JPS60179721A - Method for controlling orientation of ferroelectric liquid crystal - Google Patents

Abstract

PURPOSE:To improve the response speed of a ferroelectric liquid crystal, by forming electric fields which are uniform in direction at least in the plane of one substrate by applying a voltage across plural electrodes and orienting the liquid crystal uniformly in the course of the liquid crystal transforming from isotropic phase to smetic A phase or N phase. CONSTITUTION:When electric potential of +V is given to an electrode 22aa and electric potential of -V is given to electrodes 22aa and 22ac, lines of electric force between the plates turn their directions to ones shown in the figure with arrows of solid lines. A ferroelectric liquid crystal 23 has a spontaneous polarization P intersecting the molecular chain at almost right angles to its molecule and the direction of the vector receives force, so that the direction becomes coincident with the direction of the electric field and, therefore, the degree of freedom of the molecule is restrained only in the direction Z. When fixed electric potential is given to all of upper surface electrodes 22b thereafter, electric field are produced between top and bottom substrates. Therefore, the liquid crystal molecule is controlled even in the direction of Z-axis and, as a result, oriented in the direction of Y-axis. In this way, the liquid crystal set to isotropic phase by raising cell temperature is transformed to smectic A phase or nematic phase by gradually cooling while a voltage is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for providing a ferroelectric liquid crystal with an initial molecular arrangement state, that is, an orientation state suitable for use as a display element.

11. A type of display element that utilizes the refractive index anisotropy of ferroelectric liquid crystal molecules and controls transmitted light in combination with a polarizing element has been proposed by C1ark and Lagerwall (Japanese Patent Application Laid-Open No. 107-216-1982). This ferroelectric liquid crystal generally has a chiral smectic C phase (SmC*) or H phase (Smut) in a specific temperature range, and in this state, it is added It has the property of taking either the first optically stable state or the second optically stable state in response to an electric field and maintaining that state when no electric field is applied, that is, it has bistability, and It also responds quickly to changes in , and is expected to be widely used as high-speed and memory-type display elements. However, in order for an optical modulation element using this bistable liquid crystal to exhibit predetermined driving characteristics, the liquid crystal placed between a pair of parallel substrates must be It is necessary that the molecules be in an ordered state so that conversion between stable states can occur effectively. For example, for a ferroelectric liquid crystal with SmC" or SmH" phase, S m C" or S m
The liquid crystal molecular phase having H” phase is perpendicular to the substrate surface,
Therefore, it is necessary to form a region (monodomain) in which the liquid crystal molecular axes are arranged substantially parallel to the substrate surface. However, in conventional optical modulation elements using liquid crystals with bistability, the alignment state of the liquid crystals having such a monodomain structure was not always formed satisfactorily, and therefore sufficient characteristics could not be obtained. is the reality.

As a method for aligning such ferroelectric liquid crystal, conventional TN
As in the case of type liquid crystal display devices, first, organic thin films, inorganic vapor deposited films, and organic thin films with physical scratches are formed on the substrate surface in the liquid crystal cell by rubbing method or oblique vapor deposition, and the molecules are arranged. It gave direction. For example, as shown in FIG. 1(a), the rubbing method involves forming a transparent electrode 12 on a glass substrate 11, forming an organic polymer 11913, and rubbing it in one direction with a cloth such as velvet. This method aligns liquid crystal molecules using minute scratches on the film surface. In the oblique evaporation method, as shown in FIG. 1(b), an inorganic film 14 such as SiO is formed instead of an organic polymer film by evaporation by tilting the substrate, that is, by oblique evaporation. It is. However, when manufacturing a display element using ferroelectric liquid crystal, such conventional alignment control methods have drawbacks such as slow response time.

Unique alignment methods for ferroelectric liquid crystals include rubbing glass substrates up and down to align liquid crystal molecules, and using polyethylene terephthalate sheets as a spacer to form gaps between cells and utilizing the directionality of the edges. There are some that do, but none of them are industrial.

&Mitate 11 In view of the above-mentioned drawbacks of the conventional liquid crystal molecule alignment method, the main purpose of the present invention is to improve the response speed by reducing the restraining force exerted on the liquid crystal molecules from the electrode substrate, and to improve the response speed of the ferroelectric liquid crystal. The purpose of this invention is to provide a method for controlling the alignment of liquid crystal molecules that can fully utilize the characteristics of liquid crystal molecules.

As a result of research for the above-mentioned purpose, the present inventor found that when liquid crystal molecules are restrained on the electrode surface, the response time slows down, but if the alignment treatment on the substrate surface is eliminated, the liquid crystal molecules become stochastic. Since the liquid crystal molecules aggregate in a certain proportion and are oriented in random directions, they cannot be used as a display element, so we realized that it would be ideal if the alignment force could be removed after the liquid crystal molecules are aligned. In order to achieve these ideal conditions, during the process in which the ferroelectric liquid crystal transitions from the isotonic phase to the smectic A phase or N phase by cooling, it is necessary to It has been found that it is particularly effective to align liquid crystal molecules by applying an electric field.

The ferroelectric liquid crystal orientation control method of the present invention is based on such knowledge, and more specifically, the method for controlling the alignment of a ferroelectric liquid crystal in a liquid crystal cell in which a ferroelectric liquid crystal is sandwiched between a pair of parallel substrates. By arranging a plurality of electrodes in parallel to each other on at least one side and applying a voltage between the plurality of electrodes, an electric field is formed in a uniform direction within the plane of the at least one substrate, and by this electric field, The method is characterized in that the liquid crystal is uniformly aligned in the process of the liquid crystal transitioning from the isotoshitic phase to the smectic A phase or N phase.

Next, an example will be described.

FIG. 2 is a schematic perspective view of an example of a liquid crystal cell used to carry out the method of the present invention. This cell is a liquid crystal cell formed using a simple dot matrix method, and has striped electrode patterns 22 on each of a pair of parallel glass substrates 21a and 21b, which are substantially orthogonal to each other.
a and 22b, and a ferroelectric liquid crystal 23 is inserted between them.

Here, paying attention to the lower substrate in FIG. 2, the lower substrate 21, a
Figure 3 shows a side view of just the part taken out.

A method of controlling liquid crystal alignment using such a cell will be described.

In FIG. 3, when a potential of +V is applied to the electrode 22ab and a potential of 1 is applied to the electrodes 22aa and 22ac.

The electric lines of force between the electrode plates take the direction of the solid arrow shown in FIG.

A ferroelectric liquid crystal has a spontaneous polarization P that is almost orthogonal to the molecular chain inside its molecules.The direction of this vector matches the direction of the electric field in Figure 3.The degree of freedom of the molecule is ,
It will be suppressed only in the Z direction. Next, when a constant potential is applied to all of the upper surface electrodes 22b in FIG. 2, an electric field is generated between the upper and lower substrates as shown in FIG. 4. Here, the existence of electric lines of force roughly indicates the direction.

As a result, the liquid crystal molecules are also regulated in the X-axis direction and aligned in the Y-axis direction. Here, regarding the method of applying voltage between the electrodes, applying electric fields in the X-axis direction and the X-axis direction simultaneously and in fixed directions, as shown in Figure 4, is just an example. In order to make the arrangement of liquid crystal molecules more effective, it is also possible to adopt a method in which the electric field in the X-axis direction and the electric field in the X-axis direction are applied at different timings. Also, as shown in Figures 3 and 4, IS2
If the electrodes 22aa and 22ac on both sides of the electrode 22ab of the tomatotrix have the same potential, the electric fields between the electrodes 22aa-22ab and between the electrodes 22ac-22a'b will be in opposite directions, so the stable state of the ferroelectric liquid crystal will be different and the polarization will change. When looking through the beam, domains will be generated, so in the final stage of applying the electric field, scan is performed to keep the direction of the electric field constant as shown in FIG. That is, in the present invention, the condition of "forming an electric field in a uniform direction within the plane of the substrate" only needs to be satisfied at least in the final stage when applying an electric field sequentially to align liquid crystal molecules. .

According to the present invention, the cell temperature is raised to bring the liquid crystal into the isobathic phase, and the liquid crystal is gradually cooled while applying the voltage described above to undergo a phase transition to the smectic A layer or nematic phase.

Furthermore, in addition to the voltage application operation described in section 2 above, a method is employed in which an organic polymer thin film 24 of polyimide or the like is formed on the substrate surface, and areas other than the electrode portions are rubbed to provide a forced orientation force by the fine grooves 24a (Fig. 6). ) can also be adopted.

Figure 7 shows an example of the time variation of the voltage actually applied to the three upper and lower electrodes of the cell shown in Figure 2, and it has been empirically shown that liquid crystal molecular alignment can be effectively achieved. This is a confirmed example. In this example, during the time from T0 to T, the electric field in the X-axis direction is 22ab+22aa and 2
It goes in the direction of 2ab+22aC. 22ha~22bc
It is sufficient to electrically float the electrodes, but if this is difficult, an alternating current electric field with a peak value lower than the voltage applied to the electrodes 22aa to 22ac may be applied.

71 to T2't', electrodes 22aa to 22ac are at negative potential;
22ba to 22bc are set to a positive potential and apply an electric field in the X-axis direction. T2-T3 are T. -Same as T work, T3~T4
It is also assumed that T and -T2 are the same. In this way, time T4~
Applying the voltage as shown in the figure to T9, 22a
The polarity is shifted from a + 22 a b + 22 ac to end the orientation control.

In the present invention, when the electric field for controlling the alignment is turned off, the force for controlling the alignment of the liquid crystal molecules ceases to work, so if the alignment is disturbed due to some disturbance, it is necessary to reapply the voltage. Therefore, in order to make such disturbances less likely to occur, it is possible to form a polyimide (polyamide) thin film on the substrate surface, which has the property of arranging liquid crystal molecules parallel to the film surface. You can also rub it.

FIG. 8 shows another cell configuration for carrying out the orientation control method of the present invention, and this cell has an electrode dedicated for orientation control. That is, a pair of parallel glass substrates 31
a, 31b, striped electrode patterns 32a (32a-1, 32a-2, s・*3
2a-N) and 32b (32b-1, 32b-2,
φΦ・fist 32b-N), and each of these orientation control electrodes 32a, 32b is covered with polyimide, 5i.
Insulating films 33a and 33b made of 02 or the like are formed, display strip-like electrode groups 34a and 34b are formed thereon, and ferroelectric liquid crystal 23 is inserted between these.

In this example, the alignment control electrode 32a- provided on the lower substrate
1 to 32a-N and the alignment control electrode 32 provided on the upper substrate
The numbers b-1 to 32b-N are equal, and their positions are arranged in a 1:1 ratio.

Although not particularly illustrated, for either one of the pair of substrates, the alignment electrode 32a or 32b is connected to the display electrode 3.
4a or 34b (in this case, of course, the intermediate insulating film 33a or 33b is omitted), and the shapes and positions of the upper and lower alignment electrodes may be slightly shifted.

With this method, there is no problem even if the aperture ratio of the liquid crystal cell increases and the line spacing within the substrate decreases.

When aligning the liquid crystal in the cell of FIG. 8, an electric field is generated parallel to the cell by creating an electric field gradient that decreases sequentially from number 1 to number N on both the upper and lower substrates as shown in FIG.

As mentioned in the examples of FIGS. 2 to 7 above, the cell itself is heated, and the voltage application as described in item 1 is performed by applying a voltage during the transition from the isotropic layer to the smectic A layer. put.

When applying a voltage, it is preferable that the electric field is oriented in one direction even near the center of the thickness of the liquid crystal cell in order to uniformly align the liquid crystal molecules as a whole. Furthermore, although the cell area has become larger and the number of electrodes N has increased, due to the withstand voltage limit of the cell itself, it is not possible to provide a desired potential difference between adjacent electrodes with a uniform electric field gradient as shown in Figure 9. If there is no potential gradient, as shown in FIG. 10, if a partial potential gradient is provided and the gradient is scanned at a constant period, they will be aligned in the horizontal direction. That is, by doing this, the maximum voltage difference applied to the cell can be kept low, and the practical effect is to provide a uniform electric field gradient as shown in Figure 9, and to create a desired magnitude between adjacent electrodes. It becomes possible to give a potential difference of

In both the cases of Figures 9 and 10, when an electric field is applied intermittently in the 1 downward direction (either direction is fine, but the direction must be kept constant) at regular time intervals, the liquid crystal molecules The degree of freedom in arrangement is reduced, making it easier to align along the wall surface. More specifically, for example, an electric field in the vertical direction may be applied in a manner alternating with the electric field having the waveforms shown in FIGS. 9 and 10.

In the examples described in FIGS. 2 to 7, alignment can be performed more efficiently by applying only the electric field in the Z-axis direction using a dedicated electrode.

For this purpose, for example, as shown in FIG. 11 corresponding to FIG. 2, a vertical voltage applying electrode 36a is placed under the display and alignment electrodes 22a and 22b via an insulating film 35a or 35b. A liquid crystal cell having a two-layer electrode structure may be used, in which electrodes 36b and 36b (in this example, electrodes are formed on the entire surface) are formed. In this case as well, as in the example of FIG. 9 or 10, it is desirable that the electric field in the direction perpendicular to the substrate and the electric field in the parallel direction are applied at different timings, that is, in alternation.

As described above, in the present invention, in the process in which the ferroelectric liquid crystal transitions from the isotonic phase to the smectic A phase or N phase by cooling, the liquid crystal has a plurality of layers provided on the substrate of the liquid crystal cell. By adopting a method of aligning liquid crystal molecules by applying an electric field parallel to the substrate surface using parallel electrodes, the liquid crystal molecules are aligned during display operation, compared to conventional alignment methods using the substrate or spacer itself. It is possible to reduce the alignment force acting on the liquid crystal and improve the liquid crystal response speed. Furthermore, even if the orientation of the liquid crystal molecules is disturbed due to an accident during use, the original orientation can be easily restored by reapplying the voltage. Furthermore, by using dedicated electrodes for orientation, it is also possible to perform cell orientation processing regardless of the configuration method of the display electrodes.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are schematic cross-sectional views in the thickness direction of a cell substrate for explaining the conventional liquid crystal alignment method, respectively, and Figure 2 is a schematic diagram of an example of a liquid crystal cell for implementing the method of the present invention. A perspective view, Figure 3 is a diagram of electric lines of force around the lower substrate of the cell in Figure 2, Figure 4 is a diagram of electric lines of force when an electric field is applied both parallel and perpendicular to the substrate, and Figure 5 is a diagram of electric lines of force around the lower substrate of the cell in Figure 2. An example of voltage application to the lower electrode, FIG. 6 is a cross-sectional view of a substrate with selective rubbing treatment applied between the matrix electrodes, FIG. 7 is an example of voltage application to the matrix electrode, and FIGS. 8 and 11 9 and 10 are respectively cross-sectional views of other examples of liquid crystal cells for carrying out the method of the present invention, and FIGS. 9 and 10 are distribution diagrams of voltages applied to the alignment control electrodes, respectively. 21a, 21b, 31a, 31b--one substrate 22a (2
2aa, 22ab, 22aC-11)・IIΦ・lower electrode 22b (22ba, 22bb, 22bc) skewer・φ upper electrode 23...ferroelectric liquid crystal 24.33a, 33b, 35a, 35b --- insulation Film 32a (32a-1 to 32a-N) ψQe/horizontal voltage application electrode for orientation 32b (32b-1 to 32b-N)... Horizontal voltage application electrode for alignment 36a, 36b... Horizontal voltage application electrode for alignment 1st Ward 1 Figure 2 Figure 3 Figure 5 Inhan

Claims (1)

[Claims] By arranging a plurality of electrodes in parallel with each other on at least one of the pair of parallel substrates of a liquid crystal cell in which a ferroelectric liquid crystal is sandwiched between a pair of parallel substrates, and applying a voltage between the plurality of electrodes. , forming an electric field in a uniform direction in the plane of at least one of the substrates, and uniformly orienting the liquid crystal in the process of transitioning the liquid crystal from the isobathic phase to the smectic A phase or N phase by this electric field; A method for controlling the alignment of ferroelectric liquid crystals characterized by: