Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
2000-02-29
2003-03-11
Hjerpe, Richard (Department: 2674)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S087000, C345S092000, C345S096000, C345S098000, C345S100000, C345S090000, C345S206000, C349S042000, C349S043000, C349S045000, C257S059000, C257S072000, C257S347000, C257S350000, C257S351000
Reexamination Certificate
active
06531993
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrooptic device typified by a liquid crystal display in which a pixel matrix circuit and driving circuits disposed around thereof are provided on one substrate and to an electronic apparatus carrying such electrooptic device. It is noted that a semiconductor device refers to devices in general which function by utilizing the semiconductive characteristics including the electrooptic device and the electronic apparatus carrying the electrooptic device in the present specification.
2. Description of the Related Art
The technology for fabricating thin film transistors (TFT) on a low cost glass substrate has come to be rapidly developed in these days because the demand on an active matrix type liquid crystal display has been increasing. The active matrix type liquid crystal display is what a thin film transistor is disposed in each of several tens to several millions of pixels disposed in a matrix to control electric charge which goes in and out of each pixel electrode by the switching function of the thin film transistor.
Liquid crystal is sandwiched between each pixel electrode and an opposed electrode, thus forming a sort of capacitor. Accordingly, light transmitting through a liquid crystal panel may be controlled and an image may be displayed by changing the electrooptic characteristics of the liquid crystal by controlling the charge going in and out of the capacitor by the thin film transistor. However, the capacitor constructed as described above has had a problem that the electrooptic characteristics of the liquid crystal change and the contrast in displaying an image deteriorates because its holding voltage drops due to a leak of current.
Then, generally another capacitor called a holding capacitor also called storage capacitor) is disposed in series with the capacitor composed of the liquid crystal to supply electric charge lost due to the leak and others to the capacitor composed of the liquid crystal.
While the holding capacitor is constructed variously, typically it is constructed by sandwiching an oxide film as dielectric between a shielding film and the pixel electrode. The shielding film is a coating film having a light shielding characteristic for preventing the conductivity of the thin film transistor from fluctuating due to illumination of light in the pixel matrix section of the transmission type liquid crystal display. The shielding film may also function as a black matrix.
The holding capacitor having the structure in which the dielectric is sandwiched between the shielding film and the pixel electrode is connected to a common line to which reference potential is applied to keep the potential of the shielding film constant.
Then, in order to connect the shielding film with the common line after forming the shielding film by patterning in the process, a contact hole had to be created through an interlayer insulating film provided between the shielding film and the common line. The contact hole is created by means of photolithography using a mask.
The photolithography using the mask is carried out in the fabrication process of the active matrix type liquid crystal display in the processes of forming an active layer, of forming a gate insulating film, of forming the pixel electrode and of forming gate and source signal lines, besides the process of creating the contact hole through the interlayer insulating film provided between the shielding film and the common line. The photolithography using the mask has been the factor of increasing the number of fabrication steps of the active matrix type liquid crystal display and it has been desired to reduce the number of fabrication steps in order to achieve a high yield.
SUMMARY OF THE INVENTION
A driving method of applying inverse polar voltage to a source signal line connected to a pixel TFT is called as source line inversion in the active matrix type liquid crystal display. This source line inversion is carried out to prevent liquid crystal from deteriorating by always applying an electric field of one orientation to the liquid crystal. That is, it is possible to prevent the liquid crystal from deteriorating by always applying the electric field of one orientation by applying the signal whose polarity is inverted per line of the source signal lines and by inverting the polarity of the signals per one frame period as shown in FIG.
4
. One frame period is a period during which all pixels display one screen.
The fluctuation of the potential of the shielding film may be averaged by using this source line inversion. Therefore, because the potential of the shielding film is kept almost constant when averaged temporally even when the shielding film is not connected to the common line whose potential is kept constant (reference potential), it is possible to float the shielding film in the holding capacitor having the structure in which the dielectric is sandwiched between the shielding film and the pixel electrode. Accordingly, it becomes unnecessary to create the contact hole through the interlayer insulating film provided between the shielding film and the common line by means of photolithography using the mask to connect the shielding film and the common line after forming the shielding film by patterning. Therefore, it becomes possible to reduce the number of processes for fabricating the active matrix type liquid crystal display, to achieve a high yield and to suppress the fabrication cost thereof.
In addition to the arrangement described above, the fluctuation &Dgr;V of the potential of the shielding film is reduced by forming a large capacity coupling capacitor between the shielding film and the common line. The value of the &Dgr;V is determined by capacitive value C of the coupling capacitor formed between the shielding film and the common line and electric charge Q applied to the shielding film. However, because the electric charge Q is fixed by a number of pixels and the value of voltage of a signal inputted to the source signal line, the value of fluctuation &Dgr;V of the potential of the shielding film is actually decided by the capacitive value C of the coupling capacitor. The greater the value C, the smaller the &Dgr;V is, thus allowing the potential of the shielding film to be kept more constant.
When a large capacity coupling capacitor is formed between the shielding film and the common line by floating the shielding film, it becomes unnecessary to create any contact hole through the interlayer insulating film provided between the shielding film and the common line by means of photolithography using a mask in the same manner with the arrangement of floating only the shielding film described above. It is preferable that the capacitive value of the coupling capacitor is ten times or more of the total of the capacitive values of all holding capacitors connected to one line of the gate signal line via the pixel TFTs. Accordingly, it becomes possible to reduce the number of processes for fabricating the active matrix type liquid crystal display, to achieve the high yield and to suppress the fabrication cost thereof. In addition to that, a good contrast maybe obtained when the large capacity coupling capacitor is created between the shielding film and the common line by floating the shielding film because the potential of the shielding film may be kept more constant.
The structure of the present invention will be described below.
According to one embodiment of the invention, there is provided an active matrix type liquid crystal display having a substrate provided with a plurality of pixel TFTs, pixel electrodes electrically connected to the pixel TFTs and a shielding film; and being characterized in that the shielding film is floating and a dielectric is provided between the pixel electrode and the shielding film. It allows the above-mentioned objects to be achieved.
According to another embodiment of the invention, there is provided an active matrix type liquid crystal display having a substrate provided with a plurali
Koyama Jun
Murakami Satoshi
Tanaka Yukio
Yamazaki Shunpei
Hjerpe Richard
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
Zamani Ali
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