Joseph A. Castellano
`STANFORD RESOURCES, INC.
`SAN JOSE, CALIFORNIA
`
`ACADEMIC PRESS, INC.
`Harcourt Brace Jovanovich, Publishers
`San Diego New York Boston London
`Sydney Tokyo Toronto
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`Mercedes-Benz Ex. 1015
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`MBI_001588
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`

`

`This book is printed on acid-free paper. E>
`
`To R1
`
`Copyright© 1992 by ACADEMIC PRESS, INC.
`All Rights Reserved.
`No part 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 pem1ission in writing from the publisher.
`
`Academic Press, Inc.
`1250 Sixth Avenue, San Diego, California 92101
`
`United Kingdom Edition published by
`Academic Press Limited
`24-28 Oval Road, London NW\ 7DX
`
`Library of Congress Cataloging-in-Publication Data
`
`Castellano . .Joseph A.
`Handbook of display technology I Joseph A. Castellano.
`p. cm.
`Includes bibliographical references and index.
`ISBN 0-12-163420-5
`1. Jnfonnation display systems.
`TK7882.16C37 1992
`621.38 I '542--dc20
`
`l. Title.
`
`PRINTED TN Tl!E UNITED STATES OF AMERICA
`
`92 9J 94 95 96 97
`
`QW
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`9 8 7 6 5 4 3 2
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`l
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`91-41630
`ClP
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`MBI_001589
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`

`

`1 2 Flat Panel Displays
`
`9
`
`1.2.4 VACUUM FLUORESCENT DISPLAY (VFD)
`
`The first vacuum fluorescent displays (VFDs) were single-digit display
`tubes developed by Dr. T. Nakamura of lse Electronics Corporation in
`1967.20 The technology offered a means to provide a flat, thin CRT-like
`display that could be operated at much lower voltage. These tubes used a
`ceramic anode substrate that was scaled in a glass bulb. Later, NEC Cor(cid:173)
`poration and Futaba Corporation became major suppliers of VFDs. The
`early VFDs were used in calculators and were made in increasingly
`smaller sizes as the calculators decreased in size. The next generation
`tubes were the multidigit displays, again made with a ceramic substrate,
`but with multiple digits 10 or 12 mm high. The third-generation tube,
`introduced by Futaba Corporation, displayed multiple digits but was
`made with less expensive glass. Today, Futaba holds the largest share of
`the worldwide market with NEC a strong second and Ise third. Samsung
`Electron Devices (Suwon, Korea) makes VFOs mainly for use in the firm's
`microwave ovens and VCRs.
`In addition to the desire to produce a flat, thin light emitting display
`that could be operated at low voltage, another reason the VFD was devel(cid:173)
`oped was a rather practical one. By the mid-1960s, vacuum tube produc(cid:173)
`tion had become a high-volume, automated process. Unfortunately, by this
`time vacuum tubes were rapidly being replaced by solid-state compo(cid:173)
`nents. The VFD was seen as a new product that could be made with old
`but cost-effective equipment. Hence, this development was driven, in part
`at least, by a need to convert a factory from the production of one type of
`component to another. The message here is that sometimes it is not nec(cid:173)
`essary to shut down a plant and lay off all the workers if one can be cre(cid:173)
`ative about using the plant for another purpose.
`
`1.2.5 LIQUID CRYSTAL DISPLAY (LCD)
`
`Although liquid crystallinity was first observed in 1888 by Reinitzer, it
`was more than 30 years before Mauguin ii discovered and described the
`twisted-nematic structure that later became the basis for liquid crystal dis(cid:173)
`play (LCD) technology. During the 1920s and 1930s work on liquid crystal
`materials and the electro-optic effects that they produced was conducted
`in France, Germany, the U.S.S.R., and Great Britain. Perhaps the first pat(cid:173)
`ent on a light valve device that used liquid crystals was awarded to the
`Marconi Wireless Telegraph company (now part of GEC) in 1936.22 Then
`in the mid-1950s, researchers at the Westinghouse Research Laboratories
`discovered that cholesteric liquid crystals could be used as temperature
`sensors. It was not until the 1960s, however, that serious studies of the
`materials and the effects of electric fields on them were carried out. One
`reason for this was that liquid crystals were little known materials and, in
`fact, the first book in English to treat the subject was not published until
`
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`10
`
`H1stor1cal Development of Electronic Displays
`
`Or. George W. Gray's "Molecular Structure and the Properties of Liquid
`Crystals" appeared in 1962.2J This excellent book quickly became the de(cid:173)
`finitive work on the subject. Before its publication, students of organic
`chemistry in most U.S. universities did not know what a liquid crystal was!
`The early work on applications of liquid crystals was carried out in
`research laboratories in the United States, Europe, and Japan. During this
`period, a great deal of research and development was performed; theories
`were formulated and tested, a number of electro-optic effects were discov(cid:173)
`ered, materials with broader operating temperature ranges were prepared,
`and rudimentary fabrication techniques were developed.
`The idea of using liquid crystal materials for display applications
`was probably first conceived in 1963 by Ors. Richard Williams and George
`Heilmeier at the David Sarnoff Research Center (then the central research
`arm of RCA Corporation) in Princeton, New Jersey. 21 Later, a larger group,
`headed by Heilrneier and including Louis Zanoni, Joel Goldmacher, Lu(cid:173)
`cian Barton, anti the author, spearheaded the work to develop liquid crys(cid:173)
`tal displays for application to the fabled "TV-on-a-wall" concept, a dream
`of the late TV pioneer David Sarnoff. During the period from 1964 to 1968,
`this group discovered many of the effects that were later to be commer(cid:173)
`c:ialized, including dynamic scattering/~ dichroic dye LCDs,26 and phase(cid:173)
`change displays.2 1 One of the major breakthroughs occurred in the sum(cid:173)
`mer of 1965 when it was discovered that by mixing various pure nematic
`liquid crystalline compounds together ii was possible. for the first time,
`lo produce stable, homogeneous liquid crystal solutions that could oper(cid:173)
`ate over a broad temperature range including ordinary room temperature.2 8
`Later, cyanobiphenyl materials with improved properties and even broader
`temperature ranges were developcd;2 '1 these compounds form the basis of
`most of the liquid crystal materials used today in commercial products.
`During the mid-1960s, work on liquid crystal displays was also being
`performed by A. Kapustin and L. S. Larinova in the Soviet Union J0 and by
`George Elliott and J. G. Gibson at Marconi Electric in England.~• Later, a
`group that included Joseph Wysocki, James Adams , and Werner Haas al
`Xerox also carried out extensive liquid crystal display rescarch.;i2
`By 1969, it became clear to the RCA group and others that the devel(cid:173)
`opment of large-screen, LCD television sets would require "many years of
`research," although nobody believed il would take 16 years. Thus. an ef(cid:173)
`fort was mounted to develop simpler display devices that could be com(cid:173)
`mercialized quickly. One of these was the "point-of-purchase" display, a
`moving advertisement display used in retail stores. These segmented dis(cid:173)
`plays (produced by RCA and Ashley-Butler in the early 1970s) \11rcre made
`in sizes up to 12 x 12 inches. The system used a rotating copper drum
`patterned in such a way as to send electrical signals lo the approµriate
`segments of the display at the proper time to create the desired motion.
`Although this application proved to provide a very limited market, many
`of the techniques developed for production of these large-size LCDs were
`later used for the manufacture of smaller displays.
`
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`MBI_001591
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`1 2 Flat Panel Displays
`
`11
`
`Among the most important early applications were the wrist watch
`and portable calculator, made possible by the low power consumption of
`LCDs and the integrated circuit industry, then in its infancy. Some of the
`"products of the future" envisioned in papers published in the 1969-
`1971 period were numeric indicators for instruments, digital clocks, digi(cid:173)
`tal wrist watches, optically tuned color filters using the so-called "guest(cid:173)
`host" effect, electronically controlled " window-shades." and "displays for
`auto dashboards, aircraft cockpits, scoreboards , highway signs, and com(cid:173)
`puters." Today, we see LCDs in virtually all of these applications.
`One of the must important major breaktluoughs occurred in late
`1969 when James L. Fergason, working at a newly formed firm, Interna(cid:173)
`tional Liquid Crystal Company (TLIXCO) in Kent, Ohio, discovered the
`twisted-nematic (TN) field-effect LCIJ. which ultimately proved to be the
`most successful for the watch, calculator, and later, other applications in(cid:173)
`cluding TV. Because Mr. Fergason's patent application was not made pub(cid:173)
`lic until several years later,JJ Drs. Wolfgang Helfrich and Martin Schadt of
`F. Hoffmann LaRuche in Basel. Switzerland, published a paper on the
`same effect in 1971 34 and were awarded a patent in 1975.:•5 Needless to
`say, this sparked a long legal battle over ownership of the invention. Even(cid:173)
`tually, the issue was settled out of court. That Mr. Fergason is generally
`regarded as the inventor of the TN-LCD is exemplified by the fact that he
`was awarded the highest honor of the Society for Information Display for
`his initial discovery.
`Between 1970 and 1972 activity in the LCD field increased enor(cid:173)
`mously and many companies in the United States, Europe, and Japan be(cid:173)
`gan to exploit the development of the 1960s. The coincident development
`of large-scale integrated circuits for driving and timekeeping functions
`resulted in lhe development of the LCD wrist watch and calculator. The
`early Hl70s also saw a number of new American companies formed to
`exploit LCD technology. Among these were ILIXCO, Optel Corporation
`and Princeton Materials Science (Princeton, New Jersey), Microma (Cu(cid:173)
`pertino, California), Micro Display Systems (Dallas), and Integrated Dis(cid:173)
`play Systems (Montgomeryville, Pennsylvania). All of these firms set out
`to manufacture LCDs and the digital watches that used them.
`In those early days, it was American engineers and scientists who
`developed the first processes for the fabrication of LCDs and digital
`watches. It was an exciting but sometimes frustrating time because the
`technology was in its infancy and engineers were forced to work with
`equipment that was adapted from other industries. Although the equip(cid:173)
`menl used was crude by today's standards, the same fundamental tech(cid:173)
`niques are now being used to manufacture the hundreds of millions or
`LCDs made each year throughout the world.
`During these early years, many Japanese firms followed and copied
`the developments com ing out of the United States. However, they quickly
`began striking out on their own by developing improved fabrication and
`packaging techniques that resulted in greater reliability and lower manu-
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`MBI_001592
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`References
`
`In tc
`JapancsL '1
`busin es'
`ment. auto
`for compu1
`rope. a sm
`for 18.I'!!e-(cid:173)
`Nernrthele
`manutactu
`
`The abm
`become th
`the year.:>.,
`technolO'!l
`remain. St
`the si:x de~
`:m nounce<
`six includ
`gas-electro
`dbpla\0
`CB\ it~ di J
`and liquid
`of the
`e
`
`' . (
`
`12
`
`1 . H1stor1cal Development of Electronic Displays
`
`facturing cost. They envisioned that a large market for electronic products
`made with low-power, highly legible LCDs would be forthcoming and
`they dedicated themselves to pursuing that goal.
`The first LCD digital watches used the "dynamic scattering effect."
`However, by late 1974 this display practically vanished because of its rela(cid:173)
`tively high-voltage requirement (al least for the CMOS devices made at
`that time) and viewing angle restrictions created by the need for a specular
`(mirror) reflecting back electrode. Jt was soon replaced by tho twisted(cid:173)
`ncmatic, field-effect (TN-LCD) display, and the LCD watch began to gain
`momentum in 1976. Compact, attractive LCD calculators and watches
`made in Japan soon became household items.
`Today, manufacturing techniques and equipment arc readily avail(cid:173)
`able, and highly reliable, low-cost liquid crystal displays are being made
`by the hundreds of millions, primarily in Japan and the Far East. These
`displays arc, for the most part, driven by a low level of multiplexing (30
`to 50~o duly cycle) or directly driven with each segment receiving full
`voltage.
`The LCD technology became successful because of its " passive"
`(non-light emitting) nature that provided the combined characteristics of
`low power and viewability in bright light, factors that made miniaturiza(cid:173)
`tion and portability a reality. The Uniled States lost its leadership position
`in LCD technology because many firms were convinced that the LCD did
`nol have adequate "brightness" or contrast to meet the needs of equipment
`makers. However, the Japanese firms bdieved that only a passive display
`technology such as LCD could provide the combined characteristics of
`low power and viewabilily in bright light that would make miniaturization
`and portability a reality. By focusing on that concept, they became the
`leaders.
`In other parts of the world, LCDs were being developed more slowly
`than in the United States and Japan. The Swiss watch industry was slow
`to accept LCDs; by the time it did, the industry could not be competitive
`with the Far East at the low-priced end of lhe market. As a result, the
`Swiss abandoned the concept in favor of higher-priced analog quartz
`types with traditional faces. However, today LCD digitals and digital/ana(cid:173)
`logs are popular in both Eastern and Western Europe. LCDs are also be(cid:173)
`coming more widely used in other consumer and industrial electronic
`products throughout Europe.
`Today we sec more and more industrial and consumer products us(cid:173)
`ing liquid crystal displays. LCDs now appear in automobile dashboards,
`aircraft cockpit displays, telephones, microcomputers, word processing
`systems, gaming machines, hand-hold games, thermostats, electronic test
`equipment, monitoring and control systems in automatic machinery, and
`the list goes on. The realization that a low-cost, low-power display with
`good visibility is now available has prompted many manufacturers of elec(cid:173)
`tronics devices to incorporate LCDs in their equipment, particularly those
`that arc portable.
`
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`References
`
`13
`
`In terms of the manufacturers of LCDs, this has also changed. Most
`Japanese makers left the low-end watch and calculator merchant display
`business to other Far Eastern manufacturers, focusing instead on instru(cid:173)
`ment, auto dashboard, and large-area, high information content displays
`for computers and consumer products. ln North Amerir.a and Western Eu(cid:173)
`rope, a small group of manufacturers focuses on sper.ial types of displays
`for large-scale message displays, military systems, and custom designs.
`Nevertheless. there are still more than 50 companies throughout the world
`manufacturing and/or developing LCDs.
`
`1.2.6 OTHER EMISSIVE AND PASSIVE
`TECHNOLOGIES
`
`The above discussion has focused on the display technologies that have
`become the most successful in penetrating the market. HO\'vever, through
`the years a variety of light-emissive and light-reflective (passive) display
`technologies have appeared. Many have come and gone while others still
`remain. Still others are derivative technologies or "subtechnologies" of
`the six described above. In addition, new concepts arc continually being
`announced. Display technologies that are not subtechnologies of the major
`six include electrochromic displays. electrophorctic imaging displays,
`gas-electron-phosphor, cold cathode field emission array, incandescent
`displays, magnetic rotating spheres, electrical rotating spheres, pumped
`cavity display, ferroelectric ceramic displays, rotatable dipole displays,
`and liquid cells. More detailed descriptions of some of the most important
`of these technologies appear in Chapter 9.
`
`References
`
`1. "Fl<tt Panel Displays and CRTs," Lawrence E. Tannas. Jr., ed. Van Nostrand
`Reinhold Co., l'\ew York. 1985, page 1.
`2. Crookes, W. ·'Philosophical Translations, Part I." 1979.
`3. Shiers, G. Scienlifir. American. 230 (3). 92 (1974).
`4. "Radar Electronic Fundamentals." NAVS!IlPS 900,016, Bureau of Ships, U.S.
`Navy Department, 1944, page 4.
`5. Lyons, E. "David Sarnoff." Harper & Row, New York, 1966.
`6. "Trinitron Graffiti 1968-1988." Sony Corporation literature, 1989.
`7. Holonyak, N., and Bevacqua. Coherent (visible) light emission from a gallium
`phosphide 1unction. Applied Physics Letters. 1, 82 (1962).
`8. "flat Panel Displays and CRTs." Lawr1mce E. Tannas, Jr., ed. Van Nostrand
`Reinhold Company, New York, 19A5, page 289.
`9. "Flat Panel Displays and CRTs." Lawrence E. Tannas, Jr., ed. Van Nostrand
`Reinhold Company. New York, 19A5. page 335.
`
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`Th
`of
`
`14
`
`1. Historical Development of Electronic Displays
`
`10. Bitzer, D. L. and Slottow, H. G., The plasma display parnil-A digitally ad(cid:173)
`dressed display with inhen~nt memory. Presented at the Fall Joint Computer
`Conference, Washington, DC, 1966; AFIPS Conference Pror.., 29, 541 (1906).
`11. Nolan, ). F., "Gas Discharge Display Panel." 1969 lnternational Eler.tron Device
`Meeting, Washington. DC; J3aker, T.C., et al. U.S. Patent 3,499,167.
`12. Holz, G. E., The primed gas discharge cell. Proceedings of the SID, 13, 2
`(1972).
`13. Umeda, S., and Hirose, T. SID International Symposium. S<1n Francisco, Cali(cid:173)
`fornia (1972). Digest of Papers, page 38.
`14. Weber, L. F., and Younce, R. C. "Independent Sustain and Address Technique
`for the AC Plasma Display Panel." SID International Symposium, San Diego,
`CHlifornia (1986), Digest of Tedrnir.al Papers, page 220.
`15. Hampel, H. ). U.S. Patent 2,874,320 (1954).
`16. Maloney, T. C. IEEE Conference on Display Devices, 1972, Digest of Papers,
`page 19.
`17. Destriau, G. Journal de Chimie Physique et de Physico-Chimie Biologiques
`33, 1936, page 587.
`18. "Flat Panel Displays and CRTs." LHwrence E. Tannas. Jr .. ed. Von Nostrand
`Reinhold Company, New York, 1985, page 240.
`19. Inoguchi, T., Mito, S., ct al. SID International Symposium, San Diego, Califor(cid:173)
`ni<1, 1974, Digest of Technical Papers, page 84.
`20. Kasano, K., Masuda, M., Shimojo, T., and Kiyozumi, K. Proceedings of the
`SID, 21 (2), 107 (1980); Kiyozumi, K., ct al. SID International Symposium,
`1976, Digesl of Technk<1l Papers, page 130 and references therein.
`21. Mauguin, C. Bull. Soc. fr. Min. 34, 71 (1911).
`22. Marconi WirelP.ss Telegraph Company, British Patent 441,274 (1936).
`23. Gray, C.W. "Molecular Structure and the Properties of Liquid Crysl<1ls." Aca(cid:173)
`demic Press, New York, 1962.
`24. Williams, R. }. Chem Phys., 39, 384 (1963); Williams, R., and Heilmeier, G. H.
`J. Chem Phys. 44, 638 (1966}.
`25. Heilmeier, G. H., Barton, L. A., and Zanoni, L. A. Appl. Phys. Lett., 13, 46
`(1968); Proc. IEEE, 56, 1162 (1968).
`26. Heilmeier, G. H., C<1stellano, J. A., and Zanoni, L. A., Mul. Cryst. and Liq.
`Cryst., 8, 293 (1969).
`27. Heilmeier, G. H. and Coldmacher, J.E. Proc. IEEE, 57, 34 (1969).
`28. Goldmacher, ]. E., and Castellano, J. A. U.S. Patent 3,540,796 (1970), applied
`for June 9, 1966; Castellano, J.A. U.S. Patent 3,597,044 (1971), applied for
`September 1, 1968.
`29. Gray, G. W.. Harrison, K. )., and Nash, J. A. Electronic Lelters 9, 130 (1973).
`30. Kapustin, A. P., and Larinova, L. S., Soviet Phys. Cryst., 9, 235 (1965).
`31. Elliott, G., and Gibson, J. G. Nature, 205, 995 (1965).
`32. Wysocki, J., Adams, J., and Haas, W. Phys. Rev. Lett. 20, 1024 (1968).
`33. Fergason, J. L. U.S. Patent 3,731,986 (1973).
`34. Schadt, M., and Helfrich, W. Appl. Phys. Lett. 18, 127 (1971).
`35. Brown Boveri Company and F. Hoffmann La Roche & Co. British Patent
`1,372,868 (1975), applied for November 18, 1971.
`
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`

`CHAPTER
`
`Liquid Crystal Displays
`
`8 .1 Technology Fundamentals
`and Trends
`
`There are many ways to make displays using liquid crystal materials.
`However, only a select group of liquid crystal electro-optic effects are now
`being used in multiplexed displays or being developf!d for future displays.
`Among these <ire the twisted-nematic, field effect (TN-FE), the electroni(cid:173)
`cally controlled birefringence (ECB) effect. the supertwisted birefringent
`effect (SBE). the modulated twisted-nematic effect (MTN), the optical
`mode interforenrC' effect (OMI), and tho surface-stabilized ferroelectric
`liquid crystal effect (SSFLC). The TN-FE dis plays are the oldest, largest,
`and lowest-cost liquid crystal displays available. They also df!monstrate
`the poorest visual performance of the LC technologies when used in a
`highly multiplexed mode. SBE displays offer a substantial improvement
`in contrast and viewing angle. Supcrtwistod-nematic (STN) displays arc
`more recent entrants to the display market and arc rapidly replacing TN
`displays in large area. high information content display applications.
`These arc now made in double-layer versions (DSTN), compensated film
`types (FSTN). and triple-layer models (TSTN). Fcrroclcctric-smectic
`(SSFLC) displays are the subject of significant research activity. but no
`commercial products are available because of the difficulties encountered
`in manufacturing.
`On the following pages, the major liquid crystal display effects or
`"subtechnologies" will be described. Figurn 8.1 graphically depicts the
`breakdown of tho many LCD subtechnologies.
`
`181
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`

`182
`
`8. Liquid Crystal Displays
`
`8.1 Techl
`
`Liquid Cry1ul Dhpleys
`
`NeMatic
`
`II ·stable
`
`Smect ic A
`
`Thermal+electric
`Electro-conic
`
`Sino<:tic C
`
`Ferroelectric SSFLC
`Guest Host
`
`Direct
`
`Multiplexed
`
`Active Mltrlx
`
`2 Tenainal Devices
`
`Twisted-Nematic FE
`Guest Host
`Dynamic Sactter i ng
`Modulated TN
`Polymer Dispersed
`
`Standard TNFE
`
`Supertwisted
`
`STN
`DSTN
`FTN
`TSTN
`
`EC&
`
`CJU
`
`.Morpheus Si
`
`poly SI
`
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`
`Threshold erilanced
`
`MIN
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`
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`
`Bulk (HOS)
`
`Non·si I Icon
`
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`Ce
`Te
`
`PlaSN Addressed
`
`Figure 8 .1 The LCD subtechnologies.
`
`8.1.1 MULTIPLEXED TN-FE DISPLAY
`
`Tho first liquid crystal displays used for watches and clocks were ad(cid:173)
`dressed by connecting a driver circuit to each segment of each character.1
`This technique worked fine for small displays with less than several
`dozen addressable segments, but when higher information content dot
`matrix displays arrived, another method was required to reduce the num(cid:173)
`ber of drivers and the number of connections required as well as to sim-
`
`plify the
`nique wa
`a matrix
`on the li
`response
`rials did
`Var
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`are positi
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`themsek
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`which an
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`

`:iays
`
`8. 1 Technology Fundamentals and Trends
`
`plify the construction of the display pattern. The direct multiplexing tech(cid:173)
`nique was developed to allow the addressing of large numbers of pixels in
`a matrix format. However, the multiplexing process makes great demands
`on the liquid crystal material. It must respond quickly and have a sharp
`response threshold in order to produce a high quality image. Early mate(cid:173)
`rials did not possess these properties.
`Various liquid crystal compounds and mixtures have since been re(cid:173)
`fined in order to overcome the problems that impeded development of
`good quality, highly multiplexed displays. The primary effect of interest
`in liquid crystal displays is the interaction of the liquid crystal with an
`electric field. Nematic liquid crystals (of which there are hundreds of com(cid:173)
`positions) consist of elongated molecules held together at their ends by
`di polar (Van der Waals) forces. Microscopically, liquid crystal molecu lcs
`would look very much like polymer molecules. Physically, however, the
`molecular "chains" can slide past each other readily, resulting in the
`property of liquidity. Thus, liquid crystals have a much lower viscosity
`than conventional polymers (polymers can be liquid crystalline as well).
`As a result of microscopic structure, liquid crystals have the unique
`property of "cooperative alignment"; that is, the direction of alignment of
`one molecule influences the alignment of the others in its vicinity.i This
`cooperative alignment feature gives the material the optical properties of
`a crystal. Specifically, the optical properties are different when measured
`in the direction parallel to the optical axis from the properties measured
`perpendicular lo the optical axis. This difference between the indices of
`refraction in the parallel direction (n ) and the perpendicular direction
`(n.J is known as the optical anisotropy (delta n).
`Another important property is the dielectric anisotropy (delta c).
`Under the influence of an electric field. liquid crystals will align them(cid:173)
`selves in a direction determined by the sign of the dielectric anisotropy or
`the material. In other words, when the dielectric constant parallel to the
`long axis of the molecule (e) is greater than that in the perpendicular
`direction ( n .i) the dielectric anisotropy is positive and the molecules align
`themselves parallel to the electric field. Conversely, when the dielectric
`constant parallel to the long axis of the molecule (n 1 ) is less than that in
`Lhe perpendicular direction (e) the dielectric anisotropy is negative and
`the molecules align themselves perpendicularly (or at some angle) to the
`electric field. Liquid crystal materials used in the twisted-nematic effect
`are positive with the dielectric anisotropy of the order of 5 to 12.
`Changes in alignment under electric field excitation also change the
`optical characteristics of the material, making the display of information
`possible. Without an electric field, the liquid crystal molecules align
`themselves in a direction determined by the orientation characteristics of
`the surface to which Lhey are applied. In a typical twisted-nematic dis(cid:173)
`play, the inside surfaces of the display are treated with an alignment agent
`which anchors the liquid crystal molecules such that their long axes are
`parallel to the direction of orientation. This orientation, which involves a
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`184
`
`8 . Liquid Crystal Displays
`
`complex mechanism still not completely understood,:1 is achieved by rub(cid:173)
`bing or buffing the alignment layer (typically a polymer) coating in a spe(cid:173)
`r.ific direction. One theory, favored by the authors, states that the rubbing
`reorients the polymer chains with their long axes in the direction of the
`rubbing. It is well known that rubbing or buffing polymer materials causes
`the material lo moll and recrystallize in an oriented direction. The liquid
`crystal molecular "chains" would then align themselves in the same di(cid:173)
`rection as the polymer chains, l>eing held there l>y dipolar forces. Another
`theory assumes that lhe rubbing produces grooves into which the liquid
`crystal molecules fall. However, in many cases the alignment occurs even
`when no grooves can be seen wilh an electron microscope.
`In a conventional twisted-nematic LCD,4 the orientation on the up(cid:173)
`per plate is at an angle of 90 degrees to that on the lower plate. Because of
`the cooperative alignment characteristics of liquid crystals mentioned
`above, the molecular chains form a uniform twist from one surface to the
`olher. In order to view the electro-optic effect, polarizers are laminated
`to lhe outside surfaces of the cell with the front polarization direction
`at 90 degrees to the rear polarization direction. With no voltage applied
`(lhe OFF state), polarized light entering the front of the cell follows the
`direction of the twist and undergoes a 90° rotation as it exits the cell
`(Figure 8.2). This rotation enables the polarized light to pass through
`the rear polarizer unchanged. Wilh an applied vollagc (the ON state), the
`
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`Figure 8.2 C:onventional twisted-nematic field effect LCD operating principles.
`
`8.1 Tee
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`8.1 Technology Fundamentals and Trends
`
`185
`
`liquid crystal molecules are oriented parallel to the electric field because
`the energy of the field destroys the twisted structure. In this case, polar(cid:173)
`ized light entering the cell is not rotated and is absorbed nearly com(cid:173)
`pletely by the rear polarizer. Thus, the ON state is "black" while the OFF
`slate is clear. Importantly, however, the liquid crystal molecules attached
`to the surface arc unaffected by the electric field so that when the field is
`turned off, the twist structure is perfectly restored.
`An actual high information content display consists of rows and col(cid:173)
`umns of electrodes connected lo drivers that supply voltage. In operation,
`the display is scanned row by row from top to bottom at 60 to 100 Hz. The
`liquid crystal reacts to the average of the voltage over time instead of to
`each individual frame scan. When the proper voltage difference is gener(cid:173)
`ated across the row and column, the intersection is "selected." This is
`where the shortcomings in multiplexed displays appear. Each electrode
`(row or column) supplies the voltage required to select an element for a
`short period of time. But the nonselected elements also receive some frac(cid:173)
`tion of lhe voltage. Thus, the liquid crystal molecules in nonselected ele(cid:173)
`ments are partially oriented, thereby reducing the contrast between the
`OFF and ON elements of the display. Much of today's research in high
`information content LCDs is aimed at reducing or eliminating this so(cid:173)
`callcd "crosstalk."
`LCDs could not be used widely in portable computer applications
`until the display quality was improved substantially. Two of the prime
`features, low power and compactness, overshadowed the poor appear(cid:173)
`ance, and LCDs showed up in a few products that made a brief debut in
`the early markets for portable products. But word processors and portable
`computers suffered from stagnant growth rates until truly good displays
`arrived. The first of these enhanced displays was called supertwisted
`LCDs, and the next wave was the active matrix addressed LCDs.
`Other types of liquid crystal displays, described below, incorporate
`dyes in their composition and can present color. Mullilayered dichroics
`have demonstrated multicolor capability, but not in high information con(cid:173)
`tent displays as are required for information processing equipment.
`
`8.1.2 COLOR MULTIPLEXED TN-FE LCD
`
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`Due to the limited viewing angle and generally poor contrast ratio of mul-
`tiplexed LCDs, color is not usually purs