JPS60203920A - Driving method of liquid crystal optical element - Google Patents

Abstract

PURPOSE:To drive a liquid crystal optical element at a high speed with a low driving voltage by impressing the voltage exceeding the threshold of one polarity to selected non-linear type elements and impressing the voltage exceeding the threshold of the other polarity to the corresponding other non-linear type elements. CONSTITUTION:Elements having the voltage-current characteristic of a non-linear type are provided in accordance with the respective picture elements which are the intersected parts of a scanning electrode group 51(S1-S5,...) and a signal electrode group 52(I1-I5,...). A ferrodielectric liquid crystal is disposed between the groups 51 and 52. A scanning selection signal is made 2V0 voltage at a phase t1 and -2V0 at a phase t2. An information signal is added with a signal +V0 for image black and -V0 for white in synchronization with the scanning signal. The voltage of +3V0 in total is thus impressed to the picture element A at the phase t1 and the non-linear type element turns on by exceeding the threshould. The liquid crystal is held in one polarization state. The voltage of -3V0 is impressed to the picture element C and the liquid crystal is held in the other polarization state. The high-speed driving is thus made possible with the low driving voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical modulation element such as a display element or a light valve, and more particularly to a novel liquid crystal optical element suitable for operating a large number of pixels by time-division driving.

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, which prevents crosstalk. For this reason, it is not possible to increase the pixel capacity.

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

3. Method using non-linear elements such as MIM (metal/insulator/metal structure) corresponding to each pixel; 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 pixel capacitance can be increased to some extent, but when trying to increase the pixel density, the capacitance of each pixel's liquid crystal is small, and in order to achieve electrical matching, it is necessary to increase the pixel capacitance to some extent. The capacitance must be reduced accordingly, and in order for a nonlinear element to have a charge retention function, this becomes a major bottleneck in manufacturing along with severe driving conditions.

Regarding the liquid crystal driving method using this nonlinear element,
There are many reports. For example, I EEET Transac
tions on Electron Devices
, VoloED-28, No-6, JUNE 198
It is published in 1.

“The Op” by David R, Baraff et al.
Timization of Metal-Insula
tor→Ietal Non1inear Device
egforUse in Multiplexed L
If you are familiar with ``iquid Crystal Displays'', the current situation is that 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. be.

Therefore, an object of the present invention is to provide a new 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. The driving method is to provide the door.

The liquid crystal optical element of the present invention is produced by specifying a ferroelectric liquid crystal as the liquid crystal material, and by combining this with a nonlinear element that can also be achieved by ordinary photolithography technology. This makes it possible to provide a liquid crystal display device with a large area and high pixel density, which was previously unavailable.

The ferroelectric liquid crystal can have memory properties for each of the two polarization states, and in this case, the following great effects can be achieved.

In a conventional liquid crystal element consisting of a normal liquid crystal (for example, twisted nematic liquid crystal) and a nonlinear element, the nonlinear element is turned on by a pixel ON signal, and charges accumulate at both ends of the liquid crystal layer. A voltage is applied, and the liquid crystal corresponding to the pixel is turned on. After this, when the signal is turned off, the nonlinear element is turned off, and the charge accumulated at both ends of the liquid crystal layer is 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 charge basins at both ends of the liquid crystal layer will decrease, causing the liquid crystal corresponding to the pixel to become
It becomes impossible to keep it in the N state. 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 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, and it is difficult to construct a minute nonlinear element using normal photolithography technology. was difficult.

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 cause the liquid crystal to The voltage applied to the layer is also directly applied to the nonlinear element. 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 not be in the ON state, and the charge accumulated in the liquid crystal layer discharges through the nonlinear element. Alternatively, even if the threshold voltage of the nonlinear element is slightly higher than that 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 the nonlinear element to There is a high risk that it will return to the ON state. For this reason, in conventional liquid crystal elements that combine ordinary liquid crystals with no memory and nonlinear elements, it is necessary to make the threshold voltage of the nonlinear elements sufficiently larger than that of the liquid crystal, which increases the drive voltage. 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 two (ON, OFF') states (corresponding to the two polarization states of the ferroelectric liquid crystal) each have memory properties, then when a voltage is applied to the liquid crystal layer, the ON state can be changed, for example. When switching occurs, even if the voltage is subsequently removed, the ON state can be maintained, so the capacitance of the nonlinear element is allowed to be equal to or even lower than the capacitance of the pixel liquid crystal. It becomes possible to achieve high-speed driving with low driving voltage.

That is, the present invention provides an element (hereinafter referred to as a nonlinear element) having a nonlinear voltage-current characteristic corresponding to each pixel of a matrix electrode structure in which the intersection of a scanning electrode group and a signal electrode group are pixels. A method for driving a liquid crystal optical element in which a ferroelectric liquid crystal having two electric polarization states depending on the direction of an electric field is arranged between the scanning electrode group and the signal electrode group, the method comprising: a first phase in which a voltage exceeding a threshold value of one polarity is applied to a nonlinear element corresponding to a predetermined pixel on the scan electrode, and the ferroelectric liquid crystal corresponding to the pixel is transferred to one polarization state; , applying a voltage exceeding a threshold of the other polarity to a nonlinear element corresponding to a pixel other than the predetermined pixel on the scanning electrode to change the ferroelectric liquid crystal corresponding to the other pixel to the other polarization state; This is achieved by a driving method with a second phase of transition.

As will be explained in detail in the following examples, the driving method of the present invention is as follows:
Different from conventional nematic/cholesteric liquid crystals, 2
Since it uses ferroelectric liquid crystals that have two mutually opposite polarization states, it has a major feature in that it is essentially DC driven.

The ferroelectric liquid crystal used in the liquid crystal optical element of the present invention includes:
Chiral smectic C (SmC*) or H phase (SmH
) LCD is suitable. This ferroelectric liquid crystal,
”LE JOTIRNAL DE PffffSIQUE
LETTER8″36 (L-69) 1975,
[Ferroe/electricLiquid 0r)y
staesJ; ”Applied Pbysics
Letters”36 (11) 1980 rSu
bmicro 5econd B15tab/eEl!
electrooptic Switching in L
iquid CrystaesJ; “Solid State Physics” 16
(141) 1981 F LCD” etc.
In the present invention, the ferroelectric liquid crystals disclosed in these documents can be used. Specifically, examples of the ferroelectric liquid crystal compounds used in the method of the present invention include decyloxybenzylidene-P'-amino- 2-methylbutercinname) (
DOBAMBC), hexyloxypensyritene-P
'-Amino-2-chloropropyl cinnamate (HOB
ACPC) and 4-o-(2-methyl)-buterresol cylidene-4'-octyla-phosphorus (MBRA8
) etc.

When constructing an element using these materials, in order to maintain the temperature at which the liquid crystal compound becomes the SmC* phase or the SmE* phase, the element may be placed in a copper block etc. with a heater embedded in it, as necessary. can be supported.

Figure 3 is a schematic diagram of an example of a ferroelectric liquid crystal cell. 21 and 21 are In2O5s 5n01 and I
A substrate (glass plate) coated with a transparent electrode such as Indium-Tin Oxide (LJ) is used, and an S m C phase liquid crystal in which the liquid crystal molecular layer 22 is oriented perpendicular to the glass surface is sealed between them. There is. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment) 24 (PA) in the direction perpendicular to the molecule.
)have. When a voltage higher than a certain threshold is applied between the electrodes on the substrates 21 and 21', the helical structure of the liquid crystal molecules 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. I can do it. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction, and therefore, for example,
It is easily understood that 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 is obtained. 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 when no electric field is applied, as shown in Figure 4, and its dipole moment t-p or P′
takes 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 dark value as shown in Figure 3, the dipole moment will be directed upward in response to the electric field vector of the electric field E or E'. 34 or downward 34', and accordingly the liquid crystal molecules are in the first stable state [1
33 or the second stable state 33'. 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 may depend on the applied voltage waveform, specifically, during the pulse. Moreover, the uncertainty of this threshold value depends on the processing conditions of the substrate, the temperature of the liquid crystal material.

Therefore, when attempting to more stably drive the ferroelectric liquid crystal using a time-division method, it is necessary to maximize the memorability 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 and a method for driving the same.

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, p-n junction diodes connected in series with their directions reversed, Schottky diodes with proper reverse bias, and short-key diodes. It is also possible to use a device connected in series with the direction reversed.

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. The first illustration is a cross-sectional view of the liquid crystal element of the present invention, and FIG. 1(B) is an enlarged cross-sectional view of the MIM structure used therein. In the figure, 1 and f have opposing substrates (glass substrate, plastic substrate), 2 has a thermally oxidized Ta (Ta,05) layer with a thickness of 400 nm, and 3 has a layer 8 whose surface is anodized. Thickness 2000 people Ta
The tantalum (tantalum) layer 4 is a 1000 nm thick Cr (chromium) conductive layer. The MIM structure uses Ta as the metal layer.
The layer 3 has a laminated structure of an anodized Ta layer 8 serving as an insulator layer and a Cr conductive layer 4 serving as a metal layer. Reference numeral 5 indicates an ITO film having a thickness of 1000 mm, which defines the area of one pixel. 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 alignment treatment by rubbing or oblique vapor deposition of a material such as SiO, as necessary. 7 is a ferroelectric liquid crystal (for example, the above-mentioned D
OBAMBC), the liquid crystal layer of which can be 1.5μ thick. At this time, the temperature was controlled at 70°0.

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 format, in which each pixel is provided with the nonlinear element shown in FIG. 1. 51 (81~Ss~・
) is a scanning electrode group, and 52 (11 to 1. to . . . ) is a signal electrode group. The shaded area indicates “black” and the white area indicates “white”
shall be shown.

FIG. 6 shows the first embodiment, where S, ~S, represent electrical signals applied to each scanning electrode, ■1・-■ represent electrical signals corresponding to information applied to each signal electrode, and A , C indicates the voltage applied to each pixel (that is, the sum of the voltages applied to the nonlinear element and the liquid crystal layer).

The scan selection signal has a voltage of 2Vo in the first phase (tl).
It is a waveform of -2Vo alternating in the second phase (t2). In Figure 6, 1. = 1. An example is illustrated. Further, the non-scanning electrode receives an electrical signal O.

On the other hand, the information signal is +V for the image "black".

The -Vo signal is applied to "white" in synchronization with the scanning signal.As a result, in pixel A, at phase t1 in the figure, the nonlinear element and liquid crystal layer connected in series are connected. together +
A voltage of 3Vo is applied, and the nonlinear element exceeds the dark value and becomes O
Since the liquid crystal layer becomes N state and a high positive voltage is applied to the liquid crystal layer, it transitions to one electric polarization state (this is black). or,
In pixel C, at phase t,' in the figure, a total voltage of -3Vo is applied to the nonlinear element and the liquid crystal layer that are connected in series, and the nonlinear element exceeds the opposite threshold and is in the ON state. Since a high negative voltage is applied to the liquid crystal layer, it transitions to the other electrical polarization state (this is white). In addition, in pixels A and C, only a voltage with an absolute value of Vo is applied to the nonlinear element and the liquid crystal layer in series connection in any period other than the above-mentioned specific period, so the nonlinear The element is in the ON state and no high pressure is applied to the liquid crystal layer.
A display corresponding to "black" for A and "white" for C is achieved and maintained. With this, it is possible to obtain a still image memory by scanning all frames, and it is also possible to obtain a moving image by repeatedly scanning. FIG. 7 shows a second driving embodiment. This embodiment is exactly the same as the first embodiment except that a period Δt is provided for applying an auxiliary signal to the signal electrode side. Since this driving method is essentially a direct current driving method, the auxiliary signal applied to the signal electrode is continuously applied with a black or white voltage to each pixel on one signal electrode, resulting in non-linearity. This is to avoid the risk of deterioration of the element or inversion of the polarization state of the liquid crystal layer. In this embodiment, in the period Δt during which the auxiliary signal is applied, the write period 1. +1. A signal whose polarity is inverted from that of the signal applied to the signal electrode is applied to the signal electrode.

By changing the manufacturing parameters (area of the nonlinear element, thickness of the insulating layer, etc.), the nonlinear element can have a threshold value of 5V to 2V.
A voltage of 0V was 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.
It was about 30V to 9V for 00μsec. Under the above conditions, good operation was achieved by selecting a VoO value of 5 - 20V.

[Brief explanation of the drawing]

The first position is a cross-sectional view of the liquid crystal optical element used in the present invention.
Figure (B) is an enlarged sectional view thereof. FIG. 2 is a plan view of the liquid crystal optical element shown in FIG. 1. 3 and 4 are perspective views schematically showing a liquid crystal optical element used in the present invention. FIG. 5 is a plan view showing a matrix pixel structure used in the liquid crystal optical element of the present invention. Figures 6 and 7
The figures are explanatory diagrams each showing an embodiment of the driving method of the present invention. 1, f; 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 1 (△) Figure 7 Figure 6 Small, Figure 2 Figure B

Claims (7)

[Claims] (1) An element having a nonlinear voltage-current characteristic (hereinafter referred to as a nonlinear element) is provided 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 optical element in which a ferroelectric liquid crystal having two electric polarization states depending on the direction of an electric field is arranged between the scanning electrode group and the signal electrode group, the method comprising: a first phase in which a voltage exceeding a threshold of one polarity is applied to a nonlinear element corresponding to a predetermined pixel on the scanning electrode to transfer the ferroelectric liquid crystal corresponding to the pixel to one polarization state; Applying a voltage exceeding a threshold of the other polarity to a nonlinear element corresponding to a pixel other than the predetermined pixel on the scanning electrode to transfer the ferroelectric liquid crystal corresponding to the image collection on the side to the other polarization state. A method for driving a liquid crystal optical element characterized by having a second phase. (2) Applying electrical signals having different voltage phases to selected scanning electrodes of the scanning electrode group, and applying electrical signals having different voltages to the selected signal electrodes and unselected signal electrodes of the signal electrode group. A method for driving a liquid crystal optical element according to claim 1. (3) Applying electrical signals having different voltage polarities and phases to selected scanning electrodes of the scanning electrode group, and applying electrical signals having different voltage polarities to the selected signal electrodes and unselected signal electrodes of the signal electrode group. A method for driving a liquid crystal optical element according to claim 1, which provides a signal. (4) The method for driving a liquid crystal optical element according to claim 1, wherein the ferroelectric liquid crystal is a liquid crystal having a smectic phase. (5) The method for driving a liquid crystal optical element according to claim 4, wherein the liquid crystal having a smectic phase is a liquid crystal having a chiral smectic phase. (6) The method for driving a liquid crystal optical element according to claim 5, wherein the liquid crystal having a chiral smectic phase is a liquid crystal phase that does not form a helical structure. (7) The method for driving a liquid crystal optical element according to claim 5, wherein the liquid crystal having a chiral smectic phase is a C phase or an H phase.