Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-11-09
2002-03-05
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S043000
Reexamination Certificate
active
06353464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an active matrix type TFT array substrate on which a thin film transistor (hereinafter referred to as TFT) is mounted to serve as a switching element, to a liquid crystal display using the TFT array substrate, and to a manufacturing method of them.
2. Background Art
Researches and developments of liquid crystal display to serve as a flat panel display in substitution for CRT have been popularly carried out. In particular, making use of its feature of small power consumption and small thickness, several liquid crystal displays have been heretofore put into practical use to serve as a display for ultra small size television or for notebook type personal computer driven by battery. As a drive method of the liquid crystal display, an active matrix type TFT array is principally used in which TFT is used as a switching element from the viewpoint of high display quality.
To achieve small power consumption in the liquid crystal display, it is effective to increase an effective display area of picture element section of the liquid crystal panel, i.e., to enhance aperture ratio of picture element. As a TFT array effective for achieving a liquid crystal panel of a high aperture ratio, for example, the Japanese Patent Publication (examined) Nos. 2521752 and 2598420 and the Japanese Laid-Open Patent Publication (unexamined) No. 163528/1992, etc. disclose a structure. In this structure, after forming a TFT comprising a scanning electrode, a signal electrode and a semiconductor layer, an interlayer insulating film composed of a transparent resin is formed to coat the TFT, and a picture element electrode is formed on the uppermost layer.
A high aperture ratio is achieved in the mentioned structure principally by following two reasons. In the first place, as the picture element electrode is formed on the interlayer insulating film of transparent resin of which surface is flattened, it is possible to prevent a deficient display (domain phenomenon) caused by irregular orientation of liquid crystal molecules occurring at the step portion of the picture element electrode of the conventional structure, whereby it becomes possible to increase an effective display area. In the second place, by forming the picture element electrode on the relatively thick interlayer insulating film of 0.3 &mgr;m to 2 &mgr;m in thickness, no electric short circuit takes place between the scanning line or signal line located at the lower layer of the interlayer insulating film and the picture element electrode located on the upper layer, and therefore it becomes possible to form the picture element electrode in wider area to overlap those lines.
A manufacturing process of the mentioned known TFT array of high aperture ratio is hereinafter described. First, a TFT comprising gate electrode, gate insulating film, semiconductor layer, source electrode, and drain electrode is formed on a transparent insulating substrate such as glass substrate. Then, an interlayer insulating film composed of a transparent resin of which surface is flattened so as not to produce any step caused by the TFT is formed, and a contact hole is formed at required portions. Finally, a picture element electrode composed of a transparent conductive film such as ITO is formed, thus a TFT array being completed. The picture element electrode is electrically connected to the drain electrode on the lower layer through the contact hole formed in the interlayer insulating film.
As described in the Japanese Laid-Open Patent Publications (unexamined) Nos. 127553/1997 and 152625/1997, methods for forming a contact hole in the interlayer insulating film are classified into one method in which a photosensitive transparent resin (having a photosensitivity) is used, and into another method in which non-photosensitive transparent resin (having no photosensitivity) is used. In the case of using a photosensitive transparent resin, a required contact hole is formed by a photomechanical process, without using any resist, in which exposure and development are performed using a mask pattern of the contact hole after application and baking of the resin. On the other hand, in the case of using a non-photosensitive transparent resin, after applying and baking the resin, a resist is applied. Then, after forming a contact pattern by photomechanical process, a dry etching is performed using a gas containing at least one of CF
4
, CF
3
or SF
6
, and finally by removing the resist, a required contact hole is achieved.
On the mentioned TFT array substrate, terminals electrically connected to each of the date line, the source lines, etc. are arranged on the periphery of the image display section, whereby a terminal region for connecting each terminal to an external terminal is formed.
FIG. 18
is a partially top view showing an active matrix type TFT array substrate of high aperture ratio according to the prior art. In the drawing, reference numeral
1
indicates a glass substrate being a transparent insulating film, numeral
2
indicates a gate line formed on the glass substrate
1
, and numeral
3
indicates a source line crossing over the gate line
2
. Numeral
4
is a gate terminal electrically connected to the gate line
2
, and numeral
5
is a source terminal electrically connected to the source line
3
. Numeral
6
is an interlayer insulating film composed of a photosensitive transparent resin formed to cover the entire gate line
2
and source line
3
and end portions of the image display section side of the gate terminal
4
and the source terminal
5
. Numeral
7
is a guard resistance, numeral
8
is a contact hole of the terminal section, and numeral
9
is a contact hole of the guard resistance section. Numeral
14
is a short ring which is formed to prevent an electrostatic destruction of the TFT at the time of manufacturing thereof, and connects electrically respective terminals to each other through the guard resistance
7
.
In the active matrix type substrate of above arrangement, at the end portions on the image display section side of the gate terminal
4
and the source terminal
5
, a metallic wiring on the lower layer such as gate line
2
, source line
3
, etc. and a transparent conductive film on the upper layer are connected to each other through the contact hole
8
formed in the interlayer insulating film
6
. On the other hand, since the interlayer insulating film
6
is not formed at the end portion on the guard resistance
7
side of each terminal and at the guard resistance section
7
, a photomechanical process is separately required to form, for example, the contact hole
9
in the guard resistance section.
A transparent conductive film such as ITO is used as a picture element electrode and an upper conductive film of the gate terminal
4
and the source terminal
5
, and this ITO is formed by forming a film on the entire substrate and patterning the film. However, there is a difference in the manner of crystal growth and in etching speed between the ITO formed on the interlayer insulating film
6
which is an organic film and the ITO formed on the glass substrate
1
or on an inorganic film. For example, etching speed of the ITO formed on the organic film is faster than that of the ITO formed on the inorganic film by 2 to 5 times. In the actual manufacturing process, from the viewpoint of patterning precision of the picture element electrode, it is necessary to set an etching time to that for the image display section in which the ITO is formed on the interlayer insulating film
6
composed of an organic film. Accordingly, in the terminal region on the guard resistance
7
side where the interlayer insulating film
6
is not formed, there arises a problem that the ITO remains not etched between the adjacent two gate terminals
4
, and between two source terminals
5
. The ITO left not etched brings about a deficient leak between respective terminals.
To reduce this deficient leak, it was necessary to perform a first etching on the etching condition (at an etching
Noumi Shigeaki
Yabushita Kouji
Kabushiki Kaisha Advanced Display
McDermott & Will & Emery
Qi Mike
Sikes William L.
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