JPH0414767B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving an optical modulation element such as a liquid crystal element, and more particularly to a time division driving method for a liquid crystal element used in an optical modulation element such as a display element or a shutter array.

Conventionally, the following methods have been used to construct a liquid crystal display element in which a large number of pixels are formed in a matrix, but each method has drawbacks.

1 Method using a simple electrode matrix: Although it is extremely easy to fabricate, an electric field is also applied to non-selected points, causing crosstalk. For this reason, it is not possible to increase the pixel capacity.

2 TFT (thin film transistor) corresponding to each pixel
A method of providing active elements such as If it is applied to a liquid crystal element, the cost will be extremely high.

3 A method using nonlinear elements such as MIM (metal/insulator/metal) structures corresponding to each pixel; If good electrical matching can be achieved between each nonlinear element and the liquid crystal layer corresponding to each pixel, Crosstalk is prevented and the pixel capacitance can be increased to some extent, but when trying to increase the pixel density, the capacitance of the liquid crystal layer of each pixel becomes smaller, and in order to achieve electrical matching, each nonlinear The electrostatic capacitance of the element must be reduced accordingly, and in order for the nonlinear element to have a charge retention function, the driving conditions are also becoming stricter, which is a major hurdle in manufacturing.

There are many reports regarding liquid crystal driving methods using this nonlinear element. For example, IEEE
Transactions on Electron Devices, Vol.ED
-28, No.6, JUNE 1981
“The Optimization” by David R. Baraff et al.
of Metal−Insulator−Metal Nonlinear
Devices for Use in Multiplexed Liquid
In any case, no matter which method is used, liquid crystal elements with large pixel capacity, difficult to display on a large screen, and relatively inexpensive have not yet appeared. That is the current situation.

4. Method using ferroelectric liquid crystal; US Pat. No. 4,367,924 discloses a liquid crystal optical element in which a matrix electrode structure is incorporated into a ferroelectric liquid crystal element that exhibits a bistable state. The threshold voltage of this ferroelectric liquid crystal has a threshold characteristic that depends on the voltage application time (pulse width). For example, even if the voltage V ₁ continues to be applied, the electrical polarization state of the ferroelectric liquid crystal (for example, one state corresponding to white display) does not change;
When the voltage V ₁ exceeds the application time t ₁ , it is reversed to the other electrical polarization state (for example, corresponding to black display). Therefore, when applying a ferroelectric liquid crystal to matrix drive, - once within a frame,
For example, a pixel on a scanning electrode that has been written to white display by applying a negative voltage -V ₂ at an application time t ₀ will receive a positive voltage from the signal electrode with a pulse width t ₀ when writing a pixel on another scanning electrode. In order to receive the voltage V ₁ or the negative voltage -V ₁ , the positive voltage V ₁ with the pulse width t ₀ is continuously applied, and if the above-mentioned application time t ₁ is exceeded, the black display will be inverted. This inversion phenomenon has made it difficult to apply matrix drive.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a new liquid crystal device that overcomes the problems of the prior art, has a large pixel capacity, is capable of displaying or modulating a large screen, and can be manufactured at a relatively low cost. It is about providing law.

The liquid crystal element used in the present invention uses a material called ferroelectric liquid crystal as the liquid crystal material.
By combining this with a nonlinear element that can also be achieved using ordinary photography technology, it is possible to provide a liquid crystal display device with a large area and high pixel density that has never been available before. .

The ferroelectric liquid crystal used in the liquid crystal element of the present invention can have two polarization states each having memorability, and in this case, the following great effects can be achieved.

Ordinary liquid crystal (e.g. twisted nematic liquid crystal)
In a conventional liquid crystal device consisting of a pixel ON signal and a non-linear element, the non-linear element is turned ON by a pixel ON signal, charges are accumulated at both ends of the liquid crystal layer, and a voltage is applied to the pixel. The LCD is turned on. After this, when the signal is turned off, the nonlinear element
The charge that has accumulated significantly at both ends of the liquid crystal layer in the OFF state is capacitively divided into the capacitance of the nonlinear element and the capacitance of the liquid crystal layer. Therefore, if the capacitance of the nonlinear element is not sufficiently smaller than that of the liquid crystal layer, the amount of charge at both ends of the liquid crystal layer will decrease, making it impossible to keep the liquid crystal corresponding to the pixel in the ON state. become unable. For this reason, in conventional liquid crystal elements, the capacitance of the nonlinear element usually needs to be about 1/10 or less of that of the liquid crystal layer, and if it exceeds that, the latitude of the driving conditions becomes extremely narrow. Therefore, if you try to increase the pixel density,
Since the capacitance of the pixel liquid crystal becomes smaller, it is necessary to further reduce the capacitance of the nonlinear element, and it has been difficult to construct a minute nonlinear element using ordinary photolithography technology.

On the other hand, if the capacitance of the non-linear element is made sufficiently smaller than the capacitance of the pixel liquid crystal, when the signal is turned OFF and the non-linear element is turned OFF, the accumulated charge at both ends of the liquid crystal layer will cause the liquid crystal layer to The voltage applied to the nonlinear element is also applied almost unchanged.
Therefore, if the threshold voltage of the nonlinear element is lower than the voltage required to switch the liquid crystal layer from the OFF state to the ON state (threshold value of the liquid crystal), the nonlinear element will be in the ON state, and the charge accumulated in the liquid crystal layer will be reduced. will discharge through the nonlinear element. Alternatively, even if the threshold voltage of the nonlinear element is slightly higher than the threshold voltage of the liquid crystal, depending on the information signal voltage that is subsequently applied to the signal electrode, the voltage applied to the nonlinear element may further increase, causing nonlinearity. There is a high risk that the element will return to the ON state. For this reason, in a conventional liquid crystal element that is a combination of an ordinary liquid crystal without memory and a nonlinear element, it is necessary to make the threshold voltage of the nonlinear element sufficiently larger than the threshold voltage of the liquid crystal. This results in

In any case, in conventional liquid crystal devices, the difficulty in manufacturing nonlinear elements and the harshness of driving methods have hindered the achievement of high pixel density as commercial products. If the states (corresponding to the two polarization states of a ferroelectric liquid crystal) (ON and OFF) have memory properties, once a voltage is applied to the liquid crystal layer and switching occurs, for example, to the ON state, , even if the voltage is removed afterwards, it remains ON
Since the state can be maintained, the capacitance of the nonlinear element can be equal to or even lower than that of the pixel liquid crystal, making it possible to achieve high-speed drive with low drive voltage. Summer.

That is, the driving method of the present invention has a non-linear element corresponding to each pixel of a matrix electrode structure in which pixels are intersections of scanning electrode groups and signal electrodes, and a non-linear element is provided between the scanning electrode group and the signal electrode group. A method for driving a liquid crystal element having a ferroelectric liquid crystal, in which, in the first step, scanning electrodes are sequentially scanned, and pixels on selected scanning electrodes are simultaneously exposed to one polarity of the ferroelectric liquid crystal. A first voltage of one polarity exceeding the threshold of is applied, and in a second process, the scanning electrodes are sequentially scanned, and the pixels on the selected scanning electrode are selectively exposed to the other polarity of the ferroelectric liquid crystal. The method of driving the liquid crystal element is characterized by applying a second voltage of one polarity that exceeds a polarity threshold and a third voltage that does not exceed the threshold.

As will be explained in detail in the following examples, the driving method of the present invention differs from conventional nematic or cholesteric liquid crystals in that it uses ferroelectric liquid crystals that have two mutually opposite polarization states. The major feature of this is that it is DC driven.

The ferroelectric liquid crystal used in the liquid crystal element of the present invention includes chiral smectic C phase (SmC ^* ), H phase (SmH ^* ), F phase (SmF ^* ), I phase (SmI ^* ), or G phase.
(SmG ^* ) liquid crystal is suitable. For more information on this ferroelectric liquid crystal, please refer to “LE JOURNAL DE
PHYSIQ UE LETTERS”36 (L-69) 1975,
“Ferroelectric Liquid Crystals”; “Applied
Physics Letters” 36 (11) 1980 “Submicro
Second Bistable Eiectrooptic Switching in
``Liquid Crystals''; ``Solid State Physics'' 16 (141) 1981 ``Liquid Crystals'', etc., and the ferroelectric liquid crystals disclosed therein can be used in the present invention.

More specifically, examples of ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino-2-methylbutylcinnamate (DOBAMBC), hexyloxybenzylidene-P'-amino- 2-chloropropyl cinnamate (HOBACPC) and 4-o-(2-methyl)
-butylresolcylidene-4'-octylaniline (MBRA8) and the like.

When constructing an element using these materials,
Liquid crystal compounds include SmC ^* , SmH ^* , SmF ^* , SmI ^* and
In order to maintain the temperature at SmG ^* , the element can be supported by a copper block or the like with a heater embedded in it, if necessary.

FIG. 3 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In ₂ O ₃ , SnO ₂
It is a substrate (glass plate) coated with transparent electrodes such as ITO (Indium-Tin Oxide), etc., and SmC ^* phase liquid crystal with the liquid crystal molecular layer 22 oriented perpendicular to the glass surface is sealed between them. . A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment 24 (P⊥) in a direction perpendicular to the molecule. Boards 21 and 2
When a voltage above a certain threshold is applied between the electrodes 1', the helical structure of the liquid crystal electrons 23 is unraveled, and all dipole moments 24 are oriented in the direction of the electric field.
The alignment direction of the liquid crystal molecules 23 can be changed.
The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the major and minor axis directions. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, voltage can be applied. It is easily understood that the liquid crystal modulation element has optical characteristics that change depending on the polarity. Furthermore, when the thickness of the liquid crystal cell is made sufficiently thin (for example, 1μ), the helical structure of the liquid crystal molecules is unraveled even in the absence of an applied electric field, as shown in Figure 4, and its dipole (non-helical structure) The moment P or P' assumes either an upward 34 or downward 34' electrical polarization state. When such a cell is given an electric field E or E' with a different polarity above a certain threshold as shown in Fig. 3, the dipole moment increases upward by 3 in correspondence to the electric field vector of the electric field E or E'.
4 or downward 34', and accordingly the liquid crystal molecules are either in the first stable state 33 (one electrically polarized state) or the second stable state 33 (the other, electrically polarized state).
(electrical polarization state). Furthermore, the first and second states have a memorability property and can remain in their respective states even after the electric field is turned off.

As described above, since the ferroelectric liquid crystal has the ability to memorize the electric polarization state, it can be used as an image element with a large pixel density using a novel driving method. However, the above-mentioned threshold value is usually not extremely sharp, and moreover, it depends on the applied voltage waveform, more specifically, on the pulse width. Also, the uncertainty of this threshold is
Depends on substrate processing conditions, temperature and liquid crystal material. Therefore, when attempting to drive this more stably using a time division method, it is necessary to improve the memory performance of the ferroelectric liquid crystal by combining it with a nonlinear element to clarify the apparent threshold characteristics. It has become clear that it is possible to provide a large pixel capacitive element that can be utilized to its maximum potential and a method for driving the same.

In addition, the nonlinear elements used in the present invention include:
In addition to the above-mentioned MIM, there are also p-n junction diodes with an appropriate reverse bias, p-n junction diodes connected in series with their directions reversed, and shot key diodes with an appropriate reverse bias. Key diodes connected in series with their directions reversed can be used.

Figures 1 and 2 schematically show the structure of the liquid crystal element of the present invention.
This is an example using a structure. FIG. 1A is a sectional view of the liquid crystal element of the present invention, and FIG. 1B is an enlarged sectional view of the MIM structure used therein. In the figure, 1 and 1' are the opposing substrates (glass substrate, plastic substrate), and 2 is the thermally oxidized thickness.
400 Å Ta (Ta ₂ O ₅ ) layer, 3 2000 Å thick Ta (tantalum) with surface anodized layer 8
Layer 4 is a 1000 Å thick Cr (chromium) conductive layer. The MIM structure consists of a Ta layer 3 which is a metal layer, an anodized Ta layer 8 which is an insulator layer, and a metal layer
It has a laminated structure of Cr conductive layers 4. 5 is an ITO film with a thickness of 1000 Å, which defines the area of one pixel. Also, 6 is ITO of the counter electrode
It's a pattern. The substrate 1 on which the MIM is formed and the substrate 2 on which the conductive pattern is formed may be subjected to alignment treatment by rubbing or obliquely vapor depositing a material such as SiO, as necessary. A liquid crystal (eg, DOBAMBC as mentioned above), the liquid crystal layer of which can be 1.5μ thick. At this time, the temperature is controlled at 70°C.

FIG. 2 is a plan view of the liquid crystal element shown in FIG. 1.

Driving embodiments of the present invention are shown from FIG. 5 onwards.

FIG. 5 shows an example of a display form using a matrix electrode structure 51, in which each pixel is formed at the intersection of a scanning electrode group 52 and a signal electrode group 53, and each pixel has a ferroelectric liquid crystal. The nonlinear elements shown in FIG. 1 are arranged. In Figure 5, the black part is "black"
The display is such that white areas are displayed as "white".

FIGS. 6a and 6b respectively show the scanning signal at the time of selection and the scanning signal at the other time of non-selection.

Figures 6c and d show, respectively, an electric signal (referred to as a white signal) that orients the ferroelectric liquid crystal to the first stable state (one electric polarization state) of the bistability state and the second one of the bistability state of the ferroelectric liquid crystal. It shows an electrical signal (referred to as a black signal) that orients it to a stable state (the other electrically polarized state).

First, the driving method of the present invention applies a scanning signal to all or part of the scanning electrode group 52 in the first frame _F1 , as shown in FIG. Apply a white signal to some parts. In the next second frame _F2 , for example, as shown in FIG. 5, a black signal is applied to a predetermined location (pixel indicated by a black portion in the figure). At this time, scanning electrode groups 52, 521, 52
2,523,524,525 and signal electrode group 5
The electrical signals applied to 3,531, 532, 533, 534, and 535 and the voltage waveforms applied to pixels A and B in FIG. This is made clear in the figure.

As is clear from the figure, in the first frame _F1 , a pulse voltage of 2V ₀ is first applied to all the scanning electrodes and a white signal is applied to all the signal electrodes according to line sequential scanning. As a result, a voltage of -2V ₀ is applied to the series-coupled nonlinear elements and the liquid crystal layer in all pixels, and the nonlinear element _exceeds the threshold and enters the ON state, applying a high negative voltage to the liquid crystal layer. As a result, the liquid crystal layer is aligned to the first electrical polarization state (white).
On the other hand, at phase t ₂ , V ₀ is applied to each pixel, but
Since the nonlinear element is in the OFF state at voltage V ₀ , each pixel displays “white” which is stored as “white” at phase t ₁ during the first frame F ₁ . Next, in the second frame F ₂ , for example, in pixel B, a voltage of 2V ₀ is applied to the nonlinear element and the liquid crystal layer in series connection in a period a having a phase t ₂ ', and the nonlinear element exceeds the threshold value. When the voltage exceeds this level, the liquid crystal layer enters the ON state and a high positive voltage is applied to the liquid crystal layer, causing the liquid crystal layer to enter the second electrically polarized state (black).

In addition, in pixel A, during period b with phase t ₂ ' in the figure, only a low voltage of +V ₀ is applied to the nonlinear element and the liquid crystal layer which are connected in series.
The nonlinear element remains in the OFF state, and the liquid crystal layer maintains its "white" state. On the diagram, a
In any period other than and b, only a voltage with an absolute value of V ₀ is applied to the nonlinear element and the liquid crystal layer in series connection, so the nonlinear element is in the OFF state and the liquid crystal layer is in the OFF state. No high voltage is applied to the display, and a display corresponding to "black" for B and "white" for A is achieved.

At this time, the value of the voltage value V ₀ and the phase (t ₁ + t ₂ ) = T
The value of is usually 3 to 70 volts, although it depends on the liquid crystal material used and the thickness of the cell.
It is used in the range of 0.1 μsec to 2 msec.

In order to effectively achieve the driving method of the present invention, the electric signal applied to the scanning electrode area or the signal electrode does not necessarily have to be a simple rectangular wave signal as explained in FIG. What is good is self-evident. For example, it is also possible to drive with a sine wave or a triangular wave.

FIG. 8 shows a schematic diagram of a matrix electrode structure when applied to a liquid crystal-optical shutter. At this time, reference numeral 81 is a pixel, and the electrodes on both sides of only this part are made of transparent material. 82 represents a scanning electrode group, and 83 represents a signal electrode group.

Nonlinear elements can be manufactured by changing the manufacturing parameters (area of the nonlinear element, thickness of the insulating layer, etc.).
Threshold values of 5V to 20V were obtained. In addition, the threshold value for mutual transition between the two electric polarization states of the liquid crystal (DOBAMBC) used varies depending on the set pulse width and has a width, but the pulse width is 50 μsec ~
It was about 30V to 9V for 500μsec. Under the above conditions, good operation was shown by selecting the Vo value between 5V and 20V.

According to the present invention, it was possible to improve the dependence of the voltage application time on the threshold characteristics of the ferroelectric liquid crystal, and it was possible to realize matrix driving of the ferroelectric liquid crystal.

[Brief explanation of drawings]

FIG. 1A is a sectional view of a liquid crystal element used in the present invention, and FIG. 1B is an enlarged sectional view thereof. FIG. 2 is a plan view of the liquid crystal element shown in FIG. 1. 3 and 4 are perspective views schematically showing a liquid crystal element used in the present invention. FIG. 5 is a plan view showing a matrix pixel structure used in the liquid crystal element of the present invention. FIGS. 6a to 6d are electrical signal waveform diagrams. FIG. 7 is a voltage waveform diagram shown in time series. 8th
The figure is a schematic plan view of a liquid crystal-light shutter as an example of a preferable application target of the driving method of the present invention. 1,1'...substrate, 2...thermally oxidized Ta layer, 3...
Ta layer, 4... Cr layer, 8... anodized Ta layer,
5, 6... ITO film, 7... Ferroelectric liquid crystal layer.

Claims (1)

[Claims] 1. A two-terminal element (hereinafter referred to as a non-linear device) having a non-linear voltage-current characteristic corresponding to each pixel of a matrix electrode structure in which pixels are the intersections of crossed scanning electrode groups and signal electrode groups. A method for driving a liquid crystal element having a linear element (referred to as a linear element) as a switching element, and a ferroelectric liquid crystal having bistable properties sealed between the scanning electrode group and the signal electrode group, the method comprising: Scanning the scan electrodes sequentially and simultaneously applying a first voltage of one polarity exceeding a threshold value of one polarity of the ferroelectric liquid crystal to the pixels on the scan electrode selected for scanning, and in a second step, The scanning electrodes are sequentially scanned, and a second voltage of the other polarity exceeding the threshold of the other polarity of the ferroelectric liquid crystal and a third voltage not exceeding the threshold are selectively applied to the pixels on the scanning electrode selected for scanning. A method for driving a liquid crystal element characterized by applying a voltage.