JPH0915642A - Liquid crystal display device - Google Patents

Abstract

PURPOSE: To prevent after image, burning and the drop of a contrast ratio owing to designations by enlarging superposition between a pixel electrode forming a gate storage and a prestage gate bus line in the area easy to cause an orient defect owing to horizontal direction electric field in a pixel periphery compared with the area except that area. CONSTITUTION: The gate storage formed between the pixel electrode 14 and the prestage gate bus line 11 is used as storage capacity. On the other hand, a thin film transistor(TFT) substrate is rubbed in the right downward direction, and a color filter(CF) substrate is rubbed in the left upward direction. A superposition amount (a) of the pixel electrode onto the gate bus line 11 in the vicinity of left upward corner of the pixel electrode 14 opposite to the rubbing direction of the TFT substrate side is made larger than the superposition amount (b) of the pixel electrode onto the gate line in the vicinity of right upward corner of the pixel electrode. Thus, since the superposition of the gate storage in the area that the invasion distance of the disclination is large in the vicinity of left upward corner of the pixel is large, the invasion of the disclination onto the pixel area is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device driven by a thin film transistor in which a storage capacitor is formed between a preceding gate bus line and a pixel electrode. Liquid crystal display device.

[0002]

2. Description of the Related Art Demand for liquid crystal displays (hereinafter, referred to as LCDs) is expanding due to their compactness and low power consumption. Functionally, a large screen, high definition, and multiple gradations are being promoted. As the definition of the panel becomes higher, the horizontal electric field around the pixel electrode becomes stronger due to the reduction of the pixel size or the distance between the wiring and the pixel electrode, resulting in a problem of poor alignment caused by the horizontal electric field. ing.
This is remarkable in a multi-domain liquid crystal display device having a plurality of alignment states in a pixel, which is expected to have a higher viewing angle, due to a reduction in the alignment domain and instability.

A thin film transistor (hereinafter, referred to as TFT)
An active matrix type liquid crystal display device having an address element as an address element has a higher display performance such as a contrast ratio, a viewing angle, and a response than a simple matrix type liquid crystal display device, and thus the demand thereof is increasing.

A TFT active matrix type liquid crystal display device selects a gate line by setting a high potential by line-sequential scanning, turns on a TFT connected to the gate line, and applies a voltage corresponding to the luminance to be displayed on the data line. Then, a predetermined voltage is written to the pixel electrode. The pixel electrode is provided with a liquid crystal capacitance and a storage capacitance, and holds the pixel potential at a writing voltage for one frame period until the next writing is performed.
The difference voltage between the held pixel electrode voltage and the counter electrode voltage is applied to the liquid crystal layer, and a luminance display corresponding to the difference voltage is performed. Here, the storage capacitance added to the pixel electrode suppresses the fluctuation of the pixel electrode due to the leakage current of the TFT during the holding period, or the capacitance between the pixel electrode and the gate electrode when the TFT changes from the on state to the off state. The purpose is to reduce the amount of change (feedthrough) of the pixel electrode potential due to coupling (mainly the overlap capacitance between the gate of the TFT and the aus). The feedthrough amount differs depending on the gradation due to the dielectric anisotropy of the liquid crystal. However, if the feedthrough amount is large, a DC voltage (the liquid crystal is driven by an alternating current to prevent image sticking) is applied depending on the gradation, and It is said to cause reliability problems such as burn-in.

Conventionally, a common storage type storage capacity has been used as the storage capacity. 6 (a), (b),
(C) is a top view of a common storage type liquid crystal display device,
A sectional view and an equivalent circuit are shown, respectively. 6, a capacitor line 23 is formed in the same layer as the gate line 11, a capacitor is formed between the capacitor line and the pixel electrode 14, and a low voltage (for example, the same voltage as the counter electrode) is applied to the capacitor line 23. Are operated as storage capacitors 18. Since the capacitance line for preventing display noise is lowered and the capacitance line itself is formed in the same layer as the gate line (in order to avoid an increase in the number of steps), the capacitance line forms a pixel electrode (transparent electrode). There is a problem that the light cannot pass through the crossing area, and the display luminance is reduced.

A gate storage type storage capacitor disclosed in Japanese Patent Application Laid-Open No. Sho 59-16685 is known as a method for forming a storage capacitor by compensating for the disadvantages of the common storage type. FIGS. 7A, 7B, and 7C show a top view, a cross-sectional view, and an equivalent circuit diagram of a liquid crystal panel using a gate storage type storage capacitor. As shown in FIG.
Is overlapped with the gate line 11 in the preceding stage, thereby forming a capacitor 18 between the pixel electrode and the gate line. Since the selection of the gate line at the previous stage has already been completed and the voltage has become constant, it can be used as an electrode at one end of the storage capacitor. According to this configuration, a new capacitor wiring is not required unlike the common storage type, so that the area ratio of the region through which light is transmitted (the aperture ratio is generated) can be increased, and the display luminance increases.

On the other hand, in general, TN (twisted nematic)
Type liquid crystal is connected to the TFT substrate side and a color filter (hereinafter C
An orientation film (usually polyimide) is applied to each of the substrates on the F side, and rubbed with a roller wrapped with cloth such as rayon (rubbing treatment) to control the direction of the interface orientation of the liquid crystal molecules. By twisting the rubbing direction 90 degrees back and forth on the TFT substrate and CF substrate side, the liquid crystal is oriented in a state twisted 90 degrees forward and backward. When a rubbing treatment is performed on the alignment film, the rod-like liquid crystal molecules are aligned at an angle (pretilt angle) in the rubbing direction.

When no voltage is applied to the liquid crystal, the liquid crystal molecules are twisted while lying horizontally, but when a voltage is applied to the liquid crystal, the liquid crystal molecules rise. At this time, the rising direction of the liquid crystal molecules is the same as the pretilt angle direction in a region where the vertical electric field between the pixel electrode and the counter electrode is dominant. Usually, the rubbing direction and the liquid crystal's chiral direction (twisting rotation direction) are selected so that the rising direction of the liquid crystal molecules and the pretilt angle direction coincide with both the TFT substrate and the CF substrate. For example, in the setting of the rubbing direction as shown in FIG. 8, in the counterclockwise TN liquid crystal, since there is no influence of the horizontal electric field in the central portion of the pixel, the liquid crystal molecules rise in the pretilt angle direction.

However, the influence of the lateral electric field between the pixel electrode and the peripheral wiring becomes obvious at the pixel electrode end. At the pixel end on the extension line of the rubbing direction on the TFT substrate side, the pretilt direction of the liquid crystal molecules and the direction in which the liquid crystal molecules rise due to the lateral electric field (due to the dielectric anisotropy of the liquid crystal) coincide with each other. No liquid crystal alignment failure area occurs in the same direction, but at the pixel end opposite to the rubbing direction, the direction in which the liquid crystal molecules rise due to the lateral electric field and the pretilt angle direction are opposite, and the liquid crystal is oriented in a different direction from the pixel center. The molecules rise, and a misalignment (reverse tilt) region occurs. A transition region called a normal disclination line 24 is formed at the boundary between the poor alignment region and the normal alignment region. This is recognized as a bright bright line because the liquid crystal is in the same lying state as the white state, and the contrast is reduced. In addition, since the disclination line moves due to the fluctuation of the pixel voltage, when the disclination line appears in the display area, an afterimage or burn-in occurs, and the display performance is deteriorated. The influence of the horizontal electric field becomes a more serious problem with the phenomenon of the pixel size due to the increase in the display capacity.

In the reverse tilt alignment due to the lateral electric field, the lateral electric field is alleviated to some extent because the pixel electrode covers the gate bus line to some extent in the aforementioned gate storage type storage capacitor. If the area of the pixel electrode covering the gate bus is increased in order to increase the relaxation effect, the total capacitance associated with the pixel electrode increases, and the writing of the pixel becomes insufficient.

As a measure for preventing the influence of the lateral electric field,
A prior example is disclosed in JP-A-4-51121. As shown in FIG. 9, a storage capacitor 18 is formed above the pixel electrode 14 so as to surround the pixel electrode. Further, by setting the storage capacitor to the same potential as the potential of the counter electrode, a storage capacitor can be formed between the storage capacitor and the pixel electrode, and the lateral electric field around the pixel electrode can be suppressed. However, in order to realize this configuration, there is a problem that the film formation process, the photolithography process, and the etching process are increased, and the production cost of the TFT substrate is increased.

In recent years, in order to increase the viewing angle, a configuration (multi-domain) in which one pixel has a plurality of orientations as disclosed in Japanese Patent Application Laid-Open No. 52-21845 has been studied. It has been reported that a display device having a wide viewing angle can be obtained by compensating for asymmetric viewing angle characteristics caused by the rising direction of liquid crystal molecules. For example, many attempts have been made to divide the inside of one pixel into two as shown in FIG. 10 and to align the liquid crystal 19 so that the liquid crystal stands in the vertical direction. With such a configuration, a region where liquid crystal molecules rise downward and a region where the liquid crystal molecule rises upward when voltage is applied are formed, and the upper and lower viewing characteristics are characteristics obtained by combining the conventional upper and lower viewing characteristics. It is known that there is a tremendous effect in preventing key reversal.

One method of realizing this two-segment orientation is Dis.
play92, P591, 1992.
In this method, a high pretilt alignment film is formed on a TFT substrate,
A low pretilt alignment film is formed on the F substrate,
The rubbing direction of the alignment film on the FT substrate is made different in the alignment division region. Since the liquid crystal alignment is defined by the rubbing direction on the high pretilt alignment film, the alignment state of the liquid crystal is divided according to the rubbing direction on the TFT substrate side to realize multi-domain.

For example, when rubbing is performed in the direction shown in FIG. 10A, as shown in FIG.
Since both the T substrate and the CF substrate are in a normal alignment state in which the rising direction of the liquid crystal molecules and the rubbing direction upon application of a voltage match, disclination due to the lateral electric field is likely to occur near the upper left corner of the region B. . On the other hand, in the region A, as shown in FIG. 10B, the rubbing direction and the rising direction of the liquid crystal molecules are the same on the TFT substrate side, but the rubbing direction and the rising direction of the liquid crystal molecules are different on the CF substrate side. As a result, in the vicinity of the CF substrate, the orientation is in a splay state. In the splay alignment, the volume distortion of the liquid crystal molecules is large, the energy becomes unstable, and alignment defects such as reverse tilt and reverse twist (the liquid crystal molecules rotate in the opposite direction) are likely to occur. In the case of FIG. 10, the lower right of the region A in the splay alignment is likely to cause a reverse tilt because the rubbing direction is different from the rising direction of the liquid crystal molecule due to the lateral electric field. The reverse tilt is liable to occur even in the area of due to the influence of the lateral electric field.

As the pretilt angle of the liquid crystal molecules on the TFT substrate is larger, the influence of the lateral electric field is lessened, and the entry of disclination when the pretilt direction is different from the rising direction when a voltage is applied is suppressed. This is shown in FIG. 11 by a two-dimensional liquid crystal simulation (calculation of the liquid crystal molecular orientation from the electric field distribution in the liquid crystal). From the figure, it can be seen that increasing the pretilt angle from 3 degrees to 7 degrees reduces the disclination penetration distance from 5 μm to about 3.5 μm. When rubbing on the CF substrate side is performed in a direction in which liquid crystal molecules are in a splay alignment state, T
It can be seen that even when the FT substrate has the same pull-tilt angle of 7 degrees, the disclination penetration distance increases from 3.5 μm in normal orientation to about 5 μm.

As described above, all of the conventional gate storage capacitors have a configuration in which the overlapping portion between the gate wiring and the pixel electrode is uniform.

[0017]

When the above-mentioned conventional disclination line in a lateral electric field around the pixel electrode penetrates deeply into the pixel electrode region, the CF substrate side is used to shield the disclination line from light. It is necessary to increase the area of the black matrix (BM). As a result, BM
The opening area of the pixel decreases, the area ratio (opening ratio) through which light passes in the pixel region decreases, and the power consumption of the backlight increases to increase the power of the backlight in order to compensate for the deterioration of the visibility such as a decrease in luminance or the like. There is a problem to say.

An object of the present invention is to solve the above-mentioned problems and realize a low-power-consumption LCD with a high luminance and a high viewing angle.

[0019]

According to the present invention, there is provided an active matrix type liquid crystal display device in which a thin film transistor is used as a switching element and a storage capacitor is formed between a pixel electrode and a preceding gate bus line. The rubbing direction of the alignment film on the substrate is inclined from the first corner of the pixel electrode on the side of the preceding gate line to the direction of the next gate line, and the accumulation on the first corner side of the pixel electrode is performed. The amount of overlap between the pixel electrode forming the capacitor and the preceding gate bus line is equal to the second amount of the pixel electrode from the preceding gate wiring.
Is larger than the overlap distance between the pixel electrode forming the storage capacitor on the corner side and the preceding gate bus line. When the pretilt angle of the liquid crystal molecules on the TFT substrate side is 7 degrees or more, the overlap distance on the first corner side is 10 μm or more, the overlap distance on the second corner side is 4 μm or more, and when the pretilt angle is 3 degrees or more, , 15μm or more and 5μ respectively
m or more.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIGS. 1A, 1B, and 1C are respectively a top view of an LCD panel showing a first embodiment of the present invention and a dotted line A-.
2A is a sectional view and an equivalent circuit diagram. FIG. In the present embodiment, a gate storage formed between the pixel electrode 14 and the preceding gate bus line 11 is used as a storage capacitor. On the other hand, the rubbing direction on the TFT substrate is the lower right direction, and the rubbing direction on the CF substrate is the upper left direction. The overlap amount a of the pixel electrode 14 in the vicinity of the upper left corner of the pixel electrode 14 on the opposite side of the rubbing direction on the TFT substrate side to the gate bus line 11 is:
It is larger than the overlap distance b of the pixel electrode near the upper right corner of the pixel electrode to the gate line.

Assuming that the pretilt angle of the liquid crystal molecules on the TFT substrate side is 3 degrees, which is usually used, the overlap distance a near the upper left corner is about 15 μm or more and the overlap distance b near the upper right corner is about 15 μm from the simulation result of FIG. It is necessary to set it to 5 μm or more. Further, assuming that the high pretilt angle is about 7 degrees, which is currently possible, the overlapping distance a should be 10 μm or more and the overlapping distance b should be 4 μm or more. Here, the width Wa of the region of large overlap and the width Wb of the region of small overlap are storage area = a · Wa + b · W
Wa and Wb are set so that b can obtain a predetermined storage capacity (1/2 or more of the liquid crystal capacity). With this configuration, the overlap of the gate storages at the location where the approach distance of the disclination near the upper left corner of the pixel is large is large, so that the approach of the disclination to the pixel area can be suppressed.

FIGS. 2A, 2B, and 2C are a top view, a sectional view, and an equivalent circuit diagram of an LCD panel showing a second embodiment of the present invention. In this embodiment, the gate storage is formed by connecting a straight line with the overlap distance a at the upper left corner of the pixel electrode and the overlap distance b at the upper right corner of the pixel electrode.
In this case, assuming that the pretilt angle is about 3 degrees, a ≧
It is necessary to select the overlapping distances a and b so that 10 μm, b ≧ 4 μm and the gate storage area (a + b) · W / 2 can obtain a predetermined capacity. In this configuration, the overlap distance a can be made larger than in the first embodiment, so that the effect of further suppressing the intrusion of the disclination line into the display area can be obtained.

FIGS. 3A, 3B, and 3C are a top view, a sectional view, and an equivalent circuit diagram of an LCD panel showing a third embodiment of the present invention. This embodiment has the configuration of the first embodiment.
At the upper left corner of the pixel electrode, the pixel electrode portion on the gate bus wiring protrudes by c toward the data line side. In this configuration, the upper left corner of the pixel where the disclination easily enters is further separated by the distance c, so that the entry of the disclination line into the pixel area can be further suppressed. Here, the effect can be obtained by protruding the distance c even slightly, but a larger effect can be expected if the distance c can be protruded up to about 10 μm. However, it is necessary to determine the distance c in consideration of the balance with the layout of the data wiring. is there.

FIG. 4 shows a case in which the orientation in one pixel is divided into two in the fourth embodiment of the present invention. In the rubbing direction on the TFT substrate side, rubbing is performed toward the upper left of the pixel in the upper half A of the pixel electrode, and rubbing is performed toward the lower right of the pixel in the lower half B of the pixel electrode. On the other hand, rubbing is performed on the CF substrate side toward the upper right of the pixel. Furthermore, the overlap a of the pixel electrode near the upper right corner of the pixel with the gate bus line is larger than the overlap b of the pixel electrode near the upper left corner of the pixel with the gate bus line. In this case, the liquid crystal in the region A is oriented in the splay state, and the disclination line easily enters the pixel region near the upper right corner of the pixel.
In this structure, the pixel electrode greatly overlaps the gate bus line in that region, so that the disclination line can be prevented from entering the pixel region. According to FIG. 11, when the pretilt angle on the TFT substrate side is about 3 degrees, the overlap a needs to be 10 μm or more and the overlap b needs to be 5 μm or more. For a pretilt angle of about 7 degrees, a is 5 μm
As described above, it is understood that b needs to be 4 μm or more.

FIG. 5 shows a fifth embodiment of the present invention, in which the pixel electrode at the upper right corner of the pixel is protruded to the data line side, and the intrusion of the misalignment region due to the lateral electric field in the spray region can be suppressed. it can. In this case, a great effect can be obtained by setting the protruding distance c in the horizontal direction as large as possible within the range of the layout of the data bus lines.

According to the present invention, not only the above-described embodiment but also an essential feature is to increase the overlapping distance of the pixel electrode of the gate storage portion to the preceding gate bus line in a region where alignment failure is likely to occur due to a lateral electric field. The shape of the pixel electrode on the gate bus can be variously modified other than that described in this embodiment.

[0027]

As described above, in the liquid crystal display device of the present invention, in the case of single-domain orientation, an extension region in the direction opposite to the rubbing direction on the TFT substrate side, and in the case of two-split domain orientation, spraying is performed. In areas where the lateral electric field around the pixel tends to cause misalignment, such as an area extending in the direction perpendicular to the rubbing direction in the area, the overlap between the pixel electrode forming the gate storage and the previous gate bus line is greater than in other areas. By increasing the size, it is possible to suppress the invasion of the disclination generated in those areas into the pixel area, prevent afterimages, image sticking, and a decrease in contrast ratio due to the disclination, and obtain excellent display performance.

[Brief description of the drawings]

FIGS. 1A, 1B, and 1C are a top view, a sectional view, and an equivalent circuit diagram of a liquid crystal panel according to a first embodiment of the present invention.

FIGS. 2A, 2B, and 2C are a top view, a cross-sectional view, and an equivalent circuit diagram of a liquid crystal panel according to a second embodiment of the present invention.

FIGS. 3A, 3B, and 3C are a top view, a sectional view, and an equivalent circuit diagram of a liquid crystal panel according to a third embodiment of the present invention.

FIG. 4 is a top view of a liquid crystal panel showing a fourth embodiment of the present invention.

FIG. 5 is a top view of a liquid crystal panel showing a fifth embodiment of the present invention.

FIGS. 6A, 6B, and 6C are a top view, a cross-sectional view, and an equivalent circuit diagram of a liquid crystal panel showing a conventional example.

7A, 7B, and 7C are a top view, a cross-sectional view, and an equivalent circuit diagram of a liquid crystal panel showing a conventional example.

FIGS. 8A and 8B are a top view and a cross-sectional view of a liquid crystal panel showing a conventional example.

FIGS. 9A, 9B, and 9C are a top view, a sectional view, and an equivalent circuit diagram of a liquid crystal panel showing a conventional example.

FIGS. 10A, 10B, and 10C are a top view, a sectional view, and an equivalent circuit diagram of a liquid crystal panel showing a conventional example.

FIG. 11 is a diagram showing a relationship between a pretilt angle on a TFT substrate and a penetration distance of a disclination line.

[Explanation of symbols]

 Reference Signs List 11 gate line 12 data line 13 S / D electrode 14 pixel electrode 15 gate insulating film 16 TFT 17 liquid crystal capacitance 18 storage capacitance 19 liquid crystal layer 20 counter electrode 21 TFT substrate 22 CF substrate 23 storage capacitance line 24 disclination line 25 orientation division line

Claims (6)

[Claims] 1. In an active matrix type liquid crystal display device in which a thin film transistor is used as a switching element and a storage capacitor is formed between a pixel electrode and a gate bus line in the preceding stage, an alignment film on a substrate on which the thin film transistor is formed is formed. The rubbing direction is formed obliquely from the first corner of the pixel electrode on the gate wiring side of the previous stage to the direction of the gate wiring of the next stage, and the pixel electrode forming the storage capacitance on the first corner side of the pixel electrode is formed. It is characterized in that the amount of overlap with the preceding gate bus line is larger than the overlapping distance between the pixel electrode that forms the storage capacitance on the second corner side of the preceding gate wiring of the pixel electrode and the preceding gate bus line. Liquid crystal display device. 2. The liquid crystal display device according to claim 1, wherein
When the pretilt angle of the liquid crystal molecules on the alignment film on the substrate side on which the thin film transistor is formed is 7 degrees or more, the overlapping distance between the pixel electrode and the gate bus line on the first corner side of the pixel electrode is 10 μm or more, A liquid crystal display device, wherein an overlapping distance in the vicinity of the second corner of the pixel electrode is 4 μm or more. 3. The liquid crystal display device according to claim 1, wherein
When the pretilt angle of the liquid crystal molecules on the alignment film on the substrate side on which the thin film transistor is formed is 3 degrees or more, the overlap distance between the pixel electrode and the gate bus line on the first corner side of the pixel electrode is 15 μm or more, A liquid crystal display device, wherein an overlapping distance in the vicinity of the second corner of the pixel electrode is 5 μm or more. 4. An active matrix type liquid crystal display device in which a thin film transistor is used as a switching element and a storage capacitor is formed between a pixel electrode and a preceding gate bus line, wherein an alignment film on a substrate on which the thin film transistor is formed is formed. The rubbing direction is different between the first pixel region on the previous gate bus line side and the second pixel region on the next gate side, and the rubbing direction in the first pixel region is the same as that of the pixel electrode on the previous gate line side. And the overlap distance between the pixel electrode forming the storage capacitor on the second corner side of the pre-stage gate wiring of the pixel electrode and the pre-stage gate bus line is oblique in the direction toward the first corner, A liquid crystal display device, wherein the distance is greater than the overlap distance between a pixel electrode forming the storage capacitor on the first corner side of the pixel electrode and a preceding gate bus line. 5. The liquid crystal display device according to claim 4, wherein
When the pretilt angle of the liquid crystal molecules on the alignment film on the substrate side on which the thin film transistor is formed is 7 degrees or more, the overlap between the pixel electrode and the gate bus line near the first angle of the pixel electrode is 4 μm or more, A liquid crystal display device, wherein an overlap distance of the pixel electrode near a second corner is 5 μm or more. 6. The liquid crystal display device according to claim 4,
When the pretilt angle of the liquid crystal molecules on the alignment film on the substrate side on which the thin film transistor is formed is 3 degrees or more, the overlap between the pixel electrode and the gate bus line near the first angle of the pixel electrode is 5 μm or more, A liquid crystal display device, wherein an overlapping distance of the pixel electrode near the second corner is 10 μm or more.