JP2000019559A - Liquid crystal element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drive liquid crystals of large spontaneous polarization at a high speed by an active matrix system. SOLUTION: The pixel electrode disposed by each of pixels is divided to a first pixel electrode 50 and a second pixel electrode 51 connected to a drain electrodes 47 of a first TFT. A counter electrode 54 which is made electrically independent by each of the pixels is disposed, by which the constitution connected with two capacitances in series is obtd. and the load acting on the first TFT connected with the first pixel electrode is thereby reduced to 1/4 the ordinary load. Further, the first pixel electrode 50 and the second pixel electrode are connected by a second TFT and the charge in the pixel is discharged by turning on the second TFT and thereafter, the first TFT is turned on to supply the prescribed charge from an information signal line 6 to the first pixel electrode and to accumulate the charge to the two capacitances described above, by which the period required for writing is halved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element which performs display at high speed by an active matrix system.

[0002]

2. Description of the Related Art Various liquid crystal materials such as a nematic liquid crystal, a smectic liquid crystal, and a polymer-dispersed liquid crystal are used as a liquid crystal for a liquid crystal display device.

[0003] In particular, a liquid crystal device having spontaneous polarization and having bistability is disclosed in both Clark and Lagerwall in JP-A-56-10.
No. 7216 and U.S. Pat. No. 4,362,924. Bistable liquid crystals generally include a chiral smectic C phase (SmC ^* ) or an H phase (SmC ^* ).
mH ^* ) are used, exhibit a first optically stable state and a second optically stable state, a so-called bistable state, in response to an electric field applied in these states, and have a voltage of It is expected to be widely used in fields such as high-speed and storage-type display devices, which have a property of maintaining the state when no voltage is applied, that is, stability, and have a quick response to a change in electric field.

[0004] Antiferroelectric liquid crystals are also liquid crystals having spontaneous polarization. Such liquid crystals have a hysteresis characteristic as disclosed in JP-A-2-153322 and JP-A-8-328046. It is known that there is no threshold value as disclosed in the gazette and an intermediate alignment state is exhibited. FIG. 7 shows an example of voltage-transmittance characteristics of the two kinds of antiferroelectric liquid crystals. In the figure, (a) is a type having the above-mentioned hysteresis characteristic, and (b) is a type showing an intermediate alignment state.

More specifically, in a device using a liquid crystal having the characteristics shown in FIG. 7A, the first light transmittance state is shown when the applied voltage is 0, and a predetermined value of each polarity in absolute value is shown. By applying a voltage equal to or higher than a _first threshold value (+ V ₁ or −V ₁ ), switching to a second light transmittance state is performed. by applying a second threshold value (+ V ₂ or -V ₂₎ less voltage, switching to the second light transmitting state. Therefore, by sandwiching the liquid crystal element between the pair of polarizing plates so that the liquid crystal element is in the darkest state when the applied voltage is 0, binary white / black display is possible.

In an element using a liquid crystal having the characteristics shown in FIG. 7B, when the applied voltage value is 0, the first
Of the light transmittance state, in absolute value the polarity of a given saturation voltage (+ V ₃ or -V ₃₎ or more voltage values at present a second light transmission state, and, depending on the application voltage value It has a voltage-transmittance characteristic in which the light transmittance changes continuously between the first light transmittance state and the second light transmittance state. Therefore, when the liquid crystal element is sandwiched between a pair of polarizing plates so that the state of the first light transmittance state is the darkest state,
A continuous halftone can be displayed according to the voltage value applied between black and white.

A liquid crystal device driven by an active matrix has been studied by utilizing the high-speed and wide viewing angle characteristics of the ferroelectric or antiferroelectric liquid crystal. The following are related documents. (1) A full-color threshold
less Antiferroelectric LC
D exciting wide viewing
angle with fast response
time, T .; Yoshida et al, SID
97 (Society for Information)
on Display 97) DIGEST P841 (2) Voltage-holding property
ties of thresholdless ant
iferoelectric liquidcrys
tals driven by active mat
rices, T .; Saishu et al, SID
96 (Society for Information)
n Display 96) DIGEST P703 (3) Analytical modeling o
f active-matrix driving o
f liquid crystals withspo
ntaneous polarization T.T. V
erhurstJpn. J. Appl. Phys. Vo
l. 36 (1997) pp 720-729

[0008]

Since a ferroelectric liquid crystal or an antiferroelectric liquid crystal has spontaneous polarization, it has a higher response speed than a nematic liquid crystal and can be driven at a high speed. However, since it is necessary to supply the current required for reversing the spontaneous polarization from the outside, there is a problem that the load as viewed from the drive circuit is large. In particular, this problem becomes serious when spontaneous polarization is large. In the following, actual values will be described.

In a normal TN type liquid crystal device, the capacitance between electrodes sandwiching a liquid crystal layer is about ² nF / cm ² . Assuming that the size of one pixel is 70 μm × 210 μm, the capacitance per pixel is about 0.3 pF (= C), and the charge required for driving this at 10 V (= V) The quantity is Q = CV = 3 pC.

On the other hand, in the case of a ferroelectric liquid crystal or an antiferroelectric liquid crystal, if the magnitude Ps of spontaneous polarization is set to 100 nC / cm ² in addition to the above-described charge charge, Q = 2PsS = 29pC (S is the area of one pixel). That is, the spontaneous polarization is 100 n
When a liquid crystal of about C / cm ² is used, the amount of charge required for driving with the same cell configuration is about ten times that of the TN type liquid crystal. Therefore, when an active matrix type liquid crystal element using a liquid crystal having a large spontaneous polarization is driven in the same time as a TN type liquid crystal element, the necessary current amount is one.
The driving capability of the switching element of the pixel must be at least 10 times higher than that of the TN type liquid crystal element.

In the active matrix type liquid crystal element, for example, a thin film transistor (TFT) is used as a switching element. However, in order to increase the driving capability, that is, the conductance between the source and the drain when the gate is turned on, the TFT is sacrificed at the expense of the aperture ratio. Must be increased in size. However, it is impractical to increase it by a factor of 10 or more.

In particular, an antiferroelectric liquid crystal having a voltage-transmittance characteristic shown in FIG. 7B can display a continuous halftone by active matrix driving. Since only the one with 100 nC / cm ² or more was obtained, it was difficult to drive the active matrix with a normal TFT.

In the characteristics shown in FIG. 7B, the transition between the first light transmittance state and the second light transmittance state draws a hysteresis curve, or the transmittance of the liquid crystal before the response time is rewritten. In some cases, the state differs depending on the state. In particular, there is a problem that the disturbance of gradation display becomes remarkable in a liquid crystal having a large spontaneous polarization.

An object of the present invention is to configure a liquid crystal element which is driven at high speed by active matrix using a liquid crystal having spontaneous polarization. In particular, it is an object of the present invention to improve the stability of the domain wall and the controllability of the charge amount and realize a stable gradation display when using a liquid crystal having no threshold in the voltage-transmittance characteristic.

[0015]

According to the present invention, a plurality of scanning signal lines and information signal lines which are orthogonal to each other, and an intersection of the signal lines are defined as one pixel, and a pixel electrode and a first switching element are provided for each pixel. A liquid crystal having spontaneous polarization is sandwiched between a first substrate provided and a second substrate provided with an electrically independent counter electrode for each pixel. A plurality of regions independent of each other and interconnected by a second switching element, and a plurality of capacitances connected in series are formed as a load of the first switching element by the pixel electrode and the counter electrode. A liquid crystal element.

In the present invention, in particular, the first light transmittance state is set when the applied voltage value is 0, and the second light transmittance state is set when the applied voltage value is equal to or higher than a predetermined saturation voltage value of each polarity in absolute value. It has a voltage-transmittance characteristic in which a light transmittance state is exhibited and the light transmittance continuously changes between the first light transmittance state and the second light transmittance state according to an applied voltage value. Good gradation display can be performed using liquid crystal.

[0017]

FIG. 1 is an equivalent circuit of one pixel of an embodiment of the liquid crystal device of the present invention. In the figure, 1 is a thin film transistor (TFT) as a first switching element, 2 is a TFT as a second switching element, 3 is a first capacitance, and 4 is a second capacitance.

FIG. 2 shows a plurality of pixels (2
× 3) This is an equivalent circuit of the embodiment in which the gate electrodes of the second TFT 2 are arranged and the scanning signal line of the previous line is connected.
In the figure, 5 is a scanning signal line, and 6 is an information signal line.

FIG. 3 is a schematic plan view showing an example of the electrode structure of the embodiment having the equivalent circuit of FIG.
A cross-sectional view is shown in FIG. FIG. 3 shows only main members for convenience. In the figure, 31 to 34 are members constituting the second TFT 2, 31 is a gate electrode, and 32 is a-Si.
(Amorphous silicon) layer, 33 is a source electrode, 34
Is a drain electrode. Also, 42 to 47 are the first TFs.
42 is a gate electrode, 43 is a gate insulating film, 44 is an a-Si layer, 45 is an n ⁺ a-Si layer,
46 is a source electrode and 47 is a drain electrode. Further, 41 is a first substrate, 53 is a second substrate, 48 is an insulating film, 49 is a passivation film, 50 is a first pixel electrode, 51 is a second pixel electrode, 54 is a counter electrode, and 55 is insulating. Reference numerals 52 and 56 denote alignment films, 57 denotes a liquid crystal having spontaneous polarization, 58 denotes a storage capacitor electrode, and 7 denotes a wiring.

As the liquid crystal having spontaneous polarization used in the present invention, the above-mentioned ferroelectric liquid crystal or antiferroelectric liquid crystal is used. Among them, the voltage-transmission without threshold shown in FIG. By using a liquid crystal having a rate characteristic, good gradation display can be achieved. FIG. 7 (a)
When a liquid crystal exhibiting the above characteristics is used, white / black binary display can be performed.

In the present invention, as long as the electrode configuration according to the present invention is provided, the material, shape, manufacturing method, etc. of each member can be determined by using the technology of a general liquid crystal element, particularly, an active matrix type liquid crystal element. Can be applied.

In the present invention, a plurality of electrically independent regions are formed by dividing a pixel electrode into a first pixel electrode 50 and a second pixel electrode 51. The first pixel electrode 50 is connected to the drain electrode 47 of the first TFT 1,
The first capacitance 3 is formed between the liquid crystal 57 and the opposing electrode 54 with the liquid crystal 57 interposed therebetween. On the other hand, the second pixel electrode 51 also forms the second capacitance 4 between the second pixel electrode 51 and the counter electrode 54 with the liquid crystal 57 interposed therebetween. The counter electrode 57 is insulated for each pixel, and since the counter electrode 57 is a common electrode with respect to the first and second pixel electrodes, these capacitances 3 and 4 are connected in series as a load of the first TFT 1. It will be connected. Further, the first pixel electrode 50 and the second
Are formed to have the same area as the pixel electrode 51 and are connected by the second TFT 2.

The operation of each member in the above embodiment is shown in FIG.
This will be described below. FIG. 5 is a cross-sectional view schematically showing a main part of one pixel of the embodiment.

In the electrode configuration shown in FIG.
Is turned off, the first TFT 1 is turned on, and the first TFT 1 is turned on.
When an information signal of a predetermined potential is applied to the information signal line to which the FT1 is connected, a charge corresponding to the potential is supplied to the first pixel electrode 50 via the first TFT1, and the first pixel electrode 50 It is stored in the capacitance 3 formed by the counter electrode 54. As a result, the second pixel electrode 5 of the counter electrode 54
An electric charge equivalent to that of the first pixel electrode 50 is generated on the surface facing 1 and is accumulated in the capacitance 4 formed by the second pixel electrode 51 and the opposing electrode 54. An electric field in the opposite direction is applied to the liquid crystal by the capacitances 3 and 4.

FIG. 5A shows a pixel during display, that is, a state in which both of the capacitances 3 and 4 are charged, and both the first TFT 1 and the second TFT 2 are turned off. ing. The arrow in the figure is the direction of the electric field.

Next, the second TTF 2 is turned on to conduct the first pixel electrode 50 and the second pixel electrode 51, thereby discharging the electric charge in the pixel (FIG. 5B). When the area of these pixel electrodes is the same, an equal amount of electric charge is accumulated between the electrodes, so that the capacitances 3 and 4 are discharged instantaneously. FIG. 5C shows the pixel after the discharge. Actually, the writing of (d) may be performed after the discharging of (b) without providing a period for turning off the first and second TFTs at the same time.

Next, the second TFT 2 is turned off, and the first TFT 1 is turned on to supply a charge corresponding to the information signal to the first pixel electrode 50. At this time, when inverting the polarity of the voltage applied to the liquid crystal for each frame, as shown in (d), a charge having a polarity opposite to that of (a) is applied to the first pixel electrode 50.
To supply.

When comparing this embodiment with the conventional case where the pixel electrode is not divided, the load capacitance seen from the first TFT 1 is such that two capacitances constituted by pixel electrodes having half the area are connected in series. Therefore, the combined capacity is 1/4 of the conventional capacity. However, the drive voltage given from the information signal line must be twice the conventional voltage value V, but the amount of current flowing through the first TFT 1 is ２.

The first pixel electrode 50 for supplying electric charges
Is half of the conventional area, the inversion current due to spontaneous polarization is also halved.

Further, by discharging the pixel before writing by the second TFT 2, the writing period can be reduced to half of the conventional period, or the driving capability required for the first TFT 1 can be reduced to half. it can. Therefore, when scanning is performed in the same period as in the related art, a liquid crystal having a spontaneous polarization of 100 nC / cm ² can be displayed well with a TFT having a driving ability 2.5 times that of a TN type liquid crystal. .

Further, since predetermined writing is performed in accordance with the information signal after discharging the pixel once, the liquid crystal state at the time of writing is the same in all the pixels, so that the voltage-transmittance characteristic has a hysteresis. , Or the response time of the liquid crystal at the time of the next writing differs depending on the display state before writing, the written contents can be stabilized.

When the capacitance is connected in series by dividing the pixel electrode into two parts as in the present embodiment, the liquid crystal 57 is formed between the region of the first pixel electrode 50 and the region of the second pixel electrode 51. The direction of the applied electric field is reversed. In antiferroelectric liquid crystals, the optical axis in the bright state takes two directions tilted to the left and right with respect to the smectic axis direction, and is switched at regular intervals to drive the AC, but this is visually recognized as flicker. There is. As a method for preventing this, a so-called 1H inversion driving method in which the driving polarity is reversed for each row has been proposed (Japanese Patent Application Laid-Open No. 4-182694). Therefore, the source voltage must be alternately applied to the positive and negative sides, and a source (information signal) driver IC having a high voltage output is required. In the present embodiment, since there are two positive and negative directions in one pixel, 1H inversion is not required, and there is an advantage that the voltage of the source driver IC can be easily reduced.

In the embodiment shown in FIGS. 2 and 3,
The gate electrode 31 of the second TFT 2 is connected to the scanning signal line 5a of the previous line, and discharges the pixel using a period during which the first TFT 1 of the pixel of the previous line is turned on and writing is performed. The gate electrode 31 of the second TFT 2 in the first line
May be connected to the last scanning signal line, but since the wiring becomes long, a separate signal line is provided before the first scanning signal line to control the gate of the second TFT 2 in that line. May be. The wiring 7 is connected to the pixel electrode 51, and the wiring 7 is drawn out and grounded.

FIG. 6 is a diagram showing an equivalent circuit of another embodiment of the liquid crystal element of the present invention. In this embodiment, the gate electrode of the second TFT 2 is connected to a plurality of previous scanning signal lines. Connected, and controlled by a scanning signal applied to the previous line. Therefore, the discharge period of the pixel can be twice as long as the write period, and is effective when the response time of the liquid crystal during discharge is long. In the present embodiment, the second TFs of the pixels on the first and second lines are also used.
A separate signal line may be provided in front of the first scanning signal line for controlling the gate of T.

In this embodiment, the second TF
Since the gate electrode of T2 is connected to the plurality of scanning signal lines, by connecting a capacitor 61 between the gate electrode of the second TFT 2 and the scanning signal line as shown in FIG. When the second TFT 2 is turned on, it is possible to prevent a TFT on an unnecessary line from being turned on and malfunctioning.

[0036]

EXAMPLE A liquid crystal device having the structure shown in FIGS.
As the a-Si layers 32 and 44, hydrogen-diluted monosilane (SiH ₄ ) is glow discharge decomposition (plasma CVD).
The gate insulating film 43 was formed by glow discharge decomposition (plasma CVD) of silicon nitride (SiN _x ). Also, an n ⁺ a-Si layer 45 for ohmic contact
Was formed by doping with phosphorus. In addition, about 9p
The storage capacitor electrode 58 of f was formed. In the present embodiment, the antiferroelectric liquid crystal "CS4000" manufactured by Chisso Co., Ltd. is used as the liquid crystal, the cell thickness is 1.5 μm, the helical pitch is suppressed, and the ferroelectric alignment state and the antiferroelectric alignment state are stably realized. did. As the alignment films 52 and 56, “LP-6” manufactured by Toray Industries, Inc.
The film was rubbed so that the rubbing direction was antiparallel between the upper and lower substrates as shown in FIG.

The above-mentioned liquid crystal element is provided with a gate pulse width of 500
μs, the voltage of the scan selection signal (gate on pulse) is 16
When gradation driving was performed with V and the information signal voltage set to ± 2 to 8 V, stable display was realized.

[0038]

As described above, according to the present invention,
In the active matrix type liquid crystal element, the load on the switching element can be reduced, and the driving capability required for the switching element can be greatly reduced to 1/4. The stability of the domain wall and the controllability of the charge amount are improved, and high-speed active matrix driving can be performed without significantly sacrificing the aperture ratio. Furthermore, by using a liquid crystal having a large spontaneous polarization and capable of displaying halftones,
Stable gradation display can be performed.

[Brief description of the drawings]

FIG. 1 is a diagram showing an equivalent circuit of one pixel of one embodiment of a liquid crystal element of the present invention.

FIG. 2 is a diagram showing an equivalent circuit of one embodiment of the liquid crystal element of the present invention.

FIG. 3 is a plan view schematically showing an electrode configuration of one pixel of a liquid crystal element having the equivalent circuit of FIG.

4 is a schematic cross-sectional view of one pixel of the liquid crystal element of FIG.

FIG. 5 is a diagram for explaining the operation of the present invention.

FIG. 6 is a diagram showing an equivalent circuit of another embodiment of the liquid crystal element of the present invention.

FIG. 7 is a diagram showing a voltage-transmittance characteristic of a liquid crystal that can be used in the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 1st TFT 2 2nd TFT 3 1st capacitance 4 2nd capacitance 5, 5a, 5b Scan signal line 6 Information signal line 7 Wiring 31 Gate electrode 32 a-Si layer 33 Source electrode 34 Drain electrode 41 1 substrate 42 gate electrode 43 gate insulating film 44 a-Si layer 45 n ⁺ a-Si layer 46 source electrode 47 drain electrode 48 insulating film 49 passivation film 50 first pixel electrode 51 second pixel electrode 52 alignment film 53 Second substrate 54 Counter electrode 55 Insulating film 56 Alignment film 57 Liquid crystal 58 Storage capacitance electrode 61 Capacitance

Continuation of the front page (72) Inventor Takashi Enomoto 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Yutaka Inaba 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. F-term (reference) 2H092 JA26 JA29 JA33 JA38 JA42 JA47 JB02 JB13 JB14 JB23 JB32 JB38 JB42 JB54 JB57 JB63 JB69 KA05 KA07 KA12 KA24 MA05 MA08 MA13 MA17 MA27 MA31 MA34 MA35 MA37 MA41 NA07 NA12 NA23 NA23 NA23 NA23 NA23 NA33 NA42 NC09 NC16 NC34 NC40 NC67 ND09 ND22 ND37 ND43 NE03 NF16 NH05

Claims (7)

[Claims] A first substrate provided with a plurality of scanning signal lines and information signal lines orthogonal to each other and an intersection of the signal lines as one pixel, and a pixel electrode and a first switching element provided for each pixel; A liquid crystal having spontaneous polarization is sandwiched between a second substrate provided with an electrically independent counter electrode for each pixel. In each pixel, the pixel electrode is electrically independent and the second A liquid crystal device comprising a plurality of regions interconnected by a switching element, wherein a plurality of capacitances connected in series are formed as a load of the first switching element by the pixel electrode and the counter electrode. . 2. A signal line for controlling on / off of the second switching element is connected to a signal line for controlling on / off of a first switching element of a pixel on a previous line. Liquid crystal element. 3. A signal line for controlling on / off of said second switching element is connected to a signal line for controlling on / off of first switching elements of pixels on a plurality of previous lines. 2. The liquid crystal element according to 2. 4. A signal line for controlling on / off of the second switching element is connected via a capacitor to a signal line for controlling on / off of the first switching element of a pixel on the previous line. The liquid crystal device according to claim 2. 5. The liquid crystal device according to claim 1, wherein said first and second switching elements are thin film transistors. 6. The liquid crystal device according to claim 1, wherein the liquid crystal is an antiferroelectric liquid crystal. 7. The liquid crystal displays a first light transmittance state when an applied voltage value is 0, and a second light transmittance state when an absolute value is equal to or higher than a predetermined saturation voltage value of each polarity. It has a voltage-transmittance characteristic in which a light transmittance state is exhibited and the light transmittance continuously changes between the first light transmittance state and the second light transmittance state according to an applied voltage value. The liquid crystal device according to claim 1.