JP2000310795A - Liquid crystal element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To supply an enough charge for driving liquid crystals even in a short selection time and improve display performance by forming liquid crystals to be sandwiched in between a nonlinear two-terminal element and the second substrate having pixel electrodes in each pixel and connecting an auxiliary capacitance in parallel with the liquid crystals to the pixel. SOLUTION: A nonlinear two-terminal element 5 is connected with one end to a scanning signal line 2 and with the other end to a pixel electrode respectively and flows a current to the pixel electrode by changing a resistance corresponding to the difference between the voltage of the corresponding scanning signal line 1 and the voltage of the information electrode 2 facing the pixel electrode. Consequently a voltage corresponding to the voltage difference is applied to liquid crystals 3 sandwiched between pixel electrodes and the information electrode 2. An auxiliary capacitance 4 is arranged parallel to the liquid crystals 3 and is connected in series to one terminal of the nonlinear two-terminal element 5 and the remaining terminal is connected to an auxiliary capacitance ground line 6 in common to each pixel. With this constitution, as a necessary charge amount for liquid crystal driving can be supplied to the liquid crystals 3 in a short time, display performance can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal device used for a display of a personal computer and the like.
In particular, the present invention relates to an active matrix type liquid crystal element using a nonlinear two-terminal element.

[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.

Most of active matrix type liquid crystal elements which are put into practical use use a TN (twisted nematic) mode using a nematic liquid crystal.

On the other hand, a liquid crystal device having spontaneous polarization and having bistability is disclosed in both Clark and Lagerwall in Japanese Patent Application Laid-Open No. 56-10 / 1981.
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.

In recent years, liquid crystals having spontaneous polarization include antiferroelectric liquid crystals having two ferroelectric states and one antiferroelectric state (JJAP, 28, L126).
5, 1989), an antiferroelectric liquid crystal having no threshold value (Asia Display '95 Digest,
P. 61, 1995) and DHF (Deformed
Helix Ferroelectric liquid crystal (Li
liquid Crystals, vol. 5, No. 4,
P. 1171 and 1989).

[0006]

The liquid crystal having the above-mentioned spontaneous polarization, when switching from the first optically stable state to the second optically stable state, generates a current (hereinafter referred to as "the current") caused by the inversion of the spontaneous polarization. It is a well-known fact that the Ps current flows). The Ps current flows in a direction that obstructs an external electric field required for switching.

Accordingly, the driving of the liquid crystal having spontaneous polarization is as follows.
The charge of the liquid crystal capacitor and the consumption of the charge by the Ps current occur at the same time, and the required charge amount becomes enormous as compared with the conventional driving of the nematic liquid crystal.

The present invention has been made to solve the above problems, and an object of the present invention is to provide an active matrix type liquid crystal element which supplies a sufficient charge for driving the liquid crystal even in a short selection time, and provides a display characteristic. In particular, it is an object of the present invention to provide a configuration capable of high-speed driving even with a liquid crystal having spontaneous polarization.

[0009]

According to the present invention, there is provided a first substrate having a stripe-shaped information electrode group, a scanning signal line orthogonal to the information electrode group, and an intersection of the information electrode group and the scanning signal line. As a pixel, a liquid crystal is sandwiched between a switching element formed of a non-linear two-terminal element for each pixel and a second substrate having a pixel electrode, and an auxiliary capacitor is connected to the pixel in parallel with the liquid crystal. A liquid crystal device characterized by the following.

The liquid crystal device of the present invention has a remarkable effect particularly when driving a liquid crystal having spontaneous polarization and having a large amount of charge required for driving the liquid crystal, for example, a ferroelectric liquid crystal or an antiferroelectric liquid crystal. Although it is possible to obtain, even in the conventional nematic liquid crystal, it is possible to accumulate the electric charge required for driving the liquid crystal in a short time, so that it is possible to drive at a higher speed or to reduce the driving voltage applied to the entire liquid crystal element. Therefore, it is preferably applied to the present invention.

The operation of the present invention will be described with reference to FIG. FIG. 4 is an equivalent circuit of one pixel of the liquid crystal element of the present invention, in which 3 is a liquid crystal, 4 is an auxiliary capacitor, 5 is a non-linear two-terminal element,
41 is a signal power supply. FIG. 5 shows a conventional liquid crystal element,
That is, an equivalent circuit of one pixel of a liquid crystal element having no storage capacitor 4 is shown.

Here, the capacitance of the nonlinear two-terminal element 5 is represented by C
_v , the capacitance of the liquid crystal 3 is C _lc , the capacitance of the auxiliary capacitance 4 is C _s , and the voltage of the signal power supply 41 is V, the voltage V ₄ applied to the nonlinear two-terminal element 5 of the liquid crystal element of the present invention and the conventional liquid crystal voltage V ₅ according to the non-linear two-terminal element 5 of the element is _{V 4 = V (C s +} C lc) / (C v + C lc + C s) V 5 = V · C lc / (C v + C lc). Therefore, V ₄ > V ₅ , and even when the same voltage is applied, a higher voltage is applied to the non-linear two-terminal element of the liquid crystal element of the present invention. Since the amount increases, the required amount of charge can be supplied in a short time, and the driving frequency can be increased. Further, when the driving frequency is the same, the driving voltage applied to the entire liquid crystal element can be reduced to save power.

On the other hand, the voltage V _{4 ′} applied to the liquid crystal of the liquid crystal element of the present invention and the voltage V _{5 ′} applied to the liquid crystal of the conventional liquid crystal element satisfy V _{4 ′} <V _{5 ′} . The smaller the voltage applied to the liquid crystal is. When considering operation under a voltage such as an information signal at the time of non-selection in which the resistance of the nonlinear two-terminal element does not decrease, the smaller the voltage applied to the liquid crystal, the more the fluctuation of the liquid crystal is suppressed, and the higher the contrast. Therefore, it is preferable.

Although the voltage applied to the liquid crystal is smaller in the liquid crystal element of the present invention, the electric charge accumulated in the auxiliary capacitor 4 is also used for driving the liquid crystal. In the liquid crystal element of the present invention, the amount of charge supplied to the liquid crystal is larger.

The charge amount Q _lc required for driving the liquid crystal is as follows: P _{s is} the spontaneous polarization of the liquid crystal; S is the electrode area; d is the thickness of the liquid crystal layer (cell thickness); V _lc is the voltage applied to the liquid crystal layer; Assuming that the relative permittivity of this is ε _r and the permittivity of vacuum is ε ₀ , then Q _lc = V _lc ε ₀ ε _r （(S / d) + 2S · P _s . In other words, it is possible to drive a liquid crystal having a larger spontaneous polarization by providing an auxiliary capacitor and increasing the amount of charge stored.

Next, consider a transient response.

FIG. 9A shows the power supply 41 of the circuits shown in FIGS.
5 (b) shows the change over time of the voltage applied to point D in FIG. At the same time as the voltage is applied, the voltage applied to the liquid crystal 3 gradually increases with a time constant determined by the nonlinear two-terminal element 5 and the circuit, and rises to a point V ₀ where the resistance of the nonlinear two-terminal element 5 becomes minimum. Signal pulse width ΔT =
When T _1, when the amount of charge supplied to the liquid crystal 3 and Q _a, conditions required for driving the liquid crystal is a _{Q a = C lc · V 0} > Q lc.

Further, when ΔT = T ₁ , the charge amount Q _b supplied to the liquid crystal 3 and the auxiliary capacitor 4 through the point C in FIG.
Is a _{Q b = (C lc + C} s) V 0> Q a.

Here, the time T _{2 at which} the charging voltage V _{0 ′ at the} point C in FIG. 4 satisfies V _{0 ′} = [C _lc / (C _lc + C _s )] · V ₀ <V ₀ ((FIG. At the time of c)), the charge amount Q _{a ′} injected into the liquid crystal 3 and the storage capacitor 4 in FIG. 4 is as follows: Q _{a ′} = (C _lc + C _s ) · V _{0 ′} = C _lc ＶV ₀ = Q _a > Q _lc , which indicates that the liquid crystal can be driven with a shorter signal pulse by loading the auxiliary capacitance 4.

[0020]

FIG. 1 is an equivalent circuit of an embodiment of the liquid crystal device of the present invention. In the figure, 1 is a scanning signal line, 2 is an information electrode, 3 is a liquid crystal, 4 is an auxiliary capacitor, 5 is a non-linear two-terminal element, and 6 is an earth line of the auxiliary capacitor.

In the liquid crystal device of the present invention, a unit pixel is formed at the intersection of a scanning signal line 1 and an information electrode 2 which are orthogonal to each other, and each pixel has a non-linear two-terminal device 5, a pixel electrode, and an auxiliary capacitor 6.
Is arranged. One end of the nonlinear two-terminal element 5 is connected to the scanning signal line 2.
The other end is connected to the pixel electrode, and the resistance value changes in accordance with the difference between the potential of the corresponding scanning signal line 1 and the potential of the information electrode 2 to which the pixel electrode of the pixel is opposed, and the current flows. The voltage is applied to the pixel electrode, and as a result, a voltage having a value corresponding to the potential difference is applied to the liquid crystal 3 sandwiched between the pixel electrode and the information electrode 2. The storage capacitor 6 is arranged in parallel with the liquid crystal 3,
The non-linear two-terminal element 5 is connected in series, and one terminal is connected to an auxiliary capacitance ground line 6 common to each pixel.

FIGS. 2A and 2B are views showing a first embodiment of a liquid crystal device having the above-described equivalent circuit, wherein FIG. 2A is a schematic plan view showing an electrode configuration on the second substrate side, and FIG. It is A 'sectional drawing. In the figure, 11a and 11b are glass substrates, 12
Denotes a pixel electrode, 13 denotes a non-linear resistance film, 14 denotes a metal wiring serving as a scanning signal line, 15 denotes a dielectric layer, 16 denotes an auxiliary capacitance electrode,
17 is a stripe-shaped electrode as an information electrode, 18a and 1
8b is an alignment film, and 19 is a liquid crystal.

The first substrate side is on a glass substrate 11b.
The striped electrode 17 is formed on the metal wiring 14 on the second substrate side.
It is arranged orthogonal to. On the second substrate side, a non-linear two-terminal element 5 is formed on a glass substrate 11a by a pixel electrode 12 for each pixel, a non-linear resistance film 13 and a metal wiring 14 partially formed on the pixel electrode 12. Is formed. On the other end of the pixel electrode 12, a dielectric layer 15 and an auxiliary capacitance electrode 16 are sequentially laminated to form an auxiliary capacitance in parallel with the liquid crystal 19. The auxiliary capacitance electrode 16 is externally connected to a ground potential. The auxiliary capacitance electrode 16 may be formed simultaneously with the metal wiring 14, or may be formed of a different material.

FIG. 3 shows a second embodiment. FIG.
(A) is a schematic plan view showing an electrode configuration on the second substrate side,
(B) is the BB 'sectional drawing. The reference numerals in FIG.
Is the same as

In this embodiment, the storage capacitor is configured such that the dielectric layer 15 and the storage capacitor electrode 16 are arranged between the pixel electrode 12 and the substrate 11a. Similarly, the configuration of the non-linear two-terminal element is opposite to the configuration shown in FIG.
After the metal wiring 14 is formed thereon, the nonlinear resistance film 13 and the pixel electrode 12 may be formed.

The non-linear resistance film used in the present invention includes SiN, ZnO, Ta ₂ O ₅ , ZrO ₂ , and R _x O _y.
(R = La, Ce, Pr, Nd), Bi ₂ O ₃ , PrC
single-layer film or a laminated film is used for oxides and nitrides such oO _x, as the dielectric layer, SiO _2, SiN, T
a ₂ O ₅ , ZnO, TiO ₂ , ZrO ₂ , Al ₂ O ₃ ,
A single-layer film of an oxide or a nitride such as MgF ₂ or HfO ₂ or a laminated film thereof is used. Note that these materials are not particularly limited to the above materials as long as they have desired nonlinearity and dielectric properties. However, the dielectric layer needs to be selected so as to have higher resistance than the nonlinear resistance film.

The liquid crystal used in the present invention may be a ferroelectric liquid crystal having a spontaneous polarization or an antiferroelectric liquid crystal.
Nematic liquid crystals used in N-mode liquid crystal elements are preferably used.

As for the other members of the liquid crystal element of the present invention, the technique of the general liquid crystal element can be applied to the material, shape, manufacturing method and the like.

[0029]

EXAMPLES Example 1 and Comparative Example 1 As Example 1, a liquid crystal device having the configuration shown in FIG. The steps will be described.

The ITO is formed on the glass substrate 11a by sputtering.
(Indium Tin Oxide) 500Å
By patterning, a pixel electrode 12 of 300 μm square was formed. On top of this, a SiN film is deposited at a thickness of 1000
And a non-linear resistance film 13 was formed by patterning. The film formation conditions at this time are as follows: the substrate temperature is 250 ° C.
N ₂ : SiH ₄ = 500 sccm: 40 sccm, RF
Power is 0.05W / cm ² , pressure is 200mtorr
Met.

[0031] Next, the SiO ₂ in the DC sputtering method 150
The dielectric layer 15 was formed on the edge of the pixel electrode 12 by depositing 0 ° and patterning. The sputtering conditions at this time were as follows: the substrate temperature was 150 ° C., the DC power was 20 W / cm ² , and Ar = 2.
00 sccm and a pressure of 3 mtorr.

Next, Cr is deposited by 2000 mm by sputtering and patterned to form a metal wiring 14 and an auxiliary capacitance electrode 16.
Was formed at the same time. The overlap between the pixel electrode 12 and the auxiliary capacitance electrode 16 was 30 μm × 300 μm. The auxiliary capacitance electrode 16 was later externally connected to ground.

A SnO ₂ -dispersed siloxane film was deposited on the above substrate at 2000 ° by a spinner and baked at 200 ° C. for 2 hours to form an alignment film 18a, thereby obtaining a second substrate.

On the other hand, as a first substrate, a glass substrate 11
An electrode 17 was formed by depositing an ITO film 500b on top of b and patterning it into a stripe having a width of 300 μm. Further, a polyimide film was formed on the surface to a thickness of 50 ° and rubbed with cotton to obtain an alignment film 18b.

One of the two substrates has a particle size of 2.3 μm.
m, and the striped electrodes 17 of the first substrate and the metal wirings 14 of the second substrate are opposed to each other so as to be orthogonal to each other, and sealed with an epoxy sealant.
As a liquid crystal 19, a ferroelectric liquid crystal composition A shown below was injected.

[0036]

Embedded image

The tilt angle チ ル and the spontaneous polarization Ps are values measured by the following measuring methods.

[Measurement of tilt angle Θ] ± 30 to ± 50 V,
While applying an AC (alternating current) of 1 to 100 Hz between the upper and lower substrates of the liquid crystal element via electrodes, the liquid crystal element disposed therebetween is rotated in parallel with the polarizing plate under orthogonal crossed Nicols, and at the same time, the photomultiplier ( The first extinction position (the position where the transmittance becomes lowest) and the second extinction position are obtained while detecting the optical response with Hamamatsu Photonics. Then, a half of the angle from the first extinction position to the second extinction position at this time is defined as a tilt angle Θ.

[Method of Measuring Spontaneous Polarization]
Miyasato et al. "Direct measurement method of spontaneous polarization of ferroelectric liquid crystal by triangular wave" (Journal of the Japan Society of Applied Physics, 22, 10 (6)
61) 1983, "Direct Method wi
the Triangular Waves forMe
asuring Spontaneous Polar
ization in Ferroelectric
Liquid Crystal ”, as descri
bed by K. Miyasato et al.
(Jap. J. Appl. Phys. 22. No. 1)
0, L661 (1983))).

As Comparative Example 1, a liquid crystal device having the same structure as that of Example 1 was produced except that no auxiliary capacitance was formed (the auxiliary capacitance electrode 16 and the dielectric layer 15 were not formed).

The liquid crystal devices of Example 1 and Comparative Example 1 were driven at a substrate temperature of 30 ° C. with the driving waveforms shown in FIG. 6 to evaluate the devices. In FIG. 6, (a) is a scanning selection signal applied to the scanning signal line 1, (b) and (c) are information signals applied to the information electrodes, (b) is a white display signal, and (c) is a black display signal. It is. First, the drive voltage (| V ₂ |
+ | V ₃ |) was fixed to 30 V, and ΔT was changed to obtain the minimum ΔT at which a good display was obtained. Next, ΔT is set to 10
μs, the driving voltage (| V ₂ | + | V ₃ |) was changed to obtain the minimum driving voltage for obtaining a good display, and the contrast at that time was obtained. Comparative Example 1 was similarly evaluated. The results are shown in the table below.

[0042]

[Table 1]

As is apparent from the above results, by providing the auxiliary capacitance, ΔT can be shortened (the driving frequency can be increased). In addition, it was found that when the same ΔT was used, the driving voltage could be reduced, and effects such as an increase in contrast could be obtained.

Example 2, Comparative Example 2 As Example 2,
A liquid crystal element having the configuration shown in FIG. 2 was manufactured.

First, 500 μm of ITO is deposited on the glass substrate 11a by a sputtering method, and is patterned to 300 μm
The square pixel electrode 12 was formed. On top of that Z
An nO film was deposited at 1000 ° and patterned to form a non-linear resistance film 13. The film formation conditions at this time are as follows: the substrate temperature is 200 ° C., Ar: O ₂ = 1000 sccm: 100 sc
cm, RF power 1.8 W / cm ² , pressure 10 mt
orr.

Next, Ta ₂ O ₅ was added to 200 by DC sputtering.
A dielectric layer 15 was formed on the edge of the pixel electrode by depositing 0 ° and patterning. The sputtering conditions at this time were as follows: the substrate temperature was 150 ° C., the DC power was 10 W / cm ² , and Ar: O ₂ =
190 sccm: 10 sccm, pressure was 3 mtorr.

Next, Mo is deposited to a thickness of 2000 ° by a sputtering method, and patterned to form a metal wiring 14 and an auxiliary capacitance electrode 16.
Was formed at the same time. The overlap between the pixel electrode 12 and the auxiliary capacitance electrode 16 was 10 μm × 300 μm. The auxiliary capacitance electrode 16 was later externally connected to ground.

The following steps were performed in the same manner as in Example 1 to form a liquid crystal element. Further, as Comparative Example 2, a liquid crystal element having the same configuration as that of Example 2 except that the dielectric layer 15 and the auxiliary capacitance electrode 16 were not formed was manufactured.

The same evaluation as in Example 1 was performed on the obtained liquid crystal element. The results are shown in the table below.

[0050]

[Table 2]

As is clear from the above Table 2, ΔT can be shortened (driving frequency can be increased) by providing an auxiliary capacitor. In addition, it was found that when the same ΔT was used, the driving voltage could be reduced, and effects such as an increase in contrast could be obtained.

Example 3 As Example 3, a liquid crystal device having the structure shown in FIG. 7 was manufactured. In FIG. 7, (a) shows the second
FIG. 2B is a schematic plan view showing the electrode configuration of the substrate of FIG.
It is E 'sectional drawing. The configuration of the present embodiment is based on
5 and the metal wiring 14 are partially overlapped to provide a function as a light-shielding film between pixels.

The process up to the formation of the non-linear resistance film 13 on the second substrate side is the same as that of the first embodiment. Next, 1200 mm of Cr is deposited to form a metal wiring 14, and then the pixel electrode 12 and the metal wiring 14 are formed. A dielectric layer 15 is formed by forming an SiO ₂ film having a thickness of 2000 ° so as to overlap the dielectric layer. Further, Al is deposited thereon by 2300 ° by a sputtering method and is patterned to form an auxiliary capacitance electrode 16. The following steps were performed in the same manner as in Example 1 to form a liquid crystal element.

When the obtained liquid crystal element was evaluated in the same manner as in Example 1, the minimum ΔT was 12 μs, the minimum drive voltage was 22 V, and the contrast was 202. Also, by substituting the light shielding film on one side of the wiring with the auxiliary capacitance electrode,
The aperture ratio can be increased, and the aperture ratio is improved from 80% of the first embodiment to 85%.

Example 4 As Example 4, a liquid crystal device having the structure shown in FIG. 8 was manufactured. This element is electrically the same as the liquid crystal element of the first embodiment.

First, Cr was deposited on the glass substrate 11a by 200%.
Depositing and patterning to form an auxiliary capacitance electrode 16,
Then, a dielectric layer 15 was formed by depositing SiO ₂ at 1000 ° by a sputtering method. Further, ITO was deposited at 800 [deg.] Under the same deposition conditions as in Example 1 to replace the pixel electrode 12 with SiN.
Is deposited on the non-linear resistance film 13 by Ni
The metal wirings 14 were sequentially formed by depositing 00 mm. The following steps were performed in the same manner as in Example 1 to form a liquid crystal element.

The obtained liquid crystal element had almost the same driving characteristics as the liquid crystal element of Example 1.

[0058]

As described above, according to the present invention,
Since the amount of charge required for driving the liquid crystal can be supplied to the liquid crystal in a short time, the driving frequency can be increased or driving can be performed with a low voltage. Also, when driven at a low voltage, the influence of the fluctuation of the information signal on the pixel is reduced,
Effects such as an increase in contrast are obtained.

In the present invention, as described above, since the charge supply speed is increased, the active matrix drive can be favorably performed even in a liquid crystal having spontaneous polarization, which requires a large amount of charge for driving the liquid crystal.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram schematically showing a configuration of another embodiment of the liquid crystal element of the present invention having the equivalent circuit of FIG.

FIG. 4 is an equivalent circuit diagram of one pixel for explaining the operation of the liquid crystal element of the present invention.

FIG. 5 is an equivalent circuit diagram of one pixel for explaining the operation of a conventional liquid crystal element.

FIG. 6 is a driving waveform used in the example of the present invention.

FIG. 7 is a diagram schematically illustrating a configuration of a liquid crystal element according to a third embodiment of the present invention.

FIG. 8 is a diagram schematically illustrating a configuration of a liquid crystal element of Example 4 of the present invention.

FIG. 9 is a diagram showing voltage waveforms in the circuits of FIGS.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scan signal line 2 Information electrode 3 Liquid crystal 4 Auxiliary capacitance 5 Non-linear two-terminal element 6 Auxiliary capacitance earth line 11a, 11b Glass substrate 12 Pixel electrode 13 Non-linear resistance film 14 Metal wiring 15 Dielectric layer 16 Auxiliary capacitance electrode 17 Stripe electrode 18a , 18b Alignment film 19 Liquid crystal 41 Signal power supply

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H092 JA01 JB63 JB69 NA05 QA12 5C094 AA06 AA13 AA24 BA04 BA09 BA49 CA19 DA13 DB04 DB10 EA04 EA05 EA10 ED15 FA01 FA02 FB02 FB16 GA10

Claims (5)

[Claims] A first electrode having a stripe-shaped information electrode group;
A substrate, a scanning signal line orthogonal to the information electrode group, and a pixel having an intersection of the information electrode group and the scanning signal line as a pixel. A liquid crystal element comprising a liquid crystal interposed between a substrate and a storage capacitor connected in parallel with the liquid crystal to the pixel. 2. The liquid crystal device according to claim 1, wherein the auxiliary capacitance is formed closer to the liquid crystal than the pixel electrode. 3. The liquid crystal device according to claim 1, wherein the auxiliary capacitance is formed closer to the substrate than the pixel electrode. 4. The liquid crystal device according to claim 1, wherein the electrode of the storage capacitor also functions as a light shielding layer between pixels. 5. The liquid crystal device according to claim 1, wherein the liquid crystal has spontaneous polarization.