Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-12-08
2003-07-29
Chowdhury, Tarifur (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S042000
Reexamination Certificate
active
06600534
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix type reflective liquid crystal display device used for a computer, audiovisual (AV) equipment, etc.
2. Description of the Related Art
In general, in an electrode structure of a liquid crystal display device having switching elements, storage capacitance electrodes forming storage capacitance are provided in addition to pixel driving electrodes for driving a liquid crystal layer. In the case where pixel electrodes are provided via an interlayer insulator on a substrate, since the thickness of the interlayer insulator is large, storage capacitance electrodes are provided under the interlayer insulator.
FIGS. 24A and 24B
are diagrams showing an exemplary structure of the above-mentioned liquid crystal display device.
FIG. 24A
is a plan view thereof, and
FIG. 24B
is a cross-sectional view thereof taken along a line A-A′ in FIG.
24
A. The liquid crystal-display device shown in these figures is described in Japanese Laid-open Publication No. 9-152625.
Referring to
FIGS. 24A and 24B
, a thin film transistor (TFT)
24
is provided on a substrate
31
. An underlying electrode
25
is formed so as to be connected to a drain electrode
36
b
of the TFT
24
. In the case where the underlying electrode
25
is integrally formed with the drain electrode
36
b
, the underlying electrode
25
and the drain electrode
36
b
may be collectively called a drain electrode. An interlayer insulator
38
is formed so as to cover the underlying electrode
25
and the drain electrode
36
b
. A pixel electrode
21
formed on the interlayer insulator
38
is electrically connected to the underlying electrode
25
or the drain electrode
36
b
through a contact hole
26
. Furthermore, the underlying electrode
25
extends to a central portion of a pixel region
50
. A storage capacitance electrode
25
a
at an end of the underlying electrode
25
is opposed to a storage capacitance line
27
. The storage capacitance line
27
is formed under a gate insulating film
33
. The gate insulating film
33
covers a gate electrode
32
forming a part of the TFT
24
. Storage capacitance is formed in a portion where the storage capacitance line
27
and the storage capacitance electrode
25
a
are opposed to each other interposing the gate insulating film
33
therebetween. The underlying electrode
25
for forming storage capacitance is provided in a narrow width, except for a portion other than the storage capacitance electrode
25
a.
However, in the case where the underlying electrode
25
under the pixel electrode
21
is locally formed in the pixel region
50
(i.e., the underlying electrode
25
is formed in a shape not corresponding to that of the pixel electrode
21
), a portion of the interlayer insulator
38
where the underlying electrode
25
is present and a portion of the interlayer insulator
38
where the underlying electrode
25
is not present may be affected in a different manner from each other during a production process. Because of this, the thickness of the resultant interlayer insulator
38
becomes nonuniform, and the pixel electrode
21
may not be formed in a desired shape on the interlayer insulator
38
. In the case of a reflective liquid crystal display device, reflective electrodes (pixel electrodes) are not uniformly formed, and irregularities of reflection characteristics are observed. Particularly, in the case where the interlayer insulator
38
is formed so as to have uneven surface by heat treatment during a production process, the difference in thermal conductivity between a portion of the interlayer insulator
38
where the underlying electrode
25
is present and a portion of the interlayer insulator
38
where the underlying electrode
25
is not present is reflected onto the shape of the upper surface of the interlayer insulator
38
. This results in display unevenness.
In addition to the above-mentioned problem, there is a possibility that defects are caused in an active matrix substrate having switching elements during a production process. This results in display defects such as line defects, bright points, and flickering. Therefore, in order to enhance production yield, various defect repair techniques have been developed. Mass-production efficiency has been improved by using one of the defect repair techniques or the combination of several kinds of defect repair techniques.
FIG. 25
illustrates the first prior art defect repair technique utilizing a structure of storage capacitance (disclosed in Japanese Publication for Opposition No. No. 4-73569).
The above-mentioned technique is carried out as follows. A MOS-type transistor
208
is turned on by a currently selected scanning signal line
202
. A pixel electrode
206
is charged with a signal of a data signal line
201
. At this time, as shown in a circuit configuration diagram of
FIG. 26
, a liquid crystal capacitance
204
and a storage capacitance
205
are charged with a signal of the data signal line
201
through the MOS-type transistor
208
. Thus, in the case where the liquid crystal capacitance
204
becomes small and influence of parasitic capacitance becomes large due to miniaturization of a pixel, the liquid crystal capacitance
204
can be compensated by the storage capacitance
205
. The liquid crystal layer capacitance
204
is formed between the pixel electrode
206
and a counter electrode (not shown) on a counter substrate which is opposed to the pixel electrode
206
via a liquid crystal layer. On the other hand, the storage capacitance
205
is formed between the pixel electrode
206
and a non-selected scanning signal line
202
interposing a gate insulating film therebetween.
In the case-where pinholes are generated in the gate insulating film in the storage capacitance
205
during a production process, since the pixel electrode
206
overlaps the non-selected scanning signal line
202
via the gate insulating film in the storage capacitance
205
, the pixel electrode
206
is electrically connected to the non-selected scanning signal line
202
. Therefore, a data signal is not appropriately applied to the pixel electrode
206
. In this case, the following extreme defects are caused: the pixel remains in a light-up state, or the pixel does not light up. In order to prevent such defects, the storage capacitance electrode
207
(where the pixel electrode
206
overlaps the scanning signal line
202
) could be insulated from the other pixel electrode portions by a slit
210
except for a portion of the storage capacitance electrode
207
. Because of this, even in the case where pinholes are generated, a constricted portion provided by the slit
210
is cut with a laser beam during the later correcting step, whereby the storage capacitance electrode
207
is completely insulated from the other pixel electrode portions. Thus, the above-mentioned extreme defects are eliminated, and the pixel is controlled with a signal from the data signal line (i.e., the pixel is driven with a data signal without storage capacitance). As a result, improved effects are obtained.
Referring to
FIGS. 27A and 27B
, the second prior art technique will be described. In the second prior art technique, line defects, which are caused when a data signal line
201
and a scanning signal line
202
are short-circuited in an MOS-type transistor
208
, are corrected (disclosed in Japanese Publication for Opposition No. 3-55985)
A gate electrode
220
branched from the scanning signal line
202
is cut with a laser beam from the scanning signal line
202
along a broken line shown in FIG.
27
A. Thereafter, as shown in
FIG. 27B
, a laser beam is radiated in directions of arrows &agr; and &bgr; from above a substrate. Because of this, a source electrode
221
and a drain electrode
222
are short-circuited via the cut gate electrode
220
. As a result, an average voltage of a data signal is applied to the pixel electrode
206
, whereby the presence of defects may be made unnoticeable.
However, the above-mentioned
Ban Mariko
Fujihara Toshiaki
Nagashima Nobuyoshi
Tanaka Kyoushi
Chowdhury Tarifur
Nguyen Dung
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
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