Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-08-29
2002-07-16
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C438S149000
Reexamination Certificate
active
06420760
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of methods for manufacturing polycrystal silicon thin film transistors and thin film transistors as employed in liquid crystal display devices and input and output devices including image sensors.
BACKGROUND OF THE INVENTION
The electron mobility of a polycrystal silicon thin film transistor is greater by a factor of 100 than that of an amorphous silicon thin film transistor. The use of polycrystal silicon thin film transistors allows the miniaturization of elements and the denser mounting of driving circuits on one substrate. In the field of liquid is crystal display devices, polycrystal silicon thin film transistors are recently used in thin film transistor arrays with built-in driving circuits. These thin film transistor arrays with built-in driving circuits have been made possible by the development of technology to manufacture arrays on glass substrates which can be easily enlarged.
To form polycrystal thin film transistors at low temperatures, the development of a method for activating the dopant implanted into the polycrystal silicon thin film at low temperatures is important as well as technology to form polycrystal silicon thin film at low temperatures. Low temperature crystallization using excimer laser annealing is often employed to form good polycrystal silicon thin films on large substrates at low temperatures.
For example, IEEE Electron Device Letters, Vol. EDL-7, No. 5, May 1986, pp. 276-278, discloses technology related to excimer laser annealing. In general, thermal annealing is used for activation, but the activation rate significantly drops as a result of reducing the processing temperature.
Rapid thermal annealing (RTA) and excimer laser activation are proposed as methods for improving the dopant activation rate at low temperatures to counteract the above disadvantage. SID97 M/52: Recent Advances in Rapid Thermal Processing of Polysilicon TFT LCDs discloses RTA activation, and the Extended Abstract of the 18th (1986) International Conference on Solid State Devices and Materials, pp. 225-228, discloses excimer laser activation.
FIGS. 3A
to
3
D show process flow charts describing a conventional method of manufacturing polysilicon thin film transistors for the active matrix arrays used in liquid crystal display devices. As shown in
FIG. 3A
, a silicon oxide film which becomes a buffer layer
12
is formed on a transparent glass is substrate
11
using the plasma CVD method. Amorphous silicon (a-Si) film is then deposited using the plasma CVD method without exposing the substrate
11
, on which the buffer layer
12
is formed, to air.
Next, a thermal treatment is applied to reduce the hydrogen in the a-Si film. The a-Si film is polycrystallized by excimer laser annealing to form a poly-Si film
13
a
. Finally, the poly-Si film
13
a
is processed into the size and shape required for a TFT.
Next, a silicon oxide film which becomes a gate insulation film
14
is formed. A gate electrode
15
typically made of Al alloy is formed and dopant is implanted to form a Lightly Doped Drain (LDD) region
13
b
in the thin film transistor as shown by an arrow
100
in FIG.
3
A. As shown in
FIG. 3B
, a mask for implanting dopant into the source and drain regions is then formed using a photo resist
25
in a manner to cover the LDD region
13
b
of the thin film transistor. A large quantity of phosphorus ion, the dopant, is implanted into the source region
21
and drain region
22
by ion implantation, as shown by an arrow
100
in FIG.
3
B. The source region
21
and drain region
22
which have high concentrations of dopant are called a SD region
13
C.
Since the implanted dopant is electrically inactive, excimer laser light is applied, as shown by an arrow
101
in
FIG. 3C
, to activate it.
Then, as shown in
FIG. 3D
, a silicon oxide film which becomes an interlayer insulation film
16
is formed and contact holes
17
a
and
17
b
are opened on the insulation film in the source region
21
and drain region
22
. A layered film of Ti and Al is formed and processed to form SD wirings
18
a
and
18
b.
Finally, a protective insulation film
23
made of silicon nitride is formed, and annealed in a hydrogen atmosphere. Hydrogen annealing fills the empty ionic bonds in the polycrystal silicon thin film with hydrogen, enabling the characteristics of the thin film transistor to be improved.
However, the conventional method of activation using an excimer laser causes a high degree of thermal damage to the gate electrode
15
. More specifically, as shown in
FIG. 3C
, an irradiated excimer laser light is applied to and absorbed by the polycrystal silicon through the gate insulation film
14
at the source region
21
and drain region
22
of the thin film transistor. The laser light applied to the gate electrode
15
region is also directly absorbed by the gate metal, causing the gate electrode's temperature to rise. If metals with high melting points such as W, Mo, and Cr are used for the gate electrode
15
, cracks or peeling of the gate electrode
15
may occur as a result of thermal shock due to laser irradiation. If Al alloy is used for the gate electrode
15
, quality problems such as an increase in hillocks may occur. Hillocks are the phenomenon whereby the material surface becomes bumpy as a result of temperature rise.
The present invention provides a thin film transistor manufacturing method and thin film transistor which reduces the thermal damage to gate electrodes caused by laser irradiation during the manufacture of thin film transistors which includes the process of dopant activation by laser irradiation.
SUMMARY OF THE INVENTION
A method for manufacturing thin film transistors in accordance with an exemplary embodiment of the present invention includes the steps of forming a semiconductor thin film on a transparent substrate; forming a first insulation film having a refractive index n
1
and film thickness d
1
on the semiconductor thin film as a gate insulation film; forming a gate electrode on the first insulation film; implanting dopant into the semiconductor thin film; forming a second insulation film having refractive index n
2
and film thickness d
2
in a way to cover the first insulation film and gate electrode; and activating dopant implanted by applying laser with wavelength &lgr; after forming the second insulation film. In this configuration, the film thicknesses d
1
and d
2
practically satisfy a set of Formulae (1) and (2) as follows:
d
2
*
n
2
=2
*m
*&lgr;/4 (1)
d
1
*
n
1
+
d
2
*
n
2
=(2
*m
1
−1)*&lgr;/4 (2)
Here, m and m
1
are any given positive integer.
These film thicknesses enable the laser light to be reflected off the gate electrode and absorbed at portions other than the gate electrode. This allows a reduction in the thermal damage to the gate electrode by laser irradiation, and also achieves efficient dopant activation by the laser.
Another exemplary embodiment of the present invention refers to a method for manufacturing thin film transistors including the steps of forming the semiconductor film on the transparent substrate; forming the first insulation film having refractive index n
1
and film thickness d
1
on the semiconductor thin film as a gate insulation film; forming the gate electrode on the first insulation film; implanting dopant into the semiconductor thin film after forming the gate electrode; forming the second insulation film having refractive index n
2
and film thickness d
2
in a way to cover the first insulation film and gate electrode; and activating dopant implanted by laser irradiating with a wavelength &lgr; after forming the second insulation film.
In this configuration, the film thickness d
1
of the first insulation film and film thickness d
2
of the second insulation film fall in a range practically satisfying a set of Formulae (5) and (6) when m and m
1
are any given positive integers.
abs{d
2
*
n
2
−2
*m
*&lgr;/4}<&lgr;/8 (5); and
abs
{(
d
2
*
n
2
&p
Furuta Mamoru
Soma Koji
Matsushita Electric - Industrial Co., Ltd.
Nelms David
Ratner & Prestia
Vu David
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