Reduction in size of a display device and the accomplishment of high-definition display capability thereof are advanced together with the advancement of cellular phones and information terminals. On the other hand, a display device which has a new added value is getting attention, such as a display device that allows a viewer to view different images depending on a position from where the viewer watches the display device, i.e., a display device that provides images different from each other toward a plurality of view points, and a display device which produces a parallax image based on those images different from each other and which provides a stereoscopic image to the viewer.
A conventionally known scheme of providing images different from each other toward a plurality of view points synthesizes pieces of image data for respective view points, displays those pieces of image data on a display unit, separates the displayed synthesized images by an optical separating unit including a lens, a barrier (a light blocking plate) with slits, and provides images to respective view points. The principle of image separation is based on restriction of pixels viewable depending on a view-point direction using the optical unit, such as a barrier with slits or a lens. Examples of image separating units include a parallax barrier which is a barrier with multiple stripe-like slits, and a lenticular lens having cylindrical lenses which have a lens effect in a given direction.
A stereoscopic display device having an optical image separator is appropriate for mounting on a portable device since it does not need the use of a special eyeglass so that there is no burden of attaching the eyeglass. In practice, a portable device on which a stereoscopic display device including a liquid crystal panel and a parallax barrier is mounted is already available on the market (For example, “Nikkei Electronics, No. 838,” Nikkei Publishing, Jan. 6, 2003, pp. 26-27; Non-Patent Literature 1, hereafter).
According to the above-explained scheme, i.e., the display device that provides images different from each other toward a plurality of view points using an optical separating unit, when the view-point position of a viewer moves and an image to be viewed is changed, a boundary between the image and another image appears to be darkened in some cases. This phenomenon originates from non-display regions (a light blocking unit, so-called a black matrix in general in the case of a liquid crystal panel) between a pixel and a pixel for view points being viewed. The above-explained phenomenon inherent to the movement of the view point of the viewer does not occur in the case of general display devices having no optical separating unit. Hence, the viewer may feel strangeness or reduction of the display quality from the above-explained phenomenon that occurs on a multi-view-point display device or a stereoscopic display device having the optical separating unit.
This phenomenon is called 3D moire in general. 3D moire is periodical varying of brightness (may be the varying of color in some cases) originating from different visions displayed on different angular directions. 3D moire is luminance angular fluctuation and does not become a problem depending on a view position. However, when fluctuation of brightness in the angular direction is large, 3D moire is supposed to have undesirable effect on stereoscopic viewing.
A display device which has respective shapes and layouts of the pixel electrodes and light blocking unit of the display unit devised in order to overcome the problem originating from the optical separating unit and the light blocking unit, and which suppresses a reduction of the display quality has been provided (for example, Unexamined Japanese Patent Application KOKAI Publication No. 2005-208567; Patent Literature 1, hereafter).
The Patent Literature 1 discloses a display device as shown in FIG. 29. The display device disclosed in the Patent Literature 1 has a substantially constant ratio between the light blocking portion (a wiring 1070 and the light blocking unit 1076) and the aperture in a cross section of the display element in the vertical direction 1011 orthogonal to the arrangement direction of the cylindrical lenses 1003a at any point in the horizontal direction 1012. Hence, when the viewer moves the view point in the horizontal direction 1012 that is the direction in which the images are separated, and the viewing direction changes, the ratio of the light blocking portion to be viewed is substantially constant. That is, the viewer does not occasionally view only the light blocking portion in a specific direction, and no display appears darkly. Accordingly, reduction in the display quality originating from a light blocking region can be suppressed.
Moreover, a pixel structure suitable for the display device of the Patent Literature 1 is disclosed (for example, Unexamined Japanese Patent Application KOKAI Publication No. 2009-98311; Patent Literature 2, hereafter).
The Patent Literature 2 discloses a liquid crystal display device comprising a pixel as shown in FIG. 30. The charging capacitor line CS is arranged in the extending direction of the gate line G, i.e., is connected to the charging capacitors 4CS of respective pixels adjoining to each other in the X axis direction. In respective pixels adjoining to each other in the X axis direction, positions of the pixel thin-film transistors in the Y axis direction differ from each other, so that the charging capacitor line CS is bent and arranged in order to connect those transistors. Like the pixel thin-film transistor, the charging capacitor 4CS is arranged at the upper-bottom side of a display region in a substantially trapezoidal shape in each pixel. Accordingly, the charging capacitor 4CS can be effectively arranged between upper-bottoms of respective pixels configuring an adjoining pixel pairs 4PAIR, thereby further improving the aperture ratio.
Moreover, in the liquid crystal display device disclosed in the Patent Literature 2, an intersection between the charging capacitor line CS and the data line D is arranged at a trapezoid inclining portion so that the charging capacitor line CS and the data line D are along with each other. It is preferable to reduce wirings arranged so as to be along the image separating direction as much as possible, and the above-explained display device has the data line D only. This further improves the image quality. This is because when the charging capacitor line CS is arranged in the Y axis direction, the image of the charging capacitor line CS is enlarged by the image separating unit, resulting in a remarkable deterioration of the display quality.
That is, the display device of the Patent Literature 2 has the gate line G and the charging capacitor line CS running in the image separating direction and formed on the same layer in order to suppress an image deterioration originating from the image separating unit and the charging capacitor line CS while reducing the number of processes.
Patent Literature 2 discloses a technique of forming a scanning line and a capacitor line through the same process in order to reduce the number of production processes. In particular, there is a large demand of cost reduction for general small display devices, and it is desirable to configure a pixel array with the number of layers as small as possible.
Moreover, there is a demand for the display unit of the display device to increase the so-called aperture ratio which is defined by the area ratio between the aperture contributing to the display brightness and the light blocking portion in order to make the pixel pitch finer so as to improve the high-definition display capacity and to improve the display brightness.
However, in order to accomplish the high-definition display of an image, it is necessary to arrange a large number of pixels in a screen region which is originally small, so that it is necessary to make the size of a pixel finer. That is, how to reduce the pixel size is a technical issue. However, pixels with a finer size are almost realized together with the advancement of the microfabrication of semiconductor technologies.
As explained above, there is a tendency that pixels become finer, but it is not always enabled to reduce the size of electrical and electronic circuits, such as a switching device and an auxiliary capacitor for driving the liquid crystal in order to modulate light in proportion to the refinement of the pixel. This is because the switching device and the auxiliary capacitor are formed on a substrate like a semiconductor substrate or a glass substrate through the microfabrication technique, but there is a limit for realizable line width due to the limit of the semiconductor process. Moreover, even if finer process is technically possible, it results in the cost increase for a time from the standpoint of plant investment.
Liquid crystal display devices have a problem that because of the above-explained limit together with refinement, a region where light is blocked increases, i.e., the aperture ratio decreases, and the light use efficiency of the whole display device decreases. There is a tradeoff relationship that when it is attempted to improve the image quality by refinement of the pixel, the light use efficiency decreases. Accordingly, there is a technical issue to realize a high-image-quality and highly efficient image display device and to realize a high-definition image simultaneously.
In particular, in the case of a small display device, because of the above-explained limit together with refinement, the ratio of wirings occupying the area of a pixel and that of a contact-hole area are extremely large, and the reduction of the aperture ratio is remarkable. It is necessary for the refined pixel to reduce the number of wirings in the pixel and that of the contact holes as much as possible.
Moreover, as disclosed in the Non-Patent Literature 1, recently, the applying field of the stereoscopic image display device and the application thereof become wide. As an example, a configuration in which image separation is performed in the direction in which the data line runs may be employed depending on the application of the display device. However, the inventor of the present invention found out that the high aperture ratio and the high image quality cannot be accomplished even if the pixel structure disclosed in Patent Literature 2 is designed as the above-explained configuration while maintaining the aperture shape and the light-blocking shape of the pixel disclosed in Patent Literature 1.
What the inventor of the present invention found will be explained below in more detail. As explained above, since the direction in which the gate line runs and the image separating direction are consistent according to the conventional technologies, the running direction of the charging capacitor line formed on the same layer as that of the gate line can be drawn in the same direction as the image separating direction so as not to interfere with the image separating unit. Likewise, when the pixel structure disclosed in Patent Literature 2 is applied to a display device that separates images in a direction in which the data line runs, it is necessary to draw the charging capacitor line formed of the same material as that of the data line in the image separating direction.
In general, in order to protect the data line from any damage inherent to the process at the time of forming a switching device, the data line is often formed in a process step after the formation of the gate line, i.e., on the substrate, the data line is formed in a layer above the gate line. If the data line is formed in a layer above the gate line and the data line and charging capacitor line are formed in the same layer, the charging capacitor line has to form a charging capacitor between layers having a small relative electric permittivity per unit area, and then has to use a large area for forming a given charging capacitor. This results in an insufficient aperture ratio, and thus the transmissivity decreases.
Moreover, in the display device disclosed in Patent Literature 2, the charging capacitor 4CS can have a higher relative electric permittivity per unit area and, therefore, have the area reduced when it is formed between the silicon layer 4SI and the charging capacitor electrode in the same layer as the gate line G. In this case, however, it is necessary to newly provide a contact hole 4CONT that connects the charging capacitor electrode to the charging capacitor line CS, so that a sufficient pixel aperture ratio cannot be obtained, and thus the transmissivity decreases.
Moreover, according to the pixel structure of the display device disclosed in Patent Literature 2, the charging capacitor line CS on the same layer as that of the gate electrode is drawn so as to traverse the periphery of the switching device (TFT) in the image separating direction, so that the width in the Y axis direction of the light blocking portion located at the upper bottom of a trapezoid becomes one that is obtained by adding the line width of the charging capacitor line CS and the line drawing space to the area of the TFT. The width of the upper bottom of the substantially trapezoid in the Y axis direction cannot be reduced without the change in a process rule, so that the width of the light blocking portion in the Y axis direction covering the upper bottom of the substantially trapezoid becomes large relative to the width of the aperture region in the Y axis direction in the case of pixels with a narrow pitch. As a result, the aperture ratio drops. When the image of the light blocking portion covering the upper bottom of the substantially trapezoid is enlarged by the image separating unit, it is visually recognized as a darkened spot or stripe on the display unit by the viewer, and thus the display quality decreases.
In this specification, as explained above, the periodical varying of brightness (may be the varying of color in some cases), in particular, a luminance angular fluctuation originating from displaying of different images in different angular directions is defined as a “3D moire”. Moreover, a mixing of an image for another view point and leaking of an image to an image for a given view point are defined as “3D crosstalk”.
In general, a stripe pattern produced by an interference of structural objects having different periods is called a “moire stripe”. The moire stripe is an interference stripe produced depending on the periodicity of the structural object and the pitch thereof, and the 3D moire is a brightness varying produced due to the imaging characteristic of the image separating unit. Accordingly, the 3D moire and the moire stripe are distinguished in this specification.
The 3D moire does not become a problem depending on a view position, but when the fluctuation in brightness in the angular direction is large, an undesirable effect for stereoscopic viewing may occur, so that it is desirable to set the brightness fluctuation to be equal to or smaller than a predetermined value.