Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1997-04-11
2001-11-27
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S158000, C349S044000
Reexamination Certificate
active
06323932
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention disclosed in the specification relates to a liquid crystal display device having pixel areas arranged in a matrix form on a same substrate, particularly to an active matrix type liquid crystal display device having semiconductor devices using semiconductor thin films. Silicon films can representatively be used as the semiconductor thin films.
2. Description of Related Art
In recent times, technologies for making a semiconductor device using semiconductor thin films, for example, a thin film transistor (TFT) on an inexpensive glass substrate have been rapidly developed. The reason is that demand for an active matrix type liquid crystal display device has been enhanced.
According to an active matrix type liquid crystal display device, TFTs are arranged to each of pixel areas of several tens through several millions arranged in a matrix and electric charges inputted to and outputted from respective pixel electrodes are controlled by the switching function of the TFTs.
Here, an explanation will be given of the basic structure of an active matrix type liquid crystal display device arranged with thin film transistors in reference to FIGS.
1
(A) and
1
(B). Firstly, FIG.
1
(A) is a view showing a section cutting a liquid crystal display device illustrated by FIG.
1
(B) in a direction orthogonal to a substrate. The section corresponds to a section cut by a broken line designated by a line A-A′ of FIG.
1
(B).
Numeral
101
designates a substrate having transparency on which an insulating film (not illustrated) is formed. Numeral
102
designates an active layer of a TFT, numeral
103
designates a gate electrode, numeral
104
designates a data line, numeral
105
designates a drain electrode, numeral
106
designates an interlayer insulating film, numeral
107
designates a black matrix, numeral
108
designates a pixel electrode comprising a transparent conductive film and numeral
109
designates an alignment film.
The whole substrate having TFTs which comprise as described above is hereinafter referred to as an active matrix substrate. Although attention is paid to only one pixel area according to FIG.
1
(A), the active matrix substrate is actually constituted by several tens through several millions of the pixel areas and drive circuits driving the pixel areas.
Meanwhile, numeral
110
designates a substrate having transparency, numeral
111
designates an opposed electrode constituted by a transparent conductive film and numeral
112
designates an alignment film. The whole substrate comprising as described above and opposed to the active matrix substrate is referred to as an opposed substrate.
After performing rubbing treatment for regulating the alignment of a liquid crystal material in later steps, the active matrix substrate and the opposed substrate are pasted together as to be opposed to each other by a seal member, not illustrated.
In that case spacers, not illustrated, are interposed between the both substrates with a uniform density whereby a uniform substrate interval (referred to as cell gap) is obtained. Strictly speaking, in the case of the structure illustrated by FIG.
1
(A), a distance between the alignment film
109
on the side of the active matrix substrate and the alignment film
112
on the opposed substrate is the cell gap.
The seal member serves not only as an adhesive agent for pasting the both substrates together also as a seal member for sealing a liquid crystal material between the both substrates at an image display region comprising a plurality of pixel areas.
Thus, a liquid crystal material
113
is sealed in an image display region (each of the plurality of pixel areas) as illustrated by FIG.
1
(A). In this way, the active matrix type liquid crystal display device having the constitution as illustrated by FIG.
1
(A) is formed.
According to the pixel area illustrated by FIG.
1
(A), an image signal controlled by the thin film transistors is stored at a condenser formed between the pixel electrode
108
and the opposed electrode
111
with the liquid crystal material
113
as an insulating layer.
At this moment an electric field in correspondence with a voltage level of the image signal is formed between the pixel electrode
108
and the opposed electrode
111
in the case of an analog gray scale system. Further, various gray scales of image displays can be carried out by using the property of the liquid crystal material
113
where an optical response is varied in accordance with varying of the applied voltage.
A nematic group liquid crystal material (for example, TN (Twisted Nematic) type or STN (Super Twisted Nematic) type liquid crystal material) is generally used frequently as a liquid crystal material. According to the liquid crystal display device as illustrated by FIG.
1
(A), the nematic group liquid crystal material is provided with a property where the long axis direction of the liquid crystal is substantially in parallel to the substrate (however, a pretilt angle may be provided) when the electric field is applied thereto and the long axis direction is directed to an electric field direction when the electric field is formed.
Accordingly, the long axis direction is varied in accordance with presence or absence of the electric field applied on the liquid crystal material. Thus the image display is carried out by controlling the amount of transmittance of light by the amount of variation of the long axis direction.
However, such a behavior of the liquid crystal material is a phenomenon applicable only when the direction of the electric field formed between the pixel electrode
108
and the opposed electrode
111
is in vertical to the substrate.
For example, in a region where a horizontal electric field substantially in parallel to the substrate is formed, the alignment of the liquid crystal material is disturbed whereby alignment defect is caused and desired image is not provided.
Normally, when a cell gap is provided as to be suitable for the applied voltage on the pixel electrode
108
, a vertical electric field (electric field orthogonal to the substrate) is dominant. However, as the cell gap is increased, the influence of the vertical electric field is weakened whereas the influence of the horizontal electric field is strengthened.
Here, FIG.
1
(B) is a view showing from the top face pixel areas where alignment defects of the liquid crystal material are caused by the influence of the electric field in the horizontal direction. Incidentally, areas except for the image display area is masked by the black matrix
107
. Therefore, wirings and the like disposed below the black matrix are shown by dotted lines.
In FIG.(B), white lines formed in the image display areas (areas not masked by the black matrix
107
) show disturbances of image display caused by alignment defects of the liquid crystal material, which are referred to as disclinations. These areas are under a state where abnormality occurs, different from the inherent alignment state of liquid crystal molecules.
As one cause of the occurrence of the disclination, firstly, influence of the horizontal electric field occurred by cross talk among wirings or among the thin film transistors, is pointed out.
For example, many of the disclinations, as illustrated at the upper stage of the pixel area of FIG.
1
(B), were observed according to experiments by the inventors. It is conceived that the horizontal electric field is formed by a potential difference between the front end of the gate electrode
103
and a portion where a gate line
114
and a data line
104
intersect with each other.
This phenomenon is more manifested as the width of the pixel area (pixel pitch) is narrowed, that is, an inter-wiring distance is narrowed in pursuit of highly fine image display. Incidentally, the pixel pitch is defined by the short side of the pixel area.
Further, narrowing of the pixel pitch signifies relative enlargement of the cell gap and it is anticipated that the influence of the horizontal electric field wi
Fukunaga Takeshi
Zhang Hongyong
Fish & Richardson PC
Semiconductor Energy Laboratory Co. Ltd
Ton Toan
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