JP2721214B2 - Liquid crystal display - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a matrix type liquid crystal display device which drives each picture element of a liquid crystal layer by a large number of picture element electrodes arranged in a matrix.

(Prior Art) In an active matrix type liquid crystal display device, a liquid crystal layer is sandwiched between a pixel electrode substrate and a counter electrode substrate to form a liquid crystal display cell. Picture element electrode substrate. On a transparent substrate,
TFT (Thin Film Transistor), MIM (Metal-Insulating Layer-Metal), Transistor, which has a large number of pixel electrodes arranged in a matrix and applies a predetermined voltage to each pixel electrode.
Functional elements such as diodes and varistors are connected to the respective picture element electrodes and arranged on the transparent substrate. In such a pixel electrode substrate, each of the pixel electrodes and each functional element is prevented from being mechanically or chemically damaged, and each function is prevented from leaking between the pixel electrodes. The element and each picture element electrode are entirely covered with an insulating protective film except for a part connected to an external terminal or the like. And, in order to align the liquid crystal molecules of the liquid crystal layer on the insulating protective film,
For example, an alignment film using a polyimide resin is laminated, and the liquid crystal molecules of the liquid crystal layer laminated on the pixel electrode substrate are twist-aligned by the alignment film. A counter electrode is provided on a counter electrode substrate which is provided to face the pixel electrode substrate with the liquid crystal layer interposed therebetween.

Japanese Patent Application Laid-Open No. 62-296123 discloses that a polyimide is used as an insulating protective film on a pixel electrode substrate, and on each pixel electrode, the polyimide insulating protective film is laminated only on a part thereof. A liquid crystal display device in which a pixel electrode is covered with a liquid crystal alignment film made of polyimide is disclosed. in this case,
Polyimide used as an insulating protective film has the drawbacks that pinholes are likely to occur due to the poor film quality, and that current tends to leak due to high moisture permeability. For this reason,
When such a polyimide is used as an insulating protective film for protecting a functional element such as a TFT, a film thickness of 2 μm or more is required. In the above publication, the thickness of the polyimide insulating protective film is about 2.5 μm. In liquid crystal display devices,
Since the thickness of the liquid crystal layer is usually about 10 μm, if a polyimide insulating protective film is coated over the entire surface of the pixel electrode, the impedance of the insulating protective film is equivalent to 1/2 to 1/3 of the liquid crystal. The loss is remarkable and the liquid crystal molecules do not undergo alignment conversion unless a high voltage is applied to the liquid crystal layer. In this way, if the polyimide insulating protective film is coated over the entire surface of the pixel electrode, a high voltage is required to drive the liquid crystal layer, and the load on functional elements such as TFTs and the load on wiring are increased. Is significantly reduced. In the liquid crystal display device of the above publication, the polyimide insulating protective film on the picture element electrode is removed so that the liquid crystal layer can be driven by a relatively low voltage, and such a problem is solved.

(Problems to be Solved by the Invention) However, in the liquid crystal display device disclosed in the above publication, the thickness of the polyimide insulating protective film laminated on a part of the pixel electrode must be increased. When the impedance of the protective insulating film increases and a voltage is applied to the pixel electrode to drive the liquid crystal layer, the pixel electrode portion where the polyimide insulating protective film is laminated does not have the polyimide insulating protective film laminated. There is a problem that the applied voltage to the liquid crystal layer becomes lower than that of the other portions, so that contrast unevenness occurs in a picture element driven by one picture element electrode, and display quality is remarkably deteriorated.

In addition, when a polyimide resin is used as the alignment film of the liquid crystal layer, internal polarization easily occurs between the pixel electrode and the pixel electrode.
Although the detailed mechanism of this internal polarization is unknown, the polyimide resin Since the polarizer has many polar groups, polar molecules or ions are specifically adsorbed in or on the polyimide-based alignment film, and an electric double layer Is considered to cause internal polarization. When the internal polarization occurs, the voltage applied to the liquid crystal layer is adversely affected. Specifically, in order to drive the liquid crystal layer, a DC offset voltage is applied to an AC voltage applied between the pixel electrode and the counter electrode, and the AC voltage is brought into a non-equilibrium state. As described above, if the DC component is superimposed on the AC voltage applied to the liquid crystal layer and the DC voltage is offset, the display of the liquid crystal layer may cause flicker, decrease the contrast, and the like. The display quality is remarkably deteriorated, for example, due to the unevenness of contrast. The capacity of such internal polarization is
If an insulating protective film is interposed between the polyimide resin alignment film and the pixel electrode, the thickness is affected by the thickness of the insulating protective film, and the thickness increases as the insulating protective film becomes thicker. When polyimide is used as an insulating protective film as in the liquid crystal display device disclosed in the above publication, the film thickness needs to be 2 μm or more, and the insulating protective film is laminated on a part of the pixel electrode. Even if there is, degradation of display quality such as uneven contrast due to the occurrence of internal polarization cannot be ignored.

When a TFT is used as a functional element connected to each pixel electrode, the gate voltage due to the capacitance between the gate and the drain electrode is coupled to the drain voltage, and a DC voltage component is superimposed. As described above, when the DC voltage component is weighed in the drain voltage, the DC voltage is applied to the voltage applied to the counter electrode, and the voltage waveform of the counter electrode is offset to be superimposed on the drain voltage. A method for compensating for a DC voltage component is known. However, it is well known that the DC voltage component superimposed on the drain voltage greatly changes depending on the level of the source voltage, and in fact, it cannot be completely compensated. The DC voltage component that is not completely compensated is applied to the liquid crystal layer, and it takes a long time to increase the capacity of the above-described internal polarization and relax the polarity of the internal polarization. As a result, the strong polarity of the internal polarization generated by the source voltage applied as the image signal is stored for a predetermined time, so that the liquid crystal layer can be driven by following the change in the source voltage (image signal). It is no longer possible, and an afterimage is displayed on the liquid crystal layer. One of the causes of an afterimage in an active matrix type liquid crystal display device using a TFT as a functional element is as follows.
It was found that this internal polarization occurred.

Thus, if on the pixel electrode via an insulating protective film is laminated an alignment layer of polyimide resin, various problems caused by the generation of internal polarization, Ya inorganic material such as SiO ₂ as an alignment film When a silane coupling agent is used, the above-mentioned problems hardly occur.

When a polyimide resin is used as the alignment film, the above-described problem occurs due to the influence of the insulating protective film laminated on the pixel electrode. For example, it is conceivable that the insulating protective film is not provided. However, if the insulating protective film is removed, the resistance to mechanical and chemical damage during manufacturing will deteriorate. For example, when a polyimide resin used as an alignment film is subjected to a rubbing alignment process, the pixel electrode on a transparent substrate, etc. Peeling occurs. Further, an increase in leakage current between the pixel electrodes, for example, an increase in the off-state current of the TFT, causes display unevenness and the like, thereby deteriorating the display quality and lowering the display reliability.

An object of the present invention is to solve the above-mentioned conventional problems, and an object of the present invention is to provide a liquid crystal display device capable of preventing the influence of an internal polarization phenomenon that occurs when a polyimide resin is used as an alignment film.

(Means for Solving the Problems) A liquid crystal display device according to the present invention comprises a liquid crystal layer, a large number of picture element electrodes arranged in a matrix on a substrate for driving each picture element of the liquid crystal layer, and A liquid crystal display device comprising: a pixel electrode substrate having a functional element disposed on the substrate and electrically connected to each of the pixel electrodes; An insulating protective film made of an inorganic nitride or an inorganic oxide with a thickness of 2,000 to 5,000 mm is applied to each pixel so that the area of the part directly laminated on the electrode is 1/100 or more of the area of each pixel electrode. Each of the picture element electrode portions laminated on a part of the electrode and not covered with the insulating protective film and the insulating protective film has a thickness of 300 to 1000
A pixel electrode substrate having a polarization extinction structure covered with the alignment film having a thickness of Å is provided, whereby the object is achieved.

Preferably, the insulating protection film is interposed between the alignment film and the peripheral portion of the picture element electrode.

(Operation) According to the above-mentioned polarization elimination structure, even if a predetermined voltage is applied to each pixel electrode, ions, polar molecules, and the like are concentrated at a portion where the pixel electrode and the alignment film are directly laminated. The occurrence of internal polarization is suppressed.

In addition, the insulating protective film made of inorganic nitride or inorganic oxide can be made thin in order to have a dense film and high moisture resistance and high dielectric constant. The occurrence of internal polarization is also suppressed in the part of the alignment film that is partially laminated.

Moreover, the voltage applied to each picture element of the liquid crystal layer by each picture element electrode is uniform with little difference between the portion where the insulating protective film is laminated and the other portion where the alignment film is directly laminated. Therefore, a flicker does not occur in the driven picture element, and a high-quality image without a decrease in contrast or contrast unevenness can be obtained.

Accordingly, display unevenness due to TFT off-characteristic degradation, display quality deterioration (flicker, contrast unevenness, etc.) due to internal polarization, alignment disorder and afterimage are also eliminated, and high-quality display becomes possible.

In addition, in the case where an insulating protective film is interposed between the alignment film and the peripheral portion of the pixel electrode, resistance to mechanical and chemical damage during manufacturing is improved. For this reason, for example, when a polyimide resin used as an alignment film is subjected to rubbing alignment treatment, a film such as a pixel electrode on a transparent substrate does not peel off.

(Example) Hereinafter, the present invention will be described with reference to examples.

As shown in FIG. 1, the liquid crystal display device of the present invention comprises a pixel electrode substrate 10 on which a plurality of pixel electrodes 12 composed of ITO films are disposed.
And the picture element electrode substrate 10 and a counter electrode substrate 20 disposed to face the liquid crystal layer 30 with the liquid crystal layer 30 interposed therebetween.

As shown in FIG. 2, the pixel electrode substrate 10 has a large number of source wirings 13 made of titanium arranged in parallel with each other on a transparent substrate 11 and is insulated from each other. Many gate wires made of tantalum so as to be orthogonal
14 are wired. Each pixel electrode 12 is located in a region surrounded by each source line 13 and each gate line 14.
Are stored, and a large number of picture element electrodes are arranged in a matrix on the transparent substrate 11.

TFTs 15 as functional elements are provided in the vicinity of intersections of the gate lines 14 with the source lines 13, respectively, and the TFTs 15 and predetermined pixel electrodes 12 are electrically connected.

The specific configuration of each TFT 15 and picture element electrode 12 is as follows. As shown in FIG. 1, a gate electrode is formed by a part of a tantalum gate wiring 14 wired on a transparent substrate 11, and the portion forming the gate electrode forms a gate insulating film. It is covered with tantalum oxide 15a, and the tantalum oxide 15a
It is covered with an x (silicon nitride) film 15b. The SiNx film
15b covers the entire surface of the transparent substrate 11 between the tantalum oxide film 15a and each gate wiring 14.

The portion of the gate wiring 14 corresponding to each gate electrode is covered with an a-Si film 15c via a gate insulating film composed of two layers of a tantalum oxide film 15a and a SiNx film 15b. The a
A protective film 15d is laminated on the central portion of the Si film 15c except for each side, and a pair of a-Si (n ⁺ ) films 15e and One side portion of 15e is laminated. Each a-Si (n ⁺ ) film 15e is stacked on the a-Si film 15c except for the portion where the protective film 15d is stacked.

On one a-Si (n ⁺ ) film 15e, a source electrode 15f extending from the source wiring 13 is laminated. The source electrode 15f reaches above the SiNx film 15b as a gate insulating film.

On the other a-Si (n ⁺ ) film 15e, a drain electrode 15g made of titanium is laminated. The drain electrode 1
5g reaches above the SiNx film 15b constituting the gate insulating film, and a part of the pixel electrode 12 is laminated on the drain electrode 15g laminated above the SiNx film 15b. Pixel electrode 12
Is SiNx except for the part laminated on the drain electrode 15g.
It is laminated on the film 15b.

With such a configuration, each TFT 15 is electrically connected to the source line 13 and the gate line 14, and is also electrically connected to the pixel electrode 12.

Each of the gate lines 14, the source lines 13, the entire TFTs 15, and a part of the pixel electrode 12 wired on the transparent substrate 11 are made of, for example, Si
It is covered with an insulating protective film 16 made of Nx. The insulating protective film
Reference numeral 16 denotes a pixel electrode as shown by a dashed line in FIG. 2, for example.
On 12, it is laminated only on the peripheral portion and covers the peripheral portion. The insulating protective film 16 and the insulating protective film 16
Over the entire pixel electrode 12 portion where
An alignment film 17 made of polyimide (for example, Optmer AL manufactured by Nippon Synthetic Rubber Co., Ltd.) is laminated to a thickness of 600 mm, and the entire pixel electrode substrate 10 is covered with the alignment film 17. .

The pixel electrode substrate 10 having such a structure is formed on a transparent substrate by a known process by forming a gate wiring 14, a source wiring 13, a TF
After forming the T15 and the pixel electrode 12, for example, silicon nitride (SiNx) is formed to a thickness of 5000 mm by a CVD method, and the SiNx film is exposed by applying a photoresist and overlaying a mask (mask exposure). After that, hydrofluoric acid etching is performed to remove the SiNx film laminated on portions other than the peripheral portion of the pixel electrode 12 and the SiNx film laminated on unnecessary portions such as a connection portion with an external terminal. Is formed. Thereafter, an alignment film 17 is formed by laminating a polyimide resin over the entire surface on which the insulating protective film 16 is laminated.

Counter electrode substrate 20 disposed opposite to the pixel electrode substrate 10
The color filter patterns 22 of the three primary colors red (R), green (G), and blue (B) are opposed to the respective pixel electrodes 12 on a transparent substrate 21 by, for example, a gelatin staining method. It is arranged as follows. Then, a counter electrode 23 formed by laminating an ITO film having a thickness of, for example, 600 ス パ ッ タ リ ン グ by sputtering so as to cover all the color filter patterns 22.
Is provided. On the counter electrode 23, a polyimide alignment film 24 is laminated.

The compounded film 24 is patterned by offset printing using, for example, the same polyimide as the alignment film 17 described above, heat-treated at 200 ° C. for 1 hour, further rubbed with a nylon woven fabric, and then treated with a fluorocarbon-based material. It is obtained by washing with a solvent.

The pixel electrode substrate 10 and the counter electrode substrate 20 are disposed at an appropriate distance from each other, the periphery of the two substrates is sealed, and liquid crystal is injected into the gap to form the liquid crystal layer 30. Have been.

In the liquid crystal display device of this embodiment, each TFT 15 is turned on based on a scanning signal input from each gate wiring 14 and a display data signal input from each source wiring 13, and a predetermined voltage is applied to each pixel electrode 12. Is done. Thereby, the liquid crystal layer 30 portion (picture element portion) facing each picture element electrode 12 to which a predetermined voltage is applied is driven.

At this time, since each pixel electrode 12 is directly laminated with the polyimide resin alignment film 17 except for the peripheral portion, the occurrence of internal polarization is suppressed even when a predetermined voltage is applied. An insulating protective film 16 is interposed between the periphery of each pixel electrode 12 and an alignment film 17, and the insulating protective film 16 is made of SiNx having a dense film quality and high moisture resistance and high dielectric constant. Since it is composed of a film, the film thickness is as thin as about 2000 °, and the occurrence of internal polarization is suppressed even in the part of the alignment film 17 laminated on the insulating protective film 16. In addition, the voltage applied to each picture element of the liquid crystal layer 30 by each picture element electrode 12 has almost no difference between the portion where the insulating protective film is laminated and the other portion where the polyimide alignment film 17 is directly laminated. Because
There is no danger of flicker occurring in the driven picture elements,
A high-quality image without a decrease in contrast or uneven contrast can be obtained.

Suppresses the occurrence of internal polarization if the area where the polyimide alignment film 17 on each pixel electrode 12 is directly laminated without interposing an insulating protective film is 1/100 or more of the entire area of each pixel electrode 12 Yes, but the larger, the more effective
A sufficient effect can be obtained with / 30 or more. It is considered that the occurrence of internal polarization can be suppressed because ions, polar molecules, and the like are concentrated in a portion where the pixel electrode 12 and the polyimide alignment film 17 are directly laminated.

When the state of occurrence of internal polarization in the liquid crystal display device of this example was examined, the results shown in FIG. 3 were obtained. The internal polarization was measured by applying a predetermined DC voltage for 30 minutes and evaluating the voltage required for eliminating flicker (flicker). FIG. 3 shows that all TFTs and pixel electrodes have S
The internal polarization was measured when an insulating protective film made of iNx was provided, and the results were also shown. In this example, internal polarization hardly occurred.

By setting the film thickness of the polyimide alignment film 17 to about 300 to 1000 mm, and preferably to 600 to 800 mm, there is almost no effect on the change in the characteristics due to the thickness of the insulating protective film 16. Further, the protective insulating film is not limited to an inorganic nitride such as SiNx, but may be an inorganic oxide such as SiO ₂ , Ta ₂ O ₅ , Al ₂ O ₃ , Y ₂ O ₃ , and TiO ₂ . As the polyimide resin used for the alignment film, any polymer resin having an imide group can be applied.

The liquid crystal display device of the present embodiment shown in FIGS.
When the display unevenness due to the TFT off characteristic deterioration, the polarization display quality deterioration (flicker, contrast unevenness, etc.) due to the internal polarization, and the afterimage were examined, the results shown in Example 1 of Table 1 were obtained. For comparison, in FIG. 1, a 2000-mm-thick SiO ₂ insulating protective film was provided between the counter electrode 22 of the counter electrode substrate 20 and the polyimide alignment film 24 (Example 2). No insulating protective film is provided on the substrate, and
Insulation protection film not provided on the counter electrode substrate (Comparative Example 1), Insulation protection film of the pixel electrode substrate is coated on the entire surface of each pixel electrode, and the insulation protection film is provided on the counter electrode substrate. (Comparative Example 2) The same as Comparative Example 2 (Comparative Example 3) except that the insulating protective film was not provided on the counter electrode substrate also showed display unevenness due to TFT off-characteristic deterioration, degradation of display quality due to internal polarization, and The afterimage was examined. The results are also shown in Table 1.

As described above, in the liquid crystal display device of the present invention, good display was obtained with almost no display unevenness due to TFT off characteristic deterioration, deterioration of internal polarization display quality, and almost no afterimage.

It is to be noted that better display is obtained without interposing an insulating protective film between the counter electrode of the counter electrode substrate and the polyimide alignment film. Similar results were obtained when SiO ₂ was used as the protective insulating film on the pixel electrode substrate.

(Effect of the Invention) Since the liquid crystal display device of the present invention has the above-described polarization elimination structure, even if a predetermined voltage is applied to each pixel electrode, the portion where the pixel electrode and the alignment film are directly laminated is formed. Since ions and polar molecules are concentrated, the occurrence of internal polarization can be suppressed.

Further, the insulating protective film made of inorganic nitride or inorganic oxide can be made thinner because of its dense film quality and high moisture resistance and high dielectric constant. The occurrence of internal polarization can be suppressed even in the portion of the alignment film laminated on the portion.

In addition, the voltage applied to each pixel of the liquid crystal layer by each pixel electrode can be made uniform with almost no difference between the portion where the insulating protective film is laminated and the other portion where the alignment film is directly laminated. There is no possibility that flicker will occur in the driven picture element, and it is possible to obtain a high-quality image without a decrease in contrast or uneven contrast.

Accordingly, display unevenness due to deterioration of the off-characteristics of the TFT, display quality deterioration (flicker, contrast unevenness, etc.) due to internal polarization, alignment disorder, and afterimage are also eliminated, and high-quality display becomes possible.

In addition, in the case where an insulating protective film is interposed between the alignment film and the peripheral portion of the pixel electrode, the resistance to mechanical and chemical damage during manufacturing can be improved. As a result, for example, when a rubbing alignment treatment is performed on a polyimide resin used as an alignment film, a film such as a pixel electrode on a transparent substrate does not peel off.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a liquid crystal display device having a pixel electrode substrate according to the present invention, FIG. 2 is a plan view of the pixel electrode substrate, and FIG. 6 is a graph showing internal polarization generated in FIG. 10 ... pixel electrode substrate, 11 ... transparent substrate, 12 ... pixel electrode, 1
3… Source wiring, 14… Gate wiring, 15… TFT, 16…
Insulating protective film, 17 Alignment film, 20 Counter electrode substrate, 21
Transparent substrate, 22 color filter pattern, 23 counter electrode, 24 alignment film, 30 liquid crystal.

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-62-296123 (JP, A) JP-A-64-32234 (JP, A)

Claims (2)

(57) [Claims] 1. A liquid crystal layer, a large number of pixel electrodes arranged in a matrix on a substrate for driving each pixel of the liquid crystal layer, and said plurality of pixel electrodes electrically connected to the respective pixel electrodes. A liquid crystal display device comprising: a pixel electrode substrate having a functional element disposed on a substrate; and an area in which an alignment film made of a polyimide resin is directly laminated on the pixel electrode. An insulating protective film made of an inorganic nitride or an inorganic oxide is laminated on a part of each of the pixel electrodes in a thickness of 2,000 to 5,000 mm so that the area is at least 1/100 of the area of the pixel electrodes. 300 Å to 1000 各 on each pixel electrode part where the film is not laminated and on the insulating protective film
A liquid crystal display device comprising a pixel electrode substrate having a polarization extinction structure covered with the alignment film having a thickness of Å. 2. The liquid crystal display device according to claim 1, wherein said insulating protective film is interposed between said alignment film and a peripheral portion of said picture element electrode.