JPH06347833A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To easily adjust the VT characteristic curves of the liquid crystal display element using a ferroelectric liquid crystal to be a rectilinear characteristic line having a proper inclination by forming a liquid crystal layer having stepwise thickness changes and specifying these thickness changes. CONSTITUTION:The liquid crystal layer 2 of the liquid crystal display element having a pair of electrodes 1a, 1b and the liquid crystal layer 2 of the ferroelectric liquid crystal sealed therebetween has the stepwise thickness changes having plural step regions 3a, 3b. The thickness changes of the liquid crystal layer 2 are so set as to include at least one set of the step regions 3a, 3b in which the inversion threshold in the one region coincides approximately with the inversion saturation value in the other. The discrete VT characteristic curves A, B respectively correspond to the step region 3a of the large thickness and the step region 3b of the small thickness. The respective step regions 3a, 3b themselves have respectively analogical characteristics and further characteristic suitable for analogical threshold control are obtd. as a whole by utilizing these characteristics.

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 element used in a display device such as a television. More specifically, the present invention relates to a liquid crystal display element that uses a ferroelectric liquid crystal and is capable of gradation display.

[0002]

2. Description of the Related Art Ferroelectric liquid crystals try to stabilize in one of two bistable states, that is, in one of white and black domains, and do not take an intermediate molecular position. Special measures are required to display.

Conventionally, gradation display by a digital method, in which one pixel is divided and driven, is known.

However, in this digital method, since one pixel is divided and driven, a large number of drive electrodes are required for one pixel. Therefore, the resolution is lowered, and a complicated arithmetic circuit is required, so that the yield is lowered.

Therefore, a method of displaying halftones in an analog manner has been proposed. For example, a spontaneous unevenness of the electrode substrate or a minute mosaic pattern intentionally applied to the electrode substrate is used (Japanese Patent Laid-Open No. 59-193427),
Add a stepwise change to the thickness of the insulating layer (Japanese Patent Laid-Open No. 61-1).
No. 66590), it is proposed to create a mixed state of white and black domains in one pixel, thereby displaying a halftone.

In any of the above-mentioned analog methods, in the liquid crystal layer of one liquid crystal display element, regions in which effective voltages applied when a voltage is applied are different from each other are formed, and these regions are formed finely and in large numbers. It is intended to enable analog control.

[0007]

Although it has been confirmed that a halftone state can be created by the above conventional analog method, in order to put the gradation display by these into practical use,
Characteristics between applied voltage and obtained transmittance (hereinafter referred to as "VT characteristics"
Is required to be a characteristic that an arbitrary halftone can be easily obtained. That is, it is desirable that the VT characteristic curve of one element be a straight line having an appropriate slope.

Further, in applying the method of compensating for the characteristic fluctuation of the ferroelectric liquid crystal due to the temperature change and other factors,
It is desirable that the VT characteristic curve is linear as described above.

By the way, the conventional analog method is to form a large number of fine regions having different effective voltages in the liquid crystal layer. Therefore, in order to accurately adjust the VT characteristic curve, these fine and large regions are formed. Each needs to be adjusted, VT
It is technically extremely difficult to make the characteristic curve linear with a slope.

An object of the present invention is to make it possible to easily adjust a VT characteristic curve of a liquid crystal display device using a ferroelectric liquid crystal to a linear one having an appropriate inclination.

[0011]

Means for achieving the above object and means for achieving the above object will be described with reference to FIG.
In the invention, a pair of electrodes 1a, 1b and the pair of electrodes 1
In a liquid crystal display device having a liquid crystal layer 2 of a ferroelectric liquid crystal enclosed between a and 1b, the liquid crystal layer 2 has a stepwise thickness change having a plurality of step regions 3a and 3b, and inversion in one side The thickness change of the liquid crystal layer 2 is set so that at least one set of step regions 3a and 3b whose threshold value substantially matches the inversion saturation value of the other is included.

In the liquid crystal display element shown in FIG. 1, as shown in the figure, one surface of one electrode 1b has two steps 4.
It has a one-step shape having a and 4b. And
The liquid crystal layer 2 has a stepwise thickness change having two step regions 3a and 3b depending on the surface state of the electrode 1b.

Although the surface of the electrode 1b is stepped in FIG. 1, the surface of the electrode 1a may be stepped. Further, not only one electrode 1a or 1b, but both electrodes 1a and 1b may be stepwise.

FIG. 2 is a diagram showing the individual VT characteristic curves A and B of the step regions 3a and 3b of the liquid crystal display device shown in FIG. 1, in which the vertical axis is the transmittance and the horizontal axis is the applied voltage (however, the ln scale). ).

The individual VT characteristic curve A corresponds to the thick step region 3a, and the individual VT characteristic curve B corresponds to the small thickness step region 3b.

Both the individual VT characteristic curves A and B draw a gentle S-shaped curve in which the middle part is a linear approximation part having a slope and the upper and lower ends are curved parts. Although the reason for such individual VT characteristic curves A and B is not always clear,
The alignment state in the step portion of the thickness of the liquid crystal layer 2 is unstable, the inversion tends to spread from the region where the alignment state is unstable, and the step region 3b having a smaller thickness is It is presumed that this is based on the fact that a large effective voltage is applied in comparison with the step region 3a having a large thickness.

As is clear from the individual VT characteristic curves A and B in FIG. 2, the step regions 3a and 3b have analog characteristics themselves. The present invention utilizes the analog characteristics of each of the step regions 3a and 3b to obtain characteristics more suitable for analog gradation control as a whole.

The present invention will be further described.

As a straight line connecting the points of 10% and 90% of transmittance, the individual VT characteristic curves A and B are approximately represented by vertically extending the linear approximation part of the middle part of both individual VT characteristic curves. This applied voltage put on the intersection of the transmittance 0% line value V _th1, V _th2 each step area 3a, the inversion threshold in 3b, the applied voltage value at the intersection between the extended line and the transmission of 100% modulus line Let V _sat1 and V _{sat2 be} the inversion saturation values in the step regions 3a and 3b, respectively.

Here, if V _th1 ≈V _sat2 , both individual VT characteristic curves A and B of FIG. 2 can be drawn as shown in FIG. Further, in FIG. 3, a combined VT characteristic curve X is a combination of both individual VT characteristic curves A and B.

The composite VT characteristic curve X shown in FIG. 3 above.
Can be shown as in FIG. As is clear from FIG. 4, since V _th1 ≈V _sat2 , both individual V
The T characteristic curves A and B become a series of synthetic VT characteristic curves X. Also, the slope value γ ₁ of the individual VT characteristic curve A and the individual VT
If the slope values γ ₂ of the characteristic curve B are substantially equal, the combined VT characteristic curve X becomes one straight line having substantially the same slope.

Here, the inclination values γ ₁ and γ ₂ of the individual VT characteristic curves A and B are (inversion saturation value) / (inversion threshold) in the corresponding step regions 3a and 3b. That is, the slope values γ ₁ and γ ₂ of the individual VT characteristic curves A and B are respectively V
_sat1 / V _th1, is a V _sat2 / V _th2.

The inversion thresholds V _th1 and V _th2 are largely controlled by the effective voltage applied to the step regions 3a and 3b, and the effective voltage is the thickness of the step regions 3a and 3b (the distance between the electrodes 1a and 1b). ) Is inversely proportional to. Also, FIG.
And the thickness of the step region 3a is D,
If the step difference in the thickness between the step regions 3a and 3b is d, the thickness of the step region 3b is D−d, and therefore V
_th2 is represented by V _th1 × (1-d / D), and V _sat2 is γ.
It is represented by ₁ x V _th1 x (1-d / D).

Therefore, in order to make V _th1 ≈V _sat2 ,
It _suffices that V _th1 be substantially equal to γ ₁ × V _th1 × (1-d / D), and eventually d≈D × {1- (1 / γ
₁ )}.

On the other hand, the inclination values γ ₁ and γ ₂ change depending on the characteristics of the ferroelectric liquid crystal material, the insulating film, and the electrodes 1a and 1b, and the cross-sectional area of the corresponding step regions 3a and 3b (steps 4a and 4b). Surface area).
Therefore, the inclination values γ ₁ and γ ₂ can be adjusted by adjusting the cross-sectional area of the step regions 3a and 3b. Generally, in order to make the inclination values γ ₁ and γ ₂ substantially equal to each other, the cross-sectional areas of both the step regions 3a and 3b may be substantially equal to each other.

In order to surely perform analog gradation control, it is necessary that V _th1 ≈V _sat2 . When this is not satisfied, assuming that the number of stages attached to the liquid crystal layer 2 is n, the combined VT characteristic curve X has 2n or more bending points, and the presence of the bending points makes it difficult to perform analog gradation control. .

Further, it is preferable that the inclination values γ ₁ and γ ₂ are almost the same. If they do not match, a combined VT characteristic curve X ′ as shown by the dotted line in FIG. 4 is obtained, but there are n bending points, and if the number of stages n attached to the liquid crystal layer is the same, V _th1 ≠ V It is possible to obtain a more linear composite VT characteristic curve X ′ with fewer inflection points than in the case of _sat .

By the way, the ultimate object of the present invention is to enable analog gradation display. For that purpose, when the number of step regions 3a and 3b is N, it is (N + 1) or more, preferably (2N + 1). ) It is necessary to obtain the gradation state of.

Based on this, the index of the allowable error range of each condition required in the present invention will be considered.

First, the permissible deviation when obtaining gradations of (N + 1) or more is 1 / (N + 1) or less, and assuming that the average error in each step region 3a, 3b is σ, the following equation 1 is established. Will be needed.

Further, in order to obtain a gradation of (2N + 1) or more, the following expression 2 must be similarly established.

[0032]

[Equation 1] σ <(N + 1) N ^1/2

[0033]

[Expression 2] σ <(2N + 1) N ^1/2

The expressions 1 and 2 can be used as an index of the allowable error range of each condition in the present invention. That is, the above-mentioned conditions of V _th1 ≈ V _sat2 and γ ₁ ≈ γ ₂ mean that it is within the range of ± σ where the above formula 1 is satisfied, and within the range of ± σ where the above formula 2 is satisfied. Is preferred. The total cross-sectional area of the step regions 3a and 3b satisfying the condition of the present invention is (1-
σ) or more is preferable.

In the above description, the two step areas 3a,
Although it is the case of 3b, as shown in FIG. 5, a liquid crystal display element having a larger number of step regions 3a to 3d can be used.

In FIG. 5, 1a and 1b are electrodes, 2 is a liquid crystal layer of ferroelectric liquid crystal, 3a to 3d are step regions, and 4 are
a to 4d ... are steps and have three steps.

6 shows individual VT characteristic curves A to D ... Corresponding to the step regions 3a to 3d ... Of the liquid crystal display element shown in FIG. 5, and X is the individual VT characteristic curves A to D.
It is a synthetic VT characteristic curve which synthesize | combined ...

In the case of a liquid crystal display device having a large number of step regions 3a to 3d ... As shown in FIG. 5, there is at least one combination of step regions 3a to 3d .. It is necessary to. This combination may be any one of the step regions 3a and 3b, 3b and 3c, 3c and 3d, which are adjacent to each other, but may also be a combination of those located at distant positions such as 3a and 3c.

Of course, all adjacent step regions 3a
And 3b, 3b and 3c, 3c and 3d, ..., It is best that the inversion threshold value and the inversion saturation value are substantially the same. In addition, it is preferable that the inversion threshold value and the inversion saturation value substantially match, and at the same time, the inclination value also substantially match.

In the liquid crystal display device having a large number of step regions 3a to 3d ... As shown in FIG. 5, the settings of d ₁ to d ₃ ... To make the inversion threshold value and the inversion saturation value substantially coincide with each other, D ×. {1- (1 / γ ₁ )}, D × {1- (1 /
γ ₁ · γ ₂ )}, D × {1- (1 / γ ₁ · γ ₂ · γ
₃ )}, ... However, γ _{1 to} γ ₃ are inclination values in the step regions 3a to 3c.

[0041]

【Example】

 Example 1 FIG. 7 shows the cross-sectional shape of the liquid crystal display element of this example.

On the pair of glass substrates 5a and 5b, the IT
The electrodes 1a and 1b were formed by depositing O. Board 5a
A single layer of ITO with a thickness of 1500 Å is formed on the
For b, ITO is deposited to a thickness of 1500 Å, and then 1210 Å by a photolithography process on the ITO film.
And ITO was further formed into a film with a thickness of 1100Å.

As described above, the surface of the electrode 1b is stepped. Further, the surface area ratio of the steps 4a to 4c in a stepwise manner is set to 1: 1: 1, and the total size of the area for gradation display is 2: 1.
It was set to 00 μm × 200 μm.

An insulating film 6a, is formed on the electrodes 1a, 1b.
As 6b, a polyimide film having a thickness of 200 Å was formed and subjected to rubbing treatment. As for the rubbing direction, as shown in FIG. 8, the stepwise electrode 1b side is in the θ ₁ direction, and the opposite flat electrode 1a side is in the θ ₂ direction. The angles formed by θ ₁ and θ ₂ and the step direction are 0 ° and −10 °, respectively, with the clockwise direction as the positive direction when viewed from the flat electrode 1a side.

After that, silica beads having a diameter of 1.1 μm were applied to the surfaces of the insulating films 6a and 6b, and the substrates 5a and 5b were bonded to each other with the applied surface inside. That is, each step area 3
The cell gaps in a to 3c are 1.1 μm and 1.
21 μm and 1.33 μm. The laminating direction was such that the rubbing directions on both substrates 5a and 5b were substantially parallel.

Here, the amount of each step of the electrode 1b and each cell gap were determined as follows.

First, the electrode 1b of the comparative liquid crystal display element (cell gap = 1.2 μm) in which the electrode 1b also has no step
When a pulse voltage as shown in FIG. 9 was applied between a and 1b and the optical response characteristics were measured, Δt = 20 μs and the inclination value γ = 1.10. Therefore, the relational condition of d ₁ to d ₃ ... And D described above is set so that the inversion saturation value of the step region 3a and the inversion threshold value of the step region 3b are equal to each other, and the inversion saturation value of the step region 3b and the inversion threshold value of the step region 3c are equal to each other. Based on the above, since the surface areas of the steps 4a to 4c having the stepwise shape are equal, γ ₁ = γ ₂ was set.

The same ferroelectric liquid crystal as that used in the liquid crystal display element for comparison was injected into the above-mentioned gap and sealed. The obtained liquid crystal display element showed a good alignment state.

When a pulse voltage as shown in FIG. 9 was applied between both electrodes 1a and 1b of the liquid crystal display element thus manufactured to measure the optical characteristics, VT was obtained at Δt = 20 μs.
The characteristic curve is as shown in FIG.

As is apparent from FIG. 10, the VT characteristic curve is almost a straight line, and the inclination value γ (V
_sat / V _th ) was about 1.33, and a characteristic advantageous for gradation display was obtained.

Example 2 The liquid crystal display device of Example 1 was arranged in a simple matrix configuration and an image was actually displayed.

FIG. 11 shows the structure of the apparatus.

The apparatus comprises a 500 × 500 matrix panel 7, a clock 8, a synchronizing circuit 9, and a shift register 1.
0, the analog switch 11, the scanning waveform generator 12, and the information signal generating circuit 14 for converting and outputting the video signal from the frame memory 13 into a drive signal. In terms of mounting, it is possible to arrange and connect them on either one of the upper and lower sides of the matrix panel 7, both the upper and lower sides, one of the left and right sides, or both of the left and right sides.

This device was able to perform good gradation display regardless of whether voltage modulation, pulse width modulation or transfer modulation was adopted as the information signal waveform applying method.

[0055]

The present invention is as described above, and since the VT characteristic curve can be made linear with an appropriate inclination value, a liquid crystal display element using a ferroelectric liquid crystal is used. It makes gradation display extremely easy and
This makes temperature compensation and drive circuit design extremely easy.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a liquid crystal display element according to the present invention.

FIG. 2 is an explanatory diagram of an individual VT characteristic curve corresponding to each step region of the liquid crystal display element shown in FIG.

3 is an explanatory diagram of an individual VT characteristic curve corresponding to each step region of the liquid crystal display element shown in FIG.

4 is an explanatory diagram of a combined VT characteristic curve obtained by combining the individual VT characteristic curves of the liquid crystal display element shown in FIG.

FIG. 5 is an explanatory diagram of another liquid crystal display element according to the present invention.

6 is an explanatory diagram of an individual VT characteristic curve corresponding to each step region of the liquid crystal display element shown in FIG.

FIG. 7 is a cross-sectional view of the liquid crystal display element manufactured in Example 1.

FIG. 8 is an explanatory diagram of a rubbing direction in the liquid crystal display element manufactured in Example 1.

FIG. 9 is an explanatory diagram of drive voltage pulses according to the first embodiment.

10 is a diagram showing a VT characteristic curve of the liquid crystal display device manufactured in Example 1. FIG.

FIG. 11 is an explanatory diagram of an image display device manufactured in Example 2.

[Explanation of symbols]

 1a, 1b Electrode 2 Liquid crystal layer 3a, 3b, 3c, 3d ... Step area 4a, 4b, 4c, 4d ... Step 5a, 5b Substrate 6a, 6b Insulating film 7 Matrix panel 8 Clock 9 Synchronous circuit 10 Shift register 11 Analog switch 12 Scanning waveform generator 13 Frame memory 14 Information signal generating circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuzo Kaneko 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (4)

[Claims] 1. A liquid crystal display device having a pair of electrodes and a liquid crystal layer of ferroelectric liquid crystal enclosed between the pair of electrodes, wherein the liquid crystal layer has a stepwise thickness change having a plurality of step regions. In addition, the liquid crystal display element is characterized in that the change in thickness of the liquid crystal layer is set so that at least one set of step regions in which the inversion threshold value on one side substantially matches the inversion saturation value on the other side is included. 2. The liquid crystal display according to claim 1, wherein one of the electrode surfaces has a stepwise shape, so that the liquid crystal layer has a stepwise thickness change having a plurality of step regions. element. 3. The liquid crystal display device according to claim 1, wherein the inclination values in at least one set of step regions in which one inversion threshold value and the other inversion saturation value substantially match each other. 4. The inversion threshold value on one side and the inversion saturation value on the other side substantially match, and the slope values substantially match between all adjacent step regions. Liquid crystal display device.