FLAT PANEL DISPLAY WITHOUT TIP AND MAKING
METHOD THEREOF Technical Field [1] The present invention relates, in general, to a tipless flat panel display anda method for fabricating the same. More particularly, the present invention relates to a flat display device for presenting desired letters or characters through phosphor-coated discharge grooves formed in a flat glass plate by emitting light from the phosphor- coated discharge grooves upon discharging, which can be made slim because no tip is employed and a method for fabricating the same. Background Art [2] Various panel displays for forming and displaying pictures, letters or characters have been developed, including LED displays, PDPsand CRTs. A plasma display panel is a flat panel display technology which uses light emission produced by gas discharge to form and display the required characters. This discharge occurs between two electrodes in a small gas-filled discharge gap which is enclosed by a pair of glass plates. In this regard, after being vacuumized to a high level, the space between an upper glass plate and a lower glass plate (in which a mid glass plate may be included) is filed with an inert gas such as neon or argon or a mixture thereof, and is completely sealed. A tip is used to evacuate air out of and inject inert gas into the space through a hole. [3] For this, the tip, generally made of a glass tube, is brought into close contact with the hole. Then, air is drained and an inert gas is injected through the glass tube. Thereafter, the tip, protruding externally, is melt-sealed and trimmed so as to close the hole. [4] Korean Pat. Laid-Open Publication No. 2001-68169 discloses a technique in which a tip is used for air evacuation and gas injection in a display device. [5] This conventional technique is applied to a flat neon sign board, which generally comprises an upper glass plate having discharge electrodes on its inner surfaceand a lower glass plate in contact with the upper glass plate, in which a cavity is provided as a discharge space for forming advertising characters. [6] With reference to FIG. 1, there is a cross sectional view showing a structure of a conventional display device that uses a tip. In this display device, an upper protection glass plate 10 has transparent electrodes 12 at regular intervals on its lower surface, with an upper inner glass plate 14 in contact with the transparent electrodes 12 so as to protect them. [7] Likewise, a lower inner glass plate 14' has a lower transparent electrode 12' on its lower surface and a lower protection glass plate 10' is positioned at a distance H apart from the lower transparent electrode 12'. [8] The upper inner glass plate 14 and the lower inner glass plate 14' are bonded to each other by a sealant 16, and all of the plates provided in the display device are supported by a frame 18. [9] The lower inner glass plate 14' has a cavity therein, with a phosphor layer on the cavity. The cavity is formed by partially engraving an upper portion of the lower inner glass plate 14'. [10] Running through the lower inner glass plate 14' and the lower transparent electrode 12', a hole 20 is formed locally through which air is evacuated from the cavity to form a vacuum state and an inert gas is injected into the cavity. After the injection of the inert gas therethrough, the hole 20 is closed in order to prevent the inflow of other air. For this, a glass tube tip 22 is provided. [11] With reference to FIG. 2, there is a schematic view showing the role of the tip in the convention display device. As seen in this figure, the glass tube tip 22, which is wider in diameter than the hole 20, is inserted into the hole 20 to communicate with the space between the inner glass plates 14, 14'. In addition, the tip 22 is connected to a vacuum pump 52 and a gas injector 54, and each valve is opened in an alternating manner. [12] The space is vacummized to a pressure of 10-6 to 10-7 torr by the vacuum pump 52, and then filled with an inert gas to a pressure of 500 to 550 torr by the gas injector 54. Thereafter, the tip is clogged by melting its middle portion with a heat sealing device 56 to prevent the entrance of air. Finally, the end of the molten tip is trimmed. [13] According to the prior art, as described above, the tip 22 must protrude a con¬ siderable distance from the lower transparent electrode 12'. Accordingly, the distance H between the lower transparent electrode 12' and the lower protection glass plate 10' must be long. In fact, the upper protection glass plate 10 is about 30 mm apart from the lower protection glass plate 10' in the conventional display device. [14] This is a factor that makes the display device thick. Additionally, because it pro- trudesexternally, the tip 22 is likely to be broken, which causes air entrance and gas leakage. [15] Referring to FIG. 3, there is a schematic cross sectional view showing that the upper glass plate is bent downwards in a vacuum state of the conventional display device. As seen in FIG. 3, since the support between the upper glass plates and the lower glass plate is provided only by the sealant 16, the middle portion of the upper glassplate gets bent downwards by external pressure when the inner space is highly vacummized. Accordingly, the conventional display device has a high possibility of being be cracked or structurally damaged. [16] Furthermore, the upper glass plate, when distorted, becomes spaced apart from the frame 18, so that air or moisture may be introduced into the interior through the gap produced. Accordingly, the conventional display is not suitable for use in exterior decorations such as a signboard. Disclosure of Invention Technical Problem [17] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a flat display device which can be fabricated in a slimmer size by closing a hole through which air evacuation and gas injection is carried out, with a filling member, instead of a glass tube tip, so as to leave no material protruding externally, and a method for fabricating the same. [18] Another object of the present invention is to provide a tipless flat display device which can reach a high vacuum state within a short time and is not damaged in such a high vacuum state. Advantageous Effects [19] According to the present invention, a hole through which air evacuation and inert gas injection are carried out when fabricating a flat plasma display device can be plugged without the use of a glass tube tip, whereby the process of melt-sealing and trimming the tip, after inert gas injection, can be omitted and an unnecessarily protruding portion does not remain so as to make the display device slim. Brief Description of the Drawings [20] FIG. 1 is a schematic cross sectional view showing the structure of a conventional display device employing a tip; [21] FIG. 2 is a schematic structural view showing the closure of the tip in the conv entional display device; [22] FIG. 3 is a schematic cross sectional view showing a bent upper plate in a vacuum state in the conventional display device; [23] FIG. 4 is a view showing an upper unit of a display device according to an embodiment of the present invention; [24] FIG 5 is a view showing a lower unit of a display device according to an embodiment of the present invention; [25] FIG. 6 is a schematic cross sectional view showing the upper glass plate and the lower glass plate assembled with each other; [26] FIG. 7 is a schematic view showing a process of closing the through-hole 260 in accordance with the present invention; and [27] FIG. 8 is a view showing an example of the appearance of the display device in which spacing bars are installed. Best Mode for Carrying Out the Invention [28] The objects of the present invention could be accomplished by the provision of a tipless flat display device, comprising an upper inner glass plate and a lower inner glass plate, which are kept at a distance apart from each other and sealed along their edges to form an inner space therebetween, with an inert gas filled in the inner space, wherein the upper glass plate is covered with an upper transparent electrode made from ITO or ATO; the lower glass plate has discharge grooves engraved in letters, characters or pictures in its upper surface, each of the discharge grooves being coated with a phosphor; a through hole is formed in a portion of the upper inner glass plate or the lower inner glass plate, communicating with the inner space between the upper inner glass plate and the lower inner glass plate, and a plurality of spacing bars uniform in height is provided on the lower surface of the upper inner glass plate or on the upper surface of the lower inner glass plate in non-discharge groove regions, preventing the upper inner glass plate from being brought into close contact with the lower inner glass plate. [29] In the tipless flat display device, an upper protection glass plate and a lower protection glass plate are formed on the upper transparent electrode and beneath the lower transparent electrode, respectively, and sealed at their edges with silicon or epoxy. [30] In the tipless flat display device, the inner space between the upper inner glass plate and the lower inner glass plate is vacummized and then filled with the inert gas through the through-hole by use of an evacuation pipe equipped with a vacuum pump and an inert gas injector, which are operated in order, and the through-hole is plugged by a filling member which is UV-curable, the filling member being filled in the through-hole and cured with UV radiation. [31] Also, the present invention provides a method for fabricating a display device in which the display device comprises an upper inner glass plate having an upper transparent electrode thereon, and a lower inner glass plate having a phosphor-coated discharge groove engraved in an upper portion thereof, comprising: forming a through- hole in a portion of the upper inner glass plate or the lower inner glass plate, said through-hole communicating an external space with an internal space between the upper inner glass plate and the lower inner glass plate therethrough; providing a plurality of spacing bars on the lower surface of the upper inner glass plate or on the upper surface of the lower inner glass plate in a region which does not overlap the discharge groove, all of the spacing bars being the same in height; bonding the upper inner glass plate to the lower inner glass plate by applying an adhesive to their edges, the upper inner glass plate and the lower inner glass plate being spaced at a distance apart from each other; introducing an inert gas into the internal space in a vacuum state; and inserting a filling member into the through-hole to close the through-hole. [32] The method further comprise: forming an upper protection glass plate and a lower protection glass plate on the upper transparent electrode and beneath the lower transparent electrode, respectively, and sealing the upper protection glass plate and the lower protection glass plate at their edges with silicon or epoxy. [33] In the method, the filling member is UV-curable and, after being placed into the through-hole, is cured with UV radiation. Mode for the Invention [34] Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components. [35] FIGS. 4 and 5 are schematic views showing the structures of an upper glass plate and a lower glass plate, respectively, of a display device according to an embodiment of the present invention, and FlG. 6 is a schematic cross sectional view showing the upper glass plate and the lower glass plate, which are assembled with each other. [36] With reference to FIGS. 4 and 5, a tipless flat plasma display device (hereinafter referred to as "display device") according to an embodiment of the present invention is generally composed of an upper plate unit 100 and a lower plate unit 200. [37] The upper plate unit 100 comprises an upper protection glass plate 110 and an upper inner glass plate 130, both made from transparent glass, with an upper transparent electrode 120 therebetween. The upper inner glass plate 130 serves to prevent the upper transparent electrode 120 from being burned or damaged upon discharge. [38] When a high voltage is applied to the display device, discharge occurs through the upper transparent electrode 120 and the UV light generated by the discharge causes a phosphor coating to emit visible light. In the present invention, the upper transparent electrode 120 is distributed uniformly throughout the upper surface of the upper inner glass plate 130. [39] A material for the upper electrode 120 is required to be transparent and electrically conducting. Generally, ITO (indium tin oxide) or ATO (antimony tin oxide) is used. ITO is Sn-doped In2O3 which contains In2O3 and SnO2 in a ratio of 90: 10 to 95:5. Having similarity in various properties, but superiority in electric conductivity and optical transmittance to ATO, ITO finds a variety of applications in the electrical field, including display panels, electromagnetic shielding films, organic EL, etc. [40] At a lateral end of the upper transparent electrode 120, a bus electrode (not shown), which is a highly conductive electrode functioning to uniformly distribute high voltage across the upper transparent electrode 120 may be provided. [41] Sputtering may be conducted to coat the transparent electrode 120 on the upper inner glass plate 130. Upon discharge, the transparent electrode 120 is protected by the upper inner glass plate 130 from being burned or damaged. [42] The fabrication of the upper plate unit 100 in which to form the upper transparent electrode 120 starts with the formation of a sputtering target which is generally obtained by mixing In2O2 and SnO2 and sintering the mixture. The fabrication may undergo the processes of material mixing, molding, sintering, and bonding. [43] Having a structure similar to that of the upper plate unit 100, the lower plate unit 200 is composed of a lower inner glass plate 230 with a lower transparent electrode 220 coated on the lower surface thereof, and an underlying lower protection glass plate 210. Together presenting a desired pattern of pictures, letters, or characters through the display device, a plurality of discharge grooves 240 is formed in the upper portion of the lower inner glass plate 230. That is, because they allow light to be emitted therethrough, the discharge grooves 240 determine the shape of light visualized externally. [44] The discharge grooves 240 may be formed by engraving the lower inner glass 230 in a predetermined pattern. For the formation of the discharge grooves 240, con¬ ventional techniques may be used such as chemical etching or sandblasting techniques. [45] As far as sandblasting is concerned, for example, it may be conducted by coating the glass plate with a film that is required to be so flexible as to sustain sandblasting, cutting the film in a desired pattern, and projecting sand all over the glass plate to etch the portions which are not masked by the film. [46] The discharge grooves 240 may be patterned in the upper inner glass plate 130 as well as in the lower inner glass plate 230 in accordance with the present invention. [47] A phosphor coating is applied to the discharge grooves 240. Phosphors are uniformly coated over the entire surface of the discharge grooves 240. After being excited to a higher energy state by the UV light generated when discharge occurs, electrons of the phosphors are returned to the ground state with the concomitant generation of light rays. The light rays may differ from one another, depending on the kinds of phosphors coated. Thus, different phosphors allow various colors to be represented through the discharge grooves 240 which may be in diverse forms. Because of the presence of the phosphor coating within the discharge grooves 240 of the lower inner glass plate 230, no light is emitted by the discharging generated outside the discharge grooves 240. [48] A plurality of spacing bars 250 is provided on the lower inner glass plate 230, serving as a support to keeping the upper inner glass plate 130 spaced apart from the lower inner glass plate 230. Preferably, the spacing bars 250 are distributed at regular intervals and made from a non-conducting dielectric material so as to have no influence on the discharge of the discharge grooves 240. [49] Being as high as tens of D, each of the spacing bars 250 prevents the upper plate unit 100 from being bent to closely contact the lower plate unit 200 when in a vacuum state, so that air may be evacuated to achieve a high vacuum state in the space between the upper unit and the lower unit within a short period of time. In practice, a desired high vacuum state is achieved by operating a vacuum pump for five hours or more in the absence of the spacing bars 240. However, the spacing bars 250, which do not allow the upper plate unit 100 to contact the lower plate unit, enable the vacuum pump to operate for only about two hours to achieve the same vacuum state. [50] A through-hole 260 is provided at a side portion, running through the lower inner glass 230 and the lower transparent electrode 220. The through-hole 260 is a passage through which the inner space between the upper inner glass plate 130 and the lower inner glass plate 230 is highly vacummized and filled with an inert gas such as neon (Ne), helium (He) or xenon (Xe). [51] During the fabrication of the display device, the through-hole 260 must be closed after the air evacuation and the gas injection. [52] With reference to FlG. 6, there is a cross sectional view showing the display device to which the upper plate unit 100 and the lower plate unit 200 are assembled. Spaced apart from each other, the upper plate unit 100 and the lower plate unit 200 are attached to each other via a sealing line 300. As seen in Fig. 6, a phosphor coating 270 is applied on each of the discharge grooves 240 while the upper plate unit 100 is positioned at a distance from the lower plate unit 200 by the spacing bars 250. [53] The sealing line 300 is an adhesive made of glass. [54] The through-hole 260 is plugged by a filling member 400 to prevent air influx into or gas efflux from the display device. [55] In conventional flat display devices, the through-hole 260 is plugged by use of a glass tube tip. [56] As previously mentioned, the closure of the through-hole 260 with the glass tube tip suffers from the disadvantage of making the flat display devices thick because the glass tube tip remains protruded. [57] With reference to FlG. 7, there is a schematic view showing a process of closing the through-hole 260 in accordance with the present invention. [58] As seen in this figure, a tube that is larger in diameter than the through-hole 260 is brought into close contact with the through-hole 260. The tube is divided into two stems: one is connected to a vacuum pump 510 and the other leads to a gas injector 520. [59] By operating the vacuum pump 510, air is evacuated until the inner space of the display device is vacummized to a pressure of 10"6 to 10"7 torr. After the vacuum condition is obtained, the operation of the vacuum pump is ceased by closing the valve therefor. Thereafter, a valve for the gas injector 520 is opened to introduce an inert gas, such as neon, helium or xenon, into the inner space of the display device until the pressure of the inner space reaches 500 to 550 torr. After completion of the gas injection, the through-hole 260 is closed. [60] In the present invention, the closure of the through-hole 260 needs a filling member 400. In this regard, the filling member 400 which is in a liquid state is carried by a metal or glass strip 530 driven by an actuator and pulled into the through-hole 260. The filling member 400 is preferably UV-curable. After plugging the through-hole 260, the liquid filling member 400 is cured with UV radiation. [61] The actuator for moving the metal or glass piece 530 may be operated manually or automatically. The operation of the actuator may be carried out with the aid of an oil pressure-type or a screw-type mechanism. [62] After the filling member 400 is cured, the metal or glass piece 530 is removed from the solidified filling member, so that the exterior of the closed through-hole remains flat without additional processing. [63] Unlike the conventional display device, in which the protruding tip is sealed by melting and cut off, the present invention, in which the through-hole 260 is plugged with the filling member, does not suffer from the disadvantage of leaving the tip protruding externally. [64] Edges of both the upper protection glass plate 110 and the lower protection glass plate 210 are sealed with silicone 310. Instead of silicone 310, epoxy may be used. Sealing the upper protection glass plate 110 and the lower protection glass plate 210 with silicone or epoxy prevents moisture or external air from being introduced into the inner space. Additionally, when the display device is fabricated into forms other than circles or squares, the silicone or epoxy sealing need not additionally fabricate frames corresponding to the forms, thereby simplifying the fabrication of the display device and reducing its fabrication cost. [65] With reference to FIG. 8, there is a view showing an example of the appearance of the display device in which spacing bars are installed. As seen in FIG. 8, the set of the spacing bars may form the same pictures, letters or characters presented by the discharge grooves 240. Thus, when discharge does not occur, that is, when no electric field is applied, the display device can show pictures, letters or characters, serving as an advertising board. [66] Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. [67] For example, the through hole 260, if described above to run through the lower inner glass plate 230 and the lower transparent electrode 220, may be formed in the in the upper plate unit 100 or in both the upper plate unit 100 and the lower plate unit 200. [68] Additionally, the discharge grooves 240 may be engraved in the lower portion of the upper inner glass plate 130 and coated with phosphors so that light emission may occur in the upper plate unit 100.