GB2319854A

GB2319854A - Liquid crystal display element

Info

Publication number: GB2319854A
Application number: GB9725297A
Authority: GB
Inventors: Nobuyuki Itoh
Original assignee: UK Secretary of State for Defence; Sharp Corp
Current assignee: UK Secretary of State for Defence; Sharp Corp
Priority date: 1996-12-02
Filing date: 1997-11-28
Publication date: 1998-06-03
Anticipated expiration: 2017-11-28
Also published as: JPH10161131A; GB9725297D0; GB2319854A8; GB2319854B

Abstract

A liquid crystal display element (1), comprises an upper (11) and a lower substrate (10) which sandwich a ferroelectric liquid crystal (7). With respect to respective interfaces of a pair of alignment layers (5a,5b) a uniaxial alignment treatment is applied in the same direction. One of the interfaces of the pair of alignment layers (5a) has such an alignment control force that permits a smectic layer to exhibit C1 alignment preferentially, while, the other interface (5b) has such an alignment control force which permits a smectic layer to exhibit C2 alignment preferentially. As a result, the smectic layer of the ferroelectric liquid crystal has a stable oblique structure.

Description

LIQUID CRYSTAL DISPLAY ELEMENT The present invention generally relates to a liquid display element having a ferroelectric liquid crystal, and more particularly relates to a ferroelectric liquid crystal display element which exhibits a mono-domain alignment without defects.

The necessity of liquid crystal display devices have been very mach admitted for use in displays of information equipments such as lap-top type personal computers, word processors, portable terminals, portable televisions, etc., for their beneficial characteristics of thin, light-weight, low power consumption, low driving voltage, etc., as a flat display element.

Conventional liquid crystal display devices generally use a nematic liquid crystal except for those designed for special use such as thermal addressing smectic liquid crystals, etc. Especially, in practical applications, liquid crystal displays of two types, i.e., a super twisted nematic (STN) type and a twisted nematic (TN) type using a TFT as a driving element have been generally used in various applications.

However, it is extremely difficult to realize a display of an improved precision and capacity using a nematic liquid crystal. In the STN mode, in view of characteristics of a liquid crystal such as light transmittivity or response speed with respect to an applied voltage, etc., a number of scanning lines has already almost reached the limit in principle. On the other hand, in the TN type display adopting a TFT, it is required to provide the TFT for each pixel without defects, which make it more difficult to manufacture liquid crystal display devices as a display capacity increases. Besides, as the TFT is formed within a pixel region, it is difficult to improve an aperture ratio.

As a technique which gives a solution to the described problems and realizes a display of an improved precision and capacity, N.A. Clark and S.T.

Lagerwall proposed a liquid crystal display device adopting a chiral smectic C liquid crystal, i.e., a ferroelectric liquid crystal in 1980. Such liquid crystal display device is disclosed by U.S. Patent No.

4367924, Japanese Unexamined Patent Publication No.

107216/1981 (Tokukaisho 56-107216) and Appl. Phys.

Lett., vol. 36(1980)899., etc. The ferroelectric liquid crystals have characteristics of bistable memory (bistability) and high-speed response, and they are , .

expected to lead liquid crystal display devices of next generation.

However, the ferroelectric liquid crystals have a drawback in that it is difficult to be homogeneously aligned in a large area compared with the nematic liquid crystals. A non-homogeneous alignment state causes differences in display characteristics in the display, which results in a serious problem that a uniformity of a display screen is adversely affected.

The essential cause of hindering homogeneous alignment in the ferroelectric liquid crystals lies in its smectic layer structure. The chiral smectic C liquid crystal as a ferroelectric liquid crystal has a so-called chevron structure in which a smectic layer 14 is bent in chevron between a pair of substrates 20 as shown in Fig. 6 unlike a smectic A liquid crystal having a so-called bookshelf-type layer structure wherein a liquid crystal layer is formed perpendicular to a substrate.

Here, the reasons why a chevron layer structure appears in the chiral smectic C liquid crystal will be discussed. Fig. 7 depicts conceptual behaviors of a chiral smectic C liquid crystal molecule 15, wherein z is a smectic layer normal. In the smectic A liquid crystal, molecules are aligned parallel to the layer normal z, while in the chiral smectic C phase appeared u on the low temperature side of a smectic A phase in the phase series, the chiral smectic C liquid crystal molecule 15 is tilted by a tilt angle 8 with respect to the layer normal z. The tilt angle 8 is formed by a major molecule axis 16 and the layer normal z. The molecule 15 has a spontaneous polarization Ps perpendicular to the major molecule axis 16 and rotates on a conical locus 18 with an applied electric field.

When a phase transition occurs from the smectic.A liquid crystal to the chiral smectic C liquid crystal, an interlayer spacing is reduced as the molecule 15 is tilted with respect to the layer normal z as described above. However, as the interlayer spacing in the smectic A phase is stored and maintained in a vicinity of the substrate, a layer absorbs a distortion in the interlayer spacing, and the layer turns out to be tilted, and bent in a middle. This is believed to be a mechanism of forming a chevron layer structure.

In view of the above, it is expected in the chevron structure that the tilt angle 6 of the chevron layer is substantially equal to a tilt angle 8 of the molecules 15 shown in Fig. 7. In practice, however, the tilt angle 6 is slightly smaller than the tilt angle 0, and this has been confirmed by the inventors of the present invention and reported in a report [N.

Itoh et al.: Jpn. J. Appl. Phys., vol. 31(1992)1414.].

Fig. 8 is an explanatory view showing a crosssectional structure of a liquid crystal cell including a ferroelectric liquid crystal of a chevron structure in reference to alignment defects observed when the liquid crystal cell is seen from above. The ferroelectric liquid crystal is enclosed between two substrates 30 which are bonded together in such a manner that respective uniaxial alignments applied to the two substrates 39 have the same direction. As shown in Fig. 8, a rising direction of liquid crystal molecules 31 in an alignment layer interface formed on respective surfaces of the substrates 30 is known to be equal to a direction of the uniaxial layer alignment treatment applied to each alignment layer. An angle formed by the liquid crystal 31 and the surface of the alignment layer is defined as a pretilt angle dp.

When the chevron structure appears in the smectic layer of the ferroelectric liquid crystal, many problems arise. The chevron structure is classified into two kinds depending on a bending direction of the smectic layer: (a) C1 alignment state wherein a direction of uniaxial layer alignment treatment (rubbing direction) is opposite to a bending direction of the smectic layer 32; and (b) C2 alignment wherein a direction of the uniaxial layer alignment treatment is equal to the bending direction of the smectic layer 32.

In the smectic C liquid crystal, if it can be controlled such that only either one of the C1 alignment and C2 alignment appears stably, that would not be a problem. However, as an energy barrier generated at a transition from the C1 alignment to the C2 alignment or vice versa is not very large, it is difficult to control to achieve such a state where only either one of the C1 and the C2 appears on an entire area stably. Even if the layer structures can be aligned in one of the alignment states, it is likely that the other alignment state appears due to a change in temperature, a mechanical shock, etc., which impairs the desired layer structure.

As reported in a report [N. Itoh et al.: Liquid Crystals, vol. 15(1993)669] by the inventors of the present application, if layer structures of two kinds coexist within a liquid crystal cell, on an interface of the layer structures, alignment defects occur such as hairpin defects, zig-zag defects as shown in Fig. 8.

Moreover, as the C1 alignment and C2 alignment have mutually different optical characteristics, a uniform display cannot be obtained.

Other than the described problem, there is a serious problem that the display contrast expected in an ideal bookshelf layer structure cannot be achieved , .

in the chevron structure. This is presumably because in the portion where the bookshelf structure is bent, i.e., a so-called chevron interface, an open angle of a liquid crystal molecule with respect to the smectic layer normal is smaller than the original tilt angle of the liquid crystal molecule.

On the chevron interface, as liquid crystal molecules hit each other above and below the chevron interface when switching, a switching cannot be carried out smoothly, which lowers the response speed or impairs memory characteristics. In order to counteract the described problems, attempts have been made to realize an ideal bookshelf structure in replace of the chevron layer structure having the described problems (for example, in a report ([Takanishi et al.: Jpn. J. Appl. Phys., vol. 29 (1990) L984.], [Isogai et al.: Mol. Cryst. Liq. Cryst., vol 207 (1991) 87.]).

However, a complete solution to the described problems has not yet been achieved.

As a method of eliminating problems associated with the chevron structure, as shown in Fig. 9, an attempt has been made to achieve a tilted layer structure (oblique structure) of smectic layers 41 by applying a so-called anti-parallel alignment in which respective directions D1 and D2 of uniaxial layer alignment treatment applied to respective alignment u layers of a pair of substrates 40 which sandwich a ferroelectric liquid crystal are parallel but in opposite directions as reported (for example, in a report [Y. Ouchi et al.: Jpn. J. Appl. Phys., vol.

27(1988) L1993.]).

However, when preparing a cell in an anti-parallel alignment by rubbing an alignment layer made of polyimide, etc., for some reason, many fine linear domains (different portions in alignment) appear, and a homogeneous alignment cannot be achieved. For this reason, in many cases, despite that the chevron structure is formed, a parallel alignment wherein the direction of a uniaxial layer alignment treatment in a substrate is equal to that of the counter substrate is generally adopted.

In the report of Y. Ouchi, an oblique vaporization is adopted; however, the oblique layer structure in a homogeneous alignment state is obtained only in the inorganic column like SiO exceptionally. Needless to mention, the described inorganic vaporization technique is not suited when upsizing or mass producing the liquid display devices.

The ferroelectric liquid crystal has an advantageous characteristics of bistability as described earlier which can be appreciated when high speed driving; however, it has a drawback in that such , .

bistability makes it difficult to achieve an analog gray shade display. As a solution to the described problem, a technique of adding fine particles into the ferroelectric liquid crystal so as to distribute fine regions of different threshold voltages for switching the liquid crystal has been proposed by Japanese Unexamined Patent Publication No. 92469/1995 (Tokukaihei 7-92469). According to this technique, by adding fine particles to the liquid crystal cell having a sharp oblique layer structure wherein a smectic layer surface is substantially approximated to a plane, a quasi-oblique structure wherein a tilt angle of the layer surface varies step by step can be realized, thereby realizing an analog gray shade display.

In the described publication, an anti-parallel alignment is required when the pretilt angle of the alignment layer is not more than 4 , and an antiparallel or parallel alignment is applied when the pretilt angle is not less than 5 . Namely, it is effective to apply the anti-parallel alignment treatment irrespectively of the pretilt angle.

However, in the liquid crystal cell prepared by the normal anti-parallel alignment treatment, even when an oblique structure is formed, fine linear domains appear, and a uniform alignment state cannot be obtained. Also, in the technique disclosed in the w described Japanese Unexamined Patent Publication No.

92469/1995 (Tokukaihei 7-92469), an alignment layer by the SiO oblique vaporization which permits a uniform alignment state to be achieved with ease is adopted, and it is not suited for industrial applications.

It is an object of the present invention is to provide a ferroelectric liquid crystal display element having a smectic layer of a stable oblique structure without generating fine linear domains.

In order to achieve the above object, the first liquid crystal display element in accordance with the present invention is characterized by including: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between the pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to the respective alignment layers of the pair of light transmissive substrates in the same or substantially the same direction, and a smectic layer of the ferroelectric liquid crystal has an oblique structure.

According to the described arrangement, as the smectic layer does not have a chevron structure which II is bent in a middle but has an oblique structure which is tilted at a constant tilt angle with respect to the normal of the light transmissive substrate. As a result, improved performances in its display contrast, response speed, and bistability can be achieved. In the conventional smectic layer of the chevron structure, as the two alignment states of C1 and C2 which have mutually different bending directions coexist, it is likely that zig-zag defects or hairpin defects occur. Furthermore, when switching, as the liquid crystal molecules hit each other above and below the chevron interface at the portion where the smectic layer bends, the response speed or bistability deteriorates. In contrast, in the smectic layer of the oblique structure, a uniform monodomain alignment state is obtained, and the switching of the liquid crystal molecules is not disturbed, thereby obtaining superior characteristics over the chevron structure in view of response speed or bistability.

Furthermore, the oblique structure is obtained by such an arrangement that respective direction of the uniaxial layer alignment treatment applied to each alignment layer of the pair of light transmissive substrates is the same or substantially the same.

Unlike the anti-parallel layer alignment treatment applied to obtain a conventional oblique structure (the il uniaxial layer alignment treatments are applied to a pair of substrates in parallel and opposite directions) , a more stable homogeneous alignment state than that of the conventional oblique structure can be achieved without generating fine linear domains. As a result, a liquid crystal display element which offers excellent display stability and uniformity can be obtained.

In order to achieve the above object, the second liquid crystal display element in accordance with the present invention is characterized by including: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between the pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to the respective alignment layers of the pair of light transmissive substrates in the same or substantially the same direction, a direction of the uniaxial layer alignment treatment applied to the alignment layer of one of the pair of light transmissive substrates is in a tilt direction of the smectic layer of the ferroelectric liquid crystal, and a direction of the uniaxial layer alignment treatment applied to the alignment layer of the other of the pair of alignment layers is in an opposite direction of the tilt direction of the smectic layer of the ferroelectric liquid crystal.

According to the described arrangement, as the smectic layer has an oblique structure, or pseudo oblique structure in which the smectic layer slightly bents in the middle as shown in Fig. 14(b) of Japanese Unexamined Patent Publication No. 92469/1995 (Tokukaihei 7-92469). As a result, a uniform monodomain alignment state without generating defects like the chevron structure can be obtained. Moreover, as the switching of the liquid crystal molecules are not hindered, superior characteristics than those of the chevron structure can be achieved in view of response speed and bistability.

Conventionally, in order to form the smectic layer of the oblique structure, an alignment layer to which SiO is oblique deposited is adopted, thereby presenting the problem that it is difficult to upsize or massproduce the substrate. In contrast, as the described arrangement of the present invention permits the rubbing treatment to be suitably applied as a uniaxial layer alignment treatment to the alignment layer, a liquid crystal display element which offers a solution to the above problems can be achieved at low cost.

Furthermore, the feature of the described arrangement that respective uniaxial layer alignment treatments applied to the alignment layers of a pair of light transmissive substrates have the same or substantially the same directions offers a uniform alignment without generating fine linear domains unlike the case of applying an anti-parallel treatment (the uniaxial layer alignment treatments are applied to a pair of substrates in parallel and opposite directions). As a result, a liquid crystal display element having excellent characteristics in view of display stability and uniformity can be obtained.

In order to achieve the above object, the third liquid crystal display element in accordance with the present invention is characterized by including: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between the pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to the respective alignment layers of the pair of light transmissive substrates in the same or substantially the same direction, the alignment layer of one of the pair of light transmissive substrates has such an alignment control force that permits a smectic layer of the ferroelectric liquid crystal to have a chevron structure in which C1 alignment appears preferentially when the ferroelectric liquid crystal is enclosed in a spacing formed between two of the alignment layers placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are the same or substantially the same, and the alignment layer of the other of the pair of light transmissive substrates has such an alignment control force that permits the smectic layer of the ferroelectric liquid crystal to have a chevron structure in which C2 alignment appears preferentially when the ferroelectric liquid crystal is enclosed in a spacing formed between two of the alignment layers of the other of the pair of light transmissive substrates placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are same or substantially the same.

In the case where the smectic layer has a chevron structure, an energy barrier generated at a transition in layer structure from the C1 alignment to C2 alignment, or vice versa is not very large. Therefore, for example, even if the entire smectic layer is once aligned in the C1 alignment, the C2 alignment partially appears, and thus a uniform alignment state cannot be obtained.

However, according to the described arrangement of the present invention, the alignment layer of one of the pair of light transmissive substrates has such an alignment control force that permits the smectic layer to have C1 alignment preferentially in the chevron structure, there is a tendency that a portion in a vicinity of the alignment layer in the smectic layer is tilted in the opposite direction to the uniaxial layer alignment treatment. On the other hand, the alignment layer of the other light transmissive substrate has such an alignment control force that permits the smectic layer to have C2 alignment preferentially in the chevron structure, there is a tendency that a portion in a vicinity of the alignment layer in the smectic layer is tilted in the direction of the uniaxial layer alignment treatment.

As these alignment layers are placed opposing each other in such a manner that the uniaxial layer alignment treatments are applied thereto respectively in the same or substantially the same direction, the smectic layer sandwiched between the alignment layers generate a large energy barrier exerted in an opposite direction to the direction where the layer is preferentially tilted on each alignment surface. As a result, as shown in Fig. 2(a), an oblique structure in which the smectic layer surface is tilted with respect to the normal of the light transmissive substrate without bending like chevron structure appears stably.

, .

As shown in Fig. 5(b), even in the oblique structure, the two alignment states coexist according to the tilt direction of the layer surface with respect to the normal of the light transmissive substrate.

However, it is obvious from Figs. 5(a) and 5(b) that the energy barrier generated at a transition from one alignment state to the other is larger than that generated at a transition between the C1 and C2 in the chevron structure. Namely, as the oblique structure has high stability with respect to the chevron structure, a uniform mono domain alignment can be obtained.

The feature that the respective uniaxial alignment layer treatments are applied to the alignment layers of a pair of light transmissive substrates in the same or substantially the same directions offers a uniform alignment without generating fine linear domains unlike the case of applying an anti-parallel layer alignment treatment (the uniaxial layer alignment treatments are applied to a pair of substrates in parallel and opposite directions). Additionally, as the described arrangement permits the rubbing treatment suited for mass production to be applied to the alignment layers, a liquid crystal display element can be produced at low cost.

As described, according to the arrangement of the third liquid crystal display element in accordance with the present invention, a liquid crystal display element which has excellent characteristics in view of display contrast, response speed and bistability, and which permits a uniform display without defects can be realized at low cost.

The third liquid crystal display element may be arranged such that the alignment layer of one of the pair of light transmissive substrates has such an alignment control force that permits the smectic layer of the ferroelectric liquid crystal to have a chevron structure in which C1 alignment appears in not less than 50 percent of a total display area when the ferroelectric liquid crystal is enclosed in a spacing formed between two of the alignment layers placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are same or substantially the same, and the alignment layer of the other of the pair of light transmissive substrates has such an alignment control force that permits the smectic layer of the ferroelectric liquid crystal to have a chevron structure in which C2 alignment appears in not less than 50 percent of a total display area when the ferroelectric liquid crystal is enclosed in a spacing formed between two of the alignment layers of the other of the pair of light transmissive substrates placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are same or substantially the same.

The described arrangement permits the smectic layer of the ferroelectric liquid crystal to have a stable oblique structure, thereby providing a liquid crystal display element which has excellent characteristics in view of display contrast, response speed and bistability, and offers a uniform display without defects.

The third liquid crystal display element of the present invention may be arranged such that a pretilt angle of liquid crystal molecules of the ferroelectric liquid crystal on an interface of the alignment layer of one of the pair of light transmissive substrates is larger than a difference between a tilt angle of the liquid crystal molecule and a tilt angle of the smectic layer, and a pretilt angle of liquid crystal molecules of the ferroelectric liquid crystal on an interface of the alignment layer of the other of the pair of light transmissive substrates is smaller than the difference between the tilt angle of the liquid crystal molecules and the tilt angle of the smectic layer.

When the pretilt angle is larger than the difference in angle between the tilt angle of the liquid crystal molecules and the tilt angle of the smectic layer, C1 alignment appears preferentially. On the other hand, when the pretilt angle is smaller than the difference in angle between the tilt angle of the liquid crystal molecules and the tilt angle of the smectic layer, C2 alignment state appears preferentially. According to the described arrangement, the smectic layer of the ferroelectric liquid crystal can have a stable oblique structure, thereby providing a liquid crystal display element which has excellent characteristics in view of display contrast, response speed and bistability and which offers a uniform display without defects.

Furthermore, each of the described arrangements of liquid crystal display element may be further arranged such that fine particles are added to the ferroelectric liquid crystal, for forming fine regions having different threshold voltages for switching the liquid crystal molecules.

As a result, as fine regions having mutually different threshold voltages for switching the liquid crystal molecules are distributed, respective optical responses can be altered by applying thereto a voltage according to the threshold value. As a result, the liquid crystal display element which has excellent characteristics in view of display contrast, response speed, and bistability and offers a gray shade display can be obtained.

For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.

Fig. 1 is a cross-sectional view showing a schematic structure of a liquid crystal display element in accordance with one embodiment of the present invention; Fig. 2(a) is a cross-sectional view showing a structure of a smectic layer which has an oblique structure; Fig. 2(b) is an explanatory view which depicts an alignment state of liquid crystal molecules in a smectic layer which has an oblique structure; Fig. 3 is an explanatory view depicting a structure of a smectic layer which has a chevron structure in C1 alignment as well as an alignment state of liquid crystal molecules in the smectic layer; Fig. 4 is an explanatory view depicting a structure of a smectic layer which has a chevron structure in C2 alignment as well as an alignment state of liquid crystal molecules in the smectic layer; Fig. 5(a) is an explanatory view showing a transition between two alignment states of the smectic layer of the chevron structure (C1 alignment and C2 alignment); Fig. 5(b) is an explanatory view showing a transition between the two alignment states of the smectic layer of the oblique structure; Fig. 6 is a cross-sectional view showing a structure of a smectic layer which has a chevron structure; Fig. 7 is a typical depiction which conceptually shows behaviors of liquid crystal molecules in a chiral smectic C phase; Fig. 8 is an explanatory view showing a structure of a liquid crystal cell in which C1 alignment and C2 alignment coexist in the chevron structure referring to a state of defects that can be observed from above the liquid crystal cell; Fig. 9 is a cross-sectional view depicting a structure of a conventional ferroelectric liquid crystal cell in which the smectic layer has an oblique structure by applying an anti-parallel alignment; and Fig. 10 is a plan view showing a state in which linear domain appears in the conventional liquid crystal cell in the anti-parallel alignment.

[FIRST EMBODIMENT] The following descriptions will explain one embodiment of the present invention in reference to Fig. 1 through Fig. 5.

Fig. 1 is a cross-sectional view showing a schematic structure of a liquid crystal display element in accordance with one embodiment of the present invention. As shown in Fig. 1, a liquid crystal display element 1 includes an upper substrate 11 and a lower substrate 10 which are bonded together, and a ferroelectric liquid crystal 7 enclosed in a spacing formed between them.

On the surface of a glass substrate 2a of the lower substrate 10, a plurality of transparent signal electrodes 3a made of indium tin oxide (hereinafter referred to as ITO), etc., are formed parallel to each other. On each of the signal electrodes 3a, a transparent insulating film 4a made of SiO2, etc., is formed.

On the other hand, on the surface, facing the signal electrodes 3a, of a glass substrate 2b of the upper substrate 11 a plurality of transparent scanning electrodes 3b made of ITO, etc., are formed parallel to each other and orthogonal to the signal electrodes 3a.

Upper surface of each scanning electrodes 3b is coated with a transparent insulating film 4b made of SiO2, etc.

On the insulating films 4a and 4b, alignment layers Sa and 5b are formed respectively, which are uniaxially aligned by rubbing. For each of the alignment layers 5a and 5b, an organic polymer film is suitably used such as a polyimide film, a nylon film, a polyvinyl alcohol film, etc. Other than the above, an SiO oblique vapor deposition film may be used.

The described two glass substrates 2a and 2b are bonded together by a sealing agent 6 except for a part which serves as an injection opening, to allow the ferroelectric liquid crystal 7 to be injected through the injection opening into the spacing formed between the alignment layers Sa and 5b. After the ferroelectric liquid crystal 7 has been injected, the injection opening is sealed with the sealing agent 8.

The glass substrates 2a and 2b thus bonded are sandwiched between polarization plates 12a and 12b.

The polarization plates 12a and 12b are placed such that respective polarizing axes cross at right angle.

As described, the liquid crystal display element 1 of the present embodiment is arranged such that the ferroelectric liquid crystal 7 is enclosed in a spacing formed between (a) the lower substrate 10 including the polarization plate 12a, the glass substrate 2a, the signal electrodes 3a, the insulating film 4a and the alignment layer Sa and (b) the upper substrate 11 including the polarization plate 12b, the glass substrate 2b, the scanning electrodes 3b, the insulating film 4b and the alignment layer 5b. In the case of a large-size display area, spacers 9 are formed to maintain the lower substrate 10 and the upper substrate 11 parallel to each other and apart at a constant spacing between them (cell thickness).

Fig. 2(a) is a cross-sectional view which depicts a structure of a smectic layer in the ferroelectric liquid crystal 7 of the liquid crystal display element 1. Fig. 2(b) is cross-sectional view which depicts an alignment state of liquid crystal molecules in the smectic layer under an applied electric field from the signal electrodes 3a and the scanning electrodes 3b.

In Fig. 2(a) and Fig. 2(b), an interface P1 indicates an interface of the alignment layer 5b of the upper substrate 11, and an interface P2 indicates an interface of the alignment layer 5a of the lower substrate 10.

Arrows A1 and A2 respectively indicate a direction of the uniaxial layer alignment treatment applied to the alignment layer Sb of the upper substrate 11 and a direction of the uniaxial layer alignment treatment applied to the alignment layer 5a of the lower substrate 10.

As shown in Fig. 2(a), the liquid crystal display a element 1 is characterized in that the lower substrate 10 and the upper substrate 11 are arranged such that the uniaxial layer alignment treatment is applied to the alignment layers Sa and 5b in the same direction, and that the smectic layers 13 of the ferroelectric liquid crystal 7 is not bent like the chevron structure, and is tilted with a constant angle with respect to a normal of the upper and lower substrates 10 and 11.

A switching occurs in liquid crystal molecules in the smectic layer 13 between the two alignment states according to an electric field applied from the signal electrodes 3a and the scanning electrodes 3b as shown in Fig. 2(b). Here, one alignment state is defined to be the ON state, and the other alignment state is defined to be the OFF state.

As shown in Fig. 2(a), the smectic layers 13 of the ferroelectric liquid crystal 7 are tilted in the direction (arrow A1) of the uniaxial layer alignment treatment applied to the alignment layer 5b on the side of the upper substrate 11, and it is tilted in an opposite direction to the direction (arrow A2) of the uniaxial layer alignment treatment applied to the alignment layer 5a on the side of the lower substrate 10. To realize the described structure, for the alignment layer 5b of the upper substrate 11, those which permit a chevron structure of C2 alignment to appear preferentially may be adopted, and for the alignment layer 5a of the lower substrate 10, those which permit a chevron structure of C1 alignment to appear preferentially may be adopted.

Fig. 3 is a cross-sectional view which depicts the structure of the smectic layers of the ferroelectric liquid crystal having the chevron structure which exhibits C1 alignment to be compared with the ferroelectric liquid crystal 7 in accordance with the present embodiment. In the figure, interfaces P3 and P4 indicate interfaces of a pair of alignment layers which sandwich the ferroelectric liquid crystals in between, while arrows A3 and A4 respectively indicate a direction of the uniaxial layer alignment treatment applied to the pair of alignment layers. As shown in the figure, the smectic layer in the C1 alignment is bent in an opposite direction to the direction of the uniaxial layer alignment treatment applied to the alignment layers. Namely, in both of the interfaces P3 and P4, the smectic layer is tilted in an opposite direction to the direction of the uniaxial layer alignment treatment.

Fig. 4 is a cross--sectional view which depicts the structure of the smectic layer of the ferroelectric liquid crystals having a chevron structure in C2 alignment to be compared with the ferroelectric liquid crystal 7 in accordance with the present embodiment.

As shown in the figure, the smectic layer in the C2 alignment is bent in the same direction as the direction of the uniaxial layer alignment treatment applied to the alignment layers. Namely, in both of the interfaces P5 and P6, the smectic layer is tilted in an opposite direction to the direction of the uniaxial layer alignment treatment.

In the smectic layer of the chevron structure, which one of the C1 alignment and the C2 alignment appears preferentially depends on the pretilt angle of the liquid crystal molecules on the alignment layer interface (8p shown in Fig. 3 and Fig. 4 respectively) of the liquid crystal molecules at the interface of the alignment layer. When the pretilt angle Ep is greater than the difference between the tilt angle e (see Fig.

7) of the liquid crystal molecules of the ferroelectric liquid crystals and the tilt angle 6 of the smectic layer, the C1 alignment appears preferentially. On the other hand, when the pretilt angle Ap is smaller than the difference between the tilt angle e of the liquid crystal molecules of the ferroelectric liquid crystals and the tilt angle 6 of the smectic layer, the C2 alignment appears preferentially.

The size of the pretilt angle varies depending on various factors such as an alignment layer material, baking temperature and conditions for uniaxial layer alignment treatment, etc. Then, with appropriate selections of the alignment layer, the baking temperature of the alignment layer, and the conditions for the uniaxial layer alignment treatment, the size of the pretilt angle can be adjusted as desired.

The alignment layer Sb of the liquid crystal display element 1 in accordance with the present embodiment is provided for applying the following pretilt angle to the liquid crystal molecules. That is, when the ferroelectric liquid crystal 7 is enclosed in a spacing formed between the two alignment layers arranged such that the uniaxial alignment treatment direction applied in the same direction, the C2 alignment appears preferentially in the smectic layer of the ferroelectric liquid crystal 7. On the other hand, the alignment layer 5a applies the following pretilt angle to the smectic layers of the ferroelectric liquid crystal 7. That is, when the ferroelectric liquid crystal 7 is enclosed, the C1 alignment appears preferentially in the smectic layer of the ferroelectric liquid crystal 7.

In the state where the smectic layer has the chevron structure, an energy barrier at a transition in the smectic layer from the C1 alignment to the C2 u alignment or vice versa is not very large. Therefore, when an attempt is made to arrange the entire smectic layer in either one of the C1 alignment and the C2 alignment, it is likely that the other alignment state partially appear in the smectic layer, and thus it is difficult to achieve a uniform alignment.

In contrast, according to the liquid crystal display element 1 in accordance with the present embodiment, as the alignment layer Sa of the lower substrate 10 has an alignment control force which permits the smectic layer of the chevron structure in which the C1 alignment appears preferentially, the portion in a vicinity of the alignment layer 5a of the smectic layer is arranged in an opposite direction to the direction of the uniaxial layer alignment treatment applied to the alignment layer Sa. On the other hand, as the alignment layer 5b of the upper substrate 11 has an alignment control force which offers the smectic layer of the chevron structure in which the C2 alignment appears preferentially, the portion in a vicinity of the alignment layer 5b is likely to be tilted in the direction of the uniaxial layer alignment treatment applied to the alignment layer 5b.

By the interaction, both (i) the necessary energy for the smectic layer to be tilted in the direction of the uniaxial layer alignment treatment applied to the alignment layer 5a on the interface of the alignment layer 5a; and (ii) the necessary energy for the smectic layer to be tilted in an opposite direction to the direction of the uniaxial layer alignment treatment applied to the alignment layer 5b on the interface of the alignment layer 5b increase. As a result, the smectic layer sandwiched between the alignment layers 5a and 5b does not have the chevron structure but has the oblique structure which appears stably as shown in Fig. 2(a).

As shown in Fig. 5(b), in the oblique structure, two alignment states exist depending on the direction in which the surface of the smectic layer is tilted.

However, as is obvious from Fig. 5(a) and Fig. 5(b), the energy at a transition between one alignment state and the other alignment state in the oblique structure is larger than the energy at a transition between the C1 alignment and the C2 alignment in chevron structure.

Namely, as the oblique structure is more stable than the chevron structure, a uniform monodomain alignment can be obtained with ease.

As described, the liquid crystal display element 1 in accordance with the present embodiment includes a pair of alignment layers 5a and 5b which are placed opposing each other in such a manner that a uniaxial layer alignment treatment is applied thereto in the same direction, and that the ferroelectric liquid crystal 7 is enclosed between the alignment layers Sa and 5b. The alignment layer 5a has an alignment control force which permits the smectic layer to have the C1 alignment preferentially in the smectic layer, and the alignment layer 5b has an alignment control force which permits the smectic layer to have the C2 alignment preferentially. As a result, the oblique structure appears stably in the smectic layers of the ferroelectric liquid crystal 7. Moreover, the liquid crystal display element 1 is a so-called a liquid crystal display element of a parallel alignment wherein the alignment layers 5a and 5b are subjected to the uniaxial alignment layer alignment in the same direction. Therefore, unlike the liquid crystal display element of a so-called anti-parallel alignment wherein the uniaxial layer alignment treatment is applied in parallel and opposite directions, fine linear domains do not appear. As a result, the liquid crystal display element 1 has a stable uniform monodomain without defects.

It should be noted here that the present invention is not limited to the described arrangement of the preferred embodiment, and variations are permitted within the scope of the present invention. For &commat;-.

example, in the above example, the alignment layer 5a of the lower substrate 10 makes the C1 alignment appear preferentially, while the alignment layer 5b of the upper substrate 11 makes the C2 alignment appear preferentially. However, the present invention is not limited to the described arrangement, and it may be also arranged such that the alignment layer 5a has such an alignment control force which permits the smectic layer to have the C2 alignment preferentially, and the alignment layer Sb has such an alignment control force which permits the smectic layer to have the C1 alignment preferentially.

It is not necessarily that the uniaxial layer alignment treatment be applied to the alignment layer 5a of the lower substrate 10 in precisely the same direction as the uniaxial layer alignment treatment applied to the alignment layer 5b of the upper substrate 11, and as long as the respective uniaxial layer alignment treatments are applied in substantially the same directions in the ferroelectric liquid crystal 7.

Furthermore, as disclosed in Japanese Unexamined Patent Publication No. 92469/1995 (Tokukaihei 7-92469), by adding fine particles such as carbon black, titanium oxide, etc., fine regions in which the threshold values of the switching voltage differ can be formed, thereby permitting a gray shade display in the ferroelectric liquid crystal element 1.

The following will explain embodiments of the present invention in further details in reference to comparative examples.

[First Example] The following descriptions will explain one example in accordance with the described embodiment of the present invention. Here, members having the same function as those of the aforementioned embodiment will be designated by the same reference numerals, and thus the descriptions thereof shall be omitted here.

One example of the manufacturing method of the liquid crystal display element 1 will be explained.

First, signal electrodes 3a and scanning electrodes 3b made of, for example, ITO, etc., are formed on the glass substrates 2a and 2b respectively. On respective surfaces of the signal electrode 3a and the scanning electrode 3b, the insulating films 4a and 4b made of "OCD TYPE 2" available from Tokyo Ohka Kogyo Co., Ltd.

are formed to coat them.

On the glass substrate 2b having formed thereon the insulating film 4b, polyimide "PSI-A-2101" available from Chisso Co., Ltd. is applied, and is baked at 200 OC, and the alignment layer 5b is formed by applying a rubbingvtreatment. On the other hand, on the glass substrate 2a having the insulating film 4a formed thereon polyimide "PSI-A-2001" available from Chisso Co., Ltd. and is baked at 200 OC, and the alignment layer Sa is formed by applying thereto a rubbing treatment.

The alignment layers 5a and 5b are subjected to the rubbing treatment under the below-defined conditions: Diameter of Rubbing Roller: 50 mm Number of Rotations of Rubbing Roller: 1,000 rpm Transport Speed of Substrate with respect to Rubbing Roller: 10 mm/sec Number of Rubbing Treatment Applied: three times.

Namely, in the present embodiment, the alignment layers Sa and Sb are made of mutually different materials. The pretilt angle of the alignment layer is determined by a combination of an alignment layer material, a baking temperature and rubbing conditions.

Under the described conditions, the pretilt angle of the alignment layer 5b on the side of the upper substrate 11 is 6 , and the pretilt angle of the alignment layer 5a of the lower substrate 10 is 15 .

Next, the upper substrate 11 and the lower substrate 10 wherein the alignment layer 5a and the alignment layer 5b are subjected to the rubbing treatment in the same direction are bonded together, and the ferroelectric liquid crystal 7 "SCE 8" available from Merck Ltd. is injected in a spacing formed between them. Here, the display area of the liquid crystal display element 1 is assumed to be 100 mm x 100 mm.

As shown in Fig. 2(a), in the liquid crystal display element 1, a homogeneous oblique structure wherein a tilt angle of the smectic layer is constant is formed, and a uniform monodomain alignment state can be achieved without zig-zag defects which occur due to appearance of the chevron structure.

Generally, like the alignment layer 5a prepared under the above-defined conditions, if alignment layers having a pretilt angle which is by far greater than 10 are formed on upper and lower substrates so as to have the same rubbing directions, and the ferroelectric liquid crystal is enclosed between the substrates, the C1 alignment appears preferentially (see the belowpresented comparative examples 1-2).

On the other hand, like the alignment layer Sb, if alignment layers having a small pretilt angle are formed respectively on upper and lower substrates so as to have the same rubbing direction, and the ferroelectric liquid crystal is enclosed between the substrates, C2 alignment appears preferentially (see the below-presented comparative examples 1-1). In the present embodiment, the alignment layer 5a and the alignment layer 5b are respectively formed on the lower and upper substrates 10 and 11 so as to have the same rubbing direction, thereby forming a homogeneous oblique structure.

To be compared with the liquid crystal display element of the present embodiment, a liquid crystal display element wherein the alignment layer Sb formed on the upper substrate 11 of the first example is formed on both upper and lower substrates is prepared.

Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 1-1.

"PSI-A-2101" adopted in the first example is used as an alignment layer material, and two substrates having gone through the baking and rubbing processes under the same conditions as those of the first example are bonded together, and then a ferroelectric liquid crystal is injected in a spacing formed between them, thereby preparing a liquid crystal display element as a comparative example 1-1. In this comparative example, conditions such as an insulating film, a ferroelectric liquid crystal and a display area, etc., are the same as those of the liquid crystal display element of the first example. In the comparative example 1-1, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the display area, 70 Oo shows the C2 alignment, and 30 k shows the C1 alignment.

Similarly, to be compared with the liquid crystal display element of the present embodiment, another comparative liquid crystal display element is prepared wherein two alignment layers 5a formed on the lower substrate 10 of the first example are formed respectively on both upper and lower substrates.

Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 1-2.

"PSI-A-2001" adopted in the first example is used as an alignment layer material, and two substrates having gone through the baking and- rubbing processes under the same conditions as those of the first example are bonded together so as to have the same rubbing direction, and then a ferroelectric liquid crystal is injected in a spacing formed between them, thereby preparing a liquid crystal display element as a comparative example 1-2. In this comparative example, conditions such as an insulating film, a ferroelectric liquid crystal and a display area, etc., are the same as those of the liquid crystal display element of the first example. In the comparative example 1-2, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the entire display area, 15 shows the C2 alignment, and 85 shows the C1 alignment.

The liquid crystal display element of the first example is compared with the above comparative examples 1-1 and 1-2 respectively. As a result, it is found that as the smectic layer of the ferroelectric liquid crystal 7 has a uniform alignment state, i.e., the oblique layer structure, the liquid crystal display element of the first example shows superior characteristics over the respective comparative examples 1-1 and 1-2 in its contrast and display uniformity. Furthermore, the liquid crystal display element of the first example with -the first example shows superior characteristics over respective comparative examples also in view of bistability and response speed.

For example, with respect to each of the described liquid crystal display elements, if a bi-directional pulse having a pulse width of 500 ysec and a pulse wave-height of + 10 V is applied, a complete bistability is achieved in the liquid crystal display element of the first example. In contrast, the bistability disappears from both of the comparative examples 1-1 and 1-2 within 10 minutes. In order to achieve a complete bistability in the liquid crystal display elements of comparative examples 1-1 and 1-2, it is required to increase the pulse width to 1,000 psec.

The comparative examples 1-1 and 1-2 show deteriorations in response speed and bistability presumably because liquid crystal molecules hit each other above and below the chevron interface (at which the chevron layer structure is bent) which hinders a smooth switching. In contrast, in the liquid crystal display element of the first example, the smectic layer does not have the chevron structure but has the oblique structure having a constant tilt angle with respect to the substrates. This offers a uniform monodomain alignment state, and does not hinder the switching of the liquid crystal molecules, thereby obtaining desirable characteristics in view of response speed and bistability.

[Second Example] The following descriptions will explain another embodiment of the present invention.

Another example of the manufacturing method of the liquid crystal display element 1 will be explained.

are formed to coat them. On the glass substrates 2a and 2b having formed thereon the insulating films 4a and 4b respectively, polyimide "PSI-A-2101" available from Chisso Co., Ltd. is applied, and is baked at 200 OC, thereby forming the alignment layers 5a and 5b.

Next, as in the same manner as First Example, the alignment layer Sa is subjected to a rubbing treatment under the below-defined conditions: Diameter of Rubbing Roller: 50 mm Number of Rotations of Rubbing Roller: 1,000 rpm Transport Speed of Substrate with respect to Rubbing Roller: 10 mm/sec Number of Rubbing Treatment Applied: three times.

The above-defined conditions are hereinafter referred to as a condition 1.

On the other hand, the alignment layer 5b is subjected to a rubbing treatment using the same rubbing roller as in the above condition 1 under the belowdefined conditions: Number of Rotations of Rubbing Roller: 300 rpm Transport Speed of Substrate with respect to Rubbing Roller: 50 mm/sec Number of Rubbing Treatment Applied: three times.

The above-defined conditions are hereinafter referred to as a condition 2.

Namely, in the present example, the alignment layer 5a and the alignment layer Sb are formed by the same material at the same baking temperature but under different conditions. In this case, the pretilt angle of the alignment layer 5b on the side of the upper substrate 11 is 6 , and the pretilt angle of the alignment layer 5a of the lower substrate 10 is 12 ".

Next, the upper substrate 11 and the lower substrate 10 are bonded together in such a manner that the alignment layer 5a and the alignment layer Sb have the same rubbing direction, and the ferroelectric liquid crystal 7 "SCE 8" available from Merck Ltd. is injected in a spacing formed between them. Here, the display area of the liquid crystal display element 1 is assumed to be 100 mm x 100 mm.

As shown in Fig. 2(a), in the liquid crystal display element 1, a homogeneous oblique structure having a constant tilt angle of the smectic layer is formed, and a uniform monodomain alignment state can be achieved without zig-zag defects which occur due to appearance of the chevron structure.

Generally, like the alignment layer 5a prepared under the above-defined conditions, alignment layers having a pretilt angle which is by far greater than 10 are formed on upper and lower substrates so as to have the same rubbing directions, and the ferroelectric liquid crystal is enclosed between the substrates, thereby exhibiting C1 alignment preferentially (see the below-presented comparative examples 2-2).

On the other hand, like the alignment layer Sb, alignment layers having a small pretilt angle are formed respectively on upper and lower substrates so as to have the same rubbing direction, and the ferroelectric liquid crystal is enclosed in a spacing formed between the substrates, thereby exhibiting a C2 alignment preferentially (see the below-presented comparative examples 2-1). In the present example, the alignment layer Sa and the alignment layer Sb are respectively formed on the lower and upper substrates 10 and 11 so as to have the same rubbing direction, thereby forming a homogeneous oblique structure.

To be compared with the liquid crystal display element of the second example, a liquid crystal display element wherein the alignment layer Sb formed on the upper substrate 11 of the second example is formed on both upper and lower substrates is prepared.

Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 2-1.

"PSI-A-2101" adopted in the second example are applied as an alignment layer material on respective surfaces of the two substrates, and are baked at 200 OC in the same manner as second example. Then, the substrates are subjected to the rubbing treatment under the above-defined condition 1, and are bonded together so as to have the same rubb crystal display element of the second example. In the comparative example 2-1, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the display area, 70 k shows the C2 alignment, and 30 Co shows the C1 alignment.

Similarly, to be compared with the liquid crystal display element of this second example, a liquid crystal display element wherein the alignment layer Sa formed on the lower substrate 10 of the second embodiment is formed oh both upper and lower substrates is prepared. Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 2-2.

"PSI-A-2101" adopted in the second example are applied as an alignment layer material on respective surfaces of the two substrates, and are baked at 200 OC in the same manner as second example. Then, the substrates are subjected to the rubbing treatment under the above-defined condition 2, and are bonded together so as to have the same rubbing direction. Then, a ferroelectric liquid crystal is injected in a spacing formed between them, thereby preparing a liquid crystal display element as a comparative example 2-2. In this comparative example 2-2, conditions such as an insulating film, a ferroelectric liquid crystal and a display area, etc., are the same as those of the liquid crystal display element of the second example. In the comparative example 2-2, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the display area, 45 k shows the C2 alignment, and 65 W shows the C1 alignment.

The liquid crystal display element of the second example is compared with the above comparative examples 2-1 and 2-2 respectively. As a result, it is found that as the smectic layer of the ferroelectric liquid crystal 7 has a uniform alignment state, i.e., the oblique layer structure, the liquid crystal display element of the second example shows superior characteristics over the respective comparative examples 2-1 and 2-2 in its contrast and display uniformity. Furthermore, the liquid crystal display element of the second example shows superior characteristics over respective comparative examples also in view of bistability and response speed.

For example, with respect to each of the described liquid crystal display elements, a bi-directional pulse having a pulse width of 500 Zsec and a pulse waveheight of + 10 V is applied. As a result, a complete bistability is achieved in the liquid crystal display element of the second example. In contrast, the bistability disappears from both -of the comparative examples 2-1 and 2-2 within 10 minutes. In order to achieve a complete bistability in the liquid crystal display elements of comparative examples 2-1 and 2-2, it is required to increase the pulse width to 900 sec.

[Third Example] The following descriptions will explain still another example in accordance with the embodiment of the present invention.

A still another example of the manufacturing method of the liquid crystal display element 1 will be explained. First, signal electrodes 3a and scanning electrodes 3b made of, for example, ITO, etc., are formed on the glass substrates 2a and 2b respectively.

On respective surfaces of the signal electrode 3a and the scanning electrode 3b, the insulating films 4a and 4b made of "OCD TYPE 2" available from Tokyo Ohka Kogyo Co., Ltd. are formed to coat them.

On the glass substrate 2b having formed thereon the insulating film 4b, polyimide "PSI-A-2101" available from Chisso Co., Ltd. is applied, and is baked at 200 OC, and the alignment layer 5b is formed by applying thereto a rubbing treatment. On the other hand, on the glass substrate 2a having the insulating film 4a formed thereon, polyimide "PSI-A-2101" available from Chisso Co., Ltd. and-is baked at 150 OC, and the alignment layer 5a is formed by applying thereto a rubbing treatment.

Namely, in this example, the alignment layers 5a and 5b are made of the same material under the same condition but at different baking temperatures. The pretilt angle of the alignment layer 5b of the upper substrate 11 is 6 , and the pretilt angle of the alignment layer 5a of the lower substrate 10 is 11 .

Next, the upper substrate 11 and the lower substrate 10 are bonded together in such a manner that the alignment layer 5a and the alignment layer 5b have the same rubbing direction, and the ferroelectric liquid crystal 7 "SCE 8" available from Merck Ltd. is injected in a spacing formed between them. Here, the display area of the liquid crystal display element 1 is assumed to be 100 mm x 100 mm.

To be compared with the liquid crystal display element of the third example, a liquid crystal display element wherein the alignment layer 5b formed on the upper substrate 11 of the third example is formed on both upper and lower substrates is prepared.

Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 3-1.

"PSI-A-2101" adopted in the third example are applied as an alignment layer material on respective surfaces of the two substrates, and are baked at 200 OC. Then, the substrates are subjected to the rubbing treatment under the described condition of the third example, and are bonded together so as to have the same rubbing direction. Then, a ferroelectric liquid crystal is injected in a spacing formed between them, thereby preparing a liquid crystal display element as a comparative example 3-1. In this comparative example 3-1, conditions such as an insulating film, a ferroelectric liquid crystal and a display area, etc., are the same as those of the liquid crystal display element of the third example.

In the comparative example 3-1, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the display area, 70 t shows the C2 alignment, and 30 shows the C1 alignment.

Similarly, to be compared with the liquid crystal display element of the third example, a liquid crystal display element wherein the alignment layer Sa formed on the lower substrate 11 is formed on both upper and lower substrates is prepared. Hereinafter, the described comparative liquid crystal display element is referred to as a comparative example 3-2.

"PSI-A-2101" adopted in the third example are applied as an alignment layer material on respective surfaces of the two substrates, and are baked at 150 OC. Then, the substrates are subjected to the rubbing treatment under the same conditions as example 3, and are bonded together so as to have the same rubbing direction. Then, a ferroelectric liquid crystal is injected in a spacing formed between them, thereby preparing a liquid crystal display element as a comparative example 3-2. In this comparative example 3-2, conditions such as an insulating film, a ferroelectric liquid crystal and a display area, etc., are the same as those of the liquid crystal display element of the third example.

In the comparative example 3-2, zig-zag defects which are normally observed in the chevron structure occur, and a uniform monodomain alignment state cannot be obtained. Of all the display area, 40 % shows the C2 alignment, and 60 % shows the C1 alignment.

The liquid crystal display element of the third example is compared with the above comparative examples 3-1 and 3-2 respectively. As a result, it is found that as the smectic layer of the ferroelectric liquid crystal 7 has a uniform alignment state, i.e., the oblique layer structure, the liquid crystal display element of the third example shows superior characteristics over the respective comparative examples 3-1 and 3-2 in its contrast and display uniformity. Furthermore, the liquid crystal display element in accordance with the third example shows superior characteristics over respective comparative examples also in view of bistability and response speed.

For example, with respect to each of the described liquid crystal display elements, a bi-directional pulse having a pulse width of 500 Sm and a pulse wave-height of + 10 V is applied. As a result, a complete bistability is achieved in the liquid crystal display element of the third example. In contrast, the bistability disappears from both of the comparative examples 3-1 and 3-2 within 10 minutes. In order to achieve a complete bistability in the liquid crystal display elements of comparative examples 3-1 and 3-2, it is required to increase the pulse width to 1,100 Zsec [Fourth Example] The following descriptions will explain yet still another example in accordance with the embodiment of the present invention The upper and lower substrates prepared in the manner described in the first example are bonded together so as to have the same rubbing direction, and the ferroelectric liquid crystal 7 prepared by dispersing from 0.5 to 3 percent by weight of fine particles of carbon black or titanium oxide, etc., in "SCE 8" available from Merck Ltd., is injected in a spacing formed between the substrates, thereby preparing a liquid crystal display element.

The upper and lower substrates prepared in the manner described in the second example are bonded together so as to have the same rubbing direction, and the ferroelectric liquid crystal 7 prepared by dispersing from 0.5 to 3 percent by weight of fine particles of carbon black or titanium oxide, etc., in "SCE 8" available from Merck Ltd., is injected in a spacing formed between the substrates, thereby preparing another liquid crystal display element.

The upper and lower substrates prepared in the manner described in the third example are bonded together so as to have the same rubbing direction, and the ferroelectric liquid crystal 7 prepared by dispersing from 0.5 to 3 percent by weight of fine particles of carbon black or titanium oxide, etc., in "SCE 8" available from Merck Ltd., is injected in a spacing formed between the substrates, thereby preparing still another liquid crystal display element.

In any of the above three liquid crystal display elements, a homogeneous monodomain alignment state can be achieved without zig-zag defects occur due to an appearance of a chevron structure when observing these liquid crystal display elements with naked eye. When observing these liquid crystal display elements with a polarization microscope, however, many fine domains in which the threshold value voltages for switching vary are observed. In the characteristic curve of the amount of the transmitting light with respect to the application pulse voltage, a uniform gray shade display having a width of a threshold value of not less than 10 V can be obtained.

To be compared with each liquid crystal display element in accordance with the fourth example, the following liquid crystal display elements are prepared.

Namely, the liquid crystal display elements are prepared in the same manner as Fourth Example by dispersing from 0.5 to 3 percent by weight of fine particles of carbon black or titanium oxide, etc., in the ferroelectric liquid crystal "SCE8" available from Merck Ltd., and injecting the resulting ferroelectric liquid crystal in a spacing formed between the upper and lower substrates prepared in the same manner as comparative examples 4-1, 1-2, 2-1, 2-2, 3-1 and 3-2 respectively.

In any of these liquid crystal display elements, zig-zag defects appeared in the chevron structure occur, and a homogeneous monodomain alignment state cannot be obtained. This hinders the liquid crystal display elements from achieving a gray shade display.

To be compared with the described first through fourth examples, liquid crystal display elements wherein a uniaxial layer alignment treatment is applied to the upper and lower substrates in opposite directions are prepared. The described state of the upper and lower substrates is defined to be "antiparallel", in, for example, by Japanese Unexamined Patent Publication No. 92469/1995 (Tokukaihei 7-92469) As in the same manner as the first through fourth Examples, the upper and lower substrates having formed thereon alignment layers respectively are bonded together so as to have mutually opposite rubbing directions, thereby preparing liquid crystal display elements. These comparative examples are fabricated under the same conditions as those of the first through fourth examples except for the rubbing directions of the upper and lower substrates.

The upper and lower substrates having formed thereon alignment layers as in respective comparative examples 1-1, 1-2, 2-1, 2-2, 3-1 and 3-2 are bonded together so as to have mutually opposite rubbing directions, thereby preparing a liquid crystal display element. Here, conditions other than the rubbing direction of the upper and lower substrates are the same as those of the comparative examples 1-1, 1-2, 21, 2-2, 3-1, and 3-2 respectively.

As shown in Fig. 10, in any of the above liquid crystal display elements, many fine linear domains 19 appear in directions parallel to the rubbing directions A7 and A9 applied to the upper and lower substrates, and thus they do not function as display elements. Namely, it is found that in any of the liquid crystal display elements wherein the upper and lower substrates are bonded together so as to have opposite directions of the uniaxial layer alignment treatment, a uniform display cannot be obtained.

As is evident from the described first through fourth Examples and the comparative examples, with appropriate selections of an alignment layer material, baking temperature, and rubbing conditions, etc., alignment control forces of the alignment layers 5a and 5b can be suitably adjusted. Therefore, by arranging such that the alignment layer 5a has such an alignment control force that permits the smectic layer to have C1 alignment preferentially when the ferroelectric liquid crystal 7 is enclosed in a spacing formed between the two alignment layers 5a placed opposing each other, and that the alignment layers 5b has such an alignment control force that permits the smectic layer to have the C1 alignment preferentially when the ferroelectric liquid crystal 7 is enclosed in a spacing formed between the two alignment layers Sb placed opposing each other. As a result, the smectic layer of the ferroelectric liquid crystal 7 enclosed in the spacing between the alignment layers 5a and 5b can have a stable oblique structure.

Which one of the C1 alignment and C2 alignment appears preferentially in the smectic layer can be expressed by the ratio of the area exhibiting respective alignment states in the entire display area.

Namely, if the area indicative of the C1 alignment is not less than 50 percent, it is determined that C1 alignment appears preferentially.

It should be noted here that the above preferred embodiment of the present invention show example purposes only and do not limit the present invention.

For example, the above defined alignment layer materials, the baking temperatures, various conditions when rubbing, or other numerals are merely examples, and variations are permitted within the scope of the present invention.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modification as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims

CLAIMS:

1. A liquid crystal display element, comprising: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between said pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to said respective alignment layers of said pair of light transmissive substrates in the same or substantially the same direction, and a smectic layer of said ferroelectric liquid crystal has an oblique structure.

2. A liquid crystal display element, comprising: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between said pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to said respective alignment layers of said pair of light transmissive substrates in the same or substantially the same direction, a direction of the uniaxial layer alignment treatment applied to the alignment layer of one of said pair of light transmissive substrates is in a tilt direction of the smectic layer of said ferroelectric liquid crystal, and a direction of the uniaxial layer alignment treatment applied to the alignment layer of the other of said pair of light transmissive substrates is in an opposite direction of the tilt direction of the smectic layer of the ferroelectric liquid crystal.

.

3. A liquid crystal display element, comprising: a pair of light transmissive substrates, each having at least a transparent electrode and an alignment layer; and a ferroelectric liquid crystal enclosed in a spacing formed between said pair of light transmissive substrates, wherein a uniaxial layer alignment treatment is applied to said respective alignment layers of said pair of light transmissive substrates in the same or substantially the same direction, the alignment layer of one of said pair of light transmissive substrates has such an alignment control force that permits a smectic layer of said ferroelectric liquid crystal to have a chevron structure in which C1 alignment appears preferentially when said ferroelectric liquid crystal is enclosed in a spacing formed between two of said alignment layers placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are the same or substantially the same, and the alignment layer of the other of said pair of light transmissive substrates has such an alignment control force that permits the smectic layer of said ferroelectric liquid crystal to have a chevron structure in which C2 alignment appears preferentially when said ferroelectric liquid crystal is enclosed in a spacing formed between two of said alignment layers of the other of said pair of light transmissive substrates placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are the same or substantially the same.

4. The ferroelectric liquid crystal display element as set forth in claim 3, wherein: the alignment layer of one of said pair of light transmissive substrates has such an alignment control force that permits the smectic layer of said ferroelectric liquid crystal to have a chevron structure in which C1 alignment appears in not less than 50 percent of a total display area when said ferroelectric liquid crystal is enclosed in a spacing formed between two of said alignment layers placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are the same or substantially the same, and the alignment layer of the other of said pair of light transmissive substrates has such an alignment control force that permits the smectic layer of said ferroelectric liquid crystal to have a chevron structure in which C2 alignment appears in not less than 50 percent of a total display area when said ferroelectric liquid crystal is enclosed in a spacing formed between two of said alignment layers of the other of said pair of light transmissive substrates placed opposing each other in such a manner that respective directions of applying uniaxial alignment treatments are the same or substantially the same.

5. The ferroelectric liquid crystal display element as set forth in claim 3, wherein: a pretilt angle of liquid crystal molecules of said ferroelectric liquid crystal on an interface of said alignment layer of one of said pair of light transmissive substrates is larger than a difference between a tilt angle of said liquid crystal molecule and a tilt angle of the smectic layer, and a pretilt angle of liquid crystal molecules of said ferroelectric liquid crystal on an interface of the alignment layer of the other of said pair of light transmissive substrates is smaller than the difference between the tilt angle of the liquid crystal molecules and the tilt angle of the smectic layer.

6. The liquid crystal display element as set forth in any one of claim 1 through claim 5, wherein: fine particles are added to said ferroelectric liquid crystal, for forming fine regions having different threshold voltages for switching the liquid crystal molecules.

7. A liquid crystal display element substantially as herein described with reference to Figures 1 to 8 of the accompanying drawings.