Liquid crystal cells – elements and systems – Particular excitation of liquid crystal – Electrical excitation of liquid crystal
Reexamination Certificate
1998-10-20
2001-10-16
Sikes, William L. (Department: 2871)
Liquid crystal cells, elements and systems
Particular excitation of liquid crystal
Electrical excitation of liquid crystal
C345S087000
Reexamination Certificate
active
06304305
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to an active matrix liquid crystal display.
2. Description of the Prior Art
Presently, liquid crystal displays are extensively used as light and low-power-consuming displays in personal computers and various monitors. In particular, active matrix liquid crystal displays in which a thin film transistor is formed in each pixel are used as high-resolution displays in various applications, because the brightness of each pixel can be finely changed by voltage control.
The structure and operating principle of a conventional general active matrix liquid crystal display (to be referred to as an active matrix display hereinafter) will be described below with reference to
FIGS. 1 and 2
.
FIG. 1
is a sectional view showing this conventional active matrix display.
FIG. 2
shows an outline of the circuit configuration of a thin film transistor array formed in a matrix on a first insulating substrate shown in FIG.
1
.
In the conventional active matrix display as shown in
FIG. 1
, first and second insulating substrates
1
and
2
are opposed parallel to each other, and a liquid crystal
10
as a display material is sandwiched between these substrates. A thin film transistor array (
FIG. 2
) including pixel electrodes
6
is formed on one principal surface of the first insulating substrate
1
, which is in contact with the liquid crystal
10
.
On one principal surface of the second insulating substrate
2
, which is opposed to the first insulating substrate
1
and in contact with the liquid crystal
10
, color layers
7
,
8
, and
9
of three primary colors R (Red), G (Green), and B (Blue) are formed in portions corresponding to the pixel electrodes
6
of the first substrate. A black matrix
11
for intercepting light is formed in the boundaries between these color layers
7
,
8
, and
9
. A common electrode
12
made of a transparent conductive film is formed on the black matrix
11
.
FIG. 2
shows an outline of the circuit configuration of the thin film transistor array formed in a matrix on the first insulating substrate shown in FIG.
1
. On the principal surface of the first insulating substrate
1
, which is in contact with the liquid crystal
10
, scanning lines
3
and signal lines
4
are formed in the row and column directions, respectively. Display thin film transistors
5
are formed at the intersections of these scan and signal lines
3
and
4
. The gate, drain, and source of each display thin film transistor
5
are connected to the scanning line
3
, the signal line
4
, and the pixel electrode
6
made of a transparent conductive film, respectively.
In the active matrix display with the above arrangement, a scan pulse voltage for turning on the display thin film transistors
5
is supplied to each scanning line
3
. In synchronism with this scan pulse, a signal voltage corresponding to an image to be displayed is supplied to the signal lines
4
. Accordingly, the display thin film transistors
5
connected to the scanning line
3
operate, and a predetermined voltage is written in the pixel electrodes
6
from the signal lines
4
. The written voltage is held until a scan pulse voltage is supplied to this scanning line
3
. Consequently, an electric field corresponding to the held voltage is generated between each pixel electrode
6
and the common electrode
12
(
FIG. 1
) and changes the alignment of liquid crystal molecules. This changes the amount of light transmitted through the first insulating substrate
1
, the liquid crystal
10
, and the second insulating substrate
2
. The image is displayed by using this change in the light transmission state.
FIGS. 3A
to
3
E are sectional views showing steps of fabricating a thin film transistor used in the aforementioned conventional active matrix display in order of the steps. The thin film transistor shown in
FIGS. 3A
to
3
E has an inverse stagger structure. As shown in
FIG. 3E
, an island semiconductor film
16
opposes a gate electrode
14
via a gate insulating film
15
. A source electrode
19
and a drain electrode
18
are formed on this semiconductor film
16
via an ohmic contact layer
17
.
The fabrication steps of this conventional thin film transistor will be described below with reference to
FIGS. 3A
to
3
E. First, a first conductive film made of Al, Mo, or Cr is deposited on the entire surface of a transparent insulating film
13
such as glass by sputtering or the like. This first conductive film is coated with a photosensitive resist, and exposure, development, etching, and resist removal are performed by photolithography. Consequently, patterning of the first conductive film having a predetermined pattern of, e.g., a gate electrode
14
and a scanning line (not shown) is completed (FIG.
3
A).
Subsequently, on the entire surface of the predetermined pattern of the first conductive film, a gate insulating film
15
made of SiO
x
or SiN
x
, a semiconductor film
16
made of amorphous silicon (to be referred to as “a-Si” hereinafter), and an ohmic contact film
17
made of n-type a-Si or the like are successively formed in this order by sputtering or plasma CVD. After that, photolithography is performed to pattern the semiconductor film
16
and the ohmic contact film
17
, forming a predetermined pattern serving as a transistor channel on the gate insulating film
15
above the gate electrode
14
(FIG.
3
B).
Next, to electrically connect the first conductive film with a second conductive film for forming a source electrode, a drain electrode, a signal line, and the like by using, e.g., a scanning line input pad and a signal line input pad (neither is shown), the gate insulating film
15
is etched into a predetermined pattern by photolithography to form a hole (not shown) in the gate insulating film
15
above the first conductive film. A second conductive film made of, e.g., Al, Mo, or Cr is deposited on the entire surface by sputtering or the like. A signal line
4
(FIG.
2
), a source electrode
19
, and a drain electrode
18
are formed by photolithography (FIG.
3
C). Additionally, a transparent conductive film made of ITO or the like is deposited on the entire surface, and a pixel electrode
6
is formed by photolithography. After that, etching is performed by using the source electrode
19
and the drain electrode
18
as masks to remove the n-type a-Si, i.e., the ohmic contact film
17
from the transistor channel (FIG.
3
D). Finally, a protective film
20
made of SiN
x
or the like is deposited (FIG.
3
E). Those portions of this protective film, which are on the pixel electrode
6
and the pad for receiving external signals are removed by photolithography, thereby completing the thin film transistor.
In the fabrication process of the thin film transistor array used in this conventional active matrix display, removal charging occurs when the insulating substrate is removed from a tray or the like of the film formation apparatus or the etching apparatus in each step, or the pattern of a conductive film is charged up in, e.g., the film formation step or the etching step. Especially in the insulating film or semiconductor film formation step and the dry etching step using plasma CVD, charging readily occurs because the substrate is exposed to plasma for long time periods. In addition to this charging, abnormal discharging during the film formation step using plasma CVD sometimes instantaneously applies very large electric charge to a certain specific signal line or scanning line.
In a situation like this, as shown in
FIG. 2
, if the scanning lines
3
or the signal lines
4
are not connected and electrically isolated from each other, it is more likely that the charge amount difference between adjacent scanning lines or signal lines or the electric charge applied to a certain specific scanning line or signal line by abnormal discharging becomes larger than the breakdown voltage of an insulating film. Consequently, a sudden current flows between t
Matsumoto Seiichi
Ohi Susumu
NEC Corporation
Nguyen Dung
Sikes William L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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