1. Field of Invention
The present invention relates to a liquid crystal device whose aperture ratio is increased, a method for producing the liquid crystal device, and an electronic apparatus.
2. Description of Related Art
A liquid crystal device can include two substrates, such as a glass substrate and a quartz substrate, and liquid crystals sealed in the space between the two substrates. In the liquid crystal device, active elements, such as thin-film transistors (TFTs), are disposed in a matrix on one of the substrates, and an opposing electrode is disposed on the other substrate. By varying optical characteristics of the liquid crystal layer between both substrates in accordance with an image signal, an image can be displayed.
More specifically, a voltage can be applied to the liquid crystal layer between pixel electrodes (ITOs) and the opposing electrode based on an image signal supplied to the pixel electrodes disposed in a matrix by the TFT elements in order to change the arrangement of liquid crystal molecules. This changes the transmittance ratio of pixels. As a result, light transmitted through the pixel electrodes and the liquid crystal layer can be varied in accordance with the image signal in order to display an image.
In order to define the arrangement of the liquid crystal molecules when voltage is not applied, an alignment layer on the surface of one of the substrates (the active matrix substrate or element substrate) contacting the liquid crystal layer and an alignment layer on the surface of the other substrate (opposing substrate) contacting the liquid crystal layer is rubbed, so that, when a voltage is not applied, the liquid crystal molecules align in the rubbing direction. For example, when the alignment layers on the element substrate and the opposing substrate are rubbed at twist angles that differ by 90 degrees from each other, the liquid crystal molecules continuously change their orientation in a liquid crystal panel, so that they are aligned in such a manner that their orientations at the substrates differ by 90 degrees from each other.
Polarizers are disposed on the front and back surfaces of the liquid crystal panel in order to transmit only a predetermined polarized component of incident light. In a normally white mode, the polarization axes of the polarizers on the front and back surfaces of the liquid crystal panel can be set at angles that are different by 90 degrees from each other in order to match the respective rubbing directions at the substrates. This causes light incident upon the liquid crystal panel through the polarizer on the back surface of the liquid crystal panel to rotate 90 degrees in accordance with the orientation of the liquid crystal molecules in the liquid crystal layer and exit from the front surface of the liquid crystal panel through the polarizer on the front surface of the liquid crystal panel when a voltage is not applied, so that a bright display is produced.
When a voltage is applied, the alignment direction of the liquid crystals change, that is, the long axes of the liquid crystal molecules are tilted in accordance with the voltage. As a result, the rotation of the light in the vibration direction caused by the liquid crystals in the liquid crystal panel is restricted. Therefore, the light exiting from the front surface of the liquid crystal panel is absorbed by the polarizer on the front surface of the liquid crystal panel. Consequently, an image is displayed by applying a voltage in accordance with an image signal to the liquid crystals and transmitting light with a transmittance ratio in accordance with the image signal.
As described above, the arrangement of the liquid crystal molecules when a voltage is not applied is determined by the rubbing of the alignment layers. The alignment layers are formed by applying, for example, polyimide to a thickness of approximately a few tens of nanometers. By forming the alignment layers on the surfaces of both substrates opposing the liquid crystal layer, the liquid crystal molecules can be aligned along the surfaces of the substrates. The rubbing is performed to form small grooves in the surfaces of the alignment layers to make the alignment layers anisotropic. By rubbing the alignment layers in a certain direction, the arrangements of the liquid crystal molecules are determined.
In order for the inclination angle changing directions of all of the liquid crystal molecules to be the same when a voltage is applied, the long axes of the liquid crystal molecules are tilted by predetermined angles (pretilt angles) with respect to the substrates and aligned when a voltage is not applied.
In the liquid crystal device, application of a DC voltage to the liquid crystals causes deterioration of the liquid crystals by, for example, decomposition of liquid crystal components, contamination due to impurities produced in a liquid crystal cell, or image sticking. To overcome such a problem, in general, reverse driving is carried out to reverse the drive voltage polarities of the pixel electrodes in a certain period corresponding to, for example, one image signal field or one image signal frame.
When the drive voltage polarity of every pixel electrode forming an image display area is merely reversed at a certain period (that is, when what is called a video reverse driving method is carried out), flickering or crosstalk occurs at a certain period particularly when the number of pixels is large. To prevent flickering or crosstalk at a certain period, line reverse driving methods, such as a 1H reverse driving method or an IS reverse driving method, have been developed. The 1H reverse driving method is carried out to reverse the drive voltage polarity with every row of pixel electrodes at a certain period, and the IS reverse driving method is carried out to reverse the drive voltage polarity with every column of pixel electrodes at a certain period.
However, when a line reverse driving method is carried out, an electric field (transverse electric field) can be generated between adjacent pixel electrodes on the same substrate in the row direction or column direction in which voltages of different polarities are applied.
FIG. 12 is a schematic view illustrating the effects of transverse electric field and pretilting of liquid crystal molecules when a voltage is not applied.
As mentioned above, the liquid crystal molecules are disposed at predetermined pretilt angles. As shown by the + and − signs in FIG. 12, drive voltages of opposite polarities with respect to a reference voltage are applied to adjacent pixel electrodes 121. This causes a transverse electric field 123 shown by broken lines in FIG. 12 to be generated between the adjacent pixel electrodes 121. When such an electric field 123 is generated between the adjacent pixels, a difference occurs between the tilting direction of liquid crystal molecules at one edge of each pixel electrode 121 and the electric field direction. Therefore, the transverse electric field 123 produces an area in which the tilting direction of a liquid crystal molecule 124 at one edge of each pixel electrode 121 is different from the tilting directions of other liquid crystal molecules 122.
In areas in which the tilt angle changing directions of liquid crystal molecules are different from those disposed towards the center of the pixel electrodes 121 due to the effects of such a transverse electric field (such areas are hereinafter referred to as reverse tilt areas), the polarization of transmission light is the same as that in normal tilt areas. However, a light streak appears due to light scattering at boundary lines (disclination lines) between the normal tilt areas and the reverse tilt areas.
To overcome such a problem, the reverse tilt areas disposed outwardly of the disclination lines are set at non-open areas by a light-shielding film formed on the opposing substrate. This prevents an image to be displayed at a portion where image quality is reduced by light scattering. However, this reduces aperture ratio.
To overcome such a problem, JP2001-142089A, for example, discloses a technology for narrowing the reverse tilt areas produced by transverse electric field as a result of reducing the effects of the transverse electric field by increasing vertical electrical field at the pixel electrode edges. The vertical electric field is increased by making gaps at the pixel electrode edges narrower than the central portions of the pixel electrodes by bulging the pixel electrode edges. Narrowing the reverse tilt areas narrows the light-shielding film, so that aperture ratio is increased. However, in this technology, the pixel electrode edges may not be properly rubbed due to the bulges of the pixel electrode edges. When they are not properly rubbed, these bulges are obstacles to the rubbing, thereby reducing image quality.