The liquid crystal display device is widely used in television set, monitor for personal computer and the like because it is a display device of low power consumption and can be lightened and realize a low-profile device. However, the liquid crystal display device has an angle dependency of light transmittance since the polarization of light is usually controlled by a tilt angle of a liquid crystal molecule depending on an applied voltage. Consequently, in some directions of viewing angle, deterioration of a contrast ratio or a gray scale inversion at the time of gradation display occurs. Therefore, there was a room for contrivance in that the liquid crystal display device is generally insufficient for a viewing angle characteristic.
Technology of a domain division, in which a pixel is divided into two or more regions as well as alignment and tilt directions of liquid crystal molecules vary from region to region in the pixel, is developed. Thereby, since liquid crystal molecules are tilted in different directions in the pixel through an application of a voltage to a liquid crystal layer, it becomes possible to improve the viewing angle characteristic of the liquid crystal display device. Each region, in which alignment and tilt directions of liquid crystal molecules are different from each other, is also called a domain, and the domain division is also called a multi domain.
Examples of a liquid crystal mode of a horizontal alignment mode, in which the domain division is performed, include a multi domain twisted nematic (TN) mode, a multi domain electrically controlled birefringence (ECB) mode, a multi domain optically compensated birefringence (OCB) mode, and the like. On the other hand, examples of a liquid crystal mode of a vertical alignment mode include a multi domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, and the like, and various improvements for realizing a wider viewing angle are made to the liquid crystal display devices of various mode. Among others, a multi domain vertical alignment twisted nematic (VATN) mode is known as a liquid crystal mode which can realize a wide viewing angle and a fast response with a few number of processing. In the VATN mode, the liquid crystal molecule has a twisted structure by using vertical alignment layers in which directions of an aligning treatment on both substrates are substantially perpendicular to each other, and liquid crystal display device has a plurality of domains, in which directions of the alignment of the liquid crystal molecules are different from one another in one pixel.
Examples of a method of performing the domain division include a rubbing method and an optical alignment method. As the rubbing method, a method in which the domain division is performed by rubbing in a state of separating each pixel (subpixel) of a display region into a rubbing region and non-rubbing region by patterning with a resist is proposed. However, there was a room for contrivance in that since the alignment treatment is performed by rubbing the surface of an alignment layer with a cloth wound around a roller in the rubbing method, defects such as a fluey of the cloth, dust such as shavings, break of a switching device due to static electricity, a characteristic shift and degradation are generated. Further, there might be cases where rubbing was not sufficient at an edge portion of the resist.
On the other hand, an optical alignment method is an alignment method in which an optical alignment layer is formed by use of an optical alignment material and an alignment anchoring force is generated in the alignment layer by irradiating light such as ultraviolet light to the optical alignment layer. In accordance with the optical alignment method, generation of dirt and dust can be inhibited since an aligning treatment of an alignment layer can be performed in a noncontact manner. Further, since light can be irradiated to each region within a surface of the alignment layer under different conditions by using a photo mask at the time of exposing, a domain having a desired design can be readily formed.
As a conventional method of a domain division by an optical alignment method, for example, patent document 1 is disclosed. In patent document 1, a technology, in which a domain is formed by performing a first ultraviolet irradiation to an alignment layer by using a photo mask having light-transmitting areas and light-blocking areas, respectively, located in a stripe pattern, then shifting a position of the photo mask by half of a pitch of the stripe, i.e., by a width of the stripe, in a direction perpendicular to a direction of an orientation of the stripe, and performing a second ultraviolet irradiation from the direction opposite to the direction of the first ultraviolet irradiation, is disclosed. If the optical alignment method is thus used, the pixel (subpixel) can be readily divided into two or more domains by using a photo mask.
However, in an exposure system using a photo mask in the optical alignment method, alignment irregularities are generated in the alignment layer when dust adheres to the vicinity of a light-transmitting area of a photo mask. Then, there was a room for contrivance in that when white display is provided, a region in which alignment irregularities is recognized as a black point and therefore this causes the deterioration of display quality. Further, the optical alignment method is applied to the VATN mode (hereinafter, also referred to as a “4 VATN mode”) in which four domains are formed, the deterioration of display quality due to the influence of alignment irregularity has particularly remarkably occurred.
In association with such problems, a technology of reducing pinholes generated by contaminants on a photo mask at the time of exposing a negative photosensitive resin is disclosed (for example, see patent document 2). However, this technology is a technology concerning a method of exposing the negative photosensitive resin and is not a technology concerning the alignment layer of the liquid crystal display device, and thought of performing the domain division by using the optical alignment method is not disclosed at all.
Further, presently, an exposure device for performing a domain division by an optical alignment method is little-disclosed, and further, naturally, an exposure device which can inhibit the deterioration of display quality of a liquid crystal display device due to the dust adhering to a photo mask, is not also disclosed.
Patent Document 1: Japanese Kokai Publication Hei-11-133429
Patent Document 2: Japanese Kokai Publication Hei-5-335196