Active matrix liquid crystal display device having...

Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal

Reexamination Certificate

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C349S039000, C349S044000, C257S346000, C257S059000, C257S072000

Reexamination Certificate

active

06490014

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the circuit configuration and layout of a pixel area of an active matrix display device in which thin-film transistors are used and source lines formed above gate lines. In particular, the invention relates to the structure of an auxiliary capacitor.
2. Description of the Related Art
In recent years, techniques of forming thin-film transistors (TFTs) on an inexpensive glass substrate have been made rapid progress. This is because of increased demand for the active matrix liquid crystal display device.
In the active matrix liquid crystal display device, thin-film transistors are provided for respective ones of hundreds of thousands to millions of pixels that are arranged in matrix form and the charge entrance and exit to each pixel is controlled by the switching function of the thin-film transistor.
A liquid crystal is interposed between each pixel electrode and an opposed electrode, to form a kind of capacitor. Therefore, image display is realized by controlling the quantity of light passing through the liquid crystal panel by varying the electro-optical characteristic of the liquid crystal by controlling the entrance and exit of change to and from this capacitor with the thin-film transistor.
The capacitor having the above structure has a problem that since the voltage held by the capacitor gradually decreases due to current leakage, it changes the electro-optical characteristic of the liquid crystal and deteriorates the contrast of image display.
A common measure to solve the above problem is a configuration in which an additional capacitor called an auxiliary capacitor is provided in parallel with the capacitor including the liquid crystal and charge equivalent to charge that is lost due leakage etc. is supplied to the capacitor including the liquid crystal.
FIG. 1
is a circuit diagram of a conventional active matrix liquid crystal display device. The active matrix display circuit is generally divided into three parts: a gate driver circuit
2
for driving gate lines (i.e., gate lines, scanning lines)
4
, a data driver circuit
1
for driving source lines (i.e., data lines, source lines or signal lines)
5
, and an active matrix circuit
3
that is provided with pixels. The data driver circuit
1
and the gate driver circuit
2
are generically called a peripheral circuit.
In the active matrix circuit
3
, a number of gate lines
4
and source lines
5
are provided so as to cross each other and pixel electrodes
7
are provided at the respective intersecting points. A switching element (thin-film transistor)
6
is provided to control charge that enters or exits from each pixel electrode
7
. Selection is made between the top-gate thin-film transistor (the gate electrode is formed above the active layer) and the bottom-gate thin-film transistor (the active layer is formed above the gate electrode) in accordance with the necessary circuit structure, the manufacturing process, the required characteristics, and other factors. Further, as described above, to prevent a variation in pixel voltage due to leak current, an auxiliary capacitor
8
is provided in parallel with each pixel capacitor.
On the other hand, the conductivity of the thin-film transistor is varied by illumination with light. To prevent this phenomenon, it is necessary to cover each thin-film transistor with a light-interruptive coating (black matrix). The light-interruptive coating is formed so as to also cover the portions between the pixels to prevent color or brightness contamination between the pixels and a display failure due to a disordered electric field at pixel boundaries.
So, the light-interruptive coating assumes a matrix shape and hence is called a black matrix (BM). At first, in favor of advantages in a manufacturing process, the black matrix was provided over the substrate (opposed substrate) that opposes the substrate on which the active matrix circuit is formed. However, recently, because of the need for increasing the area of each pixel (aperture ratio), it is proposed to provide the black matrix over the substrate on which the active matrix circuit is formed.
SUMMARY OF THE INVENTION
Various proposals haven made of the structure of the auxiliary capacitor. However, it is difficult to obtain a large capacitance while maintaining the area of the open portion (light-transmissive portion) of each pixel.
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide a structure of an auxiliary capacitor which can provide a large capacitance while maintaining the area of the open portion (light-transmissive portion) of each pixel.
According to one aspect of the invention there is provided an active matrix liquid crystal display device comprising a thin-film transistor having a source region to which a pixel electrode is electrically connected; a drain electrode connected to a drain region of the thin-film transistor and formed in the same layer as a source line, the drain electrode having a pattern that covers 50% or more of an active layer of the thin-film transistor; and an auxiliary capacitor formed by using the drain electrode.
With the above configuration, the aperture ratio of the pixel can be increased because the auxiliary capacitor is formed above the thin-film transistor.
Another aspect of the invention attains the above object by forming a conductive light-interruptive film over the active-matrixside substrate, keeping it at a constant potential, and using it as one electrode of the auxiliary capacitor. Since originally the light-interruptive film does not transmit light, the aperture ratio does not decrease even if it is used as one electrode of the auxiliary capacitor.
The active matrix liquid crystal display device of the invention comprises:
(1) a thin-film transistor;
(2) a gate line and a source line formed above the gate line;
(3) a conductive film serving as a light-interruptive film and kept at a constant potential;
(4) a metal wiring connected to a drain region of the thin-film transistor and made of the same layer as the source line; and
(5) an interlayer insulating film formed between the source line and the conductive film, and comprising at least two insulating layers.
In the invention, the thin-film transistor may be of either the top gate type or the bottom gate type as long as the above conditions are satisfied. This is, since the main improvements of the invention relate to the structure above the source line, the structure below the source line (i.e., the positional relationship between the gate line and the active layer) is irrelevant. Also, the interlayer insulating layer may consist of three or more layers.
According to another aspect of the invention, in the above configuration, an auxiliary capacitor having the metal wiring and the conductive film (light-interruptive film) as electrodes and at least the lower insulating layer of the interlayer insulating film as a dielectric is formed in a region where the upper insulating layer of the interlayer insulating film is removed by etching. The dielectric may consist of two or more insulating layers.
According to a further aspect of the invention, in the above configuration, the conductive film (light-interruptive film) overlaps with the metal wiring and has a portion that is in contact with the lower insulating layer.
In the two aspects of the invention just mentioned above, it is effective to employ, as the main component of the lower insulating layer, silicon nitride that is produced stably in semiconductor processes and has a large relative dielectric constant. In this case, the dielectric of the auxiliary capacitor may be composed of only a silicon nitride layer or may have a multi-layer structure of a silicon nitride film and some other coating (for instance, a silicon oxide film).
In this case, the dielectric is made thinner and the use of silicon nitride having a large dielectric constant realizes a large capacitance. In the invention, the thickness

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