Color liquid crystal display panel, manufacturing method of...

Liquid crystal cells – elements and systems – With specified nonchemical characteristic of liquid crystal... – Within nematic phase

Reexamination Certificate

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Reexamination Certificate

active

06690446

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a color liquid crystal display panel and a manufacturing method of the same, and more particularly to a color liquid crystal display panel (hereinafter, referred to as CF-on-TFT panel) fabricated by forming a black matrix and a color filter on a TFT (Thin Film Transistor) substrate of the color liquid crystal panel made of a transparent insulating substrate having formed thereon a plurality of thin film transistors using amorphous silicon and a manufacturing method of the same. The present invention also relates to a liquid crystal display employing the CF-on-TFT panel as a color liquid crystal display panel.
2. Description of the Related Art
FIG. 1
is a cross section of a conventional liquid crystal display panel. As shown in the drawing, the conventional liquid crystal display panel is fabricated by laminating (1) a first substrate
1
(hereinafter, also referred to as TFT substrate) having switching elements such as TFTs (Thin Film Transistors) each composed of a gate electrode
2
, a gate insulating film
4
, a semiconductor layer
5
, a source electrode
6
, and a drain electrode
7
, a wiring layer (not shown) for each of the electrodes
2
,
6
, and
7
, a pixel electrode
12
in each pixel provided in a one-to-one correspondence to the TFT, a passivation film
8
covering all the foregoing components, an alignment film
18
a
, and a terminal
3
used as an electrical connection to an external circuit, and (2) a second substrate
16
having a black matrix
9
, color filters
11
R,
11
G(not shown), and
11
B respectively for three colors of R(Red), G(Green), and B(Blue), a transparent common electrode
17
such as ITO (Indium Tin Oxide), and an alignment film
18
b
to each other through a sealing material
19
applied at the circumference portion of the substrates
1
and
16
, thereby assembling a panel with their respective film forming surfaces opposing each other and spherical spacers
20
being provided in between to maintain the gap between the substrates
1
and
16
at a constant distance, and filling a liquid crystal material
21
in the panel after baking the seal.
Known as a liquid crystal filling method for the liquid crystal material
21
are two-holes method and vacuum filling method. The former method comprises the steps of providing two holes through the panel at predetermined positions, and filling a liquid crystal material
21
in through one of the holes while evacuating the panel through the other, so that the liquid crystal material
21
is drawn into the panel. The latter method comprises the steps of providing an empty panel with one filling port, placing the liquid crystal material
21
in a vacuum (1×10
−2
to 1×10
−4
Torr) to allow the liquid crystal material
21
to adhere to the filling port, and subsequently restoring the pressure gradually to atmospheric pressure, so that the liquid crystal material
21
is filled in the panel by a pressure difference between the interior and exterior of the liquid crystal panel. Presently, the latter method is used in most of the cases.
Next, after the liquid crystal is filled, the filling port is sealed. Then, polarizing plates
24
a
and
24
b
are laminated to the outside surfaces of the substrates
1
and
16
, respectively, whereby a liquid crystal panel is completed, which can be irradiated by light emitted from the back light through the first substrate
1
and second substrate
16
.
In order to achieve high definition, the liquid crystal panel of this type has to increase the density of the pixels. However, because the conventional liquid crystal panel having the color filter
11
and black matrix
9
formed on the counter substrate causes an error in the position alignment during the fabrication procedure, allowance has to be made, which makes it difficult to secure the area (numerical aperture) of the pixel opening as large as possible.
In contrast, a method of forming the color filter and black matrix on the active matrix substrate having thereon formed the switching elements such as TFTs, that is, a so-called CF(Color Filter)-on-TFT, is proposed in Japanese Patent Laid-open Publication Nos. Hei 8-122824 and 9-292633.
In either case of these publications, because the color filter and black matrix are formed on the CF-on-TFT substrate, no allowance is necessary for the position alignment, thereby simplifying the manufacturing procedure while increasing the numerical aperture of the pixel.
With the panel of the CF-on-TFT structure, however, reflection of external light is greater than the conventional liquid crystal display panel, and therefore, there occurs a problem that the display quality deteriorates under bright external light circumstances.
FIGS. 2A and 2B
are explanatory views showing external light reflection mechanisms of liquid crystal display panels. The above problem will be explained with reference to
FIGS. 2A and 2B
. Because ITO has the larger refractive index (n) (n=approx. 2.0) than those of the alignment films
18
a
and
18
b
(n=approx. 1.6) and glass substrate (n=approx. 1.4), the reflected components of external light are mainly the reflection from the ITO on the counter substrate and the reflection from the ITO of the pixel electrode
12
.
Also, reflection from the scanning lines or signal lines is noticeable. As shown in
FIG. 2A
, because reflected light passes through the RGB color filter
11
twice in the conventional TFT and CF separation type liquid crystal display panel, reflected light attenuates in a satisfactory manner. On the contrary, as shown in
FIG. 2B
, reflected light does not attenuate with the color filter
11
in the liquid crystal display panel of the CF-on-TFT structure, and a quantity of reflected light is greater than that of reflected light from the conventional panel.
In particular, the panel of the CF-on-TFT structure has a problem that reflected light of green light having the wavelength &lgr;=550 nm or so is especially noticeable under bright external light circumstances.
Incidentally, a reflection preventing film is generally provided to reduce the reflection, and Japanese Patent Laid-open Publication Nos. 6-214252 and 10-154817 disclose a technique to take out reflected light effectively by providing the reflection preventing film over and beneath the counter electrode ITO, so that reflection efficiency of the liquid crystal light bulb of the reflecting type is increased.
In this case, however, the reflection preventing film is used to take out the reflected light effectively, and for this reason, dependency on the location of the reflected light, namely the display image, becomes so great that high-quality display performance cannot be achieved unless the refractive index and film thickness of the reflection preventing film are controlled accurately. Therefore, the reflection preventing film has to be made of an inorganic material by means of sputtering or the like, which poses a problem that an additional step is added to the manufacturing procedure of the liquid crystal display.
As has been discussed and shown in
FIG. 2B
, with the liquid crystal display panel of the CF-on-TFT structure having the color filter
11
formed on the TFT substrate, reflection of external light from ITO on the counter substrate and the ITO of the pixel electrode
12
, or reflection from the signal lines and scanning lines is noticeable in comparison with the conventional TFT and CF separation type liquid crystal display panel shown in
FIG. 2A
, and in order to meet recent demand of high-quality image of the high density structure, deterioration of the image quality caused by the reflection particularly from the scanning lines and signal lines has to be prevented.
Further, in providing the reflection preventing film over and beneath ITO to prevent the deterioration of the image quality, a film of an inorganic material having predetermined refractive index and film thickness has to be formed by means of sputtering or the

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