Active matrix LCD panel

Details Active matrix LCD panel Active matrix LCD panel

2000-10-25

2004-09-07

Chowdhury, Tarifor R. (Department: 2871)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

Reexamination Certificate

active

06788355

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to an active matrix LCD panel and a method for fabricating the active matrix LCD panel. More particularly, the present invention relates to a channel protection type active matrix LCD panel in which a channel protection film is formed on a surface of an amorphous silicon (a-Si) active layer, and a method for fabricating the channel protection type active matrix LCD panel.
(b) Description of the Related Art
An active matrix mode liquid crystal display (LCD) device including an active element such as a thin-film transistor (TFT) has advantages of small thickness and light weight, and has been widely used as a high quality flat panel display unit. In general, the LCD device is either of a longitudinal electric field (twisted nematic: TN) mode wherein liquid crystal layer interposed between an active matrix LCD panel and a counter panel respectively mounting thereon transparent electrodes is driven by a voltage applied between the transparent electrodes, or of an in-plane switching mode where a liquid crystal is driven by comb-shaped electrodes formed on the active matrix LCD panel for generating transverse electric field which is parallel to the surface of both the panels. In either mode LCD device, attempts to simplify the fabrication process of the active matrix LCD panel have been made in order to realize a lower price.
Referring to
FIG. 1
showing a typical TN mode active matrix LCD panel generally includes: gate lines
12
and drain lines
14
respectively extending in directions perpendicular to each other; pixel electrodes
10
each formed in a pixel area surrounded by these lines; and thin-film transistors (TFTs)
17
each formed in the vicinity of an intersection between the gate line
12
and the drain line
14
. The source of each TFT is coupled to a gate line of next row via a gate storage capacitor having a gate storage electrode
21
and connected in parallel with the pixel capacitor formed by a liquid crystal layer. Each drain line
14
as well as each gate line
12
is protected by a protective device
18
including first and second TFTs
41
and
42
disposed in the vicinity of the electrode pad, i.e., gate terminal
15
or drain terminal
16
. A channel protection film is formed on the surface of the thin-film transistor
17
in order to secure the performance thereof. An orientation film (not shown) for aligning the liquid crystal layer in a predetermined direction is formed on each of the thin-film transistor
17
and the pixel electrode
10
of the active matrix LCD panel. The liquid crystal lays is sandwiched between the active matrix LCD panel and the counter panel, on which a color filter, a common electrode, an orientation film, and the like are formed, to implement an active matrix LCD device.
A general fabrication method for such an active matrix LCD panel will be described below. First, an ITO (Indium-Tin-Oxide) film is formed by deposition onto a transparent insulating substrate, and a resist pattern is formed on the ITO film by using a first mask pattern (or a first photolithographic step using a mask pattern). The ITO film is selectively etched by using the resist pattern to form pixel electrodes. Thereafter, a metallic film such as made of Cr, Mo, or Al to be formed as gate electrodes is formed by deposition onto the transparent insulating substrate. Another resist pattern is then formed on the metallic film by using a second photolithographic technique, followed by selective etching of the metallic film to form gate electrodes.
Subsequently, a gate insulating film such as made of SiNx is formed by deposition to cover the gate electrodes, followed by selecting etching thereof to form openings therein by using a third photolithographic step. An a-Si layer is then formed thereon by deposition. Next, the a-Si layer is selectively etched by using a fourth photolithographic step to form a plurality of island a-Si layers, and a channel protection film such as made of SiNx is formed on the island a-Si layers by deposition. Using a fifth photographic process, the channel protection film is selectively etched to leave portions of the channel protection film on the island a-Si layers. Next, in order to obtain ohmic contact with the a-Si layer, an impurity-doped n
+
type a-Si layer is deposited thereon, and a metal such as Cr, Mo, or Al is subsequently deposited thereon. Then, a sixth photolithographic step is conducted to form source/drain electrodes of the TFTs.
In the process for fabrication of the active matrix LCD panel, six photolithographic steps using six mask patterns are conducted in total for completion of the active matrix LCD panel. In view of simplification of the fabrication process for the active matrix LCD panel, it has been proposed to reduce the large number of the photolithographic steps in the fabrication process.
Japanese Patent Laid-Open Publication No. Sho 63-218925, for example, describes a fabrication process for reducing the number of the photolithographic steps, which is described below with reference to
FIGS. 2A
to
2
D schematically illustrating steps of the method for fabricating the TN mode active matrix LCD panel in sections. The left side of each of the figures shows the peripheral area of the panel where gate terminals are disposed, and the centers thereof show a single pixel disposed in the pixel area.
In the active matrix LCD panel described in the above-mentioned publication, first, an ITO film and a metallic film such as made of Cr, Mo, or Al are consecutively deposited onto a transparent insulating substrate
31
, as illustrated in
FIG. 2A
, by a sputtering technique, and a resist pattern is formed thereon using a first photolithographic step. The ITO film and the metallic film are selectively etched by using the resist pattern to simultaneously form gate electrodes
32
and pixel electrodes
10
.
Next, as illustrated in
FIG. 2B
, after a gate insulating film
34
such as made of SiNx, an intrinsic or undoped a-Si layer
35
, and a channel protection film
25
such as made of SiNx are consecutively deposited, followed by selective etching of the channel protection film
25
to leave portions of the channel protection film
25
on the channel region of the undoped a-Si layer
35
.
Next, as illustrated in
FIG. 2C
, an ohmic contact layer
36
made of impurity-doped n
+
-type a-Si is deposited thereon, and the ohmic contact layer
36
, the undoped a-Si layer
35
, the gate insulating film
34
, and the upper layer gate metallic film
32
b
are selectively etched all together using a third photolithographic step to expose pixel electrodes
10
and electrode pads for the gate electrode
32
.
Thereafter, as illustrated in
FIG. 2D
, a source/drain metallic film such as made of Al is formed thereon by deposition. Subsequently, the source/drain metallic film and the ohmic contact layer
36
above the channel region are selectively etched using a fourth photolithographic step, and the source/drain metallic film is configured to a specific shape, thereby completing the fabrication of the active matrix LCD panel.
In the method of the above publication, it is possible to fabricate the active matrix LCD panel in which the channel protection film
25
is formed in the channel region above the undoped a-Si layer
35
by using the four photolithographic steps. However, since the ohmic contact layer
36
, the undoped a-Si layer
35
, the gate insulating film
34
, and the upper layer gate metallic film
32
b
are selectively etched all together in the single step as illustrated in
FIG. 2C
after the formation of the channel protection film
25
, the side surfaces of the undoped a-Si layer
35
are not covered and thus not protected by the channel protection film
25
.
The side walls of the undoped a-Si layer
35
not protected by the channel protection film
25
made of a fine material, such as SiNx, is subjected to ingress of impurities from the liquid crystal layer through a coarse film such as a polyimide orientation film alone by d

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