JPH0795162B2 - Ferroelectric liquid crystal panel - Google Patents

Description

The present invention relates to a display device, and more particularly to a ferroelectric liquid crystal panel.

Conventional Technology Conventional technology will be described below with reference to the drawings.

First, the ferroelectric liquid crystal itself will be described.

FIG. 3 is a schematic diagram of a ferroelectric liquid crystal molecule. Ferroelectric liquid crystal is a liquid crystal having a layered structure, which is usually called smectic liquid crystal. The molecule has a structure that is inclined by θ with respect to the perpendicular direction of the layer. Further, the ferroelectric liquid crystal is usually composed of optically active liquid crystal molecules which are not racemic.

As shown in FIG. 3, the ferroelectric liquid crystal molecule has a permanent dipole moment that causes spontaneous polarization in a direction perpendicular to the long axis of the molecule, and in the cameral smectic C phase, the conical shape ( The outside of the cone) can move freely. Further, the vector dropped from the center point O of the cone to the liquid crystal molecules is called the C director. In the chiral smectic C phase, this C director is free to move outside the cone. In FIG. 3, 31 is a liquid crystal molecule, 32
Is a permanent dipole, 33 is a C director, 34 is a cone, 35 is a layered structure, 36 is a layer normal direction, and 37 is a tilt angle θ.
In addition, the ferroelectric liquid crystal molecules usually have a twisted structure because they have asymmetric atoms. This twisted structure is shown in FIG.

In FIG. 4, 41 is a liquid crystal molecule, 42 is a permanent dipole moment, 43 is a pitch (L) representing a twist period, 44 is a layer structure, 45 is a normal direction of the layer, and 46 is a tilt angle θ. When the cell thickness (d) of the ferroelectric liquid crystal panel is thicker than the pitch (d>
L) In general, the influence of the surface of the ferroelectric liquid crystal cell substrate does not extend to the central portion of the cell, so that the ferroelectric liquid crystal cell has a twisted structure. However, when the cell thickness is smaller than the pitch (d <L)
The twisted structure is unwound by the force on the substrate surface, and two regions where molecules are parallel to the substrate surface appear as shown in FIG. The two regions have the permanent dipole moments of the molecules facing in opposite directions, one from the back to the front and the other from the front to the back.
This corresponds to the tilt angle of the molecule with respect to the layer normal.

In FIG. 5, 51 is a liquid crystal molecule, 52 is a permanent dipole moment facing from the back side to the front side of the paper, 53 is a permanent dipole moment facing from the front side to the back side of the paper, 54 is a layered structure, and 55 is a layered structure. The layer normal direction, and 56 represents the tilt angle.

Next, the operation principle of the ferroelectric liquid crystal will be described with reference to the drawings. As described above, when the ferroelectric liquid crystal cell is filled with the ferroelectric liquid crystal (d <L) having a pitch larger than the cell thickness, a state having two regions as shown in FIG. 5 is obtained. At this time, when an electric field is applied from the back side of the paper to the front side, all the permanent dipole moments are oriented in the direction of the electric field and all the molecules are + as shown in Fig. 6a.
The state has a tilt angle of θ. In such a state, when the polarization axis direction of the polarizer (P) of the polarizing plate is made parallel to the long axis direction of the molecule, the polarization axis direction of the analyzer (A) is made parallel to the short axis direction of the molecule (FIG. 6 (a)). The linearly polarized light that has passed through the polarizer (P) is transmitted without being subjected to birefringence and is blocked by the analyzer (A) to obtain a dark state. When an electric field is applied in the opposite direction, all the molecules have a tilt of -θ as shown in Fig. 6b, and linearly polarized light that has passed through the polarizer passes through the analyzer due to the birefringence effect and a bright state is obtained.

As described above, bright and dark states can be obtained depending on whether the electric field is positive or negative. In addition, it is said that in a cell having a cell thickness smaller than the pitch (d <L), since the twisted structure is usually unwound, the molecule remains as it is even after the electric field is removed. 6 (a) and 6 (b), 61 is the direction of the electric field, 62 is the permanent dipole moment of the molecule, 63 is the layer structure, 64 is the tilt angle θ,
Reference numeral 65 indicates the polarization axes of the polarizer (P) and the analyzer (A), respectively. (Reference: Fukuda, Takezoe, Kondo: High-speed display using ferroelectric liquid crystal, Optronics, 9go, 64
However, in the case of a large area ferroelectric liquid crystal panel, it is necessary to show similar memory characteristics in all ranges in the above memory method, and it is necessary to delicately control the cell substrate surface. Therefore, recently, as a new memory system, a system utilizing the paraelectric anisotropy (Δε) of the ferroelectric liquid crystal material has been proposed instead of the memory based on the surface of the cell substrate. In this method, a ferroelectric liquid crystal material having a negative Δε (Δε <0) is used, and a memory property is forcibly provided by the interaction between the paraelectric anisotropy and the electric field. This memory effect due to the effect of Δε is usually called an AC stabilization effect.

(References) Jay. M. Geary., 1985 SID International Digest of Technical Paper pp.128 (JMGeary., 1985 SI
D INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPE
RS pp.128) Conventional example of oblique vapor deposition The oblique vapor deposition method was previously used as a part of the nematic liquid crystal alignment method, but the rubbing method is now predominant. The oblique vapor deposition method will be described with reference to the drawings.

The actual method of the oblique deposition method is shown in FIG. A vapor deposition source (bell jar) placed in a vacuum state has a vapor deposition source, which can be heated by passing an electric current. The cell substrate is θ from the substrate vertical direction to the vapor deposition direction.
Only tilted and set. Reference numeral 71 is a bell jar, 72 is a cell substrate, 73 is a vapor deposition source, 74 is an inclination angle θ, and by oblique vapor deposition, there can be formed a structure in which numerous column-shaped small projections 81 are present on the surface as shown in FIG. . This is usually said to be caused by an effect called self-shadowing. At this time, by changing the tilt angle θ82, a difference occurs in the orientation of the nematic liquid crystal molecules. This will be described with reference to the drawings.

(1) When the vapor deposition angle (θ) is 75 degrees to 85 degrees, θ is 75 degrees to 85 degrees.
9A, the liquid crystal molecules are oriented in parallel with the major axis direction (n) 111 of the molecules in the vapor deposition direction. Therefore, liquid crystal molecules are said to have a pretilt angle of about 15 to 30 degrees.

(2) When the vapor deposition angle (θ) is -60 °, when θ is -60 °, the liquid crystal molecules are oriented perpendicular to the vapor deposition direction, as shown in FIG. 9 (b). At this time, the pretilt angle is about 0 degree.

It is said that the difference in the orientation due to the difference in the vapor deposition angle depends on in which direction the molecules are arranged with respect to the surface column structure so that the energy of the elastic deformation is the smallest.

There are a few examples in which the oblique vapor deposition method is used as the alignment method in the smectic liquid crystal or the ferroelectric liquid crystal. However, they are used in the state where the cell thickness is thick (up to 7 μm or more), and the voltage-luminance curve (B-V curve) or the like has not been measured for complete memory property, and as a display device for pretilt angle. There is almost no mention of its usefulness.

Bibliographic References: Double. ULVAC, M.
Boyx, E. Guyon; Orientation of nematic and smectic phases on deposited film, Applied Physics
Vol. 25, No. 9, (1) 479, 1974 (W. Urbach,
M.Boix, and E.Guyon; Alignment of nematics and smect
ics on evaporated films.Applied Physics Letters, VO
L.25, No.9,1P.479 November 1974), Tsutomu Uemoto, Yasuo Iwasaki, Katsumi Yoshino, Yoshio Inuishi; Electro-optical properties of smectic ferroelectric liquid crystals (2), 4th liquid crystal discussion Collection (1978
Year) Lecture number 3R13.

Problems to be Solved by the Invention As described above, for example, in a ferroelectric liquid crystal panel having a tilt angle (pretilt angle) from the substrate surface by oblique vapor deposition, molecules tend to be large when trying to utilize paraelectric anisotropy (Δε). In the case of having a pretilt angle, if Δε is negative, AC stabilization is not effective, so that there is a problem that a memory effect does not occur and a ferroelectric liquid crystal panel with good visibility cannot be obtained.

Means for solving the problem When the ferroelectric liquid crystal molecules have a large pretilt angle in the ferroelectric liquid crystal panel, by making Δε positive, the AC stabilization effect is exerted and the memory effect is achieved. A dielectric liquid crystal panel is provided.

Action Ferroelectric liquid crystal panel having a large pretilt angle by using a ferroelectric liquid crystal material having a positive paraelectric anisotropy. Liquid crystal panel is provided.

EXAMPLE A ferroelectric liquid crystal panel of an example of the present invention will be described in detail with reference to the drawings.

First, in order to prepare a ferroelectric liquid crystal panel having a large pretilt angle, the oblique vapor deposition method described in the prior art was used, and the method shown in FIG. 7 was used.

The oblique vapor deposition method performed in this example will be described in detail. Tantalum board was used as the evaporation source, and silicon monoxide (SiO) was used as the evaporation material. The substrate used was a glass substrate on which conductive indium tin oxide was deposited (ITO substrate). Both vapor deposition angles of 85 degrees and 60 degrees were used. The deposition rate is about 20Å / sec, and the film thickness is about 3500 when viewed from the direction perpendicular to the substrate.
Å

A ferroelectric liquid crystal panel was prepared using the ITO substrate thus obliquely deposited. The structure of the oblique deposition cell is shown in FIG.

In FIG. 10, 101 is a glass substrate, 102 is an ITO layer and a diagonal vapor deposition layer thereon, 103 is a ferroelectric liquid crystal layer, 104 is a spacer for adjusting the cell thickness, and 105 is a combination of vapor deposition directions. The vapor deposition directions of the upper and lower substrates were set to be antiparallel to each other. As the ferroelectric liquid crystal material used in the experiment, a liquid crystal material having a ferroelectric property in the temperature range of 0 to 58 degrees was used. The phase transition temperature of the ferroelectric liquid crystal used below is shown.

Here, Cr: Crystal phase SmC *: Smectic C chiral phase SmA: Smectic A phase Ch: Cholesteric phase Iso: Isotropic liquid Moreover, the birefringence anisotropy (Δn) of this liquid crystal is obtained by using a Senarmont compensator. The result was 0.14. The paraelectric anisotropy (Δε) was measured by measuring the capacity of each cell of parallel alignment and vertical alignment using a Hewlett Packard LCR meter (HP4912A). The Δε of the ferroelectric liquid crystal used was +3.

The liquid crystal was injected into the above oblique vapor deposition cell, the panel was heated to 100 degrees to make an isotropic liquid, and then slowly cooled (0.6 degrees / min) to obtain a smectic C chiral phase monodomain. It was The cell thickness was 3 μm. The pretilt angle was measured with a nematic liquid crystal because it is difficult to measure the pretilt angle in the ferroelectric liquid crystal state. The measurement method was a method called null capacitance method. As a result,
It was found that the pretilt angle of the 85 degree vapor deposition cell was about 25 degrees.

Next, using this panel, a voltage-transmittance curve (hereinafter, B-
V curve) was measured. The optical experimental system used for the measurement of the BV curve is shown in FIG. In FIG. 11, white light emitted from the light source 111 passes through the polarizer 112 and enters the liquid crystal cell 113 as linearly polarized light, and then passes through the analyzer 114, is condensed by the condenser lens 115, and is detected by the photomultiplier tube 116. , Storage oscilloscope 117 measures as a BV curve. An arbitrary waveform can be added to the liquid crystal cell by the programmable pulse generator 118.

The AC stabilization effect was examined using such a measurement system. The results are shown in FIGS. 1 and 2. Here, FIG. 1 shows a ferroelectric liquid crystal panel having a vapor deposition angle of 85 degrees, and FIG. 2 shows a ferroelectric liquid crystal panel having a vapor deposition angle of 60 degrees. This will be described with reference to FIG.

FIG. 1 (a) shows the applied voltage waveform, and FIG. 1 (b) shows the corresponding luminance change.

The applied waveform is a pulse with a pulse height of 25V and a width of 1.0ms, followed by an AC continuous waveform of 20V and a width of 0.2ms, and after applying a pulse of -20V and a width of 1.0ms, an AC of + 20V and a width of 0.2ms.
A voltage waveform was used such that a continuous pulse (1 second) was applied. At this time, a pulse with a pulse width of 1.0 ms is a so-called selection pulse that determines the tilt direction of the ferroelectric liquid crystal molecules,
An AC continuous pulse with a width of 0.2 ms is related to the paraelectric anisotropy (Δε) that induces the AC stabilization effect.

In the case of the 85 degree vapor deposition of Fig. 1, when the + 25V selection pulse is applied, the brightness shows a bright state of 32%, and the brightness is kept as it is by the subsequent AC continuous pulse, and the reverse direction of -25V. The direction of the molecule is changed to the dark state (brightness 0.5%) by the selection pulse of, and this state is maintained by the AC continuous pulse. It can be seen that in a ferroelectric liquid crystal panel having such a large pretilt angle, a good memory effect can be obtained by using a ferroelectric liquid crystal material having a positive paraelectric anisotropy (Δε).

Next, the same experiment was conducted on the cell having the vapor deposition angle of 60 degrees in FIG. FIG. 2 (a) shows the applied voltage waveform,
FIG. 2 (b) shows the corresponding luminance change. As shown in Fig. 2, bright and dark states of 30% and 0.5% were obtained by the selective pulse, but no memory effect was observed by the AC continuous pulse. It is considered that this is because the pretilt angle is almost 0 degree in the case of 60 degree vapor deposition, and therefore Δε acts as a force in the direction in which the molecule stands up from the substrate surface. Since this effect is small in a ferroelectric liquid crystal panel having a small pretilt angle, it is considered that a pretilt angle of 10 degrees or more is usually required. Also, these effects are not observed with small Δε, which is greatly affected by the magnitude of Δε, and it is considered that at least +3 or more is necessary.

EFFECTS OF THE INVENTION The present invention uses a ferroelectric liquid crystal material having a positive paraelectric anisotropy (Δε) in a ferroelectric liquid crystal panel having a large pretilt angle, and has a good memory with a strong AC stabilization effect and a good display quality. A ferroelectric liquid crystal panel is provided.

[Brief description of drawings]

Figure 1 shows the AC of a ferroelectric liquid crystal panel that has been obliquely vapor-deposited at 85 degrees
FIG. 2 is a graph showing the relationship between the voltage waveform and the luminance for showing the stabilizing effect, and FIG. 2 is a graph showing the relationship between the voltage waveform and the luminance for showing the AC stabilizing effect of the ferroelectric liquid crystal panel which is obliquely vapor-deposited by 60 degrees. FIG. 3 is a schematic diagram of ferroelectric liquid crystal molecules, FIG. 4 is a schematic diagram showing a twisted structure of the ferroelectric liquid crystal, and FIG. 5 is a state in which the twisted structure is unraveled in a thin panel of the ferroelectric liquid crystal. FIG. 6 is a schematic diagram showing the operation principle in a ferroelectric liquid crystal panel having a thin cell thickness, FIG. 7 is a schematic diagram showing a vapor deposition apparatus and a vapor deposition method, and FIG. 8 is oblique vapor deposition. A schematic diagram showing the surface state at the time,
9 (a) and 9 (b) are schematic diagrams showing the alignment direction of liquid crystal molecules when the vapor deposition direction is changed, and FIG. 10 is a schematic diagram of the optical system used for the BV curve measurement of the examples. FIG. 11 is a block diagram of the obliquely vapor-deposited ferroelectric liquid crystal panel used in the examples. 82: Indicates the angle of oblique vapor deposition, 91: Indicates the orientation of molecules by oblique vapor deposition.

Front page continued (72) Inventor Hiroyuki Onishi 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. Reference JP 62-161122 (JP, A) JP 62-182720 (JP, A)

Claims (4)

[Claims] 1. A liquid crystal layer and a plurality of substrates, at least one of which is disposed so as to sandwich the liquid crystal layer, and which is transparent, and a voltage applying means attached to the substrate so that a voltage can be applied to the liquid crystal layer. In the provided liquid crystal panel, the liquid crystal has a tilt angle of 10 degrees or more with respect to the surface of at least one substrate in the panel, the liquid crystal is a ferroelectric liquid crystal, and the ferroelectric liquid crystal has a positive dielectric constant difference. A ferroelectric liquid crystal panel characterized by having memory property by having an orientation and applying an AC voltage to the panel. 2. The ferroelectric liquid crystal panel according to claim 1, wherein an inorganic substance is vapor-deposited from an oblique direction as a means for providing a tilt angle. 3. The ferroelectric liquid crystal panel according to claim 1, wherein the dielectric anisotropy is at least +3 or more. 4. A ferroelectric liquid crystal panel according to any one of claims (1), (2) and (3), characterized in that the thickness of the liquid crystal layer is 5 μm or less. .