EP0806787A1 - Fabrication of an anode of a flat viewing screen - Google Patents

Abstract

The screen has a cathode composed of microdots (2) for electron bombardment of an anode (5) provided with luminophores (7) of the primary colours on strips (9) of indium-tin oxide deposited on the glass (6). The strips are separated by insulation (20) whose surface (21) has a secondary emission coefficient less than 1 at least within the range of primary electron energies. Each insulating strip of e.g. silicon dioxide has sufficient resistivity to prevent breakdown between two neighbouring luminophores. Its surface may have a coating of low-emission material (e.g. silicon) to a thickness of about 100 Angstrom with a gap of 5-10 mu m at each edge.

Description

The present invention relates to flat display screens, and more particularly to so-called cathodoluminescence screens, the anode of which carries luminescent elements, separated from each other by insulating zones, and capable of being excited by electronic bombardment. This electronic bombardment requires that the luminescent elements are polarized and can come from microtips, from layers with low extraction potential or from a thermionic source.

To simplify the present description, below only the color microtip screens will be considered, but it will be noted that the invention relates, in general, to the various types of screens mentioned above and the like.

Figure 1 shows the structure of a color microtip flat screen.

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

The operating principle and a particular embodiment of a microtip screen are described, in particular, in the American patent n ° 4 940 916 of the French Atomic Energy Commission.

The cathode 1 is organized in columns and consists, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. The microtips 2 are produced on a resistive layer 11 deposited on the cathode conductors and are arranged inside the meshes defined by the cathode conductors. Figure 1 partially shows the interior of a mesh and the cathode conductors do not appear in this figure. The cathode 1 is associated with the grid 3 organized in lines. The intersection of a line of the grid 3 and a column of the 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 phosphor elements 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 phosphor elements 7 opposite of each of the colors.

The command to select the phosphor 7 (the phosphor 7g in FIG. 1) which must be bombarded by the electrons coming from the microtips of the cathode 1 means that selectively, the polarization of the phosphor elements 7 of the anode 5, color by color.

Generally, the rows of the grid 3 are sequentially polarized at a potential of the order of 80 volts, while the strips of phosphor elements (for example 7g in FIG. 1) to be excited are polarized under a voltage of the order 400 volts via the ITO strip on which these phosphors are deposited. The ITO bands, carrying the other bands of phosphor elements (for example 7r and 7b in FIG. 1), are at low or zero potential. The columns of cathode 1 are brought to respective potentials between a maximum emission potential and a non-emission potential (for example, 0 and 30 volts respectively). The brightness of a color component of each of the pixels of a line is thus fixed.

The choice of the values of the polarization potentials is linked to the characteristics of the phosphor elements 7 and the 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 they suffer from a short lifespan, that is to say that after a relatively short operating time (of the order of a hundred hours) , the screen brightness decreases considerably and we sometimes even see destructive phenomena appearing due to the formation of arcs between the cathode and the screen anode.

In addition, after a certain operating time, it can be seen that the color varies and no longer corresponds to the screen control instructions. This phenomenon will be called here "color drift". In practice, this means that at least one of the bands of phosphor material adjacent to the polarized bands begins to exhibit luminescence.

The origin of this phenomenon was, until now, poorly understood. We thought it was due to the fact that electrons accumulate on the insulating zones 8 between the strips of phosphor material and provide conduction to neighboring strips. To avoid this phenomenon, various techniques have been proposed in the prior art, one of which consists in separating the polarizations of the anode bands between two successive color subframes by brief time intervals, and in applying a voltage pulse. negative on the strip which has just been polarized before positively polarizing the next anode strip to be excited.

However, this method has the disadvantage of being relatively complex to implement since it complicates the supply of the anode supply voltages, which are voltages of high values (a few hundred volts) and that it harms to the brightness of the screen.

The object of the present invention is to propose a new solution to the abovementioned problems of screen life and color drift.

To achieve this object, the present invention provides a flat screen display anode of the type comprising at least two sets of alternating parallel strips of anode conductors coated with phosphor elements intended to be excited by primary electrons, said strips being separated from each other by insulation bands comprising, at least at the surface, a material having a secondary emission coefficient less than or equal to unity, at least in the energy range of said primary electrons.

According to an embodiment of the present invention, said material has a maximum secondary emission coefficient less than one.

According to an embodiment of the present invention, said insulation strips consist of a single layer of a material having a secondary emission coefficient less than unity and having a resistivity sufficient to withstand a determined potential difference between two neighboring bands of phosphor elements.

According to an embodiment of the present invention, said insulation strips consist of a first thin layer of insulating material covered with a second very thin layer of material whose secondary emission coefficient is less than unit.

According to an embodiment of the present invention, the second layer has a width less than that of the first layer to allow to remain, on either side of the second layer, an insulating space.

According to an embodiment of the present invention, said material constituting the second layer is chosen to have sufficient resistivity to withstand a determined potential difference between two neighboring strips of phosphor elements.

According to an embodiment of the present invention, said second layer is made of a conductive material, the thickness of the second layer being chosen so that it has sufficient resistance to withstand a determined potential difference between two neighboring strips of elements phosphors.

According to an embodiment of the present invention, said material with a secondary emission coefficient of less than one is chosen from chromium oxide and iron oxide.

According to an embodiment of the present invention, said material constituting the second layer is graphite carbon.

According to an embodiment of the present invention, said insulation strips are made of silicon oxide, the surface of which has been conditioned to develop a very thin layer of silicon.

According to an embodiment of the present invention, said second layer of the insulation bands is biased at a negative or zero potential.

The invention also relates to a flat display screen of the type comprising a microtip cathode and an anode made up of at least two sets of alternating strips. phosphor elements, said anode comprising insulating strips according to one of the abovementioned embodiments.

The present invention originates from an interpretation of the phenomena which give rise to the abovementioned problems in conventional screens.

The inventors consider that these problems are due, in particular, to a phenomenon of secondary emission occurring at the surface of the anode.

Figure 2 shows, schematically and in cross section, three bands of phosphor elements of an anode separated by an insulator.

For reasons of clarity, the various constituents represented in FIG. 2 will be designated by the same references as in FIG. 1. Thus, three bands, respectively 7b, 7g and 7r of phosphor elements of different colors are deposited on corresponding bands , respectively 9b, 9g and 9r of ITO, themselves deposited on a glass substrate 6 constituting the surface of the screen.

When the band 9g is polarized at 400 volts, the bands 9b and 9r are not polarized and so-called "primary" electrons ei, emitted by the microtips (not shown) of the cathode, arrive on the phosphor elements 7g. So-called electron "secondary" _century are re-emitted by the phosphor elements 7g. In addition, a number of primary electrons arrive on the edge of the insulating strips 8 separating the strip 9g from the strips 9b and 9r. Here, too, there is an emission of secondary electrons.

Any material has a secondary emission coefficient, called δ, which represents the average number of secondary electrons re-emitted for an incident electron arriving on this material. The predominant energy of the statistical distribution of secondary electrons is of the order of 30 to 50 eV, whatever the energy of the incident electrons.

The secondary emission factor of a material varies depending on the energy of the electrons touching its surface. In the case of microtip screens, the energy of primary electrons is related to the polarization potential of the anode and is, for example, of the order of 400 eV.

When the secondary emission coefficient δ is greater than 1, this means that the surface of the material re-emits more electrons than it receives and tends to charge positively. Conversely, when the secondary emission coefficient δ is less than 1, there is an accumulation of electrons.

The fact that microtip screens are produced using technologies derived from those used in the manufacture of integrated circuits, has led to the use of silicon oxide to produce the insulating strips 8. In fact, silicon oxide constitutes a common material and which we master well the use. Unfortunately, silicon oxide has a particularly high secondary emission coefficient.

FIG. 3 illustrates the characteristic of the evolution of the secondary emission coefficient of silicon oxide (SiO ₂ ) as a function of the energy of the incident electrons in eV.

Whatever the material, this characteristic has the shape of a bell, that is to say that the coefficient δ begins to increase until it reaches a level δ _max for an amount of energy U _max then decreases towards a value d 'asymptote.

The phosphor elements generally have a coefficient δ _max of the order of 2 to 2.5 for an energy U _max of the order of 500 eV.

For silicon oxide, δ _max is around 3 for an energy U _{max around} 400 eV. Conventional screens therefore operate in the maximum secondary emission region and the primary electrons which manage to touch the silicon oxide of the bands 8 generate a significant emission of secondary electrons.

The consequence of this secondary emission phenomenon on a microtip screen anode is as follows.

Initially, the tracks 8 of silicon oxide insulating material are at zero potential. The primary electrons which arrive on the edges of the neighboring insulating tracks of a polarized strip (for example 9 g) entail, by the emission of secondary electrons, a positive charge on the surface of the silicon oxide. As the screen operates, this positive charge area grows, as the primary electrons are more and more attracted to the surface as its positive charge increases, which causes a decrease in the brightness of the polarized 7g strip. The positive charge zone propagates towards the neighboring non-polarized tracks 9b and 9r and its potential can exceed the polarization potential of the anode strips.

Secondary electrons re-emitted by the phosphor elements 7g are then attracted to this zone of positive charge, which increases the phenomenon.

In addition, the surface potential of an insulating strip 8 can become such that it causes the formation of a destructive arc between the anode and the cathode.

In addition, and although the silicon oxide and the phosphor elements have a secondary emission coefficient less than 1 for an energy of the order of 30 to 50 eV which corresponds to the energy of the majority of secondary electrons, the emission of a secondary electron in turn gives rise to a re-emission of secondary electrons, which results in an avalanche effect.

Indeed, some secondary electrons have sufficient energy, the value of 30 to 50 eV corresponding to the maximum amount of a statistical distribution.

In addition, the transverse electric field between two bands of phosphor elements, respectively polarized and non-polarized, accelerates the secondary electrons which then have an energy very much higher than their initial energy (of the order of 250 eV).

As the phosphors are relatively insulating materials (they generally have a linear resistance of the order of 10 ⁸ Ω.cm), they do not discharge completely when the ITO band which supports them is no longer polarized but remains charged at a potential, generally of the order of 50 volts. Thus, the phosphors of a non-polarized band end up being excited by the secondary electrons re-emitted by the insulating tracks 8.

The secondary electron emission phenomenon has a second drawback in microtip screens. Indeed, when electrons come into contact with the material of layer 8, they can either generate a positive ion or desorb a neutral species (any molecule stuck on the surface of track 8) or even strike a neutral species and then generate a positive ion. This phenomenon leads to the formation of a microplasma on the surface of track 8. The microdots of the cathode then attract the positive ions from this plasma and are polluted by these positive ions.

In addition, these plasmas generally emit radiation. These rays translate into a bluish glow that can be seen through the surface of the screen. In addition, positive ions are likely to excite the phosphor elements of the neighboring band (non-polarized) by photoluminescence.

This phenomenon of secondary electron emission is a known phenomenon, in particular, in cathode-ray tubes where the surface of the screen also carries phosphors which are bombarded by an electron gun.

In the case of cathode ray tubes, the problem due to the phenomenon of secondary emission is resolved by coating the phosphors with a metallization, generally a thin layer of aluminum, polarized at a high positive voltage. The role of this metallization is, on the one hand, to polarize the phosphors and, on the other hand, to drain the primary charges which are not consumed as well as the secondary charges which are then collected.

This solution cannot be applied to microtip screens for several reasons.

First, it is undesirable to coat the phosphors of a microtip screen with a metallic layer. because of the relatively low energy of the primary electrons. Indeed, in a cathode ray tube, the electrons emitted by the electron gun have an energy of the order of 20 to 30 keV and therefore cross the thin metallization layer while the secondary electrons of low energy (30 eV) are collected by this metallization. In a microtip screen, the energy of the primary electrons (of the order of 400 eV) is not sufficient.

Second, in the case of a color cathode ray tube, all the phosphors are polarized to the same potential by this single layer of aluminum, whatever their color. Conversely, in the case of a color microtip screen, the anode is made up of sets of alternating parallel strips polarized by sets of strips of the same color. The strips of phosphor elements must therefore be isolated from each other to allow the screen to function.

From this analysis, the present invention proposes to suppress the appearance of the phenomenon of secondary emission on the anode of a flat display screen.

• les figures 1 et 2 qui ont été décrites précédemment sont destinées à exposer l'état de la technique et le problème posé ;
• la figure 3 représente des caractéristiques du coefficient d'émission secondaire en fonction de l'énergie d'électrons incidents pour différents matériaux ; et
• la figure 4 représente un mode de réalisation d'une anode d'écran plat de visualisation à cathodoluminescence selon l'invention.

These objects, characteristics and advantages, as well as others of the present invention will be explained in detail in the following description of particular embodiments given without limitation in relation to the attached figures among which:

• Figures 1 and 2 which have been described above are intended to show the state of the art and the problem posed;
• FIG. 3 represents characteristics of the secondary emission coefficient as a function of the energy of incident electrons for different materials; and
• FIG. 4 represents an embodiment of a cathode-luminescence flat display screen anode according to the invention.

A characteristic of the present invention is to select a surface material, for the insulating tracks separating two bands of phosphor elements from an anode provided with sets of alternating bands of phosphor elements, from materials with a low secondary emission coefficient δ.

According to the invention, the material is chosen so that its secondary emission coefficient is less than 1, at least in the energy range of the primary electrons emitted by the microtips.

The material chosen must comply with certain conditions inherent in the operation of a flat display screen of this type. In particular, this material must respect the requirements for isolation between the strips of phosphor elements of the anode, that is to say that it must withstand a potential difference of approximately 500 volts without driving (it i.e. with a low leakage current).

If necessary, a metallic material can be chosen which will then be deposited in a very thin layer to have sufficient resistance between the strips of phosphor elements. It may also be a dielectric (metal oxide), reduced so that it only has metal on the surface.

According to a first embodiment (not shown), the insulating layer, generally made of silicon oxide, is replaced by a layer of a material whose secondary emission coefficient is less than 1, at least in the energy range incident electrons emitted by the cathode (not shown). However, a material will then be chosen whose resistivity is sufficient, even though it is deposited over a thickness of the order of 1 μm.

FIG. 4 represents a second embodiment of the invention. According to the invention, the ITO strips 9 of the anode 5 ′ are separated by insulation strips 20 constituted by a first insulating layer 8 ′ (with a thickness of a few microns or even less), for example made of silicon oxide, covered with a second very thin layer 21 (of a thickness less than a μm) made of a material, having a secondary emission coefficient less than 1.

An advantage of this second embodiment is that the resistivity of the material is much easier to control on such a very thin layer. To improve the insulation between the strips of phosphor elements, provision may be made for the width of the second layer 21 to be less than the width of the first layer 8 ′ in order to allow the remaining on each side of the layer 21 , an insulating space (with a width of the order of 5 to 10 μm).

As the surface material 21 of the strips 20 has a secondary emission coefficient less than 1, it becomes negatively charged, as the screen operates, when the edge of the surface 21 receives primary electrons from microtips (not shown). This negative charge leads to the fact that the electrons are, unlike conventional screens, more and more repelled by the insulation bands 20.

This negative charge increases to a charge equilibrium point due to the positive polarization of the neighboring phosphor band.

It will be noted that this charge balance is effected with the polarized strip insofar as the other strip of phosphor elements, close to the track of the secondary material, is at a much lower potential (of the order of 50 volts) .

However, such a balance depends on the resistivity of the material 21 of the surface of the strips 20 and is difficult to control.

As a variant, the secondary bands 21 deposited on the silicon oxide are polarized at zero or negative potential. The resistance of the secondary bands is not a problem with regard to such polarization. In fact, the current flowing is very low and there are therefore few resistive losses. The potential drop caused by the band resistance on the bias is small.

An advantage of such a variant is that it makes it possible to control the negative charge level of these strips 21 and, thus, to guarantee that there is no destructive effect of the screen by a current flowing from a non-visible strip. -polarized.

An advantage of the present invention is that it eliminates any phenomenon of color drift.

Another advantage of the present invention is that it eliminates the formation of microplasmas between the strips of phosphor elements 7 and thus avoids pollution of the microdots of the cathode (not shown).

Another advantage of the present invention is that the accumulation of negative charges between the bands of phosphor elements constitutes a focusing barrier towards the polarized bands.

Four examples of materials which can be chosen to coat the surface of the first constituent layer of the strips 20 will be indicated below.

According to a first example, the surface of the first layer 8 ′ of silicon oxide (SiO ₂ ) is conditioned to develop, on the surface, a very thin layer 21 of silicon (Si) of the order of one hundred angstroms. Although silicon has a coefficient δ _max of the order of 1.1 (Figure 3), this δ _max corresponds to an energy U _max of the order of 250 eV and silicon has a coefficient of less than 1 for l energy of 400 eV of primary electrons.

It is therefore possible to choose, for layer 21, a material whose coefficient δ _max is only very slightly greater than 1, provided that its secondary emission coefficient δ is less than 1 in the energy range of the electrons primary from microtips. However, we prefer to choose a material whose coefficient δ _max is less than 1, insofar as this guarantees the absence of secondary emission independently of the energy of the primary electrons, that is to say independently of the values of polarization of the anode and the cathode.

In a second example, chromium oxide (Cr ₂ O ₃ ) is deposited by sputtering on the first layer 8 ′ of silicon oxide. This deposition is preferably carried out on a thickness of the order of 1000 to 2000 angstroms for a screen whose anode / cathode voltage is of the order of 500 volts. This gives an inter-track resistance of around 500 MΩ. The maximum secondary emission coefficient δ _max for chromium oxide is of the order of 0.95 for an energy U _max of the order of 300 eV (Figure 3).

According to a third example, iron oxide (Fe ₂ O ₃ ) is deposited by sputtering, on the layer 8 ′ of silicon oxide, whose maximum secondary emission coefficient δ _max is of the order of 0.9 for an energy U _max of the order of 350 eV. This deposition takes place over a thickness of the order of 1000 angstroms and the inter-track insulation resistance obtained is of the order of 500 MΩ.

According to a fourth example, graphite carbon (C) is deposited, the maximum secondary emission coefficient of which égal _max is equal to 1 for an energy U _max of the order of 300 eV, by sputtering on silicon oxide .

It will be noted that the implementation of the present invention is compatible with the small thicknesses (a few microns, or even less) of the constituent layers of the anode and with the conventional methods of deposition in thin layer (in particular insulation strips). which are generally used for the manufacture of conventional anodes.

Of course, the present invention is susceptible of various variants and modifications which will appear to those skilled in the art. In particular, the thickness of materials with a secondary emission factor less than 1 will be chosen according to the functional indications given above. Likewise, other materials than those mentioned above could be used to carry out the function of blocking the secondary emission and the methods of depositing these materials are within the reach of those skilled in the art.

Furthermore, the invention applies not only to a color screen, but also to a monochrome screen, the anode of which consists of two sets of alternating parallel strips of phosphor elements of the same color alternately polarized.

Claims (12)

Flat display anode (5 ′) of the type comprising at least two sets of alternating parallel strips (9) of anode conductors coated with phosphor elements (7) intended to be excited by primary electrons (ei), characterized in that said strips (9) are separated from each other by insulation strips (20) comprising, at least on the surface (21), a material having a secondary emission coefficient (δ) less than or equal to unity, at least in the energy range of said primary electrons (ei). Anode according to claim 1, characterized in that said material has a maximum secondary emission coefficient (δ _max ) less than unity. Anode according to claim 1 or 2, characterized in that said insulation strips (20) consist of a single layer of a material having a secondary emission coefficient (δ) less than unity and having sufficient resistivity to support a determined potential difference between two neighboring bands (7g, 7r) of phosphor elements. Anode according to claim 1 or 2, characterized in that said insulation strips (20) consist of a first thin layer (8 ') of an insulating material covered with a second very thin layer (21) of a material whose secondary emission factor (δ) is less than one. Anode according to claim 4, characterized in that the second layer (21) has a width less than that of the first layer (8 ') to allow an insulating space to remain on either side of the second layer. Anode according to claim 4, characterized in that said material constituting the second layer (21) is chosen to have sufficient resistivity to withstand a determined potential difference between two neighboring strips (7g, 7r) of phosphor elements. Anode according to claim 4, characterized in that said second layer (21) is made of a conductive material, the thickness of the second layer (21) being chosen so that it has sufficient resistance to withstand a determined potential difference between two neighboring strips (7g, 7r) of phosphor elements. Anode according to any one of claims 3 to 6, characterized in that said material with a secondary emission coefficient (δ) less than unity is chosen from chromium oxide (Cr ₂ O ₃ ) and oxide iron (Fe ₂ O ₃ ). Anode according to claim 4, characterized in that said material constituting the second layer (21) is graphite carbon (C). Anode according to claims 1 and 4, characterized in that said insulation strips (20) are made of silicon oxide (SiO ₂ ) whose surface has been conditioned to develop a very thin layer (21) of silicon (Si). Anode according to any one of claims 4 to 10, characterized in that said second layer (21) of the insulation strips (20) is polarized at a negative or zero potential. Flat display screen of the type comprising a cathode (1) with microtips (2) and an anode (5 ') according to any one of claims 1 to 11.

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