U.S. patent application number 13/958266 was filed with the patent office on 2013-11-28 for transflective liquid-crystal-display device.
This patent application is currently assigned to NLT TECHNOLOGIES, LTD.. The applicant listed for this patent is NLT TECHNOLOGIES, LTD.. Invention is credited to Hidenori Ikeno, Kenichi Mori, Hiroshi Nagai, Kenichirou Naka, Michiaki Sakamoto.
Application Number | 20130314455 13/958266 |
Document ID | / |
Family ID | 38478547 |
Filed Date | 2013-11-28 |
United States Patent
Application |
20130314455 |
Kind Code |
A1 |
Nagai; Hiroshi ; et
al. |
November 28, 2013 |
TRANSFLECTIVE LIQUID-CRYSTAL-DISPLAY DEVICE
Abstract
A LCD device has a LC layer sandwiched between a TFT substrate
and a counter substrate, first and second polarizing films, a first
.lamda./2 film between the first polarizing film and the counter
substrate, and a second .lamda./2 film between the second
polarizing film and the TFT substrate. Angle .theta.1 between the
direction of the optical axis of the LC layer and the polarized
direction of the light entering the LC layer satisfies the
relationship: 0 degree<.theta.1<45 degrees. The resultant LCD
device has lower leakage light and coloring.
Inventors: |
Nagai; Hiroshi; (Kanagawa,
JP) ; Sakamoto; Michiaki; (Kanagawa, JP) ;
Mori; Kenichi; (Kanagawa, JP) ; Naka; Kenichirou;
(Kanagawa, JP) ; Ikeno; Hidenori; (Kanagawa,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NLT TECHNOLOGIES, LTD. |
Kanagawa |
|
JP |
|
|
Assignee: |
NLT TECHNOLOGIES, LTD.
Kanagawa
JP
|
Family ID: |
38478547 |
Appl. No.: |
13/958266 |
Filed: |
August 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12873108 |
Aug 31, 2010 |
8502941 |
|
|
13958266 |
|
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11682646 |
Mar 6, 2007 |
7864274 |
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12873108 |
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Current U.S.
Class: |
345/690 ;
345/89 |
Current CPC
Class: |
G02F 2001/133565
20130101; G02F 1/133528 20130101; G02F 2413/03 20130101; G02F
1/133555 20130101; G02F 1/134363 20130101; G09G 3/3607 20130101;
G02F 2001/133638 20130101; G02F 2413/09 20130101; G02F 1/13363
20130101 |
Class at
Publication: |
345/690 ;
345/89 |
International
Class: |
G09G 3/36 20060101
G09G003/36 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2006 |
JP |
2006-061383 |
Claims
1-31. (canceled)
32. A method for driving a transflective LCD device having a liquid
crystal (LC) layer defining a plurality of pixels each having a
transmissive area and a reflective area, at least said transmissive
area operating in a lateral-electric-field mode; first and second
polarizing films sandwiching therebetween said LC layer, said first
polarizing film being effective common to said transmissive area
and said reflective area, said second polarizing film being
effective to said transmissive area; and a retardation film
sandwiched between said first polarizing film and said LC layer,
said method comprising the steps of: generating a first data signal
and a second data signal having therebetween a specific potential
relationship; and applying said first data signal and said second
data signal to said reflective area and said transmissive area,
respectively.
33. The method according to claim 32, wherein said relationship
between said first data signal and said second data signal is such
that said first data signal assumes a maximum gray-scale-level
potential when corresponding said second data signal assumes a
minimum gray-scale-level potential.
34. The method according to claim 32, further comprising the step
of applying a first common electrode signal to a first common
electrode disposed in said reflective areas of a plurality of said
pixels and a second common electrode signal to a second common
electrode disposed in said transmissive areas of a plurality of
said pixels, said first common electrode signal having a potential
different from a potential of said second common electrode
signal.
35. The method according to claim 32, wherein said LCD device
includes a first switching device for coupling a data line to said
first pixel electrode and a second switching device for coupling
said data line to said second pixel electrode, a first gate line
for controlling said first switching device, a second gate line for
controlling said second switching device, said method further
comprising the steps of: turning ON said first and second switching
devices in a time-division scheme, to apply a common data signal to
said first and second pixel electrodes; applying a first common
electrode signal to a common electrode during applying said common
data signal to said first pixel electrode; and applying a second
common electrode signal to said common electrode during applying
said common data signal to said second pixel electrode, said first
common electrode signal having a potential different from a
potential of said second common electrode signal.
36. The method according to claim 32, wherein said LCD device
includes a first switching device for coupling a first data line to
said first pixel electrode, a second switching device for coupling
a second data line to said second pixel electrode, said method
further comprising the step of: applying said first and second data
signals to said first and second data lines, respectively.
37. The method according to claim 36, wherein one of said first and
second data signals is supplied from outside of said LCD device,
and the other of said first and second data signals has a
gray-scale level converted from a grays-scale level of said one of
said first and second data signals by using a look-up table.
38. The method according to claim 37, wherein said look-up table is
such that similar .gamma.-characteristics are obtained for both
said reflective and transmissive areas.
39. The method according to claim 32, wherein said LCD device
includes a single common electrode for both said reflective area
and said transmissive area of a plurality of said pixels, said
method further comprising the steps of: applying said single common
electrode with a first common electrode signal at the timing of
writing said first data signal; and applying said single common
electrode signal with a second common electrode signal at the
timing of writing said second data signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention The present invention relates to a
transflective liquid-crystal-display (LCD) device and, more
particularly, to a transflective LCD device having a transmissive
area and a reflective area in each pixel of the LCD device.
[0002] 2. Description of the Related Art
[0003] LCD devices are roughly categorized into two types including
a transmissive LCD device and a reflective LCD device. In general,
the transmissive LCD device has a backlight unit and controls the
transmission amount of light from the backlight unit to thereby
represent an image on the screen. The reflective LCD device has a
reflection film that reflects light incident from the outside and
uses the light reflected by the reflection film as a light source
to represent an image. That is, the reflective LCD device does not
require the backlight unit, and is more advantageous than the
transmissive LCD device in terms of a reduction in power
dissipation, thickness, and weight. However, since the reflective
LCD device uses the background light as the light source,
visibility of the image is degraded if the background of the LCD
device is dark.
[0004] As a LCD device having both advantages of the reflective LCD
device and transmissive LCD device, a transflective LCD device is
known (refer to, e.g., JP-2003-344837A, FIGS. 4 and 20, columns
0009 to 0019, columns 0045 to 0048). The transflective LCD device
has a transmissive area and a reflective area in each pixel of the
LCD device. The transmissive area transmits the light emitted from
a backlight unit and uses the backlight unit as a light source for
representing the image. The reflective area has a reflection film
and uses the light incident thereto from the outside and reflected
by the reflection film as the light source. In the transflective
LCD device, if the background of the LCD device is light, the image
is represented on the screen by using the reflective area, with the
backlight unit being turned off, to thereby achieve a reduction in
power dissipation. On the other hand, if the background of the LCD
device is dark, the backlight unit is turned on to represent the
image by using the transmissive area, thereby enabling the image to
be represented on the screen even in a dark background.
[0005] A lateral electric-field LCD device such as an
in-plane-switching mode (IPS mode) is known as a display mode of
the LCD device. The LCD device has a pixel electrode and a common
electrode formed in each pixel on the common substrate, and these
electrodes apply therebetween a lateral electric field to a liquid
crystal (LC) layer. The IPS-mode LCD device rotates the LC
molecules in the LCD layer in the direction parallel to the surface
of the substrate so as to represent the image, thereby achieving a
wider viewing angle than a twisted-nematic(TN)-mode LCD device.
[0006] If the IPS mode is adopted in the transflective LCD device,
a problem arises in that the black image and the white image are
inverted between both the areas, as described in the technique of
the above patent publication. More specifically, if the
transmissive area is set at a normally-black mode, the reflective
area will have a normally-white mode. The problem of the image
inversion will be described hereinafter for a better understanding
of the present invention.
[0007] FIG. 22A schematically shows the sectional view of the
transflective LCD device, wherein a double-sided arrow shows the
direction of polarization axis of the polarizing film as viewed
parallel to the substrate. FIG. 22B shows the polarized direction
of light in both the reflective and transmissive areas 55 and 56
for the structure of FIG. 22A, upon emission of the light at the
first polarizing film 51, LC layer 53 and second polarizing film
52. In FIG. 22B, the double-sided arrow depicts the
linearly-polarized light, a thick arrow represents the traveling
direction of the light, a circled "R" denotes a
clockwise-circularly-polarized light, a circled "L" denotes a
counterclockwise-circularly-polarized light, and a hollow bar
denotes the direction of LC director (molecules). FIG. 22A shows
the state of a single pixel including the reflective area 55 and
the transmissive area 56. The reflective area 55 uses the reflected
light from a reflection film 54 as a light source, and the
transmissive area 56 uses the backlight unit as a light source.
[0008] The polarizing film (first polarizing film) 51 on the light
emitting side is a common polarizing film effecting on both the
reflective and transmissive areas 55 and 56, whereas the polarizing
film (second polarizing film) 52 is a dedicated polarizing film
effecting on the light incident side of the transmissive area 56.
These polarizing films 51 and 52 are arranged such that the
polarizing axes thereof cross at right angles.
[0009] In the LC layer 53, LC molecules are arranged such that the
molecular direction upon absence of applied voltage is shifted by
90 degrees relative to direction of the polarization axis (optical
transmission axis) of the second polarizing film 52. Assuming that
the direction of the polarization axis of the second polarizing
film 52 shown in FIG. 22A is the reference direction, or at 0
degree, the direction of the polarization axis of the first
polarizing film 51 is set at 90 degrees and the initial direction
of the longitudinal axis of LC molecules in the LC layer 53 is set
at 90 degrees, as shown in FIG. 22A.
[0010] In the transmissive area of the LC layer 53, the cell gap of
the LC layer 53 is adjusted such that the retardation .DELTA.nd
(.DELTA.n is the refractive index anisotropy of LC molecules and
"d" is the cell gap) assumes .lamda./2 (.lamda. is the wavelength
of light; for example, in the case of green light, .lamda. is 550
nm) and, in the reflective area 55 of the LC layer 53, the cell gap
is adjusted such that the retardation assumes .lamda./4. The image
represented on the screen of the LCD device will be described
hereinafter in the case of absence and presence of applied voltage
for the respective areas 55 and 56.
<Reflective Area Upon Absence of Applied Voltage>
[0011] The image in the reflective area 55 upon absence of applied
voltage (Voff) on the LC layer 53 will be described first with
reference to the leftmost column of FIG. 22B. In the reflective
area 55, 90-degree linearly-polarized light passing through the
first polarizing film 51 enters the LC layer 53. The direction of
the optical axis of the linearly-polarized light that has entered
the LC layer 53 and direction of the longitudinal axis of LC
molecules are aligned in this case, whereby the 90-degree
linearly-polarized light is passed through the LC layer 53 without
a change in the polarization and is reflected by the reflection
film 54. Thus, the 90-degree linearly-polarized light enters the LC
layer 53 once again without a change. The 90-degree
linearly-polarized light is thus emitted through the LC layer 53
and enters the first polarizing film 51. Since the direction of the
polarization axis of the first polarizing film 51 is set at 90
degrees, the linearly-polarized light is passed through the first
polarizing film 51. As a result, a bright image (white image) is
represented upon Voff of the LCD device.
<Reflective Area Upon Presence of Applied Voltage>
[0012] Next, the state in the reflective area 55 upon presence of
applied voltage on the LC layer 53 will be described with reference
to the second leftmost column in FIG. 22B. A 90-degree
linearly-polarized light passing through the first polarizing film
51 enters the LC layer 53. Here, the applied voltage causes the
direction of the longitudinal axis of LC molecules in the LC layer
53 to be changed from 0 degree to 45 degrees with respect to the
surface of the substrate. Since the polarized direction of the
light that has entered the LC layer 53 is deviated by 45 degrees
from the direction of the longitudinal axis of LC molecules and the
retardation of the liquid crystal is set at .lamda./4, the
90-degree linearly-polarized light that has entered the LC layer 53
assumes a clockwise-circularly-polarized light, which enters the
reflection film 54. The clockwise-circularly-polarized light is
reflected by the reflection film to be changed into a
counterclockwise-circularly-polarized state. The
counterclockwise-circularly-polarized light that has entered the LC
layer 53 passes therethrough once again to be changed into a
horizontal (0-degree) linearly-polarized light. The
horizontal-linearly-polarized light then enters the first
polarizing film 51. Since the direction of the polarization axis of
the first polarizing film 51 is at 90 degrees, the light reflected
by the reflection film 54 cannot be passed through the first
polarizing film 51, with the result that a dark image (black) is
represented on the screen.
[0013] As described above, the reflective area 55 assumes a
normally white mode in which a bright image (white) is represented
upon absence of applied voltage (Voff) and a dark image (black) is
represented upon presence of applied voltage (Von).
<Transmissive Area Upon Absence of Applied Voltage>
[0014] Next, the state in the transmissive area 56 upon absence of
applied voltage on the LC layer 53 will be described with reference
to the second rightmost column in FIG. 22B. In the transmissive
area 56, a horizontal-linearly-polarized light passing through the
second polarizing film 52 enters the LC layer 53. The polarized
direction of the incident light and longitudinal direction of LC
molecules cross each other at right angles, whereby the
horizontal-linearly-polarized light is passed through the LC layer
53 without a change in the polarization and enters the first
polarizing film 51. Since the direction of the polarization axis of
the first polarizing film 51 is at 90 degrees, the transmitted
light cannot be passed through the first polarizing film 51,
resulting in display of a dark image on the screen.
<Transmissive Area Upon Presence of Applied Voltage>
[0015] Next, the state in the transmissive area 56 upon presence of
applied voltage on the LC layer 53 will be described with reference
to the rightmost column in FIG. 22B. In the transmissive area 56, a
horizontal-linearly-polarized light passing through the second
polarizing film 52 enters the LC layer 53. Here, the applied
voltage causes the direction of the longitudinal axis of LC
molecules in the LC layer 53 to be changed from zero degree to 45
degrees with respect to the surface of the substrate. Thus, the
polarized direction of the light that has entered the LC layer 53
is shifted to 45 degrees with respect to the direction of the
longitudinal axis of LC molecules. Since the retardation of the LC
layer is set at .lamda./2, the horizontal-linearly-polarized light
that has entered the LC layer 53 is changed into a
vertical-linearly-polarized light and enters the first polarizing
film 51. As a result, in the transmissive area 56, the first
polarizing film 51 passes the backlight transmitted thereto through
the second polarizing film 52, resulting in representing a bright
image on the screen.
[0016] As described above, the transmissive area 56 assumes a
normally black mode in which a dark image is represented upon
absence of applied voltage (Voff) and a bright image is represented
upon presence of applied voltage (Von).
[0017] In the above configuration, the transflective LCD device has
a disadvantage in that a dark image and a bright image are inverted
between the reflective area 55 and the transmissive area 56 upon
both the presence and absence of the applied voltage on the LC
layer 53. A technique to solve this disadvantage is described in
the above patent publication. FIG. 23 shows a sectional view of the
LCD device described in the patent publication. In this technique,
the direction of the polarization axis of the first polarizing film
51 is shifted by 45 degrees from the direction of the longitudinal
axis of LC molecules in the LC layer 53. The mere arrangement of
the polarization axis of the first polarizing film 51 and the
longitudinal axis of LC molecules in the LC layer 53 will cause the
reflective area 55 using the reflection film 54 as the light source
to assume a normally black mode and cause the transmissive area 56
using the backlight unit 57 as the light source to assume a
normally white mode. In addition thereto, a .lamda./2 film 58 is
inserted between the second polarizing film 52 and the LC layer 53
to thereby change the transmissive area 56 into a normally black
mode, which accords the normally black mode of the reflective area
55.
[0018] The direction of the optical axis of the .lamda./2 film 58
that crosses the direction of the longitudinal axis of the LC layer
53 at right angles is set at 135 degrees. Thus, in front view of
the LCD device, optical compensation is achieved wherein the
polarization effect that the LC layer 53 having a retardation of
.lamda./2 exerts on the light compensates the polarization effect
of the .lamda./2 film. The optical compensation achieves that the
polarized state of light is not changed between the incidence and
emission thereof, in consideration of the polarization of light
effected by the LC layer 53 and .lamda./2 film 58 as a whole.
Therefore, the light passing through the second polarizing film 52
to assume the horizontal-linear-polarized light is passed through
the LC layer 53 and .lamda./2 film 58 without a change in the
polarization and cannot be passed through the first polarizing film
51 having an optical axis set at the vertical direction. That is,
the insertion of the .lamda./2 film 58 between the LC layer 53 and
the second polarizing film 52 causes the transmissive area 56 to
assume also a normally black mode.
[0019] However, in the LCD device 50a shown in FIG. 23, the
polarized direction of the light that enters the LC layer 53 and
direction of the longitudinal axis of the LC molecules in the LC
layer 53 are not parallel or perpendicular to each other in the
transmissive area 56. This leads to a disadvantage in that leakage
light cannot be suppressed sufficiently, upon display of a dark
image, in the transmissive area 56 due to wavelength dispersion
characteristics of retardation in the LC layer 53. Further, the
.lamda./2 film 58 also has wavelength dispersion characteristics,
thereby causing leakage light due to the wavelength dispersion upon
display of a dark image.
[0020] In order for solving the above problem, a configuration may
be also considered wherein absence of applied voltage in the
transmissive area 56 is achieved upon presence of applied voltage
in the reflective area by inverting the voltage applied to the
transmissive area to thereby apply the inverted voltage to the
reflective area. However, such a device scheme or drive technique
that can realize this configuration is not known in the art. In
addition, problems encountered by this configuration as well as the
countermeasures for solving the problems are not known in the
art.
[0021] Next, in an IPS-mode transflective LCD device, a
configuration will be considered in which a first polarizing film,
a first .lamda./2 film, a first .lamda./4 film, a first LC-layer
compensation film (positive or negative .lamda./4 film), an LC
layer, a second LC-layer compensation film (positive or negative
.lamda./4 film), a second .lamda./4 film, a second .lamda./2 film,
and a second polarizing film are consecutively layered one on
another from the light emitting side. In this configuration, each
of the first polarizing film, .lamda./2 film, .lamda./4 film and
second polarizing film, .lamda./2 film, .lamda./4 film are so
arranged as a broadband .lamda./4 film.
[0022] If the first and second LC-layer compensation films each are
a positive .lamda./4 film, these films are arranged such that the
optical axes thereof cross the direction of the longitudinal axis
of LC molecules at right angles. On the other hand, if the first
and second LC-layer compensation films each are a negative
.lamda./4 film, these films are arranged such that the optical axes
thereof are parallel to the direction of the longitudinal axis of
LC molecules. As a result, the LC layer is configured as a.lamda./2
film.
[0023] Accordingly, the effective retardation .DELTA.nd of the
first and second LC-layer compensation films and LC layer in total
assumes 0 in the state of initial orientation of the LC molecules,
thereby providing a dark image in a normally black mode in both the
transmissive area and reflective area. However, in this
configuration, it is impossible to completely perform phase
compensation of the polarized light if the birefringence wavelength
dispersion differs between the LC-layer compensation film and the
LC layer. Further, it is difficult to perform a gap control for the
LC layer. As a result, leakage light and/or coloring (chromaticity
shift) occurs during display of a dark image. Therefore, the
birefringence wavelength dispersion of a material used for the
compensation layer and wavelength dispersion of the LC layer need
to correspond to each other. Thus, there remains the problem that
cannot be solved only by the device configuration.
SUMMARY OF THE INVENTION
[0024] It is an object of the present invention to provide a
transflective LCD device capable of reducing coloring or leakage
light, upon display of a dark image, due to wavelength dispersion
of the LC layer.
[0025] It is another object of the present invention to provide a
transflective LCD device capable of eliminating the problems
related to inversion of bright and dark image between in the
transmissive area and in the reflective area, without requiring a
complicated structure in order for processing data signals in the
LCD device.
[0026] The present invention provides a transflective
liquid-crystal-display (LCD) device including: a liquid crystal
(LC) layer defining a plurality of pixels each having a
transmissive area and a reflective area; first and second
polarizing films sandwiching therebetween the LC layer, the first
polarizing film being effective common to the transmissive area and
the reflective area, the second polarizing film being effective to
the transmissive area; and a retardation film sandwiched between
the first polarizing film and the LC layer.
[0027] In accordance with the LCD device of the present invention,
the retardation film provided between the first polarizing film and
the LC layer compensates the wavelength dispersion of the LC layer
upon display of a dark image, thereby suppressing the coloring
and/or leakage light of the LCD device. The retardation film may
acts as a 1/2-wavelength film (or .lamda./2 film) at a wavelength
of 550 nm.
[0028] The above and other objects, features and advantages of the
present invention will be more apparent from the following
description, referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1A is a sectional view showing the structure of a
transflective LCD device according to a first embodiment of the
present invention, and FIGS. 1B and 1C are modification from the
structure of FIG. 1A;
[0030] FIG. 2 is a top plan view showing the structure of the LCD
device according to the first embodiment of the present
invention;
[0031] FIGS. 3A and 3B are waveform diagrams each showing potential
change of the pixel electrodes 35 and 36 after the pixel data
signal has been supplied to the pixel electrodes 35 and 36;
[0032] FIG. 4A is a waveform diagram showing a drive signal for the
reflective area 21 in a specific stage, and FIG. 4B is a waveform
diagram showing a drive signal for the transmissive area 22 in the
same specific stage;
[0033] FIGS. 5A and 5B are schematic sectional views showing light
polarization states in the reflective and transmissive areas when
the signals shown in FIGS. 4A and 4B are applied;
[0034] FIG. 6A is a waveform diagram showing a drive signal for the
reflective area 21 in a stage different from FIG. 4, and FIG. 6B is
a waveform diagram showing a drive signal for the transmissive area
22 in the same phase as FIG. 6A;
[0035] FIGS. 7A and 7B are schematic sectional views showing light
polarization states in the reflective and transmissive areas when
the signals shown in FIGS. 6A and 6B are applied;
[0036] FIGS. 8A and 8B are diagrams each obtained by simulation and
showing electric field distribution and light transmittance in a
dark state;
[0037] FIG. 9 is a sectional view showing the structure of the
reflection film 16 right under the pixel electrodes 35 and 36 and
common electrode 37;
[0038] FIGS. 10A to 10D show a fabrication step of process for
manufacturing the TFT substrate, wherein FIG. 10A depicts a top
plan view thereof, and FIGS. 10B to 10D depict sectional views
taken along respective lines shown in FIG. 10A;
[0039] FIGS. 11A and 11B show a fabrication step subsequent to the
step of FIGS. 10A to 10D, wherein FIG. 10A depicts a top plan view
thereof, and FIG. 11B depicts a sectional view taken along line
D-D' in FIG. 11B.
[0040] FIGS. 12A to 12D show a fabrication step subsequent to the
step of FIGS. 11A and 11B, wherein FIG. 12A is a top plan view
thereof, and FIGS. 10B to 10D depict sectional views taken along
respective lines shown in FIG. 12A;
[0041] FIG. 13A to 13D show a fabrication step subsequent to the
step of FIGS. 12A to 12D, wherein FIG. 13A is a top plan view
thereof, and FIGS. 13B to 13D depict sectional views taken along
respective lines shown in FIG. 13A;
[0042] FIGS. 14A to 14D show a fabrication step subsequent to the
step of FIGS. 13A to 13D, wherein FIG. 14A is a top plan view
thereof, and FIGS. 14B to 14D depict sectional views taken along
respective lines shown in FIG. 14A;
[0043] FIGS. 15A to 15D show a fabrication step subsequent to the
step of FIGS. 14A to 14D, wherein FIG. 15A is a top plan view
thereof, and FIGS. 15B to 15D depict sectional views taken along
respective lines shown in FIG. 15A;
[0044] FIGS. 16A and 16B show a fabrication step subsequent to the
step of FIGS. 15A to 15D, wherein FIG. 16A is a top plan view
thereof, and FIG. 16B depicts a sectional view taken along line
E-E' shown in FIG. 16A;
[0045] FIG. 17A to 17D show a fabrication step subsequent to the
step of FIGS. 16A and 16B, wherein FIG. 17A is a top plan view
thereof, and FIGS. 17B to 17D depict sectional views taken along
respective lines shown in FIG. 17A;
[0046] FIG. 18 is a schematic sectional view showing the structure
of a transflective LCD device according to a second embodiment of
the present invention;
[0047] FIG. 19 is a table showing a suitable combination of the
optical transmission axis of the polarizing films, direction of the
longitudinal axis of LC molecules in the LC layer, direction of the
optical axis of the .lamda./2 films with respect to the surface of
the substrate;
[0048] FIG. 20 is a schematic view showing the polarized state of
the light in the LCD device of the second embodiment;
[0049] FIG. 21 is a sectional view showing the reflection film 16
right under the pixel electrode 34 and common electrode 35;
[0050] FIG. 22A is a schematic sectional view showing a
conventional transflective LCD device, and FIG. 22B is a schematic
view showing the polarized state of light in the reflective and
transmissive areas of the LCD device of FIG. 22A when the light is
passed through the first polarizing film, LC layer, and second
polarizing film; and
[0051] FIG. 23 is a sectional view showing the structure and
polarized state of the light in the conventional transflective LCD
device described in a patent publication.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] Embodiments of the present invention will be described below
in detail with reference to the accompanying drawings. FIG. 1A
shows the sectional structure of a LCD device according to a first
embodiment of the present invention. The LCD device, generally
designated at numeral 10, includes a first polarizing film 11, a
first .lamda./2 film 18, a counter substrate 12, a LC layer 13, a
TFT substrate 14, a second .lamda./2 film 19, and a second
polarizing film 15, which are consecutively layered from the front
side of the LCD device 10.
[0053] The LCD device 10 is configured as a transflective LCD
device having a reflective area 21 and a transmissive area 22 in
each pixel of the LCD device 10. In the reflective area 21, a
reflection film 16 and an insulating film 17 are formed on the TFT
substrate 14. The reflection film 16 reflects the incident light
incident through the first polarizing film 11. In order to enhance
light dispersion effect, the reflection film 16 typically has a
rough surface.
[0054] A pixel electrode 35 for driving LC molecules and a common
electrode 37 for supplying a reference potential are formed on the
insulating film 17 in the reflective area 21. On the other hand, a
pixel electrode 36 and a common electrode 38 are formed on the TFT
substrate 14 in the transmissive area 22. The reflective area 21
uses the light reflected by the reflection film 16 as the light
source for representing the image on the screen. The LCD device 10
has a backlight unit (not shown in the figure) below the second
polarizing film 15, and the transmissive area 22 uses the backlight
unit as the light source for representing the image on the
screen.
[0055] In the transmissive area 22, the cell gap of the LC layer 13
is adjusted such that the retardation of the LC layer 13 assumes
nearly .lamda./2. The reason for setting the retardation to "nearly
.lamda./2" is to obtain an effective retardation of .lamda./2 by
actually setting the retardation at (.lamda./2)+.alpha.. This
margin .alpha. is necessary because, when the presence of applied
voltage on the LC layer 13 rotates LC molecules, the LC molecules
are rotated only in the central area of the cell gap, with the
rotation of the LC molecules being suppressed near both the
substrates. Assuming that retardation of the LC layer 13 is set at
.DELTA.nd=300 nm, the effective retardation upon presence of
applied voltage assumes .DELTA.nd.sub.eff=.lamda./2=550/2=275 nm.
On the other hand, in the reflective area 21, the cell gap is
adjusted such that the effective retardation of the LC layer 13
upon presence of applied voltage assumes .lamda./4 by setting a
suitable thickness for the insulating film 17.
[0056] In the transmissive area 22, upon display of a dark image,
the linearly-polarized light emitted from the backlight unit and
passing through the second polarizing film 15 is passed through the
second .lamda./2 film 19, LC layer 13, and first .lamda./2 film 18
and enters the first polarizing film 11. In the configuration of
the LCD device 10, the angle of the polarization axis and direction
of the longitudinal axis of LC molecules are set such that the
light entering the first polarizing film 11 assumes a
linearly-polarized light and the polarized direction thereof
corresponds to the direction of the absorption axis of the first
polarizing film 11. Further, angle .theta.1 between the direction
of the optical axis (or optical axis +90 degrees) of the LC layer
13 and the polarized direction of the light entering the LC layer
13 upon display of a dark image is set to satisfy the following
relationship:
0 degree<.theta.1<45 degrees,
and preferably satisfy the following relationship:
0 degree<.theta.1.ltoreq.22.5 degrees.
[0057] In the description to follow, direction of the optical
transmission axis of the first polarizing film 11 is set at 27
degrees, direction of the optical axis of the first .lamda./2 film
18 is set at 109.5 degrees, light transmission axis of the second
polarizing film 15 is set at 63 degrees, direction of the optical
axis of the second .lamda./2 film 19 is set at 70.5 degrees,
direction of the longitudinal axis of LC molecules in the LC layer
13 upon absence of applied voltage is set at 90 degrees, and angle
.theta.1 between the linearly-polarized light entering the LC layer
13 from the second .lamda./2 film 19 and direction of the optical
axis (direction of the longitudinal axis) of the LC molecules is
set at 12 degrees.
[0058] FIGS. 1B and 1C show modifications from FIG. 1A. In FIG. 1B,
the .lamda./2 film 18 is provided between the LC layer 13 and the
counter substrate 12, in both the transmissive and reflective areas
21 and 22, whereas in FIG. 1C, a retardation film 18a acting as a
.lamda./4 film in association with the LC layer 13 is provided
between the LC layer 13 and the counter substrate 12 only in the
reflective area. It is sufficient in the LCD device of the present
invention that the retardation film 18 or 18a be provided between
the first polarization film 11 and the LC layer 13.
[0059] FIG. 2 shows a top plan view of the TFT substrate 14 of the
LCD device 10 shown in FIG. 1A, depicting the configuration of a
single pixel therein. A gate line 31 and a data line 32 that cross
each other at right angles are formed on the TFT substrate 14. Near
the intersection between the gate line 31 and the data line 32,
TFTs 33 and 34 are formed in the reflective area 21 and
transmissive area 22, respectively. The gate of the TFTs 33 and 34
is connected to the gate line 31, and one of the source and drain
thereof are connected to the data line 32. The other of the source
and drain of the TFTs 33 and 34 are connected to the pixel
electrode 35 in the reflective area 21 and pixel electrode 36 in
the transmissive area 22, respectively.
[0060] The first and second common electrodes 37 and 38 correspond
to the reflective area 21 and transmissive area 22, respectively,
of each pixel and have a part that extends along the gate line 31
and a part that protrudes toward the display area of the pixel. The
first common electrode 37 is formed in the reflective area 21 at
the position opposite to the pixel electrode 35 within the
substrate plane. The second common electrode 38 is formed in the
transmissive area 22 at the position opposite to the pixel
electrode 36 within the substrate plane. A signal having a
predetermined signal waveform and common to the array of pixels in
the LCD device 10 is supplied to the first and second common
electrodes 37 and 38. In the reflective area 21, the orientation of
the LC layer 13 is controlled by an electric field corresponding to
the potential difference between the pixel electrode 35 and the
first common electrode 37. In the transmissive area 22, the
orientation of the LC layer 13 is controlled by an electric field
corresponding to the potential difference between the pixel
electrode 36 and the second common electrode 38.
[0061] The pixel electrode 35 for the reflective area 21 and pixel
electrode 36 for the transmissive area 22 are connected to the
respective TFTs 33 and 34. TFTs 33 and 34 are connected to a common
gate line 31 and to a common data line 32, whereby a common data
signal is written in the pixel electrodes 35 and 36 when the TFTs
33 and 34 are turned on. The reason for the reflective area 21 and
transmissive area 22 to use respective TFTs 33 and 34 and the
respective pixel electrodes 35 and 36, in spite of the common data
signal being written in the pixel electrode 35 for the reflective
area 21 and pixel electrode 36 for transmissive area 22, is that
the potential fluctuation is different between the pixel electrode
36 in the transmissive area 22 and the pixel electrode 35 in the
reflective area 21, after the TFTs 33 and 34 have been turned
off.
[0062] FIGS. 3A and 3B show the potential profile of the pixel
electrodes 35 and 36 after the common pixel signal has been
supplied to the pixel electrodes 35 and 36. For example, the
polarity of the drive signal is inverted line by line in a
gate-line-inversion drive scheme, whereby the potential of the
first and second common electrodes 37 and 38 is iteratively
inverted in accordance with the polarity inversion of each line
during the time period between application of a gate signal pulse
to the gate line 31 and another application of a gate signal pulse
to the gate line 31 in the next frame.
[0063] Since the TFTs 33 and 34 are off at this stage, the pixel
electrodes 35 and 36 are disconnected from the data line 32 to be
in a floating state and, as shown in FIGS. 3A and 3B, the potential
thereof is fluctuated in accordance with the potential profile of
the first and second common electrodes 37 and 38, while keeping the
potential difference upon the write of signal due to the coupling
capacitance between the pixel electrode 35 and the first common
electrode 37 and between the pixel electrode 36 and the second
common electrode 38. As described above, the potential profile of
the pixel electrodes 35 and 36 after the supply of pixel signal
differs between the reflective area 21 and transmissive area 22.
Thus, the pixel electrode should be separated between the
reflective area 21 and transmissive area 22.
[0064] FIG. 4A shows a drive signal waveform of the reflective area
21 in a specific stage, and FIG. 4B shows a drive signal waveform
of the transmissive area 22 in the same specific stage. The LCD
device 10 is driven by first and second common signals having an
alternating voltage and, in each pixel, the potential (signal)
applied to the first and second common electrodes 37 and 38 is
inverted between, e.g., 0V and 5V in each frame, as shown in FIGS.
4A and 4B. Further, the inverted signal of the first common signal
applied to the first common electrode 37 is applied to the second
common electrode 38 as a second common signal.
[0065] A specific pixel signal ranging, e.g., between 0V and 5V is
supplied to the pixel electrodes 35 and 36. Since the TFTs 33 and
34 are connected to the common data line 32, a common pixel signal
is supplied to the pixel electrodes 35 and 36. As shown in FIG. 4A,
when a 0V data signal is supplied to the pixel electrode 35 and a
5V common signal is applied to the first common electrode 37 in the
i-th frame ("i" is a natural number), the potential difference
between the pixel electrode 35 and the first common electrode 37
assumes 5V. Thus, in the reflective area 21, the LC layer 13 is
driven by an electric field corresponding to a 5V potential
difference. At this stage, a 0V signal is applied to the second
common electrode 38, whereby the potential difference between the
pixel electrode 36 and the second common electrode 38 assumes 0V.
Thus, the LC molecules in the LC layer 13 are not driven in the
transmissive area 22.
[0066] FIGS. 5A and 5B show polarization of the light in the
reflective area 21 and transmissive area 22, respectively, upon
application of the signals shown in FIGS. 4A and 4B. When the
signal shown in FIG. 4A is applied, the orientation direction of LC
molecules in the LC layer 13 in the reflective area 21 is rotated
by 33 degrees (=45 degrees-.theta.1 (=12 degrees)) due to the
electric field between the pixel electrode 35 and the first common
electrode 37. That is, the direction of the longitudinal axis of LC
molecules is changed from 90 degrees to 57 degrees.
[0067] In the reflective area 21, as shown in FIG. 5A, light that
has entered the first polarizing film 11 from the outside of LCD
device has been passed through the first polarizing film 11 to
assume a 27-degree linearly-polarized light. The polarized
direction of the 27-degree-linearly-polarized light is changed to
12 degrees after being passed through the first .lamda./2 film 18
and the 12-degree linearly-polarized light enters the LC layer 13
at an incident angle of 45 degrees with respect to the optical axis
of the LC layer. The polarization of this incident light is changed
upon being passed through the LC layer 13 to assume a
counterclockwise-circularly-polarized light.
[0068] The counterclockwise-circularly-polarized light is then
reflected by the reflection film 16 to be changed to a
clockwise-circularly-polarized light. The
clockwise-circularly-polarized light is passed through the LC layer
13 once again to assume a 102-degree linearly-polarized light,
which is then passed through the first .lamda./2 film 18 to assume
a 117-degree linearly-polarized light. Therefore, the light
reflected by the reflection film 16 cannot be passed through the
first polarizing film 11, with the result that the reflective area
21 assumes a dark state.
[0069] Although the drive voltage for maintaining the LC molecules
at a constant orientation angle is increased in the reflective area
21 due to the narrow gap on the LC layer 13, the orientation angle
to which LC molecules are driven is reduced to 33 degrees due to
the angle .theta.1 being set at 12 degrees. As a result, it is
possible to reduce the drive voltage down to 0.9 times, compared to
the case where the LC molecules are rotated by 45 degrees.
[0070] On the other hand, in the state where the signal shown in
FIG. 4B is applied, an electric field is not generated between the
pixel electrode 36 and the second common electrode 38, whereby the
direction of orientation of LC molecules in the transmissive area
22 is kept at 90 degrees. In the transmissive area 22, as shown
FIG. 5B, 63-degree linearly-polarized light passing through the
second polarizing film 15 is changed to 78-degree
linearly-polarized light after being passed through the second
.lamda./2 film 19 and then enters the LC layer 13. The direction of
the longitudinal axis of LC molecules in the LC layer is at 90
degrees. Accordingly, the 78-degree linearly-polarized light that
has entered the LC layer 13 assumes a 102-degree linearly-polarized
light after being passed through the LC layer 13. The 102-degree
linearly-polarized light then assumes a 117-degree
linearly-polarized light after being passed through the first
.lamda./2 film 18, which cannot be passed through the first
polarizing film 11 having an optical transmission axis at 27
degrees, with the result that the transmissive area 22 assumes a
dark state.
[0071] As described above, by inverting the first common signal
applied to the first common electrode 37 to apply the inverted
common signal to the second common electrode 38, the direction of
orientation of LC molecules in the LC layer 13 can be changed only
in the reflective area 21 while the common pixel signal is being
supplied to the pixel electrodes 35 and 36. As a result, a dark
state can be achieved in the transmissive area 22 when the dark
state is represented in the reflective area 21. Thus, it is
possible to display the dark state both in the reflective and
transmissive areas without the need to supply different pixel
signals to the reflective and transmissive areas 21 and 22.
[0072] FIG. 6A shows a drive signal waveform of the reflective area
21 in a stage different from the stage shown in FIG. 4, and FIG. 6B
shows another drive signal waveform of the transmissive area 22 in
the same stage as that of FIG. 6A. FIGS. 7A and 7B show
polarization of light in the reflective and transmissive areas 21
and 22 when the signals shown in FIGS. 6A and 6B are applied. In
the state where the signal shown in FIG. 6A is applied, an electric
field is not generated between the pixel electrode 35 and the first
common electrode 37, whereby the direction of orientation of LC
molecules in the reflective area 21 is kept at 90 degrees.
[0073] In the reflective area 21, as shown FIG. 7A, a 12-degree
linearly-polarized light passing through the first polarizing film
11 and first .lamda./2 film 18 is passed through the LC layer 13,
reflected by the reflection film 16, and passed once again through
the LC layer 13 to assume a -12-degree linearly-polarized light.
The -12-degree linearly-polarized light is passed through the first
.lamda./2 film 18 to assume a 51-degree linearly-polarized light,
which enters the first polarizing film 11. Since the direction of
optical transmission axis of to the first polarizing film 11 is at
27 degrees, all the components of light assuming a 51-degree
linearly-polarized light and reflected by the reflection film 16
travels toward the first polarizing film 11; however, cannot pass
through the first polarizing film 11. In this state, a highest
reflectivity achieving a highest luminance is obtained, whereby the
reflective area 21 assumes a bright state.
[0074] The angle deviation between the polarized direction of light
entering the first polarizing film 11 from the reflection film 16
in a bright state and the direction of the optical transmission
axis of the first polarizing film 11 can be represented by
.theta.1.times.2. More specifically, if the angle .theta.1 is set
at zero degree, the brightest image can be displayed and, for
example, the reflectivity obtained when .theta.1 is set at 12
degrees assumes 0.9 times the case where .theta.1 is set at zero
degree.
[0075] As described above, by increasing the angle .theta.1, the
angle by which LC molecules are driven can be reduced to thereby
reduce a drive voltage. However, there is a trade-off in the
relationship between the decrease in the drive voltage and increase
in the reflectivity. In designing a LCD device, the angle .theta.1
should be set in consideration of a balance between the drive
voltage and the reflectivity.
[0076] In a state where the signal shown in FIG. 6B is applied, the
direction of orientation of LC molecules in the LC layer 13 in the
transmissive area 22 is rotated by 45 degrees due to the electric
field between the pixel electrode 36 and the second common
electrode 38. Thus, in the transmissive area 22, as shown in FIG.
7B, a 78-degree linearly-polarized light passing through the second
polarizing film 15 and second .lamda./2 film 19 is rotated by 90
degrees relative to the angle upon display of a dark image after
being passed through the LC layer 13, to thereby assume a 12-degree
linearly-polarized light. The 12-degree linearly-polarized light
then enters the first .lamda./2 film 18 to assume a 27-degree
linearly-polarized light, which enters the first polarizing film
11. As a result, the transmissive area 22 assumes a bright
state.
[0077] As described above, by inverting the signal applied to the
first common electrode 37 to apply the inverted signal to the
second common electrode 38, a bright state can be achieved in the
transmissive area 22 upon representing the bright image in the
reflective area 21. Thus, it is possible to achieve the dark state
both in the reflective and transmissive areas by using the signals
shown in FIGS. 6A and 6B.
[0078] In the present embodiment, the common electrode is divided
into the first and second common electrodes 37 and 38 so that the
first and second common electrodes 37 and 38 correspond to the
reflective area 21 and transmissive area 22, respectively. Opposite
signals or inverted signals, which allow magnitude of the electric
fields applied to the LC layer 13 to be reversed between the
reflective area 21 and the transmissive area 22, are supplied to
the first and second common electrodes 37 and 38. This enables the
reflective area 21 and the transmissive area 22 of each pixel to
provide the same image without the need to supply different pixel
signals to the reflective area 21 and transmissive area 22, thereby
eliminating the disadvantage that the inverted data signals must be
applied between the reflective area and the transmissive area of
each pixel in the IPS-mode transflective LCD device.
[0079] In the structure of the LCD device 10C shown in FIG. 1C, the
retardation film 18a provided only in the reflective area and
acting as the .lamda./4 film in association with the LC layer 13
has a function similar to that of the LC layer 13 described in the
conventional structure. More specifically, a 90-degree
linearly-polarized light passing through the first polarizing film
51 passes the LC layer 53 and the retardation film 18a. Since the
polarized direction of the light that has passed the LC layer 53
and the retardation film 18a is deviated by 45 degrees from the
direction of the longitudinal axis of LC molecules and the
retardation of the liquid crystal and retardation film 18a in
combination is set at .lamda./4, the 90-degree linearly-polarized
light that passes the LC layer 53 and retardation film 18a assumes
a clockwise-circularly-polarized light, which enters the reflection
film 54. The clockwise-circularly-polarized light is reflected by
the reflection film to be changed into a
counterclockwise-circularly-polarized state. The
counterclockwise-circularly-polarized light that has entered the LC
layer 53 passes therethrough once again to be changed into a
horizontal (0-degree) linearly-polarized light. The
horizontal-linearly-polarized light then enters the first
polarizing film 51. Since the direction of the polarization axis of
the first polarizing film 51 is at 90 degrees, the light reflected
by the reflection film 54 cannot be passed through the first
polarizing film 51, with the result that a dark image (black) is
represented on the screen. Thus, the configuration of FIG. 1C
provides a function similar to the function obtained by the
structure of FIG. 1A.
[0080] In the structure of FIG. 1A showing the present embodiment,
the angle .theta.1 between the direction of orientation of the LC
layer 13 and the polarized direction of the light entering the LC
layer 13 upon display of a dark image is set to satisfy the
relationship:
0 degree<.theta.1<45 degrees,
and preferably the relationship:
0 degree<.theta.1.ltoreq.22.5 degrees.
As a result, it is possible for the LCD device according to the
present embodiment to reduce the adverse affect caused by
wavelength dispersion characteristics of the LC layer 13 upon
display of a dark image, as compared to the conventional
transflective LCD device 50a (FIG. 23), thereby preventing leakage
light in a dark state.
[0081] In a typical TN-mode LCD device, a reflection film is
configured by a reflection pixel electrode, to which a pixel signal
for driving the LC layer in accordance with the desired gray-scale
level is supplied. On the other hand, since the LC layer 13 is
driven by the electric field generated between the pixel electrode
35 and the common electrode (first common electrode) 37 in the IPS
mode, the potential to be applied to the reflection film 16 can
arbitrarily be determined. In the following description, the
influence that the potential of the reflection film 16 exerts on
the image in the reflective area 21 will be discussed.
[0082] FIGS. 8A and 8B, which are obtained by simulation, show the
electric field distribution in the LC layer by using iso-electric
lines and light transmittance by using iso-transmittance lines as
well as gray scale levels, both in the case of a dark state. FIG.
8A shows the case wherein the pixel electrode 35 applied with a 5V
signal and the common electrode 37 applied with a 0V signal
maintain the potential of the reflection film, i.e., reflection
electrode, 16 at an intermediate potential (2.5V) therebetween.
FIG. 8B shows the case wherein the pixel electrode 35 is applied
with a 5V signal and the common electrode 37 is applied with a 0V
signal, and the potential of the reflection film 16 is maintained
at the same potential (0V) as the common electrode 37.
[0083] If the potential of the reflection film 16 assumes an
intermediate potential between the pixel electrode 35 and common
electrode 37, as shown in FIG. 8A, although leakage light occurs on
the pixel electrode 35 and common electrode 37 to increase the
transmittance, the leakage light is suppressed at the portion
between both the electrodes to reduce the light transmittance. On
the other hand, if the potential of the reflection film 16 assumes
the same potential as the common electrode 37, as shown in FIG. 8B,
leakage light increases in the vicinity of the common electrode 37,
as indicated by a thick arrow, to thereby increase the light
transmittance in this area. This is considered due to the fact that
the electric field between the pixel electrode 35 and the
reflection film 16 is strong so that the electric lines of force,
which are supposed to converge between the pixel electrode 35 and
the common electrode 37, are directed to the reflection film 16,
with the result that LC molecules in the vicinity of the common
electrode 37 are not sufficiently driven.
[0084] The above simulation reveals that it is preferable that the
potential of the reflection film 16 be an intermediate potential
between the pixel electrode 35 and the common electrode 37. In this
respect, it is possible to control the potential of the reflection
film 16 by directly applying a desired potential to the reflection
film 16. In an alternative, it is possible to control the potential
of the reflection film 16 through capacitive coupling while
adopting a floating state thereof. For example, if the floating
potential is to be employed, an interconnect that can apply the
potential of the pixel electrode 35 and another interconnect that
can apply the potential of the common electrode 37 are formed right
under the reflection film 16 such that the area ratio between the
interconnects assumes 1:1, whereby the potential of the reflection
film 16 is controlled at the intermediate potential between the
pixel electrode 35 and the common electrode 37.
[0085] Since the leakage light occurs on the pixel electrode 35 and
common electrode 37 as shown in FIG. 8A the brightness upon display
of a dark image cannot be sufficiently reduced without using a
suitable countermeasure. In order to suppress the influence by the
leakage light, patterning should be made such that the reflection
film 16 is not formed right under the pixel electrode 35 or common
electrode 37, as shown in FIG. 9. This reduces the brightness of
reflected light observed at the positions on which the pixel
electrode 35 and common electrode 37 are formed, thereby reducing
the brightness upon display of a dark image.
[0086] Hereinafter, a fabrication process for the TFT substrate 14
shown in FIG. 1A will be described with reference to FIGS. 10A to
10D through FIGS. 17A to 17D showing respective steps of
fabrication. In each of these drawings, the drawing having "A"
character attached with the drawing number is a top plan view of
the TFT substrate in a specific step, and other figures having the
same drawing number attached with "B" to "D" are sectional views
showing the same step as the drawing of the corresponding number
and taken along lines A-A', B-B' and C--C', respectively, shown in
FIG. 10A, unless otherwise specified.
[0087] Firstly, the gate line 31 shown in FIG. 2, a first common
electrode line 37a, and a second common electrode line 38a are
formed on a substrate body of the TFT substrate, in such patterns
as shown in FIG. 10A. Sections of the reflective area 21,
transmissive area 22, and the interface (step portion) between the
reflective area 21 and the transmissive area 22 obtained at this
step are shown in FIGS. 10B to 10D, respectively. In the reflective
area 21, the first common electrode line 37a is so formed as to
project in the display area, thereby supplying a potential to the
reflection film 16. Thereafter, the gate line 31, first common
electrode line 37a, and second common electrode line 38a are
covered by an insulating film.
[0088] Subsequently, as shown in FIG. 11A, a semiconductor layer 39
for forming thereon the T T 33 is provided. As shown in FIG. 11B,
the semiconductor layer 39 is formed so as to overlap the gate line
or gate electrode 31. Thereafter, pixel electrode lines 35a and 36a
are formed in such patterns as shown in figure FIG. 12A. The pixel
electrode line 35a is connected to one of the ends of the
source/drain path of the TFT 33, and the pixel electrode line 36a
is connected to the other of the ends of the source/drain path of
the TFT 34.
[0089] Sections of the reflective area 21, transmissive area 22,
and interface between the reflective area 21 and the transmissive
area 22 obtained at this step are shown in figures FIGS. 12B to
12D, respectively. In the reflective area 21, the first common
electrode line 37a is formed between adjacent two of the pixel
electrode lines 35a. The first common electrode line 37a is formed
such that the area ratio between the pixel electrode line 35a and
the first common electrode line 37a is 1:1 in the display area.
With this configuration, the intermediate potential between the
potentials of the pixel electrode 35 and the first common electrode
37 can be applied to the reflection film 16, which is to be formed
later. After the formation of the first and second common electrode
lines 37a and 38a, these lines are covered by an insulating
film.
[0090] Subsequently, an overcoat (OC) layer 40 having a rough
surface is formed thereon, as shown in FIGS. 13A to 13D. An
aluminum (Al) layer is formed on the rough OC layer 40, and the
reflection film 16 is formed thereon in the reflective area 21 in
such a pattern as shown in FIG. 14A. Sections of the reflective
area 21, transmissive area 22, and interface between the reflective
area 21 and transmissive area 22 obtained at this stage are shown
in FIGS. 14B to 14D, respectively. As shown in FIG. 14B, in the
reflective area 21, the Al layer is selectively etched in the
region right under the pixel electrode 35a and first common
electrode 37a, to form a reflection film 16 having an opening
therein.
[0091] After the formation of the reflection film 16, a flat OC
layer 41 is formed in such a pattern as shown in FIG. 15A. The
formation of the flat OC layer 41 causes a step difference on the
interface between the reflective area 21 and the transmissive area
22 as shown in FIGS. 15B to 15D, with the result that the cell gap
is adjusted in the respective areas. Thereafter, contact holes 42
are formed in the insulating film that covers the pixel electrode
lines 35a, 36a and first and second common electrode lines 37a, 38b
at the portions shown in FIG. 16A to expose the pixel electrode
lines 35a and 36a, and the first and second common electrode lines
37a and 38a (FIG. 16B).
[0092] After the formation of the contact holes, the pixel
electrodes 35 and 36 and first and second common electrodes 37, 38
are formed on the flat OC layer 41 in such pattern as shown in FIG.
17A. Sections of the reflective area 21, transmissive area 22, and
interface between the reflective area 21 and transmissive area 22
obtained at this stage are shown in FIGS. 17B to 17D, respectively.
In this fabrication process, these electrodes are connected to the
pixel electrode lines 35a, 36a and the first and second common
electrode lines 37a, 38a through the contact holes 42. In this
manner, the TFT substrate 14 used for the transflective LCD device
10 according to the present embodiment is obtained.
[0093] FIG. 18 shows the sectional structure of a transflective LCD
device according to a second embodiment of the present invention.
The LCD device, generally designated at numeral 10a, according the
second embodiment is similar to the LCD device 10 of the first
embodiment shown in FIG. 1A except that a retardation film 20 is
disposed between the TFT substrate 14 and the second .lamda./2 film
19. In the present embodiment, the angle .theta.1 between the
polarized direction of the linearly-polarized light entering the LC
layer 13 upon display of a dark image and the direction of the
optical axis of the LC layer is set at zero degree in the
transmissive area 22.
[0094] Assuming that the retardation film 20 has a refractive index
of nx in the direction of the slow axis, a refractive index ny in
the direction of the fast axis, a refractive index nz in the
thickness direction, and a thickness of d, the following
relationship:
(nx-nz)/(nx-ny).ltoreq.0.3; or
(nx-nz)/(nx-ny)=1.0
is satisfied. Further, the value defined by:
(nx-ny).times.d
is set nearly equal to the retardation of the LC layer 13.
[0095] The retardation film 20 is disposed such that the direction
of the longitudinal axis of LC molecules in the LC layer 13 and the
direction of the fast axis of the retardation film 20 are parallel
or perpendicular to each other. If the retardation film 20
satisfies the following relationship:
(nx-nz)/(nx-ny)=1.0
and is disposed in parallel to the optical axis of the LC layer, it
is preferable to use the retardation film 20 having a
configuration, wherein the wavelength dispersion of the
birefringence is a reverse dispersion.
[0096] FIG. 19 shows a combination of polarizing films, LC layer
and retardation films having a suitable relationship of the
directions for the polarization axis and optical axis thereof. In
this table, the values entered in the columns indicate the angles
of the direction of the optical transmission axis of the polarizing
films 11 and 15, direction of the longitudinal axis of the LC
molecules in the LC layer 13, direction of the in-plane optical
axis of the first and second .lamda./2 films 18, 19, and direction
of the in-plane optical axis of the retardation film 20. In the
combination shown in FIG. 19, the polarized direction of light
passing through the second polarizing film 15, second .lamda./2
film 19 and retardation film 20 and entering the LC layer 13 is
determined to be parallel or perpendicular to the direction of the
longitudinal axis of LC molecules in the LC layer 13. This
configuration is determined for the purpose of preventing the
amount of leakage light upon display of a dark image in the
transmissive area 22 from being increased, for each case of the
refractive index Nz of the thickness direction of the retardation
film being Nz.ltoreq.0.3 and Nz=0.
[0097] The image obtained in the LCD device 10a adopting the
combination shown in FIG. 19 will be described with reference to
FIG. 20. Image of a dark image will be first described. Upon
representing the dark state, the signals shown in FIGS. 4A and 4B
may be applied, whereby the direction of the longitudinal axis of
LC molecules in the LC layer 13 in the reflective area 21 is
rotated to the direction of 45 degrees, whereas the direction of
the longitudinal axis of LC molecules in the LC layer 13 in the
transmissive area 22 is kept at 90 degrees.
[0098] In the transmissive area 22, the light emitted from the
backlight unit is passed through the second polarizing film 15
having an optical transmission axis at 75 degrees, which is equal
to the light absorption axis minus 15 degrees, to assume a
75-degree linearly-polarized light.
[0099] The 75-degree linearly-polarized light is then passed
through the second .lamda./2 film 19. At this stage, the polarized
direction of the 75-degree linearly-polarized light is rotated by
an angle equal to double the difference between the same and the
direction of the optical axis, 82.5 degrees, of the second
.lamda./2 film 19, thereby assuming a 90-degree or 270-degree
linearly-polarized light.
[0100] The 90-degree linearly-polarized light is passed through the
retardation film 20 and LC layer 13, with the polarized axis being
maintained at 90 degrees. Then, the 90-degree linearly-polarized
light is passed through the first .lamda./2 film 18 to assume a
105-degree linearly-polarized light, which enters the first
polarizing film 11.
[0101] Since the optical transmission axis of the first polarizing
film 11 is at 15 degrees, the light transmitted from the backlight
unit cannot be passed through the first polarizing film 11,
resulting in display of a dark image.
[0102] Although components of the light entering the retardation
film 20 and LC layer 13, other than the component having a
wavelength of 550 nm, assume an elliptically-polarized light due to
the wavelength dispersion by the first and second .lamda./2 films
18 and 19, the retardation film 20 compensates the wavelength
dispersion of the components of the linearly-polarized light
entering the LC layer 13, other than the component having a
wavelength of 550 nm, to thereby suppress leakage light or coloring
on the light emitting side.
[0103] In the reflective area 21, the 15-degree linearly-polarized
light passing through the first polarizing film 11 having an
optical transmission axis at 15 degrees is then passed through the
first .lamda./2 film 18 to assume a 0-degree (or 180-degree)
linearly-polarized light, which enters the LC layer 13. The
0-degree linearly-polarized light that has entered the LC layer 13
assumes a counterclockwise-circularly-polarized light upon being
passed through the LC layer 13.
[0104] The counterclockwise-circularly-polarized light is then
reflected by the reflection film 16 to assume a
clockwise-circularly-polarized light. The
clockwise-circularly-polarized light is passed through the LC layer
13 to assume a 90-degree linearly-polarized light, which enters the
first .lamda./2 film 18. The 90-degree linearly-polarized light
then assumes a 105-degree linearly-polarized light upon being
passed through the first .lamda./2 film 18. The 105-degree
linearly-polarized light cannot be passed through the first
polarizing film 11, resulting in display of a dark image.
[0105] The description will next be given of a bright image. Upon
representing a bright image, the signals shown in FIGS. 6A and 6B
may be applied, whereby the direction of the longitudinal axis of
LC molecules in the LC layer 13 in the transmissive area 22 is
rotated to the direction of 45 degrees, whereas the direction of
the longitudinal axis of LC molecules in the LC layer 13 in the
reflective area 21 is kept at 90 degrees.
[0106] In the transmissive area 22, the light emitted from the
backlight unit is passed through the second polarizing film 15
having an optical transmission axis at 75 degrees to assume a
75-degree linearly-polarized light. The 75-degree
linearly-polarized light is then passed through the second
.lamda./2 film 19 to assume a 90-degree (or 270-degree)
linearly-polarized light, which enters the retardation film 20 and
LC layer 13. Since the direction of the optical axis of the
retardation film 20 and polarized direction of the light entering
the retardation film 20 are parallel to each other, the 90-degree
linearly-polarized light is passed through the retardation film 20
with the polarized axis thereof being kept at 90 degrees. The
90-degree linearly-polarized light then assumes a 0-degree
linearly-polarized light upon being passed through the LC layer 13.
The 0-degree linearly-polarized light is then passed through the
first .lamda./2 film 18 to assume a 15-degree linearly-polarized
light. The 15-degree linearly-polarized light is passed through the
first polarizing film 11, resulting in representing a bright
image.
[0107] In the reflective area 21, the 15-degree linearly-polarized
light passing through the first polarizing film 11 having an
optical transmission axis at 15 degrees is then passed through the
first .lamda./2 film 18 to assume a 0-degree (180-degree)
linearly-polarized light, which enters the LC layer 13. The
0-degree linearly-polarized light that has entered the LC layer 13
is passed through the LC layer 13, with the polarized axis thereof
being kept at zero degree, reflected by the reflection film 16, and
is passed through the LC layer 13 once again. The 0-degree
linearly-polarized light passing through the LC layer 13 is passed
through the first .lamda./2 film 18 to assume a 15-degree
linearly-polarized light. The 15-degree linearly-polarized light is
passed through the first polarizing film 11, resulting in
representing a bright image.
[0108] In the first embodiment, the angle .theta.1 is set to
satisfy the following relationship:
0 degree<.theta.1<45 degrees,
and preferably satisfy the following relationship:
0 degree<.theta.1.ltoreq.22.5 degrees.
[0109] Thus, it is possible to cancel the phase difference between
the first .lamda./2 film and the second .lamda./2 film 19 by the
retardation of the LC layer 13. However, in the present embodiment,
the angle .theta.1 is set at zero degree, and thus coloring of the
dark image toward blue color may be observed in the transmissive
area 22 in some cases due to accumulation of the phase difference
between the first .lamda./2 film 18 and the second .lamda./2 film
19. In the present embodiment, the retardation film 20 performs the
optical compensation to thereby solve this problem.
[0110] Although, in the first embodiment, the reflection film 16 is
not formed right under the pixel electrodes 35, 36 and first and
second common electrodes 37 and 38 in the reflective area 21, the
present invention is not limited to this configuration, so long as
the portion right under these electrodes does not function as the
light source. For example, as shown in FIG. 21, part of the
reflection film 16 that is located right under the electrode 35 or
37 can be formed flat. In this case, a scatter reflection will not
occur in the portion right under the electrode, rendering the
portion darker than the other portion. As a result, it is possible
to reduce the influence by leakage light observed on the electrode.
Further, although the angle .theta.1 is more than zero degree in
the first embodiment, the angle .theta.1 may be set at zero
degree.
[0111] In a LCD device, a wide wavelength range can be achieved by
using a plurality of retardation films, which are layered one on
another so that the slow axes thereof are not parallel or
perpendicular to each other. This is described in, for example,
"Retardation film and polarizer having a wider viewing angle and a
large wavelength range" (Shingakugiho) written by Takahiro
Ishinabe, Tetsuya Miyashita, and Tatsuo Uchida, January, 2001, pp.
56. The LC layer 13 can also be considered as a kind of a
retardation film. Thus, even if the second .lamda./2 film 19 is
omitted, a wide wavelength range can be achieved by layering the
first .lamda./2 film 18 and LC layer 13. In this case, the optical
transmission axis of the second polarizing film 15 is set such that
the angle between the direction of the optical axis of the LC layer
13 and the polarized direction of the incident light assumes
.theta.1.
[0112] More specifically, if the direction of the longitudinal axis
of LC molecules is at 90 degrees and .theta.1 is 12 degrees, the
optical transmission axis of the second polarizing film 15 should
be set at 78 degrees. It is to be noted however that the wavelength
dispersion of the .lamda./2 film is generally smaller than the
wavelength dispersion of the LC layer 13, and thus it is preferable
to use a layer structure of the first and second .lamda./2 films 18
and 19 in order to achieve a wider wavelength range.
[0113] As described above, the transflective LCD device of the
present invention achieves reduction of the coloring and/or leakage
light especially upon display of a dark image.
[0114] As described above in connection with the LCD device of the
above embodiments, each of the pixels may include a pixel electrode
driven by a pixel data signal supplied common to the transmissive
area and the reflective area, a first common electrode driven by a
first common signal supplied common to the reflective areas of the
plurality of pixels, and a second common electrode driven by a
second common signal supplied common to the transmissive areas of
the plurality of pixels.
[0115] In addition, the angle .theta.1 between an optical axis of
the LC layer in the transmissive area upon display of a dark image
and a polarized direction of light incident onto the LC layer may
satisfy the following relationship:
zero degree.ltoreq..theta.1<45 degrees.
[0116] Further, the angle .theta.1 may satisfy the following
relationship:
zero degree.ltoreq..theta.1.ltoreq.22.5 degrees.
[0117] Further, the angle .theta.2 between an optical axis of the
LC layer in the reflective area upon display of a dark image and a
polarized direction of light incident onto the LC layer may satisfy
the following relationship: .theta.2=45 degrees.
[0118] Further, a polarized direction of light incident onto the
second polarizing film and an optical absorption axis of the second
polarizing film may be aligned with each other upon display of a
dark image.
[0119] Further, the LCD device may include another 1/2 wavelength
film between the second polarizing film and the LC layer.
[0120] Further, the angle .theta.1 between an optical axis of the
LC layer in the transmissive area upon display of a dark image and
a polarized direction of light incident onto the LC layer may be
zero degree, and the LCD device may further includes a retardation
film sandwiched between the another 1/2 wavelength film and the LC
layer.
[0121] Further, the retardation film may have an optical
characteristic satisfying the following relationship:
-0.3.ltoreq.(nx-nz)/(nx-ny).ltoreq.0.3; or
(nx-nz)/(nx-ny)=1.0,
where nx, ny, nz and d are refractive index of a slow axis,
refractive index of a fast axis, refractive index in a thickness
direction and thickness of the retardation film, respectively,
wherein the LC layer has a retardation substantially equal to
(nx-ny).times.d; and the fast axis of the retardation film is
parallel or perpendicular to the optical axis of the LC layer.
[0122] Further, the LC layer may include homogeneously-oriented
liquid crystal.
[0123] Further, the first and second common signals may be inverted
in synchrony with a synchronizing signal, and the first common
signal may be substantially an inverted signal of the second common
signal, and vice versa.
[0124] Further, each of the pixels may include a first pixel
electrode in the transmissive area, a second pixel electrode in the
reflective area, a first switch for supplying a data signal to the
first pixel electrode, and a second switch for supplying the data
signal to the second pixel electrode.
[0125] Further, the reflective area may include a reflection film
maintained at an intermediate potential between a potential of the
pix electrode and a potential of the first common electrode.
[0126] Further, the intermediate potential of the reflective film
may be supplied by a capacitive coupling from the pixel electrode
and the first common electrode.
[0127] Further, the intermediate potential of the reflective film
may be provided by an intermediate-potential generator.
[0128] Further, the reflective film may have an opening right under
the pixel electrode and the first common electrode.
[0129] Further, the reflective film may have a flat surface in a
region right under the pixel electrode and the first common
electrode, and a rough surface in the other region.
[0130] Further, each of said pixels may include a first pixel
electrode in said transmissive area, a second pixel electrode in
said reflective area, a first switching device for supplying a data
signal to said first pixel electrode, and a second switch for
supplying said data signal to said second pixel electrode.
[0131] Further, the transflective LCD device according to the
present invention may include a first common electrode including a
plurality of common electrodes connected in common and disposed in
said reflective areas of a plurality of said pixels, and a second
common electrode including a plurality of common electrodes
connected in common and disposed in said transmissive areas of a
plurality of said pixels.
[0132] Further, one of said first common electrode and said second
common electrode may receive a common signal obtained by inversion
of a common signal applied to the other of said first common
electrode and said second common electrode.
[0133] Further, said first and second switching devices may be
connected to a common data line, and driven by separate control
lines.
[0134] Further, said first and second switching devices may be
connected to a first data line and a second data line,
respectively.
[0135] Further, one of said first and second common electrodes may
be applied with a first data signal output from a voltage converter
for converting a second data signal applied to the other of said
first and second common electrodes.
[0136] Further, said voltage converter may include a data memory
for storing said second data signal, and a gray-scale level
converter for converting a gray-scale level of said second data
signal to output said first data signal.
[0137] Further, said voltage converter includes a look-up table for
converting said gray-scale level. The look-up may table tabulate a
maximum gray-scale level and a minimum gray-scale level in
association. The look-up table may be configured by logic
gates.
[0138] Further, the transflective LCD device may further include a
first common electrode including a plurality of common electrodes
connected in common and disposed in said reflective areas of a
plurality of said pixels, and a second common electrode including a
plurality of common electrodes connected in common and disposed in
said transmissive areas of a plurality of said pixels.
[0139] Further, a potential to be written in said first and second
common electrodes may be inverted at a timing of switching for
writing data through said first switching device or said second
switching device.
[0140] A method for driving the transflective LCD of the present
invention may include the steps of: generating a first data signal
and a second data signal having therebetween a specific potential
relationship; and applying said first data signal and said second
data signal to said reflective area and said transmissive area,
respectively.
[0141] In the method of the present invention, said relationship
between said first data signal and said second data signal may be
such that said first data signal assumes a maximum gray-scale-level
potential when corresponding said second data signal assumes a
minimum gray-scale-level potential.
[0142] The method of the present invention may further include the
step of applying a first common electrode signal to a first common
electrode disposed in said reflective areas of a plurality of said
pixels and a second common electrode signal to a second common
electrode disposed in said transmissive areas of a plurality of
said pixels, said first common electrode signal having a potential
different from a potential of said second common electrode
signal.
[0143] The LCD device may include a first switching device for
coupling a data line to said first pixel electrode and a second
switching device for coupling said data line to said second pixel
electrode, a first gate line for controlling said first switching
device, a second gate line for controlling said second switching
device, and said method may further include the steps of: turning
ON said first and second switching devices in a time-division
scheme, to apply a common data signal to said first and second
pixel electrodes; applying a first common electrode signal to a
common electrode during applying said common data signal to said
first pixel electrode; and applying a second common electrode
signal to said common electrode during applying said common data
signal to said second pixel electrode, said first common electrode
signal having a potential different from a potential of said second
common electrode signal.
[0144] The LCD device may include a first switching device for
coupling a first data line to said first pixel electrode, a second
switching device for coupling a second data line to said second
pixel electrode, and the method of the present invention may
further include the step of: applying said first and second data
signals to said first and second data lines, respectively.
[0145] In the method of the present invention, one of said first
and second data signal may be supplied from outside of said LCD
device, and the other of said first and second data signals has a
gray-scale level converted from a grays-scale level of said one of
said first and second data signals by using a look-up table.
[0146] In the method of the present invention, said look-up table
may be such that similar .gamma.-characteristics are obtained for
both said reflective and transmissive areas.
[0147] The LCD device of the present invention may include a single
common electrode for both said reflective area and said
transmissive area of a plurality of said pixels, and the method
further of the present invention may include the steps of: applying
said single common electrode with a first common electrode signal
at the timing of writing said first data signal; and applying said
single common electrode signal with a second common electrode
signal at the timing of writing said second data signal.
[0148] Although the present invention has been described with
reference to the preferred embodiment, the transflective LCD device
according to the present invention is not limited to the above
embodiments, and a transflective LCD device obtained by making
various modifications and changes in the configurations of the
above-described embodiments will fall within the scope of the
present invention.
* * * * *