1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD) using switching elements, for example amorphous silicon thin film transistors (hereinafter simply referred to as a-Si TFTs) or polycrystalline silicon thin film transistors (hereinafter, p-Si TFTs), which is particularly effective for use as an optical shutter of a projector and a method for repairing the defective portions thereof.
2. Description of the Related Art
An active matrix type LCD includes pixel electrodes arranged in a matrix fashion on a substrate and switching elements formed so as to correspond to the respective pixel electrodes. An LCD drives the liquid crystal by charging/discharging the necessary charges to/from the pixel electrodes in accordance with the operation of the switching elements.
In general, an enormous number, i.e. from several tens of thousand to several hundreds of thousand, of switching elements and pixel electrodes, are formed on the substrate of the LCD in order to accomplish satisfactory display quality. In a process for forming such elements and electrodes on the substrate, some pixels will not operate even when a voltage is applied to the liquid crystal, mainly because the switching elements cannot operate or because the pixel electrodes are abnormally formed. The existence of non-operating pixels when a voltage is applied to the liquid crystal prevents the LCD from accomplishing satisfactory display quality.
An active matrix type LCD has two display modes. One of them is a Normally-Black mode in which light is shielded when no voltage is applied and light is transmitted when a voltage is applied. The other is a Normally-White mode in which light is transmitted when no voltage is applied and light is shielded when a voltage is applied.
FIG. 3 diagrammatically shows a conventional active matrix type LCD in a Normally-White mode. The LCD 1 includes glass substrates 3 and 2 which are spaced apart from each other. On the internal surface of the glass substrate 3, p-Si TFTs 5 are formed, and on the internal surface of the glass substrate 2, a black mask pattern (B/M) 4 is formed.
The configuration of the LCD 1 will be described in detail below.
On the glass substrate 3, pixel electrodes 6 made of ITO (In.sub.2 O--SnO.sub.2 based) are arranged in a matrix fashion by performing a sputtering step, a photolithography step, and an ITO etching step. Switching elements, i.e. p-Si TFTs 5 are formed on the glass substrate 3 so as to correspond to the respective pixel electrodes 6 arranged in a matrix fashion. Over the glass substrate 3, a polyimide-based alignment film 18 is formed by an offset printing method. On the B/M 4 on the other glass substrate 2, pixel electrodes 16 made of ITO and a polyimide-based alignment film 17 are respectively formed. Each of the alignment films 18 and 17 is subjected to a rubbing treatment so as to obtain a twisted-nematic orientation. Polarizing plates 7 and 8 in a crossed-Nicols state are respectively attached to the external surfaces of the substrates 2 and 3. The LCD 1 in the Normally-White mode is configured as described above.
Next, a method for repairing the defective pixels of an active matrix type LCD in a Normally-White mode will be described.
When a sufficiently high voltage is applied to the liquid crystal 9 of the LCD 1, as indicated by the case C in FIG. 3, an incident light ray 13 transmitted through the polarizing plate 8 is shielded by the polarizing plate 7 if the pixels operate normally. This is because the polarizing direction of the incident light 13 is not changed by the liquid crystal 9. In a defective pixel, the polarizing direction of the incident light 13 transmitted through the polarizing plate 8 is rotated by the liquid crystal 10 by 90 degrees, so the incident light 13 is transmitted also by the polarizing plate 7. As a result, a leaking light ray 13' is observed so as to form a bright point, as indicated by the case E in FIG. 3. If a plurality of such defects exist, the defects are observed among the pixels as a group of bright points or line defects.
Methods for repairing such a pixel exhibiting the above-mentioned defect (hereinafter, referred to as a defective pixel) are described, for example, in Japanese Laid-Open Patent Publication Nos. 60-243635 and 3-243917.
According to the technique described in the former, as indicated by the case F in FIG. 3, a laser beam is irradiated onto a portion of an alignment film on the defective pixel so as to burn off the portion of the alignment film; and an orientation condition of the liquid crystal 11 over the defective pixel is disturbed so as to scatter the transmitted light, thereby repairing the defective pixel.
According to the technique described in the latter, as indicated by the case D in FIG. 3, a resin, ink, or the like into which a colorant including a black dye or a black pigment is mixed is applied to the position on the external surface of the glass substrate 2 which corresponds to that of the defective pixel by using a cotton needle, a pin of a nylon fiber, or the like and then the colorant is dried so as to form a light-shielding layer 12 including a light-shielding portion 12a and thereby repairing the defective pixel.
However, conventional repairing methods such as those described above for the defective pixel of the LCD in the Normally-White mode have the following problems.
According to the repairing method described in the Japanese Laid-Open Patent Publication No. 60-243635, the defective pixel is repaired by irradiating the laser beam so as to burn off the portion of the alignment film. As a result, deterioration, decomposition or alteration of the liquid crystal occurs in the defective pixel. In addition, the laser beam irradiation damages the switching elements. An active matrix type LCD is required to exhibit particularly high reliability. Therefore, such a repairing method which affects the display quality of the LCD is not preferable. Furthermore, an expensive laser repairing apparatus for repairing the defective pixel, and a detection apparatus for accurately detecting the position of the defective pixel are also required.
According to the repairing method described in the Japanese Laid-Open Patent Publication No. 3-243917, after the positions of the defective portions have been identified, the light-shielding layer 12, including a plurality of light-shielding portions 12a, is formed by applying a black resin, ink, or the like to the defective pixel portions with a pin, so that the area of each light-shielding portion 12a generally becomes larger than that of each defective pixel portion. Consequently, the light-shielding portions 12a inconveniently cover normal pixels adjacent to the defective pixels. Such a problem is particularly likely to occur in a super high-definition LCD having approximately a million and several hundreds of thousands of pixels with a pixel pitch of about 30 .mu.m.
In addition, a detection apparatus for accurately detecting the positions of the defective pixels, and an apparatus for accurately applying a black resin, or the like onto the positions of the substrate corresponding to those of the defective pixel portions are required.
Moreover, according to the repairing methods described in the Japanese Laid-Open Patent Publication Nos. 60-243635 and 3-243917, in the situation where the LCD is used as an optical shutter for a projector, bright points are likely to occur because of the following reasons: specifically, the light rays emitted from an external light source are not incident at a right angle against the surface of the substrate of the LCD, but at a certain angle against the surface, so that the deviation occurs between the positions of the defective pixels and those of the bright points. Namely, the positions of the bright points on the substrate on the light-outgoing side deviate from those of the defective pixels on the substrate on the light-incoming side. Therefore, according to the above methods, bright points still occur.