JPH0731324B2 - Liquid crystal electro-optical device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a substrate surface treatment method in a liquid crystal electro-optical device using a ferroelectric liquid crystal having a memory effect, a high-speed response, and a steep threshold characteristic.

[Conventional technology]

The substrate surface treatment method in the conventional liquid crystal electro-optical device includes
A method of providing the same type of surface treatment layer of polyimide or the like on each of the upper and lower substrates, a method of not performing any treatment, and a surface treatment layer having different surface polarities on the upper substrate and the lower substrate as in the 10th Liquid Crystal Conference Proceedings 14A17 Or a method of applying polyimide to the surface of the substrate and then rubbing the same as in the nematic liquid crystal alignment treatment.

[Problems to be solved by the invention]

The smectic C ^* liquid crystal composed of chiral molecules exhibits ferroelectricity, and as shown in FIG. 1, the liquid crystal molecule 11 is always located on a cone with a tilt of θ from the Z-axis direction.
Has a permanent dipole 12 in a direction perpendicular to and parallel to the smectic layer (XY plane). Therefore, the position of the liquid crystal molecules 11 on the cone (azimuth angle φ) can be controlled by applying an electric field to change the direction of the permanent dipole 12. However, 13 is a projection of the liquid crystal molecule 11 on the XY plane.

The orientation of liquid crystal molecules in the XY cross section of a cell having a thickness of several μm is represented as shown in FIG. 2 (a), (b),
(C) shows the alignment of liquid crystal molecules when no electric field is applied, when an electric field + E is applied from the lower substrate to the upper substrate, and when an electric field -E is applied. However, FIG. 2 shows the orientation in a cell in which polyimide 15 is applied to the substrate surface. The permanent dipoles 12 are arranged from the substrate surface to the inside of the cell when there is no electric field.By changing the type of treatment agent on the substrate surface, the permanent dipoles from the inside of the cell to the substrate when there is no electric field. Such an orientation can also be obtained. The memory effect is evaluated by the orientation state when no electric field is applied. If the same orientation as when the electric field is applied as shown in FIGS. 2B and 2C is maintained after the electric field is removed, the memory effect is excellent. Is evaluated.

The orientation state in the absence of an electric field is strongly influenced by the thickness of the cell.
As shown in the figure, it is considered that various orientations are obtained depending on the cell thickness. The orientation states of FIGS. 3 (a) to 3 (c) are twist.
The state is called, and FIG. 3 (d) is called the untwist state.

Here, depending on the cell thickness, un
The reason why the twist state cannot be maintained is considered as follows. The force that liquid crystal molecules receive from the substrate surface is 2
There are different types, the force that tries to keep the liquid crystal molecules parallel to the substrate surface (anchoring force) and the interaction between the permanent dipole possessed by the liquid crystal molecules and the polarity of the substrate surface (Coulomb force). is there. 4th if only anchoring force is considered
The two orientation states shown in FIGS. (A) and (b) are both stable, and the same untwisted state as when the electric field is applied should be maintained after the electric field is removed. However, considering the Coulomb force, the energies of the orientations of FIGS. 4 (a) and 4 (b) are different, and the orientation of FIG. 4 (a) is most stable if the polarity of the substrate surface is positive. Therefore, in this case, if the electric field is removed after the liquid crystal molecules are aligned as shown in FIG. 4 (b) by applying an electric field, the liquid crystal molecules tend to be inverted to the alignment state shown in FIG. 4 (a).

The magnitude of the influence of the Coulomb force on the orientation can be estimated by measuring the polarization reversal current. FIG. 5 shows the polarization reversal current when a triangular electric field is applied to a cell having a thickness of 4 μm. When the orientation of the permanent dipole is reversed in response to an electric field, a polarization reversal current flows and a peak appears as shown in FIG. 5 (b). As can be seen from this figure, the direction of the electric field is permanent before it is reversed. The dipole reversal has begun. That is, when the electric field strength becomes smaller than a certain value, the inversion of the permanent dipole starts in order to move from the state of FIG. 4 (b) to the energetically favorable state of FIG. 4 (a) against the electric field. Therefore, the interaction between the permanent dipole and the polarity of the substrate surface is considered to be very strong.

In order to invert the liquid crystal molecules from the state shown in FIG. 4B to the state shown in FIG. 4A, the liquid crystal molecules are not inverted in a plane parallel to the substrate surface,
Since the light is rotated about the Z axis on the cone shown in FIG. 1 so as to be inverted, one end of the liquid crystal molecules that are initially aligned parallel to the substrate surface must be lifted from the substrate surface.
(See FIG. 3) It is considered that the amount by which one end of the liquid crystal molecule can float up from the substrate surface is determined by the balance between the anchoring force and the Coulomb force that the liquid crystal molecule receives. The longer the distance between the liquid crystal molecules and the substrate surface, the smaller the anchoring force, and as the cell becomes thicker, the distance between the liquid crystal molecules located at a certain distance from one substrate surface and the other substrate surface becomes longer. The anchoring force that the molecule receives from both substrate surfaces is smaller than in the case of a thin cell. Therefore, as the cell becomes thicker, the Coulomb force becomes more dominant than the anchoring force received by the liquid crystal molecules, and various twist states shown in FIG. 3 become stable depending on the cell thickness.

Up to this point, the phenomenon in the cell using the same kind of surface for the upper and lower substrates has been described. However, when the surfaces having different polarities are used for the upper substrate and the lower substrate, the un shown in FIG.
The twist state becomes stable. For example, the upper substrate surface is SiO
_In the cell in which the _two layers 16 are provided and the polyimide layer 15 is provided on the surface of the lower substrate, the energy of the alignment state shown in FIG. 6 (a) is the smallest and the most stable, but FIG. 6 (b). The orientation state shown in 1 becomes very unstable.

In addition to this, there is a treatment method in which a polyimide layer or the like is provided on the surface of the substrate and further rubbing treatment is performed. In this case, in addition to the anchoring force and the Coulomb force, the alignment force for controlling the alignment of the liquid crystal molecules in the rubbing direction is added, so that the memory effect is further difficult to obtain.

In order to maintain the untwisted state even after the electric field is removed by using the conventional substrate surface treatment method, the cell thickness must be sufficiently thin in any treatment method. The critical cell thickness at which the twist state does not appear after the electric field is removed depends on the substrate surface treatment method, and is the largest when the surface treatment layers of the same polarity are provided on both the upper and lower substrates and no rubbing treatment is performed. However, even in such a surface treatment method, the critical cell thickness is very thin and 1 μm or less, so it is very difficult to make such a thin cell with good uniformity even at a laboratory level, and mass productivity is low. The reality is that it is extremely uncertain.

Therefore, the present invention solves such a problem, and an object of the present invention is to provide a liquid crystal electro-optical device exhibiting an excellent memory effect even in a mass-produced cell having a thickness of several μm.

[Means for solving problems]

In the liquid crystal electro-optical device of the present invention, a ferroelectric liquid crystal is enclosed between a pair of substrates having electrodes on the inner surfaces, and polarizing plates are arranged outside the pair of substrates so that their polarization axes are substantially orthogonal to each other. In the liquid crystal electro-optical device, an electrode, a platinum layer, and a ferroelectric layer are laminated in this order on each substrate, and the thickness of the ferroelectric layer is thicker than that of the platinum layer. It is characterized by being

When a polymer ferroelectric is used as the ferroelectric layer, grooves parallel to the surface of the ferroelectric layer are formed by rubbing or etching, or in any case, it is left untreated.

[Action]

With the conventional method, the polarity of the substrate surface after the electric field is removed cannot be arbitrarily controlled, but according to the above configuration of the present invention, the permanent dipole possessed by the liquid crystal molecules is changed depending on the direction when the electric field is applied. Since the polarity of the surface of the substrate is reversed and the polarity is maintained even after the electric field is removed because of its ferroelectricity, an excellent memory effect can be obtained even in a mass-producible cell having a thickness of several μm.

〔Example〕

(Embodiment 1) FIG. 7 is a sectional view of a liquid crystal electro-optical device according to this embodiment. Reference numeral 17 is a transparent electrode, 18 is a platinum layer, 19 is a ferroelectric layer, 20 is a spacer, 21 is a liquid crystal layer, and the ferroelectric 19 and the liquid crystal 21 are in contact with each other as shown in the drawing. Ferroelectric liquid crystal 21 p-de
cyloxybenzyliden-p′‐amino-2-methylbutylc-innamat
Using e (DOBAMBC), PLZT (9/65/3
5) is used and its surface remains untreated. The transparent electrode 17 is SnO ₂ . The thicknesses of the liquid crystal layer 21, the ferroelectric layer 19 and the platinum layer 18 were 4.5 μm, 0.35 μm and 0.01 μm, respectively.

The PLZT thin film was prepared by high frequency sputtering. The substrate temperature is about 500 ° C., the sputtering gas is a 25/75 mixed gas of oxygen and argon, and the target is PLZT powder containing PbO in an excess of about 8%.

Fig. 8 shows the hysteresis characteristics (60 Hz) of the PLZT thin film with a thickness of 0.5 µm prepared by this method. The horizontal axis is 8.5v /
μm / div. Compared with the hysteresis characteristic of a single crystal ferroelectric, the thin film is slightly inferior in rectangularity because the crystallinity at the boundary between the ferroelectric layer 19 and the base (platinum layer 18) is poor. . That is, the boundary layer does not exhibit ferroelectricity, and the magnitude of polarization changes in accordance with the change in electric field strength, resulting in the characteristics shown in FIG. However, in the present invention, at least the surface layer of the ferroelectric material only needs to exhibit the ferroelectricity. Therefore, the thickness of the ferroelectric material is made thicker than that of the boundary layer with poor crystallinity, and the surface of the ferroelectric material is made thicker. If the layer has a ferroelectric property, it can be used for the purpose of the present invention. However, the thickness of this boundary layer depends on the substrate temperature during sputtering.

FIG. 9 shows an alignment model of liquid crystal molecules with respect to each electric field strength in this example. 18 is a platinum layer, 19 is a ferroelectric layer, and an arrow 22 is a permanent dipole of the ferroelectric surface layer. 9 (a) to 9 (e), the electric field strength is + 3E,
It corresponds to O, −E, −3E, O. From FIG. 9, the ferroelectric layer
By providing 19 on the upper and lower substrates, the same two untwisted states as when the electric field is applied (FIG. 9 (a),
It can be seen that (d)) becomes stable.

FIG. 10 is a diagram showing whether the memory effect is good or bad. FIG. 10A shows the applied pulse voltage, and FIGS. 10B and 10C show changes in the transmitted light intensity with respect to the applied pulse voltage. The peak values V ₁ and V ₂ of the applied pulse voltage are both shown in FIG. 9 (a),
It is a value that can completely switch between the two untwist states shown in (d). The memory effect is determined by the difference in brightness between when the pulse voltage is applied and when the voltage is removed. ID-ID ', IB-IB'
The smaller the value of ID-ID ′ and IB-IB ′, the better the memory effect. FIG. 10 (b) is a change in transmitted light intensity in this example. For comparison, FIG. 10 (c) shows a thickness of 1.5 μm formed by the conventional method.
The change in transmitted light intensity in the m cell was shown. Comparing the two, it can be seen that the memory effect obtained in this example is very excellent.

The threshold characteristic for a pulse voltage with a pulse width of 100 μsec is extremely steep, and V90 / V10 = 1.1 (Vα: transmitted light intensity is α%
The voltage required to change) was about the same as the steepness of the prior art. Also, the contrast (ID ′: I
B ') was obtained 1:20 using white light.

(Example-2) With the same configuration as the first example, the thicknesses of the liquid crystal layer and the ferroelectric layer were 3.0 μm and 0.8 μm, respectively. Also in this example, the memory effect was excellent, and V90 / V10 = 1.2 and the contrast was 1:23.

(Embodiment 3) With the same configuration as in the first embodiment, the thicknesses of the liquid crystal layer and the ferroelectric layer were 1.5 μm and 1.0 μm, respectively. Also in this example, the memory effect was excellent, and V90 / V10 = 1.1 and the contrast of 1:35 were obtained.

Example 4 In this example, a BaTiO ₃ sputtered thin film having a liquid crystal layer thickness of 5.2 μm and a ferroelectric layer thickness of 1.0 μm was used. Liquid crystal material S-4-0- (2-methyl) butyl-resorcylidene
-4'-alk-ylaniline (MBRA-8). The BaTiO ₃ thin film was sputtered at a substrate temperature of about 650 ° C and heat-treated at about 850 ° C for 4 hours. Also in this example, the memory effect was excellent, and V90 / V10 = 1.15 and the contrast was 1:23.

(Example-5) In this example, p-decyloxybe-nzylidene was used as the liquid crystal material.
-P'-amino-1-methylbutylc-innamate (DOBA-1
-MBC) is used as a ferroelectric and vinylidene fluoride /
An ethylene trifluoride (VDF / TrFE) copolymer was used. The thickness is 3.5 μm and 0.3 μm, respectively. VDF / TrFE thin film is a copolymer of composition 74/26 dissolved in methyl ethyl ketone,
It was created by spin coating. After film formation, rubbing treatment was performed, and poling was performed by applying an electric field of 150 MV / m at 100 ° C. Used in this example in FIG.
The hysteresis characteristic (60Hz) of VDF / TrFE thin film is shown. The horizontal axis is 40V / μm / div. Also in this example, the memory effect was excellent, V90 / V10 = 1.1, and the contrast was 1:25.

As described above, an excellent memory effect was obtained even in a thick cell of 1 μm or more. Ferroelectric material is PLZT, BaTiO
Not limited to ₃ , VDF / TrFE, similar results can be obtained by using other ferroelectric materials, and the film thickness is not limited. When creating a ferroelectric thin film, if the substrate temperature is too low, the resulting thin film does not show ferroelectricity, but the critical temperature depends on the type of the underlying substrate, and if platinum is used as the underlying substrate, the critical temperature Is relatively low. Therefore, in the above embodiment, the platinum layer 18 is provided as shown in FIG. 7, but the platinum layer is not always necessary. Further, when applied as a matrix addressing device, the electrodes have stripe shapes, but it is not necessary to pattern the ferroelectric layer into the same shape as the electrodes, and as shown in FIG. It should be provided.

〔effect〕

As described above, according to the present invention, the memory effect which has been obtained only in a thin cell having a thickness of about 1 μm or less is several μm.
Since it can be obtained also in the cell of m, there is an effect that it is possible to easily manufacture a large-area liquid crystal electro-optical device using a ferroelectric liquid crystal, which was impossible in the conventional technique.

[Brief description of drawings]

FIG. 1 is a diagram showing a positional relationship between liquid crystal molecules, a permanent dipole and the Z axis, and FIGS. 2 (a) to 2 (c) are diagrams showing liquid crystal molecule alignment in an XY cross section of a cell having a thickness of several μm. Figure (a)-
(D) is a diagram showing the orientation of liquid crystal molecules in cells of various thicknesses, and FIGS. 4 (a) and 4 (b) are diagrams showing the interaction between the permanent dipole of the liquid crystal molecules and the polarity of the substrate surface, FIGS. 5 (a) and 5 (b) are diagrams showing polarization reversal currents when a triangular wave electric field is applied, and FIGS. 6 (a) and 6 (b) use surfaces having different polarities for the upper substrate and the lower substrate. Diagram showing the alignment state at the time,
FIG. 7 is a diagram showing the structure of a liquid crystal cell according to the present invention, FIG. 8 is a diagram showing the hysteresis characteristics of a PLZT thin film prepared by sputtering, and FIGS. 9 (a) to 9 (e) are diagrams showing a ferroelectric layer. FIG. 10 is a diagram showing an alignment state of polarization of liquid crystal molecules and a ferroelectric with respect to each applied electric field in each case, FIGS. 10 (a) to 10 (c) are diagrams showing changes in transmitted light intensity of the liquid crystal with respect to an applied pulse voltage, and FIG. FIG. 4 is a diagram showing hysteresis characteristics of a vinylidene fluoride / ethylene trifluoride copolymer. 11 …… Liquid crystal molecule 12 …… Permanent dipole 13 …… Projection of liquid crystal molecule on XY plane 14 …… Glass substrate 15 …… Polyimide 16 …… SiO ₂ 17 …… Transparent electrode 18 …… Platinum 19 …… Ferroelectric 20 Spacer 21 Liquid crystal 22 Polarization of ferroelectric surface layer

Claims (1)

[Claims] 1. A liquid crystal electro-optical device in which a ferroelectric liquid crystal is sealed between a pair of substrates having electrodes on the inner surfaces, and polarizing plates are arranged outside the pair of substrates so that their polarization axes are substantially orthogonal to each other. In, on each of the substrate, an electrode, a platinum layer,
A liquid crystal electro-optical device comprising a ferroelectric layer laminated in this order, and the thickness of the ferroelectric layer is larger than that of the platinum layer.