VERTICALLY ALIGNED HELIX-DEFORMED LIQUID CRYSTAL DISPLAY

Abstract: A vertically aligned helix-deformed ferroelectric liquid crystal display is provided which includes: the first (10) and second glass substrates (50) each of which has two surfaces, the first (10) and second glass substrates (50) facing each other; a first transparent electrode (20) having a first potential, being formed on a first surface (12) of the first glass substrate (10); a second transparent electrode (30) having a second potential different from the first potential, being formed on the first surface (12) of the first glass substrate (10); a first vertical alignment layer (40) being formed on the first surface (12) of the first glass substrate (10), on which the first and second transparent electrodes (20, 30) are formed; a second vertical alignment layer (60) formed on a first surface (52) of the second glass substrate (50); and a ferroelectric liquid crystal (70) being filled between the first and second glass substrates (10, 50) on which the first and second vertical alignment layers (40, 60) are respectively formed, facing each other, the ferroelectric liquid crystal (70) having a shorter helix pitch than the wavelength of the light, the ferroelectric liquid crystal (70) being helix-deformed in response to an electric field applied across the first and second transparent electrodes (20, 30) so that its molecules rotate in a specific direction, thereby achieving uniform alignment and the analog gray scale capability.

Description: VERTICALLY ALIGNED HELIX-DEFORMED LIQUID CRYSTAL DISPLAY Technical Field The present invention relates to a new liquid crystal display employing a ferroelectric liquid crystal and, more particularly, to a vertically aligned helix-deformed liquid crystal display in which a ferroelectric liquid crystal with a shorter helix pitch than the wavelength of rays of light incident thereon is vertically aligned, and the rotation of the molecular director is controlled through helix deformation according to the strength of an electric field applied thereto parallel to substrates thereof, to thereby obtain high contrast ratio due to uniform alignment over a wide area, to accomplish the analog gray scales, and to realize wide viewing characteristics using in-plane electrodes . Background Art A conventional liquid crystal display using the electro-optical effect of a liquid crystal is constructed in such a manner that transparent electrodes are respectively formed on two substrates, alignment layers for aligning a liquid crystal are coated thereon, the liquid crystal is filled between the two substrates, and two polarizers are respectively attached to the external surfaces of the two substrates. The conventional liquid crystal display passes or blocks rays of light incident thereon by itself, thereby displaying information on its picture. For passing or blocking the rays of light, a voltage is applied to the liquid crystal display so as to induce molecule rotation of the liquid crystal to thereby vary the molecular director. Meantime, the liquid crystals used in the liquid crystal displays are classified according to their molecule arrangement geometries into a nematic liquid crystal in which only the orientational order exists, and a smectic liquid crystal in which the orientational and positional orders exist together. The smectic liquid crystals may be further divided according to the existence of the spontaneous polarization into a ferroelectric and a paraelectric liquid crystals. The nematic liquid crystal is insensitive to the polarity of an electric field applied thereto because the spontaneous polarization does not exist therein. The nematic liquid crystal has the dielectric anisotropy. In a twisted nematic liquid crystal display using the nematic liquid crystal, the response speed of liquid crystal molecules according to the dielectric anisotropy lies from tens of msec to hundreds of msec. Thus, the nematic liquid crystal display is not suitable for high-speed applications. Moreover, the nematic liquid crystal display shows narrow viewing angle and low contrast characteristics without employing a multidomain structure or an additional optical film. In contrast to the nematic liquid crystal, a ferroelectric liquid crystal has a very high response speed for molecular rotation on the order of tens of μsec due to a direct coupling of the spontaneous polarization and the electric field applied thereto. Accordingly, the ferroelectric liquid crystal is suitable for realization of high-speed dynamic picture. However, a conventional surface-stabilized ferroelectric liquid crystal display operates in a bistable mode so that it cannot possess analog gray scales, and has a difficulty in obtaining uniform alignment over a wide area. Although the gray scale capability has been recently achieved using a time or space division driving scheme, the surface-stabilized ferroelectric liquid crystal display shows only limited gray scales in a complicated driving scheme. In case of the antiferroelectric liquid crystal display recently introduced, it has a higher response speed than the nematic liquid crystal display and continuous gray scales when the V-shape switching is used. However, it has difficulty in uniform alignment over a wide area and a problem of flickering of picture. Meanwhile, there is proposed a parallel aligned helix-deformed ferroelectric liquid crystal display employing a ferroelectric liquid crystal having a considerably shorter helix pitch than the wavelength of incident light. In such liquid crystal display, an electric field is applied across the electrodes prepared on two substrates so as to deform the helix structure in the plane of the substrate, thereby accomplishing continuous gray scales. This display device has a high response speed but it requires an additional alignment process such as shearing or an electric field treatment for uniform alignment over a wide area. Furthermore, defect structures such as striped domains are developed. As described above, the conventional ferroelectric and antiferroelectric liquid crystal displays have difficulties in achieving the wide area, uniform alignment and analog gray scales, and involve deterioration of the picture quality. Specifically, it is difficult for the ferroelectric and antiferroelectric liquid crystals with a short pitch to obtain uniform homogeneous alignment because of, particularly, stripe domains due to strong polar interactions between the liquid crystal molecules and the alignment surface. Therefore, this homogeneous geometry for a short pitch ferroelectric liquid crystal deteriorates the contrast ratio and the optical transmissivity . A high-quality large liquid crystal display requires excellent viewing angle characteristics, the wide area uniform alignment, a high response speed, and a high contrast ratio. The liquid crystal is a material having a very large optical anisotropy so that its effective refractive index is widely varied with the angle of rays of light incident on it. In case of the twisted nematic mode currently widely being used, variation of the contrast ratio according to the change in the viewing angle appears antisymmetrically for a rotation angle about a direction tilted to the axis vertical to the substrate. To solve this problem, there is widely used an optical compensation method which additionally employs a uniaxial or biaxial phase retardation film under a specific condition of compensating for the effective refractive index of the liquid crystal. Other methods include a multidomain alignment having different directions of the liquid crystal molecules in several subpixels for an individual pixel, and a method of employing the in-plane switching of the molecules on the same substrate. Among the aforementioned methods, the multidomain structure requires multi-rubbing or optical patterning with the use of an optical mask at least twice, involving complicated processes which result in deterioration of alignment reliability and high manufacturing cost. The in- plane switching mode for the nematic liquid crystal display shows a low response speed, a low transmissivity due to the low aperture ratio, and picture deterioration due to image sticking. Others such as an optically compensated bend mode and an inverse twisted nematic mode show high contrast ratio and good viewing characteristics, but they still do not have symmetrical viewing characteristics. Disclosure of Invention The present invention is directed to a vertically aligned helix-deformed ferroelectric liquid crystal display that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. An object of the present invention is to provide a vertically aligned helix-deformed ferroelectric liquid crystal display in which a ferroelectric liquid crystal having a shorter helix pitch than the wavelength of rays of light is vertically aligned between two substrates and its molecular rotation direction is controlled according to in- plane electrodes using an electric field applied parallel to the substrates, to achieve wide area uniform alignment, to maintain high contrast ratio, and to provide analog gray scales and excellent viewing characteristics. To accomplish the object of the present invention, there is provided a vertically aligned helix-deformed ferroelectric liquid crystal display, including: first and second glass substrates each of which has two surfaces, the first and second glass substrates facing each other; a first transparent electrode having a first potential, being formed on a first surface of the first glass substrate; a second transparent electrode having a second potential different from the first potential, being formed on the first surface of the first glass substrate; a first vertical alignment layer being formed on the first surface of the first glass substrate, on which the first and second transparent electrodes are formed; a second vertical alignment layer formed on a first surface of the second glass substrate; and a ferroelectric liquid crystal being filled between the first and second glass substrates on which the first and second vertical alignment layers are respectively formed, facing each other, the ferroelectric liquid crystal having a shorter helix pitch than the wavelength of the light, the ferroelectric liquid crystal being helix-deformed in response to an electric field applied across the first and second transparent electrodes so that its molecules rotate in a specific direction. For a transmissive type, the liquid crystal display further includes a first polarizer being attached on a second surface of the first glass substrate, and a second polarizer being attached on a second surface of the second glass substrate, having the optic axis perpendicular to that of the first polarizer. For a reflective type, the liquid crystal display further includes a reflecting mirror being formed on the first or second surface of the second glass substrate, a compensation film having the optical retardation characteristics of a quarter of the wavelength of the light, being formed on the second surface of the first glass substrate, and a single polarizer having the optic axis which is at the angle of 45° to that of the compensation film, being formed on the compensation film. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. Brief description of Drawings The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: In the drawings: Fig. 1 shows the configuration of a vertically aligned helix-deformed transmissive type liquid crystal display according to the present invention; Fig. 2 shows the driving principle of the liquid crystal display of Fig. 1; Fig. 3 shows the relationship between the optical transmissivity and the strength of the electric field applied to the vertically aligned helix-deformed liquid crystal display of the present invention; Fig. 4 shows the relationship between the optical transmissivity response characteristics and the square-wave voltage waveform of the electric field applied to the vertically aligned helix-deformed liquid crystal display of the present invention; Fig. 5 shows an example of a driving electrode arrangement of the vertically aligned helix-deformed liquid crystal display according to the present invention; Fig. 6 shows another example of a driving electrode arrangement of the vertically aligned helix-deformed liquid crystal display according to the present invention; and Fig. 7 shows the configuration of a vertically aligned helix-deformed reflective type liquid crystal display according to the present invention. Best Mode for Carrying Out the Invention Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The present invention basically uses the helix- unwinding phenomenon according to the electric field application to the ferroelectric liquid crystal as its driving principle. In the helix-deformed ferroelectric liquid crystal display according to the present invention, in contrast to the conventional helix-deformed ferroelectric liquid crystal display, the direction of the helix axis of the liquid crystal display is aligned vertically to the substrate, and transparent electrodes exist on only one substrate. Therefore, the analog gray scale capability is easily achieved by the electric field applied across the electrodes present in the same substrate, and high contrast ratio is accomplished due to the wide area uniform vertical alignment without employing the additional alignment process such as the rubbing process or electric field treatment. Furthermore, wide view angle characteristics are realized according to an intrinsic multidomain structure of two or four subpixels in each pixel by the direction of molecular rotation, depending on the polarity of the electric field provided by the in-plane electrodes. Fig. 1 shows the configuration of a vertically aligned helix-deformed transmissive type liquid crystal display according to the present invention. As shown in Fig. 1, the transmissive type liquid crystal display of the present invention has the first and second transparent electrodes 20 and 30 of indium-tin-oxide (ITO), being formed on a first surface 12 of a first glass substrate 10. The two transparent electrodes are alternately formed on the substrate, as shown in Figs. 5 or 6, and maintain a distance of tens to hundreds of μm therebetween, preferably 20μm in this embodiment. A vertical alignment material such as JALS-204 (Japan Synthetic Rubber Co.) is coated on first surface 12 of the first glass substrate 10 on which transparent electrodes 20 and 30 are formed, to form a first vertical alignment layer 40. The vertical alignment material of JALS-204 is also coated on a first surface 52 of a second glass substrate 50, to form a second vertical alignment layer 60. Two glass substrates 10 and 50 face each other, having a distance of several μm, preferably 5μm, therebetween so as to allow the first and second vertical alignment layers 40 and 60 to be faced each other. It is preferable that the product of the optical anisotropy of the liquid crystal by the distance between the two substrates is smaller than 720nm. The surface pretilt angle falls in the range of 75°<θs<90°, where θs is the surface pretilt angle of the vertical alignment layer to the substrate, preferably 90° in this embodiment. A ferroelectric liquid crystal 70 such as FLC-10817 (Rolic Ltd.) is inserted between two substrates 10 and 50 and sealed. The operating voltage applied to the liquid crystal becomes smaller with increasing the magnitude of the spontaneous polarization. The liquid crystal with the spontaneous polarization of 115nC/cm 2 is used in this embodiment. The molecular tilt angle is 34° which falls in the range 22.5°<θ<45°. The helix pitch is 0.2μm which is smaller than 0.35μm for the visible light in the short wavelength regime. For uniform alignment, the existence of the smectic A phase is preferred but most of materials with large molecular tilt angles have no smectic A phase in the phase sequence. Thus, a phase transition sequence of the isotropic phase- (6 .5°-62. °) -chiral nematic phase- ( 62.4 °- 61.5°) -chiral smectic C phase (ferroelectric phase) is employed in the embodiment of the present invention. For a transmissive type, a first polarizer 80 is attached to a second surface 14 of the first glass substrate 10 and a second polarizer 90 is attached to a second surface 54 of the second glass substrate 50. The optical axis of the first polarizer 80 is at the angle of 45° ± 3° to the direction of the electric field applied parallel to the substrates. The optical axis of the second polarizer 90 is at the angle of 90° to that of the first polarizer 80. The driving principle of the vertically aligned helix- deformed ferroelectric liquid crystal display will be explained in the following. The helix structure of smectic layers of the liquid crystal is maintained when the electric field is not applied to the vertically aligned structure of the present invention, in case of the central pixel of Figs. 1 and 2, so that the average optic axis direction 71 is vertical to the substrates. In Fig. 2, the projected structure of the molecule arrangement of the central pixel onto the one of the two substrates shows that the molecules point in every directions on the surface of the smetic cone. Accordingly, rays of light are completely blocked by two polarizers 80 and 90 vertical to each other. On the other hand, when an electric field above a threshold is applied to the liquid crystal display, the average optic axis direction is tilted away from the surface normal of the substrate according to the polarity of the applied electric field which directly couples with the spontaneous polarization of the ferroelectric liquid crystal. As a result, the incident light is transmitted through the polarizers which are at the angle of 45° to the electric field direction. Specifically, the liquid crystal molecules are arranged at the angle of -90° in the left pixel so that average optic axis direction 72 is tilted downward from the vertical direction. In the projected structure of the liquid crystal molecules of the left pixel of Fig. 2, the molecules are arranged downward. The liquid crystal molecules of the right pixel are arranged at the angle of 90° so that average optic axis direction 73 is tilted upward from the vertical direction. In Fig. 2, the molecules of the right pixel are arranged upward. Fig. 3 shows the relationship between the optical transmissivity and the strength of the electric field applied to the vertically aligned helix-deformed liquid crystal display of the present invention. This shows that the liquid crystal molecules continuously rotate on the surface of the smetic cone to deform the helix structure according to the strength of the applied electric field, and the magnitude of the effective birefringence of the liquid crystal is continuously varied to allow the intensity of light transmitted through the polarizers at the angle of 45° to the direction of the electric field to be successively changed. As a result, analog gray scales are achieved. The strength of the electric field applied parallel to the substrates has the maximum on the first glass substrate 10 with the electrodes, and becomes smaller on going from the first to the second glass substrate 50 with no electrodes. The magnitude of the molecular rotation is proportional to the strength of the electric field. Fig. 4 shows the relationship between the optical transmissivity response characteristics and the square-wave voltage waveform of the electric field applied to the vertically aligned helix-deformed liquid crystal display of the present invention. Referring to Fig. 4, by the application of the square wave, the liquid crystal being changed from OFF state (light blocking state) to ON state (light transmitting state) m 140μs approximately which is the rising time. By the removal of the square wave, the liquid crystal being changed from the ON state to the OFF state in 40μs approximately, which is the falling time. The optical transmissivity in the ON state is proportional to the strength of the applied electric field as shown in Fig. 3. Fig. 5 shows an example of a driving electrode arrangement of the vertically aligned helix-deformed liquid crystal display according to the present invention. The first transparent electrode 20 has the shape of letter 'n' and the second transparent electrode 30 has the shape of letter 'm' m one pixel, two electrodes 20 and 30 being alternately arranged. Thus, four subpixels are placed in the matrix of 1X4 having the branches of electrodes therebetween. Accordingly, the average optic axes of odd numbered subpixels 101 and 103 and even numbered subpixels 102 and 104 have antisymmetrical characteristics with respect to the branches of the electrodes because the directions of the electric fields applied to four subpixels 101, 102, 103 and 104 alternate in a pixel 100 in the ON state. The antisymmetrical optic axes of subpixels in one pixel bring about wide viewing angle. Therefore, the liquid crystal display of the present invention can easily secure the wide viewing characteristics according to the configuration of the transparent electrodes, compared to the conventional liquid crystal display, requiring no additional optical film. Fig. 6 shows another example of a driving electrode arrangement of the vertically aligned helix-deformed liquid crystal display according to the present invention. In this electrode structure, the second transparent electrode 30 is arranged between two first transparent electrode 20, and a plurality of branches vertically extended from the first electrode 20 and branches vertically extended from the second electrode 30 are alternately arranged. Thus, the pixel is divided into a plurality of subpixels by the electrode branches, and neighboring two subpixels have the optic axes opposite to each other in the horizontal direction. The four subpixels are arranged in the matrix of 2x2 in this electrode structure. Fig. 7 shows the configuration of a vertically aligned helix-deformed reflective type liquid crystal display according to the present invention. In this configuration, compared to the transmissive type of Fig. 1, a reflecting mirror 120 is attached on the first surface 52 or the second surface 54 of the second glass substrate 50, no polarizer is attached to the second surface 54 of the second glass substrate 50 and a compensation film 84 is placed between the first glass substrate 10 and the polarizer 80. Like components are indicated by like reference numerals in Figs. 1 and 7. The direction of the optic axis of the compensation film 84 is at the angle of 45° to that of the polarizer 80. The compensation film 84 has the phase retardation of a quarter of the wavelength of incident light. It is preferable that the phase retardation of the compensation film 84 covers from lδOxNnm to 200xNnm (N=l,2,3, ...). For the reflective type, the optical characteristics are determined by the phase retardation of the compensation film because the average optic axis of the liquid crystal is vertical to the substrate when no electric field is applied thereto. The effective phase retardation of rays of light is twice the phase retardation of compensation film 84, realizing the dark state. When the electric field is applied to the reflective type liquid crystal display, the average optic axis of the liquid crystal is tilted away from the surface normal of the substrate, thereby having the effective birefringence. Thus, the bright state is realized according to the phase retardation in terms of the magnitude of the product of the effective birefringence and the distance between the two substrates. A s described above, according to the present invention, uniform alignment and the analog gray scale capability are easily achieved, and high contrast is obtained due to the vertical alignment. Furthermore, wide viewing characteristics are realized according to an intrinsic multidomain structure based on the configuration of the electrode arrangement. Moreover, the fabrication process is simplified because the alignment layer surface treatment such as the rubbing process is not needed, reducing the manufacturing cost. It will be apparent to those skilled in the art that various modifications and variations can be made in the vertically aligned helix-deformed ferroelectric liquid crystal display of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims: What is claimed is : 1. A vertically aligned helix-deformed ferroelectric liquid crystal display, comprising: the first and second glass substrates each of which has two surfaces, the first and second glass substrates facing each other; a first transparent electrode having a first potential, being formed on a first surface of the first glass substrate; a second transparent electrode having a second potential different from the first potential, being formed on the first surface of the first glass substrate; a first vertical alignment layer being formed on the first surface of the first glass substrate, on which the first and second transparent electrodes are formed; a second vertical alignment layer formed on a first surface of the second glass substrate; and a ferroelectric liquid crystal being filled between the first and second glass substrates on which the first and second vertical alignment layers are respectively formed, facing each other, the ferroelectric liquid crystal having a shorter helix pitch than the wavelength of the light, the ferroelectric liquid crystal being helix- deformed in response to an electric field applied across the first and second transparent electrodes so that its molecules rotate in a specific direction. 2. The liquid crystal display as claimed in claim 1, wherein, when let the molecular tilt angle of the ferroelectric liquid crystal to the helix axis be θ, the molecular tilt angle falls in the range of 22.5°<θ<45° . 3. The liquid crystal display as claimed in claim 1, wherein the helix pitch of the ferroelectric liquid crystal is shorter than 0.35μm. 4. The liquid crystal display as claimed in claim 1, wherein the surface pretilt angle falls in the range of 75°<θs<90°, where θs is the surface pretilt angle of the vertical alignment layer to the substrate. 5. The liquid crystal display as claimed in claim 1, wherein the product of the distance between the substrates and the refractive index anisotropy of the ferroelectric liquid crystal is smaller than 720nm. 6. The liquid crystal display as claimed in claim 1, further comprising: a first polarizer being attached on a second surface of the first glass substrate; and a second polarizer being attached on a second surface of the second glass substrate, having the optic axis perpendicular to that of the first polarizer. 7. The liquid crystal display as claimed in claim 6, wherein the first polarizer has the optic axis which is at the angle of 45° to the direction of the electric field applied across the first and second transparent electrodes. 8. The liquid crystal display as claimed in claim 1, wherein the fist and second transparent are alternately arranged so that the optic axes of neighboring two pixels are antisymmetrical to each other. 9. A vertically aligned helix-deformed ferroelectric liquid crystal display, comprising: the first and second glass substrates each of which has two surfaces, the first and second glass substrates facing each other; a first transparent electrode having a first potential, being formed on a first surface of the first glass substrate; a second transparent electrode having a second potential different from the first potential, being formed on the first surface of the first glass substrate; a first vertical alignment layer being formed on the first surface of the first glass substrate, on which the first and second transparent electrodes are formed; a reflective mirror being attached on one side of the second glass substrate; a second vertical alignment layer formed on a first surface of the second glass substrate; a compensation film being formed on a second surface of the first glass substrate, the compensation film having the optical retardation characteristics of a quarter of the wavelength of the light; a polarizer being formed on a second surface of the first glass substrate, the polarizer having the optic axis which is at the angle of 45° to that of the compensation film; and a ferroelectric liquid crystal being filled between the first and second glass substrates on which the first and second vertical alignment layers are respectively formed, facing each other, the ferroelectric liquid crystal having a shorter helix pitch than the wavelength of the light, the ferroelectric liquid crystal being helix- deformed in response to an electric field applied across the first and second transparent electrodes so that its molecules rotate in a specific direction. 10. The liquid crystal display as claimed in claim 9, wherein the phase retardation of the compensation film covers from 160xNnm ro 200xNnm (N=l, 2 , 3, ... ) .

Inventors: LEE SIN DOO, LEE, SIN, DOO
Applicants: KIM WON DAE, LEE SIN DOO, KIM, WON, DAE, LEE, SIN, DOO
Application year: 1999
Main classification: G02F1
IPC codes: G02B5, G02F1