EP0802559B1 - Flat panel display with hydrogen source - Google Patents

Description

The present invention relates to flat screens of visualization, and more particularly of cathodoluminescence screens, with anode carrying separate luminescent elements from each other by insulating zones, and susceptible to be excited by an electronic bombardment from microtips.

The attached figure shows an example of a flat screen microtip color of the type to which this relates invention.

Such a microtip screen essentially consists a cathode 1 with microtips 2 and a grid 3 provided with holes 4 corresponding to the locations of the microtips 2. The cathode 1 is placed opposite a cathodoluminescent anode 5 including a glass substrate 6 constitutes the screen surface.

The operating principle and an embodiment particular of a microtip screen are described, in particular, in U.S. Patent No. 4,940,916 to the Commissariat for Atomic Energy.

Cathode 1 is organized in columns and is made up, on a glass substrate 10, cathode conductors organized in mesh from a conductive layer. The microtips 2 are made on a resistive layer 11 deposited on the cathode conductors and are arranged inside meshes defined by the cathode conductors. The figure partially represents the interior of a mesh and the conductors cathode do not appear in this figure. Cathode 1 is associated with grid 3 organized in lines. The intersection a row of grid 3 and a column of cathode 1 defines a pixel.

This device uses the electric field which is created between the cathode 1 and the grid 3 so that electrons are extracted from the microtips 2. These electrons are then attracted by phosphor elements 7 from the anode 5 if these are suitably polarized. In the case of a color screen, the anode 5 is provided with alternating bands of phosphor elements 7r, 7g, 7b each corresponding to a color (Red, Green, Blue). The strips are parallel to the columns of the cathode and are separated from each other by an insulator 8, generally silicon oxide (SiO ₂ ). The phosphors 7 are deposited on electrodes 9, made up of corresponding strips of a transparent conductive layer such as indium tin oxide (ITO). The sets of red, green and blue bands are alternately polarized with respect to the cathode 1, so that electrons extracted from the microtips 2 of a pixel of the cathode / grid are alternately directed towards the phosphors 7 opposite each of the colors.

The phosphor 7 selection command (the phosphor 7g in the figure) which must be bombarded by electrons from the microtips of cathode 1 requires ordering, selectively, the polarization of the phosphor elements 7 of anode 5, color by color.

Generally, the rows of grid 3 are sequentially polarized at a potential of the order of 80 volts, while the strips of phosphor elements (for example 7g) to be excited are polarized under a voltage of the order 400 volts via the ITO band on which these phosphor elements are deposited. ITO bands, carrying the other bands of phosphor elements (for example 7r and 7b), are at low or no potential. The columns of cathode 1 are brought to respective potentials between a maximum emission potential and absence potential emission (for example, 0 and 30 volts respectively). We fix thus the brightness of a color component of each of the pixels of a line.

The choice of the values of the polarization potentials is linked to the characteristics of phosphors 7 and microtips 2. Conventionally, below a potential difference of 50 volts between the cathode and the grid, there is no emission electronic, and the maximum emission used corresponds to a potential difference of 80 volts.

A disadvantage of conventional screens is that microtips gradually lose their emissivity. We can see this phenomenon by measuring the current in the cathode conductors. This results in a gradual decrease screen brightness, which affects the lifespan of classic screens.

A flat screen corresponding to the preamble of claim 1 is described in document WO 96/01492.

The present invention aims to overcome this drawback by making the emissive power of the microtips substantially constant.

The present invention also aims to provide a screen with automatic regulation of the emitting power of the microtips.

The present invention further aims to provide a process for producing a screen whose microtips have a substantially constant emissivity without modifying or screen structure, nor the screen control means.

To achieve these objects, the present invention provides a flat display screen comprising a cathode with electron bombardment microtips of an anode provided phosphor elements, the anode and the cathode being separated by a vacuum space, and containing a release source progressive hydrogen, characterized in that the progressive release source of hydrogen consists of a thin layer deposit of a hydrogenated material.

According to an embodiment of the present invention, the hydrogen source consists of a resistive layer of the cathode on which the microtips are arranged.

According to an embodiment of the present invention, the hydrogen source consists of isolation bands separating strips of phosphor elements from the anode.

According to an embodiment of the present invention, the hydrogen source is produced at the periphery of the area active anode carrying the phosphors, a source excitation of said hydrogen source being carried out, side cathode, facing said source of hydrogen.

The present invention also provides a method of manufacture of a flat display screen comprising a microtip cathode of electron bombardment of an anode provided with phosphor elements, the anode and the cathode being separated by a vacuum space, characterized in that the method comprises the step consisting in hydrogenating at the time of its deposition, at least one of the constituent thin layers formed inside this screen.

According to an embodiment of the present invention, the hydrogenated layer is obtained by chemical phase deposition plasma-assisted vapor from at least one precursor enriched in hydrogen.

The present invention originates from an interpretation phenomena that cause the above problems in classic screens.

The inventors consider that these problems are due, in particular to an oxidation of the microtips of the cathode.

In a microtip screen, the surface layers of the anode are, from a chemical point of view, oxides, that this either the phosphors 7 or the insulator 8. By cons, side cathode, the microtips are generally metallic, for example in molybdenum (Mo).

Oxide layers tend to shrink under the effect electronic bombardment, that is to say to release oxygen which oxidizes the surface of the microtips which then lose their emissive power.

From this analysis, the present invention proposes to control this oxidation phenomenon of the microtips of the cathode by introducing into the inter-electrode space the screen, partial pressure of hydrogen.

In a microtip screen, in operation, the most negative potential consists of the metallic cathode material and the H ⁺ or H ₂ ⁺ ions are therefore attracted by the microtips to reduce them if they are oxidized. On the other hand, these H ⁺ or H ₂ ⁺ ions are repelled by the anode and do not risk damaging the phosphor elements.

The water vapor (H ₂ O) formed by the recombination of the H ⁺ or H ₂ ⁺ ions is then trapped by an element for trapping impurities, generally called a "getter", communicating with the inter-electrode space.

In fact, a microtip screen is generally provided with an element for trapping impurities whose role is to absorb various pollutions resulting from the degassing of the layers of the screen in contact with the vacuum. However, in conventional screens, this getter does not succeed in effectively trapping the oxygen degassed by the phosphors 7 and the insulating layers 8 insofar as these degassings are carried out essentially in a positive ionic form (O ₂ ⁺ ) which then finds itself attracted to the microtips before it can be trapped by the getter.

Conversely, the water vapor obtained by the reduction oxygen through hydrogen ions is a molecule neutral which is then no longer attracted to microtips and can be trapped by the getter.

The partial pressure of hydrogen must not, however be too high not to interfere with the operation of the screen.

Indeed, the presence of hydrogen in the vicinity of microtips creates a microplasma of hydrogen in the vicinity of the microtips. This plasma must remain at a sufficiently low pressure and be located around the tips to do not disturb the operation of the screen. In particular, if this plasma develops, we risk seeing an arc appear between the anode and the cathode of the screen.

The partial pressure of hydrogen is according to the invention chosen according to the inter-electrode distance and the quality of the vacuum in the screen, in particular of the pressure partial of all oxidizing species combined.

As a particular example, a partial hydrogen pressure of 5.10 ^-4 millibars (5 10 ^-2 Pa) constitutes a limit pressure for an inter-electrode distance of approximately 0.2 mm.

However, the partial pressure of hydrogen must be maintained at the chosen level even though hydrogen is consumed and trapped by the getter.

A characteristic of the present invention is to provide, inside the inter-electrode space, a source of hydrogen which gradually releases H ⁺ ions as the screen operates, ie progressively degassing of oxidizing species from the anode.

Preferably, this source is placed near spikes so that the released hydrogen is not trapped by the getter before reaching the microtips.

To allow a gradual release of hydrogen, the source material must be able to give off hydrogen only under excitation.

This excitation can be thermal. In that case, the temperature rise inside the screen during its operation produces hydrogen. This excitement can also result from electronic bombardment or ionic.

According to a first embodiment of the present invention, the hydrogen source is integrated in the bands insulators 8 which separate the strips of phosphor elements from the anode. In this case, activation of the hydrogen source is essentially carried out by electronic bombardment. In Indeed, some electrons emitted by the microtips touch the edges of insulating tracks.

According to a second embodiment, the source of hydrogen is produced on the cathode side and is for example integrated to the resistive layer which supports the microtips. Activation of the source is then thermal, the cathode not being bombed.

An advantage common to both embodiments described above is that they distribute the source of hydrogen across the entire surface of the screen and thus guarantee an effect homogeneous antioxidant in the screen.

Another advantage is that they allow regulation automatic partial pressure of hydrogen in space inter-electrodes, therefore of the antioxidant means of the microtips of the cathode. Indeed, activation (thermal or by bombardment of the hydrogen source is located in the region microdots which emit and which are therefore susceptible to be oxidized.

Another advantage is that they do not require any modification of the screen structure, but only deposition conditions of the insulating tracks 8 or of the layer resistive 11, as will be seen below.

According to the invention, the deposition parameters are adjusted at least one layer chosen to cause incorporation of hydrogen in the material of this layer. Incorporation diffusion of hydrogen is adjusted according to the quantity of hydrogen that we want to see released by the material during screen operation, i.e. depending on the quality of the vacuum in the inter-electrode space, in particular the partial pressure of the oxidizing species, and of the means excitation chosen for the hydrogen source.

According to a third embodiment, the source of hydrogen is made up of dedicated zones, arranged outside the active area of the screen, for example, on the periphery of the anode. An excitation source is then produced on the cathode side opposite of these dedicated areas. The excitation source can be constituted of a microtip zone next to the hydrogen source outside the active area of the screen.

If such an embodiment requires modifying the structure of the screen, it has the advantage of providing a means antioxidant controllable independently of the functioning of the screen. Thus, it can be expected that the dedicated excitation source is ordered at regular intervals to cause regeneration microtips. We can also foresee that this dedicated source be controlled from a current measurement flowing through the cathode conductors to cause a microtip regeneration phase according to a threshold of current from which it is considered desirable to regenerate the microtips.

Several examples of materials will be indicated below. which can be chosen to constitute the source hydrogen.

The deposition of the various layers used in manufacturing of a screen is generally carried out by a chemical deposit plasma assisted vapor phase (PECVD). Such a deposit method uses mixtures of precursor compounds of the material to deposit. It is easy to control the content of added hydrogen to the precursors. This technique allows obtaining deposits highly hydrogenated and easily control the amount of hydrogen by varying the deposition parameters (temperature of deposition, self-bias voltage, deposition pressure, temperature annealing, etc.).

Among the materials that are likely to be deposited with a high percentage of hydrogen and lose this hydrogen under thermal, ionic or electronic activation, we find in particular silicon-based materials hydrogenated, hydrogenated silicon carbide, nitride hydrogenated silicon, hydrogenated silicon oxide, carbon hydrogenated, hydrogenated germanium and hydrogenated oxynitride.

The choice of material used depends, in particular, on the source of hydrogen.

If the hydrogen source is produced on the cathode side, we will be able to hydrogenate the silicon usually constituting the resistive layer 11 which dispenses hydrogen.

If the hydrogen source consists of the bands insulators 8 between the strips of phosphor elements of the anode, we will choose a material that is both dielectric and easily hydrogenated, for example, silicon carbide or silicon oxide. We can also choose nitride of silicon which also has the advantage of minimizing oxygen contained in the insulating strips so that the hydrogen released has the task of reducing the degassed oxidizing species essentially by the phosphor elements.

When this is compatible with the role of the layer chosen to also constitute the hydrogen source, we preferably choose an amorphous compound insofar as it can generate a significant amount of hydrogen because its concentration is not limited by a crystal structure.

You can also combine the antioxidant effect with a matrix anode effect which improves the contrast of the screen. Such mastering is generally designated by its Anglo-Saxon designation "black matrix" and creates black areas between the strips of phosphor elements of the anode. For this do, we will use, for example, a carbon-based compound hydrogenated to make the strips 8.

Of course, the present invention is capable of various variants and modifications which will appear to the man of art. In particular, the adaptation of the manufacturing process of a flat screen to implement the present invention is to the scope of the skilled person according to the indications functional data given above.

In addition, although the invention has been described above in conjunction with a microtip color screen, it applies also on a monochrome screen. If the anode of such a monochrome screen consists of two sets of alternating bands phosphor elements, all embodiments described above can be implemented. On the other hand, if the anode of the monochrome screen consists of a phosphor plan, the hydrogen source will be constituted either by a dedicated source external to the active area of the screen, either by the resistive layer cathode side.

Claims (6)

1. A flat display screen including a cathode with microtips for the electron bombardment of an anode (5) having phosphor elements (7), the anode (5) and the cathode (1) being separated by a vacuum space (12), and containing a progressive hydrogen release source characterized in that said source is comprised of a thin layer of a hydrogenated material.
2. A screen according to claim 1, characterized in that the hydrogen source is comprised of a resistive layer (11) of the cathode (1) on which the microtips (2) are arranged.
3. A screen according to claim 1, characterized in that the hydrogen source is comprised of insulating bands (8) separating bands of phosphor elements (7) from the anode (5).
4. A screen according to claim 1, characterized in that the hydrogen source is implemented at the circumference of the active area of the anode (5) carrying the phosphor (7), a source for energizing the hydrogen source being implemented, on the cathode side (11), facing the hydrogen source.
5. A process for manufacturing a flat display screen, including a cathode with microtips for the electron bombardment of an anode (5) having phosphor elements (7), the anode (5) and the cathode (1) being separated by a vacuum space (12) characterized in that the process includes the step of hydrogenating during its deposition at least one of the thin layers formed inside the screen.
6. A process according to claim 5, characterized in that said layer is obtained by plasma-enhanced chemical vapor deposition from at least one hydrogen-enriched precursor.

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