JPH08152636A - Production of liquid crystal display element - Google Patents

Abstract

PURPOSE: To provide a process for producing a liquid crystal display element capable of obviating the degradation in display quality by domains of the liquid crystal display element which does not require a positive orientation structure. CONSTITUTION: This process has a stage for forming a cell 3 by disposing a pair of substrates 1 including a substrate not subjected to a positive orientation treatment in at least one opposite to each other at a prescribed spacing, a stage for injecting a liquid crystal material 4 between the substrates of the cell 3, a stage for enabling making possible the free movement of liquid crystal molecules by heating the cell 3 injected with the liquid crystal material 4 and a stage of reorienting the liquid crystal molecules by cooling the cell 3 in such a manner that the temp. falling speed indicating the temp. fall per unit time attains a temp. falling rate value at which the decrease in domain sizes is substantially satd. or above in the characteristic of the change in the domain sizes of the liquid crystals with respect to a change in the temp. falling rate in the case where the temp. of the liquid crystal material 4 falls from an isotropic phase state to a liquid crystal phase state after the stage for heating the cell 3.

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 a specific molecular arrangement of liquid crystal into another different molecular arrangement by an action from the outside such as an electric field, and transmits an optical signal between them. The change in the physical characteristics is used as a visual change for display. In order to bring the liquid crystal molecules into a specific alignment state, it is common to perform an alignment treatment on the surface of the glass substrate that sandwiches the liquid crystal.

In a conventional twisted nematic (TN) type liquid crystal cell or the like, a so-called rubbing method in which a glass substrate sandwiching the liquid crystal is unidirectionally rubbed with a glass cloth is used 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.

The rubbing directions are, for example, such that the rubbing directions of the upper and lower substrates are orthogonal to each other. When the liquid crystal cell is a negative display, the polarizing plates with parallel Nicols are placed with the cell sandwiched so that the polarization axis is parallel to one of the rubbing directions. The polarizing plate is placed so that its polarization axis is parallel to the rubbing direction of the adjacent 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 the rubbing causes pretilt in liquid crystal molecules. The direction in which the liquid crystal molecules have a pretilt corresponds to the vector direction for rubbing. When a voltage is applied to the liquid crystal cell, the liquid crystal molecules rise in the pretilt direction, so when observed from the direction corresponding to the pretilt,
The optical rotatory power is easily eliminated.

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, which is a patent application by the present applicant, discloses
We propose a liquid crystal cell structure that does not have a positive alignment structure such as rubbing treatment. 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.
Is getting 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 easily move due to thermal vibration and the like, and the molecules at the interface do not melt even if heated above the NI transition point, so that the initial alignment is forever. 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 in which a cell evacuated to a predetermined vacuum degree is filled with a liquid crystal material from an injection port in a vacuum chamber. As a problem of the vacuum injection method, the liquid crystal component is easily evaporated in a vacuum, and the tendency becomes strong especially when the temperature is high. Therefore, when the liquid crystal is injected in the isotropic phase of the prior application by the vacuum injection method, the components in the liquid crystal are evaporated and it becomes difficult to inject the liquid crystal having the 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, if the liquid crystal is injected by utilizing the capillary phenomenon, the liquid crystal having a desired composition can be injected, but this method tends to leave bubbles 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 bubbles remain is also a display defect.

Similarly, in Japanese Patent Application No. 6-53639, which is a patent application by the applicant of the present application, Japanese Patent Application No. 4-2366 is used.
A liquid crystal display element that does not require a positive alignment structure like the liquid crystal display element disclosed in No. 52, and can inject liquid crystal at a low temperature without the above-mentioned problems at the time of high temperature injection, Moreover, there is disclosed a method of manufacturing a liquid crystal display element capable of eliminating the flow pattern even when liquid crystal is injected in the liquid crystal phase.

However, in the multi-domain structure of the liquid crystal display device produced by the method disclosed in Japanese Patent Application No. 4-236652 and Japanese Patent Application No. 6-53639, the arrangement direction of liquid crystal molecules in each domain is random. Although the viewing angle dependency is substantially uniform on the entire screen, the display quality partially varies depending on the viewing direction. That is, when the screen is viewed obliquely, a difference in display quality between individual domains appears.

If the size of the domain is large to some extent, it will be perceived by the naked eye as a pattern unrelated to an image such as a spotted dot based on a difference in display quality between domains. In particular, when black is displayed on a normally white screen, the appearance of the fluttering appears conspicuously. The lumpiness due to this domain causes deterioration of display quality of the liquid crystal display element.

The object of the present invention is, for example, Japanese Patent Application No.
A new liquid crystal display capable of eliminating deterioration of display quality due to domains in a liquid crystal display element which does not require a positive alignment structure such as the liquid crystal display elements disclosed in Japanese Patent Application No. 236652 and Japanese Patent Application No. 6-53639. It is to provide a method of manufacturing an element.

[0027]

A method of manufacturing a liquid crystal display element according to the present invention comprises a step of forming a cell by arranging a pair of substrates, at least one of which is not subjected to a positive alignment treatment, facing each other at a predetermined interval. After the step of injecting the liquid crystal material between the substrates of the cell, the step of heating the cell in which the liquid crystal material is injected to allow the free movement of the liquid crystal molecules, and the step of heating the cell, In the characteristic of the change in the size of the domain size of the liquid crystal with respect to the change in the temperature drop rate when the temperature drop rate indicating the temperature drop causes the liquid crystal material to drop in temperature from the isotropic phase state to the liquid crystal phase state, the decrease in the domain size So as to be equal to or higher than the temperature drop rate value at which the liquid crystal is substantially saturated, and realigning the liquid crystal molecules by cooling the cell.

[0028]

The size of the domain is controlled by controlling the cooling rate after heating the liquid crystal cell. First, a liquid crystal material is injected between substrates formed so that multi-domains are formed, and the liquid crystal cell is heated to such an extent that liquid crystal molecules can freely move. After that, when the temperature of the liquid crystal cell is gradually cooled at a predetermined lowering rate or more, a liquid crystal display element having a small domain size can be obtained.

[0029]

EXAMPLE A specific example of a method of manufacturing a liquid crystal display device according to an example of the present invention will be described below with reference to FIG. However, TFTs or ITs formed on the substrate as driving elements of display elements
Illustration of O electrodes and the like is omitted.

First, using a general active matrix polyimide material, a polyimide film having a thickness of about 50 nm is formed by printing or spin coating on the glass substrate 1 on which the ITO electrodes and the like are formed, and the alignment film 2 is formed. To do.

As the method of manufacturing this substrate, the process described in Japanese Patent Application No. 4-236652 of the prior application 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
One sheet is prepared, and both are arranged so as to face each other and adjusted so as to have a gap interval of 5 μm, and they are bonded together to form an empty cell 3 (FIG. 1A).

A chiral nematic liquid crystal material 4 is injected into the empty cell 3 by a vacuum injection method in a nematic phase state at room temperature (25 ° C.). 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)). The nematic liquid crystal is, for example, ZL manufactured by Merck Ltd.
I-2392 and fluorine liquid crystal material (SR-50 of Chisso Corporation)
03) can be used, and as the chiral agent, for example, S-811 manufactured by Merck & Co., Inc. can be used.

After the liquid crystal material 4 is completely filled in the cell 3, the cell 3 is set on a pressing jig, and the cell 3 is pressed until a uniform cell gap is obtained. (Not shown). 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 temperature is selected so as not to change the chemical properties of the alignment film, reduce the substantial alignment binding force of the alignment film 22 with respect to the liquid crystal molecules, and give thermal energy to the extent that the liquid crystal molecules can freely move. To be done.

Compared with the case where the flow pattern is extinguished by changing the chemical properties of the alignment film, it is easy to control and it is easy to obtain stable results. 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)).

Then, the cell 3 is cooled to cause the liquid crystal material 4 to undergo a phase transition from isotropic to a liquid crystal phase and be oriented to form a multi-domain (FIG. 1D). In this cooling step, the size of the domain can be reduced by controlling the temperature drop rate (cooling speed) of the cell 3. In addition,
An apparatus for controlling the cooling of the cell will be described later with reference to FIG.

FIG. 2 is an experiment showing how the domain size changes when liquid crystal cells are manufactured with various temperature drop rates in the manufacturing process of the liquid crystal display device performed by the inventor of the present invention by the above method. The result. The liquid crystal material used is fluorine-based liquid crystal or CN-based liquid crystal manufactured by Chisso Corporation.

As a quantitative standard for evaluating the size of the domain, the density of disclination lines (per unit area), which is a line defect that appears due to reverse tilt of liquid crystal molecules when a voltage is applied to the liquid crystal cell. There are disclination lines). The higher the density of the disclination lines, the finer the size of the domain becomes, so that the lumpy display becomes hard to see. Observing the number of disclination lines is easier than measuring the domain size directly.

The vertical axis of FIG. 2 represents the density value of the disclination line, and the horizontal axis represents the value of the temperature drop rate (cooling speed) when the liquid crystal material undergoes the phase transition from the isotropic state to the liquid crystal phase, in ° C. Shown in minutes. From the measurement result of FIG. 2, it was shown that the size of the domain can be controlled by controlling the temperature drop rate. That is, the higher the temperature drop rate (to the right of the horizontal axis), the higher the density of the disclination lines, so that the domain size can be made smaller and uniform.

From the measurement results of FIG. 2, it can be seen that when the temperature drop rate is increased, the density of the disclination line increases, but the increase in density tends to saturate above a certain temperature drop rate value. That is, the density of the disclination line increases rapidly from the state where the temperature drop rate is close to zero to the value where the temperature drop rate is 30 ° C./minute, but when the temperature drop rate is faster than 30 ° C./minute, The density of the disclination line is substantially saturated,
Does not increase significantly.

Therefore, in order to obtain the disclination line density (domain size) that satisfies the display quality required for a desired liquid crystal display device, the value of the temperature drop rate in the cooling step is 30 ° C. / It should be more than a minute. Preferably, the temperature drop rate is greater than 60 ° C./min, and the density of the disclination line is more saturated.

Since the saturation characteristic of FIG. 2 may change depending on the liquid crystal material used, the lower limit of the temperature drop rate is not limited to the value of 30 ° C./minute in the present invention. The cooling speed of the liquid crystal cell to be manufactured may be determined in advance based on the temperature drop rate value at which the density of the disclination line obtained by measuring the characteristics shown in FIG. 2 is substantially saturated.

Next, an example of a manufacturing apparatus for manufacturing the liquid crystal display element of the present invention will be described with reference to FIG. FIG.
[Fig. 4] is an external view of a manufacturing apparatus including a heating-cooling control device. In FIG. 3, one end of a liquid crystal cell 11 is immersed in a liquid crystal tank 10 containing a liquid crystal material. Most of both surfaces of the liquid crystal cell 11 are sandwiched by heating-cooling jigs 12. The heating-cooling jig 12 preferably has good thermal conductivity. A pipe 13 through which cooling water passes is provided inside the heating-cooling jig 12. Further, a temperature sensor 14 for monitoring and measuring the heating and cooling temperatures of the cell 11 is arranged at an appropriate portion of the cooling jig 12. Further, a seat heater 15 which is a heating device is arranged outside the jig 12.
The heating temperature can be controlled by adjusting the current passed through the seat heater 15 from a power source (not shown). The entire seat heater 15 is clamped and fixed by a pressing jig 16 from the outside.

The flow rate of the cooling water flowing from the inlet of the cooling water pipe 13 is controlled by the temperature sensor 1 so as to achieve a desired cooling speed.
Control is performed according to the cooling temperature detected in 4. For example, a known temperature control technique can be used in which the discharge flow rate of a pump (not shown) that flows cooling water according to the output of the temperature sensor is controlled to be equal to or lower than the target temperature.

By using the jig shown in FIG. 3, the liquid crystal injection step, cell pressurizing step, heating step and cooling step as described with reference to FIG. 1 can be performed. However, the heating of FIG.
If a desired cooling speed can be obtained without using a cooling jig, it is also possible to use other suitable methods such as cooling water or a cooling agent such as cooled air or liquefied nitrogen. .

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

The liquid crystal may be heated in the isotropic phase after injection or may be injected in the isotropic phase after heating. In order to prevent a flow pattern, it is preferable to inject in an isotropic phase. Rapid cooling from the isotropic phase to room temperature at a cooling rate equal to or higher than a predetermined value.

The above description has been made in relation to the techniques described in Japanese Patent Application No. 4-236652 and Japanese Patent Application No. 6-53639, which are prior applications. It is effective for the method.

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

[0051]

According to the present invention, by controlling the temperature decrease rate for cooling the liquid crystal material from the isotropic phase to the liquid crystal phase to a predetermined value or more, the uniformity of the domain size is improved and the domain Since the size becomes small, the flicker display cannot be recognized due to the difference in the viewing angle characteristics of the domains, and the display quality is improved.

At the same time, it is not necessary to inject the liquid crystal material in the isotropic phase, and the liquid crystal can be injected in the liquid crystal phase at a low temperature. Therefore, for a cell that is not suitable for high temperature heat treatment, the problem of high temperature can be reduced. Since the flow pattern can be extinguished by heating the liquid crystal display device, a high quality display device without display defects can be obtained.

[Brief description of drawings]

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

FIG. 2 is a characteristic diagram showing that the disclination line density changes when the characteristic cooling speed is changed in the method of manufacturing a liquid crystal display device according to the embodiment of the present invention.

FIG. 3 is an external view of a liquid crystal element manufacturing apparatus used in the method for manufacturing a liquid crystal display element of the present invention.

[Explanation of symbols] 1 glass substrate 2 polyimide film 3 liquid crystal cell 4 liquid crystal material 5 heating device 10 liquid crystal tank 11 liquid crystal cell 12 heating-cooling jig 13 cooling water pipe 14 temperature sensor 15 seat heater 16 pressurizing jig

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kiyoshi Ando 1-3-1 Eda Nishi, Aoba-ku, Yokohama-shi, Kanagawa Stanley Electric Co., Ltd.

Claims (8)

[Claims] 1. A step of forming a cell by arranging a pair of substrates, at least one of which includes a substrate not subjected to a positive alignment treatment, at predetermined intervals so as to form a cell, and a step of injecting a liquid crystal material between the substrates of the cell. After the step of heating the cell in which the liquid crystal material is injected to allow the liquid crystal molecules to freely move, and after the step of heating the cell, the temperature drop rate indicating a temperature drop per unit time is the liquid crystal. In the characteristics of the change in the size of the domain size of the liquid crystal with respect to the change in the temperature drop rate when the temperature of the material is dropped from the isotropic phase state to the liquid crystal phase state, the temperature drop rate at which the decrease in the domain size is substantially saturated. And a step of realigning the liquid crystal molecules by cooling the cell so that the value becomes equal to or more than a value. 2. The value of the temperature decrease rate is the number of liquid crystal disclination lines per unit area with respect to the change of the temperature decrease rate when the temperature of the liquid crystal material is decreased from an isotropic phase state to a liquid crystal phase state. In the characteristics of change of
2. The method for manufacturing a liquid crystal display element according to claim 1, wherein the increase in the number of the disclination lines per unit area is equal to or higher than the temperature drop speed value at which the disclination lines are substantially saturated. 3. The method of manufacturing a liquid crystal display device according to claim 1, wherein the value of the predetermined temperature decrease rate is substantially 30 ° C./minute or more. 4. The method for manufacturing a liquid crystal display device according to claim 1, wherein the value of the predetermined temperature decrease rate is substantially 60 ° C./minute or more. 5. The method for manufacturing a liquid crystal display element according to claim 4, wherein the heating temperature of the liquid crystal cell is a temperature equal to or lower than the thermal decomposition temperature of the liquid crystal molecules. 6. The liquid crystal material contains 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.
A method for producing the liquid crystal display element described. 7. The method for manufacturing a liquid crystal display device according to claim 1, wherein at least one of the substrates has a polyimide film on the surface. 8. The method of manufacturing a liquid crystal display device according to claim 7, wherein the polyimide film does not change its chemical property between the step of applying the energy and the step of injecting the energy.