Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
2000-01-18
2001-08-21
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
38, 38, 38
Reexamination Certificate
active
06278503
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to improvement of image display in a liquid crystal display having a storage capacitor (referred to as “SC” hereinafter).
2. Description of the Related Art
Vertical orientation type liquid crystal displays using liquid crystal with negative dielectric constant anisotropy and vertical orientation films have been developed. Devices of this type can be classified into two groups.
Devices in the first type use a vertical orientation film that has been treated by rubbing processing.
FIG. 1A
is a plan view showing an example of this type, and
FIG. 1B
shows a cross-sectional view taken along line
1
B—
1
B of FIG.
1
A. Gate lines
51
are formed on a first substrate
50
, and a gate insulating film is formed covering the gate lines
51
. Each gate line
51
includes a gate electrode
52
within a portion of a pixel. Storage capacitor electrodes (SC electrodes)
53
composed of amorphous silicon (a-Si) film are formed in discrete islands in an overlying layer so as to cross over the gate electrode
52
. The SC electrode
53
is doped with impurities, and, together with the gate electrode
52
, forms a thin-film transistor (TFT). These layers are covered by an interlayer insulating film
54
. A pixel electrode
55
composed of ITO (indium tin oxide) is formed on the interlayer insulating film
54
, and is connected to the SC electrode
53
via a contact hole opened in the interlayer insulating film
54
. Although such a contact actually is not present in this cross-section, contact is shown in the cross-sectional view illustrated in
FIG. 1B
shows this contact to help understanding. In the next overlying layer, a vertical orientation film
56
is formed. The vertical orientation film
56
has been treated by rubbing processing. The interlayer insulating film
54
is composed of two layers, and a data line
57
is disposed in the middle of the interlayer insulating film
54
. The data line
57
is connected to a source region of the TFT and supplies electric charge to the SC electrode
53
and the pixel electrode
55
when the TFT is turned on. The data line
57
is formed in a position underneath the pixel electrode
55
so as to form a vertical overlap.
On a second substrate
60
opposing the first substrate
50
, a common electrode
61
composed of ITO and other materials is formed covering the plurality of pixel electrodes
55
. Over the common electrode
61
, a vertical orientation film
62
identical to the one disposed on the first substrate
50
is deposited and treated by rubbing processing.
Liquid crystal
70
is sealed between the first substrate
50
and the second substrate
60
. The orientation of the liquid crystal molecules are controlled according to the strength of the electric field generated by a voltage applied between the pixel electrode
55
and the common electrode
61
. On the outboards of the first substrate
50
and the second substrate
60
, polarizing plates (not shown) are arranged such that their polarization axes are perpendicular to one another. The linearly polarized light passing between the polarizing plates is modulated while passing through the liquid crystal
70
controlled in different orientations in the respective display pixels, and is thereby controlled to a desired transmittance.
The liquid crystal
70
has negative dielectric constant anisotropy, i.e., its molecules tend to tilt towards the direction of the electric field. The vertical orientation films
56
,
62
control the initial orientation of the liquid crystal in the vertical direction. Accordingly, when no voltage is applied, the liquid crystal molecules are oriented vertically with respect to the plane of the vertical orientation films
56
,
62
, and linearly polarized light passing one of the polarizing plates passes through the liquid crystal layer
70
but is then obstructed by the other polarizing plate, resulting in a black display. When a voltage is applied, the molecules of the liquid crystal
70
align in the rubbing axis. Consequently, the linearly polarized light that passed one of the polarizing plates is subjected to birefringence in the liquid crystal layer
70
, is changed into an elliptically polarized light, and passes through the other polarizing plate. The display then approaches white. When both the gate line
51
and the data line
57
are turned on, a voltage is applied to the pixel electrode
55
via the TFT, and the liquid crystal positioned directly above the pixel electrode
55
is driven. An image is generated on the LCD by the application an independent voltage to each of the pixel electrodes
55
. In other words, a region in which a pixel electrode
55
is formed is defined as a pixel.
A light-blocking black matrix (not shown) is formed in regions other than pixels, i.e., gaps between the pixel electrodes
55
and the regions constituting the TFT including the SC electrodes
53
. The black matrix is disposed to prevent white inter-pixel regions from reducing contrast. When light transmitted through one of the polarizing plates and coming into the liquid crystal layer is subjected to birefringence when passing through the pre-tilted crystal, the black matrix prevents undesired light from irradiating through the other polarizing plate in the inter-pixel regions where no voltage is applied.
The function of the SC electrode
53
is next explained. In the LCD, a voltage is applied between the pixel electrode
55
and the common electrode
61
, and transmittance is controlled by orienting the liquid crystal using the electric field generated by the applied voltage, as described above. However, as liquid crystal is not an absolute insulator, a slight current flows when the voltage is applied to the pixel electrode. Consequently, the electric charge stored in the pixel electrode
55
becomes discharged, and the voltage between the pixel electrode
55
and the common electrode
61
cannot be maintained. To solve this problem, a storage capacitor (SC) line
58
made of chromium or similar material is disposed to form a storage capacitor together with the SC electrodes
53
in the portions overlapping the SC electrodes
53
, thereby supplying electric charges to the pixel electrode
55
. The SC line
58
is formed to have a large width at the portion
58
a
opposing the SC electrode
53
to provide a large capacitance together with the SC electrode
53
.
FIG. 2
shows an equivalent circuit including a pixel, a storage capacitor, a data line, and a gate line. The capacitor constituted by the liquid crystal
70
interposed between the pixel electrode
55
and the common electrode
61
, and the storage capacitor constituted by the SC electrode
53
and the SC line
58
, are connected to the data line
57
via the TFT including the gate electrode
52
.
In the second type of vertical orientation type LCD, the vertical orientation film is not treated by rubbing processing. Instead, the vertical orientation type LCD of the second type comprises a separate orientation control means for controlling the liquid crystal orientation. A vertical orientation type LCD having orientation control windows for controlling orientation is proposed in the commonly assigned Japanese Patent Application No. H05-84696 (JPA H06-301036), for example.
FIG. 3A
is a plan view illustrating an LCD having orientation control windows, and
FIG. 3B
is a cross-sectional view taken along line
3
B—
3
B of FIG.
3
A. The LCD of
FIGS. 3A and 3B
coincides with the LCD of
FIGS. 1A and 1B
in that an SC electrode
53
forming a TFT and a pixel electrode
55
connected to the SC electrode are provided on a first substrate
50
which together with a substrate
60
seals liquid crystal
70
, and that polarizing layers are disposed on the outboard. Structures of
FIGS. 3A and 3B
that correspond to structures of the LCD of
FIGS. 1A and 1B
are labeled with corresponding reference numerals and their explanations will not be repeated. The LCD of
FIGS. 3A and 3
Miyajima Yasushi
Nishikawa Ryuji
Hogan & Hartson LLP
Parker Kenneth
Qi Mike
Sanyo Electric Co,. Ltd.
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