Method for making a plasma display

Abstract: A method for fabricating plasma display wherein an organic film formed on a pattern of discharge electrodes is pre-heated to a temperature lower than a temperature, at which the organic film undergoes an exothermic phenomenon, for a given time, is described. By this, in the firing treatment after application of an insulating material inbetween organic films, a gentle change in shape of the organic film during the process of burning off the organic film is effective in suppressing a change in shape of barrier ribs formed of the insulating material.

Description: Field of the Invention This invention relates to a method for making a plasma display and more particularly, a method for forming barrier ribs of a plasma display panel (hereinafter referred to simply as PDP) used as a display device for letter information or picture information such as a bar graph in terminal equipments of computers and automatic ticket vending machines. Description of the Prior Art Fig. 1 is a schematic sectional view of a prior art PDP structure. In the figure, reference numeral 1 indicates a front transparent flat plate made of a glass sheet or an analogue thereof, reference numeral 2 indicates first discharge electrodes aligned at given intervals on the inner surface of the front transparent flat plate 1, and reference numeral 3 indicates a back flat plate in face-to-face relation with the front transparent flat plate 1 with a small gap therebetween. Reference numeral 4 indicates second discharge electrodes provided in lines to form a matrix along with the first discharge electrodes on the inner surface of the back flat plate 3, and reference numeral 5 indicates barrier ribs each of which is provided between and in parallel to the discharge electrodes on the inner surface of the front transparent flat plate 1 in order to prevent a display discharge from being spread along the second discharge electrodes 4 to an extent outside a certain region and to ensure a certain discharge space. The operation of PDP will be described. When the discharge electrodes 2, 4 connected to a discharge cell for display are appropriately selected and applied with a high voltage, a discharge light-emitting gas sealingly filled between the discharge electrodes 2, 4 is discharged to a plasma discharge P and emits light as shown in Fig. 1. The emitted light reaches a display face through the front transparent flat plate 1, thereby displaying a letter or figure. In this case, the light emission by discharge with the discharge light-emitting gas tends to spread over a non-display area along the selected discharge electrode and particularly, the discharge electrode 4 with which the scanning is effected. This is inhibited with the barrier rib 5 to limit the light emission discharge within a given area, thereby preventing an erroneous discharge or cross-talking between the discharge cells with a good display. Thus, the barrier rib 5 serves to keep a uniform discharge space by utilizing its height, width and pattern gap and also to increase mechanical strength of the panel as a whole. For the formation of the barrier rib 5, there is conventionally used a thick film printing method as shown in Fig. 2. The thick film printing method comprises providing discharge electrodes 2 in lines on a front transparent flat plate 1 [Fig. 2(a)], printing a black glass paste 6 between adjacent electrodes on the front transparent flat plate 1 by the use of a printing screen 9 and drying the paste [Fig. 2(b)], and repeating the printing and drying steps five to 10 times [Fig. 2(c) and 2(d)]. Another method for forming the barrier rib 5 includes a method using photosensitive organic films. Such a method is described in JP-A-2 165 540. Fig. 6 shows this formation method in which a photosensitive organic film 7 is formed on the discharge electrodes 2 formed in lines on the front transparent flat plate 1, on which a mask 8 having holes 8a corresponding to the positions of the discharge electrodes 2 is superposed [Fig. 6(a)], followed by exposure to light and development. Thereafter, the photosensitive organic film 7 is removed at portions which have not been exposed to light by the action of the mask 8 [Fig. 6(b)]. Subsequently, a black glass paste 6 is filled in the removed portions of a pattern formed by the exposure and development [Fig. 6(c)], dried and washed on the surface thereof, followed by firing and removal of the remaining portions of the photosensitive organic film 7 at the same time [Fig. 6(d)] and washing. The known methods for the formation of the barrier rib 5 have been carried out as described above. However, with the thick film printing method, a difficulty in involved in registration of the black glass paste 6 with the discharge electrodes 2 formed on the front transparent flat plate 1. Even though the registration becomes possible at part of the panel, the registration over the entire surface of the panel will present a problem such as by elongation of the printing screen 9. Accordingly, the five to ten repetitions of the superposed printing of the black glass paste 6 bring about disturbance of the bottom line of the barrier rib 5 or as inaccuracy of the height, as shown in Figs. 3 and 4, along with the problem that the working properties are poor. In addition, the disturbance of the bottom line of the barrier rib 5 is inevitable for the printing, so that the shape of the display cell is greatly influenced by the blurring of the barrier rib 5, with the display quality being worsened. Where the photosensitive film is used, the problem of disturbance of the bottom line of the barrier rib 5 and of accuracy of the height is not produced. However, the removal of the photosensitive organic film 7 by burning will involve a great change in shape and partial deformation or breakage of the barrier rib 5 by bonding with the black glass paste 6 as shown in Fig. 7(a). In addition, part 6a of the black glass paste 6, which has been filled in, is taken in and serves to soil the display cells as shown in Fig. 7(b). Thus, it is difficult to form barrier ribs 5 which have a given aspect ratio and are uniform and stable. In case where there is used a method wherein a glass paste is embedded in the photosensitive organic film 7, a larger aspect ratio of the barrier rib 5 has a greater tendency toward part 7a of the photosensitive organic film 7 being left in the display cell, with a poorer yield of the barrier rib 5. Thus, this method leaves a problem on the productivity of the barrier rib 5. SUMMARY OF THE INVENTION An object of the invention is to provide a method for forming barrier ribs which overcomes the problems involved in the prior art and wherein the barrier ribs can be formed in high precision with good working properties. The method according to the invention is defined in claim 1. The insulating material should preferably be a glass paste comprising a glass component which is softened at the pre-heating temperature and another glass component which is softened in the vicinity of a burning or firing temperature of the organic film. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view of a structure of a conventional plasma display panel; Fig. 2 is an illustrative view showing the steps of a thick film formation method which is one of conventional barrier rib formation methods; Fig. 3 is a perspective view of part of the barrier ribs formed by the method illustrated in Fig. 2; Fig. 4 is a plan view of the barrier ribs illustrated above; Fig. 5 is a plan view showing a discharge light-emitting state which is generated in a region partitioned with the barrier ribs of Fig. 3; Fig. 6 is an illustrative view of the steps showing another conventional formation method of barrier ribs; Fig. 7 is an illustrative view of defects of the barrier rib formed by the method of Fig. 6; Fig. 8 is an illustrative view of the steps showing a method for forming barrier ribs according to one embodiment of the invention; Fig. 9 shows a graph for a burning process of a photosensitive organic film with and without a pre-heating treatment or a preliminary burning treatment; Fig. 10 is a graph showing the relation between the aspect ratio of a barrier rib and the accepted ratio of rib; Fig. 11 is a graph showing the relation between the weight ratio of low melting glass and the ratio by percent of the low melting glass in the surface of a barrier rib; Fig. 12 is a perspective view of part of barrier ribs formed by the method illustrate in Fig. 8; Fig. 13 is a plan view showing a discharge light-emitting state generated in a region partitioned with the barrier ribs; Fig. 14 is an illustrative view of the steps showing a method for forming barrier ribs according to another embodiment of the invention; and Fig. 15 is a perspective view of part of the barrier ribs formed by the method illustrated in Fig. 14. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the invention will now be described with reference to the Fig. 8, which shows the steps according to the method of the invention. In the figure, reference numeral 1 indicates a front transparent flat plate, reference numeral 2 indicates discharge electrodes arranged in lines on the front transparent flat plate, and reference numeral 7 indicates an organic film provided on the discharge electrodes 2. In this embodiment, a photosensitive organic film is used for illustration. Reference numeral 8 indicates a mask having through-holes 8a provided at positions corresponding to the respective discharge electrodes 2. These are the same as those shown in Figs. 1 and 6. The embodiment shown in Fig. 8 according to the method of the invention is described. First, the discharge electrodes 2 are arranges lines at given intervals on the front transparent flat plate 2, on which the photosensitive organic film 7 having a uniform thickness and uniform characteristics are laminated. Then, the mask 8 having a desired barrier rib pattern is superposed on the photosensitive organic film 7 [Fig. 8(a)]. Subsequently, the film 7 is subjected to exposure to light and development, thereby leaving the photosensitive organic film 7 at portions at which the discharge electrodes 2 are provided and where a black glass paste 6 used as an insulating material has not yet been deposited [Fig. 8(b)]. The photosensitive organic film is pre-heated along with the laminated front transparent flat plate 1 for a given time, for example, of from 3 to 10 minutes, to a temperature, for example, of from 100 to 250°C, at which no exothermic reaction of the photosensitive organic film 7 occurs. If the preheating treatment is not performed, the photosensitive organic film would undergo a violent endothermic or exothermic reaction at temperatures higher than about 250°C as is particularly shown in Fig. 9(a). When the pre-heating has been effected, the reaction becomes gentle as is shown in Fig. 9(b). This is because additive components other than the photosensitive organic film component polymerized to form the photosensitive organic film are burnt off during pre-heating. Thereafter, a black glass paste 6 is printed or applied inbetween adjacent photosensitive organic films 7 which have been subjected to the pre-heating treatment [Fig. 8(c)]. The black glass paste 6 used herein is a mixture of a glass component which is softened during the pre-heating and at least one glass component which is softened in the vicinity of a burning temperature of the photosensitive organic films 7 at a certain mixing ratio. In a subsequent firing or burning step of the photosensitive organic films 7 in a temperature range where the film 7 undergoes a change in shape, the glass paste has such a viscosity that it is unlikely to suffer breakage of the shape owing to the external force caused by the change of the film. As stated above, when a low softening glass component is added, there may be the fear that in a panel bonding step where a high temperature of 400 to 550°C is applied, the barrier ribs 5 suffer a change in shape. In this connection, however, it has been found when the low softening glass is contained in an amount of 40 % by weight of the main glass component, such a shape change is negligible. However, when the content of the low softening glass exceeds 40 %, large-sized lumps of the low softening glass raise on the surface of the barrier ribs. The lumps are melted in the panel bonding step and deposited on the back flat plate 3. Accordingly, the addition of the low softening glass in larger amounts is not desirable. The black glass paste 6 provided between the patternized photosensitive organic films is heated at 100 to 200°C within a short time and dried for curing. In this state, the glass deposited on portions other than those between the organic films is removed by polishing, thereby removing the black glass paste 6 from the surfaces of the resist. The removed piece is washed so that the black glass paste 6 is embedded between any adjacent photosensitive organic films. The piece is subjected to firing or burning according to a predetermined heat application profile in an atmosphere where the content of oxygen is reduced from air or in an inert gas atmosphere such as an atmosphere of nitrogen, thereby removing the photosensitive organic film 7 by burning and firing the black glass paste 6 at the same time. By the thermal treatment in an atmosphere where burning is unlikely to occur, the photosensitive organic film 7 is suppressed from burning and the deformation of the black glass paste 6 during the firing is unlikely to occur, and the glass components during the process of changing the shape of the photosensitive organic film 7 by removal with burning are prevented from being entrained into the display cells. Subsequently, the front glass on which the barrier ribs 5 have been formed is washed to remove dirt from the discharge electrodes 2 [Fig. 8(d)]. Through these steps, stable barrier ribs 5 are formed without any discrepancy in position between the discharge electrodes and any disturbance along the width of the electrodes. As will becomes clear from the graph showing the relation between the aspect ratio (height/width of rib) and the accepted rate of rib, with a known procedure where the pre-heating is not performed, the accepted rate of 100 % is at an aspect ratio which is, at most, up to approximately 0.5 as shown in curve a , whereas with the method of the invention wherein the pre-heating is performed, the rate is up to an aspect ratio of approximately 1.5 as shown in curve b . The barrier ribs 5 formed according to the above method don't involve any irregularity in the width with a uniform height, as shown in Fig. 12, with the result that a highly accurate discharge light emission P is obtained as shown in Fig. 13. Another embodiment of the invention is described with reference to Fig. 14. The discharge electrodes 2 are arranged in lines at given intervals on the front transparent flat plate 1, on which the photosensitive organic film 7 having uniform thickness and characteristics is laminated [Fig. 14(a)]. A mask 8 having a desired barrier rib pattern is superposed on the photosensitive organic film 7 and subjected to exposure to light [Fig. 14(b)]. Subsequently, the plate 1 is subjected to developing treatment, thereby leaving the photosensitive organic film 7 at portions at which the discharge electrodes are provided and which are not to be deposited with a black glass paste 6 [Fig. 14(c)]. The pre-heating is performed under the same conditions as in the method shown in Fig. 8, and a black glass paste 6 with the same composition as used in the method is printed inbetween the patterd photosensitive organic films through a printing screen, followed by drying and curing under the same conditions as in the foregoing embodiment [Fig. 14(d)]. Then, a black glass paste 6 is printed and dried a desired number of times, to form barrier ribs 5 with a given height [Fig. 14(e)]. The resultant piece is thermally treated at a temperature of 550 to 600°C to remove the photosensitive organic film 7 by burning off and the black glass paste 6 is fired at the same time. The front glass 1 on which the barrier ribs 5 have been formed is washed to remove dirt from the discharge electrodes 2 [Fig. 14(f)]. Through the above steps, stable barrier ribs 5 are formed between any adjacent discharge electrodes without any discrepancy in position and any disturbance in width. As a result, a highly accurate discharge light emission as shown in Fig. 13 is obtained. In the above embodiments, the barrier ribs are formed on the front transparent flat plate and may be formed on the back flat plate in the same manner as described before except for etching of the discharge electrodes. Moreover, linear barrier ribs are formed in the embodiments, and barrier ribs may take a form of a lattice or other shape. The glass paste and the photosensitive organic film used to form ribs formed according to the invention have the following characteristic properties. (1) Glass paste for rib (2) Low softening point glass paste for rib The glass pastes of (1) and (2) are mixed at an appropriate ratio by weight. (3) Photosensitive organic film Dry film photoresists of an alkaline developing type (50 µm and 25 µm in thickness) are used and two or three films are superposed to obtain a desired thickness. The film is exposed to light from a high pressure mercury lamp and developed with a 1% sodium carbonate aqueous solution. During the method for forming barrier ribs according to the invention, a step of a photosensitive organic film is provided wherein the photosensitive organic film is heated to a level not higher than a temperature at which the exothermic reaction of the organic film takes place. By this, in a firing step after application of a black glass paste, the photosensitive organic film is burnt off and removed wherein its change in shape becomes gentle, thereby suppressing a change in shape of the black glass paste. Thus, the barrier ribs can be stably mass-produced in high accuracy. Moreover, the hem portion of the barrier rib which greatly influences the display quality is formed such that after uniform formation of a pattern of the photosensitive organic film by utilizing the pre-heating step, the black glass paste is repeatedly printed inbetween the photosensitive organic films of a patternized form to form the barrier ribs with a predetermined height. Thus, the barrier ribs with a high display quality and high productivity can be formed.

Claims: A method for fabricating a plasma display which comprises the steps of: a) forming a given pattern of discharge electrodes (2) on a transparent plate (1); b) forming organic films (7) on the discharge electrodes (2) so that spaces are established between any adjacent discharge electrodes (2); c) filling an insulating material (6) in each space between adjacent organic films (7); and d) firing the insulating material (6) to remove the organic film (7); characterized by e) an additional step, performed before filling the insulation material (6) into the spaces, of pre-heating the organic films (7), for a given time, at a temperature lower than a temperature at which the organic film (7) undergoes an exothermic reaction; and f) the firing temperature for the insulating material (6) being higher than the pre-heating temperature of the organic film (7). A method according to claim 1, wherein the step (b) of forming organic films (7) on the discharge electrodes (2) includes deposition of a photosensitive organic film (7) on the transparent plate (1) carrying the discharge electrodes (2), and exposure of the organic film (7) to light through a mask (8) having a desired pattern, and subsequent development. A method according to claim 1 or 2, wherein said insulating material (6) is filled into the spaces up to a thickness flush with the organic film (7) on the discharge electrodes (2). A method according to any claims 1-3, wherein said insulating material (6) is a glass paste which comprises a glass component which is softened during the pre-heating temperature and another glass component which is softened in the vicinity of the firing temperature. A method according to claim 1, characterized by applying a glass paste as insulating material (6) into the spaces between adjacent organic films (7) a plurality of times so that the glass paste is formed at a predetermined height, higher than the thickness of the organic film (7) on the discharge electrodes (2).

Inventors: ARIMOTO HIRONOBU C O MITSUBISH, ARIMOTO, HIRONOBU, C/O MITSUBISHI DENKI KK., COMM.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01J9
IPC codes: H01J17, H01J11, H01J9