JP2605610B2 - Liquid crystal display - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having an improved aperture ratio.

[0002]

2. Description of the Related Art First, a twisted nematic liquid crystal display (TN-LCD) will be described.

A liquid crystal display (LCD) is composed of two glass plates facing each other with a liquid crystal layer of several μm therebetween, and the inner surfaces of each glass plate are covered with a transparent electrode such as ITO (indium tin oxide). ing. The transparent electrode in the display screen is divided into pixels (pixels) having a shape and a size corresponding to a display target, and each of these pixels is driven from the outside through wiring running on the glass surface.

The most frequently used operation modes in practical use are:
In the twisted nematic mode, in this case, for example, the polyimide alignment film is subjected to a rubbing process in a direction orthogonal to each other on the pixel side and the counter electrode side, and accordingly, the liquid crystal molecules are respectively interfaced on the pixel surface and the counter electrode surface. Are oriented so as to be perpendicular to each other and parallel to the interface, and in the middle of both surfaces, the direction changes continuously and parallel to the interface. When no voltage is applied between the pixel and the counter electrode, the liquid crystal molecules assume the above-described orientation, and gradually become perpendicular to the electrode surface as the voltage increases.

When no voltage is applied, the polarized light incident on the liquid crystal continuously rotates in accordance with the direction of the liquid crystal molecules, and rotates 90 degrees with respect to the incident light when emitted. When a voltage is applied, the rotation of the emitted light decreases as the liquid crystal molecules become vertical, and when the voltage is sufficiently large, the direction of change of the emitted light becomes the same as that of the incident light.

By installing polarizing plates on both sides of a glass plate, it is possible to control the light transmittance in accordance with the magnitude of the applied voltage, and when the optical axes of the polarizing plates are orthogonal to each other, a normally white mode is used. (White when no voltage is applied), and a normal black mode (black when no voltage is applied) is obtained when they are set in parallel.

Next, the internal structure of the device will be described. In order to control the light transmittance of the pixel, the liquid crystal sandwiched between the transparent electrodes needs to be oriented in a specific direction. For this purpose, an alignment film is formed on the surface of the transparent electrode. The alignment film is formed of an organic polymer, an inorganic vapor-deposited film, or the like, and has a thickness of several tens to several hundreds of μ.
m is a thin film of, polyimide are often used these days. The alignment film is subjected to a treatment for aligning the liquid crystal in a predetermined direction, and rubbing is used as a typical method in the case of a polymer alignment film, and control of the evaporation direction is used in the case of an inorganic evaporation film. . Rubbing is an operation of rubbing in a specific direction with a polymer surface and a material such as cloth, and liquid crystal molecules are aligned in the rubbed direction at the interface with the alignment film in parallel with the interface.

On the other hand, there is an active matrix type LCD (AM-LCD) as another type. Among liquid crystal display elements, an active matrix type liquid crystal display element (AM-LCD) characterized by color display and high image quality display is usually divided into pixels of several tens to several hundreds μm on one glass, and each pixel is Are driven by thin film transistors (TFTs) connected to each. An integral transparent electrode is formed on the opposing electrode, and in the case of color display, a color filter is further formed in contact with the transparent electrode. TFT to drive this TFT
Data wiring (drain wiring) and address wiring (gate wiring) are provided on the side substrate. The scanning lines are sequentially addressed by the address lines, and a predetermined voltage is applied to the pixels by the data lines, whereby a predetermined voltage is written to all the pixels. The liquid crystal between each pixel and the counter electrode is driven in accordance with the voltage thus written, and the desired screen is displayed by changing the light transmittance in each pixel accordingly. An AM-LCD using such a driving method has advantages in image quality, such as a fast response speed, a good contrast, a wide field of view, no crosstalk, and a possibility of halftone display.

[0009] Further, since high image quality is demanded especially in color display which has been attracting attention in recent years, AM-LCD
Has become mainstream.

In an AM-LCD, a color filter having a transparent electrode formed on the surface is used for a color display, and a transparent electrode is formed on the surface for a monochrome display. The formed transparent substrate is used.

In the display screen of the LCD, a horizontal or oblique electric field exists between the wiring and the display electrode at a position corresponding to the periphery of the wiring on the active element side. Since this is different from the electric field based on the originally applied voltage to the liquid crystal in the pixel, the display state of the liquid crystal at this position becomes abnormal. In order to prevent the influence from this, in the AM-LCD, a light shielding area is provided at a position on the counter substrate side corresponding to a peripheral portion of the wiring. This light-shielded area is formed by a black mask (B
M), and is usually formed by fine patterning of a metal thin film such as chromium or a thin film made of black resin. The brightness of the black display of the AM-LCD is reduced by the presence of the BM, and a high contrast ratio and clear screen quality can be obtained.

FIG. 8 shows a conventional liquid crystal display device.Like
A pixel electrode 2 between wirings 1 formed in a matrix.
A portion other than the pixel electrode 2 (the wiring 1 and the TFT 3
Is shielded from light, and only the pixel electrode 2 is opened.
Thus, the black mask 4 is formed. This black
Mask 4 is almost the same as or slightly reduced in length from pixel electrode 2.
Opened in a combination of squares or rectangles, with an aperture ratio of approximately
40%.

[0013]

A problem of the AM-LCD due to the presence of the BM is that a large part of the pixel is shielded from light by the BM, so that the aperture ratio is reduced. In an LCD, light is normally emitted from the back of the LCD panel by a light source (backlight), and the effective use efficiency of the incident light is proportional to the aperture ratio. Therefore, a low aperture ratio means that the screen brightness is reduced when the light source is the same, and a higher light source energy amount is required when the screen brightness is adjusted. TFT type L
If a specific example is given in a typical example of CD, the aperture ratio is 40%.
And the utilization efficiency of the incident light is about 40%.

However, if the light-blocking area of the BM is expanded beyond the current level, the luminance in black display (black luminance) increases due to light leakage at the periphery of the wiring, resulting in a decrease in contrast ratio and deterioration in display quality. Therefore, it is impossible to further improve the aperture ratio by the BM method currently being used.

[0015]

In the present invention, the shape of the BM opening in the pixel is not formed by a rectangle or a combination of rectangles as conventionally performed, but by a conventional method.
Starting from a rectangle or a combination of rectangles, it is formed to have a notch with a convex shape on the wiring side from there.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The reason why the present invention can improve the aperture ratio without a significant increase in black luminance, unlike the prior art, will be described with reference to the drawings.

FIG. 1 shows an example of a typical BM pattern according to the present invention, wherein the aperture ratio is 50%. On the other hand, FIG. 8 shows an example of a typical BM pattern according to the prior art, in which the aperture ratio is 40%. FIG. 6 shows an AM- using this pattern.
FIG. 6 is a diagram showing a relationship between an overlay shift between upper and lower substrates in an LCD and luminance in black and white display.

The current overlay accuracy of the TFT substrate and the counter substrate is about several μm, and a deviation between the TFT element pattern in the unit pixel and the BM pattern of the counter substrate occurs within this accuracy range. The problem that arises when the overlay deviation is larger than a certain limit is that the BM cannot effectively shield the light leakage portion on the TFT side.

This example is shown as a case according to the prior art .
It is shown in FIG. Here, the TFT pattern is the same as that of FIG. 8, and the aperture ratio is increased to 50%, which is the same as FIG. FIG. 9 shows a case where there is no overlap deviation . The influence on the screen when the overlay shift occurs and the light leakage occurs is that the black luminance is not sufficiently reduced when black is displayed in the normally white mode display. Under such circumstances, the following problems occur. In the example in which the aperture ratio is set to 50% as shown in FIG. 9, the black luminance is significantly increased at the time of the misalignment, and the image quality is reduced. In this case, the calculated contrast ratio is 18, which is 100 when the aperture ratio is 40%.
Has dropped significantly.

FIG. 6 shows this relationship. Figure 6 shows BM and A
FIG. 4 is a diagram illustrating a relationship between a misalignment with an M-LCD substrate and luminance in black and white display. The vertical axis is the screen brightness,
The horizontal axis is the position coordinates with respect to the TFT substrate.
FIG. 6 (1) is based on one embodiment of the present invention, and (2) is based on the prior art. In order to make the numerical values concrete, in this example, the pixel size is 300 μm in height and 100 μm in width, and the distance between the wiring and the pixel transparent electrode is 8 μm. In addition, the current substrate overlay accuracy was set to 8 μm. B
The position of the end of M with respect to the TFT substrate is plotted on the horizontal axis in FIG. 6, and the black luminance at each position is plotted. In the prior art, as shown in FIG. 6 (2), when the BM end moves to the right, it does not change until it reaches the transparent electrode end position, but increases linearly from the right side. Here, if an overlap shift occurs, the shift amount is not constant depending on the location in the same screen. Therefore, the black luminance in the screen becomes non-uniform in accordance with the deviation. This is recognized as a display unevenness by the observer of the screen, and is recognized as a poor image quality. This is because the relationship between the deviation and the change in luminance is linear as shown in FIG. 6 (2), so that a slight deviation causes large unevenness. This is because the sensitivity of the human eye to the luminance is higher for its derivative than for the luminance itself. Therefore, in this case, the allowable range of the design of the opening of the BM is a portion inside the transparent electrode end by 8 μm of the substrate overlay accuracy, and as a result, the opening ratio cannot be increased to 40% or more.

On the other hand, FIG. 6A shows the relationship between the overlay deviation and the black luminance in the present invention. Here, the black luminance gradually increases even after the BM end reaches the position of the transparent electrode. In this case, the gradient of the black luminance increase curve becomes the same as that of the prior art because the BM moves significantly to the right and the portion of the vertical side having the maximum width (points a and a 'in FIG. Connecting straight line)
At the end of the transparent electrode. Therefore, a and a '
In a considerable part until the straight line connecting to the transparent electrode end, that is, the line connecting the points b and b ', the change rate of the black luminance corresponding to the displacement of the BM (that is, the curve of FIG. Is significantly smaller than in the prior art. This means that screen unevenness in black display caused by the overlay shift is reduced. Therefore, in the case of the AM-LCD of the present invention, even if the overlap is displaced and the opening of the BM bites into the light leaking portion of the TFT considerably, it is not recognized as unevenness on the display screen. Diagonal 1
It has been empirically known that in the case of a screen size of 0 inch, if the maximum / minimum ratio of black luminance on the display screen is 2 or less, unevenness is not recognized. In the current state of the art, the variation of the deviation on the inner surface in a 10-inch size is a maximum-minimum difference of about 4 μm, and the corresponding inner surface luminance difference in the prior art is 100% or more. (Between d-d 'in FIG. 6 (2)), which results in unevenness. On the other hand, in the present invention, such non-uniformity in the display surface does not appear, and no unevenness is recognized. FIG. 6A shows the position where the luminance difference occurs, and is ee ′. The maximum / minimum ratio of the in-plane luminance difference can be made 2 or less at the point e ′ in the figure (that is, in this example, the transparent state). Any point on the left side of the electrode end (point 11 μm right) is sufficient. That is, in this example, the BM end may be located at a maximum of 11 μm to the right of the transparent electrode end.
As shown by f in FIG. 6A, 3 μm from the end of the transparent electrode
It will be good to be located on the right. Actually, the same occurs on the left side of the pixel. When the BM is shifted to the right, light leakage is reduced on the left side. Therefore, the increase in black luminance at the time of the overlay shift is even smaller than the result discussed above.

On the other hand, in the case of the prior art,
The BM is not allowed to exist to the right of the edge of the transparent electrode.
It is only allowed on the left by more than μm in design.

Based on the allowable range of the BM end,
The allowable range of the aperture ratio corresponding to the shape of the BM is determined. According to the difference in the aperture ratios, the luminance when white display is performed is determined this time. When the aperture ratio is calculated according to the above-described example, the aperture ratio is 40% in the related art, but is 54% in the present invention.
It is 1.35 times, showing a significant improvement. The aperture ratio is proportional to the white luminance when the same backlight is used. Therefore, in the present invention, 1.35 times the luminance can be obtained. In other words, the power consumption is reduced by a factor of 1.35 in order to obtain the same luminance, resulting in a significant power reduction. When there is no misalignment, look through the BM opening T
Most of the FT substrate side is occupied by the display electrodes,
Black luminance is suppressed to a low level. On the other hand, when the superposition shift occurs on the right side (broken line), the light leakage on the right side increases, but the light leakage on the left side decreases, and the effects of both compensate for each other. Therefore, the black luminance does not increase so much. Also, there is little decrease in contrast. Incidentally, the contrast ratio in this case is 30, which is much higher than the value in the case of the prior art.

[0024]

[0025]

As described above, the LCD according to the present invention is
Can greatly increase the aperture ratio while maintaining a practical contrast ratio as a display element.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention and showing no overlap deviation.

FIG. 2 is a diagram corresponding to FIG. 1 and showing a case where an overlay shift occurs.

FIG. 3: BM pattern: Example 1

FIG. 4: BM pattern: Example 2

FIG. 5: BM pattern: Comparative example

FIG. 6 is a diagram showing the relationship between screen luminance and overlay shift, and (1)
Is the case of the present invention and (2) is the case of the prior art.

FIG. 7 shows a TFT unit element pattern used for evaluation.

FIG. 8 is a view showing a conventional example and the current aperture ratio;

FIG. 9 is a view showing a conventional example, with a high aperture ratio;

Claims (2)

(57) [Claims] A display substrate divided for each pixel on a first substrate;
A second substrate having a pole and facing the first substrate;
A liquid crystal display device having a mask opening,
The black mask is formed outside at least one side of the outer periphery of the display electrode.
The disk opening has a protruding area, and the protruding area is
A liquid crystal display device having a triangular or arc shape
Place. 2. The method according to claim 1, wherein the protruding region is in contact with the first substrate.
If the superposition of the second substrate is shifted from a predetermined position,
And the area of the protruding region is the first area.
Nonlinearly with respect to the superposition of the second substrate and the second substrate
2. The method according to claim 1, wherein the number is increased.
The liquid crystal display device as described in the above.