JPH10232397A - Liquid crystal display device and method of manufacturing the same - Google Patents

Abstract

(57) [Problem] To provide a vertical alignment type ECB mode liquid crystal display device and a method for manufacturing the same, and to improve sharpness characteristics, contrast ratio, and transmittance in a bright state. SOLUTION: An electrode is formed on a surface, and has a pair of substrates arranged facing each other at a predetermined interval, and a liquid crystal layer arranged between the pair of substrates, and no voltage is applied to the liquid crystal layer. In this state, the liquid crystal molecules are oriented perpendicular to the substrate surface, the liquid crystal layer contains a nematic liquid crystal material having a negative dielectric anisotropy, and the substrate surface is in contact with the liquid crystal of the pair of substrates. Vertically-aligned ECB mode liquid crystal display device that actively performs an alignment process in a direction parallel to.

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, and more particularly to a liquid crystal display device having excellent display quality such as contrast characteristics and responsiveness, and a method of manufacturing the same.

[0002]

2. Description of the Related Art FIG. 7 shows a vertical alignment type ECB (Elect
Rily Controlled Birefr
2 shows a basic configuration example of an LCD (liquid crystal display device). When no voltage is applied, the liquid crystal molecules 10 are oriented perpendicular to the upper and lower substrates 20 and 21. The vertical orientation is
It can be obtained by selecting a liquid crystal material and performing an alignment treatment on the substrate surface. High contrast display can be obtained by combining with the polarizing plates 23 and 24 having the orthogonal Nicols arrangement.

In this ECB mode, when a voltage is applied, FIG.
As shown in (1), the liquid crystal molecules begin to fall from the liquid crystal molecules at the center between the upper and lower substrates of the cell, and at the same time, the retardation of the liquid crystal layer changes to gradually increase the transmittance.

However, in this ECB mode, the ratio of the change in the transmittance of the liquid crystal to the change in the voltage applied to the liquid crystal, that is, the sharpness, is generally STN-LCD.
(Super twisted nematic liquid crystal display element) and the like. The sharpness is defined as a ratio of an applied voltage corresponding to a transmittance close to a predetermined dark state to an applied voltage corresponding to a transmittance close to another predetermined bright state. It is large, indicating that the transmittance in the bright state is high.

[0005] Poor sharpness means that when a display is performed by simple matrix driving with a large duty ratio, the contrast ratio is small and the transmittance in the bright state is low. It is known that the steepness can be increased by increasing the thickness of the liquid crystal cell. However, another problem arises in that the response speed decreases when the cell thickness is increased.

[0006]

An invention for improving the sharpness of a vertical alignment type ECB mode liquid crystal display device is disclosed in an embodiment of Japanese Patent Application No. 8-141910 filed earlier by the present applicant. The disclosure of the specification of the application discloses a vertical alignment type ECB mode in which liquid crystal molecules are aligned perpendicular to the substrate surface in a state where no voltage is applied to a liquid crystal layer sandwiched between a pair of substrates. In a liquid crystal display device, a liquid crystal layer contains a nematic liquid crystal material having a negative dielectric anisotropy, and when a voltage is applied to the liquid crystal layer, liquid crystal molecules gradually move between the substrates in the major axis direction. Has a helical structure that changes Specifically, by adding a chiral agent to the liquid crystal, the helical structure is given to improve sharpness.

However, in the vertical alignment type ECB mode liquid crystal display device disclosed in the embodiment of the application, there is room for further improving the sharpness, and the STN-LCD is required.
It is still slow compared to the others.

An object of the present invention is to provide a vertical alignment type ECB mode liquid crystal display device having good sharpness, a high contrast ratio, and a high transmittance in a bright state, and a method of manufacturing the same.

[0009]

The liquid crystal display device of the present invention comprises a pair of substrates having electrodes formed on a surface thereof and opposed to each other at a predetermined interval, and a liquid crystal layer disposed between the pair of substrates. A vertical alignment type ECB mode liquid crystal display element in which liquid crystal molecules are aligned perpendicular to a substrate surface in a state where no voltage is applied to the liquid crystal layer, wherein the liquid crystal layer has a negative dielectric constant difference. Including anisotropic nematic liquid crystal material,
Further, the substrate surfaces of the pair of substrates that are in contact with the liquid crystal are subjected to positive alignment processing in a direction parallel to the substrate surfaces.

According to the method of manufacturing a liquid crystal display device of the present invention, there is provided a step of preparing a pair of substrates which have been subjected to an alignment treatment so that liquid crystal molecules are vertically aligned, and the steps of: A step of performing an active alignment treatment in a direction parallel to the plane, and disposing the pair of substrates to face each other at a predetermined interval,
Injecting a nematic liquid crystal material having a negative dielectric anisotropy to which a chiral agent is added between the substrates.

[0011]

FIG. 1 is a view showing an arrangement state of liquid crystal molecules when no voltage is applied to a vertical alignment type ECB mode liquid crystal display device according to an embodiment of the present invention, and FIG. 2 is a diagram showing a voltage of the same liquid crystal display device. FIG. 4 is a diagram illustrating an alignment state of liquid crystal molecules when voltage is applied.

A nematic liquid crystal having a negative dielectric anisotropy and containing a chiral agent is injected between a pair of substrates of a vertical alignment type ECB mode liquid crystal display device. However, the substrate surfaces of the pair of substrates 1 and 2 that sandwich the liquid crystal and that are in contact with the liquid crystal are subjected to an active alignment process in a direction parallel to the substrate surface (hereinafter, referred to as a horizontal direction). In the example of FIG. 1, the direction of the arrow of the substrates 1 and 2 is the direction in which the alignment processing is performed. The illustration of the alignment film on the substrate surface is omitted.

When no voltage is applied in FIG. 1, all liquid crystal molecules are arranged almost perpendicular to the substrate surface, similarly to the conventional vertical alignment ECB mode liquid crystal display device shown in FIG. However, the liquid crystal molecules 10 at the interface are basically vertically aligned in the initial alignment when no voltage is applied by the horizontal alignment processing, but are tilted at a small angle toward the alignment direction as shown in the figure. Become. That is, a pretilt angle is given to the liquid crystal molecules at the interface. If the entire substrate surface is horizontally aligned in the same direction, a pretilt at the same angle in the same direction can be given to all liquid crystal molecules at the interface.

The cells are arranged in polarizing plates 3 and 4 in a crossed Nicols arrangement.
Observing the display surface of the substrate, when no voltage is applied, there is no birefringence in the in-plane direction of the substrate, and the liquid crystal molecules are oriented vertically, so that a good black display (low transmittance) is obtained.
Is obtained.

When a voltage is applied to this cell, as shown in FIG. 2, the liquid crystal molecules 10 fall down, the retardation increases, and a state in which light is transmitted (bright state). At this time, the liquid crystal molecules take a twisted structure (spiral arrangement) in which the major axis direction is gradually changed between the upper and lower substrates due to the chiral agent as shown by a dotted line. The addition of the chiral agent causes the liquid crystal molecules to collapse and twist, resulting in a helical structure as shown in FIG.

Due to the chirality, the sharpness, which is the ratio of the change in transmittance to the change in voltage, can have a higher value than that of a conventional device in which no chiral agent is added. This phenomenon is supported by computer simulations.

In FIG. 2, the liquid crystal molecules move from the upper substrate 2 to the lower substrate 1 with respect to the in-plane direction of the substrate.
It is rotated 80 °. This is because the alignment direction is the same in the in-plane direction of the substrate, and, for example, when the upward direction in the drawing is the reference direction, the alignment direction in the in-plane direction of the liquid crystal molecules is opposite. In this way, the direction of the liquid crystal molecules is given a direction according to the reference direction, and an angle formed when the direction is projected onto one of the reference surfaces is defined as φ.

FIG. 3 is a schematic view of the substrate viewed from an oblique direction in order to generally show the angle φ between the directions of the horizontal alignment of the liquid crystal molecules on the substrates 1 and 2. In FIG. 3, x and y indicate coordinate axes orthogonal to each other on a plane parallel to the substrate, and z indicates coordinate axes in the normal direction of the lower substrate. The orientation direction of the liquid crystal molecules on the upper substrate 2 is the direction from the inside of the liquid crystal layer to the substrate surface, and is the direction of the arrow a parallel to the substrate surface. The direction of the arrow a is opposite to the orientation (rubbing) direction. Substrate 1 below
The orientation direction of the upper liquid crystal molecules is the direction from the substrate surface into the liquid crystal layer, and is the direction of the arrow b parallel to the substrate surface. φ
Is the angle between the arrows a and b projected on the xy plane.

The angle φ is desirably set so as to match the helical structure of the liquid crystal molecules between the substrates. That is,
First, the twisted structure starts from the tilt direction of the liquid crystal molecules that matches the orientation direction of one substrate, and the liquid crystal molecules gradually twist toward the other substrate, and just at the interface of the other substrate, the orientation direction of the substrate and The orientation directions of the upper and lower substrates are set so that the inclination and the twist are aligned so as to match.

In the conventional vertical alignment type ECB mode liquid crystal display device shown in FIG. 7, since the liquid crystal molecules at the interface were almost vertical, the azimuth of the liquid crystal molecule alignment was not random and constant as a whole. For this reason, when a voltage was applied, the alignment state of the liquid crystal molecules varied greatly. Observing the dot cells on the display surface by sandwiching the conventional vertical alignment type ECB mode liquid crystal display device between polarizing plates arranged orthogonally,
A thick black pattern called a black cross as shown in the photograph of FIG. 9 can be seen.

On the other hand, in the liquid crystal display device of the vertical alignment type ECB mode which has been subjected to the alignment treatment according to the embodiment of the present invention shown in FIG. 1, the liquid crystal molecules at the substrate interface are given a pretilt in a certain direction. Since the liquid crystal molecules are initially aligned, the alignment azimuths become uniform when a voltage is applied. Therefore, no black cross occurs. 10 is a photograph of a dot cell on the display surface obtained by sandwiching the liquid crystal display device of the vertical alignment type ECB mode according to the embodiment of the present invention between polarizing plates arranged orthogonally, and is apparent from the photograph of FIG. It can be seen that no black cross occurred. In the photograph of FIG. 10, the pattern on the black line is caused by the influence of the oblique electric field due to the electrode edges of the upper and lower substrates.

In the embodiment of the present invention, since no black cross occurs, the light transmittance increases, and the image quality is improved. The addition of the chiral agent improves the sharpness characteristics without increasing the cell thickness. Furthermore, the sharpness characteristic is further improved by setting the direction of the alignment treatment performed in the direction parallel to the substrate so that the helical structure of the liquid crystal upon application of a voltage matches the pretilt direction of the liquid crystal molecules at the interface.

The empty cell is a vertical alignment type EC according to the prior art.
It can be obtained with the same manufacturing method and the same structure as the B-mode liquid crystal display element. However, by adding a chiral agent to the liquid crystal material to be injected, a liquid crystal display element having a high contrast ratio and a high transmittance can be obtained even when driven at the same duty ratio as the conventional one.

As a method for performing the alignment treatment in a direction parallel to the substrate, for example, an alignment treatment method using a photosensitive alignment film can be used. In this alignment treatment method, after forming a photosensitive alignment film on a substrate surface, a desired liquid crystal alignment is formed, for example, by irradiating the alignment film with polarized light of ultraviolet wavelength or natural light. For the photosensitive alignment film irradiated with light,
An orientation in a direction corresponding to the irradiation direction or the polarization direction of the irradiation light can be imparted to the liquid crystal molecules. By tilting the irradiation direction from the substrate normal direction, a certain pretilt is given.

As the alignment treatment, a known rubbing may be used. Further, an alignment method other than optical alignment such as oblique deposition and rubbing may be used.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The steps of actually manufacturing a liquid crystal display device according to an embodiment of the present invention and the display performance thereof will be described below.

A vertically oriented polyimide alignment film (manufactured by Nissan Chemical Industry Co., Ltd.) is applied to a glass substrate on which a display transparent electrode (dot type electrode) made of ITO (indium tin oxide) is formed by a spinner, and heated at 200 ° C. It was baked for 1 hour. The thickness of the orientation film after firing was about 60 angstroms.

Each of the two substrates manufactured in the above process was irradiated with ultraviolet light, and a uniaxial alignment process was performed so that the two substrates were aligned in a direction parallel to the substrate surface.

The two glass substrates that have been subjected to the orientation treatment in the above steps are stacked so that the cell thickness d is 4.5 μm, thereby producing an empty cell. At this time, the orientation direction of the substrates is set so that the angle (φ in FIG. 3) of the orientation direction of the liquid crystal molecules on the two substrates matches the spiral structure of the liquid crystal. That is, the orientation direction of one substrate is considered to be opposite.

A nematic liquid crystal having a negative dielectric anisotropy (manufactured by Merck) was injected into the empty cell obtained in the above step by adding a chiral agent by vacuum injection. At that time, a device was prepared in which the relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the cell thickness d was set to d / p = 0.74.

When the completed liquid crystal cell was sandwiched between two polarizing plates (manufactured by Nitto Denko) arranged in an orthogonal Nicol arrangement, and the electro-optical characteristics were measured at a 1/240 duty drive, the results shown in Table 1 were obtained. Was done. At that time, a vertical alignment type ECB mode cell (conventional cell in Table 1) according to the conventional technique was similarly manufactured and measured for comparison.

The maximum transmittance (Tmax) in Table 1 was used.
%) Is the maximum value of the transmittance (T) of the cell with respect to the change of the applied voltage (V). CR5% and CR10% are respectively 5% and 10% when the transmittance of the cell is 1/240 duty drive. % Contrast, V50 / V
5 is the voltage (V50) at T = 50% and T when the maximum transmittance and the minimum transmittance are converted to 100% and 0%, respectively.
= 5% voltage (V5) ratio.

[0033]

[Table 1]

From the measurement results shown in Table 1, the liquid crystal display element according to the embodiment of the present invention, in which the homeotropic cell was subjected to the alignment treatment in the direction parallel to the substrate surface, had higher transmittance, contrast and contrast than the conventional cell. It is clear that all items of sharpness are improved.

Next, by the same manufacturing method as in the above embodiment,
Cells in which the angles (φ in FIG. 3) of the orientation directions of the liquid crystal molecules on the upper and lower substrates were changed in various ways were manufactured. That is, as shown in FIG. 4, under a constant condition of d / p = 0.74, φ is set to 0 ° to 36 °.
A cell was prepared in which the angle was changed every 45 ° within a range of 0 °. The direction of the alignment process on the substrate viewed from the front of the display surface of the cell at each angle φ is indicated by an arrow in the right end column of FIG. The cell thus manufactured was measured in the same manner as in the example of Table 1 and evaluated. The results are shown in FIGS. 5 and 6, the horizontal axis indicates the value of the angle φ, the vertical axis in FIG. 5 indicates the sharpness, and the vertical axis in FIG. 6 indicates the contrast.

From the results of the evaluation experiments shown in FIGS. 5 and 6, φ = 1
It can be seen that the case of 80 ° is the best in both sharpness and contrast. That is, φ = 1
80 ° indicates that the relationship between the orientation directions of the upper and lower substrates matches the spiral structure of the liquid crystal, which indicates that this is the optimal orientation condition.

The optimum alignment condition can be determined by the same method for the value of d / p, which is the natural twist angle of the liquid crystal. Optimum alignment conditions vary depending on the physical properties of the liquid crystal material (such as the elastic constant K22) and the amount of the chiral agent added.

Some experimental results show that the vertical alignment type E
In a CB mode liquid crystal display device, the twist angle of the helical structure of the liquid crystal layer when voltage is applied is one rotation (360 °), and the natural twist pitch of the nematic liquid crystal material is p, and the cell interval is d. When the value is set, it is preferable that the value be smaller than the value of d / p. Further, depending on the liquid crystal material, the twist angle of the helical structure may be set in the range of 1/2 (d / p) to 5/6 (d / p) when one rotation (360 °) is one. Further, it is desirable that the relationship between the value of the natural twist pitch p and the value of the cell interval d is 0.25 ≦ d / p ≦ 1.0. Furthermore, 0.65 ≦ d / p ≦ 0.7
A condition of 5 can be recommended.

The liquid crystal molecules may be divided and aligned in the plane of the substrate. For example, after forming an alignment film on a substrate, a checkerboard-shaped first mask is formed using a photoresist layer. The first alignment process is performed in this state. Then the first
Is removed, and a second mask complementary to the first mask is formed. In this state, the second alignment process is performed in another direction, for example, a direction orthogonal to the direction. By such a plurality of types of alignment processing, a large number of divided domains having a plurality of alignment directions are formed on the substrate surface. The alignment treatment is performed by, for example, light irradiation.

For example, if one pixel is formed of two domains having different alignment directions, two regions having different alignment directions of liquid crystal molecules can be formed in one pixel. If the liquid crystal molecules are inclined in two or more different directions, the viewing angle characteristics can be improved.

Although the present invention has been described with reference to the embodiments, the present invention is not limited to these embodiments. Also,
It will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.

[0042]

According to the present invention, in a vertical alignment type ECB mode liquid crystal display device, chirality is given to a liquid crystal material to give a twisted structure to liquid crystal molecules when a voltage is applied, and alignment treatment is performed in a direction parallel to the substrate. As a result, the contrast, the transmittance, and the sharpness characteristics can all be improved, and the display quality can be improved.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an alignment state of liquid crystal molecules when no voltage is applied to a vertical alignment type ECB mode liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating an alignment state of liquid crystal molecules when a voltage is applied to a vertical alignment type ECB mode liquid crystal display device according to an embodiment of the present invention.

FIG. 3 is a perspective view showing a relationship between alignment directions of upper and lower substrates of a liquid crystal cell according to an embodiment of the present invention.

FIG. 4 is a table of cells manufactured by changing the angle φ, which is the relationship between the orientation directions of the upper and lower substrates, in the liquid crystal cell according to the embodiment of the present invention.

FIG. 5 is a graph of measurement results showing a relationship between an angle φ and sharpness of a liquid crystal display device according to an example of the present invention.

FIG. 6 is a graph of a measurement result showing a relationship between an angle φ and a contrast of a liquid crystal display device according to an example of the present invention.

FIG. 7 is a cross-sectional view showing a state of alignment of liquid crystal molecules when no voltage is applied in a conventional vertical alignment type ECB mode liquid crystal display device.

FIG. 8 is a cross-sectional view showing an alignment state of liquid crystal molecules when a voltage is applied to a conventional vertical alignment type ECB mode liquid crystal display device.

FIG. 9 is an enlarged photograph showing a fine pattern formed on a substrate on a display surface when a voltage is applied to a conventional vertical alignment type ECB mode liquid crystal display device.

FIG. 10 is an enlarged photograph showing a fine pattern formed on a substrate on a display surface when a voltage is applied to a vertical alignment type ECB mode liquid crystal display device according to an embodiment of the present invention.

[Explanation of symbols]

 1, 2 substrate 3, 4 polarizing plate 10 liquid crystal molecule

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masatoshi Horii 1-3-1 Edanishi, Aoba-ku, Yokohama-shi, Kanagawa Prefecture Inside Stanley Electric Co., Ltd. (72) Inventor Yoshihisa Iwamoto 1-3 Edanishi, Aoba-ku, Yokohama-shi, Kanagawa -1 Inside Stanley Electric Co., Ltd.

Claims (10)

[Claims] An electrode is formed on a surface of a substrate, and the substrate includes a pair of substrates disposed to face each other at a predetermined interval, and a liquid crystal layer disposed between the pair of substrates. A voltage is applied to the liquid crystal layer. In a vertical alignment type ECB mode liquid crystal display device in which liquid crystal molecules are vertically aligned with respect to a substrate surface in a state where no liquid crystal molecules are present, the liquid crystal layer includes a nematic liquid crystal material having a negative dielectric anisotropy, and A liquid crystal display device, wherein a positive alignment process is performed on a substrate surface of the substrate which is in contact with the liquid crystal, in a direction parallel to the substrate surface. 2. The liquid crystal material has a helical structure in which liquid crystal molecules gradually change its major axis direction between the substrates when a voltage is applied to the liquid crystal layer, and an alignment direction of an alignment process performed on the substrate surface. 2. The liquid crystal display device according to claim 1, wherein the liquid crystal display element is set so as to match the spiral structure. 3. The twist angle of the helical structure is smaller than the value of d / p when the natural twist pitch of the nematic liquid crystal material is p and the predetermined interval is d. 3. The liquid crystal display device according to claim 1, wherein: 4. The twist angle of the helical structure is set in a range of 1/2 (d / p) to 5/6 (d / p) when 360 ° is set to 1. Item 4. A liquid crystal display device according to item 3. 5. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined interval d is set as 0.25 ≦ d / p ≦ 1.0. Item 5. A liquid crystal display device according to any one of items 2 to 4. 6. The relationship between the value of the natural twist pitch p of the nematic liquid crystal material and the value of the predetermined interval d is set as 0.65 ≦ d / p ≦ 0.75. Item 5. A liquid crystal display device according to any one of items 2 to 4. 7. The substrate surface is divided into a number of domains,
7. The liquid crystal display according to claim 1, wherein the alignment process on the pair of substrates is performed in a plurality of directions by dividing domains, and the liquid crystal molecules are inclined in different directions. element. 8. The apparatus according to claim 1, further comprising a pair of polarizing plates sandwiching the pair of substrates, wherein the polarizing axes of the pair of polarizing plates are arranged to be substantially orthogonal to each other. 8. The liquid crystal display device according to any one of items 7. 9. A step of preparing a pair of substrates that have been subjected to an alignment treatment so that liquid crystal molecules are vertically aligned; and a step of making the pair of substrates positively contact a substrate surface in contact with the liquid crystal in a direction parallel to the substrate surface. Vertical alignment comprising the steps of: subjecting the pair of substrates to opposing at predetermined intervals, and injecting a nematic liquid crystal material having a negative dielectric anisotropy with a chiral agent added between the substrates. Type EC
A method for manufacturing a B-mode liquid crystal display element. 10. The liquid crystal material has a helical structure in which liquid crystal molecules gradually change its long axis direction between the substrates when a voltage is applied to the liquid crystal layer. 10. The method for manufacturing a liquid crystal display device according to claim 9, wherein when the substrates are arranged to face each other, processing is performed so that the alignment direction matches the spiral structure.