Fabrication method of liquid crystal display device

Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Including integrally formed optical element

Reexamination Certificate

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C438S029000, C438S071000

Reexamination Certificate

active

06376271

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a fabrication method of a liquid crystal display device, and particularly relates to a fabrication method of a liquid crystal display device of a pixel-on-passivation (POP) structure.
BACKGROUND OF THE INVENTION
Since an aperture ratio of unit pixels composing a liquid crystal display (LCD) device directly relates to brightness of a display per se, a high aperture ratio of an LCD device has been sought for conventionally. One example of means to achieve this is to make an LCD device in a POP structure as follows: as shown in FIG.
10
(
c
), an interlayer insulating film
104
is provided between a glass substrate
101
having active elements (switching elements) such as TFT (thin film transistors)
102
and pixel electrodes
103
b
(indicated by alternate long and short dash lines in FIG.
10
(
a
)), and each pixel electrode
103
b
is connected with each lower layer electrode
105
as a transparent electrode made of ITO (indium tin oxide) or the like via a contact hole
106
. In the case of the LCD device of the foregoing POP structure, regions extending to above the signal lines shown in FIG.
10
(
a
) (gate signal lines
122
and source signal lines
121
) can be used as pixel regions. Accordingly, this device has a higher aperture ratio as compared with that of a non-POP-structure LCD device shown in FIG.
10
(
b
) having a pixel electrode
103
a (indicated by alternate long and two-short dashes lines in FIG.
10
(
a
)).
Incidentally, FIG.
10
(
a
) is a plan view illustrating a region on a pixel substrate (back substrate) corresponding to one pixel and its surroundings, the pixel substrate being a substrate on which pixel electrodes are provided. The pixel substrate is a substrate on which pixels are formed. The figure illustrates the pixel electrode
103
a
of the non-POP structure and the pixel electrode
103
b
of the POP structure together, for comparison.
In order that the pixel electrode
103
b
should have a light diffusing property, fine projections and recesses are formed on a surface of the interlayer insulating film
104
as shown in FIGS.
11
(
a
) and
11
(
b
), and moreover, the pixel electrode
103
b
is formed as a reflection electrode by using a high-reflection material such as aluminum. Consequently, an LCD device of reflection type having a high aperture ratio and not undergoing parallax can be realized.
Incidentally, FIG.
11
(
a
) is a plan view of a pixel substrate provided with reflection electrodes on which projections and recesses are formed (contact holes are not shown), while FIG.
11
(
b
) is a cross-sectional view of a region of the pixel substrate corresponding to one pixel.
Furthermore, a hybrid-type LCD device as shown in FIGS.
12
(
a
) and
12
(
b
), that is capable of reflection-type display and transmission-type display both, has been also developed. To form the foregoing LCD device, projections/recesses regions (reflection regions)
107
and regions (transmission regions)
108
from which the interlayer insulating film
104
is removed are simultaneously formed, so that, in the pixel electrode
103
b,
a high-reflection material such as aluminum is applied in the reflection regions
107
, while the lower layer electrodes
105
functioning as the transparent electrodes are used as transmission regions
108
.
The foregoing interlayer insulating film
104
is required to possess the following characteristics:
(i) a sufficient film thickness;
(ii) a small variation of the film thickness in one substrate; and
(iii) good processibility.
Examples of the such interlayer insulating film
104
include an inorganic film made of SiNx or SiO
2
, and a photosensitive organic film (photosensitive resin) such as a photoresist, but the inorganic film made of SiNx or SiO
2
is difficult to be formed thick and to be processed. Therefore, it is substantially impossible to adopt the inorganic film in a reflection-type LCD device that requires shape-regulated fine projections and recesses to obtain a desired light diffusing property.
On the other hand, since the contact holes
106
and recessions and processions can be formed with respect to the foregoing photosensitive organic film by photolithography process, the photosensitive organic film is often adopted as the interlayer insulating film
104
in the LCD device of the POP structure.
However, by the foregoing conventional method, problems mentioned below arise as to (1) layer thickness distribution of the interlayer insulating film, (2) paralytic capacitance, (3) fabrication of a reflection-type LCD device, and (4) fabrication of a hybrid-type LCD device.
(1) Layer Thickness Distribution of Interlayer Insulating Film
FIGS.
13
(
a
) through
13
(
e
) illustrate a typical fabrication process of an LCD device of the POP structure. Note that TFTs and signal lines are omitted in FIGS.
13
(
a
) through
13
(
e
), so as to avoid complexity of illustration. The method of fabricating an LCD device of the POP structure is as follows.
(i) A photosensitive resin film is formed by spin-coating as the interlayer insulating films
104
on the glass substrate
101
on which the lower layer electrodes
105
are formed(see FIG.
13
(
a
)).
(ii) The foregoing photosensitive resin film is exposed so as to be formed into the interlayer insulating films
104
by means of a photo-mask
110
, so that the contact holes
106
for connecting the foregoing lower layer electrodes
105
and the pixel electrodes
103
b
that will be formed by a later step (see FIG.
13
(
b
)).
(iii) The interlayer insulating film
104
is completed through development and baking (see FIG.
13
(
c
))
(iv) An ITO film for formation of the pixel electrodes
103
b
is formed on the foregoing interlayer insulating film
104
(see FIG.
13
(
d
)).
(v) The pixel electrodes
103
b
are formed by shaping the foregoing ITO film into a predetermined shape (see FIG.
13
(
e
)).
In the foregoing step (i) shown in FIG.
13
(
a
), the interlayer insulating film
104
is formed by spin-coating. The advantage of spin coating is that it allows a relatively uniform film thickness to be easily obtained. But, since a photosensitive resin material containing a solvent is applied, a phenomenon like “dried state irregularity” occurs when the solvent vaporizes. This phenomenon of “dried state irregularity” becomes more remarkable as the interlayer insulating film
104
is thicker.
Furthermore, according to the operational principle of the spin coater, the film thickness of a peripheral interlayer insulating film
104
a
on the periphery of the substrate tends to become thicker than the film thickness of a central interlayer insulating film
104
b
on a central part of the substrate as shown in FIG.
14
(
a
), due to influences of surface tension and the like. Consequently, layer thickness distribution occurs to a certain extent in a single substrate.
Descriptions about effects achieved by thickening an interlayer insulating film are omitted here since the “Paralytic Capacitance” section, below, will mention the same, but the following three methods are deemed applicable to laminate a material of the interlayer insulating film or the like thicker by spin coating:
(i) to decrease the rate of rotation of the coater;
(ii) to apply the material repeatedly; and
(iii) to increase the viscosity of the material to be applied.
In the case of a coating process using a spin coater, a uniform film thickness is achieved by rotating a substrate, while a solvent mixed in a material is vaporized. Usually, greater effects are obtained as the substrate is rotated at a higher rate. Therefore, in the case where a film is formed by rotating the same at a lower rate, the effects decrease. For this reason, it is difficult to apply the foregoing method (i) to the process for fabricating an LCD device such as the LCD device of the POP structure typically in which “the material to be applied remains in the LCD device at the final stage”.
Problems of the method (ii) are eloquent: repetition of the sequence of “coating-photolithography-baking” l

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