FR2784225A1 - SOURCE OF ELECTRONS WITH EMISSIVE CATHODES COMPRISING AT LEAST ONE ELECTRODE FOR PROTECTION AGAINST INTERFERENCE EMISSIONS - Google Patents

Abstract

The invention concerns an electron source with field-emission cathodes having a first, so-called active region (32), with a cathode support (24) equipped with field effect elements, at least an insulating layer (34) and at least a gate layer (36), the insulating and gate layers successively covering the cathode support and enclosing said field effect elements, and having a second so-called edge region (38), peripheral to the whole of the first region (32) and without field effect elements. The invention is characterised in that the electron source further comprises at least a conductive material layer (40) called shield layer, insulated from the gate and arranged over all or part of the edge region.

Description

SOURCE OF ELECTRONS WITH EMISSIVE CATHODES

  COMPRISING AT LEAST ONE ELECTRODE FOR PROTECTION AGAINST

PARASITE EMISSIONS

DESCRIPTION

  Technical Field The present invention relates to an electron source with emissive cathodes. Emissive cathodes are generally in the form of a plurality of microtips, or studs, which are

  used as electron guns.

  The microtips, or studs, can be directed to an acceleration anode lined with luminescent material. This material, in response to electrons, emits visible light usable for display. The microtips, or studs, can also be directed towards an anode equipped with a material capable of emitting X-radiation in response to

excitation of electrons.

  Also, the invention finds applications in the manufacture of display devices with flat display screens, in the production of X-ray tubes or any other device comprising a source of electrons with microtips, or with studs, or so more general a source of electrons with elements

field effect.

  STATE OF THE PRIOR ART FIG. 1 is a schematic and simplified section of a microtip display device of known type. For the sake of clarity of the figure a

  only microtip is shown.

  The actual display device however includes a plurality of image points or "pixels" and each pixel can be equipped with one or more microtips. The display device of FIG. 1 comprises an electron source 10, called a source with emissive cathodes and a luminescent screen 12 arranged

  opposite the electron source 10.

  The screen 12 comprises a transparent substrate 14, covered with a transparent conductive layer 16 forming an anode, such as a layer of ITO (Indium-Tin Oxide) for example, and a layer 18 of luminescent material . The luminescent material emits a

  light in response to electron bombardment.

  The application of a potential difference between certain emissive cathodes, in this case certain microtips, and the screen 12 makes it possible to selectively illuminate specific areas of

screen to display an image.

  The electron source 10 comprises a cathode conductive support 24 comprising, in the example illustrated, an electrically conductive layer 26 and an electrically resistive layer 28 which supports microtips, only one of which is shown with the

reference 30.

  In a first region 32, the cathode conductive support 24 is covered in order by an insulating layer 34 and a conductive layer 36, called the gate layer. These layers surround the microtip 30 without covering it so as to allow

  the emission of electrons to the screen 12.

  The grid layer 36, when brought to an appropriate potential, can be used to modulate the intensity of an electron beam emitted by a

microtip it surrounds.

  Furthermore, according to certain possible embodiments of the electron sources, the gate layer, as well as the conductive layer 26 of the cathode support can be arranged in the form of crossed conductive tracks. These conductive tracks then form addressing lines and columns for picture elements which coincide with intersection areas of these tracks. It is also in

  these areas that are formed by microtips, that is

  say the electron emitters. In the rest of the text, an active region denotes a region encompassing all of the electron emitters located opposite

  a display area on the luminescent screen.

  In the example of FIG. 1, the first region 32 can be considered in this sense as

  being part of the active region.

  The active region is adjacent to one or more regions, called border regions. The border regions are generally located at the periphery of the active region and are devoid of microtips. In FIG. 1, a region 38, devoid of microtips and of a grid layer, corresponds to a border region. The area of the border region

  is formed by the insulating layer 34.

  For a more detailed description of the sources

  of electrons with emissive cathodes, and display screens which are equipped therewith, reference may be made to documents [1], [2], [3] and [4] whose references

  are specified at the end of this description. The

  document [4] describes an X-ray tube comprising a

  source of microtip electrons.

  Despite the absence of emissive cathodes, that is to say microtips in the border region, it is possible to observe in this region an emission of stray light. The mechanisms behind this

  lens flare are poorly understood.

  SUMMARY OF THE INVENTION An object of the present invention is to propose a source of electrons with emissive cathodes making it possible to avoid the problem mentioned above, that is to say making it possible to greatly reduce or cancel the phenomenon of spurious emission in the region or regions of

border.

  Another object is to propose such a source of electrons which is inexpensive to implement and does not require manufacturing process steps.

  additional compared to conventional sources.

  Another aim is also to provide display screens or X-ray tubes equipped with such a

source of electrons.

  To achieve these goals, the invention more specifically relates to a source of electrons with emissive cathodes comprising in a first region, called active, a cathodic support equipped with field effect elements, at least one layer of insulation and at least one grid layer, the insulation and grid layers covering in order the cathode support and surrounding said field effect elements, and comprising in a second region, called border, adjacent to the first region, the cathodic support devoid of field effect elements, and the insulating layer with a free surface. According to the invention, the electron source further comprises at

  minus a layer of conductive material called "layer-

  screen ", isolated from the grid and deposited above the insulation layer over all or part of the border region. The screen layer constitutes an electrode for

  protection against parasitic emissions.

  The emissive cathodes are formed by the field effect elements which are for example microtips. The field effect elements are carried by the cathode support, which can be arranged so as to form addressing columns of a matrix. The cathode support may be formed by a layer of electrically conductive material, designated in the following text by conductive layer. It can also comprise, according to an advantageous embodiment, a layer of resistive material such as a silicon layer which connects the conductive layer to the microtips. Furthermore, the conductive layer can be cut into a plurality of parallel tracks which form the addressing columns already mentioned. In this case, the grid layer can also be arranged in the form of tracks which are substantially parallel to one another, forming an angle with the tracks of the layer.

  conductive and forming address lines.

  Generally the cathodic support is

  arranged on a substrate, for example glass.

  The screen layer avoids a localized accumulation of charges on the surface of the

  insulating layer in the border region (s).

  It thus reduces the risk of the formation of an electric arc between the insulating layer and a luminescent screen or an X-ray emitting screen possibly associated with the electron source. Emission of stray radiation or light is

thus avoided.

  The efficiency of the screen layer can possibly be improved by polarizing it to the potential of the cathode support. Thus it can be connected to the cathode support either directly, or by means of a current limiting means such

than an electrical resistance.

  The invention also relates to a display device comprising an electron source with emissive cathodes, and a luminescent screen forming an anode, furnished with at least one luminescent material, and associated with the electron source to face the

emissive cathodes.

  Depending on the structure of the luminescent screen, the screen layer of the electron source can be located opposite an insulating region of the luminescent screen, opposite a conductive region polarized at an acceleration voltage of the electrons (anode) or, opposite a conductive region of the luminescent screen, connected to a cathode voltage terminal, that is to say

  polarized at a voltage close to the cathode potential.

  The screen layer can also be located opposite a plurality of regions of different types

mentioned above.

  The choice of the structure used for the luminescent screen can be adapted as a function of the voltages used, in order to increase the efficiency of the screen layer. Finally, the invention also relates to an X-ray emission tube comprising an electron source with emissive cathodes, and an anode plate, made of a material capable of emitting X-rays in response to impacts of electrons. , the anode plate being associated with the electron source to face the emissive cathodes. In this X-ray tube, the electron source conforms to the

description above.

  Other features and benefits of

  the invention will emerge from the description which follows, in

  reference to the figures of the accompanying drawings. This

  description is given for illustrative purposes only and

not limiting.

Brief description of the figures

  Figure 1, already described, is a schematic, simplified and partial section of a device

classic display.

  Figures 2 and 3 are schematic, simplified and partial sections of a device

  display according to the invention.

  Figure 4 is a schematic, simplified and partial section of an X-ray tube according to

the invention.

  Figures 5 and 6 are simplified top views of smaller sources

  of electrons according to the invention.

  Detailed description of procedures for implementing

  The invention For reasons of simplification, the examples described below relate to particular embodiments in which the elements

  field effect are microtips.

  The invention as defined by

  claims however is not limited to these

  achievements. Parts of Figures 2, 3 and 4, identical, similar or equivalent to parts of Figure 1 already described, are marked with the same references, to simplify reading. We can therefore, for these

  parts, also refer to the description which

  precedes. Furthermore, it should be emphasized that the figures do not represent actual display devices, but are simplified views making it possible to highlight the main

characteristics of the device.

  FIG. 2 shows a display device with a luminescent screen 12 associated with an electron source 10. A border 11 makes it possible to seal the screen on the electron source 10 so as to maintain a controlled atmosphere, such as a hole

  for example, between the source and the screen.

  The electron source comprises a conductive layer 26, made of an electrically conductive material, such as niobium for example, and a layer 28 made of an

  resistive material, such as doped amorphous silicon.

  These layers form a support, called cathode support 24, to receive the microtips 30 (including

only one is shown).

  The microtips are made of an electron emitting material such as molybdenum and are electrically connected to the conductive layer 26

  through the resistive layer 28.

  An insulating layer 34 covers the layer

  resistive 28 around the microtips.

  In a first region 32 of the source, called the active region, a layer of conductive material such as niobium is formed on the insulating layer so as to surround the microtips. This

layer forms a grid 36.

  In a second region 38, called the border region, the cathodic support is devoid of microtips and entirely covered by the layer

insulating 34.

  In this region, in accordance with the invention, a screen layer 40 is formed on the surface of the insulating layer 34 facing the screen 12, so as to

cover it at least in part.

  The screen layer is for example a niobium layer, with a thickness of between a few tens of nanometers and a few micrometers, preferably 400 nm. This layer can be formed by evaporation and be shaped by etching.

reactive ionic.

  Advantageously, the screen layer 40 and the grid 36 can be produced by etching in a

  same initial layer of conductive material.

  Only a modification of the etching mask used for the formation of the source of FIG. 1 is then necessary to separate the screen layer in the border region and the grid in the active region. Thus, no additional process steps

is not required.

  The screen layer is separated from the grid in the active region, by a distance of the order of 0.1 mm and can extend as far as the sealing edge 11. The screen layer can be polarized by connecting it to a cathode terminal K, such as for example the cathode support 24. In the example of the figure, the screen layer is polarized by means of a

  resistance R represented symbolically.

  A plurality of disjoint screen layers can be formed in multiple border regions

  adjacent to the active region. In this case, the diapers-

  screen are preferably brought to the same polarization potential. In the example of FIG. 2, the emissive cathodes of the active region are turned towards a layer of luminescent material 18 formed on a transparent conductive layer 16. The transparent conductive layer and the layer of luminescent material are deposited on a transparent support 14 already

  described with reference to Figure 1.

  However, it is observed that the layer of luminescent material 18 does not extend in the border region so that the screen layer 40 of the source 10 is opposite the transparent conductive layer 16 polarized at a potential. anode A. FIG. 3 shows an alternative embodiment of the display device in which both the layer of luminescent material 18 and the transparent conductive layer 16 are limited to the active region 32. Thus the screen layer in the border region 38 faces the insulating and transparent material

of the support 14.

  According to yet another variant, illustrated in broken broken line in FIG. 3, a conductive layer 42 can be formed on the support 14 of the screen 12 in the border region, to face the screen layer 40 of the source of electrons. This layer 42 can be connected to a terminal brought to a potential close to or equal to the cathode potential K. FIG. 4 is a simplified schematic view of an X-ray tube comprising an electron source

according to the invention.

  The electron source 10 is substantially identical to the sources described with reference to the figures

  2 and 3 and we can therefore refer to it.

  The luminescent screen 14 of FIGS. 2 and 3 is however replaced by a plate 15 of electrically conductive material, such as Mo or Cr connected to the potential of anode A. This plate when it is locally bombarded by electrons coming from the microtip (or other microtips not shown) is capable of locally emitting X-radiation. The plate 15 and the electron source 10 are contained in a tube 50, shown diagrammatically

  in broken lines, and in which a vacuum is established.

  Figures 5 and 6 are top views of electron sources that can be used in

  devices according to figures 2, 3 or 4.

  By analogy with the previous figures, in Figures 5 and 6, the references 28, 32, 34 and 40 respectively denote the resistive layer, the first active region comprising the microtips, the

  insulating layer and the screen layer.

  References 126 and 136 correspond to connection outputs of the cathode conductors and / or

grids.

  It can be observed in FIGS. 5 and 6 that the screen layer 40 can be in one (FIG. 5) or

several parts (Figure 6).

  Documents cited [1] "Microtips Fluorescent Display" by R. Meyer, A. Ghis, PH. Rambaud and F. Muller, pages 513-516, Japan Display'86, September 30-October 20, 1986, Tokyo

[2] FR-A-2 702 869

[3] US-A-5,191,217

  [4] French patent application n EN 97 07342 (Robert

BAPTIST).

Claims (12)

  1. Electron source with emissive cathodes comprising in a first region (32), called active, a cathode support (24) equipped with field effect elements (30), at least one insulating layer (34) and at least one grid layer (36), the insulation and grid layers covering in order the cathodic support and surrounding said field effect elements, and comprising in at least a second region (38), called border , adjacent to the first region, the cathode support (24) devoid of field effect elements, and the insulating layer (34), characterized in that the electron source further comprises at least one layer of material conductor (40) called screen layer, insulated from the grid and arranged above the insulation layer on all or part of the border region.   2. Electron source according to claim 1 in which the screen layer (40) is connected   electrically to the cathodic support.   3. Electron source according to claim 2 in which the screen layer (40) is electrically connected to the cathode support by   through current limiting means.   4. Electron source according to claim 1 in which the screen layer (40) is a layer of niobium (Nb).   5. Electron source according to claim 1 in which the screen layer has a thickness of between a few tens of nanometers and a few micrometers.   6. Electron source according to claim 1 comprising several border regions, each covered by a screen layer, the screen layers covering the border regions being electrically connected to the same potential.   7. Display device comprising an electron source (10) with emissive cathodes, and a luminescent screen (12), forming an anode, furnished with at least one luminescent material and associated with the electron source to face the emissive cathodes (10), characterized in that the electron source is   according to any of the claims previous.   8. Display device according to claim 7, in which the luminescent screen (12) comprises an electrically insulating region facing said border region of the source. of electrons.   9. Display device according to claim 7, in which the luminescent screen comprises an electrically conductive region (16), connected to an anode potential terminal, and facing said border region of the electron source. 10. Display device according to claim 7, in which the screen (12) comprises an electrically conductive region (42) connected to a cathode potential terminal and facing said   border region of the electron source.   11. Display device according to one   any of the preceding claims, wherein   the field effect elements are microtips.   12. X-ray emission tube comprising an electron source (10) with emissive cathodes (30), and an anode plate (15), made of a material capable of emitting X-rays in response to impacts of electrons, the anode plate being associated with the electron source to face the emissive cathodes, characterized in that the electron source   is in accordance with one of claims 1 to 6.