as shown in FIG. 11. an intensity radius BT3 for example. It may also happen that these spokes do not have to cross paving stones.

Let us now consider the case of light rays emitted by a grain in contact with the substrate and which in addition penetrate into the substrate by the point of contact of the grain and substrate.

Some of these rays do not undergo total reflection and exit, for example with an intensity B · T ₃ . Others undergo a total reflection, for example the radius of intensity CT ₃ . Such a ray can come out of the substrate with a CT ₃ ³ intensity after being reflected on another grain and having crossed twice the block supporting this grain.

Compared to FIG. 6, which relates to the use of a tinted glass support, it can be seen that the rays generated in grains other than those of the first layer are not attenuated.

This improves the gain and contrast compared to known solutions.

G ₃ and C ₃ denote the gain and the contrast of the screen according to the invention.

The calculation shows that in the hypothesis, where the transparency T ₃ of the blocks and that T ₁ of the support 1 in tinted glass are the same, the following relationships are obtained:

The above relationships show that the invention makes it possible to obtain - for T ₃ = T ₁ - a contrast C ₃ greater than that Ci obtained with a support in tinted glass, and greater than that C _o obtained without any adjustment. The invention simultaneously makes it possible to obtain a gain G ₃ greater than that obtained with a tinted glass G ₁ , but less than that G _o obtained without arrangement.

The calculation also shows that, on the assumption that a minimum gain is respected, for a screen according to the invention and for a screen with a tinted glass support, the invention allows lower transparency. As when the transparency is the same, the contrast is better with the blocks according to the invention, it is clear that the invention makes it possible, while respecting a minimum gain, to further improve the contrast.

Another advantage of the invention is that the presence of blocks does not decrease the resolution, whereas this occurs when there is an intermediate layer between the glass substrate and the first layer of grains.

A method of producing a screen according to the invention will now be described with reference to FIGS. 12a, b, c and d.

A thin layer 7 of material having the desired transparency is deposited on the substrate 1-see FIG. 12a. This deposition can be carried out, for example, by vacuum evaporation or by electrochemical means. This layer 7 may for example have a thickness of a few hundred angstroms.

The material used can be any absorbent material, for example metal or carbon.

A first layer of luminescent material is deposited on layer 7. By conventional techniques, well-individualized grains are obtained - see Figure 12b. Layer 7 is subjected to a selective plasma attack using the grains of the first layer as a mask. This attack is symbolized by vertical arrows in Figure 12b.

In the case of a layer 7 of silver or gold, attack is carried out with Argon ions for example.

It is possible to use a carbon layer produced for example by evaporation, by using a plasma comprising a hydrocarbon gas or by depositing a single layer of carbon particles with a diameter less than 0.1 μm for example, while the grains of luminescent material have a much larger diameter, of about ten µrn for example. In the case of a carbon layer 7, an attack is carried out by oxygen plasma.

Figure 12c shows the result of this attack. This attack must be stopped on the surface of the substrate in order not to frost it, and thus not to deteriorate the resolution of the screen.

Then, we deposit other layers of grains of luminescent material on the first layer and finish the screen in the usual way - see Figure 12d.

In FIGS. 13a, b, c, a grain of luminescent material 2 and its block 6 have been shown. In FIG. 13a, the block has a section substantially equal to that of the grain, in FIGS. 13b and c, the block a a significantly decreasing section, less than that of the grain. It is clear that the more the section of the block is limited to the point of contact between the grain and the block, the more the efficiency and the contrast are improved. Thus, the attenuation of intensity due to the pavement is limited to the rays created at the point of grain-pavement contact.

The production method described makes it possible to obtain blocks of section at most equal to the section of the grains. To obtain configurations such as that of FIG. 13c, one can play on the directivity of attack of the plasma.

The material used to make the pavers must have good adhesion with the glass of the substrate. It must also be able to be well attacked by plasma while the luminescent material of the grains and the glass are not very attacked. We have seen that we can use a metal such as silver or gold for example, or carbon, we can also use a layer such as that cited above and described in European patent application No. 0 018 666 This increases gain and efficiency, without reducing the screen resolution. In this case it is also necessary to use for the selective attack, a plasma which very preferably attacks this layer while the luminescent material of the grains and the support are not very attacked.

We can at the same time that we use blocks between the glass substrate and the grains of the first layer increase the thickness e of the substrate. This thickness e must remain less than the radius of the useful area, because otherwise there are no more halos but this too great thickness causes other problems.

Claims (9)

1. Luminescent screen comprising, in particular, a plurality of layers of luminescent material (2) in the form of grains deposited on a transparent carrier (1), characterized by the presence of paving-like elements disposed between the grains of the first material layer and the carrier, these paving-like elements having a section which is at the most equal to the section of the grains and having an optical transparency (T₃) smaller than 1.

2. Screen according to claim 1, characterized in that the paving-like elements (6) are of metal or carbon.

3. Screen according to claim 1, characterized in that the paving-like elements (6) are constituted of alternating layers of silicon oxide and titanium oxide.

4. Screen according to any of claims 1 to 3, characterized in that it comprises a plurality of layers of a cathodoluminescent material (2).

5. Screen according to any of claims 1 to 4, characterized in that the transparent carrier (1) is of glass.

6. Method of producing a screen according to any of claims 1 to 5, characterized in that it comprises the following steps :

a) a thin layer (7) having the desired optical transparency (T₃) is deposited on the transparent carrier (1) ;

b) a first layer of grains of luminescent material (2) is deposited on this layer (7) ;

c) a selective plasma attack of the thin layer (7) is performed by using the grains of the first layer as a mask in a manner to obtain the paving-like elements (6) ;

d) the further layers of luminescent grains are deposited, and the screen is finished in the usual manner.

7. Method according to claim 6, characterized in that the thin layer (7) is of silver and in that a selective attack is performed by means of argon ions.

8. Method according to claim 6, characterized in that the thin layer is of carbon and in that a selective attack is performed by an oxygen plasma.

9. Method according to any of claims 6 to 8, characterized in that the directivity of the plasma is adjusted to reduce the section of the paving-like elements.

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