(12) United States Patent
`Yoneya et al.
`
`(10) Patent N0.:
`
`(45) Date of Patent:
`
`US 6,242,060 B1
`Jun. 5, 2001
`
`US006242060B1
`
`ACTIVE-MATRIX LIQUID CRYSTAL
`DISPLAY
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Inventors: Makoto Yoneya, Hitachinaka; Kishiro
`Iwasaki, Hitachiota, Yasushi Tomioka;
`Hisao Yokokura, both of Hitachi;
`Katsumi Kondo, Hitachinaka;
`Yoshiharu Nagae, Hitachi, all of (JP)
`
`(73) Assignee: Hitachi, Ltd., Tokyo (JP)
`
`( * ) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/281,810
`
`(22)
`
`Filed:
`
`Mar. 31, 1999
`
`Related U.S. Application Data
`
`(63) Continuation of application No. 08/848,453, filed on May 8,
`1997, now Pat. No. 5,928,733.
`
`(30)
`
`Foreign Application Priority Data
`
`May 8, 1996
`Jun. 20, 1996
`Aug. 22, 1996
`Oct. 11, 1996
`
`(JP) ................................................. .. 8—113748
`(JP)
`8-159496
`(JP)
`8—221069
`(JP) ................................................. .. 8—269632
`
`Int. Cl.7 ................................................. .. G02F 1/1337
`(51)
`(52) U.S. Cl.
`...................... .. 428/1.23; 428/1.26; 428/1.27
`(58) Field of Search ................................ .. 428/123, 1.26,
`428/1.27
`
`5,344,916
`5,480,964
`5,856,431 *
`
`9/1994 Harris et al. .
`1/1996 Harris et al. .
`.................. .. 428/1.26
`1/1999 Gibbons et al.
`OTHER PUBLICATIONS
`
`IEICE Tranactions on Electronics, Vo.. E379—C, No. 8,Aug.
`96.
`
`Liquid Crystals, vol. 22, No. 4, pp. 391-400, Dec. 97.
`Liquid Crystals, vol. 22, No. 4, pp. 379-390, Apr. 97.
`
`* cited by examiner
`Primary Exczminer—Alexander S. Thomas
`(74) Attorney, Agent, or Firm—Antonelli, Terry, Stout &
`Kraus, LLP
`
`(57)
`
`ABSTRACT
`
`An active-matrix liquid crystal display device having a pair
`of substrates at least one of which is transparent, a liquid
`crystal layer disposed between the pair of substrates, a group
`of electrodes for applying to the liquid crystal layer an
`electric field substantially parallel to the substrate plane and
`a plural number of active elements being formed on one of
`the pair of substrates, and an alignment
`layer disposed
`between the liquid crystal layer and at least one of the pair
`of substrates. The alignment layer is a photo-reactive mate-
`rial layer, and the photo-reactive material layer is a photo-
`reactive alignment layer which has been subjected to lin-
`early polarized light
`irradiation to selectively derive a
`photochemical reaction.
`
`8 Claims, 9 Drawing Sheets
`
`8
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`
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`
`Page 1 of 26
`
`Tianma Exhibit 1018
`
`Page 1 of 26
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`Tianma Exhibit 1018
`
`

`
`
`
`U.S. PatentU.S. Patent
`
`
`
`Jun. 5, 2001Jun. 5, 2001
`
`
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`Page 2 of 26Page 2 of 26
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`Page 2 of 26
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`

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`U.S. Patent
`
`Jun. 5, 2001
`
`US 6,242,060 B1
`
`FIG.
`
`POLAR
`ANCHORING
`
`EASY AXIS
`
`TORSIONAL
`ANCHORING
`
`SUBSTRATE
`
`LIQUID
`CRYSTAL
`LAYER
`
`THICKNESS
`
`APPLIED VOLTAGE
`
`
`
`DISPLAYLUMINANCE
`
`Page 3 of 26
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`Page 3 of 26
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`

`
`U.S. Patent
`
`Jun. 5, 2001
`
`Sheet 3 of 9
`
`US 6,242,060 B1
`
`LIQUID
`CRYSTAL
`LAYER
`THICKNESS
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`
`Page 4 of 26
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`
`Page 4 of 26
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`

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`
`
`U.S. PatentU.S. Patent
`
`
`
`Jun. 5, 2001Jun. 5, 2001
`
`
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`Sheet 4 of 9Sheet 4 of 9
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`Page 5 of 26Page 5 of 26
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`

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`U.S. PatentU.S. Patent
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`Jun. 5, 2001Jun. 5, 2001
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`Page 6 of 26Page 6 of 26
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`Page 6 of 26
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`

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`U.S. Patent
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`Jun. 5, 2001
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`Sheet 6 of 9
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`US 6,242,060 B1
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`Page 7 of 26
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`Page 7 of 26
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`U.S. Patent
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`Jun. 5, 2001
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`Page 8 of 26
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`Page 8 of 26
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`

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`U.S. Patent
`
`Jun. 5, 2001
`
`Sheet 8 of 9
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`US 6,242,060 B1
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`U.S. Patent
`
`Jun. 5, 2001
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`Page 10 of 26
`
`Page 10 of 26
`
`

`
`US 6,242,060 B1
`
`1
`ACTIVE-MATRIX LIQUID CRYSTAL
`DISPLAY
`
`CROSS REFERENCE TO RELATED
`APPLICATION
`
`This is a continuation of U.S. application Ser. No. 08/848,
`453, filed May 8, 1997, now U.S. Pat. No. 5,928,733 the
`subject matter of which is incorporated by reference herein.
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates to an active-matrix liquid
`crystal display device.
`In liquid crystal display devices, displaying is performed
`by varying the optical properties of the liquid crystal layer
`disposed between the substrates by changing the alignment
`direction of the liquid crystal molecules in the layer by
`applying an electric field thereto.
`The conventional active-matrix liquid crystal displays
`have predominantly been of the twisted nematic (TN) mode
`in which the direction of the electric field applied to the
`liquid crystal molecules is set to be substantially vertical to
`the substrate plane, and display is performed by making use
`of optical rotatory power of the liquid crystals.
`On the other hand, a system which makes use of the
`birefringence effect of the liquid crystals by setting the
`direction of the electric field applied to the liquid crystals to
`be substantially parallel
`to the substrate plane by using
`interdigital electrodes (in-plane switching mode) has been
`proposed in, for instance, JP-B 63-21907 and WO 91/10936
`(JP-T 5-505247). This in-plane switching mode has the
`advantages of wide viewing angle and low load capacity in
`comparison with the conventional TN mode, and is a prom-
`ising technique for the development of active-matrix liquid
`crystal displays.
`In this in-plane switching mode, however, since it utilizes
`the birefringence effect of the liquid crystals for making
`display, it is necessary to set the gap between the substrates
`(liquid crystal layer thickness) to be about 4 gm, which is
`notably smaller than that of the TN mode (about 10 ,um), for
`obtaining a display performance equal to the TN mode.
`Generally, reduction of the layer thickness brings into relief
`the influence of display irregularity due to nonuniformity of
`the gap between the substrates, giving rise to such problems
`as deterioration of displayed image quality and reduction of
`yield resulting in lowered mass productivity.
`The gap between the substrates is controlled to a specified
`value by dispersing the uniformly sized spherical polymer
`beads as spacer of the opposing substrates between which
`the liquid crystal layer is disposed.
`In the active-matrix liquid crystal display devices, a level
`difference of up to about 1 ,um may be produced on the
`substrate surface at the active element forming section, and
`a certain degree of nonuniformity of the inter-substrate gap
`is inevitably produced at the pixel region,
`too, due to a
`delicate relation between said level difference and dispersion
`of said spacer beads.
`In the in-plane switching mode, the same degree of gap
`irregularity represents a far greater rate of gap variation than
`in the TN mode because of smaller inter-substrate gap, so
`that
`the techniques for lessening or eliminating display
`irregularity due to non-uniformity of said gap are of vital
`importance for the in-plane switching mode.
`Further, according to the known TN mode, there is no
`dependence of the threshold voltage on the gap between the
`substrates (due to voltage responsitivity), while according to
`
`Page 11 of 26
`
`2
`the in-plane switching mode, since the gap between the
`substrates independently contributes to the threshold voltage
`(due to field responsitivity) together with the gap between
`electrodes (Oh-e, et al. Appl. Phys. Lett. 67 (26), 1996, pp
`3895-3897), particularly severe control of the gap between
`the substrates is necessary.
`SUMMARY OF THE INVENTION
`
`The present invention is envisaged to solve the above
`problems, and for this purpose, it provides an active-matrix
`liquid crystal display device employing the in-plane switch-
`ing mode, which is minimized in or almost free of nonuni-
`formity or irregularity of display resulting from variation of
`the gap between the substrates, and which is also capable of
`displaying high-quality images and has excellent mass pro-
`ductivity.
`The active-matrix liquid crystal display device according
`to the present invention comprises a group of electrodes for
`applying an electric field to the liquid crystal layer disposed
`between a pair of substrates, said electric field being parallel
`to the plane of said substrates, active elements provided in
`connection to said electrodes, and an alignment layer(s)
`which aligns the liquid crystal molecules in the substantially
`same direction at the interface between said liquid crystal
`layer and at least one of the opposing substrates, wherein the
`extrapolation length, which expresses the strength of tor-
`tional anchoring of the liquid crystal molecules and said
`alignment layer surface at one or both of the interfaces
`between said liquid crystal
`layer and said opposing
`substrates, is set to be not less than 10% of the gap between
`the substrates (liquid crystal layer thickness).
`The “extrapolation length” refers to the increment of the
`apparent
`inter-substrate gap when the liquid crystal cell
`behaves like a cell having a greater inter-substrate gap than
`the actual gap in terms of the threshold characteristics on
`application of an electric field, in case the interfacial anchor-
`ing is weak and finite (de Gennes: The Physics of Liquid
`Crystal, Oxford University Press, 1974, page 75).
`The alignment layer used in the present invention may be
`one in which the tortional anchoring coefficient A2 at the
`alignment layer surface against the liquid crystal molecules
`at the interface is less than 20 ,uN/m.
`Regarding the method for changing the optical properties
`according to the status of molecular alignment of said liquid
`crystal
`layer,
`it
`is expedient
`to use a pair of polarizers
`arranged to have their axes of polarization crossed at right
`angles with each other, and to select the parameter d~An (d:
`liquid crystal layer thickness; An: refractive index anisot-
`ropy of the liquid crystal composition) so as to satisfy the
`relation of 0.2 ym<d~An<0.5 lum.
`It is desirable that the controlled alignment direction of
`the liquid crystal molecules is substantially the same at the
`two interfaces between said liquid crystal layer and said pair
`of substrates.
`
`Also, at least one of the alignment layers formed on said
`substrates is preferably made of an organic polymer con-
`taining a polymer and/or oligomer in which the polymerizate
`of long-chain alkylene groups and/or fluoro groups provided
`in the amine or acid moiety accounts for 5—30% of the total
`number of moles.
`
`The polymer and/or oligomer used in said alignment layer
`are preferably those having a weight-average molecular
`weight of 2,000—90,000. The long-chain alkylene groups
`and/or fluoro groups in the polymer may be main chain type,
`side chain type or terminal type.
`The alignment layer is preferably made of an organic
`polymer having long-chain alkylene groups and/or fluoro
`
`Page 11 of 26
`
`

`
`US 6,242,060 B1
`
`3
`groups, which includes a polymer and/or oligomer-amic
`acid imide type, polymer and/or oligomer-imide type, poly-
`mer and/or oligomer-imidosiloxane type, and polymer and/
`or oligomer-amide-imide type. It is also possible to use an
`organic polymer obtained from dehydration ring-closing
`reaction of a polymer and/or oligomer-amic acid comprising
`a single-ring rigid diamine as amine moiety and an aliphatic
`tetracarboxylic acid dianhydride and/or an alicyclic tetra-
`carboxylic acid dianhydride and an aromatic tetracarboxylic
`acid dianhydride having main chain type long-chain alky-
`lene groups and/or fiuoro groups as acid moiety.
`According to an embodiment of active-matrix liquid
`crystal display device of the present invention, at least one
`of the alignment layers formed on the substrates may be an
`inorganic material layer. This inorganic material layer is
`preferably an inorganic alignment layer which has been
`surface treated by oblique evaporation technique. In case of
`using such an inorganic alignment layer, an organic align-
`ment layer may be used as the other alignment layer. Such
`an organic alignment layer is preferably a layer of an organic
`polymer which has had a rubbing treatment.
`According to an embodiment of the electrodes and active
`elements used in the present invention, it is desirable that
`these are formed only on one of the pairing substrates, and
`that the outermost surface of this substrate is constituted by
`an inorganic material layer.
`According to another embodiment of active-matrix liquid
`crystal display device of the present invention, at least one
`of the alignment layers formed on the respective substrates
`may be a layer of a photoreactive material. Such a photo-
`reactive material layer is preferably a photoreactive align-
`ment layer which has been subjected to linearly polarized
`light irradiation treatment, and such a photoreactive align-
`ment layer is preferably made of an organic polymer con-
`taining a polymer and/or oligomer having at
`least one
`diazobenzene group.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and other objects, features and advantages of the
`present
`invention will be understood more clearly from
`reviewing the following detailed description with reference
`to the accompanying drawings, wherein:
`FIGS. 1A, 1B, 1C and 1D are the schematic illustrations
`of behavior of the liquid crystal molecules in the liquid
`crystal display device according to the present invention.
`FIGS. 1A and 1C show the states of the liquid crystal
`molecules in a situation where no electric field has been
`
`applied, and FIGS. 1B and 1D show the states of the liquid
`crystal molecules in a situation where an electric field has
`been applied.
`FIG. 2 illustrates polar anchoring and torsional anchoring
`of the liquid crystal molecules on the substrate surface.
`FIG. 3 is a graph showing the electro-optical character-
`istics of the in-plane switching mode.
`FIGS. 4A and 4B are graphic illustrations of the electro-
`optical characteristics in the device of the present invention.
`FIG. 4A shows the characteristics observed when torsional
`
`anchoring is strong, and FIG. 4B shows the characteristics
`seen when torsional anchoring is weak.
`FIG. 5 is a graph showing the relation between the
`extrapolation length/liquid crystal layer thickness ratio and
`the index of reduction of luminance variation by the weak
`torsional anchoring effect.
`FIG. 6 is a structural illustration of thin-film transistors,
`electrodes and wiring in the device of the present invention.
`
`Page 12 of 26
`
`4
`FIG. 6a is a frontal view, and FIGS. 6b and 6c are the side
`sectional views.
`
`FIG. 7 is a graph showing the results of determinations in
`an example of the present invention.
`FIG. 8 is a graph showing the results of determinations in
`another example of the present invention.
`FIG. 9 is a graph showing the results of determinations in
`still another example of the present invention.
`FIG. 10 is a graph showing the results of determinations
`in yet another example of the present invention.
`FIG. 11 is a graph showing the results of determinations
`in a comparative example of the present invention.
`FIG. 12 is a graph showing the results of determinations
`in another comparative example of the present invention.
`FIG. 13 is a graph showing the results of determinations
`in still another comparative example of the present inven-
`tion.
`
`FIG. 14 is a graph showing the results of determinations
`in yet another comparative example of the present invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Firstly, in the in-plane switching mode embodying the
`present invention, the torsional anchoring between the liquid
`crystal molecules and the alignment layer surface at their
`interface is set to such a low level that the extrapolation
`length, which is an index of torsional anchoring strength,
`will become 10% or more than 10% of the gap between the
`substrates.
`
`The working principles of the in-plane switching mode
`which underlie the present invention are explained with
`reference to a model case shown in FIG. 1. FIGS. 1A and 1B
`
`are the sectional illustrations of behavior of the liquid crystal
`molecules in a liquid crystal element of the in-plane switch-
`ing mode, and FIGS. 1C and 1D are the frontal views thereof
`(a portion corresponding to only one of the pixels is shown
`here).
`Asection on the cell side with no voltage applied is shown
`in FIG. 1A, and a frontal View thereof is shown in FIG. 1C.
`Linear electrodes 4, 1 are formed on the inside of one of the
`substrates. The surfaces of both of the pairing substrates are
`constituted by an alignment
`layer, and a liquid crystal
`composition is sandwiched between the substrates. (In this
`instance, dielectric anisotropy of the composition is sup-
`posed to be positive, but the in-plane switching mode can be
`similarly realized with a negative liquid crystal composition
`by simply interchanging the direction of the major axis and
`the minor axis of the liquid crystal molecules.)
`The rod-like liquid crystal molecules 6 are aligned in the
`direction indicated by 10, which has a slight angular differ-
`ence from the longitudinal direction (in the frontal view of
`FIG. 1C) of the electrodes 4, 1, at the interface of the two
`substrates by anchoring with the alignment layers 5. They
`stay in this state almost uniformly in the liquid crystal layer
`when no voltage is applied.
`Here, when different potentials are given to the pixel
`electrode 4 and the common electrode 1 to generate an
`electric field 9 in the liquid crystal composition layer by the
`potential difference between the two electrodes, the liquid
`crystal molecules are turned to the direction of electric field
`as shown in FIGS. 1B and ID by the interaction of the
`dielectric anisotropy of the liquid crystal composition and
`the electric field. This motivates a change of the optical
`properties of the liquid crystal element by the action of the
`refractive index anisotropy of the liquid crystal composition
`layer and the polarizer 8, and such a change prompts display.
`
`Page 12 of 26
`
`

`
`US 6,242,060 B1
`
`5
`Here, the relation between in-plane switching mode and
`interfacial torsional anchoring is discussed while making
`comparison with the conventional TN mode.
`It is known that the alignment regulating force (anchoring
`force) by anchoring of the alignment layer and liquid crystal
`molecules varies significantly depending on the material of
`the alignment layer and the rubbing conditions, but it also
`varies according to the direction in which the alignment of
`the liquid crystal molecules is changed on the alignment
`layer surface.
`In the case of a liquid crystal material having a positive
`dielectric anisotropy aligned substantially horizontally on
`the surface, the change of alignment of the liquid crystal
`molecules on the substrate surface induced by the applica-
`tion of an electric field is made in the direction rising up
`from the surface in the TN mode in which the electric field
`
`is applied substantially vertically to the substrate interface,
`while such change of alignment is made in the in-plane
`direction of the surface in the in-plane switching mode in
`which the electric field is applied substantially parallel to the
`substrate interface. Thus, the alignment regulating force at
`the interface is based on the polar anchoring illustrated in
`FIG. 2 in the conventional TN mode, while it is based on the
`torsional anchoring, also illustrated in FIG. 2, in the in-plane
`switching mode. Generally, polar anchoring is very strong
`almost unexceptionally (Proust, et al: Colloid & Polymer
`Sci., 254 , 672-673, 1976), while torsional anchoring is
`relatively weak, and it is practically possible to find out an
`alignment film which shows weak torsional anchoring as
`proposed in the present invention (Levy, et al: Journal de
`Physique Letters, Vol. 40, 1979, L-215).
`By weakly setting the torsional anchoring at the liquid
`crystal/alignment layer interface so that the extrapolation
`length will become greater than 10% of the gap between the
`substrates, it is possible, in the in-plane switching mode, to
`lessen display irregularity caused by nonuniformity of the
`gap between the substrates as compared with the case of
`strong torsional anchoring at said interface, even with a
`same degree of gap nonuniformity.
`The reason why display irregularity can be lessened by
`weakening said torsional anchoring in the in-plane switching
`mode is explained below.
`FIG. 3 graphically illustrates variation of display lumi-
`nance with change of the voltage applied across the elec-
`trodes in an in-plane switching mode liquid crystal display
`device. Graphed in FIG. 3 are the three patterns of variation
`of voltage-luminance characteristics when the inter-
`substrate gap of a liquid crystal element was changed
`slightly (:Ad) in correspondence to variation of nonunifor-
`mity of the gap.
`The threshold voltage Vc of the change of alignment
`(Fredericksz transition) for the in-plane field of the liquid
`crystal molecules in the in-plane switching mode with equal
`torsional anchoring at the interfaces of the liquid crystal
`layer and the two pairing substrates is approximately given
`by the following equation (Yokoyama: Mol. Cryst. Liq.
`Cryst., 1988, Vol. 165, pp. 265-316; Oh-e, et al: Appl. Phys.
`Lett., Vol. 67, 1995, pp. 3895-3897):
`
`vc-<ng/<d+2b>>vIm3
`
`(1)
`
`wherein d and g denote the gap between the substrates
`(liquid crystal layer thickness) and the gap between the
`electrode ends, respectively, K2 and Ae denote twist elastic
`constant and dielectric anisotropy, respectively, of the liquid
`crystal composition, and b denotes the extrapolation length
`which expresses the torsional anchoring strength of the
`
`Page 13 of 26
`
`6
`liquid crystal molecules and the alignment layer surface at
`the interface defined by the following equation using the
`torsional anchoring coefficient A2 of the alignment layer
`surface:
`
`b=K2/A2
`
`(2)
`
`The stronger the torsional anchoring of the alignment
`layer surface, the smaller becomes the extrapolation length
`b; b is supposed to be 0 when the torsional anchoring is so
`strong that the direction of alignment of the liquid crystal
`molecules on the alignment
`layer surface is considered
`fixed.
`The variation of threshold voltage AVc when the gap
`between the substrates changed by :Ad from the center
`value d is given by the following equation:
`
`Ave-<2ngAd/<<d+2b>—<Ad»vZm5
`
`<3)
`
`Let us here consider the case of gray level display where
`display irregularity appears most conspicuously. Taking the
`instance of the applied voltage V50 at which the display
`luminance shown in FIG. 3 is reduced to half (50%) of the
`maximum luminance and the amount of variation AV50
`
`induced when the gap between the substrates changed by
`:Ad, it is considered that AV50 is almost proportional to the
`afore-mentioned AVc.
`
`The ratio of “AV50weak” (in case torsional anchoring at
`the interface is weak) to “AV50str” (in case torsional anchor-
`ing is so strong that the extrapolation length is considered to
`be 0) is given by the following equation:
`
`AV50weak/AV50str=(d-d—Ad-Ad)/((d+2b) -(d+2b)—Ad-Ad)
`
`(4)
`
`When nonuniformity of the gap is Ad and Ad~Ad<d-d, then
`the above equation can be approximated by:
`
`AV50weak/AV50str~1/((1+2b/d)-(1+2b/d))
`
`(5)
`
`the above formula gives AV50weak/
`Since b>0,
`AV50str<1. It is thus seen that in case the torsional anchor-
`
`the range of variation of V50
`ing is weak (FIG. 4B),
`incidental to the change of the gap is diminished as com-
`pared with the case of strong torsional anchoring (FIG. 4A)
`as shown in FIG. 4.
`
`Regarding the range of Variation of luminance AB50
`corresponding to AV50 as shown in FIG. 3, it is considered
`that AB50 is almost proportional to AV50. Therefore, in the
`range of luminance variation of AB50 caused by gap change
`of :Ad, the ratio of luminance variation in the case of weak
`torsional anchoring at the interface to that in the case of
`strong torsional anchoring can be approximated by the
`formula (5), and when the ratio of the extrapolation length
`to the gap between the substrates is given as b*=b/d, the
`following approximation is possible:
`
`AB50weak/AB50str~1/((1+2b*)-(1+2b*))
`
`(6)
`
`Since b*>0, AB50weak/AB50str<1. It is thus seen that the
`variation of luminance resulting from gap unevenness can be
`minimized by weakening torsional anchoring at the interface
`as shown in FIG. 4. That is, when torsional anchoring is
`weakened, the range of variation (AV50) of the character-
`istic curve caused by change of the inter-substrate gap to
`d:Ad in FIG. 3 is lessened, causing a corresponding reduc-
`tion of variation (AB50) of display luminance.
`The above AB50weak/AB50str ratio can be considered as
`an index of reduction of display irregularity (variation of
`luminance) against variation of gap by the weak torsional
`anchoring effect.
`
`Page 13 of 26
`
`

`
`US 6,242,060 B1
`
`7
`
`The formula (6) is plotted in FIG. 5 with b* as abscissa
`and AB50weak/AB50str as ordinate. This graph shows that
`a slight enlargement of the extrapolation length/substrate
`gap ratio b* results in a sharp decrease of the above index
`AB50weak/AB50str, that is, produces a remarkable lumi-
`nance variation reducing effect by weak torsional anchoring.
`With reference to color vision of human being, Weber
`ratio is known as a criterion for recognizability of luminance
`difference, and it is said that the luminance difference of
`10% can be recognized by human being. Therefore, by
`controlling alignment at the interface between the substrate
`and liquid crystal layer so that nonuniformity of display
`luminance will be confined to less than 10% by availing of
`the luminance variation reducing effect incidental to gap
`variation by said weak torsional anchoring, it is possible to
`obtain a liquid crystal display device which can make
`incognizable the nonuniformity of display luminance caused
`by gap variation which may occur in the manufacturing
`process of liquid crystal elements.
`In an in-plane switching mode active-matrix liquid crystal
`display, an inter-substrate gap unevenness of about 0.5 y is
`produced in the pixel section. So, when this is combined
`with an alignment layer (e.g. a rubbed alignment layer of a
`polyimide alignment layer material PIQ which gives the
`strong torsional anchoring and available from Hitachi
`Chemical Co., Ltd.), the display irregularity becomes about
`14%.
`in order to confine the degree of display
`Therefore,
`irregularity within said threshold value (10%) of luminance
`difference visibility so that the display luminance uneven-
`ness may not be recognized, it is necessary to provide a
`value smaller than 0.7 as the reduction index (AB50weak/
`AB50str), and this can be realized by making the extrapo-
`lation length/substrate gap ratio b* (see FIG. 5) greater than
`0.1, that is, making the extrapolation length greater than
`10% of the gap between the substrates.
`Further, by adopting weak torsional anchoring for the
`alignment at the substrate/liquid crystal layer interface, it is
`possible to lower the drive voltage due to reduction of
`threshold voltage and to enhance the rise response speed.
`Secondarily, according to the present invention, the tor-
`sional anchoring coeflicient of the alignment layer surface
`for
`the liquid crystal molecules at
`the liquid crystal/
`alignment layer interface is set to be not greater than 20
`yN/m in the in-plane switching mode.
`In order to obtain, with the in-plane switching mode, the
`display performance equal to that of the TN mode,
`it is
`necessary to make the gap between the substrates (thickness
`of the liquid crystal layer) about 4 mm. In this case, for
`making the extrapolation length greater than 10% of the gap
`between the substrates, the extrapolation length b must be
`not less than about 0.4 gm. Since the twist elastic constant
`K2 of the practical liquid crystal compositions presently
`available in the art is not greater than about 8 pN, it is
`suggested to use an alignment layer material which can give
`weak torsional anchoring, with the torsional anchoring coef-
`ficient K2 at the alignment layer surface (as given from the
`equation
`being not greater than 20 ;¢N/m.
`Thirdly,
`in the present
`invention, an organic polymer
`containing an oligomer and/or polymer in which the poly-
`merizate of long-chain alkylene groups and/or fluoro groups
`given in the amine or acid moiety is 5—30% of the total
`number of moles is used as an alignment layer material for
`obtaining weak torsional anchoring such as mentioned
`above.
`
`In order to provide an extrapolation length which is not
`less than 10% of the gap between the substrates,
`it
`is
`
`Page 14 of 26
`
`8
`desirable, as mentioned above, to use an alignment layer
`material which can give weak torsional anchoring with the
`torsional anchoring coefficient at the alignment layer surface
`being not greater than 20 ,uN/m. For obtaining such weak
`torsional anchoring, it is advised to use an alignment layer
`material in which long-chain alkylene groups and/or fluoro
`groups have been introduced at a rate above a prescribed
`level (5%).
`It should be noted, however, that when the ratio of the
`copolymerized portion of long-chain alkylene groups and/or
`fluoro groups becomes higher than a certain level (30%), the
`tilt angle of the liquid crystal molecules at the interface may
`exceed 10° to cause nonuniform display due to nonunifor-
`mity of tilt angle in high-tilt alignment. Also, when the tilt
`angle exceeds 10°, the wide viewing-angle effect, which is
`one of the prominent advantages to the in-plane switching
`mode, is mostly lost.
`Further, when the ratio of the copolymerized portion of
`long-chain alkylene groups and/or fluoro groups in the
`alignment layer material is elevated, the torsional anchoring
`coefficient A2 at the alignment layer surface may become
`lower than 1.0 ‘um/N, which tends to cause improper align-
`ment and excessive lowering of decay response speed. It is
`therefore advised not to indiscreetly raise the ratio of said
`long-chain alkylene groups and/or fluoro groups.
`In view of the above effects,
`it is advised to use an
`alignment
`layer in which the copolymerized portion of
`long-chain alkylene group and/or fluoro group is 5—30% of
`the total number of moles, as in the present invention, for
`obtaining an in-plane switching mode active-matrix liquid
`crystal display which is minimized in display irregularity
`and excellent in mass productivity.
`Moreover, by use of an alignment layer comprising a
`polymer having introduced thereinto an oligomer which is
`lower in weight-average molecular weight than the conven-
`tional polymer (having a weight-average molecular weight
`of 100,000 or more) of long-chain alkylene group and/or
`fluoro group, printability is improved when a varnish is
`coated on the substrate by a printing method.
`The polymer and/or oligomer constituting the long-chain
`alkylene groups and/or fluoro groups are preferably a poly-
`mer and/or oligomer-amic acid imide type, a polymer and/or
`oligomer-imide type,
`a polymer and/or oligomer-
`imidosiloxane type, a polymer and/or oligomer-amide imide
`type or the like, which have a weight-average molecular
`weight of 2,000—90,000 (converted to standard polystyrene)
`no matter whether they are main chain type, side chain type
`or terminal type.
`Especially preferred for use in the present invention is an
`organic alignment layer in which the amine moiety com-
`prises a single-ring rigid amine and the acid moiety com-
`prises a polymer and/or oligomer-amic acid composed of an
`aliphatic tetracarboxylic acid dianhydride and/or alicyclic
`tetracarboxylic acid dianhydride and an aromatic tetracar-
`boxylic acid dianhydride having main chain type long-chain
`alkylene groups and/or fluoro gro