JP2000231109A - Liquid crystal display device and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress variance in the energy transmittance in a red region caused by minute changes in a cell gap to the min. and to suppress production of interference fringes by specifying the refractive index and film thickness of an alignment film. SOLUTION: An alignment film is formed on each of the inner face of a first transparent insulating substrate (glass 1) and a second transparent insulating substrate (glass 2) disposed at a specified distance, and a liquid crystal layer is disposed between the substrates. Plural thin film transistors, a SiN insulating film and pixel electrodes consisting of a transparent and electrically conductive film such as ITO are formed on the surface of the glass 1. Color filters(CF) and transparent electrodes comprising ITO are formed on the surface of the glass 2. By controlling the refractive index and film thickness of the alignment film to 1.64±0.06 and 105±30 nm, respectively, the optical film thickness of the alignment film is made to be 174±55.5 nm. Thereby, the generation of interference fringes in a red region produced by minute changes in a cell gap can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a liquid crystal display device, and more particularly to a method for setting the thickness of an alignment film for suppressing occurrence of interference fringes in a red region (around 610 nm).

[0002]

2. Description of the Related Art Generally, a liquid crystal display device is configured such that two transparent insulating substrates on which transparent electrodes are formed are opposed to each other at a fixed distance, and a liquid crystal layer is disposed between these substrates. The liquid crystal is driven by applying a voltage to the transparent electrode. Generally, an alignment film is formed in advance on a substrate surface that comes into contact with the liquid crystal, and an alignment process called a rubbing process is performed. As a material of the alignment film, an organic film such as polyimide or an inorganic film such as SiO is used. As a method of forming a polyimide alignment film on a large-sized substrate, an offset printing method of transferring polyimide diluted with a solvent to a rotating roller and printing the polyimide on a substrate is mainly used.

The thickness of the alignment film depends on the viscosity of the alignment film material when the applied pressure of the transfer plate, the stage speed, the temperature and the humidity are constant. Since there is a positive correlation between the viscosity and the film thickness, the film thickness can be selected by selecting the viscosity. Since the alignment film is disposed between the liquid crystal and the transparent electrode for applying a voltage to the liquid crystal, if the thickness of the alignment film is too large, the voltage drop generated in the alignment film becomes large, and the liquid crystal is not enough. No voltage is applied. On the other hand, if the film thickness is too thin, the film thickness distribution increases in the manufacturing process,
As a result, the voltage drop in the alignment film varies, and the voltage applied to the liquid crystal also varies. In addition, in the above-described rubbing step, the rubbing cloth may come off when the rubbing cloth comes into contact with the surface of the alignment film, and there may be a part where the liquid crystal cannot be aligned. Therefore, conventionally, the thickness of the alignment film is set to 40 to 1
It was formed in a range of about 00 nm.

Various studies have been made on the thickness of the alignment film. For example, Japanese Patent Application Laid-Open No. 63-52118 discloses an alignment film in order to prevent a double image from being produced in a reflective liquid crystal display device. Is set to 100 to 160 nm by a combination of the transmittance of the liquid crystal cell and the mirror surface. Further, in Japanese Patent Application Laid-Open No. Hei 10-123522, for the purpose of preventing reflection, the thickness of the alignment film is set to 100 nm or less in relation to the wavelength at which reflection is to be prevented and the refractive index of the alignment film.

[0005]

The distance between two transparent insulating substrates constituting a liquid crystal display device (hereinafter referred to as a cell gap) is kept constant by arranging a large number of spacers in a liquid crystal layer. I have. When the cell gap becomes non-uniform in the plane, it is visually recognized as interference unevenness accompanied by interference fringes.
As a cause of the non-uniform cell gap, for example,
Improper combination of seal spacer and in-plane spacer, uneven distribution density of in-plane spacer,
The spacer distribution is biased due to the degree of vacuum suction during transport,
Various causes are considered, such as uneven pressure bonding when bonding two substrates. There is a problem that the interference fringes due to the non-uniform cell gap are noticeable mainly in the red region near the wavelength of 610 nm.

[0006] The phenomenon that interference unevenness occurs due to non-uniform cell gap is considered as an active matrix type TFT-LCD.
Will be described as an example. Seven layers of optical thin films including a liquid crystal layer are formed on the pixel electrodes (ITO) inside the two glass substrates constituting the TFT-LCD (see FIG. 1). When the energy transmittance T when light passes through these seven layers is obtained by calculation, the cell gap dependence of the energy transmittance T becomes a simple oscillation as shown in FIG. Note that the calculation method will be described in detail in an embodiment described later, and thus the description thereof will be omitted. The period of this simple vibration is about 0.2 μm, and the cell gap is only 0.1 μm.
By changing m, the energy transmittance T changes from the maximum value to the minimum value. As described above, in the conventional liquid crystal display device, since the light transmittance greatly changes due to a slight change in the cell gap, in an extreme case, there is a problem that interference unevenness such that interference fringes are visually recognized occurs. was there.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and suppresses the occurrence of interference fringes in a red region (around 610 nm) caused by a slight change in the cell gap. It is an object to obtain a high-quality liquid crystal display device.

[0008]

According to the liquid crystal display device of the present invention, an alignment film is provided on each of the opposing surfaces of the first and second transparent insulating substrates which are arranged to face each other at a predetermined distance.
In a liquid crystal display device in which a liquid crystal layer is arranged between these substrates, the refractive index of the alignment film is 1.64 ± 0.06, and the film thickness is 1
By setting the thickness to 05 ± 30 nm, the optical film thickness of the alignment film is set to 174 ± 55.5 nm. The first transparent insulating substrate is provided with a plurality of thin film transistors, an insulating film made of SiN or the like, and a pixel electrode made of a transparent conductive film such as ITO on its surface. Further, the second transparent insulating substrate has a surface on which a color filter and a transparent electrode are provided.

Further, in the method of manufacturing a liquid crystal display device according to the present invention, an alignment film is provided on each of opposing surfaces of the first and second transparent insulating substrates which are arranged to face each other at a predetermined distance,
In the manufacture of a liquid crystal display device in which a liquid crystal layer is arranged between these substrates, an alignment film for minimizing a phenomenon in which the transmittance of incident light of a specific wavelength fluctuates due to a slight change in the thickness of the liquid crystal layer. A method of setting the thickness of a pixel electrode, an insulating film and an alignment film, a liquid crystal layer, and the like, disposed inside the first and second transparent insulating substrates, with incident light having a specific wavelength λ. The energy transmittance T at the time of transmission, the thickness or refractive index of the alignment film and the thickness of the liquid crystal layer as parameters, the horizontal axis represents the thickness of the liquid crystal layer, and the vertical axis represents the energy transmittance T. A step of creating a graph of simple vibration for each film thickness of the alignment film, the horizontal axis represents the film thickness of the alignment film, and the vertical axis represents the maximum value _Tmax and the minimum value T of the amplitude of simple vibration indicated by the energy transmittance T.
_A graph is prepared with the transmittance ratio _Tmax / Tmin, where _min is expressed in the form of a ratio, and the process includes a step of determining the film thickness of the alignment film that minimizes the transmittance ratio. In addition, as the specific wavelength, 610 nm which is a red region is input.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 Hereinafter, a method for manufacturing a liquid crystal display device according to an embodiment of the present invention will be described using an active matrix type TFT-LCD as an example. As shown in FIG. 1, the liquid crystal display device according to the present embodiment includes a first transparent insulating substrate (glass 1) and a second transparent insulating substrate (glass 2) which are arranged to face each other at a predetermined distance. An alignment film is provided on each of the opposing surfaces, and a liquid crystal layer is disposed between these substrates. A plurality of thin film transistors (not shown), an insulating film made of SiN, and a transparent material such as ITO are provided on the surface of the glass 1. A pixel electrode made of a conductive film is provided. Further, a transparent electrode made of a color filter (CF) and ITO is provided on the surface of the glass 2. In the present embodiment, in the manufacture of a liquid crystal display device, the thickness of an alignment film is set to minimize the phenomenon that the transmittance of incident light of a specific wavelength fluctuates due to a slight change in the thickness of a liquid crystal layer. It provides a method. In this case, 610 nm is input as the specific wavelength λ because interference unevenness accompanied by interference fringes occurring in the red region (around 610 nm) is a problem.

First, the energy required when incident light having a specific wavelength λ (610 nm) is transmitted through all layers such as a pixel electrode, an insulating film and an alignment film, and a liquid crystal disposed inside two transparent insulating substrates. The transmittance T is determined using the thickness or refractive index of the alignment film and the cell gap, which is the thickness of the liquid crystal layer, as parameters. As shown in FIG. 1, seven optical thin films including a liquid crystal layer are formed inside the two substrates. The characteristic matrix in the i-th layer is as shown in Expression (1).

[0012]

(Equation 1)

[0013] When the incident light is vertically incident is _{δ i = 2πn i d i COSθ} i / λ = 2πn i d i / λ. Here, n _i and d _i are the refractive index and the film thickness of the i-th layer, respectively. The characteristic matrix in the multilayer film from the first to the n-th layer is represented by Expression (2).

[0014]

(Equation 2)

Then, the energy transmittance T is given by the following equation (3).
It is represented as ^{_{T = 4n 0 2 / (n}} 0 m11 + n 0 2 m12 + m21 + n 0 m22) 2 ·· (3) n 0: refractive index of each layer to the refractive index formula 3 of the glass, type the thickness and wavelength lambda (610 nm), the energy The transmittance T is determined. Table 1 shows the conditions of the thickness and the refractive index of each film. In this calculation, the refractive index of the liquid crystal is a constant regardless of temperature and direction.
All thin films are our medium, do not consider absorption,
It is assumed that light is incident perpendicularly.

[0016]

[Table 1]

Next, a graph of a single vibration is prepared for each film thickness of the alignment film, with the horizontal axis representing the cell gap (the thickness of the liquid crystal layer) and the vertical axis representing the energy transmittance T obtained by the above method. (FIG. 2). As described above, when the cell gap is plotted on the horizontal axis, the energy transmittance T shows a single oscillation, and the amplitude changes by changing the thickness of the alignment film. From this, by setting the thickness of the alignment film so that the amplitude of the energy transmittance T is minimized, the specific wavelength, 610 n in the present embodiment, can be changed without changing other film configurations.
m can suppress interference fringes.

Next, the horizontal axis represents the film thickness of the alignment film, and the vertical axis represents the transmittance ratio T _{max in} the form of a ratio between the maximum value T _max and the minimum value T _min of the amplitude of the simple oscillation indicated by the energy transmittance T. A graph is created as / T _min (FIG. 3), and the film thickness of the alignment film at which the transmittance ratio becomes minimum is determined. Here, the transmittance ratio is used as a value for evaluating the amplitude of the energy transmittance. The amplitude of the energy transmittance becomes smaller as the transmittance ratio becomes minimum, that is, closer to one. According to FIG. 3, the thickness of the alignment film is 105 n.
At m, it was found that the transmittance ratio took the minimum value. If the thickness of the alignment film has a margin of ± 30 nm, the transmittance ratio falls within 4 to 5%. Further, here, the calculation was performed with the refractive index of the alignment film being 1.6.
Since similar results were obtained in the range of 1.7, the refractive index of the alignment film was set to 1.64 ± 0.06 with a margin.
However, the refractive index of the alignment film does not change so much. Even when the refractive index of the alignment film is 1.58 or less and 1.7 or more, the transmittance ratio greatly changes in the tendency of the alignment film thickness dependence. There was no. From the above, the optical film thickness of the alignment film was set to 174 ± 55.5 nm by setting the refractive index of the alignment film to 1.64 ± 0.06 and the film thickness to 105 ± 30 nm.

In order to confirm the effect of the liquid crystal display device employing the thickness of the alignment film optimized by the above method,
Using a TFT array substrate, a color filter substrate and a liquid crystal panel having the film configuration shown in Table 1, a comparison was made by changing only the thickness of the alignment film. The in-plane distribution of the orientation film thickness of the panel according to the present embodiment was 85 ± 2 nm, and that of the conventional panel used as a comparative product was 57 ± 2 nm. As a result of observing the occurrence of interference unevenness in the red region in these panels, 3 out of 1550 panels (0.2%) occurred in the conventional product, and in the panel according to the present embodiment, out of 700 panels, No failure occurred.

As described above, according to the present embodiment, a change in the energy transmittance in the red region caused by a slight change in the cell gap can be achieved by optimizing the refractive index and the film thickness of the alignment film. As a result, interference fringes in the red region, which has conventionally been a problem, can be prevented. For this reason, it has become possible to produce a liquid crystal display device with high display quality at a high yield.

Embodiment 2 FIG. In the present embodiment, in the case where the thickness and the refractive index of the color material of the color filter constituting the liquid crystal display device are different, the thickness of the alignment film in a range where interference unevenness in the red region is difficult to see is described in the first embodiment. It was calculated in the same way as. Table 2 shows the results.

[0022]

[Table 2]

When the color material specifications of the color filter are different,
Although the optimum value of the thickness of the alignment film changes, in each case, the value obtained in the above-described first embodiment is 105 ± 30 nm.
It was found that within the range, the red interference unevenness was in a range where it was difficult to see. Therefore, the thickness of the alignment film obtained in the first embodiment is not limited to the liquid crystal display device having the conditions shown in Table 1, but may be applicable to a wide range of liquid crystal display devices. it is obvious.

[0024]

As described above, according to the present invention, the refractive index of the alignment film is 1.64 ± 0.06 and the film thickness is 105 ± 30 n.
By setting m, the change in energy transmittance in the red region (around 610 nm) caused by a slight change in the cell gap could be minimized. The occurrence of interference fringes can be suppressed, and a liquid crystal display device with high display quality can be produced with a high yield.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a film formed in a pixel electrode portion of an active matrix type TFT-LCD according to the present embodiment.

FIG. 2 is a diagram showing the cell gap dependence of the energy transmittance.

FIG. 3 is a diagram showing the dependency of the transmittance ratio on the orientation film thickness.

Claims (5)

[Claims] 1. A first device arranged opposite to a predetermined distance.
And a liquid crystal display device in which an alignment film is provided on each facing surface of the second transparent insulating substrate and a liquid crystal layer is disposed between these substrates, the refractive index of the alignment film is set to 1.64 ± 0.0.
6. A liquid crystal display device wherein the optical film thickness of the alignment film is 174 ± 55.5 nm by setting the film thickness to 105 ± 30 nm. 2. The first transparent insulating substrate has a plurality of thin film transistors, an insulating film made of SiN or the like, and
2. The liquid crystal display device according to claim 1, further comprising a pixel electrode made of a transparent conductive film such as TO. 3. The liquid crystal display device according to claim 1, wherein a color filter and a transparent electrode are provided on a surface of the second transparent insulating substrate. 4. A first device disposed opposite to a predetermined distance.
And an alignment film is provided on each opposing surface of the second transparent insulating substrate.
And a liquid crystal display device having a liquid crystal layer disposed between these substrates.
In the method of manufacturing the device, the transmittance of incident light of a specific wavelength is not affected by the thickness of the liquid crystal layer.
To minimize the phenomenon that fluctuates due to kana change
A method for setting the thickness of the alignment film, wherein the alignment film is disposed inside the first and second transparent insulating substrates.
All of the pixel electrode, insulating film and the above alignment film, the above liquid crystal layer, etc.
Energy when incident light of a specific wavelength λ passes through the layer
-The transmittance T is calculated based on the thickness or refractive index of the alignment film and the liquid
The thickness of the crystal layer as a parameter, the horizontal axis represents the thickness of the liquid crystal layer, and the vertical axis represents the energy transmittance T.
A graph of the simple vibration is created for each film thickness of the alignment film.
The horizontal axis represents the thickness of the alignment film, and the vertical axis represents the energy transmittance T.
Maximum value T of vibration amplitude _maxAnd the minimum value T_minIn the form of a ratio
Transmittance ratio T_max/ T_minCreate the graph as
A step of determining the thickness of the alignment film that minimizes the transmittance ratio.
A method for manufacturing a liquid crystal display device. 5. The specific wavelength is 61 in the red region.
5. The method according to claim 4, wherein 0 nm is input.