Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-06-19
2004-09-28
Nguyen, Dung T. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S123000
Reexamination Certificate
active
06798479
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a liquid crystal display device and a process for making the same.
BACKGROUND OF THE INVENTION
FIG. 6
is a sectional view showing a principal portion of an example of conventional reflective liquid crystal display devices. This liquid crystal display device includes a first and a second substrates
21
,
22
. The first and the second substrates
21
,
22
are disposed in parallel to each other.
The first substrate
21
includes an upper surface
21
a
provided with a polarizer plate
25
and a retardation plate
26
. The polarizer plate
25
allows penetration of light rays that vibrate only in a specific direction. The polarizer plate
25
restricts entry of light from the outside to the first substrate
21
or the exit of light from the first substrate
21
. The retardation plate
26
is disposed between the first substrate
21
and the polarizer plate
25
. The retardation plate
26
compensates for interference colors caused by birefringence at the liquid crystal. This increaces the viewing angle.
The first substrate
21
includes a lower surface
21
b
on which a color filter layer
27
is provided, and a plurality of transparent electrodes
31
in the form of strips are provided thereon. The second substrate
22
includes an upper surface
22
a
provided with a plurality of reflective electrodes
33
in the form of strips extending perpendicularly to the transparent electrodes
31
.
The liquid crystal layer
23
lies between the first and the second substrate
21
,
22
. The liquid crystal layer
23
is filled with e.g. an STN (super-twisted nematic) liquid crystal. The liquid crystal layer
23
is surrounded by a seal member
35
. Pixels are provided at intersections of the transparent electrodes
31
and the reflective electrodes
33
. These pixels are arranged in a matrix. The surfaces of the transparent electrodes
31
and the reflective electrodes
33
are covered with alignment films
34
A,
34
B, respectively. The alignment films
34
A,
34
B twist liquid crystal molecules contained in the liquid crystal layer
23
.
In the above liquid crystal display device, light rays entering from the outside travel through the polarizer plate
25
, the retardation plate
26
, the first substrate
21
, the color filter layer
27
, the transparent electrodes
31
and the liquid crystal layer
23
. After travelling through the liquid crystal layer
23
, the light rays are reflected upwardly by the reflective electrodes
33
, and travel back through the same path to be emitted to the front side of the liquid crystal display device.
With a liquid crystal display device of such a structure, image display may be preferably provided using external light such as the room light or the sun light, without driving the light source (not shown) located inside the liquid crystal display device in order to keep the power consumption as little as possible. With the liquid crystal display device, the surface of each reflective electrode
33
may be a mirror so that a proper image display is obtained with the external light.
By using the reflective electrodes
33
as mirrors, the directivity of the reflected light rays is improved, consequently resulting in efficient use of light. On the other hand, the reflective electrodes
33
may give rise to the problem of a mirror imagin phenomenon which worsens the visibility.
In order to prevent the mirror imaging phenomenon, the surface of each reflective electrode
33
may be undulated. With this structure, the light rays coming from the first substrate
21
and the liquid crystal layer
23
are suitably scattered when reflected on the undulated surface of the reflective electrode
33
. Therefore, the mirror imaging phenomenon and the contrast deterioration are prevented.
The alignment film
34
B is formed to have a predetermined thickness by applying a polyimide resin or the like on the surface of each reflective electrode
33
. Then, the surface of the alignment film
34
b
may be rubbed. By this treatment, those of the liquid crystal molecules S of the liquid crystal layer
23
located closest to the alignment film
34
B are inclined at a predetermined angle (called a pre-tilt angle), as shown in the FIG.
7
. The liquid crystal molecules S are oriented in one direction by rubbing, which facilitates untwisting the twisted molecules under voltage application.
The surface of each reflective electrode
33
is undulated. The alignment film
34
B provided on the surface of the reflective electrode
33
is also undulated following the surface of the reflective electrode
33
. The greater the undulation on the surface of the reflective electrode
33
is, the larger the undulation on the surface of the alignment film
34
B will be. Therefore, the pre-tilt angle of the liquid crystal molecules S in the liquid crystal layer
23
varies depending on whether the molecules are located at a concave portion
41
or at a projection
42
on the undulated surface of the alignment film
34
B, as shown in the FIG.
8
. In other words, the pre-tilt angle of the liquid crystal molecules S differ from one display region to another. Such variations of pre-tilt angle between different display regions may result in failure of providing a desired contrast.
FIGS. 9 and 10
show an example of transmittance-drive voltage characteristics of a liquid crystal display device.
FIG. 9
shows the case where the pre-tilt angle of the liquid crystal molecules located closest to the alignment film
34
B is 2 degrees.
FIG. 10
shows the case where the pre-tilt angle of the liquid crystal molecules located closest to the alignment film
34
B is 0 degree. In both cases, the pre-tilt angle of the liquid crystal molecules located closest to the alignment film
34
A for the first substrate
21
is 2 degrees.
According to the characteristics shown in these figures, the threshold drive voltage for changing the transmittance is approximately 1.70V where the pre-tilt angle of liquid crystal molecules is 2 degrees (refer to the FIG.
9
). On the other hand, when the pre-tilt angle is 0 degree (refer to the FIG.
10
), the threshold voltage is approximately 1.72V. In this way, there is a slight difference between the threshold drive voltages in these two cases. When the pre-tilt angle of the liquid crystal molecules varies, the threshold drive voltage also varies. The greater the difference between pre-tilt angles is, the greater the difference between the threshold voltages will be.
A voltage is applied separately on the transparent electrodes
31
and the reflective electrodes
33
in the passive matrix display mode, for example. If the transparent electrodes
31
or the reflective electrodes
33
include a display regions with different pre-tilt angles, i.e., display regions requiring different threshold voltages, an accurate voltage control is substantially impossible. Therefore, a desired contrast may not be obtained with this liquid crystal display device.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a liquid display device which is capable of eliminating or alleviating the problems described above.
According to a first aspect of the present invention, there is provided a liquid crystal display device which comprises a first and a second substrates disposed in parallel to each other, a liquid crystal layer disposed between the two substrates and filled with liquid crystal, and a reflective member having a reflective surface on which light rays coming from outside through the first substrate and the liquid crystal layer are reflected towards the first substrate. The reflective surface of the reflective member is undulated. The reflective surface of the reflective member is provided with an alignment film for twisting liquid crystal molecules contained in the liquid crystal layer. The height of the undulation on the reflective surface of the reflective member is smaller than at least the thickness of the alignment film at a concave portion of the undulated reflective surface.
Preferably, the liquid cryst
Fujimoto Hisayoshi
Imamura Norihiro
Takakura Toshihiko
Merchant & Gould P.C.
Nguyen Dung T.
Nguyen Hoan Chau
Rohm & Co., Ltd.
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