Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1997-07-07
2001-02-13
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C349S187000, C349S138000, C257S059000, C438S158000
Reexamination Certificate
active
06188452
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an active matrix liquid crystal display (AMLCD), and more particularly, to an AMLCD having thin film transistors (TFTs) as switching elements.
2. Description of the Related Arts
FIG. 1
shows a conventional active matrix liquid crystal display. The conventional AMLCD includes two substrates(a first and a second substrates), in which a plurality of pixels are formed in a matrix array.
On the first substrate
3
, pixel electrodes
4
are disposed at the intersections between gate bus lines
17
and data bus lines
15
. Gate bus lines
17
are formed in the horizontal direction and include gate electrodes(not shown) branching off therefrom. The data bus lines
15
are formed in the vertical direction and include data electrodes (not shown) branching off therefrom. At the intersections between the gate bus lines and the data bus lines, TFTs
8
are formed which make electrical contact with pixel electrodes
4
.
On the second substrate
2
, a color filter layer
38
and a common electrode
37
are formed.
The first and the second substrates are bonded together with a space therebetween. The space between the substrates is filled with a liquid crystal material
40
. Polarization plates
1
are formed on the outer surfaces of the substrates before the bonding. Reference numerals
11
and
11
′ in
FIG. 1
represent transparent glass substrates.
The structure and the method of manufacturing the first substrate
3
according to the present invention is described in detail with reference to
FIGS. 2 and 3
.
FIG. 2
is a plan view showing the structure of a conventional AMLCD and
FIG. 3
is a cross-sectional view taken along the line III—III in FIG.
2
.
The structure of a conventional AMLCD is described below. On a transparent glass substrate
11
, a gate bus line
17
in the horizontal direction and a gate electrode
17
a
branching off therefrom are formed. The gate electrode may be anode-oxidized to improve insulating performance and prevent hill-locks on the surface. On the substrate
11
including the gate electrode
17
a
, a gate insulating layer
23
made of an inorganic material, such as SiN
x
or SiO
2
, is formed. A semiconductor layer
22
made of amorphous silicon (a—Si) is formed on a portion of the gate insulating layer
23
over the gate electrode
17
a
. On the a—Si semiconductor layer, ohmic contact layers
25
made of n
+
a—Si are formed to be disposed a predetermined distance away from each other. On the surface including the ohmic contact layer
25
, a data bus line
15
is formed in one direction. A source electrode
15
a
is formed to be connected to the data bus line
15
. A drain electrode
15
b
is formed a predetermined distance away from the source electrode
15
a
. The source
15
a
and the drain electrode
15
b
form electrical contacts with the corresponding ohmic contact layers.
A protection layer
26
, made of an inorganic material such as SiN
x
, is formed to cover the substrate including the source
15
a
and the drain electrode
15
b
. A pixel electrode
4
made of a transparent conductive material, such as indium tin oxide(ITO), is formed on the protection layer. The electrode
4
is electrically connected to the drain electrode
15
b
through a contact hole
31
formed in the protection layer
26
.
Since the first substrate of the conventional AMLCD results in a TFT and bus lines with a stepped surface profile as shown in
FIG. 4
, the pixel electrode
4
needs to be formed a predetermined distance away from the gate bus line
17
, data bus line
17
, and the TFT. This stepped profile appears because an inorganic material, such as SiN
x
or SiO
2
, is used for the gate insulating layer
23
and the protection layer
26
.
Moreover, these stepped TFT and bus lines cause problems in the manufacture of an AMLCD. In particular, when an alignment film is formed on the stepped surface, the initial orientation of the liquid crystal becomes inhomogeneous. This degrades the quality of the LCD because of rubbing defects at the stepped portion of the alignment film.
In order to overcome such problems, an organic material with high planarization property is used for the gate insulating layer
23
or the protection layer
26
. Then, the rubbing defects are eliminated, and the reduction in the performance of the LCD can be prevented. Moreover, an improvement in the aperture ratio can be achieved, since the pixel electrode
4
can be formed to overlap the bus lines.
However, the introduction of the organic material in a TFT structure causes new problems. The ON-characteristic of the TFT becomes unstable. For example, the ON-characteristics curve shifts toward the negative gate voltage as shown in FIG.
5
. This is due to charge traps at the surface of the semiconductor layer
22
contacting the organic layer. Accordingly, better solutions are needed to obtain good planarization while preventing ON-characteristic instability.
SUMMARY OF THE INVENTION
Accordingly, the present invention is directed to a method of manufacturing an active matrix liquid crystal display and a liquid crystal display that substantially obviate the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide an AMLCD with an organic insulating layer with stable TFT characteristics.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a method for manufacture the first substrate of an AMLCD, including plasma treatment step of the semiconductor layer using N
2
, O
2
or a gas containing N or F prior to coating the organic protection layer having the dielectric constant less than 3.0.
In another aspect, the present provides a method for manufacturing the first substrate of an AMLCD including a plasma treatment step on the organic gate insulating layer of BCB, in addition to the plasma treatment step of the semiconductor layer, using N
2
, O
2
or a plasma gas containing N or F.
In another aspect, in a liquid crystal display having a thin film transistor including a gate electrode branching off from a gate line, a first insulation layer covering the gate electrode, a semiconductor layer, an ohmic contact layer, a source and a drain electrode branching off from a data bus line, and a second insulation layer covering the semiconductor layer, the present invention provides a method of manufacturing the liquid crystal display comprising the steps of performing a surface-treatment on a surface of the semiconductor layer; and forming the second insulation layer made of an organic material on the surface-treated surface of the semiconductor layer.
In another aspect, the present invention provides a liquid crystal display comprising a substrate; a thin film transistor over the substrate, the thin film transistor including: a gate electrode contacting a gate line; a semiconductor layer; and a first insulation layer between the semiconductor layer and the gate electrode; ohmic contact layers on the semiconductor layer; source and drain electrodes each contacting corresponding ohmic contact layers, at least one of the source and drain electrodes being in contact with a data bus line; a second insulation layer made of an organic material covering the semiconductor layer; and a first surface treatment layer at an interface of semiconductor layer and the second insulation layer.
In a further aspect, the present invention provides a method for manufacturing a liquid crystal display device on a substrate, comprising forming a thin film transistor over
Kim Jeong-Hyun
Park Sung-Il
LG Electronics Inc
Morgan & Lewis & Bockius, LLP
Sikes William L.
Ton Toan
LandOfFree
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