CA1293317C - Liquid crystal electro-optical device - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A liquid crystal display comprises a pair of substrates provided with an electrode arrangement in matrix form. A chiral smectic C phase liquid crystal material is interposed between the substrates. In the liquid crystal layer, each pixel consists of a number of micro-domains which can be compared to polycrystal line structures. Interaction between adjacent pixels through well ordered liquid crystal structure is suppressed by virtue of the micro-domains.

Description

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The present invention relates to a liquid crystal device, particularly a ferroelectric liquid crystal device.
B ~EF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an explanatory view showing the condition o~
liquid crystal molecules interposed between substrates.
Fig. 2 is a schematic diagram show;ng two stable positions of a liquid cxystal molecule.
Fig. 3 is a cross sectional view of a liquid crystal device comprising a chiral smectic liquid crystal material.
Fig. 4 is a schematic view showing a micro-domain texture in a li~uid crystal layer.
Fig. 5(A) and 5(B) are microscopic photographs of micro-domains taken at two sites in a liquid crystal structure of 20 microns thickness in smectic A phase in accordance with the present invention with magnification of 200 by use of polarizing p]ates which are arranged normal to each other and sandwich the liquid crystal structure.
Fig. 6(A) and 6(B) are microscopic photographs of micro~
domains taken at two sites in a liquid crystal structure of 20 microns thickness in smectic C phase in accordance with the present invention with magnification of 200 by use of polarizing plates which are arranged normal to each other and sandwich the liquid crys-tal structure.
Figs. 7(A) and 7(B) are microscopic photographs of micro-domains taken at two sites in a liquid crystal structure of 3 microns thickness in smectic C phase in accordance with the present invention with magnification of .00 by use of polarizing plates which are arranged normal to each other and sandwich the liquid crystal structure.

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33~7 IIereloEore, it has becn known to u-tilize twisted nema-tic liquid crys-tals for electro-optical disp~ays. The liquid crys-tal materia:ls are employed in layer form, which is finely divided in-to a number of pixels by virtue oE a matrix electrode arrangemen-t contiguous to the liquid crys-tal layer. However, due -to occurrence cross--talk between adjacent pixels during operation in a -time mul-tiplexing mode, -the number or densities of pixels is subs-tantially limi-ted.

:l0 Swi-tching is performed by means of -thin film transis-tors provided for each pixel, -the driving mode being called the active matrix sys-tem. ~owever, because of -the complexities of -the manufacture process, it is very difficul-t to produce a dis-play having a large area at low cos-t.

In an attemp-t to solve the shortcomings of prior art devices, Clerk at al, proposed a ferroelectric liquid crystal device in -their U.S. Pa-tent No. 4,367,924. Fig. 1 is an explana-tory schema-tic diagram showing the action of liquid crystal mole-cules in -the prior ar-t devices. A ferroelectric liquid crystal is interposed between a pair of glass substrates 11 and 11' which is provided with an electrode arrangement made of In2O3, SnO~ or ITO (Indium Tin Oxide) on the inside thereof. The liquid crystal is arranged between the substrates so that each molecular layer 12 is formed normal to the substrates as illustrated in the Figure. The phase of the liquid crys-tal is chiral smectic C a-t which the device is driven, desirably at room temperature. The liquid crystal molecule takes two s-table positions I and II which make angles ~ and -0 with the layer normal as shown in Fig. 2.

The position of molecules switches between -the two stable positions in the light of an externally applied electric field normal to the subs-trates, whereupon visual images can be - la-I
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constructed based on differential birefringence among pixels.
One feature of this type of display devices is bistability by virtue of which the position of each liquid crystal molecule is maintained same as previous state even after the applied signal is removed until another signal is applied anew in the opposite sense. Namely, they can function as memory elements.

To such a ferroelectric liquid crystal device, it has been required to obtain a uniform state of liquid crystal without imperfections throughout the liquid crystal layer between a pair of substrates for uniform drive capability throughout the entire display area. The liquid crystal layer of this condition is referred to as ~mono-domain~ hereinafter.

Imperfections and defects are caused because of small flaws of orientation control films, unevenness , '~8--09--09 18:47 I~K ~ Y~ " 3 P.5 ~ 33~

of an electrode arrangement formed on the substrates, spacers and other causes. In order to avo~d occurrence of such imperfec~ons and defects, mono-doma-ln has been developed bY ~emPera~ure grad~ent method in which crYstalllne structure of liquid crYstal ls one-dlmensionallY developed inwardly from one end of the display area.
However, epitaxial growth of the smectlc Phase from a spacer edge under appropriate temperature gradient application of the gradient temPerature method is eFfectlve onlY when the displaY area of devices exceeds several squared cent~meters. Further-more, even if a large area mono-doma~n ls con-sturcted, the crystalline d~rect~on is not exactly aligned parallel to the substrates) mak~ng a ~re-tilted angle wlth the substrate plane. For this reason, l~quld crYstal t~olecular laYers tend to bend causlng zig-zag structures. The swltchlng due to external electr~c Fields may take place ~n reverse senses at the both sldes of a folding pl~ne in the zig-zag structure. It has been often observed that uniform display and driving performance are hindered bY the zig-zag structure.
The inventors have repeated exper~ments uslng l~quid crystal displays compr~slng a chiral smectic C
liqu~d crystal ~ferroelectric l~quid crYstal)~ How-ever, theY have ~ailed ~o sat~sFactorily drive the displays and to obtain clear images. This fail is supposed because of interaction between pixel~. The 'E~B--09--139 113:49 1<1< lt~ J~-t"--S:~*l~i"3 P.6 ~33 maln cause of lnteraction mlght be quasi-monocrYsta line (homogeneously ordered wlthout d~scontlnuitY) reglons bridsing ad~acent Pixels. In other words, the switch~ng of one p~xel m~ght influence an adiacent plxel through the mono-crYstall~ne region bridging therebetween.
SUMMARY OF THE lNVENTION
_______..________________ It ls an object of the present ~nvent~on to provide a liqui~ crystal devlce which can be driven wlth pixels dlstinctly.
In order to accompllsh the above and other objects, micro-domains are formed ~n each pixel con-slsting of a ch~ral smect~c liquid crystal layer. The micro-domain are reglons of several mlcrons width and several hundreds mlcrons length ~n wh~ch molecules are or~ented ln mono-crYstalllne form, but interfaces among the micro-domains constltute discontinuitY
whlch hinders ~nfluence of orderllness of one micro-doma~n upon others, Each pixel comPrlses a number of micro-domains. Pos~tlon of liquid crystal molecules can switch indlvisually ln each mlcro-domain by virtue of an electric f~eld exerc~sed thereuPon~ The lnteraction ~etween the mlcro-doma~ns subiected to an applied electr~c f~eld and a neighbouring micro-domalns free of the field are suppressed in the light of the lnterfaces therebetween.
The improved structure is produced by lnterpos-ing a comPosite l~quid crystal material between a Pair of substrates provided with an orlentation control ,, ~.t i 31~

surface in the inside, at a relatively higll temPera ture at whicll the 1-i~uid crYstal material is in an -Isotropic Phase~ and gradually coo.ling the liquid crystal material so that ordered arrallqenlent is deve-loPed w-ith nlicro-(lolllains.
Unlike convelltional structures in ~hich liquid crystal molecules are al~gned in a parlicular direc-tion, liqui~ crYstal molecules in accordance with the present invelltioll are aligned in diverse directions vhereby a number of micro-donlains are formed. Prefer-ably, -lhe average dimension oF said micro-domains with reference to the row dlrec-tion is one or nnuch order of magnitude smaller than that o-F said pixel.

~',.,~, :~q~ 17 DETAlLED-DEscElpTloN-oE-THE-pREEERRED-iMBoDlMENT-Rererring now to Fig.3 a linuid crystal dlsplaY
device in accor~ance with the Preserlt invention is illus-trated -in a cross sectional view. The device comPrises a Pair of glass subs-trates 2 and 2 a rirst parallel electrode strips formed on the sub-strate 2 and exten~ing in the lateral d-irection a second parallel electrode striPs formed on the sub-strate 2 and extenrling in the normal direction to the plane of lhe dra~/ing sheet a first orientatlon control Film 4 made of a Polyimide resin a second orientation control film 4 made of SiO2 and an ester -Ferroelectric liquid crYs-tal laYer 5. The ferro-'PE~-29-09 18:53 KK ~ "-J~ "3 P.9 ~3~

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electric liquld crys~al material ls a comPoslte chlral smect~c C Phase llqu~d crystal. The comblnation of the first and second electrode striPS 3 and 3' con-stitutes an electr~de arrangement ~n a matrix form compr~s~ng a number of pixels. By appropriatelY
selectton oF a suitable resin to form the orlentatlon control films, it is made Possible to relativelY rlse the threshold level of the swltch~ng signa1s aPplied to the llquid crystal laYer~ The first ~nd second orientation control ~lms 4 and 4' are given rubbins treatment, Assuming the scanning of the pixels is to be carried out ~n the directlon Parallel to the first electrode strips 3, the rubbing have to be performed in the directlon normal to the Plane of the drawlng sheet. In other words, rubblng is perfor,med along the columns in case of desisns adapted for scanning along the rows. The perlPherY of the substrates ls Provided wlth a sealing member 6 for avoiding a loss of liquid crystal, Reference numerals 1 and 7 deslgnate Polar zlng plates arranged at the right directions. The dimensions of parts of the device shown ln the flgure are only chosen for illustration but not in accordance with a real design. Although not shown in the flgure, when assembled, spacers are interposed between the substrates ln order to maintain the distance constant.
In realitY~ the d~stance between the substrate is 3 microns; the width of each electrode strip is 0.36 mm and seParated from an ad~acent str~p by 0.04 ~m intervals.

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'~18-l~9-~39 19:55 K~< J~ J~,-t"-'J~ "3 P.10 ~3317 Particularly, ~n accordance with the present inventlon~ ~he l~quid crYstal mater~al is prepared ln order to have a broad transltlon range wlthin whlch the phase of the llqu~d crysta~ ~s graduallY chansed from lts ~sotropic Phase to its ~uasl-crYstalline phase. In order to obtaln such a transition characte-ristic~ the llquid crystal matertal is Prepared bY
blending several k~nds of liquld crystals. The liqu~d crystal constituents are selected from those having d~verse transltlon temPeratures distr~buted over a wlde temperature ranse. We have ob~ained a comPosite llquid crystal mater~al having a wide transition temPerature range bY blending eight klnds of llquid crYstal esters.

What follow are e~ght l~quid crystal const~-tuents with respective Proportion ~n parenthesis which constitute a comPoslte liquid crYstal in accor~
dance with the Present invention.

NO 1 C8Hl7 ~ -C02 ~ C02 ~

Cryst<-(31.8~C)-~Smc1~<-(32.6C)-~Sm~-(53.0~C)->Iso NO 2~ ClOH21 ~ C02 ~ COz ~

Cryst<-(42.fi~C)-~Smc~<-(43.8C)->SmAc-(54.2~C)->Iso 8 ~.

9-09 1 ~1: 56 Kl< ~ j" 3 P . 1 1 3~

N0, 3: C8Hl 70~C02~0/~\
( 3 0 . 7 % ) Cryst<-(47.3C)->Smc~<~(47.80C) >SmA<-(~8.7C)->lso N0;4~ C8Hl70~C02~C02/~\

Cryst<-(75,9C)->Srn~*<-(136,3C)->SmA<-(162.2C)->lso N0 ~ ~ C l o H 2 l ~ C 0 z ~ C 2 /~/

Cryst~-(6l .5~C)~>Smc~-( l40.7C)->SmA<-( 164.3C)-~Iso o N0 . 6: C~H170 ~C2 {C~/
(0.5%) Cryst~-(83,3OC)->Smc~<-( l39~7C)->SmA<-( 152.4C)->lso O
N0 . 7 ~ C 8 H l 7 ~ C 2 ~

Cryst<-(102.0C)~>Smc~<~ 2.0C)-~SmA<-(137~C) >Iso N0 . 8: Cl oH2 1 o~C2 ~ /\
( 9 . 7 % ) cryst<-(27,0c)->sinc~<-(2g.3c) >SmA<-(55.0C)->Iso 9 , ~

'BEI-09-09 lE3:5E3 KK J~ L~JI,~ "3 P.lZ
~ 7 By b1ending the above constituents, a liquid crystal material was obta~ned wlth ~ts trans~t~on characterlstlcs such that Crystal~-(4.7C-3.5~C)->
S~ <-( 62.5C-51 .9C)->SmA<-(B9.7C-75 .4C)-~Iso It w~ll be not difficult for those skilled in the art to prepare liqu~d crystal mater~als whlch have transitlon characteristlcs suitable to the ap-plications. In accordance with exPerlmental, we pre-pared another liquld crYstal mater~al whose Phase transltion was such that lsotroplc liqu~d ~- (130C -98C) -> smectic A <- (73C - 60C) -> smectic ~C <-(10C - 0C) -> smect~c ~I <- (-10C) -> crystal.
After lnterpos~ng such a comPosite llquid crystal materlal between the or~entatlon control films pro-vided on the substrates 2 and 2' at a temPerature at whlch the liqu~d crYstal material is in its isotropic phase, the structure is graduallY cooled to a te~Pe-rature at which the entlretY of the composite liquld crystal mater~al ~s in its smectic Phase. By the way, liquid crystal molecules form micro-doma~ns whlch grow as the temPerature r~ses. Each micro-domain can be regarded to have a quasi-monocrYsta 11ne structure. The proport~on of each constltuent be~ng 5-20%. It w~ll be deslre~ ~n general to l~m~t the max~mum proportion of the constltuents up to 20 and to use many k~nds of liquld crystals at nearly equal proportlons resPectlvelY~ The ~ormation of micro-domalns in the l~quld crystal layer starts ;,,`~i 09-09 18:59 KK ~ $'~:J"3 P. 13 ~ 31"~

along the oriented surface given rubbing treatment so that the quas~-polycrystall~ne area conslst-lns o~
m~cro-domains gradua17y grows as the temPerature rlses. ~ig~4 shows a sketch topologically drafted ln accordance with a m~croscoPic photograPh of the micro-domains. The w~dth and the length of each m~cro-domaln 8 are about several microns and several hund-reds microns respectlvelY~ The signif~cant feature of the structure co~prising the micro-domalns is the absence of z~-zag defects. While the interfaces 9 between the micro-domains 8 are defects, the entirety of the liquid crystal can be regarded approximatelY
as a uniform ~uasi-crystalline structure except for the lnterfaces between the domalns which are small defects.
When a sawtooth pulse hav~ng a ~aximum voltage of ilOV ls applled between the electrode striPs 3 and 31, the molecular state of the p-ixel iust sandwlched bY the activated two electrodes is chan~ed. The state change takes place uniformly ln every mlcro-domaln 8 wtthin the plxel so that the entirety of the pixel is changed at once. In accordancc with exPer~mental, there was observed no dlfference in the chang~ng pro-cess between a central position and a Prepheral P~si~
tion o~ the pixel.
Figs.5(A) and 5(B) are microscopic Photosraphs showlng mlcro-domains taken at two sites of a liqu~d crystal layer 1n smectic A ln accordance wlth the present inventlon with ~agniflcat~on of ~OO bY use o~

11 ~i '8E3--09--09 1~:01 KK J~ y~I~J~-t"--'J~ j"3 P.14 polarizlns Plates which are arranged normal to each other and suppor~lng the liquld crystal therebetween.
The distance between a pair Gf substrates was selec-ted to be 20 microns rather than 3 mlcrons for tak~ng photosraPhs ln wh~ch mlcro-domain textures aPpear clearlY~ As shown in the Photographs~ the llquid crystal is devided ~nto a number of mlcro-domalns.
Thls ~eans that llquicl crystal molecules have been oriented unlformly in each micro-domaln but adiacent mlcro-domains have been orlented in different orien-tat~on dlrectlons. The micro-doma~ns have several m1crons in wldth and several hundreds microns in length. The m~cro-domains are formed aPProxlmately Parallel to the rubblng direct~on given to the orien-tati~n control films.
As the phase of llquld crystal ls transformed to smect~c C at a descended kemperature, strlpes appear in each micro-do~aln as seen from Figs.6(A) and 6(B) ~aken in the same conditlon as F~gs.5(A) and 5(B) except for khe temPerature. The wldth of each str~pe corresponds to one turn of a hellx of the chiral smectic C liqu~d crYstal and ls about 3 microns. As shown ln the photographs, strlPes of neishbourlns mlcro-doma~ns are aligned at the ~nter~aces. ~gs, 7(A) and 7(B) are photograPhs in the case that the dlstance between the substra~es ls about 3 m~crons, other condltions be~ng same as Flgs.5(A~ and 5(B).
The length o~ each mlcro-domalns become short as com-pared wlth Fl~s,6(A) and 6(B). Such a narrow dlstance 12 ~

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be-tween the substrates allows the liquid crystal helices unwin-ded, and therefore the response speed becomes as fast as 10 microseconds when measured ln the same conditlon as the case of 20 microns.

While several embodiments have been specifically des-cribed, it is to be appreciated that the present invention is not limited -to the particular examples described and that modifica-tions and variations can be made without departure from the scope of the invention.

Claims (9)

1. A liquid crystal device comprising a pair of sub-strates; a chiral smectic liquid crystal layer interposed between said substrates; an electrode arrangement for defining a plural-ity of cell regions in said liquid crystal layer in array form and for applying an electric field to each regions; and an orien-tation control surface formed on at least one inside surface of said substrates, wherein each region consists of a plurality of individual micro-domains in each of which liquid crystal molecules are aligned in accordance with one direction which is different from adjacent micro-domains contiguous thereto.

2. The device of claim 1, wherein said electrode arrangement defines pixels in matrix form.

3. The device of claim 2, wherein said orientation control surface is adapted to orient the molecules of said liquid crystal layer in parallel with the columns.

4. The device of claim 3, wherein the average dimen-sion of said micro-domains with reference to the row direction is one or much order of magnitude smaller than that of said pixels.

5. The device of claim 4, wherein the dimension of said micro-domains with reference to the row direction is several microns.

6. The device of claim 5, wherein the dimension of said micro-domains with reference to the column direction is of same order as that of said pixel.

7. The device of claim 5, wherein the dimension of said micro-domains with reference to the row direction is several hun-dreds microns.

8. The device of claim 1, wherein stripes are formed within each micro-domain in correspondence with the spiral pitch of the smectic liquid crystal.

9. The device of claim 8, wherein said stripes are aligned at the interface between neighboring micro-domains.