EP0625277B1 - Flat screen having individually dipole-protected microdots - Google Patents

Abstract

A flat screen having individually dipole-protected microdots and consisting of a field-emission cathode comprising microdots (12) individually protected by means of a series electrical coupling with a dipole (13) consisting of a depletion mode field effect transistor, said dipoles being designed to enable the protection threshold and the emission current level to be altered on all dots at once solely by changing the biasing of the substrate (14) common to said dipoles. Application in general to the field of display screens.

Description

The present invention relates to a screen microtip dish individually protected by dipole.

It generally concerns the field flat display or viewing screens matrix addressing of all sizes, and can apply to all industrial sectors using screens of this type: television, computers, telecommunications, control devices, monitoring facilities, etc.

Known microtip screens are vacuum tubes generally consisting of two plates of thin glass tightly sealed the plate rear or cathode plate comprising an array array of field effect transmitters formed by microtips, and the front plate or anode plate being covered with a transparent conductive layer and phosphors.

At each light point (pixel), is associated with a cathodic emissive surface located opposite screw and made up of a large number of microtips (approximately 10,000 per mm2). This emissive surface is defined by the intersection of a line (grid) and a column (cathode conductor) of the matrix.

Thanks to the short tip-grid distance (≤ 1 µm) and with the amplifying effect of the tip, a potential difference of less than 100 volts applied between row and column allows to get to the top of the peak, an electric field sufficient to cause electron emission and high luminance with a low voltage phosphor.

• Une couche d'isolation.
• Une couche résistive de silicium ou autre matériau.
• Les "conducteurs colonne" constitués d'une couche métallique qui peut être déposée soit dessous soit dessus la couche résistive.
• Une couche isolante (Si ou SiO2) qui constitue l'isolant de grille.
• Une couche métallique qui constitue la grille ou conducteurs de ligne.

The conventional structure of the cathode of a microtip screen comprises in particular, deposited successively on a glass or silicon substrate:

• A layer of insulation.
• A resistive layer of silicon or other material.
• The "column conductors" consist of a metal layer which can be deposited either below or above the resistive layer.
• An insulating layer (Si or SiO2) which constitutes the gate insulator.
• A metallic layer which constitutes the grid or line conductors.

After depositing the above layers, it is practiced in the grid and the insulating layer, by known engraving, holes in which are then made the microtips.

The main purpose of the resistive layer is to limit the current in each transmitter so to homogenize the electronic emission, and to limit the maximum current that would pass in the tip in case tip / grid short circuit.

The load characteristic which results from the placing in series, with the point, of a resistance is a line. The voltage drop in this resistance is proportional to the current flowing through it and can prove to be quite significant if the current emitted by the tip is important. The tension that must be applied to the tip-resistance protection system is increased by the same amount, which has consequences important on the consumption of the screen in particular.

The device according to the present invention proposes to solve these problems. It allows effect not only of getting an effective limitation of current flowing through each microtip by self-regulation of the emission current beyond a threshold, even if the tip is in direct contact with the grid, but also better homogeneity and efficient and simplified control of the screen luminance.

It consists of an emissive cathode field emission flat screen with microtips each protected individually thanks to an electrical coupling in series with a formed dipole a depletion field effect transistor. The current-voltage characteristic of such a dipole is not linear. These dipoles can be made of so that we can modify globally (on all points at the same time) the threshold of protection and the level of the emission current and therefore screen brightness, acting only on the polarization of the common substrate of these dipoles, or dipole groups.

EP-A-0496572 describes a microtip flat screen in which the addressing system is integrated. Tripole components such as bipolar or MOS transistors are associated with microtips to order them.

Article by U. Tietze and Ch. Schenk, Springer-Verlag 1980, pages 87-88, teaches the use of a MOS transistor as a power source.

la figure 1 est une coupe transversale illustrant le principe de fonctionnement d'un écran à micropointes connu,

la figure 2 est un schéma symbolique élémentaire d'une micropointe de la figure 1,

la figure 3 est un schéma symbolique élémentaire d'une micropointe protégée individuellement par un dipôle,

la figure 4 représente la coupe transversale d'une cathode émissive à micropointes utilisée dans l'invention,

et la figure 5 est une coupe partielle montrant en perspective le canal du transistor à effet de champ autour de la micropointe.

In the appended schematic drawings, given by way of nonlimiting example of one of the embodiments of the subject of the invention:

FIG. 1 is a cross section illustrating the operating principle of a known microtip screen,

FIG. 2 is an elementary symbolic diagram of a microtip of FIG. 1,

FIG. 3 is an elementary symbolic diagram of a microtip individually protected by a dipole,

FIG. 4 represents the cross section of an emissive cathode with microtips used in the invention,

and FIG. 5 is a partial section showing in perspective the channel of the field effect transistor around the microtip.

The basic principle of a screen microtips is shown schematically in Figure 1, on which we see successively from bottom to top (in back to front practice):

A plate 1 of glass or silicon, a coating sublayer 2, the cathode conductors or column conductors 3, a resistive layer 4, a insulating layer 5, the line or grid conductors 6, an empty space 7 and a layer of front glass 8 covered on its inner face with a conductive layer transparent constituting the anode 9, and phosphors 10.

An electron beam 11 emitted under vacuum by microtips 12 electrically connected to cathodic conductors and modulated by the potential of the grid 6 is accelerated towards the anode 9 where it excites the phosphors 10 (typical operation triode). Thanks to the short tip-anode distance, the focusing is obtained by proximity effect without no electronic optics.

In this type of cathode, each microtip 12 is protected against excess current by putting in series of a load resistor (fig 2). This resistance generally consists of a layer resistive 3 of amorphous silicon (or other material resistant).

In a flat screen with emissive cathode according to the invention, the protection of each microtip 12 is no longer by putting a series load resistance, but by the serialization of a dipole 13 whose voltage-current characteristic is not not linear. This dipole consists of a transistor with field effect (FET), preferably of the grid type depleted insulator, whose drain D is connected to the microtip 12 and source S to conductor column 3 corresponding, the door or "gate" G (or pinch electrode) of each transistor being directly connected either to the source S or to the drain D.

This arrangement allows for a complete protection of microtip 12 against frank short circuits between tip and grid 6 by complete blockage of current in the tip.

The dipoles 13 will advantageously manufactured in integrated technology, on a substrate 14 single silicon (solid or thin layer), so that we can, by polarizing said substrate, which may be common to all the dipoles 13, modify globally (on all points in same time) the protection threshold and the level of emission current (modulation of the brightness of screen).

As an example, Figure 4 shows a partial section of a microtip emissive cathode protected by dipoles 13, these being made at from a P-type substrate 14 in which are formed of N-type overdoped zones 15 obtained by diffusion or other (establishment) and constituting the sources, channel 20 (depletion transistor) formed by example by an N-type ion implantation, as well than a layer of grid insulation 16 made of silica obtained by surface oxidation or deposit. The electrode pinch 17 is created at the same time as the column conductor 3 by metallization. The point is carried out in the usual way, but is based on the transistor pinch gate. Preferably, drains located under the microtips are not overdoped, as usual in MOS structures classics.

The constituting field effect transistor the dipole 13 can advantageously have a circular geometry, its conduction channel being located all around microtip 12 (Figure 5).

The operation of dipole 13 is then next: The extraction voltage is applied to electrode 6 (grid). When this voltage is low (low enough for the voltage peak / source is less than the threshold of the transistor at depletion), the dipole in series with the tip 12 is, at roughly equivalent to the resistance of channel 20 implanted, its value is quite low. When the extraction voltage increases so that the tip / source voltage either around, or higher, at the threshold of said depletion transistor, the gate pinch 17 does its job and "pinch" channel 20 limiting the current in the dipole to a value (saturation current of the depletion transistor) which is, in the first order, more function than geometric dimensions of the assembly and of the tension of the substrate 14 relative to the source 15. The loss of voltage in the dipole no longer itself a function current in the tip, but only threshold voltage of said depletion transistor. In fact each point will be crossed by the current of saturation of the depletion transistor which protects it. The geometries of said transistors being identical, the currents in the points (whatever the specific emission characteristics) identical.

The emissive cathode can be itself realized on silicon in integrated technology. In this case, the column conductors 3, and possibly the line conductors (or grid 6) may be made up of diffused layers, buried or not, with as an alternative to double, in places, the layer diffused by a metallization (positioned in a uncluttered area for example or so minimize coupling capacities)

The positioning of the various elements constitutive gives the object of the invention a maximum useful effects that had not been, to date, obtained by similar devices.

Claims (5)

1. A flat microtip screen, characterized in that each microtip (12) is connected with a column conductor (3) through a dipole (13) constituted of a depletion field effect transistor having a drain connected with the microtip, a source connected to the column conductor and an insulated gate directly connected to said source or drain.
2. A flat microtip screen according to claim 1, characterized in that it comprises a silicon layer (14) of a first conductivity type and, in association with each microtip:

   a first region (15) of a second conductivity type having a high doping level formed in said layer (14) and connected with a cathode conductor (3);

   a second region (20) of the second conductivity type with a low doping level formed in said layer (14), adjacent to the first region (15) and coated with an insulating layer (16) provided with an aperture;

   a conductive layer (17) extending on said aperture and over said insulating layer (16) between said aperture and said first region; and

   a microtip (12) formed above said conductive layer.
3. A flat microtip screen according to claim 2, characterized in that the aperture, the microtip, the conductive layer, the second and first regions of the second conductivity type are concentric.
4. A flat microtip screen according to claim 2 or 3, characterized in that said column conductors are made of regions diffused in said silicon layer.
5. A flat microtip screen according to any of claims 2 to 4, characterized in that it further comprises means for modifying the substrate biasing of said depletion field effect transistors.

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