GB2244566A - Electroclinic liquid crystal devices - Google Patents

Abstract

An Electroclinic (EC) Liquid Crystal Device comprises a ferro electric smectic A type material in a liquid crystal cell. This cell is formed by two glass walls carrying transparent electrodes spaced 1-10 mu m apart by a spacer. The inside face of both walls is surface treated to align and provide a surface tilt to contacting liquid crystal molecules. Poor appearance, due to discontinuities within the smectic A layer, are overcome by an alignment that provides a surface tilt of 2-9 DEG . This alignment is provided by a layer of e.g. polyimide cured to about 200-300 DEG C for about 1 to 6 hours, then rubbed in a single direction with e.g. a nylon cloth. The liquid crystal director can be rotated typically by +/- 12 DEG by application of positive and negative applied voltages, typically about 30 volts. The angle of rotation varies approximately linearly with voltage. The device may be used as a spatial light modulator. <IMAGE>

Description

ELECTROCLINIC LIQUID CRYSTAL DEVICES This invention relates to smectic liquid crystal devices perticularly electroclinic liquid crystal devices.

Liquid crystal devices are well known for display devices and for light modulators. A typical devices comprises a liquid crystal cell formed by enclosing a layer of a liquid crystal material between two cell walls.

These walls carry electrode structures through which an electric field is applied across the liquid crystal layer to cause an electric field induced movement of liquid crystal molecules. In a display cell this movement of the molecules gives rise to an observable effect which is used to display information, eg digits in a time piece. In a light modulator this movement of the molecules causes a change in the amplitude, phase, birefringence, or refractive index depending upon the type of device needed.

There are three different classes of liquid crystal material, each with a distinct molecular arrangement; these classes are nematic, cholesteric, and smectic. Within the class of smectics there are many types of smectic materials, eg smectic A, smectic C, etc, some with and without chirality.

Some materials have many different liquid crystal phases between the isotropic and solid phase, others have just one liquid crystal phase. For example a material may show the following liquid crystal phases as it is cooled from the isotropic phase:- isotropic - nematic - smectic A - smectic C - solid. The description of a material as e.g. a smectic A is taken to mean the material has a smectic A phase over a useful working temperature range. Other liquid crystal phases may or may not exist above and below the working temperature range. Many display devices have been constructed using nematic and cholesteric materials. Until relatively recently few display devices had utilised smectic materials.

One type of smectic device using the ferro electric properties of chiral smectic materials (eg a chiral smectic C) is termed a bistable surface stabilized ferroelectric liquid crystal (SSFLC); this is described by N A Clark and S T Lagerwall, Appl Phys Lett 36 899 (1980). These devices require a surface alignment treatment, common in many display devices. One alignment treatment is oblique evaporation of eg silicon monoxide giving alignment to contacting liquid crystal molecules and a surface tilt of zero or about 300 depending upon the evaporation direction.Another alignment technique is deposition of e.g. a polyimide layer followed by unidirectional rubbing, giving alignment and a surface tilt of about 1 to 100 (measured using a nematic material) depending upon the polymer used and the amount of rubbing; see article by S Kuniyasu et al, Japanese J of Applied Physics vol 27, No 5, May 1988, pp 827-829.

A characteristic of SSFLC devices is that the liquid crystal molecules (or more correctly the director) switch between two different angular orientations on application of positive and negative dc pulses of appropriate magnitude; the device is bistable in these two different orientations. The angle between these two different orientations is fixed and depends upon the material used.

A recent addition to the family uses smectic A phase material and is termed a soft-mode ferroelectric liquid crystal (SIEFLC) or an electroclinic effect (EC) device first described by S Garoff and R Meyer, Phys Rev Lett 38 848 (1977). The present invention concerns such an electroclinic device.

One useful property of EC devices is an almost linear variation of the liquid crystal director orientation with applied electric field, the device is not bistable. Another useful property is the response times typically about 5 us at room temperatures although reports of 500 ns exist in the literature. As a comparison the response time of a twisted nematic display is about 50 ms. These properties make the electroclinic device suitable for many application of spatial light modulators as described in an article by G Anderson et al in J Appl Phys 66 (10), 15 Nov 1989, pp4983-4955.

One drawback with ferro electic devices is the appearance of discontinuities within the liquid crystal layer. In SSFLC devices this takes the form of zig zag lines; in EC devices roughly parallel lines appear. It is known to remove or reduce the zig-zag appearance in SSFLC devices by using high (e.g. 300) surface tilts and a long pitch cholesteric material phase above the smectic phase; this is described in G.B. 2,209,610 A, European 0,301,020- GB 2,210,468, European 0,299,970-2,210,469. The device is cooled from the isotropic phase, through the cholesteric phase where the large cholesteric pitch maintains the original alignment, and into the smectic phase with good alignment.

The present invention overcomes the poor alignment problems of EC devices by using a surface alignment that gives a surface tilt within a small range of angles. This may also be used in SSFLC devices to improve alignment and visual appearance.

According to this invention an electroclinic liquid crystal device comprises two spaced cell walls each bearing electrode structures and treated on facing surfaces with an alignment layer, a layer of a smectic A liquid crystal material enclosed between the cell walls, characterised by a surface alignment layer giving a surface tilt to contacting liquid crystal molecules of 20 to 9 , the alignment on the two walls being parallel in the same direction.

Preferably the surface tilt is in the range 4-7 .

Preferably the surface alignment is provided by a polymer layer such as a polyimide, rubbed in a single direction. The polymer layer may be cured by heating to around 150 to 3500C for 0.5 to 6.5 hours; the different curing temperatures and times vary the resultant surface tilt. The rubbing eg by a nylon cloth may be before or after the curing; different lengths of rubbing time and or pressure may be used to varying the resultant surface tilt.

The liquid crystal material has a smectic A phase at normal temperatures of operation, e.g. 5-350C or wider. At lower temperatures the material may have a smectic C or other tilted chiral phase where the electroclinic effect does not occur and the device would operate as an SSFLC device.

The invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figures 1, 2 are front and sectional views respectively of a reflective spatial light modulator drawn to different scales; Figure 3 is a diagrammatic exploded view of the modulator of Figures 1, 2; Figure 4 is a view of a layer of smectic A material showing molecular arrangement; Figure 5 is a side view of a layer of chiral smectic C material to a large scale showing two different arrangements of molecules; Figure 6 is a side view of a layer of nematic material illustrating alignment directions; Figures 7, 8 are graphs showing director angles obtained with varying applied voltages at different temperatures for two different smectic A materials.

As shown in Figures 1-3 a spatial light modulator comprises a liquid crystal cell 1 formed by two glass walls 2, 3 and a 1-10um e.g. 2.5 Fm, thick spacer 4. The inner faces of the walls carry thin transparent indium tin oxide electrodes 5, 6 connected to a variable voltage source 7. On top of the electrodes 5, 6 are surface alignment layers 8, 9 of rubbed polyimide described in more detail later. A layer 10 of a smectic liquid crystal material is contained between the walls 2, 3 and spacer 4. In front of the cell 1 is a linear polariser 11; behind the cell 1 is & quarter plate 12 and a mirror 13.

The alignment layers 8, 9 have two functions, one to align contacting liquid crystal molecules in a preferred direction, and the other to give a tilt to these molecules - a so called surface tilt - of a few degrees, typically around 4 or 5". The alignment layers 8, 9 are formed by placing a few drops of a polyimide onto the cell wall and spinning the wall until a uniform thickness is obtained. The polyimide is then cured by heating to a predetermined temperature for a predetermined time followed by unidirectional rubbing with a roller coated with a nylon cloth. Table 1 shows the surface tilt measured with different combinations of temperature and time for the material Probomide 32 (a product of the Ciba-Geigy Co).

Other materials will give different surface tilts; for example PI-130 (Nippon Chem Ind) gives a low tilt, whilst RN-369 (Nippon Chem Ind) gives high tilt, eg 4-100. The surface tilts of table 1 are given for nematic material since it is not possible to measure the tilt in the smectic phase.

Measurements are thus made on chemically similar but nematic materials, or a smectic material is measured whilst heated in the nematic phase.

The necessity for these alignment layers is as follows. A good spatial light modulator or display needs a uniform molecular ordering across the whole liquid crystal layer. The alignment layers 8, 9 provide alignment of liquid crystal molecules in contact with the alignment layer in the direction of the rubbing, R1, R2 Figure 1. In a smectic A material this results in a so called bookshelf alignment shown in Figure 4 where individual molecules form into sub-layers 14 perpendicular to the walls like books on a shelf. Within each sub-layer 14 the molecules, or more correctly the director, lie along the rubbing direction R. The rubbing provides surface anchorage and direction to the liquid crystal molecules which propagates across each sub-layer 14.

Additionally the two rubbing directions R1, R2 are in the same direction.

In a nematic material this has the effect of giving a splayed configuration to the director as shown in Figure 6.

A possible explanation, for this requirement of rubbing in the same direction, may be provided by considering alignment in a chiral smectic C material, used in SSFLC devices. Figures 5a, 5b shown alignment of sublayers 14 in a chiral smectic C material.

Within each sub-layer 14 the director in a chiral smectic material can lie along the surface of a cone; the projection of the cone axis into the cell walls is roughly along the alignment direction R. Frequently the sublayers adopt a chevron like appearance shown in Figure 5. The chevron can be pointed in either direction; when both sorts of chevron are present in a display, a series of spaced lines are observed which mar the usefulness of a device. For a good display only one chevron direction should be present.

This is achieved by arranging the rubbing directions R1, R2 parallel and in the same direction as shown in Figure 6, giving a splayed arrangement to the axes of the dones along which the director lies. If R1, R2 are antiparallel then both chevron geometrics are energetically degenerate, by rubbing R1, R2 parallel and selecting a suitable surface pretilt range this degeneracy can be broken allowing the formation of one chevron only.

It has been found that a further restriction is necessary to obtain good uniform alignment. That is, the amount of surface induced tilt must have a value high enough to give a splayed configuration as in Figure 6 but low enough so that so parts of a sub-layer 14 do not adopt a reverse chevron.

It has been found that best results are obtained when surface tilt is within 2.100, preferrably 3.70 This applies both to EC and to SSFLC devices.

The requirements for ferro electric devices, both EC and SSFLC, are:surface alignment, parallel in the same direction, with a surface tilt of 20 -i 00.

Table 1 Process (temperature, time) Pretilt O, E-7 Pretilt, mixture X 200 C 1 hour 10 - 3 Rub before cure 2000C 1 hour 3 00 - 10 Rub after cure 3000C 1 hour 4 10 - 20 Rub after cure 3000C 6 hour 7.50 30 - 40 Rub after cure The mixture X (nematic) of Table 1 = H1 + Me805F in equal portions.

Material H1 is an MBE ester mixture.

Materials E7, 764E, 870E, Me805F, are catalogue numbers of materials available from B.D.fl. Ltd., Poole, Dorset, England.

Instead of, or in addition to, the variation in temperature of curing, the amount of rubbing can be varied to vary the surface tilt.

Examples of electroclinic materials used in combination to give an S material:

F (5) R(O) g -C2H4c 2 4 R1 (H-G esters 2F (6) R e C2H4 + (O)R1 (ethyl linked)

In the above R, RI are alkyl chains Cn H2n+1 (9) BDH 764E (10) BDH 870E

Sc 150 Sa 610 N 880 I

Sc 28.80 Sa 98.50 N 115.20 Materials Details

ME 80 5F

A119 AS 500 (see below) 92.5%

I 1160 N 1040 S 64.8 S AS 500

Examples MBF

I 142.3 N 65.50 S 62 C Solid diFTP

I 1350 N 139 .5 S 105.50 S 55 C Solid Phenyl PYR

Solid 48.20 Sc 510 Sa 570 N 69.7 C I NCB

I 150.30 N 1500 Sa 118 S 520 S 81 C Solid HG ester

I 96.6 N 86.5 S 68 C Solid The molecular director is moved from the zero voltage position along the alignment direction, by an applied voltage. This movement is roughly in the plane of the liquid crystal layer, ie parallel to the cell walls 2, 3.

A negative dc voltage moves the director the opposite direction to that of a positive dc voltage. The movement and direction is due to the interaction of the applied electric field and the spontaneous polarisation coefficient Ps of the liquid crystal material. A typical value of Ps is 1.O- 100nC/sqcm or more. Typical voltages are +/- 10 to 30 volts for a layer 2.5 )rm.

Figure 7 shows how the electrically induced director tilt angle varies with applied electric field for the material A119. This material has the following phase sequence with temperature: Sc 150 Sa 610 N 880 Isotropic As shown in Figure 7 the variation of director angle with voltage is linear at higher temperatures but loses linearity towards the smectic A/smectic C phase transition temperature. However, greater tilt angles are obtained closer to this phase transition.

Figure 8 is similar to Figure 7 but for the material example (12).

Operation of the spatial light modulator is shown in Figure 3. The amplitude of reflected light is modulated by the electric voltage applied to the cell 1. Incident light is horizontally polarised by the polariser 11, then passes through the cell 1, and through the quarter wave plate 12 onto the mirror 13 and is reflected back again. The quarter wave plate 12 converts linear polarised light into eg right hand circular polarised light.

The mirror 13 reverses the sense of circular polarisation, to eg left hand circular polarisation, so that light reflected back from the quarter wave plate 12 is linear polarised and rotated by pi/2 from that entering the quarter wave plate 12. The cell 1 is arranged so that the director can be changed through +/- 11.250 by application of positive and negative voltage levels. One of the maximum deflection directions is arranged to be parallel with the polarisation axis of the polarisers 11. The cell director is deflected through a total angle of 22.5 . The net result of this and the arrangement of polariser 11, quarter wave plate 12, and mirror 13 is that the amplitude of reflected light can be electrically varied from clear to black in a linear manner.

Many other uses of the cell 1 are possible in both reflective and transmittive devices as described in J. Appl. Phy. 66 (10) 15 Nov. 1989 pp 4983/95.

Claims (4)

Claims:

1. An electroclinic liquid crystal device comprising two spaced cell walls each bearing electrode structures and treated on facing surfaces with an alignment layer, a layer of a smectic A liquid crystal material enclosed between the cell walls, characterised by a surface alignment layer giving a surface tilt to contacting liquid crystal molecules of 20 to 9 , the alignment on the two walls being parallel and in the same direction.

2. The device of claim 1 wherein the surface tilt is in the range 40to 70.

3. The device of claim 1 wherein the surface alignment layer is a cured polymer layer rubbed in a single direction.

4. The device of claim 3 wherein the polymer is a polyimide.