JPH0651283A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To simply express the gradation without reducing resolution by changing the shape of the polarization inversion region in the reduction line direction of the polarization inversion threshold value with one of counter electrodes. CONSTITUTION:When a means reducing the polarization inversion threshold value of a ferroelectric liquid crystal into a line shape reduces the threshold value of a ferroelectric liquid crystal into a line shape, the polarization inversion shape is easily changed along this line. A means reducing the shape of the polarization inversion region in the reduction line direction of the threshold value changes the primary domain together with the means reducing the polarization inversion threshold value into a line shape. These means are provided in counter electrodes. Square portions are picture elements 71 in a picture element pattern, and stripe-like irregularities 75 are provided on the counter electrodes. Picture element patterns 1201, 1202 and stripe-like projections 1203, 1204 are provided for other examples.

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 used in, for example, a liquid crystal television, and more particularly to a liquid crystal display device using ferroelectric liquid crystal capable of gradation display.

[0002]

2. Description of the Related Art Conventionally, in a liquid crystal television panel using, for example, an active matrix drive system, thin film transistors (TFTs) are arranged in a matrix for each pixel, and TF
A gate-on pulse is applied to T to energize between the source and the drain, and in this state, an image signal is applied from the source and stored in a capacitor, and a liquid crystal (for example, TN liquid crystal) is stored corresponding to the stored image signal. Gray scale display is performed by driving and simultaneously modulating the voltage of the image signal.

However, in an active matrix drive type television panel using such a TN liquid crystal,
Since the TFT used has a complicated structure, the number of manufacturing steps is large and high manufacturing cost is a bottleneck. In addition, it is difficult to form a thin film semiconductor (for example, a thin film of polysilicon or amorphous silicon) forming a TFT over a wide area, which hinders an increase in area.

On the other hand, as a display panel which can be manufactured at low cost, an active matrix drive type display panel using TN liquid crystal is known.

However, in this display panel, as the number of scanning lines (N) increases with the enlargement and definition of the screen, it is effective for one selection point while scanning one screen (one frame). Time when electric field is applied (duty ratio)
However, there is a problem that crosstalk is likely to occur and it is difficult to obtain a high-contrast image. In addition, when the duty ratio becomes smaller,
Since it is difficult to control the gradation of each pixel by voltage modulation, it is not suitable for a display panel with a high-density wiring number, especially for a liquid crystal television panel.

As a solution to such a fundamental problem of the conventional TN liquid crystal, a ferroelectric liquid crystal device having bistability is disclosed in US Pat. No. 4,367,924 of Clark and Lagavar et al. Proposed.

By the way, it has been considered that this ferroelectric liquid crystal element is not suitable for gradation expression because it does not take an intermediate molecular position in order to stabilize in either of two bistable states. In some cases, gradation display by a digital method represented by the dither method has been attempted.

[0008]

However, the gradation display using the above-mentioned ferroelectric liquid crystal has a problem that the resolution is lowered and is not suitable for a high resolution display such as HDTV compatible. Moreover, it is difficult to increase the number of luminance gradations without increasing the number of pixel divisions. In addition, a large number of drive electrodes are required for one-pixel gradation display, a complicated arithmetic processing circuit is also required, and there is a problem that the yield is reduced.

The present invention has been made in view of the above problems, and an object thereof is to make it possible to easily express gradation in a ferroelectric liquid crystal element without lowering the resolution.

[0010]

To this end, in the present invention, in a liquid crystal display element comprising a pair of counter electrodes and a ferroelectric liquid crystal disposed between the pair of counter electrodes,
A means for linearly decreasing the polarization reversal threshold value of the ferroelectric liquid crystal is provided on at least one counter electrode, and at least one counter electrode has a polarization reversal region whose shape is a line for decreasing the polarization reversal threshold value. This is a liquid crystal display element having means for changing the direction.

The principle of the present invention will be described with reference to FIGS.

In FIG. 1, 11 and 12 are glass substrates, 13 and 14 are a pair of counter electrodes composed of transparent conductive thin films, 15 and 16 are alignment films such as polyimide, and 18 is downward polarization. Part (optically white domain in the figure), 19 is a part where spontaneous polarization is upward (optically black domain in the figure), and the arrow shown in the sectional view indicates the spontaneous polarization direction of the ferroelectric liquid crystal.

As shown in the cross-sectional view, the white or black domain may be considered to be substantially uniform in the direction perpendicular to the upper and lower substrates 11 and 12, and the number of molecules with spontaneous polarization downward and the number of molecules upward. Ratio (optically, the transmittance of one pixel as a whole)
Is the area ratio of the two-dimensional white domain and black domain.

Further, as shown in FIG. 2, the white (or black) domain grows (or contracts) while the voltage from the outside is continuously applied, so that the applied voltage value or the application time is controlled. As a result, the white / black ratio within one pixel can be controlled and intermediate display can be performed.

By the way, the growth (or reduction) of the domain shown in FIG. 2 greatly differs depending on whether the domain changes its shape two-dimensionally or one-dimensionally.

FIG. 3 shows a schematic diagram of a one-dimensional change domain and a two-dimensional change domain.

The one-dimensional means that the growth / reduction of the length L shown in the figure is the main or only the growth / reduction of L, and the width D hardly changes. Two-dimensional means that R shown in the figure changes.

Here, considering the free energy G that a system including each domain has when an electric field is applied, the elastic energy at the boundary wall of the domain is proportional to the area of the boundary wall, and the electrical energy occupies the domain. Since it is proportional to the volume, G = αL−βL = in the one-dimensional domain.
k1 * L, and G = α′R−r in the two-dimensional domain
It becomes R ^ 2.

FIG. 4 shows the situation. As is clear from this graph, in the two-dimensional domain, the domain smaller than the critical value R ₀ disappears when the electric field is continuously applied.

Further, in FIG. 5, the one-dimensional domain hardly changes in the initial state of voltage application and in a steady state both when a low voltage is applied and when a high voltage is applied. The small domains that existed in the early days of time disappear. That is, in the case of driving in the vicinity of the threshold value, the transmittance does not increase easily in the case of the two-dimensional domain, and the relationship between the applied voltage and the transmittance is as shown by the dotted line in FIG. This property is disadvantageous for halftone display as compared to the one-dimensional domain.

From the above, in order to perform halftone display using the ferroelectric liquid crystal, it is preferable to control the one-dimensionally changing domain rather than the two-dimensionally changing domain. Recognize. The characteristics shown in FIG. 6 have the property of becoming more prominent when the polarization inversion threshold voltage is lowered by the method described later.

The present invention controls gradations by controlling the one-dimensionally changing domains.

First, in the present invention, the means for linearly lowering the polarization reversal threshold of the ferroelectric liquid crystal is to linearly lower the polarization reversal threshold along the line by lowering the ferroelectric liquid crystal threshold linearly. This makes it easier to change the shape. That is, it is for facilitating the above-described one-dimensional domain change.

As means for decreasing the polarization inversion threshold of the ferroelectric liquid crystal in a line shape, there are an island-shaped uneven pattern formed by arranging in a line shape and a stripe-shaped uneven pattern arranged in parallel.

The concavo-convex pattern lowers the threshold value by causing the alignment of the ferroelectric liquid crystal to be disturbed particularly near the corner and destabilizing the part. In the case of the island-shaped concavo-convex pattern, since the concavities and convexities are close to each other, the disorder of the alignment is continuous as if a series, and a portion where the threshold value is lowered can be formed in a line shape. Further, in the case of stripe-shaped unevenness, the threshold-reduced portion is formed in a line along the unevenness.

The height of the island-shaped or stripe-shaped unevenness is
It is preferably 50 to 10000 Å, more preferably 500 to 3000 Å, and the interval between the adjacent irregularities is preferably 1 to 50 μm. In the case of islands,
The planar shape may be any shape such as a circular shape as well as a circular shape, but the rectangular shape is preferably about 1 to 50 μm square. In the case of stripes, the width is 1 to 50 μm
Is preferred.

On the other hand, the means for reducing the shape of the domain-inverted region in the direction of the lowering line of the threshold value is one which causes a one-dimensional domain change in a complicated manner together with the means for reducing the above-mentioned domain-inversion threshold value in a line shape. The following means can be mentioned.

First, there is an island-shaped or stripe-shaped concave-convex pattern along the line where the threshold is lowered. In this case,
The electric field can be preferentially applied to the line with the lowered threshold value, whereby the stripe-shaped domain change can be generated.

The height of the island-shaped or striped uneven pattern is generally preferably 50 to 10000Å, more preferably 500 to 10000Å.

As another means, there is a means for gradually decreasing or gradually increasing the electric field applied to the ferroelectric liquid crystal along the direction in which the threshold voltage is lowered. In the case of the counter electrode having such a means, it is possible to obtain a shape change in which the domain-inverted region is linearly gradually extended from the region where the applied electric field is high to the region where the applied electric field is low in the threshold lowering line.

More specifically, the counter electrode having such means will be described. As one of them, an island-like structure or a structure arranged with a coarse-to-dense or dense-to-coarse spacing in the direction of the lower threshold line is used. A counter electrode having a stripe-shaped concavo-convex pattern can be used. Other counter electrodes include
Examples of the counter electrode include a counter electrode having a gradient along the direction of the lowering threshold line and a counter electrode having means for applying a bias voltage along the lowering line of the threshold value.

In the counter electrode having the above-mentioned island-shaped or stripe-shaped concavo-convex pattern, the dense region is a region to which a high electric field is applied, and the rough region is a region to which a low electric field is applied. Further, in the counter electrode having the above-mentioned gradient, a region above the gradient is a region to which a high electric field is applied, and below the gradient is a region to which a low electric field is applied. Accordingly, the electric field strength applied to the ferroelectric liquid crystal is gradually increased or decreased.

[0033]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 FIG. 7 shows a pixel pattern of this example.

A square portion indicated by 71 indicates each pixel, and 72 is a stripe-shaped concavo-convex formed on a counter electrode formed of a transparent conductive thin film.

The size of the pixel 71 is 200 μm square, the width of the stripe-shaped irregularities is 2 μm, and the interval is 10 μm.

FIG. 8 shows a cell section taken along the line aa 'in FIG.

ITO is formed on the glass substrates 81, 82.
(1500Å) is formed as the counter electrodes 83 and 84, and the pixel pattern and the stripe-shaped concavo-convex pattern are formed by two photolithography steps. The step of this stripe-shaped uneven pattern is about 200Å
Is.

Polyimide (200 Å) was applied as an alignment film 85, 86 thereon, and a rubbing treatment was applied in the direction of the stripe-shaped concavo-convex pattern.
It is pasted with 2 μm.

Ferroelectric liquid crystal (blend based on trade name "CS1014") 87 is injected into the gap. Also, by changing the cell bonding direction,
Both the parallel and anti-parallel cells shown in Figure 9 were made and both cells showed good orientation.

A pulse voltage as shown in FIG. 10 was applied between the upper and lower counter electrodes 83 and 84 of the cell thus prepared, and the optical response characteristics were measured. In this example, △
It was t = 50 μs and Vap = 15 to 30V.

As a result of observing the inversion domain shape after applying the pulse voltage Vap to 20 V in common to all pixels in the initial black state in the 2 * 2 matrix, FIG. 11 (antiparallel cell)
It became like.

For comparison, a cell without a stripe-shaped concave-convex pattern was also prepared, but the polarization reversal threshold voltage was obviously lower in the cell with the stripe-shaped concave-convex pattern. In addition, it was confirmed that the domain shape was linear, and the number and length of the line domains changed mainly by changing the applied voltage, and a one-dimensional domain change was obtained.

The variation of the transmittance between each pixel is very small, and the center of the inversion domain is constant near the center of each pixel, so that very good halftone display can be performed. Was almost the same.

Further, although the one side substrate 81 or 82 was provided with the stripe-shaped concavo-convex pattern, the decrease in the polarization reversal threshold voltage was small and the one-dimensional behavior of the domain was weaker than that of this example. .

Example 2 The upper substrate of the cell was provided with the same stripe-shaped concavo-convex pattern as in Example 1, the arrangement density of stripes was changed within the pixel on the lower substrate, and the constituent material of the stripe-shaped concavo-convex pattern was SiO _2. It is the one.

FIG. 12 shows a pixel pattern of this embodiment.

In the figure, 1201 and 1202 indicate pixel patterns, and 1203 and 1204 are stripe-shaped convex portions.

The pixel size is 100 μm square, the stripe width is 2 μm, and the stripe-shaped concave-convex patterns are orthogonal to each other on the upper substrate and the lower substrate. In the lower substrate, the densest stripe-shaped concavo-convex pattern is spaced by 2 μm,
The spacing between the coarsest parts is 15 μm.

FIG. 13 shows a sectional view taken along the line bb 'in FIG.

1301 is a glass substrate, and 1302 is I
Counter electrode made of TO (700Å), 1303 is SiO ₂
It is a stripe-shaped concavo-convex pattern formed by patterning (2500Å).

Polyimide (200 Å) was applied as an alignment film 1304 thereon, and a rubbing process was performed in the stripe direction of the upper substrate. As the ferroelectric liquid crystal material, a material having a short spiral pitch is used, and when cooling from the smectic A layer to the smectic C layer, +15 V to −15 V and 10 Hz A
When C was applied, good orientation was obtained. The cell gap is 1.6 μm, and the bonding direction is a parallel cell.

As a result of observing the inversion domain shape after applying different pulse voltage Vap to each column in the initial black state in the 4 * 4 matrix prepared as described above, FIG.
It became like.

The domain in each pixel exhibits a good one-dimensional behavior, and when the voltage is small, the inversion domain is formed only in the upper part of each pixel, and the domain is formed by the stripe-shaped concavo-convex pattern of SiO ₂ provided on the lower substrate. It was confirmed that the generation position could be controlled. That is, a regular threshold distribution could be given. Further, a variation in transmittance between pixels for each voltage value is very small, and a display device having a uniform and high gradation can be obtained.

Example 3 FIG. 15 shows a sectional view of a pixel of this example.

A transparent counter electrode 1 is formed on the upper substrate 1601.
As the layer 602, 500 liters of ITO was laminated, and the island-shaped concave-convex pattern 1603 of ITO was formed thereon by lift-off. Further, 100 Å of polyimide was laminated thereon as an alignment film 1604 and rubbed.

On the other hand, the lower substrate 1605 is provided with a gradient within a pixel, ITO of 500 Å is laminated as a transparent counter electrode 1606 on the upper substrate, and 100 Å of polyimide is further laminated thereon as an alignment film 1607 to perform rubbing treatment. It was

The upper and lower substrates 160 created in this way
1, 1605 were pasted together in the antiparallel direction via a spacer, and ferroelectric liquid crystal was injected. The direction of the gradient is parallel to the rubbing direction (the maximum inclination direction of the gradient is the rubbing direction).

FIG. 16 shows a plan view of the upper substrate 1601.

1701 is a pixel, and 1702 is an island-shaped uneven pattern.

The island-shaped uneven pattern 1702 has a side of 10 μm.
It is a square of m and the height is 1000Å.

As a result of observing the shape of the domain inversion domain when different voltage values were applied to such a cell, FIG.
It became like. As in the case of applying a low voltage, the case of applying a medium voltage was B, and the case of applying a high voltage was C, the inversion domain was linear and showed one-dimensional behavior, and good gradation characteristics were obtained.

Here, for comparison, a cell in which the island-shaped concavo-convex pattern 1702 is not provided on the upper substrate 1601 was also prepared. Ratio with Vsat Vsat /
Vth cannot be made large, and precise gradation control cannot be performed.

Embodiment 4 FIG. 18 shows a sectional view of a pixel of this embodiment.

A transparent counter electrode 19 is formed on the upper substrate 1901.
As No. 02, 500 liters of ITO was laminated, and the island-shaped uneven pattern 1903 of ITO was formed thereon by lift-off. Furthermore, a polyimide film 1904 is formed on top of it as polyimide.
00Å was laminated and subjected to a rubbing treatment.

On the other hand, a high resistance IT is formed on the lower substrate 1905.
An O film was laminated as a counter electrode 1906 with a thickness of 300Å. still,
The sheet resistance of this high resistance ITO film is 1 to 100 MΩ.

Aluminum electrodes 1907 having a thickness of 1000 Å were provided on both ends of the pixel on the counter electrode 1906, and 100 Å of polyimide as an alignment film 1908 was further laminated on the aluminum electrodes 1907 to perform rubbing treatment.

The upper and lower substrates 190 created in this way
1, 1905 were bonded to each other in an antiparallel direction via a spacer, and a ferroelectric liquid crystal was injected.

One of the aluminum electrodes 1907 is grounded, and the bias voltage Vb (-10 V) is applied to the other,
A one-dimensional potential gradient was generated within the pixel. The direction of this potential gradient was parallel to the rubbing direction.

As a result of observing the shape of the domain inversion domain when different voltage values were applied to this cell, the result was as shown in FIG. 17, which was almost the same as in Example 3, and good gradation characteristics were obtained.

[0071]

The present invention is as described above, and since gradation display can be performed by effectively utilizing even small domains with good controllability, halftone display characteristics for each pixel are improved. be able to. Moreover, since the position of the inversion domain can be controlled by forming auxiliary unevenness,
It is possible to suppress variations in halftone display characteristics between pixels to an extremely small level, and it is possible to realize a high gradation and high definition display.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a white / black domain occurrence state.

FIG. 2 is a diagram showing growth of inversion domains when a voltage is applied.

FIG. 3 is a schematic diagram of a one-dimensional domain and a two-dimensional domain.

FIG. 4 is a diagram showing free energies of a one-dimensional domain and a two-dimensional domain system.

FIG. 5 is a diagram showing changes in a one-dimensional domain and a two-dimensional domain when a voltage is applied.

FIG. 6 is a diagram showing a relationship between an applied voltage and a transmittance of a one-dimensional domain and a two-dimensional domain.

FIG. 7 is a diagram showing a pixel pattern of Example 1.

8 is a sectional view taken along line aa 'in FIG.

FIG. 9 is an explanatory diagram of a cell bonding direction.

FIG. 10 is a diagram showing a cell drive applied voltage.

11 is a diagram showing the shape of the domain inversion domain observed in Example 1. FIG.

FIG. 12 is a diagram showing a pixel pattern of Example 2;

13 is a sectional view taken along the line b-b 'in FIG.

FIG. 14 is a diagram showing the shape of a domain-inverted domain observed in Example 2.

FIG. 15 is a cross-sectional view of a pixel of Example 3.

16 is a plan view of a pixel of Example 3. FIG.

FIG. 17 is a diagram showing the shape of a domain-inverted domain observed in Example 3.

FIG. 18 is a cross-sectional view of a pixel of Example 4.

[Explanation of symbols]

 11, 12 Substrate 13, 14 Counter electrode 15, 16 Alignment film 18 White domain 19 Black domain 71 Pixel pattern 72 Striped uneven pattern 81, 82 Substrate 83, 84 Counter electrode 85, 86 Alignment film 87 Ferroelectric liquid crystal 1201, 1202 Substrate 1203, 1204 Striped uneven pattern 1301 Substrate 1302 Counter electrode 1303 Striped uneven pattern 1304 Alignment film 1601, 1605 Substrate 1602, 1606 Counter electrode 1603 Island-shaped uneven pattern 1604, 1607 Alignment film 1701 Pixel 1702 Island-shaped uneven pattern 1901, 1905 Substrate 1902 Counter electrode 1903 Island-shaped uneven pattern 1904, 1908 Alignment film 1906 Counter electrode 1907 Aluminum electrode

Claims (8)

[Claims] 1. A liquid crystal display device comprising a pair of counter electrodes and a ferroelectric liquid crystal disposed between the pair of counter electrodes, wherein a polarization reversal threshold value of the ferroelectric liquid crystal is lined on at least one of the counter electrodes. A liquid crystal display element, wherein at least one of the counter electrodes has a means for changing the shape of the domain-inverted region in the direction of the lowering line of the threshold value. 2. The liquid crystal display device according to claim 1, wherein the means for reducing the polarization inversion threshold of the ferroelectric liquid crystal in a line shape is an island-shaped concavo-convex pattern formed in line. 3. The liquid crystal display element according to claim 1, wherein the means for linearly lowering the polarization reversal threshold of the ferroelectric liquid crystal is a parallel stripe-shaped concavo-convex pattern. 4. The liquid crystal according to claim 1, wherein the means for changing the shape of the domain-inverted region in the direction of the lowering line of the domain-inversion threshold is an island-shaped or stripe-shaped concavo-convex pattern along the line of reduced threshold. Display element. 5. The means for changing the shape of the domain-inverted region in the direction of the line of decreasing the domain-inversion threshold is a means for gradually decreasing or increasing the electric field applied to the ferroelectric liquid crystal along the direction of the line of decreasing the threshold voltage. The liquid crystal display device according to claim 1, which is characterized in that. 6. Means for gradually decreasing or gradually increasing the electric field applied to the ferroelectric liquid crystal along the line direction in which the polarization reversal threshold value is lowered is an island shape arranged coarsely or densely or densely to roughly in the line direction where the threshold value is lowered. Alternatively, the liquid crystal display element according to claim 5, wherein the liquid crystal display element has a stripe-shaped uneven pattern. 7. The means for gradually decreasing or gradually increasing the electric field applied to the ferroelectric liquid crystal along the direction of the polarization inversion threshold lowering line is a gradient along the threshold lowering line direction. Liquid crystal display element. 8. The means for gradually decreasing or gradually increasing the electric field applied to the ferroelectric liquid crystal along the direction of the polarization inversion threshold decrease line is a means for applying a bias voltage to the counter electrode along the threshold decrease line direction. The liquid crystal display element according to claim 5, wherein