Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Amorphous semiconductor material
Reexamination Certificate
2000-11-09
2003-05-20
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Amorphous semiconductor material
Reexamination Certificate
active
06566683
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser heat treatment method and apparatus for forming a polycrystalline silicon film having an excellent crystalline property in order to fabricate a high-mobility thin film transistor, and a semiconductor device produced using such method and apparatus.
2. Description of the Background Art
At present, pixel portions of a liquid crystal panel produce an image by switching of thin film transistors that are formed from an amorphous or polycrystalline silicon film on a glass or synthetic quartz substrate. If a driver circuit (which is now typically independently mounted outside) for driving the pixel transistors can be simultaneously formed on the panel, significant advantages would be obtained in terms of production cost, reliability and the like of the liquid crystal panel. At present, however, since the silicon film forming an active layer of the transistor has a poor crystalline property, the thin film transistor has poor capability in terms of mobility and the like, thereby making it difficult to make an integrated circuit that requires high speed and functionality. Laser heat treatment is commonly conducted as a way to improve the crystalline property of the silicon film in order to fabricate a high-mobility thin film transistor.
The relationship between the crystalline property of the silicon film and the mobility of the thin film transistor is described as follows: in general, the silicon film resulting from the laser heat treatment is a polycrystalline film. Crystal defects are locally present at polycrystalline grain boundaries, which inhibit carrier movement in the active layer of the thin film transistor. Accordingly, in order to increase the mobility of the thin film transistor, it is only necessary to reduce the number of times for the carriers to move across the grain boundaries while moving in the active layer, and to reduce a crystal-defect density. The purpose of laser heat treatment is to form a polycrystalline silicon film having a large grain size and also having a small number of crystal defects at the crystal boundaries.
FIG. 11
is a diagram showing one example of a conventional laser heat treatment apparatus. In
FIG. 11
, a pulse laser source
501
is an excimer laser source (such as KrF (wavelength of 248 nm) and XeCl (wavelength of 308 nm)) that is a typical pulse laser source of a wavelength less than 350 nm for emitting ultraviolet light commonly used as heat treatment laser. Excimer laser light
502
is emitted from pulse laser source
501
. A beam homogenizer
503
makes an intensity distribution of excimer laser light
502
uniform. A focusing optical system
504
focuses excimer laser light
502
An amorphous silicon from
505
is disposed so that it is subjected to the laser heat treatment. Amorphous silicon film
505
is formed on an underlying silicon oxide from
506
on a glass or quartz substrate
507
.
Hereinafter, a conventional laser heat treatment method is described. Excimer laser light
502
emitted from pulse laser source
501
is directed through beam homogenizer
503
onto amorphous silicon film
505
by focusing optical system
504
. Amorphous silicon film
505
is melted in the region irradiated with excimer laser light
502
. Then, as the temperature is reduced, the melted silicon is crystallized to form a polycrystalline silicon film. Since silicon has an extremely high absorption coefficient for the excimer laser light, heat treatment can be efficiently conducted even to a thin silicon film. However, due to the excessively high absorption coefficient, the laser light will be absorbed by the time it advances about 10 nm from the surface. A melting process of amorphous silicon film
505
is shown in
FIGS. 12A
to
12
D.
FIG. 12A
shows a state of silicon film
505
upon irradiation of the laser light in the direction shown by P;
FIG. 12B
shows a state obtained several tens of nanoseconds after the irradiation;
FIG. 12C
shows a state obtained several tens of nanoseconds after
FIG. 12B
; and
FIG. 12D
shows a state after the crystal growth. Upon irradiation of the laser, silicon film
505
has a melting-depth distribution and temperature distribution corresponding to a Gaussian beam profile
601
shown in
FIG. 12A
, and a melted portion
603
of the silicon film is produced Heat is generally conducted at a certain spreading angle. Therefore, as the melting depth is increased by the heat conduction, these distributions become broader as shown in
FIG. 12B
, resulting in the uniform distributions as shown in
FIG. 12C
Thus, melted portion
603
of the silicon film is formed. Accordingly, since there is no lateral temperature distribution, recrystallization proceeds in the vertical direction, whereby the resultant crystal grains
604
have a vertically elongated shape as shown in FIG.
12
D. In other words, the crystal grain size is reduced in the direction of the plane in which the carriers move.
FIG. 13
shows dependence of the mobility (n-channel) of a MOS transistor on the irradiation energy density of laser light, wherein the MOS transistor has its active layer formed from the polycrystalline silicon film thus obtained.
FIG. 13
shows a result with the use of a KrF excimer laser source as pulse laser source
501
(FIG.
11
), and pulse duration is about 15 nsec (FWHN). In addition, silicon oxide film
506
and amorphous silicon film
505
have a thickness of 200 nm and 50 nm, respectively. Herein, a laser-irradiation area is defined as an area having an irradiation intensity that is 1/e
2
times or more of the peak value, and the irradiation energy density was calculated from the radiant laser energy. As can be seen from
FIG. 13
, under the laser-heat-treatment conditions as described above, the maximum mobility of 80 cm
2
/Vs was obtained by setting the excimer-laser irradiation energy density to 230 mJ/cm
2
, and about 80 percent or more of the maximum mobility was obtained in the range of ±5 mJ/cm
2
therefrom. However, such mobility is still insufficient to make a high-speed, high-functionality integrated circuit. Moreover, as shown in
FIG. 13
, the mobility is highly dependent on the irradiation energy density. Therefore, in introducing such a method in the production line, the produced transistors will have variation in their characteristics unless laser output and focusing capability of the optical system are highly strictly controlled. The reason for this can be considered as follows: since silicon has a high absorptance of the excimer laser light, a melting state thereof is varied with a slight change in the irradiation energy density, so that a recrystallization process is changed.
In terms of an enhancement of grain size of the polycrystalline silicon film, an attempt has been made in articles to conduct laser heat treatment with long-wavelength laser light of 350 nm or more (Reference 1 (Appl. Phys. Lett. 39, 1981, pp. 425-427), Reference 2 (Mat. Res. Soc. Symp. Proc., Vol. 4, 1982, pp. 523-528), and Reference 3 (Mat. Res. Soc. Symp. Proc., Vol. 358, 1995, pp. 915-920). Herein, a second harmonic of Nd:YAG laser (wavelength of 532 nm) is used as the long-wavelength laser light of 350 nm or more. In these reported examples, a beam profile at the irradiated position corresponds to an asymmetric Gaussian distribution. According to References 1 and 2, a recrystallization process of the laser heat treatment using the second harmonic of Nd:YAG laser is described as follows: description is herein given with reference to
FIGS. 14A
to
14
D. As shown in
FIG. 14A
, when a focused laser beam
611
having Gaussian beam profile
601
is directed from focusing optical system
504
onto silicon film
505
in the direction shown by P, a temperature distribution
612
that is very close to the Gaussian distribution is produced within silicon film
505
. Therefore, a melted portion
613
is formed in a melting state as shown in FIG.
14
B. In a shallow portion C of the melting depth in
FIG. 14B
, a longitudinal temperature d
Inoue Mitsuo
Miyasaka Mitsutoshi
Ogawa Tetsuya
Sasagawa Tomohiro
Sato Yukio
Le Thao P
Mitsubishi Denki & Kabushiki Kaisha
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