Abstract:
A microtip electron source including at least one electron emission zone composed of a plurality of microtips connected electrically to a cathode conductor. At least one gate electrode is positioned opposite the electron emission zone and pierced with apertures located opposite the microtips, to extract the electrons from the microtips. An emitted electron focusing gate is positioned opposite the gate electrode, and includes an aperture unit including at least one slit located opposite at least two successive microtips. A flat display screen can include such a microtip electron source. Further, a manufacturing process of such an electron source is disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microtip, focusing gate and high microtip density electron source. It also relates to a flat screen using such a source. 
     2. Discussion of the Background 
     The documents FR-A-2 593 953 and FR-A-2 623 013 describe field emission-excited cathodoluminescence display devices. These devices comprise a microtip emitting cathode electron source. 
     As an illustration, FIG. 1 is a transversal section view of such a microtip display screen. For simplification purposes, only a few aligned microtips have been represented. The screen is composed of a cathode  1 , which is a plane structure, positioned opposite another plane structure forming the anode  2 . The cathode  1  and the anode  2  are separated by a space in which a vacuum is produced. The cathode  1  comprises a glass substrate  11  on which the conductive level  12  in contact with the electron emitting tips  13  is deposited. The conductive level  12  is coated with an insulating layer  14 , e.g. silica, which is itself coated with a conductive layer  15 . Holes  18 , approximately 1.3 μm in diameter, have been produced through the layers  14  and  15  up to the conductive level  12  to deposit the tips  13  on said conductive level. The conductive layer  15  is used as an extraction gate for the electrons emitted by the tips  13 . The anode  2  comprises a transparent substrate  21  coated with a transparent electrode  22  on which luminescent phosphors or luminophors  23  are deposited. 
     The operation of this screen is described below. The anode  2  is brought to a positive voltage of several hundred volts with reference to the tips  13  (typically 200 to 500 V). On the extraction gate  15 , a positive voltage of several tens of volts (typically 60 to 100V) with reference to the tips  13  is applied. Electrons are then extracted at the tips  13  and are attracted by the anode  2 . The electrons&#39; paths are comprised in a top half-angle cone θ depending on different parameters, including the shape of the tips  13 . This angle induces a defocusing of electron beam  31  which increases with the distance between the anode and the cathode. However, one of the ways to increase the efficiency of phosphors, and therefore the screen brightness, is to work with higher anode-cathode voltages (between 1000 and 10,000 V), which implies increasing the distance between the anode and the cathode further to prevent the formation of an electric arc between the two electrodes. 
     In order to retain a good resolution on the anode, the electron beam must be refocused. This refocusing is obtained conventionally using a gate that can be placed between the anode and the cathode or positioned on the cathode. 
     FIG. 2 illustrates the case in which the focusing gate is positioned on the cathode. FIG. 2 takes the same example as FIG. 1 but limited to a single microtip for more clarity in the drawing. An insulating layer  16  has been deposited on the extraction gate  15  and supports a metal layer  17  used as a focusing gate. Holes  19 , of suitable diameter (typically between 8 and 10 mm) and concentric with the holes  18 , have been engraved in the layers  16  and  17 . The insulating layer  16  is used to insulate the extraction gate  15  and the focusing gate  17  electrically. The focusing gate is polarised with reference to the cathode so as to give the electron beam  32  the form represented in FIG.  2 . 
     In the case of a microtip screen without a focusing gate, such as that shown in FIG. 1, the distance between two adjacent microtips is of the order of 3 μm. For a microtip screen with a focusing gate, as represented in FIG. 2, this distance is of the order of 10 to 12 μm. In this case, the microtip density, i.e. the electron emitter density, is between 9 and 16 times lower. This results in a decrease in screen brightness. 
     In a flat screen, the luminophors are deposited on the anode in the form of parallel bands, which are successively red-green-blue, etc. For a good restored image quality, the colours must not be mixed. For this, all the electrons emitted by a pixel of a given colour must go to the corresponding luminophor and not to the adjacent luminophors. This result is obtained by the focusing phenomenon. Given the band structure of the luminophors, it is important that the focusing is carried out in the direction perpendicular to these bands to prevent mixing of colours. 
     SUMMARY OF THE INVENTION 
     The invention makes it possible to remedy the problem of low microtip density posed by prior art focusing gate electron sources. This is obtained by replacing the circular apertures of the focusing gate by slits. 
     The invention proves to be particularly effective when applied to flat screens in which the luminophors are arranged in bands. It is proposed to etch, in the focusing gate, apertures in the form of slits, with the microtips aligned on the axes of these slits. By arranging the luminophors located on the anode in the form of bands parallel to the electron source slits and just above the corresponding slits, the electrons emitted by the microtips of these slits remain concentrated on the luminophor band facing them. Therefore, there will be no mixing of colours. If the focusing is not obtained in the direction of the bands, a slight spreading of the pixel in this direction is produced, which has a relatively insignificant effect on the image quality. 
     Therefore, the focusing gate according to the present invention performs a focusing function in a single direction. 
     Therefore, the invention relates to a microtip electron source comprising: 
     at least one electron emission zone composed of a plurality of microtips connected electrically to a cathode conductor, 
     at least one gate electrode, positioned opposite said electron emission zone and pierced with apertures located opposite the microtips, to extract the electrons from the microtips, 
     an emitted electron focusing gate positioned opposite the gate electrode, and equipped with aperture means comprising at least one slit located opposite at least two successive microtips, 
     characterised in that the focusing gate is separated from the extraction gate electrode positioned opposite it by a layer of electrically insulating material with a slit aligned with the focusing gate slit, or a succession of holes aligned with the focusing gate slit, of a width less than that of the focusing gate slit. 
     According to an advantageous arrangement, the microtip electron source may comprise a plurality of electron emission zones arranged in the form of a matrix in rows and columns, with the number of cathode conductors and gate electrodes corresponding to the rows and columns to give the microtip electron source a matric access. 
     If each emission zone comprises several rows of microtips, each row of microtips has one or more corresponding slits in the focusing gate. 
     The invention also relates to a device comprising a first and second plane structure maintained opposite and at a determined distance from each other by means forming a spacer, the first plane structure comprising, on its inner device face, a microtip electron source such as that defined above, and the second plane structure comprising, on its inner device face, means forming the anode. 
     Such a device may be used to form a flat display screen, with luminophors placed between the microtip electron source and the means forming the anode. 
     The invention also relates to a flat display screen comprising a first and second plane structure maintained opposite and at a determined distance from each other by means forming a spacer, the first plane structure comprising, on its inner screen face, a microtip electron source such as that defined above, in which each emission zone comprises several rows of microtips and each row of microtips has one or more corresponding slits in the focusing gate, and the second plane structure comprising, on its inner screen face, means forming the anode, a conductive layer forming the anode and supporting luminophors arranged in alternating red, green and blue bands, with each band located parallel to and opposite a series (row or column) of electron emission zones, with the main axis of the focusing gate slits directed in the direction of the luminophor bands and each emission zone defining a pixel for the display screen. 
     Naturally, the microtip electron source according to the present invention may be used in relation with anodes of different structures, particularly conventional structures produced for cathode ray tube screens, adapted for flat screens. 
     The invention also relates to a microtip and focusing gate electron source manufacturing process, comprising: 
     a step in which the following are successively deposited on one face of an electrically insulating substrate: cathode connection means, a first electrically insulating layer of a thickness adapted to the height of the future microtips, a first conductive layer intended to form the extraction gate, a second electrically insulating layer of a thickness corresponding to the distance to separate the extraction gate from the focusing gate, 
     a step consisting of piercing the second insulating layer with holes up to the first conductive layer, with the axes of the holes corresponding to the axes of the future microtips and the diameter of these holes adapted to the size of the future microtips, 
     an electrolytic deposition step of conductive material in said holes, with the first conductive layer acting as the electrode during the electrolysis, the electrolytic deposit filling said holes from the first conductive layer and flowing onto the second insulating layer, first of all giving the electrolytically deposited conductive material the shape of mushrooms, the caps of which rest on the second insulating layer, with the electrolytic deposit subsequently producing, due to coalescence of the mushroom caps formed in adjacent and sufficiently close holes, an approximately semi-cylindrical shaped mass for each set of adjacent and sufficiently close holes, 
     a deposition step of a second conductive layer intended to form the focusing gate, with the material of this second conductive layer being different to that of the electrolytically deposited conductive material, 
     an electrolytically deposited material removal step, with this removal leaving, in the second conductive layer, one slit for each previously formed mass, the main axis of which is aligned with the holes with which it was formed, 
     a hole deepening step up to the cathode connection means, 
     an etching step of the second insulating layer to reveal the first conductive layer, 
     a microtip formation step on the cathode connection means revealed by the hole deepening step. 
     The hole deepening step may be performed by etching. This step and the second insulating layer etching step may be performed simultaneously. 
     The invention also relates to a microtip and focusing gate electron source manufacturing process, comprising: 
     a step in which the following are successively deposited on one face of an electrically insulating substrate: cathode connection means, a first electrically insulating layer of a thickness adapted to the height of the future microtips, a first conductive layer intended to form the extraction gate, a second electrically insulating layer of a thickness corresponding to the distance to separate the extraction gate from the focusing gate, a masking layer, 
     a step consisting of piercing holes through the complex formed by the masking layer, the second insulating layer and the first conductive layer up to the first insulating layer, with the axes of the holes corresponding to the axes of the future microtips and the diameter of these holes adapted to the size of the future microtips, 
     a hole deepening step in the first insulating layer up to the cathode connection means, 
     a lateral etching step of the second insulating layer to increase the diameter of the holes pierced previously to a determined value, with this lateral etching being able to render adjacent and sufficiently close holes secant, 
     a masking layer removal step, 
     an electrolytic deposition step of conductive material in said holes, with the first conductive layer acting as the electrode during the electrolysis, the electrolytic deposit filling said holes from the first conductive layer and flowing onto the second insulating layer, first of all giving the electrolytically deposited conductive material the shape of mushrooms, the caps of which rest on the second insulating layer, with the electrolytic deposit subsequently producing, due to coalescence of the mushroom caps formed in adjacent and sufficiently close holes, an approximately semi-cylindrical shaped mass for each set of adjacent and sufficiently close holes, 
     a deposition step of a second conductive layer intended to form the focusing gate, with the material of this second conductive layer being different to that of the electrolytically deposited conductive material, 
     an electrolytically deposited material removal step, with this removal leaving, in the second conductive layer, one slit for each previously formed mass, the main axis of which is aligned with the holes with which it was formed, 
     a microtip formation step on the cathode connection means through the holes produced in the first conductive layer and the first insulating layer. 
     The hole deepening step in the first insulating layer and the lateral etching step of the second insulating layer may be performed simultaneously by isotropic etching. 
     Irrespective of the process implemented, the step consisting of piercing holes may be carried out by etching. The electrolytically deposited material removal step may be carried out by chemical dissolution. The cathode connection means may be obtained by deposition of cathode conductors on the substrate, followed by deposition of a resistive layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention and the demonstration of other advantages and specific features can be obtained in the following description, given as non-restrictive examples, accompanied by the appended drawings, of which: 
     FIG. 1 illustrates a microtip flat screen according to the prior art, 
     FIG. 2 illustrate a microtip and focusing gate flat screen according to the prior art, 
     FIG. 3 is a partial and perspective view of a first variant of a microtip electron source according to the present invention, 
     FIG. 4 is a partial and perspective view of a second variant of a microtip electron source according to the present invention, 
     FIGS. 5A to  5 D illustrate a manufacturing process of a microtip electron source of the type represented in FIG. 3, 
     FIGS. 6A to  6 E illustrate a manufacturing process of a microtip electron source of the type represented in FIG. 4, 
     FIG. 7 is a top view of a first flat display screen microtip electron source according to the present invention, with this view only showing part of the electron source corresponding to a pixel of the screen, 
     FIG. 8 is a top view of a second flat display screen microtip electron source according to the present invention, with this view only showing part of the electron source corresponding to a pixel of the screen. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a partial and section view of a microtip electron source according to the invention. It has been produced using a glass substrate  40 . On this substrate  40 , the following have been successively deposited: a first layer  41  forming cathode connection means, a first insulating layer  42  and a first conductive layer  43 . In the layers  42  and  43 , holes  44  have been etched up to the first layer  41 . Electron emitters  45 , in the form of tips, have been deposited inside the holes  44  in contact with the first layer  41 . The microtips  45  are arranged in alignments. To use the electron source as a flat colour display screen cathode, the microtip alignments are parallel to the luminophor bands arranged on the screen anode. 
     The conductive layer  43  is used as an electron extraction gate. It is coated with an insulating layer  46  (second insulation layer) and a conductive layer  47  (second conductive layer). Slits  48  have been produced in the layers  46  and  47  to reach the extraction gate  43 . The axes of the slits  43  are combined with the axes of the alignments of emitters or microtips  45 . The slits  48  may have a width of 8 to 10 μm. The spacing of the slits (along their main axis) and, as a result, the spacing of the rows of emitters, is 10 to 12 μm. The distance between two emitters on the same row is of the order of 3 μm. Therefore, the solution proposed by the invention enables an emitter density that is 3 to 4 times than in the case where focusing is carried out in all directions from each of the emitters (case of FIG.  2 ). 
     The microtip electron source represented in FIG. 3 is generally intended for use as a flat display screen cathode. This flat screen is a device composed of a cathodic structure and an anodic structure facing each other, between which a vacuum is produced. The distance separating the extraction gate  43  from the focusing gate  47  is very short. In some cases of use, this may result in the risk of an electric arc in the vacuum between these two gates. 
     A solution to remedy this disadvantage is represented in FIG. 4 where the same components as for FIG. 3 are designated by the same references. In the case of FIG. 4, the slits  48  have been limited to the focusing gate. The insulating layer  46  has been etched with slits  49  centred on the corresponding emitter rows and of a width less than the width of the slits  48 . As a variant, the insulating layer  46  may be pierced with holes concentric with the holes  44 . The diameter of these concentric holes or the width of the slits  49 , depending on the case, may be two to three times the diameter of the holes  44 . In this way, the extension of the insulating layer  46  onto the extraction gate  43  provides improved protection against electric arcs. 
     The electrons emitted by the microtips corresponding to a focusing gate slit of an electron source according to the present invention are focused in the direction perpendicular to the slit axis. They only deviate very insignificantly from the plane perpendicular to the source passing through the slit axis. Therefore, the impacts of these electrons on a plane parallel to the cathode are located in a narrow band parallel to, but slightly longer than, the slit axis. 
     Electron sources such as those represented in FIGS. 3 and 4 may be produced using conventional microelectronic deposition, photolithography and etching techniques, with the microtips produced according to the prior art. However, simulation calculations demonstrate that the focusing quality depends on the centring of the focusing gate along the emitter axis and that this parameter is very sensitive. The required precision requires the use of high-performance devices, the suitability of which decreases as the screen size increases. 
     To remedy this problem, it is proposed to produce the focusing gate using a self-alignment process. 
     A first example of this process is illustrated by FIGS. 5A to  5 D. It makes it possible to obtain a microtip electron source of the type represented in FIG.  3 . 
     With reference to FIG. 5A, a metal layer, which has been etched to form columns  51 , has been deposited on a glass plate  50 . A resistive layer  52  has then been deposited uniformly in order to produce a plane surface. On the resistive layer  52 , the following have then been successively deposited: a first insulating layer  53 , a conductive layer  54  and a second insulating layer  55 . The thickness of these different layers is adapted to the required structure. The insulating layers  53  and  55  may be silica. The conductive layer  54 , intended to form the electron extraction gate, may be niobium. 
     Then, using conventional photolithography and etching techniques, holes  56 , the centres of which are aligned on parallel lines, are etched in the insulating layer  55 . The holes  56  reveal the conductive layer  54 . The distance between two successive holes on the same row is of the order of 3 μm. The distance between two consecutive rows is approximately 10 to 12 μm. For increased clarity, FIG. 5A only represents a small part of a single row of holes. 
     The next step (see FIG. 5B) consists of performing electrolytic deposition of a conductive material (e.g. an iron-nickel alloy) on the revealed parts of the conductive layer  54 , i.e. at the base of the holes  56 . The thickness of the electrolytic deposit is adjusted so as to obtain, for each hole, the formation of a mushroom, the base of which fills the hole and such that the cap is developed on the outer face of the insulating layer  55 . The formation is continued until the cap diameter reaches the required width for the focusing gate slit. Since this width is approximately 10 μm, the mushrooms will coalesce to form a semi-cylindrical shaped mass  57  of a diameter equal to the required slit width. 
     Using a vacuum deposition technique adapted to the type of material to be deposited, a second conductive layer is then deposited to form the focusing gate. This second conductive layer (metal or another resistive material) is deposited on the insulating layer  55  between the masses  57 , to form the deposit  58 , and on the masses  57  to form the deposit  59 , as represented in FIG.  5 B. Each mass  57  serves as a mask for the focusing gate aperture. Since the axis of each semi-cylinder forming a mass passes through the line joining the centres of the holes, the aperture obtained will be automatically centred on this line. 
     The masses  57  are then dissolved chemically and the structure represented in FIG. 5C is obtained. The apertures  60  produced in the focusing gate  58  are centred on the axes of the holes  56 . 
     The metal layer  54  is then etched anisotropically through the holes  56  to deepen this hole up to the first insulating layer  53 . The anisotropic etching is continued in the insulating layer  53  until the resistive layer  52  is reached. Since the insulating layers  53  and  55  are both made of silica in the example described, the etching of these two layers may be performed simultaneously. This produces, as shown in FIG. 5D, holes  61  and  64  (following the holes  56  in FIG. 5C) passing through the conductive layer  54  and the insulating layer  53 , respectively. An aperture  62  in the form of a slit is also obtained following from the slit  60 . 
     The microtips  63  are then produced conventionally, at the base of the holes  61 . Therefore, the microtips, the extraction gate holes and the focusing gate slits are self-aligned. 
     A second example of the self-alignment process is illustrated in FIGS. 6A to  6 E. It is used to obtain a microtip electron source of the type represented in FIG.  4 . 
     With reference to FIG. 6A, cathode conductor columns  71  and a resistive layer has been deposited on a glass plate  70 , as for the first process example. On the resistive layer  72 , the following have then been, successively deposited: a first insulating layer  73 , a conductive layer  74  and a second insulating layer  75  of the same type as the first insulating layer  73 . Finally, a layer of resin  85  has been deposited. The choice of layer thickness and materials used may be the same as for the first process example. 
     Holes  76  have been opened in the resin layer  85  which serves as a mask for the etching of the insulating layer  75  and the conductive layer  74 . Therefore, the holes  76  are deepened to reach the first insulating layer  73 . 
     The chemical etching of the first insulating layer  73  is then performed so as to extend the holes to the resistive layer  72 . By performing isotropic etching, significant excess etching is obtained and the holes  84  produced in the first insulating layer will have the profile shown in FIG.  6 B. Since it is of the same type as the first insulating layer  73 , the second insulating layer  75  is etched in the same way. An increase in the diameter of the holes  76 , between the conductive layer  74  and the resin layer  85  is obtained, providing cavities  82 . This increase in diameter is equal to at least twice the thickness of the first insulating layer  73 . 
     FIG. 6C represents the structure obtained after the removal of the resin layer. The second insulating layer  75  comprises holes  82  coaxial with, but of a larger diameter than, the holes  76  of the conductive layer  74 . These holes  82  may be isolated or secant (as shown in FIG. 6C) according to the thickness of the first insulating layer  73  and the distance between the holes  76  of a same row of holes. 
     Electrolytic deposition of a conductive material is then carried out from the conductive layer  74 . The deposition step is conducted so as to obtain semi-cylindrical shaped masses  77  of a diameter equal to the required width for the focusing gate slit (e.g. 10 μm). This is shown in FIG.  6 D. 
     As for the first process example, a second conductive layer is deposited to form the focusing gate. The deposit  78  between the masses  77  and the deposit  79  on the masses  77  are obtained. 
     The masses  77  are then dissolved chemically to give the structure the profile represented in FIG.  6 E. The apertures  80  produced in the focusing gate  78  are centred on the axes of holes  76 . This gate  78  is placed on the insulating layer  75 , itself comprising an aperture (formed by the succession of adjacent holes  82 ) centred on the row of holes  76 , the aperture in the second insulating layer  75  being narrower than that of the focusing gate  78 . 
     The microtips  83  are then produced conventionally at the base of the holes  84 . Therefore, the microtips, extraction gate holes and the focusing gate slits are self-aligned. 
     Viewed from above, the microtip electron source, e.g. obtained using the first self-alignment process example, may appear as shown in FIGS. 7 and 8. These figures only show part of the electron source corresponding to one pixel on the screen. The extraction gate holes  61 , at the base of which the electron emitters are placed, are aligned in the slits  60  of the focusing gate  58 . These slits may be the same length as the pixel, as in FIG.  7 . This may be split into several parts, as in FIG.  8 .