Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1998-02-13
2001-04-10
Abraham, Fetsum (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S054000, C257S055000, C257S056000, C257S057000, C257S058000, C257S059000, C257S060000, C257S066000, C257S072000
Reexamination Certificate
active
06215154
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film transistor suitable for use in an active matrix type display apparatus and a method of fabricating the same.
Liquid crystal displays (LCD) of an active matrix type which use thin film transistors (TFTs) provide a high-quality display apparatus. There are two kinds of dot matrix type LCDs each having a plurality of pixels arranged in a matrix form; a simple matrix type and an active matrix type.
The active matrix type LCD includes pixels, pixel drive elements (active elements) and signal storage elements (storage capacitors or added capacitors) and drives a liquid crystal in a quasi-static manner which permits each pixel to store data. Each pixel drive element serves as a switch which is switched on or off in response to a scan signal. When the pixel drive element is switched on, a data signal (display signal) is transmitted via that pixel drive element to an associated display electrode, so that the liquid crystal is driven by the data signal. When the pixel drive element is disabled, the data signal is stored in the form of a charge in the associated signal storage element. The liquid crystal is kept driven by the discharging of the charge until the pixel drive element is switched on again. Even though the drive time assigned to a single pixel decreases as the number of scan lines increases, the liquid crystal is sufficiently driven. This prevents the contrast from decreasing.
TFTs are generally used as pixel drive elements. A TFT has an active layer comprised of a thin semiconductor film formed on an insulator substrate. The semiconductor film preferably includes an amorphous silicon film or a polycrystalline silicon film. A TFT having an active layer comprised of an amorphous silicon film is called an amorphous silicon TFT, while a TFT having an active layer comprised of a polycrystalline silicon film is called a polycrystalline silicon TFT. The polycrystalline silicon TFT has a greater field effect mobility and higher drive performance than the amorphous silicon TFT. Because of these advantages, the polycrystalline silicon TFT can be used as a logic circuit element as well as a pixel drive element. The use of polycrystalline silicon TFTs, therefore, allows the integration of the display screen and a peripheral drive circuit, located a the periphery of the display screen, and on the same substrate. That is, the display screen and peripheral drive circuit may be formed in the same step.
FIG. 1
is a schematic block diagram of a typical active matrix type LCD. The LCD includes a display panel
101
, a gate driver
103
, and a drain (data) driver
104
. The display panel
101
has a plurality of scan lines (gate lines) G
1
, . . . , Gn, Gn+1, . . . , and Gm, a plurality of data lines (drain lines) D
1
, . . . , Dn, Dn+1, . . . , and Dm running perpendicular to the gate lines G
1
-Gm, and a plurality of pixels
102
provided at the intersections of the gate lines G
1
-Gm and the drain lines D
1
-Dm. The gate driver
103
, which is connected to the gate lines G
1
-Gm, applies a gate signal (scan signal) to the gate lines G
1
-Gm. The drain driver
104
, which is connected to the drain lines D
1
-Dm, applies a data signal (video signal) to the drain lines D
1
-Dm. Both of the gate driver
103
and the drain driver
104
form a peripheral drive circuit
105
. Either one of the drivers
103
and
104
or both are preferably formed on the same substrate on which the display panel
101
is formed. The LCD is generally called a driver-integrated (driver-incorporated) LCD. The gate driver
103
or the drain driver
104
may be provided on both sides of the display panel
101
.
FIG. 2
shows an equivalent circuit of one of the pixels
102
. The pixel
102
includes a liquid crystal cell LC having a display electrode (pixel electrode) and a common electrode. The liquid crystal cell LC is connected to both a TFT
106
and a supplemental capacitor SC. The supplemental capacitor SC has a storage electrode and an opposing electrode. The TFT
106
has a gate connected to the gate line Gn, a drain connected to the drain line Dn, and a source connected to the display electrode of the liquid crystal cell LC and the storage electrode of the supplemental capacitory SC. The liquid crystal cell LC and the supplemental capacitory SC form a signal storage element. A voltage V
com
is applied to the common electrode of the liquid crystal cell LC. A predetermined voltage signal V
R
is applied to the opposing electrode of the supplemental capacitor SC. The common electrode of the liquid crystal cell LC is common to all of the pixels
102
. The liquid crystal cell LC has a capacitory formed between the display electrode and the common electrode.
The writing characteristic and holding characteristic of the pixel
102
are important in improving the quality of displayed image. The writing characteristic shows how much the liquid crystal cell LC and the supplemental capacitor SC can write desired video signals per unit time based on the specifications of the display panel
101
. The holding characteristic shows how long the written video signals can be held. The supplemental capacitor SC is provided to increase the capacitance of the pixel to improve the holding characteristic.
When a positive voltage is applied to the gate of the TFT
106
via the gate line Gn, the TFT
106
is turned on and a data signal is applied to the drain line Dn. As a result, the capacitor of the liquid crystal cell LC and the supplemental capacitor SC are charged. If a negative voltage is applied to the gate of the TFT
106
, the TFT
106
is turned off. At this time, the capacitor of the liquid crystal cell LC and the supplemental capacitory SC hold the voltage applied to the drain lien Dn. In other words, the pixel
102
holds a data signal as the data signal is applied to the associated one of the drain lines D
1
-Dm by controlling the voltage on the associated one of the gate lines G
1
-Gm. An image is displayed on the display panel
101
in accordance with the held data signal.
FIG. 3
is a cross-sectional view of a part of the conventional LCD display panel
101
which has polycrystalline silicon TFTs
106
of a bottom gate structure. It is preferable that the display panel
101
is of a transparent type. The method of manufacturing the display panel
101
will be discussed below.
Step 1 (see FIG.
4
A): A chromium film
61
is formed on an insulator substrate
71
by sputtering.
Step 2 (see FIG.
4
B): A resist pattern
62
for forming a gate electrode
76
and a supplemental capacitor electrode
77
is formed on the chromium film
61
.
Step 3 (see FIG.
4
C): With the resist pattern
62
used as an etching mask, the chromium film
61
is locally etched off by wet etching to form the gate electrode
76
and the supplemental capacitory electrode
77
. At this time, the etching solution permeates the interfaces between both end portions of the resist pattern
62
and the chromium film
61
, thereby forming undercuts
61
a
at parts of the chromium film
61
in the vicinity of both ends of the resist pattern
62
. In cross section, therefore, the gate electrode
76
has a flat center portion (flat portion)
76
a
and a tapered end portion (tapered portion)
76
b
. The angle between the outer wall of the tapered portion
76
b
and the insulator substrate
71
is about 45°.
Step 4 (see FIG.
4
D): A silicon nitride film
78
, a silicon oxide film
79
and an amorphous silicon film
63
are formed in order on the gate and supplemental capacitor electrodes
76
and
77
and the insulator substrate
71
by plasma CVD (Chemical Vapor Deposition). The silicon nitride film
78
and the silicon oxide film
79
form a gate insulator film
80
in the region of the TFT
106
, and form a dielectric film
84
in the region of the supplemental capacitor SC. Next, the device is annealed at 400° C. to remove hydrogen from the amorphous silicon film
63
(dehydrogenation treatment). Then, excimer laser light is irradiated on the surface of the amorphous silicon
Ishida Satoshi
Kuriyama Hiroyuki
Nakahara Yasuo
Shimogaichi Yasushi
Yamada Tsutomu
Abraham Fetsum
Sanyo Electric Co,. Ltd.
Sheridan & Ross P.C.
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