JPH05333338A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To prevent the local attraction of impurity ions to the film surfaces of oriented films and to suppress the seizure of a display by attracting the impurity ions in a liquid crystal to nearly the entire part of the film planes of the oriented films and preventing the suspension thereof. CONSTITUTION:This liquid crystal display element is constituted by having a pair of transparent substrates 1, 2 facing each other, forming respectively transparent electrodes 5, 6 for display on the inside surfaces of these transparent substrates 1, 2 and the oriented films 7, 8 covering these electrodes 5, 6 for display and sealing the liquid crystal 4 in the spacing where the transparent substrates 1, 2 face each other. The parts of the non-crystallized regions of the oriented films 7, 8 of such element are removed by etching, by which the surfaces of the oriented films 7, 8 are roughened. The surface areas of the film surfaces of the oriented films 7, 8 are increased by the surface roughening.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device that displays an image through liquid crystal.

[0002]

2. Description of the Related Art Generally, a liquid crystal display device has a pair of transparent substrates arranged to face each other, and transparent electrodes for display and an alignment film covering these electrodes are formed on the inner surfaces of the transparent substrates. Moreover, liquid crystal is sealed between the opposing transparent substrates.

By the way, such liquid crystal display elements are currently mainly of TN type or STN type, but matrix liquid crystal display elements used in various OA equipments, television receivers and the like have a large screen, Since high time-division driving and high contrast are required, an active matrix system is adopted as a TN type liquid crystal display device. Further, although the STN type liquid crystal display element is a simple matrix type, in this STN type liquid crystal display element,
In order to improve the contrast when driven by a high time division, a liquid crystal having a rapid threshold characteristic is used to make the change of the light transmittance rapid with respect to the change of the applied voltage.

However, in the above active matrix TN type liquid crystal display element, the pixel electrode and its active element (thin film transistor etc.) are arranged on one of a pair of substrates facing each other with the liquid crystal layer interposed therebetween. Since the active element projects above the substrate, a step is formed on the film surface of the alignment film formed on the substrate, and the twist alignment state of the liquid crystal molecules becomes unstable. Note that some active matrix liquid crystal display devices have a non-rubbing film as an alignment film on a substrate on which an active device is formed in order to prevent dielectric breakdown of the active device due to static electricity generated during rubbing of the alignment film surface. In this liquid crystal display element, the alignment state of liquid crystal molecules becomes more unstable.

Further, since the STN type liquid crystal display element may be of the simple matrix type, no step is formed on the alignment film surface, but in this STN type liquid crystal display element, as described above, the threshold value is used as the liquid crystal. It is necessary to use a liquid crystal having rapid characteristics, and a liquid crystal having a rapid threshold characteristic has a low orientation, so that the twist alignment state of liquid crystal molecules becomes unstable.

Therefore, in the active matrix TN type liquid crystal display element and the STN type liquid crystal display element, the pretilt angle of the liquid crystal molecules is increased to stabilize the twist alignment state of the liquid crystal molecules.

[0007]

However, the liquid crystal display device in which the pretilt angle of the liquid crystal molecules is large as described above has a problem that display unevenness generally called "burn-in" occurs.

This is due to the influence of impurity ions in the liquid crystal. Impurity ions are almost evenly distributed in the liquid crystal enclosed in the liquid crystal display element. However, an alignment film that orients liquid crystal molecules at a large pretilt angle has a small surface energy and has a strong adsorption force for impurity ions. Since the liquid crystal display element using an alignment film having a large pretilt angle is weak, most of the impurity ions are distributed in a floating state in the liquid crystal, and when the liquid crystal display element drives the liquid crystal display element to display, It is locally adsorbed on the alignment film surface of the pixel portion to which the electric field is applied.

Such adsorption of impurity ions occurs especially when the liquid crystal display element is driven to display at a high temperature (45 to 50 ° C.), and if the same image is displayed for a long time at a high temperature, the Since the impurity ions adsorbed to the local part remain in the adsorbed state, the electro-optical characteristics of this part are changed, and a display burn-in phenomenon occurs.

The present invention has been made paying attention to such a point, and its purpose is to prevent impurity ions in the liquid crystal from being almost uniformly adsorbed on the entire film surface of the alignment film to prevent its floating. Therefore, it is an object of the present invention to provide a liquid crystal display element capable of preventing the local adsorption of impurity ions on the film surface of the alignment film and suppressing the occurrence of a display burn-in phenomenon.

[0011]

The film surface of the alignment film is originally in a rough surface state, but in the present invention, finer unevenness is formed in the rough surface portion to make a finer surface state. This significantly increases the total surface area of the alignment film surface, and by increasing this surface area, the impurity ions in the liquid crystal are almost uniformly adsorbed to the entire alignment film surface to prevent their floating. It was done. The means for roughening the film surface of the alignment film to increase the total surface area of the film is as follows.

Usually, a polyamic acid derivative compound is used as a material for the alignment film, a dilute solution of the polyamic acid derivative compound is applied onto the transparent electrode so as to cover the display electrode, and the applied film is dried. The polyimide film (orientation film) is formed by dehydration ring closure through the heat curing step.

Since the imidization temperature of the polyamic acid varies depending on the molecular structure of the material used, the film is formed under an appropriate curing condition, but the film is imidized or crystallized depending on the curing condition (temperature, time). Alteredly, the produced film has a crystallized region and a non-crystallized region.

The crystallized region and the non-crystallized region of the film are different from each other in chemical stability and physical strength. In the produced film, the region having higher crystallinity is the non-crystallized region. It has higher chemical stability and higher physical strength than

In the present invention, the film surface of the alignment film is roughened by utilizing such partial difference in the properties of the alignment film. That is, the non-crystallized portion of the alignment film, which has low chemical stability and weak physical strength, is removed by etching to roughen the film surface of the alignment film. It is designed to increase the total surface area.

[0016]

With the increase of the entire surface area of the film surface of the alignment film, the impurity ions in the liquid crystal enclosed in the liquid crystal display element are almost uniformly adsorbed to the entire area of the film surface of the alignment film, and the film of the alignment film is formed. Held on a face. Therefore, almost no impurity ions float in the liquid crystal. Therefore, when the liquid crystal display element is driven to display, there is no possibility that impurity ions are locally adsorbed on the film surface of the alignment film in the pixel portion to which an electric field is applied, and therefore the occurrence of the image sticking phenomenon is suppressed. be able to.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

The figure shows the structure of a liquid crystal display device. A pair of transparent substrates 1 and 2 made of glass or the like are arranged so as to face each other, and these transparent substrates 1 and 2 are adhered to each other via a sealing material 3. A nematic liquid crystal 4 is enclosed in a region surrounded by the inner surfaces of the transparent substrates 1 and 2 and the sealing material 3.
On the inner surfaces of the transparent substrates 1 and 2, display electrodes 5 and 6 formed of a transparent conductive film such as indium oxide, and polyimide alignment films 7 and 8 that cover these display electrodes 5 and 6 are provided, respectively. ing.

The display electrode 5 formed on one transparent substrate 1 is arranged at a right angle to the display electrode 6 formed on the other transparent substrate 2, and the one display electrode 5 and the other display electrode 5 are arranged. A pixel portion is a portion that vertically intersects with the electrode and faces each other. The alignment films 7 and 8 are formed of a polymer film having a small surface energy in order to increase the pretilt angle of the liquid crystal molecules and stabilize the twist alignment state of the liquid crystal molecules.

A color filter layer is formed between one transparent substrate 1 and the display electrode 5, and an insulating film layer is formed between each transparent substrate 1 and 2 and each alignment film 7 or 8. It may be

The alignment films 7 and 8 are obtained by coating a dilute solution of a polyamic acid derivative compound on the transparent electrodes 1 and 2 on which the display electrodes 5 and 6 are formed, and drying and heating the coating films. Is formed by.

The orientation films 7 and 8 formed in this way each have a myriad of minute crystallized regions and non-crystallized regions in their entirety. Since the non-crystallized region has lower chemical stability and weaker physical strength than the crystallized region, it is possible to remove the non-crystallized region by etching so that the entire film surface of the alignment films 7 and 8 is removed. The surface is roughened by forming fine irregularities. The etching process is performed by the following method, for example. (A) The non-crystallized regions on the film surfaces of the alignment films 7 and 8 are removed by etching with a hydrazine solvent. (B) The non-crystallized regions on the film surfaces of the alignment films 7 and 8 are removed by etching with a chromium mixed acid solution. (C) The non-crystallized regions on the film surfaces of the alignment films 7 and 8 are removed by plasma etching using oxygen plasma or the like. (D) Irradiation with strong UV (ultraviolet) decomposes and removes the non-crystallized regions on the film surfaces of the alignment films 7 and 8. (E) The non-crystallized regions on the film surfaces of the alignment films 7 and 8 are etched and removed by ion sputtering.

By etching and roughening the non-crystallized regions on the film surfaces of the alignment films 7 and 8 as described above, the total surface area of the film surfaces of the alignment films 7 and 8 is significantly increased. Impurity ions in the liquid crystal 4 enclosed in the liquid crystal display element are almost uniformly adsorbed on the entire film surfaces of the alignment films 7 and 8 and held on the film surfaces. Therefore, almost no impurity ions are contained in the liquid crystal 4. No longer floats.

Therefore, when the liquid crystal display device is driven for display, impurity ions are not locally adsorbed on the film surfaces of the alignment films 7 and 8 in the pixel portion to which an electric field is applied, and therefore the display is burned. The occurrence of sticking phenomenon can be suppressed.

[0025]

As described above, according to the present invention, the non-crystallized region of the alignment film is removed by etching to roughen the film surface of the alignment film. The surface area of the surface is increased and the impurity ions in the liquid crystal are almost uniformly adsorbed on the entire film surface of the alignment film. Therefore, the local adsorption of the impurity ions on the film surface of the alignment film is prevented, and the image sticking phenomenon occurs. This has the effect of suppressing the occurrence of

[Brief description of drawings]

FIG. 1 is a sectional view of a liquid crystal display element showing an embodiment of the present invention.

[Explanation of symbols]

 1, 2 ... Transparent substrate 4 ... Liquid crystal 5, 6 ... Display electrode 7, 8 ... Alignment film

Claims (1)

[Claims] 1. A pair of transparent substrates facing each other, wherein transparent display electrodes and an alignment film covering these display electrodes are formed on the inner surfaces of the transparent substrates, respectively, and between the transparent substrates. In a liquid crystal display device in which liquid crystal is enclosed, the non-crystallized region of the alignment film is removed by etching to roughen the film surface of the alignment film, and this roughening increases the surface area of the film surface of the alignment film. A liquid crystal display device characterized by the above.