U.S. patent application number 09/129323 was filed with the patent office on 2001-08-09 for liquid crystal display having thin film alignment layer that reduces image sticking.
This patent application is currently assigned to INTERNATIONL BUSINESS MACHINES. Invention is credited to CHAUDHARI, PRAVEEN, LACEY, JAMES ANDREW, LIEN, SHUI-CHIN.
Application Number | 20010012081 09/129323 |
Document ID | / |
Family ID | 22439438 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012081 |
Kind Code |
A1 |
CHAUDHARI, PRAVEEN ; et
al. |
August 9, 2001 |
LIQUID CRYSTAL DISPLAY HAVING THIN FILM ALIGNMENT LAYER THAT
REDUCES IMAGE STICKING
Abstract
A liquid crystal display device has first and second substrates,
a first electrode layer overlying one surface of the first
substrate, and a second electrode layer overlying one surface of
the second substrate. A first alignment layer having a thickness of
100 .ANG. or less overlies the first electrode layer, and a second
alignment layer overlies the second electrode layer, and a liquid
crystal material is disposed between the alignment layers. In one
preferred embodiment, the second alignment layer also has a
thickness of 100 .ANG. or less, and each alignment layers is a
polyimide layer. A method for manufacturing a liquid crystal
display device is also provided. According to the method, first and
second substrates are provided, a first electrode layer is
deposited over the first substrate, and a second electrode layer is
deposited over the second substrate. A first alignment layer having
a thickness of 100 .ANG. or less is deposited over the first
electrode layer, and a second alignment layer is deposited over the
second electrode layer. Additionally, the first and second
substrates are arranged so that the alignment layers face one
another and a space is formed between the substrates. A liquid
crystal material is disposed in the space between the first and
second substrates.
Inventors: |
CHAUDHARI, PRAVEEN;
(BRIARCLIFF MANOR, NY) ; LACEY, JAMES ANDREW;
(MAHOPAC, NY) ; LIEN, SHUI-CHIN; (BRIARCLIFF
MANOR, NY) |
Correspondence
Address: |
FLEIT, KAIN, GIBBONS,
GUTMAN & BONGINI, P.L.
ONE BOCA COMMERCE CENTER
551 NORTHWEST 77TH STREET, SUITE 111
BOCA RATON
FL
33487
US
|
Assignee: |
INTERNATIONL BUSINESS
MACHINES
|
Family ID: |
22439438 |
Appl. No.: |
09/129323 |
Filed: |
August 4, 1998 |
Current U.S.
Class: |
349/123 ;
349/117; 349/187 |
Current CPC
Class: |
G02F 1/133397 20210101;
G02F 1/133723 20130101; G02F 1/1337 20130101 |
Class at
Publication: |
349/123 ;
349/187; 349/117 |
International
Class: |
G02F 001/1337; G02F
001/13; G02F 001/1335 |
Claims
What is claimed is:
1. A liquid crystal display device comprising: a first substrate; a
first electrode layer overlying a first surface of the first
substrate; a first alignment layer overlying the first electrode
layer; a second substrate; a second electrode layer overlying a
first surface of the second substrate; a second alignment layer
overlying the second electrode layer; and a liquid crystal material
disposed between the first alignment layer and the second alignment
layer, wherein at least one of the first alignment layer and the
second alignment layer has a thickness of 100 .ANG. or less.
2. The liquid crystal display device as defined in claim 1, wherein
each of the first and second alignment layers has a thickness of
100 .ANG. or less.
3. The liquid crystal display device as defined in claim 2, wherein
each of the first and second alignment layers is a polyimide
layer.
4. The liquid crystal display device as defined in claim 3, wherein
each of the first and second substrates is a glass substrate, and
each of the first and second electrode layers is an
optically-transparent conducting layer.
5. The liquid crystal display device as defined in claim 4, wherein
each of the first and second electrode layers is an indium tin
oxide layer.
6. The liquid crystal display device as defined in claim 4, wherein
the first and second substrates are separated by approximately four
to seven microns, each of the electrode layers has a thickness of
approximately 500 to 2000 .ANG..
7. The liquid crystal display device as defined in claim 3, wherein
the liquid material is twisted nematic liquid crystal.
8. The liquid crystal display device as defined in claim 2, wherein
each of the first and second alignment layers is made from a
material in a group consisting of polyimide, hydrogenated
diamond-like carbon, amorphous hydrogenated silicon, SiC,
SiO.sub.2, glass, Si.sub.3N.sub.4, Al.sub.2O.sub.3, CeO.sub.2,
SnO.sub.2, and ZnTiO.sub.2.
9. The liquid crystal display device as defined in claim 1, further
comprising: a first polarizing film overlying a second surface of
the first substrate; and a second polarizing film overlying a
second surface of the second substrate.
10. The liquid crystal display device as defined in claim 9,
further comprising: a first optical compensating film disposed
between the first polarizing film and the second surface of the
first substrate; and a second optical compensating film disposed
between the second polarizing film and the second surface of the
second substrate.
11. The liquid crystal display device as defined in claim 1,
wherein each of the first and second alignment layers has a
thickness of 30 to 60 .ANG..
12. A method for manufacturing a liquid crystal display device,
said method comprising the steps of: providing first and second
substrates; depositing a first electrode layer over a first surface
of the first substrate; depositing a first alignment layer over the
first electrode layer, the first alignment layer having a thickness
of 100 .ANG. or less; depositing a second electrode layer over a
first surface of the second substrate; depositing a second
alignment layer over the second electrode layer; arranging the
first and second substrates so that the first surface of the first
substrate faces the first surface of the second substrate and a
space is formed between the first and second substrates; and
disposing a liquid crystal material in the space between the first
and second substrates.
13. The method as defined in claim 12, wherein the step of
depositing the first alignment layer includes the sub-steps of:
diluting polyimide with a solvent; spinning or spraying the diluted
polyimide on the first substrate; and aligning the atomic structure
of the applied polyimide.
14. The method as defined in claim 13, wherein the aligning step is
performed by ion beam bombardment.
15. The method as defined in claim 13, wherein the aligning step is
performed by at least one of neutral particle beam bombardment,
ionic particle beam bombardment, laser exposure, UV exposure,
microlithography, and mechanical rubbing.
16. The method as defined in claim 12, wherein the second alignment
layer has a thickness of 100 .ANG. or less.
17. The method as defined in claim 16, wherein each of the first
and second alignment layers is made from a material in a group
consisting of polyimide, hydrogenated diamond-like carbon,
amorphous hydrogenated silicon, SiC, SiO.sub.2, glass,
Si.sub.3N.sub.4, Al.sub.2O.sub.3, CeO.sub.2, SnO.sub.2, and
ZnTiO.sub.2.
18. The method as defined in claim 16, wherein each of the first
and second substrates is a glass substrate, and each of the first
and second electrode layers is an indium tin oxide layer.
19. The method as defined in claim 16, wherein the first and second
substrates are separated by approximately four to seven microns,
each of the electrode layers has a thickness of approximately 500
to 2000 .ANG..
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a liquid crystal display
device, and more specifically to a liquid crystal display having an
alignment film that reduces image sticking.
[0004] 2. Description of Prior Art
[0005] Flat panel displays have been become increasingly important
in the computer industry and in other industries requiring the
display of information. Such displays provide unique opportunities
for lowering the weight, size, and eventually cost of displaying
information. Presently, liquid crystal display ("LCD") devices
appear to hold the most promise as the technology that will
eventually be used for nearly all flat panel displays. Considerable
success has been achieved in using liquid crystal technology for
displaying information in small size color televisions, laptop
computers, and projection systems, and LCDs are increasingly being
used for desktop computers.
[0006] The desirability of LCDs has produced an international
industry having several billion dollars in annual sales. One
conventional LCD device includes liquid crystal cells having liquid
crystal sandwiched between transparent electrodes formed on opposed
glass substrates (i.e., plates). Another conventional LCD device
includes liquid crystal cells having liquid crystal sandwiched
between a transparent electrode on glass substrate and a reflective
electrode on a non-transparent substrate (e.g., silicon). In either
case, an electrical signal is selectively applied between the
electrodes to allow the device to modulate incident light and
display information.
[0007] To obtain contrast, the orientation of the liquid crystal
molecules must be uniformly controlled. In a field effect system
such as a nematic system (which is designed to twist the liquid
crystal molecules by about 0.degree. to 270.degree. between the
upper and lower plates), it is preferable to orient the liquid
crystal molecules parallel to the substrate surface in a
unidirectional manner. The liquid crystals can be aligned through
stretching a polymer, rubbing a polymer, depositing a polymer in
the form of a Langmuir Blodgett film, or exposing a polymer film to
UV radiation. Additionally, alignment can be achieved by depositing
particles of SiO on substrates or by etching grooves using
microlithography.
[0008] One popular alignment technique is to deposit a polyimide
alignment film on each transparent electrode to orient the liquid
crystal, and then to rub or abrade the polyimide film in a desired
direction. More specifically, a polyimide film is formed by
applying a wet coat of polyimide to a substrate using known
printing or spinning techniques. The wet coat is baked to form a
polyimide film on the substrate. After the polyimide film is formed
on the substrate, the atomic structure of the film must be aligned
in a desired direction in order to orient the liquid crystal
molecules in the desired direction. For this purpose, the polyimide
film is rubbed in the desired direction with a gigged, flocked, or
velvet cloth, and then cleaned to remove debris from the rubbing.
In this manner, an alignment film is formed as an insulating layer
with an atomic structure aligned so as to orient the liquid crystal
molecules in the desired direction.
[0009] The LCD device is formed by sandwiching a thin (e.g., five
micron) layer of liquid crystal between two glass substrates having
transparent conductors and other thin layers of materials that
provide electronics and optical filters. When a voltage is applied
across the thin layer of liquid crystal, the liquid crystal
molecules respond by rotating to minimize the electrostatic energy
of the system. This behavior is used to form a light switch that is
turned on and off by controlling the rotation of the liquid crystal
molecules using an external voltage. A large, addressable array of
such liquid crystal light switches is used in the LCD device.
[0010] While each liquid crystal light switch should switch on and
off instantaneously with the switching of the applied voltage, the
thin layer of material used for aligning the liquid crystal in
conventional LCD devices accumulates a charge that only leaks
slowly over time. For example, when the above-described process is
used to align the polyimide film, the film is altered in such a way
that the application of a voltage causes charge redistribution to
occur. This effect decays relatively slowly over time after the
removal of the voltage.
[0011] When charge accumulates on a portion the alignment film,
nearby liquid crystal molecules are exposed to a residual voltage
after the applied voltage is switched off. The slow decay of this
residual voltage results in a slow change in the alignment of the
liquid crystal molecules. Thus, with a large array of liquid
crystal switches, the LCD device can develop regions with residual
charge that keeps the liquid crystal light switches switched
locally. In such regions, the displayed image is retained after the
applied voltage is switched off. This is known as image sticking
and is a highly undesirable drawback of conventional LCD devices.
While image sticking can be reduced by carefully choosing the type
of alignment layer and the alignment technique, this prevents
optimization both in terms of display performance and manufacturing
cost.
SUMMARY OF THE INVENTION
[0012] In view of these drawbacks, it is an object of the present
invention to remove the above-mentioned drawbacks and to provide a
liquid crystal display device in which image sticking is
significantly reduced or eliminated. The LCD device is formed with
a very thin alignment film that allows for charge hopping or
tunneling. This significantly reduces charge accumulation so that
image sticking is greatly minimized or eliminated. Thus, the LCD
device can provide a very high quality display.
[0013] A first embodiment of the present invention provides a
liquid crystal display device having first and second substrates. A
first electrode layer overlies one surface of the first substrate,
and a second electrode layer overlies one surface of the second
substrate. A first alignment layer having a thickness of 100 .ANG.
or less overlies the first electrode layer. Additionally, a second
alignment layer overlies the second electrode layer, and a liquid
crystal material is disposed between the first and second alignment
layers. In one preferred embodiment, the second alignment layer
also has a thickness of 100 .ANG. or less, and both alignment
layers are polyimide layers.
[0014] A second embodiment of the present invention provides a
method for manufacturing a liquid crystal display device that has
reduced image sticking. According to the method, first and second
substrates are provided, a first electrode layer is deposited over
one surface of the first substrate, and a second electrode layer is
deposited over one surface of the second substrate. A first
alignment layer having a thickness of 100 .ANG. or less is
deposited over the first electrode layer, and a second alignment
layer is deposited over the second electrode layer. Additionally,
the first and second substrates are arranged so that the first and
second alignment layers face one another and a space is formed
between the substrates. A liquid crystal material is disposed in
the space between the first and second substrates. In a preferred
method, the first alignment layer is deposited by diluting
polyimide with a solvent, and spinning, spraying, or printing the
diluted polyimide on the first substrate. The atomic structure of
the applied polyimide is preferably aligned using ion beam
bombardment.
[0015] Other objects, features, and advantages of the present
invention will become apparent from the following detailed
description. It should be understood, however, that the detailed
description and specific examples, while indicating preferred
embodiments of the present invention, are given by way of
illustration only and various modifications may naturally be
performed without deviating from the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a cross-sectional view of a liquid crystal cell of
an LCD device;
[0017] FIG. 2 is a diagram showing the layers of a portion of an
LCD device according to a preferred embodiment of the present
invention;
[0018] FIG. 3 is a graph showing light transmission over time for
an LCD device having an ion beam-aligned polyimide film; and
[0019] FIG. 4 is a graph showing light transmission over time for
an LCD device according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Preferred embodiments of the present invention will be
described in detail hereinbelow with reference to the attached
drawings.
[0021] FIG. 1 shows a cross-sectional view of a liquid crystal cell
of a liquid crystal display ("LCD") device. As shown, a liquid
crystal cell includes a pair of glass plate substrates 10A and 10B
defining a twisted nematic cell. A transparent electrode 12 and an
alignment film 14 are provided on each of the substrates 10.
Additionally, the liquid crystal cell includes a sealing resin 15
and 16, spacers (e.g., glass beads or plastic spheres) 17 and 18,
and twisted nematic liquid crystal 19. In the preferred embodiment,
the glass substrates 10A and 10B are bonded together using an
adhesive (e.g., glue), and the spacers 17 and 18 separate the
alignment film surfaces of substrates by a space of approximately 5
.mu.m. The liquid crystal 19 is sandwiched between the two
alignment films 14A and 14B.
[0022] FIG. 2 shows the layers of a portion of an LCD device
according to a preferred embodiment of the present invention. The
LCD display 20 includes a first substrate 22 and a second substrate
24, which are both formed of a transparent material such as glass.
The two substrates are arranged parallel to one another and are
sealed at the edges (not shown) so as to define a closed interior
space. In the preferred embodiment, each substrate has a thickness
of approximately 0.7 to 1.1 millimeters and the two substrates are
separated by a distance of approximately four to seven microns. An
array of electrodes 26 that define pixels of the liquid crystal
display are deposited on the first substrate 22, and a continuous
electrode 28 is deposited on the second substrate 24. In the
preferred embodiment, a transparent film of indium tin oxide (ITO)
having a thickness of approximately 500 to 2000 .ANG. is used for
the electrodes.
[0023] On selected areas of the first substrate 22 where the
electrode film is not deposited, semiconductor devices such as
diodes and thin film transistors ("TFTs") 30 are formed. Each TFT
30 is controlled by a gate line 32 and a data line (not shown), and
the source of each TFT 30 is coupled to one of the electrodes 26.
The inside-facing surfaces of the substrates 22 and 24 are then
coated with insulating (or poorly conducting) alignment layers 38
and 40, respectively. In the preferred embodiment, each alignment
layer is a polyimide film having a thickness of approximately 50
.ANG. that is aligned using ion bombardment. During ion
bombardment, the deposited alignment film is irradiated with a beam
of atoms so as to arrange the atomic structure of the alignment
film in the desired direction in order to orient the liquid crystal
molecules. (The ion beam alignment technique is described in detail
in U.S. patent application Ser. No. 09/028,018, filed Feb. 23,
1998, which is herein incorporated by reference.)
[0024] A liquid crystal material 36 fills the space between the
alignment layers. In the preferred embodiment, twisted nematic
liquid crystal (e.g., type ZLI-6241-000, which is available from E.
Merck Darmstadt of Germany and available in the United States
through EM Industries) having a thickness of approximately four to
seven microns is used, and the liquid crystal molecules near the
alignment layers are aligned such that the long axes of the
molecules are almost parallel to the substrate with a small (e.g.,
one to five degree) pretilt angle away from the substrate surface.
Additionally, the exterior surfaces of substrates 22 and 24 are
covered with polarizing films 46 and 48, respectively. In some
embodiments, optical compensating films 42 and 44 are provided
under the polarizing films 46 and 48, respectively. When such
compensating films are omitted, light leakage around areas where
the electrode material has been removed can be prevented by using a
conventional black matrix material (i.e., for normally white
applications).
[0025] The thin film alignment layers used in embodiments of the
present invention can be formed by depositing a diluted material
and then performing an alignment technique. For example, in
preferred manufacturing processes, polyimide is diluted with a
solvent or thinner such as .gamma.-butyrolactone, bulyl cellosolue,
or N-methyl-2-pyrrolidone. The diluted polyimide is spun or spayed
onto the substrates, and after drying an alignment is performed. It
has been found experimentally that a dilution factor of 1:10
produces a very thin film of polyimide on a glass substrate, and
that such thin films provide sufficient alignment of the liquid
crystal molecules.
[0026] In embodiments of the present invention, reduced thickness
alignment layers permit rapid charge hopping or tunneling, so
charge accumulation on the alignment layers is greatly minimized or
even eliminated. More specifically, tunneling currents are
exponentially dependent on film thickness and a thin film on the
order of 50 .ANG. can tunnel charge quite effectively.
Additionally, charge can hop with the reduced thickness film.
Defects introduced into the alignment layer not only introduce the
charge that could cause image sticking, but when a thin film is
used also provides a conduction path to the adjacent electrode.
Thus, the charge is eliminated so image sticking does not
occur.
[0027] FIG. 3 shows the effect of image sticking on the display of
an LCD device formed with 600 .ANG. thick ion beam-aligned
polyimide alignment layers. As shown in FIG. 4, when 100 .ANG.
thick ion beam-aligned polyimide alignment layers are used, there
is no observable image sticking. That is, the effect on light
transmission of a voltage change is step-like or almost
instantaneous. Thus, the LCD device has significantly reduced
charge accumulation and image sticking is greatly minimized or
eliminated. This allows the present invention to provide a very
high quality display device.
[0028] The embodiments of the present invention described above
relate to LCD devices having polyimide alignment layers that are
aligned using an ion bombardment technique. However, in further
embodiments, various types of films can be employed in conjunction
with various alignment techniques to form insulating (or poorly
conducting) alignment films for the LCD device. Any film that is
optically transparent and amorphous or fine grained is suitable.
(The term amorphous means that the atomic structure of the film has
no preferred direction or orientation.) For example, a suitable
alignment film can be formed using: hydrogenated diamond-like
carbon (DLC), amorphous hydrogenated silicon, SiC, SiO.sub.2,
glass, Si.sub.3N.sub.4, Al.sub.2O.sub.3, CeO.sub.2, SnO.sub.2, or
ZnTiO.sub.2. Basically, any type of polymer or monomeric material
can be used as long as the formed film is optically transparent,
particularly in the visible spectrum.
[0029] Similarly, the alignment can be performed using any type of
particle beam, such as those employing atoms, molecules, or
clusters that are either neutral or ionic. Furthermore, laser
techniques, UV techniques, or even mechanical rubbing techniques
can be used to align the structure of the alignment layers.
Additionally, other design choices, such as the type of liquid
crystal, the layout and types of electrodes and circuit elements,
and the types of substrates could easily be adapted by one of
ordinary skill in the art. Likewise, embodiments of the present
invention may not include all of the features described above. For
example, polarizing films may not be included in all
embodiments.
[0030] While there has been illustrated and described what are
presently considered to be the preferred embodiments of the present
invention, it will be understood by those skilled in the art that
various other modifications may be made, and equivalents may be
substituted, without departing from the true scope of the
invention. Additionally, many modifications may be made to adapt a
particular situation to the teachings of the present invention
without departing from the central inventive concept described
herein. Therefore, it is intended that the present invention not be
limited to the particular embodiments disclosed, but that the
invention include all embodiments falling within the scope of the
appended claims.
* * * * *