JPH04223435A - Production of active matrix liquid crystal display panel - Google Patents

Abstract

PURPOSE:To provide the high-grade active matrix liquid crystal display panel free from surface detects by preventing the orientation defects, more particularly reverse twisted domains, to be generated in a liquid crystal panel. CONSTITUTION:The process for producing the active matrix liquid crystal display panel has a stage for cooling the panel after once raising the temp. of the panel to the liquid crystal-isotropic liquid transition temp. or above after charging liquid crystal into the panel; in addition, the cooling rate of the panel at the time of passing the liquid crystal-isotropic liquid transition temp. in the cooling process is set higher than the speed at which plural pieces of nematic nuclei are generated in at least the respective picture elements constituting the panel.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an active matrix liquid crystal display panel.

0002

[Prior art] Thin film transistor (TFT) is used for each pixel.
Active matrix liquid crystal display panels equipped with switching elements such as diodes and diodes have been applied to various displays such as liquid crystal television receivers, and have recently attracted great expectations for office automation displays and projection television receivers. .

[0003] Active matrix liquid crystal display panels are
Amid the trend of improving display quality, the trend is toward higher density and larger screens. Along with this, it has become difficult to uniformly align liquid crystals. Currently, active matrix liquid crystal display panels generally use a TN liquid crystal display mode in which the liquid crystal is twisted by 90 degrees between the upper and lower substrates. The uniform alignment treatment of the liquid crystal is performed by rubbing the surface of the polyimide film formed as the alignment film with a soft cloth. A liquid crystal panel is formed by filling a gap of several μm between two substrates whose surfaces have been subjected to alignment treatment with liquid crystal. As a filling method for liquid crystal, a vacuum injection method generally performed in a vacuum chamber is used. Since the alignment of liquid crystals is easily disturbed by the influence of the flow during injection, there is a method for uniformly aligning the liquid crystals by raising the temperature of the panel once above the liquid crystal-isotropic liquid transition temperature and then cooling it to realign the liquid crystals. It has also been taken.

0004

SUMMARY OF THE INVENTION In active matrix liquid crystal display panels, the TN liquid crystal display mode is generally used as described above. The substrate of an active matrix liquid crystal display panel has irregularities on its surface due to TFT, source, gate wiring, and the like. In addition, there are color filters and black stripes on the opposing substrate side, and similarly there are unevenness. As the density of pixels increases, the pitch of unevenness becomes finer, and as the number of pixels increases with larger screens, uniform alignment becomes difficult and alignment defects are likely to occur. This is because it becomes difficult to uniformly rub the entire panel substrate.

[0005] A particular problem in TN-oriented active matrix liquid crystal panels is the occurrence of reverse twist domains. The reverse twist domain is a domain whose twist direction is opposite to that of the surrounding normal area, and the disclination line at the boundary with the normal area causes light scattering, resulting in a display defect.

[0006] A method for preventing the occurrence of reverse twist domains is to increase the amount of chiral agent added.
There is a way to stabilize either the left or right twist. However, if the amount of chiral agent added is increased too much, characteristics deterioration such as a decrease in response speed due to an increase in viscosity occurs. Furthermore, even if the amount added is increased, the occurrence of reverse twist domains cannot be completely prevented. In addition, a method has been considered to improve the uniformity of rubbing by flattening the unevenness of the substrate surface, but the materials and processes used for flattening have not yet been established. Another problem is that the planarization film reduces the effective voltage applied to the liquid crystal layer.

Although it is an effective method to once raise the temperature of the panel to above the liquid crystal-isotropic liquid transition temperature and then cool it to rearrange the liquid crystals and uniformly align them, simply heating the panel is effective.
Even after repeated cooling, it is difficult to completely eliminate the reverse twist domain.

[0008] The present invention solves these problems, and aims to provide a high-quality active matrix liquid crystal display panel free from display defects by preventing alignment defects, particularly reverse twist domains, that occur in liquid crystal panels. shall be.

[0009]

[Means for Solving the Problems] In order to solve this problem, the present invention provides a method for, in the manufacturing process of an active matrix liquid crystal display panel, after filling the panel with liquid crystal, the temperature of the panel is changed to a liquid crystal-isotropic liquid. It has a step of raising the temperature to above the transition temperature and then cooling it, and the cooling rate of the panel when passing through the liquid crystal-isotropic liquid transition temperature in the cooling process is such that at least a plurality of pixels in each pixel constituting the panel The gist is that the speed is faster than the rate at which nematic nuclei are generated.

0010

[Operation] Normally, when a panel is filled with liquid crystal, some degree of alignment defect occurs due to the influence of the flow during filling. Once the temperature of the panel is raised above the liquid crystal-isotropic liquid transition temperature, the liquid crystal becomes an isotropic liquid state, so the original orientation is canceled, and when it is cooled, realignment occurs, resulting in a more uniform orientation than when filling. sex is improved. However, even if heating and cooling are simply repeated, it is difficult to completely eliminate the reverse twist domain.

[0011] This is due to the fact that when nematic nuclei are generated from an isotropic liquid during the recooling process, reverse twist nuclei are generated and grow with a certain probability. Reverse twist domains are difficult to exist stably in panels using flat substrates. However, in an uneven substrate panel such as an active matrix liquid crystal panel, the particles stably exist within regions separated by steps. This is thought to be due to differences in surface conditions such as surface shape and insufficient rubbing of the stepped edge portion. Observing the generation and growth of nematic liquid crystal nuclei during the cooling process from an isotropic liquid reveals the following.

For example, assume that only one nematic nucleus is generated within one pixel separated by a step in the wiring section. If this nematic nucleus is a reverse twist nucleus, the reverse twist grows over the entire pixel,
It becomes stable. When two or more nuclei occur in one pixel, there is almost no possibility that all of them are reverse twist nuclei. Rather, most of them are stable normal twisted domains. Therefore, even if a reverse-twisted nucleus is generated, it collides with the normal domain during growth and is absorbed into the stable normal domain.

The number of nematic nuclei generated during cooling tends to increase as the cooling rate increases, as in the case of general crystal growth. In the present invention, the cooling rate of the panel when passing through the liquid crystal-isotropic liquid transition temperature is set at least faster than the rate at which a plurality of nematic nuclei are generated in each pixel constituting the panel. The above effects can prevent the occurrence of reverse twist domains.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be described below with reference to the drawings. First, the present invention is characterized in that in the manufacturing process of an active matrix liquid crystal display panel, after filling the panel with liquid crystal, the temperature of the panel is raised to a temperature equal to or higher than the liquid crystal-isotropic liquid transition temperature, and then the process is cooled. It is said that

In the following embodiment, a TFT is used as an active element, and the diagonal size is 2.8 inches, 479 lines (gate) x 720 lines (source),
A matrix panel with 344,880 pixels was used. TF
The level difference in the wiring part on the T board side is about 7000 on the source line side.
A. The gate line side is approximately 1000A, and the line width is 8μ.
There are m. The opposite side was composed of a substrate having a black matrix of chromium (thickness: 1000 Å).

The generation of nematic nuclei from an isotropic liquid during cooling was investigated using the Thermosystem F manufactured by Mettler.
Observation was made under a polarizing microscope using P-82. Due to the sample size, the liquid crystal panel used was one cut from the above panel to a size of about 1 inch diagonally.

As the liquid crystal material, a PCH-based mixed liquid crystal having a nematic liquid crystal-isotropic liquid transition temperature of 99° C. was used. As a chiral agent, 0.2% of right-handed 4'-(S-2-methylbutyl)-4-cyanobiphenyl was added. The panel gap was set to be the optimum gap (approximately 5 μm) for green light of 90° twisted TN.

Next, a specific example will be described.

[Specific Example 1] Using a rayon cloth, the TFT substrate side and the counter substrate side were placed at 45° with respect to the gate line (x direction) and source line (y direction) as shown in FIG. The rubbing process was performed at an angle of 90° clockwise to produce a TN panel twisted at 90°. When the above liquid crystal material is injected in vacuum and the alignment state is observed,
There were numerous reverse twist domains along the flow direction during injection. These panels were heated to 110° C., held for 3 minutes, and then cooled to around room temperature, and the orientation state was examined in relation to the cooling rate. The cooling rate was monitored with a thermocouple attached to the panel surface. Table 1 shows the relationship between the cooling rate of the panel before and after the panel temperature passed the nematic liquid crystal-isotropic liquid transition temperature of 99° C., and the number of pixels in which reverse twist domains occurred after cooling.

[0020]

[Table 1]

Nematic liquid crystal - isotropic liquid transition temperature 9
Table 2 shows the relationship between the cooling rate of the panel before and after passing through 9°C and the number of nematic nuclei generated per pixel. The number of nuclei generated per pixel was determined by taking a video of the nematic nucleus generation, observing it by slow playback, counting the number of nuclei generated for 20 pixels, and showing the average value.

[0022]

[Table 2]

From Table 1, it can be seen that the faster the panel cooling rate is, the fewer reverse twist domains occur. This corresponds to the results in Table 2. In other words, as the cooling rate increases, the number of nematic nuclei generated from the isotropic liquid tends to increase, and along with this,
The occurrence of reverse twist domains is suppressed. The reason for this is, as mentioned above, when many nematic nuclei are generated within one pixel, even if a reverse twist nucleus is generated, it collides with a normal domain during growth and is absorbed into a stable normal domain. This is because it will be done.

The state of nucleation depends on the state of the panel, especially the state of the interface between the liquid crystal and the alignment film, and cannot be determined solely by the cooling rate. However, as shown in this example, if the cooling rate at which the nematic liquid crystal passes through the one-isotropic liquid transition temperature is -5°C/min or higher, the number of nematic nuclei generated increases and reverse twist occurs. The effect of suppressing the generation of domains can be large.

It is better not to raise the heating temperature of the panel too high above the liquid crystal-isotropic liquid transition temperature. Increasing the temperature not only has no effect on improving alignment, but also causes thermal deterioration of the liquid crystal material. Generally, the temperature may be about 10° C. above the liquid crystal-isotropic liquid transition temperature. Regarding the holding time at the isotropic liquid temperature, a short time is also preferable from the viewpoint of thermal deterioration of the liquid crystal.

[0026] The pixels in which the reverse twist domain occurs are not constant. In other words, if the panel is raised to a temperature above the liquid crystal-isotropic liquid transition temperature and then slowly cooled down,
A similar number of reverse twist domains occur with good reproducibility, but the pixels that occur are completely random, and the locations are not reproducible. This suggests that the generation of reverse twist domains occurs completely stochastically, and that increasing the number of nematic nuclei generated during the cooling process is effective in preventing the generation of reverse twist domains.

[Specific Example 2] Using a rayon cloth, the TFT substrate side and the opposing substrate side were rubbed as shown in FIG. 2 to produce a TN panel twisted 90° clockwise. As shown in FIG. 2, the TFT substrate side was first rubbed in the direction A along the source line, and then rubbed in the direction B at an angle of 45° with respect to the wiring. The liquid crystal on the surface of the TFT substrate was then rubbed with B.
A TN panel aligned in the direction of 90° clockwise was fabricated. When the above liquid crystal material was injected under vacuum and the alignment state was observed, a large number of reverse twist domains were present along the flow direction at the time of injection. These panels were heated to 110℃
The state of orientation when heated to , held for 3 minutes, and then cooled to around room temperature was examined in relation to the cooling rate. The cooling rate was monitored with a thermocouple attached to the panel surface. Table 3 shows the relationship between the cooling rate of the panel before and after the panel temperature passed the nematic liquid crystal-isotropic liquid transition temperature of 99° C., and the number of pixels in which a reverse twist domain occurred after cooling.

[0028]

[Table 3]

[0029] Specific example 1 in which rubbing was performed only from one direction
Compared to the case of Example 2, even if the cooling rate is slow, the number of reverse twist domains generated is small. This is considered to be because the number of nematic nuclei generated during cooling increases by performing the rubbing treatment from two directions. In fact, when the number of nematic nuclei generated at a cooling rate of -3°C/min was counted in the same manner as in Example 1, it was 6.7 per pixel, compared to 2.9 in Example 1. It was confirmed that there was an increase in the number of

[0030] Performing the rubbing process from different directions is particularly effective for a substrate having a step shape such as a TFT substrate. This is because it is difficult for the fibers of the rubbing cloth to contact the stepped edge portions, making it difficult to provide orientation, which causes the occurrence of reverse twist domains. This phenomenon is more likely to occur in areas with larger steps, and by performing rubbing along source lines with larger steps,
This is effective because liquid crystal orientation can be imparted even near the step edges.

[0031]

As described above, the method for manufacturing an active matrix liquid crystal display panel of the present invention prevents alignment defects, particularly reverse twist domains, from occurring in a liquid crystal panel, and produces a high-quality active matrix without display defects. A liquid crystal display panel can be realized.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing the rubbing direction of a liquid crystal panel according to a first embodiment of the present invention.

FIG. 2 is a plan view showing the rubbing direction of a liquid crystal panel according to a second embodiment of the present invention.

[Explanation of symbols]

x Gate line direction y Source line direction

Claims (1)

[Claims] 1. In the manufacturing process of an active matrix liquid crystal display panel, after filling the panel with liquid crystal, the temperature of the panel is raised to a temperature equal to or higher than the liquid crystal-isotropic liquid transition temperature, and then cooling is performed, and An active device characterized in that the cooling rate of the panel when passing through the liquid crystal-isotropic liquid transition temperature in the cooling process is faster than the rate at which a plurality of nematic nuclei are generated in each pixel constituting the panel. A method for manufacturing a matrix liquid crystal display panel.