Active matrix liquid crystal display device

Details Active matrix liquid crystal display device Active matrix liquid crystal display device

1999-10-27

2002-03-12

Ton, Toan (Department: 2871)

Liquid crystal cells, elements and systems

Particular structure

Having significant detail of cell structure only

C379S111000, C379S156000

Reexamination Certificate

active

06356330

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an active matrix liquid crystal display device. More particularly, the present invention relates to an active matrix liquid crystal display device which is configured in accordance with the IPS (In-Plane Switching) type (also called the lateral electric field type).
BACKGROUND OF THE INVENTION
In recent years, active matrix liquid crystal display devices employing active elements, represented by thin film transistors (TFTs), have been increasingly used as monitors for personal computers, workstations, and so on since they consume less power and have smaller sizes than CRT display devices while providing a high image quality equivalent to that of the CRT display devices.
One form of a liquid crystal display device suitable for monitor applications is an IPS-type active matrix liquid crystal display device. The liquid crystal display device of this type includes scanning wires, signal wires, common wires, and pairs of interdigitally formed electrodes (pixel electrode and opposing electrode) arranged on one of the two substrates, where a voltage is applied across the electrodes to drive liquid crystal. An electric field applied to the liquid crystal is substantially parallel to the surfaces of the substrates. The liquid crystal display device of IPS-type has a wider viewing angle than conventional liquid crystal display devices. This characteristic of the IPS-type liquid crystal display device makes itself more suitable for applications in direct-view type monitors.
FIG. 4
illustrates in plan view the structure of one pixel section in a conventional IPS-type active matrix liquid crystal display device as mentioned.
FIG. 5
illustrates the structure of
FIG. 4
in cross-sectional view taken along a line B-B′ in FIG.
4
.
Referring first to
FIG. 4
, the pixel section includes scanning wires
101
formed of Cr; a semiconductor layer
102
formed of amorphus silicon; a signal wire
103
formed of Cr; a pixel electrode
106
formed of Cr; opposing electrodes
107
and
107
′ formed of Cr; a common wire
401
formed of Cr; and a black matrix
402
.
The liquid crystal display device having the structure as illustrated includes a gap between the opposing electrode
107
and the signal wire
103
. An effective electric field to the pixel for display cannot be applied through this gap. In addition, this gap must be shielded because light is likely to leak from the gap due to a continuously changing voltage on the signal wire
103
.
Moreover, a gap between the scanning wire
101
and the common wire
401
must be shielded since light is likely to leak from the gap due to a direct current voltage applied at all times to these wires. Furthermore, gaps between the scanning wire
101
and ends of the opposing electrodes
107
,
107
′ must be shielded for the same reason. In addition, a thin film transistor (TFT) must be shielded over for preventing the TFT from malfunctioning due to a current possibly caused by leaked light. Thus, the liquid crystal display device has a low aperture ratio because the pixel section must be shielded by the black matrix
402
. Further, the existence of the common electrode
401
also contributes to the low aperture ratio of the liquid crystal display device.
As illustrated in
FIG. 5
, a section in which a TFT
216
is arranged presents an abruptly narrowing gap between a TFT substrate
214
and an opposing substrate
215
. This is because:
1) a thicker passivation layer
205
for the purpose of planarizing the TFT substrate
214
cannot be employed because of a resulting increase in a liquid crystal driving voltage; and
2) the black matrix
204
provided on the opposing substrate
215
in face of the TFT section cannot be omitted since the black matrix
204
is indispensable for preventing the TFT from malfunctioning due to a current generated by leaked light.
In the exemplary liquid crystal display device, a spacer bead (hereinafter simply called the “bead”)
211
positioned on the TFT section serves to define a cell gap. However, since the area of the TFT section is merely on the order of {fraction (1/100)} as much as an entire pixel, the beads
211
arranged on the TFT sections account for only {fraction (1/100)} or less of the entire area of the substrates. For this reason, in substrate regions without the TFT sections and accordingly not supported by the spacer beads
211
arranged therebetween, the cell gap suffers from non-uniformity which in turn causes a non-uniform display luminance.
Nevertheless, if an increased amount of beads
211
is dispersed in an attempt to make the cell gap more uniform, the number of beads increases not only on the TFT sections but also at openings. Thus, while the cell gap can be made more uniform, more light leaks near beads positioned at openings, resulting in a lower contract.
In the cross-sectional view of
FIG. 5
, the structure further comprises a glass substrate
201
; a gate insulating layer
202
; a contact layer
204
; an alignment film
206
; a glass substrate
207
; a color filter layer
208
; a protection film
209
which also serves as a planarizing film; an alignment film
210
; a liquid crystal layer
212
; and polarizing plates
213
,
217
.
SUMMARY OF THE INVENTION
When compared with a conventional TN-type liquid crystal display device, the IPS-type liquid crystal display device has the following two problems to be solved.
First, higher power consumption is required. This is because a low aperture ratio of the IPS-type liquid crystal display device requires higher power consumption to drive back light for providing the luminance equivalent to that of the conventional TN-type liquid crystal display device.
The low aperture ratio of the IPS-type liquid crystal display device is mainly caused by the following facts:
1) interdigital electrodes do not transmit light; and
2) a black matrix (hereinafter abbreviated as “BM”) arranged on an opposing substrate partially blocks openings from receiving light.
Regions blocked by the BM are edge portions of scanning wires and signal wires, and TFT sections.
The edge portions of scanning wires and signal wires are shielded because light may leak from these portions. The TFT sections are shielded for preventing TFTs from malfunctioning due to a current generated by leaked light.
The BM has an area which is typically set larger than the sum of possible light leaking regions and TFT regions for taking into account an allowance of the alignment of a substrate which has formed thereon the possible light leaking regions and the TFT regions to be shielded (hereinafter called the “TFT substrate”), to another substrate on which the BM is formed (hereinafter called the “opposing substrate”).
This wide area of the BM contributes to an additional reduction in the aperture ratio. The aperture ratio must be increased in order to reduce power consumption.
Another problem is that the IPS-type liquid crystal display device suffers from a low uniformity of display luminance. This is because in the IPS-type liquid crystal display device, a threshold voltage for the luminance characteristic is inversely proportional to the thickness of a liquid crystal layer sandwiched between the pair of substrates (cell gap), so that non-uniformity of the cell gap, if any, would appear in a display as corresponding non-uniformity of luminance.
In the TN-type liquid crystal display device, on the other hand, a threshold voltage does not depend on a cell gap, so that the uniformity of display luminance is relatively high. In order to improve the uniformity of display luminance in the IPS-type liquid crystal display device, the uniformity of the cell gap between the two substrates must be ensured more strictly than the TN-type liquid crystal display device. However, an attempt to make the cell gap more uniform using currently available methods would result in a lower contrast which constitutes a further problem. In the following, the cause of the second problem will be explained in detail.
The non-uniform cell gap and the reduced cont

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