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`(12) United States Patent
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`US 6,356,330 B1
`(10) Patent N0.:
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`Ando et al.
`(45) Date of Patent:
`Mar. 12, 2002
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`USOO6356330B1
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`(54) ACTIVE MATRIX LIQUID CRYSTAL
`DISPLAY DEVICE
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`JP
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`FOREIGN PATENT DOCUMENTS
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`8—62578
`3/1996
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`(75)
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`Inventors: Masahiko Ando, Hitachinaka;
`Tsunenori Yamamoto; Masatoshi
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`Wakagi, both of Hitachi, all of (JP)
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`(73) Assignee: Hitachi, Ltd., Tokyo (JP)
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`( * ) Notice:
`Subject to any disclaimer, the term of this
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`patent is extended or adjusted under 35
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`U.S.C. 154(b) by 0 days.
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`(22)
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`Filed:
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`(30)
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`(21) Appl. No.: 09/427,626
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`Oct. 27, 1999
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`Foreign Application Priority Data
`Oct. 27, 1998
`(JP)
`........................................... 10—305013
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`G02F 1/1343 G02F 1/1333
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`G02F 1/1339
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`........................ 349/141; 379/111; 379/156
`(52) US. Cl.
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`(58) Field of Search ................................. 349/141, 111,
`349/110, 191, 156
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`51
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`(56)
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`References Cited
`U.S. PATENT DOCUMENTS
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`6,069,678 A *
`5/2000 Sakamoto et a1.
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`.......... 349/141
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`* cited by examiner
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`Primary Examiner—Toan Ton
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`(74) Attorney, Agent, or Firm—Antonelli, Terry, Stout &
`Kraus, LLP
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`(57)
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`ABSTRACT
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`An active matrix liquid crystal display device is provided for
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`realizing a high uniformity of display luminance with lower
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`power consumption, a higher contrast, and a wider viewing
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`angle. In a common wire eliminated (common-less) IPS-
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`type active matrix liquid crystal display device, an opposing
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`electrode 107 is arranged above a signal wire 104 and a thin
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`film transistor (TFT) through an insulating layer. The oppos-
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`ing electrode 107 and a scanning wire 101 shield the signal
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`wire 104 or the TFT to prevent light from leaking from edge
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`portions of the signal wire 104 as well as the TFT from
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`malfunctioning due to a current generated by leaked light,
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`thereby realizing the elimination of a black matrix (BM), a
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`reduction in size of a black matrix, and planarized
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`substrates, thus resulting in a higher aperture ratio and an
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`improved uniformity of cell gap.
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`13 Claims, 8 Drawing Sheets
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`215
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`L
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`VIII/.2: .
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`103 203 216104
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`106
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`205
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`:202
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`Page 1 of 15
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`Tianma Exhibit 1006
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`Page 1 of 15
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`Tianma Exhibit 1006
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`US. Patent
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`Mar. 12, 2002
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`Sheet 1 0f 8
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`US 6,356,330 B1
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`FIG.
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`III:
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`US. Patent
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`Mar. 12, 2002
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`Sheet 2 0f 8
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`US 6,356,330 B1
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`FIG. 2
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`213
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`US. Patent
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`Mar. 12, 2002
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`Sheet 3 0f 8
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`US 6,356,330 B1
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`FIG. 3
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`213
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`US. Patent
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`Mar. 12, 2002
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`Sheet 4 0f 8
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`US 6,356,330 B1
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`F | G, 4 PRIOR ART
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`US. Patent
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`Mar. 12, 2002
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`Sheet 5 0f 8
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`US 6,356,330 B1
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`F | G, 5 PRIOR ART
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`213
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`US. Patent
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`Mar. 12, 2002
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`Sheet 6 0f 8
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`US 6,356,330 B1
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`FIG.6
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`213
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`US. Patent
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`Mar. 12, 2002
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`Sheet 8 0f 8
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`US 6,356,330 B1
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`FIG. 8
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`1
`ACTIVE MATRIX LIQUID CRYSTAL
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`DISPLAY DEVICE
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`FIELD OF THE INVENTION
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`The present invention relates to an active matrix liquid
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`crystal display device. More particularly, the present inven-
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`tion relates to an active matrix liquid crystal display device
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`which is configured in accordance with the IPS (In-Plane
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`Switching) type (also called the lateral electric field type).
`BACKGROUND OF THE INVENTION
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`In recent years, active matrix liquid crystal display
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`devices employing active elements, represented by thin film
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`transistors (TFTs), have been increasingly used as monitors
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`for personal computers, workstations, and so on since they
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`consume less power and have smaller sizes than CRT
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`display devices while providing a high image quality
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`equivalent to that of the CRT display devices.
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`One form of a liquid crystal display device suitable for
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`monitor applications is an IPS-type active matrix liquid
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`crystal display device. The liquid crystal display device of
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`this type includes scanning wires, signal wires, common
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`wires, and pairs of interdigitally formed electrodes (pixel
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`electrode and opposing electrode) arranged on one of the
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`two substrates, where a voltage is applied across the elec-
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`trodes to drive liquid crystal. An electric field applied to the
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`liquid crystal is substantially parallel to the surfaces of the
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`substrates. The liquid crystal display device of IPS-type has
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`a wider viewing angle than conventional
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`display devices. This characteristic of the IPS-type liquid
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`crystal display device makes itself more suitable for appli-
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`cations in direct-view type monitors.
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`FIG. 4 illustrates in plan view the structure of one pixel
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`section in a conventional IPS-type active matrix liquid
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`crystal display device as mentioned. FIG. 5 illustrates the
`structure of FIG. 4 in cross-sectional view taken along a line
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`B—B‘ in FIG. 4.
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`Referring first to FIG. 4, the pixel section includes scan-
`ning wires 101 formed of Cr; a semiconductor layer 102
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`formed of amorphus silicon; a signal wire 103 formed of Cr;
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`a pixel electrode 106 formed of Cr; opposing electrodes 107
`and 107' formed of Cr; a common wire 401 formed of Cr;
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`and a black matrix 402.
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`The liquid crystal display device having the structure as
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`illustrated includes a gap between the opposing electrode
`107 and the signal wire 103. An effective electric field to the
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`pixel for display cannot be applied through this gap. In
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`addition, this gap must be shielded because light is likely to
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`leak from the gap due to a continuously changing voltage on
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`the signal wire 103.
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`Moreover, a gap between the scanning wire 101 and the
`common wire 401 must be shielded since light is likely to
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`leak from the gap due to a direct current voltage applied at
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`scanning wire 101 and ends of the opposing electrodes 107,
`107' must be shielded for the same reason. In addition, a thin
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`film transistor (TFT) must be shielded over for preventing
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`the TFT from malfunctioning due to a current possibly
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`caused by leaked light. Thus,
`the liquid crystal display
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`device has a low aperture ratio because the pixel section
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`must be shielded by the black matrix 402. Further,
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`existence of the common electrode 401 also contributes to
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`the low aperture ratio of the liquid crystal display device.
`As illustrated in FIG. 5, a section in which a TFT 216 is
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`arranged presents an abruptly narrowing gap between a TFT
`substrate 214 and an opposing substrate 215. This is
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`because:
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`US 6,356,330 B1
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`1) a thicker passivation layer 205 for the purpose of
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`planarizing the TFT substrate 214 cannot be employed
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`because of a resulting increase in a liquid crystal
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`driving voltage; and
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`2) the black matrix 204 provided on the opposing sub-
`strate 215 in face of the TFT section cannot be omitted
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`since the black matrix 204 is indispensable for prevent-
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`ing the TFT from malfunctioning due to a current
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`generated by leaked light.
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`In the exemplary liquid crystal display device, a spacer
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`bead (hereinafter simply called the “bead”) 211 positioned
`on the TFT section serves to define a cell gap. However,
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`since the area of the TFT section is merely on the order of
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`1/100 as much as an entire pixel, the beads 211 arranged on
`the TFT sections account for only 1/100 or less of the entire
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`area of the substrates. For this reason, in substrate regions
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`without the TFT sections and accordingly not supported by
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`the spacer beads 211 arranged therebetween, the cell gap
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`suffers from non-uniformity which in turn causes a non-
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`uniform display luminance.
`Nevertheless,
`if an increased amount of beads 211 is
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`dispersed in an attempt to make the cell gap more uniform,
`the number of beads increases not only on the TFT sections
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`but also at openings. Thus, while the cell gap can be made
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`more uniform, more light leaks near beads positioned at
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`openings, resulting in a lower contract.
`In the cross-sectional view of FIG. 5, the structure further
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`comprises a glass substrate 201; a gate insulating layer 202;
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`a contact layer 204; an alignment film 206; a glass substrate
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`207; a color filter layer 208; a protection film 209 which also
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`serves as a planarizing film; an alignment film 210; a liquid
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`crystal layer 212; and polarizing plates 213, 217.
`SUMMARY OF THE INVENTION
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`When compared with a conventional TN-type liquid
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`crystal display device, the IPS-type liquid crystal display
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`device has the following two problems to be solved.
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`First, higher power consumption is required. This is
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`because a low aperture ratio of the IPS-type liquid crystal
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`display device requires higher power consumption to drive
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`back light for providing the luminance equivalent to that of
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`the conventional TN-type liquid crystal display device.
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`The low aperture ratio of the IPS-type liquid crystal
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`display device is mainly caused by the following facts:
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`1) interdigital electrodes do not transmit light; and
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`2) a black matrix (hereinafter abbreviated as “BM”)
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`arranged on an opposing substrate partially blocks
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`openings from receiving light.
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`Regions blocked by the BM are edge portions of scanning
`wires and signal wires, and TFT sections.
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`The edge portions of scanning wires and signal wires are
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`shielded because light may leak from these portions. The
`TFT sections are shielded for preventing TFTs from mal-
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`functioning due to a current generated by leaked light.
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`The BM has an area which is typically set larger than the
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`sum of possible light leaking regions and TFT regions for
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`taking into account an allowance of the alignment of a
`substrate which has formed thereon the possible light leak-
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`ing regions and the TFT regions to be shielded (hereinafter
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`called the “TFT substrate”), to another substrate on which
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`the BM is formed (hereinafter called the “opposing
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`substrate”).
`This wide area of the BM contributes to an additional
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`reduction in the aperture ratio. The aperture ratio must be
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`increased in order to reduce power consumption.
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`Another problem is that the IPS-type liquid crystal display
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`device suffers from a low uniformity of display luminance.
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`Page 10 of 15
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`Page 10 of 15
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`US 6,356,330 B1
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`3
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`This is because in the IPS-type liquid crystal display device,
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`a threshold voltage for the luminance characteristic is
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`inversely proportional to the thickness of a liquid crystal
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`layer sandwiched between the pair of substrates (cell gap),
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`so that non-uniformity of the cell gap, if any, would appear
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`in a display as corresponding non-uniformity of luminance.
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`In the TN-type liquid crystal display device, on the other
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`hand, a threshold voltage does not depend on a cell gap, so
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`that the uniformity of display luminance is relatively high.
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`In order to improve the uniformity of display luminance in
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`the IPS-type liquid crystal display device, the uniformity of
`the cell gap between the two substrates must be ensured
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`more strictly than the TN-type liquid crystal display device.
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`However, an attempt to make the cell gap more uniform
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`using currently available methods would result in a lower
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`contrast which constitutes a further problem.
`In the
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`following, the cause of the second problem will be explained
`in detail.
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`The non-uniform cell gap and the reduced contrast caused
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`by an attempt to make the cell gap more uniform attribute to
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`the ruggedness of the opposing surfaces of the two sub-
`strates and the beads used to form the cell gap.
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`When the cell gap is formed by sandwiching beads
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`between such rugged surfaces of two substrates, the cell gap
`is defined by those beads that are sandwiched between
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`regions in which the spacing between the substrates is the
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`narrowest. In the active matrix liquid crystal display device,
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`it is the TFT section that has the narrowest spacing between
`the substrates. This is because the TFT section has such a
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`structure that
`the most protruding TFT section on one
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`substrate faces the most protruding section on the other
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`substrate in which a black matrix for shielding the TFT
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`overlaps a color filter, so that
`the spacing between the
`substrates is the narrowest in this region.
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`The TFT section has an area approximately 1/100 a pixel
`area. Thus, when beads are dispersed over the TFT substrate,
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`beads would be sandwiched between the TFT sections of the
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`TFT substrate and corresponding sections of the opposing
`substrate with a possibility of 1/100 or lower, in additional
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`consideration of the fact that the beads are carried on the
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`TFT sections with more difficulties than on other flat
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`regions. For example, when 100 beads are dispersed, 99
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`beads will be positioned on pixel sections other than the TFT
`sections, and thus will not contribute to supporting the
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`45
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`substrates. Regions in which the substrates are not supported
`by beads are more likely to suffer from a non-uniform cell
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`gap which would give rise to the non-uniformity of display
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`luminance in the IPS-type liquid crystal display device.
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`To improve the uniformity of display luminance, the cell
`gap must be made more uniform. When a larger amount of
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`beads is dispersed to improve the uniformity of the cell gap,
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`an increased number of beads may be positioned in the TFT
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`sections, whereas the number of beads positioned in open-
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`ings is also increased in proportion.
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`Generally, near beads, light is more likely to leak due to
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`defective alignment of liquid crystal. For this reason, while
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`the increase in the number of beads might provide a more
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`uniform cell gap, the contrast would be degraded due to light
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`leaking near the beads positioned in openings. Thus, when
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`the cell gap is made more uniform by dispersing an
`increased amount of beads,
`the contrast is degraded. To
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`reduce the amount of dispersed beads required for a higher
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`uniformity of the cell gap, the ruggedness on the opposing
`surfaces of a pair of substrates must be reduced to planarize
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`the surfaces.
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`In summary,
`the IPS-type active matrix liquid crystal
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`display device has the following problems:
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`10
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`15
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`20
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`25
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`30
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`35
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`40
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`50
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`55
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`60
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`65
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`4
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`1) high power consumption; and
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`2) low uniformity of display luminance.
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`To overcome these problems,
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`1) the aperture ratio must be increased to reduce power
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`consumption for driving the back light; and
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`2) the cell gap must be made more uniform while avoiding
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`a degraded contrast.
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`It is an object of the present invention to provide an
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`IPS-type active matrix liquid crystal display device which
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`exhibits a high contrast in a displayed image.
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`It is another object of the present invention to provide an
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`IPS-type active matrix liquid crystal display device which is
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`capable of eliminating a light shielding film required in
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`conventional IPS-type active matrix liquid crystal display
`devices.
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`As a first structure to solve the above problems, in an
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`IPS-type active matrix liquid crystal display device, a por-
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`tion of an opposing electrode for driving a liquid crystal for
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`display together with a pixel electrode is formed over a
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`signal wire through an insulating film. Then, a region having
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`the signal wire formed therein, viewed from a direction
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`perpendicular to the surface of a substrate,
`is included
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`entirely within a region having the opposing electrode
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`formed therein and a region having a scanning wire formed
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`the
`therein, without protruding therefrom. Alternatively,
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`portion of the opposing electrode may be formed over an
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`active element through an insulating film.
`A first feature of this structure lies in a common wire
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`eliminated configuration (common-less configuration), as
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`described in JP-A-8-62578,
`in which the scanning wire
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`functions also as a common wire while omitting the com-
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`mon wire which is included in conventional IPS-type active
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`matrix liquid crystal display devices. By employing the
`structure based on the common wire eliminated
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`configuration, it is possible to eliminate a BM arranged on
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`the opposing substrate in conventional liquid crystal display
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`devices. This results in improving the aperture ratio as well
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`as the planarity of the opposing substrates of the liquid
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`crystal display device.
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`The followings are the reasons why the present invention
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`can eliminate BM which is required in conventional liquid
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`crystal display devices for shielding the active element and
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`possible light leaking regions in edge portions of the scan-
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`ning wire and the signal wire:
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`1) the opposing electrode arranged over the active ele-
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`ment functions as a light shielding layer;
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`2) since the signal wire is completely covered with the
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`opposing electrode and the scan electrode, light leaking
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`from edge portions of the signal wire is eliminated; and
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`3) Since the common wire eliminated configuration is
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`inherently free of light leaking from edge portions of
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`the scanning wires, light shielding is not required.
`Within the above reasons, since the signal wire is com-
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`pletely covered with the opposing electrode and the scan-
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`ning wire as set forth in 2), the structure of the present
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`invention is realized by forming the opposing electrode such
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`that its end portion is placed over the corresponding scan-
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`ning wire.
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`The light leaking from edge portions of the signal wire
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`can be completely eliminated by employing the structure as
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`described above because the present invention employs the
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`common wire eliminated configuration.
`As described in JP-A-8-62578,
`in the common wire
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`eliminated configuration, equal voltages are applied across
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`opposing electrodes and corresponding scanning wires dur-
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`ing most of a liquid crystal display operating period, so that
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`Page 11 of 15
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`Page 11 of 15
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`5
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`light will not leak even if an end portion of the opposing
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`electrode is placed over the scanning wire.
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`On the other hand, in a conventional structure having a
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`common wire, a direct current is applied between an oppos-
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`ing electrode and a corresponding scanning wire during
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`most of a liquid crystal display operating period, so that the
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`placement of an end portion of the opposing electrode in
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`close proximity to the corresponding scanning wire would
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`cause light to leak in a region between the end portion of the
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`opposing electrode and the corresponding scanning wire due
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`to the applied direct current voltage, although such place-
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`ment reduces regions of edge portions of the signal wire
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`from which light may leak.
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`Thus, even if the structure of the present invention, having
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`an end portion of an opposing electrode placed over a
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`corresponding scanning wire, were applied to the conven-
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`tional structure,
`it would not be possible to completely
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`eliminate light leaking from edge portions of the signal wire.
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`In other words,
`the present
`invention can be effective
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`exclusively in the common-less configuration which does
`not have a common electrode.
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`Also, in the foregoing structure which involves a plurality
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`of opposing electrodes in contact with a liquid crystal layer
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`through an alignment film, the plurality of opposing elec-
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`trodes are preferably made of an electro-chemically stable
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`material such as niobium (Nb) or niobium nitride in order to
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`prevent possible failures caused by electro-chemical reac-
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`tions between a liquid crystal composition and the opposing
`electrodes.
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`Since a cell gap is defined by using columnar spacers of
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`a uniform height formed on the TFT substrate, instead of
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`beads previously used for the formation of the cell gap, and
`
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`sandwiching the spacers between the pair of substrates, the
`
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`uniformity of the cell gap is improved. Preferably,
`the
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`columnar spacers are positioned at regular intervals corre-
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`sponding to periodic placement of pixel electrodes.
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`Particularly, when the spacers are positioned over the
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`active elements through the opposing electrodes, the colum-
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`nar spacers can have a minimum height because the spacing
`between the substrates is the narrowest in regions in which
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`the active elements are formed.
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`It is therefore possible to reduce the amount of materials
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`required for the columnar spacers and hence a time required
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`to form the columnar spacers.
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`In addition, since the columnar spacer is arranged over the
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`active element through the opposing electrode, the potential
`over the active element is maintained at the potential of the
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`opposing electrode and is free from the influence of the
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`columnar spacer, so that the placement of the columnar
`spacer over the active element will never cause the active
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`element to malfunction.
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`The opposing electrodes may be made of an electrically
`conductive oxide such as indium tin oxide or the like which
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`is electrochemically stable. However, since this material is
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`transparent, light shielding is required for the active ele-
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`ments or TFTs. In this event, when columnar spacers having
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`a light shielding property are arranged over the active
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`elements through the opposing electrodes, it is possible to
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`simultaneously achieve a uniform cell gap and light shielded
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`TFTs. Alternatively, a small black matrix may be formed
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`over the TFT for light shielding.
`When a TFT is used as the active element, the opposing
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`electrode arranged over the TFT through an insulating layer
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`has a function of a gate electrode (a so-called double gate
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`TFT structure), thus providing an additional effect of an
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`increased mobility and enhanced switching performance for
`the TFT.
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`10
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`15
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`US 6,356,330 B1
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`6
`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a plan view illustrating the structure seen from
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`the top of a pixel section in one embodiment of the present
`invention;
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`FIG. 2 is a cross-sectional view illustrating the structure
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`in cross-section taken along a line A—A' of the pixel section
`in FIG. 1;
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`FIG. 3 is a cross-sectional view illustrating the structure
`in cross-section of a pixel section in another embodiment of
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`the present invention;
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`FIG. 4 is a plan view illustrating the structure seen from
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`the top of a pixel section in a conventional liquid crystal
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`display device;
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`FIG. 5 is a cross-sectional view illustrating the structure
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`in cross-section taken along a line B—B‘ of the pixel section
`in FIG. 4;
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`FIG. 6 is a cross-sectional view illustrating the structure
`in cross-section of a pixel section in another embodiment of
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`the present invention;
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`FIG. 7 is a plan view illustrating the structure seen from
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`the top of the pixel section in the embodiment of FIG. 6; and
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`FIG. 8 is a plan view illustrating the structure seen from
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`the top of an embodiment according to the present invention.
`DETAILED DESCRIPTION OF THE
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`EMBODIMENTS
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`A first embodiment of the present invention will now be
`shown with reference to FIGS. 1 and 2.
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`FIG. 1 illustrates the structure of a pixel section, seen
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`from the top, in the first embodiment. The pixel section
`includes a scanning wire 101 formed of Cr; a semiconductor
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`layer 102 formed of amorphus silicon; a signal line 103
`formed of Cr; a pixel electrode terminal 104 formed of Cr;
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`a storage capacitor terminal 105 formed of Cr; a pixel
`electrode 106; and opposing electrodes 107, 107'.
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`FIG. 2 illustrates the structure in cross-section taken along
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`a line A—A' of the pixel section in FIG. 1. The illustrated
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`structure includes a glass substrate 201; a gate insulating
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`layer 202 formed of silicon nitride; an insulating layer 203
`formed of silicon oxide; a contact layer 204 formed of
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`n+-type amorphus silicon doped with phosphor; a passiva-
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`tion layer 205 formed of an organic insulating layer; an
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`alignment film 206; a glass substrate 207; a color filter layer
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`208; a protection layer 209; an alignment film 210; a spacer
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`bead 211; a liquid crystal layer 212; polarizing plates 213; a
`TFT substrate 214; an opposing substrate 215; and a thin
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`film transistor (TFT) 216.
`Description will be next made on how to manufacture the
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`active matrix liquid crystal display device according to the
`first embodiment illustrate