In recent years, liquid crystal display devices, particularly, Thin Film Transistor (TFT) liquid crystal display devices which have a twisted nematic (TN) display mode, have come into wide use. For example, they are the general-purpose display devices in personal computers.
Usually, a liquid crystal display device includes a pair of opposing substrates that are maintained with a prescribed interval, electrodes and alignment films formed on the facing surfaces of the substrates, and a liquid crystal layer inserted between the alignment films. The electrodes of one substrate are formed into a common electrode. The electrodes of the other substrate are formed into the pixel electrodes. The pixel electrodes are often provided with an active matrix. In addition, electrodes are provided only on one substrate (for example, IPS mode). A black matrix or color filter is provided on either substrate.
In conventional liquid crystal display devices, the liquid crystal molecules in the liquid crystal layer are oriented in the prescribed direction by rubbing the alignment film. The alignment film is polished by a cloth, for example rayon, which undesirably generates dust within the clean room. Moreover, the rubbing generates static electricity which could potentially result in the breakdown of the TFT of the active matrix.
The inventors of the present invention have proposed in Japanese patent application HEI 9-354940 and Japanese patent application HEI 11-72085 a technique for orienting the liquid crystal molecules through the use of ultra-violet rays. As illustrated in FIG. 37, ultraviolet light is irradiated at an angle of 45°, for example, with respect to the surface of the polyimide alignment film 501, thereby orienting the liquid crystal molecules 502.
The relationship between the pre-tilt angle and the amount of ultra-violet ray irradiation realized by the method of Japanese patent application 11-72085 is illustrated in FIG. 38.
From the relationship shown in the drawing, when the volume of ultraviolet ray irradiation is low and the pre-tilt angle is large, black points occur in locations centered around spacers used to maintain the spacing between the substrates (cell gap) of the liquid crystal display device. Correspondingly, if the ultraviolet light exposure is high, flow-induced orientations accompanying the injection of liquid crystal are produced. Both are primary causes of poor displays. In this case, an appropriate range for the pretilt angles that obtain displays having good images is a narrow range of no more than 1.0° centered near 89°.
One of the problems associated with the teachings of Japanese patent application HEI 11-72085 is strong reliance on the proper control of the angle and intensity of the ultra-violet rays. Optimum results require a maximum of a ±10% intensity deviation in order to obtain a given pre-tilt angle. Referring to the properties curve of FIG. 8, a deviation of ±0.2% commonly occurs both in the angle of irradiation and in the intensity of the ultra-violet rays, making it difficult to reliably obtain a specified pre-tilt angle. Consequently, the probability of poor display occurring increases and there is a concern that the display will be unreliable.
Accordingly, an object of the present invention is to provide an improved method for orientation of an orientation film in which a desired pre-tilt angle of liquid crystal molecules can be assured without the need for rubbing the orientation film, and which does not suffer from the aforementioned problem relating to proper control of the angle and intensity of the ultra-violet rays.
Another object of the present invention is to increase the contrast in the display surface and prevent light and dark reversal in the display, and provide an alignment technique that exposes the alignment film with ultraviolet light from different directions and produces domains in the pixels. As shown in FIGS. 39A and 39B, two domains are created in the alignment film 611 by using an optical mask 601 formed with a slit 602. The optical mask 601 is placed above the alignment film 611, and parallel ultraviolet light is irradiated at an incline from above the optical mask 601. Next, parallel light is irradiated again at an incline having a different angle (Unexamined Japanese Patent Publication (Kokai) No. Hei 11-133429). Thus, the alignment film 601 is irradiated multiple times, one time for each domain. Naturally, this leads to an increase in the number of processes.
The method disclosed in Hei 11-133429 is further problematic as the multiple irradiations tend to cause bending of the optical mask. As shown in FIG. 40, bending of the optical mask 601 causes offsets in the exposure positions of the ultraviolet light on the alignment film 611. For example, even if the ultraviolet light is irradiated in two directions with the center of the pixel as the boundary, the domain will have an offset center position. The size of the glass substrate has tended to increase each year, and the use of a 1 m2 substrate is expected. If the thickness of the optical mask is 1 cm, bending by several dozen at the center of the optical mask will be apparent based on calculations. Thus, design offsets that cannot be ignored will occur.
Consequently, if domains are created as described above, in addition to the difficulty in controlling the angle and intensity of the irradiated ultraviolet light, the processes will necessarily become more complex.
In view of the problems described above, another object of the present invention is to provide a liquid crystal display device that has a simple structure and is provided with alignment films that can very stably and easily obtain the appropriate pretilt angles for the liquid crystal molecules by a simple alignment process without rubbing.
Another object of the present invention is to provide an alignment apparatus, an alignment method able to easily and accurately create domains without increasing the number of processes.
The present invention provides an improved liquid display device and a method for creating the same. According to a first embodiment, the liquid crystal display device includes a pair of substrates in a spaced relationship with one another. A pair of alignment films are provided, one alignment film being formed on each substrate such that the alignment films face one another. A liquid crystal layer, including plural liquid crystals, is inserted between the pair of alignment films, wherein the alignment films impart a given pre-tilt angle to the liquid crystals. The alignment films are composed of a material containing at least two types of polymers having a prescribed initial alignment and different alignment variation rates in response to ultra-violet ray irradiation. The pre-tilt angle being adjusted, without rubbing the alignment films, through ultraviolet exposure of the alignment films.
Also disclosed is an alignment apparatus for adjusting the alignment of an alignment film with ultraviolet light. The alignment apparatus includes a light source to irradiate scattered ultraviolet light, and an optical mask disposed under the light source. The optical mask is formed with at least one slit. In operation the optical mask is placed above the alignment film and scattered ultraviolet light irradiates from the light source through the optical mask. Diffuse light exposes the alignment film, and produces domains in the liquid crystal that depend on the directions of diffusion of the diffuse light.