As illustrated in FIG. 1, the liquid crystal cell of the TN (twisted nematic) type is widely used for a liquid crystal display, such as liquid crystal TVs and liquid crystal displays used for computer systems. The cell consists of two substrates 1, 2, which are made of transparent materials such as glass, a layer 3 of nematic liquid crystal, which is made of positively and dielectrically anisotropic nematic liquid crystals, and which is sandwiched between the substrates, and orienting films 4, 5, which are disposed between the substrates 1, 2 and the nematic liquid crystal layer 3 for controlling the orientation of liquid crystal molecules LC. In this case, as shown by cigarette-like models, the directions of oblong liquid crystal molecules LC in the nematic liquid crystal layer 3 is twisted by 90.degree. C.
Polarizing plates 6, 7 are provided on the outsides of the substrates 1, 2. The polarizing plate 6 which is put on the substrate 1, on which light is incident, is disposed such that the direction of the polarized light therethrough is parallel with the direction of a liquid crystal molecule LC that is in the nearest position to the substrate 1. The polarizing plate 7 that is put on the substrate 2, from which the refracted light is emitted, is disposed such that the direction of the polarized light therethrough is perpendicular to that of the light polarized by the polarizing plate 6.
When no electric voltage is applied across the substrates 1, 2, the oriented direction of the liquid crystal molecules LC in the nematic liquid crystal layer 3 remains in a state twisted by 90.degree. between the substrates 1, 2, as shown in FIG. 1. Hence, the light incident on polarizing plate 6 travels along the twisted molecules LC in the layer 3. Then the light emitted from substrate 2, which is parallel with the direction polarized by polarizing plate 7, is transmitted through polarized plate 7. As a result, the cell of the TN type is in a bright state when there is no electric voltage is applied across the substrates 1, 2.
On the other hand, when an electric voltage is applied across the substrates 1, 2, the dielectric anisotropy of the molecules LC causes them to be disposed in a raised state along the direction of the electric voltage applied at that time. In this regard, the molecules LC which are located near to the substrates 1, 2 and more strongly restrained by the orienting films 4, 5, are affected to a lesser degree by the electric voltage applied.
When the amount of the voltage applied across the substrates 1, 2 becomes sufficient, the direction of substantially all the molecules LC becomes parallel with that of the voltage. At this time, the light incident on the polarized plate 6 passes through the nematic liquid crystal layer 3 in a polarized state as it is. As a result, the emitted light from substrate 2, which is polarized so that its direction is perpendicular to the polarized direction of the polarizing plate 7 on the emitting side, is not transmitted through polarizing plate 7. Accordingly,the cell of the TN type is in a dark state when there is an electric voltage between the substrates 1, 2.
A liquid crystal cell of the TN type which is in a bright state when no voltage is applied, and which is in a dark state when a voltage is applied, is called a normally-white-mode cell. On the contrary, a cell of the TN type, which is in a dark state when no voltage is applied and which is in a bright state when a voltage is applied, is called a normally-black-mode cell. This normally-black-mode cell is obtained by setting the polarized direction of the polarizing plate on the emitting side parallel with that of the polarizing plate on the incident side.
When the value of a voltage applied across the substrates 1, 2 is set at a medium value, namely, a certain value between the maximum value at which the TN-type cell is in a perfectly bright state (its transmittance is 100%) and the minimum value at which it is in a perfectly dark state (its transmittance is 0%), the transmittance of the TN-type cell can be set at a medium value to present any medium degrees of brightness (density).
However, if the TN-type liquid crystal cell is driven by using a medium degree of brightness, the brightness may change according to the direction of light passing through the cell so that the contents displayed differ, as shown in FIG. 2. Namely, the contents of the TN-type cell viewed from its front may differ from those viewed from its sides. This phenomenon is called its dependency on the optic angle. As shown in FIG. 1, the angle of .theta. indicates an angle made by light LL with the line Z normal to the substrate 2 of the TN-type cell.
In an attempt to alleviate the dependency of the TN-type cell on the optic angle, for example, Japanese Patent Early Publication No. 63-106624 discloses a method of manufacturing a cell of such type. A partial plan view of the liquid crystal display element manufactured by the method is shown in FIG. 3, and its cross-sectional view is shown in FIG. 4.
Referring to FIGS. 3 and 4, a liquid crystal display element consists of numbers of pixels, each display unit of which has a given dimension (for example, a square of 200 .mu.m). They are disposed in a plane in given spaces. This liquid crystal display element consists of a liquid crystal material 12 sealed between glass substrates 10, 11 facing each other. Each of the substrates 10, 11 has transparent electrodes 13, 14 for defining the shape of each pixel PX and orienting films 15, 16 for orienting liquid crystal molecules LC. The transparent electrode 13, 14 and the orienting film 15, 16 are laminated in order on their respective substrate. A film-type transistor 17 for driving transparent electrode 13, which controls each pixel, is mounted on glass substrate 10.
Each pixel is vertically partitioned at its center by a transparent spacer 18. The parts partitioned by adjacent transparent spacers 18 define domains DM.sub.1 and DM.sub.2 disposed by turns.
These domains DM.sub.1 and DM.sub.2 are formed as shown in FIG. 5. That is, regarding orienting film 15 on the glass substrate 10, the domain DM.sub.1 is rubbed in the rightward direction and the domain DM.sub.2 is rubbed in the leftward direction. Regarding the orienting film 16 on glass substrate 11, the domain DM.sub.1 is rubbed in the upward direction and the domain DM.sub.2 is rubbed in the downward direction.
As the direction of the orientation of a liquid crystal molecule LC follows the direction along which the orienting film is rubbed, the molecule LC in the domain DM.sub.2 has a pretilt angle different from that in the domain DM.sub.2. Further, in this case, as the same liquid crystal is sealed in both the domains DM.sub.1 and DM.sub.2, the directions of spiraling of the spiraled liquid crystal molecules LC in both the domains DM.sub.1 and DM.sub.2 are the same.
Thus, the dependency on the optic angle, which is encountered when the TN-type cell is driven by using a medium degree of brightness, can be alleviated, as shown in FIG. 6, by dividing one unit pixel PX into domains DM.sub.1 and DM.sub.2, and by differing the directions of the pretilt angles of the molecules LC in domain DM.sub.1 from those in domain DM.sub.2. The TN-type liquid crystal cell of such structure is hereinafter called a complementary-type TN liquid crystal cell.
When manufacturing such a complementary-type cell, the following seven processes are needed to form the orienting films 15, 16 on their respective glass substrates 10, 11: coating a substrate with an orienting film material (step 1), masking domain DM.sub.2 (step 2), rubbing domain DM.sub.1 (step 3), exfoliating the mask covering domain DM.sub.2 (step 4), masking domain DM.sub.1 (step 5), rubbing domain DM.sub.2 (step 6), and exfoliating mask covering domain DM.sub.1 (step 7).
These processes cause a disadvantage in that the cost of manufacturing a complementary-type TN liquid crystal cell is increased.
USPTO Backgrounds
USPTO Backgrounds