FR2798507A1 - Device for producing electric field between electrodes in field emission flat screen has series of metallic strips forming modulating electrodes, and controller applying potential difference between first and modulating electrodes - Google Patents

Abstract

Device includes means for applying potential difference between first and second electrodes (35, 37) so as to obtain, if potential difference is applied solely, pre-determined electric field value near first electrode (35). Device also has cathode plate (31) and anode plate (32). Cathode plate has glass support plate (33) on which is placed network of metallic strips (38, 39) forming modulating electrodes. Device also includes control means for applying potential difference between modulating electrodes and first electrode so as to obtain another pre-determined electric field value near first electrode. Independent claims are also included for (a) the production method of an electric field between first and second electrodes, for (b) the visualization screen, and for (c) the utilization method of a field emission screen.

Description

<B> DEVICE </ B> TO <B> PRODUCE AN </ B> ELECTRIC FIELD <B> MODULE TO THE <B> LEVEL <B> OF AN </ B> ELECTRODE <B> AND ITS APPLICATION TO < FIELD OF THE INVENTION The present invention relates to a device for producing an electric field modulated at an electrode. It applies in particular to flat screens with field emission.

 State of the Prior Art Field emission excited cathodoluminescence display devices are well known. Such a device comprises a cathode disposed opposite an anode. The cathode is a plane electron-emitting structure and the anode is another flat structure covered with a luminescent layer. These structures are separated by a space in which one has evacuated.

The cathode may be a microtip source or a source equipped with a low-output emissive material, for example carbon. Sources equipped with an emissive material are used in viewing devices that are generally in two forms: a diode type structure or a triode type structure.

Figure 1 shows, in cross section, a flat field emission screen operating in a diode type structure. The cathode 1 consists of a plate of insulating material 3 supporting metal tracks 4 parallel to each other and covered with carbon layers. The anode 2 is an insulating and transparent plate 6, for example made of glass, supporting parallel conductive tracks 7 between them and perpendicular to the tracks 4 of the cathode. The tracks 7 are made by etching a layer of transparent conductive material such as mixed tin oxide and indium (ITO). The tracks 7 are covered with phosphor layers 8.

The cathode and anode plates are placed opposite each other, the tracks facing each other to form a matrix structure. Crossing track networks forms image elements or pixels. By applying between a track 4 of the cathode and a track 7 of the anode a suitable potential difference, an electron emission occurs on the region of the track 4 corresponding to the pixel in question and the area of the phosphor 8 opposite is excited. A complete image can be obtained on the screen by successively feeding each line of the screen and by scanning the screen.

A low-output emissive material such as carbon requires, for the emission of electrons to occur, a minimum electric field of a few V / μm between an anode track and a cathode track opposite. If the spacing between these tracks is 1 mm, it is necessary to apply a potential difference of a few kV, typically 5000 to 10 000 V. this causes two main problems. The first problem is voltage withstand: there is a risk of breakdown between anode and cathode, especially between two adjacent tracks. The second problem arises from the need to switch a voltage several kV when scanning the screen. The triode type structure has been proposed to try to remedy these problems. FIG. 2 is a cross-sectional view of a flat field emission screen implementing such a structure. The cathode 11 consists of a glass plate 13 supporting metal tracks 14 parallel to each other and covered with carbon layers 15.

The tracks 14 are placed at the bottom of trenches etched in a layer of insulating material 10, this layer 10 being covered with a metal layer 19 serving as an extraction grid. anode 12 may be formed of a transparent plate 16 supporting a transparent and conductive layer 17 covered with a layer of luminescent material An emission of electrons by the emissive material can be obtained by applying, between the extraction grid 19 and the track 14, a potential difference such that the resulting electric field at the emissive material is greater threshold field this material, typically a few V / pm. Since the distance separating the extraction grid from the tracks is much smaller than the distance separating the anode from the cathode, the potential difference to be applied is reduced accordingly.

The electric field lines going from the tracks 14 to the extraction grid 19, a large part of the emitted electrons will be trapped by the grid. The triode type structure therefore has the disadvantage that very few emitted electrons reach the phosphor layer. DISCLOSURE OF THE INVENTION A first object of the invention relates to a device for producing an electric field between two electrodes, this electric field having to have a determined value in the vicinity of one of these two electrodes, the device comprising means for applying a potential difference between these two electrodes and electrode means located close to said electrode in the vicinity of which electric field must have a determined value, the device also comprising means for applying a potential difference between electrode means and said electrode located in proximity to obtain said determined value of electric field, this value being greater than that which would be due to the only potential difference between said two electrodes.

Advantageously, the electrodes the electrode means are arranged in parallel planes.

The electrode means said electrode located nearby can be located in the same plane. Alternatively, the electrode in the vicinity of which the electric field must have a determined value may be located between the other electrode and electrode means. In this case, the electrode means may consist of a single electrode.

The electrode means may comprise two electrodes surrounding said electrode in the vicinity of which the electric field must have a determined value.

A second subject of the invention relates to a method of producing an electric field between two electrodes, this electric field having to have a determined value in the vicinity of one of these two electrodes, characterized in that it comprises - the application a difference in potential between the two electrodes so as to obtain if this potential difference was applied alone, an electric field value lower than said determined value, - the application of a potential difference between the electrode in the vicinity of which electric field must have a determined value and electrode means and located in proximity to obtain, in cooperation with the electric field due to the application of the potential difference between the two electrodes, the determined value electric field.

A third object of the invention relates to a field emission display screen comprising an anode plate and a cathode plate arranged facing each other, the anode plate having on its internal face on the screen at least one electrode supporting a phosphor, the cathode plate having on its internal face to the screen at least one electron-emitting electrode at least partially facing the anode electrode, this cathode electrode becoming electron-emitting when the electric field to its vicinity exceeds a threshold value, the screen also comprising means for applying a potential difference between said anode electrode and said cathode electrode, characterized in that the screen further comprises electrode means located in proximity to the cathode electrode and means for applying a potential difference between the cathode electrode and the electro-forming means ode in order to exceed said electric field threshold value, this threshold value being greater than that which would be due to the only potential difference between said anode electrode and said cathode electrode.

The cathode screen and the electrode means may be located in the same plane. Alternatively, the cathode electrode may be located between the anode electrode and the electrode means. In this, the electrode means may consist of a single electrode.

The electrode means may comprise two electrodes flanking the cathode electrode.

Advantageously, the cathode electrode and the electrode means are separated a layer of insulating material.

Preferably, the cathode electrode comprises a conductive element on which is deposited a layer of emissive material. This layer of emissive material may be separated from the conductive element by a resistive layer. The layer of emissive material may cover only the central portion of the resistive layer.

The display screen is advantageously of the matrix type, the crossing of rows and columns defining pixels.

According to a preferred arrangement, the anode plate comprises a common electrode supporting a common phosphor, the cathode plate comprises a plate supporting conductor lines constituting the electrode means, covered with a layer of dielectric material, the layer of material dielectric carrier supporting columns of conductors, the rows and columns forming a matrix arrangement connected to addressing means and defining pixels, the conductor columns supporting an emissive material. Each pixel may correspond to the crossing of a row of several column conductors.

According to a particular arrangement, the conductor lines have windows opposite the columns of conductors, the emissive material supported by the columns of conductors being present only on the areas of the columns of conductors corresponding to the windows.

A fourth object of the invention relates to a method of using a field emission display screen comprising at least one anode electrode and at least one cathode electrode comprising an emitting material emitting electrons when the field in the vicinity of the cathode electrode exceeds a threshold value, characterized in that it comprises - the application of a potential difference between the anode electrode and cathode electrode so as to obtain, if this potential difference was applied alone, an electric field of value less than said threshold value, - the application of a potential difference between the cathode electrode and electrode means and located nearby in order to obtain, in cooperation with the electric field due to the application of the potential difference between the anode and cathode electrodes, the exceeding of said threshold value. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and other advantages and particularities will become apparent on reading the following description, given by way of nonlimiting example, accompanied by the appended drawings among which - FIG. , is a perspective view in cross section, of a first field emission flat screen according to the prior art; - Figure 2, already described, is a cross-sectional view of a second flat field emission screen according to the prior art; - Figures 3A and 3B are sectional views illustrating the operation of a device according to the invention; FIG. 4 is a cross-sectional and partial sectional view of a flat-field emission screen according to the invention; FIGS. 5 to 8 show variant embodiments of a flat field emission screen element according to the invention; FIG. 9 is a perspective view of a field emission flat screen cathode plate according to FIG. the invention. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION FIGS. 3A and 5 are sectional views illustrating the operation of a device according to the invention. The device comprises a plate 21 designated in this example as a cathode plate. The cathode plate 21 comprises a support plate 23 supporting an electrode 25 flanked by two parts 28 and 29 of the same electrode. The device also comprises a plate 22 designated in this example being that anode plate. The anode plate 22 comprises a support plate 26 supporting an electrode 27. The anode and cathode plates are arranged opposite and in parallel planes, their corresponding electrodes facing each other. They are separated by distance d.

FIG. 3A represents the case where a potential + V is applied to the electrode 27 and to the electrode 25 as well as to the parts 28 and 29 a zero potential. A uniform electric field of value V / d is established inside the device. Equipotential lines are shown in broken lines in FIG. 3A. The line shown closest to the electrode 25 corresponds to the potential intermediate between the potential of the cathode electrode 25 and that of the anode electrode 27. FIG. 3B shows where the electrode 27 is applied. a potential + V, on 'electrode 25 a zero potential and on the parts 28 and 29 a potential V1. There then occurs a movement and a deformation of the equipotentials which cause a tightening of the equipotentials above the cathode electrode 25, thus an increase of the electric field at this level. The same effect is obtained if a potential difference is set between the electrode 27 and the parts 28 and 29 and the electrode 25 is taken to a potential more negative than that of the parts 28 and 29 with respect to the electrode 27.

The electrode formed of the portions 28 and 29 may therefore be referred to as the modulator electrode.

FIG. 4 is a partial view, in transverse section, of a flat field emission screen to which the principle of the invention applies. This screen comprises a cathode plate 31 and an anode plate 32 placed vis-à-vis. according to parallel plans. They carry electrodes on their internal face. Unrepresented spacers ensure a constant spacing between the cathode plates and anode the vacuum made inside the screen.

The cathode plate 31 comprises a support plate 33 of insulating material, for example glass, on which is deposited successively a network of metal strips 38, 39 to form the modulation electrodes (which may also be called extraction electrodes ), an insulating layer 34 (for example silica) and then an array of cathode electrodes 35 placed in the intervals of the underlying network. In Figure 4, only one cathode electrode has been shown. It is either made of a material with low output work, or covered with a layer of material with low output work, for example carbon. In FIG. 4, the cathode electrode 35 supports a layer 30 of such material. The strips 38 and 39 corresponding to an electrode 35 are electrically connected together to constitute a modulator or extraction electrode.

The anode plate 32 comprises a support plate 36 of insulating and transparent material, typically made of glass, successively covered with a layer 37 of transparent and conductive material, for example ITO, and a layer 20 of a luminescent material.

The screen works as follows. A potential difference is applied between the anode electrode 37 and the cathode electrode 35 so that the resulting electric field at the emitting electrode is smaller than the electron extraction threshold field of the emissive material 30. there is thus no emission of electrons under the effect of this single field.

If the extraction electrode 38, 39 is brought to a potential intermediate between that of the anode and that of the emitting electrode, there is a movement and a deformation of the equipotentials resulting in an increase of the electric field at the level of the emitting electrode. The potential of the extraction electrode can be chosen such that the electric field at the emission electrode becomes greater than the threshold field of the emitting material. Then there will be emission of electrons. These electrons are emitted perpendicular to the emission electrode. They are then accelerated by the anode field, and strike the luminescent layer 20 covering the anode electrode 37.

By way of example, the anode 32 and cathode plates 31 may be spaced apart from the metal strips 38 and 39 may have a width of 20 μm spaced by 10 μm. The insulating layer 34 may be a silica layer 1 μm thick. The cathode electrode 35 may have a width of 5 μm and be centered in the spacing between the metal strips 38 and 39. For an emissive material having a threshold field of 5 to 6 V / μm, which is conventional, a potential of + 3000 V with respect to the cathode is applied to the anode, which gives an electric field of 3 V / μm at the emitting electrode, this field being lower than the threshold field. The cathode electrode 35 being maintained at 0 V, if the modulator electrode 38, 39 is raised to + 30 V, the electric field at the surface of the emitting electrode goes to 7 V / pm, which is greater than threshold field. It thus appears that the voltages to be switched remain low, typically a few tens of volts, which poses no problem.

The cathode plate, and in particular the distribution of the electrodes, may have different variants. Figures 5 to 8 show some of the possible variants. For the sake of clarity, only one cathode electrode has been shown in these figures.

FIG. 5 shows a cathode plate 41 comprising a plate 43 insulating material (for example glass) supporting an array of modulating electrodes each formed of two conducting strips 48 and 49 connected together. The plate 43 also supports an insulating layer 44, for example silica. On the insulating layer 44, cathode electrodes 45 are deposited in correspondence with the modulator electrodes 48, 49. Each cathode electrode is deposited above the gap separating the corresponding conductive strips 48 and 49 and symmetrically with respect to these. On these cathode electrodes 45 are successively deposited a resistive layer and a layer of emissive material 47. The function of the resistive layer 46 is to standardize the emission the surface of the emitting electrode which is formed by the superposition of the elements 45, 46 and 47. Thus, it prevents very strong punctual emissions, which can lead to breakdowns, to occur.

FIG. 6 shows a cathode plate comprising, as in FIG. 5, a support plate 53, an insulating layer 54, a cathode electrode 55, a resistive layer 56 and a layer of emissive material 57. On the other hand, the electrode Modulator 50 consists of a single conductive strip, the emitter electrode being centered on the modulator electrode.

FIG. 7 shows the cathode plate structure of FIG. 5. The cathode plate 61 comprises a support plate 63, two conductive strips 68 and 69 forming the modulating electrode, the insulating layer 64 supporting the emitting electrode constituted by by the cathode electrode 65, the resistive layer 66 and the emissive material layer 67. In this variant, the emitting electrode has the same width as the gap separating the two conductive strips 68 and 69.

FIG. 8 also shows the cathode plate structure of FIG. 5. The cathode plate 71 comprises a support plate 73, two conductive strips 78 and 79 forming the modulating electrode, the insulating layer 74 supporting the emitting electrode constituted by the cathode electrode 75, resistive layer 76 and the emissive material layer 77. In this variant, the emissive material layer 77 covers only the central part of the resistive layer 76. This arrangement makes it possible to obtain a more focused electron beam by eliminating the electrons that could be subject to the edge effects of the cathode electrode 75. This arrangement can be combined with the other variants described above.

FIG. 9 is an exploded perspective view of a matrix-type flat-field cathode plate plate according to the invention. The cathode plate 81 comprises a plate 83, for example glass, supporting a network of conductive strips forming lines, for example Yi, Yj, Yk. In these strips are openings or windows 80, rectangular example. This network of lines is covered with a layer of dielectric material 84 on which are deposited conductive strips 85 parallel to each other and perpendicular to the Y strips. The conductive strips 85 are, in this embodiment, grouped by three to constitute columns Xi, Xi, Xk. The conductive strips 85 are each covered with a layer of resistive material 86 and emissive material. In the example of Figure 9, the emitted material 87 has been deposited on the useful areas, that is to say, the areas of the columns above the windows 0 in the lines. We thus obtain two networks, one of lines and the other of columns, orthogonal to each other. A pixel is the crossing of a line and a column.

To address a pixel Xj, Yj, the anode of the screen is raised to a potential V2, the column to the potential Vo, the line Yj to a potential V1 (V1 being intermediate between Vo and V2), the other columns X are potential at potential V1 and other lines Y at potential Vo. The potential V2 is such that, if d is the distance separating the anode from the cathode, the electric field (V2 - Vo) / d is smaller than the emission threshold field of the emissive material. The potential V1 is chosen so that the increase in the electric field at the emitting electrode that results is such that the electric field becomes greater than the threshold field, resulting in the emission of electrons by the emissive material.

Claims (21)

1. Device for producing an electric field between two electrodes (25,27), this electric field to have a determined value in the vicinity of one (25) of these two electrodes, the device comprising means for applying a potential difference between these two electrodes and electrode means (28,29) located near said electrode (25) in the vicinity of which the electric field is to have a predetermined value, the device also comprising means for applying a potential difference between the electrode means (28, 9) and said electrode (25) situated in proximity to obtain said determined electric field value, this value being greater than that which would be due to the single potential difference between said two electrodes (25, 27). 2. Device according to claim 1, characterized in that the electrodes (25,27) and the electrode means (28,29) are arranged in parallel planes. 3. Device according to one of claims 1 or 2, characterized in that the electrode means (28,29) and said electrode (25) located nearby are located in the same plane. 4. Device according to one of claims 1 or 2, characterized in that said electrode in the vicinity of which the electric field must have a determined value is located between the other electrode and the electrode means. 5 Apparatus according to any one of claims 1 to 4, characterized in that the electrode means comprise two electrodes (28,29) flanking said electrode (25) in the vicinity of which the electric field must have a predetermined value. 6. Device according to claim 4, characterized in that the electrode means consist of a single electrode. 7. A method of producing an electric field between two electrodes (25,27), this electric field having to have a determined value in the vicinity of one (25) of these two electrodes, characterized in that it comprises - the application a potential difference between the two electrodes (25,27) so as to obtain, if this potential difference was applied alone, an electric field of value less than said determined value, - the application of a potential difference between the electrode (25) in the vicinity of which the electric field is to have a determined value and electrode means (28,29) and located in proximity to obtain, in cooperation with the electric field due to the application of the potential difference between the two electrodes (25,27), the determined value of the electric field. 8. A field emission display screen comprising an anode plate (32) and a cathode plate (31) arranged facing each other, the anode plate (32) having on its internal face on the screen at least one electrode (37) supporting a phosphor (20), the cathode plate (31) having on its internal face on the screen at least one electron-emitting electrode (35) at least partially facing the anode electrode ( 37), this cathode electrode (35) becoming electron-emitting when the electric field in its vicinity exceeds a threshold value, the screen also comprising means for applying a potential difference between said anode electrode (37) and said cathode electrode (35), characterized in that the screen further comprises electrode means (38,39) located proximate the cathode electrode (35) and means for applying a difference potential between the cathode electrode (35) and the electrode means (38,39) to exceed said electric field threshold value, which threshold value is greater than would be due to the single potential difference between said anode electrode (37) and said cathode electrode (35). 9. Viewing screen according to claim 8, characterized in that said cathode electrode and the electrode means are located in the plane plane. The display screen of claim 8, characterized in that said cathode electrode (35) is located between said anode electrode) and the electrode means (38,39. 11. Viewing screen according to any one of claims 8 to 10, characterized in that the electrode means comprises two electrodes (38,39) flanking said cathode electrode (35). 12. Display screen according to claim 10, characterized in that the electrode means consist of a single electrode. Viewing screen according to claim 10, characterized in that said cathode electrode (35) and the electrode means (38,39) are separated by a layer of insulating material (34). 14. Display screen according to any one of claims 8 to 13, characterized in that said cathode electrode (35) comprises a conductive element on which is deposited a layer of emissive material (30). 15. Viewing screen according to claim 14, characterized in that layer of emissive material (47) is separated from said conductive element (45) by a resistive layer 16. Display screen according to claim 15, characterized in that the layer of emissive material (77) covers only the central portion of the resistive layer (76). 17. Display screen according to any one of claims 8 to 16, characterized in that is of the matrix type, the crossing of rows and columns defining pixels. 18. Viewing screen according to claim 8, characterized in that the anode plate comprises a common electrode supporting a common phosphor, the cathode plate) comprises a plate (83) supporting conductor lines (Yi Yi, Yk) constituting the electrode means, covered with a layer of dielectric material (84), the layer of dielectric material supporting conductor columns (85), the lines and columns forming a matrix arrangement connected to addressing means and defining pixels, the columns of conductors supporting an emissive material (87). 19. Viewing screen according to claim 18, characterized in that each pixel corresponds to the intersection of a line (Yi, Yk) and several column conductors (85). 20. Display screen according to one of claims 18 or 19 characterized in that the conductor lines yj, Yk) comprise windows (80) vis-à-'columns of conductors (85), the emissive material (87) ) supported by the columns of conductors' being present only on the areas of the columns of conductors corresponding to the windows (80). 21. A method of using a field emission display screen comprising at least one anode electrode (37 and at least one cathode electrode (35) comprising an emitting material (30) emitting electrons when the electric field at the vicinity of the cathode electrode (35) exceeds a threshold value, characterized in that it comprises - the application of a potential difference between the anode electrode (37) and the cathode electrode ( 35) so as to obtain, if this potential difference was applied alone, an electric field of value less than said threshold value, - the application of a potential difference between the cathode electrode (35) and means forming electrode (38,39) and located in close proximity to obtain, in cooperation with the electric field due to the application of the potential difference between the anode (37) and cathode (35) electrodes, the overshoot of said value of threshold.