JP3529847B2 - Liquid crystal display - Google Patents

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.

[0002]

2. Description of the Related Art Liquid crystal display devices having the features of thinness, light weight, and low power consumption are widely used as display elements for office automation equipment such as Japanese word processors and desktop personal computers. Among such liquid crystal display devices, a liquid crystal display device called nematic liquid crystal, which uses a light modulation characteristic due to a twisted arrangement of liquid crystal molecules, is generally put into practical use. As the display method,
It can be roughly classified into two methods, that is, an optical rotation mode and a birefringence mode.

The LCD in the optical rotation mode is, for example, a twisted nematic (TN) having a molecular arrangement twisted by 90 degrees.
Type liquid crystal, which basically displays black and white from the operating principle, shows a high contrast ratio and good gradation display performance, and has a fast response speed (about several 10 ms), A segment type digital display panel, an image display panel driven by an active matrix drive equipped with a simple matrix drive or a switching element for each pixel used as a display device of OA equipment, and a full color display in combination with a color filter It has been applied to liquid crystal televisions and the like. On the other hand, a liquid crystal display device of a birefringence mode display method generally has a molecular arrangement twisted by 90 degrees or more and has steep electro-optical characteristics, so that a switching element such as a TFT element or a diode element is not formed in each pixel. In addition, even with a structure in which electrodes are arranged opposite to each other on a simple matrix, a large capacity display can be easily realized by the next division driving. A liquid crystal of such a mode is generally called a super twisted nematic type liquid crystal (STN type liquid crystal), but in order to make the display of such a liquid crystal display device uniform over the entire screen, liquid crystal molecules are evenly distributed over the entire surface of the substrate. Orientation is required.

At this time, the liquid crystal display device holds a liquid crystal composition in a gap between electrodes of two substrates and applies a voltage from the electrodes of the two substrates to the liquid crystal composition for display. Therefore, it is necessary to give a pretilt angle to liquid crystal molecules in advance in order to perform uniform display when a voltage is applied.

In order to give such a pretilt angle to the liquid crystal molecules, a technique is generally used in which an alignment film having a surface in contact with the liquid crystal molecules is subjected to an alignment treatment on the substrate surface. As such an alignment film, an organic polymer thin film typified by polyimide is formed on a substrate by a method such as spin coating or printing, and the upper surface of the film is lightly rubbed with a cloth or the like to impart liquid crystal alignment ability. The rubbing method is generally used as a technique suitable for mass production. Improvement of yield in the alignment treatment step by such a rubbing method and suppression of tilt reverse alignment failure,
In order to reduce the driving voltage of the liquid crystal display device, it is desirable to increase the pretilt angle to some extent.

However, in order to obtain such a relatively large pretilt angle, a polymer material having a long-chain alkyl, for example, is preferably used as the alignment film material. There is a problem that the stability against humidity and humidity is relatively low and the reliability is poor, and the workability of coating on the substrate is extremely poor.

Alternatively, a method of controlling the pretilt angle by changing the manufacturing conditions such as the forming temperature of the alignment film and the rubbing treatment conditions has been proposed. However, in such a method, the control range of the pretilt angle is small, and There is a problem that its reproducibility is poor.

Further, a method of obtaining a large pretilt angle by, for example, making two regions having different alignment capabilities separately on a substrate and vertically aligning one of them (Japanese Patent Laid-Open No. 63-141).
23, 63-14124) has been proposed, but such a method has a problem that it is extremely unpractical because it requires complicated steps for manufacturing an alignment film and alignment treatment.

Further, in the prior art, the pretilt angle is set to be almost the same on any part of the substrate. For example, in the case of a TN type liquid crystal display device, since the rising directions of liquid crystal molecules when the same voltage is applied are almost the same in any part of the display area on the substrate, the viewing angle characteristics are asymmetric with respect to the viewing direction. Therefore, there is a problem that the luminance distribution is biased and the viewing angle dependency is large.

For the purpose of solving this problem, one pixel of the liquid crystal display device is divided into a plurality of regions and the pretilt angle between the upper and lower substrates is made different for each of the plurality of regions, so that the liquid crystal molecules at the time of voltage application are A method has been proposed in which the rising direction is made different for each divided region and the viewing angle characteristics are made symmetrical with respect to the viewing direction to reduce the viewing angle dependency.

[0011]

However, in the above-mentioned conventional technique, it is actually extremely difficult to form a plurality of regions having different pretilt angles on the same substrate. That is, it is extremely difficult to form two types of alignment films, an alignment film having a low pretilt angle and an alignment film having a high pretilt angle, due to the problems of process matching and solvent resistance. In addition, by forming an alignment film with a high pretilt angle, that portion becomes less durable than an alignment film with a low pretilt angle, and the quality of the liquid crystal display device is improved in terms of reliability and durability of the completed liquid crystal display device. There is a problem of decrease.

The present invention has been made to solve such a problem, and an object thereof is to use an alignment film having a low pretilt angle with good stability which is used as a conventional alignment film material. A high pretilt angle and an improved degree of freedom in its control are realized without complicating and complicating the process, and lowering the reliability and durability of the liquid crystal display device, and the alignment stability is good and An object of the present invention is to provide a liquid crystal display device with high image quality that can be driven with a low drive voltage.

[0013]

In the liquid crystal display device of the present invention, two substrates having an alignment film on the surface of which a rubbing alignment treatment is applied are arranged so that the alignment films face each other with a gap. placement and, in a liquid crystal display device formed by sandwiching a liquid crystal composition sealed around said gap, said front Symbol rubbing orientation treatment of at least one substrate out of two substrates
Is decorated with Oriented film, wherein on the side in contact with the liquid crystal composition, in the projection plane shape with respect to the substrate principal plane rectangular, trapezoidal the cross-sectional shape and the bottom of the substrate side, is lower than the gap height,
And the edge portion is also vertical to the rubbing direction is characterized by having convex portions of the flat row.

It should be noted that the height of the above-mentioned convex portion is formed so as to be lower than the gap between the two substrates which are arranged to face each other, that is, the cell gap, and the electrodes which are respectively arranged so as to face both substrates. Needless to say, it is necessary to prevent them from touching each other, but it is particularly desirable to keep the cell gap (that is, the thickness of the liquid crystal layer) at a level that does not affect the retardation of the liquid crystal layer. .

The above-mentioned convex portion may be formed integrally with the alignment film as a part of the alignment film, or may be formed separately from the alignment film on the substrate and then the alignment film is formed thereon. Alternatively, the upper surface of the alignment film may be formed so as to have a surface protruding in a shape along the convex portion.

Further, the method of forming the above-mentioned convex portion is, for example, as follows.
A material film of an inorganic substance such as SiO _x or SiN _x or a polymer compound is formed on a substrate, a resist made of a photosensitive polymer substance such as a photosensitive resist is applied on the film, and a photomask or the like is used for this. Exposure and development are performed to form a resist pattern. Then, by using this resist pattern as a mask during etching, the material film of the above-mentioned inorganic substance or polymer compound is patterned by a photolithography method to obtain the above-mentioned convex portion, which can be formed by a manufacturing method.

The arrangement density of the convex portions on the main surface of the substrate is 1 × 10 ² pieces / mm ² to 1 × 10 ⁷ pieces / m 2.
It is desirable to arrange at a density of m ² .

The projection plane shape of the convex portion is rectangular as described above, but the three-dimensional shape, that is, the cross-sectional shape is rectangular or rectangular as long as it does not cause alignment disorder of liquid crystal molecules. It may have a rectangular cross section such as a trapezoid,
Alternatively, the cross-sectional shape may be an arc shape or a polygonal shape. Needless to say, the cross-sectional shape may be variously changed as long as it does not cause the alignment disorder of the liquid crystal molecules.

[0019]

On the substrate of the liquid crystal display device according to the present invention, a large number of projections having a rectangular projection plane shape are formed, and an alignment film having a surface subjected to a rubbing alignment treatment is formed. When a liquid crystal composition is brought into contact with a substrate provided with such an alignment film, the liquid crystal molecules generate a larger pretilt angle than in the case where the same alignment film is formed on a flat substrate and subjected to rubbing treatment. The present inventors have confirmed by experiments that it is possible.

The reason why the pretilt angle changes to a large angle according to the present invention is considered to be that the alignment regulating force of the surface of the convex portion according to the present invention is different from that of the flat portion, and the density or shape of the convex portion is changed. The pretilt angle can be controlled with a high degree of freedom. By arranging a large number of edge portions of the convex portions of the alignment film according to the present invention so as to be perpendicular or parallel to the rubbing direction, the convex portions can regularly control the alignment of liquid crystal molecules.

When the projection plane shape of the convex portion on the main surface of the substrate is not the above-mentioned rectangular shape but is, for example, a circle or a triangle, a portion which is not perpendicular or parallel to the rubbing direction as shown in FIG. Will occur. That is, alignment disorder (so-called disclination) 701 may occur in the alignment of the liquid crystal molecules in this portion, and as a result, the pretilt angle may be reduced as a whole.

Considering the simplification of manufacturing of the liquid crystal display device and the design margin in manufacturing, it is desirable that the difference in pretilt angle between the convex portion and the flat portion is large. Therefore, the shape of the convex portion is projected onto the main surface of the substrate. It can be said that it is most effective to form the planar shape into a rectangle (or a square; a square can be considered as a kind of rectangle) and then perform rubbing alignment treatment in parallel to the two opposite sides of this rectangle. .

The liquid crystal display device of the present invention having the alignment film having such a convex portion has a high pretilt stability of liquid crystal molecules and displays at a low driving voltage even though the alignment film material having a low pretilt angle is used. It is a liquid crystal display device that can realize high-quality images and has high reliability.

Further, each pixel is divided into two or more regions, and in each region, a convex portion is provided in one region and no convex portion is provided in the other region, or a convex portion is formed in each of a plurality of divided regions. It is also possible to form a dual domain liquid crystal display device, for example, by changing the pre-tilt angle in each of the divided regions by changing the aspect ratio, the size, the density, etc. of the rectangular portion.

[0025]

Embodiments of the liquid crystal display device of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a perspective view showing an outline of the structure of a substrate having an alignment film in a liquid crystal display device according to the present invention. Here, for simplification of the description, the structure will be described focusing on the alignment film according to the present invention and following the manufacturing process thereof.

The transparent electrode 2 is formed on the glass substrate 1.
A SiO _x film 3 is formed on the glass substrate 1 by sputtering to a thickness of 0.1 μm. Then, a resist material solution made of positive photosensitive polyimide is spin-coated with a spin coater, dried and pre-baked to form a resist film 4 before exposure.

Using a photomask 5 in which a rectangular repeating pattern having a long side of 20 μm and a short side of 5 μm is arranged on the resist film 4, an i-line is applied to the resist film 4 by using, for example, a proximity exposure apparatus. The mask pattern was transferred by exposure. The rectangular repeating pattern of this mask pattern was arranged at a density of about 1 × 10 ⁶ pieces / mm ² in this embodiment. That is, the arrangement density of the projections having a rectangular projection plane shape was set to about 1 × 10 ⁶ pieces / mm ² .

Thereafter, the resist film 4 is developed to obtain a resist pattern 6 having a rectangular projection plane shape, which is made of polyimide and has substantially the same shape as the mask pattern of the photomask 5. This mask pattern 6 is shown in FIG. In FIG. 2, the light-shielding pattern of the mask is shown by hatching for easy understanding of the drawing.

Next, a portion of the surface of the SiO _x film 3 to be an alignment film on the glass substrate 1 protruding from the resist pattern 6 is etched with a CF ₄ + O _{2 type} gas to form SiO _x.
On the main surface of the film 3, a convex portion 7 whose projection plane shape is substantially rectangular is formed. It goes without saying that various conditions for the above etching are set so that the convex portion 7 can be formed in the most accurate shape. The shape of one of the protrusions 7 is shown in FIG.
Shown in. As is apparent from FIG. 3, the projection 7 has a rectangular projection plane shape, and its end face (and cross section) shape is formed in a trapezoidal cross section close to a rectangle.

The rectangular convex portion 7 is formed on the glass substrate 1
As mentioned above, a large number of them are arranged at a density of about 1 × 10 ⁶ pieces / mm ² . A horizontal alignment film made of a polymer is formed on this, and the surface thereof is subjected to rubbing alignment treatment to obtain an alignment film on the counter substrate side.

On the other hand, a large number of rectangular protrusions (not shown) similar to the above are formed on the alignment film on the TFT substrate (not shown) side facing the glass substrate 1 by the same forming method as described above. Alternatively, the rectangular convex portion on the TFT substrate side is formed in a rectangular projection plane shape using SiO _x in the same manner as described above, and an organic material film such as polyimide is formed so as to cover almost the entire surface of the substrate including the rectangular convex portion. May be formed, and the surface thereof may be subjected to rubbing alignment treatment. As the alignment film, AL-1051 (manufactured by Japan Synthetic Rubber Co., Ltd.) was formed as the alignment film, and the surface thereof was rubbed to form the alignment film. Then, the TFT substrate and the counter substrate facing the TFT substrate are arranged so as to face each other with the alignment films inside so that the liquid crystal molecules are twisted by 90 °, and the periphery thereof is sealed with a sealing material to form an empty cell state. A liquid crystal cell was manufactured.

ZLI-1132 (manufactured by E. Merck) as a liquid crystal composition was injected into the liquid crystal cell in the empty cell state, and the injection port was sealed to manufacture a liquid crystal display device. The liquid crystal display device thus formed had a liquid crystal pretilt angle of about 4.0 °, and the pretilt direction was along the rubbing direction.

When the orientation of the liquid crystal molecules in the liquid crystal layer of the liquid crystal display device according to the first embodiment of the present invention as described above was confirmed, the liquid crystal molecules 8 were aligned along the convex portions 7 as shown in FIG. It was confirmed that the pre-tilt angle was regularly aligned and a uniform 90 ° twist was achieved.
Then, when such a liquid crystal display device was driven and the display performance thereof was visually confirmed, it was confirmed that a display defect such as an alignment defect was not found and a good display without tilt reverse was obtained. It was

Although the above arrangement density of the convex portions has a certain allowable range depending on the kind of the liquid crystal composition used for the liquid crystal layer, in the case of the TN type liquid crystal which is generally used for the liquid crystal display device, 1 × 10 ² pieces / mm ² to 1 × 10 ⁷ pieces / mm ² is effective.

(Embodiment 2) In the liquid crystal display device of the second embodiment, as shown in FIG. 5, the convex portion is provided on the TFT substrate side in the area A of each pixel and on the color filter side in the area B. Then, each substrate was subjected to rubbing alignment treatment in the direction of the arrow to obtain a liquid crystal display device having splay alignment in the A and B regions.

When the TFT-LCD thus manufactured was driven and its display quality was evaluated, the direction of the viewing angle was A.
The area and the area B were different by 180 °, and the vertically symmetrical isocontrast curves as shown in FIG. 6 were obtained as a whole, and a liquid crystal display device having a wide viewing angle was obtained. (Comparative example to the first embodiment) A conventional liquid crystal using the same pixel size, liquid crystal material, and driving method as in the first embodiment except that an alignment film having no convex portion 7 is used. A display device was prepared. When the pretilt angle of this conventional liquid crystal display device was confirmed, it was about 2 °. In addition, when the liquid crystal display device was driven and the display quality was visually confirmed, it was found that the display quality deteriorated because alignment defects due to tilt reverse occurred in a part of the display area and the visibility deteriorated. .

(Comparative Example with Second Example) The second example described above is used except that an alignment film having no convex portion 7 is used.
A conventional liquid crystal display device using the same pixel size, liquid crystal material, and driving method as in the above example was prepared.

When the liquid crystal display device having such a conventional structure was driven and the display performance was visually confirmed, the display angle was narrow, the visibility from the oblique direction was low, and the display was unsightly. That is, it was confirmed that the viewing angle characteristic was lower than that of the liquid crystal display device of the second embodiment.

[0040]

As described above in detail, according to the present invention, a high pretilt angle can be obtained without changing the alignment film material used, the curing temperature, the rubbing conditions, etc. , A high yield, a high reliability, a low drive voltage and a high quality display image can be obtained. Also, with a simple process, the pretilt angle can be changed within the substrate,
By using this, a liquid crystal display device having good viewing angle characteristics and high display quality can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a structure before exposure of a resist film on a counter substrate of a liquid crystal display device of a first embodiment and a photomask used for exposing the resist film.

FIG. 2 is a diagram showing a pattern of a photomask used for forming a convex portion 7 according to the present invention.

FIG. 3 is a view showing a convex portion 7 having a rectangular projection plane shape according to the present invention.

FIG. 4 is a diagram showing convex portions 7 and liquid crystal molecules arranged regularly along the convex portions 7 at a high pretilt angle.

FIG. 5 is a diagram showing a rubbing orientation direction of each pixel of the liquid crystal display device of the second embodiment and an orientation state of liquid crystal molecules induced thereby.

FIG. 6 is a diagram showing isocontrast curves of the liquid crystal display device of the second embodiment.

FIG. 7 is a diagram showing a state of occurrence of disordered alignment of liquid crystal molecules in a liquid crystal display device provided with a projection having a projection plane shape different from that of the present invention.

[Explanation of symbols]

1 ... glass substrate 2 transparent electrode 3 SiO _x film 4 resist film 5 photo mask 6 resist pattern 7 convex portion 8 liquid crystal molecules

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hitoshi Hato 8 Shinsita-cho, Isogo-ku, Yokohama-shi, Kanagawa Toshiba Yokohama Works (56) References JP-A-60-178425 (JP, A) JP-A-61 -219024 (JP, A) JP-A-3-243921 (JP, A) JP-A-63-14123 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/1337

Claims (3)

(57) [Claims] 1. A pair of substrates, each having an alignment film subjected to a rubbing alignment treatment on its surface, are arranged such that the alignment films face each other with a gap, and the periphery is sealed in the gap. in the liquid crystal display device formed by sandwiching a liquid crystal composition, prior Symbol Rabbi of at least one substrate of the two substrates
Oriented films ring alignment treatment has been performed, the liquid crystal composition in contact with the surface side, a rectangular projection plane shape with respect to the substrate principal surface, with trapezoid cross section and bottom of the substrate side, a height above
Lower than the gap, and a liquid crystal display device edge portion, characterized in that a convex portion of the flat row is vertical or <br/> other in the rubbing direction. 2. The arrangement density of the convex portions is from 1 × 10 ² to 1 ×.
The liquid crystal display device according to claim 1, wherein the number is 10 ⁷ pieces / mm ² . 3. The liquid crystal display device according to claim 1, wherein the liquid crystal composition has a splay alignment.