Title: Liquid crystal display device
Abstract: A liquid crystal display device comprises a first substrate and a second substrate facing each other and maintaining a predetermined cell gap, liquid crystals sealed between the substrates, and a pixel electrode formed on the first substrate on the side facing the liquid crystals. A common electrode is formed on the second substrate on the side facing the liquid crystals. First slits are formed in the pixel electrode, and second slits are formed in the common electrode. The second slits extend in a direction nearly at right angles with the direction in which the first slits extend, as viewed in a direction perpendicular to the substrate surface.
Patent Number: 6,992,743 Issued on 01/31/2006 to Sasabayashi
| Inventors:
|
Sasabayashi; Takashi (Kawasaki, JP)
|
| Assignee:
|
Sharp Kabushiki Kaisha (Osaka, JP)
|
| Appl. No.:
|
697713 |
| Filed:
|
October 30, 2003 |
Foreign Application Priority Data
| Oct 31, 2002[JP] | 2002-318328 |
| Current U.S. Class: |
349/143; 349/129 |
| Current Intern'l Class: |
G02F 1/13.43 (20060101); G02F 1/13.37 (20060101) |
| Field of Search: |
349/129,143,117,96
|
References Cited [Referenced By]
U.S. Patent Documents
| 6587173 | Jul., 2003 | Yoo et al.
| |
| 6600539 | Jul., 2003 | Song.
| |
| 6700635 | Mar., 2004 | Kwag et al.
| |
| 6710837 | Mar., 2004 | Song et al.
| |
| 6778244 | Aug., 2004 | Song et al.
| |
| 2005/0062924 | Mar., 2005 | Ahn et al.
| |
| Foreign Patent Documents |
| 9-211445 | Aug., 1997 | JP.
| |
| 11-242225 | Sep., 1999 | JP.
| |
| 2000/-029010 | Jan., 2000 | JP.
| |
Other References
Iwamoto et al.; "Multi-Domain Vertically-Aligned LCDs using Circular Polarizers";
Papers in the Panel Discussion, Japanese Assoc. of Liquid Crystals.
|
Primary Examiner: Ton; Toan
Attorney, Agent or Firm: Greer, Burns & Crain, Ltd.
Claims
What is claimed is:
1. A liquid crystal display device comprising:
a first substrate and a second substrate facing each other and maintaining a
predetermined cell gap;
liquid crystals sealed between the first substrate and the second substrate;
a plurality of first electrodes formed on the first substrate on the side that
faces the liquid crystals, wherein the plurality of first electrodes comprise only
a plurality of first slits extending substantially parallel with respect to each
other in a plurality of pixel region; and
a second electrode formed on the second substrate on the side that faces the
liquid crystals, wherein the second electrode comprises only a plurality of second
slits intersecting with and extending in a direction substantially at right angles
of the first slits in the plurality of pixel regions, as viewed in a direction
perpendicular to the substrate surface.
2. A liquid crystal display device according to claim 1, wherein the liquid crystal
molecules are aligned nearly perpendicularly to the surface of the substrate when
no voltage is applied across the first electrodes and the second electrode, and
are regulated for their azimuths of alignment by the first and second slits when
being tilted by the application of a voltage.
3. A liquid crystal display device according to claim 1, wherein each of the
first electrodes is a pixel electrode formed for each of a plurality of pixel regions,
and the second electrode is a common electrode formed on the display region including
the plurality of the pixel regions.
4. A liquid crystal display device according to claim 3, wherein the pixel electrode
has a rectangular shape, and the first slits extend in a direction of a long side
of the pixel electrode.
5. A liquid crystal display device according to claim 1, wherein a substantially
square shape is defined in a region where there is an overlapping of a pair of
the first slits and a pair of second slits as viewed in a direction perpendicular
to the surface of the substrate.
6. A liquid crystal display device according to claim 1, further comprising:
a first polarizer element arranged on the first substrate on the side opposite
to the side that faces the liquid crystals; and
a second polarizer element arranged on the second substrate on the side opposite
to the side that faces the liquid crystals, and having an axis of absorption nearly
at right angles with the axis of absorption of the first polarizer element.
7. A liquid crystal display device according to claim 6, further comprising:
a first ¼ wavelength plate arranged between the first substrate and the
first polarizer element; and
a second ¼ wavelength plate arranged between the second substrate and the
second polarizer element.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display device used for a display
unit of information equipment and, more particularly, to a liquid crystal display
device for which a wide viewing angle and a high brightness are required.
2. Description of the Related Art
In recent years, liquid crystal display devices of the active matrix type having
a thin film transistor (TFT) for each of the pixels have been widely used in a
variety of applications owing to their such features as small thickness, reduced
weight, operating on low voltages and consuming less electric power. Modern liquid
crystal display devices are realizing a large screen with high precision yet maintaining
a wide viewing angle, improved brightness and increased contrast. Therefore, there
are realized display characteristics comparable to those of a CRT (cathode-ray
tube), lending the liquid crystal display device well suited even for such applications
as monitors and TV receivers which have, so far, chiefly employed the CRT.
In a liquid crystal display device of the VA (vertically aligned) mode which
drives
the liquid crystal molecules in a vertically aligned manner, the liquid crystal
molecules are tilted in various directions when a voltage is applied unless the
alignment film is subjected to the alignment treatment such as rubbing. As a result,
alignment regions of different areas are formed on the pixels. In each pixel, further,
a boundary line (discrination) of the alignment region is seen as a dark line which
is differently arranged for each of the pixels. Therefore, when the display screen
is viewed from a tilted direction, there are seen shading, roughness and residual
image on the display screen causing the quality of display to be very decreased.
As a liquid crystal display device for realizing display characteristics comparable
to those of the CRT, therefore, there has been put into practical use a liquid
crystal display device realizing a wide viewing angle by utilizing an alignment
dividing system such as MVA (multi-domain vertical alignment)(see, for example,
Japanese Patent No. 2947350).
A liquid crystal panel of the MVA system has domain regulating means such as
protrusions,
dents or slits formed in the electrodes or a combination thereof on at least one
surface of either one of a pair of substrates. As a liquid crystal material, there
are used nematic liquid crystals having a negative dielectric anisotropy. When
no voltage is applied, the liquid crystals are such that the liquid crystal molecules
are aligned nearly perpendicularly to the substrate. When a voltage is applied,
the domain regulating means so works that the azimuths of alignment in which the
liquid crystal molecules are tilted are regulated to a plurality of azimuths in
each pixel. Polarizer elements are arranged on both sides of the liquid crystal
panel such that the axes of absorption are at right angles with each other.
FIGS. 8A and 8B illustrate the constitution of a pixel on a TFT substrate in
a conventional liquid crystal display device of the MVA type, FIG. 8A illustrating
the structure of a pixel electrode for realizing four-divided alignment and FIG.
8B illustrating the structure of a pixel electrode for realizing upper and lower
two-divided alignment. On the TFT substrate as shown in FIG. 8A, there are formed
a plurality of gate bus lines
112 extending in the right-and-left direction
and nearly in parallel with each other. A plurality of drain bus lines
114
are formed nearly in parallel with each other extending up and down in the drawing
and intersecting the gate bus lines
112 via an insulating film that is not
shown. Regions surrounded by the plurality of gate bus lines
112 and drain
bus lines
114 serve as pixel regions.
Further, a storage capacitor bus line
118 is formed extending nearly
in parallel with the gate bus lines
112, and traversing nearly the center
of the pixel region.
A TFT
110 is formed near a position where the gate bus line
112
and
the drain bus line
114 intersect each other. A drain electrode
122
of the TFT
110 is drawn from the drain bus line
114 so as to be positioned
on one end side of an active semiconductor layer formed on the gate bus line
112
and of a channel protection film (both of which are not shown) formed thereon.
On the other hand, a source electrode
124 of the TFT
110 is so formed
as to be opposed to the drain electrode
122 maintaining a predetermined
gap and is positioned on the other end side of the active semiconductor layer and
of the channel protection film. A region of the gate bus line
112 just under
the channel protection film works as a gate electrode of the TFT
110. Further,
the source electrode
124 is electrically connected to the pixel electrode
116 via a contact hole (not shown).
A pixel electrode
116 is formed in the pixel region. Referring to FIG.
8A,
the pixel electrode
116 includes trunk portions
128 extending nearly
in parallel with, or perpendicularly to, both bus lines
112 and
114,
branch portions
130 branching from the trunk portions
128 and extending
aslant, and slits
132 among the neighboring branch portions
130.
On an opposing substrate stuck to the TFT substrate and facing thereto maintaining
a predetermined cell gap, there are formed a transparent electrode (not shown)
on the whole surface of the display region including a plurality of pixel regions.
In the MVA-LCD fabricated by using the TFT substrate shown in FIG. 8A and the opposing
substrate that is not shown, the directions for aligning the liquid crystal molecules
are determined by the trunk portions
128 of the pixel electrode
116,
branch portions
130 and slits
132.
Liquid crystals having a negative dielectric anisotropy are sealed between
the two substrates. Liquid crystal molecules are aligned nearly perpendicularly
to the surface of the substrate due to the alignment-regulating force of vertically
alignment films (not shown) formed on the opposing surfaces of the two substrates.
The branch portions
130 and the slits
132 in FIG. 8A have widths
which are both, for example, 3 μm, and the pitches among the branch portions
and among the slits are both 6 μm. With the slit structure which is as fine
as the above-mentioned degree, the liquid crystal molecules Lc are tilted in the
directions in parallel with the directions in which the slits
132 are extending
when a voltage is applied thereto. When a predetermined voltage is applied across
the transparent electrodes of the two substrates and the liquid crystal molecules
Lc start being tilted along the directions in which the slits
132 are extending,
the tilted state propagates successively to the liquid crystal molecules Lc, and
the liquid crystal molecules Lc are tilted in the same directions among the slits
132.
Thus, upon arranging the slits
132 in the pixel electrode
116,
it is allowed to regulate the direction of tilt of the liquid crystal molecules
Lc for each of the regions. If the slits
132 are formed in two directions
which are nearly perpendicular to each other as shown in FIG. 8A, the liquid crystal
molecules are tilted in four directions in each pixel. Since the viewing angle
characteristics of the regions are mixed together, a wide viewing angle is obtained
by the MVA-LCD in the white display or in the black display. In the MVA-LCD, a
contrast ratio of not smaller than 10 is obtained even at an angle of 80 degrees
in the up-and-down right-and-left directions from a direction perpendicular to
the display screen.
As shown in FIG. 8A, therefore, when the slit electrodes are so formed that the
liquid crystal molecules are tilted in the four directions, the alignments of four
domains are realized. As shown in FIG. 8B, further, when the slit electrodes are
so formed that the liquid crystal molecules are tilted in the two directions, the
alignments of two domains are realized.
In the MVA-LCD using the pixel electrode
116 shown in FIGS. 8A and 8B,
however, a response time becomes long from the application of a voltage until the
propagation of alignment of the liquid crystal molecules Lc is completed. Therefore,
there occur in a random fashion singular points in the alignment vector of the
liquid crystal molecules Lc on the branch portions
130. Further, the positions
where the singular points are formed migrate for each of the pixels or the frames.
When the display screen is viewed from a tilted direction, in particular, there
are observed shades and roughness on the display screen, causing the display quality
to be deteriorated.
Next, described below with reference to FIGS. 8A to 9D is a relationship between
the tilting azimuth of liquid crystal molecules Lc and the directions of axes of
absorption of the two polarizing elements P and A. Referring to FIGS. 8A and 8B,
the directions of axes of absorption of the two polarizing elements P and A are
set being tilted by 45° from the azimuth of alignment of the liquid crystal
molecules Lc of when they are tilted. FIGS. 9A to 9D illustrate a relationship
between the tilting azimuth of the liquid crystal molecules Lc as seen in a direction
perpendicular to the substrate surface and the directions of axes of absorption
of the two polarizing elements P and A. FIG. 9A illustrates a case of when no voltage
is applied where the liquid crystal molecules Lc are aligned perpendicularly to
the substrate surface. On the other hand, light that has passed through one polarizing
element P passes through the liquid crystals without affected by birefringence
of the liquid crystal molecules, but is shut off by the other polarizing element
A to exhibit a black display.
When a voltage is applied, the liquid crystal molecules Lc having a negative
dielectric anisotropy are tilted with respect to the substrate surface. When a
sufficiently large voltage is applied, the liquid crystal molecules Lc become nearly
in parallel with the substrate surface. To realize an optimum white display, the
azimuth of alignment of the liquid crystal molecules Lc receives regulation relative
to the directions of axes of absorption of the polarizing elements P and A.
FIG. 9B illustrates a case where the liquid crystal molecules Lc are tilted
in an azimuth to meet in parallel with, or at right angles with, the axes of absorption
of the polarizing elements P and A. In this case, like when no voltage is applied,
light that has passed through one polarizing element P passes through the liquid
crystals without affected by birefringence of the liquid crystal molecules Lc,
but is shut off by the other polarizing element A. Therefore, white display is
not aligned.
To obtain an optimum white display as shown in FIG. 9C, the azimuth of alignment
of the liquid crystal molecules Lc must be 45° with respect to the axes of
absorption of the polarizing elements P and A. In this case, linearly polarized
beam that has passed through one polarizing element P becomes an elliptically polarized
beam being affected by the birefringence of the liquid crystal molecules Lc, producing
light that passes through the other polarizing element A. Therefore, white display
is aligned.
To obtain a favorable white display with the four-domain-divided MVA-LCD, therefore,
the azimuths in which the liquid crystal molecules Lc are to be tilted and aligned
when a voltage is applied are regulated to four azimuths shown in FIG. 9D.
Related Art documents are as follows:
JP-A-2000-29010
JP-A-9-211445
Japanese Patent No. 2947350
Papers in the Panel Discussion, Japanese Association of Liquid Crystals, by
Iwamoto, Toko, Iimura, PCa02, 2000
With, for example, the four-domain-divided MVA-LCD as described above, it is
desired that the azimuths in which the liquid crystal molecules Lc are tilted and
aligned, are four azimuths only as shown in FIG. 9D. In practice, however, due
to continuity of liquid crystals, there exist liquid crystal molecules Lc that
are tilted in the azimuths other than the four azimuths shown in FIG. 9D.
In the MVA-LCD having a 4-domain electrode structure shown in FIG. 8A, for example,
the liquid crystal molecules Lc are tilted in four different azimuths due to fine
slits
132 which are so formed as to maintain angles of 45° relative
to the axes of absorption of the polarizing elements P and A. In the regions of
boundaries where the domains are neighboring each other, however, the liquid crystal
molecules Lc are forced to be tilted in the azimuths which are in parallel with,
or at right angles with, the axes of absorption of the polarizing elements P and A.
Light does not pass through the region where the liquid crystal molecules are
tilted in the azimuths in parallel with, or at right angles with, the axes of absorption
of the polarizing elements P and A. In the case of the electrode structure shown
in FIG. 8A, therefore, a black region forms in a crossing manner on the white display,
which is a major cause that decreases the transmission factor.
To tilt the liquid crystal molecules Lc in a predetermined direction, further,
it is necessary to form a line-and-space pattern of a fine pitch as well as to
form branch portions
130 of the electrode and slits
132 as shown
in FIG. 8A. When a split exposure is employed at a step of photolithography to
meet an increase in the size of the panel, however, the branch portions
130
and the slits
132 are formed having widths which are slightly different
for each of the split regions due to a slight change in the exposure conditions,
whereby shading occurs in the brightness on the display screen when an image is
displayed on the panel, arousing a problem of a drop in the production yield.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a liquid crystal display
device
which features an improved production yield, wide viewing angle, high brightness
and favorable display quality.
The above object is achieved by a liquid crystal display device comprising:
a first substrate and a second substrate facing each other maintaining a predetermined
cell gap;
liquid crystals sealed between the first substrate and the second substrate;
a first electrode formed on the first substrate on the side that faces the liquid crystals;
a second electrode formed on the second substrate on the side that faces the
liquid crystals;
first slits formed in the first electrode; and
second slits formed in the second electrode, and extending in a direction
nearly at right angles with the direction in which the first slits are extending
as viewed in a direction perpendicular to the substrate surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view schematically illustrating the constitution of a liquid crystal
display device according to an embodiment of the invention;
FIG. 2 is a view schematically illustrating an equivalent circuit of the liquid
crystal display device according to the embodiment of the invention;
FIG. 3 is a view schematically illustrating the electrode structure of the liquid
crystal display device according to the embodiment of the invention;
FIGS. 4A to 4D are views illustrating a driving state using the electrode structure
of the liquid crystal display device according to the embodiment of the invention;
FIGS. 5A to 5D are views illustrating a driving state using another electrode
structure of the liquid crystal display device according to the embodiment of the invention;
FIG. 6 is a view illustrating a positional relationship between polarizer elements
and ¼ wavelength plates in the liquid crystal display device according to
the embodiment of the invention;
FIG. 7 is a view schematically illustrating a further electrode structure in
the liquid crystal display device according to the embodiment of the invention;
FIGS. 8A and 8B are diagrams schematically illustrating the constitution of
a TFT substrate of an MVA-LCD; and
FIGS. 9A to 9D are diagrams illustrating the operation of when a voltage is
applied to the liquid crystal molecules having a negative dielectric anisotropy.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A liquid crystal display device according to an embodiment of the invention will
now be described with reference to FIGS. 1 to 7. First, the constitution of the
liquid crystal display device according to the embodiment will be briefly described
with reference to FIG. 1. The liquid crystal display device shown in FIG. 1 has
a structure in which a TFT substrate
2 on which TFTs are formed and a CF
substrate
4 on which color filters (CFs) are formed are stuck together facing
each other, and liquid crystals are sealed between the two substrates
2
and
4.
FIG. 2 schematically illustrates an equivalent circuit of elements formed on
the TFT substrate
2. On the TFT substrate
2 are formed a plurality
of gate bus lines
12 in parallel and extending in the right-and-left direction
in the drawing. There are formed in parallel a plurality of drain bus lines
14
extending in the up-and-down direction in the drawing and intersecting the gate
bus lines
12 via an insulating film. The regions surrounded by the plurality
of gate bus lines
12 and drain bus lines
14 serve as pixel regions.
A TFT
10 and a pixel electrode
16 are formed in each of the pixel
regions arranged like a matrix. The drain electrode in each TFT
10 is connected
to the neighboring drain bus line
14, the gate electrode is connected to
the neighboring gate bus line
12, and the source electrode is connected
to the pixel electrode
16. Storage capacitor bus lines
18 are formed
nearly through the centers of the pixel regions in parallel with the gate bus lines
12. The TFTs
10, pixel electrodes
16, and bus lines
12,
14 and
18 are formed through steps of photolithography, i.e., formed
by repeating a series of semiconductor process comprising "formation of film→coating
of resist→exposure to light→developing→etching→peeling
of resist".
Reverting to FIG. 1, on the TFT substrate
2 are provided a gate
bus line drive circuit
80 mounting a driver IC for driving the plurality
of gate bus lines
12, and a drain bus line drive circuit
81 mounting
a driver IC for driving the plurality of drain bus lines
14. In response
to predetermined signals output from a control circuit
82, these drive circuits
80 and
81 send scanning signals and data signals to predetermined
gate bus lines
12 or to the drain bus lines
14. A polarizer plate
83 is arranged on the surface of the TFT substrate
2 on the side
opposite to the element-forming surface thereof, and a backlight unit
85
is mounted on the surface of the polarizer plate
83 on the side opposite
to the TFT substrate
2. On the contrary, a polarizer plate
84 is
stuck to the surface of the CF substrate
4 on the side opposite to the CF-forming surface.
FIG. 3 illustrates a portion of the electrode constitution of a pixel of the
TFT substrate
2 and of the CF substrate
4. Here, for easy comprehension,
FIG. 3 does not show TFT
10 or bus lines
12,
14 and
18
that are formed on the side of the TFT substrate
2, and does not show, either,
CFs on the side of the CF substrate
4.
In FIG. 3, a pixel electrode
16 of a transparent electrically conducting
film such as of ITO (indium tin oxide) is formed on the pixel regions of the TFT
substrate
2. The pixel electrode
16 includes a plurality (four in
the drawing) of rectangular (belt-like) electrode portions
20 having a width
a and extending up and down in the drawing. Among the neighboring electrode portions
20, there are formed slits
22 of a width b without transparent electrically
conducting film. The electrode portions
20 separated by slits
22
are electrically connected together through connection electrodes that are not shown.
On the corresponding region of the CF substrate
4 facing the pixel region
of the TFT substrate
2, there is formed a common electrode
24 of
a transparent electrically conducting film such as of ITO. The common electrode
24 includes a plurality (four in the drawing) of rectangular (belt-like)
electrode portions
26 having a width a and extending right and left in the
drawing. Among the neighboring electrode portions
26, there are formed slits
28 of a width b without transparent electrically conducting film. The electrode
portions
26 separated by slits
28 are electrically connected together
through connection electrodes that are not shown. The electrode portions
20
and
26 have a width a of, for example, 37 μm, and the slits
22
and
28 have a width b of, for example, 8 μm.
As described above, the MVA-LCD according to the embodiment comprises the TFT
substrate (first substrate)
2 and the CF substrate (second substrate)
4
facing each other maintaining a predetermined cell gap, liquid crystals sealed
between the substrates
2 and
4, the pixel electrode (first electrode)
16 formed on the TFT substrate
2 on the side facing the liquid crystals,
the common electrode (second electrode)
24 formed on the CF substrate
4
on the side facing the liquid crystals, first slits
22 formed in the pixel
electrode
16 and second slits
28 formed in the common electrode
24,
and extending in a direction nearly at right angles with the direction in which
the first slits
22 are extending as viewed in a direction perpendicular
to the substrate surface.
The liquid crystals sealed between the pixel electrode
16 and the common
electrode
24 are aligned nearly perpendicularly to the substrate surface
when no voltage is applied across the pixel electrode
16 and the common
electrode
24, and are regulated for their azimuth of alignment by the first
and second slits
22 and
28 when the liquid crystal molecules are
tilted by the application of a voltage.
Referring to FIG. 3, further, a nearly square shape is described by the
region where there are overlapped the pixel electrode
16 and the common
electrode
24 defined by the first and second slits
22 and
28
as viewed in a direction perpendicular to the substrate surface.
The polarizer plate (first polarizer element)
83 arranged on the TFT substrate
2 on the side opposite to the side facing the liquid crystals and the polarizer
plate (second polarizer element)
84 arranged on the CF substrate
4
on the side opposite to the side facing the liquid crystals, are arranged in a
cross-nicol relationship in which the axis P of absorption of the polarizer plate
83 is nearly at right angles with the axis A of absorption of the polarizer
plate
84. Further, the axes P and A of absorption of the polarizer plates
83 and
84 are tilted by about 45° with respect to the directions
in which the first and second slits
22 and
28 are extending.
The electrode structure shown in FIG. 3 has electrode portions
20,
26
which are very wider than those of the prior art and first and second slits
22,
28, and is formed without requiring fine patterning in the step of photolithography,
and can, further, be produced maintaining a high yield. Besides, the first and
second slits
22 and
28 for regulating the alignment of the liquid
crystal molecules are formed not only in the pixel electrode
16 but are
also formed in the common electrode
24, making it possible to greatly improve
the stability of alignment, uniformity and response as compared to those of the
conventional structure having slits formed in one electrode only. Further, the
first and second slits
22,
28 in the two electrodes are extending
in the directions that intersect at right angles. Therefore, there is no need of
enhancing the precision at the time of sticking the substrates together.
FIGS. 4A to 4D illustrate the alignment of liquid crystals in the liquid crystal
display device according to the embodiment of when a voltage is applied thereto.
FIG. 4A illustrates a state where the liquid crystals are aligned by applying a
voltage of 2.5 V across the electrode portions
20 and
26. Similarly,
FIG. 4B illustrates a state where the liquid crystals are aligned by applying a
voltage of 3.0 V across the electrode portions
20 and
26, and FIGS.
4C and 4D illustrate states where the liquid crystals are aligned by applying voltages
of 4.0 V and 5.0 V. As shown in FIGS. 4A to 4D, stable alignment characteristics
are obtained when any voltage is applied, the liquid crystal molecules being aligned
at a uniform angle in the regions. This improves the quality of display suppressing
flickering or rough feeling on the picture.
FIGS. 5A to 5D illustrate states of liquid crystal alignment of when a voltage
is applied in the electrode structure varying the width of the electrode portions
26. The electrode width al of the electrode portion
20 is 37 μm
and the slit width b of the slit
22 is 8 μm, while the electrode width
a
2 of the electrode portion
26 is 25 μm and the slit width
b of the slit
28 is 8 μm. As viewed in a direction perpendicular to
the substrate surface, a transversely elongated rectangular shape is described
by the region where there are overlapped the pixel electrode
16 and the
common electrode
24 defined by the first and second slits
22 and
28. FIG. 5A illustrates a state where the liquid crystals are aligned when
a voltage of 2.5 V is applied across the electrode portions
20 and
26.
Similarly, FIG. 5B illustrates a state where the liquid crystals are aligned when
a voltage of 3.0 V is applied across the electrode portions
20 and
26,
and FIGS. 5C and 5D illustrate states where the liquid crystals are aligned when
voltages of 4.0 V and 5.0 V are applied. As shown in FIGS. 5A to 5D, when a rectangular
shape is described by the region where the pixel electrode
16 and the common
electrode
24 are overlapped one upon the other, it will be learned that
the stability and uniformity of alignment of the liquid crystal molecules are lowered
as compared to those of the structure of a square shape shown in FIGS. 4A to 4D.
It is, therefore, desired that the region where the pixel electrode
16 and
the common electrode
24 are overlapped one upon the other, describes nearly
a square shape.
Here, the liquid crystal display device (MVA-LCD) according to the embodiment
described with reference to FIGS. 1 to 4D has a defect of decreased transmission
factor, since the amount of light attenuates when it is transmitted from the liquid
crystal molecules aligned in the azimuths other than 45° with respect to the
axes P and A of absorption of the polarizer plates
83 and
84. In
order to improve this defect, there has been known a method of arranging a first
¼ wavelength plate
30 and a second ¼ wavelength plate
32
on both sides of the liquid crystal panel as shown in FIG. 6 (see, for example,
Papers in the Panel Discussion, Japanese Association of Liquid Crystals, by Iwamoto,
Toko, Iimura, PCa02, 2000).
Referring to FIG. 6, the polarizer plates
83 and
84 are arranged
in a cross-nicol relationship to each other holding the liquid crystal panel (TFT
substrate
2, CF substrate
4 and liquid crystal layer held thereby)
therebetween. The ¼ wavelength plate
30 is arranged between the liquid
crystal panel and the polarizer plate
83. Further, the ¼ wavelength
plate
32 is arranged between the liquid crystal panel and the polarizer
plate
84. In order to improve the viewing angle characteristics, a layer
having a negative phase difference, such as TAC film may be arranged between the
liquid crystal panel and the ¼ wavelength plates
30,
32. In
the drawing, the upper side is the side of the observer and the lower side is the
side of the source of light.
An angle of about 45° is subtended by the optical axis (retardation axis)
C
1 of the ¼ wavelength plate
30 and the axis P of absorption
of the polarizer plate
83. Namely, a circularly polarized light is obtained
as the light emitted from the source of light passes through the polarizer plate
83 and the ¼ wavelength plate
30 in this order. Further, an
angle of about 45° is subtended by the optical axis C
2 of the ¼
wavelength plate
32 and the axis A of absorption of the polarizer plate
84. The optical axes P and A of the two ¼ wavelength plates
30
and
32 are intersecting each other nearly at right angles.
In the arrangement shown in FIG. 6, if the intensity of the incident light is
denoted by I
in, the intensity of the transmitted light by I
out,
and the retardation through the liquid crystal layer by R
LC, then, the
following relationship holds,
Iout=(½)
Iin sin
2(
RLC/2)
That is, if the intensity I
in of the incident light is presumed to
be constant, then, the intensity I
out of the transmitted light is determined
exclusively by R
LC. Namely, the intensity I
out of the transmitted
light is dependent upon the tilted angle of the liquid crystal molecules that vary
the retardation R
LC but is not dependent upon the azimuth of alignment
of the liquid crystal molecules.
Employment of the above constitution makes it possible to extinguish the
regions of low transmission factors that occur in the form of a lattice or in an
X-shape as shown in FIGS. 4A to 4D and, hence, to realize an MVA-LCD having a sufficiently
high light transmission factor.
FIG. 7 illustrates the electrode structure of a modified embodiment in the liquid
crystal display device according to the invention. Referring to FIG. 7, there are
formed, on the TFT substrate, a plurality of gate bus lines
12 nearly in
parallel with each other extending in the right-and-left direction in the drawing.
A plurality of drain bus lines
14 are formed nearly in parallel with each
other extending in the up-and-down direction in the drawing intersecting the gate
bus lines
12 via an insulating film that is not shown. The regions surrounded
by the plurality of gate bus lines
12 and drain bus lines
14 serve
as pixel regions.
There are further formed storage capacitor bus lines
18 traversing nearly
the centers of the pixel regions and extending nearly in parallel with the gate
bus lines
12.
A TFT
10 is formed near a position where the gate bus line
12 intersects
the drain bus line
14. A drain electrode
11 of TFT
10 is drawn
from the drain bus line
14 and is positioned on one end side of an active
semiconductor layer (not shown) formed on the gate bus line
12 and on one
end side of a channel protection film (not shown) formed thereon. On the other
hand, a source electrode
13 of TFT
10 is facing the drain electrode
11 maintaining a predetermined gap, and is positioned on the other end side
of the active semiconductor layer and of the channel protection film. The region
just under the channel protection film of the gate bus line
12 works as
a gate electrode of TFT
10. Further, the source electrode
13 is electrically
connected to the electrode portion
20 on the left side in the drawing of
the pixel electrode
16 that will be described later through a contact hole
(not shown).
The pixel electrode
16 of a transparent electrically conducting film such
as of ITO is formed on the pixel regions of the TFT substrate
2. The pixel
electrode
16 is formed in a rectangular shape with its long side extending
in the up-and-down direction in the drawing, and has two rectangular (belt-like)
electrode portions
20 extending up and down in the drawing maintaining the
same width. A slit
22 of a predetermined width is formed between the neighboring
electrode portions
20, the slit
22 without having no transparent
electrically conducting film and extending in the direction of long side of the
pixel electrode
16. The two electrode portions
20 separated by the
slit
22 are electrically connected together through, for example, a connection
electrode
29 formed on the storage capacitor bus line
18.
The common electrode
24 of a transparent electrically conducting film
such as of ITO is formed on the corresponding region of the CF substrate
4
facing the pixel regions of the TFT substrate
2. The common electrode
24
has a plurality of (seven in the drawing) rectangular (belt-like) electrode portions
26 having a predetermined width and extending in the right-and-left direction
in the drawing. Slits
28 are formed among the neighboring electrode portions
26, the slits
28 without having transparent electrically conducting
film. The electrode portions
26 separated by the slit
28 are electrically
connected together through a connection electrode that is not shown.
As described above, the MVA-LCD of the modified embodiment includes the TFT substrate
(first substrate)
2 and the CF substrate (second substrate)
4 arranged
facing each other maintaining a predetermined cell gap, and liquid crystals sealed
between the substrates
2 and
4. The liquid crystals are nematic liquid
crystals having a negative dielectric anisotropy. Further, the MVA-LCD of the modified
embodiment includes the pixel electrode (first electrode)
16 formed on the
TFT substrate
2 on the side facing the liquid crystals, the common electrode
(second electrode)
24 formed on the CF substrate
4 on the side facing
the liquid crystals, the first slits
22 formed in the pixel electrode
16,
and the second slits
28 formed in the common electrode
24 and stretching
in a direction nearly at right angles with the direction in which the first slits
22 are extending as viewed in a direction perpendicular to the substrate surface.
The liquid crystals sealed between the pixel electrode
16 and the common
electrode
24 are aligned nearly vertically to the substrate surface when
no voltage is applied across the pixel electrode
16 and the common electrode
24, but are regulated for their azimuths of alignment by the first and second
slits
22 and
28 when the liquid crystal molecules are tilted by the
application of a voltage.
As shown in FIG. 7, further, a nearly square shape is described by the region
where there are overlapped the pixel electrode
16 and the common electrode
24 defined by slits
22 and
28 as viewed in a direction perpendicular
to the substrate surface.
Further, the cross-nicol arrangement is employed such that the axis P of
absorption of the polarizer plate (first polarizer element)
83 arranged
on the TFT substrate
2 on the side opposite to the side that faces the liquid
crystals is nearly at right angles with the axis A of absorption of the polarizer
plate (second polarizer element)
84 arranged on the CF substrate
4
on the side opposite to the side that faces the liquid crystals. The axes P and
A of the polarizer plates
83 and
84 are tilted by about 45°
with respect to the directions in which the first and second slits
22 and
28 are extending.
The first ¼ wavelength plate
30 shown in FIG. 6 is arranged between
the TFT substrate
2 and the polarizer plate
83, and the second ¼
wavelength plate
32 shown in FIG. 6 is arranged between the CF substrate
4 and the polarizer plate
84. Further, the axis P of absorption of
the polarizer plate
83 is 45° with respect to the retardation axis
C
1 of the first ¼ wavelength plate
30, the axis A of absorption
of the polarizer plate
84 is 45° with respect to the retardation axis
C
2 of the second ¼ wavelength plate
32, and the retardation
axis C
1 of the first ¼ wavelength plate
30 is nearly at right
angles with the retardation axis C
2 of the second ¼ wavelength plate
32.
The electrode structure shown in FIG. 7 has electrode portions
20,
26
which are very wider than those of the prior art and first and second slits
22,
28, and is formed without requiring fine patterning in the step of photolithography
when the electrode is formed, and can, further, be produced maintaining a high
yield. Besides, the slits
22 and
28 for regulating the alignment
of the liquid crystal molecules are formed not only in the pixel electrode
16
but also in the common electrode
24, making it possible to greatly improve
the stability of alignment, uniformity and response as compared to those of the
conventional structure having slits formed in one electrode only. Further, the
first and second slits
22,
28 in the two electrodes are extending
in the directions that intersect at right angles. Therefore, there is no need of
enhancing the precision at the time of sticking the substrates together.
According to this modified embodiment, stabilized alignment properties
are obtained when any voltage is applied, and the liquid crystal molecules can
be aligned being tilted at a uniform angle in the regions. Therefore, the quality
of display can be improved suppressing the flickering and rough feeling in the
image. Besides, a bright display is obtained since the transmission factor is more
improved than with the MVA-LCD shown in FIGS. 1 to 4D. If the same brightness is
maintained, the source of light needs emit light in a decreased amount, and the
MVA-LCD consumes the electric power in decreased amounts.
In the modified embodiment, too, the electrodes on both substrates are patterned
to form slits, the slits on both sides intersecting at right angles with each other.
Therefore, the stability of alignment, uniformity and response are greatly improved
as compared to the case where the electrodes are patterned on the substrate on
one side only. This makes it possible to realize a liquid crystal display device
featuring a wide viewing angle and high brightness yet improving the yield of production.
The invention can be modified in a variety of ways not being limited to the above
embodiment only.
The above embodiment uses the CF substrate obtained by forming CFs on the opposing
substrate. Not being limited thereto only, however, the invention can also be applied
even to the MVA-LCD of a so-called CF-on-TFT structure forming CFs on the TFT substrate,
as a matter of course. The electrode structure of the present invention can also
be applied to the reflection-type MVA-LCD using a reflecting electrically conducting
film as the pixel electrode or to the translucent-type MVA-LCD having, for example,
a transparent electrode and a reflecting electrode in combination.
A square shape needs not necessarily be described by the region where there are
overlapped the common electrode
24 and the pixel electrode
16 one
upon the other in the above embodiment and in the modified embodiment. Pitches
of slits
22 and
28 formed in the common electrode
24 and in
the pixel electrode
16 may be suitably changed depending upon the size of
the pixels, etc.
According to this invention as described above, there is realized a liquid
crystal display device which features an improved production yield, wide viewing
angle, high brightness and favorable display quality.
*
