Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-04-09
2004-06-01
Ton, Toan (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S139000, C349S142000, C349S110000
Reexamination Certificate
active
06744482
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device and a fabrication method thereof and, particularly, to an IPS (In-Plane Switching) mode active matrix type liquid crystal display device and a fabrication method thereof.
2. Description of the Prior Art
An active matrix type liquid crystal display device (referred to as “AMLCD”, hereinafter), which uses TFT's (Thin Film Transistors) as pixel switching elements, can provide a high image quality and has been used as a display device of a portable type computer and, particularly, as a monitor of a compact desk-top computer recently.
The AMLCD is roughly classified to a type in which a display is performed by rotating a direction of molecular axis of oriented liquid crystal molecule, which is called “director”, in a plane orthogonal to a substrate thereof and a type in which a display is performed by rotating the director in a plane parallel to the substrate.
A liquid crystal display device of the TN (Twisted Nematic) mode is a typical example of the former type and that of the IPS (In-Plane Switching) mode is a typical example of the latter type.
Since, in the AMLCD of the IPS mode, a user basically looks liquid crystal molecules in only shorter axis direction even when a view point is moved, there is no dependency of the “rising” of liquid crystal molecule on a viewing angle and so it is possible to achieve the viewing angle, which is wider than that achievable in the TN mode liquid crystal display device.
In general, when a liquid crystal display device is manufactured, a patterning on a substrate is performed by photolithography using a photo mask.
Since, when the size of a liquid crystal panel becomes larger, the size of the photo mask for transferring a pattern of a liquid crystal panel onto a whole surface of the substrate becomes larger, the cost of the photo mask becomes very high. Therefore, in order to reduce the manufacturing cost, it is usual that repeated patterns to be formed in respective display regions are formed by dividing the whole display region to a plurality of sub regions and exposing the sub regions one by one with using a single small photo mask for one pattern. This technique is generally referred to as “stepper exposure”.
However, since the stepper exposure is performed in the display region within the substrate, it is required, in laminating patterned layers in the display region, to precisely pattern an underlying layer in a vertical direction in every shot and to make an error of overlapped area between adjacent exposure shots as small as possible in a horizontal direction in every exposing shot.
When the overlapped area between the adjacent exposure shots is large, the quality of formed pattern becomes different between the exposure shots, resulting in a display defect called unevenness of division.
On the other hand, the IPS mode AMLCD has the merit of wide viewing angle while has a demerit of small area of a aperture of a pixel region. Therefore, the demand of a technique for increasing the area of the aperture has become prosperous recently.
An example of the IPS mode liquid crystal display device is disclosed in JP H07-036058 A (referred to as “prior art 1”, hereinafter).
The IPS mode liquid crystal display device disclosed in the prior art 1 is constructed with a TFT array substrate, scanning lines formed on the substrate, which is formed firstly, a common electrode formed in a metal layer, which is in the same layer of the scanning lines, signal lines (referred to as “data lines”, hereinafter) formed between the common electrode and an insulating film and pixel electrodes formed in the same layer of the data lines.
Another example of the IPS mode liquid crystal display device is disclosed in U.S. Pat. No. 6,069,678 (corresponding to JP H10-186407 A and referred to as “prior art 2”, hereinafter). In one embodiment of the prior art 2, a common electrode is formed in an uppermost layer in lieu of the same layer as the initially formed scanning lines.
Since, in the latter case, it becomes possible to shield electric field generated by the data lines by the common electrode and to widen an effective display region of the pixels, it becomes possible to improve the aperture ratio of the pixel and, hence, the light utilization efficiency.
It is usual that, when a large area LCD is to be exposed by using a stepper, a very high positional accuracy is required between the exposure shots.
Describing this with reference to the stepper exposure, a pattern exposure for a substrate is performed by dividing the pattern as shown in FIG.
1
. Assuming that the size of a transparent insulating substrate is constituted with zones
37
Z, zones
1
Z to
20
Z arranged in a peripheral portion form a peripheral terminal portion for inputting voltages to a display region and the display region as a liquid crystal display is formed by zones
21
Z to
36
Z within an area defined by a thick solid line.
For example,
FIG. 2
shows a case where only the exposure shot in the zone
21
Z is deviated rightward with respect to a gate layer.
FIG. 3A
shows an ideally arranged pattern of a layout in the vicinity of a unit TFT element. As shown in
FIG. 3A
, an interlayer insulating film is formed on a scanning line
28
forming a first wiring layer and a common electrode wiring portion
26
a
and, on the interlayer insulating film, data lines
24
forming a second wiring layer and a pixel auxiliary electrode
35
are formed. In the TFT region, an amorphous silicon layer
29
is formed on the scanning line
28
and a drain electrode
30
a
connected to the data line
24
and a source electrode
30
b
connected to the pixel auxiliary electrode
35
are formed on the amorphous silicon layer
29
.
FIG. 3B
shows a case where the pattern of the data line, the drain electrode and the pixel auxiliary electrode is deviated in the rightward direction. In
FIG. 3B
, when the exposure shot of the zone
21
Z is deviated rightward with respect to the scanning line
28
(gate line), areas of the drain electrode and the source electrode, which are overlapped with the amorphous silicon layer
29
are reduced. Therefore, write characteristics and holding characteristics of the TFT, which is formed by the exposure shot of the zone
21
Z, with respect to voltage applied to liquid crystal of the TFT are varied. Therefore, a display state becomes uneven since only the region in which the exposure shots are deviated becomes dark as shown in
FIG. 5
, comparing with a uniform display state of a liquid crystal display device having no overlapping deviation between adjacent exposure shots shown in FIG.
4
.
When the data line
24
and the pixel auxiliary electrode
35
on the gate layer (scanning line
28
) are deviated with respect to the gate layer by various amounts between adjacent exposure shots, the deviation is observed as unevenness of display, which is looked as unevenness of division such as shown in FIG.
6
.
In order to achieve such high precision alignment, the second (second wiring layer) and subsequent exposures to be performed subsequent to an exposure of the first layer (first wiring layer), which is performed on absolute position with high precision, must be performed as mentioned below.
Firstly, a test exposure is performed by detecting an alignment marker formed in the first layer and, on the basis of the detected alignment marker as a reference, programming the exposure such that a designed overlapping with the pattern of the first layer is obtained.
Secondly, it is necessary to measure the positional relation of the resist pattern of the second layer to the pattern of the first layer by a fine distance measuring device, detect a deviation of the resist pattern of the first layer from an optimal position on the basis of the measurement and feeding back the detected deviation to the exposure program to thereby make the second exposure shot to the optimal position, and so on.
In the prior art 1 mentioned above, there is the common electrode in the first layer, wh
Hannuki Takahisa
Itakura Kunimasa
Koike Masashi
Matsumoto Kimikazu
Nishida Shin-ichi
Hayes & Soloway P.C.
NEC LCD Technologies Ltd.
Qi Mike
Ton Toan
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