U.S. patent application number 09/768371 was filed with the patent office on 2001-06-21 for liquid crystal display including a vertically aligned liquid crystal layer disposed between pixel electrodes and a common electrode.
This patent application is currently assigned to Sanyo Electric Co., Ltd.. Invention is credited to Koma, Norio.
Application Number | 20010004277 09/768371 |
Document ID | / |
Family ID | 17465887 |
Filed Date | 2001-06-21 |
United States Patent
Application |
20010004277 |
Kind Code |
A1 |
Koma, Norio |
June 21, 2001 |
Liquid crystal display including a vertically aligned liquid
crystal layer disposed between pixel electrodes and a common
electrode
Abstract
A vertically aligned type liquid crystal display includes a
liquid crystal layer disposed between a pixel electrode and a
common electrode and containing vertically aligned liquid crystal
molecules, the orientation of the liquid crystal molecules being
controlled by electric field. An orientation control window is
formed in the common electrode located opposite to the pixel
electrode and an aspect ration, i.e., a vertical to horizontal
length ratio of the pixel electrode is set to at least 2.
Alternatively, the pixel electrode is partitioned into at least two
electrode regions that each region represents a divided pixel
electrode. An orientation control window is formed in the common
electrode so as to correspond to each divided pixel electrode, an
aspect ratio of each divided pixel electrode is set to at least 2.
As such, the influence at the edge sections of the pixel electrode
is reduced, viewing angle characteristic and transmittance are
improved, and average response time is shortened.
Inventors: |
Koma, Norio; (Motosu-gun,
JP) |
Correspondence
Address: |
WEI-FU HSU. ESQ.
HOGAN & HARTSON, L.L.P.
BILTMORE TOWER
500 SOUTH GRAND AVENUE, SUITE 1900
LOS ANGELES
CA
90071
US
|
Assignee: |
Sanyo Electric Co., Ltd.
|
Family ID: |
17465887 |
Appl. No.: |
09/768371 |
Filed: |
January 23, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09768371 |
Jan 23, 2001 |
|
|
|
09162984 |
Sep 29, 1998 |
|
|
|
6229589 |
|
|
|
|
Current U.S.
Class: |
349/143 ;
349/139 |
Current CPC
Class: |
G02F 1/1393 20130101;
G02F 2201/121 20130101; G02F 1/134318 20210101; G02F 1/1343
20130101; G02F 1/133707 20130101 |
Class at
Publication: |
349/143 ;
349/139 |
International
Class: |
G02F 001/1343 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 1997 |
JP |
HEI 9-268973 |
Claims
What is claimed is:
1. A vertically aligned type liquid crystal display, comprising: a
vertically aligned liquid crystal layer disposed between a
plurality of pixel electrodes and a common electrode, the
orientation of said liquid crystal layer being controlled by
electric field; wherein said common electrode has an orientation
control window formed in an area corresponding to each of said
plurality of pixel electrodes, and wherein a ratio of vertical to
horizontal length of each of said plurality of pixel electrodes is
equal to or more than 2.
2. The liquid crystal display according to claim 1, wherein said
orientation control window is in the form of a slit which extends
along a longer edge of said pixel electrode in an area
corresponding to the center part of said pixel electrode.
3. The liquid crystal display according to claim 2, wherein said
orientation control window is in the form of a slit which forks at
both longitudinal ends of said pixel electrode toward corner
sections of said pixel electrode.
4. A vertically aligned type liquid crystal display, comprising: a
vertically aligned liquid crystal layer disposed between a
plurality of pixel electrodes and a common electrode, the
orientation of said liquid crystal layer being controlled by
electric field; wherein said common electrode has an orientation
control window formed in an area corresponding to each of said
plurality of pixel electrodes, and wherein each of said plurality
of pixel electrodes is divided into two or more electrically
connected electrode regions, and a vertical to horizontal length
ratio of each electrode region is larger than that of each of said
plurality of pixel electrodes.
5. The liquid crystal display according to claim 4, wherein one
orientation control window is formed for each said electrode
region.
6. The liquid crystal display according to claim 4, wherein one
orientation control window is formed for each said electrode
region, and wherein each of said orientation control windows is in
the form of a slit which extends along a longer edge of said
electrode region corresponding to the center part of said electrode
region.
7. The liquid crystal display according to claim 6, wherein said
orientation control window is in the form of a slit which forks at
both longitudinal ends of said electrode region toward corner
sections of said electrode region.
8. A vertically aligned type liquid crystal display, comprising: a
vertically aligned liquid crystal layer disposed between a
plurality of pixel electrodes and a common electrode, the
orientation of said liquid crystal layer being controlled by
electric field; wherein said common electrode has an orientation
control window formed in an area corresponding to each of said
plurality of pixel electrodes, and, wherein each of said plurality
of pixel electrodes is divided into two or more electrically
connected electrode regions, and a vertical to horizontal length
ratio of each electrode region is equal to or more than 2.
9. The liquid crystal display according to claim 8, wherein one
orientation control window is formed for each said electrode
region.
10. The liquid crystal display according to claim 8, wherein one
orientation control window is formed for each said electrode
region, and wherein each of said orientation control windows is in
the form of a slit which extends along a longer edge of said
electrode region in an area corresponding to the center part of
said electrode region.
11. The liquid crystal display according to claim 10, wherein said
orientation control window is in the form of a slit which forks at
both longitudinal ends of said electrode region toward corner
sections of said electrode region.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a liquid crystal display
(LCD) which utilizes opto-electric anisotropy of liquid crystal,
and more particularly to a liquid crystal display which achieves an
improved response speed and transmittance.
[0003] 2. Description of the Related Art
[0004] LCDs are compact, thin, and low power consumption devices
and have been developed for practical use in the field of office
automation (OA) and audio-visual (AV) equipment. In particular,
active matrix type LCDs which utilize thin film transistors (TFTs)
as switching elements are theoretically capable of static actuation
at a duty ratio of 100% in a multiplexing manner, and have been
used in large screen and high resolution type animation
displays.
[0005] TFTs are field effect transistors arranged in a matrix on a
substrate and connected to individual pixel electrodes which form
one side of pixel capacitors with a dielectric layer made of liquid
crystal. In a TFT matrix, TFTS located on a same row are
simultaneously turned on/off by a given gate line, and each TFT of
that row receives a pixel signal voltage from a given drain line. A
display voltage is accumulated in the pixel capacitors
corresponding to the on-state TFTs and designated by rows and
columns. The pixel electrodes and the TFTs are formed on the same
substrate, while a common electrode acting as the other side of the
pixel capacitors is formed on the entire surface of the second
substrate opposite to the first substrate across the liquid crystal
layer. That is, the display pixels (i.e., pixels) are defined by
partitioning the liquid crystal and the common electrode by pixel
electrodes. The voltage accumulated in the pixel capacitors is held
insulated by an off-state resistance of the TFTs for one field
period or one frame period until the TFTs are turned on again. The
liquid crystal is opto-electrically anisotropic, and its
transmittance is controlled based on the voltage applied to
respective pixel capacitors. The transmittance of each display
pixel is independently controlled, so that individual pixels are
observed bright or dark and recognized collectively as a display
image by human eyes.
[0006] Initial orientation of the liquid crystal is determined by
an orientation film disposed at the interface between the liquid
crystal and each substrate. For example, a twisted nematic (TN)
type LCD uses the liquid crystal in nematic phase which has
positive dielectric anisotropy and whose alignment vectors are
twisted 90 degrees between opposing substrates. Typically, a
polarizing plate is provided on the outside of each substrate, and
an polarizing axis of each polarizing plate coincides with the
orientation of the liquid crystal located in the vicinity of the
corresponding substrate. When no voltage is applied, linearly
polarized light passes through one polarizing plate, turns its
direction in the liquid crystal layer along the twisted alignment
of the liquid crystal, and exits from the other polarizing plate,
resulting in a "white" display. When the voltage is then applied to
the pixel capacitors, an electric field is created within the
liquid crystal and the orientation of the liquid crystal is changed
to be parallel to the direction of the applied electric field
because of dielectric anisotropy. This results in the collapse of
twisted alignment and less frequent turns of the linearly polarized
incoming light in the liquid crystal. Consequently, the amount of
light ejecting from the other polarizing plate is reduced and the
display gradually becomes black. This is known as a normally white
mode which is widely applied in the field of TN cells, in which the
display is white when no voltage is applied and changes to "black"
upon application of the voltage.
[0007] FIGS. 1 and 2 show a unit pixel structure of a conventional
liquid crystal display, wherein FIG. 1 is a plan view and FIG. 2 is
a sectional view along line G-G of FIG. 1. A gate electrode 101
made of a metal, such as Cr, Ta, or Mo, is formed on a substrate
100, and a gate insulating film 102 made of, e.g., SiNx and/or
SiO.sub.2 is formed to cover the gate electrode 101. The gate
insulating film 102 is covered with a p-Si film 103 in which an
implantation stopper 104 is used to form a lightly doped region
(LD) having a low concentration (N-) of impurities, such as P or
As, and source and drain regions (S, D) having a high concentration
(N+) of impurities located outside the LD region. A region located
immediately below the implantation stopper 104 is an intrinsic
layer which includes substantially no impurities and acts as a
channel region (CH). The p-Si 103 is covered with an interlayer
insulating film 105 made of SiNx or the like. A source electrode
106 and a drain electrode 107, both made of a material such as Al,
Mo, or the like, are formed on the interlayer insulating film 105,
each electrode being connected to the source region S and the drain
region D, respectively, via a contact hole CT1 formed in the
interlayer insulating film 105. The entire surface of the thus
formed TFT is covered with a planarization insulating film 108 made
of SOG (spin on glass), BPSG (boro-phospho silicate glass), acrylic
resin, or the like. A pixel electrode 109 made of ITO (indium tin
oxide) or the like is formed on the planarization insulating film
108 for actuating the liquid crystal, and is connected to the
source electrode 106 via a contact hole CT2 formed in the
planarization insulating film 108.
[0008] An orientation film 120 formed by a high molecular film,
such as polyimide, is disposed on the entire surface on the above
elements and undergoes a rubbing treatment to control an initial
orientation of the liquid crystal. Meanwhile, a common electrode
131 made of ITO is formed on the entire surface of another glass
substrate 130 arranged opposite to the substrate 100 across a
liquid crystal layer. The common electrode 131 is covered with an
orientation film 133 made of polyimide or the like and undergone
rubbing.
[0009] As shown herein, a DAP (deformation of vertically aligned
phase) type LCD uses a nematic phase liquid crystal 140 having
negative dielectric anisotropy, and orientation films 120, 133
formed by a vertical orientation film. The DAP type LCD is one of
the electrically controlled birefringence (ECB) type LCDs which use
a difference of refractive indices of longer and shorter axes of a
liquid crystal molecule, so-called a birefringence, to control
transmittance. In the DAP type LCD, upon application of a voltage,
an incoming light transmits one of two orthogonal polarization
plates and enters the liquid crystal layer as a linearly polarized
light, and is birefracted in the liquid crystal to become an
elliptically polarized light. Then, retardation, which is a
difference of phase velocity between ordinary and extraordinary ray
components in the liquid crystal, is controlled according to an
intensity of the electric field of the liquid crystal layer to
allow the light to be emitted from the other polarization plate at
a desired transmittance. In this case, the display is in a normally
black mode, since the display is black when no voltage is applied
and changes to white upon application of an appropriate
voltage.
[0010] As described above, the liquid crystal display displays an
image at an intended transmittance or color phase by applying a
desired voltage to the liquid crystal sealed between a pair of
substrates having predetermined electrodes formed thereon and by
controlling a turning route or a birefringence of light in the
liquid crystal. Specifically, the retardation is controlled by
changing the alignment of the liquid crystal, to thereby adjust the
light intensity of the transmitted light in the TN mode, while
allowing the separation of color phases in the ECB mode by
controlling a spectroscopic intensity depending on wavelength.
Since the retardation depends on the angle between the longer axis
of the liquid crystal molecule and the orientation of the electric
field, the retardation still changes relative to the viewer's
observation angle, i.e., a viewing angle, even when such an angle
is primarily controlled by the adjustment of the electric field
intensity. As the viewing angle changes, the light intensity or the
color phase of the transmitted light also changes, causing a
so-called viewing angle dependency problem.
[0011] Problems of decreased transmittance and slower response
speed also remain.
SUMMARY OF THE INVENTION
[0012] The present invention is made to solve the above problems
and provides a vertically aligned type liquid crystal display
including a vertically aligned liquid crystal layer disposed
between a plurality of pixel electrodes and a common electrode
facing the plurality of pixel electrodes, wherein the orientation
of the liquid crystal layer is controlled by electric field, the
common electrode has an orientation control window formed in an
area corresponding to each of the plurality of pixel electrodes,
and a ratio of vertical to horizontal length of each of the
plurality of pixel electrode is equal to or more than 2.
[0013] In another aspect of the present invention, a vertically
aligned type liquid crystal display includes an orientation control
window formed in a common electrode corresponding to each of a
plurality of pixel electrodes, wherein each of the plurality of
pixel electrodes is divided into two or more electrically connected
electrode regions, and a ratio of vertical to horizontal length of
each electrode region is larger than that of each of the plurality
of pixel electrodes.
[0014] In still another aspect of the present invention, a liquid
crystal display includes a plurality of pixel electrodes, each
pixel electrode being divided into two or more electrically
connected electrode regions having a vertical to horizontal length
ratio of equal to or more than 2.
[0015] In a further aspect of the present invention, the
orientation control window is in the form of a slit which extends
longitudinally in an area corresponding to the center part of each
pixel electrode or electrode region.
[0016] In a still further-aspect of the present invention, the
orientation control window is in the form of a slit which forks at
both longitudinal ends of the electrode or electrode region toward
corner sections of the pixel electrode.
[0017] In addition, each pixel electrode may be divided into a
plurality of electrode regions, and one orientation control window
is formed for each electrode region.
[0018] The present invention includes the above features and
reduces the influence at edge sections of the pixel electrodes by
the combination of the above-mentioned orientation control window
and the pixel electrodes, thereby achieving improved viewing angle
characteristic and transmittance and a reduced average response
time of the display.
[0019] As is apparent from the above description, the influence at
the edge sections of the pixel electrode is reduced, the viewing
angle characteristic and the transmittance are improved, and the
average response time is shortened by setting an aspect ratio (V/H)
of each pixel electrode or divided pixel electrode to at least a
predetermined value.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a plan view showing a unit pixel of a conventional
liquid crystal display;
[0021] FIG. 2 is a sectional view taken along line G-G of FIG.
1;
[0022] FIG. 3 is a plan view showing a unit pixel of a liquid
crystal display according to a first embodiment of the present
invention;
[0023] FIG. 4 is a sectional view taken along line A-A of FIG.
3;
[0024] FIGS. 5A and 5B are graphs plotting an aspect ratio of the
liquid crystal display as a function of a transmittance and an
average response time, respectively, according to the present
invention;
[0025] FIG. 6 is a plan view showing a unit pixel of the liquid
crystal display according to a second embodiment of the present
invention; and
[0026] FIG. 7 is a sectional view taken along line A-A of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Referring to FIGS. 3 and 4, a unit pixel structure of a
liquid crystal display according to the present invention is shown,
wherein FIG. 3 is a plan view and FIG. 4 is a sectional view taken
along line A-A of FIG. 3. A gate electrode 11 made of a metal, such
as Cr, Ta, or Mo, is formed on a substrate 10, and a gate
insulating film 12 made of, e.g., SiNx and/or SiO.sub.2 is formed
to cover the gate electrode 11. The gate insulating film 12 is
covered with p-Si 13 in which an implantation stopper 14 is used to
form a lightly doped region (LD) having a low concentration (N-) of
impurities, such as P or As, and source and drain regions (S, D)
having a high concentration (N+) of impurities located outside the
LD region. A region located immediately below the implantation
stopper 14 is an intrinsic layer which includes substantially no
impurities and acts as a channel region (CH). The p-Si 13 is
covered with an interlayer insulating film 15 made of SiNx or the
like. A source electrode 16 and a drain electrode 17, both made of
Al, Mo, or the like, are formed on the interlayer insulating film
15, each electrode being connected to the source region S and the
drain region D, respectively, via a contact hole CT1 formed in the
interlayer insulating film 15. The entire surface of the thus
formed TFT is covered with a planarization insulating film 18 made
of SOG (spin on glass), BPSG (boro-phospho silicate glass), acrylic
resin, or the like. A pixel electrode 19 made of ITO (indium tin
oxide) or the like is formed on the planarization insulating film
18 for actuating the liquid crystal, and is connected to the source
electrode 16 via a contact hole CT2 formed in the planarization
insulating film 18.
[0028] An orientation film 20 formed by a macro molecular film,
such as polyimide, is formed on the entire surface of the above
elements, while a common electrode 31 made of ITO is formed on the
entire surface of another glass substrate 30 arranged opposite to
the substrate 10 across a liquid crystal layer. The common
electrode 31 is covered with an orientation film 33 made of
polyimide or the like. In the present invention, the orientation
films 20, 33 and the liquid crystal 40 are selected so that liquid
crystal molecules 41 are aligned vertically.
[0029] In addition, an orientation control window 50 is formed in
the common electrode 31 facing the pixel electrode 19 and in the
form of two upper and lower Y-shaped slits connected symmetrically
to each other. More specifically, this window 50 is in the form of
a slit which extends in a straight line along a longer edge of the
pixel electrode 19 in an area corresponding to the center part of
the pixel electrode 19, and forks at an area corresponding to both
longitudinal ends of the pixel electrode 19 toward its corner
sections. Since the electric field applied to the liquid crystal
molecules 41 located below the orientation control window 50 is not
sufficiently strong to tilt those molecules 41, they have vertical
alignment. Around these molecules 41, however, the electric field
is created as indicated by a dotted line in FIG. 4, which controls
the molecules 41 to direct their longer axes perpendicular to the
applied field. This is also true at the edge sections of the pixel
electrode 19 and the longer axes of the liquid crystal molecules 41
are oriented perpendicularly to the electric field. The tilt of
these molecules is propagated to other molecules located in the
interior of the layer because of continuity of the liquid crystal.
Thus, the liquid crystal molecules are oriented in substantially
the same direction in the center part of the pixel electrode 19,
but the orientation is uneven in the vicinity of the edge sections.
It has been found that better viewing angle characteristic and
transmittance are achieved when the orientation is uniform.
[0030] To achieve this, the present invention sets an aspect ratio,
i.e., a vertical to horizontal length ratio V/H of the pixel
electrode 19 facing the orientation control window 50 to at least
2. As such, it is possible to enlarge an area where the liquid
crystal molecules are oriented in the same direction, while
decreasing the share of an unevenly oriented area. This allows the
viewing angle characteristic, the transmittance, and even the
response speed to be improved.
[0031] FIGS. 5A and 5B show the experimental results, and plot an
aspect ratio (V/H) of the pixel electrode 19 relative to its
transmittance and average response time ((.tau. on+.tau. off)/2),
respectively. As shown in the graph of FIG. 5A, the transmittance
was low until the aspect ratio reached 2, and then increased to a
preferable value and remained on that value. As shown in the graph
of FIG. 5B, the average response time was slow until the aspect
ratio reached 2, and then accelerated and generally remained
unchanged after that. Namely, at the aspect ratio of the pixel
electrode 19 equal to 2 or more, a higher transmittance and a
reduced average response time were achieved.
[0032] Referring next to FIGS. 6 and 7, a second embodiment of the
present invention will be described.
[0033] FIG. 6 is a plan view showing a unit pixel structure of the
liquid crystal display and FIG. 7 is a sectional view taken along
line A-A of FIG. 6. It is to be noted, that for the sake of clarity
the TFT structure is not shown in FIG. 7, but it is of the same
structure as that shown in FIG. 4.
[0034] In this embodiment, the vertical length of the pixel
electrode 19 corresponding to the unit pixel is longer than the
horizontal length. Thus, slits 19d and 19e are formed vertically
like a comb in the pixel electrode 19, dividing (or equally
dividing in this embodiment) it into three pixel electrode regions
19a, 19b, and 19c to set the aspect ratio V/H of each pixel
electrode region to 2 or more. It is to be noted, however, these
pixel electrode regions 19a, 19b, and 19c are partly connected to
each other under the slits 19d and 19e, because one display pixel
corresponds to one pixel.
[0035] Orientation control windows 32a, 32b, and 32c are formed in
the common electrode 31 facing the substrate 30, each window
corresponding to each pixel electrode section 19a, 19b, and 19c. In
each pixel electrode section 19a, 19b, or 19c, the liquid crystal
molecules are oriented in reverse about each orientation control
window. This increases an uniform orientation area of the liquid
crystal molecules, while decreasing an abnormal orientation area at
the edge sections of the pixel electrode. Thus, the viewing angle
characteristic, transmittance, and response time are also improved,
as in the above embodiment.
* * * * *