Title: Liquid crystal display device
Abstract: The liquid crystal display device of the present invention includes a first substrate, a second substrate, and a vertical alignment type liquid crystal layer provided between the first substrate and the second substrate, and includes a plurality of picture element regions each defined by a first electrode provided on one side of the first substrate that is closer to the liquid crystal layer and a second electrode provided on the second substrate so as to oppose the first electrode via the liquid crystal layer. The first substrate includes a first orientation-regulating structure in each of the plurality of picture element regions, the first orientation-regulating structure exerting an orientation-regulating force so as to form a plurality of liquid crystal domains in the liquid crystal layer, each of the liquid crystal domains taking a radially-inclined orientation in the presence of an applied voltage. The second substrate includes a second orientation-regulating structure in a region corresponding to at least one of the plurality of liquid crystal domains, the second orientation-regulating structure exerting an orientation-regulating force for orienting liquid crystal molecules in at least one liquid crystal domain into a radially-inclined orientation at least in the presence of an applied voltage.
Patent Number: 6,862,062 Issued on 03/01/2005 to Kubo, et al.
| Inventors:
|
Kubo; Masumi (Mie, JP);
Yamamoto; Akihiro (Nara, JP);
Ogishima; Kiyoshi (Mie, JP);
Ochi; Takashi (Nara, JP);
Maekawa; Kazuhiro (Chiba, JP)
|
| Assignee:
|
Sharp Kabushiki Kaisha (Osaka, JP)
|
| Appl. No.:
|
775175 |
| Filed:
|
February 11, 2004 |
Foreign Application Priority Data
| Oct 31, 2000[JP] | 2000-333802 |
| Current U.S. Class: |
349/129; 349/130 |
| Intern'l Class: |
G02F 001//13.37 |
| Field of Search: |
349/122,123,129,130,139,100
|
References Cited [Referenced By]
U.S. Patent Documents
| 5434687 | Jul., 1995 | Kawata et al.
| |
| 5512336 | Apr., 1996 | Yamahara.
| |
| 5594570 | Jan., 1997 | Hirata et al.
| |
| 6097464 | Aug., 2000 | Liu.
| |
| 6175398 | Jan., 2001 | Yamada et al.
| |
| 6201592 | Mar., 2001 | Terashita et al.
| |
| 6256082 | Jul., 2001 | Suzuki et al.
| |
| 6266122 | Jul., 2001 | Kishimoto et al.
| |
| 6287649 | Sep., 2001 | Fukushima et al.
| |
| 6335780 | Jan., 2002 | Kurihara et al.
| |
| 6342938 | Jan., 2002 | Song et al. | 349/143.
|
| 6384889 | May., 2002 | Miyachi et al. | 349/143.
|
| 6630975 | Oct., 2003 | Terashita.
| |
| 6661488 | Dec., 2003 | Takeda et al.
| |
| Foreign Patent Documents |
| 6-301036 | Oct., 1994 | JP.
| |
| 07-311383 | Nov., 1995 | JP.
| |
| 2000-47217 | Feb., 2000 | JP.
| |
| 2000-047253 | Feb., 2000 | JP.
| |
Other References
U.S. Appl. No. 09/983,665, filed Oct. 25, 2001.
U.S. Appl. No. 09/357,814, filed Jul. 20, 1999--with Corrected Filing
Receipt.
U.S. Appl. No. 09/923,344, filed Aug. 8, 2001--with Filing Receipt.
|
Primary Examiner: Parker; Kenneth
Assistant Examiner: Chung; David
Attorney, Agent or Firm: Nixon & Vanderhye P.C.
Parent Case Text
This application is a Divisional of Application Ser. No. 09/983,665 filed
Oct. 25, 2001, the entire content of which is hereby incorporated herein
by reference in this application.
Claims
What is claimed is:
1. A liquid crystal display device, comprising:
a first substrate, a second substrate, and a vertical alignment type liquid
crystal layer provided between the first substrate and the second
substrate; and
a plurality of picture element regions each defined by at least a first
electrode supported by the first substrate and a second electrode
supported by the second substrate so as to oppose the first electrode via
the liquid crystal layer, wherein:
the first electrode includes a plurality of unit solid portions; and
the second substrate includes an orientation-regulating structure in a
region corresponding to at least one unit solid portion among the
plurality of unit solid portions, the orientation-regulating structure
exerting an orientation-regulating force for orienting liquid crystal
molecules in the liquid crystal layer on the at least one unit solid
portion into a radially-inclined orientation at least in the presence of
an applied voltage.
2. The liquid crystal display device of claim 1, wherein a shape of each of
the plurality of unit solid portions has rotational symmetry.
3. The liquid crystal display device of claim 2, wherein each of the
plurality of unit solid portions is in a substantially circular shape.
4. The liquid crystal display device of claim 2, wherein each of the
plurality of unit solid portions is in a substantially rectangular shape.
5. The liquid crystal display device of claim 2, wherein each of the
plurality of unit solid portions has a shape with an acute angle corner.
6. The liquid crystal display device of claim 1, wherein the plurality of
unit solid portions are arranged so as to have rotational symmetry.
7. The liquid crystal display device of claim 1, wherein the first
electrode includes at least one opening.
8. The liquid crystal display device of claim 7, wherein the at least one
opening includes a plurality of openings having substantially the same
shape and substantially the same size, and at least some of the plurality
of openings form at least one unit lattice arranged so as to have
rotational symmetry.
9. The liquid crystal display device of claim 8, wherein a shape of each of
the at least some of the plurality of openings has rotational symmetry.
10. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure is provided in a region corresponding to
a vicinity of a center of the at least one unit solid portion.
11. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure is provided in a region corresponding to
each of the plurality of unit solid portions.
12. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure exerts an orientation-regulating force
for orienting the liquid crystal molecules into a radially-inclined
orientation even in the absence of an applied voltage.
13. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure is a protrusion protruding from the
second substrate into the liquid crystal layer.
14. The liquid crystal display device of claim 13, wherein a thickness of
the liquid crystal layer is defined by the protrusion protruding from the
second substrate into the liquid crystal layer.
15. The liquid crystal display device of claim 14, wherein the protrusion
includes a side surface at an angle less than 90.degree. with respect to a
substrate plane of the second substrate.
16. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure includes a surface having a horizontal
alignment power provided on one side of the second substrate that is
closer to the liquid crystal layer.
17. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure exerts an orientation-regulating force
for orienting the liquid crystal molecules into a radially-inclined
orientation only in the presence of an applied voltage.
18. The liquid crystal display device of claim 1, wherein the
orientation-regulating structure includes an opening provided in the
second electrode.
19. The liquid crystal display device of claim 1, wherein when a voltage is
applied between the first electrode and the second electrode, an inclined
electric field is produced along a periphery of each of the plurality of
unit solid portions, the inclined electric field and the
orientation-regulating structure cooperatively orient the liquid crystal
molecules in the liquid crystal layer on the at least one unit solid
portion into a radially-inclined orientation.
20. The liquid crystal display device of claim 19, wherein in the liquid
crystal layer on the at least one unit solid portion, a direction of
orientation regulation by the inclined electric field coincides with a
direction of orientation regulation by the orientation-regulating
structure.
21. The liquid crystal display device of claim 1, wherein each of the
plurality of picture element regions includes a transmission region
producing a display in a transmission mode and a reflection region
producing a display in a reflection mode.
22. The liquid crystal display device of claim 21, wherein the first
electrode or the second electrode includes a transparent electrode
defining the transmission region and a reflection electrode defining the
reflection region.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more
particularly to a liquid crystal display device having a wide viewing
angle characteristic and being capable of producing a high quality
display.
2. Description of the Background Art
In recent years, liquid crystal display devices, which are thin and light
in weight, are used as personal computer displays and PDA (personal
digital assistance) displays. However, conventional twist nematic (TN)
type and super twist nematic (STN) type liquid crystal display devices
have a narrow viewing angle. Various technical developments have been
undertaken to solve the problem.
A typical technique for improving the viewing angle characteristic of a TN
or STN type liquid crystal display device is to add an optical
compensation plate thereto. Another approach is to employ a transverse
electric field mode in which a horizontal electric field with respect to
the substrate plane is applied across the liquid crystal layer. Transverse
electric field mode liquid crystal display devices have been attracting
public attention and are mass-produced in recent years. Still another
technique is to employ a DAP (deformation of vertical aligned phase) mode
in which a nematic liquid crystal material having a negative dielectric
anisotropy is used as a liquid crystal material and a vertical alignment
film is used as an alignment film. This is a type of ECB (electrically
controlled birefringence) mode, in which the transmittance is controlled
by using the birefringence of liquid crystal molecules.
While the transverse electric field mode is an effective approach to
improve the viewing angle, the production process thereof imposes a
significantly lower production margin than that of a normal TN type
device, whereby it is difficult to realize stable production of the
device. This is because the display brightness or the contrast ratio is
significantly influenced by variations in the gap between the substrates
or a shift in the direction of the transmission axis (polarization axis)
of a polarization plate with respect to the orientation axis of the liquid
crystal molecules. It requires further technical developments to be able
to precisely control these factors and thus to realize stable production
of the device.
In order to realize a uniform display without display non-uniformity with a
DAP mode liquid crystal display device, an alignment control is necessary.
An alignment control can be provided by, for example, subjecting the
surface of an alignment film to an alignment treatment by rubbing.
However, when a vertical alignment film is subjected to a rubbing
treatment, rubbing streaks are likely to appear in the displayed image,
and it is not suitable for mass-production.
Another approach proposed in the art for performing an alignment control
without a rubbing treatment is to form a slit (opening) in an electrode so
as to produce an inclined electric field and to control the orientation
direction of the liquid crystal molecules by the inclined electric field
(e.g., Japanese Laid-Open Patent Publication Nos. 6-301036 and
2000-47217). However, the present inventors reviewed these publications
and found that with the methods disclosed therein, the orientation in
regions of the liquid crystal layer corresponding to the openings in the
electrode is not defined, whereby the orientation of the liquid crystal
molecules is not sufficiently continuous, and it is difficult to achieve a
stable orientation across each pixel, resulting in a display with
non-uniformity.
SUMMARY OF THE INVENTION
The present invention has been made to solve these problems in the prior
art, and has an object to provide a liquid crystal display device having a
wide viewing angle characteristic and a high display quality.
A liquid crystal display device of the present invention includes: a first
substrate, a second substrate, and a vertical alignment type liquid
crystal layer provided between the first substrate and the second
substrate; and a plurality of picture element regions each defined by a
first electrode provided on one side of the first substrate that is closer
to the liquid crystal layer and a second electrode provided on the second
substrate so as to oppose the first electrode via the liquid crystal
layer, wherein: the first substrate includes a first
orientation-regulating structure in each of the plurality of picture
element regions, the first orientation-regulating structure exerting an
orientation-regulating force so as to form a plurality of liquid crystal
domains in the liquid crystal layer, each of the liquid crystal domains
taking a radially-inclined orientation in the presence of an applied
voltage; and the second substrate includes a second orientation-regulating
structure in a region corresponding to at least one of the plurality of
liquid crystal domains, the second orientation-regulating structure
exerting an orientation-regulating force for orienting liquid crystal
molecules in the at least one liquid crystal domain into a
radially-inclined orientation at least in the presence of an applied
voltage. Thus, the object set forth above is achieved.
Preferably, the second orientation-regulating structure is provided in a
region corresponding to a region in the vicinity of a center of the at
least one liquid crystal domain.
Preferably, in the at least one liquid crystal domain, a direction of
orientation regulation by the second orientation-regulating structure
coincides with a direction of the radially-inclined orientation by the
first orientation-regulating structure.
The first electrode may include a plurality of unit solid portions, the
first orientation-regulating structure including the plurality of unit
solid portions, so that when a voltage is applied between the first
electrode and the second electrode, an inclined electric field is produced
along a periphery of each of the plurality of unit solid portions, thereby
forming the plurality of liquid crystal domains in regions respectively
corresponding to the plurality of unit solid portions.
Preferably, a shape of each of the plurality of unit solid portions has
rotational symmetry. Preferably, the plurality of unit solid portions are
arranged so as to have rotational symmetry in each picture element region.
Each of the plurality of unit solid portions may have a shape with an acute
angle corner.
The first electrode may include at least one opening and a solid portion;
and the first orientation-regulating structure may include the at least
one opening and the solid portion of the first electrode, so that when a
voltage is applied between the first electrode and the second electrode,
an inclined electric field is produced at an edge portion of the at least
one opening of the first electrode, thereby forming the plurality of
liquid crystal domains in regions respectively corresponding to the at
least one opening and the solid portion.
The first substrate may further include a dielectric layer provided on one
side of the first electrode that is away from the liquid crystal layer,
and a third electrode opposing at least a portion of the at least one
opening of the first electrode via the dielectric layer.
Preferably, the at least one opening includes a plurality of openings
having substantially the same shape and substantially the same size, and
at least some of the plurality of openings form at least one unit lattice
arranged so as to have rotational symmetry. Preferably, a shape of each of
the at least some of the plurality of openings has rotational symmetry.
The second orientation-regulating structure may be provided in a region
corresponding to each of the plurality of liquid crystal domains.
Alternatively, the second orientation-regulating structure may be provided
only in a region corresponding to one or more of the plurality of liquid
crystal domains that is formed in a region corresponding to the solid
portion of the first electrode.
The second orientation-regulating structure may exert an
orientation-regulating force for orienting the liquid crystal molecules
into a radially-inclined orientation even in the absence of an applied
voltage. For example, the second orientation-regulating structure may be a
protrusion protruding from the second substrate into the liquid crystal
layer. A thickness of the liquid crystal layer may be defined by the
protrusion protruding from the second substrate into the liquid crystal
layer. Preferably, the protrusion includes a side surface at an angle less
than 90.degree. with respect to a substrate plane of the second substrate.
Alternatively, the second orientation-regulating structure may include a
surface having a horizontal alignment power provided on one side of the
second substrate that is closer to the liquid crystal layer.
The second orientation-regulating structure may exert an
orientation-regulating force for orienting the liquid crystal molecules
into a radially-inclined orientation only in the presence of an applied
voltage. For example, the second orientation-regulating structure may
include an opening provided in the second electrode.
Another liquid crystal display device of the present invention includes: a
first substrate, a second substrate, and a vertical alignment type liquid
crystal layer provided between the first substrate and the second
substrate; and a plurality of picture element regions each defined by a
first electrode provided on one side of the first substrate that is closer
to the liquid crystal layer and a second electrode provided on the second
substrate so as to oppose the first electrode via the liquid crystal
layer, wherein: the first electrode includes, in each of the plurality of
picture element regions, a plurality of openings and a plurality of unit
solid portions, each of the unit solid portions being surrounded by at
least some of the plurality of openings; and the second substrate includes
an orientation-regulating structure in a region corresponding to at least
one unit solid portion among the plurality of unit solid portions and the
plurality of openings. Thus, the object set forth above is achieved.
Preferably, a shape of each of the plurality of unit solid portions has
rotational symmetry. Preferably, the plurality of unit solid portions are
arranged so as to have rotational symmetry in each picture element region.
Preferably, the orientation-regulating structure is provided in a region
corresponding to a region in the vicinity of a center of the at least one
of the plurality of unit solid portions and the plurality of openings.
The orientation-regulating structure may be a protrusion protruding from
the second substrate into the liquid crystal layer. A thickness of the
liquid crystal layer may be defined by the protrusion protruding from the
second substrate into the liquid crystal layer. Preferably, the protrusion
includes a side surface at an angle less than 90.degree. with respect to a
substrate plane of the second substrate.
The orientation-regulating structure may include a surface having a
horizontal alignment power provided on one side of the second substrate
that is closer to the liquid crystal layer.
The orientation-regulating structure may include an opening provided in the
second electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A and FIG. 1B schematically illustrate a structure of one picture
element region of a liquid crystal display device 100 having a first
orientation-regulating structure of the present invention, wherein FIG. 1A
is a plan view, and FIG. 1B is a cross-sectional view taken along line
1B-1B' of FIG. 1A.
FIG. 2A and FIG. 2B illustrate a liquid crystal layer 30 of the liquid
crystal display device 100 in the presence of an applied voltage
thereacross, wherein FIG. 2A schematically illustrates a state where an
orientation has just started to change (initial ON state), and FIG. 2B
schematically illustrates a steady state.
Each of FIG. 3A to FIG. 3D schematically illustrates the relationship
between an electric force line and an orientation of a liquid crystal
molecule.
Each of FIG. 4A to FIG. 4C schematically illustrates an orientation of
liquid crystal molecules in the liquid crystal display device 100 as
viewed in a substrate normal direction.
FIG. 5A to FIG. 5C schematically illustrate exemplary radially-inclined
orientations of liquid crystal molecules.
FIG. 6A and FIG. 6B are plan views schematically illustrating other picture
element electrodes used in the liquid crystal display device of the
present invention.
FIG. 7A and FIG. 7B are plan views schematically illustrating still other
picture element electrodes used in the liquid crystal display device of
the present invention.
FIG. 8A FIG. 8B are plan views schematically illustrating still other
picture element electrodes used in the liquid crystal display device of
the present invention.
FIG. 9A and FIG. 9B are plan views schematically illustrating still other
picture element electrodes used in the liquid crystal display device of
the present invention.
FIG. 10A and FIG. 10B are plan views each schematically illustrating a
corner of a unit solid portion of a picture element electrode used in the
liquid crystal display device of the present invention.
FIG. 11A is a graph illustrating a change in the transmittance with respect
to the angle of a polarization axis of a polarization plate in a liquid
crystal display device having a picture element electrode illustrated in
FIG. 8B and in a liquid crystal display device having a picture element
electrode illustrated in FIG. 9B, and FIG. 11B schematically illustrates
an arrangement of the polarization axis corresponding to 0.degree..
FIG. 12 is a plan view schematically illustrating still another picture
element electrode used in the liquid crystal display device of the present
invention.
FIG. 13A and FIG. 13B are plan views schematically illustrating still other
picture element electrodes used in the liquid crystal display device of
the present invention.
FIG. 14A schematically illustrates a unit lattice of the pattern
illustrated in FIG. 1A, FIG. 14B schematically illustrates a unit lattice
of the pattern illustrated in FIG. 12, and FIG. 14C is a graph
illustrating the relationship between a pitch p and a solid portion area
ratio.
FIG. 15A and FIG. 15B schematically illustrate a structure of one picture
element region of a liquid crystal display device 200 having a first
orientation-regulating structure of the present invention, wherein FIG.
15A is a plan view, and FIG. 15B is a cross-sectional view taken along
line 15B-15B' of FIG. 15A.
FIG. 16A to FIG. 16D schematically illustrate the relationship between an
orientation of liquid crystal molecules 30a and a surface configuration
having a vertical alignment power.
FIG. 17A and FIG. 17B illustrate a state in the presence of an applied
voltage across a liquid crystal layer 30 of the liquid crystal display
device 200, wherein FIG. 17A schematically illustrates a state where an
orientation has just started to change (initial ON state), and FIG. 17B
schematically illustrates a steady state.
FIG. 18A to FIG. 18C are cross-sectional views schematically illustrating
liquid crystal display devices 200A, 200B and 200C, respectively, having
different positional relationships between an opening and a protrusion.
FIG. 19 is a cross-sectional view schematically illustrating the liquid
crystal display device 200 taken along line 19A-19A' of FIG. 15A.
FIG. 20A and FIG. 20B schematically illustrate a structure of one picture
element region of a liquid crystal display device 200D, wherein FIG. 20A
is a plan view, and FIG. 20B is a cross-sectional view taken along line
20B-20B, of FIG. 20A.
FIG. 21A to FIG. 21C are cross-sectional views schematically illustrating
one picture element region of a liquid crystal display device 300 having a
two-layer electrode, wherein FIG. 21A illustrates a state in the absence
of an applied voltage, FIG. 21B illustrates a state where an orientation
has just started to change (initial ON state), and FIG. 21C illustrates a
steady state.
FIG. 22A to FIG. 22C are cross-sectional views schematically illustrating
one picture element region of another liquid crystal display device 400
having a two-layer electrode, wherein FIG. 22A illustrates a state in the
absence of an applied voltage, FIG. 22B illustrates a state where an
orientation has just started to change (initial ON state), and FIG. 22C
illustrates a steady state.
FIG. 23 is a cross-sectional view schematically illustrating one picture
element region of still another liquid crystal display device 500 having a
two-layer electrode.
FIG. 24A to FIG. 24E each schematically illustrate a counter substrate 600b
including a second orientation-regulating structure 28.
FIG. 25A and FIG. 25B schematically illustrate a liquid crystal display
device 600 including a first orientation-regulating structure and a second
orientation-regulating structure, wherein FIG. 25A is a plan view, and
FIG. 25B is a cross-sectional view taken along line 25B-25B' of FIG. 25A.
FIG. 26A to FIG. 26C are cross-sectional views schematically illustrating
one picture element region of the liquid crystal display device 600,
wherein FIG. 26A illustrates a state in the absence of an applied voltage,
FIG. 26B illustrates a state where an orientation has just started to
change (initial ON state), and FIG. 26C illustrates a steady state.
FIG. 27A and FIG. 27B schematically illustrate another liquid crystal
display device 700 including a first orientation-regulating structure and
a second orientation-regulating structure, wherein FIG. 27A is a plan
view, and FIG. 27B is a cross-sectional view taken along line 27B-27B' of
FIG. 27A.
FIG. 28A to FIG. 28C are cross-sectional views schematically illustrating
one picture element region of the liquid crystal display device 700,
wherein FIG. 28A illustrates a state in the absence of an applied voltage,
FIG. 28B illustrates a state where an orientation has just started to
change (initial ON state), and FIG. 28C illustrates a steady state.
FIG. 29A to FIG. 29C are cross-sectional views schematically illustrating
one picture element region of still another liquid crystal display device
800 including a first orientation-regulating structure and a second
orientation-regulating structure, wherein FIG. 29A illustrates a state in
the absence of an applied voltage, FIG. 29B illustrates a state where an
orientation has just started to change (initial ON state), and FIG. 29C
illustrates a steady state.
FIG. 30A and FIG. 30B schematically illustrate a liquid crystal display
device 900 including a protrusion that functions as a spacer, wherein FIG.
30A is a plan view, and FIG. 30B is a cross-sectional view taken along
line 30B-30B' of FIG. 30A.
FIG. 31A to FIG. 31C are cross-sectional views schematically illustrating
one picture element region of the liquid crystal display device 900,
wherein FIG. 31A illustrates a state in the absence of an applied voltage,
FIG. 31B illustrates a state where an orientation has just started to
change (initial ON state), and FIG. 31C illustrates a steady state.
FIG. 32 is a cross-sectional view schematically illustrating a protrusion
having a side surface whose inclination angle with respect to the
substrate plane is 90.degree. or more.
FIG. 33 is a cross-sectional view schematically illustrating a variation of
a protrusion that functions as a spacer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
First, the basic function of each element of a liquid crystal display
device of the present invention will be described.
The liquid crystal display device of the present invention includes a pair
of substrates that are arranged with a vertical alignment type liquid
crystal layer being interposed therebetween. One of the pair of substrates
has a first orientation-regulating structure capable of exerting an
orientation-regulating force such that a plurality of liquid crystal
domains are formed in each picture element region, each liquid crystal
domain taking a radially-inclined orientation (referred to also as an
"axially symmetrical orientation") in the presence of an applied voltage.
The other substrate has a second orientation-regulating structure capable
of exerting an orientation-regulating force such that the liquid crystal
molecules are arranged in a radially-inclined orientation at least in the
presence of an applied voltage, in a region corresponding to at least one
of the liquid crystal domains. Therefore, the orientation-regulating force
from the first orientation-regulating structure and that from the second
orientation-regulating structure act upon the liquid crystal molecules at
least in the presence of an applied voltage, whereby the radially-inclined
orientation of each liquid crystal domain formed in the liquid crystal
layer is more stable than that in a case where only the first
orientation-regulating structure is provided.
A preferred first orientation-regulating structure of the liquid crystal
display device of the present invention includes one of a pair of
electrodes for applying a voltage across the liquid crystal layer in each
picture element region. The electrode includes a plurality of unit solid
portions so that an inclined electric field is produced along the
periphery of each of the unit solid portions upon application of a voltage
between the pair of electrodes, thereby forming a plurality of liquid
crystal domains in regions corresponding to the unit solid portions,
respectively. The outer shape of the electrode is defined so that upon
application of a voltage between the pair of electrodes, an inclined
electric field is produced around the electrode so as to form a plurality
of liquid crystal domains each taking a radially-inclined orientation.
Herein, a portion of an electrode where a conductive film exits is referred
to as a "solid portion", and a portion of the solid portion that produces
an electric field for forming a single liquid crystal domain is referred
to as a "unit solid portion". Each solid portion is typically made of a
continuous conductive film.
It is preferred that the shape of each of the unit solid portions has
rotational symmetry. When the shape of the unit solid portion has
rotational symmetry, the resulting liquid crystal domain will also be in a
radially-inclined orientation having rotational symmetry, i.e., an axially
symmetrical orientation, thereby improving the viewing angle
characteristic.
Another preferred first orientation-regulating structure of the liquid
crystal display device of the present invention is an electrode structure
in which one of the pair of electrodes for applying a voltage across the
liquid crystal layer in each picture element region has at least one
opening (a portion of the electrode where the conductive film does not
exist) and a solid portion (a portion of the electrode other than the
opening, i.e., a portion where a conductive film exists). The solid
portion typically includes at least one unit solid portion as described
above. By providing openings in one of the electrodes, it is possible to
form a number of (e.g., four) unit solid portions that are
two-dimensionally arranged in one picture element region. In this way, it
is possible to form a larger number of liquid crystal domains than in a
case where a number of (e.g., two) unit solid portions are formed by
defining the outer shape of the electrode to be a predetermined shape
without forming openings in the electrode.
Note that while openings can be formed so that a liquid crystal domain
taking a radially-inclined orientation is also formed in a region
corresponding to an electrode opening, as will be described later, this
may not be necessary. As long as a liquid crystal domain taking a
radially-inclined orientation is formed so as to correspond to the solid
portion (unit solid portion), a continuity of the orientation of the
liquid crystal molecules in each picture element region is ensured,
thereby stabilizing the radially-inclined orientation of the liquid
crystal domain formed so as to correspond to the solid portion, even when
a liquid crystal domain formed so as to correspond to an opening does not
take a radially-inclined orientation. Particularly, when the area of an
opening is small, the opening has only a little contribution to the
display, and thus the display quality will not deteriorate significantly
even if a liquid crystal domain taking a radially-inclined orientation is
not formed in a region corresponding to the opening.
A plurality of liquid crystal domains are formed in the liquid crystal
layer. Each liquid crystal domain takes a vertical alignment in the
absence of an applied voltage, and takes a radially-inclined orientation
in the presence of an applied voltage due to an inclined electric field
that is produced at an edge portion of the electrode opening. A vertical
alignment type liquid crystal layer is a liquid crystal layer in which the
liquid crystal molecules are aligned in a direction substantially
perpendicular to the substrate plane in the absence of an applied voltage.
Typically, a vertical alignment type liquid crystal layer is made of a
liquid crystal material having a negative dielectric anisotropy, and the
orientation is regulated by vertical alignment films provided on the
opposing sides.
When a voltage is applied between the pair of electrodes, an inclined
electric field is produced in the vertical alignment type liquid crystal
layer, thereby forming liquid crystal domains in regions corresponding to
openings and solid portions of the electrode. Images are displayed by
changing the orientation of the liquid crystal domains according to the
applied voltage. Since each liquid crystal domain takes a
radially-inclined orientation (axially symmetrical orientation), there is
little viewing angle dependence of the display quality and thus a wide
viewing angle characteristic.
Moreover, a liquid crystal domain corresponding to an opening and a liquid
crystal domain corresponding to a solid portion are both formed by an
inclined electric field produced at the edge portion of the opening,
whereby these liquid crystal domains are formed adjacent to each other in
an alternating pattern, and the orientation of the liquid crystal
molecules in one liquid crystal domain and that in another adjacent liquid
crystal domain are essentially continuous with each other. Therefore, no
disclination line is formed between a liquid crystal domain formed in an
opening and another adjacent liquid crystal domain formed in a solid
portion, whereby the display quality is not deteriorated and the
orientation of the liquid crystal molecules is highly stable.
When a liquid crystal display device employs an electrode structure as
described above, the liquid crystal molecules take a radially-inclined
orientation not only in a region corresponding to an electrode solid
portion but also in a region corresponding to an opening. With such a
liquid crystal display device, as compared to the conventional liquid
crystal display device described above, the continuity in the orientation
of the liquid crystal molecules is higher while a stable orientation is
realized and a uniform display without display non-uniformity can be
obtained. Particularly, in order to realize a desirable response
characteristic (high response speed), an inclined electric field for
controlling the orientation of the liquid crystal molecules needs to act
upon a large number of liquid crystal molecules. For this purpose, it is
necessary to form a large number of openings (edge portions). In the
liquid crystal display device of the present invention, a liquid crystal
domain having a stable radially-inclined orientation is formed
corresponding to an opening. Therefore, even if a large number of openings
are formed in order to improve the response characteristic, a decrease in
the display quality (occurrence of display non-uniformity) can be
suppressed.
When at least some of the openings are provided to form at least one unit
lattice arranged so as to have rotational symmetry with substantially the
same shape and substantially the same size, a plurality of liquid crystal
domains can be arranged with a high degree of symmetry for each unit
lattice, whereby it is possible to improve the viewing angle dependence of
the display quality. Moreover, by dividing the entire picture element
region into unit lattices, it is possible to stabilize the orientation of
the liquid crystal layer across the entire picture element region. For
example, openings may be arranged so that the centers of the openings form
a square lattice. Note that where each picture element region is divided
by an opaque element such as a storage capacitance line, a unit lattice
can be arranged for each region contributing to the display.
When at least some of the openings (typically those forming a unit lattice)
each have a shape having rotational symmetry, it is possible to increase
the stability of the radially-inclined orientation of the liquid crystal
domain formed in the opening. For example, the shape of each opening (as
viewed in the substrate normal direction) may be a circular shape or a
polygonal shape (e.g., a square shape). Note that a shape that does not
have rotational symmetry (e.g., an elliptical shape) may be employed
depending upon the shape (aspect ratio) of the picture element, etc.
Moreover, when the shape of a region of the solid portion that is
substantially surrounded by the openings ("unit solid portion") has
rotational symmetry, it is possible to increase the stability of the
radially-inclined orientation of the liquid crystal domain formed in the
solid portion. For example, when the openings are arranged in a square
lattice pattern, the shape of the opening may be a generally star shape or
a cross shape, and the shape of the solid portion may be a generally
circular shape, a generally square shape, or the like. Of course, the
openings and the solid portion substantially surrounded by the openings
may both have a generally square shape.
In order to stabilize the radially-inclined orientation of the liquid
crystal domain formed in the electrode opening, it is preferred that the
liquid crystal domain formed in the opening has a generally circular
shape. In other words, the shape of the opening may be designed so that
the liquid crystal domain formed in the opening has a generally circular
shape.
Of course, in order to stabilize the radially-inclined orientation of the
liquid crystal domain formed in the electrode solid portion, it is
preferred that the region of the solid portion substantially surrounded by
the openings has a generally circular shape. A liquid crystal domain
formed in the solid portion, which is made of a continuous conductive
film, is formed corresponding to a region of a solid portion (unit solid
portion) that is substantially surrounded by a plurality of openings.
Therefore, the shape and arrangement of the openings may be determined so
that the region of the solid portion (unit solid portion) has a generally
circular shape.
With any of the alternatives described above, it is preferred that the
total area of the openings formed in the electrode is smaller than the
area of the solid portion in each picture element region. As the area of
the solid portion increases, the area of the liquid crystal layer (defined
in the plane of the liquid crystal layer as viewed in the substrate normal
direction) that is directly influenced by the electric field produced by
the electrodes increases, thereby improving the optical characteristics
(e.g., the transmittance) with respect to the voltage applied across the
liquid crystal layer.
It is preferred that whether to employ an arrangement where each opening
has a generally circular shape or an arrangement where each unit solid
portion has a generally circular shape is determined by determining with
which arrangement, the area of the solid portion can be made larger. Which
arrangement is more preferred is appropriately selected depending upon the
pitch of the picture elements. Typically, when the pitch is greater than
about 25 .mu.m, it is preferred that the openings are formed so that each
solid portion has a generally circular shape. When the pitch is less than
or equal to about 25 .mu.m, it is preferred that each opening has a
generally circular shape.
With the electrode arrangement where openings are provided in one of a pair
of electrodes, a sufficient voltage may not be applied across the liquid
crystal layer in a region corresponding to the opening and a sufficient
retardation change may not be obtained, thereby decreasing the light
efficiency. In view of this, a dielectric layer may be provided on one
side of the electrode with openings that is away from the liquid crystal
layer, with an additional electrode being provided via the dielectric
layer so as to at least partially oppose the electrode openings (i.e., a
two-layer electrode may be employed). In this way, it is possible to apply
a sufficient voltage across the liquid crystal layer corresponding to the
opening, thereby improving the light efficiency and/or the response
characteristic.
Where the electrode structure described above (i.e., the first
orientation-regulating structure) is only provided in one of the
substrates, if the radially-inclined orientation is disturbed by a stress
acting upon the liquid crystal layer, the disturbed orientation may be
maintained by the electric field effect and thus is observed as an after
image phenomenon. However, the liquid crystal display device of the
present invention includes a second orientation-regulating structure in
the other substrate, in addition to the first orientation-regulating
structure, whereby the orientation-regulating force from the first
orientation-regulating structure and that from the second
orientation-regulating structure act upon the liquid crystal molecules in
each liquid crystal domain at least in the presence of an applied voltage.
Therefore, the radially-inclined orientation of the liquid crystal domain
is stabilized and the decrease in the display quality due to a stress is
suppressed, as compared with an arrangement having only the first
orientation-regulating structure.
When the second orientation-regulating structure is provided in a region in
the vicinity of the center of a liquid crystal domain taking a
radially-inclined orientation that is formed by the first
orientation-regulating structure, it is possible to fix the position of
the central axis of the radially-inclined orientation, thereby effectively
improving the resistance of the radially-inclined orientation to a stress.
When the orientation-regulating direction of the second
orientation-regulating structure may be set in conformity with the
direction of the radially-inclined orientation by the first
orientation-regulating structure. In this way, the continuity and
stability of the orientation increase, thereby improving the display
quality and the response characteristic.
While the second orientation-regulating structure provides effect of
stabilizing the orientation as long as it exerts an orientation-regulating
force at least in the presence of an applied voltage, the orientation can
be stabilized irrespective of the level of the applied voltage if an
arrangement that exerts an orientation-regulating force also in the
absence of an applied voltage is employed. However, since a vertical
alignment type liquid crystal layer in which the liquid crystal molecules
are aligned substantially vertical to the substrate plane in the absence
of an applied voltage is employed, the display quality may decrease if a
second orientation-regulating structure that exerts an
orientation-regulating force also in the absence of an applied voltage is
employed. However, since the orientation-regulating force of the second
orientation-regulating structure is effective even if it is relatively
weak, as will be described later, the orientation can be sufficiently
stabilized even with a structure that is small with respect to the size of
each picture element, and the decrease in the display quality in the
absence of an applied voltage may be insignificant in some cases.
Depending upon the application of the liquid crystal display device (e.g.,
the magnitude of the externally applied stress) and/or the electrode
arrangement (the strength of the orientation-regulating force provided by
the first orientation-regulating structure), a second
orientation-regulating structure that exerts a relatively strong
orientation-regulating force may be provided. In such a case, a
light-blocking layer may be provided in order to suppress the decrease in
the display quality due to the second orientation-regulating structure.
Moreover, the radially-inclined orientation of each liquid crystal domain
can be stabilized as long as the orientation-regulating force by the
second orientation-regulating structure acts upon those liquid crystal
molecules in each liquid crystal domain taking a radially-inclined
orientation that is formed by the first orientation-regulating structure.
Particularly, when the second orientation-regulating structure is provided
in a region in the vicinity of the center of a liquid crystal domain, an
effect of fixing the position of the central axis of the radially-inclined
orientation is also obtained. The second orientation-regulating structure
may be realized by using any of various structures because it is only
required to exert an orientation-regulating force weaker than that exerted
by the first orientation-regulating structure.
When an electrode structure with openings as described above is employed as
the first orientation-regulating structure, liquid crystal domains are
formed both in the openings and in the solid portion. By providing the
second orientation-regulating structure for each of the liquid crystal
domains to be formed, it is possible to stabilize the radially-inclined
orientation of each liquid crystal domain. However, a practically
sufficient stability (stress resistance) can be obtained by providing the
second orientation-regulating structures only for those liquid crystal
domains that are formed corresponding to the solid portion.
Particularly, a second orientation-regulating structure that exerts an
orientation-regulating force in conformity with the radially-inclined
orientation formed in the electrode solid portion is more preferable in
terms of the production efficiency because it can be provided by a simpler
process as compared to a second orientation-regulating structure that
exerts an orientation-regulating force in conformity with the
radially-inclined orientation formed in the electrode opening. Moreover,
while it is preferred that the second orientation-regulating structure is
provided for each of the unit solid portions, a practical orientation
stability may be obtained by providing the second orientation-regulating
structure only for some of the unit solid portions depending upon the
electrode structure (the number and arrangement of openings). This is
because in the liquid crystal display device of the present invention, the
radially-inclined orientations formed in the liquid crystal layer are
essentially continuous with one another.
Moreover, in order to improve the resistance to a stress, a protrusion
including a side surface that gives the liquid crystal molecules of the
liquid crystal layer an orientation-regulating force of the same direction
as the orientation-regulating direction of the inclined electric field
described above may be provided inside the electrode opening. It is
preferred that the cross-sectional shape of the protrusion in the
substrate plane direction is the same as the shape of the opening and has
rotational symmetry as the shape of the opening described above. However,
since the liquid crystal molecules whose orientation is regulated by the
orientation-regulating force of the side surface of the protrusion have a
poor response to an applied voltage (a small retardation change in
response to the applied voltage), the protrusion may decrease the contrast
ratio of the display. Therefore, it is preferred that the size, the height
and the number of protrusions are set so as not to decrease the display
quality.
Of the electrode structures that function as the first
orientation-regulating structure of the liquid crystal display device of
the present invention, the electrode having the openings as described
above is, for example, a picture element electrode connected to a
switching element in an active matrix type liquid crystal display device
including a switching element such as a TFT in each picture element
region, while the other electrode is at least one counter electrode
opposing a plurality of picture element electrodes. Thus, by providing
openings only in one of a pair of electrodes provided so as to oppose each
other via the liquid crystal layer, it is possible to realize a stable
radially-inclined orientation. Specifically, with a production method
known in the art, it is possible to produce a liquid crystal display
device having the first orientation-regulating structure only by modifying
a photomask used in the process of patterning a conductive film into the
shape of the picture element electrode so that openings having an intended
shape are formed in an intended arrangement. Of course, a plurality of
openings may be provided in the counter electrode. Moreover, a two-layer
electrode as described above may be produced by using a method known in
the art.
Moreover, the second orientation-regulating structure of the liquid crystal
display device of the present invention is, for example, a protrusion
protruding from the counter substrate into the liquid crystal layer.
Alternatively, the second orientation-regulating structure may be a
structure having a horizontal alignment type surface provided on one side
of the counter substrate that is closer to the liquid crystal layer.
Alternatively, the second orientation-regulating structure may be an
opening provided in the counter electrode. These structures may be
produced by a method known in the art.
Moreover, the liquid crystal display device of the present invention may
have an arrangement such that one of a pair of substrates arranged so as
to interpose a vertical alignment type liquid crystal layer therebetween
("first substrate") includes an electrode having a plurality of unit solid
portions and a plurality of openings in each picture element region, with
the other substrate ("second substrate") including an
orientation-regulating structure in at least one region corresponding to a
unit solid portion among a plurality of unit solid portions and a
plurality of openings.
The electrode of the first substrate is such that an inclined electric
field is produced along the periphery of each of the unit solid portions
upon application of a voltage between the electrode and the electrode of
the second substrate, thereby forming a plurality of liquid crystal
domains each taking a radially-inclined orientation in regions
corresponding to the unit solid portions, respectively of course, the
electrode may be configured so that a liquid crystal domain taking a
radially-inclined orientation is formed also in each region corresponding
to the electrode opening. This electrode structure functions similarly to
the first orientation-regulating structure described above. A preferred
arrangement of this electrode structure is substantially the same as that
of the first orientation-regulating structure described above. For
example, the shape of each of the unit solid portions preferably has
rotational symmetry, and the unit solid portions are preferably arranged
so that they have rotational symmetry in each picture element region.
In a liquid crystal display device in which one of the substrates includes
an electrode having such a structure as described above while the other
substrate includes an orientation-regulating structure, the
orientation-regulating force from the above-described electrode structure
and that from the orientation-regulating structure act upon the liquid
crystal molecules in each liquid crystal domain at least in the presence
of an applied voltage. Therefore, the radially-inclined orientation of the
liquid crystal domain is stabilized and the decrease in the display
quality due to a stress is suppressed.
The orientation-regulating structure functions substantially similarly to
the second orientation-regulating structure as described above. A
preferred arrangement of this orientation-regulating structure is
substantially the same as that of the second orientation-regulating
structure described above. For example, by providing the
orientation-regulating structure in a region in the vicinity of the center
of each liquid crystal domain taking a radially-inclined orientation that
is formed in the unit solid portion or the opening of the electrode, it is
possible to fix the position of the central axis of the radially-inclined
orientation, thereby effectively improving the resistance of the
radially-inclined orientation to a stress. The orientation-regulating
structure may be a protrusion protruding from the second substrate into
the liquid crystal layer. Alternatively, the orientation-regulating
structure may be a structure having a horizontal alignment layer provided
on one side of the second substrate that is closer to the liquid crystal
layer. Alternatively, the orientation-regulating structure may be an
opening provided in the electrode of the second substrate.
A liquid crystal display device according to an embodiment of the present
invention will now be described with reference to the drawings.
First Orientation-regulating Structure
First, a first orientation-regulating structure, which is a preferred
electrode structure for the liquid crystal display device of the present
invention, and a function thereof will be described.
The liquid crystal display device of the present invention has desirable
display characteristics and thus can be suitably used as an active matrix
type liquid crystal display device. An embodiment of the present invention
will now be described with respect to an active matrix type liquid crystal
display device using thin film transistors (TFTs). Note, however, that the
present invention is not limited thereto, but may alternatively be used
with an MIM active matrix type liquid crystal display device or a passive
matrix type liquid crystal display device. Moreover, while the embodiment
of the present invention will be described with respect to a transmission
type liquid crystal display device, the present invention is not limited
thereto, but may alternatively be used with a reflection type liquid
cryst
