U.S. patent application number 11/103848 was filed with the patent office on 2005-08-11 for liquid crystal display using liquid crystal with bend alignment and driving method thereof.
Invention is credited to Koma, Norio, Uchida, Tatsuo.
Application Number | 20050174518 11/103848 |
Document ID | / |
Family ID | 34577393 |
Filed Date | 2005-08-11 |
United States Patent
Application |
20050174518 |
Kind Code |
A1 |
Koma, Norio ; et
al. |
August 11, 2005 |
Liquid crystal display using liquid crystal with bend alignment and
driving method thereof
Abstract
When a transition voltage, which is higher than a display
voltage for image display, is applied to liquid crystal, the liquid
crystal can transition to a bend alignment. Therefore, by applying
a transition voltage to liquid crystal prior to image display
period only for a transition time which depends on a transition
voltage so as to cause a bend transition in the liquid crystal, an
OCB mode LCD with a high speed response can be obtained. An
interval d between pixel regions is set to be less than, for
example, a transition distance of 5 .mu.m, so that a bend
transition expands over inter-pixel regions to thereby achieve a
bend transition all over the display region. In an active matrix
type LCD, a electrical field is caused to be generated between a
common electrode and data lines or gate lines disposed between
pixel electrodes due to application of a transition voltage to the
common electrode, thereby obtaining a bent transition over the
entire surface of the display screen. Further, a pretilt angle set
by an alignment film is determined to be 1.2.degree. or more, such
that a great number of transition sources for causing a bend
transition are generated to thereby secure a high speed bend
transition. Also, the pretilt angle is set to be 3.degree. or less
for accelerating a response time in a bend alignment.
Inventors: |
Koma, Norio; (Motosu-gun,
JP) ; Uchida, Tatsuo; (Sendai-shi, JP) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
|
Family ID: |
34577393 |
Appl. No.: |
11/103848 |
Filed: |
April 12, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11103848 |
Apr 12, 2005 |
|
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|
09568897 |
May 11, 2000 |
|
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Current U.S.
Class: |
349/123 |
Current CPC
Class: |
G02F 1/136286 20130101;
G09G 3/3629 20130101; G09G 2300/0491 20130101; G09G 2310/04
20130101; G02F 1/1395 20130101; G09G 2310/063 20130101; G09G
2300/023 20130101; G09G 2310/06 20130101 |
Class at
Publication: |
349/123 |
International
Class: |
G02F 001/1337 |
Foreign Application Data
Date |
Code |
Application Number |
May 14, 1999 |
JP |
HEI 11-134546 |
May 14, 1999 |
JP |
HEI 11-134547 |
May 14, 1999 |
JP |
HEI 11-134548 |
Claims
1-35. (canceled)
36. A liquid crystal display device including liquid crystal
between first and second substrates, comprising: a plurality of
first electrodes formed on a side of the first substrate facing the
second substrate, wherein the liquid crystal has a splay alignment
state and a bend alignment state, a transition factor which
triggers a bend transition from the splay alignment state to the
bend alignment state is provided to the first electrodes, and the
bend transition occurs with the transition factor as a starting
point, and the interval between adjacent electrodes among the
plurality of first electrodes, or the width of a region where a
conductive layer is not present between adjacent electrodes among
the plurality of first electrodes, is 5 .mu.m or less.
37. The liquid crystal display according to claim 36, wherein the
interval is 2 .mu.m or less.
38. The liquid crystal display device according to claim 36,
wherein an alignment film is provided on a side of the first
substrate facing the liquid crystal, an alignment film is provided
on a side of the second substrate facing the liquid crystal, and
rubbing treatment is performed on the alignment films in
substantially the same direction.
39. The liquid crystal display device according to claim 36,
wherein a second electrode is formed on a side of the second
substrate facing the first substrate, and the bend transition
starts with the transition factor as a starting point by applying a
transition voltage higher than a threshold voltage between the
first and second electrodes.
40. The liquid crystal display device according to claim 39,
wherein the transition voltage is a voltage for causing an
alignment state of the liquid crystal to transit from the splay
alignment state to the bend alignment state and is higher than a
display voltage.
41. The liquid crystal display device according to claim 39,
wherein the transition voltage is a voltage for causing an
alignment state of the liquid crystal to transit from the splay
alignment state to the bend alignment state, and the state energy
level of the liquid crystal in the bend alignment state is lower
than that of the liquid crystal in the splay alignment state in a
state wherein the transition voltage is applied to the liquid
crystal.
42. The liquid crystal display device according to claim 36,
wherein a display voltage is higher than a threshold voltage.
43. The liquid crystal display device according to claim 36,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes among said plurality of pixel
electrodes, wherein the distance between said inter-pixel electrode
and the corresponding electrode among said plurality of pixel
electrodes is 2 .mu.m or less.
44. The liquid crystal display device according to claim 43,
wherein said plurality of pixel electrodes are insulated from said
inter-pixel electrode.
45. The liquid crystal display device according to claim 36,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes of said plurality of pixel
electrodes, wherein said inter-pixel electrode and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
46. The liquid crystal display device according to claim 36,
wherein a switching element is connected to each of said plurality
of pixel electrodes, and a selection line for selecting said
switching element or a signal line for supplying a prescribed
signal to each of said plurality of pixel electrodes is provided as
an inter-pixel electrode at the interval between adjacent
electrodes among said plurality of pixel electrodes.
47. The liquid crystal display device according to claim 46,
wherein said selection line or said signal line and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
48. A liquid crystal display device including liquid crystal
between first and second substrates, comprising: a plurality of
first electrodes formed on a side of the first substrate facing the
second substrate; and a second electrode formed on a side of the
second substrate facing the first substrate, wherein the liquid
crystal has a splay alignment state and a bend alignment state, a
transition factor which triggers a bend transition from the splay
alignment state to the bend alignment state is provided to the
first electrodes, and the bend transition occurs with the
transition factor as a starting point, the interval between
adjacent electrodes among the plurality of first electrodes, or the
width of a region where a conductive layer is not present between
adjacent electrodes among the plurality of first electrodes, is 5
.mu.m or less, and a transition voltage for causing an alignment
state of the liquid crystal to transit from the splay alignment
state to the bend alignment state is applied between the first and
second electrodes, the transition voltage being higher than a
display voltage.
49. The liquid crystal display device according to claim 48,
wherein the interval is 2 .mu.m or less.
50. The liquid crystal display device according to claim 48,
wherein an alignment film is provided on a side of the first
substrate facing the liquid crystal, an alignment film is provided
on a side of the second substrate facing the liquid crystal, and
rubbing treatment is performed on the alignment films in
substantially the same direction.
51. The liquid crystal display device according to claim 48,
wherein the transition voltage is higher than a threshold voltage,
and the bend transition starts with the transition factor as a
starting point by applying the transition voltage between the first
and second electrodes.
52. The liquid crystal display device according to claim 48,
wherein the display voltage is higher than a threshold voltage.
53. The liquid crystal display device according to claim 48,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes among said plurality of pixel
electrodes, wherein the distance between said inter-pixel electrode
and the corresponding electrode among said plurality of pixel
electrodes is 2 .mu.m or less.
54. The liquid crystal display device according to claim 53,
wherein said plurality of pixel electrodes are insulated from said
inter-pixel electrode.
55. The liquid crystal display device according to claim 48,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes of said plurality of pixel
electrodes, wherein said inter-pixel electrode and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
56. The liquid crystal display device according to claim 48,
wherein a switching element is connected to each of said plurality
of pixel electrodes, and a selection line for selecting said
switching element or a signal line for supplying a prescribed
signal to each of said plurality of pixel electrodes is provided as
an inter-pixel electrode at the interval between adjacent
electrodes among said plurality of pixel electrodes.
57. The liquid crystal display device according to claim 56,
wherein said selection line or said signal line and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
58. A liquid crystal display device including liquid crystal
between first and second substrates, comprising: a plurality of
first electrodes formed on a side of the first substrate facing the
second substrate, wherein the liquid crystal has a splay alignment
state and a bend alignment state, each of the first electrodes is
formed in an individual pattern for each pixel and is connected to
a switching element provided for each pixel, and a transition
factor which triggers a bend transition from the splay alignment
state to the bend alignment state is provided to the first
electrodes, and the bend transition occurs with the transition
factor as a starting point, and the interval between adjacent
electrodes among the plurality of first electrodes, or the width of
a region where a conductive layer is not present between adjacent
electrodes among the plurality of first electrodes, is 5 .mu.m or
less.
59. The liquid crystal display device according to claim 58,
wherein the interval is 2 .mu.m or less.
60. The liquid crystal display device according to claim 58,
wherein an alignment film is provided on a side of the first
substrate facing the liquid crystal, an alignment film is provided
on a side of the second substrate facing the liquid crystal, and
rubbing treatment is performed on the alignment films in
substantially the same direction.
61. The liquid crystal display device according to claim 58,
wherein a second electrode is formed on a side of the second
substrate facing the first substrate, the bend transition starts
with the transition factor as a starting point by applying a
transition voltage higher than a threshold voltage between the
first and second electrodes.
62. The liquid crystal display device according to claim 58,
wherein a transition voltage for causing an alignment state of the
liquid crystal to transit from the splay alignment state to the
bend alignment state is higher than a display voltage.
63. The liquid crystal display device according to claim 58,
wherein the state energy level of the liquid crystal in the bend
alignment state is lower than that of the liquid crystal in the
splay alignment state in a state wherein a transition voltage for
causing an alignment state of the liquid crystal to transit from
the splay alignment state to the bend alignment state is applied to
the liquid crystal.
64. The liquid crystal display device according to claim 58,
wherein a display voltage is higher than a threshold voltage.
65. The liquid crystal display device according to claim 58,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes among said plurality of pixel
electrodes, wherein the distance between said inter-pixel electrode
and the corresponding electrode among said plurality of pixel
electrodes is 2 .mu.m or less.
66. The liquid crystal display device according to claim 65,
wherein said plurality of pixel electrodes are insulated from said
inter-pixel electrode.
67. The liquid crystal display device according to claim 58,
further comprising an inter-pixel electrode disposed at the
interval between adjacent electrodes of said plurality of pixel
electrodes, wherein said inter-pixel electrode and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
68. The liquid crystal display device according to claim 58,
wherein a switching element is connected to each of said plurality
of pixel electrodes, and a selection line for selecting said
switching element or a signal line for supplying a prescribed
signal to each of said plurality of pixel electrodes is provided as
an inter-pixel electrode at the interval between adjacent
electrodes among said plurality of pixel electrodes.
69. The liquid crystal display device according to claim 68,
wherein said selection line or said signal line and the
corresponding electrode among said plurality of pixel electrodes
have an overlapping region with an insulating layer interposed
therebetween.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
(LCD), and in particular to an OCB (Optically Compensated
Birefringence) mode LCD in which liquid crystal is driven at high
speed.
[0003] 2. Description of Related Art
[0004] LCDs with a faster response time have been in demand so as
to enhance motion reproducibility of LCDs and to achieve practical
application of field sequential LCDs (FS-LCDs).
[0005] A response time of an LCD refers to the time required for
changing a state of liquid crystal to a drive state after a drive
voltage is applied thereto. Specifically, when a voltage is applied
to liquid crystal, liquid crystal molecules are aligned in a
designed orientation to place the liquid crystal in a drive state.
A certain amount of time is required for the alignment of liquid
crystal to complete, and this time is referred to as the response
time. An LCD with a slow response time has particularly poor motion
display characteristics, because a prior image remains displayed
for some time when displaying a motion image. Accordingly, use of
an LCD employing liquid crystal with a faster response time can
smooth motion image display.
[0006] In FS-LCDs, light of three primary colors are rapidly
switched to alternately display images with respective colors in a
single pixel, thereby achieving color display. Liquid crystal used
in FS-LCDs demands a response time which is significantly faster
than that used in color filter type LCDs due to the operation
principle, and a practical application of FS-LCDs is highly
expected.
[0007] An OCB mode liquid crystal display has been known to have a
high speed response. OCB mode LCDs employ liquid crystal having a
bent alignment together with a biaxial optically compensating
layer. Referring to FIGS. 1A to 1C, an LCD comprises transparent
substrates 51, 52 made of glass or the like and opposing each
other, first and second electrodes 53, 54 and alignment films 55,
56 sequentially formed on the respective substrates, and a liquid
crystal layer 57 disposed between the alignment films. The liquid
crystal layer 57 comprises nematic liquid crystal. The alignment
films 55, 56 are subjected to a rubbing treatment in directions
substantially in parallel to each other, such that liquid crystal
molecules in the liquid crystal layer 57 are oriented at a pretilt
angle so as to face each other. Optically compensating layers (not
shown) are disposed on the respective alignment films to achieve
visible display. FIG. 1A shows an LCD when no voltage is applied to
the electrodes 53, 54, in which liquid crystal molecules 57a are
orientated in the rubbing direction (in the direction parallel to a
document surface) while liquid crystal molecules adjacent to the
alignment films 55, 56 are oriented at a pretilt angle. Referring
to FIG. 1B, a drive voltage of 5V, for example, is applied to the
electrode 53, and the liquid crystal molecules rise due to the
applied voltage whereas molecules in the center of the liquid
crystal layer 57 remain falling. A state of alignment as shown in
FIG. 1B is referred to as a splay alignment. Referring to FIG. 1C,
the alignment of the liquid crystal is changed to a bent alignment
in which, unlike the bend alignment, the liquid crystal molecules
in the center of the liquid crystal layer 57 also rise. The splay
alignment and the bend alignment are reversible phase transitions.
A transition of liquid crystal from a splay alignment to a bend
alignment is referred to as a "bend transition".
[0008] An OCB mode LCD is one example of an LCD using a bend
alignment, in which liquid crystal with a bend alignment and a
biaxial optical compensating layer are employed. Therefore, an OCB
mode LCD is suitable for motion display or FS-LCDs, because liquid
crystal with a bend alignment has a faster response than liquid
crystal used in TN type or STN type LCDs.
[0009] There is, however, a problem that the response time
significantly differs between the splay alignment before a bend
transition and the bend alignment. Therefore, when producing an LCD
which employs an OCB mode, a bend transition of the liquid crystal
within LCD cells should be secured.
[0010] Most of the physical mechanism of a bend transition remains
unknown and at present there are many problems remaining to be
solved.
SUMMARY OF THE INVENTION
[0011] The present invention therefore aims to provide an OCB mode
LCD having a fast response time, in which a bend transition of
liquid crystal is secured.
[0012] In order to achieve the above object, the present invention
has the following features.
[0013] According to one aspect, the present invention relates to a
method of achieving a transition in an alignment state of liquid
crystal, wherein liquid crystal, provided between first and second
alignment films disposed to face each other and covering first and
second electrodes, has a splay alignment state and a bend alignment
state; the splay alignment state energy level crosses the bend
alignment state energy level when a voltage applied between the
first and second electrodes reaches a prescribed threshold voltage;
and a voltage higher than said threshold voltage and higher than a
maximum in a range of voltages for driving the liquid crystal is
applied to said liquid crystal through said first and second
electrodes, to thereby achieve a transition to said bend alignment
state in said liquid crystal.
[0014] The present invention also relates to a method of driving a
liquid crystal display device, characterized in that a transition
voltage higher than a maximum in a range of applied display
voltages is applied to said liquid crystal through first and second
electrodes disposed with liquid crystal interposed therebetween, so
that said liquid crystal achieves a transition to said bend
alignment state, to thereby drive the liquid crystal in the bend
alignment state and present a display.
[0015] According to another aspect of the present invention, in the
above method of driving a liquid crystal display device, the splay
alignment state energy level crosses the bend alignment state
energy level when a voltage applied between the first and second
electrodes reaches a prescribed threshold voltage.
[0016] According to still another aspect of the present invention,
in the above liquid crystal display devices or in the methods of
driving the liquid crystal display device, the state energy level
of said liquid crystal in said bend alignment state becomes lower
than that of said liquid crystal in said splay alignment state when
the voltage applied to said liquid crystal through the first and
second electrodes reaches or exceeds a prescribed threshold
voltage.
[0017] According to a further aspect of the present invention, in
the above liquid crystal display devices or in the methods of
driving the liquid crystal display device, a display voltage is
applied between said first and second electrodes to drive said
liquid crystal and present a display in accordance with said
display voltage, and said transition voltage higher than said
display voltage is applied before applying said display
voltage.
[0018] In this manner, a transition voltage is applied to liquid
crystal prior to a shift to a display mode so that the liquid
crystal can transition to a bend alignment state before entering a
display mode. Because a display voltage is higher than said
threshold voltage, the liquid crystal remains in the bend alignment
state with a lower state energy during a normal display mode when a
display voltage is applied. An LCD in a bend alignment state can
provide a high speed response to an applied voltage as well as
preferable display with a wide viewing angle, which results in
image display at an optimum status.
[0019] According to a further aspect of the present invention, in
the above methods of driving the liquid crystal display device,
said display voltage is higher than said threshold voltage.
[0020] According to a still further aspect of the present
invention, in the above methods of driving the liquid crystal
display device, said transition voltage is continuously applied
between said first and second electrodes during a prescribed
transition period determined in accordance with the value of said
transition voltage.
[0021] With a display voltage which is higher than the threshold
voltage, it is possible to drive the liquid crystal in accordance
with a content to be displayed, while retaining the liquid crystal
in a bend alignment state which is capable of high speed response
during the display period.
[0022] Further, the length of a transition time can be
appropriately determined in accordance with a transition voltage,
such that the liquid crystal can transition to a bend alignment
with a minimum power. Also, an effective bend transition can be
performed by continuous application of a transition voltage during
the transition time.
[0023] According to a further aspect of the present invention, in
the above liquid crystal display devices or in the methods of
driving the liquid crystal display device, said first electrode is
composed of a plurality of pixel electrodes formed corresponding to
a plurality of pixels, said second electrode is formed as a common
electrode facing said plurality of pixel electrodes, and said
transition voltage is applied to said liquid crystal by making a
potential difference between said first substrate and said common
electrode greater than a potential difference between said first
substrate and said plurality of pixel electrodes.
[0024] As described, a potential difference between the common
electrode formed to extend over the whole region of the second
substrate and the first substrate is increased so as to cause a
transition voltage to be substantially applied to the common
electrode, such that the liquid crystal, when being provided with
said transition voltage, can transition to a bend alignment state
even if the transition voltage is not very high. In particular, the
transition voltage can also be applied to inter-pixel regions where
no pixel electrodes exist by the common electrode, so that the
liquid crystal can immediately transition to the bend alignment
state over the whole regions of display cells.
[0025] According to yet a further aspect, the present invention
relates to a liquid crystal display device including liquid crystal
between first and second substrates, comprising a plurality of
first electrodes formed on a side of said first substrate facing
said second substrate; a first alignment film formed to cover said
plurality of first electrodes; a second electrode formed on a side
of said second substrate facing said first substrate; and a second
alignment film formed to cover said second electrode. Said liquid
crystal is provided between said first and second alignment films,
and has a splay alignment state and a bend alignment state, and the
interval between adjacent electrodes among said plurality of first
electrodes, or the width of a region where a conductive layer is
not present located between adjacent electrodes among said
plurality of first electrodes, is 5 .mu.m or less.
[0026] According to a further aspect of the present invention, in
the above liquid crystal display device, the interval between
adjacent electrodes among said plurality of first electrodes, or
the width of a region where a conductive layer is not present
located between adjacent electrodes among said plurality of first
electrodes, is 2 .mu.m or less.
[0027] According to a still further aspect of the present
invention, in the above liquid crystal display devices, the first
electrode is formed by a plurality of stripe-type electrodes, the
second electrode is formed by a plurality of stripe-type electrodes
disposed in a direction substantially perpendicular to said first
electrodes, and the interval between adjacent electrodes among said
plurality of first stripe-type electrodes, and/or the interval
between adjacent electrodes among said plurality of second
stripe-type electrodes, is 5 .mu.m or less, or 2 .mu.m or less.
[0028] In a liquid crystal display comprising the first and second
electrodes as described above, when an interval between the
plurality of first electrodes, or an interval between the plurality
of second electrode is set to be 5 .mu.m or less or 2 .mu.m or
less, regions where no electrodes exist, namely inter-pixel
regions, do not form a barrier against a bend transition, even in a
passive matrix type LCD. Therefore, the liquid crystal can
immediately transition from a splay alignment to a bend alignment
over the entire region of the display cells.
[0029] According to a further aspect, the present invention relates
to a liquid crystal display device including liquid crystal between
first and second substrates, comprising a plurality of pixel
electrodes formed on a side of said first substrate facing said
second substrate; a first alignment film formed to cover said
plurality of pixel electrodes; a common electrode formed on a side
of said second substrate facing said first substrate; and a second
alignment film formed to cover said common electrode. Said liquid
crystal is provided between said first and second alignment films,
and has a splay alignment state and a bend alignment state. The
interval between adjacent electrodes among said plurality of pixel
electrodes, or the width of a region where a conductive layer is
not present located between adjacent electrodes among said
plurality of pixel electrodes, is 5 .mu.m or less.
[0030] According to a further aspect of the present invention, in
the above liquid crystal display devices, the width of a region
where a conductive layer is not present located between adjacent
electrodes among said plurality of pixel electrodes is 2 .mu.m or
less.
[0031] According to a further aspect of the present invention, the
above liquid crystal display devices further comprise an
inter-pixel electrode disposed at the interval between adjacent
electrodes among said plurality of pixel electrodes, wherein the
distance between said inter-pixel electrode and the corresponding
electrode among said plurality of pixel electrodes is 2 .mu.m or
less.
[0032] According to a further aspect of the present invention, in
the above liquid crystal display devices, said plurality of pixel
electrodes are insulated from said inter-pixel electrode.
[0033] According to a further aspect of the present invention, the
above liquid crystal display devices further comprise an
inter-pixel electrode disposed at the interval between adjacent
electrodes of said plurality of pixel electrodes, wherein said
inter-pixel electrode and the corresponding electrode among said
plurality of pixel electrodes have an overlapping region with an
insulating layer interposed therebetween.
[0034] According to a still further aspect of the present
invention, in the above liquid crystal display device, a switching
element is connected to each of said plurality of pixel electrodes,
and a selection line for selecting said switching element or a
signal line for supplying a prescribed signal to each of said
plurality of pixel electrodes is provided as an inter-pixel
electrode at the interval between adjacent electrodes among said
plurality of pixel electrodes.
[0035] As described above, when a distance in the direction of a
plane between a plurality of pixel electrodes formed as discrete
electrodes, or a distance between selection lines (for example,
gate lines) or signal lines (for example, date lines or auxiliary
capacitor lines) disposed in inter-pixel regions and the pixel
electrodes is set to be 5 .mu.m or less, or 2 .mu.m or less, the
liquid crystal can rapidly transition from the splay alignment to
the bend alignment over the entire region of the display cells
without forming any barriers between the pixel electrodes. In an
active matrix type LCD in which various lines are usually disposed
between the pixel electrodes, use of these conductive lines
eliminates a need for providing extra conductive layers between the
pixel electrodes for transitioning the liquid crystal to a bend
alignment.
[0036] According to a further aspect, the present invention relates
to a liquid crystal display device including liquid crystal between
first and second substrates, comprising a first electrode formed on
a side of said first substrate facing said second substrate; a
first alignment film formed to cover said first electrode; a second
electrode formed on a side of said second substrate facing said
first substrate; and a second alignment film formed to cover said
second electrode. Said liquid crystal is provided between said
first and second alignment films, and has a splay alignment state
and a bend alignment state. A pretilt angle determined by said
first and second alignment films is greater than 1.2.degree., but
not greater than 3.0.degree..
[0037] With a pretilt angle of 1.2.degree. or more, the liquid
crystal can provide still faster rising response to an applied
voltage in the bend alignment state, which is very advantageous in
applications demanding high speed drive.
[0038] Further, with a pretilt angle of 1.2.degree. or more, a
faster transition speed from a splay alignment state to a bend
alignment state (a faster expansion speed for a bend transition)
can be obtained with the same transition voltage being applied.
Accordingly, it is possible to rapidly transition the liquid
crystal from the splay alignment to the bend alignment in which a
desired high speed response can be obtained, prior to application
of a display voltage in accordance with an image to be displayed
between the first and second electrodes, for example. This can, for
example, reduce a starting time for enabling a display state of an
LCD after power is turned on.
[0039] Further, with a pretilt angle of 3.degree. or less, a rising
response time as well as a falling response time of the liquid
crystal which has once transitioned to a bend alignment is
sufficiently fast for applications demanding a high speed
operation, such as FS-LCDs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] These and other objects of the invention will be explained
in the description below, in connection with the accompanying
drawings, in which:
[0041] FIGS. 1A, 1B, and 1C are cross sections for explaining a
bend alignment and a splay alignment of liquid crystal;
[0042] FIG. 2 is a graph depicting Gibbs energy in liquid crystal
in a bend alignment state and a splay alignment state;
[0043] FIG. 3 is a graph depicting a potential barrier between a
bend alignment and a splay alignment;
[0044] FIG. 4 is a graph showing a relationship between a
transition voltage and a transition time required for a bend
transition of liquid crystal;
[0045] FIG. 5 is a graph showing an inter-electrode voltage for an
liquid crystal display according to an embodiment of the present
invention;
[0046] FIG. 6A is a plan view showing a schematic structure of an
active matrix type LCD;
[0047] FIG. 6B is a plan view showing a schematic structure of a
passive matrix type LCD;
[0048] FIGS. 7A, 7B and 7C are plan views showing how a bend
transition expands in a passive matrix type LCD;
[0049] FIG. 8 is a graph depicting a relationship among an
inter-pixel distance, an applied voltage, and a bend transition
ratio;
[0050] FIGS. 9A, 9B and 9C are plan views showing how a bend
transition expands in a passive matrix type LCD in which an
inter-pixel distance is less than a transition distance;
[0051] FIGS. 10A and 10B are a plan view and a cross section,
respectively, of an active matrix type LCD;
[0052] FIGS. 11A and 11B are a plan view and a cross section,
respectively, of another active matrix type LCD;
[0053] FIGS. 12A, 12B and 12C are plan views showing how a bend
transition expands in a passive matrix type LCD;
[0054] FIG. 13 is a graph showing a relationship between a pretilt
angle and a bent transition ratio;
[0055] FIGS. 14A and 14B are graphs each showing a relationship
between a pretilt angle and a bent transition expansion speed;
and
[0056] FIG. 15 is a graph showing a relationship between a pretilt
angle and a response time in liquid crystal with a bend
alignment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0057] Preferred embodiments of the present invention will be
described in further detail with reference to the accompanying
drawings.
[0058] FIG. 2 shows a change in Gibbs energy with respect to a
voltage applied to liquid crystal for explaining a fundamental
principle of a transition according to the present invention, in
which Gibbs energies of liquid crystal in a splay alignment state
and a bend alignment state are indicated by a solid line and a
dotted line, respectively. FIG. 3 shows Gibbs energy of the splay
and bend alignment states when a voltage of V1 which is higher than
a threshold voltage Vc is applied.
[0059] Gibbs energy is state energy which varies depending on the
alignment state of liquid crystal. The lower the state energy, more
stable is the liquid crystal. As an applied voltage increases, the
energy decreases both in the splay and bend alignment states.
Namely, liquid crystal which has been stable in the pretilt
orientation when no voltage applied is driven by application of a
voltage to assume either a splay alignment or a bend alignment.
Gibbs energy is lower in the splay alignment state when an applied
voltage is lower than the threshold voltage Vc and whereas it is
lower in a bend alignment state when an applied voltage is over Vc.
According to the nature of substances that lower the state energy,
the more stable a substance, the greater the stability of a splay
alignment over a bend alignment when an applied voltage is under
Vc, placing the liquid crystal in a splay alignments state, which
is an initial alignment state of the liquid crystal. On the other
hand, when an applied voltage exceeds Vc, a bend alignment becomes
more stable.
[0060] In an OCB mode LCD using liquid crystal with a bend
alignment, however, a transition to a bend alignment (i.e. bend
transition) is unlikely to occur, even when an applied voltage is
simply increased to V1, for example, which is higher than the
threshold voltage Vc, due to a potential barrier PB between a splay
alignment and a bend alignment as shown in FIG. 3. Namely, V1 is
not sufficient for exceeding the potential barrier PB of .DELTA.E,
failing to perform a bend transition. Once a bend transition is
performed with a voltage exceeding the potential barrier PB, the
liquid crystal remains in a bend alignment with lower Gibbs energy
as long as an applied voltage is over Vc.
[0061] Referring to FIG. 2, as an applied voltage increases beyond
V1, a difference in Gibbs energy between a bend alignment state and
a splay alignment state further increases, with the Gibbs energy
level of a bend alignment being lower. Therefore, according to the
present invention, prior to display on an LCD, a transition voltage
which is sufficiently higher than the threshold voltage Vc is
previously applied to the liquid crystal so as to cause a bend
transition to occur in the liquid crystal within the liquid crystal
cells.
[0062] An LCD according to the first embodiment of the present
invention has a schematic sectional structure which is similar to
the LCD shown in FIGS. 1A to 1C. More specifically, such an LCD
comprises transparent substrates 51, 52 opposing each other, first
and second electrodes 53, 54 and alignment films 55, 56
sequentially formed on the respective substrates, and a liquid
crystal layer 57 disposed between the alignment films. FIG. 4 shows
experimental values obtained by measuring the time period required
for a bend transition in the liquid crystal existing between the
first electrode 53 and the second electrode 54 while a fixed
voltage is continuously applied between these electrodes. When a
voltage of 10V was continuously applied between these electrodes
53, 54, a bend transition was achieved in approximately 20 seconds.
With an increase in an applied voltage, the time required for bend
transition was suddenly shortened, for example to 2 seconds when a
voltage of 18V was applied. Thus, it can be concluded that, by
applying a voltage which is sufficiently higher than the threshold
voltage Vc (which will hereinafter be referred to as "a transition
voltage"), a bend transition can be performed in liquid crystal so
that an LCD can operate in an OCB mode.
[0063] The above effect is possible because, as applied voltage is
increased, the energy difference between the bend and splay
alignment states is increased, as is the number of liquid crystal
molecules having energy sufficient for exceeding the potential
barrier PB.
[0064] Liquid crystal which has transitioned to the bend alignment
does not revert to a splay alignment state unless an applied
voltage is sufficiently lower than Vc, because a potential barrier
PB also exists when transiting to a splay alignment as clearly
shown in FIG. 3. Accordingly, liquid crystal which has achieved a
bend transition can remain in the bend alignment state such that an
LCD can operate in an OCB mode, as long as a display voltage which
is not greatly below Vc is applied to obtain image display. An
experiment performed by the present inventors confirmed that an LCD
which has achieved a bend transition retained an OCB mode for about
several hours even after a voltage of OV was applied.
Embodiment 1
[0065] Specifically, if a transition voltage is once applied when
an LCD is powered on, for example, to cause a bend transition in
liquid crystal, the bend alignment state is maintained while a
display voltage is applied (namely, while image display is
performed). Therefore, in an LCD according to this embodiment using
an OCB mode, a transition voltage Vt is first applied between the
first and second electrodes for a transition time T when the LCD is
powered on (time=0), and thereafter a display voltage in accordance
with the waveform of a video signal is applied to perform image
display, as in prior art LCDs. The transition voltage Vt and the
transition time T may be determined based on FIG. 4, for
example.
[0066] In order to secure a bend transition of liquid crystal, the
values of a transition voltage Vt and a transition time T are
determined by selecting, among the values for Vt and T plotted on a
chart of FIG. 4, those existing in a region above the solid line.
However, a long transition time T results in a long waiting time
for screen display to start, during which an applied voltage is
high and a power consumption is increased. Further, a high
transition voltage Vc also increases a power consumption and
requires a power source with a large capacity. Therefore, it is
preferable to select a transition voltage Vt and a transition time
T in the vicinity of the solid line. If a transition voltage Vt is
set to be 15V, for example, a transition time T is as short as 5
seconds. Also, when an LCD is used in a monitor of a personal
computer, for example, it is preferable to set a transition time T
to a high value, such as 15 seconds, while a transition voltage is
set to a small value such as 11V, since the necessity of viewing a
screen is low while actuating an OS (operation system) in the
waiting time. It is possible to perform a bent transition even with
a transition voltage Vt and a transition time T existing in a
region under the solid line in FIG. 4, as long as they are in the
vicinity of the solid line and are not far below the solid
line.
[0067] Although it is expected that a higher voltage applied
results in a faster bend transition, a voltage greater than a
withstanding voltage between the first and second electrode 53, 54
can not be applied because of the fine structure of LCDs. Further,
application of a high voltage requires a corresponding power
source, which leads to an upsize of a device when an LCD is used in
a monitor of a portable terminal, for example. Accordingly,
application of a voltage greater than 20V is not practical while a
transition time of at least 1 second should be secured.
[0068] FIG. 6A is a plan view showing an active matrix type LCD, in
which pixel electrodes 11 are provided for respective pixels, and
each pixel electrode 11 is connected to a data line 13 via a thin
film transistor 12. A gate electrode in each thin film transistor
12 is connected to a gate line 14 which is formed to be insulated
from the data line 13. Auxiliary capacitor electrodes shown by
dotted lines may also be provided. All of the above elements are
formed on a first substrate, and a common electrode 15 covering all
these elements is formed on a second substrate.
[0069] In order to cause the liquid crystal thus configured to
transition to a bend alignment, a transition voltage is applied
between a first electrode, namely the pixel electrode 11, and a
second electrode, namely the common electrode 15. It is possible to
apply a transition voltage by making one of the electrodes grounded
while increasing or decreasing a potential of the other electrode.
However, a region in pixel cells to which a voltage is not applied
is difficult to transition to a bend alignment, because, as already
described with respect to FIG. 2, Gibbs energy is lower in a splay
alignment state when an applied voltage is under an inversion
voltage (threshold voltage) Vc. As a result, when the common
electrode 15 is grounded while a transition voltage is applied only
to the pixel electrodes 11 on the first substrate, a bend
transition may not be performed in gap regions between the pixel
electrodes 11. In order to deal with this, it is necessary to apply
a transition voltage to all the electrodes formed on the first
substrate, including the pixel electrodes 11, the thin film
transistors 12, the data lines 13, the gate lines 14, the auxiliary
capacitor electrodes or the like. However, although not impossible,
such a solution complicates lines for applying a transition voltage
and may lead to breakdown of the thin film transistors 12 due to
application of a high voltage of 20V, being as high as a gate
voltage, to the gate electrodes. Accordingly, when a transition
voltage is applied to an active matrix type LCD, it is preferable
that each electrode disposed on the first substrate is set to have
the same potential as that of the first substrate, namely is
grounded, while a transition voltage is applied to the common
electrode 15. A potential of the common electrode 15 which covers
all the electrodes on the first substrate is varied such that a
transition voltage can be easily applied to all the region of the
liquid crystal. Since the first substrate is usually grounded, a
potential difference between the first substrate and the common
electrode is larger than a potential difference between the first
electrode and the pixel electrode 11 at the time of application of
a transition voltage.
[0070] In a passive matrix type LCD shown in FIG. 6B, on the other
hand, a first electrode 21 and a second electrode 22 are equivalent
and therefore a transition voltage may be applied to either of the
electrodes. Alternatively, a voltage of an opposite polarity may be
applied to each of the first and second electrodes 21, 22 to
together constitute a transition voltage.
[0071] Thus, this embodiment is similarly applicable in any types
of liquid crystal displays (LCDs) including a passive or active
matrix type, transmission type, reflection type, and others.
[0072] According to this embodiment, as described above, a
transition voltage which is higher than a display voltage is
applied between the first and second electrodes prior to
application of the display voltage, such that the liquid crystal
can previously transition to a bend alignment before performing
image display. During a screen display period, the liquid crystal
can be operated in a display mode with a bend alignment state which
enables high speed response, namely in an OCB mode.
[0073] Since a transition voltage is continuously applied during a
transition time period which is determined in accordance with the
applied transition voltage, a bend transition can be secured.
Further, it is possible to transition liquid crystal to a bend
alignment in a practical time while applying a minimal transition
voltage.
[0074] In an active matrix type LCD, as described above, a
transition voltage is applied to a common electrode, such that a
line for applying a transition voltage is simplified and a bend
transition can be obtained even in regions over which no pixel
electrodes are formed.
Embodiment 2
[0075] A relationship between a bend transition and a distance
between electrodes (an inter-electrode distance) in a passive
matrix type LCD will now be described. FIGS. 7A to 7C are plan
views depicting a transition from a splay alignment to a bend
alignment in a passive matrix type LCD. The LCD comprises first and
second transparent substrates facing each other which are made of
glass, and liquid crystal interposed between the cell of these
electrodes. A plurality of first electrodes 1 are formed on a
surface of the first transparent substrate facing the second
substrate so as to extend in the horizontal direction in a stripe
shape. A plurality of second electrodes 2 are formed on a surface
of the second transparent substrate facing the first substrate so
as to extend in the vertical direction in a stripe shape. Regions
where the first and second electrodes overlap with each other via
liquid crystal form respective pixel regions 3, in which an
electrical field is generated by a voltage applied between the
first and second electrodes to drive the liquid crystal.
[0076] Referring first to FIG. 7A, each of several points shown
within the cells by cross hatched lines is a transition factor
(transition source) which triggers a bend transition. Specifically,
a bend transition occurs with these transition sources 6 as a
starting point to expand in the radial directions, as shown in FIG.
7B. Referring to FIG. 7B, a bend transition has occurred in hatched
regions 5, which expands with time, with the transfer source 6 as a
center point.
[0077] It was observed, however, that a bend transition which
expanded within the pixel region did not expand any further, as
shown in FIG. 7C.
[0078] A bend transition ratio, which is a percentage of transfer
of a bend transition to an adjacent pixel region, is defined as
follows:
A bend transition ratio="observation points in which a bend
transition is transferred to an adjacent pixel region/all the
observation points"
[0079] FIG. 8 shows a change in the bend transition ratio with
respect to a transition voltage when an interval d between pixel
regions is set to 2 .mu.m, 5 .mu.m, and 11 .mu.m, respectively. In
FIG. 8, diamonds (.diamond.), squares (.quadrature.), and triangles
(.DELTA.) indicate respectively results when the interval d is 2
.mu.m, 5 .mu.m, and 11 .mu.m.
[0080] Referring to FIG. 8, when the inter-pixel distance d is 2
.mu.m, as indicated by diamonds, the transition ratio is 0 when an
application voltage is under 5V. The transition ratio starts
increasing when an applied voltages is increased to about 6V. With
application of voltage of about 8V, the transition ratio reaches 1,
where transfer of a bend transition to an adjacent pixel region is
secured.
[0081] FIGS. 9A to 9C are plan views showing how a bend transition
expands when the inter-pixel distance is 2 .mu.m. The structure of
the LCD in FIGS. 9A to 9C is similar to that shown in FIGS. 7A to
7C, except that the interval d between the pixel regions is 2
.mu.m. FIG. 9A, which corresponds to FIG. 7A, shows random
occurrence of the transition sources. Referring to FIG. 9B, the
bend transition expands with the transfer source 6 as a center
point, the bend transition region 5' expanding over to the adjacent
pixel region 3. Referring further to FIG. 9C, the bend transition
region 7 shown as a hatched region expands all over the cells. With
a high bend transition ratio, the bend transition region can expand
all over the surface as described, which secures high speed
response uniformly all over the LCD surface.
[0082] Referring back to FIG. 8, when the interval d is 5 .mu.m, as
indicated by squares, the transition ratio remains 0 when an
application voltage is 8V. The transition ratio starts increasing
when an applied voltages is about 9V. With application of voltage
of about 11, the transition ratio reaches 1. Compared to the case
where the inter-pixel distance d is 2 .mu.m, a higher transition
voltage is required for increasing the transition ratio when the
interval d is 5 .mu.m. Further, when the interval d is 11 .mu.m, as
indicated by triangles, the transition ratio remains 0 even with
application of voltage of 10V. Although it is expected that the
transition ratio will increase with a higher voltage as in the
cases of the interval of 2 .mu.m and 5 .mu.m, a pixel voltage to be
applied to an LCD is generally about 10V or less. Therefore, the
interval between the pixel regions should be 5 .mu.m or less,
preferably 2 .mu.m or less. Hereinafter, the interval between
pixels (interval between electrodes in this example) in which a
bend transition can be obtained is referred to as a transition
distance.
[0083] From the above observations, it is understood
[0084] that a bend transition is likely to transfer to an adjacent
pixel region with a higher voltage applied between the
electrodes,
[0085] that the interval region between electrodes forms a barrier
which prohibits a bend transition,
[0086] that transfer of a bend transition between adjacent pixel
regions occurs easily with a smaller interval between the
electrodes, and
[0087] that the interval between the pixel regions should be equal
to or less than a transition distance, namely 5 .mu.m or less,
preferably 2 .mu.m or less.
[0088] The above features are not limited to a passive matrix type
LCD as shown in FIGS. 9A to 9C, but are similarly applicable to an
active matrix type LCD. FIG. 10A is a plan view of an active matrix
type LCD and FIG. 10B is a cross section showing a section of an
interval between pixel electrodes 11 of the LCD. Referring to FIGS.
10A and 10B, on a first transparent substrate 10, a plurality of
data lines 13 are formed, on which gate lines 14 are further formed
via an insulating film (not shown). On the gate lines 14 are
provided pixel electrodes 11 for each pixel via an insulating film
16 which is a planarization film, and an alignment film 32 formed
thereon. On a second transparent substrate 18 disposed so as to
face the first transparent substrate 10, a common electrode 15
opposing the plurality of pixel electrodes 11 and an alignment film
30 are formed in this order. Between the first and second
substrates is disposed liquid crystal 31. An auxiliary capacitor
electrode (not shown) is also connected to the pixel electrodes
11.
[0089] In an interval d between adjacent pixel electrodes 11, the
data line 13 and the gate line 14 are disposed and also a
withstanding voltage between the electrodes should be secured.
Therefore, it is difficult to set this interval d to be equal to or
less than a transition distance. When a transition voltage is
applied only to the pixel electrodes 11 having an interval d which
is greater than a transition distance, a bend transition may not be
achieved in some pixels because of a barrier formed by a region
between the pixel electrodes 11. Therefore, when applying a
transition voltage on the side of the first substrate 10, it is
preferable to apply a transition voltage to all the electrodes and
lines formed on the first substrate including the pixel electrodes
11, the data lines 13, the gate lines 14, the auxiliary capacitor
electrodes or the like. Application of a transition voltage to the
data lines 13 and the gate lines 14 enables the liquid crystal
between the pixel electrodes 11 to transition to a bend alignment.
However, this will complicate lines for applying a transition
voltage and also may cause breakdown of the thin film transistors
due to application of a transition voltage to the gate
electrodes.
[0090] Therefore, in an active matrix type LCD, it is preferable to
apply a transition voltage to the common electrode 15. Application
of a transition voltage to the common electrode 15 which covers all
of the gate electrodes, the date lines 13 and the gate lines 14,
while various electrodes on the first substrates being grounded,
causes an electrical field to be generated not only between the
pixel electrodes 11 and the common electrode 15, but also between
the data and gate lines 13, 14 and the common electrode 15. Because
such an electrical field is generated, expansion of a bend
transition is not prevented, and a bend transition is thereby
secured over the whole surface of the display screen.
[0091] As long as a transition voltage is applied to the common
electrode 15, as described above, it is possible to set an interval
d between the pixel electrodes 11 to be greater than a transition
distance, with an interval d', d" between the pixel electrode 11
and the data line 13 or the gate line 14 being equal to or less
than a transition distance.
[0092] Although it is difficult to set an interval d between the
pixel electrodes 11 to be a transition distance, namely 2 .mu.m or
less, as described, it is easy to set an interval d' or d" between
the pixel electrodes 11 and the data line 13 or the gate line 14 to
be 2 .mu.m or less because these lines are separated from the pixel
electrodes 11 via the insulating film 16.
[0093] It is further preferable to form the pixel electrodes 11 so
as to overlap with the data lines 13 and the gate lines 14, such
that an interval d' and d" between the pixel electrode 11 and the
date line 13 or the gate line 14 becomes 0.
[0094] Disposing each electrode as an inter-pixel electrode, such
that inter-pixel regions where no electrodes exist do no extend
over a distance which is greater than a transition distance, is
important in the present embodiment. The electrodes disposed
between pixels, namely inter-pixel electrodes, are not limited to
the data lines 13 or the gate lines 14, and specific electrodes may
be provided or the auxiliary capacitor electrodes may be used for
them. In an active matrix type LCD, the date lines and gate lines
are optimum inter-pixel electrodes because they are disposed all
over the display region in a matrix shape.
[0095] According to the embodiment 2, a distance between the first
electrodes in an OCB mode LCD is set to be 5 .mu.m or less,
preferably 2 .mu.m or less, such that a bend alignment state can
expand beyond the inter-pixel regions, thereby achieving high speed
drive of the liquid crystal in each pixel in an OCB mode.
[0096] Provision of conductive layers such as the date lines 13 and
the gate lines 14 enables a bend transition to be expanded over
pixels, even when an interval between the pixel electrodes 11 is
not 2 .mu.m or less.
Embodiment 3
[0097] A relationship between a bend transition and a pretilt angle
will now be described with reference to an active matrix type LCD
as an example. FIGS. 12A to 12C are plan views of an active matrix
type LCD showing a transition from a splay alignment to a bend
alignment. The structure of the LCD shown in FIGS. 12A to 12C is
similar to the above described structure shown in FIGS. 6A and
10A.
[0098] Referring to FIG. 12A, each of several points 6 in cells
indicated by cross hatched lines is a transition factor (transition
source) which triggers a bend transition. As in the passive matrix
type LCD shown in FIGS. 9A to 9C, in an active matrix type LCD, a
bend transition occurs with these transition sources 6 as starting
points and expands in the radial directions, which is shown in FIG.
12B. Referring to FIG. 12B, a bend transition is performed in
hatched regions 5 which enlarges with time with the transfer source
6 as a center point, to finally expand over the entire surface of
the cells as shown in FIG. 12C, as long as inter-pixel regions
having no electrodes formed thereon (d', d" or d) do not extend
widely as already described.
[0099] In the transition mechanism as described above, a rapid bend
transition can be achieved by increasing the number of transition
sources generated and accelerating a transition expansion
speed.
[0100] The transition sources occur at random, as already
described, and therefore do not always occur at fixed points. When
a bend transition does not transfer between adjacent pixels, liquid
crystal on the pixel electrode 11' in which no transition sources 6
are generated does not transition to a bend alignment. A bend
transition ratio can then be defined as follows:
A bend transiting ratio="number of electrodes in which a bend
transition is achieved within 60 seconds after application of a
transition voltage/whole number of electrodes"
[0101] FIG. 13 depicts a change in a bend transition ratio for
three samples with different pretilt angles, namely three types of
LCDs each employing an alignment film having a different value set
for the pretilt angle of liquid crystal, when transfer voltages of
different values are applied. For application of the transfer
voltage, the voltage of a square wave of 30 Hz was varied from 5V
to 10V. The three samples include a sample 1 (indicated by
.diamond.) with a pretilt angle of 1.2.degree., a sample 2
(indicated by .quadrature.) with a pretilt angle of 2.7.degree.,
and a sample 3 (indicated by .DELTA.) with a pretilt angle of
3.8.degree., respectively having 10.times.10 (x.times.y) electrodes
each having a size of 5 mm.times.5 mm.
[0102] The results show that the larger the pretilt angle, the
higher the bend transition ratio.
[0103] The transition ratio is not necessarily 100% because a bend
transition transfers from an adjacent pixel. It is obvious that a
higher transition ratio can provide a faster transition. As
described above, a withstanding voltage between the first and
second electrodes is not generally set to be very large due to the
fine structure of an LCD. Further, application of a high voltage
requires a corresponding large power source, which results in
upsizing of a device when an LCD is used for a monitor of a
portable terminal, for example. Therefore, the foregoing sample 1
with a pretilt angle of 1.20, for which the bend transition ratio
is less than 50% with application of a voltage of 10V, is not
preferable. In an LCD with a bend alignment, the pretilt angle set
by an alignment film should be 1.20 or more, and preferably as
large as possible.
[0104] Now, a bend transition expansion speed is defined as
follows:
Bend transition expansion speed=a distance over which a bend
transition is performed/time required for transition
[0105] A time required for a bend transition to expand from a
transition source to a point 3 mm away from the source 6 was
measured to obtain a bend transition expansion speed. The results
are shown in FIGS. 14A and 14B, of which FIG. 14A shows a bend
transition expansion speed after isotropic treatment while FIG. 14B
shows a bend transition expansion speed in a case where another
bend transition is caused immediately after a bend transition.
[0106] These results show that the bend transition expansion speed
becomes faster as the transition voltage and the pretilt angle
increase. Also, a bend transition expands faster in a case where
liquid crystal once transitioned to a bend alignment reverts to a
splay alignment, and transitions to a bend alignment again, than in
a case where liquid crystal is subjected to an isotropic treatment
to enter an initial state before transitioning to a bend alignment.
This is observed because the state of a high pretilt angle obtained
at an interface in a bend alignment state when an transition
voltage is applied remains for a fixed time period, ranging from
several to several tens of hours in the experiments. Also, liquid
crystal, which remains undriven for a long period of time, retains
a splay alignment state. When such liquid crystal with the splay
alignment state transitions to a bend alignment, a bend transition
expansion speed is lower than when liquid crystal which remains in
a splay alignment on for a short time performs a bend transition.
Therefore, when a bend transition is required in liquid crystal
which has not been driven for a long period of time (when the power
is off, for example), it is preferable to cause the liquid crystal
to immediately transition to a bend alignment prior to application
of a display voltage.
[0107] When considering use of liquid crystal with a bend alignment
for an LCD, it is very significant that a time required for liquid
crystal to transition to an aligned state after application of a
drive voltage, namely a response time, should be fast. It should be
confirmed that the above-mentioned transition time refers to a time
required for a transition from a splay alignment state to a bend
alignment state, whereas a response time which will be described
hereinafter refers to a time required for liquid crystal to enter,
from a fundamental state, into a drive state when a drive voltage
is applied. FIG. 15 depicts a response time at 25.degree. C. in
each sample, in which a rising response time .tau.r is a time
required until liquid crystal is in a drive state after application
of a drive voltage thereto, and a falling response time .tau.d is a
time required until the liquid crystal is reverted, from a drive
state, to a fundamental state while removing a drive voltage from a
drive voltage application state. The results in FIG. 15 show that a
falling response time .tau.d is faster in an alignment film with a
smaller pretilt angle, which may result from the fact that an
alignment layer with a smaller pretilt angle has a larger anchoring
effect.
[0108] In an FS-LCD which performs image display at a frame
frequency of 60 Hz, a display time required for one color is
{fraction (1/180)} sec.=5.6 ms. Provided that a scanning time of
0.6 ms is required, a response time should be 5 ms or less.
Referring to FIG. 15, in order to obtain a falling response time
.tau.d of 5 ms or less, the pretilt angle should be 3.degree. or
less.
[0109] The foregoing example which was described using an active
matrix type LCD is similarly applicable to a passive matrix type or
any other types of LCDs.
[0110] According to this embodiment, the pretilt angle set by an
alignment film is 1.2.degree. or more in an OCB mode LCD, such that
a bend transfer in liquid crystal can be secured and a sufficiently
fast expansion speed for a bend transition can be obtained to
thereby shorten a time for applying a transition voltage.
[0111] Further, the pretilt angle is set to be 3.degree. or less,
such that not only the rising response time for liquid crystal
after bend transition, but also the falling response time thereof
can be practically effective.
[0112] While the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
* * * * *