EP0400007A1 - Ferroelectric liquid crystal screen with opacified electrodes in the non-switchable zone of the screen and methods for obtaining spacers and for treating such screen - Google Patents

Abstract

On each part of each row electrode (-column) of the screen, facing an interval separating two column electrodes (-lines) is disposed an element (40, 50) preventing the screen from being crossed by a light arriving on it towards the element. The elements arranged on the line electrodes (32) or those arranged on the column electrodes (34) also allow the spacing, without electrical connection, of the plates of the screen and the location of the zig-zag defects that can have the liquid crystal in or near the non-switchable area of the screen. As for the treatment, the screen is appropriately heated and an alternating electrical voltage is applied between the electrodes in order to locate the zigzag defects of the liquid crystal in the vicinity of the spacers. To obtain these, a layer of positive resin can be exposed, through the plate intended to carry these elements and previously provided with opaque layers on the parts corresponding to these elements. Application to the display of information.

Description

 FERROELECTRIC LIQUID CRYSTAL SCREEN WITH ELECTRODES

OPACIFIED IN THE NON-SWITCHABLE AREA OF THE SCREEN AND METHODS

OBTAINING SPACERS AND PROCESSING THIS SCREEN

The present invention relates to a ferroelectric liquid crystal screen, with electrodes opacified in the non-switchable region of the screen as well as a method for obtaining spacers for this screen and a method for processing the screen. It applies to the display of information (images, characters,

The invention is for example achievable with the chiral smectic liquid crystals C, I, F, G or H inclined (tilted according to English terminology), and in particular with the liquid crystals with chiral smectic phase C.

In document EP-A-0 032 362, a display device is described, the electrooptic display material of which is a liquid crystal with chiral smectic phase C. This display device, shown diagrammatically in longitudinal section in FIG. 1, comprises a first linear polarizer 2 and a second crossed linear polarizer 4, between which is inserted a waterproof display cell 6. A light source 8, located below the polarizer 4, allows lighting of the cell 6.

This display cell operating in transmission is formed by two walls, or plates, electrically insulating and transparent 10 and 12, generally made of glass. These walls, parallel to each other, are made integral by their edges by means of a bonding 14 serving as a seal.

The walls 10 and 12 are covered respectively with an electrode 16 and a counter-electrode 18 of a shape suitable for display, made of a transparent conductive material. The electrode and the counter electrode may each be formed of parallel conductive strips, The strips of the electrode, which can be called "column electrodes", and the strips of the counter electrode, which can be called "line electrodes", being crossed, that is to say perpendicular.

The electrode and the counter-electrode make it possible to apply to the terminals of a liquid crystal film 20 with chiral smectic phase C, contained in cell 6, an electric field continuous whose direction or value can be changed. To this end, the electrode 16 and the counter electrode 18 are connected, by means of an inverter 22, to a source of continuous electrical power 24.

In FIG. 2, a molecular scale shows the structure of a smectic phase C liquid crystal film when it is contained in the display cell 6. In order to simplify this FIG. 2 , only the walls 10 and 12 of the cell have been shown. The lower wall 12 constitutes for example a reference plane containing the two axes X and Y of an orthogonal XYZ coordinate system.

The smectic liquid crystal film C is composed of molecules 26 of elongated shape, having a longitudinal axis 28 and arranged in layers 30. These molecules each have a permanent dipole moment perpendicular to their Longitudinal axis 28.

In the ideal case, shown in FIG. 2, the smectic layers 30 are all mutually parallel and oriented perpendicular to the walls 10 and 12 of the cell.

When an electric field is applied to such a Liquid crystal, a strong coupling is obtained between the molecular orientation (Longitudinal axis 28 of the molecules) and this electric field due to the presence of the permanent dipole. This coupling is of polar type because the electric dipole is preferably oriented in a direction parallel to the electric field. The change in polarity of the electric field therefore makes it possible to change the orientation of the electric dipole and therefore the orientation of the molecules 26. In FIG. 2, the solid lines have been represented molecules 26 of the liquid crystal in a first orientation A ₁ (state 1) forming an angle -θ with respect to the direction X, the dipole moments being oriented perpendicular to the walls 10 and 12 of the cell and in the direction of the electric field going from the wall 10 towards the wall 12. The change of polarity of the electric field allows the tilting of the dipole moments in the opposite direction (from wall 12 to wall

10), causing the molecules to pivot around the Z axis by an angle of 2θ. The second orientation A ₂ of the molecules (state 2) is symbolized in phantom. It forms an angle + θ with respect to the direction X.

The molecules pass from the first to the second orientation, and vice versa, by describing a cone of angle at the top 2θ characteristic of the material (typically θ = 22.5º). FIG. 2 also shows the respective polarization directions P and P 'of the straight polarizers Lines 2 and 4.

When these two polarizers are crossed and When in State 1 the molecules 26 of the liquid crystal are parallel to the direction of polarization P 'of the polarizer 4, The optical state 1 of the liquid crystal corresponds to the absorption of Light coming from the source 8 and the optical state 2 to the transmission of this same light.

Liquid crystals with a chiral smectic C phase, oriented correctly (Figure 2), can therefore be used as display materials. They are likely, in addition to their bistability, to present interesting properties such as a rapid response or switching time, of the order of a microsecond, for low voltages applied to the electrodes (a few volts) and a wide response. electrooptical.

For the device shown in FIG. 1 to function correctly, the thickness of Liquid crystal must be extremely small, of the order of 2 micrometers for example. The spacing of the walls 10 and 12 leading to such a thickness is generally obtained by means of spacers made up of calibrated plastic balls.

These balls used as spacers are arranged randomly between the walls 10 and 12. In Figure 3, there is shown very schematically and in top view a liquid crystal display screen which comprises line electrodes 32 transparent and parallel to each other and transparent column electrodes 34, parallel to each other and perpendicular to the line electrodes. One of the important parameters of this screen is the contrast obtained between the black state displayed N and the white state B. This contrast is defined by the ratio of the intensity transmitted in the white state IB to the intensity transmitted in the black state IN. To obtain a significant contrast, it is necessary that the intensity of the black state, corresponding for example to state 1 of the device described with reference to FIGS. 1 and 2, the white state then corresponding to state 2, is as low as possible, so as to have a large IB / IN ratio.

When the screen represented in FIG. 3 uses a bistable ferroelectric liquid crystal, with chiral smectic phase C for example, the non-switchable zone 36 of the screen - it is recalled that the switchable zone, referenced 38 in FIG. 3, corresponds to all of the "overlaps" of the electrodes 32 and 34 (in top view) and that the non-switchable area (or non-addressable area) corresponds to the rest of the screen - contains substantially equal densities of states 1 and states 2. In fact, to obtain good bistability, the surface treatments which allow the orientation of the liquid crystal are such that the two states are equiprobable in the non-switchable zone. This non-switchable zone 36 therefore appears gray when an appropriate electrical voltage is established between the electrodes 32 and 34 and that straight line polarizers are suitably arranged on either side of the assembly, or cell, comprising the electrodes 32, 34 (respectively arranged on electrically insulating plates generally glass) and the layer of liquid crystal.

The fact that the non-switchable zone appears gray is very detrimental to the contrast, even if the electro-optical effect used makes it possible to obtain an excellent black state at the level of the switchable zone 38.

The dimensions of the non-switchable area cannot be reduced significantly, since in particular for large, complex screens, the yields of the etchings necessary for their manufacture impose a size limit on the non-switchable area.

This problem relating to the gray appearance of the non-switchable zone is also encountered with screens using other liquid crystals. The problem in question is then resolved by placing between the line electrodes and between the column electrodes a screen opaque to light. This screen generally consists of an electrically insulating colored material, the thickness of which must be of the order of a few microns, 1 to 2 microns for example, so that the insulating material is sufficiently absorbent. Such a thickness is incompatible with screens using a ferroelectric liquid crystal. Indeed, the thickness of the layer of this Liquid crystal does not allow the crossing of layers of colored insulating material, arranged in the intervals separating the electrodes-lines and of layers of this same material, arranged in the intervals separating the electrodes- columns.

Admittedly, one could imagine covering all the electrode-lines and -columns with an electrical insulator of suitable thickness, which would be transparent in the switchable area and opaque in the non-switchable area in order to solve the problem in question.

However, such a technique would be very unfavorable from the point of view of the functioning of the screen because a large part of the electrical energy necessary for the switching of the liquid crystal would be lost in the insulator whose thickness would be comparable to the thickness. "active" liquid crystal. Another problem resulting from the use of a ferroelectric liquid crystal, for example a liquid crystal with chiral smectic phase C, in particular for producing a screen of large dimensions, is due to the presence of characteristic alignment defects which is likely to present. such a liquid crystal. These defects are known under the name of "zig-zag" and are notably mentioned in the article by MA HANDSCHY et al., Published in the journal Ferroelectrics, 1984, vol.59, p.69 to 116.

These defects, which are in the form of lines, reduce the contrast. In addition, their distribution can be inhomogeneous on the screen surface, which produces an inhomogeneous aspect of the image displayed by this screen, an aspect which is unfavorable for obtaining a good quality screen.

The density of these defects depends on the liquid crystal used and on the surface treatments carried out on the plates between which this liquid crystal is located. One of the possible surface treatments consists in placing on each of these plates a layer of an appropriate material and rubbing the layers of this material either parallel to the line electrodes or parallel to the column electrodes.

It is very difficult to place the liquid crystal between surfaces of several dm ² without this resulting in some zig-zag defects. These defects are arranged perpendicular to the direction of rubbing of the walls and tend to catch on the balls serving as spacers which stop these defects. However, these balls are distributed randomly between the plates of the screen and in particular in the switchable zone thereof, hence a bad contrast, in a lasting manner, for such a known screen with ferroelectric liquid crystal.

The object of the present invention is to improve the contrast of a ferroelectric liquid crystal screen, for example of a screen using a chiral smectic liquid crystal inclined C, by opacifying in a particular way the non-switchable zone of the screen. and allowing to push back the defects zig-zag in the non-switchable area of the screen or in the vicinity of this area.

Specifically, the present invention firstly relates to a ferroelectric liquid crystal screen comprising an assembly comprising a layer of ferroelectric liquid crystal capable of exhibiting zigzag defects, this layer being between a group of electrodes -transparent lines, parallel and separate from each other and a group of transparent column electrodes, parallel, separate from each other and perpendicular to the line electrodes, said groups of electrodes being respectively arranged on two electrically insulating and transparent plates , screen characterized in that it further comprises, on each part of each row electrode, facing an interval separating two column electrodes, and on each part of each column electrode, facing a gap separating two electrodes -lines, an element intended to prevent the crossing of the screen by an incoming light ant on this screen in the direction of this element, and in that the elements which are arranged on the row electrodes or the elements which are arranged on the column electrodes are further provided to allow spacing, without electrical connection between the row electrodes and column electrodes, of the screen plates and have dimensions which, counted parallel to these plates, allow the localization of the zig-zag faults in the vicinity of the non-switchable zone of the screen or in this zoned.

According to the present invention, said elements are therefore arranged in the non-switchable zone of the screen, which makes it possible to suppress the gray appearance of this zone, mentioned above, the technique used in the present invention to make the non-zone opaque. switchable screen remaining compatible with the extreme thinness of the ferroelectric liquid crystal layer used.

It is indicated in this connection that the thickness of this layer, which in fact depends on the liquid crystal used, is generally of the order of 1.5 micrometers to 2 micrometers. Furthermore, because the elements allowing the spacing of the plates, and thus having the function of spacers, are placed in the non-switchable area of the screen, the zig-zag defects are no longer hampered in the switchable area of the screen and, after a more or less long time of use (of addressing) of the screen, these faults will end up abutting against the elements allowing the spacing of the plates, in the opaque non-switchable zone or in the vicinity thereof.

An improvement in visual contrast is thus obtained without degradation of the other electro-optical properties of the screen (switching time, addressing time, etc.).

The screen generally also comprises two polarization means located on either side of said assembly. These polarization means can consist of two crossed rectilinear polarizers.

According to a particular embodiment of the screen which is the subject of the invention, each element comprises an opaque layer which covers the electrode part on which this element is disposed and each element allowing the spacing of the plates comprises, in addition to this opaque layer, an electrically insulating spacer which is placed on the latter.

Preferably, the width of the opaque layer is greater than the width of the interval separating two electrodes and facing the part which is covered by this opaque layer, in particular to favor the positioning of the opaque layers situated on a plate relative to the corresponding interelectrode spaces, located on the other plate and mask the edge of the electrodes which are opposite the opaque layers, this edge being liable to exhibit imperfect switching.

Preferably also, each spacer has an elongated shape transverse to the electrode on which the element comprising this spacer is disposed. This increases in particular the rigidity of the screen compared to that of known screens, using balls as spacers.

So as not to have a problem positioning a screen plate with respect to the other, the width of each spacer may be greater than the width of the gap separating two electrodes and facing the part on which the element comprising this spacer is placed. However, preferably, in order not to have this problem, the width of each spacer is less, for example by half, than the width of the gap separating two electrodes and facing the part on which the element comprising this element is placed. spacer. Indeed, in some cases considered below, it is preferable to have such a small width. as possible for the spacers in order to best conceal the zig-zag defects.

According to another particular embodiment of the screen which is the subject of the invention, this screen further comprising two crossed line recti polarizers, on either side of said assembly, each element allowing the spacing of the plates is an electrically insulating spacer which has an elongated shape transversely to the electrode on which this element is disposed and a width greater than the width of the gap separating two electrodes and facing the electrode part on which the element is disposed, this element is made of an optically isotropic material and each element preventing said passage of the screen by light without allowing the spacing of the plates is made of an opaque layer which covers the electrode part on which the latter element is disposed.

The optically isotropic material can be a photosensitive, optically isotropic resin.

This material can be transparent or opaque.

In a particular embodiment of the screen which is the subject of the invention, said plates being further coated with orientation layers of the liquid crystal which are made anisotropic in a direction parallel to the electrodes of one of the two groups of electrodes, the elements allowing the spacing of the plates are arranged on the electrodes of this group. So, the zig-zag defects are parallel to the direction of the spacers - we mean by this, the direction which is parallel to the length of these spacers - and in such a case, it is preferable that the width of the spacers is less - of preferably much lower - than that of the inter-electrode interval relative to the other group of electrodes, in order to be able to repel the zig-zag faults in the non-switchable zone of the screen. In another particular embodiment, said plates being further coated with orientation layers of the liquid crystal which are made anisotropic in a direction parallel to the electrodes of one of the two groups of electrodes, the elements allowing the spacing of the plates are arranged on the electrodes of the other group.

Then, the zigzag defects are perpendicular to the direction of the spacers and in such a case, it is preferable that the length of each spacer is substantially equal to the width of the electrode which carries this spacer, in order to confine the zig-zag faults in the non-switchable area of the screen, whereas in the case considered above, this length may be less than or equal to this electrode width. However, in general, it is preferable that this length of spacer is of the order of the width of the electrode in question in order to obtain a screen of good rigidity. Furthermore, in the case considered above leading to defects perpendicular to the direction of the spacers, the width of these spacers is less than or equal to that of the corresponding opaque layers.

Each opaque layer can advantageously be an opaque metallic layer. Indeed, the metal layers are opaque at low thicknesses which are compatible with the low thickness of liquid crystal used in the present invention.

Thus a layer of chromium can be used and this layer of chromium can have a thickness of between approximately 30 nm and approximately 200 nm.

As already indicated, the liquid crystal can be chosen from the group comprising the chiral smectic liquid crystals C, I, F, 6, H inclined. The present invention also relates to a process for obtaining the spacers of the particular embodiment mentioned above, an embodiment in which the spacers are on opaque layers, this process being characterized in that it comprises:

a deposition of a layer of positive photosensitive resin on the plate carrying the electrodes and the opaque layers on which the spacers are intended to be located,

an exposure of the resin through this plate arranged so that the opaque layers carried by this plate serve as a mask during the exposure, and

- elimination of the insolated resin.

Finally, the present invention also relates to a method for processing the screen which is the subject of the invention. This method is characterized in that it comprises the application, between the row electrodes and the column electrodes, of an alternating electric voltage, while maintaining the screen at a temperature close to the phase transition temperature ferroelectric smectic to the immediately higher phase (from the point of view of temperature) of the liquid crystal, in order to locate the zigzag faults of the liquid crystal in the vicinity of the non-switchable area of the screen.

It is also possible to locate the faults in this way by operating the screen continuously for a sufficiently long time by applying an alternating electrical voltage between the row electrodes and the column electrodes, as already indicated above. the present invention will be better understood on reading the description which follows, of exemplary embodiments given purely by way of indication and in no way limiting, with reference to the attached drawings on which:

- Figure 1, already described, is a schematic view of a display device using a liquid crystal with chiral smectic phase C, - Figure 2, already described, represents to scale molecular, the structure of this liquid crystal in the device,

FIG. 3, already described, is a schematic view of the row electrodes and the column electrodes of a liquid crystal display screen,

FIGS. 4 and 5 are schematic and partial views of a screen according to the invention,

- Figure 6 is a top view of part of the screen shown in Figure 5, - Figures 7 and 7A are schematic and partial views of other screens according to the invention,

FIGS. 8A to 8E schematically illustrate different steps of a method for producing a screen according to the invention, FIG. 9 schematically illustrates a step of a variant implementation of this method,

FIGS. 10A and 10B schematically and partially represent a screen according to the invention, and

- Figure 11 shows schematically and partially another screen according to the invention.

FIG. 4 schematically and partially illustrates a particular embodiment of the invention: a layer 40 (respectively 42) of an opaque material, of thickness compatible with the thickness of the layer of ferroelectric liquid crystal used, is formed on each line electrode 32 (respectively column 34) in the form of a strip (see FIG. 3), opposite each of the intervals separating the column electrodes (respectively-lines).

For this purpose, for example, a layer of chromium is used, the thickness of which is a few tens of nanometers, for example, which is enough to make this layer opaque. the chromium layer is etched so as to remain only on the electrodes 32, 34 and more precisely on the parts of these, which are opposite the intervals in question, to form on each electrode-line 32 (respectively - column 34) a succession of rectangular patterns whose width is equal - or, preferably, greater - than that of the interval between two column electrodes (respectively -lines) and whose length is equal to the width of the row electrodes (respectively - columns).

The interval between two patterns corresponding to the row electrodes (respectively -columns) is equal - or preferably, less - to the width of the column-electrode electrodes (respectively -lines). The relative positioning of the glass plates 44 and 46 respectively carrying the row electrodes and the column electrodes then makes it possible to cause said patterns to optically close off the greater part of the non-switchable zone: the patterns produced on the row electrode (respectively - columns) face the intervals separating the column electrodes (respectively - lines).

This is visible in perspective in FIG. 5 and in top view in FIG. 6. It is observed that only remain as non-switchable and non-opaque domains. the parts 48 of the non-switchable zone which correspond to the "intersections" (in top view) of the inter-electrode-row intervals and of the inter-electrode-column intervals. achieving a minimum inter-electrode width, taking into account the manufacturing process described below, makes it possible to minimize these non-switchable and non-opaque parts.

To remedy the drawback associated with zigzag faults relating to ferroelectric liquid crystals, the non-switchable zone made opaque is used as explained above, seeking to locate the faults in this zone or in the vicinity of this one.

To do this, we use for example a property not known in the state of the art, namely that an alternating electrical voltage applied between the row electrodes and the column electrodes at a temperature close to the transition temperature of the ferroelectric smectic phase at the immediately higher phase (vis-à-vis the temperature) of the liquid crystal, tends to displace the zig-zag defects which are then localized in the non-switchable zone, zone which is not subjected to the alternating electric field resulting from alternating voltage. For example, for an inclined chiral smectic liquid crystal C, the latter is heated to a temperature close to the transition temperature from phase C to phase A of this liquid crystal.

A zig-zag defect (generally in the form of a line) sees its migration stopped by its encounter with a dust or a spacer. Thus, as we have seen, the use of plastic balls as spacers, these balls being distributed randomly over a pixel, is unacceptable.

The property discovered is then used in the following way: spacers 50 such as photosensitive resin pads are produced in the non-switchable region 36 made opaque of the screen (FIG. 6). In this way, the switchable area of the screen has no spacer capable of hindering the migration of faults. These spacers are respectively arranged either on the opaque patterns which are found on the line electrodes (FIG. 6) or on the opaque patterns which are found on the column electrodes (case of FIG. 7 where the spacers bear the reference 52). Each spacer has the shape of a parallelepiped block, the width of which may be less, for example by half, than that of the inter-electrode space which faces it, that is to say the space separating two electrodes- columns (respectively -lines) if the spacers are on the row electrodes (respectively -columns). The length of the block is less than or equal to the width of the electrode on which it is located.

In addition, the plates 44 and 46 (FIG. 5) being provided with liquid crystal orientation layers (not shown on

FIGS. 5 to 7A), it is known that these orientation layers must be given an anisotropy direction, for example by rubbing. However, the defects are placed perpendicular to this direction of anisotropy (which can be either parallel to the row electrodes or parallel to the column electrodes). We can therefore orient the spacers (in the direction of their length) perpendicular to the intended direction of friction D1 (Figure 6) and give them a width less than that of the corresponding opaque patterns (which is the case for the screen of the Figure 6), so that the zig-zag defects, which are then parallel to the spacers, can be blocked in the non-switchable zone, the length of the spacers being less than or equal to the length of the electrodes on which they are arranged.

Conversely, the spacers can be oriented parallel to the intended rubbing direction (case of FIG. 7 where this direction bears the reference D2) and give them a length equal to the width of the electrodes on which they are located (the column electrodes in the case of FIG. 7), so that the zigzag defects ZZ, which are then perpendicular to the spacers, can be blocked in the non-switchable zone of the screen, the width of the spacers then being equal to the width of the opaque material, or less than this width and in the latter case, it is preferably either less than or greater than the width of the opposite interelectrode space.

In FIG. 7A, an embodiment of the invention is shown which is identical to that of FIG. 7, except that the width of each spacer is greater than that of the inter-electrode space which faces it.

Various embodiments of a screen according to the invention are indicated below.

We start by forming on each glass plate or slide 44, 46 a transparent layer of indium tin oxide (ITO) 54 and then depositing on this layer 54, by evaporation under vacuum, a layer of chromium 56 d 'thickness 50 nanometers for example (Figure 8A).

A layer of positive photosensitive resin 58 is then spread over the chromium layer 56, the thickness of which is equal to that which is provided for the spacers, ie a thickness of between approximately 1.5 and 2 micrometers for liquid crystals of the type Inclined smectic chirals (Figure 8B). The resin is first exposed through the mask used to define the electrode-lines 32 (respectively -columns 34), the resin is developed and then the layers of chromium 56 and ITO 54 are etched, which defines line electrodes 32 (respectively -columns 34), coated with the chromium layer (FIG. 8C).

The resin is exposed a second time through an appropriate mask, to define the patterns of chromium 40 (respectively 42), the resin is developed and the chromium remaining unprotected by the resin is removed by etching (Figure 8D). so that there remain the row electrodes (respectively - columns) provided with chrome patterns which correspond to them and which are surmounted by a calibrated resin layer, these patterns having a width at least equal to the inter-electrode space - column (respectively -line).

Then, for the plate 44 carrying the line electrodes, the resin is annealed at 200 ° C. for approximately one hour so that the spacers 50 thus produced by these layers of resin do not deform. On the other hand, as for the plate 46 carrying the column electrodes, its resin is removed (so as not to prevent the subsequent introduction of liquid crystal between the plates 44, 46 combined).

Of course, one could on the contrary anneal the resin from the plate 46 to harden the spacers 52 made and remove the resin from the plate 44.

In an alternative embodiment relating to the glass plate on which the spacers are to be produced, in order to avoid degradation of the resin due to all of the etching steps, this resin is removed after the step comprising the second exposure of said resin, its development and etching allowing the definition of chromium patterns (FIG. 8D), after which a layer 60 of positive resin is spread whose thickness is calibrated and corresponds to that which is provided for the spacers (FIG. 9 ) and this layer 60 is exposed through the glass plate in question, so that the chrome patterns serve as a mask (FIG. 9). Then, the resin is developed and annealed for one hour at 200 ° C.

This gives "self-aligned" spacers on the glass plate in question.

In the examples relating to FIGS. 8A to 8D and 9, each spacer covers the entire pattern of corresponding chrome 40 or 42. With such spacers and in the case where the rubbing direction is perpendicular to the latter, the defects in the form of lines are located at the edge of the pixels (after for example heating and application of the electric voltage -see above), this which is visually embarrassing. It is nevertheless possible to completely locate the faults in the non-switchable zone by making spacers which each occupy a smaller surface (FIG. 6), each spacer, seen in section parallel to the plates 44, 46, having for example the shape of a rectangle whose width is less than the length and width of the corresponding pattern. To do this, an additional suitable mask is used to expose the layer 60 or 50 (52) of resin.

Once the spacers have been made, orientation layers are used. These are, for example, produced in the following manner: a layer 62 of silica, 60 nanometers thick for example, is deposited in the vapor phase on the face of each plate 44, 46 which carries the electrodes, and is formed on this silica layer an alignment layer 64 of nylon 6 ^(R) or polyamide 6, of thickness 150 nanometers for example, in a manner known in the state of the art. This layer 64 is then annealed for one hour at 120 ° C. and then rubbed in a known manner in a chosen direction which. is either parallel or perpendicular to the electrodes on which the spacers are located and in one and / or the other direction relative to this direction.

Then we deposit by screen printing on one of the plates a glue joint which will be used for sealing. After assembly of the plates, the assembly is brought for example to 160 ° C for 2 hours to allow the polymerization of the glue, then is filled in known manner, with an appropriate liquid crystal -see the examples given below- space between plates 44 and 46.

Considering for example the case of an inclined smectic chiral liquid crystal C, the screen is then heated to 120ºC and therefore the liquid crystal so that it is in its isotropic phase and, at this temperature, an alternating voltage of the order of 30V is applied between each row electrode and each column electrode to bring the zigzag defects in the vicinity of the spacers. As a purely indicative and in no way limitative, the resin is that which is marketed by the company SHIPLEY under the reference 1350J; it is developed, with the developer

Microposit 351 from the same company; the chromium is etched using a Cr etched solution marketed by the company SOPRELEC; indium tin oxide is etched using a solution containing two volumes of hydrochloric acid for one volume of ferric chloride. the liquid crystal is for example either the mixture A indicated below, or this mixture A doped with 0 to 35% by volume of the compound B also indicated below.

Next, line 66 and 68 polarizers (FIG. 5) are put in place on either side of the sealed cell obtained, so that their respective directions of polarization are perpendicular and the polarizer first encountered by the light allowing 'Illuminating the screen has its direction of polarization parallel to one of the two directions A1 or A2 of orientation of the ferroelectric liquid crystal molecules.

FIG. 5 also shows conventional control means 70 for the row electrodes and column electrodes and the liquid crystal layer is referenced 72.

The mixture A is composed, by volume, of:

The manufacturing process for 4- (4-heptyloxy-3-bromo-benzoyloxy) -4 '- ((2S, 3S) -3-methyl-2-chloropentanoyloxy) -biphenyl (compound B) is given below: a ) - Synthesis of the substituted phenol necessary for obtaining B which is a biphenyl ester

This phenol is 4- (4-heptyloxy-3-bromobenzoyloxy) - biphenyl-4'-ol of formula:

and is obtained according to the following reaction scheme:

In a 10 ml Erlen-Meyer, there are added: 205 mg of -4-heptyloxy-3-bromo-benzoic acid of formula (V), 0.7 ml of thionyl chloride (SOCI ₂ ) and 2.2 ml of benzene. The solution is brought to reflux for 4 hours. excess thionyl chloride and benzene are distilled under reduced pressure. 145 mg of 4-4'-dihydroxybiphenyl of formula (VI) are added to the crude acid chloride obtained. in 3 ml of pyridine. The solution is stirred for two days with very slight heating.

The cooled solution is then acidified with a 10% HCl solution by volume in water and extracted 3 times with ether (30 ml).

The organic phases obtained are washed with aqueous solutions:

3 x 25 ml of HCL-H ₂ O at 10% by volume of acid, 3 x 25 ml of NaHCO ₃ to 5% by weight, 3 x 25 ml of NaOH at 10% by weight,

3 x 25 ml of 10% HCl by volume,

2 x 25 ml of saturated NaCl.

Then, the organic phases are combined, dried over sodium sulfate and evaporated. The organic mixture washed on silica is chromatographed with a chloroform-ether mixture of composition 80-20% by volume as eluent. 74 mg of

4- (4-heptyloxy-3-bromo-benzoyloxy) -bipheny1-4'-ol. the reaction yield is 23%. Characteristics of the product obtained:

- in the infrared, there is an OH band at 3470 nm and a C = O band at 1715 nm,

- the clarification point is at 186ºC, and

- the coefficient Rf of chromatography is equal to 0.7. b) - Synthesis of 4 - (- 4-heptyloxy-3-bromo-benzoyloxy) -4 '- ((2S, 3S) - 3-methyl-2-chloropentanoyloxy) biphenyl by reaction of the phenol (VII) obtained in a) with an optically active acid according to the reaction scheme: y

27.3 mg of (2S, 3S) -3-methyl-2-chloropentanoic acid of formula (VIII), 58.7 mg of 4- (4-heptyloxy-3-bromobenzoyloxy) biphenyl-4'-ol, 26 mg of N, N'-dicyclohexylcarbodiimide of formula C ₆ H ₁₁ -N = C = NC ₆ H ₁₁ , 2.2 mg of 4-pyrrolidinopyridine of formula C ₄ H ₈ NC ₅ H ₄ N and 2 ml of methylene chloride of formula CH ₂ Cl ₂ sec are stirred at room temperature for at least 12 hours.

The precipitate obtained is filtered. The solution is taken up in 25 ml of methylene chloride and washed with: - 3 x 15 ml of H ₂ O,

- 3 x 15 ml of 5% acetic acid in water,

- 2 x 15 ml of a saturated solution of sodium chloride in water.

The aqueous phases are taken up twice with 25 ml of methylene chloride.

The organic phases are combined, and dried over sodium sulfate and then evaporated on a rotary steamer. the solid obtained is chromatographed on 25 g of silica with CH ₂ Cl ₂ - petroleum ether as eluent in a ratio 60/40% by volume. 58 mg of a white product are then obtained which are recrystallized from petroleum ether.

The transition temperatures of the final product are:

K represents the solid state, Se a smectic structure C, * a chiral structure, N a nematic structure, I an isotropic structure.

Another example of a screen in accordance with the invention is schematically and partially shown in FIGS. 10A (in top view) and 10B (in section). The spacers are on the line electrodes 32 and the rubbing direction D1 is parallel to these electrodes. the electrodes are made of ITO and have a width of 300 micrometers. the opaque patterns are in chrome, have a thickness of 100 nanometers and a width of 60 micrometers while the inter-electrode-column spaces are only 40 micrometers wide. Each spacer surmounting a chrome motif is made of photosensitive resin, has a width of 20 micrometers, a length of 300 micrometers and a height of 1.6 micrometers.

Another example is schematically and partially shown, in perspective, in FIG. 11. the opaque patterns, in chrome, do not carry any spacer and are all placed on the electrodes of the same group of electrodes (either the line electrodes, or the column electrodes) and the spacers 52a (electrically insulating) are carried directly by the electrodes of the other group. In the case of FIG. 11, the chrome patterns are carried by the column electrodes and the spacers by the row electrodes. the rubbing direction D2 is parallel to the spacers, the length of which is equal to the width of the electrodes which carry them so as to be able to confine the zigzag defects ZZ in the non-switchable zone. the spacers can be opaque or transparent and have a width greater than that of the inter-electrode gap facing them. They must be optically isotropic so that, even if they are transparent, they cooperate with the crossed line recti polarizers 66, 68 with which the screen of FIG. 11 is provided, to stop the light falling on the screen in the direction of spacers. In the example shown in FIG. 11, the electrodes have a width of 300 micrometers and the interelectrode space has a width of 40 micrometers; the chrome patterns have a thickness of 0.1 micrometers and a width of 60 micrometers; the spacers 52a are made of photosensitive resin, have a thickness of 1.5 micrometers, a length equal to the width of the electrodes which carry them and a width of 60 micrometers.

Claims

 CLAIMS 1. Ferroelectric liquid crystal screen comprising an assembly comprising a layer of ferroelectric liquid crystal (72) capable of exhibiting zig-zag defects, this layer being between a group of transparent, parallel electrode-lines (32) and separated from each other and a group of transparent, parallel column electrodes (34), separated from each other and perpendicular to the line electrodes, said electrode groups being respectively arranged on two electrically insulating and transparent plates (44, 46) , screen characterized in that it further comprises, on each part of each row electrode, facing an interval separating two column electrodes, and on each part of each column electrode, facing a gap separating two electrodes -lines, an element (40, 50 - 42, 52 - 52a) intended to prevent the screen from passing through a light arriving on this screen in the direction of this element, and in that the elements which are arranged on the row electrodes or the elements which are arranged on the column electrodes are further provided to allow spacing, without electrical connection between the electrodes- lines and the column electrodes, of the plates of the screen and have dimensions which, counted parallel to these plates, allow the localization of the zig-zag faults in the vicinity of the non-switchable zone of the screen or in this zone.

2. Screen according to claim 1, characterized in that each element comprises an opaque layer (40, 42) which covers the electrode part on which this element is disposed and in that each element allowing the spacing of the plates comprises, in addition to this opaque layer, an electrically insulating spacer (50, 52) which is placed thereon.

3. Screen according to claim 2, characterized in that the width of the opaque layer is greater than the width of the gap separating two electrodes and facing the part which is covered by this opaque layer.

4. Screen according to any one of claims 2 and 3, characterized in that each spacer has an elongated shape transversely to the electrode on which the element comprising this spacer is disposed.

5. Screen according to claim 4, characterized in that the width of each spacer is greater than the width of the interval separating two electrodes and facing the part on which the element comprising this spacer is disposed.

6. Screen according to claim 4, characterized in that the width of each spacer is less than the width of the interval separating two electrodes and facing the part on which the element comprising this spacer is disposed.

7. Screen according to claim 1, characterized in that, this screen further comprising two crossed rectilinear polarizers (66, 68), on either side of said assembly, each element allowing the spacing of the plates is an electrically insulating spacer (52a) which has an elongated shape transversely to the electrode on which this element is disposed and a width greater than the width of the gap separating two electrodes and facing the electrode part on which the element is disposed, in that this element is made of an optically isotropic material and in that each element preventing said passage of the screen by light without allowing the spacing of the plates is made of an opaque layer which covers the electrode part on which is placed this last element.

8. Screen according to claim 7, characterized in that the optically isotropic material is an optically isotropic photosensitive resin.

9. Screen according to any one of claims 1 to

8, characterized in that, said plates being further coated with orientation layers (62) of the liquid crystal which are made anisotropic in a direction (D1) parallel to the electrodes of one of the two groups of electrodes, the elements allowing the spacing of the plates are arranged on the electrodes of this group.

10. Screen according to any one of claims 1 to 8, characterized in that, said plates being further coated with layers (62) for orientation of the liquid crystal which are made anisotropic in a direction (D2) parallel to the elect rods of one of the two groups of electrodes, the elements allowing the spacing of the plates are arranged on the electrodes of the other group.

11. Screen according to any one of claims 4 to 10, characterized in that the length of each spacer is substantially equal to the width of the electrode carrying this spacer.

12. Screen according to any one of claims 2 to

11, characterized in that each opaque layer is an opaque metallic layer.

13. Screen according to any one of claims 1 to

12, characterized in that the ferroelectric liquid crystal is chosen from the group comprising the chiral smectic liquid crystals C, I, F, G, H inclined.

14. Method for obtaining spacers for the screen according to claim 2, characterized in that it comprises:

- depositing a layer of positive photosensitive resin (60) on the plate carrying the electrodes and the opaque layers on which the spacers are intended to be located, - exposure of the resin through this plate arranged so that the layers opaque carried by this plate serve as a mask during the sunshine, and

- elimination of the insolated resin.

15. A method of processing the screen according to any one of claims 1 to 13, characterized in that it comprises the application, between the row electrodes (32) and the column electrodes (34) of a AC voltage, while maintaining the screen at a temperature close to the transition temperature from the ferroelectric smectic phase to the next higher phase of the liquid crystal, in order to locate the zig-zag faults of the liquid crystal in the vicinity of the non-switchable area of the screen or in this area.