U.S. patent application number 09/739697 was filed with the patent office on 2001-09-13 for liquid crystal display in which at least one pixel includes both a transmissive region and a reflective region.
Invention is credited to Ban, Atsushi, Ishii, Yutaka, Katayama, Mikio, Kubo, Masumi, Narutaki, Yozo, Nishiki, Hirohiko, Shimada, Takayuki, Yoshimura, Yoji.
Application Number | 20010020991 09/739697 |
Document ID | / |
Family ID | 27548644 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020991 |
Kind Code |
A1 |
Kubo, Masumi ; et
al. |
September 13, 2001 |
Liquid crystal display in which at least one pixel includes both a
transmissive region and a reflective region
Abstract
A liquid crystal display device according to the present
invention includes a first substrate, a second substrate, and a
liquid, crystal layer interposed between the first substrate and
the second substrate. The first substrate includes: a plurality of
gate lines; a plurality of source lines arranged to cross with the
plurality of gate lines; a plurality of switching elements disposed
in the vicinity of crossings of the plurality of gate lines and the
plurality of source lines; and a plurality of pixel electrodes
connected to the plurality of switching elements. The second
substrate includes a counter electrode. A plurality of pixel
regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, and
each of the plurality of pixel regions includes a reflection region
and a transmission region.
Inventors: |
Kubo, Masumi; (Ikoma-shi,
JP) ; Narutaki, Yozo; (Yamatokoriyama-shi, JP)
; Ban, Atsushi; (Soraku-gun, JP) ; Shimada,
Takayuki; (Yamatokoriyama-shi, JP) ; Yoshimura,
Yoji; (Nara-shi, JP) ; Katayama, Mikio;
(Ikoma-shi, JP) ; Ishii, Yutaka; (Nara-shi,
JP) ; Nishiki, Hirohiko; (Funabashi-shi, JP) |
Correspondence
Address: |
Nixon & Vanderhye P.C.
1100 N. Glebe Rd., 8th Floor
Arlington
VA
22201
US
|
Family ID: |
27548644 |
Appl. No.: |
09/739697 |
Filed: |
December 20, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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|
09739697 |
Dec 20, 2000 |
|
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|
09122756 |
Jul 27, 1998 |
|
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6195140 |
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Current U.S.
Class: |
349/113 |
Current CPC
Class: |
G02F 1/136227 20130101;
G02F 1/136209 20130101; G02F 1/133638 20210101; G02F 1/13439
20130101; G02F 1/133526 20130101; G02F 1/133555 20130101; G02F
1/13712 20210101; G02F 1/13725 20130101 |
Class at
Publication: |
349/113 |
International
Class: |
G02F 001/1335 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 28, 1997 |
JP |
9-201176 |
Oct 7, 1997 |
JP |
9-274327 |
Jan 29, 1998 |
JP |
10-016299 |
Jan 30, 1998 |
JP |
10-018781 |
Mar 24, 1998 |
JP |
10-75317 |
Apr 28, 1998 |
JP |
10-117954 |
Claims
What is claimed is:
1. A liquid crystal display device comprising a first substrate, a
second substrate, and a liquid crystal layer interposed between the
first substrate and the second substrate, a plurality of pixel
regions being defined by respective pairs of electrodes for
applying a voltage to the liquid crystal layer, wherein each of the
plurality of pixel regions includes a reflection region and a
transmission region.
2. A liquid crystal display device according to claim 1, wherein
the first substrate includes a reflection electrode region
corresponding to the reflection region and a transmission electrode
region corresponding to the transmission region.
3. A liquid crystal display device according to claim 2, wherein
the reflection electrode region is higher than the transmission
electrode region, forming a step on a surface of the first
substrate, and thus a thickness of the liquid crystal layer in the
reflection region is smaller than a thickness of the liquid crystal
layer in the transmission region.
4. A liquid crystal display device according to claim 1, wherein
the occupation of an area of the reflection region in each of the
pixel regions is in the range of about 10 to about 90%.
5. A liquid crystal display device comprising a first substrate, a
second substrate, and a liquid crystal layer interposed between the
first substrate and the second substrate, wherein the first
substrate includes: a plurality of gate lines; a plurality of
source lines arranged to cross with the plurality of gate lines; a
plurality of switching elements disposed in the vicinity of
crossings of the plurality of gate lines and the plurality of
source lines; and a plurality of pixel electrodes connected to the
plurality of switching elements, the second substrate includes a
counter electrode, a plurality of pixel regions are defined by the
plurality of pixel electrodes, the counter electrode, and the
liquid crystal layer interposed between the plurality of pixel
electrodes and the counter electrode, and each of the plurality of
pixel regions includes a reflection region and a transmission
region.
6. A liquid crystal display device according to claim 5, wherein
the first substrate includes a reflection electrode region
corresponding to the reflection region and a transmission electrode
region corresponding to the transmission region.
7. A liquid crystal display device according to claim 6, wherein
the reflection electrode region is higher than the transmission
electrode region, forming a step on a surface of the first
substrate, and thus a thickness of the liquid crystal layer in the
reflection region is smaller than a thickness of the liquid crystal
layer in the transmission region.
8. A liquid crystal display device according to claim 7, wherein
the thickness of the liquid crystal layer in the reflection region
is about a half of the thickness of the liquid crystal layer-in the
transmission region.
9. A liquid crystal display device according to claim 6, wherein
each of the pixel electrodes includes a reflection electrode in the
reflection electrode region and a transmission electrode in the
transmission electrode region.
10. A liquid crystal display device according to claim 9, wherein
the reflection electrode and the transmission electrode are
electrically connected to each other.
11. A liquid crystal display device according to claim 6, wherein
each of the pixel electrodes comprises a transmission electrode,
and the reflection region includes the transmission electrode and a
reflection layer isolated from the transmission electrode.
12. A liquid crystal display device according to claim 6, wherein
the reflection electrode regions overlap at least a portion of the
plurality of gate lines, the plurality of source lines, and the
plurality of switching elements.
13. A liquid crystal display device according to claim 6, wherein
at least either of the reflection electrode regions and the
transmission electrode regions have a layer formed of the same
material as a material for the plurality of gate lines or the
plurality of source lines.
14. A liquid crystal display device according to claim 5, wherein
the occupation of an area of the reflection region in each of the
pixel regions is in the range of about 10 to about 90%.
15. A liquid crystal display device according to claim 6, wherein
the first substrate further includes storage capacitor electrodes
for forming storage capacitors with the pixel electrodes via an
insulating film, wherein the reflection electrode regions overlap
the storage capacitor electrodes.
16. A liquid crystal display device according to claim 5, further
comprising microlenses on a surface of the first substrate opposite
to the surface facing the liquid crystal layer.
17. A liquid crystal display device according to claim 6, wherein
each of the reflection electrode regions includes a metal layer and
an interlayer insulating film formed under the metal layer.
18. A liquid crystal display device according to claim 17, wherein
the metal layer has a continuous wave shape.
19. A liquid crystal display device according to claim 18, wherein
a surface of the interlayer insulating layer is of a concave and
convex shape.
20. A liquid crystal display device according to claim 17, wherein
the interlayer insulating layer is formed of a photosensitive
polymer resin film.
21. A liquid crystal display device according to claim 17, wherein
the interlayer insulating layer covers at least a portion of either
the switching element, the plurality of gate lines, or the
plurality of source lines.
22. A liquid crystal display device according to claim 9, wherein
the reflection electrodes are formed at the same level as the
plurality of gate lines or the plurality of source lines.
23. A liquid crystal display device according to claim 22, wherein
the reflection electrodes are formed at the same level as the
plurality of gate lines, and the reflection electrodes are
electrically connected to the gate lines for the pixel electrodes
adjacent to the reflection electrodes.
24. A liquid crystal display device according to claim 22, wherein
the same signals applied to the counter electrode are applied to
the reflection electrodes.
25. A liquid crystal display device according to claim 22, wherein
the reflection electrodes are formed at the same level as the
plurality of gate lines, and the reflection electrodes form storage
capacitors by overlapping drain electrodes of the switching
elements or the transmission electrodes.
26. A liquid crystal display device according to claim 9, wherein
the reflection electrode is formed of Al or an Al alloy.
27. A liquid crystal display device according to claim 26, wherein
the transmission electrode is formed of ITO, and a metal layer
interposes between the transmission electrode and the reflection
electrode.
28. A method for fabricating a liquid crystal display device
comprising a first substrate, a second substrate, and a liquid
crystal layer interposed between the first substrate and the second
substrate, the first substrate including: a plurality of gate
lines; a plurality of source lines arranged to cross with the
plurality of gate lines; a plurality of switching elements disposed
in the vicinity of crossings of the plurality of gate lines and the
plurality of source lines; and a plurality of pixel electrodes
connected to the plurality of switching elements, the second
substrate including a counter electrode, a plurality of pixel
regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, each
of the plurality of pixel regions including a reflection region and
a transmission region, the method comprising the steps of: forming
the transmission electrode regions using a material having a high
light transmittance on the first substrate; forming photosensitive
polymer resin layers; and forming reflection layers made of a
material having a high reflectance on the polymer resin layers.
29. A method according to claim 28, wherein the photosensitive
polymer resin layers have a plurality of concave and convex
portions.
30. A method for fabricating a liquid crystal display device
comprising a first substrate, a second substrate, and a liquid
crystal layer interposed between the first substrate and the second
substrate, the first substrate including: a plurality of gate
lines; a plurality of source lines arranged to cross with the
plurality of gate lines; a plurality of switching elements disposed
in the vicinity of crossings of the plurality of gate lines and the
plurality of source lines; and a plurality of pixel electrodes
connected to the plurality of switching elements, the second
substrate including a counter electrode, a plurality of pixel
regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, each
of the plurality of pixel regions including a reflection region and
a transmission region, the method comprising the steps of: forming
the transmission electrode regions using a material having a high
light transmittance on the first substrate; forming protection
films on the transmission electrode regions; and forming layers
having a high reflectance on portions of the protection films to
form the reflection electrode regions.
31. A method according to claim 30, wherein the transmission
electrode regions are formed at the same level as the plurality of
source lines.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
device and a method for fabricating the liquid crystal display
device. More particularly, the present invention relates to a
liquid crystal display device having a transmission display region
and a reflection display region in each pixel, and a method for
fabricating such a liquid crystal display device.
[0003] 2. Description of the Related Art
[0004] Due to the features of being thin and consuming low power,
liquid crystal display devices have been used in a broad range of
fields including office automation (OA) apparatuses such as
wordprocessors and personal computers, portable information
apparatuses such as portable electronic schedulers, and a
camera-incorporated VCR provided with a liquid crystal monitor.
[0005] Such liquid crystal display devices include a liquid crystal
display panel which does not emit light itself, unlike a CRT
display and an electroluminescence (EL) display. Therefore, a
so-called transmission type is often used as the liquid crystal
display device, which includes an illuminator called a backlight
disposed at the rear or one side thereof, so that the amount of the
light from the backlight which passes through the liquid crystal
panel is controlled by the liquid crystal panel in order to realize
image display.
[0006] In such a transmission type liquid crystal display device,
however, the backlight consumes 50% or more of the total power
consumed by the liquid crystal display device. Providing the
backlight therefore increases the power consumption.
[0007] In order to overcome the above problem, a reflection type
liquid crystal display device has been used for portable
information apparatuses which are often used outdoors or carried
with the users. Such a reflection type liquid crystal -display
device is provided with a reflector formed on one of a pair of
substrates in place of the backlight so that ambient light is
reflected from the surface of the reflector.
[0008] Such a reflection type liquid crystal display device is
operated in a display mode using a polarizing plate, such as a
twisted nematic (TN) mode and a super twisted namatic (STN) mode
which have been broadly used in the transmission type liquid
crystal display devices.
[0009] In recent years, there has been vigorous development of a
phase change type guest-host mode which does not use a polarizing
plate and thus realizes a brighter display.
[0010] The reflection type liquid crystal display device using the
reflection of ambient light is disadvantageous in that the
visibility of the display is extremely lower when the surrounding
environment is dark. Conversely, the transmission type liquid
crystal display device is disadvantageous when the environment is
bright. That is, the color reproducibility is lower and the display
is not sufficiently recognizable because the display light is less
bright than the ambient light. In order to improve the display
quality under a bright environment, the intensity of the light from
the backlight needs to be increased. This increases the power
consumption of the backlight and thus the resultant liquid crystal
display device. Moreover, when the liquid crystal display device
needs to be viewed at a position exposed to direct sunlight or
direct illumination light, the display quality is inevitably lower
due to the ambient light. For example, when a liquid crystal
display screen fixed in a car or a display screen of a personal
computer used at a fixed position receives direct sunlight or
illumination light, surrounding images are mirrored, making it
difficult to observe the display itself.
[0011] In order to overcome the above problems, a construction
which realizes both a transmission mode display and a reflection
mode display in one liquid crystal display device has been
disclosed in, for example, Japanese Laid-Open Publication No.
7-333598. Such a liquid crystal display device uses a
semi-transmissive reflection film which transmits part of light and
reflects part of light.
[0012] FIG. 52 shows such a liquid crystal display device using a
semi-transmissive reflection film. The liquid crystal display
device includes polarizing plates 30a and 30b, a phase plate 31, a
transparent substrate 32, black masks 33, a counter electrode 34,
alignment films 35, a liquid crystal layer 36,
metal-insulator-metal (MIM) elements 37, pixel electrodes 38, a
light source 39, and a reflection film 40.
[0013] The pixel electrodes 38, which are the semi-transmissive
reflection films, are extremely thin layers made of metal particles
or layers having sporadical minute hole defects or concave defects
therein formed over respective pixels. Pixel electrodes with this
construction transmit light from the light source 39 and at the
same time reflect light from outside such as natural light and
indoor illumination light, so that both the transmission display
function and the reflection display function are simultaneously
realized.
[0014] The conventional liquid crystal display device shown in FIG.
52 has following problems. First, when an extremely thin layer of
deposited metal particles is used as the semi-transmissive
reflection film of each pixel, since the metal particles have a
large absorption coefficient, the internal absorption of incident
light is large and some of the light is absorbed without being used
for display, thereby lowering the light utilization efficiency.
[0015] When a film having sporadical minute hole defects or concave
defects therein is used as the pixel electrode 38 of each pixel,
the structure of the film is too complicated to be easily
controlled, requiring precise design conditions. Thus, it is
difficult to fabricate the film having uniform characteristics. In
other words, the reproducibility of the electrical or optical
characteristics is so poor that control of the display quality in
the above liquid crystal display device is extremely difficult.
[0016] For example, if thin film transistors (TFTs), which in
recent years have been generally used as the switching elements of
liquid crystal display devices, are attempted to be used for the
above liquid crystal display device shown in FIG. 52, an electrode
for the formation of a storage capacitor in each pixel needs to be
formed by an electrode/interconnect material other than that for
the pixel electrode. In this case, the pixel electrode made of the
semi-transmissive reflection film, as in this conventional device,
is not suitable for the formation of a storage capacitor. Moreover,
even when the semi-transmissive reflection film as the pixel
electrode is formed over part of the interconnects and elements via
an insulating layer, the pixel electrode which includes a
transmissive component hardly contributes to an increase in the
numerical aperture. Also, if light is incident on a semiconductor
layer of the switching element such as a MIM and a TFT, an
optically pumped current is generated. The formation of the
semi-transmissive reflection film as the light-shading layer is
insufficient for the protection of the switching element from
light. To ensure light-shading, another light-shading film is
required to be disposed on the counter substrate.
SUMMARY OF THE INVENTION
[0017] The liquid crystal display device of this invention includes
a first substrate, a second substrate, and a liquid crystal layer
interposed between the first substrate and the second substrate, a
plurality of pixel regions being defined by respective pairs of
electrodes for applying a voltage to the liquid crystal layer,
wherein each of the plurality of pixel regions includes a
reflection region And a transmission region.
[0018] In one embodiment of the invention, the first substrate
includes a reflection electrode region corresponding to the
reflection region and a transmission electrode region corresponding
to the transmission region.
[0019] In another embodiment of the invention, the reflection
electrode region is higher than the transmission electrode region,
forming a step on a surface of the first substrate, and thus a
thickness of the liquid crystal layer in the reflection region is
smaller than a thickness of the liquid crystal layer in the
transmission region.
[0020] In still another embodiment of the invention, the occupation
of an area of the reflection region in each of the pixel regions is
in the range of about 10 to about 90%.
[0021] Alternatively, the liquid crystal display device of this
invention includes a first substrate, a second substrate, and a
liquid crystal layer interposed between the first substrate and the
second substrate, wherein the first substrate includes: a plurality
of gate lines; a plurality of source lines arranged to cross with
the plurality of gate lines; a plurality of switching elements
disposed in the vicinity of crossings of the plurality of gate
lines and the plurality of source lines; and a plurality of pixel
electrodes connected to the plurality of switching elements, the
second substrate includes a counter electrode, a plurality of pixel
regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, and
each of the plurality of pixel regions includes a reflection region
and a transmission region.
[0022] In one embodiment of the invention, the first substrate
includes a reflection electrode region corresponding to the
reflection region and a transmission electrode region corresponding
to the transmission region.
[0023] In another embodiment of the invention, the reflection
electrode region is higher than the transmission electrode region,
forming a step on a surface of the first substrate, and thus a
thickness of the liquid crystal layer in the reflection region is
smaller than a thickness of the liquid crystal layer in the
transmission region.
[0024] In still another embodiment of the invention, the thickness
of the liquid crystal layer in the reflection region is about a
half of the thickness of the liquid crystal layer in the
transmission region.
[0025] In still another embodiment of the invention, each of the
pixel electrodes includes a reflection electrode in the reflection
electrode region and a transmission electrode in the transmission
electrode region.
[0026] In still another embodiment of the invention, the reflection
electrode and the transmission electrode are electrically connected
to each other.
[0027] In still another embodiment of the invention, each of the
pixel electrodes includes a transmission electrode, and the
reflection region includes the transmission electrode and a
reflection layer isolated from the transmission electrode.
[0028] In still another embodiment of the invention, the reflect
ion electrode regions overlap at least a portion of the plurality
of gate lines, the plurality of source lines, and the plurality of
switching elements.
[0029] In still another embodiment of the invention, at least
either of the reflection electrode regions and the transmission
electrode regions have a layer formed of the same material as a
material for the plurality of gate lines or the plurality of source
lines.
[0030] In still another embodiment of the invention, the occupation
of an area of the reflection region in each of the pixel regions is
in the range of about 10 to about 90%.
[0031] In still another embodiment of the invention, the first
substrate further includes storage capacitor electrodes for forming
storage capacitors with the pixel electrodes via an insulating
film, wherein the reflection electrode regions overlap the storage
capacitor electrodes.
[0032] In still another embodiment of the invention, the liquid
crystal display device further includes microlenses on a surface of
the first substrate opposite to the surface facing the liquid
crystal layer.
[0033] In still another embodiment of the invention, each of the
reflection electrode regions includes a metal layer and an
interlayer insulating film formed under the metal layer.
[0034] In still another embodiment of the invention, the metal
layer has a continuous wave shape.
[0035] In still another embodiment of the invention, a surface of
the interlayer insulating layer is of a concave and convex
shape.
[0036] In still another embodiment of the invention, the interlayer
insulating layer is formed of a photosensitive polymer resin
film.
[0037] In still another embodiment of the invention, the interlayer
insulating layer covers at least a portion of either the switching
element, the plurality of gate lines, or the plurality of source
lines.
[0038] In still another embodiment of the invention, the reflection
electrodes are formed at the same level as the plurality of gate
lines or the plurality of source lines.
[0039] In still another embodiment of the invention, the reflection
electrodes are formed at the same level as the plurality of gate
lines, and the reflection electrodes are electrically connected to
the gate lines for the pixel electrodes adjacent to the reflection
electrodes.
[0040] In still another embodiment of the invention, the same
signals applied to the counter electrode are applied to the
reflection electrodes.
[0041] In still another embodiment of the invention, the reflection
electrodes are formed at the same level as the plurality of gate
lines, and the reflection electrodes form storage capacitors by
overlapping drain electrodes of the switching elements or the
transmission electrodes.
[0042] In still another embodiment of the invention, the reflection
electrode is formed of Al or an Al alloy.
[0043] In still another embodiment of the invention, the
transmission electrode is formed of ITO, and a metal layer
interposes between the transmission electrode and the reflection
electrode.
[0044] According to another aspect of the invention, a method for
fabricating a liquid crystal display device is provided. The liquid
crystal display device includes a first substrate, a second
substrate, and a liquid crystal layer interposed between the first
substrate and the second substrate, the first substrate including:
a plurality of gate lines; a plurality of source lines arranged to
cross with the plurality of gate lines; a plurality of switching
elements disposed in the vicinity of crossings of the plurality of
gate lines and the plurality of source lines; and a plurality of
pixel electrodes connected to the plurality of switching elements,
the second substrate including a counter electrode, a plurality of
pixel regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, each
of the plurality of pixel regions including a reflection region and
a transmission region. The method includes the steps of: forming
the transmission electrode regions using a material having a high
light transmittance on the first substrate; forming photosensitive
polymer resin layers; and forming reflection layers made of a
material having a high reflectance on the polymer resin layers.
[0045] In one embodiment of the invention, the photosensitive
polymer resin layers have a plurality of concave and convex
portions.
[0046] Alternatively, a method for fabricating a liquid crystal
display device of this invention is provided. The liquid crystal
display device includes a first substrate, a second substrate, and
a liquid crystal layer interposed between the first substrate and
the second substrate, the first substrate including: a plurality of
gate lines, a plurality of source lines arranged to cross with the
plurality of gate lines; a plurality of switching elements disposed
in the vicinity of crossings of the plurality of gate lines and the
plurality of source lines; and a plurality of pixel electrodes
connected to the plurality of switching elements, the second
substrate including a counter electrode, a plurality of pixel
regions are defined by the plurality of pixel electrodes, the
counter electrode, and the liquid crystal layer interposed between
the plurality of pixel electrodes and the counter electrode, each
of the plurality of pixel regions including a reflection region and
a transmission region. The method includes the steps of: forming
the transmission electrode regions using a material having a high
light transmittance on the first substrate; forming protection
films on the transmission electrode regions; and forming layers
having a high reflectance on portions of the protection films to
form the reflection electrode regions.
[0047] In one embodiment of the invention, the transmission
electrode regions are formed at the same level as the plurality of
source lines.
[0048] Thus, the invention described herein makes possible the
advantages of (1) providing a liquid crystal display device of a
type realizing both a transmission mode display and a reflection
mode display simultaneously where the light utilization
efficiencies of ambient light and light from a backlight are
improved compared with the conventional liquid crystal display
device of the same type and an excellent display quality is
obtained, and (2) providing a method for fabricating such a liquid
crystal display device. In particular, in the liquid crystal
display device according to the present invention, the display
quality obtained when the environment is bright significantly
improves.
[0049] These and other advantages of the present invention will
become apparent to those skilled in the art upon reading and
understanding the following detailed description with reference to
the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0050] FIG. 1 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 1 of the present
invention;
[0051] FIG. 2 is a sectional view taken along line a-b of FIG.
1;
[0052] FIG. 3 is a plan view of another embodiment of the active
matrix substrate according to Example 1 of the present
invention;
[0053] FIG. 4 is a plan view of a still another embodiment of the
active matrix substrate according to Example 1 of the present
invention;
[0054] FIG. 5 is a plan view partially illustrating an interlayer
insulating film and a metal film of a liquid crystal display device
according to Example 2 of the present invention;
[0055] FIG. 6 is a sectional view taken along line c-d of FIG.
5;
[0056] FIG. 7 is a sectional view of a liquid crystal display
device according to Example 3 of the present invention;
[0057] FIG. 8A is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 4 of the present
invention, and
[0058] FIG. 8B is a sectional view taken along line A-A of FIG.
8A;
[0059] FIG. 9 is a sectional view of the liquid crystal display
device according to Example 4 of the present invention;
[0060] FIG. 10 is a sectional view of an alternative embodiment of
the liquid crystal display device according to Example 4 of the
present invention, provided with microlenses;
[0061] FIG. 11A is a plan view of an alternative embodiment of the
active matrix substrate of the liquid crystal display device
according to Example 4 of the present invention, and
[0062] FIG. 11B is a sectional view taken along line B-B of FIG.
11A;
[0063] FIG. 12A is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 5 of the present
invention, and
[0064] FIG. 12B is a sectional view taken along line C-C of FIG.
12A;
[0065] FIG. 13A is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 6 of the present
invention, and
[0066] FIG. 13B is a sectional view taken along line D-D of FIG.
13A;
[0067] FIG. 14A is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 7 of the present
invention, and
[0068] FIG. 14B is a sectional view taken along line E-E of FIG.
14A;
[0069] FIG. 15 is a sectional view for explaining a
reflection/transmission type liquid crystal display device
according to Example 8 of the present invention;
[0070] FIG. 16 is a graph showing the relationship of the aperture
ratio with the transmittance and reflectance of the
reflection/transmission type liquid crystal display device of
Example 8;
[0071] FIG. 17 is a graph showing the relationship between the
aperture ratio and the light transmission efficiency of the
reflection/transmission type liquid crystal display device of
Example 8;
[0072] FIG. 18 is a plan view of a reflection/transmission type
liquid crystal display device according to Example 8 of the present
invention;
[0073] FIGS. 19A to 19F are sectional views taken along line F-F of
FIG. 18, illustrating the process of fabricating the
reflection/transmission type liquid crystal display device of
Example 8;
[0074] FIGS. 20A to 20D are sectional views illustrating the steps
of forming convex portions in the reflection regions of the
reflection/transmission type liquid crystal display device of
Example 8;
[0075] FIG. 21 is a plan view of a photomask used in the step shown
in FIG. 20B;
[0076] FIG. 22 is a sectional view illustrating a method for
measuring the reflection characteristics of pixel electrodes having
a high light reflection efficiency of the reflection/transmission
type liquid crystal display device of Example 8;
[0077] FIG. 23 is a conceptual view illustrating the generation of
interference light;
[0078] FIG. 24 is a graph showing the wavelength dependence of the
pixel electrodes of the reflection/transmission type liquid crystal
display device of Example 8;
[0079] FIG. 25 is a sectional view of a transmission/reflection
type liquid crystal display device according to Example 9 of the
present invention;
[0080] FIG. 26 is a graph showing the transmittance and reflectance
in a gray-level display in Example 9;
[0081] FIG. 27 is a chromaticity diagram of a conventional
transmission type liquid crystal display device;
[0082] FIG. 28 is a chromaticity diagram of the
transmission/reflection type liquid crystal display device of FIG.
9;
[0083] FIG. 29 is a sectional view of another embodiment of the
transmission/reflection type liquid crystal display device
according to Example 9 of the present invention;
[0084] FIG. 30 is a plan view of an active matrix substrata of a
liquid crystal display device according to Example 10 of the
present invention;
[0085] FIG. 31 is a sectional view taken along line GG of FIG.
30;
[0086] FIG. 32 is a plan view of an active matrix substrata of a
liquid crystal display device according to Example 11 of the
present invention;
[0087] FIG. 33 is a sectional view taken along line H-H of FIG.
32;
[0088] FIG. 34 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 12 of the
present invention;
[0089] FIG. 35 is a sectional view taken along line I-I of FIG.
34;
[0090] FIG. 36 is a plan view of an alternative embodiment of the
active matrix substrate of the liquid crystal display device
according to Example 12 of the present invention;
[0091] FIG. 37 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 13 of the
present invention;
[0092] FIGS. 38A to 38D are sectional views taken along line J-J of
FIG. 37, illustrating the fabrication process of the active matrix
substrate of Example 13;
[0093] FIG. 39 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 14 of the
present invention;
[0094] FIGS. 40A to 40D are sectional views taken along line K-K of
FIG. 39, illustrating the fabrication process of the active matrix
substrate of Example 14;
[0095] FIG. 41 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 15 of the
present invention;
[0096] FIGS. 42A to 42C are sectional views taken along line L-L of
FIG. 41, illustrating the fabrication process of the active matrix
substrate of Example 15;
[0097] FIG. 43 is a plan view of an active matrix substrata of a
liquid crystal display device according to Example 16 of the
present invention;
[0098] FIGS. 44A to 44F are sectional views taken along line M-M of
FIG. 43, illustrating the fabrication process of the active matrix
substrate of Example 16;
[0099] FIG. 45 is a plan view of an active matrix substrate of a
liquid crystal display device according to Example 17 of the
present invention;
[0100] FIG. 46 is a sectional view taken along line N-N of FIG.
45;
[0101] FIG. 47 is a plan view of an alternative embodiment of the
active matrix substrate of the liquid crystal display device
according to Example 17 of the present invention;
[0102] FIGS. 48A to 48C are views illustrating a construction of
Example 18 where the present invention is applied to a simple
matrix liquid crystal display device;
[0103] FIGS. 49A to 49C are views illustrating another construction
of Example 18;
[0104] FIGS. 50A to 50C are views illustrating still another
construction of Example 18;
[0105] FIGS. 51A and 51B are views illustrating still another
construction of Example 18; and
[0106] FIG. 52 is a sectional view of a conventional liquid crystal
display device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
[0107] A liquid crystal display device of Example 1 according to
the present invention includes an active matrix substrate and a
transparent counter substrate (e.g., a glass substrate), which has
a counter electrode facing pixel electrodes. A liquid crystal layer
is interposed between the active matrix substrate and the counter
substrate. A plurality of pixel regions are defined by respective
pairs of the pixel electrodes and the counter electrode for
applying a voltage to the liquid crystal layer. The pixel region
includes a pair of electrodes and the liquid crystal layer between
the pair of electrodes. This definition is also applicable to a
simple matrix type liquid crystal display device, which has a
plurality of scanning electrodes and a plurality of signal
electrodes.
[0108] The liquid crystal display device according to the present
invention has at least one transmission electrode region and at
least one reflection region in each pixel. The transmission and
reflection regions include the liquid crystal layer and the pair of
the electrodes interposing the liquid crystal layer. A region of an
electrode which defines the transmission region is referred to as a
transmission electrode region and a region of an electrode which
defines the reflection region is referred to as a reflection
electrode region.
[0109] FIG. 1 is a plan view of one pixel portion of an active
matrix substrate of the liquid crystal display device of Example 1.
FIG. 2 is a sectional view taken along line a-b of FIG. 1.
[0110] Referring to FIGS. 1 and 2, the active matrix substrate
includes pixel electrodes 1 arranged in a matrix. Gate lines 2 for
supplying scanning signals and source lines 3 for supplying display
signals are disposed along the peripheries of the pixel electrodes
1 so as to cross each other at right angles.
[0111] The gate lines 2 and the source lines 3 are overlapped by
peripheral portions of the corresponding pixel electrodes 1 via an
interlayer insulating film 19. The gate lines 2 and the source
lines 3 are composed of metal films.
[0112] Thin film transistors (TFTs) 4 are formed in the vicinity of
the respective crossings of the gate lines 2 and the source lines
3. A gate electrode 12 of each of the TFTs 4 is connected to the
corresponding gate line 2, to drive the TFT 4 with a signal input
into the gate electrode 12 via the gate line 2. A source electrode
15 of the TFT 4 is connected to the corresponding source line 3, to
receive a data signal from the source line 3. A drain electrode 16
of the TFT 4 is connected to a connecting electrode 5 which is in
turn electrically connected to the corresponding pixel electrode 1
via a contact hole 6.
[0113] The connecting electrode 5 forms a storage capacitor with a
storage capacitor electrode 8 via a gate insulating film 7. The
storage capacitor electrode 8 is composed of a metal film and
connected to a counter electrode 10 formed on a counter substrate 9
via an interconnect (not Shown). The storage-capacitor electrodes 8
may be formed together with the gate lines 2 during the same
step.
[0114] Each of the pixel electrodes 1 includes a reflection
electrode region 22 including a metal film and at least one
transmission electrode region 20 composed of an ITO film. The
reflection electrode region 22 is formed to overlie the gate line
2, the source line 3, the TFT 4, and the storage capacitor
electrode 8, while the transmission electrode region 20 is
surrounded by the reflection electrode region 22.
[0115] The active matrix substrate of Example 1 with the above
construction is fabricated in the following manner.
[0116] First, the gate electrodes 12, the gate lines 2, the storage
capacitor electrodes 8, the gate insulating film 7, semiconductor
layers 13, channel protection layers 14, the source electrodes 15,
and the drain electrodes 16 are sequentially formed on a
transparent insulating substrate 11 made of glass or the like.
[0117] Then, a transparent conductive film 17 and a metal film 18
are sequentially deposited by sputtering and patterned into a
predetermined shape to form the source lines 3 and the connecting
electrodes 5.
[0118] Thus, the source lines 3 have a double-layer structure
composed of the transparent conductive film 17 made of ITO and the
metal film 18. With this structure, even if a defect such as a
disconnection is generated in the metal film 18, the electrical
connection is maintained via the transparent conductive film 17.
This reduces the generation of disconnections in the source lines
3.
[0119] Thereafter, a photosensitive acrylic resin is applied to the
resultant substrate by a spin application method to form the
interlayer insulating film 19 with a thickness of 3 .mu.m. The
acrylic resin is then exposed to light according to a desired
pattern and developed with an alkaline solution. Only the
light-exposed portions of the film are etched with the alkaline
solution to form the contact holes 6 through the interlayer
insulating film 19. By employing this alkaline development,
well-tapered contact holes 6 are obtained.
[0120] Using a photosensitive acrylic resin for the interlayer
insulating film 19 is advantageous in the aspect of productivity in
view of the following points. Since the spin application method can
be employed for the thin film formation, a film as thin as several
micrometers can be easily formed. Also, no photoresist application
step is required at the patterning of the interlayer insulating
film 19.
[0121] In this example, the acrylic resin is colored and can be
made transparent by exposing the entire surface to light after
patterning. The acrylic resin may also be made transparent by
chemical processing.
[0122] Thereafter, a transparent conductive film 21 is formed by
sputtering and patterned, thereby forming transparent conductive
films 21. The transparent conductive films 21 are made of ITO.
[0123] Thus, the transparent conductive films 21 are electrically
connected to the respective connecting electrodes 5 via the-contact
holes 6.
[0124] A metal film 23 is then formed on the transparent conductive
films 21 and patterned so as to overlie the gate lines 2, the
source lines 3, the TFTs 4, and the storage capacitor electrodes 8,
to be used as the reflection electrode regions 22 of the pixel
electrodes 1. The portions of the transparent conductive films 21
which are not covered with the metal films 23 constitute the
transmission electrode regions 20. The transparent conductive films
21 and the metal films 23 are electrically connected with each
other. Any adjacent pixel electrodes are separated by the portions
located above the gate lines 2 and, the source lines 3 so as not to
be electrically connected with each other.
[0125] The metal films 23 are made of Al. They may also be made of
any conductive material having a high reflectance such as Ta.
[0126] In this example, as shown in FIG. 2, a liquid crystal layer
includes dichromatic pigment molecules 24 mixed in liquid crystal.
The absorption coefficient of such a dichromatic pigment varies
depending on the orientation direction of molecules thereof. The
orientation direction of the dichromatic pigment molecules 24
changes when the orientation direction of the liquid crystal
molecules 15 is changed by controlling the electric field between
the counter electrode 10 and the pixel electrodes 1. The resultant
change in the absorption coefficient of the dichromatic pigment
molecules 24 is used to generate an image display.
[0127] By using the liquid crystal display panel of Example 1 with
the above construction, the display can effectively use light which
has been emitted from a backlight and passed through the
transmission electrode regions 20 when the ambient light is low and
light reflected by the reflection electrode regions 22 when the
ambient light is high. Also, both the transmission electrode
regions 20 and the reflection electrode regions 22 can be used to
generate a display. Moreover, a liquid crystal display device
providing a bright display can be realized.
[0128] In this example, the metal films 23 of the reflection
electrode regions 22 of the pixel electrodes 1 overlie the TFTs 4,
the gate lines 2, and the source lines 3. This eliminates the
necessity of providing light-shading films for preventing light
from entering the TFTs 4 and light-shading portions of the pixel
electrodes located above the gate lines, the source lines, and the
storage capacitor electrodes. In such portions, light leakage tends
to be generated in the form of domains, disclination lines, and the
like in certain display regions. As a result, regions which are
conventionally unusable as display regions because they are blocked
by the light-shading films can be used as display regions. This
allows for effective use of the display regions.
[0129] When the gate lines and the source lines are made of metal,
they serve as light-shading regions in a transmission type display
device, and thus are unusable as display regions. In the liquid
crystal display device of this example, however, such regions which
are used as light-shading regions in the conventional transmission
type display device are usable as reflection electrode regions of
the pixel electrodes. Thus, a brighter display can be obtained.
[0130] In this example, the metal film 23 is formed on the
transparent conductive film 21. This allows the metal film 23 to
have an uneven surface in compliance with an uneven surface of the
transparent conductive film 21. The uneven surface of the metal
film 23 is advantageous over a flat surface since an uneven surface
receives ambient light at various incident angles. The resultant
liquid crystal display device provides a brighter display.
[0131] FIGS. 3 and 4 are plan views of alternative embodiment of
the liquid crystal display devices of Example 1 according to the
present invention. In these alternative examples, the ratio of the
areas of the transmission electrode region 20 to the reflection
electrode region 22 of each pixel electrode 1 is changed from that
shown in FIG. 1. In this way, a liquid crystal display device
having a desired reflectance and transmittance is obtained.
[0132] In the alternative examples shown in FIGS. 3 and 4, the
connecting electrode 5 is located in the reflection electrode
region 22. This suppresses a decrease in the brightness of light
which has passed through the transmission electrode region 20.
[0133] In Example 1, the metal film 23 of the reflection electrode
region 22 of the pixel electrode 1 is formed on the transparent
conductive film 21. Alternatively, as shown in FIG. 6, the metal
film 23 may be formed so as to overlap the transparent conductive
film 21 only partially in order to be electrically connected with
each other.
EXAMPLE 2
[0134] In Example 2, a method for forming the uneven surface of the
metal film 23 will be described.
[0135] FIG. 5 is a plan view partially illustrating the metal film
23 formed on the interlayer insulating film 19 (not shown). FIG. 6
is a sectional view taken along line c-d of FIG. 5.
[0136] The surface of the interlayer insulating film 19 is made
uneven by etching or the like, and the metal film 23 is formed on
the uneven surface.
[0137] Thus, by forming the metal film 23 on the interlayer
insulating film 19 which may be first formed flat by the spin
application method or the like, but then have the surface thereof
made uneven as described above, the metal film 23 having an uneven
surface is obtained.
[0138] In a reflection type liquid crystal display device, the
uneven surface of the metal film 23 is advantageous over a flat
surface since an uneven surface receives ambient light at various
incident angles. Thus, by forming the metal films 23 of the pixel
electrodes 1 on the interlayer insulating film 19 us as to have an
uneven surface obtained by etching or the like as shown in FIG. 6,
the resultant reflective liquid crystal display device provides a
brighter display.
[0139] The uneven surface of the metal film 23 is not limited to
the shape shown in FIG. 5, i.e., the surface having concave
portions of a circular shape in plan. Alternatively, the surface of
the metal film 23 and thus the surface of the underlying interlayer
insulating film 19 may have concave portions of a polygonal or
elliptic shape in plan. The section of the concave portions may be
of a polygonal shape, in place of the semi-circular shape as shown
in FIG. 6.
EXAMPLE 3
[0140] In Example 3, a liquid crystal display device which employs
a guest-host display method will be described.
[0141] FIG. 7 is a sectional view of a liquid crystal display
device of this example according to the present invention. The same
components as those of Example 1 are denoted by the same reference
numerals as those in FIG. 2.
[0142] When the guest-host display method is employed using a
mixture of a guest-host liquid crystal material, ZLI 2327
(manufactured by Merck & Co., Inc.) containing black pigments
therein and 0.5% of an optically active substance, S-811
(manufactured by Merck & Co., Inc.), the following problem
arises. That is, if the optical path length dt of transmitted light
from the blacklight in the transmission region using the backlight
is significantly different from the optical path length 2 dr of
reflected light from ambient light in the reflection region, the
brightness and the contrast of the resultant display are
significantly different between the case where light from the
backlight is used and the case where ambient light is used even
when the same voltage is applied to the liquid crystal layer.
[0143] Accordingly, the thickness dt of the portions of the liquid
crystal layer located on the transparent conductive films 21 of the
transmission regions and the thickness dr of the portions of the
liquid crystal layer located on the metal films 23 of the
reflection regions should be set to satisfy the relationship of dt
a 2 dr. In this example, therefore, the thickness of the metal
films 23 is changed to satisfy this relationship.
[0144] Thus, by equalizing the optical path length dt of
transmitted light from the backlight in the transmission regions
and the optical path length 2 dr of reflected light from ambient
light in the reflection region, with each other, substantially the
same brightness and contrast can be obtained irrespective of which
type of light is used (light from backlight or light from ambient
light) so long as the same voltage is applied to the liquid crystal
layer. In this way, a liquid crystal display device having better
display characteristics is obtained.
[0145] The brightness and the contrast can be made uniform to some
extent by approximating, not necessarily equalizing, the optical
path length dt of transmitted light from the backlight in the
transmission region and the optical path length 2 dr of reflected
light from ambient light in the reflection region.
[0146] The contrast can also be made uniform irrespective of which
type of light is used (light from backlight or light from ambient
light) by changing the driving voltage applied to the liquid
crystal layer, even when the optical path length dt of transmitted
light in the transmission region is significantly different from
the optical path length 2 dr of reflected light in the reflection
region.
[0147] Thus, in the liquid crystal display devices in Examples 1 to
3 above, where the transmission mode display and the reflection
mode display are realized using a single substrate, the regions
which are conventionally blocked from light by the use of a black
mask can be used as reflection electrode regions of the respective
pixel electrodes. This allows for effective use of the display
regions of the pixel electrodes of the liquid crystal panel, and
thus increases the brightness of the liquid crystal display
device.
[0148] In Examples 1 to 3, the storage capacitor electrode is
provided for forming a storage capacitor with each pixel electrode
via the insulating film, and the reflection electrode region of the
pixel electrode overlies the storage capacitor electrode.
Accordingly, the region where the storage capacitor electrode is
formed can be utilized for display as a reflection electrode region
of the pixel electrode.
[0149] The metal film of the reflection electrode region of each
pixel electrode is formed on the transparent conductive film. By
using a transparent conductive film having an uneven surface, the
resultant reflection electrode region of the pixel electrode has an
uneven surface, which makes it possible to utilize ambient light
having various incident angles as display light.
[0150] The metal film of the reflection region of each pixel
electrode may be formed on an interlayer insulating film having an
uneven surface. The resultant reflection electrode region of the
pixel electrode has an uneven surface, which makes it possible to
utilize ambient light having various incident angles as display
light.
[0151] The metal film of the reflection electrode region of each
pixel electrode is made thicker than the transparent conductive
film located in the transmission region of the pixel electrode.
This make it possible to approximate the optical path length of
ambient light which passes and returns through the portion of the
liquid crystal layer located in the reflection electrode region of
the pixel electrode and the optical path length of 5 light from the
backlight which passes through the portion of the liquid crystal
layer located on the transmission electrode region of the pixel
electrode and compare the path length to each other. By knowing the
approximate optical path lengths, changes in the characteristics of
light passing through the liquid crystal layer in the reflection
region and the transmission region can be made uniform.
[0152] The thickness of the portion of the liquid crystal layer
located on the reflection electrode region of each pixel electrode
is made one half of the thickness of the portion of the liquid
crystal layer located on the transmission electrode region thereof.
This makes it possible to approximate the optical path length of
ambient light which passes and returns through the portion of the
liquid crystal layer located on the reflection electrode region of
the pixel electrode and the optical path length of light from the
backlight which passes through the portion of the liquid crystal
layer located on the transmission electrode region of the pixel
electrode and compare the path length to each other. By knowing the
approximate optical path lengths, changes in the characteristics of
light passing through the liquid crystal layer in the reflection
region and the transmission region can be made uniform.
EXAMPLE 4
[0153] FIG. 8A is a plan view of one pixel portion of an active
matrix substrate of a liquid crystal display device of Example 4
according to the present invention. FIG. 8B is a sectional view
taken along line A-A of FIG. 8A.
[0154] The active matrix substrate of this example includes gate
lines 41, data lines 42, driving elements 43, drain electrodes 44,
storage capacitor electrodes 45, a gate -insulating film 46, an
insulating substrate 47, contact holes 48, an interlayer insulating
film 49, reflection pixel electrodes 50, and transmission pixel
electrodes 51.
[0155] Each of the storage capacitor electrodes 45 is electrically
connected to the corresponding drain electrode 44 and overlaps the
corresponding gate line 41 via the gate insulating film 46. The
contact holes 48 are formed through the interlayer insulating film
49 to connect the transmission pixel electrodes 51 and the storage
capacitor electrodes 45.
[0156] Each pixel of the active matrix substrate with the above
construction includes a reflection pixel electrode 50 and a
transmission pixel electrode 51. Thus, as shown in FIG. 8B, each
pixel is composed of the reflection electrode region, including the
reflection pixel electrode 50, which reflects light from outside,
and the transmission electrode region, including the transmission
pixel electrode 51, which transmits light from a backlight.
[0157] FIG. 9 is a sectional view of a liquid crystal display
device of this example including the active matrix substrate shown
in FIGS. 8A and 8B. The liquid crystal display device also includes
a color filter layer 53, a counter electrode 54, a liquid crystal
layer 55, alignment films 56, a polarizing plate 57, and a
backlight 58.
[0158] The regions of the transmission pixel electrodes 51
(transmission electrode region) which transmit light from the
backlight 58 do not contribute to the brightness of the panel when
the backlight 58 is off. Conversely, the regions of the reflection
pixel electrodes 50 (reflection electrode region) which reflect
light from outside contribute to the brightness of the panel
regardless of the ON/OFF state of the backlight 58. In each pixel,
therefore, the area of the reflection electrode region is desirably
larger than the area of the transmission electrode region.
[0159] In this example, the reflection pixel electrode 50 is formed
on the corresponding transmission pixel electrode 51 so as to be
electrically connected to each other so that the same signals are
input into the reflection pixel electrode 50 and the transmission
pixel electrode 51. Alternatively, the reflection pixel electrode
50 and the transmission pixel electrode 51 may not be electrically
connected to each other so as to receive different signals for
different displays.
[0160] In the liquid crystal display device shown in FIG. 9, part
of the light from the backlight 58 incident on the reflection pixel
electrode 50 is not usable as display light. In order to overcome
this problem, a modified liquid crystal display device shown in
FIG. 10 includes a microlens 59 and a microlens protection layer 60
for each pixel. With this construction, light from the backlight 58
is converged on the transmission electrode region on, which the
reflection pixel electrode 50 is not formed, via the microlens 59,
to increase the amount of light which passes through transmission
region and thus to improve the brightness of display.
[0161] FIG. 11A is a plan view of one pixel portion of an
alternative active matrix substrate of the liquid crystal display
device of Example 4 according to the present invention. FIG. 11B is
a sectional view taken along line B-B of FIG. 11A.
[0162] In the active matrix substrate shown in FIGS. 11A and 11B,
the region of the transmission pixel electrode 51 and the region of
the reflection pixel electrode 50 of each pixel are reversed from
those of the active matrix substrate shown in FIGS. 8A and 8B. The
ratio of the areas of the region of the reflection pixel electrode
50 and the region of the transmission pixel electrode 51 may be
changed appropriately.
[0163] When the active matrix substrate shown in FIGS. 8A and 8B
and that shown in FIGS. 11A and 11B are compared, the active matrix
substrate shown in FIGS. 8A and 8B is advantageous in the points
that light from outside is prevented from entering the driving
element 43 since the reflection pixel electrode 50 is formed over
the driving element 43 and that the formation of the microlens 59
for converging light is easier since the region of the transmission
pixel electrode 51 is located in the center of each pixel.
[0164] In this example, since the light reflection region and the
light transmission region are formed in one pixel, the aperture
ratio of the pixel is as large as possible. To satisfy this, a high
aperture structure is adopted in this example where the interlayer
insulating film 49, composed of an organic insulating film, is
interposed between the pixel electrodes and the levels of the gate
lines 41 and the source lines 43. Other structures may also be
adopted.
EXAMPLE 5
[0165] FIG. 12A is a plan view of one pixel portion of an active
matrix substrate of a liquid crystal display device of Example 5
according to the present invention. FIG. 12B is a sectional view
taken along line C-C of FIG. 12A.
[0166] In the active matrix liquid crystal display device of
Example 5, reflection pixel electrodes 50 are formed on tilted or
concave and convex portions of an interlayer insulating film 49.
Light from outside is therefore reflected from the reflection pixel
electrodes 50 in a wider range of directions, so that the angle of
visibility becomes wider.
[0167] The interlayer insulating film 49 in this example is formed
so as to be thickest at portions located above gate lines 41 and
source lines 42 and be completely etched away at portions located
above drain electrodes 44, forming the tilted or concave and convex
portions. This eliminates the necessity of forming contact holes
for electrically connecting the drain electrodes 44 and the
reflection pixel electrodes 50, and thus prevents a disturbance in
the orientation of liquid crystal molecules from occurring due to
sharp steps at contact holes. This contributes to an increase in
the aperture ratio.
[0168] In this example, the drain electrodes 44, which are
transparent electrodes made of ITO, serve as the transmission pixel
electrodes 51.
[0169] The tilt angle of the tilted portions or the pitch of the
concave and convex portions of the interlayer insulating film 49
should be sufficiently small so that an alignment film can be
formed on the resultant substrate and rubbed. Thus, optimal
conditions should be determined depending on the respective rubbing
conditions and the types of liquid crystal molecules.
[0170] In this example, as in Example 4, microlenses may be
provided below the drain electrodes 44 as the transmission pixel
electrodes 51, to improve the brightness of the display when the
backlight is on.
EXAMPLE 6
[0171] FIG. 13A is a plan view of one pixel portion of an active
matrix substrate of a liquid crystal display device of Example 6
according to the present invention. FIG. 13B is a sectional view
taken along line D-D of FIG. 13A.
[0172] In this example, reflection pixel electrodes 50 are formed
at the same level as gate lines 41 at and during the same step.
With this configuration, since a separate step for forming the
reflection pixel electrodes 50 is not required, the number of steps
and the production cost do not increase.
[0173] In this example, the reflection pixel electrodes 50 are not
connected to drain electrodes 44 constituting driving elements 43,
but are used only for the reflection of light from outside. Only
the transmission pixel electrodes 51 serve as the electrodes for
driving the liquid crystal. In other words, the transmittance of
light reflected by the reflection pixel electrodes 50 is controlled
by controlling the liquid crystal layer with a voltage at the
transmission pixel electrodes 51.
[0174] If no signal is input into each of the reflection pixel
electrodes 50, a floating capacitance is generated between the
reflection pixel electrode 50 and the corresponding drain electrode
44 or transmission pixel electrode 51. To avoid this problem, the
reflection pixel electrodes 50 should desirably be provided with
such a signal that does not adversely affect the display. By
connecting each of the reflection pixel electrodes 50 with an
adjacent gate line 41, the generation of a floating capacitance is
prevented, and a storage capacitor can be formed between a
reflection pixel electrode 50 and a corresponding drain electrode
44.
[0175] In this example, as in Example 4, microlenses may be
provided to converge light on the transmission pixel electrodes, to
improve the brightness of display when the backlight is on.
[0176] In this example, also, since the light reflection region and
the light transmission region are formed in one pixel, the aperture
ratio of the pixel is as large as possible. To satisfy this, a high
aperture structure is adopted where an organic insulating film is
used as the interlayer insulating film. Other structures may also
be adopted.
EXAMPLE 7
[0177] FIG. 14A is a plan view of one pixel portion of an active
matrix substrate of a liquid crystal display device of Example 7
according to the present invention. FIG. 14B is a sectional view
taken along line E-E of FIG. 14A.
[0178] In this example, reflection pixel electrodes 50 are formed
at the same level as source lines 42. With this configuration,
since the reflection pixel electrodes 50 can be formed at the
formation of the source lines 42, the number of steps and the
production cost do not increase.
[0179] In this example, since a high aperture structure via an
interlayer insulating film 49 is adopted, the reflection pixel
electrodes 50 are used only for the reflection of light from
outside. Only transmission pixel electrodes 51 serve as the
electrodes for driving the liquid crystal.
[0180] This example is different from Example 6 in that in this
example the reflection pixel electrode 50 in each pixel is
electrically connected to the corresponding drain electrode 44. In
an alternative case where the interlayer insulating film 49 is not
formed at the region above the drain electrode 44 and the drain
electrode 44 is used as the transmission pixel electrode, the
reflection pixel electrode 50 also contributes to the driving of
the liquid crystal molecules.
[0181] In this example, as in Example 4, microlenses may be
provided to converge light on the transmission pixel electrodes 51,
to improve the brightness of display when the backlight is on.
[0182] In this example, also, since the light reflection region and
the light transmission region are formed in one pixel, the aperture
ratio of the pixel is as large as possible. To satisfy this, a high
aperture structure is adopted where an organic insulating film is
used as the interlayer insulating film. Other structures may also
be adopted.
[0183] Thus, in Examples 4 to 7 above according to the present
invention, the active matrix liquid crystal display device capable
of switching between the reflection type and the transmission type
is realized.
[0184] Such a liquid crystal display device can provide a
sufficient brightness irrespective of the conditions of use, while
realizing a reduced power consumption and a prolonged use duration,
by the user's switching the mode between the transmission type and
the reflection type depending on the use conditions.
[0185] Also realized is a transmission/reflection switchable active
matrix liquid crystal display device which can be used as a
reflection type liquid crystal display device when the environment
is bright and as a transmission type liquid crystal display device
when the environment is dark.
[0186] Since the reflection pixel electrodes and the transmission
pixel electrodes are electrically connected with each other, no
interconnect is required to supply the driving signals
-independently. This simplifies the construction of the active
matrix substrate.
[0187] When the reflection pixel electrodes are formed above the
driving elements, light from outside is prevented from entering the
driving elements.
[0188] The transmission pixel electrodes do not contribute to the
brightness of the panel when the backlight is off, while the
reflection pixel electrodes contribute to the brightness of the
panel regardless of the ON/OFF state of the backlight. Accordingly,
by increasing the area of the reflection pixel electrodes, the
brightness of display can be stabilized even when the backlight is
off or emits less light.
[0189] Light from the backlight which is blocked by the reflection
pixel electrodes, the gate lines, and the like can be converged on
the transmission pixel electrodes. This makes it possible to
increase the brightness of the display device without increasing
the brightness of the backlight itself.
[0190] The reflection pixel electrodes can be made to reflect light
from outside in a wide range of directions. This allows for a wider
angle of visibility.
[0191] The reflection pixel electrodes may be formed without an
additional step for this formation. This prevents the number of
steps and the production cost from increasing.
[0192] The reflection pixel electrodes may be electrically
connected to the gate lines. This prevents the generation of a
floating capacitance and allows for the formation of a storage
capacitor with the drain electrodes.
[0193] The reflection pixel electrodes may be provided with the
same signals as those applied to the counter electrode. This
prevents the generation of a floating capacitance. Also, the
reflection pixel electrodes may be used for the formation of a
storage capacitor for the voltage applied to the pixel
electrodes.
EXAMPLE 8
[0194] In Example 8, a reflection/transmission type liquid crystal
display device according to the present invention will be
described.
[0195] First, the principle of the generation of an interference
color in the liquid crystal display device of Example 8 will be
described.
[0196] FIG. 23 is a conceptual view illustrating the generation of
an interference color. Light is incident on a glass substrate and
the incident light is reflected by a reflection film to be output
from the glass substrate.
[0197] In the above case, an interference color is considered to be
generated when light incident at an incident angle .theta.i is
reflected from a convex portion and a concave portion of the
reflection film and output at an output angle .theta.o. The optical
path difference .delta. between the two reflected light beams is
represented by expression (1) below: 1 = L sin i + h ( 1 / cos i '
+ 1 / cos o ' ) n - { L sin o + h ( tan i ' + tan o ' ) sin o } = L
( sin i - sin o ) + h { ( 1 / cos i ' + 1 / cos o ' ) n - ( tan i '
+ tan o ' ) sin o } ( 1 )
[0198] wherein .theta.i' is the incident angle at the concave
portion of the reflection film, .theta.o' is the output angle at
the concave portion of the reflection film, L is the distance
between the incident points of the two light beams on the glass
substrate, h is the height of the point on the convex portion of
the reflection film from which one of the light beams is reflected,
with respect to the point on the concave portion thereof from which
the other light beam is reflected, and n is the refractive index of
the glass substrate.
[0199] Since the calculation of expression (1) is possible only
when .theta.i=.theta.o and .theta.i'=.theta.o', the optical path
difference .delta. is simplified into expression (2) below when
.theta.i=.theta.o=.theta. and .theta.i'=.theta.o=.theta.'.
.delta.=h{2n/cos .theta.'-2 tan .theta.'-sin .theta.} (2)
[0200] When arbitrary wavelengths .lambda.1 and .lambda.2 are taken
into consideration, the output light beams reflected from the
convex portion and the concave portion are weakened by each other
when .delta./.lambda.1=m.+-.1/2 (m is an integer) and intensified
by each other when .delta./.lambda.2=m. Thus, expression (3) below
is established.
.delta.=(1/.lambda.1-1/.lambda.2)=1/2 (3)
[0201] Expression (3) above is also represented by expression (4)
below:
.delta.=(.lambda.1.multidot..lambda.2)/2.multidot.(.lambda.2-.lambda.1)
(4)
[0202] Accordingly, from expressions (2) and (4) above, the height
h can be represented by expression (5) below: 2 h = 1 / 2 { ( 1 2 )
/ ( 2 1 ) } { cos ' / ( 2 n - 2 sin ' sin ) } ( 5 )
[0203] From the above, it has been found that, in order to
eliminate the generation of an interference color, the reflection
surface of the reflection film should have a continuous wave shape.
film, at least two types of convex portions with different heights
are formed on a base plate, a polymer resin film is formed on the
base plate covering the convex portions, and a reflection thin film
made of a material having a high light reflection efficiency is
formed on the polymer resin film.
[0204] The thus-fabricated reflection thin film can be used for the
reflection portions of the reflection/transmission type liquid
crystal display device. Since such reflection portions have a
reflection surface of a continuous wave shape, light reflected from
the reflection portions is prevented from generating an
interference. When the convex portions are optically formed by use
of a photomask, they can be formed with good reproducibility by
setting the same light irradiation conditions.
[0205] In the reflection/transmission type liquid crystal display
device of this example, the convex portions are preferably not
formed in the transmission portions made of a material having a
high light transmission efficiency in order to improve the
transmission efficiency. However, the display by use of transmitted
light is possible even if the convex portions are formed in the
transmission portions.
[0206] FIG. 15 is a sectional view of a reflection/transmission
type liquid crystal display device of this example according to the
present invention.
[0207] Referring to FIG. 15, a gate insulating film 61a is formed
On a glass substrate 61. High convex portions 64a and low convex
portions 64b are formed randomly on the portions of the glass
substrate 61 located below reflection electrodes 69 having a light
reflection function. The high convex portions 64a and the low
convex portions 64b are covered with a polymer resin film 65.
[0208] Since the high convex portions 64a and the low convex
portions 64b are formed on the glass substrate 61 via the gate
insulating film 61a, the upper surfaces of the portions of the
polymer resin film 65 formed on the high convex portions 64a and
the low convex portions 64b are of a continuous wave shape. The
polymer resin film 65 is formed almost all over the glass substrate
61, not only in the regions below the reflection electrodes 69.
[0209] The reflection electrodes 69, which are made of a material
having a light reflection function, are formed on the portions of
the polymer resin film 65 having the continuous wave shape which
are formed on the high convex portions 64a and the low convex
portions 64b.
[0210] Transmission electrodes 68 are also formed on the glass
substrate 61 via the gate insulting film 61a, separately from the
reflection electrodes 69. The transmission electrodes 68 are made
of a material having a light transmission function, such as indium
tin oxide (ITO).
[0211] A polarizing plate 90 is attached to the back surface of the
thus-fabricated active matrix substrate when it is mounted as a
module. A backlight 91 is then disposed on the polarizing plate
90.
[0212] Part of light emitted form the backlight 91 and directed to
the transmission electrodes 68 passes through the transmission
electrodes 68 and thus the active matrix substrate. However, part
of light directed to the reflection electrodes 69 is reflected from
the back surfaces of the reflection electrodes 69 to return to the
backlight 91. Since the back surfaces of the reflection electrodes
69 are of a continuous wave shape, light reflected from the
reflection electrodes 69 is scattered as shown by the arrows in
FIG. 15. Such scattered light is again reflected from the backlight
91 toward the active matrix substrate. Part of such light passes
through the transmission electrodes 68 and thus the active matrix
substrate.
[0213] Thus, in the active matrix substrate including the
reflection electrodes 69 of the above-described shape, the light
from the backlight reflected by the reflection electrodes 69 can be
used for display. This allows for more effective use of light than
that expected from an actual aperture ratio, unlike the
conventional transmission type liquid crystal display device.
Specifically, if the reflection electrodes are of a flat shape,
regular reflection is mainly generated, which is difficult to be
reflected again to pass through the transmission electrodes 68. In
this example, however, the reflection electrodes 69 of a continuous
wave shape serve to return the reflected light toward the portions
of the backlight located below the transmission electrodes 68,
allowing for further effective use of light.
[0214] FIG. 16 is a graph showing the relationship of the aperture
ratio to the transmittance and reflectance observed when the
reflectances of the reflection electrodes 69 and the backlight 91
as compared with the standard white plate are about 90%, and the
transmittance of the polarizing plate 90 is about 40%. Note that
this relationship was calculated on the assumption that pixel
electrodes cover the entire display surface, not considering the
existence of bus lines and active elements.
[0215] As is observed from FIG. 16, the reflectance of the
reflection electrode 69 for light incident from outside on the side
of a counter substrate is obtained by multiplying the reflectance
of the reflection electrode 69 by the ratio of the area of the
reflection electrode 69 to the area of the entire pixel electrode.
The transmittance of the transmission electrode 68 for light from
the backlight 91 is equal to, not just the aperture ratio a (i.e.,
the ratio of the area of the transparent electrode 68 to the area
of the entire pixel electrode), but a value b, including a
component of light from the backlight reflected by the reflection
electrode 69, which can be utilized for display added to the
aperture ratio a.
[0216] Thus, since the light from the backlight 91 reflected by the
reflection electrodes 69 is also utilized, more effective use of
light than that expected from the actual aperture ratio is
possible, unlike the conventional transmission type liquid crystal
display device.
[0217] FIG. 17 is a graph showing the relationship between the
aperture ratio and the light transmission efficiency
(transmittance/aperture ratio). As is observed from FIG. 17, it has
been found from such a calculation that, when the aperture ratio is
40%, the light from the backlight 91 reflected by the reflection
electrode 69 can be utilized up to about 50% of the intensity of
the light which has directly passed through the transmission
electrode 68 from the backlight 91. From the calculation results
shown in FIG. 17, it has also been found that the greater the ratio
of the area of the reflection electrode 69 to the area of the
entire pixel electrode is, the higher the use efficiency of the
light reflected by the reflection electrode 69 becomes.
[0218] Hereinbelow, a specific example of the
reflection/transmission type liquid crystal display device of
Example 8 will be described.
[0219] FIG. 18 is a plan view of the reflection/transmission type
liquid crystal display device of Example 8 according to the present
invention. FIGS. 19A to 19F are sectional views taken along line
F-F of FIG. 18, illustrating the process of fabricating the liquid
crystal display device of this example.
[0220] Referring to FIGS. 18 and 19F, an active matrix substrate 70
of the reflection/transmission type liquid crystal display device
includes a plurality of gate bus lines 72, as scanning lines, and a
plurality of source bus lines 74, as signal lines, which are formed
to cross with each other. In each of the rectangular regions
surrounded by the adjacent gate bus lines 72 and the adjacent
source bus lines 74, a transmission electrode 68 made of a material
having a high light transmission efficiency and a reflection
electrode 69 made of a material having a high reflection efficiency
are disposed. The transmission electrode 68 and the reflection
electrode 69 constitute one pixel electrode.
[0221] A gate electrode 73 extends from the gate bus line 72 toward
the pixel electrode at a corner portion of each of the region where
the pixel electrode is formed. A thin film transistor (TFT) 71 is
formed as a switching element at the end portion of the gate
electrode 73. The gate electrode 73 itself constitutes part of the
TFT 71.
[0222] The TFT 71 is located above the gate electrode 73 formed on
a glass substrate 61 as shown in FIG. 19F. The gate electrode 73 is
covered with a gate insulating film 61a, and a semiconductor layer
77 is formed on the gate insulating film 61a so as to cover the
gate electrode 73 via the gate insulating film 61a. A pair of
contact layers 78 are formed on the side portions of the
semiconductor layer 77.
[0223] A source electrode 75 is formed on one of the contact layers
78 and electrically connected to the corresponding source bus line
74. The side portion of the source electrode 75 overlaps the gate
electrode 73 in an insulating manner, constituting part of the TFT
71. A drain electrode 76, which also constitutes part of the TFT
71, is formed on the other contact layer 78 so as to be away from
the source electrode 75 and overlaps the gate electrode 73 in an
insulating manner. The drain electrode 76 is electrically connected
to the pixel electrode via an underlying electrode 81a.
[0224] A storage capacitor is formed by forming the underlying
electrode 81a so as to overlap the gate bus line 72 used for the
adjacent pixel electrode in the next pixel row via the gate
insulating film 61a. The underlying electrode 81a may be formed
over substantially the entire region where convex portions are
formed as will be described hereinafter, so as to unify the
influence of the process of forming this layer.
[0225] High convex portions 64a and low convex portions 64b and an
overlying polymer resin film 65 are formed under each of the
reflection electrodes 69.
[0226] The upper surface of the polymer resin film 65 is of a
continuous wave shape reflecting the existence of the convex
portions 64a and 64b. Such a polymer resin film 65 is formed over
substantially the entire glass substrate 61, not only in the
regions below the reflection electrodes 69. In this example,
OFPR-800 manufactured by Tokyo Ohka Co., Ltd., for example, is used
for the polymer resin film 65.
[0227] The reflection electrode 69 is formed on the portion of the
polymer resin film 65 having the continuous wave shape which is
formed on the high convex portions 64a and the low convex portions
64b. The reflection electrode 69 is made of a material having a
high reflection efficiency, such as Al. The reflection electrode 69
is electrically connected to the corresponding drain electrode 76
via a contact hole 79.
[0228] In each pixel of the reflection/transmission type liquid
crystal display device of this example, the transmission electrode
68 is formed separately from the reflection electrode 69. The
transmission electrode 68 is made of a material having a high light
transmission efficiency such as ITO.
[0229] The method for forming the reflection electrodes 69 and the
transmission electrodes 68 which are main portions of the
reflection/transmission type active matrix substrate 70 will be
described with reference to FIGS. 19A to 19F.
[0230] First, as shown in FIG. 19A, the plurality of gate bus lines
72 (see FIG. 18) made of Cr, Ta, or the like and the gate
electrodes 73 extending from the gate bus lines 72 are formed on
the glass substrate 61.
[0231] The gate insulating film 61a made of SiN.sub.x, SiO.sub.x,
or like is formed on the entire surface of the glass substrate 61
covering-the gate bus lines 72 and the gate electrodes 73. The
semiconductor layers 77 made of amorphous silicon (a-Si),
polysilicon, CdSe, or the like are formed on the portions of the
gate insulating film 61a located above the gate electrodes 73. The
pair of contact layers 78 made of a-Si or the like are formed on
both side portions of each of the semiconductor layers 77.
[0232] The source electrode 75 made of Ti, Mo, Al, or the like is
formed on one of the contact layers 78, while the drain electrode
76 made of Ti, Mo, Al, or the like is formed on the other contact
layer 78.
[0233] In this example, as the material of the glass substrate 61,
product number 7059 manufactured by Corning Inc. with a thickness
of 1.1 mm was used.
[0234] As shown in FIG. 19B, a metal layer 81 which constitutes
part of the source bus lines 74 is formed by sputtering. The metal
layer 81 is also used to form the underlying electrodes 81a.
[0235] Subsequently, as shown in FIG. 19C, an ITO layer 80 which
also constitutes part of the source bus lines 74 is formed by
sputtering and patterned.
[0236] Thus, in this example, the source bus lines 74 are of a
double-layer structure consisting of the metal layer 81 and the ITO
layer 80. This double-layer structure is advantageous in that even
if the metal film 81 constituting the source bus line 74 is partly
defective, the electric connection of the source bus line 74 is
maintained by the ITO layer 80. This reduces the occurrence of
disconnections in the source bus line 74.
[0237] The ITO layer 80 is also used to form the transmission
electrodes 68. This makes it possible to form the transmission
electrodes 68 simultaneously with the formation of the source bus
lines 74, preventing an increase in the number of layers.
[0238] Then, as shown in FIG. 19D, rounded convex portions 64a and
64b, having substantially circular cross-sections are formed of a
resist film of a photosensitive resin over the regions on which the
reflection electrodes 69 are to be formed. Preferably, the convex
portions 64a and 64b are not formed on the transmission electrodes
68 so that a voltage is efficiently applied to the liquid crystal
layer. Optically, however, not so large influence is observed when
the convex portions 64a and 64b are formed on the transmission
electrodes 68.
[0239] Hereinbelow, the process of forming the convex portions 64a
and 64b in the reflection electrode regions will be briefly
described with reference to FIGS. 20A to 20D.
[0240] First, as shown in FIG. 20A, a resist film 62 made of a
photosensitive resin is formed on the glass substrate 61 (actually,
with the metal layer 81 and the underlying electrode 81a formed
thereon as shown in FIG. 19D) by a spin coat method. The resist
film 62 is formed of the same photosensitive resin as that used for
the polymer resin film 65 to be described hereinafter, i.e.,
OFPR-800, by spin coating at a speed preferably in the range of
about 500 to about 3000 rpm, in this example at 1500 rpm, for 30
seconds, so as to obtain a thickness of 2.5 um.
[0241] Then, the resultant glass substrate 61 with the resist film
62 formed thereon is prebaked at 90.degree. C. for 30 minutes, for
example.
[0242] Subsequently, as shown in FIG. 20B, a photomask 63 is
disposed above the resist film 62. The photomask 63 has a shape as
shown in FIG. 21, for example, which includes two types of circular
pattern holes 63a and 63b formed through a plate 63c. The photomask
63 is then irradiated with light from above as shown by the arrows
In FIG. 20B.
[0243] The photomask 63 in this example has the circular pattern
holes 63a with a diameter of 5 .mu.m and the circular pattern holes
63b with a diameter of 3 .mu.m arranged at random. The space
between any adjacent pattern holes should be at least about 2
.mu.m. If the space is too large, however, the polymer resin film
65 to be formed thereon at a later step will hardly succeed in
obtaining a continuous wave shape.
[0244] The resultant substrate is developed with a developer with a
concentration of 2.38%, e.g., NMD-3 manufactured by Tokyo Ohka Co.,
Ltd. As a result, as shown in FIG. 20C, a number of minute convex
portions 64a' and 64b' with different heights are formed on the
reflection electrode regions of the glass substrate 61. The
top-edges of the convex portions 64a' and 64b' are squared. The
convex portions 64a' with a height of 2.48 .mu.m and the convex
portions 64b' with a height of 1.64 .mu.m are formed from the
pattern holes 63a with a diameter of 5 .mu.m and the pattern holes
63b with a diameter of 3 .mu.m, respectively.
[0245] The heights of the convex portions 64a' and 64b' can be
changed by changing the sizes of the pattern holes 63a and 63b, the
light exposure time, and the developing time. The size of the
pattern holes 63a and 63b are not limited to those described
above.
[0246] Thereafter, as shown in FIG. 20D, the glass substrate 61
with the convex portions 64a' and 64b' formed thereon is heated at
about 200.degree. C. for one hour. This softens the square top
edges of the convex portions 64a' and 64b', to form the rounded
convex portions 64a and 64b having substantially circular
cross-sections.
[0247] Then, as shown in FIG. 19E, a polymer resin is applied on
the resultant glass substrate 61 by spin coating and patterned to
form the polymer resin film 65. The material OFPR-800 mentioned
above is used as the polymer resin and applied by spin coating at a
speed preferably In the range of about 1000 to about 3000 rpm. In
this example, the spin coating was conducted at a speed of 2000
rpm.
[0248] In this way, the polymer resin film 65 having an upper
surface of a continuous wave shape is obtained on the glass
substrate 61, which is flat having no convex portions.
[0249] As shown in FIG. 19F, the reflection electrodes 69 made of
Al are formed on predetermined portions of the polymer resin film
65 by sputtering, for example. Materials suitable for the
reflection electrodes 69 include, besides Al and an Al alloy, Ta,
Ni, Cr, and Ag having a high light reflection efficiency. The
thickness of the reflection electrodes 69 is preferably in the
range of about 0.01 to about 1.0 .mu.m.
[0250] A polarizing plate (not shown) is attached to the back
surface of the-thus-fabricated active matrix substrate of this
example. A backlight is then disposed on the outer surface of the
polarizing plate.
[0251] Electric corrosion is generated if the Al film is formed
after the portions of the polymer resin film 65 located on the
transmission electrodes 68 are removed.
[0252] Therefore, the portions of the polymer resin film 65 located
on the transmission electrodes 68 should be removed after the
formation of the reflection electrodes 69. This removal can be done
by ashing, together with the removal of the portions of the polymer
resin film 65 located above terminal electrodes for the connection
of drivers formed on the peripheries of the active matrix substrate
70. This improves the process efficiency and allows for efficient
voltage application to the liquid crystal layer.
[0253] If the polymer resin film 65 is not used in the process of
forming the convex portions, a layer of Mo or the like may be
formed between the transmission electrodes 68 made of ITO and the
reflection electrodes 69 made of Al, to prevent the generation of
electric corrosion.
[0254] The thus-formed reflection electrodes 69, made of a material
having a high light reflection efficiency, have an upper surface in
a continuous wave shape since the underlying polymer resin film 65
has the continuous wave shape as described above.
[0255] In this example, the transmission electrodes 68 are formed
simultaneously with the formation of the source bus lines 74. When
the source bus lines 74 are of a single-layer structure composed of
the metal layer 81, not the double-layer structure composed of the
metal layer 81 and the ITO layer 80 as described above, the
transmission electrodes 68 may be formed separately from the
formation of the source bus lines 74.
[0256] The wavelength dependence of light reflected from the
reflection electrodes 69 having a continuous wave shape and made of
a material having a high light reflection efficiency was measured
in a manner as shown in FIG. 22. An object structure for
measurement was formed by simulating conditions for the reflection
electrodes 69 equivalent to an actual liquid crystal display device
during an actual use. Specifically, a dummy glass 66 having a
refractive index of 1.5, which is substantially equal to the
refractive index of the actual liquid crystal layer is attached to
the active matrix substrate 70, with the reflection electrodes 69
and the transmission electrodes 68 formed thereon with an
ultraviolet-setting adhesive 67 having a refractive index of about
1.5.
[0257] As the measurement system, a light source L1 is disposed so
that an incident light beam L1' is incident at an incident angle
.theta.i with respect to the normal ml of the dummy glass 66, and a
photomultimeter L2 is disposed so as to capture a fixed-angle light
beam reflected at an output angle .theta.o with respect to the
normal m2.
[0258] With the above construction, the photomultimeter L2 captures
the intensity of a scattered light beam L2' which is reflected at
the output angle .theta.o among scattered light beams which are
incident on the dummy glass 66 at the incident angle .theta.i, as
the incident light beam L1'.
[0259] The above measurement was conducted under the conditions of
.theta.i=30.degree. and .theta.o=20.degree. in order to avoid the
photomultimeter L2 from capturing a regular-reflected light beam
which is emitted from the light source L1 and reflected from the
surface of the dummy glass 66.
[0260] FIG. 24 is a graph showing the wavelength dependence of
reflected light in this example.
[0261] As shown in FIG. 24, the wavelength dependence of the
reflectance is hardly recognized in this example, which proves that
a good white color display is obtained.
[0262] In this example, the shape of the pattern holes 63a and 63b
of the photomask 63 is a circle. Other shapes such as a rectangle,
an ellipse, and a stripe may also be used.
[0263] In this example, the convex portions 64a and 64b with two
different heights are formed. Alternatively, convex portions with a
single height or those with three or more different heights may
also be formed to obtain reflection electrodes having good
reflection characteristics.
[0264] It has been found, however, that reflection electrodes with
better wavelength dependence of the reflection characteristics are
obtained when convex portions with two or more different heights
are formed than when convex portions with a single height are
formed.
[0265] If it is ensured that the upper surface of a continuous wave
shape can be obtained only by the convex portions 64a and 64b, the
formation of the polymer resin film 65 is not required. Only the
resist film 62 (See FIGS. 20B and 20C) is formed to obtain the
upper surface of a continuous wave shape and then the reflection
electrodes 69 are formed thereon. In this case, the step of forming
the polymer resin film 65 can be omitted.
[0266] In this example, OFPR-800 manufactured by Tokyo Ohka Co.,
Ltd. is used as the photosensitive resin material. Any other
photosensitive resin material of the negative or positive type
which can be patterned by an exposure process may also be used.
Examples of such photosensitive resin materials include: OMR-83,
OMR-85, ONNR-20, OFPR-2, OFPR-830, and OFPR-500 manufactured by
Tokyo Ohka Co., Ltd.; TF-20, 1300-27, and 1400-27 manufactured by
Shipley Co.; Photoneath manufactured by Toray Industries, Inc.;
RW-101 manufactured by Sekisui Fine Chemical Co., Ltd.; and R101
and R633 manufactured by Nippon Kayaku K.K.
[0267] In this example, the TFTs 71 are used as the switching
elements. The present invention is also applicable to active matrix
substrates using other switching elements such as
metal-insulator-metal (MIM) elements, diodes, and varistors.
[0268] Thus, as described above, in the liquid crystal display
device and the method for fabricating the liquid crystal display
device of Example 8, the reflection electrodes made of a material
having a high light reflection efficiency are formed so as to have
a continuous wave shape. This reduces the wavelength dependence of
the reflection and thus permit realization of a good white color
display by reflection without the generation of an interference
color.
[0269] Since the convex portions are formed on the substrate by an
optical technique using a photomask, good reproducibility is
ensured. The resultant wave-shaped upper surfaces of the reflection
electrodes can also be obtained with good reproducibility.
[0270] The transmission electrodes made of a material having a high
light transmission efficiency are formed simultaneously with the
formation of the source bus lines. This allows for the formation of
the transmission electrodes of the reflection/transmission type
liquid crystal display device without increasing the number of
steps compared with the conventional liquid crystal display
device.
[0271] By forming a continuous wave shape for the reflection
electrodes, more effective use of light than that expected from the
actual aperture ratio is possible.
[0272] According to the liquid crystal display device of this
example, the reflection portion made of a material having a high
light reflection efficiency and a transmission portion made of a
material having a high light transmission efficiency are formed in
one display pixel. With this construction, when the environment is
pitch-dark, the device serves as a transmission type liquid crystal
display device which displays images utilizing light from the
backlight passing through the transmission portion. When the
environment is comparatively dark, the device serves as a
reflection/transmission type liquid crystal display device which
displays images utilizing both light from the backlight passing
through the transmission portion and light reflected from the
reflection portion composed of a film having a comparatively high
reflectance. When the environment is bright, the device serves as a
reflection type liquid crystal display device which displays images
utilizing light reflected from the reflection portion composed of a
film having a comparatively high reflectance.
[0273] In other words, according to this example, the pixel
electrode of each pixel is composed of the reflection portion made
of a material having a high light reflection efficiency and the
transmission portion made of a material having a high light
transmission efficiency. Thus, a liquid crystal display device
having a good light utilization efficiency in any of the
above-described cases and an excellent productivity is
realized.
[0274] In this example, the upper surface of the reflection portion
made of a material having a reflection function is of a continuous
wave shape. This prevents the occurrence of a mirror phenomenon
without providing a light scattering means, which is necessary when
the reflection portion is flat, thus realizing a paper-white
display.
[0275] In this example, a photosensitive polymer resin film having
a plurality of convex portions underlies the reflection portion
made of a material having a reflection function. With this
construction, even if a variation exists in the continuous smooth
concave and convex shape, it does not influence the display. Thus,
the liquid crystal display device can be fabricated with good
productivity.
[0276] The transmission portion made of a material having a high
light transmission efficiency is formed simultaneously with the
formation of the source bus lines. This greatly shortens the
fabrication process of the liquid crystal display device.
[0277] A protection film is formed between the transmission portion
and the reflection portion. This prevents the generation of
electric corrosion between the transmission portion and the
reflection portion.
[0278] The reflection material remaining on the transmission
portions and terminal electrodes is simultaneously removed when the
patterning of the reflection portions is conducted. This greatly
shortens the fabrication process of the liquid crystal display
device.
[0279] In this example, light emitted from the backlight passes
through the transmission portion to leave the substrate, while it
is reflected from the back surface of the reflection portion to be
returned to the backlight and reflected again toward the substrate.
Part of the re-reflected light passes through the transmission
portion to leave the substrate.
[0280] It is conventionally difficult to direct the rereflected
light to effectively pass through the transmission portion since
regular reflection mainly occurs when the reflection portion is
flat. In this example, however, since the reflection portion is of
a continuous wave shape, the light emitted from the backlight is
scattered, allowing the reflected light to effectively return
toward the portion of the backlight located below the transmission
portion. Thus, more effective use of light than that expected from
the actual aperture ratio is possible, unlike the conventional
transmission type liquid crystal display device.
EXAMPLE 9
[0281] FIG. 25 is a partial sectional view of a
transmission/reflection type liquid crystal display device 100 of
Example 9 according to the present invention.
[0282] Referring to FIG. 25, the liquid crystal display device 100
includes an active matrix substrate 70 shown in FIG. 18
(corresponding to the F'-F' cross section), a counter substrate
(color filter substrate) 160, and a liquid crystal layer 140
interposed therebetween. The transmission/reflection type active
matrix substrate 70 includes a plurality of gate bus lines 72, as
scanning lines, and a plurality of source bus lines 74, as signal
lines, formed on an insulating glass substrate 61 so as to cross
with each other. In each of the rectangular regions surrounded by
the adjacent gate bus lines 72 and the adjacent source bus lines
74, a transmission electrode 68 made of a material having a high
light transmission efficiency and a reflection electrode 69 made of
a material having a high light reflection efficiency are disposed.
The transmission electrode 68 and the reflection electrode 69
constitute one pixel electrode. The counter substrate (color filter
substrate) 160 includes a color filter layer 164 and a transparent
electrode 166 made of ITO or the like formed in this order on an
insulating glass substrate 162.
[0283] Vertical alignment films (not shown) are formed on the
surfaces of the substrates 70 and 160 facing the liquid crystal
layer 140. In order to define the direction of liquid crystal
molecules oriented by the electric field, the vertical alignment
films are rubbed in a direction so as to provide a pretilt angle to
the liquid crystal molecules. A nematic liquid crystal material
having a negative dielectric anisotropy (e.g., MJ manufactured by
Merck & Co., Inc.) is used for the liquid crystal layer
140.
[0284] Each pixel which is a minimum display unit of the liquid
crystal display device 100 includes a reflection region 120R
defined by the reflection electrode 69 and the transmission region
120T defined by the transmission electrode 68. The thickness of the
liquid crystal layer 140 is dr in the reflection region 120R and dt
(dt=2 dr) in the transmission region 120T, so that the optical path
lengths of light beams contributing to the display (reflected light
beams in the reflection region and transmitted light beams in the
transmission region) are substantially equal to each other.
Although dt=2 dr is preferable, dt and dr may be appropriately
determined in the relationship with the display characteristics as
far as dt>dr. Typically, dt is about 4 to about 6 .mu.m and dr
is about 2 to about 3 .mu.m. In other words, a step of, about 2 to
about 3 .mu.m is formed in each pixel region of the active matrix
substrate 70. When the reflection electrode 69 has a concave and
convex shaped surface as shown in FIG. 25, the average value of
thicknesses should be dr. In this way, the transmission/reflection
type liquid crystal display device 100 includes two types of
regions (the reflection regions and the transmission regions) where
the thickness of the liquid crystal layer 140 is different
therebetween. In this example, the active matrix substrate 70
includes the reflection regions 120R and the transmission regions
120T having different heights formed on the side facing the liquid
crystal layer 140.
[0285] A liquid crystal display device (diagonal: 8.4 inches)
having the construction shown in FIG. 25 was actually fabricated
and subjected to a 64 gray-level display to evaluate the display
characteristics (transmittance and reflectance) of the device. The
evaluation results are shown in FIG. 26. The liquid crystal display
device was fabricated under the following conditions. The ratio of
the area of the transmission region 120T to that of the reflection
region 120R in one pixel was 4:6. The transmission electrodes 68
were made of, ITO, while the reflection electrodes 69 were made of
Al. The thickness dt of the liquid crystal layer 140 in the
transmission regions 120T was set at about 5.5 .mu.m, while the
thickness of the liquid crystal layer 140 in the reflection regions
120R were set at about 3 .mu.m.
[0286] The transmittance of the liquid crystal display device in
the transmission mode using light from a backlight was measured by
MB-5 manufactured by Topcon Co., while the reflectance of the
liquid crystal display device in the reflection mode using ambient
light was measured by LCD-5000 manufactured by Otsuka Electronics
Co., Ltd. by use of an integrating sphere.
[0287] As is apparent from FIG. 26, the variations in the
reflectance and-the transmittance in the 64 gray-level display (the
solid line and the dotted line in FIG. 26, respectively)
substantially match with each other. Accordingly, a gray-level
display with a sufficient display quality is realized even if the
display in the transmission mode using light from the backlight and
the display in the reflection mode using ambient light are
conducted simultaneously. The contrast ratios in the transmission
mode and the reflection mode were about 200 and about 25,
respectively.
[0288] Hereinbelow, the evaluation results of color reproducibility
will be described. FIGS. 27 and 28 are chromaticity diagrams of a
conventional transmission type liquid crystal display .device and
the transmission/reflection type liquid crystal display device of
this example, respectively, under ambient light with different
brightnesses. The same backlight was used for these liquid crystal
display devices.
[0289] As is apparent from FIG. 27, as the illuminance on the
display screen by ambient light increases from 0 lx to 8,000 lx and
then to 17,000 lx, the range of the color reproducibility (the area
inside the triangle in FIG. 27) of the conventional liquid crystal
display device significantly decreases. This is recognized by the
observer as color blurring. In the transmission/reflection type
liquid crystal display device, however, as is observed from FIG.
28, the range of the color reproducibility when the illuminance is
8,000 lx is substantially the same as that when the illuminance is
0 lx. Moreover, only a minor decrease is observed in the color
reproducibility when the illuminance is 17,000 lx. Color blurring
is therefore hardly recognized.
[0290] In the conventional transmission type liquid crystal display
device, the contrast ratio is lower due to the reflection of
ambient light from the surface of the display panel, as well as due
to reflected light from a black mask for light shading,
interconnect lines, and the like. On the contrary, in the
transmission/reflection type liquid crystal display device of this
example, which provides a reflection mode display using ambient
light in addition to the transmission mode display, the lowering of
the contrast ratio due to the reflection of ambient light in the
transmission mode display can be suppressed by the reflection mode
display. Thus, the contrast ratio obtained by the liquid crystal
display device of this example will not become lower than the
contrast ratio which may be obtained by only the reflection mode
display irrespective of how bright ambient light becomes. As a
result, in the transmission/reflection type liquid crystal display
device of this example, the color reproducibility is hardly lowered
even under bright ambient light and thus a display with high
visibility can be obtained under any environment.
[0291] FIG. 29 shows an alternative embodiment of the construction
of this example, where a reflection electrode region 160R includes
a reflection layer (reflection plate) 169 and a portion of a
transmission electrode 168. This is unlike the construction shown
in FIG. 25, where the reflection electrode region 120R includes a
reflection electrode 69 having a reflection characteristic. The
height of the reflection electrode region 160R of the active matrix
substrate can be controlled by adjusting the thickness of the
reflection layer 169 and/or an insulating layer 170 formed on the
reflection layer 169.
EXAMPLE 10
[0292] FIG. 30 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 10 according to the
present invention. FIG. 31 is a sectional view taken along line G-C
of FIG. 30.
[0293] Referring to FIGS. 30 and 31, a plurality of gate lines 202
and a plurality of source lines 203 are formed on a transparent
insulating substrate 201, made of glass or plastic, so as to cross
with each other. Each region surrounded by the adjacent gate lines
202 and the adjacent source lines 203 defines a pixel. A TFT 204 is
disposed in the vicinity of each of the crossings of the gate lines
202 and the source lines 203. A drain electrode 205 of each TFT 204
is connected to a corresponding pixel electrode 206. The portion of
each pixel where the pixel electrode 206 is formed is composed of
two regions as is viewed from the top, i.e., a region T having a
high transmission efficiency and a region R having a high
reflection efficiency. In this example, an ITO layer 207
constitutes the top layer of the region T as a layer having a high
transmission efficiency, while an Al layer 208 (or an Al alloy
layer) constitutes the top layer of the region R as a layer having
a high reflection efficiency. The layers 207 and 208 constitute the
pixel electrode 206 of each pixel. The pixel electrode 206 overlaps
a gate line 202a for the adjacent pixel in the next pixel row via a
gate insulating film 209. During driving, a storage capacitor for
the driving of liquid crystal is formed at this overlap
portion.
[0294] The TFT 204 includes a gate electrode 210 branched from the
corresponding gate line 202 (in this case 202a), a gate insulating
film 209, a semiconductor layer 212, a channel protection layer
213, and n.sup.+-Si layers 211 which are to be source/drain
electrodes deposited in this order.
[0295] Though not shown, the resultant active matrix substrate is
provided with an alignment film, and then bonded with a counter
substrate having a transparent electrode and an alignment film
formed thereon. Liquid crystal is injected in a space between the
two substrates in a sealing manner, and a backlight is disposed on
the rear side of the resultant structure, thereby completing the
liquid crystal display device of this example.
[0296] A mixture of a guest-host liquid crystal material, ZLI2327
(manufactured by Merck & Co., Inc.) containing black pigments
therein and 0.5% of an optically active substance, S-811
(manufactured by Merck & Co., Inc.) was used as the liquid
crystal. An electrically controlled birefringence (ECB) mode may
also be used as the liquid crystal lode by disposing polarizing
plates on the top and bottom surfaces of the liquid crystal layer.
When a color display is desired, a color filter (referred to as a
CF layer) composed of red, green, and blue colored layers is
disposed on top of the liquid crystal layer.
[0297] Hereinbelow, a method for fabricating such an active matrix
substrate of this example will be described.
[0298] First, the gate lines 202 and the gate electrodes 210 made
of Ta are formed on the insulating substrate 201, and the gate
insulating film 209 is formed over the entire resultant substrate.
Subsequently, the semiconductor layer 212 and the channel
protection layer 213 are formed above each of the gate electrodes
210, followed by the formation of the n.sup.+-Si layers 211 as the
source electrodes 211 and drain electrodes 205 (or 211).
[0299] An ITO layer 203a (a lower layer) and a metal layer 203b (an
upper layer) are formed in this order by sputtering and patterned
to form the source lines 203. In this example, Ti was used for the
metal layer 203b.
[0300] This double-layer structure of the source lines 203 is
advantageous in that even if the metal layer 203b constituting each
source line 203 is partly defective, the electric connection of the
source line 203 is maintained by the ITO layer 203a, reducing the
occurrence of disconnections in the source lines 203.
[0301] The ITO layer 207 of the region T having a high transmission
efficiency is formed of the same material at the same step as the
ITO layer 203a of the source line 203, The region R having a high
reflection efficiency is formed by forming an Mo layer 214 and the
Al layer 208 by sputtering in this order and patterning. The Al
layer 208 can provide a sufficiently stable reflection efficiency
(about 90%) when the thickness thereof is about 150 nm or more. In
this example, the thickness of the Al layer 208 was 150 nm to
obtain the reflection efficiency of 90% and thus to allow ambient
light to be effectively reflected. Ag, Ta, W, and the like may also
be used in place of Al or an Al alloy for the layer (Al layer 208)
having a high reflection efficiency.
[0302] In this example, the ITO layer 207 and the Al layer 208 are
used as the pixel electrode 206 of each pixel. Alternatively,
layers of Al or an Al alloy with different thicknesses may be
formed to define a region having a high transmission efficiency and
a region having a high reflection efficiency as the regions T and
R, respectively. This makes the fabrication process simpler than in
the case of using different materials. Also, the layer having a
high reflection efficiency of the region R (the Al layer 208 in
this example) may be made of the same material as that used for the
metal layer 203b of the source line 203. This makes it possible to
fabricate the liquid crystal display device of this example by the
same process as that used in the fabrication of a conventional
transmission type liquid crystal display device.
[0303] As described above, each pixel electrode 206 is composed of
the region T having a high transmission efficiency and the region R
having a high reflection efficiency. This construction realizes a
liquid crystal display device where a transmission mode display, a
reflection mode display, and a transmission/reflection mode display
are possible by utilizing ambient light and illumination light more
efficiently, compared with the conventional liquid crystal display
device using a semi-transmissive reflection film.
[0304] The ITO layer 207 is formed, as the pixel electrode 206,
over the entire region of each pixel and above the gate line 202a
of the adjacent pixel, in the next pixel row, via the gate
insulating film 209, interposed therebetween. The Al layer 208 is
formed on the ITO layer 207 via the Mo layer 214, interposed
therebetween, to constitute the region R in the center portion of
the pixel like an island. In this way, since the ITO layer 207 and
the Al layer 208 are electrically connected with each other, the
regions T and R apply the same voltage received from the same TFT
204 to the liquid crystal. Thus, a disclination line which may
occur when the orientation of the liquid crystal molecules varies
within one pixel during the voltage application is prevented.
[0305] The interposition of the Mo layer 214 between the ITO layer
207 and the Al layer 208 serves to prevent the generation of
electric corrosion due to the contact between the ITO layer 207 and
the Al layer 208 via an electrolytic solution in the fabrication
process.
[0306] In this example, good display characteristics are obtained
by setting the ratio of the area of the region T to that of the
region R at 60:40. The area ratio is not limited to this value, but
may be appropriately changed depending on the
transmission/reflection efficiency of the regions T and R and the
use of the device.
[0307] In this example, the area of the region R is preferably
about 10 to about 90% of the effective pixel area (i.e., the total
of the area of the region T and the area of the region R). If this
percentage is below about 10%, i.e., the region having a high
transmission efficiency occupies a too large a portion of the
pixel, there arises a problem which arises in conventional
transmission type liquid crystal display devices, i.e., the problem
that the display is blurred when the environment becomes too
bright. Conversely, if the percentage of the region R exceeds about
90%, a problem arises when the environment becomes too dark to
observe the display only by ambient light. That is, even if the
backlight is turned on during such an occasion, the occupation of
the region T is so small that the resultant display is not
recognizable.
[0308] In particular, when the liquid crystal display device is
applied to an apparatus which is mainly used outdoors, battery life
is an important factor, and the device should be designed so as to
utilize ambient light efficiently to realize a lower power
consumption. Accordingly, the area of the region R, having a high
reflection efficiency, is preferably about 40 to about 90$ of the
effective pixel area. When the area occupation of the region R is
about 40%, the environment where only the reflection mode display
is sufficient for display becomes limited, and thus the amount of
time requiring light from the backlight becomes too long. This
reduces battery life.
[0309] On the other hand, when the liquid crystal display device is
applied to an apparatus which is mainly used indoors, the device
should be designed so as to utilize light from the backlight
efficiently. Accordingly, the area of the region R is preferably
about 10 to about 60% of the effective pixel area. When the area
occupation of the region R exceeds 60%, the region T for
transmitting light from the backlight becomes too small. To
compensate for this, the brightness of the backlight must be
substantially increased when compared with, for example, a
transmission type liquid crystal display device. This increases the
power consumption and lowers the backlight utilization efficiency
of such a device.
[0310] The liquid crystal display device of this example was
actually mounted in a battery-driven video camera. As a result, the
display was kept bright and recognizable regardless of the
brightness of ambient light by adjusting the brightness of the
backlight. In particular, when the device was used outdoors during
a fine weather, it was not necessary to light the backlight, thus
reducing the power consumption. Therefore, battery life is
significantly increased when compared with a device with only a
transmission type liquid crystal display device.
EXAMPLE 11
[0311] FIG. 32 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 11 according to the
present invention. FIG. 33 is a sectional view taken along line H-H
of FIG. 32.
[0312] In this example, the portion of each pixel where the pixel
electrode-.s formed is divided into two regions at the center
thereof as is viewed from the top, i.e., a region T having a high
transmission efficiency and a region R having a high reflection
efficiency.
[0313] The same components are denoted by the same reference
numerals as those in FIGS. 30 and 31 in Example 10. The pixels, the
structure of the TFTs, and the fabrication process of the device
are substantially the same as those described in Example 10.
[0314] Referring to FIGS. 32 and 33, an ITO layer 207 is formed
over the region of each pixel ranging from the center portion to a
vicinity of a corresponding gate line 202, and partly connected to
a drain electrode 205 of a TFT 204. An Al layer 208, having a high
reflection efficiency, overlaps the ITO layer 207 via an Mo layer
214 at the center portion of the pixel. The Al layer 208 extends on
the side of the pixel opposite to the region of the ITO layer 207,
to overlap a gate line 202a for the adjacent pixel in the next
pixel row via a gate insulating film 209.
[0315] Since the ITO layer 207 and the Al layer 208 are
electrically connected via the Mo layer 214, electric corrosion due
to the contact between the ITO layer 207 and the Al layer 208 is
suppressed. The overlap between the Al layer 208, i.e., the region
R and the gate line 202a, and the adjacent pixel is accomplished
via the insulating film 209. This overlap forms a storage capacitor
during the driving of liquid crystal, and this overlap portion of
the region R also contributes to the display. This significantly
increases the effective area of the pixel compared with the
conventional construction.
[0316] In order to further increase the aperture ratio of the
pixel, a film having a high reflection efficiency such as the Al
layer 208 may be formed above the TFT 204 or the source line 203,
via an insulating film, to serve as part of the pixel electrode 206
(which is electrically connected to the drain electrode 205). In
such a case, however, the thickness, the material, and the pattern
design of the insulating film should be appropriately determined so
that the degradation of image quality due to a parasitic
capacitance generated between the pixel electrode 206 and the TFT
204 or the source line 203 is minimized.
EXAMPLE 12
[0317] FIG. 34 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 12 according to the
present invention. FIG. 35 is a sectional view taken along line I-I
of FIG. 34.
[0318] This example is different from Example 11 in that a common
line 215 is formed under the region R having a high reflection
efficiency, via a gate insulating film 209.
[0319] The same components are denoted by the same reference
numerals, as those in FIGS. 30 to 33 in Examples 10 and 11. The
pixels, the structure of the TFTS, and the fabrication process of
the device are substantially the same as those described in
Examples 10 and 11.
[0320] Referring to FIGS. 34 and 35, an ITO layer 207 is formed
over the region of each pixel ranging from the center portion to a
vicinity of a corresponding gate line 202 and connected to a drain
electrode 205 of a TFT 204. An Al layer 208 having a high
reflection efficiency overlaps the ITO layer 207 via an Mo layer
214 at the center portion of the pixel. The Al layer 208 and
extends on the side of the pixel opposite to the region of the ITO
layer 201 in the vicinity of a gate line 202a for the adjacent
pixel in the next pixel row, overlapping the common line 215 via a
gate insulating film 209.
[0321] Since the ITO layer 207 and the Al layer 208 are
electrically connected via the Mo layer 214, electric corrosion due
to the contact between the ITO layer 207 and the Al layer 208 is
suppressed. The overlap between the Al layer 208, i.e., the region
R and the common line 215 via the insulating film 209 forms a
storage capacitor during the driving of liquid crystal,
contributing to an improved display. This formation of the storage
capacitor will not lower the aperture ratio.
[0322] In order to further increase the aperture ratio of the
pixel, a film having a high reflection efficiency such as the Al
layer 208 may be formed above the TFT 204 or the source line 203,
via an insulating film, to serve as part of the pixel electrode 206
(which is electrically connected to the drain electrode 205). In
such a case, however, the thickness and the material of the
insulating film should be appropriately determined so that no
parasitic capacitance is generated between the pixel electrode 206
and the TFT 204 or the source line 203. For example, after the
formation of the ITO layers 207, an organic insulating film having
a dielectric constant of about 3.6 may be deposited over the entire
resultant substrate to a thickness as large as about 3 .mu.m. Then,
the Al layer 208 may be formed in each pixel, so as to overlap the
TFT 204 or the source line 203 and to be electrically connected to
the drain electrode 205. This electrical connection can be realized
via a contact hole by forming a contact hole on the drain electrode
205 or the ITO layer 207.
[0323] In this example, the portion of each pixel where the pixel
electrode 206 is formed is divided into two regions, i.e., a region
having a high transmission efficiency (region T) and a region
having a high reflection efficiency (region R). Alternatively, the
portion may be divided into three or more regions. For example, as
shown in FIG. 36, the pixel electrode 206 may be divided into three
regions, i.e., the region T having a high transmission efficiency,
the region R having a high reflection efficiency, and a region C
having a different transmission or reflection efficiency from the
other two regions.
EXAMPLE 13
[0324] FIG. 37 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 13 according to the
present invention. FIGS. 38A to 38D are sectional views taken along
line J-J of FIG. 37, illustrating the process of fabricating the
liquid crystal display device of this example.
[0325] In this example, regions R having a high reflection
efficiency are made of the same material as that used for source
lines. The same components are denoted by the same reference
numerals as those in FIGS. 30 to 36 in Examples 10 to 12. The
pixels, the structure of the TFTs, and the fabrication process of
the device are substantially the same as those described in
Examples 10 to 12 unless otherwise specified.
[0326] In this example, each pixel includes a region T having a
high transmission efficiency formed in the center portion thereof
and a region R surrounding the region T. The outer profile of the
region R is a square along two gate lines-and two source lines. The
region R includes a layer, having a high reflection efficiency,
made of the same material as that for the source line, realizing a
high reflection efficiency.
[0327] The process of fabricating such a liquid crystal display
device will be described with reference to FIGS. 38A to 38D.
[0328] Referring to FIG. 38A, gate lines 202 (see FIG. 37) and gate
electrodes 210, a gate insulating film 209, semiconductor layers
212, channel protection layers 213, and n.sup.+-Si layers 211,
which are to be source electrodes 211 and drain electrodes 205 (or
211) are sequentially deposited on an insulating substrate 201 by
sputtering. Then, a conductive film 241 for source lines 203 (see
FIG. 37) is deposited on the resultant substrate by sputtering.
[0329] Referring to FIG. 38B, the conductive film 241 is patterned
to form layers 242 having a high reflection efficiency, drain-pixel
electrode connecting layers 243, and the source lines 203. The
regions of the layers 242 having a high reflection efficiency
correspond to the regions R.
[0330] Referring to FIG. 38C, an interlayer insulating film 244 is
formed over the resultant substrate, and then contact holes 245 are
formed through the interlayer insulating film 244.
[0331] Referring to FIG. 38D, a layer 246 having a high
transmission efficiency, made of ITO, is formed over the entire
area of each pixel. The layer 246 having a high transmission
efficiency may be made of any other material having a high
transmission efficiency. The layer 246 having a high transmission
efficiency is connected to the connecting layer 243 via the contact
hole 245 formed through the interlayer insulating film 244, thus
being electrically connected to a corresponding drain electrode
205. The layer 246 having a high transmission efficiency also
serves as the pixel electrode for applying a voltage to a liquid
crystal layer, so that the voltage is applied to the portions of
the liquid crystal layer corresponding to both the regions T and R
via the layer 246 having a high transmission efficiency. Thus, in
this example, each pixel electrode is composed of only the layer
246 having a high transmission efficiency, and are not composed of
the region T having a high transmission efficiency and the region R
having a high reflection efficiency. This construction is
advantageous over the transmission type liquid crystal display
device in that the region having a high reflection efficiency can
be formed without increasing the number of process steps and that
failure in the formation of pixel electrodes is minimized.
EXAMPLE 14
[0332] FIG. 39 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 14 according to the
present invention. Figures 40A to 40D are sectional views taken
along line K-K of FIG. 39, illustrating the process of fabricating
the liquid crystal display device of this example.
[0333] In this example, regions R (the hatched portion in FIG. 39)
having a high reflection efficiency are made of the same material
as is used for gate lines. The same components are denoted by the
same reference numerals as those in FIGS. 30 to 38 in Examples 10
to 13. The pixels, the structure of the TFTs, and the fabrication
process of the device are substantially the same as those described
in Examples 10 to 13 unless otherwise specified.
[0334] In this example, each pixel includes a rectangular region T
having a high transmission efficiency formed in the center portion
thereof and a region R substantially composed of two connected
strips surrounding the region T as is viewed from the top. The
outer profile of the region R is a square along two gate lines and
two source lines. The region R includes a layer, having a high
reflection efficiency, made of the same material as that for the
gate line, realizing a high reflection efficiency.
[0335] The process of fabricating Such a liquid crystal display
device will be described with reference to FIGS. 40A to 40D.
[0336] Referring to FIG. 40A, a conductive film is formed on an
insulating substrate 201. The conductive film is then patterned to
form gate electrodes 210, gate lines 202 (see FIG. 39), and layers
242 having a high reflection efficiency. The layers 242 having a
high reflection efficiency correspond to the regions R.
[0337] Referring to FIG. 40B, a gate insulating film 209,
semiconductor layers 212, channel protection layers 213, and no-vi
layers 211 which are to be source electrodes 211 and drain
electrodes 205 (or 211) are sequentially deposited on the resultant
substrate by sputtering. Then, metal layers 203b, used as part of
source layers 203, and drain-pixel electrode connecting layers 243
are formed during the same step. The connecting layers 243 partly
overlap drain electrodes 205 of TFTs 204.
[0338] Referring to FIG. 40C, ITO is deposited on the resultant
substrate by sputtering and patterned to form layers 246 having a
high transmission efficiency and ITO layers 203a as part of the
source lines 203. The layers 246 having a high transmission
efficiency are formed over the entire areas of respective pixels,
and the ITO layers 203a are formed on the metal layers 203b to have
the same pattern as the metal layers 203b. The layers 246 having a
high transmission efficiency partly overlap the connecting layers
243 to be electrically connected to the respective TFTs 204.
[0339] Referring to-Figure 40D, a passivation film 247 is formed
and patterned.
[0340] Thus, each pixel of the liquid crystal display device of
this example includes the region T having a high transmission
efficiency in the center portion thereof, and the region R having a
high reflection efficiency surrounding the region T in a shape of
two connected strips along the adjacent source lines. In this case,
since the ITO layers 203a of the source lines 203 and the layers
242, having a high reflection efficiency are located at different
levels, the gap between the ITO layer 203a and the layer 242,
having a high reflection efficiency, of each pixel, which is
required to prevent a leakage therebetween, can be narrowed, and
thus the aperture ratio of the pixel can be increased, compared
with the case where the regions T and R are formed in reverse
(i.e., the case where the layer having a high reflection efficiency
is located in the center portion of the pixel).
[0341] In this example, as in Example 13, each pixel electrode is
composed of only one type of electrode (i.e., the layer 246 having
a high transmission efficiency). This construction is advantageous
over the construction where the pixel electrode is composed of two
types of electrodes in that the occurrence of defects is reduced
and efficient fabrication of the device is possible.
[0342] In this example, each source line 203 is of a double layer
structure composed of the metal layer 203b and the ITO layer 203a.
Even if the metal layer 203b is partly defective, the electric
connection of the source line 203 is maintained by the ITO layer
203a. This reduces the occurrence of disconnections in the source
line 203.
EXAMPLE 15
[0343] FIG. 41 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 15 according to the
present invention. Figures 42A to 42C are sectional views taken
along line L-L of FIG. 41, illustrating the process of fabricating
the liquid crystal display device of this example.
[0344] In this example, pixel electrodes extend over gate lines
and/or source lines via an insulating film so as to increase the
effective pixel area (the area substantially functioning as a
pixel).
[0345] The same components are denoted by the same reference
numerals used in Examples 10 to 14. The pixels, the structure of
the TFTs, and the fabrication process of the device are
substantially the same as those described in Examples 10 to 14
unless otherwise specified.
[0346] As shown in FIG. 41, in this example, each pixel includes a
region T having a high transmission efficiency formed in the center
portion thereof and a region R (a hatched portion in FIG. 41) a
square formed from narrow strips, surrounding the region T as is
viewed from the top. The pixel electrode including a layer having a
high transmission efficiency overlaps adjacent gate lines 202 and
source lines 203 via an interlayer insulating film, so that a
voltage can be applied to the portions of a liquid crystal layer
located above the gate lines 202 and the source lines 203. This
ensures a larger effective pixel area than in Examples 10 to 14. In
this example, the gate lines 202 and the source lines 203 serve as
layers having a high reflection efficiency in the region R.
[0347] The process of fabricating such a liquid crystal display
device will be described with reference to FIGS. 42A to 42C.
[0348] Referring to FIG. 42A, gate electrodes 210, gate lines 202
(see FIG. 41), a gate insulating film 209, semiconductor layers
212, channel protection layers 213, and n.sup.+-Si layers 211,
which are to be source electrodes 211 and drain electrodes 205 (or
211) are sequentially deposited on an insulating substrate 201 by
sputtering. At least either of the gate lines 202 and the source
lines 203, which are to be overlapped by light transmission layers
as the pixel electrodes at a later step, are preferably made of a
material having a high reflection efficiency.
[0349] Referring to FIG. 42B, an interlayer insulating film 244 is
formed on the resultant substrate, and then contact holes 245 are
formed through the interlayer insulating film 244.
[0350] Ref erring to FIG. 42C, a material having a high
transmission efficiency such as ITO is deposited on the resultant
substrate by sputtering and patterned to form layers 246 having a
high transmission efficiency. The layers 246, having a high
transmission efficiency, are connected, via the contact holes 245,
to connecting layers 243 which are in turn connected to drain
electrodes 205 of TFTs 204. At this time, the layers 246 having a
high transmission efficiency are patterned so as to overlap at
least either of the gate lines 202 and the source lines 203. With
this construction, the gate lines 202 and/or the source lines 203
which are overlapped by the layers 246 having a high transmission
efficiency via the interlayer insulating film 244, can be used as
the layers having a high reflection efficiency.
[0351] The display device having the above construction should be
designed so that a degradation of the image quality, due to a
phenomenon such as crosstalk, does not occur due to a capacitance
generated between the layer 246, having a high transmission
efficiency, and the gate line 202 or the source line 203.
[0352] Thus, in this example, each pixel includes the region T
having a high transmission efficiency formed in the center portion
thereof and the region R having a high reflection efficiency formed
at positions corresponding to the adjacent gate lines and/or the
source lines. This eliminates the necessity of forming an
additional layer having a high reflection efficiency, and thus the
process can be shortened.
EXAMPLE 16
[0353] FIG. 43 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 16 according to the
present invention. FIGS. 44A to 44F are sectional views taken along
line M-M of FIG. 43, illustrating the process of fabricating the
liquid crystal display device of this example.
[0354] As shown in FIG. 43, each pixel of the liquid crystal
display device of this example includes a region T having a high
transmission efficiency in the center portion thereof, and a region
R (hatched portions in FIG. 43) having a high reflection efficiency
composed of two strips along adjacent source lines 203 formed on
the sides of the region T.
[0355] As shown in FIG. 44F, the region R includes high convex
portions 253a and low convex portions 253b formed randomly on an
insulating substrate 201, a polymer resin layer 254 formed over
these convex portions 253a and 253b, and a layer 242, having a high
reflection efficiency, formed on the polymer resin layer 254. The
resultant layer 242, which constitutes the surface layer of the
region R, has a surface of a continuous wave shape, and is
electrically connected to a drain electrode 205 via a contact hole
245 and an underlying electrode (not shown).
[0356] The method for fabricating such a liquid crystal display
device will be described with reference to FIGS. 44A to 44F.
[0357] Referring to FIG. 44A, a plurality of gate lines 202 (see
FIG. 43) and gate electrodes 210 branched from the gate lines 202,
made of Cr, Ta, or the like, are formed on the insulating substrate
201.
[0358] Then, a gate insulating film 209, made of SiN.sub.x,
SiO.sub.x, or the like, is formed over the insulating substrate 201
covering the gate lines 202 and the gate electrodes 210.
Semiconductor layers 212, made of amorphous silicon (a-Si),
polysilicon, CdSe, or the like, are formed on the portions of the
gate insulating film 209 located above the gate electrodes 210. A
channel protection layer 213 is formed on each of the semiconductor
layers 212. A pair of contact layers 248, made of a-Si or the like,
are formed on both side portions of the channel protection layer
extending to the side portions of the semiconductor layers 212.
[0359] A source electrode 249, made of Ti, Mo, Al, or the like, is
formed on one of the contact layers 248, while the drain electrode
205 made of Ti, Mo, Al, or the like, is formed on the other contact
layer 248.
[0360] In this example, as the material of the insulating substrate
201, a glass plate with a thickness of 1.1 mm, product number 7059
manufactured by Corning Inc. may be used.
[0361] Referring to FIG. 44B, a conductive film is formed on the
resultant substrate by sputtering and patterned, to form metal
layers 203b used as part of the source lines 203 and the underlying
electrodes 250 simultaneously. Each of the underlying layers 250
may be formed to partly overlap the gate electrode 202 for the
adjacent pixel in the next pixel row, via the gate insulating film
209, so as to form a storage capacitor therebetween.
[0362] Each of the gate lines 202 used to form a storage capacitor
may be overlapped by a layer having a high reflection efficiency,
or the reflection efficiency of the gate line 202 itself may be
made high to serve as part of the pixel region (the region R, to
further increase the aperture ratio.
[0363] Referring to FIG. 44C, ITO is deposited on the resultant
substrate by sputtering and patterned to form ITO layers 203a which
constitute the source lines 203 together with the metal layers
203b.
[0364] In this example, each source line 203 is of a double-layer
structure composed of the metal layer 203b and the ITO layer 203a.
The double-layer structure is advantageous in that, even if the
metal layer 203b is partly defective, the electric connection of
the source line 203 is maintained by the ITO layer 203a. This
reduces the occurrence of disconnections in the source line
203.
[0365] Simultaneously with the formation of the ITO layers 203a,
layer 246, having a high transmission efficiency and which
constitute the pixel electrodes, are also obtained by the
patterning. In this way, the layers 246 having a high transmission
efficiency as the pixel electrodes can be formed simultaneously
with the source lines 203.
[0366] Referring to FIG. 44D, a resist film 252, made of a
photosensitive resin, is formed and patterned, and then
heat-treated in order to round it, so that the high convex portions
253a and the low convex portions 253b, having a substantially
circular cross-section, are formed on the portions of the resultant
substrate corresponding to the regions R. Such convex portions 253a
and 253b are preferably not formed on the layers 246 having a high
transmission efficiency so that a voltage can be efficiently
applied to a liquid crystal layer. Even if the convex portions 253a
and 253b are formed on the layers 246, however, no significant
optical influence will be observed so long as the convex portions
are transparent.
[0367] Referring to FIG. 44E, a polymer film 254 is formed over the
convex portions 253a and 253b. With this film, the concave and
convex shaped surface of the region R can be made more continuous
by reducing the number of flat portions. This step may be omitted
by changing the fabrication conditions.
[0368] Referring to FIG. 44F, layers 242 having a high reflection
efficiency made of Al as the pixel electrodes are formed on
predetermined portions of the polymer films 254 by sputtering, for
example. Materials suitable for the layers 242 having a high
reflection efficiency include, besides Al and an Al alloy, Ta, Ni,
Cr, and Ag having a high light reflection efficiency. The thickness
of the layers 242 having a high reflection efficiency is preferably
in the range of about 0.01 to about 1.0 .mu.m.
[0369] Thus, each pixel of the liquid crystal display device of
this example includes the region T having a high transmission
efficiency formed in the center portion thereof, and the region R
having a high reflection efficiency formed along the adjacent
source lines. With this construction, since the ITO layers 203a of
the source lines 203 and the layers 242 having a high reflection
efficiency are located at different levels, the gap between the ITO
layer 203a and the layer 242 with a high reflection efficiency of
each pixel, which is required to prevent a leakage therebetween,
can be narrowed, and thus the aperture ratio of the pixel can be
increased, compared with the case where the regions T and R are
formed in reverse (i.e., the case when the layer having a high
reflection efficiency is located in the center portion of the
pixel).
[0370] In this example, the layers 242 having a high reflection
efficiency have a smooth concave and convex shaped surface to allow
reflected light to be scattered in a wide range of directions. When
a scattering sheet is jointly used, such convex portions need not
be formed with the resist film 252, instead the surface of the
layers 242 having a high reflection efficiency can be made flat. In
either case, the layers 242, having a high reflection efficiency,
and the layers 246 having a high transmission efficiency, exist as
individual layers with a third substance (e.g., a resin and a metal
such as Mo) interposed therebetween. With this construction, in the
specific case where the layers having a high transmission
efficiency are made of ITO and the layers having a high reflection
efficiency are made of Al or an Al alloy, Al patterning failure due
to an electric corrosion which tends to be generated at the Al
etching step can be reduced.
EXAMPLE 17
[0371] FIG. 45 is a partial plan view of an active matrix substrate
of a liquid crystal display device of Example 17 according to the
present invention. FIG. 46 is a sectional view taken along line N-N
of FIG. 45.
[0372] Referring to FIGS. 45 and 46, the active matrix substrate
includes pixel electrodes 206 formed in a matrix and gate lines 202
for supplying scanning signals and source lines 203 for supplying
display signals running along the peripheries of the pixel
electrodes 206 so as to cross with each other.
[0373] The pixel electrodes 206 overlap the gate lines 202 and the
source lines 203 at the peripheries via an interlayer insulating
film 244. The gate lines 202 and the source lines 203 are composed
of metal films.
[0374] A TFT 204 is formed in the vicinity of each of the crossings
of the gate lines 202 and the source lines 203 as the switching
element for supplying display signals to the corresponding pixel
electrode 206. A gate electrode 210 of the TFT 204 is connected to
the corresponding gate line 202 to drive the TFT 204 with signals
input into the gate electrode 210. A source electrode 249 of the
TFT 204 is connected to the corresponding source line 203 to
receive data signals. A drain electrode 205 of the TFT 204 is
electrically connected to a connecting electrode 255 and then to
the pixel electrode 206 via a contact hole 245.
[0375] The connecting electrode 255 forms a storage capacitor with
a common line 215 via a gate insulating film 209.
[0376] The common line 215 is composed of a metal film, and
connected to a counter electrode formed on a counter substrate 256
via an interconnect (not shown). The common line 215 may be formed
during the same step as the formation of the gate lines 202 to
shorten the fabrication process.
[0377] Each of the pixel electrodes 206 is composed of a layer 242
having a high reflection efficiency made of Al or an Al alloy and a
layer 246 having a high transmission efficiency made of ITO. When
viewed from the top, the pixel alectrode 206 is divided into three
regions, i.e., two regions T having a high transmission efficiency
and a region R having a high reflection efficiency (corresponding
to the hatched portion in FIG. 45). The layer 242 having a high
reflection efficiency may also be composed of a conductive metal
layer having a high reflection efficiency such as Ta as in the
above examples.
[0378] Each region R is designed to cover part of light-shading
electrodes and interconnect lines, such as the gate lines 202, the
source lines 203, the TFT 204, and the common line 215, which do
not transmit light from a backlight. With this construction, the
regions of each pixel portion which are not usable as the regions T
can be used as the region R having a high reflection efficiency.
This increases the aperture ratio of the pixel portion. The regions
T of each pixel portion are surrounded by the region R.
[0379] The method for fabricating the active matrix with the above
construction will be described.
[0380] First, the gate electrodes 210, the gate lines 202, the
common lines 215, the gate insulating film 209, semiconductor
layers 212, channel protection layers 213, the source electrodes
249, and the drain electrodes 205 are sequentially formed on a
transparent insulating substrate 201 made of glass or the like.
[0381] Then, a transparent conductive film and a metal film which
are to constitute the source lines 203 and the connecting
electrodes 255 are deposited on the resultant substrate by
sputtering and patterned into a predetermined shape.
[0382] Thus, each of the source lines 203 is of a double-layer
structure composed the ITO layer 203a and the metal layer 203b. The
double-layer structure is advantageous in that, even if the metal
layer 203b is partly defective, the electric connection of the
source lines 203 is maintained by the ITO layer 203a. This reduces
the occurrence of disconnections in the source lines 203.
[0383] Thereafter, a photosensitive acrylic resin is applied to the
resultant substrate by a spin application method to form the
interlayer insulating film 244 with a thickness of about 3 .mu.m.
The acrylic resin is then exposed to light according to a desired
pattern and then developed with an alkaline solution. Only the
light-exposed portions of the film are etched away with the
alkaline solution to form the contact holes 245 through the
interlayer insulating film 244. By employing this alkaline
development, well-tapered contact holes 245 are obtained.
[0384] Using a photosensitive acrylic resin for the interlayer
insulating film 244 is advantageous in the aspect of productivity
in view of the following points. Since the spin application method
can be employed for the thin film formation, a film as thin as
several micrometers can be easily formed. Also, no photoresist
application step is required at the patterning of the interlayer
insulating film 244.
[0385] In this example, the acrylic resin is originally colored and
can be made transparent by exposing the entire surface to light
after the patterning. The acrylic resin may also be made
transparent by chemical processing.
[0386] Thereafter, an ITO film is formed by sputtering and
patterned, to be used as the layers 246 having a high transmission
efficiency of the pixel electrodes 206. Thus, the layers 246 having
a high transmission efficiency, which constitute the pixel
electrodes 206, are electrically connected to the corresponding
connecting electrodes 255 via the contact holes 245.
[0387] The layers 242 having a high reflection efficiency, made of
Al or an Al alloy, are then formed on the portions of the layers
246 having a high transmission efficiency, which correspond to the
regions R, so as to overlie the gate lines 202, the source lines
203, the TFTs 204, and the common lines 215. The two layers 242 and
246 are electrically connected with each other, thereby forming
pixel electrodes 206. Any adjacent pixel electrodes 206 are
separated along the portions located above the gate lines 202 and
the source lines 203 so as not to be electrically connected with
each other.
[0388] As shown in FIG. 46, the thus-fabricated active matrix
substrate and the counter substrate 256 are bonded together, and
liquid crystal is injected in a space between the substrates to
complete the liquid crystal display device of this example.
[0389] As described above, the liquid crystal display device of
this example includes the layers 242, having a high reflection
efficiency, formed above the TFTs 204, the gate lines 202, and the
source lines 203 so as to constitute the regions R of the pixel
electrodes 206. This eliminates the necessity of providing
light-shading films for preventing light from entering the TFTs 204
and light-shading the portions of the pixel electrodes 206 located
above the gate lines 202, the source lines 203, and the common
lines 215. In such portions, a light leakage tends to be generated
in the form of domains, disclination lines, and the like in display
regions. As a result, the regions which are conventionally unusable
as display regions because they are blocked by the light-shading
films can be made usable as display regions. This allows for
effective use of the display regions.
[0390] When the gate lines and the source lines are composed of
metal films, they block light from a backlight in a conventional
transmission type display device and thus are unusable as display
regions. In this example, however, the region T having a high
transmission efficiency is formed in the center portion of each
pixel (as two separate portions in this example). The region R,
having a high reflection efficiency, is formed in a shape of strips
surrounding the region T That is, the region R having a high
reflection efficiency overlies the gate lines, the source lines,
the common line, and the switching element, and is used as the
reflection electrode region of each pixel electrode. This
construction increases the aperture ratio of the pixel electrode
more than the case of the reverse pattern (i.e., the pattern where
the region T surrounds the region R.
[0391] Alternatively, the region R of each pixel may be formed as
shown in FIG. 47 (hatched portion) including the connecting
electrode 255. This suppresses the decrease in the brightness of
light passing through the region T.
EXAMPLE 18
[0392] In the above examples, the present invention was applied to
the active matrix liquid crystal display device. The
present-invention can also be applied to a simple matrix liquid
crystal display device.
[0393] Hereinbelow, a basic construction of a pair of a column
electrode (a signal electrode) and a row electrode (a scanning
electrode) which face each other will be described. In the simple
matrix liquid crystal display device, the region where the pair of
the column electrode and the row electrode cross with each other
defines a pixel.
[0394] FIGS. 48A to 48C show one example of such a pixel region.
Referring to FIG. 48A, a transmission electrode region is formed in
the center portion of the column electrode in one pixel region,
while a reflection electrode region is formed in the remaining
peripheral portion thereof. The construction of the column
electrode may be as shown in FIG. 48B or 48C. The height of the
reflection electrode region can be adjusted by forming an
interlayer insulating film between the reflection electrode and the
transmission electrode as shown in FIG. 48C.
[0395] Alternatively, as shown in FIG. 49A, a reflection electrode
region may be formed in the center portion of the column electrode
in one pixel region, while a transmission electrode region is
formed in the remaining peripheral portion thereof. The
construction of the column electrode may be as shown in FIG. 49B or
49C. The height of the reflection electrode region can be adjusted
by forming an interlayer insulating film between the reflection
plate and the transmission electrode as shown in FIG. 49C.
[0396] Alternatively, as shown in FIGS. 50A, 50B and 50C and FIGS.
51A and 51B, the column electrode may have a strip-shaped
reflection electrode region. Such a strip-shaped reflection
electrode region may be formed along one, side of the column
electrode as shown in FIGS. 50A to 50C, or along the center thereof
as shown in FIG. 51A and 51B.
[0397] Hereinbelow, the features of the liquid crystal display
device according to the present invention distinguished from the
conventional reflection type or transmission type liquid crystal
display device will be described.
[0398] In the conventional reflection type liquid crystal display
device, the display is affected by use of ambient light to realize
low power consumption. Accordingly, when ambient light is lower
than a certain limit value, the display fails to be recognized even
if the device is being used in an environment where sufficient
power supply is possible. This is one of the biggest shortcomings
of the reflection type liquid crystal display device.
[0399] If the reflection characteristics of the reflection
electrodes vary at the fabrication, the ambient light utilization
efficiencies of the reflection electrodes also vary. This varies
the critical value of the ambient light intensity at which the
display becomes unrecognizable depending on the panels. At the
fabrication, therefore, the variation in the reflection
characteristics must be controlled more carefully than the
variation in the aperture ratio of which control is required for
the conventional transmission type liquid crystal display device.
Otherwise, a liquid crystal display device having stable display
characteristics is not obtained.
[0400] On the contrary, in the liquid crystal display device
according to the present invention, light from a backlight is
utilized under the environment where sufficient power supply -is
possible as in the conventional transmission type liquid crystal
display device. Accordingly, the display can be recognized
regardless of the intensity of ambient light. Thus, the variation
in the ambient light utilization efficiency due to the variation in
the reflection characteristics is not required to be controlled as
strictly as that in the reflection type liquid crystal display
device.
[0401] On the other hand, in the conventional transmission type
liquid crystal display device, when ambient light becomes bright,
the surface reflection components of the light increases, making it
difficult to recognize the display. In the liquid crystal display
device according to the present invention, when ambient light
becomes bright, the reflection regions are used together with the
transmission regions. This increases the panel brightness, and thus
improves the visibility.
[0402] Thus, the liquid crystal display device according to the
present invention can overcome both the problems that visibility is
-lowered due to surface reflection under high (i.e., bright)
ambient light in a conventional transmission type liquid crystal
display device and that display recognition becomes difficult due
to a decrease in the panel brightness under low (i.e., dark)
ambient light in a conventional reflection type liquid crystal
display device simultaneously. In addition to the above, both the
features of these devices can be obtained.
[0403] As described above, according to the present invention, each
pixel includes a region having a higher transmission efficiency and
a region having a higher reflection efficiency than in the case of
using a semi-transmissive reflection film. In each region, a layer
having a high transmission efficiency or a layer having a high
reflection efficiency serves as the pixel electrode. With this
construction, unlike the conventional liquid crystal display device
using a semi-transmissive reflection film, the utilization
efficiency of ambient light and illumination light is prevented
from decreasing due to stray-light phenomenon, for example. Good
images can be displayed regardless of the brightness of ambient
light by using either a reflection mode display, a transmission
mode display, or both a reflection mode display and a transmission
mode display. Since both light from the backlight and the ambient
light contribute to the display simultaneously and efficiently,
power consumption significantly decreases compared with the
transmission type liquid crystal display device which always uses
light from the backlight.
[0404] In other words, the shortcomings that visibility is
significantly lower under low ambient light in a conventional
reflection type liquid crystal display device and the display
recognition becomes difficult under high ambient light in a
conventional transmission type liquid crystal display device can be
overcome simultaneously by increasing the light utilization
efficiency according to the present invention.
[0405] Since the regions having a high reflection efficiency partly
cover the gate lines, the source lines, and/or the switching
elements, light incident on these portions can also be used for the
display. Therefore, the effective area of the pixel increases
markedly. This not only overcomes the problems of the conventional
device using the semi-transmissive reflection film, but also
increases the aperture ratio of each pixel even if compared with a
normal transmission type liquid crystal display device.
[0406] In the case where only a layer having a high transmission
efficiency constitutes a pixel electrode, the occurrence of a
defect caused by the pixel electrode can be reduced, compared with
the case where a layer having a high transmission efficiency and a
layer having a high reflection efficiency are electrically
connected with each other to form a pixel electrode of one pixel
and the case where a layer having a high transmission efficiency
and a layer having a high reflection efficiency partly overlap each
other to form a pixel electrode of one pixel. As a result, the
yield increases.
[0407] The layer having a high transmission efficiency or the layer
having a high reflection efficiency may be made of the same
material as that for the source lines or the gate lines. This
simplifies the fabrication process of the liquid crystal display
device.
[0408] The occupation of the area of the region having a high
reflection efficiency in the effective pixel area is set at about
10 to about 90%. This setting overcomes both the problems that the
display becomes less recognizable when ambient light is too high in
a convention transmission type liquid crystal display device and
that the display becomes completely unrecognizable when the
intensity of ambient light is extremely low in a conventional
reflection type liquid crystal display device. Thus, an optimal
display can be realized as a reflection mode display, a
transmission mode display, or both a reflection mode display and a
transmission mode display, regardless of the amount of ambient
light.
[0409] The reflection/transmission type liquid crystal display
device according to the present invention is especially effective
when it is applied to an apparatus in which the display screen is
not swingable or which cannot be moved to a better environment for
the convenience of the operator.
[0410] The liquid crystal display device according to the present
invention was actually used as a view finder (monitor screen) in a
battery-driven digital camera and a video camera. As a result, it
has been found that the power consumption was kept at a low level
while the brightness suitable for observation was maintained by
adjusting the brightness of the backlight regardless of the
brightness of the ambient light.
[0411] When the conventional transmission type liquid crystal
display device is used outdoors under bright sunlight, the display
become less recognizable even if the brightness of the backlight is
increased. Under such occasions, the liquid crystal display device
of the present invention can be used as a reflection type device by
turning off the backlight, or it can be used as the
transmission/reflection type device by lowering the brightness of
the backlight. As a result, good display quality and reduced power
consumption can be realized.
[0412] When the liquid crystal display device according to the
present invention is used indoors with bright sunlight coming
thereinto, the reflection mode display and the transmission mode
display may be switched therebetween or both may be used depending
on the directional position of the object, to obtain a more
recognizable display. When the monitor screen receives direct
sunlight, the manner described in the case of an outdoors use under
bright sunlight may be adopted. When the object is to be imaged in
a dark corner of a room, the backlight is turned on in order to use
the device as a reflection/transmission mode display.
[0413] When the liquid crystal display device according to the
present invention is used as a monitor screen in a car apparatus
such as a car navigator, also, an invariably recognizable display
is realized regardless of the brightness of ambient light.
[0414] In a car navigator using the conventional liquid crystal
display device, a backlight having a higher brightness than that
used in a personal computer and the like is used, so as to be
usable during a fine weather and in an environment receiving direct
sunlight. However, despite such a high brightness, the display is
still less recognizable under the environment described above. On
the other hand, a backlight with such a high brightness is so
bright that the user is dazzled and adversely influenced. In a car
navigator using the liquid crystal display device according to the
present invention, a reflection mode display can always be used
together with a transmission mode display. This allows for a good
display under a bright environment without increasing the
brightness of the backlight. Conversely, under a pitch-dark
environment, a recognizable display is realized by obtaining only a
low brightness (about 50 to 100 cd/m.sup.2) of the backlight.
[0415] Various other modifications will be apparent to and can be
readily made by those skilled in the art without departing from the
scope and spirit of this invention. Accordingly, it is not intended
that the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed. What is claimed is: 1. A liquid crystal
displa
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