lnsTiTuT Frongois du Péfrole Publicofions
`
`Guy fiabilloud
`Deputy Manager at the Centre d'études des matériaux organiques
`pour les technologies avancées (CEMOTA)
`Manager of the Documentation and Communication Department
`Centre d’études at de développement industriels (CEDI)
`lnstitut frangais du pélrole
`
`HIGH-PERFORMANCE
`POLYMERS E
`Mi
`CHEMISTRY AND APPLICATICMS %
`
`
`
`-_-_.........-..._._.........._-.._.
`
`2000
`
`It Editions TECHNIP 27 rue Ginoux, 75737 PARIS Cedex 15, FRANCE
`
`Page 1 of 74
`
`Tianma Exhibit 1032
`
`Page 1 of 74
`
`Tianma Exhibit 1032
`
`

`
`
`
`
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`
`FROM THE SAME PUBLISHER
`
`F.
`
`0 The Technology of Catalytic Oxidations
`1. Chemical, Catalytic and Process Aspects
`2. Safety Aspects
`A
`F.
`P.A
`«
`CAVANI, Tkmko
`RPE\TlN1ER,
`- Commodity Thcrmoplastics
`Technical and Economic Characteristics
`J.—P. ARLIE
`_
`- Synthetic Rubbers
`Processes and Economic Data
`J.—P. ARLIE
`- Petrochemical Processes. Technical and Economic Characteristics
`1. Synthesis—Gas Derivatives and Major Hydrocarbons
`2. Major Oxygenated, Chlorinated, and Nitrated Derivatives
`A. CHAUVEL, G. LEFEBVRE
`
`0 Scale-Up Methodology for Chemical Processes
`J .-P. EUZEN, P. TRAMBOUZE, J .-P. WAUQUIER
`- Chemical Reactors
`Design. Engineering. Operation
`P. TRAMBOUZE, H. VAN LANDEGHEM, J.-P. WAUQUIER
`- High—Performance Polymers. Chemistry and Applications
`
`’/
`
`'
`_ _ -. _;
`' _
`.
`I
`
`© 2000, Editions Technip, Paris
`
`All rights reserved. No pan of this publication may be reproduced or
`transmitted in any form or by any means, electronic or mechanical,
`including photocopy, recording, or any information storage and retrieval
`system, without the prior written permission of the publisher.
`
`Vol. 3 ISBN 2-7108-0720-3
`Series ISBN 2-7108-0717-3
`
`1
`l
`l
`
`77/7 3933
`
`Page 2 of 74
`
`Page 2 of 74
`
`

`
`Chapter 7
`
`ALIGNMENT LAYERS FOR
`
`LIQUID CRYSTAL DISPLAYS
`
`7.1 INTRODUCTION
`
`Information technologies generally comprise three main steps: (1) data acquisition by
`means of keyboards, video cameras, scanners, etc.; (2) data processing by electronic
`devices; and (3) data presentation to the viewer in a comprehensive form (printer and
`display devices). Polyimides are currently used at any stage ofthis chain in the fabrication
`of integrated circuits, inkjet printer heads, and flexible circuitry. Alignment layers for
`liquid crystal display devices (LCDS) constitute now an extremely active area of research,
`development and production of new polyimides. First generation polyimides have been
`successfully used to manufacture low-cost, consumer-oriented products that require battery
`operation (electronic watches, hand-held calculators, and many other applications). Search
`for flat panel displays, able to offer with an acceptable time constant a high degree of in-
`formation, has focussed on both light-emitting and light-modulating displays. The former
`category includes light emitting displays (LED), gas discharge displays (GCD), vacuum
`fluorescence displays (VFD), whereas liquid crystal panels are representative of the
`second class, also known as “passive displays” because they do not have to generate the
`necessary luminance. Initially, LCDs were manufactured by using inorganic technologies
`based on silicon dioxide orientation layers and glass sealing materials. Except for display
`devices subjected to hostile environments, the universally employed fabrication technique
`utilizes polyimides alignment layers and epoxy seals.
`
`Accordingly, the following discussion will be restricted to display devices using the
`electrooptical properties of liquid crystals. The various aspects of this topic are examined
`in this chapter by paying attention to liquid crystals in terms of chemistry, anisotropy, and
`electrooptic effects. Device fabrication and operation are then briefly reviewed to offer a
`comprehensive approach of the liquid crystal-substrate interactions in nematic and ferro-
`electric cells. Finally, a survey of the patent and scientific literature shows that the align-
`ment properties can be improved by changing the polyimide structure and mastering the
`rubbing process.
`
`Page 3 of 74
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`

`
`326
`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`7.2 LIQUID CRYSTALS
`
`7.2.1 Liquid crystallinity
`
`Liquid crystals are materials in an intermediate state of aggregation between the solid state
`and the conventional isotropic liquid phase. Unlike normal solids, mesogenic materials
`melt to form highly anisotropic fluids in which a significant part of the molecular order
`is retained. Upon heating, these solids melt and form viscous fluids exhibiting birefrin-
`gence and anisotropic physical properties. Liquid crystallinity disappear at higher temper-
`ature when the material passes into an isotropic liquid phase. The intermediate ordered
`liquid phase between these two melting temperatures, Tm and TC in Figure 7.1, is ther-
`modynamically reversible so that solid-like characteristics reappear when the isotropic
`liquid is cooled down below the second melting point. This curious behaviour was disco-
`vered by Reinitzer in 1888 [1209] with cholesteryl benzoate, a derivative of cholesterol.
`One century later, many thousand organic molecules have been discovered that exhibit
`liquid crystallinity. Thermotropic liquid crystals are composed primarily of elongated rod-
`shaped molecules exhibiting geometric anisotropy in both solid and fluid states. However,
`the molecules in liquid crystal phases are not perfectly aligned along a common axis.
`Instead, individual molecules lie at an angle 9 to the average direction described as a unit
`vector 11, called “director”. A measure of the degree of internal order is given by the order
`parameter S which is the average of the second Legendre polynomial [I210]:
`
`1
`5 = §(3cos29 -1)
`
`(88)
`
`S is readily measured by various techniques (X-ray, ultraviolet, infrared, magnetic re-
`sonance, etc.) and it is the quantity usually given to determine the anisotropy of physical
`properties. In the case of a crystalline solid, S would be 1, signifying perfect order whereas
`S is equal to 0 for the completely random distribution found in normal isotropic liquids.
`
`7.2.2 Liquid crystal phases
`
`The following is a summary ofan excellent review published by DuPré [121 1] who gives
`a number of pertinent references. Liquid crystals are divided into two major categories:
`thermotropic liquid crystals whose the properties change with temperature and lyotropic
`liquid crystals that are multicomponent mixtures in solution. Thermotropic rod-shaped
`organic compounds are subdivided into three distinctive structural categories: the sinectic,
`nematic, and cholesteric edifices. However, cholesteric substances are sometimes present-
`ed as a subclass ofnematic liquid crystals. The simplified design ofFigure 7.] shows that
`these structures differ by the nature of the residual molecular order achieved in the
`temperature range of liquid crystallinity..
`
`Page 4 of 74
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`

`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`327
`
`
`
`Smectic
`
`[Sun-opic “quid
`Nematic
`Cholesteric
`Sol id :
`
`Tm
`Temperature range of liquid crystallinity
`Tc
`Temperature
`
`Figure 7.1 Phase structures of thcrmotropic liquid crystals between first
`melting temperature Tm and clearing temperature T, leading to isotropic
`liquid. In the thermal range of liquid crystallinity, nematic, smectic or
`cholesteric phases are formed depending on the chemical structure of the
`starting molecules.
`
`7.2.2.1 The smectic phase
`
`As sketched in Figure 7.1, the three liquid crystal phases are characterized by a parallelism
`ofthe molecular long axes. The geometric arrangement of smectic liquid crystals includes
`an additional ordering of the molecular centres of gravity within two-dimensional planes
`that preexist in the organic solid precursors. Smectic liquid crystals retain a good deal of
`two-dimensional solid order; the order parameter may be as high as 0.9. Smectic structures
`are further divided into about ten classes, labelled SmA through SmK (or SA through SK),
`depending on the in-plane order and optical apparency. Smectic-A and smectic-C phases
`are frequently observed, for example, when studying ferroelectric liquid crystal molecules.
`A detailed discussion of these materials can be found in the review by DuPré and in a
`
`book by Gray and Goodby [I212].
`
`7.2.2.2 The nematic phase
`
`The parallelism of the molecular axes is preserved in nematic liquid crystals, but the posi-
`tions of the centres of gravity are not organized as they are in smectic liquid crystals. For
`most nematic materials the order parameter S lies in the range 0.6 < S < 0.8 at room
`temperature, decreasing to around 0.4 close to the nematic to isotropic phase transition
`temperature. Most liquid crystal display devices use nematic molecules because they can
`be easily aligned on rubbed glass and polymer surfaces. The alignment layer forces the
`molecules to lie parallel to the rubbing direction resulting in homogeneously aligned
`texture. The combination of adhesion strength and intermolecular forces provides an
`
`Page 5 of 74
`
`

`
`328
`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`ordered structure across a thin layer of liquid crystal from bottom to top glass slides. An
`optically active twisted structure, with the twist axis normal to the surface, is obtained
`when the upper glass substrate is rotated by 90° relative to the base support.
`
`7.2.2.3 The cholesteric phase
`
`The first liquid crystal discovered by Reinitzer is an example of the third type of meso-
`phase principally formed of cholesterol derivatives. Cholesteric liquid crystals are closely
`related to nematic substances, but exhibit spontaneous helical structure. As sketched in
`Figure 7.1, they have a screw ordering imposed so that the rotation of the director n occurs
`over a pitch length P which can be as short as 0.1 pm. All cholesteric molecules possess
`a chiral centre that imposes a bias to the director within each successive layer. It will be
`seen later on that nematic and smectic liquid crystals can be converted to cholesteric-like
`materials by adding chiral dopants that dissolve in the host liquid crystal phase.
`
`7.2.3 Liquid crystal molecules
`
`To be engaged in a mesomorphic structure—smectic, nematic, or cho1esteric~£longated
`molecules must at first meet the requirements of geometric anisotropy. But in addition,
`molecular interactions leading to mutual attraction must be established by secondary
`valence forces including dipole—dipo1e and dipole-induced dipole interactions, dispersion
`forces, and hydrogen bonding. This means that liquid crystal molecules generally contain
`polar groups or polarizable sites, aromatic rings, and double or triple bonds. As a
`guideline, many nematic and smectic liquid crystals have in common the general formula
`represented in Table 7.1 that lists some of the central linkages X between the two benzene
`rings and the most common end groups R, and R2 that are generally substituted at the 4
`and 4' positions.
`
`Table 7.1 Examples of central chemical linkage X and terminal groups R, and R2
`found in mesomorphic molecules containing aromatic rings.
`
`.,@sx-@..
`
`Central linkage X
`
`End groups R,, R2
`
`——
`
`—Cl-l=N(O)-
`
`C,,H,,,+,
`
`CH=CH-COZR
`
`-CH=CH-
`
`-N=N-
`
`O-C,,H,‘,,..
`
`F, Cl, Br
`
`-CEC-
`
`-N=N(O)-
`
`O-OC- CnH2,,,,
`
`
`
`-c0—o—-CH= — CEN
`
`
`
`Page 6 of 74
`
`

`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`329
`
`Several homologous series of chemical compounds Show stable liquid crystal phase at
`room temperature and various aromatic, aliphatic, and heterocyclic rings have been suc-
`cessfully used with a variety of linking groups. Raynes has summarized [1213] the liquid
`crystal chemistry by the general formula 376 represented in Figure 7.2 which shows that
`the elongated molecules are formed of aromatic, cycloaliphatic or heterocyclic rings con-
`nected by ester, ethylidene, and unsaturated linking units.
`
`n
`
`376
`
`n = 1. 2 or 3
`
`XI Y = CmH2m¢1i Cml-I2";->10;
`
`-<:>—= Aliphatic Q ®
`Heterocyclic -<:O>—
`
`Figure 7.2 Thermotropic liquid crystal chemistry based on the general for-
`mula 376 in which aromatic, cycloaliphatic, or heterocyclic rings connected
`by various linking units constitute the molecular backbone. Terminal groups
`are either aliphatic, ether or nitrile functions.
`
`The chemical formulas drawn in Figure 7.3 are representative examples of thermo-
`tropic liquid crystals [I214]. 4'-hexyl[1,1'-biphenyl]-4-carbonitrile 377, 4-buty1-N-[(4-
`Inethoxypheny1)methylene]benzeneamine 378, 4,4'-azoxybis(methoxybenzene) 379, bis-4-
`methoxyphenyl)— trans-1,4-cyclohexane dicarboxylate 380, and para-quinquephenyl 381
`are nematic substances covering a range of liquid crystallinity between 14 and 445°. 4-
`octyloxybenzoic acid 382 and ethyl N-[4-[1 , 1'-biphenyl]methylene]-4-aminobenzoate 383
`are smectic C and smectic A liquid crystals, whereas cholesteryl nonanoate 384 and levo—
`gyre 2-methylbutyl N-[(4-methoxyphenyl)methylene]-3-(4-aminophenyl)-2-propenoate
`385 are cholesteric materials. The first stable room temperature materials, the cyano-
`biphenyl liquid crystals were described in 1973 by Gray et al. [1215], and it has been then
`demonstrated that the cyanobiphenyls are good candidates for use with the LCD device
`technology. Most cyanobiphenyls show a nematic phase even if some longer chain deri-
`vatives exhibit smectic-A properties. Chiral—nematic or cholesteric cyanobiphenyls can
`also be obtained by replacing the linear aliphatic chain by branched alkyl substituent.
`
`Page 7 of 74
`
`

`
`3 30
`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`wO0ww@m©w
`
`377 (14-28°C)
`
`378
`
`(21-47°C)
`
`‘?
`M)%%w
`
`379
`
`(1 17-1 37° C)
`
`cM%{w©w
`
`380
`
`(143-242°C)
`
`OOOOO
`
`381
`
`(401—445‘C)
`
`w©wOOwCwm
`
`332 (1oe—147°c)
`
`333
`
`(121—131°c)
`
`CH,
`
`CH;
`
`Hac
`
`\
`
`CH3
`
`CH3-(CH2)7—o2c’
`
`384 (7a—9o°c)
`
`cH3o—©— on: NQ CH : CH—CO2-CH,-CH-C,H5
`
`CH,
`I
`
`385
`
`( 53°C - 97°C )
`
`Figure 7.3 Chemical formulas of thermotropic liquid crystals including
`nematic substances based on biphenyl 377, Schiffs base 378, azoxy 379,
`cycloaliphatic ester 380, and quinquephenyl 381 units; smectic C 382 and
`smectic A 383 compounds; and cholestcric 384, 385 substances.
`
`To illustrate the progress made since the cyanobiphenyl invention, Raynes also
`presents the general formula of a new family of liquid crystals, the fluorobiphenyl ethanes,
`which were described in 1985 by Balkwill et a1.[1216]. Whereas the cyanobiphenyl 388
`(Fig.7.5) has a nematic range of only 24 to 35°C, the fluorobiphenyl ethane 386 shown
`in Figure 7.4 is nematic from 13 to 97°C. Many liquid crystal compositions based on
`fluorinated organic compounds are now commercially available.
`
`Page 8 of 74
`
`

`
`Chapter 7. Alignment layers for liquid crystal displays
`
`33 1
`
`335 (13—97°c)
`F
`
`F
`
`337
`
`F
`
`Y
`
`x = C,H,. cF,. OCF_.,. F
`Y=H,F
`
`Figure 7.4 Chemical fonnulas of fluorinated liquid crystal molecules with
`improved thermal range of liquid crystallinity.
`
`7.2.4 Liquid crystal mixtures
`
`Commercial LCDS always utilize eutectic mixtures of liquid crystals to broaden the operat-
`ing temperature range, which is between -10 and +60°C for low end displays and of the
`order of -20 to 100°C for high performance devices. Figure 7.5 shows the chemical
`formulas of the four-component E7 mixture based on three cyanobiphenyls 388-390 and
`one cyanoterphenyl 391. This composition has been developed for wristwatch and low-
`cost calculator displays working at temperatures lower than 60°C. For active matrix LCDs,
`Plach et al. have developed [1217] liquid crystal mixtures containing different fluorinated
`compounds of general formula 387 (Fig. 7.4). A nematic phase range from -40 to 110°C
`can be achieved with these fluorinated liquid crystals. Many practical examples show the
`flexibility offered by mixtures of azoxy and cyano derivatives [I218], optically active aro-
`matic esters [I219], heterocyclic esters [1220], thioesters [122]].
`
`388 (24—35°c)
`
`389 (3o—43°c)
`
`°a“"° O 0 °“ O O 0 °“
`
`39o (54—eo°c)
`
`391
`
`(131—24o°c)
`
`Figure 7.5 Four-component eutectic mixture E7 of liquid crystals com-
`prising 4'-pent)/l[1,1'-biphenyl]-4-carbonitrile 388 (39 wt%), 4'-heptyl[1, 1'-
`biphenyl]-4-carbonitrile 389 (36 %), 4'-octyl[l ,l'-bipheny1]—4-carbonitrile
`390 (16 %), and 4"-pentyl[l,l':4',l"-terphenyl]-4-carbonitrile 39] (9 %).
`Nematic temperature range of each substance is indicated between paren-
`theses.
`
`Page 9 of 74
`
`

`
`332
`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`7.3 ANISOTROPY OF LIQUID CRYSTALS
`
`7.3.1 Electric permittivity
`
`Referring to previous publications, Raynes outlines [1213] that the behaviour of nematic
`materials is well understood by taking in consideration the anisotropy of both the electric
`permittivity and refractive index. Including anisotropy in the Onsager's theory gives:
`
`2 Pi
`3
`AE=8‘|—8i=AAa+‘k71'uL#'§_ S
`
`(89)
`
`(90)
`
`_
`
`e_1+A[a+3TT( L+|.tT)]
`
`B
`
`2
`
`2
`
`where Ae represents the anisotropy in electric permittivity, 8,, and 8* are the components
`of the electric permittivity parallel and perpendicular to the director, 8 is the mean value:
`
`e=%(e,,+2e_,_)
`
`(91)
`
`A and B are combinations of cavity and reaction field factors, or is the molecular polar-
`izability, u1_ and pr the molecular dipole moments longitudinal and transverse to the mole-
`cular axis. Raynes indicates that permittivity anisotropies for known liquid crystals lie in
`the range —1O < Ac < 50. Orientation of nematic liquid crystals can be easily achieved in
`electric or magnetic fields. Depending on the sign of the dielectric anisotropy Ac, nematic
`substances orient parallel (As: > 0) or perpendicular (As < O) to the applied field direction
`(Fig. 7.6).
`
`Spa; 0 0
`P ‘(I
`...::'.:::y
`
`I I
`—1—£—2—>n
`o -
`8" < SA
`
`E
`
`71
`
`Negative
`anisotropy
`
`Figure 7.6 Parallel and perpendicular orientation of nematic liquid crystal
`director n in applied electric field E.
`
`Page 10 of 74
`
`

`
`Chapter 7. Alignment layers for liquid crystal displays
`
`333
`
`The scheme of Figure 7.7 has been used by Penz et al. [1222] to illustrate the variation
`in orientation of the unit vector 21 describing the local optical axis of a liquid crystal of
`nematic material through the sample under the influence of the electric field E0.
`
`
`
`-T-> -j-> T) T» ?—-> -j-> Eo
`n
`
`-—?> _j-> -—> T; m; T,
`
`n / A /e
`
`v=
`
`v.>v,
`
`Figure 7.7 Geometric variation of the nematic director n orientation under
`the influence ofan applied electric field E0.
`
`The electrical torque TE on the liquid crystal making an angle 6 with respect to the
`electrode plate is equal to:
`
`TE = 3o(3II ‘ 3i)Ed9
`
`(92)
`
`where so is the permittivity of free space. The distortion of the liquid crystal results in an
`opposite elastic torque TM given by:
`
`TM -
`
`1r2K9
`
`dz
`
`(93)
`
`where K is an elastic constant of the liquid crystal and d the thickness of the dielectric
`layer. As stated above, the director is constrained by surface forces to lie in the plane of
`the glass plates. The distortion amplitude increases when raising the electric field and,
`accordingly, the dielectric torque. The condition for equilibrium between T5 and TM results
`in a critical voltage VG expressed as:
`
`K
`V‘: = E d = ——
`
`ni8o(9u—€J.)i
`
`0
`
`0.5
`
`(94)
`
`This threshold voltage for reorienting the nematic director is characteristic of liquid
`crystal displays. It includes an elastic constant K, which is related to one of the three
`deformation modes described in a following section and illustrated in Figure 7.9. K values
`are of the order of 1 to 10 pN (10" ‘-1 O"2 N), leading to threshold voltage of ~l V. The
`energy required to switch on the liquid crystal can be determined from the capacitance C
`of the sample, determined by 8", and the applied voltage V which is roughly twice the
`threshold voltage to turn on the display. In these conditions, the energy by unit of surface
`area Q is:
`
`Page 11 of 74
`
`

`
`334
`
`Chapter 7. Alignment layersfor liquid crystal displays
`
`Q:
`
`2
`
`E08"
`
`For a typical commercial device Penz et al. take values 8,, 22.3 pF, and d 10 um, lead-
`ing to an energy per unit area of about 3 nJ cm‘2 or a power of 1 p.W cm‘2 to turn on a
`device running with ac voltages at frequencies equal to or upper than 32 Hz.
`
`7.3.2 Refractive indices
`
`As underlined in chapter 3, the refractive indices are actually the high-frequency limit of
`the longitudinal and transverse permittivities. The ordinary no and extraordinary nu refrac-
`tive indices at high frequency are given by Maxwell’s relations 96, whereas the birefrin-
`gence An of a material is expressed by equation 97 and lies in the range 0.03 < An < 0.30.
`Birefringent and super-birefringent effects will be discussed in the operation f liquid
`crystal devices.
`
`n§ = e_L
`
`and
`
`I1: = 8"
`
`An = ne — no
`
`(96)
`
`(97)
`
`7.4 ELECTRO—OPTIC EFFECTS IN LIQUID CRYSTALS
`
`A review by Raynes provides [1223] an excellent summary of the major electro-optic
`effects found in the various liquid crystal phases. They are based on the formation of a
`very thin layer of liquid crystal that is oriented by the substrate surface. The initial
`orientation, and hence the optical properties, are then changed by application of an electric
`field. The construction of any LCD device follows the same technological scheme leading
`to a thin layer of nematic, smectic, cholesteric, or ferroelectric liquid crystal enclosed
`between two transparent plates covered with patterned electrodes. The second feature is
`the surface-induced alignment of liquid crystals which provides a defect-free, single-
`crystal morphology to the entire layer. The two principal alignments employed to
`manufacture LCDs are represented in Figure 7.8. They are:
`
`- The "perpendicular", or "homeotr0pic" alignment which can be achieved by coating
`the glass surfaces with surfactant molecules, silane coupling agents or chrome
`complexes.
`
`— The "planar" or "parallel", or "homogeneous" alignment which can be obtained either
`by oblique evaporation ofa dielectric layer, such as SiOx, or by rubbing a thin film of
`polyimide covering the glass plates.
`
`Page 12 of 74
`
`

`
`Chapter 7, Alignment Iayersfor liquid crystal d‘.-YPIGJ’-7
`
`335
`
`
`
`Pe|’P€ndi0U'3|’ 0|‘
`homeotropic
`alignment
`
`Parallel or
`homogeneous
`alignment
`
`
`
`Randomly dispersed
`liquid crystals
`
`Figure 7.8 Alignment of liquid crystals on glass substrates. The perpen-
`dicular or homcotropic alignment is obtained over thin films of surfactants
`or silane coupling agents, while the parallel or homogeneous alignment is
`achieved over rubbed polyimide films deposited on the glass substrates.
`
`The principle of reorientation by an electric field interacting through the liquid crystal
`permittivity, sketched in Figure 7.6, remains at the moment the most important mechanism
`involved in LCD’s electro-optic effects.
`
`7.4.1 Freedericksz transitions
`
`Raynes outlines [1213] that the average direction of nematic liquid crystal molecules is
`described by the director n and when this uniform alignment is disturbed, the free energy
`density of the distorted state takes a new form by incorporating the extra—energy density:
`
`FE = —% s0(e” — 8J_)(n E)2
`
`(98)
`
`The static continuum equation includes three elastic constants K“, K22 and K3, that can
`be identified with the splay, twist and bend deformations illustrated in Figure 7.9. They
`have values of the order of 10 pN and, for currently available liquid crystals, typical data
`for elastic constants at room temperature are:
`
`5 pN < K” < 20 pN, K22/K” = 0.5,
`
`0.5 < 1<,,/K,, < 3.0
`
`Taking the example of a planar aligned layer with permittivities 8,, > 8*, which is reor-
`iented by an electric field, Raynes indicates [1223] that the solution ofthe static continuum
`equation shows the existence of the threshold voltage Vc, which has been previously
`defined in equation 94.
`
`Page 13 of 74
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`

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`336
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`Chapter 7. Alignment layersfor liquid crystal displays
`
`iris \ \ ‘\
`
`Twist
`(K22)
`
`Bend
`(K33)
`
`Figure 7.9 Illustration ofthe three basic elastic curvature deformations of
`nematic liquid crystals: splay, twist and bend, characterized by elastic
`constants K”, K22, and K3,, respectively.
`
`Below this threshold, surface alignment dominates and there is no reorientation, and
`above it the applied field produces a progressive reorientation. This type of transition,
`illustrated in Figure 7.10 by the change in permittivity of an initially planar layer as a
`function of the applied voltage, was first observed in 1933 by Freedericksz and Zolina
`[I224]. Raynes points out that the calculation of Vc from the basic equation is straight-
`forward and, in the planar case considered above, may be written:
`
`80(9|l"8J_)Vc2 = vr2K.1
`
`(99)
`
`14
`
`.1 N
`
`Perrnittivity,e 8
`
`O
`
`1
`
`2
`
`3
`
`4
`
`5
`
`Voltage (volts)
`
`Figure 7. 10 Voltage dependence ofthe permittivity for planar aligned layer
`Of liquid crystal with positive dielectric anisotropy (adapted from [1223]
`copyright© 1987 Springer).
`
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`Chapter 7. Alignment layersfor liquid crystal displays
`
`337
`
`It will be seen later that in practical applications, the nematic director is never totally
`aligned along the electric field, and the limiting value is given by the maximum angle Gm
`achieved at the mid—plane location. Freedericksz’ transition is the basis of different electro-
`optic effects actually used to manufacture LCD devices.
`
`7.4.2 Variable birefringence
`
`The electro-optic effect resulting from birefringence variation was the first observed by
`Freedericksz and Zolina. They showed that the intensity of light polarized at an angle or
`relative to the nematic director and analysed by a second polarizer after propagation
`through the liquid crystal layer of thickness d is given by:
`
`I = [0 sin?‘ (2ot)sin2 [ 7t And]
`
`(100)
`
`where 10 represents the incident intensity, An the birefringence, and A the wavelength of
`the light. This expression implies that the transmitted intensity is related to the orientation
`of the director by an electric field through the retardation factor R expressed by:
`
`1tAnd
`
`R:
`
`9»
`
`(101)
`
`7.4.3 Guest—host pleochroic dyes
`
`Various anthraquinone derivatives have been synthesized to prepare pleochroic dyes that
`absorb only polarized light whose the polarization vector is parallel to their long molecular
`axis. Heilmeier and Zanoni have observed [1225] that a pleochroic dye dissolved into a
`liquid crystal usually align its absorbing axis along the nematic director, resulting in an-
`isotropic absorption of light. Two electro-optic effects in guest-host systems are reported
`by Raynes [I223]. The first utilizes a single polarizer to induce the off-state absorption
`which is changed by an applied electric field producing the on-state reorientation. The
`second effect uses a special cholesteric layer in which the dye absorbs all polarizations of
`incident light. This happens when the product of birefringence An times the pitch P ofthe
`nematic-cholesteric helix is far smaller than the light wavelength A (An P « A). Application
`of an electric field reorients the director and unwinds the cholesteric helix, a process called
`cholesteric-nematic phase change effect that occurs at a threshold field:
`
`e0(e.. — eJ_) E3 = —P—2—
`
`(102)
`
`where E: is the threshold field and K22 the liquid crystal elastic constant characteristic of
`the twist deformation mode (Fig. 7.9).
`
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`3 3 8
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`Chapter 7. Alignment layers for liquid crystal displays
`
`7.4.4 Twisted nematics
`
`As mentioned above, the twisted nematic device is formed of a liquid crystal layer homo-
`geneously aligned on two glass substrates that are twisted by 90°. As shown in Figure
`7.1 1, this structure rotates the nematic director and, accordingly, the plane of polarization
`of light from one boundary surface to the other provided that And » A.
`
`
`
`Transparent
`electrode
`
`Light source
`
`Rear polarizer
`
`
`
`
`
`Transparent
`electrode
`
`Light source
`
`
`
`Untwisted
`liquid crystals
`
`Figure 7.11 Operation of a conventional 90° twisted nematic cell with
`polarized light transmitted through the nematic helix in the bright or off-
`state and not transmitted when an electric field is applied (on-state).
`
`Gooch and Tarry have shown [1226 that the intensity transmitted through a structure
`twisted through an angle (1) between two polarizers oriented parallel to the entrance
`director and orthogonal to the exit director is given by:
`
`(103)
`
`The general form of the Gooch and Tarry's curve is represented in Figure 7.12 where
`light transmission is drawn versus the function:
`
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`Chapter 7. Alignment layersfor liquid crystal displays
`
`339
`
`U
`
`_ 2And
`7t
`
`(104)
`
`It clearly shows that the lowest level of light transmission is achieved when the values
`of An and d are chosen to be either on a minimum of the curve or in the region where the
`minima and maxima are no more significant. The first minimum, for example, is obtained
`with An = 0.06, d = 6 um for a wavelength of 550 nm. Most twisted nematic devices are
`operating in the so called “Gooch and Tarry minima”.
`
`Transmission
`
`(%)
`
`Figure 7.12 Relationship between the intensity of light transmitted to the
`liquid crystal layer and the function U = 2And /A.
`
`Since for nematic liquid crystals, the probability for the twist to be left- or right-handed
`is equal, a long-pitch chiral dopant is generally added to break this symmetry. The thres-
`hold voltage can be calculated from the continuum equation extended to the cholesteric
`dopant with pitch P by the relation:
`
`9o(‘6|I ‘ €J_)Vc2 = 7‘2[Kl1 + X(K33 ‘ ZK22) 4' 2K22
`
`1
`
`d
`
`(105)
`
`Raynes [1227] and Patel [1228] indicate that a complication arises in complex displays
`using shared electrodes and multiplexed addressing. The two authors illustrate this issue
`by considering a matrix of 100 X 100 pixels. Direct addressing would require 10‘ + 1 leads
`while multiplexing can be realized with a matrix of 100 row and 100 column electrodes.
`The slow electro-optic response of twisted nematic liquid crystals together with the high
`repetition rate of the voltage pulses make LCDs difficult to multiplex. As the display
`
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`340
`
`Chaprer 7. Alignment layersfor liquid crystal displays
`
`responds to the root mean square (RMS) of the applied waveform, Alt and Pleshko have
`shown [1229] that the ratio ofthe voltage between the select V3 and non-select Vns voltages
`is given by:
`
`V;
`
`l:‘/;+1:|O.5
`
`x/3-1
`
`(106)
`
`where n is the number of addressed lines. For the 100 X 100 element matrix the ratio for
`
`the RMS voltages applied on the selected and unselected pixels is only 1 .106. This means
`that a significant contrast between the off- and on-states is obtained with this addressing
`scheme only when the material exhibits a steep electro-optic response function. This point
`is discussed in the following section dealing with the development of supertwisted nematic
`displays.
`
`7.4.5 Supertwisted nematic displays
`
`It will be seen later on in this chapter that two liquid crystal tilt angles are reported in the
`literature. The surface pretilt angle 60 and the bulk pretilt angle GP which is generally mea-
`sured. Raynes presents [1223] the response of the liquid crystal director to an applied
`voltage by considering the mid-plane tilt angle Gm shown in the insert of Figure 7.13. A
`small change ofthis angle with voltage results in poor multiplexing performance with low
`contrast and restricted viewing cone. The curves plotted in Figure 7.13 illustrate the
`voltage dependence of the mid-plane tilt angle Gm as a function of the twist angle between
`90 and 315°. It can be seen that conventional nematic displays with 90° twist show only
`gradual change of 6m when the applied voltage is increased. This behaviour explains the
`modest performance of this type of device operating in RMS multiplexing scheme. A
`significant progress was accomplished in 1982 by the invention of the supertwisted
`nematic (STN) device [I230]. This patent claims that the maximum value of the director
`tilt angle Gm increases rapidly when the twist angle (1) of the liquid crystal layer is
`increased. The slope of the function Gm = f(V) becomes infinite when the value of 0,“ is
`approximately 270°.
`
`The design of a highly multiplexed supertwisted nematic liquid crystal device was
`presented in 1983 by Waters et al.[l23l] using a pleochroic dye operating in the guest-
`host mode described in section 7.4.3. One year later, Scheffer and Nehring [1232]
`developed a 270° supertwisted nematic device running in the supertwisted birefringence
`effect (SBE) with two polarizers. High—performance STN-LCDs are currently produced
`by most LCD manufacturers. Relevant references can be found in a review by Raynes and
`Waters [1233]. The authors show how the dire