(19) United States
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`(12) Patent Application Publication (10) Pub. No.: US 2001/0048498 A1
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`Tomioka et al.
`(43) Pub. Date:
`Dec. 6, 2001
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`US 20010048498A1
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`(54) LIQUID DISPLAY DEVICE
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`(52) U.s. Cl.
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`.......................................... .. 349/123; 349/141
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`(76)
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`Inventors: Yasushi Tomioka, Hitachi (JP); Kenji
`Okishiro, Hitachi (JP); Yoshiyuki
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`Umeda, Hitachi (JP); Hitoshi Oaku,
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`Hitachi (JP); Katsumi Kondo, Mito
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`(JP)
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`Correspondence Address:
`ANTONELLI TERRY STOUT AND KRAUS
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`SUITE 1800
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`1300 NORTH SEVENTEENTH STREET
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`ARLINGTON, VA 22209
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`(21) Appl. No.:
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`09/779,458
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`Filed:
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`(22)
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`(30)
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`Feb. 9, 2001
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`Foreign Application Priority Data
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`Mar. 30, 2000
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`(JP) .................................... .. 2000-097820
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`Publication Classification
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`(51)
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`Int. Cl.7 ..................... .. G02F 1/1337; G02F 1/1343
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`(57)
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`ABSTRACT
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`In a liquid crystal display device of IPS mode, occurrence of
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`a sticking-image and after-image phenomenon is prevented.
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`A liquid crystal display device comprising a pair of sub-
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`strates, at least one of the substrates being transparent; a
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`liquid crystal layer interposed between the pair of substrates;
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`and pixel electrodes and common electrodes and active
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`elements arranged on at least one substrate between the pair
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`of substrates, wherein a pair of alignment layers are made of
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`an organic polymer of polyamic acid group or polymamide
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`ester group having a relative imidization ratio above 60%; or
`at least one layer of an insulation film is interposed between
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`the alignment layer in a side of the substrate having the
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`group of electrodes formed thereon, and the pixel electrode
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`and the common electrode disposed under the alignment
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`layer to prevent occurrence of the sticking image and the
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`after image by flattening the electrode portions to moderate
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`electric field concentration.
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`Page 1 of 32
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`Tianma Exhibit 1008
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`Tianma Exhibit 1008
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 1 of 12
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`US 2001/0048498 A1
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`FIG. 1
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`Patent Application Publication
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`Dec. 6, 2001
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`Sheet 2 of 12
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`US 2001/0048498 A1
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 3 of 12
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`US 2001/0048498 A1
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`FIG. 3
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`Page 4 of 32
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`(3.4
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`B.”
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`_I/////////////////////
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`Page 5 of 32
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 5 of 12
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`US 2001/0043493 A1
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`FIG. 5
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`Page 6 of 32
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 6 of 12
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`US 2001/0048498 A1
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`FIG. 6
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 8 of 12
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`US 2001/0048498 A1
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 9 of 12
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`US 2001/0048498 A1
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`FIG. 9
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`Page 10 of 32
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 10 of 12
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`US 2001/0048498 A1
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`FIG. 10
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`26
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`VIII]!/IIIIIA
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 11 of 12
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`US 2001/0048498 A1
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`FIG. 11
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`100
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`RELATIVE IMIDIZATION RATIO OF ALIGNMENT LAYER / %
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`Patent Application Publication
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`Dec. 6, 2001 Sheet 12 of 12
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`US 2001/0048498 A1
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`FIG. 12(a)
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`1.5
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`a (2d/L)
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`Page 13 of 32
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`US 2001/0048498 A1
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`Dec. 6, 2001
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`image is estimated to be caused by that the electric potential
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`at applying voltage is not eliminated within the response
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`time, and is held and remains in the alignment layer of the
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`pixel electrode or in the interface between the liquid crystal
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`and the alignment layer, which changes the effective voltage.
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`In the TN mode, the correlation between the after image
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`phenomenon and the residual DC voltage described above
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`has been studied, and it
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`the
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`problem of after image phenomenon is solved by reducing
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`the conventional TN mode requires an alignment
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`having a property in which the DC charge is hardly accu-
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`mulated, that is, an alignment layer having a small residual
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`DC voltage component. For example, in the technology in
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`regard to the TN made liquid crystal display device dis-
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`closed in Japanese Patent Application Laid-Open No.8-
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`54631, by considering that the after image is caused by
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`absorption of ionic impurities in the liquid crystal into the
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`alignment layer due to generated driving DC voltage which
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`accumulates the electric field, a method of using a low
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`resistance alignment layer is proposed in order to moderate
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`the electric field accumulated in the alignment layer.
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`[0007] On the other hand, in the IPS mode, the following
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`technologies of suppressing the after image are proposed
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`based on the cause of after image similar to that of the TN
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`mode described above. For example,
`in the technology
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`disclosed in Japanese Patent Application Laid-Open No.7-
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`181439, by considering that the after image in the IPS mode
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`is caused by dipole of liquid crystal molecules inducing
`layer,
`polarization in the alignment
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`method of suppressing the after image by setting dipole
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`moment of side group in the liquid crystal molecules to a
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`value below 3 Debye in order to reduce the polarization
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`induced in the alignment layer. Further, in the technology in
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`regard to an IPS mode liquid crystal display device disclosed
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`in Japanese Patent Application Laid-Open No.7-159786, in
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`order to rapidly modify the charge due to polarization
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`produced in the liquid crystal layer, the alignment layer and
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`the insulation film, there is proposed a method of using an
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`alignment layer and an insulation film having a surface
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`resistance value within 3.3><1011 to 2.5><1018 Q/cm2, and
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`there is also proposed a method of reducing the after image
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`by specifying a relative relationship of relaxation times
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`expressed by a dielectric constant and a specific resistance of
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`each of the liquid crystal,
`the alignment
`layer and the
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`insulation film. Further, in the technology in regard to an IPS
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`mode liquid crystal display device disclosed in Japanese
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`Patent Application Laid-Open No.10-123526, there is pro-
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`posed a method of reducing the after image by using an
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`alignment layer having high ion adsorption in a substrate
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`opposite to a substrate on which a group of electrodes for
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`applying electric field to the liquid crystal is formed.
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`[0008] There is the correlation between the after image
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`phenomenon and the residual DC voltage in the conven-
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`tional TN mode, as described above. The correlation was
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`studied for the IPS mode, and the following results were
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`obtained.
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`[0009]
`(1) There was little significant difference in the
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`residual DC voltage between a liquid crystal display
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`element not causing the after image and a liquid crystal
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`display element causing the after image.
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`[0010]
`(2) In the IPS made, there were some liquid
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`crystal display elements in which sticking image semi-
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`LIQUID DISPLAY DEVICE
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`BACKGROUND OF THE INVENTION
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`[0001] The present invention relates to an active matrix
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`liquid display device.
`[0002]
`In general, in a liquid display device, an electric
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`field is applied to liquid crystal molecules of a liquid crystal
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`layer interposed between a pair of substrates to change an
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`alignment direction of the liquid crystal molecules. Display
`of the liquid crystal device is performed by change of the
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`optical characteristic caused by the change in the alignment
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`direction of the liquid crystal molecules.
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`[0003] The typical conventional active matrix liquid crys-
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`tal display device is twisted nematic display mode (TN) in
`which electrodes are arranged in each of a pair of substrates
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`having liquid crystal between them, and the direction of
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`electric field applied to the liquid crystal is set so as to
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`become nearly normal to the substrate interface, and display
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`is performed by using optical rotary power of the liquid
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`crystal. The most important problem in the liquid crystal
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`display device of the TN display mode is narrow view angle.
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`[0004] On the other hand, U.S. Pat. No. 4,345,249, W091/
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`10936 discloses in-plane switching (IPS) mode in which an
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`electric field having a component nearly parallel
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`substrate surface is produced using interdigital electrodes to
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`make rotational movement of liquid crystal nearly within a
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`plane, and display is performed using birefringence of the
`liquid crystal. The IPS mode is caused by in-plane switching
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`of the liquid crystal molecules, and has advantages of wide
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`viewing angle and low storage capacitance compared to the
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`conventional TN mode. Therefore, the IPS mode is a prom-
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`ising mode for a new liquid crystal display device replacing
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`the TN mode, and is one of technologies which is rapidly
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`progressing in recent years. Further, Japanese Patent Appli-
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`cation Laid-Open No.9-73101 discloses an IPS mode in
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`which at least one of electrodes to be applied with voltage
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`is formed of a transparent conductive film to improve the
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`transmittance. The bright liquid crystal display device hav-
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`ing good viewing characteristics (brightness contrast ratio,
`gray scale reversal) as described above has potential of
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`application to a monitor or a television set having a large
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`display area.
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`[0005] However, although the IPS mode liquid display
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`device has the good display performance as described above,
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`display defects such as display unevenness, a sticking image
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`called as after image and the like sometimes appear when it
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`is used for long time. Particularly, the sticking image or after
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`image is recognized as the phenomenon that one pattern is
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`being displayed for a long time, and after that when another
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`pattern is displayed, the former pattern is displayed at the
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`same time. It is also recognized that this problem is one of
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`factors to degrade the liquid display performance.
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`[0006] The mechanism of occurrence of the after image
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`phenomenon is not clear yet, but the display defect of after
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`image and sticking image in the conventional TN mode
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`liquid crystal display device is estimated to be caused by that
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`direct current charge is accumulated in the interface of the
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`liquid crystal alignment layer of each pixel by absorbing of
`ionic components to the alignment layer itself or to the
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`surface of the alignment layer to change the effective volt-
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`age. That is, the display defect of after image and sticking
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`Page 14 of 32
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`Page 14 of 32
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`US 2001/0048498 A1
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`Dec. 6, 2001
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`the liquid crystal layer is controlled to perform display by
`applying a voltage between the pixel electrode and the
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`common electrode, and a pair of alignment layers individu-
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`ally are formed on surfaces in contact with the liquid crystal
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`layer of the pair of substrates, and at least one layer of an
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`insulation film is disposed between the alignment layer in a
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`side of the substrate having the electrodes formed thereon
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`and an electrode which is closer to the alignment layer
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`between the pixel electrode and the common electrode.
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`[0019] Further, a thickness T of the insulation film is larger
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`than a value of one fourth of an interval L between the pixel
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`electrode and the common electrode. Furthermore,
`it
`is
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`particularly preferable that the thickness T of the insulation
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`film is larger than a value of one half of an interval L
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`between the pixel electrode and the common electrode.
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`Further, it is preferable that a magnitude of dielectric con-
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`stant EPAS of the insulation film is smaller than an average
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`dielectric constant ELC of the liquid crystal layer. Therein,
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`the average dielectric constant ELC of the liquid crystal layer
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`is a value which can be calculated as a dielectric constant
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`spatially averaged with respect to the liquid crystal layer
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`using the following equation.
`ELC=(EH+2EJ_)/3
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`[0020] where EH is an alignment vector of the liquid
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`crystal which is the component parallel to the molecular
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`axis, and EJ_ is an alignment vector of the liquid crystal
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`which is the component perpendicular to the molecular axis.
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`[0021] Further, the liquid crystal display device is con-
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`structed so that an interval L between the pixel electrode and
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`the common electrode within a display region where change
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`of an optical characteristic due to liquid crystal movement
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`contributes to display and a thickness D of the liquid crystal
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`layer within the display region may satisfy the relation
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`L<2D.
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`[0022] Furthermore, level difference on a surface of the
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`alignment layer in contact with the liquid crystal at the
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`uppermost portions of the pixel electrode and the common
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`electrode is flattened by the insulation film and/or the
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`alignment
`layer. Further,
`is preferable that
`the level
`it
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`difference on a surface of the alignment layer in contact with
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`the liquid crystal in the side of the substrate is flattened
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`below 50 nm.
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`[0023] Further, the insulation film is formed in a range
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`covering over all the display region where change of an
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`optical characteristic due to liquid crystal movement con-
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`tributes to display.
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`[0024] Further, according to another embodiment of a
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`liquid crystal display device in accordance with the present
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`invention, pixel electrodes and common electrodes and
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`active elements arranged on at least one of the substrates,
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`and liquid crystal of the liquid crystal layer is controlled to
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`perform display by applying a voltage between the pixel
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`electrode and the common electrode, and at least two layers
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`of insulation films are disposed between the pixel electrode
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`and the common electrode.
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`[0025] The liquid crystal display device is that at least one
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`layer of an insulation film is disposed between the alignment
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`layer in a side of the substrate having the electrodes formed
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`thereon and an electrode which is closer to the alignment
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`layer between the pixel electrode and the common electrode,
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`permanently continued,
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`decreased,
`the
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`decreased.
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`and the dark level was
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`contrast was
`substantially
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`[0011]
`It was also clarified from investigation of alignment
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`direction of liquid crystal in the regions of the after image
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`and sticking image that some alignment directions were
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`rotated from the initially set alignment direction toward the
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`alignment direction at driving by a very small angle, and not
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`completely returned to the initial alignment direction, which
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`caused decrease in contrast and gray scale reversal between
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`adjacent pixels to degrade the image quality.
`It is estimated from the above facts that the after
`[0012]
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`image and sticking image phenomenon in the IPS mode is
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`based on an after image mechanism specific to the IPS mode
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`which is completely different from the mechanism in the
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`conventional TN mode. It is required to solve the sticking
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`image and afterimage specific to the IPS mode in which the
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`liquid crystal is operated in the substrate plane to perform
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`display using the birefringence of the liquid crystal. Further,
`in the IPS mode liquid crystal display device of the high
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`transparent type in which movement of the liquid crystal on
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`the electrode surface is also used for image display by
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`employing transparent electrodes,
`the problem of after
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`image is also occurs in the same level or more severely.
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`Therefore,
`the countermeasure against these problems of
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`after image is required.
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`SUMMARY OF THE INVENTION
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`[0013] An object of the present invention is to provide a
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`high-quality liquid crystal display device with less uneven-
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`ness of display caused by sticking image and after image, the
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`liquid crystal display device being of the IPS mode in which
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`display is performed by making rotational movement of
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`liquid crystal nearly within a plane, and using birefringence
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`of the liquid crystal.
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`[0014] Another object of the present invention is to pro-
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`vide a high-quality liquid crystal display device which is
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`suitable for mass production.
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`[0015] According to an embodiment of a liquid crystal
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`display device in accordance with the present invention,
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`pixel electrodes and common electrodes and active elements
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`are arranged on at least one substrate, and liquid crystal of
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`the liquid crystal layer is controlled to perform display by
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`applying a voltage between the pixel electrode and the
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`common electrode, and a pair of alignment layers individu-
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`ally formed on surfaces in contact with the liquid crystal
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`layer of the pair of substrates are made of an organic
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`polymer of polyamic acid group or polymamide ester group
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`having a relative imidization ratio above 60%.
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`It is preferable that the alignment layer is made of
`[0016]
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`the organic polymer of polyamic acid group or polymamide
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`ester group having a relative imidization ratio above 70%.
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`[0017]
`It is preferable that the amic acid group organic
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`polymer contains at least one kind of group selected from the
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`group consisting of methyl ester group, ethyl ester group and
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`propyl ester group.
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`[0018] According to an embodiment of a liquid crystal
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`display device in accordance with the present invention,
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`pixel electrodes and common electrodes and active elements
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`are arranged on at least one substrate, and liquid crystal of
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`Page 15 of 32
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`Page 15 of 32
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`

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`US 2001/0048498 A1
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`Dec. 6, 2001
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`or that at least one layer of the insulation films interposed
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`between the pixel electrode and the common electrode is an
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`inorganic insulation film made of at
`least one kind of
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`material selected from the group consisting of silicon
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`nitride, silicon oxide and polysiloxane group materials.
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`[0026] Further, the liquid crystal display device is that the
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`insulation film is made of at
`least one kind of organic
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`polymer selected from the group consisting of polyimide
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`group polymers, acrylic group polymers and epoxy-acrylate
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`group polymers.
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`[0027] Further, the liquid crystal display device is that the
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`insulation film is composed of two layers of an organic
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`insulation film and an inorganic insulation film.
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`[0028] Further, according to the other embodiment of a
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`liquid crystal display device in accordance with the present
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`invention, the pair of alignment layers individually formed
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`on the surfaces in contact with the liquid crystal layer of the
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`pair of substrates are made of an organic polymer selected
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`from the group consisting of polyamic acid group polymers
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`and polymamide ester group polymers having a relative
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`imidization ratio above 60%. It is particularly preferable that
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`the alignment layer is made of an organic polymer selected
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`from the group consisting of polyamic acid group polymers
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`and polymamide ester group polymers having a relative
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`imidization ratio above 70%. Further, the amic acid group
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`organic polymer contains at least one kind of group selected
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`from the group consisting of methyl ester group, ethyl ester
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`group and propyl ester group.
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`[0029] Further, the liquid crystal display device is that a
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`surface elastic modulus of the pair of alignment
`layers
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`formed on the surfaces in contact with the liquid crystal
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`layer of the pair of substrates is larger than 1 GPa.
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`[0030] Further, the liquid crystal display device is that a
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`glass transition temperature Tg of interfaces between the
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`pair of alignment layers formed on the surfaces in contact
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`with the liquid crystal layer of the pair of substrates and the
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`liquid crystal layer is higher than a nematic-isotropic phase
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`transition temperature T(N-I) of a liquid crystal composition
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`forming the liquid crystal layer.
`[0031] Further, the liquid crystal display device is that the
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`glass transition temperature Tg of interfaces between the
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`pair of alignment layers formed on the surfaces in contact
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`with the liquid crystal layer of the pair of substrates and the
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`liquid crystal layer is higher than 220° C.
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`[0032] Further, the alignment layer is made of an organic
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`polymer of dehydration ring closure of polyamic acid com-
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`posed of diamine compounds expressed by a chemical
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`formula N2N—R—NH2 and tetracarboxilic acid anhydride
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`expressed by a chemical formula
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`0:0
`0:0
`O/\X/\O
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`\/\/’
`0:0
`0:0
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`and total number of bonding groups making a
`[0033]
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`molecular axis of polymer rotatable such as —O—, —S—,
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`Page 16 of 32
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`,
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`S02
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`C(CH3)2
`CH2
`, bonding
`group in the meta position, and bonding group in the ortho
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`position contained in the R position and the X position in the
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`repetitive structure of the organic polymer is equal to and
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`less three.
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`[0034] Further, in the present invention, the at least one of
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`the pair of alignment layer is a photo-reactive material layer.
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`Furthermore, the alignment layer is formed by irradiating
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`nearly linearly polarized light onto the layer made of the
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`photo-reactive material.
`[0035] Further, the present invention is effective in the
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`case where a pretilt angle of the liquid crystal layer is
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`smaller than 5 degrees.
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`[0036] Further, in the liquid crystal display device, at least
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`one of the pixel electrode and the common electrode is
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`formed of a transparent electric conductive film. The trans-
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`parent electric conductive film is an ion doped titanium
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`oxide film or an ion doped zinc oxide (ZnO) film.
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`[0037] Further, in the liquid crystal display device, the
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`alignment control directions of the liquid crystal in the two
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`interfaces between the liquid crystal layer and the pair of
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`substrates are nearly equal to each other.
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`BRIEF DESCRIPTION OF DRAWINGS
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`[0038] FIG. 1 is a cross-sectional view of a pixel portion
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`for explaining the structure of a first embodiment of a liquid
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`crystal display device in accordance with the present inven-
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`tion.
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`[0039] FIG. 2 is a plan view showing the structure of an
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`electrode of the first embodiment of the liquid crystal
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`display device in accordance with the present invention.
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`[0040] FIG. 3 is a cross-sectional view of a pixel portion
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`for explaining the structure of a second embodiment of a
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`liquid crystal display device in accordance with the present
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`invention.
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`[0041] FIG. 4 is a plan view showing the structure of an
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`electrode of the second embodiment of the liquid crystal
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`display device in accordance with the present invention.
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`[0042] FIG. 5 is a cross-sectional view of a pixel portion
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`for explaining the structure of the second embodiment of
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`another liquid crystal display device in accordance with the
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`present invention.
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`[0043] FIG. 6 is a cross-sectional view of a pixel portion
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`for explaining the structure of a third embodiment of a liquid
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`crystal display device in accordance with the present inven-
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`tion.
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`[0044] FIG. 7 is a plan view showing the structure of an
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`electrode of the third embodiment of the liquid crystal
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`display device in accordance with the present invention.
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`[0045] FIG. 8 is a view explaining the structure of a color
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`filter substrate composing the first embodied example of the
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`liquid crystal display device in accordance with the present
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`invention.
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`[0046] FIG. 9 is a diagram explaining the structure of a
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`system for driving the first embodied example of the liquid
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`crystal display device in accordance with the present inven-
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`tion.
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`Page 16 of 32
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`US 2001/0048498 A1
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`Dec. 6, 2001
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`phenomenon causes a large brightness change in the dark
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`level or the gray level range where the sense of sight of
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`human is very sensitive.
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`[0055] The after image phenomenon will be described
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`below.
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`[0056] When an electric field for displaying is applied in
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`a TN mode liquid crystal display device using a plane-
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`shaped transparent electric conductive film made of indium
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`tin oxide (ITO) or the like, a uniform electric field is formed
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`over all the display region except part of regions of electrode
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`edges. Concentration of electric field occurs in the electrode
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`edge region different from in the central portion, and accord-
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`ingly movement of the liquid crystal itself in the electrode
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`edge region is sometimes different from that in the central
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`region where the uniform electric field is formed. Further, it
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