the principles of lcd technology

8/13/2019 The Principles of LCD Technology

1/56

Clique for Applied Research in Electronic Technology, Advaita Corporation

Pragnan Chakravorty, Director, CARET, Advaita Corporation

The Principles of LCD Technology

The parallel arrangement of liquid crystal moleculesalong groovesWhen coming into contact with grooved surface in a fixed direction,liquid crystal molecules line up parallelly along the grooves.Natural state

Molecules arearranged in aloosely orderedfashion with theirlong axesparallel.

When cominginto contact witha finely groovedsurface(alignment layer).

Molecules lineup parallel alonggrooves.

When liquid crystals are sandwiched between upperand lower plates, they line-up with grooves pointing indirections 'a' and 'b,' respectively


2/56



The molecules along the upper plate point in

direction 'a' and those along the lower plate indirection 'b,' thus forcing the liquid crystals into atwisted structural arrangement./ (figure shows a 90-degree twist) (TN type liquid crystal)

Light travels through the spacing of the moleculararrangementThe light also "twists" as it passes through the twisted liquid crystals

Light passes through liquid crystals, following thedirection in which the molecules are arranged. When themolecule arrangement is twisted 90 degrees as shown inthe figure, the light also twists 90 degrees as it passesthrough the liquid crystals.

Light bends 90 degrees as it follows the twist of themolecules


3/56



Molecules rearrange themselves when voltage isappliedWhen voltage is applied to the liquid crystal structure, the twisted light passes straightthrough.

The molecules in liquid crystals are easilyrearranged by applying voltage or another external

force. When voltage is applied, moleculesrearrange themselves vertically (along with theelectric field) and light passes straight throughalong the arrangement of molecules.

Blocking light with two polarizing filtersWhen voltage is applied to a combination of two polarizing filters and twisted liquidcrystal, it becomes a LCD display.


4/56



Light passes when two polarizing filters are arranged with polarizingaxes as shown above, left.Light is blocked when two polarizing filters are arranged with polarizingaxes as shown above, right.TN type LCDsA combination of polarizing filters and twisted liquid crystal creates a liquid crystaldisplay.

When two When voltage is


5/56



polarizing filtersare arrangedalongperpendicular

polarizing axes,light enteringfrom above is re-directed 90degrees along thehelix arrangementof the liquidcrystal moleculesso that it passesthrough the lowerfilter.

applied, the liquidcrystal moleculesstraighten out oftheir helix pattern

and stop redirectingthe angle of the light,thereby preventinglight from passingthrough the lowerfilter.

This figure depicts the principle behind typical twisted nematic (TN)liquid crystal displays. In a TN type LCD, liquid crystals in which themolecules form a 90-degree twisted helix, are sandwiched between twopolarizing filters. When no voltage is applied, light passes; when voltageis applied, light is blocked and the screen appears black. In other words,the voltage acts as a trigger causing the liquid crystals to function likethe shutter of a camera.

Display SystemsDisplay principlesDisplaying letters, numbers and graphics are based on the followingthree display methods:


6/56



1. Segment system

Long display units are arranged to form a figure '8' to displaynumbers.

2. Dot matrix system (character display)

Display units are arranged in rows and columns to formcharacters.

3. Dot matrix system (graphics display)

Display units are arranged in rows and columns to depictgraphics.

The principle of color displaysA color display is made possible by placing color filters over eachdisplay unit. In dot matrix systems, red, green and blue dots areobtained through the use of filters for each of the three primary colorsred (R), green (G) and blue (B). A variety of colors can then beexpressed by combining them.Display configurationsThe light passing through the liquid crystals is merely natural or artificialambient light. The configuration of the display is categorized by therelative position of the light source. There are three types:


7/56



1. Transmissive type (LCD TV)2. Reflective type (LCD calculator, watch)3. Projection type (LCD projection)

LCD configurations

LCD Structure and Production MethodsNext is a brief description of the structure, liquid crystal materials andproduction process of a simple matrix LCD.

LCD structure


8/56



Sandwiched structureColor LCDs have a structure in which their components are formed intoa sandwich-like arrangement.1. Polarizing filter

This controls the light entering and leaving.2. Glass substrate

This stops the filtering of electricity from electrodes3. Transparent electrodes

These electrodes drive the LCD. A highly transparent material isused that will not interfere with the quality of the image's integrity.4. Alignment layer

Film is used to align the molecules in a fixed direction.5. Liquid crystals


9/56



6. SpacerMaintains a uniform space between the glass plates.

7. Color filterColor is expressed through the use of R, G and B filters.8. Backlighting

The display is lit from behind to make the screen brighter. In sometypes of monochrome LCDs, a mirror is used in place ofbacklighting so the display can be seen with ambient light.

The Evolution of LCDsLCD technology has come a long way in the lastseveral years, resulting in a steady stream of newproducts.


10/56



LCD Evolution

Improved display performanceThe first displays were segment types that could only display numbers,and then these were followed by dot-matrix systems capable ofdisplaying characters and graphics. Displays later evolved frommonochrome to color, from still images to moving images, and fromsmall to large screens.The evolution of three LCD technologies

1. Drive systems2. LCD types3. Peripheral technologies


11/56



The Evolution of Drive SystemsStatic drive system Dynamic drive system

An upper electrode isconnected to each displaysegment.

Example showing four upper electrodes and two lowerelectrodes. The display segment to be shown is selectedby segment to be shown is selected by combinations ofupper and lower electrodes.

From static to dynamic drivesA drive system 'drives' the LCD by applying voltage to specificelectrodes. In the early days of LCD development, segment systems


12/56



used a static drive system in which each segment was drivenseparately. The number of terminals required in this system increaseswith the number of display units, making it unsuitable for use with largescreens. The development of a dynamic drive made it possible to drivedisplays with fewer terminals.Simple and active matrix drivesToday, static drives are now rarely used. The most common dynamicdrive systems in use today are active matrix drives and simple matrixdrives. Active matrix drives are used for TVs and other moving pictureapplications, which require high picture quality and a fast response.Simple matrix drives are mostly used in calculators, word processors,personal computers and other still image applications.

Classification of dynamic drive systems


13/56



The Structure of Simple/Active MatrixDrive SystemsStructure of simple matrix drive systemStructure

Circuitry

The X electrodes are laid on the lower substrate of the liquid crystal cell, and the Yelectrodes are laid on the upper substrate.Electrical signals are applied to the X and Y conductors with the proper timing to selectthe target pixels.

The structure of active matrix drive systems (TFT)


14/56



Structure

Circuitry

In active matrix LCDs, switching transistors (TFTs) or diodes are attached to

each pixel to switch each one on or off. X and Y electrodes are formed on the

same substrate as TFT (or diode) arrays. The switching signals are applied to the

X electrodes. Video signals are then applied to the Y electrodes.

The Evolution of Different LCD Types

Major types of LCDsTN STN TSTN

Structure Twists nematiccrystals 90degrees

Twists nematic crystalabout 260 degrees(opposing twist directions)

Replaces DSTNcompensation cell withplastic film


15/56



Color Black/white Yellow-green/dark blue Black/white, multicolor

FeaturesLow power

consumptionThin,lightweight

Low cost

Large capacity display

Thin, lightweightLow power consumption

High contrast

Large capacitydisplay

Thin, lightweightLow powerconsumption

Color display

High contrast

Problems or

advantages Cannot handle alarge capacityBlack/white display notpossible (therefore, colordisplay not possible)

High contrast and highspeed

Main

applicationsCalculators,electronicorganizers

Word processors(monocolor)

Word processors,laptop computers

From TN to STN, TSTN, and FSTNThe very first types of LCDs were called DSM (dynamic scatteringmode), but TN (twisted nematic) has become the standard today.Almost all active matrix drive displays use TN type LCDs, and numeroustypes of active elements are being developed. The use of TN type LCDsin simple matrix drive displays causes the contrast to drop as thenumbers of scan lines of the image displayed is increased. Tocompensate for this, new types of LCDs are being researched anddeveloped. Advances in LCD R&D have already led to the developmentof STN (super twisted nematic) type LCDs, which offer high contrast,even on large screens; and TSTN (triple STN) and FSTN (film STN)LCDs, which feature a lightweight and thin body design that are optimalfor large black-and-white LCDs and precise color imaging whenequipped with a color filter.Conceptual diagrams of major LCD types


16/56



Twisted Nematic (TN) Type

Contrast tends to drop when used with large screens.

Molecules twisted 90 degrees.

Super Twisted Nematic (STN) Type

Good rise characteristic for a high contrast display

180 degrees to 260 degrees twist. Colored yellow-green and blue

Triple Super Twisted Nematic (TSTN) Type

A high polymer, double refraction film is used to create black-and-white LCDs of exceptional quality.The single-layer compensation film is called FSTN (Film SuperTwisted Nematic).

A compensation film is placed above and below the operating cell.

Classifying Active ElementsTFT structural configuration

TFT structure is composed of three electrodes--X,Y and Z--which act as switchingtransistors.


17/56



MIM structural configuration

This is structurally identical to the simple matrix system, but has only the two X and Yelectrodes. A diode with a metallic-insulated metal structure is sandwiched between thetwo terminals. The diode performs the switching function in place of the transistor, but it

is slow.

Active elementsThe following types of active elements are used in active matrixsystems: the three-terminal element, typified by thin-film transistors(TFTs) including amorphous Si-TFTs, high-temperature polycrystallineSi-TFTs, and low-temperature polycrystalline Si-TFTs; the two-terminalelement, of which metal-insulator-metal (MIM) is typical; and plasmaand address variations. The structure of the three-terminal elementoffers superior switching performance. TN is the most commonly usedLCD structure.Summary--comparing various types of LCDsBelow is a list of the differences between drive systems, LCD types andactive elements, together with their respective features. As can be seenfrom the table, an LCD with an active matrix system, TN type LCDs, andTFT LCDs offer the best overall performance.


18/56



System Structure Features Problems Applications

Simple

matrix driveX and Y electrodesare laid out in avertical andhorizontal lattice

Simpleproduction

Superior costadvantages

Picturequality drops

when thenumber ofwires isincreased

Halftoneresponsespeed

Primarily still

images such as inthe electronicorganizers, wordprocessors andpersonal computers

Active

matrix

drive

MIMAn insulator whichperforms switchingis sandwiched

between the X andY terminals

Picture quality is betweenthat of simple matrix and TFTactive matrix drive displays

Production ease and cost are

both between those of simplematrix and TFT active matrixdrive displays

Word processors,personal computers,

TVs

TFTA silicon thin-filmsemiconductorwhich performsswitching betweenthe X and Yterminals

High contrastand highpicture qualityregardless ofnumber ofconductors

Permitshalftone display

Superiorresponse

High cost

TVs, projectiondisplays and othermoving pictureapplications

Drive

system Type TerminalsDisplay performance

Response

(moving

picture

applications)Large

screenCDisplay

capacity

(high

resolution)Contrast Full

colorHalftoneViewingangle

Dutydrive

TN

STN

USTN

TSTN

Active TN TFT


19/56



matrix

drive MIM

Essential Peripheral LCD TechnologyThe impact that peripheral technologies have had on the evolution ofLCDs is to great to be ignored. The elemental technologies surroundingthe LCD play a critical role in actual cell assembly. Below are some ofthe essential technologies that help improve LCD performance.

Conceptual diagram of a TFT LCD panel


20/56



Color LCDs have a structure in which liquid crystals are sandwichedbetween the alignment layers, glass substrates, and polarizing filters.They are backlit in order to obtain the clearest possible pictures throughthe color filter. The gap between the glass substrates is several micronswide. To correctly maintain a uniform gap width, transparent spacers(beads) are required.Color filter technologyProgress is also being made in the production of RGB color filters for fullcolor displays. Putting RGB color filters in electrode substrates formscolor filter methods. Color filter methods have to be simple to make inorder to reproduce clear colors, the brightest white and when use of allRGB lights is required.

Color filter formation methods

Method name Description FeaturesResolution Color

reproduction ReliabilityGelatin dyeing methodPatterned resin is dyed

Pigment impregnation

methodResin containing pigmentsmade into a pattern

Printing methodColor ink containingpigment is printed on

No

Electroplating moleculesA resin coat is electroplatedon the pigment surface


21/56



LSI and mounting technologiesAs display capacity increases, the number of liquid crystal cell terminalsalso increases and LSI devices are required with pin numbers rangingfrom 80 to 100 or 160. Diversification in screen size and dotconfiguration likewise requires diversification in semiconductorelements. Higher cost performance, simpler system design and otherneeds must also be met. To solve these problems, advances must bemade in the mounting technology used to form the circuit substrate, aswell as in the LSI technology itself. One of these productiontechnologies, tape automated bonding (TAB), contributes to morecompact, thinner systems by permitting the tape mounting of LSIdevices with 100 to 160 film type terminals. A chip-on-glass (COG)method which mounts LSIs on LCD panel substrates is now underwayfor some products.Backlight technologySince LCDs themselves do not themselves emit light, an external lightsource is needed. In monocolor displays for calculators and otherdevices, a mirror is used to reflect ambient light, and in LCD TVs andother color displays, a backlight is used. A semi-transparent type oflighting, which integrates the features of both kinds of lighting sources,is being developed for use in vehicles. It uses a mirror when theambient light is sufficiently strong, and a backlight when it is not.Low reflection LCD technology


22/56



Conventional LCD displays could not show images clearly in light place.Low reflection LCDs are improved by 1) anti-reflection coating on thefront cover 2) incorporating low reflection rated materials to black matrixparts. The end result is brilliantly clear pictures projected because of thelow reflection to out coming light.

Structural view of a low reflective LCD

A new single-panel optical systemBecause dichroic mirrors perform color separation into red, green, andblue, no color filter is required. Adoption of an integrated flat micro lensfor gathering light makes it possible to obtain brighter, clearer high-density pictures.

Structural view of the new single-panel optical system


23/56



Defining the New LCD Age with NewLCD TechnologySuper High Aperture TFT LCDs (SuperHA)


24/56



An extraordinary level of super high picture aperture,quality and low power consumptionA screen with a high aperture of 81% (current conventional LCDaperture: 47%), high brightness, high contrast will be implemented, anda gradation of 256 levels (current number of levels: 64), which has beenregarded as almost impossible to obtain with LCDs will also beproduced. Furthermore, power consumption will be greatly reduced.These advances will allow the introduction of next-generation,multimedia-ready notebook PCs capable of handling moving picturesfrom TV broadcasts, CD-ROMs, DVDs and other video media products,offering exceptional viewing quality even in a brightly lit place, and willutilize a long-life battery. This technology will also contribute to theevolution of audio-visual equipment, car navigation and other visualsystems. Sharp's LCD technology breakthroughs:

1. Power consumption is greatly reduced by 57%*. (10.4-inchfixed intensity, 70 cd/m2 SVGA)2. Brightness levels are increased by 72% (compared with


25/56



current conventional models), and an 120 cd/m2 configurationwill be implemented. (10.4-inch SVGA with a backlight powerconsumption of 3.5 W)3. A contrast ratio 300:1 is achieved.

Compared with conventional Sharp products

Low-Temperature Polysilicon TFT LCDs(LPS)

System-on-Panel create the next generationproductsWithout LCD technology, the evolution of various products would havebeen impossible, especially in the areas of information equipment,audio-visual equipment, and consumer electronic products. In order toobtain an even higher level of LCD product performance and to furtherlessen costs, the development of new and innovative LCD technologiesis necessary. A new, encouraging technology is an LCD integrationdesign called System-on-Panel, which mounts a screen and itsperipheral circuits on the same substrate. The successfulimplementation of this technology will lead to a substantial savings in


26/56



costs by shortening the display manufacturing and inspection processeswhile maintaining a level of high reliability. System-on-Panel designs willhelp realize the implementation of high-density, multifunctional, compactLCD products, which until recently were thought to be an impossibility.As a method for implementing the System-on-Panel design, polysiliconthin film transistor (p-Si TFT) LCD technology, not the conventionalamorphous silicon thin film transistor (a-Si TFT), is showing the mostpromise. This System-on-Panel technology maximizes reliability andreduces display costs by replacing conventional high temperatureprocessing (at 1000 centigrade), with low temperature processing (500centigrade or lower), allowing mass production by using large glasssubstrates. (This will be applicable to 20-inch monitors in the future.) Inthe future, System-on-Process applications will be used in: super high-definition Hi-Vision Projectors, multimedia-ready ultra-compact PersonalInformation Tools, pocket-size audio-visual equipment, ultra-slim digitalcameras, etc.

Super Large Screen Direct-ViewingLCDs


27/56



A new joining technology for the world's largest LCDscreenSharp has developed a new LCD multipanel technology for creatinglarge screen LCDs, and trial production of the world's largest 28-inchdirect-viewing TFT LCD has begun. This technology makes the best useof technological resources including the currently used TFTtechnologies and equipment to develop large screen direct-viewingLCDs. A new joining technology has also been developed to createlarge, multipanel screens without conspicuous seams. Thisbreakthrough will make it possible for large screens to project powerfulimages, and expands the range of possible new LCD inventions in themultimedia age.Some of the features of this new technology include:

1 A uniform pixel pitch (880m) is evenly implemented throughoutthe seams.2. The high-precision glass cutting technology allows the joint


28/56



width to be reduced to 30 m or less, and the fine sealing line isreduced to a width of 150 m (which is one-fifth of theconventional sealing width) by using a new high polymer material.This polymer allows the joining distance, including the sealingwidth, to be within half of the pixel pitch (less than 440 m),making seams inconspicuous.3. A new adhesive having an optical refractivity near that of glass,with high transparency and low double refractivity is used forjoining two panels. This adhesive reduces scattering andreflection of light at the seams.4. The above-mentioned joining technology is being applied to 21-inch TFT LCDs, which has led to creation of the industry's first 28-inch LCD multipanel (using two 21-inch TFT LCDs).This multipanel technology is the most suitable for multitaskdisplay production for PCs, information boards, presentationboards and a host of other visual products.

6. Other Flat-Panel Displays


29/56



In addition to LCDs, other types of flat-panel displays are beingdeveloped. Below are descriptions of their operating principle, featuresand applications.Electro luminescent (EL) PanelsThese panels use zinc sulfide or other materials that emit fluorescencewhen voltage is applied to them. Powder electro luminescence hasbeen known since 1937, but it did not come into practical use untilaround 1981, when a thin-film EL panel was developed in whichfluorescent material was deposited on a glass substrate. These displayshave high contrast and are used in space shuttle displays as well asoffice equipment.The outstanding features of EL technologyEL displays use thin-film phosphors, which emit light when subjected toan electrical field. These displays offer the following advantages overLCD or PDP displays.

1. High resolution and large size (16-level gray scale, 1024 x 768dots)2. High contrast3. High response4. Wide viewing angle5. Lightweight, thin profile6. Low power consumption


30/56



EL display applicationsEL displays enjoy an ever widening range of applications, includingoffice automation equipment such as laptop computers and wordprocessors, displays in the space shuttle, delivery vehicle navigationterminals, train ticket vending machines, etc.Future evolutionEL displays are expected to find use in a widening range of applicationareas, but there is strong demand for multicolor or full-color versionsthat enable the display of multifaceted data. At the present stage ofdevelopment, EL displays have been created which can emit green, redor yellow light based on the use of filters and the development of newphosphor layering technologies using CVD (chemical vapor deposition)processes. As these EL displays approach a suitable level for practicaluse, we can anticipate the availability of high resolution, full-color ELdisplays in the near future. Recently, the development of material foruse in organic EL displays has proceeded very rapidly, and has becomepractical. Thin-film EL display

Thin-film EL display

Light-Emitting Diode (LED)


31/56



By joining two types of semiconductors, LEDs emit light when a currentis passed through them. These devices are capable of converting a flowof electrons into light. Due to their excellent brightness, they are oftenused on outdoor signs.

Vacuum Fluorescent Displays (VFD)This is a type of vacuum tube composed of positive, negative and grid-like electrodes. Positive electrodes and fluorescent material areseparated into display units on a glass substrate, and thermo electronsare emitted from separate negative electrode filaments in anotherlocation. These thermo electrons are accelerated by the grid locatedbetween them and impact on the target display unit, whereby it isilluminated.

Plasma Display Panels (PDP)This is a type of two-electrode vacuum tube, which operates on muchthe same principle as a household fluorescent light. An inert gas suchas argon or neon is injected between two glass plates on whichtransparent electrodes have been formed, and the glass is illuminatedby generating discharge. These have high contrast and are relativelyeasily applied to color displays. Development of 40-inch full color PDPshas developed so rapidly, that they can now be used for publicinformation signs or televisions.


32/56



Features and applications of various flat displayLCD LED EL VFD PDP

Type Non-lightemittingelements

Solidlightemittingelements

Solid lightemittingelements

Vacuumdischarge

Vacuumdischarge

Display

performance

Display

capacity

(sharpness)ContrastFull colorHalftones

Response (natural)Large screenBrightness (Backlighting)

Voltage/power

consumptionCost

Main applicationsCalculators,wordprocessors,personalcomputers,TVs, vehicledisplays

Outdoorsigns

Wordprocessors,personalcomputers,vehicledisplays

Cardashboards,audioequipment

Outdoorsigns,TVs

Field Emission Display (FED)


33/56



An FED projects pictures using the same light emitting principle asCRTs. An FED removes electrons from the cathode, and makes themcollide with fluorescent material applied to the cathode, thus emittinglight. While the cathode of a CRT uses a point electron source, an FEDuses a surface electron source. Six-inch color FED panels have alreadybeen manufactured, and research and development on 10-inch FEDs isproceeding very rapidly. When compared with TFT LCDs, FEDs offer asuperior viewing angle (160 degrees both vertically and horizontally)and are several microseconds quicker in response speed.Light emitting principle of an FED system

Digital Micro mirror Device (DMD)A DMD has a structure in which an SRAM chip is covered withaluminum mirrors measuring 16 m square. The two opposite corners ofeach mirror are supposed by columns, and rotate while keeping a }10degrees angle from the horizontal. The mirrors control the reflectingvolume of light from its source, projecting a picture on the screen.


34/56



Because a DMD operates completely under digital control, the picture isnot distorted by noise, and a seamless picture can be projected on thescreen. (DMD technology is only applicable to projectors) A DMDprojector consists of a DMD of 768 by 576 pixels, a Xe arc lamp, a colorfilter and a lens. The DMD chip is irradiated with light approximately2,000 lumens in brightness, and 640 by 480 pixel pictures are projected.Sixty 480-line pictures are projected on a 100-inch screen every 1/60 ofa second.

Configuration of a DMD projector

1. Evolution Toward the FutureCompared to cathode ray tubes (CRTs), LCDs offer the significantadvantages of greater compactness, lower operating power, and lowerpower consumption. Development to date has sought to improve


35/56



materials and drive systems while taking advantage of LCD features. Asa result, a growing number of LCDs are being used for the displayscreens of word processors and personal computers, and full-color LCDTVs. In the future, LCDs are expected to be used in larger screensoffering higher picture quality. As vacuum tubes were replaced bysemiconductors, LCDs are expected to start assuming all the roles thatCRTs are now fulfilling.Technological breakthroughs


36/56



Introducing the fourth generation ofLCDs

Since the world's first commercially practical LCD was introduced in1973, various technological innovations have been made that haveimproved picture quality, reliability, screen size, and have reduced bodysize, weight and power consumption. Every new technologicalinnovation has brought forth new products and led to the growth of LCDbusiness. Sharp regards the latest LCD breakthroughs as the fourth-generation of LCD technology and products, and plans to aggressivelypromote the further expansion of the LCD business.


37/56




38/56



New Liquid Crystal MaterialsFerroelectric LCDsEngineers have high expectations for the newest advance in LCDtechnology, ferroelectric LCD displays, and intense research efforts areunderway. Mayer first used ferroelectric liquid crystal in an LCD in 1975.These liquid crystal molecules are endowed with a positive or negativepolarity in their natural state, even without the application of an electricfield. In other words, ferroelectric LCDs utilize intrinsic polarization.Attention is being focused on ferroelectric LCDs because they offer thefollowing unique characteristics which differ from conventional LCDs:

1. MemoryFerroelectric display images are not lost when the power is cut;the image remains intact. Since the arrangement the liquid crystalmolecules had when voltage was last applied is retained, thenumber of scanning lines can be increased without sacrificingcontrast quality.2. High response rate


39/56



Very high-speed displays are possible. Speeds more than 3,000times faster than TN LCDs.3. Wide viewing anglesReduces viewing angle limitations. Since the contrast does notchange depending on the viewing angle, high resolution, large-scale LCDs are possible.

Ferroelectric LCDs do not require expensive switching elements likeactive matrix drive systems (as TFT LCDs do), making large-scale high-resolution displays with large information capacity are possible usingsimple passive matrix addressing. These displays are still in theresearch stage of development, but expectations are high that thistechnology will reveal a dramatic new LCD potential. Future challengeslie in correcting manufacturing difficulties related to improving radiationand more effectively controlling uniform cells (2 m max.), and productdevelopment.Anti ferroelectric (AFLC) LCDsAn AFLC LCD creates optimum optical switching by using a phasetransition from the anti ferroelectric phase to the ferroelectric phase thatis caused by an electrical current. In the anti ferroelectric phase, whenno electric current is applied, the optical axis is along the liquid crystallayer is normal, resulting in dark state. In the ferroelectric phase whenan electric field is applied, the optical axis tilts and causes doublerefraction, resulting in a bright state. Thus, switching between a dark


40/56



state and a bright state is controlled by using an electrical field todisplay picture images. (See Fig. 1)An AFLC offers an excellent gradation display and good imagereproduction. Its panel structure is similar to that of conventional LCDsexcept that the cell gap is thinner. In addition to having a quickresponse time, an AFLC offers other advantages including a wideviewing angle and a simple structure that leaves room to make the high-definition screen as large as possible. (See Fig. 2)Figure 1-Display principle of AFLC

Figure 2-AFLC panel structure


41/56



2. The Monitor Displays of Tomorrow(Application of Low-temperature Polysilicon TFTs: System-on-Panel)

LCD Still-Image CameraSOP technology integrates a photographic lens, an ultra-compact CCD,flash memory, and an LCD, to create a credit card-sized electroniccamera. This LCD electronic still camera can be used by anybody--anywhere, anytime.


42/56



Wristwatch-Type Personal Information ManagerThis ultra small Personal Information Tool can receive FM teletextbroadcasts and has a PHS (Personal Handy phone System) personalinformation manager functions. When equipped with a PHS portablephone function, a pen-input computer changes into a personalinformation station, which can handle various types of information.When data obtained with an LCD still camera is transmitted to theWristwatch-Type Personal Information Manager (PIM) through anoptical communication link, it also functions as a monitor.


43/56



Wristwatch-Type Multimedia-Compatible EquipmentThis Wristwatch-Type Personal Information Manager comes equippedwith multimedia-compatible applications including a built-in MMIC(mixed microwave IC) and integrated drive circuits, and a high-definitiondisplay that all fits into the units slim-profile, lightweight and narrowframe.By means of satellite data broadcasting (BS data broadcasting and CSdata broadcasting), this equipment can handle still picture data, textdata, facsimile data, etc., and can be used as a TV phone.

Hi-Vision LCD ProjectorThis compact, lightweight, Hi-Vision LCD Projector reproduces high-definition images for superb picture quality. It helps create anenvironment where products can be seen in life-like, vivid colors soviewers can conduct their entire home shopping needs while payinggreat attention to details.


44/56



Multimedia-Compatible NotebookWith this multimedia-compatible notebook, you can use the reflectivefull-color LCD, called industrial paper, exactly as a paper notebook, aswell as receive satellite broadcasted communications. This notebookhas an advanced handwriting recognition feature, which allows easycut-and-paste editing and corrections.

Panel Computer


45/56



This multimedia-compatible panel computer can be easily used byanybody, anywhere, anytime. The system can be readily expanded tohandle optical and other advanced communication functions. The PIM(Personal Information Manager) Panel Computer can be customized,making operation incredibly easy. All a user has to do is input therequired data.

Advancing LCD Technology Toward theNext CenturyHigh Polymer Dispersion LCDsIncreased brightness through the elimination ofpolarizing filtersNew developments in LCD research have made it possible to increasebrightness by eliminating the use of polarizing filters. The principle isthat fine liquid crystal particles, which are dispersed among highpolymers, when exposed to an electrical field, operate as an opticalshutter. Unlike TN LCDs, this LCD uses the scattering and non-scattering molecular states to enhance brightness intensity, thuseliminating the need for polarizing filters.


46/56



In the future, the following features will be included in projection typeLCDs:

1. High brightness without polarizing filters2. High speed operation (1 m sec.)3. The elimination of the liquid crystal injection process4. Easy cell gap control5. No rubbing is required

Ferroelectric LCDsHigh contrast, large displays featuring wide angleviewingFerroelectric LCDs are expected to usher in a new generation of highcontrast, large displays offering a wide-viewing angle.Ferroelectric LCD features:

1. A memory function and a direct matrix electrode structurewhich allows large-capacity displays to be utilized2. Quick response time of 10 sec.3. Wide-viewing angle4. High contrast picture quality

The problems:


47/56



1. The shock resistance, high-voltage resistance, high-temperature resistance and low-temperature resistance are stilltoo low2. Continuous gradation is difficult because of stability factors3. Because the picture-changing speed is low, ferroelectric LCDsare not suitable for moving pictures4. Manufacturing ferroelectric LCD panels is quite difficult

Introduction to Major Flat Panel Display Technologies

1. Liquid crystal displaysLiquid crystal displays (LCDs) are the most common type of flat panel displays (FPD

s) and have been widely used since the early 1970's. All LCDs utilize the fact that certainorganic molecules (liquid crystals, LC) can be reoriented by an electric field. As thesematerials are optically active, their natural twisted structure can be used to turn thepolarization of light by, for example, 90 degrees. Two crossed polarizers normally do not

transmit any light but if a 90-twisted LCis inserted in between, light will be transmitted

as shown to the left in. On the other hand, applying an electric field will unwind thehelical structure and the LC therefore loses its polarization-rotating characteristics. As a

result, the display turns dark as shown to the right in. An LCD consists of an array ofpicture element ("pixels"), which can be individually addressed according to the principlebelow.

Direct-viewLCDs can largely be categorized into reflectiveand transmissivedisplays,which utilize ambient light and light from a fluorescent backlight tube, respectively.


48/56



Figure 1. Principle of a passive-matrix twisted nematic liquid crystal display.

The twisted nematic(TN) type of LCs shown in are in the passive matrix configuration(i.e. simple electrodes for applying the electric field) primarily used in products such aswrist-watches, handheld calculators, pagers, pocket games, and other inexpensive devicesrequiring low power consumption and a small form factor. Its response speed is,however, insufficiently fast for high resolution and/or high frame-rate displays.

In the late 1980's, this problem was solved by the introduction of super twisted

nematic (STN) LCs which are twisted 270instead of 90. With a response speed of around

200 ms, STNenabled screen sizes of 8-10 inch with VGAresolution (640x480 pixels). It isno exaggeration to say that STNwas the enabling technology for notebook computers.

Despite this progress, though, the response speed is still not high enough fordisplaying fast moving images such as mouse movements or video. Moreover, increasedscreen-size and pixel counts negatively affects STNparameters such as contrast, gray scalecapability, and noise because of a large capacitance and limited conductivity of theelectrodes.

To solve this, a sample-and-hold circuit can be attached to each pixel, whichmaintains the voltage during one frame scan. Such a circuit was practically realized bythe advent of thin-film transistors (TFT s). At first, TFT s were extremely expensive tomanufacture and the price of a notebook with STN and TFTLCD s could differ more than

$1,000. Today, however, production technology has caught up and TFTLCDs is now themainstream technology for notebook displays. The structure of a TFTLCDis shown in.


49/56



Figure 2. Structure of a thin-film transistor liquid crystal display

The first TFTs were made from cadmium selenide (CdSe) in 1972 but an investmentmomentum in the solar cell industry convinced the Japanese to move towards amorphoussilicon (a-Si) although CdSe both has higher electron mobility and handles higher currentdensities.

The electron mobility is a crucial parameter increasing the frame rates and/or pixel

count. It is also important when downsizing TFTdevices for fine-pitch displays such as120 ppi (pixels per inch) or more. Polycrystalline silicon (p-Si), with a higher mobilitythan a-Si, is expected to play an important role in this context. Indeed, projection displaysusing tiny TFTLCD shutters (typically 2-inch diagonal) often employ p-Si because of apixel pitch of only a few tens of micrometers. Whereas these devices are grown at hightemperatures on expensive quartz substrates, direct-view displays require low-temperature processes, which are compatible with conventional glass. Low temperatureprocessing of p-Si is therefore attracting extremely much attention at the moment.

In 1998, Sharp Corp. and the Semiconductor Energy Laboratory announced a newtechnology called continuous grain silicon (CGS), which could potentially revolutionizeTFTLCDs. With mobility close to crystalline silicon, a 2.6-inch projection display device

for high-definition TV was successfully demonstrated. In addition to enabling higherresolutions, the CGS and p-Si technologies allow driver circuitry - and eventuallycomplete LSIs and CPUs - to be integrated monolithically.

Apart from being employed in notebook computers and, recently, in desktopmonitors with diagonal sizes up to 30 inch, TFTLCD

2. Displays for portable devices

s are used in reflective displays formobile terminal applications requiring video speed.


50/56



The strong customer demand for portable gadgets in Japan has triggered research onalternatives to the mainstream liquid crystal modes TN and STN, which, because of lowtransmittance and thus high power consumption, are more suitable for backlit displays fordesktop monitors.

With the goal of bringing reflective display quality close to paper print, brightness,contrast, and color saturation must be improved. Since polarizers effectively cut off 50%and color filters 66% of the incoming ambient light energy, several new modes withoutcolor filter or polarizers have been developed. One such mode is guest-hostin which anisotropic dye (guest) is incorporated in a LC (host). The applied electric field reorientsboth the LCand the dye molecules, which due to its an isotropic absorption will switchbetween transparent and opaque states. Displays employing the guest-host mode arebright, have wide viewing angle, but slow response. It is therefore mainly used inwatches and portable digital assistants (PDAs)

Another way to switch light is by controlling the amount of scattering in a mixture ofpolymers and LCdroplets. Whereas the polymer matrix is fixed, the LC droplets can be

reoriented as usual by an electric field. For certain directions, the refractive indices of thedroplets and polymer are matched and light therefore goes through without scattering. Onthe other hand, orienting the droplets in such a way that the indices are mismatchedcauses increased scattering and therefore a lower brightness. Displays based on thisprinciple still needs color filters but the brightness is greatly improved over the two-polarizer conventional TNdisplay. Applications include projectors and large-size shuttersfor window glass.

Another type of reflective displays utilizes light control via diffraction by changingthe LC phase electrically. Cholesteric LC s, for example, have a periodic structure thateither selectively backscatters light of a certain wavelength or transmit the remainingones. Applying an electric field will change the cholesteric phase to focal conic, which

efficiently scatters incoming light, and the display will appear dark. In the reflectivemode, the helical pitch determines the wavelength of the diffracted light so cholesteric

LCDs appear colored.The electrically induced phase change is reversible but has a hysteresis with built-in

bistability. Therefore, display contents will be maintained even after the power has beenswitched off. Moreover, Ch LCD

Ferro electric

s do not require active matrix driving because of thebistability and displays with extremely high resolutions (300+ ppi) are therefore withinthe reach. There are still, however, several problems that have to be solved prior tocommercialization. Switching is slow (typically hundreds of ms) and requires highvoltages (typically 40 V) incompatible with battery power. Because of the bistability,means to achieve full color are also an issue.

LC s (FLC s) play a special role in LCD s because of an intrinsic fastresponse in the microsecond range. The surface stabilized FLC (SSFLC) mode is bistableand does not require any active matrix driving. With the bistability, however, gray scale(or color) must be simulated by spatial dithering. Manufacturing SSFLC devices is verychallenging since it requires a laterally homogeneous inter-substrate spacing of less than2 micrometers. Canon has, nevertheless, commercialized a 15-inch SXGA (1280x1024)display using the SSFLCtechnology. Meanwhile, Toshiba is working on an active-matrixanti-FLC which does not suffer from bistability and, consequently, not from any lack of


51/56



gray scale capability. Similarly, Denso has developed prototypes of a passive-matrix AFLCusing a partial phase-change mode. Although FLCD

3. Field Emission Displays

s look promising from a material pointof view, unreliable production, limited operating temperature range, and lower contrastare still issues.

Field emission displays (FEDs) have many similarities with conventional cathode raytubes (CRT). In fact, one company calls its FED product "flat CRT ". As for the CRT ,electrons are accelerated in vacuum towards phosphors which then glow. The maindifference is that the electrons are generated by field emission rather than thermalemission so the device consumes much less power and can be turned on instantly. Insteadof one single electron gun, each pixel comprises several thousands sub-micrometer tipsfrom which electrons are emitted.

To achieve a low operating voltage, the tips are made of a low-work functionmatieral such as molybdenium and are shaped into very sharp tips so that the local field

strengths become high enough for even a moderately low gate voltage. The state-of-the-art FEDs can operate at gate voltages as low as 12 V. Sucked out of the tips, the electronsare accelerated towards the phosphor screen by either a low or high voltage. A lowvoltage simplifies the device design but disables the use of highly efficient and matureCRTphosphors. The design problems are rapidly being overcome and the mainstream FEDis therefore of the high-voltage type.

Since it is difficult to control the current of each individual tip, the display operates ina saturated mode with each pixel turned either on or off. Thanks to the fast response ofthe device (ns range), gray scale can be obtained by pulse-code modulation (PCM).

Much of the FED research is still focused on suitable emitter materials, which canlower the driving voltage. One of the most exciting ones is diamond which enables field

emission at voltages as low as 1-2 V. Manufacturing such tips, however, is a hurdle andthe commercial FEDs are therefore still using metal tips.

Since the FED is a vacuum device, atmospheric pressure becomes a severe problemfor large-area panels. In particular, internal support posts, which prevent the device fromimploding, must be thin enough to fit into space between pixels. This together withlifetime issues and bringing down the driving voltage are the main challenges ahead forFEDdevelopers.


52/56



Figure 3. Field emission display

Typical FED applications include portable ruggedized instruments, pocket videoplayers, mobile videophones, aircraft video displays, and, eventually, laptop computers.Compared to TFTLCDs, FEDs are far superior with a wider viewing angle, faster response,higher color saturation, and lower power consumption. Despite this, however,manufacturing and lifetime problems have prevented a full-scale commercialization ofFEDs as have furious investment in the TFTLCDproduction capacity, resulting in unhealthyprice drops.

Surface conduction emitter display (SED) is a new FED-type display type pursued byCanon. The emitter consists of a thin film of ultra small palladium oxide (PdO) particles,which is patterned into narrow gaps (10 nm) where the film has been removed. Aselectrons are driven in the surface film, they tunnel through the gap, are multiplyscattered against the other edge and finally accelerated by the anode voltage. SEDdevicesare not new but emission of previous materials (mainly metals) has proven unstable and

thus unsuitable for display applications. SEDprototypes have promisingly demonstratedluminance more than twice of that of PDP

4. Cathode Ray Tubes

s at lower power consumption so Canon isaiming at introducing the technology in consumer products.

Invented in 1897, the cathode ray tube (CRT) is still the most common display typetoday. The picture is rasterized by rapidly scanning an electron beam in a vacuum tube


53/56



whose inner front surface is covered by red, green, and blue phosphors. Electronsgenerated by a heated electron gun are accelerated towards the phosphors by a statichigh-voltage field and deflected by magnetic fields, which, together with the electronbeam current, is controlled by a video signal. As the electrons impact on the screen,

phosphors are excited and emit colored lightThe CRT is a very simple and matured device and the production costs have been

trimmed. It features advantages such as high response speed suitable for high-frame rate,high-resolution video, wide viewing angle, saturated colors, high peak luminance, andhigh contrast.

However, due to the high-voltage field, oscillating magnetic field, andBremsstrahlung (X-rays) generated by electrons hitting the screen, the CRT has beenregarded as hazardous for long-term use. During the last decade, though, several of theseproblems have successfully been solved and modern computer monitors are today beingdesigned according to strict environmental standards such as TCO-95.

Figure 4. Principle of a cathode ray tube.

Another common cause of eye fatigue is flickering which occurs from the shortemission lifetime of the phosphors. An NTSCTVsignal with a 30 Hz frame frequency istherefore interlaced at 60 Hz to reduce flicker. Because of the higher resolution ofcomputer monitors, the limited response speed of the video electronics makes it difficult

to increase the frame frequency of non-interlaced signals (standard computer output).Recent state-of-the-art video electronics can handle SXGA(1280x1024) at 75 Hz or more,though.

In addition to its size, weight, and high power consumption, traditional CRT s withcylindrically or spherically curved surfaces have suffered from geometrical distortion,particularly at the edges. Recently, however, flat-surface CRT s has been introduced bycompanies such as Sony, Sharp, and Matsushita (Panasonic), which eliminate suchdistortion.


54/56



Although several of the drawbacks above have been removed by new technologies,CRT s will eventually face problems with high-resolution displays requiring finer pixelpitches. Rather than thinness, lower power consumption and weight, it is for this reasonTFTLCDs will be a serious competitor to CRTs.

Figure 5. Principle of vacuum fluorescent displays

5. Vacuum fluorescent displaysVacuum fluorescent displays (VFDs) are another display that utilizes thermal emission

(640C filament) of electrons and phosphor excitation to generate color. In contrast to aCRT, however, the electrons are accelerated by a much lower voltage and the pixels areswitched on/off by changing the sign of the electric potential at the target anode. Apositively charged target will attract electrons whereas a negatively charge target willrepel them. Attracted electrons excite the phosphors, which thereby emit light. Contraryto a CRT, the phosphors can be patterned in any shape and VFDs are therefore suitable fordisplaying icons in consumer electronics. Due to their ruggedness and high luminance,


55/56



they are also employed in automobile dashboard- and head up displays. The two majorcompanies in Japan pursuing VFDs are Ise Electronics and Futaba.

In principle, VFDs could be used in larger displays for monitor applications but sinceit is a vacuum device, the mechanical construction could be a problem. An advantage

over FED

6. Plasma Display Panels

s, though, is that no spaces are needed between the pixels.

A plasma display panel (PDP) is essentially a matrix of tiny fluorescent tubes, whichare controlled, in a sophisticated fashion. There are two main types, DC- and AC of whichthe latter has become mainstream because of simpler structure and longer lifetime. Thissection treats theAC-type.

A plasma discharge is first induced by the positive period of an ACfield and a layer ofcarriers is shortly thereafter formed on top of the dielectric medium. This causes thedischarge to stop but is induced again when the voltage changes polarity. In this way, asustained discharge is achieved. The AC voltage is tuned just below the discharge

threshold so the process can be switched on/off by adding a relatively low voltage at theaddress electrode.

Figure 6. Principle of an AC PDP

The discharge creates plasma of ions and electrons, which gain kinetic energy by theelectric field. These particles collide at high speed with neon and xenon atoms, whichthereby are brought to higher-energy states. After a while, the excited atoms return totheir original state and energy is dissipated in the form of ultraviolet radiation. Thisradiation, in turn, excite the phosphors which glow in red, green, and blue ( RGB) colors,respectively. Since each discharge cell can be individually addressed, it is possible toswitch on and off picture elements (pixels).


56/56


To generate color shades, the perceived intensity of each RGB color must be controlledindependently. While this is done in CRTs by modulating the electron beam current, andtherefore also the emitted light intensities, PDP s accomplish shading by pulse codemodulation (PCM). Dividing one field into eight sub-fields, each with pulse weighted

according to the bits in an 8-bit word, makes it possible to adjust the widths of theaddressing pulses in 256 steps. Since the eye is much slower than the PCM, it willintegrate the intensity over time. Modulating the pulse widths will therefore translate into256 different intensities of each color. The number of color combinations is therefore 256x 256 x 256 = 16,777,216.

At first, PDP s had problems with disturbances caused by interference between thePCMand fast moving pictures. By fine-tuning the PCMscheme, however, this problem hasbeen eliminated.

While PDPs are relatively lightweight and can be manufactured at a thickness of 3-4inches, they still consume prohibitively much power. The luminous efficiency, i.e. theamount of light for a given amount of supplied electric power, is still at approximately 1

lm/W, about 10% of some other FPD technologies. Also, the discharge process causessputtering of the cells, which inevitably reduces lifetime. With a new protective dielectriclayer of MgO, however, this problem has largely been solved.

Despite these problems, PDPs are promising because of their modest requirements onmanufacturing technology. Compared to TFTLCDs, which use photolithographic and high-temperature processes in clean rooms, PDPs can be manufactured in less clean factoriesusing low-temperature and inexpensive direct printing processes. Also, PDPs feature wideviewing angles, no susceptibility to magnetic fields, and are easy to scale up for wall-hanging TV

7. Other FPD technologies

applications.

This report will be updated shortly and information added on the followingtechnologies.

1. Thin film electro luminescent displays (TFEL2. Light emitting diode arrays ( )LED3. Electro chromic displays ( )ECD4. Thermo chromic displays ( )TCD5. Organic luminescent displays ()OELD6. Plasma addressed liquid crystal displays () PALC7. Micro displays on )CMOS8. Micro-Optical Electromechanical Systems (back planes MOEMS

)

the principles of lcd technology

Documents