Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1999-12-22
2003-06-03
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C257S072000
Reexamination Certificate
active
06573954
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of liquid crystal display devices employed in information processing terminals and video appliances, and their manufacturing methods. More particularly, the present invention relates to active matrix liquid crystal display devices with a high aperture ratio using an active element substrate.
BACKGROUND OF THE INVENTION
To increase the aperture ratio of an active matrix liquid crystal display devices, pixel electrodes are formed on the uppermost layer of each thin film layer forming active elements. This type of liquid crystal display device and its manufacturing method are disclosed for example in Super-High-Aperture-Ratio TFT-LCD Structure, Digest of Technical Papers 1996 International Workshop on Active Matrix Liquid Crystal Displays (AM-LCD '96), pp. 149-152 by J. H. Kim et. al.
FIG. 5A
shows a perspective plan view of a single pixel of the disclosed liquid crystal display device.
FIG. 5B
is a sectional view taken along the dotted line
5
B—
5
B in FIG.
5
A. The liquid crystal display device shown in
FIGS. 5A and 5B
, includes a first substrate (a) which holds the active elements, a gate electrode (b), a gate insulation layer (c), a channel layer (d), source electrode wiring (e), and a drain electrode (f) forming a thin film transistor TFT. An insulation layer (g) is shown on the TFT. A contact hole (g
1
) for connecting the drain electrode (f) and pixel electrode (h) is formed in the insulation layer (g). A second substrate (i) for sandwiching a liquid crystal layer (k) between the the substrates has a light-shielding layer (j) (black matrix). The light-shielding layer (j) blocks the light from a portion of the liquid crystal layer (k) where electrical control of the pixel electrode (h) is incomplete.
For manufacturing such liquid crystal display device, first, the gate electrode (b) is formed on a glass first substrate (a). Then, the gate insulation layer (c), containing SiN and a-Si, and channel layer (d) are formed. After forming the source electrode wiring (e) and drain electrode (f), an insulation layer (g) having a contact hole (g
1
) is formed on the drain electrode (f) using low dielectric organic material (dielectric constant: 2.6-2.7) such as benzocyclobutane. A pixel electrode (h), connected to the drain electrode (f) through the contact hole (gl) is formed over the insulation layer (g) partially overlapping the source electrode wiring (e). The second substrate (i) with the light-shielding layer (j) is placed facing the first substrate (a), and the liquid crystal layer (k) is injected in-between to complete the liquid crystal display device.
The insulation layer (g) as provided above enables to extend the pixel electrode (h) up to the dotted line in
FIGS. 5A and 5B
, and up to the source electrode wiring (e) shown in
FIG. 5A
, while maintaining insulation. This allows to expand the liquid crystal driving area of the pixel electrode (h), resulting in an increased aperture ratio. Furthermore, by forming the insulation layer g with a low dielectric organic material, the parasitic capacitance between the pixel electrode (h) and source electrode wiring (e) may be reduced. This enables the achievement of a liquid crystal display device with high aperture ratio and low occurrence of cross talk.
However, the configuration of the conventional liquid crystal display device as described above may generate a crack in the pixel electrode (h), starting from the contact hole (g
1
), and cause a pixel defect in the liquid crystal display device. This phenomenon was investigated by means of a series of detailed experiments, and the following details were discovered.
As shown in
FIG. 6
, cracks (l) on the pixel electrode (h) tend to start from corners of the via hole along the contact hole (g
1
). They almost never start from a straight section. The crack is also likely to extend in the direction of the shortest distance between the contact hole (g
1
) and the edge of the pixel electrode (h). This behavior suggests that the crack occurs due to the formation of the pixel electrode (h) on the insulation layer (g) made of organic material. Since a heating process is used for the formation of the pixel electrode (h), the cracks (l) are thought to result from the difference in stress between the insulation layer (g) and pixel electrode (h).
The contact hole (g
1
) on the drain electrode (f) is normally nearly round, as shown in the conventional example in
FIG. 5A. A
regular square pattern is generally used as a photomask pattern for forming contact hole (g
1
). The pattern at the corners of the contact hole is not sharp and the corners of the contact hole become rounded as shown in
FIG. 5A
because the insulation layer g is relatively thick compared to the size of the contact hole. This is because it is necessary to minimize the area needed to create the contact hole (g
1
) on the drain electrode (f) for connecting the drain electrode (f) and the pixel electrode (h) and thus maintain the largest possible aperture ratio. The shape of the contact hole (g
1
) for electrically connecting the pixel electrode (h) and drain electrode (f) is usually minimum size, with the contact hole (g
1
) having a small radius of curvature for its open rim.
Accordingly, a crack from a via hole in the pixel electrode (h) along the contact hole (g
1
) may propagate in any direction. If several cracks (la) reach the edge of the pixel electrode (h) from almost the same starting point, for example, as shown in
FIG. 6
, these cracks may separate a portion of the pixel electrode (h) from the connecting part with the drain electrode (f), generating a defective electrode area (h
1
) which has defective electrical connection with the drain electrode (f). In the defective electrode area (h
1
), a portion which is not covered by a light-shielded area including the light-shielding layer (j), drain electrode (f), and source electrode wiring (e) becomes a defective pixel area (h
2
), shown by the slanted lines in the defective electrode area (h
1
). The defective pixel area (h
2
) becomes visibly obvious when the liquid crystal display device is being driven.
SUMMARY OF THE INVENTION
The present invention aims to provide a liquid crystal display device with a high aperture ratio which eliminates defective pixel areas even if a crack occurs in a pixel electrode. The present invention further provides a method for manufacturing such liquid crystal display device.
A liquid crystal display device of the present invention includes an active element; a contact electrode connected to the active element; a pixel electrode; an insulation layer insulating the active element, and the contact electrode from the pixel electrode; and a contact hole provided in the insulation layer for connecting the contact electrode and pixel electrode. The contact hole has more than one radius of curvature. All portions of the pixel electrode close to small radiuses of curvature out of the aforementioned multiple radiuses of curvature of the contact hole are all located in a light-shielded area.
Moreover, the liquid crystal display device of the present invention includes the active element; a contact electrode connected to the active element; a pixel electrode; an inter-layer insulation layer for insulating the active element, and the contact electrode from the pixel electrode; a contact hole provided in the interlayer insulation layer for connecting the contact electrode to the pixel electrode. The contact hole has a major axis longer than another, minor axis perpendicular to the major axis. The portion of the pixel electrode close to the extended line of the major axis of the contact hole is located in the light-shielded area.
Still more, the liquid crystal display device of the present invention has an active element containing substrate that includes an upper layer over which the pixel electrode is located and the pixel electrode is connected to the active element under an interlayer insulation layer through a contact hole provided on the interlayer insulation l
Hirose Takashi
Tsuboi Nobuyuki
Wakimoto Tatsuya
Matsushita Electric - Industrial Co., Ltd.
Parker Kenneth
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