liquid crystal display fabrication

8/13/2019 Liquid Crystal Display Fabrication

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http://liqcryst.chemie.uni-hamburg.de/lcionline/ Liquid Crystal Display Fabrication

Introduction

I. Glass Cleaning

A. Types of Contamination

1. Particulate

2. Organic films

B. Cleaning Methods

1. ltrasonic

2. ! " O#one

C. $ater %uality

&. 'CI ( Industry Procedures

II. Photolithography

A. Photoresists

B. Photomas)s

1. Chrome on Glass

2. Mylar

C. *+posure systems

1. Mas) Aligners

2. ,teppers

-. Contact s. Pro+imity

&. *tching

III. Alignment 'ayers

A. /e0uirements for &isplays

B. Polyimides

1. Application

a. ,pincoating printing
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. Typical thic)ness334553655 angstroms

2. /u ing

a. /u 7heel s. load ru ing

Glass Cleaning

Glass cleaning is one of the most important steps in li0uid crystal display fa rication.,u strates are cleaned efore processing and after many su se0uent steps and impropercleaning can result in

electrical shorts cell gap ariations poor alignment of the li0uid crystal

to name a fe7 fatal defects. It is therefore imperati e to ha e an efficient cleaning process toremo e all contamination from the glass surface.

Contamination can e classified into t7o groups8 particulate ( ( organic thin film

Particulate contamination can come from the operator 9in the form of hair s)in fla)esacteria or clothing fi ers: process e0uipment and supplies 9flec)s of dried photoresist

7iper fi ers dust etc.: or the glass itself 9fragments from cutting:. Because typical 'C& cellgaps are elo7 ten microns e en a single particle a fe7 microns in diameter can cause afatal defect. ;or this reason particulate contamination is the primary concern in the cleaningprocess.

Thin film contamination can e caused y improper cleaning and stripping procedures oroperator contamination such as s)in oils. These typically lea e a thin organic film o er all ora portion of the su strate resulting in poor adhesion9?@ : or de7etting9 D ?=EF:ofsu se0uent coatings and in some cases incomplete etching. After many procedures asimple 7ater rinse is not sufficient to remo e process chemicals. Photoresist in particularcan sometimes re0uire ultrasonic agitation9 =H H: in an organic sol ent for complete remo al.

Cleaning methods

;or 'C& fa rication a typical cleaning process in ol es ultrasonic cleaning 7ith a mild9 @ :detergent 9F EJ:follo7ed y a ! " o#one cleaning to remo e organic contamination.Production en ironments 7ill often use rush scru ing or Ket spray 7ith ! " o#onecleaning an option.

In ultrasonic cleaning the ath is agitated at ultrasonic fre0uencies in order to dislodge9

>DLD: particulate contamination ia ca itation9F F :. The ath consists of deioni#ed 7ater7ith a neutral detergent and is often heated to 453 5NC to aid in cleaning . ltrasoniccleaning is most effecti e on particles larger than -34 microns . After rinsing9 @ : thesu strates are often dried using isopropyl alcohol.


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! " o#one cleaners are e+tremely effecti e on thin organic films. ,hort 7a elength !9 elo7 -55 nm: rea)s do7n more comple+ organics and the o#one reacts 7ith the films toform car on dio+ide . !"o#one cleaning is used as the final cleaning step. This method can

e time consuming 94325 minutes " su strate: and the su strates should mo e 7ithinminutes to the ne+t process step. ! " o#one cleaning impro es adhesion of photoresistsand polyimides.

Cleaning test

The effecti eness of a cleaning process can e Kudged using se eral methods. The simplestof these is isual inspection usually under ! light illumination . ltra iolet light is scattered

y particulate contamination and thin films are readily isi le ma)ing it straightfor7ard to Kudge the cleanliness of the su strate.

An e+cellent method for testing su strate cleanliness is y measuring contact angles 7ith7ater. If a drop of 7ater is placed on an inorganic surface it should spread out andcompletely 7et the surface. If there is an organic film on the surface the 7ater 7ill tend to

ead9 =F E: and form a contact angle close to Q4N. This can e o ser ed 0ualitati ely yspraying the su strate 7ith deioni#ed 7ater if properly cleaned the 7ater should sheet9


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There are se eral manufacturers 9e.g. ,olitec and Sead7ay in .,: of spincoatinge0uipment. They pro ide units for small and large su strates 7ith arious degrees ofautomation. ,ome manufacturers supply spincoaters 7ith the spinning o7l ut these unitsare considera ly more e+pensi e.

,ome production en ironments use spincoating for photolithography polyimide and arrierlayers ut printing is more desira le ecause of its more efficient materials usage. Thedisad antage of printing is the high capital cost of the e0uipment 9typically 455 555:.

!oller coating

A popular method in small production en ironments is roller coating. This method can eused to coat photoresist for lo7 resolution photolithography. The film uniformity isconsidera ly less than for spinning or printing ut is ade0uate for direct addressed andother lo7 resolution panels.

Dip coating

Other methods of coating include dip coating and meniscus9F=J >"F= >: coating. In dipcoating su strates are dra7n out of a solution at a uniform speed. Both methods enKoylimited usage ut neither is e en close to the spinning and printing methods.

"hotolithography

The e olution in microelectronics technology and microlithography in particular hasprogressed at an astonishing rate. The con entional photolithography 7hich uses - 43Q54nm irradiation 7ere a le to print 5.435. mm features in production in the 1UU5s. Ad ancesin optics ha e ena led e+posure y shorter and shorter 7a elengths. Indeedphotolithography using 2Q6 and 1U- nm light promises to dominate production technology7ell into the ne+t century.

In semiconductor industry the three3dimensional circuit elements are fa ricated y a seriesof process collecti ely )no7n as VlithographyV. The pattern is first generated in a polymericfilm on a de ice V7aferV and this pattern is then transferred ia etching into the underlyingthin film. &ia#onaphtho0uinone3no lac materials 7ill most li)ely remain the materials of

choice for production of these de ices . The costs of introducing ne7 resist materials andne7 hard7are are strong dri ing forces pushing photolithography to its a solute limit.

The technological alternati es to con entional photolithography are largely the same as they7ere a decade ago that is

near3 and deep3 ! photolithography scanning electron eam lithography W3ray lithography.

The leading candidate for the production of de ices 7ith features as small as 5.- mm isdeep3 ! lithography. The polymer that are used as radiation3sensiti e resist films must ecarefully designed to meet the specific re0uirements of the lithography technology andde ice process. Although these re0uirements ary according to the radiation sources and


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de ice process properties such as sensiti ity contrast resolution etching resistance shelflife and purity are u i0uitous9 XYJH Z= HDR?[=>J :.

In display applications high resolution has een made possi le y decreasing the minimumfeature si#e of the circuit element. The con entional means of increasing the resolution thatis of increasing the circuit density has een to ma)e the acti e elements in the de icessmaller there y increasing the num er of acti e circuits that can e accommodated on agi en area of display.

#lectrode patterns

In order to pattern the electrodes on each su strate aphotolithographic process is used. This is the same processthat is used in printed circuit oard and integrated circuitfa rication. *ach su strate is coated 7ith a photosensiti ematerial 9photoresist: and selecti e areas are e+posed to !light 9this pattern is generated y a photomas):. A de elopingprocess lea es resist only on the desired electrodes. Thesu strate is then placed in an etch ath 7hich etches the ITO9Indium Tin Oxide : from the areas not co ered y resist. Afterstripping of the photoresist and cleaning the su strate isready for su se0uent processing.

Photoresists can e either positi e or negati e 7or)ing.Positi e resists 7hich include most commonly used li0uidresists are initially slightly solu le9L LJ : in a de eloper

solution. pon e+posure to ! light they ecome highly solu le. Therefore a Q43 5 secondde eloping 7ith agitation creates a positi e image of the photomas). \egati e resists are

often dry ] that is they are laminated9 =^[ @?F : onto the su strate rather than spun. Thesedry resists do not ha e high resolution capa ilities ut are highly suited to a productionen ironment. &e eloping is done in a cham er 7here the de eloper solution is sprayed onthe su strate.

!esist $aterials

,ingle3'e el /esist Chemistry

ype Characteristics $echanism De%eloper &d./Disad%. Sensiti%ity

'egati%eresist

less solu le inde eloper

crosslin)ing organicsol ent

image distortion high

"ositi%eresist

more solu le inde eloper

chain scission9 @F => ?_=< :or polarity change

a0ueoussol ent

high resolution

dry etchresistance

lo7

'egati%e resists

FIGURE 2 : ITO electrode pattern.
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\egati e resists are a class of materials that ecomeless solu le in a de eloper after e+posure to radiation.Generally the chemistry of negati e resists in ol essome form of radiation induced crosslin)ing. The parentpolymers are usually solu le in organic sol ents 7hichin turn are used as de elopers. Materials include inylepo+y halogen containing polymers.

"ositi%e resists

Materials that e+hi it enhanced solu ility after e+posure to radiation are defined as positi eresists. The mechanism of positi e resist action in most of these materials in ol es eithermain chain scission or a polarity change . Ordinarily the chain scission mechanism is onlyopera le at photon 7a elengths elo7 -55 nm 7here the energy is sufficient to rea) mainchain onds. The est )no7n of these so called dissolution9>=F @=F : inhi ition9 J D?F=Y J: resists is Vcon entional positi e photoresist] a photosensiti e material uses a no olac9phenol3formaldehyde: resin 7ith a dia#onaphtho0uinone photoacti e compound as adissolution inhi itor. pon irradiation the dia#onaphtho0uinone undergoes a $olffrearrangement follo7ed y hydrolysis to generate a ase solu le indene car o+ylic acid.

As de ice features mo e into the su micronregime ad anced processing techni0ues and ne7

lithographic technologies 7ill need toaccommodate high resolution high3aspect ratioimaging o er de ice topography. Thisnecessitates9_ FYJ: the de elopment of ne7 resistmaterials 7ith impro ed etching resistanceresolution and sensiti ity. A recent de elopedsensiti ity impro ing process in ol es the conceptof chemical amplification. The chemical

amplification principle has een used to design a num er of negati e resists ased on acidcataly#ed cation9 =?= : polymeri#ation of appropriate monomers 9single unjoined organic molecule: arelatively light, simple organic molecule that can join in long chains with other molecules to form a more complex molecule or polymer : or

crosslin)ing 9`: of polymers.

"hotomas(s

Photomas)s can e of se eral types depending on the resolution ( dura ility re0uired andthe type of e+posure system. Most mas) aligners re0uire chrome on glass mas)s 33these aremore e+pensi e 9 45531455 typical: ut pro ide good dura ility and high resolution. Mylaremulsion mas)s9`: pro ide medium to lo7 resolution 9do7n to a out 24 microns: ut arecheap 9less than 45: and can e used in many contact printers.

FIGURE 3

FIGURE 4


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!adiation sources

Mercury arc lamps are the most popular light source for photolithography. 'ight in the -453Q45 nm range is most effecti e 7ith con entional photoresists used for medium resolutionpatterning. $hen resolutions elo7 se eral microns are re0uired 0uasi3monochromatic light9g3 and i3line: is often used or shorter 7a elengths are used. ,ince it is difficult to relia lyetch ITO features smaller than 4315 microns con entional mercury lamps suffice for most'C& applications 9 Thin Film Transistor 3 T;T s typically re0uire the greatest resolution:.

#)posure systems

A ariety of e+posure systems are a aila le. Mostproduction lines employ some type of step and repeatphotolithography e0uipment for ma+imum throughput9

>=


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,u strates are then soa)ed in bOS solution 9to neutrali#e surface: for 5 seconds. A fullcleaning is then necessary efore proceeding to polyimide application.

Sur*ace &lignment

In the li0uid crystal de ices one of the most important pro lems is the surface alignment of

the li0uid crystal molecules. There are four asic surface alignments as sho7n elo7.In practical application a small tilt from parallel andperpendicular as sho7n in figures 9c: and 9d: namelypretilt is important for o taining domain3free orientationunder electric field.

Mechanism and method to o tain sta le surfacealignment ha e een studied y many researchers.bahn empirically descri ed the alignment is determined

y the competition et7een the surface tensions of

li0uid crystal and su strate. . Cognard Mol. Cryst. 'i0.Cryst. 6 ,upl.1.1 91U62:] '.T. Greagh and A./.bmet# Mol. Cryst. 'i0. Cryst. 2Q 4U 91U -:] ;. bahn etal. Proc. I*** 1 62- 91U -:] T. chida Mol. Cryst.

'i0. Cryst. 12- 14 91U64: 7hich is ased on the relation et7een surface energies of thesu strates and li0uid crystal 7hile se eral e+perimental results contradicting their theoryha e also een reported. I. Saller8 Appl. Phys. 'ett. 2Q -QU 91U Q:] T. chida et al. Mol.Cryst. 'i0. Cryst. 5 - 91U65: . Saller reported that

The dispersion force is considered as the only alignment factor. It is assumed that the 'Cs align perpendicular to the free surface.

Mechanisms of the parallel perpendicular and tilted homogenous alignments

"arallel &lignment

Parallel alignment is usually o tained as long as thesurface is microscopically flat and li0uid crystal doesnot contain amphiphilic impurity as 7ell as surfacepolarity is too lo7 to a sor the impurity. \ormallysurface coated 7ith fluorinated material 9to+ic reacti echemical element8 a to+ic pale yello7 gaseous element of the halogengroup that is the most reacti e and o+idi#ing agent )no7n. ,ource8fluorite cryolite. se8 7ater treatment ma)ing fluorides andfluorocar ons: gi es lo7 surface energy. Therefore sta leparallel alignment is o tained y decreasing thesurface polarity y coating polymer or surface couplingagent of 7hich molecules tend to adsor parallel tothe surface. So7e er these alignment are randomparallel alignment. In order to o tain homogenousalignment unidirectional ru ing is necessary.

Mechanism of the alignment parallel to the ru ing direction is analy#ed y Berreman Phys./e . 'ett. 26 1 6- 91U 2: .

FIGURE 6 : #asic li$!id cr%stal alignments

FIGURE ! : &arallel '&lanar( alignments
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"erpendicular &lignment

There are three proposed alignment mechanisms too tain perpendicular alignment. Amphiphilic materials9surfactants: assisted alignment i.e. amphiphilic materiala sor s perpendicular to the polar surface and 'C alignsaccording to the amphiphilic material. The secondmechanism is the use of surface coupling agents such assilanes 9silicon-hydrogen compound: a compound of silicon and hydrogenbelonging to a group analogous to the paraffin hydrocarbons. Formula: Si nH2n 2 : 7ithlong al)yl chains. The third mechanism is microscopiccolumnar structure3assisted alignment 7hich is o tained

y ,iO3rotati ely o li0ue9 H=LRDX: e aporation as reportedy Siroshima et al. apan. . Appl. Phys. 21 ' U1

91U62: . The three alignment mechanisms are illustratedelo7. Materials and process for li0uid crystals alignment

in 'C&s Alignment on the clean inorganic surfaces It haseen )no7n empirically that some li0uid crystals align

perpendicular to inorganic smooth surface such as In 2O- film. The reproduci ility anduniformity of this type of alignment is poor as the su strate surface is ill defined. Thecleaning procedures employed in the su strate preparation also play a role e.g. MBBA9limited to ,chiff ases: molecules 7ill align perpendicular to the surface of acid treatedglasses of o+ides ut non3uniform alignment parallel to the su strate surface is o tained7ith fired or detergent cleaned glass. O+idation of In 2O- coating in an o+ygen plasma lead tolayers causing parallel alignment of iphenyls. G. ,pro)el and /. M. Gi son .*lectrochem. ,oc. 12Q 44 91U :

In general the polarity of arious metal o+ides increases the tendency of perpendicularalignment of 'Cs increase. $ea) surface polarity of metal o+ides fa ors the parallelalignment. A ta le summari#ed y chida is gi en elo7.

Alignment on the surfaces organic polymers

"olymer coatings on glass substrates can be employed to align liquid crystals+ utfilm uniformity and the su strate used influence the o ser ed results. ,e eral methods ha e

een used to form the polymer layer 7hich is prefera ly thin in order to a oid an e+cessi epotential drop in the dielectric layer. The film may e transferred to the surface from a li0uid7hereas polymer casting and thermal or plasma polymeri#ation of the monomer ha e also

een used. The most common method is to form the polymer from partially polymeri#ed

solution y dipping or spin coating follo7ed y curing. Polyimide layers are the most popularpolymers in 'C& manufacturing to align 'Cs parallel 7ith some pretilt. Polymer coatings donot sustain high temperatures are not suita le as the alignment layers.

In general polymers orient the nematic director parallel to the su strate ut do not induceuniform reproduci le9


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an offset printer is often used for polyimide coatings. The ad antages of this are ettermaterials utili#ation and a ility to mas) off the gas)et9 [EXJ= : seal area. ;ilm thic)nessesare typically 4553655 angstroms.

The cured films at this point ha e no preferred alignment direction33ru ing the surface ofthe polyimide gi es it this direction. The cured films are ru ed 7ith a el et cloth 7hich isusually 7rapped around a rotating drum. Alternati ely in a research or prototyping settingload ru ing can e used. In load ru ing a flat 7eight co ered 7ith el et is dra7n accrossthe su strate at a uniform speed. The ad antage of load ru ing is its reproduci ility and0uantitati e nature. Sard ru ing is not necessary and can lead to isi le scratches 9due toscattering: in the completed cell. /u strength can e characteri#ed y a pressure ut 7hena ru 7heel is used it is often useful to discuss ru strength in terms of millimeters of pilecontact length. \issan pro ides data using this method.

/u ing is still one of the least understood aspects of 'C& fa rication. It seems clear thatthere is oth a mechanical and a chemical component to ru ed alignment in polyimides./u ing causes some groo ing of the surface 7hich is isi le y A;M and chida hassho7n that a stamped groo ed9Z F : epo+y 7ill align li0uid crystal. So7e er purelymechanical models do not accurately predict pretilts for ru ed alignment. ohn $est hasin estigated purely chemical alignment y irradiation of cured polyimide films 7ith polari#ed

! light 9245 nm:. This clearly gi es a chemical anisotropy to the film and results in goodalignment.

*arly polyimides ga e planar alignment 7ith a small 913-N: pretilt ut materials are no7a aila le that gi e pretilts up to Q5N as 7ell as homeotropic alignment. So7e er o tainingrelia le reproduci le pretilts in the 153Q5N range is not yet possi le33these materials are stillin a preliminary stage. \issan Chemical is essentially the sole supplier of these specialtypolyimides and the materials are ery e+pensi e 9\OT*8 although the \issan materials are

the est a aila le they are impractical to spincoat in a production setting33they must eprinted to e cost effecti e:. 'o7 pretilt materials are a aila le from numerous sourcesincluding &uPont and are accordingly more afforda le.

Polyimide chemical structures are not al7ays a aila le ut some insight can e gained frompretilt studies. ,ome crude9 XD JD=@?= J : classifications of the alignment mechanism can

e made ased on the eha ior of the pretilt 7ith increased ru ing strength. If pretilts aresmall ut increase 7ith ru ing strength the main chain structure is thought to eresponsi le for alignment. If pretilts are larger and decrease 7ith ru strength side chainspro a ly contri ute to the pretilt. apan ,ynthetic /u er 9 ,/: has pu lished numerouspapers in 7hich polyimides 7ith )no7n chemical structures are in estigated ut little is

)no7n a out most commercially a aila le materials especially those from \issan.

&lignment on groo%ed sur*aces and stamped morphology 9 @ @JZ=< = :

/u ing tangential e aporation or shallo7 angle ion eam etching produce a 7a y surface.It is ac)no7ledge that any ru ing material gi es good results of producing groo ed surface


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on the su strate. So7e er the reproduci ility of the ru ing process is not ery good onsu strates that are simply ru ed 9as seen in the A;M photo of ru ed PI alignment layer:.

A stamped morphology method has een introduced ychida ,I& &igest U4 91UU-: to create groo es for 'C

alignment . The non3ru ing alignment method has thead antage of accurate control of pretilt angle anda#imuthal surface anchoring energy y changing thesurface morphology. The material of the alignment layer isa isphenol A type epo+y resin 7hich 7as addedmethanphenylenediamine as the hardener and +ylene asthe sol ent. After spin coating on ITO3glass the film 7aspre a)ed at 65C o for 15 mins. The replica 7as pressed onthe su strate 7ith 455 g"cm2 force and )ept this stateuntil the epo+y resin is stiffened. Then cooled to roomtemperature efore remo ing the replica from thesu strate. In this 7ay the morphology of the replica istransferred to the su strate. 9The photos of principle ofthe stamping process and morphology are sho7n elo7:

&lignment on silane and Si, ) treated sur*aces

The silanes sometimes are considered as surface acti e agents and sometimes as polymerforming compounds. The al)o+y silanes and chlorosilanes interact strongly 7ith silanolgroups of glass surface or hydrolysis of the silane to silanol 7hich 7ill further condense into

a linear polysiol+ane layer. ,urface treatments 7ith

silanes ha e een effected y dipping during 4s to 1hin 1 34 solution of the silane in 7ater toluenediluted acetic in 7ater or acetone. $ater solution areonly sta le for a fe7 hours. In the case of 0uaternaryammonium sylil compounds these methods ga e poorresults ho7e er impro ement 7as made y dippingin hot solution 9 4C o:. ,ilane3treated surface gi e oththe homogenous and homogeneous alignmentdepending on the length of the al)yl chain.

The e+istence of hysteresis has een attri uted to

contamination of either the li0uid or the solid roughsurfaces or a sor ed surfaces film immo ility.

,blique e%aporation o* silicon o)ide

O li0ue e aporation of silicon o+ide 7as first reported y anning Appl. Phys. 'ett. 21 1 -91U 2: . The tilt angle achie ed y this method is controlled y the e aporation angle.Goodman et al. I*** Trans on *lectron &e ices *&32Q U4 91U : proposed a columnarmodel ased on o ser ations y transmission electron microscope. O li0uely depositedfilms ha e a titled columnar structure 7hich is generated y the geometric self3shado7ing ofthe incident atoms y those already present in the gro7ing film. The o li0ue e aporationmethod can control the pretilt angle and the e aporated film is compati le 7ith hightemperature process.

FIGURE + : +F, pict!re o" groo-eds!r"ace.

FIGURE ,- : +lignment on Silane treateds!r"ace.
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Langmuir- lodgett Films

I)eno et al. apan. . Appl. Phys. 2 62 91U66: demonstrated the utility of polyimide3'Bfilms for orienting 'Cs 7ithout performing ru ing . Monomolecular films of amphiphiliccompounds 7ere transferred to hydrophilic su strates y dipping a cleaned glass plate intoand out of a solution 7ith constant surface pressure y means of a motor dri e. Someotropicalignment is o tained at lo7 pac)ing density. sing this techni0ue it is possi le to uild upmonolayer or multilayer functional films of organic materials. The polyamidic acid 'B films for'C alignment are the most e+tensi ely studied material. The polyamidic acid 'B film 7ereprepared from al)ylamine salt and then remo ed the al)ylamine to o tain the polyamidicacid 'B film.

So7e er t7isted nematic 9T\: and supert7isted nematic 9,T\: 'C&s using these 'B filmsha e alignment defects such as re erse t7ist and re ere9 ^=RFD @?=RJ@=?R?F : tilt caused ythe fact the use of 'B films does not result in pretilting of li0uid crystal molecules. I)eno etal. ,I& &igest p. Q4 91U66: The 0uality of 'B film is high in microscopic region ut is poorin macroscopic areas.

"hoto-Induced &lignment o* Liquid Crystals ith "olari ed Light

The photosensiti e polymer poly9 inyl metho+ycinnamate: 9P!MC: 7hen corsslin)ed 7ith linearpolari#ed ! light 4 K"cm 2 at -25nm induces proclaim9

FR D


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illuminated 7ith an argon ion laser 941Q.4 nm: polari#ed along the ru ing a+is . After 125min e+posure at 6 $"cm2 po7er density 'Cs 7ithin the illuminated region orientedperpendicular to the laser polari#ation. The 'Cs for the guest3host system remain aligned inthe a sence of the laser light and can e reoriented again y su se0uent illumination. It 7asfound that the guest3host 'Cs aligned this 7ay assumed a t7isted nematic structure 7ithinthe illuminated region. The system may ser e as a model system to pro e t7o dimensionalphase transition at interfaces.

&lignments *or 'ematic+ Smectic+ and Cholesteric Liquid Crystals 0 . 1chida+ 2345678

In nematic li0uid crystal cells there are si+ commonly used com inations of surfacealignment8 Somogenous homeotropic t7isted hy rid 165 o ,T\ and 2 5o ,B*. As forsmectic 'C cells there are three fundamental alignments8 planar homeotropic and focalconic . $hile in cholesteric 'C cells there are three corresponding alignments8 planar 0uasi3

planar and focal conic .

&lignment *or Ferroelectric Liquid Crystals 2FLC7

One of the most important pro lems 7ith ;'Cs is to o tain homogenous and"or ista lealignment. Because chiral smectic C phases are difficult to align 7ith a uni0ue moleculara+is 9 oo)shelf te+ture: the phase sometime sho7s unfa ora le defect lines 9#ig3#ag

defects: of che ron (a heraldi" ornament in the orm o a .ide in/erted 01sha#e) structure. In order to remo e#ig3#ag defects uniform tilt alignment is achie ed y o li0uely e aporated ,iO andantiparallel sealed cell. The ista le surface sta ili#ed ;'C re0uires 7ea) surfaceanchoring. In practical ;'Cs can e made to form oo)shelf structures y lo7ering thesurface energy and applying oltage .

Spacers: &pplication

niform cell spacing is achie ed through the use of spacers. ,pacers are usually glass or

FIGURE 12


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polystyrene spheres or glass fi ers of a precisely controlled diameter ] they are usuallya aila le in half micron increments . The spacers are distri uted e enly throughout the cell.

Cell thic)nesses ary depending upon the type of display. The ta le elo7 sho7s gaps forse eral different types of displays. T7ist cell thic)nesses are usually determined from phasematching conditions. ,T\ cells in particular are e+tremely sensiti e to thic)ness differences.

ype o* Display ypical Cell Gap

T7isted nematic 4315 microns

,uper t7isted nematic Q36 microns

P&'C 15345 microns

,,CT Q34 microns

P,CT 15314 microns

,,CT8 ,urface ,ta ili#ed Cholesteric Te+ture. Bista le reflecti e displays

P,CT8 Polymer ,ta ili#ed Cholesteric Te+ture 9normal ( re erse mode:

Choice of spacer type is dependent upon desired properties as 7ell as display materials.Plastic spacers are necessary 7hen plastic su strates are used. An ad antage of plasticspacers is that their thermal e+pansion is similar to that of the li0uid crystal material. Thismeans that at higher temperatures 7hen the li0uid crystal e+pands and causes the cell to

o7 out7ard the spacers 7ill e+pand at the same rate maintaining the cell gap. Glassspacers do not e+pand as 0uic)ly and can float freely at higher temperatures . Glass fi ers

are e+tremely effecti e at maintaining cell thic)ness ut are not used ery often in acti ematri+ applications for fear of scratching 3 glass spheres are used instead.

A ariety of methods are a aila le for spacer application though most in use are ased onthe same concept. The preferred method of application is to use a controlled pulse of air todisperse the spacers 7hich then settle onto the su strate. Many industry applicators consistof a glass o+ into 7hich a fine cloud of spacers is introduced. The spacers then settle ontothe su strate. This is sometimes aided y static charge ut static in the cham er 9especiallyfrom tri oelectric effects: can cause spacers to clump9^= : together. The 'CI uses a medicalde ice called a ne uli#er9D F J: to disperse the spacers. A short pulse of air 9a hand3helddispenser unit 7or)s 0uite 7ell for this: is applied to the ne uli#er dispersing a fine cloud ofspacers from the outlet port. One ad antage to dry application is that re7or)ing is easy. Anitrogen gun can e used to remo e the spacers .

,ol ent assisted application can also e used. An air rush or spray ottle can e used 7itha safe sol ent to apply the spacers. This has the distinct disad antage of contacting thealignment layer 7ith the sol ent. This can destroy alignment if the materials are notcompati le or cause more su tle changes li)e pretilt lo7ering 9especially in high pretiltmaterials:. In addition su strates cannot usually e re7or)ed as the spacers tend to stic)0uite 7ell to the su strate.

One area of interest is in the de elopment of Vstic)yV spacers. If the spacers could e coated7ith some sort of adhesi e material adhesi e curing 7ould seal the entire area of the cellnot Kust the perimeter. This 7ould eliminate the need for pressing of the cell after filling.


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#dge-Seal &pplications

'i0uid crystal displays are sealed using a thin line of adhesi e around the perimeter of thecell. A small port is left on one side for li0uid crystal inKection. ;or capillary filling t7oopposite sides are sealed .

There is no one perfect adhesi e for sealing li0uid crystal displays. The adhesi e must ea le to maintain ond strength in ery thin layers 94315 microns: stand up to typical heatand humidity 9for outdoor displays: and ond dissimilar materials 7ell. Adhesi es that ond7ell to oth glass and polymers do not al7ays ha e the desired characteristics. One solutionis to mas) the polyimide so that the perimeter is not coated] this can e done y screenprinting 9difficult to coat thin enough films: or y photolithography for some polyimides. Manycompanies de elop custom adhesi es or use com inations of commercially a aila le

rands. Adhesi es for 'C& s should also e non3reacti e 7ith the li0uid crystal and reactclose to 155 9little or no olatiles:.

*dge3seal adhesi es can e applied y automated or handheld dispensers or 7ith smallerdisplays y screen printing. ,creen printing can affect the alignment layer in the cell interiorthrough screen contact and so is not fa ored for larger displays. Automated dispenserscom ine programming features 7ith precise mechanical control of needle speed andadhesi e flo7 rate] they gi e e+tremely reproduci le results and allo7 for fle+i le processing.&ispenser units are a aila le from Asymte) Accudyne and *;& to name a fe7.

The dispensing method is usually most effecti e for medium iscosity materials.

!3cure and thermal3cure adhesi es are oth used in the 'C& industry. !3cure adhesi es

are easy to 7or) 7ith and 7ell3suited to dispensing application. Materials are a aila le from\orland Master Bond ThreeBond and 'octite. !3cure materials can e formulated toond 7ell to glass plastics and metals ut do not usually ha e the strength or temperature

sta ility of thermal cure materials . Thermal cure materials are usually more suited to screenprinting. The 'CI currently uses \orland s \OA 6 7hich is ery easy to 7or) 7ith ut 7hichhas limited temperature sta ility.

After application of spacers and then adhesi e to one su strate the cell is assem led yplacing the other su strate on top. At this point the t7o su strates can e aligned ifnecessary efore they are pressed together for the curing process. $hile curing thesu strates must e pressed together so that they rest on the spacers. Thermal cure

adhesi es are placed in a heat press 7hile ! materials are pressed together so that theadhesi e is e+posed. One method is to place the cell on a acuum plate and co er 7ith atransparent fle+i le material 9such as Sandi3$rap: that 7ill conform to the cell and lightlypress it do7n onto the spacers. At this point monochromatic light can e used to inspect celluniformity] if nonuniform \e7ton rings 7ill e isi le.

After curing the cell 7ill usually sho7 interference fringes in 7hite light . The reason for thisis that stresses in the glass gi e it a tendency to o7 and after sealing the t7o sides of thedisplay ulge9 H< D E=D?D: out7ards. These interference fringes usually form concentricrings around the cell center.

Cell Filling and Sealing Cell filling sometimes referred to as li0uid crystal inKection is done in a acuum cham er to


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minimi#e defects caused y trapped gases in the li0uid crystal material . Capillary filling issometimes used in a research setting ecause of its ease.

&egassing of the li0uid crystal is e+tremely important. If the li0uid crystal is not properlydegassed trapped gases 7ill com ine o er time to produce u les in the displayrendering it useless in most cases. $hen the cham er is e acuated trapped gases in theli0uid crystal material oil out. This process is sometimes aided y ultrasonic agitation of theli0uid crystal as 7ell as heating to the isotropic phase for filling. Although desira le heatingis a delicate process ecause e aporation of li0uid crystal can occur. All road rangenematic li0uid crystals are eutectic 9*ormed at the lo est *ree ing point 8 descri es a mi+ture especially analloy that has the lo7est free#ing point of all com inations or constituents or the temperature at 7hich this occurs:mi+tures of many different pure li0uid crystals each ha ing different apor pressures. Caremust e ta)en to pre ent e aporation of certain components 9mi+tures containing PCS3-2in particular are suscepti le to this:. ;or the same reason ultimate acuum pressuresshould not e smaller than necessary] 153-5 millitorr is usually ade0uate.

Once the li0uid crystal material has een degassed and ultimate pressure is reached thecell is ready to e filled. The li0uid crystal is rought into contact 7ith the display filling portand the material egins to fill into the cell through capillary action. If the port is co ered yli0uid crystal the cham er acuum can e slo7ly released to speed filling33the pressuredifferential et7een the cham er and cell interior 7ill force the material into the cell.

;illing stations are commercially a aila le in a num er of configurations ut are oftencustom made. The 'CI uses a ell Kar acuum cham er . The li0uid crystal material is placedin a trough9 D


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2ro"ess% 3li$nment la'er or 2lanar 4 s2ol'imide% 78 (7ita"hi 8u#ont) 2I9555Thinner% T+-:+;ix% ,i I the #ie"es ha/eunder$one( & V) a "leanin$ #ro"edure ut .ere l'in$ in a "lean dr' #la"e then rinsin$(&W XY ZH) in I23 and a"etone is usuall' ade uate(N LVQ) I the #ie"es are "oated .ith resist then [um#to , !%

, , For uartD #ie"es .ith remnants o .ax rom the di"in$( \ \[ N\ Y) (other.ise [um# to , 9)&'ou should "lari ' in .hat "hemi"al this Aind o .ax dissol/es (4ote% Se had ex#erien"edsome Aind o .ax that .e "leaned in toluene or ?5 min in ultrasound .ith heatin$ and anotherAind that .as sim#l' dissol/ed in a"etone .ithout heatin$) 3 ter this ste# do not #er orm , 91, ? ^um# to , 5

, 9 >.a the su strates all o/er .ith "otton .ool ( QJ) soaAed(_\LV) in deter$ent, : Rinse in 8I .ater or at least :- se", ? 2ut the su strates in deter$ent ultrasoni" ath or :-min, 5 Rinse in 8I .ater or at least :-s


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, 6 2ut in ultrasoni" ath o 8I .ater or ,-min & threetimes "han$in$ the .ater ea"h time (total:-min)

, ! 2ut in ultrasoni" ath o a"etone or :-min (I the #ie"es are "oated .ith resist start here), * 2ut in ultrasoni" ath o iso#ro#'l al"ohol ( I23 ) or :-min, + Rinse .ith 8I .ater and determine ho. "lean the su strates ' o ser/in$ .hether the .ater

.ets the su strates sur a"es I the 8I .ater ilm is not uni orm /isuall' then more a$$ressi/e"leanin$ is re uired Use the "lean su strates onl' and lea/e the un"lean ones aside or uture

re1"leanin$, ,- @lo.1dr' the "lean ones .ith "lean 4 9 2urit' s#e"% ++ ++, ,, >#innin$ and s.a in$ "an o tain a hel# ul inal "leanin$ #ro"edure% 2ut the "lean

su strate on the s#inner "entered on the "hu"A .ith the side to e "oated u# >#in it at5---R2; (ram# o ,----R2; #er se") or ,min (re"i#e +) and durin$ the s#innin$ #oursome I23 and s.a it I ne"essar' re#eat .ith a"etone

2. S!)strates Inspection:

9 , ooA on the su strates under the mi"ros"o#e usin$ , 95x and 9-x o [e"ti/es or urther"on iden"e in their "leanness The un"lean ones should not e used ou ma' tr' no. to re1"lean .ith the s#inner as #er , ,,

9 9 a' the $lass su strates on the o#ti"al lat under the $reen li$ht and .at"h the sur a"e ualit' (maAe sure the lat is "lean and its ottom sur a"e on a "lean di usi/e sur a"e I not "leanthen s.a it .ith I23 "are ull') I the inter eren"e rin$es(M X) are strai$ht lines then the\ su strate "an e used& ut i the "ur/ature o the inter eren"e rin$es is lar$er than hal a rin$ethen do not use it Srite a note on the ualit' o the su strates

9 : Re#eat 9 9 .ith the >i su strates ut here the o#ti"al lat is on to# o the >i .ith the are >iside ein$ ins#e"ted

9 ? For lon$1term stora$e (more than a da')& store the "leaned su strates in a "lean dr' ox .ithsili"a $el sa"As in it For short1term stora$e (u# to e. hours)& #la"e the "leaned su strates in a"lean 2etri dish that has a "lean "o/er until 'ou start the next ste#

3. Spin coating o" pol%imide:

: , Use the #ol'imide 2I 9555 diluted in its thinner( ( ZQ,%9- ' .ei$ht The ottle is Ae#t in there ri$erator and has ma$neti" stirrer(XY\ in it ( 2ut it on the stirrer (.ithout heatin$) or ,5min e ore usin$ I the solution is #re#ared or the irst time then it needs to e stirred or ,-hours e ore use Re#la"e the ne.l' #re#ared ottle e/er' : months

: 9 Fill a "lean s'rin$e (i not& "lean it irst .ith 8I .ater then .ith a"etone and I23 and dr' it.ith 4 9) and atta"h to its end a su 1mi"ron ilter

: : 2ut the "lean su strate on the s#inner "entered on the "hu"A .ith the side to e "oated u#>#in it at 5---R2; (ram# o ,----R2; #er se") or ,min (re"i#e +) and durin$ the s#innin$ #our some I23 This ste# is /er' hel# ul to ensure "lean sur a"e #rior to the a##li"ation o the #ol'imide For the >i #ie"es s#ra' a"etone and then I23

: ? I the .i#ers around the s#inner are .et due to the I23 or a"etone s#ra'in$& then #re era l're#la"e them as the /a#ours "an arri/e to the su strate and a e"t the 2I la'er (2hoto Indu"edla'er)

: 5 For the >i #ie"es onl'& the adhesion o the 2I .as ound to e etter i 'ou heat them or,-min at +- either in the o/en or on the hot #late #rior to 2I a##li"ation

: 6 Shile the su strate on the "hu"A& #our the #ol'imide on it rom the "lean s'rin$e throu$h asu 1mi"ron ilter until it is "o/ered Use ele"troni" $rade ilter to a/oid ioni" "ontamination

: ! For the >i #ie"es onl'% s#in at ?---R2; (ram# o 5--R2; #er s") or ,min (re"i#e *) This #rodu"es a ilm o 9-nm For ea"h rand ne. ottle "he"A the thi"Aness o the 2I ilm

: * For the $lass #ie"es onl'% s#in at ?---R2; (ram# o :---R2; #er se") or ,min (re"i#e !)


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: + 7a/e a looA at the a"Aside o the su strate and "lean it rom ex"ess 2I .ith a"etone ands.a ;aAe sure the a"etone does not arri/e to the to#side

: ,- a' the su strate on a "lean 2etri dish s#e"i i"all' or 2I "oated su strates and #ut inthe o/en at ,9- or :-min then hard aAe at 9?5 or , hour then let it "ool naturall' .ith"losed door The .hole #ro"ess o heatin$ the o/en& maintainin$ it at 9?5 or , hour and"oolin$ to sa' 5- should taAe a out : hours

: ,, It is #ossi le to Aee# the su strates in a "lean dr' #la"e or u# to !9 hours e ore $oin$

to ste# ?& other.ise #re era l' re1heat at ,9- or :-min `ee# the "oated #ie"es in a "leandr' ox .ith sili"a $el inside

4. R!))ing:

Using the rubbing machine:

? , @lo. some "lean dr' 4 9 on the drum "oated .ith the Ra'on ( \ bQ XQ) "loth? 9 2ut the su strates on the sta$e o the ru in$ ma"hine and .ith a #en maAe a si$n on one ed$e

or the ru in$ dire"tion On the >i #ie"e 'ou "annot maAe a si$n .ith a #en& so use the"onta"t or an' other si$n to maAe sure o the ru in$ dire"tion 3lso this /aries de#endin$ onthe $ratin$ dire"tion and .hether the "ell is TE or T; This in ormation "an e o tained romthe .a er trans er orm

? : Turn the motor o the ru in$ .heel O4 at ? 0olts? ? >lo.l' lo.er the .heel and .at"h or the /olta$e measured .ith the /oltmeter Shen the

/olta$e [ust starts to dro# ( e. m0olts)& sto# lo.erin$ and start mo/in$ the sta$e to.ards thema"hine in a s#eed su"h that ea"h su strate $et ru ed 5-1,-- times (re/olutions o the.heel) Ex"ess #ressure ma' "ause the 2I to start #eelin$ o the sur a"e

? 5>to# the /olta$e su##l' and hi$her the .heel so that it is #ossi le to taAe the su strates out? 6 TaAe the su strates out and lo. "lean dr' 4 9 on them? ! o/er the ru in$ ma"hine .ith its a$

Using manual rubbing:

? * 2ut the su strates on the sta$e o the ru in$ ma"hine and .ith a #en maAe a si$n on one ed$eor the ru in$ dire"tion On the >i #ie"e 'ou "annot maAe a si$n .ith a #en& so use the

"onta"t or an' other si$n to maAe sure o the ru in$ dire"tion 3lso this /aries de#endin$ onthe $ratin$ dire"tion and .hether the "ell is TE or T; This in ormation "an e o tained romthe .a er trans er orm

? + For ru in$&use the la"A Ra'on "loth stret"hed on a "u i"al ox (.ith a siDe that is eas' tohandle) and maAe sure it is re#la"ed e/er' 5- "ells @lo.1dr' 4 9 on it e ore use

? ,- Ru the su strates unidire"tionall' !- times @e ore ru in$ lo. some dr' "lean 4 9 to remo/e dust 8o not a##l' an' #ressure& sim#l' let the "loth eathers tou"h the sur a"e Inorder to maAe sure that the ru in$ is done in the same dire"tion 'ou ma' use a ri$id andstrai$ht .all in the le t side to a"t as a $uide to 'our hand

? ,, TaAe the su strates out and lo. "lean dr' 4 9 on them? ,9 >tore the ru in$ tools in a "lean dr' ox .ith sili"a $el sa"As inside

. Spacers applications and assem)l%:

5 , lean the su strates assem l' holder rom an' dust5 9 Use the s#a"ers sus#ension in U06, $lue inside the s'rin$e o the dis#enser

5 : 3##l' six dots& : alon$ ea"h side o the *mm axis o the >i su strate5 ? a' the >i su strate on the assem l' holder `ee# the $lass su strate on a "lean sur a"e5 5 Usin$ "lean t.eeDers handle .ith "are the se"ond su strate and la' it on to# o the , st so that

the ru in$ dire"tions are anti parallel to ea"h other


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5 6 2ress the su strates to$ether until 'ou start to see the inter eren"e rin$es then tr' to ad[ust the #ressure to $et one uni orm rin$e ;aximum num er o allo.ed rin$es is hal rin$e or, 5um "ell ccc In order not to $et de ormations a ter remo/in$ the #ressure& 'ou should a##l'it on the s#a"ers 2ressure should e the minimum ade uate

5 ! I the num er o rin$es is lar$e& it means 'ou ha/e stron$ .ed$e (HN YZ &N\Z H) or de ormationsori$inatin$ either rom the su strates themsel/es or rom dirt There is no #oint in "ontinuin$.ith these su strates Tr' re1"leanin$ and re#la"in$ the s#a"ers and i it does not im#ro/e&

thro. the su strates a.a' and use ne. ones5 * I 'ou are satis ied .ith the assem l' then "ure the $lue .ith the U0 $un rom to# or 6-

se"onds ;aAe sure 'ou .ear the U0 #rote"tion $lasses and don t looA dire"tl' on the eamThe distan"e o the $un li$ht out#ut rom the su strate should e the minimum ade uate ( , "mis oA) I 'ou ha/e to shine U0 li$ht rom lar$er distan"e then in"rease the ex#osure timea""ordin$ to the s uare o the ra"tional in"rease o the distan"e

5 + TaAe the assem led su strates out o the holder and seal the "ell sides .ith U0>+, alon$ the*mm and #art o the 5mm sides lea/in$ inlet and outlet ,mm a#ertures or illin$3##li"ation o the $lue rom the sides ser/es also as a sealant(M\ b Q\Y)

5 ,- ure the U0>+, sealant usin$ the 7$ lam# or *min .hen the "ell is 9"m elo. thei re exit,a5e s!re %o! co-er the acti-e area o" the &I to )loc5 the U "rom irradiating it

5 ,, a' the o#ti"al lat on the "ell sur a"es under the $reen li$ht and .at"h the ualit' oea"h sur a"e Srite a note on the ualit' o the sur a"es and .hether the assem l' has

#rodu"ed an' de ormation5 ,9 lean and the dis#ensin$ tools .ith a"etone and I23 & lo. dr' .ith 4 9 and store in a

"lean dr' ox .ith sili"a $el sa"As in it

6. 0 "illing:

6 , TaAe the ottle to e used rom the re ri$erator e. minutes e ore usin$6 9 Turn the o/en O4 to start heatin$ u# to : de$rees a o/e the "learin$ tem#erature (,-- or

E??) For the /a"uum o/en it taAes 6-min to heat u# to ,-- & so 'ou ma' .ant to turn it on e ore the illin$ ' 6-min

6 : 2ut the assem led su strates in a "lean 2etri dish s#e"i i"all' or this #ur#ose and maAe surethe o/en is #re1heated to the "learin$ #oint

6 ? Sith a shar# "lean needle stored in a s#e"ial ox& taAe a small dro# o the material romthe ottle and #ut it on the ed$e et.een the t.o assem led su strates The .ill start

illin$ the $a# ' the capillar% s!ction e/en at room tem#erature6 5 Immediatel' #ut the 2etri dish .ith the su strates in it in the o/en and #um# it do.n to

,5-m@ar or 9min & then slo.l' release the #ressure to normal

6 6 Turn the o/en OFF or i 'ou ha/e an alread' #re#ared #ro$ramme& use it so that it starts"oolin$ at rou$hl' a rate o - 5 #er min et.een ,-: till 78 & then 'ou ma' in"rease the"oolin$ rate i 'ou .ish to ,19 #er min This latest rate "an e a"hie/ed ' turnin$ o theo/en and o#enin$ the door sli$htl'

6 ! ou "an taAe the sam#le out .hen the tem#erature is elo. ?-6 * lean and the illin$ tools .ith a"etone and I23 & lo. dr' .ith 4 9 and store in a "lean dr'

ox .ith sili"a $el sa"As in it

9. Finishing and ell Inspection:

! , lean "are ull' an' ex"ess material .ith a dr' s.a 8o not !se sol-ent #lease as these

"an mi$rate inside the "ell! 9 0isuall' looA at the "ell under the mi"ros"o#e I it looAs oA to 'ou in terms o uni ormit' and

"leanness& [um# to the next ste#& other.ise there is no #oint to "ontinue @est o ser/ation is.hen the #olariDers are "rossed and rotate the "ell to see that the extin"tion #osition (darA


21/21

#osition) is o tained .hen the #olariDer axis is alon$ the ru in$ dire"tion or #er#endi"ular toit Retardation "olours are est o ser/ed .ith the ru in$ dire"tion is at ?5 o to the #olariDeraxis

! : a' the o#ti"al lat on the "ell sur a"es under the $reen li$ht and .at"h the ualit' o ea"hsur a"e Srite a note on the ualit' o the sur a"es and .hether the illin$ has #rodu"edan' de ormation

! ? >eal the 5mm ed$es o the "ell .ith U0>+, and "ure it .ith the 7$ lam# or *min hile

protecting the 0 area "rom the U For #rote"tion use a metalli" sheet or a >i sheet .iththe "orre"t siDe 8istan"e o the "ell rom the i er exit should e 9"m

! 5 Remo/e the 2I rom the "onta"t area ' s"rat"hin$ the 2I .ith shar# needle! 6 onne"t the .ires usin$ sil/er #aste and some e#ox' on to# to hold them For "onsisten"'&

the la"A .ire to >i and the red to $lass ; ?@A BCD ! ! lean "are ull' the sur a"es o the su strates! * O ser/e retardation "olours A = > ?@A)) .ith , 95x o [e"ti/e& est o ser/ed .ith the

ru in$ dire"tion is at ?5 o to the #olariDer axis and the anal'ser > H )) is "rossed to the #olariDer

! + 3##l' lo. re uen"' (u# to 9-7D) ,-0##& s uare .a/e to the "ell and .at"h i it is li"Aerin$

! ,- TaAe #i"ture o the retardation "olour ma# at% -0& 50##& ,-0## at ,--A7D and at - 919A7D looAin$ or an' "olour nonuni ormit' di erent rom that at -0 2i"A the re uen"' that$i/es the .orst "olour nonuni ormit'

liquid crystal display fabrication

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