1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter referred to as a ".pi. cell") including liquid crystal molecules which have a bend orientation and are at least partially partitioned by a polymer wall so as to correspond to pixels, and a method for producing the same.
2. Description of the Related Art
First, conventional techniques will be described below.
(.pi. cells and the like)
FIGS. 14A and 14B schematically show an exemplary .pi. cell 1 disclosed in Japanese Laid-Open Patent Publication No. 61-116329. As shown in FIG. 14A, the .pi. cell 1 includes a pair of glass substrates 2 and 3, and a liquid crystal layer 4 interposed between the substrates 2 and 3. Liquid crystal molecules 5 in the liquid crystal layer 4 have a spray orientation when no voltage is applied. In this state, the liquid crystal molecules 5 are in a thermally stable state. If a voltage is applied to the .pi. cell 1 under certain conditions by using a power source 6, the liquid crystal molecules 5 are oriented so as to have a bend orientation, as shown in FIG. 14B. In a bend orientation state, the liquid crystal molecules 5 have a reverse pre-tilt angle and a high pre-tilt angle, so that the liquid crystal molecules 5 can be easily driven by an external voltage with a high response speed, and only a low driving voltage is required. Moreover, in a bend orientation, the liquid crystal molecules 5 are located in plane-symmetrical positions with respect to a plane taken in the middle of and in parallel to the substrates 2 and 3. Accordingly, the .pi. cell 1 constitutes a so-called "self-compensated cell", in which the liquid crystal molecules 5 function to compensate for one another. Thus, the .pi. cell 1 provides viewing angle characteristics which are symmetrical along the left-right direction with respect to the .pi. cell 1.
Moreover, Japanese Laid-Open Patent Publication Nos. 1-209424 and 2-306217 each disclose a structure in which a mono-axial optical compensated film composed of a mono-axially stretched polymer material is provided for a liquid crystal cell including a liquid crystal layer interposed between a pair of transparent substrates, the mono-axial optical compensated film being disposed perpendicular to longitudinal axes of liquid crystal molecules, and an effective retardation d.cndot..DELTA.n of the liquid crystal cell and the retardation d.cndot..DELTA.n of the mono-axial optical compensated film being substantially the same. This cell provides omnidirectional viewing angle characteristics.
However, since the above-mentioned liquid crystal cell has a structure in which a liquid crystal material is interposed between substrates with spacers being only partially added, a problem arises when an inputting operation is conducted by means of a pen-shaped input means for pressing a desired portion of a transparent position detection film provided on the surface of the liquid crystal cell. That is, the thickness of the cell varies owing to an external force such as that caused by an inputting operation with the pen-shaped input means (hereinafter referred to as "pen-inputting operation"), thereby leading to a display quality problem such as coarse displayed images. Moreover, the orientation state (bend state) of the .pi. cell is not thermally stable, so that, when the application of a voltage is stopped, the orientation state may slowly return to the thermally stable spray orientation state. Therefore, when a .pi. cell is adopted, an electric field is required in order to maintain the bend orientation state, thereby resulting in a high power consumption and a complicated driving method.
Japan Applied Physics Society Spring Meeting Preprints, 1p-R-8, p. 122 (1980) reports on a .pi. cell that requires no voltage-based initial orientation for realizing a bend orientation of liquid crystal molecules. This liquid crystal cell realizes a bend orientation by using high-tilt substrates capable of realizing a high pre-tilt angle of liquid crystal molecules by oblique vapor deposition of CeO.sub.2, the substrates being disposed in a non-parallel manner where their respective pre-tilt angles are not parallel to each other. However, according to a vapor deposition technique used in the above report, it is difficult to uniformly form a vapor deposited film on a large substrate with a high repeatability, so this method is not applicable to the mass-production of large, high display quality liquid crystal display devices with a high yield.
(Liquid crystal cells including polymer walls)
There has been proposed a method in which polymer walls for connecting substrates are included within a liquid crystal cell, and requires no polarizers or orientation treatment. This method utilizes the birefringence of liquid crystal so as to electrically control between a transparent state and a white state. This method amounts to the following: An ordinary refractive index of liquid crystal molecules and a refractive index of a supporting medium, i.e., a polymer, are made to be equal. As a result, a transparent state is displayed when a voltage is applied, since the liquid crystal molecules are oriented in the same direction. A white state is displayed when no voltage is applied, since the liquid crystal molecules have a random orientation, causing the scattering of light at interfaces of the liquid crystal and the polymer. Examples of such proposed techniques include a method disclosed in Japanese National Patent Publication No. 61-502128. According to this method, liquid crystal is mixed with a photopolymerizable resin or a heat-polymerizable resin and is injected into an interspace between substrates. Thereafter, the resin is cured, thus allowing liquid crystal to deposit between the substrates. As a result, liquid crystal droplets are formed within the resin, whereby polymer walls are obtained.
Japanese Laid-Open Patent Publication No. 5-27242 discloses a method for improving the viewing angle characteristics of a liquid crystal cell in a non-light-scattering mode. A polarizer is used along with the liquid crystal cell. According to this method, a complex material of liquid crystal and a polymer material is produced by a phase separation through which liquid crystal and a polymer resin are separated from a mixture of liquid crystal and a photopolymerizable resin. More specifically, liquid crystal domains are in a random orientation state due to the produced polymer, so that liquid crystal molecules stand in different directions within each liquid crystal domain when a voltage is applied. As a result, the apparent refractive index of the liquid crystal molecules becomes substantially the same irrespective of the direction in which the liquid crystal cell is viewed, i.e., the viewing angle. Thus, the viewing angle characteristics when displaying an intermediate gray tone are greatly improved. Recently, the inventors of the present invention have filed as Japanese Patent Application No. 4-286487 a liquid crystal device having remarkably improved viewing angle characteristics based on the following principle: During photopolymerization, where light is radiated onto a mixture of liquid crystal and a photopolymerizable material, the radiation of light is controlled by using a photomask or the like, so that each liquid crystal domain has an omnidirectional orientation state within a pixel region. Moreover, by controlling the liquid crystal cell by applying or not applying a voltage, the liquid crystal molecules in the liquid crystal domains behave like an umbrella being opened or closed, thereby remarkably improving the viewing angle characteristics.
Moreover, by utilizing the orientation controlling effect of a substrate surface on liquid crystal molecules, the present inventors have filed as Japanese Patent Application No. 5-30996 a method for producing a liquid crystal display device which utilizes the orientation controlling effect of a substrate surface within polymer walls. These inventions relate to liquid crystal devices that operate in a TN (Twisted Nematic) mode, a STN (Super Twisted Nematic) mode, an ECB (Electrically Controlled Birefringence) mode, or an FLC (Ferroelectric Liquid Crystal) mode.
(Stable orientation type cell incorporating a cross-linked polymer)
Japanese Laid-Open Patent Publication Nos. 6-160801 and 6-160814 disclose the fabrication of a liquid crystal display device having a stable orientation by injecting a mixture of a liquid crystal material and a small amount of photopolymerizable resin into a cell whose substrates are provided with orientations appropriate for the STN or TN mode and irradiating the cell with UV (Ultra Violet)-rays.
(Voltage-magnetic field based control of orientation of liquid crystal)
Japan Display 92 S18-4, p. 699 reports on a method for controlling the orientation of liquid crystal within a liquid crystal cell, where a mixture of liquid crystal and a photopolymerizable is irradiated with UV-rays while applying a very low voltage thereto, so as to impart liquid crystal molecules in liquid crystal droplets with a very small pre-tilt angle. According to this method, the hysteresis characteristics of the display of a liquid crystal cell are improved.