JPH0829788A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To provide a ferroelectric liquid crystal display element constituted so that the movement of a liquid crystal molecule is suppressed in the element at the driving time of liquid crystal and display definition is enhanced. CONSTITUTION:By alternately executing strong rubbing processing and weak rubbing processing in the identical direction on orientation control films 4a and 4b formed on respective upper and lower glass substrates 1a and 1b, an area where a pre-tilting angle is large and an area where the pre-tilting angle is small are alternately arranged. Besides, the pre-tilting angles in the respective areas are set to be larger than the inclination angle of the layer of the ferroelectric liquid crystal 5 between both substrates 1a and 1b. Thus, a strong boundary is formed between the respective areas and the movement of the liquid crystal molecule is suppressed. Besides, the high display definition is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device using a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art As to a liquid crystal display device using a ferroelectric liquid crystal, as disclosed in Japanese Patent Laid-Open No. 94023/1986, a pair of transparent electrodes is provided on each inner surface and an alignment treatment is performed. Glass substrate of 1 μm to 3
A structure is known in which a cell gap of about μm is maintained and they are opposed to each other in parallel, and a ferroelectric liquid crystal is injected between the substrates.

As an actual configuration, for example, as shown in the sectional view of FIG. 3A and the plan view of FIG.
ITO is formed on the inner surface of each of the glass substrates 1a and 1b.
Stripe-shaped transparent electrodes 2a and 2b, insulating films 3a and 3b, and an alignment control film 4a that has been rubbed.
4b are provided in a laminated state, and the ferroelectric liquid crystal 5 is disposed between the glass substrates 1a and 1b in a state of being sealed with a seal member 6. Each substrate 1a, 1b
The stripe-shaped transparent electrodes 2a and 2b face each other so as to form a right angle, and the substrate is a so-called simple matrix substrate. A portion corresponding to the intersection of the two transparent electrodes 2a and 2b constitutes a pixel.

The characteristic of this type of liquid crystal display element is that since the ferroelectric liquid crystal has spontaneous polarization, it can be switched by the coupling force of the spontaneous polarization and an external electric field, and the long axis of the ferroelectric liquid crystal molecule is present. Since the direction corresponds to the polarization direction of the spontaneous polarization on a one-to-one basis, the switching can be performed by switching the polarity of the external electric field.

Since a ferroelectric liquid crystal is generally a chiral smectic liquid crystal (Sm * C), the major axis of the liquid crystal molecules is twisted in the bulk state, but as described above, within the gap of about 1 μm to 3 μm. The twist of the long axis can be eliminated by inserting it in (N.
A. CLARK et al, MCLC, 1983, Vol.9
4, P213-234).

[0006]

Ferroelectric liquid crystal molecules are
It is known that it fluctuates to some extent even by a non-selection signal during matrix driving. This is also clear from the fact that when the optical response of the pixel to which the non-selection signal is applied is taken, the light amount varies in synchronization with the applied pulse.

In a liquid crystal having a so-called splay orientation in which the angle of the major axis of the molecule is largely twisted between the substrates, such a fluctuation of the molecule is displayed unless the stable position of the molecule is changed (switching). Since the contents are retained, there was no problem other than a slight decrease in contrast.

However, in a liquid crystal having an alignment in which the change in the angle of the molecular long axis direction between the substrates is relatively small, that is, a so-called uniform alignment, liquid crystal molecules move in the layer by applying a voltage (for example, a non-selection signal). You can see the phenomenon. The movement phenomenon of the liquid crystal molecules will be described in detail below with reference to FIG.

FIG. 4 (a) is a state explanatory view showing a liquid crystal state before voltage application, FIG. 4 (b) is a partial vertical sectional view of FIG. 4 (a), FIG.
4C is a partially enlarged view of FIG. 4B, and FIG. 4D is a state explanatory view showing a liquid crystal state after voltage application.

As shown in the figure, the ferroelectric liquid crystal 5 is arranged between the upper and lower glass substrates 1a and 1b in a state of being sealed by a seal member 6. In addition, the rubbing direction of the alignment control film (not shown) using polyimide on the substrates 1a and 1b is from the bottom to the top of FIGS. 1b are parallel. When such a rubbing process is performed, as shown in FIG. 4B, the smectic layer 11 is generated in a direction orthogonal to the rubbing direction.

When the ferroelectric liquid crystal molecule 12 has a film thickness sufficiently thin to release the helical pitch,
There can be two stable states, but the directions of all the liquid crystal molecules 12 in the device are aligned in one of the two stable states. This one stable state is + with respect to the layer normal as shown in FIG.
If the inclination position is θ, it is almost symmetric with respect to the layer normal, −
Another stable state exists at the tilt position of θ.

When an electric field (for example, a rectangular wave of 10 Hz and ± 8 V) is applied to the entire surface of the device in the stable state of + θ,
The liquid crystal molecules 12 are kept in the inclination of + θ, and the liquid crystal molecules 12 are shown in FIG.
Start moving in the layer from the point A side (left end) to the point B side (right end). Then, if the voltage application is continued for a long time, as shown in FIG.
As shown in (d), a void portion E without the liquid crystal molecules 12 is generated on the point A side, and the element thickness on the point B side is thicker than that on the point A side.

When the liquid crystal molecule 12 is in the stable state of -θ, the liquid crystal molecule 12 is reversed from the point B side to the point A side.
Moving in the layer toward the side, the void portion E without the liquid crystal molecules 12 is generated on the point B side.

Such a liquid crystal molecule movement phenomenon appears in a relatively short time of 20 to 50 hours, and the generated void portion E cannot be controlled electro-optically. Therefore, not only is it undesirable in display quality, but since the element thicknesses on the point A side and the point B side change with time, it becomes difficult to control the drive of the entire liquid crystal display element, and the ferroelectric liquid crystal is used. As a display element, it has been a big problem.

To solve this problem, Japanese Patent Laid-Open No. 4-247
As disclosed in Japanese Patent No. 429, there is known a means for setting a plurality of regions in which rubbing directions are alternately opposite to each other. However, it is possible to suppress movement of liquid crystal molecules and display quality of a liquid crystal display element. Both points are not completely satisfactory. In addition, an example in which the orientation control film has a plurality of pretilt angles (Japanese Patent Laid-Open No. 3-6526) is known for gradation display.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a ferroelectric liquid crystal display device having a high display quality by suppressing the movement of liquid crystal molecules in the device during liquid crystal driving. To aim.

[0017]

In order to achieve the above object, in a liquid crystal display device according to the present invention, an alignment control film on each substrate is provided with a plurality of regions having different pretilt angles, and The magnitude of the pretilt angle in each region was set to be larger than the tilt angle of the ferroelectric liquid crystal layer between both substrates.

[0018]

The present inventors have found that when a plurality of regions having different pretilt angles are set in the alignment control film, boundaries are created between the regions, and the boundaries are the ferroelectricity between the two substrates. It was found that the difference in the tilt angles of the layers of the ferroelectric liquid crystal is influenced by the chevron structure of the liquid crystal, and becomes stronger as the difference becomes larger. That is, it was found that the difference in the magnitude of the pretilt angle greatly affects the boundary strength.

Further, it has been found that the boundary between each of the above regions becomes a resistance against the flow of liquid crystal molecules and suppresses the movement of the liquid crystal molecules. The difference in the magnitude of the pretilt angle between the regions at this time is preferably 4 degrees or more. If it is smaller than this, the boundary is not sufficiently formed, and the effect of suppressing the movement of the liquid crystal molecules is small.

Further, the amount of movement of the liquid crystal molecules depends on the magnitude of the pretilt angle, and by alternately arranging the regions having a large pretilt angle and the regions having a small pretilt angle, the flow of the liquid crystal molecules becomes strong and weak. It resists the flow. In particular, it has been found that a region having a small pretilt angle becomes a resistance to a region having a large pretilt angle.

It has been found that the amount of movement of liquid crystal molecules can be controlled by appropriately setting the difference in the size of the pretilt angle and the number of regions.

It is also found that at the boundary between two regions having different pretilt angles, liquid crystal molecules are continuously raised, so that a large defect does not occur and the display quality is not adversely affected. It was

[0023]

Embodiments of the present invention will be described below with reference to the drawings. In the drawings of the embodiments, the same reference numerals are used for the same components as those of the conventional example shown in FIG. <Embodiment 1> FIG. 1 is a plan view illustrating Embodiment 1 of a liquid crystal display element according to the present invention, in which (a) is one glass substrate 1.
the state of the rubbing treatment applied to the orientation control film 4a on a.
(B) shows a state of a rubbing treatment applied to the orientation control film 4b on the other glass substrate 1b, (c) shows both substrates 1a, 1b.
1A and 1B are parallel to each other to form a liquid crystal display element.

Stripe-shaped transparent electrodes 2a and 2b made of ITO are provided under the alignment control films 4a and 4b on the substrates 1a and 1b, respectively.
Ferroelectric liquid crystal 5 is arranged in between and sealed with a seal member 6.

In this liquid crystal display element, each of the substrates 1a and 1b is
As shown in the figure, the upper alignment control films 4a and 4b are alternately subjected to a strong rubbing process and a weak rubbing process in the same direction, so that a region having a large pretilt angle and a region having a small pretilt angle are alternately arranged. It is provided in a state and is divided into six areas as a whole. Moreover, the magnitude of the pretilt angle in each region is set to be larger than the tilt angle of the layer of the ferroelectric liquid crystal 5. The substrates 1a and 1b are arranged so that the regions of the alignment control films 4a and 4b having the same rubbing direction and the same pretilt angle face each other.

As a method of forming such a plurality of regions having different pretilt angles, a hard mask made of a stainless steel plate having a thickness of 100 μm is covered on the substrate side during rubbing. This hard mask is selectively perforated and only the perforated portion is rubbed. Therefore, if the rubbing process is performed on the entire surface of the alignment control film and then the second rubbing process is performed using a hard mask, regions having different pretilt angles can be selectively provided.

As another method, a resist layer is formed on the alignment control film which has been rubbed on the entire surface, and a window is selectively opened in the resist layer by UV exposure to make a second exposure to the surface of the alignment control film. Perform rubbing treatment. After that, all the resist is removed.

In order to actually manufacture the liquid crystal display element of the present embodiment 1, stripe-shaped transparent electrodes 2a, 2b made of ITO are formed on each glass substrate 1a, 1b having a side of 75 mm.
Was formed to a thickness of about 1000Å, and the insulating films 3a and 3b using Ta ₂ O ₅ were formed thereon to a thickness of about 500Å by sputtering. Then, the orientation control films 4a and 4 are formed on the substrates 1a and 1b.
b is a fluorinated polyimide (LQ18 manufactured by Hitachi Chemical Co., Ltd.)
00) was applied and baked. As a result, the alignment control films 4a and 4b having a thickness of about 300 Å were formed. Then, each of the substrates 1a and 1b was subjected to the selective rubbing process using the above-mentioned hard mask.

As the ferroelectric liquid crystal, one having the following characteristics was used.

Temperature 30 ° C. Pretilt angle α ₁ = 23 °
α ₂ = 17 ° Spontaneous polarization Ps = 5.8 nC / cm ² Tilt angle Θ = 14.3 ° Layer tilt angle δ = 10.7 ° (ISO: isotropic phase, ch: cholesteric phase, SmA:
Smectic A phase, SmC *: Chiral smectic C phase) Liquid crystal molecules in the device of Example 1 thus manufactured and in the conventional device in which the entire surface is uniformly rubbed (pretilt angle 23 °) The difference in the migration phenomenon of the above was investigated as follows.

That is, two points A and B as shown in FIG. 4 are taken in the element, a rectangular wave of ± 5 V and 10 Hz is applied for 25 hours at 30 ° C., and the element thickness before and after that is measured. Then I investigated the difference. At the same time, the existence of the void portion E having no liquid crystal molecule as shown in FIG. 4 was examined.

As a result, in the case of the device of Example 1, point A
At -0.01 μm at point B and +0.01 μm at point B, and void E did not exist. On the other hand, in the case of the conventional element in which the entire surface is uniformly rubbed, -0.09 at the point A.
μm, +0.5 μm at point B, and void E was present.

As can be seen from these results, in the conventional device, the liquid crystal molecules move from the point A to the point B and the void E is generated on the side of the point A. Molecules hardly move, and void E does not occur. Therefore, high display quality can be obtained. <Embodiment 2> In this embodiment 2, in order to provide a plurality of regions having different pretilt angles, an insulating film containing beads is selectively formed by printing, and an orientation control film is formed thereon. As a result, smooth portions and uneven portions were selectively formed on the film surface. Then, the entire surface of the alignment control film was uniformly rubbed. In this method, the size of the pretilt angle can be controlled by changing the size and density of the beads to control the degree of unevenness on the surface of the alignment control film.

FIG. 2 is a diagram for explaining the second embodiment.
(A) is sectional drawing, (b) is a top view.

As shown in the figure, in this ferroelectric liquid crystal display element, the bead-containing insulating film 7a,
7b is selectively formed, and the orientation control films 4a, 4 and 4 are formed thereon.
b is formed. Thereby, the alignment control films 4a, 4
In b, regions having a large pretilt angle and regions having a small pretilt angle are alternately provided corresponding to the uneven portions on the bead-containing insulating films 7a and 7b and the smooth portion other than that. Further, the alignment control films 4a and 4b are uniformly rubbed.

In actually manufacturing the device having the above-mentioned structure, the bead-containing insulating films 7a and 7b have a diameter of 500Å.
A mixture of an organotitanium compound and an organosilicon compound in which fine particles of silica are previously dispersed (manufactured by Catalyst Kasei (Ti-Si
= 1: 1)) was used. And this bead-containing insulating film 7
A fluorine-based polyimide film (LQ1800 manufactured by Hitachi Chemical Co., Ltd.) was formed as the orientation control films 4a and 4b on a and 7b in the same manner as in Example 1 and uniformly rubbed. The flat portion of the alignment control films 4a and 4b had a pretilt angle of 6 °, and the uneven portion had a pretilt angle of 21 °.

Also in the case of the device of Example 2, liquid crystal molecules hardly moved and voids were not generated. Comparative Example A device was prepared and evaluated in the same manner as in Example 1 except that the magnitude of the pretilt angle by the rubbing treatment was α ₁ = 8 ° and α ₂ = 4 °. As a result, as in Example 1 above, the liquid crystal molecules hardly moved and voids were not generated. However, the contrast value is 4: 1, so that the display quality is the same as in the first embodiment.
Was higher.

[0038]

As described above, according to the liquid crystal display element of the present invention, the movement of the liquid crystal molecules in the element during the driving of the liquid crystal is suppressed, so that a high display quality can be obtained.

[Brief description of drawings]

FIG. 1 is a plan view illustrating a first embodiment of a liquid crystal display device according to the present invention, (a) shows a state of a rubbing treatment applied to an alignment control film on one glass substrate, and (b) shows the other. The state of the rubbing treatment applied to the orientation control film on the glass substrate,
(C) shows a state in which both substrates are opposed to each other in parallel to form a liquid crystal display element.

2A and 2B are views illustrating a second embodiment of the liquid crystal display element according to the present invention, in which FIG. 2A is a sectional view and FIG. 2B is a plan view.

FIG. 3 is a diagram illustrating a conventional example, (a) is a cross-sectional view,
(B) is a plan view.

4A and 4B are diagrams illustrating a conventional example, in which FIG. 4A is a state explanatory view showing a liquid crystal state before voltage application, FIG. 4B is a partial vertical cross-sectional view of FIG. 4A, and FIG. An enlarged view, (d) is a state explanatory view showing a liquid crystal state after voltage application.

[Explanation of symbols]

 1a, 1b Glass substrates 2a, 2b Transparent electrodes 4a, 4b Alignment control film 5 Ferroelectric liquid crystal 7a, 7b Bead-containing insulating film

Claims (5)

[Claims] 1. A liquid crystal display device comprising a ferroelectric liquid crystal arranged between a pair of parallel substrates each provided with a transparent electrode and an alignment control film subjected to a rubbing treatment. A plurality of regions having different pretilt angles are provided in the alignment control film, and the size of the pretilt angle in each region is set to be larger than the tilt angle of the ferroelectric liquid crystal layer between the two substrates. Characteristic liquid crystal display element. 2. The liquid crystal display element according to claim 1, wherein the difference in the magnitude of the pretilt angle between the regions of the alignment control film is set to 4 degrees or more. 3. The alignment control film according to claim 1, wherein a plurality of regions having different pretilt angles and regions having large pretilt angles are alternately arranged as the plurality of regions having different pretilt angles. Liquid crystal display device. 4. A region having a large pretilt angle and a region having a small pretilt angle are alternately arranged by alternately performing a strong rubbing process and a weak rubbing process in the same direction on the alignment control film. The liquid crystal display element according to claim 3. 5. The alignment control film is formed on an insulating film containing beads, whereby the alignment control film is provided with regions having a large pretilt angle and regions having a small pretilt angle alternately arranged. The liquid crystal display device according to claim 3.