JP2620635B2 - Liquid crystal electro-optical device manufacturing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal electro-optical device such as a liquid crystal display element or a liquid crystal optical shutter array, and more particularly, to a display by improving an initial alignment state of liquid crystal molecules. The present invention also relates to a method for manufacturing a liquid crystal electro-optical device having improved driving characteristics.

[Conventional technology]

As a conventional liquid crystal electro-optical device, a device using a twisted nemetic liquid crystal has been known. The TN liquid crystal has a problem that crosstalk occurs in time-division driving using a matrix electrode structure with a high pixel density, so that the number of pixels is limited.

An active-matrix display element in which a switching element formed by a thin film transistor is connected to each pixel and switching is performed for each pixel is known, but the process of forming a thin film transistor on a substrate is extremely complicated, and the manufacturing cost is high. There is a problem that it is difficult to produce a large-area display element due to factors such as manufacturing yield.

To solve these problems, Clark proposed a ferroelectric liquid crystal device in U.S. Pat. No. 4,367,924.

FIG. 3 schematically shows an example of a cell for explaining the operation of the ferroelectric liquid crystal. Reference numerals 11 and 11 'denote substrates (glass plates) covered with a transparent electrode made of a thin film such as In ₂ O ₂ or ITO (Indium-Tin-Oxide), between which the liquid crystal molecule layer 12 is perpendicular to the glass surface. Oriented SmC ^*
Phase or another ferroelectric liquid crystal phase.
In this liquid crystal electro-optical device, the ferroelectric liquid crystal molecules
As shown in the figure, there are a first state (I) inclined by + θ and a second state (II) inclined by −θ with respect to the normal direction of the smectic layer.

Display is performed by the difference in the birefringence effect generated by switching the ferroelectric liquid crystal molecules between the two states by applying an electric field from the outside.

At this time, the ferroelectric liquid crystal molecules are shifted from the first state (I) to the second state.
This is achieved by applying, for example, a positive electric field in a direction perpendicular to the smectic layer in order to return to the state (II). Conversely, inversion from the second state (II) to the first state (I) is performed by applying a negative electric field. That is, the two states of the ferroelectric liquid crystal molecules are changed by changing the direction of the electric field applied from the outside, and the difference in the electro-optic effect caused by the two states is utilized.

Further, even when the electric field ignited from the outside was removed, the ferroelectric liquid crystal molecules maintained the stable state, and had the first and second bistable memories.

Therefore, a signal waveform for driving the liquid crystal electro-optical device using the ferroelectric liquid crystal is a bipolar pulse train, and switching between two states taken by the ferroelectric liquid crystal molecules is performed according to a switching direction of the pulse polarity. Was.

In an electro-optical device using such a ferroelectric liquid crystal, uniform driving characteristics are naturally required in the entire device. For this purpose, a uniform liquid crystal phase, that is, a monodomain, having no defect throughout the entire liquid crystal electro-optical device is formed, that is, a plurality of layers of ferroelectric liquid crystal are arranged in one direction parallel to each other, and Technical development has been conventionally performed with the aim of achieving uniform alignment without defects.

However, liquid crystal materials, especially ferroelectric liquid crystals, have subtle scratches on the alignment film, uneven steps on the electrodes for driving the liquid crystal, spacers for maintaining a constant distance between the substrates of the liquid crystal device, or a combination of the liquid crystal material and the alignment film. In addition, defects occur due to various other causes, and a uniform monodomain cannot be obtained. Therefore, conventionally, a monodomain is grown on the entire cell from the end of the liquid crystal electro-optical cell by a method of growing a one-dimensional crystal of liquid crystal (temperature gradient), or a pair of substrates are shifted from each other by controlling a distance on the order of microns. Sharing methods and the like have been attempted.

However, when the area of the liquid crystal electro-optical device is increased, these methods cannot be applied. That is, the size of the mono-domain realized by this method is about several tens of mm square at the maximum, and it is impossible to increase the area and use it for industrial products.

Even if a monodomain having a usable size is realized, the ferroelectric liquid crystal material has a property that the liquid crystal material molecules are not arranged in parallel to the substrate but are arranged with a certain inclination (pretilt). The dielectric liquid crystal layer bends or breaks. For this reason, it is very difficult to arrange the layers in one direction in parallel, and there is a problem that zigzag defects are generated in the domain due to bending of the layers.

When the liquid crystal material changes its possible state by an external electric field, a phenomenon is observed in which the inversion process is reversed at the boundary of the zigzag defect. For this reason, there has been a problem that uniform display and drive characteristics cannot be obtained in the entire device.

[Configuration of the invention]

The present invention solves the problem that these uniform display and driving characteristics cannot be obtained by a technical idea which is different from the conventional idea of utilizing an electro-optical effect in a state where a mono-domain is obtained. Things. The present inventors have paid attention to the initial alignment of the liquid crystal material in the temperature drop process in which the liquid crystal material transitions from the isotropic liquid state (high temperature state) to the liquid crystal state (low temperature state), and the alignment is considered to be a conventional idea. By using a completely different state, the display or drive characteristics of a liquid crystal electro-optical device are realized.

That is, the present invention relates to a method of manufacturing a liquid crystal electro-optical device using a ferroelectric liquid crystal, wherein the plurality of layers are continuously arranged in parallel with each other in a phase higher in temperature than the ferroelectric liquid crystal phase. A method for manufacturing a liquid crystal electro-optical device, comprising a step of arranging in a direction and a step of cooling from the high-temperature phase to the liquid crystal phase having ferroelectricity.

In the case of the present invention, in particular, the liquid crystal is in a multi-domain state in which many small domains (having a size of about several tens of μm) are gathered.

In such a multi-domain state, the alignment defect of the liquid crystal is mitigated by the boundary between the domains, so that a zigzag defect or the like does not occur in the entire liquid crystal electro-optical device cell.

In addition, since the inside of the minute domain is in a good mono-domain state, there is no difference in the display or driving characteristics of the liquid crystal in each minute domain.
It can achieve uniform display or drive characteristics.

Further, in the case of the method of the present invention, the plurality of layers of the ferroelectric liquid crystal are parallel, but are not arranged in only one direction but arranged in multiple directions on the entire substrate. For this reason, it is not necessary to realize uniform alignment in only one direction over the entire substrate as in the past, and a liquid crystal electro-optical device with sufficiently high productivity can be manufactured even on an industrial production scale.

 An example is shown below.

〔Example〕

FIG. 1 is a schematic sectional view of a cell of the liquid crystal electro-optical device used in this embodiment. FIG. 2 shows a part of an end portion of an electrode portion arranged in a matrix in a row direction and a column direction.

The dimensions are arbitrary because they are schematic diagrams.
The cell used in this embodiment has the same cell configuration as that used conventionally. That is, the electrodes 3, 3 'for driving the liquid crystal are formed on transparent substrates (for example, glass) 2, 2'.
Are patterned and formed in a matrix in the row and column directions. In addition, orientation control is performed on the electrode.
4, 4 'are provided, and one side thereof is subjected to a known lapping process so that liquid crystal molecules are arranged. Both the alignment control films 4 and 4 'may use the same material or different materials on one side, but in this embodiment, a polyimide film is used as the alignment control film 4 and the other alignment control film is used. An SiO ₂ film was used as the control film 4 ′.

As described above, when the type of the alignment control film is changed, a relatively large threshold value can be obtained when driving the liquid crystal molecules by an external signal, which is advantageous in a matrix-type liquid crystal electro-optical device. .

A liquid crystal cell is formed at a constant interval with a spacer (not shown) interposed between the substrates 2 and 2 'superposed on each other.

In the case of the present embodiment, as can be easily understood that a plurality of layers of the ferroelectric liquid crystal are parallel to each other and further arranged in multiple directions, the helical pitch of the ferroelectric liquid crystal was set to 20 μm. The orientation of this layer can be understood by a striped pattern corresponding to. In general, it is known that a stripe pattern corresponding to a helical pitch in a ferroelectric liquid crystal coincides with the direction of a smectic layer.

For such a cell, according to a known liquid crystal injection method,
Inject liquid crystal. The liquid crystal takes a phase sequence of isotropic liquid crystal-smectic A phase-smectic C phase-crystal phase, and the transition from the isotropic liquid to the smectic A phase is 89.5.
A mixed liquid crystal performed at ℃ was used.

This mixed liquid crystal is a mixture of eight types of liquid crystal materials including an ester-based ferroelectric liquid crystal, but the ratio of the single liquid crystal material in the mixed liquid crystal is the largest at 20%. The ratio is between 5% and 20%, respectively, and there is no main liquid crystal.
Many varieties are mixed almost equally.

The transition temperature of the mixed liquid crystal used in this example is as follows.

Such a mixed liquid crystal was injected into the cell by a known injection method. At the time of injection, the mixed liquid crystal is injected in an isotropic liquid state, and after all the liquid crystal material is injected into the cell, the injection port is sealed.

Thereafter, it is gradually cooled to a liquid crystal phase state showing a ferroelectric liquid crystal. At this time, the smectic layer starts to be formed at the same time as the temperature is lowered from the liquid state to a phase higher than the liquid crystal phase of the ferroelectric liquid crystal. At this time, the smectic layers are parallel to each other in a certain region. However, in other areas, they are aligned parallel to different directions. This is shown in FIG.

FIG. 4 is a micrograph showing the state of the molecular arrangement of the liquid crystal in the smectic A phase state in this example. From the micrograph under crossed Nicols, it can be seen that the black portion and the white portion exist, the liquid crystal layers are not arranged in one direction, and the directions are slightly different.

When the temperature was further lowered and the temperature was lowered to the smectic C phase, a stripe pattern corresponding to the helical pitch was observed. This is shown in FIG.

As can be seen from this, it can be seen that the stripe patterns exist continuously in parallel and are arranged in multiple directions. Furthermore, in the process of performing the cooling, there are a part in a liquid crystal state and a part in an isotropic liquid state. At this time, the part in the liquid crystal state is subjected to a rubbing treatment applied to the alignment control film 4, As well as forming microdomains. When the temperature is further lowered, a liquid crystal portion is newly generated from a portion in a liquid state, and a micro domain is formed similarly.

In this way, the entire cell can be filled with the microdomain. The size of this micro domain is several μ
It was elongated with a m length of several 100 μm.

When such a multi-domain orientation state was observed with an optical microscope, as shown in FIG. 5, there was no zigzag defect or the like conventionally seen, but rather the boundary of each domain region contained all defects. It was in a state where the defect was small, and thus it appeared that uniform alignment was obtained in the entire cell. At a temperature near room temperature, a ± 30 V triangular wave that drives the liquid crystal by applying an external voltage between the upper and lower electrodes 3 and 3 ′ was applied to such a liquid crystal. Invert at the domain level,
There was no formation of a boat-shaped domain within a monodomain and inversion as in the prior art.

In addition, the inversion of each domain was performed almost simultaneously, and it was observed that the entire cell was inverted at the same time in the entire cell.

In the course of this reversal process, in the case of the present embodiment, it always went through the spiral formation state. In addition, when the application of the electric field was stopped, the alignment state of the liquid crystal molecules was in two states, and the state was maintained even after being left for a while.

In addition, the inversion of the liquid crystal near the center of the cell, the edge, and the liquid crystal was almost the same, and there was no difference in the inversion state depending on the location.

In this embodiment, a thick cell is used so that a plurality of layers are continuously arranged in parallel and in multiple directions. However, the same orientation is performed in a thin cell (about 3 μm). Has been confirmed. When this thin cell is used, the ferroelectric liquid crystal is in an unwound state,
Therefore, when the same voltage as that of the 20 μm cell was applied, the response speed became faster (about 10 μsec), and the threshold characteristics were improved. FIG. 6 shows the arrangement of liquid crystal molecules in a thin cell.

(Effect)

According to the present invention, a liquid crystal electro-optical device having a multi-domain orientation, which is the direction opposite to the direction of the conventional technological progress, has been put to practical use.

Defects such as zigzag defects, which have a large optical effect, did not occur, and uniform display characteristics and a high contrast ratio could be realized. Further, since the inversion characteristics of each multi-domain are uniform, it is possible to drive the liquid crystal uniformly as a whole.

In addition, complicated technical steps for forming a monodomain are not required, and industrial production is facilitated.

Therefore, the speed at which the temperature was lowered from the isotropic liquid crystal state to the ferroelectric liquid crystal phase state could be increased.

In addition, a liquid crystal electro-optical device in which a liquid crystal material is aligned by a rubbing method has a sufficiently high commercial value.

[Brief description of the drawings]

FIG. 1 schematically shows a liquid crystal electro-optical device cell used in the present invention. FIG. 2 is a diagram schematically showing a ferroelectric liquid crystal. FIG. 3 shows a possible state of the liquid crystal. FIGS. 4, 5, and 6 are micrographs showing a crystal structure of a liquid crystal array in a liquid crystal cell. 2,2 '... substrate 3,3' ... electrode 4,4 '... alignment control film 5 ... liquid crystal

Claims (2)

(57) [Claims] 1. A ferroelectric liquid crystal which is disposed between a pair of substrates and in which liquid crystal materials of a plurality of types are mixed in substantially equal amounts so that one type of liquid crystal material is 20% or less. In a phase higher in temperature than the liquid crystal phase in which the liquid crystal exhibits ferroelectricity, a domain having an elongated shape in which a plurality of layers are arranged in a predetermined direction in parallel with each other, and a plurality of layers are arranged in a direction different from the above direction. A step of forming domains having an elongated shape arranged in parallel, and a step of cooling from a phase higher in temperature than the liquid crystal phase exhibiting ferroelectricity to the liquid crystal phase exhibiting ferroelectricity, thereby forming a plurality of elongated shapes. Wherein the inversion of each of the plurality of domains is performed substantially simultaneously when an electric field is applied to the plurality of domains. 2. A method for manufacturing a liquid crystal electro-optical device according to claim 1, further comprising a step of forming microdomains in a phase higher in temperature than the ferroelectric liquid crystal phase.