(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2001/0012081 A1
`CHAUDHARI et al.
`(43) Pub. Date:
`Aug. 9, 2001
`
`US 20010012081A1
`
`LIQUID CRYSTAL DISPLAY HAVING THIN
`FILM ALIGNMENT LAYER THAT REDUCES
`IMAGE STICKING
`
`(51)
`
`Publication Classification
`
`Inventors: PRAVEEN CHAUDHARI,
`BRIARCLIFF MANOR, NY (US);
`JAMES ANDREW LACEY,
`MAHOPAC, NY (US); SHUI-CHIN
`LIEN, BRIARCLIFF MANOR, NY
`(US)
`
`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
`MACHINES
`
`BUSINESS
`
`Notice:
`
`This is a publication of a continued pros-
`ecution application (CPA) filed under 37
`CFR 1.53(d).
`
`Appl. No.:
`
`09/12 9,323
`
`Filed:
`
`Aug. 4, 1998
`
`Int. Cl.7 ........................ .. G02F 1/1337; G02F 1/13;
`G02F 1/1335
`......................... .. 349/123; 349/187; 349/117
`
`(52) U.S. Cl.
`
`ABSTRACT
`(57)
`A liquid crystal display device has first and second sub-
`strates, 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 A 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 A 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 align-
`ment layer having a thickness of 100 A 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.
`
`48
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`Patent Application Publication
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`Aug. 9, 2001 Sheet 1 of 2
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`US 2001/0012081 A1
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`Patent Application Publication
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`Aug. 9, 2001 Sheet 2 of 2
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`US 2001/0012081 A1
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`US 2001/0012081 A1
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`Aug. 9, 2001
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`LIQUID CRYSTAL DISPLAY HAVING THIN FILM
`ALIGNMENT LAYER THAT REDUCES IMAGE
`STICKING
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`[0001] Not applicable.
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`BACKGROUND OF THE INVENTION
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`[0002]
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`1. Field of the Invention
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`[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 stick-
`ing.
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`[0004]
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`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 comput-
`ers, and projection systems, and LCDs are increasingly
`being used for desktop computers.
`
`[0006] The desirability of LCDs has produced an interna-
`tional industry having several billion dollars in annual sales.
`One conventional LCD device includes liquid crystal cells
`having liquid crystal sandwiched between transparent elec-
`trodes 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.
`
`the orientation of the liquid
`[0007] To obtain contrast,
`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° to 270°
`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 poly-
`imide 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
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`Page 4 of 7
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`the liquid crystal
`a desired direction in order to orient
`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.
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`[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 rotat-
`ing 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.
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`light switch should
`[0010] While each liquid crystal
`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.
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`[0011] When charge accumulates on a portion the align-
`ment 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
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`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.
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`SUMMARY OF THE INVENTION
`
`In View of these drawbacks, it is an object of the
`[0012]
`present invention to remove the above-mentioned draw-
`backs 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 pro-
`vides 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 A or less overlies the first
`electrode layer. Additionally, a second alignment layer over-
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`US 2001/0012081 A1
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`Aug. 9, 2001
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`lies 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 A or less, and both alignment layers
`are polyimide layers.
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`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 approxi-
`mately 5 ,um. The liquid crystal 19 is sandwiched between
`the two alignment films 14A and 14B.
`
`invention
`[0014] A second embodiment of the present
`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 A 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.
`
`features, and advantages of the
`[0015] Other objects,
`present invention will become apparent from the following
`detailed description. It should be understood, however, that
`the detailed description and specific examples, while indi-
`cating 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;
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`[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;
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`[0018] FIG. 3 is a graph showing light transmission over
`time for an LCD device having an ion beam-aligned poly-
`imide film; and
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`[0019] FIG. 4 is a graph showing light transmission over
`time for an LCD device according to an embodiment of the
`present invention.
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`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`[0020] Preferred embodiments of the present invention
`will be described in detail hereinbelow with reference to the
`
`attached drawings.
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`[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
`
`[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 gITO) having a thickness of approxi-
`mately 500 to 2000 A 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 A
`that is aligned using ion bombardment. During ion bom-
`bardment, 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 incorpo-
`rated by reference.)
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`[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 avail-
`able 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. Addi-
`tionally, 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, respec-
`tively. 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 embodi-
`ments of the present invention can be formed by depositing
`a diluted material and then performing an alignment tech-
`nique. For example, in preferred manufacturing processes,
`polyimide is diluted with a solvent or thinner such as
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`N;-butyrolactone, bulyl cellosolue, or N-methyl-2-pyrroli-
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`done. The diluted polyimide is spun or spayed onto the
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`substrates, and after drying an alignment is performed. It has
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`been found experimentally that a dilution factor of 1:10
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`produces a very thin film of polyimide on a glass substrate,
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`and that such thin films provide sufficient alignment of the
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`liquid crystal molecules.
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`[0026]
`In embodiments of the present invention, reduced
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`thickness alignment layers permit rapid charge hopping or
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`tunneling, so charge accumulation on the alignment layers is
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`greatly minimized or even eliminated. More specifically,
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`tunneling currents are exponentially dependent on film
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`thickness and a thin film on the order of 50 A can tunnel
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`charge quite effectively. Additionally, charge can hop with
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`the reduced thickness film. Defects introduced into the
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`alignment layer not only introduce the charge that could
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`cause image sticking, but when a thin film is used also
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`provides a conduction path to the adjacent electrode. Thus,
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`the charge is eliminated so image sticking does not occur.
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`[0027] FIG. 3 shows the effect of image sticking on the
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`display of an LCD device formed with 600 A thick ion
`beam-aligned polyimide alignment
`layers. As shown in
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`FIG. 4, when 100
`thick ion beam-aligned polyimide
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`alignment layers are used, there is no observable image
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`sticking. That is, the effect on light transmission of a voltage
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`change is step-like or almost instantaneous. Thus, the LCD
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`device has significantly reduced charge accumulation and
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`image sticking is greatly minimized or eliminated. This
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`allows the present invention to provide a very high quality
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`display device.
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`invention
`the present
`[0028] The
`embodiments of
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`described above relate to LCD devices having polyimide
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`alignment layers that are aligned using an ion bombardment
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`technique. However, in further embodiments, various types
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`alignment techniques to form insulating (or poorly conduct-
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`ing) alignment films for the LCD device. Any film that is
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`optically transparent and amorphous or fine grained is
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`suitable. (The term amorphous means that the atomic struc-
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`ture of the film has no preferred direction or orientation.) For
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`example, a suitable alignment film can be formed using:
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`hydrogenated diamond-like carbon (DLC),
`amorphous
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`hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al2O3, CeO2,
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`SnO2, or ZnTiO2. Basically, any type of polymer or mono-
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`meric material can be used as long as the formed fiim is
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`optically transparent, particuiarly in the visible spectrum.
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`[0029] Similarly, the alignment can be performed using
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`any type of particle beam, such as those employing atoms,
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`molecules, or clusters that are either neutral or ionic. Fur-
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`laser
`techniques, UV techniques, or
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`thermore,
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`mechanical rubbing techniques can be used to align the
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`structure of the alignment layers. Additionally, other design
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`choices, such as the type of liquid crystal, the layout and
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`types of electrodes and circuit elements, and the types of
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`substrates could easily be adapted by one of ordinary slcill in
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`the art. Likewise, embodiments of the present invention may
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`not include all of the features described above. For example,
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`polarizing films may not be included in all embodiments.
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`[0030] While there has been illustrated and described What
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`the present invention, it will be understood by those skilled
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`in the art that various other modifications may be made, and
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`equivalents may be substituted, without departing from the
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`true scope of the invention. Additionally, many modifica-
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`tions may be made to adapt a particular situation to the
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`teachings of the present invention without departing from
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`the central inventive concept described herein. Therefore, it
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`is intended that the present invention not be limited to the
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`particular embodiments disclosed, but that the invention
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`include all embodiments falling Within the scope of the
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`appended claims.
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`What is claimed is:
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`1. A liquid crystal display device comprising:
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`a first substrate;
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`a first electrode layer overlying a first surface of the first
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`substrate;
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`a first alignment layer overlying the first electrode layer;
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`a second substrate;
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`a second electrode layer overlying a first surface of the
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`second substrate;
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`a second alignment layer overlying the second electrode
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`layer; and
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`a liquid crystal material disposed between the first align-
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`ment layer and the second alignment layer,
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`wherein at least one of the first alignment layer and the
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`second alignment layer has a thickness of 100 A or less.
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`2. The liquid crystal display device as defined in claim 1,
`
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`wherein each of the first and second alignment layers has a
`
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`thickness of 100 A or less.
`
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`3. The liquid crystal display device as defined in claim 2.,
`
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`wherein each of the first and second alignment layers is a
`
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`polyimide layer.
`
`
`4. The liquid crystal display device as defined in claim 3,
`
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`
`
`wherein each of the first and second substrates is a glass
`
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`substrate, and
`
`
`each of the first and second electrode layers is an opti-
`
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`
`
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`
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`cally-transparent conducting layer.
`
`
`
`5. The liquid crystal display device as defined in claim 4,
`
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`
`
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`
`
`wherein each of the first and second electrode layers is an
`
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`indium tin oxide layer.
`
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`
`
`6. The liquid crystal display device as defined in claim 4,
`
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`
`
`
`
`
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`
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`wherein the first and second substrates are separated by
`
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`approximately four to seven microns,
`
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`
`
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`each of the electrode layers has a thickness of approxi-
`
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`
`
`
`
`
`
`mately 500 to 2000
`
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`
`
`7. The liquid crystal display device as defined in claim 3,
`
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`
`
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`
`
`wherein the liquid material is twisted nematic liquid crystal.
`
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`8. The liquid crystal display device as defined in claim 2,
`
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`
`
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`
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`wherein each of the first and second alignment layers is
`
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`made from a material in a group consisting of polyimide,
`
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`
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`hydrogenated diamond-like carbon, amorphous hydroge-
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`
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`nated silicon, SiC, SiO2, glass, Si3N4, A1203, CeO2, SnO2,
`
`
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`
`
`
`
`
`and ZnTiO2.
`
`
`9. The liquid crystal display device as defined in claim 1,
`
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`further comprising:
`
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`a first polarizing film overlying a second surface of the
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`first substrate; and
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`a second polarizing film overlying a second surface of the
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`second substrate.
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`Page 6 of 7
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`

`
`US 2001/0012081 A1
`
`Aug. 9, 2001
`
`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 A.
`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
`A or less;
`
`depositing a second electrode layer over a first surface of
`the second substrate;
`
`depositing a second alignment
`electrode layer;
`
`layer over the second
`
`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 A 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, SiO2,
`glass, Si3N4, A1203, CeO2, SnO2, and ZnTiO2.
`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 approxi-
`mately 500 to 2000 A.
`
`Page 7 of 7
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`Page 7 of 7