Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Optical excitation
Reexamination Certificate
1999-07-26
2003-09-30
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Optical excitation
C349S027000
Reexamination Certificate
active
06628347
ABSTRACT:
RELATED APPLICATION DATA
The present application claims priority to Japanese Application No. P10-220694 filed Aug. 4, 1998, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical writing type liquid crystal light valve apparatus and a producing method thereof. The present invention improves sensitivity with respect to a writing light and a spatial resolving power in the optical writing liquid crystal light valve apparatus.
2. Description of the Related Art
A liquid crystal light valve apparatus (LCLV) is an optical-optical image converter.
The light valve is such an apparatus that receives a light with low light intensity and reads an optical image by means of a light from another light source in real time so as to be capable of outputting the optical image.
The LCLV has been utilized as an application to a large-sized screen for military use and commercial use. For example, an LCLV, which was announced by Rodney D. Sterling of Hughes et al. in “Video-Rate Liquid Crystal Light-Valve Using an Amorphous Silicon Photo Detector”, SID, '90 Digest, Paper No. 17A 2, pp327-329 (1990), as shown in a schematic cross-section of
FIG. 1
, is constituted so that a transparent electrode
2
is formed on a first transparent glass substrate
1
, and a photoconductive layer
3
which is made of an amorphous silicon (herein after referred to as a-Si) and whose thickness is large and continuously uniform is formed thereon, and further a light shielding layer
4
made of CdTe or the like and a dielectric mirror
5
as an optical reflective layer is laminated thereon, and an alignment layer
6
is formed on the dielectric mirror.
Meanwhile, a second transparent glass substrate
7
is prepared, and a transparent electrode
8
and an alignment layer
9
are formed thereon similarly.
The first and second glass substrates
1
and
7
are opposed to each other with a gap of about several &mgr;m with the sides where the alignment layers
6
and
9
are formed being directed inside, and liquid crystal is filled between the substrates
1
and
7
so that a liquid crystal layer
10
is formed. The LCLV is constituted in such a manner.
In this LCLV, the side of the second glass substrate
7
is set as an observation side for an optical image, and a reading light LR is irradiated on the glass substrate
7
vertically from the glass substrate
7
side by a polarized light. At this time, the reading light passes through the transparent electrode
7
and the liquid crystal layer
10
and is reflected on the dielectric mirror
5
, and passes through the liquid crystal layer
10
and the glass substrate
7
again so as to be emitted to the outside. In such a manner, the reflected light can be observed.
On the contrary, while an alternating voltage AC is being applied between the transparent electrodes
2
and
8
in the state that the reading light LR is irradiated, a writing light LW is irradiated from the first glass substrate
1
side. Then, the writing light LW passes through the first glass substrate
1
and the transparent electrode
2
and is irradiated onto the photoconductive layer
3
, and the photoconductive layer
3
is activated on this irradiated portion so that electron-hole pairs are generated. As a result, electrostatic capacity of the photoconductive layer
3
is increased, a resistance value thereof is decreased and voltages sandwiching the liquid crystal layer are increased according to a pattern corresponding to an irradiating pattern and the intensity of the writing light LW. This spatial change in the voltage becomes a change in a direction of liquid crystal molecules, and this change causes birefringence and rotation of the reading light LR which passes the liquid crystal layer, and the azimuth (polarization) of the reading light LR is modulated. Therefore, when the reading light emitted from the second glass substrate
7
is finally allowed to pass through a deflecting plate, the reading light can be observed as a change in light quantity. In other words, an optical image according to a pattern of the writing light LW, namely the optical image can be observed from the side of the second glass substrate
7
.
Here, the light shielding layer
4
is arranged between the dielectric mirror
5
, namely, the light reflective layer and the photoconductive layer
3
, so that even a slight quantity of the reading light, which has passed through the light reflective layer, is absorbed by the light shielding layer
4
. Namely, the light shielding layer
4
is disposed so as to avoid that the reading light LR reaches the photoconductive layer
3
to activate the photoconductive layer
3
and to generate an image other than a writing light, in other words, noises are produced.
Incidentally, in the above LCLV, in order to obtain high sensitivity, it is desired to make a voltage, which is applied to the liquid crystal layer according to a change in the resistance value of the photoconductive layer
3
, to be maximum. The maximum voltage is achieved when the impedance of the photoconductive layer
3
on which the light is not irradiated and the impedance of the liquid crystal layer
10
satisfy the following condition.
That is, impedance, which is generated by a parallel circuit of equivalent capacity and bulk resistance of the photoconductive layer
3
, is set so as to be substantially equal with or exceed impedance, which is generated by a parallel circuit of capacity and resistance of the liquid crystal layer
10
(this condition is referred to as a balancing relationship).
Then, the balancing relationship can be realized by, concretely, setting a film thickness of the photoconductive layer made of a-Si to become about 30 &mgr;m.
The film thickness of 30 &mgr;m is required because when the balancing relationship between the impedances of the photoconductive layer
3
and the liquid crystal layer
10
is tried to be set, a dielectric constant of the photoconductive layer
3
due to the a-Si film is higher than a dielectric constant of the liquid crystal layer
10
.
However, when the photoconductive layer made of the a-Si layer has a thickness up to 30 &mgr;m in such a manner, electric charges generated on the photoconductive layer are easily diffused in an adjacent area because of the incidence of the writing light.
Namely, an ideal LCLV is constituted so that its photoconductive layer has high resistance such that electric charges generated due to light irradiation can be prevented from diffusing in a lateral direction. However, if the thickness of an a-Si photoconductive layer becomes up to about 30 &mgr;m, sufficiently high resistance cannot be obtained, and thus the electric charges in the lateral direction (surface direction) easily diffuse. As a result, spatial contrast is lowered and resolution is lowered.
In order to avoid such inconvenience, such a trial was carried out that dopant was added to an a-Si layer composing the photoconductive layer so that resistivity was improved. In this method, when the a-Si film is deposited, since this film has a property such that n-type dopant is originally generated, the n-type dopant is canceled by doping p-type dopant such as boron.
However, according to this method, since the effect of addition of the dopant is extremely great and the occurrence of the n-type dopant varies every time the a-Si film is deposited, it is actually very difficult to accurately dope the p-type dopant for setting desired resistivity. As a result, the production cost is increased, and yield is lowered.
Accordingly, a split structure, such that a photoconductive layer is separated completely at every pixel, is suggested (U.S. Pat. No. 5,076,670). This separation is executed by pattern etching using photo-lithography, for example, but as mentioned above, when the photoconductive layer having a large thickness up to 30 &mgr;m is pattern-etched, time required for the work becomes longer, and further it is difficult to clearly pattern the photoconductive
Nakajima Hideharu
Oohata Toyoharu
Nguyen Dung
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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