Liquid Crystal Electro-Optical Device Having Micro-Domains

The present invention relates to a liquid crystal device, and more particularly relates to a ferroelectric liquid crystal device and manufacturing method therefor.

Heretofore, it is known to utilize twisted nematic liquid crystals for implementing electro-optical displays. The liquid crystal materials are employed in layer form which is finely divided into a number of pixels by virtue of a matrix electrode arrangement contiguous to the liquid crystal layer. However, due to the occurrence of crosstalk between adjacent pixels during operation in a time multiplexing mode, the number or density of pixels is substantially limited.

Switching is performed by means of thin film transistors provided for every pixel, the driving fashion being an active matrix system. However, because of complexities of the manufacturing process, it is very difficult to produce a display having a large area when cost reduction is considered.

In an attempt to solve the above shortcomings of prior art devices, Clark et al. proposed a ferroelectric liquid crystal device in their U.S. Pat. No. 4,367,924. FIG. 1 is an explanatory schematic diagram showing the action of liquid crystal molecules in the prior art devices. A ferroelectric liquid crystal is interposed between a pair of glass substrates 11 and 11' which is provided with an electrode arrangement made of In.sub.2 O.sub.3, SnO.sub.2 or ITO (Indium Tin Oxide) on the inside thereof. The liquid crystal is arranged between the substrates so that each molecular layer 12 is formed normal to the substrates as illustrated in the figure. The phase of the liquid crystal is chiral smectic C where the device is desirably driven at room temperature. The liquid crystal molecule takes two stable positions I and II which make angles .theta. and -.theta. with the layer normal as shown in FIG. 2.

The positions of the molecules switch between the two stable positions in response to an externally applied electric field normal to the substrates, whereupon visual images can be constructed based on differential birefringence among the pixels. One feature of this type of display device is bistability by virtue of which the position of each liquid crystal molecule is maintained the same as the previous state even after the applied signal is removed until another signal is applied anew in the opposite sense. Namely, they can function as memory elements.

To such a ferroelectric liquid crystal device, it has been required to obtain a uniform state of liquid crystal without imperfections throughout the liquid crystal layer between the pair of substrates for uniform drive capability throughout the entire display area. The liquid crystal layer of this condition is referred to as "mono-domain" hereinafter.

Imperfections and defects are caused because of small flaws of orientation control films, unevenness of the electrode arrangement formed on the substrates, spacers and other causes. In order to avoid occurrence of such imperfections and defects, the mono-domain condition has been developed by a temperature gradient method in which the crystalline structure of the liquid crystal is one-dimensionally developed inwardly from one end of the display area.

However, epitaxial growth of the smectic phase from a spacer edge under appropriate temperature gradient application of the temperature gradient method is effective only when the display area of the device exceeds several square centimeters. Furthermore, even if a large area mono-domain is constructed, the crystalline direction is not exactly aligned parallel to the substrates, making a pretilted angle with the substrate plane. For this reason, liquid crystal molecular layers tend to bend causing zig-zag structures. The switching due to external electric fields may take place in reverse senses at both sides of a folding plane in such a zig-zag structure. It has been often observed that uniform display and driving performance are hindered by the zig-zag structure.

The inventors have repeated experiments using liquid crystal displays comprising a chiral smectic C liquid crystal (ferroelectric liquid crystal). However, they have failed to satisfactorily drive the displays and to obtain clear images. This failure supposedly occurs because of interaction between pixels. The main cause of interaction might be quasi-monocrystalline (homogeneously ordered without discontinuity) regions bridging adjacent pixels. In other words, the switching of one pixel might influence an adjacent pixel through the mono-crystalline region bridging therebetween.

It is an object of the present invention to provide a liquid crystal device which can be driven with distinct pixel.

In order to accomplish the above and other objects, micro-domains are formed in each pixel consisting of a chiral smectic liquid crystal layer. The micro-domains are regions of several microns in width and several hundreds of microns in length in which molecules are oriented in mono-crystalline form, but interfaces between the micro-domains constitute discontinuities which hinder the influence of orderliness of one micro-domain upon others. Each pixel comprises a number of micro-domains. The position of the liquid crystal molecules can switch individually in each micro-domain by virtue of an electric field exercised thereupon. The interaction between the micro-domains subjected to an applied electric field and neighbouring micro-domains free of the field are suppressed in the light of the interfaces therebetween.

The improved structure is produced by interposing a composite liquid crystal material between a pair of substrates provided with an orientation control surface on the inside, at a relatively high temperature, at which the liquid crystal material is in an isotropic phase, and gradually cooling the liquid crystal material so that an ordered arrangement is developed with micro-domains.

Unlike conventional structures in which liquid crystal molecules are aligned in a particular direction, liquid crystal molecules in accordance with the present invention are aligned in diverse directions whereby a number of micro-domains are formed. Preferably, the average dimension of said micro-domains with reference to the row direction is one or more order of magnitude smaller than that of a pixel.

FIG. 1 is an explanatory view showing the condition of liquid crystal molecules interposed between substrates.

FIG. 2 is a schematic diagram showing two stable positions of a liquid crystal molecule.

FIG. 3 is a cross sectional view of a liquid crystal device comprising a chiral smectic liquid crystal material.

FIG. 4 is a schematic view showing the texture of a micro-domain in a liquid crystal layer.

FIGS. 5(A) and 5(B) are microscopic photographs of micro-domains taken at two sites in a liquid crystal structure of 20 microns thickness in smectic A phase in accordance with the present invention with magnification of 200 by use of polarizing plates which are arranged normal to each other and sandwich the liquid crystal structure.

FIGS. 6(A) and 6(B) are microscopic photographs of micro-domains taken at two sites in a liquid crystal structure of 20 microns thickness in smectic C phase in accordance with the present invention with magnification of 200 by use of polarizing plates which are arranged normal to each other and sandwich the liquid crystal structure.

FIGS. 7(A) and 7(B) are microscopic photographs of micro-domains taken in the same condition as FIGS. 6(A) and 6(B) except that the distance between a pair of substrates sandwiching a liquid crystal layer is 3 microns.

Referring now to FIG. 3, a liquid crystal display device in accordance with the present invention is illustrated in a cross sectional view. The device comprises a pair of glass substrates 2 and 2', first parallel electrode strips formed on the substrate 2 and extending in the lateral direction, second parallel electrode strips formed on the substrate 2' and extending in the normal direction to the plane of the drawing sheet, a first orientation control film 4 made of a polyimide resin, a second orientation control film 4' made of SiO.sub.2, and an ester ferroelectric liquid crystal layer 5. The ferroelectric liquid crystal material is a composite chiral smectic C phase liquid crystal. The combination of the first and second electrode strips 3 and 3' constitutes an electrode arrangement in a matrix form comprising a number of pixels. By appropriate selection of a suitable resin to form the orientation control films, it is possible to relatively raise the threshold level of the switching signals applied to the liquid crystal layer. The first and second orientation control films 4 and 4' are given rubbing treatments. Assuming the scanning of the pixels is to be carried out in the direction parallel to the first electrode strips 3, the rubbing has to be performed in the direction normal to the plane of the drawing sheet. In other words, rubbing is performed along the columns in the case of designs adapted for scanning along the rows. The periphery of the substrates is provided with a sealing member 6 for avoiding the loss of liquid crystal. Reference numerals 1 and 7 designate polarizing plates arranged at right directions. The dimensions of parts of the device shown in the figure are only chosen for illustration but not in accordance with a real design. Although not shown in the figure, when assembled, spacers are interposed between the substrates in order to maintain the distance constant. In reality, the distance between the substrate is 3 microns; the width of each electrode strip is 0.36 mm and separated from an adjacent strip by 0.04 mm intervals.

Particularly, in accordance with the present invention, the liquid crystal material is prepared in order to have a broad transition range within which the phase of the liquid crystal is gradually changed from its isotropic phase to its quasi-crystalline phase. In order to obtain such a transition characteristic, the liquid crystal material is prepared by blending several kinds of liquid crystals. The liquid crystal constituents are selected from those having diverse transition temperatures distributed over a wide temperature range. A composite liquid crystal material has been obtained having a wide transition temperature range by blending eight kinds of liquid crystal esters.

The following are eight liquid crystal constituents with their respective proportions in parenthesis which constitute a composite liquid crystal in accordance with the present invention. ##STR1##

By blending the above constituents, a liquid crystal material was obtained with its transition characteristics such that Crystal.rarw.(4.7.degree. C.-3.5.degree. C.).fwdarw. S.sub.C.sup.* .rarw.(62.5.degree. C.-51.9.degree. C.).fwdarw.Sm.sub.A .rarw.(89.7.degree. C.-75.4.degree. C.).fwdarw. Iso

It will not be difficult for those skilled in the art to prepare liquid crystal materials which have transition characteristics suitable to the applications. In accordance with the present invention, we prepared another liquid crystal material whose phase transition was such that isotropic liquid .rarw.(130.degree. C.-98.degree. C.).fwdarw.smectic A.rarw.(73.degree. C.-60.degree. C.).rarw.smectic .sup.* C .rarw.(10.degree. C.-0.degree. C.).fwdarw.smectic .sup.* I.rarw.(-10.degree. C.).fwdarw.crystal.

After interposing such a composite liquid crystal material between the orientation control films provided on the substrates 2 and 2' at a temperature at which the liquid crystal material is in its isotropic phase, the structure is gradually cooled to a temperature at which the entirety of the composite liquid crystal material is in its smectic phase. Liquid crystal molecules form micro-domains which grow as the temperature reduces. Each micro-domain can be regarded to have a quasi-monocrystaline structure. The proportion of each constituent is 5-20%. It will be desired in general to limit the maximum proportion of each constituent up to 20% and to use many kinds of liquid crystals at nearly equal proportions respectively. The formation of mirco-domains in the liquid crystal layer starts along the oriented surface given a rubbing treatment so that the quasi-polycrystalline area consisting of mirco-domains gradually grows as the temperature reduces. FIG. 4 shows a sketch topologically drafted in accordance with a microscopic photograph of the micro-domains. The width and the length of each micro-domain 8 are about several microns and several hundreds of microns respectively. The significant feature of the structure comprising the micro-domains is the absence of zig-zag defects. While the interfaces 9 between the micro-domains 8 are defects, the entirety of the liquid crystal can be regarded approximately as a uniform quasi-crystalline structure except for the interfaces between the domains which are small defects.

When a sawtooth pulse having a maximum voltage of .+-.10V is applied between the electrode strips 3 and 3', the molecular state of the pixel just sandwiched by the activated two electrodes is changed. The state change takes place uniformly in every micro-domain 8 within the pixel so that the entirety of the pixel is changed at once. In accordance with an experiment, there was observed no difference in the changing process between a central position and a peripheral position of the pixel.

FIGS. 5(A) and 5(B) are microscopic photographs showing micro-domains taken at two sites of a liquid crystal layer in smectic A in accordance with the present invention with magnification of 200 by use of polarizing plates which are arranged normal to each other and supporting the liquid crystal therebetween. The distance between a pair of substrates was selected to be 20 microns rather than 3 microns for taking photographs in which micro-domain textures appear clearly. As shown in the photographs, the liquid crystal is divided into a number of micro-domains. This means that liquid crystal molecules have been oriented uniformly in each micro-domain but adjacent micro-domains have been oriented in different orientation directions. The micro-domains are several microns in width and several hundreds of microns in length. The micro-domains are formed approximately parallel to the rubbing direction given to the orientation control films.

As the phase of liquid crystal is transformed to smectic C at a reduced temperature, stripes appear in each micro-domain as seen from FIGS. 6(A) and 6(B) taken in the same condition as FIGS. 5(A) and 5(B) except for the temperature. The width of each stripe corresponds to one turn of a helix of the chiral smectic C liquid crystal and is about 3 microns. As shown in the photographs, stripes of neighbouring micro-domains are aligned at the interfaces. FIGS. 7(A) and 7(B) are photographs in the case where the distance between the substrates is about 3 microns, other conditions being same as FIGS. 5(A) and 5(B). The length of each micro-domain shortens as compared with FIGS. 6(A) and 6(B). Such a narrow distance between the substrates allows the liquid crystal helices to unwind, and therefore the response speed becomes as fast as 10 microseconds when measured in the same condition as the case of 20 microns.

The size of the micro-domains can be changed by adjusting the preparation conditions. For instance, the dimensions of micro-domains were measured in the case where a rubbing treatment was performed on the orientation control film with a rolling device in the following conditions.

The figures are relative values calculated with reference to those of Case 3 being 1.0. The dimensions of the micro-domains are 25 microns.times.450 microns in Case 1; 10 microns.times.200 microns; and 8 microns.times.200 microns. The micro-domains were observed uniformly throughout the liquid crystal layer.

Instead of the use of a composite liquid crystal having a wide transition temperature range, particular liquid crystal materials have been found to expedite the formation of micro-domains. The liquid crystals are expressed in the general formula, ##STR2## where R.sub.1 is a hydrocarbon, and R.sub.2 is a hydrocarbon comprising at least two asymmetric carbon atoms. By use of a liquid crystal of this type or by use of a blended liquid crystal including this type liquid crystal, micro-domains are fabricated. For instance, R.sub.1 is --C.sub.8 H.sub.17 and R.sub.2 is a hydrocarbon comprising ##STR3##

While several embodiments have been specifically described, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departure from the scope of the invention as defined by the appended claims.