JP2635803B2 - Liquid crystal cell and method for measuring physical properties of liquid crystal and measuring apparatus using the same - Google Patents

Description

The present invention relates to a liquid crystal cell, a method for measuring physical properties of a liquid crystal, and a measuring apparatus using the same, and more particularly, to an optical position of a liquid crystal layer of the liquid crystal cell. The present invention relates to a method for measuring a phase difference, a thickness of a liquid crystal layer and a birefringence of a liquid crystal, and a measuring apparatus using the same.

(Prior Art) In a liquid crystal element, the optical retardation of the liquid crystal layer of the liquid crystal cell, the thickness d (cell thickness) of the liquid crystal layer, and the birefringence Δn of the liquid crystal are important parameters. For example, in a super twist type liquid crystal display device, these parameters are closely related to display color, response speed, alignment stability, and the like, and a more accurate measurement method is required. Knowing these values accurately is also very important in designing and evaluating devices.

Conventionally, an interference method has been used for measuring the cell thickness of a liquid crystal cell. For example, a cell thickness measuring device using a split beam system or an optical system is commercially available, and these are both cell interferometers. However, it has been reported that the cell thickness measured by the interferometry becomes larger than the actual cell thickness (KH Yang; J. Appl. Phys. 64 (9), 4780 (198
8)), the cell thickness measured by these measuring instruments does not always indicate the true cell thickness. That is, the liquid phase cell is generally formed of a multilayer film such as an ITO film, an alignment film, and a color filter, and therefore, there is a problem that interference is complicated and a correct cell thickness cannot be obtained.

Furthermore, currently commercially available cell thickness measuring instruments do not perform folding in consideration of the alignment state of the liquid crystal, and the measurement accuracy of a cell containing a liquid crystal is low. In addition, there is a problem that it is very difficult to measure a cell with a color filter which will become mainstream in the future.

In general, the optical phase difference of the liquid crystal layer of a liquid crystal element is a function of the cell thickness and the birefringence of the liquid crystal, and is usually calculated from the cell thickness and the birefringence of the liquid crystal. That was difficult.

(Problems to be Solved by the Invention) It is difficult to obtain an accurate value by the conventional liquid crystal cell thickness measurement method, and there is a problem that the accuracy is further reduced due to ITO, an alignment film, a color filter, and the like. Was.
Furthermore, the cell thickness when liquid crystal is sealed in the liquid crystal cell,
There is a problem that the optical phase difference and the like cannot be obtained accurately.

[Configuration of the invention]

(Means for Solving the Problems) In order to achieve the above-described problems, the liquid crystal cell and the method for measuring physical properties of the liquid crystal according to the present invention employ the following two methods.

The first method is to provide a transmission spectrum T (θ, λ) when a liquid crystal cell having a liquid crystal composition sandwiched between substrates is disposed between first and second polarizers, Relational expression _TLC = {[T (θ, λ) -T (θ + π) with the transmission spectrum T (θ + π / 2, λ) when the angle of at least one of the second polarizers is changed by approximately 90 degrees. / 2, λ) / [T (θ, λ) + T (θ + π / 2, λ)] + 1} / 2 to determine the optical phase difference of the liquid crystal layer of the liquid crystal cell from the transmission spectrum _TLC. This is a method for measuring physical properties of a liquid crystal cell and a liquid crystal, which is a feature.

In the second method, a transmission spectrum T (θ, λ) when a liquid crystal cell having a liquid crystal composition sandwiched between substrates is arranged between first and second polarizers, , The transmission spectrum T (θ + π / 2, λ) when the angle of at least one of the second polarizers is changed by approximately 90 degrees, and the orthogonal transmittance (T _PT ² ) of the first and second polarizers. + P _PA ² ) and the parallel transmittance (2T _PT × T _PA ), the equation T _LC (θ, λ) = {[T (θ, λ) -T (θ + π / 2, λ)] / [2T _PT × T _The liquid crystal cell and the physical properties of the liquid crystal, wherein the optical phase difference of the liquid crystal layer of the liquid crystal cell is obtained from the spectrum T _LC (θ, λ) obtained by _PA- T _PT ² -T _PA ² ] +1} / 2 It is a measuring method.

The measuring device of the present invention is a measuring device using the above-described measuring method.

(Operation) In general, the transmittance T (λ) of light having a wavelength λ that is perpendicularly incident on a liquid crystal element is expressed by the following equation.

T (λ) = T _PT ² × T ₀ × T _LC (λ) + 2T _PT × T _PA × T ₀ × {1-T _LC (λ)} + T _PA ² × T ₀ × T _LC (λ) where T _PT and T _PA is the transmittance of the polarizer, respectively T _PT: transmittance T _PA on the polarization component parallel to the transmission axis of the polarizer: a transmittance of a polarization component parallel to the absorption axis of the polarizer.

T ₀ is a term of absorption and interference of glass, ITO, an alignment film, a color filter, and the like, and T _LC (λ) is an ideal (expressed by theoretical calculation) transmittance of a liquid crystal element.

If the degree of polarization of the polarizer is sufficiently high, T _PA = 0, and the transmittance T (λ) is T (λ) = T _PT ² × T ₀ × T _LC (λ).

The transmittance T when the angle of one polarizer is, for example, θ
(Theta, lambda) and the transmittance T (θ + π / 2, λ) when the θ + π / 2, respectively, T (θ, λ) = T PT 2 × T 0 × T LC (θ, λ) T (θ + π / 2, λ) = T _PT ² × T ₀ × T _LC (θ + π / 2, λ) On the other hand, in an ideal liquid crystal element, T _LC (θ, λ) = 1−T _LC (θ + π / 2, λ) Holds, T _LC (θ, λ) = {[T (θ, λ) -T (θ + π / 2, λ) / [T (θ, λ) + T (θ + π / 2, λ) +1} / It becomes 2.

When the degree of polarization is not high, the orthogonal transmittance (T _PT ² + T _PA ² ) and the parallel transmittance (2T _PT × T _PA ) of the polarizer are preliminarily determined.
By measuring, T _LC (θ, λ) can be calculated by the following equation.

T _LC (θ, λ) = {[T (θ, λ) -T (θ + π / 2, λ)] / [2T _PT × T _PA -T _PT ² -T _PA ² ] +1} / 2 From the two measurement results obtained by shifting the angle of the polarizer by π / 2, the ideal liquid crystal element,
A transmission spectrum free of the influence of absorption and interference from ITO, alignment films, color filters, etc. can be obtained. This is because only the liquid crystal layer among these multilayer films constituting the liquid crystal element has birefringence.

By analyzing the transmission spectrum thus obtained, the optical retardation of the liquid crystal layer of the liquid crystal cell, the thickness of the liquid crystal layer, and the birefringence of the liquid crystal can be accurately determined.

Some theoretical formulas are known to express the relationship between the transmission spectrum and these values. For example, when analyzing a normal TN or ST (super twist) cell, it is necessary to use Raynes, which is easy to calculate. The reported approximate expression (EPRaynes, Mol. Cryst. Liq. Cyrst. Lett. 10, 2-4 (19
74)) may be extended so that the tilt angle can be considered. The transmittance T of an ST (or TN) cell having a twist angle φ and a tilt angle θ is given by the following equation.

T = {cosβcos (φ−Pf + Pr) + (1 + α ² ) ⁻¹ / ² · sinβsin (φ−Pf + Pr)}
² + α ² / (1 + α ² ) Sin ² βcos ² (φ−Pf−Pr) where α = π · p · d / φλ β = φ · (1 + α ² ) ^1/2 p = ne · (1 + ωsin ² θ _{^{) -1/2 + n o ω = (}} n e / n o) 2 -1 λ: wavelength of the light d: cell thickness n _e: refractive index of the liquid crystal with respect to extraordinary light n _o: a refractive index of the liquid crystal with respect to ordinary light, The definition of the angle is as follows. All angles are positively defined counterclockwise with respect to the rubbing direction 20 of the rear substrate. In FIG. 2, the first polarizer angle Pf is the first angle relative to the rubbing direction 20 of the rear substrate. The angle between the polarization axis 22 of the polarizer and the second polarizer angle Pr is different from the rubbing direction 20 of the rear substrate with respect to the polarization axis 23 of the second polarizer.
Angle, twist angle φ is the rubbing direction of the rear substrate 20
With respect to the rubbing direction 21 of the front substrate.

In this equation, the effective optical phase difference of the liquid crystal layer is p · d, and the other parameters included in the transmittance T are the twist angle and the polarizer angle, which can be easily known.
Is determined from the transmittance T. If one of the birefringence p and the cell thickness d is determined, the other is also determined. p is the effective birefringence, including the influence of the tilt angle, if the tilt angle theta = 0 ° p = Δn (= n _e -
n _o ). Since the tilt angle is almost always determined by the properties of the alignment film, the tilt angle of the alignment film and the liquid crystal to be used may be determined in advance by a magnetic field application method or the like.

Although the example of the ST type liquid crystal element has been described here, the measurement method of the present invention does not limit the display mode and the analysis method at all, and the transmission spectrum data obtained according to the measurement method of the present invention is also applicable to other display modes. However, analysis suitable for each display mode may be performed.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view of one embodiment of a measuring device according to the present invention. First and second polarizers 3 and 4 are arranged before and after a sample stage 2 that supports a measurement sample 1, and a light source 5 is further provided outside the first and second polarizers 3 and 4, and a spectrometer 6 is provided on the opposite side of the light source 5. For example, Spectra Sucan PR-702AM (PHOTO RE
(Manufactured by SEACH), and the light emitted from the light source 5 passes through the polarizer 3, the measurement sample 1, and the polarizer 4, and then enters the spectrometer 6. The first and second polarizers 3 and 4 are rotated by stepping motors 13 and 14, respectively, and the controller box 7 controls these stepping motors. A J-3100GX (manufactured by Toshiba) is installed as a personal computer 8 beside these measurement systems, and the controller box 7 and the spectrometer 6 are connected to a GP.
Controlled via IB. When the personal computer 8 enters the measurement mode, it first sends a control signal to the controller box 7 to rotate the first and second polarizers 3 and 4 to a specified angle. Next, the personal computer 8 sends a control signal to the spectrometer 6 to start the first measurement of the transmission spectrum. When the measurement of the transmission spectrum is completed, the first transmission spectrum is transmitted from the spectrometer 6 to the personal computer 8. Next, the personal computer 8 sends a control signal to the controller box 7 again to rotate the second polarizer 4 by 90 degrees. Then, the personal computer 8 sends a control signal to the spectrometer 6 and
The second measurement of the transmission spectrum is started. When the second transmission spectrum is transmitted from the spectrometer 6 to the personal computer 8, the analysis mode is set.

In the analysis mode, the personal computer 8 calculates the ideal transmission spectrum of the liquid crystal element from the two transmission spectra and graphs the transmission spectrum as described in the section of operation. Further, a value of the refractive index measured in advance is input to the personal computer 8, and the calculation is performed by the approximate expression of Raynes using the cell thickness d as a parameter. In this way, the calculated value is compared with the actually measured value to determine the cell thickness d at which the deviation is minimized.

FIGS. 3 to 7 show an example in which an ST-type liquid crystal cell is used as a measurement sample 1 and an actually measured transmission spectrum is compared with a transmission spectrum calculated by the approximate formula of Raynes by changing the cell thickness d. The calculation result when the cell thickness is 6.51 μm is most consistent with the actual measurement, and the cell thickness is determined to be 6.51 μm. The solid line is the actual measurement, and the dotted line is the calculation result.

〔The invention's effect〕

In the present invention, by utilizing the birefringence of the liquid crystal, the optical phase difference and cell thickness of the cell containing the liquid crystal and the birefringence of the liquid crystal can be accurately obtained. In addition, according to the present invention, glass, ITO,
Accurate measurement of birefringence phase difference and cell thickness can be performed without being affected by absorption and interference of a color filter or the like.

Further, the present invention can be applied even when an active element such as TFT or MIM is provided on the substrate, and it goes without saying that the present invention can be applied to liquid crystal elements of various modes as well as ST type liquid crystal display elements.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing one embodiment of a measuring device according to the present invention, FIG. 2 is a diagram showing a definition of a rubbing direction of a liquid crystal cell and an angle of a polarizer, and FIGS. 3 to 7 are ST type liquid crystal cells. FIG. 4 is an explanatory diagram comparing a measured transmission spectrum with a transmission spectrum calculated by simulation with changing the cell thickness d. DESCRIPTION OF SYMBOLS 1 ... Measurement sample, 2 ... Sample stage 3 ... First polarizer, 4 ... Second polarizer 5 ... Light source, 6 ... Spectrometer 7 ... Control box 8 ... Personal computer 13 , 14 stepping motor 20 rubbing direction of rear substrate 21 rubbing direction of front substrate 22 polarization axis of first polarizer 23 polarization axis of second polarizer

Claims (3)

(57) [Claims] 1. A transmission spectrum T (θ, λ) when a liquid crystal cell having a liquid crystal composition sandwiched between substrates is disposed between first and second polarizers. Transmission spectrum T (θ + π / 2, when the angle of at least one of the two polarizers is changed by approximately 90 degrees)
λ) and _TLC = {[T (θ, λ) -T (θ + π / 2, λ) / [T (θ, λ) + T (θ + π / 2, λ)] + 1} / 2 A method for measuring physical properties of a liquid crystal cell and a liquid crystal, wherein an optical phase difference of a liquid crystal layer of the liquid crystal cell is obtained from a transmission spectrum _TLC shown. 2. A transmission spectrum T (θ, λ) when a liquid crystal cell having a liquid crystal composition sandwiched between substrates is disposed between first and second polarizers. The transmission spectrum T (θ + π / 2, λ) when the angle of at least one of the two polarizers is changed by approximately 90 degrees, and the orthogonal transmittance (T _PT ² + T _PA ) of the first and second polarizers ² ) and parallel transmittance (2T _PT
× T _PA ), the equation T _LC (θ, λ) = {[T (θ, λ) -T (θ + π / 2, λ) / [2T _PT × T _PA -T _PT ² -T _PA ² ] +1 A method for measuring physical properties of a liquid crystal cell and a liquid crystal, wherein an optical phase difference of a liquid crystal layer of the liquid crystal cell is obtained from a spectrum T _LC (θ, λ) obtained by} / 2. 3. A liquid crystal cell and an apparatus for measuring physical properties of liquid crystals using the measuring method according to claim 1.