JPS6217751B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal-insulator-metal device (hereinafter referred to as
This invention relates to a liquid crystal display device with high multiplex drive performance based on the nonlinear characteristics of an MIM device.

In recent years, there has been a demand for liquid crystal panels that can display a large amount of information, such as dot matrix. In order to meet this demand, efforts are being made to improve the multiplex drive performance of conventional twisted nematic liquid crystal panels (hereinafter referred to as TN liquid crystal panels).

In current multiplex driving, a driving method called a voltage averaging method is mainstream in order to prevent fluctuations in the voltage applied to unselected pixel electrodes. Figure 1 shows an example of waveforms for the 1/3 bias method. FIG. 1a shows the voltage waveform applied to the scanning line, and b shows the voltage waveform applied to the signal line when liquid crystal pixel 1 in the matrix is selected. At this time, a voltage having an effective value of V ₀ /3 is applied to other liquid crystal pixels on the same signal line. Similarly, a voltage waveform that changes according to the selected pixel is applied to the other signal lines, and when the scanning line is selected, if the signal line is also selected at step 2, it is V ₀ , and if no signal line is selected, it is V ₀ /3 , if the scanning line is not selected, regardless of whether the signal line is selected or not.
A voltage of V ₀ /3 is effectively applied to each liquid crystal pixel. Each liquid crystal pixel selects these four states for several scanning lines of multiplex drive during one scanning time. However, since the scanning line is selected only once within the scanning time, each liquid crystal pixel is in the V ₀ state once when it is lit, and is in the V ₀ /3 state for the remaining scanning time, and when it is not lit, it is in the V ₀ state for the entire scanning time. /3 state.

In this way, the effective voltage applied to all non-lit pixels is always constant. Generalizing this, in the case of a duty ratio of 1/N (N is the number of scanning lines in multiplex drive) and a V/a bias method, the effective voltage applied to a lit pixel and a non-lit pixel is calculated by formula (1) and (2), respectively. ).

Maximizing the ratio of V _ON and V _OFF is a=√N+
1, and V _ON /V _OFF at that time is expressed by equation (3).

As can be seen from equation (3), as the number N of scanning lines increases, the V _ON /V _OFF ratio approaches 1. However, the current situation
Due to the voltage-transmittance characteristics of a TN liquid crystal panel, the limit of N that can maintain display quality is in the order of tens of tens.

As a method to remove this limitation, two-frequency drive method, thin film transistor (TET) matrix,
MOS transistor matrices, MIM devices, etc. have been devised. Among these, a liquid crystal display panel (SID) using an MIM device by DRBaraff et al.
International Symposium Digest of Technical
Papers vol. 11, p. 200, April 1980) has the advantage that the manufacturing process is relatively simple and that a conventional multiplex drive can be used. The configuration of the MIM device in this case consists of a Ta thin film, a _{Ta 2} O ₅ anodic oxide film, and a Ni/Cr film. When additional tests were conducted, nonlinear characteristics as shown in FIG. 2 were obtained. The current flowing through this MIM device is given by equation (4).

d, s: Thickness and area of insulating layer of MIM device n: Carrier density e: Electron charge φ: Trap level depth ε ₀ : Vacuum permittivity μ: Electron mobility κ: Boltzmann constant T: Absolute temperature ε ₁ : Optical dielectric constant (=n ² ) ORBaraff et al.'s liquid crystal panel has a configuration in which an MIM device is attached to one side of the substrate, as shown in Figure 3, and an MIM device is attached to the electrical circuit as shown in Figure 4. The device and the liquid crystal layer are connected in series to form an equivalent circuit. When this MIM liquid crystal panel is multiplex driven, a large effective voltage ratio V _ON /V _OFF applied to the liquid crystal layer can be obtained due to the nonlinear characteristics of the MIM device. As a result, it is possible to turn the liquid crystal ON and OFF even when the number of scanning lines N is about 200.

In this way, it is possible to display a large amount of information.
Although it is an MIM LCD panel, it is extremely difficult to concentrate the amount of information in a small area. The main feature of MIM liquid crystal panels is the switching of resistance using the nonlinear characteristics of the MIM device, which cleverly controls charging of the liquid crystal layer through low resistance and discharging due to leakage of the liquid crystal layer. FIG. 5 shows the MIM characteristic a and the waveform b of the voltage applied to the liquid crystal layer. The voltage waveform applied to the MIM liquid crystal panel is the waveform c for simplicity. Note that 1 and 3 are waveforms when ON, and 2 and 4 are waveforms when ON.
This is the waveform when it is OFF. According to this, the voltage applied to the liquid crystal layer is first divided by the capacitance of the liquid crystal layer C _LC and _MIM by the capacitance C MIM, and then divided by the resistance of the MIM device and the resistance R _LC of the liquid crystal layer. The voltage is changed with voltage 6 as the limit value. The time constant τ at this time is given by τ=(C _LC +C _MIM )×R _LC・R _MIM (V _MIM )/R _LC +R _MIM (V _MIM ) (5) V _MIM : voltage applied to the MIM device.

Next, when the voltage applied to the MIM liquid crystal becomes 0, the capacitance-divided voltage is similarly set as the initial value, and a voltage change occurs with voltage 0 as the limit. The time constant at this time is also given by equation (5), but R _LC ≪ R _MIM
Therefore, it can be approximately expressed as τ=(C _LC +C _MIM )R _LC (6).

FIG. 6 is a diagram showing voltage waveforms applied to the liquid crystal layer when the ratios of _CMIM and _CLC are 1:1 and 1:10. As can be seen, increasing the capacitance ratio increases the voltage. Since liquid crystals have an effective value response, it is best to simply consider the area of the diagram.

In this way, the capacitance ratio between C _MIM and C _LC greatly changes the effective value applied to the liquid crystal. ON-OFF at the same time
The effective value ratio also fluctuates, and in an optimal matching state of element constants, the larger the capacitance ratio, the higher the effective value ratio.

However, in the case of a MIM liquid crystal panel with a small pixel size aimed at displaying a large amount of information in a compact manner, it is difficult to realize a small MIM device with a C _MIM due to manufacturing constraints. As an example, Ta ₂ O ₅ in the insulator
Let us consider the case of driving with a power supply voltage of 20V.
The relative dielectric constant of liquid crystal varies from 5 to 30, but for simplicity we set it as 10, and the pixel area is 1 x 10 ^-7 m ² (approximately 0.3 mm square).
If the gap is 8 μm, _CLC is 1.1 pF.
On the other hand, if C _MIM is 1/10th of this, and the thickness of Ta ₂ O ₅ is 500 Å, the area of the MIM device is 25 μm ²
becomes. In order to equalize the characteristics of each pixel, these 5
A μ-square MIM device must be manufactured with high precision, which is proving to be extremely difficult even with current photolithography technology.

The present invention installs an auxiliary capacitor (Cs) connected in series with the MIM device and in parallel with the liquid crystal layer, thereby facilitating the production of the MIM device and producing a high-quality liquid crystal display with uniform pixel characteristics. It gives

Next, the present invention will be explained in detail based on examples.

FIG. 7 is a schematic diagram of one pixel in which _three layers of BaTiO are installed in the liquid crystal layer as an auxiliary capacitor. 2 is BaTiO ₃
Layer 3 is a liquid crystal layer. The MIM device No. 1 is made of Ta and an anodic oxide film of Ta. _BaTiO3
The layer and the liquid crystal layer are 10μm thick, and _{the three} BaTiO layers are
A mixture of _BaTiO3 powder and silicone resin with a width of 0.15
Screen printed and formed in mm. If the particle size of BaTiO ₃ powder can be adjusted to around 10μ, it can also have a gap retention function. This results in
Equivalent capacitance of the liquid crystal layer including the capacitance of _three BaTiO layers C _LC
was increased 5 to 10 times that of the case with only liquid crystal.

Figure 8 shows particles with a particle size of approximately 8 μm as an auxiliary capacity.
This is an example of spraying BaTiO ₃ powder onto a MIM liquid crystal panel, which also serves as a gap retaining material. In this case, the problem is that the amount of powder to be sprayed is large and that it is difficult to spray it uniformly, but in this example, the capacity of pixels where BaTiO 3 powder is hardly seen is the same as the capacity of pixels where BaTiO ₃ powder is hardly seen. A ratio of about 1:3 was obtained, and an increase in capacity was observed due to spraying.

The MIM liquid crystal panel manufactured in this way can be turned on and off sufficiently using the 1/5 bias method, peak voltage of 20V, and N=200. The MIM device has a Ta-Ta ₂ O ₅ -Ta structure, 10 μm square, a pixel area of 0.1 mm ² , and a gap of 10 μm.

As explained above, according to the present invention, it is possible to increase the equivalent capacitance _CLC corresponding to the pixel, which is different from the conventional
Compared to MIM devices with MIM liquid crystal panels, the minimum allowable device shape can be increased. This makes manufacturing easier and ensures that the characteristics of the MIM device are uniform. Furthermore, when applied to MIM liquid crystal panels of the same shape, a large capacitance ratio can be easily achieved, and there is the advantage that matching of the element constants of the liquid crystal layer and the MIM device results in capacitance.

Although the above embodiments utilize the large dielectric constant of BaTiO ₃ , the present invention is not limited to BaTiO ₃ in any way, and can be applied to any material having a dielectric constant larger than that of liquid crystal. It suggests that it is possible.

As described above, in the present invention, a liquid crystal is sealed in a pair of substrates, a plurality of display electrodes are formed in a matrix on one of the substrates, and each display electrode is made of metal.
In a liquid crystal display device in which an insulator-metal nonlinear thin film element is formed, a substance having a dielectric constant larger than that of the liquid crystal is interposed between the display electrode and the opposing substrate. Since it is possible to realize a liquid crystal drive electrode capacitance that is larger than the capacitance of a linear thin film element, it has the effect of greatly improving the effective value ratio between ON and OFF.

[Brief explanation of the drawing]

FIG. 1 shows waveforms of V/3 bias and multiplex drive. a is the voltage waveform applied to the scanning line, b is the voltage waveform applied to the signal line, and c is the voltage waveform applied to the pixel 1. 1...Pixel, 2...When selected (shaded area) Fig. 2 shows an example of the characteristics of the MIM device. FIG. 3 is a schematic diagram of the MIM device. 1... Upper substrate (counter electrode side), 2... Lower substrate (MIM device side), 3... Pixel FIG. 4 shows an equivalent circuit of the MIM liquid crystal panel. FIG. 5 is a diagram showing the operation of the MIM liquid crystal panel. a is the characteristic of the MIM device, b is the voltage waveform applied to the liquid crystal layer, and c is the voltage waveform input to the MIM liquid crystal panel. 1... Voltage waveform applied to the liquid crystal layer corresponding to the input voltage shown in c-3, 2... Voltage waveform applied to the liquid crystal layer corresponding to the input voltage shown in c-4, 3, 4...
Input voltage waveform, 5...Capacitance-divided voltage value, 6
...Resistance-divided voltage value Figure 6 shows the voltage waveform applied to the liquid crystal layer when the capacitance ratio is changed. A is when C _LC /C _MIM is 1, and b is when C _LC /C _MIM is 10. FIG. 7 is a schematic diagram of the vicinity of one pixel of the MIM liquid crystal panel according to the present invention. 1... MIM device part, 2... BaTiO ₃ layer, 3...
...Pixel electrode FIG. 8 is a schematic diagram of the vicinity of one pixel of the MIM liquid crystal panel according to the present invention. 1... MIM device part, 2... BaTiO ₃ particles, 3
...Pixel electrode.

Claims (1)

[Claims] 1 A liquid crystal is sealed in a pair of substrates, a plurality of display electrodes are formed in a matrix on one of the substrates, and a metal-insulator-metal nonlinear thin film element is formed on each display electrode. 1. A liquid crystal display device, characterized in that particles having a dielectric constant larger than the dielectric constant of the liquid crystal are coated or dispersed between the display electrode and the opposing substrate.