JPH03243915A - Liquid crystal light valve - Google Patents

Abstract

PURPOSE:To prevent the decay of a luminance level by providing an active element for impressing a voltage by each picture element onto a ferroelectric liquid crystal element and driving the element and a means for impressing the voltage to the ferroelectric liquid crystal element through this element in such a way that this means are alternately impressed with writing signals and non-display signals at specified cycles and specified periods. CONSTITUTION:A tantalum oxide film 12 is formed on a glass substrate 10 for a liquid crystal having a transparent conductive film 11 and further an oriented film 13 is formed. Thin-film transistors consisting of gate electrodes 17, gate insulating films 18, source electrodes 20, drain electrodes 22, and a-Si semiconductor layers 19 and electrodes 16 for display connected thereto are disposed on another substrate 22. The tantalum oxide film 15 is disposed thereon in order to provide the channel protection of the TR parts and further, an oriented film 14 for the liquid crystal is formed thereon. Writing signals and non-display signals are alternately impressed at specified cycles and specified periods by the voltage impressing means. The transmittance of a specified quantity for the given signals is obtd. over a long period of time without the decay of the luminance in this way. Since the response speed is high, the follow-up characteristic is good and the images of good sharpness are obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal light valve device that is capable of high-speed response and infinite gradation, and particularly relates to one that can be used in liquid crystal televisions and analog liquid crystal shutter arrays.

[Prior Art and Problems to be Solved by the Invention] Conventionally, in the technology related to liquid crystal televisions, thick film transistors (hereinafter abbreviated as TPT) and twisted nematics (hereinafter abbreviated as TPT) have been used.
The mainstream is the TFT/TN method, which is a combination of the two methods (abbreviated as "N"), and there are a large number of patent applications related to this method. The reason for this is: - The driving conditions of this method have good matching with TV signals, and the driving conditions of this method are 30Hz or 60Hz over the entire temperature range.
Transmittance characteristics of liquid crystal cell against voltage (■-T
One advantage of this is that because the characteristics (characteristics) are relatively smooth, it is easy to display halftones, which is essential for televisions.

On the other hand, the following relationship exists regarding the response speed of the TN system, particularly the rise speed tON.

Here, △ε, dielectric anisotropy, VOP applied voltage η,
Viscosity coefficient, Vth, two threshold voltages. Therefore, the rising speed is the applied voltage V. It can be easily inferred that it changes greatly depending on P, and becomes slower at low voltage. However, the TPT/N method uses a low voltage region to obtain halftones, so it has the disadvantage that the response speed is inevitably slow. For example, Table 1 shows the case of ZLI-195715 (E, manufactured by Merck & Co., Ltd.), which is a low viscosity liquid crystal composition.

Table 1. Example of TN system response speed In the case of television in particular, it is originally required that the desired image be displayed within one frame of 33m5 or 17m5.
As shown in Table 1, it takes several frames to 10 frames, especially at voltages to obtain halftones. Therefore, this has caused problems in terms of visual perception, such as a slowness in image switching to the naked eye or poor tracking of color relimiting. Regarding the TN system, lowering the viscosity coefficient of the liquid crystal material and reducing the cell gap are effective in improving the response speed. However, in all cases, the level shown in Table 1 seems to be the lower limit of the response speed within various constraints, so T F T/T N
It is thought that the possibility that response speed can be improved by this method is small.

On the other hand, regarding liquid crystal shutter arrays, high-speed response is similarly required due to the need to increase the number of sheets to be processed. For this reason, a dual-frequency drive system with a response speed of several milliseconds (ms) or less has already been put into practical use, and a bistable ferroelectric liquid crystal
Several prototypes of PT-driven models have also been announced. However, because the former switches between high-frequency drive and low-frequency drive, and the latter uses only two stable states of the FLC to turn on and off, both have the disadvantage that they cannot produce intermediate tones. . Therefore, although analog liquid crystal shutters have been desired in recent years, it has been difficult in principle to deal with the situation with these two methods.

On the other hand, as an attempt to obtain halftones by combining TPT/FLC, there is a domain modulation method proposed by Phlips that changes the domain inversion area by controlling the amount of charge injected into the pixel electrode. (Unexamined Japanese Patent Publication No. 63-249897
). However, since this method uses a bistable FLC, a black reset is always required, and especially when driving a TV, the signal processing becomes complicated, and when the pixels become very small, they change to white. This method has the disadvantage that the usable gradation level is determined by the minimum domain size.

Furthermore, as another attempt using the TFT/FLC method, a method has been proposed in which an FLC with a helical pitch is used to switch between a scattering state in which the spiral is wound and a transparent state by applying a voltage (P.

174, Japan Display '89 (19
89)) However, this method also has the drawback that hysteresis occurs in the V-T characteristic because the voltages used when winding and unwinding the spiral are different.

On the other hand, a surface-stabilized FLC device (abbreviated as SSFLC device) proposed by Clark and Ragawal et al.
6-107216 or USP 4.367.924)
is a bistable FLC element with two stable orientation states. Since this bistable FLC cell is reversed to the opposite state by voltage application, it has been impossible to display intermediate levels except in the domain gradation method as in the example of Philips. However, the inventor
For example, a monostable monodomain FLC cell can be obtained by asymmetrically aligning the upper and lower substrates, and if a DC voltage is applied to this cell so that the spontaneous polarization dipole is reversed, the voltage will cause the molecular axis to become fixed. I discovered a phenomenon where the position rotates. According to this method, no inversion of domains occurs during the rotation of the molecular axis, making it possible to obtain perfect halftones. However, monostables return to their original state as soon as the write pulse ends, so it is impossible to produce an image on an actual panel, so monostables such as thin film transistors (TPTs) By combining it with an active switching element, an appropriate C electric field can be created for a certain period of time, and a desired image can be obtained. In other words, in this technology, when a predetermined voltage is applied to a ferroelectric liquid crystal element that has only one stable orientation state (hereinafter referred to as a monostable FLC element), the molecular axes of FLC molecules change as shown in Figure 4. It is based on a new principle that changes depending on the voltage as shown in Figure 2, and returns to the original stable state by its own alignment force when the voltage is removed.

This technique will be explained in further detail.

FIG. 1 is a sectional view of a liquid crystal cell of a liquid crystal light valve device according to this technique. In this liquid crystal cell, a tantalum pentoxide (Ta205) film 12 with a thickness of 600 mm is formed by RF sputtering on a nightwear glass substrate 10 having a transparent conductor 11i11 to prevent short circuit between the upper and lower electrodes. , and further polyimide (for example, 5E-1 manufactured by Nissan Chemical Co., Ltd.
00 (trade name)) to a thickness of approximately 50 mm using a spinner, and baking, the alignment film 13 is formed. After firing, the alignment film 13 is subjected to a rubbing treatment according to the conventional method.

On the other substrate 22, a gate electrode 17, a gate insulating film 18, a source electrode 20, a drain electrode 21, and an a-Si
A thin film transistor made of a semiconductor layer 19 and a display electrode 8i16 connected thereto are arranged. Then, on top of this, tantalum pentoxide 1IN15 was placed for the purpose of protecting the channel of the transistor section, and then polyimide (Hitachi Chemical LQ-1802) was applied using a spinner to a thickness of about 50 mm. The liquid crystal alignment film 14 is formed by firing. After firing, the liquid crystal orientation fJ@zs was subjected to a rubbing treatment using a conventional method. The cell gap is maintained by disposing spacers 23 having a particle size of about 1.7 μm.

FIG. 2 is a plan view of the liquid crystal light valve device of this embodiment using this liquid crystal cell (display panel) 25. Here, S1 to S0 are source lines from the video signal sample and hold circuit 26, and Gl to G, l are gate lines from the vertical scanning circuit 27.

In the liquid crystal cell 25, a ferroelectric cryo-crystal ZLI-4139 manufactured by Merck is used in an isotropic (l5otro) manner.
At a grade of 7 showing pjc), place the sample in vacuum 7 and return to the chamber for about 1 hour.

The physical properties of this liquid crystal are as follows (from the catalog)
.

Ps (20℃)=13.8 (nC7cm2)
It has been confirmed that the alignment of the liquid crystal arranged in this manner is good, and that the characteristics in a test cell without TPT are completely monostable. That is, as shown in FIG. 3, when a single pulse voltage 31 is applied, the optical response 32 is sufficient and becomes bright, but when the pulse ends, it returns to its original state.

FIG. 4 shows an example of a drive waveform in this device. In this example, the gate pulse ■. While on, the information signal VOp charges the cell through the TPT. This voltage decays due to the resistance of the liquid crystal layer, and the liquid crystal molecular axes move accordingly, allowing light to pass through and generate an information signal V. When , becomes O volts, the light is shut off again.

Figure 5 shows the FL in a stable orientation state with no voltage applied.
It is an explanatory view showing the relationship between C and a polarizer. If the polarization axes of the polarizer P and the analyzer A are orthogonal to each other, and the molecular direction in a stable state matches the direction of the polarizer P, a black state is obtained when there is no voltage or a negative voltage. Next, when a positive voltage is applied,
In response to the voltage, the product molecules move smoothly to the arbitrary position indicated by the broken line, causing birefringence due to polarization and allowing light to pass through.

It can be seen that in the range exhibiting this intermediate transmittance, no domain inversion occurs and a complete intermediate tone is obtained. Furthermore, when the applied voltage is reduced to zero, the original orientation state is monostable and returns to the stable black state in a few milliseconds or less. Further, the absolute transmittance T in the birefringent cell is given by the following equation.

T=sin22θ・5in2(””””)where, θ:
Opening angle (Figure 5) ``T''r S 1 n 22 θ In this example, the opening angle θ at the point where the maximum transmittance was obtained was 36°, so
It can be seen from the equation below that the absolute transmittance T' in the seed crystal portion is about 90%.

T'=sin2 (2×36°)=0.905 Furthermore, in the black state, the opening angle does not expand further to the negative side even when a negative voltage is applied. Such an orientation state is called uniform orientation.

[Problem to be solved by the invention] However, a big problem arose here.

The problem is that when driving with the waveform shown in FIG. 4, the luminance level that is initially reached will greatly decay. Figure 6 shows this situation.

If the same driving waveform as shown in Fig. 4 is applied from a certain moment to the liquid crystal cell part as shown in Fig. 4 (a), in this case, if only the positive voltage VLC is applied to the liquid crystal cell part, the luminance decay shown in Fig. 4 (, b) will occur. I understand.

The reason for this is that the passivation film on the transparent electrode shown in FIG. 1 cuts the DC voltage, and the liquid crystal layer voltage decays due to the electrical resistance of the liquid crystal layer.

The latter is due to the action of ions in the liquid crystal, and from the perspective of an equidiscretion path, it is possible to slow down the decay speed described above by increasing the electrical resistance by increasing the purity of the liquid crystal layer.

However, the volume value of the liquid crystal used in this example was measured to be P Lc 1 xl Q l 1 (0 cm), which is a sufficiently high electrochemical purity, and no further dramatic increase in purity can be expected. It was the situation.

Moreover, the passivation 11 on the display electrode shown in FIG.
Experimentally, it is possible to slow down the decay speed by eliminating 12 or 15 (from the perspective of uniformity), but if we consider an LCD TV, the self-portrait may be displayed for several hours or more. So, I thought that the problem in this case was not something that could be handled on the liquid crystal cell side.

Therefore, an object of the present invention is to solve the above-mentioned problems by taking measures regarding driving.

[Means for Solving the Problems] In order to achieve the above object, the liquid crystal light valve device of the present invention includes a pair of opposing substrates, and is arranged between the substrates and has only one stable alignment state when no voltage is applied. a ferroelectric liquid crystal element oriented so that the ferroelectric liquid crystal element is oriented to A means for applying a voltage to the liquid crystal element is provided, and the voltage applying means alternately applies a write signal and a non-display signal at a constant cycle and for a constant period.

The non-display signal is, for example, a ground potential or a potential with a different polarity from the write signal.

For example, if the relationship between the ferroelectric liquid crystal and the polarizer is such that the pixel becomes black when no voltage is applied, the pixel becomes white when a write signal is applied, and becomes black when a non-display signal is applied. Become.

Furthermore, the light transmittance is changed by changing the voltage and period of the write signal.

[Effect] FIG. 7 shows an example of a preferred drive waveform according to the present invention.

In other words, the TPT gate voltage V at a constant period. is turned on, and at the same time the blue light signal voltage V is turned on. P or ground voltage GND (hidden signal) and +■. A voltage between P (write signal) is applied alternately depending on the amount of information. In this case, since the liquid crystal is a monostable FLC, when it reaches the ground potential GND, it returns to its original stable state, that is, becomes black in relation to the polarizing plate, resulting in black and white alternate driving. Because black and white are alternated, the time-averaged transmittance of the panel is reduced to 1/2, or the brightness level decay as described above does not occur.

[Example] Example 1 An example will be described in which the monostable FLC panel with TFT shown in FIGS. 1 and 2 is driven alternately in black and white with the drive waveform shown in FIG. 7.

The bit number of this monostable FLC panel is 240 scanning lines,
There are 480 information lines, and the panel is driven using normal NTSC television signals at the timings shown in Table 2.

Table 2 As a result, each bit becomes +VOP-GND-+Vop・
. . . The gate signal V similar to that shown in FIG. 7 is alternately applied. (
Signal gate pulse time is 63μs, +Vop (max)
5V) is input, and each pixel is driven alternately between black and white.

The VT curve at this time is shown in FIG. 8, and this characteristic is close to that of TFT/TH. In this case, since black and white were alternated, the time-averaged transmittance of the panel was reduced to 1/2, but no luminance level decay was observed as described above.

In addition, the brightness rise/fall time of each pixel is less than 2 mS at any voltage, which is sufficiently small compared to the time of interlaced drive, so scrolling screens and movement are less The outlines of fast-moving images or images with constantly changing information have become clearer.

Embodiment 2 Furthermore, an example of a preferable drive waveform based on the present invention is shown in FIG. The difference from Example 1 is V shown in FIG. , is also swinging to the negative side, which is normal T
This is exactly the same as the drive waveform of the F T/T N method. Therefore, two power systems are required, one positive and one negative, but T
This is an effective method in terms of ensuring compatibility with the FT/TN system. In this example, as in the previous example, no luminance decay is observed.

FIG. 10 shows an example in which the liquid crystal light valve device of the present invention is applied to a liquid crystal shutter array.

The basic cell configuration of a portion of the shutter array is the same as that shown in FIG. 1, and a transparent electrode 91 is arranged at a position corresponding to the shutter window. Gate lines G are placed on both sides of this transparent electrode.
There are a pair of TPT transistors 92 and 93 controlled by gate lines A and GB, and furthermore, at the information input terminal, there is a group of 256 TFTs 94 controlled by gate lines 01 to G16, respectively. are independently connected to the source lines S, -S2, 6, respectively.

Below, Figure 10 and Figure 11 showing the gate line timing are shown.
An example of a liquid crystal shutter array module having 4096 shutter window openings will be explained with reference to the drawings.

Among the serially transmitted information signals, windows 1 to 25
By turning on G1 for a time corresponding to 6, the window electrode 9
The signals of the windows 1 to 256 by 1 are charged to the capacitor, and then 62 is similarly turned on and the signals of the windows 257 to 512 are charged.
After charging the capacitor 95 with 4096 pieces of information, the gate line GA is turned on,
The information of the 1st to 4096th windows is collectively charged to the shutter window electrode 91. As a result, each shutter window (electrode) 91 is turned on while having a gradation depending on arbitrary information. Then, by turning on the gate line GB at a certain timing, the shutter window potential is grounded, and the window 91
is turned off. On the other hand, after the gate line GA is turned off, charging of each capacitor of the information signal of the next line starts. The on-timing of this gate line G8 is preferably intermediate to the on-timing of the gate line GA, or may be arbitrarily determined according to the level at which the decay of the luminance signal is allowed as described in the above-mentioned problem section.

The switching behavior of the shutter in the pseudo test cell is shown in FIGS. 12(a) and 12(b). However, if the gate lines GA and G are both +10V with a pulse width of 30 μs and the initial voltage VLC of the liquid crystal layer is as shown in Fig. 2 (a),
．． 2v, and 3.9v in the case of (b) of the same figure.

In this way, by applying any voltage that shows an intermediate tone, 2
A response speed of 00 μs or less was obtained, and since the luminance decay, which was pointed out as a problem, was not observed by using black and white alternating drive, the luminance level with respect to the information signal was significantly stabilized. In other words, a liquid crystal shutter array with 4096 shutter windows could be stably driven, including halftone output, by connecting 274 wires.

[Effects of the Invention] As explained above, according to the present invention, the following effects are achieved.

(1) When the liquid crystal light valve of the present invention is used as a liquid crystal television, a certain amount of transmittance can be obtained for a given signal over a long period of time without any luminance decay. Furthermore, since the response speed is fast, the tracking ability is good even for images with particularly rapid changes, and sharp images can be obtained.

(2) Similarly, when used as a liquid crystal shutter, a certain amount of transmittance for information signals can be obtained. Furthermore, since the response speed is fast, the process speed' becomes slow, and therefore, when used as a printer head, the number of sheets processed increases.

(3) Furthermore, since the transmittance can be changed continuously in principle,
Can be used as an analog shutter.

[Brief explanation of drawings]

Figures 1 and 2 are cross-sectional views showing an example of a TFT/monostable FLC cell and a liquid crystal light valve device using the same, Figure 3 is a graph showing the monostability of the cell, and Figure 4 is a conventional example. Timing chart showing drive waveforms, Figure 5 is an explanatory diagram showing the principle of TFT/stabilized FLC, Figures 6 (a) and (b) are graphs showing luminance decay, and Figure 7 is a preferred drive of the present invention. FIG. 8 is a graph showing V-T characteristics. FIG. 9 is a timing chart showing an example of a preferred driving waveform according to the present invention. FIG. 10 is a timing chart showing an example of a driving waveform according to the present invention. FIG. 11 is a circuit diagram showing an example, and FIG. 11 is a graph showing the tie and cross points of the drive waveform, and FIGS. 12(a) and (b) are graphs showing the optical response by the pseudo shutter cell of FIG. 10. 10.22 11,16 12 15 13 l 4 17~21 Sl ""' S 258 Gl"Gl6 1 2 3 5 Glass substrate/transparent electrode: Ta205 film: Alignment film: TFT layout diagram: Information signal line niblocked Each gate line of TPT: Shutter window electrode section (1 to 4096 pieces) TPT accessed by gate lines GA and GB: Storage capacitor for information signal

Claims (5)

[Claims] (1) A pair of opposing substrates, a ferroelectric liquid crystal element placed between the substrates and oriented so that only one stable state of alignment exists when no voltage is applied; It includes an active element for driving the dielectric liquid crystal element by applying a voltage to each pixel, and a means for applying a voltage to the ferroelectric liquid crystal element through the active element, and this voltage applying means is configured to use a write signal and a non-write signal. An active matrix type liquid crystal light valve device characterized in that a display signal is applied alternately at a fixed period and for a fixed period of time. (2) The liquid crystal light valve device according to claim 1, wherein the non-display signal is a ground potential. (3) The liquid crystal light valve device according to claim 1, wherein the non-display signal has a polarity different from that of the write signal. (4) If the relationship between the ferroelectric liquid crystal and the polarizer is such that the pixel becomes black when no voltage is applied, the pixel becomes white when a write signal is applied and becomes black when a non-display signal is applied. The liquid crystal light valve device according to claim 1, characterized in that: (5) The liquid crystal light valve device according to claim 1, wherein the light transmittance is changed by changing the voltage and period of the write signal.