Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-05-20
2002-07-02
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S005000, C349S139000
Reexamination Certificate
active
06414734
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device used in a direct viewing type or projection type display apparatus.
2. Description of Related Art
FIG. 1A
is a partial sectional view of a device showing an arrangement of a conventional reflection type liquid crystal display device. The liquid crystal display device has a basic structure in which two substrates each having electrodes formed on one surface are stuck to each other with a predetermined gap such that the electrodes are opposite to each other, and a liquid crystal is injected into the gap.
FIG. 1A
shows an arrangement of a MOS type active matrix liquid crystal display device in which transistors are arranged for pixels, respectively. A portion corresponding to one pixel is typically shown in FIG.
1
A.
As shown in
FIG. 1A
, in many reflection type liquid crystal display devices, a transparent glass substrate
210
is used as one of two stuck substrates, and a silicon substrate
290
is used as the other substrate. A transparent electrode
220
consisting of ITO (Indium Tin Oxide) or the like is formed on the inner side of the glass substrate
210
, and an alignment film
230
a
for regulating the array of liquid crystal molecules is formed on the surface of the transparent electrode
220
.
A transistor
270
or, depending on cases, as shown in
FIG. 1A
, a capacitor
280
is formed on the inner surface layer of the silicon substrate
290
, and an Al reflective electrode
250
functioning as an electrode and a light reflecting film is formed on these elements through an insulating interlayer
260
. An alignment film
230
b
is also formed on the surface of the Al reflective electrode
250
.
A predetermined gap is kept between the glass substrate
210
and the silicon substrate
290
by a spacer (not shown), and a liquid crystal layer
240
is formed in the gap by injection.
In such a reflection type liquid crystal display device, light I
0
from a light source is admitted from the glass substrate
210
side, passes through the liquid crystal layer
240
, and is reflected by the Al reflective electrode
250
on the surface of the silicon substrate
290
. When the light I
0
of incidence and reflected light I
R
pass through the liquid crystal layer, polarization directions are regulated depending on the alignment state of the liquid crystal molecules.
In general, in a transmission type liquid crystal display device, a region in which a transistor is formed serves as a light-shielding portion and cannot transmit light, and it is difficult to structurally obtain a high aperture ratio. However, in a reflection type liquid crystal display device, a transistor can be formed on the lower layer of the reflective electrode
250
. For this reason, the reflection type liquid crystal display device is more advantageous than the transmission type liquid crystal display device with respect to an aperture ratio.
FIG. 2
is a diagram simply showing an arrangement of a conventional projection type television set, i.e., a so-called liquid crystal projector, using the reflection type liquid crystal display device described above. As shown in
FIG. 2
, the liquid crystal projector has, as main components, a light source
100
, two polarizers
120
a
and
120
b
, a half mirror
130
, a liquid crystal display device
140
, a drive circuit
150
for a liquid crystal display device, an optical lens
160
, and a screen
170
.
For example, light emitted from the light source
100
passes through the polarizer
120
a
, and only a polarized light component of a predetermined direction is extracted from the light. Thereafter, the light component is changed by the half mirror
130
in a course, and is admitted on the liquid crystal display device
140
. On the liquid crystal layer in the liquid crystal display device
140
, a predetermined voltage is applied to each pixel through the drive circuit
150
, and, accordingly, the alignment state of the liquid crystal molecules is changed depending on the predetermined voltage. The polarization direction of a light component transmitted through the liquid crystal layer is regulated by the alignment state of the liquid crystal molecules. The light component reflected by the reflective electrode surface of the liquid crystal display device passes through the half mirror
130
to reach the other polarizer
120
b
. Only a polarized light component of a predetermined direction is selected by the polarizer
120
b
, and the polarized light component is enlarged through the optical lens
160
to be projected on the screen
170
.
FIG. 1B
is a partial sectional view of a device obtained by extracting a portion around the liquid crystal layer
240
in the liquid crystal display device shown in FIG.
1
A. As shown in
FIG. 1B
, a part of light I
0
being incident on the glass substrate
210
is reflected by each layer interface in the middle of the way to the liquid crystal layer
240
. In particular, since ITO or tin oxide (SnO
2
) serving as a transparent electrode has a high refractive index of about 2, reflection easily occurs on the interface between the transparent electrode and the glass substrate or the alignment film. For example, the light I
0
is admitted on the liquid crystal layer
240
, reflected light components r
11
, r
21
, and r
31
(These light components are referred to as an interface reflected light component R
1
for descriptive convenience hereinafter.) are generated by the interface between the glass substrate
210
and the transparent electrode
220
, the interface between the transparent electrode
220
and the alignment film
230
a
, and the interface between the alignment film
230
a
and the liquid crystal layer
240
, respectively.
When the reflected light I
R
reflected by the reflective electrode
250
is admitted, reflected light components r
32
, r
22
, and r
12
(These light components are referred to as interface reflected light R
2
for descriptive convenience hereinafter) are generated by the interfaces of the respective layers, respectively. The interface reflected light R
2
is reflected by the reflective electrode
250
again.
Since the interface reflected light R
1
does not pass through the liquid crystal layer
240
and is not changed in a polarization direction, in many cases, the interface reflected light R
1
is cut by the two polarizer
120
b
before reaching the screen and rarely influence the screen (see FIG.
2
). However, the interface reflected light R
2
which passes through the liquid crystal layer
240
once and is reflected by the reflective electrode
250
to be generated is rarely cut by the polarizer
120
b
, and the interface reflected light R
2
reaches the screen together with the reflected light I
R
.
FIG. 3
is a graph showing a result obtained by measuring the reflectance of light corresponding to the interface reflected light R
2
generated in the arrangement of the conventional liquid crystal display device shown in FIG.
1
B. Comparative Example 1 in
FIG. 3
is a liquid crystal display device in which an ITO film having a thickness of 400 Å and formed as the transparent electrode
220
and an SiO
2
film having a thickness of 25 Å and formed as the alignment films
230
a
and
230
b
are used. Alignment properties are given to the alignment film such that inclined vapor deposition is performed to a substrate which is inclined at 70° with respect to a vapor deposition source. Comparative Example 2 in
FIG. 3
is a liquid crystal display device in which an ITO film having a thickness of 400 Å and formed as the transparent electrode
220
and a vertical alignment polyimide film having a thickness of 700 Å and formed as the alignment films
230
a
and
230
b
are used. The vertical alignment polyimide film is applied by a printing method and thermally hardened. The surface of the polyimide film is rubbed to give alignment properties to the polyimide film.
As shown in
FIG. 3
, the interface reflected light R
2
reflected by the reflective electrode
Shigeta Masanobu
Shimizu Shigeo
Berkowitz Marvin C.
Nath Gary M.
Nath & Associates PLLC
Nguyen Dung
Sikes William L.
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