JPH0750272B2 - Method for manufacturing ferroelectric smectic liquid crystal electro-optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to an electro-optical device using a ferroelectric smectic liquid crystal.

[Prior Art] A display device using a nematic liquid crystal has many advantages such as low power consumption, low driving voltage, and thin and small size, and is widely used in various applications such as a calculator and a watch. However, its electro-optical response is slow, and its application to fields requiring high-speed response, such as optical communication and optical shutter devices such as printer heads, has been limited.

Recently, an electro-optical device using a ferroelectric liquid crystal has been reported. (For example, NAClark, STLagerwall, Appl.Phy
s. Lett. 36, p. 899, (1980)) This utilizes the fact that liquid crystals exhibit ferroelectricity in the chiral smectic C phase and H phase. In these phases, the liquid crystal molecules form a layered structure, and the major axis direction of the molecules is tilted by a certain angle with respect to the layer vertical direction. This molecule has spontaneous polarization in a direction perpendicular to the molecule and included in the plane of the layer, and attempts to align the direction of the spontaneous polarization with the direction of the electric field applied from the outside. There is a big change. The electro-optical response is 10 to 1000 times faster than the response of the conventional liquid crystal device, and it can be applied to a high-speed optical shutter device. Also, for the electric field,
Since it is possible to provide bistability, it can be applied to a large-sized display device.

However, in order to manufacture these devices, it is necessary to prepare cells in which liquid crystals are uniformly aligned. However, smectic liquid crystals are difficult to control in alignment as compared with nematic liquid crystals, which is one of the causes that prevent practical use. Conventionally, the following methods have been known as means for orienting the ferroelectric liquid crystal.

(1) Cooling from the isotropic phase while applying a strong magnetic field.

(2), The two substrates that form the cell are slightly vibrated in the shifting direction.

(3), an orientation control film is formed, and is cooled from the isotropic phase as in the single crystal growth.

The method (1) requires a large device to generate a strong magnetic field, and it is difficult to control the orientation when the cell thickness is 10 μm or less. With regard to (2) as well, a device for giving vibration is required, and there are still many problems to be solved such as a cell sealing method. Regarding method (3),
Although it is the most practical method, it has the drawbacks that the temperature of the liquid crystal must be strictly controlled, an apparatus for maintaining the cell at a constant temperature is required, and that the cooling is time-consuming.

With respect to such a problem, the present inventors have found that a chiral smectic C is formed between two substrates with electrodes subjected to an orientation control treatment.
In a liquid crystal electro-optical device that holds a ferroelectric liquid crystal layer having a phase, the liquid crystal layer has a cholesteric phase at a temperature higher than that of the chiral smectic C phase, and the length of the helical pitch in the cholesteric phase is the distance between the substrates. It has been found that a method of increasing the amount by 4 times or more is effective (Japanese Patent Application No. 59-274073), but there is a problem that it is not always sufficient depending on the application. It is a linear alignment defect called "zig / zag wall", and in applications that require a fine electrode pattern such as an optical shutter for a printer head, even if a slight zig / zag wall exists,
This causes problems such as uneven contrast.

[Problems to be Solved by the Invention] It is an object of the present invention to provide a ferroelectric smectic liquid crystal electro-optical device capable of removing a linear alignment defect which has not been solved by a conventional method. is there.

[Means for Solving Problems] The present invention has been made to solve the above-mentioned problems, and holds a ferroelectric smectic liquid crystal layer between two transparent electrode-attached substrates, In a method of manufacturing a ferroelectric smectic liquid crystal electro-optical device having a means for applying a voltage to, a surface of the two substrates with transparent electrodes in contact with liquid crystal is coated with an organic polymer film, and the surfaces are rubbed in a certain direction. Then, the substrates are combined so that the directions from the end point to the start point of the rubbing are the same, the cholesteric phase is present between the substrates at a temperature higher than the smectic phase, and the helical pitch is at a temperature just above the cholesteric-smectic phase transition point. The ferroelectric smectic liquid crystal layer made of liquid crystal having a length of 4 times or more the distance between the substrates is retained, and the smectic phase is changed from the temperature showing the cholesteric phase. The present invention provides a method for manufacturing a ferroelectric smectic liquid crystal electro-optical device, which is characterized by cooling to a temperature exhibiting a negative phase.

FIG. 1 is a sectional view of a basic liquid crystal electro-optical device of the present invention. Transparent conductive films (2a) and (2b) and an orientation control film (3a) are formed on the surfaces of two transparent substrates (1a) and (1b), respectively.
Form (3b). The conductive films (2a) and (2b) are electrodes for applying an electric field to the liquid crystal layer (4) held between the substrates, and are provided for the purpose of producing an electro-optical response. _A predetermined pattern is formed of ₂ O ₃ or SnO ₂ .

The alignment control films (3a) and (3b) are for aligning the liquid crystal in a substantially horizontal direction. As a typical example, a method of forming an organic polymer film and rubbing it with a cloth in a certain direction, or
A nematic liquid crystal orientation control method such as an oblique vapor deposition method of SiO is used. In addition, the present inventors have found that, at this time, the above-mentioned linear defects are eliminated by arranging both substrates in the same wall orientation. FIG. 2 shows a schematic diagram (a) of orientation based on the present invention and a comparative example (b).

In FIG. 2, double-lined arrow (9) indicates the orientation control direction, that is, the rubbing direction, and (8) indicates the state of liquid crystal molecules.

In the case of vapor deposition, it may be performed in the direction opposite to the rubbing direction.
In the present invention, the structure as shown in FIG. 2 (a) is adopted, and the structure shown in FIG. 2 (b), which has the same orientation as that of a general twisted nematic liquid crystal cell, produces no effect.

As the alignment film, there is a case where the organic polymer film is used in combination with rubbing as described above, and oblique vapor deposition of an inorganic compound,
A combination of an organic polymer film such as polyimide or polyamide and rubbing is preferable in consideration of stability of orientation, application to a large cell, and the like.

After performing such an alignment treatment, spacers, for example, spacers, are formed so that the substrates are held in parallel and at regular intervals.
A cell is prepared by sandwiching organic beads and alumina particles and fixing the periphery with a sealant (5). At this time, the orientation control directions of the two substrates are set to be parallel to each other.

Then, the strongly inductive liquid crystal composition is heated to a cholesteric phase or an isotropic phase, injected into the cell, and then sealed.
Two polarizing plates (6a) and (6b) are arranged outside the cell so that the polarizing plates are orthogonal to each other and form a certain angle with the orientation control direction of the substrate. This angle changes depending on the liquid crystal material, the operating temperature of the device, the driving method, etc., and the angle with the best contrast characteristics may be selected.
In some cases, the polarization axes of the polarizing plates may be slightly displaced from the orthogonal.

The light source (7) is placed on the side of the substrate (1b) so that the light is transmitted to the opposite side. In the case of using the reflection type, a reflection plate may be provided outside the polarizing plate (6b).

As the ferroelectric smectic liquid crystal of the present invention, several kinds are known, including a chiral smectic C phase (hereinafter abbreviated as SmC * phase), but easy alignment control,
Alternatively, SmC * is preferable in terms of response speed and the like. Also SmC *
It is preferable that a cholesteric phase (hereinafter abbreviated as Ch phase) is shown at a higher temperature, and that the helical pitch in the Ch phase is 4 times or more of the substrate gap. In addition, having a smectic A phase (hereinafter referred to as SmA phase) between the Ch phase and the SmC * phase
It is desirable in terms of orientation uniformity. Such a liquid crystal can be obtained by mixing an optically active substance, a smectic liquid crystal compound and a nematic liquid crystal compound in an appropriate ratio, and a non-liquid crystal additive may be added as necessary. In particular, Ch
In order to increase the pitch in the phase, it is effective to mix an optically active substance that causes a left helix and an optically active substance that causes a right helix, depending on the magnitude of the force causing the helix.

Usually, the length of the helical pitch in the Ch phase changes with temperature. In order to obtain uniform orientation, it is necessary to satisfy the condition of p> 4d just above the cholesteric-smectic phase transition point.

However, when the temperature range satisfying this condition is limited to the vicinity of the transition point, the spiral structure unintentionally transitions to the smectic phase when the temperature drop rate is high. In this case, since uniform orientation cannot be obtained, it is necessary to maintain the temperature at which p> 4d is satisfied until the helical structure is unwound, or to slow the temperature drop rate. For this reason, the temperature range in which the helical pitch p is four times or more the distance d between the substrates is 5 ° C. from the cholesteric-smectic phase transition point.
It is preferable to cover the above range, and it is more preferable to cover the entire Ch phase temperature range.

Further, it is preferable to provide a means for raising the temperature of the liquid crystal layer in case of alignment failure due to crystallization of liquid crystal or application of high voltage. As a means for this, a heater for increasing the temperature may be provided outside, but a simpler device can be obtained by applying a current to the electrode inside or outside the cell and heating directly.

In the present invention, it is more preferable to apply an AC voltage in order to completely remove the above-mentioned linear alignment defects, and therefore, the voltage application means controls the alignment as well as the operation of the electro-optical element. It is preferable to have a function for doing.

[Operation] In the present invention, the orientation state in the SmC * phase depends on the orientation in the Ch phase. In order to form a helical structure in the Ch phase without uniformly forming a helical structure, a liquid crystal having a helical pitch of 4 times the cell spacing or more is required. On the other hand, if this condition is not satisfied, a helical structure will be formed, and when the temperature is lowered to the SmC * phase, the orientation of the layers or the orientation of the liquid crystal molecules will not be uniform. Further, the reason why there is a preferable combination in the rubbing direction is not clear yet, but it is a well-known fact that the liquid crystal molecules are aligned slightly inclined depending on the rubbing direction, and both substrates are rubbed in the same direction. In this case, it is considered that the liquid crystal layer can have a structure close to horizontal as shown in FIG.

[Example] Example 1 The surface of a glass substrate provided with a patterned transparent electrode was rubbed in a certain direction with gauze, and the glass substrates were combined so that the rubbing direction was the same, and the cell gap was 1.5 μm.
m cells were constructed. Then, the mixed liquid crystal (I) shown in Table 1 having a helical pitch of 97 μm at 93 ° C. was heated to about 100 ° C. and injected. Next, when the cell was cooled to SmC * phase at 50 ° C, the alignment was good and the linear alignment defects were 5-10.
It was a book / cm ² . When a voltage of ± 5V is applied,
The remaining linear alignment defects disappeared.

Comparative Example 1 Under the same conditions as in Example 1, cells having different rubbing directions of 180 ° were constructed and the alignment state of the mixed liquid crystal (I) was compared.
It was found that the number of linear alignment defects was 10 to 20 / cm ² , which was twice as large as that in the example, and did not disappear when a voltage of ± 5 V was applied, and the defects exist stably.

Example 2 A polyimide coating (PIX-540, Hitachi Chemical Co., Ltd.) on the surfaces of a pair of substrates.
0) was formed, baked at 300 ° C. for 30 minutes, rubbed in a certain direction with gauze, and superposed so that the rubbing directions coincided with each other to form a wedge-shaped cell having a wedge-shaped thickness of 0 to 80 μm that continuously changes. Then, while heating to 100 ℃, 93
The mixed liquid crystal (I) having a helical pitch of 97 μm was injected at ℃ and the alignment state was observed.

Within the wedge cell, the uniformity of orientation was affected by the gap, being uniform in the thin region of the gap and significantly uneven in the thick region. The boundary is at 24 μm,
It turned out to be equivalent to 1/4 of the pitch.

EFFECTS OF THE INVENTION The present invention has the greatest advantage that the alignment control technology used for the conventional nematic liquid crystal can be applied as it is under limited conditions, and in particular, when the organic polymer film and the rubbing are combined, the rubbing is performed. If the direction is limited, the device can be manufactured by the same process as the nematic.

Further, as a result of eliminating the linear alignment defect, it is possible to deal with a fine electrode shape. Practically, the uniformity of the contrast is improved, or when a finely shaped electrode such as an optical shutter for a printer head is used, if there is even one linear alignment defect in each electrode part of the shutter, There is a problem in that the abnormal alignment region due to a defect exceeds the electrode shape and does not function normally. The present invention solves this problem.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing the basic structure of the present invention, and FIG. 2 is a view schematically showing an alignment state according to the present invention together with a comparative example. 1a, 1b: Substrate 2a, 2b: Conductive film 3a, 3b: Alignment control film 4: Liquid crystal layer 5: Sealant 6a, 6b: Polarizer 7: Light source 8: Liquid crystal molecule 9: Alignment control direction

Claims (3)

[Claims] 1. A method for manufacturing a ferroelectric smectic liquid crystal electro-optical device, comprising a ferroelectric smectic liquid crystal layer held between two transparent electrode-attached substrates, and means for applying a voltage to the transparent electrode. The surface of the two substrates with transparent electrodes in contact with the liquid crystal is coated with an organic polymer film, the surfaces are rubbed in a certain direction, and the substrates are combined so that the directions from the end point to the start point of the rubbing are the same, Ferroelectric smectic liquid crystal composed of a liquid crystal having a cholesteric phase at a temperature higher than that of the smectic phase, and having a helical pitch length of 4 times or more the distance between the substrates at a temperature just above the cholesteric-smectic phase transition point. Ferroelectric smectic liquid crystal electrical device characterized in that the layer is retained and cooled from the temperature showing a cholesteric phase to the temperature showing a smectic phase. Optical device manufacturing method. 2. A method of manufacturing a ferroelectric liquid crystal electro-optical device according to claim 1, wherein the liquid crystal exhibiting ferroelectricity is a chiral smectic C liquid crystal. 3. The means for applying the voltage, together with the voltage for providing a switching function as an electro-optical device,
The method for manufacturing a ferroelectric smectic liquid crystal electro-optical device according to claim 1 or 2, which has a function of applying an AC voltage for controlling orientation.