Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
1998-11-17
2003-05-13
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S130000
Reexamination Certificate
active
06563558
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), particularly to an LCD having thin film transistors for driving each pixel.
2. Description of the Related Art
Flat panel displays such as LCDs, organic electroluminescence (EL), or plasma displays have been enthusiastically developed and commercialized in recent years. Particularly, LCDs have become the most popular 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 employing 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 they are now widely used in television sets, personal computers, mobile computers, and monitors of 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
made of indium tin oxide (ITO) or Al is formed, and 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 between the color filters
61
without clearance 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
a common electrode
62
made of ITO is formed. On the common electrode
62
an alignment film
72
is formed in the same way as on the substrate
50
side and is processed by rubbing treatment as shown in FIG.
4
. 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 alignment per each display pixel, and is thereby controlled in a desired transmittance.
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 orientations 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. Thus, giving the pretilt angle and controlling the tilt direction of the liquid crystal molecule
81
to be aligned, 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 light passing due to a birefringence of the liquid crystal with the pretilt 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. However, in general, such a change of the alignment from the vertical alignment of the liquid crystal is not stable compared with the case a liquid crystal having a positive dielectric constant anisotropy such as a twist nematic (TN) liquid crystal changes from the horizontal alignment. Especially, unevenness of the alignment film
71
and
72
at the interface with the liquid crystal due to a step of the TFT or the color filter layer influences the alignment change of the liquid crystal, resulting in a deteriorated display quality.
Furthermore, as shown in
FIGS. 3 and 4
, the related art uses rubbing treatment of the vertical alignment film
71
and
72
in order to give the pretilt &thgr; to the initial alignment of the liquid crystal as shown in FIG.
2
. Therefore, when a voltage is applied, all the liquid crystal molecules
81
tilt in the direction of the pretilt (rightward in FIG.
2
). Accordingly, the tilt angle of the liquid crystal molecule
81
with respect to the optical path when viewing the LCD from upper right in
FIG. 2
is different from that when viewing the LCD from upper left, resulting in different transmittances. Thus, there is a problem that a brightness or a contrast ratio changes in accordance with a viewing direction. This is known as viewing angle dependence.
Furthermore, since the black matrix
61
BM formed on the opposing substrate
60
side should completely cover the gap region between the pixel electrodes, it is formed larger to allow for possible position shift when the black m
Hogan & Hartson LLP
Kim Robert H.
Nguyen Dung
Sanyo Electric Co,. Ltd.
LandOfFree
Liquid crystal display with light shielding film does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Liquid crystal display with light shielding film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Liquid crystal display with light shielding film will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3065347