内容案例文件

22.3/T.Kretz

22.3/T.Kretz

798•SID02DIGEST

ISSN/0002-0966X/02/3302-0798-$1.00+.00©2002SID

SID02DIGEST•

PAGE

799

22.3:A3.4-inchReflectiveColourActiveMatrixLiquidCrystalDisywithoutPolarisers

T.Kretz,G.GomezandH.Lebrun

THALESAVIONICSLCD,Z.I.Centr'Alp,38430,Moirans,France

D.CoatesandS.Reaney

CRLOpto,DawleyRoad,Hayes,Middle UB31HH

A3.4"reflectivecolourAMLCDwasdevelopedusingthestandarda-Si:HTFTtechnologyandaverticallyaligneddyedcholestericliquidcrystalmixture.Optimisedliquidcrystalmixture,de-saturatedcolourfiltersandhighpixelapertureratiowereachievedinordertodemonstrateahighreflectivitywithanacceptablecontrastratioinalargeviewingcone.

Introduction

ThedirectviewreflectiveLCDsareattractiveproductsforhandheldcomputingand municationapplicationslikePDA,phonesore-bookbutalsoforportablegamemachines.In

fact,theyneednobacklightandusetheambientlightforillumination.Theseproductspresentthereforeweightedlightandlowpowerconsumptionwithanimprovedreliability.

MostreflectiveAMLCDavailableonthemarketuseypeliquidcrystalwithnaralignmentandtwistedmolecule.TheseLCmodesrequireasinglepolariseronthefrontsurfacewhichabsorb60%ofthe inglight[1].ThereforewelookedatotherLCmodesthatdonotneedpolarisersinordertoincreasetheefficiencyoftheLCD.InfactthesedisyswillbethenextgenerationofreflectiveLCDsonthemarket[2].

TheverticallyaligneddyedcholestericnematicliquidcrystalalsonameddyedDAPliquidcrystal(DAPforDeformationAufgerichtetePhaseinGerman)waspreferredbecauseofitsverygoodperformancesanditsfullcompatibilitywithexistingdesignandprocessforactivematrixLCD.Themaincharacteristicsoftheliquidcrystalmodeareliquidcrystalmaterial,dyeanddopantconcentration,cellthickness,pitchofthecholestericmodeanddrivingvoltages[3].Theywereoptimisedinordertoachieveoptimumperformancesintermofbrightness,contrastandresponsetime.

Thispaperwillpresentthedevelopmentofapolariser-reflectivecolouractivematrixLCDusingthepreferredverticallyaligneddyedcholestericliquidcrystal.Thedisy,basedonaregular480x480avionicstransmissivea-Si:HTFTLCD,wasadaptedtoreflectiveapplicationinordertoimprovethebrightnesswhilekeeagoodlevelofcontrastandgoodcolourpurity.Thequadpixelstructurewithred,green,blueandwhitesubpixeliswelladaptedtothetargetedapplication.Thewhitepixeloffersaverybrightalphanumericinformationwhilethecolourdotscanstilldisypictureswithsaturatedcolourfilterstransmission.Thetradeoffbetweencoloursaturationandbrightnessismovedtoatradeoffbetweenbrightnessandresolutionsincetheresolutionofthequadpixelarrangementissmallerthantheresolutionofthestripepixelarrangement.ComparedtotransmissiveactivematrixLCD,thecontrastratioofthedisyislimitedtoaratherlowlevelbytheliquidcrystaleffect(5:1forreflectivecomparedto>100:1fortransmissive).Thereforetheleveloflightleakagetoleratedatthepixellevelcanbeincreased.ThedesignoftheblackmatrixwasoptimisedinordertoincreasetheapertureratioandthustoincreasethebrightnessofthereflectiveLCD.

TheopticalperformancesofthisdisyhasbeenmeasuredandcomparedtoareflectiveTNAMLCDcommerciallyavailableonthemarket.

ReflectiveAMLCDfabrication

ThereflectiveactivematrixLCDwasmanufacturedwiththemostefficientdesignfortheactivematrixLCDimplementedinatransmissivetechnology.

Activete

Figure1showsthecrosssectionofourreflectivea-Si:HTFTcellstructure.ThereflectivepixelelectrodeisrealisedwithAlmetal

Figure1:CrosssectionofthereflectiveactivematrixLCD

depositedonthetopoftheITOpixelelectrodeofourregulartransmissiveactivematrix.ThislaststructureisbasedonaITOontoptechnologywithalightshieldontheTFTside.Thisstructurehasbeendescribedseveraltimesinpreviouspapers[4-5].Thankstothistechnology,theAlmetalcouldbepatternedwithstandardAletchingsolutionwithoutdamagetheexistinglayers.TheAllayerispatternedintheactiveareausingthemaskfortheITOpixelelectrodeandcompleyremovedintheotherpartsoftheLCD.Afterprocessing,theAlelectrodespresentaphotopicreflectivityaround84%.Thisvaluewasnotoptimisedbutwasacceptabletodemonstratethefeasibilityofhighreflectivitypolariser-colourAMLCD.

Colourfilterte

Asimplewaytohaveaverylargebrightnessonreflectivecellsistoremovecompleytheblackmatrix.Sotheopenapertureratio(OAR)issetbythewholesurfaceofthepixelelectrode,i.e.90%inourcase.Unfortunay,thisleadsfirsttoacontrastlossbecauseoftheparasiticreflectionsonthemetallayeroutsidethepixelelectrode(columnandgroundne)andsecondtoabadcolouruniformityonthecellsduetocrosscoloursbetweenthepixelsintroducedbycouplingoroverlayofcolourfilters.

Figure2:Pixelarrangement,openapertureratioandcoloursaturation

Consequentlytheblackmatrixcannotbecompleyremovedonreflectivedirectviewcells.Nevertheless,itcanbeoptimisedbyachievingthecolourfillignmentonaTFTsteppertoolinsteadonasingleexposurecontactalignerusuallyusedintheproductionlinesoftheCFareas.Inthiscaseasshownonthefigure2,itresultsinanarrowblackmatrixwithawidthcloseto11µmandapixelOARaround78.5%ontheglass.

Concerningthecolourfilters,theywerefabricatedusingadyedgelatinetechnology[6].Inthiscase,de-saturatedcolourswereobtainedbyloweringtheconcentrationofdyeinthebathesaswellasbydecreasingthesteetimeinthesesbathesinordertokeepthesteeoperationtimereasonable.

Table1:PhotopictransmissionCFcomparison

Transmissiveapplication

Reflectiveapplication(twopasses)

Red

18%

45%

Green

45%

67%

Blue

13%

29%

White

77%

77%

Thetransmissionafbackandforthtravelofthelightthroughtheresultingde-saturatedcolourfiltersaremeasuredwithamicrospectrophotomendpresentedonthefigure3.Forcomparison,thespectraofthecorrespondingtransmissivedisyareaddedtothisfigure.Thephotopictransmissionforthedifferentcoloursaresummarisedinthetable1.

ReflectiveAMLCD TransmissiveAMLCD

100%

90%

Transmission(%)

80%

70%

60%

50%

40%

30%

20%

10%

0%

400 450 500 550 600 650 700

Wavelength(nm)

Figure3:ColourfilterstransmissionversusthewavelengthfortransmissiveandreflectiveAMLCDs(twopassesthroughthefilterstakeinaccountforthereflectivedisy)

Theresultingcolourcoordinatesarepresentedonthefigure4onaCIE1931colourdiagram.WeaddedtothisfigurethecolourcoordinatesofbothatransmissiveAMLCDandanexampleofnewspaperpresentedbyT.Soneharaetal.[7].Hightransmissivecolourfilterswithwideoverlapofwavelengthtransmissionregionbetweenthecolourshadbeenfabricatedforthereflectiveapplication.Thesefilterslookverypasandtheareaofcolourgamutisaround4timessmallerthanthetransmissiveone.Nevertheless,itis3timeslargerthanagivennewspaper.

Figure4:ColourcoordinatescomparisonbetweenareflectiveandatransmissiveAMLCDaswellasagivennewspaper

22.3/T.Kretz

22.3/T.Kretz

800•SID02DIGEST

SID02DIGEST•801

Cellassembly

InadditiontothedyedDAPLCmixture,onlythepolyimidematerialisspecificinthefabricationofthereflectiveAMLCDscomparedtothe ransmissiveones.AstandardverticallyalignedpolyimidematerialfromNissanwasidentifiedandusedonthiscell.Concerningtheliquidcrystalmixture,thechoicewasdrivenbyagoodtradeoffbetweenthepredictedopticalperformancesofthereflectivedisy(contrastratio,brightnessandresponsetime)andthestandardcellgap(4.7µm)anddrivingvoltages(6V)fortheTNAMLCD[3].

Opticalproperties

TheopticalmeasurementspresentedhereafterwereachievedonreflectivedyedDAPcellswithoutfrontscatteringfi.ConventionalTNdrivingvoltages(6Vrms um)wereappliedonthedatalinesfromthesecells.

Contrastratio&brightness

ThecontrastratioandthebrightnessrelativetoaLambertianstandardwhiteofthecellsweremeasuredonareflectiveELDIMsystemunderdiffuseillumination.Forcomparison,asinglepolarisercolourreflectiveTNAMLCDcommerciallyavailableonthemarketwasmeasuredinthesameconditions.

AnexampleofisocontrastplotmeasuredonthedyedDAPcellisshowninfigure5.Thefigure6presentsacrosssectionofthecontrastratioalongthehorizontalaxis(Phi=0°).ThecurveobtainedonthereflectiveTNLCDisplottedonthisfigureforcomparison.A umcontrastratioof4:1isobtainedattheta=0°onthedyedDAPcell.ThispeakcontrastisacceptableforreflectiveapplicationsbutisnotasgoodasthecommercialTNdisy.OntheotherhandawideandsymmetricalviewingangleisperformedonthedyedDAPsamplecomparedtothecommercial

consideredasanoptimisedproduct.Infact,theTNproductexhibitstwopeaksat0°andcloseto20°forbothcontrastratioandbrightness.Thiscomportmentcouldbeexinedbyinternalstructuredreflectiveelectrodesliketheonepresentedinthepapers

[8]to[10].Theseinternaldiffusingreflectorsredirectthereflectedlightforgivenangles.Inapreferreddirection,onesgetsboththespecularreflectedlightandtheredirectedlight.Thusinthispreferreddirectionthereflectedlightintensityisincreased.

ReflectivedyedDAPAMLCDReflectiveTNcommercialAMLCD

50

45

Contrastratio

0

10

05

00

-80 -60 -40 -20 0 20 40 60 80

Tetha(°)-crosssectionatPhi=0°

Figure6:ContrastratiocrosssectionatPhi=0°.DyedDAPandTNreflectiveAMLCDscomparison

ThebrightnesspredictedonthedyedDAPcellbyasimpletheoreticalmodelwhichtakesinaccountthetransmissionorthereflectionvaluemeasuredoneachindividualpartofthecellfitswellwiththemeasuredone(seetable2).Theabsorptionofthelightintheverticallyalignmentmodewasestimatedusingtransmissivetestcells.Thelowtransmissionvalueinthiscaseisrelatedtotheorderparameterofthedyeintheliquidcrystalnematicmolecules.

dis y.

40%

35%

Brightness(%)

30%

25%

20%

15%

10%

5%

0%

ReflectivedyedDAPAMLCDReflectiveTNcommercialAMLCD

-80 -60 -40 -20 0 20 40 60 80

Tetha(°)-crosssectionatPhi=0°

Figure5:IsocontrastplotmeasuredonadyedDAPcell

Thefigure7showsthebrightnessasafunctionoftheviewinganglealongthehorizontalaxismeasuredonbothAMLCDsysed.Ourproductexhibitsasymmetricalbrightnesswitha umaround24%attheta=0°.ThisvalueismuchbetterthantheonemeasuredontheTNcommercialproductwhichcouldbe

Figure7:BrightnesscrosssectionatPhi=0°.DyedDAPandTNreflectiveAMLCDscomparison

Table2:EstimatedbrightnessforthedyedDAPAMLCD

CFtransmission(twopasses)

54.5%

Alreflectivity

84.0%

LCmixture&celltransmission

67.0%

OAR

78.5%

Total

24.1%

ByapplyingthesametheoreticalbrightnessevaluationmodelontheTNcellandonthedyedDAPcell,abrightnessof18.5%inthedirectionperpendiculartothesurfacecouldonlybeachievediftheinternalstructuredAlreflectorhibitareflectivityof150%.Otherwiseonly11%ofbrightnessipected.

ThereforebyimplementingimprovedAlspecularreflectors(photopicreflectivitycloseto95%)orstructuredAlpixelelectrodes(reflectivityaround150%),thebrightnessfromthereflectivecolourdyedDAPAMLCDcouldbestillimprovedrespectivelyto28%and44%.

Responsetime

Thetable3presentstheresponsetimesfromblacktowhitestateandviceversaaswellasfromagreylevel30%toagrey70%andviceversameasuredonthereflectivedyedDAPAMLCD.

Table3:ResponsetimemeasuredonthereflectivedyedDAPAMLCD

whiteblack

15ms

blackwhite

85ms

grey30%grey70%

70ms

grey70%grey30%

150ms

Theresultingresponsetimesforthesaturatedmodeoperationareacceptableformovingapplications.

Panelstructure

Table4sumsupthemaincharacteristicsandperformancesofthedevelopedpolariser-reflectivecolourdyedDAPAMLCD.Figure4showsaviewofthereflectiveLCDmoduleandanexampleofadisyedimage.Forthedemonstrators,afrontscatteringfilmcoveredbyanantireflectivefilmwasattachedonthefrontglassoftheR-LCDsinordertocancelspecularreflectionandimprovethecontrastratioandtheviewingangleofthedisy.

Table4:SpecificationofthereflectivedyedDAPTFTLCD

Generalities

Dis

ytype

AMLCD

Numberofpixels

480x480

Activeareasize

3.4"diagonal

Pixelsize

126x126µm²

Pixelarrangement

RGBWQUAD

Thickness

2.7mm

LCDmode

DyedDAPLC

Performances

Apertureratio

78.5%

Reflectivit