K.J. Pretech Ex. 1015
`
`Pretech_000621
`
`

`
`This book is printed on acid—free paper. @
`
`Copyright © 1992 by ACADEMIC PRESS, INC.
`All Rights Reserved.
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`Urtixed Kingdom Edition published by
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`Library of Congres.-s Cataloging—ir1—}’ub|icalion Dam
`
`C2l'$1t‘.1|:JrllI. Jnsepll A.
`Hrindhook of display leclmology 1' Joseph A. Castcllano.
`p.
`cm.
`Includes bibliographical references and index.
`ISBN 0-] 2—1fi342(J—5
`I. lnfm-matimi display systenls.
`TK7B82.l6C3T
`1992
`621 .38 I '542——dc:20
`
`1. Title.
`
`l‘R|l\‘ [T-.l.) 1N TIIE UNIT}.-'.l'} .'STATES OF AMERICA
`929194959697 0W9BT65432l.
`
`Pretech_000622
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`

`
`1.2 Flat Panel Displays
`
`1.2.4 VACUUM FLUORESCENT DISPLAY {VFD}
`
`The lirst vacuum fluorescent displays [‘v'FDs} were single-digit display
`tubes developed by Dr. T. Nakamura of Ise Electronics Corporation in
`1967.2“ 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 sealed in a glass bulb. Later, NEC Cor-
`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 NEG a strong second an_d Ise third. Samsung
`Electron Devices [Suwon, Korea} makes VF Us mainly for use in the firm's
`microwave ovens and VCRs.
`
`In addition to the desire to produce a Hat, thin light emitting display
`that could be operated at low voltage, another reason the VFD was devel-
`oped was a rather practical one. By the mid-19605, vacuum tube produc-
`tion had become a high—volume, automated process. Unfortunately. by this
`time vacuum tubes were rapidly being replaced by solid-state compo-
`nents. The VFD was seen as a new product that could he 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-
`essary to shut down a plant and lay off all the workers if one can be cre-
`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“ discovered a.nd described the
`twisted-nematic structure that later became the basis for liquid crystal dis-
`play [LCD} technology. During the 1920s and ‘[9303 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-
`ent on a light valve device that used liquid crystals was awarded to the
`Marconi Wireless Telegraph company (now part of [EEG] in 1936.“ Then
`in the mid-19505. researchers at the Westinghouse Research Laboratories
`discovered that cholesteric liquid crystals could be used as temperature
`sensors. It was not until the 19605, 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
`
`Pretech_000623
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`
`'|. Historical Development of Electronic Displays
`
`Dr. George W. Grays "Molecular Structure and the Properties of Liquid
`Crystals" appeared in 1962.“ This excellent book quickly became the de-
`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 Iapan. During this
`period, a great deal of research and development was performed; theories
`were formulated a11d tested, a number of electro-optic effects were discov-
`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 Drs. Richard Williams and George
`Heilmeier at the David Sarnoff Research Center [then the central research
`arm of RCA Corporation] in Princeton. New Jersey.“ Later, a larger group,
`headed by Heilmeier and including Louis Zanoni. ]oel Goltlmacher. Lu-
`cian Barton, and the author, spearheaded the work to develop liquid crys-
`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-
`cialized, including dynamic scattering,” dichroic dye LCDs,2'3 and phase-
`chauge displays.“ One of the major hreakthroughs occurred in the sum-
`mer of 1965 when it was discovered that by mixing various pure nematic
`liquid crystalline compounds together it was possible, for the lirst time.
`to produce stable, homogeneous liquid crystal solutions that could oper~
`ate over a broad temperature range including ordinary room temperature.2*‘
`Later, cya rrobiphenyl materials with improved properties and even broader
`temperature ranges were developed?” 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 “'1 and by
`George Elliott and I. G. Gibson at Marconi Electric in England.“ Later, a
`group that included Ioseph Wysocki, James Adams, and Werner Haas at
`Xerox also carried out extensive liquid crystal display research.”
`By 1969, it became clear to the RCA group and others that the devel-
`opment ol large-screen, LCD television sets would require “many years of
`research,” although nobody believed it would take 16 years. Thus. an ef-
`fort was mounted to develop simpler display devices that could be com-
`mercialized quickly. One of these was the “point-of-purchase” display, a
`moving advertisement display used in retail stores. These segmented dis-
`plays (produced by RCA and Ashley—Butler in the early 1970s} were 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 to the appropriate
`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.
`
`Pretech_000624
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`
`‘I 2 Flat Panel Displays
`
`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 nuni.eric indicators for instrurnents. digital clocks. digi-
`tal wrist watches. optically tuned color filters using the so-called “guest-
`host" effect, electronically controlled "window-shades," and “displays for
`auto dashboards. aircraft cockpits. scoreboards. highway signs. and com-
`puters." Today. we see LCDS in virtually all of these applications.
`One of the most
`important major breakthroughs occurred in late
`1969 when Iames L. Fergason, working at a newly formed firm, Interna-
`tional Liquid Crystal Company ULIXCO] in Kent. Ohio, discovered the
`twistedmematic {TN} tield-effect LCD. which ultimately proved to be the
`most Successful for the watch, calculator, and later, other applications in-
`cluding TV. Because Mr. Fergason's patent application was not made pub-
`lic until several years later,“ Drs. Wolfgang Helfrich and Martin Schadt of
`F. I-Ioffmann LaRoche in Basel. Switzerlaml. published a paper on the
`same effect in 197134 and were awarded a patent in 1975.“ Needless to .-
`say. this sparked a long legal battle over ownership of the invention. Even—
`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-
`mously and many companies in the United States, Europe, and ]apan be-
`gan to exploit the development of the 19608. The coincident development
`of large—scale integrated circuits for driving and timekeepiiig functions
`resulted in the development of the LCD wrist watch and calculator. The
`early 1970s also saw a number of new American companies formed to
`exploit LCD technology. Among these were ILIXCO, Optel Corporation
`and i-‘rinceton Materials Science {Princeton, New }ersey], Microrna [Cu-
`pertino. California), Micro Display Systems {Dallas}, and Integrated Dis-
`play Systems ['Montgorneryvillc, 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-
`ment used was crude by today's standards, the same fundamental tech-
`niques are now being used to manufacture the hundreds of millions of
`LCDS made each year throughout the world.
`During these early years, many Iapanese firms followed and copied
`the developments coming 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-
`
`Pretech_000625
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`
`References
`
`in te
`
`Japanese
`business E
`
`for large- ’
`Neverthel
`nianufact
`
`The above
`
`displays.
`cavity dis
`and liquid
`of these i
`
`1 .
`
`H!St0{‘lCEIl Development of Electronic Displays
`
`factoring 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-
`tively high-voltage requirement (at least for the CMOS devices made at
`that time} and viewing angle restrictions created by the need for a specular
`{mirror} 1‘el"lecting back electrode. It was soon replaced by the twisted-
`nematic, field-effect [TN-LCD} display. and the LCD watch began to gain
`momentum in 1976. Compact, attractive LCD calculators and watches
`made in Iapan soon became household items.
`Today, manufacturing techniques and equipment are readily avail-
`able. and highly reliable, low-cost liquid crystal displays are being made
`by the hundreds of millions. primarily in Iapan and the Far East. These
`displays are. for the most part. driven by a low level of multiplexing (30
`to 50% duty 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-
`tion and portability a reality. The United States lost its leadership position
`in LCD technology because many firms were convinced that the LCD did
`not have adequate “hrightness” or contrast to meet the needs oi equipment
`makers. However, the Japanese firms believed that only a passive display
`technology such as LCD could provide the combined characteristics of
`low power and viewahility 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 the 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 digitalfana-
`logs are popular in both Eastern and Western Europe. I_.CDs are also be-
`coming more widely used in other consumer and industrial electronic
`products throughout Europe.
`Today we see more and more industrial and consumer products us-
`ing liquid crystal displays. LCDS now appear in automobile dashboards.
`aircraft cockpit displays, telephones, microcomputers, word processing
`systems. gaming machines, hand-held 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-
`tronics devices to incorporate LCDs in their equipment, particularly those
`that are portable.
`
`Pretech_000626
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`
`References
`
`111 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-
`ment, auto dashboard, and large-area. high information content displays
`for computers and consumer products. In North America and Western Eu-
`rope. a small group of manufacturers focuses on special types of displays
`for large-scale message displays. military systems. and custom designs.
`Nevertheless, there are still more than 5U cornpanies throughout the world
`manufacturing andfor developing LCDs.
`
`1.2.6 OTHER EM_lSSlVI£ AND PASSIVE
`TECHNOLOGIES
`
`The above discussion has focused on the display technologies that have
`become the most successful in penetrating the market. However. through
`the years a variety of light-emissive a11d 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 are continually being
`announced. Display technologies that are not subtoohnologies of the major
`six include electrochromio displays. electrophoretic imaging displays,
`gasrlectron-phosphor, cold cathode field emission array, incandescent
`displays, magnetic rotating spheres. electrical rotating spheres, pumped
`cavity display. lerroelectric 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
`
`. “Flat Panel Displays and CRTs." Lawrence E. Tannas. ]r.. ed. Van Nostrand
`Reinhold Co.. New York, 1985. page “I.
`. Crookes. W. “Philosophical Translations. Part I." 1979.
`.. Shiors. G. Scientific American. 23ll[3].92[1El74].
`. "Radar Electronic F‘unda1nentals." NAVSIIIPS 900,016. Bureau of Ships. U.S.
`Navy Department. 1944, page 4.
`. Lyons. E. “David Sarnoff." Harper 8: Row. New York, 1966.
`. “'l‘rinitron Graffiti 1‘J5B—I9Bfl." Sony Corporation literature. 1989.
`. Holnnyak, N.. and Bevacqua. Colierent [visihle] light emission from a gallium
`phosphide junction. Applied Pliysics Letters. 1. 82 [1{~]t‘>2}.
`. “Flat Panel Displays and CRTs." Lawrence E. Tannas. ]r.. ed. Van Nostrand
`Reinhold Company. New York, 1985. page 289.
`. “Flat Panel Displays and LIRTS." Lawrence E. Tannas. Ir.. ed. Van Nostrand
`Reinhold Company. New York. 1985. page 335.
`
`Pretech_000627
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`

`
`1. HlSt0f‘|CE|l Development of Electronic Displays
`
`. Bitzer, D. L. and Slottnw, H. G., The plasma display paneI—A Lli1.;italiy ad-
`dressed display with inherent rneinory. Presented at the Fall Ioinl Computer
`Conference. Washington. DC, 1966; AFIFS Conference Proc... 29. 541 ['l 965].
`. Nolan, I. t-‘., “Cas Discharge Display Panel." 1969 international ElECI|‘Dl1DH\.’l:;e
`Meeting. Washington. DC; Baker, T.(}.. et al. US. Patent 3,499,167.
`. H012. G.
`The primed gas discharge cell. Proceedings of the SID. 13. 2
`[19?2].
`and I-Iirrxse. T. SID International Symposium. San Francisco. Cali-
`'. Umeda,
`fornia E1972]. Digest of Papers, page 36.
`. Weber. L. F.. and Yonnce, R. C. “Int.Iependent Sustain and Address Technique
`for me AC Plasma Display Panel." STI) International Syniposinm, San Diego.
`California [1985]. Digest of Technical Papers, page 220.
`. I-Iampel, H. J. Us Patent 2374.320 [195-41.
`‘. Malone_v. T. C. IEEE Conference on Display Devices, ‘I972, Digest of Papers,
`page 19.
`. Destriau, G. Ieurnnl de Chimie Physique et do Ph}/sico-Claiinie Bioiogiquos
`33. 1936. page 537.
`. “Flat Panel Displays and CRTs." Lawrence E. Tannas, ]r., ed. Von Nostrand
`Reinhold Company, New York. 1985, page 240.
`. Inoguchi, T., Mito,
`et oi. SID International Symposium, San Diego. Califor-
`nia, 1974, Digest of Technical Papers, page 84.
`. Kasano, K.. Masada. M., Shimnjo, T., and Ki}-'07.ll[l'li, K. Proceedings of the
`SID. 2] [2]. 107 H5380}: Kiyozunii, K.. of al. SID International Symposium.
`1976, Digest of Technical Papers, page 130 and references therein.
`. Mauguin, C. Bull‘. Soc. fr. Min. 34, ?1 [1911].
`. Marconi Wireless Telegraph Company, British Patent -141.27-1 [I935].
`. Gray, CLW. "Molecular Strurztnre and the Properties of Liquid Crystals." Aca-
`demic Press, New York, "1962.
`. Williams. R. I. Chem Phys. 39. 334 (1963); Williams, I{., and I-Ieilmeier. G. H.
`J. Chem Phys. 44, 633 [was].
`. Heilmeier. G. H., Barton, L. A., and Zanoni, L. A. Appl. Phys. Lctt., 13, 46
`(1953): Proc. IEEE, 55, 11s2{19sa]_
`Heilmeier, G. H., Castellano, }. A., and Zanoni, L. A.. Mol. Cryst. and Liq.
`Cryst., B, 293 [19f‘JQ}.
`. Heilmeier, G. H. and Gnldmacher. I. E. Proc. IEEE, 57, 34 [1969].
`. Goldmacher. J. E., and (Iastellano. I. A. U.S. Patent 3.5-’lEI,?96 [1970]. applied
`for lune 9, 1955: Castellano, ].A. US. Patent 3,597,044 (1971), applied for
`September 1, 1955.
`. Gray. G. W.. Harrison. K. J., and Nash, I. A. Electronit; Letters 9, 130 E1973].
`. Kapustin, A. P., and Larinova, L. S., Soviet Phys. CI'ys1., 9, 235 [1EJti5].
`. Elliott. G., and Gibson. 1. G. Nature, 205, 995 {isms}.
`'. Wysocki, ]., Adams. I.. and Haas. W. Phys. Rev. Lett. 20. 1024 U968].
`'. Fergason, I. L. U.S. Patent 3.?31_.9Bti[1‘J73].
`. Schadt, M., and Helfrich, W. Appl. Phys. Lett. 18. ’127{1$J?1].
`. Brown Boveri Company and F. Hoffmann La Roche 8: Co. British Patent.
`fl,3?2,8tiB [19?5], applied for Novemher 1B,19.'r'1.
`
`'
`'
`
`.
`
`.
`
`Pretech_000628
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`

`
`lllliifiiilll
`
`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 developed for future displays.
`Among these are the twisted-netnatic, field effect [TN-FE], the electroni-
`cally controlled birefringence [ELIE] effect, the supertwisted birefringent
`effect [SBE],
`the modulated twisted-nematic effect (MTNJ, the optical
`mode interference effect [OMI], and the surface-stabilized ferroeloctric
`liquid crystal effect [SSFLC]. The TN-FE displays are the oldest, largest,
`and lowest-cost liquid crystal displays available. They also demonstrate
`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. Supertwistedmematic (STN) displays are
`more recent entrants to the display market and are rapidly replacing TN
`displays in large area, high information content display applications.
`These are now made in double-layer versions [DSTN], compensated film
`types (FSTNJ, and triple—layer models [TSTN]. Ferroelectric-sniectic
`[SSFl_.(J} 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. Figure 8.1 graphically depicts the
`breakdown of the many LCD subteclinologies.
`
`Pretech_000629
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`

`
`8.
`
`Liquid Crystal Displays
`
`Licuid crystal Displays
`
`Guest Host
`D\.rnnI11il: Sncttering
`Modulated IN
`Polymer Disp-ersrd
`
`2!! PIN
`lfiru
`lack-trhack
`
`Figure 8.1 The l..(_I|J suhtechnologies.
`
`8.1.1 MULTIPLEXED TN-FE DISPLAY
`
`The first liquid crystal displays used for watches and clocks were ad-
`dressed by connecling a driver circuit to each Segment of each character.‘
`This technique worked line 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-
`ber of drivers and the number of connections required as well as to sim-
`
`- plify the
`nique w
`a matrix
`
`respons
`rials did
`
`property
`one mo]
`cooperal
`a crystal.
`in the di
`
`perpend‘
`refractio
`
`constant
`
`the perp
`the mo]
`
`Pretech_000630
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`

`
`31 Technology Fundamentals and Trends
`
`plify the construction of the display pattern. The direct multiplexing tech-
`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-
`rials did not possess these properties.
`Various liquid crystal compounds and mixtures have since been re-
`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—
`positions) consist of elongated molecules held together at their ends by
`dipolar [Van der Waals] forces. Microscopically, liquid crystal molecules
`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.‘ 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 to the optical axis. This difference between the indices of
`refraction in the parallel direction [n,,] and the perpendicular direction
`tnl] is known as the optical anisotropy [delta :1).
`Another iniportant property is the dielectric anisotropy [delta e].
`Under the influence of an electric field. liquid crystals will align them-
`selves in a direction determined by the sign of the dielectric anisotropy of
`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 [nyj the dielectric anisotropy is pnsitive and the molecules align
`themselves parallel to the electric field. Conversely. when the dielectric
`constant parallel to the long axis of the molecule [n___) is less than that in
`the perpendicular direction [en] 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 twiste.d—ne1natic 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 they are applied. In a typical twisted-nernatic dis-
`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
`
`Pretech_000631
`
`

`
`pletely
`state is
`
`El. Liquid Crystal Displays
`
`complex mechanism still not completely understood,“ is achieved by rub-
`bing or butting the alignment layer {typically a polymer] coating in a spa;
`cific 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 bulfing polymer materials causes
`the material to melt and recrystallize in an oriented direction. The liquid
`crystal molecular “(:hain5" would then align themselves in the same di-
`rection as the polymer chains, being held there by dipolar forces. Another
`theory assumes that the rubbing produces grooves into which the liquid
`crystal molecules fall. However, in many cases the alignment occurs even
`when no grooves can be seen with an electron microscope.
`In a conventional twisted-nematic LCD,‘ the orientation on the up-
`per plate is at an angle of 90 degrees to that on the lower plate. Because oi
`the cooperative alignment characteristics of liquid crystals mentioned
`above, the molecular chains form a uniform twist from one surface to the
`Other. In order to view the electro-optic eifect, pelarizers are laminated
`to the outside surfaces of the cell with the front polarization direction
`at 90 degrees to the rear polarization direction. With no voltage applied
`[the 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 polarizor unchanged. With an applied voltage [the ON state]. the
`
`,4
`
`,1»
`
`s
`
`
`
`Ty Foam Lmanllounman
`tc-u-edumu mgacuumn
`FIELD OFF
`'WHITE'
`
`FIELD ON
`"BLACK"
`
`Figure 8.2 Conventional twistedvnernatic field effect LCD operating principles.
`
`Pretech_000632
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`

`
`B/I
`
`Technology Fundamentals and Trends
`
`liquid crystal molecules are oriented parallel to the electric field because
`the energy of the field destroys the twisted structure. In this case, polar-
`ized light entering the cell is not rotated and is absorbed nearly com-
`pletely by the rear polarizer. Thus, the ON state is ‘‘black'’ white the OFF
`state is clear. Importantly. however, the liquid crystal molecules attached
`to the surface are 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-
`umns of electrodes connected to 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-
`ated across the row and column, the intersection is “selected.” This is
`where the shortcomings i11 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-
`tion ol the voltage. Thus, the liquid crystal molecules in no11selected ele-
`ments are partially oriented, thereby reducing the contrast between the
`OFF and UN elements of the display. Much of today's research in high
`information content LCDs is aimed at reducing or eliminating this so-
`called "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-
`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 i_.(".Ds.
`Other types of liquid crystal displays, described below. incorporate
`dyes in their composition and can present color. Multilayered dichroics
`have demonstrated multicolor capability, but not in high information con-
`tent displays as are required for information processing equipment.
`
`8.1.2 COLOR MULTIPLEXED TN-FE LCD
`
`Due to the limited viewing angle and generally poor contrast ratio of mul-
`tiplexed LCDS. color is not usually pursued for products aimed at the
`business or professional market. which demands a much higher perfor-
`mance level than can be supplied by multiplexed LCDS. The standard
`technique for developing color is to use a filter layer inside the cell. The
`filter must be used in conjunction with a backlight, and the baeklight must
`be white or it must at least have the desired components of the spectrum.
`A powerful backlight must be used due to the low light transmission of
`the color filters.
`
`Some hand-held games made with multiplexed I.CDs have incorpo-
`
`Pretech_000633
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`

`
`8.
`
`Liquid Crystal Displays
`
`rated filter technology. This low-end consumer application is not reliant
`on high performance, so a limited color display is acceptable to a degree.
`
`8.1.3 ACTIVE MATRIX DISPLAYS
`
`Active matrix addressing is a technique for enhancing the addressing and
`writing of dot matrix displays. As mentioned above. it is now possible to
`address well over 200 individual rows of pixels by using multiplexing
`techniques. Multiplexing uses the timing of the signals to select and write
`a particular line of the display. As more and more lines are written, the
`amount of time the cont.roller can spend writing to each individual line
`[the duty cycle} decreases. Eventually, the molecules of liquid crystal do
`not have time to react fully to the applied voltage. and contrast dimin-
`ishes. When the addressing function in the display is separated from the
`process of writing, then each line can be written quickly, it can maintain
`its image, and the next line can then be written. This separation of ad-
`dressing and writing has been attempted by several methods. A dual input
`method where two frequencies, two volt.ages, or two different types of en-
`ergy, such as thermal and electrical, have been tried? These attempts usu-
`ally have some drawback such as slow speed, high power. or complex
`circuitry.
`The technique of active matrix addressing makes the display hard-
`ware more complex by adding a switch to each pixel. The switch can be
`turned on very rapidly [in a few microseconds] and a storage capacitor
`can then be used to maintain its condition while the other lines are being
`written. Several approaches to making individual switches have been in-
`vestigated. These include diodes, varistors, transistors, and various com-
`)hinations thereof. Not only are many different devices available, but there
`are many difiereni materials from which to make the devices.
`".- ‘-.
`The thin-film transistor [TFT] approach has emerged as the most
`successful technique for active matrix addressing in terms of the displays
`performances *3 The structure that has emerged as the most suitable lnr
`video displays is a transistor and. sometimes, a capacitor located at each
`pixel [Figure 8.3]. The capacitor is used to maintain the voltage on the
`gate of the transistor. The drain of the transistor is connected to one of the
`pixel electrodes. The source of the transistor is connected to a display
`driver. The three colors. red, green, and blue, are developed by incorpo-
`rating organic filters into the cell and back-lighting the display.
`The choice of materials to be used for the TFT array has always been
`the subject of lively debate. One of the original goals of the active matrix
`effnrt was to “integrate” the driver circuitry consisting of shift registers,
`latches, and drivers directly onto the display substrate. This would reduce
`costs by eliminating the