Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2001-05-10
2004-07-13
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000
Reexamination Certificate
active
06762802
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and, for example, to one of a pair of substrates between which a liquid crystal layer is interposed, i.e., a so-called TFT substrate on which thin film transistors (hereinafter referred to as TFTs) are formed, as well as to a fabrication method for such a TFT substrate.
2. Description of the Related Art
In the case of a related art liquid crystal display device, as described in Japanese Patent Laid-Open No. 202153/1994, its TFT substrate is fabricated by forming openings in the gate insulating film and the protective film laminated on the TFT substrate, through one photo-process (a process which includes the photolithography of forming a photomask on a work and partly removing the photomask according to a processing pattern), and carrying out patterning through a total of five photo-processes. In the TFT substrate obtained by this fabrication method, a charge holding capacitor for the TFT provided in each pixel includes a metal electrode which is formed in the same process as and of the same material as gate wiring lines and serves as a lower electrode, a metal electrode which is formed in the same process as and of the same material as signal lines for the TFT and serves as an upper electrode, and a dielectric which is disposed between the lower and upper electrodes. As the dielectric, a gate insulating film, a nondoped semiconductor (an i-type semiconductor; also called an intrinsic semiconductor) and a semiconductor containing an impurity (also called an n
+
-type semiconductor according to the conduction type of the impurity) are constructed in the form of a laminated film. The upper electrode of the charge holding capacitor is connected to a pixel electrode made of a transparent conductive film via a through-hole formed in the protective film of the TFT.
In addition, as described in Japanese Patent Laid-Open No. 232409/1998, there is a fabrication method which forms through five photo-processes a TFT substrate provided with thin film transistors each of which is of the reversed staggered type (the type in which a semiconductor layer serving as a channel is disposed on the gate electrode of a transistor) and has a channel etch structure (a structure in which a portion serving as the channel of the semiconductor layer is partly thinned by etching or the like).
In addition, there is an art which fabricates a TFT substrate for an In-Plane-Switching (hereinafter, IPS) mode of liquid crystal display device through four photo-processes by using the above-described fabrication method.
In another related art liquid crystal display device, as described in Japanese Patent Laid-Open No. 90404/1997, the upper electrode of each charge-holding capacitor is made of a metal electrode formed in the same process as and of the same material as its gate wiring lines, and the lower electrode of the charge-holding capacitor is made of a transparent electrode deposited in the same process as a metal film for signal lines of each TFT. The dielectric of the charge holding capacitor is made of a gate insulating film, and an opening (a through-hole) is formed in a protective film made of an organic material formed on the upper electrode of the charge-holding capacitor, and the upper electrode and the pixel electrode are connected to each other via the opening.
SUMMARY OF THE INVENTION
According to the arts disclosed in Japanese Patent Laid-Open No. 202153/1994 and Japanese Patent Laid-Open No. 232409/1998, at least five times of patterning (five photo-processes) are needed during the processing of the TFT glass substrate of the liquid crystal display device. Moreover, in Japanese Patent Laid-Open No. 232409/1998, although the TFT glass substrate of a lateral electric field type, i.e., the IPS display mode, of liquid crystal display device is formed through four photo-processes, the terminals of its gate and drain wiring lines are not coated with a transparent conductive film such as Indium-Tin-Oxide (hereinafter, ITO), so that the terminals suffer the problem of electrical corrosion due to humidity. In addition, since comb-teeth like pixel (source) electrodes are disposed close to the gate wiring lines, there is the problem that parasitic capacitance becomes large.
The dielectric of the charge holding capacitor described in Japanese Patent Laid-Open No. 202153/1994 has the laminated structure in which the i-type semiconductor and the n
+
-type semiconductor are laminated on the gate insulating film. Therefore, during charging for driving the TFT type liquid crystal display device, the potential of the lower electrode of the charge holding capacitor becomes higher than that of the upper electrode of the charge holding capacitor and electrons are injected into the i-type semiconductor film from the lower electrode, so that the capacitance value is determined by the thickness of the gate insulating film. During a charge-holding period in such driving, electrons are emitted from the i-type semiconductor and the capacitance value fluctuates and lowers to a capacitance value which is a value for the thickness of the i-type semiconductor, resulting in the problem that image retention occurs in the liquid crystal.
The TFT liquid crystal display device described in Japanese Patent Laid-Open No. 90404/1997 has the protective film made of the organic material, and the drain wiring lines are used as light-shielding electrodes, and pixel electrodes are disposed to overlap the drain wiring lines above the organic protective film of low dielectric constant, thereby improving the aperture ratio. However, patterning processing needs at least five photo-processes.
An object of the invention is to simplify a fabrication process for a TFT substrate compared to the above-described fabrication methods for the related art liquid crystal display devices. Another object of the invention is to improve the display contrast of a liquid crystal display device by forming a wiring structure which has high accuracy and can prevent drain wiring lines from being easily disconnected, by employing the simplified fabrication method. A further object of the invention is to increase the capacitance value per unit area of a charge holding capacitor provided in each pixel of a liquid crystal display device and increase the aperture ratio of the pixel, by using the simplified fabrication method.
Another object of the invention is to use a simple fabrication method which decreases the capacitance difference in charge-holding capacitance between on-switching and off-switching during the driving of a liquid crystal display device and so reduce image retention. A further object of the invention is to reduce the parasitic capacitance between gates and pixel (source) electrodes in IPS display mode.
To achieve the above objects, the invention provides a liquid crystal display device having a novel wiring structure.
One example of liquid crystal display devices according to the present invention comprises:
a first insulating substrate and a second substrate being disposed so that respective main surfaces thereof are opposite to one another;
a liquid crystal layer being interposed between the first and second insulating substrates;
gate wiring lines being formed on the first insulating substrate and transmitting scanning signals;
a gate insulating film being composed of the first insulating substrate and the gate wiring lines;
drain wiring lines being composed of metal films formed on the gate insulating film and transmitting video signals;
semiconductor layers being formed on the gate insulating film and at least under the drain wiring lines;
thin film transistor sections, each of which has
(1) a semiconductor channel layer composed of a part of the semiconductor layer located at least over a part of the gate wiring layer,
(2) a drain electrode composed of a part of the drain wiring line located on the semiconductor channel layer and a semiconductor contacting layer formed of a part of the semiconductor layer being conta
Kaneko Toshiki
Nakayoshi Yoshiaki
Oke Ryutaro
Ono Kikuo
A. Marquez, Esq. Juan Carlos
Duong Thoi V.
Fisher Esq. Stanley P.
Kim Robert H.
Reed Smith LLP
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