PROCESS FOR THE PRODUCTION OF FLAT-SCREEN GRIDS COATED WITH NON-EVAPORABLE GETTER MATERIALS AND GRIDS THEREBY OBTAINED

Abstract: A process is disclosed for the production of flat-screens grids coated with non-evaporable getter materials, wherein a metal sheet being as thick as the resulting grid and having at least the same surface area as the image formation zone is coated at least on one side with one or more non-evaporable getter materials and then parts of the sheet thereby coated are selectively removed. Some embodiments of grids produced with such a process are also disclosed.

Description: "PROCESS FORTHR PRODI JCTION OF FT AT-SCREEN GRIDS COATED WTTHNON- EVAPORABLE GETTER MATERIA S AND GRIDS THEREBY OBTAINED" DESCRIPTION The present invention relates to a process for the production of flat-screen grids coated with non-evaporable getter materials and to the grids thereby obtained. The flat screens are intensively studied since several years in order to replace the conventional bulky-sized television-kinescopes and computer-screens. Among the various suggested types of flat screens, the so-called field emission displays or FEDs seem to be particularly promising. Generally, a FED is obtained by welding two flat glass members along their perimeter. The welding is carried out by melting a low-melting glass paste with an operation called "frit-sealing". The resulting structure is formed of two mutually parallel surfaces at a distance ranging from a few tenths of millimeter to 2-3 millimeters. A plurality of sharpened microcathodes, made of metallic material, e.g. molybdenum, is provided on the inner surface, except for the edges, of the rear part; as well as a plurality of grid electrodes, placed in close proximity to the microcathodes, so that, by applying a small potential difference, a high electric field is obtained, capable of extracting electrons from the microcathodes. The electronic current is accelerated towards the phosphors, placed on the inner surface, except for the edges, of the front part. The zone wherein there are the phosphors, corresponding and opposite to the zone wherein there are the microcathodes, is the image formation zone. The screen image is formed by selectively exciting only some phosphors. In FEDs wherein microcathodes and phosphors are a few tenths of millimeter apart, the selective excitation of phosphors is obtained simply by selectively activating groups of microcathodes, since at these distances the electronic beam is sufficiently collimated. On the contrary, in order to selectively excite the phosphors in 2-3 millimeters thick FEDs, one or more electric grids are necessary to exactly directing the electronic beam. These grids are generally formed of metal sheets of thickness ranging from 20 to 200 μm and of the same surface area as the screen, having a plurality of pinholes of size ranging from about 30 to 200 μm and spaced about 30-300 μm apart. Further, in order to avoid the electronic beam dispersion, the FED internal space must be kept evacuated, at residual pressures not higher than 10 "3 mbar for hydrogen, and not higher than 10 "4 , preferably lower than 10 " mbar for other gases. Gases of various types may be emitted by the same FED composing materials during its working. As disclosed in patent applications WO 95/23425 and WO 96/01492 in the applicant's name, in order to maintain the necessary vacuum-degree inside the FEDs, non-evaporable getter materials, also known as NEG, may be used, being capable of fixing gases such as O 2 , H 2 O, CO, CO 2 and N 2 . In the current state of the art, the NEG devices are disposed inside the FEDs in form of little pills or thin layers on the edges of the zone wherein there are the cathodes. By thus operating, however, there is the problem of a slow gas transfer from the screen middle zone to the edges, particularly for large-sized FEDs, due to the small thickness of the FEDs void space. Accordingly gas concentration gradients are formed inside the FEDs, being harmful for their proper working. It is therefore an object of the present invention to provide getter material deposits regularly spread out on the entire screen surface. This object is achieved according to the present invention, which in its first aspect relates to a process for the production of flat-screens grids coated with getter materials, comprising the following steps: - providing a metal sheet being as thick as the resulting grid and having a surface area large at least as the image formation zone; - coating with one or more non-evaporable getter materials at least one side of the metal sheet; - selectively removing portions of the metal sheet coated with getter material. The invention will be hereinafter described with reference to the drawings, wherein: - in Fig. 1 there is shown a possible grid coated with getter material, obtained according to the invention; - in Fig. 2 there is shown a possible alternative embodiment of the grid according to the invention; - in Fig. 3 there is shown in diagrammatic form and in exploded view a FED comprising a possible grid obtained with the process according to the invention. The used sheet metal must show the usual features required for producing television-screens grids, namely must be easily formable and have a reduced vacuum gas-emission. Furthermore, it must have good adhesion properties for getter material powders. The preferred materials for this purpose are nickel and its alloys, as nickel-chromium alloys, or the alloy named INVAR, formed of about 64%ι by weight of iron and 36% by weight of nickel. The sheet thickness generally ranges from about 20 to 100 μm. The metal sheet must have at least the same surface area as the image formation zone. Preferably, the sheet surface is slightly larger than the surface of the image formation zone, such as to provide an outer edge not having holes made thereon, as hereinafter described. This edge may be useful for fixing the sheet inside the FEDs, and may be coated with the NEG material deposit, thereby providing an additional amount of this material. Alternatively, the edge may be kept free from the NEG material deposit, thereby favoring the operations for fixing the grid to the FED structure. Finally, intermediate solutions are possible, wherein the edge is only partly coated with the NEG material deposit, e.g. by coating two opposite side edges and keeping free the other two opposite side edges, thereby making a compromise between the above-mentioned advantages. It is also possible to use sheets of surface area far larger than the image formation zone, equal to about multiples thereof or of this plus an edge. In this case the resulting grids are obtained by cutting suitably sized pieces from the starting sheet, after having produced thereon the NEG material deposit. The metal sheet may be coated with getter material on one or both its sides. In order to coat the metal sheet with the NEG material deposit, all the various available techniques for producing supported thin layers of powders may be in principle used, such as the cold rolling, the spray techniques or the serigraphic technique. The coating of metal supports with NEG materials by cold rolling is well known in the metallurgical field, while the spray coating is disclosed e.g. in patent application WO 95/23425 in the applicant's name. It is preferred to use the serigraphic technique, allowing to obtain the greatest uniformity of the getter material layer when operating on large surfaces. In order to obtain a getter material layer according to the serigraphic technique, first a suspension of the material powders is prepared in a water-, alcohol- or hydroalcohol- suspending medium, wherein there are also amounts of high-boiling organic compounds, serving as viscosity adjusters, smaller than 1% of the total weight of the suspension. The suspension thereby obtained is then spread onto a net screen made of plastic material, with ports of size ranging from 10 to 200 μm; the net screen is stretched on a rigid frame and kept at a distance from the substrate ranging from 0,5 to 2 mm. By shim applying suitable rubber or metal sleekers on the upper side of the net screen, having the suspension thereon, this is forced into the net screen ports, thereby depositing on the substrate. This deposit is then dried and sintered, thus obtaining the resulting coated sheet. As to the details of the preparation of NEG material layers with the serigraphic technique, reference is to be made to International publication WO 98/03987, in the applicant's name. Another advantage of the serigraphic technique is that, by selectively obstructing the net screen ports according to selected patterns, shaped powders deposits can be obtained; it is thus particularly easy e.g. to produce metal sheet coatings having their edges totally or partly free from the NEG material for the above-mentioned purposes. Shaped deposits may be obtained also by replacing the serigraphic net screen with suitable metal plates. The thickness of the NEG material layer after sinterization ranges preferably from 20 and 100 μm. Deposits of too little thickness make little getter material available. On the other hand, too much thick deposits make the coated sheet hard to be properly cut, for obtaining the grid holes. For the sake of grid mechanical stability, the deposit is preferably not thicker than the sheet. Furthermore, if the deposit is produced by spray or serigraphic technique and the sheet is coated on both its sides, the two deposits on the opposite sides have preferably the same or at least a similar thickness, in order to prevent sheet distortions in the subsequent deposit sinterization step. The NEG material used for the sheet coating may be any of the known NEG materials, such as e.g. zirconium, titanium, niobium, hafnium, tantalum, tungsten metals, mixtures and alloys thereof comprising these or other metals, generally selected among those belonging to the first transition series and aluminium. It is preferred to use the getter alloys disclosed in patents US 3,203,901, US 4,071,335, US 4,306,887, US 4,312,669, US 4,839,085, US 5,180,568; or zirconium-cobalt alloys containing about 75-90% by weight of zirconium, or alloys therefrom obtained by adding rare earths up to 10% of the total alloy weight; or further titanium-vanadium and titanium-chromium alloys containing about 70-80% by weight of titanium. It is particularly preferred the use of the alloy containing 70% by weight of zirconium, 24,6% by weight of vanadium and 5,4% by weight of iron, produced and sold by the applicant under the tradename of St 707. Mixtures of several alloys or mixtures of alloys and the above-mentioned getter metals may also be used. Finally, if the deposit is produced by spray or serigraphic technique, powder of a metal such as nickel or titanium may be added to the mixtures of metal powders or above-mentioned getter alloys, in amount ranging from about 2 to 20% by weight of the whole mixture, in order to favor the sinterization of the layer of powders. The last process step consists of selectively removing parts of the NEG material-coated metal sheet, by making thereon the holes for the electron beams passage. The holes are generally square-, rhombohedrical-, round- or elliptical- shaped, have size ranging from about 50 to 200 μm, and are spaced apart by metal members having width ranging from 50 to 300 μm. Since for a good image quality the holes must be as even as possible and have sharp and regular edges, it is preferable, in order to make these holes, to use the chemical-etching, which allows to have a cutting accuracy of about 10 μm. The chemical-etching is the preferred technique in case of sheets coated with NEG material on a single side: in this case, the operations related to the chemical-etching technique are performed on the opposite side with respect to the side having the NEG material thereon. Alternatively, the laser-cutting technique may be employed, being the preferred technique in case of sheets coated with NEG material on both their sides. By laser- cutting, a cutting width of about 30 μm and a suitable accuracy for the production of FED grids are obtained. Furthermore, the laser-cutting causes the sheet to locally melt; this avoids the presence of cutting burrs caused by mechanical cutting; further, this local melting cooperates to fix to the cutting edge the NEG material particles, which otherwise could be detached to generate metal powders inside the FED. Both the presence of burrs at the cutting edge and of loose powders may generate spurious electric fields, thus modifying the electron beam emission or transmission and adversely affecting the image formation. Since the metal members spacing the grid holes apart are about 50-300 μm wide, it is preferable, in order to have at the end of the process a regular coating thereof, to use NEG materials powders having fine particle size, preferably smaller than about 50 μm for the grids with lower definition (holes size and metal members therebetween). As the grid definition increases, the maximum powders particle size which can be used decreases, and for the finer grids it is preferable to use powders having size smaller than about 20 μm. In its second aspect the invention relates to the grids obtained with the above-mentioned process. Some possible grids are hereinafter described, by way of non-limiting examples of the scope of the invention, with reference to the drawings. In Figure 1 a portion of a possible grid obtained with the process of the invention, wherein the NEG material covers all the available surface, is shown in a perspective view. Grid 10 is formed of a metal sheet 11 coated on both its sides (12, 12') with NEG material deposits (13, 13'). On the sheet there is a plurality of holes 14, 14', .., spaced apart by metal members 15, 15', ..., having the NEG material thereon. For the sake of simple representation, and in order to provide a clearer idea of the grid geometry, the NEG material is shown to coat only a portion of the two sides of sheet 11, but it is intended to coat the entire sheet. Furthermore, although the drawing represents a portion of the metal sheet without NEG material and with holes 14, 14', these holes are obtained, according to the process of the invention, only when the continuous sheet has been wholly coated with NEG material. Finally, the exemplified grid has square-section holes for the electron flow passage, and is coated with a NEG material deposit on both its sides, but all the combinations of possible hole geometries and coating are allowed according to the invention. In Figure 2, a portion of a possible alternative grid according to the invention, wherein the edge is free from the NEG material coating, is shown in a plan view. Grid 20 is formed of a sheet 21, having an outer edge 22 free from NEG material deposits, and middle zone 23 (enclosed by the hatched lines in the drawing) coated with NEG material deposit 24; also in this case, like in Figure 1 , deposit 24 is only partly represented. In zone 23, holes 25, 25' are made for the electron passage. In this case round holes disposed in a square screen pattern are exemplified, but all the combinations of possible hole shape and screen pattern are allowed, such as e.g. an hexagonal screen pattern of round holes. In Figure 3 the exploded view of a part of a FED comprising a grid of the invention is diagrammatically shown, having its edge free from NEG material as represented in Figure 2. In the figure the FED is formed of a front glass portion 31 and a rear portion 32; grid 20 is placed between these two portions; on inner surface 33 of portion 32 there are the microcathodes (not shown in the drawing) disposed in zone 34; on inner surface 35 of portion 31, in correspondence with zone 34, there are the phosphors disposed in zone 36, being also the image formation zone; grid 20 is disposed such that zone 23 (wherein there are the holes and the NEG material deposit) is essentially equivalent to the projection of the image formation zone on the grid itself, with edge 22 outside of such projection zone. The grids coated with getter materials of the invention perform the double task of directing the electron beam and of uniformly spreading the getter material inside the screen, thereby eliminating the mentioned problems of the prior art. These grids could not be obtained e.g. by coating with getter materials pre- perforated metal sheets; in fact, when trying to sinterize NEG materials deposits on substrates with many close holes such as those required for FED grids, the substrate itself undergoes heavy distortions, likely due to the interactions occurring at high temperature between metal and getter material.

Claims: CLAIMS 1. Process for the production of flat-screens grids coated with getter materials, comprising the following steps: - providing a metal sheet being as thick as the resulting grid and having a surface area large at least as the image formation zone; - coating with one or more non-evaporable getter (NEG) materials at least one side of the metal sheet; - selectively removing portions of the metal sheet coated with getter material. 2. Process according to claim 1, wherein the metal sheet is made of nickel or of a nickel-chromium or nickel-iron alloy. 3. Process according to claim 1, wherein the metal sheet has a thickness ranging from 20 to 100 μm. 4. Process according to claim 1, wherein the metal sheet has the same size as the resulting grid and is wholly coated with the NEG material deposit. 5. Process according to claim 1, wherein the metal sheet has the same size as the resulting grid and is partly coated with the NEG material deposit, with one or more outer edges free from such deposit. 6. Process according to claim 1, wherein the metal sheet has a size about equivalent to a multiple of the resulting grid surface and is wholly coated with the NEG material deposit. 7. Process according to claim 1, wherein the metal sheet is coated with the metal sheet on a single side. 8. Process according to claim 1, wherein the metal sheet is coated with the NEG material deposit on both its sides. 9. Process according to claim 1, wherein the NEG material deposit is produced by cold rolling. 10. Process according to claim 1, wherein the NEG material deposit is produced by spray technique. 11. Process according to claim 1, wherein the NEG material deposit is produced by serigraphic technique. 12. Process according to claim 1, wherein the NEG material deposit has a thickness ranging from 20 to 100 μm. 13. Process according to claim 12, wherein the NEG material deposit is not thicker than the used metal sheet. 14. Process according to claim 1, wherein, when the deposit is produced by spray or serigraphic technique and the sheet is coated on both its sides, the two deposits on the opposite sides have the same or a similar thickness. 15. Process according to claim 1, wherein the NEG material is selected among zirconium, titanium, niobium, hafnium, tantalum, tungsten, mixtures and alloys thereof comprising these same metals or other metals selected among those belonging to the first transition series and aluminium. 16. Process according to claim 15, wherein the NEG material is selected among Zr-Al, Zr-Fe, Zr-Ni, Zr-Co, Ti-V, Ti-Cr, Zr-V, Zr-V-Fe, Zr-Mn-Fe and Zr- Co-rare earths alloys. 17. Process according to claim 15, wherein the NEG material is a mixture of zirconium, titanium, niobium, hafnium, tantalum, tungsten and their mixtures and the alloys of claim 16. 18. Process according to claim 15, wherein, when the deposit is produced by spray or serigraphic technique, nickel or titanium powder is added to said NEG materials, in amount ranging from 2 to 20% by weight of the whole mixture. 19. Process according to claim 1, wherein the NEG materials have particle size smaller than about 50 μm. 20. Process according to claim 19, wherein the NEG materials have particle size smaller than about 20 μm. 21. Process according to claim 1, wherein the selective removal of parts of the sheet coated with NEG materials is performed by the so-called "chemical- etching" technique. 22. Process according to claim 1, wherein the selective removal of parts of the sheet coated with NEG materials is performed by laser-cutting. 23. Grids coated with non-evaporable getter materials on one or both their sides produced according to the process of claim 1. 24. Grids according to claim 23, having square-, rhombohedrical-, round- or elliptical-shaped holes. 25. Grids according to claim 24, with holes having a size ranging from about 50 to 200 μm, spaced apart by metal members having width ranging from 50 to 300 μm.. 26. Grids according to claim 24, having holes disposed in a square or hexagonal screen pattern.

Inventors: CORAZZA ALESSIO, CORAZZA, ALESSIO
Applicants: GETTERS SPA, SAES GETTERS S.P.A.
Application year: 1998
Main classification: H01J9
IPC codes: H01J7, H01J9, H01J29, H01J31