JPS6128929A - Driving method of liquid crystal element - Google Patents

Abstract

PURPOSE:To attain high-speed drive with a low driving voltage by applying a voltage of one polarity higher than a threshold value to nonlinear elements corresponding to picture elements on scanning electrodes in line sequence and then applying a voltage of the opposite polarity higher than the threshold value to nonlinear elements selected among the nonlinear elements corresponding to picture elements on the scanning electrodes in line order. CONSTITUTION:A scanning signal is applied to all or some of a scanning electrode group 52 in the 1st frame F1, and a white signal is applied to all or some of a signal electrode group 53 synchronously. A black signal is applied to specific positions in the 2nd frame F2. Namely, all the scanning electrodes are applied with the pulse voltage 2V0 firstly in line sequence in the 1st frame F1 and all signal electrodes are applied with a white signal. Consequently, a voltage -V0 is applied in phase t1 to nonlinear elements coupled with all picture elements in series and a liquid-crystal layer, and the nonlinear elements exceed the threshold value to turn on and applies a high voltage to the liquid crystal layer, which is placed in the 1st electric polarization state uniformly.

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 manufacture, 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. A method of providing active elements such as TPT (thin film transistors) corresponding to each pixel; crosstalk can be avoided because each active element performs a clear switching operation, but it is extremely difficult to fabricate active elements. This requires precise alignment technology, and if this is applied to a large screen liquid crystal element, the cost will be extremely high.

19 MIM (metal/insulator/metal) corresponding to each pixel
Method using non-linear elements such as structure: If good electrical matching can be achieved between each non-linear element and the liquid crystal layer corresponding to each pixel, crosstalk can be prevented and the capacitance of one pixel can be increased to some extent, but the pixel When trying to increase the density, the capacitance of the liquid crystal layer of each pixel becomes smaller, and in order to achieve electrical matching, the capacitance of each nonlinear element must be reduced accordingly. First, in order for a nonlinear element to have a charge retention function, driving conditions are also a major bottleneck in manufacturing.

There are many reports regarding liquid crystal driving methods using this nonlinear element0, for example, IEEE Transactions on EJle.
ctron'(Devices, Vol, ED-28, No.
6. David published in JUNE1981
The Optimizat by R. Baraff et al.
ion of Metal-Insuiator-M
etaJI NonJlinear Device
sfor Use in MuJltipex
ed LiquidCrysta sentence Disp sentence ay
Detailed disclosure is made in “s”.In any case,
No matter which method is used, it is difficult to display a large screen with a large pixel capacity, and a relatively inexpensive liquid crystal element has not yet appeared.

Therefore, an object of the present invention is to provide a new method for driving a liquid crystal element 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. Our goal is to provide the following.

The liquid crystal element used in the present invention is created by specifying a ferroelectric liquid crystal material as the liquid crystal material, and by combining this with a nonlinear element that can also be achieved by ordinary photolithography technology. Accordingly, it is possible to provide a liquid crystal display device with a large area and high pixel density.

The ferroelectric liquid crystal used in the yet-to-be-invented liquid crystal element can have two polarization states each having memorability, and in this case, the following great effects can be achieved.

In a conventional liquid crystal device consisting of a normal liquid crystal (e.g. twisted nematic liquid crystal) 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 increased. When the voltage is applied, the liquid crystal corresponding to the pixel is turned on. After this, when the signal is turned 0FFF.

The nonlinear element is turned off, and the charges accumulated at both ends of the liquid crystal layer are capacitance-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 decreases, 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 must normally 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, when trying to increase pixel density, the capacitance of the pixel liquid crystal decreases, so it is necessary to further reduce the capacitance of the nonlinear element. It was difficult to do so.

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 charges accumulated at both ends of the liquid crystal layer will be applied to the liquid crystal layer. The applied voltage is also applied almost unchanged to the nonlinear elements. Therefore, the liquid crystal layer is turned from the OFF state to the
If the threshold voltage of the nonlinear element is lower than the voltage required to switch to the N state (threshold of the liquid crystal), the nonlinear element is turned on.
state, and the charges accumulated in the liquid crystal layer are discharged 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 subsequently applied to the signal electrode,
Furthermore, there is a high risk that the voltage applied to the nonlinear element will increase and the nonlinear element will return to the ON state.For this reason, in a conventional liquid crystal element that combines a normal liquid crystal without memory and a nonlinear element, the liquid crystal It is necessary to make the threshold voltage of the nonlinear element sufficiently larger than the threshold voltage of , which results in an increase in the driving voltage.

In any case, with conventional liquid crystal elements, 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 (ON and 0FF) (corresponding to the two polarization states of the ferroelectric liquid crystal) each 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 subsequently released, the O
Since the N 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. It became.

That is, the driving method of the present invention uses an element (hereinafter referred to as a non-linear element) having non-linear voltage-current characteristics corresponding to each pixel of a matrix electrode structure in which pixels are intersections of crossed scanning electrode groups and signal electrodes. ) and having a ferroelectric liquid crystal between the scanning electrode group and the signal electrode group, the non-linear driving method corresponds to a pixel on a scanning electrode selected from the scanning electrode group. By applying a voltage exceeding the threshold value of one polarity to the element according to line-sequential scanning, the electric polarization state of the ferroelectric liquid crystal is changed to one state, and then in the next period, the electric polarization state of the ferroelectric liquid crystal on the scanning electrode is applied according to line-sequential scanning. The method is characterized in that the electric polarization state of the ferroelectric liquid crystal is transferred to another state by applying a voltage exceeding a threshold of opposite polarity to a selected nonlinear element among the nonlinear elements corresponding to a pixel. ing.

As will be explained in detail in the following examples, the driving method of the present invention uses conventional nematic or cholesteric ferroelectric liquid crystals, so it has a major feature in that it is a direct current drive. There is.

The ferroelectric liquid crystal used in the liquid crystal element of the present invention includes chiral smectic C phase (SmC''), wood H phase (SmH), F phase (SmF*), Iwamoto (SmI), or G phase (SmG*). ) liquid crystal is suitable.For this ferroelectric liquid crystal, “'LE JOUR
NAL DE PHYSIQ UE LETTER3”
36 (L-69) 1975. rFerrocl
etricLiquid CrystaJljsJ
``AppJlied Physics Letter
s” 36 (11) 1980 rsubmicro
5econdBistable Eiectroop
tic Switching 1nLiquid C
Rysta Bun S J, -1 Solid State Physics” 16 (141)
1981 R Liquid Crystal, etc., and in the present invention, the ferroelectric liquid crystal disclosed in these documents is used. More specifically, as an example of the ferroelectric liquid crystal compound used in the method of the present invention, desyloxybenzylidene -P'-amino-2-methylbutylcinnamate (DoBAMBC), hexyloxybenzylidene-P'-amino-2-chloropropylcinnamate (HOBACPC) and 4-o-(2-
Examples include methyl)-phthyl resol cylidene-4'-octylaniline (MBRA8).

When constructing devices using these materials* In this case, the liquid crystal compound is jmC, SmH*.

In order to maintain the temperature at a temperature suitable for SmF, SmI* and SmG trees, the element can be supported by a copper block or the like in which a heater is embedded, if necessary.

FIG. 3 schematically depicts an example of a ferroelectric liquid crystal cell. 21 and 21' are In2O3゜5n02 or I
T O(I n d f u m −T i n0x
Substrate (glass plate) coated with a transparent electrode such as
In between, an S m C zero-phase liquid crystal oriented such that the liquid crystal molecular layer 22 is perpendicular to the glass surface is sealed. 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. When a voltage higher than a certain dark value is applied between the electrodes on the substrates 21 and 21', the helical structure of the liquid crystal electrons 23 is unraveled, and the liquid crystal molecules 23 change their alignment direction so that all the dipole moments 24 are oriented in the direction of the electric field. be able to. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and short axis direction,
Therefore, it is easy to understand that, for example, if crossed Nicol polarizers are placed above and below a glass surface, a liquid crystal modulation element whose optical characteristics change depending on the polarity of applied voltage can be obtained. Furthermore, if the thickness of the liquid crystal cell is made sufficiently thin (for example, 1JL
), no electric field is applied as shown in Figure 4.
Even in one state, the helical structure of the liquid crystal molecules is unraveled (non-helical structure), and its dipole moment P or P' assumes either an upward (34) or downward (34') electric 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. The liquid crystal molecules are oriented downward 34', and accordingly the liquid crystal molecules are oriented either in the first stable state 33 (one electrically polarized state) or in the second stable state 33 (the other electrically polarized state S). do. 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 threshold is usually not extremely sharp;
Moreover, it depends on the applied voltage waveform, more specifically on the pulse width. In addition, the uncertainty of this threshold value may be due to substrate processing conditions,
Depends on temperature and liquid crystal material. Therefore, by combining this with a nonlinear element to clarify the threshold characteristics using a time division method, it is possible to provide a large pixel capacitive element and its driving method that can make maximum use of the memory properties of ferroelectric liquid crystals. It became clear that

Further, as the nonlinear element used in the present invention, the above-mentioned M
In addition to IM, there are p-n junction diodes with proper reverse bias, and p-n junction diodes connected in series with their directions reversed.

A Schottky diode properly reverse-biased, a Schottky diode reversed in direction and connected in series, etc. can be used.

FIGS. 1 and 2 schematically show the structure of a liquid crystal element according to the present invention, and show an example in which an MIM structure is used as a nonlinear element. FIG. 1(A) is a cross-sectional view of the liquid crystal element of the present invention, and FIG. 1(B) is a cross-sectional view of the MIM used therein.
In Figure 0, which is an enlarged cross-sectional view of the structure, 1 and 1' are opposing substrates (glass substrate, plastic substrate), 2 is a thermally oxidized Ta (Ta205) layer with a thickness of 400 nm, and 3 is an anode surface. Thickness 2000 T with oxidized layer 8
The a (tantalum) layer 4 is a cr (chromium) conductive layer with a thickness of 1000 nm. The MIM structure has a laminated structure of a Ta layer 3 serving as a metal layer, an anodized Ta layer 8 serving as an insulating layer, and a Cr conductive layer 4 serving as a metal layer. 5 is ITOII with a thickness of 1000 people! This defines one pixel area. Further, 6 is an ITO pattern of a counter electrode. The substrate 1 on which the MIM is formed and the substrate 2 on which the conductive pattern is formed may be subjected to an alignment treatment by rubbing or obliquely depositing a material such as SiO, as necessary. It is a liquid crystal (for example, the above-mentioned DOBAMBC), and the liquid crystal layer can be 1.5 mm thick. At this time, the temperature was controlled at 70''C.

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

Fig. 5 and subsequent figures show driving embodiments of the present invention.
An example of a display form using a matrix electrode structure 51,
Each pixel is formed at the intersection of the scanning electrode group 52 and the signal electrode group 53, and each pixel is provided with a ferroelectric liquid crystal and a nonlinear element shown in FIG. In FIG. 5, black areas are displayed as "black" and white areas are displayed as "white".

FIGS. 6(a) and 6(b) respectively show scanning signals during selection and scanning signals during non-selection.

Figures 6('c) and (d) show 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 bistability state, respectively. It shows an electric signal (referred to as a black signal) that orients to the second stable state S (the other electric polarization state).

First, as shown in FIG. 7, 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, and synchronizes with this to apply a scanning signal to all or part of the signal electrode group 53. Apply a white signal to the section. 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, the scanning electrode group 52 (521,
522, 523, 524, 525) and signal electrode group 53
(531, 532, 533.

534, 535) and the voltage waveforms applied to pixels A and B in FIG. 5 (the sum of the voltages applied to the nonlinear element and the liquid crystal layer) are shown in FIG.

As is clear from the figure, in the first frame F1, according to line sequential scanning, a pulse voltage of 2V (1) is first applied to all scanning electrodes, and a white signal is applied to all signal electrodes. A voltage of -2 VQ is applied to the non-linear element connected in series and the liquid crystal layer at phase t1, and the non-linear element exceeds the threshold value and turns on, applying a high negative voltage to the liquid crystal layer. On the other hand, in the first phase t2, VQ is applied to each pixel, but at the voltage vo, the nonlinear element is in the 0FFf state, so the period of the first frame F1 Then, in the second frame F2, for example, in pixel B, each pixel is displayed as "white" which is memorized as "white" at phase t1. is the phase t 2
A voltage of 2VQ is applied during a period a having ', and the nonlinear element exceeds the threshold value and becomes ON, and a high positive voltage is applied to the liquid crystal layer, so that the liquid crystal layer enters the second electric polarization state (black).

In addition, in pixel A, during the period with phase t2 in the figure, only a low voltage of +0 is applied to the nonlinear element and the liquid crystal layer that are connected in series, so the nonlinear element is turned off.
The liquid crystal layer remains in the "white" state. In the figure, in any period other than a and b, the nonlinear element and the liquid crystal layer in series connection have an absolute value of V.
Since only the voltage of O is applied, the nonlinear element is in the OFF state, and no high voltage is applied to the liquid crystal layer.
A display corresponding to "black J" for B and "white" for A is achieved.

At this time, the value of the voltage value VO and the value of the phase (tl+t2)=T depend on the liquid crystal material used and the thickness of the cell, but are usually 3 ports to 70 ports, O, lpsec.
-2 msec is used.

In order to effectively achieve the driving method of the present invention, the electric signal applied to the scanning electrode or the signal electrode does not necessarily have to be a simple rectangular wave signal as explained in FIG. 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, 81 is a pixel, and only in this part, the electrodes on both sides are made of transparent material. 82 is a scanning electrode group, 83
represents a group of signal electrodes.

FIG. 9 shows another modified example in which the waveform of the scanning signal applied to the scanning electrode group and the waveform of the information signal applied to the signal electrode group are modified.As is clear from FIG. At phase t1, a voltage of -2VQ is applied to each pixel according to line sequential scanning, and at phase t2, a voltage of v□ is applied, but the nonlinear element connected in series to one pixel is in the ON state at 2VO and the Vo-cOFF state. Therefore, the phase t1
The state displayed as "white" is memorized and displayed as white during the first frame F1. Next, in the second frame F2, which is a writing period, 2VQ, which exceeds the threshold of the nonlinear element, is applied to the pixel B at a phase t2, thereby displaying "black". After that, since a voltage exceeding the threshold of the nonlinear element cannot be applied to pixel B (the nonlinear element is in the OFF state at voltage VO or -VQ), pixel B It will be memorized and displayed in "Black".

The nonlinear element can be made to have voltages of 1, □, and 5 V by changing the manufacturing parameters (area of the nonlinear element, thickness of the insulating layer, etc.)
~20 V(7)G(7)fl161'Li,
'Also, the threshold value for transition between the two electric and polarized states of the liquid crystal (DOBAMBC) used varies depending on the set pulse width, and has a width, but the pulse width is 50g5
It was about 30V to 9V for ec ~500 pLsec. Under the above conditions, the value of VQ is 5■~20
Good operation was shown by selecting V solder.

[Brief explanation of drawings]

FIG. 1(A) is a sectional view of a liquid crystal element used in the present invention, and FIG. 1(B) 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. Fifth
The figure is a plan view showing a matrix pixel structure used in the liquid crystal element of the present invention. FIGS. 6(L) to 6(d) are electrical signal waveform diagrams. 7th
The figure is a voltage waveform diagram shown in time series. FIG. 8 shows a liquid crystal display as an example of a preferred application target of the driving method of the present invention.
FIG. 3 is a schematic plan view of an optical shutter. FIG. 9 is a voltage waveform diagram shown in time series in another specific example 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. Patent applicant Canon Co., Ltd. Figure 3 Figure 4

Claims (8)

[Claims] (1) An element that has nonlinear voltage-current characteristics corresponding to each pixel of a matrix electrode structure in which pixels are the intersections of crossed scanning electrode groups and signal electrodes (hereinafter referred to as nonlinear element)
A method for driving a liquid crystal element having a ferroelectric liquid crystal between the scanning electrode group and the signal electrode group, the non-linear element corresponding to a pixel on a selected scanning electrode from the scanning electrode group. By applying a voltage that exceeds the threshold of one polarity to the ferroelectric liquid crystal according to line-sequential scanning, the electric polarization state of the ferroelectric liquid crystal becomes one state, and then in the next cycle, pixels on the scanning electrode are applied according to line-sequential scanning. A liquid crystal element characterized in that the electric polarization state of the ferroelectric liquid crystal is transferred to the other state by applying a voltage exceeding a threshold of reverse polarity to a selected nonlinear element among the nonlinear elements corresponding to the ferroelectric liquid crystal. driving method. (2) The method for driving a liquid crystal element according to claim 1, wherein the electric polarization state of the ferroelectric liquid crystal has a memory property. (3) A method for driving a liquid crystal element according to claim 1 or 2, in which the ferroelectric liquid crystal exhibits two different electrical polarization states. (4) The method for driving a liquid crystal element according to claim 1, wherein the ferroelectric liquid crystal has a smectic phase. (5) The method for driving a liquid crystal element according to claim 1, wherein the ferroelectric liquid crystal has a chiral smectic phase. (6) The chiral smectic phase is C phase, H phase, F phase
6. The method of driving a liquid crystal element according to claim 5, wherein the liquid crystal element is in phase, I phase or G phase. (7) The method for driving a liquid crystal device according to claim 5 or 6, wherein the chiral smectic phase is a phase having a non-helical structure. (8) The method for driving a liquid crystal element according to claim 1, wherein the nonlinear element is an element having a laminated structure of a metal layer, an insulator layer, and a metal layer.