JP2001242040A - Evaluation method of liquid crystal display element - Google Patents

Abstract

(57) [Problem] To provide a method capable of correctly measuring and evaluating electro-optical characteristics of a manufactured liquid crystal display element. The method for evaluating a liquid crystal display element includes a step of measuring a spectral characteristic of light transmitted through a liquid crystal display element to be measured sandwiched between a pair of polarizing plates; A step of comparing the spectral characteristics of the liquid crystal display element with each other; and a step of determining the evaluation of the liquid crystal display element to be measured based on the comparison result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring and evaluating parameters of electro-optical characteristics of a liquid crystal display device, and more particularly, to one of important parameters for evaluating the quality of electro-optical characteristics of a liquid crystal display device. The present invention relates to a method for detecting a parameter relating to retardation and providing it for accurate evaluation as a liquid crystal display element.

[0002]

2. Description of the Related Art As a parameter governing the electro-optical characteristics of a liquid crystal display device (hereinafter, referred to as a liquid crystal cell), a product of the birefringence (Δn) of the liquid crystal and the cell thickness (d) of the liquid crystal cell, that is, It is known that retardation (Δnd) is one of the very important factors.

If the retardation value of the manufactured liquid crystal cell deviates from the designed value, display defects such as a decrease in contrast ratio and abnormal color tone occur. Also, even within a single liquid crystal cell, if the retardation value differs depending on the location, display unevenness and color tone unevenness may occur, resulting in reduced display quality and defective products.

[0004] Although the retardation value is an important parameter affecting the electro-optical characteristics of the liquid crystal cell, there is no method for directly measuring the value in the current technology.
At present, as a method for measuring a retardation value, for example, only a cell thickness (d) is measured by a liquid crystal cell thickness measuring device such as a U-6000 microscope spectrophotometer manufactured by Hitachi, Ltd. Or simply provided by a liquid crystal material manufacturer and multiplied by the value of the birefringence (Δn) of the liquid crystal to indirectly determine the retardation value (Δnd)
There is a way to find the value.

The current state of the liquid crystal cell evaluation method is to compare the retardation value obtained indirectly by this calculation with a design value to determine the amount of deviation between the two or to evaluate the variation due to the position in the cell. It is.

[0006]

However, such a conventional evaluation method has the following problems.

1) Since the liquid crystal in a normal liquid crystal cell has a pretilt angle, the effective birefringence (Δn) of the liquid crystal molecules in the liquid crystal cell is determined by the intrinsic birefringence of the liquid crystal molecules themselves. Has a value smaller than the value of. Therefore, when the birefringence of the liquid crystal cell is calculated by the conventional method as described above, an accurate value cannot be obtained, unlike a value in an actual cell.

[0008] In particular, T where the liquid crystal molecules are twist-aligned.
N-LCD (twisted nematic liquid crystal display element), S
In a TN-LCD (super twisted nematic liquid crystal display device) or the like, the liquid crystal molecules are twisted (the arrangement direction of the liquid crystal molecules is gradually changed in the thickness direction of the cell), and the tilt of the liquid crystal molecules is increased. Even more accurate retardation is obtained because the angle is not constant in the cell thickness direction but has a distribution state (usually, the tilt angle is smaller in the middle part of the cell than in the vicinity of the interface). The value cannot be determined.

2) When measuring the cell thickness (d) using a spectrophotometer, the cell thickness is measured by the reflected light at the interface between the liquid crystal and the glass on both sides of the cell. When the reflected light is weak, the measurement error increases, and an accurate measurement value of the cell thickness cannot be obtained.

3) When calculating the cell thickness from the reflected light measured by a spectrophotometer, the value of the refractive index (not the birefringence) of the medium in the cell, that is, the liquid crystal is required. In a liquid crystal cell having birefringence and having a complicated orientation such as a non-uniform distribution of twist and tilt angle, it is impossible to uniquely determine the value of the refractive index.

Normally, the inside and outside of the sealing material at the outer periphery of the liquid crystal cell (the outside is an air layer, so the refractive index is 1).
Is measured to determine the refractive index of the liquid crystal part, but it is very difficult to obtain an accurate value.

However, if a liquid crystal cell which can be determined to be finished as designed is used as a reference cell, and then the refractive index of the liquid crystal cell to be measured is determined, and thereafter the cell thickness is measured using the refractive index as a fixed value, then A relative comparison of the retardation between the measurement cell and the reference cell can be made. However, also in this case, when the reflected light at the interface is weak, a large measurement error is included, so that it cannot be said that the method is very accurate.

4) At the interface between the glass substrate and the liquid crystal layer,
Various films are formed, such as a transparent electrode for applying a voltage to liquid crystal molecules, an alignment film, and an overcoat film for preventing a short circuit between transparent electrodes between opposing substrates. Often it is not known which of these films reflected the light used in the spectrophotometer. These films are sufficiently thin compared to the thickness of the liquid crystal layer, but for accurate cell thickness measurement, these film thicknesses cannot be ignored.
In the measurement of cell thickness with a spectrophotometer, it is inevitable that these film thicknesses may cause errors in the measured values.

Due to the problems described above, measuring the retardation by the conventional method requires accurate measurement even if the measurement is relative, especially when the reflected light at the interface between the liquid crystal and the glass substrate is weak. And the evaluation of the cell was difficult.

An object of the present invention is to provide a method for accurately measuring and evaluating the electro-optical characteristics of a manufactured liquid crystal display device.

[0016]

According to the present invention, a transmitted light spectrum of light transmitted through a liquid crystal cell to be measured is measured, and the measured spectrum characteristic is compared with another transmitted light spectrum characteristic of a liquid crystal cell as a reference. Thus, the electro-optical characteristics of the liquid crystal cell to be measured are relatively evaluated.

The method for evaluating a liquid crystal display device according to the present invention comprises the steps of measuring the spectral characteristics of light transmitted through the liquid crystal display device to be measured sandwiched between a pair of polarizing plates; The method includes a step of comparing the spectral characteristics of the liquid crystal display device to be measured, and a process of determining the evaluation of the electro-optical characteristics of the liquid crystal display device to be measured based on the comparison result.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a retardation measuring method according to the present invention will be described below in detail with reference to FIGS.

FIG. 1 is a schematic diagram showing a configuration of a measuring system for implementing a measuring method according to an embodiment of the present invention. Two polarizing plates 2 are arranged opposite to each other on both sides of a liquid crystal cell 1 to be measured. Reference numeral 3 denotes a light source, and reference numeral 4 denotes a photodetector for detecting the spectrum of the light 5 transmitted through the liquid crystal cell 1 to be measured and the polarizing plate 2.

When the liquid crystal cell 1 to be measured is an ordinary TN-LCD or STN-LCD, a light source having a wavelength range of approximately visible light (380 to 780 nm) may be used. The function of dispersing light in that range is performed by the light source 3 or the photodetector 4.
May be given. In the embodiment described below, a halogen lamp is used as the light source 3, and an ultra-sensitive instantaneous multi-photometry system IM manufactured by Otsuka Electronics Co., Ltd. is used as the photodetector 4.
By using UC-7000, a spectroscopic function was provided on the detector side.

At the time of spectrum measurement, a pair of polarizing plates 2 are arranged in a parallel Nicol arrangement (polarization axis directions are parallel to each other) in a state where the liquid crystal cell 1 is not arranged in the measurement system of FIG. A reference spectrum with a rate of 100% is first measured. Furthermore, the polarizing plate 2 is arranged so as to be in an orthogonal Nicol arrangement (polarization axis directions are orthogonal to each other), and a reference spectrum having a light transmittance of 0% is measured.

These are so-called reference measurements, which are used to measure the spectrum with the wavelength characteristics of the light source and the polarizing plate removed. In order to perform the reference measurement more strictly, the measurement may be further performed by removing the influence of the wavelength characteristic of the glass substrate or the like of the liquid crystal cell. In this case, the reference measurement is performed by placing an empty cell into which no liquid crystal is injected at the time of reference measurement, or an isotropic liquid having substantially the same refractive index as that of the liquid crystal and placing the cell between the polarizing plates 2.

No matter what direction the liquid crystal cell 1 is arranged with respect to the polarization axis of the polarizing plate 2, there is no particular limitation since the peak position on the transmitted light spectrum does not change. However, the direction of the polarization axis of the polarizer on the incident light side and the orientation direction of the liquid crystal molecules at the interface of the glass substrate on the incident light side are set so that the amplitude of the spectrum (the difference between the maximum value and the minimum value of the transmittance) becomes as large as possible. It is desirable that the angle formed by the angle is 45 °.

Next, as shown in the sectional view of FIG.
The case where the TN-LCD is used as the liquid crystal cell 1 to be measured and the retardation is measured and evaluated will be described. D-ST
The N-LCD has two STN-LCD liquid crystal cells 10, 11
And one of the STN-LCDs 1
0 denotes an optical compensation cell, and the other STN-LCD 11
Operate as a drive cell that actually drives the liquid crystal.

In each liquid crystal cell, a layer of liquid crystal molecules 13 is sandwiched between glass substrates 12. In the compensating cell 10 and the driving cell 11, the liquid crystal molecules are oriented so that the twist directions are opposite to each other. Therefore, the compensation cell 10 and the drive cell 11 should have the same retardation value (Δnd). If the compensation value differs from the design value in the compensation cell 10 and the drive cell 11, the contrast ratio of the D-STN-LCD deviates from the designed contrast ratio.

In this case, the contrast ratio usually shifts in the direction of decreasing the contrast ratio. However, when the retardation value of the compensation cell 10 is slightly smaller than the retardation value of the driving cell 11, the contrast ratio increases. It may shift.

Therefore, the liquid crystal cell has such a D-STN
In the case of an LCD, the compensation cell and the drive cell must be made to have the same retardation value. Therefore, it is necessary to measure and evaluate the retardation value of the manufactured D-STN-LCD cell.

In the present embodiment, the liquid crystal cell 1 to be measured uses a D-STN-LCD having a pretilt angle of 2 ° and a 180 ° twist orientation. In the twist direction, the drive cell 11 is a left twist, and the compensation cell 10 is a right twist. The birefringence (Δn) of the used liquid crystal was 0.165.
The cell thickness (d) is 5.9 μm (design value).

Under the above conditions, as the photodetector 4, an ultra-sensitive instantaneous multi-photometry system IMU manufactured by Otsuka Electronics Co., Ltd.
The spectrum was measured using C-7000. At this time, the polarizers 14 are arranged in a crossed Nicols arrangement.

FIG. 3 shows the result of the spectrum measurement. The light transmittance in the visible light wavelength range changes like an interference fringe, and one upward peak and two downward peaks appear. That is, it is understood that the wavelength dependence (spectral characteristics) of the light transmittance can be measured. However, it is clear from the peak wavelength that this change in light transmittance is not due to reflection at the interface between the liquid crystal layer and the substrate.

Since the liquid crystal layer has birefringence, the incident light is
It could be considered separately for different types of polarization components.
It is conceivable that interference occurs when two types of polarization components are combined on the emission side. In this case, the change in light transmittance will reflect the value of cell retardation. Although the angle of the exit-side polarizer is not particularly limited, the peak is sharpest in the case of the parallel Nicol arrangement or the orthogonal Nicol arrangement.

FIG. 4 shows the results of spectrum measurement on three different liquid crystal cells to be measured. Of the three samples used in the measurement of FIG. 4, the cell A has a gap control material (spacer) of the driving cell 11 in which the application density is smaller than the design value and the cell thickness (d) is smaller than the design value. Cell that is considered to be

Cell B is a cell in which the density of the gap control material of the driving cell 11 is equal to the design value and the cell thickness (d) is considered to be the designed value.
Are cells in which the distribution density of the gap control material of the drive cell 11 is higher than the design value and the cell thickness (d) is considered to be larger than the design value. Out of the plurality of peaks in the spectrum of FIG. 4, attention was paid to the downward peak near the wavelength of 550 nm.

FIG. 5 is an enlarged graph showing the downward peak near the 550 nm wavelength and the vicinity thereof.
It can be seen that the thicker the cell thickness is, the longer the peak wavelength is on the longer wavelength side. Since these three liquid crystal cells A, B, and C are manufactured under the same conditions except for the distribution density of the gap control material, the difference between the peak wavelengths shown in FIG. 5 is caused by the cell thickness (d). This is considered to be caused by the difference in the retardation value (Δnd) of the cell. The same peak wavelength was measured for a plurality of cells having different dispersion densities of the gap control material of the compensation cell 10, and the same result as above was obtained.

Next, for some cells whose peak wavelengths were measured by the above method, U-600 manufactured by Hitachi, Ltd., which is generally used in the conventional cell thickness measurement method, was used.
The cell thickness (d) was measured and calculated using a 0 microscope spectrophotometer. The refractive index of the liquid crystal when calculating the cell thickness was 1.60. FIG. 6 shows the relationship between the peak wavelength and the cell thickness. The data plotted with white circles is the data of the driving cell 11, and the data plotted with black circles is the data of the compensation cell 10.

The driving cell data shows a clear linear relationship between the peak wavelength and the cell thickness, as shown by a straight line. However, the data of the compensating cell has a large variation, and the peak wavelength and the cell thickness are different. And no specific relationship can be found. The reason for this is that the driving cell has a transparent electrode for driving liquid crystal formed on the glass substrate surface, so that the light reflection at the time of measuring the cell thickness is large and the cell thickness can be accurately measured. This is because there is no light reflection on the glass substrate and an accurate cell thickness cannot be obtained.

Next, a first group of D's manufactured by selecting and combining the driving cells and the compensation cells so that the peak wavelengths determined by the transmitted light spectrum measurement according to the above-described embodiment of the present invention are the same for the driving cell and the compensation cell. -An STN-LCD was prepared.
At the same time, a second group of D-STN-LCDs was prepared, in which the cell thickness was measured by a conventional cell thickness measuring method, and the driving cell and the compensation cell having the same value were combined.

Conditions other than the cell thickness, that is, the birefringence, the pretilt angle, the twist angle, and the like were set to be the same between the compensation cell and the drive cell. It is premised that the samples in the second group have the same retardation value as long as the cell thickness is the same. For all D-STN-LCD cells, the contrast when displaying by 1/64 duty drive was measured and the data were compared.

D-STN-LCD of combination in which the measured peak wavelength is the same between the driving cell and the compensation cell
The average value of the contrast data of 20 cells was 46, the minimum value was 42, and the maximum value was 48.

The average contrast of the 20 D-STN-LCD cells combined so that the cell thickness values measured by the conventional method were the same was 37, the minimum value was 24, and the maximum value was 53. . The contrast of the samples of the second group varied greatly as compared with the samples of the first group.

As can be seen from the result, the peak wavelength of the transmitted light is measured, and the combination of the driving cell and the compensation cell so that the peak wavelengths are the same is obtained by the conventional driving cell thickness measuring method. And a compensating cell, the contrast value is larger and its variation can be reduced. From this, it became clear that the method of evaluating a liquid crystal cell based on the peak wavelength of the spectrum of transmitted light is superior to the conventional evaluation method based on cell thickness measurement.

When the peak wavelength of the transmission spectrum of the cell showing the maximum contrast was measured in a D-STN-LCD combined with the same cell thickness by the conventional cell thickness measurement method, the compensation cell was measured. The peak wavelength is slightly shorter than that of the driving cell, which indicates that the cell thickness on the compensation cell side is small.

As is generally known, when the retardation value (Δnd) of the compensating cell is smaller than that of the driving cell, a so-called “light transmittance vs. voltage characteristic” appears on the D-STN-LCD. This is considered to be due to the occurrence of a bounce and an increase in the contrast ratio.

By measuring the peak wavelength of the transmitted light spectrum and combining them so that the retardation value of the compensating cell is slightly smaller than that of the driving cell, the contrast is improved by utilizing the above-mentioned bound effect. It is also possible.

As described above, measuring the peak wavelength of the spectrum of the transmitted light and performing the relative evaluation of the liquid crystal cell based on the measurement is performed by setting the combination of the driving cell and the compensation cell of the D-STN-LCD. In such a case, it becomes a very effective determination means. Also, the D-STN-LCD described above
In addition to the above case, a reference cell (master cell) prepared as designed is prepared, the peak wavelength of the transmitted light spectrum is measured for the cell to be measured, and the peak wavelength of the transmitted light spectrum of the master cell is measured. And a relative evaluation can be performed. Variations in retardation within the same cell can also be evaluated.

FIG. 7 shows a twisted nematic (TN).
9 is a graph showing measurement results for a liquid crystal cell. The horizontal axis shows the wavelength in nm, and the vertical axis shows the transmittance in%. There are three types of samples, cell D, cell E, and cell F. The measurement was performed with the polarizer arrangement on both sides of the cell being parallel Nicols. Also,
The liquid crystal molecule alignment direction on the incident side and the polarization axis angle were 45 °. The liquid crystal used has a Δn of 0.12 and a designed cell thickness of 8μ.
m.

Cell D (around 560 nm peak) is a cell having a design value. The cell E is a cell finished almost as designed. The cell F is a cell in which the cell thickness does not conform to the design value and the peak wavelength is largely shifted (around 485 nm). The contrast value of these cells is 1 for cell D.
50 and cell E were 145, whereas cell F, which was found to be out of design value, had only 85.

Although the present invention has been described in connection with the preferred embodiments, the invention is not limited to these embodiments. For example, it will be apparent to those skilled in the art that various modifications, improvements, and combinations are possible.

[0049]

As described above, the transmitted light spectrum of the light transmitted through the liquid crystal cell to be measured is measured, and the measured spectrum characteristic is compared with the transmitted light spectral characteristic of the liquid crystal cell as another reference. The electro-optical characteristics of the liquid crystal cell to be measured can be simply and accurately evaluated relative to each other, and a high-quality liquid crystal display device with higher contrast and less variation can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a transmission light spectrum characteristic measuring system used in a method for evaluating a liquid crystal display device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a cross-sectional structure of a D-STN-LCD to which a method for evaluating a liquid crystal display device according to an embodiment of the present invention can be applied.

FIG. 3 is a spectrum measurement result in a method for evaluating a liquid crystal display device according to an embodiment of the present invention.

FIG. 4 shows spectrum measurement results of a plurality of different liquid crystal cells in the method for evaluating a liquid crystal display device according to an embodiment of the present invention.

FIG. 5 is a graph in which a part of the spectrum characteristics of FIG. 4 is enlarged.

FIG. 6 is a graph showing a relationship between a peak wavelength and a cell thickness according to a method for evaluating a liquid crystal display device according to an embodiment of the present invention.

FIG. 7 shows spectrum measurement results of a plurality of liquid crystal cells in a method for evaluating a liquid crystal display device according to another embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Liquid crystal cell to be measured 2 Polarizer 3 Light source 4 Photodetector 5 Transmitted light 10 Compensation cell 11 Drive cell 12 Glass substrate 13 Liquid crystal molecule 14 Polarizer

Claims (7)

[Claims] 1. A step of measuring a spectral characteristic of light transmitted through a liquid crystal display device to be measured sandwiched between a pair of polarizing plates, and a spectral characteristic of a reference liquid crystal display device and a spectral characteristic of the liquid crystal display device to be measured. And a step of determining the evaluation of the electro-optical properties of the measured liquid crystal display element based on the comparison result. 2. The liquid crystal according to claim 1, wherein, in the comparing step, a peak wavelength of a spectrum of the reference liquid crystal display element is compared with a peak wavelength of a spectrum of the liquid crystal display element to be measured. Display element evaluation method. 3. The method for evaluating a liquid crystal display device according to claim 1, wherein in the step of measuring the spectral characteristics, the polarization axes of the pair of polarizing plates are arranged in directions orthogonal to each other. 4. The method for evaluating a liquid crystal display element according to claim 1, wherein in the step of measuring the spectral characteristics, the polarization axes of the pair of polarizing plates are arranged in directions parallel to each other. 5. In the step of measuring the spectral characteristics, the direction of the polarization axis of the polarizing plate on the incident light side is set to 45 degrees with the orientation direction of liquid crystal molecules at the interface on the incident light side of the liquid crystal display device to be measured.
5. The method for evaluating a liquid crystal display element according to claim 1, wherein the liquid crystal display elements are arranged so as to form an angle of [deg.]. 6. The method for evaluating a liquid crystal display device according to claim 1, wherein the liquid crystal cell to be measured has a liquid crystal layer in a twist alignment. 7. A D-STN type liquid crystal display device in which a liquid crystal cell to be measured has a compensation cell and a driving cell overlapped with each other, wherein a peak wavelength of a spectrum of the driving cell and a peak wavelength of a spectrum of the compensation cell. 2. The method for evaluating a liquid crystal display device according to claim 1, wherein: