High luminance color screen for cathode ray tube and method for making a screen of this type

Disclosed is a high luminance color screen for cathode tubes, comprising two fluorescence luminophors of different colors, the visible trace of which, under the effect of electron bombardment, has a color which can be adjusted by the acceleration voltage of the beam of said electrons. Said voltage is variable, during operation, between two extreme values V.sub.o and V.sub.1, in which the two luminophors are arranged on the transparent support of said screen is superimposed layers formed by powders of crystals of each of the said luminophors, separated from one another by plane faces. Said screen is characterized mainly by the fact that the barrier with plane faces comprises a first layer of silicon dioxide, a layer of zinc sulfide and a second layer of silicon dioxide, the layer of zinc sulphide being included between the two layers of silicon dioxide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention concerns a screen for cathode ray tubes with traces of high 
luminance and color which can be adjusted by the acceleration voltage of 
the beam. In particular, it concerns the structure of this screen. It also 
concerns a method for making this screen. 
2. Description of the Prior Art 
In the prior art, there are so-called penetration cathode tube luminescent 
screens wherein the color of the trace varies with the acceleration 
voltage of the electron beam. 
These luminescent cathode screens have two luminophors with different 
fluorescence levels, which shall be taken to be green and red in the rest 
of the description. They are represented schematically in the form of two 
homogeneous layers, each consisting of one of the luminophors, which may 
or may not be separated by an inert or non-luminescent material emitting 
no light under the effect of an electron bombardment. 
Depending on the value of the beam acceleration voltage V, i.e. depending 
on the energy of the electrons of the beam, only the first of these 
layers, which will be assumed to be the layer with red fluorescence in the 
rest of this description, is excited by the beam, or else, if this voltage 
is sufficient, this layer or the entire second layer or a part of the 
second layer, made of a luminophor with green fluorescence, undergoes 
excitation by the beam. The green fluorescence begins to be excited only 
from a certain value of this voltage onwards, namely from the voltage 
sufficient for the energy of the electrons to enable them to penetrate the 
screen (whence the above expression `penetration screens`) up to the green 
luminophor layer, after crossing the red luminophor layer and, as the case 
may be, the inert layer. This value of the voltage shall be called V.sub.o 
in the rest of the description. The green fluorescence predominates and 
the trace takes on the color green. Between the two values, V.sub.o and 
V.sub.1, there are obtained, depending on the value of V, intermediate 
colors between green and red depending on the proportions of the two 
excited fluorescences in the trace. 
There are various known prior art penetration screen structures. 
One of them comprises layers of red and green luminophors separated by a 
layer of an inert material, all obtained by evaporation under vacuum in 
the form of thin films (Cf. patent on behalf of Feldmen, delivered in the 
United States of America with the number 3 225 238). This type of screen 
has the drawback of displaying extremely low luminance under electron 
bombardment because it is impossible for the radiation emitted in these 
films to emerge from them owing to the multiple reflections that they 
undergo. 
Another of these structures has a mixture of crystals of the two 
luminophors, red and green, the crystals of this mixture having been 
coated, prior to the mixing operation, with a film of inert material. 
Generally, with these mixtures, a luminance is obtained which is 
substantially greater than that of the previous structure. 
In a third prior art structure (see the U.S. Pat. No. 3,714,490 delivered 
in the U.S.A.), the red luminophor is integrated in the structure in the 
form of small-sized grains surrounding the inert layer coating bigger 
grains of green luminophor. The luminance of the red luminophor is thus 
substantially improved, but the problem to be resolved is really that of 
increasing the total luminance of the screen, namely luminance of all the 
traces and not only that given by the red luminophor. At the same time as 
this red luminophor luminance is increased, the luminance of the green 
luminophor should be increased in the same proportions for it is this 
proportion which sets the extent of the range of shades obtained between 
red and green from the moment when the green fluorescence begins to be 
excited by the acceleration voltage, namely from the value V.sub.o above. 
It is generally desired to have this entire range available in order to 
display all the information. This range is narrowed down if the increase 
in the luminance of the red luminophor is not accompanied by that of the 
green luminophor. 
In a fourth prior art structure, as shown in FIG. 1, the screen consists of 
superimposed layers and, as set out in detail further below, it has the 
following elements: 
a transparent support 1, made of glass, forming the screen proper; 
a layer 2 of a green luminophor made up of crystals 10, said layer being 
deposited according to a well known sedimentation technique; 
a barrier layer 3 comprising a transparent inert material. This layer is 
conventionally made either of zinc sulphide (ZnS) or silicon oxide 
(Si0.sub.2); 
a layer 4 of a red luminophor made up of crystals 40, said layer being 
deposited either by centrifugation, with the support 1 rotating on an axis 
parallel to its plane, or by fixing the screen on a spinner, the axis of 
which is the same as that of the screen. The thickness deposited depends 
on the operating voltages of the tube. The crystals are either yttrium 
vanadate or yttrium oxysulphide or gadolinium oxide doped with europium; 
and finally, a thin conductive layer, generally made of aluminium. This 
layer is taken to the high voltage of the tube and enables the removal of 
the electrostatic charges. It is also used as a reflector in order to 
obtain a one-directional radiation. 
The barrier layer 3 is deposited after the deposition of the first 
luminophor 2 in the following way: 
an organic film is made by known methods on the layer of green luminophor, 
said film acting as a temporary support for the inert material forming the 
barrier layer 3. The material used is generally a polymer, butyl 
metacrylate, and it is formed by wet deposition on the screen of a 
solution containing this product. After the water evaporates, the film is 
bonded to the layer of green luminophor. Another approach uses a 
nitrocellulose film made in a standard way according to the prior art; 
then the inert material is deposited by vacuum evaporation, either by Joule 
effect or by an electron gun. The thickness deposited is measured and 
adjusted by known means in order to achieve the desired thickness which 
makes it possible to obtain the desired threshold voltage forming a 
barrier to electrons with excessively low energy. 
The temporary organic film is removed during the subsequent heat treatments 
applied to the cathode tube. 
The deposition of this temporary supporting film is indispensable for the 
inert layer to have a plane surface and a thickness which is as constant 
as possible throughout its range, in order to obtain proper functioning of 
the screens. 
Now, following the manufacture of screens with this latter structure, it 
has been observed that the characteristics of these screens are not as 
good as expected. For, after the observation of the structure, it has been 
observed, in fact, that the barrier layer deteriorates during manufacture, 
and this deterioration takes the form of cracks in the inert material. 
For, during the vacuum evaporation, the inert material, when it is silicon 
oxide, undergoes stretching stresses which cause cracks, and also cause 
tearing in the organic film. 
When the material is zinc sulphide, the stresses undergone by this layer 
are compressive stresses, and the layer has no cracks. This 
water-sensitive material no longer has good optical transmission after 
having undergone all the screen finishing operations. 
Furthermore, the high index of zinc sulphide (n=2.3) limits optical 
transmission, especially that of red radiation coming from the red 
luminophor. 
The present invention makes it possible to overcome all these drawbacks. An 
object of the present invention is a high luminance color screen for 
cathode ray tubes, wherein the barrier layer has all the following 
characteristics: high quality optical transparency, uniform thickness 
adapted to the stopping of electrons, sound mechanical solidity and 
chemical stability under vacuum, under electron bombardment, and in the 
presence of various products such as water or organic solvents. 
SUMMARY OF THE INVENTION 
The invention thus concerns a high luminance color screen for cathode tubes 
comprising two fluorescence luminophors of different colors, for which the 
trace visible under the effect of electron bombardment has a color which 
can be adjusted by the acceleration voltage of said electron beam, said 
acceleration voltage being variable, during operation, between two extreme 
values V.sub.o and V.sub.1, wherein the two luminophors are placed on the 
transparent support of said screen in superimposed layers made of powders 
of crystals of each of said luminophors, separated from one another by a 
barrier with plane faces, wherein the barrier with plane faces has a first 
layer of silicon dioxide, a layer of zinc sulphide and a second layer of 
silicon dioxide, the zinc sulphide layer being included between the two 
layers of silicon dioxide. 
Another object of the invention is the making of a barrier in which the 
silicon dioxide layers have a thickness substantially equal to 0.1.mu.m, 
in order to use the overall refraction index of the barrier to the 
minimum, increase optical transmission, provide shielding for the zinc 
sulphide and maintain the internal compressive stresses. 
Another object of the invention is the making of a screen wherein the 
constituent powder of the deepest luminophor layer of the screen, called 
the first layer, namely the layer in contact with the support, is made of 
phosphorus P.sub.1 according to the J.E.D.E.C. specification and has a 
grain size of 6 micrometers or less, and wherein the constituent powder of 
the other luminophor layer, called the second layer, is europium-doped 
yttrium vanadate and has a grain size of 0.6 micrometers, and wherein 
these layers have thicknesses substantially corresponding to respective 
weights of 1 mg to 3 mg and 0.15 milligrams per square centimeter. 
Another object of the invention is a screen wherein the minimum operating 
voltage is 7 to 10 kV and the maximum voltage is 12 to 17 kV. 
Another object of the invention is a screen wherein the traces have the 
color green (550 nm) and the color red (610 nm) at the minimum voltage. 
The invention further consists in a method for making a high luminance 
color screen for cathode ray tubes, wherein the following steps are 
performed in succession: 
depositing the first layer of luminophor on the transparent screen; 
depositing an organic film on this first layer of luminophor; 
depositing the first layer of silicon oxide; 
depositing the layer of zinc sulphide on the layer of silicon oxide; 
depositing the second layer of silicon oxide on the layer of zinc sulphide, 
the latter three layers forming an inert material constituting a barrier 
against the electrons which have an acceleration voltage below the voltage 
threshold forming this barrier; 
depositing the first layer of luminophor on the second layer of silicon 
dioxide; 
depositing an electrically conductive layer; 
eliminating the organic layer by heat treatment of the structure.

DESCRIPTION OF A PREFERRED EMBODIMENT 
In FIGS. 1 and 2, the same references are repeated for the same elements. 
These figures show a portion of a penetration screen and, more especially, 
the constituent structure of these screens. FIG. 2 shows a thick glass 
support 1, a layer made up of a green luminophor powder, a barrier 3 made 
of an inert material, a layer made up of a red luminophor powder 4 and a 
thin conductive layer 5. 
The screen shown in this FIG. 2 is made as shall be explained below: 
On the thick glass support, there is deposited the layer 2 of type Pl green 
luminophor powder, in accordance to the J.E.D.E.C. specification as 
published by the "Electronic Industry Association", Engineering 
Department. The deposition is done according to a known technique, for 
example sedimentation. The powder in question has a grain size of about 
six micrometers. The quantity deposited is 3 milligrams per square 
centimeter. 
Then, the different layers forming the barrier 3 are deposited. 
First of all, a temporary support is deposited. This temporary support is 
made up of an organic film 35. This support 35 is used to obtain a plane 
surface for the barrier. Without this temporary surface, there would be 
infiltrations through the crystals 20 of the green luminophor when the 
barrier is being made. 
To make the organic film 35, a polymer is used. This polymer is butyl 
metacrylate in solution, which gets bonded to the layer of luminophor 2 
after the water evaporates. 
This temporary organic film 35 is removed during the subsequent heat 
treatments applied to the cathode tube. 
Thus, on this film 35, there is deposited a first layer 31 formed by an 
inert material. This material is silicon dioxide (Si0.sub.2). The 
deposition is achieved by a standard method for vacuum evaporation of 
silicon dioxide by means of an electron gun. This techniques further makes 
it possible to check the uniformity of the deposit and to obtain the 
desired thickness. 
Then, a layer 32, made up of an inert material of a different nature, is 
deposited. This material is zinc sulphide. The deposition is also achieved 
by vacuum evaporation of the zinc sulphide by Joule effect using a 
molybdenum crucible containing the zinc sulphide. The thickness is also 
controlled automatically by standard means. It is this thickness that sets 
the barrier threshold for the electrons. In controlling this thickness, 
the desired threshold V.sub.o is obtained. The electrons, which have an 
acceleration voltage smaller than the voltage V.sub.o, are stopped and the 
electrons which have a voltage greater than V.sub.o go through the 
barrier. Then, on this layer of zinc sulphide 32, a second layer of 
silicon dioxide 33 is deposited. This deposit is done in the same way as 
the first layer 31, namely by vacuum evaporation of the silicon dioxide by 
means of an electron gun. 
On top of this second laayer of silicon dioxide 33, the layer of the second 
luminophor is deposited. This is a red luminophor powder with a grain size 
substantially finer than that of the layer 2, namely, 0.6.mu.m 
approximately. This powder consists of either yttrium vanadate or an 
yttrium oxysulphide or gadolinium oxide doped with europium. 
The depositing is done by centrifugation. The support 1 rotates on an axis 
parallel to its plane, or the deposition is done by fixing the supporting 
screen to a spinner, the axis of which is identical to that of the screen. 
The deposited thickness depends on the operating voltages of the tube. For 
a voltage V.sub.o of 10 kV, and a barrier of inert material 3 with a 
thickness of 1.2.mu.m, the red luminophor layer 4 is 0.15 mg./cm.sup.2. 
Then, the conductive layer forming an aluminium film is deposited. This 
conductive layer is carried, during operation, to the high voltage of the 
tube. The screen thus formed works with a high voltage V1 of 17kV . To 
these voltages of 10 kV and 17 kV, there correspond the colors of the red 
and green traces, equal to 610 nanometers and 550 nanometers respectively. 
When the tube is finished, the penetration screen structure according to 
the invention is thus in the form of superimposed polycrystalline layers, 
between which a barrier is placed. This barrier consists of an 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 held between 
the layers of silicon dioxide. The thicknesses of the silicon dioxide 
layers are practically independent of the operating voltages V.sub.o and 
V.sub.1, and are chosen to improve the optical transmission of the zinc 
sulphide layer which has a high refraction index. By choosing a thickness 
of 1000.+-.50 angstroms, the total refraction index of the barrier is 
reduced and, consequently, optical transmission is improved.