JPH04261520A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To provide a high space resolving power and to allow stable operations by forming a photoconductive layer into a laminated structure of an inorg. insulating film and an amorphous silicon film and crimping this inorg. insulating film between one electrode substrate and the amorphous silicon film. CONSTITUTION:This element has glass substrates 1 disposed to face each other and transparent conductive films 2 formed on the inside surfaces of the glass substrates 1, by which the electrode substrate is formed. An oriented film 5 is formed on the one transparent conductive film 2 and a liquid crystal layer 4 is formed on the oriented film 5. A reading out light reflection film 6 is formed via the oriented film 5 on the liquid crystal layer 4. The inorg. insulating film 32 consisting of a silicon oxide film is formed on the other transparent conductive film 2 and the hydrogenated amorphous silicon film 31 is formed on the inorg. insulating film 32. The photoconductive layer 33 is formed of both thereof. The other surface of the hydrogenated amorphous silicon film 31 is integrated with the reading out light reflection film 6.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optically writable liquid crystal display element used in large-screen, high-definition projection displays and the like, and particularly relates to improving the quality thereof.

0002

2. Description of the Related Art FIG. 3 shows, for example, CONFERNECE
RECORD of 1990 International
nal Topical Meeting OPT
ICAL COMPUTING (1990) 9B3
, P17-18 are cross-sectional views of the conventional liquid crystal display element shown in FIG.

In the figure, 1 is a glass substrate, 2 is a transparent conductive film, 4 is a liquid crystal layer, 5 is a light distribution film, 6 is a readout light reflecting plate,
30 is a photoconductive layer made of a high resistance hydrogenated amorphous silicon film.

Next, the operation will be explained. Writing to the liquid crystal display element is performed by emitting writing light having an arbitrary light and dark spatial pattern from the left side of FIG. conductive layer 3
This is done by making it incident on 0. At this time, the photoconductive layer 30 in the area not irradiated with the writing light has a high resistance, but the photoconductive layer 30 irradiated with the writing light has a low resistance due to its photoconductivity, and the photoconductive layer 30 liquid crystal layer 4
During this period, a carrier distribution corresponding to the spatial pattern of the writing light occurs. Depending on the distribution of carriers, a spatial distribution occurs in the magnitude of birefringence or optical rotation power of the liquid crystal. For example, when a ferroelectric liquid crystal is used as the liquid crystal layer 4, a spatial distribution of birefringence of the liquid crystal layer 4 occurs depending on the spatial pattern of incident light.

[0005] The spatial distribution of birefringence and optical rotation power of the liquid crystal layer 4 written in this way can be determined by making the readout light enter the liquid crystal layer 4 from the right side of FIG. The reflected light can be read out by projecting it onto a screen again via a polarizer.

[0006]Using the above liquid crystal display element, wavelength conversion of two-dimensional images, incoherent-coherent conversion, etc. can be performed. Furthermore, since this liquid crystal display element has a higher spatial resolution than a light valve using BSO, it is possible to construct a large-screen, high-definition projection system using this liquid crystal display element.

In order to realize the writing operation of the liquid crystal display element described above, the resistance of the photoconductive layer 30 is sufficiently high compared to the liquid crystal layer 4 when not irradiated with the writing light, and the resistance of the liquid crystal layer 4 is sufficiently high when irradiated with the writing light. It is necessary to reduce the amount to a certain level. or,
In order to realize high spatial resolution of the device, in addition to the condition that the dark resistance of the photoconductive layer 30 is sufficiently large, it is also necessary that the liquid crystal layer 4 and the photoconductive layer 30 be thin.

[0008]

[Problems to be Solved by the Invention] In the conventional liquid crystal display device described above, a hydrogenated amorphous silicon film is used as the photoconductive layer 30, and in addition, CdS, crystalline silicon, BS
An O crystal plate or the like is used. Among these, the hydrogenated amorphous silicon film is formed by plasma CVD method, etc., and compared to other materials, it is relatively easy to set the resistance value, has high adhesion to the glass substrate 1, and can be used in the visible light range. It has advantages such as high photosensitivity.

However, the dark conductivity at room temperature of a hydrogenated amorphous silicon film formed by the plasma CVD method is 1×10 −9 to 1×10 −11 S/cm for an undoped film.
Even a so-called boron light-doped film containing several ppm of boron has a concentration of ~1×10-12 at most.
S/cm, and the general conductivity of liquid crystal is 1 x 10-1
It cannot be said that it is sufficiently lower than 1 to 1×10 −12 S/cm, and when a hydrogenated amorphous silicon film is used as the photoconductive layer 30, it is necessary to compensate for the high dark conductivity by some method.

Conventionally, for this purpose, the hydrogenated amorphous silicon film has been made considerably thicker than the liquid crystal layer 4 (for example, when the thickness of the liquid crystal layer 4 is 2 μm, the hydrogenated amorphous silicon film has been made considerably thicker than the liquid crystal layer 4). ), or the photoconductive film 30 has a pin structure of a hydrogenated amorphous silicon film, and writing is performed by applying a reverse bias to the formed pin photodiode. I had a method. However, increasing the thickness of the photoconductive film 30 causes a problem in that the spatial resolution of the liquid crystal display element decreases, and if the photoconductive layer 30 has a pin structure, the voltage waveform applied to the liquid crystal layer 4 differs from the driving voltage waveform. Therefore, there was a problem in that the liquid crystal deteriorated during long-time driving.

[0011] Furthermore, the operating temperature of a liquid crystal display element is limited to the temperature range in which the liquid crystal phase used is developed, but if this temperature range is higher than room temperature, the dark conductivity and Another problem is that it is difficult to make the dark resistance of the photoconductive layer 30 larger than the resistance of the liquid crystal layer 4 because the activation energy of the reverse current of the pin photodiode is larger than that of the liquid crystal.

The present invention was made to solve the above-mentioned problems, and it is possible to match the dark resistance of the entire photoconductive layer with the resistance of the liquid crystal layer, and to obtain high spatial resolution. The object of the present invention is to obtain a liquid crystal display element that can operate stably regardless of the operating temperature.

[0013]

[Means for Solving the Problems] A liquid crystal display element according to the present invention has a photoconductive layer having a laminated structure of an inorganic insulating film and an amorphous silicon film, and the inorganic insulating film is connected to one electrode substrate and an amorphous silicon film. It is sandwiched between the membrane and the membrane.

[0014]

[Operation] In this invention, when a bias is applied between the electrode substrate in a dark state, the injection of carriers from the electrode substrate to the amorphous silicon film is suppressed by the inorganic insulating film, and the dark resistance of the photoconductive layer is reduced. To increase.

[0015]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view of a liquid crystal display element according to an embodiment, in which 1 is a glass substrate disposed facing each other, and 2 is a glass substrate 21.
A transparent conductive film formed on the inner surface of the electrode substrate. 5 is an alignment film formed on one transparent conductive film 2, 4 is a liquid crystal layer formed on the alignment film 5, and a reading light reflecting film 6 is formed on the liquid crystal layer 4 via the alignment film 5. It is formed. Further, an inorganic insulating film 32 made of a silicon oxide film is formed on the other transparent conductive film 2, and a hydrogenated amorphous silicon film 31 is formed on the inorganic insulating film 32, and both of them form a photoconductive layer 33. is formed. The other surface of the hydrogenated amorphous silicon film 31 is integrated with the readout light reflecting film 6.

00
16] In manufacturing the above-mentioned liquid crystal display element, after forming an ITO film as the transparent conductive film 2 on the glass substrate 1,
An inorganic insulating film 32 was formed by sputtering on a region of the transparent conductive film 2 other than the electrode terminal extraction portion. The thickness of the inorganic insulating film 32 was 20 nm to 200 nm. Next, inorganic insulating film 3
A hydrogenated amorphous silicon film 31 was formed on 2 by plasma CVD using silane gas and diborane gas, and the film thickness was 3 μm. Between the hydrogenated amorphous silicon film 31 and the other transparent conductive film 2, a liquid crystal cell consisting of an alignment film 5, a liquid crystal layer 4, and a readout light reflecting film 6 was formed. As the liquid crystal layer 4, a ferroelectric liquid crystal (CS-1014 manufactured by Chisso Corporation) was used. For the alignment layer 5, an SiO obliquely evaporated film was used. The cell gap of the liquid crystal was 2 μm. The temperature at which this liquid crystal exhibits the SmC* phase used in the operation of display elements is 40 to 5
It is 0°C. Further, as the readout light reflecting film 6, a dielectric multilayer film formed by vapor deposition was used.

The display operation of the fabricated liquid crystal display element was carried out by keeping the temperature of the element at 45°C, but the conductivity of the liquid crystal at this operating temperature was ~3 x 10-12 S/cm, and the hydrogenation The conductivity of the amorphous silicon film 31 in the dark state is ~1×10
-11 S/cm.

Next, the operation of the above embodiment will be explained. The main operations regarding writing and reading are the same as in the conventional example. Here, due to the presence of the inorganic insulating film 32 sandwiched between the hydrogenated amorphous silicon film 31 and the transparent conductive film 2, when a bias is applied between the transparent conductive film 2 in a dark state, the transparent conductive film 2 to the hydrogenated amorphous silicon film 31 is suppressed, compared to the conventional structure, the photoconductive layer 3
The effective dark resistance of 3 increases. The resistance value of the photoconductive layer 33 in this dark state can be increased or decreased by changing the thickness of the inorganic insulating film 32. Therefore, the photoconductive layer 33 can be configured so as to be able to sufficiently cope with operation at high temperatures, which has been difficult in the past due to the reduced resistance value of the hydrogenated amorphous silicon film 31.

Under the writing light irradiation, the hydrogenated amorphous silicon film 31 can be formed by selecting an appropriate applied voltage and the thickness of the inorganic insulating film 32.
As the resistance decreases, breakdown of the inorganic insulating film 32 occurs, causing a decrease in the overall resistance of the photoconductive layer 33, and a bias is applied to the liquid crystal layer 4.

In order to clarify the features of the above-mentioned liquid crystal display element, the display characteristics were compared depending on the presence or absence of the inorganic insulating film 32 and its film thickness, and the results are shown in Table 1.

[0021]

[0022] While the parallel beam of the He-Ne laser is incident on the photoconductive layer 33 of the device created through a mask pattern for resolution evaluation, a rectangular voltage pulse with a width of 100 nsec is applied between both transparent conductive films 2. was applied to perform writing. Here, the intensity of the writing light was 10 μW/cm 2 . Table 1 shows, as display characteristics, the maximum value of spatial resolution and the voltage value range in which writing was possible when changing the voltage value of the rectangular voltage pulse during writing.

Furthermore, by sandwiching the inorganic insulating film 32 made of a silicon oxide film between the transparent electrode film 2 and the hydrogenated amorphous silicon film 31, the resolution of the display element can be increased to 100 lp/mm.
improved to a certain degree. This is considered to be because the effective dark resistance of the photoconductive layer 33 increased, thereby reducing image blurring due to lateral diffusion of carriers. Moreover, as the thickness of the inorganic insulating film 32 increases, the voltage required for writing increases, and furthermore, the voltage range that can be written increases.
Stable write operation is now possible. These effects are particularly remarkable when the thickness of the inorganic insulating film 32 is 20 nm or more.

On the other hand, the thickness of the inorganic insulating film 32 is 200 nm.
If the value exceeds 1, the resistance of the photoconductive layer 33 decreases less when irradiated with light, and writing requires a significant increase in the applied voltage or the amount of light. Furthermore, the decrease in capacitance of the photoconductive layer 33 increases image blur. For these reasons, the thickness of the inorganic insulating film 32 made of a silicon oxide film is 20 to 20 cm.
Approximately 200 nm is appropriate.

FIG. 2 shows the structure of a liquid crystal display element according to a second embodiment of the invention. In this embodiment, an inorganic insulating film 32 made of a silicon oxide film is formed on both sides of a hydrogenated amorphous silicon film 31.
is formed to constitute the photoconductive layer 34, and the other configurations are the same as in the first embodiment.

Also in this second embodiment, one of the inorganic insulating films 32 is a transparent conductive film 2 and a hydrogenated amorphous silicon film 31.
The dark resistance of the photoconductive layer 34 increases. Furthermore, since the photoconductive layer 34 exhibits approximately equivalent current-voltage characteristics regardless of whether positive or negative polarity voltage is applied, deterioration of the liquid crystal that occurs when a conventional pin photodiode is used as the photoconductive layer 34 can be prevented. can.

In each of the above embodiments, a so-called reflective liquid crystal display element having a readout reflective film 6 has been described, but the readout light reflective film 6 not only increases the utilization efficiency of readout light but also It is provided to separate the reading optical system from the reading optical system. Therefore, by not forming the readout reflective film 6 and using a wavelength outside the sensitivity of the photoconductive layers 33 and 34 as the readout light, the liquid crystal display element of the present invention can be converted into a transmissive type liquid crystal display element. It is also possible to use it as Even in that case, the photoconductive layers 33, 3
By configuring 4 as in the present invention, display characteristics can be improved. Further, although ferroelectric liquid crystal is used as the liquid crystal layer 4, twisted nematic liquid crystal, guest-host type liquid crystal, liquid crystal dispersed polymer, etc. can also be used. Further, although a silicon oxide film is used as the inorganic insulating film 32, a silicon nitride film may also be used.

[0028]

[Effects of the Invention] As described above, according to the present invention, an inorganic insulating film is provided between the electrode substrate and the amorphous silicon film of the photoconductive layer. Since carrier injection is suppressed, the dark resistance of the photoconductive layer can be increased, and a liquid crystal display element with good display characteristics can be obtained. Further, due to the increase in the dark resistance, image blur due to lateral diffusion of carriers is reduced, and high spatial resolution can be obtained. Furthermore, even if the operating temperature is high, stable operation can be performed due to the increase in dark resistance.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a liquid crystal display element according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a liquid crystal display element according to a second embodiment of the invention.

FIG. 3 is a cross-sectional view of a conventional liquid crystal display element.

[Explanation of symbols]

1 Glass substrate 2 Transparent conductive film 4 Liquid crystal layer 31 Hydrogenated amorphous silicon film 32 Inorganic insulation film 33, 34 Photoconductive layer

Claims (1)

[Claims] Claim 1: In an optical writing type liquid crystal display element in which a photoconductive layer and a liquid crystal layer are formed between a pair of electrode substrates arranged opposite to each other, the photoconductive layer is composed of an inorganic insulating film and an amorphous silicon film. A liquid crystal display element having a laminated structure and having this inorganic insulating film sandwiched between one electrode substrate and an amorphous silicon film.