1. Field of the Invention
The present invention relates to transflective liquid crystal display devices, and more particularly to a transflective liquid crystal display device having columnar spacers.
2. Description of the Background Art
A common transflective liquid crystal display device is provided with a transmissive region, which transmits light from a backlight, and a reflective region, which reflects ambient light incident on a liquid crystal layer, within each of pixels in a substrate on which TFTs (Thin Film Transistors) are formed (hereinafter may be referred to as a “TFT array substrate”). Meanwhile, a substrate on which a color filter using red, green, and blue coloring materials is formed (which hereinafter may be referred to as a “color filter substrate”) is provided opposing the TFT array substrate. The TFT array substrate and the color filter substrate sandwich the liquid crystal layer.
In this kind of transflective liquid crystal display device, the thickness of the liquid crystal layer (also referred to as a “gap between the TFT array substrate and the color filter substrate”, or a “cell gap”) is made different between the transmissive region and the reflective region within each of the pixels in order to make the optical path lengths of transmitting light and reflecting light uniform. Specifically, in the case that the thickness of the liquid crystal layer in the transmissive region is dt, the thickness of the liquid crystal layer in the reflective region is generally set at ½ dt. When the thickness of the liquid crystal layer is different between the regions in this way, the thickness of the liquid crystal layer in the reflective region particularly requires severe control. For this reason, spacers using a photosensitive organic film (hereinafter also referred to as “columnar spacers”) may be formed in some cases on the color filter substrate side or the TFT array substrate side, as a means to control the thickness of the liquid crystal layer. A transflective liquid crystal display device employing the columnar spacers is described in detail in Japanese Patent Application Laid-Open No. 2003-344838.
As a common technique for aligning liquid crystal, a rubbing technique is used in which polyimide films are formed on a color filter substrate and a TFT array substrate by a transfer method and the formed polyimide films are rubbed by a cloth in which hair is weaved. In the case of the transflective liquid crystal display device discussed above, columnar spacers are formed on the color filter substrate side or the TFT array substrate side before the substrates are subjected to rubbing; therefore, the rubbing cannot be done uniformly in the areas surrounding the spacers, causing nonuniformity in the alignment state of liquid crystal. This nonuniformity in the alignment state of liquid crystal can become a cause of such problems as degradation in contrast or development of domains in the transflective liquid crystal display device.
Generally, columnar spacers are disposed on a light-blocking film within pixel regions that do not adversely affect the display operation. However, because the columnar spacers are formed from a photosensitive organic film, their sizes are limited, and they need to have a diameter of 10 μm or greater, taking the strength originating from its pattern and the problem of peeling-off into consideration. For that reason, it has been common practice to dispose the columnar spacers in blue pixels, which have low transmissivity (in the case of a color filter that uses coloring materials of three primary colors, red, green, and blue) in order to alleviate the degree of effect to contrast degradation and domain development (the degree of noticeability in appearance).
In the color filter using coloring materials of three primary colors of red, green, and blue, the green coloring material, which has a high luminosity, is made to have good characteristics in order to increase transmissivity. Therefore, if those coloring materials are used for a transflective liquid crystal display device, a problem arises that the color tone of white shifts toward yellow in a reflective mode.
Japanese Patent Application Laid-Open No. 2003-344838 discloses a technique for improving this problem by varying a color filter in the reflective region and that in the transmissive region. Nevertheless, in the case of Japanese Patent Application Laid-Open No. 2003-344838, there has been a problem that the types of coloring materials for the color filters need to be changed from conventionally-used three colors to six colors, which necessitates high manufacturing cost.
Another technique has been proposed in which holes are formed in the coloring material of the reflective region and a transparent resin is formed therein so that the color tone of white can be adjusted in a reflective mode. The problem in using this technique, however, is that although white color can be modulated, light with other wavelengths leaks from the transparent resin portion, causing the purities of red color, green color, and blue color to deteriorate, and thus degrading the appearance in terms of display.
Moreover, another problem has been that disposing the above-mentioned columnar spacers in the blue color pixel region decreases the quantity of light from the blue color pixel region, further varying the color tone of white. In order to suppress the variation in white color tone, the light-blocking portions that are to be formed in the vicinity of the columnar spacers are formed also in the blue color pixel regions, the red color pixel regions, and the green color pixel regions. A problem with this method, however, is that the proportion of the light-blocking regions that occupy the pixel regions increases, and consequently the reflectivity lowers.
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