JPH06118418A - Optical modulation element - Google Patents

Abstract

PURPOSE:To make orientation uniform and to suppress the hysteresis and unstability of V-T characteristics intrinsic to active matrix driving and half tone display using a ferroelectric liquid crystal by using the assemblage of island structural bodies having an inclination of a specified angle for oriented films. CONSTITUTION:The oriented layers of the optical modulation element for driving picture elements in gradation by forming a ferroelectric liquid crystal layer and the picture elements clamping the oriented layers to orient this liquid crystal layer between a pair of electrodes 1 and applying the external corresponding to gradation signals between the electrodes 1 are the assemblage of the island structural bodies 2 having the specified angle theta of inclination with the electrodes 1. More preferably, the thickness of the island structural bodies 2 is <=100Angstrom , the average grain size is 50 to 300Angstrom , the presence ratio to the areas of the respective picture elements is >=5% to <=80% and the angle inclination theta with the electrodes 1 is 80 to 90 deg.. As a result, the problems, such as hysteresis and unstability, are solved and the even and uniform orientation is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulation element for gradation display, and more particularly to a gradation display liquid crystal element using a liquid crystal having at least two stable states, for example, a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art In a conventional liquid crystal television panel using an active matrix driving system, thin film transistors (TFTs) are arranged in a matrix for each pixel, and a gate-on pulse is applied to the TFTs to establish a conduction state between a source and a drain. At this time, a video image signal is applied from a source and stored in a capacitor, and a liquid crystal (for example, twisted nematic: TN-liquid crystal) is driven corresponding to the stored image signal, and at the same time, a voltage of the video signal is modulated. Is used for gradation display.

However, in an active matrix drive type television panel using such a TN liquid crystal or STN (super twisted nematic) liquid crystal, the response speed is as slow as several hundred milliseconds, and further ED which is expected to be developed in the future. When it is used for driving a television compatible with HD (high vision), there is a problem in that a defect such as an afterimage remains.

On the other hand, since a ferroelectric liquid crystal having bistability has a high response speed of several hundred microseconds, attempts have recently been made to use such a ferroelectric liquid crystal for gradation driving of a television.

An example of the method is charge control drive of a ferroelectric liquid crystal. This is between the domain inversion area (a) of the ferroelectric liquid crystal and the injected charge amount (q).

[0006]

[Equation 1] The gradation display is performed by injecting an electric charge amount corresponding to the inversion area into each pixel of the ferroelectric liquid crystal.

The charge control drive is a very effective means for gradation display using a bistable ferroelectric liquid crystal, but it may have some drawbacks at the same time.

One of them is a hysteresis phenomenon in the voltage-transmittance characteristic. The phenomenon will be described below.

FIG. 6 is a block diagram of a commonly used ferroelectric liquid crystal element. In FIG. 6, 14 and 14 'are substrates, 13 and 13' are transparent conductive layers, 12 and 12 'are insulating layers, 10 and 10' are alignment layers, and 11 is a ferroelectric liquid crystal layer.

FIG. 7 is an equivalent circuit when gradation display is performed by active matrix charge control drive using a ferroelectric liquid crystal. In the drawing, T ₁₁ , T ₁₂ , T ₂₁ , T ₂₂ , ... Show thin film transistors (TFTs) used for active matrix driving switching elements. LC ₁₁ , LC ₁₂ ,
LC ₂₁ , LC ₂₂ , ... Are respective thin film transistors T
₁₁ , T ₁₂ , T ₂₁ , T ₂₂ , ... Drain electrodes 4, 4 ',
Pixels composed of a ferroelectric liquid crystal sandwiched by 4 ″, 4 ′ ″, ... And a counter electrode 8. C ₁₁ , C ₁₂ , C ₂₁ ,
C ₂₂ ... Shows capacitors for accumulating drive signal charges formed by the gate lines 1a, 1a ', ... And the drain electrodes 4, 4', 4 ", 4 '".

Now, using the above circuit, each pixel shown in FIG.
With a drive waveform as shown in (a), one cycle is 16.7 mS (60 H).
When the reset pulse 81 of z) and the write pulse 82 corresponding to each gradation are applied, the transmittance (T) as shown in FIG. 8B is generated. At this time, the transmittance indicated by 83 is about 8
It is the transmittance corresponding to the writing pulse of 2. Now, with such a driving waveform, the transmittance of the writing pulse 82 is 8
The plot of 3 is the voltage-transmittance characteristic described above. An example thereof is shown in FIG. As is clear from the figure,
A so-called hysteresis phenomenon in which the transmittance is different between when the voltage is increased 94 and when the voltage is decreased 95 is observed.

FIG. 10 is a qualitative explanation of the above hysteresis phenomenon. FIG. 10A corresponds to 94 in FIG. That is, it is in the direction of reducing the black domain, and in terms of driving, it is necessary to write white after reset. On the other hand, FIG. 10B shows a direction in which the black domain is increased, and it is not necessary to write white after reset in terms of driving. That is, in the case of the write pulse, the former (94) requires a larger voltage than the latter (95).

As described above, when the gradation display is performed by the charge control method by the driving method as shown in FIG. 8A, a hysteresis phenomenon naturally occurs in the voltage-transmittance characteristic.

In addition, when the above active matrix driving is performed, there is a problem that the response of the liquid crystal is disturbed by the continuous application of the direct current voltage (DC) component for a long time during driving. It is considered that this is because the DC component induces uneven distribution of internal ions in the liquid crystal, which forms an electric field.

FIG. 11 shows a phenomenon that occurs when the drive waveform shown in FIG. 8A is applied. It can be clearly seen from FIG. 11 that the transmittance gradually decreases due to the repeated pulse application.

This phenomenon will be described with reference to FIG. Figure 8
In the waveform shown in (a), a positive DC component is excessively applied geometrically as viewed from the liquid crystal. FIG. 12 shows how this DC component acts on the liquid crystal. 12
At the time, by applying a positive DC component, the insulating layer portion 1
Charges are accumulated between 0, 10 'and the liquid crystal portion 11, and the partial pressure of the liquid crystal due to the charge accumulation component is in the negative direction. As a result, "white" writing becomes difficult to be performed.

The above hysteresis phenomenon and instability due to the DC component are very serious problems when the gradation display is performed by the active matrix method because the pixel display voltage cannot be uniquely determined.

In order to solve the above problems, attempts have been made to make the alignment film conductive and to reduce the resistance of the liquid crystal used. However, except for some examples (Japanese Patent Application No. 3-63200), these attempts have an adverse effect that uniform uniform orientation cannot be obtained.

[0019]

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical modulation element which has solved the above-mentioned drawbacks, and more specifically, an optical modulation element which reduces the above hysteresis phenomenon and is less unstable. In a uniform uniform orientation.

[0020]

In order to achieve the above object, in the present invention, a pixel in which a ferroelectric liquid crystal layer and an alignment layer for orienting the liquid crystal layer are sandwiched between a pair of electrodes is formed, and the electrode is formed. In the optical modulation element for driving the pixel in gradation by applying an external voltage according to a gradation signal, the alignment layer is a group of island-shaped structures having a certain inclination angle with respect to the electrode. It is characterized by

In a preferred embodiment of the present invention, the island structure has a thickness of 100 Å or less and an average diameter of 50 to 30.
0Å, existence ratio for each pixel area is 5% or more and 80%
And the angle inclination with respect to the electrode is 80 to 90 °.
Is.

[0022]

According to the present invention, in an active matrix optical modulation element for displaying gray scales, an assembly of island-shaped structures tilted at a certain angle with respect to a base electrode is used as an alignment layer. As a result, the problems such as the hysteresis phenomenon and instability described above can be eliminated, and uniform uniform orientation can be obtained.

The optical modulator that can be used in the present invention includes
Has a first optically stable state (eg, to form a bright state) and a second optically stable state (eg, to form a dark state) in response to an applied electric field; A substance having two stable states, particularly a liquid crystal having such a property, is most suitable.

As the liquid crystal having at least two stable states which can be used in the optical modulator of the present invention, a chiral smectic liquid crystal having ferroelectricity is most preferable, among which, a chiral smectic C phase (SmC *),
H phase (SmH *), I phase (SmI *), F phase (SmF
*) And G-phase (SmG *) liquid crystals are suitable. For this ferroelectric liquid crystal, please refer to "LE JOURNAL AL DE PH".
YSIQUE LETTR ") Volume 36 (L-69)
"Ferroelectric Liquid Crystals" (1975)
id Crystals ");" Applied Physics "(" Applied Physics ")
Letters ") Vol. 36, No. 11, 1980," Submicro Second Bistable Electro-Optic Switching In Liquid Crystals "(" Submicro Second B ").
istable Electro-optic Swi
tching in Liquid Crystal
s "); and Solid State Physics 16 (141) 1981" Liquid Crystal "and the like, and the ferroelectric liquid crystal disclosed therein can be used in the present invention.

More specifically, as an example of the ferroelectric liquid crystal compound used in the present invention, desyloxybenzylidene-P'-amino-2-methylbutyl cinnamate (DO
BAMBC), hexyloxybenzylidene-P'-amino-2-chloropropyl cinnamate (HOBACP
C) and 4-o- (2-methyl) -butylresorcylidene-4′-octylaniline (MBRA8) and the like.

When a device is formed by using these materials, the liquid crystal compound is SmC *, SmH *, SmI.
In order to maintain a temperature state of *, SmF *, or SmG *, the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

FIG. 13 schematically shows an example of a ferroelectric liquid crystal cell. 131 and 131 'are In ₂ O
₃ , a substrate (glass plate) coated with a transparent electrode such as SnO ₂ or ITO (Indium-Tin-Oxide), between which the liquid crystal molecular layer 132 is oriented so that it is perpendicular to the glass surface. Liquid crystal is enclosed. A thick line 133 represents a liquid crystal molecule, and the liquid crystal molecule 133 has a dipole moment (P⊥) 134 in a direction orthogonal to the molecule. Boards 131 and 13
When a voltage equal to or higher than a certain threshold is applied between the electrodes (not shown) on 1 ′, the helical structure of the liquid crystal molecules 133 is unraveled, and all the dipole moments (P⊥) 134 are oriented in the direction of the electric field. The orientation direction of can be changed. The liquid crystal molecule 133 has an elongated shape, and exhibits refractive index anisotropy in the major axis direction and the minor axis direction thereof. Therefore, for example, if polarizers arranged in a crossed Nicol positional relationship are placed above and below a glass surface. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the polarity of voltage application. When the liquid crystal cell is made sufficiently thin (for example, 1 μm), as shown in FIG. 14, the helical structure of the liquid crystal molecules is unwound (non-helical structure) even when no electric field is applied, and the dipole The moment P or P ′ takes either an upward (144) or downward (144 ′) orientation state. When an electric field E or E'having a polarity different from a certain threshold value is applied to such a cell as shown in FIG. 14, the dipole moment becomes upward 144 or downward 144 'corresponding to the electric field vector of the electric field E or E'. The liquid crystal molecules are turned to the first stable state 143 accordingly.
It is oriented in either the (bright state) or the second stable state 143 '(dark state).

There are two advantages of using such a ferroelectric liquid crystal as an optical modulator. Firstly, the response speed is extremely fast, and secondly, the alignment of the liquid crystal molecules has bistability. The second point will be described with reference to FIG. 14, for example. When an electric field E is applied, the liquid crystal molecules are in the first stable state 14
Although the first stable state 143 is maintained even when the electric field is cut off, the liquid crystal molecules are aligned in the second stable state 143 ′ when an electric field E ′ of the opposite direction is applied. It changes the direction of the molecule, but it keeps this state even when the electric field is cut off, and has a memory function in each stable state. In order to effectively realize such a high response speed and bistability, the cell is preferably as thin as possible, generally 0.5 μm to 20 μm, and particularly 1 μm.
m to 5 μm is suitable. A liquid crystal-electro-optical device having a matrix electrode structure using a ferroelectric liquid crystal of this type is disclosed, for example, in U.S. Pat. No. 4,367,924 and 4,840,46.
No. 2 specification.

[0029]

EXAMPLES Examples of the present invention will be described below.

Example 1 FIG. 2 is a basic configuration diagram of an optical modulator having an alignment film according to an example of the present invention. In FIG. 2, 23, 2
3'is a substrate, 22 and 22 'are transparent conductive films made of ITO film, 20 and 20' are alignment films made of a group of island-shaped structures having an angle inclination, which is the feature of the present invention, and 21 is a ferroelectric. It is a liquid crystal layer.

The substrates 23 and 23 'were made of 1.1 mm-thick non-alkali glass (N-made by HOYA) which was polished on both sides.
A40). The ITO films 22 and 22 'are formed to 700 Å by the reactive RF ion plating method in O ₂ .

FIG. 1 is a diagram for explaining in detail the shape of the alignment film in one pixel of the optical modulator of FIG. 1, (a) is a top view of one pixel of the optical modulator of FIG.
(B) shows a cross-sectional view of the alignment film on the ITO film. In FIG. 1, 1 is an ITO film, 2 is an island-shaped structure having an angle inclination, and is made of SiO _x in this embodiment. Here, the angle inclination means an angle θ with the ITO film in the figure.

FIG. 3 is an apparatus diagram for producing the island structure. In FIG. 3, 30 is a vacuum chamber and 31 is IT.
A glass substrate on which an O film is deposited, 32 a molecular beam source of SiO, 33 a power source of the molecular beam source, 34 an O ₂ cylinder,
35 is a roller for sending the substrate 31, 36 is a rotary pump, 37 is a mechanical booster pump, 38 is a cryopump, 39 is a valve, and 310 is a slit.

[0034] Molecular beam source SiO molecules 32 emanating from beam slit 3 to obtain a supply of O ₂ from O ₂ cylinder 34
The substrate 31 is reached via 10. On the other hand, the roller 35 rotates at an irregular angular velocity ω, which causes the substrate 31 to move at an irregular velocity. Therefore, the substrate 31 has Si
There are a part where Ox adheres and a part where there is almost no adherence. Also, the board has a tilt of θ as shown in the figure,
This causes an angle tilt. Through the above process,
An alignment film having a shape as shown in FIG. 1 is formed.

1.5 μmφ silica beads as a gap agent are dispersed in IPA (isopropyl alcohol) on a substrate on which the above-mentioned alignment film is formed by spinner coating, and after drying, a sealant is printed to overlay the same substrate. A cell was also prepared, and a ferroelectric liquid crystal was injected into the cell to prepare an optical modulator.

The alignment state of the obtained optical modulation element was observed with a polarization microscope, and it was found that uniform alignment was obtained. Further, a similar element was prepared using 80 μm thin film glass as the substrate glass, and SmA phase liquid crystal was injected,
When the pretilt angle was measured by the X-ray diffraction method, the pretilt angle was changed from 10 ° to 35 ° by changing the abundance ratio, shape and inclination angle θ of the island-shaped structure.

The write voltage-transmittance (VT) characteristic of the above optical modulator is shown by 41 in FIG. As compared with the VT characteristics (corresponding to 94 and 95 in FIG. 9) 42 of the conventional polyimide rubbing cell, it is clear that the VT characteristics 41 of the present embodiment have a clearly reduced hysteresis.

FIG. 5 shows the correlation of the all-white writing response time when the black reset leaving time is changed. In FIG.
Reference numeral 51 is the response time of the optical modulation element using the alignment film of this embodiment (corresponding to the characteristic 41 of FIG. 4), and 52 is the optical modulation element using the conventional polyimide rubbing alignment film (characteristic 42 of FIG. 4).
Response time). It can be seen that the instability is apparently eliminated by using the alignment film of this example.

Example 2 An alignment film made of an aggregate of island-shaped structures similar to that in Example 1 was produced in the same apparatus as in Example 1 using Pt metal instead of SiO _x . O ₂ was not introduced and the base pressure was 10
^It was prepared at ^-10 torr. In FIG. 3, θ was 85 ° and the thickness of the structure was 40Å.

The above alignment film was used to form a device in the same manner as in Example 1, and the alignment state, the VT characteristic hysteresis, and the instability were measured. As a result, it was confirmed that the orientation was a uniform uniform orientation, and the hysteresis and instability were significantly reduced as compared with the conventional polyimide rubbing orientation film element.

Example 3 In the same manner as in Example 1, an aggregate of island-shaped structures was subjected to SiO _x.
Was prepared by using a molecular beam of Lu-phthalocyanine metal complex instead of, and the alignment state, hysteresis and instability were measured. As a result, similar to Example 1, it was confirmed that uniform uniform orientation, hysteresis and instability were significantly reduced.

Example 4 An aggregate of Pt island-shaped structures similar to Example 2 was produced by the orthorhombic ion beam sputtering method, and the same results as in Example 2 were obtained.

Similarly, the same results were obtained when Au and W were used.

[0044]

As described above, according to the present invention,
In a half-tone display optical modulation element using a ferroelectric liquid crystal, by using an aggregate of island-shaped structures with a certain angle inclination for the alignment film, the alignment is uniform uniform alignment and the ferroelectric liquid crystal is formed. Active matrix drive used
It has an effect of suppressing the hysteresis phenomenon and instability of the VT characteristic peculiar to the halftone display.

[Brief description of drawings]

FIG. 1 is a diagram showing a shape of an alignment film according to an embodiment of the present invention.

FIG. 2 is a basic configuration diagram of an optical modulation element using the alignment film of FIG.

FIG. 3 is an apparatus diagram for producing the alignment film of FIG.

4 is a VT characteristic curve of an optical modulation element using the alignment film of FIG. 1 and a conventional element.

5 is a diagram showing a relationship between a white writing time and a black leaving time of a conventional optical modulation element using the alignment film of FIG.

6 to 10 are explanatory views of a conventional technique.

FIG. 11 is a phenomenon diagram of instability.

FIG. 12 is an explanatory diagram of instability.

13 and 14 are diagrams illustrating the principle of an optical modulator using a ferroelectric liquid crystal.

[Explanation of symbols]

1, 22, 22 ': transparent conductive film, 2: island-shaped structure having an angle inclination θ, 20, 20': alignment film composed of an assembly of island-shaped structures, 21: ferroelectric liquid crystal layer, 23, 23 ', 3
1: substrate, 30: vacuum chamber, 32: molecular beam source,
33: power source of molecular beam source, 34: O ₂ cylinder, 35:
Roller, 36: rotary pump, 37: mechanical booster pump, 38: cryopump, 39: valve, 310: slit.

Continued Front Page (72) Inventor Tomoko Murakami 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (5)

[Claims] 1. A pixel in which a ferroelectric liquid crystal layer and an alignment layer for aligning the liquid crystal layer are sandwiched between a pair of electrodes is formed,
In the optical modulation element for driving the pixel in gradation by applying an external voltage according to a gradation signal between the electrodes, a group of island-shaped structures in which the alignment layer has a constant inclination angle with respect to the electrodes. An optical modulation element characterized by: 2. The optical modulator according to claim 1, wherein the island-shaped structure has a thickness of 100 Å or less. 3. The island-shaped structure has an average diameter of 50 to 300.
The optical modulator according to claim 1, wherein the optical modulator is Å. 4. The optical modulation element according to claim 1, wherein the abundance ratio of the island-shaped structures is 5% or more and 80% or less with respect to the area of each pixel. 5. The optical modulation element according to claim 1, wherein an angle inclination of the island-shaped structure with respect to the electrode is 80 to 90 °.