Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-05-15
2001-09-18
Wu, Shean C. (Department: 1756)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S042000, C349S129000, C252S299630, C252S299670
Reexamination Certificate
active
06292247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (LCD) in which liquid crystal display is performed by driving each pixel by a thin film transistor (TFT) and, more particularly, to a liquid crystal display (LCD) using for a thin film transistor poly silicon made through a low-temperature process.
2. Description of the Prior Art
A liquid crystal display which seals in liquid crystal between a pair of substrates and applies voltage to the liquid crystal to perform a desired display has advantages of compactness, thinness, and easy reduction of power consumption. Such displays are therefore put to practical use in various office-automation, audio-visual, and portable or on-board information devices. Especially, active-matrix liquid crystal displays using thin film transistors (TFTs) as a switching device for driving each liquid crystal pixel can display higher-definition images without cross talk because liquid crystal pixels can be selectively driven.
A noncrystal silicon TFT using noncrystal (amorphous) silicon in its active layer and a poly silicon TFT using poly silicon of higher mobility in its active layer are both used in liquid crystal displays. Noncrystal silicon TFTs are often used in large-sized displays or the like because they can be formed in a wide area through a low-temperature process. On the other hand, the mobility of poly silicon is higher than that of noncrystal silicon and poly silicon can form a device by self alignment. As it is therefore easy to make the area of a TFT and a pixel with a poly silicon TFT smaller than that of a TFT and a pixel with a noncrystal silicon TFT, a poly silicon TFT is often preferred for use in high-resolution displays. Moreover, if poly silicon is used, it is easy to make a TFT CMOS structure. As a result, a display area TFT and a driver TFT for driving it can be formed on the same substrate through almost the same process.
As stated above, a poly silicon TFT has attractive characteristics and can have a driver on its substrate. Furthermore, poly silicon is known to be formed by polycrystallizing noncrystal silicon through a high-temperature (above 600° C. ) process. In this case, noncrystal silicon is exposed to a high-temperature during a process, which makes it impossible to use a cheap glass substrate. A difficulty therefore arises when a poly silicon TFT is put to practical use.
Polycrystallizing technology using annealing treatment such as laser or lamp annealing has, however, been improved to the point where it is now possible to make poly silicon through the so-called low-temperature (below 600° C. ) process. This method of forming a poly silicon TFT through a low-temperature makes it possible to use a cheap glass substrate and thereby reduce cost. Furthermore, this method makes it possible to form poly silicon TFTs in a wide area. As a result, poly silicon TFTs formed through a low-temperature process (hereinafter referred to as a low-temperature poly silicon TFT) have been put to practical use.
Although low-temperature poly silicon TFTs have been put to practical use, the most suitable liquid crystal materials and panel configuration for demonstrating the characteristics of a low-temperature poly silicon TFT as a liquid crystal display and for improving its characteristics have not been developed yet. As a result, materials and a configuration, for example, used in a liquid crystal display using a conventional noncrystal silicon TFT are directly adapted, which means that the resulting poly silicon TFTs are not able to fully demonstrate their unique characteristics.
SUMMARY OF THE INVENTION
In order to solve the above problem, an active-matrix liquid crystal display according to the present invention aims at suggesting suitable liquid crystal materials, panel configuration, and the like for obtaining a liquid crystal display enabling the best use of the characteristics of a low-temperature poly silicon TFT. In order to achieve the above object, the present invention has the following features.
First, an active-matrix liquid crystal display according to the present invention comprises a plurality of pixel electrodes formed like a matrix on the first substrate and TFTs formed to be connected to the corresponding pixel electrodes and their electrode wiring on that first substrate, wherein a display is performed by driving a liquid crystal layer sandwiched between the plurality of pixel electrodes formed on the first substrate and a common electrode formed on the second substrate placed opposite the first substrate, a poly silicon TFT, for example, using as an active layer a poly silicon layer formed at a low temperature (for example, below 600° C. ) is used as the TFTs, the initial alignment of each liquid crystal molecule in the liquid crystal layer is controlled so that it will be nearly perpendicular to the pixel electrodes, and at least one material used in said liquid crystal layer having the molecular structure given by the below chemical formulas (1)-(6) having fluorine side chains is selected.
Liquid crystal molecules having fluorine side chains have high polarity in the direction of the side chains, their minor axes, and can operate well even at a low driving voltage achieved by a poly silicon TFT. Furthermore, high polarity in the direction of the minor axes of liquid crystal molecules means that it is easy to make their initial alignment vertical by, for example, strengthening repulsion between the liquid crystal molecules and a liquid crystal alignment layer. Moreover, even at a low driving voltage suitable for a poly silicon TFT, a liquid crystal layer shows a sufficiently high voltage-holding ratio, which prevents the liquid crystal from burning. The resulting liquid crystal display can be driven at a low voltage, which makes enables reduction of power consumption.
In accordance with another aspect of the present invention, an electrode-free portion is formed in the predetermined corresponding area opposite the pixel electrode in the common electrode on the second substrate as an alignment controlling window for controlling the alignment of the liquid crystal; and more than one area having different tilt azimuths are formed in each pixel electrode area by changing the alignment of the liquid crystal molecules from the vertical direction. The alignment controlling window can stably divide an alignment area of liquid crystal molecules allowing a display to have more than one priority viewing direction, and resulting in a greater viewing angle. It can therefore reduce the dependence of a liquid crystal display on visual angle and is advantageous for large-sized displays.
In accordance with still another aspect of the present invention, a planarizing interlayer insulating layer is formed over TFTs and their electrode wiring on the first substrate and each of a plurality of pixel electrodes is formed on the planarizing interlayer insulating layer. The pixel electrodes are formed on the planarizing interlayer insulating layer so that the unevenness of the pixel electrodes will not have a bad effect on vertical alignment of the liquid crystal molecules. Furthermore, pixel electrodes are formed on the planarizing interlayer insulating layer so that the pixel electrodes will cover at least TFT-formed areas (for example, TFTs and their electrode wiring), which prevents electric fields generated by TFTs etc. from leaking into the liquid crystal layer. Again, placing the pixel electrodes at an upper layer will make it easy to more efficiently apply voltage to the liquid crystal layer.
In accordance with still another aspect of the present invention, a liquid crystal material used for the liquid crystal layer in the active-matrix liquid crystal display has negative dielectric anisotropy and vertical alignment of the liquid crystal layer is controlled by vertical alignment layers formed over the common electrode and the pixel electrodes without a rubbing process, the alignment controlling window formed in the common electrode, and. voltage appli
Koma Norio
Omura Tetsuji
Yoneda Kiyoshi
Hogan & Hartson L.L.P.
Sanyo Electric Co,. Ltd.
Wu Shean C.
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