KR100453937B1 - Method for dissolving a gray scale instability in a liquid crystal display using surface geometrical gratings and Liquid crystal display apparatus using the same - Google Patents

Abstract

The present invention relates to a method for eliminating gradation display instability of a liquid crystal display device using a surface geometry grating and a liquid crystal display device using the same. In particular, in order to derive a multi-domain in a TN liquid crystal display device by using an equipotential plane distortion phenomenon in the liquid crystal material by the surface geometry grating, an average voltage applied per unit thickness in the liquid crystal material is applied per unit thickness in the surface geometry grating. The surface geometry grating and the liquid crystal material are selected based on the value α scaled by the voltage. According to the method according to the present invention, by using a surface geometry grating having an appropriate physical property constant as the alignment layer of the TN liquid crystal display device, multi-domains are induced in the liquid crystal material without additional processing such as a rubbing process, and thus gray scale display stability is achieved. This can be greatly improved. Surface geometry gratings are fabricated using semiconductor technology, so they can be easily implemented with precise control of their parameters.

Description

Method for dissolving a gray scale instability in a liquid crystal display using surface geometrical gratings and Liquid crystal display apparatus using the same}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device and a method for solving the gray level display instability of the TN mode without adding a complicated process, and a liquid crystal display device employing the same.

In recent years, most liquid crystal display devices adopt a TN mode having excellent properties such as low voltage, low power consumption, and insensitivity to light wavelength.

However, the TN mode has a disadvantage in that the image quality is greatly deteriorated depending on the viewing angle, that is, the viewing angle. This is because the phase retardation of light used for the TN mode to act as an optical modulation device depends on the optical path. Gray scale instability, such as gray scale inversion, is a representative example.

1 is a diagram showing optical transmission characteristics at a positive vertical viewing angle of a conventional TN liquid crystal display device. Referring to FIG. 1, it can be seen that a gray scale display reversal phenomenon is observed at a viewing angle of 20 to 30 °.

In order to eliminate such gray scale display instability, new modes other than the TN mode have been studied, such as a vertical alignment mode (hereinafter referred to as a 'VA mode') or an In-Plane Switching (IPS) mode. There was also a study to improve the while adopting the same.

As a method for resolving gray scale display instability while adopting the TN mode as it is, two methods, a phase compensation method using a thin film and a mutual compensation method between domains through a multi-domain configuration, have been proposed. Among them, the multi-domain method is a method in which sub-pixel viewing angle characteristics are compensated for each other by appropriately forming subpixels having the same liquid crystal display mode but different orientations in one pixel. This multi-domain method is in principle suitable for obtaining symmetrical viewing angle characteristics. However, since subpixels having different orientation directions in each pixel are not naturally generated, in order to artificially form such subpixels, complex pixels such as a plurality of rubbing processes or a photoalignment method using a photoreactive polymer are used. The process must be added.

The present invention is to solve the problems of the prior art as described above, an object of the present invention, to solve the instability of gray scale display by applying a multi-domain in TN mode without the addition of a complicated process and applying the same A liquid crystal display device is proposed.

1 is a diagram showing optical transmission characteristics at a positive vertical viewing angle of a conventional TN liquid crystal display device;

2 is a view illustrating a one-dimensional surface geometry grating, showing two two-dimensional surface geometry gratings arranged vertically;

3 is a diagram showing an equipotential plane distortion phenomenon in a liquid crystal material by a surface geometric lattice;

4A is a photograph taken under a quadrature polarizer after driving the TN cell thus produced at a voltage of 1.82V. Figure 4b is a schematic diagram of Figure 4a,

FIG. 5 shows the optical transmission curve of the TN cell as shown in FIG. 4 implemented in Example 1 at a positive vertical viewing angle.

6 is an optical transmission characteristic chart according to the numerical analysis result according to Example 2;

7 is a diagram showing optical transmission characteristics according to Example 3. FIG.

In the present invention, in order to induce a multi-domain in the TN mode of the liquid crystal display device, the surface geometrical grating (Surface Geometrical Grating) uses the property of distorting the equipotential surface.

In addition, it is noted that the equipotential distortion of the surface geometry grating for inducing the multi-domain can be controlled by the property constants of the surface geometry grating and the liquid crystal material, and thus, the surface geometry grating and the liquid crystal for multi-domain induction. Define the relationship between each property constant of a substance.

In the present invention, in order to solve the gray scale display instability through multi-domain induction without a separate process, a liquid crystal display device using a surface geometry grating as an alignment layer of the TN mode is proposed.

Hereinafter, the method and apparatus according to the present invention will be described in detail.

FIG. 2 is a diagram illustrating a surface geometry grating, particularly showing an example of a one-dimensional surface geometry grating. One-dimensional surface geometry grating means surface curvature, which has its own period and whose thickness varies one-dimensionally. 2 shows two two-dimensional surface geometry gratings arranged vertically.

3 is a diagram showing an equipotential plane distortion phenomenon in the liquid crystal material by the surface geometry lattice. Specifically, the one-dimensional surface geometry grating as shown in FIG. 2 is disposed on both electrode substrates, and the equipotential surface in the liquid crystal material is shown in the structure composed of the liquid crystal material between each surface geometry grating. 3, it can be observed that the equipotential surface in the liquid crystal material indicated by the dotted line is distorted by the one-dimensional surface geometry lattice and the electric field indicated by the thick arrow is thus distorted.

In order to analyze the equipotential plane distortion caused by the surface geometry grating as observed in FIG. 3 and to apply it to inducing multi-domain in the TN mode of the liquid crystal display device, in the present invention, the average applied per unit thickness in the liquid crystal material It is proposed to set the voltage based on the scaled value of the applied voltage per unit thickness in the surface geometry grating. If this value is " α ", α is arranged as follows.

The total thicknesses of the liquid crystal layer and the surface geometry grating layer are referred to as l and g, respectively, the voltage applied to the liquid crystal layer is referred to as V _LC , the voltage applied to the surface geometry grating layer is referred to as V _SGG , and the total applied voltage is referred to as V _tot . . The total applied voltage V _tot is the sum of the voltage V _LC applied to the liquid crystal layer and the voltage V _SGG applied to the surface geometry grating layer. Assuming that no external source other than the total applied voltage (V _tot ) exists, and that the spontaneous polarization of the liquid crystal does not appear at the macroscopic magnitude, the electrical displacement vector is only in one direction (direction by the applied voltage, in this case the liquid crystal cell). Thickness direction), and its size (Dz) is uniform throughout the liquid crystal material.

In the surface geometry grating, since the property constant does not change due to external applied voltage, the voltage (V _SGG ) applied to the surface geometry grating layer can be obtained from the amount of charge accumulated per unit area in one substrate, and the amount of charge is the total capacitance. And the total applied voltage per unit area, and the total storage capacity can be thought of as a series connection of the storage capacity by the liquid crystal and the storage capacity by the surface geometry grating, so the voltage applied to the surface geometry grating layer (V _SGG ) is It may be represented as in Equation 1.

Similarly, the voltage V _LC applied to the liquid crystal layer may be expressed by Equation 2 below.

Accordingly, in the present invention, the value α representing the degree of equipotential surface distortion is a value obtained by scaling the average voltage applied per unit thickness in the liquid crystal material to the voltage applied per unit thickness in the surface geometry grating, that is, (V _LC / l) / ( V _SGG / g), it is summarized as Equation 3 below by Equation 1 and Equation 2 above.

In Equation 3, ε _SGG is the dielectric constant of the surface geometry lattice, and <ε _LC > is the dielectric constant of the spatially averaged liquid crystal.

In summary, according to Equation 3, in order to represent the equipotential plane distortion phenomenon in the liquid crystal material by the surface geometric lattice,? Set in the present invention is represented only by the material constants of the surface geometric lattice material and the liquid crystal material. This means that inducing a multi-domain in the TN liquid crystal display device using the equipotential plane distortion phenomenon in the liquid crystal material by the surface geometric lattice can be controlled by selecting the liquid crystal material and the material constituting the surface geometric lattice. do.

On the other hand, in the case where α becomes larger than 1 and smaller than 1, the equipotential surface distortion phenomenon increases in both cases. For example, when α is much larger than 1, the equipotential surface is deformed according to the shape of the surface geometry grating, resulting in a corresponding fringe electric field. This is the case when the surface geometry grating is made of a conductor.

However, in the case where α is smaller than 1 as shown in FIG. 3, not only the fringe electric field effect caused by the surface geometry grating but also the effective voltage applied in the liquid crystal material along the grating vector direction of the surface geometry grating is changed. It is suitable for multi-domain induction in TN liquid crystal display devices. According to Equation 3, α is expressed as the ratio of the dielectric constant of the surface geometry grating to the dielectric constant of the liquid crystal material. As the dielectric constant of the surface geometry grating is smaller than the dielectric constant of the liquid crystal material, α becomes smaller than 1 so that the liquid crystal is It is possible to make a large change in the effective voltage induced in the material.

In the TN mode the liquid crystal material is twisted 90 degrees horizontally. Under these conditions, the liquid crystal material changes the mid-plane tilt of the intermediate layer director in a small voltage region according to the typical form of cooperative phenomenon observed in statistical physics after the Freedericksz transition. Therefore, it is possible to simultaneously present a small change in the effective voltage induced in the liquid crystal by the surface geometry grating and a tilt angle of the very large difference in the intermediate layer director. Multi-domains are obtained through these simultaneous interlayer directors.

The coexistence of the tilt angles of the different interlayer directors thus derived, that is, the multi-domain, may contribute to stabilization of the gray scale display in the TN mode. The biggest cause of the deterioration of the altered display stabilization in the TN mode is that a uniform intermediate layer director angle is formed in the positive vertical viewing angle direction immediately after the predrip transition. The phase delay of the light almost disappears in such a direction that the uniform interlayer director angle is formed, and a large valley is formed in the optical transmission curve around this direction, and these valleys of the optical transmission curve according to each applied voltage It is to bring the gray scale display inversion as they cross each other (see Fig. 1).

However, if two or more interlayer director angles coexist and then maintain a large difference between the two, the valleys of the optical transmission curves by each exist simultaneously and are far from each other. The overall optical transmission curves are given by their arithmetic mean, resulting in a mutual compensation effect between the valleys. This causes the optical transmission curve not to have a special valley, consequently contributing to stabilization of the gradation display.

As previously studied as another property of the surface geometry grating, there is an orientation effect. The orientation phenomenon of the surface geometry grating is found to be due to the tendency of minimizing the elastic deformation induced in the liquid crystal material by the surface geometry grating as an effect of aligning the liquid crystal director in the vertical direction of the grating vector direction.

In the present invention, the alignment film is used in the TN mode of the liquid crystal display device by using the multi-domain effect of the surface geometry grating proposed in the present invention and the alignment effect, which is a conventional property of the surface geometry grating, in the surface geometry grating. It is possible to improve gradation display stability by replacing with.

Determining the orientation effect of the surface geometry grating is a period of the surface geometry grating, specifically, the period has an orientation force of 1 µm or less. On the other hand, since the period for determining the orientation force of the surface geometry grating is independent of the dielectric constant of the surface geometry grating, by selecting the surface geometry grating material having the appropriate dielectric constant and setting the period of the surface geometry grating, the orientation effect and multi- It is possible to set up a surface geometry grating that can achieve both domain induced effects.

In the following examples, an experiment for solving the gray scale display instability in the TN mode of the liquid crystal display device using the surface geometry grating according to the method according to the present invention will be described.

Example 1

Using a photoresist material, AZ-6612 (Clariant Co.) (a dielectric constant of 5.1 at an operating frequency of 1 KHz), a photolithography process was used to implement a one-dimensional surface geometry grating with a period of 6 µm and an amplitude of 400 nm. This property constant causes the orientation by the surface geometry grating to correspond to the "soft anchoring" region. Therefore, when the torsional elastic deformation of the liquid crystal is induced by the implemented surface geometry grating, there is a difference between the alignment axis required by the surface geometry grating and the alignment axis of the actual liquid crystal. However, when considering the orientation mechanism of the surface geometry grating, the surface geometry It is expected that by reducing the period of the lattice 2 to 3 占 퐉, the difference between the ideal alignment axis and the actual alignment axis can be eliminated. In the first embodiment, the material and period of the surface geometry grating were selected first, and according to the method of the present invention, a liquid crystal material having a dielectric constant of? Value smaller than 1 is selected. In Example 1, ZLI-4900-100 (Merck Co., horizontal and vertical dielectric constants 37.7 and 7.9, respectively) was selected. For this reason, α has a value of 0.135 to 0.645 for the distribution of all liquid crystal directors.

In order to measure the gradation display instability in the TN mode of the liquid crystal display device, instead of the alignment layer of the TN cell, the two-dimensional surface geometry gratings implemented above were subjected to a cell-spacing of 5.4 μm so that the grating vectors were perpendicular to each other. It was placed while maintaining to obtain a TN cell. Then, the liquid crystal material ZLI-4900-100 selected above was injected by capillary action to have an appropriate value of α.

4A is a photograph taken under a quadrature polarizer after driving the TN cell thus produced at a voltage of 1.82V. FIG. 4B is a schematic diagram of FIG. 4A, showing that the V domains having the largest effective liquid crystal voltage appear as the darkest regions, and the P domains having the smallest effective liquid crystal voltage appear as the brightest points. In addition, since the two one-dimensional surface geometry grid is placed vertically, it shows that the M domain of the medium brightness appears. That is, Figure 4 shows the formation of a multi-domain by the surface geometry grating in the method according to the invention.

5 is a result of measuring the optical transmission curve of the TN cell as shown in FIG. 4 implemented in Example 1 at a positive vertical viewing angle. It can be seen from FIG. 5 that the optical transmission curve at each driving voltage has a high gradation display stability without intersecting with each other up to a 60 ° vertical viewing angle region. Such gradation display stability is a level comparable with gradation display stabilization by the phase compensation film used in the conventional TN mode. That is, in the method according to the present invention, the gray scale display is selected by using only the surface geometry grating instead of the alignment layer of the TN mode without using a special process, and adjusting the used surface geometry grating and the physical properties constants of the liquid crystal material. It solves instability.

Example 2

Through the numerical analysis on the optical transmission curve of the TN cell as shown in FIG. 5 obtained as a result of Example 1, the method proposed in the present invention will be verified.

Given that light sources of liquid crystal displays generally do not have interference effects and that the response of each domain to the applied voltage is independent, the optical transmission curve of the entire TN cell is given by the arithmetic mean of the optical transmission curves of each domain. In particular, when there are no geometric singularities in the surface geometry grating, the physical properties of the M domain have a median value between the V domain and the P domain, so that the total optical transmission characteristic is calculated only by the arithmetic mean of the optical transmission characteristics of the V domain and the P domain. Can be interpreted

The V and P domains can be modeled with a twisted liquid crystal alignment structure induced by vertically arranged surface geometric gratings. In particular, since the orientation by the surface geometry grating belongs to the soft anchoring region, a difference of about 5 ° between the ideal orientation axis and the actual orientation axis is identified, so consider this. In order to model the multi-domain effect naturally induced in the liquid crystal material, it was considered that there is a difference between the liquid crystal layer thickness and the effective liquid crystal voltage between the V domain and the P domain. Since the thickness of the liquid crystal layer of the P domain is the same as the cell spacing, the thickness of the liquid crystal layer of the V domain is 6.2 μm because the size of the surface geometry grating used is 400 nm. After obtaining the relationship of the spatially averaged dielectric constant (<ε _LC >) of the liquid crystal material to the effective liquid crystal voltage (V _LC ) of the liquid crystal layer having such a thickness, substitute the corresponding applied voltage ( V _tot ) From the inverse function of this relationship, the effective liquid crystal voltage of each domain with respect to the applied voltage is obtained, and then the liquid crystal director distribution of the V domain and the P domain is obtained based on this. After applying the 2x2 extended Jones matrix method to the obtained liquid crystal director distribution, the optical transmission characteristic of each domain was calculated | required, and the arithmetic mean was performed based on appropriate weight. The weight of each domain is determined by the ratio of the area occupied by each domain. In industrial applications, appropriate values can be obtained by adjusting the parameters of the surface geometry grating (such as the ratio of peak to valley width). In the numerical analysis of the second embodiment, weights of 0.72 and 0.28 were respectively assigned to the V domain and the P domain based on the ratio of each domain region shown in FIG.

6 is an optical transmission characteristic chart based on the numerical analysis results according to Example 2. FIG. A comparison of the diagrams of FIG. 6 by numerical analysis and FIG. 5 by experiment shows a very high level of agreement. This means that the method according to the invention well describes the multi-domain effect induced in the liquid crystal material by the surface geometry grating.

Example 3

As a result of numerical analysis in Example 2, it can be seen that when using a liquid crystal material having a very high dielectric anisotropy, such as ZLI-4900-100, various side effects occur. Since the liquid crystal material having a large dielectric anisotropy generally has a large refractive index anisotropy, the phase retardation greatly changes along the optical path. This large phase delay change causes fluctuations in the optical transmission curves corresponding to 1.68, 1.82, and 2.82 (V) in FIG. 5, which is undesirable from the viewpoint of gray scale display stabilization. In addition, in the case of a liquid crystal material having a high dielectric anisotropy, a Frederician transition phenomenon is exhibited at a very low applied voltage. As the Frederician transition phenomenon appears in a region of a high applied voltage, a larger liquid crystal effective voltage has a modulation and thus mutual compensation. It has been found that the effect can be increased.

Therefore, in the third embodiment, the multi-domain is induced by ZLI-2293 (the horizontal and vertical dielectric constants of 14.1 and 4.1, respectively), which is a liquid crystal material having general dielectric anisotropy, and thus it is possible to stabilize the gray scale display of the TN mode. Whether or not, through the numerical analysis, which has been verified in Example 2 will be found.

In the method according to the present invention, α should be smaller than 1, so that the size and dielectric constant of the surface geometry grating were selected as 400 nm and 3.1, respectively. On the other hand, it is assumed that the orientation by the surface geometry grating belongs to the hard anchoring region. 7 is a diagram showing optical transmission characteristics according to Example 3. FIG. FIG. 7 shows the gradation display stability more improved than that of FIG. From the results of the third embodiment, it is possible to achieve gradation display stabilization without any additional process by adopting a properly designed surface geometry grating as an alignment layer even in TN mode employing a general liquid crystal material in the method according to the present invention. Able to know.

As described above, according to the method according to the present invention, by using a surface geometry grating having an appropriate physical property constant as the alignment layer of the TN mode, the multi-domain is induced in the liquid crystal material without any additional process such as a rubbing process. Thus, gradation display stability can be greatly improved. Surface geometry gratings are fabricated using semiconductor technology, so they can be easily implemented with precise control of their parameters.

In addition, since the multi-domain induction in the method according to the present invention takes place naturally without further processing, it may be borrowed in parallel to the conventional multi-domain method. In other words, in the TN mode adopting the conventional multi-domain method, if the alignment layer is replaced with the surface geometry grating, the multi-domain is induced in each sub-pixel by the method according to the present invention, thereby further improving the viewing angle characteristic. It is expected.

Claims (23)

A manufacturing method of a liquid crystal display device capable of inducing multi-domain of a TN cell of a liquid crystal display device by using an equipotential plane distortion phenomenon in a liquid crystal material by a surface geometry lattice, thereby eliminating gray scale display instability. (A) defining a value α obtained by scaling an average voltage applied per unit thickness in the liquid crystal material to a voltage applied per unit thickness in the surface geometry lattice by Equation 3a; (B) selecting a surface geometry lattice material and a liquid crystal material each having a dielectric constant that makes the α value less than 1; (C) forming a one-dimensional surface geometry grating having a predetermined period and amplitude in the selected surface geometry grating material; And And arranging the pair of formed surface geometry gratings so that the grid vectors are perpendicular to each other, and then injecting the selected liquid crystal material between the pair of surface geometry gratings. Manufacturing method of display device: Equation 3a In the above formula, ε _SGG is the dielectric constant of the surface geometry lattice material, <ε _LC > is the dielectric constant of the spatially averaged liquid crystal. delete delete delete The method of claim 1, In the surface geometry grating, the period of the surface geometry grating for determining the orientation force is selected independently of the dielectric constant of the surface geometry grating for determining α, the manufacturing method of the liquid crystal display device using the surface geometry grating. The method of claim 1, wherein in step (b), In order that the value of α is smaller than 1, the dielectric constant of the surface geometry grating material is first selected, and then a liquid crystal display having a dielectric constant of the α value smaller than 1 is selected. Method of manufacturing the device. The method of claim 1, wherein in step (b), In order to make the α value smaller than 1, a liquid crystal material having a specific dielectric constant is first selected, and then a surface geometry lattice material having a dielectric constant that makes the α value smaller than 1 is selected. Method of manufacturing a liquid crystal display device. The method of claim 5, The period of the surface geometry grating is less than 1㎛ level manufacturing method of the liquid crystal display device using the surface geometry grating. A liquid crystal display device using a surface geometry grating inducing a multi-domain of a TN cell of a liquid crystal display device as an alignment film in a TN mode, And a liquid crystal and a pair of surface geometry gratings selected to make the value α defined by Equation 3a less than 1 as a value obtained by scaling the average voltage applied per unit thickness in the liquid crystal material to the voltage applied per unit thickness in the surface geometry grating. , Wherein said surface geometry grating is a one-dimensional surface geometry grating: Equation 3a In the above formula, ε _SGG is the dielectric constant of the surface geometry lattice material, <ε _LC > is the dielectric constant of the spatially averaged liquid crystal. 10. The liquid crystal display device according to claim 9, wherein the pair of surface geometry gratings are arranged such that each grating vector is perpendicular to each other. delete delete delete delete delete delete The method of claim 9, The period of the surface geometry grating is characterized in that less than 1㎛ level. A liquid crystal cell having a pair of alignment layers and a layer of liquid crystal material encapsulated between the pair of alignment layers, and a driving device for applying a driving voltage for driving the liquid crystal cell to both ends of the liquid crystal cell; In the liquid crystal display device, Fine surface geometry gratings are formed on at least a part of at least one of two sides of the pair of alignment layers facing the liquid crystal layer, The liquid crystal molecules of the liquid crystal layer are oriented by the fine surface geometry gratings, The average value per unit thickness of the partial voltage applied to the liquid crystal layer among the total voltages applied to the total thickness of the pair of alignment layers in which the liquid crystal layer is encapsulated, and the partial voltage applied to the pair of alignment layers among the total voltages. A value divided by a value per unit thickness of a liquid crystal display device in which a value α defined as in Equation 3a is set to be smaller than 1: Equation 3a In the above formula, ε _SGG is the dielectric constant of the surface geometry lattice material, <ε _LC > is the dielectric constant of the spatially averaged liquid crystal. A liquid crystal cell having a pair of alignment layers and a layer of liquid crystal material encapsulated between the pair of alignment layers, and a driving device for applying a driving voltage for driving the liquid crystal cell to both ends of the liquid crystal cell; In the liquid crystal display device, Fine surface geometry gratings are formed on at least a part of at least one of two sides of the pair of alignment layers facing the liquid crystal layer, The liquid crystal molecules of the liquid crystal layer are oriented by the fine surface geometry gratings, A value obtained by dividing a value of the dielectric constant of the alignment layer by a spatially averaged value of the dielectric constant of the liquid crystal layer, wherein a value α defined as in Equation 3a is set to be smaller than 1: Equation 3a In the above formula, ε _SGG is the dielectric constant of the surface geometry lattice material, <ε _LC > is the dielectric constant of the spatially averaged liquid crystal. 20. The liquid crystal display device according to claim 18 or 19, wherein the layer of liquid crystal material is a liquid crystal layer of twisted nematic (TN) mode, and the fine surface geometry gratings are arranged one-dimensionally. 21. The method of claim 20, wherein fine surface geometry gratings are formed on both sides of the pair of alignment layers facing the liquid crystal layer, and one of the fine surface geometry gratings formed on one side of the two sides. Wherein the dimensional array direction is orthogonal to each other with the one-dimensional array direction of the fine surface geometry gratings formed on the other side of the two sides. delete 20. The liquid crystal display device according to claim 18 or 19, wherein the period of the fine surface geometry gratings is less than 1 mu m level.