KR19990005266A - Vertical alignment liquid crystal display device and manufacturing method using compensation film - Google Patents

Abstract

In the state where no voltage is applied, the liquid crystal molecules are arranged perpendicularly to the substrate, and in the state where the voltage is applied, the liquid crystal molecules remain parallel to the substrate, but the liquid crystal molecules are twisted in four regions divided by four pixels. The compensation film which has a negative retardation value is attached to a liquid crystal display device using the vertically-aligned liquid crystal which takes the form of a direction shift | deviating. In order to obtain such a liquid crystal display, ultraviolet rays are irradiated twice at different angles when forming an alignment layer, the two substrates thus formed are combined to form an angle of 90 degrees, and a compensation film is attached to the outer surface of the substrate.

Description

Vertical alignment liquid crystal display device and manufacturing method using compensation film

The present invention relates to a liquid crystal display device, and more particularly, to a wide viewing angle liquid crystal display device using a vertical alignment and pixel segmentation and compensation film.

Liquid crystals have birefringence with different refractive indices in the long axis and short axis of the molecule. The birefringence causes a difference in the refractive index of light depending on the position at which the liquid crystal display device is viewed. There is a difference in the rate of change of state, so the amount of light and color characteristics when viewed from a position away from the front are different from those seen from the front. Accordingly, in the liquid crystal display having a twisted nematic (TN) structure, changes in contrast ratio, color shift, and gray inversion occur according to the viewing angle.

In order to solve this problem, a multi-domain TN (TN), which improves a viewing angle by dividing pixels or rubbing pixels by pixel, using a compensation film, has been developed. In-plane switching mode (IPS) technology has been developed. However, in the case of a TN LCD using a negative retardation compensation film, it is possible to improve the gradation characteristics only when viewed in a direction that forms an angle of 45 degrees with the transmission axis of the polarizing plate. In some cases, the gradation characteristics become worse. IPS technology is still in the early stages of development and has many problems to solve. Multi-area TN was developed in the 80's, and its main purpose is to divide each pixel of LCD into an area with different arrangements or optical characteristics of liquid crystal molecules. In a properly designed multi-domain TN, the light characteristics of each angle are compensated for each other, resulting in a wide viewing angle and improved gray scale display performance.

The results of the optical simulations show that the four domain (4-D) TN can obtain the optimal viewing angle characteristics. However, in order to stabilize such a structure, it is necessary to maintain a very high pretilt angle. Therefore, finding an alignment film material suitable for a photolithography process with a high pretilt angle has become an important problem. For liquid crystal display devices having a vertical 4-D TN structure, an all-round SiOx deposition method can be used, which is very complicated and difficult to apply to mass production.

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal display device which can widen the viewing angle by using a compensation film for 4-D VATN-LCD, and to provide a method for manufacturing a liquid crystal display device including a method of forming an alignment film used in such a liquid crystal display device. It is to provide.

1 shows a schematic structure of a vertically aligned four region twisted nematic liquid crystal display (TN-LCD) according to an embodiment of the present invention.

2 illustrates a pretilt direction of each micro area of a vertically aligned four-zone TN-LCD according to an exemplary embodiment of the present invention.

3 illustrates an arrangement of liquid crystal molecules in an off-state state in which no voltage is applied to the vertically aligned four-region TN-LCD according to an exemplary embodiment of the present invention.

4 illustrates an arrangement state of liquid crystal molecules in a state in which a voltage is applied to a vertically aligned four-region TN-LCD according to an embodiment of the present invention.

5A to 5D illustrate a manufacturing process of a substrate for a liquid crystal display according to an exemplary embodiment of the present invention.

6 shows the chemical formula of the vertical alignment agent used in the embodiment of the present invention,

7 shows viewing angle characteristics of a vertically aligned four-zone TN-LCD using a compensation film according to an embodiment of the present invention.

8 to 9 are graphs showing eight gray scale display performances at three angles of a vertically aligned four-zone TN-LCD using a compensation film according to an embodiment of the present invention.

In order to solve this problem, in the present invention, the liquid crystal molecules are vertically arranged with respect to the substrate when no voltage is applied, and while the voltage is applied, the liquid crystal molecules are kept parallel to the substrate, but the pixels are divided into four parts. A vertically aligned liquid crystal having a form in which the twist direction of liquid crystal molecules is shifted in the divided four regions is used, and a compensation film having a negative retardation value is attached to the liquid crystal display device.

Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 shows a schematic structure of an embodiment of the present invention. Compensation film 30 is attached to one side of the cell with the vertically aligned 4-D TN liquid crystal cell 20 in the center, and the front polarizer 40 and the rear surface on both sides of the cell to which the compensation film 30 is attached. The polarizing plate 10 is attached. The liquid crystal cell 20 includes a liquid crystal material 23 enclosed between both the transparent substrate 21 and the vertical alignment layer 22 and the vertical alignment layer 22 applied inside the transparent substrate 21. The front polarizer 40 and the rear polarizer 10 are attached such that the transmission axes of the polarizers are perpendicular to each other, and the compensation film 30 is disposed between the liquid crystal cell 20 and the front polarizer 40 or between the liquid crystal cell 20 and the rear surface. It may be attached to either side between the polarizing plates 10.

2 illustrates a structure of unit pixels of a vertically aligned 4-D TN according to an exemplary embodiment of the present invention. Solid arrows and dashed arrows indicate the direction of pretilt on the upper and lower substrates, respectively.

The lower substrate is divided into two regions in the vertical direction, and the pretilt direction of the left region is from right to left, and the pretilt direction of the right region is from left to right. The upper substrate is divided into two regions in the horizontal direction so that the pretilt direction of the upper region is from top to bottom, and the pretilt direction of the lower region is from bottom to top. As such, by combining two substrates separated into two different regions to form an angle of 90 degrees, a liquid crystal cell divided into four regions having different twisting directions can be configured.

In addition, the letters R and L in the figure indicate the torsional direction of each region when voltage is applied, respectively. In the drawing, the upper right region 1 and the lower left region 3 twist the liquid crystal molecules to the right when a voltage is applied, and conversely, the liquid crystal molecules in the lower right region 4 and the upper left region 2 twist to the left.

In the case of a liquid crystal without chiral dopant added, the twisting direction of the liquid crystal molecules is determined only by the direction of the pretilt. The liquid crystal used herein is a liquid crystal having negative dielectric anisotropy, and materials such as ZLi-2806 or MJ9511-52 may be used.

3 and 4 show how the liquid crystal molecules take a form in an off-state and an off-state. Regions 1 to 4 in FIGS. 3 and 4 correspond to regions 1 to 4 in FIG. 2, respectively. In the state where no voltage is applied, the liquid crystal molecules of each region have a vertically standing structure (FIG. 3). In this case, since the light passing through the rear polarizer is transmitted through the liquid crystal layer without being affected by the polarization, the liquid crystal display device is in a dark state because the transmission axis cannot pass through the front polarizer that forms 90 degrees with the rear polarizer. When voltage is applied to the liquid crystal cell, the arrangement of the liquid crystal molecules is changed into a twisted shape as shown in FIG. 4. The incident light then passes through the liquid crystal layer while its polarization rotates along the twisted arrangement of liquid crystal molecules. The light can pass through the front polarizer according to the degree of rotation of the polarized light, and the liquid crystal display is in a bright state.

As can be seen in FIG. 2, one pixel is divided into two left twisted regions and two right twisted regions. 3 and 4, in the region 1, the pretilt direction of the upper plate is a direction coming out of the ground, and the pretilt direction of the lower plate is from left to right. In the region 2, the pretilt direction of the upper plate is the same as that of the region 1 coming out of the ground, and the lower plate is from right to left. The area 3 is a direction in which the pretilt direction of the upper plate enters the ground, and in the case of the lower plate, it is a right-to-left direction similarly to the area 2. The area | region 4 is the upper board direction to the ground, and the lower board is left to right. Between the substrates having such a pretilt direction, the liquid crystal molecules are twisted to the right in the regions 1 and 3, and the liquid crystal molecules are twisted to the left in the regions 2 and 4. Two regions having the same torsional direction are arranged with one region rotated 180 degrees with respect to the other. By doing so, the four regions can compensate for each other's optical characteristics and can improve the wide viewing angle and gradation characteristics.

When the short-axis or biaxial compensation film having an appropriate negative delay is attached to the liquid crystal display, further improved viewing angle characteristics and contrast ratio may be obtained. In particular, the contrast ratio and gray level inversion seen in the direction of 45 degrees with the transmission axis of the polarizing plate are remarkably improved. The negative retardation of the compensation film should be optimized in each case, but it is generally appropriate to have a value approximately equal to the retardation of light generated by the birefringence of the liquid crystal material, and the sum of the retardation values by the compensation film and the liquid crystal material It is preferable that the difference of the delay value due to birefringence does not exceed 40% of the delay value due to birefringence of the liquid crystal material.

In order to obtain regions having different pretilt angles, an ultraviolet irradiation method is used, which is illustrated in FIGS. 5A to 5D.

First, the vertical alignment film 52 is rotationally coated on the transparent substrate 51 and cured at an appropriate temperature (FIG. 5A). The vertical alignment film used here is polyimide, and its structure is shown in FIG. Of course, other alignment film materials may be used. Next, the linearly polarized ultraviolet rays are irradiated in a direction inclined 45 degrees with respect to the substrate normal using the mask 53 designed to irradiate only one half of the pixels onto the substrate coated with the alignment film (FIG. 5B). After the primary irradiation, the ultraviolet rays are irradiated again in a direction where the first irradiation direction and the azimuth angle are 180 degrees apart using the mask 54 that exposes the remaining half of the pixel (FIG. 5C). In this way, a substrate having two regions having different pretilt angles as shown in FIG. 5D can be formed. The two substrates formed as described above may be combined to form an angle of 90 degrees as shown in FIG. 2, to form a 4-D TN cell in the same manner as a conventional TN cell.

Electro-optical simulation results of the vertically aligned 4-D TN liquid crystal display using the compensation film are shown in FIGS. 7 to 9. The geometry and parameters used in the simulation are as follows.

6 shows viewing angle characteristics of a vertically aligned 4-D TN liquid crystal display using a compensation film. It has a very uniform viewing angle characteristics and a high contrast ratio, and it can be seen that the viewing angle characteristics in the directions (45 degrees and 135 degrees) which form an angle of 45 degrees with the transmission axis of the polarizing plate are greatly improved. FIG. 7 shows 8 gray scale display performance of a vertically aligned 4-D TN liquid crystal display using a compensation film viewed in the horizontal direction. FIG. FIG. 8 shows the eight-gradation display performance seen in a direction of 45 degrees or 135 degrees with the transmission axis of the polarizing plate. The horizontal axis of the graph represents the viewing angle, and the vertical axis represents the transmittance. As shown in Fig. 7 to Fig. 8, gray level inversion does not occur in the region inside (+-) 60 degrees.

The vertically aligned liquid crystal display according to the embodiment of the present invention is configured to take a normally black mode, which is not only better in contrast to the normally white mode but also brighter when voltage is applied. Since the display is divided, the contrast ratio is not lowered due to light leaking from the dividing surface. In this case, since the leakage light is generated only in the bright state and not in the dark state, a black matrix (BM) is not necessary to prevent this. Third, the problem of low contrast ratio in the direction of 45 degrees from the transmission axis of the polarizing plate, which is a major weakness of the vertically aligned TN liquid crystal display device, can be solved by using a compensation film. Fourth, by forming the pretilt angle by using the optical alignment technology, damage to the alignment layer due to dust particles or static electricity does not occur, and no photolithography process is required.

As described above, in the present invention, a liquid crystal display device is configured by using a liquid crystal having a negative dielectric anisotropy and a vertical alignment layer, dividing pixels into four, and attaching a compensation film, thereby widening the viewing angle.

Claims (8)

Two polarizing plates facing each other, a compensation film attached to an inner surface of any one of the polarizing plates, a pair of transparent substrates attached to an inner surface of the other one of the polarizing plates and an inner surface of the compensation film, the two transparent substrates It is applied to the inner surface of the substrate, divided into four regions, when the voltage is applied, the vertical alignment layer having a different angle of pretilt of the liquid crystal molecules in each region, and has a negative dielectric anisotropy injected between the vertical alignment layer Liquid crystal display comprising a liquid crystal material. The liquid crystal display of claim 1, wherein the compensation film is a shortening compensation film. The liquid crystal display of claim 1, wherein the compensation film is a biaxial compensation film. The liquid crystal display of claim 1, wherein the compensation film has a negative retardation. The liquid crystal display of claim 4, wherein the compensation film has a difference between a retardation value due to birefringence of the liquid crystal material and a negative retardation value of the compensation film not exceeding 40% of a retardation value due to birefringence of the liquid crystal material. Forming a vertical alignment layer on the transparent substrate, dividing each pixel on the transparent substrate into two minute regions and surface treating the liquid crystal molecules in two regions to be displaced when a voltage is applied; And combining the dogs so that the boundary lines of the minute regions of each substrate are at an angle of 90 degrees to each other. The method of claim 6, wherein the surface treatment includes firstly irradiating linearly polarized ultraviolet rays to only one of the two microregions, and performing the second irradiation in a direction in which an azimuth angle is shifted from the first irradiation direction. The manufacturing method of the liquid crystal display device. The method of claim 7, wherein the secondary irradiation direction of the ultraviolet rays forms an angle of 180 degrees with the primary irradiation direction.