JP2844659B2 - LCD light valve - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optically written liquid crystal light valve for converting an optical signal into another optical signal used for optical information processing and the like.

[Conventional technology]

Liquid crystal light valves are light modulation elements used for light projection displays and optical information processing. The device configuration of a conventional optical writing type liquid crystal light valve is disclosed in, for example, Japanese Patent Application No. 62-2527.
It is described in detail in 83 "Liquid crystal light valves". Fifth
The figure shows an equivalent circuit.

In the figure, 35 is a resistance that does not feel light, 28 is a photoconductor, 2
9, 30 are thin film transistors, 29 is for switching, and 30 is for load. 31 is a low voltage side electrode, 32 is a high voltage side electrode, 33 is a liquid crystal, and 34 is a counter electrode.

The resistance value of the photoconductor 28 changes greatly with incident light.
For example, if the resistance of the photoconductor 28 is sufficiently larger than the resistance 35 in a dark state without light incidence, the potential at the point X becomes closer to the potential _VL of the low-voltage side electrode. V _L is the switching transistor
Since the following 29 threshold, the transistor 29 is a load transistor than the potential V _H of the output of the Y points in OFF high-voltage side electrode
The potential becomes lower by the threshold voltage of 30.

In the bright state light is incident on the contrary, the potential of the point X if sufficiently smaller than the resistance value of the photoconductor 28 to the resistor 35 becomes close to the potential V _H of the high-voltage side electrode. For this reason, the threshold value of the switching transistor 29 is exceeded, the transistor is turned on, and the output at the point Y becomes almost the potential _VL of the low voltage electrode.

Thus, a large voltage change can be applied to the liquid crystal 33 by the light incident on the photoconductor 28. As the liquid crystal, a fast-response ferroelectric liquid crystal is used.

[Problems to be solved by the invention]

The problem with this conventional liquid crystal light valve is high speed. A conventional liquid crystal light valve uses a ferroelectric liquid crystal to obtain a high-speed response. However, the response speed of the liquid crystal driving unit is very slow, and the high-speed response of the ferroelectric liquid crystal cannot be utilized. Since the amorphous silicon with high resistivity is used for the photoconductor and the resistive part without photosensitivity, the resistance of these parts becomes extremely large, and the RC time constant of the liquid crystal drive part also becomes large. It is because it becomes. In fact, a time constant of a few milliseconds is the limit.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a liquid crystal light valve which eliminates the above-mentioned drawbacks of the conventional liquid crystal light valve and is excellent in high-speed operation.

[Means for solving the problem]

In the liquid crystal light valve of the present invention, a one-dimensional or two-dimensional array liquid crystal driving circuit that performs a switching operation by light is formed on a transparent insulating substrate, and a liquid crystal is formed between the glass substrate with the transparent electrode facing the liquid crystal driving circuit. With the liquid crystal light valve interposed, the liquid crystal drive circuit connects a photodiode and a capacitor in series, an analog output unit that outputs a voltage applied to the connection part, and the analog output unit that changes according to the amount of light incident on the photodiode. The output of the analog output section is used as an analog input, and an inverter circuit for converting a high / low binary output corresponding to a specific threshold value into a binary output opposite to the analog input is provided. It is characterized by being a signal of one pixel,
Suitable for use of ferroelectric liquid crystal, and has higher speed than before.

[Action]

Hereinafter, the liquid crystal light valve of the present invention will be described.
FIG. 1 shows an example of an equivalent circuit of the liquid crystal light valve of the present invention. In the figure, 1 is a photodiode, 2 is a capacitor, and 3 and 4 are thin film transistors, both of which form an inverter. 5 is a low voltage side electrode, 6 is a high voltage side electrode, 7 is a liquid crystal, and 8 is a counter electrode on the opposing substrate.

As shown in FIG. 1, a voltage determined by the low-voltage side electrode 5 and the high-voltage side electrode 6 is applied to the photodiode 1 with a reverse bias to an analog output section composed of the photodiode 1 and the capacitor 2 connected in series. When irradiated with light,
A current corresponding to the light intensity is generated in the photodiode 1,
The current is stored in the capacitor 2. The photodiode 1 has excellent linearity, and an output proportional to the amount of incident light is obtained.

This output is the input of the inverter formed by the thin film transistors 3 and 4. The switching transistor 4 can be turned ON / OFF depending on whether or not this input exceeds the threshold voltage of the switching thin film transistor 4 (hereinafter, referred to as a switching transistor) of the inverter. In the case of ON, the voltage of the low-voltage side electrode is almost output, and in the case of OFF, the voltage is lower than the potential of the high-voltage side electrode by the threshold value of the load thin film transistor 3 (hereinafter referred to as load transistor).

This output has a memory effect for a certain period of time until the capacitor 2 discharges naturally. In order to reset completely, the low voltage side electrode 5 and the high voltage side electrode 6 are short-circuited.

Photodiodes are very fast devices. Therefore, the response of this device is determined by the time required for the capacitor to be charged by the photocurrent of the photodiode and for the potential to exceed the threshold value of the switching transistor of the inverter. Although this time depends on the design, it is on the order of a few microseconds and is much smaller than the RC time constant of a conventional liquid crystal light valve. Therefore, if a ferroelectric liquid crystal is used for the liquid crystal 7, the speed becomes much higher than that of the conventional liquid crystal light valve.

〔Example〕

FIG. 2 shows a plan view of the embodiment. FIG. 3 is a sectional view taken along line AA 'of FIG. 2, and FIG. 4 is a sectional view taken along line BB' of FIG.

In the drawing, 5 is a low voltage side electrode, 6 is a high voltage side electrode, 7 is a liquid crystal, 8 is a counter electrode, 9 is a photodiode electrode, 10 is a photodiode light absorbing layer, 11 and 12 are capacitor electrodes, 13
Is a capacitor dielectric, 14 is a substrate protective film, 15 is an inter-electrode insulating film, 16 is a photodiode electrode, 17 is a connection electrode, 18 is an interlayer insulating film, 19 is a pixel electrode, 20 is a contact hole, and 21 and 2.
2 is a glass substrate, 24 and 25 are transistor active layers, and 26 and 27 are gates.

In the embodiment, the photodiode is of a Schottky barrier type, and the inverter section is formed of polysilicon. The manufacturing process is as shown below.

First, a 2000 Å silicon oxide film 14 is formed by performing low pressure vapor deposition (LPCVD) at a substrate temperature of 600 ° C.
Is formed on a glass substrate 22 to form a protective film. Continuously 2000 Å polycrystalline silicon film to silicon oxide film 14
It is formed thereon and patterned to form active layers 24 and 25 of the transistor.

Next, similarly, 2000 Å of silicon oxide is formed by LPCVD, and a 2000 Å polycrystalline silicon film is continuously formed thereon, followed by patterning to form gates 26 and 27. Further, excimer laser doping using PH ₃ gas plasma is performed on a part of the active layers 24 and 25. At this time, one electrode 12 of the capacitor is formed at the same time.

Next, silane, ammonia (NH ₃ ) and nitrogen are mixed and decomposed.
A silicon nitride film of 000 angstroms is formed and used as a dielectric 13 of the thin film capacitor.

Next, 2000 Å of ITO is formed by sputtering, and is patterned as shown in FIG. 2 to form one electrode 9 of the photodiode. After forming the contact hole 20 in the silicon nitride film 13, 3,000 angstrom chromium is formed by sputtering, and this is patterned to form the other electrode 11 of the capacitor.

Next, the silane is decomposed to 10,000 angstroms (1μ
m) The amorphous silicon is formed. Further, 3,000 Å of chromium is formed and patterned to form a photodiode electrode 16 and a photodiode light absorbing layer 10.

Next, 6000 angstrom of silicon nitride is formed to form an inter-electrode insulating film 15. After forming the contact hole 20 again, aluminum is sputtered to form a film of 6000 Å, patterned, and the high voltage side electrode 6, the low voltage side electrode 5, the electrode
Form 17.

Next, an interlayer insulating film 18 made of a silicon nitride film is formed thereon, and a hole for making a contact between the electrode 17 and the pixel electrode 19 is formed. Further, 2000 Å of aluminum is deposited thereon and patterned to form a pixel electrode 19. Further, the liquid crystal 7 is sandwiched between the glass substrate 21 having the counter electrode 8 formed of an ITO transparent electrode and the above-mentioned glass substrate 22, thereby completing the liquid crystal light valve of the present invention.

As the liquid crystal, a ferroelectric liquid crystal was used for high speed. Although not shown in this figure, a polarizing plate is used on the glass substrate 21, and the signal light is transmitted from the glass substrate 22 side.
The projection light enters from the glass substrate 21 side.

Many organic insulating materials such as polyimide can be used for the interlayer insulating film. For the production of polysilicon, there is a method in which amorphous silicon is formed by LPCVD or high vacuum deposition (MBD) at a low temperature, and then polycrystallized by solid phase growth or laser annealing.

〔The invention's effect〕

Regarding high-speed performance, which is a problem of the conventional technology, in addition to using a photodiode in a liquid crystal driving unit as shown in the embodiment of the present invention, by using a ferroelectric liquid crystal,
High-speed response several tens of times higher than that of the conventional type was confirmed.

As described above, according to the present invention, it is possible to obtain a liquid crystal light valve having excellent high-speed performance, and it is industrially very useful.

[Brief description of the drawings]

1 is a diagram showing an equivalent circuit of a liquid crystal light valve of the present invention, FIGS. 2, 3, and 4 are diagrams of an embodiment of the present invention, and FIG. 5 is an equivalent circuit diagram of a conventional liquid crystal light valve. Is shown. In the figure, 1 ... photodiode, 2 ... capacitor, 3 ... load transistor, 4 ... switching transistor, 5
…… Low voltage side electrode, 6 …… High voltage side electrode, 7 …… Liquid crystal, 8…
... counter electrode, 9 ... photodiode electrode, 10 ... photodiode light absorbing layer, 11, 12 ... capacitor electrode, 13
…… Capacitor dielectric, 14… Substrate protective film, 15… Interelectrode insulating film, 16 …… Photodiode electrode, 17… Connection electrode, 18… Interlayer insulating film, 19 …… Pixel electrode, 20… Contact holes, 21, 22 ... glass substrate, 24, 25 ... transistor active layer, 26, 27 ... gate, 28 ... photoconductor, 29 ...
... switching transistor, 30 ... load transistor, 31 ... low voltage side electrode, 32 ... high voltage side electrode, 33 ... liquid crystal, 34 ... counter electrode, 35 ... resistance.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/136 G02F 1/135 G02F 1/133 G02F 1/13 505 G09F 9/30 G09G 3/18 G09G 3 / 36

Claims (1)

(57) [Claims] 1. A one-dimensional or two-dimensional array liquid crystal drive circuit which performs a switching operation by light is formed on a transparent insulating substrate, and liquid crystal is sandwiched between a glass substrate having a transparent electrode and an opposing glass substrate. A liquid crystal light valve, wherein the liquid crystal drive circuit is configured to connect a photodiode and a capacitor in series, and output the voltage applied to the connection portion as an analog output unit, and the liquid crystal drive circuit changes according to the amount of light incident on the photodiode. An output of the analog output unit is used as an analog input, and an inverter circuit for converting a high / low binary output corresponding to a specific threshold value into a binary output opposite to the analog input is provided. A liquid crystal light valve, which is a signal of one pixel according to (1).