Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Isolation by pn junction only
Reexamination Certificate
2000-08-17
2003-04-15
Abraham, Fetsum (Department: 2826)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Isolation by pn junction only
C257S050000, C257S051000, C257S052000, C257S347000
Reexamination Certificate
active
06548370
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film using laser light (hereafter referred to as laser annealing), and to a laser apparatus for performing laser annealing (an apparatus containing a laser and an optical system for guiding the laser light output from the laser to a processing piece. In addition, the present invention relates to a semiconductor device formed by using that type of laser annealing method, and to a method of manufacturing the semiconductor device.
2. Description of the Related Art
The development of thin film transistors (hereafter referred to as TFTs) has been advancing in recent years, and TFTs using a polycrystalline silicon film (polysilicon film) as a crystalline semiconductor film have been in the spotlight. In particular, the TFTs are used as elements forming a driver circuit for controlling a pixel, or an element which switches the pixel, in a liquid crystal display device (liquid crystal display) or an EL (electroluminescence) display device (EL display).
A technique of crystallizing an amorphous silicon film into a polysilicon film is generally used as a means of obtaining the polysilicon film. In particular, recently a method of crystallizing the amorphous silicon film using laser light has been gathering attention. A means of obtaining a crystalline semiconductor film by crystallizing an amorphous semiconductor film using laser light is referred to as laser crystallization throughout this specification.
Instantaneous heat treatment of the semiconductor film is possible with laser crystallization, and laser crystallization is an effective technique as a means of annealing the semiconductor film formed on a substrate having low heat resistance, such as a glass substrate or a plastic substrate. Furthermore, the throughput is remarkably high compared to a heat treatment means using a conventional electric furnace (hereafter referred to as furnace annealing).
There are many types of laser light, but generally laser crystallization which uses laser light having a pulse emission type excimer laser as an emission source (hereafter referred to as excimer laser light) is employed. The excimer laser has the advantages of high output and being capable of repeated irradiation at a high frequency, and in addition, the excimer laser light has the advantage of having a high absorption coefficient with respect to a silicon film.
The problem drawing the most attention at present is how large can the grain size of a crystalline semiconductor film crystallized by laser light be made. Naturally, if one grain becomes large, then especially the number of grain boundaries crossing a channel forming region of a TFT will be reduced. It therefore becomes possible to improve the electric field effect mobility and the threshold voltage of the TFT, typical electrical characteristics.
Furthermore, relatively clean crystallinity is maintained within each grain, and in order to increase the TFT characteristics as stated above, it is preferable to form the TFT so as to have the channel forming region completely within one grain.
However, it is difficult to obtain a crystalline semiconductor film with a sufficiently large grain size by present techniques, and although there are reports of such films being obtained experimentally, at present this has not reached a level which can be put to practical use.
Experimental results such as those shown in Shimizu, K., Sugiura, O., and Matsumura, M., “High-Mobility Poly-Si Thin-Film Transistors Fabricated by a Novel Excimer Laser Crystallization Method”, IEEE Transactions on Electron Devices, Vol. 40, No. 1, pp. 112-7, 1993, have been obtained. A three layer structure of Si/SiO
2
+
Si is formed on a substrate in the above publication, and is then irradiated by excimer laser light on both the Si layer side and the n
+
Si layer side. It is shown that a large grain size can be achieved by this type of structure.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problems, and an objet of the present invention is to provide a method of laser annealing for obtaining a crystalline semiconductor film having a large grain size, and to provide a laser apparatus which uses the laser annealing method. Further, another object of the present invention is to provide a semiconductor device, and a method of manufacturing the semiconductor device, using the laser annealing method.
The main point of the present invention resides in that laser light is irradiated on both the top surface of an amorphous semiconductor film (the surface on which thin films are formed) and the bottom surface of the amorphous semiconductor film (the surface opposite to the top surface) at the same time when crystallizing the amorphous semiconductor film, and that the effective energy strength of the laser light irradiated on the top surface (hereafter referred to as primary laser light) and the effective energy strength of the laser light irradiated on the bottom surface (hereafter referred to as secondary laser light) differ from each other.
That is to say, when the effective energy strength of the primary laser light is taken as (I
0
), and the effective energy strength of the secondary laser light is taken as (I
0
′), the laser light irradiated is characterized in that a relationship of 0<(I
0
′/I
0
)<1, or a relationship of 1<(I
0
′/I
0
) is formed for the ratio of effective energy strength(I
0
′/I
0
). Of course, I
0
·I
0
′≠0.
Note that, throughout this specification, “effective energy strength” refers to the energy strength of the laser light when it reaches the top surface or the bottom surface of the -amorphous semiconductor film, and is defined as the energy strength after considering energy losses due to things such as reflection (the units are those of density, expressed as mJ/cm
2
). It is not possible to measure the effective energy strength, but provided that the media which exists along the laser light path is understood, the effective energy strength can be obtained by a calculation of the reflectivity and the transmittivity.
For example, a specific calculation method for effective energy strength is explained for the case of implementing the present invention in the structure shown in FIG.
6
. In
FIG. 6
, reference numeral
601
denotes a aluminum reflecting body, reference numeral
602
denotes a Coming Co. #1737 substrate (thickness 0.7 mm),
603
denotes a 200 nm thick silicon oxynitride film (hereafter referred to as an SiON film), and
604
denotes a 55 nm thick amorphous silicon film. An example of irradiating XeCl excimer laser light with a wavelength of 308 nm on this type of test piece in the air is shown.
The energy strength of the laser light (wavelength 308 nm) just before arriving at the amorphous silicon film
604
is taken to be (I
a
). At this point, the effective energy strength of the primary laser light (I
0
) is expressed as I
o
=I
a
(1−R
Si
) in consideration of the laser light reflected on the surface of the amorphous silicon film. Note that R
Si
is the reflectivity of laser light. In this case, I
0
=0.45 I
a
in the calculations.
Further, the effective energy strength of the secondary laser light (I
0
′) is expressed by I
0
′=I
a
T
1737
R
Al
T
1737
(1−R
SiON-Si
) where T
1737
is the transmittivity of the #1737 substrate R
Al
is the reflectivity of the surface of the aluminum, and R
SiON-Si
is the reflectivity when the laser light is incident on the amorphous silicon film from within the SiON film. Note that the reflectivity of the laser light incident on the SiON film from within the air, the transmittivity within the SiON film, the reflectivity when incident on the #1737 substrate from within the SiON film, and the reflectivity when incident on the SiON film from within the #1737 substrate have been shown experimentally to be ignorable, and therefore they are not included in the calculations. In this cas
Kasahara Kenji
Kawasaki Ritsuko
Ohtani Hisashi
Yamazaki Shunpei
Abraham Fetsum
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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