JPH0465368B2 - - Google Patents

Description

[Detailed Description of the Invention] <Technical Field> The present invention relates to a liquid crystal display element using ferroelectric liquid crystal;
More specifically, the present invention relates to a method for aligning a liquid crystal display element.

<Prior art and its problems> This will be explained below with reference to FIGS. 1 and 2.
FIG. 1 is a schematic diagram showing the molecular alignment of each phase of a liquid crystal. In the figure, thin arrows indicate directions that change as the temperature decreases, and thick arrows indicate orientation directions of liquid crystal molecules. The liquid crystal phase currently in practical use in liquid crystal display devices is the nematic phase 102 among the phases shown in Figure 1, in which the direction of liquid crystal molecules changes continuously by 90 degrees on the upper and lower substrates of the display section. It has a twisted-tone mask (hereinafter simply referred to as TN type) structure. This is because when an alternating electric field is applied, a change in the dominant direction of liquid crystal molecules is induced, and this is extracted and displayed as a change in the amount of light transmitted under an appropriate optical system. It's technology. A typical example of the molecular arrangement of liquid crystals is shown in FIG.

A feature of this TN type is that a change in the direction of liquid crystal molecules is induced by the effective value of the applied voltage. It has been found that in such an effective value responsive liquid crystal, when the display capacity is increased, that is, when the number of scanning electrodes is increased, the display quality (contrast, viewing angle, etc.) deteriorates. This is the biggest reason why TN-type large-capacity displays have not been realized even today. However, there is some research on achieving large-capacity, high-quality displays using effective value-responsive liquid crystals. Typical examples include TFT-driven and super-twisted type liquid crystal display devices, but both have their disadvantages, and at present it is not possible to provide a large-sized liquid crystal display capable of displaying color.

Under these circumstances, chiral smectic C ^* is a liquid crystal with completely different characteristics from nematic liquid crystal.
Liquid crystals (hereinafter referred to as SmC ^* layers) have been discovered, and their application to displays is being considered (NAClark
and STLagerwall, Appl.phys.Lett., 36, 899
(1980). ). The greatest features of this liquid crystal include pulse responsiveness, memory effect, and ultra-high-speed response, making it ideal for use in liquid crystal displays. Pulse response and memory effect are properties that are essentially different from the effective value response of TN liquid crystals, and are the fundamental factors that ensure that the image quality remains stable and does not deteriorate even when the display capacity is increased.

FIGS. 2A to 2C are explanatory diagrams of the structure of the SmC ^* phase 201. The difference from the smectic A phase 104 shown in FIG. 1 is that the molecular axis 104' of the smectic A phase is parallel to the layer direction ¹⁰⁵ ;
In this case, the molecular axis is tilted with respect to the layer direction 200. The feature is that there are two possible tilting directions, 202 and 203, and one can be selected depending on the polarity of the electric field.

When this SmC ^* phase is used as a display structure, it is necessary to develop the regular structure shown at 201 over the entire display surface. Methods for forming such an ordered structure include rubbing method and oblique evaporation method.
There are known methods such as contacting with the alignment film used in TN display elements, shearing method, and epitaxial growth method using temperature gradient. The latter two methods are not preferable methods for realizing a large-sized display element larger than A4 size, which is considered to be the original target of this display element, and are academic methods. The former rubbing method and oblique evaporation method are extremely effective in terms of mass production technology, and our experiments have shown that they provide a good oriented SmC ^* phase 201.

In either method, liquid crystal molecules are aligned using the strong unidirectional regulating force of the interface.

Here, the special characteristics of the SmC ^* phase will be described in comparison with the nematic phase.The SmC ^* phase is a solid phase close to a crystal that exists on the low temperature side of the nematic phase, and is formed by cooling. Therefore, the ferroelectric liquid crystal is treated from the high temperature side.
When we examine the phase series leading to the SmC ^* phase, we find that there are the following four series.

() I _SO →N ^* →SmA→SmC ^* () I _SO →SmA→SmC ^* () I _SO →N ^* →SmC ^* () I _SO →SmC ^* Here I _S0 is an isotropic liquid, N ^* represents the chiral nematic phase, and SmA represents the smectic A phase.
SmC ^* phase 2 in which liquid crystals belonging to the series of () and () are oriented by rubbing method or oblique evaporation method.
It is unknown whether it is possible to form 01, and it is thought to be extremely difficult, so liquid crystals belonging to () and () will be considered. The nematic phase used in the TN type corresponds here to the N ^* phase 103.
This phase, whose structure has already been schematically shown in FIG. 1, is a phase on the directly low temperature side of the isotropic liquid 101,
The degree of structural change is also small. The viscosity is liquid-like and low, and the controlling force such as the rubbing method allows it to be completely ordered.
Formation of N ^* phase is possible. Normally used liquid crystal N ^*
Since the phase develops between -20°C and 60°C, when sealing the liquid crystal into a cell, the liquid crystal and the cell are sufficiently depressurized in an atmosphere of about 60°C, and then the cell injection port is covered with the liquid crystal. Afterwards, when the reduced pressure is returned to atmospheric pressure, the pressure forces the liquid crystal into the cell. In the atmosphere,
If necessary, it was oriented by slow cooling in an oven.
N ^* is obtained. Since the degree of structural change is small, the viscosity is low, and it is easily subjected to orientation forces, slow cooling in the atmosphere is sufficient.

On the other hand, the above-mentioned method is also followed when forming the SmC ^* phase. If an isotropic phase or a nematic phase exists, the inclusion between semis can also be performed in this phase. (Due to the high viscosity of the smectic phase (SmA, SmC ^* ), it is difficult to perform the process from reduced pressure to atmospheric pressure.) In the case of (), when slowly cooled in air after encapsulation, the oriented N ^* phase is obtain. When the temperature is further lowered, it then transitions to SmA, but a major change in sequence occurs near the transition temperature. From SmA
In the transition to SmC ^* , the molecular axis also needs to be tilted with respect to the layer direction 200. In the case of (), since the N ^* phase does not exist, the structural change due to temperature decrease is more rapid, and a uniaxial restraining force is required to induce this large-scale change. From the aspect of deviceization,
The first technical requirement is to form an oriented phase of SmC ^* without disturbance in the direction of liquid crystal molecules as shown at 202 or 203 over the entire display surface.

If a liquid crystal belonging to the series () and () is sealed in an isotropic liquid state in a cell that has been subjected to alignment treatment, and slowly cooled in the atmosphere of an oven, it is possible to form usable SmC ^* . However, when the cell display area was made larger (A4 size), the following problem occurred.
This means that, unlike the preferred orientations shown in 201 to 203, macroscopic disturbances 301 and 302 of the liquid crystal molecules can be seen in some places, and Figure 3 schematically shows this situation. It is something. Such disturbance in the orientation method is undesirable because it appears as contrast unevenness under crossed nicol conditions. Also, the boundary 30 of phases with different orientation directions
3 is a transition region, which is similarly undesirable because it appears as a bright line during driving. In addition to this, in the SmC ^* phase, zigzag-shaped defects shown at 304 are also found here and there in areas that appear uniform without macroscopic disorder.
This is considered to be a type of dislocation specific to the crystal structure.

In panels for liquid crystal display elements, technology for eliminating alignment disturbances and zigzag defects 304 is most important for improving display quality and long-term stability.

<Means for Solving the Problem> In order to achieve the above object, the present invention reduces the temperature of a display element in which a ferroelectric liquid crystal is sealed and sandwiched.
When forming the chiral smectic C phase from the isotropic liquid phase or chiral nematic phase of the ferroelectric liquid crystal, the entire surface of the display element where the chiral smectic C phase is to be formed is kept at a constant temperature. In order to lower the temperature while maintaining the temperature, the temperature of the liquid is lowered while the entire display element is immersed in the liquid.
This is the main idea.

<Detailed Description of the Invention> When the size of a display element is increased and the temperature of the display element is cooled in the air in an encapsulation device or a special oven, large domains 301 and 302 with slightly different alignment directions of liquid crystal molecules are formed within the element. As mentioned above, problems such as zigzag defects 304 appearing here and there occur. According to our experiments, these structural disturbances strongly depend on the cooling conditions from the isotropic liquid phase. If the drop is steep (for example, 5℃/1min or more), the turbulence will be severe, but
On the other hand, even if the speed is very slow at 0.1/1 min, disturbances cannot be removed and productivity will deteriorate. The fact that it depends on the conditions of temperature drop means that if it is not possible to maintain temperature uniformity over the entire display surface, the structural change to the flat scratch state determined by the temperature cannot keep up with the temperature change, or It is conceivable that there may be time differences in changes depending on location.

As a result, it is suggested that discrepancies and disturbances in direction occur within the tissue. Therefore, in order to eliminate uneven orientation and defects, the following is important. First, the changes within the same phase (N ^* and SmA) are allowed to proceed completely. Then phase transitions (N ^* → SmC ^* , N ^* → SmA,
SmA→SmC ^* ) progresses completely. It is important that these two changes progress cooperatively with as little temperature difference as possible over the entire element surface. This means that when the device surface is roughly divided into two parts, there should not be a temperature difference such that one part is above the transition temperature and the other part is below the transition temperature. Disturbances in orientation and defects occur even when temperature is lowered very slowly using an oven.Using convective air as a heat medium requires a delicate operation in which the temperature is lowered over the entire element surface with no temperature difference. It is said that it is difficult.

When we actually set thermocouples in each part of an A4 size panel that was placed in an oven and monitored the temperature while it was cooling down, we found that in the worst case, there would be a momentary difference in temperature between the center and the periphery. In some cases, the temperature is around 10 degrees Celsius, and it was found that temperature differences between various parts and temperature fluctuations in the same place occur constantly. As long as air, which has a small heat capacity, is used as a heat medium, it is difficult to delicately and completely control the temperature to maintain uniformity of temperature over the entire display surface.

Therefore, the present inventor replaced the heat medium with a substance with a higher heat capacity, preferably circulating the entire medium at high speed, eliminating the temperature difference in the medium, and investigating whether it is possible to delicately and completely control the temperature of the element surface. did. The following will be explained based on examples.

<Example> An 8-inch glass substrate with a thickness of 1.6 mm was optically polished to have a flatness of the glass surface within 2 μm. 2
Two transparent substrates 403 and 409 were obtained. A 400 Å ITO film was formed on the transparent substrates 403 and 409 by sputtering, and the line width was 200 μm, the length was 160 mm, and the pitch was 300 μm using the conventional photoetching method.
Pixel electrodes 404 and 408 were formed in a line pattern of m. On the pixel electrode side of one of the transparent substrates 403, a film of silicon dioxide (SiO ₂ ) is deposited using a sputtering method.
After forming the insulating film 413 to a thickness of 600 Å, polyimide resin HL1100 (manufactured by Hitachi Chemical Co., Ltd.) was applied.
Spin coat at 2000 rpm for 30 seconds and then at 80℃ for 10 minutes.
Heated at 150°C for 30 minutes. Next, a first transparent panel A was obtained by performing a uniaxial alignment treatment using a rubbing device. Pixel electrode 40 of the other transparent substrate 409
Rubber photoresist OMR83 (Tokyo Ohka
Co., Ltd. solution for 5 seconds at 2500 rpm, and line width 80 μm according to the conventional photo etching method.
A linear resist portion 414 having a length of 170 mm is exposed with a mask at the same pitch as the pixel electrode 408 between the pixel electrodes 408 so as not to contact the pixel electrode 408, and then developed to form a second transparent panel B having an adhesive layer 414. Obtained. At this time, the film thickness of the striped adhesive layer 414 after development was 2.0 μm, which was almost uniform within the surface.

Next, the electrode group of the first transparent panel A and the electrode group of the second transparent panel B are brought into close contact with each other so that they are perpendicular to each other.
The mixture was pressurized at a pressure of Kg/cm ² and heated from room temperature to about 180° C. at a rate of 5° C./min, held for 1 hour, cooled, and the pressure was removed to obtain a matrix-driven liquid crystal cell. The cell gap at this time was 1.9 μm, slightly thinner than the film thickness after development, and was uniform over the entire surface of the substrate.

The matrix driving liquid crystal cell manufactured in this way was kept at approximately 120°C in a heating oven, and a ferroelectric liquid crystal CS1011 or CS1014 (both manufactured by Chitsuso Corporation) was heated.
After introducing the material (manufactured by Mimaki Co., Ltd.) between the substrates through the sealing port by capillary development, it was slowly cooled to room temperature. Thereafter, the sealing port was completely sealed with epoxy resin.

CS1011 and CS1014 are liquid crystals that belong to the transition series (), and their transition temperatures are: CS1011 91.3°C 78°C 55.8°C I _SO →N ^* →SmA→SmC ^* CS1014 81°C 69.4°C 54.6°C. The entire display element was immersed in a circulating constant temperature bath RKP20-D (manufactured by Lauda, Germany) using water as a heat medium. This circulation tank has a temperature difference of 0.02 within the tank.
℃ or less, and there is only this degree of temperature difference between the front and back surfaces of the display surface. First, set this temperature to 1
The temperature was raised to 95°C at a rate of °C/min, left in an isotropic phase for about 10 minutes, and then lowered to 25°C at a rate of 0.1°C/min.
In this case, no macroscopic alignment disturbance was found over the entire display surface. Next, when the cooling rate was increased to 0.5°C/min, there was almost no difference from the former. This is thought to be because the temperature difference applied to the display surface is more effective against disordered orientation and defects than the rate of descent. Next, the same element was placed in the center of a Clean Open DT-41 (manufactured by Yamato Scientific Co., Ltd.) and the temperature was lowered from 95°C to 25°C in 10 hours (0.16°C/min). However, a slight difference in orientation direction and zigzag defects were found.

When RKP20 water is used as a heat medium,
Temperature control can be performed in 0.02℃ steps, especially for N ^*
→After leaving it for about 1 hour just above the transition temperature of SmA,
Very good orientation was obtained even when the temperature was lowered at 0.1°C/min to just below the transition temperature and then rapidly at 1°C/min. Since temperature fluctuations can be reduced by using a medium with a large heat capacity, an element with less uneven orientation can be obtained. It is also clear that a large number of processes can be performed at the same time. If the transition temperature is higher than 100°C, oil or silicone oil may also be considered as a heat medium. In this case, removal of these substances remaining on the element surface is not as easy as with water, but it is not inconceivable.

<Effects> According to the present invention, ferroelectric liquid crystal display elements having a chiral smectic C phase without alignment disturbances or zigzag defects can be stably manufactured in large quantities with good reproducibility in a short time.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the molecular arrangement of each phase of liquid crystal. FIGS. 2A to 2C are explanatory diagrams of the chiral smectic C phase. The feature is that the molecular axis is tilted with respect to the layer direction 200. FIG. 3 is a plan view illustrating disturbances in alignment direction and defects as seen from the display surface of a liquid crystal element. FIG. 4A is a sectional view of the matrix driving display element, and FIG. 4B is a plan view showing the vertical relationship. 101...Isotropic liquid phase ( _ISO ), 102...Nematic phase (N), 103...Cholesteric (chiral nematic) phase (N ^* ), 104,104...
... Smectic A phase (SmA), 200... Layer direction, 201,202,203... Chiral smectic C phase (SmC ^* ), 301,302... Domains with different molecular directions, 303... 301 and 3
Boundary area of 02, 304... Zigzag defect, 40
3,409...Transparent substrate, 404,408...Pixel electrode, 405,407...Alignment film, 406...
Liquid crystal, 412... Sealing material, 413... Insulating film, 4
14...Adhesive layer.

Claims (1)

[Claims] 1. Ferroelectric liquid crystal provided with electrodes and subjected to alignment treatment. 2.
A liquid crystal element sandwiched between two substrates, which obtains a chiral smectic monodomain phase by slowly cooling the ferroelectric liquid crystal from a liquid crystal phase or liquid phase at a higher temperature than the ferroelectric liquid crystal. A method for aligning an element, characterized by lowering the temperature of the liquid with the entire liquid crystal element immersed in the liquid, and forming a chiral smectic phase by slowly cooling the liquid crystal phase or the liquid phase. A method for aligning liquid crystal elements. 2. The method for aligning a liquid crystal element according to claim 1, wherein the liquid crystal phase or liquid phase in a high temperature state is an isotropic liquid phase or a chiral nematic phase. 3. The method for aligning a liquid crystal element according to claim 1, wherein the alignment treatment is a rubbing method or an oblique evaporation method.