Abstract:
A process for manufacturing a micropoint electron source with an extraction grid and a focusing grid. This process allows for precise alignment of the holes of the extraction grid with the apertures of the focusing grid by using a single photolithography step for making the holes in the extraction grid. Such a process may find particular application for making a micropoint electron source for a flat viewing screen.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention involves a process for manufacturing a micropoint electron source with an auto-aligned focussing grid. Such a micropoint electron source can be used in particular in a device for visualisation by cathodoluminescence excited by field emission. 
     2. Discussion of the Background 
     Documents FR-A-2 593 953 and FR-A-2 623 013 disclose devices for visualisation by cathodoluminescence excited by field emission. These devices include a emitting cathode electron source with micropoint. 
     By way of illustration, FIG. 1 is a cross section view of such a micropoint viewing screen. In the interest of simplification, only a few aligned micropoints are shown. The screen is composed of a cathode  1 , which is a plane structure, oriented with respect to another plane structure which forms the anode  2 . The cathode  1  and the anode  2  are separated by a space in which a vacuum has been created. The cathode  1  includes a glass substrate  11  on which the conducting level  12  has been applied in contact with the electron emitting points  13 . The conducting level  12  is covered with a layer of insulation  14 , made of silica for example, which is itself covered by a conducting layer  15 . Holes  18  of about 1.3 •m in diameter were made through the layers  14  and  15  up to the conducting level  12  to apply the points  13  on this conducting level. The conducting layer  15  acts as an extraction grid for the electrons which will be emitted by the points  13 . The anode  2  includes a transparent substrate  21  covered by a transparent electrode  22  on which luminescent phosphors or luminophores  23  have been deposited. 
     The operation of this screen will now be described. The anode  2  is brought to a positive voltage of several hundred volts with respect to the points  13  (typically 200 to 500 V). A positive voltage of several dozens of volts (typically 60 to 100 V) with respect to the points  13  is applied to the extraction grid  15 . Electrons are then drawn from the points  13  and are attracted by the anode  2 . The trajectories of the electrons are within a half-angle cone at the peak •, depending on various factors such as the shape of the points  13 . This angle causes a defocusing of the electron beam  31  which increases as the distance between the anode and the cathode is increased. One way to increase the yield of the phosphors, and thus the luminosity of the screens, is to work with higher anode-cathode voltages (between 1,000 and 10,000 V), which implies separating the anode and the cathode further in order to avoid the formation of an electric arc between these two electrodes. 
     If good resolution on the anode is desired, the electron beam must be refocused. This refocusing is classically obtained with a grid which can either be placed between the anode and the cathode or placed on the cathode. 
     FIG. 2 illustrates the case where the focussing grid is placed on the cathode. FIG. 2 repeats the example of FIG. 1, but limited to a single micropoint for greater clarity in the drawing. An insulating layer  16  was applied to the extraction grid  15  and bears a metallic layer  17  which acts as a focussing grid. Holes  19  of an appropriate diameter (typically between 8 and 10 •m) and concentric to holes  18 , were etched in layers  16  and  17 . The insulating layer  16  electrically insulates the extraction grid  15  and the focussing grid  17 . The focussing grid is polarised with respect to the cathode in order to give the electron beam the shape shown in FIG.  2 . 
     Simulation calculations show that centering of the holes  19  of the focussing grid with respect to the holes  18  of the extraction grid is extremely important. This structure is generally made using the classic photoetching techniques used in microelectronics. For example, with a first level of photoetching, the holes  19  of the focussing grid are defined, then a second level of photoetching is used to make holes  18  in which the points will be placed. To ensure proper functioning, the second level must be positioned in an extremely precise manner with respect to the first level. This can only be done with very high-quality, expensive equipment, a serious drawback if large areas are treated. In addition, if the holes of the extraction grid are made by photolithography from a microsphere network, their arrangement is random, which rules out the use of a phototemplate for making the apertures of the focussing grid. 
     SUMMARY OF THE INVENTION 
     The invention solves the problem of precision alignment of holes located on different levels. This is achieved by a process which requires only a single photolithography step which makes the holes in the extraction grid. 
     The purpose of the invention is thus the making of a micropoint electron source with an extraction grid and a focussing grid involving: 
     the successive depositing on one side of an electrically insulating support of means of cathodic connection, a first insulating layer of thickness adapted to the height of the future micropoints, a first conducting layer to form the extraction grid, a second insulating layer of thickness corresponding to the distance which must separate the extraction grid from the focusing grid, a second conducting layer to form the focusing grid and a photosensitive resin layer; 
     the etching, by photolithography, of the photosensitive resin layer to make holes in it which exit on the second conducting layer and of which the axes correspond to the axes of the future micropoints and of which the diameter is adapted to the size of the future micropoints, these holes permitting etching of the other layers deposited on the support; 
     the etching of the second conducting layer to make holes in it which exit at the second insulating layer; 
     the etching of the second insulating layer to make cavities in it which are to be extended laterally up to a dimension corresponding to the apertures of the focussing grid and which reveal the first conducting layer; 
     etching of the first conducting layer to make holes in it for the extraction grid; 
     etching of holes in the first insulating layer until they reach the means of cathodic connection in order to make housings for the micropoints; 
     enlargement by etching of the holes of the second conducting layer to obtain apertures for the focusing grid; 
     elimination of the photosensitive resin layer remaining after the etching operations; 
     formation of micropoints in their housings on the means of cathodic connection. 
     The means of cathodic connection are preferably made by depositing cathodic conductors on the support, followed by depositing of a resistant layer. 
     A first way of etching the second insulating layer would be as follows: 
     the second insulating layer is first etched to obtain the holes in the prolongation of the holes of the photosensitive resin layer which come out on the first conducting layer; 
     the first conducting layer is then etched tof obtain the blind holes in the prolongation of the holes of the photosensitive resin layer, these blind holes constituting the beginnings of the holes of the extraction grid; 
     lastly, the second insulating layer is etched until the aforesaid cavities are obtained. 
     The etching of the holes in the first insulating layer can first be done anisotropically, the aforesaid housings then being defined by isotropic etching. 
     A second way of etching the second insulating layer is as follows. Since the first and second insulating layers can be etched simultaneously, the etching of the second insulating layer is first done isotropically to mark the places for the cavities, to reach the first conducting layer, revealing the zones allowing for making holes for the extraction grid, the holes of the extraction grid then being etched in the first conducting layer, an isotropic etching being lastly done to simultaneously obtain the aforesaid housings in the first insulating layer and the aforesaid cavities of the aforesaid dimension in the second insulating layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be better understood and its other advantages and characteristics will be clearer with a reading of the following description, which is given as a non-limiting example, accompanied by drawing in appendix among which: 
     FIG. 1, already described, illustrates a flat micropoint screen based on the prior art; 
     FIG. 2, already described, illustrates a flat micropoint screen with a focussing grid based on the prior art; 
     FIGS. 3A to  3 F illustrate the manufacturing of a micropoint electron source according to the first way of using the process of this invention; 
     FIGS. 4A to  4 D illustrate the manufacturing of a micropoint electron source according to the second way of using the process of this invention; 
     FIG. 5 is a partial and perspective view of a micropoint electron source made by the process of the present invention and in which the micropoints are arranged in lines, the distance between the adjacent micropoints of a given line being less than the diameter of the holes of the focussing grid; 
     FIG. 6 is a partial and perspective view of a micropoint electron source made by the process of the present invention, the distance between two adjacent micropoints being greater than the diameter of the holes of the focussing grid. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 3A to  3 F are cross-section views of a micropoint electron source being manufactured according to a first mode for applying the process according to the invention. 
     On a support  50  composed of a glass chip, a metallic layer is deposited (see FIG. 3A) which is etched to make cathodic conductors  51  which are parallel to each other. These cathodic conductors  51  can be used as columns for matrix display for example. A resistant layer  52  is then deposited in a uniform manner. On this resistant layer  52  are successively deposited a first insulating layer  53 , a first conducting layer  54  to form the extraction grid for the micropoint electron source, a second insulating layer  55  and a second conducting layer  56  to form the focussing grid. The thicknesses of the insulating layers  53  and  55  are chosen as a function of the chosen height of the micropoints and the distance which must separate the extraction grid from the focussing grid. A layer of photosensitive resin  57  is then deposited in a uniform manner on the second conducting layer  55 . 
     The photosensitive resin layer  57  is insolated through a template and then developed to make axis holes  58  corresponding to the axes of the micropoints to be made (see FIG. 3B in which a single hole  58  is shown). These holes allow for etching of the underlying layers. The holes  58  are prolonged by holes  59  etched in the second conducting layer  56 , which are prolonged by holes  60  etched in the second insulating layer  55 . 
     These rows of holes  58 ,  59  and  60  are then prolonged by holes  61  etched in the thickness of the first conducting layer  54 . At this stage, the holes  61  do not go through the first conducting layer  54 . 
     Still by etching, the holes  60  in the second insulating layer  55  are enlarged to a determined diameter corresponding to the diameter of the apertures to be made in the focussing grid. This gives cavities  68  as shown in FIG.  3 C. 
     Holes  61  are then etched in the first conducting layer  54  in order to reveal the first insulating layer  53 . The holes  61  are then prolonged, by etching, by holes  62  made in the first insulating layer  53  until it reaches the resistant layer  52  which is thus revealed. 
     In order to provide appropriate housings for the micropoints, the holes  62  made in the first insulating layer  53  are enlarged by isotropic etching. This gives the housings  63  shown in FIG.  3 D. The second conducting layer  56  is then etched so as to enlarge the holes of this layer to the dimension of the cavities  68  of the second insulating layer  55 . This gives the apertures  64  of the focussing grid. 
     The photosensitive resin is eliminated, giving the structure shown in FIG.  3 E. The extraction grid  65  and the focussing grid  66  are thus definitively formed. Because of the process according to the present invention, each aperture  54  of the focussing grid  66  is perfectly aligned with the corresponding hole  61  of the extraction grid  65 . 
     The last step of the process involves making micropoints by a method known to people in the field. Each micropoint  67  is thus perfectly aligned along the axis of the corresponding hole  61  of the extraction grid  65  and along the axis of the corresponding aperture of the focussing grid  66 . 
     FIGS. 4A to  4 D are cross-section views of a micropoint electron source being manufactured according to a second mode for applying the process according to the invention. This mode can be used when the two insulating layers are of the same type or are not etched in a chemically selective manner. 
     On FIGS. 4A to  4 D, the same references as those on FIGS. 3A to  3 F indicate the same elements, only the nature of the materials may change. 
     As previously, the photosensitive resin layer  57  is insolated through a template and then developed to make holes  58  in it and these holes  58  are prolonged by holes  59  etched in the second conducting layer  56  (see FIG.  4 B). 
     Then anisotropic etching of the first conducting layer  54  is done to make holes  61  in it in the prolongation of the holes  58  and  59 . These holes  61  are holes in the extraction grid. They reveal the first insulating layer  53 . 
     The first insulating layer  53  is then isotropically etched to make, in this layer, the housings  71  centred on the axis of the holes  61  (see FIG.  4 C). The two insulating layers  53  and  55  being of the same nature, this etching leads to enlargement of the cavities already made in the second insulating layer  55  to yield cavities  72 . The two etching steps for the second insulating layer  65  are designed to produce cavities  72  whose maximum dimensions correspond to the apertures of the focussing grid. 
     The second conducting layer  56  is then etched so as to enlarge the holes of this layer to the maximum dimensions of the cavities  72  of the second insulating layer  55 . The apertures  64  of the focussing grid are thus obtained. 
     The photosensitive resin is then eliminated (see FIG. 4D) and the micropoints  67  can be deposited on the resistant layer  52 . Each micropoint  67  is thus perfectly aligned along the axis of the corresponding hole  61  of the extraction grid  65  and the axis of the corresponding aperture  64  of the focussing grid  66 . 
     Depending on the nature of the materials used to make the various layers and the desired degree of precision, many variants in the invention process are possible by grouping certain steps or changing their order. 
     Various geometries for the focussing grid are possible. FIG. 5 shows an example of a micropoint electron source obtained with the first mode of applying the invention process. In this example the holes  61  of the extraction grid  65  and the micropoints  67  are arranged along parallel lines. The distance separating two successive holes  61  on a given line is less then the aperture  64  of the focussing grid  66 . The distance between two lines of adjacent micropoints is greater than this aperture. The enlargement of the holes in the layers  55  and  56  to the diameter desired for the focussing grid  66  produces intersecting holes. The apertures of the focussing grid corresponding to a given line of micropoints  67  thus constitute slits with festooned sides, the axes of these slits being the same as the lines on which the corresponding micropoints are arranged. For such a structure, the focussing of the electrons is done only in the direction perpendicular to the planes of symmetry of the slits. The luminophores placed on the anode which, in the viewing device, faces the cathode, must be arranged along lines parallel to the emitting lines. 
     FIG. 6 shows another example of micropoint electron sources obtained by the first mode of applying the present invention. In this example, the holes  61  of the extraction grid  65  are located with respect to each other at a distance greater then the diameter of the apertures  64  of the focussing grid  66 . In this case, the openings  64  of the focussing grid  66  are holes concentric to the holes  61  of the extraction grid  65 . The electrons emitted by the micropoints  67  are then focussed regardless of their emission direction.