Display device and method of manufacturing the same

Abstract: In a display device in which a display material is enclosed in a gap between bonded glass substrates and in a method of manufacturing the same, in order to use a conventional method of scratching the surfaces of the glass substrates to divide them into cells, sodium atoms on the surface of the glass substrate having a thickness of not larger than 2 mm are replaced with atoms having an ion radius larger than that of sodium to form a chemically reinforced layer having a thickness of not larger than 20 µm. According to this display device and this method of manufacturing the same, the strength of a flat display device used in a portable electric equipment can be easily improved.

Description: BACKGROUND OF THE INVENTION The present invention relates to a display device and a method of manufacturing the same, and more particularly, to a display device which has been improved in the strength of impact-resistant properties. A schematic structure of a conventional display device will be described below. A sodium passivation film made of SiO 2 is formed on the surface of a glass substrate made of soda-lime glass, borosilicate glass, or the like, and further a transparent electrode made of film-like ITO grown by a method such as evaporation or sputtering is formed thereon while being patterned. Two glass substrates, on the surface of each of which the sodium passivation film and the transparent electrode are sequentially formed as described above, are bonded to each other with a sealing agent so that they are opposite to each other, and a display material such as a liquid crystal or an electrochromic material is enclosed in the gap between the glass substrates to form the display device. Since such a display device is excellent in portability and consumes little electric power, contrary to a CRT or the like, it is widely used at present for a digital watch, a display for a computer, and so on. However, since a highly transparent substrate is desired for such a flat panel display, a glass substrate is used in the existing circumstances. However, the glass substrate has a defect that it is liable to break because it is made of glass. Especially, since the flat panel display is often used for a portable electric equipment, it is required to be strong against impact of drop. On the other hand, since the portable electric equipment is desired to be light, thin, short, and small, the thickness of the glass substrate is required to be thin, which results in deterioration in the strength. That is, the display device has such a contradictory problem. JP 62209414A describes a manufacturing method having all the steps of the preamble of claim 1 and furthermore a liquid crystal display element which uses a pair of glass substrates containing 0.3 - 7 wt% alkaline component of Na 2 O and K 2 O. The substrates have a compressive stress layer on the surface which is obtained by immersing the low alkaline content substrates in a melt of KNO 3 , whereby the alkaline ions on the surface of the glass substrates are substituted with the potassium ions, which have a larger ion radius. The use of the low alkaline glass substrates enables an insulating film, for example of silicon dioxide, for the prevention of dissolution of alkaline components from the glass substrates to the liquid crystal layer to be omitted, thereby reducing fabrication costs. SUMMARY OF THE INVENTION It is therefore an object of the present invention to realise a display device which has sufficient strength even if the thickness of a glass substrate is made thin. In order to solve the problem, according to a first aspect of the present invention, there is provided a method of manufacturing a display device, characterised by: polishing a soda lime glass substrate to provide a surface flatness of less than about 0.05 µm; immersing the polished soda lime glass substrate in a solution or a molten solution of a mixed salt of 50% of KNO 3 and 50% of KNO 2 containing potassium ions having an ion radius larger than that of sodium at a temperature low enough to maintain the surface flatness, so as to form a chemically reinforced layer having a thickness not larger than 20 µm by replacing sodium ions in the glass substrate by potassium ions; forming a sodium passivation film on the chemically reinforced layer; forming an electrode on the sodium passivation film; arranging the substrate opposite to another soda lime glass substrate having an electrode thereon and a surface flatness similar to that of said substrate so as to fabricate a plurality of cells formed from the glass substrate. The method of the present invention may be characterised by scratching the surface of the soda lime glass substrate on which said chemically reinforced layer has been formed; and dividing said glass substrates by glass cutting to provide individual cells. The soda lime glass substrate may advantageously be selected to have a thickness of not larger than 2mm. The soda lime glass substrate may, most preferably, be selected to have a thickness of 0.55 mm, the molten salt is provided at a temperature of 430°C, and a chemically reinforced layer 8 µm thick is formed. According to a second aspect of the present invention there is provided a display device fabricated in accordance with the method of the first aspect. Further, the strength of the glass of the display device can be improved by merely adding a step of immersing the glass substrate in the solution or molten solution containing the positive ions having an ion radius larger than that of sodium, which is very simple. Further, according to this method, the thickness of the chemically reinforced layer can be easily controlled by adjusting an immersion time or the like, and the device can be formed under such a condition that working of glass cutting or the like can be carried out even after reinforcement of the substrate. Still further, in the case where glass having a thickness of not larger than 2 mm, if the thickness of the chemically reinforced layer is not larger than 20 µm, the display device having sufficient mechanical strength can be easily obtained without extremely complicating a conventional step of glass cutting or the like. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing a display device according to the present invention; and Figs. 2(A) to 2(F) are views depicting manufacturing steps of the display device according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Glass has originally considerable strength in theory. However, in practice, it is easily broken because of its brittleness. The reason is that since tensile stress exists on the surface of glass, if some kind of defects are distributed on the surface, the stress is concentrated to the defects and they become break smarting points so that the glass is broken at a level considerably lower than theoretical strength. In view of such breaking mechanism of glass, there are proposed reinforced glass such as wind-cooled tempered glass, low expansion glass, high elasticity glass, crystallized glass, and chemically reinforced glass. The wind-cooled tempered glass has strength against breakage, which is improved by converting tensile stress existing on the surface of glass to compressive stress. Concretely, jets of air are blown to the surface of a glass plate heated up to about 700°C, and the glass plate is cooled while keeping the temperature difference between the interior and the surface. As a result, in the solidified state, distortion remains as compressive stress on the surface and as tensile stress in the interior according to the temperature difference, so that the strength of the glass is improved. At this time, the compressive stress on the surface reaches 900 - 1500 kg/cm 2 , and the tensile stress in the interior becomes 300 - 500 kg/cm 2 . It is said that the thickness of a practical reinforced layer is one sixth of the thickness of the glass or not less than 100 µm. The low expansion glass is devised to prevent glass from breaking due to thermal stress generated when the glass is heated or cooled. The high elasticity glass is made by introducing nitrogen, which has strong bonding force, into conventional oxide glass to reinforce the mesh of the glass. The elasticity thereof is much improved to the degree of 30 to 50 percent, and therefore the strength is improved. The crystallized glass is a kind of composite material in which crystal grains are dispersed into the matrix by reheating glass, and the progress of crack from breakage is blocked by the crystal grains. The chemically reinforced glass is, like the wind-cooled tempered glass, improved in the strength against breakage by converting the tensile stress existing on the glass surface to the compressive stress. Concretely, glass containing alkaline ions (Li, Na, etc.) having a small ion radius is immersed in alkaline molten salt having a large ion radius at a temperature (for example, 400°C) within a range not higher than the glass transition point, so that ion exchange between both is carried out on the surface of the glass. As a result, the volume is increased in the layer of glass surface, where the compressive stress exists after cooling. The compressive stress of about 100Kg/mm 2 can be obtained. Among these reinforced glasses, the wind-cooled tempered glass, low expansion glass, high elasticity glass, and crystallized glass are difficult to be applied to a display device since they have such restriction that their glass compositions themselves are changed or the reinforcing treatment is carried out during the production of glass so that it can not be carried out at a post working step. On the other hand, the chemically reinforced glass can be included in manufacturing steps of a display device since the chemical reinforcing treatment can be carried out at a relatively low temperature. That is, even after a step (polishing step, etc.) peculiar to a display device is carried out, the chemical reinforcing treatment can be carried out, so that the use of the chemically reinforced glass does not largely change and complicate conventional manufacturing steps of a display device. When a flat panel display for a liquid crystal display device or the like is fabricated, there are adopted steps in which a large number of display device cells are formed on two large glass substrates, the two large glass substrates are bonded to each other, and then the substrates are cut to divide the display device cells into individual ones. This cutting is generally carried out by scratching the surface of the glass by a hard blade of diamond or the like and then applying a pressure to the glass to break it. In the case where the glass is chemically reinforced, if the chemically reinforced layer is too thick, the scratch is hard to be transmitted into the interior of the glass, so that the cutting and dividing can not be carried out well and end faces of the divided cells do not become linear. Accordingly, in the present invention, the chemical reinforcement is selected as a method of reinforcing glass, and the chemical reinforcing step is made to be carried out in manufacturing steps of a display device so that the cutting and dividing step can be carried out like a conventional method. That is, a glass substrate having a thickness of not larger than 2 mm is immersed in a solution or a molten solution of salt containing positive ions having an ion radius larger than that of sodium, so that a chemically reinforced layer having a thickness of not larger than 20 µm is formed on the surface of the glass substrate. According to this method, since sodium atoms on the surface of the glass substrate are replaced with atoms having an ion radius larger than that of sodium so that the glass substrate is chemically reinforced, even if the thickness of the glass substrate is made thin, sufficient strength can be kept. Further, this method does not complicate conventional steps such as glass cutting. Here, it is sufficient that the glass substrate contains alkaline ions even if the quantity thereof is extremely small. Lead glass, borosilicate glass and the like can be used, not to mention soda glass. Hereinafter, examples of the present invention will be concretely described with reference to Figs. 1 and 2. (Embodiment 1) Fig. 1 is a schematic sectional view showing an example of a display device according to the present invention. Although the basic structure thereof is similar to that of a conventional display device, the surfaces of glass substrates 1 and 4 made of soda glass are chemically reinforced. Sodium passivation films 2 and 5 made of SiO 2 are formed on the chemically reinforced glass substrates, and further transparent electrodes 3 and 6 made of film-like ITO grown by a method such as evaporation or sputtering are formed on the upper surfaces of the passivation films while being patterned. The thus formed glass substrates 1 and 4 are made opposite to each other and bonded with a sealing agent 7, and a display material 8 made of an STN liquid crystal is enclosed in the gap between the substrates to form the display device. Figs. 2(A) to 2(F) depict a method of manufacturing a display device, including a chemical reinforcing step of the present invention. In Fig. 2(A), a soda glass 31 having a thickness of 0.55 mm is prepared. In Fig. 2(B), the soda glass is polished until the flatness of the surface sufficient for a display device can be obtained. In Fig. 2(C), the soda glass is immersed in a reinforcing solution 32 made of a mixed molten salt of 50% KNO 3 and 50% KNO 2 with a temperature of 430°C, so that a chemically reinforced glass substrate 33 is obtained. This immersion time was adjusted so that the thickness of a chemically reinforced layer was 8 µm from the surface. In Fig. 2(D), a sodium passivation film 34 made of SiO 2 , and transparent electrodes 35 made of film-like ITO grown by a method such as evaporation or sputtering, the number of which is equal to that of a plurality of display devices to be formed and the patterns of which are equal to each other, are formed on the glass substrate. In Fig. 2(E), a sodium passivation film 37 and a plurality of transparent electrodes 38 are formed on a glass substrate 36, the surface of which is chemically reinforced like the glass substrate 33. The glass substrate 36 is made cpposite to the glass substrate 33 and both are bonded with sealing agents 39, so that a plurality of cells are formed from the same glass substrates. In Fig. 2(F), linear scratches 40 are made on the glass substrates by a diamond blade at positions where the substrates are expected to be divided. Thereafter, the vicinities of the scratches 40 are pressurized to cut and divide the glass. A display material made of an STN liquid crystal is enclosed in the gap of individually divided cells to form the display device. In such an STN liquid crystal device, to realize the uniform thickness of a liquid crystal layer, the precision of 0.05 µm is required for the flatness of the surface of the substrate. Thus, polishing of the glass surface is an indispensable step. However, if the reinforced glass is used as a raw material from the first, polishing of the surface is hard to be performed and causes an extreme difficulty. However, as in this example, if the chemical reinforcing treatment is carried out at a relatively low temperature after polishing, the flatness of the glass surface can be easily obtained, and the warp and undulation of the glass can be avoided even in the subsequent reinforcing treatment, so that the display device improved in the strength can be realized without degrading the display performance. Further, in spite of the reinforced glass, cutting and dividing was able to be carried out almost without changing a conventional step, so that the display device was able to be fabricated very simply. Nevertheless, when the display device according to this example was fallen from the height of 2 m, no damage was observed, which showed that the device was superior in the mechanical strength. (Comparative Example) A display device was fabricated under the same conditions as example 1 except that a time of chemical reinforcement treatment in example 1 was prolonged so that the thickness of the chemically reinforced layer was 25 µm from the surface thereof. In the cutting and dividing step, the broken surface of glass become sawlike and a large number of cracks were observed. Further, when the display device was fallen from the height of 2 m, it was broken from the cracks. (Example 2) A glass substrate was a soda glass having a thickness of 0.7 mm, which was immersed in a reinforcement solution 32 made of KNO 3 molten salt heated up to 400°C so that it was chemically reinforced. Adisplay device was fabricated under the same conditions as example 1 except that an immersion time at the chemical reinforcement was adjusted so that the thickness of a chemically reinforced layer was 15 µm from the surface thereof. Also in this example, the same effects as example 1 were obtained. (Example 3) A glass substrate was a soda glass having a thickness of 1.1 mm, and an immersion time for chemical reinforcement was adjusted so that the thickness of a chemically reinforced layer was 20 µm from the surface thereof. A display device was fabricated under the same conditions as example 1 except the above. Also in this example, the same effects as example 1 were obtained. (Example 4) A glass substrate was a lead glass and a display device was fabricated under the same conditions as example 1 except the above. Also in this example, the same effects as example 1 were obtained. (Example 5) A glass substrate was a borosilicate glass, and a display device was fabricated under the same conditions as Example 1 except the above. Also in this example, the same effects as Example 1 were obtained. According to these methods, sodium atoms on the surface of the glass are replaced with atoms having an ion radius larger than that of sodium so that the chemical reinforcement was carried out. Accordingly, even if the thickness of the glass substrate is made thin, sufficient strength can be kept. Further, since the chemical reinforcing treatment can be carried out after polishing of the surface of the glass substrate, the reinforced glass having high precision of surface roughness can be easily obtained. Further, in the case where the glass substrate having a thickness of not larger than 2 mm, if the thickness of the chemically reinforced layer is made not larger than 20 µm, sufficient mechanical strength can be obtained, and further a conventional step such as glass cutting is not complicated. That is, a portable display device improved in strength against impact of drop can be easily obtained. A glass substrate especially effective in the present invention is a soda glass substrate containing a large amount of sodium.

Claims: A method of manufacturing a display device comprising the steps of immersing a glass substrate (31) in a solution (32) or a molten solution (32) of salt containing potassium ions having an ion radius larger than that of sodium, so as to form a chemically reinforced layer having a thickness not larger than 20 µm by replacing sodium ions in the glass substrate by potassium ions; forming a sodium passivation film (34) on the chemically reinforced layer; and forming an electrode (35) on the sodium passivation film (34); characterised by a first step of polishing the glass substrate (31) to provide a surface flatness of less than about 0.05 µm; and a final step of arranging the glass substrate opposite to another glass substrate having an electrode thereon and a surface flatness similar to that of said glass substrate so as to fabricate a plurality of cells formed from the glass substrates; wherein the glass substrates are soda-lime glass substrates, the salt is a mixed salt of 50% of KNO 3 and 50% of KNO 2 , and the solution or molten solution of salt is provided at a temperature low enough to maintain the surface flatness. A method according to claim 1 characterised by : scratching the surface of the soda lime glass substrate on which said chemically reinforced layer has been formed along division lines; and dividing said glass substrates (33, 36) by glass cutting to provide individual cells. A method according to any one of the preceding claims, characterised in that : the soda lime glass substrate (31) is selected to have a thickness of not larger than 2 mm. A method according to claim 3, characterised in that the soda lime glass substrate (31) is selected to have a thickness of 0.55 mm, the molten salt is provided at a temperature of 430°C, and a chemically reinforced layer 8 µm thick is formed. A display device fabricated by a method according to any one of the preceding claims.

Inventors: KUMAI SACHIKO, SHIMADA KAZUO, SUGINOYA MITSURU, KUMAI, SACHIKO, SHIMADA, KAZUO, SUGINOYA, MITSURU
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1997
Main classification: G02F1
IPC codes: G02F1, C03C21