Abstract:
An object of the present invention is to provide a method for repairing light point defects in which light points can be converted to black spots through a simple process without affecting surrounding pixels, and the repaired pixels can be maintained without their returning to light points even with the passage of time. The present invention provides a method for repairing light point defect pixels of a liquid crystal display device having a liquid crystal panel which comprises a pair of substrates, a liquid crystal layer interposed between the pair of substrates, and a pair of alignment films each of which is provided between the substrate and the liquid crystal layer restraining the orientation of the liquid crystals of the liquid crystal layer. The defect repair method includes the step of irradiating laser light onto a region of the alignment film corresponding to the light point defect pixels to locally reduce or eliminate the orientation restraining force of the alignment film, wherein the light point defect pixels are repaired by reducing the intensity of the light transmitted through the region where the orientation restraining force is reduced or eliminated when the liquid crystal is illuminated.

Description:
BACKGROUND  
       [0001]     The present invention relates to a method and apparatus for repairing pixel defects in liquid crystal display devices.  
         [0002]     Among the defects which occur in the manufacturing process of liquid crystal display devices, there are defects so serious that the subject liquid crystal display device must be scrapped. When the defects are not so serious, the device may be considered a good product and shipped even if that defect is left as is.  
         [0003]     For example, in the case of normally white display devices of which pixels are white while voltage is not applied, defective pixels that allow light pass through themselves when voltage is applied and non-defective pixels are driven black (hereinafter referred to as “light point defect” pixels) are quite easy to recognize considering human visual perception characteristics and thus become conspicuous defects. Consequently, liquid crystal display devices having light point defects are usually treated as defective products and cannot be shipped as good products.  
         [0004]     Light point defects occur due to a variety of causes, such as opened contact hole, residual ITO fragments, inter-electrode leaks in the thin film transistors and the like which make up the pixel circuits, inter-pixel shorts due to foreign matter such as metal fragments, and light scattering due to foreign matter. As a result, it is difficult to repair each defect by a method corresponding to each cause.  
         [0005]     On the other hand, defective pixels which remain black when voltage is applied and non-defective pixels are driven white (hereinafter referred to as “black spot defect” pixels) are less striking to human eyes than pixels with light point defects. As a result, even if a liquid crystal display device has black spot defects, depending on their extent, in some cases the products may be treated as good.  
         [0006]     In this regard, methods have been studied which aim, by applying some kind of processing to light point defects and converting them to black spot defects and thus repairing defective products, to increase yields and reduce production cost. For example, in Patent Laid-Open Publication No. Hei 9-90304 a method is proposed whereby conversion to black spot is accomplished by irradiating a defective pixel of a liquid crystal device with a first laser light, thus causing air bubbles to form in the liquid crystal layer peripheral to and including the defective pixel, and in this state where air bubbles have been formed, irradiating that defective pixel with a second laser light so that, through that energy, the constituent material of that defective pixel is made to spatter within the air bubble causing the adhesion and accumulation of the spattered matter onto the opposing electrodes of the liquid crystal display device.  
       SUMMARY  
       [0007]     However, with the conventional method explained above which causes bubbles to be generated, it is hard to maintain long-term the changes made to the properties of the liquid crystal layer at the site of defective pixel. There were also cases where the spattered materials that had adhered to and accumulated on the electrodes dropped on the once repaired pixels, which made the repaired pixels into light point defect pixels again.  
         [0008]     To deal with this, an object of the present invention is to provide a repair method for light point defects whereby the repaired condition can be maintained long-term.  
         [0009]     In order to solve the above-mentioned problem, the method for repair of a liquid crystal device defect of the present invention is a method for repairing light point defect pixels of a liquid crystal device having a liquid crystal panel which comprises a pair of substrates, a liquid crystal layer interposed between the pair of substrates, and alignment films each provided between the substrates and the liquid crystal layer, which restrain the orientation of the liquid crystal molecules of the liquid crystal layer; the method comprising the step of irradiating laser light onto a region corresponding to the light point defect pixels of the alignment film and locally reducing or eliminating the orientation restraining force of the alignment film, wherein the light point defect pixels are repaired by reducing, when the liquid crystal is illuminated, the intensity of the light transmitted through the region where the orientation restraining force is reduced or eliminated.  
         [0010]     Liquid crystal displays switch between light and dark by applying voltage and changing the orientation of the liquid crystal molecules. For example, in TN-type liquid crystal display devices, the liquid crystal molecules are sandwiched between two alignment films, the axes of the alignment of which are perpendicular to each other. Then, in the state where no voltage is applied to the pixels, because the liquid crystal molecules are aligned in a twisted structure in conformance to this orientation restraining force, the oscillation component of the incident light is made to rotate 90 degrees. However, with the defect repair method of the present invention, by irradiating laser light onto a region of the alignment film corresponding to the light point defect pixel, the orientation restraining force can be locally reduced or eliminated, and consequently the liquid crystal molecules will no longer be oriented in conformance to the alignment film. As a result, in the state where voltage is not applied to a pixel, the oscillation component of the incident light is not rotated, even though it is transmitted through the liquid crystal layer.  
         [0011]     In liquid crystal display devices, the liquid crystal panels are positioned between two polarizing plates. One of the polarizing plates passes through the light having oscillation components different by 90 degrees from those of the light passed through by the other polarizing plate. As a result, the light transmitted through the incident side polarizing plate, so long as its oscillation component is not rotated by the liquid crystal layer, cannot be transmitted through the polarizing plate of the light exiting side. Consequently, pixels for which the orientation restraining force is reduced or eliminated will appear as black spots.  
         [0012]     On the other hand, in the state where voltage is applied, because the liquid crystal molecules are arranged in a direction perpendicular to the substrate, the light incident to the liquid crystal layer cannot be transmitted through the light exiting side, as its oscillation component is not rotated. In the above manner, light point defect pixels which have been irradiated with laser light and the orientation restraining force of their alignment film reduced or eliminated are made to become black spots which never transmit light regardless of whether or not voltage is applied.  
         [0013]     To irradiation of the light point defect pixels, a laser light having a wavelength that the alignment film can absorb is used. For example, in the case of alignment films comprise organic films or polymer films, such as polyimide, it is preferable to use laser beam of wavelength 450 nm or less. Further, it is preferable that the wavelength of the laser beam be 200 nm or greater, so that the substrate does not absorb the laser light.  
         [0014]     Also, with the repair method for the liquid crystal device defect of the present invention, when laser light is irradiated, polarizing plates are positioned on one side of the liquid crystal panel and on the other respectively, and axes of polarization are set so that light transmitted through one of the polarizing plates can be transmitted through the other polarizing plate when the orientation restraining force of the alignment film is eliminated. It is also preferable that a process step also be included for detecting that the properties of the alignment film in the region corresponding to the light point defect pixel have changed.  
         [0015]     If the axes of polarization are set so that light transmitted through one of the polarizing plates can be transmitted through the other polarizing plate when the orientation restraining force of the alignment film is eliminated, then the irradiated laser light cannot be transmitted through that other polarizing plate until the orientation restraining force of the alignment film is sufficiently reduced. However, by irradiating the film for a certain period of time, the orientation restraining force is reduced and, in proportion, the amount of the laser light being transmitted through that other polarizing plate increases. Consequently, if the amount of the laser light transmitted through the polarizing plate positioned at the light exiting side of the liquid crystal panel is measured, the degree to which the orientation restraining force of the alignment film in the region corresponding to the light point defect pixel is reduced can be evaluated.  
         [0016]     Further, the present invention provides an apparatus for defect repair of a liquid crystal display device, comprising: a laser oscillator; a stand on which is mounted a liquid crystal display device; means for controlling the irradiation position so that laser light outputted from the laser oscillator is irradiated onto light point defect pixels of the liquid crystal display device; and a detector for detecting the amount of laser light transmitted through the liquid crystal display device. Part of or all the stand is optically transparent, and the degree of defect elimination of the light point defect pixel can be evaluated by detecting with the detector the amount of the laser light, irradiated onto the light point defect pixels, that is transmitted through the liquid crystal display device.  
         [0017]     As explained above, by positioning the polarizing plates, on the one side and the other side of the liquid crystal panel, so that light transmitted through one of the plates can be transmitted through the other polarizing plate when the orientation restraining force of the alignment film is eliminated, and by measuring the amount of the laser light transmitted through both of the two polarizing plates, the degree to which the orientation restraining force of the alignment film in the region corresponding to the light point defect pixel is reduced can be evaluated. Consequently, by configuring the system so that a part or all of the stand which holds the liquid crystal display device is optically transparent and the amount of the laser light irradiated onto the light point defect pixel that is transmitted through the liquid crystal display device can be detected by a detector, the laser light irradiated onto the light point defect pixel in order to change the properties of the alignment film can also be used to evaluate the progress toward becoming a black spot. Thus there is the benefit that it is not necessary to provide a separate light source for detecting the light transmitted through the light point defect pixel. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0018]      FIG. 1  is an explanatory diagram for explaining a light point defect pixel.  FIG. 1 (A) shows the state where voltage is not applied to the pixels and  FIG. 1 (B) the state where voltage is applied to the pixels;  
         [0019]      FIG. 2  is an explanatory diagram showing achieving a black spot through laser irradiation of a light point defect pixel.  FIG. 2 (A) shows the state where voltage is not applied to the pixels and  FIG. 2 (B) the state where voltage is applied to the pixels;  
         [0020]      FIG. 3  is an explanatory diagram showing a light point defect pixel and a black spot defect pixel.  FIG. 3 (A) shows stylistically a light point defect, and  FIG. 3 (B) a black spot defect. A black spot defect is less conspicuous than a light point defect;  
         [0021]      FIG. 4  is a summary diagram showing the defect repair method and the defect repair apparatus of the present invention;  
         [0022]      FIG. 5 (A) is a summary diagram showing one embodiment of a laser irradiation device with a condenser lens, and  FIG. 5 (B) is a diagram showing the intensity distribution and beam shape of the laser light; and  
         [0023]      FIG. 6 (A) is a summary diagram showing one embodiment of a laser irradiation device with an imaging lens,  FIG. 6 (B) is a diagram showing the intensity distribution and beam shape of the laser light.  
     
    
     DETAILED DESCRIPTION  
       [0024]     The preferred embodiments of the present invention are described below, referring to the figures.  
         [0025]     First, using  FIG. 1  and  FIG. 2 , the liquid crystal display device defect repair method of the present invention is described, taking a TN-mode liquid crystal display device as an example.  FIG. 1  and  FIG. 2  each show cross-sectional diagrams of three pixels, A, B, and C. Each pixel comprises a liquid crystal panel made up of a pair of glass substrates  20   a  and  20   b,  a liquid crystal layer  10  interposed between the substrates, and alignment films  12   a  and  12   b  each provided between the substrates  20   a  and  20   b  and the liquid crystal layer  10 . The alignment films  12   a  and  12   b  are composed of polyimide. The direction of the force which restrains the orientation, in other words the orientation restraining force, of the alignment film  12   a  is perpendicular to that of the alignment film  12   b.  Also, between the glass substrate  20   a  and the alignment film  12   a,  pixel electrodes  16   a  through  16   c  are provided, and between the glass substrate  20   b  and the alignment film  12   b  an opposing electrode  14  and a black mask  18  are provided. On the surfaces of the glass substrates  20   a  and  20   b,  polarizing plates  22   a  and  22   b  are provided, so that they respectively transmit oscillation components differing by 90 degrees.  
         [0026]     In the liquid crystal display device of  FIG. 1 , pixel B is a light point defect pixel.  
         [0027]     When voltage is not applied to the pixels, as shown in the patterns of pixels A through C of  FIG. 1 (A), due to the orientation restraining force of the alignment film, liquid crystal molecules  24 , included in the liquid crystal layer, are aligned in a twisted alignment configuration. In this way, the oscillation component of light transmitted through the polarizing plate  22   a  is rotated 90 degrees during passing through liquid crystal layer  10 , and it can be transmitted through polarizing plate  22   b.    
         [0028]     When voltage is applied to the pixels by means of the pixel electrodes  16   a  through  16   c  and the opposing electrode  14 , at non-defect pixels A and C, as shown in  FIG. 1 (B), the liquid crystal molecules  24  align themselves perpendicular to the substrate and the light transmitted through the incident side polarizing plate  22   a  passes through the liquid crystal layer  10  without its axis of polarization being changed. As a result, the light transmitted through the liquid crystal layer  10  cannot be transmitted through the exiting side polarizing plate  22   b,  and the pixels A and C display black.  
         [0029]     On the other hand, with pixel B, voltage can not be applied due to a defect, and thus the alignment of the liquid crystal molecules  24 , due to the orientation restraining force of the alignment film, remains twisted as before. Thus the oscillation component of the light transmitted through the incident polarizing plate  22   a  is rotated 90 degrees during passing through the liquid crystal layer  10 , and is transmitted through the exiting side polarizing plate  22   b.  As a result, pixel B always displays white and becomes a light point defect. As shown stylistically in  FIG. 3 (A), when the whole screen is driven back, the light point defect pixel B is readily recognized by the human eye and is extremely conspicuous, degrading the quality of the liquid crystal display device.  
         [0030]     Next, using  FIG. 2 , the state after the properties of the alignment film are changed by irradiating the liquid crystal display device shown in  FIG. 1  with laser light is described.  FIG. 2 (A) shows the state where voltage is not applied to the pixels. At this time, for non-defect pixels A and C, the liquid crystal molecules of the liquid crystal layer  10  are aligned in twisted alignment in conformance with the alignment film, and the light transmitted through the incident side polarizing plate  22   a  is transmitted through the exiting side polarizing plate  22   b.  As a result, pixel A and pixel C display white.  
         [0031]     On the other hand, with pixel B, due to a change in the properties of the alignment film, the orientation restraining force is reduced or eliminated, and the liquid crystal molecules do not align themselves in a twisted manner. As a result, the oscillation component of the light is not rotated during passing through the liquid crystal layer  10 , and the light cannot be transmitted through the exiting side polarizing plate  22   b.  The pixel B thus displays black in the state where a voltage source is not applied.  
         [0032]      FIG. 2 (B) shows the state of applying voltage to a liquid crystal display device after irradiating it with laser light. By applying voltage to pixels A through C, the liquid crystal molecules in pixels A and C align themselves perpendicular to the substrate. On the other hand, in pixel B, due to a defect, voltage is not applied. Consequently, in all pixels A through C, the liquid crystal molecules  24  do not align themselves in a twisted manner. As a result, the light transmitted through the polarizing plate  22   a  is not rotated during passing through the liquid crystal layer  10 , and thus becomes unable to be transmitted through exiting side polarizing plate  22   b,  resulting in a black display.  
         [0033]     By irradiating pixel B with laser light and changing the properties of the alignment film, it can be made a black spot defect pixel which is always black, both in the case that voltage is applied and the case that it is not applied. Due to the characteristics of the human eye, black spot defects are less conspicuous than light point defects (refer to  FIG. 3 (B)) and, in terms of product-worthiness, one could consider that by converting the defect to a black spot, the quality of the liquid crystal display will be improved because the defect can then be treated as a minor defect.  
         [0034]      FIG. 4  shows one embodiment of the defect repair apparatus for irradiating the alignment film of a defective pixel with laser light. A laser irradiation device  40  comprises a laser oscillator  400 , a table (stand)  408  on which a liquid crystal display device is mounted, and a power meter  414  which measures the laser light transmitted through a liquid crystal display device  406 .  
         [0035]     The laser oscillator  40  can generate, for example, laser light with wavelengths of 355 nm, 405 nm, and/or 441 nm which wavelengths are readily absorbed by a general polyimide alignment film. The table  408  is capable of position adjustment in the X, Y and Z directions. Laser light generated from the laser oscillator  400  is reflected by a mirror  402  and, having its focal point narrowed by a lens  404 , is irradiated onto a specified pixel of the liquid crystal display device  406 . A through-hole is provided in the center of the table  408 , so that if laser light is transmitted through the liquid crystal display device, it will be detected by the power meter  414 .  
         [0036]     With the present embodiment, it is not the final product of the liquid crystal display device that is repaired. Rather, the light point defect pixel is repaired by irradiating a laser onto the liquid crystal panel  406  before mounting the polarizing plates, in other words, onto the structure made up of the liquid crystal layer, the alignment film, the electrodes, and the black mask interposed between two glass substrates. Then, when the liquid crystal panel  406  is mounted on the table  408 , two polarizing plates  410  and  412  are positioned to hold the panel.  
         [0037]     Here, if both polarizing plates are positioned so that light oscillation components of the same direction are transmitted, light point defect pixels will not transmit light when voltage is not applied. However, by irradiating laser light onto it, the orientation restraining force of the orientation layer is reduced and, in proportion, the liquid crystal molecules of the liquid crystal layer become un-oriented so that they twist, and because the oscillation component of the laser light transmitted through the polarizing plate  410  is not rotated, it can also be transmitted through the polarizing plate  412 . The laser light transmitted through the polarizing plate  412  is further transmitted through the through-hole in the table  408  and is detected by the power meter  414 . In this way, the increase in light intensity detected by the power meter  414  becomes an indicator of the reduction in the orientation restraining force of the alignment film.  
         [0038]     In the case that the polarizing plates are arranged to transmit light oscillation components whose angles differ by 90 degrees, light transmission through the polarization plate will gradually become impossible in proportion to the change in properties of the alignment film. Consequently, in this case, by detecting the reduction in the transmitted light with the power meter, the change in the properties of the alignment film can be evaluated. Further, because in general it is easier to detect increasing light intensity, rather than detecting the weakening of transmitted light. Therefore, by arranging the two polarizing plates so that light oscillation components of the same direction are transmitted, it is possible to detect with higher sensitivity the reduction of the orientation restraining force of the alignment film, in other words, the degree to which the defect is eliminated.  
         [0039]     If the laser light is detected by the power meter  414 , the power meter  414  outputs to the laser oscillator  400  an oscillation stopping signal in order to stop the laser oscillation. Specifically, for example, a feedback control mechanism  416  is provided between the power meter  414  and the laser oscillator  400  and by means of this feedback control mechanism  416 , responsive to the strength of the light detected by the power meter  414 , a signal to control laser oscillation can be outputted to the laser oscillator  400 . By doing this, just the laser light required to reduce the orientation restraining force of the alignment film is irradiated and defects can be repaired efficiently.  
         [0040]     Further, in the laser irradiation apparatus  40 , if the two polarizing plates  410  and  412  are arranged so that they transmit light oscillation components which differ by 90 degrees and if the light irradiated by the laser and transmitted is detected by power meter  414 , it also is possible to detect the location of light point defect pixels. Once the location of a light point defect pixel is detected, if one of the polarization plates is rotated 90 degrees and that defective pixel is irradiated with laser light, then it is possible to evaluate the degree of defect repair by detecting with power meter  414  the intensity of the laser light transmitted through the polarizing plate  412 .  
         [0041]     As described above, by means of the defect repair method of the present invention, through a simple process of irradiating a laser light one time, light point defect pixels can be made into black spots, without impacting the surrounding pixels. With this method, the blackened pixels will be maintained and will not return to light points even with the passage of time. Also, with the defect repair method of the present invention, the location of defects and the degree of repair are detected and the alignment film of the defective pixels can efficiently be broken down.  
         [0042]     Further, the present invention need not be limited to the content of the above-mentioned embodiments and various forms of implementation are possible within the scope of the gist of the invention. For the lens, a condenser lens may be used or an imaging lens may be used, and for the orientation restraining force of the alignment film, instead of laser light, it is acceptable to use and detect another light source. Moreover, for specifying the location of the defective pixel a power meter or an alignment camera could be used.  
         [0043]      FIG. 5  shows one embodiment of the laser irradiation device wherein a condenser lens is used. A laser irradiation device  50  comprises a laser oscillator  500 , a mirror  502 , a condenser lens  504 , a table  508  on which is mounted the liquid crystal display device such that the position thereof in the X, Y, and Z directions can be controlled, and an alignment camera  510 . The laser light outputted from the laser oscillator  500  is reflected by the mirror  502  and is irradiated onto a liquid crystal display device  506  via the lens  504 .  
         [0044]     The location of the defective pixel can be detected by means of the alignment camera  510 , and by moving the table  508  in the XY-direction, adjustment can be made so that the laser light irradiates the defective pixel.  
         [0045]     Because the lens  504  is a condenser lens, the intensity distribution of the laser light irradiated onto the liquid crystal device  506  becomes as shown in the left diagram of  FIG. 5 (B) and the beam shape becomes as shown in the right diagram. As a result, the area of irradiation can be controlled by moving the table  508  in the Z-direction and it becomes possible to irradiate the laser light on the defective pixel only, without impacting surrounding pixels.  
         [0046]     Whether or not the properties of the alignment film have changed due to laser light irradiation can be detected by the alignment camera  510 . For example it would be acceptable to make the configuration of the table  508  such that light can be transmitted through part or all of it and to provide a light source under the table  508  so that changes in the light transmissivity of the liquid crystal display device  506  are observed by the alignment camera  510 . Through such a configuration, progress in the change of the alignment film properties can be observed, and it is possible to control the system such that when the prescribed change in properties has been reached, irradiation of the laser light is stopped.  
         [0047]      FIG. 6  shows one embodiment of the laser irradiation device wherein an imaging lens is used. A laser irradiation device  60  comprises a laser oscillator  600 , a homogenizer  610 , a rectangular mask  612 , a mirror  602 , an imaging lens  604 , a table  608  whose the position in the XY-direction can be controlled, and an alignment camera  614 . A liquid crystal display device  606  is mounted on the table  608 . The laser light transmitted through the homogenizer  610  and the rectangular mask  612  is reflected by the mirror and is irradiated onto the liquid crystal display device  606  via the imaging lens. The intensity distribution of the laser light irradiated onto the liquid crystal device  606  becomes as shown in the left diagram of  FIG. 6 (B) and the beam shape becomes as shown in the right diagram so that the exact pixel is irradiated with a uniform intensity.  
         [0048]     As a result, with laser irradiation device  60 , the pixel having a defect can irradiated by moving the table in the XY-direction only, without the need to move it in the Z direction.  
         [0049]     The other structures of the laser irradiation device  60  are the same as those of the laser irradiation device  50  and a description of them is omitted here.