Method of forming polycrystalline semiconductor film

Details Method of forming polycrystalline semiconductor film Method of forming polycrystalline semiconductor film

2001-09-18

2002-11-26

Cuneo, Kamand (Department: 2827)

Semiconductor device manufacturing: process

Formation of semiconductive active region on any substrate

Polycrystalline semiconductor

C438S150000, C438S162000, C438S166000, C438S486000, C438S487000, C438S488000, C438S795000, C438S797000, C438S798000, C438S799000

Reexamination Certificate

active

06486046

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims benefit of priority under 35USC §119 to Japanese Patent Application Nos. 2000-281353 and 2001-208028 filed on Sep. 18, 2000 and Jul. 9, 2001, respectively, in Japan, the entire contents of which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
The present invention relates to a method of forming a polycrystalline semiconductor film. Particularly, this invention relates to a method of forming a polycrystalline semiconductor film as an active layer for thin-film transistors used for liquid crystal displays, etc.
With advanced miniaturization in liquid crystal displays, thin-film transistors (TFT) having polycrystalline silicon with high mobility used for active layers have recently been used instead of known transistors having amorphous silicon as active layers.
Described with reference to
FIG. 6
is a known method of producing a transistor having polycrystalline silicon used for an active layer.
As shown in FIG.
6
(
a
), a thin film
42
of amorphous semiconductor is deposited on an insulating film
41
made of glass for example. The thin film
42
is irradiated with energy-rich beams such as the excimer laser to be melted for recrystallization, and hence changed into a polycrystalline thin film
43
as shown in FIG.
6
(
b
). The thin film
43
is patterned, followed by impurity doping to form semiconductor films
43
a
of low concentration as shown in FIG.
6
(
c
).
Next, as shown in FIG.
6
(
d
), the semiconductor films
43
a
are covered with a gate insulating film
44
. A metallic film is formed on the film
44
and pattered, thus forming a gate electrode
45
for an n-channel transistor and a metallic film
45
a
that covers the semiconductor film
43
a
for a p-channel transistor. Subsequently, p- or n-type dopant is implanted into the semiconductor films
43
a
at a high concentration via the gate electrode
45
as a mask, to form an n-type source/drain region
46
as shown in FIG.
6
(
d
).
A resist pattern
50
made of photoresist is formed as shown in FIG.
6
(
e
). The metallic film
45
a
is patterned with this resist pattern to form a gate electrode
45
a
for the p-channel transistor. P-type dopant of high concentration is implanted into the semiconductor film
43
a
for the p-channel transistor while masked with the resist pattern
50
and the gate electrodes
45
a
to form p-type source/drain regions
47
.
The resist pattern
50
is removed, followed by annealing for dopant activation, and an interlayer insulating film
48
is formed on the entire surface as shown in FIG.
6
(
f
). Contact holes are formed on insulating films
48
and
44
and are filled with an electrode-material film. The film is patterned to form source/drain electrodes
49
, thus producing the transistor.
Not only by irradiation of energy-rich beams such as the excimer laser, polycrystalline silicon films can be formed by, for example, solid phase epitaxy with annealing amorphous silicon for a long time at a temperature in the range from about 400° C. to 600° C. In general, however, polycrystalline silicon films formed by solid phase epitaxy have lower carrier mobility than those formed by irradiation of energy-rich beams for recrystallization by melting. Moreover, large substrates can easily be recrystallized by irradiation of energy-rich line beams, but not by solid phase epitaxy. Polycrystalline silicon films formed by solid phase epitaxy thus cannot be used for high-speed circuitry, and hence used only for small liquid crystal displays.
Irradiation of energy-rich beams such as the excimer laser for recrystallization of amorphous silicon by melting causes roughness on the silicon surface. Shown in
FIG. 5
is a cross-sectional view of a thin-film transistor having an active layer of polycrystalline silicon formed by irradiation of the excimer laser. It is expected from
FIG. 5
that a gate insulating film
44
, which is formed on a polycrystalline silicon
43
a
having roughness formed on an insulative substrate
41
, has thin sections formed on convex portions of the polycrystalline silicon.
Gate insulating films having thin portions cause a decrease in gate dielectric strength.
FIG. 4
indicates leak current dependency on voltage for oxide films. One oxide film was formed on a substrate of single crystalline silicon whereas the other film having the same thickness as the former was formed a polycrystalline silicon film which was formed by irradiating the excimer laser. The graphs teach that a leak current flowed through the oxide film formed on the polycrystalline silicon film at an extremely low voltage compared to the single crystal silicon. It is speculated that electric fields converged on the surface convex sections might have caused generation of current on the convex sections.
Thin-film transistors having polycrystalline silicon as active layers thus require a thick gate insulating film. An on state current for thin-film transistors are inverse proportional to the thickness of a gate insulating film. A thick gate insulating film mitigates roughness on the surface of a polycrystalline silicon film, however, it causes decrease in performance of transistors.
It is speculated that such roughness that causes decrease in performance of thin-film transistors is formed on the surface of a polycrystalline silicon due to segregation of an oxide film formed on the surface of amorphous silicon or formed with oxygen existing in the atmosphere irradiated with laser, during recrystallization by melting.
Such roughness thus can be mitigated by complete removal of an oxide film formed on the surface of amorphous silicon with lowering a partial pressure of oxygen existing in the atmosphere irradiated with laser. Laser annealing under this condition, however, causes abrasion of a silicon film at energy lower than that of laser beam irradiation required for grain of polycrystalline silicon to become large enough. Or, it causes increase in energy of laser beams for crystallization. This results in decreasing in operating rate of an apparatus for irradiating the laser beams.
Such a problem is solved by atomospheric laser annealing for enlarging grain of polycrystalline silicon, followed by removal of the surface oxide film with hydrogen fluoride (HF) and vacuum laser annealing for minimizing roughness on the surface of polycrystalline silicon. This is proposed in K. Suga et al., “The Effect of a Laser Annealing Ambient on the Morphology and TFT Performance of Poly-Si Films”, Society for Information Display 00 DIGEST, p534-537, May 2000.
This method, however, requires decompression from ambient pressure to vacuum for laser annealing or two separate systems for ambient pressure only and vacuum only. Either way requires HF treatment after atomospheric laser annealing, thus decreasing productivity.
SUMMARY OF THE INVENTION
A purpose of the present invention is to provide a method of forming a polycrystalline semiconductor film that provides high performance for thin-film transistors having this polycrystalline semiconductor film and high productivity.
A first aspect of a method of forming a polycrystalline semiconductor film according to the present invention includes depositing an amorphous semiconductor film on a substrate, a first irradiating the amorphous semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with a specific amount of oxygen, to change the amorphous semiconductor film into a polycrystalline semiconductor film, and a second irradiating the polycrystalline semiconductor film with an energy-rich beam in an atmosphere of a gas containing an inert gas as a major component with oxygen of an amount less than the specific amount.
The amount of oxygen for the first irradiating may be 5 ppm or more but less than 10% and the amount of oxygen for the second irradiating is preferably 200 ppm.
The total amount of energy for the second irradiating may be larger than the total amount of energy for the first irradiating.
It may be that the polycrystalline s

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