Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-06-07
2004-07-13
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S143000
Reexamination Certificate
active
06762805
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an active matrix type liquid crystal display device and a method of fabricating the same, and more particularly to an active matrix type liquid crystal display device which is capable of enhancing an aperture ratio and displaying images with high brightness without an increase in complexity in fabrication, and a method of fabricating such an active matrix type liquid crystal display device.
2. Description of the Related Art
In an active matrix type liquid crystal display device including a backlight source, a backlight intensity is usually enhanced for displaying images with a high brightness. However, this would consume much power, which does not satisfy small power consumption which is required in any display device.
In order to enhance a brightness in a liquid crystal display device including a backlight source, it is frequently carried out to enhance a light-transmission ratio. However, enhancement of a light-transmission ratio in a liquid crystal display device is equivalent to enhancement of a transmission ratio in a color filter which contributes to reduction in a light-transmission ratio. In order to enhance a transmission ratio in a color filter, a content ratio of pigment in a color filter is usually reduced, or a thickness of a color filter is reduced with a content ratio of pigment being kept as it is. However, if a transmission ratio in a color filter is enhanced by doing so, it would be necessary to control a color of backlight.
For the above-mentioned reason, it is usually carried out to enhance an aperture ratio of an opening in a substrate on which a thin film transistor is fabricated, in order to enhance a light-transmission ratio of a liquid crystal display device. Specifically, an area of each of pixels arranged in an opening is increased by reducing a wiring width and/or a size of a transistor, for instance.
FIG. 1
is a top plan view of a TFT substrate in a conventional liquid crystal display device including a backlight device.
In the illustrated liquid crystal display device, a plurality of scanning lines or gate lines
502
and a plurality of signal lines and drain lines
503
are formed on a transparent substrate in a matrix perpendicularly to each other. An area surrounded by each of the scanning lines
502
and each of the signal lines
503
defines a pixel area
504
.
A gate electrode
505
is formed of a portion of each of the scanning lines
502
at a corner of the pixel area
504
. On the gate electrode
505
is formed an island
506
comprised of a semiconductor layer, and on the island are formed a drain electrode
511
and a source electrode
512
. The gate electrode
505
, the island
506
, the drain electrode
511
and the source electrode
512
constitute a thin film transistor (TFT)
514
.
A transparent electrode
517
composed of indium tin oxide (ITO) is formed in the pixel area
504
. The source electrode
512
is electrically connected to the transparent electrode
517
through a contact hole
516
. The drain electrode
511
is formed integral with one of the signal lines
503
.
The scanning lines
502
, the signal lines
503
and the thin film transistor
514
are covered with a black matrix layer formed on an opposing substrate (not illustrated). An area other than the black matrix layer defines an aperture.
With reference to
FIG. 1
, a pixel area theoretically separated in a substrate on which the thin film transistor
514
is fabricated, specifically, a pixel area defined by a dotted line X is defined as a maximum aperture, and an area in which images are displayed by means of the transparent electrode
514
, specifically, an area defined by a dotted line Y is defined as an actual aperture. An aperture ratio in the illustrated liquid crystal display device is defined as a ratio of an area of the actual aperture to an area of the maximum aperture, that is, Y/X.
Hence, in order to increase an aperture ratio without changing a size of each of the pixel areas used for displaying images, a width of wirings such as the scanning and signal lines
502
and
503
covered with the black matrix layer may be reduced. However, this causes an increase in a resistance of the wirings, preventing a high-rate operation of a liquid crystal display device.
If the thin film transistor
514
and the contact hole
516
were reduced in size without changing a size of a pixel area, characteristics of the thin film transistor
514
would be deteriorated with the result of degradation in displayed images. Thus, it is quite difficult to increase an aperture ratio merely by reducing a size of wirings and/or the thin film transistor
514
.
With reduction in a size of a pixel in accordance with a requirement to displaying images at a higher density, a ratio of wirings and/or a thin film transistor to a maximum aperture becomes greater, resulting reduction in an aperture ratio. That is, displaying image at a high density causes reduction in a maximum aperture.
However, a size of wirings and a thin film transistor reaches its lower limitation. As a result, only an area of an actual aperture is reduced, and resultingly, an aperture ratio defined as a ratio of an area of the actual aperture to an area of the maximum aperture (Y/X) is reduced. Even so, if wirings, a thin film transistor and a contact hole were reduced in size in accordance with reduction in the maximum aperture, an aperture ratio remains unchanged. However, as mentioned earlier, a resistance of wirings would be increased, and characteristics of a thin film transistor would be deteriorated.
Japanese Unexamined Patent Publication No. 8-262495 (A) has suggested a liquid crystal display device for the purpose of enhancing an aperture ratio.
In the suggested liquid crystal display device, a thin film transistor is arranged in an area at which a scanning line or a gate line and a signal line or a drain line intersect with each other. By arranging a thin film transistor in such an area, it is no longer necessary to arrange a thin film transistor in a maximum aperture, ensuring enhancement of an aperture ratio.
In the suggested liquid crystal display device, a semiconductor layer such as an amorphous silicon layer is formed on a scanning line in the above-mentioned area, and then, source and drain electrodes are formed on the semiconductor layer to thereby fabricate a thin film transistor. Then, a signal line is formed so as to cover the thin film transistor therewith, and subsequently, a drain electrode is electrically connected to the signal line.
Thus, a process of fabricating the suggested liquid crystal display device includes two photolithography steps, that is, a first photolithography step of forming source and drain electrode, and a second photolithography step of forming a signal line.
In a conventional process of fabricating a liquid crystal display device, a photolithography step is carried out only once, because source and drain electrodes are formed concurrently with formation of a signal line. Thus, a process of fabricating the suggested liquid crystal display device has to carry out a photolithography step once greater than a conventional process of fabricating a liquid crystal display device, resulting in complexity in fabrication of a liquid crystal display device.
Japanese Unexamined Patent Publication No. 6-82832 (A) has suggested a liquid crystal display device including a TFT substrate comprised of an electrically insulating substrate, a plurality of gate lines formed on the substrate, a plurality of signal lines formed on the substrate perpendicularly to the gate lines, a plurality of thin film transistors, and pixel electrodes. Each of the gate lines is comprised of a gate wiring layer formed in a recess formed at a surface of the substrate.
Japanese Unexamined Patent Publication No. 10-239678 (A) has suggested a thin film transistor array substrate including an electrically insulating transparent substrate, a plurality of gate electrode lines formed on the substrate, a gate insulating film covering the
Foley & Lardner LLP
NEC LCD Technologies Ltd.
Parker Kenneth
LandOfFree
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