JPH06229921A - Method for evaluating cure degree of polyimide oriented film - Google Patents

Abstract

PURPOSE:To evaluate a cure degree of an oriented film, using a substrate of a real liquid crystal display element, by measuring the infrared absorption spectrum of a substrate in a certain region, and by using an absorption degree of an OH stretching vibration absorption band within the region. CONSTITUTION:After cure of the polyimide orientation film applied to a substrate, the infrared absorption spectrum is measured. Hereupon, as an infrared absorption spectrometer, that of Fourier transform type is preferable in the point of sensibility. Only the wave number region of at least 3100-3600cm<-1> is measured. As a reference, a substrate on which the orientation film is not applied should be used. By finding the difference between spectra of a sample coated with the orientation film and a sample not coated, only the absorption derived from the polyimide orientated film can be measured. The integrated intensity is rather more preferably measured than the peak height when the absorption degree is measured, because the OH stretching vibration absorption in this wave number region is wide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention requires accurate control of the heat-cured state of the alignment film of a liquid crystal display device.
The present invention relates to a method for evaluating the degree of cure of a polyimide alignment film used in a TFT liquid crystal display element, an STN liquid crystal display element, and the like.

[0002]

2. Description of the Related Art A liquid crystal display device is one in which electrodes and alignment films are laminated on two substrates and held so that the alignment films on the two substrates face each other. It is created by introducing. Any alignment film can be used as long as it has the ability to align liquid crystals, but polyimide is industrially used.

Polyimide, which is generally used as an alignment film, is formed by applying a solution of polyamic acid, drying, and curing by heating. The curing conditions for this polyimide are
It is necessary to find an appropriate curing condition because the molecular structures differ greatly. In order to find an appropriate curing condition, a method of performing thermal analysis to find a thermal decomposition initiation temperature and a method of analytically obtaining an imidization rate by infrared spectroscopy are often used.

In infrared spectroscopy, the absorbance of the peculiar absorption of the imide ring is measured and the degree of curing of the alignment film is expressed as the imidization ratio. Specific absorption of imide ring used in this case, approximately 1780 cm ^-1, 720 cm ^-1, is 600 cm ^-1.

[0005]

By the way, soda glass and borosilicate glass, which are used as substrates for ordinary liquid crystal display elements, strongly absorb most of the infrared light. Therefore, the above absorption band cannot be measured. Therefore, the following method is unavoidable for determining the degree of curing of the alignment film. That is, the polyimide alignment film solution is coated on a substrate such as a silicon wafer or calcium fluoride or sodium chloride that is transparent to infrared light, dried, and heated. The infrared absorption spectrum of this sample is measured, and the absorbance of the absorption band peculiar to the imide ring is examined.

However, in this case, the degree of curing was not measured in the sample in which the alignment film was applied to the actual substrate for the liquid crystal display element. Therefore, there are problems such as the fact that the degree of curing of the alignment film on the actual substrate and the degree of curing of the actual defective portion of the liquid crystal display element cannot be matched.

In view of the problems of the conventional method for evaluating the degree of cure of an alignment film, the present invention can evaluate the degree of cure of an alignment film by using a substrate of an actual liquid crystal display device. It is intended to provide an evaluation method.

[0008]

The present invention SUMMARY OF THE INVENTION may, an infrared absorption spectrum of the substrate coated with polyimide alignment film on one side, and measured at least in the region of 3100cm ^-1 ~3600cm ^-1,
The degree of hardening of the alignment film on this substrate is evaluated using the absorbance of the OH stretching vibration absorption band in this region.

[0009]

[Function] The polyimide alignment film is converted to polyimide by heat treatment with polyamic acid. In this reaction, an imide ring is formed by dehydration condensation from the amino group and the carboxyl group in the polyamic acid. That is, the carboxyl group is eliminated by this reaction.

In the polyamic acid, the absorption derived from the OH stretching vibration in the carboxyl group is 3100 cm ^{-1 to} 36.
It exists between 00 cm ^-1 . The absorbance of this absorption band is
It was found that the heat treatment reduced the degree of elimination of carboxyl groups.

The substrate material often used for the liquid crystal display element is soda glass or borosilicate glass.
These glasses do not have strong absorption at wave numbers higher than approximately 2000 cm ^-1 . Accordingly the absorption band of 3100cm ^-1 ~3600cm ^-1 derived from the OH stretching vibration, it is possible to measure also the orientation film on the glass substrate.

[0012]

EXAMPLE An example of the method for evaluating the degree of cure of the polyimide alignment film of the present invention will be described. The substrate should be at least 3100 cm
^A glass substrate or a resin substrate may be used as long as it can pass a wave number of ^{-1 to} 3600 cm ^-1 . Soda glass or borosilicate glass is optimally less absorption at 3100cm ^-1 ~3600cm ^-1.

A substrate having a polyimide alignment film coated on one side is used. Here, the polyimide alignment film is most preferably a polyimide obtained by thermosetting polyamic acid. For the polyimide alignment film of the type that is imidized before being heat-cured, it is not necessary to use the curing degree evaluation method of the present invention.
Further, a thin film other than the alignment film may be present on the substrate. Even if there is a transparent electrode, overcoat material, TFT (thin film transistor), etc., at least 3100 cm ^-1
It may be present on the substrate as long as it has a thickness that allows infrared light having a wave number of 3600 cm ^-1 to pass therethrough.

After curing the polyimide alignment film coated on the substrate, the infrared absorption spectrum is measured. Here, the infrared absorption spectrometer may be a dispersion type or a Fourier transform type, but a Fourier transform type spectrometer is preferable from the viewpoint of sensitivity. The measurements need not be performed in all frequency domain, it is sufficient measure only the wave number region of at least 3100cm ^-1 ~3600cm ^-1. Further, it is necessary to use a substrate on which an alignment film is not applied as a reference. By taking the difference between the spectra of the sample coated with the alignment film and the sample not coated with the alignment film, it is possible to measure only the absorption derived from the polyimide alignment film.

[0015] Since the OH stretching vibration absorption in the carbonyl group of the polyamic acid at a wave number region of 3100cm ^-1 ~3600cm ^-1 is broad, more preferably is better to measure the area intensity than the peak height when measuring the absorbance.

On the other hand, if there is a variation in the film thickness of the alignment film, it is necessary to measure the degree of cure by the method described below. That measuring the infrared absorption spectrum in the region of at least 2800cm ^-1 ~3600cm ^-1, by using the ratio of absorbance and CH stretching vibration absorption band of OH stretching vibration absorption bands in this region hardening of the alignment film on the substrate It is to evaluate the degree.

[0017] Among the 2800cm ^-1 ~3600cm ^-1, OH stretching vibration absorption band 3 in the carbonyl group of the polyamic acid
In the 100cm ^-1 ~3600cm ^-1. The CH stretching vibration absorption band of the polyamic acid is in a 2900cm ^-1 ~3000cm ^-1. The absorbance in the CH stretching vibration absorption band does not change before and after the imidization reaction. Therefore, OH stretching vibration absorption band and CH
The absorbance ratio of the stretching vibration absorption band may be measured. Since these often give broad peaks, more preferably the area intensity should be measured and ratioed.

Specific examples will be described below. (Examples 1 to 4) A soda glass for STN display element was spin-coated with SE150 manufactured by Nissan Chemical Co., Ltd. as a polyimide alignment film material so as to have a film thickness of 70 nm. Immediately, this substrate was left for 1 minute on a hot plate having a preset temperature of 60 ° C. to evaporate the solvent. Further, this substrate was heat-treated in a hot air dryer at a set temperature shown in (Table 1) for 1 hour to prepare samples of Examples 1 to 4 shown in (Table 1).

[0019]

[Table 1]

(Examples 5 to 8) As a polyimide alignment film material, PSI2201 manufactured by Chisso Petrochemical was spin-coated to a film thickness of 70 nm. The same heat treatment as in Example 1 was performed to prepare the samples of Examples 5-8. (Measurement of Absorbance) The infrared absorption spectra of the samples of Examples 1 to 8 were measured using a Fourier transform infrared spectrophotometer model 740 manufactured by Nikolay. The measurement conditions are the detector TG
S, resolution 8 cm ⁻¹ , total 100 times. The samples of the examples were measured using the infrared absorption spectrum of the glass substrate not coated with the alignment film as a reference.

The infrared absorption spectrum of Example 1 is shown in FIG. Among the areas of 2800cm ^-1 ~3600cm ^-1 As can be seen in Figure 1, 3150cm ^-1 ~3450cm ^-1 represents an OH absorption band, 2900cm ^-1 ~3000cm ^-1 represents CH absorption band. The area intensity of the OH absorption band from 3150 cm ^{-1 to} 3450 cm ^-1 was calculated. C of 2900 cm ^{-1 to} 3000 cm ^-1
The area intensity of the H absorption band was calculated, and the ratio with the area intensity of the OH absorption band was taken. The results are shown in (Table 1). It can be seen from the results in (Table 1) that the higher the heating temperature, the lower the absorbance in the OH absorption band. Thus, the degree of cure can be evaluated by measuring the absorbance in the OH absorption band.

The value obtained by dividing the absorbance in the OH absorption band by the absorbance in the CH absorption band also decreases as the heating temperature rises. In this way, it is possible to evaluate the degree of curing by the value obtained by dividing the absorbance in the OH absorption band by the absorbance in the CH absorption band.

Further, the following experiment was conducted in order to judge the correlation between the method for evaluating the degree of curing of the alignment film of the present invention and the conventional measurement of imidization ratio. A sample was prepared by coating the alignment film used in Examples 4 to 8 on a silicon wafer. Example 4-
It was cured under the same curing conditions as in Example 8, and the absorbance of imide ring absorption at 725 cm ^-1 was measured. Here, the heat treatment at 250 ° C. was taken as the imidization ratio of 100%, and the imidization ratio at other heat treatment temperatures was calculated from the absorbance of the imide ring absorption at 725 cm ⁻¹ . FIG. 2 shows the relationship between the imidization ratio thus obtained and the area intensity of OH absorption obtained by the alignment film on the glass substrate of the present invention. From FIG. 2, it is clear that there is a negative correlation between the imidization ratio and the area intensity of OH absorption obtained by the curing degree evaluation method of the present invention. (Comparative Example) The infrared absorption spectra of the samples used in Examples 1 to 8 were measured. 17 to measure the imidization ratio
80cm ^-1, 720cm ^-1, tried to measure the absorbance around 600 cm ^-1, absorption peaks in the spectrum at 2000 cm ^-1 smaller area in absorbance could not be observed.

The method for evaluating the degree of curing of the alignment film of the present invention can be easily incorporated into the manufacturing process because the substrate for the actual liquid crystal display device can be measured. By using the curing degree evaluation method of the present invention, the optimum curing conditions for the polyimide alignment film can be easily found at the manufacturing site and the optimum curing conditions can be maintained, which is effective in improving the quality.

[0025]

As is apparent from the above description,
According to the curing degree evaluation method of the polyimide alignment film of the present invention,
In the process of manufacturing the liquid crystal display element, the actual curing degree of the alignment film on the substrate for the liquid crystal display element can be easily known. Therefore, defective curing of the alignment film, unevenness of curing of the alignment film within the substrate surface, and the like can be easily known immediately after the curing step.

[Brief description of drawings]

FIG. 1 is a graph showing an infrared absorption spectrum used in a curing degree evaluation method of the present invention.

FIG. 2 is a graph showing the correlation between the degree of cure of one example of the present invention and the conventional imidization ratio.

Claims (3)

[Claims] 1. An infrared absorption spectrum of a substrate having a polyimide alignment film formed on one surface thereof is at least 3100 cm ^{−1 to} 3
A method for evaluating the degree of cure of a polyimide alignment film, which comprises measuring in a region of 600 cm ^-1 and using the absorbance of the OH stretching vibration absorption band in this region to evaluate the degree of cure of the alignment film on this substrate. 2. An infrared absorption spectrum of a substrate having a polyimide alignment film formed on one surface thereof is at least 2800 cm ^{-1 to} 3
It is measured in a region of 600 cm ^-1 , and the ratio of the absorbance of the OH stretching vibration absorption band to the absorbance of the CH stretching vibration absorption band in this region is used to evaluate the curing degree of the alignment film on this substrate. Evaluation method for curing degree of polyimide alignment film. 3. The method for evaluating the degree of cure of a polyimide alignment film according to claim 1 or 2, wherein the polyimide alignment film is formed by imidizing polyamic acid by heating.