NEW APPLICATIONS OF LIQUID CRYSTALS J. Borel, G. Labrunie, J. Robert

To cite this version: J. Borel, G. Labrunie, J. Robert. NEW APPLICATIONS OF LIQUID CRYSTALS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-215-C1-230. .

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JOURNAL DE PHYSIQUE

Cotloque C1, suppldment au.no 3, Tome 36, Mars 1975, page C1-215

Classification Physics Abstracts 7.130 - 0.640

NEW APPLICATIONS OF LIQUID CRYSTALS J. BOREL, G. LABRUNIE and J. ROBERT Laboratoire d'Electronique et de Technologie de l'Informatique, Laboratoire de MicroClectronique, C. E. A.-C. E. N. G., Grenoble, France

R6sum6. - Les cristaux liquides ont BtB Ctudiks tout d'abord essentiellement en raison de leurs applications potentielles dans le domaine de l'affichage. De nombreux phknom6nes physiques peuvent &tremis en ceuvre pour cette application :la diffusion dynamique, l'effet de champ, la transition de phase, le changement de structure... On pensait en gknkral que les limitations fondamentales Btaient likes B la stabilitk, la vitesse, la facilitk d'adressage. Actuellement, la plupart de ces limitations ont Ct6 surmontkes, et de nouveaux domaines d'applications apparaissent, qui sont nBs soit de nouveaux effets physiques, soit .d'une comprehension plus complete du comportement electro-optique. A titre Cexemple, un dispositif d'affichage en temps rQ1, adressC en x -y (128 x 128 points), est pr6sentB : c'est ulie premiere &ape vers 1'Ccran plat de telkvision. D'autres applications sont passBes en revue dans le domaine du traitement optique du signal, de l'optique intCgree... ~bstract.- Liquid crystals were,.at first; studied mainly for their potential applications in the display field. Various physical phenomena can be used fofsucli an application : dynamic scattering, field effect, phase change, structure change... ,The general feeling was that basic limitations were related mainly,to stability, speed, ease of addressing. Most of these limitations are now overcome and new fields of applications appear, related either to newcbasicphysical effects or increased understanding of electfo-optical behaviour. As an example a real time x -y addressed display (128 x 128) is presented, an. early 'stage to a flat TV screen. Other applications in the field of optical signal processing, integrated optics... are foreseen.

Introduction. - For a very long time liquid crystals have been investigated as laborittory curiosities and no application has been emerging from the various studies. From 1888 to 1915 [I-21 the basic work led to no practical appli&tion. Since thst ddate a lbt of efforts were, made to 'control surface alignments [3] and material quality, and physical studies were made more practical. Among the three main classes of liquid crystals [4] : nematics, smectics and cholesterics, the last ones were used, for the first time, around 1950 for thermal pattern measurements. Most promising applications issued mainly from 'research work done a t RCA from 1967 to 1971 [5-6-74-91 and much of the basic understanding came from this period [lo t o 271 dealing mainly with nematics. Smectics are now being studied [28] and will probably lead tb original applications. We restrict ourselves to nematic and cholesteric liquid crystals 'applications and- even ih this field it is hard to be exhaustive due to the great variety of devices we can built. Non destructive testing using thermal properties of cholesterics is not considered [lo29-30] because much of the work'has already been presented a t this conference in 1970 [31]. In a first part we describe the main techniques using liquid crystal cells for displays with a particular

emphasis on the driving problems and solutions. Then we present a class o f electrooptic devices which can be used in optical signal processing. At last some other peculiar applications are given. 1. Present applications of liquid crystal cells :.the display field. - Generally we are dealing with electronic displays, that is to say, we convert electrical information ilito visual information. There are two main problems to be solved ; first to use the m,osf efficient way to apply the electrical power to each element of the display (this is the addressing technique) then to 'di,splay optically the data (either using the electrical power delivered by the addressing circuitry to produce light or using ambient light). We are used to saying that the cost of a display is the cost of the driving circuitry. This is often true and care must be taken to use economical techniques minimizing, for example, the number of interconnections in the display. As far as liquid crystals are concerned they use ambient light and are fairly'readable at high ambient light levels, but not readable in the dark [37].

1.1 WHY USE LIQUID CRYSTALS FOR DISPLAYS ? First, we can ask the question : why use liquid crystal cells to display data ? The answer is in the specific properties of such a display, figure 1 :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975140

J. BOREL, G . LABRUNIE AND J. ROBERT

C1-216

-EASY

1 IONS

TO BUILD (LOW COST)

-LARGE

RANGE OF CI-IARACTER SIZES NEMATICS

-GOOD -SMALL

MOTION~FIELD EFFECT F . E.

D. S.M.

T . N.

D.S.M

CONTRAST

(kIEMORY)

POWER CONSUMPTION CHOLESTERICS

F?S=F;C.SeHS

-TRANS~~IISSIVE OR REFLECTIVE MODES

L.C. DISPLAYS

FIG. 1.

- Why use L. C. cells to display data ?

D.S.M. F.E.:

: DYNAMIC SCATTERING MODE

FIELD EFFECT

T.N. :TWISTED NEMATICS - It is very easy to build and use cheap materials requiring less care than semiconductor devices for the technology. - A large range of character sizes and presentations are easily achievable (segmented or dot-matrix characters). - A good contrast is generally obtained though there still remains some problems concerning the viewing angle. - Small power consumption is related to the fact that they do not produce light and are generally high resistivity materials. Moreover, the required driving voltage levels are fairly compatible with integrated circuits and particularly with MOSFET integrated circuits, this technology being the cheapest nowadays. - Transmissive or reflective modes of operation can be used depending on the particular application and some configurations allow building color displays.

F.C.S.

: FOCAL CONIC STRUCTURE

.

P. S : PLANAR STRUCTURE H.S. : HOMEOTROPIC STRUCTURE MAIN

PHYSICAL EFFECTS

FIG. 2. - Main physical phenomena involved..

transparent without external applied field. The driving voltage, in every case, changes the molecular arrangement : - Either by disturbing the texture by ion motion. This gives a very strong diffusion of the incident light in nematics and the effect is known as dynamic scattering mode (D. S. M.) [6a, 7, 81. If small amounts of a cholesteric liquid crystal are added, a memory of the disturbed state is seen [6b]. Generally molecules with AE < 0 are used. - Or by changing the direction of molecules alignSome of the properties can be necessary or inacceptable in peculiar applications, but the general feeling ments (with AE > 0 or AE < 0) either in a parallel is that for most of the possible uses the price is the oriented texture (field effect) or in an helicoidal texture : most sensitive criterion and in this respect liquid crystal twisted nematics (T. N.) or cholesterics. In a parallel oriented texture, birefringence is induced by the displtiys are potentially highly competitive. electric field [36] ; cholesterics are arranged in an 1.2 PHYSICAL EFFECTS. - Let us now very briefly helicoidal texture with a pitch P along which the review the main physical effects used in liquid crystal orientation of molecules changes gradually. In thin displays (Fig. 2). Basically they are of two kinds : cholesteric layers two different kinds of textures either related to ion motion within the structure can be found. (electrohydrodynamiceffects [5,6a, 181) or to collective - The focal conic texture : the orientation of the alignment of molecules in the electric field with their helix axis is parallel to the plates, but generally random. average director orientation at rest either in the - The planar texture : the helix axes are oriented direction of the field (anisotropy of the dielectric perpendicular to the glass surface. constant AE = E~~ positive) or normal to the The focal conic texture can scatter light and the field (AE < 0). The medium being strongly bireplanar texture is transparent. Texture changes from fringent (An = ne - no 0.2 to 0.5), if the director the planar structure to the focal conic structure varies from one swarm to another, the sample (AE > 0) or even to the homeotropic structure can be scatters the incident light. The anisotropic properties obtained [37]. The texture change between planar of liquid crystal layers can be used if, at rest, monocrystalline layers sandwiched between glass plates are structure and focal conic structure can be induced obtained [33, 34, 351. This is a basic condition for by ion motion (memory D. S. M.) and the reverse any application. The molecules can be arranged either change by field effect (molecules with AE < 0) [38]. Among these physical effects some are preferred parallel to the plates (homogeneous or twisted strucfor the following reasons : ture) or normal to the plates (homeotropic structure). - They can be driven by low voltages : twisted Both arrangements are used in display cells and are

-

NEW APPLICATIONS OF LIQUID CRYSTALS

Comparison of typical electrooptic effects in L. C . Type of effect Property

-

Threshold voltage (V)

D. S. M.

F. E.

T.N.

-

-

-

5 to 10

1 to 4

0.9 to 4

neg 10

neg or pos 1

POS 1

As (molecules) Current (PA cm-2) Response : time

10 to 20 ms

10 ms

cc off )> Slope at threshold

100 to 2Q0ins poor

f 45O

Viewing angle

P . S . e F . C . S . F.C.S.SH.S. -

Remarks -

-

1391.

A. C. or D. C. drive

20 V/pm

POS 1

POS 1

5 ms

30 ms

-

30 ms good

200 rns good

I ms good

f 20°

f 45"

f 45O

D. C. drive For 1. 15 V driving voltage Depend on cell thickness For multiplexing A1 At--50 % I

-- 100 ms poor

f 45O

Memory Contrast

nematics for example because the,dielectric anisotropy of L. C. can be very large. - They have a wide viewing angle : D. S . M. or T. N. - They can be used for color displays F. E. - They have an internal memory : D. S. M. using a mixture of cholesterics and nematics, P. S. to F. C . S. transition. - They can be multiplexed in their addressing. All the materials used are high resistivity materials and very low power consumption is required. The main features of the displays using these physical effects are summarized in the following table : table I.

1. 3 ADDRESSING TECHNIQUES. - Contrast is a function of the addressing technique, the electrooptical effect used and is generally greater than 20. As previously mentioned an important feature of a display, in general, is its ability to be multiplexed. This property has a direct incidence on the number of interconnecting pads for the display and on the complexity of the driving circuitry, two parameters that influence the cost per character displayed within a system. Particular care, mainly for large size displays, must be taken to use a cheap addressing technique. As far as liquid crystals are concerned several problems arise : figure 3. - The slope of the electrooptic effect at threshold is generally poor and crosstalk between addressed and non addressed dots can be significant [40]. - The rise time, for driving pulse values compatible with integrated cricuits, is relatively long (10 ms range) compared with the decay time (100 ms range) allowing few characters to be multiplexed. - The viewing angle may depend on the fact that we use multiplexing or not (for a given driving voltage level multiplexing decreases the viewing angle). - Lifetime has been found to be very sensitive to the shape of the driving voltage (D. C., A. C. low or high frequency). Reasonable values are now obtained A

,SLOPE

AT THRESHOLD

-SPEED (RISE AND DELAY TIMES) -VIEWING ANGLE

PADS

-OUTPUT NBer

OF OUTPUT

PADS

0 PARALLEL

/ 50

7 SEGMENTS

40

DISPLAYS

I

:

0

;

:

;

:

5

:

:

;

!

; 10

3

NBer

OF

CHARACTERS

PROBLEMS IN L.C. DISPLAYS ADDRESSING FIG. 3.

- The addressing technique : the problems.

when avoiding D. C . drive (from 10 000 hours for D. S. M. to 50 000 hours expected for field effect). - As mentioned, the number of output pads must be reduced to a minimum. Figure 3 also shows the incidence of the addressing technique on the number of output pads as a function of the number of characters in the display. It is seen that this number quickly becomes prohibitive for parallel addressing, a particular situation to avoid when possible. From these considerations several techniques have been proposed and are summarized figure 4. They use either a build in parallel addressing, a x - y addressing

J. BOREL, G . LABRUNIE AND J. ROBERT

C1-218

,PARALLEL

,X-Y

ELECTRONIC ADDRESSING (ELECTRON BEAM)

ADDRESSING

,PARALLEL

(L.C. MEMORY)

ELECTRONIC ADDRESSING (I.C. MEMORY)

P A R A L L E L OPTICAL ADDRESSING (RCJ

-1.C.

MOUNTED ON

ADDRESSING

FIG. 4.

L.C.

CELL

TECHNIQUES

- Addressing techniques : solutions.

or an electronic addressing directly mounted on the display. This last technique combined with a x - y addressing of the liquid crystal cell is certainly the most economical. Let us now describe in more details each of these techniques.

provided the electrical connection between the addressing electron beam of the cathode ray tube and chromium dots acting as reflective electrodes. A mosaic of 3.1 cm x 3.1 cm was used and anode potentials of 20 to 25 kV were applied. The resolution of the display is approximately 150 to 175 lines over the 3.1 cmsquare viewing area and contrast ratios of 7.511 were measured with 450 viewing angle. More recently [42] such a technique has been improved and the following results have been obtained : - mosaic : 70 mm x 95 mm with conducting wires on 160 ym centers ; - electrooptic effect : D. S. M. ; - addressing conditions : TV standard, 625 lines, 25 images per second ; - measured resolution : 300 x 400 dots ; - contrast ratio > 1011 ; - applications : TV image projection or information display in high ambient light level.

+

An example of the operation of this display is given in figure 6. The limitations of this technique are related to the fact that : - vacuum is needed for the electron beam ; - flat displays cannot be built.

1 .3.1 Electron beam parallel addressing. -The electron beam parallel addressing was first proposed by van Raalte in 1968 [41]. He used a liquid crystal cell (Fig. 5) sandwiched between a standard transparent electrode (reflective mode) and a mosaic face-plate. The mosaic was a piece of thick glass in which were embedded 25 ym wires on 100 ym centers. This mosaic MOSAIC

1

NICKEL DOTS

1

LIQUID CRYSTAL fLASS

1

FIG. 6. - 300 x 400 dots resolution E. B. addressed display.

I'

CONDUCTIVE COATING VAN RAALTE

1968

ELECTRON BEAM PARALLEL ADDRESSING

FIG. 5.

- The parallel electronic addressing using an electron beam.

I . 3 . 2 x - y-addressing. - The x - y electronic addressing allows a solution to these two problems but other limitations occur, as will be seen later. Generally, we use a strong non linearity in the optical response versus control voltage. An example of such a nonlinearity is given in figure 7 corresponding to the relative light intensity versus the applied voltage for a D. S . M. operation. A high relative light intensity is obtained when applying half the voltage to each electrode of an addressed dot (or a similar value : one-third-select addressing scheme [43 to 471...). Two problems arise : - We need a sharp threshold in the effect to avoid cross talk between adjacent dots. - Cumulative effects on a non-addressed dot alter the contrast ratio.

NEW APPLICATIONS OF LIQUID CRYSTALS

The following results have been obtained :

RELATIVE TRANSMISSION

'

Electrooptical effect

-

D. S. M. T. N. F. E. C. N. transition

fI

0

Vo

0

I

I

Vo

C1-219

Number of colunms Contrastratio

-

8 to 16 32 50 128

z 10 > 10 > 10 (color) < 10

Reference 151-401

[@I [40-511

141

2-

VOLTAGE

2

As an example we see, in figure 8, two kinds of - y addressed displays built by the Thomson-CSF laboratory in Corbeville [51]. The first one is an 8 character multiplexed D. S. M. display driven by 15V with a power-consumption of 10 mW (electronic circuitry -I- display). The second one is a x - y F. E. matrix addressed display (12 x 18 characters of 35 dots). The supply voltage is 18 V and 2 imagesls can be displayed (size 60 mm x 60 mm). x

THE X. Y. ELECTRONIC ADDRESSING FIG. 7.

- The x -y electronic addressing.

The best situation is to use a pulsed a. c. addressing avoiding D. C. current degradation ; polarity symmetry [40, 50,651 gives a constant contrast ratio whatever the displayed picture may be. Depending on the nature of the electrooptical effect used, we obtain a more or less sharp threshold and cumulative effects always limit the contrast ratio.

Improvements are still possible in this field, but for larger displays, the main limitation is related to the ratio between rise time and memory time within the liquid crystal and, as mentioned, to the cross talk between dots. To increase the number of images/second erasing techniques can be used in certain configurations by utilizing dielectric relaxation versus frequency [52] or field effect erasing of D. S. M. [24], [471, [951. 1 .3 .3 Integrated circuit parallel addressing. The I. C. parallel addressing was first proposed in 1971 [53] with a static MOSFET shift register for the parallel addressing of the L. C. display dots. Other configurations can be used for a larger,matrix needing less electronic circuitry [53-541. The liquid crystal is directly placed on top of the I. C. where metal dot eIect~o4esare evaporated using standard I. C . technology. The layer of the liquid crystal is deposited on top of t h e j . C . and sandwiched between the evaporated metal dot electrodes, driven independently by the memory elements, and an upper nesa coated cover electrode. The addressing circuitry uses either static [53-541 or dynamic memorization [53]. In this technique each dot is driven in parallel by the electronic circuitry, solving :

- the threshold problem (no voltage on unaddressed dots), - the speed problem (the electronic circuitry can be addressed in a time much shorter than the rise time of the liquid crystal).

FIG. 8.

- x -Y addressed display.

On figure 9 we have shown two particular MOSFET circuits with very few (1 or 2) devices per dot. Both schemes use dynamic storage of the address in memory capacitance CM with either a D. C . (9b) or a D. C . or A. C. drive of the liquid crystal (9a). Grey scales can be obtained with analog dynamic storage of the driving signal.

C1-220

J. BOREL, G . LABRUNIE AND J. ROBERT

These displays are particularly attractive for - watch displays (small size, very low power consumption), - projection displays (reflective mode), - good visibility under strong illumination. L.C. CELL a ) A.C.

OR

D.C. DRIVE

CELL

O - * ~ & L . C .

D.C.

b)

DRIVE

CM

'THE

1 .3.4 Optical parallel addressing. - An other parallel addressing techniques uses a sandwich of a photoconductor and a liquid crystal. Several materials have been used for displaying the data : nematics [55, 56, 571 or cholesterics [12, 21, 36, 38, 58, 59, 60, 611. Illumination with various optical wavelengths has been done : such a cell structure built by Xerox [38] is given in figure 11. The local conductivity change

MEMORY CAPACITANCE

I.C. PARALLEL ADDRESSING (WbIAMIC

FIG.9.

MEMORY l

- The I. C. parallel addressing.

As an example, figure 10, gives a view of a 1 inch x 1 inch section of the Hughes liquid crystal display using the address circuitry of figure 9b. The main characteristics are : - 100 x 100 dots (MOSFET circuits drive), - 1 inch x 1 inch size, - D. S. M. operation (D. S. M. mode). The main limitations of this technique are :

- the yield problem of very large L. S. I. arrays, - the small size of the display.

FIG. 11. - Parallel optical addressing using a photoconductor.

of the photoconductor, due to imaging light, changes the voltage locally applied to the L. C. sandwich, causes an electrooptical effect (D. S. M., F. E. or F. C. S. to P. S.) change. One of the main problems is due to chemical and electrochemical stability of the cell ; the photoconductor and the liquid crystal generally operate under D. C. for good sensitivity. The photoconductor used must be chemically inert with regard to liquid crystal and alloys containing selenium have been proposed to solve this problem [38]. A. C . operation has also been reported [60]. The speed properties do not depend on the photoconductor characteristics and are limited by the liquid crystal itself. The main results obtained, using a focal conic structure to planar structure change in a mixture of dielectrically negative nematics and cholesterics, are given table IS. TABLEI1

Characteristics of the Xerox display [38]

FIG. 10. - I. C. parallel addressing of a 100

x

100 display.

Resolution . . . . . . . . . Sensitivity . . . . . . . . . Storage time ....... Contrast ratio . . . . . . Read-out efficiency . .

40 I/mm 25 ergs/cm2 (actinic) 24 hours for 30 1/mm 711 10 %

C1-221

NEW APPLICATIONS OF HQUID CRYSTALS

Figure 12 is an image on storage panel projected into a screen. This technique of parallel addressing is very attractive for : - projection, - optical amplification, - optical frequency change (between incident and projected light), but requires an optical image or a light deflector for the source.

rogra~hy 50. Xerox troduced

loan. copyw for 1 its

have

into ing -d

from 5 output pads and act as a buffer memory with each output connected to each segment of a 9 character display (63 pads). D. C. drive was used to simplify circuit technology, but A. C . drive is possible. Such a technique can also be used to make x - y addressing on the L. C. display. This would be the solution minimizing the overall number of interconnexions : 4 output pads and (7 + N ) = 16 internal connections (N being the number of characters). Let us point out that the number of output pads is indeent of the number of characters within the display. reviously mentioned, price per digit is the most rtant parameter to compare several technologies isplays. This has to be done within acceptable limits of readability, power consumption, and ease to decode. We give in figure 14, as an example, the

of dr

PRICE PER DIGIT ($1

in th

-8 DIGITS L . C. DISPLAY c.475")

im rnenze adv be en incor] a rcodern of ma chine coa

15

5 DIGITS L E D DISPLAY (.11") .I DlGlTS LED MODULE ( 4")

....

10 ( FEBRUARY 7 3 )

FIG. 12. - Example of optically addressed L. C. display.

5

1 .3.5 Integrated circuit on L. C. displays. - For displays using a limited number of characters (say 10 to 12), the driving circuitry can be mounted directly on the glass plate carrying the electrodes pattern. Very few output pads (say 4 to 5) are needed to supply serial information to the integrated circuit. Then this data is stored in the electronic circuit and used to drive the appropriate character configuration. Due to the low cost of large scale integrated circuits this seems to be a very attractive and economical solution. Such a display using D. S. M. has been proposed by Gerristma and Lorteye [62] and is shown in figure 13. Three integrated circuits in ILL technology receive power supply and input data

0

l.B

FIG..13. - L. C. cells with I. C. drive circuitry.

COST COMPARISON O F

LED'S AND L C DISPLAYS FIG.

14.

- Cost comparison of LEDS and L. C. displays.

price per digit versus the number of units delivered for two kinds of LED's displays and a L. C. display built by two representative manufacturers. As can be seen, ,the price per digit is similar for LED's and L. C. cells and can decrease very significantly as a function of the number of units sold. The character size of L. C. cells can be varied easily without any significant increase in price. In figure 14 we give data for a liquid crystal cell with characters 0.475" in height. LED'S displays have generally (mainly for several characters) smaller sizes than 0.11". Let us mention that these prices are related either to a well established technology (LED's) or to an emerging technology (L. C.). It is believed that the tendency will be to decrease L. C. display prices by a greater amount than LED'S displays, the limiting factor for LED'S being the cost of the semiconductor material. This is just a matter of time. For large character displays or panels L. C. are now the best solution.

J. BOREL, G. LABRUNIE AND J. ROBERT

Cl-222

These economic considerations must be taken in the light of the potential market. We give, in figure 15, some information on the needs in digitslyear for different fields of application related to LED'S and L. C . displays. The most important needs are for clock radios, hand-held caIculators,. and electronic watches where both techniques are well suited. Other fields of application are instrumentation indicators, automotive, appliances.. . If we consider an asymptotic price of 1 $ per digit, this corresponds to a potential market ranging around one billion dollals a year in 1980. This is to be compared with the computer market at that time (non IBM market) evaluated to 40 billions dollars and the computer peripherals market evaluated to 15 billions dollars. FIELD

DIGITS / YEAR

AUTOS

36 MILLION

CLOCK SEVERAL RADIOS

HUNDRED MILLION (1977)

30 MILLION (1978)

5 0 0 TO 1000 MILLION DIGITS / YEAR

TOTAL

HOMEOTROPIC (A&

AN