Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2001-05-01
2004-04-13
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000, C349S147000
Reexamination Certificate
active
06721026
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a structure of an in-plane switching mode liquid crystal display (LCD), and more particularly to a structure of an in-plane switching mode TFTLCD with an improved aperture ratio of the pixel region thereof. The present invention further relates to a simplified process for forming an in-plane switching mode LCD.
BACKGROUND OF THE INVENTION
With the development and improvement on LCD-related technology, a liquid crystal display tends to substitute for a conventional cathode-ray tube display to become a main stream. Please refer to
FIG. 1A
which is a schematic circuit diagram of a unit pixel region in a liquid crystal display. In the unit pixel region, a thin film transistor (TFT)
11
functioning as a switching unit, a displaying unit
12
and a storage capacitor
13
are included. Concerning the structure of the display unit
12
, it can be classified currently into two types, i.e. a twisted nematic mode LCD (TN-LCD), and an in-plane switching mode LCD (IPS-LCD).
A general structure of the display unit
12
of a TN-LCD is schematically shown in FIG.
1
B. The display unit
12
includes a data electrode
121
and a common electrode
122
which are spaced with a cell gap d, and liquid crystal (LC) molecules
123
sandwiched between the electrodes
121
and
122
. By providing a potential difference between the data electrode
121
and common electrode
122
, the LC molecules
123
will tend to stand, and the standing angles of the LC molecules relative to Z-axis depend on the electric field applied thereonto. For illustration, three kinds of electric fields E
1
, E
2
and E
3
are applied in
FIG. 1B
to show the changes of the standing angles of LC molecules wherein E
1
>E
2
>E
3
=0. For different standing angles, the light transmittance varies, thereby controlling the brightness of individual pixels. The rotation manner of the LC molecules in such a TN-LCD result in a change of light transmittance with different viewing angles. For example, the light transmittance in the A-A′ direction and that in the B-B′ direction are different because their multiple refraction in different ways. Therefore, the viewing range of such an LCD, especially a large-size LCD, for seeing a clear image is confined.
A general structure of the display unit
12
of an IPS-LCD is schematically shown in FIG.
1
C. As shown, the data electrode
121
and the common electrode
122
are arranged at the same side of LC molecules
123
. Similar to the operations for a TN-LCD described above, a potential difference is provided between the data electrode
121
and common electrode
122
to rotate the LC molecules
123
. According to the in-plane switching mode, the LC molecules
123
will rotate about the Z-axis to a degree depending on the electric field applied thereonto. For illustration, three kinds of electric fields E
4
, E
5
and E
6
are applied in
FIG. 1C
to show the changes of the rotating angles of LC molecules wherein E
4
>E
5
>E
6
=0. For different rotating angles, the light transmittance varies, thereby controlling the brightness of individual pixels. The rotation manner of the LC molecules in such an IPS-LCD will not result in any significant change of light transmittance with different viewing angles. Therefore, it has an advantage of providing a broad viewing range, and thus is suitable for large-size display.
A general in-plane switching mode TFTLCD is schematically shown in
FIG. 2
which is a partial top plane view of the LCD structure. A conventional process for manufacturing the LCD structure principally includes the following steps:
(a) forming a first metal layer, and defining a gate conductive line
21
of TFT units and common electrodes
22
of display units;
(b) depositing a tri-layer structure which includes a gate isolation layer, a semiconductor layer, and an etch-stopper layer;
(c) defining an etch-stopper structure;
(d) forming a doped semiconductor layer, and defining source/drain regions of TFT units;
(e) forming a second metal layer, and defining a data line
23
of TFT units and data electrodes
24
of display units;
(f) depositing a passivation layer, and defining contact vias for interconnection; and
(g) forming a transparent conductive layer, and defining transparent electrodes.
In the above conventional process, the common electrodes
22
and the data electrodes
24
are formed of the first and second metal layers by different photo-masking and lithography procedures, respectively. As known, for each photo-masking and lithography step, the risks of misalignment (i.e. L
1
≠L
2
in
FIG. 2
) and contamination may be involved so as to affect the production yield. Further, the opaque feature of metal results in the reduction of light transmittance, and thus a relatively large clearance between each pair of data and common electrodes is required in order not to influence the overall light transmittance of the pixel region too much. The relatively large clearance, however, results in a relatively high operational voltage.
In order to solve these problems, a transparent conductive material substitutes for metal to form the common and data electrodes so as to enhance the light transmittance or allow the clearance to be reduced to a level less than the cell gap d. For example, indium tin oxide (ITO) can be used therefor. Nevertheless, the common and data electrodes are still defined by different photo-masking and lithography procedures, so the possibility of misalignment still exists. If such misalignment is desirably made to be tolerable, the compactness of the device cannot be achieved. Furthermore, the conductivity of the transparent conductive material is not good enough for electric conduction, so additional metal layers are required for forming the scan line and the data line. Thus the manufacturing process is even complicated.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a process for forming an in-plane switching mode liquid crystal display (IPS-LCD), which simultaneously defines pixel portions of the common and data electrodes so as to avoid misalignment.
Another object of the present invention is to provide a process for forming an in-plane switching mode liquid crystal display (IPS-LCD), which simultaneously defines the data line and the transparent pixel portions of the common and data electrodes so as to be relatively simple in manufacturing.
A further object of the present invention is to provide an in-plane switching mode liquid crystal display (IPS-LCD), in which a storage capacitor consisting of storage-capacitor portions of the common and data electrode structures is disposed outside the pixel region so as to enhance the aperture ratio of the pixel region.
The present invention relates to a process for forming an in-plane switching mode liquid crystal display (IPS-LCD), comprising steps of providing a substrate made of an insulating material; forming a first conductive layer on a first side of the substrate, and defining a gate conductive structure, and a bus portion of a common electrode; forming a tri-layer structure consisting of a gate insulation layer, a semiconductor layer, and an etch stopper layer; defining an etch stopper structure with a portion of the semiconductor layer exposed; forming a highly doped semiconductor layer, and defining a contact via for interconnection to the bus portion of the common electrode; forming a second conductive layer, and defining source/drain regions, a data line, a pixel portion of a data electrode, and a pixel portion of the common electrode with the etch stopper structure and the gate insulation layer as a stopper, wherein the pixel portion of the common electrode is interconnected to the bus portion of the common electrode through the contact via; and forming a passivation layer, and defining a pixel region for exposing the pixel portions of the data and common electrodes.
Preferably, a storage-capacitor portion of the common electrode is simultaneously defined together with the gat
Cheng Jia-Shyong
Jen Tean-Sen
Chowdhury Tarifur R.
Corless Peter F.
Edwards & Angell LLP
Hannstar Display Corporation
Jensen Steven M.
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