JP2641389B2 - Manufacturing method of liquid crystal display element - Google Patents

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 a liquid crystal display device, and more particularly to a method of manufacturing a liquid crystal display device which does not require an alignment film or alignment treatment.

[0002]

2. Description of the Related Art A liquid crystal display element used in a liquid crystal display or the like, a so-called liquid crystal cell, changes the state of a specific molecular arrangement of liquid crystal into another different molecular arrangement by an external action such as an electric field and the like, and optically intervenes between them. The change of the characteristic is used for display as a visual change. Generally, an alignment treatment is performed on the surface of a glass substrate holding liquid crystal in order to arrange liquid crystal molecules in a specific alignment state.

In a conventional twisted nematic (TN) type liquid crystal cell or the like, a so-called rubbing method in which a glass substrate sandwiching a liquid crystal is rubbed in one direction with a cotton cloth or the like is employed as an alignment treatment. When an alignment film such as a polyimide film is formed on the surface of the glass substrate, the surface of the alignment film is rubbed.

Rubbing is performed, for example, so that the rubbing directions of the upper and lower substrates are orthogonal to each other. If the liquid crystal cell is a negative display, a polarizing plate with a parallel Nicol arrangement is placed so that the polarization axis is parallel to one of the rubbing directions, and if the liquid crystal cell is a positive display, an orthogonal Nicol arrangement is used. Are arranged such that their polarization axes are parallel to the rubbing direction of the substrate.

[0005]

When the alignment treatment is performed by such rubbing, the contrast characteristic changes depending on the direction in which the screen is viewed (viewing angle) because the alignment direction of the liquid crystal molecules is uniform. In this case, there is a viewing angle characteristic in which the angle at which the display is easy to see when viewing the screen from the observer is limited to a specific angle range.

Therefore, such a liquid crystal cell has a viewing angle dependency such that it is easy to see from one direction and hard to see from another direction. When a liquid crystal cell having such a viewing angle dependence is used as a display device, the contrast is extremely lowered at a certain angle with respect to the display screen, and when it is severe, the contrast of the display is reversed.

The reason for having such a viewing angle characteristic is that rubbing causes pretilt in liquid crystal molecules. The direction in which the liquid crystal molecules have a pretilt coincides with the rubbing vector direction. When a voltage is applied to the liquid crystal cell, the liquid crystal molecules rise in the pretilt direction, so that when observed from that direction, the optical rotatory power is easily eliminated. Therefore, the end direction of the vector is most easily seen.

Furthermore, when rubbing, static electricity due to friction may be generated, causing dielectric breakdown in the alignment film, or display failure due to poor alignment at that portion. In addition, the liquid crystal cell adopts the active drive method,
When rubbing a substrate on which a driving element such as a TFT (thin film transistor) or a wiring is formed on the surface, the element or the wiring may be broken by static electricity generated by the rubbing.

Further, a large amount of fine dust is generated at the time of forming an alignment film or at the time of rubbing, and the dust adheres to the substrate by static electricity, which causes a display defect such as a gap defect of a liquid crystal cell or a black point or a white point. There are cases.

In order to solve this problem, Japanese Patent Application No. 4-236652, etc., which is a patent application filed by the present applicant, discloses:
A structure of a liquid crystal cell which does not have an active alignment structure such as a rubbing treatment is proposed. In these prior applications, the gap between the cell substrates and the chiral pitch of the liquid crystal are selected so as to have a specific relationship, and the liquid crystal is oriented using the thermo-optic effect of the liquid crystal.

That is, the liquid crystal material is heated above the phase transition temperature between the liquid crystal phase and the isotropic phase of the liquid crystal to be converted into an isotropic liquid, that is, injected into the cell in an isotropic phase, and then gradually cooled to form a liquid crystal state. In other words, the phase is changed to the liquid crystal phase and aligned. Thereby,
A large number of microscopic domains, that is, microdomains, are randomly formed. The polarizers are arranged in an orthogonal Nicol arrangement for a positive display, and a parallel Nicol arrangement for a negative display. Since there is no reference direction such as the rubbing direction in the substrate surface, the viewing angle characteristics become uniform.

In the method of the prior application, even if an alignment film is formed, a rubbing treatment for forming an alignment structure is not required, or a rubbing treatment is performed only on a substrate side which is relatively less affected by static electricity and dust. Can be.

In the embodiment of the invention of the prior application, for example, a twisted nematic liquid crystal display element (TN-LCD) having a so-called twist angle of 90 ° in which the orientation direction of liquid crystal molecules is 90 ° twisted between the upper and lower substrates. Is manufactured, the relationship between the thickness d of the liquid crystal cell and the chiral pitch p of the liquid crystal is adjusted so that d / p = Φ / 360 ° = 0.25. Φ is the twist angle (90 °) of the TN-liquid crystal cell.

That is, by using a liquid crystal having a chiral pitch p defined by the twist angle Φ of the liquid crystal cell and the cell thickness d, a TN-LCD having a twist angle of 90 ° is used.
Have gained. Specifically, it is disclosed that a desired chiral pitch p satisfying the above relationship is obtained by adding an adjusted amount of a chiral agent to a nematic liquid crystal.

According to the method of the liquid crystal display device disclosed in Japanese Patent Application No. 4-236652, the liquid crystal material is injected into the liquid crystal cell at a phase transition temperature from the nematic liquid crystal phase (N) to the isotropic phase (I). The injection is performed in a high temperature isotropic phase at a certain NI transition point or higher, and after injection, the liquid crystal is gradually cooled to room temperature to obtain a liquid crystal phase.

When the liquid crystal material is injected into the liquid crystal cell not in the isotropic phase of the prior application but in the liquid crystal phase at room temperature as in the prior art, the flow of the liquid crystal at the time of injection is reduced. The flowing pattern remains on the substrate surface.

The flow pattern is a pattern in which the flow pattern of the liquid crystal remains in stripes, and causes a large visible display defect. Once formed, this flow pattern has a property that it is difficult to erase, and cannot be easily erased by heat treatment or the like. This phenomenon is also called a memory effect.

The cause of the memory effect has not been well understood. Once the liquid crystal molecules are adsorbed on the alignment film, the molecules at the interface with the substrate do not move easily due to thermal vibration or the like, and the molecules at the interface do not melt even when heated above the NI transition point. It is thought that it will remain. It is considered that the force connecting the alignment film and the liquid crystal molecules is van der Waals force.

When the liquid crystal material is injected into the cell in an isotropic phase as in the above-mentioned prior application, it is necessary to heat the cell to a high temperature. This is because a liquid crystal display device used at room temperature (around room temperature of 25 ° C.) must have a nematic phase or a smectic phase within the operating temperature range. Therefore, the temperature above the NI point, which is an isotropic phase, is adjusted to a considerably higher temperature (for example, 90 ° C. or higher) than normal temperature. When the liquid crystal is injected into the cell at such a high temperature, there are various problems caused by the high temperature.

Furthermore, vacuum injection is widely used as a normal liquid crystal injection method. This is a method of filling a cell evacuated to a predetermined degree of vacuum with a liquid crystal material from an inlet in a vacuum chamber. As a problem of the vacuum injection method, the liquid crystal component easily evaporates in a vacuum, and the tendency is particularly strong when the temperature is high. Therefore, when liquid crystal is injected in the isotropic phase of the prior application by the vacuum injection method, components in the liquid crystal evaporate, and it becomes difficult to inject a liquid crystal having a desired composition.

In the high-temperature vacuum injection, a gas is generated from the inner wall of the chamber, a jig or the like due to the high temperature, and it takes a long time to achieve a high vacuum. Since it is necessary to perform injection and cooling while keeping the cell thickness uniform, there is a disadvantage that the production efficiency is low in that a press or the like must be performed as in the method described in Japanese Patent Application No. 4-347701. Was. The time required for normal vacuum injection and the time required for high-temperature press injection and cooling substantially differ by about 2 to 5 times.

On the other hand, by injecting a liquid crystal utilizing the capillary phenomenon, a liquid crystal having a desired composition can be injected. However, in this method, air bubbles are likely to remain in the liquid crystal cell. In particular, in the case of a cell on a substrate having an uneven surface such as a TFT,
When injecting into a large cell of the display screen, air bubbles tend to remain. The portion where the air bubbles remain also becomes a display defect.

An object of the present invention is to provide a prior application of Japanese Patent Application No. 4-236.
The liquid crystal display element does not require an active alignment structure like the liquid crystal display element disclosed in Japanese Patent No. 652, and can inject liquid crystal at a low temperature without the problem of injection at a high temperature. It is an object of the present invention to provide a method for manufacturing a liquid crystal display element capable of eliminating a flow pattern even when the liquid crystal is injected.

[0024]

According to a method of manufacturing a liquid crystal display device according to the present invention , a pair of substrates having a surface that is not subjected to an active alignment treatment is opposed at a predetermined interval so that the surfaces face each other. Arranging a cell by arranging the cell, injecting a liquid crystal material between the substrates of the cell, and injecting the liquid crystal material into the cell, liquid crystal molecules at an interface between the liquid crystal material and the substrate ; the binding action between the substrate surface is reduced, a step of providing energy to allow free movement of liquid crystal molecules in the interface, liquid crystal molecules in the interface
Is almost macroscopically in the in-plane direction of the substrate.
Orienting the liquid crystal molecules so as to be distributed in a loose direction .

[0025]

The liquid crystal material is injected between the substrates on which the films are formed so that the multi-domain is formed, and the coupling action between the liquid crystal molecules and the film at the interface between the liquid crystal material and the substrate is reduced, and the free movement of the liquid crystal molecules is achieved. When energy is applied to the extent that allows the liquid crystal molecules on the interface to be dissociated from the interface, the memory effect is lost. This causes the flow pattern of the cell to disappear.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A specific example of a manufacturing method according to an embodiment of the present invention will be described below with reference to FIG. However, TFTs, ITO electrodes, and the like formed on the substrate as drive elements for the display elements are not shown.

First, using a general polyimide material for an active matrix having a glass transition point of about 300 ° C., a polyimide film is formed on a glass substrate 1 on which an ITO electrode or the like is formed with a thickness of about 600 Å by printing. Thus, the alignment film 2 is formed.

As a method for manufacturing the substrate, the process described in Japanese Patent Application No. Hei 4-236652 can be used as it is. That is, active alignment processing, for example, rubbing is not performed.

The glass substrate 1 on which the alignment film 2 is formed is
An empty cell 3 is prepared by arranging the two to face each other and adjusting them so as to have a gap of 5 μm and bonding them together (FIG. 1A).

A chiral nematic liquid crystal material 4 is injected into the empty cell 3 at room temperature (25 ° C.) in a nematic phase by a vacuum injection method. For example, in order to obtain 90 ° twist alignment, the liquid crystal material 4 (fluorine mixed system, NI transition point 98 ° C.) has a cell gap d and a liquid crystal chiral pitch p: d / p = It is adjusted by including a chiral agent so that it becomes 1/4 (FIG. 1 (B)).

After the cell 3 is completely filled with the liquid crystal material 4, the liquid crystal injection port (not shown) of the cell 3 is sealed. In this state, a flow pattern due to the flow orientation at the time of injection can be visually observed as a defect.

Next, the cell 3 is heated by a heating device 5 such as a heater.
The liquid crystal molecules at the interface are also heated to a temperature at which free movement is possible. The heating is performed, for example, at 150 ° C. for 2 hours.
The heating temperature exceeds the phase transition temperature between the liquid crystal phase and the isotropic phase to allow free movement of liquid crystal molecules even at the interface, but the thermal decomposition of the liquid crystal does not change the chemical properties of the alignment film. The temperature should be lower than the predetermined temperature that does not exceed the temperature. That is, the liquid crystal molecules and the alignment film 22 remain unchanged without changing the chemical properties of the alignment film.
The temperature is selected so as to reduce the bonding force with the liquid crystal and to provide heat energy to such an extent that the liquid crystal molecules can freely move.

Compared with the case where the flow pattern is extinguished by changing the chemical properties of the alignment film, there is a feature that the control is easy and a stable result is easily obtained. With this heating, the liquid crystal molecules at the interface are released from the physical or chemical bond with the alignment film interface, and the flow pattern on the polyimide film 2 due to the memory effect disappears. Since the heating temperature is equal to or higher than the NI transition point of the liquid crystal, the liquid crystal material 4 is in an isotropic state (FIG. 1).
(C)).

Thereafter, the cell 3 is gradually cooled to cause the liquid crystal material 4 to undergo a phase transition from isotropic to a liquid crystal phase and to be oriented to form a multi-domain (FIG. 1 (D)). As a result of actually manufacturing the liquid crystal display element by the method of the above-described embodiment, defects due to the fluidity pattern were not visible, and when viewed under a microscope, a multi-domain structure having a random orientation like the invention of the prior application was formed. It had been.

Although the electro-optical characteristics are almost the same as those of the invention of the prior application, anchoring energy in the polar angle direction tends to be slightly weaker due to reorientation. There was a tendency that the threshold value Vth was slightly low and the fall time (Td) was slightly late. This tendency differs depending on the orientation film and the heating conditions.

Actually, a liquid crystal display device according to the embodiment and a liquid crystal display device according to a conventional example (Japanese Patent Application No. 4-347701) were prepared and their characteristics were measured. The results are shown in FIG. 2 and [Table 1].
Shown in

[0037]

[Table 1]

In FIG. 2, in the embodiment, Vth is 0 to 0.
The response characteristics in Table 1 show that the rise time Tr is ± 2 ms and the fall time Td is almost the same 0 to 0.7 ms later.

In the above embodiment, in order to eliminate the memory effect, the cell was heated to give heat energy to the liquid crystal molecules to reduce the mutual coupling with the interface. Bond between liquid crystal molecules and substrate surface
Reduces the action and allows free movement of the liquid crystal molecules.
Or irradiated with light acting urchin the liquid crystal material, thus created
A similar effect would be obtained by applying energy to the liquid crystal molecules by applying ultrasonic vibrations to the cells.

The present invention is effective not only when a polyimide film is used as an alignment film of a 90 ° twisted nematic liquid crystal but also under other conditions. further,
In the embodiment, the alignment film is formed on both of the pair of substrates. However, the same effect can be obtained when the alignment film is formed on only one of the substrates.

As described above, the present invention has been described in relation to the technique described in Japanese Patent Application No. 4-236652, but the present invention is effective for all methods of manufacturing a liquid crystal cell without performing an active alignment treatment. .

The materials and numerical values in the above description are merely examples, and the present invention is not limited to the embodiments described above. Various modifications and changes can be made by those skilled in the art based on the above disclosure. Needless to say,

[0043]

According to the present invention, it is not necessary to inject the liquid crystal material in the isotropic phase, and the liquid crystal can be injected in the low temperature liquid crystal phase. As a result, a high-quality display element free from display defects can be obtained.

Since there is no need to use a high-temperature vacuum injection or a liquid crystal injection method utilizing a capillary phenomenon, the liquid crystal can be injected by a general vacuum injection method, and therefore, it can be manufactured by a commonly used apparatus.

The effect of the present invention is particularly effective when the substrate surface has irregularities or when a large-screen display is manufactured. Further, similar to the invention described in Japanese Patent Application No. Hei 4-236652, the following effects can be obtained by eliminating the need for rubbing.

The element and wiring are not destroyed by static electricity.
Display defects due to generation and adhesion of dust can be reduced. Further, a rubbing process or an active alignment process is not required, and the manufacturing cost can be reduced.

In addition, the viewing angle characteristics are substantially uniform in all directions, and the inconvenience that the contrast is reduced depending on the specific position of the observer is reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a manufacturing process of a liquid crystal display cell according to an embodiment of the present invention.

FIG. 2 is a graph showing characteristics of a liquid crystal display cell according to an embodiment of the present invention, as compared with a conventional art.

[Explanation of symbols]

 Reference Signs List 1 glass substrate 2 polyimide film 3 liquid crystal cell 4 liquid crystal material 5 heating device

Continuation of the front page (56) References JP-A-6-194655 (JP, A) JP-A-4-119327 (JP, A) European Patent Application Publication 589264 (EP, A)

Claims (9)

(57) [Claims] A step of forming a cell by arranging a pair of substrates having a surface that is not subjected to an active alignment treatment at a predetermined interval so that the surfaces face each other ; A step of injecting a liquid crystal material between the substrates; and
Reduce the binding effect of the liquid crystal molecules and the substrate surface at the interface between the substrate, a step of providing energy to allow free movement of liquid crystal molecules in the interface arrangement direction the substrate surface of the liquid crystal molecules in the interface Inward
Macroscopically, it is distributed in almost every direction,
And a step of aligning the liquid crystal molecules. 2. In the step of injecting the liquid crystal material,
2. The method according to claim 1, wherein the liquid crystal material is injected in a liquid crystal phase. 3. The method according to claim 2, wherein the energy is given by heating the liquid crystal material, and the heating temperature is lower than a thermal decomposition temperature of the liquid crystal molecules. . 4. The heating of the liquid crystal material makes the liquid crystal material isotropic also at the interface with the substrate, and after the heating,
4. The method according to claim 3, wherein the cell is gradually cooled to align the liquid crystal molecules. 5. The liquid crystal material includes a chiral nematic liquid crystal, and the heating temperature is equal to or higher than a phase transition temperature between a liquid crystal phase and an isotropic phase of the chiral nematic liquid crystal.
The manufacturing method of the liquid crystal display element described in. 6. The method according to claim 2, wherein the energy is given by irradiating the liquid crystal material with light. 7. The method according to claim 2, wherein the energy is given by applying ultrasonic vibration to the liquid crystal material. 8. The method according to claim 1, wherein at least one of the substrates has a polyimide film on a surface. 9. The method according to claim 8, wherein a chemical property of the polyimide film is not changed between the step of applying energy and the step of injecting.