Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2002-07-05
2004-08-17
Fureman, Jared J. (Department: 2876)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S042000, C349S187000, C257S059000, C257S072000
Reexamination Certificate
active
06778232
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing an active matrix substrate employed in a liquid crystal display device, and more particularly to a method for manufacturing a Thin Film Transistor (TFT) substrate provide with a Color Filter (CF).
2. Description of the Prior Art
In a conventional twisted nematic (TN) type liquid crystal color display device, a liquid crystal is interposed between a TFT substrate and a Color Filter (CF) substrate. In such a liquid crystal display device, a black matrix is generally provided on the CF substrate to prevent degradation of images to be displayed. Taking into account misalignment between the CF substrate and the TFT substrate, the black matrix has to be formed wide enough to securely prevent a light from leaking through the liquid crystal. Accordingly, an aperture ratio of liquid crystal display device becomes small and transmittance thereof becomes low.
As one of technologies for enlarging an aperture ratio to solve the above-stated problems, the Japanese Patent Application No. 10-351637 (hereinafter, referred to as a conventional example) discloses a method for manufacturing a color filter on a TFT (the TFT substrate provided with a Color Filter).
FIGS. 1 and 2
are cross-sectional views of a TFT substrate provided with a CF in which a TFT is protected by a passivation film and used as a switching element, illustrating steps for manufacturing the TFT. The structure of a TFT provided with a CF will be described with reference to
FIGS. 1 and 2
.
First, a TFT
160
in which a channel thereof is formed by etching is formed on a transparent insulating substrate
51
, and an entire surface of the substrate including the TFT
160
is covered by a passivation film
58
. The passivation film
58
is formed by, for example, depositing a silicon nitride using plasma CVD (FIG.
1
A).
Next, a negative type photocurable color resist obtained by dispersing a red pigment in an acryl resin is spin-coated on the transparent insulating substrate
51
. The rotational speed of a spinner is adjusted to make a film thickness of the resist about 1.2 &mgr;m. Then, the substrate
51
having the resist formed thereon is heated on a hot plate for two minutes at a temperature of 80° C. in a pre-baking step, and exposed and further, developed in a TMAH solution (tetramethylammonium hydroxide) to form a red color filter
163
in an associated portion on the substrate
51
(FIG.
1
B). In this case, the red color filter
163
is formed such that the red color filter is not formed on a part
62
of the passivation film
58
in which part a third opening will be formed later in the passivation film. Then, the substrate
51
is baked in a clean oven for 60 minutes at a temperature of 220° C. to cure the red color filter
163
.
Thereafter, a green color filter
263
is formed in another pixel, in which a color filter other than the red color filter is to be formed, in accordance with the same manner as in the case where the red color filter has been formed. The substrate
51
is baked in an oven for 60 minutes at a temperature of 220° C. to obtain the green color filter
263
. A blue color filter
363
is also formed in accordance with the same manner as in the case where the red color filter has been formed.
Subsequently, after completion of formation of color filters, a black matrix
64
is formed (FIG.
1
C). The black matrix
64
is formed of a resin made of a carbon or a pigment dispersed in an acryl resin. For example, such a material having a viscosity of about 20 cp is spin-coated on the transparent insulating substrate
51
to have a film thickness of about 1.5 &mgr;m and then, the material is developed. In this case, the black matrix is not formed on a portion of the substrate
51
in which a contact through hole will be formed in a later step.
An overcoat layer
65
is coated to flatten the surface of the substrate
51
and developed to have a first opening
66
therein. The substrate
51
is baked for 60 minutes at a temperature of 220 to 230° C. to cure the overcoat layer
15
. In this case, the overcoat layer is melted by the baking to have a cross sectional profile shaped like an arch having a large curvature (FIG.
2
A).
Then, a novolac-type photoresist
67
is coated and patterned to have a second opening
68
therein. Thereafter, the passivation film
58
is etched using the novolac type photosensitive resist
67
as a mask, thereby forming a third opening
69
in the passivation film
58
(
FIG. 2B
)
After completion of formation of the overcoat layer
65
and third opening
69
, the novolac-type photoresist
67
is removed and a transparent conductive film to serve as a pixel electrode is formed covering the aforementioned components by sputtering a transparent material, and then, the transparent conductive film is patterned to form a pixel electrode
70
(FIG.
2
C). In this case, when the transparent conductive film is formed thicker, the pixel electrode
70
can more securely cover the associated portions to thereby achieve stable electrical connection between the pixel electrode
70
and the drain electrode
57
. However, in consideration of ease of operation for processing an ITO (indium-tin-oxide) film, it is preferable to deposit the ITO (indium-tin-oxide) film to a film thickness of about 100 nm.
According to this conventional example, the novolac type photosensitive resist is coated on the overcoat layer shaped like an arch and patterned to form an opening in the passivation film through the patterned resist, so that the pixel electrode and the drain electrode can be connected through the opening. In this case, the second opening
68
is designed to include the first opening
66
therein while having 1 &mgr;m alignment allowance with respect to the first opening
66
. However, actually in an alignment step, the second opening
68
is occasionally positioned inside the first opening
66
owing to variation caused during manufacturing process. This phenomenon makes an inner wall surface of the second opening
68
of the novolac type photosensitive resist stand in a direction substantially vertical to the surface of the substrate
51
along the first opening
66
of the overcoat layer at an interface between the resist and the passivation film. Therefore, in this portion, a cross sectional profile of the third opening of the passivation film is substantially vertical to the surface of the substrate
51
, contributing to degradation of cross sectional profile of the pixel electrode along the third opening
69
and then, unfavorably generating an unstable connection resistance between the pixel electrode and the drain electrode.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for manufacturing an active matrix substrate, in more detail, a TFT substrate having a color filter thereon, the method allowing connection between a source/drain electrode and a pixel electrode connected thereto to stably become low.
A method for manufacturing an active matrix substrate constructed in accordance with the present invention can be performed in the following steps. That is, a thin film transistor and a wiring are formed on a transparent insulating substrate, and a protective film covering the thin film transistor and the wiring are formed on the transparent insulating substrate, and then, a first photosensitive film is formed on the protective film exposed therefrom, the first photosensitive film having a first opening to expose a portion of the first region of the protective film. Thereafter, a second photosensitive film is formed on the first photosensitive film with a second opening positioned inside the first opening to expose a part of the protective film and a third opening is formed in the protective film. In this case, the third opening is formed by removing the part of the protective film to expose a portion of the wiring by using the second photosensitive film as a mask such that an edge of the third opening is apart from an inner wall of the firs
Ishino Takayuki
Nakata Shinichi
Yamamoto Yuji
Fureman Jared J.
NEC LCD Technologies Ltd.
Trail Allyson N.
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