Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-04-25
2003-11-11
Ngo, Julie (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S040000, C349S043000, C430S030000
Reexamination Certificate
active
06646693
ABSTRACT:
The following abbreviations are used throughout this specification, “VGA” stands for “Video Graphic Array”; “EWS” stands for “Engineering Work Station”; “CVD” stands for “Chemical Vapor Deposition”; “RIE” stands for “Reactive Ion Etching”; and “MOS” stands for “Metal-Oxide-Semiconductor”.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure of an active matrix type flat panel display device incorporating peripheral drive circuits therein.
2. Description of the Related Art
Heretofore, there has been known an active matrix type liquid crystal display device incorporating peripheral drive circuits therein. This has a structure in which an active matrix circuit constituting pixel regions formed of thin film transistors (abbreviated as a TFT) and peripheral drive circuits for driving this active matrix circuit, which are also formed of thin film transistors, are integrated on a glass substrate (or a quartz substrate).
For example, in a VGA panel, about 300,000 thin film transistors are integrated on the same glass substrate or quartz substrate. Also, in the case of an EWS panel, about 1,300,000 thin film transistors are integrated on the same glass substrate or quartz substrate.
In the above structure, if one of the thin film transistors is defective, a dot defect or a linear defect is formed.
The performance of a display device is judged visually. Therefore, when the above dot defect or linear defect is present, the display device is judged as a defective product.
When a glass substrate or a quartz substrate is used, especially, the problem of breakdown by static electricity (electrostatic breakdown) is actualized because the insulating property of the substrate is high and its area is large.
For instance, in the formation of a liquid crystal panel, a plasma using process is frequently used in the formation of various thin films and etching. In the plasma using process, pulse-form static electricity is generated as will be described hereinafter. Also, a process where static electricity is generated is existent such as a rubbing process other than the above plasma process.
As described above, in the formation of thin films constituting thin film transistors and etching, a plasma process typified by a plasma CVD method and a plasma etching method is frequently used. However, since the insulating property of a substrate used is high, there occurs such a phenomenon that discharge takes place locally in this plasma process.
A failure which is considered to be caused by this discharge occurs. Stated more specifically, the operation failure of a thin film transistor which is considered to be caused by various electrostatic breakdowns or static electricity occurs. The failure is the major cause of a reduction in the production yield of an active matrix type liquid crystal display device and other active matrix type flat panel display devices.
As the result of analyzing some examples of the occurrence of the above failures, the inventors of the present invention have reached the following findings.
Firstly, there are roughly classified into two different types of the occurrences of failures caused by static electricity or application of localized high voltage.
The first type of occurrences are caused by electrostatic pulses. The failures caused by the electrostatic pulses include a contact failure and the dielectric breakdown of an insulating film.
The contact failure is caused by the following mechanism. Firstly, at the time of forming a thin film by a plasma CVD method or plasma etching by an RIE method, localized discharge occurs. This discharge is caused by such a factor that a sample using an insulating substrate has a large area and a state where localized discharge is liable to occur is established and further such minor factors as the uneven surface of a pattern, the difference of pattern area, the slight difference of film quality, the presence of particles and the like.
As the result of the above localized discharge, high voltage is instantaneously applied to an extremely small specific region. At this point, voltage is locally induced in part of wiring and an electrostatic pulse is generated. This electrostatic pulse is generated instantaneously and a leading value of this induced voltage is extremely large.
A large current flows through a contact portion between a thin film transistor and wiring (or electrode) due to this electrostatic pulse. The instantaneous flow of a large current causes the contact portion to generate heat at a high temperature. Thereby, the contact is broken. The breakdown of this contact is permanent and is generally difficult to be repaired.
Further, the breakdown of an insulating film is due to the fact that a localized strong field is applied to the insulating film which must retain an insulation function and the insulating property of that applied portion is broken by the instantaneous flow of a large current caused by electrostatic pulses through wiring and electrodes. The breakdown of this insulating property is also permanent and is generally difficult to be repaired.
The second type of occurrences are caused by the generation of static electricity induced by plasma. This is caused by nonuniformity (such as area difference or level difference) in the shape of a wiring pattern on the substrate in the plasma using process such as film formation or etching. In this process, a localized potential difference is instantaneously induced between patterns during plasma discharge.
This localized potential difference causes localized discharge between conductive patterns or between a conductive pattern and an insulating substrate. This results in damage to a junction (such as a PI junction or NI junction) of a thin film transistor, whereby the thin film transistor malfunctions.
The damage to the junction of the thin film transistor by this localized discharge may be repaired by a heat treatment. Therefore, the failure in this case can be considered as semi-permanent.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a technology for improving the production yield of a liquid crystal panel by suppressing the occurrence of a failure caused by the above electrostatic breakdown.
The present invention is predicated upon the result of the above analysis. The present invention has basically two aspects. According to a first aspect of the present invention, there is provided means for suppressing the generation of electrostatic pulses. According to a second aspect of the present invention, there is provided means for suppressing the generation of static electricity induced by plasma.
In the present invention, to suppress the generation of electrostatic pulses, there are arranged protective capacitors for absorbing instantaneous electric pulses around a liquid crystal panel.
In the manufacturing process of an active matrix type flat panel display device typified by a liquid crystal panel, there is arranged wiring called “short ring” to eliminate a potential difference between wiring patterns. This short ring is separated from a circuit in the end. In the step of manufacturing a finished product, the short ring has no wiring function any longer.
One electrode of the above-described capacitor (protector capacitor) for absorbing electric pulses is connected to this short ring. That is, electric pulses induced by this short ring are absorbed by the above protective capacitor.
The short ring is connected to all source lines and gate lines constituting an active matrix circuit. Therefore, even if an electric pulse enters somewhere in the active matrix circuit, it is absorbed by the above protective capacitor. Even if the electric pulse is large and is not completely absorbed by the protective capacitor, its influence can be weakened.
Generally speaking, the short ring is not connected to all the gate electrodes of thin film transistors constituting peripheral drive circuits for driving the active matrix circuit. However, when the protective capacitor is arranged in an area near the peripheral drive
Teramoto Satoshi
Zhang Hongyong
Fish & Richardson P.C.
Ngo Julie
Semiconductor Energy Laboratory Co,. Ltd.
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