EP0393128A1

EP0393128A1 - Liquid crystal optical devices having a controlled surface order gradient

Info

Publication number: EP0393128A1
Application number: EP19890900833
Authority: EP
Inventors: Georges Antoine Durand; Philippe René MARTINOT-LAGARDE; Bimla Labroo; Marguerite Boix; Mohamed Monkade
Original assignee: Centre National de la Recherche Scientifique CNRS
Current assignee: Centre National de la Recherche Scientifique CNRS
Priority date: 1987-12-17
Filing date: 1988-12-16
Publication date: 1990-10-24
Also published as: FR2624985B1; FR2624985A1; WO1989005993A1; JPH03502969A

Abstract

La présente invention concerne le domaine des dispositifs à cristaux liquides. Selon l'invention une rugosité de l'ordre de grandeur moléculaire du cristal liquide est définie sur l'une au moins des plaques. Cette rugosité entraîne un gradient d'ordre qui induit une polarisation ordoélectrique associée à un champ dépolarisant qui tend à forcer les molécules de cristal liquide selon une orientation oblique par rapport à la plaque. Le contrôle de l'épaisseur et de la direction préférentielle de rugosité permet de plus de contrôler l'orientation oblique psi et l'orientation azimutale phi des molécules dans une plage continue de variation. La présente invention permet également de réaliser des interfaces non polaires pour les afficheurs à smectique C* ferroélectrique.The present invention relates to the field of liquid crystal devices. According to the invention, a roughness of the order of molecular size of the liquid crystal is defined on at least one of the plates. This roughness results in an order gradient which induces an ordoelectric polarization associated with a depolarizing field which tends to force the liquid crystal molecules in an oblique orientation relative to the plate. The control of the thickness and of the preferential direction of roughness also makes it possible to control the oblique orientation psi and the azimuthal orientation phi of the molecules in a continuous range of variation. The present invention also makes it possible to produce non-polar interfaces for ferroelectric C * smectic displays.

Description

LIQUID CRYSTAL OPTICAL DEVICES HAVING A SURFACE ORDER CONTROLLED GRADIENT.

The present invention relates to optical devices using liquid crystals. 5. The present invention was made in the Laboratory of

Solid State Physics of the University of Paris Sud, laboratory associated with the NATIONAL CENTER FOR SCIENTIFIC RESEARCH number 04 0002. Numerous research studies have been carried out for at least fifteen years on liquid crystals.

10 Different results of research carried out at

Laboratory of Solid State Physics of the University of Paris Sud are described in the French patent application filed on April 28, 1982 under number 82 07309 and published under number 2 526 177, the French patent application filed on October 23, 1984 under the number 84 16 192 and

* - ^• published under the number 2 572 210, the French patent application filed on June 18, 1985 under the number 85 09224 and published under the number 2 587 506, or the French patent application filed on May 14, 1986 under the number 86 06916 and published under number 2 598 827. In addition, work relating to liquid crystals has

- resulted in numerous publications.

We will cite for example the documents: 1) Appiied Physics Letters volume 25, N. 9, November 1974, Urbach and Al: "Alignment of nematics and smectics on evaporated films", pages 479 to 481, 2) Letters in Applied and Engineering Sciences , volume 1, 1973, pages 19-24, Guyon

25 and Al: "On different boundary conditions of nematic films deposited on obliquely evaporated plates", and 3) Physical Review Letters, volume 56, N. 19, May 1986, Durand et al: "Order electricity and surface orientation in nematic liquid crystal , pages 2056 to 2059 ".

The document 1) Applied Physics Letters, volume 25, N. 9, November 0, 1974, reports experimental tests operated on a liquid crystal device comprising a plate on which a deposit of gold or SiO, with a thickness of d from 50 to 1000 A, has been previously carried out by evaporation under oblique incidence. According to the results recorded in this document, it is possible to obtain either a planar orientation of the liquid crystal, that is to say molecules of liquid crystal oriented parallel to the plates of the device, or an oblique orientation of the liquid crystal, that is to say ie liquid crystal molecules tilted relative to the plates. According to this document 1) page 480, lines 43 to 46, the oblique inclination of the molecules relative to the plates is practically independent of the thickness of the coating deposited by evaporation. Document 2) Letters in Applied and Engineering

Sciences, volume 1, pages 19-24, 1973, offers a modeling of the results recorded in document 1). This modeling concerns the theoretical case of an evaporation deposit having a thickness of 100 to 6000 A. The document 3) Physical Review Letters, volume 56, N. 19,

May 1986, relates a theoretical analysis concerning the influence of the perturbation of the nematic order, at the level of a nematic / isotropic liquid interface. It emerges from this analysis that the order perturbation at the interface is accompanied by an energy which forces the molecules at an angle called "magic angle" defined by cos ² θ = 1/3.

The present ^* invention now aims to propose new means for defining, simply, reliably and economically, an oblique inclination of the liquid crystal molecules relative to a plate of the device. The present invention, which is based on numerous theoretical studies and experimental observations, proposes for this purpose to define on a plate ^" of the device, a roughness of the order of molecular magnitude of the liquid crystal considered.

Preferably, the roughness thus defined on the plate of the device will be of the order of magnitude from 20 to 40 A. The inventors have determined that such roughness results in an order parameter gradient which induces an ordoelectric polarization, associated with a depolarizing field which tends to force the molecules of the crystal in an oblique orientation relative to the plate. More precisely still, by extending their research, the inventors have determined that, by defining a preferential direction of roughness, for example by depositing on the plate a rough coating by evaporation in a determined direction, it was possible to obtain both. controlled oblique orientation and azimuthal orientation of the liquid crystal, relative to the plates. More precisely still, the oblique orientation and the azimuthal orientation of the liquid crystal evolve continuously, in an obi plane relative to the plates as a function of the thickness of the roughness and of its preferential direction, that is to say in the case of an evaporation as a function of the direction of incidence.

This arrangement is likely to give rise to numerous applications. Indeed, it suffices to control the thickness of roughness and its preferred direction to impose both the oblique orientation and the azimuthal orientation of the liquid crystal. Thus, for example, a bistable display with very low energy consumption can be obtained using a device comprising two parallel transparent plates, and a material comprising liquid crystal molecules placed between the two plates, at least one plates having on its surface adjacent to the liquid crystal a roughness of the order of molecular magnitude of the liquid crystal, the thickness and the orientation of the roughness being adapted to define two possible azimuthal orientations of the molecules of the liquid crystal, symmetrical with respect to to the preferred direction of roughness and of the order of 45 ° relative to this direction, and means making it possible to apply an electric field external to the device to cause the controlled tilting of the molecules from one azimuth orientation to the other. Document US-A-4 601 544 relates to a liquid crystal device having a bistable volume effect.

The documents "Conférence Accord of 1978 Biennal Display Research Conférence - Cherry Hill, NJ, 24-26 Oct 1978 pages 56-58;" Japanese Journal of Applied Physics vol 19 n. March 3, 1980 ";" J. Appl. Phys. vol 50 n. June 6, 1979 - pages 3975-3977 "relate to the observation of inclination of liquid crystal molecules relative to the plates under the effect of coatings deposited by them. The aforementioned documents briefly analyze, among other things, the influence of the thickness of the coating, but do not analyze by the influence of the thickness of the roughness, nor do they reveal the above-mentioned bistable effect.

Document FR-A-2 308 675 relates to micromachining, by means of an ion beam, of a coating deposited on the substrate of a liquid crystal cell. Other characteristics, aims and advantages of the present invention will appear on reading the detailed description which follows, and with reference to the appended drawings, given by way of nonlimiting examples and in which:

- Figures 1 to 4 show, for four experiments, the azimuthal orientation Y and the oblique bearing γ of the liquid crystal as a function of the angle of evaporation and the thickness of a deposit made on a plate,

- Figure 5 shows geometrically the oblique and azimuthal orientation of a liquid crystal molecule with respect to the direction of evaporation, - Figure 6 represents two curves illustrating the azimuthal orientation

and the oblique orientation ψ as a function of a thickness of deposit, for a determined angle of evaporation,

FIGS. 7, 8 and 9A, 9B schematically illustrate three applications of the invention _, to nematic liquid crystal devices, and

- Figure 10 shows curves illustrating the evolution of the oblique or¬ tion of a smectic liquid crystal, on each of the two faces of the device, depending on a thickness of SiO deposited.

GENERAL ANALYSIS

The present invention is based on the following theoretical analysis.

Nematic liquid crystals can be considered as liquids of oriented electric quadrupoles for which one can write: relationship in which:

_3

- -e represents the density of quadrupoles (-et \ t 10 ^~ cgs),

- S represents the order parameter of the liquid crystal (S is between O and 1),

- n is the unit vector representing the orientation of the molecules.

In the presence of spatial variations, we obtain the electrical polarization P = -V.Cj = 3/2 eV.S (hn -1/3).

We know that if, working at constant S, V acts on n, we obtain the flexo-electric effect and the polarization P becomes proportional to the curvature r. (V.rï).

If on the other hand we work with ^" n constant and S variable, we obtain the ordoelectric polarization: P = 3/2 e V S. [r> n - T / 3]. This polarization is generally oblique with respect to 7 S. With this polarization is associated a defined depolarizing field p "ar D z = O = 4TÏ P z + ε E z (where ~ Ϋ is the direction of VS).

The energy associated with a density

1/2 (ε / 4rc) E _Z ^~ = 1/2 π / e. ) P _Z ² .

This energy is minimum for P = O, that is to say for an oblique orientation θ with respect to z defined by cos ² θ = 1/3.

BASIS OF THE INVENTION AND FIRST TEST RESULTS

Starting from the general analysis above, according to which a gradient of the order parameter must lead to a stress on the liquid crystal in an oblique orientation relative to the plates, the inventors proposed the idea of controlling the order parameter S, on the surface of a liquid crystal device, by depositing on a plate of the device small grains whose size is of the order of magnitude of the molecular size of the liquid crystal considered, that is to say of l '' from 20 to 40 A.

The experiments were carried out using a nematic liquid crystal having an orientation parallel to the surface of the plate, but degenerate in the presence of a polished plate, that is to say of variable direction in a plane parallel to the surface of the plaque.

More precisely, the inventors have controlled the orientation of the liquid crystal for different pairs of evaporation angle and thickness of the deposited grains. The inventors have found that achieving a rough surface state by evaporation in a determined direction makes it possible to obtain not only an oblique orientation of the liquid crystal molecules relative to the plates, but also an azimuthal orientation which is controlled and variable with respect to to the direction of evaporation. The first results obtained are recorded in Figures 1 to 4 and 6 attached. These results are illustrated with reference to the geometric model represented in FIG. 5.

This geometric model includes a coordinate system with three axes x, y and z, orthogonal to each other. The axes x, y extend parallel to the plate P considered. The z axis extends perpendicular to this plate P, in the direction of the volume of the liquid crystal. The direction of evaporation arbitrarily coincides with the plane defined by the x and z axes. The incidence of the direction of evaporation, which corresponds to the inclination between the direction of evaporation and the z axis is defined by the angle.

The oblique orientation of the liquid crystal, which corresponds to the inclination between the z axis and the longitudinal axis of the liquid crystal molecules is illustrated by the angle θ in FIG. 5. The complement of this angle θ is referenced ψ . Finally, the azimuthal orientation of the liquid crystal, which corresponds to the inclination between the x axis and the projection on the xy plane of the longitudinal axis of the molecules is illustrated by the angle referenced ψ in FIG. 5.

Illustrated in Figure 1 attached is the evolution of the azimuthal orientation ^' -_- for MBBA liquid crystal as a function of the angle of evaporation o (and the thickness of a deposit of SiO on glass ordinary.

_ • _ ^β

For a deposit thickness less than 5 A, it can be seen that the liquid crystal molecules are oriented parallel to the plates, but in random directions. o

When the thickness of the deposit reaches 5 Å, the degeneration is lifted, that is to say that the molecules orient themselves parallel to themselves and perpendicular to the plane xz defining the direction of evaporation. At this stage the azimuthal orientation Ψ of the molecules equals 90 °, and the oblique orientation Q '- 90 °. This initial position of the molecules is illustrated diagrammatically in FIG. 5 under the reference M '.

When the thickness of the deposit still increases, we obtain as a function of the evaporation angle c (a gradual azimuthal rotation of the molecules in an oblique plane with respect to the plate P. The liquid crystal then passes from a domain for which the azimuthal orientation ψ equals 90 ° to an azimuthal domain for which can reach 0 °, that is to say a domain for which the molecules are oriented substantially parallel to the xz plane containing the direction of evaporation. azimuthal is accompanied by an increase in molecules; when T gradually goes from 90 ° to 0 °, ψ goes from 0 to about 20-30 °.

The hatched range in FIG. 1 corresponds to the range of values of the evaporation angle c. and of the thickness of evaporation for which the azimuthal orientation varies progressively between 90 ° and 0 ° and the oblique orientation ψ varies progressively between 0 and approximately 20-30 °. This progressive variation of the azimuthal orientation and the oblique orientation ψ, controllable as a function of the roughness, constitutes an essential characteristic of the invention, original compared to the prior art according to which only the terminals y "= 90 ° or 0 ° and ψ = O or 20-30 ° were known.

Comparable results are illustrated in Figure 2 attached. This FIG. 2 relates to tests carried out under the same operating conditions, that is to say deposition by evaporation of SiO on ordinary glass, for different values of evaporation angle and thickness of deposition, but with the difference of Figure 1, using liquid crystal 5 CB instead of MBBA.

Figures 3 and 4 similarly represent the value of the azimuthal orientation as a function of the angle of evaporation o (and the thickness of deposition of SiO, on an ITO plate, respectively for liquid crystal MBBA d ' on the one hand, and 5CB on the other.

Figures 2, 3 and 4, like Figure 1 above, reveal a range (hatched in the figures) of pairs of values, evaporation angle θ (/ thickness of deposit, for which the orientation azimuthal ~? gradually goes from 90 ° to 0 °.

FIG. 6 represents the simultaneous evolution of the azimuthal orientation T and of the oblique orientation Ψ as a function of the thickness of a deposit of SiO evaporated on an ITO plate with an incidence p. 74 °, using a 5CB liquid crystal.

In agreement with the results illustrated in FIG. 4, it is recognized that the azimuthal orientation γ passes progressively from 90 ° to 0 ° for a thickness of deposit substantially between 30 and 40 A. In parallel, it can be seen that the oblique orientation ψ gradually rises from 0 to 20 ° for an evaporation thickness ranging from 30 to 40 A.

In conclusion, the tests carried out by the inventors have made it possible to observe that the bare substrate, before evaporation, promotes a planar orientation, that is to say parallel to the surface, but degenerate. When the average evaporated amount (of SiO) reaches 5 A, the azimuthal degeneration is lifted, but the anchoring remains planar. Above 10 A of average evaporation, there is an increase in the nematic, which turns in a oblique plane from its initial planar position, perpendicular to the direction of evaporation, to an oblique position towards the direction of evaporation.

Of course, achieving a rough state on the plates, capable of generating an order gradient, and from there thanks to the induced ordoelectric polarization, to request a raising of the liquid crystal in an oblique orientation orientation relative to the plates, can be operated by means other than evaporation, such as for example, by chemical attack, spraying, or even ion bombardment.

"We will now discuss in the following description different examples of applications of the present invention, which are not limiting.

APPLICATION TO THE PRODUCTION OF SUPER TWIST NEMATIC DISPLAYS Nowadays, we are trying to develop nematic displays presenting anchoring directions on the two plates, inclined relatively by one. angle> greater than 90 °, with oblique orientation with respect to at least one of the plates, as illustrated diagrammatically in FIG. 7 appended, to allow the device to be multiplexed and to obtain satisfactory contrast. The rough surface state in accordance with the present invention makes it possible to easily obtain an oblique orientation of 10 to 20 ° relative to the plates, and a controlled azimuthal orientation.

APPLICATION TO THE PRODUCTION OF ANCHORAGES

BISTABLE NEMATICS.

The inventors have moreover determined that the molecules of the liquid crystal are capable of taking two symmetrical azimuthal orientations with respect to the plane xz containing the direction of evaporation.

We have thus schematically illustrated in FIG. 8 two azimuthal orientations symmetrical with respect to the plane xz defined by the direction of evaporation. By choosing the evaporation angle ^~ and ^thickness evaporation on a plate so that the azimuthal orientation with respect to the xz plane is in the range of 45 ° T, it is easy, by applying an external electric field provoke the controlled tilting of the molecules from an azimuthal orientation to the other on said plate. To cause the controlled tilting of the molecules from one azium direction to the other and vice versa, it suffices to apply alternately to the cell electric fields, for example parallel to the plates having respectively at least one component coinciding with one of these azimuthal directions if the liquid crystal has a positive dielectric anisotropy or orthogonal to one of these azimuthal orientations if the liquid crystal has a negative dielectric anisotropy. One thus obtains a modification of the azimuthal orientation of the molecules of the order of 90 °, making it possible to obtain a maximum contrast, if the device is placed between poiarisers.

In the absence of an electric field, the positions obtained from the liquid crystal remain stable.

If an electrode or plate is provided with a rough surface state, two domains are obtained: ____. γ.

If both electrodes or plates have a state of

rough surface one can obtain four domains: + ⁺ ^ κ ' ⁺ _* ~ a ~ ^ b' - ψ + ~ Ç _h ; - ψ - _. - ^considering that the indices a and b are respectively the upper and lower plates.

APPLICATION TO CRYSTAL DEVICES

NEMATIC LIQUID AT THE INSTABILITY THRESHOLD

In the context of the third envisaged application of the invention to the design of nematic liquid crystal devices, a planar and parallel anchoring is defined on the two plates of the device, as illustrated in FIG. 9A. However, on one of the plates, the angle of evaporation θ (and the thickness of evaporation of a deposit are checked to place the liquid crystal in the vicinity of the hatched transition range in FIGS. 1 to 4. the liquid crystal is thus placed in the vicinity of an instability threshold, that is to say in the vicinity of an azimuthal tilting threshold. This arrangement facilitates the tilting of the liquid crystal towards a helical formation, by application of an external electric field, as is known per se and as illustrated diagrammatically in FIG. 9B In a variant, it is possible initially to define a helical arrangement of the liquid crystal, close to a threshold of instability, and transform this helical arrangement into a parallel orientation of the molecules by application of an external electric field.

APPLICATION OF THE INVENTION TO THE CONTROL OF THE POLARITY OF SOLID-SMECTIC C * -FERROELECTRIC INTERFACES.

Ferroelectric smectic C displays, when they have a degenerate surface anchoring, theoretically allow two positions of symmetrical molecules with respect to the smectic layers. In one of these positions, the electric dipole of the molecules is oriented towards the electrode, in the other position, it is oriented towards the volume, by an external applied field. In theory, these two positions should be equivalent and allow the realization of bistable displays.

However in practice, it is found that instead of a bistable display between two symmetrical states, we obtain monostable displays on distorted textures whose contrast is poor and which cannot be multiplexed. The inventors have determined that this phenomenon is due to the fact that the electrode / liquid crystal interface is polar. This polarization seems to be chemical in nature (difference in affinity between the surface and part of the molecules).

The present invention makes it possible to cancel the chemical polarization by virtue of an ordoelectric polarization induced by the creation of a gradient of nematic order, in the smectic phase, of suitable sign and amplitude.

To obtain a good compensation for the chemical polarity, we can take highly polished interfaces, on which the nematic order is greater than the volume value, or rough interfaces on which the surface order is lower.

FIG. 10 illustrates the orientation 0, - of the liquid crystal thus obtained on the lower and upper device plates for different thickness values of a deposit of SiO operated by evaporation at an incidence θ of 81 ° on ITO plates separated by 3μm, respectively for temperatures of 55 ° C and 48.0 ° C.

Of course, the present invention is not limited to the above embodiments and applications. It extends to any variant that conforms to its spirit.

Claims

1. Method for preparing a liquid crystal device, characterized in that it comprises the step consisting in defining a roughness of the order of molecular size of the liquid crystal used on at least one of the plates.

2. Method according to claim 1, characterized in that the roughness produced on at least one of the plates of the device has a thickness between 20 and 40 A.

3. Method according to one of claims 1 and 2, characterized in that the roughness is produced on at least one of the plates of the device using one of the following techniques: deposition of a coating by evaporation, deposition of a coating by spraying, chemical attack or ion bombardment.

4. Method according to one of claims 1 to 3, characterized in that the roughness is carried out on at least one of the plates with control of a preferential direction of roughness, for example by control of a direction of evaporation, to obtain both an orientation of the molecules of the oblique liquid crystal with respect to at least one plate, and an azimuthal orientation of the molecules with respect to the preferential direction of roughness.

5. Liquid crystal device obtained by implementing the method according to one of claims 1 to 4 of the type comprising two parallel plates and a material comprising liquid crystal molecules placed between the two plates, characterized in that at least one of the plates has on its surface adjacent to the liquid crystal a roughness of the order of molecular size of the liquid crystal considered.

6. Device according to claim 5, characterized in that the roughness formed on ^the at least one of the plates has a thickness between 20 and 40 A.

7. Device according to one of claims 5 and 8, characterized in that a roughness is produced on each of the two plates of the device.

8. Device according to one of claims 5 to 7, characterized in that the liquid crystal is a nematic liquid crystal.

9. Device according to one of claims 5 a. 7, characterized in that the thickness and the orientation of the roughness on at least one of the plates are adapted to define an oblique orientation ψ of the liquid crystal with respect to at least one of the plates and directions of anchoring on the two relatively inclined plates of an angle / _> greater than 90 °, for the production of nematic displays superwisted.

10. Device according to one of claims 5 to 8, characterized in that the roughness has a preferential direction, defined for example by a direction of evaporation, that the thickness and the orientation of the roughness on the '' at least one of the plates is adapted to define two possible azimuthal orientations ψ of the liquid crystal molecules symmetrical with respect to the preferential direction of roughness and of the order of 45 ° relative to this direction, and that it comprises means allowing an external electric field to be applied to the device to cause the controlled tilting of the molecules from one azimuth orientation to the other, in order to produce bistable nematic anchors.

11. Device according to one of claims 5 to 8, characterized in that the thickness and the orientation of the roughness on at least one of the plates are adapted to place the liquid crystal in the vicinity of a threshold of azimuth tilting to facilitate this tilting when applying an external electric field.

12. Device according to one of claims 5 to 7,

_. characterized by the fact that the liquid crystal is a ferroelectric smectic C and that the thickness and the orientation of the roughness on at least one of the plates are adapted to define an ordoelectric polarization canceling the overall polarization at the surface.