Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-09-30
2002-05-28
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S059000, C257S072000, C257S347000
Reexamination Certificate
active
06396105
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor thin film having a region substantially regarded as a single crystal (hereinafter called “monodomain region”) which is formed on a substrate having an insulating surface, and to a semiconductor device using such a semiconductor thin film as an active layer. In particular, the invention relates to a thin-film transistor which uses a crystalline silicon film as an active layer.
2. Description of the Related Art
In recent years, techniques of forming thin-film transistors (TFTs) by using a silicon semiconductor thin film (thickness: hundreds to thousands of angstrom) formed on a substrate having an insulating surface attracted much attention. The thin-film transistor is widely applied to various electronic devices such as ICs and liquid crystal display devices.
The most important portions, i.e., the heart, of the thin-film transistor are the channel-forming region and the junction portions between the channel-forming region and the source and drain regions. That is, it can be said that the active layer most influences the performance of the thin-film transistor.
An amorphous silicon film formed by plasma CVD or low-pressure thermal CVD is commonly used as a semiconductor thin film for constituting the active layer of a thin-film transistor.
At present, thin-film transistors using an amorphous silicon film are in practical use. However, when higher speed operation is required, a thin-film transistor using a silicon thin film having crystallinity (called a crystalline silicon film) is needed.
Examples of known techniques for forming a crystalline silicon film on a substrate are those described in Japanese Unexamined Patent Publication Nos. Hei. 6-232059 and Hei. 6-244103, which were filed by the present assignee. In the techniques described in these publications, a crystalline silicon film that is superior in crystallinity is formed by a heat treatment of 550° C. and about 4 hours by utilizing a metal element for accelerating crystallization of silicon.
Further, Japanese Unexamined Patent Publication No. Hei. 7-321339 discloses a technique of causing crystal growth approximately parallel with a substrate by utilizing the above-mentioned techniques. The present inventors call this type of crystallized region a lateral growth region.
A lateral growth region formed by the above technique is a collection of columnar or needle-like crystals that are arranged in the same direction, and hence is superior in crystallinity. It is known that a thin-film transistor whose active layer is formed by using this type of region exhibits high performance.
However, the above technique is still insufficient for formation of thin-film transistors to constitute various arithmetic circuits, memory circuits, etc. This is because the crystallinity is still not sufficiently high to provide the necessary characteristics.
For example, peripheral circuits of an active matrix liquid crystal display device or a passive liquid crystal display device include driver circuits for driving pixel TFTs in the pixel area, a circuit handling or controlling a video signal, a storage circuit for storing various types of information, and other circuits.
Among those circuits, the circuit for handling or controlling a video signal and the storage circuit for storing various types of information are required to have performance equivalent to that of an integrated circuit formed on a known single crystal wafer. Therefore, to integrate the above circuits by using a thin-film semiconductor formed on a substrate, it is necessary to form on a substrate a crystalline silicon film whose crystallinity is equivalent to that of a single crystal.
SUMMARY OF THE INVENTION
An object of the invention is to form, on a substrate having an insulating surface, a monodomain region whose crystallinity is equivalent to that of a single crystal. A further object of the invention is to provide a semiconductor device whose active layer is constituted by such a monodomain region.
According to one aspect of the invention, there is provided a semiconductor thin film formed on a substrate having an insulating surface, said semiconductor thin film comprising a monodomain region having crystallinity that has been improved by illumination with laser light or strong light having equivalent energy thereto, the monodomain region being a collection of columnar or needle-like crystals extending generally parallel with the substrate.
According to another aspect of the invention, there is provided a semiconductor device which uses only the above monodomain region as an active layer. The monodomain region has a feature that it has substantially no grain boundaries.
According to a further aspect of the invention, there is provided a semiconductor device manufactured by a process comprising the steps of forming an amorphous silicon film on a substrate having an insulating surface by low-pressure thermal CVD; selectively forming a silicon oxide film on the amorphous silicon film; holding a metal element for accelerating crystallization of silicon adjacent to the amorphous silicon film: performing a heat treatment to convert at least part of the amorphous silicon film into a crystalline silicon film; removing the silicon oxide film; and illuminating the amorphous silicon film and/or the crystalline silicon film with laser light or strong light having equivalent energy thereto, to convert the crystalline silicon film into a monodomain region. The semiconductor device has an active layer that is constituted of only the monodomain region.
The present inventors define, as a monodomain region, a region which is obtained according to the invention by converting a lateral growth region and can substantially be regarded as a single crystal. The monodomain region has features that it contains substantially no grain boundaries and has almost no crystal defects such as dislocations and stacking faults.
“Substantially no grain boundaries” means that grain boundaries are electrically inactive even if they exist. There have been found, as examples of such electrically inactive grain boundaries, a {111} twin crystal grain boundary, a {111} stacking fault, a {221} twin crystal grain boundary, a {221} twist twin grain boundary, etc. (R. Simokawa and Y. Hayashi, Japanese Journal of Applied Physics, Vol. 27, pp. 751-758, 1987).
The inventors consider that it is highly possible that grain boundaries in a monodomain region are electrically inactive grain boundaries as mentioned above. That is, they are considered an inactive region which does not obstruct carrier movement electrically, even though they appear to exist.
The monodomain region, which is the most important concept of the invention, is formed by the following process.
First, as shown in FIG.
1
(A), crystal growth proceeds around a region
101
only in which a metal element has been introduced. The crystal growth proceeds generally parallel with a substrate, to form columnar or needle-like crystals.
The metal element for accelerating crystallization is one or a plurality of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au. Ni (nickel) is used here as an example.
A lateral growth region
102
is formed in the above manner. For example, when a heat treatment is performed at 600° C. for about 6 hours, the lateral growth length (X in FIG.
1
(A)) reaches 100-200 &mgr;m.
As shown in FIG.
1
(A), the resulting lateral growth region
102
is divided into eight portions A-H, which appear as if each were a crystal grain. This is because defects such as slips occur at locations where the portions A-H collide with each other, to form crystal boundaries.
FIG.
1
(B) is a schematic enlarged view showing a part of the portions A-H. As seen from FIG.
1
(B), microscopically each portion of the lateral growth region is a collection of columnar or needle-like crystals. Since the columnar or needle-like crystals cluster together, each portion appears like a single crystal grain macros
Fukunaga Takeshi
Koyama Jun
Miyanaga Akiharu
Yamazaki Shunpei
Fish & Richardson P.C.
Ngo Ngan V.
Semiconductor Energy Laboratory Co,. Ltd.
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