Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-11-17
2001-06-12
Dudek, James A. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000
Reexamination Certificate
active
06246458
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing a liquid crystal display (LCD).
2. Description of the Related Art
Flat panel displays such as LCD, organic electroluminescence (EL), or plasma displays have been enthusiastically developed and commercialized in recent years. Particularly, LCDs has become a main display for office automation (OA) devices and audio visual (AV) devices because LCDs have attractive features such as thin and low power consumption. Especially, active matrix LCDs having thin film transistors (TFTs) as switching elements for controlling a timing to rewrite pixel data into each pixel enable a wide screen and animation display with a high resolution, and have become widely used in various television sets, personal computers, mobile computers, and monitors for digital still and video cameras.
A TFT is a kind of field effect transistor (FET) made of metal and semiconductor layers formed in a predetermined pattern on an insulated substrate. In an active matrix LCD, each TFT is connected to a corresponding capacitor for driving the liquid crystal disposed between a pair of substrates; the capacitor is constructed between the substrates.
FIG. 1
is an enlarged plan view of a display pixel portion of an LCD, and
FIG. 2
is a cross section of the LCD along B—B line shown in FIG.
1
. On the substrate
50
a gate electrode
51
is formed that is made of Cr, Ti, Ta, or another suitable metal, over which a gate insulating film
52
is formed. On the gate insulating film
52
an amorphous silicon, i.e., a-Si film
53
is formed in an island shape so as to cross over the gate electrode
51
. On the a-Si film
53
an N
+
a-Si film
53
N is formed, each end of which is doped with impurities so as to make an ohmic layer. Above the channel region of the a-Si film
53
, an etch stopper
54
is remained. On the N
+
a-Si film
53
N a drain electrode
56
and a source electrode
57
are formed, over which an interlayer insulating film
58
is formed. On the interlayer insulating film
58
a pixel electrode
59
that is made of indium tin oxide (ITO) or Al is formed, which is connected to the source electrode
57
via a contact hole formed in the interlayer insulating film
58
. On the pixel electrode
59
an alignment film
71
made of polyimide or the like is formed, and is processed by rubbing treatment as shown in FIG.
3
. In this way, the TFT substrate is manufactured.
On another substrate
60
facing the TFT substrate
50
, red (R), green (G), and blue (B) color filters
61
are formed, each of which is made of a film resist and is disposed at a position corresponding to each pixel electrode
59
. In addition, a black matrix
61
BM which is made of a light shielding film resist is formed at a position corresponding to a gap between the pixel electrodes
59
and at a position corresponding to the TFT. On the layers of these color filters
61
and the black matrix
61
BM, a common electrode
62
made of ITO is formed. On the common electrode
62
an alignment film
72
is formed and is processed by rubbing treatment in the same way as on the substrate
50
. In this way, the opposing substrate is manufactured.
Between the TFT substrate
50
and the opposing substrate
60
, a liquid crystal layer
80
is disposed. The orientation, i.e., the alignment of the liquid crystal molecules
81
is controlled in accordance with an intensity of an electric field formed by a voltage applied between the pixel electrodes
59
and the common electrode
62
. Outsides of the substrates
50
and
60
polarizing films (not shown) with perpendicular polarizing axes are provided. Linear polarized light passing through these polarizing films is modulated when passing through the liquid crystal layer
80
that is controlled in different alignments per each display pixel, and is thereby controlled in a desired transmittance.
FIGS. 4A-4E
show a method for manufacturing the opposing substrate. First, in the step shown in
FIG. 4A
, the R, G and B color filters
61
R,
61
G and
61
B are formed on the substrate
60
. In order to form the R color filter
61
R, an R film resist is affixed, which is then exposed and developed in the shape corresponding to the R display pixels. The G color filter
61
G and the B color filter
61
B are formed in a similar manner. These color filters
61
R,
61
G, and
61
B are formed in dimensions slightly smaller than those of the corresponding display pixels
59
shown in FIG.
2
.
In the next step shown in
FIG. 4B
, a light shielding film resist
61
BM′ is affixed, and is followed by the step shown in
FIG. 4C
, in which the film resist is exposed and developed in the shape corresponding to the gap between the pixels so that the black matrix
61
BM is formed among the color filters
61
R,
61
G and
61
B. This black matrix
61
BM is formed in a dimension larger than the gap between the pixel electrodes
59
shown in FIG.
2
.
In the next step shown in
FIG. 4D
, the ITO film is formed so as to produce the common electrode
62
. In addition, in the step shown in
FIG. 4E
, a polyimide film is formed by a printing method. Then, the polyimide film is dried by baking, and processed by rubbing treatment. The film is rubbed in the arrow direction with a cloth so as to make the alignment film
72
for giving the pretilt to the liquid crystal.
In the above-mentioned example, the liquid crystal has a negative dielectric constant anisotropy. The alignment films
71
and
72
are vertical alignment films that control the initial alignment of the liquid crystal in the direction perpendicular to the substrate. In this case, when a voltage is not applied, the linear polarized light that passed through one of the polarizing films is blocked by the other polarizing films after passing through the liquid crystal layer
80
so that the display is recognized as black. When the voltage is applied, the linear polarized light that passed through one of the polarizing films is double refracted by the liquid crystal layer
80
to become an elliptically polarized light, which passes the other polarizing films so that the display is recognized as nearly white. This type is called a normally black (NB) mode. Particularly, the vertical alignment films
71
and
72
are processed by the rubbing treatment, so that the initial directions of the liquid crystal molecules
81
are aligned in the direction with a slight pretilt from the normal direction. This pretilt angle &thgr; is normally set to more than one degree, but equal to or less than five degrees. The liquid crystal molecule
81
is electrically uniaxial. The angle between the axial direction and the direction of the electric field is determined by the electric field strength, while the azimuth with respect to the direction of the electric field is not controlled. The liquid crystal molecule
81
having the negative dielectric constant anisotropy tilts in a direction different from the electric field direction. However, by providing pretilt, an applied voltage can make the liquid crystal molecule
81
tilt toward the pretilt direction. Therefore, the tilt directions are aligned so that a variation of alignments of the liquid crystal in a plane can be suppressed and deterioration of the display quality can be prevented.
The black matrix
61
BM is provided for preventing a drop of the contrast ratio due to undesired light that is emitted from the display by the birefringence of the liquid crystal with the pretilt and passes through the liquid crystal layer
80
in a region in which the voltage is not applied between the display pixels.
The liquid crystal having a negative dielectric constant anisotropy changes the alignment of its molecules upon the electric field in such a way that the alignment becomes perpendicular to the direction of the electric field. On this occasion, the liquid crystal generates an action opposing the generated electric field. Generally, however, such a change of the orientation from the vertical alignment of the liquid crystal is
Koma Norio
Omura Tetsuji
Yoneda Kiyoshi
Dudek James A.
Hogan & Hartson LLP
Sanyo Electric Co,. Ltd.
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