Liquid crystal panels are used in a broad range of applications, such as display devices in industrial appliances and medical appliances, not only in television sets and mobile phones. In general, a liquid crystal panel includes: a pair of substrates that face each other; and liquid crystal enclosed between the pair of substrates. In one of the substrates, there are formed thin film transistors for driving respective pixels, and electrodes for applying voltage to the liquid crystal. A thin film formed of an oriented material is applied on the electrodes, and an oriented film that orients the liquid crystal in a certain direction is formed by applying orientation treatment to the thin film. On the other substrate, a color filter layer is formed as necessary. The color filter layer is formed by arranging pixels including pigments of different colors, for example, red, green and blue. Adjacent pixels are partitioned by a light blocking black matrix. A common electrode is formed on the color filter layer as necessary. Furthermore, a thin film formed of an oriented film material, such as polyimide, is applied thereon, and is subjected to orientation treatment, to form an oriented film that orients the liquid crystal to a certain direction.
As one example of the orientation treatment, rubbing treatment can be cited. The rubbing treatment is treatment in which the thin film is rubbed in a predetermined orientation direction with a rubbing roller around which rubbing cloth, which is a friction member, is wound. The thin film that has undergone the rubbing treatment has an orientation regulating capability for orienting liquid crystal in a certain direction. As one example of non-contact orientation treatment, photo-alignment treatment can be cited. The photo-alignment treatment is treatment that provides an orientation regulating capability to the thin film by irradiating the thin film with ultraviolet rays such that chemical bonds of macromolecules in the oriented film material is decomposed, isomerized and dimerized.
After applying orientation treatment to both substrates, a liquid crystal layer is formed between the substrates. There are various techniques to form a liquid crystal layer. One of the techniques includes: applying a sealant on one substrate in a rectangular frame form a part of which is opened; adhering the substrates together; and injecting liquid crystal material between the substrate from the opening. Another technique includes: applying sealant on one substrate in a rectangular frame form; dropping liquid crystal in a region surrounded by the sealant; and thereafter adhering the substrates together. Then, by adhering polarization plates onto the resulting body after forming a liquid crystal layer between the substrates, a liquid crystal panel is completed. The completed liquid crystal panel undergoes a display inspection, so that a liquid crystal panel without a display defect is shipped as a product.
In recent years, the pixel size per unit area of liquid crystal panels has become small as a result of highly minute structures, and thus the number of pixels is increasing. Accordingly, the probability of pixel defect is increasing. Pixel defects in the liquid crystal panel come from breaking and short circuit of wires and electrodes, damages on the surface of the oriented film made by orientation treatment, and residue of foreign matters in the liquid crystal panel, for example. Examples of defects caused by a foreign matter in the liquid crystal panel include: a pixel defect that a foreign matter or a region around the foreign matter is seen darkly when the panel displays a white screen; and another pixel defect that the foreign matter or a region around the foreign matter is seen whitely when the panel displays a black screen. The former is called a dark spot and the latter is called a bright spot.
Here, examples of the operational modes of a liquid crystal panel include: a TN (Twisted Nematic) mode; an IPS (In Plane Switching) mode, and a VA (Vertical Alignment) mode. Among these modes, demands for the IPS mode are increasing because it has a wide viewing angle and changes in tones and colors according to the viewing angle are small.
The IPS mode is a normally black mode where a black screen is displayed when voltage is not applied. Accordingly, when there is a bright spot defect, the bright spot can always be seen because of light of the backlight provided in the liquid crystal panel even if no voltage is applied, and thus image quality is deteriorated. Accordingly, there is an increased importance in modifying the bright spots especially for a liquid crystal panel in the IPS mode.
Consequently, the cause of bright spots has been examined for modifying the bright spots. It turned out that there are two causes of bright spots. First, a bright spot results from transmission or scattering of light in a foreign matter itself. Second, a bright spot results from orientation abnormality of the liquid crystal in the circumference of a foreign matter.
The first bright spot that results from transmission or scattering of light in the foreign matter itself appears because the foreign matter, which has characteristics of transmitting light in the visible light region and/or of scattering light, makes a difference in transmittance from a portion without the foreign matter in a liquid crystal panel. In this case, since a bright spot appears as a result of light passing through the foreign matter itself or scattered in the foreign matter itself, the size of a bright spot is substantially the same as the size of the foreign matter.
The second bright spot that results from orientation abnormality of the liquid crystal in the circumference of the foreign matter appears because a foreign matter, which is organic substance, makes a difference in light transmittance from a portion without the foreign matter in a liquid crystal panel, due to occurrence of orientation abnormality coming from interaction of liquid crystal molecules in the circumference of the foreign matter with molecular chains on the surface of the foreign matter.
Usually, liquid crystal molecules orient in a predetermined direction by orientation treatment. For example, when rubbing treatment is carried out, minute unevenness is formed on a surface of the oriented film by the rubbing treatment, and thus it becomes easy to orient the liquid crystal molecules along grooves caused by the unevenness. This is because the liquid crystal molecules are affected by the unevenness and orient in a direction that results in minimum elastic free energy (direction parallel to the groove) so that elastic strain energy does not become high.
However, it can be considered that the liquid crystal molecules in the circumference of the foreign matter are not oriented along minute grooves formed by the rubbing treatment but are oriented in various directions. It can be considered that this is because the molecular chains on the surface of the foreign matter do not orient in a fixed direction but orient in various directions, and under the influence of this, the direction that the elastic free energy of the liquid crystal molecules in the circumference of the foreign matter is stable is not a fixed direction determined by the orientation treatment. As a result, the orientation abnormality arises in the circumference of the foreign matter, which makes a difference in light transmittance from a portion without a foreign matter in the liquid crystal panel, and thus a bright spot appears.
In many cases, foreign matters that have remained in the panel are: shavings of oriented film material that has remained on the oriented film as a result of, for example, rubbing treatment; or organic substances, such as protein coming from human bodies. Accordingly, there is a high probability of occurrence of a bright spot that results from orientation abnormality of the liquid crystal in the circumference of the foreign matter. In addition, even if the size of a foreign matter is so minute as to be invisible, the size of a bright spot becomes larger than the size of a foreign matter so as to be visible when orientation abnormality arises in the liquid crystal in the circumference of the foreign matter. Accordingly, it is especially important to modify a bright spot that results from orientation abnormality of the liquid crystal in the circumference of the foreign matter.
With respect to the method of modifying a bright spot of a liquid crystal panel, Japanese Unexamined Patent Application Publication (JP-A) No. 2007-065653 discloses a method of manufacturing a liquid crystal display. The liquid crystal display includes a pair of substrates arranged to face each other, and a liquid crystal layer formed between the substrates. The method includes: forming a microhole on a back surface of one of the substrates correspondingly to a portion where a bright spot produced as a result of a foreign matter remaining between the substrates is produced; forming a light blocking substance layer which blocks the bright spot according to light intensity of the bright spot inside the microhole; and irradiating the light blocking substance with ultraviolet rays in order to cure the light blocking substance. According to the method of manufacturing a liquid crystal panel, a light blocking substance layer corresponding to light intensity of the bright spot is formed. Accordingly, the modified portion is not observed in black exceptionally even if the screen is a gray screen, and thus it is possible to improve visual appreciation.
Moreover, JP-A No. 2007-171905 discloses a method of modifying a flat display panel including a first substrate and a second substrate, and at least one foreign matter arranged above either one of the first substrate and the second substrate. The method includes: forming a hole corresponding to the foreign matter on the first substrate or the second substrate; filling the hole with resin; and irradiating the resin with polarized ultraviolet rays. According to such a method of modifying a bright spot, a bright spot produced in the display panel is eliminated. Regarding ultraviolet rays to be used for irradiating the resin, a polarization component parallel to the orientation direction of the oriented film can damage bonds among the molecules of the oriented film. Therefore, damage in the oriented film can be minimized by irradiating the resin with ultraviolet rays of polarization components that are not parallel to the orientation direction of the oriented film and by blocking the polarization component parallel to the orientation direction.
Further, JP-A No. 2000-89231 discloses a method of manufacturing an optical device in which irregularity is modified by irradiating a portion where irregularity based on orientation treatment is produced with ultraviolet rays after forming an oriented film on one side of at least one substrate member of two substrate members at least one of which has translucency, or after enclosing liquid crystal between two substrate members including oriented films and completing the optical device. According to such a method of manufacturing an optical device, display quality improves since irregularity in the oriented film in the optical device is modified by irradiating the portion where irregularity in the oriented film is produced with ultraviolet rays.
The methods described in JP-A Nos. 2007-065653 and 2007-171905 are further described with reference to FIG. 7 and FIGS. 8A and 8B. FIG. 7 is a schematic diagram illustrating a situation that an observer views a liquid crystal panel from the front and a situation that an observer views a liquid crystal panel obliquely (at an angle). The illustrated liquid crystal panel includes a first transparent substrate 11, a second transparent substrate 12, a liquid crystal layer 13 and oriented films 15. FIG. 8A is a schematic diagram illustrating a case where a liquid crystal panel in which a bright spot has been modified by a conventional technique displays a black screen. FIG. 8B is a schematic diagram illustrating a case where a liquid crystal panel in which a bright spot has been modified by a conventional technique displays a white screen. FIGS. 8A and 8B illustrate pixels 51, a black matrix 52 and a light blocking substance layer formation portion 53 in the liquid crystal panel.
Both of the methods described in JP-A Nos. 2007-065653 and 2007-171905 merely shield the bright spot with light blocking substance 25 arranged in a hole 31 and do not modify liquid crystal orientation in the circumference of the foreign matter (see liquid crystal molecules 14 in FIG. 7). Accordingly, as illustrated in FIG. 7, when an observer views the liquid crystal panel around the foreign matter 17 from the front, the observer does not see a bright spot since backlight light 42 is blocked, but when viewing the liquid crystal panel from an oblique direction, the observer sees a bright spot caused by orientation abnormality of liquid crystal in the circumference of the foreign matter 17 because of backlight light 42 that entered the liquid crystal from an oblique direction. In order to block the backlight light 42 that enters the liquid crystal from an oblique direction, it is necessary to form a light blocking substance layer of an area considerably larger than that of the bright spot. In addition, since the bright spot is always shielded with the light blocking substance 25, the blocked part always has a fixed tone irrespective of the tone that should be displayed. Accordingly, as illustrated in FIG. 8A, the blocked part (light blocking substance layer formation portion 53) is not highly distinguishable when the liquid crystal display displays a black or gray screen. However, as illustrated in FIG. 8B, the blocked part can be seen as a dark spot when the liquid crystal display displays a white screen.
In addition, in the method disclosed in JP-A No. 2000-89231, in a case that the optical device is irradiated with ultraviolet rays after the oriented film is formed, a bright spot cannot be modified if a foreign matter or the like enters into the optical device after irradiation of the optical device with ultraviolet rays and before assembling the optical device, and a bright spot and irregularity resulting from, for example, dusts, stuck onto the substrate surface after the irradiation cannot be modified. In another case that the optical device is irradiated with ultraviolet rays after the optical device is assembled, irradiation with ultraviolet rays is carried out via the substrate member. Therefore, the substrate member absorbs ultraviolet rays of wavelength required for modifying the irregularity (or bright spot), and thus the irregularity (or bright spot) cannot be modified by UV irradiation. Further, it is necessary to prepare a mask dedicated for the size and the shape of the irregularity (or bright spot), and thus it is costly. In addition, JP-A No. 2000-89231 does not explicitly indicate a method of modifying a bright spot produced as a result of a foreign matter.
The present invention seeks to solve the problems.