Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-01-11
2004-02-17
Chowdhury, Tarifur R. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S043000, C349S106000, C349S110000
Reexamination Certificate
active
06693697
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active-matrix type liquid crystal display device and manufacturing method thereof. More particularly, the present invention relates to an active-matrix type liquid crystal display device of CF on TFT structure of forming both a switching element such as TFT (Thin Film Transistor) and so forth and CF (color filter) on the same substrate and its manufacturing method.
2. Description of the Related Art
Recently, development of active-matrix type liquid crystal display device of using thin film transistor and so forth as switching element is advanced. The active-matrix type liquid crystal display device is composed of TFT substrate on which switching element such as thin film transistor and so forth are formed, opposed substrate on which opposed electrode is formed, and liquid crystal put between these substrates. The TFT substrate has thin film transistor consisting of gate electrode, gate insulating film, semiconductor layer, and source/drain electrode, pixel electrode formed in every pixel, passivation film covering them, orientation film, and terminals for connecting external circuit, and so forth. In addition, the opposed substrate has black matrix for interrupting incident light heading toward thin film transistor area and wiring layer, color filter of each color of RGB (Red, Green and Blue) of conducting color display, transparent electrode made from ITO (Indium Tin Oxide) and so forth and orientation film and so forth. Further, spacers for keeping gap between both substrates by predetermined distances are put between both substrates.
About such active-matrix type liquid crystal display device, in order to improve display appearance quality, high precision fine display is required. For that reason, it is necessary to achieve pixel with high density. However, in the liquid crystal display device of structure, in which above described color filter and black matrix are arranged at the side of opposed substrate, since error occurs on position matching between both substrates in assembly process, it is necessary to form color filter and black matrix somewhat largely in anticipation of margin beforehand. For that reason, it is difficult to make rate of area of opening section to area of pixel large, namely it is difficult to make opening rate large, thus it becomes obstruction for the pixel with high density.
Accordingly, in order to improve opening rate while reducing margin of color filter and black matrix, method for forming color filter and black matrix at TFT substrate side of forming switching element such as thin film transistor and so forth, so called CF on TFT is proposed. For instance, Japanese Patent Application Laid-Open No. Hei 2-54217 and Japanese Patent Application Laid-Open No. Hei 3-237432 disclose CF on TFT.
In CF on TFT, since color filter and black matrix are formed on the TFT substrate, it is unnecessary to take into consideration position matching margin between the TFT substrate and the opposed substrate. Accordingly, it is possible to simplify manufacturing process and it is possible to achieve improvement of opening rate of pixel.
However, in CF on TFT, since pixel electrode is formed on color filter, step occurs at pixel electrode while reflecting unevenness of color filter and so forth. Then, disturbance occurs in orientation of liquid crystal caused by the step, so, there is the problem that the step causes defects such as disclination and reverse tilt domain and so forth.
In order to solve the problem, Japanese Patent Application Laid-Open No. Hei 8-122824 discloses method for forming flattened film after conducting patterning of color filter and black matrix in order to fill unevenness of color filter and black matrix.
FIG. 1
is a sectional view illustrating configuration of conventional CF on TFT described in Japanese Patent Application Laid-Open No. Hei 8-122824. It should be noted that the Japanese Patent Application Laid-Open No. Hei 8-122824 discloses technique of using polycrystal silicon TFT (p-Si TFT) as switching element, however, in the present specification, for convenience of explanation, explanation is made that channel-etch type amorphous silicon TFT (a-Si TFT) is used as switching element.
As illustrated in
FIG. 1
, in liquid crystal display device described in the Japanese Patent Application Laid-Open No. Hei 8-122824, a gate electrode
5
b
is formed on a transparent insulative substrate
4
a
, and a gate insulator
6
is formed so as to cover the gate electrode
5
b
. A semiconductor layer
15
is formed so as to overlap a gate electrode
5
b
with several times over on the gate insulator
6
. In addition, a source electrode
8
b
and a drain electrode
8
a
separated on the center section of the gate electrode
5
b
are connected to the semiconductor layer
15
through an ohmic contact layer (not illustrated), thus thin film transistor is formed. Further, a passivation film
9
is formed so as to cover the thin film transistor.
In the liquid crystal display device of CF on TFT structure, a color filter
10
and black matrix
11
are formed on the passivation film
9
, and a pixel electrode
14
is formed thereon through the overcoat layer. In the liquid crystal display device illustrated in
FIG. 1
, in order to flatten step formed by the color filter
10
and the black matrix
11
, a flattening film of thick film
24
is provided. For this measure, it is characterized in that the color filter
10
and the black matrix
11
are made to embed completely in the flattening film
24
. In addition, a contact hole
19
of penetrating the flattening film
24
and the passivation film
9
is formed, after that, the pixel electrode
14
made of transparent conductive film is formed, and the pixel electrode
14
is connected to the source electrode
8
b.
On the other hand, an opposed electrode
16
is formed on surface of the transparent insulative substrate
4
b
that stands opposite to the transparent insulative substrate
4
a
. In addition, liquid crystal
3
is filled between the transparent insulative substrate
4
a
and the transparent insulative substrate
4
b.
In formation process of CF on TFT substrate, it becomes necessary to form fine pattern of black matrix with high light shielding characteristic of degree of Optical Density (OD)=3 made of photosensitive resin as light shielding film of thin film transistor. It should be noted that OD is value defined as OD=−log
10
(T
1
/T
0
) when incident amount of light into black matrix is taken to be T
0
, and amount of projected light is taken to be T
1
. In this configuration, when black matrix is made to execute exposure, neighborhood of surface of black matrix is only executed exposure, so there is the problem that adhesion to base section becomes low.
Japanese Patent Application Laid-Open No. 2000-013571 discloses technique for forming fine pattern in such a way as to conduct exposure while providing photoconductive resin black matrix on color filter to be base section. According to this method, even though only surface neighborhood of high-OD black matrix layer is executed exposure, since adhesion is good between color filter to be base section and black matrix layer, it is possible to form fine pattern without separating black matrix from the base section.
However, there are problems indicated below in the above-described conventional technique. According to flattening technique described above, the flattening film
24
is applied so as to cover a step formed at TFT substrate. Generally, film thickness of the black matrix
11
and the color filter
10
is degree of 1 to 2 &mgr;m, thus step of degree of 2 to 3 &mgr;m is created when the black matrix
11
and the color filter
10
are overlapped. Accordingly, when the step is made to cover by the flattening film
24
, film thickness of degree of 1.5 times the step becomes necessary, so, film thickness of degree of 3 to 4.5 &mgr;m is necessary as the flattening film
24
. As a result, film thickness of the flattening film
Hidehira Masanobu
Horie Yoshitaka
Ishino Takayuki
Kimura Shigeru
Kuroha Shouichi
Chowdhury Tarifur R.
NEC LCD Technologies Ltd.
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