EP0910812A1 - Cell comprising a controlled anchoring plate for nematic liquid cristals and process for fabricating said plate - Google Patents

Abstract

The present invention relates to a liquid cristal cell comprising at least one controlled anchoring plate (100) comprising at least two superimposed layers (120, 130) having each an alignment effect on nematic molecules (150) located at the vicinity of a same surface (133) of the plate (100), both layers (120, 130) having different alignment directions (P, L).

Description

"Cell comprising a controlled anchoring plate for nematic liquid crystals and method for producing such a plate"

The present invention relates to the field of monostable and multistable devices based on nematic liquid crystals.

It has been known for a long time to obtain a uniform alignment of a nematic liquid crystal on the confinement plates thereof by means of appropriate surface treatments.

Films thermally polymerized and subjected to rubbing make it possible, for example, to obtain a direction of alignment of the nematic molecules which is coincident with the direction of rubbing, as described in [1] and [2].

This first type of treatment makes it possible to obtain surfaces with a uniform texture. There are also polymers which give an orientation perpendicular to the rubbing direction ([3]).

It is also possible to obtain oblique anchors thanks to simple layers of rubbed polymers.

These different results are very often justified by an elastic modeling proposed for a long time by D.H. Berreman ([4]).

This modeling is based on a relaxation process of excess elastic energy in the volume associated with the morphology of the anisotropic surface obtained by the rubbing process.

Anisotropic surfaces can also be obtained by means of an oblique deposition of SiO on a solid substrate, produced under vacuum and finely controlled. By this technique, it is possible to obtain different types of anchors (planar monostable, oblique monostable and oblique bistable). The type of anchoring obtained depends on the deposition parameters, essentially on the thickness of the SiO layer and the deposition angle. As described in document [5], the anchoring properties on the surfaces obtained by this latter technique can be part of a generalization of the Berreman model. A more recent surface treatment technique is linear photopolymerization (PPL). A photosensitive polymer film is, according to this technique, polymerized by means of linearly polarized UV light. The resulting anchoring direction is usually perpendicular to the polarization direction of the incident UV light, as mentioned by M. Shadt et al. ([6]).

As shown by J. Chen et al. ([7]), it is also possible to obtain nematic surface textures whose director is parallel to the direction of polarization of UV light.

The alignment of the nematic on the polymer is explained in terms of chemical transformations of the structure of the polymer, induced by linear photopolymerization. This technique does not subject the substrate to any direct mechanical stress. To date, bistable display devices have been proposed controlled by flexoelectric effect (see documents PCT / FR91 / 00496, WO92 / 00546 and US-5 357 358), or by electrochiral effect (see documents PCT / FR91 / 00052 and W091 / 11747).

An example of a bistable device can also be found in French patent application FR-95 13324 "bistable display device based on nematic liquid crystals allowing shades of gray".

This device is based on the creation of an erasable network of surface defects on a suitable substrate. It uses a weak azimuth anchor constituting a quasi-bistable planar anchor. This type of anchoring is obtained from a monostable surface having an easy axis n ₀ which corresponds to a very flat minimum of surface energy. The slightly oblique anchoring directions with respect to n ₀ therefore correspond to nematic surface states having only an energy slightly greater than the minimum energy corresponding to an orientation of the nematic along the easy axis n ₀ .

The topology of this type of anchoring allows electrical control (creation or removal) of surface defects (π walls), these defects acting as depolarizers for light. The operating mode of these different devices is based on the possibility of switching between two (or more) different optical states corresponding to different configurations of surface directors and therefore to different volume textures. The fact that the change in optical state of the cell is linked to a change in the state of the nematic on a surface makes it possible to obtain excellent performance in terms of electrical response time. Indeed, the surface dynamics are faster by an order of magnitude than the volume dynamics of the nematic liquid crystal, previously used in nematic devices.

In addition, the possibility of switching between two states as stable as each other makes it possible to obtain very wide multiplexing possibilities, which are essential in the techniques of addressing extended matrices, as can be seen in [ 8]. Thus, a person skilled in the art knows that by using the bistable anchoring properties of nematic liquid crystals, it is possible to produce electro-optical devices characterized by an electrical addressing time of the order of a few microseconds , and an optical response time of the order of a millisecond, therefore both very fast. These properties make it possible more broadly to build matrix devices of large dimensions with high resolution and infinite multiplexing capacities.

Surface treatments to obtain bistable nematic anchors on the same surface are of fundamental importance. Document [9] reports on systematic research on SiO coatings produced under vacuum, on solid substrates, to obtain nematic bistable anchors.

The methods referred to in this document, which are very appreciable in terms of anchoring properties, are not very adaptable to an industrial environment. On the one hand, it is difficult to coat extended panels of glass while ensuring suitable uniformity, and the bistable region on the other hand only exists for a fairly narrow range of deposition parameters. Furthermore, the anchoring characteristics of the SiO coating vary with temperature, and it is possible to break the limits of bistability only by modifying the temperature.

The possibility of improving the technology of nematic liquid crystal devices using bistable anchors today depends on the development of a new surface treatment which produces surface bistability and which is easily operable in an industrial process.

The present invention proposes to meet this expectation More generally, it provides a surface treatment which makes it possible to obtain azimuthal bistable anchors, planar quasi-bistable anchors or inclined anchors for nematic liquid crystals in which the anchoring direction or directions are finely controlled

This object is achieved according to the present invention thanks to a liquid crystal cell comprising at least one plate with controlled anchoring, comprising at least two superimposed layers each having an alignment effect on the nematic molecules located in the vicinity of the same surface. of the plate, these two layers not having the same direction of alignment Other techniques for obtaining surfaces with bistable anchors for nematics have been proposed

We know in particular the surfaces with bistable anchors obtained by double exposure of photo-oriented films of the Langmuir-Blodgett type to polarized ultra-violet light, as proposed in [14] We also know the surfaces obtained by two rubbings in two different directions, which have a double geometric pπdicity These surfaces have been proposed by the Defense Agency, Malveme, United Kingdom, and in particular by GP Bryan-Brown, MJ Towler, MS Bancroft, and DG Mac Donnel Another object of the present invention is to propose a surface treatment method having two anchoring sources in competition in which it is possible to control the relative anchoring forces of these sources This process is compatible with industrial constraints in terms of surface treatments.

A process proposed according to the invention is a process for producing a plate with controlled anchoring on its upper face, in which the following steps are carried out: a) production of a first layer capable of producing a first alignment effect on its upper face in a first planar direction; b) deposition on this first layer of a second layer capable of producing, optionally after an additional treatment, a second alignment effect on its upper face in a second direction different from the first, the second layer being capable of transmitting on its upper side part of the first alignment effect induced by the first layer.

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:

- Figure 1 is a sectional view of a plate according to the invention, the section plane being perpendicular to the mean plane of the plate and parallel to a main vector of undulations of a lower SiO layer according to the present invention;

- Figure 2 shows an orthonormal coordinate system linked to a nematic display device according to the invention, in which there is an orientation director of nematic surface molecules; - Figure 3 is a graph illustrating the dependence of an azimuthal anchoring direction of nematic molecules as a function of a duration of exposure of a plate according to the invention to polarized UV light;

- Figure 4 is a photograph of a bistable cell according to the invention, observed between two crossed polarizers and in which the optical axis of a straight region is aligned with one of the two polarizers; and

- Figure 5 is a photograph of the same bistable cell, rotated so that a light extinction condition is obtained for a region on the left. In the context of the present invention, the term "anchoring competition" means that the direction of the nematic surface molecules results from the effect exerted by several anchoring sources corresponding to different anchoring directions. As it is reported in the document [10], in the case of zenital anchors, a competition of anchors results in changes of orientation which are second order.

Figure 1 shows a bistable anchor plate 100 according to the present invention. In a preferred embodiment of the invention, the bistable plate 100 is one of the two opposite plates of a sandwich cell of known type. The second plate of this sandwich cell, not shown, is for example a homeotropic anchoring plate. This type of structure is commonly called a "hybrid" structure. More generally, the bistable plate 100 can also be part of any type of liquid crystal cell.

Thus, it may be the first plate of a sandwich cell whose second plate is a bistable anchoring plate which is identical to it, or whose second plate is monostable with finely controlled anchoring in accordance with the invention, the relative orientations of the anchors of the plate 100 and of the second plate of the sandwich cell which can be adapted as a function of the desired effect.

The plate 110 is preferably an isotropic confinement plate made of conductive ITO glass. The plate 110 is provided according to the invention with a two-layer coating

120/130.

In a preferred embodiment of the invention, the first layer is a layer of SiO, referenced 120, which has surface grooves which extend perpendicular to the section plane of FIG. 1, laterally offset one with respect to to the other according to a constant deviation λ.

The second layer is a film 130 of photopolymer deposited on the layer 120 of SiO. The SiO layer 120 is produced according to a known mode of vacuum deposition, with a deposition rate of 5 A / s and with an angle of 75 ° between the deposition beam and the normal to the surface of the glass plate 110 using a quartz balance perpendicular to the deposition beam and a deposition time of approximately 20 seconds.

This technique is described in [1] and [9],

As shown in FIG. 1, such a deposit gives rise to a layer consisting of an alternation of longitudinal bosses 124 and longitudinal depressions 126. The bosses 124 and the depressions 126 are directed in the same main direction contained in the plane of the glass plate 110. This main direction is perpendicular to the SiO beam during deposition.

The distribution of the longitudinal bosses 124 and the depressions 126 perpendicular to this main direction is uniform. Two consecutive boss peaks 124 are thus always separated by the same distance λ, at any point on the plate 100.

Likewise, the points of maximum depth of two consecutive longitudinal recesses 126 are separated by this same distance λ. The morphology of the layer 120 thus obtained is therefore uniform and unidirectional.

Each boss 124 and depression 126 has a substantially rounded contour, and each time has symmetry with respect to a central plane

Q, vertical and directed in said main direction. Thus, the intersection of the upper surface 123 of the layer 120 and the section plane of FIG. 1 defines a curve 122 substantially sinusoidal, of wavelength λ.

An amplitude A of this sinusoidal curve is defined as being the distance measured vertically between one of its vertices and one of its points of maximum depth.

Such a substrate 120 alone would constitute a unidirectional planar anchoring surface, in said main direction, for a usual nematic material. In other words, the anisotropy of the SiO film as well obtained could be revealed by a well defined uniform planar alignment, perpendicular to the SiO deposition beam ([9]).

A preliminary morphological analysis ([11]) carried out using an atomic force microscope (Autoprobe by Park Scientific) revealed that this SiO film has a surface roughness, that is to say an amplitude A, of the order of a few tens of Angstroems, which is compatible with the uniformity required for the polymer layer.

The upper surface of the layer 120 is therefore flat enough to allow good deposition of the polymer film produced on a roll. The photopolymer constituting layer 130 is a PVMC (Poly

Vinyl 4-Methoxy-Cinπamate by Rolic), deposited on a roller at 3000 RPM on the substrate 120, then treated according to a standard heat treatment at 120 ° C for one hour at a pressure of about 10 ^"3 mBar.

As shown in FIG. 1, the layer 130 of polymer thus deposited does not have an absolutely constant thickness. The thickness of the layer 130 is greater in the depressions 126 than at the vertices 124, so that the polymer tends to fill the depressions 126.

This phenomenon being uniform over the entire surface of the layer 120, and progressive from a given vertex towards one of its adjacent recesses, the upper surface 132 of the layer 130 also has a sinusoidal contour in the plane of the figure 1.

This sinusoidal contour 132 has an amplitude Aeff, measured in the same way as that of the layer 120 of SiO, which is substantially less than the amplitude A of the layer 120. This amplitude Aeff is uniform over the surface 133.

The upper curve 132 is, like the curve 122, substantially sinusoidal, of the same wavelength λ, Aeff is therefore the effective amplitude of the curve 132 or, in other words, of the surface grooving taking into account the fact that the Polymer film 130 smooths the initial topography of the SiO 123 surface.

The morphology of the surface 133 of the layer 130 therefore differs from that of the layer 120 in that its amplitude Aeff is smaller. The polymer layer 130 is then linearly polymerized, so as to induce a nematic attraction in the direction perpendicular to that of the coating of SiO.

Such an anchoring force in a direction perpendicular to said main direction, that is to say in a direction belonging to the plane of the confinement plate 110 and to the section plane of FIG. 1, is preferably obtained by photopolymerization linearly polarized in said main direction of the plane of the plate 110. The duration of exposure of the layer 130 to UV will depend on the anchoring force sought.

The linear photopolymerization of the PVMC film was carried out by exposure to broad spectrum ultraviolet light emitted by three 15 watt OSRAM fluorescent lamps, referenced HNS-OFR. These samples to be treated were placed at a distance of approximately 10 cm from the lamps. The incident light was linearly polarized using an Oriel 27320 UV dichroic polarizer.

In a preferred embodiment of the invention, the plate 100 constitutes the lower plate of a hybrid sandwich cell, the upper plate, not shown, is produced from a plate of conductive ITO glass, coated on its face lower DMOAP (dimethyl-octadecyl-3- (trimethoxysilil) propylammonium-chloride), so as to obtain a homeotropic anchoring.

The plate 100 and the upper plate, not shown, contain between them a nematic liquid crystal 150 which is for example a commercial 5CB (4'-n-pentyl-4-cyanobiphenyl) supplied by Merck. Of course, a bistable anchoring plate in accordance with the invention can be used in the composition of any other device using these bistable anchoring properties of nematics.

An anchoring competition is therefore obtained between two superimposed anisotropic films 120 and 130 each having an alignment effect, and coating the same isotropic substrate 110.

As shown in FIG. 1, the layer 130 of polymer has an average thickness I. We will demonstrate below the existence of a range critical I thicknesses in which there is a bistable anchor in two degenerate directions.

With reference to FIG. 2, an X axis coincides with the planar orientation direction referenced P, induced by the layer 120 of SiO, that is to say the main orientation of the plate of SiO 120. The Y axis coincides with the planar orientation induced by the linear photopolymerization process, referenced L, and the axis Z is directed towards the volume of the nematic material 150 perpendicular to the plane of the glass plate 100.

As described below, under certain conditions, the competition of anchors between the two layers 120 and 130 gives rise to a direction of azimuthal orientation resulting from the nematic surface molecules, which corresponds to a minimum of potential energy of the system. nematic 150 / plate 100. We can then define, as shown in FIG. 2, an azimuthal angle φ between the nematic director n and the x axis parallel to the planar orientation direction induced by the SiO layer.

According to the Berreman model, for such a layer of SiO 120 alone, there is an azimutable energy difference between the easy direction P and the direction perpendicular to P, which is due to the anisotropic morphology of the surface, the value of which is given by the formula w _S io = kA ² q ³ where q defines a main wave vector of the sinusoidal curve generated by the surface 123, defined above. We thus have q = 2π / λ, where λ is the wavelength of the sinusoidal curve 122, that is to say for example the distance measured in the plane of FIG. 1 between two vertices of the curve 122. A is the main amplitude of the curve 122 shown in FIG. 1, and k is the elastic constant of the volume of nematic crystal 150. The orienting effect of the polymer layer 130 after a sufficiently long exposure to UV, mainly due to Anisotropic Van der Waals interactions are characterized by an azimuthal anchoring force W _Pθ! = R where R is a constant. When the polymer 130 is deposited on the layer 120 of SiO, the amplitude A _e ff becomes smaller and smaller, while the thickness mean I of polymer film 130 increases, as shown in Figure 1

To evaluate the difference in azimuthal energy between the direction P and the direction which is perpendicular to it, due to the morphology of the anisotropic surface 120, it is necessary to take into account the amplitude attenuation generated by the geometry of the photopolymer layer 130 We therefore have in practice ws, o = kA _e ff ² q ³

Aeff varying as a function of I, w _S ιo can be modulated by varying I A critical thickness le is obtained when w _Sι o is of the same order of magnitude as wp ₀ ι, that is to say when = R

On the other hand, as mentioned in [6] and [7], the anchoring force exerted by the photopolymer film 130 on the nematic liquid crystal 150 depends on the duration of exposure to UV, which is thus used as a control parameter to modulate the azimuthal orientation of nematic liquid crystals

We hypothesize in the following reasoning that 1) the director n of the nematic liquid crystal is everywhere parallel to the plane (x, y), and

2) the whole of the liquid crystal follows the surface orientation, that is to say it is possible to neglect the elastic energy linked to the deformations of the nematic liquid crystal

The total surface azimuthal energy density for the system considered can be expressed, according to Rapini Papoular's approximation, by f = fsio + fpoi where f _s , o = 1 2 Wsio sin ² φ, and f _Pθ ι = - 1/2 w _Pθ ι sin ² φ + 1/4 b sin ⁴ φ, f _S ιo being the surface energy density corresponding to the interaction between the nematic liquid crystal 150 and the film 120 of SiO and f _Pθ ι representing the surface energy density corresponding to the interaction between the photopolymer film 130 and the nematic 150

In these equations, w _s , o> 0 means that the orientation induced by the layer 120 of SiO is parallel to the X axis (direction P), while w _Pθ ι> 0 means that the polymer layer 130 tends orient the director along the Y axis (direction L) The term 1 / 4.b.in ⁴ φ introduced by AL Alexe-lonescu et al. ([13]) is typical in the case of disordered surfaces. It is linked to the effect of the random distribution of molecules in the orientation film 130.

The total free energy per unit area of the system considered is then given by:

/ = / _* + f _M = --sin> + - sin * φ, or a = W _Pol - W _St0

The four solutions corresponding to a minimum of the total azimuthal energy density f are obtained by solving the equation: df _Λ ci. , _. b. ,. , - _. A— - 0, let sm (2φ) + —sm- φsm (2φ) ≈ 0 dφ 2 2 These solutions are: π. , a φ = 0, φ - - and sin "φ - -

2 b for a <0 (w _Pθ ι <Wsio), the stable solution is given by φ = 0 and the orientation of the nematic is imposed by the layer of SiO.

For a> b, the stable solution is given by φ = π / 2 and the nematic orientation is imposed by the photopolymer layer.

For 0 <a <b, the model gives a degenerate solution φ _τ in two directions. The azimuthal orientations + φ _τ and -φ _τ which minimize the free surface energy are between the directions P and L of planar surface attraction. It is noted that the alignment P becomes unstable when the condition a = 0, corresponding to w _Pθ ι = w _S io, is satisfied.

According to Schadt et al. ([6]) and J. Chen et al. ([7]), we analyze the effect of the duration t of exposure to UV light on the direction of the azimuthal director n using the simplest phenomenological expression for w _Pθ ι: w _Pθ ι = R (1 -e ^{"(t / x)} ) where R is the anchoring force of the polymer film 130 after a sufficiently long exposure time, and where τ is the characteristic time of the orientation dynamics of the chemical bonds. When R is greater than or equal to wsio, the condition w _Pθ | = W _S J _O , using the equation w _Pθ ι = R (1-e ^{"(t / τ)} ), defines a critical duration t _c characteristic of the orientation effect, given by R (1 -e ^{"(tc τ)} ) = w _S io-

For t <t _Cl the energy term associated with the SiO 5 substrate 120 dominates, we have aa <0, and the nematic alignment is found in the direction P, while for t> t _c (0 <a <b) , the azimuthal direction of the easy nematic axis is different from P (φ nonzero).

In the same case where R> Wsio, the relation R (1-e ^{"(t / τ)} ) - Wsio> b (or a> b), is realizable for t is greater than a second critical duration tc * 10 thus defined .

For t> t _c ^* , the energy term associated with photopolymer 130 dominates and the nematic alignment is found in the direction L.

From the previous equations, we obtain for t _c <t <t _c *, an effective azimuthal anchoring angle φ defined by:

, _R. ,. . R (\ - e ^{t, τ)} ) - Ws, o 15 sm- φ (t) - -.

According to AL Alexe-lonescu et al. ([13]), we can assume that b is of the same order of magnitude as w _Pθ ι, that is to say b = C (1-e ^{"(t τ)} ) where C is a constant, and so we can write :

sin ^: φ (t) = ^{R {~ e '} _c ^{t ^^} ,: " ^C ' ^ ™ c W _Sιo = R (l - e ^{~ *} ''>)

20 These last results indicate that all φ azimuthal surface orientations are possible, in the range (0, π / 2), using the exposure time t as a control parameter, provided that the condition given by KA ² dtq ³ = R be filled.

We therefore understand that a fine control of the two anchoring forces

25 coming from two independent surface anchoring attractors, perpendicular to each other, induces controlled surface transitions, which allow bistability.

It can be noted that the model presented here only applies to exposure times t which are not too long, for which azimuthal anisotropy is therefore an increasing function of the duration of exposure.

As reported by Schadt et al. and by Chen et al. ([6] and [7]), the azimuthal surface anisotropy induced by the linear photopolymerization (PPL) process is a non-monotonic function of the exposure time.

In the case of the embodiment given with reference to FIG. 1, the thickness of the film 130 of photopolymer was controlled by varying the concentration of PVMC in a suitable solvent such as an NMP: N-methyl Pirrolidone and the thickness actually obtained has been verified by an ellipsometry technique ([12]).

The inventors have found that with a concentration of PVMC of 0.2%, the nematic alignment always corresponds to the direction P, whatever the time of exposure to UV: the nematic anchoring is imposed by the layer of SiO .

The inventors have found a first critical thickness l _c of the layer 130 of approximately 30 A, corresponding to a PVMC concentration of 0.25%.

When the photopolymer layer is close to l _c , or slightly higher, it is possible to modulate the azimuthal orientation of the surface by changing the duration t of exposure to UV.

In addition, in the geometry used (P perpendicular to L), two azimuthal directions + φ and -φ, symmetrical with respect to the y axis, equivalent in terms of energy, are obtained. Figure 3 shows the dependence measured between the azimuth angle φ and the duration of exposure t to UV. Range of experimental values 20 are represented in the form of vertical segments, and a curve 10 produced in dotted lines has been drawn using the expression:

R \ - e ^{'tc l τ} sin ² φ (t) - (1 -) The observed values of φ start from the alignment along P (x-axis) but, under the experimental conditions adopted, seem to stagnate at φ = φ _s = 0.175 rad = 10 °. The correspondence between the theoretical curve 10 and the experimental data 20 is fairly good. In particular, the estimated critical time t _c of the order of 18.6 minutes is close to the measured value. The fairly high value of t _c indicates that the orientation of the photopolymer under linear photopolymerization is a slow phenomenon, at least in the case of a very thin layer thickness on an incipient anisotropic substrate. The experimental results can therefore be described quantitatively by the previously described model, which involves an elastic term coming from the topography of the SiO layer and an anisotropic term, of the Van der Waals type, relating to the photopolymer / nematic interaction.

In the end, the inventors found that with a concentration of PVMC of 0.3%, which defines a second critical length l _s of approximately 100 A, the nematic alignment always corresponds to the direction L after exposure to UV. Without UV treatment, a degenerate primary orientation of the non-critical liquid crystal is obtained from this thickness.

I _s and l _c depend on the polymeric material and the roughness of the anisotropic SiO layer.

Thus, for the two layers 120 and 130 both to have perceptible alignment effects on the surface nematic molecules, the average thickness must be in the case described here between 30 A and 100 A. In practice, in this type of layer assembly, a range of thicknesses of the photopolymer layer giving rise to such a result may have a lower limit l _c of a few tens of Angstroms and an upper limit l _s of a few hundred Angstroms.

Figures 4 and 5 are two photographs of a bistable plate. The invention is not limited to the specific case of plates with bistable or quasi-bistable anchors, but extends to the case of plates with controlled monostable anchoring. Thus, the inventors also carried out experiments in the case where the angle between P and L is not perpendicular.

The inventors have chosen for example an angle of approximately 45 ° between P and L. In this case they have also found a change in the angle φ which evolves from the direction P towards the direction L when the exposure time UV increases, with a thickness of photo-polymers between l _c and l _s .

In this case, however, only one surface director exists, which is thus intermediate between P and L. More precisely, the two directions P and L cutting the plane of the plate into four quadrants, the resulting anchoring direction starts from the direction P and pivots towards direction L when the duration of exposure to UV increases, traversing the two opposite quadrants with the lowest angular opening. In other words, one can obtain any angular direction included in one of the two acute angles included between the directions P and L.

The surface bistability no longer appears, indicating that the symmetry of the anchor attractors is fundamental to obtain a bistability of the surface nematic. In a similar manner to that described above, advantage is taken of the dependence of the resulting anchoring direction as a function of the duration of exposure to UV.

This possibility of controlling the direction of a monostable anchor is particularly useful in any type of cell where there is a need for a monostable anchor plate whose orientation is finely controlled.

Furthermore, by means of a similar technique, it is also possible to obtain quasi-bistable anchors, provided that the time of exposure t to UV is very short or very long. The assertion that one can obtain a quasi-bistability for very long times is based on the publications of Chen et al. ([7]), according to which the anisotropy of the polymer 130 substrate induced by the exposure from UV increases from an isotropic situation, reaches a maximum value and then decreases.

A possibility of obtaining a quasi-bistable anchoring is given, in the previously described case of a photo-polymer layer 130 on an anisotropic substrate 120, by a bistable alignment very close to that of the lower substrate 120.

In this way, it is possible to obtain anchoring properties similar to those given by an SiO substrate very close to the ideal line between the planar monostable anchor and the bistable inclined anchor. The method according to the invention for obtaining anchors of bistable or quasi-bistable nematic liquid crystals is based on the concept of anchoring competition between two alignment sources having different types of interaction with the liquid crystalline material.

The bistable anchors which can be obtained by this method are suitable for use in a bistable nematic device flexo-electrically controlled (document PCT / FR91 / 00496, document WO92 / 00546, patent US-5,357,358), in a bistable nematic device controlled by electro-chiral effect (document PCT / FR91 / 00052, document W091 / 11747) and in any other bistable electro-optical device based on the nematic surface bistability.

By this method, planar quasi-bistable nematic anchors, such as those required in the device of the document Bistable display device based on nematic liquid crystals allowing shades of gray can also be obtained. Although in the experiments, the inventors used a coating of SiO 120 deposited under vacuum as the first competitive alignment layer, the implementation of the invention does not necessarily require such treatment. Any other anisotropic substrate on which it is possible to deposit a second thin anisotropic film can be used. Mechanically rubbed substrates could be good candidates provided an appropriate roughness is reached.

On the other hand, the second competitive alignment layer is constituted by an organic coating film 130 photo-polymerized by UV (PVMC or similar), which can be sprayed or applied by spin coating on the glass substrate.

This film may be substituted by any other anisotropic film sufficiently thin to allow the anchoring competition. Although the invention has been described here in the case of planar anchors, it is of course possible thanks to a method according to the invention to produce a plate with oblique anchors.

This could for example be achieved by exposing the photopolymer film under UV light whose incident beam is oblique to the normal to the plane of the plate.

The possibility of a first substrate, not influencing the nematic, on which two or more superimposed films, anisotropic, with different anchoring attractors and with a suitable thickness to allow anchoring competition, has also been considered. It is more generally possible to envisage any number of isotropic layers, placed under the two superimposed anisotropic layers in question.

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Claims

1. Liquid crystal cell comprising at least one plate (100) with controlled anchoring, characterized in that the plate comprises at least two superposed layers (120, 130) each having an alignment effect on the nematic molecules (150) located in the vicinity of the same surface (133) of the plate (100), these two layers (120, 130) not having the same alignment direction (P, L).

2. Cell according to claim 1, characterized in that one of the layers (130) has an alignment effect of Van der forces type

Waals.

3. Cell according to claim 1 or 2, characterized in that said alignment directions (P, L) are perpendicular to each other.

4. Cell according to any one of claims 1 to 3, characterized in that the first layer (120) has an alignment effect of an elastic nature and in that the second layer (130) has an alignment effect of Van der Waals forces type.

5. Cell according to claim 4, characterized in that the second layer (130) is arranged closer to the nematic crystal (150) than the first layer (120).

6. Cell according to any one of claims 1 to 5, characterized in that the first layer (120) is a layer of anisotropic SiO.

7. Cell according to claim 6, characterized in that the SiO layer (120) has variations in altitude (A) at the surface of the order of a few tens of Angstroms.

8. Cell according to claim 6, characterized in that the SiO layer (120) is deposited on an isotropic substrate (110).

9. Cell according to claim 8, characterized in that the isotropic substrate (110) is a conductive ITO glass.

10. Cell according to any one of the preceding claims, characterized in that the second layer (130) is a layer of photopolymer.

11. Cell according to any one of claims 1 to 10, characterized in that the superimposed layers define a bistable or quasi-bistable anchoring.

12. Cell according to any one of claims 1 to 10, characterized in that the superimposed layers define a monostable anchoring.

13. Cell according to any one of claims 1 to 12, characterized in that the superimposed layers define a planar anchoring.

14. Cell according to any one of claims 1 to 12, characterized in that the superimposed layers define an oblique anchoring.

15. Method for producing a plate (100) with controlled anchoring on its upper face (133), characterized in that the following steps are carried out: a) production of a first layer (120) capable of producing a first alignment effect on its upper face (123) in a first direction (P); b) deposition on this first layer (120) of a second layer (130) capable of producing, optionally after an additional treatment, a second alignment effect on its upper face (133) in a second direction (L) different from the first direction (P), the second layer (130) being able to transmit on its upper surface (133) part of the first alignment effect induced by the first layer (120).

16. The method of claim 15, characterized in that step a) consists in depositing on an isotropic substrate (110) a layer of SiO (120) by vacuum deposition at a non-zero angle relative to the normal to the isotropic substrate surface (110).

17. The method of claim 15 or 16, characterized in that step b) consists in depositing a layer of photopolymer (130).

18. The method of claim 17, characterized in that the photopolymer (130) is deposited at an average thickness between a few tens and a few hundred Angstroms.

19. The method of claim 17 or 18, characterized in that said additional treatment consists of a photopolymerization under polarized ultraviolet light.

20. The method of claim 19, characterized in that the UV light is polarized so as to induce an anchoring attraction in a direction (L) perpendicular to the alignment direction (P) induced by said first layer (120) .

21. The method of claim 20, characterized in that the exposure to UV is brief enough for the upper face of the plate (100) to have resulting anchoring directions () oriented relative to the direction of the effect. alignment (P) induced by the first layer (120) at an angle less than 10 °.

22. Method according to any one of claims 17 to 21, characterized in that a thickness of the photopolymer layer (130) is chosen between a minimum thickness (l _c ) below which the photopolymer layer (130) has no effect on the nematic orientation, whatever the duration of exposure to UV, and less than a maximum thickness (l _s ) beyond which the alignment effect of said first layer (120) is always imperceptible, regardless of the duration of exposure to polarized UV.