Electric heating – Metal heating – By arc
Reexamination Certificate
2000-08-17
2004-06-01
Evans, Geoffrey S. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121660, C219S121740, C219S121770, C438S487000
Reexamination Certificate
active
06744008
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film with laser beams (hereinbelow referred to as laser annealing) and a laser apparatus to be used for performing the same (more specifically, an apparatus including a laser source and an optical system for guiding laser beams emitted from the laser source to an object to be processed).
2. Description of the Related Art
In recent years, developments of thin film transistors (hereinbelow referred to as TFTs) has been advanced, and in particular, TFTs employing a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film has drawn much attention. Especially, in a liquid crystal display device (liquid crystal display) or an EL (electro-luminescence) display device (EL display), these TFTs are used as elements for switching pixels and elements constituting a driver circuit for controlling the pixels.
In common techniques for obtaining a polysilicon film, an amorphous silicon film) is crystallized to obtain a polysilicon film. In particular, a method of crystallizing an amorphous silicon film with laser beams has been receiving much attention. In the present specification, the technique for crystallizing an amorphous semiconductor film with laser beams to obtain a crystalline semiconductor film is referred to as laser crystallization.
The laser crystallization enables instantaneous heating of a semiconductor film, and thus it is an effective technique for annealing a semiconductor film formed on a substrate having low heat-resistance, such as a glass substrate, a plastic substrate or the like. In addition, the laser crystallization has a significantly higher throughput, as compared to conventional heating means employing an electric furnace (hereinbelow ref erred to as furnace annealing).
Although various kinds of laser beams are available, laser beams emitted from a pulse-oscillated excimer laser (hereinbelow referred to as excimer laser beams) are generally used in the laser crystallization. The excimer laser can provide a large output power and repeat irradiation at high frequencies. Furthermore, the excimer laser beams have an advantage of a high absorption coefficient against a silicon film.
One of the most important problems to be solved in these days is how to enlarge the diameters of crystal grains in a crystalline semiconductor film crystallized with laser beams. It is clear that the larger each crystal grain (also simply referred to as a grain) is, the less number of grains traverse TFTs, in particular. channel-formation regions thereof. This enables improvements in the fluctuation of typical electrical characteristics of TFTs, such as a field effect mobility or a threshold voltage.
In addition, since relatively satisfactory crystallinity is maintained at the inside of each grain, it is desirable, when fabricating TFTs, to dispose the entire channel-formation region within a single grain so as to improve the above-mentioned various operational characteristics of TFTs.
However, it is difficult to obtain a crystalline semiconductor film having sufficiently large grain diameters with employment of presently available techniques. Although some results have been reported indicating that such a crystalline semiconductor film with sufficiently large grain diameters was experimentally obtained those reported techniques have not reached practical levels yet.
For example, in the experimental level, the results have been achieved as described in the article entitled “High-mobility poly-Si thin-film transistors fabricated by a novel excimer laser crystallization method” by K. Shimizu, O. Sugiura and M. Matsumura in IEEE Transactions on Electron Devices, vol. 40, No. 1, pp. 112-117 (1993). In this article, a three-layered structure of Si/SiO
2
+
Si is formed on a substrate, and this layered structure is irradiated with excimer laser beams from both the Si side and the n
+
Si side. The article explained that larger grain diameters can be thus obtained.
The present invention is intended to overcome the above-mentioned disadvantages In the art by providing a laser annealing method capable of providing a crystalline semiconductor film with larger grain diameters, and a laser apparatus to be used in such a laser annealing method.
SUMMARY OF THE INVENTION
In accordance with the present invention, upon crystallization of an amorphous semiconductor film, a top surface (on which a thin film is to be deposited) and a back surface (a surface opposite to the top surface) of the amorphous semiconductor film are simultaneously irradiated with laser beams while an effective energy intensity of the laser beams to be applied onto the top surface (hereinafter referred to as first laser beams) is set at a level different from that of the laser beams to be applied onto the back surface (hereinafter referred to as second laser beams).
More specifically, the irradiation conditions of the laser beams are set so that the effective energy intensity ratio Io′/Io between the effective energy intensity Io of the first laser beams and the effective energy intensity Io′ of the second laser beams satisfies the relationship of 0<Io′/I
o
<1 or 1<I
o
′/I
o
, where the product of I
o
and I
o
′ (I
o
×I
o
′) is not equal to zero.
In the present specification, the term “effective energy intensity” is defined as the energy intensity of the laser beams at the top or back surface of an amorphous semiconductor film while taking energy losses, caused by various reasons such as reflection or the like, into consideration. The unit of the effective energy intensity is the same as that of the energy density, i.e., mJ/cm
2
. Although the effective energy intensity can not be directly measured, it can be calculated based on known parameters such as a reflectance or a transmittance as long as medium present along a path of the laser beams is known.
For example, the calculation method of the effective energy intensity will be described in more detail by taking as an example the case where the present invention is applied to the structure as illustrated in FIG.
6
. In
FIG. 6
, reference numeral
601
denotes a reflector made of aluminum,
602
denotes a Corning #1737 substrate (having a thickness of 0.7 mm),
603
denotes a silicon oxynitride film (hereinbelow referred to as SiON film) having a thickness of 200 nm, and
604
denotes an amorphous silicon film having a thickness of 55 nm. This sample is irradiated with XeCl excimer laser beams having a wavelength of 308 nm in air.
The energy intensity of the excimer laser beams (with the wavelength of 308 nm) immediately before reaching the amorphous silicon film
604
is represented as I
a
. Taking into consideration the reflection of the laser beams at the surface of the amorphous silicon film, the effective energy intensity I
o
of the first laser beams can be expressed as I
o
=I
a
×(1−R
Si
), where R
Si
indicates the reflectance of the laser beams. In this case, I
o
is calculated as 0.45×I
a
.
Furthermore, the effective energy intensity I
o
′ of the second laser beams can be expressed as I
o
′=I
a
×T
1737
×R
Al
×T
1737
×(1−R
SiON-Si
) where T
1737
indicates the transmittance of the #1737 substrate, R
Al
indicates the reflectance at the aluminum surface, and R
SiON-Si
indicates the reflectance experienced by the light beams incident onto the amorphous silicon film from the SiON film. The reflectance experienced by the light beams incident onto the SiON film from air, the transmittance in the SiON film, the reflectance experienced by the light beams incident on the #1737 substrate from the SiON film, and the reflectance experienced by the light beams incident on the SiON film from the #1737 substrate are found to be negligible from the experimental results, and therefore not considered in the calculation. In this case, I
o
′ is calculated as 0.13×I
a
.
Accordingly,
Kasahara Kenji
Kawasaki Ritsuko
Ohtani Hisashi
Tanaka Koichiro
Evans Geoffrey S.
Robinson Eric J.
Robinson Intellectual Property Law Office PC
Semiconductor Energy Laboratory Co,. Ltd.
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