JPH09160049A - Liquid crystal display device and alignment layer forming method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the unevennesses of the orientation state and liquid crystal layer thickness of a liquid crystal molecule on an alignment layer and also to reduce the unevenness of the birefringent phase contrast of the alignment layer for obtaining high quality display by constituting at least one side of the alignment layer with a polyimide incorporated with a rubber component. SOLUTION: Transparent electrodes 3 and 4 for impressing respectively an electric field to the liquid crystal layer and alignment layers 5 and 6 for orienting liquid crystal molecules in a prescribed orientation state are provided at an inner surface of a pair of transparent substrates 1 and 2 placed opposite to each other with an interposed layer of a liquid crystal LC. A pair of the substrates 1 and 2 is joined by interposing a frame-like sealing material 7, and the liquid crystal LC is filled at an area surrounded by the sealing material 7 between both substrates 1 and 2. In this case, both of the oriented films 5 and 6 are formed with a polyimide incorporated with the rubber component, and surface roughnesses of these alignment layers 5 and 6, that is a center line average roughness concerning an amplitude of an unevenness specified by JIS, are kept in about <=5nm. In this way, the variation width of pretilt angle of the liquid crystal molecule is lessened.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a liquid crystal display device and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a liquid crystal display element, electrodes for applying an electric field to the liquid crystal layer are aligned on the inner surfaces of a pair of substrates facing each other across the liquid crystal layer, and liquid crystal molecules are aligned in a predetermined alignment state. The alignment film is generally formed of polyimide, and the film surface is subjected to an alignment treatment by rubbing.

[0003]

However, in the conventional liquid crystal display device, the surface roughness (center line average roughness Ra concerning the amplitude of the irregularities defined by JIS) of the above-mentioned polyimide alignment film is 10 nm to 20 nm. Therefore, the alignment state of liquid crystal molecules on the alignment film and the liquid crystal layer thickness (distance between the alignment film surfaces of both substrates) are uneven, and the birefringence phase difference of the alignment film (birefringence of the alignment film is large). Since there is unevenness in the value of the product of the film thickness and the film thickness, there is a problem in that the display is uneven.

According to the present invention, the alignment state of liquid crystal molecules on the alignment film and the unevenness of the liquid crystal layer thickness are reduced, and the unevenness of the birefringence phase difference of the alignment film is also reduced to obtain a high quality display without unevenness. It is an object of the present invention to provide a liquid crystal display device that can be manufactured and also provide a manufacturing method thereof.

[0005]

In the liquid crystal display element of the present invention, at least one of the alignment films respectively provided on the inner surfaces of a pair of substrates sandwiching the liquid crystal layer and facing each other is made of a polyimide containing a rubber component. It is characterized by.

The surface roughness of the alignment film is preferably about 5 nm or less, and the content of the rubber component of the polyimide is preferably about 5% by weight or less. According to this liquid crystal display element, since the surface roughness of the alignment film is extremely small,
It is possible to reduce the unevenness of the alignment state of liquid crystal molecules and the thickness of the liquid crystal layer on the alignment film, and to reduce the unevenness of the birefringence phase difference of the alignment film to obtain a high-quality display without unevenness.

Further, in the method for manufacturing a liquid crystal display device of the present invention, an alignment film made of polyimide containing the rubber component is applied on a substrate with a polyimide precursor containing the rubber component, and the film is formed at 50 ° C. / Min to 75 ° C / min at a temperature rising rate of 200 ° C to 250 ° C for imidization, and according to this production method, the surface roughness of the alignment film is It is possible to obtain the above liquid crystal display device having a thickness of about 5 nm or less.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of a liquid crystal display element. In this liquid crystal display element, transparent electrodes 3 and 4 for applying an electric field to the liquid crystal layer and a predetermined number of liquid crystal molecules are provided on the inner surfaces of a pair of transparent substrates 1 and 2 which face each other across the liquid crystal LC layer. The alignment films 5 and 6 for aligning in the alignment state are provided, and the pair of substrates 1 and 2 are bonded to each other via a frame-shaped sealing material 7.
It is filled in the area surrounded by the sealing material 7 between.

The molecules of the liquid crystal LC are aligned in a predetermined alignment state by controlling the alignment directions in the vicinity of the substrates 1 and 2 by the alignment films 5 and 6, and also on the outer surfaces of the substrates 1 and 2. Polarizing plates 8 and 9 are respectively arranged in the optical axis, and these polarizing plates 8 and 9 have their optical axes (transmission axis or absorption axis).
Is provided in a predetermined direction.

The liquid crystal display device of this embodiment has a TN
(Twisted nematic) type or STN (super twisted nematic) type, and the liquid crystal LC is a nematic liquid crystal having positive dielectric anisotropy,
The numerator has a predetermined twist angle between the substrates 1 and 2 (approximately 90 ° for the TN type, 180 ° for the STN type).
It is twisted at an angle of 270 ° to 270 °.

Further, this liquid crystal display element is of a simple matrix system, and a plurality of transparent electrodes 3 provided on one substrate 1 are formed in parallel with each other along the row direction (left and right direction in FIG. 1). The scanning electrodes formed as described above and the transparent electrodes 4 provided on the other substrate 2 are signal electrodes formed in parallel with each other in the column direction (direction perpendicular to the paper surface in FIG. 1).

Although not shown in FIG. 1, in a liquid crystal display device for displaying a color image, each of the pixel portions is provided on the inner surface (above or below the transparent electrode) of one of the substrates 1 and 2. Correspondingly, a plurality of color filters, for example, red, green, and blue color filters are alternately arranged.

In this embodiment, both of the alignment films 5 and 6 provided on the inner surfaces of the pair of substrates 1 and 2 are
The alignment films 5 and 6 are formed of polyimide containing a rubber component, and the surface roughness of the alignment films 5 and 6, that is, the center line average roughness Ra with respect to the amplitude of the unevenness defined by JIS is set to about 5 nm or less.

That is, the alignment films 5 and 6 are, for example,
A polyimide having a skeleton structure represented by the following structural formula [Chemical Formula 1] (hereinafter referred to as polyimide A), or a polyimide having a skeletal structure represented by the following structural formula [Chemical Formula 2] (hereinafter , Polyimide B) containing a component of natural rubber or a synthetic rubber having physical properties similar to that of the natural rubber. The content of the rubber component in each of the polyimides A and B is about 3% by weight to about 5% by weight. It is said that.

[0015]

Embedded image

[0016]

Embedded image

FIG. 2 is a diagram showing the relationship between the rubber component content of the polyimide and the surface roughness Ra of the polyimide film (orientation film), and the solid line represents the structural formula of [Chemical Formula 1] above. Polyimide A having a skeleton structure is shown, and the broken line shows a polyimide B having a skeleton structure represented by the structural formula of [Chemical Formula 2]. In addition, these polyimides A and B
Are all formed by imidizing a polyimide precursor under the conditions described below.

As shown in FIG. 2, the polyimides A and B described above are used.
In both cases, the rubber containing no rubber component (content = 0% by weight) has a considerably large surface roughness Ra of about 20 nm, but the rubber component containing the rubber component has an increased content. Along with this, the surface roughness Ra becomes small, and when the content of the rubber component becomes 3.0% by weight or more, the surface roughness Ra becomes extremely small at about 5 nm.

If the content of the rubber component is too large, the physical properties of the polyimide will be adversely affected. Therefore, it is desirable that the content of the rubber component be about 5% by weight or less. Regardless of whether polyimide A or B is used, the content of the rubber component is about 3% by weight to about 5% by weight.

The alignment films 5 and 6 made of the polyimide are subjected to alignment treatment by rubbing the film surface in a predetermined direction, and the refractive index phase difference after the rubbing (the birefringence of the alignment film and the film). The value of the product with the thickness) is 0.3 nm ± 0.0
It is 15 nm.

In this embodiment, the liquid crystal LC is
20% by weight or less of the liquid crystal substance represented by the following structural formula (1) and 25% by weight of the liquid crystal substance represented by the structural formula (2).
Below, the dielectric constant ε (parallel) in the direction parallel to the long axis of the molecule is 1
1.5 to 12.0, dielectric constant ε in the direction perpendicular to the molecular long axis
A liquid crystal material having a (vertical) of 3.5 to 4.0 and a dielectric anisotropy Δε of 8 or less and having a polarity lower than that of a normal nematic liquid crystal is used.

[0022]

Embedded image

A method of manufacturing the liquid crystal display element will be described. In this liquid crystal display element, a pair of substrates 1 and 2 on which electrodes 3 and 4 are formed and on which the alignment films 4 and 5 are formed, are used as a sealing material 7. A liquid crystal cell was assembled by bonding via a liquid crystal cell, and a liquid crystal LC was injected into the liquid crystal cell by a vacuum injection method from a liquid crystal injection port (not shown) formed by removing a part of the sealing material 7 in advance. After that, the liquid crystal injection port is sealed, and then the polarizing plates 8 and 9 are attached to the outer surfaces of the substrates 1 and 2, respectively.

In this manufacturing method, the alignment films 5 and 6 made of polyimide containing the above-mentioned rubber component are
2 is coated with a polyimide precursor containing a rubber component,
The film is heated to 200 ° C. at a heating rate of 50 ° C./min to 75 ° C./min.
It is formed by heating at ℃ to 250 ℃ and imidized, and then the alignment film surface is rubbed.

Here, the amount of the rubber component added to the polyimide precursor is selected so that the content of the rubber component when the polyimide precursor is imidized is about 3% by weight to about 5% by weight.

In this way, the polyimide precursor containing the rubber component is coated on the substrates 1 and 2, and the film is formed under the above-mentioned conditions (heating rate 50 ° C./min to 75 ° C./min, heating temperature 200).
When the film is imidized at (° C. to 250 ° C.), alignment films 5 and 6 made of polyimide with a rubber component added and having a surface roughness Ra of about 5 nm or less are obtained.

FIG. 3 is a diagram showing the relationship between the rate of temperature rise when imidizing the polyimide precursor containing the rubber component by heating and the surface roughness Ra of the formed polyimide film (alignment film). And the solid line indicates the polyimide A having the skeleton structure represented by the structural formula [Chemical formula 1], and the broken line indicates the polyimide B having the skeleton structure represented by the structural formula [Chemical formula 2]. ing.

As shown in FIG. 3, even if the polyimide contains a rubber component in an amount of about 3% by weight to about 5% by weight, the rate of temperature rise during imidization of the polyimide precursor is high. If it is fast, the surface roughness R of the formed polyimide film
Although a becomes large, if the temperature is raised to the imidization temperature (200 ° C. to 250 ° C.) at a heating rate of 75 ° C./min, the surface roughness Ra of the formed polyimide film is suppressed to about 5 nm or less. You can

The surface roughness Ra of this polyimide film is
Becomes smaller as the heating rate is slowed,
Since the imidization efficiency deteriorates when the heating rate is slowed down, in this example, the heating rate was set to 50 ° C./min to 75 ° C./min, and the imidization efficiency was not lowered so much.
A polyimide film having a surface roughness Ra of about 5 nm or less is formed.

According to the above liquid crystal display element, since the surface roughness of the alignment films 5 and 6 is as small as 5 nm or less, the alignment state of the liquid crystal molecules and the unevenness of the liquid crystal layer thickness on the alignment films 5 and 6 are not affected. In addition to reducing the number, the unevenness of the birefringence phase difference of the alignment film is also reduced, and high quality display without unevenness can be obtained.

That is, in the above liquid crystal display device,
The liquid crystal molecules are aligned in the alignment treatment direction (rubbing direction) of the alignment films 5 and 6 in a state of being pretilted to the film surfaces of the alignment films 5 and 6 to some extent.

In this case, in the conventional liquid crystal display element, since the surface roughness Ra of the alignment film is as large as 10 nm to 20 nm,
The pretilt angle of the liquid crystal molecules has a large variation depending on the roughness of the surface of the alignment film. However, in the liquid crystal display element of the above-described embodiment, the surface roughness Ra of the alignment films 5 and 6 is as small as about 5 nm or less, and therefore the liquid crystal The variation width of the pretilt angle of the molecule is small.

FIG. 4 is a diagram showing the relationship between the surface roughness Ra of the alignment film and the pretilt angle of the liquid crystal molecules. In the figure, the black dots represent the maximum value of the pretilt angle corresponding to each surface roughness, The upper and lower lines show the variation width of the pretilt angle.

As shown in FIG. 4, the pretilt angle of the liquid crystal molecules becomes smaller as the surface roughness Ra of the alignment film becomes larger.
Along with this, the variation width of the pretilt angle increases, but if the surface roughness Ra of the alignment film is about 5 nm or less, the variation width of the pretilt angle is extremely small.

Further, the stability of the alignment state of the liquid crystal molecules depends on the pretilt angle of the liquid crystal molecules, and it is desirable to increase the pretilt angle of the liquid crystal molecules to some extent in order to obtain a stable alignment state. Then, liquid crystal LC
In addition, the liquid crystal substance represented by the structural formula (1) of [Chemical Formula 3] is 2
The surface roughness of the alignment film is 0% by weight or less (70% by weight for normal nematic liquid crystal), and the liquid crystal material represented by the structural formula (2) is 25% by weight or less, which is a low-polarity liquid crystal material. When Ra is about 5 nm or less, the pretilt angle of the liquid crystal molecules is as large as about 5.5 ° to 6.5 °, so that a stable alignment state of the liquid crystal molecules can be obtained.

Moreover, in the liquid crystal display element of the above-mentioned embodiment, since the surface roughness Ra of the alignment films 5 and 6 is as small as about 5 nm or less, the thickness of the liquid crystal layer, that is, the alignment films 5 and 6 of both substrates 1 and 2 are formed. The unevenness of the spacing between the surfaces is small, and therefore the value of the product Δnd of the birefringence anisotropy Δn of the liquid crystal LC and the liquid crystal layer thickness d can be made substantially uniform over the entire display area.

Further, in the liquid crystal display device of the above embodiment, since the surface roughness Ra of the alignment films 5 and 6 is as small as about 5 nm or less, the birefringence phase difference between the alignment films 5 and 6, that is, the birefringence of the alignment films. There is little variation in the product of the film properties and the film thickness.

FIG. 5 is a diagram showing the relationship between the surface roughness Ra of the alignment film and the birefringence phase difference thereof. In the figure, the black dots represent the maximum value of the birefringence phase difference corresponding to each surface roughness. Shows,
The upper and lower lines indicate the width of variation in the birefringence phase difference.

As shown in FIG. 5, the birefringence phase difference of the alignment film increases as the surface roughness Ra of the alignment film increases, and the variation width of the birefringence phase difference increases accordingly. When the surface roughness Ra is about 5 nm or less, the birefringence phase difference is 0.3 nm ± 0.015 nm, and the variation width thereof is as small as ± 0.015 nm.

Therefore, according to the above liquid crystal display device, the alignment state of the liquid crystal molecules on the alignment films 5 and 6 and the unevenness of the liquid crystal layer thickness are reduced, and the birefringence phase difference of the alignment films 5 and 6 is also uneven. By reducing the amount, it is possible to obtain a high-quality display without unevenness.

Further, according to the manufacturing method of the liquid crystal display element of the above-mentioned embodiment, the alignment films 5 and 6 made of polyimide containing a rubber component are formed on the substrates 1 and 2 and the polyimide precursor containing the rubber component is added. Apply and coat the film at 50 ℃ / min ～ 75 ℃
Since the film is formed by imidization by heating to 200 ° C. to 250 ° C. at a heating rate of / min, it is possible to obtain the above liquid crystal display device in which the surface roughness of the alignment films 5 and 6 is about 5 nm or less. it can.

Although the liquid crystal display element of the above embodiment is of the TN type or STN type, the present invention can be applied to a ferroelectric liquid crystal display element using a ferroelectric liquid crystal or an antiferroelectric liquid crystal.

In this ferroelectric liquid crystal display element, the unevenness of the liquid crystal layer thickness has a great influence on the alignment state of the liquid crystal molecules, but according to the present invention, the unevenness of the liquid crystal layer thickness can be reduced, so The effect when applied to a liquid crystal display element is great.

Although the liquid crystal display element shown in FIG. 1 is of a simple matrix type, it goes without saying that the present invention can be applied to an active matrix type or segment display type liquid crystal display element.

Further, in the above embodiment, both the alignment films 5 and 6 provided on both the substrates 1 and 2 are alignment films made of polyimide containing a rubber component and having a surface roughness of about 5 nm or less. Such an alignment film may be formed of a normal polyimide, and even in that case, compared with a conventional liquid crystal display element in which both alignment films are formed of a normal polyimide, the liquid crystal molecules on one alignment film It is possible to reduce the unevenness of the alignment state and the liquid crystal layer thickness and the unevenness of the birefringence retardation of the one alignment film, and to obtain a high-quality display without unevenness.

[0046]

According to the liquid crystal display element of the present invention, since the surface roughness of the alignment film on at least one substrate is extremely small, the alignment state of liquid crystal molecules on the alignment film and the unevenness of the liquid crystal layer thickness are reduced. At the same time, the unevenness of the birefringence phase difference of the alignment film can be reduced, and a high-quality display without unevenness can be obtained.

Further, in the method for producing a liquid crystal display device of the present invention, an alignment film made of the polyimide containing the rubber component is coated on a substrate with a polyimide precursor containing the rubber component, and the film is formed at 50 ° C. / Min to 75 ° C / min at a temperature rising rate of 200 ° C to 250 ° C for imidization, and according to this production method, the surface roughness of the alignment film is It is possible to obtain the above liquid crystal display device having a thickness of about 5 nm or less.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display element.

FIG. 2 is a diagram showing the relationship between the content of a rubber component of polyimide and the surface roughness of the polyimide film (alignment film).

FIG. 3 is a view showing a relationship between a temperature rising rate when imidizing a polyimide precursor to which a rubber component is added by heating and a surface roughness of a formed polyimide film (alignment film).

FIG. 4 is a diagram showing the relationship between the surface roughness of an alignment film and the pretilt angle of liquid crystal molecules.

FIG. 5 is a diagram showing the relationship between the surface roughness of an alignment film and its birefringence phase difference.

[Explanation of symbols]

 1, 2 ... Substrate 3, 4 ... Electrode 5, 6 ... Alignment film LC ... Liquid crystal

Claims (4)

[Claims] 1. A liquid crystal display device characterized in that at least one of alignment films provided on the inner surfaces of a pair of substrates sandwiching a liquid crystal layer therebetween is made of a polyimide containing a rubber component. 2. The liquid crystal display device according to claim 1, wherein the surface roughness of the alignment film is about 5 nm or less. 3. The rubber component content of polyimide is about 5% by weight.
The liquid crystal display element according to claim 1 or 2, wherein: 4. A method for manufacturing a liquid crystal display element according to claim 1, wherein an alignment film made of a polyimide containing a rubber component is applied onto a substrate, and a polyimide precursor added with the rubber component is applied onto the substrate. A method for forming an alignment film, comprising forming the film by heating the film at 200 ° C. to 250 ° C. at a temperature rising rate of 50 ° C./min to 75 ° C./min to imidize the film.