Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1999-07-27
2001-01-23
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S138000
Reexamination Certificate
active
06177974
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a contact structure for electrically connecting together conducting lines formed on two opposite substrates, respectively, via conducting spacers and, more particularly, to a contact structure used in common contacts of an electrooptical device such as a liquid crystal display.
2. Description of the Related Art
In recent years, liquid crystal displays have been extensively used in the display portions of mobile intelligent terminals such as mobile computers and portable telephones including PHS (personal handyphone system). Also, active-matrix liquid crystal displays using TFTs as switching elements are well known.
A liquid crystal display comprises two substrates and a liquid crystal material sealed between them. Electrodes are formed on these two substrates to set up electric fields. A desired image or pattern is displayed by controlling the magnitudes of these electric fields. In the active-matrix liquid crystal display, TFTs (thin-film transistors) are formed on one substrate to control the supply of voltage to each pixel electrode. Therefore, this substrate is referred to as the TFT substrate. A counter electrode placed opposite to the pixel electrodes is formed on the other substrate and so it is referred to as the counter substrate.
In the active matrix display, an electric field is produced between each pixel electrode on the TFT substrate and the counter electrode on the counter substrate, thus providing a display. The potential at each pixel electrode on the TFT substrate is controlled by the TFT and thus is varied. On the other hand, the counter electrode on the counter substrate is clamped at a common potential. For this purpose, the counter electrode is connected with an extractor terminal via a common contact formed on the TFT substrate. This extractor terminal is connected with an external power supply. This connection structure clamps the counter electrode at the common potential.
The structure of the common contact of the prior art active-matrix liquid crystal display is next described briefly by referring to FIGS.
12
-
14
.
FIG. 12
is a top plan view of a TFT substrate
10
. This TFT substrate comprises a substrate
11
having a pixel region
12
, a scanning line driver circuit
13
, and a signal line driver circuit
14
. In the pixel region
12
, pixel electrodes and TFTs connected with the pixel electrodes are arranged in rows and columns. The scanning line driver circuit
13
controls the timing at which each TFT is turned on and off. The signal line driver circuit
14
supplies image data to the pixel electrodes. Furthermore, there are extractor terminals
15
to supply electric power and control signals from the outside. The substrate
11
makes connection with the counter electrode at common contact portions
16
a
-
16
d.
FIG. 13
is a cross-sectional view of the pixel region
12
and a common contact portion
16
representing the common contact portions
16
a
-
16
d
. A TFT
17
and many other TFTs (not shown) are fabricated in the pixel region
12
on the substrate
11
. An interlayer dielectric film
18
is deposited on the TFT
17
. A pixel electrode
19
connected with the drain electrode of the TFT
17
is formed on the interlayer dielectric film
18
.
A precursor for the source and drain electrodes of the TFT
17
is patterned into internal conducting lines
21
at the common contact portion
16
. The interlayer dielectric film
18
is provided with a rectangular opening. A conducting pad
22
is formed in this opening and connected with the internal conducting lines
21
. The pixel electrode
19
and the conducting pad
22
are patterned from the same starting film.
FIG. 14
is a top plan view of the known common contact portion
16
. A region located inside the conducting pad
22
and indicated by the broken line corresponds to the opening formed in the interlayer dielectric film
18
.
As shown in
FIG. 13
, a counter electrode
24
consisting of a transparent conducting film is formed on the surface of a counter substrate
23
. This counter electrode
24
is opposite to the pixel electrodes
19
in the pixel region
12
and to the conducting pad
22
at the common contact portion
16
.
Spherical insulating spacers
25
are located in the pixel region
12
to maintain the spacing between the substrates
11
and
23
. A spherical conducting spacer
26
is positioned at the common contact portion
16
and electrically connects the counter electrode
24
with the conducting pad
22
. The pad
22
is electrically connected with the internal conducting lines
21
, which in turn are electrically connected with an extractor terminal
15
. This connection structure connects the counter electrode
24
on the counter substrate
23
with the extractor terminal
15
on the substrate
11
.
In the prior art liquid crystal display, the interlayer dielectric film
18
is provided with the opening at the common contact portion
16
, as shown in FIG.
13
. Therefore, the cell gap G
c
in the common contact portion is almost equal to the sum of the cell gap G
p
in the pixel region+the film thickness t of the interlayer dielectric film
18
.
The cell gap G
p
(also known as the cell spacing) in the pixel region
12
is determined by the insulating spacers
25
. It is common practice to use standardized spacers as the insulating spacers
25
and so if the spacers
25
have a uniform diameter, the cell gap G
p
in the pixel region
12
is substantially uniform among liquid-crystal cells. However, it is difficult to avoid nonuniformity of the cell gap G
c
in the common contact portion among liquid-crystal cells.
The cell gap G
c
in the common contact portion is constant since the cell gap G
p
is constant because of the relation described above. Therefore, the cell gap G
c
in the common contact portion depends only on the film thickness t of the interlayer dielectric film
18
. Consequently, to make the cell gap G
c
uniform among liquid-crystal cells, it is necessary that the film thickness t of this interlayer dielectric film
18
be uniform among cells. However, this is impossible to circumvent.
Normally, the common contact portions of the liquid crystal display are 2 to 4 in number. The film thickness t of the interlayer dielectric film
18
may differ from location to location on the same substrate. In this case, the film thickness t may differ among different common contacts even on the same substrate.
Because of the aforementioned nonuniformity of the thickness t of the interlayer dielectric film
18
, the cell gap G
c
in the common contact portion differs among different cells or different common contacts. Furthermore, the nonuniformity of the cell gap G
c
results in the cell gap G
p
in the pixel region to be nonuniform.
The cell gap G
p
in the pixel region is affected more by the nonuniformity of the cell gap G
c
in the common contact portion as the area of the pixel region
12
becomes narrower than the area of the common contact portion. Especially, in the case of a projection display as used in a projector, the problem of above-described nonuniformity of the cell gap G
p
in the pixel region becomes conspicuous, because it is a quite accurate small-sized display of about 1 to 2 inches.
A standardized spacer is also used as the conducting spacer
26
. The diameter of this conducting spacer
26
is determined by the diameter of the insulating spacers
25
in the pixel region
12
and by the design thickness of the interlayer dielectric film
18
. Where the thickness of the interlayer dielectric film
18
is much larger than the designed value, the cell gap G
c
in the common contact portion becomes very large. This makes it impossible to connect the counter electrode with the conducting pad well by the conducting spacer
26
. In consequence, the counter electrode cannot be clamped at the common potential. As a result, a display cannot be provided.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a contact structure which is fr
Hirakata Yoshiharu
Yamazaki Shunpei
Nguyen Dung
Nixon & Peabody LLP
Robinson Eric J.
Semiconductor Energy Laboratory Co,. Ltd.
Sikes William L.
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